Andesitic Arc Research Articles

The geochemistry of high-Mg andesite and associated adakitic rocks in the Musoma-Mara Greenstone Belt, northern Tanzania: Possible evidence for Neoarchaean ridge subduction?

Geochemical data are presented for Neoarchaean metavolcanic and plutonic rocks from the Musoma-Mara Greenstone Belt (MMGB) in the northern part of the Tanzania Craton with the aim of inferring their petrogenesis and tectonic settings in which they formed. The MMGB is underlain by two unusual magmatic suites: high magnesium andesites and an adakitic suite. The high-Mg andesites are composed of intermediate volcanic rocks of andesitic composition which, when compared to normal island arc andesites, are characterised by high contents of MgO (2.42–9.47 wt%, Cr (41–797 ppm) and Ni (11–254 ppm). Ratios of Nb/Y are 0.17–0.28, La/Yb are 9.87–22.5 and Sr/Y are 16.1–48.7. These geochemical features are analogous to those shown by modern High Magnesium Andesites (HMA). The adakitic suite is composed of dacites and Na-granitoids and is characterized by Al 2O 3 contents of 14.75–18.54 wt%, variable contents of MgO (0.31–4.96 wt%), Cr and Ni (5.10–250 and 1.22–115 ppm, respectively). Their ratios of Nb/Y are 0.17–0.66, La/Yb are 11–80 and Sr/Y are 20–131. Compared to the HMA, rocks of the adakitic suite are characterized by lower MgO, Cr and Ni contents, higher contents of Al 2O 3 and Sr, higher ratios of La/Yb and Sr/Y and are compositionally similar to modern adakites. Rocks from both suites show negative anomalies of Nb, Ta and Ti and have similar ɛNd values (at 2.67 Ga) of +0.44 to +2.66. The geochemical characteristics of the HMA are consistent with the derivation of their parent magma by partial melting of mantle peridotite that has been fluxed by slab-derived aqueous fluids above a continental arc. As the slab further descended into the mantle, partial melting of the subducted oceanic crust occurred in the garnet stability field producing a melt that was depleted in HREE. The slab-derived melts percolated into the mantle wedge and reacted with mantle peridotite resulting in parental magmas of rocks of the adakitic suite. Subsequently, the parental magmas of both rock suites ascended through and were contaminated by older felsic crust forming the continental arc basement. Subsequent fractional crystallization of pyroxene and hornblende led to the range in Mg numbers, CaO, Cr and Ni contents observed in the rocks. The association of some members of the adakitic suite with locally derived clastic sedimentary rocks suggests that the latest volcanic episode in the MMGB occurred in a continental back arc basin. The rapid emplacement of volcanic and plutonic rocks in a relatively short time interval is best explained in terms of the ridge-subduction model of [Iwamori, H., 2000. Thermal effects of ridge subduction and its implication for the origin of granitic batholith and paired metamorphic belts. Earth Planet. Sci. Lett., 181, 131–144.] whereby subduction of the ridge crest results in anomalous high thermal input into the subduction zone leading to rapid arc magmatism within a few tens of kilometers from the slab–crust interface and within a time interval of 30 Ma after ridge subduction.

Tectonics of the Vosges (NE France) and the Schwarzwald (SW Germany): evidence from Devonian-Carboniferous active margin basins and their deformation

Devonian-Carboniferous sedimentary and volcanic rocks are exposed in the Badenweiler-Lenzkirch Zone (BLZ) of the Southern Schwarzwald and in the Southern Vosges (Central Europe). Several lithostratigraphic units are recognized. Sedimentary or tectonic relations between some of the units can be realized by comparing sedimentary and volcanic facies associations as well as petrographic and geochemical datasets. During the Variscan orogenesis the sediments were deposited in different kinds of basins along an active margin setting. Their deformation was studied in the Markstein area of the Southern Vosges and is presented here in detail. Some arguments favour a northward underthrusting of a narrow and oceanic pre-Upper Devonian back-arc basin beneath the crystalline units of the Central Vosges and Schwarzwald. Accretion of continental crust continued from the Upper Devonian to the late Lower Carboniferous along with a characteristical evolution of a deep marine retro-arc foreland basin. It has accumulated about 3500 m of turbidite sandstones, shales and conglomerates of volcano-plutonic origin, assembling now the Markstein Group in the Southern Vosges. To the south of the Markstein Group, deep marine mud-rich turbidite systems and a shallowing-upward succession of fluvio-deltaic sediments filled in the Oderen Basin, which subsided in the late Tournaisian and early Visean. A subduction type volcanism occurred in form of tholeiitic basalts, K-calcalkaline island arc andesites and continental arc sandstones. In contrast to former models, a genetical relationship between the Markstein and the Oderen Group is rejected here. Instead, different subduction systems in space and time have caused the development of the active margin basins. The Variscan deformation of the exposed sedimentary units started in the upper Visean. Structural analysis of kinematic indicators and macroscopic fabrics in the Markstein Group give evidence for thrusting and dextrally transpressive shearing, forming a positive flower structure along the southern border of the Central Vosges crystalline units. This was broadly synchronous with dextrally transpressive deformation along the Lalaye-Lubine/Baden Baden fault at the northern border of the Central Vosges and Schwarzwald and along the Transition Complex between the Central and the Southern Schwarzwald. The high-K-calcalkaline Metzeral granite intruded postkinematically into the principal displacement zone and has a position similar to the dextrally sheared Randgranite association along the northern border of the BLZ. A juxtaposition between the Oderen and the Markstein Groups was achieved in the upper Visean due to the dextral transpressive motion along the ESE-trending Markstein Fault. The exhumation of high grade metamorphic rocks of the Central Vosges and Schwarzwald during the upper Visean is discussed in the context of long lasting accretionary tectonics and the two-sided dextral transpression, favouring crustal extrusion as a main exhumation process. A thorough examination of the supracrustal rocks and their mode of deformation in the Southern Vosges helps us to determine the relationships with the BLZ and contributes also to the paleogeographic reconstruction of the Armorica - Proto-Alpine - Gondwana connection.

