paleoseismology along the 1980 surface rupture of the irpinia fault implications for earthquake recurrence in the southern apennines italy the irpinia fault was the source of the ms 6 9 1980 irpinia earthquake and produced the first unequivocal historical surface faulting in italy trenching of the 1980 fault scarp at piano di pecore a flat intermontane basin about 5 km south of the 1980 instrumental epicenter provides the first data on earthquake recurrence intervals slip per event and slip rate on a major normal fault in the southern apennines fault zone the trenches exposed evidence of four pre 1980 paleoearthquakes that occurred during the past 8600 years a best estimate average recurrence interval is 2150 years although the time interval between individual events varies by as much as a factor of 2 each paleo earthquake is similar to the 1980 surface rupture in amount of slip and style of deformation which suggests that the 1980 event is characteristic for the irpinia fault slip per event values average 61 cm the net vertical displacement of 2 12 2 36 m since 8600 cal year b p observed in the trenches gives a vertical slip rate of 0 25 0 35 mm yr a dip slip rate of 0 29 0 40 mm yr and an extension rate of 0 14 0 20 mm yr although fault behavior data are only available for the irpinia fault they provide a starting point for evaluating earthquake recurrence and rates of deformation in southern apennines they suggest that 1 fault specific earthquake recurrence intervals based on the historical seismic record overestimates the occurrence of large magnitude m7 earthquakes and 2 the holocene rate of extension across the apennines is 1 mm yr the 1980 earthquake and the paleoseismologic observations show that repeated and localized surface faulting occurs in southern apennines and leaves subtle but distinct geomorphic evidence that can be detected with detailed and careful investigation

a shrimp u pb chronological study of zircons from quartz veins of yangshan gold deposit gansu province the yangshan gold deposit is a supperlarge fine grained disseminated gold deposit in southern gansu province in this paper cathodoluminescence image cl and ion probe u pb dating shrimp techniques were used to determine the metallogenic age it is found that zircons from quartz veins of fine grained disseminated gold ores show characteristics of magmatic zircon with pillar idiomorphism rhythmic crystallization and main th u ratio of 0 5 1 5 three main populations of zircons were obtained giving average 206 pb 238 u ages of 197 6 1 7 ma 126 9 3 2 ma and 51 2 1 3 ma respectively the first age corresponds to k ar age of the plagiogranite dike while the last two ages indicate that buried cretaceous and tertiary intrusives exist in the ore district suggesting that the yangshan gold deposit is genetically related to the three times of magmatic hydrothermal activities by contrast zircons from quartz vein type gold ores in the ore district are much older than the host rock indicating that quartz veins were formed earlier and were not contaminated by later magmatic fluids it is concluded that coupling of multiperiodic hydrothermal activities in the ore district contributed to mineralization of the yangshan gold deposit

a geodynamic model of the southern apennines accretionary prism the apennines comprise a neogen quaternary accretionary prism that shows several anomalies with respect to classic alpine type mountain belts namely i low elevation ii a shallow new moho below the core of the belt iii high heat flow in the internal parts iv mainly sedimentary cover involved in the prism v a deep foredeep and vi a fully developed back arc basin the suction exerted by a relatively eastward migrating mantle can determine the eastward retreat of the subduction zone and an asthenospheric wedging at the retreating subduction hinge heat flow geochemical and seismological data support the presence of a hot mantle wedge underlying the western side of the apenninic accretionary prism a thermal model of the belt with foreland dipping isotherms fits with deepening of the seismicity toward the east mantle volatiles signatures are also widespread in springs along the apennines

fluid flow triggered migration of events in the 1989 dobi earthquake sequence of central afar during the august 1989 sequence of earthquakes beneath the dobi graben in central afar principal events including 10 shocks with ml 5 5 propagated 50 km north westwards in 50 h the sequence is analyzed in terms of triggering by propagation of a critical pore pressure wave in a compressible fluid through a fractured medium the macroscopic fluid diffusion tensor d in the crust beneath the graben is estimated its principal eigenvector strikes n115 e nearly parallel to the graben axis with a value it is about 10 times larger than the other two consistent with fissure anisotropy in the graben the order of magnitude of the permeability 10 8 m is compatible with characteristic fissure widths ranging from 1 mm to 3 cm estimating such parameters in other extensional regions might help to understand better the temporal evolution of seismic swarms

fluid flow during exhumation of deeply subducted continental crust zircon u pb age and o isotope studies of a quartz vein within ultrahigh pressure eclogite quartz veins in high pressure to ultrahigh pressure metamorphic rocks witness channelized fluid flow that transports both mass and heat during collisional orogenesis this flow can occur in the direction of changing temperature pressure during subduction or exhumation shrimp u pb dating of zircon from a kyanite quartz vein within ultrahigh pressure eclogite in the dabie continental collision orogen yields two age groups at 212 7 and 181 13 ma which are similar to two groups of la icpms age at 210 4 and 180 5 ma for the same sample these ages are significantly younger than zircon u pb ages of 224 2 ma from the host eclogite thus the two age groups from the vein date two episodes of fluid flow involving zircon growth the first due to decompression dehydration during exhumation and the second due to heating dehydration in response to a cryptic thermal event after continental collision laser fluorination o isotope analyses gave similar 18o values for minerals from both vein and eclogite indicating that the vein forming fluid was internally derived synchronous cooling between the vein and eclogite is suggested by almost the same quartz mineral fractionation values with regularly decreasing temperatures that are in concordance with rates of o diffusion in the minerals while the quartz veining was caused by decompression dehydration at 700 650 c in a transition from ultrahigh pressure to high pressure eclogite facies retrogression the postcollisional fluid flow was retriggered by heating dehydration at 500 c without corresponding metamorphism in either case the kyanite quartz vein formed later than the peak ultrahigh pressure metamorphic event at the middle triassic pointing to focused fluid flow during exhumation rather than subduction the growth of metamorphic zircon in the eclogite appears to have depended on fluid availability so that their occurrence is a type of geohygrometer besides geochronological applicability to dating of metamorphic events in orogenic cycles

sm nd and rb sr dating of pyroxene garnetite from north dabie in east central china problem of isotope disequilibrium due to retrograde metamorphism abstract oxygen isotopes may serve as a guide to evaluate whether equilibria are preserved in the sm nd and or rb sr systems in eclogites that experience retrograde metamorphism this study demonstrates that sm nd rb sr and o isotopes for minerals from two portions of a clinopyroxene garnetite from north dabie in east central china record both equilibrium peak metamorphic conditions and disequilibrium conditions resulting from anhydrous retrograde metamorphism the minerals are homogeneous in terms of major elements and show no variation across the sample but clinopyroxene shows variable degrees of alteration pure primary mineral separates that are in o isotope equilibrium yield triassic sm nd isochron ages of 219 11 and 229 13 ma that match previously published zircon u pb ages for peak metamorphism thus the sm nd system in these minerals equilibrated during peak metamorphism and remained unaffected by retrograde metamorphism in contrast altered and impure clinopyroxenes that display oxygen isotope disequilibria fall off the sm nd isochron defined by the pure minerals implying that like oxygen the sm nd systems in these clinopyroxene were disturbed during retrogression no meaningful isochrons are found from the rb sr dating even in the pure primary minerals implying that these systems were disturbed during retrograde metamorphism and the diffusion rate of sr is faster than those of sm nd and o in mafic minerals our results suggest that the oxygen isotopes in metamorphic minerals can be used to evaluate the validity of sm nd mineral isochrons in dating peak metamorphic events in addition variations in the sm nd concentration and isotopic compositions of altered and impure clinopyroxenes from the two portions suggest heterogeneous effects on the minerals from external and internal fluids

la icp ms u pb zircon geochronology geochemistry and kinetics of the wenquan ore bearing granites from west qinling china abstract many hydrothermal cu mo au deposits related to granitoid intrusives were recently discovered in the west qinling orogenic belt wqob these deposits were mainly formed during the late indosinian epoch ca 214 ma and the regional geological setting of cu mo ore formation in wqob during this epoch is poorly understood until now this paper describes the geochronology and geochemistry of the wenquan ore bearing pluton a composite granite body to study the geologic background of magmatic emplacement and ore formation the mo mineralisation occurs at the contact between a fine grained biotite monzogranite and a medium to fine grained porphyritic monzogranite zircon 206pb 238u ages of 223 3 ma biotite monzogranite and 225 3 ma porphyritic monzogranite were obtained geochemical analyses show that the wenquan pluton is a high k calc alkaline to shoshonite series rock with relatively high lree and low hree and a moderate to weak negative eu anomaly relatively negative anomalies of ba ti p nb ta also exist these results imply that the wenquan pluton was emplaced during a transitional process from collision to extension between the yangtze craton and north china craton during the later indosinian epoch the east qinling orogenic belt eqob and wqob had similar tectonic settings and intensive magmatic activity and mo mineralisation occurred the eqob was then involved in the mesozoic subduction of the pacific plate and its subsequent tectonic evolution was different from that of the wqob

