桂东南马山玄武岩的年代学、地球化学及岩石成因:对华南地区印支期构造背景的制约

Geochronology,geochemistry,and petrogenesis of the Mashan basalt in southeast Guangxi Province:Constraints on the Indosinian tectonic setting of South China

【研究目的】桂东南马山杂岩体北部出露的印支期玄武岩,是研究华南印支运动的关键岩石探针。目前关于马山玄武岩的研究资料较少,制约了对华南地区大地构造背景演化的认识。【研究方法】本文对马山玄武岩开展了锆石U-Pb年代学、岩石地球化学、Sr-Nd同位素研究。【研究结果】玄武岩的LA-ICPMS锆石U-Pb年龄为(246.7±1.5)Ma,MSWD=0.16。岩石富碱((K_(2)O+Na_(2)O)=5.21%~8.02%)、富钾(K_(2)O=2.59%~4.96%),为钾质粗面玄武岩,稀土元素特征为轻稀土富集型,微量元素特征为富集大离子亲石元素(Rb、Ba、K、Pb、LREE),亏损高场强元素(Nb、Ta、P、Ti、HREE),Sr-Nd同位素显示具有EMⅡ富集地幔端元的特征。【结论】马山玄武岩符合钾玄岩系列的岩石特点,其岩浆作用以分离结晶为主,无明显的地壳混染,其源区为受俯冲壳源物质释放的流体交代作用形成的含金云母石榴子石的富集地幔(>80 km)源区。马山玄武岩产于板内环境,其形成可能与印支期逆冲-推覆构造后期的伸展作用有关,由于伸展作用产生有利空间,造成玄武质岩浆上涌喷发形成玄武岩。

This paper is the result of geological survey engineering.[Objective]Indosinian basalt,exposed in the northern part of the Mashan Complex in southeast Guangxi province,is a key rock probe for the study of Indosinian movement in South China.It has been seldom researched so far,which restricts the recognition of evolution of tectonic setting in South China.[Methods]We carried out zircon U-Pb geochronology,geochemistry,Sr-Nd isotopic geochemistry and petrogenesis of the Mashan basalt.[Results]The results show that the LA-ICPMS zircon U-Pb age of the Mashan basalt is(246.7±1.5)Ma,(MSWD=0.16).The basalt belongs to potassium trachybasalt with high alkali(K_(2)O+Na_(2)O=5.21%-8.02%),especially potassium(K_(2)O=2.59%-4.96%),and is enriched in large ion lithophile elements(Rb,Th,U,K,Pb,LREE)but depleted in high field strength elements(Nb,Ta,P,Ti,HREE).The Sr-Nd isotopes of the Mashan basalt have affinity with enriched mantle(EMⅡ).[Conclusions]The geochemical characteristics of the Mashan basalt exhibit shoshonitic features,and is mainly a product through fractional crystallization without obvious crustal contamination.It was probably derived from partial melting of phlogopite-and garnet-bearing lithospheric mantle(>80 km),which was metasomatized by subducted crustal materials.The basalt is developed in an intraplate setting,likely to be shaped by magmatic eruption and intrusion upwards through favorable space created by the extension in the later stage of the Indosinian thrust-nappe structure.