Mafic Garnet Granulite Research Articles

The polyphase evolution of the mafic rocks of the Juiz de Fora Complex: The record of two supercontinent cycles

The Juiz de Fora Complex (JFC) in the Araçuaí-Ribeira orogenic system, southeastern Brazil, is a basement unit displaying a variety of rock types that is tectonically imbricated with the supracrustal rocks of the Raposos Group in the Upper Thrust System of the Occidental Terrane, Ribeira Belt. Here we report new petrological, geochemical, and zircon U–Pb and Lu–Hf isotopic data on mafic garnet-granulites from the JFC with a view to evaluate the timing and P-T conditions of the collisional event as well as protolith formation. This work presents the most comprehensive metamorphic study on the Juiz de Fora Complex rocks to date. Metamorphic P-T estimates for the metamorphic peak conditions of 800–900 °C and 8–9 kbar in basic garnet-bearing granulites were obtained for the JFC, indicating that metamorphism took place at deeper crustal levels. Geochemical, zircon U–Pb geochronology, whole-rock Sm–Nd, Rb–Sr isotopes, and Lu–Hf isotopic data of mafic granulites yielded U–Pb ages of ca. 2201 Ma, and moderately juvenile εNd(t) = −0.5 and were obtained from the JFC marking the timing of volcanic arc magmatism during the Rhyacian period. This arc-related stage was followed by the post-collisional period during the Early Paleoproterozoic with ca. 1.94 Ga E-MORB signature granulites that shows evidence of magma sourced from Neoarchean (∼2.8 Ga) crust with addition of juvenile components. The high-grade metamorphism is possibly related to the final collision of the Congo and São Francisco Cratons at 590–560 Ma, which placed the Neoproterozoic metasediments and the JFC rocks at the same level. The integrated information sheds light on two collisional episodes where the oldest is represented by arc-related granulites during the Rhyacian Orogeny and the youngest is the higher-pressure Neoproterozoic overprint during the Brasiliano Orogeny.

Evidence of Eoarchean crust beneath the Yakutian kimberlite province in the Siberian craton

The Siberian craton is one of the largest Precambrian cratons in the world. However, unlike other cratons, the Siberian craton can only provide a relatively limited amount of geochronological data since a large part of it is covered by younger sedimentary deposits. Xenoliths from numerous kimberlite and lamprophyre pipes and alkali basalts allow us to investigate the composition and evolution of the Siberian Craton crust. The available isotopic and geochemical data show that the crust of the Anabar tectonic province of the Siberian craton is not age stratified, and consists of variably reworked Paleoarchean rocks and juvenile Proterozoic rocks at all crustal levels. Hence, a detailed geochronological study of crustal xenoliths (mainly zircon U-Pb age and Hf isotopic composition) in the kimberlite pipes of the Yakutian kimberlite province is required. In this study, we present U-Pb ages and Hf isotope data of zircons derived from crustal xenoliths in the Zarnitsa (Daldyn kimberlite field) and the Nyurbinskaya (Nakyn kimberlite field) kimberlite pipes. The obtained data provided grounds for identifying four main tectono-thermal events during the evolution of the Archean crust in the Anabar tectonic province of the Siberian craton. The first event at 3.2 Ga denotes the beginning of zircon crystallisation. These zircons are eventually recrystallised during the second event at 2.9 Ga, and radiogenic Hf (formed from 176Lu decay in the host rock) was introduced into the system. The third event at 2.8–2.7 Ga featured further recrystallisation of zircons and the incorporation of additional radiogenic 176Hf. The minor addition of radiogenic Hf during the final event at 1.9 Ga indicates a relatively large range for the initial 176Hf/177Hf ratio. Our data indicate that Eoarchean or even Hadean crust, which were reworked in the Paleoproterozoic, may be present beneath the Daldyn and Nakyn kimberlite fields. Zircon grains with low 176Hf/177Hf (0.28047–0.280661) were found in the xenoliths of the mafic garnet granulite (Zar-1–02) and garnet biotite gneisses (Nrb-13 and Nrb-65).

