GRANITE FORMATION DURING INTERACTION OF MAFIC MAGMAS AND MIGMATITIC ROCKS, HELGELAND NAPPE COMPLEX, NORWEGIAN CALEDONIDES

Emplacement of the ~448 Ma dioritic Velfjord plutons caused contact melting of pre-existing (~475Ma) pelitic migmatites at mid-crustal levels, ~700 MPa. This second anatectic event affected rocks at least one km from the plutons and was followed by rapid exhumation to about 400 MPa. Regional migmatites are stromatic, whereas contact anatectic migmatites become progressively diatexitic near the plutons. Local zones of Kspar-phyric quartz monzonite ("contact granites") separate the migmatites from the plutons and reach 200m in width. Residual mineralogy in the diatexites (pl+qtz+bi+gt+sill±Kspar) is consistent with biotite dehydration melting. Pod- and dike-like leucosomes consist of early high-K granite formed by biotite dehydration melting and later low-K leucotonalite formed by H2O-saturated melting, probably as H2O exsolved from intergranular melts during exhumation. Dioritic magmas were locally injected into, and hybridized with, the diatexites and leucosomes. Although identical in mineralogy to the migmatites, the porphyritic contact granites are distinct in epsilon Nd and delta 18O (-7.6 to -9.6 and +10.9‰ to +13.5‰ in the former and -5.2 to -7.5 and +9.6‰ to +12.3‰ in the latter). This discrepancy can result from mixing high-K leucosomes with dioritic magmas in proportions of 7:1 to 7:3. We suggest that this mixing occurred in the migmatite zone around the plutons, from which the more mobile hybrid contact granite magmas migrated to zones of low strain adjacent immediately adjacent to the pluton. Because the plutons' solidi were several hundred degrees higher than the granitic magmas, the pluton walls acted as long-lived, hot, rigid surfaces along which magmas collected and migrated. This relationship shows that in middle crustal levels mafic magmas/plutons can provide heat, material, collection zones, and pathways for the formation and transfer of granitic magmas. The volume of the granite produced and transferred is a function of the volume and the dimensions of the mafic heat source as well as the fertility of the crustal source.