NEOARCHEAN STRONGLY PERALUMINOUS GRANITES OF THE WYOMING PROVINCE RECORD MODERN CRUSTAL COMPOSITIONS AND MELT-FORMING PROCESSES

Strongly peraluminous granites (SPG, ASI > 1.1) do not become abundant in the rock record until the Neoarchean. Because they typically form by partial melting of metasedimentary rocks, SPG reflect the composition of sedimentary source regions and tectonic processes that induce crustal melting. Geochemical and U-Pb zircon characteristics of 2.74-2.55 Ga SPG from the Wyoming province indicate that a modern variety of crustal sources and melting processes were present and operating by this time.

SPG occur throughout the Wyoming province, including in the:

• 2.55 Ga Mount Owen batholith (MO), a two-mica, sparsely garnet-bearing leucogranite;
• 2.62 Ga Wyoming batholith (WB), a magnesian, peraluminous biotite leucogranite intrusion that locally includes SPG;
• 2.68 Ga Webb Canyon gneiss (WC), a calcic, mainly ferroan, metaluminous to peraluminous leucogranite, locally garnet-bearing;
• 2.68 Ga Bitch Creek gneiss (BC), a magnesian, calcic, metaluminous to peraluminous leucogranite, locally garnet-bearing; and
• 2.74-2.64 Ga garnet leucogranite gneiss of the northern Laramie Mountains (NLM).

Rb-Sr-Ba characteristics indicate that some Wyoming SPG are dominated by immature, plagioclase rich sources (WC, BC), whereas others derive from clay-rich rocks (NLM). Low CaO/Na₂O ratios reflect lack of participation of plagioclase in melting reactions in WC and NLM. Inherited, 3.0-3.3 Ga zircon is abundant only in NLM; zircon saturation temperatures for these SPG exceed melting temperatures calculated from metamorphic assemblage diagrams. Al₂O₃/TiO₂ strongly correlates with temperature of melting, as represented by zircon saturation temperature, identifying both high T (>900C: WC, BC), and low T (~700C, NLM) partial melts.

These varying characteristics show that the crust that melted to form SPG consisted of immature graywacke, tonalite, and amphibolite in the west, and more mature, metapelitic sources in the east. The melting process varied from dehydration melting in response to mantle heating to decompression melting of thickened crust. All these processes are also identified in Phanerozoic SPG (Sylvester, 1998), indicating that the spectrum of crustal compositions and tectonic processes causing partial melting were well-established by the Neoarchean in the Wyoming province.