Geochronology and petrogenesis of the Shuizi pyroxenite at Danba, western Yangtze Craton

Abstract

扬子克拉通西缘在~260Ma发生短期内大规模峨眉山玄武岩溢流喷发。攀西地区发育的镁铁-超镁铁质岩被广泛认为是峨眉山大火成岩省的产物，但在北端松潘-甘孜岩区一直缺乏该类岩石的报道。本文首次报道扬子西缘丹巴水子乡单斜辉石岩的准确年龄，其锆石LA-ICP-MS U-Pb加权平均年龄为260.7±3.3Ma，表明其为峨眉山大火成岩省北端松潘-甘孜岩区镁铁-超镁铁质岩的组成部分。通过与攀枝花钒钛磁铁矿含矿岩体边缘相带苦橄岩和上部相带浅色辉长岩进行锆石微量元素对比显示，水子乡单斜辉石岩具有相近的高氧逸度，其ΔQFM为0~3，Ce_N/Ce_N^*平均为~30，该性质可能同样源自扬子西缘洋壳板片俯冲交代形成的较高氧逸度地幔源区。尽管如此，水子乡辉石岩体并未因高氧逸度而有明显的含钛磁铁矿饱和结晶，可能由其较低结晶分异程度造成。相比之下，攀枝花岩体经历了更高程度的含钛磁铁矿和斜长石分离结晶作用，伴随大规模的钒钛磁铁矿成矿。;Large-scale eruption of Emeishan flood basalt occurred in the western margin of the Yangtze Craton in a short period during~260Ma. Mafic-ultramafic rocks found in the Panxi area are widely considered to be products of the Emeishan Large Igneous Province (ELIP). However, there is a lack of reports on this kind of rocks in the Songpan-Ganzi area in the northern end of the ELIP. This paper first documents a precise age of the Shuizi pyroxenite in the Danba County. The zircon LA-ICP-MS U-Pb weighted mean age is 260.7±3.3Ma, indicating a representative mafic-ultramafic constituent of the northern end of the ELIP in the Songpan-Ganzi area. Comparing zircon trace elements of the Shuizi pyroxenite with those of picrite in the marginal zone and gabbro in the upper zone of the Panzhihua Fe-Ti-V oxide deposit were conducted. It is found that the Shuizi pyroxenite has similarly high magmatic oxygen fugacity, ΔQFM of 0~3 and average Ce_N/Ce_N^* of~30, to the Panzhihua mafic rocks. The high oxygen fugacity for both is considered to be derived from a mantle lithosphere source which had been metasomatized by subducted oceanic slab materials in the western margin of the Yangtze Craton. Although the Shuizi pyroxenite has high oxygen fugacity, there is less obvious Fe-Ti oxide saturation and precipitation here. This could be attributed to its relatively low degree of crystallization and differentiation. In contrast, the Panzhihua mafic-ultramafic intrusion experienced higher degree of titanomagnetite and plagioclase fractional crystallization, accompanied by a massive Fe-Ti-V oxide mineralization.