HTLP metamorphism & accretionary orogen geodynamics: View from the WOMB (Wagga-Omeo Metamorphic Belt)

20 Oct

HTLP metamorphism & accretionary orogen geodynamics:
View from the WOMB (Wagga-Omeo Metamorphic Belt)

 

Brad Williams ¹, Alistair Hack ¹, Glen Phillips ²

[1] School of Environmental and Life Sciences, University of Newcastle, Australia; [2] Regional Mapping & Exploration Geoscience, Geological Survey of New South Wales
This study focuses on quantifying the thermal and structural history of the WOMB and its implications for (1) models of high-temperature low-pressure (HTLP) metamorphism, and (2) contrasting geodynamic models of Tasmanides evolution. The results are based on new field observations (Fig 1), metamorphic-structural analysis (Fig 2,3), application of geochronological methods to dating tectonometamorphic fabrics (Fig 5-7). Geodynamic setting of HTLP metamorphism is further constrained using mineral equilibria, thermal modelling and analysis of modern global surface heat flow data (Fig 8,9).

Slide1

Slide2

METAMORPHIC ZONATION

Mapping metamorphic zonation and structure in the study area (Fig 1,2,4) was integrated with petrographic analysis (Fig 3) and monazite geochronology (Fig 5,6) to establish the relative and absolute timing of tectonometamorphic events in the WOMB (Fig 7).

Slide1

STRUCTURE & TECTONOMETAMORPHIC FABRICS

Slide1

HOLBROOK STRUCTURAL-METAMORPHIC MODEL

Slide2

HTLP MONAZITE-CORDIERITE FABRICS PREDATE CRUSTAL THICKENING (BENAMBRAN) & S-TYPE GRANITE EMPLACEMENT: GEOCHRONOLOGY

Slide1

A major petrographic & geochronological observation is the high-temperature fabric (S1) formed sub-parallel to bedding at 443 ± 3 Ma, prior to crustal thickening (D2/S2; maximum duration 440-430 Ma) & before emplacement of non-foliated S-type granites at ~ 430 Ma. This tectonometamorphic history conflicts with previous work on the WOMB (Morand 1990) and is not predicted by current geodynamic models.

The heat problem

One of the enduring problems with HTLP metamorphic complexes is explaining and identifying the source of the vast amount of heat in the uppermost crust that they demand. A variety of heat sources have been proposed, (a) lithospheric delamination & asthenospheric upwelling; (b) deep burial, radiogenic heating followed by rapid exhumation; (c) contact aureole eects around intrusive complexes (Loosveld & Etheridge 1990).

CRUSTAL THICKNESS & GEOTHERMAL GRADIENTS OF HTLP METAMORPHISM: PHASE EQUILIBRIA CONSTRAINTS

Analysis of phase equilibria (Fig 8) based on the most recent thermodynamic data (White et al. 2014), relevant to pelites require geothermal gradients > 80 °C km-1 and resultant surface heat flow in excess of 200 mW m-2 to produce cordierite + andalusite assemblages , such as those at Holbrook, and a minimum of 110 mW m-2 to stabilize cordierite+biotite & less than 10 km depth (sillimanite-absent). Crd-bi-sill±melt assemblages (Morand 1990) in pelites west of the TCFZ imply hotter and/or deeper crust is exposed in the Omeo zone.

GEODYNAMIC SETTING OF HTLP METAMORPHISM: HEAT SOURCES & SURFACE HEAT FLOW FINGERPRINT

Surface heat flow reflects the underlying geothermal gradient and distribution of heat sources with depth, of which both are governed by geodynamic processes. Accordingly, we have examined global surface heat flow data in order to identify locations (geodynamic setting) of sufficiently high surface heat flux, i.e. > 100mW m-2, that can be currently active zones of regional HTLP metamorphism.

A survey of global surface heat flow (Fig 9) shows only young, hot, extending oceanic crust associated with back-arc environments are capable of producing regional cordierite-bearing assemblages in HTLP terranes like the WOMB. High heat flow in this tectonic setting is a consequence of extensive mantle decompression leading to melting (due to lithospheric thinning) and mafic underplating associated with generation of new oceanic crust. Modelled cooling rates of oceanic lithosphere suggest HTLP conditions may persist for a maximum duration of ~17 My. Collisional settings on the basis of associated surface heat flow appear incapable of generating regional HTLP metamorphism.

Slide1

HTLP & TASMANIDE GEODYNAMIC EVOLUTION

SigniFIcant results of this study are that HTLP metamorphism peaking at ~443 Ma

1. Terminates during/in response to crustal thickening D2 ~435 Ma

2. Shares no temporal relation to granite emplacement ~430 Ma

3. Records high-temperature extensional geodynamic processes requiring advective mantle heat input – collisional settings for HTLP are ruled out.

This thermal and structural history supports a retreating accretionary geodynamic model in which a submarine sedimentary pile undergoes HTLP metamorphism during an early phase of extensional back-arc spreading (D1). Peak HTLP metamorphism was followed by a relatively short-lived crustal thickening event (D2) & subsequent S-type granite emplacement. This differs from existing geodynamic models for the Tasmanides.
References: Aitchison JC & Buckman S 2012. Gondwana Res, 22, 674–680. Bodorkos S et al. 2015. http://dx.doi.org/10.11636/Record.2015.002. Collins WJ 2002. Tectonics, 21, DOI 10.1029/2000TC001272 Foster DA et al 1999. Tectonics, 18, 452–485 Lagabrielle Y et al. 1997. EPSL, 149, 1–13. Loosveld RJH & Etheridge MA 1990. JMG, 8, 257–267. Morrand VJ 1990. JMG, 8, 1–12. Pollack HN et al. 1993. Rev. Geophysics, 31, 267–280. White RW et al. 2014. JMG, 32, 261–286.

Leave a Reply

Your email address will not be published. Required fields are marked *

This site uses Akismet to reduce spam. Learn how your comment data is processed.