Showing papers on "Granulite published in 1996"

Multiple zircon growth and recrystallization during polyphase Late Carboniferous to Triassic metamorphism in granulites of the Ivrea Zone (Southern Alps): an ion microprobe (SHRIMP) study

Abstract: Zircons from a metasedimentary and a meta-igneous quartz-feldspar granulite from the Val Sesia and Val Mastallone area of the Ivrea Zone (Southern Alps) differ in their response to granulite facies metamorphism with respect to crystal morphology and U/Pb ages. Detrital zircons in the metasediment developed an isometric overgrowth by the addition of Zr derived mainly from co-existing minerals, most probably biotite, decomposing during anatectic melting. The overgrowth started in the pelitic layer of the metasediment in the Late Carboniferous at approximately 296 Ma, significantly earlier than in the adjacent psammitic layer where it started only at 261 ± 4 Ma (95% confidence level). These ages are ascribed to the differential initiation of anatexis in the two layers. The delay of melting in the psammitic layer was probably due to the different position and less steep slope of its solidus in P-T-space, as compared to the solidus in the pelitic layer. Accordingly, the melting in the psammitic layer at 261 Ma was initiated by a thermal pulse and/or by a decompression event. Decompression melting is supported by a characteristic shell morphology of the zircon overgrowth in the psammitic layer, which might have grown under shear movements during high-temperature extensional faulting. The typically magmatic zircon population of the meta-igneous granulite crystallized at 355 ± 6 Ma (95% confidence level). The morphology of the zircons and the chemistry of the rock suggest that the magma was calcalkaline. A minor subpopulation of crystals is morphologically similar to the zircons in the pelitic layer of the metasediment. This points to the admixture of a minor sediment component and, thus, to a largely volcaniclastic origin of the protolith. In contrast to the detrital zircons in the metasediment, the magmatic zircons show rare and little overgrowth and, instead, have been strongly resorbed by anatectic melt. In addition, they became partially recrystallized and the rejuvenated ages from the most thoroughly recrystallized domains indicate that the rock was subject to prograde metamorphism after 279 Ma. This may correspond to the regional temperature increase prior to the climax of metamorphism or to a local thermal pulse due to nearby mafic intrusions. An Upper Triassic event at 226 ± 5 Ma is reflected by distinct peripheral zones in the overgrowths of some zircons in the metapelite. These are interpreted as a second metamorphic pulse, possibly induced by the infiltration of fluids.

Buoyancy sources for a large, unrooted mountain range, the Sierra Nevada, California: Evidence from xenolith thermobarometry

Abstract: Xenoliths hosted by Cenozoic volcanic flows and plugs from the Central Sierra Nevada and Eastern Sierra Nevada, Owens Valley, and Inyo Mountains were studied for petrography and thermobarometry. The Central Sierra Nevada suite consists of abundant lower crustal feldspathic granulites, garnet clinopyroxenites, and mantle-derived peridotites and garnet websterites. Mafic crustal assemblages occur down to ∼65–70 km, although below 35–40 km, they are mainly in the eclogite facies. In contrast, the Eastern Sierra Region suites show peridotitic, pyroxenitic, and harzburgitic assemblages at depths of ≥35–40 km. They define an adiabat in PT space (T ∼ 1180–1250°C), suggesting the presence of the asthenospheric upper mantle close to the base of the crust. The ultramafic mantle rocks from the Central Sierra Nevada also define an adiabatic slope in PT space, possibly an artifact of side heating from the east. There is xenolith evidence that the Sierra Nevada lost about half of its original crust on the eastern side of the range. Regardless of the actual mechanism of crustal thinning, the loss of the eclogitic lowermost crust and replacement by peridotite in the eastern Sierra Nevada is a process accompanied by a substantial density decrease (>100 kg/m^3). Overall, if the mechanism of eclogitic lowermost crust removal is viable, there are enough buoyancy sources to explain topographic differences between the Sierra Nevada and the adjacent Basin and Range, assuming isostatic equilibrium.

