The enhanced element enrichment in the supercritical states of granite–pegmatite systems

In this paper, we show that supercritical fluids have a greater significance in the generation of pegmatites,and for ore-forming processes related to granites than is usually assumed. We show that the supercritical melt or fluid is a silicate phase in which volatiles; principally H_2O are completely miscible in all proportions at magmatic temperatures and pressures. This phase evolves from felsic melts and changes into hydrothermal fluids, and its unique properties are particularly important in sequestering and concentrating low abundance elements, such as metals. In our past research, we have focused on processes observed at upper crustal levels, however extensive work by us and other researchers have demonstrated that supercritical melt/fluids should be abundant in melting zones at deep-crustal levels too. We propose that these fluids may provide a connecting link between lower and upper crustal magmas,and a highly efficient transport mechanism for usually melt incompatible elements. In this paper, we explore the unique features of this fluid which allow the partitioning of variouselements and compounds, potentially up to extreme levels,and may explain various features both of mineralization and the magmas that produced them.     

Theoretical models and experimental determination methods for equations of state of silicate melts: A review

Silicate melts are very active in the interior of the Earth and other terrestrial planets, and are important carriers for the transport of material and energy. The determination of the equation of state(EOS) for silicate melts and the acquisition of a precise quantitative relationship between molar volume(or density) and temperature, pressure, and composition is essential for simulating the generation, migration, and eruption processes of magmas and the evolution of the magma ocean stage during the early formation of the Earth and other terrestrial planets, for calculating and modeling the phase equilibria involving silicate melts, and for revealing the variation of the microstructure of silicate melts with pressure. However, it is experimentally challenging to determine the volumetric properties of silicate melts and the accumulated density data at high pressure are still very limited due to a series of problems such as: the high liquidus temperature of silicate rocks; proneness for silicate melts to react with sample capsules to change the melt composition; and proneness for melts to flow and leak during the high pressure and high temperature experiments. In recent years, there is rapid progress in the high pressure and high temperature experimental techniques, in terms of not only the extension of temperature and pressure ranges but also the improvement on the accuracy of measurements, and the emergence of new methods for in-situ measurements. Here, we review the widely-used theoretical models of ambient-pressure and high-pressure EOS for silicate melts, and illustrate some problems that need to be solved urgently:(1) the room pressure EOS for iron-and titanium-bearing silicate melts needs to be improved;(2) the partial molar properties of the H2 O and CO2 components in silicate melts containing volatile components may vary markedly with the melt composition, which need to be addressed in high-pressure EOS;(3) how the formulation and applicable range of EOS correspond to changes in melt structure and compression mechanism requires further study. We highlight the basic principle and applicable range of various methods for determining the EOS for silicate melts, and compare the advantages and disadvantages of doublebob Archimedes method, fusion curve analysis, shock compression experiments, sink-float method, X-ray absorption, X-ray diffraction and ultrasonic interferometry. Future trends in this field are to develop experimental techniques for in situ measurements on melt density or sound velocity at high temperature and high pressure and to accumulate more experimental data,and on the other hand, to improve the theoretical models of the EOS for silicate melts by a combination of research on the microstructure and compression mechanisms of silicate melts.     

Late Mesozoic subduction‐induced hydrothermal gold deposits along the eastern Asian and northern Californian margins: Oceanic versus continental lithospheric underflow

During late Mesozoic subduction of paleo‐Pacific lithospheric plates, numerous gold vein deposits formed in the Dabie–Sulu Belt of east‐central China plus its east‐Asian extensions, and in the Klamath Mountains plus Sierran Foothills of northern California. In eastern Asia, earlier transpression and continental collision at about 305–210 Ma generated a high pressure–ultrahigh pressure orogen, but failed to produce widespread intermediate to felsic magmatism or abundant hydrothermal gold deposits. Similarly in northern California, strike‐slip ± minor transtension–transpression over the interval of about 380–160 Ma resulted in the episodic stranding of oceanic terranes, but generated few granitoid magmas or Au ore bodies. However, for both continental margin realms, nearly head‐on Cretaceous destruction of oceanic lithosphere involved sustained underflow; reaching magmagenic depths of about 100 km, the descending mafic‐ultramafic plates dewatered, producing voluminous calc‐alkaline arc magmas. Ascent of these plutons into the middle and upper crust released CO₂ ± S‐bearing aqueous fluids and/or devolatilized the contact‐metamorphosed wall rocks. Such hydrothermal fluids transported gold along fractures and fault zones, precipitating it locally in response to cooling, fluid mixing, and/or reactions with wall rocks of contrasting compositions (e.g. serpentinite, marble). In contrast, where sialic crust was subducted to depths of about 100 km, only minor production of granitoid melts occurred, and few major coeval Au vein deposits formed. The mobilization of precious metal‐bearing fluids in continental margin and island arc environments apparently requires long‐continued, nearly orthogonal descent of oceanic, not continental, lithosphere.     

