Abstract Oceanic lithosphere is continuously destroyed by subduction, challenging our ability to develop tectonic reconstructions. One way to tackle this problem is to examine relics of once‐consumed oceanic domains preserved along convergent margins. In the New England Orogen (eastern Australia), disrupted exposures of ultramafic rocks loosely define a continuous trend. The exposures may represent vestiges of ancient oceanic lithosphere that existed along the eastern Gondwanan margin. To investigate the origin and evolution of this ultramafic belt, we obtained new petrological and geochemical data from the Marlborough Ultramafic Suite, the largest ultramafic massif in eastern Australia. It comprises harzburgite, serpentinite, and high‐grade serpentinite. Major and rare earth element systematics resemble residual abyssal peridotites from mid‐ocean ridges. The rocks preserve early lizardite serpentinization and evidence of high‐temperature fluid–rock interaction that took place at or near the ocean floor. High bulk alkali/U ratios indicate a shift into subduction, supported by the formation of prograde antigorite and talc. Magnesian olivine (Fo95) in high‐grade serpentinite is attributed to subduction‐related prograde metamorphism. Based on the petrological signatures, we suggest that abyssal peridotites from the Paleo‐Pacific Ocean were subducted beneath eastern Gondwana. Slivers of the slab were detached and mixed in the subduction channel before being exhumed and accreted onto the accretionary complex. Our results provide new constraints on the evolution of a once‐consumed oceanic domain along the eastern Gondwanan margin, offering insight into the fate of the oceanic lithosphere upon subduction.
Abstract The slab breakoff model is a preferred geodynamic interpretation for post‐collisional exhumation and associated magmatic flare‐ups. In this study, rutile U–Pb dating of retrograde eclogites revealed distinct stages of exhumation along the Paleo‐Tethyan sutures in southwest China. The studied samples yielded ages of 246.7 ± 8.5 and 239.7 ± 14.0 Ma in the Changning–Menglian suture zone, and 213.0 ± 10.0 and 205.6 ± 8.9 Ma in the Longmu Co–Shuanghu suture zone. These new rutile ages, along with published geochronological data for high‐pressure (HP) metamorphic and igneous rocks along the Paleo‐Tethyan sutures, reveal that Late Triassic tectonic exhumation was coeval with a regional magmatic flare‐up. Our results suggest that the Late Triassic tectonomagmatic activity occurred in a post‐collisional setting and was related to slab breakoff. The geochronological data for the HP metamorphic and igneous rocks show that the Late Triassic tectonomagmatic activity was diachronous along the suture. We propose that the spatiotemporal migration of these Late Triassic post‐collisional tectonothermal events was controlled mainly by the initiation and progression of slab breakoff. The results provide new insights into the tectonic evolution of the Tethyan geodynamic system and improve our understanding of post‐collisional tectonomagmatism in response to slab breakoff processes in a collision zone.
Abstract Zircon uranium–lead (U–Pb) geochronology is a cornerstone of Earth science for resolving the timing of deep‐time processes, yet the U–Pb system does not always behave as a closed chronometer. Selective loss of radiogenic Pb during fluid alteration or thermal disturbance can shift isotopic ages and generate discordant analyses. Some zircon data sets may preserve more than one Pb‐loss episode, yet many Pb‐loss modeling approaches return a single “best” time of Pb‐loss. The concordant–discordant comparison (CDC) test estimates Pb‐loss timing by finding the modeled disturbance age at which reconstructed discordant analyses best match the concordant population. Here, we extend the CDC framework to retain supported local optima across Monte Carlo realizations rather than reducing each realisation to a single optimum, and report 95% stability bounds that describe run‐to‐run repeatability in peak position rather than geological age uncertainty. This redesign can resolve the timing of multiple Pb‐loss episodes, recovering overprinting metamorphic, hydrothermal, or magmatic events. Testing the method against simulated zircon U–Pb data sets with prescribed Pb‐loss histories, we find that the ensemble CDC yields the lowest overall median absolute error of the benchmarked modeling approaches (19 Myr) and assigns age estimates to 27 of 36 true events. Errors are lowest for six single‐event benchmark examples, with a 90th‐percentile absolute error of 11 Myr, increasing to 83 Myr in overlapping multi‐event cases. Applied to an Archean Yilgarn granite, the ensemble resolves two Pb‐loss peaks at 454 and 1113 Ma, consistent with regional Pan‐African cooling and the Mesoproterozoic Warakurna Large Igneous Province respectively.
