major types and time space distribution of mesozoic ore deposits in south china and their geodynamic settings the ore deposits of the mesozoic age in south china can be divided into three groups each with different metal associations and spatial distributions and each related to major magmatic events the first event occurred in the late triassic 230 210 ma the second in the mid late jurassic 170 150 ma and the third in the early mid cretaceous 120 80 ma the late triassic magmatic event and associated mineralization is characterized by peraluminous granite related w sn nb ta mineral deposits the triassic ore deposits are considerably disturbed or overprinted by the later jurassic and cretaceous tectono thermal episodes the mid late jurassic magmatic and mineralization events consist of 170 160 ma porphyry skarn cu and pb zn ag vein deposits associated with i type granites and 160 150 ma metaluminous granite related polymetallic w sn deposits the late jurassic metaluminous granite related w sn deposits occur in a ne trending cluster in the interior of south china such as in the nanling area in the early mid cretaceous from about 120 to 80 ma but peaking at 100 90 ma subvolcanic related fe deposits developed and i type calc alkaline granitic intrusions formed porphyry cu mo and porphyry epithermal cu au ag mineral systems whereas s type peraluminous and or metaluminous granitic intrusions formed polymetallic sn deposits these cretaceous mineral deposits cluster in distinct areas and are controlled by pull apart basins along the south china continental margin based on mineral assemblage age and space time distribution of these mineral systems integrated with regional geological data and field observations we suggest that the three magmatic mineralization episodes are the result of distinct geodynamic regimes the triassic peraluminous granites and associated w sn nb ta mineralization formed during post collisional processes involving the south china block the north china craton and the indo china block mostly along the dabie sulu and songma sutures jurassic events were initially related to the shallow oblique subduction of the izanagi plate beneath the eurasian continent at about 175 ma but i type granitoids with porphyry cu and vein type pb zn ag deposits only began to form as a result of the breakup of the subducted plate at 170 160 ma along the nne trending qinzhou hangzhou belt also referred to as qin hang or shi hang belt which is the neoproterozoic suture that amalgamates the yangtze craton and cathaysia block a large subduction slab window is assumed to have formed in the nanling and adjacent areas in the interior of south china triggering the uprise of asthenospheric mantle into the upper crust and leading to the emplacement of metaluminous granitic magma and associated polymetallic w sn mineralization a relatively tectonically quiet period followed between 150 and 135 ma in south china from 135 ma onward the angle of convergence of the izanagi plate changed from oblique to parallel to the coastline resulting in continental extensional tectonics and reactivation of regional scale ne trending faults such as the tan lu fault this widespread extension also promoted the development of ne trending pull apart basins and metamorphic core complexes accompanied by volcanism and the formation of epithermal cu au deposits granite related polymetallic sn w deposits and hydrothermal u deposits between 120 and 80 ma with a peak activity at 100 90 ma

temporally and spatially uniform rates of erosion in the southern appalachian great smoky mountains we measured 1 0 be in fluvial sediment samples n 27 from eight great smoky mountain drainages 1 330 km 2 results suggest spatially homogeneous sediment generation on the 10 4 10 5 yr time scale and 100 km 2 spatial scale at 73 11 t km 2 yr 1 equivalent to 27 4 m m y of bedrock erosion this rate is consistent with rates derived from fission track long term sediment budget and sediment yield data all of which indicate that the great smoky mountains and the southern appalachians eroded during the mesozoic and cenozoic at 30 m m y in contrast unroofing rates during the paleozoic orogenic events that formed the appalachian mountains were higher 10 2 m m y erosion rates decreased after termination of tectonically driven uplift enabling the survival of this ancient mountain belt with its deep crustal root as an isostatically maintained feature in the contemporary landscape

evolving geographic patterns of cenozoic magmatism in the north american cordillera the temporal and spatial association of magmatism and metamorphic core complexes four maps are presented here that show the location and extent of magmatic fields between eastern alaska and northern mexico during the successive time intervals of 55 40 40 25 25 10 and 10 0 ma and four others show the distribution of metamorphic core complexes during the same cenozoic time intervals the maps are based on u s geological survey and canadian cordilleran data bases contining about 6000 isotopic dates and extensive literature review for nearly 60 ma the development of metamorphic core complexes has coincided with the locus of a really extensive and voluminous intermediate composition magmatic fields the association is suggestive of a close link between magmatism and core complex formation namely that magma directly and indirectly lowers the strength of the crust magmatism thus controls the location and timing of core complex formation the stresses responsible may be inherited from mesozoic crustal thickening locally created by uplift and magmatic thickening of the crust and imposed by the global pattern of plate motions and driving forces since the miocene rates of magmatism extension and core complex formation have declined the modern basin and range province is not a suitable model for the situation that existed during major magmatic culminations the singular event of early miocene time the merging of two large magmatic fields extinguishing the laramide magmatic gap explains several disconnected observations the hyperextension episode of the colorado river corridor rapid reorientation of stress patterns across much of western north america and subsequent rapid tectonic movements in california magma triggered breakup of western north america lithosphere coincided with development of the san andreas transform system thermal destruction of the laramide magmatic gap created a california microplate about 22 ma ago that moved rapidly away from north america thus two plate tectonic processes thermal destruction of the lithosphere bridge and northward growth of a transform system interacted to produce miocene and later tectonic patterns and events

late triassic melting of a thickened crust in southeastern china evidence for flat slab subduction of the paleo pacific plate abstract the dashuang complex in zhejiang province of southeast china is composed of two distinct lithologies syenite in the west and quartz monzonite in the east they record similar zircon u pb ages of 224 3 ma syenite and 226 2 ma and 227 1 ma quartz monzonite respectively but are notably different in petrography magnetic susceptibility whole rock chemistry zircon hf isotope and zircon trace element characteristics the west dashuang syenitic pluton the west body has high modal alkali feldspar high zircon saturation temperatures high whole rock and zircon mree hree ratios low fe mg ti contents and is depleted in ba sr and eu it also has low magnetic susceptibilities belongs to the ilmenite series and is a peraluminous and ferroan granitoid the east dashuang quartz monzonitic pluton the east body has abundant k feldspar megacrysts with hornblende titanite and biotite being the major ferromagnesian minerals in contrast to the west body the east body has lower zircon saturation temperatures lower whole rock and zircon mree hree ratios higher fe mg ti contents and shows no depletion in ba sr or eu the east body has higher magnetite contents high magnetic susceptibilities and belongs to the magnetite series it is a metaluminous and magnesian granitoid of arc affinity zircon hf isotopic data reveal that both bodies were derived from partial melting of paleoproterozoic igneous protoliths in the lower crust but the east body possibly incorporated subducted terrigenous sediments both bodies have higher melting temperatures and pressures than adjacent cretaceous granitoids reflecting their origin in a thickened hotter lower crust the most feasible model to explain their differences is variations in water content during crustal melting resulting in different melting and crystallization behaviors such melting in a triassic thickened crust with variable water involvement followed by cretaceous magmatism in an extensional setting is consistent with the flat slab subduction model proposed for south china the model involves crustal thickening and partial melting with mantle and lower crustal metasomatism during flat slab propagation in the triassic early jurassic and crustal thinning and extension from the mid jurassic to the cretaceous

