DOCSLIB.ORG

Implications for Compositionally Zoned Magma Bo

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
Implications for Compositionally Zoned Magma Bo American Mineralogist, Volume 80, pages 1188-1207, 1995 Simulations of convectionwith crystallization in the systemKAISiTO6-CaMgSirOu: Implications for compositionallyzoned magma bodies Fuux J. Srrnl,, Cunrrs M. Or,nnxrunc, CoNsr,l,NcrCnmsrENsEN, Mrcor, Toorsco DepartmentorGeologicar ttn::';liiff orcarirornia' 3"'1,'#*,l;,:ff:t:t:"*'universitv Ansrnlcr A model has been developedand applied to study the origin of compositional and phase heterogeneityin magma bodies undergoing simultaneous convection and phase change. The simulator is applied to binary-component solidification of an initially superheated and homogeneousbatch of magma. The model accounts for solidified, mushy (two- or three-phase),and all-liquid regions self-consistently,including latent heat effects,perco- lative flow of melt through mush, and the variation of systementhalpy with composition, temperature, and solid fraction. Phaseequilibria and thermochemical and transport data for the system KAlSirOu-CaMgSirOuwere utilized to addressthe origin of compositional zonation in model peralkaline magmatic systems.Momentum transport is accomplished by Darcy percolation in solid-dominated regions and by internal viscous stressdiffusion in melt-dominated regions within which relative motion between solid and melt is not allowed. Otherwise, the mixture advectsas a pseudofluid with a viscosity that dependson the local crystallinity. Energy conservation is written as a mixture-enthalpy equation with subsidiary expressionsthat are based on thermochemical data and phase relations that relate the mixture enthalpy to temperature, composition, and phase abundancesat each location. Speciesconservation is written as the low-density component (KAlSirOu) and allows for advection and diffirsion as well as the relative motion betweensolid and melt. Systematic simulations were performed to assessthe role of thermal boundary condi- tions, solidification rates, and magma-body shape on the crystallization history. Exami- nation of animations showing the spatial development of the bulk (mixture) composition (C), melt composition (C,), temperature (7), solid fraction ({), mixture enthalpy (h), and velocity (V), reveals the unsteady and complex nature of convective solidification owing to nonlinear coupling among the momentum, energy,and speciesconservation equations. A consequenceof the coupling includes the spontaneousdevelopment of compositional heterogeneityin terms of the modal abundancesof diopside and leucite in the all-solid parts of the domain (i.e., modal mineralogical heterogeneity)as well as spatial variations in melt composition particularly within mushy regions where phase relations strongly couple compositional and thermal fields. Temporal changesin the rate of heat extraction becauseof bursts of crystallization and concommitant buoyancy generationare also found. Crystallization of diopside, the liquidus phasein all cases,enriches residual melt in low- density K-rich liquid. The upward flow of this material near the mush-liquid interface leads to the development of a strong vertical compositional gradient. The main effect of magma-body shapeand different thermal boundary conditions is in changing the rate of solidification; in all casescompositional heterogeneitiesdevelop. The rate of formation of the compositional stratification is highest for the sill-like body becauseof its high cooling rate. Compositional zonation in a fully solidified body is found to be both radial and vertical. The most salient feature of this simple model is the spontaneousdevelopment of large-scalemagma heterogeneityfrom homogeneousand slightly superheatedinitial states, assuminglocal equilibrium prevails during the course of phasechange. INrnouucrroN the discipline itself, and it would be difficult to review the subject in a single contribution. An adequatecritique One of the most important problems in igneous pe- would encompassthe entire field of igneous petrology, trology is elucidation of the mechanismsthat give rise to include the history of planetary differentiation and crustal magmatic diversity. This problem is practically as old as growth (e.9., Galer and Goldstein, l99l; Taylor and 0003-004x/95/rtt2-1 I 88$02.00 I 188 SPERA ET AL.: SIMULATION OF CONVECTION AND CRYSTALLIZATION I 189 Mclennan, 1985),and draw upon geochemicaldata from evidence that separation of crystals from melt had oc- hundreds if not thousands of ancient and modern mag- curred, although the mechanism and rate by which this matic centers. was accomplishedare not constrainedby the geochemical Sincethe pioneering work of Bowen ( I 928), it has been data. Evidently, the lack of interphenocryst chemical generally acceptedthat crystal fractionation is one of