Late Paleoproterozoic charnockite suite within post-collisional setting from the North China Craton: Petrology, geochemistry, zircon U-Pb geochronology and Lu-Hf isotopes

摘要

Charnockites (pyroxene-bearing granitoids) of magmatic origin in diverse tectonic settings and ranging in age from Mesoarchean to Cretaceous constitute important components of the continental crust. Here we report charnockites displaying both magnesian and ferroan compositions associated with gabbros from an AMCG (anorthosite-mangerite-charnockite-granite) suite in the North China Craton. The orthopyroxene in the magnesian charnockite is characterized by moderate X_Mg of 0.63-0.65 (Wo_(1-2)En_(62-63)Fs_(35-36)), and low Al2O3 content of 0.59-0.71 wt.%. The magnesian charnockites show medium- to high-K contents, and high Mg# (~47-69) similar to that of gabbros, whereas the Mg# of the ferroan charnockites is low (~6-28). The ferroan charnockites are alkali-calcic to alkalic, and weakly peralkaline to metaluminous, whereas the magnesian charnockites are calcic to calc-alkalic, and metaluminous. Although magnesian charnockites are in general considered to have formed in subduction setting, the medium- to high-K contents, high Mg# values with a wide range, and the highly negative sHf values of the zircons in these rocks (— 8.4 to —13.6), suggest inheritance of the arc signature from the melting of ancient arc-related crustal material. The ferroan charnockites show tholeiitic affinity and define a common differentiation trend with the gabbroic anorthosites and likely represent fractionated end-members with or without crustal interaction in a post-collisional rift setting. We present U-Pb age data from zircon grains on seven samples including two ferroan charnockites, three magnesian charnockites, one gabbroic enclave in magnesian charnockite and one gabbroic anorthosite which show emplacement ages of 1748.8 ± 6.4 Ma, 1747.1 ± 9.5 Ma, 1756.4 ± 7.3 Ma, 1756.7 ± 9.2 Ma, 1731 ± 17 Ma, 1731.6 ± 8.2 Ma and 1746.5 ± 7.3 Ma respectively. The negative εHf values ( — 1.2 to —13.6) of zircon grains from these rocks and the older crustal model ages ranging from Mesoarchean to Paleoproterozoic suggest that the magma sources of these rocks involved the melting of ancient crustal components. The age data suggest that the magmatic suite was emplaced within a relatively short time interval betweenl.73 and 1.76 Ga, during late Paleoproterozoic, placing the rocks suite in a post-collisional scenario, following the amalgamation between the Eastern and Western Blocks of the North China Craton along the Trans-North China Orogen at ca. 1.85-1.80 Ga.