Zircon U–Pb and Hf isotopic study of gneissic rocks from the Chinese Altai: Progressive accretionary history in the early to middle Palaeozoic     

Min Sun  Chao Yuan  Wenjiao Xiao  Xiaoping Long  Xiaoping Xia  Guochun Zhao  Shoufa Lin  Fuyuan Wu  A. Kröner 《Chemical Geology》2008,247(3-4):352-383

Gneissic rocks in the Chinese Altai Mountains have been interpreted as either Paleozoic metasedimentary rocks or Precambrian basement. This study reports geochemical and geochronological data for banded paragneisses and associated gneissic granitoids collected along a NE–SW traverse in the northwestern Chinese Altai. Petrological and geochemical data suggest that the protoliths of the banded gneisses were possibly immature sediments with significant volcanic input and that the gneissic granitoids were derived from I-type granites formed in a subduction environment. Three types of morphological features can be recognized in zircons from the banded gneisses and are interpreted to correlate with different sources. Zircons from five samples of banded paragneiss cluster predominantly between 466 and 528 Ma, some give Neoproterozoic ages, and a few yield discordant Paleoproterozoic to Archean ages. Zircon Hf isotopic compositions indicate that both juvenile/mantle and crust materials were involved in the generation of the source rocks from which these zircons were derived. In contrast, zircons occur ubiquitously as elongated euhedral prismatic crystals in the four samples of the gneissic granitoids, and define single populations for each sample with mean ages between 380 and 453 Ma. The general absence of Precambrian inheritance and positive zircon ?Hf values for these granitoids suggest insignificant crustal contribution to the generation of the precursor magmas. Our data can be interpreted in terms of a progressive accretionary history in early to middle Palaeozoic times, and the Chinese Altai may possibly represent a magmatic arc built on a continental margin dominated by Neoproterozoic rocks.  相似文献   

新疆南天山盲起苏晚石炭世侵入岩的确定及对南天山洋盆闭合时限的限定   总被引：18，自引：5，他引：13  

朱志新  李锦轶  董连慧  张晓帆  胡建卫  王克卓 《岩石学报》2008,24(12):2761-2766

南天山额尔宾山中部出露的盲起苏侵入岩主要由花岗闪长岩组成,前人认为是泥盆纪产物,对其中锆石进行SHRIMP定年,获得侵入岩形成年龄296.9±5.4Ma和304.2±11.6Ma。其岩石学具有偏铝和过铝同碰撞花岗岩的特征,结合该带二叠纪后造山花岗岩及其它方面的区域资料,从而提出南天山古生代洋盆的闭合时限为晚石炭世。  相似文献   

Highly fractionated Late Triassic I-type granites and related molybdenum mineralization in the Qinling orogenic belt: Geochemical and U–Pb–Hf and Re–Os isotope constraints     

《Ore Geology Reviews》2014

We present new data on the highly fractionated Late Triassic I-type Liyuantang granite, which is located in the middle segment of the South Qinling Subzone of central China and is associated with molybdenum mineralization. Zircon U–Pb dating indicates that the granite was emplaced at 210.1 ± 1.9 Ma, with a single zircon containing an inherited core that yielded an age of 449.8 ± 7.1 Ma. Magmatic zircons from the granite have ε_Hf(t) values of − 4.0 to + 1.5, whereas the inherited zircon core has a ε_Hf(t) value of − 5.3. Calculated Hf model ages of crust formation are indicative of substantial contributions from melting of Proterozoic crust that ranges in age from 1501 to 1155 Ma. The granite contains high concentrations of Si, Al, Na, and K, is enriched in Rb, Th, and U, has elevated Rb/Sr and Ga/Al ratios, and is depleted in Ti, Fe, Mn, Mg, Ca, and P, with significantly negative Eu anomalies (δEu = 0.33–0.50), similar to other highly fractionated I-type granites. These data indicate that the magmas that formed the Liyuantang pluton were produced during partial melting of Proterozoic garnet-absent quartz amphibolites. The magmas then fractionated apatite, feldspar, Ti-bearing phases, biotite, and hornblende prior to emplacement.Re–Os isotope analysis of molybdenite from the study area yields a mineralization age of 200.9 ± 6.2 Ma, suggesting that the Liyuantang molybdenum deposit formed during a previously unrecognized mineralization event. The present results, together with previous data, demonstrate that highly fractionated I-type granites associated with the second pulse of magmatism in the South Qinling subzone should be considered highly prospective for mineral exploration, focusing on Triassic–Early Jurassic granitoids.  相似文献   

Tectonic evolution of the Qinling orogen and adjacent orogenic belts     

《Gondwana Research》2016

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Generation of peraluminous granitic magma in a post-collisional setting: A case study from the eastern Qilian orogen,NE Tibetan Plateau     

