Thermodynamic evolution of lithosphere of the North China craton： Records from lower crust and upper mantle xenoliths from Hannuoba

Lithospheric thinning in East China attracts much attention of geologists. In many cases where a lithospheric thinning is inferred, the trigger for instability is a preced-ing episode of crustal thickening by the continental colli-sion[1]. Lithospheric processes (e.g. lithospheric thinning) could be recorded by the thermal history of the lower crustal and upper mantle xenoliths. Xu et al.[2] suggested that the cooling (from >1200 to 850℃) recorded by spinel harzburgite from Northeast China …     

大陆下地壳和岩石圈地幔中的水:基于捕虏体的红外光谱分析

根据对华北克拉通汉诺坝和女山两个捕虏体产地的麻粒岩和橄榄岩的FTIR实验测定,分析讨论了大陆深部岩石圈内水含量和分布上的一些显著特点.结果表明,大陆下地壳和岩石圈内矿物的水含量无论在横向还是垂向分布上都是明显不均一的,造成这些不均一性的机制可能复杂多样.     

Zircon chronology and REE geochemistry of granulite xenolith at Hannuoba

The lower crustal xenolith of mafic two-pyroxene granulite (the majority) and hypersthene granulite in the Cenozoic basalt at Hannuoba have the characteristics of igneous blastic structure and granulite facies metamorphic recrystallization. Study on the zircon chronology and REE geochemistry of granulite xenolith shows that the underplating of basic magma into the lower crust during late Mesozoic led to the formation of mafic accumulate, which further through metamorphism of granulite facies formed the high-density and high-velocity crustal bottom layer at the lower crust. This suggests that the underplating of mantle magma is the important way for the vertical overgrowth of continental crust since the Phanerozoic and provides new evidence for crust-mantle interaction.     

Experimental study on the P wave velocity in rocks from lower crust and crust-mantle transitional zone beneath the Hannuoba

Cenozoic basalt-borne mafic granulite-facies plagioclase pyroxenite and eclogite-facies garnet pyroxenite xenoliths from the Hannuoba, as well as nearby Archean terrain granulites, are selected for the experimental study on the P wave velocity at high temperature and high pressure in order to reveal the present-day compositional features for the lower crust and crust-mantle transitional zone. Results show that mafic xenoliths have high Vp (7.0～8.0 km/s), in contrast, the Archean terrain granulites have low Vp (<7.0 km/s). High Vp mafic xenoliths can represent the present-day compositional features for the lower crust and crust-mantle transitional zone beneath the Hannuoba. This provides new evidence for the crust vertical growth and the formation of the crust-mantle transitional zone resulting from the magma underplating. Low Vp Archean granulite still remains the characteristics of the early lower crust.     

Paleoproterozoic lower crust beneath Nushan in Anhui Province： Evidence from zircon SHRIMP U-Pb dating on granulite xenoliths in Cenozoic alkali basalt

Zircon SHRIMP U-Pb dating was carried out for an intermediate granulite xenolith in Cenozoic alkali basalt from Nushan. The results suggest that the lower crust beneath Nushan may have formed at about 2400—2200 Ma, and have been subjected to granulite-facies metamorphism at 1915 27 Ma. The old age of the Nushan lower crust is consistent with the geochemical similarities between Nushan granulite xenoliths and Archean-Paleoproterozoic granulite terrains in the North China craton, but it is not distinguishable from high-grade metamorphic rocks in the Yangtze craton where such old ages were also reported. Significant Pb-loss occurs in the Nushan zircons, implying important influence of widespread Mesozoic to Cenozoic underplating in East China on the lower crust beneath Nushan.     

