Evidence For Crustal Contamination Research Articles

The origin of the ultrahigh-pressure Tso Morari complex, NW Himalaya: implication for early Paleozoic rifting

AbstractThe origins and age distribution of the Himalayan high-pressure (HP) and ultrahigh-pressure (UHP) metamorphic rocks are critical for understanding the pre-Himalayan history. Although the protoliths to the UHP Tso Morari eclogites in Ladakh, NW Himalaya are believed to be the Permian Panjal volcanics, the geochronological evidence is absent. Here, we demonstrate that the protoliths of the UHP Tso Morari Complex formed in a continental rift setting at the Indian margin associated with the northern East Gondwana during the Early Paleozoic. Zircon U–Pb dates from eight gneisses and one garnet amphibolite indicate the Early Paleozoic bimodal magmatism of 493–476 Ma, which could be associated with the separation of South China from North India. Except for arc-related eclogites found in the Nidar ophiolite, the eclogites and amphibolites are rift-related, exhibiting enriched light rare earth elements and high concentrations of incompatible elements, along with evidence for crustal contamination. Our findings support the previously reported diversity in the sources and ages of the protoliths of the Himalayan HP–UHP metamorphic rocks along the orogen.

Compositional and geothermobarometric analysis of the Upper Miocene tholeiitic volcanic products in the northern Andes at 5-6° N latitude: The Combia Volcanic Province

The Combia Volcanic Province (~11-5 Ma), is a volcaniclastic sequence located in northwest Colombia between the Central and Western cordilleras at 5-6° N latitude. Its source is associated with the volcanic activity of the magmatic arc produced by the subduction of the Nazca plate under the South American plate in the northern Andes. The distribution, composition, and chronostratigraphy of the province’s deposits are well-known, with the volcanic sequences characterized as compositionally bimodal. The older rocks (ca. 11-9 Ma) display tholeiitic affinity, whereas the younger (ca. 9-5 Ma) are mostly calc-alkaline, with some adakite-like signature recognized. While the magmatic system for the calc-alkaline magmas has been previously extensively studied, the processes that occurred during the magma stagnation and ascent are unknown for the tholeiitic magmas. This work bridges this gap by the study of tholeiitic lava flows outcropping at the center of the province, through petrography, mineral chemistry, whole-rock analysis, and geothermobarometry calculations of the crystallization conditions. Texturally, the rocks are porphyritic with plagioclase (An50-90) and clinopyroxene (augite and pigeonite) phenocrysts and microphenocrysts, embedded in a glassy and microcrystalline groundmass. Compositionally, the rocks vary from basaltic andesite to andesite (52.8-57.8 wt% SiO2), with relative enrichments of LILE to HFSE and REE to chondrite. Crystallization conditions, based on several plagioclase-melt and pyroxene-melt geothermobarometers, were estimated at T=1,095-1,153 °C and P=0.22-0.60 GPa for the plagioclase, T=1,046-1,131 °C and P=0.09-0.21 GPa for the augite, and T=867-1,039 °C and P=0.40-0.60 GPa for the pigeonite. These results suggest a relatively rapid magma ascent for the tholeiitic products as well as an evolution mostly through fractional crystallization. The LILE elements enrichment, the negative trend in the FeOt, TiO2 and CaO versus SiO2 content, together with some disequilibrium textures, are also evidence of crustal contamination and magma recharge. Thus it is proposed that the Combia Volcanic Province started as a simple magmatic system, where the tholeiitic products were generated by a relatively rapid magma ascent. Then, a more complex magmatic system linked to long-term magma stagnation, allowed melt evolution to form calc-alkaline magmas as previously defined.

Insight into crustal contamination and hydrothermal alteration of the Panjal Traps (Kashmir) from O-isotopes

