Multiple growth mechanisms of jadeite in Cuban metabasite

Abstract

Samples of rocks reported in the literature to be jadeite jade from the subduction-zone complex of the Escambray Massif in central Cuba have been studied by optical and transmission electron microscopy, electron microprobe and hot-cathode cathodoluminescence (CL) microscopy. Although these rocks are indeed rich in jadeite, the bulk rock composition generally conforms to MORB, with Na2O enriched by . 3 wt% and CaO depleted by .2 wt%. Al2O3 contents are unchanged. These changes are attributed to early pre-subduction spilitization of the ocean-floor protolith. Relics of magmatic augite preserving an ophitic texture are common. Disequilibrium textures are the rule. Extensively recrystallized rocks show fine, felty intergrowths of predominantly Al-rich glaucophane and jadeite, the latter with rims and patches of omphacite. TEM observations indicate extensive replacement of pyroxene by amphibole. Glaucophane developed rims of magnesiokatophorite and edenite. Chlorite and epidote are also present. Late development of actinolite, chlorite, epidote and albite is observed. Quartz is present. Less recrystallized samples with numerous large (.1.5 mm) grains of augite show several types of sodic and sodic-calcic clinopyroxene development: (1) Topotactic replacement of magmatic pyroxene by jadeite and omphacite along a broad front encroaching upon the augite grain from the rock matrix. Jadeite dominates where presumably plagioclase was formerly present. Omphacite dominates where augite is internally replaced along cleavage and fractures. Late chlorite, taramite and ferropargasite replace these pseudomorphs. (2) Former plagioclase laths of the ophitic fabric are replaced by jadeite together with lesser omphacite in epitactic relationship with the enclosing augite. Former plagioclase-augite grain boundaries remain preserved. Late pumpellyite is associated with the omphacite. (3) Jadeite þ omphacite þ pumpellyite þ chlorite with irregular grain boundaries dominate in the rock matrix between the augite relics, with idiomorphic crystals of epidote scattered throughout and in chlorite–epidote clusters. Pumpellyite is interpreted to be a late retrograde product. Quartz is present. (4) Jadeite þ omphacite þ chlorite assemblages, in which monomineralic sheaf-like jadeite aggregates are common, fill very thin (500–1500 mm) fractures criss-crossing the sample, including ophitic augite remnants. Cathodoluminescence microscopy shows that jadeite in the veins is distinctly different from CL in the other types of jadeite, showing features like oscillatory growth zoning indicative of crystallization from a fluid. Generally omphacite develops irregularly along jadeite rims, but recrystallization may lead to pairs with straight grain boundaries suggestive of phase equilibration. Comparison with published solvus relationships suggests temperatures of 425–500 C. This unusual occurrence of different types of jadeite in a metabasic rock suggests two contrasting sources. The first – in the rock matrix, as topotactic alteration of igneous pyroxene and as plagioclase replacement epitactically growing on augite – can be explained as due to local domain equilibration in a rapidly subducted ‘‘spilitized’’ gabbroic rock. The second, in very thin fracture fillings, conforms to an origin as a crystallization product from a pervasive fluid. Conceivably, ‘‘pooling’’ of the fluids flowing through the fractures in larger cavities could lead to larger masses of jadeitite. These have not yet been conclusively documented in the Escambray Massif.