Abstract Dehydration reactions within subducted oceanic crust are important for fluid‐mediated element transfer within the subducting plate and potentially to the mantle wedge. The effects of metamorphic reactions and fluid flow on element recycling that occur during retrogression and exhumation of subducted continental crust from mantle depths are poorly understood. We study two metabasite pods with fresh eclogite cores and retrogressed amphibolite‐facies rims and surrounding host gneiss within the Western Gneiss Region (WGR), Norway, to better understand element mobility and mass transfer during exhumation of subducted continental crust. Bulk‐rock data were collected from samples taken across the pod and into the host gneiss. Phengite breakdown in eclogite and epidote recrystallization in veins and/or gneiss within pod cores contributed large ion lithophile elements and REE to retrogressed eclogite closest to the pod cores. In gneiss hosting the pods, phengite and epidote breakdown provided fluid that mediated elemental transfer and redistribution to the pod rim or tail. Compared to the studied pod in the southern WGR, the pod in the northern WGR underwent higher P ‐ T conditions, partial melting and higher strain rates. This resulted in the infiltration of external fluid farther into the pod interior from the rim and facilitated larger mass gain in trace elements in the amphibolite tail of the pod relative to fresh eclogite in the core. The results show clear evidence for retrogression dehydration reactions driving significant fluid‐mediated element redistribution as observed on the outcrop scale during exhumation following ultrahigh‐pressure metamorphism, which directly impacts element signatures within the exhuming crust.
Journal Article Evaluating Consensus in Experimental K-ratios from over 40 WDS and EDS Measurement Systems Get access W O Nachlas, W O Nachlas Department of Geoscience, University of Wisconsin, Madison, WI, United States Corresponding author: nachlas@wisc.edu Search for other works by this author on: Oxford Academic Google Scholar A Moy, A Moy Department of Geoscience, University of Wisconsin, Madison, WI, United States Search for other works by this author on: Oxford Academic Google Scholar N Ritchie, N Ritchie National Institute of Standards and Technology, Gaithersburg, MD, United States Search for other works by this author on: Oxford Academic Google Scholar J Donovan, J Donovan CAMCOR, University of Oregon, Eugene, OR, United States Search for other works by this author on: Oxford Academic Google Scholar J H Fournelle, J H Fournelle Department of Geoscience, University of Wisconsin, Madison, WI, United States Search for other works by this author on: Oxford Academic Google Scholar J Allaz, J Allaz Department of Earth Sciences, ETH Zurich, Switzerland Search for other works by this author on: Oxford Academic Google Scholar R Almeev, R Almeev Institute of Mineralogy, Leibniz University Hannover, Germany Search for other works by this author on: Oxford Academic Google Scholar E S Bullock, E S Bullock Earth and Planets Laboratory, Carnegie Science, Washington, DC, United States Search for other works by this author on: Oxford Academic Google Scholar J W DesOrmeau, J W DesOrmeau Department of Geological Sciences & Engineering, University of Nevada, Reno, NV, United States Search for other works by this author on: Oxford Academic Google Scholar K Goemann, K Goemann Central Science Laboratory, University of Tasmania, Hobart, TAS, Australia Search for other works by this author on: Oxford Academic Google Scholar ... Show more R Hoffmann, R Hoffmann Institute for Geology, Mineralogy and Geophysics, Ruhr-Universität Bochum, Germany Search for other works by this author on: Oxford Academic Google Scholar P Jokubauskas, P Jokubauskas Faculty of Geology, University of Warsaw, Poland Search for other works by this author on: Oxford Academic Google Scholar N Jöns, N Jöns Institute for Geology, Mineralogy and Geophysics, Ruhr-Universität Bochum, Germany Search for other works by this author on: Oxford Academic Google Scholar T Lam, T Lam Museum Conservation Institute, Smithsonian Institution, Suitland, MD, United States Search for other works by this author on: Oxford Academic Google Scholar A Locock, A Locock Department of Earth and Atmospheric Sciences, University of Alberta, Edmonton, Alberta, Canada Search for other works by this author on: Oxford Academic Google Scholar D M Ruscitto, D M Ruscitto General Electric Research, Niskayuna, NY, United States Search for