Granitoid Types Related to Cretaceous Plutonic Au-Quartz Vein and Cu-Fe Skarn Deposits, Kitakami Mountains, Japan

Abstract. Early Cretaceous granitic intrusions are associated with Au‐quartz veins and Cu‐Fe skarns in the the Kitakami Mountains, which are underlain by the late Paleozoic of continental margin‐type sedimentary rocks and Mesozoic accretionary complexes. The plutonic rocks are divided into potassic, high‐Sr/Y calc‐alkaline and low‐Sr/Y calc‐alkaline series. All the metallic mineral deposits are spatially associated with small stocks and plugs; they show no consistent association with the larger plutonic bodies. The plutonic rocks generally belong to the magnetite series but less oxidized in the southwestern part of the Kitakami Mountains where Au‐quartz veins occur. The gold deposits are classified into high and low sulfide types. The high sulfide type contains a high volume of sulfide minerals mostly of chalcopyrite, arsenopyrite and pyrrhotite with low bulk Au/Ag ratios. This type occurs almost exclusively in and surrounding the Orikabe pluton, including two most important gold deposits (Oya and Kohoku) of the Kitakami Mountains. The pluton is composed of potassic gabbroids, potassic granitoids of the shoshonite ‐ high‐K calc‐alkaline series (Orikabe type), and less potassic Sasamori‐type granodiorite. All these rocks belong to a moderately oxidized magnetite series. The Orikabe pluton has one of the lowest initial Sr ratio (0.70392) in the Kitakami Mountains, and the Au‐Cu‐dominant ore components of the high sulfide type Au deposits are considered magmatic in origin carried by the juvenile magmas from the upper mantle. The low sulfide type is generally plain quartz vein with a low volume of sulfides and a high bulk Au/Ag ratio. The associated minerals are often scheelite and/or arsenopyrite and pyrrhotite. The ore deposits include historically famed Au‐quartz veins at Shishiori and Ogayu. They are widespread in the southwestern Kitakami Mountains and may be later than the high sulfide type in age, and are hosted most commonly in the sedimentary rocks, which surround small weakly oxidized magnetite‐series plutons of low to intermediate Sr/Y series. These less differentiated intrusions typically include quartz dior‐ite and granodiorite. Some ore components of this type may have derived from the host sedimentary rocks. Among other mineral deposit types in the region, the largest ore deposit is Kamaishi Cu‐Fe skarn (magnetite ores of 58 MT, Fe 50–64 %; Cu 143 KT). It is related to the high‐Sr/Y series Ganidake granodiorite stock, which is a strongly oxidized magnetite‐series body. In contrast, the second largest deposit in the mountains, Akagane deposit, is a similar‐type skarn but associated with an intrusion classified as less oxidized, ilmenite to intermediate series, and that is intermediate in Sr/Y of calc‐alkaline series granodiorite. Degree of magmatic differentiation appears to be not critical factor in the formation of Au‐quartz vein and Cu‐Fe skarn deposits in the region, but is definitely significant for controlling the distribution of the Mo‐mineralization to the east.     

Constraints on the depth of generation and emplacement of a magmatic epidote‐bearing quartz diorite pluton in the Coast Plutonic Complex,British Columbia

Petrology and P–T estimates indicate that a magmatic epidote‐bearing quartz diorite pluton from Mt. Gamsby, Coast Plutonic Complex, British Columbia, was sourced at pressures below ～1.4 GPa and cooled nearly isobarically at ～0.9 GPa. The P–T path indicates that the magma was within the stability field of magmatic epidote early and remained there upon final crystallization. The pluton formed and crystallized at depths greater than ～30 km. REE data indicate that garnet was absent in the melting region and did not fractionate during crystallization. This suggests that the crust was less than or equal to ～55 km thick at 188 Ma during the early phases of magmatism in the Coast Plutonic Complex. Late Cretaceous contractional deformation and early Tertiary extension exhumed the rocks to upper crustal levels. Textures of magmatic epidote and other magmatic phases, combined with REE data, can be important for constraining the P–T path followed by magmas.     

大兴安岭北段诺敏大山地区早白垩世侵入岩地球化学特征及其构造意义

笔者等选择大兴安岭北段诺敏大山地区早白垩世侵入岩进行了锆石U- Pb年代学和地球化学研究,探讨该区域侵入岩成因类型、岩浆来源及构造环境。该区侵入岩岩性主要为正长花岗岩、正长花岗斑岩和似斑状正长花岗岩,对其中正长花岗岩样品进行LA- ICP- MS锆石U- Pb测年表明,正长花岗岩侵位年龄为129. 5±0. 4 Ma,应为早白垩世岩浆活动的产物,结合区内侵入岩与地层相互接触关系,本区侵入岩形成时代为早白垩世。区内侵入岩具有富硅（SiO2= 67. 36%~74. 09%）、富碱（K2O+Na2O= 8. 88%~9. 34%）、高铝（Al2O3= 12. 56%~16. 15%）,低MgO、TiO2、CaO的特点,属于高钾钙碱性岩石系列;铝饱和指数（A/CNK）为0. 94~1. 31,为准铝质—过铝质岩石。微量元素富集Rb、U、Th、K等大离子亲石元素,强烈亏损Ti、Nb、Sr、P等高场强元素,具有明显的Eu负异常,属于高分异I型花岗岩。岩石Rb/Sr为0. 9~2. 0,Sr/Y为4. 2~7. 2,显示出高Sr、低Y的特点,指示岩浆源区为地壳物质的部分熔融。结合区域研究成果,蒙古—鄂霍茨克构造域在早白垩世之前已结束碰撞,诺敏大山地区早白垩世岩浆活动可能发生在蒙古—鄂霍茨克造山后的伸展环境。     

