Shunso Ishihara

Initial 87Sr/86Sr ratios of plutonic rocks from Japan

Initial 87Sr/86Sr rations were determined for more than 80 plutonic rocks in Japan. The 87Sr/86Sr ratios of gabbroic and granitic rocks show no significant difference in plutonic terranes where both rocks occur closely associated, implying a genetic relationship between them (e.g., Green Tuff belt) or reequilibration at deep level (e.g., Ryoke belt). Wherever granitic rocks occur independently from gabbroic rocks, the granites have higher ratios than the gabbros.Initial 87Sr/86Sr ratios of the granitic rocks are low (<0.706) in Northeast Japan but high (<0.706) in Southwest Japan, the boundary being the Tanakura Tectonic Line. Within Southwest Japan, the ratios are low along the Japan Sea side of the southernmost area. This regional variation is generally correlated with thickness of the continental crust as deduced from the Bouguer anomaly.Initial 87Sr/86Sr ratios of the granitic rocks vary from 0.7037 to 0.7124. The low group (<0.706) is considered to consist of essentially mantle-derived magmas contaminated by crustal material in lesser but varying degree, because of its geological setting and initial 87Sr/86Sr values. The high group may have been formed by contamination of a deep-seated magmas by crustal material or by generation of the main part of the magmas within the continental crust. The ratios of individual belts reflect their own history depending upon age and Rb/Sr ratio of the crustal material.Initial 87Sr/86Sr ratios of granitic rocks are generally low for the magnetite-series but high for the ilmenite-series. Thus, a negative correlation is observed between initial ratios and δ34S for most Cretaceous-Paleogene granites. However, Neogene ilmenite-series granites are low in both initial 87Sr/86Sr and δ34S indicating interaction of the granitic magma with young sedimentary rocks enriched in 32S.

Sulfur isotopic composition of the magnetite-series and ilmenite-series granitoids in Japan

Sulfur isotopic composition has been measured on 30 granitoids and 11 gabbroids from the Cretaceous and Tertiary granitic terranes of Japan. The two series of granitoids, the magnetite-series and ilmenite-series, defined by Ishihara (1977), show two specific isotope trends. The magnetite-series granioids all have positive δ (su34)S (CDT) values from +1 to +9‰, while the ilmenite-series rocks are dominated by negative values between −11 and −1‰. The trend in the ilmenite-series is consistent with the thesis that the magma has been influenced by light biogenic sulfur from the continental crust. The inferred large scale magma-crust interaction in the ilmenite-series granitoids indicates that the emplacement of this series of magma has been governed by a stoping mechanism.In contrast, the magnetite-series granitoids have little if any evidence for significant magma-crust interaction, indicating that the intrusion of this series of magma may have been more or less of fissure-filling type. Their trend towards positive δ (su3 4)S values (average ≈ +5‰) argues for the introduction of some heavy sulfur, probably of seawater origin, into the mantle derived sulfur. This is most likely to occur in an arctrench system by the subduction of an oceanic plate which accompanies the sulfate-bearing pelagic sediments.The isotopic data of gabbroids, mostly between −1 and +3‰, are close to the commonly assumed value for mantle sulfur. Nevertheless, the gabbroids from the magnetite-series granitic terranes tend to have higher δ (su34)S value than those from the ilmenite-series belts. It is inferred that the factors controlling the isotope characteristics of the granitoid sulfur have also been operative in these grabbroids at least to some extent.

Granitoid Series in Terms of Magnetic Susceptibility: A Case Study from the Barberton Region, South Africa

Abstract Magnetic susceptibilities were measured on a representative collection of Archaean granitoids of the Barberton region using a portable KT5 magnetic susceptibility meter. The studied granitoids comprise, (1) syn-tectonic tonalite-trondhjemite-granodiorite (TTG) granitoids (132 samples), (2) late-tectonic calc-alkaline granitoids (402 samples) and (3) post-tectonic low-Ca and high-Ca granitoids (12 samples). Most of the early-stage syn-tectonic granitoids (∼3450 Ma) have low magnetic susceptibilities, less than 3 × 10−3 SI units, and correspond to ilmenite-series granitoids. The late-stage Kaap Valley tonalite pluton (∼3230 Ma) contains sporadically distributed higher magnetic susceptibility values (greater than 3 × 10−3 SI units), which are less than one-third in magnetic susceptibility of typical magnetite-series TTG of the Japanese Island Arc and thus strictly belong to an intermediate series. The Barberton TTG suite is essentially derived from reduced amphibolitic lower crust that reflects the anoxic nature of the Earth surface during the Archaean Eon. The more oxidized nature of the Kaap Valley tonalite may be generated in an oxidized lower crust by fluids squeezed out of the subducting plate. Late-tectonic granodiorite - adamellite batholithic complexes (∼3105 Ma) belong mostly to the magnetite series, and seem to suggest that relatively oxidized continental crust, reflecting oxic atmosphere and subduction mechanism operating, had evolved it by this time. Post-tectonic granitic plutons formed largely between circa 2900 Ma and 2700 Ma can be subdivided into low-Ca ilmenite series and high-Ca magnetite series.

