Research Article| May 01, 2000 Two episodes of monazite crystallization during metamorphism and crustal melting in the Everest region of the Nepalese Himalaya Robert L. Simpson; Robert L. Simpson 1Department of Earth Sciences, Oxford University, Parks Road, Oxford OX1 3PR, UK Search for other works by this author on: GSW Google Scholar Randall R. Parrish; Randall R. Parrish 2NERC Isotope Geosciences Laboratory, Keyworth, Nottingham NG12 5GG, UK, and Department of Geology, Leicester University, Leicester LE1 7RH, UK Search for other works by this author on: GSW Google Scholar Mike P. Searle; Mike P. Searle 1Department of Earth Sciences, Oxford University, Parks Road, Oxford OX1 3PR, UK Search for other works by this author on: GSW Google Scholar David J. Waters David J. Waters 1Department of Earth Sciences, Oxford University, Parks Road, Oxford OX1 3PR, UK Search for other works by this author on: GSW Google Scholar Geology (2000) 28 (5): 403–406. https://doi.org/10.1130/0091-7613(2000)28<403:TEOMCD>2.0.CO;2 Article history received: 11 Oct 1999 rev-recd: 02 Feb 2000 accepted: 16 Feb 2000 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 Robert L. Simpson, Randall R. Parrish, Mike P. Searle, David J. Waters; Two episodes of monazite crystallization during metamorphism and crustal melting in the Everest region of the Nepalese Himalaya. Geology 2000;; 28 (5): 403–406. doi: https://doi.org/10.1130/0091-7613(2000)28<403:TEOMCD>2.0.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 New monazite U-Pb geochronological data from the Everest region suggest that ∼20–25 m.y. elapsed between the initial India-Asia collision and kyanite-sillimanite–grade metamorphism. Our results indicate a two-phase metamorphic history, with peak Barrovian metamorphism at 32.2 ± 0.4 Ma and a later high-temperature, low-pressure event (620 °C, 4 kbar) at 22.7 ± 0.2 Ma. Emplacement and crystallization of the Everest granite subsequently occurred at 20.5–21.3 Ma. The monazite crystallization ages that differ by 10 m.y. are recorded in two structurally adjacent rocks of different lithology, which have the same postcollisional pressure-temperature history. Scanning electron microscopy reveals that the younger monazite is elaborately shaped and grew in close association with apatite at grain boundaries and triple junctions, suggesting that growth was stimulated by a change in the fluid regime. The older monazite is euhedral, is not associated with apatite, and is commonly armored within silicate minerals. During the low-pressure metamorphic event, the armoring protected the older monazites, and a lack of excess apatite in this sample prevented new growth. Textural relationships suggest that apatite is one of the necessary monazite-producing reactants, and spots within monazite that are rich in Ca, Fe, Al, and Si suggest that allanite acted as a preexisting rare earth element host. We propose a simplified reaction for monazite crystallization based on this evidence. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
The early Mesozoic Bridge River Terrane of oceanic affinity lies in the eastern Coast Mountains of southwestern British Columbia and is one of several terranes located near the Intermontane Superterrane‐Insular Superterrane boundary. Eocene strike‐slip faulting and related extensional faulting and plutonism are largely responsible for the present upper crustal configuration of the Bridge River Terrane. This paper presents U‐Pb geochronology and a detailed structural and geochronological analysis of Eocene deformation of the Bridge River Terrane near Lillooet, British Columbia. Evidence for Eocene magmatism and deformation within the Bridge River Terrane includes 48.5–46.5 Ma U‐Pb zircon ages from four deformed granitic rocks, two of which are from the Mission Ridge pluton. All are cut by the Mission Ridge normal fault, which juxtaposes low‐grade Bridge River Group in the hanging wall against medium‐grade Bridge River Schist and associated intrusions in the footwall. The Mission Ridge fault is a low‐angle east‐dipping brittle normal fault with at least 10 km of downdip displacement and a probable dextral strike‐slip component. In the Bridge River Schist, in the footwall of the Mission Ridge fault, penetrative ductile fabrics in northwest‐trending, steeply to shallowly‐dipping mylonites with northwest‐southeast trending subhorizontal stretching lineations record a dextral sense of shear. Development of these fabrics predates normal movement on the Mission Ridge fault and is attributed to dextral movement on the Yalakom fault system within the middle crust. Two Middle Eocene syntectonic foliated intrusions, dated by U‐Pb zircon methods, have the same kinematic signature as the previously described mylonites; these constrain timing of dextral movement on the Yalakom fault system to be earlier than, contemporaneous with, and in part younger than the intrusion of the circa 47 Ma Mission Ridge pluton. The Mission Ridge fault is correlated with the Petch Creek fault, the probable extension of the Ross Lake fault near the United States border. They were broken and offset by about 100 km of dextral movement on the north‐trending Fraser fault. The Petch Creek fault hanging wall consists of the Hozameen Group, which, like the Bridge River Group, is an assemblage of low‐grade oceanic rocks. The footwall of the Petch Creek fault, the Custer Gneiss, is higher grade than the Bridge River Schist in the footwall of the Mission Ridge fault but has the same lithological and kinematic signature. Mylonitic fabrics in both the Bridge River Schist and Custer Gneiss have subhorizontal stretching lineations that were generated prior to normal movement on the Mission Ridge‐Petch Creek fault system and are attributed to earlier dextral motion along the Yalakom fault system. The clear offset of these geologic features constrains about 100 km of movement on the Fraser fault to be younger than 46.5 Ma. The Fraser fault is cut by 34 Ma phases of the Chilliwack batholith.
