Allanite and REE bearing minerals are primum fama report from migmatite gneisses,plutonic Amet granite and intrusive alkaline rocks of Rajasthan,and these minerals are important for establishing REE deposit.Allanite t...Allanite and REE bearing minerals are primum fama report from migmatite gneisses,plutonic Amet granite and intrusive alkaline rocks of Rajasthan,and these minerals are important for establishing REE deposit.Allanite taken from host migmatite and granite have been analysed along with host rocks containing REE minerals,both of which contain appreciably high amount ofΣREE and Th.Allanite phases identified are CaO and Ce2O3 rich with positive correlation of silica and alumina.As silica proportion decreases andΣREE increases,allanite changes to britholite and thorite indicating a role of fluid in its crystallization.The chondrite normalized REE in migmatite gneiss and allanite mineral is compatible indicating a primary nature of allanite.The carrier of REE minerals is envisaged as the alkaline intrusives which deposited their REE minerals during ascent of magma from deeper source.Chemico-mineralogical study of Allanite and its hosts suggest possible reaction between a REE bearing fluid rich in P and F brought up by alkaline rocks,and the silicate minerals,mainly biotite and hornblende of the migmatite gneiss and Amet granite.The allanite in migmatite gneiss and granite may prove to be a sumptuous deposit of REE minerals in India,and similar terrains elsewhere.展开更多
The plate affiliation of the North Dabie terrane （NDT） has been controversial. To address this fundamental question, an integrated study of internal structure, in-situ U-Pb dating and trace element analysis in zirco...The plate affiliation of the North Dabie terrane （NDT） has been controversial. To address this fundamental question, an integrated study of internal structure, in-situ U-Pb dating and trace element analysis in zircons and field investigation for migmatite in the NDT was carried out, which reveals par- ticipation of crustal rocks of the North China Craton （NCC） in the protolith in addition to the more common crustal rocks of the Yangtze Craton （YC）. The evidence of an NCC affinity for protolith of migmatite in the NDT is the -2.5 Ga （2 486±14 and 2 406±26 Ma） magamtic age and -1.8 Ga （1 717±79 Ma） metamorphic age in the relict zircon domains because these two age groups are characteristic for the evolution of the NCC. The evidence of a YC affinity for protolith of migmatite in the NDT is the more common 0.7-0.8 Ga （e.g., 787±12 Ma） magamtic zircon age. Mid-Neoproterozoic magmatic age （0.7-0.8 Ga） is a symbol of YC basement rocks. In view of the widely exposed YC crustal components in the NDT, we suggest that the protolith of the NDT is mainly crustal rocks from the YC with minor crustal components from the NCC. The zircon rim domains and new growth grains from all the migmatite sam- ples are characterized by anatectic zircons and have a widely concordant ages ranging from 112.2±2.8 to 159.6±4.3 Ma with several peak values, suggesting a long lasting multistage anatexis. In conclusion, the NDT has a mixed protolith origin of both the YC and the NCC crustal rocks were strongly remoulded by anatexis during orogenic collapse.展开更多
The Higher Himalayan Crystallines(HHC), in western Garhwal, Uttarakhand are located in a regionalscale intracontinental ductile shear zone(15-20 km wide) bounded by the Main Central Thrust at the base, and the South T...The Higher Himalayan Crystallines(HHC), in western Garhwal, Uttarakhand are located in a regionalscale intracontinental ductile shear zone(15-20 km wide) bounded by the Main Central Thrust at the base, and the South Tibetan Detachment System at the top. The migmatite zone in the centre has the highest grade of metamorphism in the NW Himalayas and show evidence of flowage. Zircons extracted from samples of metasediment, migmatite, biotite granite and in situ partial melt(tourmaline-bearing leucogranite) along the Bhagirathi Valley, preserve U-Pb isotopic evidence of magmatic history, magma source and effects of the Himalayan orogeny in the region. Three distinct periods of zircon growth in the leucogranite record the episodic influx of magma between 46 Ma and 20 Ma indicating a time span of more than 25 Ma between the onset of fluid-fluxed partial melting in the mid-crustal intracontinental shear zone and the emplacement of the magma into the upper crust in a post-collisional extensional setting. Metamorphic zircon growth was initiated about 46 Ma, when the partial melts were generated as the migmatite zone was exhumed.展开更多
In order to constrain temperature during subduction and subsequent exhumation of fel- sic continental crust, we carried out a Ti-in-zircon thermometer coupled with zircon internal structure and U-Pb age on migmatitic ...In order to constrain temperature during subduction and subsequent exhumation of fel- sic continental crust, we carried out a Ti-in-zircon thermometer coupled with zircon internal structure and U-Pb age on migmatitic gneisses from the Weihai region in the Sulu ultra-high pres- sure （UHP） metamorphic terrane, eastern China. The Weihai migmatitic gneisses are composed of in- tercalated compositional layers of melanosome and plagioclase （Pl）-rich lencosome and K-feldspar （Kfs）-rich pegmatite veins. Four stages of zircon growth were recognized in the Weihai migmatitic gneisses. They successively recorded informations of protolith, prograde metamorphism, decompres- sional partial melting during early stage exhumation and subsequent fractional crystallization of pri- mary melt during later stage cooling exhumation. The inherited cores in zircon from the melanosome and the Pl-rich leucosome suggest that the pro- tolith of the migmatitic gneiss is Mid- Neoproterozoic （-780 Ma） magmatic rock. Metamorphic zircons with concordant ages ranging from 243 to 256 Ma occur as over- growth mantles on the protolith magmatic zir- con cores. The estimated growth temperatures （625-717 ＂C） of the