Proterozoic Gneisses Research Articles

Recently received publications

The Ammassalik area in South-East Greenland forms a link between the Precambrian terrains of West Greenland and northwest Scotland. It consists mainly of Archaean orthogneisses reworked during a Proterozoic tectonothermal event. Proterozoic ortho- and paragneisses, major leuconorite-charnockite complexes, suites of metadolerite dykes and posttectonic granites also occur. SmNd, RbSr, PbPb and zircon UPb isotope data yield ages of ∼ 3.0–2.8 Ga for Archaean basement gneisses, 2.2-2.1 (?) Ga for Proterozoic gneisses, 1.9-1.8 Ga for the Proterozoic reworking, 1.89 Ga for the leuconorite-charnockite complexes, and 1.7-1.6 Ga for the late granites. Proterozoic reworking has disturbed most Archaean isotope systems, and contamination with Archaean material makes dating of Proterozoic rocks difficult. In several cases discordant dates are obtained from different isotope systems.The chronologies of the Lewisian complex of Scotland and the West Greenland Nagssugtoqidian mobile belt compare closely with that of the Ammassalik area, and support the correlation between these areas suggested by earlier studies. There are, however, also major differences between the Nagssugtoqidian and the mobile belt in the Ammassalik area, and precise correlation across the Inland Ice of Greenland is still ambiguous.

Early Proterozoic Sedimentary Basins of the Lake Superior Region

Studies of the complexly folded and metamorphosed early Proterozoic strata (1.8 to 2.1 b.y. B.P.; Van Schmus, 1976) of the Lake Superior region (Fig. 1) have revealed features indicating a complex sedimentary-tectonic evolution. These rocks, termed “the Marquette Range Supergroup,” and which include stratigraphic equivalents in Minnesota and Ontario (Cannon and Gair, 1970), have been divided into four groups (James, 1958), which, in ascending order, are the Chocolay, Menominee, Baraga, and Paint River Groups. In Minnesota and Ontario, the stratigraphic equivalents of the Chocolay, Menominee, and Baraga Groups are the Mille Lacs (Chocolay Group equivalent in the Cuyuna area), and lower (Menominee Group equivalent) and upper (Baraga Group equivalent) Animikie Group (Morey, 1978a) (Table 1). The stratified rocks of the Marquette Range Supergroup and stratigraphic equivalents occur in four structural settings (Fig. 1A): (1) elongate, generally synclinal, structural troughs bounded by Archean rocks; (2) extensive homoclines; (3) outliers on granitic basement; and (4) mantling rims around early Proterozoic gneiss domes (Sims, 1976). Some rims also form steep narrow troughs (for example, the Michigan River trough; Klasner and Cannon, 1974), which are here grouped with gneiss-dome rims as opposed to the other structural troughs (type 1 above) which appear to show no genetic relation to gneiss domes. Most of the data discussed herein concern the first two types of structural settings. It is the purpose of this paper to suggest a relationship between the four types of structural settings and the mode of original sedimentation. Specifically, we propose that elongate early Proterozoic sedimentary basins existed in eastern Upper Michigan at the sites of what became structural troughs later in the early Proterozoic; that is, the eastern Upper Michigan structural troughs are simply appressed sedimentary basins. As will be discussed, the Animikie basin (as reported by Morey, 1967; Morey and Ojakangas, 1970; and Alwin, 1979) represents a more “stable” type of sediment receptacle (types 1, 2 structural exposure) with a sedimentary history broadly comparable to the eastern Upper Michigan basins. Finally, it is noted that individual basin history was complex, with some more areas showing evidence of both basinal and platformal sedimentation.

Early Proterozoic isotopic ages in the East Greenland Caledonian fold belt

Rb-Sr whole rock isochron ages of 1950 m.y. and 1725 m.y. obtained on rock units of the Hagar migmatite sheet indicate that it is an early Proterozoic gneiss complex and not part of a mobile Caledonian infrastructure. New isotopic ages and recent fieldwork give no support to the concept of mobile Caledonian migmatitic masses rising into the suprastructure as envisaged in the classic “stockwerke” model for the East Greenland Caledonian fold belt. However, K-Ar mineral ages ranging from 415–462 m.y. indicate a significant Caledonian thermal event. Relationships between the Hagar sheet and bordering metasediments are discussed, including evidence for a middle Proterozoic group of sediments metamorphosed before deposition of the late Precambrian Petermann Series and Eleonore Bay Group.

Definition of Type Little Falls Dolostone (Late Cambrian), East-Central New York: GEOLOGIC NOTES

The Little Falls Dolostone has been defined and described inadequately at its type section at Little Falls, in the Mohawk Valley, New York. The main mass of the formation extends from Poland on the west to Randall on the east. I consider the immediate Little Falls area to represent the type locality, within which is a composite type section of six localities. The formation, 400 ft (122 m) thick at Little Falls, is well known for its Herkimer diamonds (quartz crystals) and cryptozoons (algal stromatolites). Dolostone predominates, but sandstones and mixed dolostone-sandstone varieties are common. Gray colors, medium to thick bedding, and vuggy beds are characteristic. Glauconite, chert, and silicified ooids are commonest in the upper part. Calcite is a minor and late paragenetic constituent. Fossils are extremely rare. Four subdivisions, informally designated as units, herein are recognized above the nonconformable contact with the underlying Proterozoic gneiss. Unit A, the lowest subdivision, is 90 to 100 ft (28 to 30 m) thick and has conglomerate and sandstone at its base overlain by coarse-crystalline dolostone. Unit B consists of about 200 ft (61 m) of dark, fine to medium-crystalline dolostone with many vuggy beds; within the unit is a unique sequence of alternating algal-stromatolitic dolostone and sandstone. Unit C, 25 to 40 ft (7.6 to 12.2 m) thick, is primarily coarsely crystalline glauconitic dolostone containing quartzose intercalations. Unit D, characterized by very fine-crystalline dolostone, is 30 to 40 ft (9.1 to 12.2 m) thick, and contains reddish-gray zones and silicified ooids; i grades upward into the Tribes Hill Formation (Lower Ordovician) at Little Falls.