Tracking decratonization process along a cratonic edge through late Permian to late Triassic magmatic flare-up in northwestern Liaoning, North China Craton

Abstract

Abstract Resolving magmatic tempos of multiple scales along convergent plate margins presents a pivotal task in any regional tectonic and supercontinental reconstructions, as is of paramount importance for the northern North China Craton (NCC) that converged with the Central Asian Orogenic belt (CAOB) during the Paleozoic to early Mesozoic. This study applies the integrated zircon U Pb dating and geochemical tracing to late Permian to late Triassic intrusive flare-up in northwestern Liaoning, with three episodes of granitic suites deciphered. Their contrasting elemental and isotopic features called for three distinct crustal end-members involved in their formation: ancient amphibolitic protolith, Paleozoic-accreted mafic meta-igneous ingredients and relaminated juvenile intermediate arc plutons. Sequential crustal anatexis of these multiply juxtaposed protoliths in a felsic hot zone tends to yield successive parental magmas for forming three granite suites. While the late Permian (260–250\u202fMa) granites, with their felsic adakitic affinity and variably evolved isotopic compositions (87Sr/86Sri\u202f=\u202f0.70637 to 0.70659, eNd(t)\u202f=\u202f−16.7 to −10.2, zircon eHf(t)\u202f=\u202f−18.1 to +1.1), are consistent with partial melts of major ancient crustal materials and minor newly-underplated mafic ingredients, the middle Triassic (242–240\u202fMa) granites show a more juvenile isotope signature (eNd(t)\u202f=\u202f−11.8 to −4.7, zircon eHf(t)\u202f=\u202f−14.7 to +4.1) and attest to distinctively higher input from newly-underplated mafic lower crust in their formation. By contrast, the late Triassic (227–223\u202fMa) granites exhibit an A-type magmatic affinity. Their radiogenic whole-rock Nd (eNd(t)\u202f=\u202f−3.7 to +1.1) and zircon Hf (eHf(t)\u202f=\u202f+0.7 to +12.0) isotopic values suggest magma derivation from high-temperature fusion of dominantly relaminated juvenile charnockitic protolith. What follows is one phase of shoshonitic mafic dykes that fingerprint a slab-melt metasomatized asthenosphere source with depleted isotopic compositions (87Sr/86Sri\u202f=\u202f0.70374 to 0.70388, eNd(t)\u202f=\u202f+3.5 to +4.1). Synthesizing these events with the previously recognized ones leads to a first-order duration of ca. 260–218\u202fMa for the flare-up and its further differentiation into three secondary pulses including ca. 260–250\u202fMa appinite and granite, ca. 242–238\u202fMa ferroan granites and ca. 227–218\u202fMa A-type granites and mafic dykes. While such primary duration is typical of post- collisional/orogenic magmatic tempos of convergent continental margins, three secondary pulses encapsulate successive geodynamic processes from crustal thickening through orogenic collapse to lithospheric dripping in the aftermath of the Paleo-Asian oceanic closure. Featuring a coupled scenario of lithospheric thinning and crustal growth, the northwestern Liaoning case of cratonic-edge destruction serves as a prelude but also a contrast to the late Mesozoic large-scale decratonization across the eastern NCC possibly due to oceanic subduction of the paleo-Pacific plate.