Granulite Facies Metapelites Research Articles

Timing and duration of discrete tectono-metamorphic events of the polymetamorphic high-grade Central zone of the Limpopo Belt (South Africa): Insight from in situ geochronology of monazite and zircon

The Central Zone of the Limpopo Belt in southern Africa is a complex polymetamorphic terrane that has experienced three metamorphic events, at 3.22 Ga (M1), 2.71–2.56 Ga (M2) and 2.02 Ga (M3). On the basis of a previous detailed petrological and geothermobarometrical study of a newly discovered supracrustal enclave within the Bulai Pluton (ca. 2.61 Ga granitoid gneiss in the Central Zone), we conducted a geochronological in situ study of texturally and chemically different monazite and zircon grains, which occur respectively as inclusions in garnet, in decompressional cordierite coronas around garnet and as matrix grains in polymetamorphic granulite-facies metapelites. Secondary-ion mass spectrometry (SIMS) in situ analyses of the monazite and zircon grains in thin sections, complemented by analyses of separated zircon grains, allowed the texturally-controlled direct determination of U-Pb and 207Pb/206Pb ages at a scale of < 10 μm. A concordant age of ca. 2.71 Ga derived from zircon included in garnet is interpreted to reflect the age of initial garnet growth during the prograde M2 metamorphism. Zircons in the cordierite coronas around garnet revealed a slightly younger concordant age of ca. 2.56 Ga. Zircon growth in the coronas is related to the retrograde breakdown of garnet (Zr-bearing) through to the decompression reaction garnet + sillimanite + quartz → cordierite + zircon and hence the ca. 2.56 Ga zircon age is interpreted to reflect the timing of post-peak decompressional uplift. The zircon ages constrain the duration of M2 metamorphism to approximately 145 Myr. A concordant 2.71 Ga age of large monazite inclusions in garnet cores corroborates the 2.71 Ga zircon age of initial garnet growth during the prograde M2 metamorphism. Core domains of the largest monazite in cordierite corona revealed ages between 2.5 and 2.0 Ga that are interpreted to reflect (multiple) Pb-loss during the M3 granulite-facies metamorphism. Other monazites in cordierite coronas and matrix showed an age of ca. 2.0 Ga, reflecting complete re-equilibration or new growth during M3 overprinting.Owing to the preservation of the original micro-textures related to the growth of zircon and monazite we were able to link the age data with the P-T record, and refine the timing and duration of the M2 granulite-facies metamorphism (approximately 2.71–2.56 Ga) in the Limpopo Belt that was previously dated at ca. 2.62 Ga by zircon separation techniques. The ca. 145 Ma duration of granulite-facies metamorphism is consistent with a tectonic model of long-lasting asthenosphere upwelling and subsequent continent–continent collision in the late Neoarchean. Decompression triggered by post-collisional extension occurred at 2.56 Ga. Paleoproterozoic M3 granulite-facies metamorphism at ca. 2.0 Ga caused the widespread re-setting of the U-Pb and Pb-Pb data that also affected most zircon and monazite grains. This study reveals that precise dating of the timing and duration of distinct tectono-metamorphic events by texturally-controlled in situ geochronology allows a much better insight into the complex geodynamic evolution of high-grade polymetamorphic terranes.

Mineral phase equilibria and zircon geochronology constraining the P–T–t path of granulite–facies metapelites of the Mashan Complex in the Shangsanyang area, Eastern Heilongjiang Province, China

The Mashan Complex is the oldest metamorphic rock series in the Jiamusi Massif, which consists mainly of amphibolite to granulite facies metamorphic rocks accompanied by various granitic rocks. Herein, we investigate the pelitic granulites from this complex as well as associated granites and identify four distinct mineral assemblages corresponding to different metamorphic stages. The prograde metamorphism (M1) is recorded by the core of the garnet and inclusions within: Grtcore–Bi–Sil–Qz. The peak granulite facies metamorphism (M2) is distinguished by the garnet mantle and the related mineral assemblages in the matrix: Bi–Kfs–Grtmantle–Ilm–Sil–Qz. The post-peak decompression stage (M3) is characterized by the formation of cordierite corona and minerals in the cordierite of the matrix: Bi–Liq–Crd–Pl–Kfs–Grt–Ilm–Sil–Qz–Mt. The retrograde stage (M4) is featured by the garnet rim and the nearby assemblages: Bi–Crd–Pl–Kfs–Grtrim–Ilm–Sil–Qz–Mt. According to the phase equilibria modeling, the metamorphic P–T conditions from M2 to M4 were estimated as 6.5–8.5 kbar/815–825 °C, 5.3–5.7 kbar/780–795 °C, and 4.5–5.3 kbar/730–775 °C, respectively, which together define a clockwise P–T path. LA–ICP–MS U–Pb dating for zircons from four metamorphic rocks shows that most of the detrital material came from middle Paleoproterozoic, middle Mesoproterozoic and early Neoproterozoic source rocks. In addition, metamorphic zircon grains in the metamorphic rocks yield two groups of ages at ca. 530 Ma and ca. 500 Ma, which are regarded as the peak of granulite-facies metamorphic (M2) and the retrograde cooling metamorphic (M4) ages, respectively.

