Mafic Granulites Research Articles

Evidence of Eocene high-temperature/high-pressure metamorphism of ophiolitic rocks and granitoid intrusion related to Neotethyan subduction processes (Doğanşehir area, SE Anatolia)

Abstract New data for regionally important granulite facies metaophiolitic rocks and cross-cutting granitoids rocks are presented and discussed. The high-temperature/high-pressure Berit metaophiolite is cut by unmetamorphosed Eocene (51–45 Ma) granitoid rocks. The highest metamorphic grade occurs in blocks of mafic granulites. Enveloping amphibolite facies rocks reflect retrograde metamorphism related to exhumation. Sm–Nd (pyroxene–garnet–amphibole–whole rock) isochron ages of 52–50 Ma for the granulite facies rocks are interpreted to represent the time of cooling of the granulite facies rocks. The over-riding Malatya metamorphic unit to the north is also intruded by Eocene granitoid rocks. The granulite facies metamorphism of the meta-ophiolitic rocks is inferred to have formed in the roots of an Eocene magmatic arc, with accentuated heat flow being provided by subduction of a spreading ridge, or rupture of the subducting slab. The high-temperature/high-pressure metamorphism was followed by exhumation, as indicated by field structural relations and the evidence of retrograde metamorphism. The Eocene arc magmatism can best be explained by northward subduction of the Southern Neotethys, which persisted after the time of latest Cretaceous regional ophiolite emplacement until the collision of the Eurasian (Anatolian) and Arabian continents during the Early–Mid Miocene. Subsequent Plio-Quaternary left-lateral strike-slip strongly affected the area. Supplementary-material: Four supplementary tables giving the whole rock geochemistry of the granitoids, mineral geochemistry of the granulite facies rocks, LA-MC-ICP-MS zircon U–Pb data belonging to granitoids and Sm–Nd data belonging to granulite facies rocks and two documents giving the detailed analytical procedures and detailed petrography of the granitoids are available at www.geolsoc.org.uk/SUP18588

Partial melting of the Archaean Thrym Complex of southeastern Greenland

High-grade Archaean rocks of the remote Thrym Complex of southeastern Greenland expose a lower crustal section that formed at a depth of >30km at temperatures reaching 1000°C and pressures approaching 1.5GPa. The complex is part of the North Atlantic Craton, which is bound to the north by the Trans Hudson–Nagssugtoqidian–Ammassalik Orogen and to the south by the Makkovik–Ketilidian Orogen. The rocks in the complex include orthogneiss, migmatitic orthogneiss with banded or nebulitic fabrics, migmatite with a breccia-like structure (agmatite), and narrow bands of lensoidal mafic and ultramafic rocks (granulites) with associated widespread garnet-bearing mafic granulite, and a minor proportion of garnet–cordierite–sillimanite–biotite gneiss. The latter high-grade rocks form part of the substrate for the emplacement of the protoliths for orthogneiss that was synchronous with the ca. 2790–2700Ma Skjoldungen Orogeny.At least two suites have been recognised in the Thrym Complex based on geochemistry. The oldest is a tholeiitic suite of mafic to ultramafic granulites, and the other is a calc-alkaline suite that includes felsic to intermediate orthogneiss. Most of the orthogneiss has a granodioritic (oldest) to quartz monzodioritic (younger) composition with rare monzogranitic, tonalitic, monzonitic and quartz dioritic protoliths. Historically, the orthogneiss units have been called TTGs, although monzogranite and tonalite are rare, and no trondhjemite is observed. In addition, they could also be called adakitic on geochemical grounds. Instead of using this terminology, which has genetic connotations, the orthogneiss is referred to as predominantly granodioritic to quartz-monzodioritic.Assuming that modern tectonics operated during the Neoarchaean, at least some of the mafic and ultramafic granulites and orthogneiss exposed in the Thrym Complex represent the root zone of a magmatic-arc that might have been relaminated around 2760Ma during the collisional Skjoldungen Orogeny.

