Mineralogy, geochemistry and petrology of an amphibolite-facies aluminum-phosphate and borosilicate (APB)-bearing quartzite from the Mesoproterozoic Itremo Group (Central Madagascar)

摘要

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-con-ditions. The Itremo Group consists from bottom to top of a micaschist formation (metapelite and metasiltstone) (approx =500m), a quartzite (approx = 1000m), and a stromatolitic dolomitic marble formation (> 1000m). 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, troll-leite, 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 deg 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 diagene-sis, 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.