Link between SSZ ophiolite formation, emplacement and arc inception, Northland, New Zealand: U–Pb SHRIMP constraints; Cenozoic SW Pacific tectonic implications

New U–Pb age-data from zircons separated from a Northland ophiolite gabbro yield a mean 206Pb/ 238U age of 31.6 ± 0.2 Ma, providing support for a recently determined 28.3 ± 0.2 Ma SHRIMP age of an associated plagiogranite and ∼ 29–26 Ma 40Ar/ 39Ar ages ( n = 9) of basalts of the ophiolite. Elsewhere, Miocene arc-related calc-alkaline andesite dikes which intrude the ophiolitic rocks contain zircons which yield mean 206Pb/ 238U ages of 20.1 ± 0.2 and 19.8 ± 0.2 Ma. The ophiolite gabbro and the andesites both contain rare inherited zircons ranging from 122–104 Ma. The Early Cretaceous zircons in the arc andesites are interpreted as xenocrysts from the Mt. Camel basement terrane through which magmas of the Northland Miocene arc lavas erupted. The inherited zircons in the ophiolite gabbros suggest that a small fraction of this basement was introduced into the suboceanic mantle by subduction and mixed with mantle melts during ophiolite formation. We postulate that the tholeiitic suite of the ophiolite represents the crustal segment of SSZ lithosphere (SSZL) generated in the southern South Fiji Basin (SFB) at a northeast-dipping subduction zone that was initiated at about 35 Ma. The subduction zone nucleated along a pre-existing transform boundary separating circa 45–20 Ma oceanic lithosphere to the north and west of the Northland Peninsula from nascent back arc basin lithosphere of the SFB. Construction of the SSZL propagated southward along the transform boundary as the SFB continued to unzip to the southeast. After subduction of a large portion of oceanic lithosphere by about 26 Ma and collision of the SSZL with New Zealand, compression between the Australian Plate and the Pacific Plate was taken up along a new southwest-dipping subduction zone behind the SSZL. Renewed volcanism began in the oceanic forearc at 25 Ma producing boninitic-like, SSZ and within-plate alkalic and calc-alkaline rocks. Rocks of these types temporally overlap ophiolite emplacement and subsequent Miocene continental arc construction.

Discovery of abundant pyroclasts in the Namunagarh Grit, South Andaman: evidence for arc volcanism and active subduction during the Palaeogene in the Andaman area

The predominant clastic rocks of the late Eocene Namunagarh Grit, South Andaman Island are coarse to fine-grained and pebbly immature greywacke sandstones. These rocks have long been known and interpreted as epiclastic deposits formed from the weathering and erosion of accreted ophiolite in a trench-slope setting. This study documents abundant juvenile vesiculated fragments, pumice clasts, shards, unbroken vesicles, cognate lithics of andesite, broken and whole euhedral fresh plagioclase, and chlorite-replaced glassy matrix in these sandstones. The sandstones evidently have a pyroclastic origin. Based on structures and textures, a coarse-grained and massive, and a finer-grained, bedded and graded sandstone facies are identified. The coarse-grained facies occurs in the lower part of the section, while the finer-grained one, interstratified with mudstone, occurs in the upper part. Major element chemistry indicates a predominantly andesitic composition for these rocks. The facies characteristics indicate that the coarse-grained facies was emplaced as debris flows while the finer-grained facies was transported by turbidity currents. Preservation of intact glassy constituents and their abundance, imply direct sedimentation, or very rapid re-sedimentation, with limited traction and limited time for alteration during transport. These pyroclast-rich sandstones were laid down in a submarine forearc environment on an accretionary complex. The petrology of the juvenile clasts indicates the subaerial or shallow subaqueous explosive eruption of crystal-rich vesicular magma from andesitic arc volcanoes located on an arc massif, inferred to have existed on the western margin of the Burma–Malaya continent during Eocene–Oligocene times. This evidence of arc volcanism indicates active subduction, accretion and emplacement of ophiolite and the associated sediments from the Andaman-Java trench. The pyroclast-rich sandstone represents a distinct and hitherto unrecorded unit in the Tertiary stratigraphy of the Andaman Islands.

