A-type,'' flood rhyolites of Trans-Pecos Texas: A major crustal melting event at 36. 8 Ma

Conference · Thu Apr 01 00:00:00 EST 1993 · Geological Society of America, Abstracts with Programs; (United States)

OSTI ID:5098389

Henry, C D; James, E W ^[1]

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1. Univ. of Texas, Austin, TX (United States). Bureau of Economic Geology

The Bracks Rhyolite, Star Mountain Formation, and Crossen Trachyte are related, voluminous, high-temperature ([ge]900 C) silicic lavas that constitute an abrupt, major eruptive event in Trans-Pecos Texas. They were emplaced rapidly at 36.8 Ma, at the onset of the main phase of subduction-related volcanism and immediately preceding a major basalt outpouring. Individual flows are as much as 55 km long, cover 1,000 km[sup 2], and have volumes of as much as 75 km[sup 3]. Cumulatively, they cover 10,000[sup 2] km and comprise 1,000 km[sup 3]. Despite areal extents comparable to those of ash-flow tuffs, outcrop and petrographic features clearly demonstrate that these rocks were emplaced as lavas. All the silicic lavas are mildly peralkaline quartz trachytes to low-SiO[sub 2] rhyolites (67%--72% SiO[sub 2]). They have mineralogic and chemical characteristics of A-type'' granites, including Fe-rich pyroxene, fayalite, and sodic amphibole; high Na[sub 2]O + K[sub 2]O, Ga/Al, Zr, Nb, and Y; and low CaO, MgO, and Sr. Most elements do not correlate with SiO[sub 2], indicating that they are not differentiation suites. Individual flows are strikingly homogeneous. Pb isotopic compositions show a narrow range that indicate either a homogeneous source with Th/U [approx] 6 or thorough mixing. These characteristics are consistent with magma generation by high degrees of partial melting of depleted, anhydrous crust. Trace-element models support crustal melting over AFC processes. Heat was supplied by major infusion of basalt that coincided with a regional flare-up in magmatism in the southern Cordillera. Thus, the lavas indicate both introduction of new crust and recycling of old crust.

• OSTI ID: 5098389
• Report Number(s): CONF-9305259-; CODEN: GAAPBC
• Journal Information: Geological Society of America, Abstracts with Programs; (United States), Vol. 25:5; Conference: 89. annual meeting of the Cordilleran Section and the 46th annual meeting of the Rocky Mountain Section of the Geological Society of America (GSA), Reno, NV (United States), 19-21 May 1993; ISSN 0016-7592
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
GEOLOGIC FORMATIONS
GEOCHEMISTRY
PETROGENESIS
PETROGRAPHY
RHYOLITES
TEXAS
CHEMICAL COMPOSITION
GEOLOGIC DEPOSITS
ISOTOPE RATIO
MINERALOGY
PLATE TECTONICS
SUBDUCTION ZONES
VOLCANISM
CHEMISTRY
DEVELOPED COUNTRIES
GEOLOGY
IGNEOUS ROCKS
NORTH AMERICA
PETROLOGY
ROCKS
TECTONICS
USA
VOLCANIC ROCKS
580000* - Geosciences