Low-pressure Granulite Facies Metamorphism Research Articles

Exploring paleoproterozoic metamorphism in the western Río de la Plata craton from drill-core evidence

Rhyacian basement is not exposed in the western part of the Río de la Plata craton, but deep borehole samples (depth of 1046–3340 m) provide information on its composition and metamorphic evolution. An amphibole schist sample from the Camilo Aldao borehole underwent two metamorphic stages: M1 and M2. The P-T conditions of M1 are not constrained whereas the M2 main metamorphic stage is characterized by conditions of 6.1–7.4 kbar and 575–605 °C. In contrast, an olivine gabbronorite sample from the Santiago Temple borehole records crystallization conditions of 1.7 ± 0.6 kbar and 1180 ± 10 °C based on clinopyroxene composition, and late magmatic to subsolidus low-pressure granulite facies metamorphism at 2.3 ± 1.2 kbar and 749 ± 85 °C based on multi-equilibrium thermobarometry. The Rhyacian evolution of the Río de la Plata craton is characterized by at least three thermo-tectonic orogenic stages: T1 (ca. 2200–2100 Ma), T2 (ca. 2100–2040 Ma), and T3 (ca. 2040–2010 Ma) that correspond respectively to: 1) early oceanic subduction and juvenile continental arc magmatism; 2) subsequent continental collision (clockwise intermediate P/T paths of metamorphism), and 3) final low-pressure metamorphism coeval with post-orogenic magmatism. The Camilo Aldao amphibole schist is representative of the T2 orogenic stage while the Santiago Temple gabbronorite is representative of T3 magmatism and metamorphism.

Neoproterozoic and middle Paleozoic geological events in the eastern Wolhyeonri complex of the southwestern Gyeonggi Massif, South Korea, and their tectonic correlations in northeastern Asia

The eastern Wolhyeonri complex in the southwestern Gyeonggi Massif, South Korea, which has a close affinity with the northern margin of the South China Craton, primarily consists of Neoproterozoic igneous and sedimentary rocks that were metamorphosed by the middle Paleozoic thermal event. In this study, we present the results from thermobarometry, whole-rock chemistry, and zircon geochronology of rocks in the eastern Wolhyeonri complex near Gwangcheon. The main rock type, such as the migmatitic biotite gneisses, was deposited after ca. 722–680 Ma, and the associated foliated amphibolites exhibit a rift-related Neoproterozoic origin. The ca. 845–820 Ma arc-related metadolerite and mafic layer in the metavolcanic-sedimentary unit are also exposed. Two Neoproterozoic magmatic events support the idea that the southwestern Gyeonggi Massif developed along the northern margin of the South China Craton. These Neoproterozoic rocks experienced upper amphibolite-facies (7–10.5 kbar and 710–735 °C) to low-pressure granulite-facies metamorphism (10.5–12.0 kbar and 810–830 °C) during 424–412 Ma, were associated with ca. 418 Ma leucogranite formation, and then underwent retrograde amphibolite-facies metamorphism. Combined with the results from previous studies, the early-middle Paleozoic orogeny in the southern Gyeonggi Massif is characterized by 470–456 Ma subduction associated with a high P/T metamorphism, and the initiation of intermediate P/T metamorphism from ca. 445 Ma followed by low-pressure granulite-facies metamorphism at 432–403 Ma, and high-pressure granulite-facies metamorphism at 392–381 Ma, indicating that the tectonic setting transitioned from a continental collision to a post-collisional setting between 432 Ma and 392 Ma. These events and the absence of Paleozoic ultrahigh-pressure metamorphism strongly correlated with the Wuyi-Yunkai orogen in the South China Craton, rather than the Qinling belt in the North China Craton. Therefore, the early-middle Paleozoic orogeny in the southern Gyeonggi Massif was caused by the subduction and subsequent collision within the South China Craton.

