Contribution Of Lithospheric Mantle Research Articles

Magnesium and zinc isotopic evidence for the involvement of recycled carbonates in the petrogenesis of Gaussberg lamproites, Antarctica

Lamproites are rare mantle-derived peralkaline ultrapotassic rocks, and they are commonly geographically associated with the ultramafic lamprophyres and kimberlites. Their unique geochemistry and mineralogy make determining their mantle source and origin important because of the significance for inferring specific geodynamic processes. In this study, we further examine lamproite petrogenesis using new Mg and Zn isotopic data for the typical Gaussberg lamproites, Antarctica, the source of which were thought to be contributed by recycled crustal materials. Results show that these lamproites have lower δ26Mg (−0.44‰ to −0.39‰) and higher δ66Zn (0.36‰ to 0.39‰) values than terrestrial mantle (δ26Mg = −0.25 ± 0.04‰, δ66Zn = 0.18 ± 0.05‰). The post-magmatic alteration and crustal contamination as well as fractional crystallization and partial melting cannot account for these anomalous Mg and Zn isotopic values. By contrast, the involvement of sedimentary carbonates which are characterized by light δ26Mg (average approximately −2.0‰) and heavy δ66Zn (average ~ +0.91‰) values in their mantle source can explain these Mg and Zn isotopic anomalies. Quantitative modelling suggests that addition of 10–15% subducted dolomite into the source of Gaussberg lamproites can well reproduce their Mg and Zn isotopic values. The source component with light Mg and heavy Zn isotopic compositions can either be sub-continental lithospheric mantle metasomatized by carbonate melts or residue of subducted carbonate-bearing sediments after deep melting in the mantle transition zone. A lithospheric mantle contribution is indeed required to explain their strongly enriched radiogenic isotopic compositions. However, in terms of carbonate component, their positive ZrHf anomalies (Hf/Hf* = 1.28–2.19), and extremely high K/U (~40, 000) and Ba/Th (~400) ratios lead us to favor the latter deep recycling model in which the recycled carbonate-bearing sediments subducted as K-hollandite and majorite underwent partial melting within the mantle transition zone.

The potential contribution to long wavelength magnetic anomalies from the lithospheric mantle

Long-wavelength magnetic anomalies (LWMA) are broad scale variations of the magnetic field that are usually observed at high altitudes with amplitudes ranging up to about 20 nT. The source of these anomalies has typically been assumed to reside within the crust with negligible contributions from the lithospheric mantle. Recent work on the magnetization of mantle xenoliths suggests a possible lithospheric mantle contribution to LWMA, depending on tectonic setting and local geotherm. Here, we investigate the potential contribution of the lithospheric mantle to LWMA across two contrasting lithospheric geotherms: cratons and hotspots. To constrain the range of possible mantle contributions to satellite-observable LWMA in these settings, we develop a Monte Carlo suite of forward models of magnetic anomalies patterned after the mantle portions of the Siberian craton, the Hawai'i hotspot, and the French Massif Central plume. The models incorporate the current geotherms, the pressure dependence of the Curie temperature of magnetite, and the statistical distributions of rock magnetic data from mantle xenoliths in the specific regions. The Siberian craton shows the highest potential contribution of the mantle to LWMA with amplitudes of a few nT over wavelengths scaling to the long-axis of the craton. The Hawai'i hotspot region displays a potential contribution from the lithospheric mantle to satellite-measured magnetic anomalies on the order of 1 nT, while the Massif Central plume regions shows an insignificant contribution to satellite magnetic anomalies from lithospheric mantle. When compared with observed lithospheric total field anomalies derived from Swarm satellite data, the results show that the lithospheric mantle may contribute a few tens of percent to LWMA observed at satellite altitudes in specific regions depending on the average remanent magnetization in mantle minerals, the thickness of magnetized mantle (which depends on the geotherm), and the spatial extent of the region under consideration.