The extent of U-series disequilibria produced during partial melting of the lower crust with implications for the formation of the Mount St. Helens dacites

The extent to which U-series disequilibria can be produced during partial melting of mafic lower crust is quantified using a simple batch melting model and both experimental and theoretical partition coefficients for U, Th and Ra. We show that partial melting of mafic lower crust can only produce small disequilibria between 238U, 230Th and 226Ra. Crystallisation of basalt and mixing between young basalt and crustal derived melts will have a similar or smaller effect. Consequently, U-series disequilibrium in arc andesites and dacites can generally only be an inherited feature derived from a mantle parent, unless the timescales of silicic magma production within the crust are short compared to the half-life of 226Ra. Our results have profound implications for several recent models of silicic magma production by thermal incubation and partial melting of the lower crust. We show that the 226Ra excess observed in most arc andesites and dacites requires extremely rapid differentiation and/or the involvement of mantle derived basalts less than a few thousand years old. Application to Mount St. Helens suggests that crystallisation of young mantle-derived magma is likely to be the dominant process in the formation of these dacites.

Geochemistry of Mesoproterozoic Volcanic Rocks in the Western Kunlun Mountains: Evidence for Plate Tectonic Evolution

Abstract Mesoproterozoic volcanic rocks occurring in the north of the western Kunlun Mountains can be divided into two groups. The first group (north belt) is an reversely‐evolved bimodal series. Petrochemistry shows that the alkalinity of the rocks decreases from early to late: alkaline→calc‐alkaline→tholeiite, and geochemistry proves that the volcanic rocks were formed in rifting tectonic systems. The sedimentary facies shows characteristics of back‐arc basins. The second (south belt) group, which occurs to the south of Yutian‐Minfeng‐Cele, is composed of calc‐alkaline island arc (basaltic) andesite and minor rhyolite. The space distribution, age and geochemistry of the two volcanite groups indicate that they were formed in a back‐arc basin (the first group) and an island arc (the second group) respectively and indicate the plate evolution during the Mesoproterozoic. The orogeny took place at ∼1.05 Ga, which was coeval with the Grenville orogeny. This study has provided important geological data for exploring the position of the Paleo‐Tarim plate in the Rodinia super‐continent.

Experimental constraints on volatile abundances in arc magmas and their implications for degassing processes

Abstract Recent phase equilibrium studies, combined with analytical and petrological data, provide rigorous constraints on the pre-eruptive P-T-f H 2 O- f O 2 - f S 2 - f CO 2 conditions of silicic to mafic arc magmas. Pre-eruptive melts show a broad negative correlation between temperature and melt H 2 O contents. Pre-eruptive melt S contents cluster around 100 ppm in residual rhyolitic liquids of silicic to andesitic magmas, and range up to 5000 ppm in more mafic ones. For the entire compositional spectrum, melt sulphur contents are almost independent of prevailing f O 2 . In contrast, they are positively correlated to f S 2 , in agreement with experimental observations. Using these intensive constraints, the composition of coexisting fluid phases has been modelled through a MRK equation of state. Pre-eruptive fluids in silicic to andesitic magmas have X H 2 O (mole fraction of H 2 O) in the range 0.65–0.95. X H 2 O decreases as pressure increases, whereas X CO 2 increases up to 0.2–0.3. Pre-eruptive fluids in hydrous mafic arc magmas, such as high-alumina basalts, generally have similar mole fractions of H 2 O and CO 2 at mid-crustal levels, with X H 2 O increasing only for magmas stored at shallow levels in the crust (<1 kbar). The sulphur content of the fluid phase ranges from 0.12 up to 6.4 wt% in both mafic and silicic magmas. For silicic magmas coexisting with 1–5 wt% fluid, this implies that more than 90% of the melt+fluid mass of sulphur is stored in the fluid. Calculated partition coefficients of S between fluid and melt range from 17 up to 467 in silicic to andesitic magmas, tending to be lower at low f O 2 , although exceptions to this trend exist. For mafic compositions, the sulphur partition coefficient is constant at around 20. The composition of both melt and coexisting fluid phases under pre-eruptive conditions shows marked differences. For all compositions, pre-eruptive fluids have higher C/S and lower H/C atomic ratios than coexisting melts. Comparison between volcanic gas and pre-eruptive fluid compositions shows good agreement in the high temperature range. However, to reproduce faithfully the compositional field delineated by volcanic gases, silicic to andesitic arc magmas must be fluid-saturated under pre-eruptive conditions, with fluid amounts of at least 1 wt%, whereas mafic compositions require lower amounts of fluid, in the range 0.1–1 wt%. Nevertheless, volcanic gases colder than 700 °C are generally too H 2 O-rich and S-poor to have been in equilibrium with silicic to andesitic magmas under pre-eruptive conditions, which suggests that such gases probably contain a substantial contribution from meteoric or hydrothermal water.

