Petrography and geochemistry of upper Triassic sandstones from the Tumengela Formation in the Woruo Mountain area,North Qiangtang Basin,Tibet: Implications for provenance,source area weathering,and tectonic setting

The petrography and major and trace element concentrations of the sandstones from the Tumengela Formation in the Woruo Mountain area, North Qiangtang Basin, are studied to determine their provenance, intensity of weathering and tectonic setting. The detrital compositions of the Tumengela sandstone samples are dominated by quartz (58.0–70.1 %, average 64.7 %) and lithic fragments (21.8–35.9 %, average 27.3 %), but low in feldspar content (4.9–12.9 %, average 8.0 %). The sandstones can be classified as litharenite and feldspathic litharenite according to their detrital compositions, which is consistent with the geochemical data. The detrital modal compositions reflect that these sandstones are probably derived from a recycled orogenic source. The index of chemical variability (ICV) and SiO₂/Al₂O₃ ratio values suggest that the compositional maturity and recycling were moderate. The weathering indices such as the chemical index of alteration (CIA), plagioclase index of alteration (PIA), chemical index of weathering (CIW), and Al₂O₃–(CaO* + Na₂O)–K₂O (A–CN–K) diagram indicate that the intensities of weathering in the source area were moderate. The Al₂O₃/TiO₂, Th/Co, La/Sc, La/Co, Th/Sc, Cr/Th ratio values and the discriminant function of the Tumengela sandstones indicate that the sediments were mainly derived from felsic source rocks, while also mixed with intermediate source rocks. The comparison of rare earth element patterns and its Eu anomalies to the probable source rocks infer that the sandstones were derived from the combination of granite, rhyolite, dacite, and gneisses. The proximal central uplift belt was probably the primary provenance area as evidenced by the petrographical and geochemical features of the Tumengela sandstones. The multidimensional tectonic discrimination diagram based on major elements show a collision setting (80 %) combined with a rift setting (20 %) for the Tumengela sandstones, which is consistent with the general geology of the study areas.     

Geochemical indicator of original eolian grain size and implications on winter monsoon evolution

Grain size of eolian deposits from the Loess Plateau in China has been widely used to reconstruct the history of the East Asian winter monsoon. However, the grain size of bulk samples is only partially indicative to the strength of the winter monsoon because post-depositional weathering processes have significantly changed the grain size of original eolian particles. Here, non-weathered loess samples were separated into eight different particle fractions, and major chemical elements were determined in order to establish a geochemical indicator of original eolian grain size. The results show that SiO₂ and Al₂O₃ contents and the SiO₂/Al₂O₃ ratio in different fractions vary regularly with grain size, and that a good linear relation exists between the SiO₂/Al₂O₃ ratio and grain size for the fractions <50 μm. Because Al and Si are among the most stable elements and pedogenic processes in the Loess Plateau cannot affect the SiO₂/Al₂O₃ ratio, this index can be used to reflect the grain size of original eolian particles. Application of this index in the Weinan and Luochuan loess sections of the last climatic cycle shows that SiO₂/Al₂O₃ is in good agreement with median grain size (Md) in the loess units. On the contrary, SiO₂/Al₂O₃ has documented a series of fluctuations in the soil units that are not clearly indicated by the grain-size changes of bulk samples.

     

Reaction microstructures in corundum‐ and kyanite‐bearing mafic mylonites from the Takahama metamorphic rocks,western Kyushu,Southwest Japan

High‐grade mylonites occur in the Takahama metamorphic rocks, a member of the high‐pressure low‐temperature type Nagasaki Metamorphic Rocks, western Kyushu, Japan. Mafic layers within the mylonites retain reaction microstructures consisting of margarite aggregates armoring both corundum and kyanite. The following retrograde reaction well accounts for the microstructures in the CaO–Al₂O₃–SiO₂–H₂O system: 3Al₂O₃ + 2Al₂SiO₅ + 2Ca₂Al₃Si₃O₁₂(OH) + 3H₂O = 2Ca₂Al₈Si₄O₂₀(OH)₄ (corundum + kyanite + clinozoisite + fluid = margarite). Mass balance analyses and chemical potential modeling reveal that the chemical potential gradients present between kyanite and corundum have likely driven the transport of the CaO and SiO₂ components. The mylonitization is considered to take place chronologically after peak metamorphism and before the above reaction, based on the following features: approximately constant thickness of the margarite aggregates, random orientation of margarite, and local modification of garnet composition at a boudin neck that formed during mylonitization. The estimated peak temperature of 640°C and the pressure–temperature conditions of the above reaction indicate that the mylonitization took place at temperature between 530 and 640°C at pressures higher than 1.2 GPa, approximately equivalent to the depth of the lower crust of island arcs.     

