highly fractionated i type granites in ne china ii isotopic geochemistry and implications for crustal growth in the phanerozoic ne china is the easternmost part of the central asian orogenic belt caob the area is distinguished by widespread occurrence of phanerozoic granitic rocks in the companion paper part i we established the jurassic ages 184 137 ma for three granitic plutons xinhuatun lamashan and yiershi we also used geochemical data to argue that these rocks are highly fractionated i type granites in this paper we present sr nd o isotope data of the three plutons and 32 additional samples to delineate the nature of their source to determine the proportion of mantle to crustal components in the generation of the voluminous granitoids and to discuss crustal growth in the phanerozoic despite their difference in emplacement age sr nd isotopic analyses reveal that these jurassic granites have common isotopic characteristics they all have low initial 87 sr 86 sr ratios 0 7045f0 0015 positive end t values 1 3 to 2 8 and young sm nd model ages 720 840 ma these characteristics are indicative of juvenile nature for these granites other late paleozoic to mesozoic granites in this region also show the same features sr nd and oxygen isotopic data suggest that the magmatic evolution of the granites can be explained in terms of two stage processes 1 formation of parental magmas by melting of a relatively juvenile crust which is probably a mixed lithology formed by pre existing lower crust intruded or underplated by mantle derived basaltic magma and 2 extensive magmatic differentiation of the parental magmas in a slow cooling environment the widespread distribution of juvenile granitoids in ne china indicates a massive transfer of mantle material to the crust in a post orogenic tectonic setting several recent studies have documented that juvenile granitoids of paleozoic to mesozoic ages are ubiquitous in the central asian orogenic belt hence suggesting a significant growth of the continental crust in the phanerozoic d 2003 elsevier science b v all rights reserved

shrimp dating of medium coarse grained granite in chaobuleng iron deposit dong ujimqin banner inner mongolia the chaobuleng iron deposit is located in the erlianhaote dong ujimqin banner polymetallic metallogenic belt of inner mongolia the medium coarse grained granite in the area has close genetic relationship with the chaobuleng skarn iron deposit and hence was chosen to determine the shrimp u pb age the results show that the granite intruded in 136 9 1 5 ma which is consistent to the mineralization age of 140 7 1 8 ma re os suggesting that the mineralization and the rock forming process occurred at the same time an analysis of regional metallogenic geological setting shows that mineralization of the metallogenic belt might have occurred in yanshanian period

ediacarian sponge spicule clusters from southwestern mongolia and the origins of the cambrian fauna carbon and strontium isotopic data are used to show that the earliest sponge spicule clusters and associated phosphatic sediments with anabarites from southwestern mongolia are of ediacarian age spicule morphologies include bundles of oxeas arranged in three dimensional quadrules linked together at junctions by tetracts pentacts hexacts or polyactines all are referred to the phylum porifera class hexactinellida these sponge spicules provide the oldest remains that can be assigned without question to an extant phylum and also the first firm evidence for filter feeding and metazoan silica biomineralization in the fossil record it is suggested that siliceous and phosphatic members of the cambrian fauna may have had their origins in eutrophic and outer shelf facies of the late proterozoic

isotopic analysis of the super large shuangjianzishan pb zn ag deposit in inner mongolia china constraints on magmatism metallogenesis and tectonic setting abstract the super large shuangjianzishan pb zn ag deposit is a newly discovered deposit located in the huanggang ganzhuermiao polymetallic metallogenic belt of inner mongolia ne china the deposit s resource includes 0 026 mt ag 1 1 mt pb and 3 3 mt zn the deposit is controlled by a nw trending ductile shear zone and ne and nw trending faults in black pelite assigned to the lower permian dashizhai formation lree enrichment hree depletion nb ta p and ti depletion and zr and hf enrichment characterize felsic magmatic rocks in the shuangjianzishan pb zn ag district the ages of porphyritic monzogranite rhyolitic crystal vitric ignimbrite and porphyritic granodiorite are 254 252 169 and 130 ma respectively pyrite sampled from the mineralization has re os isochron ages of 165 7 ma which suggest the mineralization is associated with the ca 169 ma magmatism in the shuangjianzishan district zircons extracted from the porphyritic granodiorite yield e hf t values of 11 34 to 1 41 with t dm2 dates of 1275 1901 ma the e hf t values of zircons in the rhyolitic crystal vitric ignimbrite and the ore bearing monzogranite porphyry are 7 57 16 23 and 10 18 15 96 respectively and their t dm2 ages are 177 733 and 257 632 ma respectively partial melting of depleted mantle resulted in the formation of the ca 254 252 ma ore bearing porphyritic monzogranite and the ca 169 ma rhyolitic crystal vitric ignimbrite dehydration partial melting of subducted oceanic crust resulted in the formation of the ca 130 ma porphyritic granodiorite the porphyritic monzogranite was emplaced during the late stages of closure of the paleo asian ocean during the transformation from a collisional to extensional tectonic setting the ca 170 and ca 130 ma magmatism and mineralization in the shuangjianzishan district are related to subduction of the mongolia okhotsk ocean and subduction of the paleo pacific ocean plate respectively

incipiently drowned facies within a cyclic peritidal ramp sequence early ordovician chepultepec interval virginia appalachians the chepultepec interval 145 to 260 m thick in virginia although dominated by peritidal carbonates contains thick subtidal sequences that formed during repeated incipient drowning of the early ordovician ramp cyclic peritidal sequences make up the lower member up to 150 m thick and upper member up to 85 m thick subtidal limestone and bioherms make up the middle member up to 110 m thick and pass landward into cyclic peritidal facies average long term subsidence rates were up to 4 cm 1 000 yr mature passive margin rates and ramp gradients commonly were extremely low 2 cm km computer modeling helped define mechanisms involved in formation of the stratigraphic sequence the peritidal sequences are upward shallowing cycles average period 140 000 to 200 000 yr of pellet and skeletal limestone pellet grainstone thrombolites intraclast grainstone and laminite caps these resulted from periodic sea level oscillations of several metres amplitude and possible slow long teim third order sea level fall superimposed on long term subsidence tidal flats were shifted far westward after each sea level rise but had sufficient time to prograde back across the ramp during sea level fall the middle number contains repeated sequences of subtidal skeletal limestone and pellet limestone with storm beds and burrowed units formed below fair weather wave base local rippled pellet limestone and thrombolite bioherms subwave base to wave agitated shoals that may have erosional tops overlain by thin grainstone sheets tidal flat facies are absent these sequences formed by periodic sea level rise more than 12 to 17 m amplitude which caused incipient drowning of the platform and migration of tidal flats several hundred kilometres back onto the platform these flats were unable to prograde onto the outer ramp during regression because of insufficient time and because seaward retreat of the shoreline outpaced progradation of tidal flats this left them abandoned on the inner ramp similar sequences reflecting incipient drowning are widespread in the geological record

