Fluorine and Chlorine in Granitoids from the Basin and Range Province , Western United States ERIC

Analysis of fluorine and chlorine in 228 samples of granitoids' from the Basin and Range province of the western United States suggests that at least three types of granitoids can be distinguished: (1) fluorine-poor granitoids of the northwestern Great Basin (mean F = 0.041 wt %, maximum = 0.11 wt %) intrude a variety of allochthonous oceanic or island-arc terranes that were probably accreted to North America during the Paleozoic and Mesozoic eras; (2) fluorine~rich Precambrian granites generated during "anorogenic" magmatism of Proterozoic -age in the southwestern United States (mean F = 0.118 wt %, maximum = Q.35 wt %); and (3) a large group of granitoids with moderate fluorine contents (meanF = 0.053 wt %, maximum = 0.13 wt %). Thislast group consists of granitoids of the eastern Great Basin and the southern Basin and Range province that occur throughout the autochthonous continental terrane of the western United States. These differences'in fluorine concentration do not appear to be the result of regional differences in the degree of magma evolution. Instead, this variability is attributed to magma contamination by, or generation from, a comparatively fluorine-rich reservoir in the continental crust underlying the southern and eastern portions of the -Basin and Range province and to the absence of this reservoir in the northwestern Great Basin. This interpretation is consistent with the geologically established boundaries of the"exotic" terranes and with the Sr and Nd isotope compositions of rocks from these regions. Chlorine contents (mean CI = 0.013 wt %, range = 0.005-0.050 wt %) show no regional differences and are uniformly low in these granitoid rocks. of fluorine ·in 170 specimens of unaltered glassy rhyolitic rocks, Coats et al. (1963) showed that the distribution of rocks with high contents of fluorine (>0.10 wt %) correlated well with the distribution of significant fluorite deposits. Fluorine-rich volcanic. rocks are concentrated in the eastern part of the Basin and Range province (as well as in Idaho, along the Rio Grande rift, and in the Big Bend area of west Texas). Peters (1958) suggested that these geochemical and metallogenic features were the result of a persistent geochemical anomaly in the crust across the region. Eaton (1984) has reiterated this concept. Others, also noting the distribution of fluorite deposits and their association with fluorine-rich rocks in the western United States have suggested that the fluorine enrichment occurred in the mantle-either during generation of fluorine-rich "alkalic" magmas along a subduction zone (Lamarre and Hodder, 1978) or during magma generation accompanying extensional tectonism (Shawe, 1976; Van Alstine, 1976). In addition, Bailey (1977) has questioned the existence of fluorine proviI).ces over long geologic periods and suggested instead that the regional differences in fluorine content of volcanic rocks noted by Coats et al. 9 Present address: Department of Geology, Brigham Young (1963) are the result of differences in magmatic evoUniversity, Provo, Utah 84602. lution or alkalinity. The results of our investigations