Research On The Late Mesozoic Igneous Complexes From Xinchang, Zhejiang

The Late Mesozoic igneous complexes from Xinchang, including tuffs, rhyolites, andesites, basalts, diabases, granites, were formed in the subduction setting of the Pacific Plate towards the Eurasian Plate. They exhibit bimodal features and cropouted as composite dikes, composite plutons and composite lavas. Among all kinds of igneous complexes, composite dikes and composite plutons have never been discovered before. Besides, quenched enclaves were found in Longhuangtang granites.Based on the researches in field, the characters of petrolgy, geochemstry and isotope chronology were carefully studied, and the traits of magma sources, the extents of contamination, the tectonic settings and the lithogenesis were discussed. Several conclusions were reached:1. The ⁴⁰Ar-³⁹Ar isotopic age of the diabase of composite plutons from Xinchang is 108Ma, and that of the diabase and the rhyolite of composite dikes in the same place are 109Ma and 107Ma respectively, being in accord with the ages(93-125Ma) of the other igneous rocks in eastern Zhejiang. This illuminates that all of the igneous rocks intruded or erupted at aproximate chron, and were the results of magtism in Late Mesozoic.2. The igneous complexes represent typical features of island arc rocks and belong to calc alkali series in petrochemistry. Parts of igneous rocks show apparent negative anomaly in Ba, Sr, Eu, P, Ti, et al.,implying magmatic differentiation occurred.3. Studies on the new discovered composite dikes and enclaves indicate that there existed magma mixing in Late Mesozoic in Xingchang, and this was related with underplating of basic magmas, partial melting and other crust-mantle interaction.4. The igneous complexes from Xingchang stand for the last episode of magtism in Late Mesozoic. The evolutive processes are consist of subduction of oceanic crust, replacement of fluid, enrichment of mantle, underplating of basaltic magmas, parting melting of crust and elements exchange between different magmas. The igneous complexes from Xingchang, which were formed in extention tectonics, are the products of deep magmas emplacing to shallow crust. They supply petrologic evidences of crust-mantle interaction in Late Mesozoic in SE China.