short lived polyphase deformation during crustal thickening and exhumation of a collisional orogen ribeira belt brazil abstract the ribeira belt brazil is a neoproterozoic collisional related feature that was located in a south central position in west gondwana we present quantitative data on finite strain flow vorticity and deformation temperatures for the curitiba terrane a major segment of the southern ribeira belt six deformation phases d 1 d 6 related with crustal thickening and exhumation were recognized d 1 and d 2 related microstructures are preserved exclusively within porphyroblasts in part grown during stages of high pressure 9 12 kbar isobaric heating after crustal thickening d 3 phase was active from peak metamorphism attained in contrasting crustal levels 810 400 c to the early stage of exhumation 500 400 c as indicated by petrological microstructural and quartz c axis fabric evidence kinematic vorticity results indicate that the sl 3 mylonitic fabric resulted from a simple shear dominated deformation related with westward thrusting north verging overturned d 4 folds with e w trending subhorizontal axes derived from a pure shear dominated deformation regional d 5 open folds with subvertical axes and nne ssw trending traces were produced by indentation tectonics d 6 phase comprises retrograde orogen parallel transcurrent shear zones related with scape tectonics geochronological data indicate that d 3 d 6 phases occurred between 584 and 580 ma suggesting a fast exhumation rate of 8 mm year for the deepest rocks from the southern ribeira belt

multiple mafic magmatic and high grade metamorphic events revealed by zircons from meta mafic rocks in the daqingshan wulashan complex of the khondalite belt north china craton abstract meta mafic rocks in the daqingshan wulashan complex in the central segment of the paleoproterozoic khondalite belt of the north china craton are composed predominantly of garnet bearing mafic granulites mafic granulites and amphibolites they occur mainly as irregular lenses and as a set of deformed dyke sill swarms within dioritic granitic orthogneisses and al rich paragneisses an integrated study involving laser raman and scanning electron microscope analyses of mineral inclusions from zircons cathodoluminescence imaging and la icp ms u pb dating of zircons shows that magmatic zircons in these meta mafic rocks record four age groups of 2500 2450 2300 2100 1970 1930 and ca 1850 ma indicating multiple mafic magmatic events occurred within the daqingshan wulashan complex in addition four groups of metamorphic zircons with ages of ca 2450 ca 1950 ca 1900 and ca 1850 ma are present within these meta mafic rocks zircons of the first oldest group contain medium to low pressure mlp granulite facies mineral inclusions of orthopyroxene clinopyoxene amphibole plagioclase quartz and yield ages of ca 2450 ma representing the timing of the early mlp granulite facies metamorphic event within the daqingshan wulashan reworked basement complex in contrast zircons of the second group are free of mineral inclusions and yield ages of ca 1950 ma which is considered to be the timing of the regional high grade metamorphic and structural event probably caused by the collision of the yinshan and ordos blocks zircons of the third group contain mlp granulite facies mineral inclusions of orthopyroxene clinopyoxene amphibole plagioclase quartz similar to the zircons of the first group but yield younger ages of ca 1900 ma interpreted as the timing of the regional mlp granulite facies metamorphism that most likely occurred during the early stage of post orogenic exhumation of the daqingshan wulashan complex zircons of the fourth group contain mlp amphibolite facies mineral inclusions of clinopyoxene amphibole plagioclase quartz and yield the youngest ages of ca 1850 ma which probably represent the timing of the regional amphibolite facies metamorphism during the late stage of post orogenic exhumation of the daqingshan wulashan complex combined with previous geological data of the khondalite belt it is speculated that the multiple mafic magmatism and contemporaneous high grade metamorphism of the daqingshan wulashan complex from the khondalite belt were most likely generated by an orogenic process involving early subduction collision and late exhumation in an active continental margin arc

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

deformation sequence of baltimore gneiss domes usa assessed from porphyroblast foliation intersection axes abstract the ne sw trending gneiss domes around baltimore maryland usa have been cited as classic examples of mantled gneiss domes formed by diapiric rise of migmatitic gneisses eskola p 1949 the problem of mantled gneiss domes quarterly journal of geological society of london 104 416 461 476 however 3 d analysis of porphyroblast matrix foliation relations and porphyroblast inclusion trail geometries suggests that they are the result of interference between multiple refolding of an early formed nappe a succession of six fia foliation intersection axes sets based upon relative timing of inclusion texture in garnet and staurolite porphyroblasts revealed 6 superposed deformation phases the successions of inclusion trail asymmetries formed around these fias document the geometry of deformation associated with folding and fabric development during discrete episodes of bulk shortening exclusive top to nw shear asymmetries of curvature were recorded by inclusion trails associated with the vertical collapsing event within the oldest fia set ne sw trend this strongly indicates a large ne sw striking nw verging nappe had formed early during this deformation sequence this nappe was later folded into ne sw trending up right folds by coaxial shortening indicated by an almost equal proportion of both inclusion trail asymmetries documented by the second n s trending fia set these folds were then amplified by later deformation as the following fia sets showed an almost equal proportion of both inclusion trail asymmetries

phase equilibria modelling and zircon age dating of pelitic granulites in zhaojiayao from the jining group of the khondalite belt north china craton the jining group occurs as the eastern segment of the khondalite belt north china craton and is dominated by a series of granulite facies rocks involving normal pelitic granulites recording peak temperatures of 850 c and ultrahigh temperature uht pelitic granulites recording peak temperatures of 950 1100 c the p t paths and ages of these two types of granulites are controversial three pelitic granulite samples in the jining group comprising two sillimanite garnet gneiss samples j1208 and j1210 and one spinel garnet gneiss sample j1303 were collected from zhaojiayao where normal pelitic granulites occur for determination of their metamorphic evolution and ages samples j1208 and j1210 are interpreted to record cooling paths from the tmax stages with p t conditions respectively of 870 890 c 7 8 kbar and 840 c 7 5 kbar constrained from the stability fields of the observed mineral assemblages and the isopleths of plagioclase garnet and biotite compositions in pseudosections sample j1303 is interpreted to record pre tmax decompression from the kyanite stability fields to the tmax stage of 950 1020 c 8 9 kbar and a post tmax cooling path revealed mainly from the stability field of the observed mineral assemblage the plagioclase zoning and the biotite composition isopleth in pseudosections the post tmax cooling stage can be divided into suprasolidus and subsolidus stages the suprasolidus cooling may not result in an equilibrium state at the solidus in a rock therefore different minerals may record different p t conditions along the cooling path the inferred maximum temperature is commonly higher than the solidus as well as different solidi being recorded for different samples from the same outcrop but experiencing different degrees of melt loss plagioclase compositions especially its zoning in plagioclase rich granulites are predicted to be useful for recording the higher temperature conditions of a granulite s thermal history the three samples studied seem to record the temperature range covering those of the normal and uht pelitic granulites in the jining group suggesting that uht conditions may be reached in normal granulites without diagnostic uht indicators la icp ms zircon u pb data provide a continuous trend of concordant 207pb 206pb ages from 1 89 to 1 79 ga for sample j1210 and from 1 94 to 1 80 ga for sample j1303 these continuous and long age spectrums are interpreted to represent a slow cooling and exhumation process corresponding to the post tmax cooling p t paths recorded by the pelitic granulites which may have followed the exhumation of deeply buried rocks in a thickened crust region resulted from a collision event at c 1 95 ga as suggested by the pre tmax decompression p t path

