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1.
Geophysical data illustrate that the Indian continental lithosphere has northward subducted beneath the Tibet Plateau, reaching the Bangong–Nujiang suture in central Tibet. However, when the Indian continental lithosphere started to subduct, and whether the Indian continental crust has injected into the mantle beneath southern Lhasa block, are not clear. Here we report new results from the Quguosha gabbros of southern Lhasa block, southern Tibet. LA-ICP-MS zircon U–Pb dating of two samples gives a ca. 35 Ma formation age (i.e., the latest Eocene) for the Quguosha gabbros. The Quguosha gabbro samples are geochemically characterized by variable SiO2 and MgO contents, strongly negative Nb–Ta–Ti and slightly negative Eu anomalies, and uniform initial 87Sr/86Sr (0.7056–0.7058) and εNd(t) (− 2.2 to − 3.6). They exhibit Sr–Nd isotopic compositions different from those of the Jurassic–Eocene magmatic rocks with depleted Sr–Nd isotopic characteristics, but somewhat similar to those of Oligocene–Miocene K-rich magmatic rocks with enriched Sr–Nd isotopic characteristics. We therefore propose that an enriched Indian crustal component was added into the lithospheric mantle beneath southern Lhasa by continental subduction at least prior to the latest Eocene (ca. 35 Ma). We interpret the Quguosha mafic magmas to have been generated by partial melting of lithospheric mantle metasomatized by subducted continental sediments, which entered continental subduction channel(s) and then probably accreted or underplated into the overlying mantle during the northward subduction of the Indian continent. Continental subduction likely played a key role in the formation of the Tibetan plateau at an earlier date than previously thought.  相似文献   

2.
Subduction related basalts display wide ranges in large ion lithophile element ratios (e.g., Rb/Ba and Rb/ Sr) which are unlikely to result from mixing, but suggest a role for small degree partial melting of a relatively Rb-poor mantle wedge source. However, these variations do not correlate with other trace element criteria, such as the depletions of high field strength elements (HFSE) and light rare earth elements (LREE) relative to the LILE, which characterise subduction related magmatism. Integration of radiogenic isotope and trace element data demonstrates that the elemental enrichment cannot be simply related to two component mixtures inferred from isotopic variations. Thus a minimum of three components is required to describe the geochemistry of subduction zone basalts. Two are subduction related: high Sr/Nd material is derived from the dehydration of subducted basaltic ocean crust, and a low Sr/Nd component is thought to be from subducted terrigenous sediment. The third component is in the mantle wedge, it is usually similar to the source of MORB, particularly in its isotopic composition. However, in some cases, notably continental areas, more enriched mantle wedge material with relatively high 87Sr/86Sr, low 143Nd/144Nd and elevated incompatible trace element contents may be involved Mixing of these three components is capable of producing both the entire range of Sr, Nd and Pb isotope signatures observed in destructive margin basalts, and their distinctive trace element compositions. The isotope differences between Atlantic and Pacific island arc basalts are attributed to the isotope compositions of sediments in the two oceans.  相似文献   

3.
The Dexing porphyry copper deposit, part of the circum-Pacific porphyry copper ore belt, is the largest porphyry copper deposit in China. We present new LA–ICP–MS zircon U–Pb and molybdenite Re–Os dating, bulk-rock elemental and Sr–Nd–Pb isotopic as well as in situ zircon Hf isotopic geochemistry for these ore-bearing porphyries, in an attempt to better constrain their petrogenesis. LA–ICP–MS zircon U–Pb dating shows that the Dexing porphyries were emplaced in the early Middle Jurassic (～171 Ma); molybdenite Re–Os dating indicates that the associated Cu–Mo mineralization was contemporaneous (～171 Ma) with the igneous intrusion. The rocks are mainly high-K calc-alkaline and show adakitic affinities, including high Sr and low Y and Yb contents, high Sr/Y and La/Yb ratios, and high Mg# (higher than pure crustal melts). These porphyries have initial 87Sr/86Sr ratios of 0.7044?0.7047, ?Nd(T) values of –1.5 to?+0.6, and ?Hf(T) (in situ zircon) values of?+2.6 to?+4.6. They show unusually radiogenic Pb isotopic compositions with initial 206Pb/204Pb ratios up to 18.41 and 207Pb/204Pb up to 15.61. These isotopic compositions are distinctly different from either Pacific MORB or Yangtze lower crust but are similar to the subducting sediments in the western Pacific trenches. Detailed elemental and isotopic data suggest that the Dexing porphyries were emplaced in a continental arc setting coupled with westward subduction of the palaeo-Pacific plate. Partial melting involved the subducted slab (mainly the overlying sediments), with generated melts interacting with the lithospheric mantle wedge, thereby forming the investigated high-K calc-alkaline porphyry magmas.  相似文献   

