Petrology and geochemistry of the eclogite in Heihe area of Lancang County, western Yunnan: the magmatism of the initial subduction of the oceanic crust

The Ordovician Qianmai ophiolitic melange in the Heihe area of Lancang, western Yunnan, is the southern extension of the Wanhe ophiolitic melange in Mengku region, which is the remnant of the closure of the original Tethyan oceanic basin, where some eclogite tectonic slices of different scales are also developed. The eclogite experienced many stages of retrograde metamorphism in different degrees, with abundant retrograde metamorphic structures. The metamorphic minerals of eclogite facies are garnet, omphacite, polysilicate muscovite and rutile, etc. It is chemically characterized by SiO₂ content of 49.76%~52.71%, TiO₂ content of 1.12%~1.96%, Al₂O₃ content of 14.18%~16.92%, CaO content of 7.91%~10.04%, MgO content of 5.93%~9.14%, Na₂O content of 2.81%~4.77%, K₂O content of 0.09%~1.15% and Mg^# values of 56~69, indicating that the magma has a low degree of differentiation and evolution, close to the primitive magma and similar to front arc basalt(FAB). The distribution curve of rare earth elements inclines gently to the right, and LREE is slightly enriched. The (La/Sm)_N content ranges from 1.13 to 2.05, and (Gd/Yb)_N content ranges from 1.18 to 1.56, similar to that of enriched mid-ocean ridge basalt(E-MORB). The trace elements indicate that the protolith is a set of Nb-enriched basalts from the source area of enriched mantle. In a word, the protolith of retrograde eclogite in the Heihe area belongs to FAB, which is the product of the initial subduction of the oceanic crust, and represents the petrological records of the transformation from expansion to subduction of the original Tethyan oceanic basin.