Plastic mechanism of deformation of garnet

The strongly deformed eclogites are well developed in ultra-high pressure jadeite-quartzite zone of the Dabie Mountains, Eastern China, and garnets had been deformed strongly. Observations by transmission electron microscopy identified not only structure of plastic deformation occurring as free dislocation, dislocation loops and dislocation walls, but also clusters of water molecules present in the deformed garnet. Using infrared spectroscopy, two types of hydrous components are identified as the hydroxyl and free-water in the garnet. Based on analysis of microstructure mechanism of deformation in garnets, and experimental data of petrology, the clusters of water molecules were considered to lead strong plastic deformation of garnet by dislocations because of mechanical weakening.     

U-Pb isotopic compositions of the ultrahigh pressure metamorphic (UHPM) rocks from Shuanghe and gneisses from Northern Dabie zone in the Dabie Mountains,central China: Constraint on the exhumation mechanism of UHPM rocks

The occurrence of ultrahigh pressure (UHP) minerals, such as coesite and diamond in crustal rocks in orogenic belts suggests that a huge amount of continental crust can be subducted to man-tle depth during the continental-continental collision[1—6]. This…     

Differential exhumation of tectonic units and ultrahigh-pressure metamorphic rocks in the Dabie Mountains, China

A model involving buoyancy, wedging and thermal doming is postulated to explain the differential exhumation of ultrahigh-pressure (UHP) metamorphic rocks in the Dabie Mountains, China, with an emphasis on the exhumation of the UHP rocks from the base of the crust to the upper crust by opposite wedging of the North China Block (NCB). The Yangtze Block was subducted northward under the NCB and Northern Dabie microblock, forming UHP metamorphic rocks in the Triassic (240–220 Ma). After delamination of the subduction wedge, the UHP rocks were exhumed rapidly to the base of the crust by buoyancy (220–200 Ma). Subsequently, when the left-lateral Tan–Lu transform fault began to be activated, continuous north–south compression and uplifting of the orogen forced the NCB to be subducted southward under the Dabie Orogen (`opposite subduction'). Opposite subduction and wedging of the North China continental crust is responsible for the rapid exhumation of the UHP and South Dabie Block units during the Early Jurassic, at ca 200–180 Ma at a rate of ∼ 3.0 mm/year. The UHP eclogite suffered retrograde metamorphism to greenschist facies. Rapid exhumation of the North Dabie Block (NDB) occurred during 135–120 Ma because of thermal doming and granitoid formation during extension of continental margin of the Eurasia. Amphibolite facies rocks from NDB suffered retrograde metamorphism to greenschist facies. Different unit(s) and terrane(s) were welded together by granites and the wedging ceased. Since 120–110 Ma, slow uplift of the entire Dabie terrane is caused by gravitational equilibrium.     

Distribution of hydrous components in jadeite of the Dabie Mountains

Two kinds of jadeite occur in the jadeite-quartzite from the Dabie Mountains, Eastern China: associated either with weak or strong deformation. The former shows a uniform composition while the latter shows both uniform and zoning composition. These jadeites were examined with infrared (IR) spectroscopy. All jadeite displays hydroxyl (OH) stretching bands, implying that hydrous components commonly exist in jadeite. The concentration of the hydrous components in the jadeite with weak deformation is homogenous, whereas the concentration of the hydrous components in strongly deformed samples is variable. The correlation between the intensity of OH-band absorbance and compositional zoning of jadeite with strong deformation indicates that Na-rich jadeite can accommodate more OH than Na-poor jadeite. Its interpretation is that there is more Na associated with greater OH availability in the initial crystallization environment. Our data of the comparatively high OH concentrations recorded in the core of the jadeite which contains relative high vacancies in M2 site imply that both OH content and the vacancies in M2 site could decrease during exhumation of the jadeite-quartzite. The decrease of OH solubility in jadeite would result in the formation of H₂O fluid during the early exhumation of UHP metamorphic rocks. The H₂O fluid transformed from OH during exhumation could trigger and enhance the early retrograde metamorphism of the host rocks and facilitate plastic deformation of jadeite grain by dislocation creep and diffusion creep.     

