Geology of the Pine Hill Intrusive Complex, a Layered Gabbroic Body in the Western Sierra Nevada Foothills, California

Abstract

The western Sierra Nevada metamorphic belt was subjected to several episodes of tectonism and metamorphism (Clark, 1960a, 1964; Baird, 1962; Bateman and others, 1963; Bateman and Wahrhaftig, 1966; Evernden and Kistler, 1970; Bateman and Clark, 1974; Schweickert and Cowan, 1975; Duffield and Sharp, 1975). The Foothills fault system (Clark, 1960b), which approximately parallels the regional strike of the metamorphic rocks, divides the metamorphic belt into a series of elongate structural blocks. The western Sierra Nevada metamorphic belt is intruded throughout its entire length by granitic, gabbroic, and ultramafic plutonic rocks. Ultramfic rock, which is largely serpentinized, occurs as conformable lenticular bodies which commonly coincide with major fault zones. Granitic rock which locally intrudes the ultramafic rock was emplaced mainly during the Inyo Mountains (180 to 160 m.y.) or Yosemite (148 to 132 m.y.) intrusive epochs (Evernden and Kistler, 1970). Gabbroic rock which occupies much less areal extent than either the granitic or ultramafic plutonic rocks has been genetically related to both the granitic and ultramafic rocks as well as to the enclosing metavolcanic rock (Durrell, 1940; Hietanen, 1951, 1976; Compton, 1955; Eric and others, 1955; Bateman and others, 1963; Clark, 1964; Putnam and Alfors, 1969; Olmsted, 1971). The purpose of this study is to investigate the structure, petrology, mineralogy, and chemistry of one of the largest gabbroic plutons in the western Sierra Nevada metamorphic belt, the Pine Hill intrusive complex. The origin and emplacement of this intrusion in relation to the evolution of the western metamorphic belt is also discussed. Geologic Setting East of Sacramento, California, the western Sierra Nevada metamorphic belt can be divided into at least four structural blocks, each separated by a major fault zone (Fig. 1). As most fault zones in the western Sierra Nevada metamorphic belt have been mapped on the occurrence of linear bodies of sheared serpentinite and/or highly schistose metamorphic rock and “do not systematically repeat or offset dated stratigraphic successions, the nature and amount of offset along the major fault zones” (Schweickert and Cowan, 1975) is generally unknown.