Microstructural features of garnet porphyroblasts in the granulite facies metapelites of the highland complex of Sri Lanka: evidence for a multiple growth history of garnet and early development of crenulation cleavage

Journal of the Geological Society of Sri Lanka is a peer-reviewed and open access journal published by the Geological Society of Sri Lanka. It aims to publish the most topical and highest quality papers, summarizing the results of recent research across all sub-disciplines of the Earth Science. Papers are frequently interdisciplinary covering both pure and applied fields of Geology. Contributions often refer to local, regional and/or international studies and emphasize the development of the understanding of fundamental geological processes.

Garnet trace element geochemistry as a sediment provenance indicator: An example from the Qaidam basin, northern Tibet

Abstract Major element geochemistry of detrital garnet is a widely-used approach for sedimentary provenance analysis, in particular for unravelling parent-rock lithology of siliciclastic sediments. However, garnets from felsic crystalline rocks (e.g., intermediate–acidic igneous rocks and metapelites) often have similar major element composition. This results in ambiguity of provenance interpretation in most cases. Here, we collect trace element data of garnets from different rock types to explore the feasibility of detrital garnet trace element geochemistry in sediment provenance analysis. The Qaidam basin in northern Tibet is taken as an example and provenance of Cenozoic sedimentary rocks therein is interpreted based on detrital garnet trace element composition. We find that garnets in intermediate–acidic igneous rocks have much higher heavy rare earth element (ΣHREE > 600 ppm) and yttrium (Y > 800 ppm) abundances than garnets in metapelites (ΣHREE 4 ppm) and zinc (Zn > 150 ppm) abundances than garnets in low-, medium-grade metapelites (up to amphibolite facies). Based on these findings, we suggest that Fe- and Mn-rich detrital garnets in the Cenozoic sedimentary rocks from the northern Qaidam basin were probably derived from low-, medium-grade metapelites and Mg-rich, Ca-poor detrital garnets therein were most likely derived from granulite-facies metapelites. This provenance interpretation supports that the Qilian Mountains to the north was the major source for the northern Qaidam basin and major deformation style along the Altyn Tagh Fault to west have been dominated by large-amplitude lateral offset and extrusion, rather than crustal thickening and uplift during the early Cenozoic. This study emphasizes high potential of detrital garnet trace element geochemistry in provenance analysis and reconstruction of tectono-sedimentary evolution of clastic sedimentary basins.

Paleoproterozoic amphibolite facies retrogression and exhumation of Archean metapelitic granulites in the Southern Marginal Zone of the Limpopo Belt, South Africa

Granulite-facies terrains that have undergone extensive retrogression provide insights into the recycling of volatiles in the crust during orogenesis. The timing of retrogression is key to understanding the geodynamic circumstances of such events. This study has investigated amphibolite-facies retrogression of melt-depleted granulite-facies metapelites from the South Marginal Zone (SMZ) of the Limpopo belt. The granulites formed via incongruent partial melting reactions that consumed biotite and produced garnet and rutile as prominent peritectic products at ~2.7 Ga, under conditions of ~11 kbar and 875 °C. Peak metamorphic garnet is replaced by cordierite + orthopyroxene symplectites, with cordierite subsequently partially replaced by fine-grained intergrowths of amphibolite-facies assemblages, including rutile. Zr-in-rutile thermometry yielded a temperature range of 731–846 °C for the granulite-facies rutiles and 629–690 °C for rutile in the retrograde assemblage; all rutile generations yielded a youngest U-Pb age of ~2.1 Ga. Some granulite-facies rutiles record older dates (2.55─2.25 Ga) which are non-systematic with regard to crystal size and represent non-reset relicts of metamorphic recrystallization during a potentially complex history of amphibolite-facies metamorphism between ~2.7 and 2.2 Ga. The youngest ~2.1 Ga rutile U-Pb age represents the time of final exhumation-related cooling of the rocks following a discrete Paleoproterozoic amphibolite-facies retrograde event, Coexisting orthoamphiboles in the peak metamorphic assemblage indicate a maximum peak temperature of 625 °C. The lack of age-size correlation and intragrain age zoning in rutile suggest fast cooling following the amphibolite-facies retrograde event which is inferred to be not much older. U-Pb dating of zircon from the same rocks confirms that the metapelites underwent granulite-facies metamorphism at 2695 ± 35 Ma. Thus, the metapelites of the SMZ record both an Archean granulite-facies event and a polymetamorphic Paleoproterozoic amphibolite-facies history that culminated in a mid-amphibolite facies peak inducing partial retrogression of the granulites. The Paleoproterozoic amphibolite-facies event is tentatively correlated with mantle melting documented between ~2.3 and 2.1 Ga to produce mafic and intermediate volcanic rocks in the Pretoria Group on the Kaapvaal Craton.