nature and ore formation of the erdaohezi pb zn deposit in the great xing an range ne china abstract the erguna metallogenic belt is a large erdaohezi epithermal polymetallic sulfide pb zn deposit in the northern great xing an range that is associated with andesitic porphyry mineralization mainly occurs at the edges of andesitic porphyry and adjacent jurassic quartz porphyry and volcanic strata hydrothermal alteration minerals mainly include quartz sericite adularia chlorite and calcite while ore minerals include pyrite galena and sphalerite with minor argentite chalcopyrite and tetrahedrite three hydrothermal mineralization stages are recognized three categories of fluid inclusions fis are distinguished in quartz fluorite and sphalerite veins liquid rich gas rich and a few h2o co2 inclusions the stage i fis include all three inclusion types and present high homogenization temperatures 197 233 and salinities 0 7 6 8 wt nacl eq the stage ii fis mainly comprise liquid rich inclusions with a few h2o co2 types and present higher homogenization temperatures 167 190 and slightly higher salinities 0 9 7 6 wt nacl eq than those of stage i stage iii only contains liquid rich inclusions and presents the lowest homogenization temperatures 120 157 and salinities 0 5 4 8 wt nacl eq the ore forming fluids have low temperatures and salinities in an h2o nacl co2 ch4 system the o h isotope data 2 2 to 5 0 and 160 0 to 152 5 respectively suggest that ore forming fluids gradually evolved from mantle primary water to meteoric water sulfur strontium and lead isotopes imply a mixed crustal and mantle source for the ore forming materials sulfide rb sr dating yields a well defined isochron age of 130 5 3 6 ma from the main mineralization stage which is similar to the zircon age of 134 9 0 9 ma for the andesitic porphyry related to mineralization we therefore proposed that erdaohezi is a low sulfidation epithermal pb zn polymetallic deposit and fluid immiscibility was the dominant mechanism of ore deposition mineralization occurred in an early cretaceous back arc extensional setting that was related to the subduction of the paleo pacific plate

deep water stratigraphic cyclicity and carbonate mud mound development in the middle cambrian marjum formation house range utah usa in mid middle cambrian time shallow water sedimentation along the cordilleran passive margin was abruptly interrupted by the development of the deep water house range embayment across nevada and utah the marjum formation 330 m in the central house range represents deposition in the deepest part of the embayment and is composed of five deep water facies limestone argillaceous limestone rhythmites shale thin carbonate mud mounds bioturbated limestone and cross bedded limestone these facies are cyclically arranged into 1 5 to 30 m thick parasequences that include rhythmite mound rhythmite shale rhythmite bioturbated limestone and rhythmite cross bedded limestone parasequences using biostratigraphically constrained sediment accumulation rates the parasequences range in duration from 14 to 270 kyr the mud mounds are thin 2 m closely spaced laterally linked symmetrical domes composed of massive fenestral peloidal to clotted microspar with sparse unoriented poorly sorted skeletal material calcitized bacterial filaments tubes and abundant fenestrae and stroma tactoid structures these petrographic and sedimentological features suggest that the microspar peloids clots and syndepositional micritic cement were precipitated in situ from the activity of benthic microbial communities concentrated growth of the microbial communities occurred during periods of decreased input of fine detrital carbonate transported offshore from the adjacent shallow water carbonate platform in the neighbouring wah wah range and throughout the southern great basin coeval mid middle cambrian shallow water carbonates are composed of abundant metre scale upward shallowing parasequences that record high frequency 104 105 years eustatic sea level changes given this regional stratigraphic relationship the marjum formation parasequences probably formed in response to high frequency sea level fluctuations that controlled the amount of detrital carbonate input into the deeper water embayment during high frequency sea level rise and early highstand detrital carbonate input into the embayment decreased as a result of carbonate factory retrogradation resulting in the deposition of shale base of rhythmite shale parasequences or thin nodular rhythmites followed by in situ precipitated mud mounds lower portion of rhythmite mound parasequences during the ensuing high frequency sea level fall lowstand detrital carbonate influx into the embayment increased on account of carbonate factory pro gradation towards the embayment resulting in deposition of rhythmites upper part of rhythmite mound parasequences reworking of rhythmites by a lowered storm wave base cross bedded limestone deposition or bioturbation of rhythmites by a weakened lowered o2 minimum zone bioturbated lime stone deposition this interpreted sea level control on offshore carbonate sedimentation patterns is unique to palaeozoic and earliest mesozoic deep water sediments after the evolution of calcareous plankton in the jurassic the presence or absence of deeper water carbonates was influenced by a variety of chemical and physical oceanographic factors rather than just physical transport of carbonate muds

geology hydrothermal fluids h o s pb isotopes and rb sr geochronology of the daxintun orogenic gold deposit in heilongjiang province ne china abstract the newly discovered daxintun sb au deposit heilongjiang province ne china is located at the north eastern margin of the central asian orogenic belt and it is hosted by neoproterozoic to early cambrian two mica schist and marble the antimony bearing minerals include stibnite famatinite tetrahedrite boulangerite freibergite and minor andorite gold occurs largely as native gold microparticles 0 1 20 m with high fineness 974 983 accompanied by quartz stibnite calcite and pyrite fluid inclusions in ore bearing quartz and calcite phases mainly comprise liquid vapor and minor co2 bearing three phase types and the gas phases contain co2 ch4 and h2s the daxintun fluid inclusions homogenize at 111 8 353 4 c and are of low salinity 0 53 12 51 wt nacl equiv the 18osmow of the daxintun quartz falls into a wide range 4 2 16 9 yet the dsmow falls into a narrow range 108 0 to 87 3 suggesting that the ore forming fluids formed from metamorphic fluids that mixed with meteoric water the stibnite 34s ranges between 0 9 and 5 1 mean 2 8 the stibnite has 206pb 204pb of 18 234 18 306 207pb 204pb of 15 537 15 561 and 208pb 204pb of 38 103 38 227 implying a metamorphic fluid source the conditions of gold and antimony precipitation are comprised of neutral ph and fluid mixing with decreasing fs2 and fo2 zircon u pb dating of the fine grained daxintun granite yields a late jurassic age 154 0 2 0 ma and the daxintun diorite dike yields a weighted mean age of 122 1 1 3 ma stibnite rb sr dating suggests that the sb au mineralization has an age of approximately 152 8 2 4 ma the observed ore deposit geology as well as new data on the ore forming fluids h o s pb isotopes and rb sr geochronology suggest that the daxintun sb au deposit is best described as an orogenic type gold deposit that formed during subduction of the palaeo pacific and the mongolia okhotsk oceans during the late jurassic

