Magma Mingling/mixing vs. Magmatic Fractionation: Geneses of the Shirakawa Mo-mineralized Granitoids, Central Japan

Abstract. Leucocratic biotite granites are main components in the Hatogaya pluton and the Hirase stock in the Shirakawa region of central Japan. Molybdenite‐quartz vein mineralizations are widespread in and around the Hatogaya pluton and the Hirase stock, in which the largest is vein swarm of the Hirase mine. Mafic enclaves occur abundantly with granitic to granodi‐oritic matrix in the northern part of the Hatogaya pluton, while they are rare in the Hirase granitic stock. The enclaves with generally round shape have mostly diabasic to fine plutonic textures under the microscope, and show interfingering and lobate contacts with the felsic matrix. The enclaves are quartz monzodiorite in composition containing SiO>₂ mostly around 60 %. They have felsic blebs, thus are considered a mingled magma of basaltic compositions originated in depth and a felsic magma generated from the Hida metamorphic‐plutonic complexes or their basement. The mingled magma further mixed with and reacted with the felsic magma with SiO₂ 70 %, and then formed granodiorite‐granite of the high Na group (Na₂O higher than 4.25 %). Thus, compositional variation of the northern part of the Hatogaya pluton was caused by the magma mingling. The mingling happened to be deeper level produced homogeneous granodiorite of the Mihoro pluton. Biotite granite of the low Na group (less than 4.25 %) could have originated in a granitic magma generated also from the Hida metamorphic‐plutonic complexes or their basement. Most of the granites, occurring in the southern part of the Hatogaya pluton and Hirase stock, show high Rb/Sr ratio, strong Eu negative anomalies and flat REE patterns, and are thus considered as fractionated products of the SiO₂ 70 % original magma. The strong concentration of molybdenum in the Hirase stock can be explained by high degree of magmatic fractionation which produced MoS₂‐rich residual melts, suitable fractures developed at the latest Cretaceous time, and preservation of the mineralized fractures at the present level of erosion.     

Some Pertinent Features of Mo‐Mineralized Granitoids in the Circum‐Pacific Region

Petrological characteristics of granitic rocks related to the world large molybdenum deposits are studied. The granitoids are evaluated by Fe₂O₃+TiO₂‐FeO+MnO‐MgO diagrams, and found to all plot to the magnetite‐series field. They are all high silica and high‐K series, but not A‐type, except for the Climax‐type porphyries and some others in the Colorado mineral belt. By‐product molybdenum contained in porphyry copper deposits, lower grade but huge tonnage, occurs with calc‐alkaline I‐type magnetite‐series granodiorite and monzogranite. Felsic intrusive rocks of the Climax mine are A‐type and are exceptionally high in trace elements such as F and Rb, which are generally enriched with W and Sn‐related granitoids that originated in crustal source rocks. The by‐product molybdenites in porphyry copper deposits appear to originate in adakitic granodiorite or monzogranite, having deep origins with the subducted slab or thickened juvenile mafic lower crust. Therefore, there is no single magma type but the magnetite series, which concentrates a large volume of molybdenum in the ore deposits.     

Characterization of Magnetically Zoned Pluton, Tsushima, Japan

Abstract: Systematic measurements of magnetic susceptibility were carried out at the outcrops of the Uchiyama granitic pluton, Tsushima Islands. The pluton consists of hornblende-biotite granodiorite and biotite granite. It intrudes concordantly along an anticline axis of the Taishu Group in middle Miocene (16. 10. 5 Ma), and crops out in five areas. The results show that the low magnetic susceptibility facies (LM–facies, <100A-10^-6 emu/g) always occurs in the peripheral part of the pluton, while the high magnetic susceptibility facies (HM–facies, > 250 A 10^-6 emu/g) occupies the core through the medium magnetic susceptibility zone (MM–facies). It is pointed out that the pluton forms a magnetic zoning within a single plutonic body.
Chemical compositions of the granitoids show no conspicuous differences in major elements among the LM–, MM–, and HM–facies. Biotites from the LM–, MM–, and HM-facies also indicate uniform compositions with Fe/(Fe+Mg)=0. 6.
The pluton is principally surrounded by black mudstones of the Lower Formation of the Taishu Group, which is deltaic to shallow sea sediments of the Eocene to early Miocene age, and the carbon contents in them were analyzed. The analyses show that carbon was mostly decomposed and lost in the hornfels zone, while mudstones in the non-hornfels zone usually contain 0. 5 to 0. 7 % C.
From these facts, it is concluded that the magnetically zoned pluton at Tsushima was formed by an external volatile buffer system such as CH₄–CO₂ during the solidifying magma process.     

