Ferromagnesian Trace Elements Research Articles

Geochemistry of the sedimentary rocks from the Antimonio and Río Asunción formations, Sonora, Mexico: Implications for weathering, provenance and chemostratigraphy

Geochemical (major oxides, trace and rare earth elements) studies were carried out on the siliciclastic rocks of the Antimonio and Río Asunción formations to interpret the paleoweathering conditions and provenance. The weathering indices like the Chemical Index of Alteration (CIA) and Plagioclase Index of Alterations (PIA) indicate a low to moderate intensity of chemical weathering. The chondrite normalized REE patterns of the Antimonio and Río Asunción formations have LREE enriched, flat HREE patterns with a negative Eu anomaly. However, a few samples from these formations show a positive or an absence of the Eu anomaly, suggesting that the sediments were dominantly derived from felsic source rocks with a minor contribution from mafic source rocks. The vertical distribution of trace elements and their ratios indicate that sequence I of the Antimonio Formation presents a signature for felsic source rocks, however, in sequence II the sediment source changed, i.e. mafic rocks were recorded (SAAF06 and SAAF07). Sequences III to VI indicate a felsic source rock. The basal and upper parts of sequence VII (samples SARAF02 and SARAF10) of the Río Asunción Formation indicate mafic source rocks, whereas the remaining samples were largely contributed by felsic source rocks. The lower values of ferromagnesian trace elements and their ratios in sequences VIII and IX reveal a felsic source for the upper part of the Río Asunción Formation. The Antimonio and Río Asunción formations received sediments from two different sources: Permo-Triassic granitoids of northwestern Sonora and Precambrian granitoids from Caborca and North American blocks that contributed felsic rocks. Whereas mafic elements were derived from Precambrian amphibolites and Cambrian basalts located north and south of the study area.

Geochemistry of the sandstones of Punagarh basin: Implications for two source terranes and Arabian - Nubian connection of Aravalli craton?

Sandstones of Punagarh basin of Trans Aravalli region, NW Indian shield were analyzed for their major and trace element contents in conjunction with petrographic modes. The Punagarh basin comprises four formations (Sojat, Bambolai, Khamal and Sowania), amongst which the Sojat Formation in uncoformable contact with overlying formations, is significantly older and enjoys separate status as rest of the formations constitute Punagarh Group. Petrographic attributes suggest that Sojat sandstones contain distinct modal abundances like high content of quartz and low content of feldspar, mica, matrix, rock fragments and cement. In general there is a decrease in the average modal abundance of quartz and mica with concomitant increase of feldspar, chert and rock fragments from base to top in Punagarh sandstones. Sojat sandstones are also geochemically distinct as they possess high SiO2/Al2O3, Th/U and Cr/Th ratios coupled with lowest Na2O/K2O ratio and Zr content, least fractionated LREE, more fractionated HREE and largest Eu* anomaly compared to Punagarh sandstones. The weathering indices suggest intense chemical weathering for Sojat sandstones and low to moderate for Punagarh sandstones. Compositionally all the sandstones of the Punagarh basin come under the category of quartzarenite. Compared to PAAS and UCC, both suite of sandstones are generally depleted in REE, HFSE (with exception of Zr, Hf and Ta), and enriched in ferromagnesian trace elements particularly Cr and Co. The chemical data indicate that the sediments were derived from the source(s) of mixed felsic - mafic composition, with the former being dominant. The source rocks were granites, TTG, basalts, and rhyolites. The Sojat sandstones owe their source form Archean crust while Punagarh sandstones got detritus from Mesoproterozic crust. Immobile element ratios and REE abundances of Sojat sandstones closely match with Paleoproterozoic metagreywackes of Ghana of African craton while Punagarh sandstones show near geochemical characteristic with Gogunda, Kumbalgarh and Vindhyan quartzites of Aravalli craton. This geochemical similarity of Sojat sandstones provides credence to the hypothesis that Trans–Aravalli region of India had been an integral part of Arabian - Nubian shield. The chemical data advocate the deposition of these sandstones in a tectonic setting comparable to modern back arc setting.

