Incompletely reset RbSr systems from a Cambrian red-rock granophyre terrane, Florida Mountains, New Mexico, U.S.A.

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, RbSr isochron dates from syenite and other igneous core rocks in the Florida Mountains range from ∼ 519 to ∼ 381 Ma. The RbSr 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. KAr 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 UPb 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 RbSr and KAr 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 RbSr and UPb dates, with RbSr dates scattered and systematically younger than UPb ages. We suggest that red-rock granophyres are inherently undatable by the RbSr technique because of Sr loss from K-feldspars during penetration of Fe-oxidizing solutions.