Sm-Nd and Rb-Sr ages and P-T history of the Archean Sittampundi and Bhavani layered meta-anorthosite complexes in Cauvery shear zone, South India: evidence for Neoproterozoic reworking of Archean crust

Abstract: Sittampundi and Bhavani Archean layered meta-anorthosite complexes occur as tectonic lenses within the Cauvery shear zone (CSZ), a crustal scale shear dividing the Precambrian granulite crust of south India into late Archean (> 25 Ga) and Proterozoic (c 055 Ga) blocks They and their host supracrustal-gneiss rocks record at least two stages of tectonometamorphic history The first is seen as regional scale refolded isoclinal folds and granulite metamorphism (D1-M1) while the second stage is associated with dominantly E–W dextral transcurrent shearing and metamorphic recrystallisation (D2-MCSZ) Whole rock Sm-Nd isochrons for several comagmatic rocks of the layered complexes yield concordant ages: Sittampundi – 2935±60 Ma, ɛNd + 185±016 and Bhavani – 2899±28 Ma, ɛNd + 218±014 (2σ errors) Our Sm-Nd results suggest that: (1) the magmatic protoliths of the Sittampundi and Bhavani layered complexes were extracted from similar uniform and LREE depleted mantle sources; (2) M1 metamorphism occurred soon after emplacement at c30 Ga ago P-T estimates on garnet granulites from the Sittampundi complex characterise the MCSZ as a high-P event with metamorphic peak conditions of c 118 kbar and 830°C (minimum) The MCSZ is associated with significant isothermal decompression of the order of 45–35 kbar followed by static high-temperature rehydration and retrogression around 600°C The timing of MCSZ is inferred to be Neoproterozoic at c 730 Ma based on a whole rock-garnet-plagioclase-hornblende Sm-Nd isochron age for a garnet granulite from the Sittampundi complex and its agreement with the 800–600 Ma published age data on post-kinematic plutonic rocks within the CSZ These results demonstrate that the Cauvery shear zone is a zone of Neoproterozoic reworking of Archean crust broadly similar to the interface between the Napier and Rayner complexes of the East Antarctic shield in a model Proterozoic Gondwana supercontinent

Metamorphic evolution of the Sesia-Lanzo Zone, Western Alps: time constraints from multi-system geochronology

Abstract: The Sesia-Lanzo Zone is a polymetamorphic unit containing Hercynian granulite relics overprinted by eclogite and greenschist facies metamorphism and deformation during the Alpine orogeny. Different parts of the unit record different stages on the P-T-deformation evolution, allowing multi-system isotopic studies to unravel the precise timing of the metamorphic history. New Rb–Sr white mica and U–Pb sphene data constrain the age of eclogite facies metamorphism and deformation to 60–70 Ma. This substantially alters the common view of early- to mid-Cretaceous eclogite facies metamorphism in this unit. The new results are more consistent with the established geotectonic framework for the Alpine orogeny, since they do not require a prolonged period of depressed geothermal gradient at a time when the region was in extension. It is also more concordant with recent studies of other units that demonstrate post-Cretaceous high-pressure metamorphism. Step-heated 40Ar–39Ar analysis of phengites yields good plateaux giving ages older than the corresponding Rb–Sr age. Such anomalously high ages indicate the presence of radiogenic argon-rich fluids in the grain boundary network under the fluid/pressure conditions acting during this high-pressure metamorphic event. The U–Pb sphene ages are variable in polymetamorphic rocks, and show inheritance of older Pb or sphene crystals into the high-pressure event. Two monometamorphic assemblages yield concordant ages at 66±1 Ma, reflecting crystallisation of the eclogite facies assemblage. The Gneiss Minuti Complex (GMC) lies structurally below the Eclogitic Micaschists, and its pervasive greenschist facies fabric yields tightly clustered Rb–Sr white mica ages at 38–39 Ma. This greenschist event did not affect the majority of the EMC. The 40Ar–39Ar ages of micas formed at this time were very disturbed, whereas micas surviving from an earlier higher pressure assemblage had their 40Ar–39Ar system reset. The greenschist event did not strongly affect U–Pb systematics in Hercynian age sphenes, suggesting that the GMC did not uniformly suffer an eclogite facies metamorphism during the Alpine cycle, but was juxtaposed against the EMC later in the orogeny. This model still requires that the locus of deformation and metamorphism (and possibly fluid flux) moved outboard with time, leaving the Sesia-Lanzo basement as a shear-bounded unreactive block within the orogenic wedge.

