Chlorite textures and compositions from high-pressure/low-temperature metashales and metagraywackes, Franciscan Complex, Diablo Range, California, USA
Dalla Torre, Michael; Livi, Kenneth J. T.; Frey, Martin
European Journal of Mineralogy Volume 8 Number 4 (1996), p. 825 - 846
published: Jul 29, 1996
manuscript accepted: Mar 25, 1996
manuscript received: May 29, 1995
ArtNo. ESP147050804014, Price: 29.00 €
Abstract Chlorite, chlorite/berthierine interstratifications, and mixed-layer chlorite/smectite from high-pressure/ low-temperature metashales from different areas of the Diablo Range, Franciscan subduction zone, were investigated 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 smectite to chlorite, the other includes the crystallization of chlorite from berthierine. Mixed-layer chlorite/smectite was observed in several samples. The amount as well as types and stacking order change as a function of metamorphic grade. At lower grades, chlorite and major amounts of random R0 mixed-layer chlorite/smectite plus corrensite are present. At higher grades, the metashale assemblage includes chlorite and minor amounts of random R0 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/chlorite → corrensite → mixed-layer chlorite/corrensite → chlorite as suggested by previous workers. Random R0 mixed-layer chlorite/smectite at both low and high grades may have formed metastably. Berthierine layers interstratified with chlorite layers were imaged in several Diablo Range specimens. Significant packets of berthierine have not been found in any of the samples studied, and therefore, berthierine layers within the chlorite matrix are thought to represent relicts of the precursor 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 quartz into the albite stability field at a temperature range from 230 to 350°C. EMP analyses free of contamination by other phases indicate that during this metamorphic evolution, chlorites generally become progressively less siliceous, richer in Mg and especially Fe, tetrahedral Al approaches the octahedral Al content, and the octahedral occupancy increases. These compositional variations may be described by the reverses of the SiG□Mg-2 and FeMg-i exchange vectors, where □ represents a vacant octahedral site. However, a slight increase in the total Al content is observed in our data, which indicates a deviation from the Si□Mg-2 vector.