other works by this author on: Oxford Academic Google Scholar E P Vicenzi, E P Vicenzi Museum Conservation Institute, Smithsonian Institution, Suitland, MD, United States Search for other works by this author on: Oxford Academic Google Scholar A von der Handt, A von der Handt Department of Earth, Ocean and Atmospheric Sciences, University of British Columbia, Vancouver, British Columbia, Canada Search for other works by this author on: Oxford Academic Google Scholar B Wade, B Wade Adelaide Microscopy, The University of Adelaide, Adelaide, SA, Australia Search for other works by this author on: Oxford Academic Google Scholar P Yang, P Yang Department of Earth Sciences, University of Manitoba, Canada Search for other works by this author on: Oxford Academic Google Scholar D Zhang D Zhang Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing, China Search for other works by this author on: Oxford Academic Google Scholar Microscopy and Microanalysis, Volume 29, Issue Supplement_1, 1 August 2023, Pages 225–226, https://doi.org/10.1093/micmic/ozad067.100 Published: 22 July 2023
Abstract The timing and processes of ductile deformation and metasomatism can be documented using apatite petrochronology. We integrated microstructural, U-Pb, and geochemical analyses of apatite grains from an exhumed mylonitic shear zone in the St. Barthélémy Massif, Pyrenees, France, to understand how deformation and metasomatism are recorded by U-Pb dates and geochemical patterns. Electron backscatter diffraction (EBSD) analyses documents crystal plastic deformation characterized by low-angle boundaries (<5°) associated with dislocation creep and evidence of multiple slip systems. Laser ablation–inductively coupled plasma–mass spectrometry (LA-ICP-MS) U-Pb maps indicate that dates in deformed grains reflect, and are governed by, low-angle dislocation boundaries. Apatite rare earth element (REE) and U-Pb behavior is decoupled in high-grade gneiss samples, suggesting REEs record higher-temperature processes than U-Pb isotopic systems. Apatite from (ultra)mylonitic portions of the shear zone showed evidence of metasomatism, and the youngest dates constrain the age of metasomatism. Collectively, these results demonstrate that crystal plastic microstructures and fluid interactions can markedly change apatite isotopic signatures, making single-grain apatite petrochronology a powerful tool for dating and characterizing the latest major deformation and/or fluid events, which are often not captured by higher-temperature chronometers.
Research Article| June 29, 2017 Rapid time scale of Earth's youngest known ultrahigh-pressure metamorphic event, Papua New Guinea Joel W. DesOrmeau; Joel W. DesOrmeau 1Department of Geological Sciences and Engineering, University of Nevada, Reno, Nevada 89557, USA Search for other works by this author on: GSW Google Scholar Stacia M. Gordon; Stacia M. Gordon 1Department of Geological Sciences and Engineering, University of Nevada, Reno, Nevada 89557, USA Search for other works by this author on: GSW Google Scholar Timothy A. Little; Timothy A. Little 2School of Geography, Environment and Earth Sciences, Victoria University of Wellington, Wellington 6012, New Zealand Search for other works by this author on: GSW Google Scholar Samuel A. Bowring; Samuel A. Bowring 3Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA Search for other works by this author on: GSW Google Scholar Nilanjan Chatterjee Nilanjan Chatterjee 3Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, Massachusetts 02139, USA Search for other works by this author on: GSW Google Scholar Geology (2017) 45 (9): 795–798. https://doi.org/10.1130/G39296.1 Article history received: 25 Feb 2017 rev-recd: 06 May 2017 accepted: 09 May 2017 first online: 29 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 Joel W. DesOrmeau, Stacia M. Gordon, Timothy A. Little, Samuel A. Bowring, Nilanjan Chatterjee; Rapid time scale of Earth's youngest known ultrahigh-pressure metamorphic event, Papua New Guinea. Geology 2017;; 45 (9): 795–798. doi: https://doi.org/10.1130/G39296.1 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 SocietyGeology Search Advanced Search Abstract Subduction of continental lithosphere to mantle depths results in the recrystallization of crustal material during ultrahigh-pressure (UHP) metamorphism. Knowledge of the duration and pressure and temperature (P-T) conditions at which UHP rocks equilibrate is fundamental for assessing the rates and mechanisms involved in the transfer of crustal material to and from