Metamorphic evolution of the Aileron Province in Limbla

Abstract

This Record presents thermodynamic modelling results from four samples collected in the eastern Aileron Province, Northern Territory. The Northern Territory Geological Survey undertook a 1:100 000-scale mapping campaign in the Limbla mapsheet area, eastern Aileron Province. Mapping identified high-grade metamorphic rocks of the Albarta Metamorphics and previously unrecognised Bluebush Metamorphics; both units are in close association with the Atneequa Suite, a newly defined suite of intermediate to felsic igneous rocks. Two samples of migmatite from each of the Albarta and Bluebush metamorphics were analysed to provide constraints on conditions of high-grade metamorphism.

The Albarta Metamorphics comprise stromatic metatexites from probable metamudstone protoliths. Peak-preserved metamorphism and partial melting likely occurred at high T/P or ‘Barrovian’ metamorphic conditions, which could not be accurately constrained by thermodynamic modelling. This metamorphic event may be recorded by a monazite age of 1769 ± 7 Ma. The metatexite has a high-strain foliation defined by biotite and muscovite ± sillimanite, which likely formed soon after the solidus was crossed at amphibolite-facies conditions of ~650°C and ~0.6 GPa. A second metamorphic monazite age of 1749 ± 8 Ma may represent growth of cordierite and andalusite in one of the samples at ultrahigh T/P conditions of 610°C and

The Bluebush Metamorphics comprise net- to raft-structured diatexites composed of metasedimentary rafts of paleosome surrounded by neosome. Peak-preserved metamorphism and partial melting also likely occurred at high T/P or ‘Barrovian’ metamorphic conditions, which could not be accurately constrained by thermodynamic modelling. Comingling of the Bluebush Metamorphics with a 1761 ± 21 Ma coarse-grained porphyritic granite of the Atneequa Suite may constrain timing of peak-preserved metamorphism and partial melting. A high-strain foliation defined by biotite and sillimanite likely formed within ~50°C of the solidus at amphibolite facies conditions. Subsequent decompression is implied by growth of cordierite ± andalusite at ultrahigh T/P metamorphic conditions. This event may be recorded by a metamorphic monazite age of 1743 ± 7 Ma, coeval with crystallisation of the ca 1.74 Ga Atneequa Suite. Outcrops of Bluebush Metamorphics are only found within the Atneequa Suite, consistent with entrainment of large blocks of the Bluebush Metamorphics within the Atneequa Suite as it was emplaced in the upper crust. Replacement of cordierite and K-feldspar by chlorite and muscovite suggests near isobaring cooling by ~150–200°C of the Bluebush Metamorphics in the upper crust. Cooling of the AtneequaSuite intrusion/s may have provided fluids to drive late metamorphic reactions at lower greenschist facies.