10 research outputs found
Redox preconditioning deep cratonic lithosphere for kimberlite genesis – evidence from the central Slave Craton
Attenuation of hydrogen peroxide-induced oxidative stress in SH-SY5Y cells by three flavonoids from Acer okamotoanum
Using FANP approach on selection of competitive priorities based on cleaner production implementation: a case study in PCB manufacturer, Taiwan
The Lages diatremes: mineral composition and petrological implications
Chemical data of heavy minerals from Lages diatremes in southern Brazil have been studied with the aim of characterizing the sample source(s). Three groups of minerals are recognized: I) aluminian-chromian pyroxene, pyrope garnet and chromian spinel, which represent disaggregated fragments of spinel, spinel+garnet and garnet facies peridotite; II) low-Cr aluminian pyroxene that occurs as megacrysts are high pressure phases (7-12 kb) being crystallized from an alkaline-like evolving magma; III) low-Cr aluminian diopside of crustal origin. Evidence of carbonatitic cryptic metasomatic enrichment is shown by clinopyroxenes of Groups I and II. The data do not support a kimberlitic affinity as it has been suggested for the diatremes. Rather, they are interpreted as vents related to the alkaline magmatism affecting the area in Late Cretaceous. The alkaline parental magma of the pyroxene megacrysts was generated from a metasomatized mantle at garnet facies that incorporated fragments of the surrounding still fertile mantle. Presumably at spinel-facies level the magma began to fractionate the megacrysts, whose crystallization proceeded over a large range of falling pressure and temperature. The chemical similarities between Group III clinopyroxenes and those from the differentiated lithotypes indicate that the magma carried this mineral phase on its evolution, at crustal conditions, towards a more evolved alkaline composition. Still, a non-cognate origin for the Group III clinopyroxenes cannot be discarded.<br>Dados quĂmicos de minerais pesados dos diatremas de Lages no sul do Brasil foram estudados com o propĂłsito de caracterizar as fontes das rochas. TrĂŞs grupos de minerais sĂŁo reconhecidos: I) piroxĂŞnio aluminoso-cromĂfero, granada piropo e espinĂ©lio cromĂfero, representando fragmentos desagregados de espinĂ©lio, espinĂ©lio+granada e granada da fácies peridotito; II) piroxĂŞnio aluminoso com baixo Cr, correspondendo a megacristais, com as fases de alta pressĂŁo (7-12 kb) cristalizadas a partir de magma alcalino em evolução; III) diopsĂdio aluminoso com baixo Cr e origem crustal. ClinopiroxĂŞnios dos Grupos I e II mostram evidĂŞncias de enriquecimento metassomático crĂptico de natureza carbonatĂtica. Os dados nĂŁo confirmam a afinidade kimberlĂtica sugerida para esses diatremas. Ao contrário, eles sĂŁo interpretados como condutos relacionados ao magmatismo alcalino que afetou a área no Cretáceo Superior. O magma parental alcalino dos megacristais de piroxĂŞnio foi originado a partir de um manto metassomatizado na fácies granada que aprisionou fragmentos do ainda fĂ©rtil manto adjacente. Presumivelmente na fácies espinĂ©lio teve inĂcio o fracionamento dos megacristais, cuja cristalização se deu em condições de pressĂŁo e temperatura decrescentes. As similaridades entre os clinopiroxĂŞnios do Grupo III e aqueles dos litotipos mais diferenciados sugere que essa fase mineral foi transportada pelo magma no curso de sua evolução, em condições crustais, para uma composição alcalina mais evoluĂda. Ainda, uma formação nĂŁo-cogenĂ©tica para os clinopiroxĂŞnios do Grupo III nĂŁo pode ser descartada
Petrology, bulk-rock geochemistry, indicator mineral composition and zircon U–Pb geochronology of the end-cretaceous diamondiferous mainpur orangeites, Bastar Craton, Central India
The end-Cretaceous diamondiferous Mainpur orangeite field comprises six pipes (Behradih, Kodomali, Payalikhand, Jangara, Kosambura and Bajaghati) located at the NE margin of the Bastar craton, central India. The preservation of both diatreme (Behradih) and hypabyssal facies (Kodomali) in this domain implies differential erosion. The Behradih samples are pelletal and tuffisitic in their textural habit, whereas those of the Kodomali pipe have inequigranular texture and comprise aggregates of two generations of relatively fresh olivines. The Kosambura pipe displays high degrees of alteration and contamination with silicified macrocrysts and carbonated groundmass. Olivine, spinel and clinopyroxene in the Behradih and the Kodomali pipes share overlapping compositions, whereas the groundmass phlogopite and perovskite show conspicuous compositional differences. The bulk-rock geochemistry of both the Behradih and Kodomali pipes has a more fractionated nature compared to southern African orangeites. Incompatible trace elements and their ratios readily distinguish them from the Mesoproterozoic Wajrakarur (WKF) and the Narayanpet kimberlites (NKF) from the eastern Dharwar craton, southern India, and bring out their similarity in petrogenesis to southern African orangeites. The pyrope population in the Mainpur orangeites is dominated by the calcic-lherzolitic variety, with sub-calcic harzburgitic and eclogitic garnets in far lesser proportion. Garnet REE distribution patterns from the Behradih and Payalikhand pipes display “smooth” as well as “sinusoidal” chondrite-normalised patterns. They provide evidence for the presence of a compositionally layered end-Cretaceous sub-Bastar craton mantle, similar to that reported from many other cratons worldwide. The high logfO<sub>2</sub> of the Mainpur orangeite magma (ΔNNO (nickel-nickel oxide) of +0.48 to +4.46 indicates that the redox state of the lithospheric mantle cannot be of first-order control for diamond potential and highlights the dominant role of other factors such as rapid magma transport. The highly diamondiferous nature, the abundance of calcic-lherzolitic garnets and highly oxidising conditions prevailing at the time of eruption make the Mainpur orangeites clearly “anomalous” compared to several other kimberlite pipes worldwide. U–Pb dating of zircon xenocrysts from the Behradih pipe yielded distinct Palaeoproterozoic ages with a predominant age around 2,450 Ma. The lack of Archean-aged zircons, in spite of the fact that the Bastar craton is the oldest continental nuclei in the Indian shield with an Eoarchaean crust of 3.5–3.6 Ga, could either be a reflection of the sampling process or of the modification of the sub-Bastar lithosphere by the invading Deccan plume-derived melts during the Late Cretaceous