The Petrology Of A Triassic Diabase Intrusion Near Frederick, Maryland

Abstract

Excellent exposures of an 80 meter wide, 40 kilometer long vertical diabase intrusion near Frederick, Maryland, were extensively studied. This diabase has been classified as a low-TiO2 (approx. 0.80 wt. % TiO2) quartz-normative (approx. 0.9 wt. % quartz) tholeiite and correlates with the quartz-normative Triassic diabases common in northeastern United States (Weigand and Ragland, 1970) and, more specifically, the 0.80 wt. % TiO2 falls within the 0.60-0.85 wt. % range typical of the Rossville-type of Triassic diabase in Pennsylvania (Smith, 1973). The presence of centimeter-sized plagioclase (An90+5) phenocrysts and the absence of olivine and presence of quartz in the CIPW norms also indicates a correlation with the Rossville type. The intrinsic oxygen fugacity was studied using the techniques of Buddington and Lindsley (1964) and Sato (1971). Good agreement was found between the two methods; for example, at 1100 C, the fO2 determined using both techniques was 10^-12+/-0.5 atmospheres at one atmosphere total pressure. The one atmosphere dry solidus was found to be at 1005+/-5 C and the one kbar and ten kbar wet (3 wt. % H2O) solidus were at 885+/-15 C and 800+/-15 C respectively. The one atmosphere dry liquidus was found to be at 1210+/-5 C, and the one kbar wet (3 wt. % H2O) liquidus was at 1165+/-15 C. The ten kbar wet liquidus although not within the range of the equipment used, was determined to be above 1150 C. Using the pyrolite model of Green and Ringwood (1967), the Frederick disbase is the result of differentiation of an olivine tholeiite in a magma chamber within 25+/-5 kilometers of the surface, which was subsequently intruded near the surface during the initial stages of the opening of the Atlantic Ocean in the early Mesozoic.