Reference 1570

Abstract

The major objectives of this study were to investigate the geology and geochemical characteristics of the Whitehorn stock in order to evaluate its evolution and origin. The study included detailed mapping of the stock and petrographic and geochemical analyses of collected samples. The information was used to constrain models for magma emplacement and evolution and to speculate about magma genesis. The Whitehorn stock is situated at the southern end of the Mosquito Range, northeast of Salida, in central Colorado and comprises two main intrusive bodies (named the northern and the southern) and three small satellite bodies. The main bodies are roughly elongate in a northwest-southeast direction, parallel to fold axes of Paleozoic country-rocks and are separated from each other by a paleovalley. The potassium-argon ages from biotite, 66.0+ or -1.7 to 70.8+ or -2.1 million years, indicate that the intrusion was emplaced during the Late Cretaceous (early Maestrichtian), coincident with early stages of Laramide tectonics. The Whitehorn Stock was originally intruded as a concordant, lenticular-shape laccolith. Shape, foliation, and internal contacts of the intrusion suggest that the Whitehorn Stock is a diapiric intrusion formed by emplacement of multiple pulses of magma rather than as a single injection. The compositional variation (gross zoning) within the Whitehorn Stock was produced by a combination of both in situ fractionation and injection of multiple pulses of magma. The Whitehorn Stock can be classified as a high-K calc-alkaline, I-type granitoid intrusion system. Compositions based on modal mineralogy in the Whitehorn stock and its satellite bodies range from granite to syenogabbro. The main body, which is mostly of granodioritic composition, is fine to medium grained and medium gray in color. Xenoliths of hornfels and carbonates are commonly found in the outer part of the northern body. Dark green hornblende xenocrysts are common in the southern body. Inclusions of more mafic composition are found throughout both the northern and southern bodies. Three physically distinguishable units occur in the northern body, including a mafic, a diorite-granodiorite, and a granite unit. Rocks of the mafic unit are only exposed on a small hill near the Marble Quarry. The diorite-granodiorite unit is most abundant, making up most of the main body, and the granite unit is present mainly in the central area of the northern body. The body has a gross compositional zonation from more mafic towards the stock's margin to more felsic in the interior. The northern body is strongly foliated at its margin, but unfoliated in its interior. The southern body contains two units, the granodiorite-monzonite porphyry (termed Subgroup I based on chemical composition) and the granodiorite (termed Subgroup II based on chemical composition), which are physically and chemically distinct. Typical rock compositions of this body range from quartz-monzodiorite to granodiorite and foliation is only rarely observed. Compositional trends in the northern body show smoothly decreasing Fe (sub 2) O (sub 3) (super t) , MgO, CaO, TiO (sub 2) , P (sub 2) O (sub 5) and MnO with increasing SiO (sub 2) , and smoothly increasing Na (sub 2) O, K (sub 2) O and BaO with increasing SiO (sub 2) . Trends in the southern body are similar but with inflections in the slope of Fe (sub 2) O (sub 3) (super t) , MgO, TiO (sub 2) , and P (sub 2) O (sub 5) vs. SiO (sub 2) . At about 60 wt.% SiO (sub 2) , the Na (sub 2) O vs. SiO (sub 2) trend for the southern body splits into two separate increasing trends with different slopes. The increasing K (sub 2) O vs. SiO (sub 2) trend splits into two trends as well, one increasing parallel to that observed for the northern body, the other decreasing. In the northern body Rb increases with increasing SiO (sub 2) , but in the southern body two separate decreasing trends are apparent. Comparatively, the northern body and subgroup II of the southern bo

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