Crystal Growth and Crystallization Time Scales of the Panzhihua Layered Intrusion: Constraint from Crystal Size Distribution

The Panzhihua layered intrusions is generated closely related to the Emeishan LIPs. This paper analyzes the spatial distribution of plagioclase and pyroxene. The quantitative texture analysis of 2209 plagioclase shows that the characteristic length of plagioclase is 0.54 to 0.96 mm, the intercept variation range is large, from ?0.67 to 0.96, and the slope is ?1.85 to ?1.04, the Aspect Ratio shows from 1.84 to 2.59 and fractal dimension D is 1.908–1.933. The quantitative texture analysis of 2342 pyroxene shows that the characteristic length of pyroxene is 0.38–0.64 mm, the intercept shows from 0.46 to 2.26, The slope ranges from ?2.6 to ?1.47, the Aspect Ratio value varies from 1.53 to 1.71, the fractal dimension D is 0.93 to 1.13. All the CSDs results of the Panzhihua intrusions indicate that plagioclase and pyroxene form in an open magma system and undergo four replenishment of magma injection. The plagioclase crystals do not grow as the lathlike shape, and the fractal growth leads to complex crystal surface. The plagioclase undergoes deformation compaction during the crystal process, and then is oriented. The pyroxene crystals grow along an approximately triaxial ratio and undergo texture adjustment and small crystal dissolution reabsorption. When all crystals in magma system grows up to 2 mm, the pyroxene undergoes cumulation in the Panzhihua layered intrusions. The plagioclase crystallization time scale is 171.23–304.41 years, representing that the crystallization is the more uniform in central part of the melt. The nucleation density continuously increases during the crystallization process of the magma system. The time scale to reach the final maximum crystal nucleation density is 15.28–58.98 years.     

http://www.sciencedirect.com/science/article/pii/S1674987113001370

Mineral chemistry,whole-rock major oxide,and trace element compositions have been determined for the Tuerkubantao mafic-ultramafic intrusion,in order to understand the early Paleozoic tectonic evolution of the West Junggar orogenic belt at the southern margin of the Central Asian orogenic belt.The Tuerkubantao mafic-ultramafic intrusion is a well-differentiated complex comprising peridotite,olivine pyroxenite,gabbro,and diorite.The ultramafic rocks are mostly seen in the central part of the intrusion and surrounded by mafic rocks.The Tuerkubantao intrusive rocks are characterized by enrichment of large ion lithophile elements and depleted high field strength elements relative to N-MORB.In addition,the Tuerkubantao intrusion displays relatively low Th/U and Nb/U(1.13—2.98 and 2.53—7.02,respectively) and high La/Nb and Ba/Nb(1.15—4.19 and 37.7—79.82,respectively).These features indicate that the primary magma of the intrusion was derived from partial melting of a previously metasomatized mantle source in a subduction setting.The trace element patterns of peridotites,gabbros,and diorite in the Tuerkubantao intrusion have sub-parallel trends,suggesting that the different rock types are related to each other by differentiation of the same primary magma.The intrusive contact between peridotite and gabbro clearly suggest that the Tuerkubantao is not a fragment of an ophiolite.However,the Tuerkubantao intrusion displays many similarities with Alaskan-type mafic-ultramafic intrusions along major sutures of Phanerozoic orogenic belts.Common features include their geodynamic setting,internal lithological zoning,and geochemistry.The striking similarities indicate that the middle Devonian Tuerkubantao intrusion likely formed in a subduction-related setting similar to that of the Alaskan-type intrusions.In combination with the Devonian magmatism and porphyry mineralization,we propose that subduction of the oceanic slab has widely existed in the expansive oceans during the Devonian around the Junggar block.     

A Genetic Model for the Layered Intrusions and Related V—Ti—Magnetite Deposits in Panzhihua—Xichang Region,Southwest China

The area studied had developed into an inactive continental margin on the western edge of the Yangtze plate during the Jinning period in Proterozoic time.The Yangtze plate drifted from the south northwards in Paleozoic time and reached about 9oS between Late Cambrian and Early Ordovician .The alkalic olivine basalt magma,which was produced via 2-5% partial melting of a fractionated mantle and was accumulated in a deep-seated magma chamber near the Moho.found its way upward along the reviving boundary faults into the upper crust.Two types of layered intrusions,basic and ultrabasic-basic,crystallized from the magma at different pressures.The high initial oxygen fugacity of the magma provides a favorable condition for the deposition of Fe-Ti oxides in the early stage of magmaevolution,resulting in large-sized early magmatic deposits.Magma crystallization may have been interrupted and repeated as a result of pulsative magma influx,giving rise to rhythmic cycles (including the corresponding V-Ti-magnetite ore beds).The magma was stratified owing to double diffusion at the crystallization front and convection was thus occasioned by the density gradient.This has greatly complicated the sequence as would be expected from normal crystallization.     