Sediment contamination in floodplains and alluvial terraces as an historical record of gold exploitation in the Carmo River basin,Southeast Quadrilátero Ferrífero,Minas Gerais,Brazil

This paper describes the geochemistry of sediment samples placed in floodplains and alluvial terraces downstream from gold mines in the Carmo River basin, Quadrilátero Ferrífero, Minas Gerais, Brazil. The geochemistry signature Na₂O, K₂O, SiO₂, CaO, MgO, Al₂O₃, Fe₂O₃, TiO₂, P₂O₅, Mn, As, Cu, Zn, Ba, Ni, Cr, S, Co were analyzed in different facies from stratigraphic profiles. As, Cu, Zn, and Mn anomalies are mainly associated with the clayed facies deposited in floodplains and oxbow lakes, and with coarse‐sediment facies deposited in the channel. The facies were accumulated by the gold exploitation activity in the region. The contamination of As, Cu, and Zn was controlled by minerals such as iron oxides and hydroxides (hematite, magnetite, and mainly goethite), manganese oxides, and sulfide‐rich minerals. The As‐bearing sediments of the region characterize one of the most As contaminated area of Brazil. Their main source is associated with gold exploration in the last three centuries.     

Clay mineralogy of the first and second members of the Nenjiang Formation,Songliao Basin: Implications for paleoenvironment in the Late Cretaceous

The first and second members of the Nenjiang Formation (K₂n¹⁺²) in the Songliao Basin, northeast China, are an interval of dark-colored mudstone. Paleoenvironmental studies of these strata are useful for understanding the terrestrial environment under a greenhouse climate and hydrocarbon accumulation in lake basins. In this study, clay mineralogy of the K₂n¹⁺² from four borehole or outcrop sections is investigated to understand terrestrial paleoenvironment during the depositional period in the Late Cretaceous. In the mudstone samples, smectite and illite are the predominant clay minerals, and were derived from weathering of parent rocks in a temperate, sub-humid to sub-arid climate; kaolinite and chlorite are minor clay species. The difference in the clay-mineral assemblages between the eastern and western margins of the basin was primarily controlled by provenance lithology, and the high smectite content in the western basin resulted from alteration of volcanic rocks exposed in the Greater Xing’an Range area. The increasing illite content and ratio of illite/smectite percentages in the upper part of the first member of the Nenjiang Formation indicate paleoenvironmental change. This temporal change in the clay-mineral composition was primarily caused by a regionally cooler and drier paleoclimate, consistent with previous paleoenvironmental reconstructions.     

Major-element vapor fractionation on the lunar surface: An unusual lithic fragment from the Luna 20 fines

A 250-μm fragment in the Luna 20 fines has a very fine-grained “igneous” texture and has the composition (wt.%): SiO₂, 41.1; TiO₂, 0.35; Al₂O₃, 27.2; Cr₂O₃, 0.14; FeO, 4.2; MnO, 0.06; MgO, 8.5; CaO, 17.8; Na₂O, 0.05; and K₂O < 0.02. It contains ～ 65% plagioclase An_99–100, ～ 15% olivine Fo₉₀, ～ 2% Mg-Al spinel and the remainder an unusual interstitial phase with composition SiO₂, 34.8; TiO₂, 1.78; Al₂O₃, 18.3; Cr₂O₃, 0.04; FeO, 14.1; MnO, 0.22; MgO, 5.0; CaO, 24.1; Na₂O, 0.34; K₂O < 0.02. This fragment probably represents a portion of a normal highland rock (anorthositic norite) which was heated to a very high temperature by impact, lost volatiles including SiO₂, and then partially crystallized. The observed phases and their inferred crystallization sequence are consistent with experimental results in the system CaOMgOAl₂O₃SiO₂ (Schairer and Yoder, 1969), assuming the unusual phase to be a residual glass. This type of internal fractionation, leading to silica depletion in the residuum, is different from that normally observed in lunar rocks and is attributed to slightly lower bulk SiO₂ resulting from vapor fractionation due to impact (which also results in lower Na₂O and other volatiles). Because differentiation of the type shown by this fragment is rare in lunar materials, we infer that such major-element vapor fractionation is uncommon on the surface of the moon. The experimental CaOMgOAl₂O₃SiO₂ phase relations also have a bearing on the lunar model proposed by D.L. Anderson in 1973: his “refractory” original lunar composition would differentiate to produce silica deficient liquids, like the unusual phase in our fragment, rather than the normal lunar crustal rocks.     

