Geochronology,Geochemistry and Geological Significance of Early Paleozoic Wulanaobaotu Intrusive Rocks,Inner Mongolia

Wang Shuqing; Xin Houtian; Hu Xiaojia; Zhang Yong; Zhao Hualei; Geng Jianzhen; Yang Zeli; Teng Xuejian; Li Yanfeng

doi:10.3799/dqkx.2016.046

Abstract：The study of magmatism can reveal the orogenic tectonic evolution.In this paper,zircon LA-MC-ICPMS U-Pb dat-ing,Hf isotopic compositions and whole rock geochemical data of the Early Paleozoic dioritic rocks from Wulanaobaotu region, central part of the Xing-Meng Orogenic belt are present.The rock suite consists of two major rock types,i.e.,quartz-diorite and diorite,with zircon U-Pb ages of 474.0±1.8 Ma and 487.8±1.9 Ma,respectively.In-situ zircon Hf isotopic analysis shows enriched compositions with 1 7 6 Hf/1 7 7 Hf ratios varying from 0.282 123 to 0.282 304,negativeεHf (t )values and Hf two stage model ages(T DM C )from 1 85 1 to 2 250 Ma.The results indicate that the intrusive rocks are likely derived from an ancient base-ment material.Geochemically,these rocks show moderate SiO 2 contents (56.52% - 66.06%),high and variable Na2 O (2.60%-6.81%),Al2 O 3 (14.69%-18.35%),and CaO (0.57%-6.58%,average 3.97%)contents,but low K2 O (0.73%-2.94%),TFeO and MgO contents.Most Na2 O/K2 O ratios are all higher (0.92-5.66,average 3.1 9).The trace elemental data of these rocks display enrichment of Rb,Ba,Th,U,K and Sr,but depletion of Nb,Ta,P and Ti.The total REE contents are low with moderate fractionation between LREE and HREE.Meanwhile,weakly negative Eu anormalyies (δEu ranges from 0.76 to 0.99)are observed in all samples.Thus,we propose that the Wulanaobaotu dioritic intrusive rocks were likely formed in an active continental margin setting during Late Cambrian to Early Ordovician,consistent with the existence of Early Paleo-zoic subduction-related tectonic-magmatic belt in Xing-Meng orogenic belt.The enriched zircon Hf isotopic characteristics fur-ther northern suggest the magma originated from a Precambrian basement source.