DOCSLIB.ORG

A Tale of Five Enclaves: Mineral Perspectives on Origins of Mafic Enclaves in the Tuolumne Intrusive Complex GEOSPHERE, V

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
A Tale of Five Enclaves: Mineral Perspectives on Origins of Mafic Enclaves in the Tuolumne Intrusive Complex GEOSPHERE, V Research Paper THEMED ISSUE: Different Personalities of Granites and Rhyolites: Silicic Magmas from the Lower Crust to the Surface GEOSPHERE A tale of five enclaves: Mineral perspectives on origins of mafic enclaves in the Tuolumne Intrusive Complex GEOSPHERE, v. 17, no. 2 C.G. Barnes1, K. Werts1, V. Memeti2, S.R. Paterson3, and R. Bremer2 1Department of Geosciences, Texas Tech University, Lubbock, Texas 79409-1053, USA https://doi.org/10.1130/GES02233.1 2Department of Geological Sciences, California State University, Fullerton, Fullerton, California 92834, USA 3Department of Earth Sciences, University of Southern California, Los Angeles, California 90089, USA 15 figures; 1 table; 1 set of supplemental files ABSTRACT end member. Moreover, the composition of horn- granitic rocks led other workers to propose origins CORRESPONDENCE: [email protected] blende in the immediately adjacent host rock is as “autoliths”: essentially cumulates formed in the The widespread occurrence of mafic magmatic distinct from hornblende typically observed in host magma (e.g., Pabst, 1928) or fine-grained mar- CITATION: Barnes, C.G., Werts, K., Memeti, V., Pater- son, S.R., and Bremer, R., 2021, A tale of five enclaves: enclaves (mme) in arc volcanic rocks attests to the eHD. Although primary basaltic magmas ginal facies disrupted into the host magma (e.g., Mineral perspectives on origins of mafic enclaves in hybridization of mafic-intermediate magmas with are thought to be parental to the mme, little or Bateman et al., 1963; Didier, 1973). the Tuolumne Intrusive Complex: Geosphere, v. 17, felsic ones. Typically, mme and their hosts dif- no evidence of such parents is preserved in the Another explanation for the mineralogical no. 2, p. 352– 374, https://doi.org/10.1130/GES02233.1. fer in mineral assemblage and the compositions enclaves. Instead, the data indicate that hybridiza- similarities between enclaves and host is that the of phenocrysts and matrix glass. In contrast, in tion of already hybrid andesitic enclave magmas enclaves are products of fragmentation of magmas Science Editor: Shanaka de Silva Guest Associate Editor: Guilherme Gualda many arc plutons, the mineral assemblages in mme with rhyolitic magmas in the eHD involved multi- that were undercooled when injected into the host are the same as in their host granitic rocks, and ple andesitic and rhyolitic end members, which in magma (Eichelberger, 1978; Gamble, 1979; Reid et Received 11 January 2020 major-element mineral compositions are similar turn is consistent with the eHD representing an al., 1983; Vernon, 1983, 1984; Bacon, 1986; Barnes Revision received 16 October 2020 or identical. These similarities lead to difficulties amalgamation of numerous, compositionally dis- et al., 1986). This interpretation recognizes mafic Accepted 9 December 2020 in identifying mixing end members except through tinct magma reservoirs. This conclusion applies to enclaves as potential recorders of the nature of the use of bulk-rock compositions, which them- enclaves sampled <30 m from one another. More- recharge magmas and of magma mixing processes. Published online 5 February 2021 selves may reflect various degrees of hybridization over, during amalgamation of various rhyolitic The literature now contains hundreds of studies on and potentially melt loss. This work describes the reservoirs, some mme were evidently disrupted enclave textures, mineral assemblages, and geo- variety of enclave types and occurrences in the from a surrounding mush and thus carried rem- chemical compositions, and it is generally accepted equigranular Half Dome unit (eHD) of the Tuolumne nants of that mush as their immediately adjacent that most such enclaves result from magma min- Intrusive Complex and then focuses on textural host. We suggest that detailed study of mineral gling and/or