Résumé
Multi-system geochronology was used to unravel the long and complex thermal history of the Song Chay range (Northern Vietnam), a high-grade granitic and metamorphic dome in the vicinity of the Cenozoic, Ailao Shan–Red River fault zone. It was considered to be Proterozoic South China basement, but its geological history was basically unknown. Scattered field observations suggest three episodes of high-temperature deformation: firstly at the time of granite emplacement, secondly a décollement with top to the north shear and thirdly anticlinal doming of the foliations formed during the two first stages.
P–
T estimates suggest that metamorphism coeval with the second deformation phase culminated at ∼580°C and ∼4.5
kbar (∼16
km depth). Multi-system geochronology is applied to a two-micas granite sample, slightly deformed within the décollement. U/Pb dating of zircon yields an age of 428±5
Ma (±2
σ) interpreted as the time of granite crystallization within the South China ‘Caledonian’ belt. Rb/Sr on white micas and biotite yields ages of 206±10
Ma and 176±5.3
Ma, respectively (2
σ), whereas
39Ar/
40Ar ages of the same minerals are 210±9 and 190±8
Ma (2
σ). These ages suggest an Upper Triassic episode of rapid cooling interpreted as due to doming a few million years after the end of movement on the décollement. The K-feldspar irregular
39Ar/
40Ar age spectrum can, to the first order, be explained by a cooling history with two episodes of rapid cooling: one at ∼140
Ma and a second around 41 to 25
Ma. Apatite fission tracks central age (33.6±3.6
Ma, 1
σ) confirms a Tertiary rapid cooling event interpreted as the final exhumation of the Song Chay dome.