Lithospheric Mantle Material Research Articles

Late Triassic bimodal igneous rocks in eastern Heilongjiang Province, NE China: Implications for the initiation of subduction of the Paleo-Pacific Plate beneath Eurasia

Abstract This paper reports new zircon LA–ICP–MS and SIMS U–Pb ages and Hf isotope data, and whole-rock major and trace element data for Late Triassic igneous rocks of eastern Heilongjiang Province, NE China. These data provide new insights into the timing of the initiation of subduction of the Paleo-Pacific Plate beneath the Eurasian continent. The zircon U–Pb age data indicate that a suite of Late Triassic (228–202 Ma) igneous rocks is present within the Songnen–Zhangguangcai Range Massif and within the western margin of the Khanka Massif. The Late Triassic igneous rocks within the Songnen–Zhangguangcai Range Massif consist of basalts, basaltic andesites, gabbro-diabases, and rhyolites, whereas coeval igneous rocks in the western margin of the Khanka Massif consist of hornblende gabbros and syenogranitic porphyries. These Late Triassic rocks constitute a geochemically bimodal igneous rock association that contains mafic rocks enriched in light rare earth elements (LREE) and large ion lithophile elements (LILE), and depleted in heavy rare earth elements (HREE) and high field strength elements (HFSE) such as Nb, Ta, Zr, Hf, and Ti. Zircons from these mafic rocks have eHf (t) values and TDM1 ages of +2.8 to +9.8 and 477–733 Ma, respectively, suggesting that they formed from a primary magma generated by the partial melting of depleted lithospheric mantle material that had been previously modified by subduction-related fluids. The coeval felsic rocks are characterized by enrichments in LREEs and LILEs, and depletions in HREEs and HFSEs (including Nb, Ta, and Ti), and their zircons have eHf (t) values and T DM2 ages of +0.6 to +7.9 and 766 to 1461 Ma, respectively, implying that these rocks were generated during the partial melting of juvenile crustal material. The Late Triassic bimodal igneous rocks in eastern Heilongjiang Province, combined with the regional geologic information, therefore record a post-orogenic extensional environment related to the final late Permian–Early Triassic closure of the Paleo-Asian Ocean. In addition, the presence of Late Triassic bimodal igneous rocks within the eastern margin of the Eurasian continent suggests that the subduction of the Paleo-Pacific Plate beneath the Eurasian continent began after the Late Triassic.

Magmatic evolution and metal element enrichment during formation of the Niumaoquan magnetite ore deposit, Xinjiang, China

Abstract Magmatic magnetite is an important iron resource in China; Such magnetite deposits generally formed as a result of magmatic differentiation, with continuous exchange of material and energy between the magma and the surrounding country rock during magmatic evolution. Consequently, it is important to understand whether the aggregation and formation of individual minerals have been affected by interaction with the surrounding country rock during the formation of this type of ore deposit. The Niumaoquan magnetite deposit at Harlik, Xinjiang, China, forms part of a suite of layered mafic complexes that preserve evidence of rhythmic deposition of magmatic minerals. The complex that hosts the deposit contains olivine gabbro, olivine-bearing gabbro, gabbro, and hornblende gabbro units, all of which formed from Fe-rich tholeiitic magmas that underwent fractional crystallization and assimilated minor amounts of crustal material. Their eNd (t) values and initial 87 Sr/ 86 Sr ratios vary from –3.4 to –0.50 and from –3.4 to 8.5, respectively, indicating that the primary magma was derived from partial melting of an EM1-type (enriched mantle 1) source, most probably an enriched region of the lithospheric mantle that was metasomatized by subduction-zone-derived fluids. This area of mantle wedge was located within the root of the lithosphere and was transformed by interaction with subducted lithospheric material; the delamination of this lithospheric mantle material caused partial melting as a result of heating by the surrounding asthenosphere. The enriched lithospheric mantle material ascended into the space created by the delamination of the lithosphere. The melts produced during these events resided for long periods in temporary magma chambers, enabling significant fractional crystallization and generating economic concentrations of Ti and Fe within the resulting gabbros, thereby forming the ore body. Element geochemistry shows that Ni and Cu concentrations were highest during the early stages of magma evolution indicating Ni–Cu sulfide immiscibility at this time, and the likelihood that a Ni–Cu orebody exists somewhere in the complex. Subsequently, both the complex and the associated ore deposit were formed by the intrusion of magmas in a dynamic environment.

