DOCSLIB.ORG

Deep Crustal Anatexis, Magma Mixing, and the Generation of Epizonal Plutons in the Southern Rocky Mountains, Colorado

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Deep Crustal Anatexis, Magma Mixing, and the Generation of Epizonal Plutons in the Southern Rocky Mountains, Colorado Contrib Mineral Petrol (2015) 169:7 DOI 10.1007/s00410-014-1094-3 ORIGINAL PAPER Deep crustal anatexis, magma mixing, and the generation of epizonal plutons in the Southern Rocky Mountains, Colorado Kristin H. Jacob · G. Lang Farmer · Robert Buchwaldt · Samuel A. Bowring Received: 10 June 2014 / Accepted: 5 December 2014 © Springer-Verlag Berlin Heidelberg 2015 Abstract The Never Summer Mountains in north- differentiated versions of the silicic melts parental to the central Colorado, USA, are cored by two Oligocene, Mt. Cumulus stock. Zircon U–Pb geochronology further epizonal granitic plutons originally emplaced in the reveals that the Mt. Richthofen stock was incrementally shallow levels of a short-lived (~1 m.y.), small-volume con- emplaced over a time interval from at least 28.975 0.020 ± tinental magmatic system. The younger Mt. Cumulus stock to 28.742 0.053 Ma. Magma mixing could have occurred ± (28.015 0.012 Ma) is a syenogranite equivalent compo- either in situ in the upper crust during basaltic underplating ± sitionally to topaz rhyolites. A comparison to the chemical and remelting of an antecedent, incrementally emplaced, and isotopic composition of crustal xenoliths entrained in silicic intrusive body, or at depth in the lower crust prior nearby Devonian kimberlites demonstrates that the silicic to periodic magma ascent and emplacement in the shallow melts parental to the stock were likely derived from ana- crust. Overall, the two stocks demonstrate that magma- texis of local Paleoproterozoic, garnet-absent, mafic lower tism associated with the Never Summer igneous complex continental crust. In contrast, the older Mt. Richthofen was fundamentally bimodal in composition. Highly silicic stock is compositionally heterogeneous and ranges from anatectic melts of the mafic lower crust and basaltic, man- monzodiorite to monzogranite. Major and trace element tle-derived magmas were the primary melts in the magma abundances and Sr, Nd and Pb isotopic ratios in this stock system, with mixing of the two producing intermediate vary regularly with increasing whole rock wt% SiO2. These composition magmas such as those from which Mt. Rich- data suggest that the Mt. Richthofen stock was constructed thofen stock was constructed. from mixed mafic and felsic magmas, the former corre- sponding to lithosphere-derived basaltic magmas simi- Keywords Magma mixing · Topaz rhyolite · lar isotopically to mafic enclaves entrained in the eastern Mafic underplating · Crustal anatexis portions of the stock and the latter corresponding to less Introduction Communicated by Timothy L. Grove. In many continental magmatic systems, a complex inter- Electronic supplementary material The online version of this article (doi:10.1007/s00410-014-1094-3) contains supplementary play likely exists between mafic magmas injected into the material, which is available to authorized users. crust from below, melts derived from crustal anatexis, and the crystallized products of both melt compositions, with * K. H. Jacob ( ) · G. L. Farmer this interplay varying in style with time, crustal depth, Department of Geological Sciences, CIRES, University of Colorado, Boulder, CO 80309, USA magma volume, and the overall heat budget of the system e-mail: [email protected] (McBirney et al. 1987; Wiebe et al. 2002; Acosta-Vigil et al. 2010). Geochronological, chemical, and field stud- R. Buchwaldt · S. A. Bowring ies of exposed continental igneous rocks are generally the Department of Earth, Atmospheric, and Planetary Sciences, Massachusetts Institute of Technology, Cambridge, only viable options for unraveling the plumbing and com- MA 02139, USA positional evolution of melts in such magmatic systems. 