The latest Neoarchean–Paleoproterozoic evolution of the Dunhuang block, eastern Tarim craton, northwestern China: Evidence from zircon U–Pb dating and Hf isotopic analyses

The Dunhuang block, in the easternmost segment of the Tarim craton, exposes Precambrian mafic granulite and felsic gneiss. To better understand the poorly known formation and evolution of the Tarim craton, we carried out detailed field-based petrological and geochronological investigations on the major lithologies of the Dunhuang block. U–Pb dating and Hf isotopic analyses on the mafic granulite and felsic gneiss (TTG gneiss) of the Dunhuang block reveal two distinct age populations: the Latest Neoarchaean (∼2.5Ga) and the Late Palaeoproterozoic (1.82–1.85Ga). The zircon magmatic cores and metamorphic rims of tonalitic gneisses yield similar ages of ∼2.5Ga, supporting the existence of Archean rocks in the Dunhuang block. The short time interval between Latest Neoarchean magmatism and high-grade metamorphism suggests that they were related to the same Latest Neoarchean tectonothermal event. The ∼2.5Ga zircons show Hf-depleted mantle model ages between 2.55Ga and 2.8Ga with a peak at ∼2.7Ga, suggesting that ∼2.7Ga juvenile crust of the Dunhuang block was reworked during a 200–300m.y. period at the end of the Neoarchean. The age of ∼1.85Ga obtained from the mafic granulites and felsic gneisses is interpreted as the age of high-pressure granulite facies metamorphism. Combined with the Hf isotopic data, these data suggest that the Dunhuang block experienced ∼2.7Ga crustal growth, a ∼2.5Ga magmatic–metamorphic event and a 1.82–1.85Ga (HP) granulite facies metamorphic event. This sequence of events is very similar to that of the North China Craton, implying that the evolutionary history of the Dunhuang block was consistent with that of the North China Craton from the Neoarchean to Paleoproterozoic.

Comparison of the metamorphic history of the Monapo Complex, northern Mozambique and Balchenfjella and Austhameren areas, Sør Rondane, Antarctica: Implications for the Kuunga Orogeny and the amalgamation of N and S. Gondwana

Reconstructions of Gondwana place Dronning Maud Land, Antarctica (DML) adjacent to N. Mozambique prior to fragmentation. The Monapo Complex outlier klippen overlying the Nampula Terrane in N. Mozambique has been correlated with rocks in eastern and central DML. Metamorphic assemblages and P–T conditions from the two areas are compared as well the timing of metamorphism. Granulite grade assemblages preserved in the Monapo Complex suggest P–T conditions of ∼900°C and >10kb. Textures vary with reactions typical of isothermal decompression, isobaric cooling and hydration being recognized but also include equilibrium assemblages. P–T estimates from four samples suggest initial inversion of high pressure assemblages from at least ∼10kb and ∼900°C to mid-crustal levels where near isobaric cooling and hydration at between ∼4–7kb and 550–700°C is recognized. Granulite grade assemblages from Balchenfjella in eastern Sør Rondane, DML suggest initial P–T conditions of ∼900°C and >10kb. Textures vary with reactions typical of decompression, cooling and hydration being recognized but also include equilibrium assemblages. P–T estimates from four samples suggest initial inversion of high pressure assemblages from at least ∼10kb and ∼900°C to mid-crustal levels where retrogression at between ∼6–7kb and 600–700°C is recognized.SHRIMP zircon data from six samples from eastern Sør Rondane show episodic zircon growth over a period from ∼630Ma to ∼530Ma with five metamorphic pulses being defined. Undeformed granitic intrusions with ages of ∼550Ma intrude Sør Rondane. SHRIMP zircon data from samples from alkaline intrusions and mafic granulites in the Monapo Complex yield crystallization and metamorphic ages of ∼635Ma and 570-590Ma respectively. The complex is intruded by granitic veins with ages of ∼550Ma.The similarities in metamorphic mineral assemblages, P–T estimates and zircon geochronology data indicate a common geological history between Sør Rondane and the Monapo Complex of northern Mozambique. This common history is interpreted to result from both areas being part of a mega-nappe structure which was emplaced in a transpressional setting involving collision between N. Gondwana (comprising rocks of the East African Orogeny) and south Gondwana (comprising the Nampula Terrane of northern Mozambique and the Maud Province of western Dronning Maud Land) along the Damara-Zambesi-Lurio-CDML collision boundary.Other erosional remnants of this nappe include the Mugeba Complex in northern Mozambique, the Kataragama klippe in Sri Lanka, the Urungwe klippe in Zimbabwe, the Masosa Suite and Mavhuradohna Complex in western Mozambique and NE Zimbabwe.

