TY - JOUR
T1 - Ultra-high pressure metamorphism in western Tianshan, China
T2 - Part II. Evidence from magnesite in eclogite
AU - Zhang, Lifei
AU - Ellis, David J.
AU - Williams, Samantha
AU - Jiang, Wenbo
PY - 2002
Y1 - 2002
N2 - 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.
AB - 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.
UR - http://www.scopus.com/inward/record.url?scp=0036060376&partnerID=8YFLogxK
U2 - 10.2138/am-2002-0708
DO - 10.2138/am-2002-0708
M3 - Article
SN - 0003-004X
VL - 87
SP - 861
EP - 866
JO - American Mineralogist
JF - American Mineralogist
IS - 7
ER -