[S-MP25] Oceanic and Continental Subduction Processes

[SMP25-P04] Geochemistry and geochronology of Cretaceous volcanic rocks from the Dalat Zone, Southeastern Vietnam: implications for the subduction process and crustal evolution

*Kenta Kawaguchi¹, Pham Trung Hieu^2,3, Pham Minh^2,3 (1.Graduate School of Advanced Science and Engineering, Hiroshima University, 2.Faculty of Geology, University of Science, Ho Chi Minh City, Vietnam, 3.Vietnam National University, Ho Chi Minh City, Vietnam)

Keywords:Geochemistry, Zircon U–Pb and Lu–Hf geochronology, Volcanic rock, Vietnam, Eastern margin of the Asian continent, Cretaceous

The Dalat Zone is a Late Mesozoic plutonic-volcanic orogenic belt located across southeastern Vietnam, which is part of the Indochina Block. Volcanic rocks of the Dalat Zone are divided into three complexes; the Deo Bao Loc, Don Duong, and Nha Trang Complex based on the field occurrences and petrological differences. The geochemical and geochronological characteristics of the volcanic rocks of the Dalat Zone are loosely constrained compared with those of the plutonic rocks, hence, we have conducted whole-rock geochemical investigation coupled with zircon U–Pb dating and Lu–Hf isotope analysis of the volcanic rocks of the Dalat Zone to characterize the timing of magmatism and tectonic setting. The studied volcanic rocks from three complexes consist compositionally of basaltic andesite, andesite, dacite, and rhyolite with minor trachydacite. Geochemically, the studied volcanic rocks commonly belong to high-potassium to shoshonite series with A/CNK values mostly less than 1.1 and increase of Rb is response in the increase of Y and Th, suggesting I-type affinity. Trace element contents of studied volcanic rocks, especially the depletion degrees of Nb and Ta show similar patterns with those of the subduction related arc magmatic rocks. Most of the studied volcanic rocks are characterized by non-adakitic nature represented by high Y contents (17–40 ppm) with low Sr/Y ratios (2–25), whereas minor studied volcanic rocks show adakitic nature characterized by low Y contents (10–11 ppm) and high Sr/Y ratios (46–61). Non-adakitic rocks are moderately depleted in HREEs with convex-downward patterns between MREEs and HREEs in the C1 chondrite-normalized diagram, suggesting the presence of residual hornblende, whereas the adakitic rocks are highly depleted in HREEs suggesting their residual garnet. Adakitic rocks are depleted in Ni (3–6 ppm), Cr (7–15 ppm), and MgO (1.02–1.17 wt%) while enriched in K₂O (2.70–2.87 wt%) suggesting that their compositions are similar with those of the adakitic rocks generated by partial melting of garnet-bearing thickened lower crust. Weighted mean ²⁰⁶Pb/²³⁸U ages of zircons from the Deo Bao Loc Complex show 109.0 ± 3.3 Ma, and those from the Don Duong Complex yield 100.3 ± 0.8 Ma and 96.3 ± 0.8 Ma. In addition, four rhyolites from the Nha Trang Complex give weighted mean ²⁰⁶Pb/²³⁸U ages of 99.3 ± 0.7 Ma, 95.4 ± 1.4 Ma, 90.5 ± 0.6 Ma, and 90.2 ± 1.1 Ma. Some of the studied volcanic rocks contain Precambrian–Paleozoic inherited zircons with scattered dates. The studied volcanic rocks show whole-rock ε_Nd(t) values of −6.3 to +0.9 (average = −1.2) and zircon ε_Hf(t) values of +0.6 to +11.1 (average = +5.5) and their corresponding whole-rock Nd model ages of ca. 1410–830 Ma and two-stage zircon Hf model ages of ca. 1120–450 Ma, indicate that they might mostly derived from partial melting of Precambrian crustal materials with minor input of mantle-derived melt during magma formation and differentiation during ca. 110–90 Ma. Although the basements of the Cretaceous magmatic rocks of the Dalat Zone are unclear, geochemical and isotopic characters suggest that the host crusts are the Precambrian basements of the Indochina Block.