Fractionation Of Clinopyroxene Research Articles

Petrology and geochronology of a Neoproterozoic dyke swarm from Marbat, South Oman

A suite of NW–SE trending tholeiitic dykes in the Marbat area of south Oman was emplaced into calc-alkaline gneisses and intrusives during the Neoproterozoic. A three point Sm/Nd isochron from the most primitive sample and an Rb/Sr whole rock isochron together with geological evidence, suggest an intrusion age of about 700 Ma. The dykes form a co-genetic suite ranging in composition from basalt to rhyolite. The most primitive rock has an Mg # of 67.48, a Ni content of 268 ppm and Cr of 571 ppm. All rocks in the suite are quartz normative. Initial 143Nd/ 144Nd and 87Sr/ 86Sr isotope ratios suggest some interaction between the magmas and the continental crust during ascent. Magmatic evolution was controlled by AFC processes involving fractionation of hornblende, clinopyroxene, plagioclase and magnetite. Trace element variation diagrams show negative spikes for Rb, Ta, Nb, Sr and Ti. Nb depletion results in high Th/Nb (0.9) and La/Nb ratios (3.8). Comparison of the Marbat dykes with Neoproterozoic dykes from SW Egypt, Sinai, volcanic rocks from the Andean active continental margin and the Basin and Range Province, a low-Nb dykes suite from New South Wales, Australia and a suite from the Congo shows considerable compositional overlap. These comparisons suggests that the dykes were formed by melting of subduction modified lithospheric mantle and mobilised by crustal extension. The ubiquity of dykes in the ANS showing similar signatures suggests that such modified mantle is regionally developed and results from enrichment is a supra-subduction setting during the Neoproterozoic accretionary phase of the ANS. The mantle shows compositional similarities to lithospheric mantle underlying the western USA. The calc-alkaline geochemistry of the basement gneisses and intrusive suite into which the Marbat were intruded suggest analogy with the Al-Bayda island arc terrane in Yemen. Fracture pattern analysis from Marbat suggests that intrusion of the major swarm may be related to sinistral strike-slip faulting caused by regional east–west compression. A similar stress regime can also explain the orientation of the Mukeras dyke swarms of the Al-Bayda terrane. Additional geochronological data from Marbat and the Hallaniyat Islands imply the existence of a second phase of dyke intrusion at ∼600 Ma which is again similar to the situation prevailing in the Al-Bayda terrane.

Petrology and geochemistry of monchiquites from Tchircotch� (Garoua rift, north Cameroon, Central Africa)

Summary Dykes of Cenozoic age (37.5 � 2.3 Ma) crop out in the Tchircotcharea (Garoua rift, north Cameroon). They consist of a lamprophyric (monchiquite) series with diopside, subsilicic kaersutite and apatite phenocrysts, Ba-Ti-rich biotite microphenocrysts and Cr-diopside xenocrysts scattered in a matrix of analcitic composition containing oligoclase, albite and sanidine microlites and carbonate ocelli. Major and trace element distributions are interpreted in terms of crystal fractionation of olivine, clinopyroxene, amphibole and Fe-Ti oxides. The Tchircotchmonchiquites show a relatively restricted range of initial 87 Sr= 86 Sr ratio (0.70366-0.70387), of "Nd values (þ2.5 to þ2.7), and rare earth element patterns similar to those of the least differentiated basalts of the Cameroon Line. This supports a common magma source region. This mantle source is infra-lithospheric and is strongly enriched in incompatible elements (light REE, Zr, Sr, Ba) probably transported by volatile- and halogen-rich fluids. The monchiquites appear to be derived by low degrees of partial melting as attested by steep REE patterns and high contents of other incompatible elements, suggesting the presence of residual garnet in the source. Several lines of evidence support the occurrence of phlogopite in the source region of Tchircotchamprophyre magma.

Chemical evolution, petrogenesis, and regional chemical correlations of the flood basalt sequence in the central Deccan Traps, India

The lava sequence of the central-western Deccan Traps (from Jalgaon towards Mumbai) is formed by basalts and basaltic andesites having a significant variation in TiO2 (from 1.2 to 3.3 wt%), Zr (from 84 to 253 ppm), Nb (from 5 to 16ppm) and Ba (from 63 to 407 ppm), at MgO ranging from 10 to 4.2 wt%. Most of these basalts follow a liquid line of descent dominated by low pressure fractionation of clinopyroxene, plagioclase and olivine, starting from the most mafic compositions, in a temperature range from 1220° to 1125°C. These rocks resemble those belonging to the lower-most formations of the Deccan Traps in the Western Ghats (Jawhar, Igatpuri and Thakurvadi) as well as those of the Poladpur formation. Samples analyzed for87Sr/86Sr give a range of initial ratios from 0.70558 to 0.70621. A group of flows of the Dhule area has low TiO2 (1.2–1.5 wt%) and Zr (84–105 ppm) at moderate MgO (5.2–6.2 wt%), matching the composition of low-Ti basalts of Gujarat, low-Ti dykes of the Tapti swarm and Toranmal basalts, just north of the study area. This allows chemical correlations between the lavas of central Deccan, the Tapti dykes and the north-western outcrops. The mildly enriched high field strength element contents of the samples with TiO2 > 1.5 wt% make them products of mantle sources broadly similar to those which generated the Ambenali basalts, but their high La/Nb and Ba/Nb, negative Nb anomalies in the mantle normalized diagrams, and relatively high87Sr/86Sr, make evident a crustal input with crustally derived materials at less differentiated stages than those represented in this sample set, or even within the sub-Indian lithospheric mantle.

