REPLACEMENT MECHANISMS IN ALTERED ARC VOLCANIC ROCKS: CATHODOLUMINESCENCE OF THE INYO MOUNTAINS VOLCANIC COMPLEX, EASTERN CALIFORNIA

The Inyo Mountains volcanic complex (IMVC), like metavolcanic continental arc rocks throughout east-central California, records: 1) early alkali alteration linked with syn- to slightly post-eruptive hydrothermal activity, and 2) fluid effects of contact metamorphism. Because of its relatively low metamorphic grade, albite + epidote-bearing rocks of the IMVC preserve fluid-rock features related to process (1) that are now blurred by process (2) in other exposures of Late Triassic to Middle Jurassic metavolcanic rocks in the east central Sierra Nevada. We combined cathodoluminescence (CL) petrography and spectroscopy with whole-rock and mineral chemistry to characterize the petrogenesis of the two alteration phases in the IMVC and compare these with similar data for nearby, higher-T terranes.

Abundances of Ce, Yb, Ta, and Th suggest shoshonitic affinities for the subaerial, arc flank rocks of the IMVC. However, major and trace element trends also suggest low-T alkali exchange through gain of K and Rb and loss of Na, Ca and Sr. Feldspar phenocrysts and matrix have been replaced by near-end-member K-feldspar and albite. CL images show multiple generations of K-feldspar not observed by back-scattered electron imaging or major element mineral mapping. Altered phenocrysts typically contain turbid, “brown”-CL K-feldspar. Such grains commonly preserve evidence of fracture-controlled growth. This early generation of altered feldspar is partially replaced by solid and fluid inclusion-poor, blue-CL K-feldspar along fractures and grain boundaries. Albite grains either display “brown” or red CL. Despite the marked differences in mineral types and compositions, not only are igneous textures mostly retained, but rare, relicts of igneous sanidine and intermediate plagioclase are preserved.

“Brown”-CL emitting K-feldspar is typical of low-T, K metasomatized rocks. Two examples of terranes with higher-T, pluton related overprints, the Ritter Range and Alabama Hills, contain end-member K-feldspar that displays blue CL. Some of the Inyo Mountains rocks preserve K-feldspar textures that show a transition of CL-emission from brown to blue. These data indicate that fluid-mediated dissolution and reprecipitation create both low- and high-T feldspar replacement features in metavolcanic rocks of the Cordilleran arc.