Search

The Allendale Lake stock is a representative intrusion of the Eocene alkaline igneous province in south-central British Columbia (Canada), which forms the northwestern edge of the Montana-Wyoming alkaline province. The shallow-seated intrusion (~53 Ma), about 4.5 km long and 2 km wide, is composed predominately of feldspar-phyric biotite-clinopyroxene-amphibole-bearing monzonitic/syenitic rocks. The feldspar forms distinct rhomb-shaped phenocrysts, which contain abundant exsolution of perthite and antiperthite. The Allendale Lake rocks are silica-undersaturated with SiO2 ranging from 52 to 63 wt% and high contents of alkalis, particularly K2O (4.5–6.5 wt%), typical of shoshonitic rocks. The rocks show a distinct enrichment of large-ion-lithophile elements relative to heavy rare earth elements and high-field strength elements. Their isotopic compositions are highlighted by high negative ƐNd(t) values (−4.5 to −5.5), Neoproterozoic Nd model ages (750–900 Ma) and high but relatively uniform initial 87Sr/86Sr ratios (~0.706). The rocks were generated by fractional crystallization under oxidizing conditions. The parent magma, a hydrous alkaline basaltic melt, was formed by partial melting of an amphibole-phlogopite-bearing peridotite of the subduction-modified subcontinental lithospheric mantle. The mantle source underwent Neoproterozoic and subsequently Cenozoic metasomatic enrichment events. The alkaline volcanic rocks of the Penticton Group (Marron Formation) from the nearby Eocene Challis-Kamloops volcanic belt are volcanic correlatives of these intrusions. The Eocene alkaline rocks in the south-central part of the British Columbia are related to continental arc rifting.

The evolution of the mantle source beneath the Tepla-Barrandian (TBB) and the adjacent southern part of the Saxo-Thuringian (S-STB) and northern part of the Moldanubian (N-MB) blocks of the Bohemian Massif (Czech Republic) is tracked from the Late Neoproterozoic to the Cenozoic in order to examine the coupling between the crust and underlying continental lithospheric mantle (CLM) during the Late Paleozoic Variscan orogeny. In the TBB, Late Neoproterozoic mafic rocks have highly radiogenic Nd but moderately radiogenic Sr isotopes and were derived from a spinel peridotite mantle. These within-plate rocks were emplaced in an intra-oceanic back-arc basin. Early Paleozoic (Ordovician to Early Devonian) mafic rocks are rift/extension-related, enriched in LREE and Nb relative to HREE and were probably derived from a garnet peridotite mantle source. Compared to Neoproterozoic basalts, their ƐNd(t) values are slightly lower but TDM model ages are indistinguishable suggesting derivation from a deeper portion of the same mantle source. Thus, the weak deformation and low-grade metamorphism associated with the Cadomian orogeny did not significantly decouple the crust from the underlying mantle source beneath the TBB. Early Paleozoic rift-related mafic rocks of the S-STB and N-MB have comparable isotopic signatures indicating a similar mantle source. Late Paleozoic mafic magmatism occurred during the extensional collapse phase of the Variscan orogen, in the aftermath of the collision between Gondwana and Laurussia. The TBB and S-STB basaltic rocks are within-plate, and transitional between alkaline and tholeiitic compositions but show a relative depletion of Nb. Their isotopic characteristics contrast with older basalts in that they have negative ƐNd(t) values, high initial 87Sr/86Sr ratios and older TDM model ages (1050 to 1300 Ma). Late Paleozoic mantle-derived potassic-ultrapotassic magmas occurring as lamprophyre dikes and small volume high Mg K intrusions in N-MB, S-STB and TBB have similar isotopic characteristics suggesting that all these Late Paleozoic rocks were derived from an old CLM contaminated by fluids or silicic melts derived from subducted Precambrian crustal material or alternatively, the region was underthrust by Gondwanan mantle during the Variscan collision. Cenozoic volcanic rocks in the Bohemian Massif are local representatives of the regionally extensive Cenozoic European volcanic province. The rocks are within-plate, alkaline basalts enriched in LREE and Nb and were derived from garnet peridotite mantle. They are significantly more juvenile than Carboniferous-Early Permian mafic rocks, with higher ƐNd(t) values (typically +3 to +5), lower initial 87Sr/86Sr ratios and significantly younger TDM age. These data suggest significant input from upwelling asthenospheric mantle, implying that at least a part of the Permian CLM mantle was re-fertilized.