mixing in crustal magma reservoirs. and mineral composition data on five porphy- compositions and zoning in plutonic mme provides In the volcanology community, enclaves as evi- ritic mme from the eHD. Specifically, major- and a means to identify magmatic processes that can- dence for magma influx are particularly important trace-element compositions and zoning patterns not be deciphered from bulk-rock analysis. because recharge of mafic magmas is a potential of plagioclase and hornblende were measured in trigger for eruptions (e.g., Feeley et al., 2008; Shane the mme and their adjacent host granitic rocks. In at al., 2008; Humphreys et al., 2009; Ruprecht and each case, the majority of plagioclase phenocrysts ■ INTRODUCTION Bachmann, 2010). Moreover, in volcanic systems, in the mme (i.e., large crystals) were derived from rapid quenching of eruptive products, including a rhyolitic end member. The trace-element com- Mafic magmatic enclaves are nearly ubiqui- mafic enclaves, permits assessment of original positions and zoning patterns in these plagioclase tous in arc plutons. Their origins were the subject (high-temperature) mineral assemblages and com- phenocrysts indicate that each mme formed by of significant debate over the past century (sum- positions of minerals and melt (glass), providing hybridization with a distinct rhyolitic magma. In marized by Barbarin and Didier, 1991). Proposed clear information about mixing end members (e.g., some cases, hybridization involved a single mix- origins include reworked (i.e., chemically modified) Tepley et al., 1999; Salisbury et al., 2008; Schmidt ing event, whereas in others, evidence for at least xenoliths (e.g., Bowen, 1922; Bateman et al., 1963), and Grunder, 2011; Ruprecht et al., 2012; Chad- two mixing events is preserved. In contrast, some disrupted, preexisting mafic dikes (Roddick and wick et al., 2013; Allan et al., 2017; Humphreys et hornblende phenocrysts grew from the enclave Armstrong, 1959; Cobbing and Pitcher, 1972), and al., 2019). In some instances, the compositions of magma, and others were derived from the rhyolitic residual material (restite) from the magma source quenched mafic enclaves may represent the com- This paper is published under the terms of the region (White and Chappell, 1977). The similarities position of mafic end-member magmas (but see CC-BY-NC license. Calvin Barnes https://orcid.org/0000-0002-5383-6755 in mineral assemblages of enclaves and their host Bacon, 1986). © 2021 The Authors GEOSPHERE | Volume 17 | Number 2 Barnes et al. | Origins of mafic magmatic enclaves Downloaded from http://pubs.geoscienceworld.org/gsa/geosphere/article-pdf/17/2/352/5259592/352.pdf 352 by guest on 03 October 2021 Research Paper Unlike mafic enclaves from volcanic rocks, mafic (e.g., Reid et al., 1983). However, if it can be shown are related to magma influx, (1) the new magma enclaves in plutons commonly display mineral that physical and/or chemical exchange occurred was not necessarily mafic, and (2) enclave bulk assemblages and major-element mineral compo- between an enclave and its host magma, then the compositions are likely to reflect early mixing of sitions identical to those in the adjacent host (e.g., enclave composition will not be an accurate repre- crystals and melt from the host, and in some cases Vernon, 1983; Barbarin, 1990, 2005; Barnes et al., sentation of a mafic end-member magma, whether followed by loss of melt to the host. 1990; Dorais et al., 1990; Allen, 1991), and in some parental or not (e.g., Vernon, 1990; Barbarin and instances, the enclaves contain large crystals (e.g., Didier, 1992; Browne et al., 2006). alkali feldspar megacrysts and quartz ocelli) that In this study, we analyzed major- and trace- ■ GEOLOGIC SETTING would not be expected to crystallize from a melt element compositions of calcic amphibole and of the enclave’s bulk composition (e.g., Hibbard, plagioclase in five enclaves and their adjacent The TIC (Fig. 1) is one of four Late Cretaceous 1991; Baxter and Feely, 2002). The uniformity in host rocks: the equigranular Half Dome unit of the complexes that make up the Sierra Crest suite of mineral