对超基性—基性岩中锆石成因、来源及其U-Pb年龄的探讨——以江南造山带西段为例

A discussion on genesis,provenance and U-Pb ages of the zircons from ultrabasic—basic rocks——A case study in western Jiangnan orogenic belt

对超基性—基性岩中锆石成因及其来源的解释直接关系到岩石成岩时代的准确厘定。江南造山带西段(包括桂北、黔东和湘西地区)的新元古代变质地层中分布有大量的超基性—基性火山岩和侵入岩,过去的研究认为它们都形成于新元古代,是四堡造山阶段和造山后裂谷阶段的产物。对前人近年发表相关岩石的锆石U-Pb测年数据的分析,结合笔者等对该地区多件基性岩中锆石的LA-ICP-MS测年结果,发现部分超基性—基性岩中普遍存在少量古生代,甚至中、新生代的年轻锆石。通过锆石形态及阴极发光特征分析,认为这些年轻锆石并不能一概用后期热液改造或挑样过程中混入来解释。本文抛砖引玉,旨在引起国内同行对超基性—基性岩真实成岩年龄解释的探讨。如果以往对这些超基性—基性岩中锆石成因及成岩时代的解释存在问题,将区域上多期岩浆热事件混为一谈,那么对江南造山带西段的构造演化历史需要重新解读。

Ascertaining the origin of zircons from the ultrabasic—basic rocks is the key to determine the rock-forming ages of them.A large number of ultrabasic—basic volcanic and intrusive rocks exposed in the metamorphic strata in western Jiangnan orogenic belt(including northern Guangxi,eastern Guizhou and southern Hunan).Previous studies had suggested that these ultrabasic—basic rocks were formed in Neoproterozoic relating to the Sibao orogeny and post-orogenic rift development.Re-examine the previous dating data published in recent years,along with new dating data in this text,we found that a few Paleozoic—Cenozoic zircons widespread exist in these"Neoproterozoic"ultrabasic—basic rocks.Based on the analysis of zircon morphology and cathodeluminescence(CL)characteristics,we conclude that the U-Pb ages of these young zircons cannot be completely explained by later hydrothermal transformation or mixing during sample selection.This paper is to discuss the diagenetic ages of ultrabasic—basic rocks.If the rock-forming ages of ultrabasic—basic rocks in western Jiangnan Orogen were wrongly explained in previous studies,the tectonic evolution history of the western Jiangnan orogenic belt need further discussion.