1 3 7 Page 2 of 23 Contrib Mineral Petrol (2015) 169:7 Fig. 1 Geological map of the intrusive rocks of the Never Summer igneous complex (modified from O’Neill 1981) showing sample localities. Normal faults represented by thin black lines. SLK State Line kimberlite diatremes, UH Urad–Henderson Climax-Mo deposit However, volcanic rocks only provide information on epizonal, Mid-Cenozoic intrusive igneous bodies exposed magmas that breach the surface at the uppermost levels of along the crest of the Never Summer Mountains in north- a given magmatic system. Intrusive igneous rocks, in con- central Colorado (Fig. 1). The younger Mt. Cumulus stock trast, may represent complex products of magma input, is a porphyritic syenogranite that intrudes the more hetero- recharge, and remobilization over a protracted time period geneous Mt. Richthofen stock, the latter varying composi- and over a restricted range of crustal depths (Harper et al. tionally from monzodiorite to monzogranite. We argue that 2004; Claiborne et al. 2006). Piecing together incomplete because of the small magmatic volumes likely delivered to information gathered from accessible igneous rocks into the upper crust during the history of this igneous center, the a coherent picture of the temporal evolution of a given intrusive rocks retain decipherable evidence regarding their magma system over its entire spatial extent within the crust origin. Our data demonstrate that the magmatism paren- is challenging, particularly for large-volume systems that tal to these intrusive rocks was bimodal. Silicic, anatectic involve multiple, spatially overlapping magmatic episodes. melts of mafic, lower continental crust were delivered to In this investigation, we explore the possibility that the upper crust and solidified as syenogranites such as the small-volume continental magma systems afford an oppor- Mt. Cumulus stock. Interaction between such silicic melts tunity to study fundamental aspects of magmatic evolution and mafic, mantle-derived magmas, either at depth or in that may be masked in larger volume systems. For this pur- situ in the shallow crust, led to the more intermediate com- pose, we undertook a geochronologic, chemical, and Nd, Sr position magmas from which the Mt. Richthofen stock was and Pb isotopic study of two small (~45 km2 exposed area) constructed. 1 3 Contrib Mineral Petrol (2015) 169:7 Page 3 of 23 7 Geologic background near surface level of emplacement for this stock. No mod- ern geochronology exists for the Mt. Richthofen stock, but The Oligocene Never Summer igneous complex (also low precision zircon fission track age determinations sug- referred to as the Braddock Peak Volcanic Field; Cole et al. gest that the Mt. Richthofen stock is ~28 1 Ma (Gam- ± 2008; Cole and Braddock 2009) is exposed in north-central ble 1979), an age that overlaps the range of Ar–Ar ages Colorado along the western boundary of Rocky Mountain determined for the intermediate to silicic volcanic rocks at National Park. The complex consists of ~30 km3 of pre- this center (Knox 2005). The Mt. Cumulus stock intrudes served mafic to silicic composition volcanic rocks (Knox the Mt. Richthofen stock along its southernmost margin 2005) and two epizonal stocks exposed over ~45 km2 in and so is clearly the younger of the two stocks, although the northern half of the Never Summer Mountains (Fig. 1). no absolute age determinations for Mt. Cumulus stock Volcanic activity commenced at ~29.6 Ma with the deposi- exist. The Mt. Cumulus stock must also have been shal- tion of mafic to intermediate lava flows, debris flows, and lowly emplaced, a conclusion consistent with the fact that rhyolites and culminated only ~1.5 m.y. later with the erup- portions of the stock contain abundant miarolitic cavities tion of larger volume topaz rhyolites (Christiansen et al. (Gamble 1979). Both stocks intrude Precambrian basement 1983; Knox 2005). The progression from early intermedi- rocks and Paleozoic sedimentary rocks along the Late Cre- ate composition volcanism to later silicic ignimbrite dep- taceous-Early Tertiary Never Summer thrust fault (Fig. 1), osition is similar to that observed through multiple cycles a “back thrust” presumably active during the Laramide in the southern Rocky Mountain volcanic field in southern orogeny and associated with the basement-cored uplift Colorado (Lipman 2007). The latter is generally consid- that