An extended episode of early Mesoproterozoic metamorphic fluid flow in the Reynolds Range,central Australia*

ABSTRACT The products of metamorphic fluid flow are preserved in zones within the marbles and metamorphosed semipelites of the Upper Calcsilicate Unit in the granulite portion of the Late Palaeoproterozoic Reynolds Range Group, northern Arunta Block, central Australia. The zones of retrogression, characterized by minerals such as wollastonite, grossular and clinohumite, local resetting of oxygen isotopic compositions and local major element metasomatism, were channelways for water-rich fluids derived from granulite facies metapelites. U–Th–Pb isotopic ages measured by the SHRIMP ion microprobe on zircon and monazite from a granulite facies semipelite, an early semiconcordant aluminous quartz-rich fluid-flow segregation and a late discordant quartz-rich segregation record some of the extended thermal history of the area. Zircon cores from the semipelite show its likely protolith to be an igneous rock 1812 ± 11 Ma old, itself derived from a source containing zircon as old as 2.2 Ga. Low-Th/U overgrowths on the zircon grew during granulite facies metamorphism at 1594 ± 6 Ma. Monazite cooled to its blocking temperature at 1576 ± 8 Ma. Zircon cores from the semiconcordant segregation are dominantly >2.3 Ga old, indicating that the source of the fluids was not the particular metamorphosed semipelite studied. Two generations of low-Th/U overgrowths on the zircon give indistinguishable ages for the older and younger of 1589 ± 8 and 1582 ± 8 Ma, respectively. The monazite age is the same, 1576 ± 12 Ma. Zircon from the late discordant segregation gave 1568 ± 4 Ma. Fluid flow occurred for at least 18 ± 3 (σ) Ma and ended 26 ± 3 (σ) Ma after the peak of metamorphism, suggesting a very slow cooling rate of ～3°C Ma^–1. The last regional high-grade metamorphism in the Reynolds Range occurred at ～1.6 Ga, not ～1.78 Ga as previously thought. The high-grade event at ～1.78 Ga is a separate event that affected only the basement to the Reynolds Range Group.     

Re–Os dating and sulfur isotope composition of molybdenite from tungsten deposits in western Namaqualand, South Africa: implications for ore genesis and the timing of metamorphism

Both stratiform/stratabound and granite-related models have been used to explain the genesis of W(Mo) deposits in the Okiep copper district in western Namaqualand, South Africa. Apparently, stratabound mineralization (Fe-rich wolframite with accessory molybdenite) occurs in foliation-parallel quartz veins in high-grade (∼750 °C, 5–6 kbar) metapelites of the Wolfram Formation, and less commonly in small bodies of silicified leucogranites and pegmatites. Six Re–Os ages for molybdenites from four deposits (Nababeep Tungsten Far West, Kliphoog, Narrap, Tweedam) range between 1000 ± 4 and 1026 ± 5 Ma. These molybdenites define a well-constrained ¹⁸⁷Re–¹⁸⁷Os isochron with an age of 1019 ± 6 Ma, which is interpreted as the age of W(Mo) mineralization. This age is significantly younger than Proterozoic protolith ages for supracrustal rocks and the emplacement ages for the main intrusive suites, but geologic evidence requires overlap with a period of high-grade metamorphism. We suggest that W(Mo) mineralization is genetically linked to intra-crustal magmatic processes at ∼1020 Ma, thereby precluding the ∼1060 Ma Concordia granite as the source for mineralizing fluids. A narrow range of positive δ³⁴S compositions (+3.6 to +4.5‰) for eight molybdenites from five W(Mo) mines is consistent with a SO₂-rich fluid and a granite-related genetic model. Post-peak metamorphic deformation and metamorphism of W(Mo) ores is most likely related to the retrograde stage of the Namaquan orogeny, which overlaps emplacement of late-orogenic, evolved granites and pegmatites, and the formation of W(Mo) deposits in western Namaqualand. Therefore, the effects of retrograde Namaquan metamorphism extend at least to ∼1020 Ma or, alternatively, these W(Mo) veins were affected by a poorly constrained later event (e.g. early Pan-African). Received: 12 September 1999 / Accepted: 20 April 2000     

Franciscan eclogite revisited: Reevaluation of the P–T evolution of tectonic blocks from Tiburon Peninsula, California, U.S.A.

