Granulite Facies Rocks Research Articles

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.

Geochronology and geochemistry of deep-seated crustal xenoliths in the northern North China Craton: Implications for the evolution and structure of the lower crust

The age and composition of the lower crust are critical in understanding the processes of continental formation and evolution, and deep-seated granulite xenoliths can offer direct information on the lower crust. Here, we report mineral chemistry, whole-rock major and trace elements, Sr–Nd isotopes and zircon U–Pb–Hf results for a suite of deep-seated crustal xenoliths, recently discovered in the Cenozoic basalts of the Nangaoya area in the northern part of the North China Craton (NCC). Based on the P–T estimates, these xenoliths including mafic, intermediate and felsic granulites and hornblendites were sampled from different levels of the lower crust. While a hornblendite has a flat REE pattern, all other xenoliths display LREE enrichment and depletion of Nb, Ta, Th and Ti. The mafic granulite xenolith has relatively high whole-rock εNd(t) value of −13.37, and yields Mesozoic (188–59Ma) zircons ages with high εHf(t) values from −15.3 to −9.2. The garnet-bearing intermediate granulite-facies rocks show low εNd(t) values from −16.92 to −17.48, and reveal both Paleoproterozoic (1948Ma) and Mesozoic (222–63Ma) zircon U–Pb ages. Their Mesozoic zircons have lower εHf(t) values (from −18.4 to −13.8) than those from the mafic xenolith. The remaining intermediate to felsic xenoliths show Paleoproterozoic zircon ages, and the lowest εNd(t) values (from −20.78 to −24.03).The mafic–intermediate granulites with Mesozoic zircons originated from the interaction of lower crust-derived magmas with mantle melts, with higher proportions of mantle magmas involved in the generation of mafic granulite, whereas intermediate to felsic xenoliths without Mesozoic zircons represent ancient Paleoproterozoic to Neoarchean deep crust. These deep-seated xenoliths reveal complicated crustal evolution processes, including crustal growth during Neoarchean (2.5–2.7Ga), middle Paleoproterozoic (2.2–2.1Ga) and Mesozoic, and reworking during early Paleoproterozoic, late Paleoproterozoic and Mesozoic related to magmatic underplating. The integrated analyses of lithological, geochemical and age data for a suite of deep-seated xenoliths show that the lower crust in the Nangaoya area is temporally and compositionally zoned. The upper part of the lower crust mainly comprises Neoarchean to Paleoproterozoic intermediate–felsic rocks with intercalated hornblendites, the majority of which record ~1950 and ~1850Ma metamorphism; the middle part is dominated by a Paleoproterozoic and Mesozoic intermediate garnet-bearing granulite-facies hybrid layer; and the lowermost crust is represented by a Mesozoic mafic granulite layer, which was significantly modified by episodic magmatic underplating. Such a modification induced by crust–mantle interaction can result in Mesozoic ages and more mafic components for xenolith granulites, and thus is an effective mechanism to explain the differences between exposed and xenolithic granulites.

Geochemistry, U-Pb dating, and Lu-Hf isotopes of zircon and monazite of porphyritic granites within the Jiao-Liao-Ji orogenic belt: Implications for petrogenesis and tectonic setting