assemblage is interpreted to result in part Tuolumne Intrusive Complex (TIC; Bateman and the Sierra Nevada batholith (Coleman and Glaz- from physical mixing (e.g., Vernon, 1983, 1990), but Chappell, 1979; Huber et al., 1989). Our goal was ner, 1997). It consists of five mapped units, the also from diffusive exchange between enclave and to determine whether trace-element abundances in outermost of which is Kuna Crest Granodiorite host magmas (e.g., Baker, 1991; Tepper and Kueh- amphibole and plagioclase in enclaves are distinct and equivalent rocks exposed along the margin ner, 2004; Humphreys et al., 2010). Textural features, from those in the host, and if so, to interpret the of the TIC. Successively interior units are the Half including fine-scale crystal habits and inclusion rela- reasons for these distinctions. We found that while Dome Granodiorite, which is subdivided into tionships within such enclaves preserve evidence major-element compositions of enclave minerals outer equigranular, and inner porphyritic units, of undercooling and mixing (e.g., Hibbard, 1981, are similar to those in the host, trace-element abun- Cathedral Peak Granodiorite, and Johnson Gran- 1991; Vernon, 1983, 1990), so it is interesting to ask dances and zoning patterns are generally dissimilar ite Porphyry (Bateman and Chappell, 1979; Huber whether mineral compositions and zoning patterns to the host phases. Moreover, although some large et al., 1989). Zircon U-Pb ages range from 94.86 may, in fact, also preserve a magmatic evolutionary crystals in enclaves were inherited from the adja- ± 0.29 Ma to 83.86 ± 0.31 Ma (Paterson et al., 2016). history distinct from that of the host. cent host magma, it is more common that these The enclaves described here were collected from As with mafic enclaves in volcanic rocks, the large crystals are dissimilar to equivalent crystals the eHD unit, in which U-Pb (zircon) ages range bulk compositions of mafic enclaves in plutons in typical equigranular Half Dome (eHD) rocks, with from 91.7 ± 0.2–89.6 ± 0.2 Ma (Coleman et al., have been interpreted by some to represent the the implication that some enclaves record multiple 2004; Memeti et al., 2010).
Load more

Recommended publications
  • 45. Myrmekite Formed by Na- and Ca-Metasomatism of K- Feldspar
    1 ISSN 1526-5757 45. Myrmekite formed by Na- and Ca-metasomatism of K- feldspar Jiashu RONG Beijing Research Institute of Uranium Geology P.O. Box 9818, Beijing, 100029 Peoples Republic of China Email address: [email protected] September 23, 2002 Abstract Myrmekite, an intergrowth of plagioclase and quartz vermicules, often occurs in felsic-to-intermediate, calc-alkalic plutonic and gneissic rocks. Several hypotheses for myrmekite genesis are briefly reviewed and discussed. The proportional relationship between the volume of quartz vermicules and the An value of plagioclase is consistent with the hypothesis or replacement, solid state exsolution, and recrystallization of primary plagioclase. (1) Perthitic albite and K- feldspar relicts in myrmekite and (2) swapped myrmekite between two differently oriented K-feldspar crystals obviously indicate that myrmekite is formed by nibble replacement of K-feldspar by myrmekitic plagioclase because of modification by Ca-bearing sodic emanation or fluid. Myrmekite of different morphology and spatial distribution are of the same metasomatic origin. The myrmekite that was produced during deformation should be later than that formed prior to deformation. Characteristics of myrmekite Myrmekite was first described by Michel-Lévy (1874) and named by Sederholm (1897) as an intergrowth of plagioclase and quartz vermicules. Myrmekite is commonly observed in felsic-to-intermediate calc-alkalic granitic rocks (but not in alkalic-tending plutonic rocks) and in granitic gneisses of similar composition. 2 On the basis of the geologic environment, Phillips (1974) classified myrmekite in the following ways: 1. rim myrmekite, occurring at the contact of plagioclase with another differently oriented K-feldspar; 2. intergranular myrmekite, situated at the boundary between two differently oriented K-feldspars; 3.
