The mechanism of ascent and emplacement of granite magma during transpression: a syntectonic granite paradigm

Abstract

We propose a model for syntectonic ascent and emplacement of granite magma based on structural relations in part of the northern Appalachians. In the study area in western Maine, strain was distributed heterogeneously during Devonian Acadian transpression. Metasedimentary rocks (migmatites at high grades) record two contrasting types of finite strain in zones that alternate across strike. Rocks in both types of zones have a penetrative, moderately-to-steeply NE-plunging mineral elongation lineation defined by bladed muscovite (fibrolite/sillimanite at high grades). In `straight' belts of enhanced deformation rocks have S > L fabrics that record apparent flattening-to-plane strain (apparent flattening zones, AFZs), but rocks between these belts have L > S fabrics that record apparent constriction (apparent constriction zones, ACZs). At metamorphic grades above the contemporary solidus, rocks in AFZs developed stromatic structure in migmatite, which suggests that percolative flow of melt occurred along the evolving flattening fabric. Stromatic migmatites are intruded by concordant to weakly discordant, m-scale composite sheet-like bodies of granite to suggest magma transport in planar conduits through the AFZ rocks. Inhomogeneous migmatite is found in the intervening ACZs, which suggests migration of partially molten material through these zones en masse, probably by melt-assisted granular flow. Inhomogeneous migmatites are intruded by irregular m-scale bodies of granite that vary from elongate to sub-circular in plan view and seem cylindrical in three dimensions. These bodies apparently plunge to the northeast, parallel to the regional mineral elongation lineation, to suggest magma transport in pipe-like conduits through the ACZ rocks. We postulate that the form of magma ascent conduits was deformation-controlled, and was governed by the contemporaneous strain partitioning. Magma ascent in planar and pipe-like conduits through migmatites is possible because oblique translation during contraction displaces isotherms upward in the orogenic crust to form a thermal antiform. Within this hot corridor, it is the difference in temperature between melt-producing reactions in the anatectic zone and the wet solidus for granite melt that enables magma to migrate pervasively up through the orogenic crust without congealing. Heat advected with the migrating melt promotes amplification of the thermal antiform in a feedback relation that extends the zone of plastic deformation and pervasive melt migration to shallower levels in the crust. At the wet solidus, we suggest melt flows obliquely toward axial culminations in the thermal antiform, which are sites of melt accumulation and perturbations from which magma may escape to form plutons. Batches of melt that escape from these perturbations may be trapped by tectonic structures higher in the crust, or ascent may become inhibited with decreasing depth by thermal arrest and solidification. If the rate of arrival of subsequent melt batches exceeds the rate of crystallization at the site of pluton construction, melt pressure ultimately may lead to (sub-)horizontal magma fracture, or viscous flow of wall rocks may allow lateral spreading. The resultant plutons have (sub-)horizontal tabular geometries with floors that slope down to the ascent conduit. As the thermal antiform decays, the height of sub-solidus crust that separates the deepest part of the pluton from the anatectic zone increases. Consequently, pluton inflation declines and solidification leads to infilling of the magma feeder channel to form a root zone that passes downward into migmatite, which may explain the difficulty of determining precisely the floor to these deeper segments of large plutons using gravimetry.