DOCSLIB.ORG

Thermo-Rheological, Shear Heating Model for Leucogranite Generation, Metamorphism, and Deformation During the Proterozoic Trans

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Thermo-Rheological, Shear Heating Model for Leucogranite Generation, Metamorphism, and Deformation During the Proterozoic Trans Tectonophysics 342 (2001) 371–388 www.elsevier.com/locate/tecto Thermo-rheological, shear heating model for leucogranite generation, metamorphism, and deformation during the Proterozoic Trans-Hudson orogeny, Black Hills, South Dakota Peter I. Nabelek*, Mian Liu, Mona-Liza Sirbescu Department of Geological Sciences, University of Missouri-Columbia, Columbia, MO 65211, USA Accepted 20 June 2001 Abstract This paper evaluates thermotectonic models for metamorphism and leucogranite generation during the Proterozoic Trans- Hudson orogeny, as recorded in rocks exposed in the Black Hills, SD. Intrusion of the Harney Peak Granite and associated pegmatites at 1715 Ma occurred at the waning stages of regional deformation and staurolite-grade regional metamorphism. Published Consortium for Continental Reflection Profiling (COCORP) results indicate that Proterozoic sedimentary rocks were thrust over the Archean Wyoming province during the Trans-Hudson collision. Isotopic compositions of the Harney Peak Granite suggest that the exposed Proterozoic and Archean metasedimentary rocks in the Black Hills represent source rocks of the granites. Numerical simulations of the regional metamorphism and Harney Peak Granite generation, assuming crustal thickening by thrusting coupled with erosion, show the following: (1) Doubling of the crust with normal distribution of radioactive elements does not yield sufficiently high temperatures to cause anatexis anywhere in the crust or growth of garnet in the now exposed part of the crust; (2) a 35-km drop-off length for internal heat production can yield sufficient temperature for garnet growth at the current erosion level; it is, however, insufficient to produce staurolite, and melting can occur only in the deepest parts of the crust; (3) temperatures in crust with stable 70 km thickness for 40 Ma and 35 km drop-off length for heat production could become sufficient to produce staurolite at the current erosion level, and subsequent rapid denudation of the crust could potentially trigger decompression-melting of lower crustal rocks. Although this model could potentially explain the observed temporal relationship between regional metamorphism and leucogranite generation, it is inconsistent with melting of upper crustal Proterozoic source rocks that is indicated by isotopic compositions of the granites, with lack of evidence for rapid denudation of the Trans-Hudson orogen, and with confinement of the leucogranites to the deformed Proterozoic metapelitic rocks. Production of the Harney Peak Granite and its relationship to regional metamorphism of the country rocks are best explained by shear heating at the interface between the Wyoming province and overthrusted sedimentary rocks. We suggest that with reasonable rheologic properties of metapelites and rates of plate convergence, shear heating sufficiently perturbs locally the geotherms to cause anatexis in a deep shear zone system and growth of staurolite in the overlying crust. Modeling rheology of the lithologically stratified thickened crust, with granitic basement and metapelitic upper plate shows that the currently exposed part of the crust and the granite source region were ductile through much of the orogeny, which explains regional folding of the schists and predicts ductile shear zones in the granite source region. Because of the lithologic stratification, the granitic * Corresponding author. E-mail address: [email protected] (P.I. Nabelek). 0040-1951/01/$ - see front matter D 2001 Elsevier Science B.V. All rights reserved. PII: S 0040-1951(01)00171-8 372 P.I. Nabelek et al. / Tectonophysics 342 (2001) 371–388 basement is likely to become significantly weaker during crustal thickening than the upper crust dominated by schists. A weak basement under a folded upper crust is likely to contribute to the observed relatively flat topography of high plateaus over thickened orogens. D 2001 Elsevier Science B.V. All rights reserved. Keywords: Shear heating; Leucogranites; Numerical modeling; Rheology; Black Hills; Anatexis; Metamorphism 1. Introduction phism has been focused on the Himalayas where leucogranites constitute an integral part of the orogen The source of heat leading to leucogranite gen- (e.g., Le Fort et al., 1987; Harris and Massey, 1994; eration from crustal rocks in