EVIDENCE OF MAGMA MINGLING AND SOURCE HETEROGENEITY AT MORNE PATATES VOLCANO, DOMINICA, LESSER ANTILLES

Lava dome collapses produced block-and-ash flow deposits exposed on the flanks of Morne Patates volcano. These deposits include mafic enclave-bearing dome blocks. Field and petrographic observations of the mafic enclaves reveal they are oval-shaped, have lobate margins with host phenocrysts protruding into the enclaves, irregular-shaped voids within their fine-grained matrix, and plagioclase with disequilibrium textures. The mafic enclaves contain an assemblage of pl + cpx + opx ± Fe-Ti oxides and have a common mineral assemblage with their more felsic and porphyritic hosts but differing in their mineral proportions. Geochemically, the enclaves are mostly basaltic andesite incorporated within dacite hosts; one enclave-host pair is basalt within basaltic andesite. Both enclaves and their hosts have typical subduction signatures. However, some enclaves display higher incompatible trace element concentrations than their respective host lavas.

Whole-rock Sr, Pb, and Nd isotope ratios in the Morne Patates host lavas are within the compositional range of nearby Morne Plat Pays volcano, suggesting a close petrogenetic relationship may exist between these two volcanoes. The mafic enclaves, however, are isotopically distinct from both their respective host lavas and basalts from other volcanic centers on Dominica. The mantle source beneath Morne Patates was enriched primarily by the addition of subducted sediment melts with a minor amount of hydrous slab fluids, as indicated by trace element and isotope modeling of the mafic enclaves and host lavas. Isotope ratio mixing models show that the isotopic variations exhibited amongst the mafic enclaves, between the enclaves and their hosts, and other volcanic centers on Dominica can be modeled by the mixing of depleted mantle with ~2% sediment melt from the area of the Deep Sea Drilling Project Site 144.

Collectively, these results suggest that the enclaves found at Morne Patates volcano represent undercooled globules of more mafic magmas that were injected into and mingled with a felsic host reservoir, demonstrating the importance of magma recharge in an evolving magmatic plumbing system. The isotopic variability of the mafic magmas suggests that the Morne Patates plumbing system may comprise multiple geochemically and isotopically distinct reservoirs.