Geochronology, geochemistry and Hf isotopes of andesites in the Sandaowanzi gold deposit(Great Xing’an Range, NE China): implications for petrogenesis, tectonic setting,and mineralization

The Sandaowanzi gold deposit is an extremely Au-rich deposit in the Northern Great Hinggan Range in recent years.Zircon U-Pb geochronology,Hf isotope analysis,and the geochemistry of andesites of the Longjiang Formation from the Sandaowanzi gold deposit were used to investigate the origin,magmatic evolution as well as mineralization and tectonic setting of the Early Cretaceous epithermal gold deposits in the northern Great Hinggan Range area.Zircon U-Pb dating reveals an emplacement age of 123.4±0.3 Ma,indicating that the andesites of the Sandaowanzi gold deposit was formed during the Early Cretaceous.The andesites are enriched in light rare earth elements relative to heavy rare earth elements and have weak negative Eu anomalies(δEu=0.76-0.90).The rocks are also enriched in large-ion lithophile elements,such as Rb,Ba,Th,U,and K,and depleted in the high-field-strength elements,such as Nb,Ta,and P.These characteristics are typical of volcanic rocks related to subduction.Igneous zircons from the andesite samples have relatively homogeneous Hf isotope ratios,176Hf/177Hf values of 0.282343-0.282502,εHf(t)values of-12.58 to-6.95,and two-stage model ages(TDM2)of 1743-1431 Ma.The characteristics of the andesites of the Longjiang Formation are consistent with derivation from partial melting of enriched mantle wedge metasomatized by subducted-slab-derived fluids.These rocks formed in an extensional environment associated with the closure of the Mongol-Okhotsk Ocean and subduction of the Paleo-Pacific Plate.Mineralization occurred towards the end of volcanism,and the magmatic activity and mineralization are products of the same geodynamic setting.

Generation of andesite through partial melting of basaltic metasomatites in the mantle wedge:Insight from quantitative study of Andean andesites

Continental crust in average exhibits a similar composition in both major and trace elements to andesites along active continental margins.For this reason,andesitic magmatism above oceanic subduction zones is considered to have played a key role in the generation of continental crust along convergent plate boundaries.With respect to the origin of andesites themselves,however,there is still a hot debate on how they have acquired their geochemcial compositions.The debate is mainly centralized on the relative contributions of crustal contamination,magma differentiation and source mixing,which reaches an impasse in the past decades.The essential reason for this kind of debates is that these three mechanisms only can account for some of the geochemical observations for andesites,leading to insufficient discrimination among them.Nevertheless,the geochemical features of andesites are primarily controled from early to late by the composition of their source rocks in addition to partial melting and magma differentiation processes.If source mixing and partial melting processes in the early stage of andesite magmatism can account for the first-order geochemical features of andesites,there is no need to invoke the late processes of magma differentiation and crustal contamination for andesite petrogenesis.This is illustrated by quantitative forward modeling of the geochemical data for Quaternary andesites from the Andean arc in South America based on an integrated interpretation of these data.The modeling has run with four steps from early to late:(1)dehydration of the subducting oceanic crust at forearc depths;(2)partial melting of the subducting terrigenous sediment and altered oceanic basalt at subarc depths to produce hydrous felsic melts;(3)the generation of basaltic metasomatites(e.g.,Si-excess pyroxenite)in the mantle wedge through reaction of the mantle wedge peridotite with large amounts of the hydrous felsic melts;(4)the production of andesitic melts by partial melting of the basaltic metasomatites.The results not only testify the hypothesis that the trace element and radiogenic isotope compositions of andesites can be directly produced by the source mixing and mantle melting but also demonstrate that partial melting of the basaltic metasomatites can reproduce the lithochemical composition of andesites.The compositional variations of Andean andesites within a single volcanic zone and among different volcanic zones can be explained by incorporating different amounts of heterogeneous hydrous felsic melts into their mantle sources,followed by different degree of partial melting under different pressures and temperatures.Therefore,the source mixing and partial melting processes at subarc depths can account for the first-order geochemical features of Andean andesites.In this regard,it may be not necessary for andesite petrogenesis to invoke the significant contributions from the processes of magma differentiation and crustal contamination.