Lower-Crustal Xenoliths from Jurassic Kimberlite Diatremes, Upper Michigan (USA): Evidence for Proterozoic Orogenesis and Plume Magmatism in the Lower Crust of the Southern Superior Province

Jurassic kimberlites in the southern Superior Province in northern Michigan contain a variety of possible lower-crustal xenoliths, including mafic garnet granulites, rare garnet-free granulites, amphibolites and eclogites. Whole-rock major-element data for the granulites suggest affinities with tholeiitic basalts. P–T estimates for granulites indicate peak temperatures of 690–730°C and pressures of 9–12 kbar, consistent with seismic estimates of crustal thickness in the region. The granulites can be divided into two groups based on trace-element characteristics. Group 1 granulites have trace-element signatures similar to average Archean lower crust; they are light rare earth element (LREE)-enriched, with high La/Nb ratios and positive Pb anomalies. Most plot to the left of the geochron on a 206Pb/204Pb vs 207Pb/204Pb diagram, and there was probably widespread incorporation of Proterozoic to Archean components into the magmatic protoliths of these rocks. Although the age of the Group 1 granulites is not well constrained, their protoliths appear to be have been emplaced during the Mesoproterozoic and to be older than those for Group 2 granulites. Group 2 granulites are also LREE-enriched, but have strong positive Nb and Ta anomalies and low La/Nb ratios, suggesting intraplate magmatic affinities. They have trace-element characteristics similar to those of some Mid-Continent Rift (Keweenawan) basalts. They yield a Sm–Nd whole-rock errorchron age of 1046 ± 140 Ma, similar to that of Mid-Continent Rift plume magmatism. These granulites have unusually radiogenic Pb isotope compositions that plot above the 207Pb/204Pb vs 206Pb/204Pb growth curve and to the right of the 4·55 Ga geochron, and closely resemble the Pb isotope array defined by Mid-Continent Rift basalts. These Pb isotope data indicate that ancient continental lower crust is not uniformly depleted in U (and Th) relative to Pb. One granulite xenolith, S69-5, contains quartz, and has a unique peraluminous composition. It has the lowest eNd and eHf values of the suite. Its isotopic compositions indicate that it is significantly older than the other granulites. Broken zircon cores encased by younger overgrowths suggest that this granulite includes a large component of pre-existing sedimentary rocks. Two distinct populations of zircons from S69-5 were dated by sensitive high-resolution ion microprobe. Abundant rounded zircons yield ages of 1104 ± 42 (2σ) Ma, which coincide with the Mid-Continent Rift flood basalt eruptions. Their morphology is similar to those found in lower-crustal rocks that have undergone granulite-facies metamorphism and thus they are considered to represent the age of Group 2 granulites. Also present are less abundant elongate zircon grains that yield a mean age of 1387 ± 32 (2σ) Ma. Their elongate shapes indicate growth from a melt or fluid, possibly associated with 1·3–1·5 Ga anorogenic granite magmatism exposed in the shallow crust to the south in Wisconsin, or related to an initial encroachment of the Keweenawan plume upon the lower crust. Older ages recognized in zircon cores are less well constrained but may be related to tectono-magmatic events in the southern Superior craton. Within the studied suite only S69-5 was recognized as a remnant of the Late Archean lower crust into which the Group 1 and 2 mafic granulite precursor basalts were intruded. Collectively, the data show that the lower crust beneath northern Michigan formed in Archean times and underwent a variety of tectono-magmatic processes throughout the Proterozoic, including orogenesis, partial melting and mafic magmatic underplating in response to upwelling mantle plumes.

Clockwise, low-[formula omitted] metamorphism of the Aus granulite terrain, southern Namibia, during the Mesoproterozoic Namaqua Orogeny

The Aus granulite terrain forms part of the Mesoproterozoic Namaqua Metamorphic Complex (NMC) of southern Africa. The terrain consists of pre- to syn-tectonic granitoid gneisses containing subordinate aluminous metasediments, metapsammites, mafic granulite and calc-silicate, all of which have been metamorphosed and migmatised to varying degrees. Pseudosection modelling of garnet–sillimanite–cordierite–biotite aluminous metapelitic samples constrain peak metamorphic conditions at 5.5kbar and 825°C. Pseudomorphs of sillimanite after andalusite constrain early prograde conditions of ∼550–600°C at below 4kbar, whereas the replacement of sillimanite by garnet–cordierite and subsequent partial replacement of garnet by cordierite at near-peak conditions indicates a clockwise trajectory for this segment of the P–T path. Subsequent near-isobaric cooling below the solidus resulted in secondary, texturally distinct sillimanite partially replacing cordierite. U–Pb zircon dating of pre-, syn- and post-tectonic granitoid intrusions indicates that granitoid plutonism occurred at c. 1120–1085Ma, whereas metamorphic overgrowths on these zircons constrains metamorphism to have occurred from c. 1065 to 1045Ma. A post-tectonic granite dyke intruded after the terrain cooled to subsolidus temperature has an age of 1004±6Ma. The timing and style of metamorphism is similar to that recorded in other parts of the NMC, suggesting that the various crustal fragments that constitute the NMC are not as allochtonous as previously suggested. The emplacement of voluminous granitoid magmas into this terrain triggered initial andalusite grade metamorphism through advective heating, with further slow burial of this juvenile crust leading to low-P–high-T peak metamorphic conditions. Magmatism and metamorphism appear to form a continuum, implying the presence of a long-lived (>50–70Ma), crustal-scale thermal anomaly as the cause of both. Such a primary anomaly most likely involves lithospheric delamination, although direct evidence for mantle involvement, such as contemporaneous mafic magmatism, does not occur in the Aus terrain.

