Abstract

The timing and spatial extent of mid-Cenozoic ignimbrite flare-up volcanism of the Sierra Madre Occidental silicic large igneous province of Mexico in relation to crustal extension is relatively unknown. Extension in the Sierra Madre Occidental has been variably interpreted to have preceded, postdated, or begun during Early Oligocene flare-up volcanism of the silicic large igneous province. New geologic mapping, zircon U-Pb laser ablation–inductively coupled plasma–mass spectrometry dating, modal analysis, and geochemical data from the Guazapares Mining District region along the western edge of the northern Sierra Madre Occidental silicic large igneous province have identified three informal synextensional formations. The ca. 27.5 Ma Parajes formation is an ∼1-km-thick succession composed primarily of welded to nonwelded silicic outflow ignimbrite sheets erupted from distant sources. The 27–24.5 Ma Témoris formation is interpreted as an andesitic volcanic center composed of locally erupted mafic to intermediate composition lavas and associated intrusions, with interbedded andesite-clast fluvial and debris flow deposits, and an upper section of thin distal silicic outflow ignimbrites. The 24.5–23 Ma Sierra Guazapares formation is composed of silicic vent facies ignimbrites to proximal ignimbrites, lavas, plugs, dome-collapse deposits, and fluvially or debris flow–reworked equivalents. These three formations record (1) the accumulation of outflow ignimbrite sheets, presumably erupted from calderas mapped ∼50–100 km east of the study area that were active during the Early Oligocene pulse of the mid-Cenozoic ignimbrite flare-up; (2) development of an andesitic volcanic field in the study area, likely related to rocks of the Southern Cordillera basaltic andesite province that were intermittently erupted across all of the northern Sierra Madre Occidental toward the end of and following the Early Oligocene ignimbrite pulse; and (3) the initiation of explosive and effusive silicic fissure magmatism in the study area during the Early Miocene pulse of the mid-Cenozoic ignimbrite flare-up.

The main geologic structures identified in the Guazapares Mining District region are NNW–trending normal faults, with an estimated minimum of 20% total horizontal extension. Normal faults were active during deposition of all three formations (Parajes, Témoris, and Sierra Guazapares), and bound half-graben basins that show evidence of synvolcanic extension (e.g., growth strata) during deposition. Normal faulting began by ca. 27.5 Ma during deposition of the youngest ignimbrites of the Parajes formation, concurrent with the end of the Early Oligocene silicic ignimbrite pulse to the east and before magmatism began in the study area. In addition, preexisting normal faults localized andesitic volcanic vents of the Témoris formation and silicic vents of the Sierra Guazapares formation, and some faults were reactivated during, as well as after, deposition of these formations.

We interpret extensional faulting and magmatism in the Guazapares Mining District region to be part of a regional-scale Middle Eocene to Early Miocene southwestward migration of active volcanism and crustal extension in the northern Sierra Madre Occidental. We show that extension accompanied silicic volcanism in the Guazapares region, and overlapped with the peak of mid-Cenozoic ignimbrite flare-up in the Sierra Madre Occidental; this supports the interpretation that there is a relationship between lithospheric extension and silicic large igneous province magmatism.

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