The unroofing of Archean crustal domes as recorded by detrital zircon and apatite

Abstract

This study presents in-situ U–Pb, Lu–Hf, and Sm–Nd isotopic data for detrital zircon and apatite collected from ephemeral streams of the East Pilbara Terrane, Western Australia. Given their disparate abundances in felsic versus mafic lithologies, a tandem apatite-zircon approach may offer more holistic insights into crust formation. Apatite U–Pb data define a single age peak at c. 2.9 Ga, consistent with labile, proximal, and first-cycle detritus from the Pilbara Craton. Conversely, zircon, a more refractory and durable mineral, records a more diverse geological history with U–Pb ages spanning from 3.6 to 0.2 Ga. The apatite age of c. 2.9 Ga records the timing through the Pb closure temperature during regional cooling following prograde metamorphism, while the Lu–Hf and Sm–Nd isotopic systems in the same grains yield c. 3.2 Ga isochrons, consistent with magmatic crystallisation at that time. Crystallisation age, initial ¹⁴³Nd/¹⁴⁴Nd and trace element geochemistry (Eu/Eu*) imply a chondritic or mixed (more radiogenic plus a less radiogenic) source for the apatite grains locally derived from the East Pilbara Terrane. Conversely, zircon ɛHf data reveal a broadly chondritic Paleoarchean proto-crust undergoing continual isotopic evolution punctuated by the input of juvenile, more radiogenic material on a quasiperiodic basis. Previous workers have invoked a crust-mantle overturn model triggered by stagnant-lid cooling and the episodic (re)fertilisation of the upper mantle to account for the periodic nature of crust formation in the East Pilbara Terrane. Detrital zircon grains track this process from a c. 3.8 Ga component that may have acted as a nucleus for subsequent crust formation. The oldest detrital zircon, on average, encompass less radiogenic (−ve ɛHf) components suggesting that the oldest grains preserve the unroofing of an ancient reworked crustal nucleus. Thus, the detrital zircon load arguably provides a more holistic record of the older crust in the region than the crystalline domes alone. Specifically, the less radiogenic dome cores are preferentially eroded due to their structural position and their mineral cargo lost into the detrital archive. We demonstrate that the apatite-zircon approach can be limited by the ability of apatite to be retained through crustal denudation.