THE ORIGIN OF POTASSIC GRANITE:RESULTS OF FRACTIONAL CRYSTALLIZATION EXPERIMENTS ON A HIGH-AL OLIVINE GABBRO FROM THE LARAMIE ANORTHOSITE COMPLEX, WYOMING

Dissolved water may be the key to linking gabbros of continental tholeiitic suites, such as the 1.43 Ga Laramie Anorthosite Complex (LAC) in southeast Wyoming, with commonly associated potassic granitic rocks. To test whether any of the granitic rocks can be produced by extensive fractionation of gabbroic magmas, fractional crystallization experiments using graphite capsules in a piston-cylinder apparatus were conducted on a high-Al olivine gabbro from the LAC with 0.8-0.9 wt. % water (sample GM130; see table below, all analyses normalized to 100%). A low-temperature (1020°C) crystallization experiment at 4.3 kbar produced 90% crystals and 10% residual liquid. A composition representative of the residual liquid (GM130-65 57.44 wt. % SiO₂ (table) and 4 wt. % H₂O) was synthesized for further experiments. After 80% crystallization of GM130-65 at 880°C (equivalent to 98% crystallization of the original GM130 composition) the remaining Si-rich glass (GM13065-9=72.9 wt.%SiO₂) had a composition strikingly similar to the high-K granites found in the Sherman batholith, a major anorogenic intrusion associated with the LAC (Lincoln granite, table; Frost et al 1999, JP 40, 1771-1802). Similar results were obtained when varying the water content (2 and 6 wt. %) of the synthetic mix at 4.3 kbar. Because of the peritectics encountered along the fractionation path, these high-Si compositions cannot be obtained by partial melting of GM130 at 4.3 kbar.