The system enstatite-pyrope at high pressures and temperatures and the mineralogy of the earth's mantle

In a diamond-anvil pressure cell coupled with laser heating, the system enstatite (MgSiO₃)-pyrope (3 MgSiO₃ · Al₂O₃) has been studied in the pressure region between about 100 and 300 kbar at about 1000°C using glass starting materials. The high-pressure phase behavior of the intermediate compositions of the system contrasts greatly with that of the two end-members. Differences between MgSiO₃ and 95% MgSiO₃ · 5% Al₂O₃ are especially remarkable. The phase assemblages β-Mg₂SiO₄ + stishovite and γ-Mg₂SiO₄ (spinel) + stishovite displayed by MgSiO₃ were not observed in 95% MgSiO₃ · 5% Al₂O₃, and the garnet phase, which was observed in 95% MgSiO₃ · 5% Al₂O₃ at high pressure, was not detected in MgSiO₃. These results suggest that the high-pressure phase transformations found in pure MgSiO₃ would be inhibited under mantle conditions by the presence even of small amounts of Al₂O₃ (?4% by weight). On the other hand, pyrope displays a wide stability field, finally transforming at 240–250 kbar directly to an ilmenite-type modification of the same stoichiometry. The two-phase region, within which orthopyroxene and garnet solid solutions coexist, is very broad. The structure of the earth's mantle is discussed in terms of the phase transformations to be expected in a simple mixture of 90% MgSiO₃ · 10% Al₂O₃ and Mg₂SiO₄. The seismic discontinuity at a depth of 400 km in the earth's mantle is probably due entirely to the olivine → β-phase transition in Mg₂SiO₄, with the progressive solution of pyroxene in garnet (displayed in 90% MgSiO₃ · 10% Al₂O₃) occurring at shallower depths. The inferred discontinuity at 650 km is due to the combination of the phase changes spinel → perovskite + rocksalt in Mg₂SiO₄ and garnet → ilmenite in 90% MgSiO₃ · 10% Al₂O₃. The 650-km discontinuity is thus characterized by an increase in the primary coordination of silicon from 4 to 6. A further discontinuity in the density and seismic wave velocities at greater depth associated with the ilmenite-perovskite phase transformation in 90% MgSiO₃ · 10% Al₂O₃ is expected.     

Evolution of lithospheric mantle in the north of Nain‐Baft oceanic crust (Neo‐Tethyan ophiolite of Ashin,Central Iran)

This study is focused on a plagioclase‐bearing spinel lherzolite from Chah Loqeh area in the Neo‐Tethyan Ashin ophiolite. It is exposed along the west of left‐lateral strike‐slip Dorouneh Fault in the northwest of Central‐East Iranian Microcontinent. Mineral chemistry (Mg#^olivine < ~ 90, Cr#^{clinopyroxene} < ~ 0.2, Cr#^spinel < ~ 0.5, Al₂O₃^{orthopyroxene} > ~ 2.5 wt%, Al₂O₃^{clinopyroxene} > ~ 4.5 wt%, Al₂O₃^spinel > ~ 41.5 wt%, Na₂O^{clinopyroxene} > ~ 0.11 wt%, and TiO₂^{clinopyroxene} > ~ 0.04 wt%) confirms Ashin lherzolite was originally a mid‐oceanic ridge peridotite with low degrees of partial melting at spinel‐peridotite facies in a lithospheric mantle level. However, some Ashin lherzolites record mantle upwelling and tectonic exhumation at plagioclase‐peridotite facies during oceanic extension and diapiric motion of mantle along Nain‐Baft suture zone. This mantle upwelling is evidenced by some modifications in the modal composition (i.e. subsolidus recrystallization of plagioclase and olivine between pyroxene and spinel) and mineral chemistry (e.g. increase in TiO₂ and Na₂O of clinopyroxene, and TiO₂ and Cr# of spinel and decrease in Mg# of olivine), as a consequence of decompression during a progressive upwelling of mantle. Previous geochronological and geochemical data and increasing the depth of subsolidus plagioclase formation at plagioclase‐peridotite facies from Nain ophiolite (~ 16 km) to Ashin ophiolite (~ 35 km) suggest a south to north closure for the Nain‐Baft oceanic crust in the northwest of Central‐East Iranian Microcontinent.     