metamorphic zircons have a negative correlation with their ages, indicating a progressive metamorphism in HP eciogite-facies condition during subduction. Zircon recrystal- lized rims （228-2 Ma） in the PI-rich ieucosome layers provide the lower limit of the decompress-sional partial melting time during exhumation. The ages from 228^-2 to 219~2 Ma recorded in the Pl-rich leucosome and the Kfs-rich pegmatite vein, respectively, suggest the duration of the fractional crystallization of primary melt during exhumation. The calculated growth temperatures of the zircon rims from the Pl-rich leucosome range from 858 to 739 , and the temperatures of new growth zircon grains （219±2 Ma） in Kfs-rich vein are between 769 and 529 . The estimated temperatures have a positive correlation with ages from the Pl-rich leucosome to the Kfs-rich pegmatite vein, strongly indi- cating that a process of fractional crystallization of the partial melt during exhumation.展开更多
Data from a migmatised metapelite raft enclosed within charnockite provide quantitative constraints on the pressure-temperature-time （P-T-t） evolution of the Nagercoil Block at the southernmost tip of peninsular Ind...Data from a migmatised metapelite raft enclosed within charnockite provide quantitative constraints on the pressure-temperature-time （P-T-t） evolution of the Nagercoil Block at the southernmost tip of peninsular India. An inferred peak metamorphic assemblage of garnet, K-feldspar, sillimanite, plagioclase, magnetite, ilmenite, spinel and melt is consistent with peak metamorphic pressures of 6-8 kbar and temperatures in excess of 900 ℃. Subsequent growth of cordierite and biotite record high-temperature retrograde decompression to around 5 kbar and 800 ℃. SHRIMP U-Pb dating of magmatic zircon cores suggests that the sedimentary protoliths were in part derived from felsic igneous rocks with Palae- oproterozoic crystallisation ages. New growth of metamorphic zircon on the rims of detrital grains constrains the onset of melt crystallisation, and the minimum age of the metamorphic peak, to around 560 Ma. The data suggest two stages of monazite growth. The first generation of REE-enriched monazite grew during partial melting along the prograde path at around 570 Ma via the incongruent breakdown of apatite. Relatively REE-depleted rims, which have a pronounced negative europium anomaly, grew during melt crystallisation along the retrograde path at around 535 Ma. Our data show the rocks remained at suprasolidus temperatures for at least 35 million years and probably much longer, supporting a long-lived high-grade metamorphic history. The metamorphic conditions, timing and duration of the implied clockwise P-T-t path are similar to that previously established for other regions in peninsular India during the Ediacaran to Cambrian assembly of that part of the Gondwanan supercontinent.展开更多
Migmatite-like rocks transformed from strongly metamorphosed and deformed enclave- bearing felsic plutons usually make people confuse with the true migmatites and mistake in interpreta- tion of their petrogenesis and ...Migmatite-like rocks transformed from strongly metamorphosed and deformed enclave- bearing felsic plutons usually make people confuse with the true migmatites and mistake in interpreta- tion of their petrogenesis and tectonic implications. Here we report a suite of rocks that have long been called as migmatites from the Guandi complex in Zhoukoudian region, southwest of Beijing. The rocks are dominated by felsic gneisses with garnet-free amphibolites. Field occurrence, petrography and geochemistry indicate that the felsic gneisses and amphibolites were metamorphosed from protoliths of intermediate-acid and basic igneous rocks, respectively. New LA-ICP-MS zircon U-Pb dating and geothermobarometry study further reveal that precursor magmas of the two types of rocks were emplaced at 2.54-2.56 Ga and the rocks subsequently underwent medium P/T-type metamorphism with upper amphibolite facies conditions of 0.55-0.90 GPa and 670-730℃ at -2.48-2.50 Ga. Geochemically, precursor magmas of the amphibolites were suggested to be derived from an enriched lithospheric mantle source in continental arc setting, and those of the felsic gneisses are characterized by tonalitic to trondhjemitic magmas that are usually considered to be generated by partial melting of hydrated, thickened metamorphosed mafic crust with garnet as residues, suggesting that the rock associations are not of true migmatites but migmatite-like rocks. Our study reveal that protoliths of the migmatite-like rocks from the Guandi complex, were likely formed via magmatism in a continental arc setting, followed by accretion and collision of the continental arc as well as the intro-oceanic arc terranes to the Eastern Block of the North China Craton in the transition from the Late Neoarchean to Early Paleoprnterozuic.展开更多
1 Introduction The Kangdian axis is an important polymetallic metallogenic belt in Southwest China,and it is also an area with a lot of hydrothermal uranium mineralization(Wang Hongjun,et al.,2009.The basic
This paper presents a case study of water inrush and mud burst occurring in a migmatite tunnel to study its formation mechanisms. The geological investigation and mineralogical analysis showed that water inrush and mu...This paper presents a case study of water inrush and mud burst occurring in a migmatite tunnel to study its formation mechanisms. The geological investigation and mineralogical analysis showed that water inrush and mud burst in the migmatite was closely related to the component of the host rock. High content of soluble minerals,e.g.,calcite and dolomite,would make the migmatite rock prone to be fragmentized,isintegrated and eventually form different sorts of connected or semi-connected veins. The field exploration revealed most cavities in the magmatite tunnel were eroded by groundwater and formed large interconnected networks. The two faults and the dike in the magmatite tunnel became the preferred paths and provided great convenience for plenty of precipitation and mud slurry. Due to high water pressure and blast disturbance,the cavities can soon connect each other as well as all sorts of veins,forming a complex ground channel for water inrush and mud burst. To estimate the potential occurrenceof water inrush and mud burst,the water bursting coefficient was employed. The results showed the water bursting coefficient of the magmatite tunnel was much bigger than the threshold values and it can be used to explain the accident of water inrush and mud burst occurring in the magmatite tunnel.展开更多
