Abstract

Among the many granitic plutons within the Basin and Range province of western North America, those in the Kern Mountains of eastern Nevada and western Utah are unusual structurally, mineralogically, and chemically. The two largest intrusions, which together are exposed over an area of 130 sq km (51 sq mi), have well-defined border facies of leucocratic and generally aplitic rocks, or of protoclastic rock developed from coarser core-facies material. Within the core facies are abundant aplitic, leucocratic dikes. The core facies of the larger intrusion (Tungstonia Granite) is a two-mica granite with conspicuous phenocrystic books of muscovite up to 5 cm across. The smaller intrusion (Skinner Canyon Granite) lacks muscovite, and Fe-Ti oxides and sphene are constant accessory phases in it.

Rb-Sr and K-Ar isotopic analyses suggest a wide range of initial Sr87/Sr86 ratios, which for the most part are unusually high—to nearly 0.7246 for the Tungstonia Granite—and a complex, protracted period of magmatism and isotopic adjustment that extended from possibly Mesozoic into mid-Cenozoic time. The simplest interpretation is that the final emplacement of the Tungstonia body occurred 60 m.y. ago and that the Skinner Canyon intrusion occurred between 30 and 45 m.y. ago. The data prompt the conclusion that the magmas represent remobilized sialic basement whose model dates (assuming an initial ratio typical for mantle-derived magmas) are in excess of 1.5 b.y.

The occurrence of such anatectic intrusions within the relatively intensely metamorphosed parts of the infrastructure of the Sevier orogenic belt, rather than elsewhere in the Basin and Range province, is not unexpected, and it possibly represents a more advanced evolutionary stage than do the gneiss domes previously documented within this same belt.

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