查看更多>>摘要：The Jianping Complex(JPC),located at the northwestern margin of the Eastern Block in the North China Craton(NCC),is one of the high-grade complexes that subjected to granulite facies metamorphism.It has been suggested that the JPC at least experienced two phases of Precambrian metamorphism.This study focuses on the late Neoarchean to early Paleoproterozoic metamorphic evolution of the felsic and intermediate granulites with'redeye socket'texture.Typical peak stage minerals including plagioclase,orthopyroxene,clinopyroxene and quartz,as well as post-peak growth of garnet,clinopyroxene,plagioclase,amphibole and quartz were identified.Phase equilibria modelling for two selected samples was performed using THERMOCALC 3.45.Based on the petro-graphic observations and the modelling results,anticlockwise P-T paths are recovered for the granulites.Peak stage metamorphic P-T conditions are constrained to be 940-1000 ℃/8.0-8.5 kbar,and the post-peak final solidus assemblages were stable at the conditions of 880-900 ℃/9.5-1_(0.5)kbar.U-Pb dating of three granulites yields magmatic ages of ~2.50-2.53 Ga and metamorphic ages of ~2.49 Ga,suggesting the short interval between the emplacement and the granulite facies metamorphism.Anticlockwise P-T paths can result from intrusion and underplating of mantle-derived magmas in several potential tectonic settings,among which scenarios of magmatic arc and mantle plume are both possible for the NCC at-2.5 Ga.Comprehensively considering the lithological,metamorphic,geochronological,and structural characteristics of the Neoarchean basement rocks in the NCC,we favor the mantle plume model in this study.
查看更多>>摘要：Mantle wedge peridotites are characterized by extraordinarily depleted compositions and their genesis is still controversial.In this study,we present microstructural observations and mineral chemical analyses of mantle harzburgites in the Lycian ophiolite of SW Turkey that reveal their formation in a suprasubduction zone mantle wedge.The textures of clinopyroxene in the harzburgites,particularly chromite-clinopyroxene symplectites and grains containing mineral inclusions,do not support a partial melting origin.Zoning textures and the presence of additional phases within the orthopyroxene porphyroclasts indicate that they underwent melt/fluid infiltration.Olivine crystals and interstitial grains are enclosed in orthopyroxene porphyroclasts,implying that the ortho-pyroxene grains crystallized by consuming olivine.Chromite grains in the harzburgites not only have zoned,but also contain a variety of mineral inclusions.Amphibole grains in some of the harzburgites also have compositions similar to those in the chromitites.Therefore,all mineral phases in the Lycian harzburgites have well-preserved metasomatic signatures that overprinted their partial melting features.The enrichment of fluid-mobile elements,such as Cs,Rb,U,and Pb in clinopyroxene and amphibole,coupled with their high Mg# values(92.8-95.6 and 93.7-96.4,respectively),suggest formation by Cr-and Mg-rich,hydrous metasomatic melts/fluids.The consistent compositions of clinopyroxene and chromite in the harzburgites and chromitites,together with the decreasing intensity of alteration and crystallization temperature of amphibole in the chromitite pods through dunites to harzburgites,imply that the hydrous melts/fluids were partially released from chromite surfaces during chromitite formation and/or from dehydration of the metamorphic sole,which likely attributed to metasomatism at different depths.Moreover,chromite-hosted inclusions in the harzburgites may reflect the reaction before solidification of an inclusion-rich zone possibly signifying a rapid increase of chromite-oversaturated melt.Positive correlations between Cr and Al in pyroxenes of ophiolites are widely preserved in metasomatic mantle peridotites from different tectonic settings and thus they provide a robust chemical proxy for the metasomatism.