high altitude energy source s for stable auroral red arcs stable auroral red sar arcs have been viewed across the midlatitude night sky with interest since their discovery in 1956 this relatively late discovery compared to the poleward aurora is a direct consequence of the subvisual levels of the delicate and diffuse light that makes up the sar arc except in rare instances when the emissions actually crossover the threshold to visible levels optical instruments are required to register their presence and document their morphology and occurrence frequency sar arcs are seen as relatively featureless slowly changing bands of red light that can extend across the entire night sky early observations from the ground and from satellites established the relationship between sar arcs and magnetic disturbances in near earth space involving the ring current a population of high energy ions trapped in the earth s magnetic field the long lived soft red glow of sar arcs reflects the slow energy loss from the ring current ions as they bounce back and forth in the earth s confining field geometry but the exact sequence of physical processes that feed a portion of the ring current energy to the sar arc region is a matter of continuing debate the midlatitude location of the sar arc bands mirrors the high altitude location of the outer portion of the ring current torus but exactly how this positioning is related to gradients or enhancements in the high altitude extension of the cold ionospheric plasma remained an open question our knowledge of sar arcs and associated signatures now spans more than two full solar cycles detailed observations of the high altitude space environment overlying the sar arc region have been made indicating that heavy ions from the lower atmosphere energized by the solar wind interaction with the magnetosphere play an important role in the sar arc process sophisticated models of the coupled high and low altitude regions involved in sar arc formation paint an interesting picture of the flow of energy between the atmosphere and near earth space this energy flow culminates in the sar arc s glowing red bands which act in turn as sensitive markers of the interaction regions and processes this emerging view of the sar arc process and the new observational information that provided the underpinnings of its development from a rich framework of past theoretical and observational work is the subject of the present review

structural pattern of the wutai complex and its constraints on the tectonic framework of the trans north china orogen abstract wutai complex is a key to test controversial models for the tectonic evolution of the trans north china orogen tnco along which the eastern and western blocks amalgamated to form the north china craton three stages of deformation d 1 to d 3 were identified in the wutai complex d 1 is characterized by ne sw trending foliations s 1 and nw se orientated lineations l 1 indicating an initial nw se directed compression associated with d 1 was prograde metamorphism m 1 d 2 progressively overprinted d 1 and generated z shaped folds and nee sww trending ductile shear zones d 2 resulted in a fan shaped structural pattern of the complex with top to the nw thrusting in the northwestern part and opposite top to the se thrusting in the southeastern part during d 2 the crust was doubled through thickening leading to peak metamorphism m 2 d 3 developed during post collision exhumation and deformed the early fabrics to be open folds resulting in near isothermal decompression m 3 and late cooling metamorphism m 4 the overall structure of the wutai complex is characterized by a fan shaped pattern with structural divergence structural features of the wutai complex combined with those of the adjacent hengshan and fuping complexes indicate that the tnco is characterized by a fan shaped orogenic wedge with opposite senses of thrust this orogenic wedge could have resulted from an eastward subduction and collision between the western and eastern blocks to finally form the tnco at 1 85 ga

u pb geochronological constraints on the evolution of the aspy terrane cape breton island implications for relationships between aspy and bras d or terranes and ganderia in the canadian appalachians new u pb zircon ages from nine samples of igneous and sedimentary rocks in the aspy terrane cape breton island show that neoproterozoic rocks form a major part of the terrane and confirm that the terrane was affected by a major silurian devonian tectonothermal event a rhyolitic crystal tuff a leucotonalite pluton and a felsic sheet yield ages of 618 8 0 6 ma 619 7 0 9 ma and 573 5 2 7 ma respectively a metasedimentary unit contains zircon ranging in age from ca 546 to ca 1520 ma a diorite has an age of 428 6 1 9 ma and another diorite unit and a quartz porphyry also have probable late silurian ages a third diorite yields an age of 373 0 0 5 ma part of a widespread bimodal igneous event represented by both volcanic and plutonic rocks throughout the aspy region geochronological results and field relationships indicate that the neoproterozoic rocks are similar to those in the bras d9or terrane and form the basement to the ordovician silurian rocks in the aspy terrane and that the aspy terrane is probably correlative with rocks in the hermitage flexure of southern newfoundland where silurian metasedimentary and metavolcanic rocks lie unconformably on similar neoproterozoic rocks likely correlative rocks also occur in the kingston terrane of southern new brunswick the results of this study support the idea that the aspy kingston terrane is part of ganderia and that neoproterozoic rocks in the bras d9or terrane and correlative rocks in southwestern newfoundland and elsewhere represent exposed basement to ganderia in the canadian appalachians

fold interference pattern at the top of basement domes and apparent vertical extrusion of hp rocks ambin and south vanoise massifs western alps the ambin and south vanoise brianconnais basement domes located within the hp metamorphic zone were chosen for a reappraisal of the early tectonic evolution of the internal alps in their western segment finite strain analyses have shown that hp rocks were exhumed in two main successive stages more or less expressed according to their structural position in the massifs a partitioning of deformation from core to rim of the domes is thus evidenced the d1 stage was roughly coeval with the observed peak metamorphic conditions and corresponds in the core of the ambin dome to a non coaxial regime with dominant horizontal shortening and n to nw movement direction the d2 stage well expressed in the upper part of the domes took place under low blueschist facies conditions and culminated under greenschist facies conditions it developed a retrogressive foliation and pervasive e verging shear zones at all scales that locally define major 2 tectonic contacts the d2 shearing is responsible for the development of spectacular f1 f2 fold interference structures recognised in the western part of the south vanoise dome in this area we interpret the steeply dipping hp fabric s1 defining the axial planes of the first generation of now upright folds f1 as linked to an early flat lying nappe edifice 1 subsequently steepened by the d2 shearing geometry and kinematic characteristics for the early d1 nappe forming event are strongly consistent with most evolutionary models assuming that oceanic to european and apulian crust derived nappes were initially exhumed during n to nw verging overthrusting events synthetic to the subduction they tend also to suggest that the penninic hp units may have suffered an early exhumation by low angle thrusting rather than by vertical extrusion

halaqin volcano sedimentary succession in the central northern margin of the north china craton products of late paleoproterozoic ridge subduction abstract the halaqin volcano sedimentary succession hvss is tectonically situated on top of the largely late archean to early paleoproterozoic yinshan terrane in the central northern margin of the north china craton it comprises greenschist to amphibolite facies schists quartzites marbles and amphibolites which were originally pelites pebbly sandstones sandstones limestones and volcanic rocks the presence of tight to isoclinal folds and thrusts of the hvss indicates tectonic transport from southeast to northwest present coordinates u pb ages of detrital zircon grains in the clastic rocks suggest provenance from both the fengzhen belt in the south and the yinshan terrane in the north zircon u pb ages of the hvss volcanic rocks e g 1909 10 ma 1894 6 ma 1887 6 ma and 1884 8 ma indicate volcanism in the period 1910 1880 ma detrital zircon age study of the neighbouring majiadian group reveals significant 2700 ma crust growth and 2500 ma metamorphism events in the yinshan terrane the hvss volcanic rocks have a trimodal geochemical distribution demonstrated by sio2 contents of 45 50 wt basalts 53 60 wt andesites and 70 76 wt dacites rhyolites the basalts and andesites are tholeiitic in chemistry and show distinct depletion in high field strength elements hfse e g nb zr ti compared to neighbouring elements in spider diagrams specifically low mg basalts and andesites mgo 5 wt have more or less flat ree patterns la ybn 0 45 1 76 and lile ndt t 1890 ma values of the basalts and andesites vary from 4 8 to 0 4 the rhyolites and dacites have strongly enriched light trace element patterns la ybn 8 49 47 9 and lile but depleted hfse e g nb zr ti the basalts and basaltic andesites most likely originated from the asthenosphere contaminated by melts from the lithosphere whereas the dacites and rhyolites likely originated from the crust we propose that the basalts and andesites are extrusive counterparts of the coeval and geochemically similar xuwujia gabbronorites and the dacites and rhyolites are upper crustal equivalents of the contemporaneous chemically comparable liangcheng granitoids which are in the originally underlying fengzhen belt we suggest that the hvss was most likely generated by ridge subduction below active continental margin just prior to the final amalgamation of the north china craton this ridge subduction contributed to the ultra high temperature metamorphism in this area terminal collision tectonics was responsible for the regional metamorphism and high deformation of the halaqin succession when it was transported northwestwards onto the yinshan terrane