new u pb and ar ar isotopic age constraints on the timing of eocene magmatism fort fraser and nechako river map areas central british columbia twenty three new precise eocene u pb and 40 ar 39 ar age determinations for calc alkaline volcanic rocks of the ootsa lake group and associated intrusive rocks widespread in the nechako plateau in central british columbia constrain the timing of the eocene magmatism to 53 2 47 6 ma with a local duration of as little as 2 3 million years the new dates show that magmatism in the study area is partly coeval with that responsible for the babine igneous suite newman volcanic suite 53 50 ma to the north and for the endako group 51 45 ma which overlies the ootsa lake group however locally the three magmatic suites are distinct in age and or the strata record a magmatic hiatus of as much as 7 5 million years the ages generally young from north to south 52 47 ma along the western portion of the study area the babine igneous suite newman volcanic suite represents the oldest member of this series however in the east the ootsa lake group volcanic rocks are generally older 53 51 ma the anomalously older ages may be related to the interaction of magmatism and formation of a nearby and coeval core complex which ongoing studies show was uplifted at about the same time during the eocene felsic plutonism associated with the ootsa lake group occurred between 50 5 and 47 3 ma these plutons were emplaced in an extensional setting along north northeast trending faults the new dates stratigraphic relationships and suggested correlations of eocene strata in the study area with that to the west and north require a revision of the stratigraphic nomenclature for the ootsa lake and endako groups 696

gold mineralization of the chencai suichang uplift and tectonic evolution of zhejiang province southeast china abstract a metallogenic model is proposed in which gold and base metal mineralization in se china is related to plate interactions along the pacific margin with southeast asia from about 900 to 70 ma all depositional orogenic and metallogenic processes can be explained by subduction events along northeast trending zones with progressive accretion and cratonization along the continental margin these subduction zones moved southeast for a distance of less than 150 km as a result the area is one of crustal reworking in which the rocks and mineral deposits of earlier events have been repeatedly modified the chencai suichang uplift is an uplifted crustal block in central zhejiang province of mainland china in which the proterozoic basement is exposed as tectonic windows in overlying mesozoic volcanics several gold and base metal deposits are located in these proterozoic windows as well as in the younger volcanics most economic deposits were formed during the last major tectono magmatic event affecting the area the yanshanian 160 170 ma event although there is evidence for earlier mineralization that has been remobilized during the yanshanian it is suggested that crustal reworking was particularly important in remobilising and upgrading gold mineralization the model presented is supported not only by local geology and isotopic data but also by the regional distribution of gold deposits which are arranged in a 50 km wide belt parallel to the yanshanian subduction zone three major episodes of mineralization occurred during 1 a subduction related period of metamorphism partial melting and mesothermal fluid generation and mineralization in the lower crust during the caledonian 2 subduction related volcano plutonic activity with epithermal mineralization during the early yanshanian and 3 hydrothermal mineralization during late yanshanian tectono magmatic event in the third two main categories of metallogenic control are recognized namely 1 hydrothermal fluid generation in the lower crust and 2 upper crust structural and lithological controls on mineralization

palynology and geochronology of eocene beds from cheslatta falls and nazko areas central british columbia early tertiary lavas and sediments were collected from two areas west and south of prince george british columbia and processed for k ar dating and palynoassemblages samples from cheslatta falls gave k ar dates of 36 5 and 37 7 ma and yielded a palynoassemblage very similar to that from the jackson group in alabama and mississippi this late eocene assemblage is interpreted as having developed in a humid subtropical environment prior to a climatic cooling that we believe should be assigned to early oligocene time the tertiary lavas and sediments from the nazko area west of quesnel gave three middle eocene dates and over 60 species of palynomorphs these correlate with other rocks and palynoassemblages of kamloops group equivalents that occur southward across the international boundary and north to latitude 55 n the paleoclimate was wet and in the range of very warm temperate to humid subtropical

palynological dating and paleoreconstruction of three gold and silver orebodies in the western cordillera of canada united states and mexico abstract palynological analyses of both host rock and orebody samples from three separate mining properties have yielded significant information on ages as well as paleotemperature and paleodepositional regimes the structural setting of the southernmost a silver deposit in north central mexico is complex palynological dating of pollen spores and dinoflagellates show that rocks of upper cretaceous age are thrust over the mid cretaceous orebody both of which are thrust over upper jurassic rocks the second is a gold property from the overthrust belt of southwestern montana containing a paleocene rather than a precambrian palynoassemblage resulting in an unexpected and therefore controversial age the third is a gold property from sedimentary rocks in the tintinna trench in south central yukon containing a typical mid eocene assemblage of significance in all three sites are paleotemperature data that indicate at least 270 c and probably well above 300 c such high temperatures carbonized the palynomorphs and required the development of a new processing technique referred to here as the extended schulze s technique this application of palynology is gaining interest among mining geologists and engineers to help distinguish hypo and supergene orebodies as well as to reconstruct depositional patterns

