Elemental distribution of coastal sea and stream sediments in the island-arc region of Japan and mass transfer processes from terrestrial to marine environments

A total of 49 elements have been identified in 338 coastal sea sediment samples collected from an area situated off the Ise-Tokai region of Japan for a nationwide marine geochemical mapping project. The spatial distribution patterns of the elemental concentrations in coastal seas along with the existing geochemical maps in terrestrial areas were used to define the natural geochemical background variation, mass transport, and contamination processes. The elemental concentrations of coastal sea sediments are determined primarily by particle size and regional differences. Most elemental concentrations increase with a decrease in particle size. Some elements such as Ca, Mn, and Yb are found to exist in large quantities in coarse particles containing calcareous shells, Fe–Mn oxides, and felsic volcanic sediments. Regional differences reflect the mass transfer process from terrestrial areas to coastal seas and the influence of the local marine geology. An analysis of variance (ANOVA) reveals that for many elements, the particle size effect is predominant over regional difference. The mean chemical compositions of coastal sea sediments are similar to those of stream sediments in adjacent terrestrial areas and in the upper crust of Japan. This observation supports the fact that coastal sea sediments have certainly originated from terrestrial materials. However, the spatial distributions of elemental concentrations are not always continuous between the land and coastal seas. The scale of mass movement observed in marine geochemical maps occurs at a distance of 20 km from the river mouth. A detailed examination of the spatial distribution patterns of K (K₂O) and Cr concentrations suggests that terrestrial materials supplied through rivers are deposited near the shore initially, and then gravity-driven processes shift the sediments deeper into the basin. Contamination with heavy metals such as Zn, Cd and Pb was observed in coastal bays surrounded by urban and industrial areas. It is noteworthy that the areas with the highest concentration of these elements usually do not occur near the shore (not near the contamination source) but at the center of the bay. Unexpected low concentrations of Zn, Cd and Pb near shore may either be due to a decreased anthropogenic load in the most recent sediments or to dilution by unpolluted flood sediments.     

Sediment-infill volcanic breccia from the Neoarchean Shimoga greenstone terrane,western Dharwar Craton: Implications on pyroclastic volcanism and sedimentation in an active continental margin

We report sediment-infill volcanic breccia from the Neoarchean Shimoga greenstone belt of western Dharwar Craton which is associated with rhyolites, chlorite schists and pyroclastic rocks. The pyroclastic rocks of Yalavadahalli area of Shimoga greenstone belt host volcanogenic Pb–Cu–Zn mineralization. The sediment-infill volcanic breccia is clast-supported and comprises angular to sub-angular felsic volcanic clasts embedded in a dolomitic matrix that infilled the spaces in between the framework of volcanic clasts. The volcanic clasts are essentially composed of alkali feldspar and quartz with accessory biotite and opaques. These clasts have geochemical characteristics consistent with that of the associated potassic rhyolites from Daginkatte Formation. The rare earth elements (REE) and high field strength element (HFSE) compositions of the sediment-infill volcanic breccia and associated mafic and felsic volcanic rocks suggest an active continental margin setting for their generation. Origin, transport and deposition of these rhyolitic clasts and their aggregation with infiltrated carbonate sediments may be attributed to pyroclastic volcanism, short distance transportation of felsic volcanic clasts and their deposition in a shallow marine shelf in an active continental margin tectonic setting where the rhyolitic clasts were cemented by carbonate material. This unique rock type, marked by close association of pyroclastic volcanic rocks and shallow marine shelf sediments, suggest shorter distance between the ridge and shelf in the Neoarchean plate tectonic scenario.     

Influence of surface geology and mineral deposits on the spatial distributions of elemental concentrations in the stream sediments of Hokkaido,Japan

An elucidation of the background levels of heavy metals, including certain toxic elements, is very essential to accomplish an important environmental assessment. A regional geochemical mapping in Hokkaido, Japan was undertaken by the Geological Survey of Japan, AIST as part of a nationwide geochemical mapping for this purpose. There were 692 stream sediments collected from the active channel (1 sample) / (100 km²) in Hokkaido and the fine fraction sieved through a 180 μm screen was analyzed using the AAS, ICP-AES, and ICP-MS techniques. The regional geochemical maps for 51 elements were created as a 2000 m mesh map using the geographic information system software. Spatial distribution patterns of elemental concentrations in stream sediments, particularly Neogene–Quaternary volcanic and pyroclastic rocks, are primarily determined by surface geology. The correspondence of elemental concentrations in stream sediments to parent lithology is clearly indicated by ANOVA and a multiple comparison. Sediment samples supplied from mafic volcanic and felsic–mafic pyroclastic rocks are significantly rich in MgO, Al₂O₃, P₂O₅, CaO, Sc, TiO₂, V, MnO, Total (T)-Fe₂O₃, Co, Zn, Sr, and heavy rare earth elements (REEs) (Y and Eu–Lu), but significantly lacking in alkali elements, Be, Nb, light REEs (La–Nd), Ta, Tl, Th, and U. Accretionary complexes with sedimentary rocks derived from sediments are in stark contrast to volcanic and pyroclastic rocks. Accretionary complexes with mafic–ultramafic rock have significantly elevated Nb, Ta, and Th abundances in sediments besides MgO, Cr, Ni, Co, and Cu. This inexplicable result is caused by the mixed distributions of granite and ultramafic–mafic rocks.The watersheds with mineral deposits relate to the high concentrations of certain elements such as Zn, As, and Hg. The geochemically anomalous pattern, which is a map of the regional anomalies, and a scatter diagram were applied to examine the contribution of mineral deposits to MnO, T-Fe₂O₃, Cr, Cu, Zn, As, Cd, Sb, Hg, Pb, and Bi concentrations. Consequently, they were grouped into four types: 1) Mineral deposits with no outliers resulting from mineralization (MnO, T-Fe₂O₃, and Cr), 2) sediments supplied from watersheds without metal deposits conceal high metal inputs from known mineral deposits (Cu), 3) deposits from a geochemically anomalous area that closely relates to the presence of mineral deposits (As, Sb, and Hg), and 4) deposits from the widely altered zone associated with the Kuroko as well as hydrothermal deposits corresponding to geochemically anomalous patterns (Zn, Cd, and Pb). This study provides an important regional geochemical database for a young island-arc setting and interpretational problems, such as complicated geology and active erosion, that are unique to Japan.     