北秦岭两河口岩体的地球化学特征及其成因

北秦岭西段新元古代两河口岩体,岩性为眼球状片麻状二长花岗岩,含少量黑云母、角闪石及榍石,属钾质钙碱性花岗岩类;主量元素SiO2=68.48%～72.45%,K2O/Na2O=1.35～2.07,为钾质;在K2O～SiO2关系图上投入高钾钙碱性区;A/CNK介于1.03～1.31,总体为钾质钙碱性弱过铝质花岗岩.岩石富集LILE元素（K、Th、Rb、Ba等）,亏损HFSE元素（Ta、Nb、Y、Yb等）;稀土元素总量较高（ZREE=126.82×10^-6～267.359×10^-6）,轻重稀土强烈分馏（∑LREE/∑HREE=5.447～8.894）,稀土元素分配模式为轻稀土富集型,具中等程度Eu负异常（δEu=0.417～0.621）.微量、稀土元素分配模式与北秦岭东部的碰撞型德河岩体、寨根岩体及牛角山岩体一致.Pb-Sr-Nd同位素组成具低的εNd=-3.9180～-6.0064、较高的87Sr/86Sr（t）=0.70760～0.71675、富放射性成因Pb同位素组成,模式年龄tDM=1849.73～2022.79Ma,指示两河口岩体源岩特征与秦岭岩群、马衔山群相近,岩浆源区为下地壳成因.结合区域资料,认为两河口岩体形成于同碰撞末期-后碰撞初期的构造转换时期;这一认识细化了北秦岭新元古代碰撞造山过程;其所确立的汇聚碰撞时间与我国晋宁运动时限一致;为研究中国古陆块在新元古代时期汇聚时限、过程、方式及Rodinia超大陆事件在秦岭地区的响应提供了重要证据.     

Magma mixing in the Sithonia Plutonic Complex, Greece: evidence from mafic microgranular enclaves

Summary ¶Mafic microgranular enclaves occur in most calc-alkaline granitoids, and it is widely accepted that they represent the remnants of basic magmas that interacted with more acid magmas. In this work we present new data on mafic microgranular enclaves occurring in the granodiorites of the Sithonia Plutonic Complex (Northern Greece). Enclave properties have been studied using different methods. Quantitative textural analysis has been carried out in order to decipher the crystallization history of enclaves once they have been entrained in the more acid and cooler host magma. In particular, the nucleation density (C), the mode (M) and the crystal index (n) of enclaves has been measured. Along with textural analysis, the size of enclaves has also been estimated using a method that, based on two-dimensional sections of enclaves, allows the estimation of volume of enclaves. Geochemical analyses have been performed to investigate the degree of chemical interaction that enclaves suffered from the host acid magma. The different data sets have been utilized to furnish a general evolutionary model of the magmatic interaction process between the basic and the acid magmas that led to the formation of the granodioritic host rock and related mafic microgranular enclaves. It is concluded that, as the magmatic interaction process proceeded, the crystallization of enclaves involved the nucleation of apatite and epidote (first stage of crystallization) followed by biotite, ± hornblende, plagioclase, and titanite (second stage of crystallization); the last minerals that nucleate were quartz and K-feldspar. During crystallization enclaves underwent contamination by the host acid magma through flow channels opened during the transfer of mineral phases from the host magma to the enclaves. When the two magmas attained similar rheological behaviour a two-end member mixing process was favoured inducing progressively more vigorous mixing dynamics. Volumetric analysis of enclaves indicates that the smaller ones suffered a more intense geochemical interaction compared to the larger ones. We interpret this evidence as being strictly related to the kinematics of the mixing process, the latter governed by chaotic dynamics. Enclaves are viewed as portions of the basic magma that did not mix completely with the acid host magma and survived the mixing process. Host rocks are considered as volumes of the magmatic system where the more efficient mixing dynamics produced different, generally higher, degrees of hybridisation.Received May 22, 2002; revised version accepted November 5, 2002 Published online February 24, 2003     

Chalcophile element geochemistry of the Boggy Plain zoned pluton, southeastern Australia: a S-saturated barren compositionally diverse magmatic system