zircon u pb geochronology and geochemistry of two episodes of granitoids from the northwestern zhejiang province se china implication for magmatic evolution and tectonic transition abstract granitoids 175 80 ma representing a prominent yanshanian jurassic to cretaceous magmatic event in south china widely intrude the precambrian crystalline basement and paleozoic strata here we report zircon u pb age data geochemical characteristics and sr nd isotopes of the late jurassic and early cretaceous granitoids from the northwestern zhejiang province zxb of southeastern china our results reveal two distinct episodes for the yanshanian magmatism the jiemeng and datongkeng granodiorites formed at 148 6 1 1 ma whereas the huangshitan jiuligang and ruhong aluminous a type granites were generated between 129 0 0 6 ma and 126 1 1 1 ma the two magmatic phases represent a tectonic transition from an active continental margin to post orogenic setting during the late jurassic ca 150 ma to early cretaceous ca 128 ma geochemically these intrusions are granodioritic to granitic in composition and show an affinity of s type and a type granitoids respectively the s type granodiorites of jiemeng and datongkeng are characterized by moderate sio 2 65 0 69 6 wt high k 2 o na 2 o 5 0 7 6 wt k 2 o na 2 o 1 2 1 5 zr 31 109 ppm sr 71 190 ppm and high field strength elements low to intermediate mg and moderate nb depletion the a type granites of huangshitan jiuligang and ruhong are characterized by high sio 2 72 7 77 2 wt k 2 o na 2 o 6 9 8 8 wt k 2 o na 2 o 1 3 2 1 fe t fe t mg ga 17 29 ppm 20 ppm commonly zr 96 197 ppm and sr 8 45 ppm with slight nb depletion the s type granodiorites have higher mg a nk sr sr ba sr y la yb n and lree hree and lower sio 2 k 2 o na 2 o ga and zr with weak negative eu anomalies compared to those of the a type granites with negative eu anomalies all these rocks show y nb ratios 1 2 high initial 87 sr 86 sr i sr ratios and low e nd t and are depleted in nb ti and sr indicating crustal origin with subduction zone signatures we suggested that the zxb s type granitic bodies might have been derived from the mesoproterozoic metamorphic basement rocks through partial melting induced by mantle derived magma followed by limited fractional crystallization the zxb aluminous a type granites were also derived from a similar magma source but underwent fractional crystallization at higher crustal levels the a type granites in the zxb correlate with a post orogenic tectonic setting a geological comparison between zxb and adjacent areas indicates that the geochemical features of the zxb a type granites are comparable with the baijuhuajian a type granites and also the adjacent areas a type granites but are distinct from the late cretaceous a type granites 105 90 ma distributed along the southeastern coastal area of south china we correlate the formation of the zxb s type granodiorites to inland compression associated with the subduction and collision of the paleo pacific plate in the late jurassic 170 145 ma in contrast the a type granites formed under a post orogenic setting during the early cretaceous period 145 120 ma resulting from lithospheric thinning and continent extension accompanied by slab roll back of the paleo pacific plate following the subduction collision event

silurian extension in the upper connecticut valley united states and the origin of middle paleozoic basins in the quebec embayment pre silurian strata of the bronson hill arch bha in the upper connecticut valley nh vt are host to the latest ludlow comerford intrusive suite consisting east to west of a mafic dike swarm with sheeted dikes and an intrusive complex the rocks are mostly mafic but with compositions ranging from gabbro to leucocratic tonalite the suite is truncated on the west by the monroe fault a late acadian thrust that carries rocks of the bha westward over silurian devonian strata of the connecticut valley gaspetrough cvgt dikes intrude folded strata with a pre intrusion metamorphic fabric taconian but they experienced acadian deforma tion twenty fractions of zircon and baddeleyite from three sample sites of gabbro diorite spanning nearly 40 km yield a weighted 207 pb 206 pb age of 419 1m a greenschist facies dikes sampled over a strike distance of 35 km were tholeiitic basalts formed by partial melting of asthenospheric mantle with little or no influence from mantle or crustal lithosphere the dike chemistry is similar to mid ocean ridge within plate and back arc basin basalts parent magmas originated in the astheno sphere and were erupted through severely thinned lithosphere adjacent to the cvgt extensive middle paleozoic basins in the internides of the appalachian orogen are restricted to the quebec embayment of the laurentian rifted margin and include the cvgt and the central maine trough cmt separated from the bha by a silurian tectonic hinge the ne trending comerford intrusions parallel the cvgt cmt and the tectonic hinge and indicate nw se extension during post taconian convergence the irregular margins of composite laurentia and avalon permitted continued colli sion in newfoundland st lawrence promontory and coeval extension in the quebec embayment extension may be related to hinge retreat of the northwest directed brunswick subduction complex and rise of the asthenosphere following slab break off an alternative hypothesis is that the basins originated as pull apart basins between northwest trending left stepping sinistral strike slip faults along the southern flanks of the new york and st lawrence promontories

regional stratigraphy and structural setting of the kamloops group south central british columbia the kamloops group is redefined as an assemblage of lower to middle eocene volcanic and sedimentary rocks widespread in south central british columbia in the type area west of kamloops the basal

magmatism along the southeast margin of the yangtze block precambrian collision of the yangtze and cathysia blocks of china the time and the environment of formation of igneous rocks along the southern margin of the yangtze continental block in southern anhui province and northeastern jiangxi province china are discussed sm nd dating for samples from two ophiolite suites give ages of about 1 ga late proterozoic granodiorites in the study area have ages of about 0 9 ga and have normal intrusive relations against banxi group metamorphic country rocks trace element abundances and initial nd isotope ratios for ophiolite and proterozoic granodiorites are consistent with formation in an island arc environment in contrast sr and nd isotopes for mesozoic granitoids of the region suggest that they are products of remelting of crustal sources similar to the basement of the yangtze block these relations do not support a recently proposed mesozoic overthrust tectonic model for southeast china but rather favor a late precambrian collision history for the yangtze and cathysia blocks

introduction to tectonics of china abstract the continental crust of china is a mosaic of cratonic blocks and orogenic belts containing small cratons and terranes with various tectonic settings they have diverse origins and complex histories of amalgamation and often suffered repeated reworking after multiple episodes of amalgamation in the last three decades extensive geological geochemical and geophysical investigations have been carried out on these cratonic blocks and intervening orogenic belts producing an abundant amount of new data and competing interpretations this provides important insights into understanding the formation and evolution of the chinese continents the papers assembled in this volume present a timely and comprehensive overview on major advancements and controversial issues related to the formation and evolution of continental crust in china complex tectonic histories were experienced not only by the large scale cratonic blocks and orogenic belts but also by small scale terranes and orogens between and inside these blocks nevertheless our understanding of lithotectonic units and geological processes has been greatly advanced by recent studies of zirconology and geochemistry for various rock types from major petrotectonic units in china it has been further advanced from integrated interpretations of geochemical and petrological data for petrogenesis of magmatic rocks an overview of these observations and interpretations provides new insights into understanding the continental plate tectonics and the chemical geodynamics of subduction zones

