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The present-day Mekong River is the twelfth longest river in the world. It drains the Tibetan Plateau and forms a large delta in south Vietnam. Remnants of upper Cenozoic fluvial to marginal marine proto-Mekong sediments are exposed in the Da Lat Zone in southeast Vietnam and are likely the on-land equivalents of large sediment packages offshore in the Cuu Long and Nam Con Son Basins. Provenance studies are used here to identify sources that allow reconstruction of sediment pathways. The Oligo-Miocene Di Linh Formation has a main Cretaceous zircon age population and subordinate Paleoproterozoic (c. 1.8-1.9 Ga) zircons, sourced mainly from Cretaceous plutons. In contrast, the early Pliocene to Pleistocene Song Luy Formation includes additional Permian–Triassic and Ordovician–Silurian age populations which are interpreted to be sourced from basement rocks in central and northern Vietnam. There is a significant change in provenance between these formations, interpreted as an intra-Miocene unconformity. The late Pliocene to Pleistocene Ba Mieu Formation shows a much greater abundance of Precambrian zircons, including a small age peak at c. 2.5 Ga, interpreted to be derived from old basement exposed in South China. Abundant Cretaceous zircons and a lack of Jurassic zircons in all three formations contrast with the present-day Mekong River delta. Therefore, the data are proposed to show a development of the proto–Mekong River progressing northward from the Da Lat Zone, to the Kontum Massif and northern Vietnam, to capturing of reversed rivers and/or reworking of sediments from old cratons in South China.

The Da Lat Zone in SE Vietnam consists of a Mesozoic orogenic belt that is characterised by abundant Cretaceous igneous rocks that were intruded into or deposited on Permo-Triassic rocks (represented by the Chau Thoi Formation) and Jurassic metasedimentary rocks. The three Cretaceous magmatic phases of the Dinh Quan, Deo Ca, and Ankroet suites of the Da Lat Zone were previously subdivided on the basis of geochemical and isotopic differences and related to within-plate and continental arc magmatism during Paleo-Pacific subduction. Limited dating gave varied estimates for crystallisation ages of the different suites. Here we present new whole-rock chemical data and LA-ICP-MS U-Pb zircon analyses for Cretaceous plutonic and volcanic rocks. The results indicate volcanic arc magmatism started in the Early Cretaceous, represented by the Dinh Quan suite at c. 113–135 Ma, and continued into the mid to Late Cretaceous (Deo Ca, Nha Trang, Dak Rium and Don Duong formations) with magmatism from c. 90 to 109 Ma. Granitoid basement rocks of the Cuu Long Basin yielded similar ages of c. 90 to 92 Ma, representing offshore equivalents. The Ankroet suite represents the youngest magmatic phase and is interpreted as within-plate magmatism from c. 75 to 87 Ma. The Cretaceous rocks generally have very few older zircons and their rarity is interpreted to indicate magmas were generated predominantly by melting of juvenile arc crust.

Cordilleran-type batholiths are useful in understanding the duration, cyclicity and tectonic evolution of continental margins. The Dalat zone of southern Vietnam preserves evidence of Late Mesozoic convergent zone magmatism superimposed on Precambrian rocks of the Indochina Block. The Dinhquan, Deoca and Ankroet plutons and their enclaves indicate that the Dalat zone transitioned from an active continental margin producing Cordilleran-type batholiths to highly extended crust producing within-plate plutons. The Deoca and Dinhquan plutons are compositionally similar to Cordilleran I-type granitic rocks and yield mean zircon U/Pb ages between 118 ± 1.4 Ma and 115 ± 1.2 Ma. Their Sr–Nd whole rock isotopes (ISr = 0.7044 to 0.7062; eNd(T) = − 2.4 to + 0.2) and zircon Hf isotopes (eHf(T) = + 8.2 ± 1.2 and + 6.4 ± 0.9) indicate that they were derived by mixing between a mantle component and an enriched component (i.e. GLOSS). The Ankroet pluton is chemically similar to post-orogenic/within-plate granitic rocks and has a zircon U/Pb age of 87 ± 1.6 Ma. Geobarometric calculations indicate that amphibole within the Ankroet pluton crystallized at a depth of ~ 6 kbar which is consistent with the somewhat more depleted Sr–Nd isotope (ISr = 0.7017 to 0.7111; eNd(T) = − 2.8 to + 0.6) and variable eHf(T) compositions suggesting a stronger influence of crustal material in the parental magma. The compositional change of the Dalat zone granitic rocks during the middle to late Cretaceous indicates that the tectonic regime evolved from a continental arc environment to one of post-orogenic extension. The appearance of sporadic post-90 Ma magmatism in the Dalat zone and along the eastern margin of Eurasian indicates that there was no subsequent orogenic event and the region was likely one of highly extended crust that facilitated the opening of the South China Sea during the latter half of the Cenozoic.

