Abstract:

Illite crystallinity has been used as a primary index for subdividing very low-grade metamorphic zone since 1967 after Kübler who measured illite crystallinity by use of XRD and subdivided very low-grade metamorphic zone according to illite crystallinity first. However, Kübler indexes which are used to subdivide metamorphic grade are various among geologists. Therefore a combined research with petrology study is needed in order to solve such dispute. This paper attempts to apply the conventional method of mineral paragenesis analysis to the metapelites in Well Hong-can No.1 in Songpan-Aba area within a depth of  5,975 meters. The result reveals that these rocks have undergone high anchimetamorphism. Stilpnomelane, which is believed as an indicator mineral for anchimetamorphism in metapelite, cannot occur because of the limitation of bulk composition. However, they didn't reach epimetamorphism yet, the preservation of combination gas is still possible. Comparing with petrological evidences, we found the illite crystallinity index scheme proposed by Kisch(1990) is comparatively accorded with the true situation of this area. It is believed that applying mineral paragenesis analysis in the research on very low-grade metamorphism is crucial to solve such index dispute, and moreover, it is very useful for guiding thin section observation and the X-ray diffraction phase analysis.

Key words: very low-grade metamorphism, mineral paragenesis analysis, illite crystallinity, Songpan-Aba area