thickened convergent Treloar, 1997; Harrison et al., 1998; Huerta et al., orogens is a major unresolved issue. Although under- 1998; Vance and Harris, 1999). However, analogous plating of the crust or intrusion of mafic magmas leucogranites in terms of composition, mode of em- could potentially trigger crustal anatexis, there is a placement, source and host-rock compositions, and lack of chemical and physical evidence for intrusion structural context occur in other regions where crus- of mantle-derived magmas into the source regions of tal collisions have occurred, including the Appala- leucogranites (Le Fort et al., 1987; Scaillet et al., chian Mountains of Maine (Tomascak et al., 1996; 1990; Krogstad and Walker, 1996; Tomascak et al., Pressley and Brown, 1999) and the Black Hills, SD 1996; Nabelek and Bartlett, 1998; Pressley and (Redden et al., 1990; Nabelek et al., 1992a; Krogstad Brown, 1999). Furthermore, partial melting of crustal and Walker, 1996; Nabelek and Bartlett, 1998). This protoliths requires intrusion of at least an equivalent suggests that there may be a common process lead- mass of basalt, which is likely to lead to hybrid- ing to leucogranite generation during crustal colli- ization (Grunder, 1995). Without intrusion of mafic sion. In this paper, we explore possible models for magmas, thermal relaxation within thickened crust generation of the Harney Peak Granite (HPG) in the with typical concentration of radioactive elements Black Hills during the Proterozoic Trans-Hudson cannot by itself give temperatures necessary to melt orogeny, which was responsible for coalescence of metasedimentary source rocks by dehydration-melt- much of the North American craton. Previously ing reactions, except in lower parts of the crust (e.g., published geological, geochemical, thermobaromet- England and Thompson, 1984; Thompson and Con- ric, and chronological data for the metamorphism nolly, 1995). Although pressure–temperature–time and granite generation in the Black Hills provide ( P–T–t) paths in thick orogens may intersect wa- stringent constraints for numerical models of leucog- ter-present solidus of metapelites during exhumation, ranite generation. We conclude that shear heating of thermometry, compositions, and phase relationships pelitic schists during synorogenic thrusting was most of leucogranites suggest that most were high-temper- likely responsible for generation of the HPG. The ature ( > 750 °C) magmas that formed by muscovite similarity of scales and processes in the Trans-Hud- or biotite dehydration-melting reactions in metasedi- son orogen to other large orogens suggests that shear mentary rocks (Harris and Inger, 1992; Nabelek et heating may be important for petrogenesis of leucog- al., 1992b; Nabelek and Bartlett, 1998; Patin˜o-Douce ranites in collisional settings. and Harris, 1998). Therefore, to explain the leucog- ranites, modifications of simple crustal thickening- erosion models, including decompression melting of 2. Metamorphism in the Black Hills lower-crustal rocks or deep burial of heat-producing lithologies, have been proposed (e.g., Harris and The Proterozoic Trans-Hudson orogen extends over Massey, 1994; Ruppel and Hodges, 1994; Huerta et several thousand kilometers from the southern edge of al., 1998; Jamieson et al., 1998). the Wyoming craton to northern Quebec. Following Much of the debate about the heat source for erosion and covering by Phanerozoic sediments, part leucogranite generation and associated metamor- of it was uplifted during the Laramide orogeny and is P.I. Nabelek et al. / Tectonophysics 342 (2001) 371–388 373 now exposed in the core of the Black Hills (Fig. 1a). calated gabbro sills and felsic tuffs (Redden et al., The orogenic events that are recorded by the Pre- 1990). It is likely that these sequences represent what cambrian rocks in the Black Hills have traditionally Baird et al. (1996) inferred to be a wedge of arc rocks been ascribed to collision of the Archean Superior that were thrust over the Wyoming province (Fig. 1). province with the Wyoming province. However, a The Precambrian terrane also includes exposures of a recent Consortium for Continental Reflection Profil- small Archean leucogranite body and metapelites at ing (COCORP) transect across the orogen south of Bear Mountain along the western margin of the terrane the U.S.–Canada border indicates instead that there and of a highly deformed Archean Little Elk Creek may have been a small crustal block, named the granite near the eastern margin of the terrane to the Dakota block, that collided directly with the Wyom- north of the area shown in Fig. 2 (Redden et al., 1990). ing province (Baird et al., 1996; Fig. 1b). It is thus evident that the Archean rocks, probably Metamorphic rocks and leucogranites in the Black belonging to the Wyoming province, were imbricated Hills are the products of events that occurred during with the Proterozoic formations. the orogeny. The metamorphic rocks are dominated by The metasedimentary rocks have undergone two quartzite, metapelite, and metagraywacke (Fig. 2) that regional deformation events (Redden et al., 1990). The originated as platform to deep-marine sequences de- first event resulted in northeast-trending F1 folds
