Showing papers on "Granulite published in 1997"

Partial fusion of basic granulites at 5 to 15 kbar: implications for the origin of TTG magmas

Abstract: Partial fusion experiments with basic granulites (S6, S37) believed to represent the lower crust beneath the Eifel region (Germany) were performed at pressures from 5 to 15 kbar. Water-undersaturated experiments were carried out in the presence of 1 wt% H2O plus 2.44 or 0.81 wt% CO2 equivalent to mole fractions of H2O/(H2O + CO2) of 0.5 and 0.75, respectively, of the volatile components added. At temperatures from 850 to 1100 °C the weight proportions of melt range from 7 to 30 %. Melt compositions change from trondhjemitic over tonalitic to dioritic with increasing degree of partial melting. Crystalline residua are plagioclase/pyroxene dominated at 5 kbar to garnet/pyroxene dominated at 15␣kbar. Dehydration melting was studied in granulite S35 similar in composition to S6. The magmatic precursors of the granulite xenoliths used in this study had geochemical characteristics of cumulate gabbro (metagabbro S37) and evolved melts (metabasalts S6, S35), respectively. Melts from granulite S37 match the major element compositions of natural trondhjemites and tonalites. At 5 kbar, their Al2O3 is relatively low, similar to tonalites from ophiolites. At 15 kbar, Al2O3 in the melts is high due to the near absence of plagioclase in the crystalline residua. The Al2O3 concentrations in 15 kbar melts from S6 (˜20 wt%) are higher than in natural tonalites. Depth constraints on the formation of tonalitic magmas in the continental crust are provided by REE (rare earth element) patterns of the synthetic melts calculated from the known REE abundances in metagabbro S37 and metabasalt S6 assuming batch melting and using partition coefficients from the literature. The REE patterns of tonalites from active continental margins and Archean trondhjemite-tonalite-granodiorite␣associations low in REE with LaN (chondrite normalised) from 10 to 30 and YbN from 1 to 2 are reproduced at pressures of 10 and 12.5 kbar from metagabbro S37 which displays a slightly L(light)REE enriched pattern with LaN = 8 and YbN = 3. Natural tonalites with LaN from 30 to 100 require a source richer in REE than granulite S37. At 15 kbar, H(heavy)REEN in melts from granulite S37 are depressed below the level observed in natural tonalites due to the high proportion of garnet (>30 wt%) in the residue. Melts from metabasalt S6 (enriched in REE with LaN = 38 and YbN = 16) do not match the REE characteristics of natural tonalites under any conditions.

Zirconium abundance in granulite-facies minerals, with implications for zircon geochronology in high-grade rocks

Abstract: Zircon growth in high-grade metamorphic rocks may be triggered by net transfer reactions involving the breakdown of a phase bearing zirconium (Zr). We have measured the concentration of Zr in the major minerals in granulite-facies rocks of differing bulk composition. Both garnet and hornblende contain tens of parts per million Zr, and no other major phase contains significant Zr. Simple calculations show that reaction of either garnet or hornblende to form non–Zr-bearing phases will release sufficient Zr to account for at least some new zircon growth. U-Pb ages from new zircon, grown as a result of either hornblende or garnet breakdown, are not expected to record the time of peak metamorphism, but rather will record the time of particular metamorphic reactions, allowing direct correlation of zircon ages with petrologically derived pressure-temperature-time paths. This approach offers the potential for more rigorous interpretation of the metamorphic significance of zircon ages than has previously been possible.

Do U-Pb zircon ages from granulites reflect peak metamorphic conditions?

Abstract: Granulite facies metamorphic events are constrained commonly through application of U-Pb zircon geochronometry. Zircon growth related to high-grade metamorphism is interpreted as reflecting the age of peak pressure-temperature ( P-T ) conditions. However, these ages obtained from granulites need to be interpreted with considerable care. Under conditions of high-grade metamorphism, it is important that the possible presence of melt is considered. Our modeling of partial melting and its impact on zircon stability implies that zircon crystallization in hot, isothermally uplifted granulites could postdate the pressure peak of the P-T path. In a case study of felsic granulites from the Bohemian massif of Variscan central Europe, it appears likely that most zircons in the rocks would have grown after they were exhumed to medium pressure levels. Thus, zircon growth related to high-grade metamorphism should not be automatically assumed as reflecting the age of peak P-T conditions.

