retrograde net transfer reaction insurance for pressure temperature estimates retrograde net transfer reactions significantly affect compositions of metamorphic minerals yet are rarely considered when determining pressure temperature p t conditions two natural amphibolite facies metapelites from the central himalaya of nepal exhibit extremely common compositional patterns including increases in mn and fe fe mg at the rims of garnets which are the result of retrograde garnet dissolution and fe mg exchange with biotite however typical thermobarometric approaches for these rocks result in errors of hundreds of degrees and 3 6 kbar compared with thermobarometry of nearby rocks and petrogenetic grids these large errors result because dissolution of high fe garnet has strongly affected the fe mg ratio of matrix biotite x ray maps help evaluate the extent and chemical effects of retrograde reactions in these samples by identifying mineral regions that retain highest t compositions or through a new data processing approach by permitting correction of mineral compositions to original high t values these approaches ensure against retrograde net transfer reactions and should be applied routinely in thermobarometric studies they ultimately yield p t estimates that are more petrologically reasonable and permit rapid screening of samples for those least affected by retrograde reactions reconsideration of thermobarometry in the central and eastern himalaya indicates that retrograde net transfer reactions are extremely common therefore previous thermobarometric studies based on garnet major element compositions from that region should be reevaluated

petrogenesis p t t path and tectonic significance of high pressure mafic granulites from the jiaobei terrane north china craton abstract high pressure hp mafic granulites in the jiaobei terrane are composed predominantly of garnet mafic granulites garnet hypersthene granulites and garnet amphibolites and they are found as irregular lenses or deformed dike swarms within tonalitic trondhjemitic granodioritic gneisses and granitic gneisses the hp mafic granulites contain four distinct metamorphic assemblages of which the early prograde assemblage m 1 is represented by the cores of garnets together with mineral inclusions of clinopyxene plagioclase quartz and it formed at 740 770 c and 0 90 1 00 gpa in contrast the peak assemblage m 2 consists of high ca cores in garnet high al cores in clinopyroxene and high na cores in plagioclase in the matrix which formed under p t conditions of 850 880 c and 1 45 1 65 gpa the peak metamorphism was followed by near isothermal decompression m 3 which resulted in the development of orthopyroxene clinopyxene plagioclase quartz amphibole magnetitie symplectites or coronas surrounding some garnet grains with p t conditions of 780 830 c and 0 65 0 85 gpa surrounding some garnet grains are symplectites of amphibole plagioclase quartz magnetitie which formed during a cooling retrograde stage m 4 with p t conditions of 590 650 c and 0 62 0 82 gpa an integrated study involving laser raman analysis of mineral inclusions cathodoluminescence imaging and in situ u pb dating of zircons shows that the protolith ages of the hp mafic granulites are mainly 2550 2500 ma and that the timing of the peak metamorphism of the hp mafic granulites ranges from 1900 to 1860 ma as recorded by the cores of metamorphic zircons on the other hand the medium to low pressure granulite amphibolite facies retrogression occurred mainly at 1860 1820 ma as recorded by the rims of some single zircon grains and the zircon grains that contain inclusions of garnet orthopyroxene plagioclase sphene the combination of petrography mineral compositions metamorphic reaction history thermobarometry and geochronology defines a near isothermal decompressional clockwise p t t path for the jiaobei hp mafic granulites suggesting that the jiaobei terrane underwent initial crustal thickening during 1950 1860 ma followed by relatively rapid exhumation cooling and retrogression in the period 1860 1820 ma this tectonothermal path was probably generated by subduction and collision related tectonic processes

the growth reworking and metamorphism of early precambrian crust in the jiaobei terrane the north china craton constraints from u th pb and lu hf isotopic systematics and ree concentrations of zircon from archean granitoid gneisses abstract archean granitoid tonalite trondhjemite granodiorite ttg gneisses are widely distributed in the early precambrian metamorphic basement of the jiaobei terrane the north china craton ncc and record the formation and evolution of the continental crust in the early precambrian in this study zircon cathodoluminescence cl images have assisted in obtaining u th pb and lu hf isotopic systematics and rare earth element ree concentrations from distinct zircon domains of complex zircons and the results have been integrated in order to unravel the growth reworking and metamorphism of the early precambrian crust in the jiaobei terrane zircons from seven representative archean granitoid samples collected from the jiaobei terrane record three groups of magmatic ages of 2 9 2 7 and 2 5 ga and two groups of metamorphic ages of 2 5 and 1 86 ga the 2 5 ga metamorphic event is thought to be linked to underplating of large amounts of mantle derived magma and the 1 86 ga metamorphic event to arc or continent continent collision magmatic zircons have positive hf t values 0 7 to 12 58 average 5 5 and two stage hf model ages cluster at 2 7 2 8 and 2 9 3 0 ga this indicates that the archean granitoid rocks were derived from partial melting of juvenile crust that was formed mainly during 2 7 2 8 and 2 9 3 0 ga which represent two periods of growth of juvenile crust the most significant period of crustal growth is 2 7 2 8 ga whereas 2 5 ga represents a period of reworking of the 2 7 2 8 ga juvenile crust in the jiaobei terrane as well as other areas in the ncc

hornblende garnet plagioclase thermobarometry a natural assemblage calibration of the thermodynamics of hornblende calibrations are presented for an independent set of four equilibria between end members of garnet hornblende plagioclase and quartz thermodynamic data from a large internally consistent thermodynamic dataset are used to determine the g of the equilibria then with the known mixing properties of garnet and plagioclase the non ideal mixing in amphibole is derived from a set of 74 natural garnet amphibole plagioclase quartz assemblages crystallised in the range 4 13 kbar and 500 800 c the advantage of using known thermodynamic data to calculate g is that correlated variations of composition with temperature and pressure are not manifested in fictive derived entropies and volumes but are accounted for with non ideal mixing terms the amphibole is modelled using a set of ten independent end members whose mixing parameters are in good agreement with the small amount of data available in the literature the equilibria used to calibrate the amphibole non ideal mixing reproduce pressures and temperatures with average absolute deviations of 1 1 kbar and 35 c using an average pressure temperature approach and 0 8 kbar with an average pressure approach the mixing data provide not only a basis for thermobarometry involving additional phases but also for calculation of phase diagrams in complex amphibole bearing systems

late paleoproterozoic 1900 1800ma nappe stacking and polyphase deformation in the hengshan wutaishan area implications for the understanding of the trans north china belt north china craton abstract the hengshan wutaishan area forms part of the trans north china belt developed in response to the final amalgamation of the north china craton the hengshan wutaishan domain and the adjacent fuping massif constitute the largest and the most representative rock exposure of the belt a structural study allows us to redefine the lithological and tectonic units that compose the architecture of the hengshan wutaishan domain and to propose that the trans north china belt was built up through a polyphase tectonic evolution within the period 1900 1800 ma the first event d1 corresponds to the emplacement of lower and upper nappes herein called the orthogneiss and volcanites unit ovu and the low grade mafic unit lgmu respectively the ovu mainly consists of rocks formed in an arc system and metamorphosed under amphibolite facies conditions whereas the lgmu represents oceanic crust rocks with an arc component that underwent greenschist facies metamorphism the syn metamorphic d1 deformation is characterized by a nw se stretching and mineral lineation with a top to the se sense of shear that corresponds to the sense of nappe motion u th pb epm chemical dating on monazite from three metapelites of the ovu gives isochron ages of 1884 11 ma 1886 5 ma and 1887 4 ma interpreted as the age of the prograde amphibolite facies metamorphism coeval with nappe stacking during the d1 event the d2 event corresponds to a widespread crustal melting of the ovu and underlying basement rocks in hengshan related to the exhumation of deep structures about 20 30 ma after nappe emplacement the migmatite contains blocks of retrograded eclogite or hp granulitic restites and is characterized by the development of a pervasive foliation and lineation a late d2 event represents the folding stage of the migmatitic foliation during the completion of a dome the orthogneiss and volcanics unit and the low grade mafic unit are unconformably covered by weakly metamorphosed to unmetamorphosed sediments belonging to the hutuo supergroup these rocks were deformed by a ductile d3 event characterized by a nw se trending stretching lineation south verging folds associated with an axial planar slaty cleavage and an e w crenulation lineation lastly a left lateral wrenching along the zhujiafang shear zone separates the northern and southern parts of hengshan our structural study shows that there is no significant difference between the southern part of hengshan and the lower part of wutaishan both areas belong to ovu and show a structural continuity we suggest that this polyphase deformation developed in response to a north westward subduction of an old ocean named the luliang ocean beneath the western block followed by collision with a micro continent named the fuping block at 1890 1880 ma

