THE PETROGENESIS OF THE 2.62 GA GRANITE MOUNTAINS BATHOLITH, CENTRAL WYOMING: IMPLICATIONS FOR NEOARCHEAN CRUSTAL GROWTH

The Neoarchean is thought to have been an important period of crustal growth. In the Wyoming Province, the 2.63 Ga, >1600 km² Louis Lake batholith exposed in the Wind River Mountains preserves evidence of the addition of mantle-derived magmas to the continents. The batholith is composed of a suite of calc-alkalic, metaluminous to peraluminous, diorites, granodiorites and granites, and has been interpreted as a continental arc magmatic complex. Initial Nd isotopic compositions vary from -2 to +3, recording the incorporation of different proportions of depleted mantle and crustal sources.

A second large Neoarchean batholith, the 2.62 Ga, >2000km², Granite Mountains batholith of central Wyoming, differs from the Louis Lake batholith: it consists entirely of granite that lacks amphibole, contains 1-5% biotite, trace muscovite, and accessory magnetite, epidote, sphene, apatite, and zircon. Geochemically it is peraluminous and calc-alkalic. The batholith is undeformed and mineralogically homogeneous over tens of kilometers of exposure, despite Laramide age basement-cored uplifts exposing varying depths of the batholith. Contact relations with surrounding wall rock are not preserved and the batholith contains few xenoliths and no mafic enclaves.

The Granite Mountains batholith was emplaced along a boundary between the Paleo- and Mesoarchean Sacawee Block and Neoarchean accreted terranes. At 2.62 Ga, the Sacawee block crust had εNd values averaging -8. εNd data from the Granite Mountains batholith ranges from -2 to +1, indicating that the Sacawee block was not a major magma source.

Imperfect arguments can be made for three different tectonic settings for the Granite Mountains batholith. The lack of compositional variation and the abundance of peraluminous leucogranite could suggest an origin by crustal melting in a continent-continent collisional tectonic setting. The large extent of the batholith and εNd values intermediate between preexisting crust and depleted mantle are consistent with a continental arc origin. Some post-collisional granitoid suites include early granites that resemble the Granite Mountains granite. Further investigation will help establish the most likely tectonic setting and amount of crustal addition represented by the batholith.