Petrology, phase equilibria modelling, and in situ zircon and monazite geochronology of ultrahigh-temperature granulites from the khondalite belt of southern India

The granulite facies metapelite (khondalite) belt in the Trivandrum Block of southern India has been central to investigations on extreme crustal metamorphism associated with the final assembly of the Gondwana supercontinent during late Neoproterozoic-Cambrian. Here we investigate garnet-sillimanite-cordierite-spinel-bearing metapelites from this khondalite belt using state-of-the-art petrologic, mineral phase equilibria modelling, and coupled zircon and monazite U-Pb geochronology to characterize the nature and timing of metamorphism and their tectonic implications. From textural studies and mineral phase equilibrium modelling, we infer that Sill + Grt + Crd representing prograde metamorphism was stable at 6–9 kbar and 760–790 °C. Equilibrium spinel-quartz assemblage suggests peak metamorphic conditions of 6.5–7 kbar and 1010–1030 °C consistent with ultra-high temperature metamorphism in the Trivandrum Block. Growth of cordierite and biotite at the expense of garnet correlates with retrograde metamorphism at 4.5–6.5 kbar and 770–950 °C. Our results allow an interpretation of the prograde, peak, and retrograde P–T conditions of the khondalites, where from the peak ultrahigh-temperature stage of >1000 °C, the rocks underwent isothermal decompression as indicated by garnet breakdown to cordierite, followed by isobaric cooling, corresponding to an overall clockwise P–T evolution. We present results from U-Pb geochronology of zircon as well as LA-ICPMS data on monazite grain separates. The zircon grains show two distinct age peaks, with Paleoproterozoic cores surrounded by Late Neoproterozoic-Cambrian rims or recrystallized domains. The younger ages around 550–560 Ma are similar to the lower intercept age, whereas the wide range of Paleoproterozoic ages fall along a discordia. The LA-ICP-MS monazite U-Pb data also show two distinct age populations at Paleoproterozoic and latest Neoproterozoic-Cambrian, although the Paleoproterozoic population is scarce. We correlate the late Neoproterozoic-Cambrian ages to the ultrahigh-temperature metamorphic event, associated with the assembly of the Gondwana supercontinent. The wide range of monazite ages indicate that high temperature metamorphism was long-lived.

Coupled laser Raman spectroscopy and carbon stable isotopes of graphite from the khondalite belt of Kerala, southern India

Graphite in high grade meta-supracrustal rocks provide an important proxy to characterize biogenic and abiogenic carbon sources. Here we investigate graphite in various associations within the granulite facies metapelites (khondalites) of Kerala in southern India with a view to characterize the textural varieties and their genesis through the application of laser Raman spectroscopy and carbon isotopes. We trace the increasing intensity of metamorphism from the Raman spectra of the different graphite types. The δ13C values of the graphite show two distinct origins; the relatively lighter values in the range of −17.5‰ to −20.7‰ represent a mixture of biogenic and magmatic carbon, whereas the heavier isotopes in the range of −11.7‰ to −13.8‰ are suggested to present graphite precipitation from CO2-rich fluids from sub-crustal magmatic sources. Our study provides insights on the spectral properties of graphite in relation to the intensity of metamorphism and also reveals both biogenic and abiogenic sources of carbon.

Paleoproterozoic high-pressure-high-temperature pelitic granulites from Datong in the North China Craton and their geological implications: Constraints from petrology and phase equilibrium modeling