metamorphic evolution and shrimp u pb geochronology of mafic granulites with double symplectites in the fuping metamorphic complex middle palaeoproterozoic trans north china orogen abstract garnet bearing mafic granulites occur as lenses within ttg gneiss or metapelite in the fuping complex middle palaeoproterozoic trans north china orogen they are composed mainly of garnet clinopyroxene orthopyroxene hornblende plagioclase quartz and accessary zircon and titanite four stages of metamorphic mineral assemblages have been found in these mafic granulites a the prograde assemblages m1 are inclusions preserved in the garnet porphyroblasts including clinopyroxene hornblende plagioclase and quartz b the metamorphic peak assemblages m2 consist of garnet porphyroblasts plus matrix minerals clinopyroxene orthopyroxene hornblende plagioclase quartz zircon titanite c the first retrograde assemblages m3 are represented by clinopyroxene orthopyroxene plagioclase quartz symplectites rimming the garnets and 4 the final retrograde assemblages m4 are hornblende plagioclase quartz symplectites rimming or adjacent to the garnets the retrieved metamorphic p t paths pass from 530 600 c 6 5 7 0 kbar m1 through 680 820 c 7 2 11 7 kbar m2 to 730 760 c 3 2 3 9 kbar m3 and finally to 600 640 c 3 2 3 7 kbar m4 all the p t paths of the mafic granulites are clockwise two of these even contain near isothermal decompression itd and subsequent near isobaric cooling ibc segments shrimp u pb dating of metamorphic zircons of the mafic granulites yields a mean weighted age of 1849 9ma which is consistent with the metamorphic time of surrounding ttg gneisses therefore the fuping complex records the collision event between the eastern and western blocks of the north china craton that led to the final assembly of this craton during the late paleoproterozoic

composite nature of the north china granulite facies belt tectonothermal and geochronological constraints granulite facies rocks are intermittently exposed in a roughly e w trending belt that extends for approximately 2000 km across the north china craton from the helanshan qianlishan wulashan daqingshan guyang and jining complexes in the western block through the huai an hengshan xuanhua and chengde complexes in the trans north china orogen to the jianping western liaoning eastern hebei northern liaoning and southern jilin complexes in the eastern block the belt is generally referred to as the north china granulite facies belt previously interpreted as the lowest part of an obliquely exposed crust of the north china craton recent data indicate that the north china granulite facies belt is not a single terrane instead it represents components of three separate terranes the eastern and western blocks and trans north china orogen each of these units records different metamorphic histories and reflect the complex tectonic evolution of the ncc during the late archean and paleoproterozoic mafic granulites in the eastern block and the yinshan terrane western block underwent medium pressure granulite facies metamorphism at about 2 5 ga with anticlockwise p t paths involving near isobaric cooling following peak metamorphism reflecting an origin related to intrusion and underplating of mantle derived magmas pelitic granulites in the khondalite belt western block underwent medium pressure granulite facies metamorphism at about 2 0 1 9 ga with clockwise p t paths which record the paleoproterozoic amalgamation of the yinshan and ordos terranes to form the western block mafic and pelitic granulites in the trans north china orogen experienced high to medium pressure granulite facies metamorphism at 1 85 ga with clockwise p t paths involving nearly isothermal decompression following peak metamorphism which are in accord with the final collision between the eastern and western blocks to form the north china craton at 1 8 ga the ncgb cannot therefore represent a separate unique terrane instead it reflects the amalgamation of three separate granulite terranes that evolved independently and at different times d 2005 international association for gondwana research published by elsevier b v all rights reserved

empirical garnet biotite plagioclase quartz gbpq geobarometry in medium to high grade metapelites on the basis of the net transfer reactions among garnet biotite plagioclase and quartz for both mg and fe end member models the garnet biotite plagioclase quartz gbpq geobarometer was empirically calibrated under physical conditions of p 1 0 11 4 kbar and t 515 878 c based on the input garnet biotite temperatures and garnet aluminosilicate plagioclase quartz gasp pressures of 224 natural aluminosilicate bearing metapelitic samples collated from the literature the calibrations are internally consistent with the asymmetric quaternary solid solution model of garnet the symmetric quaternary solid solution model of biotite and the al avoidance ternary solid solution model of plagioclase in calibrating the garnet biotite geothermometer and the gasp geobarometer the resulting two gbpq barometer formulae reproduce the input gasp pressures well within 1 0 kbar mostly within 0 5 kbar for both aluminosilicate bearing and aluminosilicate absent metapelites the two gbpq barometry formulae yielded identical pressures whether the sample was included or not included in calibrating the gbpq barometry the random error of the gbpq barometry may be expected as 1 2 kbar the dp dt slopes of these two gbpq formulae are close to that of the gasp barometer in p t space applications of the gbpq barometry of aluminosilicate absent metapelites to the rocks within a thermal contact aureole or rocks within a limited geographical area without post metamorphic structural discontinuity show no obvious pressure change it may be concluded that the two gbpq barometry formulae derived in this study may be used as practical tools for metamorphic pelites under the conditions of 515 878 c and 1 0 11 4 kbar in the composition range of xgros 3 in garnet xan 17 in plagioclase and x bio 3 in biotite application of the gbpq barometer beyond the calibration range should be undertaken with caution

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

p t t path and tectonic significance of pelitic migmatites from the luliang complex in xiyupi area of trans north china orogen north china craton abstract in order to reveal the metamorphic evolution of the luliang complex a combined study of petrography phase equilibria modeling and zircon u pb dating is carried out on two mesosome of pelitic migmatites from the jiehekou group in the luliang complex sample 15xyp 02 preserved the thermal peak assemblage of garnet biotite k feldspar sillimanite quartz and ilmenite without plagioclase it recorded two near isothermal decompressional indicators that rutile was moated by ilmenite and garnet was replaced by cordierite small plagioclase occurred in the rim of k feldspar and garnet four stages m1 m4 of metamorphic evolution have been identified based on the mineral assemblages and p t pseudosection it is interpreted to record pre tmax decompression from rutile stability field m1 represented by rutile relicts in matrix and biotite sillimanite quartz inclusions in garnet porphyroblasts at 8 9 kbar to the tmax stage m2 characterized by garnet rim with ilmenite and biotite k feldspar sillimanite quartz in matrix of 815 838 c the following suprasolidus post tmax cooling comprises m3 stage represented by appearance of cordierite in the assemblage of garnet biotite k feldspar sillimanite ilmenite quartz of 780 838 c 5 0 6 3 kbar and m4 stage defined by the final mineral assemblage of garnet biotite k feldspar plagioclase sillimanite cordierite ilmenite quartz of 775 c 5 0 kbar another sample 15xyp 21 preserved the thermal peak assemblage of garnet biotite plagioclase k feldspar sillimanite quartz it also records pre tmax decompression from rutile stability field at 9 0 kbar to the tmax stage of 796 821 c 6 4 7 1 kbar followed by suprasolidus post tmax cooling stage of 778 803 c 5 0 5 7 kbar the clockwise p t paths are consistent with those of orogenic belt furthermore the zircon u pb dating on mesosome and leucosome of pelitic migmatite yield metamorphic ages of 1 95 ga and 1 91 ga respectively in which the former represents the early stage of cooling after tmax nearly close to the time of pressure peak metamorphism combined with those granulites in the trans north china orogen a common long lived continent continent collision processes was proposed which may culminated at 1 95 ga and followed by a prolonged cooling process

