Geochronology, geochemistry and zircon Hf isotope of Early Paleozoic igneous rocks in Sonid Zuoqi, central Inner Mongolia and their tectonic significances

The Early Paleozoic granitic rocks and volcanic sequence outcropped in the southeastern Sonid Zuoqi area of central Inner Mongolia are important significances in constraining the tectonic evolution of the Sonid Zuoqi arc and even the Inner Mongolia-Da Hinggan Orogenic Belt. In this study, we present new zircon U-Pb ages, precise whole-rock geochemistry of major and trace elements and Hf isotope compositions for these granitic rocks and basaltic-andesite sequences. The LA-ICP-MS zircon U-Pb dating results indicate that the granitic rocks are formed in Late Ordovician and Early Silurian (441 similar to 449Ma) . The granitic rocks are dominated in granites with minor granodiorites, which are characterized by high abundances of SiO2 (69. 60% similar to 77. 36%) and Al2O3(12. 70% similar to 15. 40%) , low MgO ( 0. 19% similar to 0. 81%) and Fe2O3 (0. 94% similar to 3. 49%) contents. In addition, the granitic rocks yield slightly negative to positive epsilon(Hf) ( t) values (- 0. 81 similar to +5. 64) and young Hf two-stage model (t(DM2)) ages (1119 similar to 1478Ma). The petrological and geochemical analyses reveal that the primary magma that formed the granitic rocks was possibly originated from the partial melting of predominant juvenile crustal materials with subordinate Mesoproterozoic continental crust. The basaltic-andesite sequences are composed of basalt, trachyandesite and andesite. They are featured by low SiO2 content (49. 13% similar to 57. 82%) , high MgO (3. 31% similar to 6. 57%) and Fe2O3 (6. 54% similar to 9. 63%) contents. In addition, the Zr/Hf ratios (35. 4 similar to 37. 6) of the basaltic-andesite sequences are close to those of primitive mantle. Moreover, the basaltic-andesite sequences exhibit high abundances of Al2O3 (16. 77% similar to 18. 34% ) and Na2O (3. 46%similar to 5. 05%) , positive Sr anomaly and high Th/Ce ratios (0. 12 similar to 0. 25 ). Such features suggest that the basalticandesite magma could be derived from the partial melting of metasomatized mantle wedge. All of the studied igneous rocks belong to calc-alkaline series, exhibit enrichment of large ion lithophile elements (LILEs; e. g. , Rb, Th and K) and light rare earth elements, and exhibit depletion of high field strength elements ( HFSEs; e. g. , Nb, Ta, Ti and P) . Furthermore, their Rb/Sr ratios (0. 07 similar to 1. 0) and the abundances of Nb (4. 24 x 10(-6) similar to 15. 17 x 10(-6)) are close to those of normal continental arcs. In combination with previous research into the spatial-temporal distribution of Early Paleozoic igneous rocks in Sonid Zuoqi, we suggest that the Early Paleozoic igneous rocks in this area were probably attributed to the northward subduction of Paleo-Asian Ocean.