Phase Equilibria Constraints on Relations of Ore‐bearing Intrusions with Flood Basalts in the Panxi Region,Southwestern China

There are two types of temporally and spatially associated intrusions within the Emeishan large igneous province (LIP); namely, small ultramafic subvolcanic sills that host magmatic Cu-Ni-Platinum Group Element (PGE)-bearing sulfide deposits and large mafic layered intrusions that host giant Ti-V magnetite deposits in the Panxi region. However, except for their coeval ages, the genetic relations between the ore-bearing intrusions and extrusive rocks are poorly understood. Phase equilibria analysis (Q-Pl-Ol-Opx-Cpx system) has been carried out to elucidate whether ore-bearing Panzhihua, Xinjie and Limahe intrusions are co-magmatic with the picrites and flood basalts (including high-Ti, low-Ti and alkali basalts), respectively. In this system, the parental magma can be classified as silica-undersaturated olivine basalt and silica-saturated tholeiite. The equivalents of the parental magma of the Xinjie and Limahe peridotites and picrites and low-Ti basalts are silica-undersaturated, whereas the Limahe gabbro-diorites and high-Ti basalts are silica-saturated. In contrast, the Panzhihua intrusion appears to be alkali character. Phase equilibria relations clearly show that the magmas that formed the Panzhihua intrusion and high-Ti basalts cannot be co-magmatic as there is no way to derive one liquid from another by fractional crystallization. On the other hand, the Panzhihua intrusion appears to be related to Permian alkali intrusions in the region, but does not appear to be related to the alkali basalts recognized in the Longzhoushan lava stratigraphy. Comparably, the Limahe intrusion appears to be a genetic relation to the picrites, whereas the Xinjie intrusion may be genetically related to be low-Ti basalts. Additionally, the gabbro-diorites and peridotites of the Limahe intrusion are not co-magmatic, and the former appears to be derived liquid from high-Ti basalts.     

Early Crystallized Titanomagnetite from Evolved Magmas and Magma Recharge in the Mesoproterozoic Zhuqing Oxide-Bearing Gabbroic Intrusions, Sichuan, SW China

The ca. 1.5 Ga mafic intrusions in the Zhuqing area, predominantly composed of alkaline gabbroic rocks in the Kangdian region of SW China, occur as dykes or irregular small intrusions hosting Fe–Ti–V mineralization. All of the intrusions that intrude the dolomite or shales of the Mesoproterozoic Heishan Formation of the Huili Group are composed of three cyclic units from the base upward: a marginal cyclic unit, a lower cyclic unit and an upper cyclic unit. The Fe–Ti–V oxide ore bodies are hosted in the lower and upper cyclic units. The textural relationships between minerals in the intrusions suggest that titanomagnetite formed earlier than silicate grains because euhedral magnetite and ilmenite grains were enclosed in clinopyroxene and plagioclase. Both the magnetitess–ilmenitess intergrowths due to subsolidus oxidation–exsolutions and the relative higher V distribution coefficient between magnetite and silicate melts in the gabbros from the Zhuqing area are different from those of other typical Fe–Ti bearing mafic rocks, suggesting that the oxygen fugacity was low in the gabbric rocks from the Zhuqing area. This finding was further confirmed by calculations based on the compositions of magnetite and ilmenite pairs. The clinopyroxene, magnetite and ilmenite in the intrusions from the Zhuqing area had considerably lower Mg O than those of other typical Fe–Ti oxide-rich complexes, suggesting that the titanomagnetite from the intrusion may have crystallized at a relatively late stage of evolution from a more evolved magma. Titanomagnetite first fractionally crystallized and subsequently settled in the lower parts of the magma chamber, where it concentrated and formed Fe–Ti–V oxide ore layers at the bases of the lower and upper cycles. Moreover, the occurrence of multiple Fe-Ti oxide layers alternating with Fe-Ti oxide-bearing silicate layers suggests that multiple pulses of magma were involved in the formation of the intrusions and related Fe-Ti-V oxide deposits in the Zhuqing area.     

Rhenium-Osmium Isotope Constraints on the Origin of the Tianyu Cu-Ni Deposit in the East Tianshan Orogenic Belt, Xinjiang, NW China

The Tianyu Cu-Ni sulfide deposit occurs in the north margin of the Central Tianshan Arc in East Tianshan orogenic belt, Xinjiang, NW China. The intrusions consist of gabbro, peridotite, and olivine pyroxenite. The peridotite and pyroxenite are the main host rock for the Cu-Ni ores. Rhenium and osmium isotopic analyses of Ni- and Cu-bearing sulfide minerals from the deposit have been used to determine the source of osmium, and by inference, the sources of ore metals. Sulfide ore samples have Os and Re concentrations varying in the ranges 1.85 to 4.58 ppb and 93.56 to 146.00 ppb, respectively. An initial 187Os/188Os ratio ranges from 0.86 to 1.23 for the ores and the γOs values from 592 to 2227. Osmium isotopic data suggest that the Tianyu intrusion and associated Cu-Ni mineralization has derived from crustal-contaminated mantle melts. The intrusions early show island-arc geochemical signatures, which indicate that the Hulu mafic–ultramafic intrusions, along with the Cu-Ni deposit, formed as a result of subduction of oceanic crust in the Early Permian.     

