Petrology of Plutonic Xenoliths and Volcanic Rocks from Grenada, Lesser Antilles

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Abstract

Grenada is the southernmost island in the Lesser Antilles arc, a chain of subduction-related volcanoes distinguished by its diversity of magma composition and unusually abundant plutonic xenoliths, many with cumulate textures. We have determined the mineral compositions of a newly collected, extensive suite of plutonic xenoliths from Grenada and examined their relationship with the lavas in an attempt to explore the role of intra-crustal processes on magmatic evolution. The plutonic assemblages are dominated by mafic phases with abundant hornblende and clinopyroxene, and include the only known plagioclase-free examples in the Lesser Antilles. Bulk compositions are unlike those of natural silicate melts and are consistent with the majority of the xenoliths having a cumulate origin. Experimental and thermobarometric evidence shows that the entire cumulate suite can be generated in a narrow pressure range (0·2–0·5 GPa) with different assemblages resulting from small variations in melt chemistry and temperature. Temperature estimates are consistent with the observed crystallization sequence of olivine → clinopyroxene → hornblende → plagioclase. A spinel phase is present throughout ranging from Cr- to Fe3+-rich. The crystallization sequence requires elevated magmatic H2O contents (∼7 wt % H2O) sufficient both to suppress plagioclase crystallization and to render this phase extremely rich in anorthite upon appearance; this is a characteristic of many island arc settings. Studied lavas from the M- and C-series span picrites and ankaramites to hornblende- and orthopyroxene-bearing andesites. MELTS modelling confirms experimental hypotheses that the two lava series can be derived from a common picritic magma, with M-series differentiation occurring in the uppermost mantle (∼1·4–1·8 GPa) and C-series in the shallow crust (∼0·2 GPa). Plutonic xenoliths from Grenada are notably different from those of the neighbouring island of St Vincent, the respective assemblages and mineral chemistry demonstrating the effect of small-scale changes in melt composition and magma storage conditions between these two islands. We suggest that the unusual petrological and geochemical characteristics of Grenada magmas are a result of proximity to the South American continent and associated localized thickening of the oceanic lithosphere. This increases the depth of magma generation and is reflected in the elevated LREE/HREE of the Grenada lavas, indicating that last equilibration with a garnet lherzolite source occurred at a depth of ≥60 km.

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