Load more

Recommended publications
  • Geology of the Vardeklettane Terrane, Heimefrontfjella (East Antarctica)
    Umbruch 79-1 04.11.2009 16:20 Uhr Seite 29 Polarforschung 79 (1), 29 – 32, 2009 Geology of the Vardeklettane Terrane, Heimefrontfjella (East Antarctica) by Wilfried Bauer1, Joachim Jacobs2, Robert J. Thomas3, Gerhard Spaeth4 and Klaus Weber5 Abstract: In the southwest of the Heimefrontfjella mountain range a granu- ly, the collected samples are extensively retrogressed with lite-facies terrane is exposed in the small Vardeklettane nunataks. The Varde- abundant chlorite and sericitised plagioclase, so granulite- klettane Terrane extends, however, at least 65 km farther WNW to the nunatak group of Mannefallknausane, where comparable granulites are exposed. The facies metamorphism was not identified. For the nunatak terrane is composed of mafic and felsic granulites, leucogranite orthogneiss group of Mannefallknausane however, JUCKES (1968) reported and metasedimentary rocks. The maximum deposition age of the latter is charnockites and granulite-facies rocks. Granulite-facies rocks constrained by detrital zircon U-Pb data ranging from 2000 to 1200 Ma whereas the leucogranite gneiss gave a late Mesoproterozoic crystallization were subsequently recognised at Vardeklettane by ARNDT et al. age of 1135 Ma. The Vardeklettane Terrane is unique as being the only part of (1987), who described charnockites as well as felsic and mafic the otherwise juvenile Late Mesoproterozoic basement of Heimefrontfjella to granulites, interpreting the latter as meta-volcanic rocks. The have a Paleoproterozoic crustal component. Structurally, the terrane is sepa- rated from the main range by the Heimefront Shear Zone. following description is based on the systematic mapping at 1:10,000 scale completed during the international Heimefront- Zusammenfassung: In den kleinen Vardeklettane Nunataks im Südwesten der fjella expedition of 1994 (BAUER et al.
    [Show full text]
  • Archaean Cratonization and Deformation in the Northern Superior Province, Canada: an Evaluation of Plate Tectonic Versus Vertical Tectonic Models Jean H
    Precambrian Research 127 (2003) 61–87 Archaean cratonization and deformation in the northern Superior Province, Canada: an evaluation of plate tectonic versus vertical tectonic models Jean H. Bédard a,∗, Pierre Brouillette a, Louis Madore b, Alain Berclaz c a Geological Survey of Canada, Division Québec, 880, ch.Ste-Foy, Quebec City, Que., Canada G1S 2L2 b Géologie Québec, Ministère des ressources naturelles du Québec, 5700, 4e Avenue Ouest, Charlesbourg, Que., Canada G1H 6R1 c Géologie Québec, Ministère des ressources naturelles du Québec, 545 Crémazie Est, bureau 1110, Montreal, Que., Canada H2M 2V1 Accepted 10 April 2003 Abstract The Archaean Minto Block, northeastern Superior Province, is dominated by tonalite–trondhjemite, enderbite (pyroxene tonalite), granodiorite and granite, with subordinate mafic rocks and supracrustal belts. The plutons have been interpreted as the batholithic roots of Andean-type plate margins and intra-oceanic arcs. Existing horizontal-tectonic models propose that penetrative recrystallization and transposition of older fabrics during terrane assembly at ∼2.77 and ∼2.69 Ga produced a N-NW tectonic grain. In the Douglas Harbour domain (northeastern Minto Block), tonalite and trondhjemite dominate the Faribault–Thury complex (2.87–2.73 Ga), and enderbite constitutes 50–100 km-scale ovoid massifs (Troie and Qimussinguat complexes, 2.74–2.73 Ga). Magmatic muscovite and epidote in tonalite–trondhjemite have corroded edges against quartz + plagioclase, suggesting resorption during ascent of crystal-charged magma. Foliation maps and air photo interpretation show the common development of 2–10 km-scale ovoid structures throughout the Douglas Harbour domain. Outcrop and thin-section scale structures imply that many plutons experienced a phase of syn-magmatic deformation, typically followed by high temperature sub-magmatic overprints.
