results of la icp ms sulfide mapping from algoma type bif gold systems with implications for the nature of mineralizing fluids metal sources and deposit models quantitative laser ablation inductively coupled plasma mass spectrometry la icp ms element distribution maps combined with traverse mode analyses have been acquired on various sulfides pyrite pyrrhotite arsenopyrite from three canadian algoma type bif hosted gold deposits 4 moz au meadowbank 2 8 moz au meliadine district 6 moz au musselwhite these data in conjunction with detailed petrographic and sem eds observations provide insight into the nature and relative timing of gold events the presence and implication of trace element zoning regarding crystallization processes and elemental associations that fingerprint gold events furthermore the use of an innovative method of processing the la icp ms data in map and traverse modes whereby the results are fragmented into time slice data to generate various binary plots ag versus ni provides a means to identify elemental associations te bi not otherwise apparent this integrated means of treating geochemical data along with petrography allows multiple gold events and remobilization processes to be recognized and their elemental associations determined the main gold event in each of these deposits is characterized by the coupling of an as se te ag element association coincident with intense stratabound sulfide replacement of the fe rich host rock additionally the data indicate presence of a later remobilization event which upgraded the au tenor as either non refractory or refractory type along fracture networks due to the ingress of subsequent base metal bearing metamorphic fluids mainly a pb bi association furthermore the data reveal a stratigraphic influence as reflected in the elemental associations and the elemental enrichments observed and the nature of the sulfide phase hosting the gold mineralization arsenopyrite versus pyrite

new constraints on the timing of early proterozoic tectonism in the black hills south dakota with implications for docking of the wyoming province with laurentia a question regarding the 1900 1600 ma assembly of laurentia is whether the wyoming province west central united states was part of the hearne province west central canada prior to the early proterozoic trans hudson orogeny or a separate entity welded later to a hearne superior continent central canada new 40 ar 39 ar mica dates help to address this question by extending a middle proterozoic geochronologic front long established along the southern wyoming province into the black hills of south dakota this suggests that previously unexplained north directed fold nappes f 1 in the black hills resulted from island arc accretion to the south ca 1780 ma north northwest trending upright f 2 folds which formed during east west collision of the wyoming and superior provinces must therefore be younger new 40 ar 39 ar hornblende dates recently published age data and crustal heat flow considerations further suggest that this collision began at or before ca 1770 ma and culminated with posttectonic magmatism beginning ca 1715 ma the harney peak granite this tectonic magmatic interval is 50 60 m y younger than that reported for the hearne superior collision trans hudson orogeny in canada comparably young metamorphic dates 1810 1710 ma also typify the eastern and northern wyoming province periphery western dakotas and southwestern montana collectively these data suggest that the hearne and wyoming provinces were once separate continents that were ultimately welded to the superior province and to each other during distinct early proterozoic orogenies regional relationships further suggest that final docking of the eastern wyoming province with laurentia began during the ca 1780 1740 ma interval of island arc accretion along the southern margin of the growing craton

re os geochronology of the hitachi volcanogenic massive sulfide deposit the oldest ore deposit in japan the hitachi volcanogenic massive sulfide vms deposit in the hitachi belt is one of the largest cu zn sulfide deposits in japan however its primary formation age has not been precisely determined because regional and contact metamorphism have disturbed some radiometric isotope systems e g k ar ages on wall rocks further metamorphism has greatly reduced the fossil record which has limited the use of biostratigraphy to establish the ore deposit age we applied re os isotope dating to samples from two representative sulfide deposits of the greater hitachi vms deposit the fudotaki and fujimi deposits re os isotope data from the fudotaki deposit yields a model 3 age of 533 13 ma with an initial 187 os 188 os ratio of 0 35 0 20 in contrast re os isotope data from the fujimi sulfide ore did not exhibit a linear correlation suggesting that the re os isotope system there has been disturbed by regional metamorphism of higher grade than the fudotaki deposit as well as contact metamorphism during intrusion of cretaceous granitic rocks although the fudotaki sulfide ores have been metamorphosed to lower epidote amphibolite facies the well defined re os isochron allowed us to interpret the timing of primary sulfide deposition on the paleoseafloor some orebodies of the greater hitachi vms deposit have been estimated to be early carboniferous based on the scarce fossil records and considered to be distributed in the daioin formation but our results revealed that the fudotaki deposit were formed during the cambrian and occurs in the upper akazawa formation which strongly supports the idea that the cambrian beds are widely distributed in the hitachi belt and that a significant unconformity exists between the cambrian and early carboniferous above the fudotaki deposit the cambrian re os date classifies the hitachi vms deposit as the oldest dated ore deposit in japan and in turn provides an insight to understand the geologic framework and genesis of the proto japanese islands and the east asian region

microprobe monazite geochronology understanding geologic processes by integrating composition and chronology abstractmonazite is a light rare earth element lree bearing phosphate mineral that is present in a wide variety of rock types has an extremely variable composition reflecting host rock conditions and is a robust geochronometer that can preserve crystallization ages through a long history of geological events monazite crystals typically contain distinct compositional domains that represent successive generations of monazite which in turn can provide a detailed record of the geologic history of its host rocks the electron microprobe can be used to characterize the geometry of compositional domains analyze the composition of each domain and when carefully configured determine the u th total pb age for domains as small as 5 m in width these data allow the monazite to be linked with and place timing constraints on silicate processes in the host rocks current applications span a broad range of geologic processes in igneous metamorphic hydrothermal and sedimentary rocks

lead isotope and trace element geochemistry of paleoproterozoic metasedimentary rocks in the lead and rochford basins black hills south dakota usa implications for genetic models mineralization ages and sources of leads in the homestake gold deposit abstract mixed silicate carbonate sulfide facies banded iron formations with elevated detrital components were deposited in the northern black hills during middle paleoproterozoic time between 2 012 and 1 974 ga homestake iron formation and equivalents and at 1 887 ga rochford iron formation these iron formations deposited atop shales and basalts in intracontinental rifted basins separated in time by 80 130 myr are the hosts of extensive late paleoproterozoic gold mineralization gold sulfide ore bodies occur within dilated segments of late stage ductile brittle shears that formed after isoclinal folding related to wyoming superior continental collision lead stepwise leaching pbsl data for monazite bearing garnet separated from a sample of homestake iron formation has yielded an isochron age of 1746 10 ma 2 mswd 0 42 which represents a maximum age for both the isoclinal folding and subsequent gold mineralization monazite bearing garnet from a nearby harney peak type granitic pegmatite believed to be coeval with latest stage semibrittle shears and quartz veins that crosscut homestake ore bodies has yielded a pbsl isochron age of 1713 10 ma 2 mswd 4 1 which probably represents a minimum age for the gold event pb isotopes of sulfides from replacement ore and shear ore within the homestake iron formation cannot distinguish between these distinct ore types however both pb isotopes and rey rare earth and yttrium abundances indicate slight disequilibrium between the ore sulfides and the barren iron formation thus revealing an epigenetic sensu stricto component to the gold mineralization the pb isotope characteristics of the ore further reflect an admixed component with low 232th 204pb and variable th u ratios in addition to compositions directly derived from bif typical leads this added component corresponds to pb from the anoxic metasedimentary rocks directly underlying the iron formations and from the nearby pegmatitic granite the apparent pb isotopic match between ore and granite is further consistent with the pbsl data for shear ore type arsenopyrite which define an isochron age of 1719 45 38 ma 2 mswd 0 2 and by a striking overlap of the pb isotopic data fields defined by the homestake galenas and pegmatitic feldspars the precambrian ore leads are therefore best explained as mixtures of magmatic hydrothermal and host rock mobilized leads the lead area also contains significant gold bearing deposits of tertiary age which are hosted in both paleoproterozoic and phanerozoic rocks tertiary mineralization was temporally and spatially related to coeval plutons emplaced during laramide uplift of the black hills and represents the final mineralization event recognized in the homestake mine area like the precambrian galena leads the leads of tertiary galena hosted by middle paleoproterozoic rocks including the homestake iron formation define a trend best explained by mixtures of pb derived from magmatic fluids exsolved from tertiary intrusives and pb leached from the same country rocks that contaminated the granite derived hydrothermal fluids ascending through them during the much earlier shearing and mineralization event inferred at 1730 1715 ma

concentric slow cooling of a low p high t terrane evidence from 1600 1300 ma mica dates in the 1780 1700 ma black hills orogen south dakota u s a the crystalline core of the southern black hills south dakota exposes an extensive low p high t aureole of garnet to second sillimanite zone schists centered on the plutonic core of the 1715 ma harney peak granite hpg this paper demonstrates regional patterns of apparent ages observed for 52 40 ar 39 ar dates of muscovite and biotite in diverse rocks from across the 1000 km 2 metamorphic aureole and its plutonic center about 20 biotite dates sampled mostly near faults are influenced by excess 40 ar and obscure the regional trends the remaining mica dates reveal radial patterns of apparent younging from outer aureole toward inner granite with previously unrecognized elliptical age zones centered on the main hpg pluton and its outliers the regional pattern of 40 ar 39 ar cooling ages indicates non uniform slow cooling of the mid crust between 1600 1250 ma this scenario of delayed slow cooling from aureole to pluton is consistent with published cooling ages for muscovite rb sr and apatite u pb which range from 1690 to 1550 ma and from 1700 to 1500 ma respectively to explain these results it is likely that ambient pre granite temperatures of the country rocks were 350 c at the 10 14 km depth of granite emplacement as previously proposed and that the entire complex resided at this depth and cooled slowly from aureole to granite for hundreds of millions of years alternatively or in addition the hpg and inner aureole were not uplifted until 1480 1330 ma whereupon they finally cooled through 300 350 c

geological and isotopic geochemical constraints on the evolution of the fuping complex north china craton abstract the neoarchean to paleoproterozoic fuping complex together with the adjacent hengshan and wutaishan complexes form the middle part of the central zone that separates the north china craton into the eastern and western archean continental blocks the fuping complex comprises three major lithological assemblages the 2 50 ga fuping trondhjemite tonalite granodiorite ttg gneisses the wanzi paragneisses and the 2 08 and 2 02 ga nanying granitic gneisses the ttg gneisses contain abundant enclaves of highly metamorphosed supracrustal rocks including 2 70 ga mafic granulites and amphibolites with endt of 3 29 to 3 96 and tdm of 2 72 2 82 ga and are characterized by tdm of 2 76 3 04 ga and a linear correlation between endt 1 64 to 0 96 and 1 nd indicative of mixing of melts derived from the 2 70 ga metabasic rocks and 3 0 ga crustal materials mafic dikes along the chengnanzhuang shear zone were most likely emplaced at 2 31 ga and have endt of 2 24 to 4 78 and tdm of 2 34 2 59 ga the nanying granitic gneisses with endt of 4 04 to 5 24 were most likely related to partial melting of the 2 70 ga supracrustal rocks and or the 2 50 ga ttg gneisses these geological relationships and isotopic data suggest that the fuping complex probably formed from the following five major tectonomagmatic events 1 mafic magmatism and early crustal growth at 2 7 ga with the possible existence of older 3 0 ga crust s 2 collision related crustal thickening underplating of basaltic magma high grade metamorphism crustal anatexis and ttg magmatism at 2 5 ga 3 extensional deformation and emplacement of the mafic dikes and the nanying granitic plutons along the chengnanzhuang shear zone 2 5 to 2 0 ga 4 s type granitic magmatism and associated amphibolite facies metamorphism 2 0 to 1 80 ga and 5 emplacement of granitic pegmatite dikes at 1 80 ga

derivation of isotope ratios errors and error correlations for u pb geochronology using 205pb 235u 233u spiked isotope dilution thermal ionization mass spectrometric data a comprehensive treatment of the derivation of u pb isotope ratios and their corresponding uncertainties from isotope dilution thermal ionization mass spectrometric measurements is presented standard parametric statistical methods of error propagation are utilized to convolve uncertainties associated with instrumental mass fractionation tracer subtraction blank pb and u subtraction and initial common pb correction derivations include errors and error correlations for total sample u pb and pb isotope ratios including radiogenic and initial common pb for two and three dimensional isochron calculations radiogenic u pb and pb isotope ratios for concordia and radiogenic model age calculations and the propagation of model age errors from radiogenic isotope ratios

step leach pb pb dating of inclusion bearing garnet and staurolite with implications for early proterozoic tectonism in the black hills collisional orogen south dakota united states we report 207 pb 206 pb stepwise leaching pbsl ages of 1762 15 ma 1759 8 ma and 1760 7 ma for almandine garnet staurolite and garnet staurolite combined in an early proterozoic metapelite from the black hills collisional orogen south dakota pbsl derived 208 pb 206 pb th u trends reveal that staurolite contains inclusions of both cogenetic monazite ca 1760 7 ma and older detrital zircon 2040 ma whereas coexisting garnet contains only the zircon these results and petrologic data indicate prograde sequential growth of garnet monazite and staurolite during tectonic burial at temperatures between 400 c and 550 c i e well below pb closure temperatures we interpret the 1760 7 ma date as a maximum age for wyoming superior continental collision in the black hills which apparently postdated the 1800 1900 ma trans hudson orogeny in canada but was nearly synchronous with north directed accretion of the central plains orogen in southeastern wyoming

paleoproterozoic arc continent collision in the north china craton evidence from the zanhuang complex abstract the trans north china orogen tnco marks the major ca 300 km wide paleoproterozoic collisional suture between the eastern and western blocks of the north china craton ncc which also incorporates several microblocks or complexes here we investigate the zanhuang complex along the eastern margin of tnco we present petrological geochemical zircon u pb and lu hf data on a suite of quartz mica schist metavolcanics and diorite from the central zanhuang domain czd as well as granitoids and amphibolite that constitute the basement rocks of the western zanhuang domian wzd and eastern zanhuang domain ezd our study represents a cross section across two continental margins and covers the paleo ocean tract in between the metavolcanics show calc alkaline to tholeiitic affinity and negative correlations of la sm n vs p nd ti ti ce ce and positive with nb nb the granitoids fall within the vag field the iab and morb features of the volcanic suite are consistent with magma derivation in arc to back arc environment from an enrichment source the magmatic suite also displays lree enrichment positive rb k la pb nd anomalies and negative ta nb ce ti anomalies la cipms u pb geochronology of detrital zircon grains shows two age populations in the quartz mica schist with 207pb 206pb mean ages of 2499 14 ma and 2252 22 2216 39 ma and the timing of deposition is estimated as ca 2 3 2 2 ga zircons from the granodioritic gneiss yield 207pb 206pb mean age of 2508 14 ma representing neoarchean continental basement the metavolcanics and two diorite samples yield ages of 2302 30 ma 2089 63 ma marking the timing of an active arc the metamorphic zircons from these rocks show an age of 1882 15 ma the zircon lu hf data yield both positive and negative ehf t values suggesting depleted mantle source mixed with crustal components their crustal residence ages t dm c range from 3875 to 2224 ma and depleted model ages t dm range from 3415 to 2092 ma suggesting mixed juvenile and reworked crustal components and possible vestiges of mesoarchean basement we propose that the zanhuang complex is one of the paleoproterozoic arcs within the tnco which was accreted to the neoarchean continent along the eastern block of the ncc during the suturing along the tnco the tectonics associated with crustal growth and recycling in the tnco and the arc continent collision broadly coincided with the building of the columbia supercontinent

