Clockwise P– T paths for crustal melting and H 2O recycling in granite source regions and migmatite terrains

Abstract

Clockwise pressure–temperature ( P– T) paths of metamorphism pass through two kinds of melting reactions with increasing temperature. The first generates H 2O-saturated partial melts at the H 2O-saturated solidus, only if excess H 2O is present. Usually, this accounts for less than 3% melt. The second generates H 2O-undersaturated melts through dehydration-melting reactions (DMRs) of micas, or other hydrous minerals, and occurs at temperatures higher than the H 2O-saturated solidus. If H 2O-undersaturated melts remain in situ, and the source regions are not exhumed, then the crystallisation reactions encountered along the near-reverse P– T path may produce the same mineralogy as in the source rock. In regions of exhumation, along the decompression segments of clockwise P– T paths, two other types of crystallisation reactions occur. The better-known type occurs at the recrossing of the pelite H 2O-saturated solidus at shallower depth. This can occur at mid-crustal depths or much shallower (at pressures lower than the appropriate invariant point which relates all of these reactions), and will result in granitic pegmatites. At the other type, remaining mica can react with hydrous melt to generate anhydrous granulite with free H 2O fluid. Such suprasolidus decompression–dehydration reactions (SDDRs) result in freezing of pelitic melts at mid-crustal depth to K-feldspar+quartz assemblages, and release of H 2O which can induce H 2O-saturated melting in adjacent fertile lithologies lacking H 2O (e.g., psammite, older granite). Distinct mineral proportions and textures will result from each reaction. The actual reactions occurring are related through specific invariant points whose locations are known in the P– T space. The displacement of these invariant points due to additional components in the solid, melt and fluid phases is reasonably well known. The flat-lying SDDR (very low d P/d T) will extend to much higher pressures if a low-aH 2O fluid is present. SDDR involving amphibole or epidote will occur in mafic to intermediate compositions, over a large depth range depending on Fe/Mg, Ca/Na, etc., of the rock compositions.