孙吴—嘉荫盆地古近系物源及其地质意义

Provenance of Paleogene in Sunwu-Jiayin Basin and Its Geological Significance

为揭示孙吴—嘉荫盆地古近系物源及其地质意义,本文以古近系乌云组和孙吴组砂岩为研究对象,对样品进行了碎屑锆石LA-ICP-MS U-Pb定年和地球化学测试研究。结果表明,乌云组72颗碎屑锆石测点有7个年龄峰期:57、97、188、262、380、496和1854 Ma,孙吴组64颗碎屑锆石测点有8个年龄峰期:52、65、92、181、220、472、1884和2318 Ma,表明乌云组和孙吴组物源复杂多样。两个组样品的MgO、Al_(2)O_(3)、CaO、K_(2)O及Fe_(2)O_(3)含量均较低,化学蚀变指数(CIA)介于64.30~81.72之间,成分变异指数(ICV)介于0.44~0.70之间,两个组Th/Sc值与Zr/Sc值均呈正相关关系;乌云组和孙吴组稀土总量(∑REE)分别为(97.44~154.87)×10^(-6)和(85.65~315.62)×10^(-6),La/Yb值分别为10.94~12.84和13.26~20.74。结合碎屑锆石峰值年龄和主量微量元素特征,确定孙吴—嘉荫盆地古近系物源为松嫩—张广才岭地块东缘的古元古代地质体和各个时代岩浆活动事件产生的长英质火成岩,物源区构造背景为活动大陆边缘和大陆岛弧。盆地古近系与上白垩系物源存在明显差异,结合区域构造演化特征,推测造成这种物源差异的原因是向东亚大陆俯冲的太平洋板块发生退却或回撤和俯冲方向的变化。

In order to reveal the provenance and geological significance of the Paleogene in Sunwu-Jiayin basin,this paper studies the detrital zircon dating and geochemical characteristics of samples from Paleogene Wuyun and Sunwu Formations.The age peaks of 72 measuring points of detrital zircons in Wuyun Formation are 57,97,188,262,380,496,and 1854 Ma,and the age peaks of 64 measuring points of detrital zircons in Sunwu Formation are 52,65,92,181,220,472,1884,and 2318 Ma,indicating that the provenance of Wuyun Formation and Sunwu Formation is complex and diverse.The contents of MgO,Al_(2)O_(3),CaO,K_(2)O and Fe_(2)O_(3)in the samples are relatively low.The chemical alteration index CIA value ranges from 64.30 to 81.72,and the composition variation index ICV value ranges from 0.44 to 0.70.Th/Sc values are positively correlated with Zr/SC values.The total amount of rare earth elements(∑REE)changes from 97.44×10^(-6)to 154.87×10^(-6)and 85.65×10^(-6)to 315.62×10^(-6),respectively.La/Yb ratios vary from 10.94 to12.84 and 13.26 to 20.74.Combined with the characteristics of major,trace and rare earth elements and the age peaks of detrital zircon,it is concluded that the Paleogene sources are the Paleoproterozoic geological body in the eastern margin of Songnen-Zhangguangcai range Massif and the felsic igneous rocks produced by magmatic events in various times.The tectonic background of the source area is active continental margin and continental island arc.There are apparent differences between Paleogene and Upper Cretaceous provenance in the basin.Combined with the characteristics of regional tectonic evolution,we speculated that it was caused by the Pacific plate retreat or drawback and subducted direction changes in the process of Pacific plate subducting to the East Asian continent.