Early Paleozoic Subduction in Cathaysia (I):New Evidence from Nuodong Ophiolite

Peng Songbai; Liu Songfeng; Lin Musen; Wu Changfeng; Han Qingsen

doi:10.3799/dqkx.2016.065

Abstract：The nature of Early Paleozoic Caledonian orogeny (Guangxi movement),as one of the key geolocial problems of tec-tonic evolution in South China,has been controversial for a long time.In this study we report a suite of deformed and metamor-phosed basic rocks in tectonic slices that are exposed in epimetamorphic Ordovician strata,located at the Nuodong in the Cenxi area of the Southeast Guangxi,South China.This suite of rock is mainly composed of metamorphic basalt,diabase dike and mi-nor pyroxenite.Petrogeochemical data of the metabasalt and metadiabase dike show the content of SiO 2 =49.20%-52.30%, MgO=6.78%-9.1 1%,Mg# =55 -63,TiO 2 =1.02%-1.34%,indicating the low potassium tholeiitic series of basic rocks. The chondrite-normalized REE distribution patterns are flat type with light rare earths depleted,no obvious Eu anomaly,(La/Sm)N =0.72-1.05,(La/Yb)N =0.7-1.0,while the N-MORB normalized spider diagrams show slightly enrichment of LILEs (e.g.Rb,Ba,Th,U,K,Pb)and flat-slightly depletion of HFSEs (e.g.Nb,Ta,Zr,Hf,Ti),Ti/V = 21.30- 25.12, Nb/Th=2.1-4.2.The geological and petrogeochemical features imply that this series of rocks represent a dismembered ophio-lites formed in supra-subduction zone (SSZ)tectonic environment.The LA-ICP-MS zircon U-Pb age of the metadiabase is 437± 5 Ma,indicating that the ophiolites were formed at the Early Silurian.Based on the above comprehensive study,we propose that the Nuodong metamorphic basic rock series are Early Paleozoic ophiolite fragments formed in a forearc tectonic environ-ment in a supra-subduction zone (SSZ).The document of the Early Paleozoic Nuodong ophiolite provides key evidence for the existence of an Early Paleozoic oceanic basin and subduction-accretionary collisional orogenesis between Yangtze craton and Cathaysia blocks in South China.