origin of the alxa block western china new evidence from zircon u pb geochronology and hf isotopes of the longshoushan complex abstract the nw se trending longshoushan is in the southwestern margin of the alxa block which was traditionally considered the westernmost part of the north china craton ncc precambrian crystalline basement exposed in the longshoushan area was termed the longshoushan complex this complex s formation and metamorphism are significant to understand the geotectonics and early precambrian crustal evolution of the western ncc in this study field geology petrology and zircon u pb and lu hf isotopes of representative orthogneisses and paragneisses in the longshoushan complex were investigated u pb datings reveal three paleoproterozoic magmatic episodes ca 2 33 ca 2 17 and ca 2 04 ga and two subsequent regional metamorphic events ca 1 95 1 90 ga and ca 1 85 ga for metamorphic granitic rocks in the longshoushan complex u pb dating of the detrital magmatic zircons from two paragneisses yields concordant 207pb 206pb ages between 2 2 ga and 2 0 ga and a small number of metamorphic zircon rims provide a ca 1 95 ga metamorphic age suggesting that the depositional time of the protolith was between 2 0 and 1 95 ga and that the sedimentary detritus was most likely derived from the granitic rocks in the longshoushan complex itself zircon lu hf isotopic analyses indicate that nearly all magmatic zircons from ca 2 0 ga to ca 2 17 ga orthogneisses have positive ehf t values with two stage hf model ages tdmc ranging from 2 45 to 2 65 ga peak at ca 2 5 ga indicating that these paleoproterozoic granitic rocks were derived from the reworking of the latest neoarchean early paleoproterozoic juvenile crust detrital magmatic zircons from two paragneisses yield scattered 176hf 177hf ratios ehf t and tdmc values further indicating that the sedimentary detritus was not only derived from these plutonic rocks but also from other unreported or denuded paleoproterozoic igneous rocks the ca 2 15 ga detrital magmatic zircons from one paragneiss have negative ehf t values with tdmc ranging from 2 76 to 3 04 ga indicating another important crustal growth period in the longshoushan region these data indicate that the longshoushan complex experienced neoarchean early paleoproterozoic crustal growth approximately ca 2 3 2 0 ga experienced multiphase magmatic events and approximately ca 1 95 1 90 ga and ca 1 85 ga experienced high grade metamorphic events the sequence of tectonothermal events is notably similar to that of the main ncc together with the datasets from an adjacent area we suggest that the western alxa block was most likely an integrated component of the ncc from the neoarchean to the paleoproterozoic

formation and evolution of the paleoproterozoic meta mafic and associated supracrustal rocks from the lushan taihua complex southern north china craton insights from zircon u pb geochronology and whole rock geochemistry abstract the upper taihua complex in the lushan area is one of the best preserved paleoproterozoic metamorphic basements exposed in the southern north china craton which is mainly composed of khondalite dominated supracrustal rocks and associated meta mafic intrusions amphibolites and granitic plutons detrital zircon u pb ages of four paragneisses from the supracrustal rocks can be divided into two groups 2 39 2 10 ga and 1 97 1 83 ga concordant 207 pb 206 pb ages from the older group show typical magmatic features with a distinct peak around 2 31 2 23 ga whereas the metamorphic zircons define a remarkable age peak at 1 93 ga magmatic zircons from the amphibolite sample have a weighted mean 207 pb 206 pb age of 2128 29 ma representing the crystallization age of the protolith therefore the depositional age of the supracrustal rocks in the upper taihua complex can be well constrained at 2 23 2 13 ga geochemically the amphibolites are classified as subalkaline high fe tholeiitic basalts in compositions and characterized by slightly enrichment in lree flat hree and weakly negative eu anomalies with depleted high field strength elements e g nb ta and ti inconsistent with typical rocks derived from the asthenospheric mantle the high feo t 13 59 wt cao 6 67 wt low al 2 o 3 th e nd t values and crustal contamination sensitive elements suggest that crustal assimilation was limited during magma evolution the varied e nd t values 2 99 to 0 44 and nd model ages 2 65 2 98 ga suggest that they were likely originated from archean subcontinental lithospheric mantle sclm which have been refertilized by metasomatic processes these geochronological and geochemical signatures indicate the meta mafic rocks and associated supracrustal rocks of the upper taihua complex in the lushan area were most likely formed in an intra continental rift setting implying the extensional tectonic scenario rather than subduction related regime occurred between 2 2 and 2 0 ga at least in the southern margin of the ncc metamorphic ages suggest that the taihua complex in the lushan area was involved in subduction collision processes from 1 97 ga to 1 83 ga

metamorphic evolution and zircon u pb geochronology of the mts huashan amphibolites insights into the palaeoproterozoic amalgamation of the north china craton abstract exposed in the southernmost terminal of the palaeoproterozoic trans north china orogen tnco the garnet bearing amphibolites in the mts huashan metamorphic complex preserve three generations of mineral assemblages including the prograde peak and retrograde metamorphic stages the prograde metamorphic stage m1 is recognized as inclusion type mineral assemblages hornblende plagioclase quartz biotite ilmenite preserved in the garnet porphyroblasts the peak metamorphic assemblages m2 are represented by the garnet porphyroblasts and matrix minerals hornblende plagioclase quartz biotite ilmenite the retrograde metamorphic assemblages m3 are characterized by the symplectitic intergrowth minerals hornblende plagioclase quartz orthopyroxene biotite ilmenite rimming decomposed relict garnet porphyroblasts known as the white eye socket reaction textures calculated by geothermometers and geobarometers the p t conditions of these three metamorphic stages are estimated to be of 5 7 6 0 kbar 660 760 c for the prograde stage m1 6 6 7 7 kbar 700 820 c for the peak stage m2 and 4 5 6 7 kbar 720 760 c for the retrograde stage m3 respectively therefore these three metamorphic stages define clockwise p t paths including nearly isothermal decompression itd processes geochronological studies of zircons from the amphibolites indicate that the protolith of the amphibolites formed at 2 3 ga and the metamorphism occurred at 1 87 1 85 ga and the metamorphic period was further deciphered to be of 1 96 1 80 ga when considering our previous reports it is therefore proposed that the southernmost terminal of the tnco was also involved in the subduction and collision processes between the eastern and western blocks of the north china craton at least as early as 1 96 ga and continued as late as 1 80 ga followed by rapid uplift and exhumation the available data suggest that the palaeoproterozoic tectono metamorphic event of the southernmost terminal of the trans north china orogen not only started earlier but also continued longer than the other terranes in the middle and northern sections elsewhere in the orogen

genesis of the dianfang breccia hosted gold deposit western henan province china constraints from geology geochronology and geochemistry abstract the dianfang gold deposit located in songxian western henan province china is one of two breccia hosted gold deposits in the xiong ershan waifangshan region previous studies suggest that the breccia and host volcanic rocks of the xiong er group formed coevally the deposit consists of seven vein type orebodies occurring within the southern and hanging wall margins of the breccia pipe as well as in fractures in the rhyolites of the jidanping formation and are structurally controlled by ene trending faults their mineral assemblage includes k feldspar quartz sericite chlorite epidote carbonate adularia and sulfides formed from hydrothermal alteration related to mineralization pyrite is the dominant ore mineral and is associated with minor galena sphalerite and chalcopyrite the trace minerals include magnetite rutile cervelleite matildite and bismuthinite gold occurs as native gold and electrum inclusions in pyrite or along microfractures in sulfides and quartz four stages of hydrothermal mineralization have been identified quartz k feldspar i quartz pyrite ii polymetallic sulfide iii and carbonate adularia iv zircon u pb geochronology of the granitic matrix of the breccia constrains the timing of breccia pipe formation to 145 8 ma indicating it formed at the end of the late jurassic rather than the paleoproterozoic as suggested by previous studies hydrothermal sulfides from the main orebody yield a rb sr isochron age of 121 5 1 7 ma placing gold mineralization at the end of the early cretaceous the sulfur and lead isotope data suggest that the ore forming materials have a close connection with magmatic fluid the initial 87 sr 86 sr value of the sulfides is 0 71056 0 00012 suggesting an ancient crustal source likely the archean taihua group mixed with a juvenile mantle component the trace element characteristics of pyrite indicate the mineralizing fluid was magmatic in origin sourced from a magma derived from the partial melting of the taihua complex and may have been affected by water rock interaction at high temperatures under reducing conditions geological geochronological and geochemical data suggest the dianfang ore deposit is genetically related to a hidden late mesozoic granite and forms part of an intrusion related auriferous system gold deposits of similar ages are widespread at the southern margin of the north china craton and are coeval with the emplacement of felsic and mafic intrusions exhumation of a metamorphic core complex and formation of extensively faulted basins this suggests that the dianfang gold deposit formed under conditions of lithospheric extension caused by crustal thinning and the destruction of the lithosphere mantle beneath the craton during the late jurassic to early cretaceous

