退变质榴辉岩中角闪石的组构特征

Crystallographic preferred orientation of amphibole in retrograded eclogites

中国大陆科学钻探(CCSD)主孔的超高压榴辉岩在折返过程中普遍受到角闪岩相退变质作用改造,最终转变为斜长角闪岩。角闪石是超高压榴辉岩退变质过程的产物,在光学显微镜下没有明显的脆性破裂变形,也不显示任何指示晶体塑性变形的晶粒内部显微构造,例如不均匀消光、变形叶理、亚颗粒边界和凸出褶皱等。电子背散射衍射(EBSD)测量结果显示,角闪石具有明显的结晶学优选方位(CPO),为A型组构:[100]轴极点形成垂直线理大圆环,最大极密对应轴垂直面理;[001]轴极点形成平行面理的大圆环,最大极密对应轴向近平行线理。结合光学显微特征观察,本文认为退变质榴辉岩中的角闪石CPO主要受其显著的长柱状和针状结晶习性控制,其形成机制为差异应力条件下的溶解-沉淀定向生长,可能伴随有早期的角闪石微晶的刚性旋转变形。

Texture relations and mineral paragenesis indicate that ultrahigh pressure eclogites from the mainhole of the Chinese Continental Scientific Drilling Project have experienced different degrees of retrograde metamorphism, and some of them have been completely transformed to amphibolite. Amphibole, an important product during retrogression of UHP eclogite to amphibolite, exhibits few brittle deformation features, and does not show any intragranular microstructures indicative of crystal deformation such as undulose extinction, deformation lamellae, subgrain boundary and bulging. The crystallographic preferred orientation （CPO） of amphibole, measured using electron backscatter diffraction （EBSD）, is a common A-type. The [001 ] shows a strong point maximum near the lineation, while [ 100] form girdles normal lineation with a relatively weaker maximum perpendicular to the foliation plane. We suggest that the CPO of amphibole in retrograded eclogites is probably formed by rigid body rotation and dissolution-precipitation creep of elongate amphibole grains in a less viscous matrix of symplectite.