Distinguishing Plume and Metasomatized Lithospheric Mantle Contributions to Post-Flood Basalt Volcanism on the Southeastern Ethiopian Plateau

Abstract Magmatism in the East African Rift System (EARS) contains a spatial and temporal record of changing contributions from the Afar mantle plume, anciently metasomatized lithosphere, the upper mantle and the continental crust. A full understanding of this record requires characterizing volcanic products both within the rift valley and on its flanks. In this study, three suites of mafic, transitional to alkaline lavas, were collected over a northeast-southwest distance of ∼150 km along the southeastern Ethiopian Plateau, adjacent to the Main Ethiopian Rift. Specifically, late Oligocene to Quaternary mafic lavas were collected from Chiro, Debre Sahil and the Bale Mountains. New major element, trace element, 40Ar/39Ar ages and isotopic results (Sr, Nd, Pb, Hf, Os, He) show spatial and temporal variation in the lavas caused by dynamical changes in the source of volcanism during the evolution of the EARS. The trace element compositions of Oligocene and Miocene Chiro lavas indicate derivation from mildly depleted and nominally anhydrous lithospheric mantle, with variable inputs from the crust. Further south, Miocene Debre Sahil and alkaline Bale Mountains lavas have enriched incompatible trace element ratios (e.g. Ba/Nb = 12–43, La/SmN = 3·1–4·9, Tb/YbN = 1·6–2·4). Additionally, their 87Sr/86Sr, 143Nd/144Nd, 176Hf/177Hf and 206Pb/204Pb values trend toward a radiogenic Pb (HIMU) component. Radiogenic 187Os/188Os in these lavas correlates positively with 206Pb/204Pb and trace element indicators consistent with ancient metasomatic enrichment of their mantle source. In contrast, transitional Miocene Bale Mountains lavas have lower incompatible trace element abundances, less enriched trace element ratios (Ba/Nb ∼7, La/SmN = 2·3–2·5) and less radiogenic isotopic signatures that originate from melting garnet-bearing, anhydrous lithospheric mantle (Tb/YbN = 2·5–2·9). Pliocene and Quaternary Bale Mountains basaltic lavas are chemically and isotopically similar to Main Ethiopian Rift lavas. Trace element and isotopic indicators in both of these suites denote an amphibole-bearing source distinct from that sampled by the older Bale Mountains lavas. Isotopically, Pliocene and Quaternary Bale lavas have notably less radiogenic Sr–Nd–Pb–Hf isotopic ratios. Quaternary Bale Mountains lavas have the strongest mantle plume contribution (3He/4He = 12·1–12·5 RA), while other Bale Mountains, Debre Sahil and Chiro lavas were derived dominantly by melting of lithospheric or upper mantle sources (3He/4He = 5·1–9·1 RA). A multi-stage, regional-scale model of metasomatism and partial melting accounts for the spatial and temporal variations on the southeastern Ethiopian Plateau. Early Debre Sahil and alkaline Bale Mountains mafic lavas are melts derived from Pan-African lithosphere containing amphibole-bearing metasomes, while later transitional Bale basalts are melts of lithosphere containing anhydrous, clinopyroxene-rich veins. These ancient metasomatized domains were eventually removed through preferential melting, potentially during thermal erosion of the lithosphere or lithospheric foundering. Pliocene and Quaternary Bale Mountains lavas erupted after tectonic extension progressed throughout Ethiopia and was accompanied by increased plume influence on the volcanic products.

Late Paleoproterozoic and Mesoproterozoic magmatism of the Nico Pérez Terrane (Uruguay): Tightening up correlations in southwestern Gondwana

