辽西建平变质杂岩区蠕虫状后成合晶的成因与意义

Genesis and Significance of the Myrmekitic Symplectite from the Jianping Metamorphic Rock Complex, the Western Liaoning, China

建平变质杂岩中发育蠕虫结构的后成合晶，它受与石榴石共生的矿物组合、变质流体中水活度和变质岩原岩的制约。这种后成合晶可出现四种矿物组合：（１）Ｏｐｘ＋Ｐｌ＋少量Ｃｐｘ，（２）Ｏｐｘ＋Ｍｔ＋Ｐｌ＋Ｑ＋少量Ｃｐｘ，（３）Ｈｂ＋Ｐｌ，（４）Ｃｏｒｄ＋Ｏｐｘ±Ｐｌ。它们是在建平变质杂岩受中元古代三座庙序列侵入岩侵位作用影响发生的相当于辉石角岩相接触热变质作用中形成的，温压条件为７２０℃～７８０℃，０．６４～０．７０ＧＰａ。这些后成合晶的结构特征、矿物学特征、变质反应前后体积变化、岩体热传导模型的时间域的半定量估算等都表明它们是在压力降低，热扰动历史较为短暂（介于２．８５Ｍａ和１１．

Confined or influenced by the mineral assemblages coexisting with the early garnet, water activity of metamorphic fluid and the original rock characteristics of the metamorphic rocks, the myrmekitic symplectite outcropping in the Jianping metamorphic rock complex displays four kinds of following mineral assemblages: (1)Opx+Pl±Cpx; (2)Opx+Mt+Pl+Q; (3) Hb+Pl±Mt;(4) Cord+Opx±Pl. Geological evidences show that the myrmekitic symplectite formed while the Sanzuomiao sequence intruded into the Jianping metamorphic rock complex, cooled and crystallized due to the heat contact metamorphism with temperature between 780℃and 720℃ and pressure about 0.64￣0.70GPa. According to the textural characteristics, mineralogy, the standard volume change during the metamorphism and the calculation of metamorphic time scale based on two models of heat conduction, a conclusion can be obtained that the myrmekitic symplectite was formed under the special non equilibrium conditions of decompression and relatively short (between 2.85Ma and 11.40Ma) heat perturbation.