新生代碱性玄武岩及其蓝宝石成矿模式探讨——以鲁西昌乐地区为例

Metallogenic Mechanism of Corundum Megacrysts in Cenozoic Alkaline Basalt——A Case Investigation of Changle,Western Shandong

在全球张性构造环境中,刚玉/蓝宝石在新生代喜马拉雅期（23.3-0.73 Ma）的碱性玄武岩或碧玄岩层位中广泛赋存。来源于西太平洋地区碱性玄武岩中的刚玉/蓝宝石显示其主要形成于碱性熔体,而熔体的成分在不同地区有所差别。刚玉/蓝宝石的来源或为深源捕虏晶,或来自玄武岩浆在深部早期结晶,多数研究实例显示其来源深度可能为下地壳、壳幔边界或上地幔部位。鲁西昌乐新生代含刚玉/蓝宝石玄武岩的形成与该地区岩石圈的减薄和壳-幔物质的置换密切相关,其喷发受到深大断裂的控制,刚玉/蓝宝石为岩浆成因类型的深源捕掳晶,巨晶内部所含矿物、流体、熔体包裹体,指示其形成于高度演化的富含流体-熔体的碱性硅酸不饱和熔体中,碱金属在其结晶过程中起到了举足轻重的作用。该认识将为中国东部蓝宝石的成矿机制与成矿规律研究提供新的研究思路和证据,并为鲁西地区岩石圈演化的深部动力学研究提供实例。

In the global extensional tectonic environment, corundum/sapphire widely existed in some layers of alkali basalt or basanite of Cenozoic Himalayan period （23.3 - 0.73 Ma）. The corundum/sapphire in alkali basalts from the West Pacific continental margins indicated that they formed in alkaline inch,the ingredients of which varied a lot in different areas. The corundum/sapphire came from either the deep crystallized xenocryst or the early crystallization from deep-level basalt magma,and most studies suggested that the location of corundum/sapphire might be lower crust,boundary of crust-mantle, or upper mantle. The formation of Cenozoic basalt with corundum/sapphire in Changle,Shandong Province, was closely related with the thinning of the lithosphere and the exchange between crust and mantle,and its eruption was controlled by extensional fault. Changle corundum/sapphire belonged to deep xenocrysts of magmatic sources, and the minerals, fluids and melt inclusions in it indicated that corundum/sapphire formed in highly evolved silicate unsaturated alkaline melt which were rich in fluids and melts,and that alkali metal played a key role in the process of corundum/sapphire crystallization. This result can not only provide new ideas and evidence for the further study of metallogenic mechanism and regularity of corundum/sapphire in eastern China, but it can also provide examples for the study on deep dynamics of lithosphere evolution in western China.