Geochemistry of high-Mg andesites from the early Cretaceous Yixian Formation, western Liaoning: Implications for lower crustal delamination and Sr/Y variations

85 volcanic rocks of the Yixian Formation from the Sihetun type section were collected and analyzed for geochemical and isotopic compositions. Major element compositions indicate that the Sihetun volcanic rocks are high magnesium andesites with some basalts occurring at the bottom of section. The Sihetun high magnesium andesites (SiO2 = 52.82―59.31wt%, Al2O3 = 14.15―16.35wt%) show many characteristics of adakites such as depletion in heavy rare-earth elements (HREE; Yb = 1.03―1.88 μg/g) and Y(12―20 μg/g) and high Sr (620―1323 μg/g) and Sr/Y(32―88), with high LaN/YbN ratio (10―25). They share similar major and trace element characteristics to volcanic rocks from the Xinglonggou Formation except their lower Nd isotope ratios (143Nd/144Nd (130Ma) = 0.5118―0.5119, ε Nd (130Ma) = ?11.6―?13.8, 87Sr/86Sr (130 Ma) = 0.7058―0.7064. They were in- terpreted in a way that eclogite that formed at the base of thickened Archean lower crust of the North China craton foundered into the convecting mantle and subsequently melted and interacted with peridotite. However, compared to the Xinglonggou volcanic rocks, the source of the Sihetun magma contained more ancient continental crustal material in order to explain its evolved Nd isotopes. The age of the Sihetun Formation was 120 to 130 Ma, and this indicates that delamination lasted to the early Cretaceous period. The Sr contents and Sr/Y ratios of the Sihetun high-Mg andesites show sig- nificant negative correlations with SiO2 for samples with SiO2 > 56%. These suggest that the Sr and Sr/Y values were reduced due to fractional crystallization of plagioclase. Accordingly, the effect of crystallization on volcanic Sr and Sr/Y ratio has to be taken into account.

Comparative Study of the Rock-breaking Mechanism of a Disc Cutter and Wedge Tooth Cutter by Discrete Element Modelling

The operation of a shield tunnel boring machine(TBM)in a high-strength hard rock stratum results in significant cutter damage,adversely affecting the thrust and torque of the cutter head.Therefore,it is very important to carry out the research on the stress characteristics and optimize the cutter parameters of cutters break high-strength hard rock.In this paper,the rock-breaking performance of cutters in an andesite stratum in the tunnel of Qingdao Metro Line No.8 was investigated using the discrete element method and theoretical analysis.The rock-breaking processes of a disc cutter and wedge tooth cutter were simulated by software particle flow code PFC^(3D),and the rock-breaking degree,stress of the cutter,and rock-breaking specific energy were analyzed.The rock damage caused by the cutter in a specific section was divided into three stages:the advanced influence,crushing,and stabilizing stages.The rock-breaking degree and the tangential and normal forces of the wedge tooth cutter are larger than that of the disc cutter under the same conditions.The disc cutter(wedge tooth cutter)has the highest rock-breaking efficiency at a cutter spacing of 100 mm(110 mm)and a penetration depth of 8 mm(10 mm),and the rock-breaking specific energy is 11.48 MJ/m^(3)(12.05 MJ/m^(3)).Therefore,two types of cutters with different penetration depths or cutter spacing should be considered.The number of teeth of wedge tooth cutters can be increased in hard strata to improve the rock-breaking efficiency of the shield.The research results provide a reference for shield cutterhead selection and cutter layout in similar projects.

Geochemistry and Petrogenesis of Basic and Ultrabasic Rocks Elogo Complex in Ivindo Archean Block (Congo Craton): Geodynamic Implications