Conditions and processes leading to large-scale gold deposition in the Jiaodong province,eastern China

The gold deposits in the Jiaodong Peninsula constitute the largest gold mineralized province in China. The mineralization shows common characteristics in their tectonic setting, ore-forming fluid and metallogenic system. Sulfidation and fluid immiscibility are two important mechanisms controlling gold precipitation, both of which consume sulfur in the oreforming fluids. The escape of H_2S from the main ore-forming fluids and the decrease of total sulfur concentration not only lead to the efficient precipitation of gold, but also result in the crystallization of reducing minerals such as pyrrhotite and oxidizing minerals such as magnetite. Quartz solubility shows strong dependence on temperature, pressure, and CO_2 content. The dependence of quartz solubility on pressure is weak at low temperatures, and progressively stronger at higher temperatures.Similarly, the temperature dependence of quartz solubility is relatively low at low pressures, but becomes gradually stronger at high pressures. The results of solubility modeling can constrain the dissolution and reprecipitation behavior of quartz in the oreforming veins and the formation mechanism of different types of quartz veins. The multi-stage mineralization fluid activity resulted in the complex dissolution structure of quartz in the Jiaodong gold veins. Pyrite in the main metallogenic period in the Jiaodong gold deposits shows complex microstructure characteristics at single crystal scale. The trace elements(mainly the coupling of As-and Au-rich belt) and sulfur isotope composition also display a certain regularity. The As-rich fluids might have formed by the initial pulse of ore-forming fluids through As-rich metasedimentary strata, while the As-Au oscillation zone at the margin of pyrite grains is related to the pressure fluctuation caused by fault activity and the local phase separation of fluids. There is a temporal and spatial evolution of gold fineness in the Jiaodong gold deposits. Water/rock reaction(sulfidation) was the main ore-forming mechanism of early gold mineralization, forming relatively high fineness gold, while significant pressure drop in the shallow part accompanied by fluid phase separation promoted the late gold mineralization, forming low fineness gold. Under cratonic destruction setting, dehydration of the amphibolite and granulite facies metamorphic lower-crust resulted in the formation of Au-CO_2-rich ore-forming fluids, which rose along the deep fault and secondary structure, and formed the largescale fault-controlled gold deposits in Jiaodong.     

Low water contents in pyroxenes from spinel-peridotites of the oxidized, sub-arc mantle wedge

Pyroxene water contents measured by Fourier transform infrared spectrometry for Mexican and Simcoe (WA, USA) spinel-peridotite xenoliths range from 140 to 528 ppm in clinopyroxenes and 39 to 265 ppm in orthopyroxenes. Correlations between these water contents and major-element compositional data for the pyroxenes, associated spinels, and whole-rock xenoliths demonstrate that these water contents record mantle values that have not been perturbed since the xenoliths were brought to the surface by their host magmas. Broad positive correlations of pyroxene water contents with whole-rock Al₂O₃ are consistent with water behaving as an incompatible element during peridotite melting. The main control on the range of pyroxene water contents, however, appears to be the redox state of the peridotite, because estimates of oxygen fugacity from Mössbauer (Simcoe) and microprobe data (Mexico) on spinels are negatively correlated with water contents. This is consistent with the dominant mechanism of H incorporation into pyroxene, which is dependent on the oxidation-reduction of iron. Metasomatism of sub-arc mantle-wedge peridotites by oxidized fluids or melts rising from the slab raises the oxygen fugacity of the peridotites, and where temperature is high enough, induces them to partially melt. The oxidation, in turn, lowers the solubility of water in the peridotite minerals, causing more than half of the original water to be expelled. That water enters the hydrous partial melts and these ascend through the lithosphere to feed the arc magmatic system in the upper crust. Low water contents in pyroxenes from sub-arc mantle-wedge peridotites, such as those from Simcoe and some western Mexican sites, therefore appear to be complementary to the high water contents that characterize subduction-zone magmas and fuel their explosive eruptions. An estimate of water budget in subduction zones, however, indicates that the amount of water coming from the dehydration of mantle-wedge anhydrous minerals probably accounts for less than 5% of the total water present in subduction-related magmas. The high water contents of arc magmas thus are mainly attributed to fluids or melts from the slab proper. The relatively dry sub-arc mantle wedge appears to be an effective medium through which subducted water is transported from slabs toward the surface.     

