Structure and metamorphic petrology of the Forth metamorphic complex
Lewis, Rob (1991) Structure and metamorphic petrology of the Forth metamorphic complex. Honours thesis, University of Tasmania.
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-
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.
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|Deposited On:||11 Aug 2011 10:39|
|Last Modified:||06 Sep 2013 10:57|
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