Geochronology And Petrogenesis Of Late Carboniferous Basic Rocks In Sonid Zuoqi,Inner Mongolia

The research area,Sonid Zuoqi,central Inner Mongolia,is located in the southwestern part of the Xing'an-Airgin Sum Block.Previous researchers lacked systematic studies on the geochronology and geochemistry of the late Carboniferous magmatic rocks in the northern Sonid Zuoqi,which led to the little understanding of the origin and tectonic setting of the late Carboniferous rocks.In addition,previous petrologists did substantive research in high-aluminum basalt(HAB),however,the origin of HAB is controversial.More importantly the effect of the magma source has been ignored.In this paper,a late Carboniferous high-aluminum gabbro has been newly identified in Xing'an Mongolian Orogenic Belt,which will provide new important constraints for the origin of high-aluminum basic rocks.Therefore,systematic field geological survey,mineralogy,LA-ICP-MS zircon U-Pb dating,whole-rock major and trace elements,and zircon Hf isotopes were carried out on the amphibole gabbro at the north of Sonid Zuoqi County.We aim to determine its petrological characteristics and geochronology,clarify the nature of magma source and evolution process.These,combined with the published regional studies of igneous,sedimentation,tectonic,metamorphism,etc.,can reveal the the Late Carboniferous tectonic setting of the study area and adjacent areas.The main results obtained are as follows:The mineral assemblage of amphibole gabbro includes dominant hornblende,anorthite,and minor epidote.Large plagioclase and hornblende particles may crystallize simultaneously,and then small euhedral plagioclases crystallized,and a small amount of anhedral epidote crystallized among the hornblende and anorthite in the late stage.LA-ICP-MS zircon U-Pb dating suggests that the crystallization age of the hornblende gabbro is 314±4 Ma,the middle late Carboniferous.The late Carboniferous amphibole gabbros are characterized by low Si O₂(43.17-46.67wt.%),Mg O(4.82-6.59 wt.%),Ti O₂(0.17-0.44 wt.%),K₂O(0.16-0.41 wt.%),high Ca O(15.08-16.31 wt.%)and Al₂O₃(20.97-26.24 wt.%),which belong to the tholeiitic high-aluminum gabbro.Additionally,they show slight enrichment in rare earth elements(LREEs)and large ion lithophile elements(LILEs,such as Rb,Ba,Sr),depletion in heavy rare earth elements(HREEs)and high field strength elements(HFSEs,such as Nb,Ta and Ti).The?Hf(T)values of magmatic zircons range from 4.32 to 7.92,indicating a depleted Hf isotopic source.The late Carboniferous hornblende gabbro in Sonid Zuoqi is mainly derived from the high degree of partial melting of the garnet-bearing lherzolite.At the meantime,the characteristics of low Si,Mg and Ti,Hf/Hf*,Ti/Ti*,high Ca,Al,Sr,Ca/Al,etc.are similar to the melt produced by the melting of carbonated metasomatic peridotite.Besides,the low iron,magnesium,Cr,Co,Ni of high-aluminum gabbro in Sonid Zuoqi,imply that the primary magma could undergo early fractional crystallization of olivine and pyroxene.The weak positive Eu and Sr anomalies indicate the accumulation of plagioclase in the source.Furthermore,the existence of hornblende and epidote implies the hydrous mantle source.Based on the composition of hornblende,the calculated average water content is4.52wt.%,and the crystallization temperatures are between 770 and 730°C.Both the pressure and water content are positively correlated with the temperature,which may be related to the magma ascending,cooling and depressurization.The late crystallization of epidote indicates that the magma is richer in water and calcium in the late stage of magma evolution.On the other hand,the presence of captured zircons suggests that the magma has experienced a certain degree of crustal contamination during its ascent.The late carboniferous tholeiitic high-aluminum hornblende gabbro in Sonid Zuoqi was formed by the above geological processes,which may be the product of extensional environment.