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

evolution of ore fluids in the donggou giant porphyry mo system east qinling china a new type of porphyry mo deposit evidence from fluid inclusion and h o isotope systematics abstract the donggou mo deposit in the eastern qinling area china is a giant porphyry system discovered based on a targeting concept by using the tectonic model for collisional orogeny metallogeny and fluid flow cmf model mo mineralization is associated with the donggou aluminous a type granite porphyry and was formed during the early cretaceous in a tectonic regime of continental extension the orebodies mainly occur as numerous veinlets in the host rocks hydrothermal ore forming processes include at least three stages characterized by veinlets of 1 quartz k feldspar minor molybdenite 2 quartz molybdenite beryl and 3 quartz carbonate fluorite three types of fluid inclusions fis are distinguished in quartz and beryl in stages 1 and 2 i e aqueous w type carbonic aqueous c type and solid bearing s type with only aqueous fis observed in stage 3 minerals s type fis contain variable daughter minerals including halite chalcopyrite calcite and an unidentified transparent crystal but only halite can dissolve during heating halite bearing s type fis are mainly homogenized by halite dissolution at 182 416 c corresponding to salinities of 30 9 49 2 wt nacl equiv minor halite bearing s type fis are homogenized to liquid at 190 360 c via vapor disappearance with salinities of 29 1 36 2 wt nacl equiv other fis in minerals of stages 1 2 and 3 are homogenized at temperatures of 341 550 c 220 440 c and 125 225 c with salinities of 8 0 18 3 5 3 16 8 and 0 5 7 3 wt nacl equiv respectively the estimated minimum trapping pressures are up to 141 mpa in stage 1 and up to 81 mpa in stage 2 respectively corresponding to an initial mineralization depth of no less than 5 km the quartz in veinlets yields 18 o values of 8 5 10 0 corresponding to 18 o h 2 o values of 2 9 to 5 9 while the d h 2 o values of fluid inclusions range from 59 to 82 these data suggest that the ore fluids forming the donggou deposit changed from high temperature high salinity co 2 rich magmatic to low temperature low salinity and co 2 poor meteoritic fluids via boiling and mixing resembling those of other magmatic hydrothermal systems in qinling orogen and dabie shan this supports the notion that the porphyry systems generated in a post collisional tectonic setting were initially co 2 rich as indicated by abundant c type and co 2 bearing s type fluid inclusions

petrogenesis of early cretaceous post collisional granitoids at shapinggou dabie orogen implications for crustal architecture and porphyry mo mineralization abstract the shapinggou molybdenum deposit is located in the qinling tongbai hong an dabie orogen which accommodates the largest molybdenum ore belt in the world new laser ablation inductively coupled plasma mass spectrometry la icp ms zircon u pb dating geochemical and sr nd hf isotope data for the granitoids at shapinggou are presented in this paper two stages of early cretaceous magmatism 138 128 ma and 116 114 ma took place at shapinggou in a post collisional setting these granitoids can be divided into four groups including the early stage granites 134 129 ma sio2 62 33 to 76 27 and intermediate rocks 129 128 ma sio2 52 41 to 60 72 and the late stage quartz syenite 116 ma sio2 64 09 to 65 12 and mo bearing granite porphyry 114 ma sio2 74 95 to 77 26 the early stage granites have high sr contents sr y and lan ybn ratios but low y yb and mgo contents showing low mg adakitic chemical features they were possibly generated by partial melting of the thickened mafic lower continental crust the early stage intermediate rocks have relatively high mgo contents 2 71 to 4 84 and mg values 46 to 60 but low sr y and lan ybn ratios which were likely derived from a hybrid mantle metasomatized by melts from foundered eclogitic lower continental crust and subsequent fractional crystallization the late stage quartz syenite shows chemical features of metaluminous a type granites including high abundances of alkalis hfse zr nb and trivalent ree the late stage mo bearing granite porphyry displays high sio2 alkalis zr ce contents and a cnk ratios but low ba sr p and ti showing affinity to highly fractionated i type granites the late stage quartz syenite and granite porphyry were possibly produced by partial melting of intermediate compositions of middle crust our results support the conclusion that there were three layer continental crusts in the dabie orogen prior to early cretaceous magmatism and suggest that post collisional granitoids in the dabie orogen were derived from not only lower crust but also middle crust the giant shapinggou porphyry mo deposit was most likely formed by effective transport of volatiles and mo in a convecting granitic magma column rather than by direct melting of mo enriched sources the mo bearing rocks at shapinggou are characterized by a high degree of magma differentiation close associations with a type granite and formation at the end of regional magmatism

lithostratigraphic and tectonic framework of jurassic and cretaceous intermontane sedimentary basins of south central british columbia1this article is one of a series of papers published in this special issue on the theme of new insights in cordilleran intermontane geoscience reducing exploration risk in the mountain pine beetle affected area british columbia the south central intermontane belt of british columbia has a complex architecture comprising late paleozoic to mesozoic volcanic and plutonic arc magmatic suites marine and nonmarine clastic basins high grade metamorphic complexes and accretionary rocks jurassic and cretaceous clastic basins within this framework contain stratigraphy with hydrocarbon potential the geology is complicated by cretaceous to eocene deformation dismemberment and dislocation the eocene to neogene history of the southern intermontane belt is dominated by non arc volcanism followed by pleistocene to recent glaciation the volcanic and glacial cover makes this a difficult region to explore for resources much recent work has involved re evaluating the challenges that the overlying volcanic cover has historically presented to geophysical imaging of the sedimentary rocks in this region in light of technological advances in geophysical data collection and analysis this paper summarizes the lithological and stratigraphic fra

timing of the yuchiling giant porphyry mo system and implications for ore genesis the yuchiling mo deposit is a recently discovered giant porphyry system in the east qinling mo belt china its apparent causative intrusion i e the yuchiling granite porphyry is the youngest intrusion phase 4 of the heyu multiphase granite batholith which was emplaced between 143 and 135 ma new robust constraints on the formation of the yuchiling porphyry mo system are provided by combined zircon u pb biotite 40ar 39ar and molybdenite re os dating zircon grains from the mo mineralized granite porphyry yield weighted 206pb 238u age of 134 0 1 4 ma n 19 2 error mswd 0 30 magmatic biotite from the same sample yield a 40ar 39ar plateau age of 135 1 1 4 ma 2 error and an inverse isochron age of 135 6 2 0 ma n 7 2 error mswd 10 8 which are effectively coincident with the zircon u pb age within analytical error three pulses of mineralization can be deduced from the molybdenite re os ages namely 141 137 and 134 ma which agree well with the zircon u pb ages of granitic phases 1 2 and the yuchiling porphyry phase 4 respectively these well constrained temporal correlations indicate that mo mineralization was caused by pulses of granitic magmatism and that the ore forming magmatic hydrothermal activity responsible for the yuchiling porphyry mo system lasted about 8 ma the yuchiling mo deposit represents a unique style of porphyry mo system formed in a post collision setting and associated with f rich high k calc alkaline intrusions which differ from convergent margin associated porphyry mo deposits