petrogenesis and metallogenesis of the early cretaceous naoniushan cu dominated polymetallic deposit in the central great xing an range ne china abstract the naoniushan cu dominated polymetallic deposit in the central great xing an range is associated locally with pb zn ag or mo mineralization mineralization mainly occurs in the lower cretaceous continental volcanic rocks here we present new la icp ms zircon u pb ages whole rock geochemical data sr nd hf isotopic data and re os ages for the naoniushan deposit the emplacement of the porphyry occurred at 137 8 1 4 ma and the mineralization at 134 3 0 8 ma the granodiorite porphyry has high sio2 67 84 73 70 wt and k2o na2o 7 67 8 30 wt contents and low contents of mgo 0 60 1 04 wt mg 0 44 0 51 tfeo 1 29 2 16 wt cao 1 67 3 66 wt and k2o na2o 0 75 0 87 the porphyry is rich in large ion lithophile elements liles e g rb sr ba and k and light rare earth elements lrees without significant eu anomalies eu 0 84 1 01 and depleted in heavy ree hrees with lree hree 12 78 16 57 and la yb n 15 37 20 68 high field strength elements hfses e g nb ta and ti and siderophile elements e g cr co and ni these features are similar to those of adakitic rocks derived from thickened lower crust the naoniushan porphyry exhibits positive end t values ranging from 1 9 to 2 8 and relatively low initial 87sr 86sr ratios ranging from 0 70458 to 0 70497 most in situ zircon hf isotopic analyses show variable positive ehf t values ranging from 3 1 to 9 4 corresponding to relatively young two stage hf model ages ranging from 994 to 591 ma excluding one spot these isotopic features suggest that the primary magma of the naoniushan granodiorite porphyry was mainly derived from the partial melting of neoproterozoic juvenile lower crust comprising a mixture of 80 juvenile basalt and 20 ancient lower crust these elemental isotopic and geochronologic data combined with the regional geology deposit geology and fluid characteristics suggest that the naoniushan cu dominated polymetallic deposit formed in an evolving environment that decreased in compression over time due to slowing of the subduction of the paleo pacific plate beneath the eurasian plate and the transition from a compressional to an extensional tectonic regime following the closure of the mongol okhotsk ocean

permian magmatic sequences of the bilihe gold deposit in central inner mongolia china petrogenesis and tectonic significance abstract the bilihe gold deposit is located in the eastern section of the ondor sum yanji suture at the southern margin of the xing an mongolian orogenic belt xmob and the northern margin of the north china craton ncc central inner mongolia the magmatic rocks in the ore district are generally high k calc alkaline enriched in lree zr and hf and depleted in hree nb ta and p the magmatic evolution sequences are norite gabbro granodiorite porphyry granite or norite gabbro andesite dacite porphyry granodiorite which show a trend of decreasing tio2 feo mgo cao and p2o5 with increasing sio2 in the bilihe ore district hydrothermal processes were coeval with granitic magmatism for a period of 17 myr 272 255 ma the ages of the granite granodiorite porphyry granodiorite and dacite porphyry are 271 5 264 1 ma 269 8 255 8 ma 268 3 ma and 268 6 259 4 ma respectively the magmatic rocks contain magmatic hydrothermal and magmatic hydrothermal zircons the magmatic zircons have ce 4 la 2 5 the hydrothermal zircons have ce 3 ppm and smn lan

late ordovician brachiopod communities of southeast china the upper ordovician mid ashgill xiazhen and changwu formations in the zhejiang jiangxi border region of southeastern china contain an abundant and diverse suite of brachiopods a cluster analysis of 59 key samples from 180 collections over 23 000 specimens allowed delineation of eight distinct brachiopod communities herein named the ectenoglossa minor community the eospirifer praecursor community the ovalospira dichotoma community the antizygospira liquanensis sowerbyella sinensis community the kassinella shiyangensis community the altaethyrella zhejiangensis community the tcherskidium jiangshanensis community and the foliomena folium community these communities occur predominantly in limey clayey to silty mudstone facies similar to ziegler s early silurian brachiopod communities in the welsh borderland on the basis of lithological faunal and paleogeographical evidence the eight communities are interpreted to have distributed along an onshore offshore gradient from ba1 to ba6 setting

genesis of the zhengguang gold deposit in the duobaoshan ore field heilongjiang province ne china constraints from geology geochronology and s pb isotopic compositions abstract the zhengguang gold deposit in the duobaoshan ore field hosted in volcanic rocks of the middle ordovician duobaoshan formation is one of the largest gold deposits in the northeastern great xing an range of the central asian orogenic belt caob the deposit comprises the no i ii and iii ore zones with a total resource exceeding 35 tonnes of au 100 000 tonnes of zn and 100 tonnes of ag a genetic relationship between gold mineralization and concealed tonalite porphyry is inferred based on the characteristics of cryptoexplosive breccia and hydrothermal alteration indicative of porphyry type and epithermal mineralization zircon la icpms u pb dating reveals that the tonalite porphyry was emplaced at 462 1 1 8 ma middle ordovician the 34 s v cdt values of sulfide minerals range from 3 0 to 1 7 with an average of 2 33 indicating that sulfur was mainly derived from a magmatic source the pb isotopic compositions 206 pb 204 pb ranging from 17 572 to 17 629 207 pb 204 pb from 15 424 to 15 486 and 208 pb 204 pb from 37 206 to 37 418 suggest a major mantle component for pb and by inference for other ore metals therefore we suggest that the ore forming elements in the zhengguang gold deposit may be related to the mantle sourced tonalite porphyry on the basis of the geological characteristics and geochemical signatures documented in this study we conclude that the zhengguang gold deposit was formed in a porphyry to epithermal transitional environment associated with the concealed tonalite porphyry as part of the duobaoshan porphyry epithermal ore system that is related to the subduction of the paleo asian ocean during the ordovician

an unusual poriferan community in the albian of islares north spain palaeoenvironmental and tectonic implications abstract the discovery of a well preserved lower cretaceous siliceous sponge community in islares cantabria north spain is documented lower albian sponges crop out in great abundance as a rich necrocoenosis embedded in a marly matrix demosponges make up the bulk of an 11 m thick succession which is interpreted as having formed in a slope setting the sponges show body preservation and were buried in situ all types of growth forms are present they grew so tightly that they often fused secondarily shape polarity suggests steady low energy predominantly easterly palaeocurrents the main factor limiting the community seems to have been space competition a preliminary documentation of the systematic palaeontology is presented only taxa from the families tetracladinidae megamorinidae and corallistidae were found geometrical restoration of the platform basin transect allows estimation of a depth of approximately 40 m for the lithistid sponges sea level variations driven by block faulting controlled the available accommodation space sponges occur just below and above a sequence boundary and are associated with a major tectonic event linked with the opening of the bay of biscay

