大别山朱家冲斜长角闪岩-角闪榴辉岩-柯石英榴辉岩相转变实验研究

Expermental Study on the Transformation of Amphibolite-Amphibole Eclogite-Coesite Eclogite in Zhujiachong. Dabie Mountains

在750～1200C、1.0～4.5GPa条件下对取自于大别山朱家冲的斜长角闪岩进行了重结晶实验,并对其在高压-超高压区域的相关系进行了简要讨论。结果表明,在750 C时,斜长角闪岩随压力的前进演化经历了斜长角闪岩(Ga+Amp+Ep+Bi+Pl+Qz,1.0～1.5 GPa)、角闪榴辉岩(Ga+Cpx+Amp+Ep+Ph+Qz,1.75～2.25 GPa)、石英榴辉岩(Ga+Cpx+Zo+Ph+Qz,2.5 GPa)、柯石英榴辉岩(Ga+Cpx+Zo+Ph+Cs,3.0～3.5 GPa)和硬柱石榴辉岩(Ga+Cpx+Law+Ph+Cs,4.0～4.5 GPa)5个阶段,每个阶段之间的相关系可由一些特定的变质反应来限定。角闪石是榴辉岩演化早期和晚期阶段的主要共生矿物,它可以被看做是一种指示性矿物,其稳定上限低于2.5 GPa。黝帘石-蓝晶石-柯石英组合的出现指示大别山榴辉岩的压力上限未达到硬柱石的稳定区域,其形成压力不会超过4.0～4.5 GPa。榴辉岩中含水矿物的前进替代证明在俯冲带中有部分地表水被携带至地幔,而连续的脱水作用也为俯冲板片的部分熔融提供了水源。

The recrystallization of amphibolite from Zhujiachong in the Dabie Mountains was experimentally investigated at 750-1200℃ and 1. 0-4. 5 GPa, and the relationships of mafic rocks in high- and ultrahigh-pressure regions are briefly discussed. The results show that the progressive evolutionary process of mafic rocks with increasing pressure at 750℃ underwent five stages, i.e. amphibolite (Ga+Amp+Ep+Bi + P1+Qz, 1. 0-1. 5 GPa), amphibole eclogite (Ga+Cpx +Amp + Ep + Ph + Qz, 1. 75-2. 25 GPa), quartz eclogite (Ga + Cpx + Zo + Ph + Qz, 2. 5 GPa), coesite eclogite (Ga+Cpx + Zo+Ph+Cs, 3. 0-3. 5 GPa) and lawsonite eclogite (Ga + Cpx+Law+Ph + Cs, 4. 0-4. 5 GPa), in which phase relationships can be restricted by some specific metamorphic reactions. Amphibole is a major coexisting mineral at pre- and post-eclogite stages and can be regarded as an index mineral because its upper stability limit is less than 2. 5 GPa. The occurrence of zoisite + kyanite+coesite assemblage indicates that the upper pressure limit of eclogite in the Dabie Mountains does not reach the stability field of lawsonite, and therefore could not exceed 4. 0-4. 5 GPa. The progressive evolution of hydrous minerals in eclogite suggests that a portion of surface water may be carried deep into the mantle in a subduction zone, and that the continuous dehydration of water-bearing eclogite provides a H2O source for the partial melting of the subducting slab.