The Formation And Reworking Of The Badu Crystalline Basement In Southwest Zhejiang

The Badu Complex in southwest Zhejiang is the most representative Paleoproterozoic basement in the Cathaysia block.Li this paper,we present integrated structural,petrological,geochemical and geochronological studies on gneisses,leucosomes and granites from the Badu Complex.Our results,together with published data,aid in evaluating the growth and reworking of the Cathaysia crystalline basement and their relationship with the assembly of the Columbia supercontinent and the Indosinian orogeny in South China.The LA-ICP-MS zircon U-Pb dating of biotite gneisses and paragneisses shows that the Badu Complex xmderwent 1890-1850 Ma metamorphic reworking.The Paleoproterozoic granites that intruded into the paragneisses of the Badu Complex can be divided into S-and A-type granites.The S-type granites,formed at 1929-1861 Ma,exhibit high SiO2 and K2O contents,high A/CNK ratios(>1.1),and highly negative ?Hf(t)values of(-40-+6.9)and low Zr saturation temperatures(715?849?).In contrast,the A-type granites,formed at 1884?1832 Ma,exhibit low SiO2 and A/CNK ratios,high HFSE(such as Zr,Nb,Ce,Y,etc.)contents,10 000*Ga/Al ratios and Zr saturation temperatures(867?871?).Their ?Hf(t)values range from-37.6 to+7.5,similar to those of the S-type granites,indicating the similar source derived from the Paleoproterozoic supracrustal meta-sedimentary rocks.Based on our results and published metamorphic and magmatic data,the Paleoproterozoic tectonic evolution of the Badu complex can be divided into two stages:(1)the early stage(1.89-1.86 Ga)of granulite facies metamorphism and associated crustal anatexis(including S-type granites and leucosomes),reflecting crustal thickening associated with continental collision,and(2)the late stage(<1.85 Ga)of emplacement of A-type granites,related to post-orogenic collapse and crustal extension.In addition to the Paleoproterozoic metamorphism and magmatism event,a NE trending ductile shear zone that documents Triassic tectonic overprinting has been identified in the Badu complex.The mylonitic foliation strikes NE-SW striking,and the lineation plunges gently to SSW.Kinematic indicators suggest a dextral strike-slip shear sense.We document the shearing deformation to have occurred at?250-230 Ma under amphibolite facies conditions at temperatures of?450-600?.These results suggest that the Badu Complex underwent intensive tectonic reworking during the Early-Middle Triassic,coevally with regional generations of NE trending dextral shear zones,E-striking folds and an angular unconformity between Middle and Upper Triassic strata.The Triassic tectonic event may be related to the subduction and collision between the Indochina and South China blocks.