Extensive Crustal Contamination Research Articles

Olivines and Their Melt Inclusions in Potassic Volcanic Rocks Record Mantle Heterogeneity beneath the Southern Tibet

Abstract Constraints on mantle heterogeneity are important for understanding geochemical differentiation and recycling in Earth’s interior. Alkali basalts are of particular interest in this context due to the clear signatures of recycled components that they yield. Previous studies have focused more on oceanic basalts than on continental mafic rocks because the chemical and isotopic characteristics of the latter are often modified by crustal contamination; however, high-forsterite olivine crystals and olivine-hosted melt inclusions may record the compositions of primitive melts before extensive crustal contamination, providing valuable insights into mantle heterogeneity. Here we report the results of high-precision major-, minor-, and trace-element and Pb isotope analyses of olivine phenocrysts and melt inclusions from the Sailipu potassic alkali basalts from southern Tibet, which constrain the mantle source of continental potassic alkali basalts. The minor- and trace-element compositions of the olivine phenocrysts (e.g. Ni and Ca contents and 100 × Mn/Fe, 10 000 × Zn/Fe, and Ni/Co ratios) suggest that the primary magmas were derived from a phlogopite-bearing pyroxenite source in which peridotite residues were also present. The low- and high-Li contents of the olivine phenocrysts suggest the recycling of continental crustal material in their mantle source. Comparing the major-element compositions (e.g. SiO2, TiO2, Al2O3, MnO, CaO, and Na2O contents), FCMS and FCKANTMS parameters, and pseudo-ternary projections of the primary Sailipu magmas with experimental data suggests that the primary magmas were partial melts of olivine-poor or carbonated olivine-poor lithologies with a small amount of peridotite residue. The negative Nb and Ta and positive Pb anomalies seen in primitive mantle-normalized trace-element patterns and the extremely low Nb/U ratios of the melt inclusions suggest that recycling of continental crust was the main process involved in enriching the mantle source of the Sailipu magmas. The heterogeneous Pb isotopic compositions of the melt inclusions (208Pb/206Pb = 2.089 – 2.158; 207Pb/206Pb = 0.851 – 0.858) are consistent with the presence of peridotite residue in this source. The subduction of Indian continental crust introduced enriched material (including carbonatites) into the mantle.

Constraints of Mantle and Crustal Sources Interaction during Orogenesis of Pre‐ and Post‐collision Granitoids from the Northern Arabian‐Nubian Shield: A Case Study from Wadi El‐Akhder Granitoids, Southern Sinai, Egypt

AbstractThe Egyptian older and younger granitic rocks emplaced during pre‐ and post‐collision stages of Neoproterozoic Pan‐African orogeny, respectively, are widely distributed in the southern Sinai Peninsula, constituting 70% of the basement outcrops. The Wadi El‐Akhder, southwestern Sinai, is a mountainous terrain exposing two granitoid suites, namely the Wadi El‐Akhder Older Granites (AOG) and the Homra Younger Granites (HYG). The AOG (granodiorites with subordinate tonalite compositions) have geochemical characteristics of medium‐K calc‐alkaline, metaluminous to mildly peraluminous granitoids formed in an island‐arc environment, which are conformable with well‐known Egyptian older granitoids rocks, whereas the HYG display calc‐alkaline to slightly alkaline nature, peraluminous syeno‐, monzogranites and alkali feldspar granites matching well those of the Egyptian younger granites. With respect to the AOG granitoids, the HYG granites contain lower Al2O3, FeO∗, MgO, MnO, CaO, TiO2, Sr, Ba, and V, but higher Na2O, K2O, Nb, Zr, Th, and Rb. The AOG are generally characterized by enrichment in LILE and LREE and depletion in HFSE relative to N‐MORB values (e.g., negative Nb and Ta anomalies). The geochemical features of the AOG follow assimilation‐fractional crystallization (AFC) trends indicative of extensive crustal contamination of magma derived from a mantle source. The chemical characteristics of the AOG are remarkably similar to those of subduction‐related granitoids from the Arabian‐Nubian Shield (ANS). The compositional variations from monzogranites through syenogranites to alkali feldspar granite within HYG could not be explained by fractional crystallization solely. Correlating the whole‐rock composition of the HYG to melts generated by experimental dehydration melting of meta‐sedimentary and magmatic rocks reveals that they appear to be derived by extended melting of psammitic and pelitic metasediments, which is similar to the most of younger granitic suites in the ANS.

Identification of continental-type eclogites in the Paleo-Tethyan Changning–Menglian orogenic belt, southeastern Tibetan Plateau: Implications for the transition from oceanic to continental subduction