timing of collision of the sino korean and yangtse cratons u pb zircon dating of coesite bearing eclogites the suture zone between the sino korean and yangtse cratons in central china is marked by a series of fault bounded belts composed of crustal protoliths in the dabie mountains some of these blocks contain coesite and diamond indicating subduction to minimum depths of 120 km as a result of continental collision the blocks range from the structurally lowest ultrahigh pressure metamorphic belt and decrease in metamorphic grade upsection to the structurally highest low greenschist facies belt furthermore the faults mark large contrasts in metamorphic grade these features form a structure that is similar to metamorphic core complexes indicating extension as part of the exhumation process timing of the continental collision has previously been inferred as late triassic from regional geologic relations or caledonian from 40 ar 39 ar dating of the adjacent and possibly related qinling metamorphic belt u pb dating of zircon from ultra high pressure eclogites yields a metamorphic age of 209 2 ma this age coincides with later stages of collision and is therefore a minimum estimate of the time of collision

sulfide mineralogy in the polymetallic cassiterite deposits of dachang p r china several important mineral deposits of sn zn cu pb and other metals associated with devonian sediments and yanshanian cretaceous granitic rocks are known in the dachang district guangxi early genetic hypotheses related the origin of the deposits entirely to the yanshanian granites recently it was suggested that in devonian times an earlier syngenetic metal concentration may have occurred later overprinted by the yanshanian metallogeny this contribution is aimed at placing constraints on the physicochemical conditions during the yanshanian ore formation remobilization by studying the sulfide chemistry arsenopyrite sphalerite stannite and fluid inclusion data on the two major deposits in the area i e the polymetallic cassiterite deposit of changpo and the zn cu skarn deposit of lamo sphalerite and arsenopyrite are quite abundant in both deposits stannite is minor but fairly widespread at changpo and quite rare at lamo they are accompanied by pyrite pyrrhotite galena chalcopyrite cassiterite fluorite and a large variety of other sulfides and sulfosalts the main compositional data for sphalerite and arsenopyrite are summarized as follows

normal faulting transcrustal permeability and seismogenesis in the apennines italy abstract late pliocene pleistocene tectonic evolution of the apennines is driven by progressive eastward migration of extensional downfaulting superposed onto the late miocene early pliocene compressional thrust belt this process has led to distinct structural domains that show decreasing transcrustal permeability from conditions of pervasive mixing between deep and surface fluids in the hinterland west to conditions of restricted fluid circulation and overpressuring in the foreland east at present the highest rates of normal faulting and the strongest seismicity occur in the area bounded by stretched highly permeable crust to the west and thick poorly permeable crust to the east in this area the seismogenic sources of the largest earthquakes 5 m s

geology and geochemistry of the triassic wenquan mo deposit and mo mineralized granite in the western qinling orogen china abstract the recently discovered wenquan porphyry mo deposit hosted in the wenquan granite of the west qinling orogen has been recognized as a product of the indosinian metallogenesis three generations of mineral assemblage for the deposit are identified as follows 1 quartz biotite k feldspar 2 quartz sulfide and 3 sulfide calcite geochemical study shows that the mafic microgranular enclaves mmes in the ore bearing wenquan granite have lower sio 2 and higher mg and nb ta ratios than the host granite itself different from the granite which have zircon e hf t values of 3 6 3 0 and t dm2 of 1234 890 ma the mmes are characterized by the e hf t values of 10 1 10 8 and t dm1 of 865 441 ma this can be interpreted to indicate a mixture origin of the meso and neoproterozoic crust derived component and neoproterozoic sclm derived materials for the formation of the wenquan granite which played an essential role in the mo mineralization comparative pb isotopic data between ores and k feldspar suggest that the wenquan granitic magma originated from the middle lower crust of the south china block and the ore forming materials were incorporated by hydrothermal fluid differentiated from the triassic magmatic system with minor contribution of sedimentary rocks the 34 s values of 5 0 11 7 with a pronounced mode at 5 0 to 6 1 for the ores probably represent the sulfur incorporation of a typical magmatic hydrothermal fluid contaminated by heavy sulfur of devonian sediments the granite yielded the zircon u pb ages of 218 2 4 ma and 221 1 3 ma as the same as the ages of 217 2 0 ma and 218 2 5 ma obtained for the mmes these ages are indistinguishable with the molybdenite re os isochron age of 219 5 2 ma which is the timing for the mo mineralization tectonically the magmatic mixture processes of the wenquan granite and the mo mineralization to form the wenquan mo deposit contemporaneously occurred during the transition of tectonic regime from syn to post collision orogeny in the qinling orogen in the late triassic

fluid and metal sources of the wenquan porphyry molybdenum deposit western qinling nw china the wenquan porphyry moybdenium deposit western qinling nw china with a resource of 247 million tonnes at 0 048 mo formed during triassic collision between south china and north china blocks ore fluids at wenquan vary widely in composition from single phase low salinity aqueous to low salinity vapor and hypersaline inclusions fluid inclusion assemblages in a quartz molybdenite vein contain 60 vol vapor are equant to negative crystal shaped and form clusters or distinct inclusion trails they yield homogenization temperatures of 285 295 c with an estimated trapping temperature of 425 c suggesting a paleodepth of about 5 km calculated at a pressure correction of 100 150 mpa hydrothermal k feldspar from early stockwork veins related to potassic alteration have calculated 18ofluid values of 1 9 to 1 9 hydrothermal sericite from an overprinting phyllic alteration associated with late quartz pyrite veins has calculated dfluid values between 68 and 60 and 18ofluid values from 3 7 to 1 4 these isotopic data suggest that both early and late stage fluids are dominated by magmatic fluids with influx of meteoric water during the late stage exsolution of volatiles from magma in a late stage open system results in more variable d values than an earlier closed system molybdenite and pyrite have 34s values ranging from 1 1 to 6 6 indicating that sulfur at wenquan had a magmatic source a linear relationship between 34s values of sulfides separated from early potassic alteration and late phyllic alteration could reflect incorporation of isotopically heavy evaporate sulfate into source magmas from underlying devonian sedimentary rocks during late alteration the 56fe whole rock values of altered porphyries range from 0 08 to 0 26 similar to 56fe values of 0 15 0 32 for pyrite from quartz veins related to both the potassic and phyllic alteration assemblages the 56fe values of pyrite are positively correlated to but in general slightly lighter than those of altered porphyries indicating similar metal sources moreover both altered porphyries and the pyrite are progressively enriched in heavy fe isotopes from biotite through potassic to late phyllic alteration consistent with isotopic evolution from an early lithostatic load to a late hydrostatic load the triassic intrusive rocks and fluids responsible for mineralization were derived from a deeply sourced hybrid mantle crustal magma crystallizing at a paleodepth of 5 km under lithostatic load external late triassic meteoric or devonian formational fluids became a part of the hydrothermal system during the post fracturing final stage of ore formation under a hydrostatic regime the formation of the wenquan magmatic hydrothermal systems spans the ductile brittle transition based on a normal thermal gradient as constrained by the temperature range of 550 300 c the early high temperature potassic alteration developed under a ductile regime whereas the later low temperature phyllic alteration correlates to a brittle environment where hydraulic rock fracturing enhances permeability and fluid circulation most sulfur and metals were precipitated from vapor rich fluids resulting from cooling and expansion of a single phase fluid exsolved from a hybrid magma

compositional polarity of triassic granitoids in the qinling orogen china implication for termination of the northernmost paleo tethys abstract the qinling orogen was formed from the closure of the northernmost paleo tethys sea and the tectonic suturing of the yangtze and north china cratons the timing of this collision and the tectonic framework are debated the widely developed triassic granitoids in the western qinling orogen offer a key to understand the tectonic evolution of this region here we compile the geological geochemical and geochronological data of the triassic granitoids from the qingling orogen and conclude that the granitoids north of the mian lue suture were emplaced in an active continental margin related to the northward subduction of the mian lue oceanic plate during 248 200 ma the granitoids can be classified into i and s types with the former constituting the major variety northward from the mian lue fault an s type granite belt appears followed by and locally overlapping with an i type granite belt the i type granite belt can be subdivided into four sub belts from south to north with increasing contents of k2o k2o na2o sio2 th and u and the ratios of k2o na2o rb sr and 87sr 86sr i but decreasing contents of na2o al2o3 mg and ehf t except for the northernmost belt which occurs in the huaxiong block of the north china craton this geochemical polarity of i type granitoids and the zoned distribution of s and i types granitoids cannot be explained through continental collision orogeny including syn to post collision but can be well interpreted by considering the progressive subduction of the northernmost branch of the paleo tethys as represented by the mian lue ocean thus the termination of the northernmost paleo tethys and the onset of the continental collision between the yangtze and the north china plates are considered to have occurred at about 200 ma during the transition from triassic to jurassic our model is also supported by the available data from other studies and provides a revised framework for the timing and tectonics of assembly of the yangtze and the north china cratons

