Are subduction dynamics recorded by mafic rocks & their hosts?
Study an honours project that aims to understand the pressure-temperature-time conditions reached by the mafic rock vs the host metapelites.
During subduction, high-pressure metamorphic assemblages such as those found in eclogites and blueschists are formed in mafic rocks and are interpreted to record the behaviour of the subduction system.
However, pelitic and felsic rocks that are intercalated with subducted mafic rocks commonly lack metamorphic assemblages indicative of subduction. Therefore, the relationship between the down-going mafic oceanic slab and pelitic/felsic material (either continental crust or sediments) is enigmatic.
During the Palaeozoic, the east Gondwanan margin was the location of and long-lived (>200 Ma) subduction. In the Australian segment of the margin, this subduction was largely oceanic and consisted of a simple eastwards retreat of the subduction zone.
However, in Tasmania, continental material was incorporated into the subduction zone. These rocks now reside in western Tasmania, where mafic rocks with eclogite facies mineral occur within metapelitic host rocks that appear to lack evidence for subduction.
This poses the fundamental question that lies at the heart of how material behaves in subduction zone dynamics – did the pelitic rocks get subducted but not record high-pressures, or were the eclogites exhumed and somehow tectonically mixed into the low-pressure pelitic rocks? Thus far, there are no modern thermobarometric constrains on either the eclogite or the enclosing pelites in this system. So there is no data to examine this question.
This project will be completed using the software THERMOCALC, coupled with petrology and electron microprobe mineral compositional data.
Laser ablation ICPMS analyses will be used to determine the timing of mineral growth in both rock packages as well as the fractionation of trace elements during metamorphism between age determining minerals and their associated metamorphic assemblages. This will allow the rates of burial and exhumation of the rocks to be calculated.