Geochronology, Petrogenesis And Tectonic Setting Of Mesozoic Volcanic Rocks, Southern Manzhouli Area, Inner Mongolia

Mesozoic volcanic rocks are widely distributed in the Greater Higgnan Mountains, NEChina, and in adjacent areas such as eastern Mongolia and the Transbaikal region of Russia.The Greater Higgnan Mountains form the largest Mesozoic volcanic belt in China, from theXar Moron River to the Heilongjiang or Amur River, separating the Mongolian Plateau fromthe Songliao Basin. These mountains have been the focus of a significant amount of researchin recent decades, primarily due to their importance in determining the geodynamic andtectonic evolution of Northeast Asia. Southern Manzhouli is located in the western part of theGreater Higgnan Mountains, close to the Mongol–Okhotsk Zone, and it is an ideal place tostudy the tectonic evolution of Mongol–Okhotsk Zone during the Mesozoic, because it hasexisted for almost the entire evolution of the zone, which has included periods of oceanicsubduction, continental collision, and eventually orogenic collapse. Mesozoic volcanic rocksare well developed in southern Manzhouli area which preserve a complete record of theprocess of volcanism in the Greater Higgnan Mountains. Geochronology and geochemistry ofthese volcanic rocks provide a unique opportunity to determine the evolution of theMongol–Okhotsk Ocean. In this paper, I have undertaken major element, trace element andSr–Nd isotope analyses of volcanic rocks in southern Manzhouli area, as well as U–Pb datingand Hf isotopic analysis of their zircons, in order to determine the petrogenesis and tectonicsetting of the volcanics. Based on the interpretation of existing data and our newgeochronological and geochemical data obtained from volcanics of the southern Manzhouliarea, the following major conclusions can be drawn from the study. 1. Mesozoic volcanism in the southern Manzhouli area occurred in two stages: The firststage is represented by the Tamulangou, Jixiangfeng and Qiyimuchang formations. TheJixiangfeng Formation (162–156Ma) is a rhyolite–trachyte dominated unit that occursbetween two basalt units, namely the underlying Tamulangou (164–160Ma) and overlyingQiyimuchang (151–150Ma) formations. The second stage of magmatism is dominated byrhyolitic lavas and tuffs of the Shangkuli Formation and basaltic rocks of the YiliekedeFormation, and they yield zircon U–Pb ages of142–125and139–123Ma, respectively.2. Basaltic rocks of the Tamulangou and Yiliekede formations have a wide range ofMgO contents (1.64–9.63wt%), but are consistently depleted in Nb and Ta and enriched inincompatible trace elements such as large ion lithophile elements (LILEs) and light rare earthelements (LREEs). These basaltic samples have εHf(t) values of0.7–2.3and εNd(t) values of-0.3–1.3, which are similar to those of enstatite chondrites. Basaltic volcanic rocks insouthern Manzhouli were generated by fractional crystallisation of a common parentalmagma which was derived by partial melting of metasomatised (enriched) lithosphericmantle.3. Trachytes and rhyolites of the Jixiangfeng and Shangkuli formations are characterisedby enrichment in LILEs and LREEs relative to HFSEs and HREEs, and with negative Nb, Ta,P and Ti anomalies. They have much higher εNd(t) and εHf(t) values (0.3–6.0and3.5–10.0,respectively) than the basaltic rocks in the same area, and were probably derived by meltingof lower crustal mafic and felsic granulites, respectively.4. Geochronological data indicate that Mesozoic volcanism in southern Manzhouli wasinitiated in the Middle to Late Jurassic and continued into the Early Cretaceous. Volcanism inthis period developed on both sides of the Mongol–Okhotsk Suture Zone and was mainlyinduced by lithospheric extension after closure of the Mongol–Okhotsk Ocean.