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Vertical zonation of the Barcroft granodiorite, White Mountains, California; implications for magmatic processes

Eric C. Ferre, Karen J. Michelsen, W. Gary Ernst, John D. Boyd and Edgardo Canon-Tapia
Vertical zonation of the Barcroft granodiorite, White Mountains, California; implications for magmatic processes
American Mineralogist (July 2012) 97 (7): 1049-1059


Petrological and geochemical variations within plutons reflect their magmatic and emplacement histories. Here we present new magnetic susceptibility (K (sub m) ) data on the approximately 163 Ma Barcroft granodiorite pluton in eastern California, which is exceptionally well exposed, especially in the vertical dimension. The Barcroft pluton offers exposures over a total of 2560 m of elevation and is an appropriate target to investigate variations of magnetic susceptibility. In ferromagnetic plutonic rocks, K (sub m) reflects mainly the abundance of magnetite, whereas in paramagnetic plutonic rocks it reflects primarily the abundance of mafic silicates. Magnetic susceptibility is also determined by magmatic processes such as crystal fractionation and by intensive parameters such as oxygen fugacity. Other magmatic processes, including magma replenishment, hybridization, and host-rock assimilation, may also influence K (sub m) variations. A first data set is based on 622 core samples that were measured in the laboratory. Our second data set comes from 1960 field measurements collected at 196 stations between approximately 1600 and 4000 m elevation. Detailed surveys were performed at the outcrop scale to evaluate the impact of the approximately 100 Ma McAfee Creek intrusion on the Barcroft background magnetic susceptibility. The combined data sets display a broad positive correlation between K (sub m) and elevation. Pluton mineralogy also appears to vary with elevation but is more difficult to quantify. At the outcrop scale, small dikes of the McAfee Creek granite transect the pluton and are responsible for a decrease in K (sub m) of the host granodioritic rocks toward the dikes due to late-stage magmatic or hydrothermal alteration. A contour map of K (sub m) shows a high degree of correlation with local topographic features such as deep canyons. Magnetic susceptibility of the Barcroft mafic rocks varies at the outcrop scale as a result of presence of petrological heterogeneities. However, these small-scale variations are embedded in a broader magnetic susceptibility trend due primarily to elevation, which reflects petrologic stratification of the pluton. The late-magmatic and hydrothermal alterations described in previous studies do not affect the spatial distribution of magnetic susceptibility. We propose that vertical increase of K (sub m) was primarily caused by crystal fractionation or another magmatic differentiation mechanism rather than by an externally driven increase in oxygen fugacity toward the roof of the intrusion.

ISSN: 0003-004X
EISSN: 1945-3027
Serial Title: American Mineralogist
Serial Volume: 97
Serial Issue: 7
Title: Vertical zonation of the Barcroft granodiorite, White Mountains, California; implications for magmatic processes
Affiliation: Southern Illinois University, Department of Geology, Carbondale, IL, United States
Pages: 1049-1059
Published: 201207
Text Language: English
Publisher: Mineralogical Society of America, Washington, DC, United States
References: 41
Accession Number: 2012-067598
Categories: Igneous and metamorphic petrology
Document Type: Serial
Bibliographic Level: Analytic
Illustration Description: illus. incl. 3 tables, sketch map
N35°00'00" - N38°00'00", W118°30'00" - W117°00'00"
Secondary Affiliation: University of New Mexico, USA, United StatesStanford University, USA, United StatesBP Exploration, USA, United StatesCICESE, 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: 201235
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