Oberc-Dziedzic T., Kryza R., Pin C. and Madej S. (2013) Variscan granitoid plutonism in the Strzelin Mas sif (SW Po land): petrol ogy and age of the com pos ite Strzelin gran ite in tru sion. Geo log i cal Quar terly, 57 (2): 269-288, doi: 10.7306/gq.1083Pet ro log i cal data and re cently pub lished U/Pb zir con SHRIMP ages re veal a pro tracted Variscan mag matic evo lu tion in the Strzelin Mas sif (SW Po land), with three main stages of granitoid plutonism: 1 -tonalitic I, 2 -granodioritic and 3 -tonalitic II/gra nitic. The granitoids of the sec ond and third stages form the Strzelin in tru sion that is com posed of three va ri et ies: medium-grained bi o tite gran ite, fine-grained bi o tite gran ite and fine-grained bi o tite-mus co vite gran ite. New SHRIMP data show that the me dium-grained and fine-grained bi o tite gran ites com prise dif fer ent zir con pop u la tions that re flect com plex and prolonged plutonic pro cesses. Two dis tinct mag matic events seem to be rep re sented by well-de fined zir con pop u la tions with ap par ent 206 Pb/ 238 U ages of 303 ± 2 Ma in the me dium-grained bi o tite gran ite, and 283 ± 8 Ma in the fine-grained bi o tite granite. These dates, how ever, do not nec es sar ily re flect the true mag matic ages, pos si bly be ing "re ju ve nated" by ra dio genic lead loss in zir cons (im pos si ble to re solve based on rou tine SHRIMP data). Based on field ev i dence, the third va ri ety, the bi otite-mus co vite gran ite, post dates both types of bi o tite gran ites. The petrographic and geo chem i cal fea tures, in clud ing Nd isotope sig na ture, along with var i ous zir con in her i tance pat terns and ages, sug gest that the pa ren tal mag mas of the three gran ites orig i nated from dif fer ent crustal sources and were emplaced dur ing three suc ces sive mag matic pulses.
The Strzelin Massif in SW Poland (Central European Variscides) records a protracted igneous evolution, with three main magmatic stages: (1) tonalitic I, (2) granodioritic and (3) tonalitic II/granitic. In the northern part of this Massif, the Strzelin intrusion proper comprises three successively emplaced rock types: a medium-grained biotite granite (303 ± 2 Ma), a fine-grained biotite granite (283 ± 8 Ma) and a fine-grained biotite-muscovite granite; based on field evidence, the third variety postdates both types of the biotite granites. The structural data from the three granites, including their parallel, approximately E-W striking and steeply dipping lithological contacts and ENE-WSW trending subhorizontal magmatic lineations, suggest that the emplacement of all three successive granite varieties was controlled by an active, long-lived strike-slip fault, striking ESE-WNW, with a dextral sense of movement. After the emplacement of the youngest biotitemuscovite granite, the intrusion underwent brittle extension which produced ''Q joints'' striking NNW-SSE to N-S and dipping at 55-70°WSW to W, and showing evidence of broadly N-S directed sinistral displacements. The structural observations, supported by new geochronological data, indicate that the internal structure of the composite granitoid intrusion, including the faint magmatic foliation and lineation, formed in a long-lived strike-slip setting, different from the subsequent, post-emplacement extensional tectonics that controlled the development of brittle structures.
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