As a record of ancient subduction, eclogites preserve critical information for exploring geodynamic processes at convergent plate boundaries. The Changning–Menglian orogenic belt (CMOB) in the southeastern Tibetan Plateau is situated between the Gondwana-derived Baoshan–Tengchong Block and the Eurasia-derived Lanping–Simao Block. The belt has long been accepted as a typical cold subduction–accretion belt associated with the closure of the main Paleo-Tethys Ocean, owing to the development of oceanic-type lawsonite-bearing eclogites. However, this viewpoint is challenged by the presence of newly discovered continental-type eclogites hosted by metasedimentary rocks in the CMOB. The continental-type eclogites are depleted in high field-strength elements (e.g., Nb, Ta, and Zr), enriched in Th and U, and preserve highly negative ɛNd(t) values varying from −15.26 to −13.04, indicating derivation from continental mafic protoliths that underwent extensive crustal contamination. Inherited zircons from both the continental-type eclogites and their host schists have characteristic detrital age distributions, with dominant Pan-African age peaks at 630–505 Ma that match well with those in the Baoshan–Tengchong Block. Combined with published age data, our new geochemical and geochronological data provide convincing evidence for the presence of significant volumes of subducted continental fragments derived from the Baoshan–Tengchong Block. Thermodynamic modeling of the continental-type eclogites indicates a clockwise P–T path with a steep prograde path consistent with a geothermal gradient of 5–8 °C/km, which was followed by near-isothermal decompression. Calculations suggest the eclogite-facies assemblage of garnet + omphacite + glaucophane ± talc ± lawsonite ± phengite records peak conditions of 28.5–29.6 kbar and 580–594 °C, equivalent to depths of 85–90 km. U–Pb dating of metamorphic zircon rims from the eclogites gives an age of 234–233 Ma, which is interpreted to reflect the timing of continental subduction. The continental-type eclogites show mineral associations, paragenetic sequences and metamorphic P–T–t evolutions similar to those of low-temperature oceanic-type eclogites in the CMOB. As a result, we propose that rapid, cold subduction of continental crust, and its subsequent rapid exhumation, occurred shortly after the closure of the dense and long-lived Paleo-Tethys oceanic lithosphere. The discovery of continental-type eclogites imparts significant insights into the dynamics of Gondwana–Eurasia convergence in the Paleo-Tethyan domain. The CMOB provides a well-exposed example of the tectonic transformation from final oceanic subduction to the onset of continental subduction and collision in a cold geothermal regime.

Protracted Storage for Calc-Alkaline Andesitic Magma in Magma Chambers: Perspective from the Nageng Andesite, East Kunlun Orogen, NW China

Calc-alkaline andesitic rocks are a major product of subduction-related magmatism at convergent margins. Where these melts are originated, how long they are stored in the magma chambers, and how they evolved is still a matter of debate. In this study, we present new data of whole-rock elemental and Sr-Nd-Pb isotope compositions, and zircon U-Pb-Th isotopes and trace element contents of Nageng (basaltic-)andesites in the East Kunlun Orogen (NW China). The similar age and whole-rock elemental and Sr-Nd-Pb isotope contents suggest that the Nageng andesite and basaltic andesite are co-magmatic. Their low initial 87Sr/86Sr (0.7084–0.7086) but negative εNd(t) values (−10.61 to −9.49) are consistent with a magma source from the juvenile mafic lower crust, possibly related to the mantle wedge with recycled sediment input. The U-Pb age gap between the zircon core (ca. 248 Ma) and rim (ca. 240 Ma) reveals a protracted magma storage (~8 Myr) prior to the volcanic eruption. When compared to the zircon rims, the zircon cores have higher Ti content and Zr/Hf and Nb/Ta ratios, but lower Hf content and light/heavy rare earth element ratios, which suggests that the parental magma was hotter and less evolved than the basaltic andesite. The plagioclase accumulation likely resulted in Al2O3-enrichment and Fe-depletion, forming the calc-alkaline signature of the Nageng (basaltic-)andesites. The magma temperature, as indicated by the zircon saturation and Ti-in-zircon thermometry, remained low (725–828 °C), and allowed for the magma chamber to survive over ~8 Myr. The decreasing εHf(t) values from zircon core (avg. 0.21, range: −1.28 to 1.32) to rim (avg. −3.68, range: −7.30 to −1.13), together with the presence of some very old xenocrystic zircons (268–856 Ma), suggest that the magma chamber had undergone extensive crustal contamination.

Sr isotopes and geochemistry of Cerro Overo maar provide a unique window into arc magma genesis in the Central Volcanic Zone of the Andes

Cerro Overo maar is the most mafic lava (54 wt% SiO2, 7.4 wt% MgO) found within the modern subduction arc of the Central Andes. Structural weaknesses in the crust allowed magma to rapidly ascend while avoiding extensive crustal contamination, interactions with the mid-crustal Altiplano-Puna Magma Body, or storage in magma chambers. Cerro Overo lava thus preserves chemical and mineralogical signals from deep-crustal or upper-mantle magmatism which are otherwise absent in the modern Andes Central Volcanic zone (CVZ). Radiogenic isotope ratios of CVZ rocks are characteristically enriched, and researchers have debated for decades whether this enrichment is derived from an enriched mantle source or the result of crustal processes. Phenocrysts of high-forsterite olivine (Fo86) from Cerro Overo contain three populations of silicate melt inclusions: 1. basaltic andesite inclusions chemically identical to the host lava, 2. rhyodacite inclusions representing assimilated upper crust, and 3. basaltic melt inclusions (SiO2 = 49.0 wt%) of minimally-altered magma, reflecting an early-stage melt which may be common to the active arc of the Altiplano-Puna region. Melt inclusion 87Sr/86Sr (0.70376 to 0.70720) also form three clusters, including the most primitive Sr isotopic ratios reported for the modern CVZ arc: direct evidence that the sub-arc mantle source retains non-radiogenic Sr signatures. The least-evolved Olivine-hosted melt inclusions from Cerro Overo match the composition of postulated arc endmembers and provide direct evidence that the overall radiogenic isotopic characteristic of the modern CVZ results from lithospheric processes and not inheritance from a mantle source with an initially radiogenic 87Sr/86Sr signature.

