Components Of Continental Crust Research Articles

Geochronological constraints on the metamorphic sole of the Semail ophiolite in the United Arab Emirates

The Semail ophiolite of Oman and the United Arab Emirates (UAE) provides the best preserved large slice of oceanic lithosphere exposed on the continental crust, and offers unique opportunities to study processes of ocean crust formation, subduction initiation and obduction. Metamorphic rocks exposed in the eastern UAE have traditionally been interpreted as a metamorphic sole to the Semail ophiolite. However, there has been some debate over the possibility that the exposures contain components of older Arabian continental crust. To help answer this question, presented here are new zircon and rutile U-Pb geochronological data from various units of the metamorphic rocks. Zircon was absent in most samples. Those that yielded zircon and rutile provide dominant single age populations that are 95–93 Ma, partially overlapping with the known age of oceanic crust formation (96.5–94.5 Ma), and partially overlapping with cooling ages of the metamorphic rocks (95–90 Ma). The data are interpreted as dating high-grade metamorphism during subduction burial of the sediments into hot mantle lithosphere, and rapid cooling during their subsequent exhumation. A few discordant zircon ages, interpreted as late Neoproterozoic and younger, represent minor detrital input from the continent. No evidence is found in favour of the existence of older Arabian continental crust within the metamorphic rocks of the UAE.

The nature of xenoliths in the Novaya Melovatka intrusion, Voronezh Crystalline Massif

The paper presents petrographic and petrochemical data on metaterrigenous (comparable with rocks of the Vorontsovka Group) and metamamatic ultramafic–mafic xenoliths in the Paleoproterozoic Novaya Melovatka intrusion in the Voronezh Crystalline Massif (VCM). The intrusion was drilled through by Voronezh Stratigraphic Borehole at Site 3210 of Geotraverse 1-EV. The concordant U-Pb zircon age of a gabbrodiorite xenolith is 2138 ± 25 Ma, and a rock of this age has no analogues among any other magmatic complexes in the geological legend currently adopted for VCM. The younger zircon age value of 2052 ± 22 Ma is thought to be explained by the thermal effect of the Novaya Melovatka intrusion, which was emplaced at 2049 ± 10 to 2057 ± 12 Ma. Xenoliths from this intrusion and from norite of the Elan’ Complex are petrochemically similar to one another and comparable with oceanic-type rocks (cumulus and differentiated mafic–ultramafic series). According to circumstantial evidence, the former were emplaced at relatively shallow depths of 20–30 km in the middle and lower Paleoproterozoic crust, whereas the latter were intruded into the mantle at a depth of 40–50 km, which suggests that the Voronezh block of VCM contains no Archean continental crustal component. The paper reports dada on differences between the Ni-bearing and metallogenically barren mafic–ultramafic massifs.

Petrogenesis of the Kuangshancun and Hongshan intrusive complexes from the Handan–Xingtai district: Implications for iron mineralization associated with Mesozoic magmatism in the North China Craton

The Handan–Xingtai district in eastern China exposes numerous late Mesozoic intrusions. Among these, the Kuangshancun intrusive complex is one of the major ore-related intrusions whereas the Hongshan complex is barren, although both intrusions display similar geochemical characteristics. The Kuangshancun complex consists of diorite and monzonite, with zircon LA-ICP-MS U–Pb age of 133.7±1Ma. The Hongshan complex mainly consists of syenite and shows zircon U–Pb age of 134.5±1Ma. The mineral chemistry of plagioclase from both complexes reveals normal zoning, consistent with the fractional crystallization process. Rocks of the Kuangshancun complex show SiO2 in the range of 58.92–63.84wt.%, Na2O of 4.63–8.81wt.%, and Al2O3 of 16.14–18.18wt.%, together high Sr/Y (14–54) and high LaN/YbN (8.30–16.18) ratios. They show enrichment in LREE and depletion in HREE and HFSE, with no remarkable Eu anomalies, similar to the features of adakites. The whole rock initial 87Sr/86Sr ratios range from 0.706661 to 0.706722 and εNd (t=134Ma) between −15.26 and −15.12, which are consistent with zircon 176Hf/177Hf ratios of 0.281940–0.282059 and εHf (t) values of −27.0 to −22.7. Samples of the Hongshan complex show SiO2 in the range of 56.57–68.16wt.%, high Sr/Y (19–112) and high LaN/YbN (11.39–16.82) ratios. The zircon εHf (t) values are in the range of −15.9 to −12.8 and εNd (t=134Ma) is between −9.82 and −8.62. The Kuangshancun complex was derived through partial melting of an enriched lithospheric mantle contaminated by lower continental crust components. The Hongshan complex was also derived from the EM I-like mantle source. However, the calculated zircon Ce4+/Ce3+ and Eu/Eu∗ ratios indicate that the source magma of the Kuangshancun complex were characterized by higher oxygen fugacity as compared to that the Hongshan complex. The high oxidation states and high water contents are considered as possible key factors that led to the iron mineralization in the Kuangshancun and other intrusions in the Handan–Xingtai district.

