Precambrian Sequence Bordering the Skaergaard Intrusion

Archean tonalitic-granodioritic orthogneisses bordering theSkaergaard Intrusion contain widespread boudins and lenses ofgarnet-biotite schist, quartzite, amphibolite, and ultramaficrocks. These rocks are similar to and locally gradational withnarrow intact supracrustal belts in the region. We correlateearliest isoclinal folds in supracrustal belt rocks and in earliesttonalitic-trondhjemitic-granitic (TTG1) orthogneisses with regionallydeveloped (D₂) deformation. We also correlate strong foliation(Ssp1) in the supracrustal rocks and banding (Sbgn1) in earliestorthogneisses with D₂ deformation which followed and overlappedearliest M₁ metamorphism. Ssp1 foliation is in part axial planarwith D₂ isoclinal folds transposing compositional and subparallelmetamorphic banding in the hinge areas. Ssp1 assemblages inmetapelites consist of folia of coexisting sillimanite-biotite-quartzand correspond roughly to metamorphism at the second sillimaniteisograd. We correlate syntectonic emplacement of a later generationof orthogneisses (TTG2) with strong D₃ shearing-cataclasis associatedwith tectonic intercalation of supracrustal rocks and earliestorthogneisses. The latest metamorphic assemblages (M₂) consistof granoblastic and porphyroblastic minerals that overprintSsp1 (M₁) foliation and D₃ fabrics. These assemblages formedduring largely static regional metamorphism about 2900 Ma agoand are locally aligned with fabric elements of D₄ folding. Temperatures during M₂ metamorphism equalled or exceeded thestability of biotite-sillimanite-quartz in metapelites and chlorite-orthopyroxene-olivine-spinel-hornblendein ultramafic rocks. Fe-Mg biotite-garnet exchange, and thepressure-dependent garnet-plagioclase-sillimanite-quartz equilibriumassemblage in metapelites yield temperature and pressure estimatesfor M₂ metamorphism of 650–700?C and 3–4 kb. Thesedata suggest that M₂ assemblages formed as results of dehydrationreactions at water partial pressures that were less than thetotal pressure. The temperature-dependent equilibrium assemblagechlorite-orthopyroxene-olivine-spinel-vapor (+hornblende), adjustedfor observed phase compositions, is consistent with the Fe-Mgbiotitegarnet exchange geothermometer. Rare-earth element, Rb-Sr and Pb-Pb isotopic, and other compositionalcharacteristics of the orthogneisses are generally consistentwith a multiple stage magmatic origin of their protoliths. OlderTTG1 orthogneisses have compositions generally consistent withformation of the magmas parental to their protoliths by partialmelting of garnetiferous source rocks such as eclogite, or lowercrust. Younger TTG2 orthogneisses have compositions that areconsistent with their formation as water-saturated second meltsin equilibrium with a hornblende-rich residuum. Their formationoccurred within a few 10⁷ y after crustal emplacement of TTG1orthogneisses. The source of water for the formation of later(TTG2) melts may have been M₂ dehydration reactions deeper withinthe supracrustal pile.