山东沂水麻粒岩杂岩中的变质流体及地质意义

山东沂水麻粒岩杂岩中含有大量高密度富ＣＯ＿２流体包裹体，伴随有ＣＯ＿２－Ｈ２Ｏ，ＣＯ＿２±Ｎ＿２±ｃＨ＿４的包裹体。对这些包裹体的产状、分布和组成等特征进行了详细的岩相学研究，并选择有代表性样品进行了显微测温和激光拉曼探针分析。结合本区地质和岩石学研究成果确定沂水麻粒岩杂岩峰期变质温度为８００℃，压力为０．７５ＧＰａ。对变质流体组成、演化及其成岩意义也进行了讨论。     

<Emphasis Type="Italic">In-situ</Emphasis> trace element analyses and Pb-Pb dating of zircons in granulite from Huangtuling,Dabieshan by LAM-ICP-MS

It is revealed by CL images that there are multi-stage growth internal structures of zircons in the Huangtuling granulite, including the inherited zircons, protolith zircons, sector and planar zone zircons and retrograde zircons. In-situ trace element compositions and Pb-Pb ages have been analyzed by LAM-ICP-MS. The sector and the planar zone domains show typical trace element characteristics of granulite zircon (low Th, U, Th/U, total REEs, clear negative Eu anomalies, relatively depleted HREE and small differential degree between MREE and HREE, etc.), indicating that they formed during granulite-facies metamorphism. The protolith zircons have trace element characteristics of crustal zircon (high Th, U, Th/U, total REEs and enriched HREEs, etc.). 12 analyzed spots on granulite-facies domains give a weighted mean ²⁰⁷Pb/²⁰⁶Pb age of (2154±26) Ma (MSWD = 3.8), which is the best estimated age of granulite-facies metamorphism of this sample. The weighted mean ²⁰⁷Pb/²⁰⁶Pb age of 5 analyzed spots on protolith zircon domains is (2714 ± 22) Ma (MSWD = 1.4), which represents the protolith forming time. The discovery of ca. 3.4 Ga inherited zircon indicates that there are Palaeoarchean continental materials in this area. The interpretation of formation conditions and the ages of zircons can be constrained by simultaneous in-situ analysis of trace elements and ages.     

Late Mesozoic crust-mantle interaction and lower crust components in South China: A geochemical study of mafic granulite xenoliths from Cenozoic basalts

Mafic granulite xenoliths have been discovered in many volcanoes (especially alkali basalt and kimberlite) all over the world. They formed generally in lower crust, and recorded lots of in- formation on the lithosphere formation and crust-mantle interacti…     

中国与蒙古之地质

东昆仑中部缝合带清水泉一带发育石榴斜长紫苏麻粒岩、紫苏辉石黑云母石榴子石麻粒岩、石榴二辉斜长麻粒岩和石榴单斜辉石麻粒岩,它们与混合岩化黑云母石榴子石变粒岩、黑云母辉石变粒岩、石墨大理岩、含透辉石透闪石大理岩、透辉石大理岩、黑云斜长角闪岩和片麻岩等高级变质岩系以及纯橄岩、辉橄岩、橄长岩、辉长岩、辉绿岩和玄武岩等共同构成蛇绿混杂岩。麻粒岩相变质作用的温压条件为T=760~880℃,p=830~1200MPa,为高温中高压麻粒岩相变质作用,估算其形成深度为40~45km。麻粒岩相变质作用的SHRIMP锆石U-Pb年龄为(507·7±8·3)Ma。清水泉地区蛇绿岩形成于~520Ma,到~508Ma时俯冲至地下40~45km深处而发生中高压麻粒岩相变质作用,然后发生构造折返而剥露至地表。证实了清水泉高级变质岩和基性—超基性岩片是形成于早—中寒武世的蛇绿混杂岩,标志一个古生代早期的非常重要的板块汇聚边界,这对于进一步研究东昆仑造山带构造演化、乃至中国西部大地构造格局具有非常重要的意义。     

Mineralogical and geochemical study of a newly discovered mafic granulite, northwest China: Implications for tectonic evolution of the Altay Orogenic Belt

