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191.
Ordovician Granitoids and Silurian Mafic Dikes in the Western Kunlun Orogen, Northwest China: Implications for Evolution of the Proto-Tethys 
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下载免费PDF全文 The western Kunlun orogen in the northwest Tibet Plateau is related to subduction and collision of Proto-and Paleo-Tethys from early Paleozoic to early Mesozoic. This paper presents new LA-ICPMS zircon U-Pb ages and Lu-Hf isotopes, whole-rock major and trace elements, and Sr–Nd isotopes of two Ordovician granitoid plutons(466–455 Ma) and their Silurian mafic dikes(～436 Ma) in the western Kunlun orogen. These granitoids show peraluminous high-K calcalkaline characteristics, with(87Sr/86Sr)_i value of 0.7129–0.7224, εNd(t) values of -9.3 to -7.0 and zircon εHf(t) values of -17.3 to -0.2, indicating that they were formed by partial melting of ancient lower-crust(metaigneous rocks mixed with metasedimentary rocks) with some mantle materials in response to subduction of the Proto-Tethyan Ocean and following collision. The Silurian mafic dikes were considered to have been derived from a low degree of partial melting of primary mafic magma. These mafic dikes show initial 87Sr/86Sr ratios of 0.7101–0.7152 and εNd(t) values of -3.8 to -3.4 and zircon εHf(t) values of -8.8 to -4.9, indicating that they were derived from enriched mantle in response to post-collisional slab break-off. Combined with regional geology, our new data provide valuable insight into late evolution of the Proto-Tethys. 相似文献
192.
塔里木克拉通东北缘坡北、磁海等地二叠纪幔源岩浆活动形成了镍钴硫化物矿床和铁钴氧化物矿床,两者赋矿镁铁-超镁铁岩体的年龄相近(290～260 Ma),主、微量元素和Sr-Nd-Hf同位素组成相似,分配系数接近的微量元素比值分布于相同趋势线,揭示两者岩浆源区相同,可能为俯冲板片流体交代的亏损地幔或软流圈地幔。两类矿床镁铁-超镁铁质岩中Co与Ni含量正相关,Co主要富集在基性程度高的岩石中;块状硫化物与磁铁矿矿石中Co与Ni相关性差,Co和Ni具有不同的富集机制,Co热液富集作用明显。北山镁铁-超镁铁杂岩体是地幔柱相关软流圈上涌,诱发俯冲板片交代的亏损岩石圈地幔发生部分熔融,形成的高镁母岩浆演化过程中经历壳源混染、硫化物饱和富集镍钴形成铜镍钴硫化物矿床,富铁母岩浆氧逸度高、富水,岩浆分离结晶磁铁矿、叠加热液作用富集钴,形成铁钴氧化物矿床。 相似文献
193.
本文以阿尔金造山带南缘出露的高镁赞岐质闪长岩为研究对象,通过系统的野外地质工作、显微岩相学、年代学和全岩地球化学分析,探讨其成因类型和源区性质,限定了阿尔金造山带构造演化格架。闪长岩LA-ICP-MS锆石U-Pb测年结果表明其形成年龄为(445±3) Ma,属晚奥陶世。地球化学数据显示:闪长岩Al2O3含量为15.11%~16.19%,且具有高MgO(3.93%~5.29%)和K2O(1.68%~2.67%)含量的特征,属于高镁钙碱性准铝质岩石;闪长岩具有较高的Cr(29.3×10-6~140×10-6)、Ni(27.1×10-6~43.5×10-6)、Ba(606×10-6~936×10-6)、Sr(513×10-6~734×10-6)和Y(26.4×10-6~31.1×10-6)含量,与典型的高镁赞岐岩类地球化学特征一致;稀土配分图和微量元素蛛网图显示闪长岩样品富集轻稀土元素和大离子亲石元素(如Rb和K),而亏损高场强元素(如Nb、Ti和Zr),表现出与俯冲带环境岩浆岩相似的性质;高Ti/Zr比值(48~60)与Mg#(46.74~53.06)、高Cr(29.3×10-6~140×10-6)与高Ni(27.1×10-6~43.5×10-6)含量等暗示闪长岩岩浆源区来自遭受俯冲组分交代过的富集岩石圈地幔。岩石成因分析表明,阿尔金造山带南缘晚奥陶世高镁赞岐质闪长岩是后碰撞构造环境下俯冲板片断离诱发地幔楔发生减压熔融而形成,代表了早古生代最强烈的区域性幔源基性岩浆侵入事件。 相似文献
194.
195.
