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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, México

James F. Luhr^*,1, Christopher D. Henry^,2, Todd B. Housh^,3, J. Jorge Aranda-Gómez^#,4 and William C. McIntosh^**,5

¹ Department of Mineral Sciences, NHB-119, Smithsonian Institution, Washington, DC 20560, USA
² Nevada Bureau of Mines and Geology, University of Nevada, Reno, Nevada 89557, USA
³ Department of Geological Sciences, University of Texas, Austin, Texas 78712, USA
⁴ Unidad de Investigación en Ciencias de la Tierra, Instituto de Geología, Universidad Nacional Autonoma de México, Campus Juriquilla, A.P. 1-742, Querétaro, QRO 76001, Mexico
⁵ New Mexico Bureau of Mines and Mineral Resources, New Mexico Institute of Mining and Technology, P.O. Box 3398 Campus Station, Socorro, New Mexico 87801, USA

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 (An_26–51), 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/⁸⁶Sr_i = 0.7037–0.7041; _Nd = 4.8–2.8; ²⁰⁶Pb/²⁰⁴Pb_i = 18.91–18.77; ²⁰⁷Pb/²⁰⁴Pb_i = 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.

Key Words: Basin and Range province • contamination (magma) • Durango Mexico • extension faults • hawaiite • Miocene

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