The Grønfjellet unit – an alkaline volcanic complex of uncertain tectonic affiliation in the eastern Trondheim Nappe Complex, central Scandinavian Caledonides

The Trondheim Nappe Complex (TNC) of the central Scandinavian Caledonides is a key area for understanding the closure history of the Iapetus Ocean prior to the final collision between Laurentia and Baltica. In the western TNC, late Cambrian to early Ordovician oceanic arc formation, followed by arc–continent collision and ophiolite obduction onto a Laurentia-derived microcontinent, is well-documented. Following arc–continent collision, a mid-Ordovician phase of rifting has recently been identified, which produced a peculiar volcanic association of MORB-type basalts and a variety of alkaline, shoshonitic and ultrapotassic volcanic rocks. In the eastern TNC, the volcanic and tectonic evolution is less well constrained, but the Fundsjø Group is traditionally interpreted to represent an immature, ensimatic island arc of late Cambrian age.  

Recent field mapping, geochemistry, and air-borne geophysical work in the eastern TNC has identified a distinctive volcanic complex in the Grønfjellet area, previously mapped as part of the Fundsjø Group. The complex covers at least 7 km² and comprises a variety of rock types: (1) pyroclastic volcanic deposits with up to 20x10 cm large, subrounded, flattened, fine-grained clasts with feldspar and amphibole crystals in a matrix of similar composition, (2) fine-grained greyish rocks with mm-sized white feldspar aggregates/crystals and/or mm- to cm-sized amphibole crystals, with and without subtle compositional layering, (3) homogeneous, fine- to medium-grained feldspar- and amphibole-rich rock (“micro-gabbro texture”), and (4) very fine-grained, flinty, light-grey-greenish rocks with a homogeneous texture. Along its northern and eastern borders, the complex is associated with abundant marble layers; the western border is associated with brownish-weathering biotite-muscovite schists, whereas the southern continuation of the complex is still unclear.

Preliminary geochemical data from ten fine-grained samples of volcanic origin reveal a peculiar composition: they plot as alkaline rocks in the Nb/Y vs. Zr/Ti diagram; they are enriched in LREE as well as Th, U, Nb and Ta; they plot close to the MORB–OIB array in the Nb/Yb vs Th/Yb diagram; and they do not show significant negative Nb-Ta anomalies typical for island-arc or back-arc settings. Ranging in composition from trachybasalt, through basaltic trachyandesite to trachyandesite, they are very different from the typical island arc tholeiites and back-arc basin basalts of the Fundsjø Group metavolcanic rocks elsewhere, and are more similar to rift-related alkaline rocks from the western TNC. Age dating of the Grønfjellet rocks is ongoing, as is a comparison with newly acquired geochemical data from adjacent areas of the Fundsjø Group, in order to shed light on the tectonic affiliation of this volcanic complex.