Contrasting high-Mg, high-K rocks in Central Iberia: the appinite—vaugnerite conundrum and their (non-existent) relation with arc magmatism

The central zone of the Iberian Massif contains abundant granitic batholiths, mostly consisting of peraluminous granodiorites to granites, with minor volumes of high-Mg and high-K ultramafic, mafic, and intermediate rocks (UMI rocks), belonging to two associations: vaugnerites and appinites. The UMI rocks crop out as small stocks within anatectic domains or granite bodies, as enclaves (scattered, clusters or swarms), even as syn-plutonic dikes. SHRIMP zircon U–Pb ages range from ~ 291 Ma to ~ 328 Ma, being broadly coeval with the granites and slightly younger than the anatexites that host them. The vaugnerites are alkaline, with Na2O + K2O concentrations similar to shoshonites, whereas the appinites are comparable to high-K calc-alkaline series, but in no way can they be considered arc-rocks indicative of subduction. Compared with arc-related shoshonites and high-K calc-alkaline rocks, the Iberian UMI rocks, especially the vaugnerites, are much more enriched in mica-compatible elements such as Li, Rb, Cs, Be, Sn, Nb, etc. as well as in Th and U. They also have extraordinarily elevated production of radiogenic heat, the appinites about 1.5 µWm−3, and the vaugnerites about 3.9 µWm−3. These values contrast with the average heat production of the upper mantle (0.02 µWm−3) and the lower continental crust (0.6 µWm−3) and are even higher than the average continental crust (1.4 µWm−3) indicating extensive crustal contamination. The negative whole-rock eNdt, usually between − 2 and – 6, the elevated initial 87Sr/86Sr, usually > 0.706, and the old Nd model ages (1 Ga to 1.5 Ga), i.e., much higher than the zircon crystallization ages, reinforces this idea. Despite both rock associations being affected by crustal contamination, the processes that operated were different. Whereas appinites may have resulted from bulk contamination with anatexites plus crystal fractionation, the vaugnerites require contamination with supercritical fluids resulting from biotite breakdown. Accordingly, we propose that the production of appinites or vaugnerites depends on the fertility of the metamorphic rocks they traversed en route from the mantle to the middle crust: if the host rock was fertile and partially molten, the intrusion of mafic magma increased the melt fraction so permitting bulk contamination to produce hybrid appinite magmas; if the rock was infertile, the fluids released from micas breakdown during the metamorphism could migrate as such to the mafic magmas, contaminating them to yield vaugnerite magmas.

Petrogenesis of the early Permian A-type granites in the Halajun region, southwest Tianshan, western Xinjiang, NW China: implications for geodynamics of Tarim large igneous province

ABSTRACT Formation of a large igneous province, an explosive magmatic event in earth, is intimately related to geodynamic processes. The Tarim large igneous province, a famous representative in China, contains A-type granites. However, the petrogenesis of the A-type granites is debated. Some researchers think that the A-type granites are developed through extensive fractionation of magma derived from partial melting of newly underplated or gabbroic rocks. Others insist that the A-type granites are developed through intensive fractional crystallization of plume-derived mafic magma. Here, we present zircon U–Pb ages and whole-rock geochemical and Sr–Nd isotopic data for the Kezilekezitage, Keziertuo, Tamu, and Halajun II intrusions in northwest Tarim to elucidate their petrogenesis and geodynamic implications. Zircon U–Pb dating yielded crystallization ages of the Kezilekezitage, Keziertuo, and Halajun II intrusions of 275.1 ± 3.1 Ma (MSWD = 1.14), 274.8 ± 1.8 (MSWD = 0.46) and 275.6 ± 0.7 (MSWD = 1.04), respectively. The whole-rock geochemistry of the four intrusions is characterized by REE and HFSE enrichment, Ba, Sr, P, Ti, and Eu depletion, εNd (t) values of – 1.2 to +0.3, two-stage Nd model ages (T2DM) of 1.14–1.02 Ga, and an A1-type granite affinity. Whole-rock zircon geothermometry indicates a high formation temperature of 721–924°C. It is concluded that these A-type granites were generated by partial melting of Neoproterozoic OIB-like rocks, coupled with extensive fractional crystallization and minor crustal contamination. The production of the A-type granites at ca. 275 Ma represented the termination and greatest degree of lithospheric thinning during large igneous province formation.

U-Pb zircon geochronology, geochemistry, and Sr-Nd-Pb-Hf isotopic composition of the Late Cretaceous monzogranite from the north of the Yidun Arc, Tibetan Plateau Eastern, SW China: petrogenesis and tectonic implication