Zircon U‐Pb Ages and Geochemistry of Permo‐Carboniferous Mafic Intrusions in the Xilinhot Area,Inner Mongolia: Constraints on the Northward Subduction of the Paleo‐Asian Ocean

The Central Asian Orogenic Belt(CAOB) resulted from accretion during the Paleozoic subduction of the PaleoAsian Ocean. The Xilinhot area in Inner Mongolia is located in the northern subduction zone of the central-eastern CAOB and outcropped a large number of late Paleozoic mafic intrusions. The characteristics of magma source and tectonic setting of the mafic intrusions and their response to the closure process of the Paleo-Asian Ocean are still controversial. This study presents LA-ICPMS zircon U-Pb ages and geochemical features of mafic intrusions in the Xilinhot area to constrain the northward subduction of the Paleo-Asian Ocean. The mafic intrusions consist of gabbro, hornblende gabbro, and diabase. Their intrusion times can be divided into three stages of 326–321 Ma, 276 Ma and 254 Ma by zircon U-Pb ages. The first two stages of the 326–276 Ma intrusions mostly originated from subduction-modified continental lithospheric mantle sources that underwent a variable degree partial melting(5–30%), recording the subduction of oceanic crust. The third stage of the 254 Ma mafic rocks also show arc-related features. The primary magma compositions calculated by PRIMELT2 modeling on three samples of ～326 Ma and two samples of ～254 Ma show that these mafic samples are characterized by a variable range in SiO_2(47.51–51.47 wt%), Al_2O_3(11.46–15.55 wt%), ΣFeO(8.27–9.61 wt%), MgO(13.01–15.18 wt%) and CaO(9.13–11.67 wt%), consisting with the features between enriched mantle and lower continental crust. The source mantle melting of mafic intrusions occurred under temperatures of 1302–1351°C and pressures of 0.92–1.30 GPa. The magmatic processes occurred near the crust-mantle boundary at about 33–45 km underground. Combined with previous studies, it is concluded that Carboniferous to early Permian(～326–275 Ma) northward subduction of the Paleo-Asian oceanic crust led to the formation of the mafic magmatism in the Baolidao arc zone. The whole region had entered the collision environment at ～254 Ma, but with subduction-related environments locally. The final collision between the North China craton and the South Mongolian microcontinent may have lasted until ca. 230 Ma.     

Petrology of ultramafic to mafic cumulate rocks from the G?ksun(Kahramanmaras) ophiolite,southeast Turkey

The G?ksun(Kahramanmaras)ophiolite(GKO),cropping out in a tectonic window bounded by the Malatya metamorphic unit on both the north and south,is located in the EW-trending lower nappe zone of the southeast Anatolian orogenic belt(Turkey).It exhibits a complete oceanic lithospheric section and overlies the Middle Eocene Maden Group/Complex with a tectonic contact at its base.The ophiolitic rocks and the tectonically overlying Malatya metamorphic(continental)unit were intruded by I-type calc-alkaline Late Cretaceous granitoid(~81-84 Ma).The ultramafic to cumulates in the GKO are represented by wehrlite,plagioclase wehrlite,olivine gabbro and gabbro.The crystallization order for the cumulate rocks is as follows:olivine±chromian spinel→clinopyroxene→plagioclase.The major and trace element geochemistry as well as the mineral chemistry of the ultramafic to mafic cumulate rocks suggest that the primary magma generating the GKO is compositionally similar to that observed in the modern island-arc tholeiitic sequences.The mineral chemistry of the ultramafic to mafic cumulates indicates that they were derived from a mantle source that was previously depleted by earlier partial melting events.The highly magnesian olivine(Fo77-83),clinopyroxene(Mg#of 82-90)and the highly Ca-plagioclase(An81-89)exhibit a close similarity to those,which formed in a supra-subduction zone(SSZ)setting.The field and the geochemical evidence suggest that the GKO formed as part of a much larger sheet of oceanic lithosphere,which accreted to the base of the Tauride active continental margin,including the ispendere,K?mürhan and the Guleman ophiolites.The latter were contemporaneous and genetically/tectonically related within the same SSZ setting during the closure of the Neotethyan oceanic basin(Berit Ocean)between the Taurides to the north and the Bitlis-Pütürge massif to the south during the Late Cretaceous.     

Geochronology, Petrology and Geochemistry of Xingdi No. 3 Mafic-Ultramafic Intrusions in the Northeastern Tarim Craton, NW China