4.
We present first LA-ICP-MS U–Pb zircon ages as well as geochemical and Sr–Nd–Pb isotope data for 14 magmatic rocks collected along ca. 400 km profile across the Chatkal-Kurama terrane in the Mogol-Tau and Kurama ranges and the Gissar Segment of the Tien Shan orogen in Tajikistan. These new data from supra-subduction and post-collisional magmatic rocks of two Late Paleozoic active margins constrain a tectonic model for terrane motions across two paleo-subduction zones: (1) The 425 Ma old Muzbulak granite of the Mogol-Tau range formed in a supra-subduction setting at the northern margin of the Turkestan Ocean. The north-dipping plate was subducted from the Early Silurian to the earliest Middle Devonian. Thereafter the northern side of the Turkestan Ocean remained a passive margin until the Early Carboniferous. (2) In the Early Carboniferous, subduction under the northern margin of the Turkestan Ocean resumed and the 315 to 305 Ma old Kara-Kiya, Muzbek, and Karamazar intrusions formed in a supra-subduction setting in the Mogol-Tau and Kurama ranges. (3) At the same time, in the Early Carboniferous, rifting of the southern passive margin of the Turkestan Ocean formed the short-lived Gissar Basin, separated from the Turkestan Ocean by the Gissar micro-continent. North-dipping subduction in the Gissar Basin is documented by the 315 Ma Kharangon plagiogranite and the voluminous ca. 321–312 Ma Andean-type supra-subduction Gissar batholith. The Kharangon and Khanaka gabbro-plagiogranite intrusions of the southern Gissar range have geochemical and Sr–Nd isotopic compositions (87Sr/86Sr(t) 0.7047–0.7056, εNd of + 1.5 to + 2.3) compatible with mantle-derived origin typical for plagiogranites associated with ophiolites. The supra-subduction rocks from the Gissar batholith and from the Mogol-Tau Kurama ranges have variably mixed Sr–Nd–Pb isotopic signatures (87Sr/86Sr(t) 0.7057–0.7064, εNd of − 2.1 to − 5.0) typical for continental arcs where mantle-derived magmas interact with continental crust. (4) In the latest Carboniferous, the Turkestan Ocean and the Gissar Basin were closed. The Early Permian Chinorsay (288 Ma) and Dara-i-pioz (267 Ma) post-collisional intrusions, emplaced in the northern part of the Gissar micro-continent after a long period of amagmatic evolution, have intraplate geochemical affinities and isotopic Sr–Nd–Pb isotopic compositions (87Sr/86Sr(t) 0.7074–0.7086, εNd of − 5.5 to − 7.4) indicating derivation from Precambrian continental crust which is supported by old Nd model ages (1.5 and 1.7 Ga), and by the presence of inherited zircon grains with ages 850–500 Ma in the Chinorsay granodiorite. The post-collisional intrusions in the southern Gissar and in the Mogol-Tau and Kurama ranges (297–286 Ma), emplaced directly after supra-subduction magmatic series, have geochemical and isotopic signatures of arc-related magmas. The distinct shoshonitic affinities of post-collisional intrusions in the Mogol-Tau and Kurama ranges are explained by the interaction of hot asthenospheric material with subduction-enriched wedge of lithospheric mantle due to slab break-off at post-collisional stage. Despite origination from different tectonic environments, all magmatic rocks have relatively old Nd model ages (1.7–1.0 Ga) indicating a significant proportion of Paleoproterozoic or older crustal material in their sources and their model ages are similar to those of post-collisional intrusions from the Alai and Kokshaal Segments of the South Tien Shan.  相似文献   

