Fault on–off versus coseismic fluids reaction

The fault activation （fault on） interrupts the enduring fault locking （fault off） and marks the end of a seismic cycle in which the brittle-ductile transition （BDT） acts as a sort of switch. We suggest that the fluid flow rates differ during the different periods of the seismic cycle （interseismic, pre-seismic, coseismic and post-seismic） and in particular as a function of the tectonic style. Regional examples indicate that tectonic-related fluids anomalies depend on the stage of the tectonic cycle and the tectonic style. Although it is difficult to model an increasing permeability with depth and several BDT transitions plus independent acquicludes may occur in the crust, we devised the simplest numerical model of a fault constantly shearing in the ductile deeper crust while being locked in the brittle shallow layer, with variable homogeneous permeabilities. The results indicate different behaviors in the three main tectonic settings. In tensional tectonics, a stretched band antithetic to the normal fault forms above the BDT during the interseismic period. Fractures close and fluids are expellecl during the coseismic stage. The mechanism reverses in compressional tectonics. During the interseismic stage, an over-compressed band forms above the BDT. The band dilates while rebounding in the coseismic stage and attracts fluids locally. At the tip lines along strike-slip faults, two couples of subvertical bancls show different behavior, one in dilationJcompression and one in compressionJdilation. This deformation pattern inverts during the coseismic stage. Sometimes a pre-seismic stage in which fluids start moving may be observed and could potentially become a precursor.     

http://www.sciencedirect.com/science/article/pii/S1674987110000393

Wide-open V-shaped conjugate strike-slip faults in Asia are typically related to extrusion tectonics. However, the tectonic model based on the slip-line theory of plasticity has some critical problems associated with it. The conjugate sets of slip-lines in plane deformation, according to the theory of plasticity should be normal to each another but, in reality, the angles between the conjugate strike-slip faults, which are regarded as slip-lines in extrusion tectonics in the eastern Mediterranean, Tibet-middle Asia, China and the Indochina Peninsular regions, are always more than 90° (on average 110°) in the direction of contraction. Another problem is that the slip-line theory fails to explain how, in some cases, e.g., in the Anatolian area in the eastern Mediterranean, the extrusion rate is much higher than the indent rate. The two major problems are easy to solve in terms of the Maximum-Effective-Moment (MEM) Criterion that predicts that orientations of the shear zones are theoretically at an angle of 54.7° and practically at angles of 55°±10° with the σ1- or contractional direction. The orientations of the strike-slip faults that accommodate extrusion tectonics are, therefore, fundamentally controlled by the MEM Criterion. When extrusion is along the MEM-orientations, the extruding rate is normally higher than the indenting rate.     

http://www.sciencedirect.com/science/article/pii/S1674987110000058

<正>The thermo-electric coefficients of twenty-six magnetite samples,formed either by magmatism or metamorphism,were tested by the thermo-electric instrument BHET—06.Results showed that the coefficient is of a constant value of about -0.05 mV/℃.It is emphasized that because every magnetite grain was tested randomly,the coefficient is independent of the crystallographic direction.This fact means the thermal voltage generated from a single magnetite crystal can be accumulated,and as a result a new thermo-electric field can arise when a gradient thermal field exists and is active within the earth's crust.Because magnetite is widespread in the earth's crust(generally appearing more in the middle-lower crust),there is more-than-random probability that the additional thermo-electric field can be generated when certain thermal conditions are fulfilled.We,therefore,used the thermo-electric effect of magnetite to study the mechanism responsible for the presence of abnormal geo-electric fields during earthquake formation and occurrence, because gradient thermal fields always exist before earthquakes.The possible presence of additional thermo-electric fields was calculated under theoretical seismological conditions,using the following calcu-lation formula:E= - 0.159(σ×△T×φ×ρ_2×[(h~2-2x~2)cosα+ 3hxsinα]/ρ_1(h~2 +x~2)~(5/2)).In the above formula,σis thermo-electric coefficient of magnetite,△T is the temperature difference acting on it,φis a sectional area on a block of magnetite vertically perpendicular to the direction of the thermal current.ρ_1 andρ_2 are the respective resistivities of magnetite and the crust,and h,α,and x,respectively,h is the depth of embedded magnetite block,αmeans the angle created by the horizontal line and ligature of the two poles of magnetite block,and x is the distance from observation point to projective center point of the magnetite block on earth surface.According to simulations calculated with this formula,additional thermo-electric field intensity may reach as high as n to n×10~2 mV/km.This field is strong enough to cause obvious anomalies in the background geo-electric field,and can be easy probed by earthquake monitoring equipment. Therefore,we hypothesize that geo-electric abnormalities which occur during earthquakes may be caused by the thermo-electric effect of magnetite.     