Electrical conductivity in a partially molten crust from measurements on metasedimentary enclaves

The complex electrical impedance of a garnet–biotite–sillimanite residual enclave in the Neogene dacite of El Hoyazo (SE Spain) has been determined up to 978°C at 200–300MPa. This well studied material represents a direct sampling of the Alborán Domain thinned lower crust undergoing partial melting. The paragenesis is garnet+biotite+sillimanite+plagioclase+graphite±cordierite coexisting with widespread (~10wt.%) rhyolitic melt occurring as inclusions and interstitial glass and developed during regional anatexis at 850±50°C and 500–700MPa.The samples were used in cyclic measurements consisting in heating–cooling ramps at progressively higher maximum temperature to observe the effect of reactions on conductivity. In the first cycle up to 850°C at 2–3kbar, re-melting of the interstitial glass is achieved with no additional reactions and logarithmic conductivity (S/m) is up to −1.5. At T>950°C new melt with orthopyroxene+ilmenite is produced from biotite partial breakdown and the logarithmic conductivity (S/m) is up to −0.7. Application of mixing models as Hashin–Shtrikman bounds or Archie's Law, shows that once interconnected, melt controls the electrical conductivity of the enclave. The electrical conductivity of the rock can be simulated with the electrical conductivity data obtained on the starting Matrix glass of the enclave and with the new melt.Our experimental results obtained on a graphitic metasedimentary material evidence show that graphite does not contribute to the conductivity enhancement and that the electrical anomaly observed below the Betics can be explained as effect of partial melting of a residual crust. Comparison with previous works on mafic granulites shows that similar conductivity values can be achieved both by melting of a residual felsic crust or by melting of mafic materials and suggests that pelitic rocks can be more common at deep levels than expected.

Rheological contrast at the continental Moho: Effects of composition, temperature, deformation mechanism, and tectonic regime

The rheological contrast at the Moho is an important factor in continental tectonics. This paper explores systematically the effects of composition, temperature, deformation mechanism, and tectonic regime on the strength contrast, considering four compositions for the lower crust (felsic granulite, mafic granulite, wet diabase, dry diabase) and two for the lithospheric mantle (dry and wet peridotite). The strength contrast of the resulting eight compositional layerings is estimated as a function of Moho temperature which is varied from 600 to 1500 K. The Moho temperature can be converted to surface heat flow if the thickness and composition of the crust are known. Besides a “standard” case (crustal thickness 35 km), the cases of a thick (50 km) and thin (20 km) crust are also considered (with wet quartzite crustal rheology in the latter case). Results show a great variety of strength contrasts according to different conditions. Excluding the case of very low Moho temperature (TM ≤ 600 K), when the behaviour of both lower crust and upper mantle is frictional brittle and therefore the strength contrast vanishes, the strength contrast (at a given strain rate) is a strong function of composition, temperature, and tectonic conditions. Weak compositional stratification (e.g., soft lower crust/soft lithospheric mantle or hard lower crust/hard lithospheric mantle) results in lower contrasts than strong compositional stratification. For any given compositional combination, the absolute value of the strength contrast is higher in compressional as compared to extensional tectonic environments, and tends to decrease with increasing temperature from a maximum of hundreds of MPa at low-to-intermediate Moho temperatures (600 < TM < 1000 K) to values less than a few MPa at higher temperatures (TM > 1200 K). Therefore, rheological layering is favoured by strong intrinsic (i.e., compositional) strength contrasts between lower crust and upper mantle and relatively low Moho temperatures.

