Petrology, geochemistry and genesis of the type area trondhjemite in the Trondheim Region, Central Norwegian Caledonides.

Abstract

Trondhjemite from the Follstad district of the Trondheim region is present within allochthonous Lower Paleozoic eugenosynclinal and partly miogeosynclinal rocks of the Caledonide orogen. Four periods of trondhjemite intrusive activity have been recognized, with the main Follstad trondhjemite a product of late tectonism and metamorphism in the upper greenschist facies. Field relations, mineralogy and major and trace element abundances all attest to the high degree of homogeneity of the trondhjemite, which is of the high-Al203 type (average 16.2 %). Trondhjemites of the district average 71.5% SiO2, 5.3% Na2O3, but only 1.3%K2O. The REE abundances are depleted and highly fractionated. Barium greatly exceeds rubidium in abundance. Distinctive compositional characteristics of the Follstad trondhjemite support designation of the district as a type locality for trondhjemite. Petrogenetic models for the origin of the Follstad district trondhjemite that are based on anatexis of granitic crust or partial melting of greywacke are unlikely. Distinctive compositional gaps in the gabbro-trondhjemite suite, when compared with the other associated magmatic rocks in the Follstad district, cannot easily be explained by a fractional crystallization model. The petrogenetic model that best fits the data from the Follstad trondhjemite is based on equilibrium melting of a low-K2O tholeiitic basalt during anatexis in an orogenic zone.