Original paper

Age and origin of the Strzelin granitoids (Fore-Sudetic Block, Poland): 87Rb/86Sr data

Oberc-Dziedzic, T.; Pin, C.; Duthou, J. L.; Couturie, J. P.

Image of first page of:

Neues Jahrbuch für Mineralogie - Abhandlungen Band 171 Heft 2 (1996), p. 187 - 198

21 references

published: Dec 12, 1996

DOI: 10.1127/njma/171/1996/187

BibTeX file

ArtNo. ESP154017102003, Price: 29.00 €

Download preview PDF Buy as PDF

Abstract

The Wzgórza Strzelińskie crystalline massif (Fore-Sudetic Block, SW Poland) consists of: (a) a group of gneisses (Upper Precambrian - Cambrian), (b) an older metasedimentary-amphibolite group (Precambrian or Lower Paleozoic), and (c) a younger metasedimentary group (Lower - Middle Devonian quartzites of the Jegłowa beds). These three groups of rocks were folded and metamorphosed during the Bretonian phase. This metamorphic basement is intruded by Variscan granitoids which form isolated, small bodies, mostly stocks and flat dikes, up to tens of metres thick, within metamorphic rocks. The granitoids include granodiorites, tonalites, quartz diorites and two types of granites: medium to fine-grained biotite granite and fine-grained biotite - muscovite granite. According to the Rb - Sr isotope data, the ages of these two granite types are 347 ± 12 Ma and 330 ± 6 Ma, respectively. Their similar 87Sr/86Sr isotopic ratios of 0.7053 suggest a common origin from sources with a low time-integrated Rb/Sr ratio, and of relatively mafic composition. No linear array has been obtained for the tonalites and quartz diorites. Assuming an age of 350 Ma, their initial isotopic ratios range from 0.7051 to 0.7077. This large scatter reflects incomplete homogenization by mixing of isotopically contrasted end-members, as supported by petrographic evidence. The initial ratio of the granites (0.7053), is lower than that of most of the earlier tonalites-quartz diorites. This observation rules out derivation of these granites from the tonalites. The Sr isotopic data suggest that the granites evolved directly from low 87Sr/86Sr precursors without significant crustal contamination, as illustrated by the isochrons. It is suggested that the contamination potential of the local crust was almost entirely consumed by the earlier tonalites - quartz diorites intrusions.

Keywords

Granotoids • Rb/Sr dating • Sr isotope geochemistry • Sudetes • Variscan belt