The significance of source versus process in the tectonic controls of magma genesis

Despite the association of certain characteristic trace-element signatures with particular tectonic environments of eruption, there are accumulating data which would result in significant tectonic misassignments. Ambiguity of signals appears in active arc/back-arc systems of the southwestern Pacific and particularly in some intracontinental plate suites. Given the selective preservation of continental as opposed to oceanic lithosphere, inappropriate paleotectonic inferences are probable using trace-element criteria alone.Strong relative fractionation of the alkalis and alkaline earth elements (AEE) with respect to the rare earth elements (REE) in the majority of arc-related magmas and a number of intraplate continental basalts is strongly suggestive of the involvement of hydrous fluids at some stages in the respective petrogenetic processes occurring in these two tectonic regimes. In contrast, fractionation of high-field-strength elements (HFSE) such as Nb and Ta with respect to the REE in the same suites is most readily explained by the involvement, at some stage in the magma formation process, of high-SiO₂ melts. A number of widely applied tectonic discriminants makes use of AEE/HFSE fractionation, but the processes and sources involved in subduction-zone petrogenesis may be duplicated during interaction of mantle-derived basalt with the heterogeneous components of continental lithosphere, both mantle and crust. A significant role for both volatile-dominated fluids and silicate melts is implicated in collision and some intracontinental plate magmatism.     

Ore-forming fluid and mineralization of Caijiaying and Dajing polymetallic ore deposits

Fluid inclusions from the Dajing and Caijiaying deposits have nearly the same ho-mogenization temperature. Correlation between temperature and salinity shows that both Sn-and Cu-bearing fluids Sn and Cu were present in the Dajing deposit but only one kind of fluids continuously evolved in the Caijiaying deposit. Study on rare earth elements (REE) in ancient fluid from the inclusions indicates that the fluid of Sn mineralizing stage in Dajing was derived from remelting magma of the continental crust, and the fluid of Cu-Pb-Zn mineralizing stage in the Dajing deposit and the fluid of Cu-Pb-Zn mineralization in the Caijiaying deposit were derived from the mantle. It is concluded that the Cu-Pb-Zn mineralizations in the Dajing and Caijiaying deposits resulted from the identical tectono-thermal event of magma-fluids induced by Mesozoic tectonic transition and extension in the eastern part of North China Craton.     

A physicochemical model for deep degassing of water-rich magma

Two powerful eruptions of Quizapu vent on Cerro Azul Volcano, Chile are used as examples to discuss the problem of effusive eruptions of magmas having high preeruptive volatile concentrations. A physicochemical mechanism is proposed for magma degassing, with the volatiles being lost before coming to the surface. The model is based on the interaction of magmas residing in chambers at different depths and on the difference between the solubility of water in the melt and the water equilibrium concentration in a magma body having a considerable vertical extent. The shallower chamber can accumulate the volatiles released from the magma that is supplied from the deeper chamber. An explanation is provided of the dramatic differences in the character of the 1846–1847 and 1932 eruptions, which had identical chemical-petrographic magma compositions.     