查看更多>>摘要：Titanite and zircon from a suite of scapolite pegmatites in the Nord0yane ultra-high-pressure domain,Western Gneiss Region,Norway,were analysed by laser ablation-inductively coupled plasma-mass spectrometry(LA-ICP-MS).Titanite yielded U-Pb ages ranging from 420 to 390 Ma,with dates from prismatic crystals overlapping at 400-390 Ma.Complex zoning and resorption of older titanite grains(>415 Ma),interpreted as inherited,suggests that protracted crystal-melt reaction led to variable REE enrichment.Zircon rim zones yielded U-Pb ages of 392-385 Ma,with inherited cores yielding~(207)Pb/~(206)Pb ages of 1774-1575 Ma.In combination,the zircon and titanite U-Pb data suggest that the pegmatites crystallised at ca.400-385 Ma,consistent with the results of earlier studies.Trace element data are compatible with crystallisation at amphibolite facies conditions from melts in which fluid activity may have increased with time.Based on the ages of inherited zircon cores and titanite eNd_(400)values of-8 to-11,the melts that gave rise to the scapolite pegmatites are interpreted to have been derived from Proterozoic Baltican crust.Partial melting initiated during subduction of Baltican crust in response to an influx of mantle fluid,with melt production greatly enhanced during exhumation and decompression.Complex zoning and age signatures in both titanite and zircon point to crystal-melt-fluid interaction over a protracted interval of melt production,migration,and crystallisation.
查看更多>>摘要：Early Cretaceous lamprophyre dykes widely occurred in the southeastern margin of the North China Craton(NCC),which are the important probes to understand the enrichment and depletion processes that modified the subcontinental lithospheric mantle of the NCC.In this research,one lamprophyre site(Liguo),which is the nearest lamprophyre to the South China Block(SCB)ever discovered in the NCC,had been used to analyze the influence of the deeply subducted SCB on the lithospheric evolution of the southeastern NCC.This lamprophyre was investigated by apatite U-Pb dating with the age of 136 Ma,and it is depleted in HFSE,enriched in LILE and Sr-Nd-Pb isotope compositions(~(87)Sr/~(86)Sr(i)= 0.708038 to 0.709305,ε_(Nd)(t)=-8.0 to-9.8,~(206)Pb/~(204)Pb(t)ratios of 17.477 to 17.536,~(207)Pb/~(204)Pb(t)of 15.548 to 15.552,and 208Pb/~(204)Pb(t)of 37.944 to 38.004).Interpretations of minerals and whole rock elemental data suggest the Liguo lamprophyre was derived from 1 to 5% partial melting of garnet lherzolite; and MELTS modeling indicates fractional crystallization of-26.5% clinopyroxene,~0.1% spinel and ~ 3.5% garnet under 2.5 Gbar.Together with the Pb isotopic compositions of the contemporaneous lamprophyres in the Jiaodong Peninsula,it suggested that the mantle source of the Liguo lamprophyre was mainly modified by melts derived from partial melting of the subducted South China continental crust in the Triassic.In contrast,the magma sources for the lamprophyres in Jiaodong Peninsula were mainly modified by hydrous fluids derived from the subducted Paleo-Pacific oceanic crust in the Jurassic.Finally,it is concluded that the lithospheric mantle in the southeastern margin of the NCC is heterogeneous,and the subducted SCB could be the source of the enriched component in the southern margin of the NCC.
查看更多>>摘要：Samples of an early olivine gabbro in the lower plutonic sequence of the Troodos ophiolite were studied to constrain their thermal history and the compositions of their parental magma.These rocks exhibit typical orthocumulate textures and petrographic evidence of reaction between a migrating interstitial magma and a crystal mush,including embayed margins of olivine crystals and irregular ragged grain boundaries of clino-pyroxenes,indicating dissolution of olivine and crystallization of clinopyroxene,followed by crystallization of plagioclase within the framework of olivine and clinopyroxene.All the minerals exhibit relatively homogeneous compositions,without any clear zoning in either their major or.trace elements,indicating that the minerals and migrating magma reached equilibrium.The rare earth element(REE)distribution between clinopyroxene and plagioclase records an equilibrium temperature of about 1300 ℃,but the Mg-exchange geothermometer yielded a lower temperature of about 1000 ℃.These different closure temperatures indicate a rapid cooling history with a cooling rate of-2 ℃/year log units.The low TiO2 contents and additional trace element compositions of clinopyroxene show that the parental magma of the gabbro was related to boninitic magmas of the ophiolite.Consequently,we show that some boninitic lavas in the upper oceanic crust have been affected by melt-rock reaction process during their ascent through the lower crust,and therefore their composition cannot be directly used to constrain the composition of the primary boninites.
