@article{WillnerSepulvedaHerveetal.2009, author = {Willner, Arne P. and Sep{\´u}lveda, Fernando A. and Herv{\´e}, Francisco and Massonne, Hans-Joachim and Sudo, Masafumi}, title = {Conditions and timing of pumpellyite-actinolite-facies metamorphism in the early Mesozoic frontal accretionary prism of the Madre de Dios Archipelago (latitude 50 degrees 20'S; Southern Chile)}, issn = {0022-3530}, doi = {10.1093/petrology/egp071}, year = {2009}, abstract = {The Madre de Dios Metamorphic Complex (MDMC) in southern Chile is a fossil frontal accretionary prism, which is mainly composed of metapsammopelitic rocks, intercalations of oceanic rocks (greenstone and metachert) and platform carbonate. We concentrated on the metabasite to decipher the metamorphic evolution. This rock type contains assemblages of the pumpellyite-actinolite facies: pumpellyite +/- actinolite-chlorite +/- grandite +/- phengite +/- epidote-albite- quartz-titanite +/- K-feldspar +/- calcite. The metamorphic phases mainly grew by prograde hydration reactions during various episodes of restricted fluid influx. Fundamental phase relations of the pumpellyite-actinolite facies and adjacent facies were reproduced by pseudosections calculated for the system K2O-Na2O-CaO-FeO-O-2-MgO-Al2O3-TiO2-SiO2-H2O- CO2 at 200-400 degrees C and 1-9 kbar. The calculated stability fields of the metamorphic assemblages as realized in the MDMC metabasite indicate highest metamorphic conditions restricted to 290-310 degrees C, 4-6 kbar for the MDMC, presumably as a result of the main fluid influx at these conditions. Nevertheless, earlier local equilibria are still preserved as a result of strongly kinetically controlled mineral reactions and a lack of recrystallization and compositional homogenization at thin-section scale. Hence, thermodynamic calculations of local multivariant mineral equilibria using the entire compositional variation of minerals in the MDMC show that the prograde PT path evolved from 4 +/- 1 kbar, 200-220 degrees C to 5 +/- 1 kbar, 290-330 degrees C. The prograde PT path reflects nearly horizontal particle paths after reaching the maximum depth typical for frontal accretionary prisms. Long residence at maximum depth resulted in thermal re-equilibration. Ar-40/Ar-39 spot ages were measured by in situ UV laser ablation of local phengite concentrations in a deformed metapelite at 233 center dot 2 +/- 1 center dot 8 Ma and in an undeformed metabasite at 200 center dot 8 +/- 2 center dot 4 Ma. Whereas the first age represents an age of accretion, the latter age can be attributed to mineral growth either during a younger stage of accretion or during a retrograde stage. Ar-40/Ar-39 isotopic analyses of two further metabasite samples reflect a prominent resetting of ages at 152 center dot 0 +/- 2 center dot 2 Ma and white mica growth during external fluid access triggered by either a local intrusion or a late Jurassic extensional episode.}, language = {en} } @misc{WillnerBarrGlodnyetal.2015, author = {Willner, Arne P. and Barr, Sandra M. and Glodny, Johannes and Massonne, Hans-Joachim and Sudo, Masafumi and Thomson, Stuart N. and Van Staal, Cees R. and White, Chris E.}, title = {Effects of fluid flow, cooling and deformation as recorded by 40Ar/39Ar, Rb-Sr and zircon fission track ages in very low- to low-grade metamorphic rocks in Avalonian SE Cape Breton Island (Nova Scotia, Canada)}, series = {Postprints der Universit{\"a}t Potsdam : Mathematisch Naturwissenschaftliche Reihe}, journal = {Postprints der Universit{\"a}t Potsdam : Mathematisch Naturwissenschaftliche Reihe}, number = {584}, doi = {10.25932/publishup-41448}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-414484}, pages = {767 -- 787}, year = {2015}, abstract = {40Ar/39Ar in situ UV laser ablation of white mica, Rb-Sr mineral isochrons and zircon fission track dating were applied to determine ages of very low- to