摘要
Located in the southern Qinling Mountains of central China, the Guanjiagou Formation has been a con-troversial issue with regard to its depositional age and tec-tonic implications. Being comprised of an approximately 2050 m thick succession of texturally and compositionally immature, presumed marine turbiditic sandstones and con-glomerate, the Guanjiagou Formation consists of an overall prograding- and coarsening-upward megasequence. Al-though 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 40Ar/39Ar method. Two cooling ages of 219.690.49 and 216.460.59 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 ac-tivities (ca. 240—220 Ma) and are interpreted as the prod-ucts 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, 40Ar/39Ar and sedimen-tary analyses suggest that the Guanjiagou Formation con-tains sediments that may have filled in a remnant ocean ba-sin, which might be part of the Anyemaqen-Mianle ocean, or Tethys on the southern side of Central Orogenic Belt in China during the Late Paleozoic to Early Mesozoic.
Located in the southern Qinling Mountains of central China, the Guanjiagou Formation has been a con-troversial issue with regard to its depositional age and tec-tonic implications. Being comprised of an approximately 2050 m thick succession of texturally and compositionally immature, presumed marine turbiditic sandstones and con-glomerate, the Guanjiagou Formation consists of an overall prograding- and coarsening-upward megasequence. Al-though 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 40Ar/39Ar method. Two cooling ages of 219.690.49 and 216.460.59 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 ac-tivities (ca. 240—220 Ma) and are interpreted as the prod-ucts 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, 40Ar/39Ar and sedimen-tary analyses suggest that the Guanjiagou Formation con-tains sediments that may have filled in a remnant ocean ba-sin, which might be part of the Anyemaqen-Mianle ocean, or Tethys on the southern side of Central Orogenic Belt in China during the Late Paleozoic to Early Mesozoic.
