Asking the pikas politely…
The Elk and Sawatch Mountains of eastern Gunnison County, Colorado, contain felsic igneous rocks that were emplaced episodically during three intervals of time. The intervals were between 75 to 55 Ma, 42 to 26 Ma, and 25 to 0 Ma. This study reports the changes in magmatic style and source region that have occurred through this period of time. Four groups with distinctive geochemical signatures were identified. Group I includes the Laramide Dustin Gulch and Fossil Ridge stock, and the middle Tertiary Gold Hill intrusive. These stocks are metaluminous and have Ta decoupled from Nb. The group also possesses a normative mineralogy that is high in anorthite and pyroxene, and low in quartz and orthoclase. They formed from an oxidized, mafic source. The group is similar to rocks that contain Laramide Cu-porphyry mineralization, but the stocks in this study are barren of mineralization. Group II includes the Hillerton, Pitkin, Seeton Mine, Buck Gulch, and South Matchless Mountain stocks. They are non-mineralized and middle Tertiary in age. They are mildly- to strongly-peraluminous, have normative mineralogy high in quartz and corundum, and are low in pyroxene. Tantalum is decoupled from Nb, the concentrations Y and the heavy rare earth elements are variable and depleted. This suggests that garnet was an important mineral phase in the magmatic source. The Group II source was oxidized and meta-sedimentary. Group III is associated with precious- and base-metal mineralization and includes the middle Tertiary Crested Butte laccolith and the quartz monzonite phases of the Italian Mountain and Mount Princeton intrusive centers and the Late Tertiary Round Mountain. This group is metaluminous, has normative mineralogy high in quartz and orthoclase, but is low in anorthite and corundum. They are different from the stocks of Group I and II in that both Ta and Nb have low concentrations. These rocks formed from a less oxidized felsic meta-igneous (or gneissic) source. Group IV is limited to the Late Tertiary Boston Peak topaz rhyolite. It is highly fractionated, possesses a strong negative Eu anomaly, is light rare earth element depleted and heavy rare earth element enriched compared to Groups I, II or III. It has characteristics typical of the igneous rocks associated with stockwork Mo deposits (Climax-type) and formed from a F-enriched crustal source. The groups (and stocks) reflect the geochemical characteristics associated with the change from subduction to extensional tectonics. Laramide subduction-related magmas occurred between 76 to 71 Ma. The source for these magmas was mafic and produced the stocks of Group I. During the second magmatic pulse, between 41 to 37 Ma, magma was produced from mafic, meta-sedimentary, and meta-igneous sources (Groups I, II, and III, respectively). At about 30 Ma North America overrode the Pacific spreading center and subduction ceased. Between 33 to 29 Ma the only sources to produce magmas were from Groups II and III. Afterward, the magmatic style changed to the bimodal magmatism typical for the extensional tectonics of the Rio Grande rift. Between 14 to 10 Ma magmas were produced in this area only from meta-igneous (Group III) and F-enriched crustal (Group IV) sources. (Abstract shortened by UMI.)
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