ORIGIN OF A SCAPOLITE METAGABBRO IN BUCKS COUNTY, PENNSYLVANIA

Abstract

In Bucks County, Pennsylvania, a body of limestone was invaded by pre-Cambrian gabbro with practically no contact action. Much later, dynamic metamorphism transformed the gabbro into a metagabbro, composed chiefly of andesine and hornblende, which replaced the original augite. Lighter bands in the metagabbro, containing oligoclase feldspar, indicate the introduction of hydrothermal solutions. Within the quarry some of the metagabbro is interbanded with a scapolite metagabbro, consisting mainly of diopside and scapolite. The chemical composition of the two rock types is similar despite their distinct mineral suites. The scapolite metagabbro is considered the result of recrystallization of the metagabbro which took place at the time of dynamic metamorphism, aided by the introduction of small amounts of hydrothermal solutions. Coarse-grained, crosscutting bands of scapolite and diopside, isolated masses of scapolite and diopside, and oligoclase-diopside masses are also present within the limestone. Throughout, the composition of the scapolite is the same. All the rock masses within the limestone are boudins which lie along three parallel zones. This indicates that the original gabbro intruded the limestone in dikes, which, during later dynamic metamorphism, were drawn out into boudins and rotated in the limestone during a plastic stage of the deformation. Joints and a fault, roughly perpendicular to the regional foliation, suggest a still later brittle stage in the deformation, but the uniformity of the scapolite composition implies that the plastic and brittle stages of the deformation were connected, and both were generally contemporaneous with the hydrothermal activity.