Our first full day in St Vincent began in a dried up river valley known as the Dry Rabacca, thanks to a tip off from Dr Paul Cole at VMSG in January. Little did we know that we would spend the next 3 and a half hours in this cumulate hunting paradise! A simple look at the floor within a few metres of the entrance led to a pile of cumulate finds, and exposures from the 1902 eruption of La Soufriere volcano at the edges of the valley immediately provided in situ cumulate samples.
The finds kept on coming, including incredibly useful cumulates with their host lava attached and unusual green lithics interpreted as mafic hornfels (which could represent crustal material assimilated below the arc!) By far the best find was a very large and heavy, approximately 20cm across in situ cumulate, “King Vincy” which contains varying crystal textures. I’ll add more info on this once I return from the field and have time to study it in detail. After 3 hours, we had found so many potential samples that we had to be selective and whittle them down to those with host lava attached or unusual textures/compositions.
After a long and successful morning, we stopped for lunch and a relaxing swim at Owia salt pond in the north of the island. A glance at the geological map revealed that the mafic looking rocks around the salt pond were in fact lava flows from flank eruptions at La Soufriere. A casual look around whilst swimming and eating led to us discovering cumulates in the mafic lava flows! Of course we took the opportunity to sample these and their host lavas, which don’t appear to have been geochemically analysed before. Their age is currently unclear, though some amateur local geologists we met on the beach were convinced that they were formed during the main 1902 event.
We ended the day by collecting one last mafic lava sample, from the basalts exposed at Big Sand Bay, before heading back to the villa to enjoy the sunset and catalogue the days finds – 35 samples in total (more than we collected in 2 days in St Lucia)!