Neoproterozoic nascent island arc volcanism from the Nubian Shield of Egypt: Magma genesis and generation of continental crust in intra-oceanic arcs

The Neoproterozoic Wadi Ranga metavolcanic rocks, South Eastern Desert of Egypt, constitute a slightly metamorphosed bimodal sequence of low-K submarine tholeiitic mafic and felsic volcanic rocks. The mafic volcanic rocks are represented by massive and pillow flows and agglomerates, composed of porphyritic and aphyric basalts and basaltic andesites that are mostly amygdaloidal. The felsic volcanic rocks embrace porphyritic dacites and rhyolites and tuffs, which overlie the mafic volcanic rocks. The geochemical characteristics of Wadi Ranga volcanic rocks, especially a strong Nb depletion, indicate that they were formed from subduction-related melts. The clinopyroxene phenocrysts of basalts are more akin to those crystallizing from island-arc tholeiitic magmas. The tholeiitic nature of the Wadi Ranga volcanics as well as their LREE-depleted or nearly flat REE patterns and their low K₂O contents suggest that they were developed in an immature island arc setting. The subchondritic Nb/Ta ratios (with the lowest ratio reported for any arc rocks) and low Nb/Yb ratios indicate that the mantle source of the Wadi Ranga mafic volcanic rocks was more depleted than N-MORB-source mantle. Subduction signature was dominated by aqueous fluids derived from slab dehydration, whereas the role of subducted sediments in mantle-wedge metasomatization was subordinate, implying that the subduction system was sediment-starved and far from continental clastic input. The amount of slab-derived fluids was enough to produce hydrous magmas that follow the tholeiitic but not the calc-alkaline differentiation trend. With Mg# > 64, few samples of Wadi Ranga mafic volcanic rocks are similar to primitive arc magmas, whereas the other samples have clearly experienced considerable fractional crystallization.The low abundances of trace elements, together with low K₂O contents of the felsic metavolcanic rocks indicate that they were erupted in a primitive island arc setting. The felsic volcanic rocks are characterized by lower K/Rb ratios compared to the mafic volcanic rocks, higher trace element abundances (~ 2 to ~ 9 times basalt) on primitive arc basalt-normalized pattern and nearly flat chondrite-normalized REE patterns, which display a negative Eu anomaly. These features are largely consistent with fractional crystallization model for the origin of the felsic volcanic rocks. Moreover, SiO₂-REE variations for the Wadi Ranga volcanic rocks display steadily increasing LREE over the entire mafic to felsic range and enriched La abundances in the felsic lavas relative to the most mafic lavas, features which are consistent with production of the felsic volcanic rocks through fractional crystallization of basaltic melts. The relatively large volume of Wadi Ranga silicic volcanic rocks implies that significant volume of silicic magmas can be generated in immature island arcs by fractional crystallization and indicates the significant role of intra-oceanic arcs in the production of Neoproterozoic continental crust. We emphasize that the geochemical characteristics of these rocks such as their low LILE and nearly flat REE patterns can successfully discriminate them from other Egyptian Neoproterozoic felsic volcanic rocks, which have higher LILE, Zr and Nb and fractionated REE patterns.     

