Jackson, Trevor A.; Scott, Peter W.
Author Affiliation, Ana.
Geography and Geology; Faculty of Pure and Applied Sciences
The mineral and rock chemistry of the Above Rocks pluton, Jamaica: New information on emplacement and petrogenesis
Caribbean Journal of Science
Date of Publication
The Upper Cretaceous plutonic rocks of the Above Rocks inlier are comprised of granite, granodiorite, quartz monzodiorite, monzodiorite and diorite. Dykes, sills, and stocks of granophyre, lamprophyre and aplite pervade these rocks. Mafic microgranular enclaves (MME) contained within the host rock are generally ellipsoidal and range in size from about 5mm to 500mm. MME mineralogy is similar to that of the host granitoid but the proportion of minerals is different, with hornblende and compositionally zoned plagioclase feldspar being more abundant in the former. Microprobe analyses of the host rock minerals reveal that the amphiboles range from magnesio-hornblende through actinolitic-hornblende to actinolite. The petrography, mineralogy and geochemistry indicate that the granitoid magma appears to have formed from the mixing of at least two magmas of contrasting composition. One of these, a mafic magma, represented by the MME and the xenocrysts, mixed with a more evolved magma. Mixing of these magmas apparently commenced within the crust prior to its final emplacement at a depth of less than 10 km. It is envisaged that the underplating by mantle-derived magma to the base of the crust, contributed to crustal thickening of the volcanic arc during the late Cretaceous.....