Magma dynamics at mid-ocean ridges by noble gas kinetic fractionation: Assessment of magmatic ascent rates

Despite its impact in understanding oceanic crust formation and eruptive styles of related volcanism, magma dynamics at mid-ocean ridges are poorly known. Here, we propose a new method to assess ascent rates of mid-ocean ridge basalt (MORB) magmas, as well as their pre- and sin-eruptive dynamics. It is based on the idea that a rising magma can reach a variable degree of both CO₂ supersaturation in melt and kinetic fractionation among noble gases in vesicles in relation to its ascent rate through the crust. To quantify the relationship, we have used a model of multicomponent bubble growth in MORB melts, developed by extending the single-component model of Proussevitch and Sahagian [A.A. Proussevitch, D.L. Sahagian, Dynamics and energetics of bubble growth in magmas: analytical formulation and numerical modeling, J. Geophys. Res. 103 (1998), 18223–18251.] to CO₂–He–Ar gas mixtures. After proper parameterization, we have applied it to published suites of data having the required features (glasses from Pito Seamount and mid-Atlantic ridges). Our results highlight that the investigated MORB magmas display very different ranges of ascent rates: slow rises of popping rock forming-magmas that cross the crust (0.01–0.5 m/s), slightly faster rates of energetic effusions (0.1–1 m/s), up to rates of 1–10 m/s which fall on the edge between lava effusion and Hawaiian activity. Inside a single plumbing system, very dissimilar magma dynamics highlight the large differences in compressive stress of the oceanic crust on a small scale. Constraints on how the systems of ridges work, as well as the characteristics of the magmatic source, can also be obtained. Our model shows how measurements of both the dissolved gas concentration in melt and the volatile composition of vesicles in the same sample are crucial in recognizing the kinetic effects and definitively assessing magma dynamics. An effort should be made to correctly set the studied samples in the sequence of volcanic submarine deposits where they are collected. Enhanced knowledge of a number of physical properties of gas-bearing MOR magmas is also required, mainly noble gas diffusivities, to describe multicomponent bubble growth at a higher confidence level.     

Major changes in the post-glacial evolution of magmatic compositions and pre-eruptive conditions of Llaima Volcano,Andean Southern Volcanic Zone,Chile

Llaima is one of the most active volcanoes of the Chilean volcanic front with recent explosive eruptions in 2008 and 2009. Understanding how the volcano evolved to its present state is essential for predictions of its future behavior. The post-glacial succession of explosive volcanic eruptions of Llaima stratovolcano started with two caldera-forming eruptions at ～16 and ～15 ka, that emplaced two large-volume basaltic-andesitic ignimbrites (unit I). These are overlain by a series of fall deposits (unit II) changing from basaltic-andesitic to dacitic compositions with time. The prominent compositionally zoned, dacitic to andesitic Llaima pumice (unit III) was formed by a large Plinian eruption at ～10 ka that produced andesitic surge deposits (unit IV) in its terminal phase. The following unit V represents a time interval of ～8,000 years during which at least 30 basaltic to andesitic ash and lapilli fall deposits with intercalated volcaniclastic sediments and paleosols were emplaced. Bulk rock, mineral, and glass chemical data constrain stratigraphic changes in magma compositions and pre-eruptive conditions that we interpret in terms of four distinct evolutionary phases. Phase 1 (=unit I) magmas have lower large ion lithophile (LIL)/high field strength (HFS) element ratios compared to younger magmas and thus originated from a mantle source less affected by slab-derived fluids. They differentiated in a reservoir at mid-crustal level. During the post-caldera phase 2 (=units II–IV), relatively long residence times between eruptions allowed for increasingly differentiated magmas to form in a reservoir in the middle crust. Fractional crystallization led to volatile enrichment and oversaturation and is the driving force for the large Plinian eruption of the most evolved (unit III) dacite at Llaima, although replenishment by hot andesite probably triggered the eruption. During the subsequent phase 3 (=unit V >3 ka), frequent mafic replenishments at mid-crustal storage levels favored shorter residence times limiting erupted magma compositions to water-undersaturated basaltic andesites and andesites. At around 3 ka, the magma storage level for phase 4 (=unit V <3 ka to present) shifted to the uppermost crust where the hot magmas partly assimilated the granitic country rock. Although water contents of these basaltic andesites were low, the low-pressure storage facilitated water saturation before eruption. The change in magma storage level at 3 ka was responsible for the dramatic increase in eruption frequency compared to the older Llaima history. We suggest that the change from middle to upper crust magma storage is caused by a change in the stress regime below Llaima from transpression to tension.     

