New evidence for Late Cretaceous plume-related seamounts in the Middle East sector of the Neo-Tethys: Constraints from geochemistry, petrology, and mineral chemistry of the magmatic rocks from the western Durkan Complex (Makran Accretionary Prism, SE Iran)
Edoardo BarberoFederica ZaccariniMorteza DelavariAsghar DolatiEmilio SaccaniMichèle MarroniLuca Pandolfi
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To better constrain the petrogenesis and tectonic setting of the granitic plutons in the Guaizihu region, NW China, this study presents a new analysis of zircon U–Pb ages, whole-rock geochemistry and Hf isotopes of syenogranites. The syenogranites are medium- to coarse-grained, with moderate to high SiO2 (72.55–74.27 wt%) and Al2O3 (13.02–13.75 wt%) contents and moderate total alkalis (Na2O + K2O, 7.58–7.98 wt%), which indicate that the syenogranites exhibit calc-alkaline and peraluminous characteristics. The low Ce, 10 000 Ga/Al, and A/CNK values and the negative correlation of SiO2 and P2O5 confirm that the syenogranites are of I-type origin. The whole-rock geochemistry demonstrates that the magma was generated by partial melting and experienced fractional crystallisation. Specifically, the correlations of Rb with Ba and Sr and the weak negative Eu anomalies indicate that the plagioclase has undergone fractional crystallisation. In addition, Eu anomalies and La/Sm–La relations show that both fractional crystallisation and partial melting contributed to the magmatic process. The positive εHf(t) values (0.11–9.78); enrichments in Rb, Th and other LILEs; depletions in Nb, Sr and other HFSEs; and the ratios of geochemical elements indicate that the syenogranites were generated from the partial melting of juvenile crust. The granites are inferred to have formed in a post-collisional extensional environment. Additionally, the U–Pb zircon ages (284.6 ± 0.84 Ma and 269.4 ± 1.2 Ma) of syenogranites suggest that final closure of the Paleo-Asian Ocean occurred before 285 Ma.KEY POINTSGranites in the Guaizihu region are dated at 284.6 ± 0.84 Ma and 269.4 ± 1.2 Ma, and the Paleo-Asian Ocean, where the Engger Us Ophiolite Belt is located, closed before 285 Ma.Granites are medium- to coarse-grained and highly fractionated I-type syenogranites.Granites have been derived from magma generated by the partial melting of juvenile crust followed by fractional crystallisation.Granites were formed in an intraplate extensional environment related to a post-collisional event.
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The ophiolitic complex of Zdraljica (Central Serbia) belongs to the Eastern Branch of the Vardar suture zone. It was emp'aced during the Upper Jurassic. The complex consists predominately of a MORB/VAB-like tholeiitic suite, represented mostly by gabbros and diabases. Small occurrences of cummulitic peridotites, basalts and plagiogranites also appear. The tholeiitic suite is intruded by calc-alkaline intermediate and acid magmas. Geochemical data suggest that the ZOC tholeiitic rocks originated by partial melting of a spinel-lherzolite source. Non-modal batch melting modeling indicates 10 to 15 % of partial melting of such a source. The magmas were later modified by fractional crystallization. One-step major element modeling requires 40% (F=0.60) of fractional crystallization of a mineral assemblage: PI52 gCpxi2 5OI26 iTtn2 9Ap4.4Mgt1.0- The model is supported by the variation patterns of most trace elements.