The Late Devonian Corner Bay pluton is a part of the Sitkoh alkaline plutonic complex on the Chichagof Island, southeastern Alaska, which is located in the Alexander terrane of the North American Cordillera. The intrusion, about 12 km long and 4 km wide, is composed predominantly of amphibole-biotite-bearing syenitic/ monzonitic rocks with minor alkaline gabbros and has a U-Pb zircon age of ~367 Ma interpreted as the crystallization age of the pluton. The rocks are metaluminous and have silica contents ranging from 47 to 63 wt% with a silica gap at 52–58 wt% and have high contents of alkalis, typical of shoshonitic rocks. They are rich in large-ion-lithophile elements (including Rb, Sr, Ba, U) and their isotopic composition is characterized by relatively uniform ƐNd(t) values (+3.4 to +3.7), Neoproterozoic Nd depleted mantle model ages (650–700 Ma) and initial 87Sr/86Sr ratios (~0.704). The rocks were generated by fractional crystallization of alkali basaltic magma formed by partial melting of an amphibole-phlogopite-bearing peridotite of the sub-arc lithospheric mantle. The mantle-source underwent a Neoproterozoic metasomatic enrichment event and does not show a contribution from juvenile Devonian mantle indicating that rifting associated with the magma emplacement was of limited extent as it did not replace the old lithospheric mantle. Several geothermobarometers document emplacement of the parental magma at mid-crustal levels (8.5–10 kb) and polybaric crystallization from ~1200 °C to solidus. Crystallization of amphibole took place at ~880-980 °C and 2.4–5.5 kb. Subsequently the rocks underwent recrystallization (feldspars: 170 Ma – 40Ar/39Ar age) and re-equilibration (biotite: 129 Ma (40Ar/39Ar) at ~600-650 °C and 2 kb). Zircon, rutile and apatite saturation temperatures provide consistent results (~800 °C). The intrusion is a part of a metallogenic province of rare metals, which stretched across the islands in the southeastern most part of Alaska.

The compositionally bimodal volcanic rocks of the Eocene–Miocene Masset Formation from Queen Charlotte basin, Haida Gwaii, British Columbia, Canada, underlie an area greater than 5000 km 2 where their exposed sections are up to 1.6 km thick. The suite of mafic and felsic rocks (dacites and rhyolites) that erupted closely spaced in time, in both submarine and subaerial conditions, was associated with significant crustal extension and thin continental crust (~ 19–24 km thick), with volcanism persisting for ~ 35 Ma (from 46 to 11 Ma). Predominant mafic types (mafic:felsic ~ 2:1) are moderately enriched mid-ocean-ridge-like basalts that were derived by a partial melting of a heterogeneous spinel peridotite source. Felsic rocks are plagioclase-phyric, two pyroxene-bearing, mainly peraluminous types which have Nd, Pb and Sr isotopic compositions overlapping those of basalts including high positive Ɛ Nd(t) values (up to >+6). The chondrite-normalized REE patterns show light REE enrichment but flat heavy REE along with a variable negative Eu anomaly. Mineralogy, major and trace elements, Nd–Sr–Pb isotopic data and model calculations using MELTS are consistent with a derivation of felsic rocks from the basalts by fractional crystallization. The intercalation of basaltic and felsic rocks suggests the existence of separate, simultaneously active plumbing and feeder systems and relatively stable magma chamber(s) to generate large volumes of differentiated felsic magmas by fractional crystallization. The Masset rocks provide an example for the generation of felsic magmas of bimodal volcanic suites during rifting along a thinned continental margin.