Timing, petrogenesis, and setting of granites from the southern Beishan late Palaeozoic granitic belt, Northwest China and implications for their tectonic evolution

Late Palaeozoic granites are widely distributed in the southeastern Beishan area, which is located in the central part of the southern Central Asian Orogenic Belt (CAOB). U–Pb zircon dates of five late Palaeozoic granitic plutons from the southeastern Beishan area yield Permian ages: 285 ± 2 Ma (Shuwojing and Western Shuwojing plutons), 269 ± 3 Ma (Jianquanzi and Jiuquandihongshan plutons), and 260 ± 1 Ma (Jiujing pluton). The early Permian Shuwojing pluton, an unfractionated calc-alkaline biotite monzogranite with slightly positive εNd(t) (+0.7 and +0.6) and low initial 87Sr/86Sr (0.70722 and 0.70712), and the early Permian Western Shuwojing pluton, a high-K calc-alkaline biotite granite with slightly negative εNd(t) (−0.2 and −0.1) and low initial 87Sr/86Sr (0.70390 and 0.70919), are likely derived from a mixture of depleted (juvenile) mantle and upper continental crustal (or sedimentary) material. The mid-Permian Jianquanzi and Jiuquandihongshan monzogranites have highly fractionated potassium-rich calc-alkaline characteristics with negative εNd(t) (−4.3) and very high initial 87Sr/86Sr (0.71949), reflecting a continental crustal component. The compositionally diverse Jiujing pluton and the granodiorite and high-Sr monzogranite phases display adakite-like compositions with relatively low εNd(t) (−0.1 and −2.2) and high initial 87Sr/86Sr (0.70822 and 0.70913). The Jiujing low-Sr monzogranite has higher initial 87Sr/86Sr (0.73464) and lower εNd(t) (−2.8), indicating a significant continental crustal component in its genesis. This work, combined with the regional geology and previous studies, suggest that the early to middle Permian southern Beishan plutons formed in a post-collisional environment, probably an intracontinental rift environment linked to asthenospheric upwelling in response to the break-off of a subducted slab. In the late Permian, the southern Beishan area was in a compressive tectonic regime and thickening of the continental crust resulted in the formation of the Jiujing adakite-like granite.

Post-collisional Ultrapotassic Mafic Magmatism in South Tibet: Products of Partial Melting of Pyroxenite in the Mantle Wedge Induced by Roll-back and Delamination of the Subducted Indian Continental Lithosphere Slab

Post-collisional (25–8 Ma) ultrapotassic mafic magmatic rocks occur to the north of the India–Asia collision zone within the Lhasa terrane of the southern Tibetan Plateau, forming a near 1000 km long semi-continuous igneous belt. They include both extrusive and intrusive facies, although lava flows dominate. To understand their petrogenesis, the mineral chemistry of olivine phenocrysts and xenocrysts and whole-rock major and trace element and Sr–Nd–Pb isotope data are presented for the most primitive mafic magmatic rocks (MgO > 6 wt %) from west to east. The studied samples are characterized by high MgO (6·28–15·75 wt %), K2O (4·76–8·89 wt %), SiO2 (46·44–59·74 wt %), Ba (1368–14076 ppm), Th (69–336 ppm) and Ni (106–527 ppm) contents. Chondrite-normalized rare earth element (REE) patterns show enrichment in light rare earth elements (LREE), flat heavy REE (HREE) patterns and negative Eu anomalies. These REE patterns have a very distinctive inverted ‘spoon shape’, which appears to be a common characteristic of collision-related ultrapotassic magmas. Primitive mantle-normalized incompatible trace element patterns exhibit strong enrichments in large ion lithophile elements (LILE) relative to high field strength elements (HFSE) and strong negative Ta–Nb–Ti anomalies, which are typical of subduction-related magmas. The ultrapotassic magmatic rocks studied have extremely radiogenic initial Sr isotopic compositions (0·712379–0·737616) and low (143Nd/144Nd)i (0·511662–0·511984). Combined with their Pb isotope compositions [(206Pb/204Pb)i = 18·30–18·92; (207Pb/204Pb)i = 15·65–15·87; (208Pb/204Pb)i = 39·02–39·76] these data are consistent with the involvement of a subducted continental crustal component in their petrogenesis. The Sr–Nd–Pb isotope compositions exhibit linear trends between depleted mid-ocean ridge basalt (MORB)-source mantle (DMM) and Indian continental crust. The extreme enrichment of the upper mantle below south Tibet is considered to result from the addition of components derived from subducted Indian continental crust to the overlying mantle wedge during northward underthrusting of Indian continental lithosphere beneath the Lhasa terrane since India–Asia collision at ∼55 Ma. The post-collisional K-rich mafic magmas in south Tibet were generated by partial melting of pyroxenite in a mantle source region that was created by reaction of hydrous fluids and siliceous melts from subducted granulite–eclogite-facies Indian continental crustal rocks with the surrounding peridotitic mantle. A continuous process from slab roll-back, through break-off, to detachment of the slab may have induced partial melting of the pyroxenites. Cessation of the post-collisional ultrapotassic magmatism at ∼8 Ma may be linked to the onset of flat slab subduction beneath southern Tibet and the elimination of the wedge of Tibetan subcontinental lithospheric mantle and underlying asthenosphere; geophysical data indicate that at the present day eclogite-facies Indian continental crust directly underthrusts the crust of the Lhasa terrane with no intervening mantle wedge. The proportion of the Indian continental crustal component in the mantle source of the ultrapotassic mafic magmas decreases eastward, as do the ages and volumes of the magmatic rocks. There are no outcrops of post-collisional K-rich mafic magmatic rocks (MgO > 6 wt %) to the east of 87°E in the Lhasa terrane, which may indicate a change in subduction geometry at this longitude.