Continental lithospheric evolution: Constraints from the geochemistry of felsic volcanic rocks in the Dharwar Craton, India

Felsic magmatism associated with ocean–ocean and ocean–continent subduction processes provide important evidence for distinct episodes of crust-generation and continental lithospheric evolution. Rhyolites constitute an integral component of the tholeiitic to calc-alkaline basalt–andesite–dacite–rhyolite (BADR) association and contribute to crustal growth processes at convergent plate margins. The evolution of the Dharwar Craton of southern peninsular India during Meso- to Neoarchean times was marked by extensive development of greenstone belts. These granite-greenstone terranes have distinct volcano-sedimentary associations consistent with their geodynamic setting. The present study deals with geochemistry of rhyolites from the Chitradurga-Shimoga greenstone belts of western (WDC) and the Gadwal-Kadiri greenstone belts of eastern (EDC) sectors of Dharwar Craton to compare and evaluate their petrogenesis and geodynamic setting and their control on the continental lithospheric evolution of the Dharwar Craton. At a similar range of SiO2, Al2O3, Fe2O3, the rhyolites of WDC are more potassic, whereas the EDC rhyolites are more sodic and less magnesian with slight increase in TiO2. Minor increase in MgO content of WDC rhyolites reflects their ferromagnesian trace elements which are comparatively lower in the rhyolites of EDC. The relative enrichment in LILE (K, Rb) and depletion in HFSE (Nb, Ta, Zr, Hf) marked by negative Nb–Ta, Zr–Hf and Ti anomalies endorse the convergent margin processes for the generation of rhyolites of both the sectors of Dharwar Craton. The high silica potassic rhyolites of Shimoga and Chitradurga greenstone belts of WDC showing prominent negative Eu and Ti anomalies, flat HREE patterns correspond to Type 3 rhyolites and clearly point towards their generation and emplacement in an active continental margin environment. The geochemical characteristics of Gadwal and Kadiri rhyolites from eastern Dharwar Craton marked by aluminous compositions with low and fractionated HREE patterns and minor negative Eu anomalies are in conformity with Type 1 rhyolites and suggest that they were erupted in an intraoceanic island arc system. The overall geochemical systematics of the rhyolites from both the sectors of Dharwar Craton suggest a change in the geodynamic conditions from intraoceanic island arc of eastern Dharwar Craton and an active continental margin of western Dharwar marked by ocean–ocean subduction and migration of oceanic arc towards a continent followed by arc-continent collision that contributed for the evolution of continental lithosphere in the Dharwar Craton.

Lawsonite- and glaucophane-bearing blueschists from NW Qiangtang, northern Tibet, China: mineralogy, geochemistry, geochronology, and tectonic implications

The occurrence of high-pressure (HP) blueschists within the central Qiangtang terrane of northern Tibet has a significant bearing on plate-suturing processes. In order to contribute to the ongoing debate on whether the central Qiangtang metamorphic belt represents an in situ suture within the Qiangtang terrane, we examined lawsonite- and glaucophane-bearing blueschists from the northwest Qiangtang area (84° 10′–85° 30′ E, 34°10′–34° 45′ N). All studied rocks are metapelites, metasandstones, or metabasalts, characterized by lawsonite + glaucophane + phengite, lawsonite + glaucophane + epidote + albite + quartz, or glaucophane + phengite + quartz assemblages. The meta-mafic rocks contain very high TiO2 and low Al2O3 contents. They are typified by abundant ferromagnesian trace elements, and an absence of Eu anomalies and Nb–Ta deletions; all the above features indicate that these mafic rocks represent oceanic island basalt (OIB) protoliths. Most of the metasediments contain high SiO2, moderate Al2O3 + K2O, and low TiO2 + Na2O. They display high CIA (chemical index of alteration) values (74% ± 5%) and distinctly negative Eu anomalies (Eu/Eu* = 0.64 ± 0.05). This, along with their high field strength elemental characteristics, indicates that they were deposited in a passive continental margin environment, intercalated with OIB-type basalts. We estimate the peak metamorphic conditions for these blueschists as T = 330–415°C and P = 9–11.5 kbar. This HP event occurred at ca. 242 Ma, indicated by a well-defined 40Ar/39Ar plateau age for glaucophane. Retrograde metamorphism occurred at T = 280–370°C, P = 6.5–9.5 kbar, t = ca. 207 Ma (40Ar/39Ar dating of phengite). Therefore, a cold subduction (geotherm ∼8°C/km) attended the passive continental margin during the Triassic when the eastern Qiangtang collided with the western Qiangtang. The northwest Qiangtang HP metamorphic belt is an extension of the central Qiangtang metamorphic belt that defines the suture between eastern and western Qiangtang, and indicates an anticlockwise, diachronous closure of the Shuanghu Palaeo-Tethys.