Summary High-grade blocks in the Franciscan complex at Tiburon, California, record relatively low temperature eclogite-facies metamorphism and blueschist-facies overprinting. The eclogite-facies mineral assemblage contains prograde-zoned garnet + omphacite + epidote ± hornblende (katophoritic and barroisitic Ca–Na amphibole) ± glaucophane + phengite (∼3.5 Si p.f.u.) ± paragonite + rutile + quartz. The blueschist-facies mineral assemblage contains chlorite + titanite + glaucophane + epidote ± albite ± phengite (∼3.3 Si p.f.u.). Albite is not stable in the eclogite stage. New calculations based on garnet-omphacite-phengite thermobarometry and THERMOCALC average-P–T calculations yield peak eclogite-facies P–T conditions of P = 2.2–2.5 GPa and T = 550–620 °C; porphyroclastic omphacite with inclusions of garnet and paragonite yields an average-P–T of 1.8 ± 0.2 GPa at 490 ± 70 °C for the pre-peak stage. The inferred counterclockwise hairpin P–T trajectory suggests prograde eclogitization of a relatively “cold” subducting slab, and subsequent exhumation and blueschist-facies recrystallization by a decreasing geotherm. Although an epidote-garnet amphibolitic assemblage is ubiquitous in some blocks, P–T pseudosection analyses imply that the epidote-garnet amphibolitic assemblage is stable during prograde eclogite-facies metamorphism. Available geochronologic data combined with our new insight for the maximum pressure suggest an average exhumation rate of ∼5 km/Ma, as rapid as those of some ultrahigh pressure metamorphic terranes.     

Fluid flow in marbles at Jervois, central Australia: oxygen isotope disequilibrium and zoning produced by decoupling of mineralogical and isotopic resetting

The Jervois region of the Arunta Inlier, central Australia, contains para- and orthogneisses that underwent low-pressure amphibolite facies metamorphism (P = 200–300 MPa, T = 520–600 °C). Marble layers cut by metre-wide quartz + garnet ± epidote veins comprise calcite, quartz, epidote, clinopyroxene, grandite garnet, and locally wollastonite. The marbles also contain locally discordant decimetre-thick garnet and epidote skarn layers. The mineral assemblages imply that the rocks were infiltrated by water-rich fluids (XCO₂ = 0.1–0.3) at ∼600 °C. The fluids were probably derived from the quartz-garnet vein systems that represent conduits for fluids exsolved from crystallizing pegmatites emplaced close to the metamorphic peak. At one locality, the marble has calcite (Cc) δ¹⁸O values of 9–18‰ and garnet (Gnt) δ¹⁸O values of 10–14‰. The δ¹⁸O(Gnt) values are only poorly correlated with δ¹⁸O(Cc), and the δ¹⁸O values of some garnet cores are higher than the rims. The isotopic disequilibrium indicates that garnet grew before the δ¹⁸O values of the rock were reset. The marbles contain  ≤15% garnet and, for water-rich fluids, garnet-forming reactions are predicted to propagate faster than O-isotopes are reset. The Sm-Nd and Pb-Pb ages of garnets imply that fluid flow occurred at 1750–1720 Ma. There are no significant age differences between garnet cores and rims, suggesting that fluid flow was relatively rapid. Texturally late epidote has δ¹⁸O values of 1.5–6.2‰ implying δ¹⁸O(H₂O) values of 2–7‰. Waters with such low-δ¹⁸O values are probably at least partly meteoric in origin, and the epidote may be recording the late influx of meteoric water into a cooling hydrothermal system. Received: 29 April 1996 / Accepted: 12 March 1997     

Sapphirine-bearing orthopyroxene-kyanite/sillimanite granulites from South Harris, NW Scotland: evidence for Proterozoic UHT metamorphism in the Lewisian

Sapphirine-bearing orthopyroxene-kyanite (Opx-Ky) and -sillimanite (Opx-Sil) granulites have been found in the Lewisian complex of South Harris in northwest Scotland. In the Opx-Ky granulites, orthopyroxene and kyanite are intergrown in a stable mineral assemblage, which indicates metamorphic condition at 800–900 °C >12 kbar. Sillimanite inclusions within orthopyroxene suggest that sillimanite formed earlier; conditions are estimated at 950 ± 30 °C at 10 kbar from orthopyroxene isopleths for aluminous orthopyroxene (<9.7 wt%). In the Opx-Sil granulite, the orthopyroxene + sillimanite + garnet + sapphirine assemblage is stable at the peak metamorphic stages, indicating P-T condition of 930–950 °C, >8 kbar according to the FMAS petrogenetic grid, and similar conditions were obtained by using orthopyroxene-garnet geothermobarometers. The two types of orthopyroxene-aluminosilicate granulites indicate that the peak metamorphic conditions were over 900 °C, compatible with ultra-high temperature metamorphism. As accessory sapphirine occurs in several assemblages and with different compositions; it is interpreted to be formed at different stages of the metamorphism. These granulites were formed during Early Proterozoic high-grade metamorphism due to the emplacement of the South Harris Igneous Complex at c. 2170–1870 Ma, and are not related to the major metamorphic episode of the Badcallian/Inverian metamorphism at c. 2700–2500 Ma in the mainland Lewisian. Received: 17 July 1998 / Accepted 8 March 1999     

Evidence of Mesozoic multiple hydrothermal activity in the basement at Nonnenmattweiher (southern Schwarzwald), Germany