Geodynamic Implications of Synchronous Norite and TTG Formation in the 3 Ga Maniitsoq Norite Belt, West Greenland

We present new data for the ~3.0 Ga Maniitsoq Norite Belt of the Akia Terrane, West Greenland, with the aim of understanding its petrogenesis. The Maniitsoq Norite Belt is hosted in regional TTG and dioritic orthogneisses, intruded by later sheets of TTG and granite pegmatites, and comprises two main rock types: plagioclase-rich ‘norites’ and pyroxene-rich ‘melanorites’. Both norites and melanorites have high SiO2 contents (52 – 60 wt% SiO2), high bulk rock Mg# (0.57 – 0.83), and low TiO2 contents (0.1 – 0.7 wt%). Their trace element patterns are defined by depleted HREEs, highly enriched LREEs, negative anomalies in Nb, Ta, and Ti, and variable anomalies in Zr, Hf, and Eu. New zircon U-Pb geochronology data and previously published ages establish an emplacement age of 3013 ± 1 Ma for the majority of the Maniitsoq Norite Belt, with magmatism continuing until 3001 ± 3 Ma. This ~12 Myr period of norite magmatism is coeval with an ongoing period of TTG production in the Akia Terrane. Norite Belt emplacement was closely followed by high temperature, low pressure granulite-facies metamorphism at ~800 °C and 900 °C/GPa) and that the norite magmas were emplaced into thin crust and lithosphere. Compositions of the norites and melanorites can be explained by derivation from a single mafic parental melt (~13 wt% MgO), with the norites predominantly accumulating plagioclase and the melanorites predominantly accumulating pyroxene. Evidence from field relationships, the presence of xenocrystic zircon, major element compositions and combined trace element and Hf-isotope modelling suggests the norites were contaminated by assimilation of ~20 – 30% continental TTG crust. Geochemical and Hf-Nd isotopic constraints indicate that the norite mantle source was depleted, and that this depletion occurred significantly before the emplacement of the norite magmas. Contemporaneous production of both TTGs and norite, their emplacement in thin crust, and the rapid transition to high temperature, low pressure granulite-facies metamorphism is best explained by their formation in an ultra-hot orogeny. Formation of norites in this setting may be restricted to > 2.7 Ga, when geothermal gradients were higher on Earth.

Low–pressure granulite–facies metamorphism in the Southern Chinese Altay orogenic belt, NW China: P–T estimates, U–Pb ages and tectonic implications

The Chinese Altay orogenic belt formed in the Paleozoic is an important part of the Central Asian Orogenic Belt (CAOB), accompanying with remarkable metamorphism. Its tectono–metamorphic evolution history is still a controversial topic. In this contribution, we investigate the petrography, metamorphic peak P–T conditions and zircon geochronology of the newly discovered garnet–absent metapelitic and felsic gneisses at Wuqiagou area in the southern Chinese Altay. They are characterized by an assemblage of orthopyroxene + biotite + plagioclase + quartz + Fe–Ti oxides ± cordierite ± K–feldspar, in which most K–feldspar grains have transformed into perthites with thin albite lamellae. The Wuqiagou garnet–absent gneisses experienced pervasive partial melting, as testified by microstructures such as mineral pseudomorphs after melt films or pockets. Phase diagram modelling constrains the peak P–T conditions to 3.5–5.5 kbar and 800–900 °C, with possible geothermal gradients of 45–75 °C/km, indicative of a prominent low–pressure granulite–facies metamorphic event in the southern Chinese Altay. SIMS zircon U–Pb dating results show a weighted mean age of 255.8 ± 1.8 Ma. This age is interpreted to represent the timing of this low–pressure granulite facies metamorphism, which is highly coeval with the timing of their hosted mafic granulite lenses (∼255 Ma) and nearby mantle–derived mafic intrusions (∼257 Ma). Based on available petrological, geochemical and chronological data, we propose that the Permian low–pressure granulite–facies metamorphic event in the southern Chinese Altay was likely associated with the intrusions of deep–derived mafic magma at a relatively shallow crustal level (12–18 km) in a post–orogenic extensional setting, with a possible link with the Tarim mantle plume activity.

Zircon and Monazite Ages Constraints on Devonian Magmatism and Granulite-Facies Metamorphism in the Southern Qaidam Block: Implications for Evolution of Proto- and Paleo-Tethys in East Asia