The Paks loess-paleosol sequence: A record of chemical weathering and provenance for the last 800 ka in the mid-Carpathian Basin

The Paks loess-paleosol sequence is one of the most important terrestrial records of Middle and Late Pleistocene environmental changes in East Central Europe, spanning the last ca. 0.8Ma. While geochemical proxies demonstrate a general decreasing chemical weathering trend over the last 0.8Ma in the Carpathian Basin, mineralogy and derived indices reflect intensifying physical erosion. In theory, the observed chemical weathering trend can be accounted for both by enhanced input of relatively unweathered material and by climate deterioration during the Quaternary, as the proxies such as CIA are not capable of distinguishing between pre- and post-depositional weathering. Enhanced physical erosion of the source areas, driven by tectonism, and resulting increased sedimentation of fresh mineral dust at the depositional site are demonstrated by increasing dolomite, illite and chlorite contents and sme/ill, sme/(ill+chl) ratios from older to younger sediments in the profile, together with increasing thickness of loess layers towards the youngest part of the sequence. At the same time, constant smectite contents (30–40%) in paleosols appear to disprove progressive aridization of interglacials through time and suggest that the duration of pedogenesis played an important role in determining soil types. Further, the increasing proportion of inherited phyllosilicates (illite and chlorite) would, in theory, raise the possibility that the decreasing values of chemical weathering indices are just artifacts of enhanced physical erosion and resulting increased dust deposition by a dilution effect. The above findings highlight the fact that the general view on chemical weathering is oversimplistic, as its ‘equation’ includes two basic variables, tectonism and time beyond climate and the interplay of these equally important factors will eventually determine its final value. To get a better grasp of these processes needs further data (more age control in loess profiles, data on uplift in and around sedimentary basins) and more sophisticated proxies, as the mineralogical data presented here can be considered only semiquantitative.Regarding the provenance of sediments in the Paks profile, geochemical data demonstrate that felsic rocks dominated the source areas and there have been only very little variations in provenance over the last ca. 0.8Ma. Significant contributions from mafic/ultramafic rocks to the sediments can be ruled out as revealed by lower abundances of ferromagnesian trace elements. The appearance of amphiboles and high dolomite contents suggest that loess material was at least partly sourced from local rocks and geochemical data reveal a genetic link between floodplain sediments and loess deposits.

Geochemical systematics and precious metal content of the sedimentary horizons of Lower Gondwanas from the Sattupalli coal field, Godavari Valley, India

