Polyphase growth of accessory minerals during continental collision: Geochemical evidence from ultrahigh-pressure metamorphic gneisses in the Sulu orogen

An integrated study of petrology, geochronology and geochemistry was performed for ultrahigh-pressure metamorphic gneisses in the Sulu orogen. The results are used to elucidate the polyphase growth of such metamorphic minerals as zircon, titanite and garnet in response to pressure-temperature changes and fluid/melt action during continental collision. This provides insights into the property of metamorphic fluid/melt and their effects on trace element mobility. A combined result from REE patterns, mineral inclusions and Ti-in-zircon temperatures indicates three stages of zircon growth. Prograde growth occurred at ~ 237 Ma primarily at eclogite-facies, retrograde growth at ~ 222 Ma mostly at eclogite-facies, and the last growth at ~ 205 Ma at granulite-facies. The three stages of zircon growth are deciphered by distinct REE patterns and trace element compositions, reflecting the differences in the property of metamorphic fluid/melt. The episodic growth of metamorphic zircon is primarily dictated by the episodic concentration of Zr and Si in metamorphic fluid/melt. Relict domains of magmatic titanite are distinguished from metamorphosed and metamorphic domains by their distinctive REE patterns and trace element compositions. The metamorphic titanite exhibit variably elevated Nb contents and Nb/Ta ratios, suggesting Nb/Ta fractionation due to breakdown of amphibole and/or biotite during metamorphism. Polyphase growth of garnet is suggested by an integrated analysis of mineral inclusions, and major and trace elements in large garnet grains. Trace element abundances vary in different zones of garnet, which is ascribed to changes in the paragenesis and composition of matrix minerals involved in garnet-forming reactions at different P-T conditions. Therefore, the metamorphic growth of zircon, titanite and garnet would have occurred not only during prograde subduction but also during retrograde exhumation in the continental collision zones. Both metamorphic dehydration and partial melting would have taken place episodically during the collisional orogeny. The breakdown of hydrous minerals at high- to ultrahigh-pressure conditions is a key to fluid liberation and element supply for the growth of these accessory minerals.