New technique for the study of orogenesis and porphyroblast formation mechanisms—Application of electron backscatter diffraction on foliation inflection/intersection axes (FIA) (chief researcher: CAO Hui)
The Qilian orogenic belt, which experienced multi-phase orogenesis, is a typical research area for orogenesis. The inclusion trails in plagioclase and garnet porphyroblasts from schists of the strike slip shear zone in Tuolemuchang, North Qilian, is spectacular. A detailed study was carried out to measure the FIAs (foliation intersection axes preserved within porphyroblasts) in plagioclase. The inclusion trail preserved in plagioclase porphyroblasts from an oriented sample 0°,30°,60°,90°and 120° is clockwise and the 150° is anticlockwise. Therefore, the FIAs in plagioclase porphyroblast is between 120° and 150°, with NE-SW horizontal bulk shortening. The inclusion trails in garnet are not preserved well enough for FIA measurement. This study was also combined with garnet Lu-Hf and in-situ monazite U-Pb dating to determine the two phases of deformation recorded in garnet and plagioclase porphyroblast. The early deformation event recorded in garnet porphyroblast was at 512.3±2.7 Ma and the younger one recorded in plagioclase porphyroblast occurred not earlier than 481.0±2.3 Ma. Combined with the results of previous studies on Hf and U-Pb isotopic analysis，the 512.3±2.7 Ma garnet Lu-Hf age is considered to reflect the growth time of garnet during the subduction process in the Qilian ocean. The quartz LPO in different zones of muti-phase growth prophyroblst ws measured by electron back scattering diffraction (EBSD). The quartz LPO in different micro-tectonic positions of some samples is consistent with the macro-interpretation of the structural shear direction, which can provide quantitative data support for the measurement of FIAs. But the characteristics of quartz should be considered carefully. The data at this stage are not sufficient to provide conclusive evidence for determining whether porphyroblast rotation or not occurred during muti-phase growth. Further studies of quartz SEM-CL and OM-CL image characteristics and in-situ monazite LA-split-stream analysis for U-Pb and Sm-Nd isotopes are recommended for better understanding porphyroblast growth and orogenesis.
Research on silicified acritarchs from the Ediacaran Doushantuo Formation at Shimen, Hunan, South China (chief researcher: CHEN Shouming)
In this study, fossiliferous chert and marine seidments of the Doushantuo Formation were studied at Zhongling, Shimen City, South China. Fossil horrizons in the Zhongling section were investigated, and biostratigraphiy has been applied to establish a stratigraphic framework between Hunan and the Yangtze Gorges. In addition, various stratigraphic methods including bio-, chemical and chronostratigaphy have been used to find the marker of subdivision of the Ediacaran system, and to establish a paleoenvironmental evolutionary model on the basis of the Snowball Earth. Remarkable conclusions are as follows: Well-preserved acritarchs were found in the Zhongling section, including the Tianzhushania spinosa, which is a unique species in the lower assemblage in the Yangtze Gorges area, and it indicates that the acritarch assembage can be correlated with the lower acritarch assembage in the Yangtze Gorges area. The acritarch assemblage is a potential straitgraphic tool for regional correlation on the Yangtze Platform. For the purpose of stratigraphic correlation and understanding the causal link between biotic evolution and paleo-environmental events, bio- and chemostratigraphy have been applied, multi-stratigraphic methods are beneficial for stratigraphic correlations, especially in the same basin. The geochemical record from the Doushantuo Formation suggests that the nutritive material sourced from weathering of continental material triggered the acritarchs evolution after the Snowball Earth.
Study of criteria for Pleistocene-Pliocene division in the Nihewan Basin, Hebei Province (chief researcher: CHI Zhenqing)
The well-preserved and exposed ‘Nihewan beds’ in the Nihewan Basin defina a Cenozoic non-marine strata. The lower part of the Taiergoudong section in this basin probably records continuous Neogene-Quaternary deposition, which may be an ideal candidate for the Q/N boundary standard stratotype section. On the basis of lithostratigraphic, biostratigraphic, and magnetostratigraphic characteristics of the Taiergoudong section and Haojiatai drill core, the Cenozoic sedimentary sequence of the Nihewan paleo-lake has been reasonably well defined in this project. The sedimentary environments of the Nihewan Basin and Paleolithic sites in this basin were systemically analyzed, and a paleogeographic framework ws also reconstructed. In addition, the location and markers of the Q/N boundary in the Nihewan Basin were summarized. The Haojiatai drill core with a length of 365.82 m is subdivided into six formations downward: The Upper Pleistocene Malan Formation with a thickness of 9.64 m, consists of eolian loess, and there is a depositional hiatus at the bottom and underlying strata; The Upper Pleistocene Haojiatai Formation with a thickness of 23.46 m was deposited in a shallow shore lacustrine environment; the Middle-Lower Pleistocene Xiaodukou Formation with a thickness of 32.2 m was deposited in a delta to lakeshore environment; the Lower Pleistocene Nihewan Formation with a thickness of 74.16 m was deposited in a shallow shore lacustrine environment; the Pliocene Daodi Formation with a thickness of 129.42 m consists of lake and marsh sediments; the Pliocene Huliuhe Formation with a thickness of 73.02 m consists of floodplain sediments. The Nihewan Formation in the Shanshazui, Majuangou, Heitugou, Xiaochangliang, and Shanshenmiaozui areas is dominated by fluvial sediments, and the sedimentary environment of Paleolithic sites in these areas are channels. Paleocurrent and anisotropy of magnetic susceptibility (AMS) results demonstrate that the northern part was higher than the southern area of the Nihewan basin during the Pliocene and Pleistocene, and intermediate to mafic volcanic rocks and metamorphic rocks were the major source of the basin during the early and late evolutionary stages, respectively. Detrital zircon U-Pb ages demonstrate that Cretaceous volcanic rocks were the common and important source rocks in the development of the basin, but abundant Proterozoic and pre-Proterozoic detritus was also transported into the basin since the Pleistocene. The location of the basal boundary of the Nihewan Formation from core analysis is 139.36 m, which is consistent with the M/G boundary. Compared with the Wucheng loess, both formations are characterized by voluminous calcareous slate. Their basal age is 2.6 Ma. Mammal fossils indicate that the strata of the East Taiergoudong section below 115.85 m should belong to the Upper Pliocence, but the upward part should be Lower Pleistonce. The M/G boundary on the East Taiergoudong section is located at around 114 m.
