THE GENESIS OF CENOZOIC APISHIPA ²QUADRANGLE² DIKES NEAR SPANISH PEAKS AND DIKES FROM NORTHEASTERN NEW MEXICO

A wide variety of dikes, plugs, and sills exposed east of the Spanish Peaks magmatic complex in south-central Colorado are the focus of a continuing investigation into the origin and magmatic melting triggers for minettes intruded between about 55 and 33 Ma. Minette magmatism within this age range is documented at Spanish Peaks, Colorado, along with minettes (and similar alkalic rocks) between about 10-25 Ma. Only two of the rocks discussed here have been dated; one is approximately 10 Ma and one is approximately 22 Ma. The minettes exposed significantly east of the Peaks, in the old Apishipa quadrangle are mainly intermediate to felsic minettes, with a few unaltered mafic minettes. Similar to Navajo and Two Buttes minettes, minettes in the Apishipa area are greatly enriched in the highly incompatible trace elements with Ba abundances about 100 times Yb abundances. Dissimilar to the Navajo and Two Buttes minettes, minettes at Apishipa show a broad central hump on a rock/chondrite incompatibility plot, similar to OIBs, but with approximately 100 times greater elemental abundances (Sm, Eu,Gd, Tb) than oceanic OIBs. The Apishipa minettes do not show a significant depletion in Nb, Ta, P or Ti; however, this may not be unusual with respect to typical minettes, given that none of the minettes analyzed yet have more than 9.25 wt% MgO. More mafic minettes are likely to have the expected strong depletions in the HFSE, P, and Ti. The minettes analyzed to date appear to have a strong component from non-depleted mantle and a lesser component of the expected ancient metasomatized mantle lithosphere enriched in LILE by crust-forming events and subduction during the Proterozoic.