Browsing by Subject "geochronology"
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Item Bulletin No. 41. The Precambrian Geology and Geochronology of Minnesota(Minnesota Geological Survey, 1961) Goldich, Samuel S.; Nier, Alfred O.; Baadsgaard, Halfdan; Hoffman, John H.; Krueger, Harold W.This bulletin is an outstanding example of the cooperation of several scientists and scientific organizations. Without the cooperation of all concerned, such a comprehensive correlation of age determinations and regional geology would have been impossible short of many years of work. The results are a particularly important demonstration of cooperation among geologists, chemists, and physicists. Radioactivity dating of a large number of igneous and metamorphic rocks by the potassium-argon and the rubidium-strontium methods is the basis for revision of the classification of the Precambrian rocks of Minnesota. The major divisions of the three-fold classification are made at time boundaries of 2.5 and 1.7 billion years (b.y.), corresponding to the time of two major orogenies, the Algoman and the Penokean, respectively. The eras are referred to as Early, Middle, and Late Precambrian in preference to the older terminology of Earlier, Medial, and Later Precambrian. The division between the Early and Middle Precambrian is placed at the time of the Algoman rather than of the older Laurentian orogeny. The division between the Middle and Late Precambrian is made on the basis of the Penokean orogeny which resulted in a mountain chain that extended from central Minnesota through Wisconsin into Michigan. The Penokean Mountains formerly were assigned erroneously to late Keweenawan time. The Early Precambrian rocks are divided into the Ontarian and the Timiskamian systems. The Ontarian rocks include the Keewatin group of Minnesota and the Coutchiching metasediments which underlie the Keewatin greenstones in Ontario. Some of the gneisses in the Giants Range and the Vermilion granite regions of Minnesota probably were derived by metamorphism of ancient sediments that were deposited prior to the great outpouring of basalt flows assigned to the Ely greenstone of the Keewatin group.Item Guidebook 5. Field Trip Guide Book for Precambrian Migmatitic Terrane of the Minnesota River Valley(Minnesota Geological Survey, 1972) Grant, J.A.; Himmelberg, Glen R.; Goldich, S.S.The Minnesota River Valley provides a tantalizing window onto the Canadian Shield on the eastern margin of the Great Plains, tantalizing because of the high grade of the metamorphism, and especially because of the antiquity of the rocks there exposed. Essentially, this is a migmatitic terrane of granitic gneisses with lesser amphibolitic gneisses, commonly with pyroxene, and biotite-rich gneisses, which may contain garnet, cordierite, sillimanite, anthophyllite, or hypersthene. Some of the rocks are greater than 3.0 b.y. in age, and they have been involved in metamorphism and deformation at least 2.6 b.y. ago. These events left rocks with a metamorphic grade in the upper amphibolite or granulite facies, and with a major structure that is similar throughout most of the exposed area. Later minor intrusions, dominantly mafic, cut the older rocks, and conglomerate and quartzite of the Sioux Formation of Late Precambrian age locally overlie them. Deep weathering of the gneisses formed a regolith about 100 feet thick, a part of which was reworked in the formation of Cretaceous deposits of sand and clay. Over this came the glacial deposits of the Pleistocene. With the formation of Lake Agassiz, drainage via Glacial River Warren scoured out the precursor of the present valley leaving an underfit present-day Minnesota River and the glimpse of the Precambrian described in the following pages. The granitic gneisses in the vicinities of Morton, Granite Falls, and Montevideo are among the oldest known crustal rocks. Like very ancient rocks in other parts of the world the gneisses have had a complicated history, and metamorphic changes have masked their original characters and obscured their age. Conservatively the age may be given as 3200 or 3300 m. y. Goldich and others (1970) have attempted to probe the metamorphic history and concluded that the gneisses date back to 3550 m.y. ago. Similarly old, or older gneisses (3600 to 4000 m.y.) have been reported from the Godthaab district, West Greenland (Black and others, 1971). Field and more detailed geochronological and geochemical investigations are being continued, and the nature of this work is briefly indicated in following sections.Item M-197 Bedrock Geology of the International Falls and LittleFork 30' x 60' Quadrangles, northern Minnesota(Minnesota Geological Survey, 2014-09-25) Jirsa, Mark A.; Boerboom, Terrence J.; Chandler, V.W.The map depicts a complex history of volcanism, sedimentation, intrusion, multiple events of migmatization involving partial melting and melt dispersion, and several periods of deformation and metamorphism. It lies within the southern exposed extent of the Archean Superior Province. In northern Minnesota, the province is divided into three major tectonomagmatic terranes known as the Wabigoon, Quetico, and Wawa subprovinces. The publication shows the locations of mapped bedrock outcrops, structural measurements, interpreted bedrock geology in both map and cross-section views, and associated explanations. Ancillary imagery includes maps of regional geologic setting, index of archived mapping incorporated into this interpretation, and schematic depiction of geologic history.Item Transantarctic Mountains granitoid isotopic data(2022-05-04) Goodge, John W.; Fanning, C. Mark; Fisher, Chris M.; Vervoort, Jeff D.; jgoodge@d.umn.edu; Goodge, John W.; Department of Earth & Environmental Sciences, University of Minnesota DuluthWe obtained new U-Pb ages and O-Hf isotopic compositions in zircons from nearly 40 samples of Ross Orogen granitoids covering a large section of the modern Transantarctic Mountains in Antarctica. New age and isotopic analysis of zircons from this large suite of Ross Orogen granitoids spanning >2,000 km along the orogen provide a wealth of new geochronologic, tracer, and inheritance information, enabling us to investigate the pace of magmatism, along-strike temporal and geochemical trends, sources of melts, and tectonic mode of convergence during magmatism. Data provided to the Data Repository include primary individual sample results of in situ analysis of zircon. Age and isotopic signatures were obtained by in situ analysis of the same zircon domains to first define ages by the U-Pb method, followed by O- and Hf-isotope analysis. U-Pb and O-isotope analysis were done by sensitive high-resolution ion microprobe (SHRIMP) at Australian National University, and precise Hf-isotope analysis was done using a laser ablation system coupled to a multi-collector inductively coupled plasma mass spectrometer (LA-MC-ICPMS) at Washington State University. Summary tabulations of the sample data are included with a manuscript. Also included is a narrative summary of zircon U-Pb age results for each sample, describing zircon characteristics and age interpretations.