geochemistry of ttg and ttg like gneisses from lushan taihua complex in the southern north china craton implications for late archean crustal accretion the late archean taihua complex mainly composed of amphibolite and ttg and ttg like gneisses is volumetrically most important metamorphic rock suites scattered along the southern margin of the north china craton ncc zircon shrimp u pb dating shows two episodes 2765 13 and 2723 9 ma of archean magmatism in the lushan area with distinct geochemical features the early 2765 13 ma suite ttg like gneisses has low sio2 52 5 66 1 wt high mg 0 47 0 68 low hree ybn 3 0 5 4 and y 8 07 13 9 ppm with low to moderate la yb n 6 7 37 1 and sr y 25 9 119 3 the younger 2723 9 ma suite ttg gneisses has high sio2 63 5 74 3 wt low mg 0 13 0 52 very low ree ybn 1 1 20 2 43 both suites show pronounced negative nb ta and ti anomalies on the primitive mantle normalized spidergram the ttg like gneiss suite has similar bulk rock nd and zircon hf model ages 3 0 ga with end t 0 0 26 1 46 and is interpreted as resulting from melt of mantle interactions with the melts derived from partial melting of subducted ocean crust with a residual assemblage of garnet clinopyroxene rutile amphibole which favors subducted slab model for the late archean ttg the ttg gneiss suite has abundant relic zircons 2 95 2 80 ga with end t 0 1 31 to 0 23 which is best interpreted as derived from partial melting of thickened lower continental crust with a garnet amphibolite residue garnet amphibole rutile significant high pressure fractional crystallization garnet amphibole and accumulation plagioclase are also required in the petrogenesis the lushan amphibolite with nearly flat primitive mantle normalized trace element pattern is interpreted to represent early ocean crust metamorphism these observations suggest a possible model of late archean crustal accretion from ocean crust to continental terrain in the southern north china craton

geological geochronological pyrite geochemical and stable isotope studies of the tantou gold deposit in central china abstract the tantou au deposit is a large scale gold deposit in the xiong ershan area a geochemical study of pyrite was conducted using laser ablation inductively coupled plasma mass spectrometry two types of hydrothermal pyrite were identified in the ores and wall rocks i e the fine euhedral pyrite in brecciated ores py1 and coarse grains disseminated in the quartz sulfide veins py2 the trace elements of the two pyrite types exhibit different concentrations associations and rim core zoning implying different geneses and crystallization processes both pyrites of the two mineralization stages were hydrothermal pyrite however the au as co ni and zn were more rich in py1 while ag cu mo sb and pb were favored in py2 the growth rims of py1 suggest several pulses of fluid activities hydrothermal sulfides from the main ore forming stage have a rb sr isochron age of 124 2 3 1 ma placing the gold mineralization in the early cretaceous the initial 87sr 86sr value of the sulfides is 0 71124 0 00010 suggesting an ancient crust source the paleoproterozoic xiong er group in situ s isotope data acquired from the main sulfides suggest that the ore forming materials underwent isotopic fractionation because of oxidation of the ore forming fluids during the hydrothermal process the c o isotope values of calcite from the postore stage indicate that magmatic fluids were likely the principle sources of carbon and that low temperature alteration has had a significant effect on the levels of co2 and carbonate ions together these results suggest that the tantou au deposit is genetically related to the destruction of the north china craton as a result of lithospheric thinning during the early cretaceous

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

crystal subdivisions of the eastern and central united states and a seismic boundary hypothesis for eastern seismicity potential field and surface geologic data may be used to subdivide the precambrian and phanerozoic crust of the eastern and central united states into several blocks these subdivisions are made by using the trends amplitudes and frequencies of magnetic anomalies in conjunction with gravity and surface geologic data most blocks appear unrelated to historical seismicity but between new mexico and georgia the edges of several blocks may be connected to form the northern boundary of a broad zone of very low earthquake activity we speculate that this southern zone has been decoupled from the stress regime affecting the remainder of the central and eastern united states and should experience few if any large earthquakes in the future as long as the present stress configuration is maintained

accretion of archean oceanic plateau fragments in the abitibi greenstone belt canada the arc arc collision model is often applied to the evolution of archean greenstone belts this model predicts concordant stratigraphic relations among the various lithologies evolving generally from komatiitic tholeiitic affinities toward calc alkalic affinities new geological data from the malartic composite block of the abitibi greenstone belt indicate that this volcanic pile rather than representing a conformable sequence is composed of four distinct mafic lithotectonic domains that are overlain by a calc alkalic sequence the mafic domains are thought to be examples of accreted archean oceanic plateau material that were deformed during their collage state the younger calc alkalic sequence represents extension related volcanism that erupted through the ac creted and deformed plateau material possibly as a consequence of ridge subduction in a regime of oblique convergence the relations described here may represent a common feature of archean terranes

high sr y and la yb ratios the meaning of the adakitic signature abstract the name of adakite is used to describe a far too large group of rocks whose sole common feature is high sr y and la yb ratios however such a signature can be achieved via different processes melting of a high sr y and la yb source deep melting with abundant residual garnet fractional crystallization or afc or interactions of felsic melts with the mantle causing selective enrichment in lree and sr over hree a database of the compositions of adakitic rocks including high silica and low silica adakites martin h smithies r h rapp r p moyen j f champion d c 2005 an overview of adakite tonalite trondhjemite granodiorite ttg and sanukitoid relationships and some implications for crustal evolution lithos 79 1 2 1 24 continental adakites and archaean adakites was assembled geochemical modeling of the potential processes is used to interpret it and reveals that 1 the classical model of slab melting provides the best explanation for the genesis of high silica adakites 2 low silica adakites are explained by garnet present melting of an adakite metasomatized mantle i e at depths greater than 2 5 gpa 3 continental adakites is a term encompassing a huge range of rocks with a corresponding diversity of petrogenetic processes and most of them are different from both low and high silica adakites 4 archaean adakites show a bimodal composition range with some very high sr y examples similar to part of the ttg suite reflecting deep melting 2 0 gpa of a basaltic source with a relatively high sr y while lower sr y rocks formed by shallower 1 0 gpa melting of similar sources comparison with the archaean ttg suite highlights the heterogeneity of the ttgs whose composition spreads the whole combined range of hsa and archaean adakites pointing to a diversity of sources and processes contributing to the ttg suite

mesoarchean to paleoproterozoic crustal evolution of the taihua complex in the southern north china craton abstract the taihua complex is exposed mostly in the central western part along the southern margin of the north china craton ncc in this study field geological geochemical la icp ms zircon u pb age dating and lu hf isotopic analysis are applied to the orthorgneisses felsic leucosomes and granites of the taihua complex in order to provide constraints on its crustal evolution from the mesoarchean to paleoproterozoic ca 2 9 1 8 ga the ca 2 90 2 73 ga magmatism was an important episode of contemporaneous crustal growth and crustal reworking among them the ca 2 90 ga granodioritic enclaves have features similar to those of the archean sanukitoids and were generated by the direct partial melting of peridotitic mantle wedge in the subduction processes representing the early continental nucleus the ca 2 84 ga high mg ttgs were derived from the partial melting of subducted oceanic crust after interaction with mantle wedge whereas the ca 2 82 2 77 ga low mg ttgs were originated from the partial melting of underplated basaltic crust pointing to a series of continuous arc magmatic activities the formation of the ca 2 75 2 73 ga crust derived granites and felsic leucosomes in association with the contemporaneous metamorphism indicate the presence of a crustal anatexis which probably was the result of an important magmatic underplating event the magmatism between 2 9 ga and 2 73 ga indicates that the increased crust mantle interaction and crustal differentiation stimulated the crust in the taihua complex to become more matural the ca 2 57 2 43 ga magmatism represents another prominent magmatism in the taihua complex the ttg gneisses and dioritic gneisses aged at ca 2 57 2 50 ga were derived from partial melting of the hydrated basalts under eclogite to garnet amphibolite facies conditions with interaction between initial melts and mantle wedge indicating that incessant subduction led to the amalgamation of micro blocks the intrusion of granitic rocks felsic leucosomes ttg like gneisses accompanied by metamorphism during ca 2 50 2 43 ga indicate that a dominant crustal anatexis event occurred in association with the cratonic stabilization processes the ca 2 36 2 24 ga magmatism was also characterized by reworking of the basement rocks which are represented by the intrusion of calc alkaline monzogneisses a type granitic gneisses and mafic dykes this scenario was coeval with the formation of volcanic sedimentary sequences indicating the presence of an extension event in the taihua complex the ca 1 96 1 85 ga tectono thermal event represented the last phase of compressional deformation and consequent metamorphism leading to reworking of the pre existed basement rocks this event formed a wnw ese trending metamorphic belt that extends over 1000 km along the southern margin of the ncc and was well concordant with the world wide orogeny associated with the amalgamation of nuna supercontinent subsequently widespread a type granites mafic dykes and volcanic rocks aged of ca 1 84 1 74 ga formed marking the final consolidation of the crystalline basement at the taihua complex since then the southern ncc became stable and entered into the stage of development of platform

