Petrogenetic implications and geochronology of middle Miocene Tannayama igneous rocks,Goto Islands,Japan Sea southern margin,northwestern Kyushu,Japan

The subduction of “hot” Shikoku Basin and the mantle upwelling related to the Japan Sea opening have induced extensive magmatism during the middle Miocene on both the back-arc and island-arc sides of southwest Japan. The Goto Islands are located on the back-arc side of northwestern Kyushu, and middle Miocene granitic rocks and associated volcanic, hypabyssal, and gabbroic rocks are exposed. The igneous rocks at Tannayama on Nakadori-jima in the Goto Islands consist of gabbronorite, granite, granite porphyry, diorite porphyry, andesite, and rhyolite. We performed detailed geological mapping at a 1:10 000 scale, as well as petrographical and geochemical analyses. We also determined the zircon U–Pb age dating of the igneous rocks from Tannayama together with a granitic rock in Yagatamesaki. The zircon U–Pb ages of the Tannayama igneous rocks show the crystallization ages of 14.7 Ma ± 0.3 Ma (gabbronorite), 15.9 Ma ± 0.5 Ma (granite), 15.4 Ma ± 0.9 Ma (granite porphyry), and 15.1 Ma ± 2.1 Ma (rhyolite). Zircons from the Yagatamesaki granitic rock yield 14.5 Ma ± 0.7 Ma. Considering field relationships, new zircon data indicate that the Tannayama granite formed at ~16–15 Ma, and the gabbronorite, granite porphyry, diorite porphyry, andesite, and subsequently rhyolite formed at 15–14 Ma, which overlaps a plutonic activity of the Yagatamesaki. The geochemical characteristics of the Tannayama igneous rocks are similar to those of the tholeiitic basalts and dacites of Hirado, and the granitic rocks of Tsushima in northwestern Kyushu. This suggests that the Tannayama igneous rocks can be correlated petrogenetically with the igneous rocks in those areas, with all of them generated by the upwelling of hot mantle diapirs during crustal thinning in an extensional environment during the middle Miocene.     

青海省扎日根结扎群火山岩形成时代及构造背景分析

青海扎日根结扎群火山岩中基性岩以贫硅、钾,高钛、钙,中性岩类以低硅、中钾、钛、钙,酸性岩类以高硅、钾,中钛,低钙为特征。根据Fe＊/MgO-TiO2图解上显示出本区火山岩绝大多数火山岩落在岛弧区。微量元素中Th/Nb=0.9〉0.11,Nb/Zr〉0.04显示出其构造背景为陆-陆碰撞形成的岛弧区。扎日根结扎群火山岩时代Rb-Sr同位素等时线给出的年龄为231±28Ma和225±8 Ma,属晚三叠世。另外Sr同位素的初始比值ISr=0.70522±0.00023,小于0.719,表明岩浆（原始）来源于上地幔,并且在上升的过程中受到地壳的混染。     

Carbonatite-alkalic complex of Panwad-Kawant,Gujarat, and its bearing on the structural characteristics of the area

Carbonatite-alkalic rocks occur in the form of dykes and small volcanic plugs in the area, with major central type volcanic activity restricted to Amba Dongar. The trappean flows of Blanford and Bose are identified as plagioclasecalcite rocks. An attempt is made to explain the origin of these rocks which are extensively cut by dykes of alkaline rocks, carbonatites and dolerites. By far the dominant lavas are «fissure phonolites» (Wright, 1963) and tinguaites. The chemical analyses of these rocks show that the magma is mainly of continental sodic alkaline suite, probably turning sodi-potassic, a suggestion drawn from the occurrence of lamprophyres and pseudoleucite tinguaites, and the higher potassium contents of some rocks. Bagh sediments are mainly represented by sandstones which show mild contact effects with carbonatite, especially in the south.     