prolonged anatexis of paleoproterozoic metasedimentary basement first evidence from the yinchuan basin and new constraints on the evolution of the khondalite belt north china craton abstract discerning the history of burial residence at depth and exhumation of high grade metamorphic rocks from an orogen is essential to understanding crustal differentiation and orogenesis such high grade rocks are inferred to exist beneath the yinchuan basin which is overlain by a thick covering of younger sediments and is located at the inferred southwest boundary of the paleoproterozoic khondalite belt in the northwestern north china craton the metamorphic basement sampled from a recently completed deep borehole in the yinchuan basin is migmatitic and contains an assemblage of garnet biotite sillimanite plagioclase quartz minor k feldspar and local occurrences of muscovite phase equilibrium modelling in the na2o cao k2o feo mgo al2o3 sio2 h2o tio2 o system constrains the peak p t conditions of anatexis to 6 10 kbar at 760 810 c followed by final crystallization at 4 5 5 5 kbar and 690 710 c ti in zircon thermometry for the narrow metamorphic zircon rims provides a temperature range of 687 753 c demonstrating formation during retrogression to the solidus nanosims u pb analyses on the metamorphic overgrowths of zircon grains yields an upper intercept age of 1895 36 ma chronological and chemical analyses of monazite indicate irregular bright gray domains with relatively high th and low y and hree contents developed during prograde metamorphism 1962 8 ma conversely dark domains with low th and high y and hree overgrew during retrograde cooling 1892 14 ma that was synchronous with the development of metamorphic zircon rims la icp ms u pb analyses of detrital zircons reveal that the metamorphic basement beneath the yinchuan basin has a sedimentary protolith of paleoproterozoic age the results directly confirm the existence of paleoproterozoic basement beneath the yinchuan basin and the well constrained p t t path supports the involvement of the basement as part of the western khondalite belt in burial and exhumation processes during the 1 96 1 89 ga period during which anatexis occurred for 70 ma

stability of andalusite and sillimanite and the al_ 2 sio_ 5 triple point constraints from the ballachulish aureole scotland andalusite and and sillimanite sil occur in well defined zones in the ballachulish aureole in most of the aureole s sillimanite bearing rocks sillimanite grain diameters range from 2 m to 10 m the boundary between prograde andalusite and prograde sillimanite is constrained to a zone of max 30 m width upgrade of the muscovite quartz andalusite k feldspar h 2 o reaction isograd this boundary is interpreted to correspond to the and sil equilibrium the common occurrence of andalusite well upgrade of the first occurrence of sillimanite is ascribed to sluggish kinetics of the polymorphic inversion the p t position of the and sil equilibrium in the aureole is estimated as 645 20 c and 3 0 0 4 kbar based on a combination of 1 location of the ms qtz and kfs h 2 o reaction corrected for non end member fluid and mineral compositions and 2 independent pressure estimates obtained from equilibria in metapelites marbles calcsilcates and granitoids all in exce

geochemistry and detrital zircon u pb and hf isotopes of the paragneiss suite from the quanji massif se tarim craton implications for paleoproterozoic tectonics in nw china abstract the delingha paragneiss suite in the quanji massif southeastern tarim craton is composed of mica schist paragneiss leptynite and quartzite similar to the khondalite suites described from elsewhere in the world the mica schist is rich in al2o3 up to 26 wt and contains graphite and diagnostic minerals including sillimanite and garnet with metamorphism under amphibolite facies to locally granulite facies conditions as manifested by association with amphibolite and granulite the detrital zircon u pb ages and geochemical data indicate that the protolith materials of the delingha paragneiss suite were mainly sourced from 2 20 to 2 45 ga granites felsic volcanic rocks and ttg and were deposited at 2 17 1 92 ga the detrital zircon hf and whole rock nd isotopes document important crustal growth at 2 5 2 7 ga the detrital zircon age spectra the whole rock nd and zircon hf model ages the low maturity of the protolith and short distance transportation suggest that the detritus were derived from the underlying delingha complex and the lower dakendaban sub group the timing of magmatic activities in the source region the depositional age and metamorphic histories of the delingha paragneiss suite are all comparable to those recorded in the khondalite belt along northern margin of the ordos block in the north china craton our study shows that the 2 2 2 45 ga magmatic rocks were generated in arc or active continental margin settings suggesting a prolonged subduction and accretion history prior to final amalgamation 2 5 1 8 ga to form the unified north china craton and the assembly of the tarim craton in nw china

development of the north china craton during the late archaean and its final amalgamation at 1 8 ga some speculations on its position within a global palaeoproterozoic supercontinent abstract new data obtained over the past few years indicate that by the end of the archaean the north china craton consisted of two major crustal entities identified as the eastern and western blocks the formation of island and magmatic arcs along the western margin of the eastern block record the latest archaean events so far identified the location of the western block at this time is unknown however the occurrence of palaeoproterozoic passive margin sediments similar in age to those occurring in the eastern block indicates essentially similar conditions and possible separation by an ocean of unknown dimensions re initiation of east dipping subduction beneath the western margin of the eastern block resulted in closure of this palaeoproterozoic ocean bringing the western block into close proximity continued subduction resulted in a major continental continental collision and the western margin of the eastern block was again reactivated leading to extensive thrusting high pressure metamorphism and the generation of crustal melts this occurred between 1 88 and 1 79 ga ago resulting in the formation of the trans north china orogen and final amalgamation of the north china craton although information is sparse to non existent we have reviewed the possible location of the north china craton at the time of collision and note a similarity in terms of age lithology and distribution of major crustal components between the north china craton and baltica although speculative we present this model in the hope that it will stimulate debate and lead to further research in these areas

new geochronological evidences of late neoarchean and late paleoproterozoic tectono metamorphic events in the miyun area north china craton abstract the miyun area is located in the western margin of the eastern block of the north china craton ncc and granulite to amphibolite facies tectono metamorphic events occurred in the precambrian in this contribution high resolution secondary ion mass spectrometry sims u pb dating of metamorphic zircon from garnet pyroxene amphibolite yielded the metamorphic age of 2507 4 ma sims u pb dating of monazite from garnet biotite gneiss yielded the metamorphic ages of 2496 16 ma and 1834 21 ma respectively furthermore 40ar 39ar dating of hornblende from amphibolite yielded plateau ages of 1817 41 1795 19 ma and the biotite yielded a plateau age of 1744 12 ma combined with previous studies and the closure temperatures of the dating minerals it is inferred that the miyun area experienced a late neoarchean granulite facies metamorphic event occurred at 2 50 ga followed by a late paleoproterozoic amphibolite facies metamorphic event occurred at 1 83 1 74 ga these two tectono metamorphic events are ascribed to the precambrian orogenic related environments

valid garnet biotite gb geothermometry and garnet aluminum silicate plagioclase quartz gasp geobarometry in metapelitic rocks abstract at present there are many calibrations of both the garnet biotite gb thermometer and the garnet aluminum silicate plagioclase quartz gasp barometer that may confuse geologists in choosing a reliable thermometer and or barometer to test the accuracy of the gb thermometers we have applied the various gb thermometers to reproduce the experimental data and data from natural metapelitic rocks of various prograde sequences inverted metamorphic zones and thermal contact aureoles we have concluded that the four gb thermometers perchuk l l lavrent eva i v 1983 experimental investigation of exchange equilibria in the system cordierite garnet biotite in saxena s k ed kinetics and equilibrium in mineral reactions springer verlag new york berlin heidelberg pp 199 239 kleemann u reinhardt j 1994 garnet biotite thermometry revised the effect of alvi and ti in biotite european journal of mineralogy 6 925 941 holdaway m j 2000 application of new experimental and garnet margules data to the garnet biotite geothermometer american mineralogist 85 881 892 model 6av kaneko y miyano t 2004 recalibration of mutually consistent garnet biotite and garnet cordierite geothermometers lithos 73 255 269 model b are the most valid and reliable of this kind of thermometer more specifically we prefer the holdaway holdaway m j 2000 application of new experimental and garnet margules data to the garnet biotite geothermometer american mineralogist 85 881 892 and the kleemann and reinhardt kleemann u reinhardt j 1994 garnet biotite thermometry revised the effect of alvi and ti in biotite european journal of mineralogy 6 925 941 calibrations due to their small errors in reproducing the experimental temperatures and good accuracy in successfully discerning the systematic temperature changes of the different sequences in addition after applying the gasp barometer to 335 natural metapelitic samples containing one kind of aluminosilicate and 33 natural metapelitic samples containing two kinds of aluminosilicate as well as to rocks in limited geographic areas and thermal contact aureoles we propose that the calibrations of holdaway holdaway m j 2001 recalibration of the gasp geobarometer in light of recent garnet and plagioclase activity models and versions of the garnet biotite geothermometer american mineralogist 86 1117 1129 and newton and haselton newton r c haselton h t 1981 thermodynamics of the garnet plagioclase al2sio5 quartz geobarometer in newton r c navrotsky a wood b j eds thermodynamics of minerals and melts new york springer verlag 131 147 based on kleemann and reinhardt s kleemann u reinhardt j 1994 garnet biotite thermometry revised the effect of alvi and ti in biotite european journal of mineralogy 6 925 941 thermometer are the most valid gasp barometers other gb thermometers and gasp barometers are not recommended

