Crust-mantle hybrid magmatism reveals the trans-mantle diapiric exhumation of deeply subducted continental crust: A case study from Altyn Tagh ultrahigh-pressure terranes, NW China

The diapiric exhumation of deeply subducted slabs in orogenic belts requires the upward intrusion of exhumed rocks through the mantle wedge, resulting in significant crust-mantle interaction and partial melting. To evaluate this process, we conducted an integrated petrological, geochronological, and geochemical study of monzogranite plutons in/around the high-pressure (HP)−ultrahigh-pressure (UHP) felsic gneiss terranes of eastern South Altyn Tagh (SAT), NW China. The monzogranite suite comprises host K-rich monzogranites, mafic-intermediate enclaves, and felsic gneiss xenoliths. Felsic gneiss xenoliths recorded a protolith age of ca. 900 Ma and partial melting at ca. 480 Ma during exhumation, while the monzogranites and enclaves exhibit coeval crystallization at ca. 480 Ma, with inherited zircon U-Pb ages of ca. 900 Ma matching the protolith age of felsic gneiss xenoliths. The K-rich monzogranites exhibit geochemical and isotopic affinities with HP-UHP felsic gneisses (and the in situ granitic veins in felsic gneisses), collectively indicating origin through partial melting of the felsic gneisses. εNd(t)-εHf(t) binary mixing models and trace element metasomatic simulations suggest the mafic-intermediate enclaves originated from partial melting of a mantle wedge was metasomatized by 5−20 wt% melts derived from the anatexis of felsic gneisses. Combined with field structural relationships, we propose a diapiric exhumation mechanism for the HP-UHP felsic terranes in the eastern SAT. This process involved (1) distinct partial melting of felsic gneisses triggered by heat transfer from the mantle wedge, (2) mantle contamination by felsic gneiss-derived melts, (3) incorporation of both granitic materials (felsic gneisses and granitic melts) and mafic-intermediate melts derived from metasomatized mantle, and (4) diapiric ascent and final emplacement of the monzogranite, which transported entrained felsic gneisses and mafic-intermediate enclaves within the southern margin of the upper plate.