New age (ca 2970Ma), mantle source composition and geodynamic constraints on the Archean Fiskenæsset anorthosite complex, SW Greenland

The Archean Fiskenæsset Complex, SW Greenland, consists of an association of ca. 550-meter-thick layered anorthosite, leucogabbro, gabbro, and ultramafic rocks (peridotite, pyroxenite, dunite, hornblendite). The complex was intruded by tonalite, trondhjemite, and granodiorite (TTG) sheets (now orthogneisses) during thrusting that was followed by several phases of isoclinal folding. The trace element systematics of the Fiskenæsset Complex and associated volcanic rocks are consistent with a supra-subduction zone geodynamic setting.

The Fiskenæsset anorthosites, leucogabbros, gabbros and ultramafic rocks collectively yield an Sm–Nd errorchron age of 2973 ± 28 Ma (MSWD = 33), with an average initial ε_Nd = + 3.3 ± 0.7, consistent with a long-term depleted mantle source. Regression of Pb isotope data define an age of 2945 ± 36 Ma (MSWD = 44); and the regression line intersects the average growth curve at 3036 Ma. Slightly lower Pb–Pb errorchron age is interpreted as reflecting partial disturbance of the U–Pb system in gabbros, leucogabbros and ultramafic rocks during intrusion of TTGs.

Complex internal structures in zircons from orthogneisses reveal several episodes of zircon growth and recrystallization taking place between ca. 3200 and 2650 Ma. Zircon ages peak at about 3200, 3100, 3000, 2950, 2820, and 2750 Ma. The 3200–3000 Ma zircon cores are interpreted as inherited xenocrysts from older reworked crustal rocks. 2950 Ma is considered as an approximate intrusion age of sampled TTGs. The 2940–2650 Ma ages are attributed to metamorphic overgrowth and recrystallization in response to multiple tectonothermal events that affected the Fiskenæsset region.

On the basis of recently published trace element data, and new Nd and Pb isotope and U–Pb zircon age data, a three-stage geodynamic model is proposed to explain the evolution of the Fiskenæsset Complex. Stage 1 represents the formation of depleted shallow mantle source > 3000 Ma (ε_Nd = + 3.3 ± 0.7) for the complex. Stage 2 corresponds to the development of an intra-oceanic island arc between 3000–2950 Ma. Stage 3 is characterized by the collision of the island arc with either a passive continental margin or with an older arc between 2950–2940 Ma.