William Peck Associate Professor of Geology Past Research in Greenland Genesis of the Fiskenaesset Anorthosite Complex The 2.9 billion year old Fiskenaesset Anorthosite Complex is made up of a layered series of anorthosites and gabbros that occur as dissected layers and lenses within the Tasiusarsuaq terrane of southwest Greenland. The contact zone at Fiskenaesset Harbor displays a wide variety of unusual lithologies containing sapphirine, kornerupine, ruby corundum, hšgbomite, clintonite, tourmaline, geikielite, ilmenite, enstatite, spinel, gedrite, cordierite, biotite, hornblende, and plagioclase. These Al- and Mg-rich assemblages are confined to a thin zone between the anorthosite and overlying rocks.

Oxygen and hydrogen isotope ratios indicate that unusual rocks at the upper contact of the Fiskenaesset Anorthosite are the products of hydrothermal alteration by seawater at the time of anorthosite intrusion. Subsequent granulite-facies metamorphism of these Ca-poor and Al- and Mg-rich rocks produced sapphirine- and kornerupine-bearing assemblages. Because large amounts of surface waters cannot penetrate to depths of 30 km during granulite facies metamorphism, the isotopic signature of the contact rocks must have been obtained prior to regional metamorphism. The stable isotope and geochemical characteristics of the contact rocks support a model of shallow emplacement into Archean ocean crust for the Anorthosite.

Geology of the Fiskenaesset Anorthosite (after Myers, 1986). Fiskenaesset Harbor (blue arrow) is just one of the dozens of contact localities that host sapphirine- and kornerupine-bearing assemblages.

Related References: Peck, WH, and Valley, JW, 1996, The Fiskenaesset anorthosite complex: Stable isotope evidence for shallow emplacement into Archean ocean crust: Geology, v.24, p. 523-526.

Contact Information: Department of Geology Colgate University 13 Oak Drive Hamilton, NY 13346 Ph 315/228-7200 Fax 315/228-7187 wpeck@mail.colgate.edu

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