Middle Precambrian Rocks Research Articles

Geology and petrography of the middle Precambrian in the Southern Serra do Espinhaço, Minas Gerais, Brazil

Middle Precambrian rocks of the southern Serra do Espinhaço were examined with respect to facies, petrography, tectonics and metamorphism. An Archaean basement (Pre-Minas Supergroup) consists of an older granitic unit and a younger unit composed of schists (type: greenstone belt). An unconformity separates basement rocks from the Proterozoic Espinhaço Supergroup (approx. 2500-1600 Ma). On the eastern margin of the São Francisco Craton there has developed a NS-trending shelf-sea with shallow-water deposits (sands, conglomerates, pelites) and ferruginous and iron-rich sediments. The Espinhaço orogeny (approx. 2000-1300 Ma) caused west vergent fault and fold tectonics. Volcanic activity is mainly restricted to a late stage of orogeny. Regional dynamothermal metamorphism (?1400-1100 Ma) produced retrograde overprinting of the basement and progressive overprinting of the Espinhaço Supergroup. The Brazilian Cycle (approx. 700-450 Ma) caused moderate thermometamorphism and E-W compression.

Penokean deformation and associated metamorphism in the western Marquette Range, northern Michigan

Research Article| May 01, 1978 Penokean deformation and associated metamorphism in the western Marquette Range, northern Michigan JOHN S. KLASNER JOHN S. KLASNER 1Department of Geology, Western Illinois University, Macomb, Illinois 61455 Search for other works by this author on: GSW Google Scholar Author and Article Information JOHN S. KLASNER 1Department of Geology, Western Illinois University, Macomb, Illinois 61455 Publisher: Geological Society of America First Online: 01 Jun 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 Geological Society of America GSA Bulletin (1978) 89 (5): 711–722. https://doi.org/10.1130/0016-7606(1978)89<711:PDAAMI>2.0.CO;2 Article history First Online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation JOHN S. KLASNER; Penokean deformation and associated metamorphism in the western Marquette Range, northern Michigan. GSA Bulletin 1978;; 89 (5): 711–722. doi: https://doi.org/10.1130/0016-7606(1978)89<711:PDAAMI>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Geologic studies of Precambrian rocks in northern Michigan indicate that the Penokean orogeny, which occurred about 1.9 b.y. ago, consisted of four stages of deformation, three of which occurred during a prolonged period of regional metamorphism. The first two stages of deformation were possibly caused by gravity sliding northward off an ancestral Penokean range located in central Wisconsin. The deformation probably started while the sediments were still soft, and it produced a pervasive west-northwest–trending foliation in the middle Precambrian rocks. The third and fourth stages of deformation were caused by uplift of rigid blocks of lower Precambrian basement rocks; this uplift produced prominent grabens such as the Marquette and Republic troughs. Metamorphism began very early in the deformational sequence, peaked during the third stage of deformation, and ended in a period of retrograde metamorphism. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Geomagnetic depth sounding and magnetotelluric results from a seismically active region northeast of Quebec City: Discussion

the progress report of the Federal-Provincial Committee on Huronian stratigraphy. Canadian Journal of Earth Sciences, 7, pp. 912-918. CUMMING, . L., and SCOTT, B. P. 1976a. Rb/Sr dating of rocks from the Wollaston Lake Belt, Saskatchewan. Canadian Journal of Earth Sciences, 13, pp. 355-364. 1976b. RbISr dating of rocks from the Wollaston Lake Belt, Saskatchewan: Reply. Canadian Journal of Earth Sciences, 13, pp. 1645, 1646. DUCROT, J., and LANCELOT, J. R. 1975. Probleme de la limite Precambrien Cambrien: etude radiochronologique du volcan du Jbel Boho. Rapport d'Activite Centre Geologique et Geophysique de Montpellier, pp. 105-107. GILBOY, C. F., LEWRY, J. F.,RAY, G. E., and SIBBALD, T. I. I. 1976. Rb/Sr dating of rocks from the Wollaston Lake Belt, Saskatchewan:Discussion. Canadian Journal of Earth Sciences, 13, pp. 1643-1645. VAN SCHMUS, W. R., THURMAN, E. M., and PETERMAN, Z. E. 1975. Geology and Rb-Sr chronology of Middle Precambrian rocks in Eastern and Central Wisconsin. Geological Society of America Bulletin, 86, pp. 1255-1265. WANLESS, R. K., and EADE, K. E. 1975. Geochronology of Archean and Proterozoic rocks in the Southern District of Keewatin. Canadian Journal of Earth Sciences, 12, pp. 95-114. YOUNG, G. M. 1973. Tillites and aluminous quartzites as possible time markers for Middle Precambrian (Aphebian) rocks of North America. I n Huronian stratigraphy and sedimentation. Edited by G. M. Young. Geological Association of Canada, Special Paper 12, pp. 97-127. 1975. Geochronology of Archean and Proterozoic rocks in the southern District of Keewatin:Discussion. Canadian Journal of Earth Sciences, 12, pp. 1250-1254.

