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40Ar/39Ar 年代学证据

摘要

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/39Ar 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 39Ar 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/36Ar 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/39Ar data indicate low uplift velocities (0.1‒0.2 km/Ma) and cooling rates (3‒3.6 ℃/Ma) exhumation of the Xitieshan HP/UHP granitic gneiss after the anatexis.%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40Ar/39Ar 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40Ar。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40Ar 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。40Ar/39Ar 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。