Abstract

East-northeast extension ca. 24 Ma at Rodeo and Nazas, México, was accompanied by eruption of hawaiites, marking some of the earliest intraplate-type mafic alkalic volcanism associated with development of the southern Basin and Range Province. An earlier extensional pulse, 32.3–30.6 Ma, concurrent with subduction- related rhyolitic volcanism of the Sierra Madre Occidental, is the earliest established extension in the southern Basin and Range Province. The Rodeo hawaiites are mostly along or just west of the north- northwest−striking, west-dipping Rodeo fault, a major breakaway fault that separates moderately extended terrane to the west from less extended terrane to the east. Hawaiites and interbedded conglomerates in the Rodeo half graben are flat lying to gently tilted and cut by the Rodeo fault. Underlying Eocene−Oligocene ash-flow tuffs to the west are cut by numerous west- dipping faults and tilted as much as 40°. Nazas hawaiites are also along north-northwest−striking faults but are generally unfaulted.

Like other Miocene mafic alkalic volcanic rocks from the southern Basin and Range Province, those from Rodeo and Nazas are characterized by (1) moderately evolved hawaiitic compositions, (2) abundant megacrysts, including sodic plagioclase (An26–51), olivine (∼Fo55), Al-augite (7– 9 wt% Al2O3), and a wide variety of spinels, (3) lack of granulitic or peridotitic xenoliths, (4) variable Cs enrichments, and (5) isotopic compositions that indicate interaction with crust (87Sr/86Sri = 0.7037−0.7041; ϵNd = 4.8−2.8; 206Pb/204Pbi = 18.91−18.77; 207Pb/204Pbi = 15.57−15.60). These observations are consistent with a model whereby Miocene intraplate-type magmas rose slowly through the lithosphere, differentiating and interacting with the crust. The megacrysts are interpreted as disrupted gabbroic bodies, formed by slow cooling of mafic alkalic magmas that stagnated in the lower crust, possibly during the earlier extensional episode. Many of the elemental and isotopic parameters used to probe mantle sources of mafic volcanic rocks have been seriously obscured by crystallization, crustal interaction, and megacryst incorporation, complicating efforts to identify temporal changes in mantle source regions during development of the southern Basin and Range Province.

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