Application of multi-element statistical analysis for regional geochemical mapping in Central Japan

Some 434 stream sediment samples were collected in Central Japan for a nationwide geochemical mapping project. The resulting geochemical maps are compared with geological, mineral resource and land use maps. Spatial distribution patterns of elemental concentrations in stream sediments are determined mainly by surface geology. Elevated elemental concentrations of alkali elements, Be, Ga, Y, Cs, Ba, lanthanide (Ln), Tl, Th, and U are consistent with outcrop areas of granite, felsic volcanic rock, and accretionary complexes. High concentrations of MgO, Al₂O₃, P₂O₅, CaO, 3d transition metals, Zn, and Sr are present in sediments supplied from mafic volcanic rock, high pressure metamorphic rocks, and mafic-ultramafic rocks in accretionary complexes.A procedure is established and guidelines are set for a statistical test suite for geochemical mapping. Analysis of variance (ANOVA) and multiple comparison tests are effective for comparing means among the data subsets that are classified by parent lithological materials. Among the many procedures that have been proposed for multiple comparison tests, the Holm procedure was selected for this study. Multiple comparison statistically confirmed the correspondence of elemental abundance in stream sediments with surface geologies. However, visual interpretation of some elements is inconsistent with results of multiple comparison. According to the Holm procedure, the U content in stream sediments is affected not by granite, but by felsic volcanic rock. The Holm procedure clarifies that As, Sb, and Bi, that are not explained by the presence of mineral deposits, are enriched significantly in samples derived from accretionary complexes. Hydrothermal activity on the ocean floor might affect their levels of enrichment. Significant enrichment of Cu, Zn, Cd, Sn, Sb, Hg, and Pb observed in urban areas are also supported by the Holm procedure. The authors inferred that these sediment samples had been contaminated.     

Geochemistry of the Paleocene Clastic Rocks in Lishui Sag, East China Sea Shelf Basin: Implications for Tectonic Background and Provenance

The Lishui Sag, in the East China Sea Shelf Basin, is rich in hydrocarbons, with the major hydrocarbon-bearing layers being the Paleocene Mingyuefeng clastic rocks. Analysis of the implicit geologic background information of these Paleocene clastic rocks using petrological and geochemical methods has significant practical importance. These Paleocene sandstones are mainly lithic arenite, lithic arkose and greywacke, composed of K-feldspar, plagioclase, authigenic clays, silica and carbonates. As continental deposits, Yueguifeng clastic rocks have high aluminosilicate and mafic detritus contents, while the Lingfeng and Mingyuefeng Formations are rich in silica due to an oscillating coastal marine depositional environment. The major element contents of these Paleocene sandstones are low and have a concentrated distribution, indicating that the geochemical composition is non-epigenetic, transformed by sedimentary processes and diagenesis. The Yueguifeng detritus comprises recycled sediments, controlled by moderate weathering and erosion, while the Lingfeng and Mingyuefeng detritus is interpreted as primarily first-cycle materials due to low chemical weathering. In the Late Cretaceous to Early Paleocene, the Pacific Plate began subducting under the Eurasian Plate, causing an orogeny by plate collision and magma eruption due to the melting of subducted oceanic crust. This resulted in the dual tectonic settings of “active margin” and “continental island arc” in the East China Sea Shelf Basin. During the Late Paleocene, the Pacific Plate margin migrated eastward along with development of the Philippine Ocean Plate, and the tectonic setting of the Lishui Sag gradually turned into a passive continental margin. Detrital sources included both orogenic continental blocks and continental island arcs, and the parent rocks are primarily felsic volcanic rocks and granites.     

西昆仑东段苏巴什洋向北俯冲:来自早—中二叠世火山岩的证据

西昆仑东段苏巴什蛇绿构造混杂岩带南侧卡拉勒塔什群以大面积分布的酸性和中基性火山岩为特征,本次对卡拉勒塔什群弧火山岩代表性的岩石组合进行了LA-ICP-MS锆石U-Pb年龄、地球化学及锆石Lu-Hf同位素研究。研究结果表明,LA-ICP-MS锆石U-Pb测年获得酸性晶屑凝灰岩、蚀变玄武岩²⁰⁶Pb/²³⁸U加权平均年龄为(284.2±1.6) Ma和(262.6±2.0) Ma,表明研究区卡拉勒塔什群火山岩形成于早—中二叠世。卡拉勒塔什群火山岩具有富铝、钠、铁,富集大离子亲石元素K、Rb、Ba和亏损高场强元素Sr、Ta、Nb、Ti的地球化学特征。其中,基性火山岩属钙碱性-拉斑玄武岩系列,岩石稀土元素配分模式接近大洋拉斑玄武岩,Nd/Th和La/Nb比值为8.91~13.76和0.39~2.28,Lu-Hf同位素ε_Hf(t)值为-0.15~4.95,表现为地幔物质来源,但加入了地壳组分。酸性火山岩属于钙碱性系列,相对亏损P和Zr元素,Nd/Th和La/Nb比值为1.92~4.10和2.52~3.39,Lu-Hf同位素ε_Hf(t)值分别为0.94~3.78和8.26~12.45,二阶段模式年龄分别为1.07~1.25 Ga、0.51~0.78 Ga,表明酸性火山岩物质来源为古老地壳和新生地壳物质重熔后的混合物。卡拉勒塔什群总体地球化学特征表现为岛弧环境。卡拉勒塔什群岛弧火山岩与北侧苏巴什蛇绿构造混杂岩带在形成时代、空间分布以及基性岩地球化学特征均表现成对关系,与苏巴什蛇绿构造混杂岩带内发育的硫磺达坂砂岩组深水复理石建造共同构成造山带沟-弧-盆体系,表明苏巴什洋盆由南向北的俯冲极性,说明苏巴什蛇绿构造混杂岩形成于岛弧偏向于海沟的弧前盆地构造背景。     