Metallogenic model for the Laochang Pb–Zn–Ag–Cu volcanogenic massive sulfide deposit related to a Paleo-Tethys OIB-like volcanic center, SW China

The Laochang Pb–Zn–Ag–Cu volcanogenic massive sulfide (VMS) deposit, located within the Changning–Menglian Paleotethyan suture, SW China, is an economically significant deposit with explored reserves of 866,000 metric tons (t) Pb at 4.5%, 336,000t Zn at 3.3%, 1700t Ag at 155g/t, 116,000t Cu at 0.5–0.9%, 2.84Mt pyrite and accompany 0.8t Au. The deposit comprises three ore clusters (No. I, II and III), each of which comprises dozens of vertically stacked lenticular and stratiform orebodies. Ore clusters I and II within the Early Carboniferous volcano-sedimentary sequence erupted around 320Ma comprise most of the reserves. The Early Carboniferous sequences, situating beside the Laochang caldera, are mainly composed of basaltic to andesitic lavas and volcaniclastic rocks, and divided into three major volcanic cycles. The massive sulfides mostly accumulated in the waning time of the second and third cycles. The orebodies (e.g., orebodies I1+2 and II1) display stockwork and disseminated sulfide ores in the lower part and the semi-horizontal massive sulfide lenses and laminated carbonaceous-sulfidic chert with delicate exhalative-sedimentary textures in the upper part, and they also show vertical metal zonation transiting from Fe–Cu, Cu–Zn–Pb to Pb–Zn–Ag. The underlying rock of the semi-horizontal orebodies has pervasively sericite–pyrite–quartz alteration; comparatively, the overlying is little altered. Ores are dominantly composed of sulfides, lacking sulfate and Fe-oxides, suggesting a very low SO42− concentration in the hydrothermal solution.The host andesitic tuff and basalt show pattern of fractionated rare earth elements (REE) and the enrichment in large ion lithophile elements (LILE; e.g., Rb, Th, U and light REE) and most high field strength elements (HFSE; e.g., Nb, Ta, Hf and Zr). These features are similar to those of oceanic island basalt. The higher REE fractionation and the greater enrichment of LILE (includes light REE) in the andesitic tuffs compared to those in the basalts comply with a fractionational crystallization process in a shallow magma chamber. The δ34S‰ (CDT) of the sulfides ranges from −2.1 to 3.5‰ with an average of 0.2‰. The limited variation of δ34S‰ near zero implies that the sulfur was derived via leaching of the footwall volcanic rocks and/or degassing directly from a magma chamber. Lead isotopic compositions of sulfides also have a small range. The 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb are 18.486–18.684, 15.668–15.712 and 38.725–39.024, respectively, indicating an extremely radiogenic feature similar to the host volcanic rocks. Previous fluid inclusion investigations revealed that ore fluid had high salinity, high temperature and the H–O–C isotopic compositions close to the primary magmatic hydrothermal. Our S–Pb isotopic analysis together with the fluid inclusion study, are supportive to the magmatic degassing genesis for this deposit. The vertical zonation of metal species, occurrence of laminated carbonaceous-sulfidic chert and lack of alteration in the hanging wall imply that the orebodies were formed via seafloor sulfide precipitation instead of sub-seafloor replacement. The occurrence of black shale intercalated with sulfide lenses and the carbonaceous-sulfidic chert caps, especially the absence of sulfate and other oxidized facies in the ores indicate a compelling local anoxic environment for mineral precipitation. This anoxic environment was caused by the venting of magmatic fluids characterized by high temperature, high salinity, metal-charged (Fe, Cu, Pb, Zn and Ag) and H2S-rich. The repetitive activity of magma degassing in the waning stage of volcanic eruption corresponded to the stacked structure of massive sulfide lenses within the top layer in a volcanic cyclothem. The enormous base metal enrichment and highly radiogenic lead isotopes at Laochang are explained to reflect the incorporation of a recycled upper continental crust component into the mantle source for the ore-bearing volcanic rocks during the evolution of Tethyan ocean.

