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Petrogenesis and geodynamic implications of the late Triassic bojites in Yajiangqiao area, Hunan Province, South China
Author(s) -
Yu Yushuai,
Dai Pingyun,
Zhou Yun,
Xie Fei
Publication year - 2020
Publication title -
island arc
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.554
H-Index - 58
eISSN - 1440-1738
pISSN - 1038-4871
DOI - 10.1111/iar.12370
Subject(s) - zircon , petrogenesis , geology , geochemistry , lithophile , mafic , pluton , rare earth element , geochronology , partial melting , biotite , rare earth , radiogenic nuclide , igneous rock , tectonics , mineralogy , mantle (geology) , quartz , paleontology
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 post‐collisional 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 post‐collisional extension in the late Triassic (~ 234 Ma) and the post‐collisional extension lasted until 215 Ma.