Late Triassic tectonic inversion in the upper Yangtze Block: insights from detrital zircon U–Pb geochronology from southwestern Sichuan Basin

The Sichuan Basin and the Songpan-Ganze terrane, separated by the Longmen Shan fold-and-thrust belt (the eastern margin of the Tibetan Plateau), are two main Triassic depositional centers, south of the Qinling-Dabie orogen. During the Middle – Late Triassic closure of the Paleo-Tethys Ocean the Sichuan Basin region, located at the western margin of the Yangtze Block, transitioned from a passive continental margin into a foreland basin. In the meantime, the Songpan-Granze terrane evolved from a marine turbidite basin into a fold-and-thrust belt. To understand if and how the regional sediment routing system adjusted to these tectonic changes, we monitored sediment provenance primarily by using detrital zircon U-Pb analyses of representative stratigraphic samples from the southwestern edge of the Sichuan Basin. Integration of the results with paleocurrent and published detrital zircon data from other parts of the basin identified a marked change in provenance. Early-Middle Triassic samples were dominated by Neoproterozoic (~700-900Ma) zircons sourced mainly from the northern Kangdian basement, whereas Late Triassic sandstones recorded a more diverse range of zircon ages, sourced from the Qinling, Longmen Shan and Songpan-Ganze terrane. This change reflects a major drainage adjustment in response to the Late Triassic closure of the Paleo-Tethys Ocean and significant shortening in the Longmen Shan thrust belt and the eastern Songpan-Ganze terrane. Further, by Late Triassic time, the uplifted northern Kangdian basement had subsided. Considering the eastward paleocurrent and depocenter geometry of the Upper Triassic deposits, subsidence of the northern Kangdian basement probably resulted from eastward shortening and loading of the Songpan-Ganze terrane over the western margin of the Yangtze Block in response to the Late Triassic collision between Yangtze Block, Yidun arc and Qiangtang terrane along the Ganze-Litang and Jinshajiang sutures

A mixed source for the Late Triassic Garzê-Daocheng granitic belt and its implications for the tectonic evolution of the Yidun arc belt, eastern Tibetan Plateau

Many Late Triassic granitic plutons are present in the eastern Yidun arc belt (EYAB), and seven have been investigated in this study. From west to east, they are: the Sucuoma (235 ± 2 Ma), Ajisenduo (224 ± 2 Ma), Jiaduocuo (218 ± 1 Ma), Cuojiaoma (219 ± 1 Ma), Maxionggou (225 ± 2 Ma), Dongcuo (222 ± 3 Ma) and Daocheng (220 ± 2 Ma) plutons. Most of the plutons have granitic compositions and contain high SiO 2 , Al 2 O 3 and K 2 O + Na 2 O, but low MgO, FeO* and CaO contents. They have similar trace element patterns, with depletion in high field strength elements (HFSE, e.g., Nb and Ta) and enrichment in large-ion lithophile elements (LILE, e.g., Rb, Th and U). Samples from the Sucuoma, Jiaduocuo and Dongcuo plutons have similar zircon Hf isotopic compositions (e Hf (t) = - 1.8 to - 0.3, - 3.1 to 0.4 and - 4.6 to - 1.4, respectively), whereas those from the Ajisenduo pluton exhibit more unradiogenic Hf (e Hf (t) = - 11.9 to - 4.8). Additionally, the Sucuoma pluton has the lowest initial 87 Sr/ 86 Sr values (0.7060–0.7090) but the least negative e Nd (t) (- 4.9 to - 3.3) values, whereas samples from the Ajisenduo pluton have the highest initial 87 Sr/ 86 Sr values (0.7111–0.7160), but the most negative e Nd (t) values (- 7.9 and - 11.3). All the samples have similar high radiogenic Pb isotopic compositions ( 206 Pb/ 204 Pb = 18.6–19.4; 207 Pb/ 204 Pb = 15.7–15.8; 208 Pb/ 204 Pb = 39.1–40.4). Based on the new geochemical data, it is determined that most of the granitoids are I-type granites and the magma source was a mixture of the metamorphic Kangding Complex, which is considered to be the basement of the western Yangtze Craton, and metasediments from the basement. However, the samples from the Ajisenduo pluton are S-type granites that were derived from partial melting of basement metasediments with only limited components from the Kangding Complex. Together with an evaluation of previously-published work, these new data indicate a dominant magmatic peak at ca. 216 Ma for the Late Triassic granitoids of the YAB and that the ages of Late Triassic magmatism become younger eastward towards the Garzê-Litang suture zone. We consider that slab roll-back, with subsequent slab break-off, best explains the origin of these granitic plutons