The Yidun Arc belt, a Triassic volcanic arc, was the result of Late Triassic large-scale subduction orogenic process of Paleo-Tethys and exposed the voluminous intermediate-acid intrusions during Mesozoic. In this paper, we mainly present petrography and petrology, major and trace element, Sr-Nd-Pb-Hf isotopic compositions, and zircon U-Pb ages for the monzogranites from the Queer Mountains area at the northern of Yidun Arc. The Queer Mountains monzogranites were emplaced at 93.6 ± 0.9 Ma (MSWD = 6.8, n = 34, 2σ), revealing that the monzogranites are the products of magmatism during the period of Late Cretaceous (“Yanshanian”). The monzogranites have high SiO2, Al2O3, and K2O contents and enrichment in light rare earth elements (LREEs) and large-ion lithophile elements (LILEs) but depleted in high field strength elements contents (HFSEs), with relatively high variable (La/Yb)N values (1.38–11.79) and evidently negative Eu anomalies (Eu/Eu* = 0.07–0.35). The samples have high initial 87Sr/86Sr (0.708421–0.715698), with negative eNd(t) (− 0.53 to − 3.57) and eHf(t) (− 0.4 to − 3.2) values, and the Nd and Hf of two-stage model ages are 1.03–1.24 and 1.18–1.35 Ga, respectively. The (206Pb/204Pb)t, (207Pb/204Pb)t, and (208Pb/204Pb)t values of the monzogranites vary from 18.7234 to 18.8436, from 15.7031 to 15.7261, and from 39.0507 to 39.2780, respectively. The geochemistry signatures indicate that the granite samples may be mainly derived from the melts of upper crust. According to above geochemical signatures, the mature of monzogranites source was mainly a metapelite source, and they should be generated from the Mesoproterozoic crust material affinity to Yangtze Craton, later underwent extensive crustal contamination and fractional crystallization of K-feldspar during emplacement. These monzogranites maybe undergo crystallization temperatures from 730 to 804 °C. According to the previous studies and coeval magmatic-tectonic thermal event of the Yidun Arc area and this study, the Queer Mountains monzogranite was most likely formed in a post-collisional extensional tectonic setting after the syn-collision in Yidun Arc region.

Across‐arc versus along‐arc Sr‐Nd‐Pb isotope variations in the Ecuadorian volcanic arc

AbstractPrevious studies of the Ecuadorian arc (1°N–2°S) have revealed across‐arc geochemical trends that are consistent with a decrease in mantle melting and slab dehydration away from the trench. The aim of this work is to evaluate how these processes vary along the arc in response to small‐scale changes in the age of the subducted plate, subduction angle, and continental crustal basement. We use an extensive database of 1437 samples containing 71 new analyses, of major and trace elements as well as Sr‐Nd‐Pb isotopes from Ecuadorian and South Colombian volcanic centers. Large geochemical variations are found to occur along the Ecuadorian arc, in particular along the front arc, which encompasses 99% and 71% of the total variations in 206Pb/204Pb and 87Sr/86Sr ratios of Quaternary Ecuadorian volcanics, respectively. The front arc volcanoes also show two major latitudinal trends: (1) the southward increase of 207Pb/204Pb and decrease of 143Nd/144Nd reflect more extensive crustal contamination of magma in the southern part (up to 14%); and (2) the increase of 206Pb/204Pb and decrease of Ba/Th away from ∼0.5°S result from the changing nature of metasomatism in the subarc mantle wedge with the aqueous fluid/siliceous slab melt ratio decreasing away from 0.5°S. Subduction of a younger and warmer oceanic crust in the Northern part of the arc might promote slab melting. Conversely, the subduction of a colder oceanic crust south of the Grijalva Fracture Zone and higher crustal assimilation lead to the reduction of slab contribution in southern part of the arc.

U–Pb zircon geochronology, petrochemical and Sr–Nd isotopic characteristic of Late Neoproterozoic granitoid of the Bornaward Complex (Bardaskan-NE Iran)

The Bornaward Granitoid Complex (BGC) in the Taknar Zone is located in the northeast of Central Iranian Block. The BGC consists of granite, alkaligranite, syenogranite, leucogranite, granophyre, monzogranite, granodiorite, tonalite and diorite that have intruded into the center of Taknar Zone. These intrusive rocks affected by low grade metamorphism. Because of there are no reliable isotope dating data, for the Bornaward Granitoid Complex rocks have been proposed discordant ages (Jurassic, Cretaceous or even younger ages) by many studies. In the present study, new isotopic information based on zircon U–Pb dating has revealed the origin and time of the formation of the BGC. These new results do not confirm previously proposed ages. The results obtained from zircon U–Pb dating of the BGC rocks suggest late-Neoproterozoic (Precambrian) age (540–550Ma). The Bornaward Granitoid Complex is middle-high metaluminous to lower-middle peraluminous and belongs to tholeiite, calc-alkaline to high-K calc-alkaline rock series with enrichment in LIL (Cs, Rb and Ba, U, K, Zr, Y, Th) and depletion in HIL (Sr and Nb, Ta, Ti) elements. Chondrite-normalized Rare Earth Elements (REE) plots indicate minor enrichment of LREE compared to HREE, and strong negative anomaly of Eu compared to other Rare Earth Elements. Furthermore, initial 87Sr/86Sr and 143Nd/144Nd range from 0.70351 to 0.71689 and 0.511585 to 0.512061, respectively, and initial εNd isotope values for granite, granodiorite and diorite range from −6.73 to 2.52. These all indicate that the BGC has derived from partial melting of distinct basement source regions with very high initial 87Sr/86Sr and undergoing extensive crustal contamination (S-type granite).