The Amazon-Laurentian connection as viewed from the Middle Proterozoic rocks in the central Andes, western Bolivia and northern Chile

Abstract: Middle Proterozoic rocks underlying the Andes in western Bolivia, western Argentina, and northern Chile and Early Proterozoic rocks of the Arequipa massif in southern Peru form the Arequipa-Antofalla craton. These rocks are discontinuously exposed beneath Mesozoic and Cenozoic rocks, but abundant crystalline clasts in Tertiary sedimentary rocks in the western altiplano allow indirect samples of the craton. Near Berenguela, western Bolivia, the Oligocene and Miocene Mauri Formation contains boulders of granodiorite augen gneiss (1171±20 Ma and 1158±12 Ma; U-Pb zircon), quartzose gneiss and granofels that are inferred to have arkosic protoliths (1100 Ma source region; U-Pb zircon), quartzofeldspathic and mafic orthogneisses that have amphibolite- and granulite-facies metamorphic mineral assemblages (∼1080 Ma metamorphism; U-Pb zircon), and undeformed granitic rocks of Phanerozoic(?) age. The Middle Proterozoic crystalline rocks from Berenguela and elsewhere in western Bolivia and from the Middle Proterozoic Belen Schist in northern Chile generally have present-day low 206Pb/204Pb ( 15.57), and elevated 208Pb/204Pb (37.2 to 50.7) indicative of high time-averaged Th/U values. The Middle Proterozoic rocks in general have higher present-day 206Pb/204Pb values than those of the Early Proterozoic rocks of the Arequipa massif (206Pb/204Pb between 16.1 and 17.1) but lower than rocks of the southern Arequipa-Antofalla craton (206Pb/204Pb> 18.5), a difference inferred to reflect Grenvillian granulite metamorphism. The Pb isotopic compositions for the various Proterozoic rocks lie on common Pb isotopic growth curves, implying that Pb incorporated in rocks composing the Arequipa-Antofalla craton was extracted from a similar evolving Pb isotopic reservoir. Evidently, the craton has been a coherent terrane since the Middle Proterozoic. Moreover, the Pb isotopic compositions for the Arequipa-Antofalla craton overlap those of the Amazon craton, thereby supporting a link between these cratons and seemingly precluding part of the Arequipa-Antofalla craton from being a detached fragment of another craton such as eastern Laurentia, which has been characterized by a different U/Pb history. Pb isotopic compositions for the Arequipa-Antofalla craton are, furthermore, distinct from those of the Proterozoic basement in the Precordillera terrane, western Argentina, indicating a Pb isotopic and presumably a tectonic boundary between them. The Pb isotopic compositions for the Precordillera basement are similar to those of eastern Laurentia, and support other data indicating that these rocks are a detached fragment of North America. Finally, the distinct Pb isotopic evolution history of the Arequipa-Antofalla craton and eastern Laurentia require minor modification to tectonic models linking eastern North America-Scotland to the oroclinal bend in western South America.

Petrogenesis of a high‐temperature metamorphic terrane: a new tectonic interpretation for the north Dabieshan, central China

Abstract: The northern Dabie terrane consists of a variety of metamorphic rocks with minor mafic-ultramafic blocks, and abundant Jurassic-Cretaceous granitic plutons. The metamorphic rocks include orthogneisses, amphibolite, migmatitic gneiss with minor granulite and metasediments; no eclogite or other high-pressure metamorphic rocks have been found. Granulites of various compositions occur either as lenses, blocks or layers within clinopyroxene-bearing amphibolite or gneiss. The palaeosomes of most migmatitic gneisses contain clinopyroxene; melanosomes and leucosomes are intimately intermingled, tightly folded and may have formed in situ. The granulites formed at about 800–830 °C and 10–14 kbar and display near-isothermal decompression P–T paths that may have resulted from crust thickened by collision. Plagioclase-amphibole coronae around garnets and matrix PI + Hbl assemblages from mafic and ultramafic granulites formed at about 750–800 °C. Partial replacement of clinopyroxene by amphibole in gneiss marks amphibolite facies retrograde metamorphism. Amphibolite facies orthogneisses and interlayered amphibolites formed at 680–750 °C and c. 6 kbar. Formation of oligoclase + orthoclase antiperthite after plagioclase took place in migmatitic gneisses at T ≤ 490°C in response to a final stage of retrograde recrystallization. These P–T estimates indicate that the northern Dabie metamorphic granulite-amphibolite facies terrane formed in a metamorphic field gradient of 20–35 °C km-1 at intermediate to low pressures, and may represent the Sino-Korean hangingwall during Triassic subduction for formation of the ultrahigh- and high-P units to the south. Post-collisional intrusion of a mafic-ultramafic cumulate complex occurred due to breakoff of the subducting slab.