    [Show full text]
  • Structural Geology of the Upper Rock Creek Area, Inyo County, California, and Its Relation to the Regional Structure of the Sierra Nevada
    Structural geology of the upper Rock Creek area, Inyo County, California, and its relation to the regional structure of the Sierra Nevada Item Type text; Dissertation-Reproduction (electronic); maps Authors Trent, D. D. Publisher The University of Arizona. Rights Copyright © is held by the author. Digital access to this material is made possible by the University Libraries, University of Arizona. Further transmission, reproduction or presentation (such as public display or performance) of protected items is prohibited except with permission of the author. Download date 27/09/2021 06:38:23 Link to Item http://hdl.handle.net/10150/565293 STRUCTURAL GEOLOGY OF THE UPPER ROCK CREEK AREA, INYO COUNTY, CALIFORNIA, AND ITS RELATION TO THE REGIONAL STRUCTURE OF THE SIERRA NEVADA by Dee Dexter Trent A Dissertation Submitted to the Faculty of the DEPARTMENT OF GEOSCIENCES In Partial Fulfillment of the Requirements For the Degree of DOCTOR OF PHILOSOPHY In the Graduate College THE UNIVERSITY OF ARIZONA 1 9 7 3 THE UNIVERSITY OF ARIZONA GRADUATE COLLEGE I hereby recommend that this dissertation prepared under my direction by __________ Dee Dexter Trent______________________ entitled Structural Geology of the Upper Rock Creek Area . Tnvo County, California, and Its Relation to the Regional Structure of the Sierra Nevada ________________ be accepted as fulfilling the dissertation requirement of the degree of ____________Doctor of Philosophy_________ ___________ P). /in /'-/7. 3 Dissertation Director fJ Date After inspection of the final copy of the dissertation, the following members of the Final Examination Committee concur in its approval and recommend its acceptance:* f t M m /q 2 g ££2 3 This approval and acceptance is contingent on the candidate's adequate performance and defense of this dissertation at the final oral examination.
    [Show full text]
  • Some Uncommon Sapphire “Imitations”: Blue Co-Zirconia, Kyanite & Blue Dumortierite Dr Michael S
    Some Uncommon Sapphire “Imitations”: Blue Co-zirconia, Kyanite & Blue Dumortierite Dr Michael S. Krzemnicki Swiss Gemmological Institute SSEF [email protected] 筆者滙報數個瑞士珠寶研究院(SSEF)近期收到 in the ring showed a negative RI reading 要求鑑證的藍色寶石，經檢測後確定其中包括 (above 1.79), an isotropic optical character 一些非常罕見的藍寶石模擬石：含錮氧化鋯、 (polariscope) and thus no pleochroism at all. 藍晶石及藍線石等。 Under the microscope, we saw no inclusions, however a slightly greenish reaction under Sapphires are among the most abundant gems the LWSW and there was a weaker similar we receive at the Swiss Gemmological Institute reaction under SWUV lamps. Based on these (SSEF) for testing. From time to time, however, properties and a chemical analysis by X-ray we are quite surprised by the imitations which fluorescence (EDXRF), the blue stone was we find among the goods sent in and this can readily identified as cubic zirconia (ZrO2). then be disappointing news for the clients. In Having seen this artificial product in a wide the following short note, the author presents a range of colours, the author had not previously few uncommon imitations identified recently at seen one of such a saturated and attractive the SSEF. Identification of these imitations is blue. Based on literature (Nassau 1981) the straightforward and should be no problem for analysed traces of cobalt in that stone have any experienced gemmologist. been identified as the colouring element in this specimen. The absorption spectrum of the stone The first case is that of an attractive blue (Fig. 2) – although superposed by several rare faceted stone of approximately 1.4 ct, set in a ring with diamonds (Fig.
