Petrogenesis of the Chaihulanzi Gneiss and its Tectonic Implications for the North China Craton

The Chaihulanzi area in eastern Inner Mongolia is tectonically situated on the northern margin of the North China Craton(NCC). The main Precambrian lithologies of the area have been referred to the Archean Jianping Group metasupracrustal sequences. Based on field observations, petrographic, whole-rock geochemical, and zircon U-Pb geochronological results, a magmatic origin for the units is proposed. Our results show that the Chaihulanzi gneisses are mainly of granitic, dioritic and granodioritic compositions, and show typical magmatic rock textures and mineral assemblages. The dioritic and granodioritic gneisses show Na-rich tonalite–trondhjemite–granodiorite(TTG)-like affinity with zircon U-Pb dates of ca. 2.57–2.59 Ga, representing a juvenile continental growth for the northeastern NCC. The granitic gneiss is indeed potassic granite and yielded a zircon U-Pb date of ~2.50 Ga, which is contemporaneous with the Jining–Jiaoliao microblock collision(2.53–2.49 Ga), implying another crustal growth event. The well-developed gneissosity in 2.57–2.59 Ga dioritic and granodioritic gneisses together with the 2.5 Ga potassic granite, which crosscuts the gneissosity implies a 2.57–2.50 Ga(mainly 2.53–2.51 Ga) collisional orogeny, probably related to the Jining–Jiaoliao microblock collision. Our new geology and chronological results provide new evidence for the early Precambrian tectonic evolution of the NCC.

Early Paleoproterozoic Post-Collisional Basaltic Magmatism in Quanji Massif:Implications for Precambrian Plate Tectonic Regime in NW China

Basaltic magmas can provide important information about mantle source nature,tectonic settings and tectonic evolution for a given terrain.This paper reports geology,petrography and geochemistry of whole-rock major and trace elements and Nd-Sr isotopes for a suite of garnet amphibolites from southeastern Wulan(Ulan),Quanji Massif,northwestern China.The garnet amphibolites were likely generated from basaltic lavas,associated with both paragneisses and orthogneisses of the lower Delingha Group.The basaltic protolith of these amphibolites can be broadly constrained to be formed at~2.33 Ga in an extensional setting post-collision.The geochemistry of amphibolites shows subalkaline and highly evolved characteristics.They display high-Fe low-Ti characteristics,with TFeO of 13.1 wt.%-17.9 wt.%and TiO_(2) of 1.42 wt.%-3.09 wt.%(in most samples TiO_(2)≤2.5 wt.%).The chondrite-normalized REE patterns show enrichment of LREE and LILE and the primitive-mantle-normalized incompatible element patterns display negative P,Ti,Nb-Ta and Zr-Hf anomalies.The(^(87)Sr/^(86)Sr)t values of 0.6978-0.7123 andε_(Nd)(t)values of-2.81-5.08 respond to depleted mantle model ages(T_(DM))of 2.33-3.30 Ga.These suggest that the precursor magmas of the protolith of the garnet amphibolites were probably derived from the Early Paleoproterozoic depleted sub-continental lithospheric mantle that had been metasomatized by subduction-induced fluids and melts.The precursor basaltic magmas were contaminated by the older crustal components during magma ascending.This post-collisional basaltic magmatic event at~2.33 Ga in Quanji Massif thus enhanced the subduction shutdown or slowdown tectonic regime both in NW China and coevally with those plate tectonics in some important domains worldwide during the Early Paleoproterozoic.