Lower Crustal Granulite Xenoliths Research Articles

The lower crust of the Northern broken edge of Gondwana: Evidence for sediment subduction and syn-Variscan anorogenic imprint from zircon U-Pb-Hf in granulite xenoliths

The continental basement in the Eastern Mediterranean area represents the northern edge of Gondwana, which has been the site of repeated crustal accretion and has subsequently been modified by consecutive rifting events. We investigated the geologic and thermal history of the North Gondwana lower crust by examining the U-Pb-Hf isotope systematics in zircons within 6 mafic granulite xenoliths from Pliocene lava cone in North Israel. The lava cone protrudes through the platform cover that seals the late Neoproterozoic junction between the Arabian-Nubian basement to the South and the Cadomian basement exposed in the Taurides to the North. The mafic granulite xenoliths are composed of plagioclase + orthopyroxene + clinopyroxene ± garnet ± spinel ± secondary amphibole. U-Pb zircon ages from the granulites vary among the different samples with distinct zircon age populations at 400–1200 Ma, 170–350 Ma, and 3.6–4.2 Ma, attesting the lower crust preserves a prolonged thermal and igneous history. While 400–550 Ma U-Pb ages are interpreted to be the result of Pb loss, the wide scatter of zircon grains aged between 550 and 1200 Ma, alongside their diverse εHf(t) values (−25–+10), is an extraordinary evidence for the accretion of Neoproterozoic sediments into the North Gondwana arc root lower crust. The U-Pb-Hf signature of these zircons resembles Cadomian sediments of the Tauride block to the north, indicating southward (present coordinates) subduction under North Gondwana and possible accretion of fore-arc sediments to the lower crust through relamination in the latest Neoproterozoic. One xenolith contained metamorphic-shaped zircons aged 170–350 Ma with positive εHf values and Hf-TDM of 0.85 Ga interpreted to reflect Paleozoic recycling of the Neoproterozoic juvenile Arabian basement, which we consider to form a major component of the lower crust in the region. An overwhelming cluster of Carboniferous zircons concentrating at 305 Ma with exclusively negative εHf values around −6, was retrieved from three xenoliths. Some of these zircons portrayed igneous textures and shape. While Carboniferous igneous activity is the hallmark of Western Europe's Variscan orogeny, the latter did not affect the southern rifted edge of Neo-Tethys where our xenoliths were retrieved. The Paleozoic age-Hf composition in our xenoliths is therefore interpreted to result from syn-Variscan recycling of Neoproterozoic sedimentary remains in the lower crust, and some degree of melting in a non-orogenic environment. Rather than with horizontal plate motions and orogeny, the Carboniferous zircon ages in the xenoliths appear to coalesce with significant vertical movements that created continental scale unconformities and a broad basin and swell architecture known to develop over the entire North Gondwana margin at that time. The Carboniferous aged zircons in northern Israel lower crustal xenoliths are therefore a unique gauge of the thermal perturbation that accompanied the large-scale mantle dynamics below the then passive North African margin of Gondwana, while Variscan orogenic accretion occurred on the Eurasian margin. These lower crustal granulite xenoliths therefore contain important information with respect to the nature of the lower crust under Israel, with implications on the geodynamic setting during the Cadomian and Variscan cycles.

SHRIMP zircon U–Pb dating and Hf isotope analyses of the Muniushan Monzogranite, Guocheng, Jiaobei Terrane, China: Implications for the tectonic evolution of the Jiao–Liao–Ji Belt, North China Craton

The Paleoproterozoic granitoid rocks in the Jiaobei Terrane, which lies in the southern part of the Jiao–Liao–Ji Belt (JLJB) in China, provide valuable information on the tectonic and evolutionary history of the North China Craton (NCC). SHRIMP U–Pb dating of zircons from the Muniushan Monzogranite in the Guocheng gold deposit area, as well as in situ Hf isotope analyses of the zircons, were used to constrain the ages of emplacement and metamorphism, and to understand the petrogenesis of the granite. The geochronological data show emplacement ages of ca. 2.1Ga and metamorphic ages of ca. 1.85Ga. The magmatic cores of the zircons have εHf(t) values of −0.51 to 4.00 (average 1.98) and depleted mantle two-stage model ages (TDM2) of 2444–2703Ma (average 2563Ma). The rocks show the same trends of crustal evolution as ca. 2.5Ga tonalite–trondhjemite–granodiorite (TTG) rocks in the Jiaobei Terrane, indicating that they were formed by the remelting of those TTG rocks. The Muniushan Monzogranite is part of a linear belt of ca. 2.2–2.0Ga Paleoproterozoic granitoids within the JLJB. The granitoids are A-types, and are accompanied by coeval mafic intrusions and bimodal volcanic rocks, thus indicating widespread extension along the JLJB in the period 2.2–2.0Ga. The ca. 1.85Ga metamorphism was related to the collision of the Longgang and Nangrim blocks. The Paleoproterozoic evolution of the NCC constrains the crustal architecture and the source of Mesozoic granitoids in the Jiaobei–Sulu region. Inherited zircons of Paleoproterozoic age are widespread in the Mesozoic granitoids. In addition, Paleoproterozoic ages have been reported from lower crustal granulite xenoliths in the Sulu Orogen, and the southeastern margin of the NCC mirrors the evolution of the JLJB. It is probable, therefore, that NCC lower crust underlies the Sulu Orogen, and that the NCC was the main source for the Mesozoic granitoids in the Jiaobei–Sulu region. This view is consistent with the appearance of inherited zircons of Archean and Paleoproterozoic age on both sides of the orogen, normal δ18O values of magmatic zircons, and Paleoproterozoic to Archean two-stage Hf model ages for the granitoids.

