Formation And Exhumation Of Mesozoic Granitoids Responding To Lithospheric Destruction In Western Jiaodong Peninsula,North China Craton?NCC?

Lithospheric destruction of the North China Craton?NCC?has already been widely accepted,but lack of detailed study on crustal exhumation amount and rate which can reflect crustal eovlution history and important tectonothermal event in deep.Three distinctive stages of Mesozoic granitoids are exposed in the western of Jiaodong Peninsula,southeastern margin of the NCC,and their formation and cooling conditions can provide important clues for crustal exhumation history and crustal evolution.Those granitoids are represented by the Linglong monzogranite?LL,159-154 Ma?,Guojialing granodiorite?GJL,132 Ma?and Aishan syneogranite?AS,118 Ma?,respectively.Methods of mineral thermo-barometers,integrating published mineral ⁴⁰Ar-³⁹Ar plateau ages,are applied to set up the magmatic cooling and solidification paths for those granitoids.As a result,the normative Q-Ab-Or-H₂O and Al-in-amphibole barometer yield pressures of 4 kbar,3.8 kbar and 1 kbar,with estimated emplacement depths of 14.8 km,14.1 km and 3.7 km for the LL,GJL and AS,respectively.Temperatures from emplacement to solidification were estimated.Zircon saturation and Ti-in-zircon thermometer yield 756±22?and 701±36?for the LL,756±20?and 693±30?for the GJL,739±24?and 728±27?for the AS,respectively.The equilibrium temperature by amphibole-plagioclase thermometer is of 710±31?for the GJL.Two-feldspar and magnetite-ilmenite thermometers were used to estimate their cooling temperatures,but returned relatively low temperatures in comparison with granite consolidation temperatures.The low temperatures recorded by two-feldspar thermometer in the LL and GJL might be ascribed to equilibrium results between granitic intrusions and the wall rocks.Accordingly,geothermal gradient values are estimated to be of 33-40?/km based on two-feldspar equilibrium temperatures and emplacement depths of the GJL?561?,14.1 km?and LL?471??.Magnetite-ilmenite temperature of 471?for the LL consistent with two-feldspar equilibrium temperature,also reflects equilibrium results between intrusions and the wall rocks.Whereas,magnetite-ilmenite temperature of 566?for the AS represents the equilibrium temperature during cooling period.Consequently,we estimate that the crust of the western Jiaodong Peninsula was exhumed approximately of 10.4 km with a rate of 0.87-1.0 mm/a and cooling rates of47-59?/Ma from 130 Ma to 118 Ma ago.Our data revealed that a large scale and rapid crustal exhumation took place from 130 to 120 Ma ago,but without apparent exhumation at 160-130 Ma ago,highly consistent with peak timing of lithospheric destruction of the NCC and formation of core complex.Integrated with previous thermochronological data,we attempt to set up the cooling and exhumation P-T paths for the Mesozoic granitic intrusions in western Jiaodong Peninsula.The mechanism of the large scale crustal exhumation can be attributed to delamination of lithosphere with asthenosphere upwelling after crustal thickening.