ABSTRACT Whole rock O-isotopes of the Panjal Traps are presented in order to assess the influence of crustal contamination and hydrothermal alteration in their genesis. The basalts from the eastern Kashmir Valley and Lidder Valley have enriched δ18OVSMOW values (9.0‰ to 12.0‰). The basalt with the lowest δ18OVSMOW values (9.0‰ and 9.2‰) were likely affected by deuteric alteration but their values could be close to the original melt composition as the rocks do not show trace element (Th/NbPM ≈0.8; Nb/U ≈ 50; Th/La ≈0.1) or isotopic evidence of crustal contamination (87Sr/86Sri = 0.7043 to 0.7045; εNd(t) = +1.1 to +1.3). The δ18OVSMOW (> 12‰) values and Nd isotopes (εNd(t) < −8.6) of the silicic Panjal Traps are consistent with derivation from continental crust. The remaining mafic rocks have enriched Sr-Nd-O isotopic values that indicate crustal (10–30%) contamination (εNd(t) = −1.9 to −6.1; 87Sr/86Sri = 0.7051 to 0.7087; δ18O = 10.1‰ to 12.0‰) with the upper flows exhibiting further enrichment by hydrothermal alteration. The basalts from the Pir Panjal Range, western Kashmir Valley, have variable Nd isotopic values (εNd(t) = −6.8 to +4.3) and the lowest δ18OVSMOW values (6.8‰ to 7.9‰) of the study. The results demonstrate that the rocks from the Pir Panjal Range preserved not only differences in radiogenic isotopes but also the O-isotopes as well. The change in the Nd and O isotopes of the basalt from Guryal Ravine and Lidder Valley to the more depleted values of the Pir Panjal Range is likely due to a transition from a chondritic mantle source to a depleted mantle source as the continental rift evolved to a sea-floor spreading environment.

Pyrometamorphosed Otago Schist xenoliths cause minor contamination of Dunedin Volcanic Group basanite

ABSTRACT We examine the pyrometamorphic changes and resulting crustal contamination that occurred when fragments of Otago Schist were entrained and pyrometamorphosed in Dunedin Volcanic Group (DVG) basanite. Otago Schist greenschist facies quartz-albite laminae were replaced by fine-grained quartz grains (inverted from β-quartz or tridymite) with orthopyroxene coronas and dacitic-rhyolitic glass. Former muscovite + chlorite + epidote ± garnet segregations were converted to plagioclase (An30-80), olivine, spinel, cordierite, ilmenite, pigeonite and trachytic to trachy-andesitic glass. Minerals largely formed by mineral replacement and then by crystal nucleation from a quenched liquid as protolith minerals passed their liquidi, with the latter process generating swallow-tailed, skeletal, dendritic and ladder-structured crystal morphologies. Orthopyroxene-, olivine- and plagioclase-liquid geothermometry indicates xenoliths reached at least 940°C. Despite the molten state of the xenoliths, discernible chemical and isotopic (Sr, Nd) contamination of the basanite was inhibited by its enriched trace element budget. The only evidence for crustal contamination comes from basanite Pb isotopes, which have less radiogenic compositions within several cm of the xenoliths. The subtle variation in DVG Sr and Nd isotopes may therefore be due to different mantle source components rather than crustal contamination by the underlying Otago Schist.

Long-lived, Eocene-Miocene stationary magmatism in NW Iran along a transform plate boundary

The Eocene-Miocene Mianeh-Hashtroud igneous district in NW Iran is part of the Turkish–Caucasus–Iranian collision zone, a key region to decipher the assembly and differentiation of Gondwana-derived terranes along the Alpine-Himalayan convergence zone. Major inherited tectonic structures control in space and time the Mesozoic-Cenozoic transition from oceanic subduction to continental collision in the region. The geology of the study area is dominated by a polyphase, long-lived magmatic activity, spanning from ~45 to ~6 Ma. The igneous products are subalkaline to alkaline, with intermediate to acid compositions and a high-K calcalkaline to shoshonitic affinity. Evidence of crustal contamination is attested by inherited zircons in the oldest (Eocene-Oligocene) samples, with ages spanning from Neo-Archean to Paleocene. The Sr-Nd isotopic compositions of the Eocene-Oligocene samples plot close to the Bulk Silicate Earth estimate, whereas the Miocene samples document stronger crustal contamination. The lack of correlation between Nd-Sr isotopes and SiO2 supports a scenario of magma differentiation of different magma batches rather than crustal contamination. Major oxide and Sr-Nd isotopic variation lead us to suggest that magmatism is the consequence of re-melting of earlier underplated (Mesozoic-Tertiary) magmatic products, controlled by amphibole-dominated fractionation processes. Regional scale correlations show long-lived Cenozoic magmatism in NW Iran and Caucasus region, where the main porphyry and epithermal deposits occur. We propose that the Cenozoic collisional magmatism and the associated mineralisation at the junction between NW-Iran and Caucasus was controlled by the activity of a major, lithosphere-scale inherited boundary, transverse to the convergence zone. In such a geodynamic setting, the along-strike segmentation of the lithosphere slab generated asthenospheric melts, their upwelling into the metasomatised supra-subduction mantle wedge and the potential activation of different mantle and crustal sources, with consequent mineral endowment in the region.