geochemistry and u pb zircon ages of metamorphic volcanic rocks of the paleoproterozoic luliang complex and constraints on the evolution of the trans north china orogen north china craton abstract the paleoproterozoic luliang complex is situated in the central part of the western margin of the trans north china orogen and consists of volcanic rocks sedimentary rocks and paleoproterozoic granitoid intrusions the volcanic rocks and earlier granitoid rocks were strongly deformed and metamorphosed into the greenschist to amphibolite facies these metamorphosed volcanic rocks are dominated by basalts to basaltic andesites the parental mafic magmas of these metamorphosed volcanic rocks were mainly derived from the 5 to 30 partial melting of spinel lherzolites to spinel garnet lherzolites which had been enriched by the subduction melts mafic magma experienced fractional crystallization and crustal assimilation u pb zircon dating on two metamorphosed volcanic rocks from the yejishan and luliang groups reveals that they formed at 2210 13 ma and 2213 47 ma respectively and were metamorphosed at 1832 ma this suggests that the metamorphosed volcanic rocks in the yejishan and luliang groups formed synchronously in the paleoproterozoic these new ages integrated with recently reported u pb zircon ages for the jiehekou group and paleoproterozoic granitoids suggest that all of the lithological assemblages of the luliang complex formed and were metamorphosed in the paleoproterozoic not in the neoarchean petrological geochronological and geochemical data suggest that the geodynamic evolution of the paleoproterozoic luliang complex was involved in the development of a magmatic arc system at an active continental margin generating widespread arc related magmatism at 2 2 ga the luliang complex then underwent intense deformation and metamorphism and was incorporated into the trans north china orogen during the 1 88 1 83 ga collisional event which was followed by post collision extension at 1 80 ga

high closure temperatures of the u pb system in large apatites from the tin mountain pegmatite black hills south dakota usa abstract the cores of several large 1 2 cm apatites from the early proterozoic tin mountain pegmatite black hills south dakota have concordant u pb ages the age uncertainties of these samples are less than 5 ma the lead isotopic compositions of muscovite and k feldspar coexisting with the apatites were used for initial lead isotopic corrections the resulting u pb concordia ages agree well with u pb and pb pb isochron ages based solely on multiple fragments from single apatites this agreement justifies the use of coexisting muscovite and k feldspar for initial lead corrections the precision of these u pb ages is comparable to those of the u pb systems of more commonly analyzed minerals such as zircon monazite and sphene the estimated u pb closure temperature t c of these apatites is about 620 c this t c is in close agreement with the t c calculated for 0 5 cm diffusion radii and published data for diffusivity and activation energy of lead diffusion in apatite apatite can therefore yield high precision u pb ages that are close to its crystallization age in igneous rocks

the remarkable re os chronometer in molybdenite how and why it works the re os rhenium osmium chronometer applied to molybdenite mos2 is now demonstrated to be remarkably robust surviving intense deformation and high grade thermal metamorphism successful dating of molybdenite is dependent on proper preparation of the mineral separate and analysis of a critical quantity of molybdenite unique to each sample such that recognized spatial decoupling of 187re parent and 187os daughter within individual molybdenite crystals is overcome highly precise accurate and reproducible age results are derived through isotope dilution and negative thermal ion mass spectrometry id ntims spatial decoupling of parent daughter precludes use of the laser ablation icp ms microanalytical technique for re os dating of molybdenite the use of a reference or control sample is necessary to establish laboratory credibility and for interlaboratory comparisons the rb sr k ar and 40ar 39ar chronometers are susceptible to chemical and thermal disturbance particularly in terranes that have experienced subsequent episodes of hydrothermal magmatic activity and therefore should not be used as a basis for establishing accuracy in re os dating of molybdenite as has been done in the past re os ages for molybdenite are almost always in agreement with observed geological relationships and when available with zircon and titanite u pb ages for terranes experiencing multiple episodes of metamorphism and deformation molybdenite is not complicated by overgrowths as is common for some minerals used in u pb dating e g zircon monazite xenotime nor are re and os mobilized beyond the margins of individual crystals during solid state recrystallization moreover inheritance of older molybdenite cores incorporation of common os and radiogenic os loss are exceedingly rare whereas inheritance common pb and pb loss are common complications in u pb dating techniques therefore molybdenite ages may serve as point in time markers for age comparisons

re os geochronology and systematics in molybdenite from the endako porphyry molybdenum deposit british columbia canada precise re os molybdenite ages for the endako porphyry molybdenum deposit and the nithi mountain occurrence british columbia canada yield new information on the timing of molybdenite deposition three episodes of mineralization are indicated 1 154 ma 2 148 to 146 ma and 3 145 ma re os dates agree well with new u pb and 40 ar 39 ar data from granitoids of the nithi phase 155 2 1 5 ma the endako phase 148 4 1 5 ma and a preore dike 147 4 0 6 ma and the casey phase 145 1 0 2 ma plus hydrothermal biotite 144 5 0 75 ma villeneuve et al 2001 the excellent correlation between the re os dates from molybdenite and the ages of granitoids from the area indicates a direct genetic relationship between these plutons and molybdenite mineralization these re os data also provide further insight into the systematics and robustness of re os in molybdenite the endako molybdenum deposit is the product of two episodes of mineralization both associated with moderate salinity 5 15 wt nacl equiv and high temperature 440 c fluids a third low temperature 190 300 c hydrothermal event is associated with eocene faulting selby et al 2000 the clear distinction between two of the re os molybdenite dates in the deposit and the close agreement of these dates to the ages of granitoid intrusions suggest that the re os system in molybdenite is not reset by high temperature 440 c moderate salinity hydrothermal fluids in contrast magmatic biotite in a 147 ma preore dike has a 40 ar 39 ar age of 145 ma identical to the age of the hydrothermal biotite indicating that 440 c fluids reset the ar ar system in biotite hence we interpret that the re os isotope system in molybdenite is resistant to resetting by high temperature fluids 440 c in contrast to the 40 ar 39 ar system in biotite the re os system in molybdenite is also unaffected by postore hydrothermal fluids these data show that molybdenite can provide critical information in understanding sulfide mineral deposition in ore systems which is not recorded by other isotopic systems in alteration minerals

shrimp u pb zircon ages of granitoid rocks in the luliang complex implications for the accretion and evolution of the trans north china orogen abstract the luliang complex is situated in the central segment of the trans north china orogen tnco a continent continent collisional belt along which the discrete eastern and western blocks amalgamated to form the basement of the north china craton the complex consists of supracrustal and granitoid rocks of which the latter can be subdivided into pre tectonic ttg gneisses including the yunzhongshan guojiazhuang and chijianling guandishan gneisses syn tectonic gneissic granites represented by the huijiazhuang gneissic granite and post tectonic granites including the luyashan charnockite luchaogou porphyritic granite and tangershang guandishan massive granite the pre tectonic ttg gneisses are mostly calc alkaline and considered to have formed in a magmatic arc environment shrimp u pb analyses reveal that the yunzhongshan gneisses were emplaced at 2499 9 ma representing the earliest arc related magmatic event in the luliang complex this was followed by the intrusion of the guojiazhuang gneisses at 2375 10 ma the most widespread arc related magmatic event in the region was the emplacement of the chijianling guandishan granitoid gneisses of which the tonalitic granodioritic and monzogranitic gneisses were emplaced at 2199 11 ma 2180 7 ma and 2173 7 ma respectively metamorphic zircon overgrowth rims from a tonalitic gneiss yielded a weighted mean 207 pb 206 pb age of 1872 7 ma consistent with the metamorphic age range of 1880 1820 ma defined by metamorphic zircons from various high grade rocks in the tnco thus the evolution of the luliang complex involved emplacement of the yunzhongshan granitoids at 2499 ma the guajiazhuang granitoids at 2375 ma and the chijianling guandishan granitoids at 2199 2173 ma with the final collision between the eastern and western blocks in this area occurring at 1872 7 ma the whole series of magmatic events lasting nearly 650 ma this suggests that the trans north china orogen represents a long lived magmatic arc the new shrimp zircon data combined with structural and petrological studies also reveal the existence of both syn and post collisional granites in the luliang complex the former is represented by the huijiazhuang gneissic granite that was emplaced at 1832 11 ma whereas the latter include the 1815 5 ma luyashan charnockite 1807 10 ma luchaogou porphyritic granite 1798 11 ma guandishan massive granite and 1790 14 ma tangershan massive granite

paleoproterozoic rifting of the north china craton geochemical and zircon hf isotopic evidence from the 2137 ma huangjinshan a type granite porphyry in the wutai area abstract shrimp zircon u pb dating of the huangjinshan granite porphyry in the wutai area north china craton ncc yielded concordant ages with a weighted mean 207 pb 206 pb age of 2137 9 ma which indicates that the granite was emplaced in the middle paleoproterozoic the granite porphyry is enriched in sio 2 rees except eu zr nb ga y and zn and depleted in mgo cao al 2 o 3 sr ba v cr and ni with high feo mgo ratios with the zircon saturation temperatures ranging from 856 to 895 c and a lack on detected inherited zircon this indicates it is a hot a type granite of the type emplaced in anorogenic or extensional settings with high heat flow zircons in the granite porphyry have e hf t values ranging from 1 72 to 1 84 and hf model t dm1 ages of 2429 2554 ma the hf isotopes indicate source of the granites was pre neoarchean continental crust mixed with mafic rocks derived from depleted mantle or directly from the partial melting of ca 2 5 ga crust given that there are no syn plutonic gabbro enclaves in the huangjinshan granite porphyry it is preferred that it originated mainly from partial melting of ca 2 5 ga upper crust without mixing with mantle derived magma 2 2 2 0 ga volcanic and plutonic rocks which exhibit bimodal magma characteristics emplacing in rifting setting are abundant in the central ncc the results presented here are further evidence for that 2 2 2 0 ga is an important stage of paleoproterozoic rifting in the central ncc

shrimp u pb zircon geochronology of the fuping complex implications for formation and assembly of the north china craton abstract the fuping complex located within the central zone of the north china craton is composed of amphibolite to granulite facies orthogneisses interleaved with minor supracrustal rocks at similar metamorphic grade the oldest components recognised are hornblende gneiss enclaves within the predominant biotite orthogneiss which have a shrimp u pb zircon age of 2708 8 ma we consider these enclaves to represent fragments of 2 7 ga continental materials incorporated in the biotite gneiss the biotite gneiss has a shrimp u pb zircon age of 2513 12 ma interpreted to be time of magmatic crystallisation of the igneous precursor based on the igneous characteristics of the zircons this indicates a major magmatic episode at 2 52 ga in the fuping complex identical to the age of felsic volcanism within the low grade wutai complex which crops out immediately to the west a gneissic granite that intrudes the biotite gneisses has a poorly defined 207pb 206pb age of 2045 64 ma this is within error of the age of 2097 46 ma obtained from a fine grained gneiss interlayered with amphibolite of the wanzi supracrustal suite wss interpreted to be volcanic in origin zircons from both these samples have strong oscillatory zoning and provide the first indication of a palaeoproterozoic magmatic event in the area again similar in age to magmatic events recently recognised in the adjacent wutai complex these data indicate a comparable history for the fuping and wutai complexes and support geochemical evidence that they had a common origin and formed part of a late archaean arc affected by later palaeoproterozoic re activation low uranium zircons without oscillatory zoning separated from a sample of biotite gneiss yield data clustered at 1817 26 ma which is interpreted to reflect a period of new zircon growth during a major metamorphic event these data support the recently proposed tectonic model that amalgamation of the north china craton occurred due to collision of the eastern and western blocks along the central zone at 1 8 ga ago

petrogenesis and tectonic implications of the 2 1 2 0 ga granitoids in fuping complex north china craton constraints from petrology geochemistry and zircon u pb hf isotopes abstract mid paleoproterozoic 2 2 2 0 ga magmatism in the trans north china orogen tnco is of great significance for the paleoproterozoic geological evolution of the entire north china craton ncc in this paper we present geochemical shrimp zircon u pb and hf isotopic data on the 2 1 2 0 ga granitoids in the fuping complex of the tnco these include the nanying granitic gneiss which yielded ages of 2075 13 ma 2079 9 ma and 2073 10 ma the gangnan granitic gneiss dated at 2081 8 ma 2082 14 ma and 2063 7 ma the baiyangling granite with ages of 2077 6 ma 2070 19 ma and 2069 12 ma and the shangzhuang granite which yielded ages of 2072 12 ma and 2070 12 ma the 2 1 2 0 ga granitic rocks in fuping complex are enriched in sio2 k2o na2o rare earth elements rees except eu high field strength elements like zr nb ga and y and depleted in cao al2o3 and sr their ga al ratios and absolute zr nb y ce contents are high and their average zr saturation temperatures are as high as 890 c all of which are consistent with the features of a type granite the ehf t values of zircon in all samples are much lower than that of the depleted mantle at the same stage while hf model tdm1 tdmc ages have ranges of 2 3 2 7 ga and 2 4 2 9 ga respectively which are much older than the age of the granitoids these zircon hf model ages are similar to those of the neoarchean ttg tonalite trondhjemite granodiorite gneisses in the fuping complex therefore based on a combination of petrology geochemistry and isotopic characteristics we infer that these four 2 1 2 0 ga granitic plutons in the fuping complex are partial melts of the neoarchean ttg gneiss generated in an extensional environment based on extensive data the 2 1 2 0 ga a type granites and the mafic dikes in the fuping complex represent a bimodal magma association considering the sedimentary rocks of the gantaohe group deposited in the same period we deduce that the 2 1 2 0 ga a type granite in the tnco fuping complex was emplaced in an intra continental rift setting in the ncc this evidence indicates that the north china craton was initially stabilized at the end of the neoarchean

age and depositional setting of the paleoproterozoic gantaohe group in zanhuang complex constraints from zircon u pb ages and hf isotopes of sandstones and dacite abstract the gantaohe group located in the zanhuang area contains typical north china craton paleoproterozoic strata and is one of the best preserved examples of these rocks it is therefore a significant area for deciphering evolutionary process in the trans north china orogen in this paper we present petrologic detrital and volcanic zircon u pb ages and lu hf isotopes from three formations of the gantaohe group the zircons from a conglomerate at the base of the gantaohe group yielded a young population of ca 2 1 ga dacite from the upper section of the nansizhang formation yielded zircon 207 pb 206 pb ages of 2090 14 ma thus the initial deposition of the gantaohe group occurred at 2 1 ga the group underwent greenschist to low amphibolite facies metamorphism identical to regional 1 87 1 85 ga metamorphism in zanhuang complex therefore we propose that the gantaohe group was deposited in the period between 2 1 ga and 1 9 ga sandstones in the gantaohe group are dominated by q monocrystalline and polycrystalline quartz and f k feldspar and plagioclase with only minor lithic fragments suggesting that detrital components were mainly derived from a cratonic source detrital zircons from sandstones exhibit a dominant population of 2 55 2 5 ga grains with minor contributions of 2 7 ga and 2 1 ga grains which further indicate they were dominantly sourced from 2 5 ga ttg gneisses late archean zanhuang and late archean to paleoproterozoic guandu groups with subordinately input from 2 7 ga ttg gneisses and the paleoproterozoic xuting granite given that the sandstone clastic components chiefly comprise quartz and feldspar there is a lack of volcanic lithics and the major detrital zircons are 400 ma older than the initial deposition age we propose that the gantaohe group was deposited in a rift related setting far from any back arc basin at 2 1 ga or 1880 1810 ma syn collisional processes a few 2 5 2 7 ga zircons have positive e hf t values identical with the contemporaneous depleted mantle and the 207 pb 206 pb ages are similar to their single stage model ages t dm1 which suggests some degree of 2 5 2 7 ga crustal growth most 2 5 zircons however show 207 pb 206 pb ages that are much younger than the hf single stage model ages which indicates intensive crustal reworking in the late neoarchean the zircons collectively present two stage model ages ranging from 2 6 to 2 9 ga therefore we suggest that the 2 6 2 9 ga periods represents a time of intensive crustal growth in the zanhuang complex