Abundant porphyritic granites, including Grt-bearing and Bt-bearing porphyritic granites, and flesh-red porphyritic granites, as various scale batholiths are widely distributed within the Jiao-Liao-Ji belt (JLJB), which extends for a distance of at least 1000km, rather than being limited to sporadic outcrops in southern Liaoning and Jilin as stated in previous studies. On the basis of morphology, CL and BSE zoning patterns, and mineral inclusions of dated zircon and monazite grains from samples of various porphyritic granites, the grains exhibit evident magmatic zones with distinct cores and rims and preserve felsic mineral inclusions of Qtz+Kfs+Pl+Qtz±Ap in both domains. The magmatic zircon cores and rims from 18 dated samples yielded two distinct weighted mean age groups, from 1872±9 to 1866±7Ma and 1851±14 to 1842±10Ma, with age peaks at 1869±1.7 and 1848±2.4Ma, respectively. Similarly, the magmatic cores and rims of monazites from 10 dated samples also showed two distinct weighted mean age groups, from 1870±3 to 1864±2Ma and 1851±6 to 1842±12Ma, with age peaks at 1868±0.8 and 1847±1.2Ma, respectively. The numerous U-Pb age data of both zircons and monazites revealed that the JLJB experienced a major episode of granitic magmatism in the Paleoproterozoic with initial emplacement timing of melts at ∼1.87Ga and crystallisation end timing of melts at ∼1.84Ga as represented by the magmatic cores and rims of the zircons and monazites, respectively. A combined study of whole-rock geochemistry and zircon Lu-Hf isotopic analyses revealed that the three types of porphyritic rocks are enriched in HREE, Zr, Nb, Ga, and Y and are depleted in V, Cr, and Ni, and that most have high TFeO/MgO ratios and Zr-saturation temperatures of ∼850°C, which is suggestive of an affinity with A-type granite. Moreover, the dominant εHf(t) values of the magmatic zircons range from −4.8 to +3.3, which is consistent with those of the 2.2–2.1Ga calc-alkaline meta-granites in the JLJB. Considering the high contents of garnet, Al-rich paragneiss enclaves, and ∼1.92Ga inherited metamorphic zircons in the granites, these porphyritic rocks were most likely derived from the partial melting of the 2.2–2.1Ga calc-alkaline meta-granites with abundant involvement of Al-rich pelitic granulites from the basement of the JLJB. As the result of a newly combined study of traditional geothermobarometers and phase equilibria modelling together with geochronological U-Pb dating, a precise clockwise P-T-t path with a subsequent near isothermal decompression process has been well determined in the HP-MP granulite facies metamorphic rocks in the JLJB. Thus, the following sequence of events can be inferred. Abundant ∼2.2–2.1Ga granitic rocks and voluminous Paleoproterozoic sedimentary rocks were subducted to lower crustal depths and underwent a peak HP-MP granulite facies metamorphism at 1950–1900Ma. These granulite facies rocks were exhumed to middle crustal levels and experienced a post-peak MP-LP granulite facies metamorphism at 1875–1840Ma. The retrograde rocks continued to be exhumed to shallow crustal levels at 1830–1800Ma and were finally overprinted by late amphibolite facies retrogression. The numerous magmatic zircons and monazites from the various porphyritic granites record a distinct Paleoproterozoic granitic magmatic event with two evident age peaks of ∼1.87Ga and ∼1.84Ga, which are completely consistent with the post-peak MP-LP granulite facies metamorphism (1875–1840Ma), representing an anorogenic magmatic association formed in an extensional setting possibly resulting from mantle upwelling and lithospheric thinning and resulting in the violent partial melting of ∼2.2–2.1Ga meta-granites along with abundant Al-rich pelitic granulites. Considering these data in combination with previous whole-rock geochemistry and age data, it can be stated that similar porphyritic granites are not only limited to the entire JLJB but are also widely distributed in North and South Korea. Thus, it can be concluded that the ∼1.87–1.84Ga porphyritic granites, together with massive rapakivi granites, Kfs-megacrysts, and porphyritic monzogranites, as products of an anorogenic granitic magmatic event, may have widely formed in both the JLJB and Korean Peninsula in an extensional and thinning tectonic setting.

Zircon geochronology and Hf isotopes of the Dwalile Supracrustal Suite, Ancient Gneiss Complex, Swaziland: Insights into the diversity of Palaeoarchaean source rocks, depositional and metamorphic ages

We report the results of combined U-Pb and Lu-Hf isotope analyses of detrital and metamorphic zircon grains from a variety of metavolcano-sedimentary rocks of the Dwalile Supracrustal Suite (DSS) from the ca. 3.66 to 3.20Ga Ancient Gneiss Complex (AGC) in Swaziland. The results indicate that the DSS is made up by Palaeo- and Mesoarchaean rocks which have been affected by a polymetamorphic overprint and became tectonically assembled during the Neoarchaean. The oldest Dwalile rocks were formed during a phase of felsic to intermediate volcanism at ca. 3.46Ga, and are derived from the same magma source as the Tsawela Gneisses. Based on the combined U-Pb-Hf isotope data, two groups of metasedimentary rocks can be distinguished: an older Dwalile Group I that is similar in age to the Hooggenoeg and Kromberg Formations (3.46–3.40Ga), but with a different source area. A younger Dwalile Group II is coeval with the Fig Tree and possibly Moodies Groups (<3.23Ga) and the data suggest there was a similar provenance.The U-Pb-Hf isotope data and petrological observations demonstrate that the DSS was affected by three stages of metamorphic overprint: an event at 3.23Ga, pervasive amphibolite-facies metamorphism at 3.15Ga, and low-P granulite-facies metamorphism at 2.99Ga. Metamorphic zircon in the amphibolite-facies rocks resulted from new growth in aqueous fluids at 3.15Ga, whereas in the granulite-facies rocks new zircon was formed in the presence of melt. The 3.23Ga event coincides with the last significant metamorphic overprint documented in the Barberton Greenstone Belt (BGB), whereas as two younger events are restricted to the AGC.The data support the model suggesting amalgamation between the AGC and BGB terranes at ca. 3.23Ga. In addition, they require crustal thickening in the AGC terrane at ca. 3.15Ga, causing Barrovian-type metamorphism in the DSS and granulite-facies metamorphism in the Luboya-Kubuta Terrane. Low-P granulite-facies metamorphism at ca. 2.99Ga is most likely related to crustal extension during the onset of Pongola Basin rifting, causing mantle upwelling, mafic magmatism, and heat transfer from the mantle into the thinned crust.

