One to three whole-rock samples from each of more than a dozen discrete plutonic intrusions in the east-central Sierra Nevada batholith have been analyzed for Sr 87 /Sr 86 and Rb/Sr ratios to obtain information on initial Sr 87 abundances. The initial Sr 87 /Sr 86 ratios in the rock magmas forming this province appear to have been in the range 0.7073 ± .0010 in the majority of cases. This range is definitely higher than that found for modern alkali-type and tholeiite-type basalt magmas of oceanic regions, which commonly range between 0.703 and 0.705. However, it is much lower than the average Sr 87 /Sr 86 ratios found in Precambrian sialic regions which range from 0.71 to 0.73. It seems clear therefore that the Sierra Nevada magmas were not derived solely either from the typical source regions of oceanic basalt or from the melting of ancient crustal sial. It is possible that these magmas represent a mixture of oceanic basalt and crustal sial, as would be the case of anatexis in a geosyncline containing much volcanic material of fairly recent origin and some terrigenous sialic detritus. They may instead be of mantle derivation with admixtures of crustal material assimilated during their rise. The whole-rock Rb-Sr age results derivec from the study indicate that the Lamarck and Mount Givens Granodiorites and the alaskite of Evolution Basin and porphyritic biotite granite of Dinkey Lakes form a younger group of intrusive rocks of 90 ± 10 m.y. Although the sampling was not designed for isochron age studies, it appears that most of the remaining rock units are considerably older.
Petrographic study and modal and chemical analyses of samples collected along a traverse from the margin to the interior of the bulbous head at the north end of the Mount Givens Granodiorite show that it is compositionally zoned. Relatively mafic rock in the margin, composed of higher temperature mineral assemblages, passes inward, with only one identifiable internal contact, to more felsic rock composed of lower temperature mineral assemblages. Hornblende and biotite decrease in amount inward from the external contact, plagioclase is rather constant, and quartz and K-feldspar increase inward. The anorthite content of plagioclase and the MgO content of hornblende decrease from the cores to the margins of crystals and from the margin to the interior of the pluton. We interpret this pattern to have resulted from crystal fractionation during inward solidification with falling temperature. Fractionation occurred because residual solid material (restite) carried upward in the magma from the place of origin of the magma, together with crystals precipitating from the melt phase, was progressively cleared from the magma by settling downward and/or by accretion to the solidifying margins of the magma chamber. The kind, composition, and proportion of crystals precipitating changed with falling temperature and changing composition of the melt phase. The Mount Givens Granodiorite differs from the Tuolumne Intrusive Series in the following ways: (1) more abundant mafic inclusions and mottled cores in plagioclase suggest greater abundance of residual solid material from the source region of the magma; (2) more calcic rims of plagioclase crystals in the marginal rock indicate that the Mount Givens magma was at a higher temperature when the exposed rocks began to solidify; (3) the presence of quartz in tiny sub-equant crystals as well as in intergranular stringers in the marginal rock of the Mount Givens indicates that the Mount Givens magma was saturated in quartz as well as in plagioclase, hornblende, biotite, and magnetite when the first rocks began to solidify, whereas the Tuolumne magma was not initially saturated in quartz; (4) decreasing grain size of the mafic minerals inward precludes chilling and suggests that the wallrocks were preheated before solidification began; (5) the presence of ilmenite, absent in the Tuolumne Intrusive Series, and smaller amounts of magnetite and higher $$FeO/Fe_{2}O_{3}$$, suggest generally lower fOi in the Mount Givens magma; (6) resurgence of magma containing higher temperature settled crystals appears \o have occurred during solidification of the Mount Givens Granodiorite whereas surges of Tuolumne magma apparently contained no settled, higher temperature, minerals; (7) the fact that $$K_{2}O/Na_{2}O$$ is generally greater than one in the Mount Givens Granodiorite and less than one in the Tuolumne Intrusive Series probably accounts for plagioclase with larger An content crystallizing in the Mount Givens magma when the cotectic was intersected and K-feldspar began to crystallize.