High-temperature magma generation process and granulite-facies metamorphism can provide important information about mantle-crustal interaction and tectonic evolution. The strongly peraluminous monzonite pluton, the Jinshuikou cordierite granite on the southern margin of the Qaidam Block, can provide important information about the mantle-crustal interaction and constraints on tectonic transition from Proto-Tethys to Paleo-Tethys. This pluton develops enclaves of mafic granulite, amphibolite and quartzofeldspathic rocks, and is cut by massive monzonitic leuco-granite veins. Zircon and monazite U-Pb dating for the cordierite granite, the granulite enclaves and a massive monzonitic leuco-granite vein reveal that the cordierite granitic magma was generated from Mesoproterozoic continental crust with protolith derived from a provenance that was composed of >2.8 Ga old recycled crustal materials and recorded a ~1.7 Ga magmatic event. The continental crust underwent low-pressure granulite-facies metamorphism at ~380 Ma ago, whereas the cordierite granite magmas was generated and emplaced during 380 Ma, followed by intrusion of the massive monzonitic leuco-granite vein at circa 370–330 Ma. These data suggest that after the final closure of Proto-Tethys Ocean spreading along the southern Qaidam Block at ~420 Ma, break-off of the subducted slab or delamination of the lower crustal base and upwelling of the asthenospheric mantle beneath the southern Qaidam Block occurred before the Mid-Devonian, and that the initiation of the Paleo-Tethys tectonics might initiate near the end of Early-Carboniferous in the East Kunlun-Qaidam region, East Asia.

Mesoarchean exhumation of the Akia terrane and a common Neoarchean tectonothermal history for West Greenland

Recent geochronological and petrographic studies in the Nuuk region, West Greenland, have established that modern-style tectonic processes were active in the Neoarchean. However, more limited work has addressed Neoarchean processes elsewhere in West Greenland. In particular, conflicting models have been proposed for the tectonothermal history of the Akia terrane, to the north of the Nuuk region. In this study, the depositional and metamorphic history of supracrustal rocks in the Akia terrane have been investigated using field relationships, petrography, phase equilibria modelling, and zircon and apatite U-Pb geochronology. The Akia terrane is a Palaeo- to Mesoarchean tonalite-trondhjemite-granodiorite (TTG) gneiss terrane with subordinate supracrustal rocks, mafic to ultramafic intrusions – all metamorphosed at amphibolite to granulite facies – Proterozoic mafic dyke suites, Neoproterozoic and Jurassic kimberlitic rocks, and a Mesoarchean and a Jurassic carbonatite. Some of the supracrustal rocks are known to be Mesoarchean, whilst others lack age constraint. New detrital and metamorphic zircon ages show that the metasedimentary rocks studied – the Kangerluarsuk Supracrustal Belt – were sourced from TTG gneisses within the host Akia terrane and deposited in the period ≤2877 Ma to ≥2857 Ma; this requires exhumation of the Akia terrane after c. 3010–2970 Ma magmatism and low-pressure granulite facies metamorphism. After exhumation, the Kangerluarsuk Supracrustal Belt was buried and underwent prolonged high-temperature metamorphism, reaching ∼820–850 °C and 8–10 kbar during regional ductile deformation in the period 2857–2700 Ma. The same rocks grew metamorphic zircon and neoblastic apatite at c. 2630 Ma and >450 to <850 °C, based on U-Pb geochronology and Pb diffusion modelling respectively. This evidence of Neoarchean metamorphism of the Akia terrane shows that a large part of West Greenland was in tectonic communication and experienced common high temperature conditions in the latest Mesoarchean to early Neoarchean and by c. 2700 Ma. The recognition of prolonged Neoarchean high-temperature metamorphism and partial melting during regional ductile deformation raises questions about the veracity of textural evidence for a giant bolide impact at ≥3000 Ma in the same region as such evidence would likely have been obliterated through such pervasive metamorphic and tectonic reworking.

Shared metamorphic histories of various Palaeoproterozoic granulites from Datong–Huai’an area, North China Craton (NCC): constraints from zircon U–Pb ages and petrology

ABSTRACTDifferent tectonic interpretations have been proposed for the various spatially associated Palaeoproterozoic granulite-facies lithologies (metasedimentary rocks, metabasites, and felsic granulites) from north-central part of the North China Craton, which hinges primarily on controversies about metamorphic histories of these granulites, especially on the timing of peak metamorphism. Published data exhibit two controversial peak metamorphic ages of 1950–1900 Ma and 1850–1800 Ma. We report here LA-ICPMS U–Pb zircon ages of seven representative granulite-facies samples of different lithologies to constrain the timing of metamorphism, and then discuss their geological significance. Most zircon grains from these rocks display weak core-and-rim structures and yield two comparable group metamorphic ages of 1970–1900 Ma and 1880–1790 Ma, although their formation ages vary from Neoarchaean to Palaeoproterozoic. The older population metamorphic ages are interpreted to approximate timing of high-pressure granulite-facies metamorphism, and the younger population ages as the approximate timing of intermediate- to low-pressure granulite-facies metamorphism. Combined with recent petrological studies, we propose these granulites have shared metamorphic histories at least since ~1970–1900 Ma, and they are probably formed in one single metamorphic cycle in response to crustal-scale subduction–collision–exhumation processes involved in Palaeoproterozoic mobile belt.