5.
ABSTRACT

Intrusive rocks are well-exposed in the south Birjand around the Koudakan is herein compared to previously studied outcrops along the middle Eocene to late Oligocene Eastern Iran Magmatic Belt. This pluton is composed mainly of monzonite, quartz-monzonite, and granite with high-K calc-alkaline to shoshonitic affinities. The U-Pb zircon geochronology from monzonite and quartz-monzonite reveals the crystallization ages of 40.96 ± 0.48 to 38.78 ± 0.78 Ma (Bartonian). The monzonite, quartz-monzonite, and granite rocks show similar REEs and trace element patterns, as well as limited variations in εNd(i) and 87Sr/86Sr(i) ratio, suggesting that they are a comagmatic intrusive suite. The chondrite and primitive mantle normalized rare earth and trace element patterns show enrichment in the light rare earth elements, K, Rb, Cs, Pb, Th, and U and depletion in heavy rare earth elements, Nb, Zr, and Ti. The εNd(i) and 87Sr/86Sr(i) values range from +1.32 to +1.68 and 0.7044 to 0.7047, respectively, identical to island-arc basalt composition. The whole-rock Nd model age (TDM) for the intrusive rocks range between 0.69 and 0.73 Ga. These geochemical and isotopic signatures indicate a subduction-related sub-continental lithospheric mantle source for these rocks. Our new geochemical, isotopic, and geochronological studies integrated with previously published data indicate that the middle Eocene to late Oligocene magmatism in eastern Iran was formed in a post-collisional tectonic environment. We suggest the northeastward subduction of the Neo-Tethys ocean beneath the Lut block and the eastward subduction of the Sistan ocean beneath the Afghan block caused mantle wedge to be metasomatized by slab components. At a later stage, a collision between the Lut and Afghan blocks was accompanied by the lithospheric delamination, and the subsequent asthenospheric upwelling led to the melting of the metasomatized sub-continental lithospheric mantle and the generation of middle Eocene to late Oligocene magmatism in the Eastern Iran Magmatic Belt.  相似文献   

6.
《International Geology Review》2012,54(16):1991-2007
A series of Lower Carboniferous volcanic rocks occur in the Hatu, Darbut, and Baogutu areas of Xinjiang Province. Secondary ion mass spectrometry (SIMS) zircon U–Pb isotopic data indicate that two samples of these rocks coevally erupted at 324.0 ± 2.8 Ma and 324.9 ± 3.4 Ma. Three detailed profile measurements show that the volcanics include the Hatu basalt, the Baogutu andesite and dacite, and the Darbut andesite. Whole-rock compositions suggest that the Hatu volcanics are tholeiites and have a mid-ocean ridge basalt (MORB)-like signature with a small negative Nb anomaly, suggesting formation in a back-arc basin. Their isotopic compositions (?Nd(t) = +2.2 to +4.0, (87Sr/86Sr)i = 0.70414 to 0.70517) suggest a mixing origin from depleted to enriched mantle sources. In contrast, the Baogutu and Darbut rocks are andesite and dacite possessing a transitional tholeiite to calc–alkaline character and have E-MORB-like and OIB signatures, with a marked negative Nb anomaly and Th/Yb-enrichment, indicating that they were generated in a subduction zone setting. Isotopically, they display consistently depleted Sr–Nd isotopic compositions [(87Sr/86Sr)i = 0.70377–0.70469, ?Nd(t) = 1.0–5.2], suggesting that they were derived from a depleted mantle, and that fluid and sediments were involved in their petrogenesis. These features suggest that an early Carboniferous intra-oceanic arc and back-arc basin system generated the studied volcanic units in the West Junggar.  相似文献   

7.
华北陆块早元古代基性岩墙群及其构造意义   总被引：1，自引：0，他引：1  
华北陆块中部带的晋冀蒙地区早元古代未变形变质基性岩脉形成于1781~1765 Ma。东部陆块鲁西地区早元古代类似的基性岩脉形成时间约为1841 Ma。中部带基性岩脉依据其FeOt含量、(Nb/La)N和(Th/Nb)N值的差异能划分为组1、组2和组3三类。它们的元素-同位素组成变化表明,组1样品起源于再循环大陆玄武质组分参与的交代岩石圈地幔,组2样品源于交代富铁岩石圈地幔与MORB组分的混杂源区,组3样品则是受辉长质组分混染的、经俯冲改造而成的岩石圈地幔产物。相反,鲁西地区基性岩脉亏损HFSE,具MORB型钕同位素组成。上述地球化学特征支持华北陆块中部带约1780 Ma的基性岩脉与早期俯冲碰撞作用的关系密切,而东部陆块约1840 Ma基性岩脉类似于弧后盆地构造背景产物。  相似文献   

8.
Major and trace element and 143Nd/144Nd (0.51209–0.51216) and 87Sr/86Sr (0.70879–0.71105) isotope analyses are presented on a representative group of lavas from the Vulsini district of the Roman magmatic province. Three distinct series are identified; the high-K and low-K series are similar to those described from other Italian volcanoes, while the third is represented by a group of relative ly undifferentiated leucite basanites which are thought to be near-primary mantle melts. Major and trace element variations within the high-K series are consistent with fractional crystallisation from a parental magma similar to the most magnesian leucitites. Crustal contamination resulted in an increase in 87Sr/86Sr with increasing fractionation, but it was superimposed on magmas which had already inherited a range of incompatible element and isotope ratios from enrichment processes in the sub-continental mantle. These are reviewed using the available results from Vulsini, Roccamonfina and Ernici. Transition element abundances and Ta/Yb ratios indicate that the pre-enrichment mantle was similar to that of E-type MORB, and that these elements were not mobilised by the enrichment process. Mixing calculations suggest that three components were involved in the enrichment process; mantle comparable with the source of MORB, and two other components rich in trace elements. One, the low-K component, had high Sr/Nd, Th/Ta and Ba/Nb and no europium anomaly while the second had lower Sr/Nd, a negative europium anomaly and very high Th/Ta. It was also characterised by low Nb/Ba and high Rb/Ba ratios, similar to those reported from phlogopite-rich peridotite xenoliths. The trace element enrichment processes are therefore thought to have occurred in the mantle wedge above a subduction zone with the trace element characteristics of the high-K end-member reflecting the subduction of sediments and the stabilisation of mantle phlogopite.  相似文献   