40Ar/ 39Ar geochronology of magmatic activity, magma flux and hazards at Ruapehu volcano, Taupo Volcanic Zone, New Zealand

We have determined precise eruption ages for andesites from Ruapehu volcano in the Tongariro Volcanic Centre of the Taupo Volcanic Zone (TVZ) using 40Ar/ 39Ar furnace step-heating of separated groundmass concentrates. The plateau ages indicate several eruptive pulses near 200, 134, 45, 22 and <15 ka and, based on our and previous field mapping confirm the lavas of the Te Herenga Formation as the oldest exposed part of the volcanic edifice. The pulse at ∼134 ka includes an entire >300-m section of lavas in Whangaehu gorge as well as some lavas in Ohinepango and Waihianoa catchments on eastern Ruapehu, and this suite of lavas belongs to the Waihianoa Formation. This pulse of activity is not represented on nearby Tongariro volcano, indicating that the two volcanoes have independent magmatic systems. A younger group of lavas yields dates between 50 and 20 ka and includes lava flows from the Turoa skifield and in the Ohinepango and Mangatoetoenui catchments and is consistent with two pulses of magmatism around the time of the last glacial maximum, relating it broadly to the Mangawhero Formation. Syn- and post-last glacial activity lavas, with ages <15 ka are assigned to the Whakapapa Formation, and include the voluminous flows of the Rangataua Member on southern Ruapehu. Magma flux, integrated over 1000-yr periods, averages 0.6 km 3 ka −1 assuming a volcano lifespan of 250 ka. Fluxes for the Te Herenga, Waihianoa and Mangawhero Formations are consistent at 0.93, 0.9 and 0.88 km 3 ka −1, respectively. These fluxes are broadly comparable with those measured at other modern andesite arc volcanoes (e.g. Ngauruhoe, 0.88; Merapi, 1.2 and Karymsky 1.2 km 3 ka −1). The relatively low flux (0.17 km 3 ka −1) calculated for the Whakapapa Formation may derive from underestimates of erupted volume arising from an increase in phreatomagmatic explosive eruptions in postglacial times. However, using volume estimates for the 1995–1996 eruptions and a recurrence interval of 25 yr has yielded an integrated 1000-yr flux of 0.8 km 3 ka −1 in remarkable agreement to estimates for the prehistoric eruptions. Overall, Ruapehu shows consistency in magma flux, but at time scales of the order of one hundred to some thousands of years, field evidence suggests that short bursts of activity may produce fluxes up to twenty times greater. This is significant from the perspective of future activity and hazard prediction.

Magmatism as a probe to the Neogene shallowing of the Nazca plate beneath the modern Chilean flat-slab

Changes in the Neogene (Miocene to Recent) magmatic style of the modern Chilean flat-slab region and its margins (27–34°S) reflect both variations in Nazca–South American convergence parameters and the effects of subduction of the Juan Fernández Ridge hotspot track. These effects are superimposed on an initially thicker arc crust in a more compressional tectonic regime in the north than in the south. Distinctive magmatic periods across the region reflect relatively steep subduction from ∼27 to 20 Ma, frontal arc migration associated with forearc subduction erosion from ∼20 to 16 Ma, compressional deformation and andesitic volcanism from ∼15 to 9 Ma, a peak of slab shallowing associated with frontal arc migration and arc broadening from ∼8 to 4 Ma, and termination of volcanism over the flat-slab by ∼5 Ma. Changes in magmatic style and distinctive chemical signatures in ∼20 to 16 Ma volcanic rocks coincide with a marginwide transition from a more extensional to a more compressional tectonic regime linked to changing Nazca–South American convergence parameters. North to south differences in ∼15 to 9 Ma magmas partially reflect the southward migration of the northeast-trending arm of the subducting Juan Fernández Ridge, whose effect arrived in the flat-slab region at ∼14 Ma. The termination of andesitic arc volcanism across most of the region at ∼9 Ma and extreme shallowing of the slab beneath the central flat-slab coincides with the arrival and eastward propagation of the east–west-trending segment of the Juan Fernández Ridge. Overall, the subduction of the Juan Fernández Ridge is a perturbation that causes extreme shallowing in part of a regionally shallowing subduction zone. Chemical signatures of the erupted magmas reflect the changing mantle and crustal configuration above the shallowing subduction zone. Those with adakitic-like signatures are derived from subduction zone mafic magmas contaminated by garnet granulitic to eclogitic facies crust in the thickened crust or by crust incorporated into the mantle as a result of forearc subduction erosion. There is no evidence that any of these magmas were generated by melting of the subducted slab.

