Geochronology of Some Alkalic Rock Provinces in Eastern and Central United States
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Recent efforts to understand the ongm of igneous rocks have emphasized the association between petrographic suites and geologic environment. Plate tectonic theory has been particularly useful in relating certain types of calc-alkalic rocks to their position near active plate boundaries. In contrast, alkalic rocks are often emplaced after cessation of orogenic forces or in completely anorogenic settings of the craton. The role of plate tectonics in dictating such magmatism is not readily understandable. One obviou s prerequisite to any effort at iden tification of under lying causes is the accurate dating of the igneou s rocks. My objective in this paper is to review our prescnt state of knowledge concerning a chronology for some alkalic rocks of eastern and central United States. Although'representing less than five percent of the volume of all igneous rocks, the alkalic clan includes a variety of interesting petrographic types, such as syenite, alkalic granite, alkalic gabbro, lamprophyre, and mica peridotite. These rocks typically occupy discordant stocks, plugs, dikes, sills, and diatremes at relatively shallow levels in the crust; and they may also include eruptive counterparts. The literature abounds with contributions regarding their structure, petrography, mineralogy, and geochemistry; and I do not stray too far from my intended objective into these areas. My working definition of an alkalic rock corresponds essentially to that of Sorensen (1974), but I also include certain u ltram afic rock s (Wyllie 1967) that are often found in petrologic and spatial association. Generally, rock n ames are accepted as the original authors used them without defending the specific genetic implication sometimes att�ched to the name. This paper focuses upon several petrographically cohcrent groups of alkalic rocks, or provinces, that occur within the eastern and central U nited States. Although each province contains some felsic and some mafic members, two rather differentKeywords:
Geochronology
Abstract The Eastern Segment forms a complex crustal terrane that occupies the southeastern part of the Southwest Scandinavian Domain adjacent to the less deformed 1.65–1.85 Ga Transscandinavian Igneous Belt (TIB). New U-Pb data from the Eastern Segment indicate that orthogneisses and granitoid rocks have protolith ages equivalent to the later magmatic phases of the TIB and that the earliest thermotectonic episodes in the Eastern Segment occurred between 1.70 and 1.61 Ga during the Gothian Orogeny. The comparable ages for Eastern Segment gneisses and TIB rocks represent permissive evidence for the hypothesis that the orthogneiss protoliths intruded into the western margin of Fennoscandia and are not exotic with respect to the pre-Gothian craton. The eastern limit of Gothian deformation is thus interpreted to be an intracratonic deformation front that coincides with the later Sveconorwegian deformation front along the Protogine Zone south of Lake Vänern. The post-Gothian anorogenic period was interrupted, in at least the eastern part of the Eastern Segment, by a thermal and magmatic event at 1.47 Ga that generated granitic dykes and (re)crystallized titanite. Sveconorwegian U-Pb ages and field observations are compatible with a thrusting and exhumation of western supracrustal terranes over the Eastern Segment during the Sveconorwegian Orogeny, followed by isostatic unroofing and cooling of the lower deck at 950–930 Ma. Available, albeit limited, Sveconorwegian titanite ages in the Eastern Segment young from east to west, ranging from 950 to 930 Ma, and are substantially younger than those dated in the western terranes of SSD.
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<p>Geological background of the Chinese Loess Plateau source regions, samples and analytical methods, supplemental Figures S1–S4, and data tables S1–S4. </p>
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Re-Os molybdenite geochronology relies on the aliquant analysis taken from a mineral separate. Although suitable for coarse-grained molybdenite samples, traditional mineral separation techniques are not ideally suited for samples possessing fine-grained molybdenite ( μ m) and thus hamper the application of Re-Os geochronology for such samples. Here, we demonstrate a room-temperature hydrofluoric acid (HF) chemical separation technique that is capable of isolating ultrafine molybdenite (i.e., μ m) for Re-Os geochronology. Six Re-Os molybdenite model ages from four molybdenite in-house control and NIST reference material (RM8599) samples exposed to concentrated HF are indistinguishable from the Re-Os molybdenite model ages for aliquants not exposed to HF. Thus, HF exposure at room temperature has no effect on Re-Os molybdenite systematics. Our HF chemical separation technique was then applied to six ultrafine molybdenite samples from the Lupa goldfield, southwest Tanzania. Three samples from the Kenge deposit provide a weighted average Re-Os molybdenite model age of 1953 ± 5 Ma (MSWD = 0.6; n = 3), whereas three samples and one repeat analysis from the Porcupine deposit provide a weighted average Re-Os molybdenite model age of 1886 ± 5 Ma (MSWD = 1.5; n = 4). Our proposed analytical protocol has allowed us to determine reproducible ages from ultrafine molybdenite samples within the Lupa goldfield that were previously unsuitable for molybdenite Re-Os geochronology using conventional mineral separation techniques.
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This dataset was collected by Leah E. Morgan in the Argon Geochronology Laboratory of the USGS in Denver, Colorado in 2015. The dataset contains full raw argon isotopic data for samples presented in the manuscript: Characteristics and 40Ar/39Ar Geochronology of the Erdenet Cu-Mo Deposit, Mongolia. Imants Kavalieris, Bat-Erdene Khashgerel, Leah E. Morgan, Alexander Undrakhtamir, Adiya Borohul. Economic Geology Aug 2017, 112 (5) 1033-1053; DOI: 10.5382/econgeo.2017.4500.
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During the past 10–15 years, analytical innovations in geochronology have greatly enhanced the application of geochronological data to geological problems. The advances are mainly driven by developments in laser ablation inductively coupled plasma mass spectrometry (LA-ICPMS) which allows for rapid determination of U-Th-Pb ages of mineral grains in large sample sets. LA-ICPMS has now become the most common tool in the application of zircon geochronology to a host of different geological problems.
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This article is an individual and necessarily brief overview of the history of geochronology in Queensland with emphasis on the contributions of scientists and technicians who played major roles in the implementation of the geochronology facilities at the University of Queensland. Geochronology and isotope geochemistry research summarised here have contributed among others to our understanding of the chemical and isotopic evolution of the early Earth, mantle dynamics, the thermal history of sedimentary basins and time-scales of weathering and climate change.
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