S U M M A R YThis paper presents the mineralogy and magnetic properties of two varieties of chromitite, sampled in the same exposure of a serpentinite massif regarded to be a fragment of the Sudetic ophiolite. The varieties differ mineralogically and magnetically. One of them, labelled TaA and altered to a low degree according to scanning electron microscope (SEM) and microprobe results, comprises an unaltered Al-Cr spinel core, some secondary chromite and less abundant Cr-magnetite. It has a high magnetic susceptibility and natural remanence (NRM), and its dominating magnetic phase is ferrichromite with T b /T c of about 530 • C occurring in grains in the single domain (SD) + pseudo-single domains (PSDs) and superparamagnetic (SP) states. The second variety, labelled TaB, is highly altered and comprises, apart from Al spinel, two kinds of secondary chromites that do not differ much from the primary Al spinel and no Cr-magnetites. This chromitite variety does not reveal one well defined T b /T c , but a series ranging between 200 and 450 • C, their grain sizes correspond to the SD + SP domain states. The same pattern was observed in chromitite TaA after annealing in air at a temperature of 700 • C. Dunites associated with the chromitites contain, apart from magnetite, chromite grains. The majority of specimens reveal magnetic characteristics similar to the TaA-like variety, but in some specimens TaB-like characteristics were also observed. A standard palaeomagnetic study performed earlier on the dunites showed that their NRM has three components-Lower Devonian carried by magnetite, Permian and Tertiary-Recent. The results presented here lead us to conclude that the latter two are carried by ferrichromites similar to those found in the TaA and TaB chromitites.
S U M M A R YA complex palaeomagnetic, rock-magnetic and mineralogical study of ultrabasic rocks from the Sowie Góry Block (GSB) and Jordanów-Gogo lów Serpentinite Massif (JGSM) revealed the presence of several components of natural remanent magnetization (NRM). The authors found three groups of Palaeozoic as well as Triassic and Recent components of the geomagnetic field. The Palaeozoic components of NRM are carried mainly by magnetite of several generations formed during several serpentinization episodes. Permo-Carboniferous component (A1) present overall the Sudetes was isolated in one JGSM and two GSB exposures, whereas the Late Devonian-Early Carboniferous component (A2) was found in two exposures from the GSB. The corresponding remanent components were already revealed in palaeontologically dated sediments from other West Sudetic units. In the GSB, it was probably acquired during its unroofing dated isotopically for ca. 370-360 Ma. The newly determined group of Palaeozoic directions (A3) was found in three localities from JGSM and in two from GSB is interpreted as the oldest overprint. In JGSM, it was acquired probably shortly after the first oceanic serpentinization phase dated isotopically for ca. 400 Ma. Its acquisition in GSB corresponds to the time of emplacement of ultrabasic xenoliths dated isotopically at ca. 390 Ma. So we suppose that the mean A3 calculated for five exposures corresponds to the 380-400 Ma time span and that at that period both massifs formed one microplate. Mean inclination of A3 places this microplate at 380-400 Ma at the palaeolatitude of 23 • S, whereas the West Sudetes were situated during the Early Devonian at 16 • S. We suggest that during the Early Devonian the microplate comprising GSB and JGSM massifs was situated to the south from the West Sudetes and accreted them during Middle-Late Devonian.
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