New geochronological, isotopic and geochemical data were obtained from the late Paleoproterozoic and Mesoproterozoic magmatic units of the Nico Pérez Terrane (Uruguay). A U-Pb LA-ICP-MS zircon age of 1768 ± 11 Ma confirms the Statherian age for the Illescas rapakivi intrusion, being thus comparable with the age of the Campanero Unit felsic orthogneisses. Though both the Illescas and Campanero intrusions exhibit dominantly granitic, meta- to peraluminous compositions, the first shows a dominant shoshonitic and ferroan composition, whereas high-K calc-alkaline and magnesian to ferroan compositions characterize orthogneisses of the Campanero Unit. Additionally, Sm/Yb and LaN/YbN ratios indicate that both suites were emplaced in a thickened crust, whereas Lu-Hf zircon data of the Illescas granite together with available Sm-Nd data of the Campanero Unit point to significant recycling of an older, Archean and/or Paleoproterozoic crust of the Nico Pérez Terrane. Results suggest a common origin for both the Illescas and Campanero magmatism, probably related to a post-collisional/post-orogenic setting. On the other hand, metagabbros of the Zanja del Tigre Complex present U-Pb LA-ICP-MS zircon ages of 1479 ± 4 and 1482 ± 6 Ma and tholeiitic gabbroic compositions. Geochemical and Sm-Nd isotopic data show similarities with LIP-related intracontinental mafic magmatism, suggesting a mixed magma source derived from depleted asthenosphere and fertile subcontinental lithospheric mantle contributions. Emplacement of the metagabbros was related to lithospheric extension in a continental rifting setting and, therefore, fertilization of the lithospheric mantle probably took place during older Paleoproterozoic subduction-related events. Coeval late Paleoproterozoic and early Mesoproterozoic magmatism is also recorded in the northern Nico Pérez Terrane of southernmost Brazil, and in basement inliers of the Kaoko Belt and the Angolan Shield (southwestern Congo Craton), thus suggesting a common evolution of these blocks prior to the Neoproterozoic.

Tectonic evolution of the North Qinling Orogenic Belt, Central China: Insights from metamafic rocks of the Songshugou Complex

High‐pressure (HP) and ultrahigh‐pressure (UHP) rocks have been widely recognized in the North Qinling Orogenic Belt (NQOB). The nature and age of their protoliths can provide insights into the formation and evolution of the NQOB. The Songshugou Complex, which crops out along the Shangdan Suture Zone, mainly consists of high‐grade metamorphosed mafic and ultramafic rocks, and thus, this area is an important study site for understanding the tectonic evolution of the NQOB. The metamafic rocks that enclose the ultramafic rocks are overwhelmingly composed of amphibolites that primarily exhibit amphibolite‐facies metamorphism, and minor garnet amphibolites (retrograde eclogite) and HP mafic granulites occur as discontinuous lenses or blocks within these amphibolites. Laser inductively coupled plasma mass spectrometry (LA‐ICPMS) U–Pb dating on zircons yielded ages of 499 ± 6, 507 ± 5, and 504 ± 10 Ma for the HP mafic granulite, garnet amphibolite (retrograde eclogite), and amphibolite, respectively. Those ages were interpreted to represent timing of the HP granulite or amphibolite metamorphism that occurred during later exhumation. Relict omphacites discovered in both garnet and zircon as well as in the matrix of the garnet amphibolite indicate that the rock underwent eclogite‐facies metamorphism and subsequent rapid exhumation. However, the timing of the different metamorphic stages cannot be distinguished from the zircon ages obtained using the LA‐ICPMS technique. The protolith age of the complex is constrained to be the early Neoproterozoic. Geochemical data indicate that the protoliths of the Songshugou metamafic rocks are of continental origin. These Neoproterozoic continental mafic rocks most likely formed during the breakup of the supercontinent Rodinia and were mainly derived from low‐degree partial melting of the asthenosphere with some lithospheric mantle contribution, within an extensional regime. All the available data reveal that the NQOB was one of the components of the Rodinia supercontinent and that it was likely involved in the Grenville orogenic belt and subsequently separated from the supercontinent at approximately 830–740 Ma.

Precise U-Pb zircon ages and geochemistry of Jurassic granites, Ellsworth-Whitmore terrane, central Antarctica

