AMORPHOUS SIO2 PHASE IN A PSEUDOMORPH AFTER COESITE IN A GARNET OF THE SU-LU UHP ECLOGITE: AN INTEGRATED FIB, TEM AND RAMAN SPECTROSCOPIC STUDY

Using a focused ion beam (FIB) system, a transmission electron microscope (TEM), and Raman spectroscopy, we report for the first time the discovery of an amorphous SiO₂ phase in an anomalous pseudomorph after coesite included in a garnet of an eclogite from the Yangzhuang region in the Su-Lu ultrahigh-pressure (UHP) metamorphic belt in Eastern China. In the Yangzhuang sample, the pseudomorphs were divided into darkly colored and transparent areas under plane-polarized light, which dominate primarily at the core and margins. Radial cracks develop in garnet around the pseudomorphs. The transparent areas were composed of polycrystalline quartz grains. In contrast, the darkly colored area consisted of fine-grained aggregates of the SiO₂ and fibrous phases. The TEM observation and selected-area electron diffraction pattern showed that the internal structure of the pseudomorph consisted of the amorphous SiO₂ (APSI) phase with nano-/submicrocrystalline particles of quartz and the polycrystalline K-bearing fibrous sheet-silicate (KFSS) phase. The APSI-phase-bearing pseudomorph included in the garnet might have formed by reactions involving a supercritical fluid during the exhumation from the following processes: (1) development of radial cracks within the host garnet by the phase transition from coesite to quartz, (2) decomposition of a part of the pseudomorph by the infiltration of a supercritical fluid, (3) precipitation of KFSS phases from the SiO₂-dissolved fluid during the subsequent exhumation and cooling stage, and (4) rapid precipitation of the APSI phase under the non-equilibrium state started at a later stage of the exhumation. The occurrence of a relict amorphous phase indicates that the Yangzhuang eclogites have not experienced environments that promoted recrystallization from the APSI phase to crystalline quartz during exhumation. The presence of the amorphous SiO₂ phase in UHP metamorphic rock might provide new insights into the geodynamic phenomena occurring at continental collision zones. We petrologists should therefore focus on pseudomorphs after coesite from the nanoscale viewpoint, to understand the history of fluid activity and the cooling process during the exhumation of UHP rocks by using FIB, TEM, and other analytic techniques.