geochemical and petrological evidence of the subduction of delaminated adriatic continental lithosphere in the genesis of the neogene quaternary magmatism of central italy abstract serri g innocenti f and manetti p 1993 geochemical and petrological evidence of the subduction of delaminated adriatic continental lithosphere in the genesis of the neogene quaternary magmatism of central italy in m j r wortel u hansen and r sabadini editors relationships between mantle processes and geological processes at or near the earth s surface tectonophysics 223 117 147 the neogene quaternary magmatism of the northern apenninic arc took place in four phases separated in space and time which become progressively younger from west to east phase i 14 ma phase ii 7 3 6 0 ma phase iii 5 1 2 2 ma phase iv 1 3 0 1 ma this magmatism is the result of the activation of three physically separate sources 1 the adriatic continental crust extracted from the mantle in the late proterozoic 2 a strongly refractory recently k enriched harzburgitic mantle located in the mechanical boundary layer mbl of the lithosphere and 3 a recently metasomatized cpx rich mantle compositionally variable from iherzolite to wehrlite clinopyroxenite interpreted as an ephemerally k enriched asthenosphere the adriatic continental crust is the dominant source of the acid plutonic and volcanic rocks of the tuscan region the acid magmatism is mostly found inside an ellipsoidal area about 150 300 km centred on giglio island here defined as the tuscan crustal dome within this area mantle derived magmas unaffected by important crustal contamination processes and mixing with crustal anatectic melts have so far not been found pure crustal magmas are rare but are represented for example by some of the san vincenzo and roccastrada rhyolites virtually all the tuscan acid centres show evidence of mixing with potassic mantle derived magmas major and trace elements as well as 87 sr 86 sr and 143 nd 144 nd data on primitive rocks mg 65 reveal two groups of mantle derived magmas these define two distinct mantle enrichment trends both essentially due to the additions of k rich components which metasomatized separate compositionally diverse upper mantle sectors in both cases the most remarkable mineralogical effect of these enrichment processes is the production of variable amount of phlogopite through reaction between fluids and or melts with the mantle the rocks of group i ol hy and q normative lamproites ultrapotassic high mg latites ultrapotassic shoshonites and shoshonites saturated trend are considered to be derived by partial melting at low pressure 87 sr 86 sr 0 717 ce sr 0 3 and k 2 o na 2 o 6 7 and low ratios of 143 nd 144 nd 0 5121 0 5120 and ba la 87 sr 86 sr 0 712 and k 2 o na 2 o 8 9 values and low 143 nd 144 nd ba la 0 10 ratios these constraints do not allow to exclude a complete identity between the k rich components which metasomatized the mantle sources of the saturated and undersaturated trend magmas the geochemical and isotopic features of the components that metasomatized the mantle sources of the northern apenninic arc magmatism can be explained by a geodynamic process which causes a large amount of crustal materials to be incorporated within the upper mantle we propose that the delamination and subduction of the adriatic continental lithosphere related to the still ongoing northern apennine continental collision provide a viable mechanism to explain the genesis and eastward discontinuous migration of the magmatism in central italy the subduction of delaminated lithospheric mantle with lower crustal slivers would have exposed uppermost mantle adriatic mbl and crustal units previously imbricated in the apennine chain to the heating advected by the upwelling of a recently and ephemerally k enriched asthenospheric mantle wedge and by the underplating of magmas derived from it we consider that the diapiric uprising of a hot crustally contaminated asthenosphere occurs in the wake left above the sinking of the adriatic delaminated subducting continental lithosphere the delamination subduction process of the adriatic lithosphere has probably started in the early middle miocene but earlier than 15 14 ma ago as indicated by the age and petrologic characteristics of the first magmatic episode sisco lamproite of the northern apennine orogenesis

petrogenesis of granitoids in the dewulu skarn copper deposit implications for the evolution of the paleotethys ocean and mineralization in western qinling china abstract the tethyan tectonic domain hosts numerous world class mineral deposits among these the dewulu skarn copper deposit in western qinling china belongs to the paleotethys ore belt the skarn and orebodies here occur as stratoids or lenses at the contact between the triassic dewulu intrusive complex and permian marine clastic and carbonates alteration minerals include prograde skarns garnet diopside wollastonite plagioclase hornblende actinolite tremolite epidote chlorite calcite quartz and sericite the main ore types include early disseminated skarn type replacement orebodies and late stage quartz sulfide veins chalcopyrite is the major ore mineral along with pyrite bornite and sphalerite the dewulu intrusive complex comprises quartz diorite quartz diorite porphyry and dioritic mafic microgranular enclaves mme the mmes are spheroidal in shape and have igneous mineral assemblages acicular apatites complex oscillatory zoned plagioclase and quartz megacrysts surrounded by mafic minerals the mmes are metaluminous and calc alkaline to high k calc alkaline and possess relatively high ni cr and mgo contents and mg values they display sub parallel patterns in trace element spider diagrams and rare earth element ree plots they are also characterized by the enrichment of rb u and th depletion of ba sr nb and ta and negative eu anomaly zircon la icp ms u pb dating of the dioritic mme yields an age of 247 0 2 2 ma coeval with the host quartz diorite quartz diorite porphyry and ore related sericite 40ar 39ar plateau ages within analytical uncertainties oxygen fugacity estimated from trace element compositions of zircons from the dioritic mme shows fmq 3 3 the zircons have negative ehf t values in a range of 8 0 to 3 3 corresponding to two stage model ages ranging from 1 48 to 1 78 ga the integrated data from petrology geochronology and bulk geochemistry suggest that the early triassic granitoids associated with cu skarn mineralization at dewulu were products of arc magmatism and involved magma mixing in an active continental margin setting the magma was sourced through partial melting of enriched sub continental lithospheric mantle that had been previously modified by slab derived melt during the continuous northward subduction of the paleotethys oceanic slab

mesozoic metallogeny in east china and corresponding geodynamic settings an introduction to the special issue abstract the giant east china mesozoic metallogenic province hosts some of the world s largest resources of tungsten tin molybdenum antimony and bismuth ores of gold silver mercury lead zinc copper uranium and iron are also of major importance the province and its constituent metallogenic belts or regions south china middle lower yangtze river valley east qinling dabie interior of north china craton yan liao and north east china are the products of several pulses of igneous activity and mineralisation between 240 and 80 ma each successive stage has produced a distinctive suite of deposits that can be readily related to the geodynamic evolution of the region during the mesozoic this geodynamic evolution is linked to a complex series of tectonic events involving far field subduction plate collisions crustal thickening post collision collapse and rifting

a review of the relationships between granitoid types their origins and their geodynamic environments abstract granitoids are divided into several types according to their mineral assemblages their field and petrographical features and their chemical and isotopic characteristics this typology complements most of the recent classifications because it is not based solely on chemical and isotopic criteria but also on the field petrographical and mineralogical criteria it thus has the advantage of distinguishing the various granitoid types in the field in most cases the proposed classification shares many similarities with the twenty most used genetic classifications of granitoids both types of peraluminous granitoids are of crustal origin the tholeiitic alkaline and peralkaline granitoids are of mantle origin and both types of calc alkaline granitoids are of mixed origin and involve both crustal and mantle materials each granitoid type is generated and emplaced in a very specific tectonic setting each stage of the wilson cycle is characterised by typical associations of granitoids well typed and precisely dated granitoids can then complement structural approaches and indicate on the geodynamic environment with reference to some case studies the use of granitoids as tracers of the geodynamic evolution is also proposed and discussed

early mesozoic magmatism within the tibetan plateau implications for the paleo tethyan tectonic evolution and continental amalgamation

40ar 39ar geochronology and exhumation of high pressure to ultrahigh pressure metamorphic rocks in east central china new 40 ar 39 ar ages from rocks in the high pressure and ultra high pressure hp uhp metamorphic complex in the hong9an and dabie mountains areas east central china document a two phase cooling and exhumation history following triassic continental collision and metamorphism phengite ages from blueschist through kyanite bearing eclogite facies rocks in hong9an record initial exhumation from the collision zone between 230 and 195 ma biotite and hornblende ages from migmatites and eclogite bearing gneisses from the dabie mountains record a cooling event between 128 and 117 ma corresponding to a regional episode of crustal anatexis and emplacement of granitic plutons the triassic through early jurassic 40 ar 39 ar cooling ages corroborate u pb and sm nd metamorphic ages from previous studies and suggest that initial exhumation of these rocks was rapid emplacement of granitic melts within the hp uhp sequences 80 m y after metamorphism suggests that the metamorphic rocks were either exhumed at slower rates or became arrested at depth subsequent to the initial rapid exhumation episode

tectonic implications from re os dating of mesozoic molybdenum deposits in the east qinling dabie orogenic belt abstract the east qinling dabie molybdenum belt is part of a larger east west trending metallogenic belt in eastern china most of the molybdenum deposits occur as porphyry or porphyry skarn type but there are also some vein type deposits following systematic re os dating of molybdenite from 13 deposits and comparisons with two previously dated deposits we have recognized that the molybdenum mineralization in the east qinling dabie belt was developed during hydrothermal activity linked to magmatism and the emplacement of granitoid stocks three pulses of granitoid magmatism and mo mineralization are recognized corresponding to significant tectonic events in the east qinling dabie belt vein type deposits dated at 233 221 ma were formed in detachment fractures indicating localized extension within the collisional setting of the north china and yangtze cratons i type and transitional i and s type granites and related mineralization dated at 148 138 ma may have formed part of a continental magmatic arc with widespread magmatism and back arc extension caused by subduction of the izanagi or paleopacific plate beneath the eurasian continent in a wnw ese direction in the late jurassic early cretaceous both s type and transitional s and i type granite associated porphyry molybdenum deposits dated at 131 112 ma are part of an extensive mineralization event throughout east china that can be ascribed to regional large scale lithospheric thinning delamination and thermal erosion

