Enclaves and their bearing on the origin of the Cornubian batholith, southwest England

Abstract

Enclaves of diverse origin are present in minor amounts in the coarse-grained biotite granites of the Cornubian batholith, southwest England. The most common enclave type is layered, rich in biotite, cordierite and aluminosilicates, and has textures and compositions that reveal variable degrees of melt extraction from metasedimentary source rocks. Rare sillimanite-bearing enclaves represent residual material, either from the region of magma generation or its ascent path, but most such enclaves were probably derived from the contact aureole closer to the present level of exposure. These non-igneous enclaves (NIE) and their disaggregation products are present in all major plutons, comprising from < 2 to 5 vol.% of the granites. Enclaves of igneous origin are also present in all major plutons except Carnmenellis, generally comprising < 1 vol.% of the granites. The most common type is intermediate in composition, with microgranular texture, and mineral compositions and textures consistent with an origin by magma mixing. Large crystals of K-feldspar, plagioclase and quartz, common in these microgranular enclaves (ME) but absent in NIE, represent phenocrysts derived from the silicic end-member during magma mixing events rather than products of metasomatism as suggested previously. Although the composition of the mafic end-member (basaltic or lamprophyric) involved in the mixing process is poorly constrained, the presence of ME in the granites, and the preponderance of mantle-derived mafic rocks in the coeval Exeter Volcanics, indicate that mafic magma injection into the crust was a factor in the generation of the batholith. Advection of sub-crustal heat provides an explanation for large-volume crustal melting in regions of relatively thin crust such as southwest England.

The petrogenesis of the Early Permian Variscan granites of the Cornubian Batholith: Lower plate post-collisional peraluminous magmatism in the Rhenohercynian Zone of SW England

“…The younger granites of the G3-G4 series are considered to have formed by biotite-dominated partial melting, at higher temperatures and lower pressures, within the lower pressure, higher temperature cordierite stability field (Figure 9b) (e.g. Clark, 1995). The cordierite stability field is consistent with source tourmaline melting, which occurs at 750-775°C at 3 kbar (Holtz and Johannes, 1991); there is increased tourmaline within the G3-G4 granites relative to G1-G2 granites.…”

“…Stimac et al, 1995). However, the trends on major and trace element plots ( Figure 13) do suggest some mixing of a crustal melt with a mantle source, just not necessarily one represented by the geochemistry basalt or lamprophyres exposed within the study area.…”

“…Microgranular mafic enclaves (MME) were observed in all of the G3 granites and are described in detail by Stimac et al (1995) who tentatively suggested an origin by hybridisation of basaltic melts. Radiometric data indicate that lamprophyres, basalts and granites are contemporaneous (e.g.…”

“…Stimac et al 1995). Country rock xenoliths show sharp contacts with the granites and retain metasedimentary structures, such as a cleavage (Figure 3d).…”

Does the Meguma Terrane Extend into SW England?

“…Darbyshire and Shepherd 1994;Stimac et al 1995), this primarily represents an inherited basement signature. The data further suggest that the batholith was derived from a composite lower crustal source which, while probably older than the <1.2 Ga age estimate of Hampton and Taylor (1983) based on lead isotopes, is nevertheless likely to be of Mesoproterozoic age (Darbyshire and Shepherd 1994).…”