THE PROTOLITH FORMATIONS OF THE AILAOSHAN METAMORPHIC ROCKS AND THEIR STRUCTURAL SIGNIFICANCE

The Ailaoshan metamorphic belt, striking N4OW parallel to the Ailaoshan fault, occurs as an arc convex southwestwards in plan. According to the mineral assemblages, petrochemical characteristics, fabrics and metamorphic features, the Ailaoshan metamorphic rocks can be classified into 3 main formation types: （1） hornblende-plagioclase gneiss-granulite form- ation （CⅠ）, （2） migmatite-granite formation （c Ⅱ）. （3） muscovite-biotite schist-phyllonite formation （C Ⅲ）. The Ailaoshan metamorphic belt is paired metamorphic ones, which mainly formed in the Carboniferous to Middle Triassic and the Middle Jurassic to the end of the Early Cretaceous. The high-temperature and low-pressure metamorphic belt consists of amphibolite-granulite facies, while the low-temperature and high-pressure one consists of greenschist-blue- schist facies. According to the diagrams of （al+fm）-（c+alk） versus si, （al=alk） versus c and （FeO）/MgO versus SiO₂. （FeO） and TiO₂ and Rb-Sr isotope ages, the protoliths of the Ailaoshan metamorphic rocks are mainly volcanic rocks and less commonly sedimentary rocks. The metavolcanic rocks in C Ⅰ and C Ⅲ, which are characterized by high （FeO） and low SiO₂, （FeO）/MgO, K₂O, Na₂O and(⁸⁷Sr/⁸⁶Sr)₀ and low （FeO）, are composed mainly of calc-alkaline （CA） series and, possibly, partly tholeiite. The protoliths show that there was a wide oceanic basin （a part of the East Tethys） along the Jinsha River-Ailaoshan fault during the Precambrian to Devon- ian. Due to the shrinkage of the oceanic basin during the Carboniferous to Middle Triassic （Variscan to Indosinian） the Ailaoshan area was situated in island-arc and active continental-margin tectonic environments, which led to the evolution of the continental crust with subduction of the oceanic basin in the Upper Triassic （Indosinian）. Strong thrusting in the Carboniferous to Middle Triassic and the Middle Jurassic to the end of the Early Cretaceous （Yanshanian） resulted in the formation of the Ailaoshan paired metamorphic belts.