The western boundary between the Yangtze and Cathaysia blocks, new constraints from the Pingbian Group sediments, southwest South China Block

Abstract

The Pingbian Group of southeast Yunnan Province is the only exposed Precambrian unit in the southwestern corner of the South China Block (SCB). The nature of this group is crucial to reveal the basement component in the area, and to further understand Precambrian evolution of the SCB and to constrain the boundary between the Yangtze and Cathaysia blocks. Zircon U-Pb ages indicate that the Pingbian Group sedimentary rocks were deposited later than 772–710 Ma. “Cambrian” Langmuqiao formation sedimentary rocks near the Pingbian Group have the maximum deposition age and age spectra of detrital zircons similar to the Pingbian Group, suggesting that they probably belong to the Pingbian Group. The Pingbian Group sedimentary rocks have variable compositions with low SiO2/Al2O3 (3.7 ∼ 9.7), implying relatively low maturity. Their REEpatterns are characterized by enrichment in LREE with (La/Yb)n of 5.3–10.3. Geochemical and petrologic characteristics of these sedimentary rocks are similar to those Neoproterozoic sedimentary rocks in the southern Yangtze Block. The Pingbian Group sedimentary rocks show unimodal age spectra of detrital zircons with a peak at 844–808 Ma and minor old detritus. The age spectrum resembles those of Neoproterozoic sedimentary rocks in the southern and western Yangtze Block, but different from those in the Cathaysia and Indochina blocks, suggesting the Pingbian Group’s affinity with the Yangtze Block.The integration of U-Pb ages and Hf-isotopes of detrital zircons indicates that there were five main magmatic events occurring in the southwestern Yangtze Block. Early Paleoproterozoic, late Paleoproterozoic and late Mesoproterozoic magmatism mainly involved the reworking of ancient crusts, whereas 1.7–1.5 Ga and 0.9–0.7 Ga magmatic activities were accompanied by the input of significant mantle-derived materials. Continuous Neoproterozoic mantle-derived magmatism suggests a subduction-related tectonic setting. Geochemistry of sedimentary rocks and age distribution of detrital zircons suggest that the Pingbian Group was deposited in the back-arc foreland basin.All observations suggest that the Pingbian Group is similar to those Neoproterozoic sedimentary rocks in the western Yangtze Block, and consequently the Pingbian area should belong to the Yangtze Block. Thus, the boundary between the Yangtze and Cathaysia blocks must be located at east of the study area.