    [Show full text]
  • The Geology of the Tama Kosi and Rolwaling Valley Region, East-Central Nepal
    The geology of the Tama Kosi and Rolwaling valley region, East-Central Nepal Kyle P. Larson* Department of Geological Sciences, University of Saskatchewan, 114 Science Place, Saskatoon, Saskatchewan, S7N 5E2, Canada ABSTRACT Tama Kosi valley in east-central Nepal (Fig. 1). which were in turn assigned to either the Hima- In order to properly evaluate the evolution of the layan gneiss group or the Midlands metasedi- The Tama Kosi/Rolwaling area of east- Himalaya and understand the processes respon- ment group (Fig. 2). The units of Ishida (1969) central Nepal is underlain by the exhumed sible for its formation, it is critical that all areas and Ishida and Ohta (1973) are quite similar in mid-crustal core of the Himalaya. The geol- along the length of the mountain chain be inves- description to the tectonostratigraphy reported ogy of the area consists of Greater Hima- tigated, at least at a reconnaissance scale. by Schelling (1992) who revisited and expanded layan sequence phyllitic schist, paragneiss, The Tama Kosi valley is situated between the the scope of their early reconnaissance work. and orthogneiss that generally increase in Cho Oyu/Everest/Makalu massifs to the east Schelling (1992) separated the geology of metamorphic grade from biotite ± garnet and the Kathmandu klippe/nappe to the west the lower and middle portion of the Tama Kosi assemblages to sillimanite-grade migmatite (Fig. 1). Recent work in these areas serves to into the more traditional Greater Himalayan up structural section. All metamorphic rocks highlight stark differences between them. In the sequence (Higher Himalayan Crystallines) and are pervasively deformed and commonly Kathmandu region, the extruded midcrustal core Lesser Himalayan sequence lithotectonic assem- record top-to-the-south sense shear.
    [Show full text]
  • Northwestern Superior Craton Margin, Manitoba: an Overview of Archean
    GS-7 Northwestern Superior craton margin, Manitoba: an overview of Archean and Proterozoic episodes of crustal growth, erosion and orogenesis (parts of NTS 54D and 64A) by R.P. Hartlaub1, C.O. Böhm, L.M. Heaman2, and A. Simonetti2 Hartlaub, R.P., Böhm, C.O., Heaman, L.M. and Simonetti, A. 2005: Northwestern Superior craton margin, Manitoba: an overview of Archean and Proterozoic episodes of crustal growth, erosion, and orogenesis (parts of NTS 54D and 64A); in Report of Activities 2005, Manitoba Industry, Economic Development and Mines, Manitoba Geological Survey, p. 54–60. Summary xenocrystic zircon, and in the The northwestern margin of the Superior Province in isotopic signature of Neoarchean Manitoba represents a dynamic boundary zone with good granite bodies (Böhm et al., 2000; potential for magmatic, sedimentary-hosted, and structur- Hartlaub et al., in press). The ALCC extends along the ally controlled mineral deposits. The region has a history Superior margin for at least 50 km, and may have a com- that commences in the early Archean with the formation mon history with other early Archean crustal fragments of the Assean Lake Crustal Complex. This fragment of in northern Quebec and Greenland (Hartlaub et al., in early to middle Archean crust was likely accreted to the press). Superior Province between 2.7 and 2.6 Ga, a major period South of the ALCC, the Split Lake Block repre- of Superior Province amalgamation. Sediments derived sents a variably retrogressed and shear zone–bounded from this amalgamation process were deposited at granulite terrain that is dominated by plutonic rocks and numerous locations along the northwestern margin of mafic granulite (Hartlaub et al., 2003, 2004).
