Jackson, Trevor A.; Lewis, John F.; Scott, Peter W.; Manning, Paul
Author Affiliation, Ana.
Department of Geography and Geology
The petrology of Lamprophyre Dikes in the Above Rocks granitoid, Jamaica: Evidence of rifting above a subduction zone during the early tertiary
Caribbean Journal of Science
Date of Publication
Over 50 lamprophyre dikes, varying in width between a few centimeters to several meters, have been identified from previous work in the Above Rocks Inlier of East-Central Jamaica. Most of these dikes are amphibole-lamprophyres, with minor occurrences of mica-lamprophyres. The amphibole-lamprophyres contain phenocrysts of calcic amphibole ± albite and pyroxene. The groundmass contains calcic amphibole, plagioclase and alkali feldspar, quartz, sphene, magnetite, barite, siderite, and pyrite. The amphibole-lamprophyre dikes are classified as spessartites and the mica-lamprophyres as minettes. Their mineralogy and chemistry indicate that they are calc-alkaline lamprophyres (CAL) that are spatially and temporarily associated with the high-K, calcalkaline Above Rocks granitoid. The dikes range from primitive (minette) to highly evolved (spessartite) lamprophyres, having SiO2 from 49.47% to 60.30%, atomic Mg# from 0.65 to 0.43, Ni from 384 to 57 ppm, and Cr from 484 to 68 ppm. The lamprophyres post-date the intrusion of the granitoid which has a minimum age of 60.4 ± 3.4 Ma. The intrusion of the lamprophyres is interpreted to the onset of rifting above a subduction zone. Crustal extension continued into the early Eocene with the opening of the Wagwater Trough and the associated extrusion of the intra-plate tholeiitic basalts of the Halberstadt Volcanic Formation. The geochemistry of the lamprophyres and basalts are consistent with a source region in the subarc mantle that was subduction-modified and lithologically heterogeneous.....