low-grade metamorphic processes at 3.5±0.4 kbar, 280±30°C in the Avalonian Mira terrane of SE Cape Breton Island (Nova Scotia). The Mira terrane comprises Neoproterozoic volcanic-arc rocks overlain by Cambrian sedimentary rocks. Crystallization of metamorphic white mica was dated in six metavolcanic samples by 40Ar/39Ar spot age peaks between 396±3 and 363±14 Ma. Rb-Sr systematics of minerals and mineral aggregates yielded two isochrons at 389±7 Ma and 365±8 Ma, corroborating equilibrium conditions during very low- to low-grade metamorphism. The dated white mica is oriented parallel to foliations produced by sinistral strike-slip faulting and/or folding related to the Middle-Late Devonian transpressive assembly of Avalonian terranes during convergence and emplacement of the neighbouring Meguma terrane. Exhumation occurred earlier in the NW Mira terrane than in the SE. Transpression was related to the closure of the Rheic Ocean between Gondwana and Laurussia by NW-directed convergence. The 40Ar/39Ar spot age spectra also display relict age peaks at 477-465 Ma, 439 Ma and 420-428 Ma attributed to deformation and fluid access, possibly related to the collision of Avalonia with composite Laurentia or to earlier Ordovician-Silurian rifting. Fission track ages of zircon from Mira terrane samples range between 242±18 and 225±21 Ma and reflect late Palaeozoic reburial and reheating close to previous peak metamorphic temperatures under fluid-absent conditions during rifting prior to opening of the Central Atlantic Ocean.}, language = {en} } @article{WillnerBarrGlodnyetal.2015, author = {Willner, Arne P. and Barr, Sandra M. and Glodny, Johannes and Massonne, Hans-Joachim and Sudo, Masafumi and Thomson, Stuart N. and Van Staal, Cees R. and White, Chris E.}, title = {Effects of fluid flow, cooling and deformation as recorded by Ar-40/Ar-39, Rb-Sr and zircon fission track ages in very low- to low-grade metamorphic rocks in Avalonian SE Cape Breton Island (Nova Scotia, Canada)}, series = {Geological magazine}, volume = {152}, journal = {Geological magazine}, number = {5}, publisher = {Cambridge Univ. Press}, address = {New York}, issn = {0016-7568}, doi = {10.1017/S0016756814000508}, pages = {767 -- 787}, year = {2015}, abstract = {Ar-40/Ar-39 in situ UV laser ablation of white mica, Rb-Sr mineral isochrons and zircon fission track dating were applied to determine ages of very low- to low-grade metamorphic processes at 3.5 +/- 0.4 kbar, 280 +/- 30 degrees C in the Avalonian Mira terrane of SE Cape Breton Island (Nova Scotia). The Mira terrane comprises Neoproterozoic volcanic-arc rocks overlain by Cambrian sedimentary rocks. Crystallization of metamorphic white mica was dated in six metavolcanic samples by Ar-40/Ar-39 spot age peaks between 396 +/- 3 and 363 +/- 14 Ma. Rb-Sr systematics of minerals and mineral aggregates yielded two isochrons at 389 +/- 7 Ma and 365 +/- 8 Ma, corroborating equilibrium conditions during very low- to low-grade metamorphism. The dated white mica is oriented parallel to foliations produced by sinistral strike-slip faulting and/or folding related to the Middle-Late Devonian transpressive assembly of Avalonian terranes during convergence and emplacement of the neighbouring Meguma terrane. Exhumation occurred earlier in the NW Mira terrane than in the SE. Transpression was related to the closure of the Rheic Ocean between Gondwana and Laurussia by NW-directed convergence. The Ar-40/Ar-39 spot age spectra also display relict age peaks at 477-465 Ma, 439 Ma and 420-428 Ma attributed to deformation and fluid access, possibly related to the collision of Avalonia with composite Laurentia or to earlier Ordovician-Silurian rifting. Fission track ages of zircon from Mira terrane samples range between 242 +/- 18 and 225 +/- 21 Ma and reflect late Palaeozoic reburial and reheating close to previous peak metamorphic temperatures under fluid-absent conditions during rifting prior to opening of the Central Atlantic Ocean.