Time-scale of deformation and intertectonic phases revealed by P-T-D-t relationships in the orogenic middle crust of the Orlica-?nie?nik Dome, Polish/Czech Central Sudetes

摘要

A section of the orogenic middle crust (Orlica-?nie?nik 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 ± 3 Ma isochron age is ascribed to garnet crystallization at staurolite grade, near the end of I_(1-2). For the dated sample of amphibole-biotite granitoid sheet, a Pb-Pb single zircon evaporation age of 353 ± 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~(-1) and assumed high heating rates of c. 20 °C Ma~(-1) 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.