塔里木北缘库鲁克塔格地区南华纪花岗质火山-侵入杂岩岩石成因及地质意义

位于亚洲中部的塔里木克拉通是中国三大古老克拉通之一,经历了新元古时期罗迪尼亚(Rodinia)大陆聚合和裂解。然而,关于塔里木在该时期的构造演化仍然存在较多争议。本文对塔里木北缘库鲁克塔格地区南华系地层中流纹岩及伴生的正长花岗岩开展岩石学、地球化学、锆石U-Pb年代学和Hf同位素研究,结果表明:正长花岗岩与流纹岩是一套具有"时、空、源"一致的花岗质火山-侵入杂岩组合,锆石U-Pb年龄分别为735±10 Ma和738.9±5.4 Ma,同为新元古代南华纪岩浆作用的产物。它们具有高硅(69.85%~73.87%)、低铁(2.36%~2.96%)、贫镁(0.16%~0.75%)和富碱(Na₂O+K₂O=7.51%~9.05%)的特征,属碱性系列、过铝质岩石。富集K、Rb、Th等大离子亲石元素和轻稀土元素,亏损Nb、Ta、Ba、Sr、P和Ti等。具A₂型花岗岩的特点,是俯冲挤压向减压伸展构造转换背景下岩浆活动的产物。锆石Hf同位素组成较均一, ε_Hf(t)值大多为-17.4~-10.6之间,其二阶段模式年龄T_DM2为2732~2321 Ma,表明岩浆来源于太古宙-元古宙古老结晶基底。结合前人研究表明,大约在735 Ma塔里木北缘开始进入减压伸展的拉张环境,之后可能发生了Rodinia超大陆的裂解。

Genesis and geological significances of Nanhua granitic volcanic-intrusive complexes in Quruqtagh on the northern margin of the Tarim block,China

The Tarim Craton, one of the main three continental blocks(plates)in China, located in the center of Asia, was involved in the assembly and break-up of the Rodinia supercontinent during the Neoproterozoic. However, its tectonic evolution during these events remains more controversial. In this paper, petrology, geochemistry, zircon U-Pb age and Hf isotopic data for rhyolite and associated syenogranite were researched in Nanhua strata in the Quruqtagh block, northern Tarim Craton. The results show the uniformity in emplacement time, space and source materials for granitic volcanic-intrusive complexes. Zircon U-Pb dating results give an emplacement age of 735±10 Ma for the syenogranite and 738.9±5.4 Ma for the rhyolite. They are the same products of magma in Nanhua Neoproterozoic. The granitic volcanic-intrusive complexes are characterized by higher SiO₂(69.85%~73.87%) contents, lower Fe(2.36%~2.96%)contents, depletion in Mg(0.16%~0.75%), enrichment in alkali(Na₂O+K₂O=7.51%~9.05%), belonging to alkaline series and weakly peraluminous rocks. The geochemistry of the race elements suggests the enrichment large ion lithophile elements(K, Rb and Th) and light rare earth elements(LREEs), depletion in Nb, Ta, Ba, Sr, P and Ti, which indicate the characteristics of A₂-subtype granite, and belonging to the decompression and extension setting. Zircon Hf isotopic data indicates that ε_Hf(t)values are all negative and varying in the range of-17.4~-10.6, two-stage model ages(T_DM2) range from 2732~2321 Ma, and we propose that the rocks were most likely produced by partial melting of crystalline basement during Archean-Proterozoic which may have been derived from mantle magmas. In combination with the results of previous studies, we argue that the study area, the Quruqtagh block, northern Tarim Craton, began to entered the decompression and extension setting in 735 Ma, after that, the break-up of the Rodinia supercontinent has occurred.