Deciphering the Paleoproterozoic cooling history of the northeastern Trans-Hudson Orogen, Baffin Island (Canada), using 40Ar/39Ar step-heating and UV laser thermochronology

The previously unstudied cooling and exhumation history of mid-crustal rocks exposed on southeastern Baffin Island (Canada) provides new insights into the post-orogenic evolution of the Paleoproterozoic Trans-Hudson Orogen (THO). New 40Ar/39Ar step-heat analyses of biotite, muscovite and phlogopite and core-to-rim intra-grain 40Ar/39Ar analyses of muscovite have a range of apparent ages compatible with slow regional cooling following peak metamorphism. Twenty-nine amphibolite- to granulite-facies rocks were dated using the 40Ar/39Ar step-heating laser (CO2) method. 40Ar/39Ar spot analyses were performed across muscovite grains from three samples using an ultraviolet (UV) laser to investigate intra-grain 40Ar/39Ar age variations. Step-heating apparent ages range from ca. 1788–1622Ma for biotite, 1720–1630Ma for phlogopite and 1729–1657Ma for muscovite. UV spot 40Ar/39Ar analyses in the three muscovite grains range from ca. 1661–1640Ma, 1675–1645Ma and 1680–1652Ma, with core-to-rim apparent age gradients of 20–30Myr. Previous studies resolved peak metamorphism in this region to between ca. 1860 and 1820Ma and identified late- to post-THO zircon and monazite populations at ca. 1800–1750Ma. Numerical diffusion models for Ar in muscovite were conducted to test different Proterozoic cooling and exhumation scenarios. Comparisons with our 40Ar/39Ar ages attest to cooling rates of ~1–2°C/Myr following peak metamorphism and ~1.5–2.5°C/Myr after ca. 1740Ma. Anomalously old apparent 40Ar/39Ar ages, in cases equivalent to U–Pb zircon rim and monazite ages, likely result from incorporation of excess Ar. The results suggest that mid-crustal rocks on southeastern Baffin Island remained hotter than ~420–450°C for ~150–200Myr after peak metamorphism, with subsequent slow cooling and denudation rates that are typical of Proterozoic orogens. The apparent absence of orogenic collapse implies that, despite high temperatures and estimated maximum crustal thicknesses comparable to those of large, hot orogens, the THO remained gravitationally stable during its terminal phase.

Tectonometamorphic evolution of Seram and Ambon, eastern Indonesia: Insights from 40Ar/39Ar geochronology

The island of Seram, eastern Indonesia, experienced a complex Neogene history of multiple metamorphic and deformational events driven by Australia–SE Asia collision. Geological mapping, and structural and petrographic analysis has identified two main phases in the island's tectonic, metamorphic, and magmatic evolution: (1) an initial episode of extreme extension that exhumed hot lherzolites from the subcontinental lithospheric mantle and drove ultrahigh-temperature metamorphism and melting of adjacent continental crust; and (2) subsequent episodes of extensional detachment faulting and strike-slip faulting that further exhumed granulites and mantle rocks across Seram and Ambon. Here we present the results of sixteen 40Ar/39Ar furnace step heating experiments on white mica, biotite, and phlogopite for a suite of twelve rocks that were targeted to further unravel Seram's tectonic and metamorphic history. Despite a wide lithological and structural diversity among the samples, there is a remarkable degree of correlation between the 40Ar/39Ar ages recorded by different rock types situated in different structural settings, recording thermal events at 16Ma, 5.7Ma, 4.5Ma, and 3.4Ma. These frequently measured ages are defined, in most instances, by two or more 40Ar/39Ar ages that are identical within error. At 16Ma, a major kyanite-grade metamorphic event affected the Tehoru Formation across western and central Seram, coincident with ultrahigh-temperature metamorphism and melting of granulite-facies rocks comprising the Kobipoto Complex, and the intrusion of lamprophyres. Later, at 5.7Ma, Kobipoto Complex rocks were exhumed beneath extensional detachment faults on the Kaibobo Peninsula of western Seram, heating and shearing adjacent Tehoru Formation schists to form Taunusa Complex gneisses. Then, at 4.5Ma, 40Ar/39Ar ages record deformation within the Kawa Shear Zone (central Seram) and overprinting of detachment faults in western Seram. Finally, at 3.4Ma, Kobipoto Complex migmatites were exhumed on Ambon, at the same time as deformation within the Kawa Shear Zone and further overprinting of detachments in western Seram. These ages support there having been multiple synchronised episodes of high-temperature extension and strike-slip faulting, interpreted to be the result of Western Seram having been ripped off from SE Sulawesi, extended, and dragged east by subduction rollback of the Banda Slab.

A late Neoarchaean–early Palaeoproterozoic crustal thickening event in the eastern North China Craton: petrological and geochronological evidence from Eastern Hebei terrane

ABSTRACTDetermining an age framework for Precambrian crystalline rocks and associated granulite-facies metamorphism of the inner blocks in the North China Craton (NCC) is important for determining the tectonic setting and evolution of the craton during the Neoarchaean–Palaeoproterozoic. The Eastern Hebei terrane (EHT), located in the Eastern Block of the NCC, is composed of tonalitic-trondhjemitic-granodioritic (TTG) gneisses and potassium-rich granitoids, along with rafts of supracrustal rocks that are intruded by basic dikes. TTG gneisses in the EHT yield crystallization ages of 2516–2527 Ma. The oldest age of inherited zircons from a mylonitic TTG gneiss is ~2918 Ma. Granulite-facies supracrustal metamorphic rocks in the Zunhua high-grade meta-greenstone belt indicate an andesitic/basaltic protolith that was formed at ~2498 Ma. A syn-deformational granite in the Jinchangyu greenschist-facies shear zone yields a crystallization age of ~2474 Ma. Metamorphism of the supracrustal rocks and mylonitic greenschist took place at ~2461 and ~2475 Ma, respectively. Rare earth elements (REE) patterns and slightly negative Nb and Ta anomalies indicate that the magmatic precursors of the supracrustal rocks might be derived from partial melting of a sub-arc mantle wedge and metasomatized by fluids derived from a subducting slab. These rocks plot in the island arc basalts (IAB) field on a La/Nb vs. La diagram, further supporting this interpretation. The microstructures of a garnet–two-pyroxene granulite indicate an approximately clockwise P-T path. The crystallization ages of the TTG gneisses represent periods of the major crustal growth in the NCC, and the granulite- and greenschist-facies metamorphism indicates an orogenic event that involved crustal thickening at ~2.47 Ga.