Two botryoidal hematites from the qtz-fl-hem veins at the Nonnenmattweiher cirque (zone of Badenweiler-Lenzkirch, Südschwarzwald) were dated using the (U + Th)-He method. One hematite gives an Early Triassic age (234 ± 5 Ma) and the second straddles the Cretaceous/Jurassic time boundary (126 ± 4 Ma). The ages bracket the ∼entire age spectrum reported to date for Mesozoic vein-type mineralizations in the Schwarzwald. The maximum sedimentary overburden and associated palaeo-temperatures (>130 °C) in the basement of the southern Schwarzwald during the Jurassic did not cause severe age-resetting of the hematites. This implies that both ages probably are formation ages rather than ages of cooling, provided that the ⁴He retentivities of both hematites are similar. The fact that two hematites from the veins at Nonnenmattweiher yield highly discordant Mesozoic ages suggests multiple hydrothermal activity during the Mesozoic. A previously published age of two adularia (∼155 Ma), intermediate between the hematite ages, supports this view. Received: 1 February 1996 / Accepted: 6 September 1996     

Evidence for diamond-grade ultra-high pressure metamorphism and fluid interaction in the Svartberget Fe–Ti garnet peridotite–websterite body, Western Gneiss Region, Norway

Summary Based on mineral-chemical evidence we propose that the northernmost Scandian ultra-high pressure (UHP) metamorphic domain within the Western Gneiss Region of Norway can be extended 25 km northeastwards. A newly discovered, well preserved, fine-grained, Fe–Ti type garnet peridotite body at Svartberget, located in the Ulla Gneiss of the ‘M?re og Romsdal’ area north of Molde, is cut by a network of systematically orientated coarse-grained garnet-websterite and garnetite veins. Standard thermobarometric techniques based on electron microprobe analyses yield pressure (P) and temperature (T) estimates around 3.4 GPa, and 800 °C for the peridotite body and 5.5 GPa, and 800 °C for the websterite veins consistent with UHP conditions. In addition, polyphase solid inclusions, consisting of silicates, carbonates, sulphates and elemental carbon (including microdiamond), are randomly located in garnet and clinopyroxene of the websterite vein assemblage. Garnet-clinopyroxene mineral pairs yield a Sm–Nd cooling age of 393 ± 3 Ma for the peridotite and 381 ± 6 Ma for the vein assemblage suggesting that the Svartberget body was overprinted during the UHPM of the Scandian Orogeny. The initial ratio of the mineral isochron and Nd model ages suggest a mid-Proterozoic origin for the peridotite body. The polyphase inclusions, coupled with high ⁸⁷Sr/⁸⁶Sr ratios may indicate that the peridotite body was infiltrated by crustal-derived C–O–H melts/fluids at UHPM conditions to form the websterite veins in the diamond field. We propose that fracturing and vein emplacement were the result of local high fluid pressure during subduction of the Baltic plate. Present address: Physics of Geological Processes, University of Oslo, Oslo, Norway     

Zircon and monazite response to prograde metamorphism in the Reynolds Range, central Australia

We report an extensive field-based study of zircon and monazite in the metamorphic sequence of the Reynolds Range (central Australia), where greenschist- to granulite-facies metamorphism is recorded over a continuous crustal section. Detailed cathodoluminescence and back-scattered electron imaging, supported by SHRIMP U–Pb dating, has revealed the different behaviours of zircon and monazite during metamorphism. Monazite first recorded regional metamorphic ages (1576 ± 5 Ma), at amphibolite-facies grade, at ∼600 °C. Abundant monazite yielding similar ages (1557 ± 2 to 1585 ± 3 Ma) is found at granulite-facies conditions in both partial melt segregations and restites. New zircon growth occurred between 1562 ± 4 and 1587 ± 4 Ma, but, in contrast to monazite, is only recorded in granulite-facies rocks where melt was present (≥700 °C). New zircon appears to form at the expense of pre-existing detrital and inherited cores, which are partly resorbed. The amount of metamorphic growth in both accessory minerals increases with temperature and metamorphic grade. However, new zircon growth is influenced by rock composition and driven by partial melting, factors that appear to have little effect on the formation of metamorphic monazite. The growth of these accessory phases in response to metamorphism extends over the 30 Ma period of melt crystallisation (1557–1587 Ma) in a stable high geothermal regime. Rare earth element patterns of zircon overgrowths in leucosome and restite indicate that, during the protracted metamorphism, melt-restite equilibrium was reached. Even in the extreme conditions of long-lasting high temperature (750–800 °C) metamorphism, Pb inheritance is widely preserved in the detrital zircon cores. A trace of inheritance is found in monazite, indicating that the closure temperature of the U–Pb system in relatively large monazite crystals can exceed 750–800 °C. Received: 7 April 2000 / Accepted: 12 August 2000     

Proterozoic zircon growth in Archean lower crustal xenoliths, southern Superior craton – a consequence of Matachewan ocean opening