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

paleoproterozoic accretionary orogenesis in the north china craton a shrimp zircon study abstract the inner mongolia suture zone imsz and the trans north china orogen tnco incorporate the major paleoproterozoic accretionary orogens in the north china craton ncc with the jiao liao ji belt jljb representing the third one here we investigate the paleoproterozoic tectonics of the imsz and tnco through zircon shrimp geochronology on a representative suite of rocks comprising metasediments and arc magmatic rocks shrimp analysis of zircons with textures indicating extreme recrystallization under ultrahigh temperature uht conditions from the metapelites at heling er in the southern domain of the imsz reveals a single population with a weighted mean 207 pb 206 pb age of 1913 17 ma the zircons in another uht granulite from this locality yield a weighted mean 207 pb 206 pb age of 1910 18 ma these data correlate with the ca 1 92 ga age reported from zircons in sapphirine bearing uht granulites further north and confirm the regional extent of the paleoproterozoic uht metamorphism within the imsz zircons in a charnockite from the southern margin of the khondalite belt fringing the uht granulites in the imsz show two distinct age groups an older population with a magmatic paragenesis and a weighted mean 207 pb 206 pb age of 1932 24 ma and a younger group of metamorphic zircons with an age of 1858 26 ma we also report zircon ages from charnockites in two localities around xing he in the huangtuyao belt belonging to the huai an complex within the westernmost domain of the tnco at the junction with the imsz the charnockite from first locality carries two distinct zircon populations with the older group yielding a weighted mean 207 pb 206 pb age of 2477 2 ma and the younger population showing an age of 1807 38 ma the internal structure as revealed from cl images and the overall high th u values up to 2 42 of the older zircons suggest their magmatic affinity whereas the younger group with extremely low th u 0 02 0 09 is of metamorphic origin zircons from the charnockite in the second locality also define two distinct age clusters with a dominant older magmatic group having a weighted mean 207 pb 206 pb age of 2147 11 ma and a minor younger group with an age of 1958 25 ma the range of ages from 2477 to 2147 ma from magmatic zircons in the charnockites from the eastern periphery of the imsz within the western margin of the tnco in combination with similar ages reported in recent studies from zircons in magmatic complexes within the imsz suggest a prolonged history of subduction related arc magmatism and accretionary tectonics analogous to those in some of the phanerozoic belts such as the central asian orogenic belt and the western pacific subsequent progressive collision and suturing of the continental blocks were accompanied by the exhumation of high pressure hp and uht metamorphic rocks the available data from the imsz and tnco suggest long lived convergent margins associated with the southward subduction of the yinshan block and westward subduction of the eastern block in a double sided subduction realm prior to the final amalgamation of the ncc and its incorporation within the columbia supercontinent in the late paleoproterozoic

la icp ms u pb geochronology of detrital zircons from the jining complex north china craton and its tectonic significance abstract la icp ms u pb zircon dating and cathodoluminescene cl image analysis were carried out to determine the protolith and metamorphic ages of high grade al rich gneisses named as khondalites from the jining complex of the north china craton ncc the analytical results of more than 200 detrital zircon grains from the khondalites show three main age populations 2060 ma 1940 ma and 1890 ma these data indicate that the provenance of the jining khondalites is paleoproterozoic in age but not archean as previously suggested and the sediments were derived from a provenance different from the eastern block and the yinshan terrane of the ncc the nearly concordant youngest age of 1842 16 ma 207pb 206pb age for the detrital zircons is interpreted as the maximum depositional age of the khondalites overgrowth rims of detrital zircons yield an age of 1811 23 ma which we interpret as the metamorphic age the new age data are consistent with the recent three fold tectonic subdivision of the ncc and support that the eastern and western blocks collided at 1 8 ga to form the coherent ncc

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

quartz plastic segregation and ribbon development in high grade striped gneisses quartz microstructures and c axis fabrics formed during development of polycrystalline quartz ribbons in striped gneisses from the high grade alem paraiba shear zone in southeastern brazil are documented cluster analysis of quartz grains in samples exhibiting different degrees of shear strain revealed that formation of ribbons was a mass conservative process where isolated quartz grains became plastically segregated and then coalesced to form polycrystalline ribbons these ribbons are separated by feldspar rich domains devoid of quartz the stage at which individual stretched quartz grains start to contact each other and initiate ribbon development represents a crucial microstructural change from single grain to polycrystalline ribbon deformation mode which is reflected by an abrupt increase in the smoothness of the ribbon boundaries this change is interpreted to represent a strain softening kink in the stress strain time path progressive ribboning is accompanied by strengthening of the c axis fabric z maximum indicative of continued plastic flow by basal glide operation of basal glide at these high temperature conditions 680 700 c is interpreted to be a consequence of relatively dry deformation conditions a model is then proposed for development of straight quartz ribbons in high grade striped gneisses where scattered quartz grains are continuously stretched and segregated by crystal plastic processes the small angle misorientation of the contacting grains enables subsequent coalescence and resulting grain size enlargement pervasive grain boundary migration accounts for the straight grain boundaries and rectangular grain shapes within the ribbons

extensional collapse and uplift in a polymetamorphic granulite terrain in the archaean and palaeoproterozoic of north china abstract the archaean granulite terrain in the datong huai an area north china comprises a basement complex of felsic and mafic granulite overlain by a sedimentary sequence dominated by metapelite and metapsammite both lithological associations are separated by a tectonic contact and structural textural and stratigraphic data indicate that the terrain was affected by two prominent tectono thermal cycles c a and c b associated with two granulite facies events m1 and m2 which probably occurred 2500 ma and 1800 ma ago respectively of these m1 events were dominant and associated with all major structures in the area in the basement gneiss m1 reached peak metamorphic conditions of 12 14 kbar and 800 900 c during a foliation forming event d 2ba which transposed an earlier s 1ba foliation of unknown age and origin in the allochthonous cover sediments m1 peak conditions of about 8 kbar and 800 c were reached after a prograde thickening event d 1co associated with the formation of a bedding parallel foliation while large amounts of s type granites were emplaced the localised d 2ba and d 1co structures are interpreted as spatially distinct responses at different crustal levels to one metamorphic event juxtaposition of basement and allochthonous cover along a large low angle normal shear zone occurred during d 3 this detachment zone accommodated extension after crustal thickening and allowed unloading of the footwall resulting in a rise of the basement rocks through 4 6 kbar as deduced from syn d 3 decompression textures coeval cooling textures in the allochthonous cover sequence indicate that lateral rather than vertical displacements occurred in the hanging wall of the detachment during c b m2 peak conditions of about 4 6 kbar and 650 700 c were reached coeval with the development of discrete left lateral strike slip zones d 4 that transect both basement and allochthonous cover and accommodated some uplift the deduced c a and c b p t paths have equivalents in many precambrian granulites in general decompressional paths like the one recorded in the basement rocks are linked to processes involving crustal thickening followed by extensional collapse in the datong huai an area it can be shown that low angle detachments may have played an important role in the uplift and cooling history of granulite belts as long ago as the archaean proterozoic boundary

paleoproterozoic crustal growth in the western block of the north china craton evidence from detrital zircon hf and whole rock sr nd isotopic compositions of the khondalites from the jining complex detrital zircons from the khondalites in the western block of the north china craton were mainly derived from a 1 9 to 2 1 ga paleoproterozoic provenance most of the more than 200 detrital zircons analyzed in this study have positive initial ehf values and a wide range of hf depleted mantle model ages tdmc that range from 2 08 ga to 2 74 ga with a peak at 2 24 to 2 40 ga the whole rock nd isotopic analyses for the khondalite samples give quite uniform 143nd 144nd ratios from 0 511188 to 0 511858 corresponding to initial end 2 10ga values ranging from 1 6 to 0 6 in contrast sr isotopic compositions of these samples are more varied with initial 87sr 86sr ratios at 2 10 ga ranging from 0 7017 to 0 7076 in the end versus 87sr 86sr correlation diagram most samples plot near the mixing hyperbola of the depeleted mantle and crust with the contribution of depleted mantle about 65 to 75 percent these results suggest that a large amount of juvenile materials from the depleted mantle was accreted to the western block at 2 1 ga a period of magmatic quiescence in many cratons of the world