Consideraciones geoquímicas y petrogenéticas para establecer la evolución magmática del Complejo Volcánico Nevado del Huila

The Nevado del Huila Volcano Complex is made of rocks that have intermediate to acid composition, calc-alkaline affinity and middle content in K2O. According to SiO2 content, there are three geochemical types: andesites ss, dacitic andesites and dacites. Adakitic tendence in more recent lavas and more dacitic composition. Behaviors of mayor elements, REE, LFSE and HFSE are typical of orogenic andesites of active continental margin. Isotopic composition Sr-Nd resemble to volcanoes of ZVN Andes (Ecuador’s volcanoes, Galeras and Nevado del Ruiz volcanoes from Colombia). Homogeneous isotopic rate Sr-Nd (87Sr/86Sr = 0,7041-0,7042 and 143Nd/144Nd = 0,51279-0,51283) are typical of mantle melts, with limited taking part of derived components of subduced slab or continental crust. Compatible and incompatible trace elements indicate that fractional crystallization is petrogenetic process that has controlled magmatic evolution. Besides to reject others mechanisms: crustal contamination and magmas mixing. These rocks have probably been generated from partial melting of enriched mantle wedge by components of subduced slab. Later fractional crystallization drove magmatic differentiation of this volcano complex.

Rb–Sr and Sm–Nd isotope systematics and geochemical studies on metavolcanic rocks from Peddavura greenstone belt: Evidence for presence of Mesoarchean continental crust in easternmost part of Dharwar Craton, India

Linear, north–south trending Peddavura greenstone belt occurs in easternmost part of the Dharwar Craton. It consists of pillowed basalts, basaltic andesites, andesites (BBA) and rhyolites interlayered with ferruginous chert that were formed under submarine condition. Rhyolites were divided into type-I and II based on their REE abundances and HREE fractionation. Rb–Sr and Sm–Nd isotope studies were carried out on the rock types to understand the evolution of the Dharwar Craton. Due to source heterogeneity Sm–Nd isotope system has not yielded any precise age. Rb–Sr whole-rock isochron age of 2551 ± 19 (MSWD = 1.16) Ma for BBA group could represent time of seafloor metamorphism after the formation of basaltic rocks. Magmas representing BBA group of samples do not show evidence for crustal contamination while magmas representing type-II rhyolites had undergone variable extents of assimilation of Mesoarchean continental crust (>3.3 Ga) as evident from their initial ε Nd isotope values. Trace element and Nd isotope characteristics of type I rhyolites are consistent with model of generation of their magmas by partial melting of mixed sources consisting of basalt and oceanic sediments with continental crustal components. Thus this study shows evidence for presence of Mesoarchean continental crust in Peddavura area in eastern part of Dharwar Craton.

Late proterozoic A-type granites of the chernorechenskii massif in the Yenisei ridge: New geochemical and geochronological data

The problems in estimation of the geochronologi� cal boundaries in the history of the breakup of the Rodinia Supercontinent are mostly far from their final solution. This is explained by the lack of precise dat� ings, which limits significantly the possibilities of age correlations for global geological processes. Because of this, the problem of geochemical specifics and dat� ing of Neoproterozoic granitoid complexes, i.e., the main components of the continental crust and indica� tors of geodynamic environments of key significance for reconstruction of events in the lithosphere evolu� tion, is still important. Recently new results on the geochemistry and geo� chronology of granitoids from the Yenisei Ridge formed under contrasting geotectonic environments were obtained (1). These crystalline massifs are located in the axial part of the Central cratonic block, mostly in the Tatarsko-Ishimbinskaya suture zone. In this relation, the western part of the Central block composed of rocks of the Garevka metamorphic com� plex (GMC), the oldest in Transangaria, is the least studied. It consists of a number of intrusive massifs located within the Yenisei regional suture zone (YRSZ), the large deformational metamorphic struc� ture of the lineament type separating the Central cra� tonic block and the Isaakovskii islandarc terrane (2). This zone is a continuation of the Baikal-Yenisei fault and located on the left side of the Yenisei River along the western margin of the Siberian Craton for more than 200 km at a width from 50 to 70 km. Because of the absence of reliable geochemical and geochrono� logical data, the problem of the age and tectonic set� ting of these intrusions is debatable. This is especially important for rocks of the Chernorechenskii massif occurring in the composition of the Posol'nenskii and Ayakhtinskii magmatic complexes of various age and origin (3). To resolve these contradictions, we obtained new data on the geochemistry, age, and nature of these granitoids formed under the environment of extension along the western margin of the Siberian Craton. The Chernorechenskii massif (CM) is located in the GMC, on the western flange of the YRSZ charac� terized by contrast metamorphism, intense foliation, and dynamic recrystallization of rocks (4). This massif occurs on the right side of the Yenisei River, in the Raz� gromnaya River basin, and is located in the same tec� tonic zone as small massifs of rapakivitype granites (5) and bimodal complex of granite and basic dykes (2). The intrusive body of a lenslike shape and an area of ≈140 km 2 occurs at large submeridional faults. With account for the geophysical data (3), it is suggested that the massif has subvertical dipping, a "platelike" shape, and tectonic contacts with host rocks. Three main stages with different thermodynamic regimes and values of geothermal gradients are distin� guished in the evolution of western areas of the YRSZ (6). At the first stage related to the Grenville orogeny (1050-850 Ma), zoned lowpressure metamorphic complexes with a gradient of dT/dH = 20-30°C/km typical of orogenesis were formed. At the second stage in the nappe zone, these rocks underwent Late Riph� ean (801-793 Ma) collisional metamorphism of mod� erate pressures with low dT/dH ≤ 10°С/km. The last stage was accompanied by synexhumation metamor� phism (785-776 Ma) with dT/dH ≤ 12°С/km reflect� ing the tectonic environments of rapid uplift in the dis� location-thrust zones of the crust. The final stages of the orogen evolution in the region include the forma� tion of the bimodal dyke belt with an age of 797- 785 Ma intruding gneiss and crystalline schist of the GMC (2). These events are controlled by the processes of extension along the western margin of the Siberian Craton and consistent with the beginning of breakup of the Rodinia Supercontinent. Later a number of rif� togenic troughs and grabens filled by Neoproterozoic