Normal faulting in central Tibet since at least 13.5 Myr ago

Tectonic models for the evolution of the Tibetan plateau interpret observed east-west thinning of the upper crust to be the result of either increased potential energy of elevated crust or geodynamic processes that may be unrelated to plateau formation. A key piece of information needed to evaluate these models is the timing of deformation within the plateau. The onset of normal faulting has been estimated to have commenced in southern Tibet between about 14 Myr ago and about 8 Myr ago and, in central Tibet, about 4 Myr ago. Here, however, we report a minimum age of approximately 13.5 Myr for the onset of graben formation in central Tibet, based on mineralization ages determined with Rb-Sr and 40Ar-39Ar data that post-date a major graben-bounding normal fault. These data, along with evidence for prolonged activity of normal faulting in this and other Tibetan grabens, support models that relate normal faulting to processes occurring beneath the plateau. Thinning of the upper crust is most plausibly the result of potential-energy increases resulting from spatially and temporally heterogeneous changes in thermal structure and density distribution within the crust and upper mantle beneath Tibet. This is supported by recent geophysical and geological data, which indicate that spatial heterogeneity exists in both the Tibetan crust and lithospheric mantle.     

Petrology, geochemistry and geodynamics of basic granulite from the Altay area, North Xinjiang, China

The basic granulite of the Altay orogenic belt occurs as tectonic lens in the Devonian medium- to lower-grade metamorphic beds through fault contact. The Altay granulite (AG) is an amphibole plagioclase two-pyroxene granulite and is mainly composed of two pyroxenes, plagioclase, amphibole and biotite. Its melano-minerals are rich in Mg/(Mg Fe^2 ),and its amphibole and biotite are rich in TiO2. The AG is rich in Mg/(Mg Fe^2 ), Al2O3 and depletion of U, Th and Rbcontents. The AG has moderate ∑REE and LREE-enriched with weak positive Eu anomaly. The AG shows island-arc pattern with negative Nb, P and Ti anomalies, reflecting that formation of the AG may be associated with subduction. Geochemical and mineral composition data reflect that the protolith of the AG is calc-alkaline basalt and formed by granulite facies metamorphism having peak P-T conditions of 750℃-780℃ and 0.6-0.7 Gpa. The AG formation underwent two stages was suggested. In the early stage of oceanic crustal subduction, calc-alkaline basalt with island-arc environment underwent granulite facies metamorphism to form the AG in deep crust, and in the late stage, the AG was thrust into the upper crust.     

Discovery of mantle and lower crust xenoliths from early Cretaceous volcanic rocks of southwestern Tianshan, Xinjiang

In Tuoyun area of southwestern Tianshan, mantle and lower crust xenoliths are present in the volcanic rocks with ages of 101–123 Ma. Mantle xenoliths include mineral megacrysts such as kaersutite and pargasite, feldspar, biotite, and rare pyroxene and rock fragments such as perodotite, pyroxenite, amphibolite, and rare glimmerite. Lower crust xeno-liths are mainly banded and massive granulite. The volcanic rocks were produced by within-plate magmatism. Occurrence of hydrous and volatile mineral megacrysts, amphibolite, and some pyroxenite containing hydrous and volatile minerals indicates that mantle metasomatism was intense. Undoubtedly, this discovery is very important to understanding of the crust-mantle structure and geodynamic background in depth in southwestern Tianshan and geological correlation with adjacent regions.     

Numerical simulation of GPS observed clockwise rotation around the eastern Himalayan syntax in the Tibetan Plateau

From Global Position System (GPS) measurements, there is a clockwise rotation around the eastern Himalayan syntax in the Tibetan Plateau. This phenomenon is difficult to be interpreted by simple two-dimensional modeling from a geodynamic point of view. Because of the extremely thick crust and the lower crust with relatively high temperature in the Tibetan Plateau, the lithospheric rheology in Tibet and surrounding areas present a complex structure. In general, the tectonic structure of the Tibetan Plateau consists of brittle upper crust, ductile lower crust, high viscosity lithospheric upper mantle, and low viscosity asthenosphere, the same as the case in many other continental regions. However, the lower crust in the Tibetan Plateau is much more ductile with a lower viscosity than those of its surroundings at the same depth, and the effective viscosity is low along the collision fault zone. In this study, we construct a three-dimensional Maxwell visco-elastic model in spherical coordinate system, and simulate the deformation process of the Tibetan Plateau driven by a continuous push from the Indian plate. The results show that the existence of the soft lower crust under the plateau makes the entire plateau uplift as a whole, and the Himalayas and the eastern Himalayan syntax uplift faster. Since the lower crust of surrounding blocks is harder except in the southeastern corner where the high-temperature material is much softer and forms an exit channel for material transfer, after the whole plateau reaches a certain height, the lower crustal and upper mantle material begins to move eastward or southeastward and drag the upper crust to behave same way. Thus, from the macroscopic point of view, a relative rigid motion of the plateau with a clockwise rotation around the eastern Himalayan syntax is developed. Supported by Knowledge Innovation Project of the Chinese Academy of Sciences (Grant No. KZCX2-YW-123) and National Natural Science Foundation of China (Grant Nos. 40774048 and 90814014)     

