东南沿海早白垩世火山岩的极低级变质作用研究

摘要

中国东南沿海广泛分布的白垩纪火山岩中,发育一套具特殊岩相学特征的火山岩地层,以浙江高坞组和福建南园组为代表,其主体为熔结凝灰岩,以深灰色厚层致密块状、含大量长石和石英晶屑、大面积出露但假流动构造模糊为突出特征,而明显区别于其它白垩纪地层组的熔结凝灰岩,表明其具有独特的成岩过程.伊利石结晶度研究表明,高坞组和南园组火山岩特殊的岩相学特征与其成岩后受到极低级变质作用,从而发生一定程度重结晶有关,这一过程使其岩石结构构造趋于均一化.通过对上下火山岩系火山岩的伊利石结晶度、b0值及多型特征对比分析表明,东南沿海这一大范围的火山岩极低级变质作用,可能是早白垩世晚期“闽浙运动”后岩石圈发生强烈伸展,导致区域大规模热隆作用的反映,并与上下火山岩系之间区域性不整合面的形成、长乐-南澳构造带强烈的变质变形事件及东南沿海大规模金铜矿、非金属等成矿作用密切相关,具有重要的研究意义.%There outcropped a suit of volcanic stratum with special petrography in the widely distribute Cretaceous volcanic rocks in the Southeast Costal of China,represented by Gaowu Formation in Zhejiang and Nanyuan Formation in Fujian.They are mainly consist of ignimbrites with dark grey and thick-layer with dense tuffaceous cement,contain abundant potassium and quartz crystal fragments,these above as well as the widely distributed but with the fuzzy pseudofluidal structure are their distinguishing feature.These features are very different from the rocks of the other Cretaceous volcanic rocks.The differences between Gaowu Formation/Nanyuan Formation and the other Cretaceous volcanic rocks in SE China indicate that Gaowu Formation/Nanyuan Formation experienced special geological process.The analysis results of illite crystallinity imply that the special petrography of Gaowu Formation and Nanyuan Formation may influenced by very low-grade metamorphism,thus result in the recrystallization of Gaowu/Nanyuan Formation and which homogenized their texture and structure.The illite crystallinity,b0 and polymorph contrastive analysis of volcanic rocks from the upper and the lower volcanic series respectively,indicate that the widely very low-grade metamorphism at early stage of the Lower Cretaceous volcanic rocks may caused by the strong extension of the lithosphere after "Minzhe Movement" at late stage of the Lower Cretaceous and lead to large-scale thermal upwarping.That is closely related to the strong metamorphic and deformational events of Changle-Nanao tectonic belt and the formation of uncomformity between the upper and the lower volcanic series,also closely to the large-scale Au-Cu and nonmetal mineralization in SE China,which has fetal research significance.