Prolactine et cortisol : valeur prédictive chez les polytraumatisés

The Inner Mongolia Suture Zone marks the Paleoproterozoic collisional boundary between the Yinshan Block and Ordos Block in the North China Craton. The ultrahigh-temperature (UHT) orogen developed within this collisional suture reflects the extreme thermal conditions associated with the tectonic processes during the incorporation of the North China Craton within the Columbia supercontinent. Here we report the results from a systematic petrological and phase equilibria modeling study of sapphirine-bearing granulites from Daqingshan, a region within the central domain of the Inner Mongolia Suture Zone. The sapphirine–spinel–garnet–sillimanite–biotite-bearing granulites in Daqingshan occur typically as dark lenses or layers within spinel–garnet–sillimanite–biotite-bearing gneisses, and are associated with mafic granulites carrying orthopyroxene + clinopyroxene + plagioclase + calcic amphibole. The sapphirine in these rocks is magnesian (XMg = 0.73–0.79) and occurs in various microstructural associations such as: (1) matrix coarse-grain mineral in plagioclase aggregates, (2) subidioblastic mineral mantling spinel or in direct contact with spinel, (3) fine-grained phase in sillimanite, and (4) vermicular symplectitic aggregates with plagioclase. Garnet + plagioclase + sapphirine + sillimanite + ilmenite + inferred melt is considered to represent the peak assemblage in these rocks. We apply systematic phase equilibria analysis in the system Na2O–CaO–K2O–FeO–MgO–Al2O3–SiO2–H2O–TiO2–Fe2O3 (NCKFMASHTO) to constrain the peak P–T conditions of the Daqingshan Mg–Al granulites. Our pseudosection analysis defines the stability of the peak assemblage at T > 970 °C at P > 11 kbar. The peak temperature derived from pseudosection analysis is broadly consistent with the temperature estimate from conventional thermometry of sapphirine–spinel pairs (ca. 940 °C). The extreme metamorphism recorded from the Mg–Al granulites from Daqingshan is also consistent with the occurrence of dry orthopyroxene + clinopyroxene + plagioclase, and orthopyroxene + pargasite + plagioclase in the associated mafic granulites, although conventional thermometry yields lower temperatures from these rocks (830–840 °C) marking the post-peak retrograde stage. Our data confirm UHT conditions in Daqingshan and indicate that the extreme metamorphism is of regional extent within the Inner Mongolia Suture Zone, in agreement with the model of ultra-hot metamorphic orogen developed within a Paleoproterozoic subduction–collision system.

Links between Mesozoic intermediate–felsic igneous rocks and lower crustal granulite xenoliths, North China Craton: O and Pb isotopic evidence

We present and compare whole-rock and zircon O and Pb isotopic compositions for the Hannuoba granulite xenoliths and Mesozoic intermediate-to-felsic igneous rocks from the Zhangjiakou region, northern margin of the North China Craton, northeast China. The xenoliths have an overall Pb isotopic range similar to rocks from the regionally exposed Neoarchaean granulite terrain. Mesozoic zircons from different types of granulite xenoliths have a narrow range of δ18O values (6.0–7.7‰) higher than normal mantle δ18O values (∼5.7‰). Mesozoic intermediate–felsic igneous rocks have O and Pb isotopic compositions indistinguishable from the Hannuoba intermediate–mafic granulite xenoliths. Our new data suggest that the Mesozoic igneous rocks and granulite xenoliths are genetically linked and that both were derived from the late Neoarchaean lower crust. This argues against previous proposals that the granulite xenoliths are either products of Mesozoic basaltic underplating or formed by mixing between mantle-derived and pre-existing crustal magmas.

P–T history and geochemical characteristics of mafic granulites and charnockites from west of Periya, North Kerala, southern India

The study region forms the northern part of Kerala (south India) and constitutes part of granulite–facies rocks of the exhumed Proterozoic south Indian Granulite Terrain (SGT). The SGT consists of a large variety of rock types with a wide range of mineral parageneses and chemical compositions, namely charnockite, mafic granulite, gneiss, schist, anorthosite, granite and minor meta-sedimentary rocks. Garnet–bearing mafic granulites occur as small enclaves within charnockites. We report for the first time P–T constraints on the prograde path preceding peak metamorphism in the northernmost part of Kerala. An increase of the Mg, Fe and decrease of Ca and Mn contents from the core towards the rim of garnet in the mafic granulites suggest prograde garnet growth. The prograde path was followed by peak metamorphism at a temperature of c. 800°C and a pressure of c. 7.5kbar as computed by isopleths of XMg garnet, XCa garnet and XAn plagioclase. The resorption of garnet in various reaction textures and the development of spectacular orthopyroxene–plagioclase, biotite–quartz and hornblende–plagioclase symplectites characterize the subsequent stages of metamorphism. The PT path is characteristically T-convex suggesting an isothermal decompression path and reflects rapid uplift followed by cooling of a tectonically thickened crust. Diffusion modeling of Fe–Mg exchange between garnet and hornblende suggests a near-peak cooling rate of 10–70°C/myr. Such cooling rates are too high to be accounted for by normal isostatic uplift and erosion and suggest that the terrain was tectonically exhumed. Charnockites are richer in SiO2 and lower in MgO and CaO when compared to mafic granulites. Their REE patterns are relatively flat and show prominent negative europium anomalies. The mafic granulites are metamorphosed tholeiitic basalts as revealed by major- and trace-element geochemistry.