东天山三岔口铜矿区矿化岩体SHRIMP U-Pb年代学及锶同位素地球化学特征研究

锆石SHRIMPU Pb年代学及全岩和含矿石英脉锶同位素组成研究表明 ,东天山三岔口铜矿区矿化斜长花岗斑岩的岩浆侵位年龄为 2 78± 4Ma(95 %可信度 ) ,即成岩成矿时代为早中二叠世 ;矿化斜长花岗斑岩、英云闪长 (玢 )岩及含矿石英脉的 (87Sr/ 86Sr) i 在测定误差内一致 ,介于 0 .70 370～ 0 .70 4 37之间 ,而计算得到的εSr(t)值变化于 - 8.5～ +2 .7之间。锶同位素组成特征表明 ,成矿岩体为典型的I型花岗岩 ,可能是幔源火山岩或上地幔物质部分熔融的产物。     

Comparative Studies of the Petrochemistry of Sn–W‐Mineralized Granitoids: Continent vs. Island Arc

Granitic rocks obtained during field excursions of the famed mineralized regions of the Erzgebirge, Germany (mainly tin‐bearing), and South China (mainly tungsten‐bearing) have been geochemically analyzed and their results are compared with similar (mainly tungsten‐bearing) granites in the island‐arc setting of Southwest Japan. The studied granitoids all belong to the ilmenite‐series. The collision‐related Erzgebirge granitoids are rich in K₂O and P₂O_5, have high A/CNK ratios (1.11–1.24, i.e. S type), but are also high in Ga/Al ratio (i.e., having some A‐type characteristics). In South China, the Xihuashan granites, in contrast, are very low in P₂O₅, and have A/CNK slightly above 1.0 (1.01–1.05), indicative of I type granites. The (Sn‐) W‐related granites of southwest Japan have similarly low P₂O₅ and A/CNK ratios, indicative also of I‐type. Both in the Xihuashan and southwest Japan, the tungsten‐related granites have high whole‐rock δ¹⁸O values implying involvement of W‐rich crustal rocks. Sn and W contents of the unaltered granites are lowest in the island‐arc setting where the related Sn–W deposits are smallest in size relative to the collision and continental margin settings of the Erzgebirge and South China.     

Density model of the lithosphere in the Tsushima Basin, Sea of Japan

The 2-D density modeling of the sublatitudinal deep seismic sounding profile in the Tsushima Basin (Sea of Japan) was performed. The available data allow us to presume that the opening of the Tsushima Basin took place under conditions of an anomalously heated mantle that fostered emplacement of mantle material into the basinal crust and basin/continental margin transition zone of the Korean Peninsula attended by formation of the oceanic crust. The increased (relative to the normal oceanic crust) thickness of the basinal crust was conditioned by the accumulation of a large amount of terrigenous material and volcanogenic rocks in the upper part of the crust and underplating of mantle material in the crustal base.     