Oil and Gas Developments in North Mid-Continent in 1976

Crude oil production in Kansas during 1976 (58,714,198 bbl) was 0.66% below the 1975 level; natural gas production (836,205,709 Mcf) decreased 1.7%. Processing plants recovered approximately 28 million bbl of natural gasoline and LPG products, and 537 MMcf helium. Drilling increased 17%, with 3,977 holes; 54.3% of the holes drilled solely for oil or gas were successful. Minnesota has a unique noncommercial well that produces from middle Precambrian rocks. Its brine contains lithium and zinc, and gas from the well has an abnormally high argon content. No tests for oil or gas were drilled in the state in 1976. Missouri's drilling was 50% below the 1975 level, with 45 completions. Shallow core testing continues in western Missouri. Nebraska's drilling occurred in all 4 produ ing basins; the 323 completions were only 7% below the number of wells drilled in 1975. Kimball and Hitchcock Counties had the most activity, with 66 and 65 compeltions respectively. No tests were drilled for oil, gas, or gas-storage sites in Iowa or Wisconsin during 1976.

Rb–Sr Geochronology of the Republic Area, Marquette County, Michigan

Rb–Sr geochronologic studies have been carried out on Lower and Middle Precambrian rocks exposed in Marquette County, Michigan. These rocks were subjected to varying degrees of metamorphism during formation of the Republic metamorphic node. Samples of the Lower Precambrian granitic gneiss generally yield whole-rock model ages of 2500 to 2800 Ma for metamorphic grades ranging from chlorite through sillimanite zones, indicating relatively minor net flux of Rb and Sr on a scale of several tenths of a metre during the metamorphism. Mineral ages show partial to complete internal re-equilibration of the Rb–Sr systems about 1665 ± 50 Ma ago. Muscovite Rb–Sr ages from cross-cutting pegmatites are also about the same age. Data on minerals from metamorphosed Middle Precambrian metasediments of the Michigamme Formation are consistent with this age but are not definitive.Published structural studies indicate that the main deformation in the Republic node occurred essentially at the same time as the peak of the major mineral-forming metamorphic event. The age of 1665 Ma is inconsistent with the regional tectonics and chronology, which indicate that the major deformation and metamorphism in Upper Michigan occurred about 1900 Ma ago. However, the age of 1665 Ma does correspond to a time of more recent metamorphism prevalent throughout the western Great Lakes area, and we interpret the results from the Republic area as a general overprint younger than the main period of deformation and metamorphism.Two of the Lower Precambrian samples yield model whole-rock ages in excess of 3000 Ma. Detailed study of the rocks involved show that the apparently old ages are due to Sr and Rb redistribution on a larger scale during another event which occurred about 2100 to 2200 Ma ago.

Geology and Rb-Sr Chronology of Middle Precambrian Rocks in Eastern and Central Wisconsin