spatial distribution of 1950 1800 ma metamorphic events in the north china craton implications for tectonic subdivision of the craton abstract great progress has been made in the last twenty years in understanding the unification of the north china craton according to a prevailing model it developed by amalgamation of four sub blocks by three spatially and temporally separate orogenic belts the first at 1950 ma formed the western block the second at 1900 ma created the eastern block and the third at 1850 ma unified the eastern and western blocks however new data show that the 1950 ma orogen was affected by granulite facies reequilibration at 1800 ma and parts of the 1850 ma orogen have evidence of metamorphism at 1950 1900 ma new zircon u pb ages enable the paleoproterozoic metamorphic events to be statistically sub divided into four at 1955 ma m 1 1920 ma m 2 1885 ma m 3 and 1850 ma m 4 the m 1 2 events always appear together mainly in two belt areas including the helanshan qianlishan northern ordos yinshan liangcheng huai an lushan jiaobei and liaodong regions whereas the m 3 4 events are distributed over larger areas and were superimposed on the m 1 2 activities this age distribution is inconsistent with previous models of three separate orogens it should also be noted that the igneous rocks contemporary with the regional ultra high temperature high pressure granulite facies metamorphisms m 1 2 were previously described as arc related series i e the gabbro norites i s type granites and trimodal volcanics based on the spatial distribution of the paleoproterozoic metamorphic events m 1 4 and of the coeval igneous rocks we speculate that the assembly of the ncc was created by one orogen which formed between the eastern and western cratons in two tectonic stages two marginal arcs the korean and xuwujia developed against both eastern sides of the two sub cratons and were both accreted and deformed at 1965 1900 ma m 1 2 subsequently the amalgamation of the two sub cratons resulted in metamorphism on the western side of the eastern craton and in metamorphism that was superimposed on both accreted arcs at 1900 1790 ma m 3 4

temporal evolution of mineralization events in the bohemian massif inferred from the re os geochronology of molybdenite molybdenite is a common mineral accompanying sn w au and base metal mineralizations located in different geotectonic units of the bohemian massif but it is also widespread in granitoids and or related quartz veins pegmatites forming disseminated mo mineralization thirty re os ages were obtained for molybdenite samples from the bohemian massif to provide constraints on the timing and duration of mineralization event s within the framework of previously published geochronological data for the host and or associated rocks the obtained data for sn w li deposits in the erzgebirge metallogenetic province indicate the predominance of one and or multiple short time mineralization events taking place between 319 and 323 ma with the exception of the krupka deposit associated with the altenberg teplice caldera where the data may suggest prolonged activity until 315 ma the ages of the pb zn au mo h rky u rakovnika and fe cu as ob i d l mineralizations from the exocontacts of the cista pluton and krkonose jizera plutonic complex respectively provide evidence for synchronous emplacement of the ore and the associated granitic rocks in contrast the padr fe as mo mineralization postdates the age of the associated padr granite disseminated mo mineralization in cadomian and variscan granitoids and or related to quartz veins pegmatites provides re os ages that overlap with the previously published geochronological data for the host rocks suggesting coeval evolution molybdenite samples from the sazava suite granites of the central bohemian plutonic complex cbpc have resolvable younger ages than their host granites but similar to the age of spatially related au mineralization which is associated with the latest evolution of the cbpc

diffusion of osmium in pyrrhotite and pyrite implications for closure of the re os isotopic system abstract in order to better constrain the extent to which common sulfide minerals will retain their osmium isotopic composition subsequent to crystallization we have conducted experiments to quantify the diffusion behavior of osmium in pyrite and pyrrhotite experiments consisted of either 1 isothermal soaking of diffusion couples consisting of natural pyrite or pyrrhotite crystals packed against powdered os bearing fe sulfide or 2 relaxation of initially high near surface osmium concentrations produced in the latter experiments pyrite only osmium penetration into samples was characterized by depth profiling using rutherford backscattering spectroscopy rbs pyrite or electron microprobe analyses across sectioned run products pyrrhotite results of the first type of diffusion experiment involving pyrite show only limited osmium penetration into sample surfaces with the extent of penetration uncorrelated with run duration images of pyrite samples using atomic force microscopy show roughening of initially smooth surfaces as a consequence of step formation and suggest that osmium incorporation into the near surface occurred by solute uptake during step growth and not by volume diffusion prolonged 1000 h relaxation experiments revealed no additional osmium penetration into pyrite surfaces and based on the depth resolution for rbs a maximum diffusion coefficient of 2 5 10 23 m2 s at 500 c was calculated experiments involving pyrrhotite over the temperature range of 950 1100 c showed extensive osmium uptake and osmium concentration gradients that conform with fickian diffusion behavior we found that pyrrhotite fe s could be varied by changes in the composition of the starting material and osmium source and over the range of fe s produced in experiments molar fe s 0 83 0 90 we observed no systematic variation in the osmium diffusion coefficient diffusion coefficients measured parallel to the a crystallographic axis were on average 1 4 higher than values measured parallel to c and regression of the c axis data yielded the arrhenius relation d t 1 31 0 16 10 5 m 2 s exp 211 5 14 7 kj mol rt the application of these diffusion data to simple models of diffusive exchange during static or polythermal time temperature histories is used to assess the conditions under which radiogenic osmium will be retained during isothermal annealing calculations indicate that the cores of millimeter sized spherical pyrrhotite crystals undergoing diffusive exchange with an external osmium reservoir will have their initial compositions perturbed in 0 5 ma at temperatures exceeding 400 c pyrite undergoing the same process at 500 c requires in excess of 10 ma before crystal cores are affected the relatively short core retention time scales for pyrrhotite indicates that this mineral may be prone to isotopic resetting following relatively brief crustal thermal events thus possibly accounting for the scatter that commonly occurs in re os isochrons generated from massive sulfide samples calculated closure temperatures tc for osmium exchange in pyrrhotite yielded values of 300 400 c for grain sizes ranging from 10 to 1000 m these values of tc are similar to those calculated for ar retention in biotite and considerably lower than for sr in apatite and plagioclase for example such low closure temperatures for pyrrhotite suggest this mineral will date the final stage in the cooling of a magmatic system and possibly be susceptible to open system osmium exchange in the presence of late stage hydrothermal fluids this latter result infers that caution be applied when interpreting elevated initial osmium isotopic ratios as a product of crustal assimilation at the magmatic stage

origin of a mesozoic granite with a type characteristics from the north china craton highly fractionated from i type magmas we report geochronological geochemical and isotopic data for the mesozoic shangshuiquan granite from the northern margin of the north china craton the granite is highly fractionated with sio2 74 occurrence of annitic biotite high contents of alkalis k2o na2o rb y nb and heavy rare earth elements high feot mgo low contents of cao al2o3 ba and sr and large negative eu anomalies makes it indistinguishable from typical a type granites a mantle derived origin for the rocks of the granite is not favored because their high initial 87sr 86sr 0 706 and low end t 15 are completely different from either those of the lithospheric or asthenospheric mantle in fact their sr nd isotopes fall within the range of sr nd isotopic compositions of the archean granulite terrains and are comparable to those of mesozoic crustal derived i type granitoids in the region therefore the shangshuiquan granite is considered to be dominantly derived from partial melting of the ancient lower crust its parental magmas prove to be similar to i type magmas and to have undergone extensive fractionation during its ascent this is supported by the fact that some of the nearby hannuoba feldspar rich granulite xenoliths can be indeed regarded as the early cumulates in terms of their mineralogy chemistry sr nd isotopes and zircon u pb ages and hf isotopes it is furthermore argued that some of highly fractionated granites worldwide especially those with a type characteristics and lacking close relationship with unfractionated rocks may in fact be fractionated i type granites this suggestion can explain their close temporal and spatial associations as well as similar sr nd isotopes with i type granites our study also sheds new light on the petrogenesis of deep crustal xenoliths

characterization and origin of aluminous a type granites from the lachlan fold belt southeastern australia the metaluminous to weakly peraluminous a type granites of the lachlan fold belt are a distinctive group of igneous rocks on the basis of chemical and mineralogical criteria those granites that contain 65 72 sio2 can be distinguished from other types on the basis of higher abundances of fetotal fetotal mg high field strength elements trivalent rare earth elements ga and zn mineralogically they contain fe rich hydrous mafic minerals and primary ilmenite and hence are reduced relative to the ni nio buffer however the extremely felsic a type granites sio2 72 have the same chemical and mineralogical characteristics as felsic fractionated i type granites recent analyses indicate that the lachlan fold belt a type granites have sc f alkali element trace transition element and h2o contents similar to those of other unfractionated i type granites rb sr and nd sm isotopic compositions are highly variable probably reflecting source region heterogeneity the metaluminous to weakly peraluminous a type granites of the lachlan fold belt are distinct from peralkaline rocks in terms of chemical composition petrography and field associations although these rocks have been grouped together as a single type in current classification schemes we propose that the metaluminous to weakly peraluminous a type granites such as those of the lachlan fold belt should be defined as aluminous a type granites and should not be grouped with peralkaline granites the lachlan fold belt aluminous a type granites have relatively high calculated zircon saturation temperatures we suggest that these granites were produced by high temperature partial melting of a felsic infracrustal source

geochronological and geochemical constraints on the luliang group in the luliang complex implications for the tectonic evolution of the trans north china orogen abstract the luliang complex is located at the western margin of the middle segment of the trans north china orogen along which the western and eastern blocks amalgamated to form the basement of the north china craton the complex consists of the late neoarchean to paleoproterozoic granitic plutons and meta supracrustal rocks of which the latter are subdivided into the jiehekou luliang yejishan and heichashan lanhe groups the luliang group is composed mainly of siliciclastic rocks in the lower part and volcanic rocks in the upper part all of which have been metamorphosed from greenschist to amphibolite facies u pb ages of detrital zircons from meta sedimentary rocks in the luliang group yield four populations of 3061 2920 ma 2790 2600 ma 2599 2280 ma and 2211 2202 ma suggesting that their provenances were derived mainly from the luliang and taihua complexes in the trans north china orogen the meta volcanic rocks give igneous zircon u pb ages of 2209 2178 ma and e hf t values of 0 8 to 3 6 geochemically they are subalkaline basalts and andesites that can be classified into two groups of which one group has flat lrees weak enrichment in lrees relative to hrees and flat hrees and displays e morb like spider diagrams with the exception of insignificantly negative nb ta zr hf and ti anomalies in contrast the other group displays stronger lree fractionation stronger enrichment in lrees relative to hrees and mildly fractionated hrees and is characterized by spiky spider diagrams similar to arc volcanic rocks the first group is interpreted to have originated from a neoarchean e morb source whereas the second group shows inputs of newly subduction derived fluids in the mantle source these geochronological and geochemical signatures indicate the existence of an 2 2 2 1 ga continental back arc system in the luliang complex of the trans north china orogen which is consistent with the model that the collision between the eastern and western blocks to form the coherent basement of the north china craton in the paleoproterozoic not at the end of neoarchean

re os dating of low level highly radiogenic llhr sulfides the harnas gold deposit southwest sweden records continental scale tectonic events to obtain geologically meaningful and precise results on low level sulfides that are highly radiogenic blank corrections that account for the extreme difference in the os isotope composition of sample and blank are necessary and the resulting data is best plotted in 187 re 187 os space much like the approach used for molybdenites we here introduce the acronym llhr as a means of characterizing low level and highly radiogenic sulfides use of the traditional 187 re 188 os vs 187 os 188 os plot for llhr sulfides leads to large analytical uncertainties and highly correlated errors as 188 os is poorly determined and can generally be accounted for by the blank we now recognize that llhr sulfides such as those encountered at the harnas au deposit are a common occurrence and characteristic of many crustally affiliated ore systems the harnas au deposit in southwest sweden is located in the mjosa vanern mineral belt that transects from northwest to southeast the prominent geologic fabric of the sveconorwegian province including the mylonite zone the deposit is hosted in a strongly sheared and locally brecciated sulfide bearing quartz vein that crosscuts the foliation of orthogneisses belonging to the 1600 maa mal granitoid suite a 187 re 187 os isochron of 973 34 ma 2 for samples of pyrite and chalcopyrite from outcrops of the harnas alma and silvergruvan veins was obtained the harnas and subsidiary alma veins are au bearing whereas the silvergruvan 2 5 km to the southeast is not of the au bearing type four replicate analyses of an additional pyrite sample from the harnas vein yield a model age of 616 40 ma 95 c l in all samples re and 187 os concentrations are extremely low 0 35 to 7 1 and 0 005 to 0 074 ppb respectively and total common os 0 002 0 028 ppb is less than half the calculated radiogenic 187 os two samples of galena from the harnas vein yielded blank level re and os neither the 973 nor 616 ma time of mineralization is related to development of the mylonite zone at 920 ma or intrusion of the anorogenic bohus blomskog granites at 920 to 900 ma previously proposed sources for generating mjosa vanern mineral belt ore fluids an age of 973 ma for harnas alma silvergruvan suggests that sulfide deposition was associated with the waning stages 0 97 0 90 ga of the sveconorwegian grenvillian orogeny 1 15 0 90 ga postdating metamorphism and deformation that produced a regionally overthickened crust in the northern part of the sveconorwegian province the vein systems for these and numerous other small deposits in the mjosa vanern mineral belt could have resulted from regional exhumation in the late sveconorwegian with associated extension and the development of fracture systems at cooler middle to upper crustal levels in the southern part of the sveconorwegian province dominated by granulitic facies rocks no mineralization is present perhaps due only to increased exhumation and erosion that has exposed deeper crustal levels the late sveconorwegian age 973 ma records the main period of sulfide deposition for the harnas alma silvergruvan vein systems and indicates the robustness of the chronometer within tectonically complex terranes the late neoproterozoic age 616 ma records a later time of pyrite deposition probably associated with shearing and reopening of the harnas vein system because the au mineralization at harnas has been described as associated with shearing and occurring in fracture fillings in early pyrite we suggest that part or all of the au mineralization could have been introduced at 616 ma during reactivation of the vein system the 616 ma age for pyrite suggests a previously unrecognized and much younger mineralizing episode in the harnas region most likely reflecting continental scale extension and the late neoproterozoic opening of the iapetus ocean

zircon u pb ages of paleoproterozoic mafic granulites from the huai an terrane north china craton ncc implications for timing of cratonization and crustal evolution history abstract the north china craton ncc is considered to have been incorporated into the columbia supercontinent during late paleoproterozoic the huai an terrane located in north central part of the ncc is an important tectonic window to investigate the paleoproterozoic crustal evolution history of this craton particularly due to the wide occurrence of high grade metamorphic rocks including retrograded eclogites and high pressure granulites available data show two controversial peak metamorphic ages of these rocks as 1 95 1 90 or 1 85 1 80 ga thus determining the precise timing of metamorphism of the high pressure granulites is important in reconstructing the collisional history in this study we present zircon u pb ages bulk chemistry and mineral inclusions in zircons and matrix mineral assemblages from the high pressure granulites to evaluate the timing of metamorphism our results reveal ca 2 05 2 0 ga ages from magmatic zircons in these rocks representing the timing of emplacement of mafic magmas the metamorphic zircons yield two groups of ages at 1 95 1 90 and 1 85 1 80 ga clinopyroxene plagioclase titanite and apatite occur as common mineral inclusions within the 1 85 1 80 ga metamorphic zircons the chemical compositions of clinopyroxene and plagioclase occurring as inclusions are very similar to those of the rim compositions of those minerals in the matrix suggesting their formation during retrograde granulite metamorphism and are distinctly different from those of the core domains of the minerals in the matrix which represent high pressure granulite metamorphism these results indicate that the metamorphic zircons formed during retrograde granulite metamorphism dated as 1 85 1 80 ga and the age range of 1 95 1 90 ga corresponds to high pressure granulite metamorphism the dual nature of the geological and geochronological features suggest that the archean gray gneisses in the huai an terrane represent the basement rocks detached from the khondalite belt of the ncc after collision integrating with published data a double side collision with scissor like suturing model is proposed for the paleoproterozoic cratonization of the ncc