Geochronology, geochemistry and Sr–Nd–Hf isotopes of the Haisugou porphyry Mo deposit, northeast China, and their geological significance

The Haisugou Mo deposit is located in the northern part of the Xilamulun Mo–Cu metallogenic belt in northeastern China. The Mo mineralization mainly occurs as quartz-molybdenite veins within the Haisugou granite, which was emplaced into rocks of the Early Permian Qingfengshan Formation. Zircon U–Pb dating by LA–ICP-MS of the granite yields a crystallization age of 137.6±0.9Ma, suggesting emplacement during the peak time of Mo mineralization in eastern China, broadly constrained to ca. 150–130Ma, when tectonic stresses shifted from compression to extension. Whole-rock geochemical data suggest that the granite belongs to the high-K calc-alkaline series, and is characterized by relatively high LREE; low HREE; depletion of Ti, Ba, and Nb; and a moderate negative Eu anomaly. The zircon εHf(t) and whole-rock εNd(t) values for the intrusion range from +4.5 to +10.0 and +0.2 to +1.6, respectively, indicating that the magma originated from the juvenile lower crust source derived from depleted mantle, with some component of ancient continental crust. The granite is also characterized by initial (87Sr/86Sr)i ratios ranging from 0.7040 to 0.7074, which suggest some contamination by the upper crust during the ascent of the primitive magma. Moreover, it can be recognized from the whole-rock major and trace element data that significant fractional crystallization occurred during magmatic evolution, with the separation of plagioclase and K-feldspar. Because Mo is an incompatible element and tends to concentrate in the melt during crystallization, fractionation processes likely played an important role in the formation of the Haisugou Mo deposit.

Field, geochemistry and Sr-Nd isotopes of the Pan-African granitoids from the Tifnoute Valley (Sirwa, Anti-Atlas, Morocco): a post-collisional event in a metacratonic setting

In the Tifnoute Valley, three plutonic units have been defined: the Askaoun intrusion, the Imourkhssen intrusion and the Ougougane group of small intrusions. They are made of quartz diorite, granodiorite and granite and all contain abundant mafic microgranular enclaves (MME). The Askaoun granodi- orite and the Imourkhssen granite have been dated by LA-ICP- MS on zircon at 558±2 Ma and 561±3 Ma, respectively. These granitic intrusions are subcontemporaneous to the widespread volcanic and volcano-detrital rocks from the Ouarzazate Group (580-545 Ma), marking the post-collisional transtensional period in the Anti-Atlas and which evolved towards alkaline and tholeiitic lavas in minor volume at the beginning of the Cambrian anorogenic intraplate extensional period. Geochemically, the Tifnoute Valley granitoids belong to an alkali-calcic series (high-K calc-alkaline) with typical Nb-Ta negative anomalies and no alkaline affinities. Granitoids and enclaves display positive eNd-560Ma (+0.8 to +3.5) with young Nd-TDMbetween800and1200Maandrelativelylow 87 Sr/ 86 Sr initial ratios (Sri: 0.7034 and 0.7065). These values indicate a mainly juvenile source corresponding to a Pan-African meta- somatized lithospheric mantle pa rtly mixed with an old crustal component from the underlying West African Craton (WAC). Preservation in the Anti-Atlas of pre-Pan-African lithologies (c. 2.03 Ga basement, c. 800 Ma passive margin greenschist-facies sediments, allochthonous 750-700 Ma ophiolitic sequences) indicates that the Anti-Atlas lithosphere has not been thickened and was never an active margin during the Neoproterozoic. After a transpressive period, the late Ediacaran period (580- 545 Ma) is marked by movement on near vertical transtensional faults, synchronous with the emplacement of the huge Ouarzazate Group and the Tifnoute Valley granitoids. We pro- pose here a geodynamical model where the Tifnoute Valley granitoids as well as the Ouarzazate Group were generated during the post-collisional metacratonic evolution of the north- ern boundary of the West African craton. The convergence with the peri-Gondwanan active margin produced brittle fracturing of the cratonic boundary without thickening, allowing rising of magmas. The Tifnoute Valley granitoids display a metasomat- ized lithospheric mantle source mixed with a minor ancient (2 Ga) continental crust component from the underlying WAC.

