Late Mesozoic Volcanism And Sedimentary Provenance Study Of Typical Extensional Basins In Northeast China

Following the amalgamation of the late Mesoproterozoic-Carboniferous Central Asian Orogenic Belt with the Siberian and North China cratons, NE Asia underwent late Mesozoic lithospheric extension and in resulted in the development of a large extensional province and widespread formation of volcano-sedimentary basins. During this period, tectonic evolution of the NE China area has been influenced by processes after the closure of Paleo-Asian Ocean and convergence of the Mongol-Okhotsk and Paleo-Pacific oceans. The typical basin-and-range setting together with the widespread volcanism and geodynamic background have always been a topic of great interest and arguments to geologists. This dissertation focuses on petrology and geochemistry of late Mesozoic volcanic rocks from Songliao and Hailar basins, which situate on either side of the Great Xing’an range. Based on previous researches on volcanism of study areas and basin evolution mechanisms, we compare chemical characteristics and petrogenesis of the volcanic rocks from different basins. Combine with the results of sedimentary provenance research and investigation of spatial and temporal features of contemporary volcanism in NE Asia, our results provide new evidence for the late Mesozoic lithospheric extension, and constrains on tectonic evolution.We report zircon U-Pb ages and Hf isotopic compositions as well as geochemical data for mafic and felsic volcanic rocks from the Hailar basin, located about 1000 km north of Beijing. Zircon populations of six felsic rock samples analyzed by laser ablation ICP-MS yielded similar U-Pb age spectra ranging from 158 to 125 Ma. The youngest zircon ages are interpreted as time of magma eruption and the xenocrystic zircon-age spectra as evidence for a protracted melting of lower crust due to the underplating of mantle-derived magmas during lithospheric extension. The volcanic assemblage has a bimodal composition comprising geochemically evolved trachybasalts and felsic volcanic rocks of I- and subordinate A-type compositions. The mafic volcanic rocks have negative Nb-anomalies, high Th/Nb and Ce/Pb ratios, low initial εNd values of 0 to +3.3, and radiogenic Pb and Sr isotopes all interpreted as evidence for the melting of passively upwelling asthenosphere and lithospheric mantle previously modified by plate subduction. The xenocrystic zircon ages and chemical/isotopic data (εNd from 0.1 to 3.3) of the felsic rocks support an origin from juvenile crustal protoliths:the data of Ⅰ-type felsic rocks are consistent with the melting of underplated mafic protoliths and part of felsic components. The A-type rhyolites support the melting of a juvenile crustal source probably result from intense lithospheric extension at this point.Songliao basin is the biggest extensional sedimentary basin with voluminous volcanic rocks as a major part of the basin fill in NE China. Volcanic successions form a significant but unexposed part of the late Mesozoic magmatic province in NE China. Here we report zircon U-Pb ages and Hf isotopic compositions as well as geochemical data for mafic and felsic volcanic rocks from the Songliao basin. Zircon populations of two types of rhyolites with distinct I-and A-type affinities, analyzed by laser ablation inductively coupled plasma mass spectrometry, yielded similar U-Pb ages of 114±2 and 113±2 Ma, respectively. The mafic volcanic rocks with tholeiitic and calc-alkaline affinities are generally enriched in large ion lithophile elements (LILEs), light rare earth elements (LREEs) and depleted in high field strength elements (HFSEs), and have scattered isotopic compositions, indicating melting from enriched upper mantle source or influenced by crustal contamination. The tholeiitic rocks show relatively depleted Nd isotopic compositions as well as similar Th/Nb and Th/La ratios relative to mid-oceanic ridge basalts (MORBs), implying more contributions from depleted asthenospheric mantle. The calc-alkaline volcanic rocks have relatively evolved d values together with fractionated chemical compositions, which could result from melting of modified mantle protoliths and different degree of crustal contamination. Whole-rock Nd and zircon Hf isotope data of the felsic rocks indicate an origin from newly formed crustal protoliths. The I-type dacites and rhyolites show geochemical signatures with subduction-related affinity, indicating generation by partial melting of calc-alkaline mafic protoliths that originated from melting of subduction-modified mantle sources, whereas the A-type rhyolites have higher heavy rare earth element and high field strength element concentrations and lower