Ultra-high pressure metamorphism in western Tianshan, China: Part II. Evidence from magnesite in eclogite

Lifei Zhang*, David J. Ellis, Samantha Williams, Wenbo Jiang

*Corresponding author for this work

Research output: Contribution to journalArticlepeer-review

125 Citations (Scopus)

Abstract

Carbonates of ultrahigh-pressure metamorphic (UHPM) origin were discovered in eclogites from western Tianshan, China. In the eclogites, relict magnesite (XMg = 0.79) occurs as rounded to subidiomorphic inclusions (0.01-0.1 mm) within matrix dolomite, and also as rounded inclusions with thin reaction rims of dolomite in glaucophane. Based on the textural evidence and calculated phase relationship, these eclogites record an early UHPM assemblage overprinted by a blueschist assemblage, which can be explained by the reaction: 30MgCO3 mag + 10NaAlSi2O6 jd + 9Ca3Al2Si3O12 gr + 11SiO2 coe + 8H2O w = 15CaMg(CO3)2 dol + 5 the P-T grid involving glaucophane (gl), grossular (gr), jadeite (jd), magnesite (mag), dolomite (dol), clinozoisite (cz), coesite (coe), CO2, and H2O in a 7-component system of NCMASCH (Na2O-CaO-MgO-Al2O3-SiO2-CO2-H2O). The calculated P-T slope of the above reaction is nearly parallel to the temperature axis and suggests that it can be used as an excellent geobarometer. Using the analyzed compositions and mineral-thermodynamic database of Holland and Powell (1998), the above retrograde metamorphic reaction in Tianshan eclogites occurred at 27-28 kbar and 525-607°C calculated from Grt-Cpx geothermometers.

Original languageEnglish
Pages (from-to)861-866
Number of pages6
JournalAmerican Mineralogist
Volume87
Issue number7
DOIs
Publication statusPublished - 2002
Externally publishedYes

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