西藏西北部西昆仑造山带原特提斯洋的地球动力演化:弧下地壳和岩石圈地幔改造的影响

IF 3.5 2区 地球科学 Q1 GEOCHEMISTRY & GEOPHYSICS
Zaili Tao, Jiyuan Yin, Mike Fowler, Wenjiao Xiao, Zhiming Yang, Min Sun, Andrew C Kerr, Victoria Pease, Tao Wang, Wen Chen, Fan Yang
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The Datong syenite samples belong to the shoshonitic series and are characterized by medium SiO2 (59.5–61.4 wt.%), relatively low MgO (0.9–1.2 wt.%) and Mg# (37–42), enrichment in LILEs and depletion in HFSEs. They have high whole-rock (87Sr/86Sr)i ratios (0.7103 to 0.7105) and negative whole-rock εNd(t) values (−3.8 to −4.3), along with negative to slightly positive zircon εHf(t) values (–3.8 to +2.6), similar to coeval mafic rocks. Thermodynamic and geochemical modeling suggests that the Datong shoshonitic rocks likely originated via crystal fractionation of shoshonitic basaltic magmas in the SW Tarim Craton. The Datong monzogranites have high SiO2 (69.7–72.6 wt.%), low MgO (0.6–0.7 wt.%) and demonstrate a typical enrichment in alkalis, Zr, and Nb with depletion in Sr, P, and Ti, consistent with A-type granites. 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引用次数: 0