The Ellsworth-Whitmore Mountain terrane of central Antarctica was part of the early Paleozoic amalgamation of Gondwana, including a 13,000 m section of Cambrian−Permian sediments in the Ellsworth Mountains deposited on Grenville-age crust. The Jurassic breakup of Gondwana involved a regional, bimodal magmatic event during which the Ellsworth-Whitmore terrane was intruded by intraplate granites before translation of the terrane to its present location in central Antarctica. Five widely separated granitic plutons in the Ellsworth-Whitmore terrane were analyzed for their whole-rock geochemistry (X-ray fluorescence), Sr, Nd, and Pb isotopic compositions, and U-Pb zircon ages to investigate the origins of the terrane magmas and their relationships to mafic magmatism of the 183 Ma Karoo-Ferrar large igneous province (LIP). We report high-precision (±0.1 m.y.) isotope dilution−thermal ionization mass spectrometry (ID-TIMS) U-Pb zircon ages from granitic rocks from the Whitmore Mountains (208.0 Ma), Nash Hills (177.4−177.3 Ma), Linck Nunatak (175.3 Ma), Pagano Nunatak (174.8 Ma), and the Pirrit Hills (174.3−173.9 Ma), and U-Pb sensitive high-resolution ion microprobe (SHRIMP) ages from the Whitmore Mountains (200 ± 5 Ma), Linck Nunatak (180 ± 4 Ma), Pagano Nunatak (174 ± 4 Ma), and the Pirrit Hills (168 ± 4 Ma). We then compared these results with existing K-Ar ages and Nd model ages, and used initial Sr, Nd, and Pb isotope ratios, combined with xenocrystic zircon U-Pb inheritance, to infer characteristics of the source(s) of the parent magmas. We conclude that the Jurassic plutons were not derived exclusively from crustal melts, but rather they are hybridized magmas composed of convecting mantle, subcontinental lithospheric mantle, and lower continental crustal contributions. The mantle contributions to the granites share isotopic similarities to the sources of other Jurassic LIP mafic magmas, including radiogenic 87Sr/86Sr (0.706−0.708), unradiogenic 143Nd/144Nd (eNd < −5), and Pb isotopes consistent with a low-µ source (where μ = 238U/204Pb). Isotopes and zircon xenocrysts point toward a crustal end member of predominantly Proterozoic provenance (0.5−1.0 Ga; Grenville crust), extending the trends illustrated by Ferrar mafic intrusive rocks, but contrasting with the inferred Archean crustal and/or lithospheric mantle contributions to some basalts of the Karoo sector of the LIP. The Ellsworth-Whitmore terrane granites are the result of mafic rocks underplating the hydrous crust, causing crustal melting, hybridization, and fractionation to produce granitic magmas that were eventually emplaced as post-Ferrar, within-plate melts at higher crustal levels as the Ellsworth-Whitmore terrane rifted off Gondwana (47°S) before migrating to its current position (82°S) in central Antarctica.

The Nathrop Domes, Colorado: Geochemistry and petrogenesis of a topaz rhyolite

The four Nathrop Domes (∼30 Ma) are located near Buena Vista, Colorado, within the Arkansas Valley Graben segment of the Rio Grande Rift. The domes are largely composed of sparsely to moderately porphyritic, flow-banded rhyolite, with local vitrophyric margins. Phenocrysts include sanidine, plagioclase, quartz, biotite, and Fe–Ti oxides. Bright red Mn-garnet locally occurs in vapor-phase cavities. Two-feldspar and two-oxide temperatures assuming 1 kb pressure were, respectively, 670°C and 653°C. All four domes erupted rhyolite of essentially identical major-element chemistry, although with substantial trace-element variations, high incompatible trace-element contents (Rb up to 364 ppm; Nb up to 67 ppm), and extreme depletion of Ba, Sr, P, Eu, and Ti. These depletions are consistent with fractionation of observed phenocrysts. A rare-earth element diagram shows parallelism of the rhyolite plots, with sloping light REE values, flat heavy REE values, and a prominent, negative Eu anomaly. The narrow range of major-element compositions within the Nathrop Domes’ rhyolites precludes any fractionation modeling amongst them. Nonetheless, their elevated Rb/Sr ratios (as great as 75–120) strongly suggest that these magmas have undergone extensive fractional crystallization. Neodymium isotope analysis shows ∊ Ndt values of −10.1 for a Bald Mountain sample and −13.9 from a Precambrian granite sample (t = 29 Ma). The similarity of the ∊ Ndt of the Bald Mountain rhyolite and the Precambrian granite suggests that the Nathrop rhyolite magmas were initially formed through partial melting of Precambrian rocks. Neodymium Crustal Index (NCI) calculations were performed using the Precambrian granite to estimate the contribution of crustal sources, assuming lithospheric mantle contributions of basalt with ∊ Nd of 0 to 4, or asthenospheric mantle contributions with ∊ Nd of 5 or greater. The resulting calculations indicate NCI values of 0.727 to 0.789, assuming lithospheric mantle, and 0.799 assuming an asthenospheric mantle contribution with ∊ Nd = 5. Thus, 72.7 to 79.9 percent of the Nd present in the Nathrop rhyolite sample is likely from crustal sources, depending upon what type of mantle contribution was involved. The Nathrop rhyolites may represent the earliest phase of magmatism associated with the northern segments of the Rio Grande Rift.

