Geochronology And Geochemistry Of The Mesozoic Volcanic Rocks In The Great Xing'an Range, Northeastern China

The Great Xing'an Range in NE China is characterized by the widespread Mesozoic volcanic rocks. For the volcanic rocks in the northern segment, large number of geochronological studies has been done, which indicate that most of these volcanic rocks formed in the the Early Cretaceous, with a few of the Late Jurassic time. But for the volcanics in the middle and southern segments, systematic geochronological study is still absent till now, which hampers further studying of the geological and tectonic evolution history of the Great Xing'an Range, as well as the eastern China. On the basis of the previous studies, a systematic geochronological study has been done focusing on the volcanic rocks in the middle and southern Great Xing'an Range. And then the obtained age date was combined with the geochemistry in order to determine the magma sources and the petrogenesis of the volcanic rocks, as well as the temporal evolution of the volcanics. Importantly, here I further discussed the tectonic controlling factors and geodynamic nature of the igneous events, and the relationship between the magmatism and the lithospheric thinning in the eastern China.Based on the field relationship of the volcano strata and the features of rocks assemblages, fossil information in the sedimentary layers, as well as the episodes of the volcanism, the volcanic strata were divided into the Manketouebo, Manitu, Baiyingaolao and Meiletu Formations from bottom to top. The Manketouebo and Baiyingaolao are composed of felsic volcanics and related volcano clastic rocks, including dacite, rhyolite, trachyte and taffaceous rocks, while the Manitu and Meiletu formations comprise a suite of mafic-intermediate rocks, including basalt, andesite and trachy basealt, basaltic trachy andesite, trachy andesite, and related tuffaceous rocks. These volcanics were previously thought to be formed mainly in the Late Jurassic, but lacking of high precise age evidence. Till now, there are only few precision data reported in restricted area, nevertheless, these data can not constrain the beginning time and duration of the voluminous volcanics in the region.Systematic dating on the volcanics in the southern segment was performed by zircon U-Pb and whole rock ⁴⁰Ar/³⁹Ar methods. 17 analyzed samples from the Manketouebo Formation indicates that this formation contains two episodes of magmatism, i.e., the Late Jurassic (160-150Ma) and Early Cretaceous (136-122Ma), with the peak ages at 150Ma and 135Ma, respectively. But there is one sample have much older age of 173Ma. The obtained U-Pb ages of two samples from the location at where the Manketouebo Formation was defined are 152 and 158Ma. Dating on 10 samples from the Manitu Formation indicates that the Manitu Formation has the similer age frame to that of the Manketouebo Formation and also can be divided into two epidodes: 158Ma-150Ma and 137Ma-125Ma, with peak ages of 160Ma-155Ma and 125Ma, respectively. The only differenc is that the volcanis of the Early Cretaceous are dominant in the Manitu Formation, while the majority of the Manketouebo Formation is the Late Jurassic. In the contrast, the Baiyingaolao and Meiletu formations were formed in the Early Cretaceous with much shorter duration, during the period between 141Ma and 124Ma. LJ-Pb ages of the samples from the locations at where thses two formations were firstly founded are 139Ma and 131 Ma, respectively. All these data, combined with the previously published data, indicates that the volcanics in the middle-southern Great Xing'an Range mainly formed during the Late Jurassic-Early Cretaceous time, between 160Ma and 122Ma, with two episodes of 160Ma-150Ma and 141Ma-122Ma. Between them, there is a magmatic gap about 10Ma. Although the volcanic rocks in the both segments formed during the period of Late Jurassic-Early Cretaceous with identical peak time, the ending time and the episodes of the volcanism are different, reflecting the different magmatism sequence, which was probably caused by the different deep geodynamic procedures. These age data also suggest that the previous scheme of subdivision and regional correlation of the volcano strata need to be re-evaluated, which is also manifested by the age data obtained from the successive sections in this study. The former difined Manketouebo and Manitu formations actually contain volcanics of different time; while these four formations have obvious overlapping in age, which means that the previously defined stratigraphic sequence is not realistic.Age compilation of the igneous rocks in the Great Xing'an Range and adjacent area implies that there exist two episodes of magmatism in the Jurassic and Early Cretaceous time: the former mainly comprise intusive rocks, while the latter eruptive. Between them, the eruptive rocks have an age-decreasing trend from the west to the east; while the intrusive rocks have the oppsite trend, i.e., becoming young westward. Between them, there is a magmaic gap, which becomes shorter from the continental margin to the intracontinent, from about 60Ma in the Japanese island to less than lOMa in the Great Xing'an Range. Another important feature is that the initial time of the Early Cretaceous