Highly siderophile element behaviour during flood basalt genesis and evidence for melts from intrusive chromitite formation in the Mackenzie large igneous province

The 1.27Ga Coppermine continental flood basalt (CFB) province in northern Canada represents the extrusive manifestation of the 2.7Mkm2 Mackenzie large igneous province (LIP) that includes the Mackenzie dyke swarm and the Muskox layered intrusion. New Re–Os isotope and highly siderophile element (HSE: Re, Pd, Pt, Ru, Ir, Os) abundance data are reported together with whole-rock major- and trace-element abundances and Nd isotopes to examine the behaviour of the HSE during magmatic differentiation and to place constraints on the extent of crustal interaction with mantle-derived melts. Mineral chemistry and petrography are also reported for an unusual andesite glass flow (CM19; 4.9wt.% MgO) found in close proximity to newly recognised picrites (>20wt.% MgO) in the lowermost stratigraphy of the Coppermine CFB. Compositions of mineral phases in CM19 are similar to the same phases found in Muskox Intrusion chromitites and the melt composition is equivalent to inclusions trapped within Muskox chromites. The apparently conflicting elevated HSE contents (e.g., 3.8ppb Os) and mantle-like initial 187Os/188Os (γOs=+2.2), versus stable isotope (δ18O=+12‰) and lithophile element evidence (εNdi=−12.8) for extensive crustal contamination, implicate an origin for CM19 as a magma mingling product formed within the Muskox Intrusion during chromitite genesis. Combined with Nd isotope data that places the feeder for lower Coppermine CFB picrites and basalts within the Muskox Intrusion, this result provides compelling evidence for direct processing of some CFB within upper-crustal magma chambers. The Coppermine CFB defines a 187Re–187Os isochron with an age of 1263 +16/−20Ma and initial γOs=+2.2±0.8. The initial Os isotope composition for the Coppermine CFB is slightly higher than the near-primitive-mantle initial 187Os/188Os for the Muskox Intrusion (γOs=+1.2±0.3). This result is interpreted to reflect greater crustal contamination in extrusive CFB and the sensitivity of Os isotopes, compared with absolute HSE concentrations, for tracking crustal contributions.Modelling of absolute and relative HSE abundances in global CFB reveals that HSE concentrations decrease with increasing fractionation for melts with <8±1wt.% MgO, with picrites (>13.5wt.% MgO) from CFB (n=98; 1.97±1.77ppb) having higher Os abundances than ocean island basalt (OIB) equivalents (n=75; 0.95±0.86ppb). The differences between CFB and OIB picrite absolute Os abundances may result from higher degrees of partial melting to form CFB but may also reflect incorporation of trace sulphide in CFB picrites from magmas that reached S-saturation in upper-crustal magma chambers. Significant inter-element fractionation of (Re+Pt+Pd)/(Os+Ir+Ru) are generated during magmatic differentiation in response to strongly contrasting partitioning of these two groups of elements into sulphides and/or HSE-rich alloys. Furthermore, fractional crystallization has a greater role on absolute and relative HSE abundances than crustal contamination under conditions of CFB petrogenesis due to the dilution effect of continental crust, which has low total abundances of the HSE. Combined data for the basaltic and intrusive portions of the Mackenzie LIP indicate a mantle source broadly within the range of the primitive upper mantle. The majority of Archaean komatiites and Phanerozoic CFB also require mantle sources with primitive upper mantle to chondritic Re/Os evolution, with exceptions typically being from analyses of highly-fractionated MgO-poor basalts.

Mantle source, magma differentiation and sulfide saturation of the ∼637 Ma Zhouan mafic–ultramafic intrusion in the northern margin of the Yangtze Block, Central China

The sill-like Zhouan mafic–ultramafic intrusion in the northern margin of the Yangtze Block formed at 637±4Ma and consists of a lherzolite unit and an olivine gabbronorite unit, with a marginal zone between the lower margin of the olivine gabbronorite unit and the underlying country rocks composed of marble and schist of the Meso- to Neoproterozoic Zhujiashan Formation.Rocks from the lherzolite unit have 41.4–44.0wt.% SiO2 and 29.0–38.4wt.% MgO and contain olivine with Fo values ranging from 80 to 85mole%, indicating crystallization from a moderately evolved magma. They commonly have <0.1wt.% S and low, variable PGE (2–40ppb) and display flat primitive mantle-normalized chalcophile element patterns with relatively low Cu/Pd ratios (3900–427,415), indicating derivation possibly from S-undersaturated magmas. Rocks from the olivine gabbronorite unit contain 44.0–47.1wt.% SiO2 and 22.5–28.9wt.% MgO and contain olivine with relatively low Fo values (67–68mole%), indicating crystallization from a more evolved magma. They contain 0.03–0.9wt.% S and <15ppb PGE and have Cu/Pd ratios (462,500–10,915,500) much higher than those for the rocks from the lherzolite unit. Rocks from the olivine gabbronorite unit show relatively steep, trough-like patterns depleted in PGE relative to Ni and Cu, indicating that they formed from sulfide-saturated magmas. Rocks from the marginal zone have relatively high SiO2 (45.1–53.1wt.%) and low MgO (7.4–19.6wt.%) and have chemical composition varying between the olivine gabbronorite and the country rocks. They contain 0.57wt.% S and <10ppb PGE, with high Cu/Pd ratios (404,300) and chalcophile element patterns similar to the olivine gabbronorite unit. Rocks from the lherzolite unit have (Th/Yb)PM ratios (3.7–9.1) much lower than the upper crust (28.2) and the country rocks (41.3), indicating small degrees of crustal contamination. Their ɛNd(t) (−4.4 to +1.1) and γOs(t) (8–36) values indicate an EMI mantle source. Primary magma was PGE-poor because sulfide retained in the mantle source due to small degrees of partial melting of the enriched mantle source.We propose that moderately evolved magmas may have reached sulfide saturation due to fractionation of olivine in a deep-seated staging magma chamber, forming an olivine-laden crystal mush with minor sulfide droplets in the lower part and more evolved, PGE-depleted magmas in the upper part of the chamber. The evolved, PGE-depleted magmas were forced out of the staging chamber and entered the shallow magma chamber to form the olivine gabbronorite unit. The lherzolite unit formed from the emplacement of the olivine-laden crystal mush into the same chamber. Because of paucity of extensive crustal contamination, only minor amounts of sulfide segregation occurred in the olivine gabbronorite unit and the marginal zone. The Zhouan intrusion thus may not be a good target for economic Ni–Cu sulfide deposit.