Hadean to Neoarchean episodic crustal growth: Detrital zircon records in Paleoproterozoic quartzites from the southern North China Craton

The Archean terranes exposed in several regions of the North China Craton (NCC) provide important windows to evaluate the Hadean-Archean continental crustal evolution history. Here we report results from SIMS zircon geochronology and oxygen isotopes, as well as LA-ICPMS zircon Hf isotopic analyses on detrital zircons from Paleoproterozoic quartzites to probe the early crustal evolution in the southern NCC. Our data show episodic magmatism during the Eoarchean, Paleoarchean, Mesoarchean, Neoarchean and Paleoproterozoic as inferred from the concordant ages of 3.6Ga and 2.2Ga and the well-defined upper intercept ages of 3404±30Ma, 2919±31Ma, 2772±9.5Ma, 2698±4.7Ma, 2652±6.7Ma, and 2532±5.2Ma. The Hf and oxygen isotopic compositions of Eoarchean–Paleoarchean zircons demonstrate that the cratonic nucleus of the NCC was built in the Hadean (∼4.0Ga), similar to the timing of formation of the nuclei of old cratons elsewhere on the globe such as the Yilgarn Craton, Western Australia, and the Slave Craton, Canada, with the Eoarchean–Paleoarchean (3.8Ga, 3.6Ga and 3.4Ga) marking the first phase of crustal reworking of the Hadean crust. The salient aspect of our data is that the Meoarchean to early Neoarchean (3.0–2.6Ga with a peak at 2.7Ga) was a significant period for crustal growth from the mantle throughout the NCC, comparable with the event of major Archean crustal growth worldwide. The latest Neoarchean (∼2.5Ga) marks a period of significant crustal reworking that led to remelting of the newly-formed juvenile mafic crust (∼2.7Ga). Integration with the data obtained from the TTG (tonalite–trondhjemite–granodiorite) and granitoid rocks of the Archean terranes and the lower crustal granulite xenoliths from various parts of the NCC suggests that the episodic Eoarchean via Paleoarchean and Mesoarchean to Neoarchean crustal growth as well as crustal reworking intensely destroyed the ancient cratonic nucleus, resulting in the scarce preservation of the Hadean and Eoarchean remnants in both the upper and lower crust of the NCC. Following the Paleoproterozoic sedimentation of the Archean zircons, apart from Pb loss during the subsequent thermal events, there was no major alteration in the Hf and oxygen isotopic compositions. Thus, our detrital zircon data provide convincing evidence for the repeated crustal growth and reworking of the NCC in the Archean.

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

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

Water in minerals of the continental lithospheric mantle and overlying lower crust: A comparative study of peridotite and granulite xenoliths from the North China Craton

