Mafic magma intraplating: Anatexis and hybridization in arc crust, Bindal Batholith, Norway

The dioritic Velfjord plutons (∼ 448 Ma) were emplaced into regional migmatitic metapelitic and metacarbonate rocks at midcrustal levels, corresponding to pressures of ∼ 700 MPa. Exhumation to ∼ 400 MPa began while the migmatites were in a partly molten state. With increasing proximity to the plutons, regional stromatic migmatites change to diatexite, and diatexitic dikes are common within 500 m of the contacts. We interpret these relationships to indicate that heat from the plutons resulted in contact migmatization in a zone up to 1 km wide. Typical residual mineralogy in the diatexites is plagioclase + quartz + biotite + garnet + sillimanite ± K-feldspar, consistent with biotite dehydration melting. Pod- and dike-like leucosomes consist of two types: earlier high-K (granitic) ones with mineral assemblages identical to the migmatites and later low-K (tonalitic) ones in which sillimanite is sparse and garnet absent. The high-K leucosome magmas can be explained by biotite dehydration melting at 700 MPa. Within the aureole, mafic magmas were locally injected into, and hybridized with, the diatexites and the high-K leucosome magmas. In contrast, the low-K leucosomes are thought to result from local, late-stage remelting of H₂O-saturated diatexite. The H₂O-rich fluid was probably released from intergranular melt trapped in the diatexites during exhumation and solidification. Distinctive porphyritic 'contact granites' are common at pluton contacts. Although the mineral assemblage of these granites is identical to that of the diatexites, their isotopic compositions are distinct, with ε_Nd and δ¹⁸O in the migmatiles from - 7.6 to - 9.6 and from + 10.9‰ to + 13.5‰, and in the contact granites from - 5.2 to - 7.5 and from + 9.6‰ to + 12.3‰, respectively. Thus, the contact granites could have a source that is isotopically distinct from, but mineralogically similar to the diatexites, or they could result from mixing of magma similar to the high-K leucosomes with dioritic magmas. Mass balance calculations are consistent with the latter interpretation, with proportions of granitic to dioritic magmas from 7:1 to 7:3, Emplacement and solidification of the dioritic plutons provided zones of structural anisotropy along which high-K leucosome magmas and contact granite magmas collected. These magmas were injected by additional dioritic magma and further hybridized. Because the solidi of the plutons were several hundred degrees higher than that of the granitic magmas, the pluton walls acted as long-lived, hot, rigid surfaces along which magmas collected and migrated.