Magnesium isotope character and its constraints on the genesis of magnesite deposits in eastern Liaoning Province, China (chief researcher: DONG Aiguo)
In this study, we obtained the first Mg isotopic data from the world’s largest strata-bound magnesite deposit belt (Huaziyu deposit), hosted by the ca. 2.1 Ga Dashiqiao Formation in northeastern China. The results indicate that the δ26Mg values of most magnesite ores range from -0.95 ‰ to -0.49 ‰ (-0.75 ± 0.26 ‰, 2SD) and are heavier than most dolostone and limestone, and also heavier than magnesite (-1.14 ± 0.15 ‰, 2SD) and marble (-1.22 ± 0.35 ‰, 2SD) from the Dashiqiao Formation. Moreover, the general increase in δ26Mg values of the magnesite ores are from -0.9 ‰ at the base to -0.6 ‰ at the top of the section. Based on Mg isotopes, Fe isotopes, carbon isotopes, oxygen isotopes, element concentrations, and field observations, we interpret the Huaziyu magnesite deposit to have been dominated by two processes: (1) abundant Mg-rich carbonate precipitation from seawater in a semi-closed evaporative environment such as a lagoon; (2) magnesite formation during burial diagenesis, followed by transformation into giant magnesite ores during regional metamorphism. Giant strata-bound magnesite deposits are absent in modern and most Phanerozoic sedimentary environments and predominantly occur in Precambrian strata because Mg-rich seawater (from weathering of Mg-rich ultramafic rocks of Archean age) is a key requirement to generate Mg-rich precursor carbonate sediments that can later be transformed into giant magnesite ore deposits during burial diagenesis and regional metamorphism.
Early Mesozoic metamorphism and tectonic significance of the central and eastern Lhasa terranes (chief researcher: DONG Xin)
During 2013–2015, the project group has studied medium pressure (MP) metamorphic rocks and associated magmatic rocks from the Meldro Gongkar, Gongbogyamda, Nyingchi and Dongjiu areas in southeastern Tibet and achieved the following results: The Basongco metamorphic rocks underwent metamorphism in the Late Triassic–Early Jurassic during 204–185 Ma, and not in the Precambrian as previously considered. The metamorphic rocks including orthogneiss, paragneiss, schist, marble and amphibolite, experienced peak MP amphibolite-facies metamorphism under P–T conditions of 640–705 ºC and 6.0–8.0 kbar. Their protoliths were granodiorite, diorite and gabbro (Late Triassic–Early Jurassic at 201–198 Ma), clay and quartz sandstone (Carboniferous–Permian), and shaly-sandstone, calcareous-sandstone and linestone (Permian–Triassic). In combination with coeval MP metamorphic rocks from Yangbajain and HP metamorphic rocks from Sumdo, this study extends the early Mesozoic metamorphic belt in the mid-eastern Lhasa terrane up to 400 km. Moreover, the project also explored the genesis of early Mesozoic intrusive rocks from the Lhasa terrane, suggesting that these granitic rocks in different domains of the Lhasa terrane may have formed in different tectonic environments. This project has reported on Devonian and Cambrian magmatism in the southeastern Lhasa terrane and discussed its tectonic setting, supplying important constraints on the early-phase tectonic evolution of the Lhasa terrane. Furthermore, the project established a Mesoproterozoic–Neoproterozoic thermal-event record for the Lhasa terrane, showing that this terrane and the Indian continent had a similar Precambrian tectonic evolution, which was related to the amalgamation of supercontinents. Supported by this project, the team members published seven papers closely related in their research contents, of which three articles were published in international journals.
2.2~2.0 Ga geologic event in the Wutai and Zanhuang areas, central North China craton, and its tectonic features (chief researcher: DU Lilin)
Cratonization of North China is one of the key subjects in the early Precambrian evolution of China. The Paleoproterozoic evolution in North China is significant for unraveling its crationization process. However, major debates still occur on the Paleoproterozoic geological processes in northern China. Our study of the 2.2－2.0 Ga geological events can provide better constraints on the evolutionary process of the North China craton. In this project we studied the 2.2－2.0 Ga sedimentary basins and coeval intrusives in the Wutai and Zanhuang areas, central North China Craton, and established the tectonic setting of the 2.2－2.0 Ga geological events in order to provide robust evidence for building an objective geological evolutionary model for the North China Craton. We collected 11 samples of clastic rocks from different sugbgroups of the Hutuo Group and completed zircon U-Pb and Hf isotopic studies. We also collected 2 granite pebbles and 2 quartz pebbles from the Sijizhuang Formation and completed zircon U-Pb and Hf isotopic analyses. Additionally, we obtained geochemical and Nd isotopic data for basalts of the Hutuo Group. We collected 10 sandstone samples from different sections of the Gantaohe Group and completed zircon U-Pb age and Hf isotopis analyses. We also identified dacite from the top of the Nansizhang Formation and obtained a zircon U-Pb age, and also presented the geochemical data for basalt in the Gantaohe Group. Additionally, we obtained geochemical data, zircon U-Pb ages and Hf isotopic results for 2.2－2.0 Ga granitic intrusives in the Wutai and Zanhuang areas. Based on our research, we defined the base of the Hutuo Group at 2.2 Ga and the depositional age for the Doucun and Dongye Subgroups between 2.2 Ga and 2.0 Ga, and the Guojiazhai Subgroup after 1.9 Ga. We also established that the basic volcanic rocks of the Hutuo Group belong to the iron-rich tholeiitic series, emplaced is a within-plate setting. The youngest zircons from the base of the Gantaohe Group have a U-Pb age of ca. 2.1 Ga, and a dacite from the top of the Nansizhang Formation yielded a zircon U-Pb age of 2090±14 Ma, which suggests that the Gantaohe Group was deposited at ca. 2.1 Ga. Combined with 1.95－1.8 Ga metamorphism within the Trans-North China Orogen, we further confirmed a depostional age of 2.1 to 1.95 Ga for the Gantaohe Group. We also obtained geochemical data, zircon U-Pb ages and Hf-in-zircon isotopes and defined the Huangjinshan porphyry as an A-type granite. We also obtained zircon U-Pb ages of 2070－2090 Ma for different phases of the Xuting granite and found that potassic and sodic granites of the Xuting pluton belong to an A-type granite generation. Combining previous and our new research, we further suggested that the period 2.2－2.0 Ga represented an important tectono-thermal event in the North China Craton. 2.2－2.0 Ga magmatism and sedimentary basins in the Wutai and Zanhuang areas formed in a rift setting. Our research is significant for establishing Paleoproterozoic tectonic processes and reshaping the stabilization of the North China craton.
Zirconology of metamorphic oceanic crust, a case study from the Heilongjiang Complex in the Mudanjiang area (chief research: XIE Hangqiang)
Metamorphism of oceanic crust, in come instances, produces complex zircons, and it is necessary to work out a plausible interpretation of zircon ages based on integrated research methods. Many zircon age data have been produced for the Heilongjiang Complex in the Mudanjiang Area, but no identical interpretation has been reached. This project focused on the zirconology of this unit and to restrain the interpretation of zircon ages based on field geology, petrology, zircon O-isotopes, inclusion in zircon and Raman spectroscopy. etc. We have made following progress: 1) A garnet-biotite-quartz schist of relatively high metamorphic grade has been found in the Heilongjiang Complex in the Mudanjiang Area, and a metamorphic zircon age of 202±6 Ma has been obtained for the Heilongjiang Complex at the first time. 2) Most zircons in metagabbro or metabasalt with MORB or OIB geochemical signatures are inherited and do not record the formation age of magmatism. 3) The formation age of metasedimentary rocks, which are the dominant rock type of the Heilongjiang Complex in the Mudanjiang area, is younger than 260 Ma and suggests that the ocean was small. 4) The Huangsong Group in the Dongning area has a similar lithology to that of the Heilongjiang Complex in the Mudanjiang area, but the metamorphic age of 267±5 Ma is older than in the Heilongjiang Complex, so these two did not form in the same ocean. 5) The Heilongjiang Complex in the Yilan area is intruded by 250 Ma old granite, so its emplacement age is older than the Heilongjiang Complex in the Mudanjiang area, and probably formed in the different ocean. 6) Based on the zirconology and structural geology, we propose an Archipelago model for the tectonic interpretation in the Heilongjiang area during the Permian-Triassic period. This research solved, to a large extent, the age problem of the Heilongjiang Complex in the Mudanjiang and other area and can also be taken as an example to restrain the interpretation of zircon ages based on integrated research methods.
