SODIC TO POTASSIC ALKALINE MAGMAS AT SPANISH PEAKS CO: MAGMATIC SIGNATURE OF RIFTING ON THE SHOULDER OF THE RIO GRANDE RIFT?

At or near 22 Ma a variety of lamprophyres ranging from minette to camptonite intruded as dikes in the area surrounding Spanish Peaks CO. At the same time, silicic magmas were intruded and form the bulk of the intrusive rocks in the area. The lamprophyres are mantle-derived but formed from sources quite distinct as evidenced by K₂O/Na₂O ranging from 0.2 to 3. Although we refer to the rocks as “potassic” (K₂O/Na₂O>1) or “sodic” (K₂O/Na₂O<1), in fact there is a continuum of compositions in terms of relative alkali element abundances. Moreover, the two groups have similar patterns for normalized REE and incompatible elements although the potassic samples, which are more primitive tend to have higher incompatible element abundances. Two key differences between the two groups in addition to K₂O/Na₂O is that the potassic samples have lower Nd, Hf and Sr isotope ratios than the sodic samples (one sodic sample is essentially identical to the potassic samples, however), and the potassic samples have lower Al₂O₃ (11-14 wt%) compared to the sodic samples (Al₂O₃>14 wt%). This latter characteristic is likely imposed by source mineralogy. One possibility is that the potassic samples formed by melting of potassic richterite “cratonic” veins to produce low-Al melts and the sodic samples formed by melting of “non-cratonic” pargasite-bearing veins to produce melts with more normal Al contents following Foley et al. (1999). This idea is attractive because of the location of the area on the eastern shoulder of the Rio Grande Rift, and the western edge of the Great Plains, which presumably preserves ancient veined lithosphere beneath it. Magmatic timing suggests melting was triggered by the inception of the Rio Grande Rift. Given the continuum of magma compositions, magma mingling in the mantle probably also played a role.