Abstract The Gyeonggi Massif is one of the major Precambrian basements in the Korean Peninsula where Paleoproterozoic orthogneiss and paragneiss were widely exposed. In this study, newly identified Rhyacian to Orosirian meta‐igneous rocks from the northwestern Gyeonggi Massif (Soyeonpyeongdo and Ganghwado areas) were investigated to elucidate their petrogenesis and tectonic implications based on zircon U–Pb–Hf isotopic data and whole‐rock geochemistry. The garnet‐bearing amphibole granite gneiss and biotite granite gneiss in the Soyeonpyeongdo area emplaced at ca. 2.06 Ga and 1.88 Ga, respectively, whereas the tonalite gneiss in the Ganghwado area formed at ca. 1.93 Ga. The 2.06 Ga garnet‐amphibole granite gneiss shows both I‐type and A2‐type features enriched in LILE and depleted in Nb–Ta, indicating generation in an arc‐related extensional regime. The 1.93 Ga tonalitic gneiss exhibits adakitic characteristics, negative εHf(t) values (−8.2 to −4.9), and thickened crustal signatures (42–74 km), which are consistent with derivation from Archean mafic lower crust thickened during a regional syn‐collisional stage. The 1.88 Ga biotite granite gneiss displays metaluminous to weakly peraluminous I‐ and A2‐type features with slightly negative εHf(t) values (−1.7 to +0.6), implying formation in a post‐collisional extensional setting. These successive magmatic events (2.06–1.85 Ga) in the Gyeonggi Massif are temporally and geochemically comparable to the Rhyacian–Orosirian magmatism in the Liao–Ji Belt of the eastern North China Craton. Therefore, the northwestern Gyeonggi Massif recorded a Paleoproterozoic tectono‐magmatic evolution broadly comparable to that of the Liao–Ji Belt.
Abstract The Antarctic Ice Sheet (AIS) has recently undergone accelerating mass loss driven primarily by basal melting associated with Circumpolar Deep Water (CDW) inflow onto the continental shelf. However, temporal variability in CDW inflow and its role in mass loss from the AIS over longer timescales remain uncertain because the duration of the observational record is too short to refine future projections. Robust proxies are therefore needed to reconstruct past CDW inflow and AIS variability. Although beryllium (Be) isotopes in marine sediments have been used to trace meltwater input, recent studies have challenged this interpretation, suggesting that 10 Be may instead reflect deep‐ocean currents, such as CDW. Here, we investigate the spatial distribution of authigenic Be isotopes in surface marine sediments from two fast‐flowing ice stream regions of East Antarctica and combine satellite with in situ oceanographic observations to evaluate Be isotope sources. Our results indicate that 10 Be concentrations and 10 Be/ 9 Be ratios increase along the CDW pathway and decrease from offshore to the ice shelf front. Notably, the spatial distribution of 10 Be is not correlated with the extent of seasonal sea ice. These findings suggest that CDW inflow is the dominant control on 10 Be distributions. In contrast, 9 Be concentrations are higher near the ice shelf front, likely reflecting sediment delivery from beneath the AIS, and may serve as indicators of subglacial meltwater. These findings highlight the potential of authigenic Be isotopes in Antarctic marine sediments to serve as proxies for reconstructing past variability in both CDW and AIS dynamics.
