黑龙江杂岩中蓝片岩的地球化学特征和云母片岩的40Ar-39Ar年龄对佳木斯地块晚古生代至中生代构造演化的制约

The Late Paleozoic to Mesozoic tectonics of the Jiamusi block:Evidence from the geochemical characteristics of the blueschists and the 40Ar-39Ar age of the mica schist in the Heilongjiang complex,Northeast China

黑龙江杂岩作为存在于佳木斯地块西缘的牡丹江洋俯冲—增生的唯一直接记录,对限定佳木斯地块晚古生代—中生代的构造演化具有关键作用。本文主要对依兰和牡丹江地区出露的黑龙江蓝片岩和云母片岩分别进行了地球化学和40Ar-39Ar年代学方面的分析。研究结果显示,依兰和牡丹江地区的蓝片岩原岩均为碱性玄武岩,与OIB的地球化学性质相一致,指示其形成于洋岛环境。结合作者在依兰地区发表的蓝片岩原岩结晶年龄为~288 Ma,表明在早二叠世时期,佳木斯地块与松嫩地块之间存在古洋盆,即牡丹江洋。此外,在牡丹江地区黑龙江杂岩中获得的云母片岩白云母40Ar-39Ar年龄为187±2 Ma,其与变基性岩常常相伴产生,表明二者共同经历了牡丹江洋的俯冲消亡过程,因此代表了黑龙江蓝片岩的形成时代,即早侏罗世。综上,本文对佳木斯地块晚古生代—中生代的构造演化历史提出了新的解释,即:早二叠世,牡丹江洋一直存在于佳木斯地块与松嫩地块之间;早侏罗世,牡丹江洋向松嫩地块之下发生俯冲消减,进而造成佳木斯地块与松嫩地块发生碰撞拼贴。

The Heilongjiang complex,located in the western part of the Jiamusi block,is the only record and the key evidence to define the Late Paleozoic to Mesozoic subduction-accretion history of the Mudanjiang Ocean.In this contribution,we report new whole-rock geochemistry for the blueschists and muscovite 40Ar-39Ar age for the mica schist in the Heilongjiang complex,respectively.Our results show that the protoliths of the blueschists from Yilan and Mudanjiang areas are both alkaline basalts and have affinities to ocean island basalt(OIB)which were generated in an ocean island setting.Combined with our recently published zircon U-Pb dating result of^288 Ma which is considered as the protolith age for the blueschist in Yilan area,it is suggested that an old ocean,namely Mudanjiang Ocean,had existed between the Jiamusi and Songnen blocks since the Early Permian.In addition,the mica schist,associated with metabasalts in the Heilongjiang complex,yields a muscovite 40Ar-39Ar plateau age of 187±2 Ma,suggesting that metamorphism of the Heilongjiang blueschist occurred in the Early Jurassic.To summarize,we propose a new tectonic evolution model for the Jiamusi block,i.e.,the Mudanjiang Ocean had existed between the Jiamusi and Songnen blocks since the Early Permian,and the blueschists were formed in the Early Jurassic by the subduction of this ocean,further resulting in the collision between the Jiamusi and Songnen blocks.