Widespread late Neoarchean reworking of Meso- to Paleoarchean continental crust in the Anshan-Benxi area, North China Craton, as documented by U-Pb-Nd-Hf-O isotopes

Abstract

We present an integrated study of zircon U-Pb dating and O-Hf isotopic analyses combined with whole-rock geochemistry and Nd isotopic systematics on widespread late Neoarchean syenogranites (Qidashan Pluton) and other granitoids (granodiorite, quartz monzonite, monzogranite) in the Anshan-Benxi area, eastern North China Craton. All these rocks were emplaced at ca. 2.5 Ga according to SHRIMP zircon U-Pb dating and other indirect methods. The syenogranites are characterized by high SiO₂ and K₂O/Na₂O and low CaO, FeOt, MgO, TiO₂ and P₂O₅. However, they differ in trace element and REE compositions and can be roughly subdivided into two types. Type 1 syenogranite has strongly negative Eu anomalies and Ba depletion, with large variations of LREE to HREE differentiation; type 2 syenogranite has no negative Eu anomaly and Ba depletion. Other granitoids are also rich in K₂O and show similar trace element and REE compositional features as the type 2 syenogranite. Most syenogranite samples have whole-rock ε_Nd(t) values of –10.5 to –2.7 and t_DM(Nd) ages of 2.96 to 3.90 Ga, whereas the other granitoids have ε_Nd(t) values of –5.2 to –2.2 and t_DM(Nd) ages of 2.95 to 3.19 Ga. Magmatic zircon from the syenogranites has ε_Hf(t) values and Hf crustal model ages ranging mainly from –11.0 to +4.4 and 2.70 to 3.46 Ga, respectively; those from other granitoids have ε_Hf(t) values and Hf crustal model ages of –16.0 to +2.4 and 2.81 to 3.72 Ga, respectively. All these rocks contain Meso- to Paleoarchean xenocrystic zircon grains with ε_Hf(t) values and Hf crustal model ages ranging from –14.3 to +8.3 and 2.79 to 3.93 Ga, respectively. Decoupling of whole-rock Nd and Hf-in-zircon isotopes occurs in some samples. Most low-U magmatic zircon (U<200 ppm) from all granitoids has δ¹⁸O values in the range of +4 to +7.2 permil, whereas high-U magmatic zircon (U≥200 ppm) shows a larger variation with the lowest δ¹⁸O value being 1.6. The xenocrystic zircon grains have δ¹⁸O values in the normal range of Archean magmatic zircon. Combined with previous studies, we conclude that the 2.5 Ga granitoids formed by recycling of mature, old continental crust, including metasedimentary sources, probably in an extensional tectonic environment.