episodic crustal growth in the southern segment of the trans north china orogen across the archean proterozoic boundary abstract the dengfeng and taihua complexes are well exposed neoarchean to paleoproterozoic units in the southern segment of the trans north china orogen tnco zircon u pb dating shows that the dengfeng complex records two episodes 2568 11 ma and 2306 6 ma of tonalite trondhjemite granodiorite ttg magmatism all of the ttg rocks are characterized by high sio2 66 7 75 4 wt na2o 3 20 5 06 wt and relatively low mgo 0 40 1 78 wt the late neoarchean ttg gneisses have very low contents of hree ybn 0 69 2 75 and y 1 73 7 07 ppm with moderate la yb n 24 1 53 8 and high sr y 65 1 291 3 ratios the early paleoproterozoic ttg gneisses have low contents of hree ybn 2 93 6 37 and y 6 7 11 0 ppm with moderate la yb n 10 1 27 0 and sr y 10 6 52 1 ratios both suites show pronounced negative nb ta p and ti anomalies but positive sr and pb anomalies the late neoarchean ttg gneisses all have similar bulk rock nd and zircon hf model ages with mainly positive nd t and are interpreted as resulting from the melting of dominantly juvenile thickened lower crust with residual garnet and amphibole the early paleoproterozoic ttg gneisses have extremely variable nd t 6 23 to 4 23 and heterogeneous zircon hf t 3 3 to 3 1 which are also best interpreted as resulting from the partial melting of thickened lower crust with residual amphibole and garnet the taihua complex in the xiaoqinling area records three episodes of early paleoproterozoic ttg magmatism 2 48 ga at caotan 2 31 ga at houjiacun and 2 16 ga at bayuan younger than the taihua complex in the lushan area 2 85 2 72 ga all rocks have relatively low contents of hree ybn 1 03 8 32 and y 2 84 24 ppm with moderate la yb n 8 7 88 4 and sr y 19 8 125 8 ratios and show negative ta nb and ti anomalies and positive sr and pb anomalies the caotan gneisses at 2 48 ga and the houjiacun ttg gneisses at 2 31 ga have low mg 0 14 0 45 low cr the geochronology of the dengfeng complex in the dengfeng area and the taihua complex in the lushan xiong er and xiaoqinling areas reveals at least four magmatic episodes in the southern segment of the tnco from the late mesoarchean to early paleoproterozoic 2 85 2 72 ga 2 57 2 48 ga 2 34 2 30 ga and 2 20 2 07 ga the rocks of the two early episodes are dominantly of juvenile compositions with mostly positive whole rock nd t and zircon hf t values suggesting two episodes of crustal growth formed in a subduction tectonic setting the magmatic rocks of the third episode consist of both the juvenile and pre existing crustal materials with variable whole rock nd t and zircon hf t values which were generated in a subduction zone during the initial assembly of the ncc within the columbia supercontinent cycle the final episode of magmatism lacks juvenile materials with whole rock nd t and zircon hf t values being consistently negative these may have resulted from the orogenic collapse the episodic continental growth recorded in the southern segment of the tnco was caused by subduction and consequent orogeny consistent with global supercontinent cycles within the late archean and early paleoproterozoic

geochronology and petrogenesis of gray gneisses from the taihua complex at xiong er in the southern segment of the trans north china orogen implications for tectonic transformation in the early paleoproterozoic the taihua complex in the xiong er area along the southern segment of the trans north china orogen tnco consists of gray gneisses with minor amphibolites zircon u pb dating of the gray gneisses reveals at least two episodes of early paleoproterozoic magmatism 2 30 2 32 ga and 2 07 2 19 ga each with distinct geochemical features the tieluping ttg gneisses of the early suite have very low hree ybn 0 58 4 75 and y contents 2 05 13 2 ppm with moderate to high la yb n 14 3 192 6 and sr y ratios 38 5 220 1 pronounced negative ta nb and ti anomalies but positive sr and pb anomalies the protoliths of these gneisses were most likely derived from partial melting of thickened lower crust with residual garnet and amphibole and underwent garnet amphibole fractionation plagioclase accumulation and or fractionation the tieluping ttg gneisses have variable zircon hf model ages tdm hfc 2 48 3 06 ga and whole rock end t values 4 89 4 98 and the youngest whole rock depleted mantle nd model age tdm 2 32 ga is close to the crystallization age of the ttg 2318 8 ma the protolith of the gneisses formed from a combination of pre existing continental crust and juvenile materials implying an andean type continental margin arc environment it is suggested that crustal thickening was the result of accretionary orogenesis accompanied by basaltic underplating the ganshugou potassium rich gneisses of the later suite have low na2o and high k2o and show fractionated lree and nearly flat hree patterns with strong negative ta nb and ti anomalies they are characterized by negative end t values 5 01 to 2 33 and heterogeneous zircon ehf t values 10 65 2 10 with decoupling of zircon hf model ages tdm hfc 2 64 3 23 ga and whole rock nd model ages tdm 2 55 2 81 ga the protolith of the ganshugou gneisses was a potassium rich igneous rock which was likely the result of partial melting of pre existing crustal materials at shallow depth compatible with a post collisional setting there is therefore a tectonic transformation from an accretionary orogenesis to an extensional regime as a consequence of post collisional uplift in the southern segment of the tnco in the early paleoproterozoic

la icp ms u pb zircon ages of the qianlishan complex constrains on the evolution of the khondalite belt in the western block of the north china craton abstract the qianlishan complex is located in the westernmost part of the khondalite belt a continent continent collisional belt along which the yinshan block in the north and the ordos block in the south amalgamated to form the western block which then collided with the eastern block along the trans north china orogen to form the north china craton the complex is dominated by high grade supracrustal rocks and minor s type granites of which the supracrustals consist of graphite bearing sillimanite garnet gneiss garnet quartzite felsic paragneiss calc silicate rock and marble cl images reveal the existence of detrital and metamorphic zircons in major rocks of the qianlishan complex in most cases detrital zircons occur as either single grains with oscillatory zoning or oscillatory zoning cores typical of igneous origin which are surrounded by metamorphic overgrowth rims that are structureless high bright and low in th u ratio detrital zircons from the qianlishan complex yield nearly concordant 207 pb 206 pb ages ranging from 2 3 to 2 0 ga suggesting that the protoliths of the high grade supracrustal rocks in the qianlishan complex were deposited at some time after 2 0 ga metamorphic zircons yield two age populations with one at 1 95 ga and another at 1 92 ga of which the former is interpreted as the timing of the collision between the yinshan and ordos blocks to form the western block whereas the later is considered to be the age of subsequent post orogenic extensional event minor s type granites were emplaced at 1 88 ga as a result of partial melting of supracrustals at the stage of the exhumation of the qianlishan complex these new zircon ages combined with structural and metamorphic considerations enable resolution of the tectonothermal events involving the collision between the yinshan and ordos blocks to form the western block followed by the post collisional extension and subsequent exhumation of the khondalite belt

the latest neoarchean paleoproterozoic evolution of the dunhuang block eastern tarim craton northwestern china evidence from zircon u pb dating and hf isotopic analyses abstract the dunhuang block in the easternmost segment of the tarim craton exposes precambrian mafic granulite and felsic gneiss to better understand the poorly known formation and evolution of the tarim craton we carried out detailed field based petrological and geochronological investigations on the major lithologies of the dunhuang block u pb dating and hf isotopic analyses on the mafic granulite and felsic gneiss ttg gneiss of the dunhuang block reveal two distinct age populations the latest neoarchaean 2 5 ga and the late palaeoproterozoic 1 82 1 85 ga the zircon magmatic cores and metamorphic rims of tonalitic gneisses yield similar ages of 2 5 ga supporting the existence of archean rocks in the dunhuang block the short time interval between latest neoarchean magmatism and high grade metamorphism suggests that they were related to the same latest neoarchean tectonothermal event the 2 5 ga zircons show hf depleted mantle model ages between 2 55 ga and 2 8 ga with a peak at 2 7 ga suggesting that 2 7 ga juvenile crust of the dunhuang block was reworked during a 200 300 m y period at the end of the neoarchean the age of 1 85 ga obtained from the mafic granulites and felsic gneisses is interpreted as the age of high pressure granulite facies metamorphism combined with the hf isotopic data these data suggest that the dunhuang block experienced 2 7 ga crustal growth a 2 5 ga magmatic metamorphic event and a 1 82 1 85 ga hp granulite facies metamorphic event this sequence of events is very similar to that of the north china craton implying that the evolutionary history of the dunhuang block was consistent with that of the north china craton from the neoarchean to paleoproterozoic

melting of enriched archean subcontinental lithospheric mantle evidence from the ca 1760ma volcanic rocks of the xiong er group southern margin of the north china craton abstract the xiong er group is an important geologic unit in the southern margin of the north china craton it is dominated by the volcanic rocks dated at 1763 15 ma that have sio2 contents ranging from 52 10 wt to 73 51 wt these volcanic rocks are sub alkaline and can be classified into three subgroups basaltic andesites andesites and rhyolites they unexceptionally show enrichment of light rare earth elements lree and share similar trace element patterns depletions in nb ta sr p and ti relative to the adjacent elements are evident for all the samples the volcanic rocks are evolved with low mgo contents 0 29 5 88 wt and accordingly low mg values of 11 53 the nd isotopes are enriched and show a weak variation with nd t 7 12 to 9 63 zircon hf isotopes are also enriched with hf t 12 02 0 45 the volcanic rocks of the xiong er group are interpreted to represent fractional crystallization of a common mantle source the volcanic rocks might have been generated by high degree partial melting of a lithospheric mantle that was originally modified by the oceanic subduction in the late archean this brings a correlation with the subduction modified lithospheric mantle in an extensional setting during breakup of the columbia supercontinent in the late paleoproterozoic rather than in an arc setting the elevated sio2 contents and evolved radiogenic isotope features indicate the possible incorporation into their source of lower crustal materials that have similar nd isotopic characteristics to the subcontinental lithospheric mantle the existence of extensive xiong er volcanic rocks 60 000 km2 indicates an early large scale subduction related metasomatism in the area and probably suggest a flat subduction model for the plate margin magmatism in the late archean