Melt-rock interaction in the lower crust based on silicate melt inclusions in mafic garnet granulite xenoliths, Bakony–Balaton Highland

Major and trace element composition of silicate melt inclusions (SMI) and their rock-forming minerals were studied in mafic garnet granulite xenoliths from the Bakony–Balaton Highland Volcanic Field (Western-Hungary). Primary SMIs occur in clinopyroxene and plagioclase in the plagioclase-rich domains of mafic garnet granulites and in ilmenite in the vicinity of these domains in the wall rock. Based on major and trace elements, we demonstrated that the SMIs have no connection with the xenolith-hosting alkaline basalt as they have rhyodacitic composition with a distinct REE pattern, negative Sr anomaly, and HFSE depletion. The trace element characteristics suggest that the clinopyroxene hosted SMIs are the closest representation of the original melt percolated in the lower crust. In contrast, the plagioclase and ilmenite hosted SMIs are products of interaction between the silicic melt and the wall rock garnet granulite. A further product of this interaction is the clinopyroxene–ilmenite±plagioclase symplectite. Textural observations and mass ­balance calculations reveal that the reaction between titanite and the silicate melt led to the formation of these assemblages. We propose that a tectonic mélange of metapelites and (MOR-related) metabasalts partially melted at 0.3–0.5 GPa to form a dacitic–rhyodacitic melt leaving behind a garnet-free, plagioclase+clinopyroxene+orthopyroxene+ilmenite residuum. The composition of the SMIs (both major and trace elements) is similar to those from the middle Miocene calc-alkaline magmas, widely known from the northern Pannonian Basin (Börzsöny and Visegrád Mts., Cserhát and Mátra volcanic areas and Central Slovakian VF), but the SMIs are probably the result of a later, local process. The study of these SMIs also highlights how crustal contamination changes magma compositions during asthenospheric Miocene ascent.

Eclogite and Garnet Pyroxenite Xenoliths from Kimberlites Emplaced Along the Southern Margin of the Kaapvaal Craton, Southern Africa: Mantle or Lower Crustal Fragments?

Abstract Eclogite xenoliths, together with garnet pyroxenites and some mafic garnet granulites, found in kimberlites located along the southern margin of the Kaapvaal craton in southern Africa have been analysed by electron microprobe and mass spectrometry techniques to determine their geochemical characteristics. The majority of eclogites are bimineralic with garnet and omphacitic clinopyroxene in subequal proportions, with rutile as the main accessory phase; a few contain kyanite. Based on K2O in clinopyroxene and Na2O in garnet, the eclogites can be classified as Group II eclogites, and the majority are high-Ca in character. Garnet pyroxenites comprise garnet clinopyroxenites and garnet websterites. Major and trace element concentrations and isotope ratios of reconstituted bulk rock compositions of the eclogites and garnet pyroxenites allow constraints to be placed on depth of origin and likely protolith history. Calculated Fe–Mg exchange equilibration temperatures for the eclogites range from 815 to 1000 °C, at pressures of 1·7 ± 0·4 GPa as determined by REE partitioning, indicating that they were sampled from depths of 50–55 km; i.e. within the lower crust of the Namaqua–Natal Belt. The garnet pyroxenites show slightly lower temperatures (686–835 °C) at similar pressures of equilibration. Initial 143Nd/144Nd and 87Sr/86Sr ratios (calculated to time of kimberlite emplacement) of both lithologies overlap the field for lower crustal samples from the Namaqua–Natal Belt. Further evidence for a crustal origin is found in the similar REE patterns shown by many of the associated garnet granulite xenoliths. Garnet pyroxenites are interpreted to have a similar origin as the associated eclogites but with the mafic protolith having insufficient Na (i.e. low modal plagioclase) to allow for development of omphacitic pyroxene. Metamorphism of the mafic protoliths to these eclogites and garnet pyroxenites is inferred to have occurred during crustal shortening and thickening associated with the collision of the Namaqua–Natal Belt with the Kaapvaal craton at 1–1·2 Ga.

