期刊,Lithos 2022年420/421卷期_国家学术搜索

期刊信息/Journal information

Lithos

Elsevier Science B.V.

主办单位：Elsevier Science B.V.

国际刊号：0024-4937

Lithos/Journal LithosSCIISTP

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Partial melting caused by subduction of young, hot oceanic crust in shallow high-temperature and low-pressure environments: Indications from Middle and Late Jurassic oceanic plagiogranite in Shiquanhe, Central Tibet

Li HangWang MingZeng Xiao-Wen

21页

查看更多>>摘要：The oceanic plagiogranites that intruded into the ophiolite suite have received a great deal of attention from researchers. Some of these plagiogranites were controlled by mid-ocean ridge spreading and were formed via crystallization of basaltic magma or partial melting of the mafic gabbroic oceanic crust. The other plagiogranites were controlled by subducted oceanic crust slabs, and their formation mechanism is similar to that of the oceanic island arcs. The identification of different types of oceanic plagiogranites is of great significance for understanding the evolution of ancient ocean basins. In the Shiquanhe-Namtso melange zone in central Tibet, scholars have recently identified many rock sequences related to intra-oceanic arcs, including MORB-like basalts, boninites, high-Mg andesites, and multiple series of granitic rocks. To further explore the formation mechanism of felsic rocks in oceanic island arcs and their role in the growth of continental crustal material, we sampled a series of Middle and Late Jurassic granitoids exposed in the ophiolitic melange belt in the Shiquanhe area. Using zircon U-Pb dating, zircon Lu-Hf isotopic testing, whole-rock Sr-Nd isotopic testing and whole-rock major and trace geochemical testing, we determined the magmatic genesis and tectonic history of these samples. According to the test results and our field survey, we divided our samples into three groups. The first group consists of low-K plagiogranites (LKGs). These samples have ages that fall between 163 and 162 Ma. With their low K2O/Na2O ratios, low La/Yb ratios, low Sr/Y ratios, low Gd/Yb ratios, weak negative Eu anomalies and their significantly depleted zircon isotopic compositions (εHf(t) = 10.0-12.5; εNd(t) = 0.64-1.10), we propose that these samples are the products of partial melting of oceanic gabbro and turbidite at shallow depths in low-pressure and high-temperature conditions. The second group consists of samples from dioritic enclaves within the LKGs. These samples, which are slightly older than the LKG samples (175-173 Ma), have isotopic characteristics that are similar to those of the LKG samples (εHf(t) = 10.4-13.5; εNd(t) = 0.51-0.83). The samples in the second group may be homologous captives that formed during the early crystallization of LKG homologous magmas. The third group consists of samples with compositions of high-K granodiorites (HKGs) and ages similar to those of the LKG group (～163 Ma). With high K2O/Na2O ratios, relatively high La/Yb ratios, low Sr/Y ratios, low Gd/Yb ratios, negative Eu anomalies, significantly enriched isotopic compositions (εHf(t) = -11.1 to -15.4; εNd(t) = -13.29 to -13.43). Considering the basement exposure and known magmatic characteristics in the Shiquanhe area, we propose that HKGs samples are the result of interaction between continental subduction sediment melts and heterogeneous mantle material in high-temperature and low-pressure conditions. By comparing the oceanic plagiogranites in the Shiquanhe-Namtso melange zone with those developed in the young, hot intra-oceanic subduction zone, we determined that the oceanic plagiogranites in the Shiquanhe-Lhaguotso-Asa area are forearc granites developed in the forearc area. These plagiogranites were controlled by the young, hot subducting plate and were formed by partial melting of a multi-component source area in a high-temperature and low-pressure environment.

