Mid-crustal Magma Mixing and Forced Garnet Stability in a Caledonian Pluton, North-central Norway

The Svarthopen pluton intrudes high-grade metasedimentary rocks of the Helgeland Nappe Complex, north-central Norway. It is undated, but is considered to be part of the ~448-445 Ma suite of gabbroic to monzonitic Velfjord plutons. These plutons are associated with melting of metapelitic aureole rocks to form diatexitic migmatites and porphyritic, garnet-bearing peraluminous granites. Mixing of dioritic magma with peraluminous granitic magma is well displayed in the Svarthopen pluton. Mafic rocks in the pluton range from biotite pyroxene diorite and gabbro to pyroxene-bearing biotite hornblende quartz diorite. The pluton is intruded by, and mingled/mixed with, garnet-bearing, Kspar-megacrystic granite. Hybrid rocks in the mingling/mixing zones are predominantly quartz dioritic to tonalitic, with amphibole + garnet + biotite. In the hybrids, plagioclase cores reach An75 and rims in apparent textural equilibrium with amphibole and garnet are as low as An25. Hornblende is aluminous (2.9 to 3.5 Al pfu) and application of Al-in-hornblende barometers yields unreasonably high P (>1.0 GPa). Compared to garnets in coeval peraluminous granites, garnets in the hybrid rocks are richer in grossular component (15 to 20 versus 2 to 6 mol.%) and poorer in almandine component (66 to 70 versus 75 to 85 mol.%). Garnet-hornblende barometry gives P in the 600 to 800 MPa range. Three quartz-bearing diorites have 87Sr/86Sr values (448 Ma) from 0.7082 to 0.7101; a gabbroic sample is 0.7057. δ18O (per mil, VSMOW) values range from +8.3 to +9.3. In contrast, local K-spar megacrystic granites have initial 87Sr/86Sr and δ18O of ~0.7189 and +10 to +12 per mil, respectively. By comparison, non-hybridized diorites from the nearby Akset-Drevli pluton have initial 87Sr/86Sr as low as 0.7057 and δ18O = +6 per mil. Simulations of equilibrium crystallization of garnet-bearing hybrid rock compositions at 700 MPa predict that garnet + plagioclase + cpx should be stable well above the solidus. The "garnet rare earth element signature" in intermediate igneous rocks is commonly taken to indicate high pressures (deep crustal conditions) of melting and/or differentiation. However, garnet-bearing hybrids of the Svarthopen pluton show that garnet may be stable in such bulk compositions in the middle crust. Fractional crystallization of these hybrids should produce daughter magmas with a garnet signature, even though fractionation was in the middle crust.