Geochemical Discriminant Diagrams Research Articles

Geochemical characteristics of olivine and clinopyroxene in parental mafic–ultramafic rocks from the Yuanshishan Ni-Co laterite deposit in Qinghai Province, NW China

Yuanshishan is a Ni-Co laterite deposit that occurs in the Lajishan tectonic zone of the South Qilian orogenic belt, NW China. Ni–Co mineralization took place during the weathering of the parental mafic–ultramafic rocks. Little is known about the petrogenesis of these parent rocks. Therefore, field and petrographic investigations, together with in situ major and trace element analyses, were carried out on olivine and clinopyroxene from unweathered mafic–ultramafic rocks in the Yuanshishan Ni-Co deposit. The results show that olivine (Ni 963–2674 ppm, Co 52.7–245 ppm) is richer in Co and Ni than clinopyroxene (Ni 155–203 ppm, Co 23.1–32.5 ppm) in the Yuanshishan mafic–ultramafic rocks. The chromite (Ni 98.8–892 ppm, Co 836–1062 ppm) show higher Co and Ni than clinopyroxene, but has lower Ni but higher Co than olivine. Therefore, olivine and chromite are two main Co and Ni rich minerals in the parent rocks of Yuanshishan. A compositional comparison of olivine and clinopyroxene from the Yuanshishan deposit to those from other magmatic sulfide deposits reveals that the Yuanshishan parent magma is relatively poor in Ni and rich in Co. Olivine in the dunite of Yuanshishan has a quite high Fo value (95.2–98.3). The Yuanshishan olivine grains also show a positive correlation between Ni content and Ni/Co or Co content, and the correlation with Fo value is not obvious. The Co content shows a negative correlation with the Ni content and Fo value, which are consistent with the mantle olivine characteristics. Furthermore, the Li, Sc, V contents in olivine are lower than 10 ppm, indicating that the magma originated from the mantle. The Al, Si, Ti, Ca, and Na contents in clinopyroxene indicate that the original magma may have been subalkaline basaltic melt. Using an olivine-spinel thermometer, the Yuanshishan magma crystallization temperatures are calculated to be 1256–1376 °C. The AlⅣ:AlⅥ ratio (0.7–6.0) and Eu/Eu* value (0.9–1.4) of the clinopyroxene indicate that the Yuanshishan mafic–ultramafic rocks were formed under conditions of low to medium pressure and high oxygen fugacity. Moreover, the geochemical discriminant diagrams of clinopyroxene indicate that these rocks possibly formed in an island arc setting.

The formation environment of Carboniferous karstic bauxite deposits in the central North China Craton: Insights from nitrogen isotopes and geochemical proxies

Over 5 billion tons of karstic bauxites are hosted in the Late Carboniferous Benxi Formation in the North China Craton (NCC). However, the bauxite formation environment in the central NCC remains unknown. The geological, mineralogical, geochemical, nitrogen concentrations, and δ15N isotope evidence of the Huilong and Shangwutou bauxite deposits in the central NCC was conducted, aiming to clarify the formation environments. Typically, bauxites in the central NCC contains three layers from bottom to top: Fe-bearing claystone/clayey ironstone, bauxite ore, and bauxitic claystone. The Fe-bearing claystone/clayey ironstone is composed of kaolinite, goethite, and hematite; the bauxite ore is composed of diaspore, kaolinite, but also minor hematite, ilmenite, and svanbergite; and the bauxitic claystone consists of kaolinite, but also some diaspore and goethite. Diaspores coexisted extensively with hematite, anatase, and svanbergite, forming cryptocrystalline matrix of bauxites, suggesting that they may be formed during early diagenesis. Geochemical discriminant diagrams also indicated that two deposits were primarily affected by the hydrogenous deposition and early diagenesis, and no sedimentary recirculation. The ∑REEs concentrations and fractionation (Prn/Ybn) features along the profiles mainly concentrated in the bottom bauxite ore, which reflected that the pH values could be alkaline in bottom bauxite ore, and decreased weakly acidic for the central bauxite ore to bauxitic claystone along the profiles upwards. Combining the δ15N data with two geochemical redox proxies, and the widely coexistence of diaspore and hematite, indicated that the two deposits mainly formed under suboxic conditions, and thus denitrification was the predominant process in nitrogen cycling. The δ15N values are positively correlated with paleosalinity proxies, indicating that the higher saline bottom water may lead to a strong vertical density differential and water-column stratification, conducing to forming suboxic restricted pore-water environments.