Using teleseismic records of a broadband seismic profile across the Tarim basin from north to south and an inversion of P-wave receiver functions, we obtained the Tarim Basin crystalline basement depth and sedimentary layer shear wave velocity structure. The results show the crystalline basement thickness between 1 and 14 km. The depth of crystalline basement and the Tarim Basin secondary unit division and aeromagnetic anomalies are basically the same, and the shallowest location is in the northern part of the Kuche depression to the South Tianshan Mountain, at about 0.8-2 km depth. The Bachu uplift, Tabei uplift is relatively shallow at a depth within 5.5 km, almost between 6 and 9 km. The Awati depression contains thick sediments, the deepest godown to 13 km. In the Yecheng depression to Mengaiti slope, the sedimentary layer becomes thin, from south to north it is reduced from 11.6 km to 7 km. Using the S receiver function we obtained the MOHO depth and thickness of the lithosphere in the Tarim Basin. The Moho depth varies between 45 and 55 km beneath the Yecheng depression, but below the Maigaiti Slope and Awati sag and Tabei uplift, the depth is almost at 52-55 km and the Moho topography shows little ups and downs, in accord with craton crustal features. However, in the northern part of the Bachu uplift, the Moho shows a significantly uplift, and the shallowest section is only 42 km deep. Between the Calishaya and Tumuxiuke fracture, the Moho and the crystalline basement are the thinnest, and the southern and northern sides of the lithosphere structure is relatively stable. We speculate that the Precambrian North and South Tarim are two independent crustal blocks and were welded below the Tarim basin by the Permian mantle plume. The mantle material was weak due to eruption, forming flood basalt in the Bachu uplift.
A broadband seismic profile in northern North China to find traces of the upper mantle structure of the Paleo-Asian Ocean (chief researcher: LI Qiusheng)
We deployed 41 sets of broadband seismometers along the ‘Sinoprobe’ deep seismic reflection and wide angle reflection profile for a period of two years of observation. The collected data have been analyzed by several methods such as receiver function, and structural features at three depth levels, namerly the Moho, continental lithosphere and mantle transition zone (MTZ) have been obtained. The crustal structure and distribution characteristics of Poisson's ratio obtained in this way suggest that the crust of the XingMeng orogenic belt has nearly reached isostatic equilibrium. The geometry of the lithosphere-asthenosphere boundary (LAB) obtained from S-wave receiver functions support a geodynamic model implying that ancient Paleo-Asian oceanic crust was subducted southwards beneath the northern margin of the North China Craton and the ocean was finally closed along Linxi fault zone. The lithosphere thickness data show that the northern margin of the North China Craton experienced lithospheric thinning, but there was little effect on thr XingMeng orogenic belt. The 660 km discontinuity dropped about 25 km beneath the northern margin of North China Basin area and the MTZ thickens by 15 km compared to global models, implying that the MTZ may be affected by cold material.
Isotopic fractionation of Zn and Cu in plants (chief researcher: LI Shizhen)
The advent of high-precision, multi-collector–inductively coupled plasma–mass spectrometry (MC–ICP–MS) has made it possible to determine stable isotope abundances. Investigations of isotopic variations in transition metals such as Cu, Zn, Fe, Ca and Mg during the growth of plants represents a rapidly developing area of research. Preliminary results on fractionation processes and their mechanism in this project are in the following fields: 1) research on soil; 2) research on Cu isotopic fractionation during uptake and translocation of plants 3) research on the influence of soil conditions on Cu isotopic fractionation; 4) discussion on the mechanism of Cu isotopic fractionation during uptake and translocation of plants. The Cu isotope compositions of soil and Cu isotopic fractionation between plants and soil have also been obtained. 1) δ65Cu and δ66Zn in soil are identical at 0.25 ‰ and -0.60 ‰, respectively, and no variation in isotope composition between parent and treatment soil after the experiment (-0.06 ‰﹤Δ65Cu the treatment soil – parent soil <0.03 ‰；-0.08‰, Δ65Cu the treatment soil – parent soil <0.02 ‰); 2) fractionation of Cu and Zn between the phytoavailable component and total soil are different (Δ65Cu phytoavailable component – total soil = 0.30 ‰；Δ66Zn phytoavailable component – total soil = -0.02 ‰); 3) Cu and Zn isotopic fractionation occurs during the growth of a plant (-1.42‰ <Δ65Cu plant –soil <-0.44 ‰; -0.31‰ <Δ66Zn plant –soil <0.29 ‰). The key observations from these results are as follows. 1) The Cu and Zn isotope compositions of soils used for laboratory plant growth are uniform, and it is not necessary to consider the potential influence of variations in the Cu and Zn isotope compositions on the isotopic fractionation in these soil–plant systems; 2) Cu and Zn concentrations in plants depends on the free ions of phytoavailable components; 3) relative to total soil, there is enrichment in lighter Cu and Zn in the plant; 4) compared to Zn, the larger Cu isotopic fractionation occurs due to oxidation-reduction reactions; 5) variations in Cu and Zn isotope compositions exists in different tissues (i.e., phloem and xylem) and among bio-macromolecules; 6) Cu isotopic fractionation occurs during retranslocation, however, no isotopic fractionation occurs compared with the analytical error for Zn. In summary, in this study further Cu and Zn isotopic fractionation and mechanisms have been recognized, and it is meaningful to use Cu and Zn isotope compositions to trace the uptake and translocation process in plants.
Paleomagnetic research on the spatial-temporal variations of Cenozoic block rotation between the northeastern margin of the Pamirs and the southern margin of the southwestern Tianshan (chief researcher: LIU Dongliang)
Paleomagnetic investigations has been performed on Miocene Wuqia Group strata in the east of Wuqian Town of the southwestern Tarim Basin, which belongs to the northeastern margin of the Pamir - Southwest Tianshan. Stepwise thermal demagnetization successfully isolated high unblocking temperature characteristic directions. Using principal component analysis to obtain high temperature characteristic remanence directions, we established a geomagnetic polarity timescale. Using the Geomagnetic Polarity Time Scale (Ogg, 2012), a high-resolution magnetostratigraphy of this sectionhasl been established. The sediments of this section were deposited from ~16.6 to ~9.9 Ma. The paleocurrent had a relatively stable East-West direction (parallel to the mountains), which indicates that the area had undergone stable uplift during this time. Therefore, the time of collision for the northeastern margin of the Pamir - Southwest Tianshan collision system was earlier than ~16.6 Ma. The statistics of the magnetic declination show that there have been no significant changes, indicating that collision system was mild. In addition, there existed a growth strata in the Xiyu Conglomerate Formation at the up of this profile. This growth strata indicates that abrupt collision and uplift occurred at the northeastern margin of the Pamir - Southwest Tianshan collisional system. According to the thickness of the sediment in this profile, this abrupt uplift and collision began at ~3 Ma.