Petrology and Geochemistry of Intraplate Basalts in the South Auckland Volcanic Field, New Zealand: Evidence for Two Coeval Magma Suites from Distinct Sources

The South Auckland Volcanic Field is a Pleistocene (1·59–0·51 Ma) basaltic intraplate, monogenetic field situated south of Auckland City, North Island, New Zealand. Two groups of basalts are distinguished based on mineralogy and geochemical compositions, but no temporal or spatial patterns exist in the distribution of various lava types forming each group within the field: Group A basalts are silica-undersaturated transitional to quartz-tholeiitic basalts with relatively low total alkalis (3·0–4·6 wt %), Nb (7–29 ppm), and (La/Yb)N (3·4–7·6); Group B basalts are strongly silica-undersaturated basanites to nepheline-hawaiites with high total alkalis (3·3–7·9 wt %), Nb (32–102 ppm), and (La/Yb)N (12–47). Group A has slightly higher 87Sr/86Sr, similar εNd, and lower 206Pb/204Pb values compared with Group B. Contrasting geochemical trends and incompatible element ratios (e.g. K/Nb, Zr/Nb, Ce/Pb) are consistent with separate evolution of Groups A and B from dissimilar parental magmas derived from distinct sub-continental lithospheric mantle sources. Differentiation within each group was controlled by olivine and clinopyroxene fractionation. Group B magmas were generated by <8% melting of an ocean island basalt (OIB)-like garnet peridotite source with high 238U/204Pb mantle (HIMU) and enriched mantle (EMII) characteristics possibly inherited from recycled oceanic crust. Group A magmas were generated by <12% melting of a spinel peridotite source also with HIMU and EMII signatures. This source type may have resulted from subduction-related metasomatism of the sub-continental lithosphere modified by a HIMU plume. These events were associated with Mesozoic or earlier subduction- and plume-related magmatism when New Zealand was at the eastern margin of the Gondwana supercontinent.

Petrogenesis of post-orogenic syenites in the Sulu Orogenic Belt, East China: geochronological, geochemical and Nd–Sr isotopic evidence

The Jiazishan alkaline complex in the eastern Sulu ultrahigh pressure (UHP) metamorphic orogenic belt of eastern China is composed of potassic to ultrapotassic pyroxene syenite, quartz syenite and associated mafic dikes. A SHRIMP zircon 206Pb/ 238U age of 215±5 Ma was obtained for the quartz syenite and mineral 40Ar/ 39Ar dating gave emplacement ages of 214.4±0.3 and 214.6±0.6 Ma for the pyroxene syenite and 200.6±0.2 Ma for the mafic dike. These dates establish that the Jiazishan Complex was emplaced shortly after the UHP metamorphic event at 240 to 220 Ma due to the continental collision between the North China and Yangtze cratons. The ultrapotassic mafic dikes, with K 2O≈4.4–6.4 wt.% and K 2O/Na 2O≈3.5, have high MgO (8.06–12.44 wt.%), Ni (119–319 ppm) and Cr (477–873 ppm) and moderately low CaO/Al 2O 3 (∼0.76) and TiO 2 (∼1.12 wt.%). They also have high Sr ( 87Sr/ 86Sr∼0.7073), low Nd ( ɛ Nd=∼−16.5) isotopic ratios, enriched LILE (Ba/La=66–74), LREE [(La/Yb) N =28–33] and depleted HFSE (La/Nb=4–6). It appears that the mafic dikes were derived from a refractory, re-enriched lithospheric mantle source. The syenites have Sr and Nd isotopic compositions similar to the mafic dikes, implying a common origin. Geochemical and isotopic modeling suggests that the pyroxene syenites may have been generated by early fractionation of clinopyroxene and olivine, coupled with minor amounts of crustal contamination, of a mafic magma that had a similar composition to the mafic dikes. Subsequent fractionation of feldspar-dominated assemblages, with minor or no contamination, would result in the quartz syenites. This post-orogenic magmatism, resulting most likely in an extensional setting, provides time constraints on the major geodynamic transition from convergence to extension at the eastern margin of the North China craton. The Jiazishan potassic magmatism and geodynamic transition from convergence to extension can be explained by convective removal of the lower lithospheric mantle.