Petrogenetic relationships between peralkaline rhyolite dykes and mafic rocks in the post-Variscan gabbroic complex from Bocca di Tenda (northern Corsica, France)

The Lower Permian complex from Bocca di Tenda (Corsica island, France) consists of a gabbroic sequence crosscut by chilled dykes ranging in composition from basalt to trachyandesite and peralkaline rhyolite. The gabbroic sequence is mostly composed of olivine gabbronorites, quartz gabbronorites/diorites locally displaying high ilmenite amounts, and hornblende-rich tonalites. The quartz gabbronorites/diorites and the hornblende-rich tonalites have similar initial eNd values (+0.9 to −1.1) and record a fractional crystallization process driven by separation of plagioclase, pyroxene, and ilmenite. The olivine gabbronorites have slightly higher initial eNd than the quartz gabbronorites/diorites and the hornblende-rich tonalites, thereby documenting that the early evolution of the melts that gave rise to the gabbroic sequence was controlled by concomitant fractional crystallization and crustal assimilation. The trachyandesite dykes are rare and rich in dark mica. The selected trachyandesite has initial eNd of +0.4, which is slightly lower than the eNd of the basalt dykes. The basalt and the trachyandesite dykes are most likely genetically related through a process of fractional crystallization controlled by segregation of plagioclase, clinopyroxene and minor ilmenite, and assimilation of crustal material. The peralkaline rhyolites have initial eNd values ranging from +0.3 to −0.3. Whole-rock chemical variations and trace element compositions of Na-amphibole (arfvedsonite) indicate that the peralkaline rhyolite dykes record a process of fractional crystallization mainly controlled by separation of alkali feldspar and minor ilmenite and arfvedsonite. A plausible petrogenetic hypothesis for the genesis of the peralkaline rhyolite melts implies a protracted process of fractional crystallization from the trachyandesitic melts. This fractionation process would be initially ruled by separation of plagioclase, dark mica, and minor ilmenite. An alternative hypothesis for the origin of the peralkaline rhyolite melts implies partial melting of nearly coeval amphibole-rich mafic intrusives, which formed by crustally contaminated mantle-derived melts. The genesis of the peralkaline rhyolites is in any case correlated with mantle-derived melts that experienced extensive crustal contamination.

Geochemistry of the Emeishan flood basalts at Yangliuping, Sichuan, SW China: implications for sulfide segregation

A well-developed, 1,000 m thick basaltic sequence in the Yangliuping region, northern part of the Emeishan basalt province, includes the Lower and Middle Units of tholeiitic basalts and an Upper Unit of both tholeiites and subalkalic basalts. The basalts contain 42–55 wt% SiO2 and 4.1–8.3 wt% MgO. Most of these lavas have Gd/Yb > 2.0, Zr/Nb < 12, and ɛNd(260 Ma) values from +2.5 to +4.7. The platinum-group elements (PGE) are very mildly depleted in most of the basalts which contain 8–19 ppb Pt and 7–27 ppb Pd. However, a significant proportion of the Middle Unit basalts are strongly depleted in PGE with some samples having concentrations lower than detection limits. They have extremely high Zr/Nb ratios (up to 14.5) and low ɛNd(260 Ma) values (+3.21 to +0.65), features of extensive lower crustal contamination. Some samples in this unit have high Ni/Pd (3,965–61,198) and low Pd/Cr (410,000–3,930,000) ratios, indicating sulfide segregation and PGE depletion prior to eruption. The primary magmas were S-undersaturated and derived from partial melting at variable depths in the upper mantle. The early and late stage magmas, as represented by the Lower and Upper Units, underwent AFC processes which induced mild S-saturation and PGE depletion in some of the basalts, whereas the magmas represented by the Middle Unit experienced more extensive crustal contamination resulting in stronger S-saturation and in most cases significant PGE depletion.

Helium isotope variations in mineral separates from Costa Rica and Nicaragua: Assessing crustal contributions, timescale variations and diffusion-related mechanisms