Water contents of clinopyroxene, orthopyroxene and olivine in mantle peridotite xenoliths from the Hannuoba and Nushan basaltic volcanoes (North China Craton) were determined by Fourier transform infrared spectroscopy (FTIR). Results show that contents vary from 50 to 155 ppm (H 2O wt.) for clinopyroxene and from 20 to 55 ppm for orthopyroxene in Hannuoba peridotites, and from 5 to 355 ppm for clinopyroxene and from 5 to 140 ppm for orthopyroxene in Nushan peridotites. Water contents in clinopyroxene and orthopyroxene define a good positive correlation with a ratio of ~ 2.1. This, together with the roughly homogeneous distribution of hydrogen from core to rim region in pyroxene grains, suggests that they have preserved the initial water content of the mantle source. By contrast, olivine in these peridotites contain very little, if any, water, which were probably caused by significant loss of H during their ascent due to the faster H diffusion in olivine relative to pyroxenes and by the relatively lower water content in the mantle source of Hannuoba and Nushan. Previous studies [Xia, Q.K., Yang, X.Z., Deloule, E., Sheng, Y.M., Hao, Y.T., 2006. Water in the lower crustal granulite xenoliths from Nushan, eastern China. Journal of Geophysical Research, 111(B11202) doi:10.1029/2006JB004296; Yang, X.Z., Deloule, E., Xia, Q.K., Fan, Q.C., Feng, M., in press. Water contrast between Precambrian and Phanerozoic continental lower crust of eastern China. Journal of Geophysical Research doi:10.1029/2007JB005541] have shown that the water content ranges from 275 to 720 ppm for clinopyroxene, from 60 to 185 ppm for orthopyroxene and from 65 to 205 ppm for plagioclase in Hannuoba mafic granulite xenoliths, and from 200 to 2330 ppm for clinopyroxene, from 140 to 1270 ppm for orthopyroxene and from 145 to 900 ppm in plagioclase in Nushan mafic granulite xenoliths. The significant difference between the water content of peridotite and that of coexisting mafic granulite xenoliths, e.g. 0–100 ppm (mostly < 50 ppm) vs. 100–1000 ppm (mostly > 200 ppm) for the estimated bulk concentration, demonstrates that the distribution of water in the deep continental lithosphere is vertically heterogeneous. This may be due to the different genesis of peridotites and granulites (the residue of partial melting vs. the product of melt crystallization). Based on these water data, we define the rheological structure of the lowermost crust and uppermost mantle below the Hannuoba and Nushan regions. The viscosity (strength) of the underlying shallow mantle is similar, whereas the deep crust is relatively stronger at Hannuoba and weaker at Nushan. Difference in the viscosity structure may further influence the dynamical processes between different zones in this craton.

A Quartz-bearing Orthopyroxene-rich Websterite Xenolith from the Pannonian Basin, Western Hungary: Evidence for Release of Quartz-saturated Melts from a Subducted Slab

An unusual quartz-bearing orthopyroxene-rich websterite xenolith has been found in an alkali basaltic tuff at Szigliget, Bakony-Balaton Highland Volcanic Field (BBHVF), western Hungary. Ortho- and clinopyroxenes are enriched in light rare earth elements (LREE), middle REE and Ni, and depleted in Nb, Ta, Sr and Ti compared with ortho- and clinopyroxenes occurring in either peridotite or lower crustal granulite xenoliths from the BBHVF. Both ortho- and clinopyroxenes in the xenolith contain primary and secondary silicate melt inclusions, and needle-shaped or rounded quartz inclusions. The melt inclusions are rich in SiO 2 and alkalis and poor in MgO, FeO and CaO. They are strongly enriched in LREE and large ion lithophile elements, and display negative Nb, Ta and Sr anomalies, and slightly positive Pb anomalies. The xenolith is interpreted to represent a fragment of an orthopyroxene-rich body that crystallized in the upper mantle from a hybrid melt that formed by interaction of mantle peridotite with a quartz-saturated silicate melt that was released from a subducted oceanic slab. Although the exact composition of the slab melt cannot be determined, model calculations on major and trace elements suggest involvement of a metasedimentary component.

Water in granulites: implications for the nature and evolution of the lower continental crust

Abstract The lower continental crust is one of the most important sphere-layers in the deep earth and is the direct place where the crust-mantle interactions occur. Granulites are the dominated rocks in the lower crust and have critical implications for the knowledge of the composition, nature and evolution of the deep crust fluids are important mediums influencing many geochemical, geophysical and geodynamical characteristics of the lower crust and may also play a fundamental role in the petrogenesis of granulites and the formation of the lower crusts. In this paper we review recent advances involved with the deep continental crust granulites and fluids and some longstanding debates. Combined with the Fourier-transform infrared spectroscopy (FTIR analysis performed on the mineral assemblages (cpx, opx, plag and grt) in lower crustal granulite xenoliths and terrains (exposed section) from east China, it is suggested that structural water dominated by OH in these nominally anhydrous phases may constitute t...