Crustal structure beneath northeast India inferred from receiver function modeling

We estimated crustal shear velocity structure beneath ten broadband seismic stations of northeast India, by using H-Vp/Vs stacking method and a non-linear direct search approach, Neighbourhood Algorithm (NA) technique followed by joint inversion of Rayleigh wave group velocity and receiver function, calculated from teleseismic earthquakes data. Results show significant variations of thickness, shear velocities (Vs) and Vp/Vs ratio in the crust of the study region. The inverted shear wave velocity models show crustal thickness variations of 32–36km in Shillong Plateau (North), 36–40 in Assam Valley and ∼44km in Lesser Himalaya (South). Average Vp/Vs ratio in Shillong Plateau is less (1.73–1.77) compared to Assam Valley and Lesser Himalaya (∼1.80). Average crustal shear velocity beneath the study region varies from 3.4 to 3.5km/s. Sediment structure beneath Shillong Plateau and Assam Valley shows 1–2km thick sediment layer with low Vs (2.5–2.9km/s) and high Vp/Vs ratio (1.8–2.1), while it is observed to be of greater thickness (4km) with similar Vs and high Vp/Vs (∼2.5) in RUP (Lesser Himalaya). Both Shillong Plateau and Assam Valley show thick upper and middle crust (10–20km), and thin (4–9km) lower crust. Average Vp/Vs ratio in Assam Valley and Shillong Plateau suggest that the crust is felsic-to-intermediate and intermediate-to-mafic beneath Shillong Plateau and Assam Valley, respectively. Results show that lower crust rocks beneath the Shillong Plateau and Assam Valley lies between mafic granulite and mafic garnet granulite.

Tectonothermal evolution of the continental crust beneath the Yakutian diamondiferous province (Siberian craton): U–Pb and Hf isotopic evidence on zircons from crustal xenoliths of kimberlite pipes

U–Pb age and Hf-isotope data were collected on zircons from crustal xenoliths from the Late Devonian kimberlite pipes of Markha terrane. The xenoliths include mafic garnet granulites (Gt+Pl+Cpx±Opx±Amp±Bt±Scp) and garnet–biotite gneisses (Grt+Bt+Pl+Kfs+Qtz±Scp). The data from this study demonstrate that the crust of the Markha terrane experienced several tectonothermal events. Zircons with Archean Hf model ages (TDM=3.13–2.5Ga) are predominant in all xenoliths with one exception. These data imply that most of the lower and middle crust beneath the Markha terrane was produced in the Archean time. Later this Paleoarchean crust was significantly reworked in several tectonothermal events, including a Neoarchean stage (2.9–2.5Ga), and several Paleoproterozoic metamorphic stages (1.98, 1.9 and 1.8Ga). There is very little evidence for generation of juvenile crust associated with these metamorphic events. Importantly, crustal xenoliths from kimberlite pipes of the Markha terrane record all the same events that occurred in the adjacent Daldyn and Magan terranes.

Melting, fluid migration and fluid-rock interactions in the lower crust beneath the Bakony-Balaton Highland volcanic field: a silicate melt and fluid inclusion study

Plio-Pleistocene alkali basalt hosted mafic garnet granulite xenoliths were studied from the Bakony-Balaton Highland Volcanic Field (BBHVF) to trace fluid-melt-rock interactions in the lower crust. Two unique mafic garnet granulite samples were selected for analyses (optical microscopy, microthermometry, electron microprobe, Raman and IR spectroscopy), which contain a clinopyroxene-plagioclase vein and patches with primary silicate melt inclusions (SMI). The samples have non-equilibrium microtexture in contrast with the overwhelming majority of previously studied mafic garnet granulite xenoliths. Primary silicate-melt inclusions were observed in plagioclase, clinopyroxene and ilmenite in both xenoliths. The SMI-bearing minerals located randomly in Mi26 and in a clinopyroxene-plagioclase vein on the edge of Sab38 granulites. Petrography and the fluid and melt inclusion study suggest that at least three fluid events occurred in the deep crust represented by these xenoliths. 1. Primary CO2-dominated ± CO ± H2S fluid inclusions were observed in the wall-rock part of Sab38 xenolith. 2. The crystallization of new clinopyroxene from melt, with CO2 + H2O fluid. 3. The crystallization of new plagioclase occurred in a heterogeneous fluid-melt system with additional N2 and CH4 during crystallization. A local reaction was observed between sphene and acidic melt, which formed ilmenite + clinopyroxene + plagioclase ± orthopyroxene. The ‘water’ content of the rock forming minerals was determined by infrared spectroscopy. The calculated bulk ‘water’ content of the Mi26 xenolith is 171 ± 51 ppm wt. %. The bulk wall rock part of the Sab38 granulite contains 55 ± 17 ppm wt. % of ‘water’, whereas the bulk plagioclase-clinopyroxene vein contains 278 ± 83 ppm wt. %. These results imply a very dry lower crust, locally hydrated by percolating fluids and melts.