Pervasive assimilation of carbonate and silicate rocks in the Hortavær igneous complex, north-central Norway

The intrusive complex at Hortavær represents a magma transfer zone in which multiple pulses of gabbroic and dioritic magmas evolved along Fe- and alkali-enrichment trends. Extreme alkali enrichment resulted in nepheline-normative and sparse nepheline-bearing monzodioritic and monzonitic rocks. More evolved monzonitic and syenitic rocks are silica saturated and, in some cases, quartz bearing. Previous and current research recognized an abundance of clinopyroxene and other Ca-rich phases, such as scapolite, grossular-rich garnet, and igneous-textured calcite among the mafic and intermediate rocks. Even the most pyroxene-rich samples contain low Sc concentrations, which suggests early, intense fractionation of clinopyroxene. These features and the alkali enrichment are consistent with assimilation of carbonate-rich host rocks. Carbon isotope ratios of the igneous-textured calcite indicate an origin of the carbon from host rocks rich in calcite, consistent with assimilation. However, low ɛ Nd values (−3.4 to −10.2) and moderate initial 87Sr/ 86Sr values (0.7052 to 0.7099) indicate the need for assimilation of quartzofeldspathic rocks as well. Models of combined assimilation and fractional crystallization indicate that assimilation of simple end members, either carbonate or silicate, cannot explain the entire data set. Instead, variable proportions of carbonate and silicate materials were assimilated, with the most pronounced assimilation effects in the mafic rocks. The reasons for variable degrees of assimilation are, as yet, uncertain. It is possible that assimilation of calc-silicate rocks with variable carbonate/silicate proportions resulted in the range of observed compositions. However, the importance of carbonate assimilation in mafic rocks compared to felsic ones suggests that assimilation of carbonates was predominant at high temperature and/or mafic magma compositions and assimilation of silicates was predominant at lower temperature and/or felsic magma compositions. We suggest that the ability of the mafic magma to dissolve higher proportions of carbonate contaminants is the result of the magma's ability to form clinopyroxene as a product of assimilation. In any case, extensive carbonate assimilation was possible because CO 2 escaped from the system.

Geochemistry and petrogenesis of mafic magmatic rocks of the Jharol Belt, India: geodynamic implication

The belt consisting of deep-water meta-sediments of the Aravalli Supergroup hosts numerous mafic and ultramafic rocks occurring at two different structural levels. These are the basal Bagdunda volcanics and the Gopir magmatic rocks represented by Gopir dikes and flows associated with the ultramafic rocks along the Kaliguman lineament. The Bagdunda and Gopir mafic volcanic rocks are sub-alkaline, varying in composition from high Mg-tholeiite to basaltic komatiite. They are predominantly LREE depleted, but some flat REE patterns are also observed. Bulk geochemical data, especially the incompatible trace elements discount the possibility of crustal contamination. Gopir dikes are highly enriched in terms of their REE [(Ce/Sm) N and (Ce/Yb) N ] and other incompatible trace elements and closely resemble the basal Aravalli volcanics. Their ratio (rock)/ratio (PM) and normalized multi-element abundance patterns depict their overall enriched nature compared to N-MORB and primitive mantle. The Gopir and Bagdunda volcanics reflect trace element characteristics transitional between E-MORB and OIB. Gopir dikes show remarkable similarity with continental tholeiite and Continental Flood Basalts (CFBs) with negative Nb, P and Ti anomalies and low Nb/Ce ratio unlike the volcanics of both the suites. Petrogenetic modeling based on the compositionally corrected [Mg] and [Fe] abundances for Gopir dikes, Gopir and Bagdunda volcanics indicates (a) their derivation from non-pyrolitic sources (b) their derivation from sources that were variably enriched in [Fe/Mg] ratios with large variation in their [Fe] contents and (c) olivine was a major phase to fractionate, followed by a lesser amount of clinopyroxene in the case of Gopir volcanics, whereas in Bagdunda volcanics a combination of olivine and clinopyroxene fractionation is suggested. Based on the geochemical data, supported by field evidence, we propose a geodynamic model for the development of the Jharol Belt in which we suggest that the basement rock, i.e. the Banded Gneissic Complex (BGC) started rifting probably under influence of a mantle plume during the late Archaean–early Proterozoic period. During the opening of the basin, magma derived from asthenospheric mantle reached the surface contemporaneously with sedimentation. The first phase of volcanism is represented by Bagdunda volcanics. With continuous rifting, the crust became highly attenuated and facilitated asthenospheric upwelling, causing high degrees of melting (indicated by a large volume of mafic and ultramafic rocks) during the second phase of magmatism (Gopir volcanics) that occurred at later stages of Jharol sedimentation. Coeval melting of the sub-continental lithosphere under the adjoining BGC craton probably caused the emplacement of dikes along with the Gopir volcanics. At this stage of progressive rifting, an oceanic crust very similar to that in marginal basins developed in the Jharol Belt.

