辽宁辽阳地区辽河群酸性火山岩锆石U-Pb年代学及其地质意义

辽宁省辽阳地区出露有大面积的古元古代辽河群地层,在辽河群里尔峪组及高家峪组中识别出一套酸性火山岩。本文通过对酸性火山岩同位素年代学及地球化学研究,用以限定辽河群形成时代,并进一步探讨其形成的动力学背景。这套酸性火山岩岩性包括流纹岩、变流纹质含角砾凝灰岩、角岩化流纹岩和弱硅化流纹岩,对其中6件样品利用LA-ICP-MS锆石U-Pb方法测年。CL图像显示辽河群酸性火山岩中的锆石均具典型的岩浆振荡环带结构和较高的Th/U比值,显示为岩浆锆石。锆石U-Pb年龄可分为3期:2190~2180Ma、2110~2100Ma、1970~1960Ma,结合其中存在继承锆石的结果认为辽河群至少存在3期岩浆活动,将辽河群形成时代限制于2.19~1.96Ga,其形成时代大约持续了230Myr左右。野外接触关系的观察及酸性火山岩的年代学特征从另一个方面揭露辽河群总体为一套无序、局部有序的构造地层,各岩组间不存在上下关系。前人研究表明"辽吉花岗岩"形成年龄为2.16Ga,通过野外观察其与辽河群之间均为构造接触。D1442-1和D1444-2两个样品锆石U-Pb测年结果为2180±12Ma、2190±13Ma,均略早于"辽吉花岗岩",结合其与辽河群野外关系,认为"辽吉花岗岩"应该不是作为沉积基底,而是与辽河群为同一构造运动的产物。辽河群流纹岩Al_2O_3含量16.33%~16.90%,K_2O含量4.62%~4.77%,K_2O+Na_2O含量5.97%~7.54%,含铝指数(A/CNK)比较高为1.27~1.50,为过铝质岩石,显示富铝、高钾特征。此外流纹岩轻稀土富集,重稀土亏损,轻重稀土分馏明显,具有明显的负铕异常,亏损Ba、Sr、Ti,较高的Zr及(Zr+Nb+Ce+Y)含量、10000Ga/Al比值等特点都显示为A型花岗岩,说明其形成于地壳减薄环境。本文在辽河群中识别出一套酸性火山岩与前人在南辽河群识别出的变质流纹岩组成了酸性端元(2201~1960Ma),同时期存在的基性火成岩(2198~1869Ma)为基性端元,证明了辽河群"双峰式火山岩"的存在,暗示辽河群形成于伸展拉张环境。

Zircon U-Pb chronology and geological significance of felsic volcanic rocks in the Liaohe Group from the Liaoyang area, Liaoning Province

A set of felsic volcanic rocks have been identified in the Lieryu Formation and Gaojiayu Formation of the Liaohe Group in the Liaoyang area of the Liaoning Province. Geochronology and geochemical studies place constraints on the formation age of the Liaohe Group and its geodynamic setting. The felsic volcanic rocks include rhyolite, rhyolitic breccia tuff, hornfels rhyolite and weakly silicified rhyolite. Zircons from the 6 samples for LA-ICP-MS U-Pb dating have oscillatory zonings and high Th/U ratios indicating typical of magmatic origin. Zircon U-Pb results can be divided into 3 stages:2190~2180Ma, 2110~2100Ma and 1970~1960Ma. In consideration of inherited zircons and previous data, we suggest that the Liaohe Group has occurred at least 3 stages of magmatic activities and its formation age is limited in the range of 2.19~1.96Ga, lasting about 230Myr. Observation of field contact relations and chronological characteristics of felsic volcanic rocks reveal that the Liaohe Group is a series of disordered and locally ordered structural strata, and there is no upper and lower relation between the rock groups. Previous studies have shown that the "Liaoji granite" formed at 2.16Ga and is in tectonic contacts with the Liaohe Group based on the field observation. Zircon U-Pb dating results of samples D1442-1 and D1444-2 (2180±12Ma and 2190±13Ma) are a little earlier than the "Liaoji granite". Therefore, the "Liaoji granite" is not supposed to be the sedimentary basement, but form in the same tectonic event with the Liaohe Group. Contents of Al₂O₃, K₂O, K₂O+Na₂O in the rhyolites are 16.33%~16.90%, 4.62%~4.77% and 5.97%~7.54% respectively, and the A/CNK ratios is 1.27~1.50. Therefore, the rhyolites are characteristic of high aluminum, low alkali and high K. Moreover, these rhyolites are enriched in light rare earth elements (LREEs) and have high Zr, (Zr+Nb+Ce+Y) contents, 10000Ga/Al ratios, and are depleted in heavy rare earth elements (HREEs) and Ba, Sr, Ti, and have obvious negative Eu anomalies. All of these features are consistent with those of A-type granite, which emplacing in crustal thinning environment. A set of felsic volcanic rocks in this study and metamorphic rhyolites from former studies are the acidic composition (2201~1960Ma), and the mafic igneous rocks with the same age (2198~1869Ma) are the basic end member. This "bimodal volcanic rocks" in the Liaohe Group suggest that it formed in an extensional environment.