Upper Paleozoic (~ 365 Ma) mafic and intermediate volcanic rocks of the Piskahegan Group constitute a subordinate part of the Mount Pleasant caldera, which is associated with a significant polymetallic deposit (tungsten–molybdenum–bismuth zones ~ 33 Mt ore with 0.21% W, 0.1% Mo and 0.08% Bi and tin–indium zones ~ 4.8 Mt with 0.82% Sn and 129 g/t In) in southwestern New Brunswick (Canada). The epicontinental caldera complex formed during the opening of the late Paleozoic Maritimes Basin in the northern Appalachians. The mafic and intermediate rocks make up two compositionally distinct associations. The first association includes evolved rift-related continental tholeiitic basalts, and the second association comprises calc-alkaline andesites, although both associations were emplaced penecontemporaneously. The basalts have low Mg# ~ 0.34–0.40, smooth chondrite-normalized REE patterns with (La/Yb)n ~ 5–6, primitive mantle-normalized trace element patterns without noticeable negative Nb–Ta anomalies, and their ɛNd(T) ranges from + 2.5 to + 2.2. The basalts were generated by partial melting of a transition zone between spinel and garnet mantle peridotite at a depth of ~ 70–90 km. The calc-alkaline andesites of the second association have chondrite-normalized REE patterns that are more fractionated, with (La/Yb)n ~ 7–8.5, but without significant negative Eu anomalies. Compared to the basaltic rocks, they have lower ɛNd(T) values, ranging from + 0.5 to + 1.9, and their mantle-normalized trace element plots show negative Nb–Ta anomalies. The ɛNd(T) values display negative correlations with indicators of crustal contamination, such as Th/La, Th/Nb and SiO2. The andesitic rocks are interpreted to have formed by assimilation–fractional crystallization processes, which resulted in the contamination of a precursor basaltic magma with crustal material. The parent basaltic magma for both suites underwent a different evolution. The tholeiitic basalts experienced shallow-seated fractional crystallization and evolved along a tholeiitic trend of “early iron” enrichment (non-oxic conditions). The contaminated magma of the second association followed a calc-alkaline fractionation trend of “no iron” enrichment (oxidizing conditions) characterized by a high PH2O and PO2 environment at the mid-crust levels. The Piskahegan Group, which is associated with an important polymetallic mineral deposit, differs from the numerous non-mineralized rift-related volcanic suites of the regional Upper Devonian to Lower Carboniferous successions in the Maritimes Basin by the presence of a significant amount of coeval calc-alkaline andesite, which may be an indicator of potential mineralization.

Post-collisional volcanic rocks associated with the Variscan orogeny are preserved in relicts of Late-Paleozoic volcano-sedimentary basins in the Bohemian Massif (central Europe). They include both the Late Carboniferous calc-alkaline and the Early Permian tholeiitic suites, which were emplaced in an extensional intracontinental setting. Both suites were derived from an enriched continental lithospheric mantle and underwent early fractionation of mafic silicate minerals. A second period of fractionation occurred within the crust. Modeling indicates that the calc-alkaline rocks fractionated at a greater depth (0.55 GPa), under oxidizing (ΔFMQ +1) and hydrous conditions whereas the tholeiitic rocks fractionated at shallow depth (0.1 GPa), under moderately oxidizing (ΔFMQ +0.7) and drier conditions. During the Variscan orogeny, the lithospheric mantle under the Bohemian Massif underwent wholesale modification, where the mantle was contaminated by subducted Precambrian sediment admixture. The Carboniferous-Permian mantle-derived rocks typically have negative ɛNd(t) and Precambrian depleted mantle model ages (TDM) compared to the equivalent Early Paleozoic rocks of the Bohemian Massif, which have positive ɛNd(t) and younger TDM. Subsequently, prior to the Cenozoic, the lithospheric mantle under the whole Bohemian Massif was rejuvenated as the Cenozoic mantle-derived rocks have positive ɛNd(t) and young TDM.