Geochemistry of metavolcanics from the Neoproterozoic Tuludimtu orogenic belt, western Ethiopia

The 200 km wide Tuludimtu Belt of Western Ethiopia is one of a series of N-S trending Neoproterozoic orogenic belts of the East African Orogenic Province in southeastern Sudan and western and southern Ethiopia. The Tuludimtu Belt consists of deformed greenschist facies metasediments and metavolcanics, flanked to the east and west by gneissic terranes, all of which are invaded by pre-, syn- and post-tectonic intrusives ranging from ultramafic to felsic in composition. The Tuludimtu Belt has been subdivided into five lithotectonic domains, from east to west, the Didesa, Kemashi, Dengi, Sirkole and Daka Domains. On the basis of lithological associations, the three domains in the core of the belt, the Kemashi, Dengi and Sirkole Domains, are respectively interpreted to represent an ophiolitic terrane of oceanic crustal origin, a volcanic arc, and a fold-thrust terrane composed of interleaved thrust sheets of gneissic basement and cover strata. The Didesa and Daka Domains are composed of moderate- to high-grade gneisses, possibly representing the basement forelands to the belt. Major and trace element geochemistry for metavolcanics of predominantly basaltic and andesitic composition from the Kemashi, Dengi and Didesa Domains, are presented, and a preliminary analysis of the data undertaken. Two suites of metavolcanics are clearly differentiated by the discrimination diagrams. The samples from all three domains are predominantly tholeiitic, subalkaline basalts and andesites. Bivariate plots reveal weak negative correlations of SiO 2 and Al 2O 3 with MgO, and a positive correlation of CaO with MgO, indicating that magmas evolved by fractionation of clinopyroxene, orthopyroxene and olivine from the parent liquids. Trace element spider diagrams show a tight coherence of patterns for the Dengi and Sirkole samples, suggestive that alteration did not accompany metamorphic processes to any great extent, and a slight depletion of HFS elements possibly due to fractionation processes. On the other hand samples from the Kemashi Domain exhibit a wide variation in patterns, with large ranges in concentrations, particularly in the LFS elements, suggestive of significant alteration during metamorphism. Enrichment of Th relative to Nb, and an overall enrichment of LFS elements over HFS elements in the Kemashi samples, are characteristics of ocean floor and back-arc magmas. A Mullen plot shows the Dengi/Sirkole samples plotting predominantly within the calc-alkaline basalt field, whereas the Kemashi samples have a much wider spread, with no interpretation possible. However, a Ti–Zr–Y tectonic discrimination plot shows the Kemashi samples falling into the field of ocean floor basalts, whilst confirming the calc-alkaline basaltic character of the Dengi and Sirkole rocks. Thus the Kemashi Domain volcanics are interpreted to represent oceanic crustal material formed in a spreading environment, whilst the volcanics of the Dengi and Sirkole Domains developed in a convergent setting. Coherence of the Dengi and Sirkole samples suggests that the Sirkole Domain formed by thrusting of the Dengi Volcanic Arc over its foreland leading to imbrication of the Dengi volcanosedimentary sequence with basement gneisses.

Permo-Carboniferous extension-related magmatism at the SW margin of the Fennoscandian Shield

Abstract Permo-Carboniferous rifting in Europe was accompanied by the widespread emplacement of mantle-derived magmas forming regional dyke swarms and sills in northern England, Scotland, Norway and southern Sweden during the late Stephanian and early Autunian. The trends of the dyke swarms intersect at a focal point in the Kattegat south of the Oslo Graben, and are probably all related to a single magmatic centre that could be plume-related. The WNW- to NW-trending dyke swarm at the SW margin of the Fennoscandian Shield in southern Sweden is composed mainly of tholeiitic dolerites, with lesser amounts of alkaline mafic rocks (camptonites, alkali basalts and spessartites) and trachytes. The alkaline mafic rocks are enriched in Ba, Sr, Nb, P and CO 2 , implying a metasomatic enrichment of their upper-mantle source prior to melting. After generation of alkaline melts by relatively small degrees of partial melting, increased extension was accompanied by the formation of subalkaline tholeiitic magmas. Whole-rock compositions (Mg-numbers between 55 and 30) and mineral chemistry (olivine Fo 60 -Fo 40 ; clinopyroxene approximately Wo 30 En 45 Fs 15 ; plagioclase An 70 -An 50 ) indicate relatively evolved melts that have undergone crystal fractionation of olivine and clinopyroxene. Two groups of dolerites can be distinguished on the basis of bivariate element plots, e.g. Zr-TiO 2 and La-Sm. Although both groups show enrichment in the whole range of incompatible trace elements, slight differences in mantle-normalized trace-element patterns and different Nb/La ratios suggest that they were generated from two different sources. Group I dolerites were formed from a (re-)enriched, but isotopically mildly depleted, sublithospheric garnet-bearing mantle source (Nb/La mostly &gt;0.9, ɛNd i = +4 to +3, where ɛNd i is the initial Nd isotope ratio), whereas group II dolerites seem to indicate mixing of the asthenospheric-derived magmas with lithospheric mantle melts (Nb/La mostly &lt;0.9, ɛNd i = 0 to −1). Increasing Th/Ta ratios together with decreasing U/Nb ratios from group I towards group II dolerites further reflect progressive crustal contamination.