Inhibited eclogitization and consequences for geophysical rock properties and delamination models: Constraints from cratonic lower crustal xenoliths

Studies on lower crustal and mantle xenoliths as well as geophysical data provide important information on the cratonic lithosphere. While geothermobarometric calculations of a majority of mantle xenoliths are in agreement with the typically low surface heat flow values of a craton (~40mW/m2), P–T estimates for lower crustal xenoliths deviate significantly from the cratonic geotherms. Independent from the individual cratonic history, the temperatures are ~200–300°C higher than what is expected at the base of the lower crust (~500–600°C at ~1.3–1.6GPa). Possible explanations may be a lack of equilibration to the cratonic geotherm or a relatively recent localized heat input. The presence of granulitic rocks under eclogite-facies conditions which are expected to prevail in the lower cratonic crust has consequences for the interpretation of geophysical rock properties. A mafic granulite which has been preserved under eclogite-facies conditions has densities and P-wave velocities similar to a felsic composition equilibrated to eclogite-facies conditions. Furthermore, phase diagrams calculated from xenolith bulk compositions demonstrate that eclogitization at relatively high temperatures as required for delamination of continental crust can only be triggered at significantly higher pressures than lithostatic at the base of the lower crust. As long as P–T conditions and the rock composition entail the assemblage to be granulitic, the addition of fluid at temperatures above 800°C will not result in eclogitization, but rather in melt generation. This can also lead to an increase in density of up to 3%, however, this is strongly dependent on the amount of water saturation.

Mafic granulite rheology: Implications for a weak continental lower crust

The “jelly sandwich” model of continental rheology, long the consensus model, has recently been challenged. The controversy is based on the rheology of mafic granulite that is thought to make up most of the lower continental crust. However, because of technical issues in experimental rock deformation, there are very few data on this problem. Here we report the rheology and fabrics of a reconstituted, fine-grained nominally dry (i.e., no hydrous mineral) mafic granulite (57% plagioclase+24% clinopyroxene+14% orthopyroxene+5% opaque minerals, 0.16–0.28wt% H2O) deformed at 1.1–1.2GPa pressure in a modified Griggs-type deformation apparatus. The rheology of this mafic granulite can be described by the constitutive equation of ε̇=Aσ3.2±0.4exp(−(244±35kJ/mol)/RT) where ε̇ is in s−1, σ in MPa, T in Kelvin and A=10−2.0±1.6MPa−3.2s−1. Our results provide new experimental evidence in support of the “jelly sandwich” lithosphere strength model and imply that mafic granulite with a moderate amount of water (>0.05–0.08wt% H2O) is likely to be a weak layer in the lithosphere. Both pyroxenes and plagioclase develop pronounced fabrics in responding to axial deformation. The deformation mechanism is dislocation creep with (100)[001], and (001)[100]/(010)[100] being the dominant slip systems for pyroxenes and plagioclase, respectively. The low strength of mafic granulite is ascribed largely to the significant weakening effect of dissolved water in pyroxenes and plagioclase. A weak continental lower crust has many important implications for geodynamics of crust–mantle interactions, such as lower crust channel flow and delamination.

C . 1.85 Ga HP granulite-facies metamorphism in the Dunhuang block of the Tarim Craton, NW China: evidence from U–Pb zircon dating of mafic granulites

Mafic granulites of the Dunhuang block, eastern Tarim Craton, record high-pressure granulite-facies metamorphism followed by a medium- to low-pressure granulite-facies metamorphic overprint, suggesting a clockwise P–T path and implying an environment of collisional orogenesis. Zircon U–Pb ages of two samples are 1834 ± 12 Ma and 1842 ± 5 Ma. Mineral inclusions in zircon indicate that these ages record the high-pressure granulite-facies event, suggesting that the Tarim Craton may be related to the c . 1.85 Ga assembly of the Columbia supercontinent. Similar c . 1.85 Ga metamorphism in both the Dunhuang block and the Alxa block of the North China Craton supports their correlation across the Altyn Tagh fault. Supplementary material: Analytical procedures, mineral abbreviations, mineral chemistry and U–Pb dating data are available at www.geolsoc.org.uk/SUP18541 .