Partially migmatized rocks, banded migmatites, augen-banded migmatites and gneissic migmatites are developed successively from Xindong to Yunlu, Gaozhou, Guangdong Province in the Yunkai Caledonian orogenic belt at th...Partially migmatized rocks, banded migmatites, augen-banded migmatites and gneissic migmatites are developed successively from Xindong to Yunlu, Gaozhou, Guangdong Province in the Yunkai Caledonian orogenic belt at the border between Guangdong and Guangxi. Mass-balance calculations, statistical analysis of the textural relations and mineralogical and geochemical studies of the migmatites and the study of the metamorphlc setting of the Yunlu area indicate that the migmatites in the study area were primarily formed by anatexis without remarkable introduction of foreign components such as K, Na and Si and removal of Ca, Fe, Mg, etc.展开更多
There are some different opinions on the formation of striped migmatite. Many striped migmatites are distributed around the plutonic rocks in the core of hot dome in nature. This shows that the pressure of intrusive r...There are some different opinions on the formation of striped migmatite. Many striped migmatites are distributed around the plutonic rocks in the core of hot dome in nature. This shows that the pressure of intrusive rocks to wall rocks must play an important role in the formation of stripes. This paper, at first, discusses the effects of directional pressure (i. e. compressive stress from intrusive rock to wall rock) on particle migration and crystallization, and the physical mechanism of Ostwald maturation and its adjusting effect in crystalline growth. And then, on the basic principles of physics and chemistry, we lay down the stochastic evolutional rules of the formation of crystal nucleus, diffusion, crystallization and dissolution of leucosome (melt) particles; and,neglecting other restrictive conditions, we build an idealizedly simplified two-dimensional model of stochastic cellular automaton for the recrystallization of metamorphic rock under directional pressure, and realize it by writing a program of Windows on PC. Theoretical analyses and computer simulations show that, at the beginning, the free particles of leucosome are initially even distributed in the system and form many crystal nucleus ; then, the anisotropies of diffusion, crystallization and dissolution caused by the directional pressure together with the mechanisms of Ostwald maturation can make the nucleus successively grow to form lenses, unevenly disconnected stripes, and layer-stripe structure, which correspond to low, medium and high grades of metamorphism and migmatization, respectively; that is, the effect of the pressure of intrusive rock to wall rock alone can form streaked structure in migmatite. Moreover, cellular automaton, adopted in this paper, is a kind of discrete and local grid dynamic model, and is extremely suitable for simulating the evolution of spatiotemporal structure of real systems and analyzing micro-mechanism creating complex macro-phenomenon. We believe that cellular automata will have broad applications in the geosciences which is full of complicated natural phenomena.展开更多
Higher Himalayan Crystalline（HHC） complex of the Sikkim Himalaya predominantly consists of high-grade pelitic migmatites.In this study,reaction textures,mineral/bulk rare earth elements （REE）,trace element partiti...Higher Himalayan Crystalline（HHC） complex of the Sikkim Himalaya predominantly consists of high-grade pelitic migmatites.In this study,reaction textures,mineral/bulk rare earth elements （REE）,trace element partition coefficients and trace element zoning profiles in garnet are used to demonstrate a complex petrogenetic process during crustal anatexis.With the help of equilibrium REE and trace element partitioning model,it is shown that strong enrichment of Effective Bulk Composition（EBC） is responsible for the zoning in garnet in these rocks.The data strongly support disequilibrium element partitioning and suggest that the anatectic melts associated with mafic selvedges are likely produced by disequilibrium melting because of fast melt segregation process.展开更多
Seismic anisotropy originating within the continental crust is commonly used to determine the deformation and kinematic flow within active orogens and is attributed to regionally oriented mica or hornblende grains.How...Seismic anisotropy originating within the continental crust is commonly used to determine the deformation and kinematic flow within active orogens and is attributed to regionally oriented mica or hornblende grains.However,naturally deformed rocks usually contain compositional layers(e.g.,parallel compositional banding).It is necessary to understand how both varying mineral contents and differing intensities of compositional layering influence the seismic properties of the deep crust.In this study,we analyzed the seismic response of migmatitic amphibolite with compositional banding structures.We present the microstructures,fabrics,calculated seismic velocities,and seismic anisotropies of mylonitic amphibolite from a horizontal shear layer preserved within the Ailao Shan-Red River shear zone,southwestern Yunnan,China.The investigated sample is characterized by pronounced centimeter-scale compositional banding.The microstructures and fabrics suggest that migmatitic amphibolite rocks within deep crust may delineate regions of