Abstract Major, trace, rare earth (REE) and platinum group elements (PGE) have been analysed from the carbonaceous horizons of Permian Barakar Formation of Sattupalli coal field of Godavari Valley to understand the provenance and depositional environment. Barakar Formation of coal field consists of sandstone, siltstone, carbonaceous shale and coal seams. Coal is enriched in trace elements compared with the Clarke's background values. Ferromagnesian trace elements (Ni, Cr and Co) are similar in range in both coal and carbonaceous shales along with Large ion lithophile elements (LILE; Rb, Sr, Cs and Ba), High field strength elements (HFSE; Nb, Ta, Zr and Ti), Light REE (LREE; La-Sm) and Ag. Archean Upper Crust (AUC) normalized REE and multielement variation patterns of the coal show a flat REE with a slight depletion in Ce and Eu contents in most of the samples. Their multielement patterns exhibit enrichment of Th and U; negative Ta, Zr, Hf and positive Ti anomalies. Pronounced positive anomaly occurs at Zr, Hf and Ti that are combined with both positive and negative Nb and Ta anomalies in carbonaceous shales and clay. Sandstones have a high SiO 2 (50–80 wt.%), low Fe 2 O 3 , Al 2 O 3 and moderately high K 2 O contents. AUC normalized REE patterns are almost flat with slight negative to no Eu anomalies followed by small negative Ce anomalies (Ce/Ce* = 0.87–0.95 with one exception). Their multielement patterns exhibit positive U, small negative Nb-Ta, a pronounced positive Zr-Hf anomaly followed by negative Ti and V anomalies. Enrichment of both Zr and TiO 2 in some samples reflects on the mafic and felsic sources at the source region. The fractionated REE patterns, HREE enrichment and negative Eu anomalies are consistent with contribution of zircon through felsic source. Nb/Ta troughs endorse their inheritance from schistose rocks of Eastern Ghats complex. The high Al 2 O 3 /Yb and Th/Yb ratios along with negative Eu anomalies reflect on the TTG/felsic rocks of provenance. Based on the geochemical, available petrological and paleocurrent analysis indicate the source rocks of the Barakar Formation were granite, granitic gneiss, high grade metamorphic rocks from Eastern Ghats, and basic rocks. The provenance is located west and south east (i.e. partly Peninsular gneiss and partly Eastern Ghat Complex). The Au-PGE contents of Sattupalli coalfield have been deposited through NE–SW trending post depositional transverse faults that have affected all the Gondwana Formations during the early Cretaceous. The mafic rocks present within the basement could have been the source of these elements.

Geochemistry of lower Vindhyan clastic sedimentary rocks of Northwestern Indian shield: Implications for composition and weathering history of Proterozoic continental crust

The western margin of the ∼1600 Ma Vindhyan basin of the Indian shield is marked by the presence of lower Vindhyan Group (LVG) comprising shales and sandstone with minor limestone, conglomerate and mafic volcanic flows at the base. Major and trace element compositions of sandstones and shales of the LVG have been investigated to infer the nature of provenance, weathering processes, and tectonic setting. Provenance analysis using trace elements and petrography suggests that the Archaean Banded Gneissic Complex (BGC) of Rajasthan is most likely the source of these clastic rocks. The shale and sandstone comprising lower part of LVG namely Khardeola Formation are distinctive in having high LILE such as Th, Rb, and K 2O, high ferromagnesian trace elements and also higher values of K 2O/Na 2O, Rb/Sr and (La/Yb) n ratios in comparison to succeeding sedimentary units comprising upper formations of the LVG. The significantly different geochemical characteristics of Khardeola Formation suggest change in provenance with sediments in the lower formation being derived from proximal source and those of upper formations from distal source. Based on REE patterns and Eu/Eu ∗ values, the clastic sediments of Khardeola formation can be modeled by a mixture of 60% Berarch Granite and 40% mafic volcanic rocks of Hindoli belt. On the other hand, the upper formations are best modeled with a mixture of 60% BGC granitic gneisses, 20% BGC mafic enclaves and 20% Berach Granite. CIA (chemical index of alteration) values and A–CN–K plot suggest that the source area experienced moderate to high degree of chemical weathering under warm and humid conditions. The shift of sediment source from local in the beginning to distal during later period suggests sedimentation in a tectonically active basin.

