HYDROUS MINERAL CHEMISTRY OF THE ELKHEAD MOUNTAINS IN NW COLORADO: INSIGHTS INTO MANTLE SOURCES AND MAGMATIC PROCESSES

Mafic magmatism in NW Colorado is thought to be the result of the northernmost tectonic activity of the Rio Grande Rift. Highly potassic, mafic rocks containing hydrous-bearing minerals formed in the Elkhead Mountains during a second major regional pulse of volcanism (13.4-7.5 Ma). Geochemistry of whole rock samples and mineral phases from magmas erupted during this period are presented here. Samples investigated here have a large bulk compositional range, from evolved compositions, with MgO contents as low as 1.8 wt%, to primitive compositions with MgO contents as high as 10.3 wt%. Major and trace elements of phlogopites were determined via EMPA and LA-ICP-MS, respectively, and hydrogen isotopic compositions of phlogopites were determined with a gas source isotope ratio mass spectrometer. The phlogopite chemistry is also diverse, with many samples containing highly zoned crystals. The phlogopite Mg#s (molar Mg/Mg+Fe) predominantly range from 72 to 93. Hydrogen isotopic compositions of the phlogopites vary from -58 per mil to -144 per mil. A secondary/alteration phase, found in one sample, had a hydrogen isotopic composition of -174 per mil. This value is considered to represent the hydrogen isotopic composition resultant from post-eruption exchange. This distinction between the isotopic compositions of the phlogopites and the secondary phase suggests phlogopite has retained its magmatic signature. Phlogopite hydrogen isotopic compositions are correlated to host rock compositions and phlogopite compositions. This large range in both rock and mineral chemistry provides a unique opportunity to develop an understanding of how magmatic processes influence the hydrogen isotopic compositions of hydrous minerals, which might ultimately allow for the use of hydrogen isotopes as an invaluable geochemical tool for understanding magma genesis.