Granulite-grade, anorthositic and mafic xenoliths recovered from a Jurassic kimberlite pipe near Kirkland Lake, Ontario are fragments of the lower crust that underlies the ca. 2.7 Ga Abitibi greenstone belt of the Superior craton. Cathodoluminescence imaging and/or backscatter electron microscopy of zircon from four individual xenoliths reveals a complex crystallization history, characterized by two main stages of zircon growth. The age of the two stages has been constrained by combining imaging results with isotope dilution U-Pb dating of grain fragments and single grains. Minimum ages for the first crystallization stage in individual xeno liths are 2584 ± 7 Ma, 2629 ± 8 Ma, 2633 ± 3 Ma, whereas an approximate crystallization age for a fourth sample is 2788 ± 57 Ma. The second main stage of growth consists of chemically and isotopically distinct metamorphic zircon overgrowths. Times of solid-state zircon growth are most broadly constrained in three samples to the interval between 2.52 Ga to 2.40 Ga, and most precisely dated in a meta-anorthosite at 2416 ± 30 Ma. These complex zircons are intergrown with garnet and clinopyroxene of the host granulite-facies assemblage, and thus the Paleoproterozoic ages of the metamorphic overgrowths are interpreted to reflect an interval of isobaric, granulite-grade metamorphism of the lower crust beneath the greenstone belt approximately 150 million years after craton formation. This interval of metamorphism is broadly coeval with the intrusion of the Matachewan dyke swarm across the southern Superior craton, and with mafic magmatism and deposition of Huronian rift-margin sediments 200 km to the south during the opening of the Matachewan ocean. It is proposed that a significant volume of magma intruded the crust-mantle interface during rifting, promoting isobaric metamorphism and zircon growth in the deep levels of the Superior craton. Subsequent major rifting events along this margin apparently failed to produce a similar lower crustal response. The results have important implications for the structure of lithosphere beneath Archean continental crust. Received: 3 October 1995 / Accepted: 11 February 1997     

Sm–Nd evidence for high-grade Ordovician metamorphism in the Arunta Block, central Australia

Sm–Nd ages from the Harts Range in the south-eastern Arunta Inlier in central Australia indicate that regional metamorphism up to granulite facies occurred in the Early Ordovician (c. 475 Ma). This represents a radical departure from previous tectonic models for the region and identifies a previously unrecognized intraplate event in central Australia. Peak metamorphic assemblages (800 °C and 10.5 kbar) formed at around 476±14 Ma and underwent approximately 4 kbar of near-isothermal decompression at 475±4 Ma. A coarse-grained unfoliated garnet–clinopyroxene-bearing marble inferred to have recrystallized late in the decompressional evolution, gives an age of 469±7 Ma. Two lines of evidence suggest the Early Ordovician tectonism occurred in an extensional setting. First, the timing of the high-grade lower crustal deformation coincides with a period of marine sedimentation in the Amadeus and Georgina basins that was associated with a seaway that developed across central Australia. Second, isothermal decompression of lower crustal rocks was associated with the formation of a regional, sub-horizontal mid-crustal foliation. In the Entia Gneiss Complex, which forms the structurally lowest part of the Harts Range, upper-amphibolite facies metamorphism (c. 700 °C, 8–9 kbar) occurred at 479±15 Ma. There is no evidence that P–T conditions in the Entia Gneiss Complex were as high as in the overlying units. This implies that the extensional system was reworked during a later compressional event. Sm–Nd data from the mid-amphibolite facies (c. 650 °C and 6 kbar) detachment zone that separates the Irindina Supracrustal Assemblage and Entia Gneiss Complex give an age of 449±10 Ma. This age corresponds to the timing of a change in the pattern and style of sedimentation in the Amadeus and Georgina basins, and indicates that the change in basin dynamics was associated with mid-crustal deformation. It also suggests that compressional deformation culminating in the Devonian to Carboniferous (400–300 Ma) Alice Springs Orogeny may have begun as early as c. 450 Ma. At present, the extent of Early Ordovician tectonism in central Australia is unknown. However, granulite facies metamorphism and associated intense deformation imply an event of regional extent. An implication of this work is that high-grade lower crustal metamorphism and intense deformation occurred during the development of a broad, shallow, slowly subsiding intraplate basin.     

Relics of eclogite facies metamorphism in the Austroalpine basement,Hochgrössen (Speik complex), Austria

