Evolution of Mount Cameroon volcanism: Geochemistry, mineral chemistry and radiogenic isotopes(Pb, Sr, Nd)

Mount Cameroon volcano has erupted several times in the 20 th Century with documented eruptions in 1909,1922,1954,1959,1982,1999 and 2000.Evidence of historic volcanism is represented by several older lava flows and lahar deposits around the flanks of the volcano.This study aims to assess the evolution of Mount Cameroon volcanism through its eruptive history via interpretation of mineralogical,whole rock geochemical and Pb,Sr,Nd isotope data generated from historic and recent lava flows.In this study,samples were collected from the 1959,1982,1999 and 2000 eruptions and from several historic eruption sites with unknown eruption dates.Evaluation of major and trace element data demonstrates that Mount Cameroon is geotectonically associated with within-plate Ocean Island Basalt Settings.More than 90%of the studied historic lavas(n=29)classify as tephrites and basanites whereas the modern lavas(n=38)are predominantly trachybasalts,demonstrating evolution from primitive to evolved lavas over time typically in response to fractional crystallization.Petrographically,the lavas are porphyritic with main mineral phases being olivine,clinopyroxene,plagioclase feldspars and Fe-Ti-Cr oxides.The 1982 lavas are predominantly aphyric and dominated by lath-shaped flow-aligned plagioclase in the groundmass.Olivine chemistry shows variable forsterite compositions from Fo60-89.Clinopyroxenes vary from diopside through augite to titanaugite with chemical composition ranges from Wo45En32Fs7 to Wo51En47Fs17.Plagioclase feldspars vary from labradorite(An567 O)to bytownite(An80-87).For the Fe-Ti-Cr oxides,calculated ulvospinel component shows a wide variation from ulv38-87.CIPW-normative classification on the Di-Ol-Hy-Qz-Ne system shows that all Mount Cameroon lavas are nepheline-normative(Ne ranges from4.20 wt.%to 11.45 wt.%).Radiogenic isotope data demonstrate that Mount Cameroon lavas are HIMU(or high μ=238U/204Pb),characterized by 206Pb/204Pb=20.19-20.46,207Pb/204Pb=15.63-15.69,208Pb/204Pb=40.01-40.30,87Sr/86Sr=0.70322-0.70339(εsr=-21.37 to-18.96)and 143 Nd/144 Nd=0.51276-0.51285(εNd=+2.29 to+4.05).The historic lavas show stronger HIMU signature relative to the modern lavas,suggesting evolution towards less HIMU signatures over time.This study has revealed that Mount Cameroon volcanism has evolved from primitive magmas characterized by stronger HIMU signatures with high 206/204Pb and 208/204Pb isotopes,low SiO2 and high Mg,Ni,Cr content towards lower HIMU signatures with relatively higher SiO2,lower Mg,Cr and Ni compositions.The geochemical and isotopic changes,which account for the evolution of magmatism on Mount Cameroon occur over long periods of time because all the modern lavas erupted within the last 100 years are isotopically homogeneous,with very limited variation in SiO2 compositions.

Oligocene subduction-related plutonism in the Nodoushan area,Urumieh-Dokhtar magmatic belt: Petrogenetic constraints from U-Pb zircon geochronology and isotope geochemistry

Geochemical data and Sr-Nd isotopes of the host rocks and magmatic microgranular enclaves(MMEs)collected from the Oligocene Nodoushan Plutonic Complex(NPC) in the central part of the Urumieh-Dokhtar Magmatic Belt(UDMB) were studied in order to better understand the magmatic and geodynamic evolution of the UDMB. New U-Pb zircon ages reveal that the NPC was assembled incrementally over ca. 5 m.y., during two main episodes at 30.52 ± 0.11 Ma and 30.06 ± 0.10 Ma in the early Oligocene(middle Rupelian) for dioritic and granite intrusives, and at 24.994 ± 0.037 Ma and 24.13 ± 0.19 Ma in the late Oligocene(latest Chattian) for granodioritic and diorite porphyry units,respectively. The spherical to ellipsoidal enclaves are composed of diorite to monzodiorite and minor gabbroic diorite(SiO_2 = 47.73-57.36 wt.%; Mg# = 42.15-53.04); the host intrusions are mainly granite,granodiorite and diorite porphyry(SiO_2 = 56.51-72.35 wt.%; Mg# = 26.29-50.86). All the samples used in this study have similar geochemical features, including enrichment in large ion lithophile elements(LILEs, e.g. Rb, Ba, Sr) and light rare earth elements(LREEs) relative to high field strength elements(HFSEs) and heavy rare earth elements(HREEs). These features, combined with a relative depletion in Nb,Ta, Ti and P, are characteristic of subduction-related magmas. Isotopic data for the host rocks display ISr = 0.705045-0.707959, εNd(t) =-3.23 to +3.80, and the Nd model ages(TDM) vary from 0.58 Ga to 1.37 Ga. Compared with the host rocks, the MMEs are relatively homogeneous in isotopic composition,with Isr ranging from 0.705513 to 0.707275 and εNd(t) from -1.46 to 4.62. The MMEs have TDM ranging from 0.49 Ga to 1.39 Ga. Geochemical and isotopic similarities between the MMEs and their host rocks demonstrate that the enclaves have mixed origins and were most probably formed by interactions between the lower crust-and mantle-derived magmas. Geochemical data, in combination with geodynamic evidence, suggest that a basic magma was derived from an enriched subcontinental lithospheric mantle(SCLM), presumably triggered by the influx of the hot asthenosphere. This magma then interacted with a crustal melt that originated from the dehydration melting of the mafic lower crust at deep crustal levels. Modeling based on Sr-Nd isotope data indicate that ～50% to 90% of the lower crust-derived melt and ～10% to 50% of the mantle-derived mafic magma were involved in the genesis of the early Oligocene magmas. In contrast,～45%-65% of the mantle-derived mafic magma were incorporated into the lower crust-derived magma(～35%-55%) that generated the late Oligocene hybrid granitoid rocks. Early Oligocene granitoid rocks contain a higher proportion of crustal material compared to those that formed in the late Oligocene. It is reasonable to assume that lower crust and mantle interaction processes played a significant role in the genesis of these hybridgranitoid bodies, where melts undergoing fractional crystallization along with minor amounts of crustal assimilation could ascend to shallower crustal levels and generate a variety of rock types ranging from diorite to granite.