archean crustal evolution in the southeastern north china craton new data from the huoqiu complex abstract the southeastern margin of the north china craton ncc is characterized by zircon ages of dominantly 2 6 ga and differs significantly from most other regions in the ncc with mostly 2 5 ga zircons in order to better understand the precambrian crustal evolution of the ncc we conducted a study of in situ zircon u pb dating trace element and hf isotope analyses as well as whole rock geochemistry on a comprehensive set of core samples from three drilling holes intersecting the basement rocks of the huoqiu area at the southeastern margin of the ncc our data reveal three magmatic events at 3 02 ga 2 77 ga and 2 71 ga in addition seven analyses of possibly inherited zircon cores plot along a discordant line with an upper intercept age of 3262 35 ma indicating possible existence of a paleoarchean crystalline basement the 3 02 ga zircons have positive hf t values close to the depleted mantle growth curve and their u pb ages are only slightly younger than the tdm1 model ages hence recording a mesoarchean crust forming event the 2 77 ga zircons also register a period of crustal growth but probably with some involvement of older crust the 2 71 ga zircons have lower hf t values and older tdm1 model ages than the 2 77 ga zircons suggesting that the former were mainly derived from older continental crust in contrast to the traditional idea that 2 7 ga was a major period of crustal growth in the ncc

zircon u pb hf isotopes and geochemistry of neoarchean dioritic trondhjemitic gneisses eastern hebei north china craton constraints on petrogenesis and tectonic implications abstract dioritic trondhjemitic gneisses of neoarchean age are widely distributed throughout the gualanyu shangying area in eastern hebei north china craton these orthogneisses consist of dioritic trondhjemitic and quartz rich trondhjemitic gneisses which were subjected to amphibolite to granulite facies metamorphism la icp ms zircon u pb isotopic dating reveals that the magmatic precursors of these rocks were contemporaneously emplaced from 2535 23 to 2513 8 ma reflecting an important neoarchean dioritic trondhjemitic magmatic event followed by sequential proterozoic tectonothermal events between 2472 60 and 2307 50 ma dioritic and trondhjemitic gneisses exhibit similar zircon hf t values of 1 5 to 5 2 and 2 9 to 5 3 respectively indicating their depleted mantle affinity geochemical analysis reveals that the dioritic gneisses have low sio2 contents 55 4 63 0 various mgo contents 2 07 4 78 and mg 36 07 63 56 and high la yb n ratios 5 45 15 19 in contrast the trondhjemitic gneisses exhibit higher sio2 contents 67 4 73 8 and la yb n ratios 12 86 76 73 and lower mgo contents 0 66 2 67 although their mg 36 29 61 41 are similar to those of dioritic gneisses these geochemical and zircon lu hf isotopic features together with geochemical modeling suggest that the dioritic magmas were derived mainly from the partial melting of a depleted mantle wedge that was metasomatized by slab derived melts or fluids and that the trondhjemitic magmas were formed by fractional crystallization of the dioritic magmas quartz rich trondhjemitic gneisses exhibit extremely high sio2 contents 77 4 84 6 with zircon hf t values ranging from 3 4 to 4 1 suggesting that their precursors formed after the partial melting of a mixed magmatic source it is suggested that the precursors comprised a mafic member derived from a mantle wedge altered by oceanic slab melts or fluids and a felsic member derived from the partial melting of ancient crustal material these new data combined with previous studies of contemporaneous metavolcanic rocks in gualanyu shangying area and surrounding areas suggest that the magmatic precursors of these neoarchean dioritic trondhjemitic gneisses formed in a subduction related tectonic environment at an e w strike convergent plate margin furthermore the eastern block of the north china craton had a major lateral accretion of crustal growth in the north margin in the neoarchean whereas vertical crustal growth and recycling under plume tectonic backgrounds occurred in the interior of the eastern block

neoarchean to paleoproterozoic continental evolution and tectonic history of the north china craton a review rocks of archean age are widely distributed in the north china craton ncc and can be divided into high grade regions and granite greenstone belts even though most underwent amphibolite to granulite facies metamorphism quartzo feldspathic gneisses occupy 70 to 85 of the total exposure of archean rocks in the craton supracrustal rocks commonly consist of intercalated volcano sedimentary rocks including metabasites meta intermediate to acid rocks banded iron formations metapelites and minor marbles the oldest rocks outcrop over a small area of 4 km2 and are a quartzite amphibolite sequence that yield 3 8 3 5 ga isotopic ages chemical and geochronological data constrain the earliest main crust forming episode at ca 2 9 2 7 ga when most rocks were metamorphosed to upper amphibolite or granulite facies at moderate pressures mafic granulites can be classified into two textural types referred to as near isobaric cooling ibc and near isothermal decompression itd types high pressure granulites and retrograded eclogites occur mainly near the junction of the provinces of hebei shanxi and inner mongolia in the central ncc and in eastern shandong province of the eastern ncc as well as locally in henan province in the southern ncc these high pressure rocks show clockwise p t paths indicating decompression from 14 kb to 5 6 kb an exposed section interpreted as representing precambrian continental lower crust has also been identified in the northern part of the craton although some mafic ultramafic metamorphic rocks have been interpreted as remnants of ancient oceanic crust and mantle by a number of geologists geochemical and petrological evidence demonstrates that many supracrustal orthogneiss associations have island arc affinities therefore several tectonic models have been proposed including continent continent collision and island arc terrain accretion achieved by arc arc or arc microcontinent collision the ncc underwent multiple high grade metamorphic events of which the two most important took place during the intervals 2600 2450 ma and 1950 1750 ma which are considered to be related to early precambrian supercontinental cycles

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

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

duration and depth of faulting of the 22 june 1977 tonga earthquake the 22 june 1977 mw 8 2 tonga earthquake has the longest rupture duration ever reported for a normal fault event the 150 km depth range spanned by aftershocks of the earthquake is also unusually large there has been substantial controversy over both the depth and duration of faulting for this great event obscuring its tectonic significance we study the source process of the tonga event using long period rayleigh waves recorded by the global digital seismograph network gdsn and international deployment of accelerometers ida networks for a standard assumption of a haskell source a total duration of 84 4 sec is obtained using a least squares inversion method we introduce the use of the spectral amplitude as a weighting factor in measuring the misfit between the data and a given source finiteness model which reduces the scatter and improves the resolution of source duration determined from data ranging in period from 150 to 300 sec using a more realistic shape for the source time function in the inversion drawing upon results from body wave analysis reveals a much longer 165 sec process time component of the source process of the tonga earthquake the fundamental mode rayleigh waves do not resolve any horizontal source directivity however the centroid depth of the earthquake is well resolved as 96 km with 90 per cent confidence range 93 104 km the estimated error in the depth determination due to the uncertainties in the source finiteness and earth models is only a few kilometers the results indicate that the rupture of the earthquake excited long period seismic waves at depths somewhat greater than the 70 to 80 km depth range where the primary bodywave radiation occurred favoring rupture on the steeply dipping plane of the focal mechanism the fundamental mode rayleigh waves with periods longer than 150 sec cannot resolve vertical extent of the faulting however additional information from body wave and free oscillation analyses indicates a vertical fault extent of about 50 km with a frequency dependent variation in seismic radiation with depth

