MICROTEXTURAL ANALYSES OF INTERGRANULAR AND POIKILITIC MAFIC ENCLAVES FROM THE UTUADO PLUTON, PUERTO RICO

Mafic enclaves are essential to understanding the development and evolution of plutonic systems. Puerto Rico is a fossil island arc in the Greater Antilles with numerous plutonic bodies that record the island's extensive magmatic record. In this study, we focus on the Late Cretaceous Utuado pluton, which consists of 65% granodiorite and quartz-monzonite, 32% diorite and tonalite, and 3% gabbro while also hosting abundant mafic enclaves.

Thin sections of five enclaves from the granodiorite unit in the Utuado pluton were selected for study. Through petrographic and textural observations, we define two general types of mafic enclaves among the samples examined. Type 1 is a porphyritic micro-gabbro enclave with intergranular texture. This texture is defined by medium-grained plagioclase crystals forming triangular shapes with smaller interstitial mafic minerals such as pyroxene and biotite. Type 1 enclaves typically contain numerous large plagioclase phenocrysts (400 - 2500 µm). Type 2 is a micro-granitoid enclave with poikilitic texture. Poikilitic texture is characterized by smaller crystals (i.e., chadacrysts; 50-200 µm) enclosed by larger grains (i.e., oikocrysts). As observed in thin section, most oikocrysts are K-feldspar with pyroxene, amphibole, and/or plagioclase chadacrysts. Type 2 contains fewer phenocrysts than Type 1. While four of the samples exhibit dominantly one texture type, one enclave uniquely exhibits both Type 1 and Type 2 textures in equal proportions.

The presence of uniformly medium-grained interlocking crystals in the intergranular texture (Type 1) suggests a relatively moderate cooling rate, which enabled the plagioclase crystals to form a framework before the smaller mafic crystals were emplaced. Conversely, the poikilitic texture (Type 2) suggests a multi-stage cooling process wherein the smaller chadacrysts crystallized first. Then, interaction with a more felsic magma led to the crystallization of larger oikocrysts. The coexistence of both Type 1 and Type 2 within a singular enclave further suggests the possibility of more than two chemically distinct magmas interacting. This suggests that the Utuado pluton potentially underwent a multi-stage cooling process that incorporated at least three magmatic events as exemplified by the textural diversity recorded by the mafic enclaves.