Zircon U-Pb age and Sr-Nd-Hf isotope geochemistry of the Ganluogou dioritic complex in the northern Triassic Yidun arc belt, Eastern Tibetan Plateau: Implications for the closure of the Garzê-Litang Ocean

The Triassic Yidun arc belt (YAB) lies between the Jinshajiang suture zone to the west and the Garzê-Litang suture zone to the east, Eastern Tibetan Plateau. To study the YAB can not only help us to better understand the evolutionary history of the Garzê-Litang Ocean but can also provide some important information to constrain the evolution of the eastern Paleo-Tethys. In this paper, the geochronological and geochemical data of the Ganluogou dioritic complex were systematically investigated in order to decipher the geodynamic setting of the complex and to further determine the final closure time of the Garzê-Litang Ocean. The Ganluogou dioritic complex is located in the northern part of the YAB. It consists of ferrodiorite, diorite and a mixing zone between them and is the largest intermediate-mafic pluton in the YAB. The ferrodiorites were emplaced at 213 ± 2 Ma have low SiO2 and high Fe2O3* contents, whereas the diorites formed at 209 ± 2 Ma and have relatively higher SiO2, Na2O + K2O, Th, U, Zr, and Hf contents, but lower Al2O3, MgO, CaO, Co, and Sr contents than the ferrodiorites. Relative to the primitive mantle both the ferrodiorites and diorites are depleted in Nb and Ta. However, the ferrodiorites exhibit strong depletion in Zr and Hf, whereas the diorites contain relatively higher Th and U contents without negative Zr and Hf anomalies. Both rock-types have similar chondrite-normalized rare earth element patterns with (La/Yb)N ratios = 4.4 to 18.2, and show weak Eu anomalies, with Eu/Eu* of 0.47 to 1. They both show narrow ranges in Sr-Nd-Hf isotopic compositions. However, the ferrodiorites contain lower initial 87Sr/86Sr ratios (0.7052-0.7057) and relatively higher eNd(t) values (-3.8 to -2.4) than the diorites, which record values of 0.7062-0.7066 and -5.5 to -5.7, respectively. For the zircon Hf isotopic composition, the ferrodiorites also exhibit higher 176Hf/177Hf ratios (0.282738-0.282804) and more depleted eHf(t) values (3.4-5.6) than those of the diorites (176Hf/177Hf ratios of 0.282690-0.282728 and eHf(t) values of 1.7-2.8). These geochemical and isotopic features suggest that the ferrodiorites and diorites were derived from the different magma sources. We propose that the ferrodiorites were produced by differentiation of mantle-derived tholeiitic arc magma, whereas the diorites were derived either from dehydration melting of meta-basaltic rocks from the Kangding Complex (the basement of the west Yangtze Craton) or young arc volcanic rocks but mixed with a small amount of sediments. Both ferrodiorites and diorites were emplaced at relatively low pressure (3.3-3.9 kbar) but with quite different magma temperature (ferrodiorites: 821-960 °C; diorites: 692-788 °C). The Ganluogou dioritic complex was therefore formed under an extension setting that was associated with the closure of the Garzê-Litang Ocean. Previous studies on Late Triassic magmatism in the area have shown that there was slab-break off event at ~216 Ma. Combined with regional geological background, we conclude that the final closure of the Garzê-Litang segment of the Paleo-Tethys Ocean occurred prior to 216 Ma