滇中昆阳磷矿成矿时代及沉积环境Re-Os同位素示踪研究

The Age and Sedimentary Environment of the Kunyang Phosphate Deposit,Central Yunnan: Constraints from Re-Os Isotopes

川滇黔地区下寒武地层中广泛分布一系列磷矿床,其中滇中昆阳磷矿是我国下寒武统梅树村期黑色页岩中规模最大的海相沉积型磷矿床,其成矿时代以及沉积环境对于矿床成因研究以及区域找矿具有重要意义。Re-Os同位素不仅能够对富有机质沉积岩地层进行直接定年,而且还能为物质来源及沉积环境示踪提供一种有效的技术手段。本文对昆阳磷矿中谊村段磷块岩和石岩头段黑色页岩开展热表面电离质谱法Re-Os同位素示踪及定年探索性研究,获得了石岩头段黑色页岩Re-Os等时线年龄为521.9±5.4Ma,直接限定了昆阳磷矿黑色页岩沉积时代。磷块岩187Os/188Os初始比值为0.6576~0.7671,黑色页岩187Os/188Os初始比值为0.887,另外磷块岩187Re/188Os值为2.582~240.5,而上覆黑色页岩187Re/188Os值为96.24~341.8,而且磷块岩、黑色页岩微量元素δEu和δCe值在垂向剖面上显示由低变高的变化特征,这些证据表明昆阳磷矿形成正处于大陆快速剥蚀时期,指示大量陆源碎屑汇入海洋参与成岩-成矿过程。综合前人研究,本文认为,在寒武纪,大气氧含量快速升高,加剧陆壳风化,大量高放射性陆源Os的输入造成该时期内187Os/188Os初始比值逐渐升高。另外由于全球海平面上升,导致滇中早寒武世梅树村组沉积期古海洋环境由浅水氧化向深水还原逐渐过渡,而昆阳磷矿形成于该阶段海进旋回沉积早期。

BACKGROUND： The Kunyang phosphate deposit in central Yunnan is the largest marine sedimentary phosphorite deposit in China. The deposit is hosted in black shale of the Lower Cambrian Meishucun Formation. However,there is still some controversy on the material source of the phosphorite and the deposition age of the black shale.OBJECTIVES： To precisely date the black shale and provide direct constraint for the depositional age of the phosphorite and to better understand the successive depositional environment evolution from the phosphorite to black shale in vertical profiles.METHODS： Negative Thermal Ionization Mass Spectrometry（ NTIMS） Re-Os isotope tracer and dating studies were performed on phosphorite of Zhongyicun Member and black shale of Shiyantou Member in the Kunyang phosphate deposit.RESULTS： The Re-Os isochron age of black shale in the Shiyantou Member is 521. 9 ± 5. 4 Ma,which directly constrains the deposition age of black shale in the Kunyang Phosphate deposit. The phosphorite and black shale samples have initial187 Os/188 Os values from 0. 6576 to 0. 7671 and of 0. 887,respectively. The phosphorite and black shale samples have187 Re/188 Os values of 2. 582-240. 5 and 96. 24-341. 8,respectively. The δEu and δCe values of trace elements in the phosphorite and black shale show changes from low to high in the vertical profile.This evidence indicates that the formation of the Kunyang phosphorus deposit occurred in the period of rapid erosion in the mainland and a large amount of terrigenous debris involvement in diagenesis-mineralization also occurred.CONCLUSIONS： Based on previous studies,the atmospheric oxygen content increased rapidly,aggravating the weathering of the continental crust and the input of a large number of highly radioactive terrestrial Os caused the initial187 Os/188 Os initial ratio to increase gradually during the Cambrian Period. In addition,due to the rise of the global sea level,the paleo-marine environment of the Lower Cambrian Meishucun gradually changed from shallow water oxidization to deep water reduction. The Kunyang phosphorus deposit formed during the early stages of the sea-inward-cycle sedimentation.