Geochemistry of metasedimentary restitic rocks and implications for melting conditions and metal potential of crustal felsic magmas

Partial melting of crustal rocks, also known as anatexis, is the major mechanism of the fractionation of the continental crust and generation of felsic melts. The segregation of melts leaves residual melt-depleted rocks called restites, which are major constituents of migmatite and granulite metamorphic complexes. Restites from over 20 complexes summarized in this work, formed over a wide range of P-T conditions from low-pressure to ultra-high temperature (UHT) and ultra-high pressure (UHP) conditions. Due to protolith heterogeneity and variable melt loss, restite and protolith compositions commonly overlap, however distinctive trends are apparent. Regardless of melting conditions metasedimentary restitic rocks tend to show loss of SiO2, K2O, Na2O, and enrichment in ferromagnesian elements, often Al2O3, and TiO2. These trends can be explained readily by the extraction of significant amount of granitic melt. The trace element features of restites show systematic changes with melting temperature, pressure and fluid regime. The restites after lower crustal melting show depletion in P, B, Rb, Cs, Pb and U, and subtler features, such as Nb/Ta fractionation, which all correspond to enrichment patterns of S-type and, ultimately, rare metal granites. Lithium and Be are undepleted in the relatively shallow, low-pressure cordierite-bearing restites, and depleted from deeper lower crustal restites containing garnet. The fractionation of trace elements during anatexis contributes to the development of features of the upper continental crust such as elevated content of LILE and heat producing elements (K, Th, U), reduced Nb/Ta and overall felsic composition. The pre-dehydrated UHT terranes display characteristics consistent with minor melt loss and depletion. The compositional effect of anatexis at UHT conditions, is sensitive to dehydration history of rocks, and melting of undehydrated sediments results in depletion in LREE, Th, Ga, ±Zr, ±Nb, which is complementary to A-type granitoids enrichment patterns. It is noted that anatexis produces melts with rather limited degrees of enrichment and high-degree fractional crystallisation of granitic magmas is a prerequisite for formation of ore-grade granitic intrusions. While restite geochemistry shows multiple consistent patterns, many remain unexplained and for certain elements the data are scarce, suggesting targets for fruitful research.