Proterozoic evolution of the Mojave crustal province as preserved in the Ivanpah Mountains, southeastern California

In the western U.S., numerous exposures of Precambrian crustal rocks were exhumed from mid-crustal depths during Cenozoic tectonic events and provide a view into the deep crust. This natural laboratory provides an excellent opportunity to study orogenic and continent-forming processes at deeper levels. The Ivanpah Mountains of southeastern California contain exposures of Paleoproterozoic migmatites and banded gneisses that belong to the Mojave crustal province. New, detailed U–Pb geochronology of monazite and zircon from ortho- and paragneisses reveal the timing of the formation of these rocks and multiple periods of metamorphism and magmatism. Paragneisses have detrital zircon populations of dominantly 1.80–2.15Ga (dominant) and 2.4–2.8Ga, although the youngest detrital zircon grain is 1.754±24Ga. Values of δ18O(zircon) of the two detrital age populations are 7.0±2.2‰ and 6.3±1.4‰ (2SD), respectively, consistent with original, igneous values. Based on the 1.76Ga age of metamorphic zircon rims, the metasediments were intruded by a calc-alkaline magmatic suite including gabbro, tonalite, and porphyritic granite at 1.76Ga, although the contacts are obscured by pervasive metamorphism and deformation.U–Pb ages of metamorphic overgrowths on detrital zircons from the paragneisses indicate multiple events at ∼1.76, 1.74, 1.70 and 1.67Ga which documents a periodicity of tectonism through time. The oxygen isotope ratios of the metamorphic zircon overgrowths from the paragneisses remained relatively constant through time within each sample, consistent with growth during in situ partial melting. However, oxygen isotope ratios of zircons from 1.74Ga leucocratic material are lower in δ18O, and therefore could not have been generated by partial melting of the adjacent paragneiss. Monazite from both igneous and metasedimentary rocks preserves two periods of growth at 1.75 and 1.67Ga. In situ, U–Pb dating of monazite reveals that 1.67Ga monazite inclusions are found in major rock-forming minerals, and therefore we interpret this as the timing of fabric formation and migmitization, which is younger than previously believed and is interpreted to be the dominant metamorphic pulse in the protracted middle crustal tectonism. Thermobarometry indicates peak metamorphic conditions of ∼3.5kb and ∼740°C, which is consistent with partial melting and the metamorphic mineral assemblage in these rocks (sillimanite+K-feldspar, muscovite absent). This paper shows (1) multiple pulses identified within 100Ma orogeny, (2) zircon components and their overgrowths demonstrate mixing of Archean, 1.8 and younger reservoirs, (3) low pressure granulite facies metamorphism occurred at 1.67Ga and may have been related to continental extension of a tectonically over-thickened crust.

Reinterpretation of the tectonic context of high-temperature metamorphism in the Broken Hill Block, NSW, and implications on the Palaeo- to Meso-Proterozoic evolution

The origin of high temperatures during regional low-pressure granulite facies metamorphism within the Proterozoic Broken Hill Block, Australia, has been reinterpreted to be the result of burial of anomalously hot rock packages for which the lithospheric geothermal gradient was initially elevated during early rifting ca. 1.71–1.67 Ga, and then maintained during a ca. 1.62 Ga short-lived mid-crustal extensional event. Mid-crustal extension at ca. 1.62 Ga was associated with amphibolite facies metamorphism and elevated lithospheric geothermal gradients, and occurred prior to activity along D2 high-temperature shear zones, peak low-pressure granulite facies metamorphism (ca. 1.60 Ga) and crustal shortening during the Olarian Orogeny (ca. 1.60–1.59 Ga). This reinterpretation places the Broken Hill Block within an environment in which multiple episodes of transient extension (ca. 1.71–1.62 Ga) were followed by a switch to shortening at ca. 1.60 Ga. The interpreted tectonic environment is a continental back-arc setting located in the over-riding plate of a subduction zone along the southern margin of Palaeoproterozoic Australia.

