Geochronology Of Precambrian Basement And Adjacent Rocks Black Hills South Dakota

Author : Angela Lynn Hull
Publisher :
ISBN 13 :
Total Pages : 109 pages
Book Rating : 4.:/5 (872 download)

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Download or read book Geochronology and Thermochronology of Precambrian Basement Drill Core Samples in Nebraska and Southeastern South Dakota written by Angela Lynn Hull and published by . This book was released on 2013 with total page 109 pages. Available in PDF, EPUB and Kindle. Book excerpt: The ancient geology of midcontinent North America preserves a unique glimpse into tectonic processes that culminated in the formation of stable continental lithosphere. Much is known about the tectonic and crustal evolution in regions where Precambrian bedrock is exposed for direct observation (i.e. Rocky Mountains and the Lake Superior region). However, thick Phanerozoic sedimentary cover overlying the central Yavapai Province, between the Rockies and the Great Lakes, limits our knowledge of that important region. Drill core samples provide the only means to directly survey this zone of the midcontinent. Here I present the results of U-Pb and Lu-Hf zircon geochronology, and Ar-Ar thermochronology obtained from Paleoproterozoic basement drill core samples of the central Yavapai Province from easternmost Colorado and Nebraska, and from older rocks in southern South Dakota. LA-ICP-MS U-Pb geochronology analyses yielded six new zircon ages ranging from 1694 Ma to 1825 Ma in the central Yavapai Province and one age of 2653±11 Ma from southern South Dakota. The same zircon separates were analyzed for Lu-Hf tracer data and yielded overall, positive mean initial eHf values ranging from +1.65 to +8.45 with the exception of the Archean S.D. sample which yielded a slight negative initial eHf value of -0.28. 40Ar-39Ar mineral analyses from mainly micas yielded age spectra that varied from well-behaved to complex. Mineral ages from the central Yavapai Province were consistently younger than anticipated, ranging in age from ca. 1138-1267 Ma for micas and 1487 Ma for hornblende. Mineral ages from the South Dakota region ranged from ca. 1728-1869 Ma (micas) and 2449 Ma (hornblende), consistent with previous results in the area. The new Hf results analyzed here provide the first regional perspective of midcontinent crustal provenance. Overall positive initial eHf values from the central Yavapai samples demonstrate crust across a broad region of the Midcontinent was largely derived from a juvenile source with contributions of slightly older reworked crust. This is consistent with the long held arc accretion model for the formation of Laurentia. Ar-Ar mineral ages are the first supplied for South Dakota and the central Yavapai Province. In South Dakota, Ar-Ar results provide a thermochronologic context to the relatively sparse basement data that exists here, extending boundaries of the East-Central Minnesota Batholith farther westward, revealing Archean crust within the Proterozoic Mobile Belts, and supplying post-metamorphic cooling details to nearby Trans Hudson and Penokean Orogenic events. Anomalously young Ar-Ar mica ages across the central Yavapai Province, stretching from eastern Colorado across Nebraska, are most striking and indicate a regional Neoproterozoic thermal overprinting event. We attribute overprinting to be the result of "squeegee" tectonics in which Grenvillian thrusting caused transport of warm, hydrothermal fluids from buried margin sediments into the continental interior inducing laterally extensive thermal resetting and fluid alteration there.