Biostratigraphic division of acritarchs from the Ediacaran Doushantuo Formation in the Yangtze Gorges and their international correlation (chief researcher: LIU Pengjv)
Large acanthomorphic acritarchs are one of the most important biological types in the Ediacaran Period. Previous studies have shown that their potential for Ediacaran biostratigraphic subdivision and global correlation is becoming increasingly important. Ediacaran successions in the Yangtze Gorges area of South China, which are richly fossiliferous and have a long history of paleontological research, are one of the most important areas for the Global Standard Section and Point (GSSP) of the Ediacaran System. The systematic study of acanthomorphic acritarchs in the Yangtze Gorges area and their biostratigraphy will provide information for establishing global biostratigraphic successions (biozone) and chronostratigraphic subdivision. Based on thin section examination of chert samples collected from several typical sections, abundant large acanthomorphic acritarchs have been found in the Ediacaran Doushantuo Formation in the Yangtze Gorges area. A clear stratigraphic distribution of acritarchs has allowed two assemblages of acanthomorphic acritarchs to be established. They are a lower Tianzhushania spinosa assemblage and an upper Hocosphaeridium anozos–Hocosphaeridium scaberifacium–Tanarium conoideum assemblage. The lower T. spinosa assemblage is dominated by the taxon T. spinosa (more than 60 %), which occurs from the lowermost to the uppermost reaches of the assemblage. Apart from South China, T. spinosa has only been found in northern India (personal communication, Dr. Harshita), which shows that the lower T. spinosa assemblage may correlate with the acanthomorphic assemblage in northern India. By comparison with the lower assemblage, the upper Hocosphaeridium anozos–Hocosphaeridium scaberifacium–Tanarium conoideum assemblage preserves more acanthomorphic acritarchs, both in terms of number of individuals and in the diversity of the forms. The upper assemblage shares several species with the Ediacaran complex acritarchs palynoflora (ECAP) in Australia, and the species T. spinosa is not present in the ECAP of Australia. Hence, the upper assemblage of the Doushantuo Formation can be correlated with the ECAP of Australia, whereas the lower Tianzhushania spinosa assemblage appears to be missing in Australia. Similarly, many taxa from the upper acanthomorphic assemblage in the Yangtze Gorges area have been reported from Siberia and the East European Platform, which demonstrates that the upper assemblage is stratigraphically correlative with that of Siberia and the East European Platform. In addition, based on integrated bio- and chemostratigraphic data (carbon isotopes), we propose that the Ediacaran can be subdivided into two series and five stages, and the Chenjiayuanzi section is the best section for establishment of a GSSP in the Ediacaran because of its well exposed and continuous outcrops and most complete distribution of acanthomorphs with excellent preservation.
Petrogenesis of the Changhai khondalite series in southeastern Liaoning Province, China and tectonic implications (chief researcher: MENG En)
The geological features and petrogenesis of Paleoproterozoic geological assemblages, widely exposed near Changhai town of southeastern Liaoning Province, could provide important constraints on the tectonic developmen and geological evolution of the Paleoproterozoic Jiao-liao-Ji Belt. In this project, detailed petrological, mineralogical, and geochemical studies on Paleoproterozoic supracrustal rocks of the Changhai and other areas of the Liao-Ji belt have been carried out, and the main conclusions are: 1) The Changhai metamorphic supracrustal rocks were deposited after 1.88 Ga and do not belong to the regional South Liaohe Group, and were given a new name: the Changhai Group; 2) sodium- and potassium-rich gneisses of the Changhai ancient basement were emplaced at ca. 2543 and 2518 Ma; 3) the magmatic precursor of meta-mafic rocks of the central Liaodong Peninsula was emplaced at ca. 2154 Ma, was metamorphosed at ca.1897 Ma, and formed in a back-arc basin; 4) the protoliths of the South Liaohe Group were deposited at some time after 2035 Ma, and peak metamorphism occurred at 1885 Ma; 5) there existed a ca. 2.08 Ga potassic-rich granite emplacement event in southeastern Jilin Province, which could have formed in a tectonic setting similar to a back-arc basin.
The Altyn Tagh fault (ATF) is a large-scale sinistral strike-slip fault in Asia. As the northern boundary of the Tibetan Plateau, this fault plays an important role in accommodating the deformation resulting from the India/Eurasia collision. A study of the ATF is essential to evaluate continental deformation models of the Tibetan Plateau. On the basis of previous studies, this project focused on the key transition zone between the central and western segments of the ATF, which were poorly studied previously, to understand active tectonics of the Ashikule area. Besides, we carried out field investigation instantly after the 2014 Yutian M7.3 earthquake. Through three years of field investigation and integrated research, we determined that the late Quaternary slip rate on the Ashikule fault is 1.5-2.1 mm/yr, and we estimated that the total slip rate of the western ATF is relatively low (<10mm/yr). The large earthquake recurrence interval on the Ashikule fault was estimated to be about 500-700 years. We also determined the late Quaternary river incision rate in the Ashikule area, which is 0.2-0.35 mm/yr. This incision rate represents the average late Quaternary uplift rate of the northwestern Tibetan Plateau. The field investigations instantly conducted after the 2014 Yutian earthquake revealed that a ~29 km-long sinistral strike-slip co-seismic surface rupture zone was produced by this earthquake. The maximum horizontal displacement observed along the surface rupture zone is 1 m. This is the first instrument recorded M>7 sinistral strike-slip earthquake event along the ATF. Additionally, a study of the rupture process of the 2008 Wenchuan earthquake indicates that this earthquake consisted of two sub-events, and the late Quaternary slip rate on the Longmu Co-Gozha Co fault was estimated to have been <3mm/yr. An integrated geometry and kinematic analysis of faults around the Ashikule area indicates that the Ashikule fault, the Karakax fault, and the Longmu Co-Gozha Co fault all belong to the ATF system. These faults together controlled the tectonics and deformation of the northwestern Tibetan Plateau.