Water in the lower crustal granulite xenoliths from Nushan, eastern China

Nominally anhydrous clinopyroxene (cpx), orthopyroxene (opx), and plagioclase (pl) from 10 lower crustal granulite (two‐pyroxene granulite and hypersthene granulite) xenoliths in Cenozoic basalts from the Nushan volcano, eastern China, have been analyzed for their hydrogen content by microscopic Fourier transform infrared spectroscopy (Micro‐FTIR). The results demonstrate that hydrogen was incorporated in all these minerals in the manner of OH and that the content (H2O weight) is up to 2360 ppm for cpx, 1170 ppm for opx, and 880 ppm for pl. On the basis of the water content of constitutive minerals and their proportions, whole rock water contents of the Nushan granulites were estimated to be 150–950 ppm. Estimated equilibrium temperatures of the Nushan granulites are in the range of 810–892°C, corresponding to the lowermost crust at Nushan (about 25–30 km). Therefore this study provides direct evidence that the lower continental crust, even the lowermost part devoid of hydrous minerals, can contain a certain amount of water in nominally anhydrous minerals.

Petrology and geochemistry of granulite xenoliths beneath the Nógrád-Gömör Volcanic Field, Carpathian-Pannonian Region (N-Hungary/S-Slovakia)

Abundant upper mantle and rare lower crustal xenoliths have been found in the Plio-Pleistocene alkali basalts of the Nograd-Gomor Volcanic Field, situated in the northern Pannonian Basin, on the border between northern Hungary and southern Slovakia. A few lower crustal granulite xenoliths have been found in a small basaltic pyroclastic cone at Baglyaskő. The mafic granulite xenoliths are plagioclase-bearing hornblende clinopyroxenites, plagioclase-bearing clinopyroxene hornblendites and plagioclase-bearing clinopyroxenites. They contain unusual symplectites, composed of spinel feldspar and clinopyroxene. These symplectites are interpreted as the product of garnet breakdown. Following the breakdown reaction, the symplectite underwent in situ partial melting. Mineral constituents of these granulite xenoliths have chemical compositions similar to those of other granulite xenoliths worldwide. However, a distinctive positive Pb and Ce anomaly in mineral constituents of these granulites is characteristic.

Lower crustal granulite xenoliths from the Pannonian Basin, Hungary. Part 1: mineral chemistry, thermobarometry and petrology

Lower crustal granulite xenoliths have been found in Pliocene basaltic tuffs at two newly discovered localities in the western Pannonian Basin. Most of them are mafic meta-igneous rocks, among which garnet-rich lithologies are common. In general, clinopyroxene, garnet and plagioclase, with minor ilmenite and scapolite dominate their mineralogy. Several are amphibole-bearing, which is interpreted as evidence for a recent metasomatic event in the lower crust beneath the Pannonian Basin. Meta-sedimentary granulite xenoliths have also been identified in the region for the first time. Their mineralogy is characterised by the absence of clinopyroxene combined with a high proportion of Al-rich phases, especially plagioclase and garnet together with subordinate spinel, sphene, rutile, orthopyroxene and quartz. One sample contains biotite and several contain graphite. Thermobarometric results indicate temperatures and pressures of 800–950 °C and 8–15 kbar for all xenolith types, considerably higher than for most granulite xenoliths from other regions of Phanerozoic Europe. Given that the Pannonian Basin is currently underlain by thin crust (25–30 km thick), the pressure estimates indicate that the crust must have been considerably thicker (40–50 km) prior to the Tertiary collapse in the region. The higher pressures recorded for the meta-igneous xenoliths thus suggest that they formed at (or before) a time when the crust was thicker, i.e. pre-Tertiary. A composite meta-igneous/meta-sedimentary xenolith provides direct evidence for interaction between sediments (or metasediments) and the magmas that gave rise to the meta-igneous granulites, indicating that the metasediments are at least as old as the meta-igneous granulites. The protoliths of the meta-igneous xenoliths had primitive mafic to slightly evolved tholeiitic liquid compositions. They are dominated by LREE-depleted compositions that resemble those of mid-ocean ridge basalts. Such compositions are rare in lower crustal xenolith suites. They are also very low in large ion lithophile elements such as K and Rb, but relatively enriched in Ba and Pb. A few slightly LREE-enriched meta-igneous xenoliths are also K and Rb depleted. In contrast, the meta-sedimentary granulites are strongly LREE enriched, but have very variable LILE contents. One possible explanation for the unusual compositions is a process of subduction–accretion of oceanic crust beneath the continental crust, rather than basaltic underplating.