查看更多>>摘要：The calc-alkaline lavas of the submarine Pausanias Volcanic Field and the neighboring Methana peninsula represent the largest volcanic edifices of the western South Aegean Volcanic Arc(SAVA)in the Mediterranean Sea and erupted on 30-35 km thick continental crust.We report the first whole rock major and trace elements together with Sr-Nd-Pb isotope ratios for basaltic to andesitic lavas from the submarine Pausanias Volcanic Field consisting of six smaller,volcanic structures.The mafic lavas have the most primitive compositions of the SAVA with MgO of up to 9 wt% and Ni concentrations >100 ppm.The incompatible trace element abundance patterns of the Pausanias and Methana lavas overlap and have typical island arc patterns,but Pausanias lavas display a larger geochemical heterogeneity on a smaller spatial scale compared to those from Methana.The Pausanias lavas resemble those from Methana in Sr,Nd,and Pb isotopes reflecting higher sediment subduction in the western SAVA than in the central arc at Santorini.The variability of incompatible element concentrations at relatively constant isotope compositions suggests variable degrees of partial melting of a peridotite-sediment melange diapir in the mantle wedge.The Pausanias magmas formed from a less depleted mantle source compared to the central SAVA and their ascent in an extensional basin supported relatively rapid ascent without the extensive stagnation in the crust observed in the Methana magmas.The compositional differences between Pausanias and Methana imply differences in their composition and/or transport in the mantle wedge and the crust,allowing highly variable melts to reach the surface within a spatial distance of few kilometers.
查看更多>>摘要：Ophiolitic mantle peridotites show large variations in their geochemical compositions,which provide important information for the histories of mantle melting,melt metasomatism and prior melt depletion in the context of tectonic evolution of ancient oceans.Here we report new petrological and geochemical data for mantle peridotites from the Zhagabu ophiolite in south Tibet,which could represent fossil upper mantle of the Mesozoic Neo-Tethys oceanic lithosphere.The Zhagabu mantle peridotites,including spinel lherzolites and spinel harz-burgites,have highly heterogeneous geochemical compositions,with whole-rock Al2O3 of 0.3-2.0 wt% and CaO of 0.3-2.2 wt%,and spinel Cr# [Cr/(Cr + Al)] of _(0.2)1-0.74 and Mg# [Mg/(Mg + Fe~(2+))] of _(0.4)4-0.74.The clinopyroxenes from the studied samples show extremely depletion in light rare earth element compositions,some of which however are elevated by post-melting melt-rock interactions.Spinel and clinopyroxene compositions collectively suggest that the Zhagabu mantle peridotites have experienced-9-20% of fractional melting from a depleted mid-ocean ridge basalt mantle source.Trace element modeling of clinopyroxenes further indicates that some of the studied samples follow the refertilization-hydrous melting trends with variable fluid fluxes.Therefore,the heterogeneous compositions of the Zhagabu mantle peridotites are not comparable to those of either abyssal peridotites or forearc peridotites,but most probably record a dynamic transition from the anhydrous condition beneath the Neo-Tethyan mid-ocean ridge to the hydrous condition above the subducted slab.Considering that a forearc spreading is in contradiction with the low magma budget of Yarlung Zangbo ophiolites,the Neo-Tethyan subduction re-initiation was suggested to be achieved along oceanic detachment faults at ultraslow-spreading ridges.The Zhagabu mantle peridotites represent the very heterogeneous mantle massif in south Tibetan ophiolites,and provide an excellent window to investigate the complex mantie dynamics and tectonic evolution of the Neo-Tethys Ocean.