Geochemistry and Petrogenesis of the Wadhrai Granite Stock of the Malani Igneous Suite in Nagar Parkar Area, SE Pakistan

The Wadhrai granite stock is a part of the Nagar Parkar Igneous Complex, an extension of the Neoproterozoic Malani Igneous Suite of western Rajasthan. It is occupied by a petrographically uniform granite comprising perthite, plagioclase, quartz, with small quantities of biotite, opaque oxides, titanite, and secondary minerals. The rocks are sparingly porphyritic and contain dykes of microgranite, aplite, and rare pegmatite. In the south-central part, parallel sheets and swarms of mafic dykes, and in the western part very fine-grained felsic sheets intrude the body. The granite is metaluminous to peraluminous and characterized by high silica (73–76 wt%), and alkalis (7–9 wt%), and low CaO (0.15–1.4 wt%), MgO (0.15–0.38 wt%), Th (7–12 ppm), and U (1–2 ppm). On geochemical discriminant diagrams, it classifies mostly as A-type (with rather high Y/Nb (8.6 to 2.4, average 5.2) and low Nb/Ga and Ce (typical of A2-type), but sparingly as I-type. Chondrite-normalized patterns show enrichment in LREE over HREE, and small negative Eu anomalies, whereas mantle-normalized spidergrams display higher LILE over HFSE, distinct troughs for Nb, Sr, P, Ti, and spikes for La, Ce, Nd, Sm and Tb. The granite magma was possibly derived from a tonalite-granodiorite-dominated crustal source. Based on the above-mentioned geochemical evidence, it is interpreted that the source rocks of the magma of the Wadhrai granite likely developed initially in a continental margin subduction setting and underwent partial melting in a continental extensional environment.

Major, trace and rare earth elemental geochemistry of Santonian–Campanian onland‐offshore transition in a Gilbert‐type deltaic setting, Cauvery Basin, southern India

The spatially widely distributed and temporally protracted Santonian–Campanian stratigraphic records of the Sillakkudi Formation of the Ariyalur Group, Cauvery Basin, southern India represent an onland exposure of a Gilbert‐type delta. Here, we present geochemical, mineralogical, and petrographic analyses of two composite stratigraphic sections, and evaluate the climatic, tectonic, and other factors that contributed to the spatio‐emporal uniqueness of this formation. A range of low to intensive weathering, simultaneous exhumation/erosion, physical transport, and chemical weathering, prevalent at different parts of the source region and or the existence of multiple channels that drained varied source regions and hence varied lithologies at exhumation surfaces which in turn experienced seasonally varied flow conditions were interpreted. The prevalence of physical erosion during the initial stages of the deposition of the Sillakkudi Formation that progressed towards higher intensities of chemical weathering, in tune with the increase in atmospheric temperature and sea‐level rise during the later stages of the formation, is evidenced. Occurrences of positive and negative Eu anomalies and a progression from active to passive tectonic setting of samples, concomitant with change from tectonic control to relative sea‐level and the sediment influx controlled nature of deposition from Santonian–Campanian is recorded. Together, three sets of related factors namely, tectonics, increase of atmospheric temperature and sea‐level rise operated independently, and the enforced transience of transformation of the source terrain from active to passive margin, the transformation of depositional setting from fluvial to estuarine/intertidal to open marine, and shifting of principal loci of deposition from onland to offshore, are documented. The causal link between the progressive increase of sea‐level control over the depositional system during the Santonian–Campanian as recorded by this study could be due to the rise in atmospheric temperature that in turn might have been influenced by the movement of the Indian Plate from low latitudes towards the equatorial region. The second implication of the results is that the studies attempting to document tectonic setting, provenance, and weathering, with the help of geochemical discriminant diagrams, should also consider the spatial and temporal changes in the source area as well as depositional setting.

