Geochronological And Geochemical Constraints On The Tectonic Evolution Of The Luliang Complex, Trans-North China Orogen

Geologists gradually reached consensus that the North China Craton (NCC) could be divided into two discrete continental blocks, called the Eastern Block and the Western Block, and they collided along the Trans-North China Orogen (TNCO) at～1.85Ga. However, there is still considerable controversy about whether the NCC has experienced its cratonization in an earlier period. The Luliang Complex is located in central-western Shanxi Province and exposed as the westernmost part of the TNCO. It mainly consists of Paleoproterozoic granitoid intrusions, mafic dykes and supracrustal rocks. The Yejishan and Luliang groups, as well as three episodes of mafic dykes and three stages of gneisses are researched in this study.Both of the lower Qingyanshuwan Formation and the upper Chengdaogou Formation are composed of detrital rocks, whereas the middle Bailongshan Formation consists mainly of thick layered basalts containing several clastic interbeds. Four detrital samples from the Qingyanshuwan Formation and two from the Bailongshan Formation show similar age distribution. They generally have four main peaks of-2.1Ga,～1.95Ga,～2.35Ga and～2.45Ga on the age histogram. The youngest age peak of～1.87Ga from the lower Qingyangshuwan Formation constrains the maximum depositional age of the Yejishan Goup. The minimum age can be constrained by the～1.78Ga mafic dyke that intruded the Bailongshan Formation. Taken together, the depositional age of the Yejishan Group can be approximately bracketed between～1.87and-1.78Ga. Basalts from the Bailongshan Formation belong to the tholeiite series. They display significant fractionation between L-MREE and HREE. They are enriched in Cs, Rb, Ba, K, and depleted in Nb, Ta, Ti, Zr, Hf. They show affinity with both the within-plate basalt and continental margin on various geochemical diagrams. Thus, a continental arc setting is indicated for the Yejishan Group in this study. Rhyolites from the Liiliang Group are formed at～2.2-2.1Ga. They have highly scattered εHfy) values ranging from-5.9to5.4, and plot onto the evolution line of the-2.5-2.7Ga crust.Three episodes of mafic dykes are identified in the Luliang Complex. The two phases of metamorphosed dykes were formed at～2.11Ga and～1.94Ga, and the unmetamorphosed one were emplaced at～1.79-1.78Ga. In-situ zircon Hf isotope data of the～2.11Ga samples vary in large ranges (over fifteen epsilon units) with the highest εHf(t) value approaching the depleted mantle array. For the1.94Ga samples, almost all the εHf(t) values are positive with abundant data near or equal to the contemporary depleted mantle. Geochemical data suggest that, most of the～2.11Ga dykes are alkaline whereas almost all the-1.94Ga dykes are subalkaline. The～2.11Ga dykes show variable LREE enrichment (La/Yb=1.9-10.3), prominent Nb-Ta troughs, high Zr contents and Zr/Y ratios while the～1.94Ga dykes display no pronounced fractionation between LREE and HREE, and also significant Nb-Ta-Ti negative anomalies, but low Zr contents and Zr/Y ratios. According to the Hf isotopic and the geochemical features, we suggest that the～2.11Ga dykes were probably derived from a sub-continental lithospheric mantle in a continental rift, whereas, the～1.94Ga dykes were arc-related with little crustal contamination during forming and ascending process and the Nb-Ta troughs were probably attributed to subduction-related fluids and melts. The～1.79-1.78Ga dykes are dolerites, and belong to the tholeiite series. Most of them trend E-W, nearly perpendicular to the orogen. They are enriched in LREE and LILE and depleted in HFSE, and have negative zircon ε(t) values of-1.7to-12.2. The E-W trending mafic dykes show similar geochemical and isotopic features compare to the NW-SE trending dykes in other complexes. They were most likely originated from a lithospheric mantle metasomatised by subduction-related fluids and later emplaced along extensional fractures in a post-collisional setting. NW-SE trending fractures were formed due to gravitational collapse and thinning of the lithosphere. E-W trending fractures in the central segment of the orogen constitute a transverse accommodation belt to equilibrate the different amounts of extension between the northern and southern TNCO. The impact of the post-orogenic extension might have continued to ca.1680Ma as evidenced by the presence of abundant ca.1750-1680Ma anorthosite-gabbro-mangerite-rapakivi granite suites (AMCG-like) occurring in the northern NCC.We also studied the Yunzhongshan, Gaijiazhuang and Chijianling gneisses. The Gaijiazhuang Gneiss is formed at～2.37-2.39Ga. For zircons from the Gaijiazhuang samples, the cores are rich in uranium and have experienced increased metamictisation. The Pb-loss leads to younger age for the core than the rim. Rocks are weakly peraluminous to metaluminous. They are depleted in Ba, Nb-Ta, Sr-P, Ti, Eu, and have high Ga/Al ratios. They show affinity with the A-type granite, and plot into the A2-type granite on geochemical diagrams. They have zircon εHf(t) values ranging from1.0to5.8, similar to that of the potassium-rich granites in the TNCO. Whereas the later emplaced TTG gneisses generally have negative sHf(t) values and are thought to be formed during a crustal thickening process. In this study we suggest that the Gaijiazhuang Gneiss may be formed in a back-arc setting. The Yunzhongshan and Chijianling gneisses are formed at～2.43-2.52Ga and～2.1-2.2Ga respectively. Their Hf isotopic data indicate that there were both juvenile crust generation and old crust (>2.8Ga) reworking at the late Archean.According to our study, there might be a whole rift, subduction, collision, and post-collision cycle during the period of～2.1-1.78Ga. However, the tectonic evolution before～2.1Ga needs further research. Our study suggests that the Gaijiazhuang Gneiss may be formed in a back-arc setting, thus the evolutional history of the TNCO may be much more complicated.