Review Section

ABSTRACT

The petrology, geochronology, and geochemistry of the early Permian volcanic rocks from Houtoumiao area, south Xiwuqi County in central Inner Mongolia of China, are studied to elucidate the early Permian tectonic setting of the region. The volcanic rocks, which are interbedded with sandstone, feature both mafic and felsic compositions and show a bimodal nature. Zircon U–Pb dating reveals that the volcanic rocks formed at 274–278 Ma, similar to the ages of bimodal magmatism in neighbouring areas. The mafic rocks are composed of tholeiitic basalt, basaltic andesite, basaltic trachyandesite, and trachyandesite. They are rich in Th, U, and LILEs, depleted in HFSEs Nb, Ta, and Ti, and have positive ε_Nd(t) values (+3.6 to +7.9). Geochemical analyses indicate that the mafic rocks originated from metasomatized lithospheric mantle. The felsic volcanic rocks are mainly rhyolite, with minor trachyte and dacite. They have different evolutionary tendencies of major elements, chondrite-normalized REE patterns, and isotopic compositions from the mafic volcanic rocks, which preclude formation by fractional crystallization of mafic melts. The ε_Nd(t) values of the felsic rocks are similar to those of the Carboniferous Baolidao arc rocks in the region. It is suggested that Permian felsic melts originated from the partial melting of Carboniferous juvenile arc-related rocks. By comparison with typical Cenozoic bimodal volcanism associated with several tectonic settings, including rift, post-collisional setting, back-arc basin, and the Basin and Range, USA, the bimodal volcanic rocks in central Inner Mongolia display similar petrological and geochemical characteristics to the rocks from back-arc basin and the Basin and Range, USA. Based on the analysis of regional geological data, it is inferred that the early Permian bimodal volcanic rocks in the study area formed on an extensional continental margin of the Siberian palaeoplate after late Carboniferous subduction–accretion.     

Geochemical mapping in Shikoku, southwest Japan

Geochemical mapping of Shikoku in the SW Japan Arc was conducted using stream sediments (<0.18 mm fraction) with sample densities of 1 site per 106 km². Concentrations of 51 elements in 173 stream sediments were determined. Areas with especially high Cr, Ni, Mg and Co concentrations run intermittently and linearly through Shikoku, and coincide with exposures of the Mikabu greenstone complex composed of oceanic basaltic and gabbroic rocks with minor ultramafic bodies. Areas with higher concentrations of Fe, V, Sc, Ti, Mn, Cr, Ni, Mg, Co and Eu are distributed widely along the Shikoku Island axis; they mainly overlap with the zones of the high-pressure type metamorphic rocks (Sanbagawa Belt) and of the Jurassic accretionary complexes with sedimentary rocks (Chichibu Belt): in contrast, areas with lower concentrations spread over the northern coastal zone of Cretaceous granitoids (Ryoke Belt) and in the southern coastal zone of Cretaceous-Paleogene accretionary complexes (Shimanto Belt). Distribution patterns of these elements reflect that various amounts of mafic and ultramafic materials, possibly of oceanic origin, are associated with the rocks in the Sanbagawa and Chichibu Belts, although such components occur sparsely in the Shimanto and Ryoke Belts. Calcium, Sr, Na, Nb, Sm and Gd contents are lower along the southern coastal zone. High values of Th, U, La, Ce and Ba are associated with granitic rocks. The positive anomalies of Cu and Cd coincide with the distribution of stratabound Cu deposits in the Sanbagawa Belt. Positive anomalies of Sb in the northern area are associated with Sb deposits near the large Median Tectonic Line fault zone. A high-Hg zone is present in the southern fore-arc area. The median concentrations for stream sediments in Shikoku are higher in Hg, Cu, Ni, Cr, Rb, Li, Cs, K, slightly higher in Nb, Ta, La, Ce, Sn and lower in Ca than those for average Japanese stream sediments. The median concentrations for the four geologic zones show systematic wide variations of mafic associated elements, and narrow variations of lithophile elements generally more abundant in felsic rocks. The chemical compositions of the stream sediments in Shikoku largely reflect the concentrations in the pre-Neogene accretionary complexes and in the high-P metamorphic rocks mainly formed from clastic materials derived from continental margins, and ratios of mafic and ultramafic materials within surface rocks; they are partly influenced by sulfide mineralization and fluid migration.     

Regional spatial distribution of multiple elements in the surface sediments of the eastern Tsushima Strait (southwestern Sea of Japan)

A total of 402 coastal sea-sediment samples were collected from the continental shelf, slope, and basin off Tsushima Island in the western Sea of Japan, and were analyzed for 51 elements as part of a nationwide marine geochemical mapping project. The samples were compared to potentially related sample sets, and the results were considered from the viewpoint of the origin of marine surface sediments in the western Sea of Japan. The spatial distribution of elemental concentrations in the coastal sea sediments correspond to texture, grain size, the presence of shells and foraminifera, and the mud content of surface marine sediments. Most elemental concentrations increased with increasing mud content. Some samples located in littoral areas included sediment particles apparently supplied from nearby rivers, but their contribution was limited. Overall, the mean chemical compositions of clastic material in coastal sea sediments appear to differ from those of stream sediments in adjacent terrestrial areas. In addition, the geochemistry of the coastal sea sediments cannot be fully explained by the mixing of the material supplied from Korean and Chinese stream sediments, which are the most feasible sources. Coastal sea sediments in the study area are well mixed by transportation processes; therefore, elemental abundances in these marine sediments may be homogenized to such an extent that it is not possible to determine their origin. Alternatively, most of the clastics in the sediment may actually represent relict reworked material, originally formed in the Yellow Sea and Tsushima Strait during the last glacial stage and subsequent transgression.     

