Gravity Investigations of Subsurface Shape and Mass Distributions of Granite Batholiths

Abstract

Because granitic plutons usually show negative density contrasts with country rocks, they cause moderate to large negative gravity anomalies whose interpretation yields quantitative estimates of subsurface shape. Gravity anomalies suggest that many postkinematic granites extend to depths of about 10 km, which is one third the average continental crustal thickness, whereas some are much thinner. Granite contacts characteristically slope outward, and some granites show strong internal variations in density. Some granites, such as the Mull and Skye granites of Great Britain, are associated with large mafic plutons. Observable structural accommodation appears to be too small to explain emplacement of many large, thick postkinematic granites primarily by forcible intrusion. Stoping may, therefore, be the effective mechanism. Gravity profiles observed across granite batholiths are compared with computed profiles across a large number of hypothetical mass distributions. Negative gravity anomalies show that mass has been removed during the emplacement of most granitic plutons and indicate that the displaced mass of country rocks is neither immediately beneath the pluton nor around its margins. The gravitational effect of displaced country rocks is not apparent in observed gravity profiles. This creates a serious problem for emplacement by magmatic sloping or by granitization which may be overcome: (1) if the displaced material has sunk to great depths, (2) if it is dispersed during emplacement, or (3) if its gravitational effect is negated by a low-density root beneath the granites. Magma could originate: (1) in a granitic layer in the crust, (2) by partial fusion of the lower crust, or (3) in the mantle. Gravity profiles are computed for mass distributions which might represent each of these hypotheses, and the hypotheses are evaluated on the basis of the gravity profile. Gravity interpretation demonstrates that some areally large granitic plutons are thick (Dartmoor Granite), that some large plutons are thin (Fla Granite), and that some plutons are associated with large volumes of mafic rock (Skye granites). Gravity interpretation is a necessary approach to petrologic studies and can lead to a quantitative petrologic interpretation.