Where did the Kontum Massif in central Vietnam come from?

Wei Jiang , Jin-Hai Yu , W.L. Griffin , Fangqian Wang , Xiaolei Wang , TrungHieu Pham , DinhLuyen Nguyen

The Kontum Massif (KTM) is the largest Precambrian basement exposure in the Indochina Block, Southeast Asia. The compositions and formation ages of the KTM basement rocks and the paleo-position of the KTM in Proterozoic supercontinents have not been well constrained. Zircon U-Pb-Hf isotopic data of sixteen samples from the KTM in this study indicate that the KTM basement consists mainly of different units of metasedimentary rocks. These sedimentary rocks were deposited in five periods, late Paleoproterozoic (G1, 1.80–1.65 Ga), late Paleoproterozoic – early Mesoproterozoic (G2, 1.74–1.45 Ga), Mesoproterozoic (G3, 1.4–1.1 Ga), late Mesoproterozoic – early Neoproterozoic (G4, 1.1–0.81 Ga), and late Neoproterozoic – early Paleozoic (G5, 0.61–0.51 Ga), and underwent three phases of metamorphism in the early Paleozoic (501–415 Ma), late Paleozoic (371–331 Ma), and Indosinian (272–235 Ma). The G1 and G2 sedimentary rocks have abundant Neoarchean (2.63–2.48 Ga) and late Paleoproterozoic (1.83–1.74 Ga) detritus, and the G3 and G4 sedimentary rocks consist mainly of late Paleoproterozoic (1.74–1.72 Ga) and middle Mesoproterozoic (1.41–1.35 Ga) clastic materials. The G5 sedimentary rocks are characterized by consecutive Mesoproterozoic detrital zircon age spectra with a wide age peak at 1.06–1.01 Ga, much different from the G1 to G4 sedimentary rocks. Most Archean detrital zircons show juvenile Hf isotopes, whereas late Paleoproterozoic, middle Mesoproterozoic, and Grenvillian zircons show large Hf-isotope variations. Most detrital zircons are exotic because coeval igneous rocks have not been identified in the KTM (or elsewhere in Indochina), except ∼1.44 Ga ones.

Comprehensive comparisons of detrital zircon U-Pb, Hf isotopes and sedimentary environments of these five units of sedimentary rocks with the extensive magmatism and coeval siliciclastic rocks in different cratons and microcontinents suggest that most of the Precambrian sediments in the KTM were derived from southwestern Laurentia, and the KTM has maintained a long-time connection with southwestern Laurentia for about one billion years from the G1 to the G4. The KTM was adjacent to Hainan Island and Tasmania during the G3-G4 deposition. The U-Pb ages and Hf isotopes of detrital zircons and sedimentary environment of the G5 sedimentary rocks indicate that the KTM was located on the northern margin of East Gondwana and near South China, India and western Australia, consistent with previous configurations for the Indochina Block. Therefore, the KTM, together with South China and Hainan Island, experienced quick northward movement from southwestern Laurentia to the northern margin of East Gondwana during the G4 (ca 1.10–0.81 Ga) to the G5 (0.61–0.51 Ga), which is also supported by new paleomagnetic data.

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