How the delamination and detachment of lower crust can influence basaltic magmatism

Formation of enriched mantle components by recycling of upper and lower continental crust

The basaltic basement of the large igneous province formed by the Kerguelen Plateau and Broken Ridge in the southeastern Indian Ocean has been sampled by the three Ocean Drilling Program cruises (Legs 119, 120 and 183). Although the Cretaceous parts of this plateau formed in the embryonic Indian Ocean basin, presumably by melting associated with the Kerguelen plume, trace element abundances and isotopic ratios of Sr, Nd and Pb of Cretaceous basalt from several drill sites indicate that continental lithosphere was involved in their petrogenesis. On the basis of relative depletions in Nb, Ta and Th, and isotopic characteristics similar to those of EMI ocean island basalt, lavas from Leg 120 Site 747 in the Central Kerguelen Plateau contain a component derived from the lower continental crust. On the basis of relative abundances of Sr and Eu and EMI-like Pb isotopic ratios, the source of basalt from Leg 120 Site 750 in the northeastern part of the Southern Kerguelen Plateau also contained a component derived from lower continental crust; in this case, the crustal component formed as a plagioclase-rich, clinopyroxene-bearing cumulate. Basalts from Leg 120 Site 749 define two distinct isotopic (Sr, Nd and Pb) groups which differ from the isotopic fields for Site 747 and 750 basalts. Among Site 749 lavas, there is subtle evidence for a continental component, broadly similar (i.e. moderate 200Pb/201Pb ~ 18.0) to that expressed more obviously in basalt from Leg 119, Site 738 on the southern edge of the Southern Kerguelen Plateau and Leg 183 Site 1137 on Elan Bank. The continental components in the Kerguelen Plateau basalts may have resided in a heterogeneous mantle plume that was formed, in part, by deep recycling of crust. It is more likely, however, that slivers of Gondwana lithosphere reside within the lithosphere and athemosphere of the Indian Ocean mantle where they contaminate both plume-derived and mid-ocean ridge basaltic magmas.

Involvement of continental crust in the formation of the Cretaceous Kerguelen Plateau: New perspectives from ODP Leg 120 sites

Ocean Drilling Program (ODP) Leg 183 was designed to investigate the origin and evolution of the large igneous province composed of the Kerguelen Plateau and Broken Ridge. Of the eight sites drilled, basalt was recovered from seven, five on the plateau and two on Broken Ridge. We present results from four of these sites, 1136, 1138, 1141 and 1142. Although this large igneous province is interpreted as being derived from the Kerguelen mantle plume, the geochemical characteristics of basalt from some parts of the province indicate a role for continental lithosphere. The 118–119 Ma basalt flows recovered in the Southern Kerguelen Plateau (Site 1136) have a more subtle continental signature than shown by basalt at Leg 119 Site 738. A continental signature is absent in the 100–101 Ma tholeiitic basalts at Site 1138 in the Central Kerguelen Plateau (CKP); their age-corrected Nd–Sr–Pb isotopic values and incompatible element ratios are similar to those estimated for primitive mantle. These flows may represent a major mantle source in the Kerguelen starting-plume head. The 20 basalt units identified are a product of magma chamber replenishment, fractional crystallization, and resorption of crystallizing phases. The topmost unit, Unit 1, is a dacite that evolved from a basalt magma similar to those represented by Units 3–22; unlike the basalts the dacite magma was probably influenced by continental material. Middle Cretaceous (∼95 Ma) lavas of Sites 1141 and 1142 on Broken Ridge (originally part of the CKP) are alkalic, with one exception (a tholeiite at the base of Site 1142). The alkalic lavas may represent a late-stage cap or carapace of relatively low-degree partial melts that overlies a thick tholeiitic lava pile. The tholeiite and pebbles from the top of a probable talus deposit (Unit 2) at Site 1142 have geochemical signatures consistent with a minor contribution from continental material. This signature is absent in the other units from these two sites, which have ocean-island-like incompatible element ratios and age-corrected isotopic characteristics similar (but not identical) to those proposed for the post-30 Ma Kerguelen plume. These alkalic basalts may be the purest representatives of the Cretaceous plume tail composition yet found.

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Mantle sources and the highly variable role of continental lithosphere in basalt petrogenesis of the Kerguelen Plateau and Broken Ridge LIP : Results From ODP Leg 183

