Chapter 12 - Archean Trondhjemites of the Southwestern Big Horn Mountains, Wyoming: A Preliminary Report

Abstract

The Archean terrane of the southwestern Big Horn Mountains includes two generations of trondhjemitic rocks. The older generation (E-1) consists of trondhjemitic-tonalitic gneiss that shows irregular banding, was twice metamorphosed to upper amphibolite facies, and contains 5-10 percent of irregularly dispersed pegmatite stringers and 1-5 percent of lenses of metabasalt. Geochronology of these rocks by the methods of U-Pb in zircon and whole-rock Rb-Sr gives respective ages of 2,972 ± 180 (2δ) and 3,007 ± 68 (2δ) m.y. (T. W. Stern, J. G. Arth and M. F. Newell, unpub. results, 1978). The younger generation (E-2) is found as a pluton of 45 km 2 area that consists of synkinematically intruded trondhjemite and minor tonalite and granodiorite of probable 2,800-m.y.-age (Arth, unpub. results, 1978). Major-element and minor-element contents of the E-2 pluton mostly are identical to those of the E-1 trondhjemitic-tonalitic gneiss: 14 samples range from 69.5-72.5 percent SiO 2 , 14.6-16.3 percent Al 2 O 3 , and 1.20-1.65 percent K 2 O (one sample showed 2.64 percent K 2 O). Six samples analyzed show 45-55 ppm Rb and 299-527 ppm Sr. Thus the trondhjemitic rocks of both E-1 and E-2 events are of the common high-Al 2 O 3 type (Al 2 O 3 ± 15 percent). The trondhjemitic rocks of both events also are markedly depleted in heavy REE's (rare-earth elements): La ranges from 27-97 times chondrites and Lu 1-3.3 times. The E-2 trondhjemite, however, is slightly enriched in light REE's relative to the E-1 type. Amphibolite lenses in the E-1 gneiss have the compositions of tholeiitic basalts and basaltic andesite. Thin dikes of hornblende-biotite gneiss of late E-l age show major-element composition of andesite, but are anomalously high in light REE's (La = 249-485 times chondrites).