Concentrations of Te, As, Bi, Sb and Se in the Marginal Zone of the Bushveld Complex: Evidence for crustal contamination and the nature of the magma that formed the Merensky Reef

The association of platinum-group elements (PGE) and the chalcophile elements Te, As, Bi, Sb and Sn (TABS) has been extensively documented in several magmatic sulfide deposits over the past years. However, understanding the roles of TABS during the formation of magmatic sulfide deposits partially depends on constraining the concentration of TABS on the liquids from which they crystallized. This study presents the distribution of TABS (apart from Sn) in rocks of the Marginal Zone of the Bushveld Complex. These rocks record the composition of the parental magmas from which the Bushveld Complex have crystallized. The major and trace elements of the marginal rocks have been modelled as mixtures of komatiite, continental crust and a plagioclase-rich residuum. Similar mixtures are required to model the TABS in the marginal rocks, with the continental crust component contributing a large part of the As, Sb and Bi budgets in these melts. The concentrations of the TABS in the Merensky Reef can be modelled as a mixture of two of the magma types present in the Marginal Zone (the B-1 and B-2). The modelling also reveals that the distributions of Se, Te and Bi in the reef are essentially controlled by the presence of sulfide minerals, whereas As and Sb distributions are controlled by both sulfide minerals and melt component. This is because Se, Te and Bi are moderately to strongly chalcophile elements, but As and Sb are only slightly chalcophile elements. Thus, whole-rock As and Sb concentrations are not upgraded by the formation of the sulfide minerals, and may still be used to trace crustal contamination.

Geochemical constraints on the petrogenesis of Triassic alkaline basalts of Sierra de Valle Fértil, Western Sierras Pampeanas, Argentina: implications for their origin, evolution and tectonic setting

The first geochemical and Sr-Nd isotope data of a series of Triassic basaltic rocks from Sierra de Valle Fértil, Western Sierras Pampeanas, Argentina, are herein presented. The mafic rocks are the result of continental intraplate magmatism related to an extensional tectonic setting after the climax of the Gondwanide orogeny (Upper Carboniferous-Permian). Major minerals phases of these rocks are olivine, clinopyroxene, plagioclase and Fe-Ti oxides. Olivine and clinopyroxene occur both as phenocrysts and as groundmass, while feldspar is mostly restricted to the groundmass. The rocks belong to a typical alkaline series displaying SiO2 contents of 43.9–51.6 wt%, Na2O between 3.1 and 5.4 wt%, K2O between 0.9 and 2.9 wt% and high TiO2 2.2–3.6 wt% and P2O5 0.5–1.2 wt% contents. Nepheline on its CIPW normative composition indicates that it crystallized from silica-undersaturated magmas. Samples have moderate to low Mg# (58.1–15.9) values and Cr and Co contents that indicate they are not primary magmas but have undergone fractional crystallization with earlier fractionation of olivine and clinopyroxene. Geochemical patterns are similar to those found in the ocean island basalts (OIBs), with enrichment of alkalis and incompatible elements: LILE, HFSE and LREE, strong fractionation of HREE relative to LREE [(La/Yb)N = 9.6–22.4], no depletion of Nb and Ta and absence of negative Eu anomaly. Observed LILE-enrichment is indicated by the alkaline nature of the basaltic rocks and suggests melting of an enriched mantle. The Eu and Sr positive anomalies, not related to plagioclase accumulation, argue against significant plagioclase fractionation and are original geochemical characteristics of the primary magma. The chemical features, together with high incompatible-element ratios, are consistent with low degrees of partial melting of a dominantly garnet-pyroxenite mantle source. The trace-element patterns and isotopic data with 143Nd/144Nd(i) (0.51257–0.51267), positive εNd(i) and low 87Sr/86Sr(i) (0.70292–0.70339) values, suggest a mantle source influenced by an enriched-component. Active subduction-related mechanisms promoted the conditions for decompression melting in the asthenospheric mantle. Primary basaltic magma experienced subsequent compositional modifications by fractional crystallization in shallow-level low-pressure magma storage reservoirs. There is no evidence of crustal contamination during magma ascent.