Ba/Nb ratios that are interpreted as evidence for melting juvenile crustal sources in an intraplate tectonic environment. Some of the I-type felsic rocks show typical adakitic characteristics with high Sr/Y and La/Yb ratios, also suggesting complicated crustal melting processes. Typical geochemical compositions of A-type rhyolites indicate anorthite-rich plagioclase as a residual magmatic phase and imply melting at shallow crustal levels. The A-type felsic rocks closely connect with tholeiitic basalts in the field, and they show similar geochemical features like relative high Fe2O3 concentrations and Nb/La ratios. We suggest this rock assemblage were generated by the same process that results from asthenospheric upwelling during a period of maximum lithospheric extension below Songliao area around 110 Ma.It is noteworthy that the Mesozoic tectonic evolution and related volcanism of NE Asia resulted from overprinting of multiple tectonic events like closure of Paleo-Asian Ocean and convergence of the Mongol-Okhotsk and Paleo-Pacific oceans and these transforming regimes played the key role in different regions and periods. Similar late Mesozoic volcanic rock assemblages with mantle-derived mafic rocks at the base and mainly I- and A- type felsites on the top were generated in both Hailar and Songliao areas. But the mafic rocks of Hailar and Songliao basins show different geochemical characters with alkaline and subalkaline affinities, respectively. The differences indicate these mafic volcanic rocks were generated from different degree or depth of mantle melting that consistent with distinct initial mechanism of basin development and underlying mantle structure of Hailar and Songliao basins. According to the geodynamic background, we suggest that the initial evolution of Hailar basin that predominated by passive rifts was mainly induced by subduction and closure processes of Mongol-Okhotsk Ocean, while the typical active rift Songliao basin was generally controlled by subduction of Paleo-Pacific plate. The A-type rhyolites of Hailar and Songliao basins, which represent a maximum degree of lithospheric extension under the two regions show different ages from ca.120 Ma to ca.110 Ma, indicating a migration trend of lithosphere thinning and mantle upwelling from west to east in NE China. The migration trend can be also identified from the character of spatial and temporal distribution of Mesozoic volcanic rocks in NE China and Far East area of Russia, which we suggest to be mainly controlled by subduction and roll-back of Paleo-Pacific plate. Based on recent results of research on early Cretaceous volcanic rocks from eastern Mongolia, we speculate that the Mongol-Okhotsk suture could have been reactivated by processes of Paleo-Pacific plate subduction and resulted in the early Cretaceous magmatism along the suture belt.After gradual cessation of rift and volcanic activities, a continuous sedimentary sequence that deposited during Songliao basin transformed from syn-rift stage to post-rift stage is preserved in the upper part of the basin (Denglouku and Quantou formations). The lack of isotopic and geochemical data from these sedimentary rocks has hindered a better understanding of the depositional-tectonic history of the basin so far. These rocks provide clues to depositional processes operating in this region during the late Cretaceous. Dominant zircon populations of the underlying Denglouku Formation are largely Paleozoic (270-250 Ma) and Mesozoic (190-170 Ma) in age, whereas the overlying Quantou Formation contains mostly older zircon populations of ～1800 Ma and ～2500-2900 Ma. The youngest detrital zircons place the deposition ages of both rock formations at about ～117 Ma and ～103 Ma, respectively. The Denglouku Formation is characterized by less negative initial εNd values (-6.3 to-2.8), younger Nd model ages (1.05-1.39 Ga) and lower initial 87Sr/86Sr ratios (0.7079 to 0.7088), compared to the Quantou Formation (-12.8,1.75 Ga and 0.7129), confirming that both formations were derived from different source areas. Our data suggest that Paleozoic and Mesozoic magmatic rocks were widespread in the northern periphery of the basin, and served as potential sources of the Denglouku Formation. In the south-east, old basement rocks of the North China craton provided sedimentary material for the Quantou Formation. The south-eastward migration of erosion center(s) in the Songliao basin in the late Mesozoic probably resulted from an abrupt change in basin uplift and subsidence. These processes could be driven in response to regional tectonic events such as rapid direction and velocity change of subducting Paleo-Pacific plate or collision of the Okhotomorsk Block with East Asia during Cretaceous.