摘要

放射性同位素是研究地壳演化的重要工具,在揭示岩浆来源和岩石成因方面发挥着关键作用。然而,它们在区分具有相似放射性同位素组成的不同岩浆源方面可能效果不佳,这在自然界中很常见。本文在西昆仑造山带(WKOB)奥陶纪火成岩中解决了这一难题,利用基于矿物和全岩地球化学的适当热力学和地球化学模型,区分了具有相似同位素组成的两个潜在岩浆源(即塔里木克拉通和天水海陆地)。通过锆石 U-Pb 测定,普寿辉长岩、大同单斜岩和正长岩的年龄分别为 483 ± 3 Ma、469 ± 2 Ma 和 461 ± 2 Ma。普寿辉长岩的二氧化硅含量低(47.4-49.1 wt.%),氧化镁含量高(5.5-6.9 wt.%),大离子亲岩元素(LILEs,如Rb、Ba、Th和K)含量高,高场强元素(HFSEs,如Nb、Ta、Zr、Hf、P和Ti)含量低,这表明它们起源于俯冲改造地幔。它们显示出较高的全岩(87Sr/86Sr)i 比值(0.7156 至 0.7192)、负的全岩εNd(t)值(-7.1 至 -7.8)以及较高的锆石δ18O 值(7.6-7.9 ‰)和富集的锆石 Hf 同位素组成(εHf(t) = -5.3 至 -7.7),与富集地幔源中 1-5% 的俯冲沉积物相一致。痕量元素模型进一步证实,辉长岩很可能来自尖晶石-石榴石面的低度(~15%)部分熔融的俯冲改造塔里木地幔,而不是来自天水海地幔。大同正长岩样品属于闪长岩系列,具有中等二氧化硅含量(59.5-61.4 wt.%)、相对较低的氧化镁含量(0.9-1.2 wt.%)和镁含量(37-42)、富含锂辉石(LILE)和贫乏高频闪长岩(HFSE)的特征。它们具有较高的全岩(87Sr/86Sr)i比值(0.7103至0.7105)和负的全岩εNd(t)值(-3.8至-4.3),以及负到略正的锆石εHf(t)值(-3.8至+2.6),与共生岩浆岩类似。热力学和地球化学建模表明,大同闪长岩很可能起源于塔里木克拉通西南部的闪长玄武岩浆的晶体分馏。大同闪长岩具有高SiO2(69.7-72.6 wt.%)、低MgO(0.6-0.7 wt.%)的特征,并表现出典型的碱金属、Zr和Nb富集以及Sr、P和Ti贫化的特征,与A型花岗岩一致。它们的特征是全岩(87Sr/86Sr)i 比值高(0.7321 至 0.7323)、全岩εNd(t)为负(-11.3 至 -11.8)、锆石εHf(t)为负(-11.0 至 -16.5)和锆石δ18O 高(7.2-8.0 ‰),表明它们是由古地壳源重熔而成的。热力学、主要元素和痕量元素建模表明,它们的母岩浆可能是在高温(T&t;gt; ~950 °C)、低压(P = 5-8 kbar)条件下,由塔里木克拉通西南部而非天水海陆带下的古地壳物质缺水(~2 wt.%)部分熔融生成的。根据西昆仑台地的构造框架,我们推测在奥陶纪时期,南昆仑台地下原有的地幔和地壳可能已被西南塔里木克拉通下的地幔和地壳所改变或部分取代。因此,塔里木岩浆活动的证据与区域沉积和构造记录相结合,表明南昆仑地层的奥陶纪岩浆活动与前特提斯洋在约485Ma时开始的偶发性向北俯冲最为吻合。中奥陶世板块断裂可以解释A型花岗岩的形成,但奥陶世晚期大通正长岩的产生则需要重新向北俯冲。
本文章由计算机程序翻译,如有差异,请以英文原文为准。
Geodynamic Evolution of the Proto-Tethys Ocean in the West Kunlun Orogenic Belt, NW Tibet: Implications from the Sub-Arc Crust and Lithospheric Mantle Modification
Radiogenic isotopes serve as a crucial tool for investigating crustal evolution, playing a pivotal role in revealing magma sources and petrogenesis. However, they can be ineffective in distinguishing distinct magmatic sources with similar radiogenic isotopic compositions, a common occurrence in nature. This paper addresses this challenge in the Ordovician igneous rocks from the West Kunlun orogenic belt (WKOB), to distinguish between two potential magmatic sources (i.e., the Tarim Craton and the Tianshuihai terrane) with similar isotopic compositions, by using appropriate thermodynamic and geochemical modeling based on mineral and whole-rock geochemistry. Zircon U–Pb dating yields ages of 483 ± 3 Ma, 469 ± 2 Ma and 461 ± 2 Ma for the Pushou gabbros and the Datong monzogranites and syenites, respectively. The Pushou gabbros exhibit low SiO2 (47.4–49.1 wt.%), high MgO (5.5–6.9 wt.%), high large ion lithophile elements (LILEs, e.g., Rb, Ba, Th, and K) and low high field-strength elements (HFSEs, e.g., Nb, Ta, Zr, Hf, P, and Ti), which suggest an origin in subduction-modified mantle. They display high whole-rock (87Sr/86Sr)i ratios (0.7156 to 0.7192), negative whole-rock εNd(t) values (−7.1 to −7.8), as well as high zircon δ18O values (7.6–7.9 ‰) and enriched zircon Hf isotopic compositions (εHf(t) = −5.3 to −7.7), consistent with 1–5% subducted sediments in an enriched mantle source. Trace element models further confirm that the gabbros are most likely derived from low-degree (~15 %) partial melting of subduction-modified Tarim mantle in the spinel-garnet facies, rather than from the Tianshuihai mantle. The Datong syenite samples belong to the shoshonitic series and are characterized by medium SiO2 (59.5–61.4 wt.%), relatively low MgO (0.9–1.2 wt.%) and Mg# (37–42), enrichment in LILEs and depletion in HFSEs. They have high whole-rock (87Sr/86Sr)i ratios (0.7103 to 0.7105) and negative whole-rock εNd(t) values (−3.8 to −4.3), along with negative to slightly positive zircon εHf(t) values (–3.8 to +2.6), similar to coeval mafic rocks. Thermodynamic and geochemical modeling suggests that the Datong shoshonitic rocks likely originated via crystal fractionation of shoshonitic basaltic magmas in the SW Tarim Craton. The Datong monzogranites have high SiO2 (69.7–72.6 wt.%), low MgO (0.6–0.7 wt.%) and demonstrate a typical enrichment in alkalis, Zr, and Nb with depletion in Sr, P, and Ti, consistent with A-type granites. They are characterized by high whole-rock (87Sr/86Sr)i ratios (0.7321 to 0.7323), negative whole-rock εNd(t) (−11.3 to −11.8), negative zircon εHf(t) (−11.0 to −16.5) and high zircon δ18O (7.2–8.0 ‰), indicating derivation from the remelting of an ancient crustal source. Thermodynamic, major, and trace element modeling indicate that their parent magma may have been generated by water-deficient (~2 wt.%) partial melting of ancient crustal material beneath the SW Tarim Craton rather than that of the Tianshuihai terrane, under high-temperature (T > ~950 °C), low-pressure (P = 5–8 kbar) conditions. Based on the tectonic framework of the WKOB, we propose that the original mantle and crust beneath the southern Kunlun terrane may have been modified or partially replaced by that beneath the SW Tarim Craton during the Ordovician. Therefore, this evidence for Tarim-derived magmatism, when combined with regional sedimentary and structural records, indicates that Ordovician magmatism in the southern Kunlun terrane is most consistent with episodic northward subduction of the Proto-Tethys Ocean, commencing at ~485 Ma. Middle Ordovician slab break-off can explain the formation of the A-type granites, but re-instated northward subduction is required for the generation of late Ordovician Datong syenites.
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来源期刊
Journal of Petrology
Journal of Petrology 地学-地球化学与地球物理
CiteScore
6.90
自引率
12.80%
发文量
117
审稿时长
12 months
期刊介绍: The Journal of Petrology provides an international forum for the publication of high quality research in the broad field of igneous and metamorphic petrology and petrogenesis. Papers published cover a vast range of topics in areas such as major element, trace element and isotope geochemistry and geochronology applied to petrogenesis; experimental petrology; processes of magma generation, differentiation and emplacement; quantitative studies of rock-forming minerals and their paragenesis; regional studies of igneous and meta morphic rocks which contribute to the solution of fundamental petrological problems; theoretical modelling of petrogenetic processes.
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