Yangshan A-Type Granites in the Lower Yangtze River Belt Formed by Ridge Subduction:Radiogenic Ca and Nd Isotopic Constraints

The Early Cretaceous aluminous A-type granites in the Lower Yangtze River belt(LYRB)can provide important insights into the Mesozoic magmatism in eastern China,but their origin remains highly controversial.In this study,radiogenic Ca-Nd isotopic analysis was performed for syenite porphyry and alkali-feldspar granite porphyry of the Yangshan pluton,a typical aluminous A-type granitic intrusion in the LYRB,to constrain its source and geodynamic setting.The results show thatε_(Ca)(126 Ma),ε_(Nd)(126 Ma)and K/Ca_(source) of the syenite porphyry range from-0.24 to+0.96,-7.2 to-6.0,and 0.31 to 1.26,respectively.The corresponding values for the alkali-feldspar granite porphyry range from 0.26 to 0.84,-8.0 to-6.1,and 0.79 to 1.08,respectively.Binary mixing modeling indicates that they were originated from the same sources with different proportion,namely,a mixing of 50%to 75%Neoproterozoic crust and 50%to 25%asthenospheric mantle.Together with previous works,we propose that the Early Cretaceous subduction of the ridge between the Pacific and Izanagi plates was responsible for the formation of the aluminous A-type granites in the LYRB.

Geochemistry of Enclaves and Host Granitoids from the Kashan Granitoid Complex, Central Iran: Implications for Enclave Generation by Interaction of Cogenetic Magmas

The major and trace elements and Sr-Nd-Pb isotopes of Miocene host granitoid rocks and their mafic microgranular enclaves（MMEs） were studied to understand the petrogenesis of MMEs in the Kashan complex, which is part of the Urumieh-Dokhtar magmatic belt（Iran）. The host rocks consist of quartz-diorite and tonalite associated with a dioritic intrusion. The enclaves show microgranular texture and the same mineralogy as their respective host with plagioclase, quartz and biotite. MMEs have a diorite to quartz-diorite composition and show geochemical characteristics mostly between their granitoid host and the diorite intrusion. Chondrite-normalized REE patterns of all samples are moderately fractionated [（La/Yb）N=2.1 to 12.9]. The MMEs display in part small negative Eu anomalies（Eu/Eu＊=0.54 to 0.99）, with enrichment of LILE and depletion of HFSE. The enclaves show emplacement depth of -4 to 6 km which is comparable with the host rocks. Moreover, the Hornblende-plagioclase equilibrium temprature of MMEs yields average temperatures of 795℃ which is slightly higher than the host ones. Identical mineral compositions and Nd-Sr-Pb isotopic features of MME-host granitoid pairs indicate interactions and parallel evolution of MME and enclosing granitoid in the Kashan plutons. Additionally, the geochemical and isotopic investigations of host and dioritic intrusions suggest a common source for their genesis. A thermal anomaly induced by underplated basic magma into a hot crust would have caused partial melting in the lower crust to generate Kashan granitoid rocks.

Forensic isoscapes based on intra-individual temporal variation of δ^(18)O and ^(206)Pb/^(207)Pb in human teeth

Isotopic signatures used in the georeferencing of human remains are largely fixed by spatially distinct geologic and environmental processes.However,location-dependent temporal changes in these isotope ratios should also be considered when determining an individual’s provenance and/or trajectory.Distributions of the relevant isotopes can be impacted by predictable external factors such as climate change,delocalisation of food and water sources and changes in sources and uses of metals.Using Multi-Collector Inductively-Coupled Plasma Mass Spectrometer(MC-ICP-MS)analyses of ^(206)Pb/^(207)Pb in tooth enamel and dentin from a population of 21±1-year-old individuals born circa 1984 and isotope ratio mass spectrometry(IRMS)of δ^(18)O in their enamel,we examined the expected influence of some of these factors.The resulting adjustments to the geographic distribution of isotope ratios(isoscapes)found in tooth enamel and dentin may contain additional useful information for forensic identification,but the shifts in values can also impact the uncertainty and usefulness of identifications if they are not taken into account.