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

• Main Author: Dymek, Robert F.
• Format: Thesis
• Language: English
• Published: 1977
• Subjects: Greenland; Godthåb
• Online Access: https://thesis.library.caltech.edu/14012/; https://thesis.library.caltech.edu/14012/1/Dymek_RF_1977.pdf; https://resolver.caltech.edu/CaltechTHESIS:12042020-191103512

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 As a result of MII, cordierite and garnet have been replaced by kyanite + biotite (green).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 As a result of MII, pyroxene has been replaced by calcic amphibole that differs in color (pale green to blue green) and composition (Na, K, Al, Ti are lower; Si, Mg, Fe³⁺ are higher) from the MI varieties.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 ...