Palaeomagnetism of Palaeozoic ultrabasic rocks from the Sudetes Mts (SW Poland): tectonic implications

Abstract

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.

Mineralogy and magnetism of Fe-Cr spinel series minerals from podiform chromitites and dunites from Tąpadła (Sudetic ophiolite, SW Poland) and their relationship to palaeomagnetic results of the dunites

“…Dunites outcrop close to the abandoned chromitite mine. Their mineralogy, magnetic properties and palaeomagnetic results have been already discussed in Kądziałko‐Hofmokl et al (2006a, b). Here we will show that magnetic varieties of chromite grains in the dunites contribute to their NRM.…”

“…Their initial serpentinization was directly dated at 400 +4/−3 Ma based on an integrated study of hydrothermal zircon from the rodingite blackwall (Dubińska et al 2004). Palaeomagnetic studies suggest that the JGSM together with GSB has become the part of West Sudetes not earlier than 390 Ma (Kądziałko‐Hofmokl et al 2006b).…”

“…Two hand samples of chromitite, labelled TaA and TaB, were collected from a waste dump of the abandoned chromitite mine near the village of Tąpadła; the dunites (seven hand samples, see Kądziałko‐Hofmokl et al 2006b) were sampled nearby. For our study, we prepared three cylinders for standard palaeomagnetic measurements and about a dozen (as many specimens as was necessary for respective experiments, often repeated on fresh sister‐specimens) from each chromitite hand sample.…”

“…In a previous paper (Kądziałko‐Hofmokl et al 2006b), we discussed the mineralogy and palaeomagnetism of ultramafic rocks from the Góry Sowie Block (GSB) and the Jordanów‐Gogołów Serpentinite Massif (JGSM), the latest supposed to be fragment of a dismembered Sudetic ophiolite. Dunites (labelled Ta dunites) exposed at Tąpadła Pass in the JGSM, contain magnetite and grains of chromite (Cr‐bearing spinel minerals).…”

“…The purpose of the present paper is to show that chromite grains present in Ta dunites are partly altered to ferrichromites of a similar type to those found in associated chromitites, and are carriers of Permian and Tertiary‐Recent components of natural remanent magnetization natural remanence (NRM) (see Kądziałko‐Hofmokl et al 2006a). The oldest, Lower Devonian, component of NRM isolated in Ta dunites and labelled A3 (see Kądziałko‐Hofmokl et al 2006b) is carried by magnetite, formed probably during an initial stage of oceanic serpentinization of the Sudetic ophiolite.…”