High‐pressure granulites from the Sudetes (south‐west Poland): evidence of crustal subduction and collisional thickening in the Variscan Belt

Abstract: Two high-grade gneissic complexes of the Western Sudetes, the Gory Sowie Block and the Śnieznik area complex, contain small, predominantly felsic granulitic inliers with minor Cpx-bearing intercalations. The P–T conditions of the granulite facies events and of the subsequent re-equilibration are estimated using the ternary feldspar thermometer and the Geo-Calc computer program (version TWQ, Jan 92). In the Gory Sowie granulites, the peak granulitic event occurred at c. 18–20 kbar and 900 °C, and the late decompressive re-equilibration within a range of 4–10 kbar and temperatures decreasing to 600–700 °C. The latter event is thought to have coincided with the main metamorphic phase in the surrounding gneisses. The P–T estimates are more scattered in the Śnieznik granulites, but the peak conditions for the granulitic event are estimated at pressure over 22 kbar (possibly around 30 kbar) and temperature exceeding 900 °C. The analysed samples from the Śnieznik area bear no significant evidence of lower-pressure re-equilibration. Integrating the thermobarometric data and some age constraints indicates that the Gory Sowie granulites belong to the early stage ‘type I’ granulites of the Variscan Belt (c. 400 Ma old), which are interpreted as fragments of continental crustal materials subducted to mantle depths in the earliest stages of the Variscan orogeny. The Śnieznik granulites are more problematic; they may belong to a ‘younger high-P suite’ (c. 350 Ma old), widespread in the southern and eastern parts of the Bohemian Massif, and possibly related to the climax of the Variscan continent–continent collision.

Rare earth element mobility during prograde granulite facies metamorphism : significance of fluorine

Abstract: Mafic gneisses occur as lenses or thin layers in spatial association with tonalitic leucosomes in a granulite zone of the Quetico subprovince of the Superior Province, Ontario, Canada, and exhibit concentric zoning with a biotite-rich margin, orthopyroxene-rich outer zone, clinopyroxene-rich central zone, and, occasionally, patches of relict amphibolites within the clinopyroxene-rich zone. The granulites (biotite-, orthopyroxene- and clinopyroxene-rich zones) in the mafic gneisses are characterized by significant amounts of rare earth element (REE)-bearing fluorapatite (1–10 vol.%) and other REE-rich minerals (allanite, monazite and zircon). Fluorapatite shows an increase in modal abundance from the biotite- and orthopyroxene-rich zones to the clinopyroxene-rich zone, but is rare in the relict amphibolites. Textural evidence and element partitioning indicate that fluorapatite (and other REE-rich minerals) was part of the peak metamorphic assemblages. Whole-rock geochemical analyses confirm that the granulites in the mafic gneisses contain anomalously high contents of REE and high field strength elements (HFSE), whereas the relict amphibolites are geochemically typical of tholeiitic basalts. Mass-balance calculations reveal that REE and HFSE were introduced into the mafic gneisses during the prograde granulite facies metamorphism, pointing to REE mobility under granulite facies metamorphic conditions. The presence of high F contents in the REE-rich minerals and their associated minerals (e.g. biotite and hornblende) suggests that REE and HFSE may have been transported as fluoride complexes during the granulite facies metamorphism. This conclusion is supported by previously published results of hydrothermal experiments on the partitioning of REE between fluorapatite and F-rich fluids at 700°C and 2 kbar.

Zircon geochronology of anatectic melts and residues from a highgrade pelitic assemblage at Ihosy, southern Madagascar: evidence for Pan-African granulite metamorphism

Abstract: We report U—Pb and 207Pb/206Pb zircon ages for a granulite facies gneiss assemblage exposed in a large quarry at Ihosy, southern Madagascar. The granulites are derived from pelitic to arkosic sediments and attained equilibrium conditions at 650–700°C and 4–5 kbar. Higher P—T conditions of 750–800°C and 6 kbar in the presence of low water activities have led to dehydration melting processes. The formation of granitic melts, which (partly) moved away from their source region, intruded into upper parts of the metapelitic gneisses as small granitic veins and left behind granulitic garnet-cordierite-quartz bearing rocks. Detrital zircons in a sample of metapelite and a sample of quartzofeldspathic gneiss yielded ages between ˜720 and ˜1855 Ma, suggesting a chronologically heterogeneous source region and a depositional age of less than ˜720 Ma for these rocks. High-grade metamorphism and anatexis are documented by zircon ages between 526 ±34 and 557 ±2 Ma with a mean age of about 550 Ma. The broad lithologies, metamorphic grades and ages recorded in the Ihosy rocks are similar to those in the Wanni Complex of northwestern Sri Lanka and in high-grade assemblages of southernmost India and support the contention that all these terrains were part of the Mozambique belt which formed as a result of collision of East and West Gondwana in latest Precambrian time.

