USING RARE EARTH ELEMENT DATA TO DETERMINE THE DOMINANT MECHANISM FOR THE ORIGIN OF THE SPANISH PEAKS INTRUSIVE ROCKS OF SOUTH-CENTRAL COLORADO

The Spanish Peaks are located just east of the Sangre de Cristo Mountains on the far western edge of the Great Plains in South-central Colorado within the confines of the Rio Grande Rift (RGR) zone. Being within the RGR, the Spanish Peaks exhibit rift-related transitional chemistry straddling the line of Irvine and Baragar (1971) between alkaline and subalkaline compositions. Physiographically, the area is dominated by the East and West Spanish Peaks stocks. The most dramatic features are the hundreds of exposed dikes, some up to 20 km in length and rising several 10s of meters above the surrounding terrain. Many of the dikes lie in radial patterns focused on West Spanish Peak and Silver Mountain; others are part of the N80E trending subparallel dikes. A small number of additional dikes are independent of the radial or subparallel dike systems. Except for the West Spanish Peak, which is a holocrystalline phaneritic granular quartz syenite, the remaining rocks are pophyritic with the East Spanish Peak being a composite stock of granodiorite porphyry surrounded by granite porphyry; radial dikes composed of syenite and monzonite porphyry; and the subparallel dikes tend to be mafic to ultramafic camptonite to minette lamprophyre. After the methods outlined by Hanson (1989), comparing plots of trace elements with D_i <<1 versus those elements with D_i >>1. The range in concentrations for elements with D_i >>1 are fairly large for fractional crystallization and narrow for partial melting. Using this approach it was determined that the rocks in the Spanish Peaks region were largely derived from partial melting of either lower crustal or upper mantle sources.