云南禄劝县朗茂二叠纪玄武玢岩的岩石学特征及其对高钛玄武岩成因的指示意义

Petrology of the Permian Langmao Basaltic Porphyry,Luquan County,Yunnan Province: Implications for the Petrogenesis of High-Ti Basalts

云南禄劝县朗茂次火山岩体位于峨眉山大火成岩省的东部,由下部的玄武玢岩和上部的多斑玄武玢岩组成。全岩主要元素分析结果表明,其TFe2O3和TiO2含量较高,分别为14.05%~14.86%、3.12%~4.69%,属于高钛玄武岩的范围,但是两者的主要元素也存在一定的差别,表现在前者有相对低的S iO2、TiO2、A l2O3和Na2O含量以及相对高的MgO和CaO含量。矿物电子探针分析结果表明,单斜辉石均为相对富铁钛的普通辉石,斜长石为中-拉长石,而钛磁铁矿以异常高的TiO2含量（24.25%~26.65%）为特征。根据单斜辉石温度计和压力计以及斜长石的温度计估算结果,玄武玢岩和多斑玄武玢岩的形成温度和压力均有明显的差别,后者温度和压力较低,分别为1 143~1 176℃、0.23~0.86 MPa;而前者较高,分别为1 316~1 370℃、1.98~2.25 MPa。另外,钛磁铁矿中异常高的XUsp含量以及单斜辉石中低的Fe3＋指示岩浆的低氧逸度,估算的氧逸度为1.013×10-6~1.013×10-10Pa。因此可以推测,由于低的氧逸度抑制了钛磁铁矿在早期结晶,导致晚期岩浆富集铁和钛,所以高钛玄武岩可以通过正常的低钛玄武岩岩浆在早期发生硅酸盐矿物的分离结晶演化而成。但是由于有一部分铁和钛在单斜辉石结晶过程中进入单斜辉石晶体中,导致在残留岩浆中没有高度富集铁钛,因而在该地区没有形成钒钛磁铁矿矿床。

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.