T227 A Matter of Perspective: Integrating Macro- and Micro-Scale Evidence in Investigations of Magmatic Origins and Histories (Posters)

Tuesday, 3 November 2015
9:00 AM - 6:30 PM
Exhibit Hall

TRACE ELEMENT VARIATION IN PLAGIOCLASE PHENOCRYSTS FROM IGNIMBRITES FROM THE MOGOLLON-DATIL VOLCANIC FIELD, SOUTHERN NEW MEXICO, USA: INSIGHTS INTO THE PROCESSES AND TIMESCALES OF MAGMATISM

Paper number: T227-7
Program: Technical Sessions
Time: 9:00 AM – 6:30 PM

Authors

Abstract

The Mogollon-Datil volcanic field in southern New Mexico represents the last volcanism associated with continental arc magmatism in the southwest United States. In the 13 m.y. history of the volcanic field as few as 15 calderas erupted homogeneous andesite to rhyolite ignimbrites and lava flows between 38 Ma and 25 Ma. Here we present bulk-rock major- and trace- element compositions, Sr isotopic ratios and single crystal major- and trace- element data from phenocrysts for two volcanic sequences near the Sierra Cuchillo Mountains.

The Dacite-Rhyolite sequence (36.5 Ma) is a crystal-rich, high-K, calc-alkaline rhyolite consisting of six tuffs and tuff-breccias. The basal member is a densely welded, tuff-breccia and the remaining members are vitric-crystal tuffs. Mineralogically, this sequence contains plagioclase>quartz>biotite>>hornblende and trace orthopyroxene. The youngest member also contains abundant sanidine.

The Latite-Andesite sequence (36.2 Ma) is a crystal-rich, high-K, calc-alkaline suite of four volcanic units. The basal member consists of a crystal-rich, trachy-andesite lava. The remaining members of the sequence contain blocks of the underlying lava and lithics from the Paleozoic sedimentary rocks. Mineralogically, the tuff-breccia’s contains plagioclase>alkali feldspar>quartz> biotite>hornblende>orthopyroxene. The lava flow contains plagioclase>alkali feldspar> hornblende>biotite>>quartz.

Geochemically, the plagioclase phenocryst populations of the two volcanic sequences are distinct. Within the six analysis from the dacite-rhyolite sequence there is no correlation between crystal population and whole-rock magma chemistry. However, within the latite-andesite sequence there are three distinct plagioclase populations; low An(0-10), An(50-60), and one population of high An(95-100) concentration. These data suggest a complex magma reservoir erupting the latite-andesite sequence.