We report new He abundance and isotope measurements of phenocryst phases in volcanic tephra and lavas from the Nicaragua-Costa Rica section of the Central American arc, where significant variations in crustal thickness have been inferred. Helium isotope values range from 4.6 R A to 7.5 R A, with no evidence for crustal thickness influencing measured 3He/ 4He ratios. A comparison of He abundances and isotopes measured in mafic phenocrysts from tephra vs. lavas from two separate eruptions at Cerro Negro show that both sampling media preserve phenocrysts with high 3He/ 4He values. 3He/ 4He ratios measured in phenocryst phases show good agreement with He isotope values of geothermal fluids from the same volcanoes. However, we note that the pyroxenes tend to have lower 3He/ 4He ratios (4.6–7.0 R A) than the olivines ( 3He/ 4He = 6.1–7.5 R A) over a range of concentration values and are consistently lower in cogenetic phenocryst pairs at all locations sampled. In order to assess how this difference arises, we explore two alternative mechanisms: (1) diffusion-related isotopic fractionation, and (2) late-stage radiogenic 4He additions, preferentially influencing pyroxene grains. In the first case, we reject diffusion-related fractionation of He isotopes since lower 3He/ 4He ratios are not accompanied by a decrease in He concentration values. The second scenario is evaluated on the basis of Mg numbers in cogenetic phenocryst pairs and by petrological modeling of the crystallization sequence. Mg numbers and modeling results at low pressure conditions (= 1 kbar) suggest that olivine crystallization preceded pyroxene crystallization. However, since lavas do not show evidence for extensive crustal contamination, we suggest that the best explanation for the lower 3He/ 4He ratios in pyroxenes is related to the closure temperatures of the phenocryst phases. Given its lower closure temperatures and higher He diffusion rates, we suggest that pyroxenes would be more susceptible to late-stage He exchange with a low 3He/ 4He source during ascent, presumably the surrounding crust.

Laacher See revisited: High-spatial-resolution zircon dating indicates rapid formation of a zoned magma chamber

High-spatial-resolution ( 230 Th)/( 238 U) disequilibrium dating reveals rapid zircon crystallization prior to the 12,900 yr B.P. Laacher See (Germany) phonolite eruption. Zircons from Lower Laacher See tephra (LLST) and syenitic subvolcanic nodules share REE and low U/Th characteristics and define an isochron age of 17.1 ± 1.3 ka (1σ). Zircons also show higher initial ( 230 Th)/( 232 Th) 0 and oxygen isotopic disequilibrium compared to their host phonolite. Thus, LLST zircons shortly predate the eruption and yet are not strictly part of the phonolite crystallization sequence. Based on their similarity to zircons in subvolcanic nodules, they are instead interpreted to be scavenged from a precursor syenite intrusion, which implies short (a few k.y.) filling and differentiation timescales for the Laacher See magma chamber. Thorium and oxygen isotopes in zircon as well as evidence for abundant crustal xenocrysts further indicate extensive crustal contamination, which allows for considerably faster basanite-phonolite differentiation than the ∼100 k.y. time span previously estimated from bulk U-series data.

Oxygen and Hydrogen Isotope Stratigraphy of the Rustenburg Layered Suite, Bushveld Complex: Constraints on Crustal Contamination

New dO values for plagioclase, pyroxene and olivine, and limited whole-rock dD values are presented for samples from the Rustenburg Layered Suite of the Bushveld Complex, South Africa. In combination with existing data, these provide a much more complete composite O-isotope stratigraphy for the intrusion. Throughout the layered suite, mineral dO values indicate that the magmas from which they crystallized had dO values that were about 7 1‰, that is, 1 4‰ higher than expected for mantle-derived magmas, suggesting extensive crustal contamination. More limited H-isotope data suggest that the OH present within whole rocks, regardless of the degree of alteration, is of magmatic origin and not an alteration phenomenon. There appears to be no systematic change in dO value with stratigraphic height and this requires the contamination to have taken place in a ‘staging chamber’ before emplacement of the magma(s) into the present chamber. Large amounts (30–40%) of contamination by the lower to middle crust are needed to explain these dO values, which is in general agreement with previous estimates based on Srand Nd-isotope data. Alternatively, smaller amounts of contamination ( 20%) by sedimentary rocks, or their partial melts, represented by the country rock can explain the data, but it is not apparent how such material could have been present at the depth of the ‘staging chamber’ in the lower to middle crust.

Geochemical Constraints on the Petrogenesis of Cenozoic I-Type Granitoids in Northwest Anatolia, Turkey: Evidence for Magma Generation by Lithospheric Delamination in a Post-Collisional Setting

Extensive magmatic activity followed major plate convergence and Eocene continent-arc collision in northwest Anatolia, Turkey. This produced a considerable volume of eruptive products, as well as a large proportion of plutonic bodies along the suture zone. The main plutonic bodies in northwest Anatolia are exposed in the Armutlu, Kapidag, and Lapseki peninsulas where they form an E-W trending magmatic belt that comprises three individual intrusive suites. The plutonic bodies intrude Paleozoic—Mesozoic metamorphic basement rocks overthrust by Upper Cretaceous—Tertiary ophiolite fragments. Petrographic and geochemical characteristics of these three plutons are remarkably similar, indicating that the magmas that formed each of them were generated from the same source and experienced similar petrogenetic processes. The plutons are generally calc-alkaline, metaluminus, and I-type, and range in composition from hornblende-monzogranite and granite to granodiorite. The rocks are characterized by enrichment in LILE and LREE and depletion in HFSE relative to N-MORB values (e.g., negative Nb and Ta anomalies). They follow assimilation and fractional crystallization (AFC) trends indicative of extensive crustal contamination of magma derived from a mantle source that had been modified by earlier metasomatic events. The chemical characteristics of the plutonic rocks are remarkably similar to those of subduction-related or active continental margin granites. Subduction-related dehydration metasomatism may therefore be inferred as the likely process that modified the mantle wedge source region from which the initial melts were generated. Late orogenic delamination of either the subducting slab or the lowermost part of the mantle lithosphere (e.g., the thermal boundary layer) and concomitant rise of hot mantle asthenosphere appears to be a suitable explanation of the heat that triggered partial melting within the metasomatized part of the mantle lithosphere after subduction waned