article i nfo The Early Permian Panjal Traps of northern India represent a significant eruption of volcanic rocks which occurred during the opening of the Neotethys Ocean. Basaltic, basaltic-andesites, dacitic and rhyolitic rocks collected from Guryal Ravine and Pahalgam show evidence for subaerial and subaqueous eruptions indicating that they are contemporaneous with the formation of a shallow marine basin. The major and trace element geochemistry of the basalts is consistent with a within-plate setting and there are basalts which have high-Ti (TiO2 N 2.0 wt.%) and low-Ti (TiO2 b 1.8 wt.%) compositions. The 'high-Ti' basalts are similar to OIB whereas the 'low-Ti' basalts are similar to continental tholeiites. The identification of 'high- and low-Ti' basalts within the Panjal Traps is analogous to other large igneous provinces (e.g. Karoo, Deccan, Parana, Emeishan). The Sr-Nd isotopic values (eNd(T) = −5.3 to +1.3; ISr = 0.70432 to 0.71168) of both types of basalts overlap indicating that the rocks may have originated from the same ancient subcontinental lithospheric (i.e. EMII-like) mantle source (TDM = ~2000 Ma). The two groups of basalts can be modeled by using a primitive mantle source and different degrees of partial melting where the high-Tirocksare producedby ~1% partial melting of a spinel peridotite source where- as the low-Ti rocks are produced by ~8% partial melting. Trace elemental and isotope modeling indicatesthat some of the basalts assimilated ≤ 10% crustal material. In contrast, the basaltic-andesites are likely formed by mixing be- tween basaltic magmas and crustal melts which produced rocks with higher SiO2 (~55 wt.%) content and enriched isotopic signatures (eNd(T) = −6.1; ISr = 0.70992). The Panjal Trap volcanism was likely due to partial melting of the SCLM within a passive extensional setting related to the rifting of Cimmeria from Gondwana. Contemporaneous volcanic and plutonic granitic rocks throughout the Himalaya are probably not petrogenetically related but are likely part of the same regional tectonic regime.

The Early Jurassic (ca. 177 Ma) Bokan Mountain granitic complex, located on southern Prince of Wales Island, southernmost Alaska, cross-cuts Paleozoic igneous and metasedimentary rocks of the Alexander terrane of the North American Cordillera and was emplaced during a rifting event. The complex is a circular body (~ 3 km in diameter) of peralkaline granitic composition that has a core of arfvedsonite granite surrounded by aegirine granite. All the rock-forming minerals typically record a two-stage growth history and aegirine and arfvedsonite were the last major phases to crystallize from the magma. The Bokan granites and related dikes have SiO2 from 72 to 78 wt.%, high iron (FeO (tot) ~ 3–4.5 wt.%) and alkali (8–10 wt.%) concentrations with high FeO(tot)/(FeO(tot) + MgO) ratios (typically > 0.95) and the molar Al2O3/(Na2O + K2O) ratio

The Yarlung Zangbo Suture Zone (YZSZ), southern Tibet, is a discontinuous belt that is more than 2000 km long, composed of the remnants of Neo-Tethyan Mesozoic ocean. One of these relicts is the Xiugugabu ophiolitic massif which is a mantle thrust sheet of more than 260 km2 overlying the Cretaceous tectonic melange south of the YZSZ in SW Tibet. The massif is composed of harzburgites and clinopyroxene–harzburgites with porphyroclastic and porphyromylonitic textures. In the southern part of the massif, peridotites were intruded by amphibole-bearing microgabbro and microgabbronorite sills. A diabase unit which is overlaid by a sedimentary sequence crops out on the NE flank of the massif. Mineral chemistry in harzburgites and clinopyroxene–harzburgites indicates compositions similar to abyssal and forearc peridotites. Peridotites are slightly LREE depleted to enriched with [La/Yb]CN 0.06–2.8 and [La/Sm]CN 0.34–2.64. These ultramafic rocks are inferred to be the residues of 5–25% of partial melting of a depleted mantle that has been enriched by percolating metasomatic melts in a suprasubduction environment. Amphibole–microgabbro and amphibole–microgabbronorite sills are mostly composed of brown to green amphibole, calcic plagioclase, clinopyroxene, ilmenite and orthopyroxene in gabbronorite. Textures and compositions of the brown amphiboles indicate a near-solidus high temperature hydrothermal origin (> 800 °C). These intrusive rocks are tholeiitic and show N-MORB type REE patterns ([La/Yb]NC 0.35–0.90), a LILE (mainly Th) enrichment and noticeable Nb, Ta and Ti negative anomalies. They have a suprasubduction affinity and were formed in a back-arc basin setting. The diabase unit outcropping to the NE of the massif is not directly related to the ultramafic and mafic ophiolitic rocks. The diabase shows LREE enriched patterns ([La/Yb]NC 8–8.9) and slight Nb, Ta and Ti negative anomalies. The diabase has an intraplate affinity and could have been derived from a mantle source enriched by subduction-related fluids. The absence of continental crustal assimilation indicates that these rocks were probably emplaced in the Jurassic, in an oceanic environment after the Triassic disaggregation of the Indian plate. The data are consistent with the recent geodynamic model proposed for the central part of the suture for the closure of the Neo-Tethys and suggest that the geodynamic evolution of the western part of the basin was comparable to the central part.

