Geochemistry and mineralogy of the Eocene-Ogligocene volcanic sequence, Southern Sierra Madre Occidental, Juchipila, Zacatecas, Mexico

Abstract

The Sierra Madre Occidental (SMO) volcanic province of western Mexico is a remnant of a broad continental-margin orogenic belt formed as a result of mid-Tertiary subduction of the Farallon plate beneath Mexico. The most voluminous rocs exposed in the SMO are rhyolitic ash-flow tuffs (AFT) erupted throughout most of the providence from 36 to 27 Ma. Two other magmatic episodes recorded in the SMO include an Eocene event (53-40 Ma) characterized by mafic-intermediate composition lava flows, and a post-ignimbrite event (29-20 Ma) characterized by the eruption of extension-related basaltic-andesites. This paper presents new data for an area in the southem SMO wich has evidence for both Eocene and Oligocene volcanism. Te stratigraphic sequence established for the area near Juchipila, Zacatecas, consists of Eocene basaltic andesite to andesite lava flows wich are uncomfortably overlain by Oligocene basalt to basaltic lava flows and/or Oligocene rhyolitic ash-flow tuffs. An upper sequence of basalt to andesite lava flows caps the section. The high-K calc-alkaline Oligocene sequence is composed predominantly of rhyolitic AFT (~25 Ma) wich are younger than most other reported SMO occurrences. This suggest a younging to the south of SMO volcanism. Although Juchipila AFT are phenocryst poor, one widespread AFT unit is characterized by alkali feldspar with unusually Ba-rich cores (up to 14% celsian component). This is the first reported occurrence for this composition feldspar in the SMO.