Hotspot volcanism and highly siderophile elements

Abstract Hotspot volcanic rocks are formed under conditions that differ from conventional plate tectonic boundary magmatic processes and are compositionally distinct from mid-oceanic ridge basalts. Hotspot volcanic rocks include – but are not limited to – ocean island basalts (OIB), continental flood basalts (CFB), komatiites, oceanic plateau and some intraplate alkaline volcanic rocks. Studies of the highly siderophile element (HSE) geochemistry of hotspot volcanic rocks have provided new perspectives into mantle convection, mantle heterogeneity, core-mantle interactions, crustal and mantle lithospheric recycling, melting processes and crust-mantle interactions. The HSE, comprising Os, Ir, Ru, Rh, Pt, Pd, Re and Au, have strong affinities for metal and sulphide relative to silicate. These elements also have variable partitioning behaviour between highly compatible Os, Ir, Ru and Rh relative to compatible Pt and Pd and to moderately incompatible Re and Au during melting and crystallisation. This exceptional geochemical behaviour, combined with the existence of the long-lived 187 Re- 187 Os and 190 Pt- 186 Os decay systems embedded within the HSE, make these elements powerful tracers of processes acting on magmas and their volcanic products. The HSE can be utilised to understand sub-aerial volcanic degassing and crustal assimilation processes in hotspot volcanic rocks such as CFB and OIB, as well as for quantitative assessment of fractional crystallisation. Mantle melting studies have highlighted the strong control of sulphide in the mantle prior to exhaustion of S and generation of Os ± Ir ± Ru metal alloys at ~ > 25% partial melting; a behaviour of the HSE that is fundamental to understanding terrestrial hotspot volcanism. Perhaps the most exciting utility of the HSE, however, lies in their ability to reveal both short- and long-term fractionation processes acting on hotspot volcanic sources from inter-element HSE fractionations and 187 Os/ 188 Os- 186 Os/ 188 Os systematics. The growing database for HSE abundances and 187 Os/ 188 Os in hotspot volcanic rocks is consistent with their generation from a heterogeneous upper mantle generated by melt differentiation and recycling of crust and mantle lithosphere during plate subduction. 187 Os/ 188 Os variations (ratios up to 0.175) in high-Os abundance (> 50 ppt) HIMU (high 238 U/ 206 Pb) OIB indicate high long-term (> 1 to 2 Ga) Re/Os, supporting models for recycled oceanic lithosphere in the source of these volcanic rocks. In contrast, and despite elevated 87 Sr/ 86 Sr and low 143 Nd/ 144 Nd, enriched mantle (EM) OIB can have 187 Os/ 188 Os that dominantly reflect contributions from peridotite with only minor contributions from recycled sediment or continental crust and/or lithospheric mantle materials. The HSE provide geochemical evidence for how lithological and chemical heterogeneities are sampled within the mantle. Modeling of HSE abundances and Os isotopes show that large apparent recycled contributions (50% to 90%) in some OIB can be explained by the preferential melting of volumetrically minor ( 186 Os/ 188 Os data for hotspot volcanism are limited, but the combined variations in long-term Re/Os and Pt/Os retained in some mantle sources may reflect either the long-term fractionation of Re and Pt from Os between the inner and outer core, or ancient sulphide segregation and lithological variations in the mantle caused by convection but unrelated to core-mantle interaction. Either mode-of-origin is important for a firmer understanding of processes occurring in Earth, and both suggest that some hotspot volcanic mantle sources have been isolated and have evolved with supra-chondritic Pt/Os over > 2 to 3 Ga time-scales. Study of the HSE in hotspot volcanic rocks from Solar System bodies also informs on planetary-scale processes, indicating that Earth, the Moon, Mars and fully differentiated asteroids all have HSE abundances in their mantles that are higher than expected from low-pressure metal-silicate partitioning. Furthermore, the HSE are in broadly chondritic-relative abundances for these planetary bodies, at ~ 0.0002 (Moon), to ~ 0.007 (Mars), to ~ 0.009 (Earth) × carbonaceous chondrite Ivuna (CI) composition. The timing of addition of the HSE to planetary bodies preserved in their magmas and volcanic products is consistent with Solar-System-wide late accretion no later than 3.8 Ga for Earth, and even earlier based on evidence from the Moon (~ 4.4 Ga), Mars (~ 4.5 Ga) and asteroids (> 4.56 Ga).

U–Pb geochronology and geochemistry of the bedrocks and moraine sediments from the Windmill Islands: Implications for Proterozoic evolution of East Antarctica

Abstract Zircon LA-ICP-MS U–Pb geochronology and geochemistry on representative bedrocks and moraine sediments in the Windmill Islands in Wilkes Land, East Antarctica were performed to characterize the age and origin of main rocks as well as the lithology and age of ice-covered basement in this sector of East Antarctica. Zircon U–Pb dating on two garnet-bearing granite gneisses and one foliated garnet-bearing granite from Bailey Peninsula yield weighted mean 207Pb/206Pb ages of 1247 ± 13 Ma, 1258 ± 12 Ma and 1242 ± 13 Ma, respectively, which are interpreted as emplacement ages of these rocks. Igneous inherited zircons in these rocks yield a weighted mean 207Pb/206Pb age of 1372 ± 13 Ma, indicating existence of a ca. 1.37 Ga igneous activity in the Windmill Islands. Both garnet-bearing granite gneisses and foliated garnet-bearing granites from Bailey Peninsula are characterized by similar geochemical and isotopic compositions and were likely produced by partial melting of predominantly Paleoproterozoic ancient crustal materials at 1.24–1.25 Ga. Zircon U–Pb analyses on two charnockite samples from Robinson Ridge yield weighted mean 207Pb/206Pb ages of 1196 ± 8 Ma and 1205 ± 13 Ma, respectively, indicating emplacement of the Ardery Charnockite at ca. 1.20 Ga. The charnockites were likely produced by partial melting of the ancient mafic lower crust heated by underplated basaltic magma during the D2 cycle at late stage of the granulite facies metamorphism, and some lithospheric mantle materials could be involved during formation of these rocks. U–Pb analyses of detrital zircons from sand-sized moraine sediments yield concordant 207Pb/206Pb ages mainly ranging from ca. 1368 Ma to ca. 1107 Ma with main magmatic zircon populations at ca. 1107 Ma, 1150–1160 Ma, ca. 1196 Ma, 1231–1255 Ma, ca. 1317 Ma and ca. 1368 Ma, respectively. Some old zircon populations at ca. 1411 Ma to ca. 2360 Ma, which are considered to be derived from recycling of the Proterozoic metapsammitic gneiss rocks, have been identified. Their zircon Hf isotopic model ages ( T DM C ) are mainly around 1.6–2.4 Ga. The absence of Archean zircons in moraines suggests that the crust of Law Dome and inland areas of Wilkes Land are dominated by Palaeo-Mesoproterozoic rocks, which is consistent with the vast majority of the Nd–Hf isotopic data presented. Abundant Mesoproterzoic zircons from ca. 1107 Ma to ca. 1533 Ma in moraines indicate the crustal rocks in the Windmill Island and its inland areas are mainly formed in the Mesoproterozoic. The ages and Hf isotopic compositions of these moraine zircons are comparable with those of the bedrocks in the Windmill Islands, indicating that subglacial rock compositions of Law Dome and the inland areas of Wilkes Land are similar to the rock composition of the Windmill Islands area. Law Dome and the inland areas of Wilkes Land are probablly a similar Mesproterozoic high-grade metamorphic terrane as the Windmill Islands in East Antarctica. Our results also confirmed that there is no evidence for Pan-African tectonothermal events and the Windmill Islands and its inland areas are not affected by Pan-African tectonism and magmatism.