Geothermal and rheological regime in the Po plain sector of Adria (Northern Italy)

We present a thermo-rheological study of the Adriatic crust in the central Po plain (Northern Italy), which records the convergence of the Alpine and Apennine external fronts. The present thermal regime of the crust is inferred from geological and geophysical data from oil exploration. A set of temperature data from 37 deep boreholes (bottom hole, drill stem, and production test temperatures) is used to estimate undisturbed formation temperatures down to 6-7 km of depth. Best quality data are used to estimate upper crustal geotherms, which are subdivided into four groups taking into considerations the following factors: i) presence/absence of relevant fluid circulation; ii) conductive thermal refraction; iii) recent sedimentation, and iv) other disturbing tectonothermal processes (e.g., recent tectonics). Assuming average corrections for temperature data affected by recent sedimentation effects, geotherms fall within the same range and provide a homogeneous thermal signal. Temperature measurements in the upper ~7 km are used to constrain a 1-D thermal model at the crustal scale. The best fitting geotherm gives a thermal gradient of ~23 ± 5 mK m-1 (over 7 km of depth), a surface heat flow of ~46 ± 9 mW m-2, and an estimated Moho temperature of 620 ± 80 °C.Using the estimated crustal structure and composition and the best-fitting geotherm, a representative strength envelope is obtained.For hydrostatic and supra-hydrostatic pore pressures, the calculated total crustal strength of this sector of the Adria microplate (not including the upper mantle contribution) is in the 1.8-2.5 TN m-1 range. Considering also the lithospheric mantle contribution, the total lithospheric strength is estimated to be between 23 (wet peridotite)and 39 (dry peridotite) TN m-1.

A-type granitoids from the Eastern Pontides, NE Turkey: Records for generation of hybrid A-type rocks in a subduction-related environment

The Eastern Pontides is characterized by the many of intrusive bodies formed throughout the late Mesozoic–early Cenozoic. Most of these are I-type granitoids, but here, we present for the first time an A-type pluton from the region to assess source characteristics and geodynamic implications. The A-type Pirnalli pluton has a SHRIMP zircon U–Pb age of 81.2±1Ma. It is composed of granite, syenite and quartz monzonite, and its enclaves are monzonitic in composition with elevated Ga/Al ratios and low Mg# (<42). The pluton is dominantly metaluminous (A/CNK=0.88 to 1.00) and belong to shoshonitic and ultra-potassic series. The samples are highly enriched in LREE and show significant negative Eu (Eu/Eu*=0.33 to 0.92), Ba, Nb, Sr and Ti anomalies in the spidergrams. The host rocks and their enclaves posses quite similar Sr–Nd isotopic compositions (ISr=0.70693 to 0.70736, εNd (81Ma)=−2.6 to −2.0, TDM=0.94 to 1.12Ga), pointing to a lower crustal parental magma with a minor contribution of lithospheric mantle. We hypothesize that upwelling of asthenosphere triggered melting of chemically enriched upper mantle and basic magma formed. Melting of lower crust was provided by the underplating of this basic magma. Mixing between lower crust- and mantle-derived melts at depths of lower crust appears to be most reasonable petrogenetic process responsible for generation of the pluton. Sr–Nd isotope modeling suggests mixing of 82–90% of the lower crustal-derived melt with ~10–18% of the mantle-derived melt. Then the hybrid melt ascended to shallower crustal level and underwent a limited fractionation process to generate a variety of rock types. Our data also suggest that the A-type Pirnalli pluton likely formed at an extensional environment of active continental margin throughout the late Cretaceous. Ongoing extension then led to opening of Black Sea as a back-arc basin further north of the Eastern Pontides.

