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Early Palaeozoic alkaline trachytes in the North Daba Mountains, South Qinling Belt: petrogenesis and geological implications
International Geology Review ( IF 2.6 ) Pub Date : 2020-09-17 , DOI: 10.1080/00206814.2020.1818302
Hang Yang 1 , Shao-Cong Lai 1 , Jiang-Feng Qin 1 , Ren-Zhi Zhu 1 , Shao-Wei Zhao 2 , Yu Zhu 1 , Fang-Yi Zhang 1 , Ze-Zhong Zhang 1 , Xing-Ying Wang 1
Affiliation  

ABSTRACT

Early Palaeozoic alkaline magmatism is widely preserved in the North Daba Mountains, South Qinling Belt (SQB), predominately composed of basalt–diabase and trachyte–syenite. The petrogenesis of the felsic rocks and their genetic connections with the mafic members remain controversial. Here, an integrated investigation, combining geochronology and whole-rock and Sr–Nd isotopic geochemistry, is conducted to further constrain the origin and tectono-magmatic evolution of the trachytes in Pingli–Zhuxi area, North Daba Mountains. Zircon U–Pb dating for the Pingli–Zhuxi trachytes yield ages of 406.0 to 427.9 Ma, which are close to those of the mafic rocks (420–455 Ma) in research area. The trachytes show relatively high SiO2 (60.88–63.87 wt.%) and total alkali (Na2O + K2O = 10.19–12.24 wt.%) contents, and are characterized by pronounced enrichment in LREEs and HFSEs, with insignificantly negative Eu anomalies and significantly negative Sr anomalies. All of the samples display low and variable initial 87Sr/86Sr ratios of 0.6959 to 0.7083 and narrow range initial 143Nd/144Nd ratios of 0.5123 to 0.5125 with positive εNd(t) values of +3.0 to +7.3, suggesting a depleted source. Together with the published data from coeval SQB diabases, our geochemistry evidences show regular and linear variations between mafic and felsic end-members, suggesting their closely genetic link. A plausible petrogenetic hypothesis for the genesis of the Pingli–Zhuxi trachytes implies a protracted process of fractional crystallization driven by separation of K-feldspar, plagioclase, biotite, apatite, and Ti-magnetite from a basaltic melt. Enrichment in REEs and HFSEs and similar geochemical characteristics point to that the Pingli–Zhuxi trachytes and the coeval mafic rocks originate from a cogenetic metasomatized lithosphere source in rift setting. Asthenospheric upwelling is a key factor for continental break up and lithosphere metasomatism. Collectively, melting of the metasomatized source, followed by protracted K-feldspar-dominated fractional crystallization, leading to the occurrence of the Pingli–Zhuxi trachytes in the North Daba Mountains.



中文翻译:

南秦岭北大巴山早古生代碱性粗面岩的成因及地质意义

摘要

早古生代碱性岩浆作用广泛保存于南秦岭带(SQB)北大巴山脉,主要由玄武岩-辉绿岩和粗面-正长岩组成。长英质岩石的成因及其与镁铁质成员的成因联系仍然存在争议。在这里,结合年代学和全岩以及Sr-Nd同位素地球化学的综合调查,进一步限定了北大巴山平利-竹溪地区粗面体的成因和构造-岩浆演化。平利-竹溪粗面岩锆石U-Pb定年406.0~427.9 Ma,与研究区基性岩(420~455 Ma)相近。粗面岩显示出相对较高的 SiO 2 (60.88–63.87 wt.%) 和总碱 (Na 2 O + K 2O = 10.19–12.24 wt.%) 含量,其特征是 LREE 和 HFSE 显着富集,Eu 负异常不显着,Sr 负异常显着。所有样品都显示出低且可变的初始87 Sr/ 86 Sr 比率为 0.6959 至 0.7083 和窄范围的初始143 Nd/ 144 Nd 比率为 0.5123 至 0.5125,具有正ε Nd(t) +3.0 至 +7.3 的值,表明资源耗尽。连同来自同时代 SQB 辉绿岩的已发表数据,我们的地球化学证据表明,镁铁质和长英质最终成员之间存在规律和线性的变化,表明它们之间存在密切的遗传联系。平利-竹溪粗面岩成因的一个似是而非的岩石成因假说暗示了一个由钾长石、斜长石、黑云母、磷灰石和钛磁铁矿从玄武岩熔体中分离驱动的分步结晶过程。REEs 和HFSEs 的富集以及相似的地球化学特征表明,平利-竹溪粗面岩和同时期的基性岩起源于裂谷背景下的共生交代岩石圈源。软流圈上升流是大陆分裂和岩石圈交代作用的关键因素。集体,

更新日期:2020-09-17
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