an upper ordovician sponge bearing micritic limestone and implication for early palaeozoic carbonate successions abstract a potentially new type of non reef sponge bearing micritic limestone is reported from the upper ordovician xiazhen formation of south eastern china the sponges are preserved as incomplete skeletons that consist of curved bifurcated and trifurcated spicules embedded in dark micrite and can only be recognized under a petrographic microscope the characteristics of the spicule networks suggest that the sponges are probably belonging to demosponges however based on the absence of features such as desma zygome a distinct dermal layer and a canal system they are not considered to be lithistids the majority of the sponges are found in a lime mudstone facies together with some micritic portions of wackestone to grainstone facies comprising approximately 13 of the 50 m thick micritic limestone successions it is interpreted that the non lithistid demosponges flourished on soft substrates over shallow marine carbonate platform such sponge bearing carbonates are similar to spiculites and spongolites in terms of being a major constituent of the sedimentary rocks and their potential contribution as sediment producers but affinity and modes of preservation of the xiazhen sponges are significantly different to those of the spiculites and spongolites in light of the present finding it is suggested that non lithistid demosponges may have been more widespread in early palaeozoic non reef carbonates than has previously been recognized thus indicating the critical need for further detailed studies if we are to understand their distributions and sedimentological contributions

sm nd dating and ree composition of scheelite for the honghuaerji scheelite deposit inner mongolia northeast china abstract sm nd analyses of seven scheelite samples from scheelite quartz veins in the honghuaerji scheelite deposit produce a well defined linear array on an isochron diagram with a mean square weighted deviation mswd of 0 87 corresponding to an age of 178 4 2 9 ma with end t 1 50 this age is interpreted to represent the age of scheelite mineralization the scheelite sm nd age is in good agreement with u pb ages obtained from a mineralization related granite 179 4 178 6 ma indicating that there is no apparent age gap between granite crystallization and ore formation rare earth element ree abundances in the scheelite were determined by in situ laser ablation inductively coupled plasma mass spectrometry la icp ms and the scheelite samples contain elevated ree concentrations with total ree y contents in the range of 3339 to 6321 ppm the chondrite normalized ree distribution patterns of all scheelites are middle ree mree enriched with strong negative eu anomalies eu eu 0 09 0 23 the ree characteristics of the honghuaerji scheelite suggest that ree 3 substituted into the ca site along with na and nb dominated by na whereas eu is predominantly present as eu 2 in the scheelite and may have crystallized from relatively reduced fluids all sulfur isotope data 34 s for sulfide separates range from 2 0 to 3 8 with an average of 3 2 measured 206 pb 204 pb 207 pb 204 pb and 208 pb 204 pb ratios vary from 18 243 to 18 451 15 494 to 15 574 and 37 933 to 38 340 respectively on the basis of common sr nd pb hf isotopic characteristics between the scheelite and the host granite and the positive initial nd isotope ratios 1 46 1 52 low 87 sr 86 sr ratios 0 704983 0 705297 in the scheelites it is inferred that the hydrothermal fluids responsible for tungsten mineralization at honghuaerji were of magmatic origin with a mantle derived signature based on the regional geology tectonic evolution and geochemical evidence the honghuaerji granite is thought to have been derived from the partial melting of thickened lower crust produced during post collisional orogenic events associated with the closure of the mongol okhotsk ocean

rocket and balloon observations of electric field in two thunderstorms instruments that measure the intense electric field strengths in thunderclouds 100 kv m 1 are designed to minimize the production of ions by small electrical discharges coronas emanating from the instruments themselves the nearby charge of these ions would unpredictably disturb the natural field of the cloud in an attempt to assess this disturbance two different instruments one carried by a rocket and one carried by a balloon were launched on two occasions into thunderstorms in spite of differing trajectories the soundings were similar which gives us some confidence in both instruments in addition the measurements revealed some interesting features of the two storms each storm appeared to have six significant and distinct regions of charge the balloon soundings also revealed that lightning flashes temporarily increased the electric field strength above the thunderclouds at altitudes from 9 7 to 14 3 km by amounts up to 10 kv m 1 after which the fields decayed away in 50 to 125 s one pair of ascent and descent rocket soundings separated in time by a maximum of 60 s and horizontally by 1 to 3 km showed little change in the thunderstorm electric field between ground and 7 5 km altitude

new paleomagnetic results from northern china collision and suturing with siberia and kazakhstan abstract we report new results from our paleomagnetic studies of the inner mongolia autonomous region in northeastern china and the xinjiang autonomous region in northwestern china primary magnetizations are isolated in tertiary cretaceous late jurassic and late permian rocks and secondary components of magnetization of late jurassic and recent ages are seen in strata of inner mongolia western junggar basin and part of the tarim block suffered widespread complete tectono thermal remagnetization at about 280 ma permian the paleomagnetic evidence suggests that the sampled areas in inner mongolia have been part of the north china block at least since late permian time and the junggar block collided with the tarim block by late permian time our own paleomagnetic results together with available data from other parts of eastern eurasia suggest a tectonic model in which kazakhstan europe and siberia collided with each other and formed the europe siberia kazakhstan plate eur sib kaz in late permian time to the east mongolia outer mongolia inner mongolia and the north china block were also wedged together forming the mongolia north china plate mon nc by this time these two composite continental plates were not a stable unit in the late permian the geometrical relationship of the paleomagnetic poles for these two plates suggests that collision initially occurred in the late permian near the western end 50 n 75 e of the southern siberian fold belt and progressed eastward as the mon nc plate rotated and clockwise about 117 relative to the eur sib kaz plate during the early mesozoic suturing and rotation were probably complete by the late jurassic significant relative motion between the tarim and siberia blocks continued from early triassic to late jurassic as tarim moved northeastward 1400 km relative to siberia

untangling intricate microbial sponge frameworks the contributions of sponges to early ordovician reefs abstract the early ordovician witnessed an increased influence of skeletal organisms on the construction of microbial dominant reefs which preceded the middle to late ordovician expansion of skeletal dominant reefs skeletal frameworks of early ordovician reefs built by stromatoporoids bryozoans pelmatozoans and calathiids have recently been reported in this study we document the occurrence of localized archaeoscyphia frameworks and additional unidentified non anthaspidellid spiculate sponges that played diverse roles in the construction of microbial siliceous sponge reefs preserved in the early ordovician dumugol formation korea which have not been previously described in coeval reefs elsewhere two types of boundstone textures are recognized microbial dominated boundstone and sponge microbial boundstone microbialites were primarily responsible for reef construction making up about one third of reefs both as framework builders and as encrusters of other reef components the archaeoscyphia and spiculate sponges contributed in various ways to reef development archaeoscyphia constituting about one fourth of the dumugol reefs mainly constructed sponge microbial boundstone together with encrusting microbialites clusters of archaeoscyphia individuals sporadically formed centimeter sized sponge framestone 3 of reefs with primary cryptic spaces spiculate sponges 4 of reefs a previously unknown component of early ordovician reefs also played diverse roles such as encrusting other constituents dwelling in cryptic spaces stabilizing reef flank sediments and even building frameworks such framework building siliceous sponges together with previously reported skeletal frameworks in other early ordovician reefs reflect an early phase of the transition from microbial to skeletal dominated reefs in the early paleozoic

