MAFIC ENCLAVES AND EVIDENCE FOR MAGMA MINGLING AT MORNE PATATES VOLCANO, DOMINICA, LESSER ANTILLES

Morne Patates is the youngest volcano on the island of Dominica, Lesser Antilles and is situated amongst a cluster of 12 lava domes. Approximately 28 ka ago, the nearby, larger Plat Pays stratovolcano underwent a catastrophic collapse of its southwestern flank. Subsequently, from within the sector collapse scar, Morne Patates volcano and several lava domes emerged. Together, with the Plat Pays stratovolcano, they comprise the Plat Pays volcanic complex (PPVC) at the southern end of the island. Volcanic activity in the PPVC represents the youngest eruptions on Dominica, the most recent of which emanated from Morne Patates 450-685 years ago.

Dome collapse block-and-ash flow deposits are exposed on the flanks of Morne Patates. Mafic enclaves (2 to 10 cm) are commonly found within the lava dome blocks. Field and petrological observations include oval shapes, sieve and embayment textures in plagioclase, crenulated 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 plagioclase + orthopyroxene + clinopyroxene +/- Fe-Ti oxides, exhibiting a common mineral assemblage with their more mafic enclaves but differing in their mineral proportions. Geochemically, the enclaves are basalts and basaltic andesites having 48 to 57 wt. % SiO₂ with correlative variations in other major and trace elements with their felsic hosts (dacites; 63 to 64 wt. % SiO₂). Host dacites and their enclaves have almost identical REE patterns characterized by enrichment of LREE over HREE, flat and weakly fractionated HREE, and no Eu anomaly. Together, these observations suggest that the enclaves are globules of a more mafic magma that was injected into and mingled with the host felsic magma. Ongoing Sr, Nd and Pb isotopic analyses of enclave and host pairs will further constrain their potential genetic relationship and help determine whether the enclaves represent a mafic recharge event during the life of the volcano or if they are derived from another source, such as the larger Plat Pays volcano or from one of the nearby mafic volcanic centers, thus providing insight into the structure of the magmatic system operating beneath Morne Patates.