拉萨地块南缘大竹卡组富铯硅质角砾岩的发现及其地质意义

Discovery of cesium-rich siliceous breccia in Dazhuka Formation at southern margin of Lhasa Block and its geological significance

大竹卡组是分布在拉萨地块南缘的一套陆相磨拉石沉积,研究程度较低。文章以该套地层中新发现的富铯硅质角砾岩为研究对象,开展了镜下鉴定、全岩Sr-Nd同位素、主微量元素地球化学以及角砾中挑选出的岩浆锆石LA-ICP-MS U-Pb测年和Lu-Hf分析。研究结果显示,大竹卡组硅质角砾岩中岩浆锆石的形成时代为(54.92±0.75)Ma,与帕那组年龄相近证实始新世岩浆作用产物可能为大竹卡组提供了部分物源。锆石ε_(Hf)(t)值介于7.71~10.17、全岩Sr-Nd同位素与林子宗群火山岩相似,但其(^(87)Sr/^(86)Sr)i明显区别于区域以上地壳来源为主的铯硅华矿床。Fe-Mn-Al、Cr-Zr、SiO_(2)-(K_(2)O+Na_(2)O)、SiO_(2)-Al_(2)O_(3)等图解皆显示大竹卡组硅质角砾岩为热液交代成因。综上所述,文章认为大竹卡组富铯硅质角砾岩原岩可能为碰撞造山初期形成的火山碎屑岩,该套火山碎屑岩在大竹卡组沉积之前,经历了富铯、富硅的热液流体交代事件。文章发现了拉萨地块南缘早于中新世的富铯硅质角砾岩,证实了在印度—欧亚大陆碰撞造山过程中发生了铯的富集,并可能是现代铯硅华矿床的潜在物质源区。这些富铯硅质角砾岩数量多、铯含量高,具有一定的铯矿找矿潜力。

The Dazhuka Formation is a set of continental molasse deposits distributed in the southern margin of Lhasa Block,which has been poorly studied.In this paper,the new cesium-rich siliceous breccia found in the Dazhuka Formation is studied.The microscopic identification,Sr-Nd isotope,major and trace element geochemistry of the whole rock,LA-ICP-MS U-Pb dating and Lu-Hf analysis of magmatic zircons selected from the breccia are car-ried out.The results show that the formation age of magmatic zircons in the siliceous breccia of the Dazhuka For-mation is(54.92±0.75)Ma,which confirms that the products of Eocene magmatism provide part of the prove-nance of the Dazhuka Formation.ZirconεHf(t)values are between 7.71~10.17 and whole rock Sr-Nd isotopes are similar to those of the Linzizong Group volcanic rocks,but(^(87)Sr/^(86)Sr)i is obviously different from the cesium gey-serite deposits with crustal origin in the region.Fe-Mn-Al,Cr-Zr,SiO_(2)-(K_(2)O+Na_(2)O),SiO_(2)-Al_(2)O_(3) and other dia-grams show that the siliceous breccia of the Dazhuca Formation is of hydrothermal metasomatism origin.It is concluded that the protolith of the cesium-rich siliceous breccia in the Dazhuka Formation may be pyroclastic rocks formed in the early stage of collision orogenesis,and the pyroclastic rocks underwent a cesium-rich and sili-ca-rich hydrothermal fluid metasomatism event before the deposition of the Dazhuka Formation.The discovery of cesium-rich siliceous breccia in the southern margin of Lhasa Block prior to the Miocene confirms that cesium enrichment occurred during the India-Eurasia continental collision orogeny,and may be a potential source area for modern cesium geyserite deposits.These cesium-rich siliceous breccia are abundant and have high cesium content,which is a new cesium ore prospecting prospect area.