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Eclogite facies metarodingites – phase relations in the system SiO 2 ‐Al 2 O 3 ‐Fe 2 O 3 ‐FeO‐MgO‐CaO‐CO 2 ‐H 2 O: an example from the Zermatt‐Saas ophiolite
Author(s) -
LI XP.,
RAHN M.,
BUCHER K.
Publication year - 2008
Publication title -
journal of metamorphic geology
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 2.639
H-Index - 114
eISSN - 1525-1314
pISSN - 0263-4929
DOI - 10.1111/j.1525-1314.2008.00761.x
Subject(s) - geology , eclogite , diopside , geochemistry , epidote , metamorphic facies , ophiolite , mantle (geology) , oceanic crust , subduction , facies , chlorite , tectonics , quartz , paleontology , structural basin
Eclogite facies metarodingites occur as deformed dykes in serpentinites of the Zermatt‐Saas ophiolite (Western Alps). They formed during the subduction of the Tethys oceanic lithosphere in the Early Tertiary. The metarodingites developed as a consequence of serpentinization of the oceanic mantle. Three major types of metarodingites (R1, R2 & R3) can be distinguished on the basis of their mineralogical composition. All metarodingites contain vesuvianite, chlorite and hydrogrossular in high modal amounts. In addition they contain: R1 – diopside, tremolite, clinozoisite, calcite; R2 – hydroandradite, diopside, epidote, calcite; and R3 – hydroandradite. Both garnets contain a small but persistent amount of hydrogarnet component. The different metarodingites reflect different original dyke rocks in the mantle. In each group of metarodingite, textural relations suggest that reactions adjusted the assemblages along the P–T path travelled by the ophiolite during subduction and exhumation. Reactions and phase relations derived from local textures in metarodingite can be modelled in the eight‐component system: SiO 2 ‐Al 2 O 3 ‐Fe 2 O 3 ‐FeO‐MgO‐CaO‐CO 2 ‐H 2 O. This permits the analysis of redox reactions in the presence of andradite garnet and epidote in many of the rocks. Within this system, the phase relations in eclogite facies metarodingites have been explored in terms of T – X CO2 , T –μ(SiO 2 ), μ(Cal)–μ(SiO 2 ) and P–T sections. It was found that rodingite assemblages are characterized by low μ(SiO 2 ) and low X CO2 conditions. The low SiO 2 potential is externally imposed onto the rodingites by the large volume of antigorite‐forsterite serpentinites enclosing them. Moreover, μ(SiO 2 ) decreases consistently from metarodingite R1 to R3. The low μ(SiO 2 ) enforced by the serpentinites favours the formation of hydrogarnet and vesuvianite. Rodingite formation is commonly associated with hydrothermal alteration of oceanic lithosphere at the ocean floor, in particular to ocean floor serpentinization. Our analysis, however, suggests that the metarodingite assemblages may have formed at high‐pressure conditions in the subduction zone as a result of serpentinization of oceanic mantle by subduction zone fluids.