Characterising the P–T–t histories and effects of melt loss in high thermal gradient terranes

Abstract

Zircon U–Pb and Lu–Hf isotopes, in situ U–Pb monazite geochronology and calculated metamorphic phase diagrams are used to explore the tectonic settings of regional high thermal gradient metamorphism as well as the consequences of melt loss on the bulk composition and reactivity of residual rock packages. Case studies are presented from four high thermal gradient terranes: the Windmill Islands in Wilkes Land, east Antarctica; the central Aileron Province in central Australia, the Rayner Complex in east Antarctica and the southern Gawler Craton in South Australia. The Windmill Islands region records two stages of high thermal gradient metamorphism between c. 1320–1300 Ma and c. 1240–1170 Ma. The first stage of metamorphism occurred at conditions of 3.5–4 kbar and 700–730 ºC and was associated with the formation of a horizontal fabric. The second stage of metamorphism is most strongly recorded in the southern Windmill Islands where it reached conditions of ~4 kbar and 850 ºC, coincident with the emplacement of voluminous isotopically juvenile granitic and charnockitic magmas. The metasedimentary rocks of the Windmill Islands contain both arc- and craton-derived detrital zircon grains, suggesting that they formed in a back-arc setting. An extensional setting is consistent with the high thermal gradients and the formation of a regional horizontal fabric during the first stage of metamorphism. The intrusion of juvenile charnockite further suggests that the overall tectonic regime was extensional and that the crust beneath the Windmill Islands contained little evolved material. The central Aileron Province records long-lived high thermal gradient anatectic conditions between c. 1590 and 1520 Ma. Peak temperatures were in excess of 850 ºC with pressures of 6.5–7.5 kbar, corresponding to a thermal gradient of >130–140 ºC/kbar. The retrograde evolution involved minor decompression and then slow cooling, culminating with the development of andalusite. The absence of any syn-metamorphic magmatism and the development of contractional structures during metamorphism suggest that long-lived high thermal gradient metamorphism was likely to have been driven to a significant extent by the burial of high heat producing pre-metamorphic granitic rocks that volumetrically dominate the terrane. The Rayner Complex in east Antarctica was extensively deformed and metamorphosed during the Rayner Orogeny between c. 1020 and 900 Ma. Metamorphism was associated with voluminous granitic and charnockitic magmatism. The earliest phase of metamorphism is recorded in the southern Rayner Complex and involved pressures of >7.5 kbar. Pervasive metamorphism at 950–900 Ma affected the whole Rayner Complex and involved temperatures of 850–880 °C and lower pressures of 6–7 kbar. The Rayner Complex is interpreted to be a back-arc basin that was closed during two-stage collision between the Archean Antarctic cratons to the south and the arc, followed by collision with the Indian Craton. High thermal gradient metamorphism can occur in both collisional and extensional regimes and in both plate margins and intracontinental settings. The primary thermal driver in the Windmill Islands and the Rayner Complex was likely to have been the thinned lithosphere resulting from back-arc extension, whereas in the central Aileron Province, the primary thermal driver was likely to be anomalously high heat producing crust. However, in all three terranes, the attainment of regional high temperatures was facilitated by the preconditioning (dehydration) of the crust by prior melt loss events and slow erosion rates. In all four studied terranes, high thermal gradient metamorphism resulted in melt loss that significantly altered the compositions and reactivity of the residual rocks. One implication of melt loss during regional high temperature metamorphism is that it creates a terrane comprising anhydrous, residual rock compositions that are relatively resistant to reworking during subsequent metamorphic events. As demonstration of this, the Rayner Complex records a metamorphic event at c. 540–500 Ma that reached peak conditions of 800–870 °C and 5.5–6.5 kbar. However, high-T mineral growth at 540–500 Ma is only recorded in some locations. The spatial distribution of this mineralogical reworking was controlled by localised rock reactivity that may reflect domains that had undergone hydrous retrogression at the end of the Rayner Orogeny, locally enhancing the responsiveness of the rock mass during the Cambrian. In the southern Gawler Craton, forward modelling of an Fe-rich phyllite sequence shows that melt loss can also have economic implications by increasing the concentration of iron in the residual rock package, leading to enrichment in Fe-oxide minerals (magnetite and hematite). Muscovite-rich rocks with lower iron content are more fertile, produce more melt and therefore show a more significant increase (up to 35%) in the Fe-oxide content in the residual (melt depleted) rock package. Rocks with primary Fe-rich compositions are less fertile, lose less melt and therefore do not experience the same relative increase in the amount of Fe-oxides in the residuum. The economic implications of the modelling are that the more fertile horizons with lower primary iron contents may be significantly upgraded as a result of melt loss, thereby improving the overall grade of the ore system. In the case of southern Gawler Craton, melt loss-driven Fe enrichment has contributed to the formation of one of Australia’s largest known magnetite resource systems.