摘要

Major and trace element, whole-rock Sr-, Nd- and Hf-isotope, zircon U-Pb age and Hf-O isotope data are reported for the intrusive rocks from the Matsu Islands in the coastal area of southeastern (SE) China, in order to study the ages, sources and petrogenesis of these rocks and evolution of the lower crust. The rocks include gneissic granite, massive granite, brecciated granite and diabase. Secondary ion mass spectrometer (SIMS) zircon U-Pb dating reveals that the rocks in the Matsu Islands were emplaced at similar to 160 Ma, similar to 130 Ma and similar to 94 Ma. The Jurassic granites (similar to 160 Ma) have high SiO2 (74.1-74.5 wt%) and K2O + Na2O (8.32-8.33 wt%) contents and high Rb/Srratios of 0.6-1.2 and (La/Yb)(cN) ratios of 12.6-19.4. Their relatively high initial Sr-87/Sr-86 ratios (0.7074-0.7101), variable and negative epsilon(Nd)(t) values (-9.2 to 5.4), and variable zircon epsilon(Hf)(t) (-17.0 to + -5.2) and delta O-18 (4.7-8.1%0) values indicate they were mainly derived from an ancient lower crustal source, but with involvement of high epsilon Hf(t) and low delta O-18 materials. The Early Cretaceous diabase (similar to 130 Ma) has SiO2 content of 56.5 wt%, relatively high MgO concentration, low initial Sr-87/Sr-86 ratio and negative epsilon(Nd)(t) value, similar to geochemical features of other Cretaceous mafic rocks in the coastal area of SE China. Zircons from the diabase have high epsilon(HF)f(t) values (-5.5 to + 0.2) and relatively low delta O-18 values of 4.2-5.0%0. These characteristics indicate that the parental magma of the diabase was generated by partial melting of enriched lithospheric mantle, which have been metasomatised by altered oceanic crust-derived low-delta O-18 fluids. For the Cretaceous granitoids (similar to 130 Ma and 94 Ma), they have relatively low SiO2 (68.0-71.3 wt%) an K2O +Na2O (5.30-7.55 wt%) contents and low Rb/Sr ratios and (La/Yb)(cN) ratios of 5.8-7.1. They have low initial Sr-87/Sr-86 ratios (0.7071-0.7082), homogeneous epsilon(Nd)(t) (-4.3 to -4.5) and relatively high zircon epsilon(Hf)(t) values (-3.7 to + 1.2) and low delta O-18 values (4.6-5.9%0). Their isotopic compositions are similar to those of the diabases in this study as well as other Cretaceous mafic rocks in the coastal area of SE China, suggesting that the sources of the Cretaceous granitoids might be the newly formed lower crust related to the underplated mafic rocks. Whole -rock geochemical, Sr-Nd and zircon Hf-O isotopic compositions indicate that the Jurassic granitoids are most likely generated by partial melting of relatively ancient basement rocks, whereas the Cretaceous granitoids were generated by partial melting of relatively young lower crustal rocks with addition of mantle -derived magma. This distinction implies that the pre-existing ancient lower crust beneath the coastal area of SE China has been modified by large-scale mafic magma underplating. Therefore, underplating of mantle -derived mafic magma would result in modification of the pre-existing ancient lower crust and formation of the relatively juvenile lower crust.