ISOTOPIC SIGNATURES
compositions of ~4 Ma plutons, some of which are associated with molybdenite mineralization. There is no good control, however, on the exact melt sources of the ~4 Ma rocks. Contributions from felsic Proterozoic basement or re-melting of Laramide igneous rocks at ~4 Ma are feasible. The S-type characteristics, distinguished by normative corundum, of the Calico Peak sample (Fig. 1, Table 1) may also indicate melting of sedimen- tary or metasedimentary rocks in the crust (e.g., phyllite in the Proterozoic Uncompahgre Formation).
The results of this investigation offer new insights into the generation of magmas in the Rico Mountains at ~68 and ~4 Ma, from subduction-driven mag- matism during the Laramide orogeny to regional rifting in the middle to late Cenozoic. The isotopic evidence (Figs. 2-3, Tables 2-3) reveals a shift from CLM or Proterozoic basement sources at ~68 Ma to those dominated by upper crustal rocks at ~4 Ma. These trends are similar those documented in 28 to 26 Ma volca- nic rocks in the San Juan Mountains in which early magmas were created by melting of mantle and lower crustal sources, and then modified by melting and assimilation of upper crustal rocks at higher levels (e.g. Riciputi et al., 1995). Variations in magma sources through time in the area also influenced the type and trends of mineralization in the western San Juan Mountains.
Acknowledgements
We thank Andrea Kirkpatrick for assisting with many different aspects of this project. A special thanks is given to Mark Pecha and Nicky Giesler at the University of Arizona for assisting with the Lu-Hf isotopic analyses at the University of Arizona LaserChron Center. We are also grateful for the assis- tance and guidance of Emily Verplanck at the University of Colorado for the Sr and Nd analyses. We want to thank John Berry for his review of the manuscript.
This project was funded in part by Fort Lewis College, the Geological Society of America, and the Colorado Springs Mineralogical Society. Otto would like to thank Patrick Lang and Swantje Quarder for supporting his undergradu- ate education and senior thesis research. He also thanks to his classmates Peyton Valko and Tanner Morgan for their assis- tance and company he received during his time in the field.
References Cited
Armstrong, L.A., 1969, K-Ar dating of laccolithic centers of the Colorado Plateau and vicinity: Geological Society of America Bulletin, v. 80, p. 2081-2086.
54 TPG •
Jan.Feb.Mar 2019
Barnes, R., 1985, The mines and minerals of Rico: The Mineralogical Record, v. 6, p. 203-247.
Chapin, C.E., 2012, Origin of the Colorado Mineral Belt: Geosphere, v. 8, p. 28-43.
Chapin, C.E., Wilks, M., and McIntosh, W.C., 2004, Space-time patterns of Late Cretaceous magmatism in New Mexico - comparison with Andean volcanism and potential for future volcanism: New Mexico Bureau of Geology & Mineral Resources, Bulletin 160, p. 13-40.
Coney, P. J., and Reynolds, S. J., 1977, Flattening of the Farallon slab: Nature, v. 270, p. 403–406.
Cross, W., and Spencer, A.C., 1900, Geology of the Rico Mountains, Colorado: U. S. Geological Survey Twenty First Annual Report, Part II, Chapter 1, p. 7-167.
Cunningham, C.G., Naeser, C.W., Marvin, R.F., Luedke, R.G., and Wallace, A.R., 1994, Ages of selected intrusive rocks and associated ore deposits in the Colorado mineral belt: U.S. Geological Survey Bulletin 2109, 31 p.
Easley, E., and Morgan, P., 2013, Fluid, gas, and isotopic variation of ther- mal springs in the southern Rocky Mountains, Colorado: Geothermal Resources Council Transactions, v. 37, p. 385-392.