The behavior of the platinum group elements (PGE) and Re in felsic magmas is poorly understood due to scarcity of data. We report the concentrations of Ni, Cu, Re, and PGE in the compositionally diverse Boggy Plain zoned pluton (BPZP), which shows a variation of rock type from gabbro through granodiorite and granite to aplite with a SiO₂ range from 52 to 74 wt %. In addition, major silicate and oxide minerals were analyzed for Ni, Cu, and Re, and a systematic sulfide study was carried out to investigate the role of silicate, oxide, and sulfide minerals on chalcophile element geochemistry of the BPZP. Mass balance calculation shows that the whole rock Cu budget hosted by silicate and oxide minerals is <13 wt % and that Cu is dominantly located in sulfide phases, whereas most of the whole rock Ni budget (>70 wt %) is held in major silicate and oxide minerals. Rhenium is dominantly hosted by magnetite and ilmenite. Ovoid-shaped sulfide blebs occur at the boundary between pyroxene phenocrysts and neighboring interstitial phases or within interstitial minerals in the gabbro and the granodiorite. The blebs are composed of pyrrhotite, pyrite, chalcopyrite, and S-bearing Fe-oxide, which contain total trace metals (Co, Ni, Cu, Ag, Pb) up to ~16 wt %. The mineral assemblage, occurrence, shape, and composition of the sulfide blebs are a typical of magmatic sulfide. PGE concentrations in the BPZP vary by more than two orders of magnitude from gabbro (2.7–7.8 ppb Pd, 0.025–0.116 ppb Ir) to aplite (0.05 ppb Pd, 0.001 ppb Ir). Nickel, Cu, Re, and PGE concentrations are positively correlated with MgO in all the rock types although there is a clear discontinuity between the granodiorite and the granite in the trends for Ni, Rh, and Ir when plotted against MgO. Cu/Pd values gradually increase from 6,100 to 52,600 as the MgO content decreases. The sulfide petrology and chalcophile element geochemistry of the BPZP show that sulfide saturation occurred in the late gabbroic stage of magma differentiation. Segregation and distribution of these sulfide blebs controlled Cu and PGE variations within the BPZP rocks although the magma of each rock type may have experienced a different magma evolution history in terms of crustal assimilation and crystal fractionation. The sulfide melt locked in the cumulate rocks must have sequestered a significant portion of the chalcophile elements, which restricted the availability of these metals to magmatic-hydrothermal ore fluids. Therefore, we suggest that the roof rocks that overlay the BPZP were not prospective for magmatic-hydrothermal Cu, Au, or Cu–Au deposits.     

大兴安岭北段诺敏大山地区早白垩世侵入岩地球化学特征及其构造意义

本文选择大兴安岭北段诺敏大山地区早白垩世侵入岩进行了锆石U-Pb年代学和地球化学研究,探讨该区域侵入岩成因类型、岩浆来源及构造环境。该区侵入岩岩性主要为正长花岗岩、正长花岗斑岩和似斑状正长花岗岩,对其中正长花岗岩样品进行LA-ICP-MS锆石U-Pb测年表明,正长花岗岩侵位年龄为129.5±0.4 Ma,应为早白垩世岩浆活动的产物,结合区内侵入岩与地层相互接触关系,本区侵入岩形成时代为早白垩世。区内侵入岩具有富硅（SiO2= 67.36%~74.09%）、富碱（K2O+Na2O= 8.88%~9.34%）、高铝（Al2O3= 12.56%~16.15%）,低MgO、TiO2、CaO的特点,属于高钾钙碱性岩石系列;铝饱和指数（A/CNK）为0.94~1.31,为准铝质—过铝质岩石。微量元素富集Rb、U、Th、K等大离子亲石元素,强烈亏损Ti、Nb、Sr、P等高场强元素,具有明显的Eu负异常,属于高分异I型花岗岩。岩石Rb/Sr为0.9~2.0,Sr/Y为4.2~7.2,显示出高Sr、低Y的特点,指示岩浆源区为地壳物质的部分熔融。结合区域研究成果,蒙古—鄂霍茨克构造域在早白垩世之前已结束碰撞,诺敏大山地区早白垩世岩浆活动可能发生在蒙古—鄂霍茨克造山后的伸展环境。     

Lithospheric thinning and ignition of a Cordilleran magmatic flare-up: Geochemical and O-Hf isotopic constraints from Cretaceous plutons in southern Korea

