Petrogenesis of Early Paleozoic I-type granitoids in the Longshoushan and implications for the tectonic affinity and evolution of the southwestern Alxa Block

IF 3.2 2区 地球科学 Q1 GEOCHEMISTRY & GEOPHYSICS Solid Earth Pub Date : 2024-06-26 DOI:10.5194/egusphere-2024-1145
Renyu Zeng, Hui Su, Mark B. Allen, Haiyan Shi, Houfa Dua, Chenguang Zhange, Jie Yan
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Abstract

Abstract. In the Paleozoic, the Alxa Block was situated between the Central Asian Orogenic Belt and the North Qilian Orogenic Belt, and experienced intense magmatic activity. Thus, the Alxa Block is an important area for understanding the tectonic framework and evolution of these two orogenic belts. However, there has long been debate regarding the tectonic affinity and tectonic evolution of the Longshoushan, located in southwestern margin of the Alxa Block, during the Paleozoic. In this study, we present zircon U-Pb ages, whole-rock major and trace elements, and Hf isotopic data for the granitoids from the east of the Longshoushan to investigate these issues. Bulk-rock analyses show that these granitoids are weakly peraluminous, with high SiO2 and K2O, but low MgO, TFe2O3 and P2O5. They are also characterized by enrichment in LREE and LILE, depletion in HREE and HFSE, and a large range of variation in εHf(t) values (monzogranite: -0.37 to -16.28; K-feldspar granite: 3.53 to -7.74). These geochemical features indicate that these granitoids are highly fractionated I-type granite, which were formed by crust and mantle-derived magma mixing. LA-ICP-MS zircon U-Pb dating constrains that the monzogranite and K-feldspar granite were formed at 440.8 ± 2.1 Ma and 439.4 ± 2.0 Ma, respectively. Combining these results with previous chronological data, the geochronology framework of Paleozoic magmatic events in the Longshoushan is consistent with the North Qilian Orogenic Belt to the south, but significantly differs from other parts of the Alxa Block and the Central Asian Orogenic Belt to the north. This result indicates that the Longshoushan was primarily influenced by the North Qilian Orogenic Belt during the Early Paleozoic. Integrated with previous studies, a three-stage tectonic model is proposed of Early Paleozoic accretion and arc magmatism leading to collision in the Longshoushan. (1) 460–445 Ma: Arc magmatism on an active continental margin with the northward subduction of the North Qilian back-arc basins (NQ bab). (2) 445–435 Ma: Magmatic rocks, dominated by I-type granites, were formed in a continent-continent collision setting. Significant crustal thickening is interpreted to result from compressional stress and/or magmatic additions. (3) 435–410 Ma: The development of abundant A-type granites and mafic dikes in response to intraplate extension, supported by a change in trace element chemistry indicating crustal thinning at this stage. This sequence of events and their timings is similar to other parts of the Central China Orogenic Belt, and requires either a coincidence of several oceanic plates closing at the same time, or an along-strike repetition of the same system.
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龙首山早古生代I型花岗岩的岩石成因及其对阿拉善西南地块构造亲缘关系和演化的影响
摘要在古生代,阿拉善地块位于中亚造山带和北祁连造山带之间,经历了强烈的岩浆活动。因此,阿拉善地块是了解这两个造山带构造框架和演化的重要区域。然而,位于阿拉善地块西南边缘的龙首山在古生代的构造亲缘关系和构造演化一直存在争议。在本研究中,我们展示了龙首山东部花岗岩的锆石U-Pb年龄、全岩主要元素和微量元素以及Hf同位素数据,以探讨这些问题。大块岩石分析表明,这些花岗岩为弱过铝质,SiO2和K2O含量高,但MgO、TFe2O3和P2O5含量低。它们还具有 LREE 和 LILE 富集、HREE 和 HFSE 贫乏以及 εHf(t)值变化范围大的特点(单斜花岗岩:-0.37 至 -16.28;K 长石花岗岩:3.53 至 -7.74)。这些地球化学特征表明,这些花岗岩属于高度分馏的I型花岗岩,是由地壳和地幔岩浆混合形成的。LA-ICP-MS锆石U-Pb年代测定表明,单斜花岗岩和K长石花岗岩分别形成于440.8 ± 2.1 Ma和439.4 ± 2.0 Ma。将这些结果与之前的年代学数据相结合,龙首山古生代岩浆事件的地质年代框架与南面的北祁连造山带一致,但与阿拉善地块的其他部分和北面的中亚造山带有显著差异。这一结果表明,龙首山在早古生代主要受到北祁连造山带的影响。结合以往的研究,提出了龙首山早古生代增生和弧形岩浆作用导致碰撞的三阶段构造模式。(1) 460-445 Ma:随着北祁连背弧盆地(NQ bab)的向北俯冲,活跃大陆边缘的弧岩浆活动。(2) 445-435 Ma:岩浆岩形成于大陆-大陆碰撞环境中,以I型花岗岩为主。地壳的显著增厚被解释为压缩应力和/或岩浆作用的结果。(3) 435-410 Ma:由于板内延伸,形成了丰富的A型花岗岩和岩浆岩尖峰,痕量元素化学成分的变化表明在这一阶段地壳变薄。这一系列事件及其发生时间与华中造山带的其他地区相似,要么是几个大洋板块同时闭合,要么是同一系统沿走向重复。
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来源期刊
Solid Earth
Solid Earth GEOCHEMISTRY & GEOPHYSICS-
CiteScore
6.90
自引率
8.80%
发文量
78
审稿时长
4.5 months
期刊介绍: Solid Earth (SE) is a not-for-profit journal that publishes multidisciplinary research on the composition, structure, dynamics of the Earth from the surface to the deep interior at all spatial and temporal scales. The journal invites contributions encompassing observational, experimental, and theoretical investigations in the form of short communications, research articles, method articles, review articles, and discussion and commentaries on all aspects of the solid Earth (for details see manuscript types). Being interdisciplinary in scope, SE covers the following disciplines: geochemistry, mineralogy, petrology, volcanology; geodesy and gravity; geodynamics: numerical and analogue modeling of geoprocesses; geoelectrics and electromagnetics; geomagnetism; geomorphology, morphotectonics, and paleoseismology; rock physics; seismics and seismology; critical zone science (Earth''s permeable near-surface layer); stratigraphy, sedimentology, and palaeontology; rock deformation, structural geology, and tectonics.
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