Oceanic mafic magmatism in the Siletz terrane, NW North America: Fragments of an Eocene oceanic plateau?

The Siletz terrane, a predominantly mafic accreted oceanic terrane, is located in the Cascadia forearc region of Oregon, Washington and Vancouver Island. The terrane represents a late Palaeocene–Eocene large igneous province that consists of pillow lavas, massive flows and intrusive sheets. Previously it has been proposed that the Siletz terrane represents either an accreted oceanic plateau, hotspot island chain, backarc basin, island arc, or a sequence of slab window volcanics. A province-wide geochemical reassessment of the terrane, including new high precision Sr-Pb-Nd-Hf isotope data, has been used to assess the validity of the proposed tectonomagmatic models for the Siletz terrane. The trace element data show little evidence of crustal contamination, or an arc signature, and the samples have rare earth element (REE) patterns that are flat to light REE enriched. These features are similar to other oceanic plateaus such as the Ontong Java and the Caribbean. Initial isotope ratios range from 206Pb/204 Pb: 18.751 to 19.668, 207Pb/204Pb: 15.507 to 15.661, 208Pb/204Pb: 38.294 to 39.2128, 176Hf/177Hf: 0.28300 to 0.28316 (εHf: 9.0 to 14.5), 143Nd/144Nd: 0.51282 to 0.51299 (εNd: 5.0 to 8.1) and 87Sr/86Sr: 0.70302 to 0.70380. These data are consistent with a mantle source of the Siletz terrane that appears to have been heterogeneous and slightly enriched. The enriched signature has characteristics of both EM2 and HIMU components and this, combined with a calculated mantle potential temperature well above ambient mantle, indicates derivation of the Siletz magmatism from a mantle plume, possibly the Yellowstone Hotspot. We therefore conclude that the Siletz terrane represents an accreted oceanic plateau.

Late Paleozoic to Mesozoic extension in southwestern Fujian Province, South China: Geochemical, geochronological and Hf isotopic constraints from basic-intermediate dykes

The tectonic evolution of SE China block since late Paleozoic remains debated. Here we present a new set of zircon U-Pb geochronological, Lu-Hf isotopic data and whole-rock geochemistry for two stages of basic-intermediate dykes from the southwestern Fujian. The samples were collected from the NE-trending (mainly diabases) and NW-trending (mainly diabasic diorites) dykes and yielded zircon U-Pb ages of 315 and 141 Ma, with εHf (t) values of −8.90 to 7.49 and −23.39 to −7.15 (corresponding to TDM2 values of 850 to 1890 Ma and 737 to 2670 Ma), respectively. Geochemically these rocks are characterized by low TiO2 (0.91–1.73 wt.%) and MgO (3.04–7.96 wt.%), and high Al2O3 (12.5–16.60 wt.%) and K2O (0.60–3.63 wt.%). Further they are enriched in LREEs and LILEs (Rb, Ba, Th and K), but depleted in HFSEs (Nb, Ta and Zr). The tectonic discrimination analysis revealed that the dykes were formed in an intraplate extensional environment. However, the NW trending dykes show crust-mantle mixed composition, which indicate an extensional tectonic setting with evidence for crustal contamination. The SE China block experienced two main stages of extensional tectonics from late Carboniferous to early Cretaceous. The tectonic evolution of the SE China block from late Devonian to Cretaceous is also evaluated.

Geochemistry and Petrogenesis of Tertiary Volcanic Rocks of the Eastern Roodbar, Alborz Mountain, North of Iran

In the Alborz Mountains of the eastern Roodbar (north of Iran), Tertiary volcanic rocks have a variety of composition between olivine basalt, basaltic andesite, pyroxene andesite and andesite. The presence of different xenoliths and xenocrysts is among the evidence of crustal contamination of these rocks. The positive correlations of CaO/Al2O3 vs to MgO and Al2O3 vs. SiO2 are of signs of the olivine and clinopyroxene fractionation in the variation trend of the area rocks. Positive correlations K2O/P2O5 vs with SiO2 and La/Sm vs. K2O/P2O5 demonstrate contamination of magma with the continental crust. The incompatible trace element patterns and their comparison with crustal contents indicate contamination of the rocks of the area with the lower and upper continental crust. Linear trends in the variation diagram of Nb/Y vs. Zr/Y, introduce two different source regions: a MORB source and the other continental crust for the rocks which are the genesis. The variations of Y/Nb vs. Zr/Nb and Rb/Y vs. Nb/Y reveal a crustal contamination of the magma originated from the MORB source. Geochemical studies represent that the area’s rocks were derived from the 15% melting of a mantle source of MORB type with spinel facies within a continental environment, which was contaminated by varying degrees of continental crustal rocks.

