The Geochronology,Geochemistry And Tectonic Implications Of Mafic Rocks In Helan Mountains

The Helan Tectonic Belt is tectonically located in the triangle area that formed among the Orodos basin,the Alxa Massif and the Qilian Tectonic Belt.Due to its significant geotectonic location,the Helan Tectonic Belt was severely affected by these surrounding tectonic units via the process of a series of tectonic evolution,and thus is deemed to record abundant information of the evolutionary history.Although its magmatism-metamorphism of the basements,paleontological composition,stratigraphic boundaries and tectonic deformation have been well-documented,there are still controversies about its evolution processes during Phanerozoic time,especially from the Late Paleozoic to Early and Mesozoic interval.Such disputes are considered to limit the understanding the relationship between Alxa Terrence and North China Block.A sequence of Paleozoic-early Mesozoic mafic rocks outcrop in the Helan tectonic belt are identified with their late Paleozoic to early Mesozoic nature.On the basis of our field geological works,these mafic rocks have two kinds of occurrences,including the hypabyssal intrusive and eruptive.The mafic dikes are mainly occurred in the North and Middle parts of the Helan tectonic belts,while the basalts are erupted in the Rujigou area.The LA-ICPMs zircon U-Pb age of Zongbieli diabase,Baisikou diabase and Rujigou basalt are 304±3 Ma,242±1 Ma,191±3 Ma,respectively.Geochemical analysis shows that the Zongbieli diabase belongs to the high-Fe tholeiitic basalt series.The SiO₂ content ranged from 51.55% to 52.28%,the MgO content is 5.34% to 5.71%,the Al₂O₃ content is 14.21% to 14.52%,and the TiO₂ content varied from 1.50% to 1.72%.The samples are depleted of HFSE,with obvious negative anomalies of Nb and Ta.They show significant LREE enrichment with?La/Yb?N=6.39 9.39.We proposed that the diabase is derived from an enriched lithospheric mantle modified by subduction fluids.The geochemical characteristics indicate that Baishikou diabase is high-Fe tholeiitic basalt.SiO₂ range of 45.61% to 47.43%,MgO range of 5.97% to 6.76%,TiO₂ range from 1.94% to 2.32%.The samples have a significant difference between LREE and HREE,and relatively LREE enriched in right-dipping patterns,?La/Yb?N=4.70 6.10.The ratio of incompatible elements indicates that Baishikou diabase is the product of partial melting of 10% in the lithospheric mantle source area of garnet+spinel lherzolite.SiO₂ contents of the Rujigou basalts ranged from 46.24% to 48.15%,MgO is from 2.70% to 5.71%,and TiO₂ varied from 1.49% to 1.66%.Cs,Ba,Sr are relatively enriched,and it lack the negative anomalies of Nb and Ta.The difference between LREE and HREE is weak,showing slightly LREE enriched in normalized pattern,?La/Yb?N=3.93 4.40.It is speculated that the source region is the asthenospheric mantle,which is a partially melted product in the region where the transition from the spinel to the garnet takes place about 20%.It is concluded that the Baishikou diabases and Rujigou basalts are both formed within the intra-continental rift setting and represent a significant extension event during the Middle Triassic to Early Jurassic interval in the Helan Tectonic Belt.According to our studies,the enriched lithospheric mantle beneath the North Helan tectonic belt was more likely influence by the subduction and closure of the Palaeo-Asian Ocean to the north.Therefore,we proposed that the amalgamation of the axle and North China block are not occurred in the Middle-Late Triassic.