The effect of recycling on provenance determinations
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Sedimentary rocks and modern sediments sample large volumes of the Earth’s crust, and preserve units that vary greatly in age and composition. Determining the provenance of component minerals is complicated by the ability of some minerals to be recycled through multiple sedimentary cycles, so minerals from completely unrelated sources may end up in the same sedimentary basin. To untangle these multi-stage signals, two or more chemical signatures measured in minerals with different stability are required. For instance, labile minerals, such as feldspar, can break down rapidly during sedimentary transport, while refractory minerals, such as zircon, can be much more resilient and survive repeated recycling. One sedimentary succession suitable for testing this hypothesis is the Upper Carboniferous Millstone Grit Group, a fluvio-deltaic, upward-coarsening sequence of mudstones, sandstones and conglomerates deposited in the Pennine Basin of northern England over c. 14 myr. New isotopic data have been measured in detrital K-feldspar and zircon from five of the seven stages, complementing previous work in the area [1,2,3]. Two K-feldspar Pb isotope peaks at 206Pb/204Pb = 12.5–15.5 and c. 18.4 indicate derivation from Archaean–Proterozoic basement and Caledonian granites, respectively. Zircon U–Pb age peaks at c. 2700, 1000–2000 and 430 Ma reflect a mixture of Archaean basement, Proterozoic sediments and Caledonian granites, while Hf model ages form two broad peaks at c. 4500–3000 and 2300–1500 Ma, indicating contributions from both juvenile and reworked crust. Strong similarities between potential sources in this complicated region mean no one mineral or isotopic system can provide a unique provenance determination. Instead, comparing first-cycle and multi-cycle minerals with different hydrodynamic properties is necessary to untangle the full story. Combining these results with published garnet, monazite and muscovite data demonstrates the power of multi-proxy provenance work, indicating a primary source area in the Greenland Caledonides, with minor contributions from Norway and Scot-land. Comparisons between zircon U–Pb distributions in Palaeozoic sediments suggest long-lived sedimentary systems recycled material around the North Atlantic over c. 100 myr, much of it ultimately derived along the Grenvillian margin of Laurentia. This consistency is interrupted only by regular variations in palaeoflow direction, reflecting tectonic evolution in the region.Keywords:
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Combining U-Pb SHRIMP zircon geochronology with cathodoluminescence imaging enables the resolution of temporally closely-spaced geological events important for understanding tectonothermal processes in the Paleoproterozoic Halls Creek Orogen of northern Australia. The youngest detrital zircon grains from a low-grade quartz-muscovite psammite of the Tickalara Metamorphics have a ^207^ Pb/ ^206^ Pb SHRIMP age of 1864+ or -4 Ma, defining a maximum depositional age for the unit. Zircon crystals from a high-grade garnet-biotite metapelite ( approximately 5-10 volume percent leucosome) from the same sequence are considerably more complex, and SHRIMP analyses form a single, large concordant group in the range approximately 1885 to 1830 Ma. The zircon crystals contain three distinct CL zoning patterns, and individual SHRIMP spots show a corresponding variation in ^207^ Pb/ ^206^ Pb ages. Concentric oscillatory-zoned zircon dominates pre-1850 Ma ages and is interpreted as detritus from igneous source rocks. Narrow structureless zircon rims infrequently overgrow and truncate the oscillatory zoning. These rims comprise most of the post-1850 Ma analyses and are inferred to be the product of uppermost amphibolite facies metamorphism, reflecting the interaction of a robust pre-existing detrital zircon suite with a very limited melt volume. In addition, some zircon cores of uncertain geological affinity contain large areas devoid of oscillatory zoning with individual analyses clustering around 1850 Ma. Dividing the data into detrital and \"metamorphic\" suites solely on the basis of CL imaging yields an older group of 18 analyses ( ^207^ Pb/ ^206^ Pb age = 1867+ or -4 Ma) and a younger group of 10 analyses ( ^207^ Pb/ ^206^ Pb age = 1843+ or -4 Ma), not including five SHRIMP spots within areas of unzoned zircon. Mixture modeling of all 33 analyses in the post-1900 Ma data set resulted in a best-fit solution composed of two distinct components: (1) an older group of 19 analyses with an age of 1867+ or -4 Ma, and (2) a younger group of 14 analyses with an age of 1845+ or -4 Ma. These results suggest that the unzoned patches of zircon might be related to metamorphism rather than being detrital cores. Importantly, the ages and proportions of populations predicted by mixture modeling are otherwise very similar to those derived from analysis of CL zoning patterns. These data imply that high-temperature metamorphism occurred in the metasedimentary rocks less than 25 my after the crystallization of the igneous detrital source. Such rapid rates of erosion, deposition, and burial have rarely been proposed for Proterozoic rocks, despite evidence for analogous orogenic processes in the Mesozoic and Cainozoic on a comparable timescale. Careful evaluation of geological and geochronological data in Proterozoic provinces elsewhere may reveal similar patterns, with potential implications for the possible rates of Proterozoic orogenesis and crustal evolution.
