A new approach to modelling differentiation (with particular focus on granitic magmatism): Equilibrated Major Element Assimilation with Fractional Crystallisation (EME-AFC)

2020 ◽  
Author(s):  
Alex Burton-Johnson ◽  
Colin Macpherson ◽  
Christopher Ottley ◽  
Geoff Nowell ◽  
Adrian Boyce
Keyword(s):  
Major Element ◽  
Crustal Contamination ◽  
Mafic Magma ◽  
Fractional Crystallisation ◽  
Geochemical Data ◽  
Magmatic Differentiation ◽  
New Approach ◽  
Mount Kinabalu ◽  
High K ◽  
Granitic Magmatism

<p>We present the new approach to AFC modelling published as Editor’s Choice in the July 2019 issue of Journal of Petrology [1].</p><p>Our new, Equilibrated Major Element – Assimilation with Fractional Crystallisation (EME-AFC) approach simultaneously models the major element, trace element, and radiogenic and oxygen isotope compositions during such magmatic differentiation (including a new approach to oxygen modelling); addressing the lack of current AFC modelling approaches for felsic, amphibole- or biotite-bearing systems. We discuss the application of this model to granitic magmatism in SE Asia and Antarctica, with particular focus on the Mt Kinabalu granitic intrusion of Borneo. We discuss the background to the model, and explain how it can be freely accessed via GitHub [2], and applied to other scenarios of magmatic differentiation; not just granitic magmatism.</p><p>We present new geochemical data for the composite units of the Mount Kinabalu, and use this to explore the discrimination between crustal- and mantle-derived granitic magmas. The isotopic data (oxygen, Hf, Sr, Nd, and Pb) indicate that the magma cannot be the result only from fractional crystallisation of a mantle-derived magma. Alkali metal compositions show that crustal anatexis is also an unsuitable processes for genesis of the intrusion. Using the new EME-AFC modelling approach, we show that the high-K pluton was generated by fractional crystallisation of a primary, mafic magma followed by assimilation of the partially melted sedimentary overburden. We propose that Mt Kinabalu was generated through low degree melting of upwelling fertile metasomatised mantle driven by regional crustal extension in the Late Miocene.</p><p>[1] Burton-Johnson, A., Macpherson, C.G., Ottley, C.J., Nowell, G.M., Boyce, A.J., 2019. Generation of the Mt Kinabalu granite by crustal contamination of intraplate magma modelled by Equilibrated Major Element Assimilation with Fractional Crystallisation (EME-AFC). J. Petrol. 60, 1461–1487.</p><p>[2] https://github.com/Alex-Burton-Johnson/EME-AFC-Modelling</p>

scholarly journals Generation of the Mt Kinabalu Granite by Crustal Contamination of Intraplate Magma Modelled by Equilibrated Major Element Assimilation with Fractional Crystallization (EME-AFC)

2019 ◽  
Vol 60 (7) ◽  
pp. 1461-1487 ◽  
Author(s):  
A Burton-Johnson ◽  
C G Macpherson ◽  
C J Ottley ◽  
G M Nowell ◽  
A J Boyce
Keyword(s):  
Oxygen Isotope ◽  
Fractional Crystallization ◽  
Major Element ◽  
Crustal Contamination ◽  
Mafic Magma ◽  
Isotope Ratios ◽  
Geochemical Data ◽  
Magmatic Differentiation ◽  
Clear Trend ◽  
Mount Kinabalu