is the precambrian basement of the tarim craton in nw china composed of discrete terranes abstract the precambrian evolution of the tarim craton in ne china in particular during the early precambrian stage remains enigmatic in this contribution we report field observation petrology geochemistry zircon lu hf isotopes and u pb ages of the major rock formations of the aketage area in the southeastern section of the tarim craton the milan group in aketage is dominantly composed of 2 7 2 5 ga gneissic amphibolite ttg complex with minor paragneiss both the mafic and silicic rocks exhibit geochemical features consistent with an arc affinity the arc signature of the 2 01 2 03 ga gneissic granites and gabbros which intrude the archean basement as well as the major 2 0 ga metamorphic event revealed by zircon u pb dating suggest an important subduction collision event possibly related to the assembly of the paleoproterozoic columbia supercontinent the ca 1848 1856 ma massive potassic granites 1867 ma mafic dyke swarm and 1844 ma massive leucogranite dykes reveal magmatism in a post collisional extensional setting a comprehensive synthesis of the major orogenic events and continental crust growth process from the different precambrian terranes in tarim craton show significant discrepancy in time related to the late neoarchean crust formation ages and the paleoproterozoic orogenic events for example the major orogenic event took place at 1 85 ga in quruqtagh dunhuang terrane at 1 90 ga in the southwest tarim craton and at 2 0 ga in the aketage qaidam terrane these different terranes exhibit distinct periods of continental crust growth in the early precambrian continental growth in the aketage area took place during 2 7 4 3 ga the 3 6 ga xenocrystic zircons as well as the peak of 4 2 ga zircon hf model ages indicate the possible existence of paleoarchean and even hadean crust in the aketage area in the quruqtagh dunhuang terrane the growth of early precambrian continental crust took place at 2 6 3 3 ga with peaks at ca 2 6 2 7 ga and 3 0 ga the diachronous late paleoproterozoic orogenic events and the significant difference in continental growth process suggest that the precambrian basement of the tarim craton is composed by independent continental terranes possibly detached from the cores of discrete ancient cratonic nuclei which were not unified until the early neoproterozoic during assembly of the rodinia supercontinent

precise determination of phanerozoic zircon pb pb age by multicollector sims without external standardization 1 zircon has long been recognized as the best geochronometer and the most important timekeeper in geosciences modern microbeam techniques such as sims and la icpms have been successfully applied to in situ u pb zircon age determinations at spatial resolutions of 20 30 m or less matrix matched calibration by external standardization of well characterized natural zircon references is a principal requirement for precise microbeam u pb zircon age determination due to fractionation effects between pb and u which usually result in an external age error exceeding 1 alternatively zircons with a closed u pb system can be directly dated by measurement of 207pb 206pb isotopic ratio without external standardization which has been a common practice for zircons older than 1 0 ga but not for relatively young 1 0 ga and particularly phanerozoic ones because of limitations of analytical precision we describe in this paper a method of 207pb 206pb measurement on phanerozoic zircons using a new generation of large radius magnetic sector multicollector cameca ims 1280 sims in combination with multicollector mode a nuclear magnetic resonance nmr magnet controller and oxygen flooding techniques we achieve precisions of 207pb 206pb ratio of 0 1 and 0 1 0 2 propagating to pb pb age errors 0 4 and 1 3 excluding u decay constant uncertainties for zircons of latest neoproterozoic and late paleozoic to mesozoic age respectively therefore the multicollector sims is capable of direct determination of zircon pb pb ages as young as mesozoic age with uncertainties of geological significance this technique is useful for direct dating of zircons in thin sections moreover it has significance for dating of some other u rich minerals i e baddeleyite and zirconolite that are not suitable for sims u pb dating by external standardization

late mesozoic felsic magmatism and mo au pb zn mineralization in the southern margin of the north china craton a review abstract the widespread large scale mo au pb zn mineralization in the southern margin of the north china craton ncc is spatially and temporally associated with the voluminous late mesozoic granitic magmatism relationship between the granitic magmatism and ore mineralization has been extensively investigated in the last decade however controversy still exists regarding the magma source metallogeny and geodynamic settings this paper systematically reviews the major features of the mesozoic granitic magmatism and associated mo au pb zn mineralization in the southern margin of the north china craton including temporal and spatial variations of magmatic rocks composition of ore fluids sources of ore metals and metallogeny the granitic rocks were mainly formed in late mesozoic and emplaced in two stages the early stage 158 128 ma granites are of typical i type derived from remelting of ancient crust of the southern margin of the ncc with input of mantle material under lithospheric thinning tectonic regime the late stage 127 112 ma granites include a type and or highly fractionated i type granites which commonly contain more fractions of mantle materials as exhibited by more depleted isotopic signatures suggestive of an intracontinental extensional setting mo mineralization 155 130 ma related to the early stage granitic magmatism includes mainly of porphyry or porphyry skarn type with ore metals mainly derived from the mixed source of the mantle and crust whereas the late stage mo mineralization 127 116 ma is mainly of porphyry type with the ore metals from similar sources with more input of mantle materials pb zn deposits 144 130 ma associated with the early stage granitic magmatism are mainly of hydrothermal vein and skarn types whereas those 125 117 ma related to the late stage granites are of hydrothermal vein type the pb zn deposits are most likely distal products of the mo deposits together forming a metallogenic system au deposits are located in the xiaoqinling and xiong ershan areas and were formed during middle jurassic to early cretaceous 163 118 ma mostly from 132 to 120 ma the ores mainly occur in quartz veins and altered rocks and are hydrothermal magmatic in origin with au derived from a mixed source of crust and mantle

archean paleoproterozoic crustal evolution of the ordos block in the north china craton constraints from zircon u pb geochronology and hf isotopes for gneissic granitoids of the basement abstract the ordos block has been considered as an archean micro block in north china craton however its formation and evolution are poorly understood for a long time due to lack of available basement rocks our la icpms and sims zircon u pb dating of gneissic granitoids from the ordos block basement identified two distinct periods of granitic magmatism at 2 5 ma and 2 2 2 0 ga and two phases of metamorphisms at 1 95 ga and 1 85 ga respectively of which most zircons of 2 5 ga granitoids show positive hf t values ranging from 0 1 to 4 1 with a peak t dm of 2 7 ga minor zircons have negative hf t values from 3 8 to 0 02 and t hf c of 2 8 3 1 ga moreover the 3 40 ga inherited zircon and zircons with t hf c of 3 1 4 2 ga were found from 2 2 ga to 2 0 ga granitoids these suggest that there existed a neoarchean crust in the ordos block which underwent a major crustal growth at 2 7 ga and granitic magmatism at 2 5 ga highly consisting with 2 7 ga crustal growth and 2 5 ga magmatism resulted from juvenile crustal reworking in north china craton the zircons from 2 2 ga to 2 0 ga granitoids in ordos block basement display variable hf isotopic composition of positive to negative hf t values much similar to those of 2 2 2 0 ga granitoids related to continental margin arc setting in the trans north china orogen they were subsequently subjected to 1 95 and 1 85 ga metamorphisms which were widespread throughout the trans north china orogen based on the highly coherent zircon geochronology and hf isotopic composition between ordos block and other micro blocks of north china craton we propose that there is a neoarchean crust in ordos block which underwent a tectonic evolution similar to those in east block in later neoarchean and then experienced 2 2 2 0 ga granitic magmatism related to continental margin arc and 1 95 1 85 ga metamorphism resulted from collision between the east and west blocks along tnco

repeated crustal thickening and recycling during the andean orogeny in north chile 21 26 s posttectonic granitoids 2 lower crustal p wave velocities of 7 3 7 7 km s 1 compatible with underplated mafic crust and 3 results from recent experimental petrologic work showing garnet stability in mafic mineralogies 12 kbar 40 km crustal thickness analogous to older andean magmatic episodes in north chile newly underplated basaltic crust may account for the remaining deficit in neogene crustal thickening similar evolutionary patterns in geochemistry and initial sr and nd isotopic characteristics of andean 200 ma to present magmatic rocks suggest that the andean orogeny in this region evolved by a combination of processes of repeated arc migration tectonic and magmatic crustal thickening and igneous recycling which was controlled by periodically changing plate convergence rates and obliquity and corresponding changes in the rheologic behavior of the continental crust index terms 1020 geochemistry composition of the crust 1030 geochemistry geochemical cycles 0330 8159 evolution of the earth rheology crust and lithosphere 9360 information related to geographic region south america keywords andean orogeny continental crust recycling adakite ttg crustal thickening

th u pb monazite geochronology of the luliang and wutai complexes constraints on the tectonothermal evolution of the trans north china orogen abstract recent lithological metamorphic and geochronological studies show that the basement of the north china craton ncc formed by collision of two discrete archean to paleoproterozoic blocks eastern and western blocks along a paleoproterozoic orogen named the trans north china orogen however the timing of the collision is controversial and post collisional exhumation has not been precisely dated this study applies the electron microprobe th u pb monazite dating technique to determine the ages of the peak metamorphic event and subsequent exhumation of the low to medium grade luliang and wutai complexes in the trans north china orogen electron microprobe th u pb monazite data for the luliang complex reveal five tho 2 pbo age ranges 1 1940 1938 ma 2 1880 1847 ma 3 1795 1755 ma 4 1720 1703 ma and 5 1648 ma of these age ranges the first four have also been recorded in the wutai complex which yields tho 2 pbo ages of 1 1930 ma 2 1838 1822 ma 3 1793 ma and 4 1719 ma the oldest age range of 1940 1930 ma is interpreted to record the widespread emplacement of mafic dikes in rocks of the luliang and wutai complexes the age range of 1882 1822 ma is in accord with the shrimp u pb ages of metamorphic zircons and mineral sm nd and 40 ar 39 ar ages obtained for other complexes in the orogen and is interpreted as the age of the major metamorphic event caused by amalgamation between the eastern and the western blocks the age range of 1795 1755 ma is consistent with emplacement of large scale unmetamorphosed mafic swarms at 1800 1765 ma interpreted as the time of post orogenic extension the 1720 1703 ma and 1648 ma ages are considered to date later multiple stages of hydrothermal alteration since monazites with such young ages only occur along fractures in older monazite grains and at the rims these new monazite tho 2 pbo ages support the tectonic model for the evolution of the north china craton that envisages discrete eastern and western blocks colliding along the trans north china orogen at 1 85 ga and then undergoing post collisional extension in the period 1795 1755 ma