9.
Post-3Ma volcanics from the N Luzon arc exhibit systematic variations in 87Sr/86Sr (0.70327–0.70610), 143Nd/144Nd (0.51302–0.51229) and 208Pb*/206Pb* (0.981–1.035) along the arc over a distance of about 500 km. Sediments from the South China Sea west of the Manila Trench also exhibit striking latitudinal variations in radiogenic isotope ratios, and much of the isotopic range in the volcanics is attributed to variations in the sediment added to the mantle wedge during subduction. However, Pb-Pb isotope plots reveal that prior to subduction, the mantle end-member had high 8/4, and to a lesser extent high 7/4, similar to that in MORB from the Indian Ocean and the Philippine Sea Plate. Th isotope data on selected Holocene lavas indicate a source with unusually high Th/U ratios (4.5–5.5). Combined trace element and isotope data require that three end-members were implicated in the genesis of the N Luzon lavas: (1) a mantle wedge end-member with a Dupal-type Pb isotope signature, (2) a high LIL/HFS subduction component interpreted to be a slab-derived hydrous fluid, and (3) an isotopically enriched end-member which reflects bulk addition (<5%) of subducted S China Sea terrigenous sediment. The 87Sr/86Sr ratios in the volcanics show a restricted range compared with that in the sediments, and this contrasts with 143Nd/144Nd and 208Pb*/206Pb*, both of which have similar ranges in the volcanics and sediments. Such differences imply that whereas the isotope ratios of Nd, Pb and Th are dominated by the component from subducted sediment, those of Sr reflect a larger relative contribution from the slab-derived fluid.  相似文献   

10.
The Tabar–Lihir–Tanga–Feni (TLTF) islands of Papua New Guinea mainly comprise high-K calc-alkaline and silica undersaturated alkaline rocks that have geochemical features typical for subduction-related magmatism. Numerous sedimentary, mafic, and ultramafic xenoliths recovered from Tubaf seamount, located on the flank of Lihir Island, provide a unique opportunity to study the elemental and isotopic composition of the crust and mantle wedge beneath the arc and to evaluate their relationships to the arc magmatism in the region. The sedimentary and mafic xenoliths show that the crust under the islands is composed of sedimentary sequences and oceanic crust with Pacific affinity. A majority of the ultramafic xenoliths contain features indicating wide spread metasomatism in the mantle wedge under the TLTF arc. Leaching experiments reveal that the metasomatized ultramafic xenoliths contain discrete labile phases that can account for up to 50% or more of elements such as Cu, Zn, Rb, U, Pb, and light REE (rare-earth elements), most likely introduced in the xenoliths via hydrous fluids released from a subducted slab. The leaching experiments demonstrated that the light REE enrichment pattern can be more or less removed from the metasomatized xenoliths and the residual phases exhibit REE patterns that range from flat to light REE depleted. Sr–Nd isotopic data for the ultramafic residues show a coupled behavior of increasing 87Sr/86Sr with decreasing 143Nd/144Nd ratios. The labile phases in the ultramafic xenoliths, represented by the leachates, show decoupling between Sr and Nd with distinctly more radiogenic 87Sr/86Sr than the residues. Both leachates and residues exhibit very wide range in their Pb isotopic compositions, indicating the involvement of three components in the mantle wedge under the TLTF islands. Two of the components can be identified as Pacific Oceanic mantle and Pacific sediments. Some of the ultramafic samples and clinopyroxene separates, however, exhibit relatively low 206Pb/204Pb at elevated 207Pb/204Pb suggesting that the third component is either Indian Ocean-type mantle or Australian subcontinental lithospheric mantle. Geochemical data from the ultramafic xenoliths indicate that although the mantle wedge in the area was extensively metasomatized, it did not significantly contribute to the isotopic and incompatible trace element compositions of TLTF lavas. Compared to the mantle samples, the TLTF lavas have very restricted Pb isotopic compositions that lie within the Pacific MORB range, indicating that magma compositions were dominated by melts released from a stalled subducted slab with Pacific MORB affinity. Interaction of slab melts with depleted peridotitic component in the mantle wedge, followed by crystal fractionation most likely generated the geochemical characteristics of the lavas in the area. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.  相似文献   

