Petrogenesis of the Early Cretaceous adakitic intrusive rocks in Chuzhou and Guandian, Anhui Province: Constraints from zircon U-Pb geochronology and Sr-Nd-Hf isotopic compositions

This paper reports new geochemical data involving whole -rock major, trace and Sr-Nd isotopic compositions, and zircon U-Pb ages with Hf isotopic data for the Chuzhou dioritic porphyry and Guandian quartz diorite in the Early Cretaceous, with the purpose of constraining their petrogenesis and tectonic significance. Zircons in the dioritic porphyry display typical oscillatory zoning and in quartz diorite show clear striped absorption in cathodoluminescence images, both of which have magmatic origin. LA-ICP-MS zircon UPb dating yield age of 128 1Ma in dioritic porphyry and 130 2Ma in quartz diorite respectively, indicating that they were both emplaced during the Early Cretaceous. The dioritic rocks show intermediate Si02 ( 58. 55% - 60. 12%) and high Mg0 contents (4. 43% - 5. 10%, Mg# = 57 - 61) with Na20/K20 = 1. 20 -1. 47, which are classified as high -K cala-alkaline series. In addition, they are enriched in LREE and LILE ( e. g., Rb and Ba), depleted in HREE and HFSE ( e. g., Nb, Ta, Zr and Hf) with negative Eu anomalies (8Eu = 0. 85 - 0. 93), and exhibit high Sr (575 x 10 -6 - 1035 x 10 -6) and low Y contents ( 10. 2 x 10 -6 - 15.3 x 10 -6) with significantly high Sr/Y ratios (57.-69), being typical of adakitic rocks. Moreover, the Chuzhou dioritic porphyry has uniform (87Sr/86Sr), ratios of O. 7061 - 0. 7066, eNd ( t) values of - 17. 7 - - 16. 9, and eHf ( t) values of - 23. 1 - 19. 1, and Guandian quartz diorite has similar (87Sr/86Sr), ratios of 0. 7046 - 0. 7059 and variable ENd ( t) ( - 18. 7 - - 15. 8) and EHf t) ( - 25. 6 20. 0) values. As a consequence, the adakitic intrusive rocks in Chuzhou and Guandian are derived from the partial melting of delaminated lower crust in contamination with mantle peridotite, and we supposed that the rocks are formed in an extension setting during the Early Cretaceous delamination in eastern North China Block.