The origin of rapakivi feldspar by a fluid-induced coupled dissolution- reprecipitation process

Abstract

The rapakivi texture found in many granitoids comprises alkali feldspar megacrysts mantled by plagioclase, usually of oligoclase composition. The existing models for their genesis involve magmatic or dry subsolidus processes. Here, we describe the occurrence of rapakivi feldspars in A-type granites from the Malani Igneous Suite in western India and use microtextural and geochemical evidence to argue that rapakivi textures can form by subsolidus deuteric alteration of feldspar megacrysts through a coupled dissolution-reprecipitation replacement process. The feldspars in granites from the Malani Igneous Suite crystallized at temperatures >720°C and subsequently underwent coherent exsolution, producing strain-controlled braid microperthite/antiperthite. At temperatures of 465-490°C, the feldspar megacrysts reacted with deuteric fluids, which led to the dissolution of the braid perthite/antiperthite along an inward-moving reaction interface and coupled precipitation of an oligoclase/albite mantle. As the rapakivi replacement front progressed inward, the fluids infiltrated into the interiors of the relict megacrysts along fractures and braid boundaries and reacted with the braid perthite via a dissolution-reprecipitation replacement process. This resulted in the formation of patch perthite/antiperthite. The replacement reactions were incomplete, preserving patches of the unreacted braid perthite. At temperatures of 253-283°C, the feldspars were partially albitized, whereby the oligoclase patches and the plagioclase mantle were partially pseudomorphically replaced by albite. Mass-balance constraints indicate that the replacement processes leading to the formation of the plagioclase mantle and the patch perthite/ antiperthite were not isochemical. The fluid composition was externally buffered for many of the elements, but internally controlled by feldspar dissolution-reprecipitation reactions for those elements that are normally incorporated in the feldspar structure. These results conclusively demonstrate for the first time that in addition to magmatic processes, rapakivi feldspars can form by subsolidus, fluid-induced, dissolution-reprecipitation replacement reactions.