INVESTIGATING CARBONIFEROUS TO JURASSIC CYCLES OF MOUNTAIN BUILDING RECORDED BY DETRITAL ZIRCON U-Pb GEOCHRONOLOGY AND (U-Th)/He THERMOCHRONOLOGY IN THE MONGOLIAN ALTAY
We performed detrital zircon double dating (U-Pb and (U-Th)/He, ZHe) on Cretaceous to Oligocene sandstones sampled within basins along the eastern margin of the Altay to reconstruct the orogenic evolution of source terranes and improve future provenance interpretations.
Our data contain four U-Pb-He age modes that we correlate with major cooling phases in the Mongolian Altay. The youngest cooling age corresponds to middle Jurassic (~150 Ma) ZHe ages with 160-165 Ma crystallization ages, which are interpreted as rapidly exhumed grains. A late Triassic (~220 Ma) cooling phase is recorded by grains with Permian, Devonian, and Cambrian crystallization ages. The most dominant cooling signal is early Permian (~280 Ma) in age and is characterized by rapidly exhumed grains with U-Pb ages at ~290 Ma, and non-rapidly exhumed grains with Devonian and Cambrian crystallization ages. The early Permian cooling phase is accompanied by volcanogenic grains that are recognized by their overlapping U-Pb and (U-Th)/He ages. Finally, the oldest cooling signal is recorded by Carboniferous (~320 Ma) ZHe ages with Devonian and Neoproterozoic-Cambrian U-Pb ages.
Our preliminary data suggest that the Permian and Carboniferous ZHe age modes coincide with plate reconstructions that place the Altay in the eastward closure of the Mongol-Okhotsk Ocean by that time. We also propose that the Mongolian Altay were proximal to active volcanism during the early Permian based on the presence of volcanogenic zircons.