Petrogenesis and geodynamic implications of the late Triassic bojites in Yajiangqiao area, Hunan Province, South China

Abstract

The mafic igneous activities in the late Triassic play an important role in the tectonic evolution of the South China Block (SCB). In this paper, relevant data of the bojites in Yajiangqiao area, eastern Hunan Province, South China were provided, including zircon U-Pb ages, whole-rock geochemical data, Sr-Nd-Pb isotopic data, and zircon Hf isotope data. These data indicate that the bojites were crystallized at 215.7 Ma, and thus are coeval with biotite monzonitic granites of Yajiangqiao pluton. The bojite samples feature a low SiO 2 content of 48.84-49.94 wt%, a low ALK (K 2 O + Na 2 O) content of 5.13-6.13 wt%, a relatively high Al 2 O 3 content of 17.64-21.22 wt%, a moderate MgO content of 4.32-7.07 wt%, and a moderate FeO T content of 5.66-7.33 wt%. In addition, the samples have a total rare earth element (REE) content of 108-163 ppm. They display right-declined REE patterns, enriched in light rare earth elements and large-ion lithophile elements (such as Rb, K, Th, and U) and depleted in high field strength elements (such as Nb, Ta, P, and Ti). All the samples have homogeneous Sr-Nd-Pb and zircon Hf isotopic compositions, with the (87 Sr/ 86 Sr) i ratio varying from 0.708 202 to 0.709 515, (143 Nd/ 144 Nd) i ratio from 0.512 156 to 0.512 229, (206 Pb/ 204 Pb) t ratio from 18.185 to 18.264, (207 Pb/ 204 Pb) t ratio from 15.667 to 15.672, (208 Pb/ 204 Pb) t ratio from 38.510 to 38.587, and initial 176 Hf/ 177 Hf ratio ranging from 0.282 428 to 0.282 530. Meanwhile, the calculated ε Nd (t) and ε Hf (t) values range from −4.0 to −2.6 and from −7.9 to −4.1, respectively, with two-stage Nd model ages of 1.20-1.32 Ga and two-stage Hf model ages of 1.35-1.56 Ga, respectively. As indicated by geochemical data, the primitive magmas of the bojites in Yajiangqiao area were derived from lithospheric mantle in postcollisional extensional environment, which underwent low-degree partial melting (1-5 %) of spinel-garnet lherzolites (spinel > garnet) and was subjected to metasomatism of slab-derived fluids. Furthermore, it can be concluded from these data and previous data that the SCB was transformed from syn-collisional compression to postcollisional extension in the late Triassic (234 Ma) and the post-collisional extension lasted until 215 Ma.

Mineral Chemistry and Chronology Investigation of Uraninite in the Jinguanchong Uranium Deposit in Eastern Hunan Province and the Implications for Geological Significance

“…It is characterized by an overall stock-like intrusion into the Lengjiaxi Group and Banxi Group, representing a multistage composite granite pluton. The diagenetic age can be categorized into two stages: 244–251 and 212–213Ma . The main geological structures within the mining area include the Dazhang–Hejiaqiao inclined fault, two deep faults F1 and F2, and numerous interlayer and intralayer fracture zones.…”

Provenance analysis of Jurassic basins alongChaling–Chenzhou–LinwuFault, South China: Implications for palaeogeographic reconstruction and Mesozoic tectonic transition

“…A watershed or mountain chain could be induced by thrust, strike-slip, or normal faulting (Bollinger, Henry, & Avouac, 2006;Brunel, Arnaud, Tapponnier, Pan, & Wang, 1994;Cunningham, 2005;Hennig et al, 2016;Sachau, Koehn, & Passchier, 2013). A-type granites and bimodal volcanic rocks imply Late Triassic to Early Jurassic extension occurred in the South China Block (Cen et al, 2016;Chen, Hua, Zhang, Lu, & Fan, 2002;Gan et al, 2017;Gao et al, 2017;Shu et al, 2009;Xu, Li, et al, 2019;Yu et al, 2010Yu et al, , 2020Zhou et al, 2006). There is no NNEstriking large-scale half-graben basin that occurred along the Chaling- Cawood, & Du, 2016).…”

