The solidification of the Shonkin Sag laccolith: Mineralogy, petrology, and experimental phase equilibria.

摘要

The Shonkin Sag laccolith is a differentiated sheet-like body of mafic igneous rock 70 m thick. The composition of chilled margins (CM) suggest that it formed from an initially crystal-rich magma containing 35% diopside, leucite (altered to pseudoleucite), olivine, minor apatite and trace biotite. In-situ gravitative processes produced several distinct rock types characterized by Pearce diagrams and mass-balance calculation: Shonkinite, by accumulation of approximately 80% diopside and 20% olivine; syenite, by sinking of mafic phenocrysts and accumulation of about 20% leucite by flotation; transition rock (TR), by upward leakage of residual liquid from the lower cumulate pile through a system of vertical "pipes" observable as cylindrical salic rock in a mafic matrix occurring from 13 m above base to the TR at +40 m; and soda-syenite, by concentration of residual magma in shrinkage cracks formed near the solidus.;Crystal size distribution (CSD) analyses show linear curves for the CM, concave upward for the lower shonkinite and concave downward for the syenite, as expected for accumulation and depletion of original phenocrysts. CSD analysis therefore appears to be a useful tool for investigating fractionation processes in hypabyssal intrusions.;Liquidus (1240;Harker variation diagrams of TR analyses reveal linear trends which represent the entire liquid line of descent from calculated initial liquid to final soda-syenite. The interstitial TR melts evolved as leucite dissolved in an increasingly sodium-rich melt, in which it was not stable.