Early extension and associated mafic alkalic volcanism from the southern Basin and Range Province: Geology and petrology of the Rodeo and Nazas volcanic fields, Durango, Mexico

East-northeast extension ca, 24 Ma at Rodeo and Nazas, Mexico, was accompan ied by eruption of hawaiites, marking some of the earliest intraplate-type mafic alkalic volcanism associated with development of the southern Basin a nd Range Province. An earlier extensional pulse, 32.3-30.6 Ma, concurrent w ith subduction-related rhyolitic volcanism of the Sierra Madre Occidental, is the earliest established extension in the southern Basin and Range Provi nce. The Rodeo hawaiites are mostly along or just west of the north-northwe st-striking, west-dipping Rodeo fault, a major breakaway fault that separat es moderately extended terrane to the west from less extended terrane to th e east. Hawaiites and interbedded conglomerates in the Rodeo half graben ar e 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 faul ts and tilted as much as 40 degrees, Nazas hawaiites are also along north-n orthwest-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 (I) modera tely evolved hawaiitic compositions, (2) abundant megacrysts, including sod ic plagioclase (An(26-51)), olivine (similar to Fo(55)), Al-augite (7-9 wt% Al2O3), and a wide variety of spinels, (3) lack of granulitic or peridotit ic xenoliths, (4) variable Cs enrichments, and (5) isotopic compositions th at indicate interaction with crust (Sr-87/Sr-86(i) = 0.7037-0 7041; epsilon (Nd) = 4.8-2.8; Pb-206/Pb-204(i) = 18.91-18.77; Pb-207/Pb-204(i) = 15.57-1 5.60), These observations are consistent with a model whereby Miocene intra plate-type magmas rose slowly through the lithosphere, differentiating and interacting with the crust, The megacrysts are interpreted as disrupted gab broic 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 ma fic 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 southe rn Basin and Range Province.