The age of conglomerates in the lower part of the XiufengFormation in the northern Da Hinggan Mountains,NE China, and their tectonic implications
LI Jinyi1, HE Zhengjun1, MO Shenguo1, Andrey A. Sorokin2(1 Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China;2 Division of Regional Geology and Hydrogeology, Far-East Branch of Russian Academyof Sciences, Blagoveshchensk 675000, Russia)
Field investigations into conglomerate in the lower part of the Xiufeng Formation on the southern margin of the Mohe basin have revealed that clastic materials in conglomerate also include volcanic pebbles in addition to granitic pebbles reported previously from the supposed Neoproterozoic granite batholith, and that the conglomerate and the Middle Jurassic clastic sedimentary strata are in fault contact rather than are continuously deposited. Granite as the principal source rocks of the conglomerate has zircons evaporation 207Pb/206Pb ages of 354-164 Ma, SHRIMP ages of 180-190 Ma, whole rock-feldspar-biotite internal Rb-Sr isochron ages of 177-191Ma and biotite 40Ar/39Ar plateau ages of 171-175 Ma, indicating that the batholith was intruded in the Early Jurassic and had not been exposed on the surface in the Middle Jurassic. The volcanic pebbles display similarities to volcanic rocks in the Late Jurassic Tamulangou Formation in respect to the petrological and petrochemical features. These data, coupled with the regional geological data, imply that the age of the conglomerate is not Middle Jurassic but possibly Early Cretaceous, and that the Mohe basin was a foreland basin related to the Mongolian-Okhotsk orogenic belt, rather than a Jurassic intracontinental down-faulted basin.