西藏拉萨地体冈底斯岩基紫苏花岗岩中的高密度CO_2包裹体成因及其地质意义

Origin and geological significance of high-density CO_2 fluid inclusions in Charnockites from the Gangdese batholith,Lhasa terrane,southern Tibet

产于西藏拉萨地体东南部冈底斯岩基中的紫苏花岗岩侵入体具有低水活度的矿物组合,富含CO2流体包裹体.形成紫苏花岗岩所需要的低水活度条件主要是受到含CO2流体带入的控制.通过对产于冈底斯岩基紫苏花岗岩中CO2流体包裹体的岩相学观察、显微测温和激光拉曼光谱分析,确定了其组成和密度.结果表明碳质(CO2)流体包裹体是在紫苏花岗岩结晶过程中捕获的,其等容线通过了由矿物温压计估算的P-T条件(850℃～950℃,0.75～1.0GPa),随后经历了短期近等压降温作用和地体快速隆起的影响.石英中的高密度碳质流体包裹体(密度最高可达1.14g/cm3)提供了证据表明,拉萨地体麻粒岩相变质和紫苏花岗岩侵位期间有富CO2流体的参与.富CO2流体包裹体的产出及其形成的高温高压条件可能说明在紫苏花岗岩的形成机制与洋中脊俯冲过程中的脱水熔融和渗滤有关.

Charnockits, occurring as intrusive rocks in the Gangdese batholith in the southeastern Lhasa terrane, are characterized by the presence of dry mineral assemblage. The anhydrous conditions required for the formation of charnockites are thought to be controlled by the lowering of water activity through the influx of CO2 -- bearing fluids. Through petrographic, microthermometric studies and Raman analysis of CO2- rich fluid inclusions in the eharnockite samples from the Gangdese batholith, we have characterized the composition and density of the fluids during the crystallization of the charnockites. Our results indicate that the carbonic fluid inclusions were trapped during the crystallization of the charnockites and its isochores pass through the P T conditions around 850 ℃ -950 ℃ and 0.75- 1.0 GPa as estimated by mineral thermobarometry data. High-density carbonic fluid inclusions （up to 1.14 g/cm^3 ） in quartz provide potential evidence for the involvement of CO2 -- rich fluid during granulite facies metamorphism in the Lhasa terrane. Occurrence of CO2 --rich inclusions and elevated P--T conditions open up the possibility favoring the mechanism of vapour-deficient dehydration melting accompanied by CO2 infiltration to account for charnockite formation in the ocean ridge subduction process.