山东昌乐新近纪玄武岩中刚玉巨晶反应边的成因  

Coronas of corundum megacrysts in the Neogene Changle basalt and its forming model

作　　者：董泽龙[1] 陈小明[1] 胡文瑄[1] 王汝成[1] 赵明[1] 

机构地区：[1]内生金属矿床成矿机制研究国家重点实验室,南京大学地球科学系南京210093

出　　处：《岩石学报》2007年第4期805-816,共12页Acta Petrologica Sinica

基　　金：国家自然科学基金项目(No.40673023;No.40221301和No.49972025);高等学校博士学科点专项科研基金项目(20020284033)资助课题

摘　　要：通过对山东昌乐方山新近纪碱性玄武岩中原生刚玉巨晶的观察研究,发现在刚玉巨晶与玄武岩间存在反应边,可见新近纪碱性玄武岩中的刚玉巨晶为捕虏晶。电子探针下观察,反应边有两类:第1类由尖晶石带和钛磁铁矿+熔体带构成,第2类由尖晶石带和长石+钛磁铁矿带构成。在第1类反应边中,尖晶石带很窄(<40μm),钛磁铁矿+熔体带较宽(100～120μm);而且尖晶石的成分变化很大,从靠近刚玉一侧(内侧)向外,Al_2O_3逐渐降低,从85.08%～58.94%,MgO和FeO逐渐升高,分别从5.14%～14.82%和8.08%～28.24%。在第2类反应边中,尖晶石带较宽且稳定(100μm左右);长石+钛磁铁矿带不稳定,特别是钛磁铁矿不但窄(<40μm),而且分布断断续续;尖晶石的成分变化较小,Al_2O_359.25%～62.82%、MgO 12.34%～13.50%、FeO 23.25%～27.37%。本文对两类反应边的形成模式进行了探讨,认为刚玉巨晶与玄武岩浆反应持续时间的长短是导致形成两类反应边的主要因素,第1类反应边是反应没有达到平衡的产物,第2类反应边为反应平衡或者是接近平衡的产物。Coronas surrounding the primary corundum megacrysts captured by the Neogene Fangshan alkali basalt in Changle area, Shandong Province, is discovered through detailed electron microprobe observations. As an indicator of xenocryst for the corundum megacrysts, the coronas, made of two layers, could be divided into two types based on their characteristics. One type coronas is composed of a narrow spinel zone near the megacryst body and a wide titanomagnetite zone （ 100-120μm） with some alumosilicate as the exterior layer near the host basalt, and the other is composed of a spinel zone as the interior layer adjacent to the corundum and a feldspar zone （ 100μm） with a few titanomagnetite. The spinel belt, as the first reaction rim of all the primary corundums, is less than 40μm width in the former type coronas in which Al2O3 content decreases gradually from interior to exterior from 85.08% - 58.94% while contents of MgO and FeO vary reversely （5.14% - 14.83% and 8.08% -28.24% respectively）, but that of the second type is much wider （ about 100μm） and has the more stable composition compared to the first （ Al20359.25% -62.82% , MgO 12.34% 13.50%, FeO 23. 25% -27.37%）. However, the titanomagnetite zone in the second type, less narrower than 40μm width distributes uncontinuously between the spinel zone and the feldspar zone or inside of the feldspar zone. The coronas-forming models of the corundums were studied, it is believed that the first type corona is the result of the unequilibrium reaction between the corundums and host alkaline magma, and the second one is resulted from equilibrium or near equilibrium reaction.

关 键 词：碱性玄武岩 刚玉巨晶 反应边 形成模式 山东昌乐 

分 类 号：P588.145[天文地球—岩石学]