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

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 FeOVMgO ratios are relatively low ( =3. 44 ∼5. 04) , and 10⁴ × 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°C 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( ε_Nd (t) = -5. 49 ∼ -4. 20) , however, zricon Hf isotopic compositions of them are highly variable with a ε_Hf(t) variation range over 10 ε_Hf 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.