Geochemistry of Metamafic Dykes from the Quanji Massif: Petrogenesis and Further Evidence for Oceanic Subduction, Late Paleoproterozoic, NW China

A suite of ~1.84-1.92 Ga metamafic dykes within the paragneiss suite（khondalite） of the Quanji massif in NW China, has been chosen in this study for further understanding the tectonic evolution and possible links to the global Columbia supercontinent. Occurrence and field relations suggest that they were formed coevally with a previous studied ~1.83-1.85 Ga metamafic dyke swarms. Whole-rock major and trace elemental geochemistry suggests precursor magma of the amphibolites being generated from a volcanic arc-related tectonic setting rather than a back-arc environment where the metamafic dyke swarms were emplaced. The metamafic dykes show enrichment of LREE and strongly negative anomalies for Ta-Nb, Zr-Hf and Ti, have high SiO_2（49.3 wt.%-52.5 wt.%） but low MgO（6.40 wt.%-7.76 wt.%） contents and Mg~#（Mg~#=[100×（MgO/40.3）]/[MgO/40.3＋FeO/71.8]） values（45.7-52.1）, suggesting evolved precursor magma. The high values of La/Ta（22.2-42.8） and La/Nb（1.71-2.47）, mildly negative εNd（t） values（-2.51-0.15）, with depleted mantle model ages（TDM） of 2.45-2.84 Ga, suggest that their precursor magmas were possibly derived from a subduction-related fluid metasomatized Archean sub-continental lithospheric mantle. This study provides further evidence for oceanic plate subduction prevailing before or around ~1.85 Ga, which confirms a prolonged subduction-accretion-collision history in the NW China which is possibly linked to the assembly of the Columbia supercontinent.

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.