re os sulfide chalcopyrite pyrite and molybdenite systematics and fluid inclusion study of the duobaoshan porphyry cu mo deposit heilongjiang province china abstract the duobaoshan porphyry deposit located in the northwestern part of the lesser hinggan range is one of the biggest porphyry cu mo deposits in the central asian orogenic belt in china the duobaoshan porphyry deposit occurs in granodiorite and volcanic rocks of the middle ordovician duobaoshan formation six types of veins have been identified in three ore forming stages as follows a quartz potassic feldspar vein in the early ore forming stage an early stage quartz molybdenite vein late stage quartz molybdenite and quartz chalcopyrite pyrite veins in the middle ore forming stage and quartz pyrite and calcite quartz veins in the late ore forming stage the following four types of fluid inclusions are distinguished from various quartz veins two phase aqueous pure gas phase co 2 bearing and daughter mineral bearing inclusions the ore forming fluid for the early ore forming stage belongs to the h 2 o co 2 nacl system which is characterized by high temperatures 550 c intermediate salinities 16 2 18 1 wt nacl eqv and high co 2 content the ore forming fluid from the middle ore forming stage evolved to the h 2 o co 2 nacl system which is characterized by intermediate to high temperatures 230 450 c and high low salinities 0 8 to 65 3 wt nacl eqv and is also rich in co 2 and metals the ore forming fluid finally reached cool temperatures 110 200 c low salinities 3 9 8 4 wt nacl eqv and was co 2 poor intensive fluid immiscibility or boiling occurred when the ore forming fluid with temperatures of 230 450 c and pressures of 10 41 mpa ascended to 4 1 km inducing the escape of co 2 depressing the solubility of fluid and depositing abundant metal sulfides the total re and os concentrations of chalcopyrite and pyrite range from 0 15 to 2 95 g g and 0 74 to 15 01 ng g respectively analyses of seven chalcopyrite and pyrite samples yielded isochron ages of 482 486 ma and the model age of one molybdenite sample is 485 6 3 7 ma the mineralization age of the duobaoshan deposit is between 482 and 486 ma and mantle fluid and materials were involved in the mineralization process the duobaoshan deposit is a classic porphyry cu mo deposit that formed in an early palaeozoic island arc tectonic setting

deep water hexactinellid sponge mounds from the upper triassic of northern sichuan china a belt of upper triassic carnian sponge mounds was discovered in northern sichuan in the transition zone between a shallow shelf sea in the se chuan dian shelf and a deeper marine trough ganzi aba trough in the nw individual sponge mounds are up to 60 m high and 150 m wide and interfinger laterally with dark shales main frame builders of the mounds are lychniscosa and lyssacinosa accompanied by much rarer hexactinosa lithistida annelid worms occasional crinoids and bryozoa the reef dwelling fauna consists of brachiopods ostracods foraminifera pelecypods gastropods and ammonites typical shallow water reef builders scleractinian corals and calcareous algae are lacking or extremely rare coralline sponges this supports the interpretation of the mounds as buildups in deeper water of several tens up to one hundred meters the sponge mounds of northern sichuan are thus an equivalent of similar buildups of late jurassic age in central and western europe they provide the first well established record of lychniscosa in the triassic and their first reported pre tertiary occurrence outside of europe

ore geology fluid inclusion and 40ar 39ar geochronology constraints on the genesis of the yingchengzi gold deposit southern heilongjiang province ne china abstract the yingchengzi gold deposit located at the eastern margin of the zhangguangcai range southern heilongjiang province ne china is the only high grade gold deposit in the region the deposit is hosted in mylonitized paleozoic granite and metavolcanic rocks and is structurally dominated by brittle ductile shear zones and faults the orebodies consist of gold bearing quartz veins and gold bearing altered mylonites the hydrothermal ore forming process can be divided into early middle and late stages characterized by quartz pyrite veins quartz gold polymetallic sulfide veins and quartz carbonate veins or veinlets respectively ore minerals and gold mainly occurred in the middle stage four types of fluid inclusions were identified in the vein minerals based on petrography and laser raman spectroscopy nacl h 2 o w type co 2 h 2 o c type pure co 2 pc type and less common calcite bearing s type inclusions the early stage minerals contain c type and rare pc type fluid inclusions which have homogenization temperatures of 262 400 c and low salinities of 2 20 8 51 wt nacl equiv only the middle stage minerals contain all four types of fluid inclusions with homogenization temperatures of 173 316 c and salinities of 2 20 12 42 wt nacl equiv the late stage quartz and calcite crystals contain only w type fluid inclusions which have homogenization temperatures of 104 207 c and salinities of 1 05 6 58 wt nacl equiv fluid boiling and the mixing caused the rapid precipitation of sulfides and gold these indicate that the ore forming fluid system evolved from co 2 rich metamorphogenic to co 2 poor due to the boiling and input of meteoric water the trapping pressures estimated from c type fluid inclusions are 98 245 mpa in the main ore forming stage giving a reasonable estimate that an alternating lithostatic hydrostatic fluid system controlled a fault valve activity at a depth of 9 10 km 40 ar 39 ar dating of hydrothermal sericite from gold bearing altered rock shows that gold mineralization occurred at 248 ma we assume that the yingchengzi gold deposit was formed in a late permian early triassic continental collision regime following closure of the paleo asian ocean the regional geology ore geology fluid inclusions mineralization age and tectonic setting infer that the yingchengzi gold deposit is a mesozonal orogenic gold system making it the first orogenic gold deposit discovered in the zhangguangcai range ne china