crustal growth and intracrustal recycling in the middle segment of the trans north china orogen north china craton a case study of the fuping complex the most important crustal growth on earth occurred at 2 7 ga but the north china craton ncc is characterized by prevalent development of 2 5 ga juvenile crust with relatively rare records of 2 7 ga crustal growth the fuping complex in the middle segment of the trans north china orogen tnco between the eastern and western blocks of the ncc is composed mainly of 2 5 ga fuping tonalitic trondhjemitic granodioritic ttg gneisses and longquanguan augen gneisses 2 1 ga nanying granitic gneisses and the wanzi supracrustal rocks previous studies have suggested one major phase of crustal growth at 2 5 ga possible intracrustal recycling at 2 1 ga and the presence of older rocks in the fuping complex but there has been no record of 2 7 ga crustal growth the fuping ttg gneisses are dominated by stromatic migmatite and new u pb dating of magmatic zircons from two stromatic migmatite samples yielded three different ages 1 2 75 ga which is the oldest age obtained from the fuping ttg gneisses 2 2 54 ga which just falls in the published zircon u pb age range of 2 53 to 2 47 ga for the fuping ttg gneisses and 3 2 11 ga which is almost the same as the age of the nanying granitic gneisses therefore there are two generations of ttg gneisses in the fuping complex importantly both of the 2 75 and 2 54 ga zircons have the highest e hf t values almost equal to the contemporaneous depleted mantle this indicates high contributions of juvenile material to the two generations of ttg gneisses in contrast the 2 11 ga zircons have apparently low e hf t values of 0 47 to 2 04 just falling in between 2 55 and 2 75 ga continental crust values this strongly suggests the reworking of the two generations of ttg gneisses at 2 1 ga zircon u pb and hf isotopes convincingly reveal two major phases of crustal growth in the fuping complex at 2 7 and 2 5 ga the same as in the northern and southern segments of the tnco and also confirm one major phase of intracrustal recycling at 2 1 ga which may be responsible for the nanying granitic gneisses

a hybrid origin for the qianshan a type granite northeast china geochemical and sr nd hf isotopic evidence abstract major and trace element whole rock sr and nd isotope and zircon hf isotope data are reported for a suite of a type granites and mafic microgranular enclaves from the early cretaceous 126 2 ma qianshan pluton liaodong peninsula northeast china with the aim of investigating the sources and petrogenesis of a type granites the qianshan pluton includes hornblende alkali feldspar granite graphic biotite granite and mafic microgranular enclaves the hornblende alkali feldspar granites have high sio 2 fe 2 o 3t mgo k 2 o na 2 o rb zr and lree contents and low ba and sr concentrations with strongly negative eu anomalies their high rb sr 87 rb 86 sr 16 76 24 15 and initial 87 sr 86 sr ratios 0 7215 to 0 7283 negative e nd t values 14 1 to 16 5 and zircon e hf t values 18 9 to 11 5 indicate they were mainly derived from a crustal source but with involvement of high e nd t and e hf t materials graphic biotite granites have similar geochemical features and sr nd hf isotopic compositions to enclaves indicating they were the result of crystal fractionation of evolved mafic magmas but with involvement of low e nd t and e hf t materials the mafic enclaves have an igneous texture and contain acicular apatite suggesting quenching of mafic magmas that have co mingled with the host granites they have low initial 87 sr 86 sr ratios 0 7097 0 7148 negative e nd t 14 5 to 11 9 and zircon e hf t 17 1 to 6 9 values and are enriched in liles and lrees and depleted in hfses when coupled with the high mgo mg up to 54 this indicates derivation from an enriched lithospheric mantle source but contaminated by crustal materials geochemical and sr nd and zircon hf isotopic compositions rule out simple crystal liquid fractionation or restite unmixing as the major genetic link between enclaves and host rocks instead magma mixing of mantle derived mafic and crustal derived magmas coupled with crystal fractionation is compatible with the data this example shows that at least some a type granites formed through a complex process involving mantle and crustal derived magma mixing crystal fractionation and infracrustal melting

zircon u pb ages and lu hf isotope compositions from clastic rocks in the hutuo group further constraints on paleoproterozoic tectonic evolution of the trans north china orogen abstract the hutuo group as one of the classic examples of paleoproterozoic strata plays an important role in establishing evolutionary processes in the trans north china craton in this contribution we present petrologic detrital zircon u pb ages and lu hf isotopes from three subgroups of the hutuo group sandstones in the doucun and dongye subgroups are dominated by q monocrystalline and polycrystalline quartz and f k feldspar and plagioclase with minor lithic fragments suggesting that the detrital components were mainly derived from both the continental block and recycled orogen in contrast clastic components in the guojiazhai subgroup are mainly q and lithic fragments indicating they were derived predominantly from the recycled orogen the ages of detrital zircons from sandstones in the doucun and dongye subgroups are mainly concentrated at ca 2 5 ga and 2 2 2 1 ga with minor 2 7 ga zircons also present which indicates they were dominantly sourced from 2 5 ga wutai fuping and zanhuang complexes and paleoproterozoic intrusives the guojiazhai subgroup displays a different zircon age population of ca 2 4 ga 2 2 2 1 ga and 2 0 1 9 ga which indicates likely derivation from paleoproterozoic intrusives in the wutai luliang and hengshan areas based on the observation that sandstone clastic components in the doucun and dongye subgroups are dominantly quartz feldspar and sedimentary lithic fragments but not volcanic lithics we propose that they were deposited in a rift related setting zircons from the lower sequence of the hutuo group yielded a young population of 2140 ma considering the volcanic rocks of 2140 14 ma at the base of the group and 2 2 2 0 ga magmatism along the tnco we propose that doucun and dongye subgroups formed at 2 2 2 0 ga the youngest ca 1 9 ga zircons in the guojiazhai subgroup indicate this group was deposited during closure of the rift at 1 9 1 8 ga the two stage model ages of the detrital zircons mainly range from 2 6 to 2 9 ga with a minor 2 5 ga contribution therefore we infer that 2 6 2 9 ga represents a period of intensive crustal growth in the trans north china orogen but that some degree of crustal growth continued to 2 5 ga

geochronological and geochemical constraints on the petrogenesis of the early paleoproterozoic potassic granite in the lushan area southern margin of the north china craton abstract we conducted a geochronological and geochemical study on the paleoproterozoic potassic granites in the lushan area southern margin of the north china craton ncc to understand the tectonic regime of the ncc at 2 2 2 1 ga this rock suite formed at 2194 29 ma the rocks are rich in sio2 76 10 77 73 wt and k2o 5 94 6 90 wt with high k2o na2o contents from 7 56 wt to 8 48 wt but poor in cao 0 10 0 28 wt p2o5 0 02 0 05 wt and mgo 0 01 0 30 wt mg 1 08 27 3 indicating they experienced fractional crystallization major element compositions suggest the potassic granites share an affinity with high k calc alkaline granite even though the lushan potassic granitic rocks have high a cnk ratios 1 11 1 25 which can reach peraluminous feature the very low p2o5 contents and negative correlation of p2o5 and sio2 ruling out they are s type granites different from peralkaline a type granites the lushan potassic granites have variable zr concentrations 160 344 ppm 226 ppm on average and 10 000 ga al ratios 1 76 3 00 together with high zircon saturation temperatures tzr 826 885 c indicating they are fractionated aluminous a type granites enriched lree la yb n 9 72 81 8 negative eu anomalies and low sr y with no correlations in sr y and sr zr versus cao suggest the possible presence of ca rich plagioclase and absence of garnet in the residual magmatic zircon grains have variable ehf t values 2 4 to 7 3 with zircon two stage hf model ages tdmc varying from 2848 ma to 2306 ma mostly around ca 2 5 ga and are plotted in the evolution line of crustal felsic rock we propose that the rocks mainly formed by partial melting of ca 2 50 ga tonalitic granodioritic crust as a result of upwelling mantle derived magmas which provided thermal flux and source materials in an intra continent rifting the ca 2 2 ga magmatism suggests that intra continental rifting occurred at 2 35 1 97 ga at least in the southern margin of the ncc after its final cratonization in the late neoarchean

re os isotopic ages of pyrite and chemical composition of magnetite from the cihai magmatic hydrothermal fe deposit nw china the eastern tianshan orogenic belt of the central asian orogenic belt and the beishan terrane of the tarim block nw china host numerous fe deposits the cihai fe deposit 90 mt at 45 6 fe in the beishan terrane is diabase hosted and consists of the cihai cinan and cixi ore clusters ore minerals are dominantly magnetite pyrite and pyrrhotite with minor chalcopyrite galena and sphalerite gangue minerals include pyroxene garnet hornblende and minor plagioclase biotite chlorite epidotite quartz and calcite pyrite from the cihai and cixi ore clusters has similar re os isotope compositions with 14 to 62 ppb re and 10 ppt common os pyrrhotite has 5 to 39 ppb re and 0 6 ppb common os pyrite has a mean re os model age of 262 3 5 6 ma n 13 in agreement with the isochron regression of 187os vs 187re the re os age 262 ma for the cihai fe deposit is within uncertainty in agreement with a previously reported rb sr age 268 25 ma of the hosting diabase indicating a genetic relationship between magmatism and mineralization magnetite from the cihai deposit has mg al ti v cr co ni mn zn ga and sn more elevated than that of typical skarn deposits but both v and ti contents lower than that of magmatic fe ti v deposits magnetite from these two ore clusters at cihai has slightly different trace element concentrations magnetite from the cihai ore cluster has relatively constant trace element compositions some magnetite grains from the cixi ore cluster have higher v ti and cr than those from the cihai ore cluster the compositional variations of magnetite between the ore clusters are possibly due to different formation temperatures combined with regional tectonic evolution of the beishan terrane the re os age of pyrite and the composition of magnetite indicate that the cihai fe deposit may have derived from magmatic hydrothermal fluids related to mafic magmatism probably in an extensional rift environment

age and source constraints for the giant muruntau gold deposit uzbekistan from coupled re os he isotopes in arsenopyrite the muruntau gold deposit uzbekistan is one of the largest gold deposits known worldwide but its origin remains controversial we used re os arsenopyrite geochronology to precisely determine the age of main stage gold mineralization at muruntau to be 287 5 1 7 ma which overlaps the emplacement of proximal post tectonic granitoid magmatism additionally we suggest that arsenopyrite growth may have occurred over an interval of at least 2 m y os initial ratios derived from arsenopyrite were coupled with he isotopic data from fluid inclusions within arsenopyrite to constrain the source of ore metals and fluids muruntau arsenopyrite yields relatively unradiogenic initial os 0 37 0 27 and elevated 3he 4he ratios 0 23 0 33 r a relative to purely crustal os he reservoirs these data suggest the presence of a mantle derived component in the ore system that was probably introduced during the generation of the granitoid magmas these new timing and source constraints provide important new insight into the generation of this giant gold deposit and they necessitate reexamination of genetic models for muruntau and potentially other giant orogenic gold deposits worldwide

besshi type vms deposits of the rudny altai central asia for the first time in the rudny altai we propose to distinguish besshi and kuroko type deposits previously considered as members of a single massive sulfide polymetallic group the ores of the two deposit types differ in mineral composition with pyrite pyrrhotite chalcopyrite sphalerite mineralization in the besshi type deposits versus pyrite sphalerite galena chalcopyrite gold mineralization in the kuroko type deposits the besshi type deposits are restricted to the irtysh terrane and associated with deformed and metamorphosed greenschist to mid amphibolite facies mafic siliciclastic rock sequences this contrasts with the well preserved kuroko type deposits hosted in bimodal volcanic sequences in the devonian carboniferous rudny altai magmatic arc our u pb 40ar 39ar and re os isotope dating of sulfides and host rocks of the besshi type deposits suggests that they were probably formed during the early paleozoic in a back arc rift setting they were then metamorphosed during devonian times within the accretionary wedge in front of the rudny altai arc synchronously with formation of the kuroko type deposits

shrimp u pb zircon geochronology of the huai an complex constraints on late archean to paleoproterozoic magmatic and metamorphic events in the trans north china orogen the huai9an complex is situated in the northern segment of the trans north china orogen tnco a continent continent collisional belt along which the discrete archean eastern and western blocks amalgamated to form the basement of the north china craton the complex consists of six distinct lithologic units the huai9an ttg gneisses the manjinggou high pressure mafic granulites the khondalite series the dongjiagou granitic gneiss the huai9an charnockite and the dapinggou k feldspar granite shrimp u pb geochronology combined with th and u data and cathodoluminescence cl imaging of zircons enables resolution of magmatic and metamorphic events that can be directed towards understanding the late archean to paleoproterozoic history of the tnco cl images reveal the coexistence of magmatic and metamorphic zircons in most lithologies of the huai9an complex of which the metamorphic zircons occur as either single grains or overgrowth rims surrounding and truncating oscillatory zoned magmatic zircon cores shrimp u pb analyses on magmatic zircons reveal that the tonalitic trondhjemitic and granodioritic protoliths of the huai9an ttg gneisses were emplaced at 2515 20 ma 2499 19 ma and 2440 26 ma respectively much earlier than the emplacement of the dongjiagou granitic gneiss dated at 2036 16 ma however their metamorphic zircons yield similar concordant 207pb 206pb ages of 1847 17 ma 1842 10 ma and 1847 11 ma for the tonalitic trondhjemitic and granodioritic gneisses respectively and 1839 46 ma for the dongjia granitic gneiss these ages demonstrate that the huai9an complex underwent a regional metamorphic event at 1850 ma which is further supported by a mean 207pb 206pb age of 1848 19 ma for metamorphic zircons in the manjinggou high pressure mafic granulite and 1849 10 ma and 1850 17 ma for igneous zircons in the anatectic huai9an charnockite and dapinggou garnet bearing s type granite respectively the timing of late archean to paleoproterozoic magmatism and regional metamorphism in the huai9an complex is in general agreement with recent shrimp zircon data for other metamorphic complexes in the tnco these data prove that the high grade gneiss complexes were not the basement to the low grade granite greenstone terranes in the tnco furthermore the lithologies of the orogen are considered to have developed as a long lived magmatic arc that was subsequently tectonically disrupted and juxtaposed during the collision of the eastern and western blocks at 1 85 ga leading to final assembly of the north china craton

craton scale distribution of archean greenstone gold deposits predictive capacity of the metamorphic model genetic models should not only attempt to explain features of archean gold mineralization on the scale of mines or districts but should also explain its marked spatial and temporal heterogeneity in greenstone belts the application of genetic models developed with such a philosophy can have predictive capacity in gold exploration the metamorphic model one of a number of current genetic models for archean gold deposits involves generation of auriferous h 2 o co 2 fluids by devolatilization of the stratigraphically lowermost greenstones during metamorphism and broadly synchronous synkinematic granitoid intrusion crustal scale fault shear systems are required to induce channeled fluid flow and to focus ore fluids toward suitable host rocks and structural sites declining temperature below the amphibolite greenschist transition combined with sulfidation of fe rich host rocks is a particularly efficient au depositing mechanism the model suggests that the major controlling factors in declining order of importance are 1 high temperature low pressure generally low strain metamorphic regimes 2 crustal scale fault systems 3 unaltered fe rich rocks tholeiites banded iron formation at a suitable stratigraphic metamorphic level and 4 abundant au enriched komatiites and s rich sediments in source regions the low pressure metamorphic conditions combined with the occurrence of elongate high strain zones in most if not all generally low strain greenstone belts may explain the ubiquitous occurrence of gold deposits in archean terranes all four favorable parameters appear to act in concert to produce the largest gold deposits and most abundant gold mineralization in 2 7 or 0 1 b y old rift phase greenstone belts interpreted to have formed under conditions of high crustal extension and extensive crustal thinning i e with predicted high geothermal gradients high fault density limited synvolcanic alteration and abundant komatiites and sulfidic sediment horizons more widespread commonly older platform phase greenstone belts which formed under conditions of lesser crustal extension and thinning are less mineralized as predicted by the metamorphic model the ability of the model to explain the regional distribution of known archean gold deposits lends credence to its predictive capacity for example 1 greenstone belts in any one craton can be broadly ranked in order of likely relative gold potential 2 the likelihood of discovery of large deposits in poorly exposed or poorly explored belts of known type can be estimated 3 areas with the highest potential can be defined in rift phase greenstone belts by the coincidence of critical parameters such as specific host rock sequences and metamorphic facies 4 prospective structural sites for gold mineralization can be postulated in platform phase greenstone belts affected by diapiric tectonics and 5 the possible spacing of large deposits can be estimated further the potential use of the model in exploration emphasizes serious gaps in our knowledge of parameters crucial to understanding greenstone belt metallogeny including alteration patterns metamorphic p t paths and regional scale structural regimes

