Petrogenesis of Late Paleoproterozoic Liangcheng charnockites and S-type granites in the central-northern margin of the North China Craton: Implications for ridge subduction

摘要

The 1930-1890 Ma, garnet-bearing, peraluminous, Liangcheng granitoids are mainly distributed in the Fengzhen belt in the North China Craton (NCC). They consist of early porphyritic, coarse-grained charnockites (magmatic orthopyroxene granitic rocks) and late, medium- to coarse-grained S-type, crustal-melt granites. The charnockites are distinguished from the S-type granites by their higher TiO_2, Al_2O_3, FeO_t, MgO, CaO, and trace element contents, especially the heavy rare earth elements (REE). The charnockites show varied light-REE enrichments (La/Yb_N=1.92-18.0), negative Eu anomalies (Eu/Eu*=0.2-0.9), and negative anomalies in high field strength elements (HFSEs, e.g., Nb, Ta, Ti) and in Sr, compared with the neighbouring elements in spidergrams. Their ~(87)Sr/~(86)Sr~t (t=1900Ma) ratios vary from 0.704 to 0.707, with one at 0.74, and εNd~t (t=1900Ma) values range from -4 to +1. In contrast, the Liangcheng S-type granites show strong light-REE enrichments (La/Yb~N=13.3-230), positive Eu anomalies (Eu/Eu*=1.1-3.6), negative anomalies in HFSEs (e.g., Nb, Ta, Ti) and negative or positive anomalies in Sr, compared with neighbouring elements in spidergrams. Their ~(87)Sr/~(86)Sr~t (t=1900Ma) ratios vary from 0.703 to 0.712, and εNd~t (t=1900Ma) values range from -3 to +1. Both the charnockites and S-type granites originated by melting of in situ meta-sediments with significant (charnockites) or little (S-type granites) amounts of additional melts from the parental magma of the adjacent Xuwujia gabbronorites. High-Mg (19-23wt.%) samples of the Xuwujia gabbronorites have whole-rock Re contents of 0.16-1.93ppb, Os contents of 1.6-9.8ppb, ~(187)Re/~(188)Os ratios of 0.08-1.76, and ~(187)Os/~(188)Os ratios of 0.1168-0.1186 (with one exception at 0.1845). These values are comparable with those of typical komatiites, and agree with a depleted source for these rocks. The charnockites have received 10-30wt.% or even larger amounts of melt contribution from the Xuwujia gabbronorites, whereas the S-type granites received only few melts from the gabbronorites, and they possibly had lower melting temperature than the charnockites. We consider that the charnockites and S-type granites had surface counterparts, represented by two types of rhyolitic to dacitic volcanics in the Halaqin succession, which also contains basalts to basaltic-andesites that are geochemically comparable to the Xuwujia gabbronorites or the charnockites (some andesites). From their field relations and geochemical and thermal characteristics we propose that the peraluminous charnockites and S-type granites were most likely generated by ridge subduction at ~1930Ma prior to the final cratonization of the NCC at ~1850Ma, and that this ridge subduction was also responsible for the regional 1950-1880Ma gabbronorites-charnockites-S-type granites-volcanics, as well as the ~1930Ma ultrahigh-temperature metamorphism, in the NCC.