First evidence of ultrahigh-temperature decompression from the granulite province of southern India

Abstract: Sapphirine-bearing, orthopyroxene-sillimanite ± garnet granulites of the Palni Hill Ranges record evidence of ultrahigh-temperature metamorphism during which primary mineral assemblages were partially overprinted by a succession of reaction coronas and symplectites that together suggest decompression. Model system petrogenetic grids allow constraints to be placed on the evolution of P–T conditions as recorded by reaction relationships of successively developed mineral assemblages; thermobarometry on garnet-bearing assemblages allows minimum constraints to be placed on P–T conditions for the primary mineral assemblages. Primary mineral assemblages suggest P–T conditions of c. 12 kbar and c. 900-1000 "C. Secondary mineral assemblages in reaction textures show a two-stage decompression to probable P–T conditions of c. 5 kbar and c. 850 °C. The inferred P–T path is clockwise.

Cr-Spinels from Brazilian Mafic-Ultramafic Complexes: Metamorphic Modifications

Abstract: Chemical compositional data on the Cr-spinels from selected Brazilian mafic-ultramafic complexes are presented and compared. The massive stratiform chromitites from the Pium-Hi and Serro complexes and massive podiform chromitites from the Abadiânia and Morro Feio bodies were strongly deformed and metamorphosed from greenschist to epidote-amphibolite facies. The massive stratiform chromitites from Campo Formoso, Medrado-Ipueiras, and Luanga were metamorphosed to the upper amphibolite facies and variably deformed. The large Barro Alto, Niquelândia, and Canabrava (and the small Americano do Brasil, Mangabal-I and -II, and Caraiba) stratiform complexes were polydeformed and metamorphosed to granulite and amphibolite facies. Regardless of their metamorphic and deformational changes, the chromitites still retained some primary features, such as: (1) texture, (2) structures, and (3) original chemical composition. It was observed that the chemical changes imparted to the Cr-spinels during metamorphicdeformational...

An example of ultrahigh-temperature metamorphism: orthopyroxene-sillimanite-garnet, sapphirine-quartz and spinel-quartz parageneses in Al-Mg granulites from In Hihaou, In Ouzzal, Hoggar

Abstract: Quartz Al–Mg granulites exposed at In Hihaou, In Ouzzal (NW Hoggar), preserve an unusual high-grade mineral association stable at temperatures up to 1050°C, involving the parageneses orthopyroxene–sillimanite–garnet–quartz, sapphirine–quartz and spinel–quartz. The phase relationships within the FMAS system show that a continuum exists between the earlier prograde reaction textures and those of the later decompressive event. The following mineral reactions involving sillimanite are deduced: (1) Grt+QtzOpx+Sil, (2) Opx+SilGrt+Spr+Qtz, (3) Grt+Sil+QtzCrd, (4) Grt+SilCrd+Spr, (5) Grt+Sil+SprCrd+Spl, (6) Grt+SilCrd+Spl, (7) Grt+Crd+SilSpl+Qtz and (8) Grt+SilSpl+Qtz. Minerals in quartz Al–Mg granulites display compositional variations consistent with the observed reactions. The Mg/(Mg+Fe2+) range of the main minerals is as follows: cordierite (0.81–0.97), sapphirine (0.77–0.88), orthopyroxene (0.65–0.81), garnet (0.33–0.64) and spinel (0.23–0.56). The reaction textures and the evolution of the mineral assemblages in the quartz Al–Mg granulites indicate a clockwise P–T trajectory characterized by peak conditions of at least 10 kbar and 1050°C, followed by decompression from 10 to 6 kbar at a temperature of at least 900°C.

