Time‐scale of deformation and intertectonic phases revealed byP–T–D–trelationships in the orogenic middle crust of the Orlica‐Śnieżnik Dome, Polish/Czech Central Sudetes

Abstract

A section of the orogenic middle crust (Orlica-Snie_ znik Dome, Polish/Czech Central Sudetes) was examined to constrain the duration and significance of deformation (D) and intertectonic (I) phases. In the studied metasedimentary synform, three deformation events produced an initial subhorizontal foliation S1 (D 1 ), a subsequent subvertical foliation S2 (D 2 ) and a late subhorizontal axial planar cleavage S3 (D 3 ). The synform was intruded by pre-, syn-and post-D 2 granitoid sheets. Crystallization-deformation relationships in mica schist samples document I 1-2 garnet-staurolite growth, syn-D 2 staurolite breakdown to garnet-biotite-sillimanite/andalusite, I 2-3 cordierite blastesis and late-D 3 chlorite growth. Garnet porphyroblasts show a linear Mn-Ca decrease from the core to the inner rim, a zone of alternating Ca-Y-and P-rich annuli in the inner rim, and a Ca-poor outer rim. The Ca-Y-rich annuli probably reflect the occurrence of the allanite-to-monazite transition at conditions of the staurolite isograd, whereas the Ca-poor outer rim is ascribed to staurolite demise. The reconstructed P-T path, obtained by modelling the stability of parageneses and garnet zoning, documents near-isobaric heating from~4 kbar/485°C to~4.75 kbar/575°C during I 1-2 . This was followed by a progression to 4-5 kbar/580-625°C and a subsequent pressure decrease to 3-4 kbar during D 2 . Pressure decrease below 3 kbar is ascribed to I 2-3 , whereas cooling below~500°C occurred during D 3 . In the dated mica schist sample, garnet rims show strong Lu enrichment, oscillatory Lu zoning and a slight Ca increase. These features are also related to allanite breakdown coeval with staurolite appearance. As Lu-rich garnet rims dominate the Lu-Hf budget, the 344 AE 3 Ma isochron age is ascribed to garnet crystallization at staurolite grade, near the end of I 1-2 . For the dated sample of amphibolebiotite granitoid sheet, a Pb-Pb single zircon evaporation age of 353 AE 1 Ma is related to the onset of plutonic activity. The results suggest a possible Devonian age for D 1 , and a Carboniferous burialexhumation cycle in mid-crustal rocks that is broadly coeval with the exhumation of neighbouring HP rocks during D 2 . In the light of published ages, a succession of telescoping stages with time spans decreasing from c. 10 to 2-3 Ma is proposed. The initially long period of tectonic quiescence (I 1-2 phase, c. 10 Ma) inferred in the middle crust contrasts with contemporaneous deformation at deeper levels and points to decoupled P-T-D histories within the orogenic wedge. An elevated gradient of 30°C km À1 and assumed high heating rates of c. 20°C Ma À1 are explained by the protracted intrusion of granitoid sheets, with or without deformation, whereas fast vertical movements (2-3 Ma, D 2 phase) in the crust require the activity of deformation phases.

Monazite stability and the maintenance of Th-U-total Pb ages during post-magmatic processes in granitoids and host metasedimentary rocks: A case study from the Sudetes (SW Poland)

“…There fore, in di vid ual monazite dates ob tained from compositional do mains of larger grains or sin gle do mains in smaller grains po ten tially pro vide a re cord of the meta mor phic ep i sodes that ac com pany suc ces sive de for mation and intertectonic stages be tween 360 and 320 Ma. This inter pre ta tion agrees with the re cent P-T-t-d re con struc tions of Skrzypek et al (2014). On the other hand, the spec trum of ca.…”

“…The re versed un com pleted tran si tion of monazite to al la nite in the paragneiss and si mul ta neous par tial al ter ations of monazite in the granitoid doc u mented in our study were re lated to the cool ing be low ca. 500°C dur ing the ret ro gres sive stage (Skrzypek et al, 2014). These tem per a ture con di tions are con sis tent with pre vi ously reported con di tions of sim i lar monazite al ter ations and dem onstrate that the sil i cate as sem blage mainly con trolled the compo si tion of fluid in ter act ing with monazite and the vari abil ity of the al ter ation prod ucts.…”

“…Nev er the less, the age dif fer ence be tween the two studies sam ples is con sis tent with pre vi ous struc tural stud ies demon strat ing that the Jawornik granitoids rep re sent an ear lier mag matic pulse pre dat ing a late duc tile fold ing in the Orlica-OEnie¿ nik Dome (e.g., Cwojdziñski, 1977;Woj cie chow ska, 1993). A com par i son of the pres ent study to the re sults of pre vious geo chron ol ogy stud ies on Jawornik granitoids (Bia³ek and Werner, 2004;Bia³ek, 2014;Skrzypek et al, 2014) in di cates that the ob tained monazite age of ca. 343 Ma re flects the tim ing of the so lid i fi ca tion of the Jawornik granitoids.…”

“…Ther mo dy namic mod elling us ing a bulk com po si tion of Shaw's (1956) av er age pelite re vealed that the al la nite to monazite tran si tions un der con ditions of mid dle am phi bo lite-fa cies oc cur at 525-600°C, with the tem per a ture sub se quently in creas ing as the bulk CaO con tent in creases (Spear, 2010). P-T con straints of metasedimentary rocks host ing Jawornik granitoids es ti mated max i mum tem pera tures of meta mor phism of 625-640°C dur ing de for ma tion stage D 2 (Murtezi, 2006;Skrzypek et al, 2014). The re versed un com pleted tran si tion of monazite to al la nite in the paragneiss and si mul ta neous par tial al ter ations of monazite in the granitoid doc u mented in our study were re lated to the cool ing be low ca.…”

“…The pre vi ous geo chron ol ogy per formed on Jawornik granitoids in di cated a Pb-Pb evap o ra tion zir con age of ca. 353 Ma (Skrzypek et al, 2014) and Ar-Ar dates of bi o tite and mus co vite rang ing between 351 and 343 Ma (Bia³ek and Werner, 2004).…”