http://www.sciencedirect.com/science/article/pii/S1674987110000319

The 3-D geometry of the seismicity in Hindu Kush–Pamir–western China region has been defined by seismic records for 1975–1999 from the National Earthquake Information Center, the U.S. Geological Survey, and over 16,000 relocated earthquakes since 1975 recorded by the Xinjiang seismic network of China. The results show that most Ms ≥ 5.0 hypocenters in the area are confined to a major intracontinental seismic shear zone (MSSZ). The MSSZ, which dips southwards in Pamir has a north-dipping counterpart in the Hindu Kush to the west; the two tectonic realms are separated by the sinistral Chaman transform fault of the India–Asia collisional zone. We demonstrate that the MSSZ constitutes the upper boundary of a south-dipping, actively subducting Pamir continental plate. Three seismic concentrations are recognized just above the Pamir MSSZ at depths between 45–65 km, 95–120 km, and 180–220 km, suggesting different structural relationships where each occurs. Results from focal mechanism solutions in all three seismological concentrations show orientations of the principal maximum stress to be nearly horizontal in an NNW–SSE direction. The south-dipping Pamir subduction slab is wedge-shaped with a wide upper top and a narrow deeper bottom; the slab has a gentle angle of dip in the upper part and steeper dips in the lower part below an elbow depth of ca. 80–120 km. Most of the deformation related to the earthquakes occurs within the hanging wall of the subducting Pamir slab. Published geologic data and repeated GPS measurements in the Pamir document a broad supra-subduction, upper crustal zone of evolving antithetic (i.e. north-dipping) back-thrusts that contribute to north-south crustal shortening and are responsible for exhumation of some ultrahigh-pressure rocks formed during earlier Tethyan plate convergence. An alternating occurrence in activity of Pamir and Chaman seismic zones indicates that there is interaction between strike-slip movement of the Chaman transform fault system and deep-subduction of the Pamir earthquake zone. Pamir subduction-related seismicity becomes shallower in depth with increasing distance east of the transform fault. Therefore, sinistral movement of the Chaman transform fault appears to be influencing continental deep-subduction in the Pamir region and may provide an explanation for the unusual south-dipping geometry of the intracontinental Pamir plate.     

http://www.sciencedirect.com/science/article/pii/S1674987110000149

The structural stability of manganese titanate MnTiO3 at high pressure was investigated by X-ray diffraction and Raman spectroscopy with diamond anvil cells. Ilmenite-type MnTiO3 is stable at least to 26.6 GPa, and lithium niobate type MnTiO3 reversibly transforms at room temperature to perovskite at 2.0 GPa. Bulk moduli (K300) of ilmenite, lithium niobate and perovskite are 174(4) GPa, 179(8) GPa, and 208(5) GPa, respectively (at fixed first pressure derivative K′ = 4). The Grüneisen parameter γ has been estimated to be 1.28 for ilmenite and 1.75 for perovskite. In ilmenite phase, TiO6 octahedra become more regular with increasing pressure. In perovskite phase structural distortion increases with pressure increase.     

http://www.sciencedirect.com/science/article/pii/S1674987111000466

The Ordos Basin of North China is not only an important uranium mineralization province, but also a major producer of oil,gas and coal in China.The genetic relationship between uranium mineralization and hydrocarbons has been recognized by a number of previous studies,but it has not been well understood in terms of the hydrodynamics of basin fluid flow.We have demonstrated in a previous study that the preferential localization of Cretaceous uranium mineralization in the upper part of the Ordos Jurassic section may have been related to the interface between an upward flowing,reducing fluid and a downward flowing,oxidizing fluid.This interface may have been controlled by the interplay between fluid overpressure related to disequilibrium sediment compaction and which drove the upward flow,and topographic relief,which drove the downward flow.In this study,we carried out numerical modeling for the contribution of oil and gas generation to the development of fluid overpressure,in addition to sediment compaction and heating.Our results indicate that when hydrocarbon generation is taken into account,fluid overpressure during the Cretaceous was more than doubled in comparison with the simulation when hydrocarbon generation was not considered.Furthermore,fluid overpressure dissipation at the end of sedimentation slowed down relative to the no-hydrocarbon generation case.These results suggest that hydrocarbon generation may have played an important role in uranium mineralization,not only in providing reducing agents required for the mineralization,but also in contributing to the driving force to maintain the upward flow.     

