An oxygen and hydrogen isotope study of high-grade metamorphism and anatexis in the Ruby Mountains-East Humboldt Range core complex, Nevada

Abstract

Oxygen isotope analyses were made of 91 whole rock and mineral separate samples from the Ruby Mountains-East Humboldt Range metamorphic core complex and other nearby areas in eastern Nevada. In addition, ten hydrogen isotope analyses were made of the hydrous minerals muscovite, biotite, and hornblende. The samples include plutonic igneous rocks, high-grade metasedimentary rocks, and low-grade sediments that are thought to be their protoliths. The δ^(18)O values vary systematically in each of these categories. Most of the metasediments were shifted to lower δ^(18)O values than their protoliths, but the magnitude and pervasiveness of this ^(18)O-depletion varies in different geographic areas, becoming most intense at Lizzies Basin in the East Humboldt Range where the deepest structural levels in the area are exposed. Within the area sampled at this locality, all silicate rock-types are isotopically homogenized over scales of at least tens of meters and shifted to δ^(18)O values close to +6. Quartz from contrasting igneous and metasedimentary rock-types has a uniform oxygen isotope composition implying that this mineral was very well equilibrated (see also WICKHAM and PETERS, 1990). Elsewhere, exchange was less effective as illustrated by contrasts in the δ^(18)O values of adjacent marble, quartzite, and leucogranite layers. The δ^(18)O values of chemically similar leucogranites vary widely within the area studied. The most ^(18)O-rich samples are from the northern Ruby Mountains (δ^(18)O > + 12), and these must have been derived by anatexis from sedimentary material. The lower-^(18)O leucogranites were probably derived from similar material that had been isotopically altered at an earlier, metamorphic stage. ^(18)O-depletion in the deep-seated metasediments and igneous rocks was probably the result of exchange with fluids derived from primitive mantle-derived intrusive rocks or with the rocks themselves. This conclusion is supported by the "normal" plutonic igneous δD values shown by these same samples. At high structural levels, mylonites and associated rocks within the low-angle fault zone exposed at Secret Creek gorge were variably affected by meteoric water infiltration, which probably occurred during Tertiary extensional deformation, resulting in anomalously low D/H and ^(18)O/^(16)O ratios. Low-grade sedimentary rocks from other areas are also sporadically ^(18)O-depleted, and this is similarly attributable to exchange with meteoric waters. These new isotopic data from Nevada add to, and complement, data from other terranes, such as the Pyrenees (WICKHAM and TAYLOR, 1985), that demonstrate that profound ^(18)O- depletions and isotopic homogenization of metasedimentary rocks can take place during certain types of regional metamorphism. However, it is also becoming clear that these metamorphic ^(18)O-depletions are probably produced by a variety of different mechanisms.