structural evolution and 40ar 39ar dating of the zanhuang metamorphic domain in the north china craton constraints on paleoproterozoic tectonothermal overprinting abstract the zanhuang domain in the hinterland of the north china craton consists of archaean paleoproterozoic mesoproterozoic and palaeozoic rocks and was previously considered a mesozoic metamorphic core complex our reappraisal of regional structural history and mineral 40 ar 39 ar geochronology shows that the domain was subjected to a number of deformation events and had a heterogeneous uplift cooling history during the paleoproterozoic four primary deformation events have been recognized in the domain the first event reflects the earliest deformation in the region however only closed and rootless folds are preserved d2 was dominated by wnw ese shortening and top to ese thrusting in a collisional environment d3 was a divergent extensional and ductile shearing event driven by the post collisional extensional collapse and exhumation of the thickened crust in the domain d4 was characterized by localized ductile shearing with the development of three nne trending ductile shear zones the 40 ar 39 ar plateau ages derived from the thermochronology analyses of the biotite concentrates from the metamorphic rocks are interpreted as cooling ages through biotite 40 ar 39 ar closure temperature 300 c and this allows us to constrain the timing of deformation events in conjunction with the evidence from structural analyses and previous geochronological data we conclude that 1870 and 1870 1826 ma define the timing of d1 and d2 events respectively while 1826 1793 ma represents the age of d3 the last event d4 occurred at 1689 1633 ma as constrained by 40 ar 39 ar plateau ages of biotite concentrates from the mylonites within the ductile shear zones these data together with k feldspar 40 ar 39 ar geochronology allows us to depict the cooling history of the region during the paleoproterozoic and mesoproterozoic it appears that the uplift cooling rate varies temporally and spatially in the region

the origins of granulites a metamorphic perspective although many recent reviews emphasize a uniformity in granulite pressure temperature p t conditions and paths granulites in reality preserve a spectrum of important petrogenetic features which indicate diversity in their modes of formation a thorough survey of over 90 granulite terranes or occurrences reveals that over 50 of them record p t conditions outside the 7 5 1 kbar and 800 50 c average granulite regime preferred by many authors in particular an increasing number of very high temperature 900 1000 c terranes are being recognized both on the basis of distinctive mineral assemblages and geothermobarometry petrogenetic grid and geothermobarometric approaches to the determination and interpretation of p t histories are both evaluated within the context of reaction textures to demonstrate that the large range in p t conditions is indeed real and that both near isothermal decompression itd and near isobaric cooling ibc p t paths are important amphibolite granulite transitions promoted by the passage of co 2 rich fluids as observed in southern india and sri lanka are exceptional and not representative of fluid related processes in the majority of terranes it is considered on the contrary that fluid absent conditions are typical of most granulites at or near the time of their recorded thermal maxima itd granulites are interpreted to have formed in crust thickened by collision with magmatic additions being an important extra heat source erosion alone is not however considered to be the dominant post collisional thinning process instead the itd paths are generated during more rapid thinning 1 2 mm yr exposure related to tectonic exhumation during moderate rate or waning extension ibc granulites may have formed in a variety of settings those which show anticlockwise p t histories are interpreted to have formed in and beneath areas of voluminous magmatic accretion with or without additional crustal extension ibc granulites at shallow levels very rapid 5 mm yr extensional thinning subsequent to collision it is suggested that the preservation of ibc paths rather than itd paths in many granulites is primarily related to the rate and timescale of extensional thinning of thickened crust and that hybrid itd to ibc paths should also be observed most ibc granulites and probably many itd granulites have not been exposed at the earth s surface as a result of the tectonic episodes which produced them but have resided in the middle and lower crust for long periods of time 100 2000 ma following these events the eventual exhumation of most granulite terranes only occur through their incorporation in later tectonic and magmatic events unrelated to their formation

shrimp u pb zircon geochronology of palaeoproterozoic metasedimentary rocks in the north china craton evidence for a major late palaeoproterozoic tectonothermal event abstract although metasedimentary successions are widely distributed across the north china craton those of palaeoproterozoic age are restricted to three orogenic belts namely the trans north china orogen the khondalite belt and the jiao liao ji belt prior to this work some of those successions were considered to be archaean in age the palaeoproterozoic metasedimentary rocks contain detrital igneous zircon grains with ages ranging from 3 0 to 2 1 ga with the majority recording an age between 2 3 and 2 0 ga this latter group define a series of magmatic events that occurred between formation of the archaean basement and deposition of the metasedimentary rock protoliths the metasedimentary rocks occur as two associations older volcano sedimentary units that formed between 2 37 and 2 0 ma and younger sedimentary units that formed mainly between 2 0 and 1 88 ga all these rock associations were then subjected to metamorphism during the late palaeoproterozoic 1 88 and 1 85 ga tectonothermal event named the luliang movement in early chinese literature which was a continent continent collision that led to the amalgamation 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

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

high pressure granulites in the sanggan area north china craton metamorphic evolution p t paths and geotectonic significance high pressure basic granulites are widely distributed as enclaves and sheet like blocks in the huaian ttg gneiss terrane in the sanggan area of the central zone of the north china craton four stages of the metamorphic history have been recognised in mineral assemblages based on inclusion exsolution and reaction textures integrated with garnet zonation patterns as revealed by compositional maps and compositional profiles the p t conditions for each metamorphic stage were obtained using thermodynamically and experimentally calibrated geothermobarometers the low ca core of growth zoned garnet along with inclusion minerals defines a prograde assemblage m1 of garnet clinopyroxene plagioclase quartz yielding 700 c and 10 kbar the peak of metamorphism at about 750 870 c and 11 14 5 kbar m2 is defined by high ca domains in garnet interiors and inclusion minerals of clinopyroxene plagioclase and quartz kelyphites or coronas of orthopyroxene plagioclase magnetite around garnet porphyroblasts indicate garnet breakdown reactions m3 at conditions around 770 830 c and 8 5 10 5 kbar garnet exsolution lamellae in clinopyroxene and kelyphites of amphibole plagioclase around garnet formed during the cooling process at about 500 650 c and 5 5 8 kbar m4 these results help define a sequential p t path containing prograde near isothermal decompression itd and near isobaric cooling ibc stages the clockwise hybrid itd and ibc p t paths of the hp granulites in the sanggan area imply a model of thickening followed by extension in a collisional environment furthermore the relatively high pressures 6 14 5 kbar of the four metamorphic stages and the geometry of the p t paths suggest that the hp granulites together with their host huaian ttg gneisses represent the lower plate in a crust thickened during collision the corresponding upper plate might be the tectonically overlying khondalite series which was subjected to medium to low pressure mp lp 7 4 kbar granulite facies metamorphism with a clockwise p t path including an itd segment both the hp and the mp lp granulite facies events occurred contemporaneously at c 1 90 1 85 ga in a collisional environment created by the assembly process of the north china craton

thermobarometry in granulites pitfalls and new approaches abstract computer models of cooling granulite facies rocks with the assemblage garnet biotite sillimanite k feldspar plagioclase quartz are presented that incorporate simultaneous reaction net transfer and exchange and multicomponent diffusion in garnet in models where cooling is accompanied by exchange reactions plus diffusion only the calculated garnet core matrix biotite temperature is always lower than the metamorphic peak by an amount that depends on the cooling rate the size of the garnet and the volumetric ratio of garnet to biotite maximum calculated core temperatures of approximately 700 750 c are to be expected from typical granulite facies garnet biotite assemblages and calculated core temperatures can easily be as low as 600 c in models where both net transfer and exchange reactions are possible calculated garnet core matrix biotite temperatures are typically higher than the metamorphic peak temperatures by more than 150 c for large garnets r 2 mm but may be lower than the metamorphic peak for small garnets r the composition of matrix plagioclase in equilibrium with garnet is very sensitive to the amount of modal plagioclase that reacts and hence calculated p t trajectories are very sensitive to the petrologic interpretation of this recrystallization history for models in which all the modal plagioclase reacts the core and rim pressures may be faithfully recovered provided that the temperature estimate for the core is reliable for models in which only a small percentage of the modal plagioclase reacts very small modal amounts of plagioclase may be in equilibrium with garnet rim which if incorrectly interpreted may lead to grossly erroneous p t path interpretations a method to test an inferred p t path model is presented that involves attempting to match the observed garnet and plagioclase zoning information with a forward model that involves simultaneous reaction and diffusion an example from western new hampshire usa is presented that closely matches the measured profile

zircon u pb geochronology of the zanhuang metamorphic complex reappraisal of the palaeoproterozoic amalgamation of the trans north china orogen amphibolites and metapelites exposed in the zanhuang metamorphic complex situated in the south middle section of the trans north china orogen tnco underwent upper amphibolite facies metamorphism and record clockwise p t paths including retrograde isothermal decompression high resolution zircon u pb geochronological analyses indicate that the metamorphic peak occurred during 1840 1860 ma which is in accordance with the ubiquitous metamorphic ages of 1850 ma retrieved by miscellaneous geochronologic methods throughout the metamorphic terranes of the northern tnco confirming that the south middle section of the tnco was involved in the amalgamation of the eastern and western blocks of the north china craton during the palaeoproterozoic