early mesozoic mo mineralization in the qinling orogen an overview abstract as one of the most important mo provinces in the world the qinling orogen is host to mainly late mesozoic late jurassic early cretaceous porphyry mo systems recently additional early mesozoic mainly triassic plus minor early jurassic mo deposits have been identified which occur as fault controlled quartz veins or as carbonatite veins and as such they are fundamentally different from the commonly observed porphyry systems furthermore early mesozoic porphyry systems are also present but they are different from those of late mesozoic age thus early mesozoic mo systems not only offer a new and comprehensive view of the mo metallogenesis but also provide unique insight into the tectonic evolution of the qinling orogen taking into consideration the role of mineral deposits as ideal probes of the geodynamic evolution the known early mesozoic mo deposits in qinling can be classified into three types based on their geological and geochemical features porphyry carbonatite and orogenic systems porphyry mo systems including porphyry porphyry skarn and intrusion related veins are spatially and genetically associated with granitic porphyries they display typical porphyry type alterations and are formed by magmatic water carbonatites are considered as mantle derived igneous rocks and mo mineralization is associated with magmatic fluids orogenic mo systems occur as fault controlled quartz lodes and the ore forming fluids are medium high temperature co2 rich metamorphic fluids spatially early mesozoic porphyry mo systems are concentrated in the western and middle part of the qinling orogen whereas carbonatite veins and orogenic mo systems are located in the northeastern part temporally orogenic mo deposits were the earliest mainly between 220 and 250 ma followed by carbonatite veins mainly between 205 and 225 ma and porphyry mo systems mainly between 190 and 205 ma with some exceptions there is generally a trend of decreasing deposit ages toward the western part of the orogen detailed comparison reveals that the early mesozoic porphyry mo systems in the qinling orogen are different from the late mesozoic ones in terms of their economic importance spatial distribution ore causative intrusions hydrothermal alteration and the ore forming fluids to explain the above geological and geochemical features the triassic qinling is considered as an active continental margin caused by the northward subduction of the mian lue oceanic plate a transition from continental arc to back arc basin is suggested for the formation of triassic orogenic and carbonatite vein mo deposits whereas continental arc is host to the early mesozoic porphyry mo systems

u pb zircon dating geochemical and sr nd hf isotopic compositions of early indosinian intrusive rocks in west qinling central china petrogenesis and tectonic implications the qinling dabie sulu orogenic belt is the junction between the north and south china blocks which resulted from the final amalgamation of china continents during the indosinian indosinian granitoids are widespread in the qinling orogen and their geneses can thus constrain the evolution of china continent we carried out a combined u pb zircon dating and geochemical study for the shuangpengxi granodiorite pluton and the xiekeng diorite granodiorite pluton in the middle part of the west qinling orogen u pb zircon dating shows that the magma crystallization ages of 242 3 ma for the shuangpengxi pluton and 244 242 ma for the xiekeng pluton geochemical and sr nd hf isotopic compositions reveal that the magma of the shuangpengxi granodiorite was derived from partial melting of crustal materials the xiekeng diorites can be divided into high al diorite and high mg diorite both of them resulted from partial melting of enriched lithospheric mantle but their mantle source had been modified by previous slab derived melt the high al diorite was formed by fractional crystallization of olivine pyroxene and or preferential accumulation of plagioclase and the high mg diorite was formed by fractional crystallization of olivine and or preferential accumulation of pyroxene the xiekeng granodioritic porphyry was formed by mixing of crust derived and mantle derived melts we propose that the early indosinian magmatism resulted from break off of subducted oceanic slab after collision the slab break off model can well explain the linear distribution of the early indosinian plutons and rapid crustal uplift during the middle triassic in the west qinling

fluid inclusion geochemistry and magmatic oxygen fugacity of the wenquan triassic molybdenum deposit in the western qinling orogen china abstract the wenquan mo deposit in gansu province china is one of the large porphyry mo deposits in the western qinling orogen that associated with the triassic continental collision between the north china block and south china block in this study detailed fluid inclusions fis analyses were performed on early stage quartz biotite k feldspar veins middle stage quartz polymetallic sulfide veins and late stage sulfide calcite veins four compositional types of fis are observed including pure co2 pc type co2 h2o c type h2o nacl w type and daughter mineral bearing polyphase s type with no halite daughter mineral the early stage quartz contains all the four types which are mainly homogenized between 285 c and 495 c with salinities ranging from 4 2 to 17 6 wt nacl eqv however in the middle stage pc type fis are not observed and s w and c types fis yield homogeneous temperatures th of 200 397 c and salinities of 2 4 13 1 wt nacl eqv late stage quartz contains only w type fis with th of 102 246 c and salinities of 1 2 10 1 wt nacl eqv results indicate that the fluid system evolved from an initial co2 rich magmatic hydrothermal fluid to a late co2 poor meteoric fluid in the early and middle stages it is possible that heterogeneous fis associations may have been trapped from a fluid immiscibility system the dramatically decreasing percentage of pc and c types fis in the ore stage indicates that co2 escaped which is possibly the result of earlier fluid immiscibility and phase separation in addition there are double peaks of th in this stage that are obviously lower than in the early stage showing that meteoric water flowed into and mixed with magmatic fluids thereby cooling the hydrothermal system these results suggest that metal precipitation resulted from fluid immiscibility co2 escape and inflow of meteoric water occurred in the middle stage the initial fluids of the wenquan deposit are characterized by relatively high temperature high salinity high fo2 and are co2 rich and nacl poor indicating that the wenquan deposit is a collisional type porphyry mo deposit the oxygen fugacity fo2 constrained by trace element compositions of zircons from the wenquan mo bearing porphyry is uneven with fmq 10 3 4 68 average of 4 32 which could result from the input of mafic magma and its inhomogeneous mixing with felsic magma such values are still much higher than those of the adjacent contemporaneous mo barren lujing baijiazhuang jiaochangba luchuba and zhongchuan granitic plutons which confirms the affinity between mo mineralization and the oxidized melt system and implies that the wenquan porphyry has relatively high fo2 characteristics for its tectonic location and epoch fluid inclusion initial mineralization depth 8 6 km magma fo2 and the zircon high u yb ratio of the wenquan mo deposit strongly suggest that mo mineralization was related to continental collision rather than oceanic crust subduction this study therefore proposes that the wenquan mo deposit was formed in a rapid crust uplifting setting caused by syn orogenic crustal thickening followed by post orogenic delamination of the lithospheric root during the late triassic

pyrite re os and zircon u pb dating of the tugurige gold deposit in the western part of the xing an mongolia orogenic belt china and its geological significance abstract the tugurige gold deposit in the northeastern margin of the north china craton ncc and western part of the xing an mongolia orogenic belt xmob is a large tonnage quartz vein type deposit with a resource of 24 t at about 4 g t au gold mineralization is hosted by porphyritic syenogranite and controlled by nw trending faults our study provides the first precise age for the tugurige gold mineralization and host porphyritic syenogranite the u pb age of zircons from the syenogranite is 276 2 ma and re os dating of six pyrite separates from the deposit yields an isochron age of 268 15 ma the results show that the ages of syenogranite and gold mineralization are the same within error furthermore geological and geochronological evidence indicate that the syenogranite and gold mineralization have very close genetic relationships and that they are products of permian magmatism related to post collisional extension of the central asian orogenic belt caob the initial os isotopic compositions 187 os 188 os 1 26 0 69 of pyrite indicate that the source of os and by inference other ore related materials are derived from the crust the late paleozoic is an important period in which economic gold mineralization was deposited in the caob

u pb hf and o isotope evidence for two episodes of fluid assisted zircon growth in marble hosted eclogites from the dabie orogen abstract a combined study of internal structure u pb age and hf and o isotopes was carried out for metamorphic zircons from ultrahigh pressure eclogite boudins enclosed in marbles from the dabie orogen in china cl imaging identifies two types of zircon that are metamorphically new growth and recrystallized domain respectively the metamorphic zircons have low th and u contents with low th u ratios yielding two groups of 206 pb 238 u age at 245 3 to 240 2 ma and 226 4 to 223 2 ma respectively anomalously high 18 o values were obtained for refractory minerals with 9 9 to 21 4 for garnet and 16 9 for zircon this indicates that eclogite protolith is sedimentary rocks capable of liberating aqueous fluid for zircon growth during continental subduction zone metamorphism most of the zircons are characterized by very low 176 lu 177 hf ratios of 0 000001 0 000028 indicating their growth in association with garnet recrystallization a few of them falling within the older age group have comparatively high 176 lu 177 hf ratios of 0 000192 0 000383 suggesting their growth prior to the formation of garnet in the late stage of subduction the variations in the lu hf ratios for zircons can thus be used to correlate with garnet growth during eclogite facies metamorphism in either case the zircons have variable e hf t values for individual samples suggesting that their protolith is heterogeneous in hf isotope composition with localized fluid availability in the bulk processes of orogenic cycle nevertheless a positive correlation exists between 206 pb 238 u ages and lu hf isotope ratios for the metamorphically recrystallized zircons suggesting that eclogite facies metamorphism in the presence of fluid has the identical effect on zircon lu hf and u th pb isotopic systems we conclude that the zircons of the older group grew in the presence of fluid during the subduction prior to the onset of peak ultrahigh pressure metamorphism whereas the younger zircons grew in the presence of fluid released during the initial exhumation toward high pressure eclogite facies regime

