Petrogenesis of Eocene to early Oligocene granitic rocks in Phan Si Pan uplift area, northwestern Vietnam: Geochemical implications for the Cenozoic crustal evolution of the South China Block

Abstract

The Phan Si Pan uplift area of NW Vietnam is a part of the Archean to Paleoproterozoic Yangtze Block (SW China) that was displaced along the Ailaoshan-Red River Fault shear zone and adjacent structures (i.e., Song Da zone). The region is of interest as it is at the forefront of the debate regarding the extrusion of SE Asia during the Early Paleogene. Two Cenozoic silicic plutonic complexes of contrasting composition intruded the basement rocks of the Phan Si Pan uplift area. The Ban Xeo granite is ferroan (A-type), alkalic and yielded a zircon weighted-mean 206Pb/238U age of 49 ± 0.9 Ma. The major and trace element compositions of the Ban Xeo granite are similar to the spatially associated Late Permian Muong Hum granites. The εNd(t) values (−2.5 to −1.4), 87Sr/86Sr initial ratios (0.70793, 0.70989, and 0.75341), and zircon εHf(t) values (−1.1 to +3.0) indicate that the Ban Xeo granite is likely derived by differentiation of a mafic parental magma that had EMII-like mantle source characteristics. The younger Ye Yen Sun (YYS) granite is magnesian, alkali-calcic to calc-alkalic, and yielded zircon U–Pb weighted-mean ages of 35.3 ± 0.5 to 34.1 ± 0.4 Ma. The YYS granites have 87Sr/86Sr initial ratios of 0.70636 to 0.70860, εNd(t) values from −8.7 to −2.6, and zircons εHf(t) values from −9.3 to +0.6 which indicate a crustal origin. The YYS dioritic rocks range from syeno-diorite to granodiorite, and have similar Sr (87Sr/86Sr = 0.70627) and εNd(t) (−3.8) values as the YYS granites. However, the zircon Hf isotopes (εHf(t) = +0.3 to +3.2) of the syeno-diorite are slightly more chondritic than the granites and indicates that the parental magma was likely derived from a mantle source. Rhyolite–MELTS modeling results show that the YYS granites were likely generated by fractionation of a parental magma similar in composition to the granodiorite. Furthermore, the YYS syeno-diorites may be representative of a cumulate rock that developed during the early stages of emplacement or crystallization. The parental magma of YYS igneous rocks likely formed by partial melting of mafic Mesoproterozoic juvenile rocks (lower crust/uppermost mantle) by heat from the upwelling lithospheric mantle during a period of crustal relaxation. The formation of the Ban Xeo granites and Ye Yen Sun igneous rocks are likely related to lateral melt migration along the eastern edge of the India-Asia collision zone. It is possible that repeated episodes of Cenozoic magmatism was pivotal in the development and initiation of fault movement of the Ailaoshan-Red River shear zone.