Geological significance of the newly discovered Middle Permian ocean island basalt‐type gabbros in Ewulang, Nianqing‐Sumdo area, Tibet

Numerous research data have been obtained about the Sumdo Palaeo‐Tethys, but the reports on the occurrence of Permian Ocean islands are relatively scarce. Here, we present the zircon U–Pb geochronology, geochemistry, and in situ zircon Hf isotopic characteristics of the ocean island basalt (OIB)‐type gabbros from Ewulang in Nianqing‐Sumdo, Tibet. The zircon U–Pb age obtained from Ewulang OIB‐type gabbros is 263 Ma (the Middle Permian), representing the formation age of the gabbros. The geochemical characteristics of the samples show no contamination by crustal material. Meanwhile, all samples are enriched in light rare earths (LREEs) and large‐ionic lithophile elements (LILEs), which are similar to the typical OIB. In various geochemical discriminant diagrams, all samples fall into the fields of OIB and related tectonic settings, indicating that the rocks were formed in the oceanic island environment within the oceanic plate. Various geochemical indicators imply that the OIB‐type gabbros may originate from mantle plume and the in situ zircon εHf(t) values of Ewulang gabbros range from −0.34 to +4.30, suggesting that the magmas derived from an enriched mantle source. Combined with regional data and previous studies, this article further confirms that Sumdo Palaeo‐Tethys extends west to Nianqing‐Sumdo area and the Tangjia (Zhikong)‐Sumdo Palaeo‐Tethys was still an ocean basin with a certain scale during the Middle Permian.

O Stock Serra da Acauã: um exemplo de granito tipo I fracionado oxidado no Domínio Rio Piranhas-Seridó, NE do Brasil

O Stock Serra da Acauã está inserido no contexto da atividade plutônica ediacarana, uma das mais importantes feições geológicas na Província Borborema, Nordeste do Brasil. O corpo está dividido em duas fácies: uma equigranular fina a média e outra inequigranular média a fracamente porfirítica com fenocristais de K-feldspato. Determinações U-Pb em zircão apontaram uma idade ediacarana de c.a 578 Ma para a sua cristalização. Petrograficamente, o stock consiste de um biotita monzogranitos hololeucocráticos a leucocráticos (M = 1,3 – 9,9%), com biotita sendo o máfico principal (0,5 a 8,1%). Minerais opacos, titanita, allanita, apatita e zircão são acessórios e as fases minerais mais precoces. As rochas que formam o stock são geoquimicamente similares, de caráter fracamente peraluminoso (1,00 ≤ A/CNK ≤ 1,04), com coríndon normativo e de assinatura transicional entre rochas alcalinas e calcioalcalinas. A variação do conteúdo de óxidos e elementos traço sugere o fracionamento de plagioclásio, biotita, titanita, apatita e magnetita durante a cristalização do magma. O diagrama para elementos terras raras mostra enriquecimento dos elementos terras raras leves em relação aos elementos terras raras pesados, com razões LaN/YbN entre 16,10 e 54,75, e anomalias negativas de Eu (Eu/Eu* de 0,66 a 0,90) compatíveis com fracionamento de plagioclásio. Dados referentes à temperatura de saturação de zircão somados a diagramas discriminantes geoquímicos permitem associar essas rochas a granitos tipo I altamente fracionados. O conjunto de dados permite correlacionar o magmatismo no contexto de granitoides calcioalcalinos de alto potássio, tardi a pós-tectônicos do Domínio Rio Piranhas-Seridó.

