Eclogite, serpentinite, mélange and mafic intrusive rocks : manifestation of long-lived Palaeozoic convergent margin activity, Port Macquarie, eastern Australia

Publication Type:
Thesis
Issue Date:
2007
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NO FULL TEXT AVAILABLE. This thesis contains 3rd party copyright material. ----- Nine units, six formally defined here, have been identified along the coastal tract between Port Macquarie and Tacking Point in the NSW Mid-North Coast part of the New England Fold Belt. All show the imprint of convergent margin tectonics. The oldest rocks are the (?)650 Ma MORB protolith of eclogite phacoids embedded in the chlorite-actinolite schist matrix of the Rocky Beach Metamorphic Mélange that occurs as a slab within the Port Macquarie Serpentinite Mélange. The last is a product of alteration of cumulate rocks of a c.530 Ma fore-arc ophiolite. The Watonga Formation comprises broken formation that includes Late Ordovician pelagic rocks, the MORB substrate on which these were deposited, younger basalt and olistostromes of ocean island origin, and tuff, siltstone and sandstone inferred to be trench fill. These were off-scrapped in the Late Ordovician-Carboniferous interval and form part of the Palaeozoic New England accretionary subduction complex. In the Permian the Tacking Point Gabbro and the Town Beach Diorite, calc-alkaline plutons geochemically allied with the Clarence River Supersuite of the New England Batholith, mark the establishment of a magmatic arc. Dykes of Karikeree Metadolerite and Nobbys Beach Lamprophyre, geochemically allied to the plutons, comprise later products of arc activity. Uncommon felsic dykes are considered minor components of the Late Triassic leucoadamellite suite of the New England Batholith. Little-altered Shelly Beach Dolerite dykes characterised by high Ti and Fe were subsequently emplaced in the coastal tract. Their geochemistry suggests a source influenced by subduction-derived fluids and they were probably emplaced in a backarc setting following eastward relocation of the magmatic arc Several deformational and metamorphic episodes have been identified. Eclogite shows syn-deformational prograde (De₁, Me₁) and retrograde (De₂, Me₃) (469 Ma) blueschist facies metamorphism separated by static eclogite recrystallisation (Me₂) (c.570 Ma), and a later greenschist overprint (De₄, Me₄). Accretion of the Watonga Formation was accompanied by deformations D₁ and D₂ that led to stratal disruption and local folding and foliation development and low-grade metamorphism. Subsequently expansion-related deformation and protrusion of the Port Macquarie Serpentinite led to its multiply deformed mélange character (Ds₁, Ds₂) that is also shown by the matrix of the related Rocky Beach Metamorphic Mélange (Dr₁, Dr₂). Serpentinisation occurred under conditions such that chrysotile and lizardite were stable and the altering ultramafic protolith, early mafic dykes, and eclogite were boudinaged during expansion, producing mélange. Blocks of the Watonga Formation were incorporated into the mélange during its emplacement as a series of lenses probably in the late Carboniferous. Emplacement of Permian-Triassic magmatic arc rocks was accompanied by shearing, faulting and large-scale open folding (D₃, D₄). Elevated temperatures were maintained and the intrusive rocks and the Watonga Formation were affected by low-grade regional alteration and local thermal metamorphism. Mylonite zones formed in the cooling plutons during D₃. The Tacking Point coastal tract contains a history, albeit fragmentary, of convergent margin activity extending over at least 370 Ma (c.570 Ma–c.200 Ma). Over this interval it was successively the site of forearc underplating, subduction accretion, arc magmatism and backarc magmatism.
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