characteristics and origin of coarse gold in late pleistocene sediments of the cariboo placer mining district british columbia canada the cariboo placer mining district 1000 km2 sited in the interior plateau of central british columbia canada is the premier placer gold mining district of the province gold is recovered from three late pleistocene sedimentary facies postglacial fluvial gravels 25 ka this study reports the morphology size roundness sphericity of 1636 gold grains ranging in size from 0 25 to 17 mm recovered from 19 placer mines older gravels contain the smallest gold grains mean grani size 1 53 mm grains of intermediate size occur in till 2 23 mm and the coarsest gold occurs in postglacial gravels 2 34 mm with a mean of 1 93 mm for the mining district as a whole the most common grain shapes are sub rounded discoidal 14 73 of the grain population sub angular discoidal 10 88 and sub rounded sub discoidal 9 59 the most angular grains occur in postglacial gravels in situ growth of coarse angular grains is indicated by a composite grain structure consisting of aggregates of gold particles welded together by high grade ag 2 filamentous gold in situ coarsening may be reliant on organic complexing agents produced below a dense forest cover an evolutionary sequence of grain form from angular aggregates to rounded pumpkin seed grains is suggested rounded grains commonly show a crystalline structure which may result from the cold hammering of gold during transport fracturing along crystal boundaries is common gold grains may undergo cycles of coarsening rounding diagenesis and breakup in response to repeated recycling through pleistocene sedimentary environments

unroofing maine relating pressure of crystallization thermochronological data tectonics and topography abstract we gathered data on the age and depth of crystallization of mid paleozoic igneous and metamorphic rocks from 64 sites in and near maine and also a time series of rock temperatures from the casco bay area of maine by fitting a trend surface to some of our geobarometric data we estimated total erosion at the casco bay site in the last 284 m y and thus the geothermal gradient at that time the temperatures could thus be converted to a time series of erosion assuming airy isostasy we then obtained a time series of the mean height of the land surface above present sea level represented by h of four erosion laws fit to the latter a power law e phu where e is the erosion rate and p and u are fitting parameters provides the best fit without excessive mathematical complexity however the exponent u is unexpectedly 6 km and that it likely began in the late middle paleozoic thus extending earlier estimates of this component of displacement backward in time and upward in magnitude a decrease in denudation rate in the early mesozoic is interpreted as reflecting suturing of gondwana and laurentia to form pangea the denudation rate increased again following mesozoic rifting this latter increase occurs later to the northwest suggesting retreat of a rift flank escarpment at a rate of a few tenths of a millimeter per year using our full geobarometric data set we calculated an average value of u of 1 2 we then generated topographic maps of the state at 380 320 and 200 ma at 380 ma the mountains of western maine were likely somewhat higher than today s rocky mountains drainage then was to the northeast contributing sediment to the maritimes basin the alleghanian collision at 320 ma raised comparable mountains somewhat nearer the present coast in western maine following mesozoic rifting drainage was also to the southeast into rift basins bordering the opening atlantic ocean since the devonian an average of 9 km of rock have been eroded from maine

the distribution style and intensity of alleghanian metamorphism in south central new england petrologic evidence from the pelham and willimantic domes abstract alleghanian metamorphism in south central new england is characterized by late pennsylvanian documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepackage ot2 ot1 fontenc newcommand cyr renewcommand rmdefault wncyr renewcommand sfdefault wncyss renewcommand encodingdefault ot2 normalfont selectfont declaretextfontcommand textcyr cyr pagestyle empty declaremathsizes 10 9 7 6 begin document landscape mathrm kyanite mathrm muscovite mathrm staurolite end document grade heterogeneous reconstitution of migmatitic documentclass aastex usepackage amsbsy usepackage amsfonts usepackage amssymb usepackage bm usepackage mathrsfs usepackage pifont usepackage stmaryrd usepackage textcomp usepackage portland xspace usepackage amsmath amsxtra usepack

erosion of an ancient mountain range the great smoky mountains north carolina and tennessee analysis of 10be and 26al in bedrock n 10 colluvium n 5 including grain size splits and alluvial sediments n 59 including grain size splits coupled with field observations and gis analysis suggest that erosion rates in the great smoky mountains are controlled by subsurface bedrock erosion and diffusive slope processes the results indicate rapid alluvial transport minimal alluvial storage and suggest that most of the cosmogenic nuclide inventory in sediments is accumulated while they are eroding from bedrock and traveling down hill slopes spatially homogeneous erosion rates of 25 30 mm ky 1 are calculated throughout the great smoky mountains using measured concentrations of cosmogenic 10be and 26al in quartz separated from alluvial sediment 10be and 26al concentrations in sediments collected from headwater tributaries that have no upstream samples n 18 are consistent with an average erosion rate of 28 8 mm ky 1 similar to that of the outlet rivers n 16 24 6 mm ky 1 which carry most of the sediment out of the mountain range grain size specific analysis of 6 alluvial sediment samples shows higher nuclide concentrations in smaller grain sizes than in larger ones the difference in concentrations arises from the large elevation distribution of the source of the smaller grains compared with the narrow and relatively low source elevation of the large grains large sandstone clasts disaggregate into sand size grains rapidly during weathering and downslope transport thus only clasts from the lower parts of slopes reach the streams 26al 10be ratios do not suggest significant burial periods for our samples however alluvial samples have lower 26al 10be ratios than bedrock and colluvial samples a trend consistent with a longer integrated cosmic ray exposure history that includes periods of burial during down slope transport the results confirm some of the basic ideas embedded in davis geographic cycle model such as the reduction of relief through slope processes and of hack s dynamic equilibrium model such as the similarity of erosion rates across different lithologies comparing cosmogenic nuclide data with other measured and calculated erosion rates for the appalachians we conclude that rates of erosion integrated over varying time periods from decades to a hundred million years are similar the result of equilibrium between erosion and isostatic uplift in the southern appalachian mountains

fluid evolution of the yuchiling porphyry mo deposit east qinling china the yuchiling mo deposit east qinling china belongs to a typical porphyry mo system associated with high k calc alkaline intrusions the pure co2 pc co2 bearing c aqueous h2o naci w and

diagenesis and anchimetamorphism in an overthrust belt external domain of the taconian orogen southern canadian applachians ii paleogeothermal gradients derived from maturation of different types of organic matter abstract 3hermal maturation patterns have been established for the external domain of taconian orogen of the southern quebec appalachians between quebec city and the u s border reflectance of three types of organic matter om measured in the same sample shows that graptolite reflectance r g is higher than pyrobitumen reflectance r b which in turn is higher than reflectance of amorphous organic matter r a r g r b and r a are converted to vitrinite equivalent reflectance from which maximum paleotemperatures were calculated inverted diagenesis is preserved in northwest part of the chaudiere nappe to the southeast original burial diagenetic grade was upgraded by later synorogenic heating in the st lawrence lowlands paleogeothermal gradients at maximum burial were 33 37 c km a gradual southwestward increase in maturation level suggests increasing post taconian burial depth from 4 6 to 6 7 km indicating an increasing thickness of eroded rocks from northeast to southwest along logan s line one of the most important contributions of this study is the proof that the acadian orogeny has affected the successor basin of the taconian orogeny in the st lawrence lowlands the thermal effect of the mid cretaceous intrusion of the monteregian hills on their country rocks is limited to one diameter or less of the intrusive body as shown by the narrow

