Correlating metamorphic mineral assemblages with metamorphic ages in rocks recording multiple tectonothermal events: A case study of the Jiaobei terrane, eastern North China Craton

Abstract

Interpretations of high-grade terranes recording more than one episode of metamorphism can often be problematic due to the difficulties in assigning the metamorphic conditions to the time of metamorphism. The Jiaobei terrane as a typical Precambrian metamorphic terrane in the North China Craton (NCC) records multiple high-grade metamorphic events. Resolving the timing and features of each metamorphic event can cast light on the above issue and also play a key role in understanding the geological evolution of the Jiaobei terrane and the NCC. This study reports paired U-Pb dating with in-situ trace elements and oxygen isotope compositions of metamorphic zircons in meta-mafic rocks from the Qixia area of Jiaobei terrane. The zircons develop core-rim structures with both of them exhibiting metamorphic features. Zircon rims, although widely developed, are mostly too thin for in-situ age dating. LA-ICPMS as well as SIMS zircon U-Pb analysis of zircon cores yielded metamorphic ages of ∼ 2500 Ma, whereas the rims yielded 207Pb/206Pb ages of 1915–1717 Ma. The rare earth elements (REE) patterns and δ18O values measured in the Paleoproterozoic metamorphic zircon rims are distinct from those measured in the Archean cores. Distinct features of the cores and rims imply that the garnet-bearing (high-pressure) mineral assemblages were formed during late Paleoproterozoic metamorphic event and absent during the Neoarchean event. These new geochronological data further confirm that the Jiaobei terrane records both the late Neoarchean metamorphism and the late Paleoproterozoic tectonic thermal event. Also, our study argues for the existence of global subduction network geological records in the NCC during Paleoproterozoic, rather than during Neoarchean. The influence of the overprinted late Paleoproterozoic metamorphism should be fully considered when estimating P-T conditions of the earlier tectonothermal event (Neoarchean metamorphism).