Summary Retrograde eclogites and serpentinites from the Hochgr?ssen massif, Styria, are parts of the Speik complex in the Austroalpine basement nappes of the Eastern Alps. They are in tectonic contact with pre-Alpine gneisses, amphibolites, and Permo-Triassic quartz phyllites (Rannach Series). The eclogites are derived from ocean-floor basalts with affinities to mid-ocean ridge and back-arc basin basalts. Fresh eclogites are rare and contain omphacite with a maximum of 39 mol% jadeite content, garnet (Py_15–19) and amphibole. Retrograde eclogites consist of amphibole and symplectites of Na-poor clinopyroxene (5–8 mol% Jd) + albite ± amphibole. Amphiboles are classified as edenite, pargasite, tschermakite, magnesiohornblende and actinolite. In relatively fresh eclogite, edenite is a common amphibole and texturally coexists with omphacite and garnet. An average temperature of 700 °C was obtained for eclogite facies metamorphism using garnet-pyroxene thermometry. A minimum pressure of 1.5 GPa is indicated by the maximum jadeite content in omphacite. Thermobarometric calculations using the TWEEQ program for amphibole in textural equilibrium with omphacite and garnet give pressures of 1.8–2.2 GPa at 700 °C. The equilibrium assemblage of Na-poor clinopyroxene, albite, amphibole and zoisite in the symplectites gives a pressure of about 0.6–0.8 GPa at 590–640 °C. ⁴⁰Ar/³⁹Ar radiometric dating of edenitic amphibole in textural equilibrium with omphacite gave a plateau age of 397.3 ± 7.8 Ma, and probably indicates retrograde cooling through the closure temperature for amphibole (∼500 °C). The age of the high-pressure metamorphism thus must be pre-Variscan and points to one of the earliest metamorphic events in the Austroalpine nappes known to date. Received June 11, 2000; revised version accepted January 2, 2001     

Modeling of retrograde diffusion zoning in garnet: evidence for slow cooling of granulites from the Highland Complex of Sri Lanka

Summary ?Diffusion modeling of zoning profiles in garnet rims from mafic granulites is used to estimate cooling rates in the Proterozoic basement of Sri Lanka, which represents a small, but important fragment of the Gondwana super-continent. Metamorphic peak temperatures and pressures, estimated with two-pyroxene thermometry and garnet–clinopyroxene–plagioclase–quartz (GADS) barometry, yield 875±20 °C and 9.0±0.1 kbar. These peak metamorphic conditions are slightly higher than results obtained by garnet-biotite Fe–Mg exchange thermometry of 820±20 °C. Reset flat zoning profiles were observed in most garnets. Only narrow garnet rims touching biotite exhibit retrograde zoning in terms of Fe and Mg exchange. The garnet zoning observed requires a slow cooling history. Equilibrium was achieved along grain boundaries during or close to peak metamorphism. During subsequent cooling to lower temperatures, only local exchange between garnet and biotite occurred. A cooling rate of 1–5 °C/Ma is estimated. The estimated temperature-time history from garnet profiles is in good agreement with the cooling history inferred from mineral radiogenic ages in the literature. Received December 11, 2001; revised version accepted August 28, 2002     

Internal morphology, habit and U-Th-Pb microanalysis of amphibolite-to-granulite facies zircons: geochronology of the Ivrea Zone (Southern Alps)

Several types of growth morphologies and alteration mechanisms of zircon crystals in the high-grade metamorphic Ivrea Zone (IZ) are distinguished and attributed to magmatic, metamorphic and fluid-related events. Anatexis of pelitic metasediments in the IZ produced prograde zircon overgrowths on detrital cores in the restites and new crystallization of magmatic zircons in the associated leucosomes. The primary morphology and Th-U chemistry of the zircon overgrowth in the restites show a systematic variation apparently corresponding to the metamorphic grade: prismatic (prism-blocked) low-Th/U types in the upper amphibolite facies, stubby (fir-tree zoned) medium-Th/U types in the transitional facies and isometric (roundly zoned) high-Th/U types in the granulite facies. The primary crystallization ages of prograde zircons in the restites and magmatic zircons in the leucosomes cannot be resolved from each other, indicating that anatexis in large parts of the IZ was a single and short lived event at 299 ± 5 Ma (95% c. l.). Identical U/Pb ages of magmatic zircons from a metagabbro (293 ± 6 Ma) and a metaperidotite (300 ± 6 Ma) from the Mafic Formation confirm the genetic context of magmatic underplating and granulite facies anatexis in the IZ. The U-Pb age of 299 ± 5 Ma from prograde zircon overgrowths in the metasediments also shows that high-grade metamorphic (anatectic) conditions in the IZ did not start earlier than 20 Ma after the Variscan amphibolite facies metamorphism in the adjacent Strona–Ceneri Zone (SCZ). This makes it clear that the SCZ cannot represent the middle to upper crustal continuation of the IZ. Most parts of zircon crystals that have grown during the granulite facies metamorphism became affected by alteration and Pb-loss. Two types of alteration and Pb-loss mechanisms can be distinguished by cathodoluminescence imaging: zoning-controlled alteration (ZCA) and surface-controlled alteration (SCA). The ZCA is attributed to thermal and/or decompression pulses during extensional unroofing in the Permian, at or earlier than 249 ± 7 Ma. The SCA is attributed to the ingression of fluids at 210 ± 12 Ma, related to hydrothermal activity during the breakup of the Pangaea supercontinent in the Upper Triassic/Lower Jurassic. Received: 7 July 1998 / Accepted: 4 November 1998     

