New major and high-precision ICP-MS trace element data on the Neoarchean (ca. 2970 Ma) layered Fiskenæsset Complex and associated volcanic rocks, southern West Greenland, provide new constraints on the petrogenesis and geodynamic setting of the complex. The complex appears to have been emplaced as multiple sills of magma and crystal mush into oceanic crust (tholeiitic basalts and gabbros), forming an association of ca. 550 m thick anorthosite, leucogabbro, gabbro, and peridotite layers. The Fiskenæsset Complex and the associated volcanic rocks were intruded by Neoarchean tonalite, trondhjemite, and granodiorite (TTG) sheets during thrusting that was followed by several phases of isoclinal folding. Despite the intense deformation and amphibolite to granulite facies metamorphism, primary cumulate textures and igneous layering are locally well preserved throughout the complex. The presence of calcic-plagioclase (An 75-95) and igneous amphibole in anorthosites, gabbros and leucogabbros, and hornblendite veins in peridotites suggests a hydrous magma source(s). The major and trace element compositions of tholeiitic basalts (amphibolites) suggest that they are petrogenetically related to the Fiskenæsset Complex by fractional crystallization. The trace element systematics of the least-altered anorthosites, gabbros, leucogabbros, peridotites, tholeiitic basalts, and calc-alkaline high-magnesian andesites (HMA) is collectively consistent with a supra-subduction zone geodynamic setting. On the log-transformed tectonic discrimination diagram, including La/Th, Sm/Th, Yb/Th, and Nb/Th ratios, tholeiitic basalts display a trend projecting from mid-ocean ridge basalt (MORB) field to island arc basalt (IAB) field. This trend is interpreted as reflecting a transition from the Neoarchean depleted upper mantle to a subarc mantle wedge following the initiation of intra-oceanic subduction and arc migration. Collectively, on the basis of field relationships, petrographic features, and geochemical characteristics, the Fiskenæsset Complex is interpreted as a fragment of a Neoarchean oceanic island arc. On the basis of REE patterns, anorthosites are divisible into four major groups: (1) Group 1 displays moderately depleted to slightly enriched LREE and HREE patterns; (2) Group 2 possesses strongly enriched LREE and moderately depleted HREE patterns; (3) Group 3 has strongly enriched LREE and depleted HREE patterns; and (4) Group 4 (garnet anorthosite) exhibits concave-upward REE patterns; that appear to have resulted from contamination of anorthositic magma by basaltic rocks or magma. The different REE patterns in Group 1, 2, and 3 anorthosites are interpreted to reflect various depths of partial melting and variably enriched (in incompatible elements) source compositions. The majority of anorthosites and leucogabbros were derived from depths above the garnet stability field.
- Programområde 4: Mineralske råstoffer