Zonal Distribution of Variations in Structural State of Alkali Feldspar within the Rader Creek Pluton, Boulder Batholith, Montana

The granodioritic Rader Creek pluton of the composite Boulderbatholith contains microperthitic alkali feldspar of bulk compositionOr₆₅ to Or₈₆ with a structurally variable potassic phase. Completecell parameters, 2V measurements, and bulk composition are givenfor 11 feldspar samples. The 131 and 131 reflections for theseand 58 additional samples show the following structural typesin the potassic phase: orthoclase only; orthoclase with subordinatemaximum 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 plutondifferent feldspar structural types occur in zones whose boundariesare approximately parallel to contacts with younger intrusiverocks cutting the Rader Creek pluton but are, in places, nearlyperpendicular to zonation within the pluton defined by rockcomposition. In general, the orthoclase zone is closest to thecontact with younger intrusives; the intermediate microclinezone is the most distant. Bulk compositions of alkali feldsparare more potassic in the orthoclase zone than elsewhere. Thedata suggest a complex history for the alkali feldspar, involvingat least two stages: 1. Exsolution and partial inversion oforthoclase to intermediate microcline during cooling of theRader Creek pluton; 2. Transformation of the intermediate-microclineassemblage to orthoclase during reheating of the pluton at thetime of intrusion of younger plutons of the batholith. The transitionalstage in this transformation is characterized by orthoclaseco-existing with subordinate microcline, whose obliquity usuallyapproaches that of maximum microcline.     

Magma Mixing, Recharge and Eruption Histories Recorded in Plagioclase Phenocrysts from El Chichon Volcano, Mexico

Consistent core-to-rim decreases of ⁸⁷Sr/⁸⁶Sr ratios and coincidentincreases in Sr concentrations in plagioclase phenocrysts ofvarying size (