The Velfjord plutons are mafic-intermediate plutons in the Bindal Batholith that were emplaced into pelitic and calcareous rocks of the Helgeland Nappe Complex of the Uppermost Allochthon at ~448 Ma. Al-in-hornblende barometry suggests emplacement in the range of 6 to 8 kbar, which is consistent with the presence of magmatic epidote. Pelitic rocks in the aureoles underwent partial melting and leucosomes locally accumulated adjacent to the plutons to form "contact granites". The result of the garnet-aluminosilicate-silicia-plagioclase (GASP) thermobarometry combined with a recently published petrogenetic grid for dehydration partial melting of pelites suggest a clockwise P-T -t path for the wall rocks. The GASP results, plus the presence of early kyanite, suggest initial melting in the 7 to 8 kbar range. Temperatur is close to the plutons isothermal decompression to ~4 to 5 kbar resulted in the hottest wall rocks reaching cordiente stability while they were still partly molten. Cooler wall rocks could not reach conditions of cordierite stability because of the positive slope of the cordierite-in equilibrium. The state of HNC crust prior to pluton emplacement cannot yet be determined. However, rapid exhumation of the plutons and their wall rocks suggests that the youngest plutons of the Bindal Batholith were probably emplaced at lower pressures.
Calvin G. Barnes, Department of Geosciences, Texas Tech University, Lubbock, TX 79409-053, USA;
Tore Prestvik, Department of Geology and Mineral Resources Engineering, Norwegian University of Science and Technology, Høgskoleringen 6, N-7491 Trondheim, Norway