The accretion history of the South China Block at its northwest margin in the Neoproterozoic: Records from the Changba complex in the Mianlue zone

Abstract

To constrain the accretion history of the South China Block (SCB), a geochronological and geochemical study was carried out for the Changba Complex within the Mianlue zone in the northwestern margin of the SCB. Zircon U-Pb dating reveals that the metagabbros and metatonalites were formed at ca. 985–950 Ma, while the bt-amp gneisses have a younger protolith age of ca. 830 Ma. The metagabbro and metatonalite are low- to medium-K tholeiitic series and are significantly enriched in LILEs (Rb, Ba, Th, U, K) and slightly enriched in LREEs, but depleted in HFSEs such as Nb, Ta and Ti. In addition, they have highly positive zircon εHf(t) values (+11.3~ +16.9) and whole-rock εNd(t) values (+3.9~ +5.5) and εHf(t) values (+11.0 ~ +11.8). These geochemical features resemble to those of the magmatic rocks in juvenile oceanic arcs. Zircon δ18O value ranges from 3.0 to 6.2‰ for metagabbro and 4.6 to 5.34‰ for metatonalite, which are slightly lower than the mantle values. Hydrothermally altered gabbros in the lower oceanic crust often have lower δ18O values than mantle and can be primary components of arc lower crust. Therefore the Changba tonalite is interpreted to have been formed by partial melting of the hydrothermally altered gabbroic lower crust. The ca. 830 Ma gneisses are enriched in LREEs and incompatible elements and depleted in HFSEs, similar to the trace element pattern of the average bulk continental crust. They have enriched Sr-Nd-Hf isotopic compositions (0.7055 ~ 0.7060, −5.1 ~ −6.3, and −8.5 ~ −8.8, respectively), which is distinct from the juvenile arc crust. Besides, variable zircon εHf(t) values of −40.6 to 0 and slightly low to elevated zircon δ18O values of 4.2 to 6.6‰, suggesting derivation from reworking of a heterogeneous crustal source. In this regard, the Changba complex records the accretion of juvenile island arc and subsequent reworking of both juvenile and ancient crustal materials, which highlights the crustal growth and geodynamical evolution of the northern margin of the South China Block during the Neoproterozoic in response to the assembly of Rodinia.