Zircon U–Pb and Hf isotope constraints on crustal melting associated with the Emeishan mantle plume

Abstract

SHRIMP zircon U–Pb dates, combined with in-situ Hf isotopic data, provide new constraints on the petrogenesis and protolith of peralkaline, metaluminous and peraluminous intrusions and rhyolitic tuffs in the Emeishan large igneous province, with significant bearing on crustal melting associated with mantle plumes. Syenite and A-type granitic intrusions from Huili, Miyi and Taihe in the center of this large igneous province yield U–Pb dates at ∼260 Ma, consistent with the ages obtained for mafic layered intrusions in the same province. Zircon from these rocks exhibits a wide range of initial Hf isotope ratios ( ε Hf( t) = −1.4 to +13.4), with corresponding T DM1 of 400–900 Ma. The highest ε Hf( t) value is only marginally lower than that of depleted mantle reservoir at 260 Ma, suggesting that their source is primarily juvenile crust added during Emeishan volcanism, with incorporation of variable amounts of Neoproterozoic crust. The trigger of crustal melting is most likely related to advective heating associated with magmatic underplating. In contrast, the 255–251 Ma peraluminous granites from Ailanghe and 238 Ma rhyolitic tuff from Binchuan, have negative initial ε Hf values of −1.3 to −4.4, and of −7.7 to −14, respectively. Hf isotopic model ages and presence of inherited zircons indicate their derivation from Mesoproterozoic and Paleoproterozoic crust, respectively. Given the time lag relative to the plume impact (∼260 Ma) and insignificant mantle contribution to 255–238 Ma magmatism, conductive heating is suggested as the trigger of crustal melting that resulted in formation of delayed felsic magmas. The involvement of older crust in younger felsic magmas is consistent with upward heat transfer to the lithosphere during plume impregnation, if the age of crust is inversely stratified, i.e., changes from Paleoproterozoic to Mesoproterozoic to Neoproterozoic to Permian with increasing depth. Such crust may have resulted from episodic, downward crustal growth during the evolution of the western Yangtze Craton.