U-Pb zircon dating, geochemistry and Sr-Nd-Pb isotopic ratios from Azna-Dorud Cadomian metagranites, Sanandaj-Sirjan Zone of western Iran

An integrated study of U–Pb zircon dating, whole-rock geochemistry and Sr-Nd-Pb isotopes was accomplished on metagranites from the Azna-Dorud region, Sanandaj-Sirjan Zone (SaSZ) of Iran. U–Pb zircon dating display that the crystallization of the protolith was occurred at age of 525.6 ± 4 Ma (Early Cambrian). The new dating reveal that Iranian basement rocks with ages Late Neoproterozoic-Early Cambrian from SaSZ are similar to the Central Iran region. Zircons from the rocks show oscillatory zoning and high Th/U ratios (0.36–1.23) that imply their magmatic source. The zircons are considered as late magmatic phase on the basis of the geochemistry of their trace elements. The petrographic investigations show the mineral assemblage mainly consisting of quartz, plagioclase, alkali feldspar (microcline) and biotite, as well as accessory minerals of titanite, allanite, epidote and zircon. Based on the chemical composition, the rocks have high SiO2 (71–75 wt%), total alkali contents (Na2O = 5.77–6.22%; K2O = 0.34–0.39 wt%) and low CaO and Al2O3 and are classified as metaluminous to slightly peraluminous (A/CNK = 0.95–1.01). The metagranites are depleted in Sr, Ba, P and Ti and enriched in Th and Ce relative to primitive mantle. These characteristics together with various geochemical discriminant diagrams offer a sodic A2-type nature for them. Initial 87Sr/86Sr and 143Nd/144Nd ratios vary from 0.7049 to 0.7121 and from 0.51189 to 0.51190, respectively. Also, the εNd(t) values of the metagranites vary from −1.2 to −1.5. The lead isotopic ratios are as (206Pb/204Pb)t = 18.135–18.596, (207Pb/204Pb)t = 15.628–15.655 and (208Pb/204Pb)t = 37.717–37.939. As a consequence, the Sr-Nd-Pb isotopic evidence reveals the Cadomian metagranites of the Azna-Dorud region generated from partial melting of a metabasite (i.e. amphibolite), which had its source in an enriched metasomatic mantle.

High-pressure granulite-facies metamorphism in central Dronning Maud Land (East Antarctica): Implications for Gondwana assembly

Central Dronning Maud Land (DML; East Antarctica) is located in a key region of the Gondwana supercontinent. The Conradgebirge area (central DML) consists of orthogneisses, derived from both volcanic and plutonic protoliths, and minor metasedimentary rocks, intruded by Cambrian syn- to post-metamorphic plutons and dykes.Mafic-ultramafic boudins in the metavolcanic and metaplutonic gneisses from Conradgebirge consist of amphibolites and high-grade garnet-bearing pyroxene- and amphibole-rich granofels. They occur either as discontinuous levels or as pods boudinaged within highly-strained and strongly-migmatized gneisses. Bulk-rock major and trace-element compositions, together with geochemical discriminant diagrams (e.g., Th/Yb versus Ta/Yb and V versus Ti), suggest derivation from enriched mantle source for the mafic rocks boudinaged in metaplutonic gneisses, whereas a calc-alkaline signature is common for the mafic boudins in metavolcanic rocks. The microstructural study and P–T modelling of an ultramafic metagabbroic rock reveal a prograde metamorphic evolution from amphibolite-facies (ca. 0.5GPa; 500°C) up to high-P granulite-facies conditions (ca. 1.5–1.7GPa; 960–970°C). Partial melting is testified by “nanogranitoid” inclusions enclosed in garnet. An almost isothermal decompression down to ca. 0.4GPa and 750–850°C produced well-developed An+Opx-bearing symplectites around garnet. A final isobaric cooling at nearly 0.4GPa is testified by Grt coronas around high-T symplectites.The above reconstruction traces a clockwise loading-heating P-T evolution with a peak metamorphism at high-P granulite-facies conditions suggesting crustal thickening at nearly 570Ma, followed by a tectonically assisted rapid exhumation, and then, by an isobaric cooling. 40Ar-39Ar dating of amphibole and biotite at ~505–480Ma testify mineral re-equilibration at upper crustal level (T<650°C) during the isobaric cooling.This tectono-metamorphic scenario seems representative of the evolution resulting from the Neoproterozoic/Early Palaeozoic (600–500Ma) collision between parts of East- and West-Gondwana blocks that led to the final assembly of Gondwana.

