Bostonite

Abstract: (C) Many inclusions have risen.Shelburne Point, Vermont: On Shelburne Point and on Nash's Point, near-by, a few miles south of Burlington, Vermont, are several inclusion-bearing dikes of great interest. They have been described by Hitchcock' and by Kemp and Marsters.2 On both sides of Shelburne Point are outcrops of a 20-foot bostonite dike filled with angular inclusions of Middle Ordovician shale and red Cambrian quartzite. These inclusions have sharp edges and do not appear to have been noticeably altered by the bostonite. On Nash's Point is another bostonite dike, about 12 feet wide and vertical, with chilled margins 2 feet and i foot wide on the respective sides of the dike, and a mass of fragments in the middle. A part of the fragments are quite angular and a part decidedly rounded. They vary in size from a fraction of an inch to 4 or 5 inches in length, but Kemp and Marsters found one piece of norite 18 feet in diameter. They consist of garnetiferous hornblende schist, pre-Cambrian norite, quartz, grey sandstone, red Cambrian sandstone, Trenton shale, and black limestone, cemented by a bostonite groundmass. Kemp and Marsters add: "Under the microscope, sections of norite show plagioclase and garnet, all exhibiting the results of dynamic action. Sections of the red quartzite have the usual fragmental character, with the evidences of strain less developed." The presence of the norite and schist inclusions show that the dikes have come through an indefinite amount of the pre-Cambrian as well as through Cambrian sandstone and Ordovician limestone into

Abstract: Nusab El Balgum mass complex represents one of peralkaline volcanic activity phenomena in the south Western Desert of Egypt, which is typical for within-plate event, which formed in Mesozoic period. It consists of acidic volcanic (rhyolite and their pyroclastics) and sub-volcanic granitic rocks (incomplete ring, small stock and dyke of a peralkaline aphanites) as well as dykes (trachyte, bostonite, rhyodacite, rhyolite and porphyritic rhyolite) variable in thickness and the most of run in NNE-SSW trend. The peralkaline granitic rocks, especially those located at the southwestern part of this mass are characterized by extreme enrichments in HFSE (rare metals such as Zr, Nb, U and Th and REEs) which are the highest concentrations (e.g., >1% Zr, 0.5% Nb and 2.6% total REEs, Y up to 1%, eU up to 300 ppm and eTh up to 1100 ppm). The rare metal bearing minerals are thorite, uranothorite, autunite, amorphous secondary uranium, zircon and ferrocolumbite, while the REEs bearing minerals are bastnaesite, monazite and xenotime. The positive relations in all of the binary diagrams of Zr versus Nb, Y, eU and eTh, Nb versus Y, eU and eTh, Y versus eU and eTh in post magmatic intensely hematised peralkaline granites indicated that, this process is responsible for the enrichment in these HFSE. The chondrite-normalized pattern of high-altered peralkaline granites indicates: 1) higher LREEs enriched pattern (La/Gd = 11.34 and 12.25) means the alteration processes taking place under open system and these rocks evolved from magma of lithospheric rifting, 2) ΔCe 2O, and thus very low viscosity, despite its low temperature (ments, as indicated by strong negative Eu anomalies; and c) it had abundances of HFSE cations. Redistribution of elements took place by post magmatic hydrothermal solutions.

Abstract: In Stara planina Mts., Bulgaria the Buhovo-Seslavtzi pluton is built up of potassic monzonites and quartzsyenites, which contain calcic pyroxenes and amphiboles, plagioclase and potassium feldspar. The plutonic rocks are accompanied by dykes of different composition: lamprophyres, syenite-aplites, syenite-porphyries, bostonite-porphyries and peralkaline syenite and granite porphyries. In the dykes of bostonite porphyries and peralkaline syenite and granite porphyries calcic pyroxenes and amphiboles are replaced by alkaline mafic phases: aegirine-augites, sodic-calcic and sodic amphiboles. Aegirine-augites are Ti-rich up to 5.95 wt. % TiO 2 and Zr-rich up to 0.83 wt. % ZrO 2 . Ti and Zr present antipathetic relationships in aegirine-augite, as their repartition depends on the agpaicity. Ti enters pyroxene structure under strongly agpaitic conditions, whereas Zr enters aegirine-augite in mildly agpaitic conditions. Composition of sodic-calcic and sodic amphiboles evolves through ferrowinchite, ferrorichterite, richterite toward magnesio-arfvedsonite. Evolution ends with crystallization of potassic magnesio-arfvedsonite, which contains up to 4.67 wt. % K 2 O, as K totally fills [A] site. Pyroxenes and amphiboles show persistent magnesian character during the evolution toward sodic-calcic and sodic varieties. This trend is not common for silica-oversaturated rocks, and indicates high fO 2 conditions and crystallization between NNO and HM buffers. The high activities of alkalis and silica are considered as the most important factors leading to the formation of these peculiar aegirine-augite and potassic magnesio-arfvedsonite.