METAMORPHOSED VOLCANICS AND RELATED MAFIC INTRUSIONS OF THE SCANDIAN STØREN NAPPE, NORWAY: STRATIGRAPHIC AND GEOCHEMICAL CORRELATION INTO HIGHLY DEFORMED PARTS OF THE WESTERN GNEISS REGION

The Støren Nappe of the Upper Allochthon is a thrust complex emplaced onto the Baltica margin during the Silurian-Early Devonian Scandian Orogeny. It is thought to be composed of Ordovician outboard arc and ophiolite fragments, accreted to Laurentia prior to the Scandian collision with Baltica. Where nappe sequences are preserved in deep synclines in the Western Gneiss Region southwest and west of Trondheim, we sampled 16 localities (87 samples) over a distance of ~400 km to test proposed correlations. The samples are lower amphibolite facies mafic volcanics including pillow lavas, and gabbros, with rare andesite, dacite, and diorite. West and north of Trondheimsfjord much of the Støren consists of deformed intermediate to felsic plutons, not reported on here. Where stratigraphic control was available, there was no consistent variation across strike. In the Moldefjord and Surnadal synclines the amphibolites follow a MORB-like incompatible element trend, with some scatter toward arcs, with Nb_n/La_n ratios of 0.3-1.6, approximately paralleling LREE-enrichment, and with La_n/Sm_n ratios of 0.2-1.6, akin to modern back-arc basin volcanics. Real negative Nb anomalies, corrected for incompatible-element slope, range from none to moderate. Some samples are extremely depleted in incompatible elements, rivaling the most-depleted MORB and arc volcanics known. In contrast, mafic Støren from north of Trondheimsfjord follows an arc-like incompatible element trend, LREE-enriched with La_n/Sm_n ratios of 1.0-2.2, higher than most MORB-like samples, but with Nb_n/La_n ratios of 0.1 to 0.3, lower than most MORB-like samples. Some of the arc-like rocks are intimately associated with felsic plutons having arc signatures that are also part of the Støren Nappe. The Støren gabbros seem to be derived from the more LREE-depleted back arc-like magmas, and contain as little as ~1% model trapped liquid. Støren Nappe equivalents studied by others from areas nearer the foreland, where deformation/metamorphism is less, are broadly similar to rocks we report but in general are more evolved, and highly depleted rocks are rare. Others report two areas, Hølanda and Snåsa, where alkaline mafic rocks occur, which so far are absent to the west. This work confirms extension of the Støren Nappe into narrow synclines of the Scandian hinterland.