Compositions of Paleozoic and Lower Mesozoic volcanic rocks of western Nevada and adjoining areas reflect the tectonic condition of their time and place of eruption. Despite structural complication and metamorphic overprinting, a general change can be observed from continental, cratonic, volcanic assemblages in the east to oceanic assemblages in the west. In Paleozoic (pre-Sonorria) time, an ensimatic basin was present west of the craton margin and east of correlative island-arc assemblages. In early Mesozoic time, continental assemblages passed westward and northwestward into continental margin assemblages in far western Nevada and into island-arc assemblages farther west. These arc assemblages may have formed on a crust that was compositionally intermediate between oceanic and continental crust.

The North American continent grew westward by accretion of one or more island arcs in late Paleozoic–early Mesozoic (Sonoma) time and by general segregation of sial along the margin. Crustal thickening occurred during and probably following the Sonoma orogeny. The sialic material must have been newly formed from the mantle and represented significant westward continental growth in the past 300 m.y.

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