Quantitative constraints on the rates of tectonic processes underpin our understanding of the mechanisms that form mountains.In the Sikkim Himalaya, late structural doming has revealed time-transgressive evidence of metamorphism and thrusting that permit calculation of the minimum rate of movement on a major ductile fault zone, the Main Central Thrust (MCT), by a novel methodology.U-Th-Pb monazite ages, compositions, and metamorphic pressure-temperature determinations from rocks directly beneath the MCT reveal that samples from ~50 km along the transport direction of the thrust experienced similar prograde, peak, and retrograde metamorphic conditions at different times.In the southern, frontal edge of the thrust zone, the rocks were buried to conditions of ~550°C and 0.8 GPa between ~21 and 18 Ma along the prograde path.Peak metamorphic conditions of ~650°C and 0.8-1.0GPa were subsequently reached as this footwall material was underplated to the hanging wall at ~17-14 Ma.This same process occurred at analogous metamorphic conditions between ~18-16 Ma and 14.5-13 Ma in the midsection of the thrust zone and between ~13 Ma and 12 Ma in the northern, rear edge of the thrust zone.Northward younging muscovite 40 Ar/ 39 Ar ages are consistently ~4 Ma younger than the youngest monazite ages for equivalent samples.By combining the geochronological data with the >50 km minimum distance separating samples along the transport axis, a minimum average thrusting rate of 10 ± 3 mm yr À1 can be calculated.This provides a minimum constraint on the amount of Miocene India-Asia convergence that was accommodated along the MCT.
Geochemical and geochronological analyses provide quantitative evidence about the origin, development and motion along ductile faults, where kinematic structures have been overprinted. The Main Central Thrust is a key structure in the Himalaya that accommodated substantial amounts of the India–Asia convergence. This structure juxtaposes two isotopically distinct rock packages across a zone of ductile deformation. Structural analysis, whole-rock Nd isotopes, and U–Pb zircon geochronology reveal that the hanging wall is characterized by detrital zircon peaks at c . 800–1000 Ma, 1500–1700 Ma and 2300–2500 Ma and an ϵ Nd(0) signature of −18.3 to −12.1, and is intruded by c . 800 Ma and c . 500–600 Ma granites. In contrast, the footwall has a prominent detrital zircon peak at c . 1800–1900 Ma, with older populations spanning 1900–3600 Ma, and an ϵ Nd(0) signature of −27.7 to −23.4, intruded by c . 1830 Ma granites. The data reveal a c . 5 km thick zone of tectonic imbrication, where isotopically out-of-sequence packages are interleaved. The rocks became imbricated as the once proximal and distal rocks of the Indian margin were juxtaposed by Cenozoic movement along the Main Central Thrust. Geochronological and isotopic characterization allows for correlation along the Himalayan orogen and could be applied to other cryptic ductile shear zones. Supplementary material: Zircon U–Pb geochronological data, whole-rock Sm–Nd isotopic data, sample locations, photomicrographs of sample thin sections, zircon CL images, and detailed analytical conditions are available at www.geolsoc.org.uk/SUP18704 .
Visean conglomerates of the Drahany Uplands contain clasts of
granulites, granites and durbachites. Detailed petrological
study of granulites enabled to reconstruct their P-T path. U-Pb
geochronological dating of the rocks yielded late Variscan
ages. The exhumation of the rocks from lower crust to the
surface was very fast.
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