the compositional variation of i type granites constraints from geochemical analyses and phase equilibrium calculations for granites from the qinling orogen central china abstract there is a large compositional variation of i type granites although partial melting and fractional crystallization are the two primary mechanisms for the variation it is intriguing to know how they jointly work this issue is addressed by a combined study of geochemistry and phase equilibrium modelling for a triassic pluton from the qinling orogen the pluton consists of high k calc alkaline i type granites geochemical results suggest that the granites are likely sourced from triassic mafic intermediate igneous rocks compared with experimental melts it is inferred that the natural granites are not the crystallized product from pure magmatic melts this is confirmed by phase equilibrium calculations that have utilized the triassic mafic intermediate igneous rocks as the source yielding modelled melts that contain too low feot mgo and mg to match the target granites the addition of restitic mineral assemblages to melts can relieve the discrepancy but the modelled magmas have much higher cao than the granites replacing equilibrium melting by disequilibrium melting in terms of the plagioclase behavior can lead to a satisfactory match furthermore by taking the average of the target granites as a parental magma the modelled crystalline products can well reproduce the variation trends in the high k calc alkaline i type granites from the qinling orogen in this regard the crustal anatexis would produce the parental magma at first which experienced fractional crystallization to form the granitic pluton with the compositional variation as observed therefore these two processes would jointly lead to the compositional variation of i type granites in nature

structural and fluid chemical properties of seismogenic normal faults abstract structures in extensional fault zones can be classified using the configuration of fault branch lines and tip lines important classes include the segment bend segment termination segment branch cross fault intersection and segment offset the effect of these structures on rupture history is not necessarily consistent neither between individual earthquakes nor between different fault zones rupture behavior is dependent on several other factors including loading conditions regional and localized rupture tip stress fields fluid mechanical processes and chemical processes fracture toughness is partly controlled by the angular discordance between slip directions on adjacent fault segments greater discordance between slip directions and intersection lines of fault segments results in greater strain incompatibility an internal fracture network generally evolves within a segment boundary to maintain compatibility and transfer slip between the segments the dimensions and structure of this fracture network may also partly control rupture propagation presumably activation of a fault network with large angular discordances between slip directions and intersection lines will generate numerous asperities as the subsidiary faults mutually interact and offset each other the geometry of a segment boundary may change with depth and the three dimensional nature of the structure may be important in controlling rupture history fluids influence rupturing via fluid pressure effects and time dependent chemical processes fracture propagation by stress corrosion may favor instability and chemical alteration may produce minerals of lower strengths allowing time dependent creep sealing and healing of fractures however may remove damage and increase strength elementary computations indicate that representative times for sealing and chemical alteration are between 1 and 1000 years for reasonable physical conditions well within the recurrence intervals of most large earthquakes time to failure for stress corrosion cracking is more highly variable and strongly sensitive to applied stress and fluid pressure

late orogenic post orogenic and anorogenic granites distinction by major element and trace element chemistry and possible origins granites classified into four categories based solely on tectonics of occurrence and associated rock types also have compositional characteristics that are consistent within groups and different am

topaz albite granites and rare metal mineralization in the limu district guangxi province southeast china the topaz albite granites of the limu district are ultra acidic peraluminous li f na rich and sn ta nb mineralized a distinct vertical zonation is developed in the granite stocks there is an upward systematic transition from leucocratic microcline albite granite through albite microcline granite topaz albite granite pegmatite stockscheider and layered pegmatite aplite dikes to k feldspar quartz veins and lepidolite fluorite stringers in the country rocks snow ball textures homogeneous distribution of rock forming and accessory minerals disseminated mineralization and melt inclusions in quartz topaz and albite are typical features indicative of their crystallization from the late stage li f na rich and sn ta nb bearing residual granitic melts at a higher intrusion level a comparison with rare metal bearing pegmatite ongonite topaz rhyolite and obsidian glass from other regions shows the worldwide existence of these specialized residual melts their emplacement and crystallization in a variety of geological environments result in the formation of a series of chemically similar rocks with different petrographic textures and mineral associations the topaz albite granites and associated mineralization in the limu district provide a good example of highly evolved magmatic fractionation in the f rich granite system and fluid melt partitioning behavior of rare metal elements during magmatic hydrothermal evolution

an f rich sn bearing volcanic intrusive complex in yanbei south china the igneous complex that hosts the yanbei porphyry tin deposit is situated along the eastern margin of the southeast china caledonian fold belt it intrudes a late jurassic granitic batholith and was unconformably covered by late cretaceous red beds the complex consists mainly of volcanic ignimbrites and minor rhyolitic lavas tuffs and subvolcanic rhyolite porphyry and intrusive rocks topaz bearing granite porphyry and granite these rocks are generally peraluminous with f and vi al rich biotite as the main mafic mineral and f rich and oh poor topaz as an accessory mineral in the granite porphyry and granite and as an essential mineral in the topazite topaz in the granites is usually interstitial and crystallized during the late magmatic stage the volcanic rocks and granites are rich in si k f sio 2 75 wt na 2 o k 2 o 0 03 0 72 f 0 06 6 48 wt and incompatible trace elements rb zr nb w sn and poor in mg ti ca p and compatible trace elements sr eu the chondrite normalized rare earth element ree patterns for the granites generally have a distinct v shape with strong eu depletion eu eu super whitesunwithrays 0 04 0 20 the main rocks of the complex have low epsilon nd t values 9 3 to 8 9 for volcanic rocks 4 0 to 4 8 for granites and high delta 18 o values of quartz from dacite 9 5ppm and granites 9 6 10 1ppm the difference in epsilon nd t between the volcanic rocks and granites indicates that they are not comagmatic the source of the volcanic rocks probably contains a greater metasedimentary component than that of the granites the yanbei porphyry tin deposit is located along the contact zone between topaz bearing granite porphyry and rhyolite porphyry and consists principally of one orebody which makes up 99 percent of the total reserves of the metallic minerals 5 by volume cassiterite pyrite and chalcopyrite are dominant sphalerite wolframite and galena are less important and siderite molybdenite and stannite are minor cassiterite accounts for 96 percent of the total sn reserves and stannite is the subordinate sn bearing mineral that occurs as rims around chalcopyrite cassiterite occurs mostly in disseminated form and as veinlets the wall rock alteration is intense and can be roughly grouped into three zones spatially arranged around the granite porphyry namely topaz quartz chloritic and sericitic zones most of the orebody occurs in the topaz quartz and chloritic zones hydrothermal alteration caused a decrease in k and na and an increase in al fe ca sn w and cu the delta 18 o values of quartz and cassiterite from the mineralized zones vary from 6 0 to 11 per mil and the calculated delta 18 o sub h 2 o values of the ore forming fluid range from 4 2 to 7 8 per mil the altered rocks have elevated delta 18 o sub h 2 o values 14 5 9 7ppm relative to fresh rocks the pyrite from the ore has a limited range of delta 34 s values 0 3 1 5ppm these data suggest a magmatic source for the ore forming hydrothermal fluid

the role of disseminated calcite in the chemical weathering of granitoid rocks accessory calcite present at concentrations between 300 and 3000 mg kg 1 occurs in fresh granitoid rocks sampled from the merced watershed in yosemite national park ca usa loch vale in rocky mountain national park co usa the panola watershed ga usa and the rio icacos puerto rico calcite occurs as fillings in microfractures as disseminated grains within the silicate matrix and as replacement of calcic cores in plagioclase flow through column experiments using de ionized water saturated with 0 05 atm co2 produced effluents from the fresh granitoid rocks that were dominated by ca and bicarbonate and thermodynamically saturated with calcite during reactions up to 1 7 yr calcite dissolution progressively decreased and was superceded by steady state dissolution of silicates principally biotite mass balance calculations indicate that most calcite had been removed during this time and accounted for 57 98 of the total ca released from these rocks experimental effluents from surfically weathered granitoids from the same watersheds were consistently dominated by silicate dissolution the lack of excess ca and alkalinity indicated that calcite had been previously removed by natural weathering the extent of ca enrichment in watershed discharge fluxes corresponds to the amounts of calcite exposed in granitoid rocks high ca na ratios relative to plagioclase stoichiometries indicate excess ca in the yosemite loch vale and other alpine watersheds in the sierra nevada and rocky mountains of the western united states this ca enrichment correlates with strong preferential weathering of calcite relative to plagioclase in exfoliated granitoids in glaciated terrains in contrast ca na flux ratios are comparable to or less than the ca na ratios for plagioclase in the subtropical panola and tropical rio icacos watersheds in which deeply weathered regoliths exhibit concurrent losses of calcite and much larger masses of plagioclase during transport limited weathering these results indicate that the weathering of accessory calcite may strongly influence ca and alkalinity fluxes from silicate rocks during and following periods of glaciation and tectonism but is much less important for older stable geomorphic surfaces

u pb zircon ages constrain the architecture of the ultrahigh pressure qinling dabie orogen china new shrimp and tims zircon ages 40ar 39ar ages and eclogite locations contribute significantly to our understanding of the ultrahigh pressure dabie shan 1 the geographic extent of the yangtze craton that was subducted to ultrahigh pressure extends to the northern edge of the dabie shan 2 the northern half of the dabie shan is a magmatic complex intruded over a 10 myr interval between 137 and 126 ma that accommodated 100 n s stretching of the pre existing collisional architecture 3 granitic orthogneisses and enclosing ultrahigh pressure paragneisses have indistinguishable zircon populations the population of triassic zircon ages ranges from 219 to 245 ma leading us to question the prevailing assumption that 219 ma zircons formed at ultrahigh pressure and to propose instead that they reflect late retrogression at crustal pressures following the bulk of exhumation

