Low delta O-18 zircons, U-Pb dating, and the age of the Qinglongshan oxygen and hydrogen isotope anomaly near Donghai in Jiangsu Province, China

摘要

Zircons from metamorphosed granites exposed near Qinglongshan have delta(18)O(VSMOW) values of -7 to 0parts per thousand in both grain rims and cores. The concordant U-238/Pb-206 ages of zircon cores are 684 to 754 Ma with rims at 221 Ma. Discordant U-238/Pb-206 ages range from 242 to 632 Ma. Results demonstrate a Neoproterozoic age for the origin of the Qinglongshan oxygen and hydrogen isotope anomaly. The low delta(18)O values were imprinted on the rocks by a hydrothermal system charged with meteoric water from a cold climate. Groundwater circulation was driven by heat from cooling granitic magma. The geologic age of the hydrothermal system correlates with that of the Nantuo tillite in the Sinian strata of the South China block, suggesting that Qinglongshan's cold climate may be a manifestation of Neoproterozoic "snowball Earth." Copyright (C) 2002 Elsevier Science Ltd. [References: 47]