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

episodic mantle melting crustal reworking in the late neoarchean of the northwestern north china craton zircon ages of magmatic and metamorphic rocks from the yinshan block abstract the yinshan block in the northwestern north china craton comprises late neoarchean greenstones high grade metamorphic rocks granulite charnockite enderbite and amphibole gneiss diorites sanukitoid and mg adakite and granitoids predominantly trondhjemite tonalite a greenstone terrane comprises a volcanic sequence of predominant basalt and dacite with minor andesite zircons from a high mg andesite and an andesite have weighted mean 207pb 206pb ages of 2533 5 ma and 2510 7 ma respectively which reflect the timing of two distinct volcanic episodes these two either predated or postdated a major volcanic episode that formed a basalt dacite bimodal suite during 2515 10 basalt to 2516 10 ma dacite magmatic zircons from the high grade metamorphic rocks yielded weighted mean 207pb 206pb ages of 2544 5 ma for a mafic granulite 2521 6 ma for a garnetiferous gneiss 2535 5 ma for a charnockite and 2526 5 ma for an amphibole gneiss these ages reflect the minimum emplacement time of rocks that make up the protoliths of the high grade metamorphic complex the metamorphic ages of the high grade complex were determined at 2515 18 ma for mafic granulite and 2503 12 ma for garnetiferous gneiss zircons from a high mg diorite i e sanukitoid have a weighted mean age of 2520 6 ma which is significantly younger than the 2556 14 ma age of a magnesian diorite mg adakite from the same diorite massif these ages reflect distinctive magmatic pulses that generated the diorite massif a tonalite has a zircon age of 2516 7 ma indistinguishable from the 2520 9 ma age of a trondhjemite from the same trondhjemite tonalite dominated batholith a granitic compositional layer in the 2516 7 ma tonalite has zircon ages of 2515 6 ma high u zircon domains and 2512 22 ma low u zircon domains which reflect an intracrustal melting event the presence of an older crustal remnant ca 2600 ma prior to the formation of the yinshan block is indicated by a 2594 4 ma leucogranite xenolith in the diorite massif and by the widespread presence of ca 2583 3485 ma mostly ca 2700 ma zircon xenocrysts in a variety of volcanic and plutonic rocks an undeformed pegmatite dike in high mg andesite has an emplacement age of 2508 17 ma zircon an undeformed trondhjemite dike that cuts the supracrustal greenstone sequence yielded a zircon age of 2502 6 ma similar to that of the pegmatite dike and representing the latest archean magmatic event in the yinshan block these ages of undeformed granitoid dikes delimit the termination of thermo tectonic development of the yinshan block based on these new ages and published geological and geochronological data we conclude that the yinshan block is a typical neoarchean cratonic block not a segment of the so called paleoproterozoic inner mongolia north hebei orogen episodic mantle melting and resultant crustal reworking account for the thermo tectonic development of the yinshan block in a brief period ca 2556 2502 ma in the late neoarchean and based on their regional chronological correlations we argue for their craton scale significance

polymetamorphism and isobaric cooling of pelitic schists from the annex sulphide deposit gordonia belt south africa muscovite poor pelitic schists in the wallrocks of the proterozoic annex sulphide deposit near prieska south africa contain peak metamorphic assemblages including crd bt sil st sil bt crd st bt and rarely ky st crd all rocks include oligoclase quartz and commonly fe mn garnet with or without muscovite peak assemblages assigned to m2 regional metamorphism in the gordonia belt namaqua province are syn to post kinematic with respect to the main s2 fabric although larger staurolite grains contain s1 inclusion trails garnet biotite thermometry utilizing corrections for fe3 mn alvi and ti yields peak temperatures of 571 624 c at pressures of 4 5 6 0 kbar consideration of the sympathetic variation of xmn in garnet with xmg in biotite and the preserved zoning patterns in prograde garnets together with the inferred prograde transition from kyanite to sillimanite indicates that heating occurred during mild decompression to the m2 metamorphic peak sillimanite and cordierite grew last in the prograde sequence possibly related to a pulse of thermal metamorphism m3 that is found along the margin of the keimoes suite batholith to the north retrograde assemblages including ms ky chl qtz after crd bt ky ms after sil and chl ms after st indicate a period of isobaric cooling m4a terminated by rehydration in the kyanite stability field at about 500 c the size difference between prograde 1 2 mm and retrograde 0 05 0 1 mm mineral grains indicates substantial undercooling below equilibrium positions of relevant retrograde reactions prior to rehydration and explains why cordierite that grew during m2 is almost completely destroyed post m4a regrowth of staurolite and garnet m4b is spatially linked to sites of m4a rehydration it reached temperatures of 510 530 c remaining within the stability field of kyanite a best fit of the observed textural history to the namaqua orogenic cycle involves collision and heating m2 d2 followed by granite intrusion m3 rifting m4a and renewed heating due to crustal loading during volcanism m4b the p t path for the annex region is consistent with those derived from elsewhere in the gordonia belt and with modification to that published already for the nearby prieska copper mines

h t deformation of garnet an ebsd study on granulites from sri lanka india and the ivrea zone abstract garnets from three regional granulite terrains sri lanka eastern ghats ivrea zone have been analyzed and the results indicate that garnet can be plastically deformed by dominant dislocation glide creep with variable dominant slip systems garnets from a granulite facies fold hinge in a quartzite of the highland complex of sri lanka unequivocally attest to plastic deformation of garnet outside the fold hinge the garnets form flat discs elongated parallel to a pronounced stretching lineation and show deformation microstructures like boudinage and pinch and swell structures in the fold hinge these garnet discs are folded within the quartzitic matrix at temperatures of about 800 c in spite of the preservation of deformed shapes intracrystalline deformation microstructures are scarce the garnets have distinct crystallographic preferred orientations which can be interpreted by a dominant 1 2 111 110 slip system approaching easy slip positions these are reached rapidly under moderate strains as necessary lattice rotations are small similar slip systems were derived for a garnet bearing quartzite from the eastern ghats deformed at temperatures of 750 800 c but with more influence of initial orientation of garnets completely different textures indicative of dominant 100 010 slip were found in a restitic garnet sillimanite granulite from the ivrea zone the variable deformation features and variable slip systems with 1 2 111 and 100 burgers vectors prove that garnets bear significant potential for contributing additional information on the h t deformation of rocks

first application of the revised ti in zircon geothermometer to paleoproterozoic ultrahigh temperature granulites of tuguiwula inner mongolia north china craton the revised titanium in zircon geothermometer was applied to paleoproterozoic ultrahigh temperature uht granulites at tuguiwula inner mongolia north china craton the tuguiwula granulites contain diagnostic uht mineral assemblages such as sapphirine quartz and high alumina orthopyroxene sillimanite quartz suggesting formation under temperatures of ca 1 000 c and pressures of up to 10 kbar here we report detailed petrographic studies and icp ms data on titanium concentration in zircons associated with the uht assemblages the zircons associated with sapphirine spinel sillimanite magnetite assemblages have the highest ti concentration of up to 57 ppm yielding a temperature of 941 c and suggesting that the growth of zircons occurred under ultrahigh temperature conditions the maximum temperatures obtained by the revised ti in zircon geothermometer is lower than the equilibrium temperature of sapphirine quartz indicating an interval of cooling history of the granulites from uht condition to ca 940 c many of the zircons have ti concentrations ranging from 10 to 33 ppm indicating their growth or recrystallization under lower temperatures of ca 745 870 c these zircons are interpreted to have recrystallized during the retrograde stage indicated by microstructures such as cordierite rim or corona between spinel and quartz and orthopyroxene cordierite symplectite around garnet previous geochronological study on the zircons of the tuguiwula uht granulites gave a mean u pb shrimp age of 1 92 ga however based on the ti in zircon geothermometer results reported in this work and considering the relatively slow thermal relaxation of these rocks we infer that the timing of peak uht metamorphism in the tuguiwula area could be slightly older than 1 92 ga