Boron in central Andean ignimbrites: implications for crustal boron cycles in an active continental margin

Calc-alkaline pumices and lavas from the Neogene–Pleistocene central Andean Altiplano–Puna Volcanic Complex (APVC) contain quartz phenocrysts with abundant primary melt inclusions. Melt inclusions represent the melt composition of dacitic to rhyodacitic magmas prior to eruption and they are widely protected from eruptive degassing, low-temperature vapor-phase alteration and post-depositional contamination with boron-rich dusts and aerosols. The average boron isotopic compositions of melt inclusion and matrix glasses are uniform in the units studied (overall average δ 11B=−3.8±2.8‰, 1 SD) and overlap with the range of local basement rocks ( δ 11B=−11‰ to −5‰). B is enriched in glasses (55±14 ppm, 1 SD), whereas whole-rock contents range between 10 and 40 ppm. At similar B contents, incompatible trace element ratios like B/Nb are generally higher in the ignimbrites and lavas (B/Nb=2.4 and 4.0) than in crustally derived Paleozoic to Cretaceous granitoids from NW-Argentina (B/Nb=1.0–2.7). This suggests a minor contribution of high-B/Nb arc andesites and is consistent with petrogenetic models in which APVC dacites originated from hybridization of dominantly crustally derived melts with andesitic magmas. Boron-rich hydrothermal brines and salt-lake water from within the APVC have δ 11B=−4.1 to +5.7‰. These values are close to the APVC ignimbrite values and contrast with hydrothermal brines from the eastern Puna and eastern Cordillera, which have lower δ 11B=−10‰ to −17‰ and occur in an area of little volcanic cover. This regional pattern suggests that borate enrichments resulted from the leaching of local country rocks: ignimbrites in the W Altiplano–Puna and Paleozoic basement in the eastern Puna and eastern Cordillera.

O magmatismo alcalino Neoproterozóico na Ilha do Cardoso, Sudeste do Estado de São Paulo

This work focuses on the geology and geochronology of rocks cropping out on Cardoso Island, on the southeastern coast of Sao Paulo State, close to the boundary with Parana State. The island, with an area of about 151 km² is a protected area administered by the Institute of the Secretariat for the Environment of the State of Sao Paulo. It is mountainous, with a peak at 814 m, and is covered by denseAtlantic Forest vegetation. The island is made up mainly of an igneous complex with light grey leucocratic, inequigranular, medium to coarse-grained syenites. The Tres Irmaos Syenite (STI), composed of pyroxene, hornblende, and perthitic to mesoperthitic microcline, predominates has magmatic flow structures, and is cut by the pinkish grey, leucocratic medium-grained Cambriu alkali-feldspar granites (GC). Geochemical analysis of STI and GC demonstrate their metaluminous alkaline nature and late orogenic to anorogenic character. The bodies formed between 620 and 570 Ma according to U-Pb dating of zircons and cooled between 597 and 531 Ma (K-Ar in amphiboles). Whole rock Sm-Nd analyses yield Meso - and Paleoproterozoic TDM ages (1,500 - 2,200 Ma). A belt of low-grade metasedimentary rocks occurs in the northern part of the island. Quartz schist, quartz-mica schist and mica-quartz schist, often-containing andalusite and cordierite, predominate.Geochemical and geochronological data suggest that the sources of the metasediments were continental arc andesites of whose protoliths separated from the mantle between 1,800 and 2,200 Ma during the Paleoproterozoic. These metasediments probably continue on the continent in the Taquari region and extend southwards in narrow strips between the granitoids of the Paranagua Domain.

Arc magmatism and hydrocarbon generation in the northern Taranaki Basin, New Zealand

A Middle–Late Miocene andesitic arc named the Mohakatino Volcanic Centre is buried beneath younger sediments in the northern part of the Taranaki Basin, New Zealand’s primary oil production province. Volcanoes of the centre cover an area of about 3200 km 2 . An estimated 7000±3000 km 3 of andesite were erupted from the centre and between 1000 to 2500 km 3 of magma were intruded into the basement beneath the volcanic cones. The key element of the petroleum system altered by magmatism is the maturity of source rocks and the timing of expulsion, although volcanism also contributes to the formation of potential reservoirs and traps. In the northern Taranaki Basin, two periods of hydrocarbon expulsion occurred: following magmatism (14 Ma to about 8 Ma), and following renewed burial (since about 4 Ma). Thermal models indicate that Late Cretaceous terrestrial source rocks close to large magmatic intrusions became fully mature during magmatism. Overlying marine source rocks are modelled to still be generating and expelling hydrocarbons to the present day. Hence hydrocarbon expulsion and the charge history of this basin is partly governed by Miocene magmatism. Results are also relevant to many petroleum basins that contain similar andesitic arc volcanic rocks.