《Gondwana Research》2016

Early Paleozoic peraluminous granites are abundant in the eastern part of the Qilian orogen, northeastern margin of the Tibetan Plateau. A combined study involving geochronology, whole-rock geochemical and Sr–Nd–Hf isotopic compositions for three Early Paleozoic peraluminous granitic plutons (Jishishan, Ledu and Shichuan plutons) from the eastern Qilian orogen was carried out to evaluate the causes of chemical variations and generation mechanisms of peraluminous granitic magmas. These granitic plutons have magma crystallization ages of 455–427 Ma and are moderately to strongly peraluminous (A/CNK = 1.03–1.18). Geochemical and Sr–Nd–Hf isotopic data indicate that they consist substantially of crust-derived melts. The Jishishan and Ledu peraluminous granites were dominantly produced by partial melting of Precambrian orthogneisses. The Shichuan monzogranites, with low HREE contents (e.g., Yb = 0.80–1.83 ppm) and slightly negative ε_Nd(t) (− 5.3 to − 2.3) and positive ε_Hf(t) (+ 1.6 to + 3.4), could be derived from immature crustal materials. Relatively high average zircon saturation temperatures (> 750 °C for each pluton), obvious negative Eu anomalies (Eu/Eu* = 0.28–0.80) and low Pb/Ba ratios (0.03–0.16) for the Jishishan, Ledu and Shichuan granites are consistent with crustal melting involving biotite breakdown under fluid-absent conditions. Our results suggest that compositional variations of moderately to strongly peraluminous granitic magmas are mainly controlled by source compositions and melting conditions, while the processes such as mixing with mantle-derived magma, fractional crystallization, restite unmixing and peritectic assemblage entrainment were insignificant (or only play secondary roles) in their genesis. Late Ordovician to Middle Silurian crustal anatexis in the eastern Central Qilian was probably linked with slab break-off which may be an important mechanism in addition to lithospheric delamination for the generation of moderately to strongly peraluminous granites in a post-collisional setting.  相似文献   

Petrogenesis of a late-Variscan rhyodacite at the Ossa Morena-Central Iberian zones boundary,Iberian Massif,Central Portugal: Evidence for the involvement of lithospheric mantle and meta-igneous lower crust     

《Chemie der Erde / Geochemistry》2016,76(3):429-439

A late-Variscan rhyodacite is exposed at the contact between the Ossa Morena Zone and the Central Iberian Zone of the Iberian Massif, Central Portugal. Dykes of rhyodacite intruded the Série Negra Unit and the Sardoal Complex that are part of the Cadomian basement. The igneous crystallization age of the rhyodacite (308 ± 1 Ma) was obtained on igneous monazite by the ID-TIMS U-Pb method. It is broadly coeval with the emplacement of late-Variscan granitoids during the last deformation phase of the Variscan Orogeny (ca. 304–314 Ma) and with the development of the large late-Variscan strike-slip shear zones (ca. 307 Ma). The rhyodacite samples are calc-alkaline, show identical composition and belong to the same magmatic sequence. The rhyodacite isotopic signatures (Sm-Nd and δ¹⁸O) are consistent with depleted-mantle juvenile sources and the contribution of the meta-igneous lower crust. The input of mantle juvenile sources is related to Variscan reactivation of lithospheric fractures. The inherited Neoproterozoic (ca. 619 Ma) and Mesoproterozoic (ca. 1054 Ma) zircon ages, are similar to those of the Central Iberian Zone. This suggests that lower crust of the Central Iberian Zone was involved in the magma generation of the rhyodacite. Coeval late-Variscan magmatic rocks display a larger contribution from ancient crustal components, which may be attributed to the smaller volume and faster cooling rate of the rhyodacite and consequent lower melting of the crust. Mixing of juvenile mantle-derived melts with melts from the lower continental crust was followed by fractional crystallization of garnet and amphibole that remained in the source. Fractional crystallization of plagioclase, biotite, quartz and zircon occurred in shallower magma chambers. Fractional crystallization of zircon was not significant.  相似文献   

Geochemistry and petrogenesis of Early Carboniferous volcanic rocks in East Junggar,North Xinjiang: Implications for post-collisional magmatism and geodynamic process     