deformation-assisted,channelized,reactive,porous melt flow.The origin of compositional banding in the studied migmatitic amphibolite is attributed primarily to partial melting together with some horizontal shearing deformation.The microfabrics and structures investigated in this study are considered to be typical for the base of active horizontal shear layers in the deep crust of southeastern Xizang.Seismic responses are modeled by using crystal preferred orientations for minerals of the migmatitic amphibolite by applying the Voigt-Reuss-Hill homogenization method.Calculated P-wave and S-wave velocities are largely consistent in the various layers of the migmatite.However,seismic anisotropies of P-wave(AV_(p))and S-wave(AVs)are higher in the melanosomes(AV_(p)=5.6%,AV_(s)=6.83%)than those in the leucosomes and the whole rock(AV_(p)=4.2%–4.6%,AV_(s)=3.1%–3.2%).In addition,there is pronounced,S-wave splitting oblique to the foliation plane in the migmatitic amphibolite.The multiple parallel compositional layers generate marked variation in the geometry of the seismic anisotropy(Vs1 polarization)in the whole rock.Combined with the macroscale geographical orientation of fabrics in the Ailao Shan-Red River shear zone,these compositional banding effects are inferred to generate significant variations in the magnitude and orientation of seismic anisotropy,especially for shear-wave anisotropy(AV_(s))in the deep crust.Hence,our data suggest that layering of various origins(e.g.,shear layers,partial-melting layers,and compositional layers)represents a new potential source of anisotropy within the deep crust.展开更多
Based on the geological and geochemical information about migmatites, the followinglines of evidence have been proposed for discriminating the anatectic leucosome in the DabieshanComplex: (1) its width is larger than ...Based on the geological and geochemical information about migmatites, the followinglines of evidence have been proposed for discriminating the anatectic leucosome in the DabieshanComplex: (1) its width is larger than that derived from the subsolidus genesis, cutting acrossregional foliation, thus giving rise to complicated folds and wider selvages; (2) it is composedof melanic and accessory minerals in addition to quartz and feldspars; (3) the significant difference in anorthites of plagioclase between paleosome and leucosome; (4) temperature and pressure (P/T) conditions revealed by the mineral compositions and assemblages are over those forthe onset of anatexis; and (5) it is enriched in the major elements (e. g. AiZO3, FeZO3 andTiO2) and immobile and incompatible elements (e. g. LREE, Th, Hf and Zr). Finally, bycombining the geological and geochemical features with the statistical data for the spatial distribution of minerals and mass-balance in the migmatites, it is concluded that anatexis is the cardinal mechanism of migmatization in the Dabieshan Complex.展开更多
The regional migmatites in the Yunkai Block were formed under low\|pressure metamorphism. The majority of their protolith are biotite\|rich peraluminous gneisses. Detailed field observations, and studies of petrology,...The regional migmatites in the Yunkai Block were formed under low\|pressure metamorphism. The majority of their protolith are biotite\|rich peraluminous gneisses. Detailed field observations, and studies of petrology, spatial distribution of minerals and geochemistry suggest that the leucosomes were derived from anatexis. The single grain zircon U\|Pb dating data indicate that a pulse of migmatization occurred at 394～449 Ma and may have resulted from the large\|scale Caledonian magmatism in the Yunkai Block.展开更多
The petrographic characteristics of Jingshan "migmatitic granite" and the occurrence of the magmatic zircons indicate that the granite was formed by normal crystallization of felsic melts. All zircons in the...The petrographic characteristics of Jingshan "migmatitic granite" and the occurrence of the magmatic zircons indicate that the granite was formed by normal crystallization of felsic melts. All zircons in the granite have inherited cores and fine-scale oscillatory zoning rims of magmatic origin. It is realized that the granite was formed at 160.2±1.3 Ma through dating magmatic zircons. The generation of the granitic magma could be related to the lithospheric mantle and/or lower crust delamination after the ultrahigh pressure metamorphism (UHPM) in Triassic. Most inherited zircons yield the ages of 217.1±6.6 Ma, which is consistent with the peak UHPM in the Dabie-Sulu orogenic belt. Some of the inherited zircons (433-722 Ma) constitute a discordia line with the upper intercept age of 850+85/-68 Ma and a lower intercept age of 261+100/-140 Ma. These ages imply that the granite could be derived from the partial melting of the crustal materials of the South China Block that was intensively superimposed by the UHPM. The UHPM could be the reason for the major Pb loss at ±220 Ma.展开更多
Extensive migmatization and large-scale post-collisional intrusions occurred in the Dabie orogen, east-central China,during the Early Cretaceous, characterized by distinct deformation preserved in migmatites in the No...Extensive migmatization and large-scale post-collisional intrusions occurred in the Dabie orogen, east-central China,during the Early Cretaceous, characterized by distinct deformation preserved in migmatites in the North Dabie Complex unit.The North Dabie Complex unit can be subdivided into three areas based on detailed field observations： the north of the Tiantangzhai pluton, the Luotian area and the Yuexi area. Banded migmatites crop out in the north of the Tiantangzhai pluton while anisotropically deformed migmatites occur in the Luotian area, and both types coexist in the Yuexi area. Microscopy reveals similar micro-structures are in migmatites from the north of the Tiantangzhai pluton, the Yuexi area and border of the Luotian area, while static recrystallization appears in migmatites from the core of the Luotian area. The Lattice-Preferred Orientation of