The water and trace element contents of melt inclusions across an active subduction zone

Water concentrations of olivine-hosted melt inclusions show no consistent variation across the northern part of the Central American subduction zone in southeastern Guatemala. Magmatic water contents remain moderately high (~2 wt%) throughout the back-arc region. Melt inclusions from some of these back-arc basalts also have notably high CO2 contents (>900 ppm CO2). The B and B/Ce ratios of melt inclusions systematically decline across the arc, the first parameters to exhibit systematic changes across southeastern Guatemala. It appears, therefore, that dehydration-driven, flux-melting persists across the arc, although decompression melting is of approximately equal importance in the back-arc region. Dehydration of the slab/wedge region is regarded as semi-continuous down-dip, to depths of at least 175–200 km. Moderate water contents are maintained by stepwise dehydration reactions, while truly incompatible fluid mobile elements are progressively stripped from the Cocos plate. The notably high CO2 contents of some back-arc basalts may indicate increasing devolatilization of subducted carbonate sediments with slab depth. The moderate H2O contents of back-arc basaltic magmas has likely contributed to their early fractionation of clinopyroxene around the Moho.

Geochemical provinciality in the Cretaceous basaltic magmatism of Northern Madagascar: mantle source implications

The northern part of the Late Cretaceous flood-basalt province of Madagascar is formed by lava flows, dykes and intrusive complexes. These range from tholeiitic to weakly alkaline and have composition from basalts to basaltic andesites (with rare, more evolved, rocks). The magmatic evolution is dominated by low-pressure crystal fractionation of olivine, Cr-spinel, plagioclase and clinopyroxene. The erupted rocks form two subprovinces, in terms of incompatible element concentrations or ratios and geographical position. The mafic and intermediate samples from the western subprovince have low abundances of high field strength elements (e.g. Nb 2–9 ppm; Zr 50–230 ppm), negative Nb anomalies on mantle-normalized diagrams, and a wide range of initial (88 Ma) 143 Nd/ 144 Nd (0.51293–0.51197; ε Nd +7.9 to −10.9). These elemental and isotopic variations are compatible with melting of predominantly N-MORB-like mantle and variable low-pressure crustal contamination. The mafic samples of the eastern subprovince are mildly enriched in incompatible elements (e.g. Nb 6–25 ppm; Zr 109–330 ppm), with the highest values found in tholeiites from the east Mahajanga basin, Tampoketsa Kamoreen and Tamatave. They also have a comparatively narrow range of initial 143 Nd/ 144 Nd (0.51276–0.51258; ε Nd +5.1 to +1.0), consistent with derivation from incompatible element enriched mantle sources and minor subsequent crustal contamination. The marked differences in the chemical and isotopic characteristics of the samples imply variable degrees of partial melting of hotter than normal, chemically heterogeneous, mantle sources at pressures where spinel to garnet are residual phases. No clear evidence is seen for a chemical component similar to that represented at present by the Marion hotspot lavas.

Les monchiquites de Tchircotché, vallée de la haute Bénoué (Nord du Cameroun)

Des filons de monchiquites d'âge Cénozoı̈que (37,5±2,3 Ma) recoupent les grès crétacés à Tchircotché dans la vallée de la haute Bénoué (Nord du Cameroun). La différenciation de la série des monchiquites s'explique par la cristallisation fractionnée d'olivine, de clinopyroxène, de magnétite, d'ilménite et d'apatite. Une roche étudiée possède une faible valeur de rapport isotopique initial du strontium (0,70369±10), semblable à celles des basaltes de la Ligne du Cameroun. La croûte continentale ne semble pas avoir joué de rôle significatif dans la genèse des monchiquites. Dykes of monchiquites of Cainozoic age (37.5±2.3 Ma) are intrusive in Cretaceous sandstones in the Tchircotché area, Upper Benue valley (northern Cameroon). The differentiation of the monchiquites is explained by crystal fractionation of olivine, clinopyroxene, magnetite, ilmenite and apatite. A studied rock has low Sr-isotope ratio (0.70369±10), which is similar to those of the alkali basalts of the Cameroon Line. Therefore the continental crust appears to have no significant role in the genesis of the monchiquites.

The geochemistry and significance of sills within the Ordovician Borrowdale Volcanic Group around Black Combe, SW English Lake District

SUMMARY The Borrowdale Volcanic Group (BVG) in the SW of the English Lake District (SWLD) consists predominantly of subaerial pyroclastic rocks that host syn-volcanic intrusions ranging in composition from basalt to dacite. The petrology and geochemistry of the sills verify a broad compatibility with the BVG in the central Lake District. The geochemistry indicates a marginal continental arc setting, a petrogenesis involving c. 15% partial melting of a lherzolite mantle source and subsequent fractionation of olivine, ortho- and clinopyroxene, plagioclase and Fe–Ti oxide. Three groups of basaltic sills are recognized in the SWLD, one of which bears a close resemblance, geochemically, to the lower BVG (Birker Fell Formation) basaltic lavas and appears to have been emplaced early in the SWLD succession. The other two groups, which may be cogenetic, appear to be some what geochemically distinct from the first, with trace element and rare-earth characteristics suggesting a source slightly more enriched in incompatible elements. It is suggested that the fault-dominated tectonic setting in the Millom Park area of the SWLD may account for the least-evolved magmas being allowed to migrate directly to a high structural level, accounting for a higher proportion of basic lithologies in the upper BVG succession of the SWLD than is found in other parts of the Lake District.