Unraveling the record of successive high grade events in the Central Zone of the Limpopo Belt using Pb single phase dating of metamorphic minerals

Dating of relic metamorphic assemblages can provide important information about the timing and character (metamorphic grade and/or PT-evolution) of early high grade episodes in polymetamorphic provinces. Using Pb stepwise leaching of metamorphic silicates, we have dated multiple granulite facies metamorphic episodes in the Central Zone (CZ) of the Limpopo Belt. Ages of 2.52 Ga were obtained from sillimanite and cogenetic garnet and ages of about 2.01 Ga from titanite, garnet and clinopyroxene. Together with new and published conventional age data from accessory phases and in the context of combined petrological and structural data, these results lead us to a reinterpretation of the tectono-metamorphic history of the CZ. Three distinct high grade events at about 3.2−3.1 Ga, 2.65−2.52 Ga and 2.0−0.05 Ga are recognized. Each of these is suggested to correspond to a tectonic episode of distinct character: (a) for the 3.2 Ga event magmatic activity can mainly be identified (best represented, for example, by the Sand River Gneisses or the Messina Layered Intrusion). The field relationships concerning the tectonometamorphic history of this Early-Archean event are largely erased by at least two high grade metamorphic overprints. (b) Late-Archean (∼2.6-2.52 Ga) low pressure granulite facies metamorphism was associated with voluminous granitic and charnockitic plutonism. The anticlockwise P-T evolution of these granulites probably reflects deep crustal processes, associated with magmatic underplating (or in-plating), contemporaneous with vertical crustal growth of the Zimbabwe craton around 2.6 Ga. (c) During the Proterozoic event (∼2.05-1.95 Ga) tectonic thickening was caused by the collision of the Kaapvaal and Zimbabwe cratons. The CZ was squeezed between these two cratons and as a consequence underwent high pressure granulite facies metamorphism with a clockwise P-T evolution. The structural, metamorphic and geochronological data can be best explained with a tectonic model that describes this final event as a dextral transpressive orogeny.

Palaeoproterozoic granulite-facies metamorphism and granitoid intrusions in the Ubendian-Usagaran Orogen of northern Malawi, east-central Africa

The Paleoproterozoic basement of northern Malawi shows evidence for granulite-facies metamorphism older than the 1930±30 Ma-old Nyika Granite. Low-pressure granulite-facies metamorphism in regionally coherent and extensively exposed cordierite-garnet granulite reached 750–850°C and 5-5.5 kbar. The Chelinda Granite intruded during this event and has been dated by single zircons at 1995±0.4 Ma (207Pb206Pb age; 2σ-mean errors). Anatectic melt which formed concurrently with cordierite growth in the cordierite-garnet granulite yielded a 207Pb206Pb zircon age of 1988±0.6 Ma. These ages are interpreted to date the peak of regional low-pressure granulite-facies metamorphism. The Nyika Granite, and other smaller collisional-type two-mica granites, intruded after the peak of granulitefacies metamorphism. Their zircon ages range from 1930±30 Ma to 1969±0.4 Ma and constrain the end of regional high-temperature conditions. Enderbitic gneiss, which is tectonically intercalated with the cordierite-garnet granulite, records peak-metamorphic conditions of ca 850–880°C and 9–11 kbar. Metamorphic zircons from the enderbitic gneiss yielded a 207Pb206Pb age of 2002±0.3 Ma. This age for high-pressure granulite-facies metamorphism in northern Malawi is similar to the recently reported age for eclogite-facies metamorphism in the Usagaran Belt of central Tanzania and indicates that orogenic activity during the Ubendian-Usagaran Orogeny culminated at ca 2000 Ma.Pre-2000 Ma magmatic events encompass the intrusion of the enderbitic gneiss precursor at 2093±0.6 Ma. Furthermore, a late granite intrusion of the Rumphi Igneous Complex (RIC) yielded a single-zircon age of ca 2048±0.7 Ma. The chemistry of those late-stage granites of the RIC displays an affinity to volcanic-arc granites. Therefore, the age of 2048 Ma is interpreted to date a mature stage of volcanic-arc magmatism. The nature of the pre-2000 Ma magmatic events remains open, however; the granitoids might have been emplaced in a long-lived Andean-type subduction zone. Tectonic juxtaposition of both granulite-facies rocks and of the latter two with the RIC occurred after Paleoproterozoic granulite-facies metamorphism in northern Malawi.