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

tectonic history of subduction zones inferred from retrograde blueschist p t paths many phanerozoic convergent plate junctions are marked by discontinuous blueschist belts reflecting relatively high pressure p prograde trajectories common blueschist paragneisses such as those of the western alps exhibit widespread overprinting by greenschist and or epidote amphibolite facies assemblages for this type of high p belt retrograde metamorphism involved fairly rapid nearly isothermal decompression some terranes underwent continued heating during early stages of pressure release uplift probably occurred as a consequence of the entrance of an island arc oceanic plateau or segment of continental crust into the subduction zone collision resulting in marked deceleration or cessation of underflow and buoyant approximately adiabatic rise of the stranded recrystallized subduction complex other high p belts such as the franciscan of western california preserve metamorphic aragonite and lack a low p overprint retrogression approximately retraced the prograde p t temperature path or for early formed high grade blocks occurred at even higher p t ratios parts of this type of metamorphic belt evidently migrated slowly back up the subduction zone in response to isostatic forces during continued plate descent and refrigeration upward motion took place as tectonically imbricated slices as laminar return flow in melange zones and perhaps partly a lateral spreading extension of the underplated accretionary prism retrograde p t trajectories of high pmore belts therefore provide important constraints on the tectonic evolution of convergent plate junctions less

u pb dating and hf isotope study of detrital zircons from the zhifu group jiaobei terrane north china craton provenance and implications for precambrian crustal growth and recycling abstract the zhifu group exposed in the northeastern jiaobei terrane is subdivided from base to top into the laoyeshan bingying and dongkou formations and comprises mainly quartzites and muscovite and tourmaline bearing quartz schists in order to define its provenance and depositional age and to constrain the crustal growth and recycling of the source regions we have conducted la icp ms u pb dating and in situ hf isotopic study of detrital zircons from three quartz schists samples collected from the zhifu group most of the analyzed zircon grains have oscillatory zoning and th u ratios 0 4 suggesting igneous origins 270 detrital zircons u pb analyses yield u pb ages of 1709 3679 ma that cluster into three major age populations of 1 7 1 9 ga with a peak at 1844 ma 1 9 2 0 ga with a peak at 1936 ma and 2 4 2 55 ga with a peak at 2465 ma and two minor age populations of 2 2 2 35 and 2 6 2 7 ga the peaking age of 1844 ma for the youngest age population indicates that the zhifu group must be deposited after 1844 ma detrital zircons with ages of 1 7 2 35 ga can be linked to pre and post tectonic polyphase magmatic events along the jljb and most of the zircons have negative hf t values from 1 76 to 18 02 and two stage hf model ages t dm2 concentrating at 2 7 3 0 ga indicating that source rocks of these zircons were derived from recycling of ancient crusts detrital zircons with ages of 2 4 2 9 ga may have been sourced from the archean granitoid mainly ttg gneisses in the jiaobei terrane and these zircons have hf t values from 6 4 to 7 53 and t dm2 of 2 6 3 4 ga peak at 2 7 3 0 ga suggesting that source rocks of these zircons were mainly derived from juvenile crusts associated with a major crustal growth at 2 7 3 0 ga and a limited amount of ancient crustal component the two detrital zircons with ages of 3631 and 3679 ma have negative hf t values of 4 42 and 1 51 and t dm2 of 4137 and 4000 ma respectively implying the recycling of ancient continental crust 3 6 ga and crustal growth prior to 4 1 ga in the ncc

lithotectonic elements and geological events in the hengshan wutai fuping belt a synthesis and implications for the evolution of the trans north china orogen the hengshan wutai fuping belt is located in the middle segment of the trans north china orogen a palaeoproterozoic continental collisional belt along which the eastern and western blocks amalgamated to form the north china craton the belt consists of the medium to high grade hengshan and fuping gneiss complexes and the intervening low to medium grade wutai granite greenstone terrane and most igneous rocks in the belt are calc alkaline and have affinities to magmatic arcs previous tectonic models assumed that the hengshan and fuping gneiss assemblages were an older basement to the wutai supracrustal rocks but recent studies indicate that the three complexes constitute a single long lived neoarchaean to palaeoproterozoic magmatic arc where the wutai complex represents an upper crustal domain whereas the hengshan and fuping gneisses represent the lower crustal components forming the root of the arc the earliest arc related magmatism in the belt occurred at 2560 2520 ma marked by the emplacement of the wutai granitoids which was followed by arc volcanism at 2530 2515 ma forming the wutai greenstones extension driven by widespread arc volcanism led to the development of a back arc basin or a marginal sea which divided the belt into the hengshan wutai island arc japan type and the fuping relict arc at 2520 2480 ma subduction beneath the hengshan wutai island arc caused partial melting of the lower crust to form the hengshan tonalitic trondhjemitic granodioritic ttg suites whereas eastward directed subduction of the marginal sea led to the reactivation of the fuping relict arc where the fuping tonalitic trondhjemitic granodioritic suite was emplaced in the period 2360 2000 ma sporadic phases of isolated granitoid magmatism occurred in the hengshan wutai fuping region forming 2360 ma c 2250 ma and 2000 2100 ma granitoids in the hengshan complex the c 2100 ma wangjiahui and dawaliang granites in the wutai complex and the 2100 2000 ma nanying granitoids in the fuping complex atc 1920 ma the hengshan wutai island arc underwent an extensional event possibly due to the subduction of an oceanic ridge leading to the emplacement of pre tectonic gabbroic dykes that were subsequently metamorphosed together with their host rocks to form medium to high pressure granulites at 1880 1820 ma the hengshan wutai fuping arc system was juxtaposed intensely deformed and metamorphosed during a major and regionally extensive orogenic event the l uliang orogeny which generated the trans north china orogen through collision of the eastern and western blocks the hengshan wutai fuping belt was finally stabilized after emplacement of a mafic dyke swarm at 1780 1750 ma

geochemical ar 40 ar 39 geochronological and sr nd isotopic constraints on the origin of paleoproterozoic mafic dikes from the southern taihang mountains and implications for the ca 1800 ma event of the north china craton abstract the wide occurrence of paleoproterozoic mafic dikes in the basement of the trans north china orogen provides important constraints on the major 1800 ma tectonothermal event of the north china craton these mafic dikes form a tholeiitic series dominated by basalts and basaltic andesites and can be geochemically classified into three groups the groups 1 and 2 rocks are characterized by high total feo contents 12 69 15 61 of which the group 1 rocks are more enriched in lile and lree contents and depleted in hfse th nb n 2 5 3 8 la yb cn 3 0 5 8 nb la n 0 27 0 38 and have more radiogenic nd isotopic ratios nd t 3 40 to 5 14 than the group 2 rocks th nb n 0 8 1 2 la yb cn 1 5 2 1 nb la n 0 65 0 87 nd t 0 60 to 1 67 in contrast the group 3 rocks are typified by low feot 7 86 11 04 and high mgo 5 62 9 56 contents higher la yb cn 4 22 7 13 ratios less radiogenic nd isotopic ratios nd t 2 75 to 5 52 and more significant th u and nb ta depletion are also apparent features 40ar 39ar geochronology of three representative samples from each group yielded plateau ages of 1780 7 0 5 ma 1765 3 1 1 ma and 1774 7 0 7 ma respectively these elemental and isotopic data suggest that the geochemical variations of these rocks cannot be simply explained by crystallization from a common parental magma involving crustal contamination during emplacement instead they most likely originated from variable sources under different degrees of partial melting the group 1 rocks were derived from relatively low degrees of partial melting of a refractory lithospheric mantle previously metasomatised by subduction related fluids whereas the group 2 rocks originated from a hybridized source involving ca 40 subduction modified lithospheric mantle similar to the group 1 and ca 60 n morb component in contrast the group 3 rocks show significant th u depletion that may mark the involvement of portions of gabbroic lower crust trapped in the source region during subduction based on all the available data we propose a tectonic model of a full subduction syncollision postcollision rifting cycle for the evolution of the ncc between 1870 and 1765 ma a variety of source regions beneath the southern taihang mountains initially developed during the period of subduction collision and subsequent extension rifting resulted in the melting of the subduction modified lithospheric mantle which produced widespread paleoproterozoic mafic dikes on the trans north china orogen in response to upwelling of convective mantle

