2015 GSA Annual Meeting in Baltimore, Maryland, USA (1-4 November 2015)

Paper No. 142-5
Presentation Time: 9:00 AM-6:30 PM


MATTINSON, Chris G.1, YOSHIDA, Kenta2, HIRAJIMA, Takao3, KAWAKAMI, Tetsuo3 and ZHANG, J.X.4, (1)Geological Sciences, Central Washington University, 400 E University Way, Ellensburg, WA 98926-7418, (2)Department of Geosciences, Osaka City University, Osaka, 558-8585, Japan, (3)Department of Geology and Mineralogy, Kyoto University, Kyoto, 606- 8502, Japan, (4)Chinese Academy of Geological Sciences, Institute of Geology, Beijing, 100037, China, mattinson@geology.cwu.edu

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.