http://www.sciencedirect.com/science/article/pii/S1674987112001065

It has been thought that granitic crust,having been formed on the surface,must have survived through the Earth’s evolution because of its buoyancy.At subduction zones continental crust is predominantly created by arc magmatism and is returned to the mantle via sediment subduction,subduction erosion, and continental subduction.Granitic rocks,the major constituent of the continental crust,are lighter than the mantle at depths shallower than 270 km,but we show here,based on first principles calculations, that beneath 270 km they have negative buoyancy compared to the surrounding material in the upper mantle and transition zone,and thus can be subducted in the depth range of 270-660 km.This suggests that there can be two reservoirs of granitic material in the Earth,one on the surface and the other at the base of the mantle transition zone(MTZ).The accumulated volume of subducted granitic material at the base of the MTZ might amount to about six times the present volume of the continental crust.Our calculations also show that the seismic velocities of granitic material in the depth range from 270 to 660 km are faster than those of the surrounding mantle.This could explain the anomalous seismic-wave velocities observed around 660 km depth.The observed seismic scatterers and reported splitting of the 660 km discontinuity could be due to jadeite dissociation,chemical discontinuities between granitic material and the surrounding mantle,or a combination thereof.     

http://www.sciencedirect.com/science/article/pii/S1674987110000034

<正>We synthesize significant recent results on the deep structure and origin of the active volcanoes in mainland China.Magmatism in the western Pacific arc and back-arc areas is caused by dehydration of the subducting slab and by corner flow in the mantle wedge,whereas the intraplate magmatism in China has different origins.The active volcanoes in Northeast China(such as the Changbai and Wuda-lianchi) are caused by hot upwelling in the big mantle wedge(BMW) above the stagnant slab in the mantle transition zone and deep slab dehydration as well.The Tengchong volcano in Southwest China is caused by a similar process in the BMW above the subducting Burma microplate(or Indian plate). The Hainan volcano in southernmost China is a hotspot fed by a lower-mantle plume which may be associated with the Pacific and Philippine Sea slabs' deep subduction in the east and the Indian slab's deep subduction in the west down to the lower mantle.The stagnant slab finally collapses down to the bottom of the mantle,which can trigger the upwelling of hot mantle materials from the lower mantle to the shallow mantle beneath the subducting slabs and may cause the slab—plume interactions.     

http://www.sciencedirect.com/science/article/pii/S1674987110000137

<正>This study attempts to acquire information on tectonic activity in western China from land surface temperature(LST) field data.On the basis of the established relationship between heat and strain,we analyzed the LST distribution in western China using the satellite data product MODIS/Terra.Our results show that:1. There are departures from annual changes of LST in some areas,and that these changes are associated with the activity of some active tectonic zones.2.When annual-change background values caused by climate factors are removed,the long-period component(LST_(LOW)) of temperature residual(△T) of the LST is able to serve as an indicator for tectonic activity.We have found that a major earthquake can produce different effects on the LST fields of surrounding areas.These effects are characterized by both rises and drops in temperature.For example, there was a noteworthy temperature decline associated with the Sumatran M9 earthquake of 2004 in the Bayan Har-Songpan block of central Tibetan Plateau.3.On the other hand,the LST field of a single area may respond differently to major shocks occurring in different areas in the regions surrounding China.For instance,the Kunlun M 8.1 event made the LST on the Longmen Mountains fault zone increase,whereas the Zaisan Lake M 7.9 quake of 2003,and the Sumatran M9 event of 2004.caused decreases in the same area's LST.4.The variations of land surface temperature(LST) over time are different in different tectonic areas.These phenomena may provide clues for the study of tectonic deformation processes.On the basis of these phenomena,we use a combination of temperature data obtained at varied depths,regional seismicity and strain results obtained with GPS measurements,to test the information related to tectonic activity derived from variations of the LST field,and discuss its implications to the creation of models of regional tectonic deformation.     

