Search results for "Biotite"
showing 5 items of 25 documents
Protomylonite, Mylonite and Ultramylonite
2009
The objective of this chapter is to show how variation of strain intensity can be judged in thin section. Usually this kind of variation can best be observed in low-grade mylonites where the percentage of porphyroclasts decreases progressively with strain intensity. However, the percentage of matrix is highly dependent on mineralogical composition (e.g. quartz and biotite tend to convert to matrix readily). Compositional banding in gneiss can therefore result in mylonitic banding of apparent strain variation, which in fact only reflects variation in composition of the parent rock. Several examples of ultramylonite are derived from quartzitic rocks that tend to form few or no porphyroclasts …
The Proterozoic P–T–t Evolution of the Kemp Land Coast, East Antarctica; Constraints from Si-saturated and Si-undersaturated Metapelites
2007
Integrated metamorphic and geochronological data place new constraints on the metamorphic evolution of a Neoproterozoic orogen in east Antarctica. Granulite-facies rocks from a 150 km stretch of the Kemp Land coast reflect peak conditions involving T � 870^9908 Ca t P� 7� 4^10 kbar, with pressure increasing westward towards an Archaean craton. Electron microprobe-derived (Th þ U)^Pb monazite ages from metapelitic assemblages indicate that the major mineral textures in these rocks developed during the c. 940 Ma Rayner Orogeny. Complex compositional zoning in monazite suggests high-T recrystallization over c. 25 Myr. Diversity in metapelitic reaction textures reflects silica and ferromagnesia…
Age and isotopic evidence for the origin of the Archæan granitoid intrusives of the Johannesburg Dome, South Africa
1999
Abstract Results of RbSr, PbPb and SmNd whole rock, Rbr biotite and PbPb zircon evaporation analyses are presented for certain granitoid rocks from the Johannesburg Dome. These data indicate that the granodiorite, granite and leucosome from migmatite were emplaced ∼ 3090 Ma ago, were genetically related and were derived primarily from a source between ∼ 3300 and ∼ 3500 Ma old. A portion of the granodiorite and granite might have been derived from a source between ∼ 4000 and ∼ 4300 Ma old. The tonalite was emplaced ∼ 3170 Ma ago and was derived from a source between 3.3 and 3.5 Ga old. RbSr biotite-whole rock ages, ranging between about ∼ 2614 and ∼ 2080 Ma, probably reflect complete r…
Pre-3000 Ma thermal history of the Archean Kaap Valley puton, South Africa
1992
The Kaap Valley pluton is one of several early Archean (3200-3500 Ma) tonalitetrondhjemite plutons that surround the Barberton Greenstone belt, southern Africa. Precise dating using single-grain 207 Pb/ 206 Pb evaporation of zircon and 40 Ar/ 39 Ar laser step-heating of hornblende and biotite indicates that, in its interior, the Kaap Valley pluton preserves a memory of its initial intrusion and cooling, which spanned a time from 3225 to 3142 Ma. The pluton also records the effect of a low-temperature thermal event at its margin as seen by a 40 Ar/ 39 Ar biotite age of 3035 Ma, which is perhaps related to hydrothermal activity and gold mineralization in the adjacent Barberton Greenstone belt…
Pore and mineral structure of rock using nano-tomographic imaging
2015
ABSTRACTIn order to better understand the micrometer-scale structure of rock and its transport properties which arise from it, seven monomineral samples from two sites (Olkiluoto and Sievi, Finland) were studied with micro- and nanotomography and scanning electron microscopy. From the veined gneiss of Olkiluoto we studied biotite, potassium feldspar, plagioclase (composition of oligoclase) and cordierite, and from Sievi tonalite biotite and two grains of plagioclase (albite). These minerals were the main minerals of these samples. Samples were carefully separated and selected using heavy liquid separation and stereomicroscopy, their three dimensional structure was imaged using X-ray tomogra…