EVIDENCE FROM VOLCANIC ROCK XENOCRYSTS FOR ONSET OF EXTENSIONAL TECTONISM IN NORTHERNMOST RIO GRANDE RIFT SYSTEM, COLORADO

The Oligocene (~27 Ma to ~23 Ma) Rabbit Ears volcanic field in north central Colorado was active contemporaneously with extensional basin development and sediment infilling in this northernmost expression of the Rio Grande Rift system. However, the occurrence of an older (~33 Ma) olivine lamproite pluton suggests that the generation of carbonate-rich silicate melts in the continental lithospheric mantle (CLM) was the earliest extension-related magmatism (Thompson et al., Mineral. Mag., 1997). Evidence for this earlier event is provided by megacrysts in basaltic dikes from the Rabbit Ears volcanic field (e.g. Corral Peak) which contain ~ 20% olivine and clinopyroxene glomerocrysts and megacrysts up 2 cm in length. Electron microprobe data show that ol megacrysts are unzoned but collectively have Mg# ranging from 89 to 76 and Ca contents from 1,200 ppm to 1,500 ppm. Ol megacryst rims have lower Mg# (78 to 75) and higher Ca contents (1,500-2,600 ppm) formed by interaction with the host basalt. Cpx megacrysts are low Cr (<0.5 wt. % Cr₂O₃) diopsides, with higher Wo content rims. Both ol and cpx are likely mantle-derived xenocrysts but are not disaggregated CLM peridotite. Olivine in mantle xenoliths from the ~8 Ma Herring Park basalt further south in South Park are more magnesian (Fo_91-93) and less calcic (~500ppm) than the Rabbit Ears xenocrysts (Melton, MSc thesis Col. Sch. Mines 2007). Instead, the bimineralic xenocryst assemblage and the ol and cpx compositions are consistent with an origin via disaggregation of wehrlite present along melt channel walls in the CLM. The wehrlite likely formed by interaction between peridotite and earlier, ascending Si-undersaturated, carbonated melts, a conclusion consistent with microRaman hyperspectral analyses of 200-300m diameter melt pockets from both ol and cpx xenocrysts which are nephelinitic and contain microcrystalline ol, phl, cpx, ne, ap, alkali feldspar and oxides that contrast with the cpx-dominated fine-grained matrix of the host basalt. The preservation of Si-undersaturated silicate melts in the xenocrysts and the wehrlitisation, itself, are consistent with the mobilization of low degree partial melts of the CLM at the onset of extensional tectonism in this northernmost part of the Rio Grande Rift system.