In Situ Assimilation Tracked By Trace Element Variation in Augite: Sausfjellet Pluton, North-Central Norway

The Sausfjellet pluton is a sub-circular, 445 Ma pluton in the Bindal Batholith. It was intruded into mid-crustal level (~700 MPa), high-grade rocks during Taconian assembly of the Helgeland Nappe Complex. Stage 1 of the pluton is massive pyroxene hornblende gabbro with poikilitic hornblende. Stage 2, the subject of this study, occupies the central and western parts of the pluton and is gradationally zoned from central, layered, xenolith-rich hornblende biotite two-pyroxene diorite + anorthosite, to western, massive, xenolith-poor hornblende biotite three-pyroxene quartz monzonite. Bulk-rock compositions are consistent with widespread accumulation of pyroxene + plagioclase.

In order to separate effects of accumulation from assimilation, trace element zoning profiles in augite were used as a proxy for changes in melt composition. Augite is normally zoned, with incompatible elements increasing from cores to rims. However, trace element variation among samples plots as multiple trends that can be resolved into two broad groups that correspond to the central and western zones. Augite in the layered central zone has lower abundances of REE, Pb and Hf and higher abundances of Cr, V, Ni, Sr, and P compared to the western zone. The two trends intersect at low concentrations of incompatible elements such as Zr.

Calculation of melts in equilibrium with augite, combined with AFC-type modeling, suggest that variation in augite Zr contents requires assimilation of Zr-rich rocks in all Stage 2 rocks. In the western zone, increases in Zr, Hf, REE and Pb suggest assimilation of Neoproterozoic gneisses that are host to the western zone; this conclusion is consistent with bulk-rock δ¹⁸O values.

We conclude that assimilation of host rocks was widespread in this mid-crustal magma body, and that augite preserves a trace element record of differentiation (AFC) that is difficult to identify using bulk-rock compositions alone.