T142 Bridging Two Continents (Posters): Advances in Crustal Subduction and Ultrahigh-Pressure Metamorphism

Monday, 2 November 2015
9:00 AM - 6:30 PM
Exhibit Hall

GARNET-HOSTED MULTIPHASE INCLUSIONS REFLECT FLUID COMPOSITIONS DURING SYN-EXHUMATION GARNET GROWTH, DULAN REGION, NORTH QAIDAM UHP TERRANE, WESTERN CHINA

Paper number: T142-5
Program: Technical Sessions
Time: 9:00 AM – 6:30 PM

Authors

Abstract

Multiphase inclusions occur in garnet rims in a paragneiss (Grt + Ms + Tur + Qtz + Pl, with minor Bt, Kfs, Rt, Ilm, Zrn, and Chl). X-ray maps distinguish Grt cores (high Y + Mn ± P), mantles (lower Y, Mn, P), and a discontinuous inner rim (higher Ca + Ti) interpreted to record UHP conditions; low Ca + Ti, high Mn outer rims (and small matrix Grt) contain abundant inclusions (most <5 um, with a negative crystal shape). Rim thermobarometry yields 6-7 kbar, 550-600°C, suggesting inclusion trapping at these conditions or earlier.

Raman inclusion analyses (106 spots) identify chlorite and carbonate (~40% each), with lesser rutile and mica (~10% each). Cristobalite (4 spots) may be a metastable precipitate. One Grt has an inner, higher Ca zone of smaller inclusions, and an outer, low Ca, high Mn zone of larger inclusions that contain more carbonate + mica, and less chlorite than the inner zone. Three inclusions contain N2 + CH4 ± H2O, and two other inclusions contain CO2, but no spatial pattern is evident. SEM-EDS analyses (80 spots) identify Fe-rich Chl (~70%), Ca-carbonate (~50%), a Si-rich phase (~40%), and minor rutile + mica (~5% each); one analysis contains a Na peak. SEM imaging shows that most inclusions contain multiple solid phases and an open cavity that may have hosted fluid and/or phases plucked during sample polishing.

We interpret that fluid containing Si, Ca, Ti, K triggered Grt rim growth and inclusion trapping during exhumation from UHP to the mid-crust; the apparent correlation of inclusion minerals with Grt zoning suggests changing fluid compositions. Minor marble in the UHP unit could supply Ca + CO2; phengite breakdown could supply K + Ti + H2O, and can also release ammonia, which could react with CO2 to produce the observed N2 + CH4. This fluid could be partially generated within the exhuming UHP terrane, and similar fluids may also be responsible for eclogite retrogression and associated amphibole + carbonate veins.