吉林省“大黑山条垒带”晚古生代火山事件及其构造意义

Late Paleozoic volcanic events in “Daheishan Horst” in Jilin,China and their tectonic significance

对出露于吉林省"大黑山条垒带"、行政区辖属伊通县发展乡一带的"大黑山火山岩"-安山质凝灰熔岩、片理化流纹岩分别采样并挑选锆石,经LA-ICP-MS定年,查明安山质凝灰熔岩74颗锆石的^(206)Pb/^(238)U年龄介于1 873~240 Ma,其中^(206)Pb/^(238)U年龄值介于317~240 Ma的69粒锆石均显示出岩浆成因特征,表明研究区自晚石炭世晚期便存在火山事件,火山喷发活动于早、中二叠世发展至峰期,该火山岩形成于晚石炭世晚期至晚二叠世,并于早、中三叠世遭受了岩浆作用;古元古代、中元古代、中泥盆世及早石炭世锆石具碎屑锆石特征,可能为被捕获的古老锆石。片理化流纹岩38粒锆石的^(206)Pb/^(238)U年龄介于307~240 Ma,且均显示出岩浆成因特征,表明该火山岩形成于早二叠世早期至晚二叠世,并遭受了印支期岩浆作用。所研究的"大黑山火山岩",前人多认为其与同地区桃山组地层时代相当,为早志留世或中志留世,但本文所获该火山岩的年龄表明二者年龄时限完全不同,故大黑山火山岩与桃山组地层应完全要剥离开来,分属不同时代残留的构造岩片。上述火山岩富集轻稀土元素和大离子亲石元素,亏损高场强元素,表现出类似岛弧或活动大陆边缘火山岩的特征;在Th-Ta-Hf/3三角图和Nb/Th-Zr/Nb和Nb/Yb-Th/Yb构造环境判别图中投点在火山弧火山岩、活动大陆边缘附近,表明该火山岩可能与当时古亚洲洋板块向松嫩地块俯冲所形成的火山弧有关。

The Daheishan volcanic rocks including the andesitic tuff lava and the foliated rhyolite are exposed at ＂Daheishan horst＂,in Fazhan area,Yitong,Jilin Province.The LA-ICP-MS U-Pb dating of 74 zircons from the andesitic tuff lava yield the 206Pb/238 U age between 1873-240 Ma,69 grains of them showing magmatic origin yield 206Pb/238 U age of 317-240 Ma and indicate that the volcanic events were started from Late Carboniferous and reached the peak period in Early-Middle Permian.The volcanic rocks formed between Late Carboniferous to Late Permian,and suffered magmatic emplacement during Early-Middle Triassic.The other older zircons of Paleoproterozoic,Mesoproterozoic,Middle Devonian and Early Carboniferous showing the characteristics of detrital origin may be captured zircons.The dating of 38 grains of zircons from the foliated rhyolite yield 206Pb/238 U age ranging between 307-240 Ma,showing that the rocks are formed from near Early to Late Permian and suffered magmatic emplacement during the Indo-Sinian epoch.Previous authors thought that the age of ＂Daheishan volcanic rocks＂ is Early or Middle Silurian,close to that of the Taoshan Formation in the same area,however,the data of the present paper indicates that the ages between them are totally different,so we consider that the Daheishan volcanic rocks and Taoshan Formation are remnant tectonic slices of different ages,and should be separated.The above mentioned volcanic rocks are enriched in LREE and LILE,depleted in HSFE,showing the characteristics of island arc or active continental margin volcanic rocks.The plots in diagrams of tectonic discrimination show that the volcanic rocks fall into the areas of island arc or active continental margin.The above evidences indicate that the volcanic rocks may be related to the volcanic arcs formed by the subduction of the Paleo-Asian Ocean Plate to Xilinhaote-Songnen-Jiamusi massif.