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Petrogenesis of M esozoic volcanic rocks in the Z ha'ertai area, I nner M ongolia: C onstraints from trace elements and S r– N d– P b isotopes
Author(s) -
Kang Ming,
Siqin Bilige,
Tong Ruiling,
Tang Zijian,
Yue Changcheng,
Gao Chao,
Yuan Ye,
Liu Xiaofeng,
Chen Yu
Publication year - 2018
Publication title -
geological journal
Language(s) - English
Resource type - Journals
SCImago Journal Rank - 0.721
H-Index - 54
eISSN - 1099-1034
pISSN - 0072-1050
DOI - 10.1002/gj.3237
Subject(s) - geology , volcanic rock , rhyolite , geochemistry , petrogenesis , pyroclastic rock , lithophile , lava , volcano , basalt , partial melting
The Mesozoic acid volcanic rocks of the Zha'ertai area, located in Inner Mongolia, were formed in the late Early Cretaceous and resulted in polymetallic deposits of gold, silver, and other precious metals with huge commercial value. The more developed acid volcanic rocks include subvolcanic and pyroclastic rocks and volcanic lava, with SiO 2 of 73.18–83.82%, Na 2 O + K 2 O of 6.17–9.03%, and K 2 O/Na 2 O of 1.28–70.27%. These rocks are enriched in large‐ion lithophile elements and high‐field strength elements. They are also enriched in light rare earth elements (REEs), with light to heavy REE ratios of 2.91–14.30; light to heavy REE fractionation is significant, and there are obvious negative Eu anomalies (δEu = 0.02–0.25). The acid volcanic rocks have extremely low TiO 2 , MgO, P 2 O 5 , and TFe 2 O 3 contents, exhibit significant enrichment in Th, U, and Pb, and are strongly depleted in Ba and Sr, showing the trace elements feature of A‐type rhyolite. The rocks have smaller ( 87 Sr/ 86 Sr) i , lower ε Nd (t), younger T DM2 , lower ( 206 Pb/ 204 Pb) i ,( 207 Pb/ 204 Pb) i , and ( 208 Pb/ 204 Pb) i . The acid volcanic rocks differ considerably from the intermediate to basic volcanic rocks in geochemical composition, and there is a compositional hiatus present in the succession. This indicates that neither endmember of these Mesozoic bimodal volcanic rocks is cogenetic and that rhyolitic rocks might not be a product of basaltic magma crystallization differentiation. Instead, they may result from partial melting of crust‐derived rock, stemming from partial melting of rocks in the pre‐existing crust underplated by mantle‐derived magma. This suggests that the acid volcanic rocks were formed in a lithospheric extensional setting during the Early Cretaceous.