Paleoproterozoic granulite-facies metapelites are widespread in northern part of the North China Craton (NCC), and their metamorphic conditions and P-T paths provide important constraints for tectonic evolution of the NCC. However, it is still controversial on the metamorphic evolution of these rocks due to variations of retrograde modification. In combination with detailed petrological studies, geological mapping, and geochronology, we here present new evidence from Gushan, Datong, conjunction zone of the Ji’ning Complex and Huai’an Complex to constrain P-T evolution of the metapelites. Field investigations reveal that the pelitic granulites are spatially associated with high-pressure (HP) mafic granulites, showing concordant characteristics of deformation and metamorphism. Fine mineralogical and microstructural studies reveal that kyanite-bearing HP granulite-facies indicators preserved in garnet porphyroblasts of the pelitic granulites. It suggests the previously assumed moderate- to low-pressure (MP-LP) and high- to ultrahigh-temperature (HT-UHT) pelitic granulites have experienced HP granulite-facies metamorphism.Four metamorphic generation assemblages (M1–M4) are recognized in the studied samples. The pre-Pmax assemblages (M1) are characterized by mineral inclusions within garnet core, including Bt-Qz-Rt±Ms±Liq, representing the late stage of prograde metamorphism. The Pmax assemblages (M2, HP granulite-facies) are represented by garnet mantle and its inclusion-type minerals, i.e., Grt-Ky-Kfs-Bt-Rt-Liq-Qz±Ms±Zn Spl. It is followed by heating until the maximum of temperature stage (M3, Tmax), featured by disappearance of biotite and transition from kyanite into sillimanite, with mineral assemblages of Grt-Sil-Kfs-Liq-Qz-Ilm±Zn Spl. The cooling stage (M4) is featured by breakdown of garnet to Bt+Qz±Pl±Sil±Ilm. Phase equilibrium modeling and titanium-in-biotite thermometer are combined to constrain the P-T evolution of the studied samples. The P-T conditions for the M1–M4 are respectively estimated around ∼11–11.5kbar/∼770–790°C, ∼10.3–13.8kbar/∼820–835°C, ∼6.5–9.5kbar/∼850–925°C, ∼6kbar/∼645–761°C, which defined a clockwise P-T path. Zircon U-Pb ages of two representative samples demonstrated comparable age clusters of ∼1950Ma and ∼1850Ma, which are interpreted to represent the timing of HP and MP-LP granulite-facies stages, respectively. Combining with other studies of the NCC, we propose that the pelitic granulites and associated HP mafic granulites have experienced the common metamorphic processes from peak stage (HP-HT granulite-facies) to retrograde metamorphism. Due to intensive and pervasive overprint of sillimanite-type granulite-facies metamorphism, the HP granulite-facies mineral assemblages of the pelitic granulites are partially or completely reworked. The metamorphic peak conditions and relatively high thermal gradients of these granulite-facies rocks suggest that their formation is probably in response to a crustal-scale subduction-collision process during the late Paleoproterozoic.

Hydrothermally altered volcanic rocks metamorphosed at granulite-facies conditions: an example from the Grenville Province

A 1.2 Ga association of aluminous gneisses, garnetites, and white felsic gneisses of andesitic composition in the southern Manicouagan area (central Grenville Province) provides evidence consistent with protolith formation and hydrothermal alteration in a submarine volcanic environment. In addition to field relations, potential relics of quartz phenocrysts in the aluminous gneisses, revealed by SEM–MLA (scanning electron microscope with a mineral liberation analysis software) imaging, are consistent with a volcanic precursor. Moreover, in these rocks, aluminous nodules and seams of sillimanite are considered to represent metamorphosed hydrothermal mineral assemblages and to reflect former pathways of hydrothermal fluid. These features are preserved despite the Grenvillian granulite-facies metamorphic overprint and evidence of partial melting. In addition, the garnetites are inferred to represent hydrothermally altered products of the white gneisses, based on the gradational contacts between the two rock types. The compositional ranges of minerals are generally similar to those of granulite-facies metapelites, but moderately elevated contents of Mn in garnet from the garnetites, and Zn in spinel from the aluminous gneisses, are consistent with hydrothermal addition of these elements to the protolith. The most prominent alteration trends are an increase in Fe–Mg–Mn from the white gneisses to the aluminous gneisses and the garnetites, and a trend of increasing alumina index in some white gneisses, suggesting mild argillic alteration. The new findings highlight the preservation of early hydrothermal alteration in high-grade metamorphic belts in the Grenville Province, and these altered rocks are potential targets for exploration.

Thermal gradient and geochronology of a Paleozoic high-grade terrane in the northeastern Cathaysia block, South China

The Chencai terrane in the northeastern Cathaysia Block forms part of the northeast extension of the Paleozoic orogeny in South China. Recent field investigations have revealed the occurrence of high grade rocks in this terrane including granulite facies metapelites (khondalites) and orthopyroxene-bearing granulites. However, the thermal gradients and metamorphic history of this region have not been well constrained, thus impeding a better understanding of the geodynamic history of the Paleozoic orogeny in South China. Here we report results from a systematic study on the metamorphic PT conditions and geochronology of the orthopyroxene granulites and metasedimentary rocks (metapelites) from the Chencai area. The orthopyroxene granulites preserve granulite facies peak assemblage of Opx+Grt+Pl+Kfs+Qtz and a retrograde P-T trajectory. Peak PT conditions are 845–860°C and 0.83–0.86Gpa, corresponding to a thermal gradient of ~30°C/km which is much hotter than the normal thermal gradient of continental crust. Mineral compositions of the metasedimentary rocks re-equilibrated during retrograde metamorphism so that the peak PT conditions recorded by these rocks are much lower than those obtained from the orthopyroxene granulite. Peak PT conditions are 710–720°C, 0.57–0.58Gpa for garnet-biotite gneiss and 760–780°C, 0.60–0.62Gpa for biotite garnetite. Zircon U-Pb dating shows that this episode of granulite facies metamorphism occurred at 430–450Ma and the detrital population shows minor or no contribution from the basement rocks of the Cathaysia Block. The Neoproterozoic rocks occurring along the Jiangnan belt are considered to have provided the sedimentary source as indicated by the two main age peaks of 0.79Ga and 0.83Ga from the detrital zircon cores. Lu-Hf isotope data on the zircons indicates extensive alteration and little new zircon growth during the Paleozoic metamorphism. The εHf(t) values and model ages (TDM1) of zircons suggest that the source rocks involved juvenile crustal components with Hf model ages of 1.1–1.5Ga. Sedimentation of such a continental shelf sequence as well as the high thermal gradient of ~30°C/km confirm the previously proposed continental arc formation and maturation model in the Chencai region.

