Extensive crustal melting during craton destruction: Evidence from the Mesozoic magmatic suite of Junan, eastern North China Craton

摘要

The cratonic destruction associated with the Pacific plate subduction beneath the eastern North China Craton (NCC) shows a close relationship with the widespread magmatism during the Late Mesozoic. Here we investigate a suite of intrusive and extrusive magmatic rocks from the Junan region of the eastern NCC in order to evaluate the role of extensive crustal melting related to decratonization. We present petrological, geochemical, zircon U-Pb geochronological and Lu-Hf isotopic data to evaluate the petrogenesis, timing and tectonic significance of the Early Cretaceous magmatism. Zircon grains in the basalt from the extrusive suite of Junan show multiple populations with Neoproterozoic and Early Paleozoic xenocrystic grains ranging in age from 764 Ma to 495 Ma as well as Jurassic grains with an age range of 189-165 Ma. The dominant population of magmatic zircon grains in the syenite defines three major age peaks of 772 Ma, 132 Ma and 126 Ma. Zircons in the granitoids including alkali syenite, monzonite and granodiorite yield a tightly restricted age range of 124-130 Ma representing their emplacement ages. The Neoproterozoic (841-547 Ma) zircon grains from the basalt and the syenite possess epsilon Hf (t) values of -22.9 to -8.4 and from -18.8 to -17.3, respectively. The Early Paleozoic (523-494 Ma) zircons from the basalt and the syenite also show markedly negative eHf(t) values of -22.7 to -18.0. The dominant population of Early Cretaceous (134-121 Ma) zircon grains presented in all the samples also displays negative eHf(t) values range from -31.7 to -21.1, with TOM of 1653-2017 Ma and TDMC in the range of 2193-3187 Ma. Accordingly, the Lu-Hf data suggest that the parent magma was sourced through melting of Mesoarchean to Paleoproterozoic basement rocks. Geochemical data on the Junan magmatic suite display features similar to those associated with the arc magmatic rocks involving subduction-related components, with interaction of fluids and melts in the suprasubduction mantle wedge. From the data presented, we propose that the Late Mesozoic intrusive and extrusive suites of Junan represent extensive lower and middle crustal melting, possibly triggered by mantle upwelling during the back-arc spreading associated with the Pacific plate subduction beneath the NCC. Intense asthenospheric upwelling resulted in lithospheric thinning and partial delamination during the Early Cretaceous, marking one of the peak decratonization stages of the NCC.