Layered gabbros and peridotites from Piton des Neiges volcano, La Réunion Island

Abstract

We conducted a petrological study of the layered gabbros and peridotites outcropping in the cirque of Salazie, combining field investigations, geochemistry, petrography and, mineralogy to constrain the magmatic evolution of Piton des Neiges volcano (La Réunion). Three new hectometer-long outcrops are reported here (Sites 1, 2 and 3) in addition to the gabbroic series (hereafter named Site 4) previously described in the literature. The Sites 1 and 2 are located in the upper parts of the Mât River and show dunite and wehrlite units with no apparent foliation, lineation, or layering. The Sites 3 and 4 are located in the lower parts of the river and are made of olivine-gabbro, gabbro and ferrogabbro units showing marked foliation and lineation, and abundant layering. The mean composition of primary phases varies between Sites 1–2 and Sites 3–4: Fo89.5 to Fo69.8 in olivine, Mg# of 90 to 69 in clinopyroxene, and Mg# of 44 to 0 in Cr-spinel, titanomagnetite and, ilmenite, respectively. Here we show that the layered gabbros and peridotites exposed at the four sites represent multiple levels of magma storage. Based on the results of previous gravimetric studies and a drill hole related to geothermal exploration, we propose that the gabbroic series belongs to a magmatic system predating the building of Piton des Neiges and centered underneath the cirque of Salazie. The high volume of erupted materials in the NE quadrant of the volcanic edifice, and the southward magmatic flow direction in the layered gabbros suggest that the initial eruptive center of Proto-Piton des Neiges was above the cirque of Salazie, ~3 km north of the present-day summit. The landscape of Piton des Neiges may have changed when its NE flank experienced a major lateral collapse, after complete solidification of the gabbroic bodies. The magmatic activity, then, shifted southward after the flank collapse. Finally, we suggest that peridotites outcrops are associated with the present-day Piton des Neiges.