dynamically recrystallized quartz grains in the migmatites are measured using electron backscattered diffraction,revealing prism slip or slip in migmatite from the north of the Tiantangzhai pluton and the Yuexi area and in one sample from the Luotian area. A Type I crossed girdle is developed in another sample from the Luotian area, indicating top-to-the-SE shearing that developed under greenschist facies conditions. Zircon U-Pb dates from four migmatites reveal that mainly Early Cretaceous ages are from the north of the Tiantangzhai pluton, only four Early Cretaceous ages are from the Luotian area, and all zircons from the Yuexi area record Indosinian metamorphic ages. Although zircon U-Pb results show multiple migmatization events, all samples record a migmatization age of about 132 Ma, suggesting an orogen-scale event at that time. The subsolidus deformation in migmatites indicates that deformation were soon after migmatization. Overall, this study shows that deformation in migmatites of the North Dabie Complex unit occurred somewhat later than the migmatization（132 Ma） at about 131 Ma. The most likely mechanism for thinning of the thickened crust in the Dabie orogen involved removal of the upper along the detachment fault and ductile adjustment of lower crust during development of the North Dabie metamorphic core complex.展开更多
Ⅰ. GEOLOGICAL BACKGROUND Covering an area of 1200 km^2, the Wugongshan migmatite-granite batholith is located on the northwestern margin of South China Caledonian folded region and the southern flank of Pingxiang-Gua...Ⅰ. GEOLOGICAL BACKGROUND Covering an area of 1200 km^2, the Wugongshan migmatite-granite batholith is located on the northwestern margin of South China Caledonian folded region and the southern flank of Pingxiang-Guangfeng deep fault. It is characterized by the trinity of meta-sediments,展开更多
文摘Allanite and REE bearing minerals are primum fama report from migmatite gneisses,plutonic Amet granite and intrusive alkaline rocks of Rajasthan,and these minerals are important for establishing REE deposit.Allanite taken from host migmatite and granite have been analysed along with host rocks containing REE minerals,both of which contain appreciably high amount ofΣREE and Th.Allanite phases identified are CaO and Ce2O3 rich with positive correlation of silica and alumina.As silica proportion decreases andΣREE increases,allanite changes to britholite and thorite indicating a role of fluid in its crystallization.The chondrite normalized REE in migmatite gneiss and allanite mineral is compatible indicating a primary nature of allanite.The carrier of REE minerals is envisaged as the alkaline intrusives which deposited their REE minerals during ascent of magma from deeper source.Chemico-mineralogical study of Allanite and its hosts suggest possible reaction between a REE bearing fluid rich in P and F brought up by alkaline rocks,and the silicate minerals,mainly biotite and hornblende of the migmatite gneiss and Amet granite.The allanite in migmatite gneiss and granite may prove to be a sumptuous deposit of REE minerals in India,and similar terrains elsewhere.
基金supported by the National Basic Research Program of China (No. 2015CB856101)the National Natural Science Foundation of China (Nos. 41372076 and 41572039)the Natural Science Foundation of Hubei Province (No. 2015CFB190)
文摘The plate affiliation of the North Dabie terrane （NDT） has been controversial. To address this fundamental question, an integrated study of internal structure, in-situ U-Pb dating and trace element analysis in zircons and field investigation for migmatite in the NDT was carried out, which reveals par- ticipation of crustal rocks of the North China Craton （NCC） in the protolith in addition to the more common crustal rocks of the Yangtze Craton （YC）. The evidence of an NCC affinity for protolith of migmatite in the NDT is the -2.5 Ga （2 486±14 and 2 406±26 Ma） magamtic age and -1.8 Ga （1 717±79 Ma） metamorphic age in the relict zircon domains because these two age groups are characteristic for the evolution of the NCC. The evidence of a YC affinity for protolith of migmatite in the NDT is the more common 0.7-0.8 Ga （e.g., 787±12 Ma） magamtic zircon age. Mid-Neoproterozoic magmatic age （0.7-0.8 Ga） is a symbol of YC basement rocks. In view of the widely exposed YC crustal components in the NDT, we suggest that the protolith of the NDT is mainly crustal rocks from the YC with minor crustal components from the NCC. The zircon rim domains and new growth grains from all the migmatite sam- ples are characterized by anatectic zircons and have a widely concordant ages ranging from 112.2±2.8 to 159.6±4.3 Ma with several peak values, suggesting a long lasting multistage anatexis. In conclusion, the NDT has a mixed protolith origin of both the YC and the NCC crustal rocks were strongly remoulded by anatexis during orogenic collapse.
文摘The Higher Himalayan Crystallines(HHC), in western Garhwal, Uttarakhand are located in a regionalscale intracontinental ductile shear zone(15-20 km wide) bounded by the Main Central Thrust at the base, and the South Tibetan Detachment System at the top. The migmatite zone in the centre has the highest grade of metamorphism in the NW Himalayas and show evidence of flowage. Zircons extracted from samples of metasediment, migmatite, biotite granite and in situ partial melt(tourmaline-bearing leucogranite) along the Bhagirathi Valley, preserve U-Pb isotopic evidence of magmatic history, magma source and effects of the Himalayan orogeny in the region. Three distinct periods of zircon growth in the leucogranite record the episodic influx of magma between 46 Ma and 20 Ma indicating a time span of more than 25 Ma between the onset of fluid-fluxed partial melting in the mid-crustal intracontinental shear zone and the emplacement of the magma into the upper crust in a post-collisional extensional setting. Metamorphic zircon growth was initiated about 46 Ma, when the partial melts were generated as the migmatite zone was exhumed.