Metallogeny, Minerogenic Series, and Gold Mineralization of the Qinling Orogen, China

The Qinling orogen extends 1650 km E-W and has a width of 95-255 km. It has a long evolutionary history of ∼3.02 Ga and a mineralization history of approximately 2.5 Ga. There are more than 400 mineral deposits of Au, Ag, Pb, Zn, Sb, Hg, Mo, W, Fe, Ni, Mn, rare elements, REE, U, V, P, As, Ba, Sr, Ti, Al, pyrite, gypsum, trona, halite, glauberite, mirabilite, blue asbestos, fluorite, rock crystal, mica, vermiculite, graphite, kyanite, andalusite, talc, fireclay, and K-feldspar in the Qinling orogen and adjacent areas. The deposits were formed in six tectono-minerogenic cycles and have been grouped into 21 regional minerogenic series. The metallogeny of the Qinling orogen is closely associated with the tectonic evolution of the orogen, and most of the ore deposits were formed in two important metallogenic periods: (1) Middle Proterozoic to Early Paleozoic, which was characterized by spreading and rifting-related minerogenetic processes; during this period, the minerogenic series are directly associated with basic-ultrabasic magmatism and marine volcanism resulting from spreading-rifting and deep faulting; (2) Mesozoic, when mineralization was controlled by intracontinental subduction and postorogenic relaxational taphrogenesis or extension; the minerogenetic series are associated with intermediate-acid magmatism and terrestrial volcanism. Gold is one of the most important ore-forming commodities in the Qinling orogen. Over 80% of those gold deposits were formed in the Mesozoic and Cenozoic and are related genetically to magmatism, terrestrial volcanism, and alluvial sedimentation. Most gold deposits in the Qinling orogenic belt are of hydrothermal origin. The Dashui-type gold deposits may be a new category of high-sulfi-dation epithermal gold deposits hosted by carbonate rocks of the Upper Paleozoic and Triassic in the southwestern part of the Qinling orogen. Shuangwangtype gold deposits in the middle of the Qinling orogen are characterized by low-grade (but large in terms of tonnage) ores hosted by brecciated and banded albitites, which are thought to have been formed by Yanshanian (late Mesozoic) magmatic-hydrothermal fluids injected into the Devonian sedimentary rocks. Carlin-type, quartz vein-type, and placer-type deposits also are important in this continental orogenic belt and adjacent areas.

Multiphase Melt Inclusions in the Jinchuan Complex, China: Implications for Petrogenic and Metallogenic Physico-Chemical Conditions

Three types of melt inclusions were identified in pyroxenes, olivine, and chromite from the Jinchuan intrusion–sulfide inclusions (I), crystallized polyphase inclusions (II), and fluid inclusions (III). Sulfide inclusions in chromite are spherical and occur parallel to chromite growth planes. Polyphase inclusions are found in olivine and consist of olivine, orthopyroxene, chrome-spinel, amphibole, and sulfide daughter minerals. Primary fluid inclusions are usually found in olivine and pyroxene with spherical to tubular shapes. Laser Raman analyses were used to identify daughter minerals and fluid phases in the inclusions. The result shows that fluids consist mainly of CO2, with minor amounts of SO2, H2S, and CH4. This indicates that the parental magma of the Jinchuan intrusion contains abundant volatiles. Microthermometry shows that Type-II inclusions in dunite and sulfide-rich lherzolite have higher trapping temperatures (1000° to 1050°C), whereas trapping temperatures in sulfide-poor lherzolite are lowe...

