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A study of sampling requirements to determine modal variability within the Lakeview Mountains pluton, northern Peninsular Ranges batholith, southern California

By
A.K. Baird
A.K. Baird
Department of Geology, Pomona College, Claremont, California 91711, USA
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W.B. Wadsworth
W.B. Wadsworth
Stagecoach Road, Marshfield, Maine 04654, USA
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D.M. Morton
D.M. Morton
U.S. Geological Survey and Department of Earth Sciences, University of California, Riverside, California 92521, USA
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Published:
January 01, 2014

Tonalite of the Lakeview Mountains pluton is extremely heterogeneous in modal composition at both locality and outcrop scales. We applied an X-ray diffraction (XRD) method of modal analysis to 335 whole-rock powders previously used to study variability in major-element chemistry. We developed sampling levels appropriate for XRD modal study, mapped compositional gradients, recognized contrasts between chemical and modal variability, and tested the XRD method's suitability for petrologic investigations. Modal variability was determined at differing scales ranging from drill-core samples and hand specimens to 610 m2 subareas of the pluton. Compositing of three hand samples reduced variance within subareas of this size sufficiently for mapping modal composition; the equivalent number found by previous studies for major-element chemical composition was nine. For this pluton, the X-ray method permits a single machine analysis to substitute for point counting at least 18 standard thin sections in each subarea in order to map compositional gradients. Because potassium occurs significantly only in biotite, close similarity of trend surfaces for potassium and biotite provides confirmation of the efficacy of XRD modal analysis in petrologic studies.

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Contents

GSA Memoirs

Peninsular Ranges Batholith, Baja California and Southern California

Douglas M. Morton
Douglas M. Morton
U.S. Geological Survey and Department of Earth Sciences, University of California, Riverside, California 92521, USA
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Fred K. Miller
Fred K. Miller
U.S. Geological Survey, 904 West Riverside Ave., Spokane, Washington 99201, USA
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Geological Society of America
Volume
211
ISBN print:
9780813712116
Publication date:
January 01, 2014

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