Clay mineral formation in DSDP Leg 34 basalt

A blue-green smectite (iron-rich saponite) and green mica (celadonite) are the dominant sheet silicates in veins within the 10.5 m of basalt cored during DSDP Leg 34, Site 321, in the Nazca plate. Oxygen isotopic analyses of these clays, and associated calcite, indicate a formation temperature of≤25°C.Celadonite contains appreciable Fe₂O₃, K₂O and SiO₂, intermediate MgO, and very little Al₂O₃. Celadonite is commonly associated with goethite and hematite, which suggests that this phase formed by precipitation within a dominantlyoxygenated environment of components leached from basalt and provided by seawater. A mass balance estimate indicates that celadonite formation can remove no more than 15% of the K annually transported to the oceans by rivers. In contrast, iron-rich saponite containing significant Al₂O₃ appears to have precipitated from anon-oxidizing, distinctly alkaline fluid containing a high Na/K ratio relative to unmodified seawater.Seawater-basalt interaction at low temperatures, resulting in the formation of celadonite and smectite may explain chemical gradients observed in interstitial waters of sediments overlying basalts.     

Geochemistry of Archaean spinifex-textured peridotites and magnesian and low-magnesian tholeiites

Major and trace element (Rb, Sr, Ba, Zr, Y, Nb, Ni, Co, V, Cr) data are presented for 11 spinifex-textured peridotites (STP) and a number of high-magnesian and low-magnesian tholeiitic basalts. The STP, representing high-magnesian liquids, come from the Yilgarn Block of Western Australia, Munro Township in the Abitibi Belt of Canada and one sample from the Barberton area of South Africa. All of the basaltic samples come from the Yilgarn Block.The STP and high-magnesian rocks are considered to belong to the komatiite suite (1, 2) despite their low CaO/Al₂O₃ ratios. It is argued that the high values (about 1.5) reported for this ratio from the Barberton area can be explained by a combination of factors, viz. garnet separation, Al loss or Ca addition during metamorphism. The processes can be evaluated using CaO/TiO₂, Al₂O₃/TiO₂ ratios, the REE group and trace elements (e.g. Y, Sc). It would appear that most STP from other Archaean belts do not have abnormal CaO/Al₂O₃ ratios.The STP display close to chondritic ratios for Ti/Zr, Zr/Nb, Zr/Y, and TiO₂/Al₂O₃ and are considered to represent liquids produced by large amounts of partial melting of the Archaean mantle. The data suggest that virtually all phases other than olivine were removed by melting during the production of STP liquids. In the STP, Ti/V, Ti/P ratios are non-chondritic, suggesting original depletion and/or incorporation into the core.For lower levels of partial melting, including mid-ocean ridge basalts (MORB) non-chondritic ratios are exhibited by Zr/Y, TiO₂/Al₂O₃, TiO₂/CaO, suggesting controlling phases in the residue for Y, Ca, Al. It is apparent that for STP, Cr is not being controlled, indicating the absence of chromite in the residual. However, at about 15% MgO the data suggest that chromite becomes a residual phase.The transition metals, with the exception of Mn, have higher abundances in Archaean basaltic rocks than in MORB. This is interpreted as being mainly due to more extensive partial melting of the mantle in the Archaean, as a result of higher temperatures.It is suggested that the generation of STP liquids with about 32% MgO is due to upwelling mantle diapirs which probably originated at depths greater than 400 km and at temperatures in excess of 1900°C.Modern equivalents to Archaean greenstone sequences are lacking. The closest tectonic analogue would be the development of oceanic crust within a rifted continental block.     