shrimp u pb zircon ages of the fuping complex implications for late archean to paleoproterozoic accretion and assembly of the north china craton the fuping complex is situated in the central part of the north china craton and consists of four major lithological assemblages fuping tonalitic trondhjemitic granodioritic gneisses longquanguan augen gneisses wanzi supracrustal assemblage and nanying granitic gneisses shrimp u pb geochronology combined with u th and cathodoluminescence cl imaging of zircon enables resolution of magmatic and metamorphic events that can be directed towards understanding the late archean to paleoproterozoic history of the fuping complex cl images reveal the coexistence of magmatic and metamorphic zircons in nearly all rock types of the fuping complex the metamorphic zircons occur as either single grains or overgrowth or recrystallization rims surrounding and truncating oscillatory zoned magmatic zircon cores and are all characterized by nebulous zoning or being structureless with extremely high luminescence and very low th contents these features make them distinct from magmatic zircons that are characterized by concentric oscillatory zoning comparatively low luminescence and high th and u contents shrimp u pb analyses on magmatic zircons reveal that the tonalitic trondhjemitic and granodioritic plutons of the fuping gneisses were emplaced at 2523 14 ma 2499 10 ma and 2486 8 ma respectively whereas the monzogranitic and granitic plutons of the longquanguan augen gneisses were intruded respectively at 2510 22 ma and 2507 11 ma prismatic and oscillatory zoned zircons dominate in the pelitic rocks of the wanzi supracrustal assemblage and are interpreted as detritus from igneous source rocks the concordant and discordant u pb ages of 2502 5 ma and 2507 14 ma obtained from two pelitic rock samples indicate these rocks must have been deposited no earlier than 2507 ma ago in addition a zoned zircon grain in one pelitic rock sample has a near concordant age of 2109 5 1 ma which may provide a maximum depositional age for the wanzi supracrustal rocks shrimp results also reveal that granitic magmatism assigned to the nanying granitic gneisses occurred over a protracted interval from 2077 13 ma to 2024 21 ma the nebulously zoned zircon grains and overgrowth recrystallization zircon rims from different rocks yielded similar concordant 207pb 206pb ages in the range 1875 to 1802 ma interpreted as approximating the age of regional metamorphism of the fuping complex timing of primary zircon crystallization and regional metamorphism of the fuping complex is in general agreement with recent u pb zircon ion probe results for the wutai and hengshan complexes that bound the fuping complex to the northwest these areas are characterized by the emplacement of major granitoid bodies at around 2 50 ga to 2 48 ga ago deposition of supracrustal rocks in the paleoproterozoic intrusion of paleoproterozoic granitic bodies at 2 1 to 2 0 ga and regional metamorphism at 1 875 to 1 802 ga these data indicate that the fuping and hengshan complexes do not represent an older crystalline basement to the wutai complex as suggested in previous tectonic models but together with the wutai complex represent elements of a single late archean to paleoproterozoic magmatic arc system that has been subsequently tectonically disrupted and juxtaposed during the collision of the eastern and western north china blocks at 1 85 ga which resulted in the final assembly of the north china craton

crystallization conditions and evolution of magmatic fluids in the harney peak granite and associated pegmatites black hills south dakota evidence from fluid inclusions abstract a microthermometric study of inclusions in granites and pegmatites in the proterozoic harney peak granite system identified four types of inclusions type 1 inclusions are mixtures of co2 and h2o and have low salinities on average 3 5 wt nacleq type 2 inclusions are aqueous solutions of variable salinities from 0 to 40 wt nacleq type 3 inclusions are carbonic dominated by co2 with no detectable water and type 4 inclusions consist of 20 to 100 solids with the remaining volume occupied by a co2 h2o fluid many inclusions have a secondary character however a primary character can be unambiguously established in several occurrences of the type 1 inclusions these inclusions were trapped above the solidus and represent the exsolved magmatic fluid the secondary populations of types 1 2 and 3 probably formed as a result of reequilibration and unmixing of the type 1 fluid that progressively changed composition and density with decreasing temperature and pressure and was finally trapped along healed microfractures under subsolidus conditions type 4 inclusions are primary and are interpreted to be trapped fluid bearing complex silicate melts that subsequently solidified or underwent other posttrapping changes it is demonstrated that primary type 1 fluid inclusions that coexist with crystallized melt inclusions in the complex li bearing tin mountain pegmatite were trapped along the two fluid phase boundary in the system co2 h2o nacleq consequently the temperature and pressure conditions of trapping are identical to the bulk homogenization conditions on average 340 c and 2 7 kbar these conditions indicate that this li cs rb p and b rich pegmatite crystallized at some of the lowest known temperatures for a silicate melt in the crust an internally consistent empirical solvus surface in p t xco2 coordinates was generated for the pseudobinary co2 h2o 4 3 wt nacleq pegmatite fluid system distribution coefficients for the major species co2 h2o nacl and ch4 between the immiscible co2 rich and h2o rich fluid phases as a function of pressure and temperature were extracted from data for the two cogenetic fluid inclusions types

late and mid cretaceous mineralization in the northern canadian cordillera constraints from re os molybdenite dates new re os molybdenite dates are presented from three deposits casino cash and mt nansen in the dawson range yukon northern canadian cordillera the re os molybdenite dates provide constraints on the timing of sulfide mineralzation and together with 40 ar 39 ar and k ar mineral dates yield estimates for the longevity of the porphyry hydrothermal system at the casino deposit re os molybdenite dates associated with the potassic 75 05 0 28 74 69 0 28 ma alteration are nominally older to overlapping with re os molybdenite dates 74 4 0 28 ma n 2 with phyllic alteration and suggest that it is likely that potassic and phyllic mineralization alteration was contemporaneous the re os molybdenite dates are significantly older than hydrothermal k feldspar 40 ar 39 ar and k ar biotite dates 72 ma which most likely reflect the cessation of the porphyry hydrothermal system the molybdenite age data combined with silicate hydrothermal mineral dates suggest a porphyry hydrothermal system longevity of 4 16 to 1 94 casino 8 66 to 6 44 cash and 2 47 to 1 48 ma mt nansen these longevity estimates for porphyry hydrothermal systems are greater than previously proposed 50 c km may control the disparate re os molybdenite and 40 ar 39 ar k feldspar dates of the porphyry systems of this study the contrasting re os and 40 ar 39 ar dates clearly show the re os isotope system in molybdenite is resistant to prolonged 1 ma interaction with a high temperature 400 500 c hydrothermal fluid and that the re os isotope systematics in molybdenite unambiguously record directly the timing of sulfide mineralization this ability to directly date sulfide mineralization has identified mid pattison idaho creek 102 104 ma and late cretaceous zappa koffee 74 5 ma mineralization which is hosted by the mid cretaceous 104 102 ma casino plutonic suite the late cretaceous date is very similar to that for molybdenite mineralization at casino and we suggest that the zappa koffee mineralization is an area of late cretaceous porphyry mineralization for which the host pluton has not yet been discovered

2090 2070 ma a type granitoids in zanhuang complex further evidence on a paleoproterozoic rift related tectonic regime in the trans north china orogen abstract the xuting pluton located at the north section of the zanhuang complex contains potassic and sodic granite in both of which shallow level emplacement is shown by graphic texture feldspar and quartz intergrowth two potassic granite and one sodic granite samples yielded zircon 207 pb 206 pb ages of 2066 17 ma 2092 14 ma and 2071 14 ma respectively suggesting that the xuting pluton emplaced between 2070 ma and 2090 ma which is coeval with the paleoproterozoic gantaohe group dominated by mafic volcanic and sedimentary rocks the potassic granite is enriched in sio 2 total feo rees except eu zr nb ga and y and depleted in mgo cao al 2 o 3 sr v cr and ni with high tfeo mgo ratios consistent with the features of a type granite except for the high mgo and low k 2 o contents the sodic granite also presents most of the features of a type granites the zircon saturation temperatures from the potassic and sodic granites range from 855 to 998 c and 853 to 933 c respectively indicating that they are from the high temperature magma the potassic granites have the e hf t ranging from 5 9 to 1 33 and hf model ages of 2 5 2 8 ga which are identical with the ages of neoarchean ttg gneisses in the zanhuang complex we propose that the potassic granite in the xuting pluton derived from the partial melting of neoarchean ttg gneisses the sodic granites also present negative e hf t values from 4 81 to 0 76 and hf model ages of 2 5 2 8 ga suggesting that they come from the partial melting of the neoarchean crust however with relative enrichment in mgo and total feo and lower k 2 o and rb the sodic granite was derived from a mafic source in the lower crust the likely heat source was high heatflow associated with mantle upwelling the a type xuting pluton and mafic volcanic rocks in the gantaohe group constitute a bimodal lithologic association from a paleoproterozoic continental rift environment 2 2 2 0 ga magmas are widely distributed along the trans north china orogen tnco and also present a bimodal character we propose that 2 2 2 0 ga bimodal magmas throughout the tnco were likely formed in a paleoproterozoic rifting setting considering the widespread 2 2 2 0 ga magmatic rocks in the north china craton ncc we infer that the magmatism likely occurs in tensional environment after the initial amalgamation of the ncc in the late archean

comparative isotopic and chemical geochronometry of monazite with implications for u th pb dating by electron microprobe an example from metamorphic rocks of the eastern wyoming craton u s a polygenetic monazite grains in diverse precambrian crystalline rocks from the black hills south dakota have been analyzed in situ by ion and electron microprobe methods shrimp and emp to evaluate the accuracy and precision of emp ages determined using a new analytical protocol that incorporates improved background acquisition and interference corrections parallel evaluations were conducted by comparing emp chemical and shrimp isotopic ages at regional rock and grainscales the monazite data set includes 354 emp chemical analyses from 26 grains in six metamorphic rocks which resolve into 54 age composition domains and 31 shrimp isotopic ages from 13 grains in one of the rocks with six grains microanalyzed in common by the two methods the data set also includes monazite bearing garnets in two of the rocks whose isotopic compositions were analyzed using pb stepwise leaching pbsl methods both the emp and shrimp data sets reveal a continuum of apparent monazite ages spanning a 1790 1680 ma timeframe with a relatively high probability of ages at 1755 and 1715 ma that correspond spatially to core and rim domains pbsl ages of 1742 and 1734 ma obtained from monazite bearing garnet in two rocks are intermediate compared to the corresponding emp ages and are thereby interpreted as mixed ages emp data for two grains in the structurally deepest of the six rocks record 1785 and 1755 ma ages in the cores and higher y and lower th rims respectively and these results are duplicated by shrimp ages in these and or other grains from the same rock overall the emp shrimp and pbsl ages are internally consistent at the various scales of observation which serves to validate emp chemical dating as an accurate and precise method of discerning monazite age populations in polymetamorphic terrains the emp data set is interpreted geologically as res ecting multiple episodes of monazite growth that are provisionally related to known metamorphic events in the black hills taking the most precise emp data at face value it is possible to resolve the timing of the two older events at 1784 4 ma or 1786 6 ma and 1756 3 ma or 1753 4 ma with 95 con dence these events are considered to be related to sequential episodes of n directed thrusting and e w compression associated with paleoproterozoic crustal assembly in the mid continent a younger metamorphism related to granite intrusion known to have occurred at 1715 3 ma is dated independently at 1717 2 ma from the emp monazite ages

electron probe ultrachron microchronometry of metamorphic monazite unraveling the timing of polyphase thermotectonism in the easternmost wyoming craton black hills south dakota a metapelite from the easternmost wyoming craton black hills south dakota has been analyzed by microstructural methods to unravel polyphase deformational history associated with 1800 1700 ma assembly of southern laurentia three deformational fabrics are recognized in oriented thin sections an ene trending s 1 fabric preserved as oblique inclusion trails in garnet porphyroblasts a nnw trending s 2 fabric preserved as microlithons in the rock matrix and a flattening fabric s 3 which transposed s 1 s 2 and dominates the matrix a complex monazite porphyroblast has been analyzed in situ with the electron microprobe ultrachron to constrain the timing of s 1 s 3 fabric formation associated with monazite growth the core of this grain uniquely preserves the s 1 s 2 fabrics as sigmoidal inclusion trails the mean total pb age of this domain is 1750 10 ma all dates reported at 95 confidence n 39 spots which is equivalent to the published 2 0 7 pb 2 0 6 pb age for the same domain these results validate the total pb dating method in general and the ultrachron in particular for reliable age determination in low th monazite and are interpreted as 1750 ma minimum ages for the s 1 s 2 fabrics and sequential d 1 d 2 collisional events that imposed them n directed arc accretion and e w continental collision respectively a higher th y rim of this same rosetta grain truncates the s 1 s 2 sigmoid and is associated with resorption textures in garnet porphyroblasts coupled release of y and an s 3 fabric that pervasively overprinted s 1 s 2 in the rock matrix the mean ultrachron date of this domain is 1692 5 ma n 17 spots which is slightly younger that the published isotopic age for all monazite rims combined these results support a 1715 1690 ma timeframe for localized doming d 3 related to granite magmatism the onset of which has been dated independently at 1715 3 ma the timing of post d 3 cooling through 350 and 300 c is constrained by 4 0 ar 3 9 ar dates of 1610 and 1480 ma obtained for separates of d 3 matrix muscovite and biotite respectively which are interpreted as closure ages this study shows that fabrics in poly deformed rocks can be dated by linking monazite spot ages to key microtextures further the results of this micrometer scale study enhance previous knowledge of local thermotectonism black hills and regional terrane assembly laurentia

archean geodynamics in the central zone north china craton constraints from geochemistry of two contrasting series of granitoids in the fuping and wutai complexes abstract the archean to paleoproterozoic central zone of the north china craton is situated between the eastern and western archean continental blocks and contains two contrasting series of neoarchean granitoids the 2523 2486 ma tonalite trondhjemite granodiorite ttg gneisses in the fuping complex and the 2555 2525 ma calc alkaline granitoids tonalite granodiorite granite and monzogranite in the wutai complex the fuping ttg gneisses most likely formed from partial melting of 2 7 ga basalts at 50 km with an involvement of 3 0 ga crustal material the wutai granitoids have higher k 2 o lile and rb sr but lower sr y and la n yb n than the fuping ttg gneisses are characterized by nd t dm from 2 5 to 2 8 ga and e nd t from 0 49 to 3 34 and are derived from partial melting of a juvenile source at the geochemistry of these two contrasting series of neoarchean granitoids provides further evidence that the wutai complex originated and evolved separately from the fuping complex the wutai complex most likely formed as an oceanic island arc with volcanism and synvolcanic granitoid intrusions at 2555 2525 ma the wutai complex was subsequently accreted onto the eastern archean continental block and was probably responsible for crustal thickening and ttg magmatism at 2523 2486 ma in the fuping complex as part of the taihangshan hengshan block at the western margin of the eastern archean continental block

tectonothermal history of the basement rocks in the western zone of the north china craton and its tectonic implications the basement of the north china craton can be divided into the eastern central and western zones based on lithological structural metamorphic and geochronological data the western zone comprises two different petrotectonic units archaean tonalitic trondhjemitic granodioritic ttg grey gneisses and metamorphic mafic rocks and palaeoproterozoic khondalite series the former is characterized by isobaric cooling ibc type anticlockwise p t paths in the north northwestern part of the zone and near isothermal decompression itd type clockwise p t paths in the eastern part adjacent to the central zone on the other hand the tectonothermal evolution of palaeoproterozoic khondalite series rocks is characterized exclusively by nearly isothermal decompression following the peak of metamorphism and then cooling defining clockwise p t paths the archaean ttg gneisses and associated mafic rocks with anticlockwise metamorphic p t paths reflects an origin related to underplating and intrusion of mantle derived magmas which may be derived from mantle plumes they represent a late archaean continental block in the western part of the north china craton the palaeoproterozoic khondalite series rocks represent passive continental margin deposits they were metamorphosed and deformed in the late palaeoproterozoic during the amalgamation of the western continental block with another continental block in the east part of the north china craton the itd type clockwise p t t paths of the palaeoproterozoic khondalite series rocks record the tectonothermal histories of the collision of the western and eastern continental blocks which resulted in the final assembly of the north china craton at c 1800 ma

