The geochronological and geochemical constraints on the petrogenesis of the Early Mesozoic A-type granite and diabase in northwestern Fujian province

摘要

Early Mesozoic igneous rocks are rarely found in the area east of Wuyi Mountain, southeast China. Our study of petrography and geochronology reveals that the Dayinchang (DYC hereafter) granitic pluton and Xiamao diabases are Early Mesozoic igneous rocks. The DYC pluton can be divided into two parts: the western and eastern parts. Zircon U-Pb dating revealed that the crystallization age of the western part is 237 ± 2 Ma, and the age of the eastern part is 228 ± 2 Ma (Table 1). The crystallization age of the Xiamao diabase is 224 ± 1 Ma. The DYC granites belong to metaluminous to peraluminous high Kcalc-alkaline A-type granite that is depleted in Ba, Nb, Sr, P and Ti and enriched in Rb, Th, and U. The total rare earth elements (REE) contents of the DYC granites range from 134.85 ppm to 510.68 ppm, with light rare earth elements (LREE) enrichment and evident negative Eu anomalies. These granites have A-type geochemical signatures with high Na2O+K2O, FeO~T/MgO and Ga/Al ratios and high Zr+Nb+Ce+Y contents. They display bulk rock ε_(Nd)(t) values in the range of —11.58 to —8.38 and Nd isotope two-stage model ages (T_(NdDMC)) of 1.68 Ga to 1.95 Ga. The ε_(Hf)(t) of the DYC granites are -9.99 to -4.76, with Hf isotope two-stage model ages (T_(NdDMC)) of 1.57 Ga to 1.89 Ga. These characteristics suggest that the Early Mesozoic DYC pluton is oxidized A-type granite and may have been derived from partial melting of the Meso-Paleoproterozoic amphibolite. The geochemical difference between the eastern and western parts may have been due to the difference in their Meso-Paleoproterozoic source materials. The Xiamao diabase rocks are rich in large iron lithosphere elements (LILE) and LREE and depleted in high field strength elements (HFSE). Their initial Sr isotope composition is -0.7080, and their ε_(Nd)(t) values are — 8.12 to — 7.95 with a T_(DM) of 1.35 Ga to 1.49 Ga The geochemical and isotopic data suggest that the Xiamao diabase was derived from the enriched mantle produced by the paleo-Paciflc subduction. After examining these rocks and other Early Mesozoic igneous rocks near the research area, we suggest that the influence of the Early Mesozoic paleo-Pacific subduction may have affected Fujian and Zhejiang provinces but may have been limited to the area east of Wuyi Mountain.