Geochronology and geochemistry of Mesozoic igneous rocks of the Hunjiang basin, Jilin Province, NE China: Constraints on regional tectonic processes and lithospheric delamination of the eastern North China block

Abstract The Hunjiang basin in Jilin Province, China, is host to a NE-trending sequence of sedimentary rocks located along the northeastern margin of the North China block in the Paleoproterozoic Jiao-Liao-Ji belt. The North China block is the largest and oldest cratonic block in China, but unlike other Precambrian cratons, its eastern part has been decratonized by multiple Phanerozoic orogenic events. No comprehensive geochronology study has been conducted on the rocks of the Hunjiang basin, and both the mechanisms and timing of lithospheric delamination beneath the eastern part of the North China block are largely unconstrained. In the Hunjiang basin, Proterozoic to Paleozoic sedimentary rocks are intruded by Mesozoic granitoids and are overlain unconformably by sedimentary and volcaniclastic rocks. Twenty igneous rocks (granites, porphyritic rocks, and volcaniclastic rocks) from the basin were analyzed for whole-rock geochemistry and dated by U-Pb laser ablation-inductively coupled plasma-mass spectrometry of magmatic zircons. The samples collected are felsic to intermediate in composition and of calc-alkaline affinity, and the granitic and porphyritic rocks show an adakite-like geochemistry. With the exception of one Neoarchean granite dike with a 207 Pb/ 206 Pb age of 2524 ± 9 Ma that intrudes the basement along the northeastern margin of the basin, all igneous rocks dated in the present study have Mesozoic crystallization ages between 180 ± 1 Ma and 90 ± 2 Ma. The Precambrian inherited zircons show age peaks at ~2700 Ma, ~2650 Ma, and ~2500 Ma. The presence of inherited zircons with ages as old as 3387 ± 38 Ma ( 207 Pb/ 206 Pb ages) shows that Paleoarchean crust exists beneath the Proterozoic to Paleozoic sedimentary rocks of the basin. The geochronological results suggest that magmatic activity and associated formation of hydrothermal ore deposits in the Hunjiang basin were prevalent from the Middle Jurassic to the early Late Cretaceous. Furthermore, structural constraints on the cessation of major tilting in the basin are provided, as an Early Cretaceous (127 ± 1 Ma) flat-lying volcaniclastic rock unit unconformably overlies steeply dipping sedimentary rocks. The adakite-like geochemistry of most granitic and porphyritic rocks suggests partial melting of a delaminated eclogitic lower crust, whereas the higher MgO, Ni, and Cr content of the Early Cretaceous porphyritic rocks indicates reaction between adakitic melts and mantle peridotite. Many of the volcaniclastic rocks have more evolved, less adakitic signatures and show negative Eu anomalies, indicating these formed from more evolved melts at shallower crustal depths, accompanied by assimilation-fractional crystallization processes. The results of this study indicate that major delamination was completed in the Hunjiang basin by the Middle Jurassic, while delamination was ongoing into the Early Cretaceous in many other parts of eastern China.