Cenozoic anorogenic volcanism of the Bohemian Massif is an integral part of the Central European Volcanic Province. The temporal and spatial distribution of volcanic rocks in the Bohemian Massif, their geochemistry and mineralogy as well as their tectonic setting and paleostress data are used to characterize and classify this volcanic activity. Three main volcanic periods can be distinguished based on K–Ar data and known paleostress fields: (i) pre-rift (79–49 Ma), (ii) syn-rift (42–16 Ma) and (iii) late-rift (16–0.3 Ma), with the youngest period further subdivided into three episodes. The dominant mafic rock types (N7 wt.% MgO) of all periods are of nephelinite–basanite/tephrite composition. The exceptions are suites of melilitic ultramafic rocks of the pre-rift period in northern Bohemia and of the final episode of the late-rift period in western Bohemia. The most voluminous are volcanic rocks of the syn-rift period occurring in the Ohře Rift Graben. The initial 87Sr/86Sr (0.7032 to 0.7050) and 143Nd/144Nd (0.51264 to 0.51301) ratios of the mafic volcanic rocks of the Bohemian Massif are characteristic of magmas derived from a sub-lithospheric mantle source. The isotopic ratios resemble those of the HIMU mantle source (206Pb/204Pb ca. 19 to 20). These rocks have themost isotopically depleted compositions among the Central European Volcanic Province volcanics.

The Rheic Ocean formed at ca. 500 Ma, when several peri-Gondwanan terranes (e.g. Avalonia and Carolinia) drifted from the northern margin of Gondwana, and were consumed during the Late Carboniferous collision between Laurussia and Gondwana, a key event in the formation of Pangea. Several mafic complexes ranging in age from ca. 400–330 Ma preserve many of the lithotectonic and/or chemical characteristics of ophiolites. They are characterized by anomalously high eNd values that are typically either between or above the widely accepted model depleted mantle curves. These data indicate derivation from a highly depleted (HD) mantle and imply that (i) the mantle source of these complexes displays time-integrated depletion in Nd relative to Sm, and (ii) depletion is the result of an earlier melting event in the mantle from which basalt was extracted. The extent of mantle depletion indicates that this melting event occurred in the Neoproterozoic, possibly up to 500 million years before the Rheic Ocean formed. If so, the mantle lithosphere that gave rise to the Rheic Ocean mafic complexes must have been captured from an adjacent, older oceanic tract. The transfer of this captured lithosphere to the upper plate enabled it to become preferentially preserved. Possible Mesozoic–Cenozoic analogues include the capture of the Caribbean plate or the Scotia plate from the Pacific to the Atlantic oceanic realm. Our model implies that virtually all of the oceanic lithosphere generated during the opening phase of the Rheic Ocean was consumed by subduction during Laurentia–Gondwana convergence.