Petrology and metamorphic P–T path of high-pressure mafic granulites from the Jiaobei massif in the Jiao-Liao-Ji Belt, North China Craton

High-pressure mafic granulites from the Jiaobei massif in the southern segment of the Jiao-Liao-Ji Belt (JLJB) situating in the Eastern Block of the North China Craton (NCC) were examined based on an integrated study of petrology, mineral chemistry and pseudosection modeling. Microstructural relationships of the high-pressure mafic granulites indicate four metamorphic stages, including the pre-peak (M1) stage indicated by garnet core+inclusion-phase minerals of hornblende+clinopyroxene+plagioclase+quartz±epidote±rutile, the peak (M2) stage featured by garnet mantle/rim+matrix-phase minerals of clinopyroxene+plagioclase+quartz±rutile, the post-peak (M3) stage characterized by the clinopyroxene+plagioclase+orthopyroxene symplectite, and the retrograde (M4) stage represented by the hornblende+plagioclase symplectite and symplectic clinopyroxene+ilmenite+quartz. By applying the THERMOCALC (Version 3.33) technique in the NCFMASHTO system, the P–T conditions are estimated at 9.4–9.8kbar and 690–702°C for the pre-peak (M1) stage, 13.1–15.1kbar and 780–890°C for the peak (M2) stage, 7.8–8.4kbar and 820–880°C for the post-peak (M3) stage and the 6.4–7.2kbar and 690–760°C for the retrograde (M4) stage. These mineral assemblages and their P–T conditions define a clockwise P–T path for the high-pressure mafic granulites in the Jiaobei massif, suggesting that the rocks underwent near-isothermal decompression and subsequent cooling after the peak metamorphism, which implies that the southern segment of the JLJB must have experienced subduction-/collision-related tectonic processes at ca. 1.93–1.90Ga. Therefore, the Jiao-Liao-Ji Belt may represent another Paleoproterozoic collisional belt along which the Longgang and Langrim Blocks amalgamated to form the Eastern Block of the North China Craton.

Petrogenesis of Mafic to Felsic Lavas from the Oligocene Siebengebirge Volcanic Field (Germany): Implications for the Origin of Intracontinental Volcanism in Central Europe

Magmatism in the Cenozoic Central European Volcanic Province (CEVP) has been related to two geodynamic scenarios, either extensional tectonics in the north Alpine realm or upwelling of deep mantle material. The Oligocene ( 30^19 Ma) Siebengebirge Volcanic Field (SVF) is a major part of the German portion of the CEVP and consists of erosional remnants of mafic to felsic volcanic edifices. It covers an area of 35 km (NW^SE) by 25 km (SW^NE) with eruptive centres concentrated near the eastern shore of the Rhine river in the vicinity of the city of Bonn. Mafic rocks in the SVF comprise strongly SiO2-undersaturated basanites to alkaline basalts. Occurrences of alkaline basalts are confined to an inner NW^SE-striking zone, whereas the more SiO2-undersaturated basanites dominate the western and eastern periphery of the SVF. Radiogenic isotope compositions (Sr/Sr 0·70335^ 0·70371; eNd þ3·1 to þ4·5; eHf þ6·5 to þ8·0; Pb/Pb 19·46^19·69; Pb/Pb 15·63^15·66; Pb/Pb 39·34^ 39·62) indicate a common asthenospheric mantle end-member with HIMU-like characteristics for all mafic rocks, similar to the European Asthenospheric Reservoir (EAR). A lithospheric mantle source component with a residual K-bearing phase (phlogopite or amphibole) is inferred from negative K anomalies. Incompatible trace element modelling indicates that melting took place in the spinel^garnet transition zone with low degrees of melting at higher pressures generating the basanitic magmas (LaN/YbN1⁄4 20^25), whereas the alkaline basalts (LaN/YbN1⁄414^18) are the result of higher melting degrees at shallower average melting depths. Differentiation of basanitic primary melts generated tephritic to tephriphonolitic magmas that, for instance, erupted at the Lo« wenburgVolcanic Complex in the central SVF. Latites and trachytes, such as the prominent Drachenfels andWolkenburg protrusions, are more common in the central portion of the SVF.These compositions originate from parental alkaline basaltic melts. All differentiated samples show evidence for crustal contamination, possibly with lowerto mid-crustal material comprising mafic granulites as found in Eifel basalt xenoliths and metapelites. Based on the spatial and temporal distribution of the various volcanic rock types, a model for the temporal evolution of the SVF can be proposed. During the initial phase of volcanism, low-degree basanitic melts were generated as a result of decompression following tectonic rifting and formation of the Cologne Embayment, a northward extension of the Rhine Graben. In a second stage, alkali basalts were generated at shallower depths and higher degrees of melting as a result of continued lithospheric thinning and passive upwelling of asthenospheric mantle. These conclusions strengthen previous models suggesting

Evidence for middle Triassic to Miocene dual subduction zones beneath the Shan–Thai terrane, western Thailand from magnetotelluric data

