Chromian-spinel compositions from the Bo Xinh ultramafics, Northern Vietnam: Implications on tectonic evolution of the Indochina block

The Bo Xinh ultramafics, north of the Song Ma fault zone are parts of the isolated bodies that have long been considered as remnants of Paleotethyan oceanic lithosphere between the Indochina and South China blocks. The core compositions of the early igneous stage Cr-spinels are used to deduce the petrogenesis and tectonic environments for the Bo Xinh ultramafics. These spinels are characterized by medium Cr₂O₃content (37.03-39.41wt.%), high contents of Al₂O₃ (29.08-32.01wt.%), FeO (15.08-17.80wt.%) and MgO (14.09-16.13wt.%), and very low TiO₂ content (<0.12wt.%) with medium Cr# (0.44-0.47) and high Mg# (0.62-0.71). They also show low Fe³⁺# and high Fe²⁺/Fe³⁺ ratios of 0.018-0.040 and 5.52-11.24, respectively. The Cr-spinel compositions along with forsterite number (Fo_88-92) suggest that the parental magma was lherzolite to harzburgite in composition, indicating the forearc tectonic environment for the Bo Xinh ultramafics. Thus, the supra-subduction zone nature of ophiolites in the Bo Xinh ultramafics, combined with available information on magmatism, metamorphism and sedimentary environment, suggest the presence of a southward subduction zone since Cambrian and subsequent collision of the Indochina and South China blocks in Late-Silurian to Early-Devonian. This further indicates that Permian to Triassic magmatic and metamorphic events, recorded widely within the Indochina block, should be linked to the northward subduction of the Paleo-oceanic plate beneath the Indochina block, following by Middle to Late-Triassic collision between the Sibumasu and Indochina blocks.