Northeast Asian continental margins contain the products of magma emplacement driven by prolonged subduction of the (paleo-)Pacific plate. As observed in many Cordilleran arcs, magmatic evolution in this area was punctuated by high-volume pulses amid background periods. The present study investigates the early evolution of the Cretaceous magmatic flare-up using new and published geochronological, geochemical, and O-Hf isotope data from plutonic rocks in the southern Korean Peninsula. After a long (～50 m.y.) magmatic hiatus and the development of the Honam Shear Zone through flat-slab subduction, the Cretaceous flare-up began with the intrusion of monzonites, granodiorites, and granites in the inboard Gyeonggi Massif and the intervening Okcheon Belt. Compared to Jurassic granitoids formed during the former flare-up, Albian (～111 Ma) monzonites found in the Eopyeong area of the Okcheon Belt have distinctly higher zircon ε_Hf(t) (?7.5 ± 1.3) and δ¹⁸O (7.78‰ ± 0.25‰) values and lower whole-rock La/Yb and Sr/Y ratios. The voluminous coeval granodiorite and granite plutons in the Gyeonggi Massif are further reduced in Sr/Y and to a lesser extent, in La/Yb, and have higher zircon ε_Hf(t) values (?13 to ?19) than the Precambrian basement (ca. ?30). These chemical and isotopic features indicate that Early Cretaceous lithospheric thinning, most likely resulting from delamination of tectonically and magmatically overthickened lithospheric keel that was metasomatized during prior subduction episodes, and consequent asthenospheric upwelling played vital roles in igniting the magmatic flare-up. The O-Hf isotopic ranges of synmagmatic zircons from the Albian plutons and their Paleoproterozoic and Jurassic inheritance attest to the involvement of lithospheric mantle and crustal basement in magma generation during this decratonization event. Arc magmatism then migrated trenchward and culminated in the Late Cretaceous, yielding widespread granitoid rocks emplaced at shallow crustal levels. The early Late Cretaceous (94–85 Ma) granites now prevalent in Seoraksan-Woraksan-Sokrisan National Parks are highly silicic and display flat chondrite-normalized rare earth element patterns with deep Eu anomalies. Synmagmatic zircons in these granites mimic their host rock’s chemistry. Delamination-related rejuvenation of crustal protoliths is indicated by zircon ε_Hf(t) values of granites (?6 to ?20) that are consistently higher than the Precambrian basement value. Concomitant core-to-rim variation in zircon O-Hf isotopic compositions reflects a typical sequence of crustal assimilation and fresh input into the magma chamber.     

内蒙古阿拉善地区二叠纪曼德林乌拉岩体年龄、成因及其地质意义

内蒙古阿拉善地块北缘及其邻区广泛出露早古生代—早中生代侵入岩,其时空分布、源区物质组成及成因对研究阿拉善北部地区构造演化乃至整个中亚造山带南缘晚期的演化具有重要意义。曼德林乌拉岩体位于阿拉善地块北部雅布赖-诺尔公-洪古尔玉林带西段,岩体以二长花岗岩为主,广泛发育岩浆暗色包体。这些镁铁质包体为岩浆结构,大多具有塑性外形,并具有多种不平衡结构和矿物组合,如斜长石环带、针状磷灰石等。LA-ICP-MS锆石U-Pb测年结果显示,曼德林乌拉二长花岗岩年龄为271±3Ma,花岗岩中发育的包体年龄为271±2Ma,表明该岩体形成于二叠纪,而非之前认为的中生代。二长花岗岩的锆石ε_(Hf)(t)值为-18.4～-10.1,相应的二阶段Hf模式年龄为1.8～2.3Ga;暗色包体中的13颗二叠纪锆石相应的ε_(Hf)(t)值为-23.6～-9.1,相应的二阶段Hf模式年龄为1.7～2.5Ga。锆石Hf同位素特征表明,形成花岗岩和镁铁质暗色包体的这2种岩浆均来自以古老地壳物质为主的源区,这与东段诺尔公—红古尔玉林地区的中酸性侵入岩相同。曼德林乌拉岩体花岗质岩和镁铁质暗色包体的岩石学、地球化学及同位素研究表明,它们可能也具有岩浆混合成因。这为阿拉善地块北缘区域在二叠纪发生广泛的壳幔相互作用提供了进一步证据。     

The evolution of a silicic magma system: isotopic and chemical evidence from the Woods Mountains volcanic center,eastern California

The isotopic compositions of Nd and Sr and concentrations of major and trace elements were measured in flows and tuffs of the Woods Mountains volcanic center of eastern California to assess the relative roles of mantle versus crustal magma sources and of fractional crystallization in the evolution of silicic magmatic systems. This site was chosen because the contrast in isotopic composition between Precambrian-to-Mesozoic country rocks and the underlying mantle make the isotope ratios sensitive indicators of the proportions of crustal- and mantle-derived magma. The major eruptive unit is the Wild Horse Mesa tuff (15.8 m.y. old), a compositionally zoned rhyolite ignimbrite. Trachyte pumice fragments in the ash-flow deposits provide information on intermediate composition magma types. Crustal xenoliths and younger flows of basalt and andesite (10 m.y. old) provide opportunities to confirm the isotopic compositions of potential mantle and crustal magma sources inferred from regional patterns. The trachyte and rhyolite have _Nd values of -6.2 to -7.5 and initial ⁸⁷Sr/⁸⁶Sr ratios mostly between 0.7086 and 0.7113. These magmas cannot have been melted directly from the continental basement because the _Nd values are too high. They also cannot have formed by closed system fractional crystallization of basalt because the ⁸⁷Sr/⁸⁶Sr ratios are higher than likely values for parental basalt. Both major and trace element variations indicate that crystal fractionation was an important process. These results require that the silicic magmas are end products of the evolution of mantle-derived basalt that underwent extensive fractional crystallization accompanied by assimilation of crustal rock. The mass fraction of crustal components in the trachyte and rhyolite is estimated to be between 10% and 40%, with the lower end of the range considered more likely. The generation of magmas with SiO₂ contents greater than 60% appears to be dominated by crystal fractionation with minimal assimilation of upper crustal rocks.     