magmatism is similar in different region, at about 135Ma, not older than 140Ma.In the northern part of the Great Xing'an Range, the mafic-intermediate volcanics are mainly alkaline series, with minor of sub-alkaline; on the contray, those in the middle-southern segment are sub-alkaline series. In the northern part, the mafic-intermediate rocks are characterized by enrichment in LREE, LILE and depletion in HSFE, and can be divided into two types of high Ti and low Ti types. Compared to the low Ti rock, the high Ti rocks are relatively enriched in LREE and Ba-Sr, with high abundances of P and Ti; while the low Ti rocks have more enrichment in Th and depletion in P and Ti. In the middl-southern segment, the mafic-intermediate rocks have relatively low but variable degrees of LREE enrichment. They are enriched in the LILE and depleted in HFSE and 1Mb, Ta. Based on the features of trace elements, these rocks can be divided into the high K and low K types. The former is characterized by the relatively high LREE, Rb, Ba, Th, and Sr abundances and obvious enrichment of K; while the latter has low abundances of LREE, clear positive Ba and negative K, Zr and Hf anomalies. Geochemical and isotopic features of the mafic-intermediate volcanics in the Great Xing'an Range imply the heterogeneity and complexity of the magma sources, that is, the mantle sources include both enriched and depleted. The magma source of the bolcanic rocks in the northern segment is mainly enriched, while in the middle-southern segment includes both enriched and depleted mantle sources. The Nd and Hf isotopic ages show that the formation of the the enriched mantle was closely related to the subduction of the oceanic crust of the Paleo-Asian ocean.The felsic rocks include two distinct groups with different REE and trace elements patterns. Rocks of the first group have relatively low HREE abundances and high Ba-Sr abundances with weak Eu anomalies, while rocks of the second group have relatively high HREE abundances and obvious negative anomalies of Eu, Ba and Sr. They are nemed as high Ba-Sr rocks and low Ba-Sr rocks, respectively. The high Ba-Sr rocks mainly distribute in the northern segment, while the low Ba-Sr rocks in the whole range. Temporally, the felsic rocks of low Ba-Sr abundances mainly generated in late stage of the volcanism. The low Ba-Sr rocks in the northern segment have obviously high generation temperatures, indicating the high isothermal surface when they formed; while rocks of the same kind in the middle-southern segment have low temperatures. The different temperatures of the rocks probably reflect the different deep processes. Some of the felsic rocks formed in the Late Jurassic time have the geochemical features of adakitic rocks, indicating the existence of thichened crust at that time.For the tectonic environment of the Early Cretaceous, it is consensus that they formed in an extensional environment. The evidences supporting this conclusion include the widespread A-type granite and other kind of alkaline rocks, metamorphic core complexes and mafic-felsic dyke swarms of the same time. Whist, the rock assemblages, features of regional tectonics as well as the well developed accretionary complex along the continental margin indicate that the compressive environment during the Late Jurassic time. That is, during the period of Late Jurassic-Early Cretaceous, the tectonic environment of Great Xing'an Range and adjacent region transformed from compressive to extensional. And the transformation corresponds to the magmatic gap.Inspite of the Late Jurassic volcanic rocks in the Great Xing'an Range, the peak time of the magmatism is the Early Cretaceous time, which is identical to that of the Early Cretaceous giant igneous events. This means the huge voluminous volcanic rocks in the Great Xing'an Range is related to the lithospheric thinning event. The temporal-spatial distribution of the Mesozoic igneous rocks in the eastern margin of the continent indicates that they were controlled by the subduction of the Paleo-Pacific plate. During the Jurassic time, the flat subduction of the oceanic plate beneath the continental plate induced the igneous assemblages of active continental margin affinity and hickened the continental crust. Along the subduction, the igneous rocks become youngling westward from the continental margin to the intracontinent. In the Early Cretaceous time, the delamination of the thickened crust induced the upwelling of the asthenospheric mantle, and subsequent huge magmatism. The delamination migrated eastward from the intracontinent to the continental margin, and the corresponding migration of the volcanism. At the climax of the delamination, the asthenosphere contacted with the crust directly, and induced the voluminous felsic rocks. Delamination also induced the change of the subduction angle of the oceanic slabs, resulting in the present high angle subduction revealed by the geophysic data. Although controlled by the same tectonic background, the northern and middle-southern parts of the Great Xing'an Range had different deep processes. In the northern part, the lithospheric thinning was remarkable; while in the middle-southern part underplating of was dominant. That is, the lithospheric thinning is heterogeneous spatially.