Yanyan Zhou

The Paleoproterozoic Continental Evolution in the Southern North China Craton: Constrains from Magmatism and Sedimentation

Recently, increasing lines of evidence for the 2.45–2.00 Ga magmatic rocks and Paleoproterozoic low-grade metasedimentary sequences have been identified, which can provide diagnostic constrains on the debate on the Paleoproterozoic (2.45–2.00 Ga) evolutional regime of the NCC. The widespread 2.45–2.20 Ga magmatisms occurred in the southern NCC mainly include TTG, dioritic-gabbroic gneiss, amphibolite, and high-K granites. The 2.45–2.20 Ga TTG or TTG-like gneisses show variable Mg# values, low Cr, Ni, and high Rb/Sr ratios, suggesting that they most likely derived from partial melting of basaltic lower crust with juvenile materials addition. The 2.45–2.20 Ga dioritic-gabbroic gneisses show the similar geochemical characteristics with adakitic rocks from thickened lower crust. Their e Hf(t) and e Nd(t) values are variable, and have weak Ta enrichment, and not obvious negative Nb anomalies, suggesting they were produced by partial melting of metasomatized lithospheric mantle. The 2.45–2.20 Ga amphibolites are consistent with magma derivation from MORB-like mantle wedge. The 2.45–2.20 Ga (high-K) calc-alkaline granites are representative of syn-collisional granites, and derived from older crust with variable mixing of a juvenile melt in a subduction-collision related setting. The 2.20–2.00 Ga magmatism reveals a major period of crustal reworking, rather than one of crustal addition. The 2.20–2.00 Ga monzonites have mixed IAB- and OIB-like geochemical signatures, possibly related to extension and thinning of the lithosphere and upwelling of asthenosphere. The 2.20–2.00 Ga potassic granites belong to highly fractionated aluminous A-type granite, and formed in an extensional-rift setting. As indicated by the zircon in situ Hf isotopic compositions, the injection of basaltic melt into the crust has been widely considered as an important mechanism to generate silicious melts. The 2.20–2.00 Ga tonalite would be derived from partial melting of delaminated lower crust. The temporal change from mostly 2.45–2.20 Ga low-K igneous rocks (TTG) to 2.20–2.0 Ga mostly high-K igneous rocks in the southern segment of the NCC indicates a tectonic transformation from accretionary orogenesis (ca. 2.30 Ga) to extensional regimes (ca. 2.10 Ga). On the other hand, provenances, depositional ages and tectonic settings of low-grade Paleoproterozoic metasedimentary units in the NCC can also provide rigorous constraints on the tectonic evolution in the period between 2.45 and 2.00 Ga. In the Henan-Shaanxi province on the southern NCC, the Paleoproterozoic sedimentary sequences include the Shangtaihua Group, the Songshan Group, the Yinyugou Group, and the Tietonggou Formation. The low-grade metasedimentary Songshan Group deposited after 2.35 Ga and before 1.78 Ga and sourced from felsic rocks including major 3.00–2.40 Ga TTG gneisses, 2.40–1.95 Ga granitoid plutons and meta-rhyolites of the Dengfeng, Zhongtiao and Taihua complexes in the southern NCC, and minor 3.70–3.00 Ga transported exotic Paleoarchean and Mesoarchean crustal materials because no lithologies or zircons with such age founded in study areas. The Tietonggou Formation deposited at 1.91–1.80 Ga, with detrital zircon age peak of ~2.10 Ga which possibly sourced from ~2.10 Ga lithologic units in the south of NCC. The depositional ages of the Paleoproterozoic low-grade metasedimentary units in the NCC are constrained at a certain period of 2.35–1.78 Ga, which overlaps with the stage of subduction-collision related 2.45–2.20 Ga magmatisms and rift setting related 2.20–2.00 Ga magmatisms. The detrital zircon Hf isotopes of the low-grade sediments varied mainly toward the reduction of the radiogenic Hf isotope and gradually show a similar trend of the isotope trajectories of crustal evolution. Like previous studies, all these groups were deposited in basin settings which were not simple long-lived foreland basins. Combining with 2.45–2.00 Ga igneous rocks, they may evolved from back-arc or intra-arc basins developing at the subduction-collision stage (from ∼2.45 Ga) to rift stage (from ∼2.20 Ga) and then to foreland basins at the collision stage (from ∼1.85 to ∼1.80 Ga).