小笠原群島母島に産する初生的ソレアイトおよびカルク・アルカリ系列岩

Hahajima, Bonin archipelago, is located approximately 1050 km away to the south-south east of Tokyo. Hahajima is the Tertiary volcano mainly composed of evolved island arc andesites and primitive volcanics also occur. The primitive volcanics can be classified into tholeiitic basalts and calc-alkaline basaltic andesites. Their genetical relationships were considered. Primitive tholeiites (TH-P) and calc-alkaline rocks (CA-P) show magnesian compositions (FeO*/MgO 700ppm, Ni>150ppm). They can be distinguished, however by assortment of groundmass pyroxenes. Euhedral olivines with Mg#>90 exist in the TH-P. The olivine should be the phenocrysts which crystallized from the liquid. Olivines in the CA-P could not be analyzed because of the serpentinized alteration. Their augite and Cr-spinel compositions are significantly magnesian. The CA-P is not distinguished from the TH-P by the Sr isotopic ratio. Cr-spinels in olivines in CA-P are systematically enriched in Cr than those in the TH-P. Therefore, the TH-P and CA-P would have originated from different primitive magmas. Judging from Cr-spinel compositions and REE modelings, the CA-P's parental mantle might be more depleted than the TH-P's. Hydrous partial melting experiments of lherzolite imply that the CA-P would have been produced under more hydrous condition than the TH-P. This inference is not contradictory to the CA-P's alkali and alkali-earth elements enrichment. The TH-P might have been produced at the deeper region of oceanic mantle where was not so refractory, while the CA-P might be produced at the shallower region where is relatively deplete. Incompatible trace elements characteristics of the TH-P and CA-P are very similar to those of the tholeiite and calc-alkaline volcanics from Funagata volcano (North east Japan arc). This feature might imply that the geochemical characteristics of relatively unevolved island arc volcanics could be explained by immutable process not according to the age or region of subduction.

ＸＲＦによる１：５希釈ガラスビードを用いた全岩主要成分・微量成分の分析精確度および精密度

Analytical results of major and trace elements (V, Cr, Ni, Rb, Sr, Ba, Y, Zr, Nb) for bulk rocks by XRF using 1 : 5 dilution glass beads were evaluated. The results show relatively good accuracy and precision for major and trace elements. Analytical conditions, the accuracies and precisions are reported. If a rock contains V>40 (ppm), Cr>20, Ni>10, Rb>5, Sr>5, Ba>75, Y>4, Zr>10, Nb>5, the relative standard deviations would be below 10%. This method could be suitable to arc andesites.

Evolution of Mesozoic Sandstone Compositions, Southern Junggar, Northern Tarim, and Western Turpan Basins, Northwest China: A Detrital Record of the Ancestral Tian Shan

ABSTRACT Sandstone compositional data can be a powerful tool in the interpretation of tectonic and climatologic influences on sedimentary basin fill, in addition to yielding important information about porosity in sandstone petroleum reservoirs. In order to explore these relationships, a modified Gazzi-Dickinson point-counting technique was used to analyze the composition of 143 Mesozoic sandstone samples from the southern Junggar, northern Tarim, and western Turpan basins of northwestern China. Results indicate that a Mesozoic, ancestral version of the Tian Shan physiographically separated the Junggar and Tarim basins and provided sand of very different composition to each basin. Mesozoic sandstone from the northern Tarim basin is diverse in composition and lithic-rich (Qm41F14L45; Qp42Lvm26Lsm32), is locally micaceous, and contains common radiolarian-chert grains and few dense accessory minerals. Inferred source rocks include upper Paleozoic alkali granite and metamorphic complexes, thick Silurian bedded-chert sequences, and lower Paleozoic strata of a passive continental margin. In contrast, sandstone from the southern Junggar and western Turpan basins is uniformly volcanic-rich (Qm21F21Lt58; Qp13Lvm68Lsm19), and contains abundant dense accessory minerals and only local radiolarian chert and mica. Inferred principal source rocks are Devonian-Carboniferous andesitic arc volcanics. The effect of sampling scale on sandstone composition outweighs that of plate-tectonic setting. Samples were derived mostly from medium- and coarse-grained fluvial systems that likely drained only portions of the ancestral Tian Shan and hence preserve local source-rock signatures, rather than an integrated compositional signal that can be directly compared to plate-tectonic petrofacies models. In addition, though Mesozoic basins of western China were most akin to broken foreland basins, Mesozoic sandstone is considerably more compositionally diverse and lithic-rich than that of modern or ancient broken foreland basins because of the variety of accreted terranes constituting the ancestral Tian Shan. Temporal changes in sandstone composition are consistent with episodes of Mesozoic deformation in the Tian Shan. Each deformational episode increased physiographic relief of the ancestral range, produced renewed downcutting and erosion of source rocks, and resulted in the deposition of compositionally very immature sandstone in adjacent basins. Although a regional early Mesozoic megamonsoon and an Early Cretaceous rain shadow cast across the northern Tarim basin are interpreted from regional facies and paleontologic data, neither paleoclimatic phenomenon appears to have significantly modified sandstone composition in the study area. Calculations of intergranular volume (% porosity + % cement) indicate that porosity in sandstone from the Tarim and Junggar basin depocenters was reduced principally by burial compaction and that the rate of porosity reduction was highest for lithic-rich samples.