Abstract A mafic granulite body was newly discovered in the Altay Orogenic Belt, northwest China. The rocks comprise a suite of coarse‐grained and fine‐grained granulites. Orthopyroxenes (hypersthenes) in the rocks have high X_Mg and low Al₂O₃ contents, whereas clinopyroxenes have low TiO₂ and Al₂O₃ contents. Amphiboles and biotites have a high Mg/(Mg + Fe²⁺) ratio and low contents of F and Cl. The peak metamorphic pressure–temperature (P–T) conditions are estimated as 750–780°C and 6–7 kbar, and retrograde P–T conditions are in the range of 590–620°C and 2.3–3.7 kbar, indicating significant decompression. Metamorphic reactions and P–T estimates define a clockwise P–T path. Geochemically, the rocks are high in Mg/(Mg + Fe) and Al₂O₃, depleted in U, Th, K and Rb, and characterized by light rare earth element enrichment and a weak positive Eu anomaly. The Altay mafic granulite shows depleted Nb, P and Ti contents in the mid‐oceanic ridge basalt normalized spider diagram. The geochemical characteristics suggest that the protolith of the Altay mafic granulite was calc‐alkaline basalt and andesite with an island‐arc affinity. The rock has a high ¹⁴³Nd/¹⁴⁴Nd ratio with ?Nd(0) > 0, indicating derivation from a mantle‐depleted source. In the present study, a two‐stage model for the evolution of the Altay mafic granulite is proposed: an early stage in which calc‐alkaline basalt and andesite with island‐arc affinity were subducted into a deeper level of the crust and subjected to granulite‐facies metamorphism generating the mafic granulite, followed by the later stage exhumation of the system into the upper crust by the late Paleozoic thrusting.     

青藏高原北羌塘新生代火山岩中的麻粒岩捕虏体

北羌塘枕头崖地区新生代火山岩主要岩石类型为安山岩和英安岩类。其中,安山岩在一定程度上显示了埃达克质火山岩的特征,如高Sr(>1000×10-6)、Sr/Y>50以及低Yb(<2×10-6),表明其应源于榴辉岩相的青藏高原加厚陆壳中下部(>45km深度)。而英安岩类富集LILE,如Rb、Ba、Th、U和K等,亏损HFSE,如Ti、Nb、Ta和Sr等,尤其是Sr显著亏损,表明其应源于斜长石稳定的麻粒岩相源区。该区新生代安山岩和英安岩中麻粒岩捕虏体可分为两种类型,即二辉石麻粒岩和单斜辉石麻粒岩。二辉石麻粒岩平衡温度为783~818℃,单斜辉石麻粒岩形成压力在0·845~0·858GPa之间,来源深度约为27·9~28·3km,表明它们是来自青藏高原加厚陆壳中部的岩石样品,代表了本区英安岩类火山岩的源区物质组成。     

SHRIMP U-Pb zircon age,geochemistry and Nd-Hf isotope of Neoproterozoic mafic dyke swarms in western Sichuan:Petrogenesis and tectonic significance

Neoproterozoic magmatic rocks are widespread in the western margin of the Yangtze block, and their origin and genesis have significant implications for understanding the evolution of the Rodinia super- continent. However, there are currently two opposing interpretations for their petrogenesis and tectonic setting: mantle plume-related and island arc origin. To further verify these two competing models, SHRIMP U-Pb zircon age determinations and geochemical and Nd-Hf isotopic analyses are conducted on the mafic dykes in the Kangdian Rift, western Sichuan. U-Pb dating suggests that these mafic dykes were emplaced at 780―760 Ma, spatially and temporally coeval with the Kangding granitoid complex. The parental magmas of these dykes were derived from a depleted asthenosphere mantle source likely triggered by an anomalously-hot mantle plume. Despite some arc-geochemical features caused by variable degrees of contamination of young island arc crust during magma ascending and emplace- ment, they show general geochemical and Nd-Hf isotopic features similar to those of the intraplate basalts. Our results support the reconstruction model of Rodinia in which the South China block was located between Australia and Laurentia.     

Ultrahigh-pressure eclogite transformed from mafic granulite in the Dabie orogen, east-central China

Although ultrahigh‐pressure (UHP) metamorphic rocks are present in many collisional orogenic belts, almost all exposed UHP metamorphic rocks are subducted upper or felsic lower continental crust with minor mafic boudins. Eclogites formed by subduction of mafic lower continental crust have not been identified yet. Here an eclogite occurrence that formed during subduction of the mafic lower continental crust in the Dabie orogen, east‐central China is reported. At least four generations of metamorphic mineral assemblages can be discerned: (i) hypersthene + plagioclase ± garnet; (ii) omphacite + garnet + rutile + quartz; (iii) symplectite stage of garnet + diopside + hypersthene + ilmenite + plagioclase; (iv) amphibole + plagioclase + magnetite, which correspond to four metamorphic stages: (a) an early granulite facies, (b) eclogite facies, (c) retrograde metamorphism of high‐pressure granulite facies and (d) retrograde metamorphism of amphibolite facies. Mineral inclusion assemblages and cathodoluminescence images show that zircon is characterized by distinctive domains of core and a thin overgrowth rim. The zircon core domains are classified into two types: the first is igneous with clear oscillatory zonation ± apatite and quartz inclusions; and the second is metamorphic containing a granulite facies mineral assemblage of garnet, hypersthene and plagioclase (andesine). The zircon rims contain garnet, omphacite and rutile inclusions, indicating a metamorphic overgrowth at eclogite facies. The almost identical ages of the two types of core domains (magmatic = 791 ± 9 Ma and granulite facies metamorphic zircon = 794 ± 10 Ma), and the Triassic age (212 ± 10 Ma) of eclogitic facies metamorphic overgrowth zircon rim are interpreted as indicating that the protolith of the eclogite is mafic granulite that originated from underplating of mantle‐derived magma onto the base of continental crust during the Neoproterozoic (c. 800 Ma) and then subducted during the Triassic, experiencing UHP eclogite facies metamorphism at mantle depths. The new finding has two‐fold significance: (i) voluminous mafic lower continental crust can increase the average density of subducted continental lithosphere, thus promoting its deep subduction; (ii) because of the current absence of mafic lower continental crust in the Dabie orogen, delamination or recycling of subducted mafic lower continental crust can be inferred as the geochemical cause for the mantle heterogeneity and the unusually evolved crustal composition.     

