Geochronology and origin of the Laoniushan complex in the southern margin of North China Block and their implications: New evidences from zircon dating, Hf isotopes and geochemistry
华北地块南缘老牛山杂岩体时代、成因及地质意义——锆石年龄、Hf同位素和地球化学新证据

The Laoniushan granitoid complex is located in the southern margin of North China Block.Field work and LA-ICP-MS zircon dating suggest that this complex consists of Late Triassic(Indosinian)and Late Jurassic(Yanshanian)granitoids.The rock types of the Indosinian granitoids are adamellite,quartz diorite and coarse-grained biotite monzogranite with ages of 223±1 Ma,223±1 Ma and 21 4±1 Ma,respectively.The Yanshanian granitoids are mid-coarse-grained,mid-grained biotite monzogranite and fine- grained biotite monzogranite with ages of 1 52±1 Ma and 1 46Ma,respectively.The Indosinian adamellite and quartz diorite are low in SiO2 but rich in Al2 O3 and Na2 O+K2 O,showing Shoshonite series and metaluminous I-type granite;coarse-grained biotite monzogranite are rich in SiO2,Al 2 O3 and Na2 O+K2 O,but poor in MgO with the features of high-K calc-alkalic series and metaluminous to peraluminous I-type granite.The Yanshanian biotite monzogranite with high SiO2,Al 2 O3 and Na2 O+K2 O also show the nature of high-K calc-alkalic series and metaluminous I-type granite.From Late Triassic to Late Jurassic SiO2 content of these granitoids increase,but MgO,CaO and Na 2 O decrease.The total REE contents of the granitoids are very high,especially LRRE, suggesting obvious LRRE and HRRE fractionation,and weakly negative Eu anomalies.The granitoids of Indosinian and Yanshanian are all rich in large ion lithophile elements(K,Rb,Ba,Sr),and relative depleted in high-field strength elements(Nb,Ta,P).The Indosinian granitoids withεNd(t)=-1 1.3～-1 4.87,t DM =1.7～1.9Ga,ε Hf (t)=-9.57～-25.11,t DM2 =1 863～2841 Ma,and the YanshanianεNd(t)=-1 3.32～-1 6.83,t DM =1.7～1.9Ga,ε Hf (t)=-18.28～-24.79,t DM2 =2360～2767 Ma,indicate that their sources are dominated by old crust component related to the Archaean Taihua Group and the sources of the Indosinian granitoids is with more juvenile component contributed.