2014 GSA Annual Meeting in Vancouver, British Columbia (19–22 October 2014)

Paper No. 110-6
Presentation Time: 9:30 AM

CRUSTAL GROWTH THROUGH PROTRACTED MAGMATISM FROM 3.5-3.2 GA IN THE SPANISH PEAKS, NORTHERN MADISON RANGE, MONTANA


MOGK, D.W., Dept. Earth Sciences, Montana State Univ, Bozeman, MT 59717, MUELLER, Paul A., Department of Geological Sciences, University of Florida, 241 Williamson Hall, Gainesville, FL 32611-2120, WOODEN, J.L., U.S Geological Survey, Menlo Park, CA 94025 and BOWES, D.R., Department of Geographical and Earth Sciences, University of Glasgow, Glasgow, G12 8QQ, Scotland

Quartzofeldspathic gneisses occur in a series of tabular, sill-like bodies in the Spanish Peaks, Northern Madison Range, Montana. The main rock units (and age, U-Pb zircon SHRIMP) include: porphyritic hornblende granodiorite (3.53 Ga), biotite trondhjemite (3.35, 3.27, 2.56 Ga), hornblende monzodiorite (3.21Ga), leucogranite (3.22 Ga), biotite tonalite (3.27, 3.29, 3.30 Ga). A mylonitic biotite granite (2.77 Ga) has also been intruded in the Big Brother Shear Zone. Zircons in the hornblende granodiorite unit also have developed rims on zircons that record a 1.73 Ga age of metamorphism. Whole rock geochemistry of these units generally demonstrate a calc-alkaline affinity, high concentration of LILE’s (Ba, Sr), HFSE depletion, and depletion of HREEs characteristic of igneous rocks formed deep in a modern continental arc environment. These units were intruded into the middle crust in lit-par-lit fashion as a series of discrete magmas that are not genetically related (i.e., deriving from different sources) over a time interval of ~ 3.5-3.2 Ga; this corresponds to the peak of detrital zircon ages determined for quartzites from across the northern Wyoming Province. Conspicuously absent are ages of ~2.8 Ga, the age of the major arc-related crust forming event that occurred in the Beartooth-Bighorn Magmatic Terrane to the east. The suite of 3.5-3.2 Ga rocks, in aggregate, are similar to those exposed as pendants in the eastern Beartooth Mountains. Protracted generation of silicic magmas semi-continuously over a time interval of 300 Ma (3.5-3.2 Ga) reflect melting of a mafic source, in some cases leaving a garnet-bearing residuum. The arc-signature evident in trace element abundances (e.g., elevated LILs and low HREE) suggest short episodes of subduction, which apparently stimulated melting in thick mafic crust Sm-Nd model ages suggest this mafic crust formed several hundred million years prior to the oldest part of the rock record in northern Madison Range. Crustal evolution then proceeded as subductio-driven magmatism became the dominant crust-forming mechanism in the northern Wyoming Province.