Gaëlle Plissart

Deformation-driven Differentiation during in situ Crystallization of the 2·7 Ga Iguilid Mafic Intrusion (West African Craton, Mauritania)

The 2 7 Ga Iguilid mafic intrusion is a post-metamorphic plutonic body made of cumulate gabbronorite and norite characterized by limited variations in modal proportions but significant disparities in trace element contents. The cumulates display microtextural evidence for syn-magmatic deformation, leading to reorientation of cumulus minerals, indented contacts at plagioclase grain boundaries, local bending and fracturing of euhedral cumulus plagioclase and interstitial melt migration/ segregation structures. All these features suggest that the Iguilid intrusion formed after in situ crystallization and syn-magmatic deformation of an unconsolidated mush within solidification fronts. Quantitative investigations of syn-magmatic fabrics together with geochemical estimations of the trapped melt fraction reveal a strong inverse correlation between the anisotropy degree P and the volume of trapped liquid retained in the cumulates. We interpret the dominantly vertical planar and linear fabrics in the Iguilid rocks as a result of horizontal shortening of the cumulates induced by filter pressing of a partly solidified mush against the vertical walls of the intrusion. Flattening of the cumulates occurred in response either to a syn-magmatic rotation of the Iguilid body relative to the tectonic main stress or to stages of overand under-pressurization owing to cycles of magma recharge or discharge. Deformation of the unconsolidated cumulates led to migration of the silicic interstitial melt and controlled their bulk content in incompatible elements. This migrating liquid had similar trace element and isotopic compositions to synchronous 2 7 Ga dioritic to granodioritic plutons, forming a bimodal, post-metamorphic, magmatic suite with the Iguilid body. The interstitial melt extracted from the Iguilid mafic intrusion could have fed synchronous metaluminous intermediate to felsic plutons.