Kanchanaburi province, western Thailand, is in the middle of Shan–Thai terrane. Paleomagnetic and geological data support the hypothesis of dual subduction zones beneath the Shan–Thai terrane. During the late Triassic there was a westward subduction in the east where the Lampang-Chiang Rai block subducted beneath the Shan–Thai terrane. In addition, in the early Tertiary, the western Burma terrane subducted underneath the Shan–Thai in the west resulting in an eastward subduction. A pioneer survey of this region was therefore conducted using magnetotelluric (MT) techniques with the aim of finding deep structures associated with these ancient subduction zones. 39 MT stations were deployed to cover most of the province. Resistivity structures are obtained from the 3-D inversion. The near surface resistivity structures correspond well with the surface geology. The mid and lower crusts are conductive and this is interpreted as the crusts being composed of mafic granulites of 3% porosity in which the remnant dehydrated fluid was accumulated during the subduction. In addition, the westward and eastward subduction zones generated mafic/ultramafic intrusions which appear as two conductive zones on the east and west, respectively. These two conductors and the conductive lower crust support the hypotheses of the middle Triassic to Miocene dual subduction zones.

Role of H2O in the formation of garnet coronas during near‐isobaric cooling of mafic granulites: the Tasiusarsuaq terrane, southern West Greenland

AbstractThe Mesoarchaean Tasiusarsuaq terrane of southern West Greenland consists of Tonalite–trondhjemite–granodiorite gneisses and, locally, polymetamorphic mafic and ultramafic rocks. The terrane experienced medium‐pressure granulite facies conditions during M1A in the Neoarchean, resulting in the development of two‐pyroxene melanosome assemblages in mafic granulites containing garnet‐bearing leucosome. Reworking of these rocks during retrogression introduced garnet to the melanosome in the form of overgrowths, coronas and grain necklaces that separate the mafic minerals from plagioclase. NCFMASHTO pseudosection modelling constrains the peak metamorphism during M1A to ∼850 °C and 7.5 kbar at fluid‐saturated conditions. Following M1A, the rocks retained their M1A H2O content and became fluid‐undersaturated as they underwent near‐isobaric cooling to ∼700 °C and 6.5–7 kbar, prior to reworking during M1B. These low H2O contents allowed for the formation of garnet overgrowths and coronas during M1B. The stability of garnet is greatly increased to lower pressure and temperature in fluid‐absent, fluid‐undersaturated mafic rocks, indicating that fluid and melt loss during initial granulite facies metamorphism is essential for the introduction of garnet, and the formation of garnet coronas, during retrogression. The occurrence of garnet coronas is consistent with, but not unique to, near‐isobaric cooling paths.

Age of mafic granulites from the early precambrian metamorphic complex of the Angara-Kan terrain (Southwestern Siberian Craton): U-Pb and Lu-Hf isotope and REE composition of zircon

Age of mafic granulites from the early precambrian metamorphic complex of the Angara-Kan terrain (Southwestern Siberian Craton): U-Pb and Lu-Hf isotope and REE composition of zircon

Generation of new continental crust by sublithospheric silicic-magma relamination in arcs: A test of Taylor's andesite model

The paradox of the Earth's continental crust is that although this reservoir is generally regarded as having differentiated from the mantle, it has an andesitic bulk composition that contrasts with the intrinsic basaltic composition of mantle-derived melts. Classical models for new crust generation from the mantle in two-stage processes fail to account for two fundamental facts: the absence of ultramafic residues in the lower crust and the hot temperature of batholith magma generation. Other models based on the arrival of already-fractionated silicic magmas to the crust have not received the necessary attention. Addition of new crust by relamination from below of subducted materials has been formulated as a process complementary to delamination of mafic residues. Here we show important support to relamination from below the lithosphere as an important mechanism for new crust generation in magmatic arcs of active continental margins and mature intraoceanic arcs. The new support is based on three independent lines: (1) thermo-mechanical modeling of subduction zones, (2) experimental phase relations and melt compositions of subducted materials and (3) geochemical relations between mafic granulites (lower crust) and batholiths (upper crust). The mineral assemblage and bulk geochemistry of lower crust rocks are compared with solid residues left after granite melt segregation. The implication is that an andesite magma precursor is responsible for the generation of new continental crust at active continental margins and mature oceanic arcs. According to our numerical and laboratory experiments, melting and eventual reaction with the mantle of subducted oceanic crust and sediments produce the andesite magmas. These ascend in the form of mantle wedge diapirs and are finally attached (relaminated) to the continental crust, where they crystallize partially and produce the separation of the solid fraction to form mafic granulites (lower crust) and granitic (sl) liquids to form the batholiths (upper crust).