Abstract:

The Langmao subvolcanic intrusion in the village of Luquan County，Yunnan  Province is located in the east of Emeishan large igneous province, which is composed of basaltic porphyries at the lower part and polyphyric basaltic porphyries at the upper part. The wholerock major element analyses show that total Fe2O3 and TiO2 contents are relatively high, from 1405% to 1486%, and 312% to 469% respectively, corresponding to those of highTi basalt. However, there are also some different features in the two rock types: the former is characterized by relatively lower content of SiO2, TiO2, Al2O3, Na2O and relatively higher content of MgO and CaO. Electric microprobe analyses show that the clinopyroxenes, which are relatively rich in iron and titanium, are augite, and plagioclase are andesine and labradorite, whereas titanomagnetites are characterized by extremely high TiO2 content (2425%-2665%). Based on clinopyroxene geothermometer and geobarometer and plagioclase thermometer, the two rock types formed in significantly different temperature and pressure. The temperature and pressure of the basaltic porphyries is estimated to be in the range of 1143-1176 ℃ and 023-086 MPa respectively, whereas those of the polyphyric basaltic porphyries is estimated to range from 1316 to 1370 ℃ and 198 to 225 MPa respectively. In addition, the titanic magnetite with very high XUsp and low Fe3+ in clinopyroxene suggests low oxygen fugacity. The oxygen fugacity is estimated to be 1013×10-6-1013×10-10  Pa. It can be inferred that low oxygen fugacity caused no crystallization of titanic magnetite in the early stage, and resulted in the enrichment of Fe and Ti in the late stage. Conclusively, highTi basalts can be formed from normal lowTi basalts by early crystal fractionation of silicate minerals. However, owing to some Fe and Ti are hosted in clinopyroxene crystals, no very high Fe and Ti enrichment has been recognized in the residual magma, and thus VTi magnetite deposits were not formed in this district.

Key words: basaltic porphyry, titanomagnetite, oxygen fugacity, physicochemical condition, Emeishan large igneous province, Luquan County, Yunnan Province