Geochemistry and chronology of large-superlarge nonmetal deposits in eastern Zhejiang Province: A case study

From geology and geochemistry studies on four typical large-superlarge nonmetal deposits in eastern Zhejiang Province, it is concluded that nonmetal mineralization is an in situ reduction process in which SiO₂ and Fe₂O₃ are lost, and Al₂O₃ and K₂O are enriched. Moreover, incompatible elements are depleted in zeolite but enriched in alunite. Rb-Sr isotopic age of zeolite and K-Ar isotopic age or micro-area⁴⁰Ar-³⁹Ar isochron age of alunite are coincident with their geological data. respectively. Comparison studies of various chronology methods indicate that K-Ar and Rb-Sr methods are effective for zeolite and alunite, but should be carefully used for pyrophyllite, dekite and illite. From isotopic ages of ore and wall rocks of zeolite and alunite, it could be deduced that the age difference of metallogenism and volcanism is about 10–20 Ma.     

Geochemistry of the Late Paleozoic radiolarian cherts within the NE Jiangxi ophiolite melange and its tectonic significance

Major and trace elements are presented for the late Paleozoic radiolarian cherts, which were spatially associated with the NE Jiangxi ophiolite melange. These chert samples show relatively low SiO₂ (78.40%-89.28%) and high Al₂O₃ (3.42%-11.02%). Low Si/Al ratios (6.3-23) and tight negative correlation between Si/Al and Al₂O₃ of the samples indicate that they are muddy cherts containing high and variable contents of pelitic detritus. Geochemically, they are characterized by Al₂O3/(Al2O3+Fe₂O3) = 0.51-0.90, shale-normalized La_n/Ce_n = 0.76-1.11, Ce/Ce* = 0.91-1.22, V<20_μg/g, V/Y<2.6 and Ti/V>40, resembling those of cherts formed in the continental margin regimes. It is therefore concluded that these late Paleozoic radiolarian muddy cherts were most likely formed in a continental margin regime, and not genetically related to the ophiolite suite in NE Jiangxi. It is also unlikely that an oceanic basin existed between the Yangtze and Cathaysia blocks during the late Paleozoic.     

Zoned Cr,Fe-spinel from the La Perouse layered gabbro,Fairweather Range,Alaska

Zoned spinel of unusual composition and morphology has been found in massive pyrrhotite-chalcopyrite-pent-landite ore from the La Perouse layered gabbro intrusion in the Fairweather Range, southeastern Alaska. The spinel grains show continuous zoning from cores with up to 53 wt.% Cr₂O₃ to rims with less than 11 wt.% Cr₂O₃. Their composition is exceptional because they contain less than 0.32 wt.% MgO and less than 0.10 wt.% Al₂O₃ and TiO₂. Also notable are the concentrations of MnO and V₂O₃, which reach 4.73 and 4.50 wt.%, respectively, in the cores. The spinel is thought to have crystallized at low oxygen fugacity and at temperatures above 900°C, directly from a sulfide melt that separated by immiscibility from the gabbroic parental magma.     

A Comparative Study of Ozonation,Homogeneous Catalytic Ozonation,and Photocatalytic Ozonation for C.I. Reactive Red 194 Azo Dye Degradation

In this study, the decolorization, dearomatization, and mineralization efficiencies of different advanced oxidation processes (AOPs; namely O₃, O₃/Fe(II), O₃/Fe(II)/UVA, and O₃/TiO₂/UVA) were investigated for the azo dye C.I. Reactive Red 194 (RR194). The effects of pH (3–11), amount of TiO₂ (0.05–1 g/L), and concentration of Fe(II) (0.1–1.6 mM) were investigated for the applied methods. The decolorization and mineralization efficiencies of the photocatalytic ozonation system (O₃/TiO₂/UVA) were increased by decreasing the pH of the dye solution in contrast to the ozonation system (O₃). Decolorization of RR194 was increased in the photocatalytic ozonation system with an increasing amount of catalyst however, a decreasing was occurred for the homogeneous catalytic system (O₃/Fe(II)) when increasing the concentration of catalyst. The decolorization efficiency of the homogeneous catalytic system (O₃/Fe(II)) was enhanced when combined with UVA light. In our study, the most efficient method for dearomatization and mineralization was the O₃/TiO₂/UVA among the applied AOPs.     