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

sm nd and shrimp u pb zircon geochronology of high pressure granulites in the sanggan area north china craton timing of paleoproterozoic continental collision abstract high pressure hp mafic granulites are widely distributed as sheet like and lenticular bodies within highly deformed tonalitic gneisses in the sanggan area north china craton they have clockwise isothermal decompression and isobaric cooling p t paths implying continental collision tectonics whole rock analyses of 15 samples define a sm nd isochron yielding an age of 2547 87 ma with e nd t 2 7 corresponding nd depleted mantle model ages show a similar range and are interpreted as the age of the magmatic protolith to the high pressure granulites a combined sm nd mineral whole rock isochron from one of these samples yields an age of 1813 23 ma taken to represent the metamorphic age at an early stage on the cooling path shrimp u pb zircon analyses of three hp mafic granulite samples give uniform weighted mean 207 pb 206 pb ages of 1817 12 1817 12 and 1819 16 ma these ages are interpreted to date metamorphism of the hp granulites and with the sm nd mineral whole rock result strongly suggest that the time of continental collision was at ca 1 82 ga an additional sample records two age populations one at 1803 9 ma and an older one at 1872 16 ma the latter may indicate the timing of either an earlier metamorphic event or possibly the earliest stage of a single prolonged metamorphic episode that peaked at 1 82 ga these results confirm the paleoproterozoic age for assembly of the north china craton

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

petrogenesis of adakitic porphyries in an extensional tectonic setting dexing south china implications for the genesis of porphyry copper mineralization the dexing adakitic porphyries quartz diorite granodiorite porphyries associated with giant porphyry cu deposits are located in the interior of a continent south china they exhibit relatively high mgo cr ni and sr contents high la yb and sr y ratios but low yb and y contents similar to adakites produced by slab melting associated with subduction however they are characterized by bulk earth like nd sr isotope compositions end t 1 14 to 1 80 and 87 sr 86 sr i 0 7044 0 7047 and high th 12 6 27 2ppm contents and th ce 0 19 0 94 ratios which are different from those of cenozoic slab derived adakites sensitive high resolution ion microprobe shrimp geochronology studies of zircons reveal that the dexing adakitic porphyries have a crystallization age of 171 3ma this age is contemporaneous with middle jurassic extension within the shi han rift zone and within plate magmatism elsewhere in south china indicating that the dexing adakitic porphyries were probably formed in an extensional tectonic regime in the interior of the continent rather than in an arc setting their high th contents and th ce ratios and middle jurassic age argue against an origin from a neoproterozoic 1000ma stalled slab in the mantle taking into account available data for the regional metamorphic magmatic rocks and the present day crustal thickness 31km in the area we suggest that the dexing adakitic porphyries were most probably generated by partial melting of delaminated lower crust which was possibly triggered by upwelling of the asthenospheric mantle due to the activity of the shi hang rift zone moreover the dexing adakitic magmas must have interacted with the surrounding mantle peridotite during their ascent which elevated not only their mgo cr and ni contents but also the oxygen fugacity fo2 of the mantle the high fo2 could have induced oxidation of metallic sulfides in the mantle and mobilization of chalcophile elements which are required to produce associated cu mineralization therefore the cu metallogenesis associated with the dexing adakitic porphyries is probably related to partial melting of delaminated lower crust similar to the metallogenesis accompanying slab melting

contrasting sources for upper and lower continental crust the greenstone connection abstract greenstones may be useful in tracking the role of oceanic plateaus in the growth of continents lithologic proportions th ta ratios and ni concentrations in greenstone basalts of all ages show that arc related greenstones greatly exceed oceanic plateau and morb related greenstones in abundance this distribution may be accounted for by the preferential obduction of arcs formed on top of oceanic plateaus during collision of plateaus with continents because thick oceanic plateaus resist subduction a significant volume may be accreted to continental margins during collisions and over time these plateaus may evolve into lower continental crust this idea has important implications for continental development in that the lower continental crust may come chiefly from accreted oceanic plateaus while upper continental crust forms by subduction related processes consistent with this model are high seismic wave velocity layers in the lower crust of proterozoic cratons similar to high velocity layer

late neoarchean magmatism and tectonic evolution recorded in the dengfeng complex in the southern segment of the trans north china orogen abstract the dengfeng complex exposed in songshan area in the southern segment of the trans north china orogen tnco mainly consists of late neoarchean tonalite trondhjemite granodiorite ttg gneiss metadiorite and amphibolite the sanhuangzhai amphibolite records the earliest mafic magmatism 2658 5 ma in the region which is enriched in lree and lile with negative nb ta and ti anomalies and positive initial e nd t and e hf t values suggested to be the remnant of an arc related juvenile mafic crust the shipaihe amphibolite at the end of neoarchean 2492 7 ma is characterized by flat ree pattern slightly negative nb ta and ti anomalies and positive initial e nd t and e hf t values which is interpreted as large degree partial melt of metasomatised mantle source in a back arc basin zircon u pb dating shows two episodes of ttg magmatism 2 57 2 55 ga and 2 55 2 50 ga within the dengfeng complex all ttg gneisses have high sio 2 and na 2 o but low mg cr ni yb n and y with high la yb n and sr y the early ttg gneiss 2 57 2 55 ga in the eastern songshan area has variable whole rock hf nd isotopes e hf t 2 14 4 64 e nd t 0 11 3 35 respectively and its protolith is interpreted as the result of partial melting of a thickened lower crust with residual garnet and amphibole in the source but the later ttg gneiss 2 55 2 50 ga in the western songshan area has depleted whole rock hf nd isotopes e hf t 5 59 6 08 e nd t 2 42 3 86 respectively suggesting an overall juvenile source and its protolith is interpreted as the partial melt of flat subducting oceanic crust the shipaihe metadiorite is characterized by high mg cr ni sr ba highly fractionated ree patterns and strong negative nb ta and ti anomalies resembling to sanukitoid suite which is interpreted as the partial melt of a mantle wedge metasomatised by subduction derived fluids melts integrated geochronology and geochemical data of the dengfeng complex in songshan area reveal the late neoarchean crustal accretion process from subduction to arc continental collision accretionary orogeny followed by post collisional extension back arc basin implying an active continental margin arc environment in the southern segment of the north china craton

geochronology and geochemistry of the late paleoproterozoic aluminous a type granite in the xiaoqinling area along the southern margin of the north china craton petrogenesis and tectonic implications abstract the occurrence of extensional related a type granites with a high k nature in some early precambrian blocks hints a possible link with the orogeny and may imply the ending of the orogenic processes and the beginning of craton stabilization the newly recognized late paleoproterozoic a type granite the guijiayu pluton composed of monzogranites in the xiaoqinling area of the southern margin of the north china craton ncc is a promising locus to investigate the late paleoproterozoic tectonic evolution of the ncc la icp ms zircon u pb dating shows that the guijiayu pluton was emplaced at 1 80 ga petrologically the guijiayu granites contain perthite annite and calcic amphibole but are lack of alkali mafic minerals geochemically all the rocks are enriched in silicon and total alkali and depleted in mgo cao and p2o5 with high feot mgo tio2 mgo and 10 000 ga al ratios and high hfses i e zr nb ce y concentrations as well as negative eu sr p and ti anomalies they are also characterized by high melting temperatures and metaluminous to weakly peraluminous natures taken together the guijiayu granites show an affinity to aluminous a type granite their geochemical characteristics e g obviously negative eu and sr anomalies along with the low zircon ehf t values 13 9 to 7 0 and whole rock end t values 5 81 to 5 65 indicate that the guijiayu aluminous a type granite was derived from partial melting of the crustal calc alkaline granitoids dominated by the granodiorite of the taihua group in the study area combined with the regional data especially for the identification of the clockwise p t paths of paleoproterozoic metamorphism in the southern margin of the ncc even coeval granites and regional retrograde metamorphism in other areas in the ncc it is most likely that the peak of continent continent collision had finished in the study area at least before 1 80 ga and that the generation of the guijiayu aluminous a type granite was related to the underplating of mafic magmas in a post collisional regime

u pb and hf isotopic study of detrital zircons from the wulashan khondalites constraints on the evolution of the ordos terrane western block of the north china craton abstract detrital zircons from the wulashan khondalites in the ordos terrane western block of the north china craton give u pb ages between 1 84 and 2 32 ga showing that their provenance was dominated by paleoproterozoic rocks much younger than those exposed in the eastern block these zircons have oscillatory growth zoning and possess e hf values between 8 and 9 suggesting derivation of their precursor magmas from old crust and juvenile materials from the mantle the lowest e hf values of zircons with different ages give a line that intersects the depleted mantle line at about 2 6 ga in a e hf vs time diagram this may imply that the precursor magma source in the deep level of the crust was largely separated from the mantle 2 6 ga ago and therefore 2 6 ga may be an important crustal formation period for the terrane one khondalite sample has detrital zircons with a single age population at 2 0 ga and positive e hf values from 1 to 9 clearly recording the significant addition of juvenile materials at 2 0 ga in the source region this may inspire us to seek whether there was a connection between a mantle superevent and the formation of columbia supercontinent data of this study exclusively demonstrate that the ordos terrane was developed independently from the eastern block until assembly of the columbia supercontinent therefore tectonic models involving that the whole north china craton was cratonized 2 5 ga ago should be dismissed