A model of ocean-crust accretion for the Superior province, Canada

One of the keys to understanding the origin of Archaean greenstone belts lies in the geological relationships between mafic and ultramafic greenstones, felsic to intermediate volcanic rocks and terrigenous sediments. Traditional models for greenstone belt evolution have been based on in-situ stratigraphic relationships. Most of these models, for example an oceanic island-arc developed on oceanic basement, back-arc basins, and the recently popular plume model, predict concordant stratigraphic relationships among the various greenstone belt lithologies. However, rather than being depositional in nature, several authors have indicated that many of the relationships between the different lithologies in greenstone belts are in fact tectonic, suggesting an allochthonous origin for most greenstone sequences. All of these latter models make analogies to Phanerozoic tectonic processes involving accretion of oceanic materials with volcanism related to both plate subduction and rifting.

In this paper, we have evaluated the geological relationships between volcanic rocks and sediments in three regions in the Superior province, where the accretion of oceanic material can be documented, and direct comparisons are made to geological processes in Phanerozoic accretionary complexes. In the Malartic area in the southeastern Abitibi Subprovince, 3 to 4 km thick slices of komatiite and tholeiite, with intercalated terrigenous sediment, are tectonically imbricated and are overlain by calc-alkaline volcanics which postdate tectonic stacking. In both the Larder Lake region of the southwestern Abitibi belt and in the Beardmore-Geraldton belt, at the south-eastern limit of the Wabigoon belt, slices of iron-rich tholeiite and chemical sediments of an oceanic origin are tectonically imbricated with terrigenous sediment.

The Malartic-Val d'Or area is considered to be an example of accretion of an Archaean oceanic plateau, while the Larder Lake and the Beardmore-Geraldton regions are potentially typical of accretion of normal oceanic crust in an arc-environment. Phanerozoic accretion of oceanic crust is accompanied by a step-back in subduction, and in this paper we suggest that oceanic crust accretion may have been the principal mechanism by which the locus of subduction migrated towards the south of the Superior province. Asthenospheric upwelling associated with the isolated sinking plate may have been responsible for widespread late-magmatism. This scenario requires that magmas be erupted through previously accreted volcanic, plutonic and sedimentary material. Furthermore, later ridge subduction will result in transpressional tectonics and eruption of mafic sequences over mature and immature volcano-plutonic sequences. The combined result of the plate tectonic scenario envisaged would result in the well-described “cyclic stratigraphy” of many granite greenstone sequences.     

Likely “mantle plume” activity in the Skellefte district,Northern Sweden. A reexamination of mafic/ultramafic magmatic activity: Its possible association with VMS and gold mineralization

Most attention has been given to the geology of the extensive VMS and subordinate precious metals mineralization in the Skellefte district. Less attention has been given to indications of deep-seated origins of felsic and mafic/ultramafic volcanic rocks; of VMS and precious metals mineralizing fluids; and the primary origins of these metals. A holistic view of the significance of mafic/ultramafic volcanic rocks to both the geotectonic evolution of the area and the existence of its important base and precious metals deposits has never been presented. These subjects are discussed in this investigation.Primitive mantle normalized spider diagrams of rare-earth-elements (REE) distinguish two groups of mafic/ultramafic volcanic rocks, each with distinct geochemical characteristics: a mid-ocean-ridge “MORB”-type, and a geochemically unusual and problematic calc–alkaline–basalt “CAB”-type which is the main subject of this investigation. The “MORB”-type mafic volcanic rocks are mostly older than the Skellefte Group felsic volcanic rocks hosting the VMS deposits, whereas the more primitive “CAB”-type mafic/ultramafic volcanic rocks are mostly younger.A common source for these “CAB”-type, mafic-(MgO wt.% < 14%) and ultramafic-(MgO wt.% > 14%) volcanic rocks is suggested by their similar and distinctive geochemical features. These are near-chondritic (Al-undepleted) Al₂O₃/TiO₂ ratios; moderate to strong high-field-strength-element (HFSE) depletion; light-rare-earth-element (LREE) enrichment and moderate heavy-rare-earth-element (HREE) depletion. They outcrop throughout an area of at least 100 × 100 km. Gold mineralization is spatially associated with ultramafic volcanic rocks.Zr and Hf depletion has been shown to be associated with Al-depletion in mafic/ultramafic volcanic rocks elsewhere, and has been attributed to deep-seated partial melting in ascending mantle plumes. Zr and Hf depletion in “CAB”-type Al-undepleted mafic/ultramafic volcanic rocks is therefore unusual. The solution to this dilemma is suggested to be contamination of an Al-depleted mantle plume by felsic crustal rocks whereby Al-depleted ultramafic magmas become Al-undepleted. It will be argued that this model has the potential to explain previous observations of deep-seated origins; the spatial association of ultramafic volcanic rocks with occurrences of gold mineralization; and even the primary origin of metals in VMS deposits.     