age and evolution of a late archean to paleoproterozoic upper to lower crustal section in the wutaishan hengshan fuping terrain of northern china abstract the taihangshan wutaishan area forms part of the central north china craton and consists of three main components the hengshan and fuping complexes containing predominantly ductilely deformed late archean to paleoproterozoic high grade granitoid orthogneisses intruded by mafic dykes of gabbroic composition and the low grade late archean wutai greenstone belt developed between the high grade terrains and consisting of bimodal volcanic rocks and metasediments associated with coeval granitoids zircon dating shows that both the hengshan and fuping complexes were intruded by major granitoid bodies between 2 52 and 2 48 ga with rare occurrences of 2 7 ga gneisses wutai granitoids adjacent to the fuping complex also contain 2 7 ga xenocrysts all possibly reflecting the same older basement rare granitic gneisses in the hengshan have emplacement ages of 2 35 ga and show the same structural features as the older rocks indicating that the main deformation occurred after 2 3 ga deposition of supracrustal rocks and emplacement of mafic dykes occurred in the paleoproterozoic together with intrusion of granitic bodies at 2 1 2 0 ga and all underwent regional metamorphism at 1 88 1 85 ga we interpret the hengshan and fuping granitoid gneisses as the lower plutonic part of a late archean to early paleoproterozoic japan type magmatic arc with the upper volcanic part represented by the wutai complex components of this arc may have evolved at a continental margin as indicated by the 2 7 ga zircons virtually all magmatic events are recorded in all three complexes however the 1 85 ga metamorphic event is only recorded in the higher grade portions of the wutai since low grade and presumably high crustal level precluded metamorphic zircon growth shear zones in the hengshan are interpreted as major lower crustal discontinuities post dating the peak of high pressure granulite metamorphism we suggest that they formed during orogenic collapse and uplift of the hengshan complex in the late paleoproterozoic 1800 ma

petrologic and geochemical links between the post collisional proterozoic harney peak leucogranite south dakota usa and its source rocks abstract the proterozoic terrane of the black hills south dakota includes the composite harney peak leucogranite and associated pegmatites that were emplaced into metamorphosed pelites and graywackes available dates indicate that granite generation post dated regional metamorphism and deformation that have been attributed to collision of the wyoming and superior cratons at 1760 ma previous radiogenic and stable isotope work indicates that the exposed metasedimentary rocks are equivalent to sources of the leucogranites in this study whole rock and mineral compositions of the metasedimentary rocks were used to calculate the likely average residue mineralogies and melt fractions that would be generated by muscovite dehydration melting of the rocks these were then used to model observed trace element compositions of the granites using published mineral melt distribution coefficients model trace element melt compositions using pelitic and graywacke protoliths yield similar results the models reproduce well the observed depletion of transition metals and ba in the granites relative to metasedimentary protoliths the depletion is due mainly to high proportion of biotite with variable amounts of k feldspar in the model residue sr is also depleted in the granites compared to source rocks but to a lesser relative extent than ba this is because of the low biotite melt distribution coefficient for sr and because high proportion of plagioclase in the residue is compensated by high sr concentrations in protoliths rubidium cs and ta behaved as slightly compatible to incompatible elements and therefore were not strongly fractionated during melting of the considered elements only b appears to have been highly incompatible relative to residue during melting the protoliths had sufficient b to allow tourmaline crystallization in those parts of the harney peak granite in which ti concentration was sufficiently low not to enhance crystallization of biotite the reproducibility of observed trace element concentrations in the harney peak granite by the models supports the often made proposition that metapelites and metagraywackes are common sources for leucogranites this argues against mass input from the mantle into metagraywacke and metapelitic crustal sources or melting of amphibolites to generate the post collisional harney peak and other similar peraluminous granite suites

relationship between copper tonnage of chilean base metal porphyry deposits and os isotope ratios re os isotopes in pyrite and chalcopyrite from early high temperature hypogene alteration assemblages in chilean porphyry copper deposits identify the source of os and by inference cu in these ore systems typical concentrations for os in both pyrite and chalcopyrite are between 7 and 30 ppt 10 12 g g and for re between 0 200 and 10 ppb 10 9 g g re os isochrons yield ages and initial 187 os 188 os ratios for the sulfides the isotopic data reflect the relative contributions of copper from the mantle and crust in chilean porphyry copper deposits seven ore deposits that reside in different tectonic terranes and represent distinct epochs of mineralization in chile were studied the initial osmium ratios of the first stage of mineralization at each of the deposits range from 0 15 to 5 these values are more radiogenic than the present chondritic mantle 0 13 and indicate significant crustal contributions of os to the magmatic and or hydrothermal systems there is a strong correspondence between the total copper content and initial os isotopic ratios in base metal porphyry deposits the larger deposits have lower initial os ratios than the smaller less significant deposits this relationship implies that larger deposits acquire a greater proportion of os from the mantle the initial os ratio of samples in the central segment of porphyry copper deposits of northern chile also decreases with decreasing age of the deposit a plausible interpretation of the re os data is that the later and larger deposits use regional tectonic and structural features that allow sampling of deeper more primitive magmatic sources

u pb geochronology and hf isotope geochemistry of detrital zircons from the zhongtiao complex constraints on the tectonic evolution of the trans north china orogen abstract the zhongtiao complex is located in the south segment of the trans north china orogen a continent continent collisional belt along which the discrete eastern and western blocks amalgamated to form the north china craton the complex consists of the neoarchean to paleoproterozoic granitoid plutons and low grade supracrustal rocks of which the latter are subdivided into the songjiashan jiangxian lower zhongtiao upper zhongtiao and danshanshi groups the songjiashan jiangxian and lower zhongtiao groups are composed of metaclastic rocks carbonates and metavolcanic rocks interpreted as the products of a back arc basin whereas the upper zhongtiao and danshanshi groups consist of metaconglomerates and metasandstones interpreted as molasses type siliciclastic rocks deposited in a retro arc foreland basin the u pb ages of detrital zircons from the songjiashan lower zhongtiao upper zhongtiao and danshanshi groups form four populations of 3600 3100 ma 2850 2750 ma 2630 2200 ma and 2150 1850 ma of which the population of 2630 2200 ma is the most dominant and its major sources are considered to be derived from the 2560 to 2300 ma granitoids and metavolcanics in the zhongtiao complex the source rocks of the subordinate 3600 3100 ma detrital zircons were probably derived from the paleo mesoarchean crust of the eastern block since similar aged rocks have not been found in the western block and the trans north china orogen whereas the 2850 2750 ma detrital zircons are interpreted to be derived from the 2850 to 2752 ma ttg gneisses and amphibolites in the adjacent taihua complex the age population of 2150 1850 ma is coincident with the ages of paleoproterozoic granitoids in the trans north china orogen the youngest detrital zircons in the songjiashan and lower zhongtiao groups constrain their maximum depositional ages at 2253 ma and 2168 ma respectively whereas the maximum depositional age of the overlying upper zhongtiao and danshanshi groups is defined at 1848 ma the late paleoproterozoic shift from a back arc basin to a retro arc foreland basin is consistent with the model that the collision between the eastern and western blocks to form the trans north china orogen occurred at 1 85 ga

leucogranites in the black hills of south dakota the consequence of shear heating during continental collision leucogranites are common features of convergent orogens however the heat necessary for producing leucogranite magma from pelitic sources in convergent orogens is a major unresolved issue we suggest that the generation of the harney peak granite black hills south dakota during the proterozoic trans hudson orogeny is best explained by shear heating during thrusting of a proterozoic sedimentary sequence over the archean wyoming basement our thermotectonic model successfully reproduces pressure temperature time constraints on early regional metamorphism of schists and subsequent generation of the late orogenic harney peak granite the model is consistent with isotopic data for the granite which require both proterozoic and archean source rocks and its emplacement as small batches of melt the similarity of geologic relationships in the black hills to those of other collisional orogens suggests that shear heating may be a viable mechanism for leucogranite generation in thickened crusts

constraints on the genesis of gold mineralization at the homestake gold deposit black hills south dakota from rhenium osmium sulfide geochronology the homestake gold deposit located in the black hills south dakota usa is one of the largest known hydrothermal gold deposits globally with total mining production exceeding 40 moz au rhenium osmium geochronology of ore associated arsenopyrite and pyrrhotite was performed in an effort to delineate the timing of gold mineralization in relation to known tectonothermal events in the northern black hills arsenopyrite yields a rhenium osmium re os age of 1 736 8 ma mean squared weighted deviation 1 6 consistent with existing age constraints for gold mineralization whereas re os pyrrhotite data are highly scattered and do not yield a meaningful mineralization age this is taken to indicate that the re os arsenopyrite chronometer is robust to at least 400 c whereas the re os pyrrhotite chronometer is likely disturbed by temperatures of 300 350 c the re os arsenopyrite age and initial os ratio 0 28 0 15 are interpreted to indicate that gold was introduced at ca 1 730 ma coincident with the onset of exhumation of crustal blocks and possibly the earliest intrusive phases of harney peak granite magmatism new in situ u pb monazite analyses from an aplite dike in the east central black hills indicate that granite magmatism was a protracted event persisting until at least ca 1 690 ma

petrogenesis of the 2 1 ga lushan garnet bearing quartz monzonite on the southern margin of the north china craton and its tectonic implications abstract the lushan garnet bearing quartz monzonite was emplaced into the western taihua complex at 2134 17 ma on the southern margin of the north china craton ncc the rocks contain sio 2 contents of 56 98 59 05 wt with high k 2 o na 2 o 7 46 9 14 wt and al 2 o 3 15 87 17 58 wt and low cao 2 38 2 77 wt and mgo 1 18 1 50 wt contents and belong to an intermediate shoshonitic series magmatic garnet grains in the rocks are chemically homogeneous with relatively high cao 6 36 7 92 wt but low mno 1 45 1 94 wt and mgo 2 47 3 45 wt contents and high fe mn ratios they contain 61 94 66 39 mol almandine 18 60 23 40 mol grossular 10 06 15 11 mol pyrope and 1 09 4 32 mol spessartine similar to high pressure garnets formed from m type magma but different from those found in s type granites and metapelites phase equilibrium diagrams in combination with trace element distribution coefficients between zircon and garnet suggest whole rock zircon saturation temperatures varying from 891 c to 951 c and the mineral assemblage of garnet plagioclase amphibole or biotite in these rocks is stable at 15 kbar and 890 950 c with h 2 o t 800 c all of the rocks are enriched in lile and lree with low nb la ratios 0 24 0 61 similar to those of lithospheric mantle which has a nb la ratio of 0 5 therefore the garnet and whole rock chemical compositions indicate that the lushan quartz monzonite possibly formed by small degrees of partial melting of a phlogopite bearing lithospheric mantle the variable zircon hf t values 3 5 to 4 1 with t dm c from 2 34 ga to 2 51 ga suggest involvement of crustal materials the rocks have slightly negative nb and ta anomalies characteristics of island arc basalts iab however their high zr 598 926 ppm and zr y ratios 17 1 21 8 suggest an affinity with ocean island basalt oib hence the mixed iab and oib like geochemical signatures are indicative of continental intra plate rifting possibly related to extension and thinning of the lithosphere and upwelling of asthenosphere our study proposes that an intra continental rifting event occurred at 2 2 2 0 ga on the southern margin of the ncc after the final amalgamation of ncc

juvenile magmatism and crustal recycling at the end of the neoarchean in western shandong province north china craton evidence from shrimp zircon dating western shangdong province experienced major crustal growth as a result of 2 75 to 2 50 ga tectonothermal events different from other archean areas in the north china craton besides early neoarchean tonalite trondhjemite granodiorite ttg and supracrustal assemblages there are large volumes of late neoarchean gabbroic dioritic and granitic rocks in western shandong shrimp zircon dating of thirty one samples of different rock types from late neoarchean rocks yielded a narrow age range from 2560 to 2490 ma based on these data and previously published results the following conclusions can be drawn 1 the archean basement can be divided into three belts a late neoarchean 2525 2490 ma crustally derived granite belt in the northeast an early neoarchean 2 75 2 60 ga rock belt in the center and a late neoarchean 2550 2500 ma belt of juvenile rocks in the southwest 2 the tectonic regime in western shandong province probably changed from compressional to extensional at around 2525 ma between 2530 and 2520 ma 3 crustal recycling with addition from the mantle occurred at the end of the neoarchean 4 the late archean magmatic rocks were probably formed in an arc environment

a late archean foreland fold and thrust belt in the north china craton implications for early collisional tectonics abstract the archean north china craton is divided into the western and eastern blocks along the central orogenic belt a 1600 km long archean foreland basin and thrust belt fringes the eastern side of the central orogenic belt rocks in the orogen form tectonically stacked east vergent fold and thrust sheets including foreland basin sediments 2 50 ga ophiolitic melange and an island arc complex foreland basin sediments overlie a passive margin sequence and include a 2 50 ga deep water turbidite sequence that grades upward and westward into shallow water molasse now disposed in structurally imbricated east verging thrusts and asymmetric folds that gradually migrated craton ward with deformation uplift and erosion of the orogen there is a strong linked relationship of the formation of the foreland basin to collision of the east and west blocks of the north china craton along the central orogenic belt at 2 50 ga the qinglong foreland basin and central orogenic belt of the north china craton represents one of the best preserved archean orogen to craton transitions in the world its classic internal to external zonation and flexural response to loading demonstrate that convergent tectonics in the archean were broadly similar to phanerozoic convergent margin processes

u pb and hf isotopic study of detrital zircons from the yejishan group of the luliang complex constraints on the timing of collision between the eastern and western blocks north china craton abstract the trans north china orogen is a recently recognized continent continent collisional belt along which the discrete eastern and western blocks amalgamated to form the coherent basement of the north china craton however there is a hot debate on the timing of the collision between the two blocks with one school of thought arguing the collision occurred at 2 5 ga whereas others believe it took place at 1 85 ga u pb ages and hf isotopic data of detrital zircons from foreland basins in the trans north china orogen can provide rigorous constraints on this controversial issue one of such foreland basins is represented by the yejishan group in the luliang complex in the middle sector of the western margin of the trans north china orogen the group consists of conglomerates quartz wackes sandstones and minor limestones most of which contain large amounts of detrital zircons cl images reveal that nearly all detrital zircons from the yejishan group possess oscillatory zoning with th u ratios of 0 2 2 34 typical of an igneous origin the 207 pb 206 pb ages of detrital zircons range between 3 24 and 1 81 ga with most having paleoproterozoic ages of which the youngest group ages are around 1 87 1 81 ga this indicates that the foreland basin in which the yejishan group was deposited must have formed not earlier than 1 87 1 81 ga thus supporting the collision between the eastern and western blocks at 1 85 ga not at 2 5 ga

single zircon grains record two paleoproterozoic collisional events in the north china craton abstract the western block of the north china craton consists of the yinshan block in the north and the ordos block in the south which were amalgamated along the east west trending khondalite belt at 1 95 ga the western block then collided with the eastern block to form the coherent basement of the north china craton along the north south trending trans north china orogen at 1 85 ga the huaian complex a high grade terrrane located at the conjunction of the khondalite belt and trans north china orogen records metamorphic events associated with both collisions the complex consists of lithologies from both the khondalite belt and trans north china orogen of which the former consist of graphite garnet sillimanite gneiss garnet quartzite felsic paragneiss calc silicate rock and marble together called the khondalite series zircons in the graphite garnet sillimanite gneiss can be divided into three types 1 spherical grains without internal structures 2 grains with a core and rim structure and 3 grains with a dark core surrounded by double rims except for the dark core in type 3 all other types of zircon domains are structureless and highly luminescent with very low th u ratios typical of a metamorphic origin analyses on the cores of type 2 and the inner rims of type 3 from two samples yield upper intercept ages of 1946 26 and 1947 22 ma similar to previously determined metamorphic ages from the khondalite belt and thus interpreted as the time of collision between the yinshan and ordos blocks analyses on type 1 zircons rims of type 2 and the outer rims of type 3 from the same two samples give ages of 1850 15 and 1857 16 ma interpreted as the time of collision between the eastern and western blocks thus zircons in the graphite garnet sillimanite gneiss of the huaian complex record both of the paleoproterozoic collisional events in the north china craton