“…There is no NNEstriking large-scale half-graben basin that occurred along the Chaling- Cawood, & Du, 2016). Palaeocurrent directions compiled from provenance analysis of detrital zircons: Grimmer et al, 2003;Deng et al, 2008;Yang et al, 2010Yang et al, , 2012Yan et al, 2011;She et al, 2012;Yao et al, 2012;Cui et al, 2013;Li et al, 2013;Yang & He, 2013;Luo et al, 2014;Su et al, 2014;Hu et al, 2015;Meng et al, 2015;Zhang et al, 2015; (Gao et al, 2017;Liu, Wang, Li, Zi, & Huangfu, 2018;Thanh, Hieu, Minh, Nhuan, & Thuy, 2019;Wang et al, 2013Wang et al, , 2018 and late Late Triassic A-type granites and mafic dikes related to oceanic subduction of the Palao-Pacific regime (Cai et al, 2015;Li, Ma, Liu, & Robinson, 2012;Sun, Ling, Shen, Wang, & Huang, 2017;Wang et al, 2013;Xu, Li, et al, 2019;Yu et al, 2020;Zhu, Li, Xu, Wilde, & Chen, 2016). More and more data imply that the initial subduction of Pale-Pacific Plate commenced at Late Triassic to Early Jurassic at the eastern margin of Asia (Li et al, , 2019Liu, Gurnis, Ma, & Zhang, 2017;Suo et al, 2019;Wang et al, 2013;Xu et al, 2017;Xu, Li, et al, 2019;Xu, Tang, & Lin, 2016;Zhou et ...…”

“…The South China Block was always located along the periphery of a supercontinent and underwent different oceanic subduction since the Neoproterozoic collage of the Yangtze and Cathaysia blocks (Cawood et al, 2018;Metcalfe, 2013;Wang et al, 2013Wang et al, , 2018Zhao et al, 2018;Zhou & Li, 2000). Especially during the Mesozoic period, the South China Block was located in the transition zone of the Tethys and Palaeo-Pacific oceans and changed from the Tethys to the Palaeo-Pacific tectonic regimes (Mao, Cheng, Chang, & Pirajno, 2013;Wang et al, 2013;Xu, Li, Gui, Su, & Zhang, 2019;Xu, Tang, & Lin, 2016;Xu, Zhang, Jia, Shu, & Wang, 2009;Yu, Dai, Zhou, & Xie, 2020;Zhou, Sun, Shen, Shu, & Niu, 2006).…”

“…Mesozoic tectonic transition from the Tethys to Palaeo-Pacific regimes in the South China Block was thought to have occurred in the Middle to Late Jurassic on the base of traditional divisions of Mesozoic igneous rocks, depositional basins, and structural deformation (Xu et al, 2019, c;Zhou et al, 2006). Recently, new ages of initial subduction of the Palaeo-Pacific Plate were proposed, such as Late Triassic to Early Jurassic (Li et al, , 2019Suo et al, 2019;Wang et al, 2013;Xu et al, 2017;Xu, Li, et al, 2019;Xu, Tang, & Lin, 2016;Yu et al, 2020), Middle Permian to Early Triassic Li, Li, Li, & Wang, 2006;, and even to Late Carboniferous (Hu et al, 2012;Shen, Yu, O'Reilly, & Griffin, 2018). Initial subduction ages of the Palaeo-Pacific Plate deduced from igneous rocks and sedimentary rocks in the southeastern coastal area are suspect, because this coastal area may be a Mesozoic exotic terrane (Lin et al, 2018(Lin et al, , 2020Lu et al, 1994;Niu et al, 2015;Yang, 2013).…”