The Elogo complex is a greenstone belt portion located on the Eastern edge of the Archean Congo craton at the junction with the Paleoproterozoic to Neoproterozoic Sembe Ouesso basin. This study was carried out on this complex to determine the context of the placement of basaltic rocks. Metaluminous tholeiitic basalts (basic and ultrabasic), calc-alkaline basalts, andesitic basalts, and peraluminous calc-alkaline dacites represent greenstones. Tholeiitic and calc-alkaline basalts come from deep enriched and depleted mantle sources, including garnet in fusion residues [Al2O3/TiO2 > 16 (16.5 to 35.12) and in some samples between 12.45 to 14.48;CaO/Al2O3 1 (1.04 to 1.35) in ten samples and (Gb/Yb)PM > 1]. The calc-alkaline dacites come from a shallow depleted mantle source [Al2O3/TiO2 > 16;CaO/Al2O3 1]. Tholeiitic and calc-alkaline basalts have a negative Rb, Ba, Ce, and Nb anomaly without negative Ti anomaly, positive Ta, Pb anomalies, and a lack of significant REE [(La/Yb)n = 0.36 to 0.97 and 1 to 2.15;(Ce/Yb)n = 0.27 to 0.96 and 1.04 to 1.72, respectively] fractionation. High Nb/Th (2 to 10) and Nb/U (1.82 to 26) ratios and low La/Ta (5 to 27) ratios are characteristic of divergent margin magmatic sources. Tholeiitic and calc-alkaline basalts correspond to an extensive back-arc basin-type tectonic setting. Calc-alkaline andesitic basalts and dacites show positive Ba, U, Th, K, La, Ce, Pb, and Li anomalies and negative Nb, Ta, and Ti anomalies reflecting crustal contamination and hydrothermal alteration in a compressive tectonic context as a volcanic arc in a subduction regime marking the interruption of the meso-neoarchean Elogo’s opening. Elogo’s opening and closing are probably associated with the emplacement of the greenstone of the meso-neoarchean Gabon Belinga group and the relics of the Mesoarchean greenstones of the Cameroun Ntem complex.

Eocene Crustal Thickening and Uplifting of the Tibetan Plateau:Evidence from Adakitic Volcanic Rocks

Changes in oceanic O-Sr-C isotopic compositions, global cooling and Asian continental aridification beginning in the Middle-Late Eocene(47-34 Ma) are considered to have been caused by the uplift of the Tibetan plateau.The specific timing and uplift mechanism,however,have long been subjects of debate.We investigated the Duogecuoren lavas of the central-western Qiangtang block,which form the largest outcrops among Cenozoic lavas in north-

Acoustic experimental study of two types of rock from the Tibetan Plateau under the condition of freeze-thaw cycles

Under the condition of freeze-thaw cycles, two types of rocks （granite and andesite）, used as slope protection for the Qinghai-Tibet Railway, were tested according to the special climatic conditions in the Tibetan Plateau, and their various damage processes in ap- pearance were carefully observed. Observation results show that damage of andesite was more serious than that of granite. Using an acoustic instrument, ultrasonic velocity was tested. The changing trends of velocity with the number of freeze-thaw cycles were analyzed, and the freeze-thaw cycle damaging the physical and mechanical properties of rocks can be seen. According to the changing trends of ultrasonic velocity with the number of freeze-thaw cycles, mechanical parameters of rocks, such as dynamic elasticity modulus, Poisson＇s ratio, and dynamic bulk modulus were analyzed. It is found that they all have declining trends as the number of fi＇eeze-thaw cycles increases, and in particular, when the cycle number reaches a certain extent, the Poisson＇s ratio of rocks begins to become negative.

The off-crust origin of granite batholiths

Granitod batholiths of I-type features （mostly granodiorites and tonalites）, and particularly those forming the large plutonic associations of active continental margins and intracontinental collisional belts, represent the most outstanding magmatic episodes occurred in the continental crust. The origin of magmas, however, remains controversial. The application of principles from phase equilibria is crucial to understand the problem of granitoid magma generation. An adequate comparison between rock com- positions and experimental liquids has been addressed by using a projected compositional space in the plane F（Fe ＋ Mg）-Anorthite-Orthoclase. Many calc-alkaline granitoid trends can be considered cotectic liquids. Assimilation of country rocks and other not-cotectic processes are identified in the projected diagram. The identification of cotectic patterns in batholith implies high temperatures of magma segregation and fractionation （or partial melting） from an intermediate （andesitic） source. The com- parison of batholiths with lower crust granulites, in terms of major-element geochemistry, yields that both represent liquids and solid residues respectively from a common andesitic system. This is compatible with magmas being formed by melting, and eventual reaction with the peridotite mantle, of subducted mOlanges that are finally relaminated as magmas to the lower crust. Thus, the off-crust generation of granitoids batholiths constitutes a new paradigm in which important geological implica- tions can be satisfactorily explained. Geochemical features of Cordilleran-type batholiths are totally compatible with this new conception.