Os–Nd–Sr isotopes in Miocene ultrapotassic rocks of southern Tibet: Partial melting of a pyroxenite-bearing lithospheric mantle?

Miocene post-collisional ultrapotassic rocks in the southern and central parts of the Lhasa Terrane of southern Tibet provide an opportunity to explore the deep processes and lithospheric evolution of the Tibetan Plateau. The magmatic source of the ultrapotassic rocks is still debated. However, the source can be identified using the Re–Os isotopic system. In this paper, we provide comprehensive data on the Re–Os isotopic compositions of ultrapotassic rocks from Mibale and Maiga areas in southern Tibet, and we refine the age of the Mibale ultrapotassic rocks to 12.5Ma. The Os isotopic data demonstrate that crustal assimilation affected the Os isotopic compositions of some ultrapotassic rocks with low Os contents, but samples with high Os contents have little or no evidence of crustal contamination. The initial 187Os/188Os ratios of the least-contaminated ultrapotassic rocks are higher than those of primitive upper mantle (PUM). The ultrapotassic rocks show a weak correlation between initial 187Os/188Os ratios and Mg# values, a negative correlation between εNd(t) and Mg# values, and high Ni contents and FeO/MnO ratios. These observations indicate that the ultrapotassic rocks were derived from a pyroxenite-bearing lithospheric mantle. Simple calculations indicate <20% pyroxenite in the lithospheric mantle, which is consistent with the pyroxenite xenoliths found in the ultrapotassic rocks of southern Tibet. The Os model ages for the ultrapotassic rocks in the south Lhasa Terrane range from 75 to 541Ma, indicating that the lithospheric mantle beneath southern Tibet underwent multiple magmatic events. We conclude, therefore, that convective removal of a pyroxenite-bearing lithospheric mantle or break-off of the Indian continental lithospheric mantle could have resulted in the generation of the ultrapotassic rocks in southern Tibet.

Geochemistry and petrogenesis of Ghohroud Igneous Complex (Urumieh–Dokhtar zone): evidence for Neotethyan subduction during the Neogene

The Oligo–Miocene Ghohroud granitoid is located in the north of Isfahan, in the Urumieh–Dokhtar magmatic arc (UDMA). The rock composition ranges from granodiorite to tonalite. Rocks mainly include quartz, plagioclase, and alkali feldspar with minor biotite and amphibole. Dacitic and basaltic rocks outcrop as dykes. Geochemical data show that these rocks are subalkaline, calc-alkaline, and metaluminous. Microgranular enclaves, sieve texture, oscillatory zoning in plagioclase, and clear roundness of quartz pieces demonstrated magma mixing. The magma has experienced fractional crystallization that has led to the presence of different petrological units. Large-ion lithophile element (LILE) and Pb enrichment is evidence of crustal contamination. Considering the clear negative anomaly of Nb and Ti, the magma has been formed in a volcanic arc environment. The geochemistry of major and rare earth elements reveals that initial magma has been formed by partial melting of lower crustal protoliths. An underplating magma can supply heat source for the dehydration melting of lower crust and generation of Ghohroud granitic melt by melting of a mafic source during the subduction of the Neotethyan oceanic plate and the Central Iran continental crust.