Alkali-rich igneous rocks and related Au and Cu large and superlarge deposits in China

The alkali-rich igneous rocks in China occur as fifteen linear distributed belts of each extending several hundreds to several thousands kilometers in length. These include most types of alkali-rich igneous rock categories discovered worldwide. The related Au, Cu large and superlarge deposits or metallogenic focus-areas include Dongping, Guilaizhuang, Yulong, etc. Direct and indirect genetic links have been found between alkali-rich igneous rocks and Au, Cu mineralization. The petrogenesis and metallogenesis of the alkali-rich igneous rocks are mainly controlled by (1) mantle enrichment, (2) strong interaction between mantle and crust, (3) lower contens of sulfur, high and high contents of volatiles, (4) the significant turn of regional tectonic framework from compress to extension and (5) the strong change of regional lithosphere structure.     

Sulfur and chlorine degassing from primitive arc magmas: temporal changes during the 1982–1983 eruptions of Galunggung (West Java, Indonesia)

The 1982–1983 eruptions of Galunggung represent a nine-month period of intermittent volcanic activity with significant changes in explosivity and emission of volatiles. Eruptions started with Vulcanian explosions but changed gradually to Strombolian activity. Compositions of juvenile material changed from basaltic andesite to high-Mg basalt, which are among the most primitive rock types known in the Indonesian arc system. Although bulk compositions suggest a single evolution trend, we infer from the compositions of melt inclusions in olivine phenocrysts that the magmas represent derivatives of a complex spectrum of primary melts. Primitive inclusions in olivine phenocrysts from magma erupted during the Strombolian phase contain up to 2000 ppm sulfur, but concentrations decrease rapidly with increasing SiO₂ down to matrix glass values (50–100 ppm). ‘Vulcanian’ inclusions appear to be degassed before eruption (200 ppm S). Chlorine concentrations increase from 750 to 2200 ppm in Strombolian, and from 800 to 1500 in Vulcanian magmas, whereas matrix glass contains about 1000 ppm in both cases. Ash leachates show two cycles of decreasing S/Cl ratios: from 9.7 to 5.6 at the start of the activity, and from 12.2 to 2.0 after four months. As the second cycle follows upon increased seismic activity at shallow depth, it probably reflects degassing of fresh sulfur-rich magma arriving in the shallow Galunggung reservoir. In contrast to the degassed state of Vulcanian magma, the significant amounts of adsorbed sulfur on the ashes point to an excess source of sulfur, which was most likely derived from intruding Strombolian magma. Hence, the observed sulfur flux of 2 Mt is not in accordance with a petrologic estimate of 0.09 Mt. Using a published value of 550 Mt of erupted material about 0.34 km³ fresh undegassed magma is needed to account for the observed sulfur flux. This is close to the erupted volume of Vulcanian magma (0.26 km³), which presumably was replaced completely by Strombolian magma during the eruption. Using the petrologic method, we calculate a total release of 0.3 Mt chlorine, which agrees well with an output of 0.47 Mt estimated independently from S/Cl ratios of the ash leachates and TOMS sulfur yields. Ash leachates show that about 35% of the sulfur and 30% of the chlorine was scavenged from the eruption plumes. Our results suggest that sulfur and chlorine were largely decoupled during degassing, which resulted in considerable variations in S/Cl ratios during the Galunggung eruptions. We infer that sulfur degassing reflects the arrival of fresh magma at shallow depth, whereas chlorine is largely derived from simultaneously erupted material. As a consequence, the petrologic estimates are more consistent with observed emissions for chlorine than for sulfur.     