Evolution and genesis of calc-alkaline magmas at Filicudi Volcano, Aeolian Arc (Southern Tyrrhenian Sea, Italy)

Petrological, trace element and Sr, Nd, Pb isotopic data are reported for volcanic rocks from the island of Filicudi, Aeolian Arc, Southern Tyrrhenian Sea. The volcano consists of several monogenic and polygenic centres built up through four major phases of explosive and effusive activity started before 1 Ma. Rock composition ranges from calc-alkaline basalts to high-K andesites. There is a negative correlation between silica and MgO, CaO, TiO 2, FeO total, and a positive trend for K 2O, Na 2O and P 2O 5. LILE and HFSE increase with silica, whereas ferromagnesian trace elements have an opposite tendency. Incompatible elements, such as Zr, Ba, Rb, La, display well-defined positive correlations on elemental variation diagrams; weak correlations are shown by the other incompatible elements; Sr and compatible elements define negative, roughly curvilinear trends with incompatible elements. 87Sr/ 86Sr is poorly but significantly variable (0.704016–0.704740) and shows overall higher values in the mafic than in the sialic rocks. Nd isotope ratios range from 0.512670 to 0.512760 and are negatively correlated with 87Sr/ 86Sr. Pb isotope ratios cluster around 206Pb/ 204Pb=19.31–19.67, 207Pb/ 204Pb=15.64–15.69, 208Pb/ 204Pb=39.11–39.47. Major, trace element and isotopic variations reveal complex, multistage polybaric evolutionary processes for the Filicudi magmas. It is clear that crystal-liquid fractionation processes determined many of the petrologic and geochemical characteristics of these magmas. However, elemental variations when coupled with isotopic variations (in particular Sr isotopes) demonstrate that mixing processes and interaction of the magmas with older crustal material also played an important role. When compared with other Aeolian arc volcanoes, Filicudi shows petrological and geochemical characteristics similar to those of the nearby islands of Salina and Alicudi. The three islands consist of calc-alkaline rocks, but the degree of magma evolution increases going from the Alicudi to Salina. These variations are likely related to the plumbing system of the three volcanoes. However, trace element and isotopic evidence also suggests significant variations of primary magmas, which reveal a zoned source which suffered different types of metasomatism.

Provenance and Tectonic Setting of the Proterozoic Turbidites in Hunan, South China: Geochemical Evidence

The geochemical composition of the Middle-Upper Proterozoic turbidites in Hunan, South China is related to provenance and tectonic setting of the sedimentary basin. The turbidites are characterized by moderate SiO 2 contents (Al 2 O 3 /SiO 2 typically 0.1-0.3) and moderate K 2 O/Na 2 O ratios (generally 0.1-0.3) but relatively high Fe 2 O 3 * + MgO contents (5-10 wt %) and unusually low contents of CaO (typically < 1 wt %). Abundances of ferromagnesian trace elements and incompatible elements are moderate. The REE patterns are uniform and similar to the upper continental crust and typical post-Archean shales, with LREE enrichment, flat HREE, and significant negative Eu-anomalies. In general, the slates show higher contents of Fe 2 O 3 * + MgO and ferromagnesian trace elements, and lower ratios of La/Sc and Th/Sc than the associated graywackes, suggesting that more mafic materials were incorporated into the clay-size fraction. Compared to the Upper Proterozoic Banxi Group, the Middle Proterozoic Lengjiaxi Group has higher concentrations for most ferromagnesian trace elements and lower La/Sc ratios, implying more mafic components in the provenance of the latter. The relationship among alkali and alkaline earth elements, the Chemical Index of Alteration (CIA), and the Th/U ratios indicate that the source area was affected by a moderate weathering history. The main source was old continental crust (the Yangtze craton) dominated by felsic rocks of magmatic origin, with a variable admixture of mafic components from continental island arcs (e.g., the Fangjingshan-Dayong and the Sibao-Yiyang island arc chains). The flysch deposition took place in a back arc basin, rather than a failed intracontinental rift as proposed by some workers. We suggest that suturing between the Yangtze and Cathaysia blocks did not occur until late Proterozoic time and that the final assembly of the supercontinent Rodinia in South China was probably marked by the Jingning orogeny at ∼0.8 Ga.

