Geochemistry of peridotite xenoliths in alkali basalts from Jeju Island, Korea

Abstract   Ultramafic xenoliths in alkali basalts from Jeju Island, Korea, are mostly spinel lherzolites with subordinate amounts of spinel harzburgites and pyroxenites. The compositions of major oxides and compatible to moderately incompatible elements of the Jeju peridotite xenoliths suggest that they are residues after various extents of melting. The estimated degrees of partial melting from compositionally homogeneous and unfractionated mantle to form the residual xenoliths reach 30%. However, their complex patterns of chondrite-normalized rare earth element, from light rare earth element (LREE)-depleted through spoon-shaped to LREE-enriched, reflect an additional process. Metasomatism by a small amount of melt/fluid enriched in LREE followed the former melt removal, which resulted in the enrichment of the incompatible trace elements. Sr and Nd isotopic ratios of the Jeju xenoliths display a wide scatter from depleted mid-oceanic ridge basalt (MORB)-like to near bulk-earth estimates along the MORB–oceanic island basalt (OIB) mantle array. The varieties in modal proportions of minerals, (La/Yb)_N ratio and Sr-Nd isotopes for the xenoliths demonstrate that the lithospheric mantle beneath Jeju Island is heterogeneous. The heterogeneity is a probable result of its long-term growth and enrichment history.     

Evidence for Modal Metasomatism in the Orogenic Spinel Lherzolite Body from Caussou (Northeastern Pyrenees, France)

Metasomatic mineral-bearing and/or trace element-enriched ultramaficassemblages have been reported from very few Alpine-type massifs.The small ultramafic body from Caussou (Ari?ge, northeasternPyrenees) compared with other north Pyrenean ultramafic complexesshows distinctive features which are similar to those of modallymetasomatized mantle xenoliths found in alkali basalts. It ismainly composed of clinopyroxene-rich spinel lherzolites (cpx/opxratios 1), with subordinate titanian pargasite-rich peridotites,both greatly depleted in orthopyroxene. Moreover the Caussouperidotites differ from other Ari?ge peridotites in the presenceof ilmenite, the abundance of sulfide inclusions in pyroxenesand amphiboles, higher Al, Ca, Na, K, Ti, and lower Mg contents,and enrichment in incompatible trace elements (ITE). Such mineralogicaland geochemical features are interpreted as resulting from modalmetasomatism produced by influxes of silicate melt into theperidotites. At Caussou, the metasomatic assemblage comprisesTi-pargasite+Ti-bearing clinopyr oxene+ilmenite+Ti-phlogopite+sulphide+fluid,suggesting that K, Ti, Na, ITE (including S, H₂O CO₂ and possiblyFe and Ca, were introduced by the metasomatizing agent. Thismetasomatism was probably imposed on an ultramafic associationdominated by LREE-depleted peridotites similar to the northPyrenean spinel lherzolites. These features indicate that, underupper lithospheric mantle conditions, a mafic melt locally infiltratedlherzolites by a grain-boundary percolation process and reactedwith the original mineral assemblage. The infiltration of alkali-basalticliquids into spinel peridotite led to: (1) partial dissolutionof orthopyroxene and, locally, spinel; (2) crystallization ofclinopyroxene directly from introduced melts; and (3) re-crystallization/equilibrationof pre-existing clinopyroxene with these magmatic liquids. Inthe last stage of the metasomatism, segregation of more fractionatedsilicate liquids, coexisting with a (CO₂+H₂O) fluid phase, mayhave been responsible for the crystallization of titanian pargasite,possibly by means of hydro-fracturing mechanism. The pervasive modal metasomatism at Caussou was contemporaneouswith the segregation of amphibole-bearing dykes in the Lherz-Freychin?debodies (northeast Pyrenees) (101–103 Ma). They representtwo manifestations of the same magmatic event in the lithosphericmantle, probably related the Middle Cretaceous alkaline magmatisrnof the Pyrenees.     

Brittle tectonism in relation to the Palaeogene evolution of the Thulean/NE Atlantic domain: a study in Ulster

The tectonic effects of the Thulean mantle plume on the opening of the North Atlantic Ocean is still poorly understood. An analysis of the brittle deformation affecting the Late Cretaceous Chalk and Lower Tertiary igneous formations cropping out in Ulster (Northern Ireland), part of the Thulean Province, leads to the recognition of two tectonic phases. Each of these phases is characterized by different stress regimes with similar trends of the horizontal maximum principal stress σ_H. The first phase, syn-magmatic and dominated by NE–SW to ENE–WSW extension, occurred during the Palaeocene. It is followed by a second post-magmatic phase, characterized initially by a probably Eocene strike-slip to compressional palaeo-stress regime with σ₁ (=σ_H) trending NE–SW to NNE–SSW associated with the partial reactivation (as reverse faults) of normal faults formed during the first phase NE–SW extension. This episode is postdated by an Oligocene extension, with σ_H (=σ₂) still striking NNE–SSW/NE–SW, which reactivated Eocene strike-slip faults as nearly vertical dip-slip normal faults. This Palaeogene tectonic evolution is consistent with the tectonic evolution of similar age in western Scotland and in the Faeroe Islands. In particular, the post-magmatic NE–SW compression is here related to the ‘Faeroe compressive event’, which is related to the earliest stages of drift of the Greenland plate.