11.
The concentrations and isotopic compositions of Sr, Nd, Pb, He and C have been determined for suites of xenoliths from Bullenmerri (Australia), Ichinomegata (Japan), Geronimo (Arizona), and East Africa. The wehrlites and pyroxenites from Bullenmerri have Sr, Nd and Pb isotopic compositions that are generally similar to those found for alkali basalts in the region. The spinel lherzolites, in contrast, have higher 87Sr/86Sr and 206Pb/204Pb and lower 143Nd/144Nd ratios. Whereas the isotopic compositions of He are generally within the range of mid-ocean-ridge basalts (MORB) and do not covary with those of other trace elements, there is an apparent correlation between the 13C/12C and 143Nd/144Nd ratios for each of the two petrologic groups. These relationships, if substantiated for other xenolith suites, greatly limit the possible mechanisms for generating lithophile and volatile isotopic variations in the continental lithosphere. The helium isotopic compositions for all of the xenoliths fall within the range for MORB. This includes those from Ichinomegata, suggesting that the lower 3He/4He ratios found for He sampled at the surface at subduction zones result from mixing mantle He with near-surface crustal He rather than with subducted radiogenic He. Measured C isotopic compositions (relative to Peedee belemnite) for the Ichinomegata xenoliths include values that are both lighter and heavier than those in MORB, and are compatible with contributions from subducted carbon. The Nd and Sr isotopic compositions of the Ichinomegata xenoliths exhibit a correlation over a substantially greater range of values than typically observed for other light-rare-earth-element (LREE)-depleted xenoliths, and include more radiogenic Sr and less radiogenic Nd compositions. The carbon isotopic compositions found for the East African and Geronimo xenoliths extend to values that are lighter than those typically found for MORB.  相似文献   

12.
The geologic evolution of the New Zealand microcontinent was characterised by intermittent Cretaceous to Quaternary episodes of intraplate volcanism. To evaluate the corresponding mantle evolution beneath New Zealand with a specific focus on the tectonic evolution, we performed a combined major and trace element and Hf, Nd, Pb, Sr isotope investigation on a suite of representative intraplate volcanic rocks from both main islands and the Chatham Islands. Isotopically, the data set covers a range between “HIMU-like” end member compositions (206Pb/204Pb: 20.57, 207Pb/204Pb: 15.77, 87Sr/86Sr: 0.7030, εHf: + 3.8, εNd: + 4.2), compositions tending towards MORB (206Pb/204Pb: 19.01, 207Pb/204Pb: 15.62, 87Sr/86Sr: 0.7028, εHf: + 9.9, εNd: + 7.0) and compositions reflecting the influence of subducted sediments (206Pb/204Pb: 18.99, 207Pb/204Pb: 15.67, 87Sr/86Sr: 0.7037, εHf: + 4.4, εNd: + 3.9). Whereas volcanism on the Chatham Islands constitutes the HIMU end member of our data set, intraplate volcanic rocks from the North Island are dominated by MORB-like compositions with relatively radiogenic 206Pb/204Pb signatures. Volcanic rocks from the South Island form a trend between the three end members. Assuming a polybaric melting column model, the primary melt compositions reflect variations in the degree of melting, coupled to variable average melting depths. As the three isotope and trace element end members occur throughout the volcanic episodes, the “HIMU-like” and the sediment influenced signatures most likely originate from a heterogeneous subcontinental lithospheric mantle, whereas an asthenospheric origin is inferred for the MORB-like component. For the South Island, affinities to HIMU wane with decreasing average melting depths whereas MORB and sediment-like signatures become more distinct. We therefore propose a polybaric melting model involving upper asthenospheric mantle and a lithospheric mantle source that has been modified by subduction components and veins of fossil “HIMU-like” asthenospheric melts. The proportion of asthenospheric versus lithospheric source components is controlled by variations in lithospheric thickness and heat flow, reflecting the different tectonic settings and rates of extension. Generally, low degree melts preferentially tap enriched vein material with HIMU signatures. The widespread occurrence of old Gondwana-derived lithospheric mantle beneath intraplate volcanic fields in East Gondwana is suggested by overall similarities between New Zealand intraplate volcanic rocks and volcanic rocks in East Australia and Antarctica. The petrogenetic model proposed here may therefore serve as a general model for the petrogenesis of Cretaceous to Recent intraplate volcanic rocks in former East Gondwana. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.  相似文献   