Microgranular enclaves are common within intermediate to felsic granitic rocks that have I- and S-type affinity however they are rare within alkaline anorogenic granitoids of A-type affinity. The Permian (~ 260 Ma) Emeishan large igneous province (ELIP) of southwest China contains two peralkaline silica saturated A-type granitic plutons that host microgranular enclaves. The enclaves from the Baima pluton are intermediate in composition and have lower SiO 2 and higher TiO 2 , CaO and Mg# (SiO 2 = 57.2 to 63.0 wt.%; TiO 2 = 0.8 to 1.8 wt.%; CaO = 1.7 to 3.3 wt.%; Mg# = 28 to 44) than their host (SiO 2 = 62.6 to 67.8 wt.%; TiO 2 = 0.5 to 1.4 wt.%; CaO = 0.4 to 1.8 wt.%; Mg# = 15 to 31). The enclaves from the Taihe pluton are more felsic (SiO 2 = 63.8 to 71.3 wt.%; TiO 2 = 0.3 to 0.6 wt.%; CaO = 0.6 to 2.3 wt.%; Mg# = 8 to 22) but are still less evolved than their host (SiO 2 = 69.8 to 75.1 wt.%; TiO 2 = 0.2 to 0.6 wt.%; CaO = 0.4 to 0.8 wt.%; Mg# = 3 to 12). In both cases, the enclaves have very similar eNd (T) values (Baima eNd (T) = + 2.8 to + 3.2; Taihe eNd (T) = + 1.0 to + 2.0) as their hosts (Baima eNd (T) = + 3.0 to + 3.2; Taihe eNd (T) = + 1.5 to + 1.9). The major and trace element trends of the enclave-host pairs suggest that fractional crystallization occurred and that element diffusion was likely minimal. The enclaves are interpreted as entrained accumulations of early formed crystals of a silicic magma which was originally produced by fractional crystallization of a mafic magma.

The Saga and Sangsang ophiolites are located about 600 and 450 km west of Lhasa and represent a western extention of the central portion of the Yarlung Zangbo Suture Zone (YZSZ) ophiolite belt. The Saga massif comprises fresh mantle lherzolite and cpx-harzburgite, an ophiolite melange (± amphibolite), metamorphosed mafic crustal rocks (meta-gabbro, meta-basalts and amphibolites) and a sequence of uppermost crustal rocks (chert, basaltic lavas, diabase sills and dikes). The Sangsang ophiolite consists of an ophiolite melange (harzburgite) and upper mantle harzburgite with minor lavas and gabbro. Peridotites from both massifs show variable degrees of serpentinization. Their Mg# varies between 0.89 and 0.91. All peridotites show distinct flat REE (rare earth elements) patterns with La/YbN ratios close to 1, probably indicative of a refertilized mantle. The Olivine-Spinel equilibrium and the spinel chemistry for the Saga (Cr# ~ 0.10–0.22) and Sangsang (Cr# ~ 0.30–0.55) peridotites suggest that the Saga peridotites have a deeper mantle provenance (> 20 kbar) and have undergone lower degrees of partial melting (5–12%) than the Sangsang peridotites (