Variation in parental magmas of Mt Rouse, a complex polymagmatic monogenetic volcano in the basaltic intraplate Newer Volcanics Province, southeast Australia

Monogenetic volcanoes have long been regarded as simple in nature, involving single magma batches and uncomplicated evolutions; however, recent detailed research into individual centres is challenging that assumption. Mt Rouse (Kolor) is the volumetrically largest volcano in the monogenetic Newer Volcanics Province of southeast Australia. This study presents new major, trace and Sr–Nd–Pb isotope data for samples selected on the basis of a detailed stratigraphic framework analysis of the volcanic products from Mt Rouse. The volcano is the product of three magma batches geochemically similar to Ocean–Island basalts, featuring increasing LREE enrichment with each magma batch (batches A, B and C) but no evidence of crustal contamination; the Sr–Nd–Pb isotopes define two groupings. Modelling suggests that the magmas were sourced from a zone of partial melting crossing the lithosphere–asthenosphere boundary, with batch A forming a large-volume partial melt in the deep lithosphere (1.7 GPa/55.5 km); and batches B and C from similar areas within the shallow asthenosphere (1.88 GPa/61 km and 1.94 GPa/63 km, respectively). The formation and extraction of these magmas may have been due to high deformation rates in the mantle caused by edge-driven convection and asthenospheric upwelling. The lithosphere–asthenosphere boundary is important with respect to NVP volcanism. An eruption chronology involves sequential eruption of magma batches A, C and B, followed by simultaneous eruption of batches A and B. Mt Rouse is a complex polymagmatic monogenetic volcano that illustrates the complexity of monogenetic volcanism and demonstrates the importance of combining detailed stratigraphic analysis alongside systematic geochemical sampling.

Evidence for crustal contamination in intra-continental OIB-like basalts from West Qinling, central China: A Re–Os perspective

The magnitude of crustal contamination in intra-continental OIB-like basalts is commonly difficult to assess in terms of major, trace-element and Pb–Sr–Nd isotopes. However, the Re–Os isotope system is a very sensitive tracer of the crustal component during formation of basaltic rocks. Here we report new Sr–Nd–Pb–Os geochemical and isotopic data from the Cretaceous Duofutun Volcanic Rocks (DVRs) of West Qinling, central China. The DVRs exhibit low Os abundances (4.46–42.02ppt) with Re abundances ranging from 45.58 to 812.98ppt, variably high Re/Os (3–126), and high initial 187Os/188Os ratios (0.2112–0.6784). We propose that the degree of partial melting is responsible for the first-order control on the variation of Os concentrations. Even the least radiogenic sample still possesses a higher initial 187Os/188Os ratio than the primitive upper mantle, implying a non-negligible contribution from high Re/Os and 187Os/188Os reservoir(s). Both the crust and pyroxenite-rich mantle are reliable candidates, but the latter cannot be reconciled with the Os mass balance between pyroxenite and ambient peridotite. Hence, we contend that these rocks were contaminated by crustal materials en route to the surface. Assimilation/fractional crystallization modeling manifests the observed isotopic variations can be produced by minor assimilation of the Proterozoic mafic lower crust. In addition, the intra-continental OIB-like basalts show a relatively wider range of 187Os/188Os ratios in comparison with MORB and OIB, confirming the continental crust can have a higher concentration of radioactive Os, which is induced by time-integrated growth.

Zircon U–Pb ages and Hf isotopic compositions of alkaline silicic magmatic rocks in the Phan Si Pan-Tu Le region, northern Vietnam: Identification of a displaced western extension of the Emeishan Large Igneous Province

In-situ zircon U–Pb and Hf isotope analyses were carried out for alkaline silicic magmatic rocks from the Phan Si Pan-Tu Le region in northern Vietnam to constrain their possible sources and to determine their petrogenetic relationship with the Emeishan Large Igneous Province (ELIP), SW China. Nine granites and nine rhyolites yield zircon 206Pb/238U ages from 262Ma to 249Ma, coinciding with the timing of silicic magmatism in the Panxi area of the ELIP. The zircon εHf(t) values (+14 to +3) of these granites and rhyolites suggest a moderately depleted mantle source and overlap with those of peralkaline and metaluminous granites in the Panxi area [i.e. εHf(t)=+14 to +4]. The zircon Hf isotope ratios show that the zircons probably record the original source characteristics whereas whole-rock Nd isotope data indicate an evidence for crustal contamination that may have occurred at lower temperatures during magma emplacement. The synchroneity, coupled with petrological and geochemical similarities, indicate that silicic rocks from the Phan Si Pan-Tu Le region are cogenetic with the Panxi silicic plutonic rocks and that they are likely derived by similar petrogenetic processes (i.e. fractionation of mafic magmas or partial melting of mafic rocks). Therefore, we propose that the Phan Si Pan-Tu Le region represents a displaced portion of the ELIP inner zone.

The Upper Triassic alkaline magmatism in the NW Iberian Chain (Spain).