Rb-Sr geochronologic data have been obtained from a large variety of plutonic, metavolcanic, and metamorphic rocks that crop out throughout eastern and central Wisconsin. Most of the data are consistent with an apparent age of approximately 1,650 m.y., although for some samples the data suggest somewhat older ages. Comparison of the Rb-Sr results with other published geochronologic data and with recently determined U-Pb zircon ages for some of the same rocks indicates that the 1,650-m.y. apparent age obtained by Rb-Sr methods on rocks from this region does not represent their true age of extrusion or intrusion. Instead, U-Pb data for zircons from these rocks indicate that the true ages are in the range 1,800 to 1,900 m.y. Rocks that yield Rb-Sr whole-rock or mineral isochrons having 1,650-m.y. apparent ages are also found throughout the western Great Lakes area (Michigan, Minnesota, and Ontario), indicating that a widespread, low-grade metamorphic event of some kind occurred throughout this region about 1,650 to 1,700 m.y. ago. As yet no major intrusive or extrusive rocks having true ages of 1,650 to 1,700 m.y. have been found in the region, so the exact cause of the alteration of the Rb-Sr geochronologic systems is still unknown. One major result of these studies is the demonstration that most of the older Precambrian rocks in Wisconsin are in the age range of 1,800 to 1,900 m.y. old and represent extensive crustal development along the southern edge of the Superior Province during that time.

Ambient Pyrite in Precambrian Chert: New Evidence and a Theory

Ambient pyrites of two distinct types were described from middle Precambrian rocks of the Lake Superior area. A new class of this phenomenon is here described from middle Precambrian chert from western Australia. The newly found ambient pyrites are quite minute and characteristically occur in groups forming a "starburst" pattern. All three types of ambient pyrite may be explained in terms of pressure solution initiated by gas evolution from organic material attached to the pyrite. Thermal degradation of the kerogen produces the gases which, due to the impermeability of the encompassing chert, build up the pressures necessary to initiate solution. Pyrite appendages bear a striking resemblance to micro-organisms and, thus, constitute the smallest pseudofossils known.

Buried Precambrian Rocks of South Dakota

Early Precambrian granitic gneiss, mafic schist, and granite, outlined by prominent east-northeast-trending magnetic and gravity anomalies, are extensive in the buried basement of northeastern South Dakota, the southwestern-most extension of the Superior geological province. These rocks were involved in widespread orogenesis about 2500 m.y. ago. In central and western South Dakota equally prominent geophysical anomalies define the trend of middle Precambrian rocks. A middle Precambrian sedimentary succession in the Black Hills was deformed and intruded by granite approximately 1700 m.y. ago. Four distinct metamorphic belts which possibly date mainly from middle Precambrian time are recognized. A large granite terrane in southern South Dakota is tentatively interpreted as having formed 1430 to 1460 m.y. ago. The basement complex in southeastern and central South Dakota is overlain by a sheet of Sioux Quartzite. A new Rb-Sr age determination on a rhyolite suggests that the Sioux Quartzite is at least 1470 m.y. old.

K-Ar Ages of Mafic Dikes and Evidence for Low-Grade Metamorphism in Northeastern Minnesota

K-Ar mineral and whole-rock ages for mafic dikes and one sill that intrude the early and middle Precambrian rocks of northeastern Minnesota and adjacent areas in Ontatio suggest times of intrusion at more than 2200, 1300, and 900 to 1100 m.y. ago. Samples which give K-Ar ages greater than 1300 m.y. show extensive alteration characteristic of low-grade metamorphism (prehnite-pumpellyite facies). For these samples, the whole-rock K-Ar ages have been lowered and suggest that the metamorphism occurred at about 1600 m.y. ago. The samples which give whole-rock K-Ar ages of 1300 m.y. or younger do not show the same type or extent of alteration, nor do the whole-rock K-Ar ages appear to have been appreciably lowered.

A Revision of Stratigraphic Nomenclature for Middle Precambrian Rocks in Northern Michigan

The name Marquette Range Supergroup is proposed to supplant the term Animikie Series for middle Precambrian strata of the Northern Peninsula of Michigan and adjacent areas of Wisconsin. The Marquette Range Supergroup consists of the Chocolay, Menominee, Baraga, and Paint River Groups, as defined in previous literature. We feel that this new name to apply to Northern Peninsula rock units is appropriate, as continued investigations have failed to show unequivocal correlation between middle Precambrian rocks of Michigan and the Huronian Supergroup of Ontario. Although the equivalence of the Animikie Group in Ontario and Minnesota with parts of the Michigan rocks is likely, the Stratigraphic complexity of the Michigan sequence requires supergroup rank. The inherent confusion of an Animikie Group in Ontario and Minnesota, and an Animikie Supergroup in Michigan, makes a local name such as Marquette Range Supergroup preferable to Animikie for the middle Precambrian rocks of Michigan.