Geodynamic significance of the Kontum massif in central Vietnam: CompositeAr-40/Ar-39 and U-Pb ages from Paleozoic to Triassic

The Kontum massif of south-central Vietnam has long been classified as lowe r continental crust of Precambrian (Archean) age. U-Pb zircon and Ar-40/Ar- 39 biotite ages from charnockites and amphibolite facies rocks from this ba sement, however, yield much younger ages and imply at least two Paleozoic t hermal events. Syntectonic charnockites from the Kannack complex and the so uthern Kontum massif record Permo-Triassic magmatism (U-Pb: 249 +/-2[2 sigm a] Ma and 253 +/-2 Ma). The former sample yields an Ar-40/(39) Ar age of 24 3 +/-4 Ma (2 sigma), implying rapid cooling to the 250 degreesC isograd fol lowing emplacement into the lower crust. The ages confirm the importance of Permo-Triassic magma-generating orogenesis throughout a large part of east ern Indochina probably related to the closing of the Paleo-Tethys Sea. Olde r ages in amphibolite facies igneous and metamorphic rocks from the western Kontum massif record magmatic emplacement at 451 +/-3 Ma (U-Pb) and possib ly a Paleozoic tectonometamorphic event (similar to 340 Ma) that partially to completely reset the argon systems (40Ar/39Ar: 424 +/-5 Ma, 380 +/-3 Ma, and 339 +/-4 Ma). Crustal thickening and regional heating during suturing of the Indochina block to the South China block along the Song Ma suture zo ne may have produced the 340 Ma low-temperature thermal event, although slo w variable cooling could also explain the argon data. The Kontum massif and Kannack metamorphic complex should henceforth be considered exposures of d eep crustal levels of the Permo-Triassic orogeny in contact with Paleozoic midcrustal rocks. Evidently, the Kontum massif did not rift from a Precambr ian granulite belt in Gondwana.