Petrogenesis of the 2.1 Ga Lushan garnet-bearing quartz monzonite on the southern margin of the North China Craton and its tectonic implications

Abstract

The Lushan garnet-bearing quartz monzonite was emplaced into the western Taihua Complex at 2134±17Ma on the southern margin of the North China Craton (NCC). The rocks contain SiO2 contents of 56.98–59.05wt.%, with high K2O+Na2O (7.46–9.14wt.%) and Al2O3 (15.87–17.58wt.%), and low CaO (2.38–2.77wt.%) and MgO (1.18–1.50wt.%) contents, and belong to an intermediate shoshonitic series. Magmatic garnet grains in the rocks are chemically homogeneous, with relatively high CaO (6.36–7.92wt.%), but low MnO (1.45–1.94wt.%) and MgO (2.47–3.45wt.%) contents and high Fe/Mn ratios. They contain 61.94–66.39mol.% almandine, 18.60–23.40mol.% grossular, 10.06–15.11mol.% pyrope and 1.09–4.32mol.% spessartine, similar to high-pressure garnets formed from M-type magma but different from those found in S-type granites and metapelites. Phase equilibrium diagrams in combination with trace element distribution coefficients between zircon and garnet suggest whole-rock zircon saturation temperatures varying from 891°C to 951°C and the mineral assemblage of garnet+plagioclase+amphibole or biotite in these rocks is stable at >15kbar and 890–950°C with H2O<3wt.% in the magma. This temperature range permits the breakdown of biotite and Ca-amphibole (T>800°C). All of the rocks are enriched in LILE and LREE with low Nb/La ratios (0.24–0.61), similar to those of lithospheric mantle, which has a Nb/La ratio of ∼0.5. Therefore, the garnet and whole-rock chemical compositions indicate that the Lushan quartz monzonite possibly formed by small degrees of partial melting of a phlogopite-bearing lithospheric mantle. The variable zircon ɛHf (t) values (−3.5 to +4.1) with TDMC from 2.34 Ga to 2.51Ga suggest involvement of crustal materials. The rocks have slightly negative Nb and Ta anomalies, characteristics of island arc basalts (IAB). However, their high Zr (598–926ppm) and Zr/Y ratios (17.1–21.8) suggest an affinity with ocean island basalt (OIB). Hence, the mixed IAB- and OIB-like geochemical signatures are indicative of continental intra-plate rifting, possibly related to extension and thinning of the lithosphere and upwelling of asthenosphere. Our study proposes that an intra-continental rifting event occurred at 2.2–2.0Ga on the southern margin of the NCC after the final amalgamation of NCC.