赣南大洲塘中生代高镁安山岩地球化学特征及年代学意义

Geochemical Characteristics and Chronological Significance of the Mesozoic High Magnesium Andesite in Dazhoutang, Southern Jiangxi

为研究赣南地区中生界石溪组中基性火山岩地球化学特征及年代学特征,揭示中国东南部中生代岩浆活动及构造演化,对赣南大洲塘石溪组的高镁安山岩开展LA-ICPMS锆石U-Pb定年和地球化学分析。结果显示,锆石206Pb/238U表面年龄主要集中于4个年龄区间:1754.6 Ma、769.0~770.8 Ma、410~440 Ma、153.9 Ma,代表了4次岩浆热事件,但均不能代表高镁安山岩的形成年龄。综合前人的同层位测年结果,推测该套高镁安山岩形成于早白垩世。地球化学数据显示,该高镁安山岩为高钾钙碱性安山岩,轻稀土元素明显富集,未见Eu、Ce异常,富集大离子亲石元素Rb,Th,K,亏损高场强元素Nb,Ta,Ti等。元素和构造环境图解显示,高镁安山岩具有富集地幔特征,为地幔与下地壳混染作用产物,构造环境属于板内伸展环境。结合高镁安山岩地球化学特征的特殊性,推测中国东部燕山期可能存在高原。

In order to study the geochemical and chronological characteristics of Mesozoic medium basic volcanic rocks in Shixi Formation of Southern Jiangxi, and reveal the Mesozoic magmatic activity and tectonic evolution in Southeast China, LA-ICPMS zircon U-Pb dating and whole-rock geochemical analysis were carried out on the high magnesium andesite developed in the Shixi Formation of Dazhoutang, Southern Jiangxi. The results show that the surface age of zircon206Pb/238U is mainly concentrated in four age ranges: 1 754.6 Ma, 769.0~770.8 Ma, 410~440 Ma, 153.9 Ma, which are representing four magmatic thermal events, however, they cannot reveal the age of andesite formation. Based on the previous dating results of the same horizon, it is speculated that the suite of high magnesium andesite was formed in the Early Cretaceous. Geochemical data show that the andesite is a high potassium calc alkaline high magnesium andesite, with obvious enrichment of light rare earth elements, no Eu and Ce anomalies, enrichment of large ion lithophile elements Rb, Th, K, and loss of high field strength elements Nb, Ta, Ti. The diagram of element and tectonic environment indicate that the high magnesium andesite has the characteristics of enriched mantle, which was the product of the mixing of mantle and lower crust, and formed in an intraplate extensional environment. Based on the geochemical characteristics of high magnesium andesite, it is speculated that there may be a plateau in the eastern China during the Yanshanian period.