Adakitic-like magmatism in western Ossa–Morena Zone (Portugal): Geochemical and isotopic constraints of the Pavia pluton

Granitic rocks are a major component of the Earth's continental crust and occur in a wide variety of tectonic settings. Their chemical and isotopic characterization is crucial to the recognition of the potential sources and mechanisms involved in their generation. In this study, we present the first whole rock chemical and isotopic (Sr–Nd–O) data for the Pavia pluton (328–317Ma), located near the western border of the Ossa–Morena Zone (Évora Massif, Portugal). Major and trace element geochemistry suggests that the different granitic phases composing this intrusive body (enclaves, granites (s.l.) and crosscutting dikes) represent independent magma pulses and the majority is similar to TTGs and adakites. The little Sr–Nd–O isotopic variation, with (87Sr/86Sr)328=0.70428–0.70560, εNd328 ranging between −3.4 and +0.4 and δ18O varying from +5.6‰ to +8.4‰ implies an isotopically similar protolith for all phases. The most viable mechanism for the generation of the Pavia pluton adakitic-like magmatism is assimilation–fractional crystallization of a mantle-derived magma. This mechanism was also invoked to explain the genesis of other plutons within the Évora Massif but they have a distinct chemistry (typical arc calc-alkaline rocks). The chemical differences between them and the Pavia pluton granitic rocks are interpreted as the result of lower degrees of crustal assimilation and higher degrees of contamination of mantle-derived magmas by the sinking slab (after subduction blocking and subsequent slab break-off).

Hafnium isotope evidence from Archean granitic rocks for deep-mantle origin of continental crust

Combined whole-rock and zircon MC-ICP-MS Lu–Hf isotope data are reported for a large collection of Archean granitoids belonging to typical tonalite–trondhjemite–granodiorite (TTG) suites. Our data demonstrate that the time-integrated Lu/Hf of the mantle source of TTGs has not significantly changed over the last 4Gy. Continents therefore most likely grew from nearly primordial unfractionated material extracted from the deep mantle via rising plumes that left a depleted melt residue in the upper mantle. The deep mantle could retain its primitive relative element abundances over time because sinking plates are largely stripped barren of their oceanic and continental crust components at subduction zones; this process results in only small proportions (<15–25%) of present-day continental mass getting recycled to great depths. Zircon populations extracted from the analyzed TTGs have Hf isotopic compositions broadly consistent with those of their host whole-rocks, whereas the U–Pb system in the same grains is often disturbed, causing a discrepancy that creates spurious initial εHf values. This problem is endemic to the Archean detrital zircon record and consistent with experimental results bearing on the relative retentivity of Hf vs. U and Pb in zircon. We argue that this behavior biases the Archean zircon record toward negative εHf values, which are at odds with the present TTG data set. If Hadean Jack Hills zircons are considered in light of these results, the mantle source of continents has remained unchanged for the last 4.3Gy.

Cenozoic Volcanism of the Caucasian Mobile Belt in Georgia, its Geological-Petrological Peculiarities and Geodynamic Conditions

The Caucasian mobile belt is situated in the area of Late Cenozoic collision of the large Afro-Arabian and Eurasian lithospheric plates. Extensive volcanic activity in the Georgian part of the Caucasian mobile belt took place during the Late Miocene-Holocene. Five volcanic regions have been identified in Georgia; each of them reveals in a greater or lesser degree similarity of tectonic and magmatic processes. Volcanic products are represented by basaltic, doleritic, andesitic basaltic, andesitic dacitic, rhyolitic lavas and their pyroclastics with andesites and dacites prevailing. Using petrochemical and geochemical data the volcanics can be divided into two main rock groups: calc-alkaline and subalkaline series. The marker petrogeochemical series is presented by the medium- to high-K calc-alkaline volcanics. Relative to the heavy elements (HFSE) Y, Nb they are enriched in some large ion lithophile elements (LILE) Rb, Sr, Ba and light rare earth elements (REE) La, Ce. This confirms the leading role of fractional crystallization in forming the volcanics of the study area. These volcanics have the characteristics of pre-collision subduction (increased LILE content and high La/Nb ratios) geodynamic regimes. Volcanic rocks derived from sources displaying different tectonic environments show close petrogeochemical resemblance, indicating the similarity of the melting substrates of magmatic chambers. The findings also allow us to give priority to the magma generation conditions, to its periodical renewal and depths of inception in comparison with the geodynamical factors. Isotopic data (^8^7Sr/^8^6Sr) have confirmed that the subduction-enriched lithospheric mantle material was more important than that of the continental crust components. Sr isotopic ratios do not show marked dependence on the values of the petrochemical composition of the enclosing rocks and on the time of their formation.