AbstractNew geochemical data are presented for the composite units of the Mount Kinabalu granitoid intrusion of Borneo and utilised to explore the discrimination between crustal- and mantle-derived granitic magmas. The geochemical data demonstrate that the units making up this composite intrusion became more potassic through time. This was accompanied by an evolution of isotope ratios from a continental-affinity towards a slightly more mantle-affinity (87Sr/86Sri ∼0·7078; 143Nd/144Ndi ∼0·51245; 206Pb/204Pbi ∼18·756 for the oldest unit compared to 87Sr/86Sri ∼0·7065, 143Nd/144Ndi ∼0·51250 and 206Pb/204Pbi ∼18·721 for the younger units). Oxygen isotope ratios (calculated whole-rock δ18O of +6·5–9·3‰) do not show a clear trend with time. The isotopic data indicate that the magma cannot result only from fractional crystallization of a mantle-derived magma. Alkali metal compositions show that crustal anatexis is also an unsuitable process for genesis of the intrusion. The data indicate that the high-K units were generated by fractional crystallization of a primary, mafic magma, followed by assimilation of the partially melted sedimentary overburden. We present a new, Equilibrated Major Element -Assimilation with Fractional Crystallization (EME-AFC) approach for simultaneously modelling the major element, trace element, and radiogenic and oxygen isotope compositions during such magmatic differentiation; addressing the lack of current AFC modelling approaches for felsic, amphibole- or biotite-bearing systems. We propose that Mt Kinabalu was generated through low degree melting of upwelling fertile metasomatized mantle driven by regional crustal extension in the Late Miocene.

The chemical character of the Late Caledonian Donegal Granites, Ireland, with comments on their genesis

2006 ◽  
Vol 97 (4) ◽  
pp. 437-454 ◽  
Author(s):  
Azman A. Ghani ◽  
Michael P. Atherton
Keyword(s):  
Major Element ◽  
Large Range ◽  
Tectonic Event ◽  
Major Element Chemistry ◽  
Iapetus Ocean ◽  
High K ◽  
Granitic Magmatism ◽  
Pacific Margin ◽  
Chemical Character ◽  
Very High

ABSTRACTThe Late Caledonian granites of Donegal are all intruded into metasediments of the Dalradian Supergroup of Neoproterozoic age, which were metamorphosed and deformed during the Grampian Phase of the Caledonian orogeny at c. 470–460 m.y. They were intruded in a singular pulse well after the main tectonic event, apparently peaking at 407–402 m.y.; importantly after the strong collision of Laurentia with Baltica on closure of the Iapetus Ocean. The plutons are mainly made up of granodiorite and granite, and are all 'I' type, but different to Cordilleran ‘I’ types of the eastern Pacific margin. Major element chemistry indicates they are high-K calc-alkaline rocks with a large range in SiO2 content. However three of the plutons (Fanad, Thorr, Ardara), have very high Ba and Sr contents, even higher than Mainland Scotland counterparts; they are high Ba–Sr plutons. Three plutons (Barnesmore, Rosses, Trawenagh Bay) are evolved and are low-Ba–Sr types, while one (Main Donegal) has atypical, intermediate characteristics. The origin of the magmas is still much debated; here we suggest slab breakoffon Iapetus Ocean closure accounts for the special compositions of these magmas and the other major features of Late Caledonian granitic magmatism, including the singular intrusion peak and the associated appinite–lamprophyre suite.

Formation of ultrapotassic magma via crustal contamination and hybridization of mafic magma: an example from the Stomanovo monzonite, Central Rhodope Massif, Bulgaria

2021 ◽  
pp. 1-16
Author(s):  
Peter Marchev ◽  
Raya Raicheva ◽  
Stoyan Georgiev ◽  
Ivan P. Savov ◽  
Danko Jelev
Keyword(s):  
Crustal Contamination ◽  
Mafic Magma ◽  
Primitive Mantle ◽  
Crustal Assimilation ◽  
Isotopic Data ◽  
Basement Rocks ◽  
High K ◽  
Metamorphic Basement ◽  
High Alkalinity ◽  
Continental Lithospheric Mantle

