Abstract Kornerupine, (□, Mg, Fe)(Al, Mg, Fe) 9 (Si, Al, B) 5 O 21 (OH, F), is known from only five mafic or ultramafic settings worldwide (of the >70 localities overall). We report a sixth occurrence from Akarui Point in the Lützow-Holm Complex, East Antarctica, where two ruby corundum (0.22–0.34 wt% Cr 2 O 3 )–plagioclase lenses are found at the same structural level as boudinaged ultrabasic rocks in hornblende gneiss and amphibolite. Ion microprobe analyses of kornerupine give 13–59 ppm Be, 181–302 ppm Li, and 5466–6812 ppm B, corresponding to 0.38–0.47 B per 21.5 O; associated sapphirine also contains B (588–889 ppm). Peak metamorphic conditions are estimated to be 770–790 °C and 7.7–9.8 kbar. Kornerupine encloses tourmaline and plagioclase, which suggests the prograde reaction tourmaline (1) + plagioclase (>An34)+ sapphirine±spinel→kornerupine+corundum (ruby)+plagioclase (<An82)±(fluid or melt). Alternatively, kornerupine and tourmaline could have formed sequentially under nearly constant P – T conditions during the infiltration of fluid that was originally B-bearing, but then progressively lost Na (or gained Ca) and B through reaction with mafic rocks. Kornerupine later reacted with H 2 O–CO 2 fluid in cracks at P – T conditions in the andalusite stability field: kornerupine+plagioclase+(Na, K, ± Si in fluid)→tourmaline+biotite+corundum (sapphire)± magnesite±andalusite+(Ca in fluid). Secondary tourmaline differs from the included tourmaline in containing less Ti and having a higher Na/(Na+Ca+K) ratio. There are two possible scenarios for introducing B into the lenses: (1) infiltration of boron-bearing aqueous fluids released by prograde breakdown of muscovite in associated metasedimentary rocks; (2) hydrothermal alteration of mafic and ultramafic rocks by seawater prior to peak metamorphism. The latter scenario is consistent with an earlier suggestion that Akarui Point could be part of an ophiolite complex developed between the Yamato–Belgica and Rayner complexes.

Abstract Research on the Precambrian basement of North America over the past two decades has shown that Archean and earliest Proterozoic evolution culminated in suturing of Archean cratonic elements and pre-1.80-Ga Proterozoic terranes to form the Canadian Shield at about 1.80 Ga (Hoffman, 1988,1989a, b). We will refer to this part of Laurentia as the Hudsonian craton (Fig. 1) because it was fused together about 1.80 to 1.85 Ga during the Trans-Hudson and Penokean orogenies (Hoffman, 1988). The Hudsonian craton, including its extensions into the United States (Chapters 2 and 3, this volume), formed the foreland against which 1.8- to 1.6-Ga continental growth occurred, forming the larger Laurentia (Hoffman, 1989a, b). Geologic and geochronologic studies over the past three decades have shown that most of the Precambrian in the United States south of the Hudsonian craton and west of the Grenville province (Chapter 5) consists of a broad northeast to east-northeast-trending zone of orogenic provinces that formed between 1.8 and 1.6 Ga. This zone, including extensions into eastern Canada, comprises or hosts most rock units of this age in North America as well as extensive suites of 1.35- to 1.50-Ga granite and rhyolite. This addition to the Hudsonian Craton is referred to in this chapter as the Transcontinental Proterozoic provinces (Fig. 1); the plural form is used to denote the composite nature of this broad region. The Transcontinental Proterozoic provinces consist of many distinct lithotectonic entities that can be defined on the basis of regional lithology, regional structure, U-Pb ages from zircons, Sr-Nd-Pb isotopic signatures, and regional geophysical anomalies.

Abstract The Harney Peak Granite is located in the south-central portion of the Precambrian core of the Black Hills, South Dakota. This field study includes five sites near Keystone, South Dakota (Figs. 1 and 2). All sites are accessible by passenger car. At Site 1, the Harney Peak Granite is exposed at the Mount Rushmore National Memorial. (Large groups visiting at Mount Rushmore are expected to check in at the memorial headquarters.) Hammers or sample collecting are not allowed at the Rushmore site. A suite of Harney Peak Granite maybe collected to the west of the memorial (Site la). Sites 2 through 5 are pegmatites (Diamond Mica, Etta, Peerless, and Dan Patch) associated with the Harney Peak Granite. Permission for access to the Etta pegmatite may be obtained from the Pacer Corporation, Box 912, Custer, South Dakota 57730 (605/673-4458). Permission for access to the Peerless pegmatite maybe obtained from the Rushmore Borglum Museum in Keystone, South Dakota (605/666-4449). The other sites are on public land.