}, language = {en} } @article{WillnerMassonneRingetal.2012, author = {Willner, Arne P. and Massonne, Hans-Joachim and Ring, Uwe and Sudo, Masafumi and Thomson, Stuart N.}, title = {P-T evolution and timing of a late Palaeozoic fore-arc system and its heterogeneous Mesozoic overprint in north-central Chile (latitudes 31-32 degrees S)}, series = {Geological magazine}, volume = {149}, journal = {Geological magazine}, number = {2}, publisher = {Cambridge Univ. Press}, address = {New York}, issn = {0016-7568}, doi = {10.1017/S0016756811000641}, pages = {177 -- 207}, year = {2012}, abstract = {In the late Palaeozoic fore-arc system of north-central Chile at latitudes 31-32 degrees S (from the west to the east) three lithotectonic units are telescoped within a short distance by a Mesozoic strikeslip event (derived peak P-T conditions in brackets): (1) the basally accreted Choapa Metamorphic Complex (CMC; 350-430 degrees C, 6-9 kbar), (2) the frontally accreted Arrayan Formation (AF; 280-320 degrees C, 4-6 kbar) and (3) the retrowedge basin of the Huentelauquen Formation (HF; 280-320 degrees C, 3-4 kbar). In the CMC, Ar-Ar spot ages locally date white-mica formation at peak P-T conditions and during early exhumation at 279-242 Ma. In a local garnet mica-schist intercalation (570-585 degrees C, 11-13 kbar) Ar-Ar spot ages refer to the ascent from the subduction channel at 307-274 Ma. Portions of the CMC were isobarically heated to 510-580 degrees C at 6.6-8.5 kbar. The age of peak P-T conditions in the AF can only vaguely be approximated at >= 310 Ma by relict fission-track ages consistent with the observation that frontal accretion occurred prior to basal accretion. Zircon fission-track dating indicates cooling below similar to 280 degrees C at similar to 248 Ma in the CMC and the AF, when a regional unconformity also formed. Ar-Ar white-mica spot ages in parts of the CMC and within the entire AF and HF point to heterogeneous resetting during Mesozoic extensional and shortening events at similar to 245-240 Ma, similar to 210-200 Ma, similar to 174-159 Ma and similar to 142-127 Ma. The zircon fission-track ages are locally reset at 109-96 Ma. All resetting of Ar-Ar white-mica ages is proposed to have occurred by in situ dissolution/precipitation at low temperature in the presence of locally penetrating hydrous fluids. Hence syn-and postaccretionary events in the fore-arc system can still be distinguished and dated in spite of its complex heterogeneous postaccretional overprint.}, language = {en} } @misc{WillnerMassonneRingetal.2012, author = {Willner, Arne P. and Massonne, Hans-Joachim and Ring, Uwe and Sudo, Masafumi and Thomson, Stuart N.}, title = {P-T evolution and timing of a late Palaeozoic fore-arc system and its heterogeneous Mesozoic overprint in north-central Chile (latitudes 31-32° S)}, series = {Postprints der Universit{\"a}t Potsdam : Mathematisch-Naturwissenschaftliche Reihe}, journal = {Postprints der Universit{\"a}t Potsdam : Mathematisch-Naturwissenschaftliche Reihe}, number = {597}, issn = {1866-8372}, doi = {10.25932/publishup-41490}, url = {http://nbn-resolving.de/urn:nbn:de:kobv:517-opus4-414909}, pages = {31}, year = {2012}, abstract = {In the late Palaeozoic fore-arc system of north-central Chile at latitudes 31-32 degrees S (from the west to the east) three lithotectonic units are telescoped within a short distance by a Mesozoic strikeslip event (derived peak P-T conditions in brackets): (1) the basally accreted Choapa Metamorphic Complex (CMC; 350-430 degrees C, 6-9 kbar), (2) the frontally accreted Arrayan Formation (AF; 280-320 degrees C, 4-6 kbar) and (3) the retrowedge basin of the Huentelauquen Formation (HF; 280-320 degrees C, 3-4 kbar). In the CMC, Ar-Ar spot ages locally date white-mica formation at peak P-T conditions