Problems with inferring P–T–t paths in low‐P granulite facies rocks

Abstract: Microstructural evidence commonly is used to infer metamorphic reactions, which are used to infer pressure–temperature–time (P–T–t) paths However, this approach in low-P/high-T (LPHT) granulite facies metamorphic terranes has two main problems (1) Microstructural evidence may be inconclusive, so that reactions cannot be inferred with confidence In particular, relative timing of mineral growth inferred from inclusions, moulding relationships and foliation–porphyroblast relationships is commonly ambiguous or invalid The most reliable indicators of metamorphic reactions are partial pseudomorphs and corona structures, especially if symplectic intergrowths (indicating simultaneous growth of two or more minerals) are involved (2) Even reactions that can be inferred with confidence do not indicate unique P–T trends, owing to P–T slopes of reaction curves Where successive reactions can be shown to have occurred in the same rock, a line or curve joining reaction-curve intersections gives an apparent single-event path However, isotopic evidence is needed to prove that polymetamorphism (involving more complex paths making fortuitous intersections with the apparent single-event path) did not occur Although these problems are well known, their importance is not always emphasized in metamorphic investigations The difficulties are illustrated by published work on P–T–t paths for Proterozoic LPHT granulite facies rocks of central Australia and Antarctica Recent work in Antarctica has shown that P–T–t paths may be episodic and more complex than the simple, single-event paths commonly inferred from microstructural evidence alone

High‐pressure metamorphism caused by magma loading in Fiordland, New Zealand

Abstract: Cretaceous granulite facies metamorphism in the Fiordland area of New Zealand has distinctive mineralogical, textural and structural features that set it apart from most other regional metamorphic belts. The metamorphism, developed over a 30×150-km area and the consequence of a 20-km-thick increment to crustal thickness, is closely associated in space and time with a large plutonic complex, the Western Fiordland Orthogneiss (WFO). Although temperatures and pressures as high as 700 °C and 12 kbar were attained, the metamorphic overprint on earlier low-pressure assemblages is weak and incomplete. Little strain accompanied the metamorphism. The temperature threshold at which metamorphic recrystallization is recorded is over 500 °C. Zoned garnets are preserved at unusually high temperatures, indicating duration of metamorphism on the order of 10 times shorter than in most other regional terranes. This pattern of features bears close similarity to metamorphism in the Coast Plutonic Complex in North America, where a mechanism of ‘magma loading’ has been invoked. In Fiordland, the high-pressure metamorphism can be explained by depression of country rock under a crustal zone that is inflated by intrusion of the WFO. Regional structure of the WFO as a horizontally sheeted complex suggests that the pluton was emplaced by vertical displacement of country rock, and supports the magma loading model.

Melting of albite and dehydration of brucite in H 2 O–NaCl fluids to 9 kbars and 700–900°C: implications for partial melting and water activities during high pressure metamorphism

Abstract: The melting reaction: albite(solid)+ H2O(fluid) =albite-H2O(melt) has been determined in the presence of H2O–NaCl fluids at 5 and 9.2 kbar, and results compared with those obtained in presence of H2O–CO2 fluids. To a good approximation, albite melts congruently at 9 kbar, indicating that the melting temperature at constant pressure is principally determined by water activity. At 5 kbar, the temperature (T)- mole fraction (X (H2O) ) melting relations in the two systems are almost coincident. By contrast, H2O–NaCl mixing at 9 kbar is quite non-ideal; albite melts ∼70 °C higher in H2O–NaCl brines than in H2O–CO2 fluids for X (H2O) =0.8 and ∼100 °C higher for X (H2O) =0.5. The melting temperature of albite in H2O–NaCl fluids of X (H2O)=0.8 is ∼100 °C higher than in pure water. The P–T curves for albite melting at constant H2O–NaCl show a temperature minimum at about 5 kbar. Water activities in H2O–NaCl fluids calculated from these results, from new experimental data on the dehydration of brucite in presence of H2O–NaCl fluid at 9 kbar, and from previously published experimental data, indicate a large decrease with increasing fluid pressure at pressures up to 10 kbar. Aqueous brines with dissolved chloride salt contents comparable to those of real crustal fluids provide a mechanism for reducing water activities, buffering and limiting crustal melting, and generating anhydrous mineral assemblages during deep crustal metamorphism in the granulite facies and in subduction-related metamorphism. Low water activity in high pressure-temperature metamorphic mineral assemblages is not necessarily a criterion of fluid absence or melting, but may be due to the presence of low a (H2O) brines.