ordovician reef systems and settings in south china before the late ordovician mass extinction abstract ordovician reefs of the south china block occur chiefly in three stratigraphic units 1 the middle tremadoc fenhsiang formation and the upper part of the lunshan formation 2 the late tremadoc hunghuayuan formation and 3 the middle ashgill xiazhen sanjushan and daduhe formations the region therefore records part of the early palaeozoic reef expansion episode and permits a broader assessment of change in ordovician reef facies during middle tremadoc time lithistid sponge bryozoan calathium calcimicrobially dominated patch reefs of the fenhsiang formation occur in the high energy belt of the central yangtze platform columnar nonskeletal stromatolites of the lunshan formation coeval with fenhsiang formation occur on the southeastern margin of the platform where low diversity reef attached organisms and the generally fine grained character of the sediment are consistent with a depth below normal wave base calathium and lithistid sponges were the principal reef builders of late tremadoc reefs and were widespread in the areas of yichang hubei province and dongzhi anhui province from the platform centre to its margin settings bryozoan reef builders occur only in the platform centre with a high diversity of reef dwellers such as brachiopods trilobites and nautiloids however during the same time at the platform margins microbes played an important part in reef building together with calathium and lithistid sponges the middle late tremadoc lithistid sponge bryozoan calathium microbial community was replaced by a middle ashgill coral stromatoporoid community and shows that the succession of community replacement during the ordovician took a relatively long time restricted by the black shales of the main part of the yangtze region middle ashgill reef complexes can be found only on the northeast platform of cathaysian land between the yushan jiangxi and changshan zhejiang regions carbonate mudmounds are present on the western margin of the yangtze platform on the northeast platform of cathaysian land patch reefs some higher relief with talus and biostromes of the xiazhen formation consist of high diversity biotas of corals stromatoporoids calcimicrobes brachiopods and gastropods the sanjushan formation is age equivalent to the xiazhen formation and contains carbonate mudmounds that are composed of abundant calcareous algae and calcimicrobia uplift forced a northward extension of cathaysian land and caused a regional relative sea level fall eliminating this reef complex prior to the first extinction event of the late ordovician the daduhe formation carbonate mudmounds occur in the nearshore belt of kangdian land western margin of the yangtze platform and are paraconformably overlain by the hirnantian nancheng formation in total the ordovician reefs of south china show a range of habitats and controls on growth and demise

a review of the genesis geochronology and geological significance of hydrothermal copper and associated metals deposits in the great xing an range ne china abstract the great xing an range located in the eastern mongolia hinggan orogenic belt ne china hosts numerous endogenous base and precious metal mineral deposits the hydrothermal copper and associated metals deposits in this area can be divided into three genetic types based on their geology and geochronology porphyry cu mo high sulfidation cu ag and cu sn epithermal and cu fe skarn all of these mineral deposits barring the cu sm epithermal deposits are closely related to high k calc alkaline i type granitic magmatism the eastern part of the area hosts porphyry copper deposits that generally formed around 180 170 ma with a few deposits formed at 485 ma whereas high sulfidation epithermal copper silver deposits in the central great xing an range formed at 245 ma the west of the study area hosts high sulfidation epithermal cu sn deposits that formed at 150 135 ma and are related to a type granites skarn deposits are associated with the epithermal and porphyry deposits described above and formed at 180 160 and 150 135 ma the geodynamic setting of the region during these mineralizing events is consistent with early paleozoic collision between the xing an massif and the songnen terrane late permian collision between the north china craton ncc and the heilongjiang plate middle jurassic collision between the siberian plate and the ncc epicontinental aggradational belt and crustal extension and thinning during an early cretaceous collisional orogenic event this indicates that the mineral deposits formed in an intracontinental transitional orogenic or post orogenic extensional tectonic setting

origin of the wunugetushan porphyry cu mo deposit inner mongolia ne china constraints from geology geochronology geochemistry and isotopic compositions abstract the wunugetushan porphyry cu mo deposit is located in the southeastern margin of the mongol okhotsk orogenic belt and in the northwestern segment of the great xing an range ne china the orebodies of this deposit are mainly hosted in the monzogranitic porphyry stock and in contact with the granitic porphyry dyke and biotite granite batholith the shrimp zircon u pb dating of the granitic porphyry dyke yielded ages of 201 4 3 1 ma 2 mswd 1 5 these results indicate that the magmatism in the wunugetushan area might have occurred at ca 201 ma in the early jurassic and that the mineralization age ca 181 ma of this deposit is later than the age of intrusive granitic porphyry in the area geochemically the wunugetushan granitoids belong to high k calc alkaline and shoshonitic series enriched in k rb nd and pb and depleted in sr nb ti and p with negative eu anomalies in situ hf isotopic analyses of zircons using la mc icp ms indicate that the ehf t values for zircons from a granitic porphyry sample vary from 2 4 to 11 8 and that the corresponding crustal model ages tdmc vary from 483 to 1088 ma the least altered monzogranitic porphyry granitic porphyry and biotite granite yielded relatively uniform end t values from 1 0 to 0 6 and low 87sr 86sr i ratios ranging from 0 704387 to 0 708385 the geochemical and sr nd hf isotopic data for the granitoids indicate that the source magma for these rocks could be derived from a juvenile lower crust the 34s values of sulfides show a narrow range 0 76 to 3 20 similar to those of magmatic sulfur further implying a lower crust origin based on the results of this study and the regional geodynamic evolution it is proposed that the formation of the wunugetushan deposit and associated granitoids should be linked to the southeastward subduction of the mongol okhotsk oceanic plate beneath the erguna massif during the early jurassic and that the monzogranitic porphyry intrusions in wunugetushan area probably provided important ore metals responsible for the large scale cu mo mineralization

isotopic c o s geochemistry and re os geochronology of the haobugao zn fe deposit in inner mongolia ne china abstract the haobugao zn fe deposit is a typical skarn deposit located in the southern part of the great xing an range that hosts polymetallic mineralization over a large region the main ore minerals at the deposit include sphalerite magnetite galena chalcopyrite and pyrite and the main gangue minerals include andradite grossular garnet hedenbergite diopside ilvaite calcite and quartz there are broadly two mineralizing periods represented by the relatively older skarn and younger quartz sulfide veins in detail there are five metallogenic stages consisting of an early skarn late skarn oxide early quartz sulfide and late quartz sulfide calcite stages electron microprobe analyses show that the garnet at the deposit varies in composition from and97 95gro0 41pyr1 64 to and30 69gro66 69pyr2 63 and pyroxene is compositionally in the diopside hedenbergite range i e di90 63hd8 00jo1 37 hd88 98di4 53jo6 49 petrographic observations and electron microprobe analyses indicate that the sphalerite has three generations zn0 93fe0 08 s zn0 75fe0 24 s the zn associated with the first generation sphalerite replaced cu and fe of early xenomorphic granular chalcopyrite i e cu1 01fe1 03 s2 cu0 99fe0 99 s2 and part of the first generation sphalerite is coeval with late chalcopyrite i e cu0 96fe0 99zn0 03 s2 cu1 00fe1 03zn0 01 s2 magnetite has a noticeable negative ce anomaly ce 0 17 to 0 54 which might be a result of the oxidized ore fluid thirty 34sv pdb analyses of sulfides from the ore range from 2 3 to 0 1 in value which are indicative of a magmatic source the 13c and 18o values for calcite from the ore formed at quartz sulfide calcite stage vary from 9 9 to 5 5 and from 4 2 to 1 1 respectively contrasting with 13c 2 9 4 8 and 18o 9 8 13 9 values for calcite from marble it is suggested that the ore forming fluid associated with late stage of mineralization was predominantly magmatic in origin with some input of local meteoric water molybdenite from the haobugao deposit defines an isochron age of 142 1 ma which is interpreted as the mineralization age being synchronous within error with the zircon u pb ages of 140 1 141 2 and 141 1 ma for granite at the deposit these data and characteristics of lithology and mineralization further show that the zn fe mineralization is temporally and spatially related to the emplacement of granite in an extensional tectonic setting during the mesozoic