Timing of anatexis and melt crystallization in the Socorro–Guaxupé Nappe, SE Brazil: Insights from trace element composition of zircon, monazite and garnet coupled to U[sbnd]Pb geochronology

The timing of partial melting and melt crystallization in granulite facies rocks of the Socorro–Guaxupé Nappe (SGN), Brazil is constrained using a combination of imaging techniques, LA-ICP-MS and EPMA dating, trace element geochemistry and thermobarometry. (Orthopyroxene)-garnet-bearing migmatite that records extensive biotite dehydration melting shows evidence for a clockwise P–T–t path. UHT peak conditions were attained at 1030±110°C, 11.7±1.4kbar, with post-peak cooling to 865±38°C, 8.9±0.8kbar. Cryogenian igneous inheritance of ca. 720–640Ma is identified in oscillatory zoned zircon cores (n=167) with steep HREE patterns. Resorbed, Y-rich monazite cores preserve a prograde growth stage at 631±4Ma prior to the partial melting event, providing an upper age limit for the granulite facies metamorphism in the SGN. REE-rich, Th-depleted monazite related to apatite records the initial stages of decompression at 628±4Ma. Multiple monazite growth episodes record melt crystallization events at 624±3Ma, 612±5Ma and 608±6Ma. Stubby, equant “soccer ball” zircon provide evidence for melt crystallization at 613±2Ma and 607±4Ma. The excess scatter in zircon and monazite age populations between 629±4 and 601±3Ma is interpreted as discontinuous and episodic growth within this age range, characterizing a prolonged metamorphic event in the SGN lasting ca. 30m.y. The development of Y+HREE-rich monazite rims at ca. 600Ma documents retrograde garnet breakdown, extensive biotite growth and the final stages of melt crystallization. Th-rich, Y+HREE-poor monazite rims at ca. 590Ma record monazite recrystallization.

Phase equilibria modelling and zircon age dating of pelitic granulites in Zhaojiayao, from the Jining Group of the Khondalite Belt, North China Craton

AbstractThe Jining Group occurs as the eastern segment of the Khondalite Belt, North China Craton and is dominated by a series of granulite facies rocks involving ‘normal’ pelitic granulites recording peak temperatures of ~850 °C and ultrahigh‐temperature (UHT) pelitic granulites recording peak temperatures of 950–1100 °C. The P–T paths and ages of these two types of granulites are controversial. Three pelitic granulite samples in the Jining Group comprising two sillimanite–garnet gneiss samples (J1208 and J1210) and one spinel–garnet gneiss sample (J1303) were collected from Zhaojiayao, where ‘normal’ pelitic granulites occur, for determination of their metamorphic evolution and ages. Samples J1208 and J1210 are interpreted to record cooling paths from the Tmax stages with P–T conditions respectively of ~870–890 °C/7–8 kbar and &gt;840 °C/&gt;7.5 kbar constrained from the stability fields of the observed mineral assemblages and the isopleths of plagioclase, garnet and biotite compositions in pseudosections. Sample J1303 is interpreted to record pre‐Tmax decompression from the kyanite‐stability fields to the Tmax stage of 950–1020 °C/8–9 kbar and a post‐Tmax cooling path revealed mainly from the stability field of the observed mineral assemblage, the plagioclase zoning and the biotite composition isopleth in pseudosections. The post‐Tmax cooling stage can be divided into suprasolidus and subsolidus stages. The suprasolidus cooling may not result in an equilibrium state at the solidus in a rock. Therefore, different minerals may record different P–T conditions along the cooling path; the inferred maximum temperature is commonly higher than the solidus as well as different solidi being recorded for different samples from the same outcrop but experiencing different degrees of melt loss. Plagioclase compositions, especially its zoning in plagioclase‐rich granulites, are predicted to be useful for recording the higher temperature conditions of a granulite's thermal history. The three samples studied seem to record the temperature range covering those of the ‘normal’ and UHT pelitic granulites in the Jining Group, suggesting that UHT conditions may be reached in ‘normal’ granulites without diagnostic UHT indicators. LA‐ICP‐MS zircon U–Pb data provide a continuous trend of concordant 207Pb/206Pb ages from 1.89 to 1.79 Ga for sample J1210, and from 1.94 to 1.80 Ga for sample J1303. These continuous and long age spectrums are interpreted to represent a slow cooling and exhumation process corresponding to the post‐Tmax cooling P–T paths recorded by the pelitic granulites, which may have followed the exhumation of deeply buried rocks in a thickened crust region resulted from a collision event at c. 1.95 Ga as suggested by the pre‐Tmax decompression P–T path.

