| Abstract: |
The Kontum massif of south‐central Vietnam has long been classified as lower continental crust of Precambrian (Archean) age. U‐Pb zircon and 40Ar/39Ar biotite ages from charnockites and amphibolite facies rocks from this basement, however, yield much younger ages and imply at least two Paleozoic thermal events. Syntectonic charnockites from the Kannack complex and the southern Kontum massif record Permo‐Triassic magmatism (U‐Pb:249±2[2σ] Ma and 253±2 Ma). The former sample yields an 40Ar/39Ar age of 243±4 Ma (2σ), implying rapid cooling to the 250°C isograd following emplacement into the lower crust. The ages confirm the importance of Permo‐Triassic magma‐generating orogenesis throughout a large part of eastern Indochina probably related to the closing of the Paleo‐Tethys Sea. Older ages in amphibolite facies igneous and metamorphic rocks from the western Kontum massif record magmatic emplacement at 451±3 Ma (U‐Pb) and possibly a Paleozoic tectonometamorphic event (∼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 zone may have produced the 340 Ma low‐temperature thermal event, although slow variable cooling could also explain the argon data. The Kontum massif and Kannack metamorphic complex should henceforth be considered exposures of deep crustal levels of the Permo‐Triassic orogeny in contact with Paleozoic midcrustal rocks. Evidently, the Kontum massif did not rift from a Precambrian granulite belt in Gondwana.
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