Effective bulk composition changes due to cooling : a model predicting complexities in retrograde reaction textures

Abstract: Diffusive processes are a strong function of temperature. Thus, during cooling of rocks, mineral grains may develop zoning profiles as successively larger parts of the grain “close” to the diffusive exchange with the rock. One of the consequences of this process is that, during cooling, successively larger parts of zoned minerals (depending on grain size) are effectively removed from the reacting part of the rock volume. Thus, the effective bulk composition of metamorphic rocks changes during cooling and the rate of its change will be a function of grain size. Because the sequence of metamorphic reactions seen by a given rock is a strong function of its bulk composition, this process may have the consequence that two rocks of identical overall bulk composition, but of different grain size, may experience a different sequence of reactions. Qualitatively identical peak paragenesis may therefore react to form qualitatively different retrograde reaction textures. The model is applied to examples in the pelitic system. There, garnet is usually the slowest diffusing phase developing zoning profiles during cooling and the effective removal of garnet from the reacting rock volume will cause changes of the effective bulk composition. It is shown that, during cooling of pelitic rocks from amphibolite facies conditions, typical aluminous peak parageneses of garnet-muscovite-kyanite ± biotite may react to form either staurolite, chlorite or muscovite (or different combinations thereof), depending on grain size. During cooling from the granulite facies, aluminous peak parageneses of garnet-cordierite-sillimanite may form biotite, either on the expense of cordierite or garnet, also depending on grain size. The two examples are illustrated with a series of reaction textures reported for amphibolite and granulite terrains in the literature.

Ultra‐high‐temperature metamorphism and multistage decompressional evolution of sapphirine granulites from the Palni Hill Ranges, southern India

Abstract: Sapphirine granulites from a new locality in the Palni Hill Ranges, southern India, occur in a small enclave of migmatitic, highly magnesian metapelites (mg=85–72) within massive enderbitic orthogneiss. They show a variety of multiphase reaction textures that partially overprint a coarse-grained high-pressure assemblage of Bt+Opx+Ky+Grt+Pl+Qtz. The sequence of reactions as deduced from the corona and symplectite assemblages, together with petrogenetic grid considerations, records a clockwise P–T evolution with four distinct stages. (1) Equilibration of the initial high-P assemblage in deep overthickened crust (12 kbar/800–900 °C) was followed by a stage of near-isobaric heating, presumably as a consequence of input of extra heat provided by the voluminous enderbitic intrusives. During heating, kyanite was converted to sillimanite, and biotite was involved in a series of vapour-phase-absent melting reactions, which resulted in the ultra-high-temperature assemblage Opx+Crd+Kfs+Spr±Sil, Grt, Qtz, Bt, coexisting with melt (equilibration at c. 950–1000° C/11–10 kbar). (2) Subsequently, as a result of decompression of the order of 4 kbar at ultra-high temperature, a sequence of symplectite assemblages (Opx+Sil+Spr/Spr+CrdOpx+Spr+CrdOpx+CrdOpx+Crd+Spl/Crd+Spl) developed at the expense of garnet, orthopyroxene and sillimanite. This stage of near-isothermal decompression implies rapid ascent of the granulites into mid-crustal levels, possibly due to extensional collapse and erosion of the overthickened crust. (3) Development of late biotite through back-reaction of melt with residual garnet indicates a stage of near-isobaric cooling to c. 875 °C at 7–8 kbar, i.e. relaxation of the rapidly ascended crust to the stable geotherm. (4) A second period of near-isothermal exhumation up to c. 6–5 kbar/850 °C is indicated by the partial breakdown of late biotite through volatile phase-absent melting reactions. Available isotope data suggest that the early part of the evolutionary history (stages 1–3) is presumably coeval with the early Proterozoic metamorphism in the extended granulite terrane of the Nilgiri, Biligirirangan and Shevaroy Hills to the north, while the exhumation of the granulites from mid-crustal levels (stage 4) occurred only during the Pan-African thermotectonic event, which led to the accretion of the Kerala Khondalite Belt to the south.