Lower crustal granulite xenoliths from the Pannonian Basin, Hungary, Part 2: Sr–Nd–Pb–Hf and O isotope evidence for formation of continental lower crust by tectonic emplacement of oceanic crust

Mafic granulite xenoliths from the lower crust of the Pannonian Basin are dominated by LREE-depleted bulk-rock compositions. Many of these have MORB-like 143Nd/144Nd but 87Sr/86Sr is elevated relative to most MORBs. Their δ18O values cover a wide range from +3.8 to +9.5‰. A group of LREE-enriched mafic granulites have higher 87Sr/86Sr (0.704–0.708) and lower 143Nd/144Nd (0.5128–0.5124), with higher δ18O values on average (+7.8 to +10.6‰) than the LREE-depleted granulites. The LREE-enriched granulites are, however, isotopically similar to newly discovered metasedimentary granulite xenoliths. A sublinear correlation in eHf–eNd isotope space has a shallower slope than the crust–mantle array, with the metasedimentary rocks forming the low eHf end member; the radiogenic end is restricted to the LREE-depleted granulites and these overlap the field of MORB. Pb isotopes for the LREE-depleted samples are less radiogenic on average than those of the LREE-enriched and metasedimentary xenoliths, and metasedimentary granulites have consistently higher 208Pb/204Pb. The wide range in δ18O over a restricted range in Nd and Sr isotope values, in combination with the predominance of LREE-depleted trace-element compositions, is consistent with an origin as a package of hydrothermally altered oceanic crust. The existence of δ18O values lower than average MORB and/or mantle peridotite requires that at least some of these rocks were hydrothermally altered at high temperature, presumably in the oceanic lower crust. The low 143Nd/144Nd of the LREE-enriched mafic granulites cannot be explained by simple mixing between a LREE-depleted melt and an enriched component, represented by the recovered metasediments. Instead, we interpret these rocks as the metamorphic equivalent of the shallowest levels of the ocean crust where pillow basalts are intimately intercalated with oceanic sediments. A possible model is accretion of oceanic crustal slices during subduction and convergence, followed by high-grade metamorphism during the Alpine orogeny.

Proterozoic zircon ages from lower crustal granulite xenoliths, Kola Peninsula, Russia: evidence for crustal growth and reworking

Lower crustal garnet granulite xenoliths from beneath the Kola–Karelian domain of the Fennoscandian Shield (Russia) were brought to the surface in Devonian lamprophyre diatremes. To establish the relationships between processes in the lower crust and geological events recorded in the upper crust, zircons from two of these xenoliths were dated by ion microprobe. One xenolith contains mainly primary magmatic zircon grains that yielded an average age of 2.47 Ga. The Kola–Karelia domain formed part of a large basaltic igneous province in the earliest Proterozoic ( c. 2.45 Ga); thus, such xenoliths probably represent deep-seated equivalents of this magmatism. A second garnet granulite xenolith contained only internally featureless zircons. Most of these yielded concordant ages between 1.77–1.61 Ga. This age distribution indicates that at c. 1.7 Ga the protolith was migmatized during an event that may be related to emplacement of granites and pegmatites in the region. However, two zircon grains in the second xenolith gave much younger ages of 1.47 and 1.45 Ga. This spread in the ages is related to an unknown younger event around 1.4 Ga (or perhaps later) when the c. 1.7 Ga ’migmatite‘ zircons were reset to variable degrees. These zircon ages are evidence for the complex evolution of the lower crust, recording multiple events, some of which are not currently expressed on the surface.

Rb–Sr and Sm–Nd isotopic studies of mafic igneous rocks from the Ryoke plutono‐metamorphic belt in the Setouchi area, Southwest Japan: implications for the genesis and thermal history

Abstract Rb–Sr and Sm–Nd isochron ages were determined for whole rocks and mineral separates of hornblende‐gabbros and related metadiabases and quartz‐diorite from Shodoshima, Awashima and Kajishima islands in the Ryoke plutono‐metamorphic belt of the Setouchi area, Southwest Japan. The Rb–Sr and Sm–Nd whole‐rock‐mineral isochron ages for six samples range from 75 to 110 Ma and 200–220 Ma, respectively. The former ages are comparable with the Rb–Sr whole‐rock isochron ages reported from neighboring Ryoke granitic rocks and are thus due to thermal metamorphism caused by the granitic intrusions. On the contrary, the older ages suggest the time of formation of the gabbroic and related rocks. The initial 87Sr/86Sr and 143Nd/144Nd ratios of the gabbroic rocks (0.7070–0.7078 and 0.51217–0.51231 at 210 Ma, respectively) are comparable with those of neighboring late Cretaceous granites and lower crustal granulite xenoliths from Cenozoic andesites in this region. Because the gabbroic rocks are considered to be fragments of the lower crustal materials interlayered in the granulitic lower crust, their isotopic signature has been inherited from an enriched mantle source or, less likely, acquired through interaction with the lower crustal materials. The Sr and Nd isotopic and petrologic evidence leads to a plausible conclusion that the gabbroic rocks have formed as cumulates from hydrous mafic magmas of light rare earth element‐rich (Sm/Nd &lt; 0.233) and enriched isotopic (ɛSr &gt; 0 and ɛNd &lt; 0) signature, which possibly generated around 220–200 Ma by partial melting of an upper mantle. We further conclude that they are fragments of refractory material from the lower crust caught up as xenoblocks by granitic magmas, the latter having been generated by partial melting of granulitic lower crustal material around 100 Ma.

