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Western Australia – the Musgrave province

These two models reproduce aspects of the tectono-metamorphic and magmatic evolution of the Mesoproterozoic Musgrave province, Western Australia.  The Musgrave Province developed at the nexus of the North, West and South Australian cratons and its Mesoproterozoic evolution incorporates a 100 Ma period of ultra-high temperature (UHT) metamorphism from ca. 1220 to ca. 1120 Ma.  This was accompanied by high-temperature A-type granitic magmatism over an 80 Ma period, sourced in part from mantle-derived components and emplaced as a series of pulsed events that also coincide with peaks in UHT metamorphism. The tectonic setting for this thermal event (the Musgrave Orogeny) is thought to have been intracontinental and the lithospheric architecture of the region is suggested to have had a major influence on the thermal evolution.

The model features a 1200km-wide, 100km-thick, terrane with a 100km-wide post-collisional suture zone in the centre (i.e. zone of lower plastic strength).  This central terrane is flanked to either side by two older, thicker (250km) cratonic blocks.  A temperature of 1344°C is used for the base of the lithosphere (Moho).  The convergence rate is symmetric; each side is pushed with the same constant velocity of 4 cm/a for the first 6 Ma.  For both models, the convergence rate is set to zero after 6 Ma and deformation is entirely self-driven.  Model II uses a mean cratonic lithosphere density of 3300kg/m3; Model III uses a mean cratonic lithosphere density of 3290kg/m­3

Musgrave Province - Model A

In the initial stage of Model A, localization of deformation occurs very rapidly due to introduction of the weak suture zone in the centre of the Mesoproterozoic province.  This results in subduction of the mantle lithosphere by 5 Ma, and detachment of the down-going material by 36 Ma, which leads to introduction of hot asthenospheric material into the area left of the weak zone.  As decompression melting begins, ultra-high temperatures are introduced into shallow levels and extensive melting of mafic and felsic material commences in the initiation zone.  The lateral propagation of the hot/partially molten asthenosphere leads to the peeling of the remaining material to the left of the weak zone up to the craton boundary, followed by the lateral propagation of hot material to the right.  As the hot material flows to the right, it underplates the “pre-molten” zone (initial weak zone) and only 400 km further to the right the next epicenter of decompression melting occurs.  The central (underplated) zone has been mostly subjected to HT metamorphism and UHT metamorphism and melting of felsic rocks.  It can be seen that the 900 °C isotherm does not reach levels below 40 km depth in the central parts of the model.  As the second epicenter of decompression melting begins at ~67 Ma the remaining thin lithosphere is delaminated and a second episode of extensive melting begins and continues for ~20 Ma.  Melting initiates from a “central” point with the highest temperatures and extension, and leads to the formation of limited and short-lived oceanic crust (by 70 km).  The spreading center closes after ~10 Ma as the system starts to cool down.  At around 90 Ma, melting reaches steady state and the system begins to cool and a new Moho is established.  The highest meting activity is observed for 70 Ma, and by the end of the experiment at 123 Ma, melts comprise 44% crustal input and 56% mantle derived melts.

Musgrave Province - Model B

In Model B, after delamination of the mechanically thickened lithosphere (weak zone), the remaining part of the thickened lower crust undergoes extensive melting (at a depth of 70 km with temperatures exceeding 700°C) from ~20 Ma and lasting for almost 20 Ma.  As delamination continues, asthenospheric material is introduced into shallow levels at ~39 Ma and extensive mafic melting begins, with further delamination as the lithosphere “peels off” in both directions from the melting center. The main UHT melting event is extensive, occurring along the entire Mesoproterozoic zone and lasting for ~25 Ma. The occurrence of such elevated temperatures (exceeding 900°C) promotes melting of all types of rocks. Due to the localized input of hot material, rocks undergo melting under different thermal conditions - from the center of the heat influx to its flanks. Three main bursts of melt production are observed at 39 Ma, 44 Ma and 47 Ma, which are associated with induction of three melting centers that develop in the model. Large, growing magma chambers form, and magma mixing and mingling accounts for multiple melting events that strongly affect the petrological and geochemical evolution of these rocks. Since high temperatures are retained for over 30 Ma, rocks are exposed to periodic re-melting and freezing episodes. Strong mixing and mingling of mafic and felsic material also takes place. A zone of metamorphosed crustal rocks rims the melting zones. Crystallized rocks may sink into partially molten melt reservoirs and mobile melt may rise to upper levels where it solidifies before reaching densities high enough for foundering. All subsequent melting events are related to re-melting processes, as temperatures remain high enough to allow for crustal anatexis.

Further information on metamorphism and P-T trajectories are available in Gorczyk et al., 2015.