浙江瑶坑碱性花岗岩体的年代学、地球化学及其成因与构造指示意义

Geochronology and geochemistry of the Yaokeng alkaline granitic pluton in Zhejiang province: Pelrogenetic and tectonic implications

瑶坑岩体位于闽浙交界沿海地区,是闽浙沿海晚中生代A型花岗岩带中的一个典型碱性花岗岩体.锆石LA-ICPMS U-Pb同位素定年表明,该岩体的形成年龄为91.3±2.5 Ma（MSWD=3.7,2σ）,属晚白垩世岩浆活动的产物.岩石发育特征的微文象结构和晶洞构造,含有典型的碱性铁镁矿物钠铁闪石.地球化学上,该岩体富硅、碱,贫钙、镁,富K、Rb、Cs、Th、U,贫Sr、Ba、P、Ti、Sc.其Nb、Ta、Zr、Hf和Ga等元素的含量也较高,（Ga/Al）×10^4值变化于3.64～4.41,并具显著的铕负异常（δEu=0.15～0.20）.与闽浙沿海A型花岗岩带中的其它岩体相比,瑶坑岩体具有偏高的εNd（t）值（=-0.7～-1.4）和偏低的二阶段Nd模式年龄（t2DM=0.95Ga～1.01Ga）,指示幔源组分对该岩体的形成具有更重要的贡献.在A型花岗岩亚类的判别上,瑶坑岩体以较低的Y/Nb比值和较高的Nb/Ta比值相似于Eby（1992）区分的A.型花岗岩,而明显有别于岩带主体属A2型的特点.岩石产出构造背景及地球化学特征的综合分析表明,多组深断裂交汇,进而引发多量幔源组分参与成岩过程,是导致瑶坑岩体元素-同位素组成与区内其它同类岩体有较明显差别的主要原因.

Located at the coastal juncture of Zhejiang and Fujian provinces, the Yaokeng pluton is a typical alkaline granitic intrusion in this Late Mesozoic A-type granite belt. Zircon LA-ICP-MS U-Pb dating yields an age of 91.3 ± 2.5Ma （ MSWD = 3.7, 2σ）, indicating that it was formed in Late Cretaceous. The granites of the Yaokeng pluton develop characteristic micrographic texture and miarolitic structure, and contain typical alkaline ferromagnesian minerals, such as arfvedsonite. Geochemically, the granites are rich in silicon, alkalis, poor in calcium and magnesium, and show enrichments of K, Rb, Cs, Th, U and depletions of Sr, Ba, P, Ti, Sc. They also have higher concentrations of Nb, Ta, Zr, Hf,Ga, and display high （Ga/Al） × 10^4 values （ = 3.64 -4.41 ） and strong europium depletions （δEu = 0.15 -0.20）. Compared with the other A-type granitic plutons in the coastal areas of Zhejiang and Fujian provinces, the Yaokeng granites have higher end （t） values （ = - 0.7 - - 1.4） and younger two-stage Nd model ages （ t2DM=0.95Ga -1.01Ga）, indicating that mande-derived components have made greater contributions to the magma genesis. The rocks also show lower Y/Nb and higher Nb/Ta ratios, suggesting that they belong to the Al type proposed by Eby （ 1992）, whereas the main A-type granites in this belt are of A2 type. By integrated study of the tectonic setting and geochemical characteristics, we suggest that the elemental and isotopic differences between the Yaokeng and the other A-type granites in this belt were mainly resulted from more mantle material involvements in the magraa generation, which was induced by intersections of multi-oriented deep faults in the Yaokeng pluton.