Geochemistry of the Mesoarchean Fiskenæsset anorthosite complex and associated tonalite-trondhjemite-granodiorite (TTG) gneisses, southwestern Greenland
Abstract
This study presents new field, petrographic, and high precision geochemical data for the Mesoarchean Fiskenæsset Complex and spatially associated Mesoarchean to Neoarchean tonalite-trondhjemite-granodiorite (TTG) gneisses, in southwestern Greenland. On the basis of textural evidence, whole-rock geochemical data, and the trace element compositions of hornblende, plagioclase and clinopyroxene, it is suggested that hornblende has a magmatic origin. On the basis of whole-rock major and trace element characteristics and the trace element compositions of hornblende grains, two distinct geochemical suites (Suite A and Suite B) are recognized in the Fiskenæsset Complex. Field observations, whole-rock geochemistry, and hornblende, plagioclase and clinopyroxene trace element compositions indicate that the Fiskenæsset Complex originated through fractionation of olivine, pyroxene and hornblende, and late crystallization of plagioclase. Injection of new mafic magmas into variably solidified anorthositic magmas and crystal mushes resulted in the formation of leucogabbros via magma mixing. The rocks of the Fiskenæsset Complex are characterized arc signature, consistent with a subduction zone geodynamic setting. The occurrence of magmatic hornblende throughout the sequence is consistent with a hydrous mantle source. Orthogneisses in the Fiskenæsset region are composed of an older suite of metamorphosed TTGs, and a younger suite of high-K granites. The TTGs have high Al2O3, Na2O and Sr but low Y contents. They also have highly fractionated REE patterns, enrichment of Sr and Pb, and depletion of Nb and Ti. The geochemical characteristics of the TTGs can be explained by partial melting of hydrous basalts at the base of a thickened oceanic island arc, leaving a rutile-bearing eclogitic residue. Field observations and geochemical modelling suggest that the spatially and temporarily associated tholeiitic basalts (now amphibolites) in the Fiskenæsset region might have been the sources of TTG melts. The high-K granites have steep REE patterns and display variably negative Eu anomalies. Relatively high K2O/Na2O ratios in the granodiorites indicate that they were the source of the granites. Delamination of the eclogitic residue led to the loss of the lower crust and thereby triggering the subsequent basaltic underplating. Melting of the granodiorites in response to the basaltic underplating resulted in the generation of high-K granitic melts.