Petrology,geochemistry and Sr-isotope characteristics of lavas from the area of Commenda (Mts. Vulsini,Italy)

Major, trace element and Sr-isotope compositions are reported for a suite of lavas coming from the area of Commenda in the SE Vulsinian district. The analyzed samples have all low silica contents and variable but generally high CaO, MgO and FeO_t. Based on K₂O% and K₂O/Na₂O ratio, the rocks from Commenda can be classified as belonging to the Potassic Series (KS) and the High-potassium Series (HKS). The HKS rocks appear to have derived by cristal/liquid fractionation from the most mafic types with separation of olivine and clinopyroxene and then of clinopyroxene + leucite. The most primitive HKS rocks have aphyric texture and high Mg-values, Cr and Ni contents which are close or within the range of values of magmas formed by partial melting of periodititic mantle sources. The KS rocks have lower incompatible element contents as the HKS rocks with similar degree of evolution.The variations of Sr-isotopic ratios of the analyzed rocks and of other Vulsinian lavas, indicate that the basic HKS Vulsinian rocks did not interact significantly with the continental crust. Instead, the KS appears to have evolved by combined crystal fractionation and assimilation processes, starting from parental magmas which had⁸⁷Sr/⁸⁶Sr ratio not significantly lower than that found in the less evolved rocks of the suite.The most primitive HKS rocks from Commenda have hygromagmatophile element distribution pattern characterized by high ratio of LILE/HFSE with negative anomalies of Ta and Ti, resembling closely those of other Roman mafic volcanics. The primitive geochemical characteristics of the Commenda rocks exclude that these features are the products of interaction with the crust and provide a further support to the hypothesis of a genesis within a subduction-modified mantle source.     

Petrochemistry and tectonic setting of mafic volcanic rocks in the Chon Daen–Wang Pong area,Phetchabun, Thailand

The mafic volcanic rocks and hypabyssal rocks in the Chon Dean‐Wang Pong area are possibly the southern extension of the western Loei Volcanic Sub‐belt, Northeast Thailand. They are least‐altered, and might have been formed in Permian–Triassic times. The rocks are commonly porphyritic, with different amounts of plagioclase, clinopyroxene, orthopyroxene, amphibole, Fe–Ti oxide, unknown mafic mineral, and apatite phenocrysts or microphenocrysts, and are uncommonly seriate textured. The groundmass mainly shows an intergranular texture, with occasionally hyalophitic, intersertal and ophitic–subophitic textures. The groundmass constituents have the same minerals as the phenocrysts or microphenocrysts and may contain altered glass. The groundmass plagioclase laths may show a preferred orientation. Chemically, the studied rock samples can be separated into three magmatic groups: Group I, Group II, and Group III. These magmatic groups are different in values for Ti/Zr ratios. The averaged Ti/Zr values for Group I, Group II, and Group III rocks are 83 ± 6, 46 ± 12, and 29 ± 5, respectively. In addition, the Group I rocks have higher P/Zr, but lower Zr/Nb relative to Group II and Group III rocks. The Group I and Group II rocks comprise tholeiitic andesite–basalt and microdiorite–microgabbro, while the Group III rocks are calc‐alkalic andesite and microdiorite. According to the magmatic affinities and the negative Nb anomalies on normal mid‐oceanic ridge basalt (N‐MORB) normalized multi‐element plot, arc‐related lavas are persuasive. The similarity between the studied lavas and the Quaternary lavas from the northern Kyukyu Arc, in terms of chondrite‐normalized rare earth element (REE) patterns and N‐MORB normalized multi‐element patterns, leads to a conclusion that the mafic volcanic rocks and hypabyssal rocks in the Chon Daen–Wang Pong area have been formed in a volcanic arc environment.     

Timescales and the role of inheritance in long-term landscape evolution,northern New England,Australia

Landscape evolution in northern New England is characterized by the persistence of landforms over long time periods. The topography in the study area was initiated by Paleozoic diapiric intrusion of granitic rocks as highs within metasedimentary and volcanic rocks. This framework was probably reinforced by differences in the resistance of the massive igneous and the fractured metasedimentary rocks to erosion. As a result, both the Tertiary and present-day relief of the area consists of uplands developed on granitic rocks partly overlain by Paleozoic volcanic rocks, and lowlands developed on Paleozoic metasediments. Extensive volcanism in the Tertiary had only a minimal effect on this relationship. A significant impact of Tertiary volcanism, however, was the diversion of major streams across topographic highs. Despite 19-35 Ma of incision, these streams are still far from establishing equilibrium profiles and dissecting the highs. The persistence of topography for a timescale of the same order of magnitude as that conventionally thought to be required for planation is due to reduced denudation rates in a passive tectonic setting and the presence of erosion-resistant Paleozoic igneous units. The latter influence is indicated by knickpoint persistence for 19-35 Ma on streams diverted across these rocks. Since it is generally considered that such disequilibria are short-lived features formed by Quaternary or ongoing deformation, this knickpoint persistence has important implications for landscape analyses that use disequilibria to assess recent tectonism in a landscape.     

