Geochemistry and Sr-Nd isotopic characteristics of volcanic rocks in the Dazuozishanin copper polymetallic deposit, Inner Mongolia, and their geological implications

This paper first reports the characteristics of volcanic rocks in the Dazuozishan copper polymetallic deposit of Inner Mongolia. The deposit is located on the northern margin of the North China Platform, between Sila Mulun River great fault and Shaolang River great fault. The rhyolites and trachytes in the deposit are closely related to the mineralization. The rhyolites contain 74.93%-76.36 % of SiO₂ and 11.22%-11.70% of Al₂O₃. The rhyolites exhibit slightly alkali-rich form and the ratio between K₂O and Na₂O is approximately between 1.29 and 1.54. Meanwhile, the trachytes contain 65.52%-68.04% of SiO₂ and 15.05%-16.38% of Al₂O₃. They are of the alkali-rich type and the ratio between K₂O and Na₂O is approximately between 0.85 and 1.64. Both of the two rocks belong to the metaluminous-weak peraluminous series. Meanwhile, the rhyolites belong to the high potassium calc alkali series. The trachytes, on the other hand, belong to the high potassium calc alkali ~ shoshonite series. The rare earth element (REE) patterns of these two volcanic rocks are generally rightly-inclined. The rhyolites show strong Eu negative anomaly, while the trachytes show weak Eu positive anomaly. Except the elements of Ba, Sr and Ti whose values are notably lower in rhyolites than in trachytes, the trace elements in these two rocks are comparable in composition. In addition, they are both enriched in LILE and are depleted in HFSE. These two rocks have similar initial ratios of (⁸⁷Sr/⁸⁶Sr)_i and (¹⁴³Nd/¹⁴⁴Nd)_i. The ratio of (⁸⁷Sr/⁸⁶Sr)_i is approximately between 0.7046 and 0.7063, while the ratio of (¹⁴³Nd/¹⁴⁴Nd)_i is between 0.5119 and 0.5122 (t=155 Ma) , and ε_Nd is in the range of -4.24 to-10.37. These two rocks are the products of the anatexis of plagioclase amphibolite in the middle Proterozoic lower crust. The environment for the formation of the rhyolites and the trachytes in the Dazuozishan ore district is the intraplate environment of post-orogenic extension. In other words, under the condition of the post-orogenic extension of Mongolia-Okhotsk scissor-type closed orogeny and lithospheric thinning in late Jurassic (150-160 Ma), the anatexis of plagioclase amphibolite in the middle Proterozoic lower crust formed the volcanic rocks of metaluminous-weakly peraluminous series.