Microstructures, deformation mechanisms and seismic properties of a Palaeoproterozoic shear zone: The Mertz shear zone, East-Antarctica

The Mertz shear zone (MSZ) is a lithospheric scale structure that recorded mid-crustal deformation during the 1.7Ga orogeny. We performed a microstructural and crystallographic preferred orientation (CPO) study of samples from both mylonites and tectonic boudins that constitute relics of the Terre Adélie Craton (TAC). The deformation is highly accommodated in the MSZ by anastomosed shear bands, which become more scattered elsewhere in the TAC. Most of the MSZ amphibolite-facies mylonites display similar CPO, thermal conditions, intensity of deformation and dominant shear strain. Preserved granulite-facies boudins show both coaxial and non-coaxial strains related to the previous 2.45Ga event. This former deformation is more penetrative and less localized and shows a deformation gradient, later affected by a major phase of recrystallization during retrogression at 2.42Ga. Both MSZ samples and granulite-facies tectonic boudins present microstructures that reflect a variety of deformation mechanisms associated with the rock creep that induce contrasted CPO of minerals (quartz, feldspar, biotite, amphibole and orthopyroxene). In particular, we highlight the development of an “uncommon” CPO in orthopyroxene from weakly deformed samples characterized by (010)-planes oriented parallel to the foliation plane, [001]-axes parallel to the stretching lineation and clustering of [100]-axes near the Y structural direction.Lastly, we computed the seismic properties of the amphibolite and granulite facies rocks in the MSZ area in order to evaluate the contribution of the deformed intermediate and lower continental crust to the seismic anisotropy recorded above the MSZ. Our results reveal that (i) the low content of amphibole and biotite in the rock formations of the TAC, and (ii) the interactions between the CPO of the different mineralogical phases, generate a seismically isotropic crust. Thus, the seismic anisotropy recorded by the seismic stations of the TAC, including the MSZ, must be due to mantle rather than crustal structures.

A systematic evaluation of the Zr-in-rutile thermometer in ultra-high temperature (UHT) rocks

The Zr-in-rutile geothermometer is potentially a widely applicable tool to estimate peak metamorphic temperatures in rocks from diverse geological settings. In order to evaluate its usefulness and reliability to record and preserve high temperatures in granulite facies rocks, rutile from UHT rocks was investigated to assess different mechanisms of Zr (re-)distribution following cooling from high temperature. Granulite facies paragneisses from the lowermost part of the Ivrea Zone, Italy, incorporated as thin sheets into the extensive basaltic body of the Mafic Complex were selected for this study. The results show that Zr-in-rutile thermometry, if properly applied, is well suited to identify and study UHT terranes as it preserves a record of temperatures up to 1190 °C, although the thermometer is susceptible to partial post-peak metamorphic resetting by Zr diffusion. Texturally homogeneous rutile grains preserve Zr concentrations corresponding to temperatures of prograde rutile growth. Diverse rutile textures and relationships between some rutile host grains and included or adjacent Zr-bearing phases bear testimony to varying mechanisms of partial redistribution and resetting of Zr in rutile during cooling and link Zr-in-rutile temperatures to different steps of the metamorphic evolution. Rutile grains that equilibrated their Zr concentrations at temperatures above 1070 °C (i.e. 1.1 wt% Zr) could not retain all Zr in the rutile structure during cooling and exsolved baddeleyite (ZrO2). By subsequent reaction of baddeleyite exsolution lamellae with SiO2, zircon needles formed before the system finally closed at 650–700 °C without significant net loss of Zr from the whole host rutile grain. By reintegration of zircon exsolution needles, peak metamorphic temperatures of up to 1190 °C are derived for the studied rocks, which demonstrates the suitability of this solution thermometer to record UHT conditions and also confirms the extraordinary geological setting of the lowermost part of the Ivrea Zone.

Age and provenance constraints on seismically-determined crustal layers beneath the Paleozoic southern Central Asian Orogen, Inner Mongolia, China

We present 110 ages and 51 in-situ δ18O values for zircon xenocrysts from a post-99Ma intraplate basaltic rock suite hosted in a subduction–accretion complex of the southern Central Asian Orogenic Belt in order to constrain a seismic profile across the Paleozoic Southern Orogen of Inner Mongolia and the northern margin of the North China Craton. Two zircon populations are recognized, namely a Phanerozoic group of 70 zircons comprising granitoid-derived (ca. 431–99Ma; n=31; peak at 256Ma), meta-granitoid-derived (ca. 449–113Ma; n=24; peak at 251Ma) and gabbro-derived (436–242Ma; n=15; peaks at 264 and 244Ma) grains. Each textural type is characterized by a distinct zircon oxygen isotope composition and is thus endowed with a genetic connotation. The Precambrian population (2605–741Ma; n=40) exhibits a prominent age peak at 2520Ma (granulite-facies metamorphism) and four small peaks at ca. 1900, 1600, and 800Ma. Our new data, together with literature zircon ages, significantly constrain models of three seismically-determined deep crustal layers beneath the fossil subduction zone–forearc along the active northern margin of the North China Craton, namely: (1) an upper arc crust of early to mid-Paleozoic age, intruded by a major Permian–Triassic composite granitoid–gabbroic pluton (8–20km depth); (2) a middle crust, predominantly consisting of mid-Meso- to Neoproterozoic felsic and mafic gneisses; and (3) a lower crust composed predominantly of late Archean granulite-facies rocks. We conclude that the Paleozoic orogenic crust is limited to the upper crustal level, and the middle to lower crust has a North China Craton affinity. Furthermore, integrating our data with surface geological, petrological and geochronological constraints, we present a new conceptual model of orogenic uplift, lithospheric delamination and crustal underthrusting for this key ocean–continent convergent margin.

