Abstract
A zircon separate from a syenite from the Florida Mountains, near Deming, southwest New Mexico, yields a 514 ± 3-Ma concordia upper-intercept age. However, RbSr isochron dates from syenite and other igneous core rocks in the Florida Mountains range from ∼ 519 to ∼ 381 Ma. The RbSr dates are based on scattered linear arrays suggestive of open-system conditions for the rocks. Calculated initial 87 Sr 86 Sr ratios are highly variable for presumably coeval rocks, and some are anomalous values below 0.699. KAr dates from these igneous core rocks range from 555 to 418 Ma, while an andesite dike, possibly associated with Tertiary mineralization in the area, yields a date of 2.7.9 Ma. The zircon UPb concordia age can be reconciled with the available stratigraphic controls that require a pre-Middle-Ordovician age for some granite and syenite core-rock exposures. Collectively, the geochronologic data argue for Early Paleozoic plutonism at ∼ 510–550x Ma, with a resetting event affecting the RbSr and KAr systematics in the syenite and alkali granite in the Tertiary. The resetting mechanism may have involved Sr loss, possibly through ion-exchange reactions between feldspar and moderately heated fluids, as well as loss of Ar and/or addition of K in the syenites, alteration of the Th/U ratio in the syenite and granite, and production of brick-red K-feldspars due to the incorporation of minute hematitic inclusions. δ 18O-values for several whole-rock samples range between + 5.8 and + 9.2‰, consistent with a high water/rock ratio and alteration temperatures of ⪅ 300°C. World-wide, red-rock granophyre terranes commonly yield discordant RbSr and UPb dates, with RbSr dates scattered and systematically younger than UPb ages. We suggest that red-rock granophyres are inherently undatable by the RbSr technique because of Sr loss from K-feldspars during penetration of Fe-oxidizing solutions.
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