滇西剑川苦橄玢岩中单斜辉石的矿物学特征及其地质意义

摘要

The Jianchuan picritic porphyrite consists predominantly of olivine, clinopyroxene and plagioclase.In this paper, the authors determined chemical composition of clinopyroxene from the picritic porphyrite and picritic annulate from Erlong and Manyuan areas in detail by electron microprobe analysis.The results show that most clinopyroxenes in Jianchuan area are composed of augite, which contains relatively low TiO2(0.62％ ～ 2.84％), Al2O3(1.86％～5.15％) andNa2O(0.23％～0.61％).Compared with those in the matrix, clinopyxenes of phenocrysts from Erlong and Manyuan areas are enriched in more magnesium but have poorer iron.The parental magma of the Jianchuan picritic porphyrite should belong to tholeiitic series, characterized by low alkali content, high CaO content and very low oxygen fugacity.During the formation of the clinopyroxene, cationic isomorphic substitution commonly occurs, and the main equations are Mg + Si = AlⅣ + AlⅥ and Mg + Si = AlⅣ + Fe3+ , in which Ti and Na tend to be incorporated into the titanic augite.The crystallization temperature and pressure of clinopyroxene in Jianchuan picritic porphyrite were estimated by the clinopyroxene-melt equilibrium.The results show that the crystallization temperature (1324 ～ 1428℃) and pressure (1.5 ～ 2.9 GPa) of clinopyroxenes in Erlong and Manyuan picritic porphyrite were predominantly lower than the crystallization temperature (1439～1462℃ ) and pressure (3.0～3.2 GPa) of clinopyroxenes in Manyuan picritic cumulate, and the formation depth of clinopyroxene in Manyuan picritic cumulate might have been larger than that in Erlong and Manyuan picritic porphyrite.Generally, the formation temperature and pressure of the Jianchuan picritic porphyrite were high, suggesting the derivation of materials from a relatively great depth.One possibility is that the rocks were formed by partial melting of lithospheric mantle resulting from upwelling of asthenosphere along the deep faults, whereas the other possibility is the involvement of mantle plume, i.e., the Permian Emeishan mantle plume was reactivated, probably resulting in large-scale magmatic activities.The evidence is as follows: the temperature of the clinopyroxene in Jianchuan picritic porphyrite is moderately higher than that of asthenosphere and the clinopyroxene in Lijiang picrite; the Mg# both in bulk-rock and in clinopyroxene of Jianchuan picritic porphyrite is close to that in Lijiang picrite, suggesting that Jianchuan picritic porphyrite might have shared a common source region with the Lijiang picrite, or the characteristics of their sources were largely inherited from the Permian plume.%利用电子探针对滇西剑川的二龙和满元两个地区苦橄玢岩和苦橄质堆晶岩中的单斜辉石进行了详细的矿物学研究,结果表明绝大多数单斜辉石属于普通辉石,具有贫硅、贫碱、高钙和低氧逸度的特征.在单斜辉石形成过程中,阳离子间普遍发生类质同像替代.单斜辉石温压计算结果显示,二龙和满元苦橄玢岩的形成温度(1 324～1 428℃)和压力(1.5～2.9GPa)明显低于满元苦橄质堆晶岩的温度(1439～1462℃)和压力(3.0～3.2GPa),二龙和满元苦橄玢岩的形成深度也比满元苦橄质堆晶岩的深度(99～106 km)浅,整体看来,剑川苦橄玢岩形成的温度和压力较高,来源较深,可能是软流圈沿断裂带上涌导致上地幔部分熔融的结果,也可能是地幔柱活动所导致.