Cenozoic Volcanism of the Caucasian Mobile Belt in Georgia, its Geological-Petrological Peculiarities and Geodynamic Conditions

The Caucasian mobile belt is situated in the area of Late Cenozoic collision of the large Afro-Arabian and Eurasian lithospheric plates. Extensive volcanic activity in the Georgian part of the Caucasian mobile belt took place during the Late Miocene-Holocene. Five volcanic regions have been identified in Georgia; each of them reveals in a greater or lesser degree similarity of tectonic and magmatic processes. Volcanic products are represented by basaltic, doleritic, andesitic basaltic, andesitic dacitic, rhyolitic lavas and their pyroclastics with andesites and dacites prevailing. Using petrochemical and geochemical data the volcanics can be divided into two main rock groups: calc-alkaline and subalkaline series. The marker petrogeochemical series is presented by the medium- to high-K calc-alkaline volcanics. Relative to the heavy elements (HFSE) Y, Nb they are enriched in some large ion lithophile elements (LILE) Rb, Sr, Ba and light rare earth elements (REE) La, Ce. This confirms the leading role of fractional crystallization in forming the volcanics of the study area. These volcanics have the characteristics of pre-collision subduction (increased LILE content and high La/Nb ratios) geodynamic regimes. Volcanic rocks derived from sources displaying different tectonic environments show close petrogeochemical resemblance, indicating the similarity of the melting substrates of magmatic chambers. The findings also allow us to give priority to the magma generation conditions, to its periodical renewal and depths of inception in comparison with the geodynamical factors. Isotopic data (^8^7Sr/^8^6Sr) have confirmed that the subduction-enriched lithospheric mantle material was more important than that of the continental crust components. Sr isotopic ratios do not show marked dependence on the values of the petrochemical composition of the enclosing rocks and on the time of their formation.

Petrogenesis of Davidson Seamount lavas and its implications for fossil spreading center and intraplate magmatism in the eastern Pacific

Seafloor spreading causes abundant magmatism along active ocean spreading centers, but the cause of magmatism along fossil spreading centers is enigmatic. Samples collected from Davidson Seamount, a typical volcanic ridge along an abandoned spreading center in the eastern Pacific, consist of an alkalic basalt to trachyte lava series; transitional basalts were sampled from another part of the abandoned axis, 20 km from the seamount. All samples experienced complex fractional crystallization prior to eruption, but they all share a common, compositionally heterogeneous mantle source. The parental magmas of the transitional basalts were produced from this source at higher degree of melting than those of the alkalic lava series. The composition of Davidson lavas overlaps with those of ridges along other fossil spreading centers and isolated near‐ and off‐ridge seamounts in the eastern Pacific. Together they define a compositional continuum ranging from tholeiitic, normal mid‐ocean ridge basalt (MORB)‐like to alkalic, ocean island basalt (OIB)‐like, similar to lavas that form linear island chains and ridges. We propose that this entire compositional spectrum of intraplate lavas that do not form linear volcanic chains in the eastern Pacific results from variations in the degree of partial melting of a common, compositionally heterogeneous mantle source. This source consists of more easily melted, geochemically enriched components of varying sizes and amounts embedded in a depleted lherzolitic matrix. Large degree of partial melting produces normal MORB–like melts represented by some near‐ridge seamount lavas, whereas small degree of melting produces OIB‐like fossil spreading center lavas. The small degree of partial melting beneath recently abandoned spreading centers results from either buoyancy‐driven decompression melting of the hot lithospheric and asthenospheric mantle material beneath active spreading centers or rapid motion, with respect to the underlying asthenosphere, of abandoned spreading axes that are thickening over a fertile mantle. Mid‐Tertiary volcanic rocks in coastal California, which are compositionally akin to intraplate lavas, are interpreted to be small degree partial melts of the same compositionally heterogeneous sub‐Pacific mantle that has upwelled through windows in a subducted slab.

