Compositional diversity of three subduction-related basaltic to andesitic metavolcanic units along the Paleoproterozoic Tampere Belt (Finland)

Here, we report on compositional variation of 1.90-1.89 Ga metavolcanic units along the E-W-trending Tampere Belt. These volcanics have been suggested to have formed during subduction under the inferred ca. 2 Ga Keitele microcontinent in the north. This study is based on 74 samples, of which all were analysed for major and trace elements by XRF, 54 for selected trace elements also by ICPMS, and 17 for Nd isotopes. The studied units are at Vihteljärvi (in the west), at Hoivasvuori (ca. 25 km southeast of Vihteljärvi), and near Palvajärvi (80 km west of Vihteljärvi). In general, the rocks are characterized by basaltic to andesitic compositions and Ta&Nb depletion with respect to LILE, Th and LREE. Dispersions are mostly wide for major and LILE on Harker diagrams but all the units show increase in Th and Nb with increasing Zr. The ɛNd(1.9Ga) values vary from -1.1 to +1.7 and are similar to those in the bulk of 1.89-1.87 Ga plutonic rocks of southern and central Finland. Our data agree with the view that subduction-related volcanism of the Tampere Belt took place at or close to an active continental margin rather than at an oceanic island arc. The rocks from Vihteljärvi and Palvajärvi resemble basalts and andesites of present-day volcanic arcs. Those at Vihteljärvi are medium-K rocks. The rocks from Palvajärvi are, due to prominent secondary alteration, dispersed from low-K/medium-K boundary to shoshonitic field but, probably, most of them were originally of high-K type. These units show steep decrease in Cr with increasing Zr and Y, convex-upward P vs. Zr patterns, and Eu depletions. At similar Zr, the Palvajärvi rocks are significantly higher in P, LILE, LREE, Nb, and Th than those at Vihteljärvi. The ɛNd(1.9Ga) values at Vihteljärvi (two samples analysed) are ~ -1.1 while those from Palvajärvi (11 samples analysed) range from 0.2 to 1.4. The Hoivasvuori rocks are medium-K to high-K basalts and basaltic andesites. They compose a miscellaneous group with confusing features including wide dispersions on P, LREE and LILE vs. Zr diagrams and partly cross-cutting LREE patterns. The Ti and Nb contents increase with increase in Zr. Compared to the Palvajärvi rocks with similar Zr, the Hoivasvuori rocks are lower in Th but mostly higher in P, Ti, V, Sc, Y and Yb while LREE are relatively low. The contents of Cr are consistently quite high. The ɛNd(1.9Ga) values are 1.5 and 1.7. Compositional variations within the Palvajärvi and Vihteljärvi units may largely be attributed to fractional crystallization. The contrast in composition of these units can be mainly be explained by variable enrichment of mantle wedge by subduction fluxes and heterogeneous source regions of the magmas prior to subduction. The Nd isotopes possibly suggest a larger role for pre-1.9 Ga material at Vihteljärvi than at Palvajärvi. The high Y and Yb contents in the Hoivasvuori basalts, compared to those at Vihteljärvi and Palvajärvi, indicate lower degree of melting or different mantle composition at Hoivasvuori. The relatively high contents of Ti and P in the Hoivasvuori basalts might hint to slight “within plate” enrichment.