Petrology, phase equilibria and monazite geochronology of granulite-facies metapelites from deep drill cores in the Ordos Block of the North China Craton

Among the various Precambrian crustal blocks in the North China Craton (NCC), the geology and evolution of the Ordos Block remain largely enigmatic due to paucity of outcrop. Here we investigate granulite-facies metapelites obtained from deep-penetrating drill holes in the Ordos Block and report petrology, calculated phase equilibria and in-situ monazite LA-ICP-MS geochronology. The rocks we studied are two samples of cordierite-bearing garnet–sillimanite–biotite metapelitic gneisses and one graphite-bearing, two-mica granitic gneiss. The peak metamorphic age from LA-ICP-MS dating of monazite in all three samples is in the range of 1930–1940Ma. The (U+Pb)–Th chemical ages through EPMA dating reveals that monazite occurring as inclusions in garnet are older than those in the matrix. Calculated metamorphic phase diagrams for the cordierite-bearing metapelite suggest peak P–T conditions ca. 7–9kbar and 775–825°C, followed by decompression and evolution along a clockwise P–T path. Our petrologic and age data are consistent with those reported from the Khondalite Belt in the Inner Mongolia Suture Zone in the northern part of the Ordos Block, suggesting that these granulite-facies metasediments represent the largest Paleoproterozoic accretionary belt in the NCC.

Constraints on the thermal evolution of the Adriatic margin during Jurassic continental break-up: U–Pb dating of rutile from the Ivrea–Verbano Zone, Italy

The Ivrea–Verbano Zone (IVZ), northern Italy, exposes an attenuated section through the Permian lower crust that records high-temperature metamorphism under lower crustal conditions and a protracted history of extension and exhumation associated partly with the Jurassic opening of the Alpine Tethys ocean. This study presents SHRIMP U–Pb geochronology of rutile from seven granulite facies metapelites from the base of the IVZ, collected from locations spanning ~35 km along the strike of Paleozoic fabrics. Rutile crystallised during Permian high-temperature metamorphism and anatexis, yet all samples give Jurassic rutile U–Pb ages that record cooling through 650–550 °C. Rutile age distributions are dominated by a peak at ~160 Ma, with a subordinate peak at ~175 Ma. Both ~160 and ~175 Ma age populations show excellent agreement between samples, indicating that the two distinctive cooling stages they record were synchronous on a regional scale. The ~175 Ma population is interpreted to record cooling in the footwall of rift-related faults and shear zones, for which widespread activity in the Lower Jurassic has been documented along the western margin of the Adriatic plate. The ~160 Ma age population postdates the activity of all known rift-related structures within the Adriatic margin, but coincides with extensive gabbroic magmatism and exhumation of sub-continental mantle to the floor of the Alpine Tethys, west of the Ivrea Zone. We propose that this ~160 Ma early post-rift age population records regional cooling following episodic heating of the distal Adriatic margin, likely related to extreme lithospheric thinning and associated advection of the asthenosphere to shallow levels. The partial preservation of the ~175 Ma age cluster suggests that the post-rift (~160 Ma) heating pulse was of short duration. The regional consistency of the data presented here, which is in contrast to many other thermochronometers in the IVZ, demonstrates the value of the rutile U–Pb technique for probing the thermal evolution of high-grade metamorphic terrains. In the IVZ, a significant decoupling between Zr-in-rutile temperatures and U–Pb ages of rutile is observed, with the two systems recording events ~120 Ma apart.

