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

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.