Metamorphism of carbonaceous material in the Tono contact aureole, Kitakami Mountains, Japan

Abstract Optical and X-ray studies of carbonaceous material in the Tono contact metamorphic aureole, Kitakami Mountains, northeast Japan, have revealed that metamorphic graphitization proceeded through two discontinuous changes: first, optically anisotropic domains develop within the coaly phytoclast, forming transitional material, and then, ordered graphite crystallizes by the decomposition of pre-existing carbonaceous materials. Coaly material disappears in the uppermost chlorite zone. Transitional material appears in the middle of the lower chlorite zone. Graphite appears in the upper chlorite zone and its modal abundance increases across the andalusite iso-grad to the cordierite isograd where all the carbonaceous materials have converted to graphite. The apparently continuous variation in the crystallographic parameters of the bulk carbonaceous material during graphitization is largely due to variation in the modal proportions of three types of carbonaceous materials. The temperature of graphitization in the present area is at least 100°C higher than the temperature in the Sanbagawa and New Caledonia high-pressure metamorphic terrains, probably due to the slow reaction rate of metamorphic graphitization and to the short duration of contact metamorphism.     

内蒙古中部查干诺尔辉长岩体地球化学特征及其地质意义

查干诺尔辉长岩体由角闪辉长岩和辉长闪长岩组成,呈岩株状侵入于晚石炭世花岗闪长岩和上石炭统宝力高庙组中。角闪辉长岩LA-ICP-MS锆石U-Pb年龄为302Ma,表明该岩体的形成时代为晚石炭世。查干诺尔辉长岩具有高铝钙、低钛和贫碱的特征,属于钙碱性系列岩石。相对富集大离子亲石元素(LILE),亏损高场强元素(HFSE),具明显的Nb、Ta、Ti、Zr、Hf负异常和Pb正异常,与典型的弧岩浆类似。岩石稀土总量较低且变化范围大(ΣREE=63.4~135μg/g),轻、重稀土元素分异程度中等,无明显的Eu异常;稀土元素分布模式表现为轻稀土富集、重稀土平坦且向右缓倾型式。Sr-Nd同位素组成为低(87Sr/86Sr)i(0.70407~0.70428)、正εNd(t)(1.62~3.47),显示了亏损地幔源区的同位素特征;有较高的Pb同位素比值,(206Pb/204Pb)i为17.816~18.040,(207Pb/204Pb)i为15.472~15.484,(208Pb/204Pb)i为37.242~37.534。上述地球化学特征表明查干诺尔辉长岩形成于晚石炭世大陆弧环境,是交代岩石圈地幔含角闪石二辉橄榄岩在低压条件下低程度部分熔融的产物,交代组分来自古亚洲洋闭合期间俯冲板片释放的流体。查干诺尔辉长岩的厘定可能表明内蒙古中部贺根山洋晚石炭世往北俯冲的时间持续至302Ma,并在早二叠世发生闭合。     

Metamorphic Environments around the Cretaceous Kurihashi Pluton in the Kitakami Mountains, NE Japan: in Comparison to the Ganidake Pluton - Kamaishi Skarn Metasomatism

Abstract. Many granitic plutons of Early Cretaceous age are intruded on various scales in the Kitakami Mountains. The stock‐type Ganidake pluton accompanies enormous Fe‐Cu mineralization of the Kamaishi deposits, whereas the Kurihashi pluton accompanies less mineralization. To elucidate the cause of these differences, the metamorphic conditions and redox state of the contact metamorphic aureole around the Kurihashi pluton have been examined by the petrochemical study and gas analysis of the metamorphic rocks. A typical mineral assemblage in the pelitic rocks in the lowest‐grade part is biotite‐muscovite‐chlorite‐quartz‐plagio‐clase‐graphite, which occur more than 2 km away from the contact point with the Kurihashi pluton. Graphite disappears at the 1550 m point, and cordierite and garnet appear in the middle and highest‐grade parts, respectively. A typical mineral assemblage in the tuffaceous rocks in the lower‐grade part is chlorite‐actinolite‐biotite‐quartz‐plagioclase. Actinolite changes into hornblende near to the pluton. The CO₂/CH₄ ratios obtained in measurements by gas chromatography exceeds 100 in the pelitic rocks at the contact point with the pluton. The ratios decrease and become less than 0.1 with distance from the pluton. Equilibrium temperatures calculated from a garnet‐biotite pair in the pelitic rock and a hornblende‐plagioclase pair in the tuffaceous rock are 640d? and 681 d?C at the contact point, respectively. The log?o₂ values among these metamorphic aureoles estimated from the CO₂/CH₄ ratios are slightly lower than the FMQ‐buffer. Redox states of the contact metamorphic aureole are kept in an intermediate condition between oxidized magma of the Kurihashi pluton and graphite‐bearing pelitic country rocks. Judging from these metamorphic conditions around the Kurihashi pluton and from the re‐evaluation of the previous knowledge about contact metasomatism around the Ganidake pluton, the Kurihashi metamorphism has occurred at higher temperatures and dry conditions than the Ganidake metasomatism. These differences in the metamorphic conditions and presence or absence of a large limestone mass around the pluton might be the principal reasons why the Kurihashi pluton accompanies less mineralization and the Ganidake pluton accompanies gigantic Kamaishi skarn mineralization.     