mantle depths. High-precision U-Pb zircon geochronology combined with thermobarometry, P-T pseudosection modeling, and Zr-in-rutile thermometry of the youngest known UHP eclogite from eastern Papua New Guinea (PNG) documents metamorphism at mantle depths on a sub-million year time scale. The P-T analyses based on garnet, omphacite, phengite, and rutile compositions show the crustal rocks were subducted to conditions of ∼27–31 kbar and ∼715 °C. Individual zircons containing inclusions of the peak UHP mineral assemblage, with identical composition as those used in P-T estimates, yield chemical abrasion–isotope dilution–thermal ionization mass spectrometry (CA-ID-TIMS) 206Pb/238U (Th-corrected) dates of 6.0 ± 0.2 Ma to 5.2 ± 0.3 Ma. These results more precisely and accurately define the youngest UHP metamorphism on Earth and provide compelling evidence of rapid (3.2–2.3 cm/yr) exhumation of the Pliocene PNG terrane. Subsequent retrogression of the terrane near the base of the crust and final emplacement within the upper crust occurred in less than ∼3 m.y. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
Abstract Metamorphic core complexes (MCCs) are considered to be a hallmark of large-magnitude crustal extension, with their characteristic high-strain mylonitic fabrics attributed to simple-shear strain downdip of a detachment fault. However, some MCCs exhibit pure-shear–dominated mylonitic fabrics temporally decoupled from regional extension, which may be related to magmatically enhanced buoyant doming of the lower crust. We tested the viability of buoyant doming for the formation of MCCs in the North American Cordillera by investigating the kinematics and conditions of mylonitic shear in the Ruby Mountains–East Humboldt Range (REH) MCC. Field observations and geochronology demonstrate an ~10 m.y. gap between midcrustal attenuation and regional extension in the brittle upper crust. Mylonites in the REH record general shear strain with >80% bulk attenuation at strain rates of 10–13 to 10–12 s–1 and temperatures of 400–600 °C. The REH mylonites developed at the culmination of 40–29 Ma magmatism involving mantle-derived mafic intrusions and leucogranite crustal melts prior to post–17 Ma detachment faulting. We posit diapirism driven by thermal and melt buoyancy could have generated shear zones along the diapir flanks at our documented strain rates. Characteristics of the buoyant doming model are expressed in many low- to moderate-melt-fraction MCCs globally, and the pre-extensional high-strain mylonitic fabrics may therefore be an important mechanism for localizing temporally decoupled brittle detachment faulting.
Abstract Strongly deformed footwall rocks exposed in metamorphic core complexes (MCC) of the North American Cordillera were exhumed via ductile attenuation, mylonitic shearing, and detachment faulting. Whether these structures accommodated diapiric upwelling or regional extension via low‐angle normal faulting is debated. The Ruby Mountains‐East Humboldt Range MCC, northeast Nevada, records top‐west normal‐sense exhumation of deformed Proterozoic‐Paleozoic stratigraphy and older basement. We conducted 1:24,000‐scale mapping of the southwestern East Humboldt Range, with integrated structural, geochemical, and geochronological analyses to characterize the geometry and kinematics of extension and exhumation of the mylonitized footwall. Bedrock stratigraphy is pervasively intruded by Cretaceous, Eocene, and Oligocene intrusions, but observations of a coherent stratigraphic section show >80% vertical attenuation of Neoproterozoic to Ordovician rocks. These rocks are penetratively sheared with top‐west kinematics. The shear zone thus experienced combined pure‐ and simple‐shear (i.e., general shear) strain. We argue that this shear zone was syn‐/post‐kinematic with respect to Oligocene plutonism because: (a) mylonitic shearing spatially corresponds with preceding Oligocene intrusions; (b) thermochronology reveals that the shear zone experienced substantial cooling and exhumation after Oligocene plutonism; and (c) the mylonites are crosscut by undated, but likely late Oligocene, leucogranite. We propose that Eocene mantle‐derived magmatism and thermal incubation led to Oligocene diapiric upwelling of the middle crust, with ductile stretching focused on the flanks of this upwarp. Regional Basin and Range extension initiated later in the middle Miocene. Therefore, the development of the East Humboldt Range shear zone was not driven by regional extension and coupled detachment faulting.