Discussion of “Kimberlites and aillikites as probes of the continental lithospheric mantle”, by D. Francis and M. Patterson (Lithos v. 109, p. 72–80)

Abstract The origin of kimberlites and volatile-rich alkaline ultramafic magmas such as aillikites has been a matter of considerable scientific interest and the recent paper by Francis and Patterson (Francis and Patterson, 2009. Kimberlites and Aillikites as probes of the continental lithospheric mantle. Lithos 109, 72–80) attempts to summarize advances in this research field. They introduce new major element geochemical discrimination diagrams and present a petrogenetic model that allegedly has implications for diamond exploration. Contrary to the statements of Francis and Patterson we argue that kimberlite bulk chemical compositions are not determined by contamination with lithospheric mantle material alone and cannot be used to predict reliably the diamond grade of new discoveries. We dispute their petrogenetic model, specifically the proposal that calls for carbonate-rich aillikite melts being parental to kimberlite magmas. Rather, we promote the idea of carbonated ultramafic lamprophyre magmas, such as aillikites, being parental to many carbonatite intrusions in areas of rifted cratonic lithosphere, from which archetypal kimberlites appear to be absent. Furthermore, we believe that the geochemical discrimination diagrams (Si versus K and Fe, respectively) introduced by Francis and Patterson do not effectively distinguish between kimberlites and aillikites because there is significant data overlap and bias. Of particular concern is the omission of relevant compositional data for the ultramafic lamprophyre group. In this discussion we re-iterate that bulk chemical analyses of kimberlites and alkaline ultramafic rocks need to be accompanied by petrographic descriptions and mineralogical data. Only by the inclusion of such data can the petrogenesis of the rocks considered be discussed.

Identifying the asthenospheric component of kimberlite magmas from the Dharwar Craton, India

Bulk-rock samples of kimberlites from the Narayanpet and Wajrakarur kimberlite fields of the Dharwar Craton, India have been analysed for major and trace element composition and Nd and Hf isotope ratios. Despite the effect of post-emplacement alteration on a number of elements, the dataset allows inferences to be made regarding kimberlite petrogenesis and the nature of the magma source. It is argued that no significant compositional differences exist between kimberlites of the Narayanpet and Wajrakarur kimberlite fields, despite evidence for significantly different sub-continental lithospheric mantle domains beneath each field. In addition, Nb/U and Ce/Pb ratios are distinct from lithospheric xenoliths, but similar to sub-lithospheric mantle-derived rocks, lending support to models of kimberlite genesis involving direct derivation from asthenospheric depths. We argue that the observed Nd and Hf isotopic compositions of Indian kimberlites were not detectably impacted by the assimilation of sub-continental lithospheric mantle material, and faithfully record the characteristics of an asthenospheric source. Further, based on low eHfi relative to eNdi of samples we conclude that the asthenospheric component must have originated from within or below the transition zone, and may represent ancient subducted oceanic crust.

Primitive off-rift basalts from Iceland and Jan Mayen: Os-isotopic evidence for a mantle source containing enriched subcontinental lithosphere

Abstract New measurements of Os, He, Sr and Nd isotopes, along with major and trace elements, are presented for basalts from the three volcanic flank zones in Iceland and from Jan Mayen Island. The 187Os/188Os ratios in lavas with 30 ppt Os (n = 17) range between 0.12117 and 0.13324. These values are surprisingly low for oceanic island basalts and include some samples that are less than putative present-day primitive upper mantle (PUM with 187Os/188Os of 0.1296). These low 187Os/188Os preclude significant shallow-level contamination from oceanic crust. The 187Os/188Os ratios for Jan Mayen lavas are less than PUM, severely limiting the presence of any continental crust in their mantle source. A positive correlation between 143Nd/144Nd and 187Os/188Os ratios in Iceland and Jan Mayen lavas likely reflects the presence in their source of ancient subcontinental lithosphere that has undergone incompatible trace element enrichment that did not affect the Re–Os system. In addition, the Jan Mayen lava isotopic signature cannot be explained solely by the presence of subcontinental lithospheric mantle, and the influence of another geochemical component, such as a mantle plume appears required. Combined 87Sr/86Sr, 143Nd/144Nd, 3He/4He and 187Os/188Os data indicate a genetic relationship between Jan Mayen Island and the Iceland mantle plume. Material from the Iceland mantle plume likely migrates at depth until it reaches the tensional setting of the Jan Mayen Fracture Zone, where it undergoes low-degree partial melting. At a first-order, isotopic co-variations can be interpreted as broadly binary mixing curves between two primary end-members. One end-member, characterized in particular by its unradiogenic 187Os/188Os and 143Nd/144Nd, low 3He/4He and high 87Sr/86Sr, is represented by subcontinental lithospheric mantle stranded and disseminated in the upper mantle during the opening of the Atlantic Ocean. The second end-member corresponds to a hybrid mixture between the depleted-MORB mantle and the enriched Iceland mantle plume, itself resulting from mixing between recycled oceanic crust and depleted lower mantle. This hybrid accounts for the high 3He/4He (∼28 Ra), high 143Nd/144Nd (∼0.5132), high 187Os/188Os (∼0.14) and low 87Sr/86Sr (∼0.7026) composition observed in Iceland. Two different models may account for these observed mixing relationships between the end-members. In this first model, the Iceland mantle entrains pristine depleted material when rising in the upper mantle and allows refractory sub-lithospheric fragments to melt because of excess heat derived from the deep plume material. A second model that may better account for the Pb isotopic variations observed, uses the same components but where the depleted-MORB mantle is already polluted by subcontinental lithospheric mantle material before mixing with the Iceland mantle plume. Both cases likely occur. Though only three principal components are required to explain the isotopic variations of the Iceland–Jan Mayen system, the different possible mixing relationships may be accounted for by potentially a greater number of end-members.

