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A petrochronological approach to alkaline ring complexes: new U/Pb ages for the Lake George Ring Complex, Pikes Peak batholith, Colorado

Tobias Hendrickx1, Julien Allaz1, Tyler Wickland2, Olivier Bachmann1

  • Affiliations: 1Department of Earth and Planetary Sciences, ETH Zurich, Zurich, Switzerland; 2Department of Geological Sciences, University of Colorado Boulder, Boulder, CO, USA

  • Presentation type: Poster

  • Presentation time: Thursday 16:30 - 18:30, Room Poster Hall

  • Poster Board Number: 212

  • Programme No: 1.9.19

  • Theme 1 > Session 9

Abstract

The Pikes Peak batholith (PPB) hosts multiple alkaline intrusions categorized as a 'sodic' or a 'potassic' series. They were interpreted as late-stage intrusions based on field relationships (Smith et al., 1999). This study investigates the formation and geological history of the Lake George Ring Complex (LGRC) and its relation to the PPB through petrological and petrochronological analyses. The LGRC is at the PPB's western margin and displays concentrically zoned lithologies of both types. As ring structures are often linked to ancient caldera systems, the LGRC and other alkaline intrusions may represent the remnants of volcanic activity associated with the emplacement of the PPB. The LGRC outer rim is a monzogranite (74wt.% SiO2, 9.4wt.% Na2O+K2O) crosscut and mingled by high-Na mafic dikes (58wt.% SiO2, 10.5wt.% Na2O+K2O). Next is a quartz syenite (66wt.% SiO2, 10.5wt.% Na2O+K2O) and finally a 'sodic' core of syenomonzonite (64wt.% SiO2, 11.2wt.% Na2O+K2O), with a few olivine (Fo36-39) gabbro bodies in the center (46wt.% SiO2). New zircon U/Pb ages clarify the relative ages of the concentric lithologies, shedding light on their petrogenesis. The current consensus favors a late emplacement of smaller intrusions relative to the parent pluton, though ages by Guitreau et al. (2016) challenge this interpretation for the LGRC. We hypothesize that some LGRC lithologies are cumulates and seek evidence of crystal-melt separation. The release of fluids by the crystallizing magmas may have altered the lithologies of the LGRC. Guitreau, M. et al. (2016). Precambrian Research,280, 179--194. Smith, D. R. et al. (1999). Precambrian Research,98, 271--305.