middle cambrian siliceous sponge calcimicrobe buildups daegi formation korea metazoan buildup constituents in the aftermath of the early cambrian extinction event abstract numerous decimetre to metre scale carbonate buildups dominated by siliceous sponges and the calcimicrobe epiphyton are reported from the middle cambrian series 3 daegi formation of korea these siliceous sponge epiphyton buildups consist predominantly of grey micritic boundstones with dark clots and or white clumps the boundstones contain sponge spicule networks interpreted as the calcified remains of siliceous sponges the white clumps and dark clots in the boundstones represent variously preserved epiphyton siliceous sponges form constructional pore space and are commonly encrusted by epiphyton the sponges were probably the primary frame builders providing substrates for the attachment and subsequent growth of epiphyton epiphyton is considered to be a binder when covering the surface of siliceous sponges and a subordinate frame builder when filling depositional voids created by siliceous sponges or growing on top of other epiphyton growth bundles the siliceous sponge epiphyton buildups of the daegi formation show similarities to previously described late cambrian furongian anthaspidellid calcimicrobe buildups from iran and the usa together with recently reported examples from the zhangxia formation of eastern china the sponge epiphyton buildups from korea represent some of the oldest metazoan calcimicrobe buildups after the extinction of most archaeocyaths at the end of the early cambrian series 2 this implies that the incorporation of metazoans in middle cambrian series 3 carbonate buildups occurred much earlier than previously known the buildup forming siliceous sponges described in this study demonstrate that their role in early phanerozoic carbonate buildups has been grossly underestimated

sources of phanerozoic granitoids in the transect bayanhongor ulaan baatar mongolia geochemical and nd isotopic evidence and implications for phanerozoic crustal growth abstract the central asian orogenic belt caob is renowned for massive generation of juvenile crust in the phanerozoic mongolia is the heartland of the caob and it has been subject to numerous investigations particularly in metallogenesis and tectonic evolution we present new petrographic geochemical and sr nd isotopic analyses on phanerozoic granitoids emplaced in west central mongolia the data are used to delineate their source characteristics and to discuss implications for the phanerozoic crustal growth in central asia our samples come from a transect from bayanhongor to ulaan baatar including three tectonic units the baydrag cratonic block late archean to middle proterozoic the eo cambrian bayanhongor ophiolite complex and the hangay hentey basin of controversial origin the intrusive granitoids have ages ranging from ca 540 to 120 ma the majority of the samples are slightly peraluminous and can be classified as granite s s including monzogranite syenogranite and alkali feldspar granite most of the rocks have initial 87sr 86sr ratios between 0 705 and 0 707 late paleozoic to mesozoic granitoids 250 ma are characterized by near zero end t values 0 to 2 whereas older granitoids show lower end t values 1 5 to 7 the data confirm the earlier observation of kovalenko et al geochemistry international 34 1996 628 who showed that granitoids emplaced outside of the pre riphean basement rocks are characterized by juvenile positive end t values whereas those within the pre riphean domain and the baydrag cratonic block as for the present case show a significant effect of contamination by precambrian basement rocks nevertheless mass balance calculation suggests that the granitoids were derived from sources composed of at least 80 juvenile mantle derived component despite our small set of new data the present study reinforces the general scenario of massive juvenile crust production in the caob with limited influence of old microcontinents in the genesis of phanerozoic granitoids

the lithosphere structure of the great xing an range in the eastern central asian orogenic belt constrains from the joint geophysical profiling abstract the great xing an range is characterized by a complex crustal history involving the successive development of several paleozoic to mesozoic tectonic domains a set of new different geophysical data including deep seismic reflection profile seismic refraction wide angle reflection profile and magnetotelluric survey were conducted recently to establish the lithosphere structure and geodynamic framework of the great xing an range and adjacent basins results based on the interpretation of the integrated geophysical data include the discovery of the crocodile style assembly fabric of the songnen and xing an blocks the intrusion of the great xing an range continental crust into the songnen block while the mantle lithosphere of the songnen block beneath the xing an block the great xing an range experienced the delamination and the underplating which is very common in the eastern part of central asian orogenic belt the building of the great xing an range could divide into two phases a the crustal shortening and overlapping resulted from the compression between the songnen and xing an blocks in late devonian to early carboniferous b the vertical building by the upwelling and underplating of the magmas in mesozoic and cenozoic

a late ordovician high energy temperate water carbonate ramp southern quebec canada implications for late ordovician oceanography the trenton group late ordovician the youngest carbonate unit in the taconic foreland basin of southern quebec is a tripartite unit with a distinctive coarse grained middle part the deschambault formation lithofacies of the deschambault formation are dominated by coarse grained bioclastic intraclastic limestones finer grained lithofacies are ubiquitous but subordinate the complete spectrum of lithofacies indicates sedimentation ranging from above fairweather to below storm wave base skeletal components are indicative of the modern temperate water bryomol association non skeletal elements are represented by peloids and intraclasts accretion rates from areas of continuous sedimentation were low 14 cm 103 years from sedimentological and faunal evidence it is proposed that the late ordovician deschambault ramp was bathed by temperate waters the model compares favourably with modern cool water shelves rimming the southern edge of the australian continent palaeomagnetic data locate southern quebec in a low latitudinal setting during the late ordovician upper ordovician facies distribution in eastern canada and progressive disappearance of some faunal provinces through late ordovician time are used to conclude that the initiation of the late ordovician glaciation that covered most of gondwana was instrumental in easing northward movement of cold oceanic currents this resulted in the rapid contraction of the southern hemisphere warm water tropical belt from a 30 latitudinal wide zone in the early caradoc to a 15 zone in the late caradoc