高温高压下碳酸盐熔体对地幔岩电导率的影响

为了观测含碳酸盐地幔岩部分熔融过程中电导率的变化,厘清碳酸盐熔体在金伯利岩岩浆形成过程中所起的作用,并探讨Slave克拉通中部Lac de Gras地区约80～120km深处的高导成因,我们利用DS 3600t六面顶压机和Solartron 1260阻抗/增益-相位分析仪在1.0～3.0GPa、673～1873K温压条件下分别测量了含碳酸钠(Na_2CO_3)、碳酸钙(CaCO_3)和大洋中脊玄武岩(MORB)的地幔岩样品的电导率.实验结果表明,地幔岩样品的电导率主要受到温度和组分的影响,而压力对其影响较小.在温度低于1023K时,含Na_2CO_3地幔岩样品的电导率明显高于含同比重CaCO_3和MORB的;温度达到1023K时,含Na_2CO_3地幔岩样品开始熔融;但在之后的200K温度区间内,该部分熔融样品的电导率随温度的增加几乎不发生变化.这一现象或许揭示:地幔深部的碳酸质岩浆在快速上升过程中会同化吸收岩石圈地幔中的斜方辉石(Opx),进而形成金伯利岩岩浆,期间岩浆的电导率几乎不发生变化.含CaCO_3和MORB的地幔岩样品分别在1723K和1423K开始熔融,其部分熔融样品的电导率随温度的增加而快速增加.依据前人的研究结果和我们的实验结果,我们认为可以用含碳酸盐的部分熔融样品来解释Slave克拉通中部Lac de Gras地区约80～120km深处的异常高导现象,并推测熔体中碳酸盐的熔体比例小于2wt.%.     

The basaltic to trachydacitic upper Diliman Tuff in Manila: Petrogenesis and comparison with deposits from Taal and Laguna Calderas

The basaltic to trachydacitic (50–65 wt.% SiO₂) upper Diliman Tuff is the youngest deposit of a sequence of tuffaceous deposits in Metro Manila. The deposit is located north of Taal Caldera and northwest of Laguna Caldera, which are both within the Southwest Luzon Volcanic Field. Chemical variations in the pumice fragments within the upper Diliman Tuff include medium-K basalt to basaltic andesite, high-K basaltic andesite to andesite and trachyandesite to trachydacite. Magma mixing/mingling is ubiquitous and is shown by banding textures in some pumice fragments, considerable range in groundmass glass composition (54 to 65 wt.% SiO₂) in a single pumice fragment, and zoning in plagioclase phenocrysts. Simple binary mixing modeling and polytopic vector analysis were used to further evaluate magma mixing. Trace-element variations are inconsistent with the medium-K and high-K magmas being related by crystal fractionation. The medium-K basalts represent hotter intrusions, which induced small degrees of partial melting in older crystallized medium-K basaltic material within the crust to produce the high-K magmas. All melts likely differentiated in the crust but the emplaced and new basaltic intrusions originated from the mantle wedge and were generated by subduction zone processes. The volcanic source vent for the upper Diliman Tuff has not been identified. In comparisons with the deposits from adjacent Taal and Laguna Calderas it is chemically distinct with respect to both major- and trace-element concentrations.     

Alkali-rich igneous rocks and related Au and Cu large and superlarge deposits in China

The alkali-rich igneous rocks in China occur as fifteen linear distributed belts of each extending several hundreds to several thousands kilometers in length. These include most types of alkali-rich igneous rock categories discovered worldwide. The related Au, Cu large and superlarge deposits or metallogenic focus-areas include Dongping, Guilaizhuang, Yulong, etc. Direct and indirect genetic links have been found between alkali-rich igneous rocks and Au, Cu mineralization. The petrogenesis and metallogenesis of the alkali-rich igneous rocks are mainly controlled by (1) mantle enrichment, (2) strong interaction between mantle and crust, (3) lower contens of sulfur, high f _O2 and high contents of volatiles, (4) the significant turn of regional tectonic framework from compress to extension and (5) the strong change of regional lithosphere structure.     

Ore-forming mechanism for the Xiaoxinancha Au-rich Cu deposit in Yanbian,Jilin Province,China: Evidence from noble gas isotope geochemistry of fluid inclusions in minerals