Ancient heat flow, crustal thickness, and lithospheric mantle rheology in the Amenthes region, Mars

Surface heat flow calculations for the Amenthes region of Mars can be independently performed using the depth to the brittle–ductile transition and the effective elastic thickness of the lithosphere estimated for the Late Noachian/Early Hesperian (equivalent to an estimated absolute age of ∼3.6–3.8 Ga). This, along with crustal heat production rates estimated from heat-producing elements abundances, permits us to put constraints, for that particular place and time, on both the thermal and mechanical properties of the lithosphere and the crustal thickness. The depth to the brittle–ductile transition deduced from modeling of the topography of Amenthes Rupes is 27–35 km, and the associated surface heat flow is 26–37 mW m−2. On the other hand, the effective elastic thickness in this region is between 19 and 35 km: the surface heat flow deduced by considering crustal and lithospheric mantle contributions to the total lithospheric strength, as well as wet or dry olivine for lithospheric mantle rheology, is 31–49 mW m−2. The relatively limited overlap among Te- and zBDT-based heat flow values implies a surface heat flow of 31–36 mW m−2 (with a high fraction originated from crustal heat sources) and a wet mantle rheology. The so obtained local crustal thickness is 43–74 km, which suggests an average thickness of ∼40–75 km for the Martian crust; for the frequently used crustal density of 2900 kg m−3, our results suggest a crustal thickness of 50–63 km for the Amenthes region, and an average crustal thickness of ∼45–65 km for Mars.

Recycling of deeply subducted continental crust in the Dabie Mountains, central China

Post-collisional mafic–ultramafic intrusive rocks from the North Dabie zone show evidence for derivation from a mantle source that was overprinted by partial melts derived from subducted crust of the South China Block. All the samples are characterized by enrichment of large ion lithophile elements (LILE, e.g., Ba, Th) and depletion of high field strength elements (HFSE, e.g., Nb, Ti). The Zhujiapu intrusion developed in the North Dabie zone has moderately enriched 87Sr/ 86Sr i (0.70605–0.70827), negative ε Nd( T) values (− 2.3 to − 18.8), and unradiogenic Pb isotopes. Such typical “continental” geochemical features do not result from crustal contamination or magma mixing, but reflect the properties of the mantle source. Because the trace element and Sr–Nd isotopic compositions of the post-collisional mafic–ultramafic intrusive rocks from the North Dabie zone are similar to those of the Mesozoic mantle-derived rocks and lower crust granulites and xenoliths from the North China Block, it is unclear whether the “continental” features are due to the contribution of lithospheric mantle and lower crust from the North China Block or deeply subducted materials from the South China Block. However, for a given 206Pb/ 204Pb i or Δ7/4, the 208Pb/ 204Pb i or Δ8/4 (159–180) of the post-collisional mafic–ultramafic intrusive rocks from the North Dabie zone is close to the Mesozoic granitoids, eclogites, and gray gneisses from the Dabie–Sulu orogen, but significantly higher than the Mesozoic mantle-derived rocks and lower crust granulites and xenoliths from the North China Block (Δ8/4 < 160). This indicates that the subducted slabs of the South China Block experienced a higher time-integrated Th/U but similar U/Pb as the deep lithosphere of the North China Block. We therefore propose that the mantle source of the post-collisional mafic–ultramafic intrusive rocks from the Dabie–Sulu orogen result from the depleted upper mantle metasomatised by melts from recycled deeply subducted crust. Due to the continent–continent collision between the South China Block and North China Block in the late Triassic, the lithosphere of the Dabie orogen was over-thickened resulting in eclogitic metamorphism in the mafic lower crust, producing a gravitational instability. The subducted eclogitic lower crust foundered and was recycled into the upper mantle beneath the orogen, resulting in mountain root removal, less mafic composition of unusually evolved continental crust, and upper mantle heterogeneity.

