Petrogenesis of the Beijie pluton from the northwestern Jiaodong Peninsula: Constraints from zircon U-Pb age, petrogeochemistry and Sr-Nd-Pb isotopes.

The northwestern Jiaodong Peninsula is the most important gold mineralization area in China. The majority of gold resources (> 95%) in Jiaodong are hosted in the Late Jurassic Linglong-type and Early Cretaceous Guojialing-type granitoids. However, there is still controversy over the geotectonic setting of Guojialing-type granitoids and its relationship with gold mineralization, especially its petrogenesis. This paper systematically investigated the Beijie pluton which belongs to Guojialing-type granitoids to analyse its petrography, petrogeochemistry, zircon U-Pb age, and Sr-Nd-Pb geochemistry. Fine-grained granodiorite, porphyritic finegrained granodiorite, porphyritic medium -grained granodiorite, porphyritic coarse -grained granodiorite and granite pegmatite are identified in Beijie pluton. They have the similar geochemical characteristics and isotope compositions, and present continuous evolution tendency. All these suggest the five intrusive rocks are the product of the same magmatic activity. Electron probe analysis shows the biotites in granodiorites have high MgO, FeOT, Al2O3 and low TiO2, indicating the Fe -rich biotite is magmatic origin. Negative correlation with Mg and Al implicit they may be displaced during crystallization. Petrogeochemistry studies show that: (1) the - [ Negative correlation with Mg and Al implicit they may be displaced during crystallization. Petrogeochemistry studies show that: (1) the - 6. 05 x 10(-6)), Yb (0.08 x 10(-6) - 0.40 x 10(-6)), enrichment in LREEs, relative depletion in HREEs (LREE/HREE = 13. 13 21. 40); (2) the fractional distillation of the LREEs and HREEs is obvious [ (La/Yb), = 18. 39 45. 76]; (3) the LILE (e. g., Rb, Sr, Ba and Pb) is enriched, while the HFSE (e. g., Nd, Ta, Zr and Hf) is obviously depleted. All these features suggest that the rocks are similar to the Ba-Sr granites. Zircon LA-ICP-MS U-Pb dating yield 131. 53 0. 86Ma for the granodiorite and 127. 4 1. 3Ma for the granitic pegmatite, indicating they are formed in Early Cretaceous. The granitic pegmatite should be the product of the late magmatic activity. Sr-Nd isotope analysis shows that they have ISr and ENd (t) values of O. 710658 - 0. 711568 and - 18. 53 Beijie granodiorites possess high total alkalis (K2O + Na2O = 6. 92% 9. 31%), Ba (518 x 10 -6 3330 x 10 -6), Sr (305 x 10 -6 1371 x10,with low A1203(14. 41% -15. 82%), MgO (O. 19% -O. 77%), Rb (39. 3 x10(-6) -105 x10(6) , Y (1. 2 x10(-6) -11. 17 respectively, with t(2) of 2107 -2425Ma which have the same age with the metamorphic rocks of the Jiaodong Group. This indicates that the partial melting of Jiaodong Group is involved in the primordial magma. Pb isotope analysis shows their Pb compositions are 17. 047 -17.945 ((206 Pb/ 2" 208Pb))) and 37.744 -38.389 Pb/ 204 Pb)1), implying a large amount of lower crust material involved in the diagenesis. The above data analysis shows the Beijie pluton might be generated by acidic magmas from lower crust formed by partial melting of the metamorphic rocks in Jiaodong Group mixed with basic magma derived from mantle. Upwelling of asthenosphere material and partial melting of the lower crust under the tectonic background of subduction of the Pacific Plate to the Eurasian Plate may be the mechanism of the formation of this granite. It is part of the volcanic arc at the edge of the Eurasian Plate created by subduction. In terms of regional gold mineralization, it provides favorable occurrence space for gold mineralization, rather than the source of metallogenic fluid or thermal dynamic conditions.