Discovery of the 1.8 Ga Granite on the Western Margin of the Songnen Masiff, China

The Songnen masiff is located to the east of the Central Asian Orogenic Belt. The existence and rather the scale and nature of the Precambrian crystalline basement is controversial. Based on recent study of zircon U-Pb chronology, besides a minor number of Neoproterozoic strata, most of the so-called Precambrian metamorphic rock series, such as the Dongfengshan Group and the Zhangguangcailing Group, were formed in the Mesozoic and Paleozoic.

New Discovery of the Quaternary Glaciation Remains in Tala Mountain of Ar Horqin Banner, Inner Mongolia

Objective Northeast China is located in a cold region with high latitude. In the mid-low mountain areas, the existence of Quaternary glacier as well as its range and nature are always controversial. The glacial paleogeomorphology of Northeast China had been extensively studied and explored. The results revealed that Quaternary glaciers was developed in the northern part of Great Hinggan Ranges and Changbai Mountains. However, some scholars believed that the development of the Quaternary glaciers in China resulted from tectonic coupling and close relation with the elevation of mountains. In Northeast China, glaciation did not occur in any mid-low mountain areas during the Quaternary except the Changbai Mountains where the elevation is more than 2600 m. The question is whether glaciation occurred in the mid-low mountain areas of Northeast China during the Quaternary？ In order to clarify this question, this paper reports the Quaternary glacial remains of Tala Mountain, which were newly discovered in Ar Horqin Banner, lnner Mongolia and their age dating.

Geochemistry and tectonic significance of volcanic rocks from Qushenla Formation in Gerze area of northern Tibetan Plateau

By using petrological,isotope chronological,and geochemical methods,the authors studied the volcanic rocks in the studied area,mainly including dacites and trachytes. The results show that they formed during the late Early Cretaceous. Geochemically,the volcanic rocks are relatively enriched in large-ion lithophile elements( Rb,K,and Th) and depleted in high field strength elements( Nb,Ta,and Ti),and rich in light rare earth elements,and depleted in heavy rare earth elements. The fact indicates that the main body of the volcanic rocks in the Qushenla Formation was derived from the partial melting of lower crust. The lithological assemblages are characterized by continental high-K calc-alkaline and shoshonitic series,suggesting that the southward-subducting oceanic slab in southern Bangong Lake had break off and that the Bangong Lake-Nujiang Ocean had closed before 107 Ma. The main dynamic mechanisms for the genesis of this set of intermediate-acidic volcanic rocks were upwelling of the asthenosphere and partial melting of the lower crust caused by slab break-off.

Geochemical Characteristics and Geological Significance of Meta-Volcanic Rocks of the Bainaimiao Group, Sonid Right Banner, Inner Mongolia, China

The Bainaimiao Group, which crops out in the Sonid Right Banner area of Inner Mongolia, China, comprises mainly metamorphosed volcano-sedimentary rocks. This group can be divided into two formations: a lower formation characterized by intermediate-felsic volcanic rocks, and an upper formation of intermediate-mafic volcanic rocks. Zircon dating indicates that biotite leptynite from the lower formation and chlorite-sericite schist from the upper formation crystallized at 499±2 and 478±2 Ma, respectively, corresponding to different volcanic events. Meta-volcanic rocks of the Bainaimiao Group belong to the calcalkaline series, and the SiO2 concentrations suggest that their protoliths were mainly basalts and rhyolites. Greenschist rocks of the group are enriched in light rare earth elements(LREEs) relative to heavy rare earth elements(HREEs), with(La/Yb)N ratios of 3.08–10.9. In addition, they are enriched in large-ion lithophile elements, including Rb, Ba and K, and depleted in the high-field-strength elements Nb, Ta and Ti. The felsic meta-volcanic rocks also exhibit relative enrichments in LREEs compared with HREEs, with high(La/Yb)N ratios of 2.92–9.89, and are enriched in Rb, Ba, Zr and Hf, and depleted in Sr, Nb and Ta. These geochemical characteristics indicate that the meta-basic volcanic rocks were originated from partial melting of sub-arc mantle wedge that had been previously metasomatized by subducted-slab-derived fluids, whereas the meta-felsic volcanic rocks were generated by partial melting of continental crust. These results suggest that the meta-volcanic rocks of the Bainaimiao Group are the products of oceanic plate subduction magmatism along an active continental margin, which can be attributed to Early Paleozoic subduction of the PaleoAsian Ocean plate beneath the northern margin of the North China Craton.