福建迪口地区前坑村岩体和罗村岩体成因及其构造背景

Petrogenesis and tectonic background of Qiankengcun and Luocun plutons in Dikou area,Fujian Province

前坑村岩体和罗村岩体位于福建东北部的迪口地区,侵位于建瓯构造混杂岩中。文章对前坑村岩体的花岗斑岩和罗村岩体的花岗岩进行地质年代学、地球化学及同位素研究。锆石U-Pb定年显示,花岗斑岩和花岗岩的侵位年龄分别为(140.75±0.92)Ma与(131.70±1.70)Ma,属于燕山晚期岩体。地球化学特征显示,前坑村岩体与罗村岩体的w(SiO_(2))、w(Na_(2)O+K_(2)O)过高,稀土四分组效应较强,高场强元素富集,Ba、Sr、Ti元素亏损,Eu负异常明显,锆石的饱和温度为730~776℃,这些特征表明其为高分异I型花岗岩。微量元素及同位素特征显示,前坑村岩体与罗村岩体的δEu为0.10~0.12,ε_(Nd)(t)为-10.0~-8.3,Nd模式年龄低于华夏基底变质岩,这些特征意味着岩体的形成存在二阶段成岩过程,即首先经历了幔源岩浆与其诱发地壳物质熔融产生的长英质岩浆在地壳深部混合,随后这一混合岩浆又经历进一步分异演化。研究结果表明,前坑村岩体与罗村岩体可能形成于太平洋板块俯冲引起的岩石圈伸展的构造环境。

The Qiankengcun and Luocun plutons are located in Dikou area,northeastern Fujian Province.The plutons intruded into the Jian’ou tectonic mélange.In the study,the combined zircon U-Pb geochronological,whole-rock geochemical and isotopic studies on the Qiankengcun and Luocun plutons were carried out.The zircon U-Pb dating of the granodiorite porphyries and granites are(140.75±0.92)Ma and(131.70±1.70)Ma,respectively,corresponding to the magmatism during Late Yanshanian stage.The whole geochemical results show that the Qiankengcun and Luocun plutons have high w(SiO_(2)),total alkalis w(Na_(2)O+K_(2)O),strong tetrad effect,enriched high field strength elements(HFSE),depletion of Ba,Sr,Ti elements,strong negative Eu anomalies and lower zircon saturation temperatures(730-776℃),which indicates that they are highly fractionated I-type granites.TheδEu(0.10-0.12),negativeε_(Nd)(t)(-10.0--8.3)of Qiankengcun and Luocun plutons and lower T_(DM2) relative to that of the Cathaysian metamorphic basement suggest that they might generate via a two-stage process including the formation of parental magma by mixing of a depleted mantle-derived magma and an induced crustal-melted felsic magma in the deep crust,and the further differentiation during magma ascent.The results show that the Qiankengcun and Luocun plutons might be formed in the tectonic environment of lithospheric extension as a consequence of the subduction of Pacific subducted slab.