Apatite Composition of Representative Magnetite‐series and Ilmenite‐series Granitoids in Japan

Apatites of representative magnetite‐series and ilmenite‐series granitoids were studied in the Japanese Islands. Concentrations of the volatile components F, Cl and SO₃ are differently distributed in apatites of these granitoid series. Apatites are always fluoroapatite. They have weakly higher F content in the ilmenite series than in the magnetite series. In contrast, Cl and SO₃, are significantly concentrated in apatites of the magnetite series compared to the ilmenite series. These characteristics reflect the original concentrations of these components in the host granitic magmas. A high fO₂ seems most important for the S‐concentration as sulfate in apatite of the magnetite series. REE and Y are only erratically high in the studied apatites.     

东天山三岔口铜矿区矿化岩体SHRIMP U-Pb年化学及锶同位素地球化学特征研究

锆石SHRIMP U-Pb年代学及全岩和含矿石英脉锶同位素组成研究表明,东天山三岔口铜矿区矿化斜长花岗斑岩的岩浆侵位年龄为278±4 Ma(95%可信度),即成岩成矿时代为早中二叠世;矿化斜长花岗斑岩、英云闪长(玢)岩及含矿石英脉的(87Sr/86Sr)i在测定误差内一致,介于0.70370~0.70437之间,而计算得到的εSr(t)值变化于-8.5~+2.7之间.锶同位素组成特征表明,成矿岩体为典型的I型花岗岩,可能是幔源火山岩或上地幔物质部分熔融的产物.     

Thermochronology for the Granitic Pluton Related to Lead-Zinc Mineralization in Tsushima, Japan

Abstract: K–Ar datings and oxygen isotope analyses revealed a cooling history of the Uchiyama granitic pluton, which is genetically related to the Pb-Zn deposits (Taishu mine) in the Tsushima Islands, Japan. The pluton intrudes into the Paleogene Taishu Group to form the biotite-hornfels zone, while the Taishu vein-type Pb–Zn deposits occur in fissures developed in the non-hornfels zone about 1 to 3 km westward from the contact. Amphibole and biotite K-Ar ages of the pluton have a wide range from 19 to 13 Ma. Oxygen isotopes of the biotite and coexisting quartz grains indicate that isotopic exchange reactions have occurred under subsolidus conditions, and that the K-Ar ages are affected by various cooling rates in the pluton. The mineralization age of the Taishu ore deposits is obtained for the first time to be 15.40.8 Ma by a K-Ar age of 2 M ₁–muscovite in a calcite–quartz–muscovite–chlorite veinlet of the Shintomi orebody. Whole rock K-Ar ages of biotite-hornfels near the pluton represent similar ages to the ore deposits. Using blocking temperature calculated from reported diffusivity for argon, the pluton was cooled from 560 to 350C in the period of 17 to 14 Ma. The vein formation took place after the time when temperature in wall rocks of the pluton had dropped below the brittle-plastic transition (about 400C). These results imply that the cooling of the pluton has caused injection of magmatic fluids into meteoric hydrothermal systems, and the Pb-Zn mineralization has occurred due to this mixing at the age of about 15 Ma.     

巴尔哲地区碱性花岗岩的成岩和成矿作用:矿化和未矿化岩体的比较

巴尔哲地区碱性花岗岩由801和802两个岩体组成:801岩体为典型的稀有稀土矿化碱性花岗岩,其中锆和稀土元素的储量已达超大型矿床规模;802岩体稀土元素和某些稀有元素含量也较高,但未形成工业矿床。对巴尔哲地区碱性花岗岩进行了主量元素、微量元素和Sr-Nd同位素地球化学研究,研究结果表明:矿化和未矿化碱性花岗岩体为同源岩浆,具有富集地幔源的特征,在其侵位过程中没有受到同化混染作用的影响。主量元素和微量元素地球化学特征显示,在脱离源区时802岩体的原始岩浆演化程度明显高于801岩体的原始岩浆,因此802岩体的成矿潜力应大于801岩体。801岩体具有明显的岩相分带,可以分上下2个岩相带:上部为钠长石花岗岩带,下部为钠闪石花岗岩带。在上部岩相带中分布着相当数量的伟晶岩块(壳)和晶洞,在该岩相带的石英(斑晶和伟晶岩)中存在着大量的流体-熔体包裹体以及岩石具有明显的稀土四分组效应,这些地质地球化学特征均暗示801岩体的原始岩浆在侵位后发生充分的分异演化;802岩体具有典型的浅成-超浅成侵入相岩石的结构特征,且该岩体岩石的主量元素和微量元素组成相对均匀,表明802岩体的原始岩浆在侵位后没有经历明显的分异演化过程。显然,岩浆侵位后分异演化程度对巴尔哲地区碱性花岗岩的稀有稀土元素成矿作用具有明显的制约。     