produced the Colorado Front Range (O’Neill 1976, ered to be the easternmost expression of the mid-Tertiary 1981). Post-intrusion, N–S oriented normal faults, pos- “ignimbrite flareup”, a period of intense magmatic activity, sibly related to extension associated with the Rio Grande primarily centered in the western USA, that took place ~25 rift, cut both stocks although total offset on these faults is to 40 Ma (Coney and Reynolds 1977; Armstrong and Ward unknown (O’Neill 1976). Exhumation of the stocks may 1991). The Never Summer igneous complex could repre- have been initiated by this young extensional tectonism but sent a significantly smaller and shorter-lived manifestation was accelerated by Late Cenozoic glaciation that resulted of this same event. However, the complex is also roughly in the extensive present-day exposures of the stocks along coincident in time and space with the development of the the crest of the Never Summer Mountains. northernmost Rio Grande rift system in Colorado (Kellogg 1999). Small-volume granite porphyries associated with Climax-style porphyry molybdenum deposits and topaz Samples and analytical techniques rhyolites that occur in the late stages of Oligocene magma- tism elsewhere in Colorado are likely related to lithospheric The basic objective of the study was to use field, petro- extension associated with the Rio Grande rift (Christiansen graphic, mineral and whole rock chemical data, U–Pb zir- et al. 2007; Ludington and Plumlee 2009), and this setting con geochronology, and whole rock Nd, Sr and Pb isotopic may also be relevant for the formation of the Never Sum- analyses to assess the origin of these plutonic rocks and to mer igneous complex. define the compositions of melts and magmas that were Our concern in this study is the origin of the epizonal delivered to the upper continental crust during the Oligo- stocks exposed in the Never Summer Mountains which cene igneous activity. We collected intrusive rock samples consist of the intermediate composition Mt. Richthofen across accessible portions of complex at ~1 km intervals. stock and the Mt.
Load more

Recommended publications
  • The Segregation and Emplacement of Granitic Magmas
    Journal of the Geological society, London, Vol. 144, 1987, pp. 281-297, 13 figs. Printed in Northern Ireland The segregation and emplacement of granitic magmas S. M. WICKHAM Division of Geological and Planetary Sciences California Institute of Technology, Pasadena, California 91125 USA Abstract: The segregation of granitic magma from residual crystals at low melt-fraction is strongly dependent on the viscosity of the melt. Theoretical considerations imply that for the typical range of granitic meltviscosities (104Pa S to 10” Pa S) only verylimited separation will bepossible by a compaction mechanism over the typical duration of a crustal melting event (c. 106years). Small-scale segregations (millimetre to metre) of the type observed in migmatite terranes may be generated by compaction (possiblyassisted by -continuousdeformation), or byflow of melt into extensional fractures, but low melt-fraction liquids are unlikely to be extracted to form large (kilometre-size) granitic plutons because of the limited separation efficiency. At higher melt-fractions (>30%) the rapid decrease in strengthand effectiveviscosity during partial meltingallows other segregation processes tooperate. Calculations and experiments indicate that in granitic systems the effective viscosity of partially melted rocks,having a very narrow melt fraction range of 30-50% will fall rapidly to levels at which convective overturn of kilometre-thick zones can occur. Convective motion within anatectic regionsis capable of generating large (kilometre-size) homogeneous, highcrystal- fraction, crustally-derived magma bodies, which are orders of magnitude greater insize than low melt-fraction segregates. Before convective instability is reached, small (centimetre- to metre-sized) pods of granitic liquid may rise buoyantly through, and pond at the top of such partly molten zones; such a process is consistent with the observation that some granulites appear to be residue rocks, chemically depleted in a minimum melt component.
