Late Paleozoic-Early Mesozoic Tectonic Evolution In The Eastern Heillongjiang Province, NE China: Constraints From Detrital Zircons And Volcanical Events

This thesis studies the provenance and maximum age of deposition of the Late Paleozoic sedimentary strata and the formation time of the Late Paleozoic-Early Mesozoic volcanic rocks in eastern Heilongjiang Province by using LA-ICP-MS zircon U-Pb chronology. Besides, we discuss the nature of magma sources, and their tectonic setting based on major-and trace-elements, and Sr-Nd isotope as well as zircon Hf isotope for these volcanic rocks. These chronological and geochemical data, together with regional tectonic analysis, provide constraints on the Late Paleozoic-Early Mesozoic tectonic evolution in eastern Heilongjiang Province. Main achievements are as follows:1. Maximum age of deposition of the Late Paleozoic basins in eastern Heilongjiang ProvinceThe dating results of detrital zircons from quartz sandstones of the Early Devoniar Heilonggong (D1hl) (in eastern Harbin), the Early Devonian Baoquan (D1b) (in the Lesse: Xing'an Range), and the Early-middle Devonian Heitai formations (D1-2ht) (in eastern margir of the Jiamusi Massif) as well as the formation times of the volcanic rocks within the overlying stratas, reveal that:(1) rocks from the D1hl Formation is deposited after 403 Ma; (2) the Dih and Dib formations are deposited between 413～386 Ma, and 484～390 Ma, respectively.2. Remnants of ancient crystalline basementDetrital zircon dating results from the Dihl and D1b formations reveal that they wer sourced from regions with rocks as old as～2500,～1833, and 903～800 Ma in addition to th Phanerozoic material. Taking into account the paleomagnetic data, the timing of the final closure of the Paleo-Asian ocean and the researches on the Phanerozoic granites within the CAOB, it appears that these Archean/Proterozoic zircons were derived from a local source close to these Late Paleozoic basins, i.e., several ancient Precambrian crustal remnants exist within the Xing'an, Erguna, and Songnen-Zhangguangcai Range massifs in the Paleozoic.3. Timing of the amalgamation of the Songnen-Zhangguangcai Range and Jiamisi massifsThe dating results show that the D1hl and D1b formations of the Songnen-Zhangguangcai Range Massif contain numerous detrital zircons with ages of 489～551 Ma, similar to the age features of the Mashan Complex, Early Paleozoic gneissic granitoids, and the detrital zircons of the D1-2ht Formation in the Jiamusi Massif. This finding suggest that the collision between the Songnen-Zhangguangcai Range and Jiamisi massifs most possibly took place prior to the deposition of the D1hl and D1b formations, i.e., before 403～413 Ma. Combination with the existence of the north-south-trending Caledonian I-type granitoid belt (445 Ma), we propose that the amalgamation of the Jiamusi and Songnen-Zhangguangcai Range massifs probably occurred between the Late Ordovician and early stage of the Early Devonian (413～445 Ma).4. Spatial and Temporal Features of the Late Paleozoic-Early Mesozoic Volcanisms in eastern Heilongjiang ProvinceWe undertook LA-ICP-MS zircon U-Pb dating for the Late Paleozoic-Early Mesozoic volcanic rocks in eastern Heilongjiang Province. Zircons from these volcanic rocks are euhedral-subhedral in shape and display striped absorption and fine-scale oscillatory growth zoning as well as their high Th/U ratios, implying a magmatic origin. The dating results show that the Late Paleozoic-Early Mesozoic volcanic rocks in the study area can be divided into four stages, i.e., the Middle-late Devonian (-386 Ma), Early Permian (～291 Ma), Middle Permian (～268 Ma) and Late Triassic volcanisms (201～217 Ma). Moreover, the two former volcanisms widespreadly exist in the eastern margin of the Jiamusi Massif, Lesser'Xingan and Zhangguangcai ranges; the Middle Permian volcanisms mainly occur in southeastern margin of the Jiamusi Massif; whereas the Late Triassic volcanisms chiefly outcropped in western margin of the Khanka Massif. 5. Determinations of the passive continental-margin setting in eastern margin of the Jiamusi Massif and an intra-continental extensional environment in the Lesser Xing'an Rang in Middle-late Devonian.The Middle-late Devonian volcanic rocks in eastern margin of the Jiamusi Massif are composed mainly of major rhyolite and minor basalt, displaying the features of the typical bimodal volcanism. Combined with the coeval sedimentary characteristics, we propose that there exist a passive continental-margin setting in eastern margin of the Jiamusi Massif in Middle-late Devonian. Furthermore, the coeval volcanic rocks occurred in the Lesser Xing'an Range have the features of A-type rhyolite, indicating an intra-continental extensional environment. The above findings are also consistent with the extensional environment existed after the amalgamation of the Songnen-Zhangguangcai Range and Jiamisi massifs in the Late Caledonian.6. Determinations of the active continental-margin setting in eastern margin of the Jiamusi Massif and an extensional environment in the Zhangguangcai Range in Early Permian.The Early Permian volcanic rocks in eastern margin of the Jiamusi Massif are composed mainly of basalt, basaltic-andesite, and minor dacite. They belong to the subalkaline series, and display a calc-alkaline evolutionary trend, indicating that an active continental-margin setting could exist in the eastern margin of the Jiamusi Massif in the Early Permian. Besides, there exist voluminously coeval basalt, andesite-basalt and rhyolite in the Zhangguangcai Range. The geochemical data show that they are typical of bimodal volcanism. Together with the coeval A-type granites, the early Permian bimodal volcanic rocks in the study area indicate an extensional environment in the Zhangguangcai Range. Combined with the regional geological data and an active continental setting in eastern margin of the Jiamusi Massif, we propose that the bimodal volcanism in the Zhangguangcai Range could form in an extensional tectonic setting, similar to a back-arc basin, and its formation could be related to subduction of the Paleo-Asian oceanic plate beneath the Songnen-Zhangguangcai Range and Jimusi massifs.7. Timing of the amalgamation of the Jiamisi and Khanka massifs The Middle Permian volcanic rocks in eastern and southeastern margin of the Jiamusi Massif consist mainly of rhyolite and minor dacite. They are characterized by high SiO2, low MgO, enrichment in K2O, Rb, Th and U, and depletion in Eu, Sr, P and Ti, implying a crust-derived origin and similar to the syn-collisional rhyolites. Therefore, it is proposed that the Middle Permian volcanic rocks could have formed under the collision of the Jiamusi and Khanka massifs.8. Determinations of the intra-continental extensional environment in western margin of the Khanka massif in Late Triassic.The Late Triassic volcanic rocks in western margin of the Khanka Massif consist mainly of rhyolite and minor dacite. These geochemical features are similar to those characteristics of A-type rhyolite, suggesting an extentional setting. Combined with the regional geological data, the Late Triassic volcanic rocks could form in an inter-continental extensional environment related to the final closure of the Paleo-Asiatic ocean and the collision of the NCC and Jiamusi-Khanka Massif in the late Paleozoic-Early Triassic.