ZIRCON U-PB AGES AND GEOCHEMISTRY OF QIAN’AN CHARNOCKITES IN EASTERN HEBEI, NORTH CHINA CRATON: IMPLICATIONS FOR PETROGENESIS AND TECTONIC SETTING

The North China Craton (NCC), has experienced a variety of tectono-thermal events in the Archean, forming large volumes of TTG suites and charnockites. The latter ones play an important role in the differentiation of crust. We report here an example from the Qian’an charnockites in the eastern part of the NCC.

A total of 10 samples were collected for petrogenetic investigation from Qian’an area. They exhibit SiO₂ (59.14–66.97%), MgO (1.62–2.79%), with enrichment in Ba (574.2-1572ppm), LREE (La_N/Yb_N =6.64–78.96) and depletion in HESE, little or no negative Eu anomalies (Eu_N/ Eu_N^*=0.8-3.4). In general, their high Sr/Y (16.93-155.79) and La_N/Yb_N values indicate TTG affinities, which are also supported by the presence of residual plagioclases in the charnockites, and peritectic garnets in the nearby TTG gneisses.

There are 17 zircon spots selected for LA-ICP-MS U-Pb dating from the charnockites. The results can be divided into two groups. The first group shows ²⁰⁷Pb/²⁰⁶Pb mean age of 2503±60 Ma (N=9, MSWD=0.0104). The Th/U ratios are 0.31–1.25, indicating magmatic origin. The second group defines a ²⁰⁷Pb/²⁰⁶Pb mean age of 2457±50 Ma (N=8, MSWD=0.0090), with Th/U ratios of 0.22–0.91, also implying that they crystallized from melts.

Combined geochemical and zircon U-Pb age analysis, with field and petrographic observations, a mantle plume model is favored, which can reasonably interpret the geological features of Late Archean basement rocks in the Eastern Block of the NCC. In conclusion, the relatively cooler mantle-plume head heated the crust initially, and added a large volume of mostly mafic magmas, which promoted partial melting of the lower crust to form TTG granitoids. Subsequently, the relatively hotter mantle-plume tail heated the crust, causing granulite-facies metamorphism as well as charnockitic magmatism in the eastern part of the NCC during the late Archean.