Geochemistry of post-collisional mafic lavas from the North Anatolian Fault zone, Northwestern Turkey

Abstract Extensive magmatic activity developed at the northwestern part of the Anatolian block and produced basaltic lavas that are situated along and between the two segments of the North Anatolian Fault zone. This region is a composite tectonic unit formed by collision of continental fragments after consumption of Neotethyan ocean floor during the late Cretaceous. Northwestern Anatolian basalts and evolved lavas exhibit both tholeiitic and calc-alkaline characteristics. Mafic lavas are moderately enriched in LILE (except depleted part of Yuvacik and Iznik samples) and depleted in HFSE (but not Zr, Hf) relative to primitive mantle values, suggesting derivation from a MORB-like mantle source that is unexpected in this subduction environment. Sr and Nd isotopes are close to the mantle array and vary beyond analytical error ( 87 Sr/ 86 Sr 0.70404–0.70546, 143 Nd/ 144 Nd 0.51270–0.51289). These geochemical features may result from two possible processes: (1) melting of a MORB-like mantle source that was modified by subduction-released fluids and melts or (2) modification of mafic liquids derived from a dominantly MORB-like source by crustal or lithospheric mantle material. Geochemical characteristics of the lavas (e.g., Ba/Rb, Rb/Sr, Ba/Zr, 87 Sr/ 86 Sr, Sr/P) vary systematically along the fault zone from east to west, consistent with a decrease in the degree of melting from east to west or a change in the nature of the source composition itself. Thus, the difference in incompatible elements and Sr–Nd isotopic ratios seems to result from small-scale mantle heterogeneity in a post-collisional tectonic environment.

Os, Pb, and Nd isotope geochemistry of the Permian Emeishan continental flood basalts: Insights into the source of a large igneous province

Abstract The nature of the source of continental flood basalts (CFB) is a highly debated topic. Proposed mantle sources for CFBs, including both high- and low-Ti basalts, include subcontinental lithospheric mantle (SCLM), asthenospheric mantle, and deep, plume-related mantle. Re–Os isotope systematics can offer important constraints on the sources of both ocean island basalts (OIB) and CFB, and may be applied to distinguish different possible melt sources. This paper reports the first Re–Os isotope data for the Late Permian Emeishan large igneous province (LIP) in Southwest China. Twenty one CFB samples including both low- and high-Ti basalts from five representative sites within the Emeishan LIP have been analyzed for Os, Nd, and Pb isotopic compositions. The obtained Os data demonstrate that crustal assimilation affected Os isotopic compositions of some Emeishan basalt samples with low Os concentrations but not all of the samples, and the Emeishan basalts with high Os contents likely experienced the least crustal contamination. The low and high-Ti basalts yield distinct Os signatures in terms of 187Os/188Os and Os content. The low-Ti basalt with the highest Os concentration (400 ppt) has a radiogenic Os isotopic composition (γOs(t), +6.5), similar to that of plume-derived OIB. Because the Os isotopic composition of basalts with relatively high Os concentrations (typically >50 ppt) likely represents that of their mantle source, this result implies a plume-derived origin for the low-Ti basalts. On the other hand, the high-Ti basalts with high Os concentration (over 50 ppt) have unradiogenic Os isotopic signatures (γOs(t) values range from −0.8 to −1.4), suggesting that a subcontinental lithosphere mantle (SCLM) component most likely contributed to the generation of these magmas. Combining Pb and Nd isotopic tracers with the Os data, we demonstrate that the low-Ti basaltic magmas in the Emeishan CFB were mainly sourced from a mantle plume reservoir, whereas the high-Ti basaltic magmas were most likely derived from a SCLM reservoir or were contaminated by a significant amount of lithospheric mantle material during plume-related magma ascent through the SCLM.

Geochemical and Pb–Sr–Nd isotopic composition of the ultrapotassic volcanic rocks from the extension-related Çamardı-Ulukışla basin, Niğde Province, Central Anatolia, Turkey

