厄立特里亚Koka花岗岩锆石U-Pb年代学、地球化学特征及其地质意义

Koka花岗岩位于厄立特里亚Nakfa地区以西，是Koka金矿床的主要赋矿围岩.岩体具有富SiO₂（67.94%~78.40%）、Na₂O+K₂O（5.86%~8.76%）、Al₂O₃（11.05%~16.51%）、FeOT（2.46%~3.80%），弱过铝质－强过铝质（A/CNK为1.09~1.55），低CaO（0.06%~1.85%）、MgO（0.15%~0.39%）的主量元素特征，同时轻稀土富集，重稀土相对亏损，强烈亏损Sr、P、Ti元素，REE分配曲线呈现燕式分布和明显的负铕异常，表明岩体具有A型花岗岩的特征.岩体锆石LA-ICP-MS U-Pb年龄显示其成岩年龄为851.2±1.9 Ma，属早新元古代，不同于区域上广泛分布的与造山后伸展作用相关的A型花岗岩（650~540 Ma），结合区域研究成果认为，其可能形成于由俯冲作用而引起的弧后拉张环境.岩体锆石具有一定的Ce正异常，Ce4+/Ce3+变化范围为3.86~146.31，平均为32.4，指示岩浆的氧逸度相对较低，结合岩浆源区为较“干”的体系特征，暗示该岩体成矿潜力较低，难以形成相关的大型、超大型矿床. 

Zircon U-Pb Geochronology and Geochemistry of Koka Granite and Its Geological Significance,Eritrea

Koka granite, located in the west of the Nakfa region in Eritrea, is the main host rock of Koka gold deposit. The major elements of the granite are characteristiced by high SiO₂ (67.94%-78.40%), Na₂O+K₂O (5.86%-8.76%), Al₂O₃ (11.05%-16.51%) and FeOT (2.46%-3.80%), weakly peraluminous to strongly peraluminous (A/CNK is 1.09-1.55), and low CaO (0.06%-1.85%), MgO (0.15%-0.39%). It is also enriched in LREEs and relatively depleted in HREEs, strongly depleted in Sr, P, Ti. The REE distribution curve shows characteristics such as swallow distribution and apparent negative europium anomalies. All of these indicate that the Koka granite has an affinity of A-type granite. Zircon LA-ICP-MS U-Pb dating shows the granite formed in early period of the Neoproterozoic with the diagenetic age of 851.2±1.9 Ma. This is different from the A-type granite which is widely distributed and related to the extension after collisional orogen (650-540 Ma). Combined with the regional geological research results, it is suggested that Koka granite was formed in a back-arc extensional environment which resulted from subduction. The zircon has a certain Ce positive anomaly, and variation range of the Ce4+/Ce3+ is 3.86-146.31, with an average of 32.4, indicating low degree of magmatic oxygen fugacity. Meanwhile the parental magma of Koka granite is "dry", suggesting that the metallogenic potentiality is low, and it is hard to form related large or super large deposits.