冈底斯西段麻木矽卡岩型铅锌矿化区岩浆岩锆石U-Pb年龄及地质意义

西藏革吉麻木铅锌矿化区是中冈底斯成矿带(中拉萨地块)西段南缘近年发现的一个矽卡岩型铅锌矿化区,矿化产于花岗斑岩与灰岩外接触带的矽卡岩内.金属矿物有方铅矿、闪锌矿、磁铁矿、镜铁矿、黄铁矿;脉石矿物有石榴子石、透辉石、蔷薇辉石、阳起石、绿帘石、石英、长石、方解石;蚀变包括硅化、黏土化、大理岩化、绿泥石化;赋矿围岩是捷嘎组灰...     

Geneses of Two Types of Mafic Rocks to Carry Placer‐Magnetite Ores in the Sanin Granitic Belt,SW Japan

Two types of mafic rocks from the central Sanin district, and their mafic minerals, were studied chemically and microscopically. They are classified into pyroxene‐containing gabbroid and hornblende–biotite quartz diorite. The gabbroid had higher color index but lower magnetite content; while the quartz diorite had lower color index, but higher magnetite content. The magnetite contents are also related to the amounts of hydrous mafic silicates. The gabbroic magma having pyroxene–amphibole assemblage, originated in the upper mantle, was considered essentially anhydrous, but became partly hydrous on the way to the site of solidification in the continental crust, and crystallized some magnetites with hypersthene and amphibole. The quartz dioritic magma was formed by partial melting of possibly subducting ocean‐floor basalts, once exposed to the sea‐floor then altered; thus the magmas became hydrous and oxidized originally, and precipitated abundant magnetite and hydrous mafic silicates from the early crystallization stage onward. Their weathered parts provided the most placer magnetite ores in the history.     

Oxygen Isotopic Compositions of the Cape Ashizuri Granitoids in Southwest Japan

Oxygen isotopic composition was determined on representative samples of the Cape Ashizuri plutonic rocks, in order to estimate the genetic background of the biotite granite and alkaline granitoids. The biotite granite (70.1–76.1% SiO₂) ranged from 8.61 to 9.30‰ δ¹⁸O and averaged as 8.9‰ δ¹⁸O (n = 3), which is much smaller than the same Miocene granitoids of the Okueyama (avg. 10.1‰) and Takakumayama (11.6‰) granitic bodies, which are associated with tin mineralization. Among the alkaline granitoids, quartz syenites also have values as low as 7.14–8.70‰, with an average of 8.0‰ (n = 3), and monzonite and gabbro vary from 6.14 to 7.86‰, with an average of 7.0‰ (n = 3). These alkaline granitoids may be lower crustal in origin. The gabbroids containing 12.5% MgO at the maximum with low Sr initial ratio, are considered to be derived from the upper mantle through the fore‐arc tectonic break‐up on the subducting slab.     