Chloé Loury Yann Rolland Stéphane Guillot Pierre Lanari Clément Ganino Raphael Melis Anthony Jourdon Carole Petit Olivier Beyssac Sylvain Gallet Patrick Monié 《Journal of Metamorphic Geology》2018,36(8):959-985
The South Tien Shan (STS) belt results from the last collision event in the western Central Asian Orogenic Belt (CAOB). Understanding its formation is of prime importance in the general framework of the CAOB. The Atbashi Range preserves high‐P (HP) rocks along the STS suture, but still, its global metamorphic evolution remains poorly constrained. Several HP units have been identified: (a) a HP tectonic mélange including boudins of mafic eclogites in a sedimentary matrix, (b) a large (>100 km long) high‐P metasedimentary unit (HPMU) and (c) a lower blueschist facies accretionary prism. Raman Spectroscopy on carbonaceous material combined with phengite and chlorite multiequilibria and isochemical phase diagram modelling indicates that the HPMU recorded homogeneous P–T conditions of 23–25 kbar and 560–570°C along the whole unit. 40Ar/39Ar dating on phengite from the HPMU ranges between 328 and 319 Ma at regional scale. These ages are interpreted as (re‐) crystallization ages of phengite during Tmax conditions at a pressure range of 20–25 kbar. Thermobarometry on samples from the HP tectonic mélange provides similar metamorphic peak conditions. Thermobarometry on the blueschist to lower greenschist facies accretionary prism indicates that it underwent P–T conditions of 5–6 kbar and 290–340°C, highlighting a 17–20 kbar pressure gap between the HPMU‐tectonic mélange units and the accretionary prism. Comparison with available geochronological data suggests a very short time span between the prograde path (340 Ma), HP metamorphic peak (330 Ma), the Tmax (328–319 Ma) and the final exhumation of the HPMU (303–295 Ma). Extrusion of the HPMU, accommodated by a basal thrust and an upper detachment, was driven by buoyant forces from 70–75 km up to 60 km depth, which directly followed continental subduction and detachment of the HPMU. At crustal depths, extrusion was controlled by collisional tectonics up to shallow levels. Lithological homogeneity of the HPMU and its continental‐derived character from the North Tien Shan suggest this unit corresponds to the hyper‐extended continental margin of the Kazakh continent, subducted southward below the north continental active margin of the Tarim craton. Integration of the available geological data allows us to propose a general geodynamic scenario for Tien Shan during the Carboniferous with a combination of (a) N‐dipping subduction below the Kazakh margin of Middle Tien Shan until 390–340 Ma and (b) S‐dipping subduction of remaining Turkestan marginal basins between 340 and 320 Ma. 相似文献
196.
Yves Feisel Richard W. White Richard M. Palin Tim E. Johnson 《Journal of Metamorphic Geology》2018,36(6):799-819
In this study, we investigate the metamorphic history of the Assynt and Gruinard blocks of the Archean Lewisian Complex, northwest Scotland, which are considered by some to represent discrete crustal terranes. For samples of mafic and intermediate rocks, phase diagrams were constructed in the Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O2 (NCKFMASHTO) system using whole‐rock compositions. Our results indicate that all samples equilibrated at similar peak metamorphic conditions of ~8–10 kbar and ~900–1,000°C, consistent with field evidence for in situ partial melting and the classic interpretation of the central region of the Lewisian Complex as representing a single crustal block. Melt‐reintegration modelling was employed in order to estimate probable protolith compositions. Phase equilibria calculated for these modelled undepleted precursors match well with those determined for a subsolidus amphibolite from Gairloch in the southern region of the Lewisian Complex. Both subsolidus lithologies exhibit similar phase relations and potential melt fertility, with both expected to produce orthopyroxene‐bearing hornblende granulites, with or without garnet, at the conditions inferred for the Badcallian metamorphic peak. For fully hydrated protoliths, prograde melting is predicted to first occur at ~620°C and ~9.5 kbar, with up to 45% partial melt predicted to form at peak conditions in a closed‐system environment. Partial melts calculated for both compositions between 610 and 1,050°C are mostly trondhjemitic. Although the melt‐reintegrated granulite is predicted to produce more potassic (granitic) melts at ~700–900°C, the modelled melts are consistent with the measured compositions of felsic sheets from the central region Lewisian Complex. 相似文献
197.
198.