Abstract Major, trace element and Sr–Nd–Pb isotope data are presented for the ultrapotassic lavas and dykes from the Late Cretaceous-Early Tertiary Ulukisla Basin in the Central Anatolia. All samples have geochemical characteristics belonging to Group III ultrapotassic rocks (Foley, S.F., Venturelli, G., Green, D.H., Toscani, L., 1987, The ultrapotassic rocks: characteristics, classification and constraints for petrogenetic models, Earth Science Reviews, 24, 81–134.). These rocks have unusually high contents of large-ion-lithophile elements (LILE) (e.g. Ba up to 5900 ppm, K2O up to 8 wt% in lava and 10 wt% in dykes). Negative Nb and Ti anomalies and LREE enrichments relative to HREE on chondrite normalized trace and rare earth element patterns indicate that subduction related material is present in the mantle source region. Their high initial 87Sr/86Sr (0.70798–0.70917) and low 143Nd/144Nd (0,512109–0,512239) ratios suggest that they originated from an enriched lithospheric mantle source with low Sm/Nd ratios. The elevated 207Pb/206Pb (15.743–15.797), low 143Nd/144Nd ratios and geochemical features such as low Nb/La and elevated Ce/Sr ratios may reflect the involvement of sediments as a metasomatic agent for the source region. The steep trend on the 207Pb/204Pb vs 206Pb/204Pb diagram also imply that the metasomatic component represents recycled continent-derived material in the source region. Integration of the geochemistry with regional and local geological data suggest that the ultrapotassic volcanic rocks from the Camardi-Ulukisla basin were derived from a lithospheric mantle material in a post-collisional extension-related geodynamic setting following Late Mesozoic continental collision between the Eurasian plate and Tauride-Anatolide platform, as a result of convergence between the Eurasian and Afro-Arabian plates.

Cenozoic lithospheric extension induced magmatism in Southwest Japan

Island chains off western Kyushu are the surface exposure in the northern margin of the Taiwan–Sinzi Folded Zone that spreads along the arc–trench system in the back-arc side from SW Japan to Taiwan. Intermittent igneous activity between the Middle Miocene and Holocene occurred on these islands and widely covered or intruded sedimentary rocks of Early–Middle Miocene. Geochemistry of the volcanic rocks from the Hirado, Ikitsuki and Takushima islands believed to relate to the back-arc opening along the East China and Japan Seas shows a temporal change in source material. Submarine to sub-aerial volcanism occurred on Hirado Island at 15 Ma during the final opening stage of the East China Sea producing tholeiitic basalt and associated andesite–dacite. These eruptives show low incompatible element contents and high FeO*/MgO ratios and reflect a tholeiitic differentiation trend. High Sr and Pb and low Nd isotopic ratios suggest the involvement of EM2-like lithospheric mantle and crustal material in the formation of these syn-opening volcanic rocks. Post-opening alkali basalt volcanism occurred at 9–6 Ma on the islands is characterized by OIB-like higher large ionic lithophile elements (LILE) and high field strength elements (HFSE) compared to 15 Ma basalts in this region and Quaternary basalts along the volcanic front. They have variable range of incompatible element concentrations and ratios along with variable Sr, Pb and Nd isotopic ratios suggesting the involvement of both lithospheric and asthenospheric sources at variable melting degrees (from 4% to less than 15%). The observation that the isotopic compositions of Quaternary alkali basalts south of the studied area are even more depleted suggests an increase in the involvement of asthenospheric source with time.

159 Ma Kjakebeinet lamproites (Dronning Maud Land, Antarctica) and their implications for Gondwana breakup processes

Lamproite dykes that cross-cut Middle Jurassic Karoo-related flood basalts in Vestfjella, Dronning Maud Land, Antarctica, have been dated at 158.7±1.6 Ma by 40Ar/39 Ar incremental heating of phlogopite. Compared to most lamproites, these dykes are unusually low in SiO2 (40.09–40.18 wt%) and high in CaO (12.35–16.24 wt%) and P2O5 (3.03–3.81 wt%). They have broad affinities to spatially and temporally related group II kimberlites of southern Africa, but the geochemically closest correlatives are CaO- and P2O5-rich diopside madupitic lamproites from Leucite Hills, USA. The Kjakebeinet lamproites are notably high in incompatible elements, show negative initial εNd values (−6.0, −6.7) and positive initial εSr values (+14, +17) with TDM model ages of c. 1.1 Ga, and probably record melting of metasomatized subcontinental lithospheric mantle beneath western Dronning Maud Land. Their emplacement records the youngest recognized magmatic event in the region, coincides with the proposed second and final stage of Gondwana breakup between Africa and Antarctica, and may be associated with deep-seated coast-parallel strike-slip faulting in Dronning Maud Land. Survival of old, easily fusible lithospheric mantle material 20 Ma after the eruption of voluminous flood basalt magmas suggests that the latter were emplaced within narrow zones of lithospheric thinning and that the source of lamproites remained largely intact between these zones.

Primordial helium isotope signature from Plio–Quaternary alkaline basalts in Yemen

Abstract Isotopic compositions of He, Ne and Ar were measured on Plio–Quaternary alkaline basalts of Marib–Sirwah and Shuqra volcanic fields in Yemen, south‐western Arabian Peninsula. Very high 3He/4He isotope ratios were found in olivine phenocrysts of some Quaternary alkaline basalts in both volcanic fields, located on the margin of the dispersed Afar mantle plume, compared with the Afar–Ethiopian province in the center of the mantle plume. This suggests that the Afar mantle plume source may consist of common component (C or focal zone (FOZO)) with variable primordial 3He/4He ratio rather than high μ mantle (HIMU) component. The three component mixing C as the Afar mantle plume, depleted mantle (DM) as upper mantle and lithospheric mantle with a hybrid enriched mantle I–II (EM I–EM II) characteristics may be adequate to explain He–Sr–Nd–Pb isotope variation for the Afar–Arabian Cenozoic volcanics. The occurrence of high 3He/4He ratios in the Marib–Sirwah volcanic field appears to show that the primitive basaltic magma, derived from the margin of the dispersed trous‐like Afar mantle plume during 15–0 Ma, was not by contamination of lithospheric and upper mantle materials in comparison with that from the center of the Afar mantle plume as a result of relatively low thermal anomaly.