Relationships between tectonite and cumulate in ophiolites: the Miyamori ultramafic complex,Kitakami Mountains,northeast Japan

The Miyamori ultramafic complex forms the basal ultramafic portion of an ophiolite. The complex consists of a tectonic member which is composed dominantly of harzburgite and dunite, and a cumulate member which is composed of interlayered wehrlite, dunite and clinopyroxenite. The tectonite member is overlain by the cumulate member and characterized by tabular granular or porphyroclastic textures, a strong lineation and magnesian olivine (Mg/Mg + Fe = 0.88–0.93). In contrast, the cumulate member exhibits igneous textures and shows no evidence of a penetrative deformation. The olivine is less magnesian than that of the tectonite member (Mg/Mg + Fe = 0.82–0.89). At the boundary of the two members, harzburgite xenoliths have been found in wehrlite of the cumulate member. The minerals at the core of a few large harzburgite xenoliths preserve the compositional characteristics of typical harzburgites in the tectonic member. The occurrence of the harzburgite xenolith in vehrlite and the structural and textural features of the two members indicate that the tectonite member had already been deformed before a magma intruded into the tectonite member and formed a magma chamber in which cumulates were deposited together with harzburgite fragments on the floor of the tectonite. The xenoliths show a fine grained mosaic texture, which may be attributed by the heat of the intruded magma. This hiatus implies that the magma which made the cumulate member did not originate directly from the underlying harzburgite.     

Magma–host interactions during differentiation and emplacement of a shallow-level, zoned granitic pluton (Tarçouate pluton, Morocco): implications for magma emplacement

The Tarçouate pluton (Anti-Atlas, Morocco) is an inversely zoned laccolith emplaced 583 Ma ago into low-grade metasediments, with the following succession: leucocratic granites, biotite–granodiorites (±monzodiorites), hornblende–granodiorites (±monzodiorites) and monzodiorites syn-plutonic dykes. These rocks form two distinct, chemically coherent, units:

(1) A main unit consists of layered (572<59 wt.%) and homogeneous (632<67%) hornblende–granodiorites, biotite–granodiorites (672<72%) and aplites (702<76%). All these rocks are metaluminous to peraluminous and display fractionated HREE depleted patterns (La/Yb_N=14–61; Yb_N=0.7–6.8). Initial ⁸⁷Sr/⁸⁶Sr ratios (0.7072 to 0.7080) increase, whereas _Nd(t) values (−1.7 to −2.8) decrease from the hornblende– to the biotite–granodiorites. Monzodiorites occur as mafic microgranular enclaves or syn-plutonic dykes.

(2) A subordinate unit consists of leucocratic, distinctly peraluminous, muscovite-bearing granites (722<75%) occurring at the northern edge of the pluton and as dykes in the surrounding schists towards the top of the pluton. These rocks are free of monzodioritic enclaves. They display less fractionated patterns with higher HREE contents (La/Yb_N=2–19; Yb_N=11–18), a distinct _Nd(t) value (−11.8) and a ⁸⁷Sr/⁸⁶Sr initial ratio (0.7480) within those of the surrounding schists (0.7393–0.7819).

Magma–host interactions are closely related to differentiation and occurred at different levels, but mainly before emplacement. Field relationships and petrogenetic modelling show that the bt–granodiorites formed at levels deeper than the level of emplacement, by fractional crystallisation (0.65

These data preclude any significant material transfer process for the emplacement of the Tarçouate pluton, but rather suggest assembly of successive pulses of variably differentiated crystal-poor magmas. These shallow level granitic plutons can be considered as an end-member of magma emplacement with minimum interactions with the country rocks.     