Late Mesozoic granitoids of the Dalat zone are of sub-alkaline affinity, belong to the high-K calc-alkaline series and display features typical of I-type granites. The Dinhquan suite consists of hornblende-biotite granodiorites, diorites, and minor granites emplaced at ∼110 My. These rocks have relatively low initial 87 Sr/ 86 Sr ratios (0.7049–0.7061) and moderate e Nd(T) values (−0.8 to −2.0). Chondrite-normalized REE patterns are fractionated and have small negative Eu anomalies (Eu/Eu*=0.55–0.97). All these characteristics, combined with low Al 2 O 3 /(FeO+MgO+TiO 2 ) and (Na 2 O+K 2 O)/(FeO+MgO+TiO 2 ) ratios and high Mg# values, suggest an origin through dehydration melting of alkaline mafic lower crustal source rocks. The Cana suite contains biotite-bearing granites poor in hornblende. The rocks are 96–93 My old in age, having higher initial 87 Sr/ 86 Sr ratios (0.7060–0.7064) and nearly constant e Nd(T) (−2.5 to−2.7) values. These characteristics, in conjunction with the chemical signatures and old T DM model ages, indicate that crustal material played a very important role in their petrogenesis. The granites are further characterized by strong negative Eu anomalies (Eu/Eu*=0.04–0.39) and Sr, suggesting melting with residual plagioclase and/or a high degree of fractional crystallization. The Deoca suite is made up of 92–88 My old pink porphyritic hornblende-biotite-bearing granodiorites, monzogranites and diorites. Initial isotopic compositions ( 87 Sr/ 86 Sr=0.7055–0.7069; e Nd(T) =+0.9 to −3.3) and chemical features suggest derivation by dehydration melting of heterogeneous metagreywacke-type sources with additional input of mantle-derived material. Furthermore, the Deoca rocks have concave-upward REE patterns indicating that amphibole played a dominant and garnet an insignificant role during magma segregation.

Mantle xenoliths entrained in the Neogene basalts at central and southern Vietnam provide important constraints on the nature of the lithospheric mantle beneath the eastern Indochina block. The basalts contain mantle xenoliths (garnet lherzolite, spinel lherzolite, harzburgite, and eclogite), cumulate xenoliths (wehrlite, websterite, and pyroxenite) and megacrysts (olivine, Al-rich clinopyroxene, orthopyroxene, anorthoclase, Ti-amphibole, phlogopite, sapphire, and zircon). The mineral compositions of spinel lherzolite xenoliths studied are typical of mantle phases: Fo-rich olivine, En-rich orthopyroxene, Di-rich clinopyroxene and Cr-rich spinel. Concentrations of trace elements for clinopyroxene were determined in situ by laser ablation ICP-MS. Clinopyroxenes exhibit large compositional variations ranging from LREE-depleted (LaN/YbN = 0.5–0.7) to cryptically metasomatized LREE-enriched (LaN/YbN = 6.7–20.3). Slightly LREE-depleted REE patterns with YbN of 10 indicate these peridotites were originated from very low degree of partial melting. However, most REE patterns present much lower HREE concentrations with YbN of 2–6 suggesting these peridotites were products of either high degree of partial melting or the residual from the removal of garnet. Spidergrams of most clinopyroxenes show Ba, Nb–Ta, Hf–Zr and Ti depletion. Therefore, the hydrous silicate melts/fluids of subduction zone origin are likely candidates for the required metasomatic agent. The subcontinental lithospheric mantle beneath central and southern Vietnam was complicated and heterogeneous.