The mafic-silicic layered intrusions of Saint-Jean-du-Doigt (France) and North-Guernsey (Channel Islands), Armorican Massif: Gabbro-diorite layering and mafic cumulate-pegmatoid association

The Saint-Jean-du-Doigt (France) and North-Guernsey (Channel Islands) Intrusive Complexes (hereafter referred to as SJIC and NGIC, respectively) are examples of mafic-silicic layered intrusions in the Armorican Massif. Both are characterized by the occurrence of (1) a basal/peripheral gabbroic unit interlayered with sheets (generally dioritic in composition, occasionally gabbroic) and crossed by leucocratic diapirs and pipes (from monzodioritic to Q-monzonitic in composition), (2) peripheral pegmatoids associated with mafic cumulates and (3) coeval granitoids. Beside these main similarities, some contrasted features lead us to propose two distinct models of formation. The Variscan SJIC includes tholeiitic mafic rocks (monzogabbro) that locally mingle and mix with leucocratic components (monzonite or Q-monzonite). The Cadomian NGIC is calc-alkaline. The SJIC sheet-bearing gabbro is homogeneous from a petrologic point of view, whereas the NGIC exhibits gabbroic macrorhythmic sequences with mineral layering. The Sr-Nd isotopic compositions of the SJIC gabbros are significantly different from those of the associated dioritic layers. This is not the case in the NGIC where the magmas could be cogenetic. We argue that the SJIC gabbro was a liquid that crystallized in situ without significant crystal settling. By contrast, the rhythmic sequences of the NGIC are consistent with crystal accumulation. Subsequently, both can be seen as mafic reservoirs which were repeatedly invaded by magmas of intermediate composition. We interpret the sheets in the SJIC as the result of horizontal spreading of dioritic metastable magmas into a gabbroic reservoir crystallizing from below, at levels of neutral buoyancy. Injections and convection in the central part of the reservoir possibly resulted in spectacular mixing/mingling structures. In the NGIC, the emplacement of the dioritic sheets was rather controlled by pre-existing rhythmic cumulative structures. In both intrusions, late differentiated diapirs were extracted from the dioritic sheets. Associated peripheral pegmatoids are thought to result from the crystallization of liquids issued from a mafic intercumulus melt in the presence of a fluid phase. This extraction might have been enhanced by the disruption of the peripheral cumulate stack, perhaps following pressure