zircon u pb ages and geochemistry of the wenquan mo bearing granitioids in west qinling china constraints on the geodynamic setting for the newly discovered wenquan mo deposit abstract east qinling is the largest porphyry molybdenum province in the world these mo deposits have been well documented in west qinling however few mo deposits have been discovered although granitic rocks are widespread recently the wenquan porphyry mo deposit has been discovered in gansu province which provides an insight into mo mineralization in west qinling in this paper we report pb isotope compositions for k feldspar and sulfides s isotope ratios for sulfides the results obtained from petrochemical study and from in situ la icp ms zircon u pb dating and hf isotopes the granitoids are enriched in lile and lree with ree and trace element patterns similar to continental crust suggesting a crustal origin the mg 40 05 to 56 34 and cr and ni contents are high indicating a source of refractory mafic lower crust the e hf t values of zircon grains from porphyritic monzogranite range from 2 9 to 0 6 and from granitic porphyry vary from 3 3 to 1 9 the zircons have t dm2 of 1014 to 1196 ma for the porphyritic monzogranite and 954 to 1224 ma for the granitic porphyry implying that these granitoids were likely derived from partial melting of a late mesoproterozoic juvenile lower crust the pb isotope compositions of the granitoids are similar to granites in south china showing that the magma was sourced from the middle lower crust in the southern qinling tectonic unit the pb isotopic contrast between the mo bearing granitoids and ores shows that the pb in the ore forming solution was derived from fractionation of a triassic magmatic system 34 s values of sulfides are between 5 02 and 5 66 similar to those associated with magmatic hydrothermal systems la icp ms zircon u pb dating yields crystallization ages of 216 2 1 7 and 217 2 2 0 ma for the granitoids consistent with a previously reported molybdenite re os isochron age of 214 4 7 1 ma this suggests that the mo mineralization is related to the late triassic magmatism in the west qinling orogenic belt in view of these geochemical results and known regional geology we propose that both granitoid emplacement and mo mineralization in the wenquan deposit resulted from the triassic collision between the south qinling and the south china block along the mianlue suture since triassic granitoid plutons commonly occur along the qinling orogenic belt the triassic wenquan mo bearing granitoids highlight the importance of the triassic tectono magmatic belt for mo exploration in order to apply this metallogenic model to the whole qinling orogen further study is needed to compare the wenquan deposit with other deposits

geological and geochemical characteristics and formation mechanisms of the zhaishang carlin like type gold deposit western qinling mountains china abstract located in the western part of the min li metallogenic belt within the western qinling mountains the zhaishang gold deposit is a giant carlin like disseminated gold deposit discovered recently the ore deposit is present both in rocks of low grade metamorphic middle devonian and lower permian clastic formation which is composed of quartz sandstone siltstone calcareous slate and argillaceous limestone gold mineralization is strictly controlled by a fault zone minerals in ores are quite complex and consist of sulfides sulfosalt oxides sulfate carbonate tungstate telluride native metals and polymetallic alloys the diversity of mineral in the ores and the existence of microscopic visible native gold constitute the outstanding features of the gold deposit we believe that the mineral source of the ore deposit has a close connection with the host rocks because all samples show the light of the ree distribution patterns the s c pb h and o isotopic compositions show that the ore forming elements were mainly derived from the country rocks the underlying rocks and partially deep sources data obtained suggest that the ore forming fluids were derived predominantly from an active meteoric groundwater system mineralization and related alteration have features of a low water rock ratio ore forming temperatures are estimated to have been in the range from 120 to 240 c the dissolution of ferruginous limestone in the host rocks and the sulfidation of the dissolved iron by h2s introduced by ore fluids constituted not only the most important depositional mechanisms for the existence of microscopic visible gold grains but also favorable conditions for gold enrichment in the zhaishang gold deposit

geologic framework and tectonic evolution of the qinling orogen central china abstract the geologic framework of the qinling orogen was built up through interplay of three blocks the north china block including the north qinling the south qinling and the south china block separated by the shangdan and mianlue sutures the shangdan suture resulted from middle paleozoic collision of the north china block and the south qinling the mianlue suture resulted from late triassic collision of the south qinling and the south china block present upper crust of the qinling is structured dominantly by thrust fold systems the north qinling displays thick skinned deformation with crystalline basement involved whilst the south qinling is characterized by thin skinned thrusts and folds detached above the lower sinian two types of precambrian basement crystalline and transitional are defined according to lithology and metamorphic grade and different in age stratigraphic and sedimentary architecture is characterized by distinct zonation the qinling orogen experienced a prolonged continental divergence and convergence between blocks during the period from late neoproterozoic to early paleozoic times the south qinling was the northern margin of the south china block and the north qinling was the southern margin of the north china block separated by a proto tethyan qinling ocean the north qinling evolved into an active margin when the proto tethyan qinling ocean subducted northward during ordovician time collision of the south and north qinling took place in middle paleozoic along the shangdan suture synchronous with the collision rifting occurred at the southern rim of the south qinling and was followed by the opening of the paleo tethyan qinling ocean during the late paleozoic resulting in the splitting of the south china block from the south qinling collision of the south qinling and the south china block came about in the late triassic along the mianlue suture the late triassic collisional orogeny caused extensive fold and thrust deformation and granitoid intrusions throughout the qinling and led to final amalgamation of the north and south china blocks

relationships between ultrapotassic and carbonate rich volcanic rocks in central italy petrogenetic and geodynamic implications the pleistocene intra apennine volcanic iav centres occurring east of the potassium rich roman comagmatic province show variable petrological and geochemical composition some rocks have a strongly undersaturated ultrapotassic kamafugitic affinity with k2o na2o 8 20 whereas the rocks from the southern center of mt vulture are still strongly undersaturated in silica but are enriched in both na2o and k2o with k na around unity carbonate rich pyroclastic rocks believed to represent carbonatitic magmas are found in the iav centers kamafugites have high abundances of lile and high lile hfse ratios and their incompatible element patterns resemble closely those of ultrapotassic rocks from the adjoining roman province the vulture volcanics also display high contents of lile but their lile hfse ratios are intermediate between intraplate alkaline rocks and kamafugites the carbonate rich rocks exhibit an exotic mineralogy and high enrichments in lile which speaks for a carbonatitic affinity however they have similar incompatible element patterns but consistently lower abundances of almost all the elements than the associated silicate volcanics these data favour the hypothesis that the iav carbonate rocks may represent mixtures of silicate magmas and geochemically depleted carbonate material the sedimentary carbonates that crop out extensively along the apennine chain may be the source of barren carbonate material overall geochemical data of iav centres and of the rocks from the roman province display strong geochemical and isotopic evidence of being generated in an upper mantle that was modified by addition of upper crustal material brought down by subduction processes a possible exception is represented by mt vulture which however occurs east of the main axis of the apennines on the western margin of the foreland adria plate the occurrence of strongly undersaturated alkaline rocks requires magma generation at high pressures and xco2 this is in agreement with the hypothesis that subduction processes under the apennines occurred by consumption of poorly hydrated thinned or delaminated continental crust

depth and modes of pliocene pleistocene crustal extension of the apennines italy since early pliocene times the apenninic chain has been dissected by normal faults propagating towards the adriatic foreland in the tyrrhenian sea extension involved deep crustal sections whereas in the central apennines downfaulted area it affected the shallow crust the tyrrhenian back arc domain is connected to the overall flexural retreat of the adriatic apulia plate in front of the apenninic collisional wedge in the outer apenninic belt thin skinned delamination and gravitational collapse occurred in the hanging wall of a thickened wedge overthrusting the uplifted buoyant crust of the apulia foreland differential sinking velocity of the foreland plate results from the inherited competence contrast between the rigid gargano apulia carbonate platform to the south and the low competence lithology of the pelagic sequence of the adriatic basin to the north during late messinian early pliocene times this palaeogeographical boundary acted as a lithospheric tear separating segments of the apulia plate subjected to different subduction modes

late tertiary reorganizations of deformation in northeastern tibet constrained by stratigraphy and provenance data from eastern longzhong basin the deformation of the tibetan plateau is central to unraveling the process and mechanism of continental tectonics although most agree that crust shortening and plateau growth were protracted throughout the cenozoic indo asian collision particular deformation histories relating to tectonic kinematics and dynamics are still incomplete due to sparseness of diagnostic geological information from plateau margin here we present combined investigation of stratigraphy magnetostratigraphy and provenance for the eastern margin of longzhong basin to show two reorganizations of basin formation and tectonic regime during the late tertiary first the depocenter migrated from the dispersed paleogene sequences to the wushan tianshui foreland sequence during the earliest miocene circa 22 ma accompanied by shift of sedimentary provenance from double sources including the eastern qilian block and eastern west qinling terrain to single source within the west qinling it suggests reorganization of deformation from nw se extension to ne sw contraction and initial uplift of the eastern west qinling second massive coarse grained fluvial beds were revived in the wushan basin during the late miocene circa 10 ma associated by eastward depositional expansion and another shift of sedimentary provenance toward northeast it reflects thrusting up of the northern edge of the west qinling and liupan shan mountains linked with relocation of crust shortening from ne sw direction to ene wsw direction and accelerated deformation of northeastern tibet these transitions of deformation regimes imply variation of geodynamic mechanisms during the process of plateau growth