U-Pb geochronologic constraints on the Paleoproterozoic crustal evolution of the upper Granite Gorge, Grand Canyon, Arizona

Abstract: New field observations and U-Pb zircon and monazite ages are used to outline a geologic history of Paleoproterozoic rocks beneath the Colorado Plateau in the Grand Canyon of northern Arizona. The Upper Granite Gorge of the Grand Canyon exposes three lithotectonic units, including supracrustal rocks, mafic to intermediate-composition plutons, and peraluminous granite dikes, that are variably deformed and metamorphosed. New U-Pb ages indicate that layered supracrustal rocks were deposited or erupted over at least 8 m.y., between 1750 ± 2 and 1741 ± 1 Ma. Mafic to intermediate-composition plutons intruded the supracrustal rocks at 1741 ± 1 Ma (Zoroaster granite), 1737 ± 1 Ma (Grapevine Camp granite), 1730 ± 3 Ma (Trinity granodiorite), 1717 ± 1 Ma (Ruby granodiorite), and 1713 ± 2 Ma (Horn diorite). Both supracrustal and plutonic rocks are intruded by 1698–1662 Ma peraluminous granite and pegmatite dikes that locally form 50% of the rock volume. U-Pb ages of foliated plutonic rocks and crosscutting granite dikes bracket the timing of fabric development for three groups of fabrics. Group 1 fabrics formed between 1730 ± 3 and 1698 ± 1 Ma. Group 2 fabrics formed between 1713 ± 2 and 1685 ± 1 Ma. The timing of group 3 fabric formation is poorly constrained, although the 1662 Ma Phantom pluton contains shear bands that offset pegmatitic dikes that crosscut the pluton. Metamorphic grade varies across the transect from lower amphibolite facies to lower granulite facies. We suggest that single-crystal U-Pb ages (1706–1697) of metamorphic monazite, from a 1741 Ma supracrustal rock, directly date the timing of lower granulite-facies metamorphism in the eastern Upper Gorge. U-Pb ages (1685–1680 Ma) of granite dikes that postdate the development of leucosomal pegmatite pods in migmatitic supracrustal rocks in the eastern Upper Gorge are consistent with this suggestion. The Paleoproterozoic geologic history of the Upper Granite Gorge is similar to that of the Yavapai province in central Arizona. However, unlike the juvenile crust that characterizes the Yavapai province, rocks in the western Upper Gorge preserve evidence for pre-1.8 Ga crust. First, the 1840 ± 1 Ma Elves Chasm pluton, the oldest known rock from the southwestern United States, has a Pb isotopic signature similar to that of rocks in the Mojave province. Second, the Tuna Creek granodiorite, a member of the 1710–1750 Ma arc-related plutonic suite, contains inherited zircons with minimum ages of 1998 and 2178 Ma and exhibits whole-rock Pb isotopic signatures similar to rocks of the Mojave province. These data indicate that pre-1.8 Ga crustal components participated, at least locally, in the development of the 1740–1710 Ma arc-related magmatic suite and suggest that a boundary between reworked crust and juvenile crust may be present in the western Upper Gorge.

SHRIMP II dating of zircons and monazites: reassessing the timing of high‐grade metamorphism and fluid flow in the Reynolds Range, northern Arunta Block, Australia

Abstract: Key insights into the timing of tectonometamorphic events in a complex high-grade metamorphic terrane can be obtained by combining results from SHRIMP II ion microprobe studies of individual monazite grains with SHRIMP II studies and scanning electron microscope (SEM)-based cathodoluminescence (CL) imaging of zircons. Results from the Reynolds Range region, Arunta Block, Northern Territory, Australia, show that the final episode of regional metamorphism to high-T and low-P granulite facies conditions is most likely to have occurred at c. 1580 Ma, not at 1785–1775 Ma, as previously accepted. The previous interpretation was based on zircon studies of structurally controlled granitoids, without SEM-based CL imaging. Monazites in a 1806± 6 Ma megacrystic granitoid preserve rare cores that are interpreted to be inherited magmatic monazite, but record no evidence of another high-T event prior to 1580 Ma. Most monazites from the region record only a single high-T metamorphic event at c. 1580 Ma. Zircon inheritance is very common. Zircons or narrow overgrowths of zircon dated at c. 1580 Ma have only been found in two types of rocks: rocks produced by metasomatic fluid flow at high temperatures (≤750°C), and rocks that have undergone local partial melting. Previous explanations that attributed these 1580 Ma zircon ages to widespread hydrothermal fluid fluxing associated with post-tectonic pegmatite emplacement at amphibolite facies conditions are not supported by the available evidence including oxygen isotope data. The observed high regional metamorphic temperatures require the involvement of advective heating. However, contrary to a previous tectonic model for the formation of this and other low-P, high-T metamorphic belts, the granites that are exposed at the present structural level do not appear to be the source of that heat, unless some of the granites were emplaced at c. 1580 Ma.