Quartz and clinoenstatite exsolutions in clinopyroxene of garnet-pyroxenolite from the North Dabie Mountains, eastern China

The exsolution lamellae of quartz and clinoen-statite are idenfied in diopside of garnet-pyroxenolite from the North Dabie Mountain by transmission electron microscopy, which is interpreted that the lamellae are originally exsolved from a former ultra-high-pressure clinopyroxene due to decreasing of pressure. Study of petrography shows that there is compositional zoning hi the diopside itself. It is implied that the garnet-pyroxenolite had undergone intensive high-temperature granulite fades and high-amphibolitic fades retrogressive metamorphism, while the peridotite (the garnet-pyroxenolite's host rock) emplaced the crust     

Garnet granulite facies xenoliths from Yingfengling Cenozoic basalt in Leizhou, Guangdong Province

The granulite xenoliths are first found in Yingfengling pyroclastic rocks of Leizhou region, Guangdong Province. Of them high_pressure garnet granulite xenolith found is very sparse in China. Garnet granulite is different from pyroxene granulite in mineral assemblage and composition. \%P_T\% calculation shows that garnet granulite was formed at 1 130-1 160℃and 1.4-1.7 GPa, and pyriclasite at about 800℃and 0.65-0.80 GPa. High xenolith_derived paleogeotherm indicates Cenozoic rifting in Leizhou area. Granulites with varied mineral assemblages were formed at different depths by the metamorphism of the underplated basaltic melt.     

Zircon U-Pb ages of olivine pyroxenite xenolith from Hannuoba: Links between the 97—158 Ma basaltic underplating and granulite-facies metamorphism

U-Pb zircon dating by LA-ICP-MS andSHRIMP for one olivine pyroxenite yields complex agepopulations including Mesozoic ages of 97-158 Ma and 228±8.7 Ma, Early Paleozoic ages of 418--427 Ma, Paleoprotero-zoic age of 1844±13 Ma, Neoarchean age of 2541±54 Ma andmiddle Archean age of 3123±4.4 Ma. The 97--158 Ma and228±8.7 Ma zircons show typical igneous oscillatory zona-tion in CL images, suggesting two episodes of magmaticevents. Overlapping of the 97-158 Ma ages with that ofgranulite xenoliths indicates that the Mesozoic granu-lite-facies metamorphism was induced by heating from thebasaltic underplating at the base of the lower crust. Bothprocesses lastcd at least from about 158 to 97 Ma. Ages of 418--427 Ma could be records of the subduction of Mongoliaoceanic crust under the North China craton. Ages of 1.84 Ga,2.54 Ga and 3.12 Ga correspond to the three importantcrust-mantle evolutionary events in the North China craton,and imply preservation of Precambrian lower crust in thepresent-day lower crust.     

Evidence of crustal ＇channel flow＇ in the eastern margin of Tibetan Plateau from MT measurements