paleoproterozoic p t t evolution in the hengshan wutai fuping area north china craton evidence from petrological and geochronological data abstract this contribution carries out zircon dating and a review of the previous metamorphic and geochronological work on the hengshan wutai fuping area in the central trans north china orogen for elucidating its paleoproterozoic orogenic evolution the hengshan complex can be subdivided into north hengshan and south hengshan complexes separated by the strike slip zhujiafang shear zone high pressure hp mafic granulite and garnet absent amphibolite collected from the north hengshan complex yield metamorphic ages of 1963 1769 ma and 1882 1793 ma respectively both with weighted mean ages of 1 85 ga in the hp mafic granulite zircons of 1 90 ga and those of

lithological units at the boundary zone between the jining and huai an complexes central northern margin of the north china craton a paleoproterozoic tectonic melange abstract in the central northern margin of the north china craton the tectonic boundary between the jining and huai an complexes is well exposed in the gushan area in this boundary zone exotic blocks of orthopyroxene bearing monzonitic gneiss garnet bearing mafic granulite marble and calc silicate were identified in the meta psammitic and meta pelitic matrix the likely nature of the rock units is a tectonic melange which is a key to understand the tectonic evolution of the craton in this paper geochronological and geochemical studies have been carried out on representative lithologies from the gushan area to discuss their petrogenesis and geodynamic settings the opx bearing monzonitic gneisses u pb zircon emplacement age of 2 17 ga are high k calc alkaline to shoshonitic i type rocks with trace element magmatic arc signature the trace elements whole rock sm nd and zircon lu hf isotopes suggest that they were possibly derived from partial melting of neoarchean lower crust garnet amphibolite in a continental arc the protoliths of the garnet bearing mafic granulites are inferred to have formed between 2 37 ga and 1 95 ga possibly at 2 2 ga they show n morb geochemical affinity and were most likely produced by partial melting of spinel lherzolite in a mid ocean ridge setting detrital zircons from one metapsammite sample garnet bearing quartzofeldspathic gneiss yield nearly concordant 207 pb 206 pb ages ranging from 2 1 ga to 2 0 ga in situ u pb and hf isotopic data of zircons from the metapsammite sample support that the metasedimentary protoliths were probably derived from a mid paleoproterozoic continental arc three metamorphic age groups 1 95 1 93 ga 1 86 1 83 ga and 1 80 ga are revealed by the opx bearing monzonitic gneisses garnet bearing mafic granulites garnet bearing quartzofeldspathic gneiss and sillimanite garnet k feldspar gneiss the predominant group 1 95 1 93 ga is considered to record the age of near peak granulite facies metamorphism the 1 86 1 83 ga group may result from decompression or the thermal imprint of the collision between the eastern and western blocks the 1 80 ga group is consistent with the age 1 81 ga of the ductile deformation of the garnet k feldspar mylonite and is interpreted as the timing of a relatively rapid tectonic extension thus the paleoproterozoic gushan melange formed and evolved in a subduction collision exhumation process and finally situated in a likely suture zone between the eastern and western blocks or sub cratons

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

the neoarchean ultramafic mafic complex in the yinshan block north china craton magmatic monitor of development of archean lithospheric mantle abstract neoarchean ultramafic mafic complexes are critical for characterizing the embryonic continental lithosphere and hence for arbitrating the extrapolation of plate tectonics into the archean geology this geochronological and geochemical study documents the newly identified amphibolite to granulite facies xiwulanbulang xwlbl ultramafic mafic complex in the yinshan block north china craton ncc it is composed mainly of meta gabbro hornblendite and serpentinite sims u pb zircon dating on the later granite constrains a minimum formation age of 2581 ma for the complex whereas the metamorphic zircons from the hornblendite and coexisting kyanite garnet gneiss yield a metamorphic age of 2 50 ga the dominant meta gabbros in the complex show an morb composition and characterized by an lree depleted to flat ree pattern yet possessing prominent nb zr and ti depletions which suggest dual geochemical features combined with their depleted nd t isotope compositions 1 77 to 2 23 and regional high temperature high mg basaltic magmatism we suggest that they may have been produced by interaction of the upwelling asthenosphere with the metasomatized lithospheric mantle minor hornblendites pyroxenites that occurred in the complex display high mgo 13 6 20 2 wt cao 7 5 13 8 wt cr 475 2049 ppm and ni 154 859 ppm contents reflecting they have originated either as a crystal mush magma or high degree partial melts of a mantle source they show moderately fractionated ree patterns la yb n 2 9 10 7 but depleted nd t 1 48 to 3 53 and zircon hf t up to 7 3 values considering their negative anomalies for nb zr and ti relative to neighboring ree we suggest that the mantle source of the hornblendites was refractory depleted peridotite that experienced prior basaltic magma extraction followed by metasomatism of subduction related fluids systematic variations trend in both major and trace elements between the hornblendites and the associated serpentinites support their genetic linkage high cao 0 7 6 9 wt and al2o3 3 9 6 9 wt contents but low mg 79 85 imply that the serpentinites were likely the cumulates segregated from the coexisting hornblendite pyroxenite magma instead of mantle peridotite taking into full account the regional tectono thermal events we conclude that the xwlbl ultramafic mafic complex was produced in a neoarchean subduction related zone which was more likely trigged by the upwelling asthenosphere mantle as a result of rollback of subducted oceanic lithosphere

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

deformation history of the hengshan wutai fuping complexes implications for the evolution of the trans north china orogen abstract the trans north china orogen separates the north china craton into two small continental blocks the eastern and western blocks as one of the largest exposure in the central part of the orogen the hengshan wutai fuping complexes consist of four lithotectonic units the wutai hengshan and fuping complexes and the hutuo group the hengshan complex contains high pressure mafic granulites and retrograded eclogites structural analysis indicates that most of the rocks in these complexes underwent three distinct episodes of folding d 1 to d 3 and two stages of ductile thrust shearing stz 1 between d 1 and d 2 and stz 2 after d 3 the d 1 deformation formed penetrative axial planar foliations s 1 mineral stretching lineations l 1 and rarely preserved small isoclinal folds f 1 in the hengshan and fuping complexes in the wutai complex however large scale f 1 recumbent folds with sw vergence are displayed by sedimentary compositional layers penetrative transposition resulted in stacking of thrust sheets which are separated by ductile shear zones stz 1 the kinematic indicators of stz 1 in the hengshan and wutai complexes show top to the s230 w thrusting likely related to northeastward oblique pre collisional subduction d 1 resulted in crustal thickening with resultant prograde peak metamorphism the hutuo group did not undergo the d 1 deformation either because sedimentation was coeval with the d 1 deformation or because it was at a high structural level and was not influenced directly by the early deformation the d 2 deformation produced nw verging asymmetric and recumbent folds the d 2 deformation is interpreted to have resulted from collision between the eastern and western blocks of the north china craton in the hutuo group and the fuping complex the development of ese verging asymmetric tight folds is associated with d 2 the structural pattern resulting from superimposition of d 1 and d 2 is a composite synform in the hengshan wutai fuping complexes all four lithotectonic units were superposed during the later d 3 deformation the d 3 deformation developed nw trending open upright folds ongoing collision led to development of transpressional ductile shearing stz 2 forming the transpressional zhujiafang dextral ductile shear zone between the northern hengshan complex and the southern hengshan complex and generating the sinistral longquanguan ductile shear zone between the fuping complex and the wutai complex respectively the stz 1 and d 2 deformation were possibly responsible for fast syn collisional exhumation of the high pressure mafic granulites and retrograded eclogites the structural patterns and elucidation of the deformation history of the hengshan wutai fuping complexes places important constraints on the tectonic model suggesting that an oceanic lithosphere between the eastern and western blocks underwent northeastward directed oblique subduction beneath the western margin of the eastern block and that the final closure of this ocean led to collision between the two blocks to form the coherent basement of the north china craton

