Pegmatites as geological expressions of spontaneous crustal flow localisation

Abstract

Amongst the silicate-rich crystalline rocks that are produced in the continental crust, pegmatites are characterised by their large crystals which give them both an aesthetic and economic interest. Pegmatites crystallise either from fractionated magma derived from a parent granitic body or from the partial melting of meta-sediments or meta-igneous rocks (e.g. amphibolite). The mechanism of residual magma (or fluid) extraction from the parent granitic body has been thoroughly studied, but pegmatitic melt extraction after partial melting has received less attention. We present here a series of non-dimensional numerical experiments using a two-phase flow formulation that couples the Stokes problem to/with non-linear Darcy flow. This approach makes it possible to predict the movement of fluid inclusions (named porosity) in a deformable of a viscous rocks (named porous matrix). We find that the simulation produces either clusters or an isolated body of fluid inclusion depending on the compaction/decompaction ratio of the effectively viscous matrix in which they rise. Using a review of pegmatite natural properties, we propose a scaling of our numerical simulations that describes the ascent of a pegmatite-forming melt produced by partial melting. We then discuss possible travel distances and temperature effects. To discuss our results in light of field observations, we assume that the compaction-decompaction ratio is an accurate proxy for the influence of brittle processes at a scale smaller than the representative volume element, and therefore corresponds structural level variations at which pegmatites are emplaced. We find that our numerical simulation explain the statistical organisation, in terms of level of emplacement, of real fields of pegmatites possibly derived from partial melting of meta-sediments. Pegmatites in fact tend to organise as clusters around brittle faults in upper crustal levels, whereas they present a scattered distribution at mid to lower crustal levels. Our results therefore show that porosity waves are a possible mechanism for rapidly extracting and transporting pegmatite melts formed during low-degree (ca. 10%) partial melting at distances up to a few kilometres in the crust.