Steady State to Flare-Up ARC Magmatism in the Largest Cenozoic Silicic Igneous Province on Earth: The Sierra Madre Occidental (MEXICO)

University Of California-Santa Barbara, Santa Barbara CA

Investigators

Abstract

Steady state to flare-up ARC Magmatism in the largest Cenozoic silicic iIgneous province on Earth: The Sierra Madre Occidental (MEXICO) (project jointly supported by EAR/Petrology & Geochemistry and OISE/Americas) The Sierra Madre Occidental (SMO) of western Mexico is the least deformed silicic large igneous province (LIP) on Earth, as well as the largest Cenozoic one. The SMO volcanic rocks also host one of the largest epithermal precious-metal deposits in the world. Yet less than 10% of it has been mapped or dated, leaving a big hole in our knowledge about silicic LIPs in general, and the evolution of the Basin and Range province in particular. The SMO may be the best field area on Earth for studying the origin of silicic LIPs, because it is very well exposed at a variety of structural levels, due to syn- to post-volcanic extension, followed by deep erosional dissection. The PI and her collaborators have discovered an extensive, previously unmapped and undated area of andesite arc rocks ("Lower Volcanic Complex", LVC) beneath the silicic LIP rocks (Upper Volcanic Complex", UVC). The LVC may represent an essential precursor to the ignimbrite flareup, and provides a record of the strain regime and conditions extant at the beginning of the flareup (UVC). New field and geochronological data are presented to show that extension began immediately before the flareup, rather than during or after it as proposed by previous workers. The extensional structures that formed in the precursor andesite arc were partially reactivated during the flareup, and at least in part appear to have controlled the distributions of fissure-like silicic intrusions and vents during the flareup. The UVC appears to contain some unusual vent and near-vent features that may be related to very high peak magma fluxes, such as surge-like ignimbrites, with large-scale cross-bedded lithic lag breccias, as well as more widespread distinctive deposits, such as mega-surge deposits. The proposed work will address the following questions: What is the nature and timing of volcanological and structural events that accompany the transition from a "normal", steady-state arc system to a "flare-up" arc system? What is the strain regime present at the start of flareup, and is precursory extension essential? How do structures that developed during the steady-state arc phase (LVC) correlate with, and possibly control, the location, size, and geometry of vents and intrusions in the UVC? How do eruptive processes, volcanic compositions, areal extent, duration of activity, and magmatic production rates in this very large supervolcano field compare with with other, smaller but much better-studied supervolcano fields? What are the volcanic and structural settings of epithermal precious metal deposits in the proposed field area? Due to the great size and complexity of the area, as well as the frontier nature of the field research, partial support is requested for a postdoctoral researcher and for two PhD students. Fieldwork will include construction of geologic maps that cover 8,000 km2, at a variety of scales, with measured sections and cross sections. Modal and textural analysis will be done to select samples suitable for dating and geochemistry. LA-ICPMS zircon geochronology (for silicic volcanic rocks) will be done by Luigia Solari and Luca Ferrari at UNAM, and Ar/Ar geochronology (on mafic- to intermediate volcanic rocks, and for correlation of widespread unaltered ignimbrites) will be done by Paul Renne at the Berkeley Geochronology Center. Pumice samples (on ignimbrites) and whole rock samples (on other rock types) will be collected for reconnaisance geochemical studies. Intellectual merit - This work will contribute significantly to: (1) understanding the age, stratigraphy and structure of the SMO; (2) understanding the formation of silicic LIPs, as well as the relationships between extensional tectonics and magmatism; and (3) understanding the relationship between LIPs and mineralization, in a region famous for its mineral deposits. Broader impact - The work will strengthen collaborations between the University of California and the National Autonomous University of Mexico, including one female professor at each insitute. The PI has aggressively recruited female students, and one of the PhD students will likely be female (TBD). The research will also benefit the precious metal exploration industry in Mexico.