    [Show full text]
  • Full-Text PDF (Final Published Version)
    Pritchard, M. E., de Silva, S. L., Michelfelder, G., Zandt, G., McNutt, S. R., Gottsmann, J., West, M. E., Blundy, J., Christensen, D. H., Finnegan, N. J., Minaya, E., Sparks, R. S. J., Sunagua, M., Unsworth, M. J., Alvizuri, C., Comeau, M. J., del Potro, R., Díaz, D., Diez, M., ... Ward, K. M. (2018). Synthesis: PLUTONS: Investigating the relationship between pluton growth and volcanism in the Central Andes. Geosphere, 14(3), 954-982. https://doi.org/10.1130/GES01578.1 Publisher's PDF, also known as Version of record License (if available): CC BY-NC Link to published version (if available): 10.1130/GES01578.1 Link to publication record in Explore Bristol Research PDF-document This is the final published version of the article (version of record). It first appeared online via Geo Science World at https://doi.org/10.1130/GES01578.1 . Please refer to any applicable terms of use of the publisher. University of Bristol - Explore Bristol Research General rights This document is made available in accordance with publisher policies. Please cite only the published version using the reference above. Full terms of use are available: http://www.bristol.ac.uk/red/research-policy/pure/user-guides/ebr-terms/ Research Paper THEMED ISSUE: PLUTONS: Investigating the Relationship between Pluton Growth and Volcanism in the Central Andes GEOSPHERE Synthesis: PLUTONS: Investigating the relationship between pluton growth and volcanism in the Central Andes GEOSPHERE; v. 14, no. 3 M.E. Pritchard1,2, S.L. de Silva3, G. Michelfelder4, G. Zandt5, S.R. McNutt6, J. Gottsmann2, M.E. West7, J. Blundy2, D.H.
    [Show full text]
  • On Charnockites ⁎ B
    Available online at www.sciencedirect.com Gondwana Research 13 (2008) 30–44 www.elsevier.com/locate/gr GR Focus On charnockites ⁎ B. Ronald Frost , Carol D. Frost Department of Geology and Geophysics, University of Wyoming, Laramie, WY 82071, USA Received 4 June 2007; received in revised form 20 July 2007; accepted 24 July 2007 Available online 7 August 2007 Abstract Charnockitic rocks form extensive orthogneiss plutons in many granulite terranes and are less commonly found in unmetamorphosed plutons, which have formed in various tectonic regimes. Geochemically, clearly igneous charnockites cover nearly the whole range of granite chemistry, from magnesian to ferroan and from calcic to alkalic. Pyroxenes from unmetamorphosed charnockitic rocks have compositions ranging from magnesian to very iron-rich and record temperatures as high as 1000 °C. Oxygen fugacities for these plutons range from below FMQ to Δ log FMQN+2, values that cover nearly the whole range found in other granitic rocks. This range in bulk chemistry and intensive parameters is a reflection of the many mechanisms that produce charnockites. They may form in rifting environments, where they are ferroan, alkali-calcic to alkalic and metaluminous. Many of these ferroan charnockites are isotopically primitive, suggesting that they have been derived largely or entirely from differentiation or melting of tholeiitic melts. Charnockites are also found in deeply eroded arcs, where they are magnesian, calcic to calc-alkalic and metaluminous. Some charnockitic magmas may form by crustal melting or have incorporated a large component of crustal melt; these plutons tend to be weakly to moderately peraluminous and to have intermediate values of FeO/(FeO+MgO).
