浙江洪公铝质A型花岗岩类的岩石地球化学及其构造环境

Geochemical Characteristics of the Honggong Aluminous A-type Granite Pluton in Zhejiang Province and Its Tectonic Setting

初步研究表明，以往被认为是典型的Ⅰ型花岗岩质岩石的浙江洪公岩体应为铝质A型花岗岩质岩石。该岩体以高钾为特征，K2O达5％以上，K2O／Na2O〉1．2，准铝-过铝质（A／NKC=0．80～1．14）；FeO^＊／MgO比值大（平均14．20），高于M型、S型和Ⅰ型花岗岩；富含稀土元素（∑REE=313．09×10^-6～523．73×10^-6），具有较高的Ga／Al（×10^4）值（2．92～4．29）和Zr＋Nb＋Ce＋Y元素组合值（551．5×10^-6～987．4×10^-6），而亏损Ba，Sr，P，Ti等；Nd同位素模式年龄为1．3～1．6Ga，反映洪公岩体主要起源于地壳物质的部分熔融。区域背景、构造被动定位特点和地球化学综合分析表明，洪公岩体形成于拉张的构造环境。

A preliminary study shows that the Honggong pluton, formally considered as typical Ⅰ-type granite rock, is proved to be of aluminous A-type. This pluton is characterized by rich potassium, with K2O contents more than 5% and K2O/ Na2O ratios over 1.2. The rocks are metaluminous to slightly peraluminous, with A/NKC values ranging from 0. 80 to 1.14. They also have higher FeO^＊/MgO ratios, with the mean value of 14.20, which is higher than that of the M- , S- and Ⅰ-type granites. The granites have higher rare earth element abundances, with ZREE values of 313.09×10^-6- 523.73 ×10^-6, and have higher Ga/Al（ ×10^4 ） ratios （2.92 - 4.29） and （ Zr ＋Nb＋Ce ＋Y） values （551.5 ×10^-6 -987.4 ×10^-6）, but depleted in Sr, Ba, P, Ti, etc. The Nd istope model ages vary between 1.3 Ga and 1.6 Ga, from which it is deduced that the Honggong granite pluton is on the whole originated from the partial melting of crustal rocks. Integrated geological, geochemical data as well as its features of tectonic passive emplacement suggest that the Honggang granite pluton was formed under a tensile tectonic setting.