Mineralogic and Petrologic Studies of Archaean Metamorphic Rocks from West Greenland, Lunar Samples, and the Meteorite Kapoeta

In Part I of this thesis, petrographic and electron microprobe data are reported for samples from a series of high-grade Archaean gneisses, collected on Langø, an island in the northwest corner of the Godthåb District of West Greenland. Rocks with a wide variety of bulk compositions occur that all preserve evidence for two distinct episodes of metamorphic mineral growth. Syn- to post-tectonic hornblende granulite grade metamorphism (MI) was followed by post-tectonic amphibolite grade metamorphism (MII) at a later time. In potassium feldspar bearing gneisses, MI assemblages, in addition to quartz and plagioclase are: garnet + biotite (brown) ± sillimanite; cordierite + biotite (brown) ± sillimanite; and biotite (brown). In potassium feldspar- free gneisses, MI assemblages, in addition to quartz and plagioclase are: cordierite + garnet + biotite (brown); and orthopyroxene + biotite (brown) ± cordierite. As a result of MII, cordierite and garnet have been replaced by kyanite + biotite (green). Brown biotite is Ti-rich (>1.0 wt% TiO₂), whereas green biotite is Ti-poor (≾1.0 wt% TiO₂), and an examination of biotite analyses suggests that Ti is incorporated in that mineral via a vacancy-forming substitution. In mafic rocks, MI assemblages include various combinations of hornblende (green-brown) + clinopyroxene ± orthopyroxene ± garnet ± biotite, with plagioclase and rarely quartz. However, garnet and clinopyroxene do not occur together. As a result of MII, pyroxene has been replaced by calcic amphibole that differs in color (pale green to blue green) and composition (Na, K, Al, Ti are lower; Si, Mg, Fe³⁺ are higher) from the MI varieties. MI hornblende and plagioclase have an extremely large, compositional range, which correlate with one another in terms of Al/(Al+Si) and Na/(Na+Ca), and suggest that their compositions are controlled by rock composition, and not by metamorphic grade at these conditions. The composition of coexisting pyroxenes, the position of the kyanite-sillimanite boundary, and the composition of garnet (cores) and cordierite constrain the pressure and temperature for MI to lie between ~7-8 kb and ~700-800°C. Temperatures calculated from Fe-Mg partitioning between garnet (rims) and MII biotite suggest ~450°C for MII, and the occurrence of kyanite would indicate a pressure of 3-4 kb for this temperature. The MI event can be ascribed with certainty to regional metamorphic-magmatic activity at ca. 2.8 AE (Black et al., 1973), whereas MII may correspond to regional heating during the emplacement of the Qôrqut Granite at ca. 2.5 AE (Baadsgaard and Collerson, 1976), or to a regional thermal event at ca. 1.6 AE identified in isotopic studies (Pankhurst et al., 1973). The pressure and temperature estimated for MI imply a thermal gradient of ca. 30°C/km, and a minimum crustal thickness of 20 km at the time of metamorphism. The petrologic characteristics of MI are similar to medium-P facies series metamorphism found in younger orogenic belts, and nothing "unique" can be ascribed to this Archaean metamorphic event, based on the presently-available data and observations. Parts II and III of this thesis are a series of published papers that involve mineralogic and petrologic studies of Apollo 15, 16, and 17 lunar samples and the meteorite Kapoeta. Parts II and III of this thesis are a series of published papers that involve mineralogic and petrologic studies of Apollo 15, 16, and 17 lunar samples and the meteorite Kapoeta.