攀西地区红格层状岩体上部带中单斜辉石的环带结构特征及其成因

Origin of complex zoning of clinopyroxene from the upper zone of the Hongge layered intrusion in the Panxi region, SW China

我国攀西地区出露的红格镁铁-超镁铁质层状岩体赋含世界级的钒钛磁铁矿矿床。该岩体自下而上被分为下部橄辉岩带、中部辉石岩带和上部辉长岩带。厚层磁铁矿主矿体主要赋存在中部辉石岩带中,而下部带和上部带发育浸染状和海绵陨铁结构矿石。通过对上部带中含浸染状磁铁矿的辉长岩样品的岩相观察发现,在单偏光镜下单斜辉石发育多种类型环带结构,表现为由强烈出溶铁钛氧化物的暗色区和弱或无出溶的明亮区组成的复杂环带特征。弱或无出溶区相对强出溶区具有略高的Mg~#值(76~79)和较低的FeO、TiO_2、Al_2O_3、REE和Zr含量。在弱或无出溶区和强出溶区的边界,TiO_2和Al_2O_3含量发生突变,表明单斜辉石经历了增生生长的过程。强出溶区富Fe、Ti的特征表明其结晶于富Fe熔体,可能与岩浆房晶粥中粒间熔体不混溶作用产生的富Fe熔体有关。弱或无出溶区可能结晶较早、并受到富Fe熔体不同程度的溶解,随后增生再生长,形成铁钛氧化物强烈出溶区。单斜辉石单颗粒中强出溶区的REE和Zr含量变化较大,可能与粒间熔体的成分随分异的进行而持续演化有关。

The Hongge mafic-ultramafic layered intrusion in the Panxi region,SW China,hosts a world-class Fe-Ti-(V)oxide ore deposit.The intrusion is divided into the lower,middle and upper zones,which are mainly composed of wehrlite,clinopyroxenite,and gabbro,respectively.The middle zone hosts thick,major ore bodies,whereas the lower and upper zones contain disseminated ores and net-textured ores,respectively.In this study,a detailed petrographic investigation was carried out on the gabbro samples with disseminated Fe-Ti oxides in the upper zone.The clinopyroxene grains in the gabbro show diverse zoning patterns,which are mainly composed of dark domains that contained intense exsolution lamellae of Fe-Ti oxides and bright domains that contained less or no exsolution lamellae.Compared with the dark domains,the bright domains of clinopyroxene had relatively high Mg^#and low FeO,TiO2,Al2O3,REE,and Zr contents.There are sharp variations of TiO2and Al2O3contents between the bright and dark domains,indicating that the clinopyroxene grains experienced overgrowth process.The dark domains with abundant exsolution lamellae of Fe-Ti oxides may have crystallized from Fe-rich melt,which resulted from unmixing within a crystal mush.The bright domains with less or no lamellae may have crystallized earlier and then were dissolved by the Fe-rich melt.The complex zoning of the clinopyroxene grains may indicate the dissolution of the bright domains followed by the re-precipitation of Fe-rich melt forming the dark domains.The dark domains with highly variable REE and Zr contents may be related to the progressive fractionation of the Fe-rich melt.