The Lithogeochemical Characteristics And Magma Evolution Of The Mafic-ultramafic Intrusions In Panxi District

Mafic-ultramafic layered intrusions are located in the western of Yangtze craton, the evolution of which is controlled by the regional north-south trunk fracture tectonic trending fault belts. The study region belongs to and consisting important components in the western part of the Emeishan large igneous province in SW China. The formations of these intrusions were almost coevally formed with the basalt of Emeishan large igneous province(257.7 ± 2Ma). Three representative intrusions, Panzhihua, Baima and Hongge, were chosen for major element, trace element, geologic feature, and petrographical study, in order to interpret the progress of magma evolution fractionation crystallization and ore formationed. Although the three mineralized intrusions have little distinction in dividing the rock lithofacies belt, all the intrusions show the characteristic of rhythmic cycle structures. Whole rock major element data show that all the three intrusions display the characteristics of relatively high Ti, Fe, Mg, Ca contents and low Si, Na, K contents. On the diagram of Zr/Ti O2*0.0001 vs. Si O2, Nb/Y vs. Zr/Ti O2*0.0001, and AFM, almost all the samples belong to sub-alkaline tholeiite series. The trace element geochemical characteristics display that The trace element geochemical characteristics display that the majority of the samples are enriched in LREEs and some LILEs(e.g., Th, U), and depleted in HREEs and some HFSEs(e.g., Zr, Hf). These features, together with the plot trending on the diagrams, imply that all the three mafic-ultramafic intrusions may have the same magmatic sources and have close affinities with OIB. The characteristics of low Mg O(0.85–9.29%) content, low Cr(6.43-295ppm) content and relatively wide range of Si O2(32.59–65.81%) content show that the intrusions may be derived from some evolved magma instead of parental magma of mantle. Trace elements of the samples display that they have the characteristics similar with the fractionated magmas. This, in combination with the enrichments of incompatible elements and the spider diagram of trace element, suggests a close affinity with the Lijiang picritic lava. The Lijiang picrite is almost the representative of the parental magma of Emeishan flood basalt, and been interpreted as the product of fractionation crystallization from partial melting of mantle plume. Therefore, we propose that the mafic-ultramafic intrusions in the Panxi area were derived from the mantle plume. However, the intrusions in Panxi display no negative Nb, Ta anomalies, suggesting that the plume of magmatic source has not undergone modest contamination with crustal materials. Moreover, the negative Zr and Hf anomalies of the samples also indicate that they have no contamination with crustal materials. The high contents of Ti O2(>2.5%, some even reach 8.44%) suggest the similar characteristic of high Ti basalt in the Emeishan large igneous province. The correlations between Mg O contents and major oxide contents(e.g., Mgo vs. Ca O and Fe O, showing positive correlation; Si O2 vs. Al2O3, showing negative correlation) reflect the fractional crystallization of monoclinic pyroxene during magma evolution. The negative correlations between Mg O and Na2 O, Al2O3, Ti O2, Si O2 show that the period of crystallization of ultramafic rock appear large amounts of accumulating olive. The existence of accumulating olive didn’t lead the main process and only occurred in the early stages of magma evolution. The positive correlations between Mg O and Fe O and Ti O2, and no correlation between Mg O and Al2O3 and Ca O, suggest that there is no fractional crystallization of plagioclase during the evolution of magma. Therefore, it is proposed that the mafic-ultramafic intrusions in Panxi were derived from the high Ti basaltic magma of surrounding mantle plume. During the early stage of magmatic evolution, Fe-Ti oxide liquids concentrate at the bottom. Then the mafic-ultramafic rocks and vanadium-titanium magnetite ores form fractional crystallization of magma resulting from liquid immiscibility at the late stage. Ultimately, vanadium-titanium magnetite ores concentrate by action of gravity.