Genesis and Mixing/Mingling of Mafic and Felsic Magmas of Back-Arc Granite: Miocene Tsushima Pluton, Southwest Japan

The Middle Miocene Tsushima granite pluton is composed of leucocratic granites, gray granites and numerous mafic microgranular enclaves (MME). The granites have a metaluminous to slightly peraluminous composition and belong to the calc‐alkaline series, as do many other coeval granites of southwestern Japan, all of which formed in relation to the opening of the Sea of Japan. The Tsushima granites are unique in that they occur in the back‐arc area of the innermost Inner Zone of Southwest Japan, contain numerous miarolitic cavities, and show shallow crystallization (2–6 km deep), based on hornblende geobarometry. The leucocratic granite has higher initial ⁸⁷Sr/⁸⁶Sr ratios (0.7065–0.7085) and lower ε_Nd(t) (?7.70 to ?4.35) than the MME of basaltic–dacitic composition (0.7044–0.7061 and ?0.53 to ?5.24), whereas most gray granites have intermediate chemical and Sr–Nd isotopic compositions (0.7061–0.7072 and ?3.75 to ?6.17). Field, petrological, and geochemical data demonstrate that the Tsushima granites formed by the mingling and mixing of mafic and felsic magmas. The Sr–Nd–Pb isotope data strongly suggest that the mafic magma was derived from two mantle components with depleted mantle material and enriched mantle I (EMI) compositions, whereas the felsic magma formed by mixing of upper mantle magma of EMI composition with metabasic rocks in the overlying lower crust. Element data points deviating from the simple mixing line of the two magmas may indicate fractional crystallization of the felsic magma or chemical modification by hydrothermal fluid. The miarolitic cavities and enrichment of alkali elements in the MME suggest rapid cooling of the mingled magma accompanied by elemental transport by hydrothermal fluid. The inferred genesis of this magma–fluid system is as follows: (i) the mafic and felsic magmas were generated in the mantle and lower crust, respectively, by a large heat supply and pressure decrease under back‐arc conditions induced by mantle upwelling and crustal thinning; (ii) they mingled and crystallized rapidly at shallow depths in the upper crust without interaction during the ascent of the magmas from the middle to the upper crust, which (iii) led to fluid generation in the shallow crust. The upper mantle in southwest Japan thus has an EMI‐like composition, which plays an important role in the genesis of igneous rocks there.     

Permian High Ba-Sr Granitoids: Geochemistry, Age and Tectonic Implications of Erlangshan Pluton, Urad Zhongqi, Inner Mongolia

Abstract: Erlangshan Pluton from Urad Zhongqi, central Inner Mongolia, is located in the middle segment of the northern margin of the North China Plate. The rocks consist mainly of diorites with gneissic structure. Petrochemical characteristics reveal that the diorites belong to metaluminous, high-potassium calc-alkaline series, with chemical signatures of I-type granites. They are characterized by low SiO2 contents (56.63%–58.53%) and A/CNK (0.90–0.96), high Al2O3 contents (17.30%–17.96%) and Na2O/K2O ratios (1.20–1.70), enrichment in large ion lithophile elements (LILE, e.g., Ba=556–915 ppm, Sr=463–595 ppm), and relative depletion in high field strength elements (HFSE, e.g., Nb, Ta, Ti) in primitive mantle-normalized spidergram, and right-declined rare earth element patterns with slightly negative Eu anomalies (d?Eu=0.72–0.90). They have Sr/Y ratios (20–25) evidently less than Kebu Pluton (49–75) to its east. Sensitive high resolution ion micro-probe U-Pb zircon dating of the diorites has yielded an intrusive age of 270±8?Ma. This leads us to conclude that Erlangshan diorites were formed by mixing between the middle or lower crustal-derived magma and minor mantle-derived mafic magma, followed by fractional crystallization, which was trigged by crustal extension and fault activity in post-collisional setting.     