Basanites related to Late Eocene extension from NE Oman

Six small basanite intrusions were emplaced into the allochthonous Batain Nappes of NE Oman along approximately north–south to NNW–SSE lineaments. They contain olivine, clinopyroxene, plagioclase, analcite, ulvöspinel and apatite ± biotite ± calcite. The basanites are primitive, with Mg numbers (100Mg/(Mg + Fe 2+ )) from 63.18 to 69.37, and are nepheline normative. Compositional variation suggests fractionation of olivine and clinopyroxene. Thermobarometric studies on rare spinel lherzolite xenoliths suggests re-equilibration at shallow depth. 143 Nd/ 144 Nd and 87 Sr/ 86 Sr isotopic data and REE modelling indicate that the magmas were formed by 1–2% melting of an ocean-island basalt (OIB)-like source containing about 3–4% garnet. Deviations from OIB are explained by mobilization of incompatible elements from potassic layers in the lithosphere during ascent. Sedimentological studies and three new whole-rock Ar/Ar ages (39.77 ± 0.26, 37.63 ± 0.46 and 36.51 ± 0.21 Ma, Lutetian to Priabonian time) suggest intrusion during maximum deepening of the Abat Trough. Movements were related to dextral strike-slip movement along major tectonic structures during the initial stages of the opening of the Gulf of Aden. The basanites were formed by decompressional melting of an asthenospheric mantle source facilitated by thinned lithosphere and depression of the solidus by H 2 O- and CO 2 -rich fluids.

The petrogenesis of the Alberta Complex within the Rehoboth Basement Inlier of Namibia

Research Article| June 01, 2002 The petrogenesis of the Alberta Complex within the Rehoboth Basement Inlier of Namibia Thomas Becker; Thomas Becker Geological Survey of Namibia, P.O. Box 2168, Windhoek, 9000, Namibia, e-mail: tom@mme.gov.na Search for other works by this author on: GSW Google Scholar Arnd Brandenburg Arnd Brandenburg 2 DESY Theory Group, D-22603 Hamburg, Germany, e-mail: Arnd.Brandenburg@desy.de Search for other works by this author on: GSW Google Scholar Author and Article Information Thomas Becker Geological Survey of Namibia, P.O. Box 2168, Windhoek, 9000, Namibia, e-mail: tom@mme.gov.na Arnd Brandenburg 2 DESY Theory Group, D-22603 Hamburg, Germany, e-mail: Arnd.Brandenburg@desy.de Publisher: Geological Society of South Africa First Online: 07 Mar 2017 Online ISSN: 1996-8590 Print ISSN: 1012-0750 © 2002 Geological Society of South Africa South African Journal of Geology (2002) 105 (2): 147–162. https://doi.org/10.2113/105.2.147 Article history First Online: 07 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Thomas Becker, Arnd Brandenburg; The petrogenesis of the Alberta Complex within the Rehoboth Basement Inlier of Namibia. South African Journal of Geology 2002;; 105 (2): 147–162. doi: https://doi.org/10.2113/105.2.147 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietySouth African Journal of Geology Search Advanced Search Abstract The Alberta Complex is the most prominent member of a mafic to ultramafic igneous suite within the Rehoboth Basement Inlier/Namibia, being intrusive into sediments and mafic volcanics of the Paleoproterozoic Elim Formations. Subsequent amphibolite grade metamorphism of pre-Kibaran age obscured the original mineral assemblage. Two main units have been distinguished in the past within the Alberta Complex. (1) A thick pile of various amphibolites represents original norite, hyperite, gabbro and anorthosite and constitutes the Layered Sequence. It has been subdivided into four zones, thought to represent individual magmatic pulses. (2) Later stage magmatism comprises pegmatite veins as well as pyroxenite, harzburgite and dunite, emplaced in plugs and a cone sheet.In contrast to field observations, geochemical profiles of compatible elements across the complex indicate more complex layering and subdivision of the Layered Sequence into at least seven units. Incompatible trace elements are largely decoupled from compatible elements, which probably is the result of variable amount of interstitial liquid within cumulate phase. However, the shape of REE patterns has been controlled chiefly by fractionation of clinopyroxene and plagioclase. Finally, decoupling of hydrophile LILE from both compatible and incompatible elements may result from hydrothermal overprint or from redistribution during metamorphism.Gabbroic rocks from the Marginal Zone of the Alberta Compex display affinity to mafic volcanic rocks of the Elim Fm. Modeling based on in situ fractional crystallisation or, alternatively, mixing of evolved liquid with cumulate supports this hypothesis. In contrast, for samples within the magma chamber, only REE could be matched closely by these models, while modeled incompatible trace elements commonly deviate from the measured data. Additional complexity of local magma chamber probably best explains these deviations. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

Geochemistry and Petrogenesis of Kunduru Betta Calc-Alkaline Ring Complex in the Dharwar Craton, Southern India