Abstract Generally all orogenic ultrapotassic rocks are formed after melting of metasomatized sub-continental lithospheric mantle via subducted crustal mica-bearing lithologies. Here we present another possible model, based on the study of the small Stomanovo ultrapotassic monzonite porphyry intrusion in the Central Rhodope Massif, Bulgaria. The monzonite dated at 30.50 ± 0.46 Ma is intruded into the voluminous Oligocene (31.63 ± 0.40 Ma) Bratsigovo–Dospat ignimbrite. The monzonite hosts both normally and reversely zoned clinopyroxene phenocrysts. The normally zoned clinopyroxene is characterized by gradually diminishing core-to-rim Mg no. (89–74), whereas the reversely zoned clinopyroxene has green Fe-rich cores (Mg no. 71–55) mantled by normally zoned clinopyroxene (Mg no. 87–74). Neither the core of the normally zoned clinopyroxene nor the Fe-rich green cores are in equilibrium with the host monzonite. This ultrapotassic monzonite shows more radiogenic Sr isotopes ((87Sr/86Sr)i = 0.71066) and ϵNd(t) = −7.8 to −8.0 that are distinct from the host ignimbrites with (87Sr/86Sr)i = 0.70917–0.70927 and ϵNd(t) = −4.6 to −6.5. The Sr–Nd isotopic data and the presence of copious zircon xenocrysts from the underlying metamorphic basement suggest extensive crustal assimilation. Our observations indicate that the Stomanovo ultrapotassic monzonite formed after extensive lower or middle crustal fractional crystallization from an evolved magma producing cumulates. The process was followed by hybridization with primitive mantle-derived magma and subsequent continuous crustal contamination. We suggest that instead of inheriting their high K2O and large-ion lithophile element enrichments from slab-derived/metasomatic fluids, the Stomanovo ultrapotassic monzonite may owe some of its unusually high alkalinity to the assimilation of potassium-rich phases from the Rhodope Massif basement rocks.

Pétrologie des roches volcaniques de sillon de roches vertes archéennes de Matagami–Chibougamau à l'ouest de Chapais (Abitibi est, Québec), 2. Le groupe hautement potassique d'Opémisca

1986 ◽  
Vol 23 (8) ◽  
pp. 1169-1189 ◽  
Author(s):  
Christian Picard ◽  
Michel Piboule
Keyword(s):  
Fractional Crystallization ◽  
Crustal Contamination ◽  
Major Elements ◽  
Geochemical Data ◽  
Continental Margins ◽  
Garnet Lherzolite ◽  
Calc Alkaline ◽  
Volatile Elements ◽  
High K ◽  
Active Continental Margins

In the western part of the Chapais syncline (Abitibi East, Quebec), the Opemisca Group unconformably overlies the Roy Group at a low angle. It consists of a thick turbidite sequence covered by an interdigitated sequence of lavas and alluvial cone sediments. The subaerial lavas include two sequences evolving from porphyric metabasalts to metatrachyandesites and porphyric metatrachytes (lower sequence) or to K-rich aphanitic meta-andesites (upper sequence). These lavas, with calc-alkaline to shoshonitic affinity, have high K2O, Ba, Sr, and Th contents and show highly enriched LREE spectra.The behaviour of major elements, trace elements, and lanthanides suggests an origin from partial melting of a mantle source consisting of a garnet lherzolite enriched in K, Sr, Rb, Ba, and Th by volatile elements and also by crustal contamination and by fractional crystallization mechanisms. The evolution of the lavas of the lower sequence, progressively deficient in Y, seems to have been controlled by fractional crystallization of a plagioclase, clinopyroxene, and olivine mixture; this was followed for the metatrachyandesites and metatrachytes by high H2O activity of a feldspar, amphibole, titanomagnetite, and apatite mixture. The evolution of lavas enriched in Y in the upper sequence seems to have been controlled by weak H2O activity of the anhydrous plagioclase, clinopyroxene, and olivine assemblage.The petrographic and geochemical data suggest an emplacement similar to that occurring on the active continental margins of the central Andes and implies the existence of a late-stage ensialic arc. [Translated by the journal]

Early Cretaceous crust–mantle interaction linked to rollback of the Palaeo-Pacific flat-subducting slab: constraints from the intermediate–felsic volcanic rocks of the northern Great Xing’an Range, NE China