possible southwestward extrusion of the ordos block in the late paleoproterozoic constraints from kinematic and geochronologic analysis of peripheral ductile shear zones abstract the paleoproterozoic is a pivotal period of the tectonic evolution of the north china craton ncc various models have been postulated by previous researchers through the study of petrology and metamorphism but the lack of structural analyses has hampered further understanding of the tectonic evolution of the ncc a series of well exposed ductile shear zones developed on the northern northwestern and eastern edges of the ordos block in the western part of the ncc we carried out detailed field studies along with isotopic dating focused on the kinematics and geochronology of the ductile shear zones on the north margin of the ordos block the wulashan daqingshan ductile shear zone is characterized by an e w trending shearing foliation that steeply dips 75 80 to the north or south with subhorizontal stretching lineation plunging 10 15 toward the east or west and dextral strike slip shearing kinematics on the west margin of the ordos block is the zongbieli ductile shear zone with top to the nw thrusting kinematics on the east margin of the ordos block is the xueling ductile shear zone with top to the se thrusting accompanied by sinistral strike slip shearing kinematics the 40ar 39ar ages of deformed minerals from mylonitic high grade metamorphic rocks and la icp ms u pb dating results of zircons from syn tectonic anatectic granites reflect that the ductile shear zones surrounding the ordos block deformed between 1 85 and 1 81 ga the similar deformation ages indicate that the ductile shear zones might be genetically correlated the geometry kinematics and geochronology characteristics of the wulashan daqingshan ductile shear zone on the north margin the zongbieli ductile shear zone on the west margin and the xueling ductile shear zone combined with the zhujiafang sinistral ductile shear zone on the east margin defined the possible southwestward extrusion of the ordos block in the late paleoproterozoic which might have resulted from the westward subduction collision of the eastern block under the western block at approximately 1 85 ga and which is correlated with the amalgamation of the columbia supercontinent

promontory promontory collision in the canadian appalachians in cape breton island and southwestern newfoundland the appalachian orogen is extremely narrow the rocks were strongly deformed and underwent high grade barrovian type metamorphism in silurian time and the orogen parallel silurian thrusts are dominantly west vergent antithetic to the westward subduction this area is bounded by two transverse dextral wrench faults the canso fault to the southwest and the gunflap hills fault to the northeast these tectono stratigraphic structural and metamorphic features are explained by silurian collision between the st lawrence promontory on the laurentian margin and the cabot promontory on the avalon margin during the final closure of the iapetus ocean the west vergent antithetic thrusting is interpreted to be related to tectonic wedging while the two transverse wrench faults accommodated differential movement from promontories to reentrants during the collision promontory promontory collision should be a common process and probably played a significant role in shaping the tectonic framework of many orogenic belts

episodic paleoproterozoic 2 45 1 95 and 1 85 ga mafic magmatism and associated high temperature metamorphism in the daqingshan area north china craton shrimp zircon u pb dating and whole rock geochemistry abstract in the daqingshan area within the khondalite belt of the north china craton paleoproterozoic gabbro and dolerite intrusions show amphibolite to granulite facies metamorphism u pb zircon dating and whole rock geochemistry were undertaken on these rocks in order to understand the timing of mantle magmatism and thermal processes ten samples yielded zircon u pb formation ages of 2 46 2 44 ga 1 97 1 92 ga and 1 84 ga and metamorphic ages of 1 95 1 83 ga they are variable in geochemical composition with some being enriched in light rare earth and large ion lithosphere elements but depleted in nb ta combined with previous results it is concluded that mafic magmatism at 2 45 2 37 ga and 1 97 1 92 ga indicates two paleoproterozoic extensional phases with the latter accompanied by ht uht metamorphism as a result of underplating of mantle magma

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

the precambrian khondalite belt in the daqingshan area north china craton evidence for multiple metamorphic events in the palaeoproterozoic era high grade pelitic metasedimentary rocks khondalites are widely distributed in the northwestern part of the north china craton and were named the khondalite belt prior to the application of zircon geochronology a stratigraphic division of the supracrustal rocks into several groups was established using interpretative field geology we report here shrimp u pb zircon ages and hf isotope data on metamorphosed sedimentary and magmatic rocks at daqingshan a typical area of the khondalite belt the main conclusions are as follows 1 the early precambrian supracrustal rocks belong to three sequences a 2 56 2 51 ga supracrustal unit the previous sanggan group a 2 51 2 45 ga supracrustal unit a portion of the previous upper wulashan group and a 2 0 1 95 ga supracrustal unit including the previous lower wulashan group a portion of original upper wulashan group and the original meidaizhao group the units thus do not represent a true stratigraphy 2 strong tectono thermal events occurred during the late neoarchaean to late palaeoproterozoic with four episodes recognized 2 6 2 5 2 45 2 37 2 3 2 0 and 1 95 1 85 ga with the latest event being consistent with the assembly of the palaeoproterozoic supercontinent columbia 3 during the late neoarchaean to late palaeoproterozoic 2 55 2 5 2 37 and 2 06 ga juvenile mantle derived material was added to the crust

p t t evolution of pelitic gneiss from the basement underlying the northwestern ordos basin north china craton and the tectonic implications abstract using petrography phase equilibria modeling and in situ u th pb monazite geochronology we show that pelitic gneiss from close to the bottom of the qitan1 borehole in the northwest of the ordos basin records a four stage metamorphic evolution the m1 m3 stages which were suprasolidus are represented by rutile relicts in the matrix and biotite k feldspar plagioclase sillimanite quartz inclusions in the cores of garnet porphyroblasts m1 inclusion free mantles on garnet with ilmenite partially replacing rutile and biotite k feldspar plagioclase sillimanite quartz in the matrix m2 and the appearance of cordierite in the assemblage biotite k feldspar plagioclase sillimanite ilmenite quartz m3 the final subsolidus mineral assemblage is garnet biotite k feldspar plagioclase sillimanite cordierite ilmenite quartz m4 the p t conditions for the mineral assemblage evolution m1 m4 are constrained by a p t pseudosection constructed in the na 2 o cao k 2 o feo mgo al 2 o 3 sio 2 h 2 o tio 2 o 2 chemical system the p t t path is clockwise involving slight heating during near isothermal decompression from 8 8 kbar at 769 c to 5 5 kbar at 785 c followed by close to isobaric cooling across the solidus two cores of monazite and three unzoned grains that have similar populations of dates yield robust ages of 1 96 1 94 ga interpreted to date the timing of late prograde to peak metamorphism m1 m2 in addition one rim and two unzoned monazite grains with similar populations of dates yield robust ages of 1 90 1 88 ga interpreted to date the timing of post decompression slow cooling across the solidus m3 m4 on the basis of these results the pelitic gneiss is interpreted to have formed by crustal thickening m1 during collision between the yinshan and ordos terranes the latter with the khondalite belt protoliths along its northern margin present co ordinates this collision culminated at ca 1 95 ga and was followed by decompression m2 m3 and slow cooling m4 to ca 1 89 ga during post collisional extension the similarity between the p t t path determined in this study and those from hp mp granulites in the khondalite belt confirms that they experienced a common tectono metamorphic history the p t t path records an evolution from an apparent thermal gradient of 75 c kbar at peak pressure to 150 c kbar after decompression together with the occurrence of mid paleoproterozoic magmatism in the basement of the ordos basin these features indicate that the ordos terrane was the upper plate at a convergent margin with the khondalite belt representing the forearc sediments along its northern edge collision with the yinshan terrane part of the subducting plate led to shallow slab breakoff and the development of a two sided hot collision and formation of a wide plateau like hot orogen similar to those in the numerical models of sizova et al 2014

is the ordos block archean or paleoproterozoic in age implications for the precambrian evolution of the north china craton the ordos block is a large but poorly exposed crustal segment in the western part of the north china craton little is known about its precambrian basement rocks and their evolution because of an extensive cover by younger sedimentary rocks we report shrimp zircon ages and hf in zircon isotopic compositions for five samples recovered from drill holes that penetrated into the ordos basement based on these data two age groups of rocks can be distinguished an early paleoproterozoic group that is represented by one sample from the northern margin of the ordos block and a late or middle late paleoproterozoic group consisting of the remaining four samples the early paleoproterozoic sample contains zircon cores and metamorphic rims with ages of 2 4 ga and 2 28 ga respectively the cores have ehf t values of 8 8 to 6 2 and hf crustal model ages of 2606 to 3221 ma the remaining rocks of late or middle late paleoproterozoic age contain zircons that may also show a core rim structure but with the cores having ages of 2 08 ga and the metamorphic rims recording ages of 1 9 ga the zircon cores have ehf t values of 9 1 to 10 1 and hf crustal model ages of 2039 to 3068 ma combined with data from earlier studies we draw the conclusion that late paleoproterozoic metasedimentary rocks are widespread in the ordos basement there is no evidence from our samples of an archean component and this calls into question the prevailing view that the ordos basement is an archean cratonic block the fact that the ordos basement was involved in a widespread late paleoproterozoic tectono thermal event indicates that most current models for the tectonic evolution of the western portion of the north china craton need to be revised