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Petrogenesis and tectonic implication of Neoproterozoic I-Type Granitoids and orthogneisses in the Goa-Mandja area, Central African Fold Belt (Cameroon)

Bovari Syprien YomeunWei WangJean Pierre Tchouankoue

27页

查看更多>>摘要：The geological evolution of the Central African Fold Belt (CAFB) remains debated. The granitoids, orthogneisses and granitic veins from the Goa-Mandja area (Adamawa-Yade domain) are considered to constrain this belt. These rocks are consistent with late-D2 regional emplacement and synchronous with the magmatism at ～600 Ma within the CAFB in Cameroon and its extension into northeastern Brazil, as the zircon U-Pb dating points to 603-598 Ma crystallization ages. Analytical data reveal slighdy elevated δ~(18)O_(zrn) (6.45-7.17‰) and δ~(18)O_(WR) (8.20-8.61 ‰), moderate to high SiO2 (59.70-77.23 wt%), low MgO (0.07-2.77 wt%), Cr (0.29-55.12 ppm), Ni (0.45-33.6 ppm), V (1.52-116.62 ppm) and Mg~# (0.02-0.36) contents, indicating crustal origin. The low to medium ~(87)Sr/~(86)Sri (0.703328-0.722670) and positive to negative ε_(Nd(t)) (+5.7 to -9.9), coupled with the diversity of Nd model ages (TDM2) support the heterogeneity of crustal protoliths. The high positive ε_(Nd(t)) (+5.7) of the granitic vein indicates derivation from juvenile Neoproterozoic (Nd-T_(DM2) at 854 Ma) crustal protolith. T_(DM2 (Nd)) and T_(DM2(Hf)) ages of 1188-2120 Ma and 2173-2480 Ma also suggest the involvement of Mesoproterozoic to Paleoproterozoic crustal protoliths. Samples show metaluminous to weakly peraluminous, high-K calc-alkaline to shoshonitic and I-type granitoid features and are similar with crustal rocks in regard to LILE (Rb, K) and LREE enrichments and HFSE (Nb, Ta, Ti) depletions. These combined features indicate slab subduction related setting for the magma generation. Slab subduction may have generated the heat necessary to provoke partial melting at the lower crustal level. Considering their position close to the Tchollire-Banyo Fault corridor, at the transition between the Adamawa-Yade and northern domains, the subduction derived features shown by the Goa-Mandja I-type granitoids could be related to the subduction of an old plate boundary during amalgamation of Gondwana. Therefore, the Goa-Mandja area records dominant magmatic accretion along the northern margin of the Congo craton (active margin) associated with a possible northwestern subduction.

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New insights into the origin of Middle to Late Permian volcaniclastics (Nb-Zr-REY-Ga-rich horizons) from eastern Yunnan, SW China

Ning WangShifeng DaiXibo Wang

21页

查看更多>>摘要：Volcaniclastic horizons near the Guadalupian-Lopingian Boundary (G-LB) from southwest China have attracted much attention, not only due to the End-Guadalupian mass extinction near the boundary, but also to the unique and regionally extensive Nb-Zr-REY-Ga mineralization with potential economic significance (REY = rare earth elements and Y). Previous studies have suggested that these horizons were derived from volcanism associated with the terminal phases of the Emeishan mantle plume, which however was not the only contemporaneous volcanic activity in southeast Asia at this time (～260 Ma). Here we present a systematic analysis focusing on the petrological, mineralogical, and geochemical features of the volcaniclastics from the Yiliang and Weixin boreholes from the lowest strata of the Late Permian Xuanwei and Longtan formations, respectively, eastern Yunnan, SW China. In addition, previous geochemical data obtained for the correlative layers near the investigated boreholes were used for reference. The mineralogy of the volcaniclastics is dominated by mixed-layer illite/ smectite, illite, berthierine, kaolinite, and quartz, and to a lesser extent, albite, calcite, siderite, anatase, and hematite, along with traces of chamosite, rutile, and florencite. Additionally, zircon, pyrite, chalcopyrite, sphalerite, galena, rhabdophane, parisite, goyazite, and barite were identified under SEM-EDS. The values of Al2O3/TiO2 in the volcaniclastic samples are mostly greater than eight, consistent with the characteristics of intermediate-felsic rocks. The Nb/Yb and Zr/TiO2 vs. Al2O3/TiO2 diagrams, and the chondrite-normalized REY distribution patterns of the samples collectively indicate that the Nb-Zr-REY-Ga mineralization was not only derived from the ELIP alkaline rocks, but also from the arc-related intermediate-felsic magmatic rocks from the Paleo-Tethys. However, the latter source for Nb-Zr-REY-Ga mineralization in SW China has not been previously reported. The two sources of clastic materials, i.e., from both within-plate/anorogenic magmatism and arc-related/orogenic magmatism, agree with previously-reported discrimination diagrams of ε_(Hf(t)) vs. age and Hf/ Th vs. Th/Nb in the zircon grains from the correlative claystones. In addition, the modes of occurrence of critical metals (Nb, Zr, REY, Ga) are identified in detail in this study. The results show that most of the HREE and almost all the Zr and Hf are in zircon; the LREE are dominantly in REE-bearing carbonates and phosphates; the Nb is largely in former Fe-Ti oxide phases; and the Ga is predominantly in the clays.

