Subduction zone metamorphism: Timescales and emergence in the geological record

Abstract

Eclogites and blueschists are the primary recorders of processes that occur within subduction zones. Their mineralogy can be used to determine their pressure-temperature-time (P–T–t) evolution during burial to mantle depths, which in turn illuminates information about plate tectonics, subduction channel dynamics, and the thermal state of the mantle. This thesis uses high-pressure rocks as a tool to investigate subduction systems through time, from blueschist formed in a currently active subduction zone to the emergence of eclogites ca. 2 billion years ago. Of ongoing debate are the durations that materials can remain trapped in subduction channels, and the P–T–t paths they experience. Numerical modelling has predicted complex behaviour, such as accumulation and cycling of high-pressure low-temperature (HP–LT) rocks within the subduction channel, but there are few rock systems preserved clearly documenting this process. This thesis presents work on two locations of long-lived entrapment of HP–LT rocks, from a currently active subduction zone, the Izu-Bonin-Mariana system, and a palaeosubduction system in Eastern Australia. In the Mariana system, a fragment of blueschist was subducted at ca. 50 Ma and entrapped within the subduction system until its recent eruption in a serpentinite-volcano. In Eastern Australia, blocks of lawsonite eclogite and garnet blueschist were formed and accumulated at ca. 490 Ma and 470 Ma respectively. Further work identified two cycles of metamorphism in the lawsonite eclogite, with a second burial event taking place after partial exhumation within the subduction channel over 40 My. Subduction in the recent Earth can involve very cool thermal gradients (< 250 °C/GPa), although the earliest recognised instances of modern-style subduction occured along warmer thermal gradients. Eclogites from the Usagaran Belt in central Tanzania are amongst the oldest in the world, forming at ca. 2 Ga. They were buoyantly exhumed within metapelitic lithologies within ca. 6 Myr. They, along with other eclogites across the African Palaeoproterozoic orogens, demonstrate the transition from a warm subduction regime to the establishment of modern-style subduction on Earth. While the P–T–t evolutions of HP–LT rocks hold information about geodynamics, a fascinating topic is the emergence of subduction-style rocks in the geological record. Although there is debate around the timing of the first HP–LT rocks, eclogites widely emerge in the geological record from ca. 2.2–1.8 Ga, and then disappear until the Mesoproterozoic. Mantle temperatures were possibly too warm before ca. 2.2 Ga to allow the creation and/or preservation of eclogites. However, the subsequent ca. 0.7 Ga gap in the geological record is of interest. Coincident with this emergence, the global granite record shows in-creases in Y and heavy rare earth elements (HREEs), and decreases in Sr and Eu at ca. 2 Ga. These changes are explained by increasing temperatures associated with granite generation. This warming event is proposed to have been driven by crustal growth from ca. 2–1.8 Ga, which increased thermal insulation of the mantle. Warmer temperatures in the mantle prohibited the preservation of eclogites, until secular cooling of the mantle overcame the insulating effect at ca 1.1 Ga.