Late Permian to Early Oligocene granitic magmatism of the Phan Si Pan uplift area, NW Vietnam: their relationship to Phanerozoic crustal evolution of Southwest China

Abstract

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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). This area is of particular interest because it experienced a number of Phanerozoic crustal building events, including the Emeishan large igneous province, the India-Eurasia collision, and Ailaoshan – Red River Fault displacement. In the Phan Si Pan uplift area, there are at least three different geochronological complexes, including (1) Late Permian, (2) Eocene, and (3) Early Oligocene. (1) The Late Permian silicic plutonic and volcanic rocks are correlated with the Emeishan large igneous province (ELIP). The silicic rocks of the Phan Si Pan uplift area, and Tu Le basin are ferroan alkalic and have zircon CA-ID-TIMS weighted-mean ages of 256.3 ± 0.4 Ma to 257.3 ± 0.1 Ma for granite and 257.1 ± 0.6 Ma to 257.9 ± 0.3 Ma for rhyolite. The high precision CA-ID-TIMS of Phan Si Pan – Tu Le silicic rocks suggest that Emeishan lavas erupted over a period of ~6 million years with plutonism ending shortly thereafter. Granitic rocks from Phan Si Pan uplift and Tu Le basin have variable ISr values ranging between 0.70214 and 0.70357 that could be the result of significant fractional crystallization (i.e., highly depleted Sr) or mobility of Rb, and εNd(t) values are broadly chondritic (εNd(t) = 0 ± 1). Zircon εHf(t) values range from +3.3 to +8.8 suggesting the granitic rocks of Phan Si Pan uplift and Tu Le basin are derived from a mantle source. Reported fractional crystallization modeling suggests the Phan Si Pan – Tu Le silicic rocks can be produce from Emeishan high-Ti basaltic rocks. (2) 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. (3) 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 zircon 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.