THE DAULET METAMORPHISM: A CONTACT METAMORPHISM BY SOLID INTRUSION OF THE KOKCHETAV UHP-HP METAMORPHIC SLAB

Metapelites in the andalusite-sillimanite type Daulet Suite provide a good opportunity to reveal a new-type of metamorphism. The Daulet Suite, underlying the Kokchetav ultrahigh-pressure - high-pressure (UHP-HP) massif of northern Kazakhstan, is composed of pelitic-psammitic gneisses or schists and quartz schist with minor amounts of metacarbonate and metabasite. About 300 thin sections were examined from the Daulet metapelites; they were divided into two mineral zones. Zone A is characterized by an assemblage of andalusite + cordierite + biotite and zone B by sillimanite + cordierite + biotite with excess K-feldspar, quartz and plagioclase in both zones. Thermal structure of the Daulet Suite is subhorizontal and metamorphic grade increases upwards, i.e., towards the boundary with the UHP-HP unit. Observations in this study indicate that heat source for the Daulet metamorphism is not the Devonian granitoid but the "hot" Kokchetav UHP-HP unit. Since K-feldspar first appears with andalusite within zone A prior to appearance of sillimanite, pressure-temperature conditions of the Daulet metamorphism range from 580 to 680 ^oC at a nearly constant pressure of about 2 kbar; tectonic juxtaposition occurred at notably shallow crustal levels. Many UHP-HP rocks are partially hydrated during exhumation and are overprinted by the Barrovian-type metamorphism. It was considered that fluids from a subducting slab have will exert important effects on metasomatic and melting relations (e.g. Selverstone et al.,1984; 1992). The tectonic juxtaposition of dehydrated UHP-HP units on hydrous low-grade rocks would allow infiltration of fluids into the UHP-HP unit. Such a tectonic overlap would transport sufficient fluids from the underlying unit to the overlying UHP-HP unit and would effectively obliterate the UHP-HP mineralogy.