Localized occurrences of granulite: P–T modeling, U–Pb geochronology and distribution of early-Sveconorwegian high-grade metamorphism in Bamble, South Norway

New localities of granulite in the Bamble lithotectonic domain of south Norway illustrate localized distribution of granulite facies rocks in an amphibolite facies gneiss terrain. U–Pb zircon SIMS data constrain the magmatic age of the protoliths to be both 1545±7 to 1542±5Ma and 1149±8Ma. They provide a robust date of 1139±11Ma for metamorphic zircon, supporting available estimates for early-Sveconorwegian metamorphism in Bamble. P–T modeling using Grt+Opx±Cpx+Pl+Bt+Qz-assemblages yields pressures up to 1.15GPa and T>850°C. Clinopyroxene-free granulite assemblages are preserved in low-strain lenses, while high-strain granulite shows crystallization of clinopyroxene. The replacement of orthopyroxene by amphibole illustrates hydration and reequilibration at amphibolite facies. The new findings in Bamble show that granulite-facies rocks occur unevenly distributed in high-grade metamorphic terrains and that relationship between granulite and amphibolite facies is complex and irregular. It suggests that factors as fluid availability, deformation and whole-rock composition are important for metamorphic evolution in addition to pressure and temperature gradients.

Deep scientific drilling results from Koyna and Killari earthquake regions reveal why Indian shield lithosphere is unusual, thin and warm

The nature of crustal and lithospheric mantle evolution of the Archean shields as well as their subsequent deformation due to recent plate motions and sustained intraplate geodynamic activity, has been a subject of considerable interest. In view of this, about three decades ago, a new idea was put forward suggesting that out of all shield terrains, the Indian shield has an extremely thin lithosphere (∼100 km, compared to 250–350 km, elsewhere), apart from being warm, non-rigid, sheared and deformed. As expected, it met with scepticism by heat flow and the emerging seismic tomographic study groups, who on the contrary suggested that the Indian shield has a cool crust, besides a coherent and thick lithosphere (as much as 300–400 km) like any other shield. However, recently obtained integrated geological and geophysical findings from deep scientific drillings in 1993 Killari (Mw: 6.3) and 1967 Koyna (Mw: 6.3) earthquake zones, as well as newly acquired geophysical data over other parts of Indian shield terrain, have provided a totally new insight to this debate. Beneath Killari, the basement was found consisting of high density, high velocity mid crustal amphibolite to granulite facies rocks due to exhumation of the deeper crustal layers and sustained granitic upper crustal erosion. Similar type of basement appears to be present in Koyna region too, which is characterized by considerably high upper crustal temperatures. Since, such type of crust is depleted in radiogenic elements, it resulted into lowering of heat flow at the surface, increase in heat flow contribution from the mantle, and upwarping of the lithosphere-asthenosphere boundary. Consequently, the Indian shield lithosphere has become unusually thin and warm. This study highlights the need of an integrated geological, geochemical and geophysical approach in order to accurately determine deep crust-mantle thermal regime in continental areas.

High-temperature metamorphism and crustal melting: Working with melt inclusions

The application of melt inclusion (MI) studies to migmatitic and granulitic terranes is a recent, small-scale approach for a better understanding of melting in the continental crust. In order to show the role of anatectic MI in providing a wealth of microstructural and compositional information on high-temperature metamorphism and crustal anatexis, we review a series of studies on the crustal footwall of the Ronda peridotites (Betic Cordillera, S Spain), which consists of an inverted metamorphic sequence with granulite-facies rocks showing extensive melting on top and amphibolites-facies rocks at the bottom. We studied the microstructures and geochemistry of small (2-10 µm) primary MI hosted in peritectic garnet of metatexites at the bottom of the migmatitic sequence and of diatexites close to the contact with the mantle rocks. The occurrence of MI is a proof that the investigated rocks were partially melted at some time in their history, despite other microstructures indicating the former presence of melt in mylonite were erased by deformation. MI show a variable degree of crystallization ranging from totally glassy to fully crystallized (nanogranites), consisting of Qtz+Pl+Kfs+Bt+Ms aggregates (often modal Kfs > Pl in mylonites). Piston cylinder remelting experiments led to the complete rehomogenization of nanogranites in metatexites at the conditions inferred for anatexis. Compositions of investigated MI are all leucogranitic and peraluminous and differ from those of coexisting leucosomes and from melts calculated by phase equilibria modeling. Systematic compositional variations have been observed between MI in metatexites and mylonites: the former commonly show higher H 2 O, CaO, Na 2 O/K 2 O and lower FeO. The compositions of MI in metatexites and mylonites are interpreted to record the composition of the anatectic melts produced from a peraluminous greywacke i) on, and immediately after crossing, the fluid-saturated solidus of this metasedimentary rock, and ii) during anatexis via biotite dehydration melting at increasing temperature, respectively. While partial melting at the bottom of the migmatitic sequence likely started in the presence of an aqueous fluid phase, MI data support the fluid-absent character of the melting event in mylonites. Anatectic MI should therefore be considered as a new and important opportunity to understand the partial melting processes.