13.
L. Borsi  R. Petrini  F. Talarico  R. Palmeri 《Lithos》1995,35(3-4):245-259
Mafic dykes cutting the gneisses and migmatites in the Deep Freeze Range high-grade metamorphic complex of northern Victoria Land, Antarctica, have undergone strong recrystallization and deformation during amphibolite-facies metamorphism. Metamorphic mobility mostly affected the large-ion lithophile elements (LILE). Rare Earth (RE) and the high field-strength elements (HFSE) were essentially immobile during metamorphism. Together with the major-element geochemistry, this suggests primary characteristics of evolved tholeiitic magmas and mafic cumulates. No precise ages of intrusion are available for the dykes, but geological evidence suggest emplacement during the time interval 800 to 900 Ma. The Rb-Sr isotopic system in some of the dykes were also variably affected by a later thermal event, probably coincident with the time of amphibolite metamorphism, ca. 500–550 Ma ago. This event can be correlated with the Ross Orogeny in the Transantarctic Mountains. Nd isotopes and trace element abundances indicate that the dykes were derived by different degrees of partial melting and fractionation of heterogeneous sub-continental lithospheric mantle. The Nd isotopic compositions range from depleted to enriched signatures (εNd computed back to 850 Ma = +4.5 to −11.61), and are coupled to different trace element normalized patterns characterized by a slight positive Nb anomaly in the former case to a strong negative Mb anomaly for the latter samples. On isotopic and chemical ground the depleted signature of the mantle source resembles that reported for E-type MORB. The nature of the enriched components cannot be uniquely stated; nevertheless, on the basis of isotopic and geochemical data, it could be represented by sediments recycled into the sub-continental mantle or by crustal contamination during underplating of mafic magmas, or a combination of the two processes.  相似文献   

14.
~(40)Ar/~(39)Ar and zircon U-Pb geochronological and whole-rock geochemical analyses for the Laozanggou intermediate-acidic volcanic rocks from the western Qinling orogenic belt,Central China,constrain their petrogenesis and the nature of the Late Mesozoic lithospheric mantle.These volcanic rocks yield hornblende or whole-rock ~(40)Ar/~(39)Ar plateau ages of 128.3-129.7 Ma and zircon U-Pb age of131.3±1.3 Ma.They exhibit Si02 of 56.86-66.86 wt.%,K_2 O of 0.99-2.46 wt.% and MgO of 1.03-4.47 wt.%,with Mg# of 42-56.They are characterized by arc-like geochemical signatures with significant enrichment in LILE and LREE and depletion in HFSE.All the samples have enriched Sr-Nd isotopic compositions with initial ~(87)Sr/~(86)Sr ratios ranging from 0.7112 to 0.7149 and ε_(Nd)(t) values from 10.2 to 6.3.Such geochemical signatures suggest that these volcanic rocks were derived from enriched lithospherederived magma followed by the assimilation and fractional crystallization(AFC)process.The generation of the enriched lithospheric mantle is likely related to the modification of sediment-derived fluid in response to the Triassic subduction/collision event in Qinling orogenic belt.The early Cretaceous detachment of the lithospheric root provides a reasonable mechanism for understanding the petrogenesis of the Laozanggou volcanic sequence in the western Qinling orogenic belt.  相似文献   

