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：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 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.
查看更多>>摘要：This study combines zircon U-Pb ages,whole-rock major and trace elements,and Sr-Nd isotope data to investigate the petrogenesis and tectonic implications of late Paleozoic magmatic rocks in central Inner Mongolia of the southeastern Central Asian Orogenic Belt.The results indicate the presence of two stages of bimodal magmatism from the late Carboniferous to early Permian.The late Carboniferous(ca.323 Ma)bimodal associations consist mainly of gabbroic and granitic intrusive rocks,whereas the early Permian(ca.273 Ma)bimodal suites are dominated by dolerite and granite porphyry dikes.The gabbroic intrusive rocks and dolerite dikes are calc-alkaline,slightly enriched in light rare earth elements and depleted in Nb,Ta,Ti,and have low initial ~(87)Sr/~(86)Sr ratios(0.70317-0.70510)and high ε_(Nd)(t)values(+3.1 to +7.4).They are both interpreted to result from partial melting of a depleted spinel-lherzolite mantle source that was metasomatized by subduction-related components.The granitic intrusive rocks show I-type granite affinity and similar geochemical features to coeval gabbroic rocks,suggesting they were derived from a melt-induced anatexis of basaltic lower crust.The early Permian granite porphyry dikes show characteristics of LREE enrichment,negative Nb-Ta anomalies and positive Zr-Hf anomalies.They have higher La/Yb and Th/Zr ratios and ε_(Nd)(t)values,and lower initial ~(87)Sr/~(86)Sr ratios than the coeval dolerite dikes.These features suggest that the early Permian felsic dikes were most likely generated by partial melting of a juvenile lower crust mixing with minor mantle-derived components.An integrated study with regional geological data suggests that the episodic bimodal suites in central Inner Mongolia formed in a post-collisional to within-plate rift setting after the closure of the Paleo-Asian Ocean.
查看更多>>摘要：Exploring magmatic'flare-up'events can be used to unravel dynamic Earth processes including plate subduction and crustal growth.However,the cause and mechanism of magmatic'flare-up'remain unresolved.The abundant magmatic records that are preserved in the Lhasa Terrane on the southern Tibetan Plateau make it an important region to study the episodic nature of the secular magmatism.This paper presents geochronological,mineral-ogical,and geochemical data that describes the Dazhuka diorites in the central-western and Sangri granites in the eastern parts of the southern Lhasa Terrane.Zircon LA-ICP-MS U-Pb data indicate that these rocks were generated around ~95-90 Ma.Both are enriched in large ion lithophile elements and depleted in high field strength elements,indicating geochemical affinity with subduction-related magmatic rocks.The Dazhuka diorites have low SiO2 contents(54.49-56.72 wt%),with moderate Mg#(41.7-42.7)and rare earth element fractionation((La/Yb)N = 6.9-8.6),and homogeneous whole-rock ~(87)Sr/~(86)Sri(0.704204-0.704480),ε_(Nd)(t)(+3.5-h4.1),and zircon ε_(Hf)(t)(+1_(0.2)-+13.3).These features suggest that the Dazhuka diorites originated in the spinel-garnet transition zone within the lithospheric mantle of a mantle wedge environment.The Sangri granites have high SiO2(64.36-69.54 wt%),Al2O3(14.27-15.61 wt%),and(La/Yb)N(4.6-49.6)and Sr/Y(34.7-119.3)ratios,indicating geochemical affinity with adakites.Low MgO(1.3-2.22 wt%)and depleted Sr-Nd-Hf isotopes(whole-rock ~(87)Sr/~(86)Sri = 0.704099-0.705453,ε_(Nd)(t)= +2.9-+3.5,zircon ε_(Hf)(t)= +7.1-+9.3)suggest that the granites were derived from oceanic crust.The types of magmatic rocks that are distributed over the central-western and the eastern parts of the southern Lhasa Terrane differ,with higher magmatic temperatures in the east as compared to the central-west.These results indicate that different areas in the southern Lhasa Terrane may have experienced different geodynamic processes,with oceanic ridge subduction in the eastern part of the southern Lhasa Terrane and normal oceanic plate subduction in the central-western part.These different types of subduction processes are likely to have caused the magmatic'flare-up'and variation in the magmatism of southern Tibet.
查看更多>>摘要：Numerous studies have shown that the mid-ocean-ridge basalts(MORBs)distributed in the southern Central Indian Ridge(CIR)exhibit the DUPAL anomaly; yet,the corresponding temporal variations of this signal remain underexplored in this region.Here,we report new Sr-Nd-Pb isotope data for the MORB which forms the southernmost segment of the CIR(CIR-1)and interpret these together with previously reported elemental data.Our results indicate that the MORB from the current spreading axis of the CIR-1 segment is produced by a mixture of the partial melt of the depleted MORB mantle and melt derived from the lower continental crust(LCC).This interpretation is consistent with the previously proposed model for Indian MORBs that exhibit the DUPAL anomaly.In contrast,the MORB from the of f-axis area shows a depleted trace element composition,particularly for highly incompatible elements.This can be explained by the mixture of the partial melt of a highly trace element-depleted mantle that has not been previously defined in this area and melt from the LCC and C component.Furthermore,high ~(87)Sr/~(86)Sr,A8/4,and A7/4 values indicate that the source mantle of the MORB in the of f-axis area of the CIR-1 segment has undergone at least two melting events,including the formation of the current MORB.The first melting event occurred approximately 500 Ma and resulted in lower parent/daughter ratios(Th/Pb and U/Pb)in the residual mantle,yielding an unradiogenic lead isotope composition.Overall,our results suggest that a highly depleted mantle domain that underwent an ancient melting event lies under the northern CIR-1 segment,contributing to the formation of MORBs with an unradiogenic lead isotopic signature.
查看更多>>摘要：The Paleo-Tethyan arc belt is host to a small number of porphyry Cu deposits the key controls on which remains poorly understood.It has been hypothesized that the Paleo-Tethyan arc magmas were reduced and therefore less fertile for porphyry Cu formation but few studies have been conducted to directly assess the magmatic oxygen fugacity.We investigate the magma fertility of eastern Paleo-Tethyan arc igneous rocks in the Sanjiang region,SW China,represented by the Maoding(268.3 ± 2.3 Ma),Suwalong(275.6 ± 1.4 Ma to 264.7 ±1.4 Ma)and Baimaxueshan(255.7 ± 1.5 Ma to 249.3 ± 1.1 Ma)granitoids.They have geochemical features consistent with typical continental arc rocks,formed from partial melting of the mantle wedge,with increasing contamination of crust-derived melts over time.The presence of amphibole,high Sr/Y ratios(35 ± 20,n = 17),and lack of negative Eu anomalies(Eun/Eun* = 1.04 ± 0.3,n = 17)are consistent with derivation from hydrous magmas.Zircons from the intrusive rocks have relatively low Eun/Eun*(mostly<(0.4))ratios and calculated AFMQ values(-2.1 ± 1.2,n = 86).Combined with apatite microphenocrysts with extremely low sulfur contents(<0.12 wt% SO3),it suggests that the intrusions originated from water-rich but reduced arc magmas,that were less fertile for porphyry Cu formation.Our study highlights that magmatic fO2 is probably a critical control on the prospectivity of arc systems and confirms that zircon and apatite compositions can be used to evaluate porphyry Cu fertility.
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