Genesis of post-collisional calc-alkaline and alkaline granitoids in Qiman Tagh, East Kunlun, China

The post-collisional magmatism of Qiman Tagh is characterized by the intrusion of voluminous intermediate to felsic granitoids, including syenogranite, monzogranite, granodiorite, tonalite and diorite. The granitoids can be divided into two magmatic suites: Calc-alkaline (CA) and alkaline (Alk), which were emplaced from ~236Ma to ~204Ma. The CA suite contains metaluminous granodiorites and monzogranites. Typical Qiman Tagh CA granodiorites show moderately fractionated REE patterns ((La/Yb)N=4.35–25.11) with significant negative Eu anomalies (Eu/Eu*=0.54–1.34), and the primitive mantle-normalized spidergrams show strong depletion of Nb and Sr. The Qiman Tagh CA monzogranites show similar fractionated REE patterns ((La/Yb)N=2.70–13.5) with less prominent negative Eu anomalies, and the chondrite-normalized spidergrams show strongly depleted Ba, Nb and Sr. The Alk suite, including syenogranite, is highly potassic (K2O/Na2O=1.09–3.56) and peraluminous (A/CNK=0.91–1.06). Compared to typical Qiman Tagh CA granodiorites, the Qiman Tagh Alk granitoids can be distinguished by their higher Rb, Nb, Ga/Al, FeO*/MgO, Y/Sr and Rb/Sr, as well as their lower Mg#, MgO, CaO, Al2O3, Sr, Co, V, Eu/Eu*, Ba/Nb, La/Nb, Ba/La and Ce/Nb. The Qiman Tagh CA rocks were most likely to be derived from the partial melting of garnet-amphibolite-facies rocks in the lower crust, leaving behind anhydrous granulite-facies rocks with plagioclase and garnet in the residue. The Alk rocks may have formed by the continued partial melting of granulite-facies rocks at elevated temperatures (>830°C).

Coupled mass transfer through a fluid phase and volume preservation during the hydration of granulite: An example from the Bergen Arcs, Norway

The Precambrian granulite facies rocks of Lindås Nappe, Bergen Arcs, Caledonides of W. Norway are partially hydrated at amphibolite and eclogite facies conditions. The Lindås Nappe outcrops over an area of ca. 1000km2 where relict granulite facies lenses make up only ca. 10%. At Hillandsvatnet, garnetite displays sharp hydration fronts across which the granulite facies assemblage composed of garnet (55%) and clinopyroxene (45%) is replaced by an amphibolite facies mineralogy defined by chlorite, epidote and amphibole. The major element bulk composition does not change significantly across the hydration front, apart from the volatile components (loss on ignition, LOI) that increases from 0.17wt.% in the granulite to 2.43wt.% in the amphibolite. However the replacements of garnet and of clinopyroxene are pseudomorphic so that the grain shapes of the garnet and clinopyroxene are preserved even when they are completely replaced. The textural evolution during the replacement of garnet by pargasite, epidote and chlorite and of pyroxene by hornblende and quartz in our rock sample conforms to that expected by a coupled dissolution–precipitation mechanism.SEM and electron microprobe analysis coupled with the software XMapTools V 1.06.1 were used to quantify the local mass transfer required during the replacement processes. The element losses and gains in replacing the garnet are approximately balanced by the opposite gains and losses associated with the replacement of clinopyroxene. The coupling between dissolution and precipitation on both the grain and whole rock spatial scale preserves the volume of the rock throughout the hydration process. However, the hydration involves reduction of rock density and mass balance calculations, together with volume preservation (isovolumetric reaction) require a significant loss of the mass of the rock to the fluid phase. This suggests a mechanism for coupling between the local stress generated by hydration reactions and mass transfer, dependent on the spatial scale over which the system is open where the fluid infiltration is pervasive along grain boundaries.

Paleoproterozoic multistage evolution of the lower crust beneath the southern North China Craton

