277 Recent Advances in Mineralogy and Petrology

Wednesday, 3 November 2010
1:30 PM - 5:30 PM
Mile High Ballroom 4AB

MAJOR AND TRACE ELEMENT GEOCHEMISTRY AND 87SR/86SR AND εNd ISOTOPIC COMPOSITIONS OF VOLCANIC CLASTS FROM THE SHINARUMP MEMBER, UPPER TRIASSIC CHINLE FORMATION

Paper number: 277-2
Program: Technical Sessions
Time: 1:45 PM – 2:00 PM

Authors

Abstract

Major and trace element geochemistry coupled with isotopic data may help resolve the long standing enigma of the origin of volcanic clasts found in the Upper Triassic Chinle Formation. This study is the first to document the major and trace element geochemistry as well as the 87Sr/86Sr and εNd isotopic compositions of volcanic clasts from the basal Shinarump Member of the Chinle Formation. Clasts are cobble-sized, silicified, porphyritic, rhyolitic to dacitic tuffs and lavas. Most clasts sampled from Holbrook and Cameron, Arizona, exhibit abnormally high SiO2 (84.46-95.40 wt. %), highly depleted Na2O (< 0.02 wt. %), moderately depleted K2O (< 2.0 wt. %) and depleted CaO (< 1.0 wt %.) attributed to varying degrees of silicification. Three samples collected from the Cameron section retain normal silica values (~ 76.0 wt. %) along with an enrichment in Na2O (~ 6.0 wt. %) and depletion in K2O (< 1.0 wt. %). It is possible that these samples have undergone less severe alkali alteration, providing a window into the other clasts’ original geochemistry.

Refractory trace element abundances and isotopic data may offer insight into the clasts’ igneous provenance. Y/Nb, Y/Zr, Ti/Y vs. Zr/Y, and Nb/Y vs. Zr/Y form coherent trends suggesting limited diffusion during alkali exchange. εNd values of volcanic clasts from the Shinarump Member range from -6 to -11, which correlate well with three plutonic suites in the Mojave Desert; the San Bernardino (εNd = -7 to -10), the San Gabriel (εNd = -2 to -7), and the Granite Mountain (εNd = -6 to -8). These igneous provinces were chosen for comparison because preliminary data of volcanic clasts from the Sonsela Member of the Chinle Formation display geochemical, isotopic, and geochronologic affinities to these plutonic suites. Initial 87Sr/86Sr ratios of the volcanic clasts are highly variable ranging from 0.68754 to 0.71751, highlighting the mobility of Sr and Rb during silicification greatly diminishing their effectiveness as source tracers. While the origin of the clasts still remains uncertain, the development of an in-depth geochemical and isotopic profile will help in locating their source by allowing for more confident correlations to made between the volcanic clasts and suspected plutonic sources.