Petrogenesis And Geological Implications Of The Mashan Shoshonitic Complex In NE Guangxi(SW China)

Middle Jurassic magmatic rocks are widely distributed in the South China Block. Previous studies mainly focus on the Middle Jurassic granitic rocks in South China Block, and the Middle Jurassic mafic rocks, however, effective carriers for probing the mantle nature, have been poorly reported. Thus, what dynamic processe the deep mantle had undergone during Middle Jurassic in the South China Block is still a pending problem. The Mashan complex is located in the Liuwan Mountain and composed of ultramafic, mafic, intermediate and acid igneous rocks. In this study, we conducted new zircon U-Pb geochronological, whole-rock elemental and Sr-Nd isotopic analyses for the basalt-andesite and monzonite-syenite from the Mashan complex, to provide more evidences restricting the tectonic setting of South China Block during Middle Jurassic.The Mashan basalt-andesite yield a zircon U-Pb age of 158.6±3.0 Ma. They are typical shoshonitic rocks with high Na2O+K2O contents(5.29-8.81 wt.%) and K2O/Na2 O ratios(0.5-2.0). The Mashan basalt-andesite show OIB-like primitive mantle-normalized spider diagram and chondrite-normalized REE patterns with insignificant Nb-Ta negative anomalies. They possess uniform Sr-Nd isotopic compositions with(87Sr/86Sr)i= 0.706512~0.70693 and εNd(t)= +1.01-+2.35. These OIB-like rocks are generated by a low degree partial melting of spinel-facies peridotite.The Mashan monzonite-syenite yield a zircon U-Pb age of 157.2±2.4 Ma. The Monzonite samples show low SiO2(54.22-55.54 wt.%) contents,but high Al2O3(16.71-17.92 wt.%), K2O(4.07-4.48 wt.%) and K2O+Na2O(7.93-8.33 wt.%) contents. The syenite samples display higher SiO2(60.08-61.20 wt.%), Al2O3(18.64-19.34 wt.%) and K2O(5.12-5.39 wt.%) contents. Both of them are classified as metaluminous shoshonitic rocks. The Mashan monzonite-syenite show OIB-like primitive mantle-normalized spider diagram and REE patterns with obvious Nb-Ta and Ti negative anomalies. The monzonite possess Sr-Nd isotopic compositions of(87Sr/86Sr)i= 0.706083~0.706965 and εNd(t)= +1.01~+2.35. The syenite possess Sr-Nd isotopic compositions of(87Sr/86Sr)i= 0.705001~0.706355 and εNd(t)= +1.12~+2.00. The Mashan monzonite and syenite samples show similar elemental and Sr-Nd isotopic compositions, suggesting the same mantle source for them. Based on our data and previous studies, we argue for the partial melting of spinel-facies lithospheric mantle for the petrogenesis of the Mashan Monzonite-Syenite.We summarized the previous age data in the South China block and recognized a significant Middle Jurassic magmatic flare-up. In combination with previous studies, we proposed that the Mashan complex was generated in an intra-plate setting and related to the asthenosphere upwelling in response to the thinning of the South China lithospheric mantle. The zircon U-Pb ages(157-159 Ma) of the Mashan complex indicate the massive asthenosphere upwelling in South China block has begun in the Middle Jurassic.