Petrogenesis of Mg-Al granulite from Huatugou ultrahigh-temperature metamorphic terrane in the western Qaidam Block and its relationship with the behavior of the melts
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 meta-morphic conditions and metamorphic evolution history of ultra-high temperature(UHT)metamorphism,our knowl-edge 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 investiga-ted 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 distri-bution 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 gran-ulites,the whole-rock compositions show descending trends in aluminum,calcium,potassium,and sodium,with increases in iron and magnesium.Mg-Al granulites have high XMg values(0.51～0.69),which distinguish them from other pelitic gneisses(XMg=0.34～0.43).According to the phase equilibrium modeling and related calcula-tion on the metapelite,the partial melting and melt loss account for most of the chemical trends,but basically do not affect the whole-rock XMg value;only the removal of partial melts with garnet during prograde heating can trans-form the metapelite into Mg-Al granulite.In addition,compared with the nearby leucosome-bearing pelitic gneis-ses,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 geo-chemical 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 hypothe-sis,and we believe that evidence for the migration of garnet-bearing melts and the transformation of the Mg-Al gran-ulite probably have been preserved within other HT-UHT metamorphic regions.
western Qaidam BlockHT-UHT metamorphismMg-Al granulitepetrogenesisgarnet-bearing melt
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