Geochemical, Sr–Nd–Pb, and Zircon Hf–O Isotopic Compositions of Eocene–Oligocene Shoshonitic and Potassic Adakite-like Felsic Intrusions in Western Yunnan, SW China: Petrogenesis and Tectonic Implications

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Abstract

Coeval potassic adakite-like and shoshonitic felsic intrusions in the western Yunnan province of SW China are spatially and temporally associated with Eocene–Oligocene shoshonitic mafic volcanic rocks. The shoshonitic syenite and quartz monzonite intrusions are characterized by high K2O contents (4·9–6·8 wt %) and K2O/Na2O ratios (1·1–1·7), high Y (1·7–34·8 ppm) and Yb (1·50–3·16 ppm) contents, nearly flat heavy rare earth element (HREE) patterns and moderate Eu anomalies (Eu/Eu* = 0·65–0·78). The potassic adakite-like granite and quartz monzonite intrusions are characterized by enrichment in light rare earth elements (LREE), depletion in HREE and fractionated HREE patterns, high Sr (328–1423 ppm), Sr/Y (38–243) and La/Yb (23–62), and low Y and Yb contents. The shoshonitic syenite and quartz monzonites have the same Sr–Nd–Pb isotope compositions as the shoshonitic mafic volcanic rocks. They define linear trends on Harker diagrams, and have similar REE and trace element patterns to the shoshonitic mafic volcanic rocks. These observations suggest that the shoshonitic syenite and quartz monzonite magmas were differentiated from parental shoshonitic mafic melts by fractional crystallization of olivine, clinopyroxene and feldspar. The parent magmas originated from a metasomatized lithospheric mantle source. The shoshonitic syenite and quartz monzonites have higher magmatic zircon δ18O values (6·26–7·05‰) than the mantle, which suggests some 18O enrichment during earlier subduction-related metasomatism of their lithospheric mantle source. The potassic adakite-like granites have Sr–Nd–Pb isotopic compositions that overlap those of lower-crustal amphibolites. They have low Mg#, MgO, Ni and Cr contents, abundant inherited zircons, high zircon εHf (0–5·5) and mantle-like δ18O (4·78–6·25‰) values. These granites were plausibly derived by partial melting of a thickened, potassic, mafic, lower crust with minor input from an older igneous felsic component. The potassic adakite-like quartz monzonites contain abundant mafic microgranular enclaves, and have transitional major and trace element characteristics between the adakite-like granite and the shoshonitic mafic magma. The quartz monzonites generally have higher Mg#, MgO, Ni and Cr contents than the lower crust-derived adakite-like rocks. They have no inherited zircons and have uniform zircon εHf and δ18O values. It is suggested that they were derived by variable degrees of mixing between lower-crustal melts and shoshonitic mafic magmas. The coeval shoshonitic and potassic adakite-like rocks appear to be associated with thinning of overthickened lithospheric mantle along the trans-lithospheric Jinsha suture following the collision between India and Asia. This lithospheric thinning could have resulted in the upwelling of the asthenosphere beneath western Yunnan, which induced partial melting of the residual metasomatized lithospheric mantle as well as the thickened lower crust in the Eocene.

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