The spatial and temporal patterning of the deep crust and implications for the process of melt extraction

Abstract

Volumetrically significant melt production requires crustal temperatures above approximately 800• C. At the grain scale, the former presence of melt may be inferred based on various microstructures, particularly pseudomorphs of melt pores and grainboundary melt films. In residual migmatites and granulites, evidence of melt-extraction pathways at outcrop scale is recorded by crystallized products of melt (leucosome) and residual material from which melt has drained (melanosome). These features form networks or arrays that potentially demonstrate the temporal and spatial relations between deformation and melting. As melt volume increases at sites of initial melting, the feedback between deformation and melting creates a dynamic rheological environment owing to localization and strain-rate weakening. With increasing temperature, melt volume increases to the melt connectivity transition, in the range of 2-7 vol% melt, at which point melt may escape in the first of several melt-loss events, where each event represents a batch of melt that left the source and ascended higher in the crust. Each contributing process has characteristic length and time scales, and it is the nonlinear interactions and feedback relations among them that give rise to the dissipative structures and episodicity of melt-extraction events that are recorded as variations in the spatial and temporal patterning of the crust. Focused melt flow occurs by dilatant shear failure of low-melt fraction rocks creating melt-flow networks that allow accumulation and storage of melt, and form the link for melt flow from grain boundaries to veins allowing drainage to crustal-scale ascent conduits. Preliminary indications suggest that anatectic systems are strongly self-organized from the bottom up, becoming more ordered by decreasing the number and increasing the width of ascent conduits from the anatectic zone through the overlying subsolidus crust to the ductile-to-brittle transition zone, where the melt accumulates in plutons.

Organizing Melt Flow through the Crust

“…The range of fertility for melt production among common crustal rocks (greywackes, pelites, amphibolites, and granites sensu lato) at granulite facies P-T conditions requires that the volume of crust involved in granite production was much greater-perhaps by an order of magnitude-than the volume of orogenic granite. The spacing of plutons in orogens suggests sources of broadly similar volume in any one setting (Brown 2007(Brown , 2010. When combined with evidence from inversion of gravity anomalies related to a root of granite extending to depth beneath plutons (Vigneresse 1995a), which represents the infi lled ascent conduit, these observations require that lateral fl ow of melt must be a common occurrence in the source (Brown 2007(Brown , 2010.…”

“…The spacing of plutons in orogens suggests sources of broadly similar volume in any one setting (Brown 2007(Brown , 2010. When combined with evidence from inversion of gravity anomalies related to a root of granite extending to depth beneath plutons (Vigneresse 1995a), which represents the infi lled ascent conduit, these observations require that lateral fl ow of melt must be a common occurrence in the source (Brown 2007(Brown , 2010.…”

“…It is these gradients in pressure that drive local melt fl ow, resulting in the formation of a network of channels. The spacing of the compositional layering, the strength of the fabric and the compaction length control the scale of these networks (Rutter and Mecklenburgh 2006;Brown 2010).…”

Melt Reintegration Modelling: Testing against a Subsolidus Reference Assemblage

“…The melt connectivity transition approximately 7%, [46] was used as a maximum for the melt loss that would have occurred in any one melt extraction event. However, we acknowledge that in the presence of syn-anatectic deformation, melt extraction may well occur at a lower threshold [5,18,81,82]. Figure 8.…”

Introduction to a virtual special issue on crustal melting

“…The connection between small-scale features, such as networks of leucosomes that can be mapped in an outcrop of migmatite or granulite, to medium-scale features, such as metric granites (Fig. 1f), that transport melt longer distances may enable distinction among the several mechanisms proposed for the large-scale movement of anatectic melts through the continental crust (Brown, 2010). Papers concerning melt extraction and ascent have been published frequently in the JMG, particularly during the past decade, and a selection is included in this virtual special issue.…”