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

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

intra continental transpression and gneiss doming in an obliquely convergent regime in se asia abstract the tengchong terrane comprises a sequence of linear dome like zones cored by granite and migmatitic layers these cores are mantled by predominantly gneiss and subordinate schist sequences decreasing in deformation intensity outward from extensive mylonitization to weak mylonitization the pre doming deformation was characterized by the formation of large scale top to the east shearing d1 in the gneiss terrane locally preserved flat lying foliation s1 weak folding f1 and emplacement of the mangbang granite during the cretaceous 114 104 ma the second stage of deformation d2 consisted of map scale east verging folds f2 dome amplification and minor lateral strike slip shear zones between the anticlines in the gneiss and migmatitic sequences extensive partial melting and emplacement of 67 30 ma synkinematic granitoid bodies veins occurred leading to the emplacement of wedges of granite into the easterly directed f2 fold cores these wedges formed kilometer scale granitoid domes the post doming d3 deformation with transpression recorded strain partitioning with simple shear dominated high strain zones along the gaoligong and nabang dextral lateral strike slip shear zones active during 30 11 ma late transtensional deformation d4 during cooling of the entire terrane involved the localized low temperature gaoligong west and east detachment faults that controlled the late exhumation of the gaoligong metamorphic zone since 10 ma our structural observations combined with previous studies suggest that this style of doming is a representative type of intra continental deformation in the cenozoic during the oblique india asia collision the actual dome shapes reflect formation of antiforms during compression dominated transpression prior to localized strike slip shearing in the accommodation belt around the eastern himalayan syntaxis vertical exhumation of crustal material by contractional doming played an important role in absorbing the vast majority of the internal deformation of crustal fragments during oblique collision

p t t paths and tectonic history of an early precambrian granulite facies terrane jining district south east inner mongolia china abstract the widespread khondalite series of south east inner mongolia consists largely of biotite sillimanite garnet gneiss and quartzo feldspathic gneiss with some marble and mafic granulite layers it has experienced two metamorphic events at c 2500 and 1900 2000 ma a pre peak stage of the first metamorphism at t 600 700 c and p 6 7 kbar is recognized by the relict amphibolite facies assemblage ky grt bt pl qtz and protected inclusions of biotite hornblende sodic plagioclase and quartz in garnet or orthopyroxene the peak stage with t c 800 50 c and p 8 10 kbar is characterized by the widespread granulite facies assemblages sil grt bt kfs pl qtz in gneiss and opx cpx pl hbl grt in granulite the p t t path suggests that the supracrustal sequence was buried in the lower crust by tectonic thickening during d1 d2 the beginning of the second metamorphism is characterized by further temperature rise to 700 c or more at lower pressure this stage is manifested by the appearance of cordierite after garnet fibrolite sil2 after biotite in gneiss and transformation of hbl1 into opx2 and cpx2 in granulite coronas of symplectitic opx2 pl2 surrounding grt1 and cpx1 in mafic granulite are interpreted as products of near isothermal decompression the p t t path may be related tectonically to waning extension of the crust by the end of the early proterozoic

structural evolution of the thor odin gneiss dome abstract the thor odin dome is a large structural culmination along the eastern margin of the shuswap terrain previous workers have presented divergent models for the origin of the dome some have proposed that it is a product of diapiric uprise of material from the lower crust alternatively the dome is interpreted to have formed from the interference of later folding this study presents the results of a detailed analysis of new data on the structure pattern of strain variation and metamorphic fabric of the dome these data together with new geochronological and metamorphic phase assemblage information provide the basis for a revised interpretation of the evolution of the thor odin gneiss dome the pre doming evolution of the thor odin gneiss dome was characterized by the formation of large scale nappe structures and the imbrication of archean basement rocks with the cambrian and younger cover rock sequence the first period of deformation phase one consisted of large scale infolding of the cover rock sequence into the basement rocks the pingston fold in the core of the dome is a product of this event the second period of deformation phase two was marked by the forcing of wedges of basement into the cores of northerly directed nappes these structures had no observable effect on the central core zone which is here termed the autochthonous core gneiss domain the third period of deformation phase three was co axial with phase two and consisted of imbrication and refolding of the upper levels of the stack of phase two nappes the actual domal shape was formed by fold interference of phases four five and six which were upright chevron style flexural slip folds with northwest southwest and northerly trending axial planes the doming events occurred at upper crustal levels probably during the tertiary analysis of the strain pattern suggests that little of the total deformational strain is related to these dome forming deformations this outline of the events that formed the dome is consistent with the new geochronological results and interpretation of metamorphic phase assemblages cordierite gedrite sillimanite assemblages previously interpreted as indicating 1000 mpa pressure probably formed under reduced water fugacities and 500 700 mpa total pressure the thor odin gneiss dome thus provides no evidence for lower crustal upwelling related either to dome formation or to the formation of the rocky mountain thrust belt

deformation history of the qianlishan complex khondalite belt north china structures ages and tectonic implications abstract the qianlishan complex is a multiply deformed granulite terrane of the khondalite belt that represents a paleoproterozoic continent continent collisional orogen in the western block of the north china craton two major stages of deformation d1 and d2 were identified in the qianlishan complex d1 mainly produced nww trending overturned to recumbent intrafolial isoclinal folds f1 sub horizontal penetrative foliations s1 nne ssw mineral lineations l1 associated with top to the ssw thrust shear reflecting crustal thickening due to the nne ssw compression d1 was developed with the prograde m1 and peak m2 granulite facies metamorphism d2 successively superposed the d1 structures and was characterized by nww see trending doubly plunging open to tight upright folds f2 and minor sub vertical axial planar foliations cleavages s2 d2 was accompanied by post peak near isothermal decompression m3 probably related to exhumation following the crustal thickening sims u pb zircon ages of deformation related leucocratic dykes demonstrate that d1 broadly occurred at 1976 1936 ma and d2 subsequently happened at 1936 1854 ma these results indicate that the khondalite belt underwent long term 100 myr polyphase deformation and high grade metamorphism associated with magmatism timing constraints and kinematic compatibility suggest that the deformation sequence resulted from the nne ssw collision between the yinshan and ordos blocks to form the western block at 1 95 ga

carbonic metamorphism at ultrahigh temperatures evidence from north china craton abstract the north china craton ncc constitutes one of the fundamental precambrian nuclei of asia the paleoproterozoic khondalite belt along the northern margin of the ncc shows evidence for extreme crustal metamorphism at temperatures exceeding 1000 c and pressures of 10 12 kbar with the formation of diagnostic ultrahigh temperature uht mineral assemblages including sapphirine quartz high alumina orthopyroxene sillimanite and high temperature mesoperthites here we record for the first time the common occurrence of co2 rich fluid inclusions in peak uht minerals in the mg al granulites of ncc and trace the fluid evolution history from microthermometric and laser raman spectroscopic studies we show that the syn metamorphic fluid in the uht rocks was pure co2 while very high density carbonic fluids are preserved in some cases corresponding to entrapment at peak conditions many of the inclusions underwent density modification during the rapid and near isothermal exhumation of the rocks following peak uht metamorphism and initial cooling at depth the present study area within the northern margin of the ncc defines a collisional orogenic belt where continental collision and deep subduction were possibly followed by extensional collapse of the orogen evidence for the extensional phase is provided by the several mafic magmatic intrusions of paleoproterozoic age that are seen emplaced within the granulite facies supracrustals the thermal anomaly displayed by the uht granulites may thus correlate with asthenospheric upwelling and emplacement of mafic magmas possibly associated with plume activity which provided the heat and co2 input to achieve extreme crustal metamorphism and generation of dry granulite facies assemblages our study is the first report of syn metamorphic co2 rich fluids in uht granulites from a paleoproterozoic ca 1 92 ga terrane and provide evidence for carbonic metamorphism at ultrahigh temperatures as north china craton joined the columbia supercontinent amalgam

