U-Pb zircon ages from crustal xenoliths in a Pliocene basalt flow from the southern Rio Grande rift: Implications for the timing of extension and magmatism
Jeffrey M. Amato, Carmen Athens, William C. McIntosh, and Lisa Peters

Abstract:

Metaigneous xenoliths with granulite facies assemblages from the Pliocene Elephant Butte volcanic field basalts flows were derived from the lower crust. Protolith compositions are inferred to be diorite and granite based on mineralogy and color index. The metadiorite has orthopyroxene (opx) compositions of En68–En55 and plagioclase compositions of An48–An45, whereas the metagranite has opx compositions of En55–En30 and plagioclase compositions of An40–An35. Augite is also more magnesian in the metadiorite relative to the metagranite. A 40Ar/39Ar date on the host basalt is 2.71 ± 0.06 Ma. Cathodoluminescence imaging indicates that xenolith zircons have both oscillatory zoned cores as well as altered cores and low-U rims. The core dates are complicated by high common Pb and discordance, but the best core ages from the four samples dated are 1664 ± 14 Ma, 1452 ± 32 Ma, ~1450 Ma, and 1432 ± 14 Ma. The core age of 1664 Ma is similar to other dates from Mazatzal province basement, and the ~1450 Ma dates from the remaining samples is similar to other dates from surface exposures of granites that are part of the ~1.4 Ga granite-rhyolite province. These dates indicate that ~1.4 Ga granites are found at all depths of the crust indicating a significant volume of ~1.4 Ga granite in the crust. The rim dates for two samples are similar at 9.8 ± 1.7 Ma and 11.0 ± 2.2 Ma. A combined age using three samples is 10.4 ± 1.1 Ma. This age is coeval with the Selden basalt flow >60 km to the south and coincides with the onset of extension and magmatism following a 8 m.y. hiatus. The lack of surface flows at 10 Ma at Elephant Butte suggests that mantle derived magmas provided heat to the lower crust, possibly by underplating, but that extension was not yet significant enough to allow magmatism to occur. Volcanism at 2.7 Ma at Elephant Butte brought up the xenoliths and was facilitated by extension in the transfer zone between two offset north-striking and west-dipping normal faults. The Miocene zircon rim ages in Proterozoic xenoliths provide indirect evidence for the onset of extension-related heating of the lower crust in the absence of surface eruptions.


Citation:

  1. Amato, Jeffrey M.; Athens, Carmen; McIntosh, William C.; Peters, Lisa, 2012, U-Pb zircon ages from crustal xenoliths in a Pliocene basalt flow from the southern Rio Grande rift: Implications for the timing of extension and magmatism, in: Geology of the Warm Springs region, Lucas, Spencer G.; McLemore, Virginia T.; Lueth, Virgil W.; Spielmann, Justin A.; Krainer, Karl, New Mexico Geological Society, Guidebook, 63rd Field Conference, pp. 273-284.

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