Timing and setting of Guanjiagou conglomerate in South Qinling and their tectonic implications

Located in the southern Qinling Mountains of central China, the Guanjiagou Formation has been a controversial issue with regard to its depositional age and tectonic implications. Being comprised of an approximately 2050 m thick succession of texturally and compositionally immature, presumed marine turbiditic sandstones and conglomerate, the Guanjiagou Formation consists of an overall prograding- and coarsening-upward megasequence. Although bounded by faults on both its northern and southern margins, it is weakly metamorphosed and deformed. To the north is the Devonian Sanhekou Group and to the south is the Neoproterozoic Hengdan Group. The lower portion of the sequence contains granitic and volcanic clasts (Guanjiagou conglomerate). The feldspars from these clasts were dated using the 40 Ar/ 39 Ar method. Two cooling ages of 219.69±0.49 and 216.46±0.59 Ma, for K-feldspar from a granitic clast and plagioclase from a volcanic clast, respectively, were obtained. These ages are identical to the time of regional igneous activities (ca. 240–220 Ma) and are interpreted as the products of magmatism associated with collision in the Qinling orogenic belt in the Early Mesozoic, suggesting that the Guanjiagou Formation was deposited in the Norian of the Late Triassic, ca. 220 Ma. Therefore, 40 Ar/ 39 Ar and sedimentary analyses suggest that the Guanjiagou Formation contains sediments that may have filled in a remnant ocean basin, which might be part of the Anyemaqen-Mianlüe ocean, or Tethys on the southern side of Central Orogenic Belt in China during the Late Paleozoic to Early Mesozoic.