The Xingdi mafic-ultramafic intrusions occur in the northeastern margin of the Tarim craton. The Xingdi No. 3 intrusion is the smallest of four intrusions, with an exposed area of 1.7 km~2, and the zircon U-Pb age of the intrusion is 752±4 Ma. The intrusion consists of gabbros, pyroxenites and peridotites, and exhibits a crystallization sequence of the main rock-forming minerals as olivine, orthopyroxene, clinopyroxene and plagioclase. Mineralization occurred at or near the boundaries of the intrusion between pyroxenites and peridotites, and appears as a layered or lenticular shape about 500 m long and 4–15 m wide. The primary sulfides have a relatively simple mineralogy dominated by pyrrhotite-pentlandite-chalcopyrite assemblages, which occur as droplet, star-like and graphic texture and locally sideronitic structures. Geochronological and geochemistry investigations suggest that the Xingdi mafic-ultramafic intrusions and coeval volcanic rock in the Kuluktag area of the Tarim craton formed in an intracontinental breakup environment. Based on the composition of the dominant rockforming minerals and covariant relationships of other oxides versus Mg O, the parental magma of the Xingdi No.3 intrusion belongs to high-Mg tholeiitic basaltic magmas with Mg O of 10.78 wt%. The Xingdi No.3 intrusive rocks are characterized by light REE enrichment relative to heavy REE, negative Nb-Ta anomalies, low ~(143)Nd/~(144)Nd ratios(from 0.511183 to 0.511793) and high initial ~(87)Sr/~(86)Sr ratios(from 0.7051 to 0.7113). The magma was derived from the enriched-lithospheric mantle and was contaminated during emplacement. According to rock assemblages, mineralization, olivine characteristics, geochemical characteristics and mass balance, there are better copper-nickel ore prospects in the Xingdi No.3 intrusion than in the other three intrusions in the area.     

Iron Isotopic Fractionation and Origin of Chromitites in the Paleo-Moho Transition Zone of the Kop Ophiolite, NE Turkey

The Kop ophiolite in NE Turkey is a fragment of Neo-Tethyan forearc.It can be mainly divided into a paleo-Moho transition zone(MTZ)in the North and a harzburgitic mantle sequence in the South.Dunites are predominant in the MTZ of the Kop ophiolite,and they are locally interlayered with chromitites and enclose minor bodies of harzburgites near the petrological Moho boundary.Large Fe isotopic variations were observed for magnesiochromite(-0.14‰to 0.06‰)and olivine(-0.12‰to 0.14‰)from the MTZ chromitites,dunites and harzburgites.In individual dunite samples,magnesiochromite usually has lighter Fe isotopic compositions than olivine,which was probably caused by subsolidus Mg-Fe exchange between the two mineral phases.Both magnesiochromite and olivine display an increasing trend ofδ56Fe along a profile from chromitite todunite.This trend reflects continuous fractional crystallization in a magma chamber,which resulted in heavier Fe isotopes concentrated in the evolved magmas.In each cumulative cycle of chromitite and dunite,dunite was formed from relatively evolved melts after massive precipitation of magnesiochromite.Mixing of more primitive and evolved melts in the magma chamber was a potential mechanism for triggering the crystallization of magnesiochromite,generating chromitite layers in the cumulate pile.Before mixing happened,the primitive melts had reacted with mantle harzburgites during their ascendance;whereas the evolved melts may lie on the olivine-chromite cotectic near the liquidus field of pyroxene.Variable degrees of magma mixing and differentiation are expected to generate melts with differentδ56Fe values,accounting for the Fe isotopic variations of the Kop MTZ.     

Petrology and Geochemistry of the Dangqiong Ophiolite,Western Yarlung-Zangbo Suture Zone,Tibet,China

The Dangqiong ophiolite, the largest in the western segment of the Yarlung-Zangbo Suture Zone(YZSZ)ophiolite belt in southern Tibet, consists of discontinuous mantle peridotite and intrusive mafic rocks. The former is composed dominantly of harzburgite, with minor dunite, locally lherzolite and some dunite containing lenses and veins of chromitite. The latter, mafic dykes(gabbro and diabase dykes), occur mainly in the southern part. This study carried out geochemical analysis on both rocks. The results show that the mantle peridotite has Fo values in olivine from 89.92 to 91.63 and is characterized by low aluminum contents(1.5–4.66 wt%) and high Mg# values(91.06–94.53) of clinopyroxene. Most spinels in the Dangqiong peridotites have typical Mg# values ranging from 61.07 to 72.52, with corresponding Cr# values ranging from 17.67 to 31.66, and have TiO2 contents from 0 to 0.09%, indicating only a low degree of partial melting(10–15%). The olivine-spinel equilibrium and spinel chemistry of the Dangqiong peridotites suggest that they originated deeper mantle(20 kbar). The gabbro dykes show N-MORB-type patterns of REE and trace elements. The presence of amphibole in the Dangqiong gabbro suggests the late-stage alteration of subduction-derived fluids. All the lherzolites and harzburgites in Dangqiong have similar distribution patterns of REE and trace elements, the mineral chemistry in the harzburgites and lherzolites indicates compositions similar to those of abyssal and forearc peridotites, suggesting that the ophiolite in Dangqiong formed in a MOR environment and then was modified by late-stage melts and fluids in a suprasubduction zone(SSZ) setting. This formation process is consistent with that of the Luobusa ophiolite in the eastern Yarlung-Zangbo Suture Zone and Purang ophiolite in the western Yarlung-Zangbo Suture Zone.     