Swelling characteristics of andesite foundation induced by water immersion and their influence on ballastless track subgrade

To study the mechanical and deformation characteristics of ballastless track subgrade filled with micro-expansion fillers in a water-immersed environment, a physical model of ballastless track subgrade was constructed on a 1:2 scale with expansive andesite fillers. A water immersion test was carried out to model the soaking of the expansive soil foundation caused by rising groundwater. The swelling behaviors of the foundation and their influences upon the mechanics and deformations of the subgrade were analyzed. The lateral swelling pressure of the foundation and the heave of the subgrade obviously increased due to the water immersion, and the values were closely related to the overlying load and lateral restraint. The heave deformation of the double-line ballastless track subgrade showed significant nonuniformity along the lateral direction, causing the track slab to incline with a maximum inclination angle of 1.55×10^-3 deg. The heave of the foundation caused a heave in subgrade, but this transferred heave was significantly attenuated. The attenuation rate of the heave at the midline of the track slab was up to 13.38%. The attenuation characteristic can be fully utilized for the anti-heave deformation measures of railway subgrade in expansive soil areas.

Subaqueous early eruptive phase of the late Aptian Rajmahal volcanism,India:Evidence from volcaniclastic rocks,bentonite,black shales,and oolite

The late Aptian（118-115 Ma） continental flood basalts of the Rajmahal Volcanic Province（RVP） are part of the Kerguelen Large Igneous Province,and constitute the uppermost part of the Gondwana Supergroup on the eastern Indian shield margin.The lower one-third of the Rajmahal volcanic succession contains thin layers of plant fossil-rich inter-trappean sedimentary rocks with pyroclasts,bentonite,grey and black shale/mudstone and oolite,whereas the upper two-thirds consist of sub-aerial fine-grained aphyric basalts with no inter-trappean material.At the eastern margin and the north-central sector of the RVP,the volcanics in the lower part include rhyolites and dacites overlain by enstatite-bearing basalts and enstatite-andesites.The pyroclastic rocks are largely felsic in composition,and comprise ignimbrite as well as coarse-grained tuff with lithic clasts,and tuff breccia with bombs,lapilli and ash that indicate explosive eruption of viscous rhyolitic magma.The rhyolites/dacites（〉68 wt.%） are separated from the andesites（〈 60 wt.%） by a gap in silica content indicating their formation through upper crustal anatexis with only heat supplied by the basaltic magma.On the other hand,partially melted siltstone xenoliths in enstatite-bearing basalts suggest that the enstatite-andesites originated through mixing of the upper crust with basaltic magma,crystallizing orthopyroxene at a pressure-temperature of ~3 kb/1150℃.In contrast,the northwestern sector of the RVP is devoid of felsic-intermediate rocks,and the volcaniclastic rocks are predominantly mafic（basaltic） in composition.Here,the presence of fine-grained tuffs,tuff breccia containing sideromelane shards and quenched texture,welded tuff breccia,peperite,shale/mudstone and oolite substantiates a subaqueous environment.Based on these observations,we conclude that the early phase of Rajmahal volcanism occurred under predominantly subaqueous conditions.The presence of grey and black shale/mudstone in the lower one-third of the succession across the entire Rajmahal basin provides unequivocal evidence of a shallow-marine continental shelf-type environment.Alignment of the Rajmahal eruptive centers with a major N—S mid-Neoproterozoic lineament and the presence of a gravity high on the RVP suggest a tectonic control for the eruption of melts associated with the Kerguelen plume that was active in a post-Gondwana rift between India and Australia-Antarctica.

Petrology and Geodynamic Context of Metabasic Rocks of Nemba Complex in the West Congo Fold Belt (Republic of Congo)

The West Congo belt contains Paleoproterozoic and Neoproterozoic Units which are deformed during Panafrican event. The Neoproterozoic Unit contains rift and post rift deposits. The rift formation is made of metabasite, volcanic and volcano-sedimentary rocks. The metabasite constitutes the Nemba Complex which is considered put into place in a continental or oceanic context. Samples from Nemba Complex collected along the “Congo ocean railway realignment” are analyzed. Major elements indicate a low potassic subalkaline affinity of the samples. Classification diagrams based of major elements shows that the rocks are a tholeiitic serie constituted of andesitic basalts, sub-alkaline andesitic basalts and a sub-alkaline to alkaline basalts. Some samples with a calc-alkaline signature indicate a crustal contamination. The traces elements plots indicate that the rocks are the MORB. The spectrum of multi-elements is characteristic of continental rocks with a lithospheric origin. This is an agreement with many works on the Nemba Complex and the alkali plutons and acid volcanic rocks associated which attribute there a rift context.

