THE ARCHEAN WYOMING BATHOLITH: AN INTRUSIVE EQUIVALENT OF VERY LARGE VOLUME HIGH-SILICA IGNIMBRITES?

An enduring question in granite petrology is whether there are intrusive equivalents of large-volume, chemically-homogeneous, silicic ignimbrites. Typical batholiths contain rocks that vary from diorite to granite, which range in silica content from under 55% to over 70%. These rocks record magmatic processes such as differentiation, assimilation, and magma recharge. By contrast, a Neoarchean batholith that extends over >6,900 km²in central Wyoming is composed solely of homogeneous, true granite. This ~2.62 Ga granite is exposed throughout the Granite and Pedro Mountains where it is known as the Granite Mountains batholith. Granite of the same age and composition also occurs in the Shirley Mountains, Wind River Mountains (the Bears Ears pluton), and the Laramie Mountains (the Laramie batholith). Together these granites compose what we refer to as the Wyoming batholith.

This medium-grained equigranular biotite granite has high silica content of 74.2 ± 1.8% (1 SD). The batholith is uniformly calc-alkalic to alkali-calcic, slightly magnesian, and peraluminous. REE patterns are uniformly LREE-enriched with negative Eu anomalies and flat HREE. Initial epsilon Nd values are also quite restricted, with a mean of -3.1 ± 2.4 (1 SD). The homogeneity of the batholith is striking because Laramide and later faulting has surely exposed different structural levels across its extensive outcrop. The only textural variation observed is a banded structure to the rocks adjacent to the margins of the batholith. A contact aureole to the granite is not exposed, nor does the granite contain assemblages appropriate for geothermobarometry. Thus the depth of emplacement is very poorly constrained. The absence of miarolitic cavities or vugs suggests that the batholith was emplaced at greater than 1 kb depth.

One possible explanation for the observed homogeneity is that the Wyoming batholith was formed by extraction of granitic interstitial melt from a large crystal mush in much the same way that has been proposed for the origin of rhyolite magmas (i.e., Bachman and Bergantz, 2008). However, instead of ascending to the surface and being erupted, some or all of the extracted magma stalled to form a huge, silicic batholith within the upper crust.

Bachmann and Bergantz, 2008, J. Petrology 49, 2277-2285.