Contact metamorphism by solid-state emplacement of the ultrahigh-pressure-high-pressure metamorphic rocks

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The andalusite-sillimanite type low-pressure unit, called Daulet Suite, underlies the Kokchetav ultrahigh-pressure-high-pressure (UHP-HP) massif of northern Kazakhstan. The Daulet Suite 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, and 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. K-fedlspar appears with andalusite at lower temperatures than first formation of sillimanite. Pressure-temperature conditions of the Daulet metamorphism range from 580 to 680 ℃ at a nearly constant pressure of about 2 kbar. Comparison of composition of solid-solution minerals with a buffered assemblage indicate that the metamorphic grade increases towards the boundary with the overlying UHP-HP unit. Detailed mapping revealed that the primary structure of both the Kokchetav UHP-HP massif and its surrounding units is sub-horizontal; the UHP-HP unit has been thrust over the Daulet Suite with a top-to-the-north movement. The thermal structure of the Daulet Suite is sub-horizontal and metamorphic grade increases upwards, I.e. towards the boundary with the UHP-HP unit, whereas the thermobaric maximum of the UHP-HP unit occurs at an intermediate structural level. The upward increase in metamorphic grade within the Daulet Suite imply a contact metamorphism by the tectonic juxtaposition of the 'hot' Kokchetav UHP-HP unit at shallow crustal level.