Research Article| October 01, 1973 Fusion Relations in the System NaAlSi3O8-CaAl2Si2O8-KAlSi3O8-SiO2-H2O and Generation of Granitic Magmas in the Sierra Nevada Batholith D. C. PRESNALL; D. C. PRESNALL 1Institute for Geological Sciences, University of Texas at Dallas, Dallas, Texas 75230 Search for other works by this author on: GSW Google Scholar P. C. BATEMAN P. C. BATEMAN 2U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025 Search for other works by this author on: GSW Google Scholar GSA Bulletin (1973) 84 (10): 3181–3202. https://doi.org/10.1130/0016-7606(1973)84<3181:FRITSN>2.0.CO;2 Article history first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation D. C. PRESNALL, P. C. BATEMAN; Fusion Relations in the System NaAlSi3O8-CaAl2Si2O8-KAlSi3O8-SiO2-H2O and Generation of Granitic Magmas in the Sierra Nevada Batholith. GSA Bulletin 1973;; 84 (10): 3181–3202. doi: https://doi.org/10.1130/0016-7606(1973)84<3181:FRITSN>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Chemical analyses of 167 typical specimens indicate that about 95 percent of the intrusive rocks of the central Sierra Nevada contain more than 79 percent normative Ab + An + Or + Qz. If the composition of the lower continental crust is similar to or slightly more felsic than andesite, as seems likely, the system NaAlSi3O8-CaAl2Si2O8-KAlSi3O8-SiO2-H2O provides an excellent chemical model for testing various schemes of fusion of the lower crust and crystallization of the resulting magmas. From consideration of this system in conjunction with field and petrographic data, we conclude that the intrusive rocks are best explained by repeated episodes of equilibrium fusion corresponding to magmatic sequences defined by field, petrologic, chemical, and geochronologic data. Fractional crystallization of the crystal-liquid mush generated by equilibrium fusion, coupled with periodic upward or lateral movement of the less crystallized central part of the magma, would produce the characteristic mafic to felsic sequence of intrusion; each mafic to felsic sequence corresponds to a separate equilibrium fusion event. In contrast, a close approach to fractional fusion of the lower crust is inadequate for obtaining most of the plutonic rocks, because rock compositions capable of being produced by this process do not match those observed. Normal amounts of conductive heat from the mantle and from radioactive decay in the crust may have been capable of causing fusion in the deepest parts of a thickened crust under the central part of the Sierra Nevada without the aid of a transient heat source from the mantle, but would have been inadequate where the crust was thin in the western Sierra Nevada. However, upward transport of andesitic and basaltic magmas generated along a Mesozoic subduction zone dipping beneath the Sierra Nevada would have provided sufficient additional heat to make fusion of the lower crust unavoidable. This implies that a major portion of the present batholith must have been derived from the lower crust. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
Decades of field and microscope studies, and more recent quantitative geochemical analyses have resulted in a vast, and sometimes overwhelming, array of nomenclature and terminology associated with igneous rocks. This book presents a complete classification of igneous rocks based on all the recommendations of the International Union of Geological Sciences (IUGS) Subcommission on the Systematics of Igneous Rocks. The glossary of igneous terms has been fully updated since the first edition and now includes 1637 entries, of which 316 are recommended by the Subcommission. Incorporating a comprehensive bibliography of source references for all the terms included in the glossary, this book is an indispensable reference guide for all geologists studying igneous rocks, either in the field or the laboratory. It presents a standardised and widely accepted naming scheme that will allow geologists to interpret terminology in the primary literature and provide formal names for rock samples based on petrographic analyses. It is also supported by a website with downloadable code for chemical classifications.
Research Article| May 01, 1979 Crystallization, fractionation, and solidification of the Tuolumne Intrusive Series, Yosemite National Park, California PAUL C. BATEMAN; PAUL C. BATEMAN 1U.S. Geological Survey, 345 Middlefield Road, Menlo Park, California 94025 Search for other works by this author on: GSW Google Scholar BRUCE W. CHAPPELL BRUCE W. CHAPPELL 2Australian National University, Department of Geology, P.O. Box 4, Canberra, A.C.T. 2600, Australia Search for other works by this author on: GSW Google Scholar GSA Bulletin (1979) 90 (5): 465–482. https://doi.org/10.1130/0016-7606(1979)90<465:CFASOT>2.0.CO;2 Article history first online: 01 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation PAUL C. BATEMAN, BRUCE W. CHAPPELL; Crystallization, fractionation, and solidification of the Tuolumne Intrusive Series, Yosemite National Park, California. GSA Bulletin 1979;; 90 (5): 465–482. doi: https://doi.org/10.1130/0016-7606(1979)90<465:CFASOT>2.0.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract Study of the Tuolumne Intrusive Series, a concentric texturally and compositionally zoned plutonic sequence in the eastern part of Yosemite National Park, was undertaken to develop and test a model for the origin of comagmatic plutonic sequences in the Sierra Nevada batholith. The granitoid units that make up the sequence are progressively younger and more felsic inward. The bulk of the rocks are granodiorite, but the outermost formation is quartz diorite, and the innermost one is granite porphyry. The compositional gradient changes both gradually within formations and abruptly between them. The change is greatest in the outer 1 km and lower toward the center of the sequence. Hornblende and biotite, abundant in the marginal rocks, decrease rapidly inward for 1 km as K-feldspar and quartz increase, but farther inward, they decrease slowly. The most conspicuous chemical changes are shown by the elements that are enriched in the mafic minerals.The compositional zoning indicates that with decreasing temperature, the sequence solidified from the margins inward. Solidification was interrupted repeatedly by surges of fluid core magma. The magma eroded the adjacent solidifying rock, and it expanded the area of the magma chamber at the exposed level by crowding the wall and roof rocks outward and upward and by breaking through the solidifying carapace into the wall rocks. The compositional zonation resulted from crystal fractionation that could have involved (1) preferential accretion of crystalline material present in the magma to the margins of the magma chamber, thus displacing the melt phase progressively inward, and/or (2) downward settling of crystals, probably accompanied by upward movement of melt and volatiles; the residual magma solidifying to form the granitoids. Although either mechanism can explain the observed relations, they lead to very different interpretations of the composition of the magma when the first exposed granitoids solidified at the margins of the magma chamber and as the sequence solidified inward. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.