查看更多>>摘要：The Zambezi Belt located in southern Zambia and northern Zimbabwe forms part of the E-W-trending Kuunga orogen associated with the amalgamation of Gondwana supercontinent.The belt consists of a crystalline basement unconformably overlain by a supracrustal sequence of meta-pelites/psammites,calc-silicates,meta-car-bonates,and magmatic rocks.Monazite-bearing meta-pelites are distinguished by petrography,geothermobarometry,monazite chemistry,and detrital provenance.The studied samples include six garnet-bearing schists with biotite,plagioclase,quartz,muscovite ± staurolite ± kyanite from the Nega Formation,and three kyanite-mica schists with chlorite,quartz,and accessory rutile,ilmenite,tourmaline from the Chi-pongwe Formation.Phase equilibria modelling and isopleth geothermobarometry identified Barrovian-type medium-T/medium-P facies peak conditions of 570-665 ℃/6.5-9 kbar on garnet-bearing schists,and 600-663 ℃/~8.8-12 kbar on kyanite-mica schists.Evidence from detrital geochronology suggests that the two formations are stratigraphically equivalent,but electron microprobe dating on monazite suggests they underwent different tectonothermal histories.Monazite ages indicate metamorphism and crustal growth from ~650-579 Ma in 3 of the garnet-bearing schists of the Nega Formation,and ~550-500 Ma in one of the kyanite-mica schists of the Chipongwe Formation.Dating of detrital zircon by LA-ICP-MS indicates local and distal sediment sources including Neoarchean to Paleoproterozoic(2633-1800 Ma)and Meso-to Neoproterozoic(1700-852 Ma).A tectonic cycle of burial metamorphism to exhumation is indicated by clockwise P-T-t paths along different geothermal gradients.The rock units were brought together during collision between the Congo and Kalahari Cratons and final assembly of Gondwana.
查看更多>>摘要：Late Mesozoic uplift of the Gangdese arc occurred prior to the India-Asia collision and was important in the evolution of the Tibetan plateau,but the geodynamic mechanisms that drove this process are unclear.In this paper,we report geochronological,petrological,geochemical,and Sr-Nd-Hf isotopic data for intermediate-acidic igneous rocks from the western Gangdese arc.These igneous rocks were emplaced at 87-67 Ma and are characterized by enrichments in light rare earth and large-ion lithophile elements,depletions in high-field-strength elements,high(~(87)Sr/~(86)Sr)i ratios(0.7074-0.7116),and low ε_(Nd)(t)(-7.5 to-4.1)and ε_(Hf)(t)values(-16 to 0.8).These magmas were most likely derived by high-temperature(960-1200 ℃)partial melting of the thickened crustal materials,with minor mantle input,related to post-orogenic lithospheric delamination.The increasing ε_(Nd)(t)and ε_(Hf)(t)values and decreasing initial Sr isotope ratios from the ca.87 Ma diorites to ca.67 Ma diorites and rhyolites indicate an increasing mantle contribution as delamination proceeded.Estimated pressure conditions for the high-Sr/Y gabbroic diorites indicates the magmas were derived from crust at pressures of up to 19 kbar(i.e.,a depth of-66 km).Therefore,the western Gangdese arc had been thickened during the Late Cretaceous,possibly due to the flat-slab subduction of Neo-Tethyan oceanic lithosphere and coeval collision of the Qiangtang-Lhasa terranes.
查看更多>>摘要：Phosphorus(P)is one of the most critical limiting nutrients for biological evolution,and volcanic rocks are a crucial source of available P for terrestrial ecosystems.Volcanic-sedimentary records indicate that massive volcanism on the North China Craton in the late Mesozoic supported the evolution of terrestrial biotas.The variation of P content in the Mesozoic mafic volcanics mainly results from melt-garnet reactions during litho-spheric thinning.The high P contents in most mafic volcanics worldwide could be related to the contents of olivine and clinopyroxene.In contrast,P variation in felsic volcanics could depend on apatite saturation.In the Phanerozoic volcanics,P content increases with the decrease of rock ages,resulting from the Earth's cooling.The contents of P and SiO2 in these Phanerozoic volcanics have similar variation trends.However,this is not the case for intrusive rocks,which could have experienced more complex and protracted magmatism.Therefore,deep processes and associated magmatism control the variation of P in volcanics,ultimately reflected in the evolution of surface ecosystems.
NSTL