TECTONOGEOCHEMISTRY OF THE CORDILLERA DE LA COSTA ECLOGITE BELT, VENEZUELA

In 1979, Bernard Evans published a creative and inspirational paper (American Mineralogist 64: 15-32) that, among other things, showed the great power of linking geochemistry to petrology in studies of metasomatized rocks. Nowadays, if the geochemical signatures of metabasites appear to be disrupted by metasomatism, a standard approach uses the "least mobile elements" to guess at the protolith, and ignores "unreliable" data for elements readily mobilized in fluids. But some "mobile element" data for altered rocks are a geochemical complement to a P-T path. In such cases, contrasts in the styles and processes of alteration can be linked to different P-T-t stages in the host tectonic environment. The Cordillera de la Costa (CdlC) eclogite belt of Venezuela provides an example.

The CdlC eclogite belt, exposed along the Caribbean coastline near Puerto Cabello, consists of lensoid bodies and boudins of high P/T metabasite in a matrix of mica schist and metacarbonate rocks. The metabasite bodies are eclogite and eclogite retrogression products. Less mobile element values indicate that metabasite protoliths likely varied from NMORB to EMORB to cumulate gabbros. In some metabasites, LILE were enriched by phengite deposition during retrogression. This style of alteration is seen in eclogites of the Samana Peninsula, Dominican Republic, but not those on Isla Margarita, an eclogite terrane exposed ~250 mi away, off the Venezuelan coast. Other CdlC metabasites have apparently been stripped of K, Rb, Ba, and U, some to values below XRF and INAA detection limits. Some of the metasedimentary host rocks of the LILE-poor metabasites also show extreme LILE depletion, most notably a lens that records P>20 kb at T~600°C. This schist has Al/Si ratios comparable to shale, but <0.3 Wt% K₂O.

Rocks of the CdlC eclogite belt record LILE expulsion, probably at "peak" P-T conditions, and LILE enrichment during retrogression. The distinctive geochemical signatures that appear to have been produced by different fluid-rock processes operating during different stages of metamorphism illustrate the value of examining even "unreliable" mobile element data for patterns related to P-T-t.