the paleoproterozoic north hebei orogen north china craton s collisional suture with the columbia supercontinent abstract understanding the geologic history and position of the north china craton in the paleoproterozoic columbia supercontinent has proven elusive paleoproterozoic orogenic episodes 2 00 1 85 ga are temporally associated with ultimate stabilization of the north china craton ncc followed by the development of extensive craton wide rift systems at 1 85 1 80 ga the age difference between the sedimentary cover and the metamorphic basement is up to 500 700 ma suggesting that uplift and doming of cratonic basement occurred in the latest paleoproterozoic mafic dike swarms 1 80 1 77 ga and anorogenic magmatism 1 80 1 70 ga record the extensional breakup and dispersal of the north china craton during this stage the late paleoproterozoic tectonic framework and geological events documented provide important constraints for reconstruction of the ncc within the late paleoproterozoic supercontinent of columbia an east west striking thousand kilometer long belt of khondalites granulite facies metapelites stretches along the northern margin of the north china craton on the cratonward side of the northern hebei orogenic belt this granulite belt includes mg al sapphirine bearing granulites that reached ultrahigh temperature peak metamorphic conditions of 1000 c at 10 kbars at 1927 11 ma following peak ultrahigh temperature conditions the rocks underwent initial isobaric cooling and subsequent isothermal decompression and these trajectories are interpreted to be part of an overall anti clockwise p t evolution indicating that the northern margin of the craton experienced continental collision at 1 93 1 92 ga the position of the khondalite belt south of the northern hebei orogenic belt makes it analogous to tibet a continental collision related plateau characterized by double crustal thicknesses and granulite facies metamorphism at depth we suggest that the tectonic evolution of the ncc during this period was closely related to the assembly and break up of the columbia supercontinent and that the ncc was adjacent to the baltic and amazonian cratons in the period 2 00 1 70 ga craton wide extension occurred within 100 150 ma of collision along the northern margin of the craton at 1 93 1 92 ga it is concluded that mantle upwellings are chiefly responsible for the breakup of the ncc from the paleoproterozoic supercontinent

re os sm nd u pb and stepwise lead leaching isotope systematics in shear zone hosted gold mineralization genetic tracing and age constraints of crustal hydrothermal activity abstract a combined re os sm nd u pb and stepwise pb leaching pbsl isotope study of hydrothermal mo w bearing minerals and base metal sulfides from two adjacent shear zone hosted gold deposits ran kimberley in the harare shamva greenstone belt zimbabwe constrain the timing of the mineralizing events to two periods during an initial late archean event 2 60 ga a first molybdenite scheelite bearing paragenesis was deposited in both shear zone systems followed by a local reactivation of the shear systems during an early proterozoic 1 96 ga tectono thermal overprint during which base metal sulfides and most of the gold was re deposited while pbsl has revealed an open system behavior of the u pb systematics in molybdenite and wolframite from the ran mine initial archean re os ages are still preserved implying that this system in these minerals was more resistant to the overprint a similar retentivity could be shown for the sm nd system in scheelite and powellite associated with the above ore minerals re os isotopic data from the proterozoic mineralization in the kimberley mine point to a recent gain of re most pronouncedly affecting fe rich sulfides such as pyrrhotite a significant re loss in powellitic scheelite an alteration phase of molybdenite bearing scheelite coupled with a marked loss of u in w mo ore minerals complements the observation of a major re uptake in fe sulfides during oxidizing conditions in a weathering environment pyrrhotite under these conditions behaves as an efficient re sink lead isotope signatures from pbsl residues of molybdenite powellite and quartz indicate a continental crustal source and or contamination for the mineralizing fluid by interaction of the fluids with older sedimentary material as represented by the direct host country rocks our investigation reveals the potential of the re os isotopic system applied to crustal hydrothermal ore minerals for genetic tracing and dating purposes the simplified chemical separation of re and os from geological material used in this study together with improvements of chemical yields will enable high precision data to be collected rapidly on crustal material with low os concentrations in the future

genesis of the hengling magmatic belt in the north china craton implications for paleoproterozoic tectonics abstract the 2200 1880 ma igneous rocks in the central and eastern parts of the north china craton ncc constitute a new hengling magmatic belt hmb which includes the 2147 ma hengling mafic sill dyke swarm the 2060 ma yixingzhai mafic dyke swarm and the 1973 ma xiwangshan mafic dyke swarm the three swarms are contiguous and have experienced variable degrees of metamorphism from greenschist to low amphibolite facies hengling medium granulite facies yixingzhai and medium high pressure granulite facies xiwangshan they are all tholeiitic in composition typically with 47 52 wt sio 2 and 4 10 wt mgo and all show light rare earth element enrichments and nb and ta depletion their nd t dm ages are in the range of 2 5 3 0 ga specifically the hengling and yixingzhai dykes sills are depleted in th u zr hf and ti whereas the xiwangshan dykes are enriched in u and weakly depleted in other elements variable sr anomalies indicate significant feldspar accumulation positive anomalies or fractionation the end t values of the three swarms are 3 2 3 0 hengling 1 7 1 8 yixingzhai and 1 4 1 0 xiwangshan these mafic representatives of the hmb originated from the 2 5 ga sub continental lithospheric mantle of the ncc and with a type granites and other igneous associations in this belt they likely evolved in an intra continental rift the progressive changing compositions of the three swarms are interpreted in terms of their source regions at different depths i e shallower and shallower through time and the decrease in scale and size of the intrusions and their magma volumes indicate the progressive weakening of magmatism in this rift the rocks in this belt are different chronologically petrologically and chemically from those in the xuwujia magmatic belt xmb we propose that the two magmatic belts represent two different magmatic systems in different blocks of the ncc i e an eastern block with the hmb and a western block with the xmb terminal collision was possibly a result of ridge subduction between the two blocks which led to exhumation of the igneous rocks in the two belts from different crustal levels distinguishable by their different grades of metamorphism

shrimp data on zircons from the wanzi series constraints on the rock formation time and implications of migmatization at 2 1 2 0 ga in the fuping complex north china craton in most of the rocks of the fuping complex in the trans north china orogen a large quantity of leucocratic veins dykes and granitic intrusions are present as migmatites the incongruent partial melting of biotite to hornblende suggests the water fluxed anatexis of the migmatization of which the melt is manifested by the euhedral crystallization of some quartzofeldspathic minerals and presence of albite rim on earlier plagioclase or k feldspar importantly some melt may be segregated in the sillimanite gneisses or extracted and injected as faint dispersed melt into the competent metasedimentary quartzite of the wanzi series rocks the quartzite has plenty of zircon ages cluster at c 2 5 ga and some at c 2 1 2 0 ga the former may be detrital zircon age while the latter represents the age of migmatization or crystallization of the injected melt not detrital zircon ages as conventionally believed the data have constrained that the sedimentation time of the protolith of the quartzite of the wanzi series was between c 2 5 ga and c 2 1 2 0 ga not younger than c 2 1 2 0 ga as once proposed therefore it must be cautious in applying detrital zircon ages in constraining the time of sedimentation if the rocks have undergone middle high grade metamorphism especially when migmatization was involved in the metamorphism process

ore genesis of the fancha gold deposit xiaoqinling goldfield southern margin of the north china craton constraints from pyrite re os geochronology and he ar in situ s pb isotopes abstract the fancha gold deposit is one of the representative lode type deposit in the xiaoqinling goldfield four stage mineralization process were identified namely i the barren quartz stage ii the pyrite dominated stage iii the quartz polymetallic sulfides stage and iv the quartz carbonate stage correspondingly pyrite crystal can be divided into three generations i e py1 py2 and py3 according to their different textures and paragenesis during the first three mineralization stages in order to better understand the ore genesis an integrated analysis on re os he ar s pb multi isotopes was carried out for each mineralization stage to delineate the timing of the mineralization and the origins of the ore forming materials in detail gold bearing pyrite yields a re os isochron age of 124 3 2 6 ma mswd 1 9 overlapping with the previous hydrothermal mica 40ar 39ar ages suggesting that gold mineralization occurred in the early cretaceous the 3he 4he r ra and 40ar 36ar values of the py2 are 0 68 to 1 17 and 656 55 to 7384 2 and those of the py3 are 0 20 to 0 33 and 647 67 to 8913 55 respectively which reveal a significant contribution of mantle component in the ore forming fluids and the content of crust fluid is increasing in the later evolution in situ 34s values of the sulfides and tellurides in the auriferous quartz veins including the pyrite py1 py2 and py3 chalcopyrite sphalerite galena bismuthinite and tetradymite display a relatively narrow range 4 9 to 3 6 consistent with magmatic sulfur which are quite different from those of the pyrite in surrounding rock 7 8 to 9 3 in situ pb isotopic compositions of the galena and bismuthinite of stage iii are homogeneous with values of 208pb 204pb 207pb 204pb and 206pb 204pb being 37 482 37 508 15 392 15 523 16 995 17 006 and 39 279 39 287 15 588 15 590 18 283 18 288 respectively indicating a mixed pb sources with obvious proportions of mantle combined with previous data we suggest that the ore forming fluids and metals of the fancha gold deposit stemmed directly from the regional mantle derived magmatic hydrothermal system which is coupled with the large scale lithospheric extension and thinning in the eastern north china craton during the early cretaceous

tectonic setting and significance of 2 3 2 1ga magmatic events in the trans north china orogen new constraints from the yanmenguan mafic ultramafic intrusion in the hengshan wutai fuping area abstract a detailed study of the 2 2 ga yanmenguan mafic ultramafic intrusion in the hengshan area is presented with the aim of clarifying the tectonic history of the trans north china orogen in the north china craton during the period 2 3 2 1 ga shrimp u pb dating of magmatic zircons from a gabbro indicates emplacement at 2193 ma the whole rock and mineral geochemistry of this intrusion are akin to alaskan type complexes and are typical of subduction related magmas the parental magma likely originated from the depleted mantle wedge above a subduction zone and experienced fractional crystallization of olivine and clinopyroxene the continental arc represented by the mafic ultramafic intrusions and contemporary granitic and volcanic rocks together with the luliang back arc basin constitute a marginal arc back arc basin system that evolved between 2 3 and 2 1 ga based on geochronological datasets and available data on magmatism metamorphism and structural deformation a comprehensive new tectonic model for the evolution of the tnco during the neoarchean to paleoproterozoic is constructed 1 2 5 ga intra oceanic arc back arc basin system and its accretion at the end of the neoarchean 2 a long hiatus in magmatism and tectonism between 2 5 and 2 3 ga 3 2 3 2 1 ga marginal arc back arc basin magmatism developed in the accreted hengshan wutai fuping arc terranes and the luliang terrane 4 2 1 ga collision resulting in closure of the 2 3 2 1 ga luliang back arc basin accretion of the luliang hengshan wutai fuping arc terranes and their attachment to the western block of the north china craton 5 the intrusion of 1970 1910 ma post collisional mafic dykes in an extensional event 6 final collision between the western and eastern blocks between 1 9 and 1 8 ga resulting in extensive magmatism tectonism and metamorphism and lastly 7 post 1 8 ga crustal extension

petrogenesis of syenite granite suites from the bryansky complex transbaikalia russia implications for the origin of a type granitoid magmas abstract two syenite granite suites metaluminous and peralkaline that form the bryansky complex in transbaikalia russia have been studied with the aim to constrain the existing models of a type granitoid magma generation the bryansky complex is a large intrusive body of about 1600 km2 emplaced in the central part of the mongolian transbaikalian granitoid belt which extends for more than 2000 km and is 200 300 km wide the belt comprises about 350 a type granitoid plutons and numerous volcanic fields u pb and rb sr isotope dating revealed that all the intrusive rocks of the complex and closely associated comendites were emplaced within a narrow time span 279 285 ma the isotope characteristics are rather similar for all main rock types the metaluminous suite has a 87sr 86sr t value of 0 7050 0 001 end t from 1 9 to 3 0 and the peralkaline suite has 87sr 86sr t 0 7053 0 0008 end t 2 1 and 2 4 comendites and trachyandesites have similar end t values from 2 2 to 3 5 but a slightly higher 87sr 86sr t value of 0 7062 0 0002 the systematic change in chemical and mineralogical composition from syenitic to granitic rocks in both suites and the similar isotopic ratios suggest that the granites were formed by fractional crystallization of the syenite magmas several lines of evidence suggest that metaluminous syenite is the parental magma for the whole bryansky complex study of melt inclusions in quartz phenocrysts from the peralkaline granite and in pyroxene from the nordmarkite indicates that fractional crystallization has resulted in significant f enrichment in the granitic magma up to 1 5 1 7 wt the syenite magmas crystallized at rather high temperature 940 c whereas the near liquidus temperature of the peralkaline granite was lower 760 790 c very high homogenization temperatures of the melt inclusions in quartz phenocrysts from comendites 1000 1100 c suggest that the alkali rich silicic magma formed at a depth of 50 60 km far exceeding the normal crust thickness the sr nd isotope data advocate the main role of mantle derived material in the source region from which the alkali rich syenitic and granitic magmas were produced

proterozoic mineral deposits near plate margins of the archean wyoming province usa abstract proterozoic mineral deposits located near the margin of the archean wyoming province can be interpreted in a plate tectonic context the southern margin of the archean wyoming province is exposed in southeastern wyoming where early proterozoic island areas have been sutured to an early proterozoic passive margin developed during rifting of the archean wyoming craton mineral deposits include uranium thorium gold bearing quartz pebble conglomerate deposited during rifting stratiform sulphide in black shale of the foredeep basin anomalous platinum group element values in layered complexes of the island arcs and a variety of mineral deposits that formed during closure of the arcs and passive margin the eastern margin of the wyoming province is probably located under phanerozoic cover just east of the black hills of south dakota the black hills is a segment of the early proterozoic trans hudson orogen which is interpreted here as an area exhibiting evidence of opening and closing of an intracratonic basin in the black hills domes underlain by archean rocks are overlain by an early proterozoic clastic successions that preserves evidence of rifting these clastic successions are succeeded by marine quartzite which is in turn overlain by a eugeosynclinal succession containing iron formation graywacke and a variety of volcanic units the basal early proterozoic clastic succession contains uranium and gold bearing quartz pebble conglomerate formed during rifting the eugeosynclinal succession contains the homestake iron formation which is host to the early proterozoic homestake type gold deposits geologic and geochemical evidence can be marshalled to support either an epigenetic or syngenetic origin for the homestake type gold deposits if the gold deposits are syngenetic they may have formed in a foredeep basin developed during closure of the intracratonic basin

geochronology and petrogenesis of neoarchean potassic meta granites from huai an complex implications for the evolution of the north china craton abstract neoarchean potassic meta granites occurring in the huai an complex within the north central part of the north china craton ncc comprise leucocratic syenogranites biotite monzogranites and garnet bearing syenogranites high pressure garnet granulites occur as lenses sheets or dismembered dikes with the granitoids here we present shrimp zircon u pb data from the leucocratic syenogranites which show an age of 2493 6 ma mswd 1 and la icp ms u pb data from the biotite monzogranites and garnet syenogranites revealing their formation ages of 2437 10 ma mswd 2 0 and 1977 2003 ma respectively we also report metamorphic ages of ca 1800 ma from zircons in the garnet syenogranites geochemical studies show low mgo 0 5 0 06 wt cr 6 19 0 53 ppm and ni 27 66 2 01 ppm with varying sio2 62 3 to 76 8 wt contents indicating their derivation from a crustal source the leucosyenogranites exhibit elevated positive eu anomalies with decreasing sr th u and bulk ree concentrations and th u ratio whereas rb and ba values increase with decreasing th u ratio indicating fluid melt interaction the leucosyenogranites have high sr and ba contents and sr y and la yb n ratios and low y and hrees with zircon ehf t values ranging from 1 7 to 5 8 and hf model ages of 2 60 2 73 ga reflecting their derivation by partial melting of overthickened juvenile lower crust of eclogitic composition or an eclogitic mineral assemblage in the residue geochemical compositions of the biotite monzogranites with positive end t values 2 3 to 2 5 and model ages

discovery of the huronian glaciation event in china evidence from glacigenic diamictites in the hutuo group in wutai shan abstract paleoproterozoic glacigenic diamictites have provided key evidence for the great oxidation event goe at 2 3 ga associated with the widespread huronian glaciation event hge here we report for the first time paleoproterozoic glacigenic diamictite from the sijizhuang formation of the hutuo group in wutai shan china formed during the late stage of hge the sijizhuang formation comprises diamictite sandstone and siltstone mudstone facies conforming to a glaciomarine association the diamictites show weak sorting and roundness and carry a few striated and faceted boulders typical dropstones and associated structures are present in the sandstone and siltstone mudstone facies diamictite facies which characterizes the lower portion of the sijizhuang formation displays an upward decrease with a concomitant increase in the proportions of sandstone and siltstone mudstone facies the lithofacies associations of diamictite with sandstone and siltstone mudstone indicate a frequently changing hydrodynamic environment the petrochemical compositions of different lithofacies show that the clasts were multi sourced and generated by limited weathering in keeping with a cold climate available isotope age data constrain the glacigenic deposition of the sijizhuang formation to between 2 40 and 2 13 ga and most likely from 2 2 to 2 1 ga this estimate compares well with the 2 4 2 1 ga age of the widespread hge but the sijizhuang diamictite appears to be younger than the last glacial unit gowganda formation in the type area of the hge in canada and suggests a locally protracted glacial event

re os dating of sulfides associated with gold mineralization in central victoria australia we report re os isotope ages that are the first for sulfides closely associated with gold mineralization at the bendigo and maldon goldfields in central victoria two arsenopyrite samples from bendigo contain 11 ppb re and 0 08 ppb highly radiogenic os llhr samples stein et al 2000 and have model ages of 444 7 and 447 7 ma with a mean model age of 446 5 ma a five point isochron based on these arsenopyrite analyses plus three pyrite analyses including one replicate gives an age of 438 6 ma molybdenite associated with au te cu and sb mineralization at maldon gives an re os age of 376 2 ma similar to the age of the nearby har court granite these results support previous 40 ar 39 ar data indicating that initial emplacement of gold mineralization in central victoria occurred in the late ordovician during the earliest stages of accretionary development of the southern lachlan fold belt the molybdenite data are also consistent with the mobilization and or introduction of gold during late devonian magmatism

