Zircon petrochronology and mineral equilibria of the eclogites from western Tasmania: Interrogating the early Palaeozoic East Gondwana subduction record

The metamorphic footprint of western Laurentia preserved in subducted rocks from southern Australia

“…Metapelitic samples FMC‐1b and FMC‐2c enclose metre‐ to decimetre‐scale mafic eclogites which were subducted in the Cambrian (e.g., Brown, Hand, & Morrissey, 2021), with FMC‐1b preserving an apparent high‐pressure assemblage containing garnet, kyanite, muscovite, rutile, and quartz. Garnet porphyroblasts in FMC‐1b record c. 1240 Ma metamorphism, while texturally distinct fine‐ to medium‐grained garnet in the same sample records Cambrian metamorphism.…”

“…The Western Tasmanian Terrane includes southern Australia's western Tasmania and King Island and contains Mesoproterozoic strata unconformably overlain by Neoproterozoic strata and mafic volcano‐sedimentary sequences (Figure 2; Berry et al, 2008). The Proterozoic strata of western Tasmania were metamorphosed and deformed during the middle to late Cambrian Tyennan Orogeny—an expression of broader subduction orogenesis along the early Palaeozoic East Gondwana margin (Figure 1a; Brown, Hand, & Morrissey, 2021; Meffre et al, 2000; Mulder et al, 2016). The Tyennan Orogeny involved the collision of the east‐facing passive margin of the Western Tasmanian Terrane with an intraoceanic island arc, resulting in c. 515–500 Ma high‐pressure metamorphism, emplacement of an ophiolite onto the continental margin, and ultimately the distribution of several metamorphic complexes across western Tasmania (Figure 2; Berry & Crawford, 1988; Berry et al, 2007; Brown, Hand, & Morrissey, 2021; Mulder et al, 2016; Palmeri et al, 2009; Turner & Bottrill, 2001).…”

“…The Proterozoic strata of western Tasmania were metamorphosed and deformed during the middle to late Cambrian Tyennan Orogeny—an expression of broader subduction orogenesis along the early Palaeozoic East Gondwana margin (Figure 1a; Brown, Hand, & Morrissey, 2021; Meffre et al, 2000; Mulder et al, 2016). The Tyennan Orogeny involved the collision of the east‐facing passive margin of the Western Tasmanian Terrane with an intraoceanic island arc, resulting in c. 515–500 Ma high‐pressure metamorphism, emplacement of an ophiolite onto the continental margin, and ultimately the distribution of several metamorphic complexes across western Tasmania (Figure 2; Berry & Crawford, 1988; Berry et al, 2007; Brown, Hand, & Morrissey, 2021; Mulder et al, 2016; Palmeri et al, 2009; Turner & Bottrill, 2001). As a consequence of Tyennan orogenic and post‐orogenic processes (the latter involving rapid exhumation of the high‐pressure products and the development of syn‐sedimentary normal faults and felsic extrusive magmatism; Mortensen et al, 2015; Noll & Hall, 2005) and subsequent overprinting during the Devonian Tabberabberan Orogeny, much of the Proterozoic history recorded in western Tasmania has been obscured (Berry et al, 2007; Brown et al, 2022).…”

“…In the Franklin Metamorphic Complex, quartz‐rich metapelitic schist is the most common rock type, together with lesser quartzite and phyllite (Figure 1c). The garnet–kyanite‐bearing metapelitic schists in the Franklin Metamorphic Complex enclose metre‐ to decimetre‐scale mafic boudins which reached eclogite‐grade conditions of ~18–21.5 kbar and ~650–700°C during subduction orogenesis along the East Gondwana margin (Brown, Hand, & Morrissey, 2021). The mafic boudins preserve HREE‐rich c. 1495–1410 Ma zircon ( 207 Pb/ 206 Pb date of 1457 ± 16 Ma), which is either detrital or magmatic or reflects metamorphism in a system without garnet (Brown, Hand, & Morrissey, 2021).…”

“…The garnet–kyanite‐bearing metapelitic schists in the Franklin Metamorphic Complex enclose metre‐ to decimetre‐scale mafic boudins which reached eclogite‐grade conditions of ~18–21.5 kbar and ~650–700°C during subduction orogenesis along the East Gondwana margin (Brown, Hand, & Morrissey, 2021). The mafic boudins preserve HREE‐rich c. 1495–1410 Ma zircon ( 207 Pb/ 206 Pb date of 1457 ± 16 Ma), which is either detrital or magmatic or reflects metamorphism in a system without garnet (Brown, Hand, & Morrissey, 2021). The presence of garnet–kyanite‐bearing mineral assemblages indicates the schists were subducted in the Cambrian together with the mafic rocks (e.g., Chmielowski & Berry, 2012).…”