Metamorphic evolution of garnet amphibolite in the western Dabieshan eclogite belt, Central China: Evidence from petrography and phase equilibria modeling

The western Dabieshan terrane in central China involves numerous blocks of garnet amphibolite and eclogite enclosed in felsic gneiss and schist. Two representative garnet amphibolite samples SHS22 and BSW05 were selected for detailed study. They are composed of garnet, homblendic amphibole, sodic plagioclase, epidote, quartz, biotite and/or muscovite, and small amounts of rutile, sphene and ilmenite. A few phengite flakes are found as inclusions in garnet in sample SHS22. Garnet varies in grain size and rim composition. The coarse-grained garnet porphyroblasts are mostly anhedral and exhibit compositional zoning with increasing pyrope and decreasing or constant grossular content from core to rim. Pseudosections calculated in the system NCKMnFMASHO suggest that this garnet zoning reflects prograde P-T paths of early-stage eclogite facies from 1.9 GPa at 490 degrees C to 2.3 GPa at 555 degrees C for sample SHS26 and from 1.8 GPa at 450 degrees C to 2.2 GPa at 540 degrees C for sample BSW05. The peak P-T conditions are constrained by the garnet rim compositions with the maximum pyrope (X-prp) and minimum grossular (X-grs) contents. The Si contents in phengite from sample SHS22 were modeled to yield pressure conditions similar to that constrained by garnet. The other garnet rim compositions with decreasing X-prp and increasing X-grs contents were modeled in P-M (H2O) pseudosections to reflect different degrees of the modification of eclogite facies to amphibolite fades in fluid-unsaturated conditions, indicating decreasing pressures from 0.7 to 0.5 GPa and increasing water contents from 3.6 to 5.2 mol.% at T = 575 degrees C for sample SHS22, and decreasing pressures from 0.8 to 0.55 GPa and increasing water contents from 3.5 to 4.7 mol.% at T = 530 degrees C for sample BSW05, respectively. The garnet rims with minimum X-prp and maximum X-gis were predicted to be close to an equilibrium state with the matrix minerals involving hornblendic amphibole, plagioclase, epidote, biotite and quartz. The constituting assemblage was closely saturated with water. As it is hard to evaluate if the relevant minerals are in equilibrium or not in the garnet amphibolite with multi-generation assemblages, the conventional thermobarometers are not very useful for estimating the P-T conditions for this type of rock. (C) 2012 Elsevier Ltd. All rights reserved.