Tectonic evolution of the ultrahigh-pressure (UHP) and high-pressure (HP) metamorphic belts from central China

Abstract In the Triassic suture between the Sino-Korean and Yangtze cratons, the Dabie metamorphic Complex in central China includes three tectonic units: the northern Dabie migmatitic terrane, the central ultrahigh-P coesite- and diamond-bearing eclogite belt, and the southern high-P blueschist-eclogite belt. This complex is bounded to the north by a north-dipping normal fault with a Paleozoic accretionary complex and to the south by a north-dipping reverse fault with Yangtze basement plus its foreland fold-and-thrust sequence. Great differences in metamorphic pressure suggests that these units reached different depths during metamorphism and their juxtaposition occurred by wedge extrusion of subducted old continental fragments. These units were subsequently subjected to (i) Barrovian type regional metamorphism and deformation at shallow depths; (ii) intrusion of Cretaceous granitic plutons; and (iii) doming and segmentation into several blocks by normal and strike-slip faults. A new speculative model of tectonic exhumation of UHP rocks is proposed.     

Hydrogen and oxygen isotope compositions of eclogites from the Dabie Mountains and geodynamic implications

Heterogeneous δ¹⁸O values as low as - 2.6‰ to+7.0% are observed for ultrahigh pressure eclogites from the Dabie Mountains in East China. Oxygen isotope equilibrium has been approached between the eclogite minerals, suggesting that the rocks would have acquired the unusual δ¹⁸O values prior to ultrahigh pressure metamorphism by interaction with¹⁸O-depleted fluid. δD values of hydroxyl-bearing are between — 51% and - 83‰, precluding the possibility of paleoseawater involvement. The only likely fluid is ancient meteoric water that exchanged oxygen isotopes with the eclogite precursor (a kind of basaltic rocks) formerly resident on the continental crust. This suggests a crustal recycling process in the suture zone of late subduction. Because silicate minerals undergo rapid oxygen isotope exchange at mantle pressures, preservation of the isotopic signature of meteoric water in the eclogites indicates limited crust-mantle interaction and thus a short residence time (<20 Ma) when the plate containing the eclogite precursor was subducted to mantle depths. The agreement in oxygen isotope temperatures for different mineral pairs suggests a rapid cooling and ascent process for the eclogites subsequent to their formation at mantle depths. Project supported by the National Natural Science Foundation of China and the Chinese Academy of Sciences.     

Deformation features of garnet-bearing granites from Huwan,western Dabie Mountains

Foliated garnet-bearing granite, usually associated with high pressure and ultrahigh -pressure (UHP) metamophic rocks, is a particular rock-type extensively exposed in the Mesozoic Dabie-Sulu orogenic belt of China. This study focuses on deformation features and SHRIMP zircon dating of foliated garnet granite in a high-pressure metamorphic unit from Huwan, western Dabie Mountains in order to resolve discrepancies in current versions of its petrogenesis and structural evolution. SHRIMP dating reveals a zircon age of 762 ± 15 Ma (MSWD=1.7) for Huwan granites, representing the Middle to Late Neoproterozoic age of intrusion and crystallization. Field and microstructural studies show that the Huwan granite body underwent multiple-stage deformation. The deformation was manifested by an early stage of rootless folding and imposition of relict foliation (S₁); an Indosinian main stage marked by imposition of north-dipping penetrative gneissosity (S₂) and development of ductile shear zones under NNE-SSW directed compression; and a final Indosinian stage of southward thrusting of the Huwan high-pressure unit. Shallow level extension prevailed after the Late Triassic, giving rise to south-dipping thrust faults and north-dipping normal faults. Supported by the National Natural Science Foundation of China (Grant Nos. 40802046 and 40334037) and the Project of Science & Technology Research and Development from Sinopec (Grant No. P02009)     

Two episodes of subduction and collision events at the northern foot of Dabie Mountains: Evidence from petrology and structural geology of granitoid rocks

The intrusive bodies studied include Mafan diorites ((462.7±1.5) Ma,⁴⁰Ar/³⁹Ar amphibole plateau age), Duhudian granites ((293±12) Ma, U-Ph zircon age) and Suxianshi granites ((146.2±0.9) Ma) in Beihuaiyang area at the northern foot of Dabie Mountains, central China. Petrological studies indicate that all of them belong to I-type granitoid rocks. Among them, the Mafan and Duhudian stocks were formed by arc magmatism, while the Suxianshi pluton is a post-collisional granitic body. Three intrusive bodies have distinctive characteristics of structural deformation. The Mafan stock has a rather complicated structure pattern resulting from polyphase deformation during the Caledonian and Mesozoic, the Duhudian stock has been pronouncedly deformed during the Hercynian-Yanshanian events, while regional foliation is not pronounced within the Yanshanian Suxianshi stock. Combination of regional stratigraphic, regional structural and geochronological data shows that the Yangtze plate has experienced two episodes of subduction northward beneath the North China plate during the Paleozoic and following collisional events. The first phase of collision at about 400 Ma resulted in the formation of the Beihuaiyang crystalline basement and the Caledonian high-pressure metamorphism in Dabie orogenic zone, and a late phase of continent-continent collision (~230 Ma) is responsible for the Triassic ultrahigh- and high-pressure metamorphism in Dabie Mountains and for orogenic uplift of the Dabie Mountains. It is suggested that the Beihuaiyang tectonic belt at the northern foot of the Dabie Mountains is a multicyclic suture.     

