THE GABBRO-DIORITE LAYERED INTRUSIONS OF SAINT-JEAN-DU-DOIGT (FRANCE) AND BEAUCETTE (GUERNSEY, CHANNEL ISLANDS), ARMORICAN MASSIF: AN EMPLACEMENT MODEL

The gabbro-diorite bodies of Saint-Jean-du-Doigt (France) and Beaucette (North Guernsey) belong to the Mafic-Silicic Layered Intrusions (MASLI; cf Wiebe, 1996) group, defined by Wiebe (1996). Both are located in the Armorican Massif, but their formation is ascribable to different orogenies. The 570 Ma Northern Complex of Guernsey, has calc-alkaline affinities and is associated with a Cadomian volcanic arc. The 300 Ma Saint-Jean-du-Doigt intrusion, shows continental tholeiitic affinities and is related to late-Variscan extension.

At the base of the Saint-Jean-du-Doigt intrusion, diorite occurs as sheets within homogeneous gabbro. Sr and Nd isotopes data show that the diorite is not related to the gabbro by simple fractional crystallisation, which implies the existence of at least two contrasted parent magmas. To account for the field observations, we propose that several batches of diorite replenished the crystallizing gabbroic reservoir. Dioritic batches rose through the gabbroic mush to levels of neutral buoyancy, where they turned into horizontal sheets. Emplacement of these sheets was progressively higher as the gabbro crystallized. As the diorite layers cool, evolved residual liquid accumulated at the upper boundary of the sheet and then formed small granitic/pegmatitic diapirs.

The layered lower part of the Northern Guernsey intrusion (Beaucette) is more complex. Dark and grey gabbroic macrorythmic layers, which are lacking in Saint-Jean-du-Doigt, are present. The layers are crossed by dioritic veins, sheets, and inclined pipes. We propose that the macrorythmic units correspond to sequences of accumulation of ferro-magnesian phenocrysts (dark units), locally coupled with plagioclase crystals (grey units). The leucocratic veins and pipes from Beaucette should derive from Saint-Peter-Port–type layered cumulates. Both formations have An80 plagioclase, which suggests crystallization under high water pressures, as does the association of mafic pegmatoids with cumulate facies.

References.

Wiebe, R.A. (1996) Mafic-silicic layered intrusions: the role of basaltic injections on magmatic processes and the evolution of silicic magma chambers. Transactions of the Royal Society of Edinburgh: Earth Sciences, 87: 233-242.