2021 ◽  
pp. 1-22
Author(s):  
Jia-Hao Jing ◽  
Hao Yang ◽  
Wen-Chun Ge ◽  
Yu Dong ◽  
Zheng Ji ◽  
...  
Keyword(s):  
Early Cretaceous ◽  
Volcanic Rocks ◽  
Geochemical Data ◽  
Calc Alkaline ◽  
Ne China ◽  
Asthenospheric Mantle ◽  
High K ◽  
Wide Range ◽  
Great Xing’An Range ◽  
Subducting Slab

Abstract Late Mesozoic igneous rocks are important for deciphering the Mesozoic tectonic setting of NE China. In this paper, we present whole-rock geochemical data, zircon U–Pb ages and Lu–Hf isotope data for Early Cretaceous volcanic rocks from the Tulihe area of the northern Great Xing’an Range (GXR), with the aim of evaluating the petrogenesis and genetic relationships of these rocks, inferring crust–mantle interactions and better constraining extension-related geodynamic processes in the GXR. Zircon U–Pb ages indicate that the rhyolites and trachytic volcanic rocks formed during late Early Cretaceous time (c. 130–126 Ma). Geochemically, the highly fractionated I-type rhyolites exhibit high-K calc-alkaline, metaluminous to weakly peraluminous characteristics. They are enriched in light rare earth elements (LREEs) and large-ion lithophile elements (LILEs) but depleted in high-field-strength elements (HFSEs), with their magmatic zircons ϵHf(t) values ranging from +4.1 to +9.0. These features suggest that the rhyolites were derived from the partial melting of a dominantly juvenile, K-rich basaltic lower crust. The trachytic volcanic rocks are high-K calc-alkaline series and exhibit metaluminous characteristics. They have a wide range of zircon ϵHf(t) values (−17.8 to +12.9), indicating that these trachytic volcanic rocks originated from a dominantly lithospheric-mantle source with the involvement of asthenospheric mantle materials, and subsequently underwent extensive assimilation and fractional crystallization processes. Combining our results and the spatiotemporal migration of the late Early Cretaceous magmatic events, we propose that intense Early Cretaceous crust–mantle interaction took place within the northern GXR, and possibly the whole of NE China, and that it was related to the upwelling of asthenospheric mantle induced by rollback of the Palaeo-Pacific flat-subducting slab.

Genesis of the Wuxing Pt–Pd-rich Cu–Ni sulfide deposit in the eastern Central Asian Orogenic Belt: evidences from geochronology, elemental geochemistry, and Sr–Nd–Hf isotopic data

2019 ◽  
Vol 56 (4) ◽  
pp. 380-398 ◽  
Author(s):  
Jing-gui Sun ◽  
Yun-peng He ◽  
Ji-long Han ◽  
Zhong-yu Wang
Keyword(s):  
Crustal Contamination ◽  
Early Period ◽  
Orogenic Belt ◽  
Central Asian Orogenic Belt ◽  
Geochemical Data ◽  
Sulfide Deposit ◽  
Late Period ◽  
Enriched Mantle ◽  
Central Asian ◽  
T Values

