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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

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Plutonic xenoliths from St Vincent, Lesser Antilles, act as ‘windows into the deep’

Coarse-grained igneous rocks, sourced directly from a sub-volcanic magma chamber, provide glimpses into the prevailing conditions beneath an active volcano.

Tollan and co-workers used several different techniques to analyse the major and isotopic composition of mineral phases in cumulates sourced from the active volcanic island of St Vincent in the Lesser Antilles. Cumulates are igneous rocks comprise the first fractionating minerals that form from a crystallising melt. The combinations of minerals and their composition are modulated by the conditions imposed upon the magma when it is cooling, and thus each rock represents a unique ‘snapshots’ of magmatic evolution.

The study revealed that the rounded cumulate nodules are distinctively rich in anorthitic plagioclase and pargasitic hornblende, accompanied by fresh olivine and pyroxenes. The composition of the minerals indicates the cumulates formed at ~970 – 1150°C at 5-6 km below the Earth’s surface.

Cumulate xenoliths on the island of Bequia, Lesser Antilles.

Cumulate xenoliths on the island of Bequia, Lesser Antilles.

The paper concludes that all the cumulates collected from St Vincent formed from relatively evolved melts rich in calcium, aluminium and water. These types of magmas are produced from early crystallisation of mafic phases (olivine, clinopyroxene and Cr-rich spinel) before their low density facilitates ascent through the crust, where they stall and deposit the observed cumulus minerals in shallow magma chambers. Evidence from oxygen isotopes suggests the cumulates have a residence time of ~50,000 years before being entrained and fragmented by newly injected magmas, and transported to the surface during explosive volcanic eruptions.

Full reference: Tollan, PME, Bindeman, I & Blundy, JD (2012) ‘Oxygen and hydrogen isotope compositions of plutonic xenoliths from St. Vincent, Lesser Antilles Island Arc.’ Contributions to Minerology and Petrology, no. 163, pp. 189-208. http://dx.doi.org/10.1007/s00410-011-0665-9

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