Improved calibrations for Raman-spectroscopic determinations of CO2 in cordierite using three excitation wavelengths (488, 515 and 633 nm)

Ten natural cordierite single crystals with X Fe in the range 0.044–0.76 and 0.12 to 1.97 wt% CO 2 were investigated with Raman microspectroscopy using three different wavelengths. Improved calibration diagrams for CO 2 determinations in single crystals and thin sections for the 488, 515 and 633 nm laser excitation wavelengths show significantly lower standard deviations for CO 2 determinations (±0.04 to 0.07 wt%) compared to an earlier study done solely at 633 nm. The diagrams are based on the linear relationship between CO 2 in wt.% and the ratios of the Raman peak of CO 2 to two Si-O-stretching vibrations. The consideration of orientation dependencies and the improvement of background corrections and spectra fitting procedures also contributed to the significantly higher precision. Because Fe-incorporation in cordierite leads to structural changes that result in Raman peak shifts and peak broadening, including peaks considered in this investigation, a correction was introduced for precise CO 2 determination on Fe-bearing samples. This improved CO 2 determination method was then applied on a 2D spatial scale to two cordierite grains in a thin section of a granulite-facies metapelite from Kosseldorf in the Sauwald area (southern Bohemian Massif, Upper Austria), which yielded CO 2 contents of 0.27–0.31 ±0.07 wt%.

Hafnium isotopes and Zr/Hf of rutile and zircon from lower crustal metapelites (Ivrea–Verbano Zone, Italy): Implications for chemical differentiation of the crust

We have used granulite facies metapelites from the classic Ivrea–Verbano Zone (IVZ) lower crustal section, northern Italy, to investigate the behaviour of the hafnium (Hf) isotope system in rutile and zircon under conditions of high temperature (700–1050 °C) metamorphism and partial melting. The Hf isotope composition of rutile is shown to be robustly preserved from these conditions, and is also unaffected by retrogression, making it a powerful new tool for metamorphic petrology. Rutiles from our suite of IVZ metapelite samples have crustal Hf isotope signatures (εHf(i)=−6±1 to −13±1 at 288 Ma), with the most strongly crustal signature (εHf(i)=−9.2±0.3 to −12.9±1.0) recorded by narrow slivers of metapelite incorporated within a gabbroic body. The Hf isotope composition of rutile from these metapelite slivers is the direct result of dissolution of ancient detrital zircon with strongly unradiogenic 176Hf/177Hf at ultra-high temperatures associated with the emplacement of the gabbro. Rutile is an important host of Zr and Hf in high temperature metapelites, containing up to 11 000 ppm Zr and 400 ppm Hf. It has subchondritic Zr/Hf (13–32) that is significantly lower than the Zr/Hf of both zircon (35–52) and the starting bulk rock composition (36–40). Dissolution of zircon into extracted melts is expected to lead to a lowering of bulk rock Zr/Hf towards that of rutile. In the IVZ samples this effect is balanced by the strongly superchondritic Zr/Hf of garnet (40–70). The IVZ case study demonstrates the strong control that dissolution of detrital or inherited zircon can exert on the available Hf isotope budget, whether in a metamorphic or magmatic setting. The link between dissolution of accessory phases into melt and changes in trace element and Hf isotope chemistry points to important differences between high- and ultra-high temperature metamorphism in terms of chemical and isotopic differentiation of the crust.

Early Mesoproterozoic metamorphism in the Barossa Complex, South Australia: links with the eastern margin of Proterozoic Australia

LA-ICP-MS U–Pb geochronological data from metamorphic monazite in granulite-facies metapelites in the Barossa Complex, southern Australia, yield ages in the range 1580–1550 Ma. Metapelitic rocks from the Myponga and Houghton Inliers contain early biotite–sillimanite-bearing assemblages that underwent partial melting to produce peak metamorphic garnet–sillimanite-bearing anatectic assemblages. Phase equilibrium modelling suggests a clockwise P–T evolution with peak temperatures between 800 and 870°C and peak pressures of 8–9 kbar, followed by decompression to pressures of ∼6 kbar. In combination with existing age data, the monazite U–Pb ages indicate that the early Mesoproterozoic evolution of the Barossa Complex is contemporaneous with other high geothermal gradient metamorphic terranes in eastern Proterozoic Australia. The areal extent of early Mesoproterozoic metamorphism in eastern Australia suggests that any proposed continental reconstructions involving eastern Proterozoic Australia should share a similar tectonothermal history.

The robustness of the Zr-in-rutile and Ti-in-zircon thermometers during high-temperature metamorphism (Ivrea-Verbano Zone, northern Italy)

This study investigates the behaviour of the Zr-in-rutile and Ti-in-zircon thermometers in granulite facies metapelites from the Ivrea-Verbano Zone lower crustal section. U–Pb ages of zircon constrain the timing of regional amphibolite–granulite facies metamorphism to 316 ± 3 Ma and record zircon recrystallisation and resetting of U–Pb ages at 276 ± 4 Ma and 258 ± 3 Ma. Zr-in-rutile thermometry records peak contact metamorphic temperatures related to intrusion of mafic magmatic rocks and gives peak temperatures between 900–930 °C and 1,000–1,020 °C that are consistent with the geological settings of the samples. Ti-in-zircon temperatures of 700–800 °C and 810–870 °C record growth or re-equilibration of zircon after cooling from peak temperatures. Ti-in-quartz thermometry for one sample records both peak and retrograde temperatures. Some rutiles in all samples record resetting of Zr-in-rutile temperatures at ~750–800 °C. Electron microprobe profiles across individual rutiles demonstrate that Zr expulsion occurred by recrystallisation rather than by diffusive exchange. Exsolution of small needles of baddelyite or zircon from rutile is an important method of Zr redistribution, but results in no net Zr loss from the grain. The demonstration that Zr-in-rutile thermometry can robustly record peak temperatures that are not recorded by any other thermometer emphasises the relevance of this technique to investigating the evolution of high-grade metamorphic terranes, such as those that characterise the lower crust.

