Dating of zircon and monazite from diamondiferous quartzofeldspathic rocks of the Saxonian Erzgebirge - hints at burial and exhumation velocities.

In order to better understand the formation and evolution processes of ultrahigh pressure (UHP) felsic rocks, we determined the ages of various domains of zircon and monazite crystals from the diamondiferous quartzofeldspathic rocks of the Saxonian Erzgebirge. According to cathodoluminescence imagery and Th/U ratios, three zircon zones were distinguished. Each was dated using several spot analyses from a sensitive high-resolution ion microprobe (SHRIMP) analysing Pb, U and Th isotopes. The results were: (1) core zone - 21 analyses: Th/U less/equal 0.023 and 337.0 plus/minus 2.7 Ma (2 sigma, combined 206Pb/238U-207Pb/235U age); (2) diamond-bearing intermediate zone - 23 analyses: Th/U greater/equal 0.037 and 336.8 plus/minus 2.8 Ma; and (3) rim zone-12 analyses: Th/U = 0.0150.038 (plus one analysis of 0.164) and 330.2 plus/minus 5.8 Ma. The U-Pb obtained ages are virtually concordant. Furthermore, two oscillatory zoned zircon cores (Th/U greater/equal to 0.8) yielded (~concordant) ages of ~400 Ma. Six SHRIMP analyses of monazites gave an age of 332.4 plus/minus 2.1 Ma. In addition, Pb, Th and U contents in monazite were analysed with an electron microprobe (EMP). A mean age of 324.7 plus/minus 8.0 (2σ) Ma was acquired from 113 analyses.By combining the defined ages with previously published P-T conditions, minimum velocities for burial and exhumation were estimated. In addition, we present a likely geodynamic scenario involving age data from the literature as well as this study: beginning 340 million years ago, gneisses at the base of a thickened continental crust (~1.8 GPa, 650C) were transported to depths of at least 130 km, possibly as deep as 250 km. Here they were heated (>1050C) and partially melted and as a result began to rise rapidly. The burial and subsequent ascent back to a depth of 50 km, where zircon rims and monazite formed, took only a few million years and perhaps significantly less

Heat capacity and phase equilibria of hollandite polymorph of KAlSi 3 O 8

The low-temperature heat capacity ( C p ) of KAlSi 3 O 8 with a hollandite structure was measured over the range of 5–303 K with a physical properties measurement system. The standard entropy of KAlSi 3 O 8 hollandite is 166.2±0.2 J mol −1  K −1 , including an 18.7 J mol −1  K −1 contribution from the configurational entropy due to disorder of Al and Si in the octahedral sites. The entropy of K 2 Si 4 O 9 with a wadeite structure (Si-wadeite) was also estimated to facilitate calculation of phase equilibria in the system K 2 O–Al 2 O 3 –SiO 2 . The calculated phase equilibria obtained using Perple_x are in general agreement with experimental studies. Calculated phase relations in the system K 2 O–Al 2 O 3 –SiO 2 confirm a substantial stability field for kyanite–stishovite/coesite–Si-wadeite intervening between KAlSi 3 O 8 hollandite and sanidine. The upper stability of kyanite is bounded by the reaction kyanite (Al 2 SiO 5 ) = corundum (Al 2 O 3 )  + stishovite (SiO 2 ), which is located at 13–14 GPa for 1,100–1,400 K. The entropy and enthalpy of formation for K-cymrite (KAlSi 3 O 8 ·H 2 O) were modified to better fit global best-fit compilations of thermodynamic data and experimental studies. Thermodynamic calculations were undertaken on the reaction of K-cymrite to KAlSi 3 O 8 hollandite +  H 2 O, which is located at 8.3–10.0 GPa for the temperature range 800–1,600 K, well inside the stability field of stishovite. The reaction of muscovite to KAlSi 3 O 8 hollandite + corundum + H 2 O is placed at 10.0–10.6 GPa for the temperature range 900–1,500 K, in reasonable agreement with some but not all experiments on this reaction.Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/46910/1/269_2006_Article_63.pd