The 473 + 5/− 3 Myr Asbestos ophiolite complex of the Quebec Appalachians was formed in a forearc basin and obducted on a margin of Laurentia ~ 460 Myr ago. The complex together with its sedimentary cover is well exposed at Burbank Hill (~ 130 km SW of Quebec City) where eight distinct lithologies have been identified: 1) pyroxenites and wehrlites with minor dunitic layers, (2) fractured gabbros, (3) breccia containing gabbro and diabase fragments, (4) polygenic conglomerates, (5) red mudstone/chert/sandstone succession, (6) tuffs with intercalated grey chert, (7) greenish grey mudstones and (8) slates and sandstones of the Saint-Daniel Melange. The gabbroic and mafic volcanic rocks show the characteristics of arc tholeiites (TiO2 ~ 0.8–0.85 wt.%) and boninites (TiO2 45; (La/Sm)N ~ 1.9–3.3). These rock-types also occur as mafic fragments in the polygenic conglomerate. The mafic fragments (crystals and rocks) within the conglomerate and the ophiolitic sandstones also show the same greenschist facies metamorphism as the mafic igneous rocks of the complex. The conglomerate was probably formed after the forearc crust was fragmented by pre-obduction normal faults. These fault scarps would have promoted the erosion of the oceanic crust, leading to the accumulation of diverse detritus in grabens. The sedimentary lithologies which overlie the conglomerate exhibit a gradual increase in continent-derived material ((La/Sm)N ~ 5 and La/Yb ~ 20) and a decrease in ophiolitic material moving upward in the stratigraphic sequence. The sedimentary rocks at the bottom of the sequence (group 3) are mostly composed of ophiolite material with only 20% of crustal material. They also have a high content of Cr and MgO with values of 350–480 ppm and 5.7–8%, respectively. The rocks of the uppermost part of the pile (group 1) which contain up to 80% crustal material have low abundances of Cr and MgO (30–100 ppm and 2.7–4.5 wt.%). Continental detritus on the top of the Asbestos ophiolite suggests the complex formed near the Laurentian continental margin and that the eroded continental material had access to the basin where the ophiolite was formed. This is consistent with the obduction of the Asbestos and Thetford–Mines ophiolites on the Laurentian margin shortly after their formation. The proximity of thick Laurentian continental crust near the trench could explain why the subduction zone blocked-up soon after the ophiolites formation.

The volcanic rocks of the Ohře/Eger Rift (northern Bohemia, Czech Republic), the easternmost part of the Cenozoic Volcanic Province of western and central Europe, include rare occurrences of Late Cretaceous to Paleocene (68 to 59 Ma) ultramafic melilitic rocks in the Osecna Complex and associated Devil's Dyke swarm. These melilitic suites, related to the initial stage of rifting, occur in the outer parts of the rift zone. Magmatism during the main stage of the rifting is represented by a voluminous Eocene to Miocene (40 to 18 Ma) bimodal suite of basanites and phonolites which is present in the inner part of the Ohře Rift zone. The Osecna Complex is a lopolith-like subvolcanic intrusion, composed mainly of olivine melilitolite with rare pegmatoids, ijolites and glimmerites, accompanied by numerous cone-sheets and dykes of olivine micro-melilitolite and melilitic lamprophyre. The NNE–SSW trending Devil's Dyke swarm consists predominantly of melilite-bearing olivine nephelinite. The primitive melilitic rocks have a primary olivine + melilite + spinel ± clinopyroxene association and are characterized by low contents of SiO2, Al2O3 and total alkalis but high CaO, MgO, Cr, Ni, CO2 and strongly incompatible trace elements including light REE. High initial ɛNd values of + 3.2 to + 5.2 accompanied by variable 87Sr/86Sr ratios of ∼ 0.7033 to ∼ 0.7049 are interpreted as evidence for melting of a heterogeneous veined mantle. A portion of a depleted mantle source was overprinted by carbonate-rich fluids with enriched Sr isotopic composition. Mantle metasomatism was probably related to carbonatitic magmatism associated with incipient rifting of the Bohemian Massif lithosphere.

Upper amphibolite–granulite facies gneisses and granites of the Liscomb Complex (Nova Scotia, Canada), which are exposed in a core complex within the Cambro–Ordovician Meguma Group of southern Nova Scotia, yielded concordant U–Pb zircon/monazite ages of 377 ± 2 and 374 ± 3 Ma, respectively. Geochronological and geochemical data suggest a single Devonian high-grade metamorphic event, which generated the granitic magma by partial melting of the fertile Liscomb gneisses at a depth of ∼ 30 km. The melting was also synchronous with an extensional event during which the gneisses were uplifted in a core complex associated with the intrusion of granitoids to a depth of ∼ 10 km. Subsequently, the gneisses and granites underwent rapid exhumation before the deposition of unconformably overlying late Fammenian rocks at ∼ 364 Ma. These events took place during terminal stages of the Acadian Orogeny and the onset of extensional tectonics in Atlantic Canada during the Middle–Late Devonian. The close temporal and spatial association of Liscomb gneisses/granulites and granites, their major and trace element compositions, and their overlapping isotopic characteristics confirm the hypothesis that high-grade metamorphism and generation of granitic melt are complementary processes. As the Liscomb granites are of similar age, mineralogy and chemistry to the voluminous granitoid plutons found throughout the Meguma Terrane, a similar process is indicated for the rest of the terrane.

