The Bengal Fan provides a Neogene record of Eastern and Central Himalaya exhumation. We provide the first detrital thermochronological study (apatite and rutile U-Pb, mica Ar-Ar, zircon fission track) of sediment samples collected during International Ocean Discovery Program (IODP) Expedition 354 to the mid−Bengal Fan. Our data from rutile and zircon fission-track thermochronometry show a shift in lag times over the interval 5.59−3.47 Ma. The oldest sample with a lag time of 6 m.y.) has a depositional age of 5.59−4.50 Ma, and the zircon and rutile populations then show a static peak until >12 Ma. This interval, from 5.59−4.50 Ma to >12 Ma, is most easily interpreted as recording passive erosion of the Greater Himalaya. However, single grains with lag times of <4 m.y., but with high analytical uncertainty, are recorded over this interval. For sediments older than 10 Ma, these grains were derived from the Greater Himalaya, which was exhuming rapidly until ca. 14 Ma. In sediments younger than 10 Ma, these grains could represent slower, yet still rapid, exhumation of the syntaxial antiform to the south of the massif. Lag times <1 m.y. are again recorded from 14.5 Ma to the base of the studied section at 17 Ma, reflecting a period of Greater Himalayan rapid exhumation. Mica 40Ar/39Ar and apatite U-Pb data are not sensitive to syntaxial exhumation: We ascribe this to the paucity of white mica in syntaxial lithologies, and to high levels of common Pb, resulting in U-Pb ages associated with unacceptably high uncertainties, respectively.
Research Article| February 01, 1990 Northern Monashee Mountains, Omineca Crystalline Belt, British Columbia: Timing of metamorphism, anatexis, and tectonic denudation J. H. Sevigny; J. H. Sevigny 1Department of Geology and Geophysics, University of Calgary, Calgary, Alberta T2N 1N4, Canada Search for other works by this author on: GSW Google Scholar R. R. Parrish; R. R. Parrish 2Lithosphere and Canadian Shield Division, Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario K1A 0E8, Canada Search for other works by this author on: GSW Google Scholar R. A. Donelick; R. A. Donelick 3Department of Geology, Rensselaer Polytechnic Institute, Troy, New York 12180-3590 Search for other works by this author on: GSW Google Scholar E. D. Ghent E. D. Ghent 1Department of Geology and Geophysics, University of Calgary, Calgary, Alberta T2N 1N4, Canada Search for other works by this author on: GSW Google Scholar Author and Article Information J. H. Sevigny 1Department of Geology and Geophysics, University of Calgary, Calgary, Alberta T2N 1N4, Canada R. R. Parrish 2Lithosphere and Canadian Shield Division, Geological Survey of Canada, 601 Booth Street, Ottawa, Ontario K1A 0E8, Canada R. A. Donelick 3Department of Geology, Rensselaer Polytechnic Institute, Troy, New York 12180-3590 E. D. Ghent 1Department of Geology and Geophysics, University of Calgary, Calgary, Alberta T2N 1N4, Canada Publisher: Geological Society of America First Online: 02 Jun 2017 Online ISSN: 1943-2682 Print ISSN: 0091-7613 Geological Society of America Geology (1990) 18 (2): 103–106. https://doi.org/10.1130/0091-7613(1990)018<0103:NMMOCB>2.3.CO;2 Article history First Online: 02 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 J. H. Sevigny, R. R. Parrish, R. A. Donelick, E. D. Ghent; Northern Monashee Mountains, Omineca Crystalline Belt, British Columbia: Timing of metamorphism, anatexis, and tectonic denudation. Geology 1990;; 18 (2): 103–106. doi: https://doi.org/10.1130/0091-7613(1990)018<0103:NMMOCB>2.3.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 SocietyGeology Search Advanced Search Abstract The northern Monashee Mountains lie at the northern termination of a north-south zone of Eocene extension in the southern Omineca Crystalline Belt of British Columbia. Isotopic, structural, and petrogenetic studies in the northern Monashee Mountains document middle to Late Cretaceous regional metamorphism, two episodes of crustal anatexis, and a period of rapid cooling coincident with middle Eocene extension. Partial melting of geochemically and isotopically similar pelitic sources produced peraluminous granites during and postdating regional metamorphism at 100 Ma and 63 Ma, respectively. Rb-Sr muscovite-whole-rock isochrons from the younger granites give an age of 62.5 ±2.0 Ma. Rb-Sr isochron and U-Pb mineral ages are similar, indicating that the 63 Ma granites were emplaced into a terrane that had cooled to 500 °C or less by 63 Ma. Hornblende, muscovite, and biotite K-Ar and apatite fission-track mineral cooling ages from the northern Monashee Mountains are are 76.3 ±5.8 (n = 7), 51.4 ±3.5 (n = 6), 55.1 ± 2.7 (n = 10), and 45.0 ±2.9 (n = 3) Ma, respectively (mean age and standard deviation). Accelerated cooling, beginning with the emplacement of 63 Ma granites and ending at 45 Ma, occurred in response to ramping along a crustal-scale thrust fault followed by offset on the middle Eocene offset on the middle Eocene North Thompson normal fault. 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.