Anticlockwise P-T evolution of amphibolites from NE Sardinia, Italy: Geodynamic implications for the tectonic evolution of the Variscan Corsica-Sardinia block

In the Migmatite Complex from NE Sardinia, a large lensoid body of coarse-grained, dark-green amphibolite with a schistose to weakly massive aspect crops out. Within this amphibolite centimetre-sized layers locally occur which contain millimetric porphyroblastic garnet. We investigated the amphibolite and the layers applying microstructural analyses and thermodynamic modelling in the NCKFMASH+Ti+Mn system in order to reconstruct the pressure-temperature (P-T) metamorphic evolution. The amphibolite underwent a burial path, recorded by the compositional zoning of garnet, that started at pressures of 0.8 GPa and showed only a slight increase in temperature leading to peak P-T conditions. The garnet rim records peak P-T conditions of 1.3-1.4 GPa at 690-740 °C. As the early exhumation of the amphibolites occurred already at lower temperatures than the burial, an anticlockwise P-T path results which is in contrast to the typical clockwise P-T paths reported for several high-pressure metamorphic rocks from NE Sardinia. We interpret the anti-clockwise path by the location of the studied rocks in the lowermost part of the upper plate and their burial to depths of around 45 km during the Variscan continental collision between Laurussia and Gondwana. This process could have affected some rock slices of the upper plate only owing to tectonic erosion by the downgoing plate. The subsequent uplift occurred in an exhumation channel where these slices were continuously cooled by the upper portion of the lower continental plate

Minerali inclusi in zirconi provenienti dalle rocce crostali di alta pressione della Zona d'Ultimo (Trentino-Alto Adige, Italia): alla ricerca dei primi eventi della orogenesi Varisica (300-400Ma)

L'interesse per lo studio di inclusioni solide in zirconi separati dalle rocce crostali di alta pressione della Zona d'Ultimo (UZ) \ue8 motivato dal fatto che \ue8 stato dimostrato come i primi stadi dell'evoluzione metamorfica di una roccia possano lasciare relitti di minerali all'interno dello zircone. Quest'ultimo ha caratteri fisico-chimici tali da poter preservare le inclusioni anche attraverso pi\uf9 cicli tettonici e metamorfici. L'integrazione tra immagini elettroniche (per localizzare le inclusioni in funzione della zonatura dello zircone ospite), microanalisi quantitative (microsonda) e metodi di calcolo termobarometrici forniranno gli elementi per rispondere a questioni fondamentali riguardanti le prime fasi della evoluzione metamorfica Varisica, ora sconosciuta, della crosta dell'UZ

H2O content of deep-seated orogenic continental crust: the Ulten Zone, Italian Alps

To estimate the amount of H2O stored at lower crustal levels after burial, we considered the pile of migmatitic paragneisses in the Variscan Ulten Zone as a case study area. We constructed a pseudosection in the system K2O-Na2O-CaO-FeO-MnO-MgO-Al2O3-SiO2-TiO2-H2O for an average paragneiss, a relevant prograde PT path (8.5 kbar, 600◦C; 11.5 kbar, 750◦C; 14.0 kbar, 1000◦C) and H2O contents between 0 and 10 wt.%. Based on an assemblage of garnet + biotite + white mica + kyanite + 20\u201330 vol.% former melt (now represented mainly by leucosomes composed of plagioclase+quartz), a bulk H2O content of 3.2 \ub1 1.1 wt.% was estimated for a peak temperature ranging between 770 and 800◦C. Before melting, somewhat less than 1.8 wt.% H2O was stored in minerals. Thus, a considerable amount of H2O must have either resided in pore spaces along grain boundaries or, much less likely, infiltrated the paragneisses from below. Evidently, significant quantities of H2O as a free phase may be stored in buried sialic crust, resulting in considerable melting of deep-seated rocks during continent\u2013continent collision