incremental emplacement and syn tectonic deformation of late triassic granites in the qinling orogen structural and geochronological constraints abstract granitoids are important components of major orogenic belts and provide important information about the regional geodynamic evolution the emplacement mechanism of granite plutons and its relationship with regional tectonics has long been discussed although it still remains debated the qinling orogen within the central china orogen was marked by the emplacement of numerous late triassic granitic plutons although the petrology geochemistry and geochronology of these intrusions have been addressed in various studies their tectonic setting remains controversial particularly since the structural aspects not been evaluated in detail in this study we attempt to reconstruct the emplacement process of the late triassic dongjiangkou pluton in the south qinling belt field observations show extensive syn plutonic deformations both in the pluton and its contact zones microstructural observations demonstrate that fabrics in the pluton were mainly acquired during submagmatic flow to high t solid state deformation zircon u pb ages reveal that the pluton is a composite intrusion which is composed of two juxtaposed small plutons with distinct ages 210 ma and 200 ma al in hornblende thermobarometer indicates that the pluton was formed at depths ranging from 4 7 km to 8 8 km with an increasing depth trend from the inner unit to the outer unit distribution of the internal fabrics shows two concentric patterns which are concordant with pluton margins at the pluton scale and were probably induced by the regional sinistral transpression integrating these analyses an incremental emplacement model is proposed for the syn tectonic pluton this model not only solves the room problem but also accounts for the zoned petrological features of the pluton combined with previous studies we suggest that the late triassic granite plutons in the qinling orogen were emplaced under a syn collisional convergence setting and that the granite magmatism was probably controlled by regional tectonics additionally the incremental emplacement model may be a common mechanism for the late triassic granite plutons

formation of voids and veins during faulting abstract we have developed a quantitative model of void and vein development in fault zones based on observations of fault roughness and the contact characteristics of rough directionally anisotropic fractal surfaces this model includes progressive dilation of the fault during slip events and the elastic deformation of the surfaces normal to the fault plane closure as new void space develops the model predicts vein geometries that are qualitatively similar to those observed in fault controlled mineral deposits the statistics of the vein system are described and strategies for sampling of such structures by drilling are developed the results have significant implications for evaluation of ore reserves and evaluating the fluid transport properties of faults

heterogeneous sources of the triassic granitoid plutons in the southern qinling orogen an e w tectonic division in central china 1 the qinling orogen is an important orogenic belt formed by the collision between the north and south china blocks along the mianlue suture during the triassic the orogen is customarily divided into the western and eastern qinling terranes however the boundary has long been a matter of debate there are many triassic granitoid plutons in the orogen especially in south qinling along the southern part of the shangdan suture systematic analysis of u pb ages and hf isotopes in zircons frxom six granitoid plutons including guangtoushan gaoqiaopu xiba laocheng dongjiangkou and zhashui from west to east allows us to trace their formation ages and source regions all plutons yield ages that vary only from 218 to 211 ma however zircon hf isotope data for these triassic plutons cluster in two distinct groups granitoids from the western segment i e guangtoushan gaoqiaopu and xiba of the south qinling belt have negative ehf t values 20 9 to 5 2 and relatively old two stage hf model ages 1 58 to 2 57 ga in contrast those from the eastern segment i e laocheng dongjiangkou and zhashui show higher ehf t values 5 4 to 6 8 and younger two stage hf model ages 0 82 to 1 60 ga integrating these analyses with published sr nd pb hf isotopic data we suggest that the division between the western and eastern south qinling segments is located near the taibai chenggu line between the baoji chengdu railway and 108 e longitude the integrated data suggest that the western south qinling segment separated from the north china block during the paleoproterozoic and early mesoproterozoic and then switched into continental convergence with the eastern south qinling segment northern margin of the yangtze block during the late mesoproterozoic to early neoproterozoic finally the two segments amalgamated during the late neoproterozoic

rate of diffuse carbon dioxide earth degassing estimated from carbon balance of regional aquifers the case of central apennine italy central italy is characterized by an anomalous flux of deeply derived co2 in the western tyrrhenian sector of central italy co2 degassing occurs mainly from focused emissions vents and strong diffuse degassing and thermal springs whereas in the eastern apennine area deep co2 is dissolved in cold groundwaters of regional aquifers hosted by mesozoic carbonate evaporite formations influx of deep co2 into 12 carbonate aquifers 12 500 km2 of the central apennine is computed through a carbon mass balance that couples aquifer geochemistry with isotopic and hydrogeological data mass balance calculations estimate that 6 5 1010 mol yr 1 of inorganic carbon are dissolved in the studied aquifers approximately 23 of this amount derives from biological sources active during the infiltration of the recharge waters 36 comes from carbonate dissolution while 41 is representative of deep carbon sources characterized by a common isotopic signature 13c 3 the calculated deep co2 influx rate ranges from 105 to 107 mol yr 1 km 2 increasing regionally from east to west in the study area

mechanisms of thrust faulting in the gran sasso chain central apennines italy abstract the gran sasso chain in central italy is made up of an imbricate stack of eight thrust sheets which were emplaced over the upper miocene lower pliocene laga flysch the thrust sheets are numbered from 1 to 8 in order of their decreasing elevation in the tectonic stack and their basal thrusts are numbered from t 1 to t 8 accordingly on the basis of their different deformation features the major thrust faults fall into three groups 1 thrust faults marked by thick belts of incoherent gouges and breccia zones t 1 t 2 t 3 2 thrust faults characterized by a sharp plane which truncates folds that had developed in the footwall rocks t 5 t 6 and 3 thrust faults truncating folds developed in both the hangingwall and footwall units and bordered by foliated fault rocks t 7 the deformation features observed for the different faults seem to vary because of two combined factors 1 lithologic changes in the footwall and hangingwall units separated by the thrust faults and 2 increasing amounts of deformation in the deepest portions of the imbricate stack the upper thrust sheets from 1 to 6 are characterized by massive calcareous and dolomitic rocks they maintain a homoclinal setting and are truncated up section by the cataclastic thrust faults the lowermost thrust sheets 7 and 8 are characterized by a multilayer with competence contrasts which undergoes shear induced folding prior to the final emplacement of the thrust sheets bedding and axial planes of folds rotate progressively towards the t 5 t 6 t 7 and t 8 thrust boundaries and are subsequently truncated by propagation of the brittle thrust faults the maximum deformation is observed along the t 7 thrust fault consistent with horizontal displacement that increases progressively from the uppermost to the lowermost thrust sheet in the tectonic stack the axial planes of the folds developed in the hangingwall and footwall units are parallel to the t 7 thrust fault and foliated fault rocks have developed field data and petrographic analysis indicate that cleavage fabrics in the fault rocks form by a combination of cataclasis cataclastic flow and pressure solution slip associated with pervasive shearing along subtly distributed slip zones parallel to the t 7 thrust fault the development of such fabrics at upper crustal levels creates easy slip conditions in progressively thinner domains which are regions of localized flow during the thrust sheet emplacement

late oligocene to recent episodes of magmatism in and around the tyrrhenian sea implications for the processes of opening in a young inter arc basin of intra orogenic mediterranean type abstract five episodes of magmatic activity as deduced from geochronological data affected the tyrrhenian and peri tyr rhenian areas from the late oligocene 32 ma to recent times the episodes of w e migration of magmatism from the islands of sardinia and corsica to the eolian islands and the roman comagmatic region of central italy are related to the anti clockwise drift movements of the apenninic chain and clockwise drifting of the sicilian part of the maghrebian chain magmatic activity in the tyrrhenian sea initiated in late miocene times 8 5 6 5 ma in concomitance with a significant change of the geodynamic regime from compressional to tensional the tyrrhenian bathyal plain was affected by two episodes of spreading the first episode occurred around 7 ma and the second from about 1 9 to 1 3 ma these spreading events apparently developed during times of magmatic quiescence in the peri tyrrhenian areas two subsequent episodes of volcanism of the seamount type occurred in the bathyal plain one from 5 to 1 8 and the other from 1 3 to recent they are contemporaneously with extensive manifestations in the peri tyrrhenian areas the different types of magmatism of the late miocene back arc spreading in the central bathyal plain and acidic manifestations of intra crustal anatectic origin in the northern continental platform and slope i e islands of capraia elba and montecristo and seamount vercelli are possibly related to movements of upwelling of the astenosphere which in the southern tyrrhenian sea developed with a much stronger intensity timing of the volcanism in relationship with the overall structural evolution through time in the basin and neighbouring lands is considered

