Article navigation zonal distribution of variations in structural state of alkali feldspar within the Rader Creek pluton, Boulder Batholith, Montana

Journal of Petrology
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Abstract

The granodioritic Rader Creek pluton of the composite Boulder batholith contains microperthitic alkali feldspar of bulk composition Or65 to Or86 with a structurally variable potassic phase. Complete cell parameters, 2V measurements, and bulk composition are given for 11 feldspar samples. The 131 and 131 reflections for these and 58 additional samples show the following structural types in the potassic phase: orthoclase only; orthoclase with subordinate maximum or near-maximum microcline (obliquity = 0.75–1.00); orthoclase with subordinate intermediate microcline (obliquity = 0.64–0.71); and intermediate microcline (obliquity = 0.56–0.77) with subordinate orthoclase. Within the pluton different feldspar structural types occur in zones whose boundaries are approximately parallel to contacts with younger intrusive rocks cutting the Rader Creek pluton but are, in places, nearly perpendicular to zonation within the pluton defined by rock composition. In general, the orthoclase zone is closest to the contact with younger intrusives; the intermediate microcline zone is the most distant. Bulk compositions of alkali feldspar are more potassic in the orthoclase zone than elsewhere. The data suggest a complex history for the alkali feldspar, involving at least two stages: 1. Exsolution and partial inversion of orthoclase to intermediate microcline during cooling of the Rader Creek pluton; 2. Transformation of the intermediate-microcline assemblage to orthoclase during reheating of the pluton at the time of intrusion of younger plutons of the batholith. The transitional stage in this transformation is characterized by orthoclase co-existing with subordinate microcline, whose obliquity usually approaches that of maximum microcline.

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Additional publication details

Publication type Article
Publication Subtype Journal Article
Title Article navigation zonal distribution of variations in structural state of alkali feldspar within the Rader Creek pluton, Boulder Batholith, Montana
Series title Journal of Petrology
DOI 10.1093/petrology/9.3.331
Volume 9
Issue 3
Year Published 1968
Language English
Publisher Oxford Academic
Contributing office(s) California Volcano Observatory
Description 28 p.
First page 331
Last page 357
Country United States
State Montana
Other Geospatial Boulder Batholith, Montana
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