Acta Petrologica et Mineralogica

Mg-Al granulite is a collective term for a class of granulite-facies metamorphic rocks that are enriched in magnesium and aluminum. Although its petrographic characteristics have been widely used to infer the peak metamorphic conditions and metamorphic evolution history of ultra-high temperature (UHT) metamorphism, our knowledge about the protolith and petrogenesis of Mg-Al granulite is still very limited. Taking the Huatugou UHT meta-morphic terrane of the western Qaidam Block as an example and based on the field observation, this study investigated the petrography and whole-rock geochemistry of Mg-Al granulites and pelitic gneisses. Mg-Al granulites and leucosome-bearing pelitic gneisses have similar contents of SiO₂, TiO₂, and P₂O₅, with overlapped variation ranges of ^TFe₂O₃, Al₂O₃, MnO, CaO and Na₂O. Meanwhile, Mg-Al granulites and pelitic gneisses exhibit similar distribution patterns of trace and rare earth elements. Combined with the similar mineral assemblages in parts of Mg-Al granulites and pelitic gneisses, we concluded that the protolith of the Huatugou Mg-Al granulite is similar to that of pelitic gneisses. From the lower-amphibolite facies metapelite to leucosome-bearing pelitic gneisses and Mg-Al granulites, the whole-rock compositions show descending trends in aluminum, calcium, potassium, and sodium, with increases in iron and magnesium. Mg-Al granulites have high X_Mg values (0.51~0.69), which distinguish them from other pelitic gneisses (X_Mg=0.34~0.43). According to the phase equilibrium modeling and related calculation on the metapelite, the partial melting and melt loss account for most of the chemical trends, but basically do not affect the whole-rock X_Mg value; only the removal of partial melts with garnet during prograde heating can transform the metapelite into Mg-Al granulite. In addition, compared with the nearby leucosome-bearing pelitic gneisses, the garnet-rich pelitic residue is depleted in silicon, sodium, and potassium, and enriched in aluminum, iron, magnesium, manganese, and calcium, with significantly higher contents of heavy rare earth elements. These geochemical features require the addition of garnet-bearing melts and then the removal of melts, which supports the movement of garnet-bearing melts observed in the field. Finally, we provide a hypothesis on why Mg-Al granulite in the Huatugou region only occurred as lenses. It is speculated that, because it is difficult for the melt to move with garnet for a long distance, only metapelite surrounded by felsic orthogneiss can get rid of garnet-bearing melts and transform into Mg-Al granulite during the prograde heating process. More studies are needed to verify this hypothesis, and we believe that evidence for the migration of garnet-bearing melts and the transformation of the Mg-Al granulite probably have been preserved within other HT-UHT metamorphic regions.

western Qaidam Block; HT-UHT metamorphism; Mg-Al granulite; petrogenesis; garnet-bearing melt