Age and geochemical constraints for partial melting of granulites in Estonia

Summary The rocks of the crystalline basement of the East European Craton in southern Estonia show effects of partial melting under granulite facies conditions. Zircons extracted from partial melting products (tonalite from the Tapa Zone – 1824 ± 26, tonalite from the South Estonian Zone – 1788 ± 16 Ma and charnockite from the Tapa Zone – 1761 ± 11 Ma) yield U–Pb crystallisation ages that span over approximately 80 Ma, suggesting a prolonged high-grade metamorphism or several separate events. U–Pb zircon age of one sample of charnockite is concordant with the Nd model age of partial melting of its host mafic granulite facies gneiss (intercept at 1.76 Ga). Linear geochemical trends and similar initial Nd isotopic compositions of mafic granulites and charnockites suggest their possible genetic relationship. From our new and previously published data it follows that the peak granulite metamorphic conditions and formation of tonalites and charnockites (850 °C and 6 kbar) in the Estonian basement occurred at 1788–1778 Ma. Then, the rocks cooled down, passing through the garnet closure temperature of approximately 650–700 °C at 1728 ± 24 Ma. The age of metamorphism of the Estonian granulites is lower than the metamorphic ages known from southern Finland, but it is similar to the age of metamorphism reported from the Belarus-Baltic Granulite Belt in Latvia.     

Anatexis at 700 to 1000 MPa in the aureole of the Marcy anorthosite, Adirondack Highlands, New York

High temperature (>900 °C) metamorphism affected the New Russia gneiss complex in the aureole of the Marcy anorthosite, Adirondack Highlands, New York. Dehydration melting of pargasitic hornblende and plagioclase in metagabbro during contact metamorphism produced garnet among other phases, an indicator that pressure exceeded 700 MPa during anatexis. Partial melting also occurred in mangerite and charnockite. Minerals that equilibrated during melting yield barometric estimates of 970 ± 100 MPa (garnet–orthopyroxene–plagioclase–quartz in metagabbro and mangerite) and 735 ± 100 and 985 ± 100 MPa (garnet–hornblende–plagioclase–quartz, metagabbro and mangerite, respectively). From these results we infer that the Marcy anorthosite was emplaced at a depth of at least 23 km and probably near 32 km. Received: 9 February 2000 / Accepted: 4 April 2000     

Structural-metamorphic evolution of the Southern Yenisey Range of Eastern Siberia: implications for the emplacement of the Kanskiy granulite Complex

Summary ?The Southern Yenisey Range of Eastern Siberia consists of the granulite facies Kanskiy Complex bordered in the west by the lower-grade Yeniseyskiy and Yukseevskiy Complexes. Three deformational events were recognized in each of the three complexes along the Yenisey River cross-section: a D1 fabric forming event, a D2 shear and folding event, and a D3 shear event. Thrust kinematics across the Southern Yenisey Range suggest that during the D2 event the Kanskiy Complex was thrusted along a regional ductile shear zone onto the lower-grade complexes. This resulted in shearing and folding as well as the development of a dynamic metamorphic zonation. In the low-grade greenstone belt part of the cross section (Yukseevskiy complex) D2 shearing is associated with peak prograde (T ∼ 660 °C and P ∼ 5.8 kbar) metamorphism. The retrograde P-T path of the Yukseevskiy Complex coincides with minimum T of the near-isobaric cooling P-T paths for the adjacent granulites of the Kanskiy Complex (Perchuk et al., 1989). The metamorphism can therefore be attributed to deformation and heat transfer caused by exhumation of the Kanskiy Complex in the time period 2000–1800 Ma which also defines the most significant tectono-thermal event in the Southern Yenisey Range. The tectono-metamorphic pattern and evolution of the low- to high-grade metamorphic complexes of the Southern Yenisey Range is very similar to that described for the ∼ 2600 Ma Limpopo Complex of Southern Africa and the ∼ 1900 Ma Lapland Complex of the Kola Peninsula. Similar geodynamic processes were therefore possibly responsible for the formation of these high-grade terrains suggesting that their formation is linked to a general geodynamic model.