Trace element geochemistry of a thick till and clay-rich aquitard sequence, Saskatchewan, Canada

Samples collected from a 170 m borehole penetrating till, bentonitic clay, and sand at the Birsay site, Saskatchewan, Canada, has been studied as a first step for understanding the controls on dissolution and transport of trace elements, including REEs, in porewaters of a clay-rich aquitard. The REE budgets of the till, clay, and sand are dominated by apatite and monazite, with sporadic zircon and Y-phosphate. Apatites contain 0.3 to 1.7 wt.% REE, and monazites 12 to 55 wt.%. All three units are characterized by unusual bulk sample REE patterns, with maxima at Eu–Gd, convex down patterns from La–Sm with minima at Ce, and variably fractionated HREE. Apatites feature maxima at Eu–Gd. Consequently the complex patterns arise from variable modal proportions of apatite, monazite, and plagioclase feldspar. Relative to Post-Archean Average Shale (PAAS), the till is SiO 2 and CaO rich with about half the contents of Al 2O 3, TiO 2, and Fe 2O 3. These compositional relationship stem from relatively high proportions of quartz and carbonate, but low modal abundances of ferromagnesian and Ti-oxide minerals. Similarly, the till has Cs, Co, Ni, Sc, V, Cu, Pb and Zn contents at about half PAAS. The bentonite clay has slightly higher SiO 2 and CaO contents than PAAS, with lower abundances of most other trace elements, including REE which plot at 0.7 to 0.9 PAAS. The major and trace element composition is similar to continental arc andesites, and the clay is interpreted to have a mixed provenance of arc andesite and eroded underlying sands. The composition of the till appears to reflect mixed provenance: erosion of the underlying Cretaceous clay and sand, Paleozoic carbonate rocks, and the Precambrian Shield. The silicious Ardkenneth sand has lower concentrations of REE and most trace elements compared to the clay, but the most fractionated REE patterns. The clay and till have inherited more subdued REE patterns in part from the underlying sand unit. Compositional variations between the units are marked by an upturn of Nb, Th, U and REE concentrations at the till–clay interface (80.2 m), and decrease of these elements at the clay–sand boundary (156.2 m). Ce and Eu anomalies vary erratically with depth: the anomalies appear to reflect the degree of sedimentary sorting, which in turn controls the modal abundances of detrital monazite, apatite, zircon, and plagioclase feldspar. Zr/Zr*, Hf/Hf* and Zr/Hf ratios are higher in the till than the clay and sand, indicating an additional Zr (Hf)-bearing phase with a distinctively high Zr/Hf ratio. The observed small compositional variations in the till and clay are not reflected in variations of geotechnical properties. Given that the REE budget of the till and clay are dominated by highly insoluble trace minerals, it is unlikely that the REE patterns of porewaters will be controlled by bulk sample REE concentrations. Weathering of the upper 3 m of the till has oxidized Fe 2+-bearing silicates, oxides, and sulphides, and dissolved carbonates relative to the underlying unoxidized till, weathering has not disturbed major, REE or other trace element compositions.

Evidence for Late Miocene to Recent contamination of arc andesites by crustal melts in the Chilean Andes (25–26°S) and its geodynamic implications

Chemical and isotopic data from 12 volcanic centers of the southern Central Volcanic Zone (CVZ) in Chile, whose ages of 20, 16, 11, 8, 5, 2 and <1 Ma bracket the peak of shortening and crustal thickening in the mid-Miocene, show systematic differences with age. The composition of andesites erupted before and after crustal thickening are similar in terms of most major and trace elements, but the post-Miocene andesites show enrichments in Th, U, Cs and Rb, as well as high 87Sr/ 86Sr and 206Pb/ 204Pb ratios coupled with low εNd values which indicate greater crustal contamination compared with the older equivalents. Comparison of contamination indicators with age shows that contamination was low from 20 Ma to 8 Ma, increased sharply between 8 and 5 Ma, and remained at a high level into the Quaternary. Constant ratios of fluid-mobile vs immobile elements (Cs/Th or Ba/Zr) in even the most contaminated rocks indicate that fluid interaction was negligible. The contaminated andesites display disequilibrium textures and contain phenocrysts with a mixed population of melt inclusions. We suggest that the main process of crustal contamination was mixing with crustal melts. This is supported by geophysical evidence for a zone of partial melting in the mid and lower crust under the magmatic arc and by the presence of late Miocene to Pliocene crustal-derived felsic ignimbrites in the CVZ.

Hornblende, the continent maker—Evolution of H2O during circum-Pacific subduction versus continental collision

