JUXTAPOSITION OF WINDERMERE AND BELT STRATA, BIG CREEK, IDAHO

Windermere and Belt rocks, variably intruded by Idaho batholith, Tertiary plutons, and a Tertiary dike swarm, are juxtaposed across newly recognized faults in the Big Creek area, central Idaho. A central block consists of a well-preserved amphibolite-grade homoclinal Windermere section (Lund and others, 2003) having no exposed base and with upper parts intruded in simple cross-cutting fashion by biotite-muscovite granite of the Idaho batholith. Tertiary dikes are rare and Tertiary shearing is nearly absent.

The central block is juxtaposed on the Fawn Meadow fault against a western block of distinctive micaceous quartzite and migmatized quartzofeldspathic gneiss. The western block is devoid of recognizable Windermere strata and is presumably of Belt age. It hosts not only intimate lit-par-lit anatectic leucogranite, but also the N-S Big Creek shear zone, the N-S Tertiary dike swarm, and most of the mineralization of the Big Creek mineral belt. Mineralization post-dates anatexis, but pre-dates shearing, dike emplacement and the pronounced left-lateral offset on the Fawn Meadow fault.

Continuous northerly to northwesterly foliation through anatectic granite and gneiss in the western block indicates the granite and gneiss were deformed simultaneously. Two new U-Pb zircon ages on the leucogranite (75.3 ± 0.4 and <85 Ma) demonstrate that the anatexis is of Idaho batholith age, and suggest that the western block once resided in the root zone of the Idaho batholith.

Belt strata and a segment of lowermost Windermere strata lie east of the central block across the Hogback fault. Although relations are complicated by thrusting, the thick lowermost quartzite unit of the Windermere section may lie unconformably on the Belt rocks.

The Windermere strata represent a basin-filling sequence comprised of clean cross-bedded quartzite in the lowermost part of the section, carbonate, conglomerate (diamictite), siltstone and bi-modal volcanics in the central portion, and increasingly pelitic rocks in upper parts. Regionally, the Windermere strata may be distributed in a synclinorium between the Big Creek and Stibnite areas.