Partial melting mechanisms of peraluminous felsic magmatism in a collisional orogen: An example from the Khondalite belt, North China craton

Abstract

Sedimentary-derived (S-type) granites are an important product of orogenic metamorphism, and a range of subtypes can be recognized by differences in field occurrence, mineralogy and geochemistry. These subtypes can reflect variations of initial protolith composition, partial melting reactions, pressure and temperature of anatexis, or magmatic processes that occur during ascent through the crust (e.g. mineral fractional crystallization or crustal assimilation). Together, these diverse factors complicate geological interpretation of the history of peraluminous felsic melt fractions in orogenic settings. To assess the influence of these factors, we performed integrated field investigation, petrology, geochemistry, geochronology and phase equilibrium modelling on a series of leucosomes within migmatite associated with different S-type granites within the Khondalite belt, North China craton (NCC), which is an archetypal collisional orogen. Three types of leucosome are recognized in the east Khondalite belt: leucogranitic leucosome, K-feldspar (Kfs)-rich granitic leucosome and garnet (Grt)-rich granitic leucosome. Phase equilibrium modelling of partial melting and fractional crystallization processes indicate that the leucogranitic leucosomes were mostly produced through fluid-present melting, Kfs-rich granitic leucosomes are produced through muscovite dehydration melting with 3 vol.% garnet fractional crystallization, and Grt-rich granitic leucosomes are produced through biotite dehydration melting with 20–40 vol.% K-feldspar fractional crystallization and up to 20 vol.% peritectic garnet entrainment. Mineral fractional crystallization and peritectic mineral entrainment occur in the source during melting, and play equally important roles in partial melting mechanisms in terms of affecting the geochemical compositions of granitic melts. Thus, we suggest that peraluminous felsic magmas preserved in collisional orogens are dominantly produced by fluid-absent melting in the middle to deep continental crust, although extraction of low-volume melt fractions from an anatectic source region at shallower depths during fluid-present melting can also generate small amounts of S-type granites that subsequently crystallize at high structural levels in the crust.