We have carried out a complete study of the Upper-Triassic alkaline magmatism on the northwestern margin of the Iberian Chain. This magmatism is composed of mafic sills, which intrude the Keuper facies in two geographic sectors: the Cameros Massif (NW) and the Moncayo Massif (SE). In both of these sectors, the rocks are characterised by an intense alteration (spilitisation). The field relationships point to an Upper Triassic age for the studied sills, given that: 1) interactions are recognised between the magma and wet, unconsolidated host sediments, suggesting that the magma was emplaced during or shortly after the deposition of the Keuper facies; 2) in the Moncayo sector a conglomerate bed that includes igneous clasts overlaies the sills and passes upwards to the Norian-Rhaetian Imon Fm. According to the petrological and geochemical features of the sills, an internal differentiation can be recognised inwards, with the developments of three zones: chilled margins, central facies and pegmatoid facies. The rocks are porphyritic and are mainly composed of phenocrysts of pseudomorphed olivine and microcrysts of plagioclase, opaque minerals and minor clinopyroxene. Vesicles and xenoliths of the host sedimentary rocks are also recognised, especially at the chilled margins. The mineral assemblage is constant across the sills but the mineral proportions vary among the different zones. The sills are basic to intermediate rocks. They are classified as basalts to dacites with an alkaline geochemical affinity. According to the trace element contents, this magmatism is related to an enriched sub-lithospheric mantle source affected by crustal contamination. Considering the Upper Triassic magmatisms from southwestern Europe, two types can be distinguished. On the one hand, the magmatisms in the Catalonian Coastal Ranges and SE France are paleogeographically located in the inner platform and are related to deep, astenospheric mantle sources with no evidence of crustal contamination. On the other hand, the magmatisms in the NW Iberian Chain –this study-, the External Betics and the Brescian Prealps are situated in a more litoral position and are related to a shallower, lithospheric mantle with evidence of crustal contamination. These differences could be related to the thickness of the continental crust, which might be thinner in the inner areas of the platform.

Alkali feldspar syenites with shoshonitic affinities from Chhotaudepur area: Implication for mantle metasomatism in the Deccan large igneous province

Two petrologically distinct alkali feldspar syenite bodies (AFS-1 and AFS-2) from Chhotaudepur area, Deccan Large Igneous Province are reported in the present work. AFS-1 is characterized by hypidiomorphic texture and consists of feldspar (Or55Ab43 to Or25Ab71), ferro-pargasite/ferro-pargasite hornblende, hastingsite, pyroxene (Wo47, En5, Fs46), magnetite and biotite. AFS-2 exhibits panidiomorphic texture with euhedral pyroxene (Wo47-50, En22-39, Fs12–31) set in a groundmass matrix of alkali feldspar (Or99Ab0.77 to Or1.33Ab98), titanite and magnetite. In comparison to AFS-1, higher elemental concentrations of Ba, Sr and ∑REE are observed in AFS-2. The average peralkaline index of the alkali feldspar syenites is ∼1 indicating their alkaline nature. Variation discrimination diagrams involving major and trace elements and their ratios demonstrate that these alkali feldspar syenites have a shoshonite affinity but emplaced in a within-plate and rifting environment. No evidence of crustal contamination is perceptible in the multi-element primitive mantle normalized diagram as well as in terms of trace elemental ratios. The enrichment of incompatible elements in the alkali feldspar syenites suggests the involvement of mantle metasomatism in their genesis.