SHRIMP U-Pb dating of diagenetic xenotime in sedimentary rocks - a case study of the Changzhougou Formation (chief researcher: SHI Yuruo)
SHRIMP U-Pb dating of diagenetic xenotime from sedimentary rocks has provided robust minimum ages for sediment deposition. Xenotime (YPO4) may grow during early diagenesis and is typically present as a trace constituent in siliciclastic sedimentary rocks in the form of syntaxial outgrowths on detrital zircon grains. Diagenetic xenotime occurs in a wide variety of rock types, including conglomerate, sandstone, siltstone, shale, phosphorite and volcaniclastic rocks, that vary in age from the early Archean to the Mesozoic. Xenotime can be an isotopically robust U–Pb chronometer because it contains elevated levels of U (generally >1000 ppm) and very low concentrations of initial common Pb, and commonly yields concordant and precise ages. Authigenic-diagenic xenotime in Precambrian rocks from the Ming Tombs district are favorable for geochronological determination of the Precambrian strata of the North China Craton. There are abundant grains of authigenic-diagenic xenotime, forming irregular or pyramidal outgrowths on detrital zircon grains dispersed throughout siltstone close to the suture of the Changzhougou Formation. The xenotime is fine-grained, commonly occurring with a maximum dimension of <10 μm, and this makes it almost impossible to extract this mineral from these rock samples using traditional crushing and separation techniques. Therefore, the rock was crushed to a ~500 μm particle size, and this material was mounted on more than 20 epoxy discs, polished, photographed in reflected light, and the xenotime grains were identified using backscattered electron (BSE) imaging and Raman spectroscopy. Because of the small size of diagenetic xenotime crystals, an in-situ isotopic technique with a spatial resolution of <10 μm is required to date these minerals. Six analyses of xenotime that did not overlap zircon (no ZrO2+ counts were observed during analysis) yielded a weighted mean age of 1363 ± 25 Ma. Monazite and rutile grains were discovered by using energy spectrum analysis to study the silty mudstone which was collected from the lower part of the Chuanlinggou Formation in the Changping area, Beijing. The maximum particle size of monazite is up to 88 microns whereas the largest rutile is 20 microns. According to many back-scatter images, the monazite grains show jagged edges and irregular shapes, and appear as fishes, birds, flowers, worms and so on, whereas rutile is arranged as a string of beads. The overwhelming majority of the monazite and rutile grains occur in quartz or hematite veins. In places far away from these veins, however, monazite or rutile was only rarely found. These monazite grains with an age of ca. 152 Ma are were dated using a SHRIMP II instrument at the Beijing SHRIMP Center. According to the preliminary age data, we suggest that these monazite grains belong to a secondary monazite generation that is associated with a late hydrothermal event and did not form during diagenesis.
Petrogenesis of two periods of TTG gneiss formation in the Zanhuang area, Hebei Province (chief researcher: SONG Huixia)
Studies on the formation and evolution of the North China Craton (NCC) are currently hotspots for research in Precambrian geology. TTG gneisses mainly studied in this project are located in the Zanhuang area, Hebei Province, which belongs to the southern section of the Central Orogenic Belt in the NCC. Through research on the TTG gneisses in this area, more information can be provided on the formation and evolution process of the NCC. The gneisses in the research area can be divided into compositionally banded gneiss and homogeneous granodioritic gneiss. We dated the two types of gneiss and found that they were mainly generated during two periods. The age of the banded tonalitic gneiss is 2703 Ma, whereas the age of the banded dioritic gneiss is 2723 Ma. The age of the weakly banded dioritic gneiss is 2689 Ma, and the age of the homogeneous granodioritic gneiss is 2521 Ma. We also studied the geochemical characters of these two types of gneiss and found them to be similar to a certain degree. Both types are rich in Al, Si, Na and poor in K; the LREEs are enriched, and the HREEs are depleted, and fractionation between LREEs and HREEs is remarkable; an Eu anomaly is not obvious; the LILE are enriched, and the HFSE such as Nb, Ta, Ti, Yb and Y are depleted. In primitive mantle normalized spidergrams, Nb, Ta, Ti, P and Sm show negative anomalies. Both types of gneiss have similarities with Archaean high-Al TTG and adakite. Therefore we infer that both the 2703 Ma banded tonalitic gneiss and the 2521 Ma homogeneous granodioritic gneiss were derived from partial melting of juvenile crust. We also consider that they formed during two independent magmatic events. Through the geochemical study and field observations, no evolutionary relationship was found between them. After formation of the protoliths of the 2.7 Ga TTGs, these rocks experienced anatectic processes and the gneiss with light and dark bands was formed. Combining the results of 2.7 Ga TTGs in the Luxi, Jiaodong Qixia, Wanbei Huoqiu, and Hebei Fuping areas, we suggest that magmatic events in the NCC had began on a large scale prior to 2.7 Ga, and this caused extensive crustal growth during this period. The widespread 2.5 Ga magmatic events in the NCC may offer support to the hypothesis of amalgamating microcontinents and that formed the basement of the NCC in the Paleoproterozoic.
New records on the unique eight-armed Ediacaran fossil Eoandro- meda octobrachiata, preserved in contrasting taphonomic windows from southern China and Australia, have been deduced to reflect an early comb jelly (ctenophore). The earliest animal macrofossils have not only bridged the international gaps in stratighraphy between the Sinian Doushantuo Fomation and the relevant strata of Ediacara biota, but also extend back the first appearance of ctenophores back into Ediacaran biota by more than 30 Ma. Moreover, new discoveries would again crop the evolutionary tree of animals. The recent paleontological study is also more consistent with the molecular phylogenomic hypothesis, supporting to answer the puzzle of early animal evolution through new fossil evidence. This paper presents one of the well-preserved carbonaceous compressions from the Doushantuo Formation, Guizhou, South China – an abundant, presumed alga, Gesinella, with a complete rhizoid holdfast structure. The rhizoid base of this taxon is similar to Hiemalora, one of the most widely reported Ediacaran fossils, in shape and size. As the Ediacaran macroscopic carbonaceous compressions with the largest thallus, these new specimens of Doushantuo Gesinella offer insights into the benthic Ediacaran biota with casts and molds in the ascendant: the disc-shaped Hiemalora may be holdfast discs of multicellular algae rather than a medusoid organism or an attachment of other frond-like Ediacaran life. In recent years, newly described macrofossil algae, trace fossils and more complex possible Ediacaran-type fossils (“Jiangchuan Biota”), as well as numerous examples of the ribbon-like macrofossil Shaanxilithes have been found in strata underlying the base of the early Cambrian phosphatic layer. Relevant boundary sections from the Dengying Formation up to the base of phosphatic strata (Zhongyicun Member) suggest continuous deposition and contain diverse macrofossil assemblages (assemblage zones) which may be useful for defining the Dengyingxian Stage of the upper Ediacaran, followed by the Jinningian and Meishucunian Stages of the lowermost Cambrian. These sections are candidates for a potential Chinese standard stratotype section. An abundant and diversified assemblage of benthic fossils from the Ediacaran Doushantuo black shales in the Wenghui section, Guizhou Province, South China, contains two discoidal carbonaceous forms, Kullingia rotadiscopsis sp. nov. and Eoaequorea xingi gen. & sp. nov. The fossils have well-preserved concentric rings and radiating lines and resemble many circular casts and moulds in Ediacaran clastic and carbonate rocks worldwide. The Doushantuo carbonaceous macrofossils help us to enquire into the current functional identifications of circular disks as the holdfasts of unknown organism or scratch circles. We suggestnthat these carbonaceous compressions are prone to be the ancestor of pelagic jellyfish-like organisms with medusoid hydrozoan affinity.
