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Pluton assembly and the genesis of granitic magmas; insights from the GIC pluton in cross section, Sierra Nevada Batholith, California

Keith D. Putirka, Joe Canchola, Jeffrey Rash, Oscar Smith, Gerardo Torrez, Scott R. Paterson and Mihai N. Ducea
Pluton assembly and the genesis of granitic magmas; insights from the GIC pluton in cross section, Sierra Nevada Batholith, California
American Mineralogist (July 2014) 99 (7): 1284-1303

Abstract

The approximately 151 Ma Guadalupe Igneous Complex (GIC) is a tilted, bi-modal intrusion that provides a rare view into the deeper, mantle-derived portions of a granitic pluton. Major oxide relationships show that GIC granitic rocks formed by in situ differentiation. Assimilation of sedimentary country rock is precluded, as GIC alumina saturation indices (ASI) are too low by comparison, while TiO (sub 2) and P (sub 2) O (sub 5) contents disallow partial melting of metavolcanic lower/middle crust. In contrast, Rb-Sr systematics support in situ magmatic differentiation, as unaltered GIC whole rock samples fall on a single 151 Ma isochron (initial (super 87) Sr/ (super 86) Sr = 0.7036) matching zircon age dates (Saleeby et al. 1989). Crystal/liquid segregation, though, was not continuous: mafic and felsic samples form discordant compositional trends, with a gap between 60-66% SiO (sub 2) . We posit that crystal/liquid segregation is continuous between 50-60% SiO (sub 2) , and leads to the genesis of intermediate composition liquids that are then too viscous to allow further continuous liquid segregation. Further crystal/liquid separation thereafter occurs discontinuously (at F nearly equal 45-50%), to yield a mafic crystalline (52-59% SiO (sub 2) ) residue and a silicic (70-75% SiO (sub 2) ) liquid (Bachmann and Bergantz 2004), which are, respectively, preserved in the Meladiorite and Granite/Granophyre units of the GIC. Outcrops in the gabbroic section support this view, where mafic crystalline layers feed directly into granitic dikes, and intermediate compositions are absent; mass balance calculations at the outcrop scale also support this model. It is unclear, though, to what extent this model applies to larger Sierran plutons; the smaller GIC may represent an end-member process, where rapid cooling limits mixing, due to rapid increases in mafic/felsic melt viscosity contrasts.


ISSN: 0003-004X
EISSN: 1945-3027
Coden: AMMIAY
Serial Title: American Mineralogist
Serial Volume: 99
Serial Issue: 7
Title: Pluton assembly and the genesis of granitic magmas; insights from the GIC pluton in cross section, Sierra Nevada Batholith, California
Affiliation: California State University, Department of Earth and Environmental Sciences, Fresno, CA, United States
Pages: 1284-1303
Published: 201407
Text Language: English
Publisher: Mineralogical Society of America, Washington, DC, United States
References: 121
Accession Number: 2014-058165
Categories: Igneous and metamorphic petrology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 2 tables, sketch map
N37°10'00" - N37°49'60", W120°25'00" - W119°19'60"
Secondary Affiliation: University of Southern California, USA, United StatesUniversity of Arizona, USA, United States
Country of Publication: United States
Secondary Affiliation: GeoRef, Copyright 2017, American Geosciences Institute. Abstract, copyright, Mineralogical Society of America. Reference includes data from GeoScienceWorld, Alexandria, VA, United States
Update Code: 201431
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