Structural-metamorphic evolution of late proterozoic ophiolites and precambrian basement in the central asian foldbelt of Mongolia

Abstract

Abstract The late Proterozoic and Palaeozoic Central Asian foldbelt is regarded as one of the best examples which demonstrates that processes of lateral crustal growth and obduction—accretion tectonics took place since at least late Precambrian times. In Mongolia the Precambrian and early Cambrian rocks were formed in two different environments: continental and oceanic. The Archaean to early Proterozoic complexes are autochthonous and constitute a metamorphic continental basement. The late Proterozoic to early Cambrian ophiolite complexes represent remnants of obducted oceanic crust and occur as intensely deformed nappes and tectonic melanges overthrust on the older basement during the Caledonian orogeny. Distinctly differentiated tholeiitic, calc-alkaline and boninite rocks series together with the lithological characteristics of associated volcanogenic-sedimentary formations indicate that the Mongolian ophiolite complexes were generated in a forearc-marginal basin setting. The culmination of ophiolite formation occurred in late Precambrian to early Cambrian time. Five phases of progressive regional metamorphism have been identified in the basement complex and overlying ophiolite association. Each of these was accompanied by folding and granitoid plutonism and is characterised by a distinct composition of fluid inclusions.