Geochronology and geochemistry of the Manxin ophiolitic melange in the Changning-Menglian Suture Zone, southwest China: Implications for the tectonic evolution of the Proto-Tethys Ocean

To date, the evolutionary history of the Proto-Tethys Ocean that separated the Gondwana and Laurasia continents remains ambiguous. The Changning-Menglian Suture Zone assumingly represents the Proto- and Palaeo-Tethys suture in the Sanjiang area (the Yunnan Province, southwestern China). The Palaeozoic Proto-Tethys Ocean in the Manxin area in the southmost Changning-Menglian Suture Zone is widely recognized, but the timing and formation mechanism of the Manxin ophiolitic melange are poorly understood. In this study, we present petrographical, geochronological and geochemical data of the Manxin ophiolitic melange situated in the Changning-Menglian Suture Zone. We found that this ophiolitic melange is composed of pillow basalt, metamorphic gabbro, diabase dike, and altered peridotite. The mafic rocks could be divided into two groups based on geochemical characteristics: (1) Mid-ocean ridge basalt (MORB) [formed at 446.0 +/- 3.2 Ma and exhibiting depleted light rare earth element (LREE) patterns and high sigma(Nd)(t) (+5.7 to +8.4)] Trace element and isotope analyses indicated that the MORB was derived from a spinel-facies depleted mantle. (2) Ocean island basalt (OIB) [crystallized at 460.2 +/- 6.0 Ma]. sigma(Nd)(t) (+5.7) and significant enrichments in large-ion lithophile element (LILE) and light rare earth element (LREE) were identified, while the trace element and isotope systematics suggested that the OIB rocks originated from partial melting of the garnet-spinel lherzolite mantle. Overall, we clearly constrained the timing and nature of the Manxin ophiolitic melange using the proposed data, thus further supporting the notion that the Manxin ophiolites represent remnants of the Proto-Tethys Ocean.