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Zusammenfassung ?Strukturelle und metamorphe Entwicklung des südlichen Jenissei-Gebirges in Ost-Sibirien: Bedeutung für die Platznahme des Kanskiy Granulit-Komplexes Das südliche Jenissei-Gebirge in Ost-Sibirien besteht aus dem granulit-faziellen Kanskiy Komplex, der im Westen durch die niedrig-gradigen Jenisseiski und Jukseevski-Komplexe begrenzt wird. Drei Deformations-Phasen k?nnen in jedem der drei Komplexe l?ngs eines Profiles am Jenissei-Fluss beobachtet werden: Eine Phase, die zur Entwicklung des D1 Gefüges führte, eine Phase D2 mit Scher- und Faltvorg?ngen und eine D3 Scher-Phase. Die Kinematik von überschiebungen über das südliche Jenissei-Gebirge deuten an, dass w?hrend der D2-Phase der Kanskiy-Komplex l?ngs einer regionalen duktilen Scherzone auf die niedriggradigeren Komplexe überschoben wurde. Dies führte zu Scherung und Faltung, sowie zur Entwicklung einer dynamischen metamorphen Zonierung. In dem niedriggradigen Grünsteingürtel innerhalb des Profils (Jukseevski-Komplex) ist D2-Scherung mit dem H?hepunkt der prograden Metamorphose (T ≈ 660 °C und P ≈ 5,8 kbar) zusammengefallen. Der retrograde P-T-Pfad des Jukseevski-Komplexes f?llt mit der Minimum-Temperatur der fast isobaren Abkühlung der P-T-Pfade für die benachbarten Granulite des Kanskiy-Komplexes zusammen (Perchuk et al., 1989). Die Metamorphose kann deshalb auf Deformation und W?rmefluss zurückgeführt werden, die durch die Freilegung des Kanskiy-Komplexes zwischen 2.00 und 1.80 Ma verursacht wurde; letztere f?llt auch mit der wichtigsten tektono-thermalen Phase im südlichen Jenissei-Gebirge zusammen. Das tektono-metamorphe Muster und die Entwicklung von niedrig- zu hochgradigen metamorphen Komplexen des südlichen Jenissei-Gebirges ?hnelt in vielf?ltiger Weise dem ungef?hr 2.600 Ma alten Limpopo-Komplex im südlichen Afrika und dem 1.900 Ma alten Lappland-Komplex der Kola-Halbinsel. ?hnliche geodynamische Prozesse waren deshalb m?glicherweise für die Entstehung dieser hochgradig metamorphen Terrains verantwortlich; dies wiederum weist darauf hin, dass ihre Entstehung einem allgemeinen geodynamischen Modell entspricht.

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Received April 27, 1999;/revised version accepted July 14, 1999     

P–T–t evolution of spinel–cordierite–garnet gneisses from the Sauwald Zone (Southern Bohemian Massif, Upper Austria): is there evidence for two independent late-Variscan low-P/high-T events in the Moldanubian Unit?

The Sauwald Zone, located at the southern rim of the Bohemian Massif in Upper Austria, belongs to the Moldanubian Unit. It exposes uniform biotite + plagioclase ± cordierite paragneisses that formed during the post-collisional high-T/low-P stage of the Variscan orogeny. Rare metapelitic inlayers contain the mineral assemblage garnet + cordierite + green spinel + sillimanite + K-feldspar + plagioclase + biotite + quartz. Mineral chemical and textural data indicate four stages of mineral growth: (1) peak assemblage as inclusions in garnet (stage 1): garnet core + cordierite + green spinel + sillimanite + plagioclase (An_35–65); (2) post-peak assemblages in the matrix (stages 2, 3): cordierite + spinel (brown-green and brown) ± sillimanite ± garnet rim + plagioclase (An_10–45); and (3) late-stage growth of fibrolite, muscovite and albite (An_0–15) during stage 4. Calculation of the P–T conditions of the peak assemblage (stage 1) yields 750–840°C, 0.29–0.53 GPa and for the stage 2 matrix assemblage garnet + cordierite + green spinel + sillimanite + plagioclase 620–730°C, 0.27–0.36 GPa. The observed phase relations indicate a clockwise P–T path, which terminates below 0.38 GPa. The P–T evolution of the Sauwald Zone and the Monotonous Unit are very similar, however, monazite ages of the former are younger (321 ± 9 Ma vs. 334 ± 1 Ma). This indicates that high-T/low-P metamorphism in the Sauwald Zone was either of longer duration or there were two independent phases of late-Variscan low-P/high-T metamorphism in the Moldanubian Unit.     

Sub-solidus Oligocene zircon formation in garnet peridotite during fast decompression and fluid infiltration (Duria, Central Alps)

Summary A garnet peridotite lens from Monte Duria (Adula nappe, Central Alps, Northern Italy) contains porphyroblastic garnet and pargasitic amphibole and reached peak metamorphic conditions of ∼830 C, ∼2.8 GPa. A first stage of near isothermal decompression to pressures <2.0 GPa is characterised by domains where fine grained spinel, clinopyroxene, orthopyroxene and amphibole form. The newly formed amphibole contains elevated levels of fluid mobile elements such as Rb, Ba and Pb indicating that recrystallization was assisted by infiltration of a crustal-derived fluid. Further decompression and cooling to ∼720 °C, 0.7–1.0 GPa associated with limited fluid influx is documented by the formation of orthopyroxene-spinel-amphibole symplectites around garnet. Zircon separated from this garnet peridotite exhibits two distinct zones. Domain 1 displays polygonal oscillatory zoning and high trace element contents. It contains clinopyroxene and amphibole inclusions with the same composition as the same minerals formed during the spinel peridotite equilibration, indicating that this domain formed under sub-solidus conditions during decompression and influx of crustal fluids. Domain 2 has no zoning and much lower trace element contents. It replaces domain 1 and is likely related to zircon recrystallization during the formation of the symplectites. SHRIMP dating of the two domains yielded ages of 34.2 ± 0.2 and 32.9 ± 0.3 Ma, respectively, indicating fast exhumation of the peridotite within the spinel stability field. We suggest that the Duria garnet peridotite originates from the mantle wedge above the tertiary subduction of the European continental margin and that it was assembled to the country rock gneisses between 34 and 33 Ma. Third author was Deceased     

