Abstract The boron-bearing minerals grandidierite, werdingite, serendibite and sinhalite are common in high-grade rocks of the Tranomaro belt in southeastern Madagascar. The mutual occurrence of these phases allows a new understanding of the role of boron-rich fluids in the crustal evolution of Gondwana, and we provide critical borosilicate data to constrain that development. We distinguish two types of grandidierite depending on their B 2 O 3 and Al 2 O 3 contents and on their relations with associated borosilicate phases. (1) At Vohibola the presence of sinhalite and serendibite associated with phlogopite lenses in metasedimentary diopsidites indicates an evaporitic origin from calc-silicate sediments. (2) At Cape Andrahomana borosilicates are associated with pegmatites and granites that were emplaced along shear zones on the boundary of the Tranomaro belt. The shear zones acted as conduits for boron-bearing fluids and for granitic partial melts, which had derived their boron from calc-silicate sedimentary protoliths. Using geothermometry and geobarometry of minerals from associated rocks, we calculate that ambient pressures and temperatures changed in time from 7.5 to 4.0 kbar and from c . 800 °C to 700 °C. Our results confirm the important role of shear zones in channelling the fluid flow of boron-bearing fluids that were derived from crustal melt granites in the same shear zones, but that ultimately derived their boron from early metasediments. We provide new information on the mineralogy, phase assemblages and paragenetic history of multiple borosilicates.
Abstract Sapphirine-bearing assemblages occur in paragneisses in a 200 km long block in the Grenville province in Labrador-Quebec. The occurrence of some of these rocks was previously known, but their considerable extent is now recognised from regional mapping. The mineral assemblages, reactions, and compositions and the tectonic structure in the paragneisses of this block are surprisingly uniform. Within feldspar-quartz layers we recognise assemblages with sapphirine, quartz, iron titanium oxides, spinel, corundum, diaspore, orthopyroxene, sillimanite, cordierite, garnet, and biotite in metre to millimetre-thick layers. These minerals reacted with their matrix, especially quartz, during cooling and uplift. At least 11 retrograde reactions gave rise to spectacular corona textures and define a P-T -time trajectory from c. 8 kbar at 900 °C to 6 kbar at 700 °C which changed from early isobaric to late isothermal. Based on successive generations of sapphirine and orthopyroxene with constant X Mg and decreasing Fe 3+ /Fe 2+ ratio in recalculated formulae, we deduce an accompanying change from high to low oxygen fugacity along this trajectory. The isothermal section of the trajectory is consistent with predicted rapid uplift and with field evidence for thrust tectonics and mylonitisation.
Transition metals (Fe, Co, Ni, Cu, Zn, Ti, Cr, and Mn) forming sulfides and oxides are found in a large variety of lava flows erupted on the sea floor of the South Pacific at the Pitcairn Hotspot and the Pacific Antarctic Ridge. These volcanic rocks consist of an enriched alkaline series (basanite-alkali basalt-trachybasalt-trachyandesite) and a depleted tholeiitic series (MORB -basaltic andesite-andesite-dacite). The distribution of the sulfides and oxides found in globules and as disseminated (< 1-5%) crystals in the rocks is controlled by the degree of fractional crystallization. The formation of these constituents reflects the decrease of the bulk transition metal content in the rock. Sulfide globules of pentlandite, pyrrhotite and cubanite are prominent in the intermediate and evolved rocks. Ti-bearing chromite and titanomagnetite are mainly concentrated in the least evolved (picritic alkali basalt, basanite, alkali basalt) and in the silica-rich (trachyte, andesite, dacite) lavas, respectively. The paragenesis of sulfide globules and oxides also correlates with the magma evolution during crystal-liquid fractionation.
Abstract Grandidierite occurs in the outer margin of a biotite-rich pelitic xenolith in norite close to the contact of a tourmaline-bearing granite in the Haddo House norite-gabbro complex. The boron for the grandidierite was derived by metasomatism from the granite. The assemblage is (decreasing order): An 55–51 plagioclase (65%), biotite (10%), spinel (10%), cordierite (5%), corundum (5%), ilmenite, grandidierite (< 1%), rutile, sphene, zircon. X Mg range is grandidierite (0.80–0.78) > cordierite (0.78) > biotite ( c . 0.59) > spinel (0.35–0.27). The grandidierite has little chemical variation: SiO 2 , 19.9–20.5%; Al 2 O 3 , 51.3–51.9%. A compilation of all grandidierite analyses in the literature shows that the only substitutions are Fe 2+ ⇌ Mg and Al ⇌ Fe 3+ , the X Mg range being 0.98 to 0.19.
The present paper reports the whole rock and mineral chemistry, textural relations and the mineral reactions under greenschist to amphibolite facies conditions of the Aluminum-Phosphate and -Borosilicate (APB)-bearing Mesoproterozoic quartzite of the Itremo Group (central Madagascar) and discusses the genesis of the protolith as well as the metamorphic P-T-conditions. The Itremo Group consists from bottom to top of a micaschist formation (metapelite and metasiltstone) (approximate to 500 m), a quartzite (approximate to 1000 m), and a stromatolitic dolomitic marble formation (> 1000 m). In the quartzite cross bedding and ripple marks document a deposition in a shallow continental shelf environment. Lazulite, augelite, trolleite, svanbergite, goyazite, crandallite, berlinite, tourmaline, apatite, celestine, anhydrite, muscovite and dumortierite with the minor constituents zircon, monazite and xenotime in addition to quartz are the rock-forming minerals. Boron and Sr have been supplied to the protolith of the quartzite by submarine hydrothermal fluids or by rivers fed by B and Sr-rich thermal springs. Using textures, stability data for augelite, trolleite, berlinite, lazulite, scorzalite, dumortierite, muscovite, kyanite and temperature data from the literature the P-T conditions of Panafrican metamorphism, which produced the prograde APB-parageneses can be approximated as 3.8 kbar and about 475 degrees C. Deposition and metamorphism of the protolith can be summarized as: i.) deposition of clay-bearing sand in a shallow continental shelf environment, ii.) chemical precipitation of phosphates and of phosphorus-rich iron hydroxides by oxidizing marine pore waters, iii.) reductive dissolution of the Fe-oxyhydroxides with crystallization of authigenic Al-phosphates in early diagenesis, iv) metamorphic overprint and crystallization during the Panafrican tectonothermal event, v) hydration reactions, element mobilisation and consequent retrograde resetting of prograde mineral equilibrium by postorogenic fluid circulation at the end of the Panafrican tectonothermal event.
