Geochemistry Characteristic Of Carbonates Influenced By Magmatism And The Mg Isotope Fractionation

The study area is located in Bachu district of the northwestern Tarim basin,where the Ordovician carbonate rocks were widely effected by the magmatic activities and marbleization process.The characteristics of marble and micritic limestone in petroloty and minerology,homogenization temperature of fluid inclusions,major and trace elements and C,O,Mg isotope show regular changes with the distance of limestone to intrusion varied.The closer the carbonate rocks to the intrusion,the higher degree of marbleization they would experienced and the coarser mineral grains would be produced.The marble and micritic limestone have relatively consistent geochemical characteristics:LREE enrichment and HREE depletion suggested by the right deviation of REE distribution pattern diagram,prepondence of Co,Ni,Ba,W and lighter carbon and oxygen isotope compositions,the high homogenization temperature of fluid inclusions in marble and mudstone but gradually decrease with the distance to the intrusion increased,indicating that the marble and mudstone are influenced by hydrothermal fluid.The diabase located in the middle part of the intrusion body shows the same magnesium isotope composition(?²⁶Mg=-0.28‰)with the mantle rocks,while the diabase near the contact zone has more negtive magnesium isotope composition(?²⁶Mg=-0.53-0.35‰),indicating that the interaction between thermal fluid and carbonate rocks promotes the magnesium isotope fractionation.Magmatic activity could affect the magnesium isotope fractionation in carbonate rocks.Where near the intrusive body distance <1.5-2m the fractionation of the magnesium in marble is driven by temperature difference,and the high temperature area will rich in light magnesium isotope composition,whereas the low temperature area will rich in heavy magnesium isotope.Beyond the influence of the temperature?>1.5-2m?,the interaction between carbonate rocks and the thermal fluid derived from the magmatic activity will result in the positive magnesium isotope values of the precipitated calcite.The stratas beneth the intrusion have a relatively positive ?²⁶Mg?-2.43‰^-2.89‰?,while The stratas lie upon the intrusion have a negative ?²⁶Mg?-3.23‰^-3.81‰?.Dolomization developed well in the l stratas beneth the intrusion,and ?²⁶Mg have a positive correlation with dolomitization,indicating that mineral transformation between calcite and dolomite in dolomitization may promote the magnesium isotope fractionation in carbonate rocks.The influence and migration of fluids are closely related to faults,thus indirectly influenced the magnesium isotope composition of carbonate rocks.