Changing tectonic settings through time: Indiscriminate use of geochemical discriminant diagrams

No one method should be used to identify ancient tectonic settings, and this is especially true for rocks older than 2.5Ga, when Earth was changing from stagnant lid to plate tectonics. The most useful application of geochemical discriminant diagrams should be to constrain mantle source compositions, mantle heterogeneity, the effects of degree of melting, mixing of melts, and crustal contamination. It is only the convergence of evidence from greenstones that includes rock association, stratigraphic variation, sediment types and provenances, information from syntectonic granitoids, and geochemistry of all rock types that we can make progress in understanding evolving tectonic settings on planet Earth.

Definição de suítes magmáticas em corpos ediacaranos no extremo NE da Província Borborema (estado do Rio Grande do Norte): Plútons Pitombeira, Taipu e Gameleira

The Pitombeira, Taipu and Gameleira plutons, located in the northeasternmost portion of the Sao Jose do Campestre Domain within the Borborema Province (NE, Brazil), are subjects of lithogeochemical studies in the present work. The Pitombeira Pluton comprises a monzo- to syenogranitic facies defined by K-feldspar phenocrysts, and a quartz-dioritic to tonalitic facies, mainly as enclaves, with laths of plagioclase showing compositional zoning and chilled rims. The Taipu Pluton is made of equigranular, locally porphyritic, monzo- to syenogranites, whereas the Gamaleira Pluton encompasses tonalitic and granodioritic rocks with porphyritic textures and compositionally zoned plagioclase crystals. Geochemical discriminant diagrams reveal that the rocks forming the Gameleira Pluton are metaluminous, with a calc-alkaline signature and magnesian character, contrasting with the subalkaline (high-K calc-alkaline) signature and ferroan character of the Taipu and Pitombeira Plutons, where the former is highly peraluminous and the latter is metaluminous to slightly peraluminous. Rare Earth Elements patterns are similar to all plutons and register negative Eu anomalies and a relative enrichment of LREE over HREE, with LaN/YbN ratios between 9.38 to 16.20 (Gamaleira Pluton), 17.99 to 31.39 (granitic facies of the Pitombeira Pluton), 14.15 to 21.81 (dioritic facies of the Pitombeira Pluton) and 15.17 to 175.41 (Taipu Pluton). Based on the combined investigation of structural, textural and geochemical data, a late- to post-collisional tectonic environment is suggested for the plutons here studied.

Continental origin of eclogites in the North Qinling terrane and its tectonic implications

Eclogites in an orogen, serving as a credible marker for deep subduction of oceanic or continental crust, are a key to decipher the process and fate of plate subduction. The nature of eclogite precursors bears important information on orogenic mechanism and pre-subducted geological history. Despite their importance, the nature of ultra-high pressure (UHP) eclogites in the North Qinling (NQ) terrane is still poorly constrained, which leads to conflicting tectonic scenarios for the evolution of the Qinling orogenic belt. In this paper, we provide a combined study of zircon U–Pb ages, major and trace element data, and Sr–Nd–Pb–Hf isotope compositions for eclogites from the NQ terrane. U–Pb age determinations on magmatic zircon cores of two eclogite samples yield similar ages of ca. 800 Ma, indicating that the protoliths of the NQ UHP eclogites formed in the Neoproterozoic. The NQ eclogites have tholeiitic compositions and are characterized by high TiO2 (1.40–2.40%), and low P2O5 (0.09–0.22%), Na2O (1.10–2.49%) and K2O (0.17–1.04%) contents. They are moderate fractionation in REEs (LaN/YbN=2.31–4.20) and show negligible Nb, Ta, Ti, and Eu anomalies (Eu/Eu*=0.82–1.10). The eclogites have high positive whole-rock ɛNd(t) (2.81–5.53) and zircon ɛHf(t) (10.5–11.9) values, indicating their derivation from a relatively depleted mantle source. On geochemical discriminant diagrams based on immobile element ratios, the NQ eclogites fall in the field of continental basalts. Moreover, both the geochronological and geochemical features of the eclogite protoliths are quite different from those of the Erlanpging basaltic rocks, but comparable to those of the coeval continental basalts in South Qinling (SQ) orogen and the northern margin of the South China Block (SCB). As the mid-Neoproterozoic basaltic rocks in the SQ orogen and the northern margin of the SCB were well documented to form in a continental rift setting in response to the global-scale breakup of the Rodinia supercontinent, it is inferred that the NQ eclogite precursors might also form in a continental rift setting at ca. 800Ma, which resulted in the splitting of the NQ terrane from the SCB. As the protoliths of the NQ eclogites have a continental property, we suggest that the ca. 490Ma UHP metamorphism of the NQ unit might result from its northward subduction beneath the Erlangping arc and thus manifested an arc-continent collision process.