Major element chemistry of ocean island basalts — Conditions of mantle melting and heterogeneity of mantle source

We estimate average compositions of near-primary, ‘reference’ ocean island basalts (OIBs) for 120 volcanic centers from 31 major island groups and constrain the depth of lithosphere–asthenosphere boundary (LAB) at the time of volcanism and the possible depth of melt–mantle equilibration based on recently calibrated melt silica activity barometer. The LAB depth versus fractionation corrected OIB compositions (lava compositions, X, corrected to Mg# 73, X_OIB^#73, i.e., magmas in equilibrium with Fo₉₀, if olivine is present in the mantle source) show an increased major element compositional variability with increasing LAB depths. OIBs erupted on lithospheres < 40 km thick approach the compositions (e.g. SiO₂^#73, TiO₂^#73, [CaO/Al₂O₃]^#73) of primitive ridge basalts and are influenced strongly by depth and extent of shallow melting. However, X_OIB^#73 on thicker lithospheres cannot be explained by melt–mantle equilibration as shallow as LAB. Melt generation from a somewhat deeper (up to 50 km deeper than the LAB) peridotite source can explain the OIB major element chemistry on lithospheres ≤ 70 km. However, deeper melting of volatile-free, fertile peridotite is not sufficient to explain the end member primary OIBs on ≥ 70 km thick lithospheres. Comparison between X_OIB^#73 and experimental partial melts of fertile peridotite indicates that at least two additional melt components need to be derived from OIB source regions. The first component, similar to that identified in HIMU lavas, is characterized by low SiO₂^#73, Al₂O₃^#73, [Na₂O/TiO₂]^#73, and high FeO?^#73, CaO^#73, [CaO/Al₂O₃]^#73. The second component, similar to that found in Hawaiian Koolau lavas, is characterized by high SiO₂^#73, moderately high FeO?^#73, and low CaO^#73 and Al₂O₃^#73. These two components are not evenly sampled by all the islands, suggesting a heterogeneous distribution of mantle components that generate them. We suggest that carbonated eclogite and volatile-free, silica-excess eclogite are the two most likely candidates, which in conjunction with fertile mantle peridotite, give rise to the two primitive OIB end members.     

The join grossularite-calcite through the system CaO—Al2O3—SiO2—CO2 at 30 kilobars: Crystallization range of silicates and carbonates on the liquidus

At 30 kbar, calcite melts congruently at 1615°C, and grossularite melts incongruently to liquid + gehlenite (tentative identification) at 1535°C. The assemblage calcite + grossularite melts at 1450°C to produce liquid + vapor, with piercing point at about 49 wt.% CaCO₃. Vapor phase is present in all hypersolidus phase fields except for those with less than about 7% CaCO₃ or 8% Ca₃Al₂Si₃O₁₂. These results, together with known liquidus data for CaO—SiO₂—CO₂ and inferred results for CaO—Al₂O₃—CO₂ and Al₂O₃—SiO₂—CO₂, permit construction of the position of the CO₂- saturated liquidus surface in the quaternary system, and estimation of the positions of liquidus field boundaries separating some of the primary crystallization fields on this surface. The field of calcite is separated from those for grossularite and quartz by a field boundary with about 50% dissolved CaCO₃. Crystallization paths of silicate liquids in the range Ca₂SiO₄—Ca₃Al₂Si₃O₁₂—SiO₂, with some dissolved CO₂, will terminate at a quaternary eutectic on this field boundary, with the precipitation of calcite together with grossularite and quartz, at a temperature below 1450°C. Addition of Al₂O₃ to CaO—SiO₂—CO₂ in amounts sufficient to stabilize garnet thus causes little change in the general liquidus pattern as far as carbonates and silicates are concerned. With addition of MgO, we anticipate that silicate liquids with dissolved CO₂ will also follow liquidus paths to fields for the precipitation of carbonates; we conclude that similar paths link kimberlite and some carnbonatite magmas.     

Volcanism in the Sumisu Rift, I. Major element, volatile, and stable isotope geochemistry

