Zircon SHRIMP U–Pb dating of metamorphic complexes in the conjunction of the Greater and Lesser Xing’an ranges, NE China: Timing of formation and metamorphism and tectonic implications

Abstract

Metamorphic complexes, including the Luomahu, Xinkailing and Fengshuigouhe groups, are scattered in the conjunction of the Greater and the Lesser Xing’an ranges, NE China, and have long been interpreted to represent the Precambrian basement of the so-called “Xing’an” and “Songnen” blocks although reliable evidence is lacking. Thin-section examination and mineral assemblage indicate that the protoliths of these metamorphic rocks are mainly meta-sedimentary and meta-volcanic rocks and have experienced greenschist- to amphibolite-facies metamorphism and strong deformation. Zircon SHRIMP U–Pb dating results of the metamorphic complexes and associated pre- or syn-kinematic veins constrained the formation and metamorphic ages of the Luomahuo Group at 175±3Ma and 159±3Ma, the Xingkailing Group at 200±2Ma and 158±3Ma, and the Fengshuigouhe Group at between 230–310Ma and 170±3Ma, respectively. Additionally, all these rocks contain some Late Paleozoic detrital or inherited zircons with ages between 290–390Ma, but no Precambrian, even if Early Paleozoic, age information was detected from these metamorphic rocks. These new data suggest that the metamorphic complexes were formed during Mesozoic-Late Paleozoic period, rather than in Precambrian as previously inferred, and that all of them underwent metamorphism and deformation during Middle Jurassic between 170–160Ma. Consequently, these metamorphic complexes are not basement rocks of the so-called Precambrian continental blocks. Instead, there are likely metamorphosed Late Paleozoic–Mesozoic accretionary complexes and/or arc terranes. These results, in combination with published data, illustrate an overall young trend from north to south in the tectonic evolution of the northern Xing’an region. Significantly, the Jurassic (170–160Ma) metamorphism and deformation event firstly identified by this study from the metamorphic complexes likely recorded the coinstantaneous Mongol–Okhotsk collisional and thrusting event, implying the northern Xing’an region was in the fore-land setting during Middle Jurassic. The metamorphic complexes in the study area are either remnants of the detachment zones of the thrusting system or results of the far-field response of the collision event.