Petrogenesis of post-collisional A-type granitoids from the Urumieh–Dokhtar magmatic assemblage, Southwestern Kerman, Iran: Constraints on the Arabian–Eurasian continental collision

Three plutons (Deh-Siahan, Bande-Bagh and Baghe-Khoshk Sharghi, collectively referred to as the DBB hereafter) in southwestern Kerman, in the southeastern part of the Urumieh–Dokhtar magmatic assemblage (UDMA) of the Zagros orogenic belt differ from the typical calc-alkaline metaluminous, I-type intrusions of the region. The DBB intrusions have a distinct lithological assemblage varying from diorite through monzogranite and monzonite to alkali feldspar syenite and alkali granite. The DBB granitoids are metaluminous to slightly peraluminous, alkaline to shoshonitic in composition and have high total alkali contents with K 2O > Na 2O, high FeO T /MgO values, and low CaO and MgO contents. They are enriched in some LILEs (such as Rb and Th) and HFSEs (such as Zr, Y and REEs except Eu) and depleted in Sr and Ba relative to primordial mantle, and have low concentrations of transitional metals. These features along with various geochemical discriminant diagrams suggest that the DBB granitoids are post-collisional A-type granitoids, which had not been recognized previously in the UDMA. The chondrite-normalized REE patterns of the DBB granitoids show slightly enriched light REEs [(La/Sm) N = 2.26–4.13], negative Eu anomalies [(Eu/Eu*) N = 0.19–0.74] and flat heavy REE patterns [(Gd/Yb) N = 0.80–1.87]. The negative Eu anomaly indicates an important role for plagioclase and/or K-feldspar during fractional crystallization. Whole-rock Rb–Sr isotope analysis yields an isochron age of 33 ± 1 Ma with an initial 87Sr/ 86Sr value of 0.7049 ± 0.0001. Whole-rock Sm–Nd isotope analysis gives ε Nd t values from + 2.56 to + 3.62 at 33 Ma. The positive ε Nd t and low I Sr values of the DBB granitoids together with their T DM of 0.6–0.7 Ga suggest their formation from partial melting of a lithospheric mantle source, modified by fluids or melts from earlier subduction processes. Melting of lithospheric mantle occurred via a dehydration melting process at pressures below the garnet stability field, as a consequence of lithospheric mantle delamination or break-off of a subducted slab and melting of the lithospheric mantle by upwelling of hot asthenosphere. On the basis of Rb/Sr age dating and the post-collisional geochemical signatures of the DBB granitoids, along with extensive pre-collisional volcanic eruptions in Middle Eocene, we suggest Late Eocene for the time of collision between the Arabian and Central Iranian plates. This also implies that the calc-alkaline I-type intrusions in the southwestern Kerman and in other parts of the UDMA may have formed in a post-collisional context.

GEOCHEMICAL SIGNATURE OF THE BRASILIANOAGE PLUTONISM IN THE SERIDÓ BELT, NORTHEASTERN BORBOREMA PROVINCE (NE BRAZIL)