http://www.sciencedirect.com/science/article/pii/S167498711000037X

The tried and tested multianvil apparatus has been widely used for high-pressure and high-temperature experimental studies in Earth science. As a result, many important results have been obtained for a better understanding of the components, structure and evolution of the Earth. Due to the strength limitation of materials, the attainable multianvil pressure is generally limited to about 30 GPa (corresponding to about 900 km of the depth in the Earth) when tungsten carbide cubes are adopted as second-stage anvils. Compared with tungsten carbide, the sintered diamond is a much harder material. The sintered diamond cubes were introduced as second-stage anvils in a 6–8 type multianvil apparatus in the 1980s, which largely enhanced the capacity of pressure generation in a large volume press. With the development of material synthesis and processing techniques, a large sintered diamond cube (14 mm) is now available. Recently, maximum attainable pressures reaching higher than 90 GPa (corresponding to about 2700 km of the depth in the Earth) have been generated at room temperature by adopting 14-mm sintered diamond anvils. Using this technique, a few researches have been carried out by the quenched method or combined with synchrotron radiation in situ observation. In this paper we review the properties of sintered diamond and the evolution of pressure generation using sintered diamond anvils. As-yet unsolved problems and perspectives for uses in Earth Science are also discussed.     

http://www.sciencedirect.com/science/article/pii/S1674987111001022

This study proposes three models to explain the mechanism of the three major types of mafic dyke swarms.Parallel dyke swarms form in response to a regional stress field,e.g.the mafic dyke swarms in the...     

http://www.sciencedirect.com/science/article/pii/S1674987114000371

Dissolution of fluorite(CaF_2) and/or fluorapatite(FAP)[Ca_5(PO_4)_3F],pulled by calcite precipitation,is thought to be the dominant mechanism responsible for groundwater fluoride(F~-) contamination.Here,one dimensional reactive—transport models are developed to test this mechanism using the published dissolution and precipitation rate kinetics for the mineral pair FAP and calcite.Simulation results correctly show positive correlation between the aqueous concentrations of F and CO_3~(2-) and negative correlation between F~- and Ca~(2+).Results also show that precipitation of calcite,contrary to the present understanding,slows down the FAP dissolution by 10~6 orders of magnitude compared to the FAP dissolution by hydrolysis.For appreciable amount of fluoride contamination rock-water interaction time must be long and of order 10~6 years.     

http://www.sciencedirect.com/science/article/pii/S1674987111000491

Based on the interpretations of three seismic profiles and one wide-angle seismic profile across the Northwest Sub-basin, South China Sea, stratigraphic sequences, deformation characteristics and an extension model for this sub-basin have been worked out. Three tectonic-stratigraphic units are determined. Detailed analyses of extension show that the event occurred mainly during the Paleogene and resulted in the formation of half-grabens or grabens distributed symmetrically around the spreading center. Sediments are characterized by chaotic and discontinuous reflectors, indicating clastic sediments. Farther to the southwest, the sub-basin features mainly continental rifting instead of sea-floor spreading. The rifting would have been controlled by the shape of the massif and developed just along the northern edge of the Zhongsha-Xisha Block, rather than joined the Xisha Trough. After 25 Ma, a southward ridge jump triggered the opening of the Southwest Sub-basin. The NW-directed stress caused by the sea-floor spreading of the Northwest Sub-basin may have prevented the continuous opening of the sub-basin. After that the Northwest Sub-basin experienced thermal cooling and exhibited broad subsidence. The deep crustal structure shown by the velocity model from a wide-angle seismic profile is also symmetrical around the spreading center, which indicates that the Northwest Sub-basin might have opened in a pure shear model.     

http://www.sciencedirect.com/science/article/pii/S1674987112000254

The applications of intelligent techniques have increased exponentially in recent days to study most of the non-linear parameters.In particular,the behavior of earth resembles the non-linearity applications.An efficient tool is needed for the interpretation of geophysical parameters to study the subsurface of the earth.Artificial Neural Networks(ANN) perform certain tasks if the structure of the network is modified accordingly for the purpose it has been used.The three most robust networks were taken and comparatively analyzed for their performance to choose the appropriate network.The single-layer feed-forward neural network with the back propagation algorithm is chosen as one of the well-suited networks after comparing the results.Initially,certain synthetic data sets of all three-layer curves have been taken for training the network,and the network is validated by the Held datasets collected from Tuticorin Coastal Region(78°7′30″E and 8°48′45″N),Tamil Nadu.India.The interpretation has been done successfully using the corresponding learning algorithm in the present study.With proper training of back propagation networks,it tends to give the resistivity and thickness of the subsurface layer model of the field resistivity data concerning the synthetic data trained earlier in the appropriate network.The network is trained with more Vertical Electrical Sounding(VES) data,and this trained network is demonstrated by the field data.Groundwater table depth also has been modeled.     