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The Luxi Terrane (eastern China) exposes widespread Early Cretaceous alkaline rocks, whereas their petrogenesis remains controversial, including fractional crystallization, partial melting and crustal contamination regime. Here, we present petrology, geochemistry, sphene U-Pb geochronology and trace element data from the syenogranite, quartz syenite and quartz monzonite of the Guandimiao alkaline complex rocks to investigate their petrogenesis. Geochemical data suggest that these alkaline rocks show alkalic and peralkaline characters, and high Ga/Al ratios, SiO2, light rare-earth element (LREE), Zr and Nb, and low MgO, CaO, Eu contents, corresponding to A-type granites. Sphene trace elements in syenogranite and quartz monzonite show obvious fractionation between LREE and heavy rare-earth element (HREE) and high Th/U ratios, indicating a magmatic origin. They yield U-Pb lower intercept ages of 128 ± 2.3 Ma and 127 ± 1.3 Ma, representing the crystallization ages of these alkaline rocks. The negative correlations between CaO, Fe2O3 (Total), MgO, P2O5, TiO2, MnO and the pronounced depletion in Nb, Ta and Ti suggest that the alkaline rocks were formed by fractional crystallization. Additionally, the positive correlation between La/Hf and La, Th and Th/V, Ce/Yb and K2O, and Tb/Yb and Yb suggest that the alkaline melts are generated by partial melting. Such high Rb/Nb, (Th/Nb)N and Nb/Th ratios indicate crustal contamination during the magma emplacement. We, therefore, propose the magma source of the alkaline rocks in the Guandimiao complex originated by partial melting of lithospheric mantle, which experienced fractional crystallization and crustal contamination processes during its emplacement. Such complex alkaline rocks were probably formed in an extensional back-arc setting induced by the retreat of the subducting Izanagi plate.
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The Canary Island Seamount Province forms a scattered hotspot track on the Atlantic ocean floor ~1300 km long and ~350 km wide, perpendicular to lithospheric fractures and parallel to the NW African continental margin. New 40Ar/39Ar datings show that seamount ages vary from 133 Ma to 0.2 Ma in the central archipelago and from 142 Ma to 91 Ma in the southwest. Combining 40Ar/39Ar ages with plate tectonic reconstructions, I find that the temporal and spatial distribution of seamounts is irreconcilable with a deep fixed mantle plume origin, or derivation from passive mantle upwelling beneath a mid-ocean ridge. I conclude that shallow mantle upwelling beneath the Atlantic Ocean basin off the NW African continental lithosphere flanks produced recurrent melting anomalies and seamounts from the Late Jurassic to Recent, nominating the Canary Island Seamount Province as oldest hotspot track in the Atlantic Ocean and most long-lived preserved on earth.
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Evidence for open-system magmatic processes is abundant in igneous rocks from most tectonic settings and with ages spanning most of geologic time. Accurately documenting these processes is critical for understanding magma reservoir dynamics, including the processes that lead to compositional diversity in igneous rocks, and for deciphering the thermochemical evolution of the crust and mantle. Quantitative models describing open-system processes such as assimilation–fractional crystallization (AFC) have provided significant insight into all of these, but, nevertheless, suffer from several serious deficiencies. Foremost among these are the absence of energy conservation and the lack of consideration of country rock partial melting. For a magma body undergoing AFC, a new quantitative model, Energy-Constrained Assimilation Fractional Crystallization (EC-AFC), self-consistently balances energy, species and mass while also tracking compositional variations generated in anatectic melt as country rock undergoes partial melting. EC-AFC represents a significant improvement to existing AFC models for several reasons. First, the inclusion of energy conservation provides a direct and crucial link between thermal parameters and volcanological or geological data. Second, unlike 'classical' AFC that models mass and chemical properties only, EC-AFC models mass, chemical and thermal properties of a magma body, thus allowing the energetics of the open-system magma reservoir to be linked to the geochemical evolution. Third, compared with 'classical' AFC models, EC-AFC geochemical trends are distinct, exhibiting non-monotonic behaviors that are directly linked to the effects of energy conservation and country rock partial melting. Comparison of EC-AFC trends with data from natural systems indicates that some of the criteria currently used to demonstrate the efficacy of AFC require modification. Finally, comparison of 'classical' AFC and EC-AFC results for data from well-documented volcanic centers demonstrates that EC-AFC does a superior job of tracking the compositional trends, provides a plausible physical context for the process of AFC, and allows geologically relevant predictions to be made about particular magmatic systems.
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Wadi
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