Structure and metamorphic petrology of the Forth metamorphic complex

Abstract

The Forth Metamorphics outcrop in the lower reaches of the Forth River, Northern Tasmania and comprise a banded garnetiferous schist and quartzite, interlayered with sub-ordinate orthoamphibolites of tholeiitic MORB-type affinity. Geochemical and sedimentological constraints are consistent with a passive continental depositional environment and the sequence has been regionally metamorphosed during a two-phase tectono-metamorphic event. An early isoclinal fold phase (D1) produced a penetrative muscovite foliation but is largely overprinted by the dominant S2 schistosity. High temperature, relatively low strain quartz mylonites developed in narrow zones during west-directed D2 transport and are separated by domains of west vergent isoclinal F2 folds. Microprobe analyses of stable pelitic and metabasite assemblages have been used in conjunction with traditional/dataset thermobarometric methods and phase equilibrium constraints to estimate P-T conditions during D2. Peak conditions of 700°C +/- 50° C and 13 kb +/- 2 kb for kyanite-garnet-biotite schists in the Forth Valley are matched by independant estimates for garnet amphibole plagioclase assemblages and the latter preserve an early garnet-clinopyroxene-albite assemblage indicating conditions of 660° C and 11 kb during core growth. These results are supported by semi-quantitative P-T modelling of local calcite-altered garnet clinopyroxene zoisite intetbands, which formed in a locally H20-poor environment during compression/heating from 675° C 9-11 kb to peak conditions of 740° C and 13- 15kb. Significant P-T zonation is indicated by the spatial distribution of pelitic assemblages and peak temperatures some 100° C lower are inferred for staurolite-chloritoid bearing schists in the western half of the area. Paragonite and chloritoid textures in these units are consistent with breakdown of glaucophane, and may indicate an early high P-low T history. Late sphene and possibly albite developed during decompression but the preservation of substantially unretrogressed high grade assemblages indicates rapid late-D2 uplift and cooling. In the Forth Valley, garnet amphibolite assemblages preserve geochemical and textural evidence of late-D2 Kmetasomatism. The alteration is confined to a 300m wide local high strain zone along the contact with adjacent pelitic schists and thermometry results indicate post-peak conditions of around 600° C. The P-T history of the Forth Metamorphics is similar to that of the Collingwood River eclogites and may have developed during Precambrian partial subduction of a passive continental margin. Serpentinite bodies, enclosed within the metamorphics have minor structures consistent with early west-directed emplacement. The structural setting and geometry of these bodies has been modified by an east-directed thrust event of probable Devonian age, which produced a spatially restricted crenulation cleavage and minor folds in the metamorphics.