Decoding multiple tectonothermal events in zircons from single rock samples: SHRIMP zircon U–Pb data from the late Neoarchean rocks of Daqingshan, North China Craton

Abstract

A complex sequence of tectonothermal events shaped the Neoarchean–Paleoproterozoic crustal evolution in the North China Craton (NCC). Here we present a systematic SHRIMP zircon U–Pb investigation on a suite of metamorphic rocks from the Hademengou area of Daqingshan within the Khondalite Belt of the Inner Mongolia Suture Zone. We analyzed zircons from two meta-volcanic rocks of the Sanggan “Group” and four meta-igneous rocks of late Neoarchean to early Paleoproterozoic. Zircons from these rocks display typical core–rim or core–mantle–rim structures. The relict magmatic zircons record crystallization ages of ca. 2.5Ga followed by variable Pb loss, showing a range of ages in the concordia up to 2.1Ga. The recrystallized domains of zircons show sector zoning or are composed of inner dark domains, and outer lighter domains. The overgrowth mantles are homogeneous. The recrystallized zircons and overgrowth mantles show similar age data distribution along a concordia from ~2.5 to ~2.1Ga. The overgrowth rims are poorly developed but record ages (1.96–1.84Ga) similar to those displayed by metamorphic zircons in the late Paleoproterozoic “Upper Wulashan Subgroup” as well as the charnockites and UHT granulites reported in previous studies from this region. A meta-syenogranite intruding the Sanggan “Group” and Hypersthene dioritic gneiss contains 1.95Ga magmatic and 1.85Ga metamorphic zircons. Magmatic zircons of the late Neoarchean–early Paleoproterozoic metavolcanic rocks have εHf(t) of 1.2 to 7.0, tDM1(Hf) model ages of 2687 to 2532Ma and tDM2(CC)(Hf) model ages of 2837 to 2559Ma. The recrystallized zircons and mantle domains display Hf isotope composition similar to that of the magmatic domains. Our study is the first report from this region that identifies multiple tectonothermal events ranging from late Neoarchean through early to late Paleoproterozoic from zircons within single rock samples in the NCC. A strong overprint of late Paleoproterozoic metamorphism traced commonly in all the rock units indicates that these rocks were involved in the tectonothermal event associated with the final amalgamation of the NCC.