Petrology of the Rainy Lake area,Minnesota, USA-implications for petrotectonic setting of the archean southern Wabigoon subprovince of the Canadian Shield

The Rainy Lake area in northern Minnesota and southwestern, Ontario is a Late Archean (2.7 Ga) granite-greenstone belt within the Wabigoon subprovince of the Canadian Shield. In Minnesota the rocks include mafic and felsic volcanic rocks, volcaniclastic, chemical sedimentary rocks, and graywacke that are intrucded by coeval gabbro, tonalite, and granodiorite. New data presented here focus on the geochemistry and petrology of the Minnesota part of the Rainy Lake area. Igneous rocks in the area are bimodal. The mafic rocks are made up of three distinct suites: (1) low-TiO₂ tholeiite and gabbro that have slightly evolved Mg-numbers (63–49) and relatively flat rare-earth element (REE) patterns that range from 20–8 x chondrites (Ce/Yb_N=0.8–1.5); (2) high-TiO₂ tholeiite with evolved Mg-numbers (46–29) and high total REE abundances that range from 70–40 x chondrites (Ce/Yb_N=1.8–3.3), and (3) calc-alkaline basaltic andesite and geochemically similar monzodiorite and lamprophyre with primitive Mg-numbers (79–63), enriched light rare-earth elements (LREE) and depleted heavy rare-earth elements (HREE). These three suites are not related by partial melting of a similar source or by fractional crystallization of a common parental magma; they resulted from melting of heterogeneous Archean mantle. The felsic rocks are made up of two distinct suites: (1)low-Al₂O₃ tholeiitic rhyolite, and (2) high-Al₂O₃ calc-alkaline dacite and rhyolite and consanguineous tonalite. The tholeiitic felsic rocks are high in Y, Zr, Nb, and total REE that are unfractionated and have pronounced negative Eu anomalies. The calcalkaline felsic rocks are depleted in Y, Zr, and Nb, and the REE that are highly fractionated with high LREE and depleted HREE, and display moderate negative Eu anomalies. Both suites of felsic rocks were generated by partial melting of crustal material. The most reasonable modern analog for the paleotectonic setting is an immature island arc. The bimodal volcanic rocks are intercalated with sedimentary rocks and have been intruded by pre- and syntectonic granitoid rocks. However, the geochemistry of the mafic rocks does not correlate fully with that of mafic rocks in modern are evvironments. The low-TiO₂ tholeiite is similar to both N-type mid-ocean-ridge basalt (MORB) and low-K tholeiite from immature marginal basins. The calc-alkaline basaltic andesite is like that of low-K calc-alkaline mafic volcanic rocks from oceanic volcanic arcs; however, the high-TiO₂ tholeiite is most similar to modern E-type MORB, which occurs in oceanic rifts. The conundrum may be explained by: (1) rifting of a pre-existing immature arc system to produce the bimodal volcanic rocks and high-TiO₂ tholeiite; (2) variable enrichment of a previously depleted Archean mantle, to produce both the low- and high-TiO₂ tholeiite and the calc-alkaline basaltic andesite, and/or (3) enrichment of the parental rocks of the high-TiO₂ tholeiite by crustal contamination.     

The Quebradagrande Complex: A Lower Cretaceous ensialic marginal basin in the Central Cordillera of the Colombian Andes

The Quebradagrande Complex of Western Colombia consists of volcanic and Albian–Aptian sedimentary rocks of oceanic affinity and outcrops in a highly deformed zone where spatial relationships are difficult to unravel. Berriasian–Aptian sediments that display continental to shallow marine sedimentary facies and mafic and ultramafic plutonic rocks are associated with the Quebradagrande Complex. Geochemically, the basalts and andesites of the Quebradagrande Complex mostly display calc-alkaline affinities, are enriched in large-ion lithophile elements relative to high field strength elements, and thus are typical of volcanic rocks generated in supra-subduction zone mantle wedges. The Quebradagrande Complex parallels the western margin of the Colombian Andes’ Central Cordillera, forming a narrow, discontinuous strip fault-bounded on both sides by metamorphic rocks. The age of the metamorphic rocks east of the Quebradagrande Complex is well established as Neoproterozoic. However, the age of the metamorphics to the west – the Arquía Complex – is poorly constrained; they may have formed during either the Neoproterozoic or Lower Cretaceous. A Neoproterozoic age for the Arquía Complex is favored by both its close proximity to sedimentary rocks mapped as Paleozoic and its intrusion by Triassic plutons. Thus, the Quebradagrande Complex could represent an intracratonic marginal basin produced by spreading-subsidence, where the progressive thinning of the lithosphere generated gradually deeper sedimentary environments, eventually resulting in the generation of oceanic crust. This phenomenon was common in the Peruvian and Chilean Andes during the Uppermost Jurassic and Lower Cretaceous. The marginal basin was trapped during the collision of the Caribbean–Colombian Cretaceous oceanic plateau, which accreted west of the Arquía Complex in the Early Eocene. Differences in the geochemical characteristics of basalts of the oceanic plateau and those of the Quebradagrande Complex indicate these units were generated in very different tectonic settings.     

