With our work on St Lucia done, we made our way bright and early to the airport for our early flight to the next target island, St Vincent. After a quick stop at the airport beach we boarded our mere 20 minute flight and arrived at our new accommodation overlooking the sea on the south coast of St Vincent.
We decided to spend the afternoon at Black Point, the site of the most mafic basalt in the Lesser Antilles arc. A trip through the tunnel on the headland, previously used to transport sugar, led to a spectacular exposure of the basaltic lava flows and deposits from older explosive eruptions. The lowermost unit in the sequence consists of multiple pyroclastic flow deposits separated by thin ashy horizons, followed by the Yellow Tephra formation (believed to have been erupted between 3600 and 4500 years bp) and finally the mafic basalt lava flow.
As well as sampling the basalts, which may represent magmas from near-mantle depths below St Vincent, we were able to collect several in situ cumulates and blocks of juvenile magma from the pyroclastic deposits. Majority of the cumulates found were of the “salt and pepper” type, consisting of white plagioclase and black amphibole crystals of similar sizes. Most of these cumulates also contained a small amount of olivine. These samples could potentially represent “non cumulate gabbros” – frozen equivalents of the final erupted magmas. Comparing the chemistry of the plagioclase in the cumulates and juvenile magmas from the same deposits will allow me to test this.
After another successful day of sampling (and avoiding being forced into buying coconuts from enthusiastic local salesmen) we retreated to the villa for a well deserved glass of local rum.