Granites simmering in their own juices: alkalic centers and the longevity of the Pikes Peak batholith, Colorado

The classic A-type Pikes Peak batholith (PPB) hosts multiple alkaline intrusions, traditionally interpreted as late-stage features, and divided into a ‘sodic’ and a ‘potassic’ series (Smith et al., 1999). A-type granites are known for their gemstones (e.g., amazonite, topaz, aquamarine) and rare-earth element (REE) deposits, which crystallize from late magmatic fluids, for example, in pegmatites. Here, we investigate the concentrically zoned Lake George Ring Complex (LGRC) at the western PPB margin to constrain its temporal relationship to the main granite of the batholith (i.e., the Pikes Peak granite; PPG) and to evaluate the role of late magmatic fluids in its formation.
Mineral textures and compositions were characterized using backscatter electron images and quantitative electron microprobe mapping. Zircon trace-element analysis and U/Pb dating using LA-ICP-MS provide insight into the evolution of these lithologies. Concordant zircons from the LGRC were selected for high-precision CA-ID-TIMS dating. Our CA-ID-TIMS ages show that the LGRC postdates the local PPG by ca. 5 Myr, overturning previous results (Guitreau et al., 2016) that placed the LGRC early in the magmatic sequence. The PPG yields Th-corrected 206Pb/238U ages between 1082.61 ± 0.68 Ma and 1086.29 ± 0.66 Ma. The syenomonzonite core yields ages of 1078.25 ± 0.70 Ma to 1079.45 ± 0.85 Ma; the inner syenogranite ring, 1078.5 ± 1.4 Ma to 1079.05 ± 0.93 Ma; and the second, syenitic ring, 1079.08 ± 0.53 Ma to 1083.9 ± 1.0 Ma. These data fill a temporal gap in existing datasets (Fonseca Teixeira et al., 2025) and reveal continuous magmatism in the Pikes Peak batholith lasting up to 15 Myr. 
Zircons from all LGRC lithologies typically exhibit high U (>1000 μg/g) and low Ti (<20 μg/g), indicating crystallization below the granite solidus (Fonseca Teixeira et al., 2023) and at a highly evolved stage. This contrasts with higher-Ti, lower-U zircon from the main PPG. Zircon from other alkaline complexes of the PPB, such as the Redskin granite (Fonseca Teixeira et al., 2025) and the Mount Rosa complex, shows U and Ti signals similar to those in the LGRC, suggesting a shared influence of late magmatic fluids. In contrast, PPG zircon preserves a predominantly magmatic signature.
All ‘rings’ of the LGRC contain large volumes of near end-member orthoclase and albite (ca. An₅), more evolved than feldspars in the PPG (ca. An₂₅). These feldspars occur as perthite exsolution, albite overgrowths, and ‘patch perthite’ (Norberg et al., 2013), indicating extensive (re)crystallization from a cool (< 500°C), fluid-saturated environment, rich in flux elements such as F. Albitization of Ca-rich plagioclase by Na-rich fluids generated abundant fluorite, which is coeval with the crystallization of REE-rich minerals in associated pegmatites (Hendrickx et al., 2024). 
We interpret the LGRC as a late-stage intrusion with extensive crystallization from and alteration by late magmatic fluids. These results highlight the potential for prolonged (>10 Myr) magmatism in large A-type systems, driven by late magmatic fluids and persisting well beyond the traditionally inferred granite solidus (~700 °C).