    [Show full text]
  • Shallow-Crustal Metamorphism During Late Cretaceous Anatexis in the Sevier Hinterland Plateau: Peak Temperature Conditions from the Grant Range, Eastern Nevada, U.S.A
    Shallow-crustal metamorphism during Late Cretaceous anatexis in the Sevier hinterland plateau: Peak temperature conditions from the Grant Range, eastern Nevada, U.S.A. Sean P. Long1*, Emmanuel Soignard2 1SCHOOL OF THE ENVIRONMENT, WASHINGTON STATE UNIVERSITY, PULLMAN, WASHINGTON 99164, USA 2LEROY EYRING CENTER FOR SOLID STATE SCIENCE, ARIZONA STATE UNIVERSITY, TEMPE, ARIZONA 85287, USA ABSTRACT Documenting spatio-temporal relationships between the thermal and deformation histories of orogenic systems can elucidate their evolu- tion. In the Sevier hinterland plateau in eastern Nevada, an episode of Late Cretaceous magmatism and metamorphism affected mid- and upper-crustal levels, concurrent with late-stage shortening in the Sevier thrust belt. Here, we present quantitative peak temperature data from the Grant Range, a site of localized, Late Cretaceous granitic magmatism and greenschist facies metamorphism. Twenty-two samples of Cambrian to Pennsylvanian metasedimentary and sedimentary rocks were analyzed, utilizing Raman spectroscopy on carbonaceous material, vitrinite reflectance, and Rock-Eval pyrolysis thermometry. A published reconstruction of Cenozoic extension indicates that the samples span pre-extensional depths of 2.5–9 km. Peak temperatures systematically increase with depth, from ~100 to 300 °C between 2.5 and 4.5 km, ~400 to 500 °C between 5 and 8 km, and ~550 °C at 9 km. The data define a metamorphic field gradient of ~60 °C/km, and are corroborated by quartz recrystallization microstructure and published conodont alteration indices. Metamorphism in the Grant Range is correlated with contemporary, upper-crustal metamorphism and magmatism documented farther east in Nevada, where metamorphic field gradients as high as ~50 °C/km are estimated.
    [Show full text]
  • The Archean Geology of Montana
    THE ARCHEAN GEOLOGY OF MONTANA David W. Mogk,1 Paul A. Mueller,2 and Darrell J. Henry3 1Department of Earth Sciences, Montana State University, Bozeman, Montana 2Department of Geological Sciences, University of Florida, Gainesville, Florida 3Department of Geology and Geophysics, Louisiana State University, Baton Rouge, Louisiana ABSTRACT in a subduction tectonic setting. Jackson (2005) char- acterized cratons as areas of thick, stable continental The Archean rocks in the northern Wyoming crust that have experienced little deformation over Province of Montana provide fundamental evidence long (Ga) periods of time. In the Wyoming Province, related to the evolution of the early Earth. This exten- the process of cratonization included the establishment sive record provides insight into some of the major, of a thick tectosphere (subcontinental mantle litho- unanswered questions of Earth history and Earth-sys- sphere). The thick, stable crust–lithosphere system tem processes: Crustal genesis—when and how did permitted deposition of mature, passive-margin-type the continental crust separate from the mantle? Crustal sediments immediately prior to and during a period of evolution—to what extent are Earth materials cycled tectonic quiescence from 3.1 to 2.9 Ga. These compo- from mantle to crust and back again? Continental sitionally mature sediments, together with subordinate growth—how do continents grow, vertically through mafi c rocks that could have been basaltic fl ows, char- magmatic accretion of plutons and volcanic rocks, acterize this period. A second major magmatic event laterally through tectonic accretion of crustal blocks generated the Beartooth–Bighorn magmatic zone assembled at continental margins, or both? Structural at ~2.9–2.8 Ga.