the variation and correlation between phenocryst composi- most effective magmatic differentiation processes.By tion and enclosingmelt (glass)composition arguesagainst some means, crystals become separatedfrom the melts simple crystal settling. Similar conclusionswere reached in which they crystallize. Despite the generalsignificance by Dunbar and Hervig (Dunbar and Hervig, 1992; Her- of this problem, the mechanism of crystal fractionation vig and Dunbar, 1992) in their detailed study of melt- and the associatedquestion ofthe vigor ofconvection in inclusion traceelement abundances and comparisonswith magma bodies remain controversial(e.g., Marsh, 1989; enclosingpumice and whole-rock compositions. Martin and Nokes, 1989; Gibb and Henderson, 1992., A popular scenario whereby compositionally evolved Huppert and Worster, 1992). The problem is challenging melt becomes separated from associatedcrystals is by mainly becauseof the enormous changesin magma vis- maryinal upflow, a mechanismfirst hypothesizedby Shaw cosity that occur in the crystallization interval. Although (1974) and extensively investigated theoretically, exper- adequatemodels exist for the calculation of melt viscos- imentally, and by numerical simulation since then (e.g., ity as a function of temperature and composition (e.g., Turner, 1980;Chen and Turner, 1980;Turner and Gus- Shaw,1972;Bottinga and Weill, L972;Urbainet al., 1982; tafson,198 l; Speraetal., 1982,1984,1989; Huppert and see also Stein and Spera, 1993), accurate description of Sparks,1984; Turner, 1985; Nilson et al., 1985; Mc- the complex rheological behavior of magmatic mixtures Birney et al., 1985;Lowell, 1985;Huppert et al., 1986; is a far more difrcult task (Lejeune and Richet, 1995). Bergantzand Lowell, 1987;Trial and Spera,1988; kitch, Because,in general,there can be relative motion between 1989;Langmuir, 1989;Huppert, 1990).The essentialidea the constituent solid, melt, and vapor phasesand because is that in systemswhere phasechange occurs along a sub- component phases have such widely different physical vertical sidewall (e.g., solidification or anatexis),the ini- properties, the characterization of the rheology of mag- tial downward flow of cooled marginal magma can be matic mixtures presents a formidable problem. Briefly reversed if phase change enriches melt in low-density stated, it is difficult to describe simply and'accurately components(e.g., SiOr, NarO, KrO, and HrO) sufficiently both the rheological properties of magma and the rela- to overcome negative thermal buoyancy effects. In this tionship between phase fraction (e.9., fraction solid, f,) model, accumulation by advective transport of the cooler and permeability of magmatic multiphasemixtures across but chemically enriched (and buoyant) melt at the top of the wide range of conditions relevant to magma transport the magma body produces a thermally and chemically and evolution. The problem is particularly acute in silicic zoned layer. Additional in situ crystallization within the and intermediate-composition systemsfor which there is roofward layer then producesa magmatic mixture with a overwhelming and incontrovertible evidence that large phenocryst content inconsistent with any single-stage, volumes of evolved magma, sometimesexceeding several closed-systemfractionation scheme,but consistent with thousand cubic kilometers, exist within the crust. Fur- trends expected from crystal-fractionation processesas thermore, differentiation may occur at geologicallyrapid commonlynoted (Michael, l9B3; Wolffand Storey,1984). rates(e.g., lO-'zkm3/yr) prior either to solidifyingto form If HrO is significantly enriched upward, then the local mineralogically zoned or layered plutons or other com- (compositionally dependent)liquidus to solidus temper- posite batholiths or to erupting to form compositionally ature interval, as well as the liquidus temperature itself, zoned ignimbrites or lava flows (Smith, 1979; Hildreth, varies with depth within the magma body. This leads to l98l; Trial and Spera,1990; Wiebe, 1993). A classicex- variation in phenocryst content with location within the ample is the 600 km3 Bishop Tuff in eastern California body independent of the effectsof crystal settling. (Hildreth, 1979) emplaced as a plinian pumice fall, fol- Although the simple version of the marginal upflow lowed by a sequenceof pyroclastic flows. Hildreth con- hypothesisis conceptually pleasingand there is no doubt cluded that no mixing of crystalsby gravitational settling that marginal upwelling occursin laboratory experiments had occurred prior to the eruption. His conclusion was with aqueous solutions, its relevance to magma differ- basedprincipally on the observationthat individual hand- entiation has been questioned (see review by Trial and sized pyroclasts had compositionally uniform popula-
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]