To decipher the petrogenesis of chromitites from the Moho Transition Zone of the Cretaceous Oman ophiolite, we carried out detailed scanning electron microscope and electron microprobe investigations of ∼500 silicate and chromite inclusions and their chromite hosts, and oxygen isotope measurements of seven chromite and olivine fractions from nodular, disseminated, and stratiform ore bodies and associated host dunites of the Maqsad area, Southern Oman. The results, coupled with laboratory homogenization experiments, allow several multiphase and microcrystal types of the chromite-hosted inclusions to be distinguished. The multiphase inclusions are composed of micron-size (1–50 μm) silicates (with rare sulphides) entrapped in high cr-number [100Cr/(Cr + Al) up to 80] chromite. The high cr-number chromite coronas and inclusions are reduced (oxygen fugacity, fO2, of ∼3 log units below the quartz–fayalite–magnetite buffer, QFM). The reduced chromites, which crystallized between 600 and 950°C at subsolidus conditions, were overgrown by more oxidized host chromite (fO2 ≈ QFM) in association with microcrystal inclusions of silicates (plagioclase An86, clinopyroxene, and pargasite) that were formed between 950 and 1050°C at 200 MPa from a hydrous hybrid mid-ocean ridge basalt (MORB) melt. Chromium concentration profiles through the chromite coronas, inclusions, and host chromites indicate non-equilibrium fractional crystallization of the chromitite system at fast cooling rates (up to ∼0·1°C a−1). Oxygen isotope compositions of the chromite grains imply involvement of a mantle protolith (e.g. serpentinite and serpentinized peridotite) altered by seawater-derived hydrothermal fluids in an oceanic setting. Our findings are consistent with a three-stage model of chromite formation involving (1) mantle protolith alteration by seawater-derived hydrothermal fluids yielding serpentinites and serpentinized harzburgites, which were probably the initial source of chromium, (2) subsolidus crystallization owing to prograde metamorphism, followed by (3) assimilation and fractional crystallization of chromite from water-saturated MORB. This study suggests that the metamorphic protolith assimilation occurring at the Moho level may dramatically affect MORB magma chemistry and lead to the formation of economic chromium deposits.

A new view on the petrogenesis of the Oman ophiolite chromitites from microanalyses of chromite-hosted inclusions

(16·9–17·2) and Sr/Sr (up to 0·706), whereas the second In this work we investigate the olivine-phyric basalt suite of the group with low La/Th (5–9) has higher Pb/Pb (17·7–18·1) Aphanasey Nikitin Rise, an intraplate volcanic structure formed and Sr/Sr (up to 0·713). The tholeiite composition is likely during the Late Cretaceous in the Indian Ocean. The parental melt to be controlled by contamination of the parental tholeiitic melts by of the basalt suite has a hypersthene-normative tholeiitic composition continental crust derived from cratonic Gondwanan lithosphere. This with low H2O content (0·3–0·5 wt %) and high SiO2/Al2O3 conclusion, combined with other evidence for ancient crustal material (3·5). The basalt suite is characterized by Nb, Ta, Th and U in the Indian and Atlantic Oceans, indicates the impact of continental depletion, and uniquely low Pb/Pb and Nd/Nd among crust in the oceanic lithosphere. the Cretaceous tholeiitic basalts of the Indian Ocean. Our modelling demonstrates that fractional crystallization of depleted mantlederived melt and lower continental crust assimilation is a suitable model for the genesis of the parental magma of this suite. The

Petrogenesis of Olivine-phyric Basalts from the Aphanasey Nikitin Rise: Evidence for Contamination by Cratonic Lower Continental Crust

The paper presents the very first data on concentrations of major and trace elements; Sr, Nd, and Pb isotopic ratios of rocks; and the composition of olivine phenocrysts of 38-Ma basalts recovered by Hole 513a (DSDP Leg 71) in the South Atlantic. The bulk-rock samples and the chilled glasses are mildly magnesian (7–8 wt % MgO) and bear elevated FeO and low Na2O concentrations, as is typical of MORB of the TOR-1 type. Olivine phenocrysts (Fo84.5–88) in these rocks contain concentrations of trace elements (Ni, Mn, Cr, and Zn) that are typical of classic MORB, which are produced by partial melting of mantle peridotite. The rocks are strongly depleted in incompatible elements [(La/Sm)n ~ 0.6] but have elevated Ba/Nb, K/Nb, and Pb/Ce ratios and Cu, Ag, and Au concentrations that are 1.5–4 times higher than in typical depleted MORB (N-MORB) and in most rift basalts in the South Atlantic. Isotope compositions of the basalts (average ratios 206Pb/204Pb ~ 18.0; 207Pb/204Pb ~ 15.6, 208Pb/204Pb ~ 38.0, 143Nd/144Nd ~ 0.5130, and 87Sr/86Sr ~ 0.7040) are close to those in modern tholeiites from the southern MAR segment (SMAR) north of the Agulhas Fracture Zone. The data indicate that the magmas were derived from a strongly depleted mantle source that contained a minor (~3%) admixture of an enriched component, which is discernible in the magmas of the Discovery hotspot. The composition of the source, which is more depleted than DM, and the high degrees of melting of this source explain why the basalts from DSDP Hole 513a are enriched in chalcophile elements. It is believed that spreading magmatism at 45°–48° S in SMAR as far back as 40 Ma was already affected by the Discovery hotspot. This hotspot might be related to the Tristan plume system, and its origin and long-lasting influence on spreading magmatism in the South Atlantic are regarded as evidence of the extensive effect of the Tristan plume.

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B. V. Belyatsky

Papers

Petrogenesis of Olivine-phyric Basalts from the Aphanasey Nikitin Rise: Evidence for Contamination by Cratonic Lower Continental Crust

Long-Lasting Influence of the Discovery Plume on Tholeiitic Magmatism in the South Atlantic: Data on Basalts Recovered by Hole 513a, DSDP Leg 71