contrasted tectonic styles for the paleoproterozoic evolution of the north china craton evidence for a 2 1 ga thermal and tectonic event in the fuping massif structural analysis along with 40ar 39ar and u pb datings in the fuping massif provide new insight into the evolution of the eastern part of the trans north china belt north china craton from 2 7 ga to 1 8 ga d1 is responsible for the development of a dome and basin structure coeval with crustal melting giving rise to migmatite and nanying gneissic granites at 2 1 ga this dome and basin architecture resulted from the interference between a n s compression of a weak ductile crust and gravity driven vertical flow in a high thermal regime the next events involved flat lying ductile thrusting d2 and normal faulting d3 dated at around 1880 ma and 1830 ma respectively the d2 and d3 events belong to the trans north china orogeny that results in the final amalgamation of the north china craton the d1 deformation is considered as evidence for an earlier orogen developed around 2 1 ga prior to the trans north china orogeny the change in the deformation style between the 2 1 ga and 1 8 ga could be viewed as a consequence of the cooling of the continental crust in the north china craton

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

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

pressure temperature time paths and a tectonic model for the evolution of granulites petrologic studies and application of well calibrated mineralogic thermometers and barometers reveal several important features common to many granulite terrains 1 peak metamorphic pressure and temperature conditions cluster around values of 7 5 pm 1 kbar and 800 pm 50 c implying average geothermal gradients of 30 35 c km 2 in comparison with upper amphibolite facies rocks especially in paired amphibolite granulite terrains it is evident that granulites are distinguished from amphibolites by the former s higher metamorphic temperatures but not higher pressure 3 initial cooling of granulite terrains from peak conditions is nearly isobaric for garnet closure temperatures of 600 pm 50 c the dp dt retrograde paths inferred from compositionally zoned garnet rims are 2 8 b c in most granulites 4 primary coarse sillimanite is found in most granulites and retrograde kyanite is known in a few these data suggest that granulites form as a result of anomalous thermal gradients

neoarchean sanukitoids and associated rocks from the tengzhou pingyi intrusive complex north china craton insights into petrogenesis and crust mantle interactions abstract quartz monzodiorites granodiorites and monzogranites constitute the dominant lithological assemblage in the neoarchean tengzhou pingyi intrusive complex tpic in the southwestern part of western shandong province wsp in the eastern part of the north china craton ncc and mafic magmatic enclaves mmes are widely developed only in granodiorites of the tpic la icp ms zircon u pb isotopic dating reveals that the tpic was emplaced at 2 54 2 52 ga the 2 54 ga monzogranites display geochemical affinities with high k granites and they were likely derived from the partial melting of metamorphic greywackes at lower crustal levels the 2 54 2 52 ga quartz monzodiorites and granodiorites show significant geochemical similarities to the archean sanukitoids of the western karelian province of finland with higher mg 43 57 values cr 24 88 ppm sr 547 1154 ppm and ba 662 1961 ppm contents and k2o na2o 0 42 1 00 ratios therefore we define the quartz monzodiorites and granodiorites as neoarchean sanukitoids based on their lithological assemblages and geochemical characteristics these sanukitoids can be subdivided into sanukitoid quartz monzodiorites sqms and porphyritic sanukitoid granodiorites psgs the sqms exhibit higher positive hf t2 values from 1 9 to 3 6 with tdm values of 2 70 to 2 77 ga and are strongly enriched in light rare earth elements lrees and large ion lithophile elements liles implying a mantle source that was enriched by either fluids or melts the psgs have a wide range of hf t2 values from 3 4 to 3 8 suggesting that they were formed by crust mantle magma mixing this interpretation is also supported by the petrological textural and compositional disequilibrium of plagioclase phenocrysts from the psgs moreover the mmes and their host psgs have identical crystallization ages 2 54 ga similar mineral chemistries and zircon lu hf isotopic compositions indicating that the mmes are cumulates from the earlier host psgs magma petrogenetic studies and zircon lu hf isotopic features of these lithological assemblages indicate that the interval of 2 55 2 52 ga in the tpic represents an important period of crustal growth and crust mantle interactions considering the spatial temporal relationships and the petrogenesis of the diverse lithological assemblages we propose that the neoarchean tpic was most likely formed in an active continental margin setting

zircon geochronology and metamorphic evolution of mafic dykes in the hengshan complex of northern china evidence for late palaeoproterozoic extension and subsequent high pressure metamorphism in the north china craton abstract magmatic and metamorphic zircons have been dated from ductilely deformed gabbroic dykes defining a dyke swarm and signifying crustal extension in the northern part of the hengshan complex of the north china craton these dykes now occur as boudins and deformed sheets within migmatitic tonalitic trondhjemitic granodioritic and granitic gneisses and are conspicuous due to relics of high pressure granulite or even former eclogite facies garnet pyroxene bearing assemblages shrimp ages for magmatic zircons from two dykes reflect the time of dyke emplacement at 1915 ma whereas metamorphic zircons dated by both shrimp and evaporation techniques are consistently in the range 1848 1888 ma the youngest granitoid gneiss yet dated in the hengshan has an emplacement age of 1872 17 ma these results complement recent geochronological studies from the neighbouring wutai and fuping complexes to the se of the hengshan showing that a crustal extension event occurred in the late palaeoproterozoic this preceded a major high pressure collision type metamorphic event in the central part of the north china craton that occurred in the palaeoproterozoic and not in the late archaean as previously thought our data support recent suggestions that the north china craton experienced a major craton wide orogenic event in the late palaeoproterozoic after which it became cratonized and acted as a stable block

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

the petrology of two distinct granulite types in the hengshan mts china and tectonic implications abstract the archean to proterozoic hengshan complex north china craton comprises tonalitic and granodioritic gneisses with subordinate mafic lenses pegmatites and granites amphibolite facies assemblages predominate although granulite facies relics are widespread and greenschist facies retrogression occurs in km wide shear zones mafic lenses locally abundant occur as strongly deformed amphibolite hornblende plagioclase boudins or sheets in contrast to previously published models we find two series of mafic rocks with distinctly different granulite facies evolutions in the north of the complex relict high pressure mafic granulites are garnet clinopyroxene bearing rocks with a secondary development of orthopyroxene around both garnet kelyphites and clinopyroxene coronas south of the newly defined central e w trending zhujiafang shear zone numerous mafic boudins and less deformed dykes exhibit a macroscopically visible magmatic texture with coronitic growth of metamorphic garnet full of quartz inclusions between the magmatic plagioclase and pyroxene domains additional orthopyroxene after magmatic augite and sodic rims to magmatic plagioclase clearly indicate medium pressure granulite facies metamorphism these findings suggest tectonic juxtaposition in this area of three different structural levels of the same proterozoic imprinted crust high pressure granulite grade in the northern hengshan medium pressure granulite grade in the southern hengshan and amphibolite to greenschist facies grade in the wutaishan to the se

amphibolite facies metamorphism and geochronology of the paleoproterozoic aketashitage orogenic belt northwestern china abstract garnet amphibolite and semi pelitic gneiss preserving three stages of metamorphic mineral assemblages i e the prograde metamorphic inclusion assemblage m1 preserved in the garnet porphyroblast metamorphic peak assemblage m2 and retrograde symplectic assemblage m3 have been recognized in the aketashitage area northwestern china geothermobarometric computation shows that the m1 m2 and m3 assemblages record the metamorphic p t conditions of 600 700 c 4 2 5 3 kbar 730 790 c 6 3 7 1 kbar and 770 c 5 6 kbar respectively these three metamorphic stages define clockwise p t paths including nearly isothermal decompression itd process indicating typical orogenic metamorphism the peak metamorphic p t conditions reach the transition zone between the high amphibolite facies and granulite facies and belong to intermediate p t facies series the shrimp and la icp ms u pb dating of metamorphic zircon implies that the subduction might occur as early as 1 97 ga and last as late as 1 84 ga combined with literature data it is concluded that the aketashitage area was a paleoproterozoic orogenic belt which is possibly related to the assembly of the basements of the columbia supercontinent