藏南洛扎地区基性岩墙群锆石U-Pb定年、地球化学特征及构造意义

藏南洛扎地区广泛分布有近东西向产出的基性岩墙群,主要侵位于晚侏罗世—早白垩世及其以前的地层中,锆石U-Pb同位素定年获得成岩年龄138.0Ma±3.5Ma。基性岩墙可以划分为2种主要类型:一类低K(K2O=0.06%～0.54%),稀土元素总量和轻、重稀土元素分异程度低[∑REE=58.95×10-6～115.5×10-6,(La/Yb)N=0.23～2.94],亏损LILE;另一类富K(K2O=1.22%～1.67%),稀土元素总量和轻、重稀土元素分异程度较高[∑REE=199.97×10-6～381.47×10-6,(La/Yb)N=6.57～11.5],富集LILE而亏损HFSE。地球化学研究结果表明,基性岩墙为具大陆拉斑玄武岩特征的大陆板内岩浆侵入体,可能分别源于亏损的软流圈地幔和富集的岩石圈地幔。基性岩墙群是新特提斯洋晚期大规模扩张的产物,标志着喜马拉雅被动大陆边缘在晚侏罗世—早白垩世处于强烈的拉张、裂离和岩石圈伸展减薄、软流圈上涌的构造环境与动力学背景。     

Eocene tonalite–granodiorite from the Havza (Samsun) area,northern Turkey: adakite-like melts of lithospheric mantle and crust generated in a post-collisional setting

ABSTRACT

Eocene intermediate to felsic plutons of different sizes and compositions are widespread in the Eastern Pontides Orogenic Belt in northern Turkey. Of these, the Ta?l?k Tepe pluton in the Havza (Samsun) area is fine-to-medium-grained, with granular, porphyritic, and micrographic textures, and include mafic microgranular enclaves (MMEs). LA-ICP-MS U-Pb zircon dating yielded emplacement ages of 42.9 (± 1.4) and 40.5 (± 1.3) Ma for the host granodioritic pluton and the dioritic MMEs, respectively. Petrochemically, the host pluton has I-type, high-K calc-alkaline, and metaluminous-to-slightly peraluminous features (A/CNK = 0.95–1.06). The host pluton also shows geochemical features of adakite-like rocks with high SiO₂ (67–68 wt%) and Al₂O₃ (15.5–16.0 wt%) content and Ba/La (17–23), Sr/Y (40.7–61.6), and La_N/Yb_N (14.4–23.7) ratios and low Y (8.2–9.9 ppm) and Yb_N (3.1–4.4) contents. Whole-rock major and trace element variations suggest that fractional crystallisation played a significant role in the pluton evolution. The N-MORB normalised trace element patterns of the pluton are similar to those of MMEs with enrichment in large-ion lithophile elements, Th and Ce, and negative Nb and Ti anomalies. Chondrite-normalised rare earth element plots show moderate-to-highly enriched concave patterns (La_N/Lu_N = 14.2–21.6) with insignificant negative Eu anomalies (Eu_N/Eu* = 0.86–1.14), all of which imply hornblende fractionation during magmatic evolution. The pluton samples have ⁸⁷Sr/⁸⁶Sr ratios of 0.704767 to 0.704927, ¹⁴³Nd/¹⁴⁴Nd ratios of 0.512767–0.512774, εNd values of (+2.52) – (+2.65), and δ¹⁸O values of 7.9–9.7‰. The isotopic compositions of the host pluton and MMEs are similar to I-type granitoids derived from mantle sources. The MMEs show incomplete magma mixing/mingling, representing small bodies of mafic parental magma. Combined with regional studies, these new data suggest that the parental magma of the studied adakite-like pluton was generated from the lithospheric mantle and then modified by fractional crystallisation and assimilation in a post-collisional setting.