    [Show full text]
  • The Origin of Himalayan Anatexis and Inverted Metamorphism: Models and Constraints
    Journal of Asian Earth Sciences 17 (1999) 755±772 The origin of Himalayan anatexis and inverted metamorphism: Models and constraints T. Mark Harrison*, Marty Grove, Oscar M. Lovera, E.J. Catlos, Jessica D'Andrea Department of Earth and Space Sciences and Institute of Geophysics and Planetary Physics, University of California, Los Angeles, CA 90095-1567, USA Abstract The key to comprehending the tectonic evolution of the Himalaya is to understand the relationships between large-scale faulting, anatexis, and inverted metamorphism. The great number and variety of mechanisms that have been proposed to explain some or all of these features re¯ects the fact that fundamental constraints on such models have been slow in coming. Recent developments, most notably in geophysical imaging and geochronology, have been key to coalescing the results of varied Himalayan investigations into constraints with which to test proposed evolutionary models. These models fall into four general types: (1) the inverted metamorphic sequences within the footwall of the Himalayan thrust and adjacent hanging wall anatexis are spatially and temporally related by thrusting; (2) thrusting results from anatexis; (3) anatexis results from normal faulting; and (4) apparent inverted metamorphism in the footwall of the Himalayan thrust is produced by underplating of right-way-up metamorphic sequences. We review a number of models and ®nd that many are inconsistent with available constraints, most notably the recognition that the exposed crustal melts and inverted metamorphic sequences not temporally related. The generalization that appears to best explain the observed distribution of crustal melts and inverted metamorphic sequences is that, due to speci®c petrological and tectonic controls, episodic magmatism and out-of-sequence thrusting developed during continuous convergence juxtaposing allochthonous igneous and metamorphic rocks.
    [Show full text]
  • Geochemical Characterisation of Anatexite Within High Grade Migmatite Complex Terrain from Ogbomoso, Southwest of the Nigerian Precambrian Basement Complex
    American Scientific Research Journal for Engineering, Technology, and Sciences (ASRJETS) ISSN (Print) 2313-4410, ISSN (Online) 2313-4402 © Global Society of Scientific Research and Researchers http://asrjetsjournal.org/ Geochemical Characterisation of Anatexite within High Grade Migmatite Complex Terrain from Ogbomoso, Southwest of the Nigerian Precambrian Basement Complex Adegoke Olukayode Afolabia*, Rukayat Omobolanle Lawalb, Esther Olufunmilayo Ogunmiyidec, Kolade Adeosund a,b,c,dDepartment of Earth Sciences, Ladoke Akintola University of Technology, Ogbomoso, 210214, Nigeria aEmail: [email protected] bEmail: [email protected] cEmail: [email protected] dEmail: [email protected] Abstract Migmatite rocks are complex rocks largely due to the degree of partial melting and nature of parent rocks of this rock type. Partial melting or anatexis yield metatexite and diatexite components with variable mineralogical and chemical compositions. The nature of fractionation of elements during anatexis is not clearly understood. The migmatite quartzite gneiss complex of southwestern Nigeria exposed at Ogbomoso was studied in order to constrain the geochemical character of its metatexite and diatexite components. Field mapping revealed migmatite with stromatic, surreitic and dictyonitic structures represented the metatexite while parts of the migmatite with schollen structure and granitic component represent the diatexite. Several sections of representative samples were cut and examined for mineralogical compositions. Thin section
    [Show full text]
  • Granitoid Rocks Granitoids Common Features Anatexis? Evidence for Anaatexis Inclusions
    Granitoids “Granitoids” (sensu lato): loosely applies to a Granitoid Rocks wide range of felsic plutonic rocks This lecture focuses on non-continental arc Reading: intrusives Winter (2001) Chapter 18 Associated volcanics are common and have same origin, but are typically eroded away Common Features Anatexis? • Most large granitoid bodies occur in areas • Because the crust normally is solid, some thermal where the continental crust was thickened disturbance is required to form granitoids by orogeny • Most workers believe that the majority of • Formed by either continental arc granitoids are derived by crustal anatexis, but that subduction or collision of sialic masses. the mantle may also be involved in the process. • Many granites, however, may post-date the • The mantle contribution may range from being a thickening event by tens of millions of source of heat for crustal anatexis to being the years. source of material as well. Evidence for Inclusions Anaatexis Table 18-1. The Various Types of Enclaves Name Nature Margin Shape Features Backscattered electron image of a Xenolith piece of country sharp to angular contact metamorphic rocks gradual to ovoid texture and minerals zircon from the Strontian Granite, Xenocryst isolated foreignsharp angular corroded Scotland. The grain has a rounded, crystal reaction rim Surmicaceous residue of melting sharp, lenticular metamorphic texture un-zoned core (dark) that is an Enclave (restite) biotite rim micas, Al-rich minerals inherited high-temperature non- Schlieren disrupted enclave gradual oblate coplanar orientation Felsic Micro- disrupted sharp to ovoid fine-granied melted crystal from the pre-granite granular Enclave fine-grained margin gradual igneous texture source.
