Isochronous Miocene volcaniclastic horizon in the SW Tarim Basin: Composition, ages, origins, and implications for tephrochronology

Abstract Volcaniclastic rocks offer insights into magmatic and tectonic activity and a means to constrain depositional age by radioisotopic dating. However, these tasks can be challenging. An archetypal example is offered by the Upper Miocene strata of the SW Tarim Basin, where intercalated volcaniclastic intervals may decisively improve the existing chronostratigraphy, but detailed multidisciplinary analyses are indispensable to obtain robust chronostratigraphic constraints. Here, we coupled sedimentary facies analysis with X-ray diffraction, scanning electron microscopy–energy-dispersive spectrometry, heavy-mineral, whole-rock geochemical, and single-mineral isotopic analyses to determine the composition, eruption age, formation mechanism, and provenance of the volcaniclastic rocks. This case study demonstrates how the integration of multiple analytical methods can be applied in the identification, dating, and interpretation of volcaniclastic rocks. The studied volcaniclastic sandstones, laterally distributed over more than 200 km, were deposited in a sandy braided-river setting. Mineralogical and geochemical analyses reveal the dominance of volcanic-derived glass, clinopyroxene, sanidine, and/or analcime. The single-grain total fusion method on biotite/sanidine for 40Ar/39Ar dating yielded Upper Miocene (Tortonian) eruption ages of ca. 11 Ma. Zircon U-Pb age spectra suggest that volcanic detritus in most areas (between Qipan and Qimugan) was sourced from the eastern Pamir, whereas that in the Kekeya area was sourced from the western Kunlun region. All combined evidence suggests that these volcaniclastic intervals were deposited contemporaneously or penecontemporaneously with volcanism at ca. 11 Ma and constitute a regionally isochronous marker horizon. Volcanism was plausibly associated with the major Karakoram and Karakax strike-slip faults in the western Tibetan Plateau. Our results suggest that the accuracy of maximum depositional ages based on zircon U-Pb and biotite/sanidine 40Ar/39Ar ages is affected by several factors, including mineral fertility in volcanic source rocks. Selecting minerals with abundant autocrysts, applying high-resolution geochronometers, and/or integrating multiple geochronometers are recommended to enhance the accuracy of age constraints of volcanic/volcaniclastic rocks.