15.
Northeastern Queensland, a part of the Phanerozoic composite Tasman Fold Belt of eastern Australia, has a Paleozoic to Mesozoic history dominated by subduction zone processes. A suite of 13 peridotite xenoliths from the <3 Ma Atherton Tablelands Volcanic Province, predominantly from Mount Quincan, comprise fertile (1.8-3.4 wt.% Al2O3 and 38.7-41.9 wt.% MgO) spinel lherzolites free from secondary volatile-bearing phases and with only weak metasomatic enrichment of incompatible trace elements (SmN/YbN = 0.23-1.1; LaN/YbN = 0.11-4.9). The suite is isotopically heterogeneous, with measured Sr (87Sr/86Sr = 0.7027-07047), Nd (143Nd/144Nd = 0.51249-0.51362), and to a lesser extent, Os (187Os/188Os = 0.1228-0.1292) compositions broadly overlapping MORB source mantle (DMM) and extending to more depleted compositions, reflecting evolution in a time-integrated depleted reservoir. Major and rare earth element systematics are consistent with mantle that is residual after low to moderate degrees of melt extraction predominantly in the spinel facies, but with a few samples requiring partial melting at greater pressures in the garnet field or near the garnet-spinel transition. In contrast to most previously studied suites of continental lithospheric mantle samples, the incompatible trace element contents and Sr and Nd isotopic systematics of these samples suggest only minimal modification of the sampled lithosphere by metasomatic processes.Five of six Mount Quincan xenoliths preserving depleted middle to heavy REE patterns form a whole rock Sm-Nd isochron with an age of ∼275 Ma (εNdi = +9), coincident with widespread granitoid emplacement in the overlying region. This isochron is interpreted to indicate the timing of partial melting of a DMM-like source. Xenoliths from other Atherton localities scatter about the isochron, suggesting that the sampled mantle represents addition of DMM mantle to the lithosphere in the Permian, when the region may have broadly been within a subduction zone setting. A sixth middle to heavy REE-depleted Mount Quincan xenolith has a distinct Nd and Os isotopic composition consistent either with an earlier, possibly Precambrian melt extraction event, or with Permian derivation from a mantle source with a less depleted (time-averaged lower Sm/Nd) Nd isotopic composition, but a more depleted (low Re/Os) Os isotopic composition.The range in measured whole rock Os isotopic compositions cannot solely be the result of time-integrated effects of variable melt extraction, especially considering the coherent Sm-Nd systematics of the suite. The Os heterogeneity more likely reflects either a heterogeneous ∼275 Ma DMM source that would have a present-day Os composition (187Os/188Os ∼ 0.1265-0.1287) overlapping both abyssal peridotites and chondrites, or significant and variable enrichment within the lithospheric mantle by secondary sulfides carrying radiogenic Os in a cryptic chalcophile enrichment event. Regardless of the origin of the Os isotopic variability, these data highlight the mantle Re-Os isotopic heterogeneity that may be present over small length scales where the lithophile Sm-Nd system may be relatively homogeneous.  相似文献   

16.
Olivine and clinopyroxene grains have been separated from four large Fe-Ti-V oxide ore-bearing intrusions (Panzhihua, Hongge, Baima and Taihe) in the Panxi area, Emeishan large igneous province, Southwest China, for He and Ar isotope studies. The samples examined revealed extremely low 3He/4He ratios (0.078-4.34 Ra with the mean value 0.78 Ra) for gases extracted by stepwise heating. This feature, combined with low 40Ar/36Ar ratios can be interpreted as due to addition of subduction-related fluids and melts that had been stored in the lithospheric mantle for long periods. Considering the regional geologic history, such addition can be attributed to the paleo subduction that occurred along the western margin of the Yangtze Block during the Neoproterozoic. The subducted oceanic crust beneath the Panxi area underwent eclogite-facies metamorphism and subsequent exhumation. The infiltration of subduction-related melts and fluids into the lithospheric mantle led to enriched isotopic signatures from that of the slightly depleted asthenopheric mantle which has been suggested by the Sr, Nd and Pb isotopic data of the Emeishan basalts and picrites. In addition, considerable amounts of eclogitic melts produced by partial melting of eclogite-facies oceanic crust extensively contaminated the lithospheric mantle. During the late Permian, partial melting of an upwelling mantle plume that contained an eclogite or pyroxenite component generated the parental Fe-rich magma that supplied the ore-bearing intrusions. The combination of these factors may have been the crucial reason that many world-class Fe-Ti-V oxides deposits are clustered in the Panxi area.  相似文献   

17.
We examined seven ultramafic xenoliths from 1~3 Ma alkali olivine basalt reefs near the Eurasian continent and one sample of the host alkali basalt to identify the mantle wedge material and to constrain the origin and evolution of mantle beneath SW Japan. Six xenoliths are from Kurose and one xenolith is from Takashima, northern part of the Kyushu islands, SW Japan. The Sr and Nd isotopic ratios vary from 0.70416 to 0.70773 and from 0.51228 to 0.51283, respectively. The Kurose and Takashima xenoliths have higher Sr isotopic ratios and lower Nd isotopic ratios than those of the peridotite xenoliths from the other arc settings such as Simcoe and NE Japan.

The Kurose xenoliths have less radiogenic Os isotopic ratios (187Os/188Os = 0.123–0.129) than the primitive upper mantle (PUM) estimate and limited variation compared to the other arc xenoliths. Their Os isotope compositions are rather similar to the ultramafic xenoliths from NE and east China. In addition, the samples of the Kurose and Takashima xenoliths plot along a mixing line between ultramafic xenoliths from SE and NE China and a slab component in Sr–Nd–Os isotopic space. Our results suggest that fragments of continental lithospheric mantle from the China craton may exist beneath Kurose and Takashima after the Sea of Japan expansion when the Japanese islands were rifted away from the Eurasian continent during Miocene. Later magmatism due to subduction of the Philippine Sea Plate beneath the SW Japan arc around 15 Ma ago may have introduced fluids or melts derived from slab component, interpreted to be oceanic sediments rather than altered oceanic crust, that possibly modified the original composition of the lithospheric mantle sampled by the peridotite xenoliths from Kurose and Takashima.  相似文献   


