藏东金沙江古特提洋闭合时间:来自昌都地块东缘早-中三叠世不整合及碰撞型岩浆岩的约束

Timing of closure of the Jinshajiang Paleo-Tethys Ocean in eastern Tibet:Constraints from the Early-Middle Triassic unconformity and collision-related igneous rock in the eastern margin of Qamdo block

藏东金沙江古特提斯洋的闭合时间还存在争议。藏东昌都地块东缘下-中三叠统马拉松多组及其底部不整合为金沙江古特提斯洋闭合时间提供了新的约束。本文报道了马拉松多组的地层特征、时代及其中火山岩的地球化学特征。马拉松多组为一套酸性火山岩-碎屑岩地层,其角度不整合覆盖在古生界最上部,又被上三叠统甲丕拉组角度不整合覆盖。锆石U-Pb年龄、化石及马拉松多组下伏的最年轻地层综合限定马拉松多组的时代是早三叠世-中三叠世早期(T_(1)-T_(2)^(1),252~242Ma)。马拉松多组火山岩属于强过铝质高钾钙碱性流纹岩,具低的ε_(Nd)(t)值(-13.2~-12.6)和高的(^(86)Sr/^(87)Sr)_(i)值(0.717774~0.720643),并有古老的Nd模式年龄(2.0~2.1Ga)。流纹岩锆石具有负的ε_(Hf)(t)值(-16.2~-13.0)和古老的Hf同位素地壳模式年龄(2.1~2.3Ga)。马拉松多组流纹岩是昌都地块与中咱-扬子地块碰撞背景下昌都地块古老/成熟地壳物质重熔的产物。通过对昌都地块东缘和金沙江结合带的沉积作用、岩浆作用和高压变质作用的时空分析,认为金沙江古特提斯洋闭合时间为早三叠世-中三叠世早期(T_(1)-T_(2)^(1),252~242 Ma)。

The timing of closure of the Jinshajiang Paleo-Tethys Ocean remain controversial.The unconformity between Lower-Middle Triassic Malasongduo Formation and Paleozoic strata in the eastern margin of Qamdo block offers new insights into closure of the Jinshajiang Paleo-Tethys.This paper reports the stratigraphic characteristics,age,and volcanic rocks geochemical data of the Malasongduo Formation.The Malasongduo Formation is a set of acidic volcanic-clastic sedimentary rocks unconformably overlies the uppermost of Palaeozoic strata and angularly unconformable with the overlying Upper Triassic Jiapila Formation.Zircon U-Pb age,fossils and the youngest strata under the Malasongduo Formation jointly give an Early Triassic-early Middle Triassic(T_(1)-T_(2)^(1),252~242Ma)age to the Malasongduo Formation.The volcanic rocks in Malasongduo Formation are characterized by peraluminous high potassium calc-alkaline rhyolites with lowε_(Nd)(t)values(-13.2~-12.6),high initial^(87)Sr/^(86)Sr ratios(0.717774~0.720643),and ancient Nd model ages of 2.0~2.1Ga.The zircons extracted from the rhyolite show negativeεHf(t)values of-16.2~-13.0 and Proterozoic Hf isotopic crustal model ages of 2.1~2.3Ga.The rhyolites of the Malasongduo Formation are products of the remelting of ancient/mature crustal materials in the Qamdo block under the background of the collision between the Qamdo block and the Zhongza-Yangtze block.Base on temporal-spatial analysis of sedimentation,magmatism and high-pressure metamorphism in the eastern margin of the Qamdo block and the Jinshajiang suture zone,it is concluded that the closure time of the Jinshajiang Paleo-Tethys Ocean range from the Early Triassic to the early Middle Triassic(T_(1)-T_(2)^(1),252~242Ma).