Geochemical constraints on the provenance and depositional setting of Neoproterozoic volcaniclastic rocks on the northern margin of the Yangtze Block, China: Implications for the tectonic evolution of the northern margin of the Yangtze Block

Abstract

Neoproterozoic volcanic and sedimentary rocks, cropping out along the Chengkou–Fengxi Fault on the northern margin of the Yangtze Block in China, consist primarily of volcaniclastic deposits and sandstone–conglomerate successions. The volcaniclastic deposits (termed the Longtanhe Formation) are composed of volcanogenic sedimentary rocks with several primary volcaniclastic rock interlayers. The Longtanhe Formation provides a good opportunity to investigate the Neoproterozoic tectonic evolution of the northern margin of the Yangtze Block. In this paper, geochemistry and zircon U–Pb geochronology are used to infer the provenance and tectonic setting of the Longtanhe Formation volcaniclastic rocks. Two volcanic sandstone samples, collected from the base and top part of the Longtanhe Formation respectively, and one tuff sample were studied for zircon U–Pb dating. They show three consistent age populations with peaks at ∼825Ma, ∼770Ma and 703Ma, indicating that the Longtanhe Formation was deposited after 703Ma and there were no changes in the sources during the period of sedimentation. Chondrite-normalized REE patterns are uniform with slight LREE enrichment (LaN/YbN, average 3.8) and flat HREE patterns without a distinct Eu anomaly (Eu/Eu*, average 0.99), indicating a magmatic arc source. Sc–V enrichment, high La/Th (average 9.4), Cr/Ni (average 2.1) and low Th/Sc (commonly <0.1) suggest that the volcaniclastic rocks were mainly derived from basic to intermediate rocks. Large volumes of basaltic to andesitic clasts in volcaniclastic rocks also suggests that the source rocks were predominantly mafic rocks. A few samples have higher Th/Sc (0.12–0.24) ratios and Hf (4.70–4.90) contents implying minor felsic rocks in the source area. A small amount of quartz clasts in matrix for these six samples also supports a minor contribution from felsic rocks. Not only the lapilli in tuff but also the pebbles and lithic grains in volcanic sandstones are characterized by poor rounding, indicating that the Longtanhe Formation was deposited near-source. Both major- and trace-element discrimination diagrams suggest an oceanic island arc-related setting for the Longtanhe Formation. All these lines of evidence suggest that a contemporaneous (703Ma) magmatic arc, primarily consisting of basalt to andesite, was the main source for the Longtanhe Formation volcaniclastic rocks. The older and minor felsic components were probably derived from mafic to felsic rocks with an arc signature that cropped out widely in the Hannan Complex (840–746Ma) and/or Yaolinghe Group (845730 Ma). Based on its arc-related geochemical affinity, the Longtanhe Formation is proposed to have been deposited in an oceanic arc-related setting, indicating that oceanic crust subduction beneath the northern margin of the Yangtze Block had not ended before 703Ma.