It is generally recognized that the North China Craton (NCC) resulted from the amalgamation of several micro-blocks during the Paleoproterozoic period. However, how the lower crust beneath the craton evolved during the amalgamation of these blocks is less well-known. Four garnet-bearing felsic granulite xenoliths entrained by the Mesozoic Xinyang volcanic rocks at the southwestern part of the Eastern Block of the craton were selected for analysis of whole-rock major- and trace elements, and the U–Pb ages, trace-element patterns and Lu–Hf isotopic composition of zircons. These xenoliths are igneous in origin but with distinct sources. The relict cores of magmatic zircon in the xenoliths give ages including ca. 3.61Ga, ca. 2.22Ga and ca. 2.05Ga, and Hadean–Neoarchean Hf model ages (Tcrust=2.4–4.3Ga), implying the existence of the Hadean crustal components and a complex history in Archean–Paleoproterozoic. The metamorphic zircons from the xenoliths define three thermal episodes at ca. 2.33Ga, 1.86–1.91Ga and 1.72–1.77Ga, and exhibit low but flat HREE patterns and negative Eu anomalies, indicating that they formed in garnet-bearing granulite-facies rocks. Combined with the published data, a vertical section of the lower crust can be built: the upper part consists mainly of 3.4–3.6Ga garnet-free felsic granulites and rare pyroxenites, the middle part of 1.8–2.2Ga garnet-bearing felsic granulites and meta-gabbros, and the lowermost part of 1.7–2.0Ga intermediate and mafic granulites and Paleozoic eclogites. These observations suggest that the lower crust beneath the area underwent multistage modification in the Paleoproterozoic, which was linked to the successive geological processes between the Western and Eastern Blocks, such as the subduction, collision and rifting.

Petrology and zircon U–Pb geochronology of metagabbro from the Highland Complex, Sri Lanka: Implications for the correlation of Gondwana suture zones

The basement in the Sri Lankan fragment of Gondwana is composed of upper amphibolite- to granulite-facies rocks that underwent latest Neoproterozoic – Cambrian high-grade metamorphism. A recent tectonic model suggests that the Highland Complex in the central part of the island corresponds to a suture zone formed by the closure of a Neoproterozoic ocean between the Wanni and Vijayan Complexes. In order to test the model and further unravel the complex tectonic evolution of this region, we present new petrological, pressure–temperature, and zircon U–Pb geochronological data on garnet-bearing metagabbros from the Highland Complex. The metagabbros, which occur as blocks or lenses within metasediments of the Highland Complex, contain granulite-facies mineral assemblages of garnet+clinopyroxene+orthopyroxene+ilmenite+hornblende+plagioclase+quartz (sample HC7) and garnet+plagioclase+clinopyroxene+orthopyroxene+pargasite+quartz+rutile+ilmenite (sample HC8). Application of phase equilibria modeling in the system NCFMASHTO indicates peak P–T conditions of 900–950°C/10.5–11kbar and 800–830°C/8.5–9kbar, respectively, and clockwise P–T paths as inferred from decompression textures and mineral compositional isopleths. These conditions are significantly lower than the high-pressure (>17kbar) and ultrahigh-temperature (>1000°C) conditions obtained previously for similar metagabbros from the Highland Complex, whereas our results are consistent with those of similar rocks from the Palghat-Cauvery Suture Zone in India and the Lützow-Holm Complex in East Antarctica. The latest Neoproterozoic to Cambrian metamorphic ages (574±28Ma and 534±14Ma) as well as the available petrological data suggest that the metagabbros in the Highland Complex could be correlated with those in the Lützow-Holm Complex in East Antarctica. The results of this study confirm that the Highland Complex is an extension of the Lützow-Holm Complex, and that both these terranes involve a package of accreted trench sediments and remnant oceanic lithosphere metamorphosed during the final collision related to the Gondwana assembly.

Tracking the thermal properties of the lower continental crust: Measured versus calculated thermal conductivity of high-grade metamorphic rocks (Southern Granulite Province, India)

In this study, the bulk thermal conductivity (TC) of 26 rock samples representing different types of granulite-facies rocks, i.e., felsic, intermediate and mafic granulites, from the Southern Granulite Province, India, is measured at dry and saturated conditions with the optical-scanning method. Thermal conductivity is also calculated from modal mineralogy (determined by XRD and EPMA), applying several mixing models commonly used in thermal studies. Most rocks are fine- to medium-grained equigranular in texture. All samples are isotropic to weakly anisotropic and possess low porosities (<2%). Measured TC values range between 2.5 and 3.0Wm−1K−1 for felsic granulites, between 2.5 and 3.5Wm−1K−1 for intermediate granulites and between 2.4 and 2.7Wm−1K−1 for mafic granulites. Considering this data and literature compilations, rocks representative for the lower continental crust typically display values between 2 and 3Wm−1K−1 at ambient temperature and pressure conditions. Depending on the mixing model and the mineral TC value used in the calculations, measured and calculated bulk TC could be properly fitted. For mean values of mineral TCs, the harmonic mean provides an almost perfect fit, with a mean deviation of −1±6% (1σ). However, the implication of that correspondence would be that minerals and pores are predominantly aligned parallel, which is in apparent contrast to the texture of the rocks studied here. The geometric mean, which does not consider any layering of minerals or pores in the rock and, thus, should be in better harmony with the textural characteristics of the studied high-grade rocks, matches the measured TC data very well, if minimal minerals TCs reported in the literature are applied (mean deviation 5±8%). Thus, if samples appropriate for laboratory measurements (in terms of sample size or physical-chemical-mechanical condition) are not available, bulk TC of high-grade metamorphic rocks with low anisotropy and porosity could be satisfactorily good assessed from modal mineralogy, using the data sets for mineral TC applied in this study. Further work is required on the applicability of mixing models to compute TC of other rock types, e.g., of igneous and sedimentary rocks.