和龙百里坪银、多金属矿化集中区白垩纪岩浆岩地球化学特征与成矿

百里坪银、多金属矿化集中区白垩纪岩浆岩主要有花岗质与闪长质两个岩石系列。前者为富钾质钙碱性岩石,后者为富钠质碱钙性岩石,二者具有不同的变异曲线特征;花岗质岩石Co/N i值平均为2.644,闪长质岩石平均为0.498;花岗质岩石R b/S r值平均为0.223,闪长质岩石平均为0.020;在R b、S r、B a演化趋势上,花岗质岩石趋向富集R b,闪长质岩石趋向富集S r、B a,二者明显不同;两个岩系代表性岩石在稀土含量、稀土比值以及稀土模式上都不相同。两者的成矿专属性为:花岗质岩石对A g、Cu、P b、Zn矿化有利,闪长质岩石对A g、A u矿化有利。     

Geochemical evolution of a shallow magma plumbing system during the last 500 years, Miyakejima volcano, Japan: Constraints from U-Th-Ra systematics

In order to unravel magma processes and the geochemical evolution of shallow plumbing systems beneath active volcanoes, we investigated U-series disequilibria of rocks erupted over the past 500 years (1469-2000 AD) from Miyakejima volcano, Izu arc, Japan. Miyakejima volcanic rocks show ²³⁸U-²³⁰Th-²²⁶Ra disequilibria with excess ²³⁸U and ²²⁶Ra, due to the addition of slab-derived fluids to the mantle wedge. Basaltic bombs of the 2000 AD eruption have the lowest (²³⁰Th/²³²Th) ratio compared to older Miyakejima eruptives, yielding the youngest ²³⁸U-²³⁰Th model age of 2 kyr. This reinforces our previous model that fluid release from the slab and subsequent magma generation in the mantle wedge beneath Miyakejima occur episodically on a several-kyr timescale. In the last 500 years, Miyakejima eruptives show: (1) a vertical trend in a (²³⁰Th/²³²Th)-(²³⁸U/²³²Th) diagram and (2) a positive linear correlation in a (²²⁶Ra/²³⁰Th)₀ − 1/²³⁰Th diagram, which is also observed in lavas from some of the single eruptions (e.g., 1940, 1962, and 1983 AD). The variations cannot be produced by simple fractional crystallization in a magma chamber with radioactive decay of ²³⁰Th and ²²⁶Ra, but it is possibly produced by synchronous generation of melts in the mantle wedge with different upwelling rate or addition of multiple slab-derived fluids. A much more favorable scenario is that some basaltic magmas were intermittently supplied from deep in the mantle and injected into the crust, subsequently modifying the original magma composition and producing variations in (²³⁰Th/²³²Th) and (²²⁶Ra/²³⁰Th)₀ ratios via assimilation and fractional crystallization (AFC). The assimilant of the AFC process would be a volcanic edifice of previous Miyakejima magmatism. Due to the relatively short timescales involved, the interaction between the assimilant and recent Miyakejima magmatism has not been recorded by the Sr-Nd-Pb isotopic systems. In such cases, Th isotopes and (²²⁶Ra/²³⁰Th) ratio are excellent geochemical tracers of magmatic evolution.     

Fractional crystallization and magma mixing: evidence from porphyritic diorite-granodiorite dykes and mafic microgranular enclaves within the Zhoukoudian pluton,Beijing

The Zhoukoudian pluton in the North China craton is a circular granodiorite intrusion containing porphyritic diorite dykes (PDDs), porphyritic granodiorite dykes (PGDs) and abundant mafic microgranular enclaves (MMEs), which provide an excellent opportunity to study fractional crystallization and magma mixing. The PDDs and PGDs are located in the western part of the pluton with the PDDs intruded by the PGDs. The dykes have similar mineral assemblages although plagioclase in the PDDs has higher anorthite content than the PGDs. Linear relationships between the SiO₂ and most major and trace element contents, as well as a positive trend of initial ⁸⁷Sr/⁸⁶Sr ratios and a negative trend of epsilon Nd values with increasing SiO₂ contents for the dykes suggest that both types were formed by assimilation and fractional crystallization of a common parental magma. Major oxide mass balance and trace element Rayleigh fractionation modeling points to early separation of garnet (11 %), clinopyroxene (27 %), orthopyroxene (16 %), plagioclase (25 %), biotite (19 %), and apatite (2 %) and late fractionation of hornblende (25 %), plagioclase (46 %), biotite (25 %), apatite (1 %), and magnetite (3 %). Most MMEs occur within the transitional granodiorite of the Zhoukoudian pluton. Zoned MMEs, dyke-like MME swarms, local presence of concave margins, veins and enclaves of host granodiorite within some MMEs, and several MMEs surrounded by the biotite-rich granodiorite support their formation by multiple magma mixing events, which finally resulted in different whole-rock major oxides and compatible elements, but homogeneous mineral major oxides (except zoned plagioclase), whole-rock incompatible elements and Sr-Nd isotopes between the MMEs and their host granodiorite. We suggest that multiple magma mixing events might also cause complexly zoned plagioclase in the Zhoukoudian pluton. Relative calcic, irregular or patchy cores and dusty zoned mantles from the zoned plagioclase crystals and their relatively low anorthite content indicate multiple mixing events between mafic/intermediate and felsic magmas. The mafic/intermediate end-members could be represented by the diabase dykes and the PDDs. Therefore, the dykes and MMEs in the Zhoukoudian pluton are genetically linked.     

