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Late Silurian to Early Devonian volcanics in the East Kunlun orogen, northern Tibetan Plateau: Record of postcollisional magmatism related to evolution of the Proto-Tethys Ocean
Journal of Geodynamics ( IF 2.3 ) Pub Date : 2020-10-01 , DOI: 10.1016/j.jog.2020.101780
Ruibao Li , Xianzhi Pei , Zuochen Li , Daniel Patias , Zhenguo Su , Lei Pei , Guochao Chen , Youxin Chen , Chengjun Liu

Abstract The late Early Paleozoic volcanic rocks in the East Kunlun orogen are crucial to determining collisional processes related to the evolution of the Proto-Tethys Ocean. We report new zircon U-Pb ages, whole-rock geochemistry and Sr-Nd isotopes from these volcanic rocks and constrain their tectonic setting and magmatic processes. This volcanic suite consists mainly of foliated metarhyolites and minor metabasalts. Zircon U-Pb ages indicate that the bimodal volcanic rocks formed from Late Silurian to Early Devonian (ca. 420−409 Ma). The basalts show tholeiitic geochemical signatures characterized by low contents of SiO2 (47.19–54.83 %), MgO (2.21–7.52 %), and K2O/Na2O ratios (0.32−0.77%) and high contents of TiO2 (1.80–2.91 %). Their chondrite-normalized REE patterns are characterized by enrichments in LREEs (LREE/HREE = 3.68–6.09) with slight Eu anomalies (δEu = 0.67–1.00); their patterns are similar to the reference line of oceanic island basalt (OIB). The primitive mantle-normalized trace element diagram also shows similarities with OIBs and features overall enrichments in large ion lithophile and high field strength elements, except for Nb and Ta. The eNd(t = 422 Ma) values range narrowly from −1.79 to +1.32. These features suggest that the basalts were most likely derived from an asthenospheric mantle that was contaminated by small volumes of subcontinental lithospheric mantle (SCLM)/crust. The variations in major and trace elements show that the basalts experienced fractional crystallization of olivine, pyroxene and plagioclase. In contrast, the metarhyolites have high SiO2 (65.85–70.83 %), Na2O + K2O (6.71–10.09 %), K2O/Na2O ratios (1.69–2.80 %) and low MgO, Ni, and Cr contents. Their trace element signatures show enrichments in LILEs and LREEs (e.g. Cs, Rb, Ba), depletion of HFSEs (e.g. Nb, Ti, Ta) and high negative Eu anomalies. These features suggest a crustal origin lacking interactions with mantle melts. Considering all the geochemical and tectonic events of this area, it is concluded that the Maoniushan Formation volcanic rocks in EKOB formed in a postcollisional extensional setting related to asthenospheric mantle upwelling and continental delamination processes during the Late Silurian to Early Devonian. These tectonic-magmatic events, together with previous data, suggest that the Proto-Tethys Ocean had closed and evolved to postcollisional collapse stage since the Late Silurian.

中文翻译:

青藏高原北部东昆仑造山带晚志留世至早泥盆世火山岩:与原特提斯洋演化相关的后碰撞岩浆活动记录

摘要 东昆仑造山带早古生代晚期火山岩对于确定与原特提斯洋演化相关的碰撞过程至关重要。我们报告了这些火山岩的新锆石 U-Pb 年龄、全岩地球化学和 Sr-Nd 同位素,并限制了它们的构造环境和岩浆过程。该火山岩组主要由叶状变晶岩和次要变玄武岩组成。锆石U-Pb年龄表明双峰火山岩形成于晚志留世至早泥盆世(约420-409 Ma)。玄武岩显示出拉斑地球化学特征,其特征是 SiO2 (47.19–54.83%)、MgO (2.21–7.52%) 和 K2O/Na2O 比率 (0.32–0.77%) 和高含量 TiO2 (1.80–2.91)。它们的球粒陨石归一化 REE 模式的特征在于 LREE 的富集 (LREE/HREE = 3.68–6。09) 有轻微的 Eu 异常 (δEu = 0.67–1.00); 它们的图案类似于大洋岛玄武岩(OIB)的参考线。原始地幔归一化微量元素图也显示了与 OIB 的相似性,并具有大离子亲石剂和高场强元素的整体富集,除了 Nb 和 Ta。eNd(t = 422 Ma) 值的范围很窄,从 -1.79 到 +1.32。这些特征表明玄武岩很可能来自被少量次大陆岩石圈地幔 (SCLM)/地壳污染的软流圈地幔。主要元素和微量元素的变化表明玄武岩经历了橄榄石、辉石和斜长石的分级结晶。相比之下,变晶石具有高 SiO2 (65.85–70.83 %)、Na2O + K2O (6.71–10.09 %)、K2O/Na2O 比率 (1.69–2.80 %) 和低 MgO、Ni、和 Cr 含量。它们的微量元素特征显示出 LILEs 和 LREEs(例如 Cs、Rb、Ba)的富集、HFSEs(例如 Nb、Ti、Ta)的消耗和高负 Eu 异常。这些特征表明地壳起源缺乏与地幔熔体的相互作用。综合考虑该地区所有的地球化学和构造事件,认为EKOB的猫牛山组火山岩形成于晚志留世至早泥盆世与软流圈地幔上涌和大陆拆沉过程相关的后碰撞伸展环境。这些构造岩浆事件连同先前的数据表明,自晚志留纪以来,原始特提斯洋已经闭合并演化到碰撞后塌陷阶段。HFSE 的消耗(例如 Nb、Ti、Ta)和高负 Eu 异常。这些特征表明地壳起源缺乏与地幔熔体的相互作用。综合考虑该地区所有的地球化学和构造事件,认为EKOB的猫牛山组火山岩形成于晚志留世至早泥盆世与软流圈地幔上涌和大陆拆沉过程相关的后碰撞伸展环境。这些构造岩浆事件连同先前的数据表明,自晚志留纪以来,原始特提斯洋已经闭合并演化到碰撞后塌陷阶段。HFSE 的消耗(例如 Nb、Ti、Ta)和高负 Eu 异常。这些特征表明地壳起源缺乏与地幔熔体的相互作用。综合考虑该地区所有的地球化学和构造事件,认为EKOB的猫牛山组火山岩形成于晚志留世至早泥盆世与软流圈地幔上涌和大陆拆沉过程相关的后碰撞伸展环境。这些构造岩浆事件连同先前的数据表明,自晚志留纪以来,原始特提斯洋已经闭合并演化到碰撞后塌陷阶段。认为EKOB的猫牛山组火山岩形成于晚志留世至早泥盆世与软流圈地幔上涌和大陆分层过程相关的后碰撞伸展环境。这些构造岩浆事件连同先前的数据表明,自晚志留纪以来,原始特提斯洋已经闭合并演化到碰撞后塌陷阶段。认为EKOB的猫牛山组火山岩形成于晚志留世至早泥盆世与软流圈地幔上涌和大陆分层过程相关的后碰撞伸展环境。这些构造岩浆事件连同先前的数据表明,自晚志留纪以来,原始特提斯洋已经闭合并演化到碰撞后塌陷阶段。
更新日期:2020-10-01
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