Mantle xenoliths from Tallante (Betic Cordillera): Insights into the multi-stage evolution of the south Iberian lithosphere

Extreme compositional heterogeneities have been recorded in mantle xenoliths from Tallante, attracting an intense petrological interest that is reflected in the impressive number of scientific studies. In this contribution we present new isotopic analyses carried out by MC-ICP-MS and TIMS on minerals separated from samples previously characterized by Beccaluva et al., 2004 (Lithos 75, 67–87). The new Sr–Nd–Hf isotopic analyses, combined with the previously published data, highlight that anhydrous cpx-poor peridotites display enrichment trends towards Enriched Mantle (EM) components, related to metasomatic interaction between alkaline melt and a depleted peridotite matrix that suffered melt extraction in the garnet stability field. In contrast, anhydrous cpx-rich lherzolites (containing up to 5% plagioclase) cluster around the Depleted Mantle (DM) component and are possibly related to a pervasive refertilization induced by tholeiitic melts that permeated the Betic lithosphere. These metasomatic reactions possibly occurred during the Mesozoic tectono-magmatic cycle that ultimately led to the opening of the central Atlantic and the westward propagation of the Neotethys rifting and oceanization. Further metasomatic reactions within the lithosphere occurred in the Neogene, during the Alboran back-arc development in connection with the Betic orogenic event. This cycle is documented by composite peridotite xenoliths crosscut by plagioclase and orthopyroxene ± quartz ± phlogopite ± amphibole veins that are characterized by crustal Sr–Nd–Hf isotopic signatures. This indicates hydrous silica-oversaturated melts as causative metasomatic agents, in turn implying the recycling – via subduction – of continental crust components within the mantle.

The Yermakovsky deposit, Western Transbaikal Region, Russia: Isotopic and geochemical parameters and sources of beryllium-bearing granitoids and other rocks

An isotopic study of igneous and metamorphic rocks has been carried out at the Yermakovsky bertrandite-phenakite-fluorite deposit. It has been established that the model age of the schists pertaining to the Zun-Morino Formation is 1360–1260 Ma. In Nd and Sr isotopic composition, these schists deviate from the isotopic composition of the continental crust and are close in this respect to the enriched mantle reservoir (EM-II). The model age of carbonate rocks of the Zun-Morino Formation is 1330–1020 Ma. The Middle Riphean model age of the Zun-Morino Formation is interpreted as the age of its protolith. According to the Sr and Nd isotopic data, all preore igneous rocks (granitic dikes, gabbroic rocks, and gneissose granite of the Tsagan Complex) were formed with the participation of continental crustal material. Synore basic dikes, alkali leucogranite stock, and syenite intrusion are considered to be mixtures of mantle components (DM+HIMU) and various continental crustal components (Tsagan gneissose granite, crystalline schists, the mean composition of granitoids of the Angara-Vitim batholith as an estimate of average composition of the regional continental crust). Synore igneous rocks are genetically cognate and related to the magmatic activity in the Western Transbaikal Rift Zone presumably formed in the Triassic under effect of a mantle plume.

Provenance of Ordovician clastic sequences of the San Rafael Block (Central Argentina), with emphasis on the Ponón Trehué Formation

The Ordovician Ponón Trehué Formation is the only early Palaeozoic sedimentary sequence known to record a primary contact with the Grenvillian-age basement of the Argentinean Cuyania terrane, in its southwards extension named the San Rafael block. Petrographic and geochemical data indicate contributions from a dominantly upper continental crustal component and a subordinated depleted component. Nd isotopes indicate ε Nd of − 4.6, ƒ Sm/Nd − 0.36 and T DM 1.47 Ga in average. Pb-isotope ratios display average values for 206Pb/ 204Pb, 207Pb/ 204Pb, and 208Pb/ 204Pb of 19.15, 15.69 and 38.94 respectively. U–Pb detrital zircon ages from the Ponón Trehué Formation cluster around values of 1.2 Ga, indicating a main derivation from a local basement source (Cerro La Ventana Formation). The Upper Ordovician Pavón Formation records a younger episode of clastic sedimentation within the San Rafael block, and it shows a more complex detrital zircon age population (peaks at 1.1 and 1.4 Ga as well as Palaeoproterozoic and Neoproterozoic detrital grains). Detailed comparison between the two Ordovician clastic units indicates a shift with time in provenance from localized basement to more regional sources. Middle to early Upper Ordovician age is inferred for accretion of the Cuyania terrane to the proto-Andean margin of Gondwana.

