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Cumulate xenoliths betray small scale changes in melt composition and magma storage conditions

A texturally diverse suite of cumulates beneath Grenada, Lesser Antilles, are produced at shallow depths and show marked differences from comparable rocks in the same volcanic arc

Primitive melts produced beneath island arc volcanoes are rarely erupted at the surface in their original form, instead charting a huge variety of evolved compositions and testifying to the influence of intracrustal processing during magmatic ascent. The study of cumulates (coarse-grained igneous rocks) that sample directly from magma storage regions offers a chance to glimpse a ‘snapshot’ of this magmatic evolution.

A new CRITMAG-funded study by Stamper and co-workers combines major element analysis of mineral compositions in plutonic xenoliths and volcanic rocks with data from previous experimental studies. The data is used to explore the differentiation of mantle-derived magmas beneath volcanic island of Grenada, Lesser Antilles.

Photomicrograph (PPL) of poikilitic hornblende gabbro. Hornblende oikocrysts containing inclusions of clinopyroxene, spinel and iddingsitised olivine, with interstitial plagioclase.

Photomicrograph (PPL) of poikilitic hornblende gabbro. Hornblende oikocrysts containing inclusions of clinopyroxene, spinel and iddingsitised olivine, with interstitial plagioclase.

They find that observed diversity in cumulate assemblage and texture is caused by variability in parental melt composition and post-cumulus interaction with hydrous evolved melts. The whole plutonic suite is produced in a narrow pressure window (P = 0.2 – 0.5 GPa) at ∼ 850 – 1050◦C, tracing a shallow (depth ≤15km) section of a vertically extensive volcanic system. Major element barometers and experimental phase relations indicate that the source magma underwent equilibration with a garnet lherzolite source at depth of ≥55 km.

Grenada cumulates are notably different from those found on the neighbouring island of St Vincent, which lies only 120 km to the north. At Grenada, lower magmatic H2O contents are manifest are in plagioclase-rich cumulates and aluminous spinels. The contrast in assemblages and mineral chemistry of cumulate xenoliths from the two islands demonstrate the effect of small scale changes in melt composition and magma storage conditions.

Stamper CC, Blundy JD, Arculus RJ, & Melekhova E. (2014) ‘Petrology of Plutonic Xenoliths and Volcanic Rocks from Grenada, Lesser Antilles’. Journal of Petrology, 55(7), 1353-1387. http://dx.doi.org/10.1093/petrology/egu027

SCIENTIFIC 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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