Titanite evidence for Triassic thickened lower crust along southeastern margin of North China Craton

Abstract Titanite U–Pb isotopic and major and trace element compositions of one mafic garnet granulite from a rare suite of lower crustal xenoliths (e.g., eclogite, garnet pyroxenite, and mafic garnet granulite) hosted in Early Cretaceous dioritic porphyries in the Xu–Huai area along the southeastern margin of the North China Craton (NCC) were analyzed by laser ablation ICP-MS. Titanite occurs as granular grains or coronary rims on rutile. The coronary titanite is clearly a secondary product of rutile decomposition. The granular titanite exhibits zonation in U–Pb age and chemical composition. Petrographic and geochemical evidence suggests that the zonation was formed by thermal diffusion and later fluid-assisted recrystallization. Occurrences of granular titanite between garnet grains point to a pressure of > 10 kbar, while inclusions of rutile inside granular titanite rims imply that the pressure might have reached 15 kbar. Granular titanite cores give U–Pb ages of 237–241 Ma and Zr-temperatures of 794–831 °C at 10 kbar and 850–892 °C at 15 kbar, indicating high-pressure granulite-facies metamorphism. Together with previous P–T estimates of coeval eclogite-facies xenoliths, a geotherm of above 60 mW m− 2 is implied. The geotherm plots below the temperature field of amphibole dehydration melting, consistent with presence of abundant amphibole. This geotherm is similar to that of the Kohistan arc, which has preserved a 12-km-thick dense lower crust, but significantly cooler than the geotherm of the Talkeetna arc, where most of the dense lower crust has been foundered. Our results provide new evidence for Triassic thickened dense lower crust along the southeastern margin of the NCC. By comparison with the Kohistan and Talkeetna arc crusts, we suggest that this dense lower crust was not hot enough to be foundered in the Triassic. Foundering must have occurred in the Jurassic–Cretaceous in order to explain the present-day seismic velocity structure characterized by a sharp Moho, overall slow velocities in the lower crust, and a thin crustal thickness in the Xu–Huai area and other parts of the eastern NCC. We suggest that the Jurassic–Cretaceous foundering was related to the Pacific subduction. On one hand, the Jurassic subduction may have further thickened the southeastern margin of the NCC prior to Cretaceous extension, leading to greater instability of the lower crust. On the other hand, the subduction-related magma provided heat and water that weakened the lower crust, resulting in the final foundering. The large contrast in mineralogy between the Xu–Huai eclogite-facies xenoliths and nearby Nushan garnet-free granulite xenoliths entrained by Quaternary basalts indicates > 20 km removal of the lower crust along the southeastern margin of the NCC.

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.

Crustal composition model across the Bangong–Nujiang suture belt derived from INDEPTH III velocity data

The Bangong–Nujiang suture (BNS) belt separates the Lhasa and Qiangtang plates and, as indicated from previous teleseismic tomography, receiver functions, and active source seismic experiments, record the convergence and collision between the Indian and Eurasian plates. In this study, we construct a crustal composition model along the 400 km long NNW–SSE-oriented INDEPTH III profile by comparing the P-wave velocity (VP ), S-wave velocity (VS), VP /VS ratio, density (ρ) and Lam´ e impedances (ρλ, ρμ) to those determined by laboratory measurements of different crustal rock types. Corrections for pressure and temperature are performed in order to directly compare the physical properties determined from the INDEPTH III data with these laboratory measurements made at room temperature and variable pressures. Inversion of the INDEPTH III data set to rock compositions indicates that there are a number of differences in the crustal composition of the Lhasa and Qiangtang plates. For example, while the average composition of the upper crust in both plates is mainly characterized by granite–granodiorite and granite gneiss, these lithologies are much thicker beneath the Lhasa plate. Metagreywacke provides a best fit lithology for the upper crust beneath the BNS. The middle crust in both plates is somewhat similar, with diabase being the best fit for the Lhasa plate and basalt for the Qiangtang plate. The lower crust is quite distinctive for the two plates, with mafic garnet granulite, gabbronorite or troctolite providing a best fit for the Lhasa plate, whereas mafic garnet granulite, marble and anorthositic granulite being the best fit for the Qiangtang plate. While the difference in composition between the two plates may be affected by the different thermal regimes determined for them, it nevertheless appears that the BNS clearly forms a boundary between the physical properties and therefore the composition of the Lhasa and Qiangtang plates.