江西修水县梅子坑钼矿床地质特征及成因初探

梅子坑钼矿位于九岭钨钼成矿带,为中型石英脉型钼矿床。矿区内地层和岩浆岩钼元素含量分别是克拉克值的43倍和21倍。矿体赋存于双桥山群修水组浅变质岩系中及北西向断裂控制的裂隙密集带中;矿石主要类型为石英脉型,矿石有益组分为辉钼矿,形成于石英-黄铁矿-辉钼矿-黄铜矿早期矿化阶段。矿床可能与隐伏的燕山期细粒花岗岩、花岗斑岩岩脉有成因关系,属与燕山期岩浆活动有关的中-高温热液矿床。北西向断裂密集带,硅化、云英岩化、黄铁矿化围岩蚀变,燕山期花岗岩类及双桥山群浅变质岩系,是其主要找矿标志。     

江西省广丰县李家膨润土矿床成因及找矿标志

广丰县李家膨润土矿床赋存于白垩系上统南雄组第二岩性段酸性火山岩的顶底部。膨润土的主要矿物成分为蒙脱石,矿石结构构造与成矿原岩有关。矿石化学成分与酸性熔岩相近,矿体形态及空间分布受酸性熔岩流相带的控制。因此认为,膨润土矿床成矿原岩为酸性玻质熔岩,矿床是由酸性玻璃质熔岩经水解脱玻化而形成,其成因类型为风化淋滤蚀变岩型矿床。膨润土可自然改型,由原来的钠基膨润土改型成钙基膨润土和偏镁的钙基膨润土。对时代较新的酸性火山岩,在玻质熔岩发育处都有可能找到有利用价值的膨润土矿。由于膨润土常由珍珠岩、沸石脱玻化形成,三者常共生出现,是很好的找矿标志。     

Geochemistry of the Early Miocene volcanic succession of Northland,New Zealand,and implications for the evolution of subduction in the Southwest Pacific

Latest Oligocene and Early Miocene volcanic rocks occur on the Northland Peninsula, New Zealand, and record the inception of Cenozoic subduction-related volcanism in the North Island that eventually evolved to its present manifestation in the Taupo Volcanic Zone. This NW-striking Northland Arc is continuous with the Reinga Ridge and comprises two parallel belts of volcanic centres ca. 60 km apart. A plethora of tectonic models have been proposed for its origins. We acquired new trace element and Sr–Nd isotope data to better constrain such models. All Northland Arc rocks carry an arc-type trace element signature, however distinct differences exist between rocks of the eastern and western belt. Eastern belt rocks are typically andesites and dacites and have relatively evolved isotope ratios indicating assimilated crustal material, and commonly contain hornblende. Additionally some eastern belt rocks with highly evolved isotope compositions show fractionated REE compositions consistent with residual garnet, and some contain garnetiferous inclusions in addition to schistose crustal fragments. In contrast, western belt rocks are mostly basalts or basaltic andesites with relatively primitive Sr–Nd isotope compositions, do not contain hornblende and show no rare earth element evidence for cryptic amphibole fractionation. Eastern and western belt rocks contain comparable slab-derived fractions of fluid-mobile trace elements and invariably possess an arc signature. Therefore the difference between the belts may be best explained as due to variation in crustal thickness across the Northland Peninsula, where western belt centres erupted onto a thinner crustal section than eastern belt rocks.The consistent arc signature throughout the Northland arc favours an origin in response to an actual, if short-lived subduction event, rather than slab detachment as proposed in some models. No Northland Arc rocks possess a convincing adakite-like composition that might reflect the subduction of very young oceanic lithosphere such as that of the Oligocene South Fiji Basin. Therefore we favour a model in which subduction of old (Cretaceous) lithosphere drove subduction.     