Microscopic pore characteristics of andesite and implication from mineral content:A case study in Huoshiling Formation of Changling fault depression,Songliao Basin

In order to study the microscopic pore characteristics of andesite reservoir and the effect of mineral content on the andesite pore,this study takes the andesite of the Huoshiling Formation in Longfengshan of Changling fault depression,Songliao Basin as a subject.The andesite reservoir space was discerned through the observation of cores and casting thin sections.Besides,the pore size distribution of andesites and their mineral contents were quantitatively characterized by high-pressure mercury injection,nitrogen adsorption and XRD,respectively.The results show that:(1)There are various types of reservoir space in andesites,including vesicles,amygdule,intergranular pores,matrix dissolution pores and dissolution pores of amygdala,and three types of fractures including dissolution,structural and explosion fractures.(2)The pore size distribution of andesite is complex.The main pore size of andesite is mid-pore(10-20 nm)with some large-pores(>50 nm).Mid-pore and large-pore provide the main specific surface area,which are the main space for gas storage.(3)The andesite reservoir space in the study area is mainly controlled by dissolution,as supported by the relationship between the change of mineral content and porosity evolution.The porosity of andesites decreases with the increase of quartz and chlorite content,but increases with the increase of soluble mineral,e.g.,feldspar content.

GEOCHEMICAL CHARACTERISTICS OF VOLCANIC ROCKS FROM TANGBALE OPHIOLITE IN THE WESTERN JUNGGAR OF XINJIANG

An INAA technique was applied to determine simultaneously abundances of rare-earth, transitional metal, large-ion lithophile and high field strength elements in volcanic rocks from Tangbale ophiolite belt. The detailed study on trace element geochemistry shows that the volcanic rocks were erupted in the back-arc basin. The volcanic rocks of early and middle stages of the expanding period of the basin have low REE and other incompatible element contents. At early and late stages of closing period of the basin, alkalic basalts, basaltic andesites and andesites were erupted in which light REE and other incompatible elements were enriched.

Evolution History of Gassan Volcano, Northeast Japan Arc

Evolution history of the volcano is essential not only to characterize the volcano, but also consider magma genesis beneath the volcano. Most of the stratovolcanoes in northeast Japan follow a general evolutional course: cone building, horse-shoe shaped caldera forming collapse, and post-caldera stages. However, the detailed history of each stage is not well investigated. We investigated evolution history of young edifice of Gassan volcano, representative stratovolcano in rear side of northeast Japan arc. Most of the products are lavas, which are divided into two groups by geomorphologic and geologic features. The former (Gassan lower lavas) is composed of relatively thin and fluidal lavas, whose original geomorphology remains a little, while the latter (Gassan upper lavas) is composed of relatively thick and viscous lavas, whose original geomorphology is moderately preserved. Based on geologic features, the upper lavas can be further divided into Gassan upper north lavas and upper summit lavas in ascending order. After the formation of the thick lavas, horse-shoe shaped caldera was formed by the instability of the edifice, probably triggered by fault activity. No evidence of post caldera activity inner part of it is observed. Based on K-Ar data, estimated age of Gassan lower lavas is ca. 0.75 to ca. 0.6 Ma, those of Gassan upper north and upper summit lavas are ca. 0.60 to ca. 0.55 Ma and ca. 0.55 to ca. 0.45 Ma. The eruption rate is estimated to be ca. 0.0004 km3/1000 years in Gassan lower lavas and ca. 0.02 km3/1000 years in Gassan upper summit lavas. These values are lower than the eruption rate of representative Japanese stratovolcanoes.

Petrology and Tectonic Setting of Volcanic Rocks in West and South West of Salafchegan, Qom, Iran

In the west and south-west part of Salafchegan near Zavarian village in Iran (in central Iranian volcano plotonic belt), there are some volcanic rocks. Based on petrography and geochemistry findings, these volcanic rocks are Basalt, Andesite, Tracky-Andesite, Trackyte, Latite, Dacite. Tectono-magmatic diagram Y versus Zr shows the magmatic arc setting and Zr/TiO2 versus Ce/P2O5 diagram contrasts post colligenal magmatic arcs. The HFSE depletion in the MORB spider diagram shows significant volcanic arc magmatism. The high enrichment of Eu, Sr, Th, Rb, Cs, K (LFS elements) and Y, Zr, Hf shows negative anomaly and subduction tectonic setting. Based on MORB, Chondrite and primitive mantle spider diagrams, LREE have enriched to HREE in this area. The results of petrography, geochemistry and tectonic setting studies in this area, indicate that neogene magmatism occurred in post colligenal tectonic setting—subduction of Neo-thetise ocean under central Iranian plate in neogen era.

