Metamorphic Ages and PT Conditions of Amphibolites in the Diebusige and Bayanwulashan Complexes of the Alxa Block, North China Craton

The metamorphism and geological significance of amphibolites in the Diebusige and Bayanwulashan Complexes of the eastern Alxa Block, North China Craton, were poorly understood until now. This study presents the results of petrology, laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS) zircon U-Pb analysis, phase equilibrium modeling and geothermobarometry for these rocks. The peak mineral assemblage of clinopyroxene + hornblende + plagioclase + K-feldspar + ilmenite + quartz + melt is inferred for amphibolite sample ALS2164 in the Diebusige Complex. Correspondingly, the peak mineral assemblage of clinopyroxene + hornblende + plagioclase +/- K-feldspar + ilmenite + quartz + melt is identified for amphibolite sample ALS2191 in the Bayanwulshan Complex. Phase equilibrium modelling constrained the peak metamorphic condition of amphibolite sample ALS2164 in the Diebusige Complex to be 825-910 degrees C/7.2-10.8 kbar, which is similar to that (800-870 degrees C/7.0-10.7 kbar) of amphibolite sample ALS2191 in the Bayanwulashan Complex. Hbl-pl-qz thermobarometry yielded the metamorphic PT conditions of 732-810 degrees C/3.0-6.7 kbar for these amphibolites, which are consistent with the average temperatures of 763 degrees C, 768 degrees C and 780 degrees C calculated by Ti-zircon thermometry. As a result, phase equilibrium modelling yielded wide PT condition ranges of 800-910 degrees C/7.0-10.8 kbar, the lower limit of which is consistent with the upper limit of estimates by the hbl-pl-qz thermobarometer. In addition, LA-ICP-MS U-Pb analysis on metamorphic zircons yielded weighted mean Pb-207/Pb-206 ages of 1901 +/- 22-1817 +/- 21 Ma, which represent the timing of amphibolite-facies metamorphism. As a whole, the PT estimates display a high geothermal gradient, which is consistent with coeval ultrahigh-temperature metamorphism and associated mantle-derived mafic-ultramafic rocks in the Diebusige Complex. Combing this information with the previously published data from the Diebusige Complex, an extensional setting after continental collision is inferred for the eastern Alxa Block during the late Paleoproterozoic. The HREE enrichment patterns of metamorphic zircons from the amphibolites in this study are in agreement with that these amphibolites formed at relatively shallower crust than the garnet-bearing mafic granulites in the Diebusige Complex.