A bimodal volcanic suite with KAr ages of 0.05–1.40 Ma was collected from the Sumisu Rift using alvin. These rocks are contemporaneous with island arc tholeiite lavas of the Izu-Ogasawara arc 20 km to the east, and provide a present day example of volcanism associated with arc rifting and back-arc basin initiation. Major element geochemistry of the basalts is most similar to that of basalts found in other, more mature back-arc basins, which indicates that back-arc basins need not begin their magmatic evolution with lavas bearing strong arc signatures.Volatile concentrations distinguish Sumisu Rift basalts from island arc basalts and MORB. H₂O contents, which are at least four times greater than in MORB, suppress plagioclase crystallization. This suppression results in a more mafic fractionating assemblage, which prevents Al₂O₃ depletion and delays the initiation of Fe₂O_3^(tot) and TiO₂ enrichment. However, unlike arc basalts,Fe^3+/ΣFe ratios are only slightly higher than in MORB and are insufficient to cause magnetite saturation early enough to suppress Fe₂O_3^(tot) and TiO₂ enrichment. Thus, major element trends are more similar to those of MORB than arcs.H₂O, CO₂ and S are undersaturated relative to pure phase solubility curves, indicating exsolution of an H₂O-rich mixed gas phase. HighH₂O/S, highδD, and low (MORB-like)δ³⁴S ratios are considered primary and distinctive of the back-arc basin setting.     

Mineralogy and chemistry of modern orogenic lavas — some statistics and implications

Island arc and continental margin (i.e. western Americas) lavas are divided (based on raw data from literature) into basalts (defined by absence of Ca-poor pyroxene, dominated by quartz-normative tholeiites); basaltic andesites and andesites (subdivided on basis of breaks in SiO₂ histogram and taken as <56% and 56–63% SiO₂; Ca-poor pyroxene present; amphibole and biotite absent); and hornblende (±biotite) lavas, which prove to be mainly relatively silicic andesites. Relative proportions of these types are (576 samples): 23% basalts, 29% basaltic andesites; 30% andesites; 18% hornblende andesites. The compilation emphasizes the dominance of calcic plagioclase (labradorite-anorthite) amongst the phenocryst phases. Pyroxenes are largely augite and hypersthene (En_60–75); olivine (Fo_65–85) is common through all compositions. There is an overall close similarity in chemistry and mineralogy between continental margin and island arc lavas, although small consistent differences are apparent (e.g. K₂O, TiO₂, P₂O₅).Modal data indicate that 70% of lavas are phenocryst-rich (20–60 vol.%), and that phenocryst contents show a bimodal distribution. Statistically and petrologically significant correlations are found between mineralogy and rock chemistry, most notably between total rock Al₂O₃ and modal phenocrystic plagioclase (found in all data groups, except hornblende andesites). This, and related data and correlations, indicate that the majority of orogenic magmas are modified by crystal fractionation (including crystal accumulation) processes dominated by plagioclase, and interpreted to occur under relatively low pressures. Dominance of plagioclase suggests phenocryst precipitation occurs typically in water-undersaturated magmas.     

Major element variation of the Skaergaard pyroxene and its petrologic implications

Pyroxenes from the Layered Series (LS), Upper Border Series (UBS), and Marginal Border Series (MBS) of the Skaergaard intrusion were analyzed using electron microprobe and mineral separation techniques to examine geochemical variations. In general, pyroxenes from all three series show similar trends in major elements vs % crystallization: SiO₂, MgO, Al₂O₃, and TiO₂ progressively decrease, FeO and MnO progressively increase, and CaO, Fe₂O₃, and P₂O₅ do not change systematically with differentiation. Pyroxenes in the LS and MBS follow a trend similar to that reported by Wager and Brown. The estimated crystallization temperatures closely follow the general trends of published temperature estimates. Major element variation in Skaergaard pyroxenes shows smooth variations with increasing differentiation, indicating that there was no volumetrically significant injection of new magma into the chamber after the initial emplacement. These results strongly support the idea that the Skaergaard intrusion represents in situ crystallization under a closed system magma chamber.     

The fragment of Paleo-Tethys ophiolite from central Qiangtang, Tibet:Geochemical evidence of meta-basites in Guoganjianian

N-MORB-type metabasites are discovered in the Guoganjianian area, central Qiangtang, Tibet, which are mainly metagabbro with cumulate structure and metabasalt. The rocks are distributed nearly from west to east unconformably underlying the Wanghuling Group of upper Triassic. On the basis of geo- chemical analysis, we find that the content of SiO2 is 43.03%―53.42%, and TiO2 1%―2.67%, Al2O3 16.75%―21.52%, CaO 7.03%―11.13%, K2O 0.05%―0.38%; the REE pattern is slight depletion or flat, and the trace spider diagram is like that of N-MORB, so we consider that the metabasite was formed under the setting of mid-ocean ridge or adult back-arc basin, and it is the fragment of Paleo-Tethys ophiolite.