Uplift-and Exhumation Of The Hongol Molybdenum Ore Field,Northern Erenhot,Inner Mongolia

The Hongol Molybdenum ore field,located in the west of the Erenhot-East Ujimqin metallogenic belt,contains porphyry ore bodies that are important mineral exploration targets such as Wulandele Mo deposit and Zhunsujihua Cu polymetallic deposit.The petrochemistry and zircon U-Pb geochronology age of plutonic rocks were used to further our understanding of the magmatic and tectonic history of the Central Asian Orogenic Belt(CAOB),which were integrated with low-temperature thermochronology and geothermobarometer to constrain the uplift and exhumation of the studied area quantitatively.New zircon U-Pb data are presented for granites:Carboniferous granites(340.0±1.4Ma,317.6±1.4Ma,301.3±1.0Ma)and Late Mesozoic granites(146.7 ±0.89 Ma)that agree with previous work which identified the magma emplacing during a great time span from Late Paleozoic to Late Mesozoic,representing emplacement timing of magma.Whole-rock geochemical analyses show that granites of different ages were of homogeneity in geochemical character,displaying an A-type magmatic affinity with typical enrichment in large ion lithophile elements(LILE)and light rare earth element(LREE),depletion of high field strength elements(HFSE),with fractionated REE patterns showing strong negativeEu anomalies.Given by the rock assemblages and geochemistry,the Carboniferous granites were A2-type granites derived from partial melting of juvenile continental crust resulted from the upwelling of asthenosphere material in the post-collision environment,while the Late Mesozoic granites were A1-type granites developed under the intraplate extensional system,indicating that the closure time of Paleo-Asian Ocean laied before Carboniferous in this area.Geothermobarometers show the emplacement depths of the Early Carboniferous,Late Carboniferous and Late Mesozoic granites were 1.34±1.64 km,7.59±3.15 km and 6.18±0.73 km,respectively.Apparent cooling rates were therefore calculated using zircon U-Pb ages,ZFT ages,AFT ages,and the emplacement depths indicating that the vertical motion of the crust of the studied area was heterogeneous temporally and spatially.By means of thermal history modeling,three following apparent cooling stages from Late Carboniferous to Cenozoic have been recognized: stage one was a relatively slow cooling period at 300-160 Ma with cooling rate from 0.26 to 1.15 ?/Myr;stage two was a rapid cooling period occurred between 160 Ma and 110 Ma with cooling rate from 2.90 to 3.69 ?/Myr with the greatest amount of cooling;the latest stage was a slow erosion period since 110 Ma with cooling rate from 0.21 to 0.23 ?/Myr which bring the studied samples to the Earth's surface.The estimated minimum amount of denudation occurred in the central part is about 2.95 km,and a modest amount,7.54 km of denudation in the western part,while the maximum amount of denudation is upmost to 10.40 km occurred in the eastern with a significant portion of the eroded succession being Late Cretaceous in age.The variation of denudation suggests that the uplift and exhumation of the study area was localized and the resource prospecting should pay close attention to the central region regarding of the shallow level emplacement of Hercynian porphyry deposits.