阿尔巴尼亚布尔齐泽壳-幔过渡带豆荚状铬铁矿成因及其对富Ti熔体交代作用的记录

Petrogenesis of chromitites and its records of Ti metasomatism in crust-mantle transition zone, Bulqiza ophiolite massif, Albania

关键词:  布尔齐泽岩体  壳-幔过渡带  豆荚状铬铁矿  熔体交代  金红石

Key words:  Bulqiza ophiolite massif  crust-mantle transition zone  podiform chromitites  melt metasomatism  rutile

摘要:

豆荚状铬铁矿是关键金属铬的重要来源之一，尽管豆荚状铬铁矿的研究取得了诸多进展，但对于发育于蛇绿岩壳-幔过渡带的铬铁矿成因却涉及较少。阿尔巴尼亚布尔齐泽岩体壳-幔过渡带中产出的Cerruja豆荚状铬铁矿矿床，其矿体及纯橄岩围岩普遍被辉石岩脉穿切，辉石岩脉与矿体接触带以及辉石岩脉中的铬尖晶石强烈破碎，在铬尖晶石的裂隙和包裹体中发育大量富Ti矿物相，如金红石、钛铁矿和榍石等，是研究壳-幔过渡带铬铁矿成因的理想对象。Cerruja豆荚状铬铁矿及纯橄岩围岩中铬尖晶石Cr#分别为0.56~0.58和0.52~0.55，属于高铝型铬铁矿。接触带及辉石岩脉中的铬尖晶石Cr#明显升高（分别为0.57~0.67和0.72~0.83），且Ti、V、Mn、Sc、Co、Zn和Ga含量也升高。本文依据铬尖晶石的结构及矿物化学成分变化特征，提出布尔齐泽壳-幔过渡带铬铁矿经历多阶段演化叠加：首先，Mirdita-Pindos洋盆在侏罗纪（约165 Ma）发生洋内初始俯冲，软流圈物质上涌生成的MORB-like弧前玄武质熔体随着俯冲的进行逐渐向玻安质熔体演变，期间产生的过渡型熔体与地幔橄榄岩反应生成高铝型铬铁矿；然后，部分MORB-like弧前玄武质熔体随着堆晶间隙分离结晶往富Fe和Ti的方向演化，改造早期形成的高铝型铬铁矿并结晶高铬型铬铁矿，同时生成金红石、钛铁矿和榍石等富Ti矿物相。

Abstract:

Podiform chromitites are one of the important sources of the key metal chromium. A lot of progress has been made on the research of podiform chromitites, but little referred to the genesis of the chromitites located in crust-mantle transition zone of ophiolite. The Cerruja podiform chromitites and dunite wall-rocks were intruded by pyroxenite dikes in the crust-mantle transition zone of the Bulqiza ophiolite massif, Albania. Highly brecciated spinel and Ti-bearing minerals such as rutile, ilmenite and titanite were found in pyroxenite dikes and in the interaction zone between pyroxenite dikes and chromitites. Such characteristics make them an ideal subject for the study of the chromitites in the crust-mantle transition zone. Cerruja podiform chromitites are high-Al variety with Cr# of chromitites ore varying from 0.56 to 0.58 and of dunite wall-rocks varying from 0.52 to 0.55. Spinels in the interaction zone between pyroxenite dikes and chromitites and in the pyroxenite dikes are characterized by the obviously higher Cr# value (0.57 to 0.67 and 0.72 to 0.83, respectively) than chromitites ores. Contents of Ti、V、Mn、Sc、Co、Zn and Ga of spinels in the interaction zone are higher with the closer distance to the pyroxenite dikes. According to the texture characteristics of spinel and the variations of mineral chemical composition, we propose that the chromitites in the crust-mantle transition zone of Bulqiza massif is the result of a multi-stage process:First, high-Al chromitites were produced by the reaction between peridotites and the transition melts which has the geochemical properties both of MORB-like and boninitic, formed during the evolution of initial subduction of the Mirdita-Pindos ocean basin (~165 Ma); and then, Ti-Fe-rich residual melts were produced by intercumulus crystal fractionation of the MORB-like melt in a crystal-melt mush, metasomatizing and transforming the surrounding high-Al chromite into high-Cr chromite, and also crystallizing Ti-rich minerals such as rutile, ilmenite and titanite.