Abstract
Detailed analytical studies of mafic phyllosilicates from volcanic and hypabyssal intrusive rocks from the upper portion of the Point Sal remnant of the California Coast Range ophiolite reveal a notable correlation between phyllosilicate mineral parageneses and ophiolite pseudostratigraphy. Five phyllosilicate phases have been identified by systematic X-ray diffraction studies on clay size fractions: (1) smectite occurs in minor amounts in the upper volcanic zone (1A lavas), (2) randomly mixed-layered chlorite/smectite is ubiquitous in the 1A lavas but less common in the lower volcanic zone (1B lavas), (3) regularly interstratified chlorite/smectite occurs in both volcanic units, (4) chlorite is present in the 1B lavas and dike and sill complex, and (5) celadonite occurs sporadically throughout the 1A lavas. With increase in depth, smectite layers progressively transform to chlorite so that chlorite is the predominant phyllosilicate in the dike and sill complex.
The phyllosilicate zonation correlates with the calc-silicate mineral parageneses. Discrete smectite and randomly interlayered chlorite/smectite occur in both zeolite- and pumpellyite-grade volcanic rocks. Regularly interlayered chlorite/smectite is more common in pumpellyite-grade volcanic rocks than in the zeolite-grade volcanic rocks, but discrete chlorite is restricted to epidote-grade volcanic rocks and dikes/sills. The authigenic mineral zonation at Point Sal is similar to that reported for the Del Puerto ophiolite and from some active geothermal systems.
Phyllosilicates in the Point Sal remnant exhibit compositional trends correlative to pseudostratigraphic depth. The Si content of phyllosilicates decreases with depth from 7.8 to 5.5 cations/28 oxygens because of decrease in smectite interlayers with depth. Ca contents of mixed-layered chlorite/smectite, correlated with presence of smectite, also decrease with depth.
Mafic phyllosilicates from hydrothermally recrystallized metavolcanic rocks exhibit systematic structural and compositional variations. Chlorite from the Coast Range ophiolite, Troodos ophiolite, DSDP Hole 504B, and Onikobe and Icelandic geothermal fields that has been identified with X-ray diffractometry does not have Si cation totals greater than 6.25 cations/28 oxygens. Detailed X-ray diffraction studies of many layer silicates from low-grade metabasaltic rocks, previously identified as chlorite on the basis of compositional information alone, may lead to their re-identification as various interlayered phases.
