Late Indosinian extensional event in Central Zhejiang Province:Evidence from the NNE-trending A-type alkaline granite belt

The NNE-trending granites distributed along the Jiangshan-Shaoxing fault zone, Zhejiang province, was formed in230~220 Ma of late Indosinian according to zircon U-Pb dating.Geochemical data indicates that the igneous rocks are high in Al₂O₃ and K₂O and low in Na₂O and MgO, and belong to a type of metaluminous-peraluminous high-K calcalkaline shoshonitic granites.The oxides and their negative correlation with SiO₂ demonstrate that the late Indosinian granites in Zhejiang Province should be from a homologous magma.They are concentrated in LILE (Th、La、Ce) and LREE, but depleted in high field-strength elements, such as Ba, Sr, P, Ti.Whole-rock LREE/HREE ratios are ((La/Yb) _N=12.87~140.02) withδEu=0.1~0.7.And the 10⁴)×Ga/Al ratios and (Zr+Nb+Ce+Y) contents are also high.In addition, the characteristics of Sr-Nd isotopic show high value of I_Sr (0.7069~0.7203) and low value of εNd (t) (-14.9~-4.1) with two-stage depleted mantle Nd model ages of 1.33~2.21 Ga.The geochemical characteristics indicate that the Indosinian granites are intraplate A-type granites formed in an extensional environment under combined crust-mantle effect and partial melting of crust material.In consideration of the contemporaneous mafic dyke swarms in some places, it is supposed that the tectonic transformation was completed 230 Ma in Zhejiang Province.With the intracontinental rifting along the ancient collision zone, the underplating invasion of ultrabasic-basic-alkaline magma caused the crust-mantle interaction and partial melting of the crust.