    [Show full text]
  • Igneous Garnet and Amphibole Fractionation in the Roots of Island Arcs: Experimental Constraints on Andesitic Liquids
    CORE Metadata, citation and similar papers at core.ac.uk Provided by Serveur académique lausannois Contrib Mineral Petrol (2009) 157:541–558 DOI 10.1007/s00410-008-0351-8 ORIGINAL PAPER Igneous garnet and amphibole fractionation in the roots of island arcs: experimental constraints on andesitic liquids Raquel Alonso-Perez Æ Othmar Mu¨ntener Æ Peter Ulmer Received: 23 January 2008 / Accepted: 6 October 2008 / Published online: 30 October 2008 Ó Springer-Verlag 2008 Abstract To evaluate the role of garnet and amphibole for ‘slab melting’, or dehydration melting in the deep arc. fractionation at conditions relevant for the crystallization of Garnet fractionation, either in the deep crust via formation magmas in the roots of island arcs, a series of experiments of garnet gabbros, or in the upper mantle via formation of were performed on a synthetic andesite at conditions garnet pyroxenites remains an important alternative, ranging from 0.8 to 1.2 GPa, 800–1,000°C and variable despite the rare occurrence of magmatic garnet in volcanic H2O contents. At water undersaturated conditions and fO2 rocks. established around QFM, garnet has a wide stability field. At 1.2 GPa garnet ? amphibole are the high-temperature Keywords Experimental petrology Á Hydrous andesite liquidus phases followed by plagioclase at lower tempera- liquids Á High-pressure crystallization Á Amphibole Á ture. Clinopyroxene reaches its maximal stability at H2O- Garnet fractionation contents B9 wt% at 950°C and is replaced by amphibole at lower temperature. The slopes of the plagioclase-in boundaries are moderately negative in T-XH2O space. At Introduction 0.8 GPa, garnet is stable at magmatic H2O contents exceeding 8 wt% and is replaced by spinel at decreasing Major and trace element similarities between estimates of dissolved H2O.
    [Show full text]
  • Reworked Restite Enclave
    Reworked restite enclave: Petrographic and mineralogical constraints from the Tongshanling intrusion, Nanling Range, South China Xu-Dong Huang, Jian-Jun Lu, Stanislas Sizaret, Ru-Cheng Wang, Jin-Wei Wu, Dong-Sheng Ma To cite this version: Xu-Dong Huang, Jian-Jun Lu, Stanislas Sizaret, Ru-Cheng Wang, Jin-Wei Wu, et al.. Re- worked restite enclave: Petrographic and mineralogical constraints from the Tongshanling intru- sion, Nanling Range, South China. Journal of Asian Earth Sciences, Elsevier, 2018, 166, pp.1-18. 10.1016/j.jseaes.2018.07.001. insu-01856863 HAL Id: insu-01856863 https://hal-insu.archives-ouvertes.fr/insu-01856863 Submitted on 10 Dec 2019 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. Reworked restite enclave: Petrographic and mineralogical constraints from the Tongshanling intrusion, Nanling Range, South China Xu-DongHuangab Jian-JunLua StanislasSizaretb Ru-ChengWanga Jin-WeiWua Dong-ShengMaa A : State Key Laboratory for Mineral Deposits Research, School of Earth Sciences and Engineering, Nanjing University, Nanjing 210023, China B : Institut des Sciences de la Terre d’Orléans, UMR 7327-CNRS/Université d’Orléans/BRGM, Orléans 45071, France Abstract Microgranular enclaves, which can provide important petrogenetic indications for the host granitoids, are commonly observed in the Middle-Late Jurassic Cu-Pb-Zn-bearing granodiorites in the Nanling Range of South China.
    [Show full text]
  • HK Fancy Sapphire
    Hong Kong, March 2011 Fancy Coloured Sapphires: The Beauty beyond "Blue" of Sapphire and "Red" of Ruby Dr. Michael S. Krzemnicki Swiss Gemmological Institute SSEF Switzerland All photos © M.S. Krzemnicki, SSEF except where indicated. The range of colours... © Swiss Gemmological Institute SSEF 1 The range of colours... © Swiss Gemmological Institute SSEF The range of colours... © Swiss Gemmological Institute SSEF 2 The range of colours... © Swiss Gemmological Institute SSEF The range of colours... © Swiss Gemmological Institute SSEF 3 The range of colours... © Swiss Gemmological Institute SSEF The range of colours... © Swiss Gemmological Institute SSEF 4 The range of colours... © Swiss Gemmological Institute SSEF The range of colours... © Swiss Gemmological Institute SSEF 5 The range of colours... © Swiss Gemmological Institute SSEF The range of colours... © Swiss Gemmological Institute SSEF 6 The range of colours... © Swiss Gemmological Institute SSEF The range of colours... © Swiss Gemmological Institute SSEF 7 The range of colours... © Swiss Gemmological Institute SSEF The range of colours... © Swiss Gemmological Institute SSEF 8 The range of colours... © Swiss Gemmological Institute SSEF The range of colours... Fancy sapphires: The colour range beyond red of rubies and blue of sapphires © Swiss Gemmological Institute SSEF 9 The range of colours... Photo: © SilkenEast Ltd, Bangkok © Swiss Gemmological Institute SSEF The range of colours... Collection: SilkenEast Ltd, Bangkok © Swiss Gemmological Institute SSEF 10 Jewellery with fancy sapphires Photos © Luc Phan, SSEF © Swiss Gemmological Institute SSEF © Swiss Gemmological PhotoInstitute © Luc SSEF Phan, SSEF 11 Corundum Chemical composition: aluminium oxide, Al2O3 Chemical pure aluminium oxide is colourless © Wikipedia In nature always with trace elements (chemical impurities), usually: - Mg, Ti, V, Cr, Fe, Ga - and occasionally rare HFS-elements such as Nb, Sn, Ta, Th Not all trace elements are affecting the colour (e.g.
