Magma flow within the Tavares pluton, northeastern Brazil: Compositional and thermal convection

Crystallization coupled with gravity removal of depleted interstitial melt has long been recognized as a mechanism of magma differentiation. Similarly , heat released by synplutonic basaltic magma intrusions has long been reco gnized as capable of driving convection in granite chambers. Direct evidenc e of these processes has seldom been described in granites. In the Tavares pluton, we mapped a number of melt extraction structures from pores of a cr ystal-liquid mush (an effectively solid magma where crystals form an interc onnected skeleton) and a variety of flow structures such as (1) meter-scale tear- or mushroom-shaped blobs representing within-chamber diapirs; (2) me ter-scale ellipsoids representing frozen thermal plumes of granite, driven by heat released from disrupted diorite intrusions; and (3) ladder dikes an d snail structures representing cross sections of several superposed cylind rical magma channels (possibly feeders of diapirs and plume heads). A funda mental feature of the structures in the Tavares pluton is that they are wel l delineated by mafic schlieren developed at active channel margins. We pos tulate a new model for the origin of marginal schlieren, which combines she ar flow sorting and melt escape from the flowing magma into an effectively solid surrounding mush. Extraction structures (representing melt extraction from mush pores into melt pockets) and schlieren (representing regions whe re melt escaped into surrounding mush pores) are both favored by magmas tha t form an interconnected solid framework at low crystal fractions (similar to 50%), because these mushes are ductile and permeable. Favorable magmas a re those with a high wetting angle between melt and solid (similar to 60 de grees) and a propitious crystal size and shape distribution. We propose a m odel of compositional and thermal convection that accounts for all describe d structures.