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Structure and metamorphic petrology of the Forth metamorphic complex


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Lewis, Rob 1991 , 'Structure and metamorphic petrology of the Forth metamorphic complex', Honours thesis, University of Tasmania.

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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.

Item Type: Thesis - Honours
Authors/Creators:Lewis, Rob
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