Petrology and Geochemistry of Granulite Xenoliths from Udachnaya and Komsomolskaya Kimberlite Pipes, Siberia

Lower crustal xenoliths from the Udachnaya and Komsomolskaya kimberlite pipes in Siberia are mainly meta-igneous mafic garnet granulites, with subordinate feldspar-rich garnet granulites. Pressure and temperature estimates are interpreted as the conditions in the lower crust at the time of the last granulite-facies metamorphic event (800–890°C) followed by cooling to 610–720°C with a pressure decrease from 1·2 to 0·8 GPa. Most of the xenoliths show minor alteration. Leaching experiments demonstrate that their isotopic, major and trace element compositions have been affected by interaction not only with the host kimberlite but also with a fluid mobilized from local sedimentary country rocks. To obtain unambiguous compositional data we have calculated the composition of selected samples using modal analyses, electron microprobe and laser ablation inductively coupled plasma mass spectrometry data for constituent minerals. The reconstructed protoliths of most of the xenoliths were Fe-tholeiites of intraplate affinity, similar to some Archean basalts, whereas the others show characteristics of subduction-related magmas. However, the mafic granulites are strongly depleted in Rb, Th and U, which were removed by a small-degree partial melt. A protolith age of c. 3 Ga is supported by a disturbed Sm–Nd isochron, Nd and Hf model ages, and published U–Pb ages of zircon cores.

Oriented growth of garnet by topotactic reactions and epitaxy in high‐pressure, mafic garnet granulite formed by dehydration melting of metastable hornblende‐gabbronorite (Jijal Complex, Kohistan Complex, north Pakistan)

AbstractGarnet growth in high‐pressure, mafic garnet granulites formed by dehydration melting of hornblende‐gabbronorite protoliths in the Jijal complex (Kohistan palaeo‐island arc complex, north Pakistan) was investigated through a microstructural EBSD‐SEM and HRTEM study. Composite samples preserve a sharp transition in which the low‐pressure precursor is replaced by garnet through a millimetre‐sized reaction front. A magmatic foliation in the gabbronorite is defined by mafic‐rich layering, with an associated magmatic lineation defined by the shape‐preferred orientation (SPO) of mafic clusters composed of orthopyroxene (Opx), clinopyroxene (Cpx), amphibole (Amp) and oxides. The shape of the reaction front is convoluted and oblique to the magmatic layering. Opx, Amp and, to a lesser extent, Cpx show a strong lattice‐preferred orientation (LPO) characterized by an alignment of [001] axes parallel to the magmatic lineation in the precursor hornblende‐gabbronorite. Product garnet (Grt) also displays a strong LPO. Two of the four 〈111〉 axes are within the magmatic foliation plane and the density maximum is subparallel to the precursor magmatic lineation. The crystallographic relationship 〈111〉Grt//[001]Opx,Cpx,Amp deduced from the LPO was confirmed by TEM observations. The sharp and discontinuous modal and compositional variations observed at the reaction front attest to the kinetic inhibition of prograde solid‐state reactions predicted by equilibrium‐phase diagrams. The P–T field for the equilibration of Jijal garnet granulites shows that the reaction affinities are 5–10 kJ mol.−1 for the Grt‐in reaction and 0–5 kJ mol.−1 for the Opx‐out reaction. Petrographic and textural observations indicate that garnet first nucleated on amphibole at the rims of mafic clusters; this topotactic replacement resulted in a strong LPO of garnet. Once the amphibole was consumed in the reaction, the parallelism of [001] axes of the mafic‐phase reactants favoured the growth of garnet crystals with similar orientations over a pyroxene substrate. These aggregates eventually sintered into single‐crystal garnet. In the absence of deformation, the orientation of mafic precursor phases conditioned the nucleation site and the crystallographic orientation of garnet because of topotaxial transformation reactions and homoepitaxial growth of garnet during the formation of high‐pressure, mafic garnet‐granulite after low‐pressure mafic protoliths.

