Integrated O-Sr-Nd isotope constraints on the evolution of four important Fe-Ti oxide ore-bearing mafic-ultramafic intrusions in the Emeishan large igneous province, SW China

摘要

In the Emeishan large igneous province, SW China, there are many layered mafic-ultramafic intrusions such as the Baima, Hongge, Panzhihua and Taihe intrusions that host world-class Fe-Ti oxide ore deposits. Despite numerous studies, the origin of these deposits still remains elusive. This includes the role of crustal contamination, especially addition of external CO2 from carbonate country rocks during contact metamorphism, in triggering Fe-Ti oxide crystallization from high-Mg basaltic magma. To address this important issue, we have carried out an integrated O-Sr-Nd isotope study of these ore-bearing intrusions and the country rocks. Our results show that in these intrusions clinopyroxene is much less susceptible to fluid-mineral oxygen isotope exchange than coexisting plagioclase and Fe-Ti oxides, which is similar to other intrusions worldwide (e.g., Taylor, 1967; Gregory and Taylor, 1981). Our calculations based on the least exchanged clinopyroxene oxygen isotope data show that the mean delta O-18 values for the parental magmas of these intrusions are Baima = 5.7 parts per thousand, Panzhihua = 6.1 parts per thousand, Taihe = 5.9 parts per thousand. The estimated mean delta O-18 value for the parental magma of the Upper and Middle Zones of the Hongge intrusion is 6.2 parts per thousand, which is similar to those for the parental magmas of the other intrusions (Baima, Panzhihua and Taihe). By contrast, the estimatedmean delta O-18 value for the parental magma of the Lower Zone of the Hongge intrusion is higher (6.9%). This difference, together with higher initial Sr-87/Sr-86 ratios (0.7057 to 0.7076) and lower epsilon Nd-t values (-2.82 to -0.07) for this zone, can be attributed to higher degrees of contamination with siliceous crustal materials in this zone than elsewhere in this intrusion. Comparison of O-Sr-Nd isotope compositions between the intrusions and country rocks reveals that bulk assimilation of carbonate country rocks is negligible in all of these intrusions. Mixing calculations using the O-Sr-Nd isotope data are consistent with variable degrees of contamination with siliceous crustal materials in the intrusions: Panzhihua, <5%, Baima and Taihe, <10%, the Middle and Upper Zone of the Hongge intrusion, <10%, the Lower Zone of the Hongge intrusion, <15%. These percentages are maximum values and may be reduced if contamination was selective in nature, involving Sr-and Nd-bearing fluids or partial melts. Based on the oxygen isotope results, an iterative calculation with a CO2/magma mass ratio = 1/1000 for each increment reveals that the Panzhihua magma reacted with <1 wt.% of CO2 released from the footwall during contact metamorphism. This amount is not sufficient to increase the oxidation state of the magma to the level that Fe-Ti oxides would crystallize alone from the magma. Therefore, we conclude that external CO2 did not play a major role in the formation of the Fe-Ti oxide ores in the Panzhihua intrusion. (C) 2015 Elsevier B.V. All rights reserved.