南辽河群盖县组的重新厘定:来自辽南地区黄花甸-苏子沟一带变质砂岩碎屑锆石U-Pb年代学证据

传统认为辽河群顶部的盖县组广泛分布于辽南地区。本文对岫岩县黄花甸-苏子沟一带的南辽河群盖县组变质长石石英砂岩和变质石英砂岩进行了LA-ICP-MS碎屑锆石U-Pb年代学研究。其中变质长石石英砂岩中碎屑锆石普遍发育核-边结构,核部具有清晰的振荡环带;变质石英砂岩中碎屑锆石基本无核-边结构,具有清晰或者略模糊的振荡环带。锆石微量元素分析结果显示,具有振荡环带锆石微区具有轻稀土元素(LREE)亏损、重稀土元素(HREE)相对富集的配分曲线特征,Ce正异常和Eu负异常明显,Th/U和Zr/Hf比值较高,表明其岩浆成因。变质长石石英砂岩岩浆成因锆石微区~(207)Pb/~(206)Pb谐和年龄集中于2506～1748Ma之间,且呈现～2178Ma和～1863Ma两个年龄主峰,表明主要物质来源为同时期花岗质岩石(条痕状花岗岩和斑状花岗岩);变质石英砂岩岩浆成因锆石微区~(207)Pb/~(206)Pb谐和年龄集中于3546～1950Ma之间,呈现～2149Ma年龄主峰值,表明主要物质来源为条痕状花岗岩,另有少量太古宙基底物质的加入。研究区变质长石石英砂岩～1.86Ga的年龄峰值与辽河群其它变沉积岩明显不同,表明沉积时代一定晚于～1.86Ga,即形成于辽河群古元古代变质作用之后。综合研究区盖县组变质长石石英砂岩与辽河群其它变沉积岩碎屑锆石U-Pb年龄的差异,我们建议将盖县组部分变沉积岩从辽河群中解体出来。

Redefinition of the Gaixian Formation of the South Liaohe Group: Evidence from the detrital zircon U-Pb geochronology of metamorphosed sandstone in Huanghuadian-Suzigou area, the southern Liaoning Province

The Gaixian Formation is widely distributed in the southern Liaoning Province and traditionally defined as the coping of the Liaohe Group. In this paper, we applied LA-ICP-MS zircon U-Pb dating techniques to determine the depositional ages of a metamorphosed feldspathic quartz sandstone and a metamorphosed quartz sandstone from the Gaixiain Formation. Detrital zircon grains separated from the metamorphosed feldspathic quartz sandstone are characterized by core-rim structures, with strongly oscillatory zoned cores. However, most detrital zircon grains selected from the metamorphosed quartz sandstone are structureless, with distinctly or weakly oscillatory zoning. Trace element compositions of zircons from the two samples have similar chondrite-normalized REE (rare earth element) patterns, strongly depleted in LREE (light rare earth element) and enriched in HREE (heavy rare earth element). They are characterized by pronounced positive Ce anomalies and negative Eu anomalies, suggesting their magmatic provenance. ²⁰⁷Pb/²⁰⁶Pb ages of the metamorphosed feldspathic quartz sandstone yielded by detrital zircons with igneous features scatter ranging from 2506Ma to 1748Ma, with two evident age peaks of~2178Ma and~1863Ma, indicating that their sediments were transported from the similar-aged granites (monzogranitic gneisses and porphyritic monzogranites). Whereas those detrital zircons of the metamorphosed quartz sandstone yield ²⁰⁷Pb/²⁰⁶Pb ages from 3546Ma to 1950Ma, and major age peak at~2149Ma, indicating their provenance from monzogranitic gneisses and minor Archean basement rocks of the adjacent complex. The depositional age of the metamorphosed feldspathic quartz sandstone is significantly different from that of other meta-sedimentary rocks of the Liaohe Group with respect to the age peak of~1.86Ga. It is inferred that the metamorphosed feldspathic quartz sandstones deposited at some time after~1.86Ga. Based on these discriminations, it is suggested that part of meta-sedimentary rocks from the Gaixian Formation should be separated from the Liaohe Group.