假蓝宝石(Sapphirine)的矿物学特征及其在超高温变质作用研究中的应用

Mineralogical characteristics of sapphirine and application in investigating ultrahigh-temperature(UHT) metamorphism

假蓝宝石是Mg-Al质麻粒岩中一种特殊的高温矿物,对超高温变质作用的研究有重要的意义。本文通过对全球66个超高温麻粒岩中47个含假蓝宝石麻粒岩地区的文献调研,总结了几种最常见的含假蓝宝石矿物组合产出的结构位置和变质反应关系,以及假蓝宝石的矿物化学特征。假蓝宝石的化学成分一般位于7∶9∶3端元左右,X_(Mg)大于0. 7,XFe_(3+)变化范围很宽,为0～0. 7。含假蓝宝石矿物组合的形成和演化指示了岩石经历的P-T轨迹。岩石中保留的假蓝宝石取代尖晶石、Grt/Opx+Sil取代Spr+Qz组合,以及随后的Spr+Crd±Opx后成合晶取代Grt/Opx+Sil组合的结构,一般可能指示了逆时针P-T轨迹中冷却和随后减压的部分;岩石中Grt/Opx+Sil/Ky或富Mg十字石反应形成Spr+Qz组合的结构可能指示了顺时针P-T轨迹中减压升温的部分。超高温变质岩不同的P-T轨迹暗示着它们的成因机制并不单一,前者可能是幔源基性岩浆底侵或增生作用的结果,后者可能与长期的热造山作用相关。

Sapphirine is a specific high-temperature mineral in Mg-Al-rich granulites,and it is of significance in investigating ultrahigh-temperature( UHT) metamorphism. This study has tracked 47 sapphirine-bearing granulite localities among the worldwide 66 UHT granulite localities,and summarized the textural settings and metamorphic reaction sequences of several most commonly-observed sapphirine-bearing mineral assemblages,as well as the typical features of sapphirine mineral chemistry. Sapphirine compositions are usually localized close to the 7∶9∶3 end-member,and their XMg is usually higher than 0. 7,and XFe3+shows a large range of 0 ~0. 7. Formation and evolution of the sapphirine-bearing mineral assemblages in the rocks indicate the experienced P-T paths. Textures of sapphirine replacing spinel,Grt/Opx + Sil replacing Spr + Qz assemblage,and subsequent Spr + Crd ± Opx symplectite replacing Grt/Opx + Sil assemblage,generally indicate the cooling and subsequent decompression parts of an anticlockwise P-T path. Whereas,the texture of Grt/Opx + Sil/Ky or Mg-rich staurolite producing Spr + Qz assemblage,suggests that the rocks have probably experienced the heating and decompression stages in a clockwise P-T path. Different shapes of P-T paths imply that the origin mechanism of the UHT metamorphism is multiple,with the former probably resulting from underplating or accretion of the mantle-derived mafic magma,and the latter probably related to the long-lived hot orogenesis.