    [Show full text]
  • The Himalayan Leucogranites: Constraints on the Nature of Their Crustal Source Region and Geodynamic Setting
    Gondwana Research 22 (2012) 360–376 Contents lists available at SciVerse ScienceDirect Gondwana Research journal homepage: www.elsevier.com/locate/gr The Himalayan leucogranites: Constraints on the nature of their crustal source region and geodynamic setting Zhengfu Guo a,⁎, Marjorie Wilson b a Key Laboratory of Cenozoic Geology and Environment, Institute of Geology and Geophysics, Chinese Academy of Sciences, P.O. Box 9825, Beijing 100029, China b School of Earth and Environment, University of Leeds, Leeds LS2 9JT, UK article info abstract Article history: Late Oligocene–Miocene leucogranites within southern Tibet form part of an extensive intrusive igneous Received 10 February 2011 province within the Himalayan orogen. The main rock types are tourmaline leucogranites (Tg) and two-mica Received in revised form 18 July 2011 87 86 leucogranites (2 mg). They have high SiO2 (70.56–75.32 wt.%), Al2O3 (13.55–15.67 wt.%) and ( Sr/ Sr)i Accepted 25 July 2011 (0.724001–0.797297), and low MgO (0.02–0.46 wt.%) and (143Nd/144Nd) (0.511693–0.511906). Chondri- Available online 11 August 2011 i te-normalized rare earth element (REE) patterns display strong negative Eu anomalies. Whole-rock major and trace element and Sr–Nd isotope data for the leucogranites suggest that their source region was a Keywords: fl Leucogranites two-component mixture between a uid derived from the Lesser Himalayan (LH) crustal sequence and the Decompression melting bulk crust of the Higher Himalayan (HH) sequence. Trace element and Sr–Nd isotope modeling indicate that Himalayan orogen the proportion of fluid derived from the LH sequence varied from 2% to 19% and the resulting metasomatised Steepening of the subducted Indian slab source experienced 7–16% melting.
    [Show full text]
  • Emerald Mineralization and Amphibolite Wall-Rock Alteration at the Khaltaro Pegmatite-Hydrothermal Vein System
    AN ABSTRACT OF THE THESIS OF Brendan M. Laurs for the degree of Master of Science in Geology presented on January 17. 1995. Title: Emerald Mineralization and Amphibolite Wall-Rock Alteration at the Khaltaro Pegmatite-Hydrothermal Vein System. Haramosh Mountains. Northern Pakistan. Redacted for Privacy Abstract approved: John H. Di lles Emeralds form within 0.1 to 1 m thick hydrothermal veins and pegmatites which have intruded amphibolite within layered Proterozoic gneisses in the Nanga Parbat- Haramosh massif. Simply-zoned albite-rich miarolitic pegmatites and hydrothermal veins containing various proportions of massive quartz, albite, tourmaline, muscovite, and beryl are associated with a small >3 m thick heterogeneous leucogranite sill of presumed Tertiary age, which discordantly intrudes the amphibolite and is locally albitized. Emeralds form within thin <30 cm wide massive quartz veins and tourmaline- albite veins, and more rarely in pegmatites, near the contacts with altered amphibolite. Electron microprobe analyses of colorless, pale blue, and emerald green beryl indicate that coloration is due to Cr3+ ± Fe2++Fe34-. Emerald green color is produced by Cr2O3 >0.20 wt%; Fe oxide varies from 0.62 to 0.89 wt% in emerald, 0.10 to 0.63 wt% in pale blue beryl, and 0.07 to 0.28 wt% in colorless beryl. The amphibolite adjacent to the pegmatites/veins is hydrolytically altered into ca. 20 cm-wide symmetrically zoned selvages. A sporadic inner zone containing biotite + tourmaline + fluorite ± albite ± muscovite ± quartz ± beryl assemblages typically extends to ca. 3 cm from the pegmatite/vein contacts; an intermediate zone containing biotite ± plagioclase + fluorite ± quartz assemblages extends to ca.
