Zircon U–Pb ages of Permian–Triassic igneous rocks in the Song Hien structure, NE Vietnam: The Emeishan mantle plume or the Indosinian orogeny?

The Permian to Triassic tectonic evolution of the Song Hien structure in NE Vietnam is poorly understood and if the Emeishan plume or the Indosinian orogeny contributed to Song Hien magmatism is controversial. Reliable estimates of the timing of magmatic activity in the region could help in clarifying this disputed history. However, very few U-Pb zircon ages are available, particularly for mafic–ultramafic intrusions. This study aimed to provide additional information on the timing of magmatism in this region. Sensitive-high-resolution-ion-microprobe (SHRIMP) and laser-ablation-inductively-coupled-mass-spectrometry (LA-ICP-MS) were used to obtain U-Pb ages for zircons from ultramafic to mafic, mafic to intermediate, and felsic intrusions and felsic volcanic suites within the Song Hien structure. Ultramafic to mafic intrusions consist of plagioclase-bearing lherzolite and olivine gabbro. Mafic to intermediate intrusions are composed of monzogabbro and quartz monzonite. There are granodioritic and granitic intrusions and rhyolite suites. Ultramafic to mafic rocks have ages of 252.2 ± 2.5 Ma to 246.5 ± 1.0 Ma, whereas mafic to intermediate rocks have ages of 250.1 ± 1.9 Ma to 246.7 ± 1.6 Ma. Felsic rocks have ages of 248.4 ± 2.6 Ma to 247.6 ± 1.5 Ma.. Our new zircon U-Pb ages indicate a peak of magmatic activity at 247–248 Ma, that generated both mafic and felsic units. Additionally, U-Pb ages of single zircon grains from both mafic and felsic suites show two similar peaks at 250 Ma and 241–243 Ma. These peaks coupled with data from the internal structure of age-patterned grains may reflect the continuous growth of zircons in magma chambers. The results further confirm that igneous activities in the Song Hien structure were 12 Myr younger than the Emeishan mantle plume. Therefore, this magmatism in the Song Hien structure is not temporally correlated with the Emeishan plume. We suggest that the studied magmatism in the Song Hien structure traced the Triassic subduction/collision inversion related to the Indochina–South China amalgamation (the Indosinian orogeny). The 247–252 Ma interval could correspond to its pre-collision stage. Based on previously published and newly received regional age data, we suggest that the timing of events linked to the Indosinian orogeny in northern Vietnam may be described as: pre-collision (245–255 Ma), syn‐collision (230–245 Ma, with the main peak at 230–240 Ma), and post-collision (200–230 Ma).