Formation and evolution of an Archean granite-greenstone belt in western Shandong Province: geology, geochemistry and zircon dating (chief researcher: WAN Yusheng)
We carried out geological investigations, SHRIMP zircon dating, Hf-in-zircon isotope analysis and a whole-rock geochemical study of the Neoarchean granite-greenstone belt in western Shandong Province. We also worked on other areas of the North China Craton (NCC) for comparison. The main results are as follows. 1) The Archean basement in western Shandong can be divided into three belts. Belt A is a ate Neoarchean (2525-2490 Ma) crustally-derived granite belt in the northeast, Belt B is an early Neoarchean (2.75-2.60 Ga) rock belt in the center, and Belt C is a late Neoarchean (2550-2500 Ma) belt of juvenile rocks in the southwest. Ca. 2.7 Ga rocks have also been discovered in Belts A and C, besides Belt B. This indicates that early Neoarchean rocks occur more widely in western Shandong than thought before. 2) Ten samples of different magmatic rock types, including hornblendite, gneissic tonalite, gneissic trondhjemite and gneissic granite, contain magmatic zircons with 207Pb/206Pb ages ranging from 2598 to 2667 Ma. They have εHf(t) values and Hf crustal model ages of 1.1 to +11.3 and 3.02–2.4 Ga, respectively. The middle Neoarchean rocks mainly occur together with early Neoarchean rocks in the northeastern portion of the central belt. Juvenile additions to continental crust and crustal recycling played important roles, and the entire Neoarchean tectonic evolution can be subdivided into middle to early Neoarchean (2.75–2.6 Ga) and late Neoarchean (2.6–2.5 Ga) events. 3) Different types of rocks (8 samples) in Belt B have metamorphic and anatectic zircons varying in age from 2.63 to 2.59 Ga, with an age peak at 2.61 Ga, suggesting that western Shandong experienced a regional tectono-thermal event at that time. Most zircons, whether magmatic or metamorphic, have δ18O values of 4.5–6.5 ‰ (V-SMOW). However, one sample has zircons showing low δ18O (V-SMOW) values, suggesting crustal extension and influx of meteoric water during the evolution of the rocks at least in local areas. 4) Western Shandong Province underwent a very strong tectono-thermal event at the end of the late Neoarchean. More than 20 samples from belts A, B and C contain metamorphic or anatectic zircons with ages of ~2.5 Ga. The protoliths of the rocks are 2.6-2.7 and 2.52-2.55 Ga in age. The late Neoarchean magmatism and metamorphism were closely associated in space and time, which was also identified in other areas of the NCC. 5) Late Neoarchean magma mingling has been identified in Belt C. The mingling belt extends for more than 10 km, and the rocks include syenogranite, monzogranite, granodiorite, quartz diorite and amphibolite with magmatic zircon ages of ~2.5 Ga. 6) We reviewed the spatial distribution, rock types, geochemical and Nd-Hf isotopic compositions of 2.7 Ga granitiods that are widely identified in the NCC. These granitoids are mainly tonalitic in composition. Our Nd-Hf isotope study indicates that the strong 2.7 Ga tectono-thermal event mainly involved juvenile additions to the continental crust.
Iron isotope fractionation during fluid exsolution of skarn-type deposits: a case study of a polymetallic deposit in the Middle-Lower Yangtze valley (chief researcher: WANG Yue)
Skarn-related mineralization is one of major types for high grade iron, copper, tungstern and tin ores and have important scientific and economic sifnificance. Lots of studies on this espect have been carried out in past decades, however, the source of metallic elements is still in hot controversal. With the development of method on high precision iron isotope mesurement, a new tool to solve this problem provided. Fluid exsolution is a critical process during skarn-type mineralization and whether Fe isotope fractionate during this process is a key issue as this kind of information is crutial in tracing metal source using Fe isotope. Meanwhile, this kind of information also has important significance for developing the theory of isotopic geochemistry. Fe isotope investigation of intrutive rocks, altered rocks, ores and sedimental rocks from typical deposits in the Middle-Lower Yangtze valley under various of metallogenic geological settings and component systems has been carried out in this project to reveal Fe isotope behavior during fluid exsolution process, to illustrate regularity of variations in Fe isotope composions, and to study whether the influence from distinguishing component systems on that regularity exist or not. Based on the above studies, a working model for directly tracing the metal source of skarn-type deposit by using Fe isotopic system is going to be built. In this project, a number of high-quality data has been accquired, which showing a tendency that endoskarn and magneites are enriched in light iron isotopes relative to igneous rocks. Together with Fe isotopic compositions of wall-rocks of skarn deposit, this tendecy can not be resulted by component mixing of wall-rock, which suggesting that Fe isotopes fractinated during fluid exsolution process and fluid exslututed from igneous rocks is enriched in light iron isotopes. Comparations among Tongling, Edongnan and Ningwu districts, sulfide-dominated skarn deposits and Fe-dominated skarn deposits revealed a similiar regulatiron of Fe isotope, indicating that igneous component and background of ore deposits may not change the regularity of Fe isotopes fractionation during fluid exsolution process. Based on above studies, we constraint metallic source of skarn deposit as magmatic source tracing by Fe isotopes.In this project, we also found the temporial and spatioal zonation of Fe isotopes as fluid evluiton. Fe isotope compositions of ore-forming fluid will evolve with time as minerals precipitaion progroceed. This study provide a further comprehensive on metallic source and mineralisaion processes directly tracing by metallic element, and the recoganitions accquried in this project also provied a basic foundation on further studies on ore-deposit and exploration.
Detailed FIB and TEM studies of unusual mineral inclusions in chromite and mantle peridotite from the Kangjinla region of Tibet (chief researcher: XU Xiangzhen)
A series of unusual mantle minerals has been discovered from chromite and mantle peridotite in the Kangjinla region of the Yarlung Zangbo ophiolite zone. This project focuses on in-situ mineral inclusions together with petrological and mineralogical features of chromite and mantle peridotites. We collected data about the inner structure, spatial relationship, formation mechanism and stress environment of corundum, Fe-Ni alloys and in-situ inclusions, utilizing analytical methods such as focused ion beam (FIB), transmission electron microscope (TEM), cathodouminescence (CL) imaging, and Fourier transform infrared spectroscopy (FTIR), to determine the origin, formation depth, and physical/chemical settings of corundum and Fe-Ni alloys in ophiolite, and we carried out preliminary research on the genesis of the Luobusa chromite deposit. The project has achieved the following preliminary results after three years of detailed research. (1) Podiform chromitites and their host peridotites in the Kangjinla mining district of the Luobusa ophiolite contain similar associations of ultrahigh pressure (UHP), highly reduced and crustal-type minerals. Abundant diamonds have been recovered from both lithologies, and these are associated with a wide range of base metal alloys, native elements, carbides, oxides, silicates and others. The presence of UHP and highly reduced minerals in these rocks indicates that at least some of the chromite must have crystallized deep within the mantle as well as in a shallow mantle wedge in a supra-subduction zone (SSZ) environment. The unusual minerals were encapsulated in chromite grains and then carried upwards by mantle convection. The peridotite of Luobusa was trapped in the mantle wedge where it was modified by SSZ fluids and melts. Partial melting and mobilization of the chromite grains allowed them to be carried to shallow levels in melt channels and eventually deposited as chromitites near the crust-mantle boundary. The unusual minerals were preserved during this process because they were encapsulated in chromite grains, either during crystallization or during later fluid fluxing. (2) We identified two types of chromitite from the Luobusa ophiolite, eastern part of the Yarlung Zangbo suture zone, of which one is massive chromitite as harzburgite envelope and the other is disseminated chromitite as an envelope of a dunite shell. There is a large difference between the spinel chemical composition, PGE and Re-Os isotopic characteristics of the two types of chromitite, which suggests different evolutionary processes. (3) Mineral inclusions (TiN and Mg-Al-oxide) in corundum from the Kangjinla chromitite were investigated by TEM. Mg-Al-oxides indicate characteristics of ultra-high pressure with a Cmcm structure. (4) All diamonds contain nitrogen between 151 and 589 ppm and are, within error, type Ib by means of FTIR. They are different from most diamonds occurring in kimberlites and impact craters.