and during early exhumation at 279-242 Ma. In a local garnet mica-schist intercalation (570-585 degrees C, 11-13 kbar) Ar-Ar spot ages refer to the ascent from the subduction channel at 307-274 Ma. Portions of the CMC were isobarically heated to 510-580 degrees C at 6.6-8.5 kbar. The age of peak P-T conditions in the AF can only vaguely be approximated at >= 310 Ma by relict fission-track ages consistent with the observation that frontal accretion occurred prior to basal accretion. Zircon fission-track dating indicates cooling below similar to 280 degrees C at similar to 248 Ma in the CMC and the AF, when a regional unconformity also formed. Ar-Ar white-mica spot ages in parts of the CMC and within the entire AF and HF point to heterogeneous resetting during Mesozoic extensional and shortening events at similar to 245-240 Ma, similar to 210-200 Ma, similar to 174-159 Ma and similar to 142-127 Ma. The zircon fission-track ages are locally reset at 109-96 Ma. All resetting of Ar-Ar white-mica ages is proposed to have occurred by in situ dissolution/precipitation at low temperature in the presence of locally penetrating hydrous fluids. Hence syn-and postaccretionary events in the fore-arc system can still be distinguished and dated in spite of its complex heterogeneous postaccretional overprint.}, language = {en} } @article{MassonneO'Brien2003, author = {Massonne, Hans-Joachim and O'Brien, Patrick J.}, title = {The Bohemian Massif and the NW Himalayas}, year = {2003}, language = {en} } @article{WillnerGerdesMassonneetal.2011, author = {Willner, Arne P. and Gerdes, Axel and Massonne, Hans-Joachim and Schmidt, Alexander and Sudo, Masafumi and Thomson, Stuart N. and Vujovich, Graciela}, title = {The geodynamics of collision of a microplate (Chilenia) in Devonian times deduced by the pressure-temperature-time evolution within part of a collisional belt (Guarguaraz Complex, W-Argentina)}, series = {Contributions to mineralogy and petrology}, volume = {162}, journal = {Contributions to mineralogy and petrology}, number = {2}, publisher = {Springer}, address = {New York}, issn = {0010-7999}, doi = {10.1007/s00410-010-0598-8}, pages = {303 -- 327}, year = {2011}, abstract = {The Guarguaraz Complex in West Argentina formed during collision between the microplate Chilenia and South America. It is composed of neritic clastic metasediments with intercalations of metabasic and ultrabasic rocks of oceanic origin. Prograde garnet growth in metapelite and metabasite occurred between 1.2 GPa, 470 degrees C and 1.4 GPa, 530 degrees C, when the penetrative s(2)-foliation was formed. The average age of garnet crystallization of 390 +/- 2 Ma (2 sigma) was determined from three four-point Lu-Hf mineral isochrones from metapelite and metabasite samples and represents the time of collision. Peak pressure conditions are followed by a decompression path with slight heating at 0.5 GPa, 560 degrees C. Fluid release during decompression caused equilibration of mineral compositions at the rims and also aided Ar diffusion. An Ar-40/39 Ar plateau age of white mica at 353 +/- 1 Ma (1 sigma) indicates the time of cooling below 350-400 degrees C. These temperatures were attained at pressures of 0.2-0.3 GPa, indicative of an average exhumation rate of >= 1 mm/a for the period 390-353 Ma. Late hydrous influx at 0.1-0.3 GPa caused pervasive growth of sericite and chlorite and reset the Ar/Ar ages of earlier coarse-grained white mica. At 284-295 Ma, the entire basement cooled below 280 degrees C (fission track ages of zircon) after abundant post-collisional granitoid intrusion. The deeply buried epicontinental sedimentary rocks, the high peak pressure referring to a low metamorphic geotherm of 10-12 degrees C/km, and the decompression/heating path are characteristics of material buried and exhumed within a (micro) continent-continent collisional setting.}, language = {en} }