Petrogenesis of the Sugarloaf syenite, Pikes Peak batholith, Colorado

Abstract

Sugarloaf Peak is one of seven sodic plutons that lie within or adjacent to the ca. 1.08-Ga Pikes Peak batholith in central Colorado. This report represents the first study of the Sugarloaf pluton. Major element and modal analyses from the other six plutons (Lake George, Tarryall, Rampart Range, West Creek, Mt. Rosa, and Spring Creek), together with data presented here, indicate that sodic and potassic rocks from all of them were produced by fractional crystallization of mantle-derived basaltic magmas. The Sugarloaf pluton is composed of fine-grained, medium-grained, coarse-grained, and pegmatitic syenite. The syenites lack quartz and are dominated by perthitic feldspar and ferrorichterite amphibole. The fine-grained syenite intrudes the medium- and coarse-grained syenites. The Sugarloaf pluton is surrounded by coarse-grained Pikes Peak Granite, which is the predominant rock type in the batholith. The linear arrangement of six of the seven sodic plutons parallel to major Precambrian fault trends suggests that the emplacement of Sugarloaf pluton may be rift-related. The Sugarloaf syenites have high total alkalis, high FeO (total), low CaO, and low MgO concentrations. They are also enriched in rare earth elements (REE) and high field strength elements (HFSE). Pronounced trace element variation among the Sugarloaf syenites can be explained partially by models of fractional crystallization. A plot of Ba versus Sr shows that compositions of the syenites closely follow modeled fractionation vectors for potassium feldspar. The fractional crystallization trends show that fine-grained syenite is the most chemically evolved, consistent with field relations that show the fine-grained syenite intruded the medium- and coarse-grained syenites. Accessory mineral fractionation, release of volatiles, or removal of pegmatitic fluids also may have influenced geochemical variations among the Sugarloaf syenites.