Granitic Rocks of the Mt. Barcroft Quadrangle, Inyo Batholith, California-Nevada

Abstract

Granitic rocks underlie 120 square miles of the Mt. Barcroft quadrangle portion of the Inyo batholith, California-Nevada. The Barcroft and Cabin Granodiorite plutons make up 18 per cent of this granitic area; the Cottonwood Porphyritic Adamellite pluton, 28 per cent; the McAfee Ad-amellite pluton, which is texturally divisible into a fine-grained, a medium-grained, and a porphyritic facies, 51 per cent; and the isolated Leidy and Sage Hen Adamellite plutons, 3 per cent. These units, sampled on a 1-mile grid and by traverses across contacts, are in sharp intrusive contact with each other and with the sedimentary rocks. The plutons were intruded in a mafic-to-felsic sequence into low-grade metasedimentary and metavolcanic rocks. The Pellisier Granite, which became a classic example of granitization after G. H. Anderson9s 1937 article on the northern Inyo Range, consists of several separate units in this area. These units include some that are metamorphic but not granitic (schists and gneisses) and the two distinctive plutons of granodiorite. The “pseudo-sedimentary structure in granite” described by Anderson (1937, p. 10) is not related to any sedimentary structure but is the result of banding due to repeated uplift on a recent fault. The granodiorite portions of the Pellisier Granite are separated from the sedimentary rocks by a sheared zone and a narrow aureole of contact metamorphism rather than by broad transitional zones as described by Anderson. Plagioclase compositions are homogeneous within plutons and range from an average of An 35 in the granodiorites to An 23 in the major adamellite. Variations in modal compositions and normative compositions calculated from modes are directly related to the experimentally located thermal valley which controls the compositions of liquids in the system SiO 2 –NaAlSi 3 O 8 –KAlSi 3 O 8 . Only limited areas of migmaties and gneisses occur, and these are regarded as by-products of magmatic intrusion. It is apparent that this classic area of “extensive” granitization can no longer be cited as evidence of the metamorphic origin of granite.