黑龙江省依兰地区大顶子山林场岩体形成时代及构造环境探讨

Discussion on the formation age and tectonic environment of Dadingzishan forest farm rock mass in Yilan area,Heilongjiang Province

东北地区中生代构造一直是地质学者研究的热点之一,目前存在着不同的认识。笔者对黑龙江省东部依兰地区大顶子山林场岩体进行了锆石LA-ICP-MS U-Pb测年和岩石化学样品分析,结果显示:锆石呈柱状,环带清晰,具岩浆锆石特征,206Pb/238U数值多位于谐和线上,加权平均年龄为(204. 23±0. 88) Ma,该数值应为该岩体的结晶年龄。岩石化学特征表现为钙碱性;稀土元素特征为轻稀土富集,具负铕异常;微量元素U、Cr元素相对富集,而Th、La、Y、Sr元素相对亏损。晚三叠世研究区可能仍处于古亚洲洋闭合晚期阶段,处于陆壳增生造山-伸展环境,在佳木斯地块与松嫩地块之间的古亚洲洋板块向松嫩地块俯冲,形成了大面积的主要分布于牡丹江西侧南北展布的三叠纪花岗岩带。

Mesozoic tectonics in Northeast China has always been one of the key points of geologists,but there are different understandings at present.LA-ICP-MS U-Pb dating and petrochemical analysis of zircon from the Dadingzishan forest farm rock mass in Yilan area of Eastern Heilongjiang Province were carried out.The results show that the zircon is columnar,with clear zoning and magmatic zircon characteristics.The 206 Pb/238 U values are mostly on the concordant curve and the weighted average age is(204.23±0.88)Ma,which must have been the crystallization age of the rock mass.The geochemical characteristics of the rocks are calc-alkali;the REE is characterized by LREE enrichment with negative Eu anomaly;trace elements U and Cr are enriched relatively while Th,La,Y and Sr are depleted relatively.The study area might have been the late stage of palaeo-Asian Ocean closure and the continental accretionary orogenic and extensional environment during the late Triassic epoch.The palaeo-Asian Ocean plate between Jiamusi block and Songnen block subducted toward Songnen block,and formed a large area of Triassic granite belt which mainly distributed in the west of Mudanjiang area.