palaeoproterozoic metamorphic evolution and geochronology of the wugang block southeastern terminal of the trans north china orogen abstract amphibolites of the wugang block in the taihua metamorphic complex which is located in the southeastern terminal of the palaeoproterozoic trans north china orogen tnco preserve three generations of metamorphic mineral assemblages i e the prograde m 1 peak m 2 and retrograde m 3 assemblages forming during the palaeoproterozoic tectono metamorphic event these amphibolites record metamorphic p t conditions of 4 8 6 7 kbar 720 750 c m 1 9 0 10 6 kbar 710 780 c m 2 and 7 1 8 5 kbar 700 780 c m 3 which define clockwise p t paths involving isothermal decompression itd the metamorphic peak reached upper amphibolite facies and high resolution in situ sims u pb dating of metamorphic zircons suggests that the peak metamorphism took place at 1 96 1 92 ga in the wugang block the present new data suggest that the wugang block was also involved in the continental collision between the eastern and western blocks of the north china craton ncc in the palaeoproterozoic combining metamorphic and geochronological data concerning the tnco obtained by previous scholars it may be anticipated that the middle and southern sections of the tnco were involved in a long lived continental collision and the collision event can be possibly traced back to as early as 1 96 ga and lasted to as late as 1 80 ga

deformation history of the hengshan complex implications for the tectonic evolution of the trans north china orogen structural analysis indicates that the hengshan complex underwent five distinct episodes of deformation d1 d5 the d1 episode formed small isoclinal folds f1 penetrative axial planar foliations s1 and mineral stretching lineations l1 d1 fabrics were reworked by following d2 deformation associated with d2 was the development of nw verging asymmetric folds and accompanying thrust faults both d1 and d2 resulted from crustal thickening and coherent with prograde and peak metamorphism d2 deformation was followed by transpressional dextral shearing d3 including top to nw oblique slip shearing and nne ssw dextral strike slip shearing ongoing collision led to development of the zhujiafang ductile shear zone d4 a near e w trending high strain belt across the hengshan complex d5 deformation is characterized by f5 open folds and associated normal faults probably related to exhumation of the complex structural patterns of the hengshan complex can place important insights into a recently proposed tectonic model suggesting that an ocean between the eastern and western blocks underwent eastward directed subduction beneath the western margin of the eastern block and closure of this ocean finally led to collision between the two blocks to form the coherent north china craton

recalibration of the gasp geobarometer in light of recent garnet and plagioclase activity models and versions of the garnet biotite geothermometer the garnet al silicate plagioclase gasp geobarometer has been recalibrated using four recent garnet activity models four analogous garnet biotite temperature models and two recent plagioclase activity models a typical sillimanite bearing sample that formed at about 5 25 kbar 575 c shows a possible p range of 0 7 kbar due to t error 1 kbar due to range of garnet activity model 0 9 kbar due to range of plagioclase activity model and 5 4 kbar due to range of experimental end member reversals extended by one sigma calibrations were further constrained with the kyanite sillimanite k s phase boundary such that the best fit of 76 pelitic schist samples from 11 localities provides an individual end member calibration for each of the eight possible combinations of garnet and plagioclase activity models with the appropriate geothermometer samples with low grossular or anorthite component were rejected the end member calibrations are constrained to pass through the the best determined portion of the gasp experimental reversals at 1230 c 26 6 kbar these individual end member calibrations provide self consistent models that tend to compensate for error in the garnet and plagioclase activity expressions the models were also tested on a set of 59 samples from the alps the recommended calibration is the average garnet activity model and average garnet biotite t model of holdaway 2000 the fuhrman and lindsley 1988 plagioclase activity model and h grs 6628521 s grs 258 76 to combine with the remaining phases in the berman database to produce the optimum end member gasp curve these thermodynamic data are for the gasp geobarometer only error is about 0 8 kbar absolute and about 0 6 kbar relative geological error is the largest component of error in many of these samples care should be taken to be sure that analyzed plagioclase and biotite are near analyzed garnet that the peak t portions of garnet and plagioclase are selected that the peak t al silicate is determined and that the t calculated is the most accurate possible these calibrations represent an improvement over previous published gasp calibrations these eight models are available for distribution as three programs t p p t intersection for the dos based personal computer

p t t evolution of orogenic belts and the causes of regional metamorphism barrow 1893 introduced three important ideas that furthered understanding of metamorphic processes i the use of critical index minerals in argillaceous rocks to define metamorphic zones and elucidate spatial features of regional metamorphism ii the concept of progressive metamorphism and iii the concept of magmatic advection of heat as a possible cause of regional metamorphism this article expands upon these themes by reviewing our understanding of the dynamic evolution of orogenic belts as interpreted from the p t t paths of metamorphic rocks and by considering the likely causes of the different kinds of regional metamorphism that we observe within orogenic belts understanding metamorphic rocks allows the distinction of two fundamentally different types of orogenic belt defined by relative timing of maximum t and maximum p orogenic belts characterized by clockwise p t paths achieved maximum p before maximum t the metamorphic peak normally post dated early deformation within the belt and additional heating above the normal conductive flux has been related to the amount of overthickening by contrast orogenic belts characterized by counterclockwise p t paths achieved maximum t before maximum p the metamorphic peak normally pre dated or was synchronous with early deformation within the belt and additional heating above the normal conductive flux has been related to the emplacement of plutons techniques used to constrain portions of p t t paths include the use of mineral inclusion suites in porphyroblasts and reaction textures thermobarometry the use of fluid inclusions thermodynamic approaches such as the gibbs method radiogenic isotope dating fission track studies and numerical modelling we can utilize specific mineral parageneses in suitable rocks to determine individual p t t paths and a set of p t t paths from one orogenic belt allows us to interpret the spatial variation in dynamic evolution of the metamorphism recent advances are reviewed with reference to collision metamorphism high temperature low pressure metamorphism granulite metamorphism and subduction zone metamorphism and some important directions for future work are indicated

polyphase deformation of the fuping complex trans north china orogen structures shrimp u pb zircon ages and tectonic implications abstract the fuping complex is located in the central part of the trans north china orogen tnco a paleoproterozoic collisional belt along which the eastern and western blocks were assembled to form the north china craton three stages of deformation d 1 d 3 have been identified in the fuping complex d 1 formed tight to isoclinal folds f 1 penetrative foliation s 1 and mineral lineation l 1 which resulted from the early nw se orientated thrusting and shearing d 2 progressively overprinted d 1 and was dominated by mostly nw se shortening and top to the se e thrusting under a compressive environment d 1 and d 2 occurred in association with the peak metamorphism m 1 d 3 occurred during the exhumation after the crustal thickening producing regional scale nww see trending open folds f 3 and associated low angle detachment faults and was responsible for the isothermal decompression m 2 metamorphism of the complex following the d 3 deformation the complex was exhumed to shallow crustal levels where the rocks underwent the retrogressive cooling m 3 metamorphism syn tectonic leucocratic dykes are widespread in the fuping complex two leucocratic dykes that did not experience d 1 but underwent d 2 yield shrimp zircon ages of 1843 12 ma and 1844 18 ma which indicate that d 1 must have occurred at some time earlier than 1844 ma whereas d 2 must have occurred at some time later than this age two weakly deformed post d 2 leucocratic dykes yield shrimp zircon ages of 1817 14 ma and 1815 45 ma suggesting that the d 2 deformation occurred in the period 1843 1815 ma the new shrimp data indicate that the collision related deformation of the fuping complex must have occurred in the paleoproterozoic not in the late archean structural and geochronological data presented in this study in association with previous lithological and geochemical data suggest that the fuping complex may have been located in the hinterland of an eastward subduction zone and underwent intensive deformation and metamorphism involved in the final collision between the western and eastern blocks at 1 85 ga