    [Show full text]
  • Geochemical Characteristics of Crustal Anatexis During the Formation of Migmatite at the Southern Sierra Nevada, California
    Contrib Mineral Petrol (2005) DOI 10.1007/s00410-005-0010-2 ORIGINAL PAPER Lingsen Zeng Æ Jason B. Saleeby Æ Mihai Ducea Geochemical characteristics of crustal anatexis during the formation of migmatite at the Southern Sierra Nevada, California Received: 6 October 2003 / Accepted: 10 March 2005 Ó Springer-Verlag 2005 Abstract We provide data on the geochemical and iso- tively high Rb, Pb, Ba, and K2O contents, Rb/Sr ratios topic consequences of nonmodal partial melting of a (0.15<Rb/Sr<1.0), and initial eNd values . Group B thick Jurassic pelite unit at mid-crustal levels that pro- leucosomes have relatively low Rb, Pb, Ba, and K2O duced a migmatite complex in conjunction with the contents, Rb/Sr ratios (<0.15), and initial eNd values. intrusion of part of the southern Sierra Nevada batho- The low Rb concentrations and Rb/Sr ratios of the lith at ca. 100 Ma. Field relations suggest that this pelitic group B leucosomes together suggest that partial melting migmatite formed and then abruptly solidified prior to was dominated by water-saturated or H2O-fluxed melt- substantial mobilization and escape of its melt products. ing of quartz + feldspar assemblage with minor Hence, this area yields insights into potential mid-crustal involvement of muscovite. Breakdown of quartz and level contributions of crustal components into Cordil- plagioclase with minor contributions from muscovite leran-type batholiths. Major and trace-element analyses resulted in low Rb/Sr ratios characterizing both group A in addition to field and petrographic data demonstrate and group B leucosomes. In contrast, group A leuco- that leucosomes are products of partial melting of the somes have greater contributions from K-feldspar, pelitic protolith host.
    [Show full text]
  • Geochemical Characteristics of Crustal Anatexis During the Formation of Migmatite at the Southern Sierra Nevada, California
    Contrib Mineral Petrol (2005) DOI 10.1007/s00410-005-0010-2 ORIGINAL PAPER Lingsen Zeng Æ Jason B. Saleeby Æ Mihai Ducea Geochemical characteristics of crustal anatexis during the formation of migmatite at the Southern Sierra Nevada, California Received: 6 October 2003 / Accepted: 10 March 2005 Ó Springer-Verlag 2005 Abstract We provide data on the geochemical and iso- tively high Rb, Pb, Ba, and K2O contents, Rb/Sr ratios topic consequences of nonmodal partial melting of a (0.15<Rb/Sr<1.0), and initial eNd values . Group B thick Jurassic pelite unit at mid-crustal levels that pro- leucosomes have relatively low Rb, Pb, Ba, and K2O duced a migmatite complex in conjunction with the contents, Rb/Sr ratios (<0.15), and initial eNd values. intrusion of part of the southern Sierra Nevada batho- The low Rb concentrations and Rb/Sr ratios of the lith at ca. 100 Ma. Field relations suggest that this pelitic group B leucosomes together suggest that partial melting migmatite formed and then abruptly solidified prior to was dominated by water-saturated or H2O-fluxed melt- substantial mobilization and escape of its melt products. ing of quartz + feldspar assemblage with minor Hence, this area yields insights into potential mid-crustal involvement of muscovite. Breakdown of quartz and level contributions of crustal components into Cordil- plagioclase with minor contributions from muscovite leran-type batholiths. Major and trace-element analyses resulted in low Rb/Sr ratios characterizing both group A in addition to field and petrographic data demonstrate and group B leucosomes. In contrast, group A leuco- that leucosomes are products of partial melting of the somes have greater contributions from K-feldspar, pelitic protolith host.
