安徽繁昌盆地中生代火山岩锆石LA-ICPMS U-Pb年龄及其意义

繁昌盆地是长江中下游地区重要的陆相火山断陷盆地之一。本文对繁昌火山岩盆地中分村组粗安岩、赤沙组黑云母粗安斑岩、蝌蚪山组流纹岩、三梁山组黑云母粗面岩进行了锆石LA-ICPMSU-Pb测年,获得其年龄分别为134.4±2.9、131.3±1.8、130.8±2.2、128.1±3.1Ma,显示盆地内火山岩浆活动的时限约在135～128Ma之间,火山岩均为早白垩世岩浆活动的产物。同时在火山岩中测得了多颗继承或捕获锆石的古老年龄,其中最大的为2894Ma,可能对应了2.7Ga峰值年龄的全球性岩浆活动和大陆生长,一定程度上为繁昌盆地可能存在太古宙基底提供了证据。对比显示,繁昌盆地中生代火山岩与长江中下游地区其它断陷盆地火山岩几乎形成于同一时期,与庐枞、宁芜盆地的4个旋回火山活动可以完全对应,而135Ma可以作为长江中下游地区陆相火山断陷盆地早白垩世火山岩浆活动全面展开的时间。盆地内中生代火山活动期(135～128Ma)处于中国东部岩石圈减薄的高峰期,火山岩正是这一地球动力背景下深源岩浆活动的产物。

LA-ICPMS U-Pb ages of zircons from Mesozoic volcanic rocks and their significance in Fanchang basin, Anhui Province, China.

The Fanchang basin is one of the important continental volcanic fault basins in the Middle-Lower Reaches of Yangtze River, where the volcanic rocks can be generally subdivided into four formations, i.e., Zhongfencun Formation, Chisha Formation, Kedoushan Formation and Sanliangshan Formation. The LA-ICPMS U-Pb ages of zircons from trachyandesite of Zhongfencun Formation, biotite trachytic porphyry of Chisha Formation, rhyolite of Kedoushan Formation and biotite trachyte of Sanliangshan Formation are 134.4±2.9Ma, 131.3±1.8Ma, 130.8±2.2Ma and 128.1±3.1Ma respectively. Hence, the ages of volcanic rocks in the Fanchang basin range from 134~128Ma, corresponding to the Early Cretaceous. Many inherited or captured zircons occur in the volcanic rocks, the oldest one is 2894Ma age which may correspond to the 2.7Ga peak age of global magma activities and continental developments. It is implies the existence of Archean basement under the Fanchang basin. The forming period of volcanic rocks in the Fanchang basin is nearly similar to those in the other continental volcanic basins in Middle-Lower Reaches of Yangtze River, and the four volcanic circles of Fanchang basin can entirely correspond with those of Luzong basin and Ningwu basin. The 135Ma is considered as the beginning of the Early Cretaceous volcanic activities in the continental volcanic fault basins in Middle-Lower Reaches of Yangtze River. The period of Mesozoic volcanic activities, 135~128Ma, in Fanchang basin, correspond to the fastigium of the lithospheric thinning in eastern China, which constrained the volcanic activities in Fanchang basin.