SUMMARY

New interpretations on mineral composition of serpentinite massifs in the
Sông Mã Suture Zone in Sơn La area: Supra-subduction zone ophiolite type

Ngô Xuân Thành, Mai Trọng Tú

The serpentinites in the Sông Mã Suture Zone in Sơn La area are parts of the ultramafic massifs along the Song Ma Suture Zone. They have been considered as parts of the ophiolitic complex formed due to the collision between the Indochina and South China Plates. The rocks are highly serpentinitized and strongly deformed. The chromites remaining in the rocks usually appear in podiform texture that is typical of mantle peridotite, their compositions are analyzed and used for interpreting tectonic environment of the peridotite. The chromites are characterized by medium Cr# and high Mg# (0.43-0.48 and 0.61-0.70, respectively), low Fe³⁺# while high Fe²⁺/Fe³⁺ ratios (0.018-0.040 and 6.52-14.24, respectively), very low TiO₂ content (<0,12 wt%). Olivine compositions show high Mg#. These characteristics together with textures of chromites and serpentines indicate that the serpentinites in the Sơn La area of Song Ma Suture Zone are of mantle part of ophiolites, forming in a supra-subduction zone (SSZ) rather than in that of MOR tectonic setting.

Người biên tập: GS.TS Trần Văn Trị.