a shear zone origin for alleghanian permian multiple deformation in eastern massachusetts the area around worcester massachusetts has been used to determine which deformational and metamorphic features are due to the alleghanian permian orogeny and which are pre alleghanian isolated fault bounded inliers of carboniferous rocks display evidence of a single metamorphism and the formation of two prominent cleavages the first cleavage formed synchronously with metamorphism pre carboniferous metasedimentary rocks have been affected by two metamorphisms and contain three prominent cleavages the initial cleavage formed during the first metamorphism and the second cleavage formed during the second retrogressive metamorphism thus the second two cleavages and the second metamorphism in pre carboniferous rocks are interpreted as alleghanian normal displacements on two distinct faults are bracketed by the formation of the first and second alleghanian cleavages the initial faulting along the newly defined wachusett mylonite zone wmz formed a wide zone of ductile mylonites which dips moderately to the northwest and contains elongation lineations which trend northwest the second faulting along the clinton newbury fault cnf occurred along a steeply inclined plane which cuts the wmz mylonites and contains a thin zone of phyllonites and mylonites which have elongation lineations which trend west alleghanian cleavages and metamorphism are confined to a mappable zone which is approximately 15 km wide where well defined this zone is interpreted as a ductile shear zone which moved twice forming the first and second alleghanian cleavages early formed pre alleghanian metamorphic minerals are retrograded to hydrous phyllosilicates and new hydrous minerals formed in the shear zone indicating that it was a pathway for fluid flow the initial motion was left lateral with a thrust component the second cleavage formed during top to the northwest normal displacements thus the history of alleghanian tectonism in this area began with sinistral faulting and then included three displacements along normal faults or shear zones this suggests that the strain history included two distinct episodes with the second experiencing slightly different strain orientations at different times resulting in first wmz normal motion then cnf normal motion and finally normal displacements related to the second alleghanian cleavages this history agrees well with other work on the nature of the alleghanian orogeny in the northeast united states

palynology paleoclimatology and correlation of middle miocene beds from porcupine river locality 90 1 alaska abstract beds in the upper ramparts canyon of the porcupine river alaska 67 20 n 141 20 w yielded a flora rich in pollen of hardwood genera now found in the temperate climates of north america and asia the beds are overlain or enclosed by two basalt flows which were dated to 15 2 0 1 ma by the 40 ar 39 ar method fixing the period of the greatest abundance of warm loving genera to the early part of the middle miocene the assemblage is the most northern middle miocene flora known in alaska organic bed 1 underlies the basalt and is older than 15 2 ma but is of early to middle miocene age the pollen assemblage from organic bed 1 is dominated by conifer pollen from the pine and redwood cypress yew families with rare occurrences of temperate hardwoods organic bed 2 is a forest floor containing redwood trees in life position engulfed by the lowest basalt flow a pine log has growth rings up to 1 cm thick organic beds 3 and 4 comprise lacustrine sediment and peat between the two basalt flows their palynoflora contain conifers and hardwood genera of which about 40 have modern temperate climatic affinities hickory katsura walnut sweet gum wingnut basswood and elm pollen are consistently present and beech and oak alone make up about 20 of the pollen assemblage a warm high latitude climate is indicated for all of the organic beds but organic bed 3 was deposited under a time of peak warmth climate data derived by comparison with modern east asian vegetation suggest that at the time of deposition of organic bed 3 the mean annual temperature mat was ca 9 c the warm month mean temperature wmmt was 20 c and the cold month mean temperature cmmt was ca 2 c in contrast the modern mat for the region is 8 6 c wmmt is 12 6 c and cmmt is 28 c organic beds 3 and 4 correlate to rocks of the middle miocene late seldovian stage of cook inlet and also probably correlate to and more precisely date the lower third of the suntrana formation in the alaska range beds at unalaklect part of the upper mackenzie bay sequence in the beaufort mackenzie basin and the mary sachs gravel of banks island this suggests that forests with significant percentages of temperate deciduous angiosperms existed between latitudes 60 and 72 n during the early middle miocene

sedimentological controls on gold in a late pleistocene glacial placer deposit cariboo mining district british columbia canada abstract it is a widely perceived notion that glaciation results in dispersal of mineralized bedrock and that sedimentary concentrates of economic minerals placers rarely occur in glaciated basins this paper describes economic gold placers within late pleistocene glacial and related fluvial sediments of the cariboo mining district in central british columbia canada the area has been defined as a giant gold placer total production since 1858 is over 93 000 kg the oldest and volumetrically largest placers occur in fluvial gravels and valley side fan deposits deposited during a long non glacial interval from as early as 125 000 to 30 000years b p the richest placers are found along bedrock gutters in the deepest parts of valleys indicating repeated fluvial reworking of the valley infills braided and wandering gravel bed fluvial facies can be identified glacial placers that overlie the fluvial placers occur within lodgement till complexes deposited below the late wisconsin cordilleran ice sheet after 30 000 years b p the basal portions of lodgement tills are commonly enriched in gold as a result of incorporation from older gravels subglacial meltwaters created a highly effective sluicing system and left lucrative pay zones along meltwater cut channels on bedrock benches within intraformational gravels in lodgement till and within lee side deposits down ice of bedrock highs lee side deposits are essentially water worked talus slopes that accumulated in subglacial cavities finally postglacial wandering gravel bed rivers have repeatedly reworked older placers resulting in rich pay zones at the base of extensive bar platform deposits similar sedimentological controls on gold distribution can be identified in other glacial placers of late cenozoic and paleozoic age in north america southern africa and australia a distinction is drawn between these placers all characterized by coarse grained nuggety gold and the more well known precambrian and paleozoic placers where finely comminuted gold is dispersed through large thicknesses of rock episodes of glaciation typically occur after long periods of tropical and subtropical weathering when supergene processes were active and glaciers were able to remove and concentrate coarse gold in contrast gold in non glacial placers of precambrian and paleozoic age has been through many cycles of erosion and transport and coarse gold is uncommon

the gang ranch big bar area south central british columbia stratigraphy geochronology and palynology of the tertiary beds and their relationship to the fraser fault tertiary rocks in the gang ranch big bar area south central british columbia consist of 1 early or middle eocene 50 ma lavas breccias and tuffs capped by a mappable unit of conglomerate a