2480 ma mafic magmatism in the northern black hills south dakota a new link connecting the wyoming and superior cratons the laramide black hills uplift of southwest south dakota exposes a precambrian crystalline core of 2560 2600 ma basement granitoids nonconformably overlain by two paleoproterozoic intracratonic rift successions in the northern black hills a 1 km thick layered sill the blue draw metagabbro that intrudes the older rift succession provides a key constraint on the timing of mafic magmatism and of older rift basin sedimentation ion microprobe spot analyses of megacrysts of magmatic titanite from a horizon of dioritic pegmatite in the uppermost sill portion yield a 207pb 206pb upper intercept age of 2480 6 ma all age errors 2 comparable to two point 207pb 206pb errorchron ages obtained by pb stepwise leaching of the same titanites nearly concordant domains in coexisting magmatic zircon yield apparent spot ages ranging from 2458 16 to 2284 20 ma i e differentially reset along u pb concordia and hornblende from an associated metadiorite yields a partially reset date with oldest apparent a

re os geochronological evidence for multiple paleoproterozoic gold events at the scale of the west african craton the archean proterozoic craton of west africa hosts numerous gold deposits which are spatially and temporally related to the eburnean orogeny that took place between 2250 and 1980 ma and included multiple deformation events the majority of these gold deposits are located along shear zones the structural history is relatively well established for most gold deposits but absolute timing of the mineralization is commonly lacking five deposits hosted in the baoule mossi domain located in the southwestern part of the west african craton were studied to better constrain timing of gold events nassara and kiaka in burkina faso and wassa damang and obuasi in southern ghana gold mineralization was structurally characterized at each deposit and dated by the re os method on pyrite arsenopyrite and pyrrhotite grains that were coeval with the gold deposition combined structural and re os geochronological constraints allow two groups of gold deposits to be distinguished early orogenic gold formed during the eoeburnean orogeny i e between 2190 and 2125 ma as represented by kiaka1 at 2157 24 ma and wassa1 at 2164 14 ma these early gold occurrences are expressed as early disseminated gold enrichment 1 3 g t au at kiaka and gold hosted in folded and boudinaged quartz veins that are transposed along the primary s foliation at wassa these re os ages on the early sulfides at wassa and kiaka are significant because they definitively provide the first direct age constraints on gold only eoeburnean mineralization in the birimian of west africa late orogenic gold hosted by brittle structures and with higher gold grades up to 60g t at kiaka formed during late eburnean deformation d d events between 2120 and 2000 ma as represented by the re os dating of these ores at wassa at 2055 18 ma and obuasi at 2045 40 ma similar style ores dated at nassara at 2131 99 ma and damang at 2080 135 ma but both with large uncertainties and an undated younger event at kiaka are also suggested to be part of this later mineralizing episode the high uncertainties on the nassara and damang ages may be directly linked to the low rhenium and osmium contents of the studied samples these results highlight the polyphase character of the widespread gold mineralization in the west african craton although the late stages of the eburnean orogeny constitute a prolific period for the formation of high grade gold mineralization identification of less well studied early stage gold deposits which can also contain large quantities of gold is critical for mineral exploration in the west african craton

40ar 39ar evidence for middle proterozoic 1300 1500 ma slow cooling of the southern black hills south dakota midcontinent north america implications for early proterozoic p t evolution and posttectonic magmatism 40ar 39ar total gas and plateau dates from moscovite and biotite in the southern black hills south dakota provide evidence for a period of middle proterozoic slow cooling early proterozoic 1600 1650 ma mica dates were obtained from metasedimentary rocks located in a synformal structure between the harney peak and bear mountain domes and also south of bear mountain metamorphic rocks from the dome areas and undeformed samples of the 1710 ma harney peak granite hpg yield middle proterozoic mica dates 1270 1500 ma two samples collected between the synform and bear mountain dome yield intermediate total gas mica dates of 1550 ma we suggest two end member interpretations to explain the map pattern of cooling ages 1 subhorizontal slow cooling of an area which exhibits variation in mica ar retention intervals or 2 mild folding of a middle proterozoic 1500 ma 300 c isotherm according to the second interpretation the preservation of older dates between the domes may reflect reactivation of a preexisting synformal structure and downwarping of relatively cold rocks during a period of approximately east west contraction and slow uplift during the middle proterozoic the mica data together with hornblende data from the black hills published elsewhere indicate that the ambient country rock temperature at the 3 4 kbar depth of emplacement of the hpg was between 350 c and 500 c suggesting that the average upper crustal geothermal gradient was 25 40 c km prior to intrusion the thermochronologic data suggest hpg emplacement was followed by a 200 m y period of stability and tectonic quiescence with little uplift we propose that crust thickened during the early proterozoic was uplifted and erosionally thinned prior to 1710 ma and that the hpg magma was emplaced into isostatically stable crust of relatively normal thickness we speculate that uplift and crustal thinning prior to hpg intrusion was the result of differential thinning of the subcrustal lithosphere beneath the black hills if so this process would have also caused an increase in mantle heat flux across the moho and triggered vapor absent melting of biotite to produce the hpg magma this scenario for posttectonic granite generation is supported in part by the fact that in the whole of the black hills the hpg is spatially associated with the deepest exposed early proterozoic country rock

petrogenesis of the 2090 ma zanhuang ring and sill complexes in north china a bimodal magmatism related to intra continental process abstract it is widely debated as to whether the 2100 ma bimodal magmatic events in the eastern north china craton represent intra continental rifting or marginal processes the genetic relationship between the two components of bimodal suites is the key solution to this issue and this is investigated on a group of newly identified ring complexes as well as the apparently associated sills in zanhuang southern taihang mts these ring complexes comprise varied components pyroxenite gabbro anorthosite k rich a type granite and minor na rich a type granite u pb analyses on baddeleyite and zircon grains liberated from a gabbro sample both yield a 2090 ma age which represents crystallization timing and is contemporaneous with a type granites of the ring complexes as well as the sills in the gantaohe group the mafic rocks of the ring complexes and sills show similar chemical features they were characterized by enriched light rare earth elements compared to middle and heavy ones la ybn 1 4 2 8 enriched large lithosphile elements but depleted high field strength elements and enriched sr nd isotopes endt t 2090 ma 1 9 0 6 nd tdm 2 7 ga they were most likely originated from the same magma source resided in the sub continental lithospheric mantle which formed in the late archean the compositional variation of the mafic sills may be resulted from the fractionation of olivine and plagioclase the k rich a type granite as well as the dacitic sills could be originated from this same source with firstly olivine plagioclase dominated fractionation in the lower magma chamber to more felsic compositions and then with significant removal of clinopyroxene plagioclase in the upper magma chamber which at the same time produced pyroxenite gabbro and minor anorthosite cumulates in the ring complexes na rich a type granite was likely formed locally under relatively high water fugacity with fractionation of k feldspar plagioclase clinopyroxene from a parental magma similar to the k rich a type granite this bimodal zanhuang ring and sill complexes and other comparable bimodal suites widely distributed in the eastern north china craton were likely intra continental representing a craton scale extension at 2100 ma

understanding the microscale spatial distribution and mineralogical residency of re in pyrite examples from carbonate hosted zn pb ores and implications for pyrite re os geochronology abstract accurate and precise geochronology using the re os isotopic system in pyrite is an invaluable tool for developing and confirming genetic models of ore systems however as a bulk method the results produced by pyrite re os geochronology are commonly complex and many imprecise isochrons exist in the literature using la icpms methods it is now possible to map and quantify re distribution at the ppb level allowing an unprecedented look into the re os systematics of pyrite bearing ore two samples from the lisheen zn pb ore deposit in ireland showing disparate re os isotopic behavior were investigated in situ sulfur isotope measurements using sims an analytical technique not previously attempted on the irish deposits was used to supplement the re os dataset a massive pyrite sample from the main zone produced a precise low scatter isochron 346 6 3 0 ma mswd 1 6 the re distribution in this sample is relatively homogeneous with the re budget dominated by pyrite containing 1 5 ppb re but the 34s varies significantly from 45 2 to 8 2 a second more paragenetically complex sample from the derryville zone produced a younger age with high scatter 322 11 ma mswd 206 and this also displays a large variation in 34s 53 to 4 the cores of grains of main stage iron sulfide are depleted in trace elements and show low re abundances a second re os dataset from zn pb mineralization at hawker creek nunavut canada was produced from massive pyrite that displays low re concentrations in general therefore the complexities of pyrite re os geochronology can result from impurities in mineral separates attempts to eliminate impurities through different mineral separation techniques e g crushing heavy liquid separation magnetic separation acid leaching are frequently only partially successful and therefore full characterization of any resulting mineral separates is extremely important we conclude that la icpms mapping of re and mo distributions is essential for the identification of such impurities although other trace element la icpms maps in situ sulfur isotope measurements and petrographic evidence were of limited use in assessing the re budget of a sample they are invaluable in linking the documented re distribution obtained through la icpms to re os geochronological results

rhenium osmium geochronology of arsenopyrite in meguma group gold deposits meguma terrane nova scotia canada evidence for multiple gold mineralizing events rhenium osmium geochronology using arsenopyrite was undertaken for three gold deposits in the meguma terrane nova scotia canada in order to better constrain their age of formation and to assess the utility of arsenopyrite for dating similar deposits globally analyses of arsenopyrite from bedding concordant veins from the ovens locality southwest meguma terrane yield a precise re os isochron age of 409 5 ma arsenopyrite analyses from a bedding discordant vein at this locality indicate an identical re os age of 407 4 ma saddle reef veins from the dufferin deposit northeast meguma terrane contain arsenopyrite with a precise re os isochron age of 380 3 ma the lack of common os in some arsenopyrite samples from both the ovens and dufferin permit calculation of single mineral model ages for each deposit which are identical to those determined using the isochron method initial os compositions for the two vein types at the ovens suggest a predominately crustal source of os in the mineralizing fluids whereas a less radiogenic initial os composition for arsenopyrite from dufferin does not as clearly define a crustal metal source at a third locality the touquoy deposit the re os systematics of arsenopyrite associated with disseminated gold mineralization do not define a precise formation age possibly as a result of mixing of re and os derived from the mineralizing fluid and the shale host rock the re os ages of arsenopyrite indicate that there were at least two distinct periods of gold deposition in the meguma terrane coinciding with widespread tectonothermal events regional deformation and metamorphism associated with acadian orogenesis and widespread generation of meta and peraluminous granites and high grade metamorphism within the basement rocks under the meguma terrane the ca 407 ma age for the ovens arsenopyrite provides the best estimate for the timing of regional acadian deformation in the meguma terrane and is slightly older than previous estimates based solely on 40ar 39ar dating

are iron rich sedimentary rocks the key to the spike in orogenic gold mineralization in the paleoproterozoic paleoproterozoic rocks with ages from 2100 to 1800 ma are recognized as belonging to one of the three main periods in earth history that host orogenic gold deposits in contrast to older archean orogenic gold deposits which are hosted predominantly in greenstone terranes supracrustal sedimentary rocks became increasingly important as hosts in the paleoproterozoic unusually iron rich 1840 ma marine mudstones in the tanami region host one world class gold deposit and many other large gold deposits fluid rock modelling at 300 and 325 c suggests a strong correlation between gold grade and these iron rich fine grained sedimentary rocks new regional stratigraphic correlations for similar iron rich rocks to those in the tanami region are suggested with 1860 ma gold bearing stratigraphy in the pine creek region these northern australian paleoproterozoic iron rich sedimentary rocks could be linked globally to iron rich and gold bearing sedimentary rocks deposited between 2100 to 1800 ma in homestake south dakota united states and in ghana and brazil worldwide from about 2400 to 1800 ma the paleoproterozoic is also marked by the deposition of mainly superior style bifs which are attributed to the progressive oxygenation of the deep oceans resulting in the global scrubbing of iron from the oceans the high iron concentrations noted in pre 1800 ma marine sediments in northern australia could also be related to this same process occurring globally and help explain a worldwide peak in orogenic au deposits hosted in rocks aged from 2100 to 1800 ma

geochemistry of the paleoproterozonic nanying granitic gneisses in the fuping complex implications for the tectonic evolution of the central zone north china craton abstract the nanying granitic gneisses represent a major 2 0 ga magmatic event in the fuping complex of the central zone of the north china craton they occur in the chengnanzhuang shear zone which is chiefly developed along the contact between the neoarchean fuping tonalite trondhjemite granodiorite ttg gneisses and the wanzi paragneisses the nanying granitic gneisses were derived from monzogranite and syenogranite with minor amounts of quartz diorite and granodiorite and belong to the metaluminous to peraluminous high potassium calc alkaline series correlations between major and trace elements and similarities in chondrite normalized rare earth element ree patterns support field relationships e g gradational contacts indicating that the nanying granitic gneisses originated from similar magmas the granodioritic monzogranitic and syenogranitic gneisses have similar ranges of initial 87sr 86sr values 0 7030 0 7093 0 7021 0 7193 and 0 7049 0 7111 respectively although these variations are partly attributable to local resetting of the isotopic system during late deformation hydrothermal alteration the whole rock 208pb 204pb 206pb 204pb and 207pb 204pb values of the nanying granitic gneisses are 37 32 76 8 15 24 23 82 and 14 92 17 92 respectively these sr pb isotope systematics together with previously published nd isotope compositions suggest that the nanying granitic gneisses were derived from partal melting of the 2 50 ga fuping ttg gneisses with local assimilation involving the wanzi paragneisses the nanying granitic gneisses are geochemically similar to the cca and ccl groups of granitoids except for a few samples which plot in the hlo group the nanying granitic gneisses exhibit depletion in rb sr ba ti and u and enrichment in th nb zr hf and ree similar to those of syn shear high potassium calc alkaline granitoids and late shear alkaline granitoids they therefore represent the product of a syn collisional to post collisional magmatic event this collisional event which is significantly older than the 1 84 1 80 ga event that has been proposed for the juxtaposition of the eastern and western archean continental blocks in the final stabilization of the north china craton supports a tectonic model involving multiple collisional events for the amalgamation of the internal lithotectonic domains within the central zone and the assembly of the north china craton as a whole