Crust‐mantle boundary in the central Fennoscandian shield: Constraints from wide‐angle P and S wave velocity models and new results of reflection profiling in Finland

Our paper presents analysis of the crust‐mantle boundary in the central Fennoscandian shield based on new P and S wave two‐dimensional velocity models of the SVEKA'81, SVEKA'91 and FENNIA wide‐angle reflection and refraction profiles and on results of a new seismic reflection experiment in Finland (Finnish Reflection Experiment (FIRE)). In this area, the crust is extremely thick (50–60 km), and the Moho boundary is difficult to detect using methods based on interpretation of P waves only (near vertical profiling and wide‐angle experiments). However, the S wave reflections from the Moho boundary (SmS) are frequently more pronounced in wide‐angle data than the P wave reflections (PmP). In order to infer the crust‐mantle transition, we developed new P and S wave velocity models using reprocessing of the old data and compared them to record sections of collocated reflection profiles and to published values of Vp and Vp/Vs for the main types of lower crustal and upper mantle rocks. On the basis of the lateral variations of Vp, Vp/Vs, and reflectivity in the lower crust and upper mantle, three main types of the crust‐mantle boundary were distinguished. The first type corresponds to eclogitized lower crust that overlies peridotitic upper mantle. In this case, the Moho coincides with the lithological crust‐mantle boundary. The second type corresponds to lower crust composed of mafic garnet granulites overlying the peridotitic upper mantle, for which the Moho and the lithologic crust‐mantle boundary coincide as well. The third type corresponds to the mafic garnet granulites underlain by a layer of eclogitic upper mantle. In the latter case, the lithological crust‐mantle boundary is deeper than the Moho.

Modal mineralogy and chemical composition of the Uralide lower crust determined from physical properties data

Modeling of rock physical properties of the Uralide lower crust is carried out using the Physprops Excel worksheet, which calculates its P-wave (Vp), S-wave (Vs), Poisson's ratio ( σ), and density ( ρ). The mineral modal abundance and composition of mafic granulite, mafic garnet granulite, amphibolite, and gabbronorite is modeled, and the composition of each is determined by converting the aggregate mineralogy into wt.% oxides. The modeled wt.% oxide composition of mafic (+ garnet) granulite fit well with those estimated for post-Archean terrains, and all are within 1 σ of the average of all lower crustal xenoliths. The modeled gabbronorite also shows a reasonable fit with these estimates. The modeled amphibolite compositions show variations in the wt.% SiO 2, FeO and MgO with respect to the estimates of mafic granulite and amphibolite. The differences in the composition across the Uralide lower crust are largely due to the presence or absence of garnet in the mafic granulite, which may be a result of higher temperatures having been reached in the interior part of the orogen.

Petrogenesis of Mafic Garnet Granulite in the Lower Crust of the Kohistan Paleo-arc Complex (Northern Pakistan): Implications for Intra-crustal Differentiation of Island Arcs and Generation of Continental Crust

We report the results of a geochemical study of the Jijal and Sarangar complexes, which constitute the lower crust of the Mesozoic Kohistan paleo-island arc (Northern Pakistan). The Jijal complex is composed of basal peridotites topped by a gabbroic section made up of mafic garnet granulite with minor lenses of garnet hornblendite and granite, grading up-section to hornblende gabbronorite. The Sarangar complex is composed of metagabbro. The Sarangar gabbro and Jijal hornblende gabbronorite have melt-like, light rare earth element (LREE)-enriched REE patterns similar to those of island arc basalts. Together with the Jijal garnet granulite, they define negative covariations of LaN, YbN and (La/Sm)N with Eu* [Eu* 1⁄4 2 · EuN/(SmN þ GdN), where N indicates chondrite normalized], and positive covariations of (Yb/Gd)N with Eu*. REE modeling indicates that these covariations cannot be accounted for by high-pressure crystal fractionation of hydrous primitive or derivative andesites. They are consistent with formation of the garnet granulites as plagioclase–garnet assemblages with variable trapped melt fractions via either high-pressure crystallization of primitive island arc basalts or dehydration-melting of hornblende gabbronorite, provided that the amount of segregated or restitic garnet was low (<5 wt %). Field, petrographic, geochemical and experimental evidence is more consistent with formation of the Jijal garnet granulite by dehydration-melting of Jijal hornblende gabbronorite. Similarly, the Jijal garnet-bearing hornblendite lenses were probably generated by coeval dehydration-melting of hornblendites. Melting models and geochronological data point to intrusive leucogranites in the overlying metaplutonic complex as the melts generated by dehydration-melting of the plutonic protoliths of the Jijal garnetbearing restites. Consistent with the metamorphic evolution of the Kohistan lower arc crust, dehydration-melting occurred at the mature JOURNAL OF PETROLOGY VOLUME 47 NUMBER 10 PAGES 1873–1914 2006 doi:10.1093/petrology/egl030