青藏高原东北缘岩石圈变形及其机理

为了解青藏高原东北缘岩石圈变形特征,进一步研究该地区地壳运动的壳-幔耦合机理,本文通过处理分析该地区1999～2007年多期GPS观测数据、1972~2000年水准测量数据和1992年及2007年相对重力测量资料,获得了该区域地壳水平运动速度场、较长时间段的垂直形变场和相对重力变化场.分析发现青藏高原东北缘东西部的变化特征存在明显差异：西部以北东向地壳缩短运动为主,而东部以顺时针旋转为主;东部以地壳隆升为主,速率在2.1 mm/a左右,而西部隆升的速率小于1 mm/a;相对重力变化则表现为在整体增大的背景下东部升高速率较大,平均为9.0×10-8 m·s-2·a-1,而西部较小,平均值为3.1×10-8 m·s-2·a-1.我们还发现,地壳不同变形形式的转换不是渐变的,而是发生在较窄的一个转换带内.这个转换带的整体走向为NEE,北部位于金昌与武威之间,中部在祁连山东部、门源以西,南部位于德令哈以东青海湖以西.最后结合前寒武纪构造格架、重力均衡异常资料和地震SKS分裂结果对形成这种运动态势的机理进行了探讨,我们认为岩石圈物质侧向流动、岩石圈结构及壳-幔耦合方式差异可能是导致东部与西部岩石圈变形差异主要动因.     

Structure of the crust and uppermost mantle from broadband seismic array in the western Dabie Mountains,east central China

A broadband seismic array of 7 stations was set up in the western Dabie Mountains (31°20′-31°50′N, 114°30′-115°E). Teleseismic events from May 2001 to November 2001 were collected and analyzed by radial receiver function to determine the S-wave velocity structure of the crust and uppermost mantle. The crustal thickness is 32-38 km beneath the array. The crust-mantle boundary appears as a gently north-dipping velocity discontinuity, but turns to be a velocity gradient beneath a station near the Qiliping shea...     

Oxygen and carbon isotope composition from the UHP Shuanghe marbles, Dabie Mountains, China

Investigations on the oxygen and carbon isotope compositions from the ultrahigh-pressure (UHP)-metamorphosed Shuanghe marbles, that occur as a member of a UHP slab, show that the δ¹⁸ O values range from +11.1% to+20.5% SMOW, and δ¹³ C from+1.0% to+5.7% PDB, respectively. The variations in isotope compositions show a centimeter scale of homogeneity and a heterogeneity of regional scale larger than 1 meter. In contrast to the eclogite marbles from Norway, the Shuanghe marbles have inherited the carbon isotope compositions from their sedimentary precursor. The δ¹³C shows positive correlation to the content of dolomite. The depletion in¹⁸O, compared with the pmtolithic carbonate strata, might result from three possible geological processes: 1) exchanging oxygen isotope with meteoric water before the UHP metamorphism, 2) decarbonation during the UHP metamorphism, and 3) exchanging oxygen isotope with country gneiss at local scale during retrograde metamorphism. It seems that the advection of fluid in the orogenic belt was very limited during subduction and exhumation of UHP rocks. Project supported by a U. S. -China cooperative project led by Prof. Cong Bolin of the Institute of Geology. Chinese Acade-my of Sciences, and Prof. J. G. Liou of the Department of Geological and Environmental Sciences, Stanford University and by the National Natural Science Foundation of china (Grant No. 49794042). Chinese Academy of Sciences (Grant No. KZ951-A1-401r, and National Science Foundation (Grant No. EAR-95-26700).     

