Eclogites from the Sunndal-Grubse Ultramafic Mass, Almklovdalen, Norway and the T-P History of the Almklovdalen Masses

Abstract

An orthopyroxene eclogite from the Sunndal-Grubse mass contains clinopyroxenes with lamellae of garnet and orthopyroxene. Differences in mineral and bulk chemistry distinguish this, and adjacent eclogites, from eclogites in other Almklovdalen ultramafic masses. Analyses are used in conjunction with experimental data to assign three T-P points in the complex T-P path of the ultramafic masses. Other T-P points are assigned on a basis of petrographic and structural arguments. The inferred T-P path offers support to earlier suggestions that the masses are tectonically controlled introductions within the surrounding gneisses. The survival of eclogite facies assemblages within the ultramafic masses is attributed to a gneissic metamorphism where PH₂O < P_Total. The T-P path also suggests an extended history of subsolidus re-equilibration under mantle conditions possibly complicated by open system reactions. The eclogites of the Sunndal-Grubse mass are thought to be cumulates and differentiates of relatively high pressure (25–28 kb) melting in ultramafic rocks. The complex T-P path can be explained in terms of convection cell mechanisms.