    [Show full text]
  • Himalayan Leucogranites and Migmatites: Nature, Timing and Duration of Anatexis
    J. metamorphic Geol., 2016, 34, 821–843 doi:10.1111/jmg.12204 Himalayan leucogranites and migmatites: nature, timing and duration of anatexis R. F. WEINBERG School of Earth, Atmosphere and Environment, Monash University, Clayton, Vic. 3800, Australia ([email protected]) ABSTRACT Widespread anatexis was a regional response to the evolution of the Himalayan-Tibetan Orogen that occurred some 30 Ma after collision between Asia and India. This paper reviews the nature, timing, duration and conditions of anatexis and leucogranite formation in the Greater Himalayan Sequence (GHS), and compares them to contemporaneous granites in the Karakoram mountains. Himalayan leucogranites and associated migmatites generally share a number of features along the length of the mountain front, such as similar timing and duration of magmatism, common source rocks and clock- wise P–T paths. Despite commonalities, most papers emphasize deviations from this general pattern, indicating a fine-tuned local response to the dominant evolution. There are significant differences in – – P T XH2O conditions during anatexis, and timing in relation to regional decompression. Further to that, some regions underwent a second event recording melting at low pressures. Zircon and monazite ages of anatectic rocks range between c. 25 and 15 Ma, suggesting prolonged crustal melting. Typi- cally, a single sample may have ages covering most of this 10 Ma period, suggesting recycling of accessory phases from metamorphic rocks and early-formed magmas. Recent studies linking monazite and zircon ages with their composition, have determined the timing of prograde melting and retro- grade melt crystallization, thus constraining the duration of the anatectic cycle. In some areas, this cycle becomes younger down section, towards the leading front of the Himalayas, whereas the oppo- site is true in other areas.
    [Show full text]
  • Geology and Tectonic Implications of Tourmaline Bearing Leucogranite of Bastipadu, Kurnool, Andhra Pradesh, India
    J. Earth Syst. Sci. (2018) 127:87 c Indian Academy of Sciences https://doi.org/10.1007/s12040-018-0990-8 Geology and tectonic implications of tourmaline bearing leucogranite of Bastipadu, Kurnool, Andhra Pradesh, India Kiran Jyoti Mishra, Santanu Bhattacharjee* , M S Reddy, M N Praveen, A D Bhimte and N Mahanta Geological Survey of India, Southern Region, Hyderabad 500 068, India. *Corresponding author. e-mail: [email protected] MS received 17 October 2017; revised 2 January 2018; accepted 19 January 2018; published online 2 August 2018 Tourmaline bearing leucogranite occurs as a pluton with pegmatitic veins intruding the Archaean granodiorite in the Bastipadu area, Kurnool district of Andhra Pradesh. We present field and petrographic relations, mineral chemistry and geochemical data for the leucogranite. It is essentially a two-mica granite, composed of quartz, perthite, microcline, albite, tourmaline and muscovite along with minor biotite and titanite. The euhedral tourmalines are regularly distributed in the rock. The geochemical studies show that the leucogranite is calc-alkaline, peraluminous to metaluminous which formed in a syn-collisional to volcanic arc-related setting. It displays strong ‘S’ type signatures with high K/Na ratios, moderately fractionated light rare earth elements, relatively flat heavy rare earth elements with [Ce/Yb]N ≤ 27.8 and a strong negative Eu anomaly. The geochemical characteristics indicate that the leucogranite melt might have been generated from partial melting of metasediments. Electron probe microanalyser data show the presence of alkali group tourmaline in leucogranite represented by schorl and dravite. Tourmaline compositions plot in the Li-poor granitoids and associated pegmatites and aplites and metapelites/metasammites fields.