Water contents and deformation mechanism in ductile shear zone of middle crust along the Red River fault in southwestern China

Using Fourier transform infrared spectroscopy (FTIR), we measured water contents of quartz and feldspar for four thin sections of felsic mylonite and two thin sections of banded granitic gneiss col- lected from a ductile shear zone of middle crust along the Red Rivers-Ailaoshan active fault. The ab- sorbance spectra and peak position suggest that water in quartz and feldspar of granitic gneiss and felsic mylonite occurs mainly as hydroxyl in crystal defect, but also contains inclusion water and grain boundary water. The water contents of minerals were calculated based on the absorbance spectra. Water content of feldspar in granitic gneiss is 0.05 wt%-0.15 wt%, and that of quartz 0.03 wt%-0.09 wt%. Water content of feldspar ribbon and quartz ribbon in felsic mylonite is 0.095 wt%-0.32 wt%, and those of fine-grained feldspar and quartz are 0.004 wt%-0.052 wt%. These data show that the water content of weakly deformed feldspar and quartz ribbons is much higher than that of strongly deformed fine-grained feldspar and quartz. This suggests that strong shear deformation leads to breakage of the structures of constitutional water, inclusion and grain boundary water in feldspar and quartz, and most of water in minerals of mylonite is released to the upper layer in the crust.     

Differences of granitic weathering at the northern and southern feet of Dabie Mountains,Central China: Implication for tectonic and climatic environments

Previous studies of weathering generally started with geochemistry[1—8] and mineralogy[9—12], and have been focused on chemical weathering rates[1—3], removability-enrichment of elements[3—6] during chemical weathering, and the age of weathering profi…     

Geochronology and geochemistry of gneissic metagranites in eastern Dabie Mountains:Implications for the Neoproterozoic tectono-magmatism along the northeastern margin of the Yangtze Block

The gneissic metagranites with Neoproterozoic protolith ages are widely exposed along the eastern margin of the Dabie ultra-high-pressure(UHP) metamorphic belt.In this paper,five representative plutons,including Huangzhen,Daba,Shima,Shuanghe,and Sanzusi,were selected for a detailed chronological and geochemical study aiming to identify the nature of the protoliths and to reveal their implications for the Neoproterozoic tectono-magmatic evolution along the northeastern margin of the Yangtze Block.These gneissic metagranites consist mainly of quartz,albite(oligoclase) and K-feldspar,and minor amphibole and biotite,as well as some metamorphic minerals due to the UHP metamorphism,such as phengite,epidote and minor kyanite and garnet.Mafic alkaline minerals(e.g.,aegirine or aegirine-augite) have been observed in some of these plutons.Zircon LA-ICP-MS U-Pb dating results indicate that the protoliths of the gneissic metagranites have been generated in Mid-Neoproterozoic(770-780 Ma),and suffered both ultrahigh-pressure metamorphism in Early Mesozoic and subsequently high-pressure eclogite-facies recrystallization at about 215 Ma.Although the gneissic metagranites generally have high SiO2 contents(70.23%-77.23%) and show metaluminous-weakly peraluminous signatures(ASI=0.90-1.05),there are still some geochemical variances between different plutons.Compared with the Sanzusi pluton,the metagranites from the Huangzhen,Daba,Shima,and Shuanghe have high K2O+Na2O contents(7.76%-9.45%),FeOtotal/(FeOtotal+MgO) ratios(0.82-0.96),HFSEs and Ga concentrations with an average 104×Ga/Al ratio up to 3.07.Combined with the features that the four plutons commonly contain aegirine-augite and have high zircon saturation temperatures(816-918°C),it is suggested that their proto-liths belong to peralkaline A-type granites.The Sanzusi pluton generally contains biotite and epidote generated from metamorphic reaction of amphibole,and is chemically enriched in calcium and depleted in potassium with K2O/Na2O ratios ranging from 0.42 to 0.54,suggesting their protoliths should be ascribed to calc-alkaline granitoids(I-type).The gneissic metagranites in the eastern margin of Dabie Mountains show both arc-and rift-like geochemical signatures,and their protoliths likely have been generated by reworking of the preexisting arc-related crust under an extensional setting,suggesting that the northeastern margin of the Yangtze Block during the Middle Neoproterozoic likely have been under the initial stage of a passive rifting rather than in an active rifting setting typically induced by the upwelling of a mantle plume.     

Neogene deformation of the Kuqa-Tianshan Basin-range System

The Neogene deformation of the Kuqa-Tianshan Basin-range System is characterized by discrepancy in paleostress patterns on the basin-range boundary and in the basin interior, and by discrepancy in deformation styles of the basement and the cover. Measurement and paleostress reconstruction of the brittle faults display a stress pattern with NW-SE and/or NNW-SSE extension on the basin margin and NW-SE compression in the basin interior. The basement was cut into blocks separated by boundary faults and upthrusts that were recognized in the seismic reflection profile. The block-faulting could have caused vertical uplift of the basement and gravitational sliding in the overlying sedimentary cover. Theoretical calculation indicates two generations of potential decollement folds within the basin, with one being mudstone in the Triassic Huangshanjie formation and the other the Cenozoic salt-gypsum layers.     

