The Bikou volcanic terrane is predominated by subalkaline tholeiitic lavas. Rock samples display lower initial ratios of Sr and Nd, 0.701248-0.704413 and 0.511080-0.512341 respectively. 207Pb and 208Pb are significant...The Bikou volcanic terrane is predominated by subalkaline tholeiitic lavas. Rock samples display lower initial ratios of Sr and Nd, 0.701248-0.704413 and 0.511080-0.512341 respectively. 207Pb and 208Pb are significantly enriched in the lavas. Most samples have positive εNd, which indicates that the magma was derived from EM-type mantle source, while a few samples with negative εNd indicate that there was contamination in the magma evolution. Magma differentiation is demonstrated by variations of LREE and LILE from depletion to enrichment. Additionally, normalized REE patterns and trace elements showed that lavas from the Bikou volcanic terrane have similar characteristics to those of basalts in arc settings caused by subduction and collision. Analyses showed that the Bikou volcanic terrane is a volcanic arc. New evidence proved that the Hengdan Group, north of the Bikou arc, is a turbidite terrane filling a forearc basin. Consequently, the Bikou volcanic terrane and the Hengdan turbidite terrane construct an arc-basin system. New SHRIMP ages showed that this arc-basin system developed on the northern margin of the Yangtze craton in the Neoproterozoic (846-776 Ma), and this arc-basin system is in agreement with the tectonic processes of Rodinia in the Neoproterzoic.展开更多
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 ...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.展开更多
基金supports from the National Natural Science Foundation of China(grants 40172071 and 40211120151 to Yan Quanren)the Ministry of Science and Technology of the People’s Republic of China(grant 2202CB412608 to Wang Zongqi)+2 种基金the China Geological Survey(grant DKD2001002 to Wang Zongqi)the University of Nevada Las Vegas(to Hanson)and the Geological Society of America(to Druschke)are gratefully appreciated.
文摘The Bikou volcanic terrane is predominated by subalkaline tholeiitic lavas. Rock samples display lower initial ratios of Sr and Nd, 0.701248-0.704413 and 0.511080-0.512341 respectively. 207Pb and 208Pb are significantly enriched in the lavas. Most samples have positive εNd, which indicates that the magma was derived from EM-type mantle source, while a few samples with negative εNd indicate that there was contamination in the magma evolution. Magma differentiation is demonstrated by variations of LREE and LILE from depletion to enrichment. Additionally, normalized REE patterns and trace elements showed that lavas from the Bikou volcanic terrane have similar characteristics to those of basalts in arc settings caused by subduction and collision. Analyses showed that the Bikou volcanic terrane is a volcanic arc. New evidence proved that the Hengdan Group, north of the Bikou arc, is a turbidite terrane filling a forearc basin. Consequently, the Bikou volcanic terrane and the Hengdan turbidite terrane construct an arc-basin system. New SHRIMP ages showed that this arc-basin system developed on the northern margin of the Yangtze craton in the Neoproterozoic (846-776 Ma), and this arc-basin system is in agreement with the tectonic processes of Rodinia in the Neoproterzoic.
文摘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.