geologic and geochemical insights into the formation of the taiyangshan porphyry copper molybdenum deposit western qinling orogenic belt china abstract taiyangshan is a poorly studied copper molybdenum deposit located in the triassic western qinling collisional belt of northwest china the intrusions exposed in the vicinity of the taiyangshan deposit record episodic magmatism over 20 30 million years pre mineralization quartz diorite porphyries which host some of the deposit were emplaced at 226 6 6 2 ma syn collisional monzonite and quartz monzonite porphyries which also host mineralization were emplaced at 218 0 6 1 ma and 215 0 5 8 ma respectively mineralization occurred during the transition from a syn collisional to a post collisional setting at ca 208 ma a barren post mineralization granite porphyry marked the end of post collisional magmatism at 200 7 5 1 ma the ore bearing monzonite and quartz monzonite porphyries have a e hf t range from 2 0 to 12 5 which is much more variable than that of the slightly older quartz diorite porphyries with t dm2 of 1 15 1 23 ga corresponding to the positive e hf t values and t dm1 of 0 62 0 90 ga corresponding to the negative e hf t values molybdenite in the taiyangshan deposit with 27 70 to 38 43 ppm re suggests metal sourced from a mantle crust mixture or from mafic and ultramafic rocks in the lower crust the 34 s values obtained for pyrite chalcopyrite and molybdenite from the deposit range from 1 3 to 4 0 0 2 to 1 1 and 5 3 to 5 9 respectively suggesting a magmatic source for the sulfur calculated 18 o fluid values for magmatic k feldspar from porphyries 13 3 hydrothermal k feldspar from stockwork veins related to potassic alteration 11 6 and hydrothermal sericite from quartz pyrite veins 8 6 to 10 6 indicate the taiyangshan deposit formed dominantly from magmatic water hydrogen isotope values for hydrothermal sericite ranging from 85 to 50 may indicate that magma degassing progressively depleted residual liquid in deuterium during the life of the magmatic hydrothermal system alternatively d variability may have been caused by a minor amount of mixing with meteoric waters we propose that the ore related magma was derived from partial melting of the ancient mesoproterozoic to neoproterozoic middle to lower continental crust this crust was likely metasomatized during earlier subduction and the crustal magmas may have been contaminated with lithospheric mantle derived magma triggered by mash e g melting assimilation storage and homogenization processes during collisional orogeny in addition a significant proportion of the metals and sulfur supplied from mafic magma were simultaneously incorporated into the resultant hybrid magmas

subcrustal earthquakes in the northern apennines italy evidence for a still active subduction previously unreported subcrustal earthquakes have been located up to 90 km depth beneath the northern apennines italy these earthquakes occur beneath a zone of abundant upper crustal seismicity mostly confined to the upper 20 km of the crust although there are relatively few well located subcrustal earthquakes there appears to be a general southwestward deepening of the hypocenters from the adriatic to the tyrrhenian sea consistent with other geophysical data which suggest that the adriatic lithosphere is presently subducting beneath the northern apennines

ages of ultrahigh pressure metamorphism and protolith orthogneisses from the eastern dabie shan u pb zircon geochronology the dabie shan contains two rare geologic features one the occurrence of ultra high pressure metamorphic rocks peak p 2 7 gpa uhp in the east central part of this belt and two some of these uhp rocks are characterized by anomalously 18o depleted oxygen isotopic compositions geochronological data are presented that constrain the age of the uhp metamorphism the ages of protoliths and potentially the age of hydrothermal alteration believed to be responsible for the anomalous oxygen isotopic compositions the age of uhp metamorphism in the eastern dabie shan is dated at 218 5 1 7 ma using u pb in zircon separated from host gneisses this age is identical within uncertainty with a less precise age using u pb in zircon separated from uhp eclogite 225 5 3 6 ma and with previously published ages from uhp eclogites of the dabie shan map relationships strongly support an in situ as opposed to exotic orgin of the uhp eclogites in this region the identical ages of uhp metamorphism in the gneisses and eclogites provides additional support for this interpretation the age of the protolith gneisses is dated as 772 5 9 5 ma based on the upper intercept of the zircons which accords well with other dates of granitic gneisses in the qinling belt the association of mafic and felsic magmatism is interpreted to reflect rifting along the northern margin of the yangtze block at this time these upper intercept ages may date the hydrothermal circulation of 18o depleted meteoric water which gave rise to the anomalously 18o depleted uhp protoliths of the dabie shan and sulu region the coincidence of these protolith ages with the early sinian glaciation of south china is consonant with the suggestion that the highly anomalous oxygen isotopic compositions in this region date from this time the upper intercept age of the eclogite is imprecisely determined at 447 82 79 ma but is younger than the host gneisses and is not characterized by anomalous oxygen isotopic compositions

the geochemical characteristics of granitoids in contrasting arcs and comments on magma sources geochemical variations in modern subduction related igneous suites with respect to arc maturity in time and space are illustrated using data for both volcanic suites basalt andesite and dacite and intrusive granitoid suites diorite tonalite granodiorite and granite from circum pacific arcs using trace element data we suggest that two groups of processes control the natural variation in the parental magmas of both suites a subduction zone enrichment of lithospheric mantle locally coupled with crustal assimilation allied with fractional crystallization afc in zones of thickened crust all of which yield magmas with enhanced concentrations of the large ion lithophile lil elements k rb th u lrees etc b with increasing distance from the active trench contributions from within plate sub continental lithosphere producing mantle derived magmas with enhanced levels of high field strength hfs elements among which nb ta hf and y are particularly distinctive thus even for the evolved granitoids of intrusive arc series ratios of hfs lil elements not significantly affected by crystal fractionation such as ta nb k rb la may throw some light on the origin of mafic intermediate precursor magmas in terms of these elements we suggest the following groupings for interpreting the tectonic associations of granitoid suites 1 primitive calcic arc granitoids with low lil and hfs element abundances 2 normal calc alkaline continental arc granitoids with enhanced lil element abundances and low hfs lil ratios 3 mature alkali calcic arc granitoids with high levels of lil and hfs elements and higher hfs lil ratios 4 back arc anorogenic alkaline granitoids with the highest levels of hfs elements

styles of back arc extension in the central mediterranean in the central mediterranean two back arc basins the liguro provencal lpb and the tyrrhenian basin tb opened progressively and consecutively from the late eocene oligocene to the present evolution in space and time of rifting and drifting processes along three different transects across these basins shows differences in the style of extension lpb opened with the formation of a narrow single rift while in the tb deformation and magmatism is spread over a wide area moreover at the northern end of the tb the locus of extension progressively migrated towards the east whereas in the southern tb the locus of extension and magmatism migrated inside the basin inducing continental break up and drifting of the previously formed older conjugate basins we propose that these different styles of back arc extension depend upon internal conditions such as prerift rheology linked with its geological heritage and external conditions e g the style of subduction

zircon morphology geochronology and trace element geochemistry of the granites from the huangshaping polymetallic deposit south china implications for the magmatic evolution and mineralization processes abstract the huangshaping polymetallic deposit is a large scale deposit located in the central nanling region south china different types of mineralization have been classified in the area which are attributed to three types of granitoids from the composite granitic pluton quartz porphyry granophyre and granite porphyry in order to study the magmatic genesis and evolutionary process as well as constrain the crystallization and mineralization events 100 zircon separates from different intrusive stages of the pluton have been analyzed for trace element contents and u pb isotopic concentrations the results show that zircons from the huangshaping granitoids are enriched in th u hf ti ree and other trace elements indicating hydrothermal fluid alteration and transformation additionally there is a large variation between different types of granitoids in terms of trace element concentrations compared to the zircons from the quartz porphyry and the granophyre zircons from the granite porphyry are more enriched in th u hf ti and p but strongly depleted in eu this suggests that the magma that formed the granite porphyry possessed higher temperatures possibly experienced stronger crystal fractionation and originated from a more evolved complex magmatic source region with higher degree of crustal maturity combined with characteristics of crystal morphology and internal texture hydrothermal zircons were distinguished from magmatic zircons in each granitoid to reveal mineralization processes the u pb geochronology results show that there are two major stages of mineralization 190 ma weak cu pb zn mineralization related to the quartz porphyry and the granophyre and 155 ma strong w sn mo pb zn mineralization associated with the granite porphyry compositions of these hydrothermal zircons suggest that the earlier stage mineralization could be close to single relatively cold reduced and quickly ascended fluids whereas the later stage might be related to hot oxidized strongly fractionated and highly evolved mixed fluids this study also indicates that hydrothermal zircons from typical composite granitic plutons could be a good tool to reveal the multi stage magmatic and mineralization process in middle to late jurassic south china

zircon u pb geochronology sr nd hf isotopic composition and geological significance of the late triassic baijiazhuang and lvjing granitic plutons in west qinling orogen the qinling orogen was a consequence of continental collision of the south china craton with the north china craton in the triassic and caused widespread granitoid magmatism however the petrogenesis of these granitoids remains controversial in this paper we choose the baijiazhuang bjz and lvjing lj plutons in the west qinling orogen for a combined study of the zircon u pb geochronology whole rock major and trace element compositions and sr nd hf isotopic characteristics we obtained zircon crystallization ages of similar to 216 ma and similar to 212 ma for the bjz and the lj plutons respectively the granitoid samples from both plutons have high k2o metaluminous to peraluminous compositions they are enriched in large ion lithophile elements liles light rare earth elements lrees and depleted in high field strength elements hfses with significant negative eu anomalies the bjz samples have initial sr isotopic ratios of 0 7032 to 0 7078 epsilon nd t of 10 99 to 8 54 and epsilon hf t of 1022 to 6 41 the lj granitoids have initial sr isotopic ratios of 0 7070 to 0 7080 epsilon nd t of 5 37 to 4 58 and epsilon hf t of 3 64 to 1 78 the enriched isotopic characteristics of the two plutons are consistent with their source being dominated by ancient continental crust however two bjz samples show depleted sr isotope compositions which may infer possible involvement of mantle materials mantle derived melt which formed from partial melting of mantle wedge peridotite facilitated by dehydration of the subducted subducting mianlue ocean crust provide the required heat for the crustal melting while also contributing to the compositions of these granitoids that is the two granitic plutons are magmatic responses to the closure of the mianlue ocean basin and the continental collision between the yangtze and south qinling crustal terranes c 2016 elsevier b v all rights reserved