Multi-stage origin of the lower crust of the Karelian craton from 3.5 to 1.7 Ga based on isotopic ages of kimberlite-derived mafic granulite xenoliths

Mafic garnet granulite xenoliths recovered from ∼500 to 600 Ma old eastern Finland kimberlites provide direct information on the petrology and physical properties of the lower cust below the Archean Karelian craton. Mineral thermobarometry, together with isotopic, petrological and seismic velocity constraints, imply that the xenolith suite is derived from a geophysically-determined, dense, high-velocity layer at the base of the crust (40–58 km depth). Single grain zircon U–Pb dates and Nd model ages ( T DM) imply that this is a hybrid layer consisting of both Archean and Proterozoic mafic granulites. Zircon ages of up to ∼3.5 Ga and Nd T DM model ages ∼3.7 Ga of the xenoliths are equivalent to those of the oldest upper crustal assemblages. During the Proterozoic, transient heating of these Archean granulites by voluminous basic magmatic intrusions resulted in multiple younger zircon generations within individual xenoliths. The most important post-Archean lower crust growth took place during ∼1.9 Ga accretion of the Svecofennian arc complex to the craton margin, when underplating Proterozoic basaltic magmas became mingled with pre-existing Archean mafic granulites. Later, post-orogenic (∼1.80–1.73 Ga) transient heating of the lower crust occurred as a response to magmas ponded at the uppermost lithospheric mantle. In conclusion, Karelian lower crust records the geological evolution of the craton margin environment through a period of nearly 4 billion years. Our data add a new piece to the emerging picture that emphasises the global importance of the 3.5 Ga crustal growth episode. We suggest that it represents a major mantle-plume event – more widespread than previously recognized – when a significant fraction of Paleoarchean continental crust was formed.

Lithological interpretation of crustal composition in the Fennoscandian Shield with seismic velocity data

In this study, we report the results of an investigation of lithological interpretation of the crust in the central Fennoscandian Shield (in Finland) using seismic wide-angle velocity models and laboratory measurements on P- and S-wave velocities of different rock types. The velocities adopted from wide-angle velocity models were compared with laboratory velocities of different rock types corrected for the crustal PT conditions in the study area. The wide-angle velocity models indicate that the P-wave velocity does not only increase step-wise at boundaries of major crustal layers, but there is also gradual increase of velocity within the layers. On the other hand, the laboratory measurements of velocities indicate that no single rock type is able to provide the gradual downward increasing trends. Thus, there must be gradual vertical changes in rock composition. The downward increase of velocities indicates that the composition of the crust becomes gradually more mafic with increasing depth. We have calculated vertical velocity profiles for a range of possible crustal lithological compositions. The Finnish crustal velocity profiles require a more mafic composition than an average global continental model would suggest. For instance, on the SVEKA'81 transect, the calculated models suggest that the crustal velocity profiles can be simulated with rock type mixtures where the upper crust consists of felsic gneisses and granitic–granodioritic rocks with a minor contribution of amphibolite and diabase. In the middle crust, the amphibolite proportion increases. The lower crust consists of tonalitic gneiss, mafic garnet granulite, hornblendite, pyroxenite and minor mafic eclogite. Assuming that these rock types are present in sufficiently extensive and thick layers, they would also have sufficiently high acoustic reflection coefficients for generating the generally well-developed reflectivity in the crust in the central part of the shield. Density profiles calculated from the lithological models suggest that there is practically no density contrast at Moho in areas of the high-velocity lower crust. Comparison of reflectors from FIRE-1 and FIRE-3 transects and the velocity model from SVEKA'81 wide-angle transect indicated that the reflectors correlate with velocity layering, but the three-dimensional structures of the crust complicate such comparisons.