Lower crustal granulite xenoliths from the Arkhangelsk kimberlite pipes: petrological, geochemical and geophysical results

Five mafic garnet granulite xenoliths have been obtained from Devonian kimberlite hosts from the Pachuga field, Arkhangelsk, NW Russia. Whole rock major and trace element data, Sr and Nd isotopic analyses, mineral chemistry, calculated compressional wave velocities and densities of the xenoliths are reported here. The xenoliths are medium grained and are reasonably fresh with well-developed granoblastic fabrics. Primary mineralogy consists of almandine–pyrope-rich garnet, diopside, oligoclase plagioclase and scapolite; minor phases include rutile and ilmenite. One sample contains only garnet and metasomatic pargasitic amphibole, pseudomorphing primary diopside. All analysed samples have sub-alkaline basaltic compositions. REE patterns show variable LREE-enrichment (La N/Yb N=13 to 1.4) and small negative and positive Eu anomalies (0.87–1.32), indicating that both plagioclase fractionation and accumulation have occurred. Present-day 87 Sr/ 86 Sr and 143 Nd/ 144 Nd isotope ratios are very low (0.70272–0.70378 and 0.511736–0.512225, respectively), suggesting that both assimilation of Archaean continental crust and removal of Rb, possibly related to granulite-facies metamorphism, took place early in the history of these rocks. P/ T estimates (670–730°C, 1.4–1.6 GPa) indicate equilibration depths of up to 50 km. Calculated compressional wave velocities range from 6.94 to 7.70 km/s and densities from 3.2 to 3.5 Mg/m 3, assuming P/ T conditions of 500°C and 1.4 GPa. These values support an origin in the deep continental crust. T DM model ages are 1.7 to 1.9 Ga and may represent the time of extraction from the mantle during the mid-Proterozoic. These xenoliths may represent the in situ mafic lower crust beneath Arkhangelsk, formed as a result of basaltic underplating during the mid-Proterozoic.

Agpaitic magmatism in the northeastern Baltic Shield: a study of the Niva intrusion, Kola Peninsula, Russia

The recently discovered Niva agpaitic syenite intrusion is located 50 km southwest of the Khibina complex, central Kola Peninsula. It forms a lense-like body 1.5–2 km in size that outcrops over an area of 300×200 m on the southern bank of the Imandra lake. Rb–Sr (379±21 Ma) and Sm–Nd (383±58 Ma) isochron ages of the intrusion coincide with the age of the Khibina and Lovozero complexes. A characteristic feature of the Niva rocks is their extremely high agpaicity and enrichment in Sr, Ba, Y, Zr, Nb, Rb, LREE, and volatiles. The mineral association in the agpaitic syenite is K-feldspar, aegirine-diopside, biotite, lamprophyllite, aenigmatite and shcherbakovite. Isotopic compositions indicate the absence of contamination by continental crust during formation of the Niva alkaline magmas. A depleted asthenospheric mantle reservoir mixed with a slightly enriched mantle component is suggested to be the main source of the Niva magma. A metasomatically altered garnet granulite xenolith from the western contact of the Niva intrusion contains a primary paragenesis of diopside, garnet and plagioclase with chemical characteristics similar to those in the granulite suite of the adjacent Lapland zone and lower crustal granulite xenoliths from Devonian pipes in the Kandalaksha area. Additionally, the granulite contains vinogradovite, neptunite, pectolite and natrolite which partially replace both primary minerals of the granulite assemblage and secondary minerals which were formed during a later regional metamorphism. PT estimates support the lower crustal origin of the xenolith. The U–Pb zircon age of the Niva garnet granulite is 2711±7 Ma.