The Xiaoxinancha Au-rich copper deposit is one of important Au-Cu deposits along the continental margin in Eastern China. The deposit consists of two sections: the Beishan mine (North), composed of altered rocks with veinlet-dissemination sulfides and melnicovite-dominated sulfide-quartz veins, and the Nanshan mine (South), composed of pyrrhotite-dominated sulfide-quartz veins and pure sulfide veins. The isotope compositions of noble gases extracted from fluid inclusions in ore minerals, i.e. ratios of 3He/4He, 20Ne/22Ne and40Ar/36Ar are in the ranges of 4.45―0.08 Ra, 10.2―8.8 and 306―430, respectively. Fluid inclusions in minerals from the Nanshan mine have higher 3He/4He and 20Ne/22Ne ratios whereas those from the Beishan mine have lower 3He/4He ratios. The analysis of origin, and evolution of the ore fluids and its relations with the ore-forming stages and the ages of mineralization suggests that the initial hydrothermal fluids probably come from the melts generated by partial melting of oceanic crust with the participation of fluids from the mantle (mantle-plume type)/aesthenosphere. This also corresponds to the continental margin settings during the subduction of Izanagi ocaneic plate towards the palaeo-Asian continent (123―102 Ma). The veinlet-dissemination ore bodies of the Beishan mine were formed through replacement and crystallization of the mixed fluids generated by mixing of the ascending high-temperature boiling fluid with young crustal fluid whereas the melnicovite-dominated sulfide-quartz veins were formed subsequently by filling of the high-temperature ore fluid in fissures. Pyrrhotite-dominated sulfide-quartz veins in the Nanshan mine were formed by filling-deposition-crystallization of the moderate-temperature ore fluids and the pure sulfide veins were formed later by filling-deposition-crystallization of ore substance-rich fluids after boiling of the moderate-temperature ore fluids. The metallogenic dynamic processes can be summarized as: (1) formation of fluidand ore substance-bearing Adakitic magma by degassing, dewatering and partial melting during subduction of the Izanagi plate; (2) separation and formation of ore fluids from the Adakitic magma; and (3) success-sive ascending of the ore fluids and final formation of the Au-rich Cu deposit of veinlet-dissemination and vein types by secondary boiling.     

The influence of crustal structure on compositions of subduction-related magmas

A survey of Sr isotopic ratios and other compositional features of subduction-related magma suites reveals significant correlations between these averaged parameters and characteristics of the underlying crust (i.e., thickness, composition, and age). These observations lead to the conclusion that crust and(or) mantle rocks in the hanging walls of subduction zones are involved in modification of primary mafic magmas (typically basalt or basaltic andesite). It is proposed that mafic magmas will stagnate within the crust or uppermost mantle where they may differentiate and react with wall rocks. The extent to which such processes manifest themselves will depend upon details of the local crustal structure. In particular, the composition and age of the crust will strongly influence such parameters as Sr, Nd and Pb isotopic compositions. Such data strongly indicate the involvement of crustal rocks in locales underlain by old sialic crust (e.g., central Andes). Depending upon the level of magma stagnation and evolution within the crust, different trends in isotopic composition may result. These isotopic trends may be enhanced by partial melting of the wall rocks to produce relatively silicic anatectic magmas, and locally they may reflect subduction of continental sediments. Interpretation of the isotopic data may be more ambiguous in locales underlain by younger and more mafic continental crust (Cascades, E Eleutians) and those underlain by oceanic crust owing to the similarity in isotopic composition of primary magmas and the latter crustal materials. Yet some degree of crustal involvement in magmatic evolution seems highly probable even in these more primitive terranes. Consequently, most island arc magmas, and especially those more evolved than basalt, are probably not primary in the sense that they do not represent direct melts of the upper mantle. Studies of arc volcanic rocks may yield misleading conclusions concerning processes of magma generation related to subduction unless evolutionary processes are defined and their effects considered. It appears that modern volcanic arcs provide a poor analog for models of early crustal development because the modern mantle-derived magmatic components are more mafic in composition than average continental crust.     

Fluid/melt in continental deep subduction zones: Compositions and related geochemical fractionations

Plate subduction is the most magnificent process in the Earth. Subduction zones are important sites for proceeding matter- and energy- transports between the Earth's surface and the interior, continental crust growth, and crust-mantle interactions. Besides, a number of geological processes in subduction zones are closely related to human beings' daily life, such as volcanic eruptions and earthquakes, formation of mineral deposits. Subduction process thus has long been the centric topic of Earth sciences. The finding in 1980 s that continental crust could be subducted to mantle depths is a revolutionary progress in plate tectonic theory. Compared to oceanic crust, continental crust is colder, drier, lighter, and much more geochemically/isotopically heterogeneous. Hence, continental subduction process would affect the structure, compositions and evolutions of the overlying mantle wedge even more. During continental subduction and subsequent exhumation, fluids and melts can be generated in the(de)hydration process and partial melting process, respectively. These melts/fluids play important roles in crust-mantle interactions, elemental migrations, isotopic fractionations, and mantle metasomatism. By summarizing recent research works on subduction zones in this paper, we present a review on the types, physicochemical conditions and compositions of fluids/melts, as well as the migration behaviors of fluid-related characteristic elements(Nb-Ta-V) and the fractionation behaviors of non-traditional stable isotopes(Li-Mg) in subduction zones. The aim of this paper is to provide the readers an update comprehensive overview of the melt/fluid activities in subduction zones and of Li-Mg isotope systematics in subduction-related rocks and minerals.     