Geochemical and mineralogical characteristics of recent clastic sediments from lower Godavari river: Implications of source rock weathering

The major, trace and rare earth elements geochemistry and clay mineral compositions in the river bed sediments from lower reaches of Godavari river suggest that they are derived from weathering of felsic rocks. Trace and rare earth elemental compositions indicate evidence of sedimentary sorting during transportation and deposition. Lower concentrations of transition elements, such as V, Ni and Cr imply enrichment of felsic minerals in these bed sediments. The REE pattern in lower Godavari sediments is influenced by the degree of source rock weathering. The light rare earth elements (LREE) content are indicating greater fractionation compared to the heavy rare earth elements (HREE). A striking relationship is observed between TiO₂ and gZREE content suggesting a strong control by LREE-enriched titaniferous minerals on REE chemistry. Shale-normalized REE pattern demonstrate a positive Eu anomaly, suggesting weathering of feldspar and their secondary products, which are enriched in Eu. Chondrite-normalised REE pattern is characteristic of felsic volcanic, granites and gnessic source rocks. Trace elemental compositions in sediments located near urban areas suggest influence of anthropogenic activity. Chemical Index of Alteration (CIA) is high (avg. 65.76), suggesting a moderate chemical weathering environment. X-ray diffraction analysis of clay fraction shows predominance of clay minerals that are formed because of the chemical weathering of felsic rocks.     

The use of the “immobile” elements Zr and Ti in lithogeochemical exploration for massive sulphide deposits in the precambrian pecos greenstone belt of northern New Mexico

Mineralogical and geochemical data of surface rocks of the Mid-Proterozoic Pecos greenstone belt in Northern New Mexico and results of drill core data of the Jones Hill deposit are presented and discussed within the geological framework derived from detailed geological mapping. Over 800 rock samples were analyzed for 25 elements (all major and several trace elements) and the lithologies were mineralogically and petrographically determined. The results show that the stratigraphic sequence of the Jones Hill deposit is composed of felsic pyroclastics, exhalites and pyroclastics of mafic and mixed felsic-mafic origin, upon which are superimposed certain alteration effects.In volcanic rocks of felsic origin Mg, Fe, Mn, Cu, Pb, Zn, As are enriched and Na, Ca, Sr are clearly depleted when compared with their unaltered source rocks (Table I). Zirconium and Ti are found to be relatively immobile during alteration and subsequent intrusive, deformational and metamorphic events.Alteration trends, best defined by the Na₂O/MgO ratios and systematic metal zoning, confirm that the stratigraphic sequence is overturned. Lithogeochemical anomalies outline a target which is bigger (over 300 m) and more precisely defined than do other methods, mapping of alteration assemblages or soil geochemistry.New important aspects have evolved from this study, which are applicable to rock geochemical exploration and the recognition of alteration/mineralization anomalies in general. Ratios of Zr/TiO₂ can be shown to reflect the primary pre-alteration-premetamorphic composition of metavolcanic rocks (including shallow intrusives, flows, tuffs, and volcaniclastics). This ratio also enables the determination of the original composition of variably altered rocks of identical parentage (source rock). This determination is a fundamental prerequisite for the recognition and quantification of alteration trends. Direct applications are, for example, that apparently similar chlorite schists derived through hydrothermal alteration, metamorphism, and deformation Zr/TiO₂ ratio of 0.04 (and greater); however chlorite schists derived from mafic volcanic sources and metamorphosed without much hydrothermal alteration have a much lower Zr/TiO₂ ratio of 0.0075 (and less). The altered rock related to massive sulphide alteration can thus be recognized and distinguished from similar rocks of different origin. In addition, the relative concentrations of the immobile elements Zr and Ti are useful in making stratigraphic correlations in structurally complex terrain such as the Pecos greenstone belt.The assumption of a relative immobility of these elements in samples from Jones Hill is supported by the following observation:

1. (1) The TiO₂ and Zr values of Jones Hill rocks fall in narrow clusters (Fig. 1) as opposed to a random scatter plot.

2. (2) The element abundances (ratios) are very similar to: (a) granite (Type A), fresh felsic volcanics (Type A) of the same area and same age (Table I) and basalt (Type B) of the same area; and (b) fresh basalts and rhyolites reported from young volcanic island arcs.

3. (3) Highly altered chlorite schists directly underlying the main ore horizon and within the ore horizon, display Zr/TiO₂ ratios identical to less altered felsic rocks in the foot wall.

These rock geochemical methods can be demonstrated to be applicable at all scales ranging from regional reconnaissance through local prospect evaluation down to detailed drill core interpretations during deposit delineation stages.     

