⁴⁰Ar/³⁹Ar and K–Ar ages: early Paleozoic metamorphism and deformation in the Narooma accretionary complex, NSW

Abstract

White micas in slates from an imbricated sequence of deformed turbidites, and clast-in-matrixmelanges that form the Narooma accretionary complex at Batemans Bay, record apparent 40 Ar/ 39 Ar whole rock ages in the range 450 to 500 Ma, with intermediate-temperature average ages of 471 + 13 Ma and 476 + 11 Ma (2 standard deviations). By contrast, two K-Ar whole rock ages are 430.9 + 8.6 Ma and 449.2 + 9.0 Ma. Fractions (52, 3-5 and 10-20 mm) from the same samples show a progressive increase in age with grainsize (52 mm ¼ 413.1 + 8.3-404.7 + 8.1 Ma; 3-5 mm ¼ 417.1 + 8.3 Ma; 10-20 mm ¼ 434 + 8.6-437.5 + 8.8 Ma 2 standard deviations). The older whole rock 40 Ar/ 39 Ar ages are attributed to recoil loss of 39 Ar. The cleavage forming event is believed to have occurred at *460-440 Ma (Upper Ordovician to Lower Silurian). The finest fractions possibly record a younger thermal event at approximately 413 Ma. Kü bler Index values obtained from the Wagonga and Adaminaby Groups indicate lower epizonal (greenschist facies) metamorphic conditions. b 0 lattice parameter values (x ¼ 9.033; n ¼ 21) are characteristic of the intermediate pressure facies series.Assuming a temperature of approximately 3008C and P ¼ 4 kb suggests a depth of burial of approximately 15 km was reached by these rocks and that a geothermal gradient of 208C/km operated during subduction.

Underplated seamount in the Narooma accretionary complex, NSW, Australia

“…Early east-west trending folds are present and represent either slump folds formed on the slopes of the accretionary prism or early soft-sediment deformation folds formed during subduction (Fergusson and Frikken, 2003;Powell, 1983;Prendergast, 2007). Cleavage formed during subduction occurred between 460 and 440 Ma (Fergusson and Phillips, 2001;Prendergast et al, 2011). The grade of metamorphism is lower greenschist based on KI studies (Offler et al, 1998;Prendergast et al, 2011).…”

“…Further, subduction models for the Lachlan Orogen are well supported (Collins, 2002;Fergusson and Frikken, 2003;Foster and Gray, 2000;Foster et al, 1999;Gray, 1997;Gray and Foster, 1998;Prendergast, 2007;Prendergast et al, 2011;Spaggiari et al, 2004), with subduction being active from at least 460-440 Ma. However, alternative models for the eastern margin of the Lachlan Orogen have been proposed.…”

“…Subsequently, the basalts were subjected to epizonal (lower greenschist facies) conditions and burial to a depth of 15 km based on previous K-white mica studies of the rocks in the Wagonga Group at Batemans Bay (Prendergast et al, 2011). The assemblages developed under these conditions overprinted the lower grade assemblages, evidence for which is observed in the highly deformed clasts in which cleavage defined by white mica is flattened against relict hydrothermal quartz or amygdules (Fig.…”

“…Cleavage formed during subduction occurred between 460 and 440 Ma (Fergusson and Phillips, 2001;Prendergast et al, 2011). The grade of metamorphism is lower greenschist based on KI studies (Offler et al, 1998;Prendergast et al, 2011).…”

“…Gray, 1996, 1997;Offler et al, 1998;Prendergast et al, 2011), (v) underplating to at least 15 km based on KI (Kübler Index) and b cell parameter analysis of K-white mica (Offler et al, 1998;Prendergast et al, 2011) and (vi) tectonic slices of Adaminaby Group turbidites in the Wagonga Group (Fergusson and Frikken, 2003;Prendergast, 2007).…”