Petrogenesis and Stratigraphy of the High-Ti/Y Urubici Magma Type in the Parana Flood Basalt Province and Implications for the Nature of 'Dupal'-Type Mantle in the South Atlantic Region

The high-Ti/Y Urubici (or Khumib) magma type of the Paraná–Etendeka large igneous province has a restricted spatial extent, near the southeast Brazilian coast and in the northern Etendeka (Namibia). Urubici flows are interbedded with low-Ti/Y Gramado flows. Flow correlations indicate that local topographic relief was important in controlling emplacement of flows, and that lavas near the coast have undergone up to 1 km of post-magmatic uplift relative to inland areas. Urubici magmas have undergone extensive fractional crystallization (MgO <5.5 wt %). Stratigraphic variations highlight complexities of mixing and minor crustal assimilation indicative of open-system magmatic plumbing. The least contaminated samples have high La/Nb (∼1.5) and (Tb/Yb)N (∼2.5), Sr–Nd isotopes close to Bulk Earth (87Sr/86Sri ∼0.7050; εNdi −2.7), and Dupal Pb isotopes with unradiogenic 206Pb/204Pb (∼17.6). These features are similar to those of the Walvis Ridge DSDP (Deep Sea Drilling Project) Site 525A basalts that define the EM1 oceanic mantle component, and many are also shared with local Cretaceous alkalic magmas that are inferred to be lithospheric mantle melts. Low 206Pb/204Pb material found in the Urubici and Site 525A basalts is not seen as a mixing end-member within the modern Tristan plume system or in South Atlantic mid-ocean ridge basalt. An origin from lithospheric mantle material, delaminated and dispersed within the asthenosphere following continental break-up, is preferred. Thus the South Atlantic Dupal mantle anomaly cannot be considered as a single entity: Urubici flood basalts and Walvis Ridge Site 525A basalts have a relatively shallow origin within originally lithospheric mantle, whereas the Tristan plume is a deep mantle upwelling.

Plume‐ridge interactions of the Discovery and Shona mantle plumes with the southern Mid‐Atlantic Ridge (40°‐55°S)

We report on 66 Pb, Sr, and Nd isotope analyses of basalts dredged along the Mid‐Atlantic Ridge (MAR) from 40° to 55°S. The results strongly indicate interaction and mixing between the off‐ridge Discovery and ridge‐centered Shona mantle plumes and the ambient asthenosphere beneath the MAR. In addition, the Bouvet mantle plume appears to be feeding the southernmost portion of the MAR as suggested earlier by le Roex et al [1987]. The Discovery and Shona plumes have enriched mantle and high‐μ(μ = 238U/204Pb) affinities, respectively. Their proximity to one another suggests a genetic relationship, probably associated with subducted altered oceanic crust recycled through the mantle with some sediment (Discovery) or without sediment (Shona). The Discovery Ridge Anomaly exhibits Pb, Sr, and Nd isotopic discontinuities resulting from southward preferential plume flow across the Agulhas transform beginning ∼13 Ma. The presence of a component with unusually low 206Pb/204Pb accompanied by high 87Sr/86Sr and low 208Pb/204Pb and 143Nd/144Nd in the Discovery Ridge Anomaly and to a lesser extent in the Shona Ridge Anomaly indicates three‐component mixing between the ambient asthenosphere, the Discovery and Shona plumes, and this low‐μ (LOMU) component which possibly represents subcontinental lithospheric mantle material. We also note that in Pb, Sr, and Nd isotopic space, ocean island basalts from the Tristan, Gough, and Discovery family of plumes could be interpreted as resulting from binary mixing between a generic plume component similar to Bouvet or the “C” component [Hanan and Graham, 1994] and the LOMU component, which progressively increases southward. The LOMU component seems to be a characteristic feature of the South Atlantic and Indian Ocean mantles and is thought to reside passively in the shallow mantle because of delamination of subcontinental lithospheric mantle following the breakup of Gondwana.

A joint study of experimental deformation and experimentally induced microstructures of pretextured peridotites

A series of deformation tests was performed on samples from natural xenoliths and dunites with well‐defined textures and microstructures. The rock samples were deformed perpendicular to the foliation, 45° to the foliation, and parallel to the foliation at temperatures from 1000°C to 1200°C, at a confining pressure of 300 MPa, and a constant strain rate of 10−5/s to obtain insight into the rheological behavior of anisotropic lithospheric mantle material. The experiments showed strong work hardening for all samples with a large increase of the flow stress at relatively low strain. In all samples, both brittle and plastic mechanisms were operative. We observed nonlocalized‐dilatant and localized‐dilatant deformation features. Nonlocalized and localized plastic deformation created subgrain boundaries, deformation lamellae, and an increase in dislocation density in the olivine crystals. Heterogeneous dislocation activities occurred on the [100] (0kl) system at 1000°C, and on the [100] (010) and [101] (010) systems at 1200°C. Strong crystallographic fabrics developed in samples with olivines that were initially oriented with low resolved shear stresses on their slip systems. The postdeformation fabric intensity was lower in specimens with strong initial petrofabrics. Interactions between dislocations of different type led to high work hardening rates during high‐temperature deformation, favoring intragranular microcracking. Fractures opened parallel to the [100] screw dislocations in the (010) plane, and in the {hk0} planes oblique to the (100) plane. The strength of the deformed samples was temperature and orientation dependent. Rocks deformed perpendicular to the foliation are strongest, and those deformed parallel to the foliation have intermediate rock strength similar to those deformed at 45° to the foliation. Based on the laboratory results, we find important implications for the rheology of the lithosphere: (1) Peridotites develop semibrittle rheology even at high temperature, and show characteristic microstructures and fabric changes. (2) The orientation of compressive stress and initial petrofabric are important parameters for defining a deformation regime if pressure, temperature, and strain rate are fixed. (3) Interaction of slip systems produces strong work hardening and intragranular microcracking.