Petrology of high-pressure granulite facies metapelites and metabasites from Tcholliré and Banyo regions: Geodynamic implication for the Central African Fold Belt (CAFB) of north-central Cameroon

The Tcholliré and Banyo high-pressure granulites occur mainly as highly strained small lenses, bands or elongated bodies interbedded with gneisses and migmatites in the Central African Fold Belt (CAFB) in north-central Cameroon. They were previously attributed to Palaeoproterozoic but are now shown to be Pan-African. These granulites are made up of two occurrences of metapelites with garnet-kyanite-sillimanite-cordierite-biotite-quartz-plagioclase and metabasites containing garnet-clinopyroxene-orthopyroxene-hornblende-quartz-plagioclase.Eight samples were analysed in detail by electron microprobe for mineral chemistry.Parageneses in both metapelites and metabasites highlight three main stages witnessed by prograde, peak and retrograde mineral assemblages. The prograde stage is preserved as well-defined inclusion trails of kyanite, biotite, plagioclase, quartz, and rutile within porphyroblasts of garnet in metapelites; or with more or less clinopyroxene, hornblende, plagioclase, quartz, ilmenite, rutile and apatite, within porphyroblasts of garnet in metabasites. The peak stage, displaying heterogranular granoblastic texture is characterised by porphyroblastic garnet-kyanite-K-feldspar-biotite in metapelites and garnet-clinopyroxene-plagioclase-quartz in metabasites. This was followed by decompression and cooling during the retrograde stage marked by aggregate of sillimanite prisms presumably after kyanite and cordierite corona around garnet in metapelites and by symplectites or vermicular structures of orthopyroxene-plagioclase and orthopyroxene corona around garnet in metabasites. Geothermobarometric study shows that granulite facies metapelites and metabasites from both Tcholliré and Banyo regions recrystallised under peak pressure-temperature conditions of 13–14kbar and 800–900°C. They experienced similar clockwise P-T path with nearly isothermal decompression.Our results provide evidence for a substantial crustal thickening during the Pan-African continent–continent collision but show that there is no significant isothermal decompression as commonly observed in recent collisional orogens. The reconstructed P-T paths are in some ways reminiscent of the ones reported in Precambrian “mixed-hot orogens”.

Crustal shear (S) velocity and Poisson's ratio structure along the INDEPTH IV profile in northeast Tibet as derived from wide-angle seismic data

SUMMARY From the S-wave data collected along a 270-km-long profile spanning the Kunlun mountains in NE Tibet, 14595 Sg phase arrivals and 21 SmS phase arrivals were utilized to derive a whole-crustal S velocity model and, together with a previously derived P velocity model, a Poisson's ratio (σ) model beneath the profile. The final tomogram for the upper 10–15 km of the crust reveals the lower velocities associated with the predominantly Neogene-Quaternary sediments of the Qaidam basin to the north and the higher velocities associated with the predominantly Palaeozoic and Mesozoic upper crustal sequences of the Songpan-Ganzi terrane and Kunlun mountains to the south. This study finds no evidence that the Kunlun mountains are involved in large-scale northward overriding of the Qaidam basin along a shallow south-dipping thrust. The σ in the upper 10–15 km of the crust are often lower than 0.25, indicating a preponderance of quartz-rich rocks in the upper crust beneath the profile. Below 10–15 km depth, the remainder of the crust down to the Moho has an average σ of 0.24 beneath the Songpan-Ganzi terrane and Kunlun mountains and 0.25 below the Qaidam basin. These low σ are similar to other low σ found along other profiles in the northeastern part of the plateau. Assuming an isotropic situation and no significant variation in σ between 10–15 km depth and the Moho, then the lower crust between 25–30 km depth below sea level and the Moho with P velocities varying from 6.6 km s−1 at the top to around 6.9 km s−1 at the base and σ of 0.24–0.25 should comprise intermediate granulites in the upper part transitioning to granulite facies metapelites in the lower part. As the pre-Cenozoic Qaidam basin crust has probably not lost any of its lower crust during the present Himalayan orogenic cycle in the Cenozoic and only has a σ of 0.245–0.25, then it appears that the pre-Cenozoic Qaidam basin crust involved in the collision is more felsic and thus weaker and more easily deformable than normal continental crust with a global average σ of 0.265–0.27 and the Tarim and Sichuan basin crusts. This situation then probably facilitates the collision and promotes the formation of new high plateau crust at the NE margin of Tibet. South of the Qaidam basin, the crust of the Songpan-Ganzi terrane and Kunlun mountains has an even lower average crustal σ of 0.23–0.24 and is thus presumably even weaker and more easily deformable than the crust beneath the Qaidam basin. This then supports the hypothesis of Karplus et al. that ‘the high Tibetan Plateau may be thickening northward into south Qaidam as its weak, thickened lower crust is injected beneath stronger Qaidam crust'.