Monazite–xenotime miscibility gap thermometry. I. An empirical calibration

Abstract: Rare earth element (REE) and yttrium concentrations of coexisting monazite and xenotime were determined from a suite of seven metapelites from the Variscan fold belt in NE Bavaria, Germany. The metapelites include a continuous prograde, mainly low-P (3–5 kbar) metamorphic profile from greenschist (c. 400 °C) to lower granulite facies conditions (c. 700 °C). The LREE (La–Sm) are incorporated preferentially in monoclinic monazite (REO9 polyhedron), whereas the HREE plus Y are concentrated in tetragonal xenotime (REO8 polyhedron). The major element concentrations of both phases in all rocks are very similar and do not depend on metamorphic grade. Monazite consists mainly of La, Ce and Nd (La0.20–0.23, Ce0.41–0.45, Nd0.15–0.18)PO4, all other elements are below 6 mol%. Likewise, xenotime consists mainly of YPO4 with some Dy and Gd solid solutions (Y0.76–0.80, Dy0.05–0.07, Gd0.04–0.06). In contrast, the minor HREE concentrations in monazite increase strongly with increasing metamorphic grade: Y, Dy and Gd increase by a factor of 3–5 from greenschist to granulite facies rocks. Monazite crystals often show zonation with cores low in HREE and rims high in HREE that is interpreted as growth zonation attained during prograde metamorphism. Similarly, Sm and Nd in xenotimes increase by a factor of 3–4 with increasing metamorphic grade. Prograde zonation in single crystals of xenotime was not observed. The XHREE+Y in monazite and XLREE in xenotime of the seven rocks define two limbs along the strongly asymmetric miscibility gap from c. 400 °C to 700 °C. The empirical calibration of the monazite miscibility gap limb coexisting with xenotime is appropriate for geothermometry. Due to its contents of U and Th, monazite has often been used for U–Pb age determination. The combination of our empirical thermometer on prograde zoned monazite along with possible age determination of zoned single crystals may provide information about prograde branches of temperature–time paths.

Late Archaean to early Proterozoic granitoid magmatism and high-grade metamorphism in the central Limpopo belt, South Africa

Abstract: The Central Zone of the Limpopo belt in southern Africa has previously been interpreted as a segment of Archaean crust which experienced its main deformation and metamorphism around 2.7 Ga ago. We report new single zircon U/Pb and Pb/Pb ages for granitoid gneisses, supracrustal rocks and anatectically derived granitic melt patches in the area around Messina, South Africa. The Sand River Gneiss is a composite suite of tonalitic to trondhjemitic rocks with protolith ages between 2.6 and 3.2 Ga. The Singelele gneiss, a heterogeneous granodioritic to quartz monzonitic rock, has protolith ages between 2.55 and 2.58 Ga. Since both the Sand River and Singelele gneisses experienced polyphase high-strain ductile deformation this must have occurred later than 2.55 Ga ago. Granulite-facies pelitic gneisses of the Beit Bridge Complex contain abundant spherical, multifacetted zircons which reflect new zircon growth near or at the peak of metamorphism. These zircons provide ages with a mean at 2026.5±6.3 Ma which is interpreted as reflecting a high P – T event (>10 kbar, 825±25°C). Granitic melt patches in the metapelites as well as in the Sand River Gneiss and anatectic granites are probably related to rapid near-isothermal decompression to below 3–5 kbar and 600–750°C). These rocks contain new magmatic zircons which yielded a mean age of 2005.6±4.4 Ma and probably reflect a crustal melting event resulting from rehydration of the granulitic assemblage. Our zircon data support previous suggestions for only one single granulite-facies event in the Central Zone, and we suggest that this event occurred c. 2027 Ma ago. Since most of the deformation seen in the gneisses of the Messina area must have occurred later than 2.55 Ga ago, it is likely that the ‘Limpopo Orogeny’, at least in the Central Zone, is not an Archaean event but took place in the early Proterozoic.

Proterozoic Events in the Eastern Ghats Granulite Belt, India: Evidence From Rb‐Sr, Sm‐Nd Systematics, and Shrimp Dating

Abstract: Metamorphic and protolith ages of five rock types (mafic granulite, orthopyroxene granulite, leptynite, sillimanite granite, and metapelite) from Rayagada, in the north‐central part of the Eastern Ghats Granulite Belt (EGGB), India, were determined from Rb‐Sr and Sm‐Nd whole rock and mineral isochrons in combination with SHRIMP U‐Pb zircon data. Most of the whole rock isochron ages in both Sm‐Nd and Rb‐Sr systems point to either ∼1450 or ∼1000, Ma, and the mineral isochron ages are ∼1000, ∼800, and ∼550 Ma. SHRIMP U‐Pb zircon ages of ∼940 Ma were obtained from metapelite, which are in close agreement with the Sm‐Nd and Rb‐Sr isochron ages. From all these data, four age clusters (∼1450, ∼1000, ∼800, and ∼550 Ma) have been noted. The 1450 Ma ages are interpreted to represent igneous protolith formation of mafic granulite and leptynite. The 1000 Ma age cluster is regarded as the intrusion ages of sillimanite granite, and charnockite, and associated granulite facies metamorphism. Two other age clusters (800 a...

