95Ma是华南晚中生代岩浆演化转折点吗?——来自浙闽典型花岗岩体的证据

Is 95 Ma the turning point of the Late Mesozoic Magmatic Evolution in South China?——Evidence from typical granitoids in Zhejiang and Fujian provinces

中国东南沿海地区出露的大面积中生代岩浆岩与矿化作用与区域构造域转换密切相关,在100~90 Ma左右浙闽地区壳幔作用强烈,前人大多数的研究工作是针对单个岩体或几个复式岩体展开的相应研究工作,缺少对浙闽沿海中生代花岗岩类的跨区域研究,特别是具体时间节点的对比和剖析。本研究选取浙闽地区100~90 Ma形成的典型花岗岩体为研究对象,通过对其进行地球化学数据分析和整理,并与前人在华南地区的研究数据相结合,认为100~90 Ma的浙闽花岗岩不同于日本和安第斯型的典型岛弧酸性岩。浙闽花岗岩可能起源于古太平洋板块俯冲引起基性岩浆上涌,少量基性岩浆与花岗质岩浆以不同比例混合形成了这一时期东南沿海中生代花岗岩类的母岩浆。地壳组成的差异造成浙闽一带形成的母岩浆的不同,而花岗质岩体则是在这种不同母岩浆基础上高度演化的结果。利用Rhyolite—MELTS方法计算浙闽地区在100~90 Ma的岩浆过程,以福建长泰岩体为例,在含水量为6%,压力为300 MPa的条件下进行模拟,计算结果显示98 Ma岩体的分离结晶程度约37%,而从95 Ma开始,岩浆演化开始趋向于无明显规律。浙闽中生代花岗岩体显示以斜长石、钾长石、黑云母的分离结晶为主导的结晶过程,不同岩体的分离结晶程度有所不同。特别是在95 Ma时,浙闽一带的岩体同时出现了斜长石的分离结晶作用,随后又进入由钾长石和黑云母的分离结晶控制的阶段,结合Sr、Ba的变化特征和Nd同位素等变化,认为95 Ma左右存在地幔物质的介入,从而导致浙闽地区在晚中生代岩浆演化上出现了转变,而且95 Ma很可能是浙闽晚中生代花岗岩形成的转折期,岩浆温度的计算结果也证实了这一点。

Objectives:Large amounts of Mesozoic magmatic rocks are exposed in the coastal area of the southeastern China.The data of geochemistry and zircon U-Pb ages of granitoids from typical plutons in the coastal area of Zhejiang and Fujian are studied systematically in order to explore the reason for the complexity of magmatism during the period of 100~90 Ma.Methods:Comparative analyses of regional geochemical characteristics,U-Pb zircon geochronology and Rhyolite-MELTS method to calculate magma processes at 100~90 Ma in Zhejiang—Fujian area.Results:Plagioclase is as the main mineral in Zhejiang—Fujian plutons at 98 Ma,while typical minerals of aegirine and large amounts of perthitic feldspar appear at 95 Ma and 92 Ma respectively.The Late Mesozoic high alkaline granites in Zhejiang—Fujian area are different from those typical island arc granites in the world.They are enriched in LILE,depleted in HFSE Nb-Ta,and Ba and Sr,strong Eu anomalies and varied LREE/HREE ratios.Most of them during this period are derived from partial melting of continental crust materials and existing the transition in chemical composition at 95 Ma.Conclusions:The parental magma of Zhejiang—Fujian Late Mesozoic granites during the period of 100~90 Ma are derived from partial melting of the mantle materials caused by the subduction of the paleo-Pacific plate.Magmatic evolution in Zhejiang—Fujian area showed a magmatic transition due to the intervention of mantle materials at 95 Ma,and the transition period of the granitic formation in Zhejiang—Fujian area is probably about 95 Ma.