Early Mesozoic tectonic evolution of Beishan Orogenic Belt : Constraints from chronology and geochemistry of the Late Triassic diabase dyke in Liuyuan area, Gansu Province

Beishan Orogenic Belt (BOB), located in the southern section of Central Asia Orogenic Belt (CAOB), is an important region to discuss the accretionary evolution of the CAOB, and it is also one of the most important Triassic metallogenic belts in the northern China. Regionally, the Triassic sediment is absent, while, many granitoid plutons and dikes exposed with minor mafic dikes during the Late Paleozoic to Early Mesozoic. Therefore, it is an ideal area to probe the evolution of BOB. U-Pb zircon dating yields magma crystallization age of the Late Triassic (weighted Pb-206/U-238 ages of 227. 5 +/- 1. 4Mo) for a diabase dyke. Geochemically, the diabase has relatively high Mg-# (65 similar to 70) and Ni (110 x 10 (6 )similar to 157 x 10(-6)) with weakly fractionated REE patterns ((La/Yb)(N) = 1. 42 similar to 1. 94), flat HREE patterns, no or weakly positive Eu anomaly, pronouncedly negative Nb and Ta anomalies and enrichment of Zr and Hf. Isotopically, the diabase dikes display positive epsilon(Hf)(t) values of - 2. 21 similar to + 4. 02 with an average of + 1.08. Their relatively high ratios of (Th/Nb)(N) and Th/Yb, and low ratios of Nb/La and Th/U, and constant La/Nb and Th/Nb ratio suggest that slab-derived components were probably involved in the magmatic source with minor crustal contamination. In addition, the diabase show positive epsilon(Hf)(t) values, low ratios of Nb/Yb and Th/U, and depletion of Nb and Ta with flat HREE pattern, which indicates that it is mainly derived from a depleted asthenosphere mantle, but not an Ocean Island Vasalts (OIB) -like source . Accordingly, it is more likely that the mafic magma originated from partial melting of a depleted asthenosphere mantle overprinted by slab-derived components, with minor crustal contamination during the ascending through continental lithosphere. Integrated the geochemistry of the Permian mafic dykes in this region, it is considered that the Late Triassic diabase of this study was formed in a lithospheric extensional tectonic setting which leads to melt uplift of depleted asthenosphere with overprinting of slab-derived components and experienced interaction with crustal material. By statistic analysis upon the zircon Hf isotopic data of felsic and mafic intrusions from the Late Permian to the Early Triassic (from 280Ma to 220Ma) , we found that the felsic rocks show a concave shape for their epsilon(Hf)(t) values, while the mafic ones display a downward trend. Combined with the structural deformation, source of basin sediment, magmatic evolution and metallogenesis events, we holds that the Hf isotope change could be related to the tectonic evolution of BOB which experienced a collisional compression stage during the Early to Middle Triassic and a post-orogenic extension environment in the Late Triassic. To summarize, this work further improved the BOB tectonic evolution from the Late Paleozoic to the Early Mesozoic.