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Mid-Neoproterozoic tectonic evolution of the northern margin of the Yangtze Block, South China: New insights from high-temperature magma events

Chang-Cheng HuangHao ZouLeon Bagas

15页

查看更多>>摘要：Numerous Neoproterozoic intrusive rocks are distributed along the northern margin of the Yangtze Block in South China, and their petrogenesis and tectonic setting remain the subject of debate. This study systematically compiled data from igneous rocks from the northern margin of the Yangtze Block, including abundant felsic plutons with formed temperatures of >800 °C. The data reveal two high-temperature rifting-related events at ～830-780 and 750-700 Ma. Significant changes in Hf-O isotopes appeared at ～790 Ma, with later rocks exhibiting positive ε_(Hf(t)) and low δ~(18)O. To clarify the nature and tectonic significance of these high-temperature events, we sampled plutonic rocks from the Lianghekou (LHK) and Dalanggou (DLG) areas of Hannan, on the northern margin of the Yangtze Block. The samples from LHK are ～785 Ma monzogranite emplaced towards the end of the first high-temperature event and derived from partial melting of thickened juvenile crust contaminated by older crustal material. These rocks have ε_(Hf(t)) values between +4.6 and + 11.5, δ~(18)O values between +6.2‰ and + 7.3‰, and contain residual combinations of garnet, clinopyroxene, rutile, and amphibole. The genesis of the monzogranite is interpreted as being emplaced in an initial rifting environment and related to a super mantie plume. The DLG samples are ～743 Ma alkali feldspar granite representing the second event; they have the geochemical characteristics of a reduced A-type granite enriched in Fe and poor in Mg, FeO~T/(FeO~T + MgO) values close to 1, and emplacement temperatures of 813 °C-848 °C. The alkali feldspar granite originated from partial melting of a tholeiitic source and was influenced by extensive fractionation. It has ε_(Hf)(t) values of +2.5 to +10.9, δ~(18)O values of +6.6‰ to +7.4‰, and contains residual plagioclase. The DLG alkali feldspar granite and nearby reduced A-type granite indicate that the northern margin of the Yangtze Block experienced a low oxygen fugacity environment at ～817-743 Ma. The second high-temperature event possibly reflected rifting that gradually matured after 790 Ma, at which point the mantle changed from a source of heat alone into a source of both heat and material.

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Nature of the Late Cretaceous mantle source beneath the western Lhasa terrane, southern Tibet: Insights from the newly discovered mafic intrusion