Mineralized and unmineralized calderas in Spain; Part I, evolution of the Los Frailes Caldera

The Cabo de Gata volcanic field of southeastern Spain contains several recently-recognized calderas. Some of the calderas are mineralized with epithermal gold, alunite, and base metal deposits, and others are barren, and yet they formed under generally similar conditions. Comparison of the magmatic, geochemical, and physical evolution of the Los Frailes, Rodalquilar, and Lomilla calderas provides insight into the processes of caldera evolution that led to precious-metal mineralization. The Los Frailes caldera formed at 14.4 Ma and is the oldest caldera. It formed in response to multiple eruptions of hornblende dacite magma. Following each eruption, the area collapsed and the caldera was invaded by the sea. Dacite domes fill the lower part of the caldera. Pyroxene andesites were erupted through the solidified core of the caldera and were probably initially responsible for magma generation. The Los Frailes caldera did not evolve to rhyolites nor was it subjected to the amount of structural development that the younger, mineralized Rodalquilar and Lomilla calderas were.     

冀东太平寨地区单斜辉石的地质特征和成因

冀东太平寨地区的各类变质岩石中普遍含有单斜辉石，按成因可分为岩浆成因的变余单斜辉石和变质成因的变晶单斜辉石。后者又可分为三个世代，它们分别形成于该区变质作用的不同阶段。第一世代和第二世代单斜辉石的形成温度分别为８８０±６０℃和８２０±６０℃，相当于辉石麻粒岩亚相。第三世代单斜辉石的形成温度为７７５℃左右，压力为０．９１４Ｇｐａ，相当于角闪麻粒岩亚相。岩石中的变余单斜辉石主要是普通辉石，变晶单斜辉石大多属次透辉石。另外，原岩成分也是控制单斜辉石成分的主要因素。     

湖南东坡矿田花岗岩类岩石学基本特征

东坡矿田所处湘南区域,岩浆岩比较发育,由老到新依次为花岗岩、花岗斑岩脉、辉绿玢岩脉,主要分燕山早期和晚期两期五次侵入就位。在统计前人所做的K-Ar、Rb-Sr同位素测定的基础上,笔者对各期依次所形成的主要矿体特征进行总结概述。     

新疆萨吾尔山花岗岩类的形成时代

新疆萨吾尔山存在两期不同成因的花岗岩类,其Ｒｂ－Ｓｒ同位素年龄值分别为３１４．９－３２０．３Ｍａ和２９５－２９６Ｍａ。结合其侵位的地层及其与火山岩的关系等确定这两种不同类型花岗岩的时代分别为海西中期和晚期。     

Granitoids in the Rozvadov Pluton,Western Bohemia and Oberpfalz

The Rozvadov Pluton is a complex of mainly Variscan granitoid rocks situated near the Bohemian-Bavarian border between Bärnau, Tachov, Rozvadov and Waidhaus, 25 km ESE of the KTB site. Five mappable units can be distinguished, which intruded as folows: (1) slightly deformed leucocratic meta-aplite/metapegmatite dykes with garnet and tourmaline; (2) a complex of cordierite-bearing granitoids, which have been divided into three facies (a) biotite granite with cordierite (at the margin of the complex), (b) biotite-cordierite granite and (c) cordierite tonalite (in the centre of the complex; (3) fine-grained biotite granite of the Rozvadov type with associated pegmatite bodies; (4) two-mica Bärnau granite; and (5) geochemically specialized albite-zinnwaldite-topaz granite (Kríový kámen/Kreuzstein granite) with indications of Sn-Nb-Ta mineralization and associated phosphorus-rich pegmatite cupolas. Rare earth element data suggest that meta-aplite/pegmatite dykes are the result of a batch partial melting process, whereas the compositional variation of the other rock types was mainly controlled by fractional crystallization. The genesis of the cordierite granitoid suite is best explained in terms of a batch melting of metapelitic source followed by crystallization of a cordierite-rich cumulate and K-feldspar enriched melt. The leucocratic pluton constituents — the meta-aplites and the Bärnau and Kíový kámen granites are rich in phosphorus (0.5–0.8%). The main carriers of phosphorus are alkali feldspars, especially K-feldspar (up to 0.8% P₂O₅). The presence of P-rich leucocratic granites is one of the features distinguishing the Variscan granitoids within the Moldanubian zone from the nearly contemporaneous granitoids in the Saxothuringian zone.