Magmatic Underplating, Extension, and Crustal Reequilibration: Insights From A Cross-Section Through the Ivrea Zone and Strona-Ceneri Zone, Northern Italy

Abstract: The thermal impact of magmatic underplating at various crustal levels is studied along a traverse through the Ivrea‐Verbano Zone and Strona‐Ceneri Zone in northern Italy. Geochronological and petrologic data are compared to a two‐dimensional thermal‐kinematic model. Field data and numerical simulation show the strong disturbance of the temperature field in the lower and intermediate crust in relation to magmatic underplating leading to granulite‐ to amphibolite‐facies metamorphism as well as reequilibration of mineral chemical and isotopic systems. Magmatic underplating leaves a crust with an apparently heterogeneous tectonometamorphic evolution, as information on the earlier history is preserved only at upper crustal levels.

Petrology of high-pressure granulites from the eastern Himalayan syntaxis

Abstract: The eastern Himalayan syntaxis, situated at the eastern terminus of the Himalayas, is the least-known segment of the Himalayas. Recent research in this area has revealed that the syntaxis consists of the Gangdise, the Yarlung Zangbo, and the Himalayan units, each of which is bounded by faults. The Himalayan unit, the northernmost exposed part of the Indian plate, mainly contains amphibolite facies rocks, marked by the assemblages staurolite+kyanite+plagioclase+biotite+muscovite±sillimanite and garnet+amphibole+plagioclase, in the south; to the north, low- to medium-pressure granulite grade pelitic gneisses and marbles are present and are characterized by the assemblages garnet+sillimanite+K-feldspar+plagioclase or antiperthite+biotite+quartz±spinel±cordierite±orthopyroxene in gneisses, and anorthite+diopside±wollastonite and plagioclase+diopside+quartz+phlogopite+calcite in marbles. Within this unit, the Namula thrust system is a series of moderately north-dipping structures that displaced the granulite facies rocks southwards over the amphibolite facies rocks. High-pressure granulites occur as relics within these granulite facies rocks and contain garnet–kyanite granulite and garnet clinopyroxenite. The peak assemblage of the garnet–kyanite granulite includes garnet (core part)+kyanite+ternary feldspar+quartz+rutile. Sillimanite+garnet (rim part)+K-feldspar+ oligoclase+ilmenite+biotite and spinel+albite+biotite or spinel+cordierite±orthopyroxene, which are coronas around sillimanite and garnet, are retrograde products of this peak assemblage. Another peak assemblage includes very-high-Ca garnet (CaO 32–34 wt%, Alm10±Grs>80) and diopside (CaO 22–24 wt%), scapolite, meionite, quartz, and accessory Al-bearing titanite (Al2O3 4–4.5 wt%). The diopside has kink bands. Partial or complete breakdown of Ca-rich garnet during post-peak metamorphism produced pseudomorphs and coronas consisting of fine-grained symplectic intergrowths of hedenbergite and anorthite. Thermobarometric estimates in combination with reaction textures, mineral compositions, and recent experimental studies indicate that these peak assemblages were formed at P=c. 1.7–1.8 GPa, T =c. 890 °C, and the retrograde assemblages experienced near-isothermal decompression to P=0.5±0.1 GPa, T =850±50 °C. The whole-rock compositions indicate that marble and pelite are plausible candidates for the protoliths. These facts suggest the following (1) sedimentary rocks were transported to upper-mantle depths and equilibrated at those conditions to form these high-pressure granulites, which were then emplaced into the crust quickly. During the rapid exhumation of these rocks, the earlier high-pressure assemblages were overprinted by the later low- to medium-pressure assemblages, that is, the high-pressure granulite belt formed in the syntaxis. (2) The Namula thrust system is an important tectonic boundary in the syntaxis, or even in the Higher Himalaya more generally.

