Study On Metamorphism And Geochronology Of The Douling Complex, South Qinling Orogen

The Douling Complex, one of important geological unites between the North and Yangtze cratons, is located in the South Qinling tectonic belt to the southern side of the Shangdan Fault. This poorly studied complex is a crystalline basement that developed in the Neoproterozoic-Paleozoic weakly metamorphosed to non-metamorphosed strata in the South Qinling tectonic belt. Due to inadequate geochronological research and inaccurate age data, no conclusive results could be drawn yet regarding the protolith and metamorphic ages of this complex. In this paper, we present original results of our field investigation, and metamorphic petrological, geochronological studies on the Douling Complex, which is very important for understanding the tectonic evolution of the northern margin of the Yangtze craton.The detailed investigation in the field reveals that the Douling Complex mainly consists of banded dioritic-granitic gneiss (including amphibole-plagioclase gneiss, biotite-plagioclase gneiss, plagioclase gneiss and two-feldspar gneiss) with intercalated amphibolite, metapelite, calc-silicate rock and marble, where the amphibolite sometimes interbeds with gneiss. Based on microstructure characteristics and co-existing relationship between minerals in the Douling Complex, mineral assemblages and metamorphic evolution were determined. Metapelite mainly comprises garnet, biotite, staurolite, kyanite, sillimanite, plagioclase and quartz, in which sillimanite formed later than other minerals. Amphibolite consists of garnet, amphibole, plagioclase and quartz. Lower-grade minerals such as chlorite, sericite, epidote and albite occurred during retrograde stage. Using conventional geothermobarometeries, P-T conditions of the peak metamorphism was estimated to be650-730℃and0.80-1.10GPa. This suggests that the peak metamorphism of the rocks reaches upper amphibolite facies conditions, equivalent to the geothermic gradient of20℃/km. Compositional zoning of garnet and transition from kyanite facies to sillimanite facies indicate a clockwise P-T path for the rocks.LA-MC-ICPMS zircon U-Pb dating on different rock types of the Douling Complex has provided two important infromations. Firstly, the protolith emplacement ages of the dominant dioritic-granitic gneisses occurred between2.51-2.47Ga, and an important peak age of～2.48Ga was also obtained for a metasedimentary rock. These geochronological results suggest the occurrence of an igneous event of2.51-2.47Ga at the northern margin of the Yangtze craton. The age-corrected εHf(t) values obtained from in situ zircon Hf isotopic analysis are mainly between-5.5and+0.3, and the corresponding two-stage Hf model ages range from3.30to2.95Ga. Considering two important periods of～3.3-3.2Ga and-2.95-2.90Ga for the continental crustal growth in the Yangtze craton, we infer that the dioritic-granitic gneisses from the Douling Complex are the products of reworking of Palec-to Mesoarchean crust at the northern margin of the Yangtze craton at～2.5Ga. There may be a small contribution from juvenile materials. Meanwhile, detrial zircons from a metasedimentary rock also show two peak ages of-1.96Ga and-1.39Ga, suggesting that the Yangtze craton not only experienced Paleoproterozoic reworking and Neoproterozoic continental crustal growth, but also underwent Mesoproterozoic magmatic activity.Sencondly, the metamorphic ages of820-780Ma were obtained for garnet amphibolites and metapelites, and an age of837±8Ma on zircon overgrowth rims was also obtained for a amphibole-plagioclase gneiss. This suggests that the main phase amphibolite facies metamorphism of the Douling Complex occurred during the Neoproterozoic (840-780Ma), which is still unreported from the Yangtze craton. The more recent research indicates that widespread arc magmatism related to oceanic subduction may have occurred at the northern margin of the Yangtze craton, we hence infer that this metamorphism is associated with this accretionary orogenic event. Futhermore, the Douling Complex was intruded by voluminous diorite, granodiorite and granite intrusives, which are generally subjected to late deformation. Three diorite, granodiorite and granite samples yielded protolith ages of735-705Ma, indicating the importance igneous event during the Neoproterozoic. Based on the available geochemical data, these Neoproterozoic intrusives may have formed in an active continental margin setting.