The Ahmeyim Great Dyke of Mauritania: A newly dated Archaean intrusion

A precise U–Pb baddeleyite age of 2733±2Ma has been obtained for the Ahmeyim Great Dyke of Mauritania that intruded into the Tasiast–Tijirit Terrane of the Reguibat Shield, NW Mauritania. This dyke is approximately 1500m wide at the sampling area and extends for more than 150km NNE/SSW. Major and trace element geochemistry of the dyke indicates that the magmas that formed this intrusive body were sub-alkaline, tholeiitic and boninitic, and the presence of a negative Nb anomaly indicates the involvement of subducted oceanic lithosphere during magma genesis, most likely an inherited signature from earlier subduction events and the Mesoarchaean collision of the Tasiast–Tijirit and Choum–Rag el Abiod Terranes. A palaeomagnetic study was also undertaken on samples collected from two different sections across the dyke. However, no within- or inter-site grouping of any palaeomagnetic directions could be identified, thus precluding any palaeographic interpretation. The Ahmeyim Great Dyke is interpreted to be part of the feeder system for a 2733Ma Large Igneous Province (LIP); tholeiitic–komatiitic greenstone belts of this age are absent in the West African Craton (WAC) but are present on many other blocks. However, additional constraints are required to reliably link the Ahmeyim Great Dyke with any other such LIP-type greenstone belts in late Archaean supercontinent reconstructions. The magmas that formed the Ahmeyim Great Dyke were boninitic; this, combined with evidence of crustal contamination, the scale of the dyke and its potential link (as a feeder) to greenstone belts of tholeiitic–komatiitic affiliation within other crustal blocks suggests that it, and cogenetic magmatic units elsewhere, may be prospective for magmatic Ni–Cu–PGE sulphide exploration.

Petrogenesis of basaltic shergottite Northwest Africa 5298: Closed‐system crystallization of an oxidized mafic melt

Abstract– Northwest Africa (NWA) 5298 is an evolved basaltic shergottite that has bulk characteristics and mineral compositions consistent with derivation from an oxidized reservoir in Mars. Chemically zoned clinopyroxene (64.5%, augite and pigeonite), with interstitial lath‐shaped plagioclase (29.4%, An40 to An55), constitutes the bulk of this meteorite. The plagioclase has been converted by shock to both isotropic maskelynite and spherulitic, birefringent feldspar representing a quenched vesicular melt. The remainder of the rock consists of minor amounts of Fe‐Ti oxides (ilmenite and titanomagnetite), phosphates (merrillite and apatite), silica polymorph, fayalite, pyrrhotite, baddeleyite, and minor hot desert weathering products (calcite and barite). Oxygen fugacity derived from Fe‐Ti oxide thermobarometry is close to the quartz‐fayalite‐magnetite (QFM) buffer indicating that the late stage evolution of this magma occurred under more oxidizing condition than those recorded in most other shergottites. Merrillite contains the largest abundances of rare earth elements (REE) of all phases, thereby controlling the REE budget in NWA 5298. The calculated bulk rock REE pattern normalized to CI chondrite is relatively flat. The evolution of the normalized REE patterns of the bulk rock, clinopyroxene, plagioclase, and phosphate in NWA 5298 is consistent with closed‐system chemical behavior with no evidence of crustal contamination or postcrystallization disturbance of the REE contents of these phases.

Contrasting komatiite belts, associated Ni–Cu–(PGE) deposit styles and assimilation histories

The Agnew–Wiluna greenstone belt in the Yilgarn Craton of Western Australia is the most nickel-sulfide-endowed komatiite belt in the world. The Agnew–Wiluna greenstone belt contains two mineralised units/horizons that display very different volcanological and geochemical features. The Mt Keith unit comprises >500 m-thick spinifex-free adcumulate-textured lenses, which are flanked by laterally extensive orthocumulate-textured units. Spinifex texture is absent from this unit. Disseminated nickel sulfides, interstitial to former olivine crystals, are concentrated in the lensoidal areas. Massive sulfides are locally present along the base or margins of the lenses or channels. The Cliffs unit is locally >150 m thick and comprises a sequence of differentiated spinifex-textured flow units. The basal unit is the thickest, and contains basal massive nickel-sulfide mineralisation. The Mt Keith and Cliffs units display important common features: (i) MgO contents of 25–30% in inferred parental magmas; and (ii) Al/Ti ratios of ∼20 (Munro-type). However, the Mt Keith unit is highly crustally contaminated (e.g. LREE-enriched, high HFSEs), whereas the Cliffs unit does not display evidence of significant crustal assimilation. We argue that the distinct trace-element concentrations and profiles of the two komatiite units reflect their different emplacement style and country rocks: the Mt Keith unit is interpreted to have been emplaced as an intrusive sill into dacitic volcanic units whereas the Cliffs unit was extruded as lava flow onto tholeiitic basalts in a subaqueous environment. The mode of emplacement and nature of country rock is the single biggest factor in controlling mineralisation styles in komatiites. On the other hand, evidence of crustal contamination does not necessarily provide information of the prospectivity of komatiites to host Ni–Cu–(PGE) mineralisation, despite being a good proxy for the style of komatiite emplacement and the nature of country rocks.