大兴安岭北部雄关地区倭勒根岩群大网子岩组火山岩地球化学特征

GEOCHEMISTRY OF THE VOLCANIC ROCKS OF DAWANGZI ROCK FORMATION IN NORTHERN DAXINGANLING MOUNTAINS

雄关地区的倭勒根岩群大网子岩组主要为变碎屑岩夹变流纹岩和变粗面安山岩.利用ICP-MS仪器对变粗面安山岩进行锆石U-Pb年龄测定,结果显示变粗面安山岩锆石206Pb/238U年龄为443.8~783.8 Ma,获得ICP-MS锆石U-Pb同位素谐和年龄为463.7±2.1 Ma,加权平均年龄为463.6±6.4 Ma(n=10,MSWD=0.073).所测锆石振荡环带较发育,Th/U值一般为0.42~1.76,具有岩浆成因特征.该年龄代表粗面安山岩形成年龄,认为其形成于中奥陶世,与区域大网子岩组形成时代(寒武纪-早志留世)一致.大网子岩组粗面安山岩属碱钙系列准铝质岩石,铕异常不显著,相对亏损高场强元素,富集大离子亲石元素.岩石具有壳幔混源特征,形成于陆缘环境,与古亚洲洋壳俯冲有关.

The Dawangzi rock formation of Wolegen rock group in Xiongguan area,northern Daxinganling Mountains,is composed of mainly meta-clastic rocks with metarhyolite and meta-trachyandesite. The zircon U-Pb dating of the trachyandesite by ICP-MS yields the206 Pb/238 U age of 443.8-783.8 Ma,with the zircon U-Pb concordia age of 463.7 ±2.1 Ma,and the weighted mean age of 463.6 ± 6.4 Ma(n=10,MSWD = 0.073). The tested zircons show developed oscillation rings,with the Th/U ratio generally ranging from 0.42 to 1.76,indicating a magmatic origin. This age represents the forming age of the trachyandesite,which is believed to be formed in Middle Ordovician,consistent with the formation time of regional Dawangzi rock formation(Cambrian-Early Silurian). The trachyandesite,belonging to alkali-calcic quasi-aluminous rocks,is characterized by unobvious Eu anomaly,depletion of HFSEs and enrichment of LILEs. The rocks have the features of crust-mantle mixed source,which was formed in the continental margin environment and related to the subduction of Paleo-Asian Ocean crust.