CHLORITE TEXTURES AND COMPOSITIONS FROM HIGH-PRESSURE LOW-TEMPERATUREMETASHALES AND METAGRAYWACKES, FRANCISCAN COMPLEX, DIABLO RANGE, CALIFORNIA, USA/

Chlorite, chlorite/berthierine interstratifications, and mixed-layer c hlorite/smectite from high-pressure/low-temperature metashales from di fferent areas of the Diablo Range, Franciscan subduction zone, were in vestigated using X-ray powder diffraction (XRPD), electron microprobe (EMP), transmission electron microscopy (TEM), and analytical electron microscopy (AEM). Our data indicate that chlorite in the Diablo Range formed in two different ways. One involves the transformation from sm ectite to chlorite, the other includes the crystallization of chlorite from berthierine. Mixed-layer chlorite/smectite was observed in sever al samples. The amount as well as types and stacking order change as a function of metamorphic grade. At lower grades, chlorite and major am ounts of random RO mixed-layer chlorite/smectite plus corrensite are p resent. At higher grades, the metashale assemblage includes chlorite a nd minor amounts of random RO mixed-layer chlorite/smectite plus mixed -layer chlorite/corrensite. The occurrence of corrensite at lower, and mixed-layer chlorite/corrensite at higher grade may represent a part of the normal prograde sequence smectite --> mixed-layer smectite/chlo rite --> corrensite --> mixed-layer chlorite/corrensite --> chlorite a s suggested by previous workers. Random RO mixed-layer chlorite/smecti te at both low and high grades may have formed metastably. Berthierine layers interstratified with chlorite layers were imaged in several Di ablo Range specimens. Significant packets of berthierine have not been found in any of the samples studied, and therefore, berthierine layer s within the chlorite matrix are thought to represent relicts of the p recursor mineral berthierine. Thus, we suggest that the berthierine -- > chlorite transition in the Diablo Range samples investigated in this study is almost complete. Once the chlorite structure is established, two generations of chlorites formed: an older Mg-rich, and a younger Fe-rich generation. Both types are thought to have crystallized after the high-pressure event when the rocks passed from the jadeite plus qu artz into the albite stability field at a temperature range from 230 t o 350 degrees C. EMP analyses free of contamination by other phases in dicate that during this metamorphic evolution, chlorites generally bec ome progressively less siliceous, richer in Mg and especially Fe, tetr ahedral Al approaches the octahedral Al content, and the octahedral oc cupancy increases. These compositional variations may be described by the reverses of the Si square Mg--2 and FeMg-1 exchange vectors, where square represents a vacant octahedral site. However, a slight increas e in the total Al content is observed in our data, which indicates a d eviation from the Si square Mg--2 vector.