The petrogenetic relationship between migmatite and granite in the Himalayan orogen: Petrological and geochemical constraints

Partial melting of crustal rocks contributes to the formation of granites in collisional orogens. It is generally recognized that residual granulites are compositionally complementary to crustal-derived granites, yet the petrogenetic relationship between migmatites and granites remains obscure. To shed light on this issue, a comprehensive study of field geology, petrographic observations, whole-rock major and trace elements, mineral major elements, zircon and monazite U-Th-Pb isotopes and trace elements, and monazite Sm-Nd isotopes was carried out for the Higher Himalayan leucosomes, leucocratic dikes, and leucogranites from the Cona area in eastern Himalaya. These rocks were formed by partial melting of the Higher Himalayan metasedimentary rocks and granitic gneisses in the Miocene, and are characterized by distinct petrological and geochemical features. Leucosomes and leucocratic dikes, which were derived from different source rocks (granitic gneisses and metasedimentary rocks, respectively), share many similarities. Both of them are rich in K-feldspar and have high K2O, Ba, and Sr contents and CaO/(CaO + Na2O) ratios, but low Rb/Sr and Sm/Nd ratios; LREE contents correlate positively with Th contents, but negatively with P2O5 contents; zircon and monazite have low MREE/ HREE ratios. In contrast, leucogranites, which originated from mixed sources of metasedimentary rocks and granitic gneisses, are rich in plagioclase and have low K2O, Ba, and Sr contents and CaO/(CaO + Na2O) ratios, but high Rb/Sr and Sm/Nd ratios; LREE contents correlate positively with Th and P2O5 contents; zircon and monazite have high MREE/HREE ratios. The K2O-rich leucosomes and leucocratic dikes and the Na2O-rich leucogranites do not show geochemical signatures indicative of a link owing to melt evolution processes, suggesting that the two main groups are the products of distinct and independent anatectic processes in the source. The leucosomes and leucocratic dikes are associated with water-fluxed (hydration) melting at lower temperatures and pressures, whereas the leucogranites are related to water-absent (dehydration) melting at higher temperatures and pressures. In this regard, the leucosomes and leucocratic dikes in migmatites and the leucogranites likely represent two different types of anatectic products, which were respectively derived from partial meltig of the hydrated crustal rocks at the shallower level and the dehydrated crustal rocks at the deeper level. The hydrated-dehydrated crustal architecture is ascribed to the influx of water into the shallower crustal rocks caused by metamorphic dehydration at the deeper crustal level. The spatiotemporally coupled dehydration-hydration anatectic processes may be a common mechanism for the formation of coherent granite, migmatite, and granulite in collisional orogens.