Emplacement and metamorphism of the mafic rocks from the Chencai terrane within the Cathaysia Block: Implications for the Paleozoic orogenesis of the South China Block
Understanding the Early Paleozoic tectonic evolution of the South China Block (SCB) is critical in constraining its relationship with (super-) continent configurations, such as the Rodinia and Gondwana. High-grade mafic and supracrustal rocks occurring in the Chencai terrane of the Cathaysia Block are the uplifted lower crustal components of the Kwangsian (Early Paleozoic) Orogen which extensively reshaped many parts of the SCB. Two amphibolites, three biotite-bearing amphibolites, one norite and one mafic granulite were collected from the Chencai terrane. In previous studies, these rocks (or their protoliths) have been suggested to belong to the same cumulate series and the mafic granulite records metamorphic P-T conditions of a high-geothermal-gradient environment (∼30℃/km). Our SIMS zircon U-Pb results indicate that protoliths of these meta-mafic rocks were emplaced at ∼434-445 Ma and most of them have experienced subsequent (almost synchronous) metamorphism at 428-439 Ma. The norite sample (434 Ma), which experienced the least crustal contamination and the least metamorphic alteration, has the lowest zircon δ18O values of ∼ +6.7‰ (V-SMOW), whereas the mafic granulite which experienced the highest grade metamorphism has the highest zircon δ18O values of ∼ +9.4‰ (V-SMOW). Zircon δ18O values of other samples mostly scatter between them. Most metamorphic zircons have younger U-Pb ages and higher δ18O values than magmatic zircons. High δ18O crustal materials (e.g. sedimentary rocks) have possibly been involved during the metamorphism which resulted in the contrasting δ18O values of magmatic and metamorphic zircons. The large variations in δ18O values shown by magmatic zircons of each sample (like the amphibolite and biotite-bearing amphibolite) indicate that the high-δ18O-value crustal materials (sedimentary rocks) must have started to affect these mafic rocks since the magmatic stage. Zircon 176Hf/177Hf values of each of these samples scatter within small ranges. Initial ɛHf(t) values of most zircons fall in between the CHUR line and the DM linear model line, indicating that these mafic rocks were derived from an enriched mantle source. Combined with previous geochronological, metamorphic and geochemical data, we propose that these Paleozoic mafic magmatism might have contributed thermal energy to the later thermal relaxation of the orogeny. This episode of mafic magmatism may represent a transition of the Kwangsian Orogen from early compression to post-orogenic extension. Metamorphism of these mafic rocks and the synchronous anatexis indicated by the high grade supracrustal rocks from Chencai terrane, might be related with extension rather than orogenic compression.