江西南部东坑盆地流纹岩地球化学特征及其地质意义

东坑盆地位于南岭构造带东段,其中的流纹岩为该带燕山期最早的“流纹岩—玄武岩”双峰式火山岩组合的酸性端元.主量元素、微量元素、Sr-Nd-Pb-O-Hf同位素研究表明,流纹岩富硅、钾,贫镁、钙、钛,属亚碱性弱过铝质岩石;稀土元素富集,轻重稀土分异和铕负异常明显,表现典型的M型稀土元素4分组效应,富集高场强元素Ta、Hf、Zr、Nb、Ce、Y和大离子亲石元素Rb、Th、U、Ba、Ga,亏损大离子亲石元素Sr,具有A型流纹岩和高Sr-Ba流纹岩的微量元素特征;(87Sr/86Sr)i较高,(206Pb/204Pb)i、(207Pb/204Pb)i和(208Pb/204Pb)i较低,εNd(t)、εHf(t)和δ18OV-SMOW较高,TDM2(Nd)和TDM2(Hf)较小.这些特征表明,东坑盆地流纹岩是拉张构造环境下源于新元古代亏损地幔和少量古老下地壳物质混合而成年轻下地壳部分熔融的产物,为早侏罗世早期南岭构造带东段处于拉张构造环境、地壳属正常厚度提供了岩石学证据.

Geochemistry and tectonic implications of the rhyolite in Dongkeng Basin,southern Jiangxi Provinc

The Dongkeng Basin is located in the eastern section of Nanling tectonic belt. The rhyolite as the acidic end-menber of bimodal volcanic rock association, is the most ancient rhyolite that distinguish from the southern China during the Yanshanian period. The rhyolites in the study area is characterized by high SiO₂, K₂O, and low MgO, CaO, TiO₂, and plot in the field of sub alkaline and weakly peraluminous series in the SiO₂ vs K₂O and A/CNK vs A/NK diagrams. The primitive mantle（PM）-normalized trace element patterns of all of these rocks are enriched in large ion lithophile elements（LILE, such as Rb, Ba, Th, U）and depleted in high field strength elements（HFSE, such as Nb, Ta, Hf, Zr）. The rhyolite are characterized by light REE（LREE）enrichments, with weakly differentiated heavy REE（HREE）and significant negative Eu anomalies, which shows typical M-type tetrad effects. Moreover, the Sr-Nd-Pb-O-Hf isotopic compositions show low ；（⁸⁷Sr/⁸⁶Sr）_i,（²⁰⁶Pb/²⁰⁴Pb）_i,（²⁰⁷Pb/²⁰⁴Pb）_i and（²⁰⁸Pb/²⁰⁴Pb）_i, high ε_Nd（t）、ε_Hf（t）and δ¹⁸O_V-SMOW‰ value, and low T_DM²（Nd）and T_DM²（Hf）. The comprehensive analysis of rhyolite's geochemical characteristics indicates that Dongkeng Basin rhyolite originates from the Neoproterozoic of depleted mantle and enriched mantle type-II and mixed with a small amount of ancient light lower crust during the Early Jurassic and early products of partial melting of extensional tectonic environment, providing the petrology of evidence for the early stage of Early Jurassic in Nanling belt which crust is normal thickness.