geochronology and geochemistry of the paleoproterozoic yinyugou group in the southern north china craton implications for provenance and tectonic evolution abstract the paleoproterozoic yinyugou group in the southern north china craton is a littoral facies sedimentary sequence which is mainly composed of feldspar quartzites and mica schists with minor carbonate rocks u pb isotopic dating of detrital zircons bracket the depositional age of the yinyugou group at 2 43 2 30 ga chemical index of alteration cia values of most feldspar quartzite and schist samples are lower than 75 indicating generally weak to moderate source weathering conditions for the yinyugou rocks the majority of the studied yinyugou samples give a large range of index of compositional variability icv values of 0 93 2 59 revealing a chemically immature source which deposited in an active continental margin setting chondrite normalized ree patterns and discrimination diagrams involving la th hf co and sc reveal that the yinyugou group dominantly received detritus from felsic sources with minor contribution from mafic igneous rocks u pb ages of detrital zircons from the yinyugou group can be divided into two groups 2 90 2 40 ga and 3 50 3 20 ga with a predominant age peak at 2 50 ga the 3 50 3 20 ga detrital zircons have negative ehf t values ranging from 11 2 to 3 8 and old crustal model ages tdmc of 4246 to 3909 ma indicating a mature crustal source most 2 50 ga zircons show sub depleted mantle positive ehf t values and tdmc ages mainly ranging from 3 10 to 2 70 ga representing the time when juvenile materials were extracted from the depleted mantle taking all these data into account the yinyugou group was most likely deposited in a back arc setting in combination with previous studies on other paleoproterozoic volcanic sedimentary sequences and magmatic rocks in the ncc a cycle of subduction collision extension rift subduction processes is proposed to explain the tectonic evolution of the ncc during paleoproterozoic

origin of the late paleoproterozoic low δ18o a type granites on the southern margin of the north china craton and their geodynamic mechanism abstract a type granites with 18o depletion signatures are rather enigmatic as regards not only their typology and petrogenesis but also their rock forming geodynamic mechanism here we report a suite of late paleoproterozoic low 18o a type granites from the longwangzhuang pluton on the southern margin of the north china craton sims zircon u pb dating showed that the longwangzhuang pluton was emplaced at 1613 4 ma combined with previous data it is clear that the mineral components of longwangzhuang granites are variable and consist of perthite quartz plagioclase biotite and amphibole calcic hastingsite ferroedenite and sodic ferrogedrite geochemically they are metaluminous to peraluminous and have high alkali contents k2o na2o 8 76 wt k2o na2o 1 feot feot mgo 0 95 ratios zr nb ce y 993 ppm contents and 10000 ga al 4 31 ratios resembling a type granites in situ oxygen isotope analysis displayed low zircon 18o values between 4 0 and 4 9 with a weighted average of 4 4 based on the archean two stage zircon hf and whole rock nd model ages coupled with the low la yb cn ratios negative anomalies of eu and sr and high zr saturation temperatures and the presence of low pressures 3 3 5 7 kbar amphiboles the longwangzhuang a type granites were generated by self cannibalizing pre existing low 18o old rocks i e the taihua complex and emplaced at shallow depth combined with regional geological observations i e coeval tuffites a passive rift setting was the most plausible explanation for the generation of the late paleoproterozoic low 18o a type granites on the southern margin of the north china craton such a rifting could likely have lasted at least till 1 6 ga

petrogenesis and tectonic implications of the iron rich tholeiitic basalts in the hutuo group of the wutai mountains central trans north china orogen abstract the hutuo group located in the wutai mountains area is characteristic of paleoproterozoic strata in the north china craton and plays an important role in establishing evolutionary processes in the trans north china orogen we present petrological observations along with geochemical and nd isotopic data for basalts in the qingshicun and hebiancun formations of the hutuo group these basalts are enriched in tfeo 11 97 18 01 wt consistent with occurrences of iron rich basalt world wide they also display relatively high mgo 4 95 12 25 wt ni 79 121 ppm and cr 37 101 ppm and low in sio 2 41 37 51 95 wt therefore the iron rich character of the hutuo group basalts originated with their parental magma rather than via shallow crustal fractionation at low oxygen fugacity as inferred for many high iron basalts the hutuo group basalts exhibit weak to moderate differentiation of light ree over heavy ree la lu cn 2 61 6 79 with minor or slight eu anomalies eu eu 0 77 1 41 and also show the nb and ti troughs without obvious zr and hf anomalies the nd t values of the basalts range from 1 23 to 0 81 with t dm of 2489 2664 ma the basalts most plausibly originated from the fertile subcontinental lithosphere mantle and in the case of the qingshicun formation basalts also underwent the crustal contamination basalts in the hutuo group also have the high zr hf ratios and present high ti v values of 24 55 and zr y ratios ranging from 4 6 to 13 6 suggesting that they were emplaced in a within plate setting considering the sedimentary associations and detrital fragments in the hutuo and gantaohe groups and bimodal magmatism along the trans north china orogen we propose that 2 2 2 0 ga geological events within the orogen were rift related but did not result from subduction or island arc processes

the xiong er volcanic belt at the southern margin of the north china craton petrographic and geochemical evidence for its outboard position in the paleo mesoproterozoic columbia supercontinent abstract the xiong er volcanic belt covering an area of more than 60 000 km 2 along the southern margin of the north china craton has long been considered an intra continental rift zone and recently interpreted as part of a large igneous province formed by a mantle plume that led to the breakup of the paleo mesoproterozoic supercontinent columbia however such interpretations cannot be accommodated by lithology mineralogy geochemistry and geochronology of the volcanic rocks in the belt lithologically the xiong er volcanic belt is dominated by basaltic andesite and andesite with minor dacite and rhyolite different from rock associations related to continental rifts or mantle plumes which are generally bimodal and dominated by mafic components however they are remarkably similar to those rock associations in modern continental margin arcs in some of the basaltic andesites and andesites amphibole is a common phenocryst phase suggesting the involvement of h 2 o rich fluids in the petrogenesis of the xiong er volcanic rocks geochemically the xiong er volcanic rocks fall in the calc alkaline series and in most tectono magmatic discrimination diagrams the majority of the xiong er volcanic rocks show affinities to magmatic arcs in the primitive mantle normalized trace element diagrams the xiong er volcanic rocks show enrichments in the lile and lree and negative nb ta ti anomalies similar to arc related volcanic rocks produced by the hydrous melting of metasomatized mantle wedge nd isotope compositions of the xiong er volcanic rocks suggest that 5 15 older crust has been transferred into the upper lithospheric mantle by subduction related recycling during archean to paleoproterozoic time available shrimp and la icp ms u pb zircon age data indicate that the xiong er volcanic rocks erupted intermittently over a protracted interval from 1 78 ga through 1 76 1 75 ga and 1 65 ga to 1 45 ga though the major phase of the volcanism occurred at 1 78 1 75 ga such multiple and intermittent volcanism is inconsistent with a mantle plume driven rifting event but is not uncommon in ancient and existing continental margin arcs taken together the xiong er volcanic belt was most likely a paleo mesoproterozoic continental magmatic arc that formed at the southern margin of the north china craton similar paleo mesoproterozoic continental magmatic arcs were also present at the southern and southeastern margins of laurentia the southern margin of baltica the northwestern margin of amonzonia and the southern and eastern margins of the north australia craton which are considered to represent subduction related episodic outbuilding on the continental margins of the paleo mesoproterozoic supercontinent columbia therefore in any configuration of the supercontinent columbia the southern margin of the north china craton could not have been connected to any other continental block as proposed in a recent configuration but must have faced an open ocean whose lithosphere was subducted beneath the southern margin of the north china craton

shrimp and la icp ms zircon geochronology of the xiong er volcanic rocks implications for the paleo mesoproterozoic evolution of the southern margin of the north china craton the paleo mesoproterozoic xiong er volcanic rocks along the southern margin of the north china craton are lithologically and geochemically similar to those formed in subduction related continental margin volcanic arcs the volcanic rocks are primarily composed of basaltic andesites and andesites with minor dacites and dacitic rhyolites traditionally the xiong er volcanic rocks have been divided from lower to upper into the xushan jidanping and majiahe formations but the ages of volcanic rocks in these formations have not been well constrained which has hindered further understanding the tectonic significance of the xiong er volcanic belt at the southern margin of the north china craton shrimp and la icp ms u pb zircon analyses combined with cathodeluminescence cl images have enabled resolution of xenocrystic and magmatic zircons that can be directed toward determination of the ages of the xiong er volcanic rocks shrimp and la icp ms u pb analyses on magmatic zircons from two basaltic andesite samples one dacite sample and one rhyolite sample of the xushan formation known as the lowest sequence of the xiong er volcanic rocks indicate that the volcanic eruption of this formation occurred at 1 78 ga but most xenocrystic inherited zircons in these samples yielded 207pb 206pb ages ranging from 2 55 ga to 1 91 ga of three samples collected from the jidanping formation two rhyolite samples 05xe015 and 05xe100 yielded weighted mean 207pb 206pb ages of 1778 5 5 ma and 1751 14 ma respectively similar to the ages of the volcanic rocks in the xushan formation whereas one dacite sample 05xe066 gave a weighted mean 207 pb

