东喜马拉雅构造结岩体冷却的^(40)Ar/^(39)Ar年代学研究  

^(40)Ar/^(39)Ar geochronology studies of rocks in eastern Himalaya syntaxis

作　　者：龚俊峰[1] 季建清[1] 陈建军[1] 桑海清[2] 李宝龙[1] 刘一多[2] 韩宝福[1] 

机构地区：[1]北京大学地球与空间科学学院造山带与地壳演化教育部重点实验室,北京100871 [2]中国科学院地质与地球物理研究所岩石圈演化国家重点实验室,北京100029

出　　处：《岩石学报》2008年第10期2255-2272,共18页Acta Petrologica Sinica

基　　金：国家自然科学基金项目(编号:40472100)成果

摘　　要：对采自东喜马拉雅构造结核心地段雅鲁藏布大峡谷地区的13件标本中的20件矿物样品进行了系统的常规^(40)Ar/^(39)Ar年代学研究。数据显示,样品的(^(40)Ar/^(36)Ar)_i值均接近尼尔值(295.5±5),且绝大部分样品的坪年龄与其反等时线年龄在误差范围内一致。从数据统计结果来看,所测样品的^(40)Ar/^(39)Ar年龄大都集中在1.3Ma和2.5Ma左右,表明南迦巴瓦地区在上新世中期和更新世早期均经历了快速冷却抬升事件。本次测试的样品采自不同的高程及不同的构造单元,且样品原岩的成因及岩性各异,但沿着大峡谷由北向南不同地段的样品的不同矿物(角闪石、黑云母、白云母、钾长石)的^(40)Ar/^(39)Ar年龄相近,而同一样品中不同矿物的^(40)Ar/^(39)Ar年龄大小又并非完全按照矿物对氩同位素体系的封闭温度高低来分布,表明该地区在上新世以来的岩体冷却速率很大,以致该地区的矿物对氩同位素体系的封闭过程与处于缓慢冷却环境中的封闭过程明显不同。以本文报道的数据估算,南迦巴瓦地区的岩体在最近3Ma以来的冷却速率达120～240℃/Ma,岩体抬升速率达3.4～6.9mmn/a。Conventional 40Ar/^39Ar geochronology studies have been done on 20 separate minerals in 13 samples which are from the Yarlung-Tsangpo River gorge, eastern Himalaya syntaxis. The 40Ar/^39Ar geochronology data show that ： （ 1 ） the values of （ 40Ar/^39Ar） of all the minerals tested are close to 295.5 ; （2） most plateau ages are accord with their respective inverse isochron ages within error range. Age statistics indicate that most 40Ar/^39Ar ages are focused on 1. 3Ma and 2. 5Ma approximately, indicating that Namjagbarwa region has undergone two fast cooling-uplift events in middle Pliocene and early Pleistocene respectively. The samples are collected from the northern to the southern parts of the gorge, they are from various altitude and belong to different tectonic units, and the petrogenesis and lithology of their protolith are different, but the 40Ar/^39Ar Ar ages of most minerals （including hornblende, biotite, muscovite and K-feldspar etc. ） from various samples are similar, and the age of different minerals from a single sample are not distributed according to the value of the closure temperature to argon isotopic system. Therefore, it seems that the cooling rate of the rocks in Namjagbarwa region has being so fast since Pliocene that the cooling process to argon isotopic system of minerals from this area are apparently different from ones from slow cooling environment. The cooling rate of Namjagbarwa region has being 120 - 240℃/Ma and so an uplift rate of 3.4 -6. 9 mm/a since the latest 3Ma according to the data published in this paper.

关 键 词：东喜马拉雅 南迦巴瓦 ^40AR/^39AR年代学 

分 类 号：P588.346[天文地球—岩石学] P597.3[天文地球—地质学]