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

Abstract

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, hornblendic 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 1.9GPa at 490°C to 2.3GPa at 555°C for sample SHS26 and from 1.8GPa at 450°C to 2.2GPa at 540°C for sample BSW05. The peak P–T conditions are constrained by the garnet rim compositions with the maximum pyrope (Xprp) and minimum grossular (Xgrs) 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 Xprp and increasing Xgrs contents were modeled in P–M (H2O) pseudosections to reflect different degrees of the modification of eclogite facies to amphibolite facies in fluid-unsaturated conditions, indicating decreasing pressures from 0.7 to 0.5GPa and increasing water contents from 3.6 to 5.2mol.% at T=575°C for sample SHS22, and decreasing pressures from 0.8 to 0.55GPa and increasing water contents from 3.5 to 4.7mol.% at T=530°C for sample BSW05, respectively. The garnet rims with minimum Xprp and maximum Xgrs 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.