Northeastern Section - 36th Annual Meeting (March 12-14, 2001)

The Hillstadfjellet pluton is the northernmost of the Velfjord plutons of the Bindal Batholith. It is part of the Helgeland Nappe Complex (HNC). Its U-Pb (zircon) age is 448± 2 Ma. The pressure of emplacement was ~7 kbar (middle crust). The Hillstadfjellet pluton was emplaced in two major stages. The first stage is mainly gabbroic. The younger, more voluminous Stage 2 is monzonitic but ranges from diorite to quartz monzonite. Contact relations between Stage 1 and Stage 2 are best exposed on the eastern side of the pluton at Lundvangbukta. Here, the transition southward from Stage 1 to Stage 2 consists of a contact breccia of Stage 1 gabbro with veins of Stage 2 monzonite and monzodiorite, then a zone of angular Stage 1 blocks, and then a zone of predominantly Stage 2 monzonite and monzodiorite with rounded Stage 1 blocks. In contrast, the contact between the high SiO2 and high K2O units of Stage 2 were not observed in the field. The Stage 1 rock has low SiO2 (<52%) and consists mainly of metagabbro and metadiorite. Stage 1 samples show greatly variable Al2O3 contents (12-25 wt%). Abundances of V, TiO2, Al2O3, and P2O5 also vary widely. Stage 2 contains two compositional units. One shows relative K2O enrichment and the other, relative SiO2 enrichment. The SiO2-enriched trend suggests hybridization of Hillstadfjellet magma with contact granites. K-enrichment is suggestive of a shoshonitic trend; however, unlike the typical shoshonites, the Hillstadfjellet pluton also shows Fe enrichment relative to Mg and has more variable K2O/Na2O than type shoshonites. This compositional trend is similar to shoshonites from the Marianas, rather than type shoshonites from Independence Volcano, Montana.