palaeogeographical influence on late eocene biosiliceous sponge rich sedimentation southern western australia late eocene nearshore shallow marine environments within the bremer and western eucla basins of southern western australia were characterized by the thick deposition of spongolite and spiculite deposits epibenthic sponge communities dominated estuaries and topographically complex basin margin embayments archipelagos while cool water carbonates with up to 10 sponges accumulated in open shelf environments the transition from a biosiliceous to calcareous epibenthos was related to the degree of palaeogeographical protection within basement protected embayments there was an offshore gradation from shoreface spongolite and pure spiculite to a muddy spiculite facies towards central embayment areas calcareous fossils are rare throughout embayment facies but rapidly increase in more open outer archipelago areas this depositional relationship occurred along 2000 km of the late eocene southern australian coastline palaeogeographical protection from strong currents acted in concert with 1 a planar low gradient inland topography with sluggish run off supplying fine grained sediment nutrients and abundant dissolved silica and 2 a microtidal setting weak to moderate swells and opposing wind and coriolis surface current forcing which inhibited water exchange between embayments estuaries and the open shelf this situation led to an embayment water chemistry that encouraged prolific sponge growth calcareous spiculites record the mixing front between these embayment waters and normal open shelf waters supporting cool water carbonates

geochronology and geochemistry of the wunugetushan porphyry cu mo deposit in ne china and their geological significance abstract the wunugetushan porphyry cu mo deposit is located in the manzhouli district of ne china on the southern margin of the mesozoic mongol okhotsk orogenic belt concentric rings of hydrothermal alteration and cu mo mineralization surround an early middle jurassic monzogranitic porphyry the cu mo mineralization is clearly related to the quartz potassic and quartz sericite alteration molybdenite re os and groundmass 40 ar 39 ar of the host porphyry dates indicate that the ore formation and porphyry emplacement occurred at 177 6 4 5 ma and 179 0 1 9 ma respectively geochemically the host porphyry of the deposit is characterized by strong lree hree fractionation enrichment in lile ba rb u th and pb and depletion of hfse nb ta ti and hree the sr nd pb isotopic compositions of the porphyry display an varied initial 87 sr 86 sr i ratio a positive e nd t values and high 206 pb 204 pb t 207 pb 204 pb t and 208 pb 204 pb t ratios these data indicate that the magmatic source of the host porphyry comprised two end members lithospheric mantle metasomatized by fluids derived from the subducted slab and continental crust we infer that the primitive magma of the host porphyry was derived from crust mantle transition zone based on regional geology and geochemistry of the host porphyry the wunugetushan deposit is suggested to form in a continental collision environment after closure of the mongol okhotsk ocean

tetradiid siliceous sponge patch reefs from the xiazhen formation late katian southeast china a new late ordovician reef association decimetre scale domal to tabular patch reefs consisting of dendroid tetradiids are found in the late ordovician late katian xiazhen formation southeast china it is intriguing to observe that siliceous sponges occur together with the tetradiids in many patch reefs as this has not been previously reported the tetradiid rhabdotetradium jiangshanense are preserved in situ and the siliceous sponges where present as spicule networks calcimicrobe ortonella encrusting tetradiid tubes and burrows filled with faecal pellets also constitute as subordinate components of the patch reef the tetradiid tubes are regarded as frame builders that are commonly surrounded and encrusted by the siliceous sponges the sponges wherever observed form the bulk of the patch reef and are regarded as a primary constructor as well as binder of the patch reef burrows filled with faecal pellets often penetrate the area between tetradiids and sponge spicule networks in the boundstone and these are considered to be the loci of dweller where the sponges and tetradiids co occur the tetradiid tubes are sparsely distributed and commonly display non quadripartite fission i e bipartite or tripartite in contrast where the sponges are rare in the patch reef the tetradiid tubes are densely spaced and predominantly display quadripartite fission this indicates that the tetradiid growth was influenced by the presence of siliceous sponges this finding of the new late ordovician reef association and their mutual interplay extend our understanding of the early palaeozoic carbonate reefs it reaffirms the need for multidisciplinary studies of carbonate buildups where similar interactions with siliceous sponges might have been overlooked

a type granites in northeastern china age and geochemical constraints on their petrogenesis abstract a type granites are widely distributed in northeastern china ne china they were emplaced during three major episodes the permian late triassic to early jurassic and early cretaceous and evolved in different tectonic regimes according to their mineralogical and geochemical characteristics two subgroups of a type granites aluminous and peralkaline can be recognized the peralkaline subgroup contains alkali mafic minerals such as riebeckite arfvedsonite and sodic pyroxene while the aluminous subgroup contains annite and fe rich calcic or sodic calcic amphibole with respect to the aluminous subgroup the peralkaline granites contain higher rb ga and total rare earth elements ree but lower mgo cao al 2 o 3 ba and sr based on the discrimination criteria of eby geology 20 1992 641 the permian and late triassic to early jurassic a type granites belong to the a 2 post orogenic type whereas the early cretaceous granites are of the a 1 anorogenic type nd isotopic compositions of these a type granites indicate their derivation from a dominantly juvenile crustal source their origin is thought to have involved partial melting of an underplated lower crustal source because the generation of a type granites requires high melting temperature we propose models involving slab break off lithospheric delamination and extension the permian a type granites have the same age range as those in eastern junggar southern mongolia and central inner mongolia they occur along a major suture and form a narrow belt between the north china and siberian cratons we suggest that their formation was associated with post collisional slab break off the late triassic to early jurassic a type granites are likely to be the product of lithospheric delamination after the final collision of the major crustal blocks in the late paleozoic to early triassic the early cretaceous a type granites have an anorogenic affinity and were possibly associated with rifting in eastern china at this time associated with the onset of paleo pacific subduction consequently we conclude that the a type granites in ne china were generated at three different times involving multiple processes operative in different tectonic environments

mio pliocene adakite generation related to flat subduction in southern ecuador the quimsacocha volcanic center abstract new geochemical and geochronological data on the miocene pliocene quimsacocha volcanic center qvc have led to the recognition of adakitic lavas generated by slab melting related to the flat slab subduction in southern ecuador and northern peru the qvc located in the presently inactive southern part of the ecuadorian arc was built up during three distinctive volcanic phases the first phase generated a basal edifice with mainly andesitic lava flows while the second phase is characterized by the emplacement of cryptodomes domes and related outflow breccias comprised of andesites and some dacites the last phase released rhyolitic ignimbrites associated with the formation of a large caldera which was later partly filled by dacitic rhyolitic domes geochemical data for the qvc indicate higher al2o3 tio2 na2o zr and sr contents and lower fe2o3 mgo y mree and hree abundances compared to other eruptive rocks of the plio quaternary volcanic front of ecuador such geochemical features as well as the frequent presence of an associated epithermal gold deposit are characteristic of the involvement of slab melts also known as adakites 1 2 in the generation of these magmas after a calc alkaline arc magmatism phase slab horizontalization in response to the subduction of a buoyant oceanic plateau results in increased involvement of a slab melting component in the magmas produced however pristine adakites were generated and emplaced during a relatively short period as indicated by zircon fission track ages then volcanic activity stopped and a volcanic gap formed the identification of these adakites their location and age support a model of slab melting associated with flat slab subduction m a gutscher et al geology 28 2000 535 538