Genesis and Tectonic Implications of the Kabr El-BonayaUltramafic Rocks,Sinai Peninsula,Egypt: Constraints fromMineralogical and Geochemical Characteristics

The Kabr El-Bonaya mafic–ultramafic intrusion is exposed along the southeastern border of the Sinai Peninsula and the northernmost segment of the Arabian-Nubian Shield(ANS). It occurs as an elliptical intrusive body that is located along the major NE–SW trending fracture zones that prevail in the Kid metamorphic complex. The ultramafic rocks in the complex comprise ultramafic cumulates of peridotites(dunite, harzburgite and wehrlite) and pyroxenite. These rocks are generally unmetamorphosed and have intrusive contacts with the country rock. Mineral chemistry and whole-rock chemical compositions of these ultramafic rocks are mostly consistent with those of residual mantle peridotites from refractory suprasubduction tectonic settings. Based on the variations of the major elements, the studied ultramafic rocks are consistent with those of a supra-subduction zone mantle, as it seems to have melted at 1–2 GPa and 1300–1450°C. Linear variations of Al2 O3, CaO, V and Ni with MgO, coupled with incompatible and rare-earth-element depletion and mineral compositions,suggest prior events of partial melting in both wehrlites and harzburgites. The LREE enrichment in the harzburgite, as well as the development of Cr-rich spinel, is consistent with a history of melt–peridotite interaction. The calculated(Sm/Yb)N variations for the studied peridotites indicate a general increase in the addition of fluids with an increasing degree of melting from the wehrlite(～13–15 wt% of fluid) in the source, after initial spinel peridotite melting to the harzburgite(～20–25 wt% of fluid) in the same source, which is contrary to normal abyssal peridotites. The estimated equilibration temperature ranges from 1214 to 1321°C for the studied wehrlites and from 1297 to 1374°C for harzburgites. The Mg-rich nature of the analysed olivines from the studied ultramafic rocks(Fo = 81.41 to 91.77) reflect their primary composition and are similar to olivines in Alaskan-type ultramafic rocks. The Fo content of the analyzed olivines decrease slightly from the dunite to the harzburgite to the wehrlite and to pyroxenite, reflecting a fractional crystallization trend. The high Cr# and very low TiO2 contents(0.03–0.12 wt%) of the Cr-spinels from the studied peridotites are mostly consistent with modern highly refractory fore-arc peridotites, indicating that these peridotites developed in a supra-subduction zone environment.     

Ore Genesis of the Kalatongke Cu–Ni Sulfide Deposits, Western China: Constraints from Volatile Chemical and Carbon Isotopic Compositions

The Kalatongke Cu–Ni sulfide deposits located in the East Junggar terrane, northern Xinjiang, western China are the largest magmatic sulfide deposits in the Central Asian Orogenic Belt (CAOB). The chemical and carbon isotopic compositions of the volatiles trapped in olivine, pyroxene and sulfide mineral separates were analyzed by vacuum stepwise-heating mass spectrometry. The results show that the released volatiles are concentrated at three temperature intervals of 200–400°C, 400–900°C and 900–1200°C. The released volatiles from silicate mineral separates at 400–900°C and 900–1200°C have similar chemical and carbon isotopic compositions, which are mainly composed of H2O (av. ~92 mol%) with minor H2, CO2, H2S and SO2, and they are likely associated with the ore-forming magmatic volatiles. Light δ13CCO2 values (from ?20.86‰ to ?12.85‰) of pyroxene indicate crustal contamination occurred prior to or synchronous with pyroxene crystallization of mantle-derived ore-forming magma. The elevated contents of H2 and H2O in the olivine and pyroxene suggest a deep mantle-originated ore-forming volatile mixed with aqueous volatiles from recycled subducted slab. High contents of CO2 in the ore-forming magma volatiles led to an increase in oxygen fugacity, and thereby reduced the solubility of sulfur in the magma, then triggered sulfur saturation followed by sulfide melt segregation; CO2 contents correlated with Cu contents in the whole rocks suggest that a supercritical state of CO2 in the ore-forming magma system under high temperature and pressure conditions might play a key role in the assemblage of huge Cu and Ni elements. The volatiles released from constituent minerals of intrusion 1# have more CO2 and SO2 oxidized gases, higher CO2/CH4 and SO2/H2S ratios and lighter δ13CCO2 than those of intrusions 2# and 3#. This combination suggests that the higher oxidation state of the volatiles in intrusion 1# than intrusions 2# and 3#, which could be one of key ore-forming factors for large amounts of ores and high contents of Cu and Ni in intrusion 1#. The volatiles released at 200–400°C are dominated by H2O with minor CO2, N2+CO and SO2, with δ13CCO2 values (?25.66‰ to ?22.98‰) within the crustal ranges, and are considered to be related to secondary tectonic– hydrothermal activities.     

Geochronology, Geochemistry and Sr-Nd-Pb-Hf Isotopes of No. I Complex from the Shitoukengde Ni–Cu Sulfide Deposit in the Eastern Kunlun Orogen, Western China: Implications for the Magmatic Source, Geodynamic Setting and Genesis

