Tertiary alkaline magmatism in Trans-Pecos Texas

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Authors:Barker, Daniel S.
Author Affiliations:Primary:
Univ. Tex. at Austin, Austin, TX, United States
Volume Title:Alkaline igneous rocks
Volume Authors:Fitton, J. G., editor; Upton, B. G. J.
Source:Geological Society Special Publications, Vol.30, p.415-431; Alkaline igneous rocks, Edinburgh, United Kingdom, Sept. 1984, edited by J. G. Fitton and B. G. J. Upton. Publisher: Geological Society of London, London, United Kingdom. ISSN: 0305-8719
Publication Date:1987
Note:In English. 101 refs.; illus. incl. 2 tables, sketch maps
Summary:Alkaline magmatism in far W Texas extended from 48 to 16 m.y. ago, peaking in volume during the interval 37-26 m.y. when all the felsic rocks were emplaced. Primitive magmas are rare, if not entirely lacking, but ne- and ol + hy-normative hawaiite and mugearite liquids (Ni <120 ppm; 100 Mg/(Mg + Fe) <66) erupted through the entire time span. These evolved through the ne- normative and Q-normative trachytes to phonolite and rhyolite, including peralkaline varieties of both. Mafic Q-normative rocks are distinctly subordinate. Transition from a contractional to an extensional regime occurred between 32 and 30 m.y. ago, after which Trans-Pecos magmatism was almost exclusively mafic. Major- and trace-element compositions show gradation south-westward to the calc-alkaline rocks of the Sierra Madre Occidental in W Mexico, which were erupted 35-28 m.y. ago. This compositional gradient suggests that the alkaline and calc-alkaline rocks were all related to subduction of the Farallon plate. The Trans-Pecos province is not a continental rift, and probably is not a product of back-arc spreading. Peralkaline phonolites and rhyolites in Texas and calc-alkaline rhyolites in Mexico were erupted at the same time and probably in the same stress regime, and their compositional differences must reflect differences in source rocks and conditions of melting. (Authors' abstract) [J.M.H.]
Subjects:Alkalic composition; Cenozoic; Genesis; Geochemistry; Igneous rocks; Isotopes; Major elements; Metals; Rare earths; Tertiary; Trace elements; Brewster County Texas; Culberson County Texas; El Paso County Texas; Hudspeth County Texas; Jeff Davis County Texas; Pecos County Texas; Presidio County Texas; Reeves County Texas; Terrell County Texas; Texas; Trans-Pecos; United States; West Texas
Abstract Numbers:88M/2801
Record ID:1988021092
Copyright Information:GeoRef, Copyright 2019 American Geosciences Institute. Reference includes data from Mineralogical Abstracts, United Kingdom, Twickenham, United Kingdom
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