The Wuxing Pt–Pd-rich Cu–Ni sulfide deposit in Heilongjiang Province, Northeast China, is located to the northeast of the Dunhua–Mishan fracture of the eastern Central Asian Orogenic Belt. The mafic–ultramafic complex consist of early-period hornblende–olivine pyroxenite, diopsidite, and hornblende pyroxenite and late-period gabbro and diabase units. An early-period hornblende pyroxenite yielded a zircon U–Pb age of 208.2 ± 2.6 Ma and a late-period diabase yielded a U–Pb age of 205.6 ± 1.1 Ma, with zircon εHf(t) values of +1.24 to +8.13. The early- and late-period lithofacies are relatively enriched in LILE (Rb, Ba, and Sr) and LREE, and variably depleted in HFSE (Nb, Ta). The whole-rock and single-mineral analyses of the early-period lithofacies yield (87Sr/86Sr)i ratios of 0.7055–0.7083 and εNd(t) ratios of −7.98–+3.10. These geochemical data suggest that the parental magmas of the Wuxing complex are high-Mg subalkaline basaltic in nature and were derived from an enriched mantle source. The magmas chamber formed after the injection of magma into the crust along with crustal contamination, producing early crystalline minerals and ore-bearing magmas. The rupturing of the magma chamber released evolved magmas, which then ascended and generated Pt–Pd-bearing lithofacies and Cu–Ni sulfide orebodies by fractional crystallization, accumulation, and liquation. During the late period, the residual magma invaded the early lithofacies and Cu–Ni orebodies. The fluids exsolved from the gabbroic magmas concentrated the mineralized metal elements and enhanced the precipitation of Pt–Pd-bearing veinlet-disseminated orebodies and Pt–Pd–Cu–Ni orebodies.

scholarly journals Geochemical Characteristics and Tectonic Setting of Amphibolites in Ifewara Area, Ife-Ilesha Schist Belt, Southwestern Nigeria

2016 ◽  
Vol 6 (1) ◽  
pp. 43 ◽  
Author(s):  
Anthony Temidayo Bolarinwa ◽  
Adebimpe Atinuke Adepoju
Keyword(s):  
Rare Earth ◽  
Rare Earth Elements ◽  
Major Element ◽  
Tectonic Setting ◽  
Crustal Contamination ◽  
Geochemical Evolution ◽  
Schist Belt ◽  
Southwestern Nigeria ◽  
Eu Anomaly ◽  
Ree Patterns

Trace and Rare Earth Elements (REEs) data are used to constrain the geochemical evolution of the amphibolites from Ifewara in the Ife-Ilesha schist belt of southwestern Nigeria. The amphibolites can be grouped into banded and sheared amphibolites. Major element data show SiO2 (48.34%), Fe2O3 (11.03-17.88%), MgO (5.76-9.90%), CaO (7.76-18.6%) and TiO2 (0.44-1.77%) contents which are similar to amphibolites in other schist belts in Nigeria. The Al2O3 (2.85-15.55%) content is varied, with the higher values suggesting alkali basalt protolith. Trace and rare earth elements composition reveal Sr (160-1077ppm), Rb (0.5-22.9ppm), Ni (4.7-10.2ppm), Co (12.2-50.9 ppm) and Cr (2-7ppm). Chondrite-normalized REE patterns show that the banded amphibolites have HREE depletion and both negative and positive Eu anomalies while the sheared variety showed slight LREE enrichment with no apparent Eu anomaly. The study amphibolites plot in the Mid Oceanic Ridge Basalts (MORB) and within plate basalt fields on the Zr/Y vs Zr discriminatory diagrams. They are further classified as volcanic arc basalt and E-type MORB on the Th- Hf/3- Ta and the Zr-Nb-Y diagrams. The amphibolites precursor is considered a tholeiitic suite that suffered crustal contamination, during emplacement in a rifted crust.

scholarly journals Miocene and Pliocene Silicic Coromandel Volcanic Zone Tephras from ODP Site 1124-C: Petrogenetic Applications and Temporal Evolution

2021 ◽  
Author(s):  
◽  
Matthew Thomas Stevens
Keyword(s):  
Trace Element ◽  
Mantle Wedge ◽  
Major Element ◽  
Volcanic Glass ◽  
Fractional Crystallisation ◽  
Volcanic Zone ◽  
Arc Magmas ◽  
Glass Shard ◽  
Tephra Layers ◽  
Glass Shards