The Shitoukengde Ni-Cu deposit, located in the Eastern Kunlun Orogen, comprises three mafic–ultramafic complexes, with the No. I complex hosting six Ni-Cu orebodies found recently. The deposit is hosted in the small ultramafic bodies intruding Proterozoic metamorphic rocks. Complexes at Shitoukengde contain all kinds of mafic-ultramafic rocks, and olivine websterite and pyroxene peridotite are the most important Ni-Cu-hosted rocks. Zircon U-Pb dating suggests that the Shitoukengde Ni-Cu deposit formed in late Silurian(426–422 Ma), and their zircons have εHf(t) values of-9.4 to 5.9 with the older T_(DM1) ages(0.80–1.42 Ga). Mafic-ultramafic rocks from the No. I complex show the similar rare earth and trace element patterns, which are enriched in light rare earth elements and large ion lithophile elements(e.g., K, Rb, Th) and depleted in heavy rare earth elements and high field strength elements(e.g., Ta, Nb, Zr, Ti). Sulfides from the deposit have the slightly higher δ~(34)S values of 1.9–4.3‰ than the mantle(0 ± 2‰). The major and trace element characteristics, and Sr-Nd-Pb and Hf, S isotopes indicate that their parental magmas originated from a metasomatised, asthenospheric mantle source which had previously been modified by subduction-related fluids, and experienced significant crustal contamination both in the magma chamber and during ascent triggering S oversaturation by addition of S and Si, that resulted in the deposition and enrichment of sulfides. Combined with the tectonic evolution, we suggest that the Shitoukengde Ni-Cu deposit formed in the post-collisional, extensional regime related to the subducted oceanic slab break-off after the Wanbaogou oceanic basalt plateau collaged northward to the Qaidam Block in late Silurian.     

Assimilation of carbonate country rock by the parent magma of the Panzhihua Fe-Ti-V deposit（SW China）：Evidence from stable isotopes

The Panzhihua intrusion in southwest China is part of the Emeishan Large Igneous Province and host of a large Fe-Ti-V ore deposit.During emplacement of the main intrusion,multiple generations of mafic dykes invaded carbonate wall rocks,producing a large contact aureole.We measured the oxygen-isotope composition of the intrusions,their constituent minerals,and samples of the country rock.Magnetite and plagioclase from Panzhihua intrusion haveδ¹⁸O values that are consistent with magmatic equilibrium, and formed from magmas withδ¹⁸O values that were 1-2‰higher than expected in a mantle-derived magma.The unmetamorphosed country rock has highδ¹⁸O values,ranging from 13.2‰（sandstone） to 24.6-28.6‰（dolomite）.The skarns and marbles from the aureole have lowerδ¹⁸O andδ¹³C values than their protolith suggesting interaction with fluids that were in exchange equilibrium with the adjacent mafic magmas and especially the numerous mafic dykes that intruded the aureole.This would explain the alteration ofδ¹⁸O of the dykes which have significantly higher values than expected for a mantle-derived magma.Depending on the exactδ¹⁸O values assumed for the magma and contaminant, the amount of assimilation required to produce the elevatedδ¹⁸O value of the Panzhihua intrusion was between 8 and 13.7 wt.%,assuming simple mixing.The exact mechanism of contamination is unclear but may involve a combination of assimilation of bulk country rock,mixing with a melt of the country rock and exchange with CO₂-rich fluid derived from decarbonation of the marls and dolomites.These mechanisms,particularly the latter,were probably involved in the formation of the Fe-Ti-V ores.     

牛头山地区玄武岩类的高压分异作用

The Nutoushan basaltic cone, eonsisting of subalkali (quartz-tholeiite and olivinetholeiite) and alkali basalts, is Late Tertiary in age. Its major characteristics are generalized as follows: (1) Both early subalkali and late alkali basalts are formed under the same geological environment. (2) The continuity in chemical composition from snbalkali to alkali and the low FeO/MgO in alkali basalts show that they arc the products of cognate magmatic diffcrcntiation. (3) The change from low REE abundance and weak enrichment of LREE in subalkali to high REE abundance and strong enrichment of LREE in alkali basalts indicates obvious REE cnrichment and fractionation during magmatic differentiation.Weak positive Eu anomalies in the REE patterns arc indicative of their formation under low oxygen fugacity conditions. (4) According to the calculated values, 70-75% of the primary olivine tholeiitic magma had been separated as subalkaline basalic magma, the rest residual magma became alkaline basaltic magma. This result is consistent to the field observation that the outcrop area of subalkali basalts is four times as much as that of alkali basalts. (5) The basaltic rocks of Nutoushan show an S-type distribution straddling the thermal barrier on Ol‘-Ne‘-Qu‘ diagram and an evolution tendency for Ne to increase with increasing FeO/MgO. This is in agreement with the melting experimental data on olivine basalts at 10-20 kb. (6) Mantle-derived inclusions (spinel lherzolite) in this area occur in both alkali olivine basalts and olivine tholeiites. The latter is of extremely rare oceurrence. The formation temperature and pressure of the inclusions in alkali basalts and olivine tholeiites have been calculated. The results silow that the alkaline basaltic magma was separated from the subalkaline basaltic magma at about 20 kb. Basaltic rocks in Nutoushan were formed through the soealled “high pressure differentiation”, that is, at about 20 kb the crystallization of clinopyroxene and orthpyroxene resulted in the separation of subalkaline basaltic magma from the primary olivine tholeiitic magma, and then the residue gradually became alkaline olivine basaltic magma.     