Strength Developing in Clay-Andesite Brick

The objective of this work was to develop a more and better understanding of the strength developing in clay-andesite fired brick. The purpose was to improve the quality of the clay bricks that are handmade in the southern region of Ecuador to make them suitable for more widespread use in the local construction industry. To achieve our goal, we first physically, chemically, and mechanically characterized the “clayey” and “sandy” materials used in the fabrication of handmade bricks in the region. Second, the optimal mixture (OM): the optimal proportion between the amounts of “clayey” and “sandy” material was sought. Third, clay bricks were prepared using the OM, baked at 950°C, and characterized. In addition, bricks produced by regional artisans were characterized, and the results were compared with the results obtained for the bricks prepared using the OM. Our data reveal that the optimal mixture is 50% “clayey” material and 50% “sandy” material and that with this mixture, an average improvement in brick quality of 300% can be achieved;thus, the use of the OM makes it possible to expand the use of these bricks in the local construction industry and enables an environmentally friendly production process by reducing the intensive exploitation of regional clay deposits. This improvement is achieved by virtue of the anorthite enrichment that occurs in the solid solution, which results from the evolution of andesine. The anorthite contributes to the formation of a stronger matrix among the different grains of the material. Conditions are favorable for this enrichment process to occur when “sandy” materials with high contents of andesite, which is common in Ecuadorian soils, are used.

A Comparison between the ~1.08–1.13 Ga Volcano-Sedimentary Koras Group and Plutonic Keimoes Suite: Insights into the Post-Collisional Tectono-Magmatic Evolution of the Eastern Namaqua Metamorphic Province, South Africa

Along the eastern margin of the Mesoproterozoic Namaqua metamorphic province(NMP) of southern Africa are a bimodal volcano-sedimentary succession, the ~1.13–1.10 Ga Koras Group, composed of rhyolitic porphyries and basaltic andesites, and the ~1.11–1.07 Ga late-to post-tectonic granitoids of the Keimoes Suite. This review examines existing whole-rock major-and trace-element data, along with isotope chemistry(with some new isotopic data), to investigate the role these two magmatic successions played in terms of post-collisional magmatism of the eastern NMP near the boundary with the Archean Kaapvaal Craton. The Keimoes Suite comprises variably porphyritic biotite monzogranites and granodiorites, with a charnockitic member. They are metaluminous to weakly peraluminous, ferroan, and calc-alkalic. They exhibit large ion lithophile(LIL) element enrichment relative to the high field strength elements(HFSE) with depletions in Ba, Sr, Nb, P, Eu and Ti, and enrichments in Th, U and Pb. Isotopic values(ε_(Nd)(t): 2.78 to-2.95,but down to-8.58 for one granite, depleted mantle Nd model ages(T_(DM)): 1.62–1.99 Ga, but up to 2.55 Ga;initial ^(87)Sr/^(86)Sr: 0.652 82–0.771 30) suggest derivation from weakly to mildly enriched(and radiogenic)sources of Meso-to Paleoproterozoic age, the former of more juvenile character. The Koras Group is characterized by a bimodal succession of calcic to calc-alkalic, magnesian and tholeiitic basaltic andesites and mostly metaluminous to peralkaline rhyolitic porphyries. Two successions are recognised, an older, lower succession that extruded at ~1.13 Ga, and a younger, upper succession at ~1.10 Ga. The rhyolitic porphyries of both successions show similar LILE/HFSE enrichment and the same element enrichments and depletions as the Keimoes Suite granitoids. The upper succession is consistently more fractionated in terms of both whole-rock major and trace element chemistry, and, isotopically, has a greater enriched source component(ε_(Nd(t):-0.69 to-4.26;T_(DM): 1.64–2.44 Ga), relative to the lower succession(ε_(Nd(t): 0.74–5.62;T_(DM): 1.28–2.12 Ga). Crystal fractionation of plagioclase and K-feldspar appears to have played a role in bringing about compositional variation in many of the granites. These were derived from partial melting of mainly igneous with subordinate sedimentary sources from mostly lower crustal depths, although some granitoids have indications of a possible mantle source component. The lower succession of the Koras Group was derived by partial melting of subduction-influenced enriched mantle giving rise to mafic magmas that fractionated to give rise to the rhyolitic porphyries. The upper succession rhyolites were derived by crustal melting due to the input of mafic magmatism. Crystal fractionation was the main compositional driver for both successions. The Keimoes Suite granitoids and the Koras Group are associated with extensional regimes subsequent to the main deformational episode in the eastern NMP.

