Abstract

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 = 0.35 wt %); and (3) a large group of granitoids with moderate fluorine contents (mean F = 0.053 wt %, maximum = 0.13 wt %). This last 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 Cl = 0.013 wt %, range = 0.005-0.050 wt %) show no regional differences and are uniformly low in these granitoid rocks.

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