The Kunduru Betta Ring Complex (KRC), at the southern margin of Dharwar craton, South India, comprises metaluminous sub-solvus syenites and quartz monzonite with a concentric disposition younging towards the center. An outer mafic syenite (of lamprophyric affinity) is followed by porphyritic monzonite, quartz monzodiorite and finally a quartz monzonitic stock at the centre. SiO 2, Al 2O 3 and Na 2O increase from the primitive lamprophyric mafic syenite to the quartz monzonite through the intermediate members, while CaO, MgO, Fe 2O 3 T, TiO 2, P 2O 5 and MnO show an opposite trend suggesting fractionation of hornblende, clinopyroxene, biotite, apatite, sphene, and iron oxide minerals. Rb, Th and U increase with a complementary decrease in Sc, V, Cr, Co, Cu, Sr and Ba from the outer mafic syenite to the inner quartz monzonite. Y, Zr and Hf decrease from lamprophyric mafic syenite to quartz monzodiorite and the trend is reversed in the final quartz monzonite phase. However, the suite is characterised by a compositional gap between quartz monzodiorite and quartz monzonite. Total REE gradually decrease from the mafic syenite to quartz monzonite and the REE distribution patterns show LREE-enriched and HREE-depleted parallel distributions with negligible Eu anomalies. The geochemical data suggest that the rock types were formed as products of progressive differentiation by crystal fractionation of calc-alkaline lamprophyric parent magma which was derived by partial melting of metasomatically enriched mantle in the Kabini lineament. Although the quartz monzonites conform to the trend of differentiated Kunduru Betta suite, the compositional gap between them and the quartz monzodiorite precludes their origin by simple differentiation. It is suggested that convective liquid fractionation might have resulted in the discrete body of quartz monzonite.

Coupled evolution of back-arc and island arc-like mafic crust in the late-Neoproterozoic Agardagh Tes-Chem ophiolite, Central Asia: evidence from trace element and Sr-Nd-Pb isotope data

We report major-element, trace-element and isotopic data of volcanic rocks from the late-Neoproterozoic (570 Ma) Agardagh Tes-Chem ophiolite in Central Asia, south-west of Lake Baikal (50.5°N, 95°E). The majority of samples are high-alumina basalts and basaltic andesites having island-arc affinities. They were derived from an evolved parental magma (Mg#≥0.60, Cr~180 ppm, Ni~95 ppm) by predominantly clinopyroxene fractionation. The parental magma developed from a primary mantle melt by fractionation of about 12% of an olivine+spinel assemblage. The island-arc rocks have high abundances of incompatible trace elements (light rare-earth element abundances up to 100 times chondritic, chondrite-normalised (La/Yb)n=14.6–5.1) and negative Nb anomalies (Nb/La=0.37–0.62), but low Zr/Nb ratios (7–14). Initial eNd values are around +5.5, initial Pb isotopic compositions are 206Pb/204Pb=17.39–18.45, 207Pb/204Pb=15.49–15.61, 208Pb/204Pb=37.06–38.05. Enrichment of large-ion lithophile elements within this group is significant (Ba/La=11–130). Another group of samples consists of back-arc basin-related volcanic rocks. They are most likely derived from the same depleted mantle source as the island-arc rocks, but underwent higher degrees of melting (8–15%) and are not influenced by slab components. They have lower abundances of incompatible trace elements, flat rare-earth element patterns [(La/Yb)n=0.6–2.4] and higher eNd values (+7.8 to +8.5). Negative Nb anomalies are absent (Nb/La=0.81–1.30), but Zr/Nb is high (21–48). At least three components are necessary to explain the geochemical evolution of the volcanic rocks: (1) an enriched (ocean island-like) component characterised by a high Nb concentration (up to 30 ppm), an absent negative Nb anomaly, a low Zr/Nb ratio (~6.5), a low eNd value (around 0), and radiogenic 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb; (2) a back-arc basin component similar to N-MORB with a flat rare-earth element pattern and a high eNd value (around +8.5); and (3) an island-arc component from a mantle source which was modified by the downgoing slab. Crystal fractionation superimposed on mixing and source contamination by subducted sediments is suitable to explain the observed geochemical data. The most likely geodynamic environment to produce these characteristics is a young, intra-oceanic island-arc system and an associated back-arc basin.

Petrologic and geochemical constraints on the petrogenesis of Permian–Triassic Emeishan flood basalts in southwestern China

