冈底斯南部打加错地区鸭洼基性杂岩的年代学及地球化学特征

Chronology and Geochemical Characteristics of Yawa Mafic Complex in the Dajiacuo Area,Southern Gangdese

前人对西藏冈底斯构造带古生代地质构造属性认识仍然存在多种解释。本文报道了位于冈底斯南部打加错地区的鸭洼基性岩浆杂岩的锆石U-Pb定年和地球化学数据,以期对这一问题进行约束。鸭洼基性杂岩单斜辉石岩的加权平均年龄为263.5±4.0Ma(MSWD=0.34),2件辉长岩分别为262.7±2.3Ma(MSWD=2.8)和263.9±2.4Ma(MSWD=3.8),反映冈底斯带存在二叠纪深成岩浆活动。鸭洼基性杂岩体的两类岩石属于碱性岩系列,具有高的TiO2含量,区别于冈底斯二叠纪岛弧型火山岩类,类似于板内玄武岩;稀土配分型式呈∑LREE富集型,Eu﹡值为0.98～1.07;高场强元素丰度与板内玄武岩平均丰度相近,Zr/Nb比值、Hf/Th比值分别变化在3.07～4.94和1.25～1.98范围内,类似于板内玄武岩;微量元素原始地幔标准化的蜘网图和MORB标准化的蜘网图显示岩浆形成于板内构造环境,微量元素构造环境判别图解也支持这一结论。结合区域上石炭——二叠纪沉积特点、火山活动和变形变质作用,认为冈底斯南部二叠纪时期为被动陆缘环境,以北地区属于主动陆缘环境。

There has been different understanding of geological natures of the Gangdese belt in Tibet during the Paleozoic. This paper presents the data of geochemistry and U-Pb dating of Yawa mafic complex exposed in Dajiacuo area, southern Gangdese so as to put constrain on this issue. A weighted average age of one clinopyroxene sample from the Jawa mafic rock is 263.5±4.0Ma (MSWD=0.34) and the ages of two gabbro samples are 263.9±2.4Ma(MSWD=0.38) and 262.7±2.3 Ma(MSWD=2.8)respectively, revealing that the Permian plutonism existed in the Gangdese belt. Two kinds of rocks in the Yawa complex belong to alkaline rock series characterized by high content of TiO2, and are different from island-arc volcanic rocks in the Gangdese belt, similar to intraplate basalt. The REE patterns of the samples exhibit enrichment of light REE, with Eu=0.98~1.07. The HSE abundance of the samples is equal to that of intraplate basalt, and the ratios of Zr/Nb and Hf/Th are 3.07~4.94 and 1.25~1.98, respecticely. All this features indicate that the Yawa mafic complex is similar to intraplate basalt. The patterns of chondrite- normalized REE and MORB- normaized trace element indicate that the Yawa complex developed in a intraplate setting, and this is supported by the trace element identification figure of tectonic setting. Based on the regional sedimentary features, volcanics and deformation and metamorphism during Carboniferous and Devonian, it is suggested that southern Gangdese was in passive continental margin, and northern area in active continental margin during the Devonian.