Magnetotelluric （MT） survey has been carried out in the eastern margin of the Tibetan Plateau and its neighboring Shimian-Leshan area, Sichuan Province. Analysis of this MT data reveals that the electric structure of the Tibetan Plateau differ much from that of the Sichuan block. In general, the electric resistivity of crust beneath the Sichuan block in the east is larger than that of the eastern margin of the Tibetan Plateau in the west. The crust of the plateau is divided into upper, middle, and lower layers. The middle crust is a low resistivity layer with minimum down to 3-10Ωm about 10-15 km thick. It presumably contains partial melt and/or salt-bearing fluids with low viscosity, prone to deform and flow, producing a ＂channel flow＂ under the southeastward squeeze of the eastern Tibetan Plateau. This low-resistivity layer makes the upper crust decoupled mechanically from the lower crust. In the brittle upper crust, faults are dominated by left-lateral strike-slip and thrust motions, leading to surface rising and shallow earthquakes. The low-resistivity layer also cut the Xianshuihe-Anninghe fault zone into two sections vertically. In this region, the thicknesses of upper, middle, and lower crust vary laterally, producing a transitional zone in the eastern margin of the Tibetan Plateau characterized by thicker crust and higher elevation in the west and thinner crust and lower elevation in the east.     

New geological evidence of crustal thickening in the Gangdese block prior to the Indo-Asian collision

Recent mapping in the Gangdese block has revealed many leucogranites that are similar to those in the High Himalaya. These leucogranites formed at ～140 Ma as indicated by monazite Th-Pb ion-microprobe dating and cooled at ～130 Ma as indicated by muscovite ^40Ar/^39Ar dating. In conjunction with previous structural and paleogeographic studies, the new data indicate that the Gangdese block underwent crustal thickening and associated exhumation during ～140—130 Ma. In this regard, the southern margin of Eurasia continent was comparable to the modern South American Altiplano-Puna plateau, the prime example of active ocean-continent subduction and associated thickened crust. Specifically, the early stages of crustal thickening and uplifting of the Gangdese block may result from subduction of the Neo-Tethyan Ocean. If the Tibetan Plateau would form by accretion of a series of blocks with thickened crust, an elevated topographic plateau similar to the Altiplano-Puna plateau had formed before collision between the Indian and Eurasian plates. Then the Tibetan Plateau would have quickly thickened, uplifted, and begun to extend soon after onset of the collision. Thus, the deformational mechanism of the Tibetan Plateau is not distributed shortening, but rather concentrating deformation within regions of thin crust between the accreted blocks.     

Sr, Nd and Pb isotopic characteristics of granulite and pyroxenite xenoliths from Hannuoba Basalts in five-dimensional space and their geological implications

Sr, Nd and Pb isotopic characteristics of granulite and pyroxenite xenoliths from Han-nuoba Basalts in five-dimensional space are studied. Combined with the distribution of xenoliths, it is suggested that the isotopic relationship between various xenoliths can be well explained by the processes of delamination.     

Water in granulites: implications for the nature and evolution of the lower continental crust

The lower continental crust is one of the most important sphere-layers in the deep earth, and is the direct place where the crust-mantle interactions occur. Granulites are the dominated rocks in the lower crust, and have critical implications for the knowledge of the composition, nature and evolution of the deep crust; fluids are important mediums influencing many geochemical, geophysical and geodynamical characteristics of the lower crust, and may also play a fundamental role in the petrogenesis of granulites and the formation of the lower crusts. In this paper, we review recent advances involved with the deep continental crust, granulites and fluids, and some long-standing debates. Combined with the Fourier-transform infrared spectroscopy (FTIR) analysis performed on the mineral assemblages (cpx, opx, plag and grt) in lower crustal granulite xenoliths and terrains (exposed section) from east China, it is suggested that structural water, dominated by OH, in these nominally anhydrous phases may constitute the most important water reservoir in the deep crust. This structual water may help to understand many lower crustal geological processes and phenomena (e.g. seismic activities and electrical conductive anomalies), and influences from these water must be taken into consideration.     