Research Article| August 01, 1999 Hornblende, the continent maker—Evolution of H2O during circum-Pacific subduction versus continental collision W. G. Ernst W. G. Ernst 1Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305-2115, USA Search for other works by this author on: GSW Google Scholar Geology (1999) 27 (8): 675–678. https://doi.org/10.1130/0091-7613(1999)027<0675:HTCMEO>2.3.CO;2 Article history first online: 02 Jun 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation W. G. Ernst; Hornblende, the continent maker—Evolution of H2O during circum-Pacific subduction versus continental collision. Geology 1999;; 27 (8): 675–678. doi: https://doi.org/10.1130/0091-7613(1999)027<0675:HTCMEO>2.3.CO;2 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGeology Search Advanced Search Abstract Andesitic arcs are sited over convergent lithospheric plate junctions. Aqueous fluids driven off the downgoing slab are responsible for partial fusion of the warming subducted plate and/or the overlying mantle wedge, and the ascent of calc-alkaline melts. Various hydrous minerals have been proposed to be the source of this H2O. Experimental equilibrium studies show that, under subduction-zone geothermal gradients of 5–7 °C/km, clinoamphibole constitutes a major phase in deep-seated (>75 km) metabasalts; other hydrous minerals are absent or are of very minor abundance. Clinoamphiboles dehydrate at pressures of ∼2.2–2.4 GPa under equilibrium conditions (pressure overstepping is probable), so mafic blueschists and amphibolites expel H2O at magmagenic depths. Partly serpentinized mantle beneath the oceanic crust devolatilizes at comparable to slightly higher pressures. However, micas remain stable in pelitic and granitic gneisses to pressures far exceeding 4.0 GPa, so at subduction depths >100 km, micaceous lithologies characterizing the upper and middle sialic crust fail to evolve significant H2O. Deep underflow of hydrated oceanic lithosphere thus generates most of the volatile flux along and/or above a subduction zone prior to continental collision. As large masses of quartzofeldspathic material enter a suture zone, volatile evolution at deep levels nearly ceases. While small amounts of peraluminous, S-type anatectic melts may be produced, I-type calc-alkaline arcs—and the continents—owe their formation over geologic time to the sustained underflow of oceanic lithosphere. This content is PDF only. Please click on the PDF icon to access. First Page Preview Close Modal You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Petrographic temper provinces of prehistoric pottery in Oceania

The mineralogical compositions and petrological character of non-calcareous mineral sand tempers in prehistoric potsherds from Pacific islands are governed by the geographic distribution of geotectonic provinces controlled by patterns of plate tectonics. As sands from different islands are not mingled by sedimentary dispersal systems, each temper sand is a faithful derivative record of parent bedrock exposed on the island of origin. Tempers are dominantly beach and stream sands, but also include dune sand, colluvial debris, reworked volcanic ash, broken rock, and broken pottery (grog). From textural relations with clay pastes, most tempers were manually added to clays collected separately, but naturally tempered clay bodies occur locally. Calcareous temper sands derived from reef detritus are widely distributed, but ancient potters commonly preferred non-calcareous sands for temper. Consequently, beach placer sand tempers rich in diagnostic heavy minerals are typical of many temper suites. Distinctive temper classes include oceanic basalt, andesitic arc, dissected orogen, and tectonic highland tempers characteristic of different geologic settings where contrasting bedrock terranes are exposed. Most Oceanian sherd suites contain exclusively indigenous tempers derived from local island bedrock, but widely distributed occurrences of geologically exotic tempers document limited pottery transfer over varying distances at multiple sites.

Nature and growth rate of the Northern Izu–Bonin (Ogasawara) arc crust and their implications for continental crust formation

The bulk composition of the continental crust throughout geological history is thought by most previous workers to be andesitic. This assumption of an andesitic bulk composition led to an early hypothesis by Taylor (1967) that the continental crust was created by arc magmatism. This hypothesis for the origin of continental crust was challenged by several authors because: (i) the mean rate of arc crust addition obtained by Reymer and Schubert (1984) is too small to account for some certain phases of rapid crustal growth; and (ii) the bulk composition of ocean island arcs, the main contributor to the Archean and early Proterozoic crust, is basaltic rather than andesitic (Arculus 1981; Pearce et al. 1992). New data from the Northern Izu–Bonin arc are presented here which support the Taylor (1967) hypothesis for the origin of the continental crust by andesitic arc magma. A geological interpretation of P wave crustal structure obtained from the Northern Izu–Bonin arc by Suyehiro et al. (1996) indicates that the arc crust has four distinctive lithologic layers: from top to bottom: (i) a 0.5–2‐km‐thick layer of basic to intermediate volcaniclastic, lava and hemipelagite (layer A); (ii) a 2–5‐km‐thick basic to intermediate volcaniclastics, lavas and intrusive layer (layer B); (iii) a 2–7‐km‐thick layer of felsic (tonalitic) rocks (layer C); and (iv) a 4–7‐km‐thick layer of mafic igneous rocks (layer D). The chemical composition of the upper and middle part of the northern Izu–Bonin arc is estimated to be similar to the average continental crust by Taylor and McLennan (1985). The rate of igneous addition of the Northern Izu–Bonin arc since its initial 45‐Ma magmatism was calculated as 80 km3/km per million years. This rate of addition is considered to be a reasonable estimate for all arcs in the western Pacific. Using this rate, the global rate of crustal growth is estimated to be 2.96 km3/year which exceeds the average rate of crustal growth since the formation of the Earth (1.76 km3/year). Based on this estimate of continental growth and the previously documented sediment subduction and tectonic erosion rate (1.8 km3/year, von Heune &amp; Scholl 1991), several examples of growth curves of the continental crust are presented here. These growth curves suggest that at least 50% of the present volume of the continental crust can be explained by arc magmatism. This conclusion indicates that arc magmatism is the most important contributor to the formation of continental crust, especially at the upper crustal level.