Petrological and Isotopic Studies on Palaeozoic High-pressure Granulites, Gory Sowie Mts, Polish Sudetes

Abstract: 2154·7-1·4 Ma, whereas 19 analyses of grain fractions and single grains of multi-faceted zircons interpreted, from their char- acteristic spherical 'football' shape, as newly crystallized meta-

A revised Archaean chronology for the Napier Complex, Enderby Land, from SHRIMP ion-microprobe studies

Abstract: The long and complex Archaean evolution of the Napier Complex of Enderby Land, characterized by high-grade metamorphism and several strong deformations, is reassessed in the light of new SHRIMPU–Pb zircon dating results bearing on the ages of protoliths and possible regional extents of distinct Archaean tectonothermal events. Initial felsic igneous activity occurred over a significant time interval c. 3800 Ma ago. An age of 2980±9 Ma for the emplacement of charnockite at Proclamation Island might date the oldest tectonothermal event to be recognized in the Napier Complex. An ensuing, very-high grade, previously imprecisely dated tectonothermal event occurred at 2837±15 Ma. U–Pb zircon ages ranging from 2456+8/−5 Ma to 2481±4 Ma date a subsequent, protracted high-grade tectonothermal event. Whereas the ~2840 Ma event is of regional importance in the Amundsen Bay-Casey Bay area, it is possible that the ~2980 Ma event was of only moderate grade, minor importance, or even absent, in that part of the Complex. If so, the apparent trend to very-high temperature metamorphism in the Tula and Scott mountains compared with the Napier Mountains may reflect two distinct metamorphic events rather than a simple baric and thermal gradient. The oldest crustal component in the Napier Complex appears to have been of igneous derivation. Zircon populations in paragneisses at Mount Sones are similar to those in the nearby orthogneisses, which therefore may have been basement. Another paragneiss, in the Casey Bay area, yields no zircons older than 2840 Ma, probably indicating that pre-3000 Ma crust, which is now located nearby, was not exposed at the time of sedimentation there. The isotopic data are quite complex, particularly in rocks that experienced postcrystallization metamorphic temperatures of 1000°C or more. It is postulated that this complexity, which was largely the product of migration of radiogenic Pb within the zircon grains in ancient times, and produced local excesses of this element with respect to its parent U, was caused by volume diffusion at these abnormally high regional crustal temperatures.

Pressure-Temperature and Fluid Evolution of Quartzo-Feldspathic Metamorphic Rocks with a Relic High-Pressure, Granulite-Facies History from the Central Erzgebirge (Saxony, Germany)

Abstract: by (1) high-pressure (HP) equilibration, followed by near-isothermal The Gneiss–Eclogite Unit is a composite tectonometamorphic unit decompression at high temperatures, during which rocks from different within the Variscan Erzgebirge mega-antiform. It comprises migdepths were amalgamated, and (2) extensive hydration and rematitic paraand orthogneisses, high-temperature (HT) mylonites, equilibration at medium pressures, followed by rapid cooling during kyanite-bearing granulites, eclogites and garnet peridotites. Four continued uplift, when the entire unit came into contact with cooler, different quartzo-feldspathic assemblages are recognized, in which now overand underlying units. This scenario is attributed to maximum conditions of up to 830°C and 21 kbar were determined. continent collision, orogenic collapse and disintegration of the HP The assemblages are characterized by the nearly complete prograde unit during continuing collision, crustal stacking and uplift controlled breakdown of biotite, by high grossular content (23–47 mol %) by extension. of garnet in the presence of albite, and high Si contents of phengite [3·3–3·4 per formula unit (p.f.u.)]. Water activities at this stage are variable and range from 0·4. The maximum pressures indicated for individual rock volumes may vary considerably between 12 and 24 kbar at 700–800°C, so that non-coherency

U-Pb isotopic evidence for the accretion of different crustal blocks to form the Lewisian Complex of northwest Scotland

