Genesis of zircon megacrysts in Cenozoic alkali basalts and the heterogeneity of subcontinental lithospheric mantle, eastern China

摘要

Zircon megacrysts are found in alluvial deposits associated with Cenozoic basalts from Changle in Shandong Province, Mingxi in Fujian Province and Penglai in Hainan Province within the coastal area of eastern China. They are colourless, transparent to light brown-maroon, and some of them are up to 16 mm long. U-Pb ages of zircon megacrysts from Changle, Mingxi and Penglai are 19.2±0.7 Ma, 1.2± 0.1 Ma and 4.1 ±0.2 Ma respectively, slightly older than the eruption ages of their corresponding host rocks (16.05-18.87 Ma, 0.9-2.2 Ma, 3 Ma). eHf(t) values of zircon megacrysts are 9.02±0.49, 6.83±0.47, 4.46±0.48 for Changle, Mingxi and Penglai, respectively, which indicates their mantle origin. We suggest that the zircon megacrysts originated from metasomatised lithospheric mantle and were later brought up quickly by the host basaltic magma. The euhedral forms, uniform internal structure and chemical homogeneity within a single grain suggest crystallization under stable conditions. Pronounced positive Ce anomalies and negligible Eu anomalies suggest oxidizing conditions and little or no fractional crystallization of plagioclase.