contrasting styles of taconian eastern acadian and western acadian metamorphism central and western new england the two major early to middle palaeozoic tectonic metamorphic events in the northern appalachians were the taconian middle to late ordovician in central to western areas and the acadian late silurian to early middle devonian in eastern to west central areas this paper presents a model for the acadian orogenic event which separates the acadian metamorphic realm into eastern and western belts based on distinctively different styles we propose that the acadian metamorphism in the east was the delayed consequence of taconian back arc lithospheric modification east of the taconian island arc thick accumulations of late ordovician and silurian sediments coupled with plutons rising along a magmatic arc produced crustal thermal conditions appropriate for anomalously high t low p metamorphism accompanied by major crustal anatexis in this zone upward melt migration was coupled with subsequent e w crustal shortening possibly due to outboard collision with the avalon terrane to produce mechanical conditions that favoured formation of fold and thrust nappes and resultant tectonic thickening to the west and probably to the east as well the basis for the distinction between the eastern and western acadian events lies in the contrasting styles of metamorphism accompanying each evidence for contrasting metamorphic styles consists of 1 estimated metamorphic field gradients mfgs based on thermobarometric studies and 2 petrological evidence for contrasting p t trajectories west of the acadian metamorphic front the taconian zone has an mfg in which peak temperatures of 400 600 c were reached at pressures of about 4 6 kbar with both p and t increasing to the east near its western edge the western acadian metamorphic overprint has a similar mfg to the taconian and is mainly discriminated by 40ar 39ar dating and microtextural evidence east of this narrow zone the western acadian overprint is characterized by progressively higher temperatures 600 725 c and pressures 6 5 10 kbar or more to the east yielding an overall mfg that lies along or slightly above the kyanite sillimanite boundary on a p t diagram there is little or no plutonism accompanying western acadian metamorphism in contrast thermobarometry in the eastern acadian east of the bronson hill belt yields high t intermediate p conditions for the highest grade rocks known in new england t 650 750 c p 4 5 6 5 kbar for granulite facies assemblages which apparently formed along an anticlockwise p t path the bronson hill belt lies geographically between the eastern and western acadian zones and shows transitional petrological behaviour anomalously high temperatures at intermediate pressures but a clockwise path with decompression cooling radiometric dating indicates peak taconian conditions may have been achieved as early as 475 ma in the taconian hinterland and as late as 445 ma in the taconian foreland including the taconic allochthons eastern acadian magmatism may have started as early as 425 ma and most nappe stage deformation and metamorphism in the eastern acadian zone appears to have ended by about 410 ma tectonic thickening in the western acadian including the western counterparts of the nappe stage deformation documented in the eastern acadian must pre date attainment of peak metamorphic conditions dated at 395 385 ma dome stage deformation clearly post dates peak metamorphism and deforms metamorphic isograds the end of western acadian deformation is well constrained by 370 375 ma radiometric ages of late pegmatites and granitoids which cross cut all structures

zircon u pb geochronology and hf isotopic compositions of the mesozoic granites in southern anhui province china abstract mesozoic magmatism is extensively developed in eastern china along the eastern asian continental margin but the geochronological framework is not well understood for each area in this study seventy eight samples were collected from the mesozoic igneous rocks in south anhui province east central china in order to precisely constrain their formation ages and contributions to crustal growth zircon u pb age determination using ion probe and laser ablation techniques indicate that magmatism in the area can be divided into two separate stages at 150 136 and 136 120 ma respectively the first stage of granitic rocks not documented in other areas of eastern china is mostly composed of hornblende bearing i type granodiorite and monzogranite and is considered to be formed by slab tearing during pacific subduction the second stage of magmatism coeval to that in other areas of eastern china is characterized by the occurrence of a type granite and syenite and their volcanic counterparts although i type granitic rocks are also developed locally these rocks are mostly distributed around and within the fault controlled volcanic basins and some are closely related to detachment shearing indicating their formation in an extensional setting zircon hf isotopic analyses indicate that archean material might be present in the area but that the main episode of crustal growth took place during the mesoproterozoic with very little juvenile material in the mesozoic

re os isotopic dating of molybdenites in the xiaoliugou w mo deposit in the northern qilian mountains and its geological significance abstract the xiaoliugou is a large scale w mo deposit newly explored in the northern qilian mountains caledonian fold belt northwestern china it is a vein and skarn type deposit genetically associated with the xiaoliugou caledonian granodiorite stock re os isotopic dating for the molybdenite in the deposit yields an isochron age of 462 13 ma 2 and model ages of 436 to 496 ma these results suggest that w mo mineralization occurred before the collision of plates and mineralized substances mainly originated from the crust

cretaceous tectonic evolution of south china a preliminary synthesis abstract the cretaceous tectonic evolution of south china is characterized by widespread extensional basin and dome generation voluminous magma intrusion eruption and associated polymetallic mineralization all of which are of world wide interests that have stimulated the attention of geologists for more than half a century due to the lack of a comprehensive understanding of regional tectonic evolution and geodynamics of south china many controversies regarding the nature and origin of these features remain this paper attempts to make a review by synthesizing existing structural petrological geochronological and geochemical data of the cretaceous structures and magmas which guides us to propose a three stage tectono thermal evolutionary history of south china during the cretaceous period the earliest cretaceous 145 137 ma tectonic stage was characterized by syn orogenic shortening deformation and metamorphism under a nw se compressional setting which generated voluminous porphyry cu au ore bearing adakitic rocks in the lower yangtze river belt and gneissic granites in the coastal area its geodynamic origin was interpreted as combined effects of the pacific izanagi ridge subduction beneath the lower yangtze river belt and the pacific subduction beneath the cathaysia block this syn orogenic shortening stage was followed by a post orogenic stage 136 86 ma comprising two episodes of alternate extensional and shortening events the extension 136 118 ma in the earlier episode was dominated by a nw se extensional regime it led to significant taphrogenesis manifested by large scale extensional basins and voluminous magma intrusion eruption this extensional event was associated with a combination of the slab window opening during the ridge subduction and the rollback of the subducted pacific slab the subsequent nw se transpressional event led to cessation of active marginal magmatism and resulted in tectonic inversion of previous rift basins this transpression lasted from 117 ma to 108 ma and its driving mechanism was attributed to the collision between the eastern asian margin and the west philippine block the extension 107 86 ma in the later episode was dominated by a wnw ese extensional regime which led to a second phase of basin subsidence and produced numerous a and i type granites and bimodal volcanoes the tectonic regime then changed at ca 85 ma to wnw ese oriented transpression causing inversion of the late cretaceous rift basins and cessation of extension related magmatism the change of stress field from wnw ese extension to wnw ese transpression was related to the variations of the subducted slab dynamics i e the transition from either the ese ward retreat to wnw ward subduction of the pacific slab or the alternate stress during slab break off during the latest cretaceous a drastic change of regional tectonic stress orientation from wnw ese to n s occurred in south china the third stage predominated by n s extension was developed which caused a new phase of regional crustal subsidence along the youngest e w trending extensional structures

