Catastrophic slab loss in southwestern Pangea preserved in the mantle and igneous record

The study addresses the long-debated origin of the mid-Permian–Triassic Choiyoi Magmatic Province (CMP) in southwestern Pangea. The central research question is whether the CMP’s voluminous silicic magmatism was generated by processes related to subduction (e.g., arc magmatism) or by catastrophic modifications to the subducting lithosphere, specifically large-scale slab break-off and loss. The authors motivate the work by noting that direct documentation of slab break-off processes in pre-Cenozoic time has been limited and often contradictory when relying solely on surface geological records. By integrating plate-kinematic reconstructions with lower mantle slab imaging and geochemical discrimination, the study aims to provide a geophysically constrained explanation for the CMP and to clarify the role of slab loss in generating silicic large igneous provinces (SLIPs).

Previous studies have proposed localized slab break-off in late stages of certain SLIPs (e.g., Sierra Madre Occidental), but a SLIP generated primarily by massive slab break-off has not been documented. Tomographic and plate reconstruction frameworks (e.g., UU-P07 model; Atlas of the Underworld; global plate boundary evolution since the late Paleozoic) have been developed to link present-day lower mantle structures with past subduction zones. Work on SLIPs such as the Chon Aike Magmatic Province and the Okhotsk–Chukotka belt indicates active subduction contexts, whereas the Whitsunday SLIP has been associated with arrested subduction or post-convergent breakup settings. Geochemical discrimination schemes (e.g., Hildebrand and Whalen) provide trace-element criteria to distinguish arc, slab-failure, and within-plate granitoids. Theoretical and numerical studies suggest slab break-off dynamics depend on multiple triggers and may be favored during supercontinent assembly/breakup when upper mantle temperatures are elevated.

Tomotectonic analysis: The authors integrated geology, mantle tomography, and plate kinematics using GPlates 2.0. They employed the global UU-P07 seismic tomography model and resolution tests from the Atlas of the Underworld. Slab gaps in the tomography were delineated using amplitude thresholds (>0.2% for slab boundaries) following established criteria. Plate reconstructions were overlain to assess paleogeographic correspondence between imaged lower mantle structures and surface magmatic provinces. Vote maps: Positive wave-speed vote maps were generated using plotting tools from the SubMachine portal and 26 global P- and S-wave models that vary in data selection, parameterization, and inversion regularization. Lower mantle vote maps were constructed applying a standard deviation threshold (per Shephard et al.), with additional high-velocity maps using a zero threshold. Maps were built at depths between 1800 and 2800 km in 200 km steps, targeting depths linked to late Paleozoic–Mesozoic subduction beneath southwestern Pangea. Geochemical analysis: A compiled geochemical dataset of Upper Carboniferous–Jurassic igneous rocks was filtered by SiO2 content (55–70 wt%) and aluminum saturation index (ASI < 1.1), yielding n = 379 samples. This filtered set was plotted on tectono-magmatic discrimination diagrams to distinguish arc, slab break-off (slab failure), and within-plate signatures. Geochronological information and paleogeographic data were compiled to place geochemical signals in space and time relative to reconstructed slab geometries. Comparative tomotectonics of other SLIPs: For context, the authors interpreted tomographic slices and reconstructions for the Jurassic Chon Aike, Upper Cretaceous Okhotsk–Chukotka, and Cretaceous Whitsunday provinces to assess whether their mantle structures imply active or arrested subduction during SLIP emplacement.

- The Choiyoi Magmatic Province was produced by a large-scale slab loss event (massive slab break-off), rather than solely by typical arc or within-plate processes. - A reconstructed 2,800–3,000 km wide lower mantle slab gap spatially coincides with the CMP, indicating removal of a substantial segment of subducted lithosphere beneath southwestern Pangea. - Geochemical discrimination of 379 filtered samples shows a slab break-off fingerprint in CMP magmas, consistent with slab failure magmatism. - The slab break-off is compatible with Permian paleogeographic modifications along the southwestern Pangea margin. - CMP represents the oldest example of a slab loss event constrained jointly by geophysical imaging (mantle tomography), plate reconstructions, and the igneous record. - Compared to typical SLIPs, the CMP includes anomalously abundant intermediate (andesitic) magmatism in its lower portions and distinct geochemical signatures, consistent with slab break-off origin. - The timing (~285–250 Ma) aligns with a distinct mineralization event (porphyry, epithermal, polymetallic, and intrusion-related deposits) coeval with CMP emplacement.

The integrated tomotectonic and geochemical evidence addresses the origin of the CMP by linking surface magmatism to a lower mantle slab gap, indicating catastrophic slab break-off. This reconciles the unusual lithological and geochemical attributes of the CMP with a convergent margin undergoing slab failure, explaining differences from more typical rhyolite-dominated SLIPs. The study suggests that SLIPs driven by massive slab break-off are exceptional and likely require multiple triggers, with increased likelihood during supercontinent assembly or breakup when elevated mantle temperatures and thermal insulation enhance melting. Comparative analyses of other pre-Cenozoic SLIPs indicate that active-subduction contexts (e.g., Chon Aike, Okhotsk–Chukotka) differ from arrested-subduction or post-convergent settings (e.g., Whitsunday), reinforcing the uniqueness of CMP’s massive slab-loss scenario. The findings have broader implications for interpreting abrupt paleogeographic changes along convergent margins and for understanding the temporal-spatial distribution of ore deposits associated with slab failure magmatism.

By integrating lower mantle tomography, plate-kinematic reconstructions, and geochemical-chronological datasets, the study identifies massive slab break-off and slab loss as the driver of the Choiyoi Magmatic Province, establishing the oldest geophysically constrained example of slab loss. The paleogeographic alignment of a 2,800–3,000 km slab gap with the CMP and slab failure geochemical fingerprints substantiate this model. The work highlights that slab break-off–driven SLIPs are rare and likely tied to supercontinent cycles with elevated mantle temperatures. Future research should rigorously test the slab-loss hypothesis by reproducing the southwestern Pangea mantle structure through subduction numerical models that assimilate plate kinematics and seismic tomography, and explore implications for ore genesis and the geodynamic settings of other ancient silicic provinces.

Detection of slab break-off in pre-Cenozoic times has been limited and often contradictory when based solely on surface geology; while the present study adds geophysical constraints, the conclusions rely on tomographic model interpretations, amplitude thresholds, and plate reconstructions that carry inherent uncertainties. The hypothesis would benefit from forward geodynamic modeling that reproduces the observed mantle structure, to further validate the proposed slab loss scenario.