archean blocks and their boundaries in the north china craton lithological geochemical structural and p t path constraints and tectonic evolution an examination of lithological geochemical geochronological structural and metamorphic p t path data suggests that the basement of the north china craton can be divided into eastern and western blocks separated by major crustal boundaries that roughly correspond with the limits of a 300 km wide zone called the trans north china orogen the eastern block consists predominantly of late archean domiform tonalitic trondhjemitic granodioritic ttg batholiths surrounded by anastomosing networks and linear belts of open to tight synforms of minor volcanic and sedimentary rocks metamorphosed from greenschist to granulite facies at 2 5 ga with anticlockwise p t paths some early to middle archean rocks are locally present in the eastern block but their tectonic history is unclear due to reworking by the 2 5 ga tectonothermal event the western block has a late archean assemblage structural style and metamorphic history similar to that of the eastern block but it differs in the absence of early to middle archean assemblages and in being overlain by and interleaved with paleoproterozoic khondalites which were affected by a 1 8 ga metamorphic event involving clockwise p t paths a mantle plume model is proposed for the formation and evolution of late archean basement rocks in the eastern and western blocks based on a combination of extensive exposure of ttg gneisses affinities of mafic rocks to continental tholeiitic basalts presence of voluminous komatiitic rocks dominant diaprism related domiform structures anticlockwise p t paths and a short time span from the primary emplacement of ttg and ultramafic to mafic rocks until the onset of regional metamorphism between the two blocks is the trans north china orogen which is bounded by two major fault systems and is composed of late archean to paleoproterozoic ttg gneisses and granitoids interleaved with abundant sedimentary and volcanic rocks that are geochemically interpreted as having developed in magmatic arc and intra arc basin environments these rocks underwent multiple phases of compressional deformation and peak high pressure metamorphism followed by rapid exhumation during the late paleoproterozoic at 1 8 ga as a result of collision between the eastern and western blocks resulting in the amalgamation of the north china craton

formation of an archaean continent about 30 of the earth is covered by continents but only about 10 small kernels of these continentsae known as archaean cratonsae are continental fragments formed before 2 5 gyr ago the kaapvaal craton of south africa which formed and stabilized between 3 7 and 2 7 gyr ago is one of the oldest reasonably sized examples of these continental fragments it consists of a mosaic of subdomains that have been welded together by processes similar to those of modern day plate tectonics the earliest subdomains may have owed their origin to the onset of efficient recycling from the earth s hydrosphere into the mantle

episodic growth of the gondwana supercontinent from hafnium and oxygen isotopes in zircon it is thought that continental crust existed as early as 150 million years after planetary accretion but assessing the rates and processes of subsequent crustal growth requires linking the apparently contradictory information from the igneous and sedimentary rock records for example the striking global peaks in juvenile igneous activity 2 7 1 9 and 1 2 gyr ago imply rapid crustal generation in response to the emplacement of mantle super plumes rather than by the continuous process of subduction yet uncertainties persist over whether these age peaks are artefacts of selective preservation and over how to reconcile episodic crust formation with the smooth crustal evolution curves inferred from neodymium isotope variations of sedimentary rocks detrital zircons encapsulate a more representative record of igneous events than the exposed geology and their hafnium isotope ratios reflect the time since the source of the parental magmas separated from the mantle these model ages are only meaningful if the host magma lacked a mixed or sedimentary source component but the latter can be diagnosed by oxygen isotopes which are strongly fractionated by rock hydrosphere interactions here we report the first study that integrates hafnium and oxygen isotopes all measured in situ on the same precisely dated detrital zircon grains the data reveal that crust generation in part of gondwana was limited to major pulses at 1 9 and 3 3 gyr ago and that the zircons crystallized during repeated reworking of crust formed at these times the implication is that the mechanisms of crust formation differed from those of crustal differentiation in ancient orogenic belts

u pb and hf isotopic study of zircons of the helanshan complex constrains on the evolution of the khondalite belt in the western block of the north china craton abstract the helanshan complex is located at the westernmost segment of the khondalite belt a continent continent collisional belt along which the yinshan block in the north collided with the ordos block in the south to form the western block of the north china craton the complex consists of s type granites and high grade pelitic granulite gneiss felsic paragneiss quartzite calc silicate rock and marble together called the khondalite series rocks la q icp ms u pb geochronology and la mc icp ms lu hf isotopic data of zircons combined with cathodoluminescence cl imaging enable the resolution of magmatic and metamorphic events that can be directed towards understanding the formation and evolution of the khondalite belt in the western block of the north china craton cl images reveal the coexistence of magmatic type detrital zircons and metamorphic zircons in most of the khondalite series rocks of which the metamorphic zircons occur as either single grains or overgrowth rims surrounding and truncating magmatic type detrital zircon cores la q icp ms u pb analyses on magmatic type detrital zircons reveal two distinct age populations with one in the archaean 2 85 2 53 ga and the other in the palaeoproterozoic 2 2 2 0 ga suggesting that the sedimentary protoliths of the khondalite series rocks in the helanshan complex must have been deposited at some time after 2 0 ga the hf analyses show that the archaean detrital zircons possess negative ehf t values from 7 64 to 0 15 with depleted mantle model ages ranging from 3 34 to 3 10 ga this implies the existence of a paleo mesoarchaean 3 34 3 10 ga crust in the western block which underwent a recycling event in the period 2 85 2 53 ga nearly all palaeoproterozoic 2 2 2 0 ga detrital zircons possess positive ehf t values 0 86 9 38 with depleted mantle model ages of 2 45 2 15 ga suggesting a significant crustal growth event in the western block in the palaeoproterozoic metamorphic zircons yield two groups of age with one at 1 95 ga and the other at 1 87 ga of which the former is considered as the timing of the collision between the yinshan and ordos blocks to form the western block whereas the latter is consistent with the age of 1 86 ga from two s type granites which are considered to have formed by the partial melting of pelitic granulites gneisses at the stage of the exhumation of the helanshan complex

geochronology and petrogenesis of the neoarchean paleoproterozoic taihua complex ne china implications for the evolution of the north china craton abstract the north china craton ncc preserves important records of archean paleoproterozoic crustal evolution here we present results from petrology geochemistry zircon u pb dating and lu hf isotopes of the taihua complex thc from the southern part of the ncc zircon u pb dating of the amphibolite yields the ages of 2776 2771 ma representing the formation age marking a major phase of late archean crustal growth in the ncc the 1922 1915 ma ages represent the paleoproterozoic metamorphism associated with the collisional assembly between the eastern and western blocks the amphibolites are characterized by low sio2 na2o k2o and nb y indicating a sub alkaline basaltic protolith these rocks show relatively high lile hfse and lree hree ratios similar to e morb low nb la pm negative nb ta anomalies and positive ehf t values indicating partial melting of a metasomatic mantle source the ttg gneisses are characterized by high sio2 sr y la yb n nb ta zr sm and zr hf ratios and low mgo cr ni y gd yb and dy yb ratios with positive eu anomalies indicating partial melting of juvenile lower crust with garnet amphibolite residue our data are consistent with a tectonic setting within a volcanic arc in a convergent margin our study on the thc contributes to the archean paleoproterozoic crustal accretion and reworking history along the southern margin of the ncc

complex evolution of the lower crust beneath the southeastern north china craton the junan xenoliths and xenocrysts abstract in our paper we suggested that the junan granulite xenoliths and xenocrysts record evolution of the precambrian lower crust beneath the southeastern north china craton ncc yuan and xia 2015 disagree with us however they have not fully considered the evolutional histories of the ncc lithosphere and geochemical and isotopic compositions of the junan xenoliths we also contend that they have misinterpreted the available geophysical data synthesizing the geochronological characteristics of the ncc lower crust nature of the junan granulite xenoliths and reinterpretation of the resistivity profile we again emphasize that the junan granulite xenoliths are tectonically affiliated to the ncc lower crust and the junan zircon data could reflect the complex evolution of the lower crust beneath the southeastern ncc

metamorphic p t paths of the zanhuang amphibolites and metapelites constraints on the tectonic evolution of the paleoproterozoic trans north china orogen garnet bearing metapelites and amphibolites are exposed in the south and middle parts of the zanhuang complex which is located in the central segment of the nearly ns striking trans north china orogen these rocks preserve three metamorphic mineral assemblages forming at the prograde peak and post peak decompression stages the prograde metamorphic stage m1 is represented by mineral inclusions within garnet porphyroblasts the peak metamorphic stage m2 is represented by garnet rims and matrix minerals whereas the retrograde stage m3 is represented by amphibole plagioclase symplectite rimming garnet porphyroblasts in the amphibolites and biotite plagioclase symplectite rimming garnet porphyroblasts in the metapelites all garnet porphyroblasts in the metapelites preserve prograde chemical zoning except for the ubiquitous quite narrow zones from the underwent post peak decompression it has been determined through thermobarometric computation that the metamorphic conditions are 650 710 c at 8 2 9 2 kbar for the m1 inclusion assemblages 810 c at 12 5 kbar for the metamorphic peak m2 matrix assemblages and 660 680 c at 4 4 4 5 kbar for the retrograde m3 symplectite assemblages these rocks are thus determined to have undergone metamorphism with clockwise p t paths involving nearly isothermal decompression itd segments which is inferred to be related to the amalgamation of the eastern and western blocks to form the coherent basement of the north china craton along the trans north china orogen in the late paleoproterozoic 1 88 1 85 ga