Composition of the Uralide crust from seismic velocity (Vp, Vs), heat flow, gravity, and magnetic data

P-wave velocity (Vp), S-wave velocity (Vs), Poisson’s ratio (σ), heat flow, potential field, and surface geological data are integrated to constrain a model for the composition of the Uralide crust along the URSEIS transect. The model is constructed using published laboratory measurements of Vp, Vs, σ and density for a variety of crustal rock types. These laboratory data have been corrected for depth (pressure) and the Uralides temperature–depth function. The model shows clear differences between the composition of the old continental crustal nucleus of the East European Craton and the newly added crust of the accreted arc terranes to the east. The crust of the East European Craton is more felsic than that of the Magnitogorsk and East Uralian zones, and the latter two have a lowermost crust whose characteristics indicate a high garnet content (mafic garnet granulite) and/or the presence of hornblendite. The overall composition of the arc terranes is basaltic. Physical properties suggest that eclogite is not present in the arc terranes, or if present it exists in small amounts that are below the resolution of the data set. The lack of eclogite in the lower crust favours an intracrustal differentiation model for the evolution of the bulk composition of the continental crust. Nevertheless, the absence of surface uplift, the lack of metamorphism and late orogenic mantle melts, and the current crustal thickness indicate that crustal thinning did not affect the bulk composition of the Uralide crust.

Seismic and laboratory constraints on crustal formation in a former continental arc (ACCRETE, southeastern Alaska and western British Columbia)

The ACCRETE project studies continent formation by assemblage of exotic terranes and arc magmatism. A marine‐land seismic profile crosses the terranes with dense sampling and remarkably strong S waves as well as P waves. Unique, high‐quality S‐wave seismic data provide further constraints on interpretation and are used together with new laboratory P‐ and S‐wave velocity measurements (corrected for high temperatures within and at the base of the crust) to make a lithologic and petrologic interpretation. VP/VS ratios indicate three distinct terranes that were assembled to form continental crust. These terranes are the outboard, accreted Alexander‐Wrangellia terrane, the Coast Mountains Batholith (CMB) magmatic arc, and the inboard Stikinia terrane. Below the CMB, VP and VP/VS to a depth of 10–15 km are appropriate for tonalite and diorite. The VP and VP/VS increase with depth indicating that the rocks become more mafic, but VP/VS is not high enough for gabbro. The VP and VP/VS of the lower crust are consistent with a mixture of mafic garnet granulite and restite, whose quartz content lowers the VP/VS ratio below that of gabbro. When corrected for high temperature and uplift due to exhumation, the crust under the CMB exhibits seismic properties of an average continental crust. These results suggest that gabbro could have intruded a metasedimentary pile in the deep crust to cause melting of the metasediments to form intrusions of the CMB, leaving behind a mixture of mafic garnet granulite and sillmanite‐garnet‐quartz restite and generating the deeper part of what becomes an average continental crustal section.

Garnet Granulite Xenoliths from the Northern Baltic Shield--the Underplated Lower Crust of a Palaeoproterozoic Large Igneous Province?

Garnet granulite facies xenoliths hosted in Devonian lamprophyres from the Kola Peninsula are interpreted to represent the high-grade metamorphic equivalents of continental flood tholeiites, emplaced into the Baltic Shield Archaean lower crust in early Proterozoic time. Geochronological data and similarities in major and trace element geochemistry suggest that the xenoliths formed during the same plume-related magmatic event that created a widespread Palaeoproterozoic large igneous province (LIP) at 2·4–2·5 Ga. They are, thus, the first samples of the lower crust of a Palaeoproterozoic LIP to be studied in petrological detail. The suite includes mafic granulites (gar + cpx + rutile ± plag ± opx ± phlog ± amph), felsic granulites (plag + gar + cpx + rutile ± qtz ± Kspar ± phlog ± amph) and pyroxenites (± phlog ± amph), but mafic garnet granulites predominate. Although some samples are restites, there is no evidence for a predominance of magmatic cumulates, as is common for Phanerozoic lower-crustal xenolith suites. Metasediments are also absent. Phlogopite and/or amphibole occur in xenoliths of all types and are interpreted to be metasomatic in origin. The K-rich metasomatic event occurred at ∼2·0 Ga, and led to substantial enrichment in Rb, K, LREE/HREE, Th/U, Th/Pb and, to a lesser extent, Nb and Ti. The fluids responsible for this metasomatism were probably derived from a second plume that arrived beneath the region at this time. Evidence for partial melting of mafic crust exists in the presence of migmatitic granulites. The timing of migmatization overlaps that of metasomatism, and it is suggested that migmatization was facilitated by the metasomatism. The metamorphism, metasomatism and migmatization recorded in the Kola granulite xenoliths may be representative of the processes responsible for converting Archaean LIP-generated proto-continents into continental crust.