Early Paleozoic (Pan African) thermal event of the Larsemann Hills and its neighbours, Prydz Bay, East Antarctica

The early Paleozoic (Pan African) thermal event of the Larsemann Hills and its adjacent areas, East Antarctica is discussed based upon the isotope ages we obtained. An Sm-Nd internal isochron for a representative mafic granulite yields an age of 540 Ma±75 Ma. Another Sm-Nd internal isochron, which is made up of the assemblage of the peak metamorphism and its whole rock as well, gives an age of 497 Ma ± 7 Ma The isotopic chronological data of single zircon stepwise evaporation dating and 40Ar-39Ar analysis provide further evidence for the early Paleozoic event of high-grade metamorphism in the region. The data from the field geological investigation in the Larsemann Hills also show that there is not only strong regional partial melting but also low-pressure granulite-facies metamorphism accompanied by it in the region. The early Paleozoic (Pan African) thermal event of the region may be related to the final formation of the East Antarctica craton, even of Gondwanaland.

40Ar/39Ar thermochronology and thermobarometry of metamorphism, plutonism, and tectonic denudation in the Old Woman Mountains area, California

Discrimination of individual tectonometamorphic events in polymetamorphosed terranes requires a comprehensive understanding of the relative timing and conditions of metamorphism and plutonism. We have applied a combination of 40 Ar/ 39 Ar thermochronology, petrology, and thermobarometry to reconstruct the complex Early Proterozoic through early Cenozoic tectonic and metamorphic evolution of continental crust in the Old Woman Mountains area, south-eastern California. Strong Mesozoic thermal events obscure the earlier history in much of the Old Woman Mountains area. In those areas where Early Proterozoic rocks underwent only lower-greenschist-facies metamorphism during the Mesozoic, thermobarometry of pelitic schists indicates that Proterozoic metamorphism occurred at 9 to 11 kbar and ∼700 °C. 40 Ar/ 39 Ar ages of hornblende from samples of interbedded Proterozoic amphibolite indicate that this high-grade metamorphism took place before 1600 Ma. The relatively high-pressure conditions of Early Proterozoic metamorphism in the Old Woman Mountains area contrast with the low-pressure granulite-facies metamorphism that occurred elsewhere in the Mojave Desert at this time. 40 Ar/ 39 Ar analyses of hornblende from Proterozoic rocks within Mesozoic shear zones and hornblende barometry from Jurassic intrusive rocks suggest that tectonism and burial of Paleozoic strata to >10 km began between 170 and 150 Ma. This tectonism resulted in regional greenschist-facies metamorphism. Late-stage mineral assemblages in Proterozoic and Paleozoic pelitic rocks in the Old Woman Mountains area indicate an increase in metamorphic grade from greenschist to upper amphibolite facies toward Late Cretaceous plutons of the 73 Ma Old Woman-Piute batholith. Metamorphic temperatures from garnet-biotite thermometry increase from 650 °C, and 40 Ar/ 39 Ar ages of hornblende from Proterozoic rocks decrease from 1600 to 73 Ma toward these plutons. Barometric calculations from garnet-bearing metamorphic rocks suggest that this Cretaceous metamorphism took place at 3.5 to 5.0 kbar in the Old Woman Mountains. Similar pressures are obtained from hornblende barometry of Late Cretaceous granodiorite. 40 Ar/ 39 Ar and apatite fission-track data from the 73 Ma batholith indicate that the rocks exposed in the Old Woman Mountains cooled rapidly to below 350 to 300 °C by 70 ± 1 Ma and that moderately rapid cooling at rates of 10 to 30 C°/m.y. continued until after 60 Ma. The presence of maximum Mesozoic temperatures on the borders of plutons, hornblende ages of ca. 73 Ma, and similar pressure estimates for plutons and country rocks indicate that the high temperatures needed to cause ductile deformation and amphibolite-facies metamorphism were related to heating from the intrusion of the 73 Ma Old Woman-Piute batholith. Amphibolite-facies metamorphism along pluton contacts was superimposed on the regional greenschist-facies metamorphism related to thrusting and crustal thickening. The extremely rapid cooling in the Old Woman Mountains between 73 and 71 Ma was due largely to conduction of heat into the country rocks, whereas protracted rapid cooling to temperatures below ambient temperatures estimated for the depth of emplacement was due to tectonic denudation at 1 to 2 km/m.y. until about 68 to 65 Ma.