Characteristics of lower crustal granulite xenoliths from the east Qinling orogenic belt and their tectonic significance

The Tongbai granulites are present mainly as xenoliths in granodioritic gneisses. The xenoliths with a zircon age of 470Ma are older than the host rocks of granodioritic gneisses which yield a zircon age of 435Ma. It is suggested that the granulites were transported from the lower crust to the upper level along with granodioritic magma. Geothermometrical and geobarometrical studies based on the coexisting minerals (Opx-Cpx and Opx-Gar) show that the granulites were crystallized at 818 –840 °C and 9.5−9.8 × 108 Pa corresponding to the lower crust. Tectonically, the Shangdan suture zone constitutes the boundary between the North China and Yangtze plates. The zone is char acterized by the occurrence of ophiolites in the western part and by that of granulites in the eastern part. So the western part marks the upper crustal level of the Qinling belt, while the eastern part represents the exposure of a deeper level. The results of isotopic dating and the geochemical characteristics of the xenoliths are consistent with those of metatholeiites of the ophiolites in the western part. Therefore, it is assumed that both ophiolites found in the west and granulites found in the east all represent the remnants of the ancient Qinling ocean plate. The difference is that the ophiolites are pieces of obducted fragments from the ocean floor during the subduction in the Early Palaeozoic. However, in the Tongbai area, when the ocean floor was subducting towards the lower crust, it underwent a granulite fades metamorphism. Subsequently, granodioritic magma formed by partial melting trapped some fragments of granulite upwards.

The La Bre�a ? El Jag�ey Maar Complex, Durango, Mexico: II. Petrology and geochemistry

A suite of 16 basanitic volcanic rocks, representing all stages in the evolution of the La Brena — El Jaguey (LBEJ) Maar Complex, has been studied petrographically and analyzed for mineral compositions and whole-rock major element, trace element, and Sr−Nd−Pb isotopic compositions. Two feldspathic granulite xenoliths were also studied as possible lower-crustal contaminants to the LBEJ magmas. The volcanic rocks contain the stable minerals olivine, plagioclase, augite, and titanomagnetite±ilmenite, plus a diverse suite of xenocrusts derived from disaggregation of mantle xenoliths of spinel lherzolite (olivine, orthopyroxene, spinel) and lower-crustal granulite xenoliths (plagioclase, quartz, augite, ilmenite). Late-stage interstitial melts rich in Fe and Ti migrated into vesicles in several samples, forming coarse-grained segregation vesicles that are dominated by ilmenite blades up to 2 mm long. The whole-rock elemental data are typical of intra-plate basanitic rocks, with strong enrichments in large ion lithophile elements (i.e. K, Th, U) as well as high field strength elements (i.e. Nb, Ta) relative to mid-ocean ridge basalts (MORB) and estimates of primordial mantle abundances. Mg# increased systematically with time during the evolution of the LBEJ Maar Complex, from 57.0–58.2 in the pre-maar lavas to 59.1–63.8 in the post-maar lavas. Compatible elements (Ca, Sc, Cr, Co, Ni) correlate positively with Mg#, whereas a large group of incompatible elements (Al, Na, K, P, Rb, Sr, Zr, Nb, Ba, La, Ce, Sm, Hf, Ta, Th, U) correlate negatively with Mg#. These trends can be closely reproduced by simple models of fractional crystallization, provided that the incompatible element abundances of the parental, high-Mg# magmas are allowed minor variability. All successful fractionation models demand an important role for augite, despite its presence in the LBEJ volcanic rocks as only a late-stage microphenocrystic and groundmass mineral. Minor garnet fractionation is necessary to produce depletion of heavy rare earth element (REE) abundances in the pre-maar lavas, whose REE patterns cross those for the rest of the suite. The importance of augite and garnet fractionation indicate that the differentiation of the LBEJ magmas took place within the upper mantle, a conclusion that is supported by the presence of spinel lherzolite xenoliths in magmas from all stages in the evolution of the maar complex. Isotopic data for seven LBEJ volcanic rocks show the following ranges: 87Sr/86Sr 0.70327–0.70347, ɛNd 4.2–5.0, 206Pb/204Pb 18.60–18.81, 207Pb/204Pb 15.58–15.65, 208Pb/204Pb 38.19–38.58. Sr-Nd values are negatively correlated and form a trend parallel to the mantle array, overlapping the field for ocean island basalts (OIB). The LBEJ rocks have similar 87Sr/86Sr values but lower ɛNd compared to basanitic rocks from the US Basin and Range Province (BRP). Pb isotopic ratios are positively correlated and overlap the braod fields for MORB and OIB and the small fields for Mexican ore deposits and volcanic rocks from the active subduction-related Mexican Volcanic Belt. The LBEJ rocks have slightly more radiogenic Pb than basanitic rocks from the US BRP. Despite correlations among the isotopic ratios of the LBEJ suite, none of these ratios correlate with position in the eruption sequence, Mg#, or any other compositional parameter. The two lower-crustal xenoliths have high 87Sr/86Sr values (0.707, 0.710) and low ɛNd (-1.5,-8.0) compared to the LBEJ volcanic rocks, but their Pb isotopic compositions are only slightly more radiogenic than the volcanic rocks. These data do not support the widely held view that the lower crust is a major reservoir of unradiogenic Pb. In order to further constrain the role played by crustal contamination in generating the isotopic diversity in the LBEJ suite, we conducted an extensive investigation of Sr−Nd−Pb isotopic ratios for scoria clasts from different levels of a single scoria-fall horizon in the pyroclastic sequence related to the formation of La Brena Maar. Our results do not support an important role for crustal contamination in the LBEJ magmas. Rather, we conclude that minor isotopic variability exists in the mantle source regions beneath the maar complex.