Quartzofeldspathic rocks of the Gfohl gneiss from the Moldanubian of the Czech Republic span amphibolite-to granulite-facies, and are associated with eclogite. Protomylonitic fabrics related to terminal tectonic emplacement and reworking of the gneiss are common. Some non-mylonitic rocks, however, preserve early, prograde features (e.g., Opx-rimmed Hbl in metabasites), whereas others have characteristics generally associated with near-isothermal decompression (e.g., Pl-Opx moats separating Grt and Qtz in metabasites; Crd ± Spl coronas on Grt and aluminosilicates in metapelites); the unequivocal distinction between prograde and decompressional features in these rocks, however, may not be possible or even justified. For example, some metapelites contain growth-zoned (i.e., rimward increase in XMg) garnets that also record evidence (i.e., rimward decrease in XCa, compensated by the presence of reversely-zoned plagioclase in the same rock) of decompression. In rare instances, eclogitic rocks (P > 11 kbar) interpreted as tectonic enclaves within the gneiss also record mineralogic evidence of decompression (e.g., Crd-Opx-Spr coronas on pyrope). In metapelites, plagioclase-cored coronal garnets with high PrpGrs ratios (~ 2.5) record near-isobaric cooling from near the thermal maximum at a relatively shallow but undetermined crustal level. Unlike Gfohl gneisses elsewhere (e.g., in Austria), the rocks described here do not preserve evidence of extreme metamorphic conditions. Texturally stable Grt-Bt pairs in non-mylonitic samples give Tmax < 750 °C. Pmax is not known, but prograde metamorphism apparently progressed from the kyanite to sillimanite fields, implying P ~ 8 kbar at the maximum Grt-Bt temperature. At these conditions, dehydration of mafic gneiss occurred in the presence of a CO2-rich (XCO2 ~ 0.85) pore fluid

Rare-earth element abundances of calc-alkaline andesitic rocks from southern Peru show that these rocks cannot be produceed by a single stage process. The high content of LILE, particularly LREE requires their derivation from a source already enriched in these elements and having a distinctly fractionated REE pattern. It is suggested that ascending hydrous fluids, released from the subducted oceanic lithosphere, enriched the upper mantle in LILE by zone refining. The partial melting of such an enriched upper mantle, followed by fractional crystallization, could produce andesitic rocks. REE data indicate that shoshonitic rocks from southern Peru can be derived from an unfractionated garnet-bearing peridotite by a low degree of partial melting.

The U content of major rock-forming minerals in a suite of calc-alkaline volcanic rocks from Sardinia is very low and their partition coefficient of U (D U = C S /C L ) is smaller than 0.1. The values of D U of the mineral phases decrease from basalt to dacite and, in all rocks, the bulk of U and K is present in the groundmass. The apparent close association between K and U in basic and intermediate rocks probably reflects the fact that most of their rock-forming minerals are relatively ‘inert’ with respect to both of these elements.

A sequence of amphibolite to granulite facies metasedimentary and mafic metaigneous rocks from the western Italian Alps has been analysed for rare earth elements (REE). In this sequence, the metasedimentary granulites have probably been affected by a melting event while the metaigneous granulites remained unaffected. Metasedimentary granulites have a less fractionated chondrite-normalized REE pattern than equivalent amphibolite facies rocks. The granulites tend to have a higher content of heavy REE and lower abundances of light REE (LREE). The leucosomes of migmatitic granulites have lower REE content than the melanocratic bands and both these rock types have variable relative abundances of Eu. The mafic granulites have LREE enriched patterns while the amphibolites are slightly depleted in LREE. The differences between the mafic granulites and amphibolites are probably of pre-metamorphic origin.