喜马拉雅中段高压麻粒岩变质作用、地球化学与年代学

Metamorphism, geochemistry and U-Pb zircon SHRIMP geochronology of the high-pressure granulites in the central Greater Himalayas.

研究的高压麻粒岩发现于西藏亚东以北约40公里的(Zherger-La)、出露在藏南拆离系(STDS)主构造面下盘的高喜马拉雅结晶岩系中,是继喜马拉雅东西构造结的Nanga Barbat、Namjag Barwa和喜马拉雅中段Khatra & Marina地区、定结地区发现的榴辉岩或高压麻粒岩之后,在青藏高原上新近发现的高压麻粒岩。该麻粒岩呈岩片被包裹于花岗质片麻岩中。麻粒岩记录了两期变质作用,早期矿物组合为Grt+Cpx+Pl+Qz,属麻粒岩相变质产物,矿物成分分析显示早期矿物组合达到了平衡,并且没有表现成分扩散;后期矿物组合为Hbl+Pl+Bio或Opx+Pl,指示了较高温但相对压力较低的麻粒岩相退变变质作用,矿物成分分析和结构显示了退变作用没有达到变质平衡。显微结构可以观察到多组变质反应:Grt+Cpx+Qtz=Opx+Pl,Grt+Qtz=Opx+Pl,Grt+Cpx+L=Hbl+Pl+Bio+Mt,和Cpx+L=Hbl+Mt。根据矿物平衡关系,利用Grt-Cpx温度计和Grt-Cpx-Pl-Qz压力计估算的早期变质作用温压为T=780～850℃,P=12～15kbar,相对应的地温梯度16℃～18℃/km。借用Hbl-Pl温度计和Al^(tol) in Hbl压力计估算的晚期变质作用温压为T=730～760℃:P=4～6kbar,相当的地温梯度为38℃～50℃/km。变质作用P-T演化呈等温降压轨迹,指示麻粒岩地体从增厚(或俯冲)地壳到减薄增温(或部分熔融)地壳,进而被快速剥露地表的构造过程。初步的地球化学结果表明高压麻粒岩原岩可能相当于大陆拉斑玄武岩。麻粒岩锆石SHRIMP年代学有两组相对集中的年龄分别为98±5Ma(5 spots)和17.0±0.3Ma(13 spots)。高压麻粒岩的两期变质作用的温度都在700℃以上,略高于锆石U-Pb同位素体系计时封闭温度,推断17Ma是高压麻粒岩变质后发生折返,随高喜马拉雅结晶岩系剥露冷却的年龄;98.4Ma的测年结果被推测是高压麻粒岩原岩形成的年龄。在喜马拉雅山,高压麻粒岩记录了类似增厚地壳到减薄地壳的转变一方面可能是地壳深部作用机制的转变,另一方面,这种机制与喜马拉雅南坡巨大的降雨量和去顶作用有密切关系,意义重大。

The studied high pressure mafic granulite (HP-granulite) outcrops in the Greater Himalayas 40 kilometers north to Yadong City in the southern Tibetan Plteau. It exposes in the footwall of the southern Tibetan detachment system (STDS) as the enclaves of the parageiness. The HP-granulite is marked by the early mineral assemblages garnet (Grt) + clinopyroxene (Cpx) Plgioclase (Pl) and the later assemblages amphibole + Plgioclase + biotite(Bio) orthopyroxene(Opx). The metamorphic reaction textures indicate the following reactions: Grt + Cpx + Qtz = Opx + Pl, Grt + Qtz = Opx + Pl, Grt + Cpx + L = Hbl + Pl + Bio + Mt,Cpx + L = Hbl + Mt. Thermobarmetric estimates based on equilibrium texture and mineral chemistry indicate that the early mineral assemblages formed at P = c.1.2 similar to 1.5 GPa, T = c. 780 similar to 850degreesC with the geothermal gradient of 15degreesC similar to 18degreesC/km, and the later retrograde assemblages experienced near-isothermal decomposition to P = 4 similar to 6kbar, T = 730 similar to 760degreesC with the geothermal gradient of 38degreesC similar to 50degreesC /km. Metamorphic texture, P and T evolution shows HP-granulite had experienced a rapid change of tectonic background from thicken crust to thinning or heating crust. The U-Pb SHRIMP ages of zircons collected from the HP-granulite show most ages are around 17 Ma, and other mainly around 98 Ma. Because the estimated metamorphic temperatures are higher than the closure temperature of zircon U-Pb chronological system, the later age of 17 Ma should be the cooling age during rapid exhumation related to the activity of STDS and MCT in the Greater Himalayas. The Greater Himalayan belt as well as the lesser Himalayan belt is regarded to be the part of Indian continent. The HP-granulite must have been the continental thoelitic basalt metamorphosed due to the collision between Indian and Eurasian continents. The HP-granulite provides a new clue to study the process of collision and convergence between India and Asia, and how and why the Greater Himalayas uplift.