Pikes Peak batholith (Colorado, USA) revisited: A SIMS and LA-ICP-MS study of zircon U–Pb ages combined with solution Hf isotopic compositions

Abstract

The Lake George ring complex (LGRC) is an epizonal intrusion that is part of seven alkalic stocks distributed throughout the Mesoproterozoic Pikes Peak batholith (PPB) in central Colorado. Despite having been studied for more than a century, details about the emplacement history of the PPB lithologies have remained elusive. To parse out the relative sequence of intrusive events and magma petrogenesis, we analyzed zircons from different LGRC intrusive phases for their U–Pb ages and Lu–Hf isotopic compositions. We found that the LGRC is composed of a 1428±8Ma syenomonzonite at its center, which has a Hf isotopic composition indistinguishable from that of coeval wall rocks related to the Berthoud plutonic suite. We hence interpret it as a roof pendant. The three ring dikes that surround the LGRC center have ages of 1085±5Ma, 1115±12Ma, and 1078±11Ma from innermost to outermost, respectively. The local phase of Pikes Peak granite that surrounds LGRC was emplaced at 1066±10Ma. Therefore, LGRC represents an early stage in PPB emplacement, and evidently formed over a protracted period of time. The LGRC zircon εHf values (+4.8±0.2 at 1115Ma, +0.4±0.9 at 1078Ma, and −0.8±0.2 at 1066Ma) indicate input of shallow mantle-derived melts and direct involvement of the Proterozoic Colorado Front Range (CFR) crust with the latter gradually prevailing over time. The results presented here provide new insights into the petrogenesis of the PPB and call for a general reassessment of emplacement ages and source characteristics of the other alkalic intrusive centers in the batholith.