<p>The Coromandel Volcanic Zone (CVZ) was the longest-lived area of volcanism in New Zealand hosting the commencement of large explosive rhyolitic and ignimbrite forming eruptions. The NW trending Coromandel Peninsula is the subaerial remnant of the Miocene-Pliocene CVZ, which is regarded as a tectonic precursor to the Taupo Volcanic Zone (TVZ), currently the most dynamic and voluminous rhyolitic volcanic centre on Earth. This study presents new single glass shard major and trace element geochemical analyses for 72 high-silica volcanic tephra layers recovered from well-dated deep-sea sediments of the SW Pacific Ocean by the Ocean Drilling Program (ODP) Leg 181. ODP Site 1124, ~720 km south and east from the CVZ, penetrated sediments of the Rekohu Drift yielding an unprecedented record of major explosive volcanic eruptions owing to the favourable location and preservation characteristics at this site. This record extends onshore eruptive sequences of CVZ explosive volcanism that are obscured by poor exposure, alteration, and erosion and burial by younger volcanic deposits. Tephra layers recovered from Site 1124 are well-dated through a combination of biostratigraphic and palaeomagnetic methods allowing the temporal geochemical evolution of the CVZ to be reconstructed in relation to changes in the petrogenesis of CVZ arc magmas from ~ 10 to 2 Ma. This thesis establishes major and trace element geochemical "fingerprints" for all Site 1124-C tephras using well-established (wavelength dispersive electron probe microanalysis) and new (laser ablation inductively coupled plasma mass spectrometry) in situ single glass shard microanalytical techniques. Trace element analysis of Site 1124-C glass shards (as small as 20 um) demonstrate that trace element signatures offer a more specific, unequivocal characterisation for distinguishing (and potentially correlating) between tephras with nearly identical major element compositions. The Site 1124-C core contains 72 unaltered Miocene-Pliocene volcanic glass-shard-bearing laminae > 1 cm thick that correspond to 83 or 84 geochemical eruptive units. Revised eruptive frequencies based on the number of geochemical eruptive units identified represent at least one eruption every 99 kyr for the late Miocene and one per 74 kyr for the Pliocene. The frequency of tephra deposition throughout the history of the CVZ has not been constant, rather reflecting pulses of major explosive eruptions resulting in closely clustered groups of tephra separated by periods of reduced activity, relative volcanic quiescence or non-tephra deposition. As more regular activity became prevalent in the Pliocene, it was accompanied by more silicic magma compositions. Rhyolitic volcanic glass shards are characterised by predominantly calc-alkaline and minor high-K enriched major element compositions. Major element compositional variability of the tephras deposited between 10 Ma and 2 Ma reveals magma batches with pre-eruptive compositional gradients implying a broad control by fractional crystallisation. Trace element characterisation of glass shards reveals the role of magmatic processes that are not readily apparent in the relatively homogeneous major element compositions. Multi-element diagrams show prominent negative Sr and Ti anomalies against primitive mantle likely caused by various degrees of plagioclase and titanomagnetite fractional crystallisation in shallow magma chambers. Relative Nb depletion, characteristic of arc volcanism, is moderate in CVZ tephras. HFSEs (e.g. Nb, Zr, Ti) and HREEs (e.g. Yb, Lu) remain immobile during slab fluid flux suggesting they are derived from the mantle wedge. LILE (e.g. Rb, Cs, Ba, Sr) and LREE (e.g. La, Ce) enrichments are consistent with slab fluid contribution. B/La and Li/Y ratios can be used as a proxy for the flux of subducting material to the mantle wedge, they suggest there is a strong influence from this component in the generation of CVZ arc magmas, potentially inducing melting. CVZ tephra show long-term coherent variability in trace element geochemistry. Post ~ 4 Ma tephras display a more consistent, less variable, chemical fingerprint that persists up to and across the CVZ/TVZ transition at ~ 2 Ma. Initiation of TVZ volcanism may have occurred earlier than is presently considered, or CVZ to TVZ volcanism may have occurred without significant changes in magma generation processes.</p>

scholarly journals An Overview of the Tectonic Evolution of the Indochina block and Granitoid Emplacement, particularly in the central and south Vietnam