A geologic problem has been addressed in the Fraser-Strathcona Deep Copper mine, Sudbury, wherein diabase dikes and the chalcopyrite vein system have a mutually crosscutting relationship. In any situation where crosscutting relationships are mutual there is ambiguity, and it is important to independently test and verify the ages of the events in question, thus avoiding erroneous conclusions about the controls on mineralization. To this end detailed underground geologic mapping and U-Pb dating have been applied to a diabase dike that crosscuts the 1850 Ma Sudbury Igneous Complex and footwall Cu-Ni-PGE vein mineralization. The dike has a minimum 1.5 km strike length, 2.2 km dip length, and a width of 20 m. Where the dike cuts the Cu-Ni-PGE vein system there are (1) sharp contacts with the dike clearly cutting chalcopyrite veins, (2) laminated chalcopyrite-pyrrhotite-pentlandite veins along the hanging-wall or footwall contacts of the dike, and (3) back veining in the form of 0.5- to 2 m-wide remobilized sulfide veins and stockworks that cut the dike.
Magmatic zircons from the diabase have a concordant 206Pb/238U age of 506 ± 4 Ma, whereas abundant xenocrystic zircons have ages of 1810 to 1840 Ma and appear to have been derived from the Sudbury Igneous Complex. Magmatic titanite from the dike defines a highly discordant array of points with an upper intercept at 530 ± 21 Ma, although the titanite analyses are not precise. The discordance results from recent Pb loss, which may have been related to brittle fracturing and fluid infiltration during exhumation and mild deformation.
The diabase dike is interpreted to be part of the 500 to 600 Ma extension and rifting that formed the Ottawa-Bonnechere graben and related features during rifting of eastern North America prior to the opening of the Iapetus Ocean. It represents the youngest intrusive event within the post-impact geologic history of the Sudbury structure.
Abstract The Bengal Fan provides a Neogene record of Eastern and Central Himalaya exhumation. We provide the first detrital thermochronological study (apatite and rutile U-Pb, mica Ar-Ar, zircon fission track) of sediment samples collected during International Ocean Discovery Program (IODP) Expedition 354 to the mid–Bengal Fan. Our data from rutile and zircon fission-track thermochronometry show a shift in lag times over the interval 5.59–3.47 Ma. The oldest sample with a lag time of <1 m.y. has a depositional age between 3.76 and 3.47 Ma, and these short lag times continue to be recorded upward in the core to the youngest sediments analyzed, deposited at <1 Ma. We interpret the earliest record of short lag times to represent the onset of extremely rapid exhumation of the Eastern Himalayan syntaxial massif, defined as the syntaxial region north of the Nam La Thrust. Below the interval characterized by short lag times, the youngest sample analyzed with long lag times (>6 m.y.) has a depositional age of 5.59–4.50 Ma, and the zircon and rutile populations then show a static peak until >12 Ma. This interval, from 5.59–4.50 Ma to >12 Ma, is most easily interpreted as recording passive erosion of the Greater Himalaya. However, single grains with lag times of <4 m.y., but with high analytical uncertainty, are recorded over this interval. For sediments older than 10 Ma, these grains were derived from the Greater Himalaya, which was exhuming rapidly until ca. 14 Ma. In sediments younger than 10 Ma, these grains could represent slower, yet still rapid, exhumation of the syntaxial antiform to the south of the massif. Lag times <1 m.y. are again recorded from 14.5 Ma to the base of the studied section at 17 Ma, reflecting a period of Greater Himalayan rapid exhumation. Mica 40Ar/39Ar and apatite U-Pb data are not sensitive to syntaxial exhumation: We ascribe this to the paucity of white mica in syntaxial lithologies, and to high levels of common Pb, resulting in U-Pb ages associated with unacceptably high uncertainties, respectively.
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