New Data on the Structural Setting and ~(40)Ar/~(39)Ar Age of the MP-LP Metamorphism of the Daulet Formation, Kokchetav Metamorphic Belt, Northern Kazakhstan, and Their Tectonic Interpretation

摘要

Ultra-high-pressure (UHP) and high-pressure (HP) metamorphic complexes in continental subdue -tion zones are spatially associated with younger medium-pressure (MP) and low pressure (LP) metamorphic rocks. It is a controversial question whether these MP-LP rocks are formed by the contact effect of exhumed hot nappe stack of UHP-HP rocks on the underlying rocks (by analogy with metamorphic base of obducted ophiolite allochthon) or result from later syncollisional metamorphism related to crustal thickening and granite magmatism.MP and LP metamorphic complexes with inverted metamor-phic zoning are known in the collisional Himalaya nappe-thrust system. Structurally, they are confined to the overthrusting of the Himalayas onto the Indian continent along the Main Central Thrust. The MP (garnet-kyanite-staurolite) and LP (sillimanite-biotite and biotite) rocks were formed at the initial stage of thrusting (25-15 Ma) and exhumed to the surface by later thrusts and denudation.