NEW CA-TIMS AGES FOR FELSIC VOLCANIC ROCKS OF THE AMBLER DISTRICT, SOUTH CENTRAL BROOKS RANGE: IMPLICATIONS FOR REGIONAL CORRELATIONS AND SUBDUCTION INITIATION IN THE CORDILLERA

Felsic volcanic rocks associated with volcanogenic massive sulfide (VMS) deposits in the Ambler district play a key role in establishing the Paleozoic tectonic setting for the Coldfoot subterrane of the Arctic Alaska terrane (also Schist belt) in the southern Brooks Range, Alaska. Similarity of the Ambler VMS occurrences with those in the Yukon-Tanana terrane have long supported views that mid-Paleozoic magmatism in pericratonic arc terranes of the northern Cordillera is related to subduction initiation. The felsic volcanic units that host VMS mineralization in the southern Brooks Range occur with mafic metavolcanic units, variably chloritic, graphitic and calcareous metaclastic lithologies and carbonate rocks of the Ambler sequence, which is structurally interleaved with regionally extensive Proterozoic-Devonian metaclastic units. Previous age estimates obtained by secondary ion mass spectrometry (SIMS) for felsic volcanic units across the Ambler district range from 380 ± 2 Ma to 376 ± 3 Ma with one outlier at 386 ± 3 Ma. Two samples of massive rhyolite porphyry from the Arctic deposit previously analyzed by SIMS were analyzed by chemical abrasion isotope dilution mass spectrometry (CA-TIMS). Results from both methods document the presence of inherited components and Pb-loss associated with the widespread Jurassic-Cretaceous blueschist and amphibolite facies metamorphic overprint in the region. The two samples define ²⁰⁷Pb/²⁰⁶Pb weighted mean ages of 377.49 ± 1.21 Ma (n=4/6) and 377.09 ± 0.78 Ma (n=6/7) and ²⁰⁶Pb/²³⁸U dates that agree and are up to 6 Ma younger due to the overprint. These ²⁰⁷Pb/²⁰⁶Pb ages are consistent with previous SIMS ages of 376 ± 3 Ma and 379 ± 3 Ma. Additional CA-TIMS analyses are needed to evaluate the range in VMS-related magmatism across the Ambler district defined by the SIMS data. Nevertheless, the current data clearly differentiates the age of arc to back-arc magmatic rocks in the Arctic Alaska terrane and younger units in the Yukon-Tanana terrane that yield CA-TIMS ages of 363 – 361 Ma (Manor et al., 2022). These and future high precision geochronological datasets will help evaluate the rate of southward propagation of subduction initiation and back arc evolution in the northern Cordillera.