浙江大陈岛花岗岩的成因--来自地球化学、锆石U-Pb年代学和Sr-Nd-Hf同位素的证据

Petrogenesis of granites in Dachen Island,Zhejiang Province:constraints from geochemistry,zircon U-Pb ages and Sr-Nd-Hf isotope

大陈岛地处浙闽晚中生代构造岩浆带的中部,出露钾长花岗岩、基性岩脉和酸性火山岩。对大陈岛地区钾长花岗岩进行岩相学、主量和微量元素、Sr-Nd-Hf同位素及锆石U-Pb年龄分析,结果显示大陈岛地区钾长花岗岩锆石U-Pb年龄加权平均值为95.9±1.8 Ma,形成于晚白垩世早期。大陈岛的花岗岩主要由钾长石、石英及少量黑云母、斜长石组成,发育典型的文象结构。花岗岩富SiO_(2)、K_(2)O,贫CaO、MgO,呈弱过铝质(A/CNK=1.0~1.1)和高分异(DI=96.0~96.9)特征,具较高的Ga/Al值(2.6~2.8),为高钾钙碱性系列。稀土元素配分曲线呈右倾,负Eu异常明显(0.32~0.39),微量元素蛛网图显示,岩石相对富集Rb、K、Th、U,亏损Sr、Ti、Nb、Ta,属于A型花岗岩。大陈岛钾长花岗岩I_(Sr)值为0.7092~0.7094,Nb/U和Ta/U值较低,ε_(Nd)(t)与ε_(Hf)(t)值分别为-7.07~-6.93和-11.1~-6.7,表明钾长花岗岩的源区主要为浙东南古老的富铝地壳物质且成岩过程中有新生物质的参与。结合前人的认识,认为大陈岛钾长花岗岩是在晚中生代古太平洋板块俯冲消减过程中弧后伸展背景下,伴随深部基性岩浆上涌,地壳基底减压部分熔融形成长英质岩浆并与基性岩浆发生少量的混合作用,幔源物质比例在35%左右,随后伴随较强的矿物结晶分异作用,最终在地壳浅部侵位形成。

Dachen Island is located in Zhejiang-Fujian Late Mesozoic tectonic magmatic belt.K-feldspar granites mafic dike rock and volcanic rock are wide distribution in the Dachen Island.In this study petrography major and trace elements Sr-Nd-Hf isotopes and zircon U-Pb geochronology of the granites are studied systematically.The zircon U-Pb weighted average age shows the magmatic event at 95.9±1.8 Ma representing the Early Late-Cretaceous.The Dachen Island granite is mainly composed of K-feldspar quartz plagioclase and few of biotite.The major and trace elements show that the granite has high SiO2 K2O and Ga/Al ratios 2.6~2.8 low CaO MgO and weakly peraluminous 1.0~1.1 of A/CNK and highly heterogeneous 96.0~96.9 in DI belonging to high-K calc-alkaline series.The rare earth element is obvious with a rightward REE distribution pattern and negative Eu anomaly 0.32~0.39.The granite enriches in LILEs Rb K Th U at different degree and depletes relatively to Sr Ti Nb and Ta.The ISr 0.7092~0.7094 low Nb/U and Ta/U values andε_(Nd)(t)=-7.07~-6.93 of whole rockε_(Hf)(t)==-11.1~-6.7 of zircon indicate that the granite is mainly derived from crust materials in the southeast Zhejiang and mantle materials inputting to the magmatic process.Combining with the previous study the Dachen granites were generated by the subduction process of the Paleo-Pacific plate in the Late Mesozoic which resulted in partial melting of the crust materials to form felsic magmas and mixing proportion of mafic material about 35%.Felsic magmas experienced strong crystallization differentiation and then formed the Dachen Island A-type granite.