Litholog and structure of the Grenville-aged (≈1.1 Ga) basement of heimefrontfjella (East Antarctica)

Abstract: The Heimefrontfjella mountains, Western Dronning Maud Land (East Antarctica), are dominantly composed of Grenville-aged (≈ 1.1 Ga) rocks, which were reworked during the Pan -African orogeny at ≈500 Ma. Three discontinuity-bounded Grenville-aged terranes have been recognized namely (from north to south) the Kottas, Sivorg and Vardeklettane terranes. The terranes contain their own characteristic lithological assemblages, although each is made up of an early supracrustal sequence of metavolcanic and/or metasedimentary gneisses, intruded by various (predominantly granitoid) suites. No older basement upon which the protoliths of these older gneisses were deposited has been recognized. In each terrane the older layered gneisses were intruded by various plutonic suites ranging in age from ≈ 1150 to ≈1000 Ma. The Vardeklettane terrane is characterized by abundant charnockites and two-pyroxene granulite facies parageneses in metabasites, whereas the Sivorg and Kottas terranes were metamorphosed to amphibolite facies grade. P-T estimates show that peak metamorphic conditions changed from ≈600°C at 8 kbar in the south, to ≈700 °C at 4 kbar in the northern Sivorg terrane. Regional greenschist retrogression of high-grade assemblages may be of Pan-African age. The Heimefrontfjella terranes were juxtaposed and pervasively deformed during a complex and protracted period of E-W collision orogenesis in a transpressive regime at ≈ 1.1 Ga. This is manifest as early, gently dipping thrust-related shear fabrics (D1), succeeded by the initiation of an important (D2) steep dextral shear zone (Heimefront shear zone, HSZ), during which the early fabrics and structures were steepened and rotated in an anticlockwise sense. The HSZ is a curvilinear structure which changes from a dextral oblique strike-slip lateral ramp in the north to a steep dip-slip frontal ramp in the south, where it forms the boundary between the Sivorg and Vardeklettane terranes. The Pan-African event is manifested as discrete, low- to medium-temperature ductile to brittle shears (D3) and numerous K/Ar cooling ages.

Thick-skinned versus thin-skinned thrusting: Rheology controlled thrust propagation in the Variscan collisional belt (The southeastern Bohemian Massif, Czech Republic - Austria)

Abstract: The Variscan nappe assembly within the southeastern Bohemian Massif includes (1) crystalline nappes which were transported from initial granulite to amphibolite facies conditions to uppermost crustal levels, (2) nappe assembly of footwall units under very low to low-grade metamorphic conditions, and (3) frontal, thin-skinned imbrication of flysch sequences which were deposited within a deep and narrow foreland basin during loading of the lower plate. Thick-skinned thrusting of hinterland tectonic units resulted in an inverted metamorphic zonation and contrasts with the thin-skinned tectonic character of foreland units. 40Ar/39Ar hornblende and muscovite plateau ages from hanging wall units reflect complete rejuvenation of intracrystalline argon isotopic systems during Late Variscan thrusting (between ∼ 340 Ma and 325 Ma). The 40Ar/39Ar mineral ages together with previously published 40Ar/39Ar and Sm/Nd data record systematically decreasing Variscan mineral ages structurally downward into footwall units. This is explained by rapid exhumation and cooling synchronously with thrusting along crustal-scale ramps during foreland-directed nappe propagation. The location of decollement zones within specific structural levels was apparently controlled by crustal-scale rheological con-trasts. Late Variscan cooling ages contrast with Cadomian 40Ar/39Ar hornblende and muscovite ages recorded within the structural basement immediately below the major Variscan thrust plane where intracrystalline isotopic systems were only partially rejuvenated during Variscan thrusting. These record a sequence of Cadomian magmatic and metamorphic events in foreland units including (1) pre intrusive metamorphism and deformation under amphibolite facies metamorphic conditions (>610 Ma), (2) greisen formation during a Cadomian collisional event (∼ 600 Ma), and (3) intrusion of calcalkaline granitoid bodies with I-type affinities (∼ 590 Ma).