柴北缘锡铁山花岗质片麻岩深熔作用年代和冷却历史:来自浅色体^(40)Ar/^(39)Ar年代学证据  

Anatexis and Cooling History of Granitic Gneiss in the Xitieshan Terrane, North Qaidam: Evidence from ^(40)Ar/^(39)Ar Geochronological Study of Leucosome

作　　者：胡荣国[1,2,3] 邱华宁[2] Jan R.WIJBRANS Fraukje M.BROUWER 何立言[2] 王敏[2] 

机构地区：[1]桂林理工大学广西隐伏金属矿产勘查重点实验室,广西桂林541004 [2]中国科学院广州地球化学研究所同位素地球化学国家重点实验室,广东广州510640 [3]Department of Petrology,VU University Amsterdam

出　　处：《大地构造与成矿学》2016年第1期125-135,共11页Geotectonica et Metallogenia

基　　金：荷兰皇家文理科学院基金(06CDP002);中-荷联合培养博士生计划(08Ph D-08)联合资助

摘　　要：长英质浅色体广泛出露于柴北缘造山带中段锡铁山地体花岗质片麻岩中,多呈脉状、不规则透镜状顺区域片麻理走向产出,主要由钾长石、斜长石、角闪石、石英以及少量黑云母组成,是黑云母花岗质片麻岩深融作用的产物。温压估算获得其形成条件为P=(6.5~9.6)′102 MPa,T=640~690℃,达麻粒岩相。选取浅色体角闪石和钾长石单矿物进行激光阶段加热^(40)Ar/^(39)Ar定年。角闪石共进行了17个阶段,其中3~17阶段数据形成平坦年龄谱,坪年龄为442.5±4.0 Ma;构成年龄坪的数据对应的等时线年龄为441.6±3.9 Ma,相应初始捕获氩比值为303±4,暗示该角闪石样品不含过剩^(40)Ar。坪年龄~442.5 Ma解释为角闪石初始结晶年龄,代表了锡铁山地区黑云母片麻岩发生深融(混合岩化)的时代。共生钾长石阶段加热获得一个低温和一个高温坪,坪年龄分别为307.5±2.9 Ma和323.3±3.0 Ma;等时线图谱暗示该钾长石样品未受过剩^(40)Ar干扰。坪年龄分别代表浅色体冷却到钾长石封闭温度~200℃和~250℃的时间。^(40)Ar/^(39)Ar定年结果显示锡铁山黑云母片麻岩深熔作用后经历了低速抬升(0.1~0.2 km/Ma)和缓慢冷却(3~3.6℃/Ma)的演化历史。Felsic leucosome veins are widely distributed in biotite granitic gneiss in the Xitieshan terrane, North Qaidam, western China. The leucosome veins are derived from anatectic magmatites and consist mainly of plagioclase, K-feldspar, quartz and amphibole with minor biotite. Thermobarometric study indicates that the leucosome records granulite-facies metamorphic conditions of P=（6.5-9.6）′102 MPa and T=640-690 ℃. Amphibole and K-feldspar separates from the leucosome veins were analyzed by the laser stepwise heating ~40Ar/~39 Ar method. The release pattern of the amphibole is characterized by older apparent ages for the first two steps, which account for ~3% of the cumulative ~39 Ar released. The remainder of the experiment produced younger and concordant apparent age with a plateau age of 442.5±4.0 Ma. The data points constituting the age plateau yields a well-defined isochron with an intercept age of 441.6±3.9 Ma, and an initial ~40Ar/~36 Ar ratio（303±4） indistinguishable from that of atmosphere. Considering the leucosome was directly produced by crystallization of melt in the anatexis region, the amphibole plateau age of ca. 442.5 Ma provides an effective constraint on anatexis of the Xitieshan biotite-bearing granitic gneiss. K-feldspar produces an increasing staircase-shaped age spectrum with a younger plateau age of ca. 307.5 Ma in the first three steps and an older plateau age of ca. 324.3 Ma in the last segment. These ages are interpreted in terms of cooling through closure temperatures of ~200 ℃ and ~250 ℃, respectively. ~40Ar/~39 Ar data indicate low uplift velocities（0.1-0.2 km/Ma） and cooling rates（33.6 ℃/Ma） exhumation of the Xitieshan HP/UHP granitic gneiss after the anatexis.

关 键 词：浅色体 花岗质片麻岩 深熔作用 40Ar/39Ar定年 冷却历史 锡铁山地体 

分 类 号：P597[天文地球—地球化学]