Abstract: Single zircon and titanite U-Pb SHRIMP data presented for tonalite-trondhjemite-granodiorite (TTG) suite gneisses and an ultramafic rock from the northern and central regions of the Lewisian Complex of northwest Scotland, show that protolith ages of tonalitic gneisses in the northern region (2800–2840 Ma) are significantly younger than those in the central region (2960–3030 Ma). Further evidence of a major (2490–2480 Ma) metamorphic event in the central region is documented by a metamorphic zircon associated with a granulite facies ultramafic body. A dioritic gneiss from the northern region has also been dated at c. 2680 Ma. The northern region therefore does not comprise reworked central region rocks and consequently the old models for the evolution of the Lewisian which were based upon this concept need replacing. It is instead proposed that two distinct crustal blocks, now the northern and central regions, were tectonically juxtaposed along a boundary corresponding to the Laxford Front. Juxtaposition would appear to have occurred in Proterozoic times, as it must have postdated the 2490–2480 Ma (?Inverian) metamorphism recorded only in the central region, and the emplacement of granite sheets restricted to the northern side of the boundary. The first recorded event common to both regions is resetting of titanite ages associated with c. 1750 Ma Laxfordian amphibolite facies metamorphism. Zircon inheritance in rocks of both regions is scarce. Within one zircon from the northern region a c. 3550 Ma core was found. This represents the oldest known material from the region.

Transect across the northwestern Grenville orogen, Georgian Bay, Ontario: Polystage convergence and extension in the lower orogenic crust

Abstract: The Grenville orogenic cycle, between ∼ 1190 and 980 Ma, involved accretion of magmatic arcs and/or continental terranes to the Laurentian craton. A transect across the western Central Gneiss Belt, Georgian Bay, Ontario, which crosses the boundary between parautochthonous and allochthonous units at an inferred orogenic depth of 20–30 km, offers some insights on the thermal and mechanical behavior of the lower crust during the development of the Grenville orogen. Prior to Grenvillian metamorphism, this part of Laurentia consisted largely of Meso-proterozoic (∼ 1450 Ma) granitoid orthogneisses, granulites, and subordinate mafic and supracrustal rocks. Grenvillian convergence along the transect began with transport of the previously deformed and metamorphosed (∼ 1160 Ma) Parry Sound domain over the craton sometime between 1120 Ma and 1080 Ma. This stage of transport was followed by out-of-sequence thrusting and further convergence along successively deeper, foreland-propagating ductile thrust zones. A major episode of extension at ∼ 1020 Ma resulted in southeast directed transport of allochthonous rocks along the midcrustal Shawanaga shear zone. The final stage of convergence involved deformation and metamorphism in the Grenville Front Tectonic Zone at ∼ 1000–980 Ma. Peak metamorphism along most of the transect at 1065–1045 Ma followed initial transport of allochthonous rocks over the craton by 15–35 m.y. Regional cooling, which postdated peak metamorphism by >70 m.y., was probably delayed by the combined effects of late-stage extension and convergence. Transport of allochthons at least 100 km over the craton was accomplished along a weak, migmatitic decollement; further propagation of the orogen into the craton followed partial melting and weakening of parautochthonous rocks below this decollement. Extensional deformation was associated with distributed ductile flow, the formation of regional transverse folds with axes parallel to the stretching direction, and reactivation of the allochthon-parautochthon thrust boundary as an extensional decollement. The extensional lower crustal flow was likely the primary cause of the subhorizontal attitude of many structures and seismic reflectors in this part of the Central Gneiss Belt.

Partial melting during tectonic exhumation of a granulite terrane: an example from the Larsemann Hills, East Antarctica

Abstract: Anatectic migmatites in medium- to low-pressure granulite facies metasediments exposed in the Larsemann Hills, East Antarctica, contain leucosomes with abundant quartz and plagioclase and minor interstitial K-feldspar, and assemblages of garnet–cordierite–spinel–ilmenite–sillimanite. Qualitative modelling in the system K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–O2, in conjunction with various P–T calculations indicate that the high-grade retrograde evolution of the terrane was dominated by decompression from peak conditions of c. 7 kbar at c. 800 °C to 4–5 kbar at c. 750 °C. Extensive partial melting during decompression involved the replacement of biotite by the assemblage cordierite–garnet–spinel within the leucosomes. These leucosomes represent the site of partial melt generation, the cordierite–garnet–spinel–ilmenite assemblage representing the solid products and excess reactants from the melting reaction. The extraction and accumulation of this decompression-generated melt led to the formation of syntectonic pegmatites and extensive granitic plutons. Leucosome development and terrane decompression proceeded during crustal transpression, synchronous with upper crustal extension, during a progressive Early Palaeozoic collisional event. Subsequent retrograde evolution was characterized by cooling, as indicated by the growth of biotite replacing spinel and garnet, thin mantles of cordierite replacing spinel and quartz within metapelites, and garnet replacing orthopyroxene and hornblende within metabasites. P–T calculations on late mylonites indicate lower grade conditions of formation of c. 3.5 kbar at c. 650 °C, consistent with the development of late cooling textures.

