Paleozoic and Mesozoic magmatism in the Gaodi porphyry Mo-Cu deposit: Implications for the evolution of the Mongol-Okhotsk Ocean in the northern Great Xing'an Range, NE China

Abstract

The newly discovered "460 Gaodi porphyry Mo–Cu deposit" (with Re-Os age of 179.6 ± 3.6 Ma) is located 30 km west of Hanjiayuanzi town in the northern Great Xing’an Range (GXR), NE China. The intrusive rocks in the deposit area can be classified into four periods: (1) Early Ordovician, which includes a high-K calc-alkaline monzodiorite (480 Ma) indicating significant crustal contamination, and a “syntectic” I-type granodiorite (479 Ma) with low Mg# values (34–38) and εHf(t) values (−4.4 to +0.9) show mixing of mantle-crust derived materials, which is related to post-orogenic event between the Ergun and Xing’an blocks; (2) Late Triassic, which includes an adakitic granodiorite porphyry (220 Ma) related to partial melting of the lower part of a thickened crust (C-type adakitic) with low Na2O/K2O ratio (1.27–1.49), Cr and Ni contents, with εHf(t) values of −0.7 to +3.3; (3) early Early Jurassic, which includes a differentiated I-type monzogranite (198 Ma) and syenogranite (196 Ma) with negative Eu anomaly (0.06–0.74), low Mg# (9–37) and high Rb/Sr (0.84–5.07). The εHf(t) values of them are −3.3 to 1.8 and −3.0 to +2.3, respectively, indicating the same source areas originated from partial melting of juvenile lower crust; (4) late Early Jurassic, which includes an ore-related C-type adakitic granite porphyry (182 Ma) with relative low Mg# (36–38), and low Na2O/K2O (0.65–0.81), strongly peraluminous quartz porphyry (181 Ma) with highly differentiated characteristics, and post-collisional gabbroic diorite (180 Ma). The ore-related granite porphyry yields εHf(t) values ranging from −4.1 to −0.4, suggesting partial melting of juvenile lower crust. The 460 Gaodi deposit formed in a late-collisional setting related to the Mongol-Okhotsk Ocean system. A post-collisional stage about 182 to 163 Ma immediately following the main collision is proposed.