Accessory minerals and subduction zone metasomatism: a geochemical comparison of two mélanges (Washington and California, U.S.A.)

Chemical Geology
By:  and 



The ability of a subducted slab or subducted sediment to contribute many incompatible trace elements to arc source regions may depend on the stabilities of accessory minerals within these rocks, which can only be studied indirectly. In contrast, the role of accessory minerals in lower-T and -P metasomatic processes within paleo-subduction zones can be studied directly in subduction-zone metamorphic terranes.

The Gee Point-Iron Mountain locality of the Shuksan Metamorphic Suite, North Cascades, Washington State, is a high-T mélange of metamafic blocks in a matrix of meta-ultramafic rocks. This mélange is similar in geologic setting and petrology to the upper part of an unnamed amphibolite unit of the Catalina Schist, Santa Catalina Island, southern California. Both are interpreted as shear zones between mantle and slab rocks that formed during the early stages of subduction. Some garnet amphibolite blocks from the Gee Point-Iron Mountain locality display trace-element enrichments similar to those in counterparts from the Catalina Schist. Some Catalina blocks are highly enriched in Th, rare-earth elements (REE), the high-field-strength elements Ti, Nb, Ta, Zr and Hf (HFSE), U and Sr compared to mid-ocean ridge basalt (MORB), and to other garnet amphibolite blocks in the same unit. Textural and geochemical data indicate that accessory minerals of metamorphic origin control the enrichment of Th, REE and HFSE in blocks from both areas. The Mg-rich rinds around blocks and the meta-ultramafic matrix from both mélanges are highly enriched in a large number of trace elements compared to harzburgites, dunites and serpentinites. Evidence for recrystallization or formation of accessory minerals in the former rocks suggests that these minerals control some of the trace-element enrichments.

Data from the Gee Point and Catalina mélanges suggest that the accessory minerals titanite, rutile, apatite, zircon and REE-rich epidote play a significant role in the enrichment of trace elements in both mafic and ultramafic rocks during subduction-related fluid-rock interaction. Mobilization of incompatible elements, and deposition of such elements in the accessory minerals of mafic and ultramafic rocks may be fairly common in fluid-rich metamorphic environments in subduction zones.

Publication type Article
Publication Subtype Journal Article
Title Accessory minerals and subduction zone metasomatism: a geochemical comparison of two mélanges (Washington and California, U.S.A.)
Series title Chemical Geology
DOI 10.1016/0009-2541(93)90258-K
Volume 110
Issue 1-3
Year Published 1993
Language English
Publisher Elsevier
Description 29 p.
First page 269
Last page 297
Online Only (Y/N) N
Additional Online Files (Y/N) N
Google Analytic Metrics Metrics page
Additional publication details