Continental hydrothermal sedimentary siliceous rock and genesis of superlarge germanium (Ge) deposit hosted in coal: A study from the Lincang Ge deposit,Yunnan, China

The study on marine hydrothermal sedimentationtends to be perfect at present, and a suit of hydrother-mal sedimentary distinguishing criterions (includingstructure, conformation, geochemistry, etc.) has beenfound[1—12]. While it is unsubstantial on the study ofhydrothermal sedimentation in continental deposi-tional environment, and little was known about therelation between continental hydrothermal sedimenta-tion and metal mineralization. The Lincang Ge deposit,hosted in coal with independent…     

Li,Pb and Tl abundances in the K-alkaline rocks from the Middle Latina Valley volcanoes (southern Latium,Italy) and their petrological significance

Li, Pb and Tl contents of 15 primitive lava samples from the Middle Latina Valley volcanoes (southern Latium, Italy) are higher in the high-K than in the K-Series rocks, the enrichment factors roughly following the increase of the ionic size of the elements.The abundances in both series are higher than those of typical alkali basalts from oceanic environments, but similar to those of granitoid rocks. Such anomalous features may be explained as due to either crustal contamination of the parental magmas or derivation of these latter from a lithophile-enriched mantle region due to metasomatism by a lower crust-derived fluid. In both hypotheses, however, a larger involvement of crustal materials is suggested for the HKS rocks.Comparison of Li, Pb and Tl abundances in the primitive rocks of the Middle Latina Valley and Roccamonfina volcanoes shows that, unlike the KS rocks from the two districts show comparable levels and probably originated under similar conditions, the HKS rocks from the Middle Latina Valley are enriched in Li, Pb and Tl relative to their analogues from Roccamonfina. This suggests either a higher involvement of crustal materials in their genesis, or an evolution at shallower depth in the crust.     

The Kari Kari caldera, Bolivia, and the Cerro Rico stock

The rocks comprising the Kari Kari massif southeast of the city of Potosi, Bolivia, consist entirely of welded ignimbrites. It is argued that the massif constitutes the resurgent centre of a 20-m.y.-old resurgent caldera. Plutonic rocks are exposed in the south, but volcanic rocks of the caldera rim are exposed in the north, and indicate a shallower erosion level there. The volcanic rocks consist of a coarse moat deposit, consisting of angular fragments of basement material and juvenile clasts, overlain by an extensive garnet-bearing ignimbrite. A plant-fossil-bearing lacustrine deposit was laid down in a lake within the caldera. The Cerro Rico stock, noted for its silver-tin mineralisation, may be a late intrusion along the caldera ring fractures.     

内蒙古乌兰浩特索伦镇地区中生代侵入岩特征研究

内蒙古乌兰浩特索伦镇地区位于大兴安岭中南段。该区中生代侵入岩广泛发育。开展侵入岩的年代学、地球化学特征研究能为深入探讨区域构造背景提供依据。3个样品K—Ar全岩法测年结果（93．39Ma-108．48Ma）证明侵入岩形成于早白垩世,略晚于其南侧同一构造带上的杜尔基地区侵入岩年龄（100．15Ma-120．29Ma）,符合太行山-大兴安岭早白垩世侵入岩时代自南向北由老变新的演化趋势。岩石地球化学分析结果显示岩石属于碱性系列,具有高硅、高钾、富碱、过铝、贫钙、镁、铁等特征。稀土元素总量为（77．96-241．29）μg／g,平均值为158．92μg／g,与杜尔基地区早白垩世侵入岩的稀土总量平均值（146．85μg／g）及华北克拉通内碱性系列岩石的稀土总量平均值（163．32μg／g）都非常接近。稀土元素球粒陨石标准化曲线整体右倾,因Eu负异常而呈“V”型形态,与太行山一大兴安岭地区早白垩世典型岩体的稀土配分型式极为相似。侵入岩的成因类型为A-型,形成于伸展构造环境。     

Tectonic setting and formation of the Setouchi volcanic rocks in the western Seto Inland Sea,Japan

