TWO NEOARCHEAN LEUCOGRANITE SUITES FROM THE TETON RANGE, WYOMING: EVIDENCE FOR COLLISIONAL PLATE TECTONIC PROCESSES

Continent-continent collisional orogens are a hallmark of modern plate tectonics; their apparent absence throughout much of the Archean geologic record has been offered as evidence that Archean crust was too weak to form rigid plates and support mountain belts. Here we describe two suites of 2.68 Ga leucogranites from the Teton Range, Wyoming, USA that have compositions best explained by partial melting processes that accompany modern-style collisional plate tectonic processes.

The northern Teton Range exposes granulite-facies pelitic gneiss and ultramafic rocks that reached P >10 kb and T > 800°C at 2695 Ma. These rocks are in contact with qz-pl-bi gneisses that experienced P < 6 kb and T ~700-750C. These relations suggest that the pelitic rocks were overthrust and buried to depths of 36 km or more, then underwent cooling at high pressure before decompression during tectonic assembly with the qz-pl-bi gneisses. Both of these units were intruded by leucogranitic gneisses during extension immediately following tectonic assembly.

Two leucogranites, Webb Canyon gneiss and Bitch Creek gneiss, intruded immediately following tectonic assembly, from 2675-2685 Ma. They are indistinguishable in age and initial epsilon-Nd. Both are peraluminous and calcic. Both contain biotite, but hornblende is not present in the Bitch Creek gneiss. There are additional important differences: Bitch Creek gneiss is high-Al (15-18% Al₂O₃), magnesian, high Sr, low Y and Zr and its REE pattern is LREE-enriched with no Eu anomaly. Webb Canyon gneiss is ferroan, low-Al (11-13% Al₂O₃), low Sr, high Y and Zr, and its REE pattern is gull-winged with deep Eu anomalies.

The geochemistry of the Bitch Creek gneiss is consistent with water-excess melting at pressures sufficiently high to leave garnet in the restite. By contrast, the Webb Canyon gneiss magmas most likely formed by dehydration melting at higher temperatures but lower pressures. These two different partial melts could form from the same source rocks and at the same time during a collisional event. Hot-over-cold thrusting released water from the lower plate. Migration of this water produced water-excess melting at the base of the over-riding plate to form the Bitch Creek gneiss. Coeval extension in the upper portion of the plate caused decompression melting, forming the Webb Canyon gneiss.