STRONTIUM ISOTOPES IN AUTHIGENIC FELDSPAR FROM K-METASOMATIZED TUFFS, HARCUVAR MOUNTAINS, WEST-CENTRAL ARIZONA

Carbonate Sr isotopic data is commonly used to discern paleogroundwater flow paths and sources, but such studies of silicate alteration phases are rare. Cathodoluminescence (CL) guided physical microsampling of silicate alteration phases combined with Sr isotopic data, can potentially determine both flow paths and the age of the alteration. Low-T K metasomatism is a common alteration style throughout the southwestern United States and is commonly associated with detachment faulting. We investigated K-metasomatized Tertiary tuffs from the hanging wall of the Bullard detachment fault in the Harcuvar Mountains, west-central Arizona. Samples were collected in a horizontal traverse away from the Bullard detachment fault along Aguila Ridge, from the most- to the least-altered rocks. Igneous feldspars in the K-metasomatized Tertiary tuffs have been replaced by relatively pure adularia and albite, resulting in whole rock gain of K and Rb and loss of Na, Ca and Sr. Guided by CL and BSE imaging, primary and authigenic minerals were microsampled for Rb-Sr isotopic analysis. Authigenic adularia define an errorchron that yields an absolute age of metasomatism at 18.45 ± 5.1 Ma, within the age range given by K-Ar whole rock data (17.35 ± 0.36 Ma; Scarborough and Wilt, 1979, USGS Open File Report 79-1429). The errorchron defines an initial ⁸⁷Sr/⁸⁶Sr value of adularia of 0.7103 ± 0.0021, which overlaps the isotopic compositions of unaltered igneous feldspars (⁸⁷Sr/⁸⁶Sr = 0.7090 ± 0.0005) at the time of metasomatism. This similarity implies that the Sr incorporated into secondary adularia was derived directly from the feldspars that the adularia replaced. Alternatively, adularia Sr could have been derived from metasomatizing fluids, but these fluids must have had uniform Sr isotopic compositions, identical to that of igneous feldspars, throughout the study area. In either case, the large flux of Sr liberated from igneous feldspars likely controlled the Sr isotope composition of the secondary adularia and may represent a common feature of similar groundwater alteration systems.