high pressure granulites formation recovery of peak conditions and implications for tectonics high pressure granulites are characterised by the key associations garnet clinopyroxene plagioclase quartz in basic rocks and kyanite k feldspar metapelites and felsic rocks and are typically orthopyroxene free in both basic and felsic bulk compositions in regional metamorphic areas two essential varieties exist a high to ultrahigh temperature group and a group representing overprinted eclogites the high to ultrahigh temperature type formerly contained high temperature ternary feldspar now mesoperthite coexisting with kyanite is associated with garnet peridotites and formed at conditions above 900 c and 1 5 gpa clinopyroxene in subordinate basic rocks is al rich and textural evidence points to a high pressure high temperature melting history the second variety contains symplectite like or poikilitic clinopyroxene plagioclase intergrowths indicating former plagioclase free i e eclogite facies assemblages this type of rock formed at conditions straddling the high pressure amphibolite high pressure granulite field at around 700 850 c 1 0 1 4 gpa importantly in the majority of high pressure granulites orthopyroxene is secondary and is a product of reactions at pressures lower than the peak recorded pressure in contrast to low and medium pressure granulites which form at conditions attainable in the mid to lower levels of normal continental crust high pressure granulites of nonxenolith origin mostly represent rocks formed as a result of short lived tectonic events that led to crustal thickening or subduction of the crust into the mantle short times at high temperature conditions are reflected in the preservation of prograde zoning in garnet and pyroxene high pressure granulites of both regional types although rare are known from both old and young metamorphic terranes e g c 45 ma namche barwa e himalaya 400 340 ma european variscides 1 8 ga hengshan china 1 9 ga snowbird saskatchewan and 2 5 ga jianping china this spread of ages supports proposals suggesting that thermal and tectonic processes in the lithosphere have not changed significantly since at least the end of the archean

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

dynamic recrystallization of quartz correlation between natural and experimental conditions quartz veins in the eastern tonale mylonite zone italian alps were deformed in strike slip shear due to the synkinematic emplacement of the adamello pluton a temperature gradient between 280 c and 700 c was effected across this fault zone the resulting dynamic recrystallization microstructures are characteristic of bulging recrystallization subgrain rotation recrystallization and grain boundary migration recrystallization the transitions in recrystallization mechanisms are marked by discrete changes of grain size dependence on temperature differential stresses are calculated from the recrystallized grain size data using paleopiezometric relationships deformation temperatures are obtained from metamorphic reactions in the deformed host rock flow stresses and deformation temperatures are used to determine the strain rate of the tonale mylonites through integration with several published flow laws yielding an average rate of approximately 10 14s 1 to 10 12s 1 the deformation conditions of the natural fault rocks are compared and correlated with three experimental dislocation creep regimes of quartz of hirth tullis linking the microstructures of the naturally and experimentally deformed quartz rocks a recrystallization mechanism map is presented this map permits the derivation of temperature and strain rate for mylonitic fault rocks once the recrystallization mechanism is known

paleoproterozoic ultrahigh temperature granulites in the north china craton implications for tectonic models on extreme crustal metamorphism abstract ultrahigh temperature uht metamorphic rocks associated with the khondalite belt within the inner mongolia suture zone imsz provide robust evidence for extreme thermal metamorphism in the north china craton ncc the imsz marks the collisional suture between the yinshan block to the north and the ordos block to the south as the ncc was incorporated within the columbia supercontinent amalgam during paleoproterozoic here we present a synthesis of the salient features of the uht rocks from the ncc including petrologic indicators fluid characteristics and monazite and zircon chronometry on the extreme crustal metamorphism the granulites carry diagnostic uht mineral assemblages including sapphirine quartz low zn fe3 spinel quartz high alumina orthopyroxene sillimanite quartz and high temperature mesoperthite the stability fields of the typical mineral assemblages conventional geothermobarometry and phase equilibria modeling using pseudosections as reported in a number of recent studies converge to indicate that these uht rocks experienced metamorphic temperatures up to or in excess of 1000 c at ca 10 kbar followed by an isobaric cooling segment the rocks were exhumed along a near isothermal decompression path microstructures mineral reactions and phase equilibria modeling suggest an anti clockwise p t path similar to those displayed by metamorphic orogens developed in subduction collision settings the dominant category of fossil fluids preserved within the major uht minerals is co2 consistent with the stability of the broadly anhydrous mineral assemblage in these rocks both chemical and radiogenic isotopic ages from monazite and zircon chronometry suggest the timing of the uht event as around 1 92 ga the paleoproterozoic high grade metamorphism younging from 1 95 ga in the western domain to 1 92 ga in the eastern domain of the khondalite belt might suggest a scissor like closure of oblique collision between the yinshan and the ordos blocks the salient features of the uht metamorphism in the ncc include 1 extreme metamorphic temperatures at moderate pressures 2 dominantly anhydrous nature of the mineral assemblages typically the stability of orthopyroxene 3 common presence of co2 rich fluid inclusions as the trace of the ambient fluid 4 regional extent of the uht granulites and 5 the association of the uht orogen with an accretionary belt in a continental collisional suture we evaluate the diverse models on the generation of uht orogens including their formation in thickened and inverted back arcs orogen self heating through heat producing elements heat and co2 input by plume impingement below a carbonated tectosphere and asthenospheric upwelling through ridge subduction and slab window process or during post collisional slab break off the ultra hot and dry uht rocks in the ncc provide one of the well preserved examples from the paleoproterozoic globe for investigating extreme metamorphism and related tectonic processes within the plate tectonic paradigm

evolution of a svecofennian mantled gneiss dome in sw finland with evidence for thrusting abstract the structure and petrology of a dome in svecofennian migmatitic gneisses schists amphibolites and granitoids on kemlo island is described the structure resembles classical mantled gneiss domes in that it comprises a core and mantle but differs in that intrusive granitoids occur in the mantle rather than the core four generations of folds are recognised and structural and petrological evidence is presented for an f 2 related thrusting origin for the discontinuity that separates core and mantle the metamorphic evidence suggests that the core was subject to low pressure amphibolite facies conditions at a time prior to thrusting when the mantle was subject to higher pressure amphibolite facies conditions it is pointed out that metamorphic jumps elsewhere on kemio can be better explained by thrusting than by normal faulting as previously suggested the structure is shown to be adequately explained as an interference structure resulting from superposition of f 3 and f 4 folding the possibility of diapirism cannot be eliminated but there is no necessity to invoke such a process and no evidence for it it is suggested that the interference pattern interpretation plus a thrusting origin for the core mantle contact and the folded foliation may have more general applicability the structural and metamorphic history is very similar to that of phanerozoic mountain belts that are interpreted as a product of plate tectonics it is suggested that the same model can be applied to the svecofennian

two types of precambrian high grade metamorphism inner mongolia china archaean and proterozoic granulite facies complexes of inner mongolia differ in lithological association tectonic style mineral assemblage and metamorphic p t path a nearly isobaric cooling path for archaean high grade metamorphic rocks is suggested by reaction textures and geothermobarometry early proterozoic metamorphic rocks show nearly isothermal decompression archaean metamorphism may have been caused by magmatic accretion whereas early proterozoic metamorphism suggests a major continental thickening event followed by exhumation

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