浙闽地区新元古代变火山岩系岩石地球化学特征及其地质意义

GEOCHEMICAL CHARACTERISTICS OF THE NEOPROTEROZOIC METAVOLCANIC ROCKS IN ZHEJIANG AND FUJIAN PROVINCES AND ITS GEOLOGICAL SIGNIFICANCE

浙闽地区华夏地块前寒武纪上基底主要由出露于闽西北和浙西南的马面山群、万全群和龙泉群组成。对比研究表明,以上3个岩群具有相似的构造演化、岩石组合、元素地球化学特征及形成时代,它们共同组成了统一的华夏地块变质上基底,在构造上是相连的,主要由经历了高绿片岩相-低绿帘角闪岩相变质变形作用的绿片岩和变粒岩组成,其原岩为一套双峰式火山岩,形成于新元古代中期。绿片岩的稀土配分型式右倾,轻重稀土分异较大,具洋岛玄武岩(OIB)的特征,其微量元素表现为左部"大隆起"式的特征,大离子亲石元素(LILE)K,Rb,Ba,Th明显富集,Nb相对La富集,表现出大陆裂谷玄武岩的特征。绿片岩和变粒岩具有统一的岩浆演化趋势,为同源岩浆演化的产物,地球化学特征和构造判别图解显示它们形成于板内拉张的大陆裂谷环境,可能是新元古代Rodinia泛大陆裂解在华南的记录,与地幔柱的活动有关。马面山群第二类绿片岩表现出岛弧特征,可能为陆壳物质混染所致。

The upper Precambrian basement of the Cathaysia block in southwestern Zhejiang and northwestern Fujian Provinces was mainly composed of Longquan Group, Mamianshan Group and Wanquan Groups. The contrastive study suggested that the assemblage,geochemical characteristics and formation age of the three groups were similar,and therefore,they can be considered as a unified upper metamorphic basement in Cathaysia block. The rock types of greenschists and granulite indicated that the basement experienced upper-greenschist to lower-amphibolite facies metamorphism and intense deformation. Their protolith which formed in the middle of the Neoproterozoic was bimodal volcanic rocks. REE pattern of the greenschists were right dipped and LREEenriched,similar to OIB. The greenschists are characterized by ＂humped＂ trace element patterns, and were highly enriched in K,Rb and Ba,and Th,Nb was enriched relative to La,similar to the intra-continental rift basalts. Greenschists and granulites showed the uniform chemical trends of magma evolution and belonged to a comagmatic system. The geochemical characteristics as well as trace discrimination diagrams reflected that the greenschists and granulite formed in the tectonic environment of intraplate rift,which might be related to Neoproterozoic breakup of the Rodinia supercontinent in South China and the activity of mantle plume. Some greenschist samples from Mamianshan group displayed geochemical characteristics similar to IAB, which might be due to the continental crust contamination.