The Tam Ky-Phuoc Son Shear Zone in central Vietnam: tectonic and metallogenic implications

The Tam Ky-Phuoc Son Shear Zone (TKPSSZ) is one of the most pronounced crustal structures in central Vietnam and juxtaposes high-grade rocks of the Kontum Massif against lower grade rocks of the Truong Son Fold Belt. An internal zone comprises highly strained rocks, including ultramafic–mafic tectonites, bounded by mylonitic zones that straddle tens to hundreds of kilometers. An external zone comprises mainly high-grade metamorphic complexes (southern flank), and weakly metamorphosed siliciclastic and carbonate rocks with subordinate greenstone (northern flank). Detailed structural analysis reveals that the shear zone is a multi-deformed terrane: D₁ produced a regional high grade schistosity and mylonitization; D₂ generated regional northwest–southeast trending fold arrays and brittle–ductile shear zones; D₃ locally produced northeast-trending folds; brittle faulting occurred during D₄–D₅. U–Pb dating of zircon and monazite suggests that D₁ involved metamorphism and felsic magmatism at ca. 430 Ma, recording part of a regional collisional orogeny. Monazite and titanite growth at ca. 250–240 Ma in basement rocks is synchronous with widespread syn- to post-D₂ magmatism between ca. 260 and 245 Ma suggesting a second major collisional event during the Indosinian Orogeny. D₃ may have occurred as part of (or soon after) this Permo-Triassic event. Deformation during post-collisional stages, perhaps in response to extrusion/wedging and oroclinal rotation of terranes, led to post-D₃ structures (D₄–D₅). The E–W trending TKPSSZ is here shown to be a continuation of the N–S trending Po Ko Shear Zone (PKSZ). Most of the significant lode gold occurrences in central Vietnam occur along this TKPSSZ–PKSZ structure and are associated with, and controlled by, D₁ ductile to ductile–brittle high-strain zones. Mineralized later-stage structures support remobilization and reconcentration of ore during subsequent events at ca. 240 Ma (Re–Os molybdenite age). The TKPSSZ–PKSZ D₁ structure likely represents part of a paleosuture zone, marking the closure of an ancient ocean basin through terrane assembly in the Early Paleozoic.