The Emeishan flood basalt is a large igneous province erupted during the Permian–Triassic period in southwestern China. Based on petrographic, major and trace element, and Sr–Nd isotope data, the Emeishan basalts can be classified into two major magma types. These are: (1) a low-Ti (LT) type that exhibits low Ti/Y (<500), Fe 2O 3* (<12%), Nb/La (0.6–1.4), ε Nd( t) (−4.8 to +1.4) and relatively high SiO 2 (48–53%) and Mg# (0.52–0.64); (2) a high-Ti (HT) type that has high Ti/Y (>500). The HT lavas can be further divided into three subtypes. HT1 lavas exhibit significantly high TiO 2 (3.65–4.7%), Fe 2O 3* (12.7–16.4%), Nb/La (0.75–1.1), coupled with higher ε Nd( t) (1.1–4.8) and lower SiO 2 (45–51%); HT2 lavas are compositionally similar to the HT1 lavas but show conspicuous depletion in U and Th. The HT3 type has higher Mg# (0.51–0.61) than the HT1 and HT2 lavas. It differs from the LT type in having higher TiO 2 (∼3%) at comparable Mg#. Elemental and isotopic data suggest that the chemical variations of the LT and HT lavas cannot be explained by crystallization from a common parental magma. Instead, they may originate from different mantle sources under various melting conditions and underwent distinct differentiation and contamination processes. REE inversion calculations indicate that the HT magmas were generated by low degrees of partial melting (1.5%) of a mantle source that has ε Nd( t) of ∼+5 and 87Sr/ 86Sr( t) of ∼0.704 within the garnet stability field. These magmas were then subjected to shallow level gabbroic fractionation, which led to larger chemical variations. In contrast, parental magmas of the LT type were generated by higher degree of partial melting (16%) of a distinct mantle source ( ε Nd( t)≈+2, 87Sr/ 86Sr( t)≈0.705) around the spinel–garnet transition zone. The chemical evolution of the LT lavas is controlled by an olivine (ol)+clinopyroxene (cpx) fractionation. The Emeishan flood basalts may result from a starting mantle plume. The petrogenesis of both the LT and HT magmas was further complicated by contamination of upper crust and lithospheric mantle. While the HT1 lavas have experienced an AFC style of contamination in the upper crust, the HT2 lavas that mark with U–Th depletions may result from additional interaction with melts derived from a gabbroic layer near the crust–mantle boundary. In contrast, a temperature-controlled style of contamination was associated with the LT lavas. Our data show that both temporal and spatial geochemical variations exist in the Emeishan flood basalt province. The occurrence of thick LT lavas in the western part of the province may record the main episode of the flood basalt emplacement. In contrast, the less abundant overlying HT basalts may imply a waning activity of the plume. In fact, the HT basalts are the dominant magma type in the periphery of the province. The lower degrees of mantle melting of the HT lavas may be a result of relatively thicker lithosphere and lower geotherm.

A large K-foiditic hydromagmatic eruption from the early activity of the Alban Hills Volcanic District, Italy

In this paper we discuss the uncommon case of an energetic, pyroclastic-flow-forming eruption with a SiO2-poor (42–45 wt.%), K-foiditic magma composition. The Trigoria-Tor de' Cenci Tuff (TTC; 561 ka) is the product of the first large-scale explosive event (of the order of 1–10 km3 of erupted products) in the Alban Hills Volcanic District, near the city of Rome, Italy. After an initial Plinian phase that produced a scoria fall horizon, pyroclastic current activity emplaced ash deposits with leucite-bearing juvenile scoria lapilli. The abundance of accretionary lapilli, the most distinctive feature of these deposits, together with the high degree of fragmentation, the abundance of minute lithic inclusions and the morphology of ash particles, indicates a hydromagmatic character for the most part of the eruption. The absence of vent-derived carbonate lithic clasts from the deep regional aquifer and the abundance of cognate lithic fragments suggest that the interaction with external water involved a surficial aquifer in the older Alban Hills volcanic terrains. Perhaps the most striking aspect of the TTC is the K-foiditic composition of the pre-eruptive melt, which, to our knowledge, is unique among explosive events of comparable size elsewhere in the world. The pre-eruptive magma system feeding the TTC was controlled mainly by leucite+clinopyroxene fractionation under aH2O<1 conditions. The low SiO2 activity prevented plagioclase and K-feldspar crystallization. The depth of the magma chamber can be estimated at 3–6 km within the carbonate substrate. In contrast to the other major pyroclastic-flow-forming eruptions of the Alban Hills, the juvenile volatile exsolution due to magma crystallization is not seen as the main mechanism driving the TTC eruption. We suggest that the explosive behaviour of the TTC magma in the early magmatic phase resulted from a rapid decompression due to a regional seismic event and from magma–water interaction in the succeeding phase.

Almandine megacrysts from Yingfengling Cenozoic basalt in Leizhou Peninsula and their parental magma origin

The garnet megacrysts from Yingfengling basalts are characterized by high FeO (>20%), CaO (7.02% –8.16%) and low MgO (5.88%–10.87%). Significant composition variations are observed in these megacrysts, of which Ni, V, Sc, Co, and HREE are positively correlated with their Mg#, and Zr, Hf, Ga, Y, Sr, Nb, Zn and LREE-MREE are negatively correlated with Mg#. Megacryst parent magma is a highly evolved residual melt with strongl depletion in Ti, Sr, Hf, Nb and HREE. This parental magma was generated by more than 60% of crystallization fractionation of clinopyroxene, garnet, plagioclase and ilmenite from quartz tholeiitic magma. It has not erupted to the surface, but stayed at the upper mantle and formed the megacrystic cumulate. Megacrysts and their host basalt are in disequilibrium.