Oxygen isotope evidence for large-scale hybridization of the lower crust during magmatic underplating

Lower crustal granulite xenoliths from eight localities worldwide exhibit an extremely large range in O-isotope ratios ( δ 18O = +5.4 to + 13.5%.) with a concentration of δ 18O values around +7.0%.. Mafic granulites with Mg numbers greater than 0.7 (group Ml) exhibit the smallest range and lowest average δ 18O value ( +6.7 ± 0.6%.) of all granulite types. Mafic granulites with Mg numbers less than 0.7 (groups M2 and M3) and intermediate composition granulites have higher average δ 18O values that are statistically indistinguishable at +8.0 ± 1.1%. (M2), +7.7 ± 1.2%. (M3) and +8.1 ± 1.8%., respectively. Silicic metaigneous granulites ( + 10.0 ± 1.9%.) and metasediments ( + 10.2 ± 2.2%.) are significantly enriched in 18O relative to other granulite groups and exhibit the widest range of δ 18O values (6.9%.). The overall range in 18O/ 16O ratios of ca. 8%. emphasizes the O-isotope heterogeneity of the present-day deep continental crust and demonstrates that pervasive deep crustal fluid flow and isotopic homogenization is not a major process operating on a regional scale. Contamination of the order of 20–50% by an 18O-enriched component (such as the metasedimentary and/or silicic metaigneous granulites) with an average δ 18O value of ca. +10%. is required to account for the 18O/ 16O ratios of most mafic xenoliths. Although such an explanation may apply to some M2 and M3 granulites, it is not applicable to M1 and M2 group granulites with strongly mafic major element compositions. Instead, many of these samples may represent the culmination of a large-scale hybridization process involving differentiating mantle-derived mafic melts and mafic restite residual after metasediment anatexis and granite genesis. This hypothesis can explain the rarity of identified restites in lower crustal xenolith suites because such rocks have been modified and/or consumed by interaction with mafic magmas during and/or following episodes of large-scale intracrustal melting. Delamination of mafic lower crust following transient overthickening events at convergent plate margins may be an important means of introducing surficially derived oxygen into the region of the crust/mantle boundary.

Lower crustal granulite xenoliths in carbonatite volcanoes of the Western Rift of East Africa

AbstractSporadic small garnet granulite and two-pyroxene granulite xenoliths found in the carbonatite tuffs and lavas near Fort Portal, South West Uganda, are chiefly silica-saturated and rich in Al2O3(&gt; 20 wt. %) and Na2O (c.2 to 4 wt. %). ThreeREEpatterns are distinguished:LREEenriched—HREEdepleted with a positive Eu anomaly;LREEdepleted—HREErelatively enriched and flat; andLREEslightly enriched with a very weak Eu anomaly and high overallREE. The xenoliths are considered to represent original basaltic melts and fractional crystallisation products, varying with the dominance of the clinopyroxene, plagioclase or olivine crystallising phase. It is thought thatREEabundances were established before metamorphism.The clinopyroxenes are low-jadeitic augites, the orthopyroxenes, aluminous hypersthenes and the garnets, pyrope-almandine with constant grossularite. Plagioclase varies with increasing metamorphic grade from labradorite to andesine-oligoclase. Scapolite (meionite), alkali-feldspar, quartz, mica, amphibole, rutile and apatite are minor phases and some appear to be metasomatic.Calculated temperatures of metamorphic equilibration range from 580 to 800°C at pressures &gt; 4 kbar for the two-pyroxene granulites and &gt; 6 kbar for the garnet granulites. A known geophysical discontinuity marking a density change at 16 km in the Western Rift may be due to the presence of two-pyroxene granulite, calculated to become garnet-bearing at depths greater than 23 km. The absence of proven omphacite-bearing eclogite xenoliths (with no plagioclase) indicates that the greatest depth of crustal sampling by the carbonatite in the Fort Portal field is about 25 km which could be the depth of the Moho in this area.