Continental dynamics and resource potential of the Qinghai-Tibet Plateau (chief researcher: XU Zhiqin)
Based on continental dynamics and according to the indentation record between India and Asian, this project investigates the evolution of the Neo-Tethyan Ocean and the mechanism transforming the India-Asia system from collision to strike-slip, as well as its effects on resource formation and environmental changes as a response to continental dynamics. Based on previously research, our research team has achieved considerable progress in understanding continental dynamics of the Tibetan Plateau, especially the evolution of Tethys and the growth of the Tibetan Plateau during the past decade. These achievements are as follows: (1) A hypothesis on formation of the Tibetan Plateau as a orogenic plateau was proposed; (2) reconstruction of the tectonic framework of the Tibetan-Tethys system; (3) discovery of in-situ diamonds and a deep mantle-derived mineral group in ophiolites distributed along the Neotethyan suture zone; (4) understanding of the subduction mechanism of the Neotethys oceanic basins; (5) understanding the role of magmatism in the early phase of the Indo-Asian collision for exhumation of the Himalaya; (6) establishments of 3D models for the collisional orogeny and exhumation of the Himalaya; (7) new proposal for the extrusion of the SE Tibetan Plateau: ‘crustal bending and decoupling’; (8) subduction- and collision-related continental gneiss domes in the Tibetan Plaeau; (9) tectonic setting and the Wenchuan Earthquake mechanism at the eastern margin of the Tibetan Plateau; (10) Tibetan plateau collision mineralization model; (11) Numerical modeling of the Indo-Asian collisional process. This program aims to communicate with and stimulate interest amongst global geologists to better understand continental dynamics of the Tibetan Plateau.
Volcano-sedimentation and tectonic setting of mineralization of the Caotangou Group in the western section of the North Qinling Mountains (chief researcher: YAN Zhen)
The Caotangou Group is a suite of volcano-sedimentary rocks that experienced slight deformation and low-grade greenschist-facies metamorphism and is the important host strata of VMS-type deposits in the North Qinling Mountains. The volcano-sedimentary sequence of this unit has so far been overlooked, and its tectonic setting using geochemical data is hotly debated. On the basis of field mapping and typical section measuring, the rock-types and distribution, the ore-bearing lithology, and the volcanic sequence and genesis of the Caotangou Group were studied in detail in this project. The age, tectonic setting, mineralization, and relationship between the formation and the Proto-Tethyan Ocean were also studied by systematic investigations in paleontology, zircon geochronology, petrochemistry, Sr-Nd-Pb and Lu-Hf isotopic tracing, and fluid inclusions. These results do not only provide reliable evidence for the tectonic framework, mineral exploration and metallogenic prognosis in the North Qinling Mountains but also serve as an example for similar areas. Systematic studies on the facies and geochemistry demonstrate that the Caotangou Group represents a bimodal volcanic sequence formed in a backarc setting with massive sulfide horizons closely associated with a siliciclastic-felsic supracrustal sequence. Spatially, the proportion of felsic lavas decreases westwards but andesite increases. Mafic and felsic sequences mainly occur in the Laochang Mine, the dacite and dacitic sedimentary assemblage of which is the host lithology of the mineralization. SHRIMP and LA-ICP-MS zircon U-Pb ages for rhyolite and tuff of the Caotangou Group indicate that volcanism occurred during ca. 440-406 Ma, which is similar to dacite and andesitic basalt of the Xieyuguan Group. Precambrian xenocrystic zircon grains suggest that the subduction-related crustal source of these volcano-sedimentary rocks is closely related to the Qinling Group. In combination with previous results on the age of fossils, U-Pb ages of volcanic rocks, and the geochemistry of lavas and sulfides, we suggest that a southward-facing subduction-accretion system developed along the southern margin of the North China plate during 490-410 Ma, and subduction-related calc-alkaline magmatism did not end prior to ca. 400 Ma.
Geochronological and geochemical study of mafic-intermediate dikes in West Junggar, NW China (chief researcher: YIN Jiyuan)
The Carboniferous to Permian was a critical period for the tectonic evolution and ore-forming processes in the Central Asian Orogenic Belt (CAOB). However, as a key to understand the development of the CAOB, metallogenetic geodynamic settings are still hotly debated. We investigated late Paleozoic mafic-intermediate dikes and granitoids and carried out petrology, geochronology, elemental geochemistry and Sr-Nd-Hf isotope studies to investigate their ages, magma sources and petrogenesis and thus elucidate the geodynamic processes for the formation of the dikes and granitoids with implication for continental growth. Our results are summarized as follows: (1) We established the rocks association and formation ages of the mafic-intermediate dikes in the southern part of West Junggar, and we propose that events associated with ridge subduction likely played an important role in crustal growth in the CAOB; (2) we established the petrogenetic mechanism of two type sanukitoids in the West Junggar during the late Paleozoic; (3) we propose a new evolutionary model for the northern part of West Junggar in the early Carboniferous; (4) we established the formation ages and tectonic setting of late Paleozoic rocks in the West Junggar; (5) we established the rocks association and formation ages for plutonic rocks during the transitional period from the early to late Paleozoic in the West Junggar, and we propose that a slab roll-back model can account for their petrogenesis and geodynamic processes. The above results have led to the publication of 10 peer-reviewed papers on international scientific journals such as Lithos, Tectonophysics and Gondwana Research.
Relationship between high-pressure granulite and adakitic rocks - a case study in the Dulan area, North Qaidam Mountains (chief researcher: YU Shengyao)
During the past three years (2014-2016), this project focused on high-pressure granulites and associated anatectic rocks from the early Paleozoic North Qaidam collisional orogen in northern Tibet. On the basis of fieldwork, petrology, mineralogy and geochronology, the following important results have been obtained: (1) On the basis of large-scale mapping, a high-pressure granulite unit has been recognized in the Dulan area of the North Qaidam UHP metamorphic terrane. The high-pressure granulite probably formed in a thickened lower crust, and penecontemporaneous metamorphic ages but different geothermal gradients between HP granulites and related UHP eclogite define a possible paired metamorphic belt generated in a subduction–collision setting. (2) An adakite-like leucosome has been recognized in high-pressure granulites of the Dulan area. The petrography, geochronology and geochemistry suggest that high-pressure granulite formation and anatexis occurred in a thickened lower crust, which provides a natural laboratory for observing the formation and emplacement mechanism of adakite. (3) An integrated study of the petrology, geochronology and geochemistry demonstrates that partial melting of the UHP metamorphic rocks in the NQD UHP terrane mainly occurred during the initial phase of retrogression, and anatectic melts probably crystallized under granulite-facies conditions. (4) Metapelite and metamafic rocks commonly experienced granulite-facies metamorphism, accompanied by anatexis and granitoid magmatism. Geochronological data indicate that granulite-facies metamorphism, anatexis and magmatism occurred simultaneously but later than the peak of UHP metamorphism.