Ming LeiJianlin ChenYing Luo

13页

查看更多>>摘要：Recent studies indicate that the mantle source of eastern Lhasa terrane was still depleted during the late stage of oceanic subduction (100-50 Ma). However, the mantle source beneath the western Lhasa terrane during this period was poorly constrained mainly due to the rare exposure of mafic rocks in this region. Here, we report the detailed petrological, geochronological, geochemical, and Sr-Nd-Hf-Pb isotopic data for recently identified gabbroic rocks in the Taruocuo area, western Lhasa terrane, southern Tibet. The zircon U-Pb dating results indicate that the Taruocuo gabbroic rocks were emplaced during the Late Cretaceous (～90 Ma). These gabbroic rocks have low SiO2 (50.9-55.0 wt%), and moderately high MgO (4.10-5.60 wt%) contents with arc-like trace-element patterns, such as enrichment in large ion lithophile elements, light rare earth elements, and depletion in high field strength elements. Meanwhile, they have high Th/La, low Ba/La values, and exhibit relatively enriched and uniform Sr-Nd-Hf-Pb isotopic compositions (~(87)Sr/~(86)Sr{0 = 0.7048-0.7058, εNd_((t)) = -1.8 to -1.2, εHf_((t)) = +3.1 to +3.8, and ~(206)Pb/~(204)Pb = 18.60 to 18.66). Such geochemical features of the Taruocuo gabbroic rocks suggest that they would originate from an enriched and fertile mantle source that was generated by the meta-somatic reaction between the mantle wedge peridotite and the subducted Neo-Tethyan oceanic sediment-derived melts. Taking the regional tectonic evolution into consideration, we favor the model of Neo-Tethyan oceanic-ridge subduction having taken place to account for the Late Cretaceous (～90 Ma) magmatic event in the Lhasa terrane. Under such geological background, partial melting of this enriched mantle would have produced the Taruocuo gabbroic rocks in the western Lhasa terrane. Given that the Late Cretaceous Taruocuo gabbroic rocks have the relatively enriched Sr-Nd-Hf isotopic compositions, distinctive from the contemporaneous mafic rocks in the eastern Lhasa terrane that have the depleted Sr-Nd-Hf isotopic compositions, we propose that the mantle source beneath the Lhasa terrane was probably isotopically segmented during the Late Cretaceous (～90 Ma).

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Generation of continental crust by remelting of enriched oceanic crust in accretionary orogen: Geochemical evidence of granitoids in the Tongbai Orogen, Central China

Yadi ZhuangYuanbao WuWenxiang Zhang

13页

查看更多>>摘要：Oceanic arcs are known as the critical sites to generate new continental crust. However, additional processes are needed to transform mafic oceanic crust to mature continental crust. Ascertaining the petrogenesis of granitoids therein is vital to understand such a maturation process. In this study, we carried out a combined study of in-situ zircon U-Pb dating and Hf-0 isotopic analyses, as well as whole-rock geochemical and Sr-Nd isotopic compositions for the Changwan granitoids in the Erlangping oceanic arc unit of the Tongbai orogen. This intrusion mainly consists of biotite granite with subordinate trondhjemite. LA-ICPMS and SIMS zircon U-Pb dating yielded identical U-Pb ages of ca. 460 Ma, representing the intrusion ages. Whole-rock analyses demonstrated that the Changwan granitoids have relatively high SiO2 (72.71-77.14 wt%) and Al2O3 (12.61-14.24 wt%), but low MgO (0.31-0.74 wt%) contents and Mg# (32.0-40.2). The relatively high K2O (2.81-3.63 wt%) contents and K2O/Na2O values (0.76-1.03) of the granitic samples suggest they belong to middle-K weakly peraluminous I-type granite, while the trondhjemitic samples with low K2O (1.06-1.96 wt%) and K2O/Na2O values (0.23-0.42) are tholeiitic to middle-K series. In both cases, the samples show arc-type trace element distribution patterns. They are characterized by depleted Sr-Nd-Hf isotopic compositions with (~(87)Sr/~(86)Sr); of 0.7038-0.7050, ε_(Nd)(t) of +0.52 ～ +2.85, zircon ε_(Hf)(t) of +3.4 ～ +9.6, and relatively low zircon δ~(18)O values of 4.71 ± 0.17-4.97 ± 0.18‰. These isotopic fingerprints suggest the Changwan granitoids were derived from a relatively depleted source that had been hydrothermally altered under high temperature conditions. The most likely candidate is the accreted Erlangping lower oceanic crust which might have been heterogeneously modified in a supra-subduction zone. This is further verified by the trace element modeling on potential mafic source rocks enriched in incompatible elements in the Erlangping unit. We suggest that the high K2O/Na2O granitic samples were derived from K-rich segment of the Erlangping lower oceanic crust whilst the low K2O/Na2O trondhjemites from K-poor part. Therefore, our results show that partial melting of enriched oceanic crust in supra-subduction zone plays an important role for crust maturation in accretionary orogens.

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