2020 ◽  
Vol 23 (3) ◽  
pp. First
Author(s):  
Tuan Anh Nguyen ◽  
Ngo Tran Thien Quy ◽  
Vu Thi Hao ◽  
Pham Minh
Keyword(s):  
Geochemical Data ◽  
Language Program ◽  
R Language ◽  
Magmatic Rocks ◽  
South Vietnam ◽  
Granitic Magmatism ◽  
Granitoid Emplacement ◽  
Rock Analysis ◽  
Geochemical Data Analysis ◽  
Southwest Part

Introduction: Vietnam is mainly located within the Indochina block in Southeast Asia. Asmall northern part of Vietnam belongs to the South China block, the southwest part liesadjacent to the Sibumasu block and opens to the East Sea on the east side. Tectonicactivities in Vietnam were very complicated they relate to intense interactions betweenmany geological blocks at different times. Magmatic emplacement is the final and instantproduct of tectonic activities. Methods: Geochemical data analysis from rock samples withinVietnam collected by other researchers has been reused in the scope of this study to verifythe relation between tectonic evolutions and their granitic magmatism. GCD (GeochemicalData Toolkit), an R language program for handling and recalculation of geochemical data. Results: Geochronology and geotectonic model derived from rock analysis have beenascertained main tectonic evolutions of the Indochina. The current granitoidclassification in Vietnam mostly based on petrographical studies. The Nui Cam granitoid isbeing classified as Deo Ca, Dinh Quan granitoid. However, based on trace elements, they aredifferent. They may belong to different granitoid system. Conclusion: Major tectonic eventswithin the Indochina block are well supported by the nature of granitoid emplacements. Petrological studies of these magmatic rocks would bring out valuable information toconfirm and clearly understand the tectonic evolutions of the region. Igneous rocksclassification must based on tectonic fundamental instead of petrographical studies.

On the volcanic architecture, petrology and geodynamic setting of the 3.48 Ga Barberton komatiite suite, South Africa

2021 ◽  
Author(s):  
E.G. Grosch ◽  
J. Slama
Keyword(s):  
South Africa ◽  
Mantle Plume ◽  
Source Region ◽  
Crustal Contamination ◽  
Primitive Mantle ◽  
Geochemical Data ◽  
Geodynamic Setting ◽  
Crustal Material ◽  
Type Section ◽  
Formation Type

Abstract This study presents new field and petrological observations combined with geochemical data on a range of komatiitic to tholeiitic volcanic rocks from the ca. 3.48 Ga mid-lower Komati Formation type-section of the Barberton Greenstone Belt, South Africa. A range of mafic-ultramafic rocks is identified across a 1.44 km profile, leading to the proposition of a new preliminary volcanic architecture for the mid-lower Komati Formation type-section. Major, trace and rare earth element (REE) data in conjunction with Lu-Hf isotopic constraints indicate that the tholeiites, newly recognized high-magnesium basalts, basaltic komatiites and komatiites in the volcanic sequence have a primitive mantle signature with no geochemical affinity to Archaean or modern-day supra-subduction zone boninites. The whole rock initial εHf values of spinifex and massive komatiite flows in the lowermost part of the Komati type-section are negative, ranging between -1.9 and -3.1, whereas the second overlying spinifex and massive flow unit records positive initial εHf values between +0.5 and +4.7. A new geodynamic model involving crustal contamination of the mafic-ultramafic lavas is proposed for the Barberton mid-lower Komati Formation type-section, involving mantle plume-crust interaction. The new observations and data indicate that the komatiites erupted as a result of a mantle plume from a hot (&gt;1 600oC) mid-Archaean mantle, in which the earliest volcanic flows were variably affected by crustal contamination during their ascent and eruption. The possibility of incorporation of lower crustal material and/or recycled crust residing in the mantle source region cannot be excluded. This indicates that modern-style plate tectonic processes, such as subduction, may not have been a requirement for the formation of the 3.48 Ga Barberton komatiite suite, with implications for the hydration state, geodynamic processes and secular thermal evolution of the Archaean mantle.

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