The Voisey’s Bay Ni-Cu-Co Sulfide Deposit, Labrador, Canada: Emplacement of Silicate and Sulfide-Laden Magmas into Spaces Created within a Structural Corridor

The Voisey’s Bay Ni-Cu-Co sulfide deposit is hosted in a 1.34 Ga mafic intrusion that is part of the Nain Plutonic Suite in Labrador, Canada.The Ni-Cu-Co sulfide mineralization is associated with magmatic breccias that are typically contained in weakly mineralized olivine gabbros, troctolites and ferrogabbros, but also occur as veins in adjacent paragneiss.The mineralization is associated with a dyke-like body which is termed the feeder dyke.This dyke connects the shallow differentiated Eastern Deeps chamber in the east to a deeper intrusion in the west termed the Western Deeps Intrusion.Where the conduit is connected to the Eastern Deeps Intrusion, the Eastern Deeps Deposit is developed at the entry line of the dyke along the steep north wall of the Eastern Deeps Intrusion.The Eastern Deeps Deposit is surrounded by a halo of moderately to weakly mineralized Variable-Textured Troctolite (VTT) that reaches a maximum thickness above the ENE-WSW axis of the Eastern Deeps Deposit. At depth to the west, the conduit is adjacent to the south side of the Western Deeps Intrusion, where the dyke and intrusion contain disseminated magmatic sulfide mineralization.The Reid Brook Zone plunges to the east within the dyke, and both the dyke and adjacent paragneiss are mineralized.The Ovoid Deposit comprises a bowl-shaped body of massive sulfide where the dyke widens near to the present-day surface.It is not clear whether this deposit was developed as a widened-zone within the conduit or at the entry point into a chamber that is now lost to erosion. The massive sulfides and breccia sulfides of the Eastern Deeps are petrologically and chemically different when compared to the disseminated sulfides in the VTT; there is a marked break in Ni tenor (Ni content in 100% sulfide, abbreviated to [Ni]100) and Ni/Co of sulfide between the two.The boundary of the sulfide types is often marked by strong sub-horizontal alignment of heavily digested and metamorphosed paragneiss fragments, development of barren olivine gabbro, and by a change from typically massive sulfides and breccias sulfides into more typical variable-textured troctolites with heavy to weak disseminated sulfide.Sulfides hosted in the feeder dyke tend to have low metal tenors ([Ni]100=2.5%-3.5%); sulfides in Eastern Deeps massive and breccia ores have intermediate Ni tenors ([Ni]100=3.5%-4%) and disseminated sulfides in overlying rocks have high Ni tenors ([Ni] 100=4%-8%) . Conduit-hosted mineralization and mineral zones in the paragneiss adjacent to the Reid Brook Deposit tend to have lower Ni tenor than the Ovoid and Eastern Deeps Deposits.The tenor of mineral hosted in the country rock gneisses tends to be the same as that developed in the conduit ; the injection of the sulfide into the country rocks likely occurred before formation of monosulfide solid solution.The Ovoid Deposit is characterized by coarse-grained loop-textured ores consisting of 10cm-2msized pyrrhotite crystals separated by chalcopyrite and pentlandite.A small lens of massive cubanite surrounded by more magnetite-rich sulfide assemblages represents what appears to be the product of in-situ sulfide fractionation. Detailed exploration in the area between the Reid Brook Zone and the Eastern Deeps has shown that these intrusions and ore deposits are connected by a branched dyke and chamber system in a major westeast fault zone.The Eastern Deeps chamber may be controlled by graben-like fault structures , and the marginal structures appear to have controlled dykes which connect the chambers at different levels in the crust.The geological relationships in the intrusion are consistent with emplacement of the silicate and sulfide laden magma from a deeper sub-chamber (possibly a deep eastward extension of the Western Deeps Intrusion where S-saturation was initially achieved) .The silicate and sulfide magmas were likely emplaced through this conduit into the Eastern Deeps intrusion as a number of different fragment laden pulses of sulfide-silicate melt that evolved with different R factors and in response to some variation in the degree of evolution of the parental magma.S isotope and S/Se data coupled with geological evidence point to a crustal source for the sulfur , and the site of equilibration of mafic magma and crustal S is placed at depth in a sulfidic Tasiuyak Gneiss. The structural control on emplacement of small intrusions with transported sulfide is a feature found in different nickel sulfide deposits around the world.Champagne glass-shaped openings in sub-vertical chonoliths are a common morphology for this deposit type (e.g.the Jinchuan , Huangshan , Huangshandong , Jingbulake , Limahe , Hong Qi Ling deposits in China , the Eagle deposits in the United States , and the Double Eagle deposit in Canada) .Some of the structures of the Midcontinent Rift of North America also host Ni-Cu-(PGE) deposits of this type (e.g.the Current Lake Complex in the Quetico Fault Zone in Ontario , Canada and the Tamarac mineralisation in the Great Lakes Structural Zone of the United States) .Other major nickel deposits associated with flat structures adjacent to major mantle-penetrating structures include the Noril’sk , Noril’sk II , Kharaelakh , NW Talnakh , and NE Talnakh Intrusions of the Noril’sk Region of Russia , the Kalatongke deposit in NW China , and Babel-Nebo in Western Australia.These deposits are all formed in mantle-penetrating structural conduits that link into the roots of large igneous provinces near the edges of old cratons.     