High-pressure melting of metapelite and the formation of Ca-rich granitic melts in the Namche Barwa Massif, southern Tibet

A detailed geochemical and geochronological study of anatectic migmatites from the Namche Barwa Massif (NBM), southern Tibet, has been carried out to place important constraints on the thermal and tectonic evolution of the eastern Himalayan syntaxis. SHRIMP zircon U/Pb dating indicates that the granulite-facies metapelite underwent metamorphism at 21.8 ± 0.7 Ma and 24.5 ± 0.7 Ma, respectively. The latter is similar to the timing of partial melting and the formation of Ca-rich leucosomes at ~ 24–25 Ma. These leucosomes are characterized by (1) high CaO, Na 2O, and Na/K ratios; (2) radiogenic Sr ( 87Sr/ 86Sr(t) = 0.7407–0.7904) but unradiogenic Nd (ε Nd(t) = − 7.0 to − 21.2) isotope compositions; (3) depleted HFSE, and (3) variable but depleted HREE relative to their host pelites. Some of the leucosomes show large degrees of Nd isotopic disequilibrium, up to 10 epsilon units with respect to their hosts. These high CaO and Na 2O leucosomes were derived from fluxing melting of metapelite at high pressures. A similar process could have operated during the formation of the Himalayan leucogranites and contributes to the heterogeneities in such granites.

The behavior of lithium in amphibolite- to granulite-facies rocks of the Ivrea–Verbano Zone, NW Italy

To investigate the behavior of Li during high-grade metamorphism and the Li composition of the lower continental crust, the Li concentrations ([Li]) and isotopic compositions (δ 7Li) of amphibolite- to granulite-facies metapelites, leucosomes and metabasites from the Ivrea–Verbano Zone, NW, Italy were determined. The average [Li] of amphibolite facies kinzigites (79 ± 69 μg/g, 2σ) is higher than that of granulite facies stronalites (8 ± 6 μg/g, 2σ) that experienced partial melting via biotite dehydration reactions. Biotite abundance and the [Li] in metapelites correlate positively, reflecting the importance of Mg-bearing phyllosilicates in controlling the Li budget of the metapelites. Despite the loss of Li following biotite breakdown, there is no significant change in δ 7Li, which averages − 1.4 ± 2.0 (2σ, excluding an anomalously light sample) in the kinzigites (amphibolite facies) and + 0.9 ± 2.9 (2σ) in the stronalites (granulite facies). Both average δ 7Li values are comparable with those of other pelitic sediments and likely reflect the δ 7Li of their protoliths. An anomalous kinzigitic sample, with the lowest δ 7Li (− 8.4) and the highest [Li] (120 μg/g), has extremely low Ba and Sr concentrations and the highest CIA value, all indicators of a highly weathered protolith, which was likely isotopically light prior to significant metamorphism. This sample may also have experienced Li addition via diffusion, increasing [Li] and lowering δ 7Li. The leucosomes in the kinzigites and stronalites are interpreted to have formed in different ways; the former precipitated from hydrothermal fluids, have an average [Li] = 13 ± 2.6 μg/g (2σ) and are isotopically similar to the kinzigites (δ 7Li = − 1.0 ± 0.2, 2σ); the latter formed by dehydration melting and have lower [Li] (3 ± 2.6 μg/g, 2σ) that is heavier (δ 7Li = + 6.0 ± 6.3, 2σ) than that of the stronalites or kinzigites. The mineralogical and compositional characteristics of the stronalites are consistent with substantial Li loss during partial melting. A melting model indicates that Li removed from the stronalites may contribute to the formation of S-type granites or Li-enriched pegmatites. Three granulite-facies metabasites have relatively constant [Li] of 3.0–4.2 μg/g (average 3.6 ± 1.2, 2σ) and variable δ 7Li of − 3.2 to + 3.3 (average 0 ± 6.5, 2σ). While these concentrations are typical of those of possible basaltic protoliths, the highly variable δ 7Li suggests Li mobility during metamorphism. Collectively, the Li signatures in granulite facies stronalites and metabasites indicate the [Li] of this section of lower continental crust is close to 8 μg/g or less, and the concentration weighted δ 7Li of this section of the lower continental crust is + 1.0, which is similar to a previous estimate (8 μg/g, + 2.5).