Petrology and geochemistry of Early Permian volcanic rocks in South Tian Shan,NW China: implications for the tectonic evolution and Phanerozoic continental growth

Situated in the southwest of the Central Asian Orogenic Belt (CAOB), the South Tian Shan (STS) Block is a key area for understanding the final accretion of the CAOB. A suite of volcanic rocks interbedded with continental sediments from the Xiaotikanlike Formation lies along the southwestern edge of the Tian Shan orogen. Laser-ablation-inductively coupled plasma-mass spectrometer U–Pb dating provided a crystallization age of 295.0 ± 2.8 Ma (MSWD = 1.3), suggesting an Early Permian magmatic event. The volcanic rocks show a variable composition, with dominant rhyolites and dacites, subordinate basaltic andesites and few basalts. The felsic rocks are enriched in K and exhibit remarkably negative anomalies in Ba, Sr, Eu, P and Ti. These anomalies associated with their high negative ε _Nd(t) values and old Nd model ages suggest that they are most likely sourced from ancient lower crustal rocks. The mafic rocks are characterized by high Mg#, Cr, Ni contents, negative Nb, Ta anomalies and pronounced enrichment in light rare earth elements as well as mild enrichment in large-ion lithophile elements. The mafic rocks are thus inferred to derive from enriched subcontinental lithospheric mantle. The petrographic and geochemical characteristics of the Xiaotikanlike Formation volcanic rocks indicate that they were generated under a post-collisional regime. Therefore, the final collision between the Tarim Craton and the Kazakhstan–Yili terrane took place before Early Permian, most probably at Late Carboniferous. Differing from other tectonic units of the CAOB, the recycling of ancient lithospheric crust played a significant role in the continental growth of the STS Block.     

Geochemistry of the Boil Mountain ophiolitic complex,northwest Maine,and tectonic implications

A coherent ophiolitic complex of pyroxenite, serpentinite, metagabbro, mafic volcanics, felsic volcanics and sediments crops out in NW Maine, adjacent to the Chain Lakes massif. The complex (here informally referred to as the Boil Mountain ophiolitic complex) is about 500 m.y. old. The volcanic sequence is not typical of ophiolites in that it contains a large proportion of felsic volcanics. The mafic volcanics are divided into two geochemical groups. A stratigraphically lower group is depleted in Ti, Zr, Y, Cr and REE contents similar to basalts from supra-subduction zone ophiolites. An upper mafic group has trace element contents similar to normal mid-ocean ridge basalts. The felsic volcanics are mostly rhyolitic and similar to low-K rhyolites found in the forearc of the Marianas trench and in an island arc sequence in the Klamath Mountains, California. The flat REE patterns of the felsic volcanic rocks are similar to those found in siliceous rocks in the Oman ophiolite. The presence of thick sequences of felsic volcanics, the abundance of pyroxenite, the low Ti, Zr and REE contents of some mafic rocks, the flat REE patterns of the felsic volcanics, and the composition of clinopyroxene all suggest the complex was formed in the vicinity of a subduction zone. The complex may be correlated with ophiolitic fragments in the eastern part of the Dunnage Zone in Newfoundland, rather than the main ophiolite belt of the western Appalachians.     

福建北部近岸东台山岛火山岩形成时代、地球化学与古太平洋板块俯冲的响应

福建北部沿岸岛屿岩石组合以晚中生代火成岩为主,研究认为是古太平洋俯冲消减的产物,对反演洋盆构造演化具有重要的指示意义。本次对其中的福建北部海域东台山岛上广泛发育的酸性火山岩进行了锆石U-Pb定年以及全岩主微量地球化学分析工作。2件年代学样品分别获得了92 Ma和86 Ma的锆石U-Pb年龄,确定东台山岛火山岩形成于晚白垩世。全岩地球化学特征指示火山岩样品以酸性钙碱性岩石为主,整体富集Rb、Ba等元素,亏损Nb、Ta、Sr、Eu等元素,显示弧型岩浆岩的地球化学组成。研究认为东台山岛火山岩起源于古老下地壳变沉积岩熔融,并在浅层岩浆房内经历了不同程度的结晶分异过程。结合区域上晚中生代岩浆作用由陆向海的时空迁移特征,福建北部沿岸岛屿火山岩形成的深部动力学机制应该与古太平洋俯冲过程中的板片回转过程相关。     

Mineralogical and geochemical features of alluvial sediments from the Lobo watershed (Southern Cameroon): Implications for rutile exploration

This paper is focused on the morphological, mineralogical, and geochemical features of alluvial sediments from the Neoproterozoic Pan-African belt to explore rutile. The fine-grained sediments, which contain a large proportion of rutile, are made up of quartz, rutile, zircon, brookite, tourmaline, andalusite, and kyanite. The high SiO₂ and TiO₂ contents highlight the predominance of silica minerals in the alluvia from the humid tropical zone. La/Sc, La/Co, Th/Sc and Zr/Cr ratios reflect the contribution of felsic and mafic sources. The highest Ti contents, which occur at the outlet of the Lobo watershed, indicate the resistance of rutile. The REE distribution could be linked to the heavy mineral sorting. The low (La/Yb)_N ratios and high Zr contents are attributed to the high proportion of zircon. Chondrite-normalized REE patterns indicate high felsic sources, which are the regional rocks. Ultimately, the Yaoundé Group constitutes a favorable potential target for further rutile exploration.