Geodynamics in the Bamble area during Gothian- and Sveconorwegian times : a comparative petrological study of two gabbros

Abstract

Continents started to grow already at an early stage of the earth history. Some of these early continents were preserved from major destroying forces and are now recognized as cratons. What make these cratons unique is that they contain useful information about the continent building mechanism at an early time of the earth history. I focus in this thesis on such continent building mechanism occurring in the southwestern part of the Baltic Shield, the Bamble area, in a period between 1.8 Ga and 0.9 Ga ago. This area is one of the youngest parts of the Baltic Shield and exhibits two distinct periods of continental growth. The oldest is known as the Gothian period (between 1.75 Ga and 1.50 Ga); the youngest as the Sveconorwegian (between 1.25 and 0.9 Ga). The method I used to investigate this continent building mechanism at late proterozoic times is by comparing the geology of two gabbros i.e. the Jomasknutene and Dale gabbro. The Jomasknutene gabbro has been dated at 1.77 Ga ±0.19 Ga and must contain information about the Gothian and Sveconorwegian period. The Dale gabbro has been dated at 1.11 Ga ±0.14 Ga and must contain information about the Sveconorwegian period only. In the period the Bamble area was subjected to continental growth, between 1.75 Ga and 0.9 Ga ago, the reigning metamorphic conditions in this area were characterized by a continuous high heat flow from the area below. In terms of geotectonic setting, this can be interpreted as a discontinuous active rift system. At least two periods of gabbroic intrusions were initialized in this rift environment. True rifting was probably opposed by, compressional, orogenic activity. That the geotectonic setting in the Bamble area did not change much between 1.75 Ga and 0.9 Ga ago is accentuated by the geological similarity between the Gothian Jomasknutene and the Sveconorwegian Dale gabbro. They appear to have a similar whole rock chemistry, magmatic fractionation trend, magmatic and, in less extent, metamorphic mineralogy. The most prominent results of this study, based on the comparison of the two gabbros, are listed below.