A barometric response to late compression in the Strangways Metamorphic Complex, Arunta Block, central Australia

Evidence for four major deformation events ( D 1– D 4) and two metamorphic events ( M 1, M 2) is shown by granulite- and amphibolite-facies rocks of the Strangways Metamorphic Complex, central Australia, which is divided into two domains: the Ongeva Granulites to the east, and the Anamarra Granite domain to the west. D 1 and D 2 produced layer-parallel foliations, with many of the effects of D 1 overprinted by intense non-coaxial shear during D 2. Low-pressure granulite-facies metamorphism, M 1, accompanied D 1–2, at conditions of P = 5.3 ± 1.2 kbar and T > 750°C. The effects of M 1 are characterized by S 1 spinel + quartz-bearing assemblages, with overprinting S 2b sillimanite + biotite + magnetite assemblages, implying approximately isobaric cooling. Effects of M 2 are characterized by S 1–2 cordierite pseudomorphed by orthopyroxene + sillimanite + biotite + magnetite symplectites, at conditions of P = 7.5 ± 0.8 kbar and T ≊ 800° C . This retrograde increase in pressure was probably the result of crustal thickening during D 3, which produced NE-plunging F 3 and F 4 folds in the Ongeva Granulites, and a widespread shear foliation in the Anamarra Granite domain. The increase in pressure with M 2 may reflect the beginning of a clockwise P- T- t path typical of collisional-style tectonics. Sillimanite and orthopyroxene-bearing ultramylonites were produced during D 4, an extension event parallel to the D 3 transport axis and probably related to crustal collapse after thickening during D 3. Wide N-dipping shear zones ( D 5) containing sillimanite, kyanite, staurolite and overprinting greenschist-facies assemblages bound the Strangways Metamorphic Complex. These zones indicate a S-directed sense of shear and were probably responsible for bringing the Strangways Metamorphic Complex to higher crustal levels, during both a probable late Proterozoic event and the Mid Carboniferous Alice Springs Orogeny.

Geological history of Adélie Land and King George V Land, Antarctica: Evidence for a polycyclic metamorphic evolution

Abstract Metamorphic and structural studies in Adelie Land and King George V Land, East Antarctica, show that this part of the East Antarctic Shield can be divided into four different sections. The areas east of Commonwealth Bay show evidence for a polycyclic metamorphic evolution in which low-pressure granulite facies M 1 fabrics (4–4.5 kbar; ∼ 750°C) are overprinted by retrograde hydrated decompression textures of amphibolite facies during M 2 and by late, high-pressure, low-temperature coronas during M 3 (5.7–7.0 kbar, 500–600°C). The regional structure and relative timing of events in the other three areas is consistent with the recognition of these three metamorphic events. The Cape Denison orthogneiss in Commonwealth Bay was intruded after the pervasive low-pressure M 1 event but before the major ductile deformation phases and M 2 . The Cape Denison orthogneiss therefore only suffered the M 2 amphibolite facies metamorphic event and subsequent overprinting by M 3 mid-greenschist facies assemblages involving lawsonite. The Cape Hunter phyllites were deposited after M 2 but before M 3 and consequently were only affected by the mid-greenschist metamorphic event, which also overprinted the amphibolite facies fabrics at Cape Denison. The areas west of Commonwealth Bay underwent the low-pressure granulite facies metamorphism during M 1 , decompression during M 2 but no late M 3 overprint. The variation of the grade of M 3 in the different areas from granulite facies pressures east of Commonwealth Bay to no effect west of Cape Hunter is interpreted as a consequence of differential uplift along shear zones with a north-south strike, which exposed successively deeper levels towards North Victoria Land to the east. This spatial association and the age of M 3 may suggest that there is a connection between M 3 in King George V Land and Adelie Land and the Ross orogeny in North Victoria Land. The recognition of three metamorphic events can be correlated with three radiometric ages previously established for the Cape Denison orthogneiss.