Petrology And Geochemistry Of Mesozoic Volcanic Rocks In Southern Jilin Province

IntroductionThe nature and evolution of the Mesozoic lithospheric mantle and the lowercrust beneath the North China Craton (NCC) is the key to understand the time andmechanism of the Phanerozoic lithospheric thinning. Therefore, the thesis studiesthe Mesozoic volcanic rocks in southern Jilin province, northeast China. Detailedstudies on their petrology and geochemistry shed light on the tectonic backgroundand the nature of the magma sources, which is of important significance toconstrain the time and dynamic mechanism of the Phanerozoic lithosphericthinning.1 Forming time and rock associations of Mesozoic volcanic rocks in southernJilin province The⁴⁰ Ar/³⁹Ar plateau age of hornblende in Guosong formation volcanic rockis 130.2±0.3Ma, suggesting that they were formed in the early stage of the earlyCretaceous. The volcanic rocks of the Guosong formation are composed ofbasalt-basaltic trachyandesite-trachyandesite-dacite. The whole-rock K-Ar age ofShankeyushu formation volcanic rock is 118.3±1.87 Ma, indicating that they wereerupted at the late stage of the early Cretaceous. The volcanic rocks of theShankeyushu formation mainly consist of trachyandesite-trachydacite.2. Petrology and geochemistry of the Mesozoic volcanic rocks in southernJilin ProvinceThe early Cretaceous volcanic rocks in southern Jilin province belong tosub-alkaline series on the total alkali-silica diagram and show the evolution trendof calc-alkaline series. They are mainly composed of the high-K calc-alkalineseries rocks. The SiO2 and the total alkali (K2O+ Na2O) contents of GuosongFormation lavas range from 46.7 to 64.1 wt% and from 6.1 to 7.9wt%,respectively. The SiO2 and (K2O+ Na2O) contents of Shankeyushu Formationlavas range from 46.7 to 64.1 wt% and from 6.1 to 7.9 wt%, respectively. The K2O,Na2O, K2O+ Na2O, FeOT, and P2O5 contents and Mg# values of the volcanic rocksincrease, but SiO2, Al2O3, TiO2, and CaO contents decrease from the early to thelate stages of the early Cretaceous. The ∑REE abundances of Guosong Formation lavas range from 166.8ppmto 267.3ppm. They are rich in light rare earth elements (LREE), poor in heavy rareearth elements (HREE) (LaN/YbN=14.16²3.68), and slightly negative Euanomalies or no Eu anomaly. The ∑REE abundances of the SankeyushuFormation lavas range from 235.5 to 357.7ppm. Their LaN/YbN ratios range from25.05 to 47.86. They have slightly negative Eu anomalies. From the early to thelate stages of the early Cretaceous, the REE total contents and the LREE/HREEratios of the volcanic rocks increase. The REE distribution patterns are consistentwith those of the active continental margin and island arc basalts. But their LREEabundances are lower than those of the Qingshan formation volcanic rocks inwestern Shandong, and higher than those of the early Craterous volcanic rocks inYanbian areas. The early Cretaceous lavas in southern Jilin Province are strongly enriched inlarge ion lithophile elements (LILEs, i.e., Rb, Ba, Sr, K, Th), depleted in the highfield strength elements (HFSEs, i.e., P, Ti, Nb, Ta, Zr, Hf), which are similar to thebasalts in the active continental margin, island arc, western Shandong, Yanbianareas. But their LILE contents are lower than those of the Qingshan Formationvolcanic rocks in western Shandong, but higher than those of the basaltic rocksfrom the active continental margin, island arc, and Yanbian area. The distributionpatterns of their trace elements are strongly different from those of the oceanisland basalts. Primitive mantle-normalized transitional element patterns are similar to 'M'type, i.e. enriched in Ti, V and Mn, and depleted in Ni and Cr. The Ni and Crcontents in the volcanic rocks are related to their SiO2 contents. Initial (87Sr86/Sr)i andεNd( values of the volcanic rocks from the eastern part t)of southern Jilin province range from 0.7065 to 0.7077 and -2.67 to -4.34,respectively. Initial (87Sr86/Sr)i andεNd( values of the volcanic rocks from the t)western part of southern Jilin province range from 0.7056 to 0.7069 and -6.71 and-19.71, respectively. It is suggested that the deep lithospheric compositions insouthern Jilin province are inhomogeneous.3 Tectonic settings of formation of the Mesozoic volcanic rocks in southernJilin province The rock association, major and trace element data of the early Cretaceousvolcanic rocks in southern Jilin imply that they could formed under the tectonicsetting of the active continental margin. The coexistence of the early Cretaceous gabbro and syenite porphyry dykesin the southern Jilin indicates that a strongly extensional environment could takeplace. Above all, we consider that the early Cretaceous volcanic rocks in southernJilin could form under the volcanic arc extensional setting within the activecontinental margin, resulting from Izanagi plate subduction in the Mesozoic.4 Magma source of Mesozoic volcanic rocks and its nature in southern Jilin The field relationship between Guosong and Shankeyushu Formationssuggests that they could come from the different parental magmas, which isconsistent with their geochemical features. According to the geochemical characteristics of the mantle-derived originalmagma, the early Craterous volcanic rocks in southern Jilin are not the originalmagmas derived from the mantle. It is proved by the lack of the mantle-derivedinclusions, and lower Mg# values (<65), and lower abundances of Sc (Sc=13¹9ppm), Co (Co= 17³5ppm), and Ni (<60ppm). Meanwhile, they areobviously different from the Cenozoic basalts in geochemical compositions. The estimated pressures for clinopyroxene cumulates from GuosongFormation volcanic rocks range from 3.9 to 6.1kb, meaning the depths from 13 to20 km. The estimated pressures for clinopyroxene phenocrysts range from 0.6 to11.8kb, meaning the depths of 2 to 39 km. The clinopyroxene phenocrysts fromShankeyushu Formation lavas could form under pressures of 2.0¹3 kb, implyingthey could be derived from the depths of 7⁴3 km. On the basis of the biggest...