Water in granulites: implications for the nature and evolution of the lower continental crust

The tower continental crust is one of the most important sphere-layers in the deep earth, and is the direct place where the crust-mantle interactions occur. Granulttes are the dominated rocks in the lower crust, and have critical implications for the knowledge of the composition, nature and evolution of the deep crust; fluids are important mediums influencing many geochemical, geophysical and geodynamical characteristics of the lower crust, and may also play a fundamental role in the petrogenesis of granulites and the formation of the lower crusts. In this paper, we review recent advances involved with the deep continental crust, granulites and fluids, and some longstanding debates. Combined with the Fourier-transform infrared spectroscopy (FTIR) analysis performed on the mineral assemblages (cpx, opx, plag and grt) in lower crustal granulite xenoliths and terrains (exposed section) from east China, it is suggested that structural water, dominated by OH, in these nominally anhydrous phases may constitute the most important water reservoir in the deep crust. This structual water may help to understand many lower crustal geological processes and phenomena (e.g. seismic activities and electrical conductive anomalies), and influences from these water must be taken into consideration.     

Imaging the Indian subcontinent beneath the Himalaya

The rocks of the Indian subcontinent are last seen south of the Ganges before they plunge beneath the Himalaya and the Tibetan plateau. They are next glimpsed in seismic reflection profiles deep beneath southern Tibet, yet the surface seen there has been modified by processes within the Himalaya that have consumed parts of the upper Indian crust and converted them into Himalayan rocks. The geometry of the partly dismantled Indian plate as it passes through the Himalayan process zone has hitherto eluded imaging. Here we report seismic images both of the decollement at the base of the Himalaya and of the Moho (the boundary between crust and mantle) at the base of the Indian crust. A significant finding is that strong seismic anisotropy develops above the decollement in response to shear processes that are taken up as slip in great earthquakes at shallower depths. North of the Himalaya, the lower Indian crust is characterized by a high-velocity region consistent with the formation of eclogite, a high-density material whose presence affects the dynamics of the Tibetan plateau.     

Continuing Colorado plateau uplift by delamination-style convective lithospheric downwelling

The Colorado plateau is a large, tectonically intact, physiographic province in the southwestern North American Cordillera that stands at ～1,800-2,000?m elevation and has long been thought to be in isostatic equilibrium. The origin of these high elevations is unclear because unlike the surrounding provinces, which have undergone significant Cretaceous-Palaeogene compressional deformation followed by Neogene extensional deformation, the Colorado plateau is largely internally undeformed. Here we combine new seismic tomography and receiver function images to resolve a vertical high-seismic-velocity anomaly beneath the west-central plateau that extends more than 200?km in depth. The upper surface of this anomaly is seismically defined by a dipping interface extending from the lower crust to depths of 70-90?km. The base of the continental crust above the anomaly has a similar shape, with an elevated Moho. We interpret these seismic structures as a continuing regional, delamination-style foundering of lower crust and continental lithosphere. This implies that Pliocene (2.6-5.3?Myr ago) uplift of the plateau and the magmatism on its margins are intimately tied to continuing deep lithospheric processes. Petrologic and geochemical observations indicate that late Cretaceous-Palaeogene (～90-40?Myr ago) low-angle subduction hydrated and probably weakened much of the Proterozoic tectospheric mantle beneath the Colorado plateau. We suggest that mid-Cenozoic (～35-25?Myr ago) to Recent magmatic infiltration subsequently imparted negative compositional buoyancy to the base and sides of the Colorado plateau upper mantle, triggering downwelling. The patterns of magmatic activity suggest that previous such events have progressively removed the Colorado plateau lithosphere inward from its margins, and have driven uplift. Using Grand Canyon incision rates and Pliocene basaltic volcanism patterns, we suggest that this particular event has been active over the past ～6?Myr.     

Evidence for mechanical coupling and strong Indian lower crust beneath southern Tibet

How surface deformation within mountain ranges relates to tectonic processes at depth is not well understood. The upper crust of the Tibetan Plateau is generally thought to be poorly coupled to the underthrusting Indian crust because of an intervening low-viscosity channel. Here, however, we show that the contrast in tectonic regime between primarily strike-slip faulting in northern Tibet and dominantly normal faulting in southern Tibet requires mechanical coupling between the upper crust of southern Tibet and the underthrusting Indian crust. Such coupling is inconsistent with the presence of active 'channel flow' beneath southern Tibet, and suggests that the Indian crust retains its strength as it underthrusts the plateau. These results shed new light on the debates regarding the mechanical properties of the continental lithosphere, and the deformation of Tibet.