The Role of Earth's Deep Volatile Cycling in the Generation of Intracontinental High-Mg Andesites: Implication for Lithospheric Thinning Beneath the North China Craton

A key role of water has been emphasized in the generation of Early Cretaceous high-Mg andesites (HMAs) and lithospheric thinning of the eastern North China Craton (NCC). However, the source of water and when it hydrated the NCC lithospheric mantle are still unclear. To address these issues, here we investigate the petrography, mineral chemistry, and the chemical and Sr-Nd-Pb isotopic compositions of two suites of Early Cretaceous HMAs from Western Liaoning, China, combined with new Sr-Nd-Pb isotopic data of the Early Cretaceous primitive basalts from the same area. Petrographic features and variations of chemical and isotopic compositions suggest a petrogenesis of mixing between mantle-derived mafic and crust-derived felsic melts for these HMAs and most of other Early Cretaceous HMAs in the NCC. The mantle-derived primary melts are indicated to have been derived from the refractory lithospheric mantle beneath the NCC. Fluid-rich melt inclusions in olivines, absence of plagioclase phenocryst, enrichments of fluid-indicative trace elements (e.g., Rb, Ba, K, Pb, and Sr), high water content in clinopyroxenes, and low Ca in olivines indicate high water contents in the magmas and their mantle sources. The EM1-like Sr-Nd and unradiogenic Pb isotopic compositions of these lavas unravel a Mantle Transition Zone source for the fluids that hydrated the NCC lithospheric mantle. The wet upwelling of the Mantle Transition Zone components would be triggered by the penetration of deep-subducted Paleo-Pacific slab. Our study provides key evidence for the transport of water from deep Earth into the cratonic lithosphere and its role in lithospheric thinning of cratons.