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岩石学报 2010
Geochemistry and Sr-Nd-Pb istopic features of the Ganlongtang-Nongba ophiolite from the Changning-Menglian suture zone
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Abstract:
This paper presents the major- and trace elements and Sr-Nd-Pb isotopes of the meta-basalts from the Ganlongtang-Nongba ophiolitic mélange which is situated at the middle part of the Changning-Menglian Paleo-Tethys suture zone. The Nongba basalts are mainly composed of alkaline and tholeiitic basalts. The tholeiitic basalts have high TiO2 and Th/Yb, low K2O and Zr/Nb and depleted light rare earth elements (LREEs), and show(La/Yb)N ratio (1.87~2.38) and Sr-Nd-Pb isotopes similar to those of enriched mid-ocean ridge basalt (E-MORB), suggesting that they may originate from an enriched mantle source or a mixture of depleted asthenosphere mantle and mantle plume. The Nongba alkaline basalts have high TiO2 (2.38%) and K2O (2.37%) and (La/Yb)N (~11), similar to that of the oceanic island basalts (OIB), indicating that they were derived from a hot spot through the oceanic crust. The Ganlongtang tholeiitic basalts show typical characteristics of N-MORB in terms of TiO2, Mg#, K2O and LILEs, but have variable Sr-Nd-Pb isotopic compositions probably resulted from crustal contamination. It is proposed that the Changning-Menglian ancient ocean basin experienced a complete growth process from an embryonic to a mature ocean basin.
