Implications of compositions of plagioclase and olivine on the formation of the Panzhihua V-Ti magnetite deposit, Sichuan Province
Implications of compositions of plagioclase and olivine on the formation of the Panzhihua V-Ti magnetite deposit, Sichuan Province
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作者：
Zhang Xiao
攀枝花岩体中下部的矿物组合及伴生结构表明，斜长石、橄榄石和铁钛氧化物在相似的温度范围内结晶。因此，利用斜长石和橄榄石的成分，可以估算攀枝花层状岩体中不同旋回的磁铁矿辉长岩和辉长岩形成过程中的温度、氧逸度（fO 2）和岩浆成分的变化。电子探针数据表明，斜长石从低层到中层的成分变化很小（An58.0-52.5）。而在一个旋回单元内，橄榄石中镁橄榄石的含量（Fo）则从磁铁矿辉长岩向辉长岩显著降低。这些特征说明攀枝花岩体经历了多次富铁钛的岩浆的补充。斜长石成分变化较小且规则，说明岩浆中fO 2和Fe 3 +/Fe 2+的变化对斜长石的结晶作用影响不大。因此，斜长石的成分可以用来估算Fe-Ti氧化物的晶化温度。橄榄石的Fo值在一个旋回单元内变化较大，表明橄榄石的成分与其结晶岩浆的Fe 3 +/Fe 2+和Fe 2 +/Mg值有关。因此，利用橄榄石的Fo值重建了磁铁矿辉长岩和辉长岩形成过程中fO 2的变化。攀枝花岩体中、下带斜长石的结晶温度为1079 ~ 1121℃，推测Fe-Ti氧化物的结晶温度也可能在此范围内。另一方面，在一个旋回单元内，橄榄石从磁铁矿辉长岩到辉长岩的Fo降低，表明在新补充岩浆的分离结晶过程中，fO 2降低。这一结论与前人在氧封闭条件下分步结晶过程中fO 2变化规律的实验研究结果相一致。
Mineral assemblages and associated textures in the low and middle part of the Panzhihua intrusion indicate that plagioclase, olivine and Fe-Ti oxides were crystallized at a similar temperature range. Thus, it is possible to estimate variation of temperature, oxygen fugacity (fO2) and magma composition during the formation of the magnetite gabbro and gabbro in different cycles of the Panzhihua layered intrusion by means of compositions of plagioclase and olivine. Electron microprobe data indicate that plagioclase has small compositional variation (An58.0-52.5) from the low zone to the middle zone. Whereas, forsterite percentages of olivine (Fo) decrease significantly upwards from magnetite gabbro to gabbro within a single cycle unit. These features indicate that the Panzhihua intrusion was developed by replenishment of many pulses of Fe-Ti enriched magmas. Small and regular composition variations of plagioclase suggest that variations of fO2 and Fe3+/Fe2+ of the magma have little effect on crystallization of plagioclase. Therefore, compositions of plagioclase can be used to estimate temperature of crystallization of Fe-Ti oxides. In contrast, large variations of Fo of olivine within a cycle unit suggest that the compositions of olivine depend on values of Fe3+/Fe2+ and Fe2+/Mg of the magma, from which the olivine crystallized. Thus, Fo of olivine was used to rebuild variation of fO2during the formation of the magnetite gabbro and gabbro. Crystallizations of plagioclase of the lower and middle zones of the Panzhihua intrusion occurred between 1079℃ and 1121℃, we estimate that Fe-Ti oxide probably crystallized at the same temperature range as well. On the other hand, decrease of Fo of olivine from magnetite gabbro to gabbro within a cycle unit indicate that fO2 decreased during fractional crystallization of a new replenished magma. This conclusion is consistent with previous experimental studies about the fO2 variations during fractional crystallization process at a system closed to oxygen.