The Seridó Belt (Borborema Province, NE Brazil) displays a strong imprint of the late Neoproterozoic Brasiliano orogenic cycle. The most important megascopic structures in the Seridó Belt are strike-slip shear zones, coeval with the emplacement of several granitoid bodies. The paper is based on 217 chemical analyses of the granitoid plutons grouped into five different geochemical suites (Shos - Shoshonitic, PKCAlk - Porphyritic K-Calc-Alkaline, EqKCAlk - Equigranular K-Calc-Alkaline, Alk - Alkaline and ChAlk - Charnockitic Alkaline). They are distinguished by petrographic and textural features, as well as Harker’s and geochemical discriminant diagrams. The Shosis easily distinguished from the others by its low silica content, while the Alk and ChAlk suites have high alkali contents. The PKCAlk and EqKCAlk show similar geochemical behaviour; but they can be separated mainly by petrographic and textural aspects. Field relationships and geochemical affinities suggested that the Shos, PKCAlk, EqKCAlk and Alk are approximately coeval, syntectonic plutons, whereas the ChAlk is interpreted as lateto post-tectonic as regards to the Brasiliano orogeny.

GEOCHEMICAL SIGNATURE OF THE BRASILIANOAGE PLUTONISM IN THE SERIDÓ BELT, NORTHEASTERN BORBOREMA PROVINCE (NE BRAZIL)

The Serido Belt (Borborema Province, NE Brazil) displays a strong imprint of the late Neoproterozoic Brasiliano erogenic cycle. The most important megascopic structures in the Serido Belt are strike-slip shear zones, coeval with the emplacement of several granitoid bodies. The paper is based on 217 chemical analyses of the granitoid plutons grouped into five different geochemical suites ( Shos - Shoshonitic, PKCAlk - Porphyritic K-Calc-Alkaline, EqKCAlk - Equigranular K -Calc-Alkaline, Alk - Alkaline and ChAlk - Charnockitic Alkaline). They are distinguished by petrographic and textural features, as well as Marker's and geochemical discriminant diagrams. The Shos is easily distinguished from the others by its low silica content, while the Alk and ChAlk suites have high alkali contents. The PKCAlk and EqKCAlk show similar geochemical behaviour; but they can be separated mainly by petrographic and textural aspects. Field relationships and geochemical affinities suggested that the Shos, PKCAlk, EqKCAlk and Alk are approximately coeval, syntectonic plutons, whereas the ChAlk is interpreted as lateto post-tectonic as regards to the Brasiliano orogeny.

Geochemical Types, Petrology, and Genesis of Late Cenozoic Volcanic Rocks from the Kurile-Kamchatka Island-Arc System

The Kurile-Kamchatka Late Cenozoic volcanic rocks can be categorized as magmatic series of island arc and within-plate geochemical types, respectively. These types clearly can be identified on geochemical discriminant diagrams. Across-arc geochemical, mineralogical, and Sr-Nd-isotope zoning is characteristic of island-arc volcanics, but is not found for within-plate volcanics. Such zoning probably is the result of modification of fluid-phase composition, which originates in the underlying subducting plate (as a result of dehydration of water-bearing secondary minerals) and rises into the zone of island-arc magma generation in the mantle wedge. Along-arc Sr, Be, H., and 0 isotopic zoning is noted for island-arc volcanics. Probably it is associated with various levels of island-arc magma contamination. 10Be data for modern island-arc lavas attest to the fact that young pelagic sediments from the subducting plate participate in island-arc magma genesis. Within-plate lavas are found in the region north of Ava...

Geochemistry and tectonic discrimination of Late Proterozoic arc-related volcaniclastic turbidite sequences, Antigonish Highlands, Nova Scotia

The Late Proterozoic (ca. 618–610 Ma) Georgeville Group of northern mainland Nova Scotia lies within the Avalon Composite Terrane and consists of subgreenschist- to greenschist-facies mafic and felsic volcanic rocks overlain by volcaniclastic turbidites that were deposited in an ensialic basin within a rifted volcanic arc. Geochronological data indicate that the volcanic and sedimentary rocks are coeval. The geochemical and isotopic signatures of the sedimentary rocks are attributed to erosion of the coeval Avalonian volcanic rocks that flank the basin and are consistent with synorogenic deposition. There is no evidence of significant chemical contribution from Avalonian basement.Knowledge of the tectonic setting facilitates the testing of published geochemical discriminant diagrams for clastic sedimentary rocks. Discrimination diagrams using ratios such as K2O/Na2O and Al2O3/(CaO + Na2O) give inconclusive results, probably due to elemental mobility during secondary processes. Plots involving MgO, TiO2, and Fe2O3 detect the chemical contribution of mafic detritus, give much tighter clusters of data, and plot between Aleutian- and Cascade-type arc-derived sediments, suggesting a moderate thickness of continental crust beneath the arc.The arc-related signature of the Georgeville sedimentary rocks is clearly recognizable on ternary plots involving inter-element ratios of high field strength elements (e.g., Ti–Y–Zr, Nb–Y–Zr, and Hf–Ta–Th) in which the samples plot as mixing trends between mafic and felsic end members. Diagrams of this type may have widespread application to tectonic discrimination of sedimentary rocks because in most suites these ratios are relatively insensitive to sedimentary and metamorphic processes.