Highly fractionated granites: Recognition and research

Granite is one of the most important components of the continental crust on our Earth; it thus has been an enduring studied subject in geology. According to present knowledge, granite shows a great deal of heterogeneity in terms of its texture, structure, mineral species and geochemical compositions at different scales from small dike to large batholith. However, the reasons for these variations are not well understood although numerous interpretations have been proposed. The key point of this debate is whether granitic magma can be effectively differentiated through fractional crystallization, and, if so, what kind of crystallization occurred during the magmatic evolution. Although granitic magma has high viscosity because of its elevated SiO₂ content, we agree that fractional crystallization is effectively processed during its evolution based on the evidence from field investigation, mineral species and its chemical variations, and geochemical compositions. These data indicate that crystal settling by gravitation is not the only mechanism dominating granitic differentiation. On the contrary, flow segregation or dynamic sorting may be more important. Accordingly, granite can be divided into unfractionated, fractionated (including weakly fractionated and highly fractionated) and cumulated types, according to the differentiation degree. Highly fractionated granitic magmas are generally high in primary temperature or high with various volatiles during the later stage, which make the fractional crystallization much easier than the common granitic melts. In addition, effective magmatic differentiation can be also expected when the magma emplaced along a large scale of extensional structure. Highly fractionated granitic magma is easily contaminated by country rocks due to its relatively prolonged crystallization time. Thus, granites do not always reflect the characteristics of the source areas and the physical and chemical conditions of the primary magma. We proposed that highly fractionated granites are an important sign indicating compositional maturity of the continental crust, and they are also closely related to the rare-elemental (metal) mineralization of W, Sn, Nb, Ta, Li, Be, Rb, Cs, REEs, etc.     

Variations in the chemical and isotopic composition of fluids discharged from the Taupo Volcanic Zone, New Zealand

Contents of H₂O, CO₂ and Cl in well discharges from six explored geothermal systems of the Taupo Volcanic Zone, New Zealand, point to the existence of two distinct source fluids. The fluid present in discharges from systems along the eastern boundary is characterised by high CO₂ contents, 1.6 ± 0.5 , at mole ratios of 3.9 ± 1.5. High (0.06) and (12) weight ratios in these waters suggest that all four constituents are derived from associated andesitic rock. Geothermal discharges in the western parts of the TVZ, dominated by rhyolitic magmatism, are characterised by low CO₂ contents, 0.12 ± 0.05 , and low (0.14 ± 0.1) ratios. Again, relative Cl, B, Li and Cs contents agree with those of this potential source rock. High and ratios in the east are typical of fluids affected by the addition of volatiles released from subducted marine sediments. For the western systems, these ratios resemble more closely those expected for mantle-derived volatiles. The isotopic compositions of all deep waters point to the presence of variable amounts of a magmatic component, some 14 ± 5% in the eastern and 6 ± 2% in the western systems. The observed variations are explained in terms of interaction of volatiles released from the subducted sediments with material of the mantle wedge to form a volatile-charged, high-alumina basalt. Its convective rise, in a direction opposite to that of the down-going slab, leads to high enrichment in volatiles of the magmas generated beneath the eastern parts of the TVZ and increases their ability to intrude the continental crust. Further fractional crystallisation and assimilation leads to the formation of volatile-rich andesitic melts, partly extruded to form the volcanoes of the andesitic arc, partly intruded to act as source rocks for the high-gas geothermal systems. Batches of high-alumina basalt, depleted in subducted volatiles, travel farther west to pond beneath a zone of crustal extension. Following extensive fractionation, highly siliceous melts, carrying predominantly mantle-type volatiles, rise beneath the western part of the TVZ to supply both heat and volatiles to the geothermal systems there.