Eclogite-melt/peridotite reaction: Experimental constrains on the destruction mechanism of the North China Craton

To study the mechanism of melt-peridotite reaction pertinent to the destruction of the North China Craton (NCC) lithosphere, a series of experiments were performed at a pressure of 2.0 GPa and temperatures from 1250 to 1400°C using Bixiling eclogite and Damaping peridotite as starting materials. The experimental results show that the reaction between eclogite melt and peridotite causes dissolution of olivine and orthopyroxene and precipitation of clinopyroxene in the melt. The experimental run products, characterized by a lherzolite/pyroxenite/garnet-pyroxenite sequence, are consistent with the mantle xenoliths in the Neogene Hannuoba basalt of the NCC found by Liu et al. (2005). It suggests that the mafic lower continental crust was probably recycled into the mantle during the Mesozoic Era. In the experiments conducted at 1300 and 1350°C, the resulting melts have a high Mg# andesite signature, indicating that the melt-peridotite reaction may have played a major role in the generation of high Mg# andesite. Our experimental results support the hypothesis that melts derived from foundered eclogite in the asthenosphere will consume the lithospheric peridotites. Therefore, melt-peridotite reaction is an important mechanism for the destruction/thinning of the lithosphere.

New insights into the origin of the bimodal volcanism in the middle Okinawa Trough： not a basalt-rhyolite differentiation process

In the middle Okinawa Trough （MOT）,rhyolites have been typically considered as products ofcrystallization differentiation of basaltic magma as afeature of bimodal volcanism. However, the evidence isinsufficient. This paper compared chemical trends ofvolcanic rocks from the MOT with fractional crystal-lization simulation models and experimental results andutilized trace element modeling combined with Rayleighfractionation calculations to re-examine fractional crystal-lization processes in generating rhyolites. Both qualitativeand quantitative studies indicate that andesites, rather thanrhyolites, originate by fractional crystallization frombasalts in the MOT. Furthermore, we established twobatch-melting models for the MOT rhyolites and proposedthat type 1 rhyolites are produced by remelting of andesiteswith amphiboles in the residue, while type 2 rhyolites arederived from remelting of andesites without residualamphiboles. It is difficult to produce melts with a SiO2content ranging from 62% to 68% either by magmaticdifferentiation from basalts or by remelting of andesites,and this difficulty might help account for the compositionalgap （Daly gap） for bimodal volcanism in the OkinawaTrough.

Identification of the Original Tectonic Setting for Oceanic Andesite Using Discrimination Diagrams:An Approach Based on Global Geochemical Data Synthesis

Systematical analyses of data from GEOROC and PetDB database show that large amount of Cenozoic andesites occurred in the various oceanic environments such as mid-oceanic ridge,plumerelated island and oceanic arc.In this study,we employed the geochemical data of 351 mid-ocean ridge andesites(MORA),2539 plume-related andesites(PRA)and 3488 oceanic arc andesites(OAA)from the database to discuss the relationship between andesite tectonic settings and their geochemical features,thereby making an attempt to construct tectonic discrimination diagrams.Based on the data-driven pattern,all available elements were employed to derive logratios for the possible coordinates,and the overlap-rate calculation was adopted to evaluate the discrimination effect of more than 330000 prospective diagrams.Finally,four tectonic discrimination diagrams have been successfully established to identify MORA,PRA and OAA,which can be utilized to identify the original settings of andesite with an age range from Cenozoic to Archean a certain extent.Of these diagrams,PRA is mainly distinguished by high LREE/HREE ratio due to enriched mantle source.Whereas,OAA is mainly characterized by high LILE/HFSE ratio,which reveals that fluids derived from subducted slab play an important role in forming oceanic arc andesites.Consequently,the petrogenesis of andesites is closely related to their tectonic settings.However,it should be noted that those andesites formed in both continental and oceanic environments cannot be effectively distinguished using these diagrams.We strongly recommend integrating the discrimination diagrams result with other geological information to reach a comprehensive interpretation of evolution history with those ancient andesites.This paper presents a case study which suggests that data-driven method is a powerful tool for solving geological problems in this’big data’era.