Erratum to A new style of Ni-Cu mineralization related to magmatic breccia pipes in a transpressional magmatic arc, Aguablanca, Spain

The Aguablanca deposit represents a new style of magmatic Ni-Cu mineralization in discordant sulfide-rich pyroxenitic breccia pipes. The orebody is hosted by Variscan calc-alkaline diorites and gabbros which intruded during an oblique subduction/collision event. Transpressional-transtensional left-lateral structures facilitated the intrusion of primitive magmas to shallow depths in the crust. A two-stage genetic model is proposed. In the first stage, a transitional deep magma chamber formed. The primitive magma interacted at depth with wall rocks, resulting in extensive crustal contamination, concomitant sulfide magma immiscibility and settling of orthopyroxene-, clinopyroxene- and sulfide-rich cumulates to form a layered magmatic complex. Geochemical and mineralogical evidence, including the virtual disappearance of olivine, heavy 34S values (7.4‰), distinctive Nd, Sr and Pb signatures, high Au contents, and the presence of spinel and graphite indicate a major interaction with the upper crust, probably with pyrite-rich carbonaceous slates of Late-Proterozoic age. The second stage was related with the emplacement of residual calc-alkaline gabbroic to noritic melts and the development of an intrusive breccia containing fragments of the consolidated layered complex rocks and associated disseminated to massive sulfides.

Post-collisional Neogene magmatism of the Mediterranean Maghreb margin: a consequence of slab breakoff

A 1 200 km-long linear magmatic belt extends along the Mediterranean coast of the Maghreb from Eastern Tunisia to Morocco. This belt is mainly composed of Langhian calc-alkaline metaluminous to peraluminous granitoids and associated andesites/dacites in Central and Eastern Algeria. In Tunisia and Oranie/Western Morocco, calc-alkaline activity started later (during the Serravallian) and was followed by the emplacement of alkali basalts and basanites since the Tortonian to the Pliocene and, in some places, the Pleistocene. Available data on the tectonic setting, petrology, age and geochemistry of this belt show that most of its striking features, e.g. (1) very low magma production rate, subduction-related geochemical imprint, extensive crustal contamination for the calc-alkaline magmatism and (2) progressive magmatic change from calc-alkaline to alkaline, are consistent with magma generation during a slab breakoff process as proposed by Carminati et al. in 1998. The magmatism associated with this breakoff started in Central Eastern Algeria at 16 Ma, then propagated eastwards and westwards. The upward flow of asthenospheric enriched plume-type mantle through the tear in the downgoing slab first triggered melting of the overlying lithospheric mantle which had been metasomatised during a previous subduction period. Heat supply from this uprising asthenosphere may have warmed up the continental crust and made its involvement in assimilation processes easier. As the asthenosphere ascended through the ‘window’ in the slab, partial melting occurred at the uprising boundary between asthenosphere and lithosphere, generating basalts with transitional characteristics between those of calc-alkaline and alkaline basalts. As the asthenospheric upwelling proceeded, partial melting then occurred in the sole asthenospheric mantle, producing alkali basalts. La marge septentrionale du Maghreb comporte une chaîne magmatique, qui s'étend sur 1 200 km des Mogods (Tunisie) à Ras Tarf (Maroc). En Algérie centrale et orientale, elle se compose principalement de granitoïdes métalumineux et peralumineux langhiens et d'andésites et dacites calco-alcalines associées. En Tunisie, en Oranie et au Maroc, l'activité calco-alcaline débute plus tard, au Serravallien. Elle est, par la suite, relayée par un magmatisme alcalin d'âge Tortonien à Pliocène et, localement, Pléistocène. La synthèse des données tectoniques, géochronologiques, pétrologiques et géochimiques disponibles nous permet de montrer qu'un processus de rupture de lithosphère plongeante proposé par Carminati et al. en 1998 rend compte de l'essentiel des particularités magmatiques de cette chaîne (comme le très faible taux de production magmatique, l’empreinte géochimique de subduction des roches calco-alcalines et la forte contamination crustale qu'elles ont subie, ainsi que la transition calco-alcalin–alcalin au cours du temps). Le magmatisme lié à cette délamination lithosphérique a débuté vers 16 Ma en Algérie centrale et orientale, puis s'est propagé vers l'est et vers l'ouest. Le flux ascendant de manteau asthénosphérique enrichi (de type panache) au travers de la déchirure lithosphérique a d'abord provoqué la fusion du manteau lithosphérique sus-jacent, métasomatisé lors d’une période antérieure de subduction, ce qui a entraîné la formation de magmas calco-alcalins. Ce flux asthénosphérique a également contribué à réchauffer la croûte continentale, facilitant ainsi son assimilation par les magmas d'origine mantellique. Par suite de la remontée progressive de l'asthénosphère dans la déchirure lithosphérique aux deux extrémités de la chaîne, la fusion partielle s’est produite à la limite lithosphère–asthénosphère, engendrant des magmas basaltiques, dont la signature géochimique est transitionnelle entre les types calco-alcalin et alcalin. Consécutivement à l’élargissement de la déchirure lithosphérique, la remontée asthénosphérique s’est poursuivie ; la fusion partielle s’est alors exercée dans le seul manteau asthénosphérique enrichi, produisant des liquides basaltiques alcalins.