Fossil crust‐to‐mantle transition, Val Malenco (Italian Alps)

Abstract: An exhumed, undisturbed fossil lower crust to upper mantle section is preserved in Val Malenco, Italian Alps, and is now exposed along the boundary between Penninic and Austroalpine nappes. Lower-crustal metapelitic rocks are welded to upper-mantle ultramafic rocks by a mid-Permian gabbro intrusion. The underplating of gabbro caused granulite metamorphism and partial melting of the metapelites. In the crust-to-mantle transition zone of at least 1 km thickness, gabbros, large xenoliths of restitic metapelites and ultramafic rocks occur, with densities of 2.95-3.14, 3.25 and 3.27 g/cm 3 , respectively. The seismic Moho therefore did not coincide with the boundary between peridotites and crustal rocks but was situated above the upper limit of the peridotitic mantle. The whole complex underwent cooling with only moderate decompression within the kyanite field. This process started at 1 GPa and ∼800°C and ended at 0.85 GPa and 600°C and is interpreted as thermal relaxation after the gabbro intrusion. Later, during Jurassic rifting, the crust-to-mantle section was exhumed at the Adria margin of the Tethys ocean.

Mafic Granulite Xenoliths in Neogene Alkali Basalts from the Western Pannonian Basin: Insights into the Lower Crust of a Collapsed Orogen

Abstract: Mafic granulite xenoliths from Pliocene alkali basalts of western melt. This can be explained by mixing between mafic back-arcHungary represent the lower crust of the Pannonian Basin. The basin-like tholeiitic melts and pre-existing lower crust that has high xenolith suite from two localities, Szigliget and Bondorohegy, includes Sr, Pb and O isotopes but low Nd/Nd. The most crustally medium-pressure mafic granulites (plagioclase+pyroxene±garnet) contaminated rocks have Nd/Nd ratios that can only be that range from gabbroic (i.e. orthopyroxene-free) to two-pyroxene explained by assimilation of an old component (>1 Ga). Thus, and garnet-bearing lithologies. Two groups can be distinguished on although the crustal block accreted to Europe in the Mesozoic was the basis of chemistry: light rare earth element (LREE)-depleted predominantly oceanic in character, it must have contained slivers granulites that have Sr, Nd and O isotope ratios typical of depleted of Precambrian crust. mantle, and LREE-enriched granulites that have higher Sr/ Sr (0·706–0·709), lower Nd/Nd (0·5128–0·5123) and higher dO (+7·5 to +10·4‰). The LREE-depleted group also has lower Pb/Pb and Pb/Pb for a given Pb/