    [Show full text]
  • Metamorphic and Metasomatic Kyanite-Bearing Mineral
    Metamorphic and Metasomatic Kyanite-Bearing Mineral Assemblages of Thassos Island (Rhodope, Greece) Alexandre Tarantola, Panagiotis Voudouris, Aurélien Eglinger, Christophe Scheffer, Kimberly Trebus, Marie Bitte, Benjamin Rondeau, Constantinos Mavrogonatos, Ian Graham, Marius Etienne, et al. To cite this version: Alexandre Tarantola, Panagiotis Voudouris, Aurélien Eglinger, Christophe Scheffer, Kimberly Tre- bus, et al.. Metamorphic and Metasomatic Kyanite-Bearing Mineral Assemblages of Thassos Island (Rhodope, Greece). Minerals, MDPI, 2019, 10.3390/min9040252. hal-02932247 HAL Id: hal-02932247 https://hal.archives-ouvertes.fr/hal-02932247 Submitted on 7 Sep 2020 HAL is a multi-disciplinary open access L’archive ouverte pluridisciplinaire HAL, est archive for the deposit and dissemination of sci- destinée au dépôt et à la diffusion de documents entific research documents, whether they are pub- scientifiques de niveau recherche, publiés ou non, lished or not. The documents may come from émanant des établissements d’enseignement et de teaching and research institutions in France or recherche français ou étrangers, des laboratoires abroad, or from public or private research centers. publics ou privés. minerals Article Metamorphic and Metasomatic Kyanite-Bearing Mineral Assemblages of Thassos Island (Rhodope, Greece) Alexandre Tarantola 1,* , Panagiotis Voudouris 2 , Aurélien Eglinger 1, Christophe Scheffer 1,3, Kimberly Trebus 1, Marie Bitte 1, Benjamin Rondeau 4 , Constantinos Mavrogonatos 2 , Ian Graham 5, Marius Etienne 1 and Chantal Peiffert
    [Show full text]
  • Ocean Drilling Program Scientific Results Volume
    Von Herzen, R. P., Robinson, P. T., et al., 1991 Proceedings of the Ocean Drilling Program, Scientific Results, Vol. 118 22. PLASTIC DEFORMATION AT AN OCEANIC SPREADING RIDGE: A MICROSTRUCTURAL STUDY OF THE SITE 735 GABBROS (SOUTHWEST INDIAN OCEAN)1 Mathilde Cannat2 ABSTRACT Microstructural analysis suggests that some Site 735 gabbros were deformed in the crystal mush stage, before they had completely crystallized. Later, solid-state deformation began in granulite-facies metamorphic conditions. At this stage, temperature-dependent diffusion processes may have controlled the plastic deformation of these gabbros, and mylonites formed only in intervals containing more than 10% Fe-Ti oxides. Most oxides in these mylonites are interpreted as magmatic in origin. A sharp change of deformation processes occurred as temperature decreased to the stability conditions of amphibolite-facies metamorphic assemblages: dislocation slip in plagioclase may have become the leading plastic flow mechanism. This change of deformation processes coincides with increased availability of hydrothermal water and with a marked decrease of the plagioclase recrystallized grain size. This reduction in recrystallized grain size is thought to result from an increase of the deviatoric stress, and thus from an increase of the yield strength of the gabbros. In amphibolite-facies metamorphic conditions, dynamic recrystallization of plagioclase locally caused strain-softening, leading to the formation of mylonites. Relict porphyroclasts (plagioclase, olivine, and pyroxenes) in these mylonites are fractured and boudinaged. The cracks produced by this brittle failure are filled with green to brown hydrothermal hornblende. INTRODUCTION the Site 735 deformed gabbros. These results are used to interpret the tectonic evolution of these gabbros (Cannat et A total of 500 m of gabbro was drilled at Site 735 during al., this volume).
