Tectonic significance of mantle sources for mafic synextensional dikes in metamorphic core complexes, southern Arizona and northern Sonora

Conference · Wed Jan 01 00:00:00 EST 1992 · Geological Society of America, Abstracts with Programs; (United States)

OSTI ID:5975444

Wright, J E ^[1]; Haxel, G B ^[2]; Simmons, A M

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1. Rice Univ., Houston, TX (United States)
2. Geological Survey, Flagstaff, AZ (United States)

Synextensional microdiorite and lamprophyre dikes in the metamorphic core complexes (MCCs) of southern Arizona represent magmatism unambiguously associated with middle Tertiary crustal extension. These 24--20 Ma dikes are shoshonitic and primitive. Petrographic and geochemical data and AFC modeling suggest absence of an essential crustal source component. Dikes from individual MCCs have closely similar sr and Pb isotopic compositions that do not correlate with degree of fractionation, indicating little or no crustal assimilation. Minimal crustal interaction is consistent with emplacement of the dikes in a strongly extensional tectonic regime. Samples from the two MCC belts differ isotopically and chemically. Isotopic and trace element data indicate a minimum of three mantle sources, two lithospheric and one asthenospheric. The predominant lithospheric source for dikes in the northern belt was LILE-depleted Proterozoic lithosphere (EM1-like), whereas in the southern belt it was LILE-enriched lithosphere (trending toward EM2) strongly influenced by Proterozoic to Tertiary subduction. The asthenospheric source was available under both MCC belts but more widely or frequently tapped beneath the more highly extended northern belt. The presence of an asthenospheric component in some of the MCC dikes is the only known exception to the observation that asthenosphere-derived Cenozoic basalts first appeared in Arizona ca Ma. By the later stages of middle Tertiary crustal extension, the subduction zone beneath Arizona evidently had fragmented or became inactive, so that it did not form a barrier to ascent of asthenosphere-derived magmas. Continued study will evaluate the belief that these data support tectonic models for middle tertiary crustal extension in which the asthenospheric mantle is active rather than passive.

• OSTI ID: 5975444
• Report Number(s): CONF-921058-; CODEN: GAAPBC
• Journal Information: Geological Society of America, Abstracts with Programs; (United States), Vol. 24:7; Conference: 1992 annual meeting of the Geological Society of America (GSA), Cincinnati, OH (United States), 26-29 Oct 1992; ISSN 0016-7592
• Country of Publication: United States
• Language: English

Related Subjects

58 GEOSCIENCES
ARIZONA
GEOLOGIC FORMATIONS
MAGMATISM
CONTINENTAL CRUST
DIKES
EARTH MANTLE
GEOCHEMISTRY
GEOLOGIC MODELS
MAGMA
ORIGIN
PETROGRAPHY
SUBDUCTION ZONES
TECTONICS
TERTIARY PERIOD
CENOZOIC ERA
CHEMISTRY
DEVELOPED COUNTRIES
EARTH CRUST
GEOLOGIC AGES
GEOLOGIC STRUCTURES
GEOLOGY
NORTH AMERICA
USA
580000* - Geosciences