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

Dome collapse block-and-ash flow deposits are exposed on the flanks of Morne Patates with mafic enclaves (2-10 cm) preserved within the dome blocks. Field and petrologic observations of the enclaves reveal oval shapes, disequilibrium textures in plagioclase, lobate margins with host phenocrysts protruding into the enclaves, and irregular-shaped voids within the fine-grained matrix of the enclaves. The felsic dome blocks are porphyritic, containing an assemblage of pl + cpx +/- Fe-Ti oxides, exhibiting a common mineral assemblage with their more mafic enclaves but differing in their mineral proportions. Geochemically, the enclaves are basaltic andesites having ~ 57 wt. % SiO₂ with correlative variations in other major and trace elements with their dacite (~ 64 wt. % SiO₂) hosts. One host-enclave pair is basaltic andesite and basalt in composition with 57 and 48 wt. % SiO₂, respectively. Mafic enclaves display enriched REE patterns with some enclaves displaying higher concentrations than their respective host rocks.

Whole-rock Pb, Sr, and Nd isotope ratios indicate that the Morne Patates host dacites fall within the range of those of nearby Morne Plat Pays volcano, suggesting a petrogenetic relationship between the two volcanoes. However, the mafic enclaves are isotopically distinct from both their respective host rocks and from basalts from other eruptions on Dominica. Incompatible trace element ratios of the mafic enclaves and host rocks such as Th/Nb and Ba/Th indicate that the magma source beneath Morne Patates was enriched by subducted local sediment addition rather than by fluids from the dehydrating subducting slab. Isotope mixing models illustrate that a mixture of depleted mantle with ~1% sediment of varying proportions from the area of the Deep Sea Drilling Project Site 543 and from the island of Barbados can explain the isotopic variations exhibited amongst the mafic enclaves, between the enclaves and their hosts, and other volcanic centers on Dominica.

Together, these results suggest that the enclaves represent globules of a more mafic magma that was injected into and mingled with the host felsic reservoir. The isotopic variability of the mafic magmas suggests that multiple geochemically and isotopically distinct reservoirs may comprise the Morne Patates plumbing system.