Mesozoic basin-fill records in south foot of the Dabie Mountains: Implication for Dabie Orogenic attributes

For the Triassic continental collision, subduction and orogenesis in the Dabie-Sulu belt, a lot of data on petrology, geochemistry and chronology have been published[1]. However, so far no depositional records on the Triassic syn-collisional orogenesis of…     

Metamorphic evolution of garnet–clinopyroxene–amphibole rocks from the Proterozoic Songshugou mafic–ultramafic complex, Qinling Mountains, central China

Garnet–clinopyroxene–amphibole rocks from the Songshugou mafic–ultramafic complex ( ca 1029 Ma) in the Qinling Mountains, central China, occur in a metabasite unit that is situated structurally beneath the peridotite body. These rocks have many records testifying to a multistage metamorphic history for the metabasites. Through petrographic and mineralogic investigations, at least three metamorphic stages were recognized: (i) a pre-eclogite stage of the amphibolite-facies condition as inferred from inclusions in garnet; (ii) an eclogite-facies stage as inferred from Pl-Cpx symplectites and from their reconstructed omphacite compositions (Jd_17–35); and (iii) a post-eclogite stage characterized by various reaction textures such as coronas and reaction zones around garnets, clinopyroxenes and fine-grained mineral aggregates after garnets and clinopyroxenes. A textural relationship observed in the field and thin-section scales indicates that a breakdown of the Grt–Cpx–Amp assemblage led to the Grt-free and the Cpx-free assemblages, that is, Amp + Pl assemblage at the end. The eclogite-facies metamorphism of the metabasites, discussed in the present report, was related to subduction at the age intervening between ca 983 and ca 1029 Ma. Regional geologic data indicate that the eclogite-related subduction took place during the opening of the Kuanping marginal basin in the southern margin of the North China craton, and slightly before the formation of the Danfeng magmatic arc ( ca 984 Ma). The exhumation and emplacement of the eclogite-facies domain probably resulted from a collision between the Danfeng magmatic arc terrain and a fragment of the North China craton (i.e. the Qinling Complex) during 790–890 Ma.     

Microstructure and magnetic fabric in the Shuanghe pluton: A synkinematic granite in Eastern Dabie Mountains,China

Strain analyses for the Shuanghe pluton show that the main strain planes suffered distinct deformation. The main strain value (XZ) is up to 1.59-2.18, and the value of Flinn index (K) ranges from 0.11 to 0.82. Anisotropy of magnetic susceptibility (AMS) measurements reveal that the orientations of the magnetic foliation and lineation gently dip SE, consistent with the macroscopic foliation of the pluton. The value of anisotropy degree (P) ranges from 1.109 to 1.639, and the shape parameter (7) from 0.079 to 0.534. These studies prove that the pluton was deformed under strong compression. Quartz c-axis textures, defined by monoclinic or triclinic asymmetry, usually developed the high maxima paralleling the b-axis, which is defined by the developed in the high-ultrahigh pressure rocks (UHP) which were captured in the pluton or country rocks. It is concluded that the Shuanghe pluton emplaced under regional compression slightly after the formation of UHP, and it is characterized by synkinematic granitic deformation.     

Occurrence of “monalbite” in nature: a TEM study

Natural “monalbite” (MA) has been observed on a submicron-scale in jadeite along with high albite (HA) and low albite (LA) in jadeite quartzite in the Dabie ultrahigh-pressure (UHP) metamorphic terrane, China. Transmission electron microscope (TEM) observation of inclusions in jadeite crystals has revealed mineral phases with C2/m and C1? structures. The cell parameters of the C2/m crystal structure correspond to those of MA, whereas the cell parameters of C1? structure are identical to those of HA and LA. The existence of the C2/m structure shows that the jadeite underwent partial change to MA on a submicron-scale during retrograde metamorphism. Albite transformation twinning of HA suggests the transformation of MA into HA. MA occurrence provides powerful evidence of high-temperature metamorphism during exhumation, probably above 930 °C. The preservation of MA may be due to the armouring by the jadeite and implies a very rapid cooling (quenching) during retrogression.