粤东地区侏罗纪代表性剖面火山岩成因:对古太平洋板块俯冲早阶段的地质响应

Petrogenesis of the Jurassic Representative Volcanic Rocks in Eastern Guangdong: Response to the Early Stage of the Paleo-Pacific Subduction

通常认为中国东南部大规模分布的白垩纪火成岩是古太平洋板块俯冲作用晚阶段的产物,而俯冲过程早阶段的地质响应形式尚不明确。研究针对中国东南部侏罗纪火山岩规模最大的粤东地区代表性剖面开展了年代学及岩石成因研究,锆石U-Pb年代学研究表明该地区侏罗纪火山岩断续形成于192~183 Ma(嵩灵组)、177~163 Ma(吉岭湾组)和162~156 Ma(高基坪群)。结合前人研究数据,中国东南部晚中生代火山岩表现出沿粤东-闽东-浙东南方向的年轻化趋势。锆石Lu-Hf同位素分析表明,粤东地区侏罗纪火山岩主要源于古元古代地壳基底物质的重熔,并有不同程度的亏损幔源物质贡献,从早侏罗世到晚侏罗世亏损幔源物质贡献的比例呈现出由高到低的规律性变化。粤东地区侏罗纪火山岩岩石成因的规律性变化暗示了地壳应力环境的伸展程度逐渐减弱,对应了古太平洋板块的前进式俯冲。综合分析表明,粤东地区侏罗纪火山岩是古太平洋板块俯冲早阶段的地质产物,反映古太平洋板块俯冲作用开始于早侏罗世左右。

It is widely accepted that the widespread Cretaceous volcanic rocks in SE China were formed at the late stage of the paleo-Pacific subduction. On the contrary, the geological response of the early stage of the subduction remains unclear. This study focuses on the representative Jurassic volcanic sections in eastern Guangdong, which are the most widespread in SE China.Zircon U-Pb dating results reveal that the Jurassic volcanic rocks were formed intermittently at 192-183 Ma(Songling Fm.), 177-163 Ma(Jilingwan Fm.) and 162-156 Ma(Gaojiping Gp.). Thus, a younging trend along eastern Guangdong to eastern Fujian to southeastern Zhejiang could be identified for the entire Late Mesozoic volcanic rocks in SE China. Zircon Lu-Hf isotopic analyses show that the Jurassic volcanic rocks were mainly derived by remelting of the Paleoproterozoic crustal basement, with decreasing involvement of juvenile components in their origin over time from Early to Late Jurassic. The petrogenesis of the Jurassic volcanic rocks in eastern Guangdong suggests decreasing crustal extension, corresponding to the forward subduction of the paleo-Pacific slab. Thus, the Jurassic volcanic rocks in eastern Guangdong record the early stage of the paleo-Pacific subduction, and further suggest that the subduction started at around Early Jurassic.