Geology and Petrogenetic Constraints of a Peraluminous Biotite Granite, North-Central Norway

The island of Ylvingen in north-central Norway is underlain by ~476 Ma (Barnes et al., 2007) plutonic rocks and their host amphibolite-grade metasedimentary rocks of the Helgeland Nappe Complex. These granites are part of a larger plutonic system that is dominated by a ~475 Ma (Barnes et al., 2007), enclave-rich peraluminous granodiorite that underlies the nearby island of Vega. On Ylvingen, biotite ± muscovite granite and garnet biotite ± muscovite granite are present. Most samples are peraluminous and magnesian, and range from alkali-calcic to calc-alkalic. Silica contents are 64.5–69.5 wt % and overlap between the two units. The contact between biotite granite and garnet biotite granite is gradational, whereas the contact between biotite granite and Vega granodiorite is sharp and oriented subparallel to magmatic foliation.

Host rocks consist of interbedded psammites, semipelites, and conglomerates and calc-silicate schist interbedded with marble. Prograde assemblages in the semipelites include quartz + feldspar ± biotite ± garnet ± muscovite ± sillimanite ± kyanite ± staurolite, The contact between the plutonic and metamorphic rocks is covered, but is subparallel to foliation in the host rocks. Enclaves in the pluton consist of diatexite, calc-silicate schist, marble, gneiss, and surmicaceous enclaves, range in size from a few centimeters to several meters, and have lenticular to rectangular shapes. Diatexite consists of quartz + feldspar + biotite ± garnet ± muscovite ± kyanite ± sillimanite ± staurolite. The presence of metacarbonate xenoliths and similar prograde assemblages in the diatexite and host rocks suggest that the pluton–host-rock contact is intrusive.

Bivariate plots generally produce scattered arrays, but some are linear. Arrays on element ratio plots are not hyperbolic, thus ruling out magma mixing. The degree of scatter on many element-element plots suggests that parental melts segregated from multiple sources and accumulated at the level of exposure.