峨眉山大火成岩省内带黑谷田含钒钛磁铁矿层状岩体成因

Petrogenesis of the Heigutian Ti-V-magnetite ore-bearing layered intrusion,the inner zone of the Emeishan large igneous province

黑谷田岩体产于峨眉山大火成岩省内带，是一个小型含钒钛磁铁矿辉长岩体。与区内其它典型大型基性-超基性层状岩体具有多个旋回岩相的特征不同，黑谷田层状岩体分为下部、上部两个岩相带：下部岩相带从底到顶依次为橄榄辉石岩、磁铁辉长岩、含磷灰石辉长岩和中粒辉长岩，上部岩相带为细粒辉长岩，二者呈突变接触关系。黑谷田岩体的锆石SHRIMPU-Pb年龄为2634-5Ma，表明其是-260Ma峨眉山地幔柱岩浆主活动期的产物。岩石的矿物组合（主要为单斜辉石、斜长石，磁铁矿，少量橄榄石等）、元素地球化学（富Fe2O2、TiO2、P2O5，高Sm／Yb及低La／Sm）及低的初始87Sr／86Sr值和亏损的εNd（t）值特征一致指示黑谷田岩体与峨眉山高Ti玄武岩具有密切的内在成因联系。岩体的岩相学及地球化学特征暗示下部岩相带是富Fe-Ti岩浆侵入发生橄榄石、单斜辉石、磁铁矿、斜长石、磷灰石等矿物分离结晶、堆积固结的产物，而上部岩相带是另一期岩浆上侵较为快速冷却固结的结果，矿物堆晶作用不显著，但是二者起源于相同的母岩浆。下部岩相带比上部岩相带具有相对低的初始。87Sr／89Sr值（分别为0．7041～0．7051和0．7050～0．7056）和略高的εNd（t）值（分别为2．1～4．4和0．6～1．3），表明后者比前者经历了稍微强烈的地壳物质同化混染。下部岩相带仅有橄榄辉石岩及辉长岩而缺少正长岩和花岗岩、以及较厚的氧化物矿体赋存在岩体底部下凹部位说明黑谷田钒钛磁铁矿形成于岩浆通道系统中，磁铁矿在流动过程中由于重力作用堆积成矿。黑谷田含钒钛磁铁矿岩体的发现表明小型层状岩体也具有重要的Fe—Ti氧化物成矿潜力，在勘探找矿中不容忽视。

The Heigutian intrusion is a small-scaled gabbroic intrusion containing Ti-V-magnetite oxides and is located in the central part of the Emeishan large igneous province. In contrast to other typical large layered intrusions characterized by several cyclic units in a petrographic zone, the Heigntian intrusion is subdivided into a lower zone and an upper zone. The lower zone consists of olivine pyroxenite, magnetite gabbro, apatite gabbro and medium-grained gabbro from the base upward, whereas the upper zone consists of primarily fine-grained gabbro, and demonstrates a sharp contact between these two zones. The zircon SHRIMP U-Pb age dating result displays that the Heigutian intrusion was intruded at 263 ± 5Ma, and may be the product of the main stage of plume-related magmatism at -260Ma. The Heigutian intrusion shows a close genetic relationship to the Emeishan high-Ti basalts, evidenced consistently by the mineral assemblages （mainly by clinopyroxene, plagioclase, magnetite, and small amounts of olivine, apatite）, geochemical features （enriched in Fe2 O3, TiO2, and P2O5, high Sm/Yb and low La/Sm ratios）, and low initial 87Sr/86 Sr ratios as well as high εNd （t） values in the mafic rocks. The accumulation sequences and geochemical features reveal the lower zone was resulted from accumulation of olivine, clinopyroxene, magnetite, apatite and plagioclase crystallized from a single pulse of Fe-Ti-enrich magma, whereas the upper zone was generated by slightly rapid cooling consolidation of a new influx of magma. However, both the lower and upper zones were originated from the same parental magma genetically related to the Emeishan mantle plume. The lower zone has lower initial 87Sr/86Sr （0. 7041 - 0. 7051 ） and higher εNd （t） values （ 2. 1 - 4.4） than the upper zone （0. 7050 - 0. 7056, and 0. 6 - 1.3, respectively）, suggesting the later had experienced slightly more extensive crustal contamination relative to the former. Only the olivine pyroxenite and gabbro occur in the lower zone but absence of granites and synenites. The thick stratiform Fe-Ti oxide layers are located at the concave part of the base of the Heigntian intrusion. These observations imply that the Heigutian intrusion occurred in a magma plumbing system, and the Fe-Ti oxide layers resulted from coupling of gravity settling and sorting of the crystallized Fe-Ti oxides from Fe-Ti- enriched magmas. The discovery of the Heigutian Fe-Ti oxide bearing intrusion indicates that the small-scaled intrusions can also be very significant targets to host Fe-Ti oxide deposit and should not be ignored during exploration.