Precambrian Sequence Bordering the Skaergaard Intrusion

Archean tonalitic-granodioritic orthogneisses bordering the Skaergaard Intrusion contain widespread boudins and lenses of garnet-biotite schist, quartzite, amphibolite, and ultramafic rocks. These rocks are similar to and locally gradational with narrow intact supracrustal belts in the region. We correlate earliest isoclinal folds in supracrustal belt rocks and in earliest tonalitic-trondhjemitic-granitic (TTG1) orthogneisses with regionally developed (D2) deformation. We also correlate strong foliation (Ssp1) in the supracrustal rocks and banding (Sbgn1) in earliest orthogneisses with D2 deformation which followed and overlapped earliest M1 metamorphism. Ssp1 foliation is in part axial planar with D2 isoclinal folds transposing compositional and subparallel metamorphic banding in the hinge areas. Ssp1 assemblages in metapelites consist of folia of coexisting sillimanite-biotite-quartz and correspond roughly to metamorphism at the second sillimanite isograd. We correlate syntectonic emplacement of a later generation of orthogneisses (TTG2) with strong D3 shearing-cataclasis associated with tectonic intercalation of supracrustal rocks and earliest orthogneisses. The latest metamorphic assemblages (M2) consist of granoblastic and porphyroblastic minerals that overprint Ssp1 (M1) foliation and D3 fabrics. These assemblages formed during largely static regional metamorphism about 2900 Ma ago and are locally aligned with fabric elements of D4 folding. Temperatures during M2 metamorphism equalled or exceeded the stability of biotite-sillimanite-quartz in metapelites and chlorite-orthopyroxene-olivine-spinel-hornblende in ultramafic rocks. Fe-Mg biotite-garnet exchange, and the pressure-dependent garnet-plagioclase-sillimanite-quartz equilibrium assemblage in metapelites yield temperature and pressure estimates for M2 metamorphism of 650–700°C and 3–4 kb. These data suggest that M2 assemblages formed as results of dehydration reactions at water partial pressures that were less than the total pressure. The temperature-dependent equilibrium assemblage chlorite-orthopyroxene-olivine-spinel-vapor (+hornblende), adjusted for observed phase compositions, is consistent with the Fe-Mg biotitegarnet exchange geothermometer. Rare-earth element, Rb-Sr and Pb-Pb isotopic, and other compositional characteristics of the orthogneisses are generally consistent with a multiple stage magmatic origin of their protoliths. Older TTG1 orthogneisses have compositions generally consistent with formation of the magmas parental to their protoliths by partial melting of garnetiferous source rocks such as eclogite, or lower crust. Younger TTG2 orthogneisses have compositions that are consistent with their formation as water-saturated second melts in equilibrium with a hornblende-rich residuum. Their formation occurred within a few 107 y after crustal emplacement of TTG1 orthogneisses. The source of water for the formation of later (TTG2) melts may have been M2 dehydration reactions deeper within the supracrustal pile.