Garnetite, garnet-quartz (‘coticule’) and calc-silicate layers in high-pressure metapelitic rocks, Venezuela: metamorphosed exhalites in a Cretaceous back-arc basin

ABSTRACT

Massive, boudinaged, rose-coloured layers up to one metre in thickness composed of 60–80 vol% spessartine-rich garnet and 15–40 vol% quartz occur in high-pressure, garnet-mica schist on Margarita Island, Venezuela. In the past, such spessartine-garnet-rich rocks have often, although inaccurately, been called ‘coticule’. Apatite can be significant, reaching 20 vol%. Accessory minerals are allanite, rutile (commonly rimmed by ilmenite) and rare phengite. Within the same mica-schist sequence, intercalated calc-silicate layers from one to two metres in thickness exhibit alternating layers, on a sub-mm- to cm-scale, of quartz and greenish layers rich in zoisite, epidote-supergroup minerals and diopside. Hyalophane and barite are present in small amounts. Calc-silicates are commonly pitted, with yellow to dark-brown, gossanous, botryoidal crusts. The composition of garnet in the garnet-rich layers, the calc-silicate layers and their host metasediments is distinctly different. In the garnet-rich layers, compositions are almandine-dominant (Alm_57-63), followed by spessartine (Sps_18-24), grossular (Grs_2-16), pyrope (Prp_6-11) and andradite (Adr_0-6). Garnet in calc-silicate layers can range from almost pure grossular to Grs_45-49Alm_32-34Sps_13-15, and in the enclosing metasediments is typically Alm_71-73Prp_14-15Grs_3-8Sps_4-7And₂. Chondrite-normalized REE patterns of all three rock types are similar in shape, differing only in absolute contents, so that a common sedimentary precursor is indicated. Major and trace element chemistry of the garnet-rich layers are best explained if their pre-metamorphic protolith originated as a mixture of 60–70% normal detrital input and a precipitate from a distal source of hydrothermal activity. The term meta-exhalite is appropriate. The calc-silicate layers are more enigmatic, but an origin as a mixture of a hydrothermal component with a Ca-rich sediment also appears likely. Numerous lines of evidence indicate that the protoliths of the high-pressure rocks of Margarita Island originated in an Early Cretaceous back-arc basin in NW Colombia, so that an exhalite origin for garnet-rich and calc-silicate rocks is a plausible scenario.

KEYWORDS:

• Garnetite
• “coticule“
• spessartine-rich rock
• calc-silicate
• meta-exhalite
• Margarita island

Acknowledgments

We thank Jim Pindell for fruitful discussions, helpful comments and for drafting and putting Fig 1 at our disposal. Paul G. Spry, Frédéric Hatert and an anonymous reviewer provided helpful and perceptive reviews that helped us improve presentation and logic. Chief Editor R.J. Stern’s comments and suggestions are appreciated. Niels Jöns supported us during electron-microprobe analysis.

Disclosure statement

No potential conflict of interest was reported by the authors.

Supplementary material

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