Farmer, G.L., Broxton, D.E., Warren, R.G., and Pickthorn, W., 1991, Nd, Sr, and O isotopic variations in metaluminous ash-flow tuffs and related volcanic rocks at the Timber Mountain/Oasis Valley Caldera Complex, SW Nevada: implications for the origin and evolu- tion of large-volume silicic magma bod- ies: Contributions to Mineralogy and Petrology, v. 109, p. 53-68.
Farmer, L.G., Bailley, T., and Elkins- Tanton, L.T., 2008, Mantle source volumes and the origin of the mid-Ter- tiary ignimbrite flare-up in the southern Rocky Mountains, western U.S: Science Direct, Lithos, v. 102, p. 279-294.
Gonzales, D.A., 2015, New U-Pb zircon and 40Ar/39Ar age constraints on the late Mesozoic to Cenozoic plutonic rocks in the western San Juan Mountains: The Mountain Geologist, v. 52, no. 2, p. 5-14.
Gonzales, D.A., 2017, A review and revi- sion of late Mesozoic to Cenozoic plu- ton chronology in the Rico Mountains, Colorado in The Geology of the Ouray- Silverton Area, Karlstrom, Karl E.; Gonzales, David A.; Zimmerer, Matthew J.; Heizler, Matthew; Ulmer-Scholle, Dana S., New Mexico Geological Society 68th Annual Fall Field Conference Guidebook, p. 91-96.
Gonzales, D.A., and Lake, E.T., 2017, Geochemical constrains on mantle-melt sources for Oligocene to Pleistocene
mafic rocks in the Four Corners region, USA: Geosphere, v. 13, no. 1, p. 1-26.
Humphreys, E.D., 1995, Post-Laramide removal of the Farallon slab, west- ern United States: Geology, v. 23, p. 987–990.
Humphreys, E., Hessler, E., Dueker, K., Farmer, G.L., Erslev, E., and Atwater, T., 2003, How Laramide-age hydra- tion of North American lithosphere by the Farallon slab controlled subse- quent activity in the western United States, in Klemperer, S.L., and Ernst, W.G., eds., The George A. Thompson Volume: The lithosphere of Western North America and its geophysical characterization: Geological Society of America, International Book Series, v. 7, p. 524–542.
Lake, E.T., and Farmer, G.L., 2015, Oligo-Miocene mafic intrusions of the San Juan volcanic field, southwestern Colorado, and their relationships to voluminous, caldera-forming magmas: Geochimica Et Cosmochimica Acta, v. 157, p. 86-108.
Lipman, P.W., Steven, T.A., and Mehnert, H.H., 1970, Volcanic history of the San Juan Mountains, as indicated by potas- sium-argon dating: Geological Society of America Bulletin, v. 81, p. 2329-2352.
McKnight, E.T., 1974, Geology and ore deposits of the Rico district, Colorado: U.S. Geological Survey Professional Paper 723, 97 p.
Mueller, P., and Vervoort, J., 2017a, Inductively coupled plasma mass spectrometer (ICP-MS): https://serc.
carleton.edu/research_education/geo- chemsheets/techniques/MCICPMS. html (accessed January 20, 2018).
Mueller, P., and Vervoort, J., 2017b, Thermal ionization mass spectrometry:
https://serc.carleton.edu/research_edu- cation/geochemsheets/techniques/
TIMS.html (accessed January 20, 2018).
Pratt, W.P., 1968, Summary of the geol- ogy of the Rico region, Colorado in New Mexico Geological Society 19th Annual Field Conference Guidebook: New Mexico Geological Society, p. 83-87.
Pratt, W.P., McKnight, E.T., and DeHon, R.A., 1969, Geologic map of the Rico quadrangle, Dolores and Montezuma counties, Colorado: U.S. Geological Survey Map GQ-797, scale 1:24,000, 1 sheet.
Riciputi, L.R., Johnson, C.M., Sawyer, D.A., and Lipman, P.W., 1995, Crustal and magmatic evolution in a large multi- cyclic caldera complex: isotopic evidence from the central San Juan volcanic field: Journal of Volcanology and Geothermal Research, v. 67, p. 1-28.
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