Magmatic processes at rifted margins: Preliminary results from peridotites of the Diamantina zone (SW Australia)

Keywords: magma-poor rifted margin, refertilization, partial melting, mantle-melt interaction

Although magmatic processes are of primary importance for the understanding of lithospheric breakup, many first order questions remain, such as: how, when, where and how much magma is produced during final rifting; what are the conditions and controlling processes of magma production; how does magma percolate and interact with the lithospheric mantle; and how and when does magma focus, how is it extracted and how does it interact with the extensional processes during final rifting and breakup? Answering to these questions is a prerequisite to understand lithospheric breakup and formation of a new plate boundary, which is among the least understood plate tectonic processes at present.

In this study we present preliminary petrological results from mantle rocks dredged from the SW Australia ocean-continent transition (OCT, Diamantina zone). We analyzed pyroxene and spinel compositions from these peridotites to identify mantle domains and mantle-melt reactions during rifting and breakup. The chemical composition of clinopyroxenes shows two distinct populations: a first generation characterized by low (Sm/Yb)_N ratios and no Eu anomalies, while a second generation shows interstitial textures and flat HREE patterns with a deep negative Eu anomaly. These two populations of clinopyroxenes suggest that the peridotites from the Diamantina zone record two distinct events: a first cooling event that is followed by magma infiltration. This is further supported by equilibrium temperatures calculated on the two clinopyroxene generations showing that the first population equilibrated at lower temperatures (900°C ± 30°C) corresponding to a subcontinental geotherm, while the second generation equilibrated at higher temperatures (1100°C ± 100°C), and was likely liked to the entrapment of MORB-type melts in the plagioclase stability field at low pressure (~5kbar) during magma infiltration.  

The exhumation path of the Diamantina peridotites determined in our study is similar to those of refertilized peridotites from the present-day Iberia and fossil Alpine Tethys OCTs, suggesting that refertilisation processes occurring at magma-poor rifted margins during final rifting and breakup are not dependent from the inherited nature of the subcontinental mantle.