Petrogenesis of Early Devonian intrusive rocks in the east part of Eastern Kunlun Orogen and implication for Early Palaeozoic orogenic processes

Large scale of Early Devonian magmatism was found in both east and west parts of Eastern Kunlun Orogen.But few studies were focused on its origin,geodynamic mechanism and relationship with Proto-tethys.This paper reports a typical Early Devonian intrusion from the east part of the Eastern Kunlun Orogen,which named Yuejinshan intrusion.The intrusion is mainly composed of monzogranite,granodiorite,some gabbro and cordierite-bearing granite,in which the nature of the first three rock types is the study object of this paper.Monzogranite and granodiorite samples are absence of hornblende and cordierite,and are characterized by relatively high contents of silicon and potassium,low contents of iron,magnesium and calcium,with moderate alumina saturation index(A/NCK = 1.0 ～ 1.1) and enrichment of large ion lithophile elements(LILE:Rb,Th and K) and distinct depletion of Ba,Sr,Nb,Ta,Ti,P,Eu,etc.,belonging to peraluminous I-S transition type granites.Gabbros are characterized by significantly high contents of iron and titanium(the contents of FeOT and TiO2 are 8.17% ～ 12.70% and 4.50% ～ 6.54%,respectively),relatively high Cu contents(11.5 × 10-6 ～ 30.6 × 10-6) and Cu/Ni values(1.41 ～ 6.41),enriched in LREE(the values of(La/Yb) N and LREE/HREE are 1.94 ～ 3.15 and 2.65 ～ 3.48,respectively),relatively low Mg#(48 ～ 50),Cr(3.8 × 10-6 ～ 60.4 × 10-6) and Ni(1.8 ×10-6 ～12.5 ×10-6),Nb-Ta-Ti anomalies relative to primitive mantle.Monzogranites and granodiorites have relatively high ISr values(0.710 ～ 0.740 and 0.710,respectively),low eNd(t) values(-4.05 ～-5.80 and-3.54 ～-3.71,respectively) and old Nd istope model ages(the t2DM values are 1.47 ～ 1.62Ga and 1.43 ～ 1.45Ga,respectively).But the granodiorites have relatively higher eHf(t) values(-3.00 ～ 0.86).Gabbros have relatively high ISr values(0.711 ～ 0.714),low eNd(t)(-3.44 ～-6.82) and more concentrated eHf(t) values(-2.19 ～ 1.05),which show the characteristics of an enriched mantle.The LA-ICP-MS zircon U-Pb analyses suggest that the formation ages of granodiorites and gabbros are 407 ± 3Ma and 406 ± 3Ma,respectively.With a comprehensive consideration on petrology,geochemistry,whole rock Sr-Nd isotopes and zircon Lu-Hf isotope analyses,and so on,we consider the formation of Yuejinshan pluton had experienced the following processes.Mantle derived magmas rose to the crust and provided enough heat for partial melting of crust materials(meta-greywacke) which produced S-type cordierite-bearing granites.Meanwhile,they mixed with crust derived magmas and produced I-S transition type monzogranite and granodiorite.The Yuejinshan and other Early Devonian magmatisms in the Eastern Kunlun orogeny have the same rock assemblages as typical post-collisional magmatisms and show obvious marks of mantle magmatism,which suggests the Eastern Kunlun orogeny at least begun its post-collisional extensionin Early Devonian.Combining with regional studies,this paper suggests that Early Devonian is a critical period for the tectonic transformation of the Central orogenic belt(especially for Eastern Kunlun,North Qinling and Northern margin of Qaidam basin).During this period,the merging of terranes had finished and the tectonic framework begun to transfer from compression to extension.