基金supported by the National Key Basic Research Program of China (No.2009CB825001)the National Natural Science Foundation of China (Nos.40603002,41072046,and 41090371)the Fundamental Research Funds for the Central Universities,China University of Geosciences,Wuhan (No.CUG120121)
文摘In order to constrain temperature during subduction and subsequent exhumation of fel- sic continental crust, we carried out a Ti-in-zircon thermometer coupled with zircon internal structure and U-Pb age on migmatitic gneisses from the Weihai region in the Sulu ultra-high pres- sure （UHP） metamorphic terrane, eastern China. The Weihai migmatitic gneisses are composed of in- tercalated compositional layers of melanosome and plagioclase （Pl）-rich lencosome and K-feldspar （Kfs）-rich pegmatite veins. Four stages of zircon growth were recognized in the Weihai migmatitic gneisses. They successively recorded informations of protolith, prograde metamorphism, decompres- sional partial melting during early stage exhumation and subsequent fractional crystallization of pri- mary melt during later stage cooling exhumation. The inherited cores in zircon from the melanosome and the Pl-rich leucosome suggest that the pro- tolith of the migmatitic gneiss is Mid- Neoproterozoic （-780 Ma） magmatic rock. Metamorphic zircons with concordant ages ranging from 243 to 256 Ma occur as over- growth mantles on the protolith magmatic zir- con cores. The estimated growth temperatures （625-717 ＂C） of the metamorphic zircons have a negative correlation with their ages, indicating a progressive metamorphism in HP eciogite-facies condition during subduction. Zircon recrystal- lized rims （228-2 Ma） in the PI-rich ieucosome layers provide the lower limit of the decompress-sional partial melting time during exhumation. The ages from 228^-2 to 219~2 Ma recorded in the Pl-rich leucosome and the Kfs-rich pegmatite vein, respectively, suggest the duration of the fractional crystallization of primary melt during exhumation. The calculated growth temperatures of the zircon rims from the Pl-rich leucosome range from 858 to 739 , and the temperatures of new growth zircon grains （219±2 Ma） in Kfs-rich vein are between 769 and 529 . The estimated temperatures have a positive correlation with ages from the Pl-rich leucosome to the Kfs-rich pegmatite vein, strongly indi- cating that a process of fractional crystallization of the partial melt during exhumation.
基金Funding for analyses and fieldwork was provided through Australian Research Council(ARC)Discovery and DECRA projects DP0879330 and DE1201030(to CC)Future Fellowship Scheme#FT120100340(to ASC)+1 种基金the Australia-India Strategic Research Fund project#ST030046(to CC and ASC)support from Curtin University Strategic Research Funding
文摘Data from a migmatised metapelite raft enclosed within charnockite provide quantitative constraints on the pressure-temperature-time （P-T-t） evolution of the Nagercoil Block at the southernmost tip of peninsular India. An inferred peak metamorphic assemblage of garnet, K-feldspar, sillimanite, plagioclase, magnetite, ilmenite, spinel and melt is consistent with peak metamorphic pressures of 6-8 kbar and temperatures in excess of 900 ℃. Subsequent growth of cordierite and biotite record high-temperature retrograde decompression to around 5 kbar and 800 ℃. SHRIMP U-Pb dating of magmatic zircon cores suggests that the sedimentary protoliths were in part derived from felsic igneous rocks with Palae- oproterozoic crystallisation ages. New growth of metamorphic zircon on the rims of detrital grains constrains the onset of melt crystallisation, and the minimum age of the metamorphic peak, to around 560 Ma. The data suggest two stages of monazite growth. The first generation of REE-enriched monazite grew during partial melting along the prograde path at around 570 Ma via the incongruent breakdown of apatite. Relatively REE-depleted rims, which have a pronounced negative europium anomaly, grew during melt crystallisation along the retrograde path at around 535 Ma. Our data show the rocks remained at suprasolidus temperatures for at least 35 million years and probably much longer, supporting a long-lived high-grade metamorphic history. The metamorphic conditions, timing and duration of the implied clockwise P-T-t path are similar to that previously established for other regions in peninsular India during the Ediacaran to Cambrian assembly of that part of the Gondwanan supercontinent.
基金supported by the National Natural Science Foundation of China (No. 41672060)the Undergraduate Teaching Projects of China University of Geosciences (Nos. ZL201610 and 2018G36)
文摘Migmatite-like rocks transformed from strongly metamorphosed and deformed enclave- bearing felsic plutons usually make people confuse with the true migmatites and mistake in interpreta- tion of their petrogenesis and tectonic implications. Here we report a suite of rocks that have long been called as migmatites from the Guandi complex in Zhoukoudian region, southwest of Beijing. The rocks are dominated by felsic gneisses with garnet-free amphibolites. Field occurrence, petrography and geochemistry indicate that the felsic gneisses and amphibolites were metamorphosed from protoliths of intermediate-acid and basic igneous rocks, respectively. New LA-ICP-MS zircon U-Pb dating and geothermobarometry study further reveal that precursor magmas of the two types of rocks were emplaced at 2.54-2.56 Ga and the rocks subsequently underwent medium P/T-type metamorphism with upper amphibolite facies conditions of 0.55-0.90 GPa and 670-730℃ at -2.48-2.50 Ga. Geochemically, precursor magmas of the amphibolites were suggested to be derived from an enriched lithospheric mantle source in continental arc setting, and those of the felsic gneisses are characterized by tonalitic to trondhjemitic magmas that are usually considered to be generated by partial melting of hydrated, thickened metamorphosed mafic crust with garnet as residues, suggesting that the rock associations are not of true migmatites but migmatite-like rocks. Our study reveal that protoliths of the migmatite-like rocks from the Guandi complex, were likely formed via magmatism in a continental arc setting, followed by accretion and collision of the continental arc as well as the intro-oceanic arc terranes to the Eastern Block of the North China Craton in the transition from the Late Neoarchean to Early Paleoprnterozuic.