Chemistry, petrology and origin of banded iron-formation lithologies from the 3800 MA isua supracrustal belt, West Greenland

Data are presented on the phase petrology of 60 samples of banded iron-formation (BIF) and related rock types from 35 localities spanning the known stratigraphic units in the 3.8-Ga Isua Supracrustal Belt, West Greenland. We report representative microprobe analyses for all silicate and carbonate minerals found in these samples. In addition, complete chemical analyses for major, minor and trace elements are presented for a subset of 28 samples. In order to make our data base more representative of actual field occurrences, we have deliberately included in our sample set certain quartz-rich rocks that have iron contents lower than the nominal limit of 15 weight percent ordinarily ascribed to BIFs. On the basis of field setting, mineral assemblage and rock composition, we recognize six iron-formation lithologies: (I) quartz-magnetite IF; (II) magnesian IF; (III) aluminous IF; (IV) graphitic IF; (V) Mg-Fe carbonate IF; and (VI) Ca-Mg carbonate IF. Although rocks containing abundant oxide, carbonate and/or silicate minerals occur, we consider it inappropriate to assign a ‘facies classification’ onto the Isua occurrences, given the extreme state of deformation and high grade of metamorphism experienced by these rocks. The most common assemblage is: quartz+magnetite±cummingtonite/grunerite±actinolite/hornblende±ripidolite (ferroan chlorite)±calcite; almandine-rich garnet, ferroan talc, or minor sulfide (pyrite±pyrrhotite) occur in a few samples. Two unique assemblages are: quartz+magnetite+grunerite+stilpnomelane; and quartz+magnetite+actinolite+grunerite+ferrosalite. Carbonate-rich samples contain the assemblages: ±quartz±magnetite±cummingtonite/grunerite± actinolite±ripidolite±ferroan dolomite ± siderite/ferroan magnesite. Minor graphite may be present in some of the carbonate-rich samples, as well as in several of the oxide- and silicate-rich samples. In most samples, evidence of retrogression is rare, although the status of ripidolite as a prograde mineral is ambiguous. Overall, the iron-formation assemblages are compatible with amphibolite-facies P-T conditions based on studies of pelitic rocks. An interesting feature of the Isua BIFs is the very wide range of compositions revealed for coexisting Ca-poor and Ca-rich amphiboles, reflecting the variable nature of the samples studied. Nevertheless, the Isua assemblages are very similar to those found in younger Archaean and Proterozoic BIF suites that have been metamorphosed to amphibolite facies. The average major element composition for the 28 analysed samples is (in wt.% on an H 2O- and CO 2-free basis): SiO 2, 55.9; TiO 2, 0.06; Al 2O 3, 1.36; Fe 2O 3, 11.8; FeO, 19.3; MgO, 6.7; MnO, 0.62; CaO, 4.0; Na 2O, 0.01; K 2O, 0.20; P 2O 5, 0.16. These values are in the normal range of averages for younger Archaean and Proterozoic BIFs, when one allows for dilution by quartz in our data set. The average abundances of ferromagnesian trace elements are all low (in ppm: Sc, 2.5; V, 33; Cr, 32; Co, 11; Ni, 58; Cu, 28; Zn, 84), and similar to data on younger BIFs, although the Ni contents of the Isua samples appear to be slightly higher. The relative distributions of these elements qualitatively resemble those reported in ‘Algoma-type IF’ as opposed to values in ‘Superior-type IF’. These results support the contention that, as early as 3800 Ma, restricted basins of deposition were established that had the appropriate physical-chemical conditions for acumulation of Fe-rich chemical sediments from a probable volcanogenic source. Abundances of the rare-earth elements (REE) span two orders of magnitude concentration, ranging from ∼ 0.1 to 20 times chondrites. Most samples show negative Ce anomalies, and all but one have a positive Eu anomaly. Average values are (in ppm): La, 2.34; Ce, 3.92; Nd, 2.03; Sm, 0.51; Eu, 0.369; Tb, 0.118; Yb, 0.585; Lu, 0.102. Abundances of most other trace elements are quite low with the exception of Sr in Ca-Mg carbonate IF (up to ∼ 100 ppm), and Zr in aluminous IF (up to ∼ 100 ppm). A ‘crustal’ Zr/Hf ratio of ∼40 characterizes the sample suite as a whole, but an evaluation of La-Sc-Zr-Hf systematics reveals that most samples are free of ‘contamination’, based on comparisons with data for contemporaneous Isua clastic metasediments and amphibolites. Attention is drawn to the close similarity between the abundances of the REE and ferromagnesian trace elements in the Isua samples and those found in present day deep-sea hydrothermal deposits, such as the Galapagos Mounds. Mixing calculations using present-day seawater REE (with its negative Ce anomaly), and REE in high-temperature mid-ocean ridge hydrothermal fluid (with its pronounced positive Eu anomaly) faithfully reproduce the average REE pattern of the Isua BIFs using a mixture of 100 parts seawater and 1 part hydrothermal fluid. Accordingly, we interpret the Isua BIFs as the products of precipitation from very dilute hydrothermal solutions that had passed through (mafic to ultramafic?) volcanic rocks, stripping them of their iron, silica and other trace elements, particularly Eu. A further implication of our results is that the Archaean seawater component possessed a negative Ce anomaly, suggesting that ancient oceans may have been substantially more oxidized than was heretofore recognized.