Evolution of Moldanubian rocks in Austria: review and synthesis

Abstract: The Moldanubian zone in Austria comprises three major lithological units. Despite general agreement that nappe tectonics contributed to its current structure, the number and position of tectonic boundaries, or continental pieces that were involved in its evolution, as well as the age, extent and position of oceanic sutures are disputed. Recent models ascribe the Moldanubian tectonostratigraphic structure to its oblique, N- to NE-directed collision with Moravia only. The rocks of the Moldanubian Bunte series and Gfohl unit experienced a common, intensive overprint in the range 700–800 °C and 8–11 kbar. Textural evidence suggests that this overprint was attained during nearly isothermal decompression, so the rocks experienced higher pressures prior to this overprint. These conditions constrain a continent–continent collision environment that contributed to the formation of the Moldanubian granulites. The estimated metamorphic temperatures are close to Tmax. During this Hercynian, high-T overprint, the minerals underwent extensive diffusion-controlled homogenization of elements. The early stages of retrogression of these units were characterized by isobaric cooling at c. 6 kbar in the range 650–500 °C that is related to the oblique collision of the Moldanubian and Moravian zones. Cooling to c. 400 °C is demonstrated by unstrained, diasporized corundum inclusions in garnet of common Moldanubian granulites. The available age data (including cooling ages) from metamorphic rocks show a very wide variation between 490 and 280 Ma that depends on sample characteristics and the dating method used. They demonstrate clearly, however, that the metamorphic overprint is Hercynian. The possibility that the large variation in ages reflects homogenization, resetting and closure of the isotopic systems attained at different, sample- and method-specific times is discussed. Age data varying between c. 370 and c. 346 Ma tentatively date different stages during the Hercynian, high-T decompression. The majority of zircon and monazite U/Pb ages as well as the hornblende and muscovite Ar/Ar cooling ages cluster between c. 345 and c. 326 Ma and date the effective closure conditions and the onset of rapid, nearly isobaric cooling. The continent–continent collision that formed the granulites pre-dates c. 370 Ma. The intra-Moldanubian nappe-stacking pre-dates thrusting of the Moldanubian zone over the Moravian zone. The range c. 340–335 Ma is the lower limit for completion of tectonic activity in the Moldanubian zone. The Moldanubian series are post-tectonically intruded by granitoids of the Southern Bohemian Pluton. Recent age determinations and geochemical evidence suggest that the formation of the early granitoid types took place in the lower crust in connection with the Hercynian high-grade overprint. The Moldanubian Monotone series in Austria is separated from the other Moldanubian units by a conspicuous tectonic horizon. It also differs from them by its characteristic high-T , low-P overprint, which is best demonstrated by a widespread cordierite gneiss.

Terranes and terrane boundaries in the Sudetes, Northeast Bohemian Massif

Abstract: In the Sudetes, seven distinct lithostratigraphic terranes exhibit a symmetric distribution A central region of basinal/oceanic and ophiolitic rocks, the Central Sudetic terrane is bordered, respectively to the northwest and southeast, by the sialic Saxothuringian and Moldanubian terranes These exhibit contrasting metasedimentary/metavolcanic successions and tectonic-metamorphic sequences, but both are characterized by Palaeozoic plutonism These are in turn bordered (again respectively to the northwest and southeast) by the Lusatian and Moravian terranes, which are also sialic, but contain Cadomian granitoids and represent rifted and now widely separated fragments of Gondwana Along the southwestern flank of the Sudetes, the Barrandian terrane, largely covered by younger sediments, extends to the southwestern margin of the Bohemian Massif The Sowie Gory terrane forms a klippe of high grade gneisses tectonically emplaced on top of low-grade, sheared ophiolites of the Central Sudetic terrane The Sowie Gory terrane exhibits a history of three distinct, probably multi-orogenic, regional metamorphic events: an early high-pressure granulite/eclogite metamorphism followed by medium- to low-pressure granulite, and in turn by amphibolite facies metamorphism All the terrane boundaries are complex zones of ductile to brittle shearing, modified by later brittle movements Some, such as the Leszczyniec shear zone, mark lines of old, pre-Variscan rift and suture zones, reactivated and overprinted during a series of Variscan ductile to brittle events of extensional shearing with related metamorphism and plutonism

Timing and exhumation of eclogite facies shear zones, Musgrave Block, central Australia

Abstract: Timing constraints on shear zones can provide an insight into the kinematic and exhumation evolution of metamorphic belts In the Musgrave Block, central Australia, granulite facies gneisses have been affected, to varying degrees, by mylonitic deformation, some of which attained eclogite facies The Davenport Shear Zone is a dominant strike-slip system that formed at eclogite facies conditions (T ≈650 °C and P≈120 kbar) Sm–Nd mineral isochrons obtained from equilibrated high-pressure assemblages, as well as 40Ar–39Ar data, show that the eclogite and greenschist facies high-strain overprints were coeval, at c 550 Ma Mylonitic processes do not appear to have reset the U–Pb system in zircon, but may have partially disturbed it The thermal gradient in the Musgrave Block crust at c 550 Ma was c 16 °C km−1 and at c 535 Ma was c 18 °C km−1, based on P–T estimates of eclogite and greenschist facies shear zones, respectively These estimates are similar to present-day geothermal gradients in many stable continental shield areas, suggesting that the region did not undergo a significant transient perturbation of the geotherm Therefore, in the Musgrave Block, cooling subsequent to eclogite facies metamorphism appears to have been controlled by exhumation, rather than by the removal of a heat source Estimated exhumation rates in the range 02 to ≥15 mm year−1 are comparable with other orogenic belts, rather than cratonic areas elsewhere