    [Show full text]
  • Scientific Communication
    SCIENTIFIC COMMUNICATION NOTES ON FLUID INCLUSIONS OF VANADIFEROUS ZOISITE (TANZANITE) AND GREEN GROSSULAR IN MERELANI AREA, NORTHERN TANZANIA ELIAS MALISA; KARI KINNUNEN and TAPIO KOLJONEN Elias Malisa: University of Helsinki, Department of Geology, SF-00170 Helsinki, Finland. Kari Kinnunen and Tapio Koljonen: Geological Survey of Finland, SF-02150 Espoo, Finland. Tanzanite is a trade name for a gem-quality has been reported in Lalatema and Morogoro in vanadiferous zoisite of deep sapphire-blue colour Tanzania and in Lualenyi and Lilani in Kenya discovered in Merelani area, Tanzania in 1967. (Naeser and Saul 1974; Dolenc 1976; Pohl and This mineral was first described as a strontium Niedermayr 1978). -bearing zoisite by Bank, H. & Berdesinski, W., Crystals of tanzanite occur mainly in bou- 1967. Other minor occurrences of this mineral dinaged pegmatitic veins and hydrothermal frac- Fig. 1. Tanzanite-bearing horizon in the graphite-rich diopside gneiss. The yellow colour indicates hydrothermal alteration, which can be used in pros- pecting for tanzanite. Length of photo ca. 8 m. 54 Elias Malisa, Kari Kinnunen and Tapio Koljonen given as Ca2Al3Si30120H (Ghose & Tsang 1971). The chemical compositions of tanzanites studied are given in Table 1. Unit cell dimensions, measured by X-ray dif- fraction, are a = 16.21, b = 5.55, c = 10.03 ± 0.01 Å in agreement with Hurlbut (1969). Zoisite shows diffraction symmetry mmmPn-a, which limits the possible space groups to Pnma if centric or Pn2, if acentric (Dallace 1968). The most striking property of tanzanite is its pleochroism, which changes from trichroic to dichroic on heating; normally its pleochroism varies: X = red-violet, Y = c = deep blue, Z = a = yellow- Fig.
    [Show full text]
  • VOLUME 45, NO. 80 PLEOCHRONIC MINERALS Wednesday July 28 7:00—9:00 Pm Makiki District Park Administration Building NEXT MONTH
    VOLUME 45, NO. 80 JULY 2010 PLEOCHRONIC MINERALS MEETING BY DEAN SAKABE Wednesday Pleochroic minerals are miner- July 28 als that show different colors 7:00—9:00 pm depending on what direction Makiki District you are observing the crystal. Park In order to view pleochroism Administration you need an individual transpar- Building ent crystal. This effect can be very dramatic. Many minerals NEXT MONTH are technically pleochroic, but Wednesday most often the color change is August 25, 2010 so small that it can be barely detected. For those few other LAPIDARY minerals, the color change is very, very obvious. The great- Every Thursday est change is limited to three 6:30-8:30pm colors and is called trichroic(1-3). Second-floor Arts A two color change occurrence and Crafts Bldg is called dichroic (4-5). Pleo- Makiki District chroic, which means "many col- Park ors", is used to cover both of 1-3 Tanzanite with all 3 colors of the natural these color changes. Most of trichroic crystal present and strongly show- ing down different axes of view the time, the color change is MEMBERSHIP limited to shade changes such COSTS as from pale pink to dark pink. 2008 Single: $10.00 Family: $15.00 Rock and Mineral Society of Hawai‛i INC. PLEOCHRONIC MINERALS , PAGE 2 rhombic, monoclinic, and triclinic minerals that can be trichroic. This is because they have three unique axes of symmetry and therefore three unique directions that can absorb light in three different ways. The most famous dichroic mineral is Cordierite, a Magne- sium Aluminum Silicate.