    [Show full text]
  • The Role of Partial Melting and Extensional Strain Rates in the Development of Metamorphic Core Complexes
    Tectonophysics 477 (2009) 135–144 Contents lists available at ScienceDirect Tectonophysics journal homepage: www.elsevier.com/locate/tecto The role of partial melting and extensional strain rates in the development of metamorphic core complexes P.F. Rey a,⁎, C. Teyssier b, D.L. Whitney b a Earthbyte Research Group, School of Geosciences, University of Sydney, NSW2006 Sydney, Australia b Department of Geology and Geophysics, University of Minnesota, Minneapolis, Minnesota 55455, USA article info abstract Article history: Orogenic collapse involves extension and thinning of thick and hot (partially molten) crust, leading to the Received 11 September 2008 formation of metamorphic core complexes (MCC) that are commonly cored by migmatite domes. Two- Accepted 12 March 2009 dimensional thermo-mechanical Ellipsis models evaluate the parameters that likely control the formation and Available online 20 March 2009 evolution of MCC: the nature and geometry of the heterogeneity that localizes MCC, the presence/absence of a partially molten layer in the lower crust, and the rate of extension. When the localizing heterogeneity is a Keywords: normal fault in the upper crust, the migmatite core remains in the footwall of the fault, resulting in an Continental extension Migmatite asymmetric MCC; if the localizing heterogeneity is point like region within the upper crust, the MCC remains Metamorphic core complex symmetric throughout its development. Therefore, asymmetrically located migmatite domes likely reflect the Gneiss dome dip of the original normal fault system that generated the MCC. Modeling of a severe viscosity drop owing to the Partial melting presence of a partially molten layer, compared to a crust with no melt, demonstrates that the presence of melt Modeling slightly enhances upward advection of material and heat.
    [Show full text]
  • Dictionary of Geology and Mineralogy
    McGraw-Hill Dictionary of Geology and Mineralogy Second Edition McGraw-Hill New York Chicago San Francisco Lisbon London Madrid Mexico City Milan New Delhi San Juan Seoul Singapore Sydney Toronto All text in the dictionary was published previously in the McGRAW-HILL DICTIONARY OF SCIENTIFIC AND TECHNICAL TERMS, Sixth Edition, copyright ᭧ 2003 by The McGraw-Hill Companies, Inc. All rights reserved. McGRAW-HILL DICTIONARY OF GEOLOGY AND MINERALOGY, Second Edi- tion, copyright ᭧ 2003 by The McGraw-Hill Companies, Inc. All rights reserved. Printed in the United States of America. Except as permitted under the United States Copyright Act of 1976, no part of this publication may be reproduced or distributed in any form or by any means, or stored in a database or retrieval system, without the prior written permission of the publisher. 1234567890 DOC/DOC 09876543 ISBN 0-07-141044-9 This book is printed on recycled, acid-free paper containing a mini- mum of 50% recycled, de-inked fiber. This book was set in Helvetica Bold and Novarese Book by the Clarinda Company, Clarinda, Iowa. It was printed and bound by RR Donnelley, The Lakeside Press. McGraw-Hill books are available at special quantity discounts to use as premi- ums and sales promotions, or for use in corporate training programs. For more information, please write to the Director of Special Sales, McGraw-Hill, Professional Publishing, Two Penn Plaza, New York, NY 10121-2298. Or contact your local bookstore. Library of Congress Cataloging-in-Publication Data McGraw-Hill dictionary of geology and mineralogy — 2nd. ed. p. cm. “All text in this dictionary was published previously in the McGraw-Hill dictionary of scientific and technical terms, sixth edition, —T.p.
    [Show full text]