18.
Cenozoic lamprophyres (minettes, spessartites, kersantite) from the Western Alps, northern Italy, represent small volume, mafic melts with high Mg#s and high Ni and Cr contents. All the lamprophyres show light REE enrichment, high incompatible element contents, and Ta, Ti and Nb troughs on chondrite-normalized diagrams. Age-corrected 87Sr/86Sr isotopic ratios (assuming t = 30 Ma) are highly variable and range from 0.70590 to 0.71884; 143Nd/144Nd ratios range from 0.51203 to 0.51242. Pb isotopic ratios are: 206Pb/204Pb = 18.669–18.895, 207Pb/204Pb = 15.605–15.689 and 208Pb/204Pb = 38.224–39.134. 87Sr/86Sr ratios show a negative correlation with 143Nd/144Nd, and a positive correlation with K, Ba, and Rb as well as with Ti, Th, Ta, Nb and Zr abundances. The primitive nature of the lamprophyres, coupled with their enriched incompatible trace element and isotopic signatures, suggest derivation from a metasomatized upper mantle source. Linear arrays in isotope space and elemental data plots suggest mixing between two distinct end-members in the Italian mantle; an enriched end-member that is isotopically similar to pelagic sediments, and a significantly less enriched end-member that approaches Bulk Earth values. New isotopic data indicate that the mantle source(s) of the lamprophyres from the Western Alps contain a very high proportion of the enriched end-member. The geochemical signature of the enriched end-member is attributed to fluids or melts derived from pelagic sediments subducted during the closure of the Tethyan Ocean in the late Cretaceous to early Tertiary.  相似文献   

19.
We present the first report of geochemical data for submarine basalts collected by a manned submersible from Rurutu, Tubuai, and Raivavae in the Austral Islands in the South Pacific, where subaerial basalts exhibit HIMU isotopic signatures with highly radiogenic Pb isotopic compositions. With the exception of one sample from Tubuai, the 40Ar/39Ar ages of the submarine basalts show no significant age gaps between the submarine and subaerial basalts, and the major element compositions are indistinguishable at each island. However, the variations in Pb, Sr, Nd, and Hf isotopic compositions in the submarine basalts are much larger than those previously reported in subaerial basalts. The submarine basalts with less-radiogenic Pb and radiogenic Nd and Hf isotopic compositions show systematically lower concentrations in highly incompatible elements than the typical HIMU basalts. These geochemical variations are best explained by a two-component mixing process in which the depleted asthenospheric mantle was entrained by the mantle plume from the HIMU reservoir during its upwelling, and the melts from the HIMU reservoir and depleted asthenospheric mantle were then mixed in various proportions. The present and compiled data demonstrate that the HIMU reservoir has a uniquely low 176Hf/177Hf decoupled from 143Nd/144Nd, suggesting that it was derived from an ancient subducted slab. Moreover, the Nd/Hf ratios of the HIMU basalts and curvilinear Nd–Hf isotopic mixing trend require higher Nd/Hf ratios for the melt from the HIMU reservoir than that from the depleted mantle component. Such elevated Nd/Hf ratios could reflect source enrichment by a subducted slab during reservoir formation.  相似文献   

20.
New geochemical data from the Cocos Plate constrain the composition of the input into the Central American subduction zone and demonstrate the extent of influence of the Galápagos Hotspot on the Cocos Plate. Samples include sediments and basalts from Ocean Drilling Program (ODP) Site 1256 outboard of Nicaragua, gabbroic sills from ODP Sites 1039 and 1040, tholeiitic glasses from the Fisher Ridge off northwest Costa Rica, and basalts from the Galápagos Hotspot Track outboard of Central Costa Rica. Site 1256 basalts range from normal to enriched MORB in incompatible elements and have Pb and Nd isotopic compositions within the East Pacific Rise MORB field. The sediments have similar 206Pb/204Pb and only slightly more radiogenic 207Pb/204Pb and 208Pb/204Pb isotope ratios than the basalts. Altered samples from the subducting Galápagos Hotspot Track have similar Nd and Pb isotopic compositions to fresh Galápagos samples but have significantly higher Sr isotopic composition, indicating that the subduction input will have a distinct geochemical signature from Galápagos-type mantle material that may be present in the wedge beneath Costa Rica. Gabbroic sills from Sites 1039 and 1040 in East Pacific Rise (EPR) crust show evidence for influence of the Galápagos Hotspot ∼100 km beyond the morphological hotspot track.  相似文献   

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