Origins and Scales of Compositional Variations in Crustally Derived Granitic Rocks: The Example of the Dartmoor Pluton in the Cornubian Batholith of Southwest Britain

The ca. 280 Ma, postorogenic, S-type Dartmoor pluton was assembled from numerous sheets of granitic magma, emplaced into the shallow crust. The main magma source lies in the middle crust and is most probably Proterozoic metagreywackes, with minor metapelites and metavolcanic or plutonic rocks, possibly formed in a syncollisional environment. Partial melting of this source may have occurred under fluid-deficient conditions, and the magmas most likely had relatively high initial H2O contents. The pluton contains substantial, whole-rock-Sr and quartz-O isotope heterogeneities on scales down to a meter or less, and such small-scale heterogeneities are probably common in granitic intrusions derived from heterogeneous protoliths. Thus, variations in source terranes may not be fully captured with the sample numbers and scales commonly applied in studies of granitic plutons. The preservation of both large- and small-scale isotopic heterogeneities suggests that the Dartmoor magmas were never efficiently homogenized by flow-driven mechanical mixing. This implies a source terrane with lithological variations on scales of tens of meters or less. The granitic rocks form five texturally, chemically, and isotopically distinct groups, each of which had somewhat different sources or mixtures of sources. The main chemical variations cannot have been formed through fractionation of any combination of the major minerals in the rocks. Instead, entrainment of variable proportions of peritectic plagioclase, orthopyroxene, and ilmenite was responsible, together with local crystal fractionation. Low-density, late-magmatic melts and aqueous fluids produced patchy enrichment in light elements and extreme enrichment in some of the highly silicic, two-mica microgranites. However, although they are also enriched in light elements, the “aplites” were not produced through fractionation and seem to have had independent magmatic origins.