Geochronology, geochemistry, Hf isotope, and electron probe analysis of the diorite in the Jianhei Mountain, East Kunlun Orogenic Belt, China, and their geological significance

摘要

The Eastern Kunlun orogenic belt (EKOB) is located in southern margin of the Qaidam Basin. After the evolution of the Proto-Tethys Ocean and the Paleo-Tethys Ocean, the geological structure is complex and diverse in the EKOB. Herein, the results of geochronological, geochemical, Hf isotope, and electron probe analyses are presented for the Jianhei Mountain diorite in the EKOB. The results of the geochemical studies show that the rock is enriched in Al, Ca, and Fe, depleted in Ni and Cr, and contains moderate amounts of Mg. The samples exhibit a high degree of fractionation of light and heavy rare earth elements (REEs), with the light REEs being enriched, and the heavy REEs being depleted. Furthermore, the samples are enriched in the large ion lithophile elements, and depleted in the high field strength elements. The Jianhei Mountain diorite yields a weighted mean zircon U-Pb dating age of 251.3 +/- 1.8 Ma, which is in the Early Triassic. The Zircon Hf isotope analysis gives a zircon (Eu/Eu-;)(N) value that ranges from -5.2 to -3.3, and the two-stage model age is 1244-1342 Ma. Combined with other geochemical characteristics, it can be concluded that the source area of the Jianhei Mountain diorite is dominated by Mesoproterozoic crustal materials, with a small amount of mantle material involved in the petrogenetic process. In addition, the results of electron probe micro-analysis show that the MgO content of the hornblende is 9.57-10.40%, the Mg/(Mg + Fe2+) ratio is 0.49-0.52, the Mg content is 2.15-2.31, the Ca content is 1.71-1.80, and the total of Fe3+ + Fe2+ is 2.04-2.17. The MgO content of the biotite is 8.47-9.31%, and the Fe2+/(Mg + Fe2+) ratio is 0.50-0.52. In accordance with previous studies, the mineral compositions of the hornblende and biotite are closely related to their genesis. It can be concluded that the hornblende and biotite of the Jianhei Mountain diorite have the characteristics of a crust-mantle mixed source. Overall, the present authors believe that the Jianhei Mountain diorite is the product of partial melting of the lower crust in the tectonic background of the northward subduction of the Paleo-Tethys Ocean, with a small amount of mantle material involved in the petrogenetic process.