喜马拉雅中段哲古拉花岗岩中高压麻粒岩包体及其主岩的^(40)Ar/^(39)Ar年代学研究

^(40)Ar/^(39)Ar geochronology of high-pressure granulite xenolith and its surrounding granite in central Himalaya

为了深入了解喜马拉雅构造带的地质演化历史,同时为了探讨作为建立高压变质条件下的同位素年代学方法的重要前提的同位素体系特征,对出露于喜马拉雅中段西藏哲古拉地区的高压麻粒岩包体中的峰期矿物和退变质矿物及其主岩花岗岩中的富钾矿物进行了常规^(40)Ar/^(39) Ar 年代学研究。花岗岩中的黑云母坪年龄为11.48±0.18Ma,钾长石坪年龄为12.63±0.19Ma,二者的等时线年龄与之相当,分别为11.63Ma 和12.58Ma。高压麻粒岩峰期矿物黑云母的坪年龄为48.5±0.54Ma,等时线年龄为48.95±0.83Ma,(^(40)Ar/^(36)Ar)_i 为285;退变质矿物角闪石的坪年龄谱呈马鞍形,坪区数据对应的等时线年龄为31.1±5.4Ma,(^(40)Ar/^(36)Ar)_i 为394,显示有过剩^(40)Ar 的存在。结合前人的研究,推定高压麻粒岩经历了一个快速的退变质作用过程,不仅变质作用没有达到平衡,早期与晚期变质矿物之间也没有达成氩同位素交换平衡,在标本尺度上或高压麻粒岩包体与主岩花岗岩之间均是如此。根据^(40)Ar/^(39)Ar 年代学结果也可以了解到,在高压麻粒岩的退变质过程中,早期与晚期变质矿物之间的氩同位素体系有明显不同,这种氩同位素体系在不同变质阶段的不平衡记录为帮助建立不同变质地质事件的年代学序列提供了研究途径。依照获得的^(40)Ar/^(39)Ar 年代学数据,可以建立喜马拉雅中段高压麻粒岩包体形成和演化的动力学过程:推定高压麻粒岩经受了两期变质作用的叠加,峰期变质老于48.5Ma,晚期变质发生在31Ma 前后;大约在17Ma 前后为其主岩花岗岩捕虏,并在11Ma～12Ma 之间被带至地表。文中对前人锆石 U-Pb 年龄进行了再分析,认为在高压变质作用条件下,由于熔体或流体从变质岩石中被抽提出去而限制了变质锆石的生长,因此,高压麻粒岩包体中的锆石 U-Pb 年龄没有能够记录高压变质事件。

To make a further study of geological evolution history of Himalaya structure zone, and to study the isotopic system, which is an important foundation of isotopic geochronology in high pressure metamorphism, conventional ^40Ar/^39 Ar geochronological research of peak and retrograde minerals in high-pressure granulite xenolith and potassium-rich minerals of its surrounding granite in central Himalaya have been done. In the surrounding granite, the biotite have a plateau age of 11. 48 ±0. 18 Ma and an isochron age of 11.63 Ma, while the K-feldspar have a plateau age of 12. 63 ±0. 19Ma and an isochron age of 12. 58Ma. in the HP-granulite xenolith, the peak metamorphic biotite, whose plateau age and isochron age are respectively 48. 5± 0. 54 Ma and 48. 95 ±0. 83 Ma, together with its initial ^40Ar/^36Ar value of 285, shows that there is no extra ^40Ar in it, The retrograde hornblende has an isochron age of 31. 1 ± 5. 4Ma, but have a saddle-shaped age spectrum and an initial ^40Ar/^36Ar value of 394, showing the obvious existence of extra ^40Ar. Following former research, it is presumed that the high-pressure granulite xenolith had undergone a rapid retrograde metamorphism, both the metamorphism and the exchange of argon isotope in various minerals of different phases are non-equilibrium, where both in rock samples and between the high-pressure granulite xenolith and its surrounding granite. It is certain that the argon isotopic system among peak and retrograde minerals are various according to the ^40Ar/^39Ar geochronological data, which is useful in dating geological events, Based on the ^40Ar/^39 Ar geochronological data, a likely formational and evolutional geodynamic process of the high-pressure granulite xenolith in central Himalaya can be recoveried： The HP-granulite xenolith have undergone two respective metamorphism, the peak metamorphism was earlier than 48. 5 Ma, while the retrograde metamorphism was at around 31 Ma ago, then have been caught by its surrounding granite at around 17 Ma ago, and brought to the earth＇ s surface at about 11Ma - 12Ma ago, According to the Zircon U-Pb geochronological data, it is considered that the growth of metamorphic zircon might be restricted as a result of the lacking of melt or liquid under high-ultrahigh pressure metamorphic condition, which making it possible that the high-pressure metamorphic event not recorded by Zircon U-Pb age,