http://www.sciencedirect.com/science/article/pii/S167498711400067X

The Proterozoic Bamble Sector, South Norway, is one of the world＇s classic amphiboliteto granulite- facies transition zones. It is characterized by a well-developed isograd sequence, with isolated ＇granulite-facies islands＇ in the amphibolite-facies portion of the transition zone. The area is notable for the discovery of C02-dominated fluid inclusions in the granolite-facies rocks by Jacques Touter in the late 1960＇s, which triggered discussion of the role of carbonic fluids during granulite genesis. The aim of this review is to provide an overview of the current state of knowledge of the Bamble Sector, with an emphasis on the Arendal-Froland-Nelaug-Tvedestrand area and off shore islands （most prominantly Tromay and Hisoy） where the transition zone is best developed. After a brief overview of the history of geological research and mining in the area, aspects of sedimentary, metamorphic and magmatic petrology of the Bamble Sector are discussed, including the role of fluids. Issues relevant to current geotectonic models for SW Scandinavia, directly related to the Bamble Sector, are discussed at the end of the review.     

http://www.sciencedirect.com/science/article/pii/S167498711500002X

Discovery rates for all metals,including gold,are declining,the cost per significant discovery is increasing sharply,and the economic situation of the industry is one of low base rate.The current hierarchical structure of the exploration and mining industry makes this situation difficult to redress.Economic geologists can do little to influence the required changes to the overall structure and philosophy of an industry driven by business rather than geological principles.However,it should be possible to follow the lead of the oil industry and improve the success rate of greenfield exploration,necessary for the next group of lower-exploration-spend significant mineral deposit discoveries.Here we promote the concept that mineral explorers need to carefully consider the scale at which their exploration targets are viewed.It is necessary to carefully assess the potential of drill targets in terms of terrane to province to district scale,rather than deposit scale,where most current economic geology research and conceptual thinking is concentrated.If otogenic,IRGD,Carlin-style and 10 CG gold-rich systems are viewed at the deposit scale,they appear quite different in terms of conventionally adopted research parameters.However,recent models for these deposit styles show increasingly similar source-region parameters when viewed at the lithosphere scale,suggesting common tectonic settings.It is only by assessing individual targets in their tectonic context that they can be more reliably ranked in terms of potential to provide a significant drill discovery.Targets adjacent to craton margins,other lithosphere boundaries,and suture zones are clearly favoured for all of these gold deposit styles,and such exploration could lead to incidental discovery of major deposits of other metals sited along the same tectonic boundaries.     

http://www.sciencedirect.com/science/article/pii/S167498711100137X

This study presents promising variants of genetic programming (GP),namely linear genetic programming (LGP) and multi expression programming (MEP) to evaluate the liquefaction resistance of sandy soils....     

http://www.sciencedirect.com/science/article/pii/S1674987113000315

The Panzhihua gabbroic intrusion,part of the plumbing system of the Emeishan large igneous province, intruded late-Proterozoic dolomites and marls about～263 Ma ago.The dolomites in the contact aureole were converted to brucite marbles and a diverse suite of forsterite,diopside and garnet skarns.The variation in mineralogy is explained in part by differences in the composition of the protolith,particularly the proportion of silica minerals and clay,and in part by transfer of elements from intruding magmas.The trace element compositions of most marbles and skarns are very similar to those of unmetamorphosed dolomites and marls,but some contain high Si,Ti,and Fe contents that are interpreted to have come from a magmatic source.Three brucite marbles sampled～10 m from the contact of the intrusion and named "enriched brucite marble" have trace element compositions very different from their dolomitic protolith:their rare earth elements are strongly enriched whereas levels of Nb-Ta,Zr-Hf and Ti are very low.These characteristics resemble those of carbonate liquid in equilibrium with silicate liquid or more probably with silicate minerals in the case of Panzhihua,a similarity we take to indicate that the sample underwent partial melting.Samples taken up to 300 m from the contact contain brucite indicating that high temperatures persisted well into the country rocks.However,other samples collected only tens of metres from the contact are only slightly recrystallized indicating that conditions in the aureole were highly variable.We suggest that temperatures within the aureole were controlled by conduction of heat from the main intrusion and by supply of additional heat from abundant small dykes within the aureole.Circulation of fluids derived from deeper levels in the aureole flushed the carbon dioxide from the dolomite,lowering temperature needed to partially melt carbonate to the temperatures attained near the intrusion.Irregular but extensive heating destabilized the carbonates of the aureole and decarbonation reactions associated with carbonate breakdown and melting emitted a large volume of CO₂,with potential impact on global climate.     

