Caution in linking localized metamorphic P–T paths to tectonic settings

Metamorphic pressure-temperature-time (P-T-t) paths are critical records of crustal thermomechanical evolution, yet the growing documentation of both clockwise (CW) and counterclockwise (CCW) paths within single high-grade terranes complicates straightforward correlations with specific tectonic settings. The Eastern Hebei Complex in the eastern North China Craton, which preserves coexisting Neoarchean CW and CCW P–T paths, offers an ideal natural laboratory to investigate such seemingly contradictory metamorphic features and to reconcile the ongoing debate over its tectonic setting.

In the eastern part of the complex (Eastern Tectonic Domain, ETD), Neoarchean mafic and felsic granulites from the Taipingzhai, Qianan, and Caozhuang areas record 2.52–2.48 Ga CCW P–T paths with peak ultra-high temperature (UHT) conditions (>900°C), interpreted as evidence for a sagduction setting. In contrast, the western part (Western Tectonic Domain, WTD) contains identified Neoarchean ophiolitic mélanges. Geochemical data from ultramafic-mafic blocks in the Zunhua and Shangying areas indicate that these mélanges incorporate both Neoarchean forearc oceanic lithosphere fragments and exhumed subducted slab materials. Metamorphic blocks within the mélange (mafic granulite, garnet amphibolite) and the surrounding pelitic granulite matrix record different peak P–T conditions, defining CW P–T paths ranging from 715–850°C and 9.4–13.6 kbar at 2.48–2.46 Ga. Furthermore, Neoarchean UHP peridotite and eclogite-facies garnet clinopyroxenite have been documented in this belt. These metamorphic features are analogous to those recorded in tectonic mélanges of well-established Phanerozoic orogens. The coexistence of petrogenetically diverse blocks, derived from varying depths, indicates their entrainment, mixing, and exhumation within a subduction channel during plate convergence, followed by tectonic juxtaposition with the gneissic matrix.

Thermodynamic modeling indicates that these blocks experienced near-isothermal decompression, interpreted as rapid exhumation from varying depths within a subduction channel. The coeval CCW paths documented elsewhere in the complex are interpreted to result from the downward advection of isotherms during incipient subduction. The full spectrum of metamorphic P–T conditions and paths (CW and CCW) documented in the complex closely resembles patterns in post-Archean orogens, revealing the spatiotemporal evolution of thermal structure and perturbations during subduction-accretion. Our findings demonstrate that the lithological assemblages and diverse metamorphic records can be reconciled within a unified plate tectonic model, without invoking sagduction. The study emphasizes the necessity of integrating litho-tectonic unit classification and regional structural-metamorphic analyses over reliance on isolated P–T path before making tectonic interpretations.