The Setouchi volcanic rocks include high-Mg andesites (HMAs) and garnet-bearing dacite–rhyolite, and are sporadically distributed along the Median Tectonic Line, Japan. New U–Pb zircon ages and geological and geochemical data are presented for those rocks in the Western Setouchi region (W-Setouchi). Previous studies referred to the altered andesite in the W-Setouchi as “pre-Setouchi volcanic rocks.” However, on the basis of the new U–Pb age (14.4 Ma ± 0.3 Ma) and geochemical characteristics, we redefine it as the Jikamuro Formation, part of the Setouchi volcanic rocks. Incompatible elements are more enriched in the Jikamuro Formation rocks than in the Setouchi HMAs. The characteristic element compositions may be explained by mixing of compositionally different magmas, including subducted sediment melts, plus a contribution from crustal contamination. A stress-inversion technique with Bingham distribution method was applied to the orientations of felsic and mafic dikes within the Setouchi volcanic rocks, and indicates paleo-stress conditions during the period of Setouchi volcanism in the W-Setouchi. The analysis reveals NNW-extensional stresses and a strike-slip stress. We infer that the former represents extensional conditions during the main period of volcanism and the latter represents a stress transition during the most recent period of volcanism (after 12 Ma).     

Provenance of sandstones from the Wakino Subgroup of the Lower Cretaceous Kanmon Group, northern Kyushu, Japan

Abstract The Wakino Subgroup is a lower stratigraphic unit of the Lower Cretaceous Kanmon Group. Previous studies on provenance of Wakino sediments have mainly concentrated on either petrography of major framework grains or bulk rock geochemistry of shales. This study addresses the provenance of the Wakino sandstones by integrating the petrographic, bulk rock geochemistry, and mineral chemistry approaches. The proportions of framework grains of the Wakino sandstones suggest derivation from either a single geologically heterogeneous source terrane or multiple source areas. Major source lithologies are granitic rocks and high‐grade metamorphic rocks but notable amounts of detritus were also derived from felsic, intermediate and mafic volcanic rocks, older sedimentary rocks, and ophiolitic rocks. The heavy mineral assemblage include, in order of decreasing abundance: opaque minerals (ilmenite and magnetite with minor rutile), zircon, garnet, chromian spinel, aluminum silicate mineral (probably andalusite), rutile, epidote, tourmaline and pyroxene. Zircon morphology suggests its derivation from granitic rocks. Chemistry of chromian spinel indicates that the chromian spinel grains were derived from the ultramafic cumulate member of an ophiolite suite. Garnet and ilmenite chemistry suggests their derivation from metamorphic rocks of the epidote‐amphibolite to upper amphibolite facies though other source rocks cannot be discounted entirely. Major and trace element data for the Wakino sediments suggest their derivation from igneous and/or metamorphic rocks of felsic composition. The major element compositions suggest that the type of tectonic environment was of an active continental margin. The trace element data indicate that the sediments were derived from crustal rocks with a minor contribution from mantle‐derived rocks. The trace element data further suggest that recycled sedimentary rocks are not major contributors of detritus. It appears that the granitic and metamorphic rocks of the Precambrian Ryongnam Massif in South Korea were the major contributors of detritus to the Wakino basin. A minor but significant amount of detritus was derived from the basement rocks of the Akiyoshi and Sangun Terrane. The chromian spinel appears to have been derived from a missing terrane though the ultramafic rocks in the Ogcheon Belt cannot be discounted.     

图们江流域新生代火山岩Sr、Nd同位素初步研究

中国地震局地质研究所; 中国地震局地质研究所 北京     

Basaltic ignimbrite-like rocks on Saikhan Volcano, northeastern Khangai, Mongolia: Mineralogic and geochemical evidence

Rocks having a pseudofluidal ignimbrite texture have been found on Saikhan Volcano in northeastern Khangai, Mongolia. The rocks have a typically nodular banded texture. The fiamme and the bands vary in width between a few millimeters to a few centimeters. These rocks have the same bulk composition as trachybasalts and do not differ from the ordinary trachybasalts found on this volcano in the form of dikes and lavas. The difference consists in the composition of glasses and minerals, as well as in the concentration of CO₂ (which is higher in the ignimbrite-like rocks). The glasses in the ignimbrite-like rocks show a trend from basaltic trachyandesites to tephriphonolites and foidites, thus indicating the liquidus crystallization of clinopyroxene. The glasses in the lavas and dikes have a trachyte composition, indicating a residual origin following the crystallization of olivine and Ti-magnetite. Much of the pyroxenes (∼20%) in the ignimbrite-like rocks show calculated pressures during their generation to have been in the range of 6.5–14 kbars, while all pyroxenes in the ordinary lavas and dikes crystallized at pressures below 0.3 kbars. It thus follows that the magmas that have produced the ignimbrite-like rocks began crystallizing in the subcrustal magma chamber under fluid-saturated conditions, whence they were rapidly transported to the surface.     

