EXPERIMENTAL INVESTIGATION OF CRYSTALLIZATION TEXTURES: INTERPRETING THE JUWA PASS RHYOLITES AND THE SALIÑA MATIJS BASALTIC TRACHYANDESITES, FROM BONAIRE, LEEWARD ANTILLES

Bonaire, an island within the dissected Cretaceous arc in the Leeward Antilles, has a complex geological history (Wright & Wyld, 2011). Bonaire has a starkly different magmatic history than that of its current sister islands, Aruba and Curacao. Bonaire has more silicic/intermediate compositions within its bedrock whereas the other islands’ basements are mainly mafic. This study compares textures of igneous rocks found at Juwa Pass and Saliña Matijs, Washington Slagbaai National Park, northwestern Bonaire, to those formed in experiments in order to recreate the emplacement, cooling and crystallization environments of rhyolite and basaltic trachyandesite. Experiments ranged in time from 11 to 1,346 hours, with cooling rates ranging from to 2.24°C/hr. The samples created were analyzed by SEM (SE & BSE imagery) to study crystal growth patterns, and then compared with natural textures seen in thin sections. Samples were then compared to one another, finding which cooling rate best matched each sample. Bonaire’s Washikemba Group has no distinction on historical geologic maps (Westermann & Zonneveld, 1956) between the basaltic trachyandesite intrusions and rhyolite hypabyssal or surface flows, and my study aimed to find a similarity between the two through microtextural analysis. While surface flows tend to show more aphanitic textures than those of intrusions, the rhyolite has an aphanitic matrix composed of a felty, microlite textures, and the basaltic trachyandesites from Bonaire have an aphanitic matrix as well, however it consists of more phenocrysts throughout the matrix. Both the rhyolite and the basaltic trachyandesite have allotriomorphic matrixes, indicating a similarity in cooling rates, despite chemical differences and emplacement geometries. The rocks have more similarities to experimental samples with slower cooling rates, and the basaltic trachyandesite sample would need more time in cooling rate experiments to better mimic textures found in the natural rocks.