Oxidized Granitic Magmas and Porphyry Copper Mineralization

Absence or presence of modal magnetite and whole-rock Fe2O3/FeO ratio of granitoids have been potentially used to recognize the bimodal series of granioids in Japan viz. magnetite-series and ilmenite-series granitoids corresponding to oxidized and reduced granitoids respectively. Oxidized granitoids are mostly responsible for the origin of metallic (copper and gold) sulfide deposits particularly of porphyry nature. Oxidation status of some copper-hosting Precambrian and Phanerozoic granitoids have been assessed in order to understand intrinsic properties of source regions and prevailed oxygenated environment evolving through time. Chemistry of hydrous mafic silicates (amphibole and biotite) and apatite has also been discussed specifically to characterize the mineralized, oxidized granitoids and explaining the key role of F, Cl and SO3 as potential carrier of ore metals in hydrothermal system emanated from granitic magmas respectively.

Mesozoic tectonics and related metallogenesis in northeast Asia

Granitoids and related mineralization in the circum-Japan Sea region and its environs in Northeast Asia were reviewed from the view point of the evolution of the crust. Redox state of granitoid magmas, linked strongly with metallogeny, is influenced by sedimentary materials under various tectonic settings in the eastern margin of Eurasian continent during the Mesozoic to Early Tertiary time. The style of magmatism and mineralization contrasts with that of western North America which was situated in a relatively monotonous convergent margin. Magmatism in a given area tends to be of the oxidized-type with time due to the depletion of crustal carbon by previous magmatism. Stress conditions may also play an important role for the genesis of granitoid types; the oxidized-type granitoids may form even in a sedimentary-dominated crust under specific extensional environments without significant crustal input.

西南日本，岡山市の採石場に見られる還元型／酸化型花崗岩類

Two different granites in their oxygen fugacity during the formation, are seen in the drill cores from the northwestern part of Okayama City. One is coarse-grained granite having no or little rock-forming magnetite, while the other is fine-grained aplitic granite occurring in sheet-like form and containing coarse-grained magnetite in miarolitic aggregates of K-feldspar and quartz. The major and trace elements chemistry indicates that the aplitic granite is crystallized from fractionated melts of the coarse-grained granitic magma. It is suggested that the fractionated magma became rich in H2O which dissociated into O2 and H2 and the hydrogen diffused out to the roof rocks; then the oxygen fugacity was increased to form magnetite. Löllingite was discovered along cracks of the coarse-grained granite at one quarry, reflecting a low sulfur fugacity of the post-magmatic hydrothermal activity of these granites.

Comparative geochemical, magnetic susceptibility, and fluid inclusion studies on the Paleoproterozoic Malanjkhand and Dongargarh granitoids, Central India and implications to metallogeny

The Malanjkhand granodiorite (MG) hosting economic copper mineralization and the hitherto barren Dongargarh granitoids (DG) have subtle differences in their petrographic and bulk geochemical features. The two plutons are contiguous and occur in the northern part of the Bhandara Craton in Central India with intervening volcanosedimentary sequence of the Dongargarh Supergroup amidst older gneisses. The Dongargarh granitoids studied in two smaller units have higher bulk magnetic susceptibility than the Cu-bearing MG; the majority of samples studied from the latter being ilmenite-series rocks. DG crystallized at higher pressures compared to MG. Plagioclase composition ranges from albite to high bytownite in MG, whereas its compositional range is restricted to high andesine in DG. However, both intrusions give identical temperature ranges estimated by binary feldspar thermometry. Biotite in MG shows higher Fe/Mg ratios, as well as a greater range of compositional variation, than that in DG. MG has a moderately fractionated rare earth element distribution pattern without any significant Eu anomaly, showing depletion in mid-range rare earth elements (REE) and no depletion in heavy REE. DG is characterized by a prominent negative Eu anomaly. Geochemical features indicate subtle differences in the nature of source rocks and/or melting processes responsible for the generation of the two granitoids. MG displays more consistent bulk chemical features and is possibly a result of crystallization from a homogeneous granodioritic melt. DG displays a greater diversity and possibly incorporated a significant felsic crustal component that contributed to the parent melt. A fluid inclusion study of quartz grains from the granitoids and barren quartz veins occurring in MG indicates identical low-temperature nature of the fluid in both cases. They differ from the fluid in the mineralized zone in MG in the absence of a high-temperature component and CO2 in the fluid. Late-stage fluids in DG and associated barren quartz veins compare well with those from MG. The hydrothermal activity following the granite emplacement seems to have operated under identical temperature conditions, and the aqueous fluid at the two occurrences seems to have been broadly similar. In both cases, internal evolution of the exsolved fluid to low temperatures and moderate salinity are visualized. Based on the existing information, the lack of ore potential in DG may be attributed to the metal and volatile (water + halogens) deficient nature of the parental granitic melt.