Petrogenesis Of Early Late Jurassic Rhyolite From Zhirui Basin In Southern Great Xing’an Range: Constraints From Chronology And Geochemistry

The early Late Jurassic volcanic basin called Zhirui Basin is located in Hexigten Banner, Inner Mongolia, at the southern tip of the Great Xing’an Range. Its tectonic position lies in the Liaoyuan block between the Xar Moron-Changchun suture zone and Chifeng-Kaiyuan suture zone. The felsic volcanic series in Zhirui basin occurs unconformablely above the underlying Dashizhai Formation, and below the overlving basalt of Hannuoba Formation, and has been intrusived by granitic porphyry. According to the rock assemblage features, and combining the age data of rhyolite obtained in this paper, this rock assemblage should belong to Xinmin Formation.The SHRIMP zircon 206*Pb/238U age estimates for the rhyolite yield weighted an average age of 156.9Ma±1.7Ma, therefore, the Zhirui rhyolites were formed in the early Late Jurassic. And according to the International Chronostratigraphic Chart (2015/01) and the strati graphic Chart of China(2014), the boundary line of Middle Jurassic and Early Cretaceous is 163.5±1.0Ma, and the boundary line of Late Jurassic and Early Cretaceous is 145.0Ma±0.8Ma, indicating the rhyolites at Zhirui basin are the product of volcanic activities in early Late Jurassic.The whole-rock geochemistry of the rhyolites show an A-type magmatic affinity, with a typical enrichment in SiO2(73.92%-75.79%), K2O(4.55%-6.44%), Ga, Zr, Nb, Y and a high ratio of FeOT/MgO(20.37-40.69) but low contents of A12O3(11.91%-12.58%), CaO (0.10%～0.17%)and MgO(0.07%-0.13%), A/CNK=0.99-1.09,K2O/Na2O=1.07-2.24, Na2O-2<K2O, A.I range from 0.90 to 1.00,σ range from 2.08 to 2.77, belonging to metaluminous high-K calc-alkaline series rocks.Furthermore, they also have significantly higher content of trace elements such as Ga, Zr, Nb, Y but low levels of Ba and Sr. Relative enriched in Rb, Th, U, Pb, Zr, Hf and depletion in Ba, Sr, P, Ti and Nb, Ta, and fractionated REE patterns showing strong negative Eu anomalies, Zr+Nb+Ce+Y=937-1139×10-6(>350×10-6), and the 10000Ga/Al ratio of rhyolites range from 3.73 to 4.55, the co(Y)/co(Nb) ratio of the rhyolite samples range from 1.3 to 1.93, show the characteristics of A2-type garnite that demonstrate an extensionally tectonic setting. The calculated zircon saturation temperature range from 936～954±(average 944±), match the A-type magmatic affinity with high temperature. In the trace element variation diagrams, the EEE、Rb、Ba、Zr、Hf、Nb、Th、La、Yb shows positive coorelation against SiO2, and the Sr、U、Pb、(La/Yb)N、Zr/Hf、Nb/Ta shows discrete distribution with the growing of SiO2 contents. The Rb/Sr、Ti/Y and Ti/Zr ratio of rhyolites are7.56～11.1 (averaging at 8.75),19.7-23.7(averaging at 21.0) and 1.85-2.15(averaging at 1.94), belonging to the range of crustal sourced magma; Th/Ce ratio of rhyolites range from 0.10 to 0.13, Th/Nb=0.41-0.60, both ratios are same to the ratio of the continental crust, combine with the results showing in the discrimination diagrams of magma source, we guset that the source of the thyolitic magma may come from the continental crust.The rhyolites share such features as relative high (87Sr/86Sr)i valeus(0.706510-0.709821), low εNd(t) values(εNd(t)=-6.32-4.44) and relatively young Nd-modle age (1304-1457Ma). They exhibit low radiogenic Pb isotopic compositions ((206Pb/204Pb)t= 17.15-17.80, (207Pb/204Pb)t=15.41-15.48, (208Pb/204Pb)t=37.38-37.63. The projection point are located between the lower crust and depleted mantle as shown in the diagram of sNd(t)-(87Sr/86Sr)i, and located in the lower crust areaclose to the Enriched mantle in the plumbotectonics model diagram of lead isotopes.These elemental and isotopic characters argue for parental magmas from partial melting of Mid-Mesoproterozoic crust materials which were drived from the Enriched mantle and their subsequent fractional crystallization. In the major elements and trace elements discrimination diagrams of the tectonic settings, all rhyolites indicative that they belong to extensionally tectonic setting within the plates. As viewed from regional correlation with the coeval volcanic rocks, together with the Late Jurassic A-type intrusions widely scattered in NE China, the Zhirui rhyolite represent typical post-orogenic magmatism developed under an intra-continental extensional tectonic regimete. They were the product of a regional extensional environent associated with the evolution of the northern Mongol-khotsk suture in the Jurassic, may be accompanied by mantle-derived magma underplating.