    [Show full text]
  • 40Ar/39Ar Age Constraints on Deformation and Metamorphism in the Maine Central Thrust Zone and Tibetan
    TECTONICS, VOL. 8, NO. 4, PAGES 865-880, AUGUST 1989 40Ar/39Ar AGE CONSTRAINTSON DEFORMATION AND METANDRPHISM IN THE MAIN CENTRAL THRUST ZONE AND TIBETAN SLAB, EASTERN NEPAL HIMALAYA Mary S. Hubbard1 Department of Earth, Atmospheric and Planetary Sciences, Massachusetts Institute of Technology, Cambridge T. Mark Harrison Department of Geological Sciences, State University of New York at Albany Abstract. In eastern Nepal, structural 6.4+0.8 Ma (Tc=225ñ20ø C). An adjacent and petrologic observations suggest that gneiss yields a 9.1ñ0.2 Ma biotite movement on the Main Central Thrust (MCT), isochron and a K-feldspar minimum age of the development of an inverted metamorphic 3.6_+0.2 Ma (Tc=210-+50øC). In the upper sequence, and leucogranite plutonism were Tibetan Slab, samples collected >200 m genetically related. Samples from across from visible intrusives yield a biotite the MCT zone and its hanging wall, the isochron age of 20.2!0.2 Ma and a complex Tibetan Slab, were collected from four hornblende age spectrum suggestive of locations in the Everest region for excess argon. At the fourth location an 40Ar/39Ar studies: (1) the MCT zone, (2) augen gneiss, a pegmatite, and a the lower Tibetan Slab, (3) the upper tourmaline-bearing leucogranite, all in Tibetan Slab away from intrusive rocks, mutual contact, yield indistinguishable and (4) the upper Tibetan Slab adjacent to mineral ages. The average biotite and and including leucogranitic material. muscovite isochron ages for these samples Within the MCT zone, muscovite and are 17.0_+1.4 and 16.6ñ0.2 Ma, hornblende yield cooling ages of 12.0-+0.2 respectively.
    [Show full text]
  • Petrological Description of the Leucogranite in Örsviken
    UNIVERSITY OF GOTHENBURG Department of Earth Sciences Geovetarcentrum/Earth Science Centre Petrological description of the leucogranite in Örsviken Johanna Engelbrektsson Hannah Berg ISSN 1400-3821 B775 Bachelor of Science thesis Göteborg 2014 Mailing address Address Telephone Telefax Geovetarcentrum Geovetarcentrum Geovetarcentrum 031-786 19 56 031-786 19 86 Göteborg University S 405 30 Göteborg Guldhedsgatan 5A S-405 30 Göteborg SWEDEN Abstract For a deeper understanding of the different processes that can create, alter and destroy magnetic minerals in rocks it is important to integrate rock magnetism with conventional petrology. Bulk composition, hydrothermal alteration, redox state, metamorphism and tectonic setting influence the magnetic properties of igneous rocks. An investigation of a part of a cape that consists of a leucogranite in Örsviken, 20 kilometres south of Gothenburg, was of interest after high susceptibility values had been discovered in the area. Five samples from the leucogranite were collected in order to do a whole rock major element analysis, find the cause of the high susceptibility, classify and interpret the origin of the leucogranite. Magnetite is the cause of the high susceptibility values across the leucogranite. The leucogranite is considered to be an A-type alkali granite with an anorogenic or a post-orogenic petrogenesis. The leucogranite shows quite similar patterns and chemistry to the Askim- and Kärra Granites, which may indicate that the leucogranite is associated with them. Keywords: Magnetic minerals, leucogranite, Örsviken, susceptibility, whole rock major element analysis, magnetite, A-type granite. Sammanfattning För att få en djupare förståelse för de olika processer som kan skapa, förändra och förstöra magnetiska mineral i berg är det viktigt att integrera bergarters magnetism och konventionell petrologi.
