TRACE ELEMENT AND RADIOGENIC (ND AND PB) ISOTOPE SYSTEMATICS OF THE MESOARCHEAN FISKENAESSET ANORTHOSITE COMPLEX, SW GREENLAND: IMPLICATIONS FOR ARCHEAN CONTINENTAL GROWTH

The Archean Fiskenaesset Complex, southern West Greenland, composed of ca. 550-meter-thick layered anorthosite, leucogabbro, gabbro, dunite, peridotite, pyroxenite, and hornblendites. The Fiskenaesset Complex appears to have been intruded as multiple sills of magma and crystal mush into Mesoarchean oceanic crust. The complex was intruded by tonalite, trondhjemite, and granodiorite (TTG) sheets (now orthogneisses) during thrusting that was followed by several phases of isoclinal folding. Despite polyphase deformation and amphibolite to granulite facies metamorphism, primary cumulate textures and igneous layering are well preserved throughout the complex.

Trace element (e.g., REE, HFSE) systematic of the Fiskenaesset Complex and associated volcanic rocks are consistent with a supra-subduction zone geodynamic setting. On recently developed new petrogenetic discrimination diagrams, based on the log-transformed ratios of immobile elements, such as La/Th, Sm/Th, Yb/Th , Nb/Th, Fiskenaesset volcanic rocks display a trend projecting from mid-ocean ridge basalt (MORB) to island arc basalt (IAB) field. This trend is interpreted as reflecting the entrainment by induced convection of Archean depleted upper mantle (i.e., the source of Archean MORB) into a subarc mantle wedge following the initiation of intra-oceanic subduction and arc migration.

On several variation diagrams, samples from three differentiated (dunite to anorthosite) sequences plot along a well-defined liquid line of descent, consistent with in situ fractional crystallization. Bulk chemical compositions of these sequences are used to constrain the approximate compositions of parental magmas. Normative olivine (85 to 4 wt.%) and magnetite (2.5 to 0.1 wt.%) decrease, whereas anorthite (5 to 80 wt.%) increases upward in all sequences. Petrographic observations and geochemical data suggest that Sequences 2 and 3 were solidified from evolved magmas that underwent olivine fractionation prior to their intrusion. In contrast, Sequence 1 appears to have derived from a near-primary picritic parental magma (MgO=20 wt.%, Ni=780 ppm, Mg-number=81).

If the relative thickness of ultramafic layers, the sum of dunite, peridotite and pyroxenite layers, in differentiated sequences is taken as an analog for the original complex emplaced into Archean oceanic crust, the Fiskenaesset Complex might have had a minimum thickness of 1000 m, with a 500 m thick ultramafic unit at the bottom. The thickness of ultramafic unit in the preserved complex is less than 50 m, suggesting that more than 90% of the original ultramafic unit was detached and recycled back into the mantle through either delamination or subduction zone processes.

The Fiskenaesset anorthosites, leucogabbros, gabbros and ultramafic rocks yield a Sm-Nd isochron age of 2973±28 Ma (MSWD=33), with an initial epsilon-Nd = +3.3, consistent with a long-term depleted mantle source. Regression of Pb isotope data defines an age of 2945±36 Ma (MSWD=44); and the regression line intersects the average growth curve at 3036 Ma. Despite multiple phases of deformation and amphibolite to granulite facies metamorphism, the Sm-Nd and U-Pb isotope systems in the Fiskenaesset Complex appear to have not been significantly disturbed.

Complex internal structures in zircon from TTGs reveal several episodes of zircon growth and recrystallization 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. The 2950 Ma is considered as an approximate intrusion age of TTGs. The 2940-2650 Ma ages are attributed to metamorphic overgrowth and recrystallization in response to multiple tectonothermal events that affected the Fiskenaesset region.

On the basis of trace element, Nd and Pb isotope, and U-Pb zircon age data, a three-stage geodynamic model is proposed to explain the evolution of the Fiskenaesset Complex. Stage 1 represents the formation of depleted shallow mantle source > 3000 Ma (epsilon-Nd = +3.3) 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. Archean continental crust in southern West Greenland was grown by accretion of Eoarchean to Mesoarchean oceanic island arcs and intrusion of granitoid magmas into these island arcs.