composition and evolution of the archaean crust in central henan china the upper archaean in central henan province is composed chiefly of rocks of the dengfeng complex and the taihua complex rocks of the dengfeng complex show essential features of a granite greenstone terrane they consist of greenstone belts and surrounding granitic and migmatite gneiss terranes when the late superposed structures are removed the archaean conspicuously exhibits alternating gneiss domes and synformal basins which are occupied by granitic rocks and greenstone belts respectively the succession of the greenstone belts is divided into three groups 1 1 the lower group consists of ultramafic complexes which are only locally exposed 2 2 the middle group which comprises most of the greenstone belts contains tholeiitic metabasalts and felsic volcanics which are bimodal in character 3 3 the upper sedimentary group consists of molasse and turbidite formations the metamorphic grade in the dengfeng complex is up to the medium pressure almandine amphibolite facies the mafic volcanics are characterized by flat to slightly enriched ree patterns that resemble the th2 type of condie in most other elements the rocks are also similar to the tholeiitic metabasalts in typical greenstone belts throughout the world however they possess certain unique characteristics as for the granitic rocks the early plutons tend to be na rich and are geochemically similar to tonalitic rocks while the later intrusives tend to be k rich the taihua complex is composed primarily of felsic gneisses and metamorphosed supracrustal rocks interbedded with mafic ultramafic rocks the metamorphic grade in the complex ranges from granulite to upper amphibolite facies geochemical characteristics of the rocks are also similar to those of other high grade terranes throughout the world in general an overall structural grain characterizes the taihua complex which constitutes an elongate fold belt with characteristic features of linear mobile belts that are frequently found in high grade terranes the extensive dengfeng granite greenstone terrane and the linear taihua high grade gneissic terrane are bounded by thrusts or contacts of unknown nature and together make up the late archaean crust in central henan they may have developed more or less contemporaneously on a pre existing sialic crust in this region controlled by shallow convection cells in the upper mantle their development may have occurred in two different tectonic settings the rift type and the subsiding type setting respectively

linking thermodynamic modelling lu hf geochronology and trace elements in garnet new p t t paths from the sevier hinterland major element trace element and lu hf geochronological data from amphibolite facies pelitic schist in the raft river and albion mountains of northwest utah and southern idaho indicate that garnet grew during increasing pressure interpreted to be the result of tectonic burial and crustal thickening during sevier orogenesis garnet growth was interrupted by hiatuses interpreted from discontinuities in major element zonation pressure temperature paths were determined from the pre hiatus portions of the garnet chemical zoning profiles and indicate an increase of 2 kbar and 50 c in the western raft river mountains garnet lu hf dates of 150 1 ma in the western raft river mountains and 138 7 0 7 ma and 132 5 ma in the southern albion mountains indicate the timing of garnet growth lutetium garnet zoning profiles indicate that the lu hf ages are biased towards the post hiatus or outer pre hiatus segments indicating that the determined ages likely post date the recorded p t path history or date the tail end of the paths crustal thickening associated with sevier orogene sis in the western raft river mountains thus began slightly before 150 1 ma in the late jurassic this study shows that integrating p t paths determined from garnet growth zoning with lu hf gar net geochronology and in situ garnet trace element analyses is an effective approach for interpreting and dating deformation events in orogenic belts

adakites without slab melting high pressure differentiation of island arc magma mindanao the philippines abstract new geochemical data for pleistocene magmatic rocks from the surigao peninsula eastern mindanao the philippines demonstrate typical adakitic traits including elevation of sr y and depletion of the heavy rare earth elements 87sr 86sr and 143nd 144nd ratios of the adakites do not support melting of the subducted philippine sea plate but resemble pliocene arc lavas generated in the same subduction zone excepting the heavy rare earth elements the adakites and arc lavas also possess similar ratios of incompatible elements suggesting that the adakites were ultimately derived from melting of the mantle wedge the wide range of sio2 in the adakites and its strong correlation with trace element concentrations and ratios indicate two possible mechanisms for generating the adakitic signature 1 adakitic melt was produced from basaltic arc magma by fractional crystallisation of a garnet bearing assemblage 2 solidified basaltic rock containing garnet melted to yield adakitic magma in either case the basaltic precursor was generated from fluid modified mantle then differentiated within the garnet stability field in surigao this requires differentiation within mantle the surigao example suggests that any subduction zone has the potential to produce adakitic magma if basalt crystallises at sufficient depth this has important implications for the geodynamics of modern and ancient subduction zones that have generated similar rocks

hf isotopic compositions of the standard zircons and baddeleyites used in u pb geochronology abstract zircon and baddeleyite u pb geochronological dating is widely used in solid earth sciences and the advent of rapid in situ methods of analysis such sims and icp ms has re emphasized the importance of having uniform standards recently it has been shown that hf isotopic data can provide important information on these minerals since they contain high concentrations of hf but have low lu hf ratios which results in negligible age correction however the complex internal structures that result from multiple thermal events such as inherited cores and metamorphic overgrowths require that the hf isotopic data be measured with high spatial resolution however the isobaric interferences of 176 yb and 176 lu on 176 hf hamper the precise determination of the 176 hf 177 hf ratio during in situ laser ablation mc icpms analysis it is shown here that mass biases of yb yb and hf hf change with time during analyses and behave differently for solutions and solid material therefore it is suggested that the mean yb value of the individual spot be used to obtain the precise isotopic composition for in situ zircon and baddeleyite hf isotopic analyses for low yb hf 176 yb 177 hf yb value have little effect on the accuracy of the hf isotopic composition however the appropriate yb isotopic abundance is necessary for high yb hf 176 yb 177 hf 0 001 zircons and baddeleyites since the interference of 176 yb on 176 hf is significant using the mean yb value of the individual spot and newly published yb isotopic abundance data six standard zircons and two standard baddeleyites have been investigated using a neptune mc icpms with 193 nm laser for zircons the obtained 176 hf 177 hf ratios are 0 282307 31 2sd for 91500 0 282680 31 2sd for temora 0 281729 21 2sd for cz3 282177 17 2sd for cn92 1 0 282983 17 2sd for fm0411 and 281234 11 2sd for phalaborwa the baddeleyites from phalaborwa and sk10 2 have 176 hf 177 hf ratios of 0 281238 12 and 0 282738 13 2sd these results agree well with the values obtained by the solution method and indicate that these standards have different hf isotopic compositions in which the extremely low 176 lu 177 hf and 176 yb 177 hf values of cz3 zircon and phalaborwa baddeleyite make them excellent standards for machine calibration during in situ zircon hf isotopic measurement with the other standards being more suitable for the development of the correction method

mo rich source and protracted crystallization of late mesozoic granites in the east qinling porphyry mo belt central china constraints from zircon u pb ages and hf o isotopes abstract in the east qinling orogenic belt central china there are numerous mo deposits with over 6 mt mo metal the giant nannihu shangfanggou and yuchiling porphyry mo deposits are hosted in porphyry stocks associated with the heyu batholith whereas the giant donggou porphyry mo deposit is hosted in a stock associated with the taishanmiao batholith zircon grains from the heyu batholith have concordant 238u 206pb ages scattered from 150 3 to 130 2 ma with an age interval of nearly 20 m y whereas ore hosting granitic porphyry stocks from the nannihu shangfanggou and yuchiling deposits have 206pb 238u ages ranging from 151 1 to 135 1 ma 143 1 to 132 1 ma and 143 5 to 131 5 ma respectively it is likely that the heyu batholith has a prolonged history of incremental assembly and the ore bearing granitic porphyry stocks may have originated from the same magma reservoir and emplaced in different stages likewise zircon grains from the taishanmiao batholith have concordant 238u 206pb ages spanning in a period from 130 2 to 111 3 ma whereas those from the donggou granitic porphyry stock have 238u 206pb ages ranging from 125 2 to 110 3 ma nearly coeval with the taishanmiao batholith the molybdenite re os model ages for each deposit are coincident with relatively young zircon 238u 206pb ages of the host stock indicating that mo mineralization is likely related to late stage magmatism zircon grains from the batholiths and granitic porphyry stocks have ehf t ranging from 10 to 30 tdm2 ages from 1 6 to 2 5 ga and 18o from 5 0 to 8 7 which are taken to suggest the involvement of the subducted continental crust of the yangtze block in the source we thus propose that the extensive mo mineralization in the east qinling porphyry mo belt is genetically related to the prolonged granitic magmatism in the late mesozoic and a mo rich source that was related to the subducted continental crust of the yangtze block stagnating beneath the southern margin of the north china craton

combined u pb hafnium and oxygen isotope analysis of zircons from meta igneous rocks in the southern north china craton reveal multiple events in the late mesoarchean early neoarchean abstract we report the results of a comprehensive isotopic investigation of zircons from metamorphosed tonalites and amphibolites from lushan in the far south of the trans north china orogen of the north china craton that reveal the oldest rocks yet dated from the orogen a detailed investigation of the internal structures of zircons combined with targeted sims u pb and oxygen isotope analyses and icp ms hf zircon analyses has resulted in the discovery of previously unknown geological events in the north china craton magmatic zircon from the tonalite and amphibolite was found to be essentially coeval with a small spread of ages from 2829 18 ma to 2832 11 ma in the tonalites and from 2838 35 ma to 2845 23 ma in amphibolite hafnium isotope data suggest derivation of both tonalite and amphibolite from a depleted mantle source with minor crustal contamination oxygen isotope ratios in zircons range from 5 1 to 7 3 vsmow with high 18 o results recorded only from metamorphic domains the earliest stage of metamorphic zircon growth present in all four samples analyzed in this study occurred between 2772 17 22 in the tonalites and 2776 20 and 2792 12 ma in the amphibolites a younger metamorphic event at 2638 61 in the tonalite and between 2671 13 and 2651 13 ma in the amphibolites is recognized in zircons from three of the four samples investigated this is the first time that metamorphic events at either of these times have been recorded in the north china craton importantly there is no evidence for the 1 8 ga high grade metamorphic event that is manifest in the northern and central parts of the trans north china orogen