Petrology and geochronology of the charnockite from the southern Gangdese belt, Tibet: Evidence for the Andean-type orogen

Abstract

The Gangdese batholith emplaced in southern Tibet in the Cretaceous has been widely regarded as the major component of an Andean-type convergent margin resulting from the northward subduction of the Neo-Tethyan oceanic lithosphere beneath Asia. While the Gangdese batholith consists predominantly of calcalkaline and adackitic ganodiorites, we have recognized a suite of chamockites in the eastern part of the Gangdese belt. They consist of andesine, microcline, enstatite, diopside and Ti-rich biotite, with FeO/(FeO + MgO) ratios of 0. 47 ∼ 0. 50, MALI of - 3. 97 ∼ + 0. 11 , and ASI of 0. 83 ∼ 0. 92, suggesting a typical characteristics of magnesian, alkali-calcic and metaluminous granitic rocks. The estimated temperature and pressure conditions for the crystallization of chamockites are 850 ∼950°C and >0. 4GPa, and the speculated formation depth is below 13 ∼ 15km. Zircons from the chamockites are typical of magmatic origin, and give the weighted mean ages of 87Ma. This demonstrates that the chamockites may have formed in a continental magmatic arc. The presence of chamockites and associated high-temperature granulites in the southern Gangdese indicates that the Gangdese belt has been subjected to an Andean-type orogenic event in the Late Cretaceous.

青藏高原南部白堊紀形成的岡底斯巖基是新特提斯大洋巖石圈向北部的歐亞大陸之下俯沖形成的安第斯型會聚板塊邊緣的重要組成部分。已經有研究證明,岡底斯巖基主要由鈣堿性和埃達克質的花崗閃長巖組成,但本文報道了在岡底斯帶東南緣產出的紫蘇花崗巖。這種巖石主要由中長石、微斜長石、頑火輝石、透輝石、石英、富鈦的黑云母和角閃石組成。在化學成分上,其鐵鎂指數FeO/(FeO+MgO)=0.47～0.50,鈣堿指數MALI為-3.97～+0.11,鋁飽和指數ASI為0.83～0.92,是典型的鎂質、鈣堿性、準鋁質紫蘇花崗巖。計算出的結晶溫度為850～950℃,壓力>0.4GPa,形成深度為13～15km。紫蘇花崗巖中的鋯石為巖漿結晶成因,兩個樣品的U-Pb定年分別給出了87Ma的相同加權平均年齡。巖石學和地球化學特征表明,岡底斯帶南緣晚白堊紀的紫蘇花崗巖形成在典型的大陸巖漿弧環境。紫蘇花崗巖和伴生高溫麻粒巖的存在為岡底斯帶在晚中生代經歷的安第斯型造山作用提供了更加確鑿的證據。