福建北东沿海高分异Ⅰ型花岗岩的成因:锆石U-Pb年代学、地球化学和Nd-Hf同位素制约

Petrogenesis of highly fractionated I-type granites in the coastal area of northeastern Fujian Province:Constraints from zircon U-Pb geochronology,geochemistry and Nd-Hf isotopes.

以福建北东沿海的南镇、大层山、三沙和大京四个岩体为对象，系统研究了它们的年代学和地球化学特征，并据此探讨了岩体的成因。锆石LA—ICP—MSU—Pb定年结果表明，南镇、大层山、三沙和大京岩体的成岩年龄分别为96．1±2．7Ma、93．1±2．4Ma、91．5±1．5Ma和93．8±1．8Ma，指示它们均为晚白垩世岩浆活动的产物。这些岩体的组成矿物主要为石英和条纹长石，并含有一定量斜长石（〈15％），铁镁矿物主要为少量黑云母，未出现碱性铁镁矿物，副矿物组合中普遍出现榍石而未见铝过饱和矿物。化学组成上表现为高硅、富碱、准铝，贫钙、镁、铁，Rb／Sr比值高，K／Rb比值低，富Cs、Rb、Th、u、Pb，贫Ba、Sr、P、Ti、Eu。岩石的FeO^＊／MgO比值较低（=3．44—5．04），10^4×Ga／Al比值和Zr＋Nb＋Ce＋Y含量均低于A型花岗岩的下限值，其锆石饱和温度也较低（730—779℃），综合地质地球化学特征指示这些岩体应属高分异的Ⅰ型花岗岩。四个岩体的Nd同位素组成较均-（εNd（t）=-4．2～-5．5），而锆石Hf同位素则具较大的变化范围（εHf（t）=-11．6～4．5），εHf（t）值散布于正值与负值之间，变化幅度可达10个εHf单位以上，指示岩体的形成存在不同来源物质的贡献。岩石的形成极可能经历了壳幔物质在源区混合形成原始岩浆，随后这一壳幔混源岩浆又经高程度分异演化的二阶段成岩过程，而并非起源于幔源岩浆底侵形成的初生地壳与古老基底地壳混合的地壳原岩的部分熔融。

Late Mesozoic granites, with genetically being predominantly of I-type and subordinately of A-type, are widespread along the coastal area of Zhejiang and Fujian provinces, SE China. Previous research have focused mainly on A-type granites in this belt, however, the I-type granites were less concerned. In this paper, four I-type plutons （ Nanzhen, Dacengshan, Sansha and Dajing） from the northeastern coastal area of Fujian Provice were selected as examples for a detailed geochronological and geochemical study in order to elucidate their petrogenesis. LA-ICP-MS U-Pb zircon ages for Nanzhen, Dacengshan, Sansha and Dajing are 96. 1 ± 2.7Ma, 93.1± 2. 4Ma, 91.5 ± 1.5Ma and 93.8 ± 1.8Ma respectively, indicating that they were all generated in Late Cretaceous. These plutons consist mainly of quartz and perthite, and frequently contain some plagioclase （ 〈 15% ） and minor amounts of biotite. No mafic alkaline minerlals have been found. Titanite is common in their accessory mineral assemblage and no aluminum oversaturation minerals have been observed. Chemically, these plutons are metaluminous （ A/NKC 〈 1.0）, and show high contents of silica and alkalis, low abundances of calcium, magnesium and iron, and have high Rb/Sr and low K/Rb ratios. They are also enriched in Cs, Rb, Th, U, Pb, and depleted in Ba, Sr, P, Ti, Eu. Their FeO^＊/MgO ratios are relatively low （ =3.44 -5.04）, and 10^4 × Ga/Al ratios and Zr ＋ Nb ＋ Ce ＋ Y contents are less than that of the low-limit values of the A-type granites. Zircon saturation temperatures of them range from 730 to 779℃ which are also lower than that of the typical A-type granites. Integrated geological and geochemical data suggest that the four plutons described in this study are highly fractionated Ⅰ-type rather than A-type. Nd isotopic compositions of the four plutons are relatively uniform（ δNd （t） = -5.49 - 4. 20）, however, zricon Hf isotopic compositions of them are highly variable with a δHf（t） variation range over 10 ~f units （ =-11.6 -4. 5 ）. Such large variation in zircon Hf isotopic compositions suggests that different source materials have contributed to magma genesis. The granites were most likely generated via a two-stage process including formation of parental magma by mixing of a mantle-derived basaltic magma and a crustal felsic magma in the deep crust and subsequently extensive magmatic differentiation of the parental magmas, rather than originated from partial melting of a mixed crustal protolith with both pre-existing ancient basement crust and juvenile crust formed by underplating of mantle-derived magma.