Petrology of the Precambrian intrusive center at Lake George, southern Front Range, Colorado

The intrusive center at Lake George, at the western margin of the Pikes Peak batholith (1030 m.y.) of central Colorado, contains rocks of both the potassic and sodic differentiation trends recognized in the batholith. Finer grained variants of the Pikes Peak Granite initially formed a texturally zoned stock 8 kilometers in diameter that cut the dominant coarse-grained granite of the batholith. This granite stock (potassic trend) was later intruded by partial ring dikes of quartz syenite to fayalite granite and by a central stock of quartz syenite to syenomonzonite (sodic trend). Xenoliths of alkali gabbro, diorite porphyry, and granodiorite are included within rocks of both differentiation trends. All major rock types of the intrusive center are thought to be derivatives of alkali basaltic magma from the mantle, modified by fractional crystallization and reaction melting of crustal rocks. The geometry of the intrusive center was controlled by ring-dike/cauldron-subsidence mechanisms.