Redox evolution of differentiating hydrous basaltic magmas recorded by zircon and apatites in mafic cumulates: The case of the Malayer Plutonic Complex,Western Iran

Mafic-ultramafic cumulates can provide records of basaltic magma chambers' conditions and processes, which are often difficult to determine in areas dominated by crustal-derived felsic intrusions, such as the Malayer Plutonic Complex (MPC), Western Iran. New U-Pb zircon ages for mafic cumulates in the MPC confirm the presence of isolated magma chambers of contrasting compositions during Middle Jurassic. Mafic cumulates found in seven separate zones across the MPC vary from olivine gabbro to anorthosite. While the mineralogical, textural, and geochemical lines of evidence recorded in mafic cumulates indicate pH₂O controls on the liquidus phases, the estimated oxygen fugacity (logfO₂) using zircon and apatite chemistry suggests a smoothly rising redox state during the fractionation process, consistent with the trend expected for late-stages differentiation of hydrous arc magmas. This trend is further confirmed by sulfur speciation in apatites determined from microbeam sulfur K-edge X-ray absorption near edge structure (μ-XANES) spectra (S⁶⁺/∑S = 0.93–0.98 ~ FMQ + 2 to 0.99 ~ FMQ + 3, where ∑S = S⁶⁺+S⁴⁺+S²⁻). The low S content and increasing redox state of the fractionating basaltic melts most likely resulted from preferential removal of sulfur en-route to the magma chambers along with effective assimilation of oxidizing crustal components. The reduced condition in the early basaltic melt is also evidenced by the presence of pyrite and magnetite inclusions in olivines in mafic cumulates. The shift in the prevailing fO₂ from sulfide-saturated to sulfate-bearing recorded by MPC mafic cumulates, similar to that in other magmatic arcs, is accompanied by changes in the differentiation path from transitional tholeiitic to calc-alkaline.     

辽西义县组玄武岩中环带状单斜辉石的成因及其对岩浆演化的约束

辽西四合屯地区早白垩世义县组玄武岩包含大量具环带结构单斜辉石。本文对这些单斜辉石进行了系统的矿物学、主微量元素和原位Sr同位素分析,并探讨了义县组玄武岩的岩浆成因和演化历史。四合屯义县组玄武岩中的单斜辉石主要发育正环带结构,少量为反环带结构。反环带单斜辉石核部主要为透辉石,并具有低的Mg^#(65.5~79.7)和Cr₂O₃(< 0.48%),以及高的Na₂O (0.34%~1.33%)和Al₂O₃(1.02%~3.20%)。它们具有轻稀土元素富集的配分型式,具有明显的Eu和Sr负异常,说明其与斜长石平衡过。在Mg^#-Na₂O/Al₂O₃相关图中,反环带单斜辉石核部总体落入华北古老下地壳麻粒岩中单斜辉石的成分范围,因此其可能是来自下地壳麻粒岩的捕虏晶。反环带单斜辉石幔部和正环带单斜辉石核部主要为普通辉石,它们具有高度一致且变化范围相对窄的化学组成,包括高的Mg^#(84.7~89.2)和Cr₂O₃(0.13%~1.41%),以及低的Na₂O (0.17%~0.48%)和Al₂O₃(1.44%~2.57%)。它们的轻重稀土元素强分馏,与玄武岩全岩组成吻合。利用单斜辉石和基性岩浆之间Fe-Mg及微量元素的分配系数,我们计算了与反环带单斜辉石幔部和正环带单斜辉石核部平衡的岩浆组成,结果表明它们与剔除橄榄石捕虏晶的全岩组成达到平衡,因此这部分单斜辉石是岩浆成因的斑晶。它们相比全岩具有变化范围较宽和整体略高的原位⁸⁷Sr/⁸⁶Sr比值(0.7058~0.7085)。岩浆成因单斜辉石结晶温压计算结果(1171~1221℃、0.54~1.08GPa)表明义县组玄武岩的岩浆演化主要发生在下地壳浅部(18~36km)。四合屯义县组玄武岩具有壳幔双重属性,捕虏晶矿物的同化混染和下地壳榴辉岩的拆沉作用并不能合理解释其大陆下地壳属性。本次研究中反环带单斜辉石核部捕虏晶具有较高的HREE含量,它们可能作为源区残留相记录了华北东部古老下地壳在中生代的部分熔融过程,该过程中对应的熔体相则可以解释义县组玄武岩的下地壳属性。因此,我们认为四合屯义县组玄武岩是壳幔相互作用(岩浆混合)的结果。义县组玄武岩的主体来自受俯冲洋壳改造的岩石圈地幔,在下地壳岩浆房或幔源岩浆上升过程中存在少量下地壳来源酸性熔体的混合作用。