Deep crustal anatexis, magma mixing, and the generation of epizonal plutons in the Southern Rocky Mountains, Colorado

  • Kristin H. Jacob
  • G. Lang Farmer
  • Robert Buchwaldt
  • Samuel A. Bowring
Original Paper

Abstract

The Never Summer Mountains in north-central Colorado, USA, are cored by two Oligocene, epizonal granitic plutons originally emplaced in the shallow levels of a short-lived (~1 m.y.), small-volume continental magmatic system. The younger Mt. Cumulus stock (28.015 ± 0.012 Ma) is a syenogranite equivalent compositionally to topaz rhyolites. A comparison to the chemical and isotopic composition of crustal xenoliths entrained in nearby Devonian kimberlites demonstrates that the silicic melts parental to the stock were likely derived from anatexis of local Paleoproterozoic, garnet-absent, mafic lower continental crust. In contrast, the older Mt. Richthofen stock is compositionally heterogeneous and ranges from monzodiorite to monzogranite. Major and trace element abundances and Sr, Nd and Pb isotopic ratios in this stock vary regularly with increasing whole rock wt% SiO2. These data suggest that the Mt. Richthofen stock was constructed from mixed mafic and felsic magmas, the former corresponding to lithosphere-derived basaltic magmas similar isotopically to mafic enclaves entrained in the eastern portions of the stock and the latter corresponding to less differentiated versions of the silicic melts parental to the Mt. Cumulus stock. Zircon U–Pb geochronology further reveals that the Mt. Richthofen stock was incrementally emplaced over a time interval from at least 28.975 ± 0.020 to 28.742 ± 0.053 Ma. Magma mixing could have occurred either in situ in the upper crust during basaltic underplating and remelting of an antecedent, incrementally emplaced, silicic intrusive body, or at depth in the lower crust prior to periodic magma ascent and emplacement in the shallow crust. Overall, the two stocks demonstrate that magmatism associated with the Never Summer igneous complex was fundamentally bimodal in composition. Highly silicic anatectic melts of the mafic lower crust and basaltic, mantle-derived magmas were the primary melts in the magma system, with mixing of the two producing intermediate composition magmas such as those from which Mt. Richthofen stock was constructed.

Keywords

Magma mixing Topaz rhyolite Mafic underplating Crustal anatexis 

Notes

Acknowledgments

Support for this study was provided by The Geological Society of America, the Colorado Scientific Society, Rocky Mountain Association of Geologists, and the University of Colorado Department of Geological Sciences. The manuscript was greatly improved in response to reviews by Eric Christiansen and Calvin Miller. We thank Timothy L. Grove for his handling of the paper as executive editor. Laboratory assistance from Emily Verplanck was extremely helpful. We thank Judy Visty at Rocky Mountain National Park for granting us access to the Grand Ditch Road which made the remote Never Summer Mountains readily accessible. Electron microprobe work was carried out with the help of Julian Allaz at the University of Colorado. U–Pb geochronology carried out at MIT was made possible by NSF EAR-0931839.

Supplementary material

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Copyright information

© Springer-Verlag Berlin Heidelberg 2015

Authors and Affiliations

  • Kristin H. Jacob
    • 1
  • G. Lang Farmer
    • 1
  • Robert Buchwaldt
    • 2
  • Samuel A. Bowring
    • 2
  1. 1.Department of Geological Sciences, CIRESUniversity of ColoradoBoulderUSA
  2. 2.Department of Earth, Atmospheric, and Planetary SciencesMassachusetts Institute of TechnologyCambridgeUSA

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