基金supported by the China Nuclear Industry Geological Bureau Foundation (No.201637 and 201638)
文摘1 Introduction The Kangdian axis is an important polymetallic metallogenic belt in Southwest China,and it is also an area with a lot of hydrothermal uranium mineralization(Wang Hongjun,et al.,2009.The basic
基金support of the National Natural Science Foundation of China (Grant Nos.51379007,41130742)the support of the Chinese Fundamental Research (973)Program through the Grant No.2013CB036006
文摘This paper presents a case study of water inrush and mud burst occurring in a migmatite tunnel to study its formation mechanisms. The geological investigation and mineralogical analysis showed that water inrush and mud burst in the migmatite was closely related to the component of the host rock. High content of soluble minerals,e.g.,calcite and dolomite,would make the migmatite rock prone to be fragmentized,isintegrated and eventually form different sorts of connected or semi-connected veins. The field exploration revealed most cavities in the magmatite tunnel were eroded by groundwater and formed large interconnected networks. The two faults and the dike in the magmatite tunnel became the preferred paths and provided great convenience for plenty of precipitation and mud slurry. Due to high water pressure and blast disturbance,the cavities can soon connect each other as well as all sorts of veins,forming a complex ground channel for water inrush and mud burst. To estimate the potential occurrenceof water inrush and mud burst,the water bursting coefficient was employed. The results showed the water bursting coefficient of the magmatite tunnel was much bigger than the threshold values and it can be used to explain the accident of water inrush and mud burst occurring in the magmatite tunnel.
文摘Partially migmatized rocks, banded migmatites, augen-banded migmatites and gneissic migmatites are developed successively from Xindong to Yunlu, Gaozhou, Guangdong Province in the Yunkai Caledonian orogenic belt at the border between Guangdong and Guangxi. Mass-balance calculations, statistical analysis of the textural relations and mineralogical and geochemical studies of the migmatites and the study of the metamorphlc setting of the Yunlu area indicate that the migmatites in the study area were primarily formed by anatexis without remarkable introduction of foreign components such as K, Na and Si and removal of Ca, Fe, Mg, etc.
文摘There are some different opinions on the formation of striped migmatite. Many striped migmatites are distributed around the plutonic rocks in the core of hot dome in nature. This shows that the pressure of intrusive rocks to wall rocks must play an important role in the formation of stripes. This paper, at first, discusses the effects of directional pressure (i. e. compressive stress from intrusive rock to wall rock) on particle migration and crystallization, and the physical mechanism of Ostwald maturation and its adjusting effect in crystalline growth. And then, on the basic principles of physics and chemistry, we lay down the stochastic evolutional rules of the formation of crystal nucleus, diffusion, crystallization and dissolution of leucosome (melt) particles; and,neglecting other restrictive conditions, we build an idealizedly simplified two-dimensional model of stochastic cellular automaton for the recrystallization of metamorphic rock under directional pressure, and realize it by writing a program of Windows on PC. Theoretical analyses and computer simulations show that, at the beginning, the free particles of leucosome are initially even distributed in the system and form many crystal nucleus ; then, the anisotropies of diffusion, crystallization and dissolution caused by the directional pressure together with the mechanisms of Ostwald maturation can make the nucleus successively grow to form lenses, unevenly disconnected stripes, and layer-stripe structure, which correspond to low, medium and high grades of metamorphism and migmatization, respectively; that is, the effect of the pressure of intrusive rock to wall rock alone can form streaked structure in migmatite. Moreover, cellular automaton, adopted in this paper, is a kind of discrete and local grid dynamic model, and is extremely suitable for simulating the evolution of spatiotemporal structure of real systems and analyzing micro-mechanism creating complex macro-phenomenon. We believe that cellular automata will have broad applications in the geosciences which is full of complicated natural phenomena.
文摘Higher Himalayan Crystalline（HHC） complex of the Sikkim Himalaya predominantly consists of high-grade pelitic migmatites.In this study,reaction textures,mineral/bulk rare earth elements （REE）,trace element partition coefficients and trace element zoning profiles in garnet are used to demonstrate a complex petrogenetic process during crustal anatexis.With the help of equilibrium REE and trace element partitioning model,it is shown that strong enrichment of Effective Bulk Composition（EBC） is responsible for the zoning in garnet in these rocks.The data strongly support disequilibrium element partitioning and suggest that the anatectic melts associated with mafic selvedges are likely produced by disequilibrium melting because of fast melt segregation process.
基金supported by the National Natural Science Foundation of China(No.41772207)the Second Tibetan Plateau Scientific Expedition and Research Program(No.2019QZKK0703).
文摘Seismic anisotropy originating within the continental crust is commonly used to determine the deformation and kinematic flow within active orogens and is attributed to regionally oriented mica or hornblende grains.However,naturally deformed rocks usually contain compositional layers(e.g.,parallel compositional banding).It is necessary to understand how both varying mineral contents and differing intensities of compositional layering influence the seismic properties of the deep crust.In this study,we analyzed the seismic response of migmatitic amphibolite with compositional banding structures.We present the microstructures,fabrics,calculated seismic velocities,and seismic anisotropies of mylonitic amphibolite from a horizontal shear layer preserved within the Ailao Shan-Red River shear zone,southwestern Yunnan,China.The investigated sample is characterized by pronounced centimeter-scale compositional banding.The microstructures and fabrics suggest that migmatitic amphibolite rocks within deep crust may delineate regions of deformation-assisted,channelized,reactive,porous melt flow.The origin of compositional banding in the studied migmatitic amphibolite is attributed primarily to partial melting together with some horizontal shearing deformation.The microfabrics and structures investigated in this study are considered to be typical for the base of active horizontal shear layers in the deep crust of southeastern Xizang.Seismic responses are modeled by using crystal preferred orientations for minerals of the migmatitic amphibolite by applying the Voigt-Reuss-Hill homogenization method.Calculated P-wave and S-wave velocities are largely consistent in the various layers of the migmatite.However,seismic anisotropies of P-wave(AV_(p))and S-wave(AVs)are higher in the melanosomes(AV_(p)=5.6%,AV_(s)=6.83%)than those in the leucosomes and the whole rock(AV_(p)=4.2%–4.6%,AV_(s)=3.1%–3.2%).In addition,there is pronounced,S-wave splitting oblique to the foliation plane in the migmatitic amphibolite.The multiple parallel compositional layers generate marked variation in the geometry of the seismic anisotropy(Vs1 polarization)in the whole rock.Combined with the macroscale geographical orientation of fabrics in the Ailao Shan-Red River shear zone,these compositional banding effects are inferred to generate significant variations in the magnitude and orientation of seismic anisotropy,especially for shear-wave anisotropy(AV_(s))in the deep crust.Hence,our data suggest that layering of various origins(e.g.,shear layers,partial-melting layers,and compositional layers)represents a new potential source of anisotropy within the deep crust.
