氧逸度对攀枝花岩体成岩成矿作用的制约:来自锆石微量元素的证据

Constraints of oxygen fugacity on the formation of the Panzhihua layered intrusion and its mineralization:Evidence from trace element in zircon

本文报道了攀枝花钒钛磁铁矿含矿岩体边缘岩相带中的苦橄玢岩和岩体中淡色辉长岩的锆石微量元素特征。结果表明二者所含锆石都具有明显的Ce正异常和Eu负异常,以及轻稀土元素亏损和重稀土元素富集的特征,其Th/U比值为0.35~3.23,都属于典型的岩浆锆石。本次研究利用最新实验标定的锆石氧逸度计对苦橄玢岩和淡色辉长岩的氧逸度进行了估算。估算结果表明苦橄玢岩和淡色辉长岩均具较高的氧逸度,分别为QFM+0.3~QFM+2.5和QFM+0.7~QFM+3(QFM为石英-铁橄榄石-磁铁矿缓冲剂)。苦橄玢岩作为来自深部岩浆房侵入到攀枝花主岩体的富橄榄石"晶粥体",其高氧逸度的特征反映出攀枝花岩体的原生岩浆以及地幔源区是相对氧化的,而导致这一结果的原因很可能与古老俯冲事件导致的地幔交代作用有关。通过地幔柱-岩石圈相互作用,在较高氧逸度下发生部分熔融形成了铁质苦橄岩及其堆晶作用产物苦橄玢岩。此外,淡色辉长岩的氧逸度也显示出较高的特征,这说明这种氧化的特征很可能是贯穿了整个成岩过程的,对钒钛磁铁矿成矿,特别是导致铁钛氧化物早期结晶起到了不可忽视的作用。

This study reports trace elemental characterisitics of zircons from leucogabbro and picritic porphyry in the marginal zone of the Panzhihua layered intrusion which hosted Fe-Ti oxide deposit.The studied zircons are of magmatic origination and characterized with positive Ce anomalies,negative Eu anomalies,depletion of light rare earth elements and enrichment of heavy rare earth elements,with Th/U ratios ranging from 0.35 to 3.23.The oxygen fugacity values of the picritic porphyry and the leucogabbro are estimated by the latest experimentally calibrated zircon oxybarometer.The results show that both of the oxygen fugacities of the picritic porphyry and the leucogabbro are high,ranging from QFM+0.3~QFM+2.5 and QFM+0.7~QFM+3(QFM is quartz-fayalite-magnetite buffer),respectively.According to previous studies,the Panzhihua picritic porphyry was crystallized from an olivine crystal mush which had derived from a deep magma chamber that intruded into the Panzhihua layered intrusion.The high oxygen fugacity of the studied samples reflects that the primary magma and mantle source of the Panzhihua layered intrusion is relatively oxidized,which is probably related to the mantle metasomatism caused by the ancient subduction events.The interaction of the lithospheric mantle with the mantle plume leads to partial melting under the high oxygen fugacity to form the ferropicritic primary melts and its cumulation product of the picritic porphyry.The leucogabbro also shows high oxygen fugacity characteristics,which indicates the layered intrusion and Fe-Ti oxide mineralization are formed under oxidizing condition.Therefore,such an oxidizing condition could account for the early crystallization of Fe-Ti oxides and thus plays an important role in the Fe-Ti oxide mineralization.