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Research Article| July 01, 2012 Provenance of the Lower Ocoee Supergroup, eastern Great Smoky Mountains Suvankar Chakraborty; Suvankar Chakraborty 1Department of Earth and Environmental Sciences, University of Kentucky, Lexington, Kentucky 40506-0053, USA Search for other works by this author on: GSW Google Scholar David P. Moecher; David P. Moecher † 1Department of Earth and Environmental Sciences, University of Kentucky, Lexington, Kentucky 40506-0053, USA †E-mail: moker@uky.edu Search for other works by this author on: GSW Google Scholar Scott D. Samson Scott D. Samson 2Department of Earth Sciences, Syracuse University, Syracuse, New York 13244-1070, USA Search for other works by this author on: GSW Google Scholar Author and Article Information Suvankar Chakraborty 1Department of Earth and Environmental Sciences, University of Kentucky, Lexington, Kentucky 40506-0053, USA David P. Moecher † 1Department of Earth and Environmental Sciences, University of Kentucky, Lexington, Kentucky 40506-0053, USA Scott D. Samson 2Department of Earth Sciences, Syracuse University, Syracuse, New York 13244-1070, USA †E-mail: moker@uky.edu Publisher: Geological Society of America Received: 22 Jul 2011 Revision Received: 15 Nov 2011 Accepted: 29 Nov 2011 First Online: 08 Mar 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 © 2012 Geological Society of America GSA Bulletin (2012) 124 (7-8): 1278–1292. https://doi.org/10.1130/B30578.1 Article history Received: 22 Jul 2011 Revision Received: 15 Nov 2011 Accepted: 29 Nov 2011 First Online: 08 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Suvankar Chakraborty, David P. Moecher, Scott D. Samson; Provenance of the Lower Ocoee Supergroup, eastern Great Smoky Mountains. GSA Bulletin 2012;; 124 (7-8): 1278–1292. doi: https://doi.org/10.1130/B30578.1 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 Provenance and tectonic setting of clastic rocks of the Neoproterozoic Ocoee Supergroup were assessed using modes of detrital framework minerals, detrital feldspar chemistry, whole-rock geochemistry, heavy mineral assemblages and chemistry, and detrital zircon U-Pb geochronology. Putative source rocks comprising locally exposed Grenville basement were analyzed for comparison. Samples studied are primarily arkosic to subarkosic siltstones, sandstones, and minor conglomerate. Ternary tectonic discrimination diagrams indicate a continental basement uplift setting, although the mineralogically most mature Longarm quartzite indicates a continental interior setting. The range of detrital feldspar compositions among Ocoee units is comparable to feldspar in basement rocks. Orthoclase-rich detrital alkali feldspar predominates over detrital albitic plagioclase. Whole-rock normative Qz-Or-Pl values and trace-element abundances closely match those of local basement. Snowbird Group siltstones and sandstones contain high abundances of zircon, apatite, titanite, ilmenite, and epidote, which locally occur as concentrations of heavy minerals in discrete laminae. Zircon-tourmaline-rutile indices and titanite compositions are most consistent with a granitic source for these sediments. The majority of detrital zircon U-Pb ages are late Mesoproterozoic, with scattered late Neoproterozoic and Paleoproterozoic grains. All but one of the ∼1200 detrital zircons (at ca. 2000 Ma) can be accounted for by a local Blue Ridge basement magmatic source terrane. Archean detrital zircon ages are absent from the Ocoee detrital zircon age population. This absence is consistent with the general paucity of Archean grains in other Neoproterozoic Southern Appalachian clastic sequences. All data are consistent with a lithologically and geochronologically restricted, proximal, flank source for sediment filling a marginal continental rift. The Ocoee represents the first pulse of sediment dominated by Grenville-age zircon on the eastern Laurentian margin and serves as a major source of recycled zircon for Paleozoic clastic sequences in eastern Laurentia. The youngest detrital zircon age obtained from the basal Ocoee Wading Branch Formation (639 ± 8 Ma) provides a maximum depositional age for the Ocoee Supergroup and onset of rift-filling sedimentation in the intracontinental Ocoee basin. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
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In an attempt to clarify the significance of Pb model ages in Pb-Zn sedimentary deposits, we report high-precision Pb isotopic compositions for 64 galenas and 52 K-feldspars, the former from ores and the latter separated from granites. All samples are from Spain and the French Pyrenees. Lead from galena ores is of unequivocal continental origin. With few exceptions, Pb model ages systematically exceed emplacement ages by up to 400 Ma, a gap which is well outside the uncertainties of ~ 30 Ma assigned to the model. The histogram of the new high-precision Pb isotope data shows prominent peaks of galena Pb model ages at 94 ± 38 Ma and 392 ± 39 Ma. When the data are consolidated with literature data and examined in 3-dimensional Pb isotope space, cluster analysis identifies five groups. The model ages of the peaks occur, in order of decreasing peak intensity, at 395 ± 40 (Middle Devonian), 90 ± 34 Ma (Middle Cretaceous), and 613 ± 42 Ma (Neoproterozoic), with two minor peaks at 185 + 26 Ma (Jurassic) and 313 ± 41 (Upper Carboniferous). To a large extent, the model ages centered around these peaks correspond to distinct localities. The ages of the peaks do not coincide with any of the Betic, Variscan, or Pan-African tectonic events, which are the main tectonic episodes that shaped Iberian geology, but instead match well-known global oceanic anoxic events. It is argued that surges of metals weathered from continental surfaces scorched during anoxic events accumulated and combined in anoxic water masses with unoxidized marine sulfide released by submarine hydrothermal activity to precipitate the primary Pb-Zn stock. Frozen Pb isotope compositions require that galenas from black shales are the source of the final ores. The sulfides were later remobilized by large-scale convective circulation of basinal and hydrothermal fluids. The peaks of K-feldspar Pb model ages are distinct from those of galenas and do not correlate with magmatic emplacement ages. It is suggested that they instead reflect local circulation in Paleozoic sediments surrounding individual plutons. While Pb isotopes can be used as a regional provenance tool, such an approach requires that the data are considered in a fully 3-dimensional space.
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Abstract Detrital zircon U-Pb age and Hf isotope studies are useful for identifying the chemical evolution of the continental crust. Zircon, however, is typically a magmatic mineral and thus often fails to document the timing of low-grade metamorphism, and its survival through multiple sedimentary cycles potentially biases the crustal evolution record toward older events. In contrast, monazite typically records metamorphic events and is less likely to survive sedimentary recycling processes, thus providing information not available by zircon. Here, we demonstrate that monazite apparently faithfully records the Sm-Nd isotope composition of the bulk rock and can therefore track the record of crustal evolution and growth, similar to Hf isotopes in zircon. We examine the utility of detrital zircon and monazite for studies of crustal evolution through a comparison of age and tracer isotope information using sediments from two large rivers draining the South China block (SCB). Monazite and zircon grains yield mostly Mesozoic and Paleozoic U-Pb ages and depleted mantle model age peaks at ca. 1900–1300 Ma, indicating that both minerals preserve similar, yet critical, information on the crustal evolution of the catchment area. In contrast, zircon yields abundant Neoproterozoic and older U-Pb ages with a very large spread of model ages, preserving a history strongly skewed to older ages. Based on the lack of known rocks of this age in the catchments, ancient zircon was likely sourced from sedimentary rocks within the catchment area. This combined data set presents a more complete history of crustal evolution and growth in the SCB and demonstrates the advantages of an integrated approach that includes both detrital monazite and zircon.
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Massif
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Abstract Quartz, the most common mineral in most sandstones, rarely is used in provenance studies. This study demonstrates the provenance-discriminatory potential of combining cathodoluminescence (CL) color wavelength spectra of quartz with morphology and in situ U-Pb ages of zircon. The Cambrian Meson Group in northwestern Argentina is used for the test of this combined methodology, because it is composed of nonmetamorphosed sandstones with > 90% quartz and a dominance of zircon among the heavy minerals. Correlation of the results from the two methods is evident: (1) both CL spectra typical for bright-luminescent quartz (red to blue; > 90%) and oscillatory zircon growth zoning (80–90%) indicate a dominance of magmatic detritus. (2) Upsection, diminishing input from euhedral 510–600 Ma zircons (70% at the base) correlates with lower amounts of volcanic quartz (40% red and violet grains at the base). Instead, plutonic quartz and abraded zircons of 550–700 Ma age prevail. The data point to the occurrence of a (nearly) synsedimentary volcanic phase, which has not been known previously. Furthermore, initial short transportation paths from nearby magmatic bodies (100–200 km) can be assumed, because of the low degree of grain abrasion and an age correlation with magmatic bodies in the region, such as the Santa Rosa de Tastil batholith, here dated at 513 +4/−5 Ma. The proximity to the source area at an initial stage indicates first-cycle sand. The maturity was probably reached by intense chemical weathering. The upsection-diminishing young, volcanic input indicates erosion of the volcanic source within a time period of millions of years. At that stage, mainly up to 1000-km-long coast-parallel transport from the Sierras Pampeanas in the south fed the depositional basin. The change in—and expansion of—the main source areas record a change to production of multicycle sand. Hence, the maturity was partly caused by sedimentary recycling. More generally, the provenance-discriminative effect of the CL of quartz, which has been doubted by some researches, can be confirmed. Therefore, the results imply that both quartz and zircon should be taken into account in provenance studies of quartz arenites.
Margin (machine learning)
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