    [Show full text]
  • Internal Framework and Geochemistry of the Carboniferous Huaco Granite Pluton, Sierra De Velasco, NW Argentina
    Andean Geology ISSN: 0718-7092 ISSN: 0718-7106 [email protected] Servicio Nacional de Geología y Minería Chile Internal framework and geochemistry of the Carboniferous Huaco granite pluton, Sierra de Velasco, NW Argentina Sardi, Fernando; Grosse, Pablo; Murata, Mamoru; Lozano Fernández, Rafael Pablo Internal framework and geochemistry of the Carboniferous Huaco granite pluton, Sierra de Velasco, NW Argentina Andean Geology, vol. 45, no. 2, 2018 Servicio Nacional de Geología y Minería, Chile Available in: https://www.redalyc.org/articulo.oa?id=173955659006 DOI: https://doi.org/10.5027/andgeoV45n2-3015 PDF generated from XML JATS4R by Redalyc Project academic non-profit, developed under the open access initiative Fernando Sardi, et al. Internal framework and geochemistry of the Carboniferous Huaco granite plu... Artículos de Investigación Internal framework and geochemistry of the Carboniferous Huaco granite pluton, Sierra de Velasco, NW Argentina Estructura interna y geoquímica del plutón granítico carbonífero de Huaco, Sierra de Velasco, NW de Argentina Fernando Sardi DOI: https://doi.org/10.5027/andgeoV45n2-3015 INSUGEO, Argentina Redalyc: https://www.redalyc.org/articulo.oa? [email protected] id=173955659006 Pablo Grosse CONICET, Argentina [email protected] Mamoru Murata Naruto University of Education, Japón [email protected] Rafael Pablo Lozano Fernández Instituto Geológico y Minero de España, España [email protected] Received: 28 November 2016 Accepted: 11 November 2017 Abstract: e A-type Huaco granite pluton of the Velasco range (Sierras Pampeanas of northwest Argentina) is formed by three coeval granitic facies and contains subordinate coeval-to-late facies, as well as enclaves, dikes and stocks that show different temporal relations, textures and compositions.
    [Show full text]
  • Intrusive Igneous Rocks, Part 2
    GLY 4310C LAB 7 INTRUSIVE IGNEOUS ROCKS, PART 2 Granite, Alkali Feldspar Granite, Granodiorite, Quartz Monzonite, and Monzonite These intrusive (plutonic) rocks correspond to fields on diagram 2-2 of Winter. These rocks are generally richer in silica and alkali feldspar than those studied in Lab 6. More information about these rocks is included in Chapter 13 and 14 of Moorhouse. GRANITE - Intrusive igneous, plutonic. Medium to coarse-grained plutonic rock containing between 20% to 60% quartz, feldspar (the plagioclase to plagioclase plus alkali feldspar (P/A+P) ratio is between 10 to 65%), and small amounts of biotite, hornblende or other silicates. K-spar may be orthoclase and/or microcline. The plagioclase is sodic, either oligoclase or andesine. Rocks of this composition are scarce and the definition is often broadened, for example by including quartz monzonite. The alkali feldspar is commonly microcline perthite (mixture of microcline and plagioclase). In some perthites, the albite and K-feldspar are completely separated, possibly due to recrystallization. The quartz is almost always anhedral in granites. The quartz grains often contain inclusions. Biotite is usually brown or brownish green and often contains inclusions. Hornblende is dark green and pleochroitic. Biotite may form a rim around the hornblende. If pyroxene (diopside) is present, hornblende may form a rim around the pyroxene. Muscovite may occur in patches around the biotite. Many granites are foliated. The name is from the Latin granum, meaning grain. ALKALI FELDSPAR GRANITE - Intrusive igneous, plutonic. Medium to coarse-grained plutonic rock containing between 20% to 60% quartz, feldspar (the plagioclase to plagioclase plus alkali feldspar (P/A+P) ratio < 10), and small amounts of biotite, hornblende or other silicates.
    [Show full text]