Geochemical characteristics of the south Tuscany (Italy) volcanic province: Constraints on lava petrogenesis

Three suites of volcanic rocks from Radicofani, Mts. Cimini and Mt. Amiata (south Tuscany, Italy) were analyzed for major- and trace-element contents and Sr isotopic composition. All samples are silica saturated, hypersthene normative and some are peraluminous. Samples from Radicofani and the less evolved Mts. Cimini samples have high Mg numbers (74.5–66.3), consistent with the fosteritic-rich nature of their olivine phenocrysts and high bulk-rock abundances of ferromagnesian trace elements. All samples are highly enriched in light REE (150–315 × chondrites) compared to heavy REE (9–15 × chondrites) and have negative Eu anomalies ( Eu Eu★ = 0.92-0.44 ). 87 Sr 86 Sr ratios are high and variable: Radicofani = 0.7137-0.7150; Mts. Cimini = 0.7128-0.7138; Mt. Amiata = 0.7119-0.7131. A distinctive feature of the Radicofani samples is that with decreasing age, the rocks increase in K 2O content but decrease in La/Ce. These south Tuscany rocks have geochemical characteristics of the calc-alkaline and alkaline series: namely, in addition to the high K, Rb, Cs, Ba and LREE contents, they have high ratios of incompatible elements relative to the high-field-strength ions such as Ti, Zr, Hf, Nb and HREE when compared to an estimated primitive mantle composition. In general, the geochemical characteristics of south Tuscany volcanics are consistent with mixing between crustal-derived material and mantle-derived magma. The mantle-derived component was unusually rich in radiogenic Sr and incompatible elements and had geochemical features similar to basic lavas of the high-K series in the Roman Province.

Geochemistry of some unusual early Archaean sediments from Dharwar Craton, India

Abstract Metamorphosed Archaean sediments comprise ∼ 35% of the 3.5 Ga old Holenarasipur schist belt (HNSB) and 90% of the 3.2 Ga old Javanahalli schist belt (JSB) of the Dharwar Craton. The high-alumina sediments of the HNSB are variable in their major, LIL and ferromagnesian trace elements. The REE patterns are variable, with both positive and negative Eu anomalies. Cr and Ni are more abundant than in most of the detrital sediments, at corresponding silica contents. Al and Cr show positive correlation. The Th/U, La/Th, Ba/La and Zr/La ratios and abundances show a large-scale scatter. The paragneisses of JSB also have a variable major element composition, but uniform REE patterns and abundances, with strong negative Eu anomalies similar to some Archaean tonalitic gneisses and Phanerozoic shales and quartzites. The para-amphibolites, stratigraphically above these gneisses, have depleted flat-REE patterns and positive Eu anomalies. The HNSB sediments are probably derived from a source consisting of mafic, ultramafic, anorthositic and acid volcanic rocks. In contrast, the source area for JSB sediments had considerable acid plutonic rocks. This indicates that the processes of sial formation started soon after the HNSB group had been deposited and continued through the development of the younger greenstone belts. Rapid nucleation and development of continental crust occurred during the interval 3.5 to 3.2 Ga.