Pb–Nd–Sr isotope and trace element geochemistry of Quaternary extension-related alkaline volcanism: a case study of Kula region (western Anatolia, Turkey)

The Quaternary alkaline volcanism of Kula is located in a western Anatolian graben system that resulted from an Aegean extensional regime. The typically silica-undersaturated, alkaline lavas of Kula are distinguished in three different sequences, namely Burgaz, Elekçitepe and Divlittepe, with a range of basanite, tephrite and phonotephrite compositions. To evaluate the source characteristics of the Kula lavas, rare earth elements as well as Sr, Nd and Pb isotopic compositions were determined. The volcanic rocks show significant enrichment in high field strength elements (such as Nb and Ta) and large ion lithophile elements. Furthermore, they exhibit high Nb/Y (>3) ratios, which are typical characteristics of within-plate alkaline volcanic rocks. However, the Rb, Ba, Sr, Nb and Ta contents and Rb/Nb and K/Nb ratios of the Kula lavas are considerably higher than in typical ocean island basalts (OIB), implying a contribution of lithospheric mantle in their genesis. Isotopic compositions of 87Sr/86Sr, 143Nd/144Nd, 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb for the Kula lavas range from 0.703029 to 0.703490, from 0.512773 to 0.512941, from 18.689 to 19.064, from 15.606 to 15.683 and from 38.560 to 39.113, respectively. Assimilation and fractional crystallisation modelling indicates that crustal contamination does not play a significant role in the evolution of the Kula lavas but that they evolved via fractional crystallisation processes. Elemental variations are also related to fractional crystallisation. The geochemical characteristics of the Kula lavas suggest a contribution from two mantle sources: (1) mainly an OIB-like asthenospheric component and (2) a limited contribution from a lithospheric mantle component. The volcanism of Kula is linked to an extensional setting in western Anatolia that developed during Late Miocene–Pliocene. It is correlated to the thinning of the crust in a post-collision period and concomitant with the upwelling of the underlying upper mantle.

Re-Os isotope characteristics of postorogenic lavas: Implications for the nature of young lithospheric mantle and its contribution to basaltic magmas

Re-Os isotopes have been measured on postorogenic potassic lavas from the Tibetan Plateau, the Betic domain of southeastern Spain, and the Colorado Plateau of the southwestern United States. Previous work has established that these lavas were all derived from parts of the subcontinental lithospheric mantle that had undergone metasomatic enrichment in incompatible elements, following various degrees of melt depletion. Cratonic depleted subcontinental lithospheric mantle peridotites typically have subchondritic 187 Os/ 188 Os; however, the postorogenic lavas are characterized by radiogenic 187 Os/ 188 Os ratios (0.139‐0.559). Simple modeling shows that only very large degrees of melt depletion (>25%‐30%) can lower source Re/Os ratios sufficiently to permit timeintegrated development of subchondritic 187 Os/ 188 Os ratios. Such processes may have been largely restricted to the older Precambrian, and the peridotite component of the postorogenic lavas source was probably depleted by <25%. The more radiogenic values may reflect increasing contributions from metasomatic components or possibly crustal contamination. Our findings imply the need for caution in the use of Os isotopes as a diagnostic tracer of subcontinental lithospheric mantle contributions to lavas erupted through younger Proterozoic and Phanerozoic lithosphere.

Geochemistry of late Caledonian minettes from Northern Britain: implications for the Caledonian sub-continental lithospheric mantle

AbstractThe geochemistry of late Caledonian minettes from across the orogenic belt is compared in order to constrain the composition of the Caledonian sub-continental lithospheric mantle (SCLM). All the minettes are similar petrographically and chemically and several samples have characteristics typical of near primary mantle melts. Samples from the Northern Highlands and the Caledonian foreland show enrichment in many trace elements (notably LILE and LREE) relative to those from the Grampians, the Southern Uplands and northern England, coupled with distinct Nd and Sr isotope characteristics. Processes such as fractional crystallization, crustal assimilation, and partial melting played a negligible role in creating the differences between the two groups which reflect long-term, time-integrated differences in the compositions of their SCLM sources. The Great Glen Fault appears to represent the boundary between these two lithospheric mantle domains. Other currently exposed Caledonian tectonic dislocations cannot be correlated directly with compositional changes within the SCLM. The chemical provinciality displayed by the minettes shows some resemblance to that within other late Caledonian igneous suites, including the newer granites, suggesting that the minettes may represent the lithospheric mantle contributions to these rocks.