Origin of the inverted metamorphism of the lower Himalayas, Central Nepal

Abstract

The southern slope of the central Nepal Himalayas is divided geologically by the Main Central Thrust zone into the underlying Midland metasediment zone (Lower Himalayas) in the south and the overlying Himalayan gneiss zone (Higher Himalayas) in the north. The former zone consists of a thick pile of various kinds of metasediments folded into an anticlinorium, and the latter zone consists of high-grade crystalline rocks with a north-dipping homoclinal structure and is further overlain by the Tethys Himalayas, composed of non- or slightly metamorphosed Tethyan sediments. The Main Central Thrust zone shows a characteristic lithology such as calcareous schist, quartzite, phyllitic schist and augen gneiss, and is intensely mylonitized. The mineral composition and paragenesis of the rocks in the present area indicate an inverted metamorphism from the Midland metasediment to the Main Central Thrust zones, that is, the metamorphic grade of the Midland metasediment zone increases upward toward the thrust zone from the chlorite to the biotite zones of the greenschist facies, and the greenschist-amphibolite transitional zone is attained in the thrust zone. The Himalayan gneiss zone is apparently much higher in metamorphic grade than the thrust zone. On the basis of the compositional zoning of garnets and muscovite compositions, however, it is concluded that the Himalayan gneisses suffered a polymetamorphism. The younger metamorphism resulted in a retrograde metamorphism of the Himalayan gneisses together with the above-mentioned inverted metamorphism and an anatectic melting in depth. This was caused by shear heating due to the thrust movement along the Main Central Thrust zone during the Alpine tectonism. The older metamorphism, which seems to be Precambrian in age, was much higher in metamorphic grade than the younger one and formed the pyrope-rich cores of garnets in the Himalayan gneisses. The maximum metamorphic temperature of 550°C, estimated from the mineral paragenesis, in the thrust zone, is compatible with that caused by shear heating due to the thrusting with thrusting velocity of 5–10 cm/yr, maximum stress of 300 bars and thrusting distance of 250 km (Bird, 1978).