Dredged basaltic rocks from the seaward extensions of the Reykjanes and Snaefellsnes volcanic zones,Iceland

Data for basaltic rocks dredged on the Reykjanes Ridge and Jökulbanki off the southwest coast of Iceland are presented. The rocks from the Reykjanes Ridge, which are identified as recent lavas, are olivine tholeiites and quartz tholeiites very similar to those formerly described from this region. The heterogeneous Jökulbanki dredges were identified as erratics.In the light of the present and other recent data on tholeiitic rocks from Iceland and the Reykjanes Ridge, a distinct chemical gradient, together with increased scatter across Iceland is demonstrated for certain incompatible elements. This is in accordance with the mantle plume theory.     

Occurrence of oceanic plagiogranites in the older tectonic zone,Southwest Japan

K, Rb and Sr concentrations and Sr isotopic compositions were determined for the Dai granitic rocks of trondhjemitic composition occurring in a serpentinite mass in the Nagato tectonic zone formed in the Late Paleozoic era, and for the granitic rocks of quartz dioritic composition recently dredged from the seamount of the Kyushu-Palao Ridge. Both granitic rocks are characterized by low abundances of K and Rb, low K₂O/Na₂O ratios, high K/Rb ratios, low Rb/Sr ratios and low initial⁸⁷Sr/⁸⁶Sr ratios. These characteristics suggest that strong similarities may exist between the Dai granitic rocks and the dredged granitic rocks, and that the Dai granitic rocks may be classified as oceanic plagiogranite. These oceanic plagiogranites may plausibly represent single-stage mantle-derived granites, possibly from the suboceanic mantle.     

Geology, ore deposits and hydrothermal venting in Bahía Concepción, Baja California Sur, Mexico

Abstract Bahía Concepción is located in the eastern coast of the Baja California peninsula and it is shaped by northwestern–southeastern normal faults. These are associated with a 12–6 Ma rifting episode, although some have been reactivated since the Pliocene. The most abundant rocks correspond to the arc related Comondú Group, Oligocene to Miocene, which forms a mainly calc‐alkaline volcanic and volcaniclastic sequence. There are less extensive outcrops of sedimentary rocks, lava flows, domes and pyroclastic rocks of Pliocene to Quaternary ages. The Neogene volcanism in the area indicates a shift from a subduction regime to an intraplate volcanism related to continental extension and the opening of an oceanic basin. The Bahía Concepción area contains numerous Mn ore deposits, being the biggest at El Gavilán and Guadalupe. The Mn deposits occur as veins, breccias and stockworks, and are composed by Mn oxides (pyrolusite, coronadite, romanechite), dolomite, quartz and barite. The deposits are hosted in volcanic rocks of the Comondú Group and, locally, in Pliocene sedimentary rocks. Thus, the Mn deposits formed between the Middle Miocene and the Pliocene. The mineralized structures are associated with Miocene northwestern–southeastern fault systems, which are analogous to those associated with the Cu‐Co‐Zn‐Mn deposits of El Boleo. The Bahía Concepción area also bears subaerial and submarine hot springs, which are associated with the same fault systems and host rocks. The submarine and subaerial geothermal manifestations south of the bay are possibly related with recent volcanism. The geothermal manifestations within the bay are intertidal hot springs and shallow submarine diffuse venting areas. Around the submarine vents (5–15 m deep, 87°C), Fe‐oxyhydroxide crusts with pyrite and cinnabar precipitate. In the intertidal vents (62°C), aggregates of opal, calcite, barite and Ba‐rich Mn oxides occur covered by silica‐carbonate stromatolitic sinters. Some 10–30 cm thick crustiform veins formed by chalcedony, calcite and barite are also found close to the vents. The hydrothermal fluids exhibit mixed isotopic compositions between δ¹⁸O‐enriched meteoric and local marine water. The precipitation of Ba‐rich Mn oxides around the vent sites could be an active analog for the processes that produced Miocene to Pliocene hydrothermal Mn‐deposits.