Pb isotope disequilibrium in metasediment-derived granitic melts

Radiogenic isotope compositions of magmatic rocks have been widely used to infer the nature of their sources. However, updated studies have demonstrated that crustal derived granites can carry a large magnitude of isotope (Sr, Nd, and Hf) disequilibrium which could be used to infer the melting reactions as well as the nature of sources. The Himalayan Cenozoic granites are typical products from melting of crustal sources.  Experimental results and theoretical calculations suggest that the Himalayan leucogranites are characterized by Sr, Nd, Hf isotope disequilibrium. Exception for the reported trace element compositions and ratios and the initial Sr isotopic ratios, radiogenic Hf and Pb isotopic ratios are heterogeneity. Leucogranites from Malashan-Gyirong area consist of two groups of granites formed fluid-absent melting of muscovite (Group-A) and fluid-fluxed melting of muscovite (Group-B), respectively. Except for substantial differences in key trace element compositions and their ratios, and Sr isotope compositions, follow-up studies show that as compared to Group-A granites, Group-B granites have much higher Th, Th/U, and ²⁰⁸Pb/²⁰⁴Pb ratios. However, their ²⁰⁶Pb/²⁰⁴Pb and ²⁰⁷Pb/²⁰⁴Pb ratios are similar. Such characteristics could be explained by enhanced solubility of monazite (high Th/U and ²⁰⁸Pb/²⁰⁴Pb phase) relative to zircon during fluid-fluxed melting of metasedimentary rocks. Our findings suggest that zircon and monazite play an critical role in shaping Pb isotope systematics in crustal-derived melts.