The boring billion? – Lid tectonics, continental growth and environmental change associated with the Columbia supercontinent

The evolution of Earth＇s biosphere,atmosphere and hydrosphere is tied to the formation of continental crust and its subsequent movements on tectonic plates.The supercontinent cycle posits that the continental crust is periodically amalgamated into a single landmass,subsequently breaking up and dispersing into various continental fragments.Columbia is possibly the first true supercontinent,it amalgamated during the 2.0-1.7 Ga period,and collisional orogenesis resulting from its formation peaked at 1.95-1.85 Ga.Geological and palaeomagnetic evidence indicate that Columbia remained as a quasi-integral continental lid until at least 1.3 Ga.Numerous break-up attempts are evidenced by dyke swarms with a large temporal and spatial range; however,palaeomagnetic and geologic evidence suggest these attempts remained unsuccessful.Rather than dispersing into continental fragments,the Columbia supercontinent underwent only minor modifications to form the next supercontinent （Rodinia） at 1.1 -0.9 Ga; these included the transformation of external accretionary belts into the internal Grenville and equivalent collisional belts.Although Columbia provides evidence for a form of ‘lid tectonics’,modern style plate tectonics occurred on its periphery in the form of accretionary orogens.The detrital zircon and preserved geological record are compatible with an increase in the volume of continental crust during Columbia＇s lifespan; this is a consequence of the continuous accretionary processes along its margins.The quiescence in plate tectonic movements during Columbia＇s lifespan is correlative with a long period of stability in Earth＇s atmospheric and oceanic chemistry.Increased variability starting at 1.3 Ga in the environmental record coincides with the transformation of Columbia to Rodinia; thus,the link between plate tectonics and environmental change is strengthened with this interpretation of supercontinent history.     

http://www.sciencedirect.com/science/article/pii/S1674987112000643

Incipient charnockites represent granulite formation on a mesoscopic scale and have received considerable attention in understanding fluid processes in the deep crust.Here we report new petrological data from an incipient charnockite locality at Rajapalaiyam in the Madurai Block,southern India,and discuss the petrogenesis based on mineral phase equilibrium modeling and pseudosection analysis. Rajapalaiyam is a key locality in southern India from where diagnostic mineral assemblages for ultrahigh-temperature（UHT） metamorphism have been reported.Proximal to the UHT rocks are patches and lenses of charnockite（Kfs + Qtz + Pl + Bt + Opx + Grt + Ilm） occurring within Opx-free Grt-Bt gneiss（Kfs + Pl + Qtz + Bt + Grt + Ilm + Mt） which we report in this study.The application of mineral equilibrium modeling on the charnockitic assemblage in NCKFMASHTO system yields a p-T range of～820℃and～9 kbar.Modeling of the charnockite assemblage in the MnNCKFMASHTO system indicates a slight shift of the equilibrium condition toward lower p and T（～760℃and～7.5 kbar）. which is consistent with the results obtained from geothermobarometry（710—760℃,6.7—7.5 kbar）. but significantly lower than the peak temperatures（>1000℃） recorded from the UHT rocks in this locality,suggesting that charnockitization is a post-peak event.The modeling of T versus molar H₂O content in the rock（M（H₂O）） demonstrates that the Opx-bearing assemblage in charnockite and Opxfree assemblage in Grt-Bt gneiss are both stable at M（H₂O） = 0.3 mol%-0.6 mol%.and there is no significant difference in water activity between the two domains.Our finding is in contrast to the previous petrogenetic model of incipient charnockite formation which envisages lowering of water activity and stabilization of orthopyroxene through breakdown of biotite by dehydration caused by the infiltration of CO₂-rich fluid.T-X_Fe³⁺（= Fe₂O₃/（FeO + Fe₂O₃） in mole） pseudosections suggest that the oxidation condition of the rocks played a major role on the stability of orthopyroxene:Opx is stable at X_Fe³⁺ <0.03 in charnockite.while Opx-free assemblage in Grt-Bt gneiss is stabilized at X_Fe³⁺ >0.12.Such low oxygen fugacity conditions of X_Fe³⁺ <0.03 in the charnockite compared to Grt-Bt gneiss might be related to the infiltration of a reduced fluid（e.g.,H₂O + CH₄） during the retrograde stage.