《Gondwana Research》2016,29(4):1466-1481

Early Carboniferous volcanic rocks in the Batamayineishan Formation overlie unconformably the molasse deposits and the ophiolitic mélanges and are restricted in narrow zones along both sides of the Kalamaili orogenic belt in North Xinjiang, southern Central Asian Orogenic Belt. These rocks demonstrate the post-collisional setting in East Junggar commenced in Tournaisian and also mark an important transitional period from the final amalgamation to late Paleozoic voluminous juvenile granitoids in East Junggar. The volcanic rocks are composed of basalt, basaltic andesite, andesite, trachyte and rhyolite. Both mafic and felsic rocks are characterized by enrichments in large ion lithophile elements, light rare earth elements and depletion in Nb and Ta, low initial ⁸⁷Sr/⁸⁶Sr and high, positive ɛ_Nd(t). Three groups of mafic rocks have been identified: Shoshonitic group 1 has the highest MgO, CaO, Ni and Cr and the lowest Na₂O, Al₂O₃, La, Ba, La/Yb and Ba/Th with primary magma features; group 2 calc-alkaline and high-K calc-alkaline mafic rocks have the lowest K₂O, P₂O₅, Th and Th/Nb, and the highest TiO₂; and group 3 (shoshonitic to potassic alkaline) has the highest K₂O, P₂O₅, La, Ba, La/Yb and Th/Nb, and the lowest TiO₂. The A-type-like felsic rocks were derived from the differentiation of the mafic magma. Geological and geochemical evidences indicate that the Batamayineishan Formation was generated from the process of slab breakoff (detachment). Group 1 samples are produced by decompressional melting of the upwelling asthenosphere mainly composed of spinel and garnet (50:50) lherzolite which has been enriched by overlying metasomatized lithosphere during ascent. Group 2 is derived from 5–10% partial melting of shallower spinel-bearing lithospheric mantle induced by the hot rising asthenosphere, where the contribution of slab-derived fluid is predominant. Low partial melting (3–5%) of the mantle wedge and/or thickened lithospheric mantle enriched by slab-derived components generates group 3. Slab breakoff as an important geodynamic process accounts for the post-collisional magmatism between 343.5 Ma–330 Ma, providing a model for post-collisional crust–mantle interaction in the CAOB.  相似文献   

柴达木盆地西部地区咸化湖盆天然气地球化学特征及成因类型判识指标——以古近系—新近系为例     

刘军  刘成林  孙平  张林  李浩涵  张艳芳  康艳丽  席勤 《地质通报》2016,35(203):321-328

根据天然气地球化学基本理论,对柴达木盆地西部（柴西）地区天然气的地球化学特征进行分析。测试和收集该地区18个油气田及含油气构造的83个天然气样品,分析其组分和C同位素数据,结果显示,该区天然气以烃类气体为主,烃类含量介于79.24%~99.81%之间,天然气比重介于0.72~1.36之间,天然气干燥系数（C1/C2+）介于1.04~617.8之间,甲烷C同位素值介于-51.4‰~-24.2‰之间,乙烷C同位素值介于-34.2‰~-17.4‰之间。在此基础上,对该区不同构造带的天然气成因类型进行研究。研究结果表明,柴达木盆地西部地区天然气类型可划分为腐泥型气、腐殖型气、混合气3类,柴西北部天然气主要为腐泥型气和混合气,少量腐殖型气;柴西南部主要为腐泥型气,少量混合气。同时,建立了天然气成因类型的判识指标,对该区的天然气地球化学特征有了较为深入的认识,对今后的天然气勘探具有指导意义。  相似文献   

塔里木盆地塔中隆起奥陶系碳酸盐岩油气运聚主控因素及成藏模式     

沈卫兵  庞雄奇  陈践发  王阳洋  张科  高兆富  贾颖超  陈泽亚 《沉积学报》2018,36(5):1008-1022

从多元地质条件及其空间匹配关系角度出发,综合地质剖析及地球化学示踪等方法,对塔里木盆地塔中隆起奥陶系油气输导体系、运移方向及分布特征等进行的精细研究。结果显示:断裂、源储接触关系及盖层等共同控制着油气垂向运移的距离及层位;构造背景、不整合面、渗透性输导层、断裂的配置关系等控制着油气侧向运移的方向、通道和距离;海平面升降旋回、优质储层的配置关系控制着油气垂向聚集层位及平面分布规律。进一步结合研究区烃源岩条件、储层类型等研究成果,基于源储纵向接触关系及油气垂向运移距离,目的层油气成藏模式可分为近源混合成藏模式与远源混合成藏模式;根据油气来源区域及油气横向运移距离,又可分为内部源岩叠加混合成藏模式与内外部源岩交叉混合成藏模式。  相似文献   

Discovery of the fossiliferous Cu Brei Formation (Lower Devonian) in the Kon Tum Block (South Viet Nam)     

《Journal of Asian Earth Sciences》2007,29(1):127-135

Lower Devonian corals and stromatoporoids have recently been discovered in limestones among low grade metamorphic rocks on the western margin of the Kon Tum Block (South Viet Nam). This unit has been identified as the Cu Brei Formation. Coral and stromatoporoid species have been described including Squameofavosites aff. spongiosus, Parallelostroma cf. multicolumnum, Amphipora cf. rasilis, A. cf. raritalis, Simplexodictyon cf. artyschtense, Stromatopora cf. boriarchinovi and Stromatopora sp. indet. The Cu Brei Formation is exposed in a small area 6 km in length and 3 km wide at the foot of Cu Brei Mountain (Sa Thay District, Kon Tum Province). As this formation is in marine shelf facies it is probable that further exposures of Lower Devonian sediments may be discovered in the Kon Tum Block. This discovery raises the question of the tectonic history of the metamorphic Kon Tum Block. It is possible that the block was not an area of positive uplift from the beginning of Paleozoic as has been supposed, but was submerged in a marine environment, at least on its outer margins, in the Devonian, and possibly even earlier, in Early Paleozoic.  相似文献