PETROGENETIC RELATIONSHIPS BETWEEN POTASSIC AND SODIC ALKALINE MAGMAS AT SPANISH PEAKS CO

Mildly ne-normative, alkaline lamprophyric rocks crop out as sills, independent, radial and sub parallel dikes near the famous radial dike locality of Spanish Peaks CO. We focused on lamprophyric dikes and sills north and west of Spanish Peaks. These rocks are moderately mafic (5-9 wt.% MgO) and range from relatively sodic (Na2O>K2O) to relatively potassic (K2O>Na2O). Available geochronologic and isotopic data for the rocks of our study indicate that the sodic and potassic magmatism originated at approximately the same time and from isotopically similar mantle sources. The potassic rocks which include true minettes typically have phlogopite + diopside +/-olivine as phenocrysts whereas the relatively sodic rocks which include true sannaites have kaersutitic to pargasitic amphibole (including megacrysts up to 2 cm in diameter) + diopside +/- altered feldspar as typical phenocrysts. Rare diopside grains in the potassic rocks have the greenish, Fe- and Al-rich cores that are found around the world in potassium-rich, mafic igneous rocks. Analysis of major and trace element abundances in the dike rocks indicate that the two magma series formed independently from distinct sources. For example, at similar MgO contents, the potassic magmas are characterized by lower Al2O3 (typically <13 wt.%) and higher TiO2 (typically > 2 wt.%) than the relatively sodic magmas (typically > 14 wt. % Al2O3, < 2 wt. % TiO2) and these differences are independent of MgO content. Apatite compositions were analyzed from both magma series: apatites in both potassic and sodic magmas are primarily fluor-apatites, but those in the potassic magmas are nearly anhydrous whereas the apatites from the sodic rocks have significant inferred H2O contents. None of the apatites have a Cl-rich signature that could be attributed to a subduction zone component except apatites from an amphibole-apatite xenolith possibly related to the sodic lineage.