In this study, we report the first occurrence of Archean Niobium-enriched mafic dikes in the southern Huangling dome within the Yangtze Block, southern China. Zircon U–Pb dating of three mafic dike samples yielded weighted mean 207Pb/206Pb ages of 2,858 ± 24 Ma (MSWD = 1.14), 2,889 ± 11 Ma (MSWD = 0.98), and 2,840 ± 11 Ma (MSWD = 0.17), respectively, which are interpreted as the intrusion ages of the dikes. Zircons from the host granodioritic gneiss show different trace element patterns and variable Eu anomalies, and yield an upper intercept age of 2,891 ± 32 Ma. On the basis of previous studies, the 2,891 Ma age is interpreted as the metamorphic recrystallization time caused by late tectono-thermal events. The mafic dikes possess high Nb contents ranging from 4.7 to 18.4 ppm, which are similar to those reported from Archean, Proterozoic, and Phanerozoic Nb-enriched basaltic rocks occurring worldwide. The Nb-enriched mafic dikes show HFSE-depleted patterns with significant negative Zr, Hf, and Ti anomalies, and higher Nb/U (8.29–48.42), Nb/Thpm (0.59–1.71), and Nb/Lapm (0.76–1.34) ratios than “normal” island arc volcanic rocks that are consistent with a subduction zone geochemical affinity. Positive εNd(t) values (+5.0 to +11.8) of the Nb-enriched mafic dikes point to an extended depleted mantle characterized by elevated Sm/Nd ratio, whereas εNd(t) values (+0.3 to −2.3) and tectonic environment of the host granodioritic rocks, as well as the most widely accepted petrogenetic models of the TTG series indicate enriched mafic crustal sources. Therefore, we propose that the Nb-enriched mafic dikes were generated from a depleted mantle with an extended history of elevated Sm/Nd ratio that was metasomatized by slab-derived melts at ca. 2.90–2.85 Ga. 相似文献
199.
The eastern Tianshan is an important Ni–Cu mineralization belt that contains several ore-bearing mafic–ultramafic intrusions. The eastern part of this belt has been well studied, whereas the western part is poorly studied. Here, we present studies of two mafic intrusions (DC3 and DC4) from the Dacaotan area in the western part of the eastern Tianshan. The gabbrosof the DC3 and DC4 intrusions contain plagioclase and clinopyroxene, and are characterized by light rare earth element enrichment, negative Nb anomalies, depleted platinum group element contents, high and variable Cu/Pd ratios, positive εNd (6.0–7.5) values, and low initial 87Sr/86Sr ratios (0.7033–0.7036). The two intrusions have geochemical affinities with intrusions in the Huangshan area, despite mineralogical differences, with orthopyroxene and amphibole being present in the Huangshan intrusions. The two studied mafic intrusions formed from a high-Mg basaltic magma derived from anhydrous and depleted mantle. Due to the low degree (<8%) of partial melting, the magma was PGE-depleted and sulphide remained in the mantle source. The PGE contents were further depleted by early magmatic sulphide segregation. Given the limited contamination (<10%) by the upper crust and coeval A-type granites, these two intrusions have poor potential for Ni–Cu sulphide mineralization. 相似文献
200.
The determination of metamorphic pressure–temperature (P–T) conditions of granulites provides us a natural window into the composition, structure, and P–T conditions of the lower crust and process of continental growth. Here, we constrain the P–T evolution of a suite of mafic granulite from the eastern part of Madurai Block. Integrated results from mineral reactions, conventional thermobarometry, and pseudosection analysis suggest that the studied mafic granulites, with a peak assemblage of coarse-grained garnet + coarse-grained clinopyroxene + plagioclase + quartz + [rutile], were metamorphosed under granulite facies conditions of 830 ± 50°C and 9.5 ± 1 kbar. Subsequently, the rocks underwent near isothermal decompression leading to the formation of symplectic assemblages of clinopyroxene + orthopyroxene + plagioclase and orthopyroxene + ilmenite, within a P–T range of 9.0–5.5 kbar and 750–800°C. The estimated peak temperature conditions are somewhat lower than those obtained using characteristic ultra-high temperature (UHT) mineral assemblages and non-conventional thermometers (~1,000°C at ~10 kbar) from the co-metamorphosed metapelitic granulites of the Madurai Block. This may be attributed to the high diffusion rate of divalent Fe and Mg, which often results in retrograde re-equilibration leading to considerable underestimation of peak temperatures in mafic granulites. To overcome this problem, we have additionally determined peak metamorphic temperatures of the studied rocks using a recently developed garnet–clinopyroxene rare earth element (REE)-based thermometer that takes advantage of much lower diffusion rate of trivalent REEs. Using the REE thermometer, we have obtained a peak temperature of 1,050 ± 50°C for the studied rocks, which is in reasonable agreement with the peak metamorphic temperature obtained from previous studies. The constrained P–T conditions and the clockwise P–T path suggest that the studied rocks were buried to lower crustal depths at UHT conditions in a convergent margin setting. The near isothermal decompression suggests rapid exhumation, most likely due to an extensional event. The peak UHT metamorphic conditions could have been attained at the core of a long-lived orogenic plateau, as suggested by other studies. 相似文献