PETROGRAPHY OF THE MAFIC ENCLAVES WITHIN THE UTUADO PLUTON, PUERTO RICO

The Utuado pluton of Puerto Rico covers an area of ~250km² and contains abundant mafic enclaves. The pluton is composed of approximately 65% granodiorite and quartz-monzonite, 32% diorite and tonalite, and 3% gabbro. While previous studies have sought to explain the origin of the pluton, the mafic enclaves have been mostly neglected, described as identical in composition to the granodiorite [1] and as fragments of andesitic wall-rock [2]. Due to textural and compositional variability observed in the field, an investigation of the origin these mafic enclaves is warranted.

Cores of eleven different xenoliths were drilled in the field and constructed into thin sections. From these, textural observations, petrographic observations, and point count data were obtained. All enclaves demonstrated a poikilitic/heteradcumulate texture consistent with an accumulation of minerals at the bottom of the magma chamber that cooled slowly and formed a cumulate pile. Point counting data showed a large variability in the number of cumulate phases within the enclaves, inconsistent with previous suggestions of compositional homogeneity and consistent with a layered cumulate origin.

The results of this study indicate that the mafic enclaves found within the Utuado pluton are not the same composition as the parent body and are not wall-rock xenoliths. Instead, it appears that the mafic enclaves in the Utuado pluton were formed from a cumulate pile at the bottom of the magma chamber which was disrupted prior to the solidification of the pluton. This research has contributed much-needed insight to fulfill the gap regarding the formation of the xenoliths in this area. The conclusions obtained in this investigation can be extrapolated and compared with other local intrusive bodies to provide new knowledge of the magmatic evolution of the island. It also opens the way for future studies in this field that may leads us to a better understanding of the geologic evolution of Puerto Rico and possibly, the Caribbean.

[1] Weaver 1958, GSAB [2] Chen 1967