cretaceous magmatism and mo mineralization in the south china mo province u pb and re os geochronology constraints from the sanzhishu porphyry mo deposit abstract compared with the mesozoic w sn cu au pb zn ag mineralization in the south china mo province scmp detailed studies on mo deposits especially cretaceous mo deposits are rare due to limited exposures the sanzhishu porphyry mo deposit located in the scmp is characterized by disseminated and veinlet type molybdenite quartz veins within monzogranite zircon sims u pb analysis of the monzogranite yields a concordia age of 112 3 ma indicating that the emplacement of the sanzhishu monzogranite was in the early cretaceous six molybdenite samples yield re os ages between 111 and 112 ma with an isochron age of 111 6 4 ma which is consistent with the zircon age of the ore bearing monzogranite a new compilation of reliable re os isotope age data indicate that cretaceous mo mineralization in the scmp occurred during two major episodes at 130 145 ma and 90 120 ma with most deposits forming in the later episode during the cretaceous a back arc extensional setting in se china is evidenced by the occurrence of numerous high f mo deposits widespread a type granites and the development of pull apart basins the increasing trend in re contents of molybdenite from the early mo mineralization period to the late mo mineralization period in the scmp indicates that the area experienced stronger crust mantle interaction in the later mineralization episode based on geological evidence we propose that the early episode of mo mineralization formed in a tectonic setting associated with a metamorphosed subducted slab that experienced an increase in density and an increase in the gravitational pull on the slab itself which induced rollback of the subducting paleo pacific slab the late episode of mo mineralization developed in an extensional tectonic setting caused by the continued rollback and the eventual slab break off

u pb zircon geochemical and sr nd hf isotopic constraints on age and origin of jurassic i and a type granites from central guangdong se china a major igneous event in response to foundering of a subducted flat slab abstract the mesozoic geology of se china is characterized by widespread jurassic to cretaceous igneous rocks consisting predominantly of granites and rhyolites and subordinate mafic lithologies however the tectonic regime responsible for the inland jurassic granites remains controversial we report here u pb zircon ages geochemical and sr nd hf isotopic data for the nankunshan alkaline granite and the fogang granitic batholith in central guangdong mineralogical and geochemical features suggest that the fogang and nankunshan rocks are i and aluminous a type granites respectively shrimp u pb zircon analyses yield consistent ages ranging from 159 3 ma to 165 2 ma for four samples from the fogang batholith and an age of 158 5 ma for the nankunshan granite the fogang granites having i sr 0 7098 0 7136 e nd t 4 3 to 12 2 and e hf t 11 5 to 3 1 for the magmatic zircons were derived from paleoproterozoic mafic intermediate igneous protolith with minor addition of mantle derived magmas the nankunshan rocks have relatively lower i sr 0 706 0 708 higher e nd t 0 3 to 2 4 and e hf t 5 7 to 1 1 for the magmatic zircons and some oib like trace element ratios they were likely generated through extensive fractional crystallization of mantle derived alkaline parental magma associated with crustal assimilation these 160 ma i and a type granites in central guangdong were emplaced coeval with the widespread 165 155 ma i and a type granites and syenites following the initiation of intraplate basaltic and or bimodal igneous magmatism at 180 170 ma in the adjacent regions we interpret these jurassic igneous rocks as anorogenic magmatism formed during a major igneous event in response to foundering of an early mesozoic subducted flat slab beneath se china continent

petrogenesis and tectonic significance of a mesozoic granite syenite gabbro association from inland south china article i nfo a granite syenite gabbro association from inland south china has been studied for zircon u pb ages and hf isotopic compositions as well as whole rock elemental and sr nd isotopic compositions to constrain their sources and tectonic settings these rocks have distinctive crystallization ages the longyuanba biotite granites have a mean age of 240 ma the longyuanba two mica granites pitou alkali feldspar granites and amphibole bearing alkali feldspar granites tabei and huangbu syenites and chebu and chenglong gabbros have a similar age of 178 ma whereas the dafengnao syenite gives a mean age of 165 ma the longyuanba biotite granites and two mica granites exhibit s type characteristics probably derived from a neoproterozoic crustal source the chenglong gabbro has oib like trace element and highly depleted isotope compositions ehf t 10 0 1 3 end t 5 2 suggesting its parental melt to be of asthenospheric origin with insignificant crustal assimilation however crustal contamination is required to explain the isotopic compositions of the chebu gabbro the syenites are shoshonitic in composition and have depleted sr nd hf isotopic signatures which we interpret to have resulted from a mixed source of asthenospheric mantle and metasomatized lithospheric mantle as expected the signals of crustal assimilation are conspicuous in the petrogenesis of the more evolved syenites the pitou alkali feldspar granite and amphibole bearing alkali feldspar granite exhibit i type and a type characteristics respectively they are isotopically more depleted than the s type longyuanba granites we suggest that they may have formed through magma mixing of predominantly mantle derived melts with the neoproterozoic crust the jurassic granite syenite gabbro association was the product of asthenosphere lithosphere crust interactions which records the primary role of asthenospheric mantle upwelling in magma generation both in the mantle and in the crust in the early yanshanian time in south china they probably occurred in an intraplate rift like environment as a tectonic response to far field stress at plate margins during the early stage of the paleo pacific plate subduction the tectonic transition from the tethys orogenic regime to the paleo pacific regime was accomplished in the early jurassic and the early yanshanian magmatism should be genetically associated with the paleo pacific tectonic regime

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

chronostratigraphy of eocene volcanism central british columbia onset and termination of eocene felsic volcanism in the chilcotin plateau of central british columbia is constrained between 54 6 and 46 6 ma by 33 new u pb and 40ar 39ar isotopic age determinations dates were obtained from representative felsic coherent and fragmental volcanic rocks that comprise the ootsa lake group the resulting chronostratigraphy shows that magma compositions evolved from felsic to intermediate with no spatial migration of the volcanic activity rhyolitic compositions are oldest and are overlain by dacitic rocks with varied phenocrysts assemblages in many parts of the chilcotin plateau the eocene stratigraphy is capped by distinctive vitreous black dacite lavas which are contemporaneous with andesitic lavas of the endako group in the nechako plateau to the north crystallization ages from ootsa lake group rocks of the chilcotin plateau overlap age determinations from correlative rocks of the nechako plateau and southern bc collectively this geochronological dataset supports p