An Experimental Study of Partial Melting of Metafelsic Rocks: Constraints on the Feature of Anatectic Melts and the Origin of Garnets in Collisional Orogens

Crustal anatexis in continental subduction zones has great bearing on chemical differentiation of the continental crust at convergent plate boundaries. This was experimentally investigated for ultrahigh-pressure (UHP) metafelsic rocks at 0.5–3.0 GPa and 650–900 °C. The results show that partial melting begins at about 750 °C when pressure drops from 3.0 to 2.0 GPa, corresponding to decompressional exhumation of the deeply subducted continental crust. As the pressure further decreases to 1.0 GPa, the partial melting degree reaches the maximum of ∼25% at 900 °C. Partial melts produced in these experiments are rich in silica and alkali, and poor in iron, manganese and magnesium. As the degree of partial melting increases, the composition of partial melts gradually converges toward homogeneous one. In the absence of free water, the partial melting of metafelsic rocks were triggered by the breakdown of hydrous minerals. At low temperatures of ∼750 °C at 1.0–2.0 GPa, phengite dehydration melting occurs at first, giving rise to small amounts of felsic melts and peritectic K-feldspar. As the temperature rises up to 850–900 °C, biotite begins to break down and gives rise to large amounts of felsic melts and peritectic minerals such as garnet, K-feldspar and orthopyroxene. It is noted that peritectic garnet is much different from anatectic garnet crystallized from anatectic melts and metamorphic garnet formed through metamorphic dehydration reaction under subsolidus conditions. The peritectic garnet is characterized not only by anhedral shapes with many multiphase crystal inclusions but also by compositions poor in spessartine and grossular but rich in almandine and pyrope. On the other hand, the anatectic garnets are characterized not only by euhedral shapes with few inclusions but also by compositions rich in grossular and spessartine but poor in almandine and pyrope. These observations provide experimental constraints on the origin of garnets in UHP metamorphic rocks, which have great bearing on understanding of anatectic metamorphism in collisional orogens.