Eclogite and garnet peridotite with different protoliths and ages of formation are preserved in the North Qaidam ultrahigh pressure metamorphic (UHPM) belt, providing us with a good opportunity to study geodynamic process on a plate boundary and rock associations, mineral structures and element mobility, etc. during subduction and exhumation in an orogenic belt. This project focuses on petrological, mineralogical, phase equilibrium and geochronological work on eclogites and associated rocks in the North Qaidam UHPM belt and the Sumdo area, eastern Tibet, in order to constrain the metamorphic processes of eclogite and garnet peridotite formation with different geochemical characteristics and different geological settings. We further speculate on the geodynamic setting and the interaction of oceanic and continental material in different types of subduction and collision. The results show that there are two types of eclogite with different mineral assemblages and geochemical features in the Xitieshan terrane, North Qaidam UHPM belt. Similar PTt characters show that these rocks formed during the same orogeny. Compared with typical oceanic eclogite in the Sumdo area, it shows that the eclogite from different tectonic settings attained strikingly different mineral assemblages and P-T paths during retrogression, providing the basis for studies on chemical element mobility. In a comparative study on garnet peridotite from both North Qaidam and the Scandinavian Caledonides, we developed a simplified conceptual model to classify orogenic garnet peridotite with their P-T conditions during formation and the time lag of the formation and subduction and future constrain the evolutionary of orogenic garnet peridotite.
Provenance and deformation of early Paleozoic sediments along the southern and eastern margins of the Alax Block and their tectonic implications (chief researcher: ZHANG Jin)
Detrital zircon ages of the early Paleozoic Xiangshan Group and the Dahuangshan Formation in the southeastern Alxa Block show that both sequences are late Cambrian in age, but the top parts of both successions may be Early Ordovician in age. The detrital zircon age spectra and paleocurrents analysis also show that the provenance of both sequences were in the eastern Ghats–Rayner orogenic belt and the Wilkes–Albany–Fraser orogenic belt in eastern Gondwana. The North China Block and the North Qilian orogenic belts are not the source regions for the Dahuangshan Formation and the Xiangshan Group. Similar paleontology, paleomagnetic data and detrital zircon age spectra indicate that the Alxa Block was located to the north of the eastern Gondwanan continent (present coordinates). A weighted mean age of 277.2±2.8 Ma for the diabase emplaced into the Xiangshan Group was obtained, indicating that the diabase is early-middle Permian in age. The diabase was a product of an intraplate extension event. The data suggest that the study region was once in a back-arc extensional setting in the early-middle Permian caused by northward subduction of the Yangtze Block beneath the North China Block. Emplacement of the late Paleozoic diabase in the study region argues against the idea that the so-called “Helan aulacogen” did not develop during the early Paleozoic and a “collision-induced rift” also did not develop during the late Paleozoic. There may have been a back-arc extension environment in the study region during the early-middle Permian during convergence between the NCP and the Yangtze Plate. Detrital zircon LA-ICP-MS U-Pb ages and paleocurrent analysis of the Paleozoic strata in the southeastern Alxa Block and structural analysis along the eastern Alxa Block show that amalgamation of the Alxa and North China Blocks occurred between the Late Devonian and early Carboniferous. An important north-south trending deformation belt of Late Devonian age was found along the eastern Alxa Block, which indicates nearly east-west compression during the late Paleozoic. In addition to the above east-west compression, a Triassic ductile left-lateral strike-slip fault, a Late Jurassic top-to-the southeast thrusting event, an Early Cretaceous extension event, an early Cenozoic top-to-the northwest thrusting event, a middle Cenozoic left-lateral strike-slipping event and a late Cenozoic extension event were documented in the eastern Alxa Block. All these deformation events were related to multi-phase intraplate tectonics occurring in China since the Triassic. The boundary of the Triassic Ordos Basin was not extensional but is a large strike-slip fault. The discovery of a large Cenozoic left-lateral strike-slip fault along the eastern Alxa Block may indicate that the Altyn Tage fault may already have been connected with the eastern Alxa Block during the early Cenozoic.
Ages, characteristics and tectonic implications of ophiolites from the southern Xiemisitai Mountain in West Junggar, Xinjiang (chief researcher: ZHAO Lei)
This project investigates the components, rock assemblages and field occurrence of ophiolites in the southern Xiemisitai Mountain and obtained geochronological and geochemical data for the ophiolite. The results enable us to discuss a comparison and connection of ophiolites in West Junggar and eastern Kazakhstan, and the distribution of paleo-oceanic and paleo-continental domains in Central Asia. The southern Xiemisitai Mountain contain ophiolite fragments consisting of serpentinized peridotite, gabbro, basalt, spilite, chert blocks and homologous pyroclastic rocks. LA-ICP-MS zircon U–Pb ages for gabbro in the Chagantaolegai ophiolite are 517 ± 3 and 519 ± 3 Ma. Serpentinized peridotites in the ophiolite show low rare earth element (REE) abundances (0.61–0.94 μg/g) and a slight loss of middle REEs. Chagantaolegai metagabbro samples have flat to slightly light REE depleted patterns with a narrow range of REE abundances. The Chagantaolegai spilites and metagabbros display similar flat patterns and show a geochemical signature similar to mid-ocean ridge basalt (MORB) between Yb and Zr, and diverge from MORB towards the most incompatible elements in MORB-normalized multi-element variation diagrams. The Th/Yb vs. Nb/Yb diagram for the spilite samples also suggests a mid-ocean ridge (MOR) setting. All geochemical characteristics of the Chagantaolegai ophiolite point to a MOR origin. Positive εNd(t) values (0.78 to +9.85) suggest that the gabbros and spilites were derived from a chemically inhomogeneous depleted mantle source. The Chagantaolegai ophiolite spatial-temporal characteristics correlate well with the Kujibai and Hongguleleng ophiolites in the northern West Junggar, and the Zhaur and Balkybek ophiolites in eastern Kazakhstan. These data show that early Palaeozoic oceanic crust began to develop in the early to middle Cambrian in the northern West Junggar. The Darbut ophiolite in the Sartohay region is covered unconformably by sedimentary-volcanic series of the Tailegula Formation. LA-ICP-MS U-Pb dating of zircons from a dacite yielded a 206Pb/238U weighted mean age of 349 ±2 Ma. The dacites show linear REE patterns with LREE enrichment and negative Eu anomalies. The average Sr, Sr/Y, and La/Yb of dacites in the Tailegula Formation are 195×10-6, 5.2 and 4.6, respectively, much less than those of classic arc dacites and adakites. Combined with low-grade deformation and metamorphism, the dacites in the Tailegula Formation are suggested to have erupted in a rift environment. Based on regional geological data, nine representative column maps were prepared for tectonic subdivision, and the South Saur Fault is suggested to be the northern boundary of the Chingiz–Tarbagatai early Palaeozoic orogen in the West Junggar, whereas an inferred buried fault in the Heshituoluogai depression separates the Chingiz–Tarbagatai early Palaeozoic orogen from the West Junggar- East Kazakhstan residual ocean in the south.
Institute Of Geology, Chinese Academy Of Geological Sciences
No. 26 Baiwanzhuang Street 100037 Beijing,China, Email:geoinst@cags.ac.cn