Discovery of a Miocene Mafic Dyke from the Western Hills of Beijing and its Geological Implications

Abstract: The present study is the first report of a Miocene mafic dyke from the Dahuichang, in the Western Hills of Beijing. The dyke cuts the fossil-dated Changxindian Formation of Eocene sequences and yields K-Ar ages of 14–15?Ma. The dyke is fine-grained diabase and has 49.84%–50.81% SiO2 and 3.56–3.97% Na2O+K2O, high TiO2 (1.65%–1.93%) and MgO (7.36%–9.85%), and low K2O (<1.22%) contents, with Na2O>K2O and slightly varied magnesium numbers (Mg#=55.54–62.74). In trace elements geochemistry, the dyke is very similar to the Miocene basalts from Jining and Hanuoba. The enrichment of light rare earth elements ([La/Yb]N=5.03–6.12) and large ion lithophile elements (LILEs?), no negative Eu anomalies, relatively high Cr (265–326 ppm) and Ni (155–262 ppm), and almost constant V concentrations (194–213 ppm) reveal that the composition close to the primary basaltic magma from ?an enriched-mantle source, with little crustal contamination and fractional crystallization. The basaltic magma was possibly derived from the upwelling asthenosphere mantle beneath eastern China during the Miocene lithospheric thinning.     

Geochemistry of mafic rocks and melt inclusions and their implications for the heat source of the 14.0°S hydrothermal field,South Mid-Atlantic Ridge

Fresh rocks sampled from the 14.0°S hydrothermal field of the South Atlantic Ridge can be divided into two categories： olivine-gabbro and basalt. The olivine-gabbro is composed mainly of three types of minerals： olivine, clinopyroxene and plagioclase, while a multitude of melt inclusions occur in the plagioclase phenocrysts of the basalts. We analyzed the whole-rock, major and trace elements contents of the basaks, the mineral chemistry of phenocrysts and melt inclusions in the basalts, and the mineral chemistry of olivine-clinopyroxene-plagioclase in the olivine-gabbro, then simulated magma evolution within the crust using the COMAGMAT program. The whole-rock geochemistry shows that all the basalts exhibit typical N-MORB characteristics. In addition, the mineral chemistry characteristics of the olivine-gabbro （low-Fo olivine, low-Mg# clinopyroxene, high-TiO2 clinopyroxene, low-An plagioclase）, show that strong magma differentiation occurred within the crust. Nevertheless, significant discrepancies between those minerals and phenocrysts in the basalts （high-Fo olivine, high-An plagioclase） reflect the heterogeneity of magma differentiation. High Mg# （-~0.72） melt inclusions isobaric partial crystallization simulations suggest that the magma differentiation occurred at the depth shallower than 13.03 km below the seafloor, and both the vertical differentiation column shows distinct discrepancies from that of a steady-state magma chamber. Instead, a series of independent magma intrusions probably occurred within the crust, and their corresponding crystallized bodies, as the primary high-temperature thermal anomalies within the off-axis crust, probably act as the heat source for the development of the 14.0°S hydrothermal system.     

Exsolution Lamellae in Olivine Grains of Dunite Units from Different Types of Mafic‐Ultramafic Complexes

Exsolution microstructures in olivine grains from dunite units in a few selected tectonic environments are reported here. They include lamellae of clinopyroxene and clinopyroxene-magnetite intergrowth in the Gaositai and Yellow Hill Alaskan-type complexes, clinopyroxene-magnetite intergrowth in the K?z?lda? ophiolite, and chromite lamellae in the Hongshishan mafic-ultramafic intrusive complex. These lamellae commonly occur as needle-or rod-like features and are oriented in olivine grains. The host olivine grains have Fo contents of 92.5–92.6 in the Gaositai complex, 86.5–90.1 in the Yellow Hill complex, 93.2–93.4 in the K?z?lda? ophiolite and 86.9–88.3 in the Hongshishan complex. Clinopyroxene in the rod-like intergrowth exsolved in olivine grains in the Gaositai and Yellow Hill is diopside with similar major element compositions of Ca O(23.6–24.3 wt%), SiO_2(52.2–54.0 wt%), Al_2O_3(0.67–2.15 wt%), Cr_2O_3(0.10–0.42 wt%) and Na_2O(0.14–0.26 wt%). It falls into the compositional field of hydrothermal clinopyroxene and its origin is thus probably related to reaction between dunite and fluids. The enrichment of the fluids in Ca~(2+), Fe~(3+), Cr~(3+) and Na+, resulted in elevated concentrations of these cations in olivine solid solutions via the reaction. With decreasing temperature, the olivine solid solutions altered to an intergrowth of magnetite and clinopyroxene. The Fe~(3+) and Cr~(3+) preferentially partitioned into magnetite, while Ca~(2+) and Na+ entered clinopyroxene. Since the studied Alaskan-type complexes and ophiolite formed in a subduction environment, the fluids were probably released from the subducted slab. In contrast, the exsolved chromite in olivine grains from the Hongshishan complex that formed in post-orogenic extension setting can be related to olivine equilibrated with Cr-bearing liquid. Similarly, these lamellae have all been observed in serpentine surrounding olivine grains, indicating genetic relations with serpentinization.