Geology of the epithermal Ag–Au Huevos Verdes vein system and San José district, Deseado massif, Patagonia, Argentina

The San José district is located in the northwest part of the Deseado massif and hosts a number of epithermal Ag–Au quartz veins of intermediate sulfidation style, including the Huevos Verdes vein system. Veins are hosted by andesitic rocks of the Bajo Pobre Formation and locally by rhyodacitic pyroclastic rocks of the Chon Aike Formation. New ⁴⁰Ar/³⁹Ar constraints on the age of host rocks and mineralization define Late Jurassic ages of 151.3 ± 0.7 Ma to 144.7 ± 0.1 Ma for volcanic rocks of the Bajo Pobre Formation and of 147.6 ± 1.1 Ma for the Chon Aike Formation. Illite ages of the Huevos Verdes vein system of 140.8 ± 0.2 and 140.5 ± 0.3 Ma are 4 m.y. younger than the volcanic host rock unit. These age dates are among the youngest reported for Jurassic volcanism in the Deseado massif and correlate well with the regional context of magmatic and hydrothermal activity. The Huevos Verdes vein system has a strike length of 2,000 m, with several ore shoots along strike. The vein consists of a pre-ore stage and three main ore stages. Early barren quartz and chalcedony are followed by a mottled quartz stage of coarse saccharoidal quartz with irregular streaks and discontinuous bands of sulfide-rich material. The banded quartz–sulfide stage consists of sulfide-rich bands alternating with bands of quartz and bands of chlorite ± illite. Late-stage sulfide-rich veinlets are associated with kaolinite gangue. Ore minerals are argentite and electrum, together with pyrite, sphalerite, galena, chalcopyrite, minor bornite, covellite, and ruby silver. Wall rock alteration is characterized by narrow (< 3 m) halos of illite and illite/smectite next to veins, grading outward into propylitic alteration. Gangue minerals are dominantly massive quartz intergrown with minor to accessory adularia. Epidote, illite, illite/smectite, and, preferentially at deeper levels, Fe-chlorite gangue indicate near-neutral pH hydrothermal fluids at temperatures of >220°C. Kaolinite occurring with the late sulfide-rich veinlet stage indicates pH < 4 and a temperature of <200°C. The Huevos Verdes system has an overall strike of 325°, dipping on average 65° NE. The orientations of individual ore shoots are controlled by vein strike and intersecting north-northwest-striking faults. We propose a structural model for the time of mineralization of the San José district, consisting of a conjugate shear pair of sinistral north-northwest- and dextral west-northwest-striking faults that correspond to R and R′ in the Riedel shear model and that are related to master faults (M) of north-northeast-strike. Veins of 315° strike can be interpreted as nearly pure extensional fractures (T). Variations in vein strike predict an induced sinistral shear component for strike directions of >315°, whereas strike directions of <315° are predicted with an induced dextral strike–slip movement. The components of the structural model appear to be present on a regional scale and are not restricted to the San José district.     

Polymetamorphism of the Variscan Basement of the Moldanubian Black Forest （Germany） Documented in Zircon and Garnet Minerals from Gneisses

High-grade metamorphic Variscan basement is exposed in the Moldanubian zone of the Black Forest (BF), being the internal zone of the European Variscan belt. Zircon grains from K-rich felsic orthogneisses and an anatectic paragneiss in the Moldanubian Black Forest demonstrate a multi-stage crystallization at ～ 600 Ma, ～ 480 Ma, ～ 400 - 380 Ma, and ～350 Ma. The last three stages of crystallization probably represent metamorphic overprint during pre-Variscan and Variscan metamorphism.Using stepwise leaching procedures, garnet minerals from felsic orthogneisses as well as paragneisses in the Moldanubian Black Forest yielded Early Carboniferous Sm-Nd ages (～ 330- 340 Ma), which are consistent with the well-constrained Variscan HT metamorphic event,and Early Palaeozoic ( ～480 Ma) to Devonian ( ～400 - 370 Ma) Pb-Pb ages. The coincidence of growth time for zircon and garnet minerals at Early Palaeozoic is interpreted as dating a metamorphic event. These garnet data demonstrate that the Moldanubian BF basement underwent at least two metamorphic events during the Early Palaeozoic and Early Carboniferous.During the Variscan HT metamorphism, the Sm-Nd system of garnet was disturbed, but not the U-Pb system, implying the peak metamorphic temperature was lower than ～800℃.