Peritectic assemblage entrainment (PAE) model for the petrogenesis of Neoproterozoic high-maficity I-type granitoids in the western Yangtze Block, South China

Abstract

Under the condition of paucity of mantle-derived ferromagnesian components, peritectic assemblage entrainment (PAE) model can properly explain a large variation of Fe, Mg, Ti, and Ca contents as well as positive linear trends for Ti and Ca plotted against maficity (molar Fe + Mg) exhibited by more mafic I-type granitic melts. Herein we present a comprehensive study of zircon U–Pb–Hf isotopes, whole-rock major and trace elements, and Sr–Nd isotopes for newly identified Neoproterozoic high-maficity I-type granitoids collected from the Yonglang area in the western Yangtze Block, South China, in order to evaluate PAE process acting on the petrogenesis of these more mafic granitoids. Zircon U-Pb geochronological results show that the Yonglang granitoids, including the granodiorites, monzogranites, and biotite granites, yield the weighted mean 206Pb/238U ages of ca. 850–825 Ma. They are geochemically characterized by high SiO2 (64.63–72.62 wt.%) and K2O (3.49–6.59 wt.%) contents as well as variable A/CNK (0.94–1.09) and molar Fe + Mg values (0.0452–0.1210), and display a negative correlation of A/CNK and Fe + Mg, indicating an I-type affinity. Isotopically, they display negative whole-rock εNd(t) (−4.06 to−6.38) and highly evolved zircon εHf(t) values (−9.36 to +1.92) as well as old two-stage Nd model ages (1.69–1.85 Ga) and crustal Hf model ages (1608–2139 Ma), implying a crustal source. In comparison with the geochemical composition of fluid-absent experimental melts, the Yonglang I-type granitoids have Fe, Mg, Ti, and Ca contents that are too high to equilibrium with those of pure crustal melts, indicating the additional incorporation of a ferromagnesian, Ti-, and Ca-rich component into primary melts. Like the Cotoncello–Monte Capanne granitic complex in Elba Island and Verdenburg I-type granite in South Africa, the Yonglang I-type granitoids exhibit highly positive linear correlations for Ti vs. maficity and Ca vs. maficity as well as negative correlation for A/CNK vs. maficity, suggesting that these high-maficity I-type granitoids predominantly underwent the peritectic assemblage entrainment of Ca-Fe-Mg-Ti bearing minerals (e.g., clinopyroxene, plagioclase, ilmenite, etc.) during coupled melting of biotite and hornblende phases in crustal source. The process of peritectic assemblage entrainment thus play a vital role in moulding geochemical compositions of more mafic I-type granites.