Chronology,Geochemistry and Tectonic Settings of the Hadamiao Granodiorite on the Northern Margin of the North China Platform

The Hadamiao granodiorite,located on the northern margin of the North China platform and acting as the country rock of gold deposits in the Hadamiao region,was formed in the same age and similar tectonic settings with the Hadamiao gold deposit and the large-scale Bilihe gold deposit in the same area.By using the LA-ICP-MS method,the U-Pb age obtained is 267±1.3 Ma,which represents the crystallized age of the granodiorite,and that of the xenolithic zircon is 442.8±5 Ma. Base on the main elements,it exhibits the features of calc-alkaline to high-potassium calc-alkaline series,low silicon,and quasi-aluminous I-type granites,and with high magnesium（Mg~#=0.45-0.57） and high sodium contents（Na_2O/K_2O=0.98-2.29）.The SREE values（81.6-110.15 ppm） are relatively low,the fractionations between LREE and HREE are obvious,showing a right-inclined dispersion in the REE distribution diagram.Compared with the primitive mantle,the rock is relatively rich in LREE（La and Ce）,LILE（K,Sr,and Th）,and intensively depleted in HFSE（Ti,P,Nb and Ta）.The ratios of Sr/Y and（La/Yb）_N and the contents of Rb,Nb and Y are relatively low,the Sr values are high （436.35-567.26 ppm）,and the Yb contents of most samples are low（1.25-1.8）,which indicate the features of typical continental margin arc and adakitic rocks.According to the values ofε_（Nd）（t）（-2.4 to ＋0.2） and I_（Sr）（0.7028-0.7083）,and variations of the La/Sm ratios,the Hadamiao granodiorite was formed from mixing of the thickened molten lower crust and the mantle wedge substances.The rock was related to the southward subduction and accretion of the Paleo-Asia Ocean in the Late Paleozoic, being Late Paleozoic magma of the continental margin arc formed on the basement of the Early Paleozoic accretion complexes,and showing a trend of turning into adakitic rocks,which indicates their great metallogenic（Au） potential.

Discovery of the Early Paleozoic Boin Sum-Ordor Sum Island Arc in the Hadamiao Gold Ore District, Inner Mongolia and its Significance to the Evolution of the Paleo-Asian Ocean

Early Paleozoic granodiorite has been identified on the northern margin of the North China craton in the east section of the central-Asian orogenic belt, which was previously known as early Indosinian in age. By using the LA-ICP-MS method, the obtained zircon U-Pb age is 445.6 2.7 Ma, which represents the crystallization age of the granodiorite. The granodiorite near the east of the large-sized Bilihe gold deposit is of the tholeiite series with low potassium. It is quasi-aluminous I-type granite, enriched in sodium （Na2 O/K2O=7.29 9.77） and magnesium （Mg # =0.51 0.67）. The ΣREE value is relatively low, obvious differentiation is shown between LREE and HREE and within LREE, and the Eu anomaly is low and negative （δEu=0.74 0.91）. In the primitive-mantle normalized spider diagrams of trace elements, the granodiorite is relatively rich in LREE and LILE （Ba, Sr, Th）, and strongly depleted in HFSE （Nb, Ta, Ti and P）, which shows features of subduction zone components （SZC）. In the discrimination diagrams of tectonic settings of granite for Rb vs. （Nb＋Y）, Rb vs. （Ya＋Ta）, La/Nb vs. Ba/Nb and Th/Nb vs. Ba/Nb, the granodiorite exhibits typical features of island arc granite. The normalized values of K and Rb are extremely low, while the values of Sr and Eu are very high, which are similar to those of island arc magma that has undergone metasomatism of fluid from the oceanic crust. The granodiorite is relatively depleted in ε Hf （t） （5.1 7.1） and low in ε Hf （t） model ages （1089 921 Ma）. In the ε Hf （t） vs. age （T） diagram, the distribution area of the granodiorite is accordant with the field of the Xing’anling-Mongolia orogenic belt, which indicates that the magmatic sources are mainly the mixture of partial melting of wedged mantle subjected to metasomatism of fluid from the oceanic crust and young substance from the crust. The granodiorite is similar to the felsic arc magma in the Damao Banner, Bate Obon, Boin Sum and Ordor Sum regions, and they altogether constitute an early Paleozoic accretionary island arc magmatic belt on the northern margin of the North China craton. A number of early Paleozoic zircons trapped in late Paleozoic intrusions in the Hadamiao and Bilihe regions and the discovery of the early Paleozoic island arc magmatic belt near the east of the Bilihe gold deposit suggest that the late Paleozoic volcanic-intrusive rocks have a basement of early Paleozoic arc accretionary complexes. This is just the evident of the multiphase subduction and accretion model of the Paleo-Asian Ocean （PAO）. Paleozoic structures and magmas on the northern margin of the North China craton are shown from south to north as the late Paleozoic Andes-type arc magmatic belt in the Inner Mongolia plateau, the Chifeng-Bayan Obo fault and the late and early Paleozoic arc magmatic belt, which shows that after the early Paleozoic arc-continent collisional orogeny and at the stage of the late Paleozoic accretionary orogeny, the PAO plate was likely to continuously pulsate and underthrust beneath the early Paleozoic island arc accretionary complex belt and its front, i.e. the North China craton. During the early Paleozoic collisional orogeny, the PAO plate might not experience large-scale breakup or delamination. The characteristics of the early Paleozoic island arc accretionary complex basement have a significant control on late Paleozoic diagenesis and metallization in the Hadamiao and Bilihe gold concentrated areas.