A tectonics test of the most commonly used geochemical discriminant diagrams and patterns

During the last two decades many tectonomagmatic geochemical discriminant diagrams were created and are now being widely used in paleotectonic reconstructions. The sources of original data and discriminating quality of fourteen of the most commonly used diagrams are examined herein. Over 200 Jurassic rift basalts, diabases and chilled diabase margins from eastern North America are plotted on these diagrams. The inconsistent results from other regions with known tectonic settings are also discussed. It is found that most diagrams fail to identify continental basalts (CB), the majority of which plot in every field except within-plate basalts (WPB). Because continental basalts do plot in other fields, such as MORB and arc basalts, the discriminating ability of these diagrams is insufficient without additional criteria. Large amounts of chemical data (especially trace- and rare-earth element data) and intensive studies of continental basalts were not available until the late 1980's. Lack of representative data at the time these diagrams were created is a major reason for their failure to identify continental basalts. Moreover, the genesis of continental basalt magma involves a series of complicated processes that may vary from place to place even within the same tectonic province. As a result, the geochemical patterns of continental basalts cover a wide range in compositions of isotopes, trace elements, REE, as well as major and minor elements. It is, therefore, difficult to create a universal chemical diagram that can discriminate continental basalts from all others. Our conclusion is that the reconstruction of paleotectonic settings should not be done by using geochemical data alone. Although this may seem obvious to many, perusal of the literature shows that tectonic setting assignments frequently are based almost entirely on geochemistry. Tectono-stratigraphic analysis based on knowledge of field relations, structure and petrology of epiclastic, volcaniclastic and igneous protoliths is an essential adjunct to geochemistry in determining ancient tectonic environments.

The Karoo igneous province — A problem area for inferring tectonic setting from basalt geochemistry

Tholeiitic basalts and associated intrusives are the major component of the Karoo igneous province. They are of Mesozoic age and constitute one of the world's classic continental flood basalt (CFB) provinces. It has been argued that most Karoo basalts have not undergone significant contamination with continental crust and that their lithospheric mantle source areas were enriched in incompatible minor and trace elements during the Proterozoic. The only exceptions to this are late-stage MORB-like dolerites near the present-day continental margins which are considered to be of asthenospheric origin. When data for the “southern” Karoo basalts are plotted on many of the geochemical discriminant diagrams which have been used to infer tectonic setting, essentially all of them would be classified as calc-alkali basalts (CAB's) or low-K tholeiites. Virtually none of them plot in the compositional fields designated as characteristic of “within-plate” basalts. There is little likelihood that the compositions of the Karoo basalts can be controlled by active subduction at the time of their eruption and no convincing evidence that a “subduction component” has been added to the subcontinental lithospheric mantle under the entire area in which the basalts crop out. It must be concluded that the mantle source areas for CAB's and the southern Karoo basalts have marked similarities. In contrast, the data for “northern” Karoo basalts largely plot in the “within-plate” field on geochemical discriminant diagrams. Available data suggest that the source composition and/or the restite mineralogy and degree of partial melting are different for southern and northern Karoo basalts. There is no evidence for any difference in tectonic setting between the southern and northern Karoo basalts at the time they were erupted. This appears to be clear evidence that specific mantle source characteristics and/or magmatic processes can vary within a single CFB province to an extent that renders at least some geochemical discriminant diagrams most unreliable for classifying tectonic environment with respect to continental volcanic rocks.