烧变岩矿物相特征及其转化

Characteristics and transformations of mineral phases in burnt rocks

【目的】烧变岩作为一类由煤层自燃烘烤围岩形成的特殊岩石,在鄂尔多斯盆地东北缘延安组广泛发育,该类岩石中烧变相关的矿物相特征及形成机理尚不清楚。【方法】对陕西张家峁煤矿井下烧变岩进行分析测试和原岩高温加热模拟,进行烧变岩矿物相特征分析。【结果和结论】研究发现:井下烧变岩中铁镁质成分富集,Zn、Rb、Sr、Zr等微量元素相较原岩异常迁移,稀土元素Eu迁出,La/Yb−ΣREE分配模式更接近花岗岩。显微镜下烧变岩发育绿泥石化、磁铁矿化和赤铁矿化,发育变余结构和矿物烧熔现象,同时发育高温变质结晶矿物,如方石英、鳞石英、莫来石和堇青石。烧变岩变质作用类型主要为重结晶作用和熔融作用,缺少典型热变质岩中的交代蚀变或同化混染作用,也不发育含水或含羟基矿物,这种特征可能是由于烧变作用条件为近地表低压和开放环境,其变质作用体系内无H2O流体作用于矿物间的物质交换。结合烧变作用条件和矿物组合特征,认为烧变作用是不同于其他热变质作用的特殊变质类型。综合前人和本次研究,将烧变岩变质相作为一类特殊的热接触变质作用提出,按变质温度由低到高将其分为4个阶段:(1)低温脱水相阶段(绿泥石化、含水矿物被破坏)、(2)中温氧化相阶段(磁铁矿化、赤铁矿化)、(3)高温烧熔相阶段(莫来石、堇青石、方石英和鳞石英)、(4)完全熔融相阶段(辉石、假玻璃质长石)。研究对烧变岩矿物相及其转化做了系统总结,并在变质岩相系及其变质条件研究的基础上总结烧变相系,认为温度和近地表低压开放体系是不同烧变岩相形成的原因。

[Objective]Burnt rocks are special rocks formed by the surrounding rocks that are baked due to the spontaneous combustion of coal seams.They are extensively distributed in the Yan’an Formation on the northeastern margin of the Ordos Basin.However,the characteristics and formation mechanisms of burning-associated mineral phases in these rocks remain unclear.[Methods]This study conducted analytical tests and high-temperature heating simulations of protolith to analyze the mineral phases of underground burnt rocks in the Zhangjiamao Coal Mine,Shaanxi Province.[Results and Conclusions]The results indicate that the underground burnt rocks exhibit enrichment in mafic components,abnormal migration of trace elements like Zn,Rb,Sr,and Zr compared to protoliths,outward migration of rare earth ele-ment(REE)Eu,and a La/Yb-ΣREE distribution pattern closer to that of granites.Microscopic observations reveal that the burnt rocks feature chloritization,magnetization,and hematitization,as well as palimpsests and mineral melting,with the presence of high-temperature metamorphic crystalline minerals such as cristobalite,tridymite,mullite,and cordierite.Primary metamorphism types of the burnt rocks include recrystallization and melting,while the metasomatic alteration(or assimilation and contamination)observed in typical thermally metamorphosed rocks and hydrous or hydroxyl-bear-ing minerals are absent.This characteristic may be due to the fact that the burning process occurred in a near-surface low-pressure and open system,where no H2O fluids acted on the material exchange between minerals within the metamorph-ism system.Based on the burning conditions and mineral assemblages,it can be inferred that the burning process is a special metamorphism type different from other thermal metamorphisms.In combination with previous results,this study proposes that the burnt rocks are characterized by special contact thermal metamorphism,which can be divided in-to four stages based on metamorphic temperatures:low-temperature dehydration(chloritization and the destruction of hydrous minerals),moderate-temperature oxidation(magnetization and hematitization),high-temperature melting(mullite,cordierite,cristobalite,and tridymite),and complete melting(pyroxene and pseudo-vitreous feldspar).This study systematically summarizes the mineral phases and their transformations in burnt rocks,providing a summary of the metamorphic facies series of burnt rocks based on the analysis of the metamorphic rock facies series and their meta-morphic conditions.It can be concluded that temperature and a near-surface low-pressure and open system are the causes of the formation of different burnt rocks.