Paper No. 9
Presentation Time: 4:35 PM
CORRELATING SEGMENTS OF THE STØREN NAPPE, CENTRAL CALEDONIDES, NORWAY: DOES GEOCHEMISTRY WORK AS A LONG-DISTANCE RECONSTRUCTION AID?
In the Scandinavian Caledonides, a series of allochthons were thrust long distances southeastward onto Baltica during the Late Silurian-Early Devonian Scandian Orogeny. The Lower and most of the Middle Allochthons are derived from the Baltica margin, and the Uppermost Allochthon is derived from Laurentia. Between, the Upper Allochthon is a complex of low metamorphic grade arc, back arc, and ocean floor fragments from the Iapetus Ocean and its margins. These rocks are exposed along 1600 km of the Caledonides in Norway and Sweden. This work stems from a project to trace relatively undeformed Upper Allochthon rocks from eastern Norway and western Sweden, where fossils and lava pillows are recognizable, into strongly deformed rocks in deep synclines in the Western Gneiss Region (WGR), southwest of Trondheim. From this area we have 87 new analyses (51 chemical components) of Upper Allochthon volcanic and plutonic rocks. The mafic volcanics define two trends: 1) MORB-like greenstones having flat to ultra-depleted REE patterns and small to no Nb anomalies. These follow an oceanic-type mantle melting array. A LREE-enriched, E-MORB-like variety may be a distinct group. 2) Slightly LREE-enriched concave-upward REE patterns with large negative Nb anomalies. These follow a depleted mantle-slab enrichment arc-type array. Intermediate rocks between these trends are common. Our analyses have been compared to over 1100 analyses from the literature of Upper Allochthon rocks from the rest of the Scandinavian Caledonides to help characterize along-strike regional differences and potential correlation with the more deformed rocks in the Western Gneiss Region. Three problems stand out: 1) Insufficient comparable data for whole igneous complexes. The literature represents spot projects, and analyses spanning 50 years of analytical technology. 2) Difficulty identifying stratigraphic or structurally equivalent subunits. 3) Apparently real geographic differences in unit compositions and inferred tectonics between regions. Nonetheless, greenstones in deep synclines in the WGR to the southwest are remarkably similar to ophiolitic rocks in the Løkken-Resfjell area near Trondheim. Similarities include the same two limiting compositional trends and intermediates, and a separate group of more alkaline rocks, the Hølonda sills.