Geological significance of the former Xiong'er Volcanic Belt on the southwestern margin of the North China Craton

The rock association of low-grade metasedimentary rocks and greenschists located within the Meso- Cenozoic Liupanshan Fault system on the southwestern margin of the North China Craton (NCC)is regarded as part of the Paleoproterozoic Xiong'er Group.These low- grade rocks are separated by normal faults,with the greenschist located in the hanging wails.Zircon LA-ICP- MS U-Pb ages of the greenschists range from 2455 to 423 Ma,suggesting that they are not Paleoproterozoic in age. The protolith ages (206-194 Ma)of the greenschists were determined by LA-ICP-MS U-Pb dating of zircons from two siltstone inteflayers.The petrology and geochemistry of the greenschists reveal that their protolith was continental tholeiitic basalt that formed in an extensional environment such as a continental rift.Thus,it is proposed that the protolith of the greenschists was a mafic volcanic rock of Late Triassic-Early Jurassic age and was metamorphosed during the Jurassic due to tectonism within the Liupanshan tectonic belt.These results show that the greenschists should be reclassified and removed from the Xiong'er Group,and explains why they differ so much from those of typical Xiong'er Group successions in other areas.The formation of the mafic volcanic rocks under conditions of continental rifting differs from that of coeval granitic rocks in the western Qinling Orogen,where the extension occurred during a post-collisional stage in the Late Triassic,which further suggests that the southwestern margin of the NCC became an extensional setting after the Late Triassic.

The evidence of Sm-Nd isochron age for the early Paleozoic ophiolite in Mangya area,Altun Mountains

The REE patterns of the basic volcanic rocks in Mangya area, Altun, are slight rich in LREE with (La/Ya)-N=1.69-3.20, (La/Sm)-N=1.37-1.87, other trace element ratios of the rocks are Th/Ta≈1 (for a few samples greater than 1.5), Nb/Y=0.34-0.62, Ti/Y=310-443 (on the average: 381), Ti/V=37-62, Zr/Nb=9.4-12.4, Sr/Rb=12-80 (on the average: 37), and Nb/Th=7.7-16.8. These features are similar to that of E-MORE or OIB. The ε Nd(t) value, being 3.95- 4.12, shows that the source of the volcanic rocks is derived from depleted asthenosphere mantle mixed with materials from enriched mantle. These, together with the information of geological setting and rock assemblages, indicate that the basic volcanic rocks are of ophiolite. The Sm-Nd isotope ages for the eight basic volcanic rock samples construct a straight line with good correlation, and the calculated isochron age is (481.3±53) Ma. Besides, the eight calculated ε Nd(t) and model ages are close to each other, which suggests that they are homologous, so the isochron is not a mixed line. In the meantime, the isochron age ((481.3±53) Ma) is lower than the model ages (T DM=1 004-1 534 Ma) of the samples, suggeting that the isochron age represents the formation age of the basic volcanic rocks and the ophiolite belt in Mangya area, Altun is formed in the early Paleozoic (Cambrian-Ordovician). In spite of the greater uncertainty of the age, it is still reliable because it is consistent with the age constrained by the regional strata.