A deep mantle source for carbonatite magmatism: evidence from the nephelinites and carbonatites of the Buhera district, SE Zimbabwe

Abstract In the Buhera district of SE Zimbabwe the Dorowa and Shawa complexes are composed of dolomitic carbonatites with MgO and Mg# values comparable to those anticipated for mantle melts. These carbonatites are associated with olivine-bearing nephelinitic plugs and dykes which are correlatives of the picritic and nephelinitic lavas of the basal Karoo volcanic succession in the Nuanetsi–North Lebombo (NNL) region. The NNL volcanics are isotopically anomalous. Previous detailed Sr, Nd, Pb and Os isotopic studies indicate that the anomalous signatures result from mixing between enriched lithospheric mantle and deep mantle plume material. New geochemical and isotopic data are presented which reveal that the Buhera nephelinites have signatures similar to, but more enriched, than those of the NNL attaining e Nd values of −22 to −15 at e Sr of +8 to +20. By contrast the Shawa carbonatites are not isotopically anomalous with e Nd (0 to +1) and e Sr (+3 to +6) comparable to the `mildly depleted mantle' signature typical of carbonatites globally. A single analysis of a Dorowa carbonatite has an intermediate signature ( e Sr +7; e Nd −10). These relationships provide unequivocal evidence that the Buhera nephelinites and carbonatites were derived from discrete, unrelated parental magmas derived from different sub-cratonic mantle segments. Clearly, the source of the Shawa carbonatites must lie below the zone of ancient LREE enriched lithosphere sampled by the Buhera and NNL silicate magmas and their isotopic signatures more closely approximate the asthenospheric component identified in the NNL picrites. As such these results require the existence of discrete carbonatite magmas at mantle depths in excess of those producing nephelinitic magmatism.

Ophiolitic magmatism in the Ligurian Tethys: an ion microprobe study of basaltic clinopyroxenes

Ion microprobe data (REE, Na, Sc, Ti, V, Cr, Sr, Zr) of unaltered clinopyroxenes in the ophiolitic basalts from the Northern Apennines have been used in a epx-based geochemical modelling of MORB magmatism from both External (EL) and Internal (IL) sectors of the Ligurian Tethys (i.e. Jurassic Ligure-Piemontese basin), alternative to the more common whole-rock approach. Clinopyroxenes from EL basalts display slightly fractionated LREE (CeN/SmN∼0.5) and HREE (GdN/ YbN∼1.5) patterns and large variations in the REE composition (up to 6 times from microphenocryst cores to interstitial clinopyroxenes). Interstitial clinopyroxenes in IL basalts are similar to the microphenocrysts from the most primitive EL basalts. By contrast, IL microphenocrysts are characterized by greater LREE (CeN/SmN ∼0.3) and lesser HREE (GdN/YbN<1.2) fractionation. The comparison of trace element variations in wholerocks and clinopyroxenes clearly shows that the olivine and plagioclase portion of the fractionation sequence is poorly represented by the EL and IL basalts. In fact, ophiolitic basalts mainly consist of a minor interstitial glass (now deeply altered) associated with a prevailing plagioclase-clinopyroxene assemblage crystallized from liquids significantly evolved along the olivine-plagioclase-clinopyroxene saturation boundary. Thus, bulk rock chemistry is largely governed by clinopyroxene composition. This, in addition to alteration, indicates that the bulk rock chemistry does not provide reliable chemical information to constrain the composition and the generation of the parental magmas. Unfortunately, most clinopyroxenes are characterized by complex zoning, probably caused by disequilibrium partitioning during crystal growth as a result of kinetic factors. On this ground, estimation of melt chemistry and inferences about the origins of these basalts are only allowed by the core compositions of microphenocrystic clinopyroxenes. Modelling of (Nd/Yb)N and Ti/Zr in the parental magmas, as deduced from the clinopyroxene compositions, indicates thata EL and IL basalts do not represent products of different mantle source composition. Rather, they were generated by varying degrees of fractional melting in the spinel stability field, lower for the EL (a few percent) relative to IL, totalling no more than 10% of an asthenospheric MORB source, and leaving in the residua clinopyroxene with REE patterns similar to those shown by IL suboceanic type peridotites. Accordingly, these latter are interpreted as refractory residua after MORB-generating fractional melting occurred during rifting and opening of the Ligure-Piemontese basin. By contrast, residual clinopyroxenes from the EL subcontinental type peridotites are not consistent with low degrees of fractional melting in agreement with the current interpretation that EL peridotites are unrelated to the MORB magmatism in the Ligure-Piemontese basin and represent lithospheric mantle material already emplaced towards the surface by a tectonic denudation mechanism during the early stages of oceanic rifting.