文摘Based on the geological and geochemical information about migmatites, the followinglines of evidence have been proposed for discriminating the anatectic leucosome in the DabieshanComplex: (1) its width is larger than that derived from the subsolidus genesis, cutting acrossregional foliation, thus giving rise to complicated folds and wider selvages; (2) it is composedof melanic and accessory minerals in addition to quartz and feldspars; (3) the significant difference in anorthites of plagioclase between paleosome and leucosome; (4) temperature and pressure (P/T) conditions revealed by the mineral compositions and assemblages are over those forthe onset of anatexis; and (5) it is enriched in the major elements (e. g. AiZO3, FeZO3 andTiO2) and immobile and incompatible elements (e. g. LREE, Th, Hf and Zr). Finally, bycombining the geological and geochemical features with the statistical data for the spatial distribution of minerals and mass-balance in the migmatites, it is concluded that anatexis is the cardinal mechanism of migmatization in the Dabieshan Complex.
文摘The regional migmatites in the Yunkai Block were formed under low\|pressure metamorphism. The majority of their protolith are biotite\|rich peraluminous gneisses. Detailed field observations, and studies of petrology, spatial distribution of minerals and geochemistry suggest that the leucosomes were derived from anatexis. The single grain zircon U\|Pb dating data indicate that a pulse of migmatization occurred at 394～449 Ma and may have resulted from the large\|scale Caledonian magmatism in the Yunkai Block.
基金supported by MST(Grant No.TG1999075502)the National Natural Science Foundation of China(Grant No.40172030)Jilin University(Grant No.2002CX004).
文摘The petrographic characteristics of Jingshan "migmatitic granite" and the occurrence of the magmatic zircons indicate that the granite was formed by normal crystallization of felsic melts. All zircons in the granite have inherited cores and fine-scale oscillatory zoning rims of magmatic origin. It is realized that the granite was formed at 160.2±1.3 Ma through dating magmatic zircons. The generation of the granitic magma could be related to the lithospheric mantle and/or lower crust delamination after the ultrahigh pressure metamorphism (UHPM) in Triassic. Most inherited zircons yield the ages of 217.1±6.6 Ma, which is consistent with the peak UHPM in the Dabie-Sulu orogenic belt. Some of the inherited zircons (433-722 Ma) constitute a discordia line with the upper intercept age of 850+85/-68 Ma and a lower intercept age of 261+100/-140 Ma. These ages imply that the granite could be derived from the partial melting of the crustal materials of the South China Block that was intensively superimposed by the UHPM. The UHPM could be the reason for the major Pb loss at ±220 Ma.
基金supported by the National Natural Science Foundation of China (Grant Nos. 41572186, 41541045)
文摘Extensive migmatization and large-scale post-collisional intrusions occurred in the Dabie orogen, east-central China,during the Early Cretaceous, characterized by distinct deformation preserved in migmatites in the North Dabie Complex unit.The North Dabie Complex unit can be subdivided into three areas based on detailed field observations： the north of the Tiantangzhai pluton, the Luotian area and the Yuexi area. Banded migmatites crop out in the north of the Tiantangzhai pluton while anisotropically deformed migmatites occur in the Luotian area, and both types coexist in the Yuexi area. Microscopy reveals similar micro-structures are in migmatites from the north of the Tiantangzhai pluton, the Yuexi area and border of the Luotian area, while static recrystallization appears in migmatites from the core of the Luotian area. The Lattice-Preferred Orientation of dynamically recrystallized quartz grains in the migmatites are measured using electron backscattered diffraction,revealing prism slip or slip in migmatite from the north of the Tiantangzhai pluton and the Yuexi area and in one sample from the Luotian area. A Type I crossed girdle is developed in another sample from the Luotian area, indicating top-to-the-SE shearing that developed under greenschist facies conditions. Zircon U-Pb dates from four migmatites reveal that mainly Early Cretaceous ages are from the north of the Tiantangzhai pluton, only four Early Cretaceous ages are from the Luotian area, and all zircons from the Yuexi area record Indosinian metamorphic ages. Although zircon U-Pb results show multiple migmatization events, all samples record a migmatization age of about 132 Ma, suggesting an orogen-scale event at that time. The subsolidus deformation in migmatites indicates that deformation were soon after migmatization. Overall, this study shows that deformation in migmatites of the North Dabie Complex unit occurred somewhat later than the migmatization（132 Ma） at about 131 Ma. The most likely mechanism for thinning of the thickened crust in the Dabie orogen involved removal of the upper along the detachment fault and ductile adjustment of lower crust during development of the North Dabie metamorphic core complex.
基金Project supported by the National Natural Science Foundation of China
文摘Ⅰ. GEOLOGICAL BACKGROUND Covering an area of 1200 km^2, the Wugongshan migmatite-granite batholith is located on the northwestern margin of South China Caledonian folded region and the southern flank of Pingxiang-Guangfeng deep fault. It is characterized by the trinity of meta-sediments,