    [Show full text]
  • Petrogenesis of Slab-Derived Trondhjemite-Tonalite-Dacite/ Adakite Magmas M
    Transactions of the Royal Society of Edinburgh: Earth Sciences, 87, 205-215, 1996 Petrogenesis of slab-derived trondhjemite-tonalite-dacite/ adakite magmas M. S. Drummond, M. J. Defant and P. K. Kepezhinskas ABSTRACT: The prospect of partial melting of the subducted oceanic crust to produce arc magmatism has been debated for over 30 years. Debate has centred on the physical conditions of slab melting and the lack of a definitive, unambiguous geochemical signature and petrogenetic process. Experimental partial melting data for basalt over a wide range of pressures (1-32 kbar) and temperatures (700-1150=C) have shown that melt compositions are primarily trondhjemite- tonalite-dacite (TTD). High-Al (> 15% A12O3 at the 70% SiO2 level) TTD melts are produced by high-pressure 015 kbar) partial melting of basalt, leaving a restite assemblage of garnet + clinopyroxe'ne ± hornblende. A specific Cenozoic high-Al TTD (adakite) contains lower Y, Yb and Sc and higher Sr, Sr/Y, La'/Yb and.Zr/Sm relative to other TTD types and is interpreted to represent a slab melt under garnet amphibolite to eclogite conditions. High-Al TTD with an adakite-like geochemical character is prevalent in the Archean as the result of a higher geotherm that facilitated slab melting. Cenozoic adakite localities are commonly associated with the subduction of young (<25Ma), hot oceanic crust, which may provide a slab geotherm (*9-10=C km"1) conducive for slab dehydration melting. Viable alternative or supporting tectonic effects that may enhance slab melting include highly oblique convergence and resultant high shear stresses and incipient subduction into a pristine hot mantle wedge.
    [Show full text]
  • A Crystal Size Distribution (CSD) Study of Microcline in the Cathedral Peak Granodiorite, Sierra Nevada, California
    Origin of megacrysts in granitoids by texturai coarsening: a crystal size distribution (CSD) study of microcline in the Cathedral Peak Granodiorite, Sierra Nevada, California MICHAEL D. HIGGINS Sciences de la Terre, Université du Québec à Chicoutimi, Chicoutimi, G7H 2Bl, Canada (e-mail: [email protected]) Abstract: Microcline megacrysts in the Cathedra! Peak Granodiorite and other parts of the Tuolumne Intrusive Suite were formed by texturai coarsening (Ostwald Ripening) of earlier formed crystals. The early-formed crystals nucleated and grew in an environment of increasing undercooling, probably during the ascent of the magma. Emplacement of the magma into warm host rocks promoted texturai coarsening. Crystals smaller than a certain size (the critical size) dissolved in the interstitial melt whilst larger crystals grew. Microcline was most sensitive to this effect as the magma temperature was buffered close to its liquidus for a long period by the release of latent heat of crystallization. Positive feedback between texturai coarsening and magma permeability channelled the flow of interstitial melt to produce a heterogeneous distribution of megacrysts. Megacryst growth was halted when cooling resumed at the end of the intrusive cycle. K-feldspar nucleation was then renewed and K-feldspar crystals grew to form part of the groundmass. lt was the particular thermal history of this pluton that promoted texturai coarsening-chemically similar plutons that lack megacrysts probably did not have the pause during cooling that was necessary for the development of this texture. Euhedral K-feldspar megacrysts up to 20 cm Swanson (1977) and Fenn (1977) measured the long are a striking and relatively common nucleation and growth rates of plagioclase, component of many granitoid plutons, but K-feldspar and quartz crystallizing from glassy their origin is still controversial.
    [Show full text]