Summary

The petrographic - Geochemical characteristics of ultramafic rocks in Tan Lap area, Kon Tum and their geological implication

Phạm Trung Hiếu, Trần Duân, Trần Xuân Hòa, Phạm Minh, Bùi Kim Ngọc

The ultramafic rocks in Tan Lap area consists peridotite, pyroxenite, serpentinite-apodunite and ultramafic- related rocks such as gabbro-amphibolite, anthophyllite, chrysotile. They often reveal lenses-shape or fit with surrounding rocks, ranging from 1.5 to 15 m in wide, between 40 and 150 m long, mainly in the Northeast-Southwest direction.

The geochemistry of Tan Lap ultramafic rocks has related to the upper mantle and the alpine-type ultramafic rocks, which are high MgO, low SiO₂, poor alkali, and very low TiO₂. Behaviors of rare- trace elements in Tan Lap ultramafic rocks are like those of supra-subduction zone (SSZ) rather than Mid-ocean ridges (MOR). The analytical results on the mineral chromespinel show that they are derived from the upper mantle of oceanic lithosphere, which has undergone partial melting with a high melting range ~ 30-35%. Their formation is the fore-arc context of the subduction zone, supporting and supplementing the results of petrographic and geochemical analyses. The Tan Lap ultramafic rocks do not have much meaning in minerals. They can be used for the purpose of mixing fused phosphate fertilizer and protecting the environment.

Keyword: Ultramafic, Tan Lap area, upper mantle, subduction zone, chromspinel.