Detrital chromian spinels from Upper Ordovician deposits in the Precordillera terrane, Argentina: A mafic crust input

The study of heavy minerals from the Caradocian Pavón Formation (central Argentina) suggests that MORB and continental intraplate flood basalts were among the source rocks for the detrital record. The Pavón Formation was deposited on the disputed exotic Precordillera terrane. Provenance studies on this formation revealed sedimentary rocks characterized by an upper continental crustal component mixed with a mafic source, where the latter could not be described. The detrital chromian spinels provide more information about this component. The detrital spinels were chemically separated into two groups. Group 1 shows characteristics typical for host rocks related to MORB (Cr# values are between 0.4 and 0.6, Fe 2+# range from 0.2 to 0.4, and they have low TiO 2 and low Fe 3+# contents). Conversely, Group 2 has Cr# of 0.7, Fe 2+# ranging from 0.5 to 0.8, higher Fe 3+# and TiO 2 contents up to 4.7%, and they are related to continental flood basalts. The detrital spinels are chemically and texturally compared to those hosted in probable source rocks. Although the mafic source still remains unknown, the presence of detrital chromian spinels within the Pavón Formation implies the existence of an oceanic crustal component closely related to the Precordillera terrane.

The Variscan gabbros from the Spanish Central System: A case for crustal recycling in the sub-continental lithospheric mantle?

The gabbroic intrusions that crop out along the Spanish Central System (SCS) are geochemically heterogeneous, including primitive and evolved rocks. Differentiation is mainly related to fractionation of Cr-spinel and olivine, but mixing with coeval granitic magmas or crustal assimilation may have also played a role in the evolution of the most differentiated rocks. The most primitive uncontaminated gabbros show arc-like trace element chondrite and primitive-mantle normalised patterns, characterised by large ion lithophile elements (LILE)-light rare earth elements (LREE) enrichment, Sr and Pb positive and Nb–Ta–Ti negative anomalies. However, paleogeographic constraints suggest that the SCS was located far from subduction zones, so these geochemical signatures could be better explained by a recycling of continental crustal components within the mantle. The most primitive SCS gabbros expand the Sr–Nd isotopic compositional range of the Variscan basic magmatism in the Central Iberian Zone to more depleted values. This reflects a heterogeneous sub-continental lithospheric mantle under central Spain ranging from a depleted mantle (εNd = + 3.1, 87Sr/ 86Sr = 0.704) towards an isotopically enriched component (εNd = − 1.6, 87Sr/ 86Sr = 0.706). Geochemical modelling suggests that mantle enrichment could be explained by minor lower crustal metapelitic granulite contamination (~ 2%). Additionally, the Sr–Nd–Pb isotopic ratios of the most primitive gabbros match the composition of the European subcontinental lithospheric mantle recorded in ultramafic xenoliths from western and central Europe.

Geochemistry of the Panjal Traps basalts (NW Himalaya): records of the Pangea Permian break-up

AbstractThe late Lower to Middle Permian Panjal Traps (NW Himalaya, India-Pakistan) represent the greatest magmatic province erupted on the northern Indian platform during the Neotethys opening. New geochemical and isotopic analyses were performed on basalts from the eastern borders of the traps (SE Zanskar-NW Spiti area) in order to characterize this volcanism, to discuss its compositional variations in comparison to Panjal counterparts and its relationships with the opening of Neotethys. Lavas show features of tholeiitic low-Ti (&amp;lt; 1.6%) continental flood basalts with LREE, Th enrichments and Nb-Ta negative anomalies. Trace element ratios combined with εNdi values (−3.6 to +0.9) and high Pb isotopic ratios suggest that these tholeiitic basalts were derived from an OIB-like mantle contaminated at various degrees by a continental crust component. Previous geochemical features are broadly similar to those of the coeval Panjal volcanic sequences identified westwards (Ladakh, Kashmir and Pakistan). Present geochemical constraints obtained for the Panjal Traps basalts suggest they originated from rapid effusion of tholeiitic melts during opening of the Neotethys Ocean. Similar magmatism implying an OIB-type reservoir is contemporaneously recognized on and along the adjacent Arabian platform. Both Indian and Arabian Permian volcanics were emplaced during coeval syn-rift to post rift transition. These Lower to Middle Permian south Neotethyan continental flood magmatism are regarded as associated to a passive rifting. In this scheme, OIB-type isotopic signature would be related either to a melting episode of syn-rift up-welling mantle plumes or to a melting of a regional abnormally hot and enriched mantle.