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Yoshiyuki Tatsumi - One of the best experts on this subject based on the ideXlab platform.
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Operation of subduction factory and production of andesite
Journal of Mineralogical and Petrological Sciences, 2006Co-Authors: Yoshiyuki Tatsumi, Toshiro TakahashiAbstract:The subduction factory processes raw materials such as oceanic sediments and basaltic crust, selectively extracts particular subduction components and manufactures magmas, their solidified materials and continental crust as products. The waste materials from the factory, such as chemically modified oceanic materials and delaminated mafic arc lower crust are transported down to the deep mantle modified their compositions and ultimately recycled as mantle plumes. Andesite composes the bulk continental crust and therefore is the major product in the subduction factory. Two types of Andesites, calk-alkalic and tholeiitic series, are commonly recognized in a single arc volcano. We propose a new mechanism for production of these two magma series on the basis of data obtained by Sr isotopic micro-analyses of plagioclase in volcanic rocks from Zao Volcano, NE Japan. Tholeiitic magmas having constant and enriched isotopic signatures are produced by anatexis of the preexisting mafic lower crust, whereas calc-alkalic magmas, having compositions similar to the bulk continental crust, are products of mixing a mantle-derived, hence isotopically depleted, basaltic magma and crust-derived felsic tholeiites.
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high mg Andesites in the setouchi volcanic belt southwestern japan analogy to archean magmatism and continental crust formation
Annual Review of Earth and Planetary Sciences, 2006Co-Authors: Yoshiyuki TatsumiAbstract:AbstractThe occurrence of unusual high-Mg andesite (HMA) characterizes the Setouchi volcanic belt in SW Japan, which was activated at 13.7±1.0 Ma by subduction of the young and hot Shikoku Basin lithosphere into the high-temperature upper mantle. This tectonic setting may be analogous to the thermal regime during Archean times, which suggests more ubiquitous production of HMA. A plausible process that can comprehensively account for the petrological and geochemical characteristics of Setouchi HMAs involves partial melting of subducting lithosphere, subsequent melt-mantle interactions, and final equilibration with the upper-most mantle. HMAs and more differentiated Andesites, which are coined sanukitoids, are distinct in that they are phenocryst-poor (<10%), compact, and nearly anhydrous, despite HMA magmas originally containing ∼7 wt% H2O, and commonly form composite lava flows. One mechanism for explaining these features is formation of a mostly solidified HMA pluton, remelting of the HMA pluton by intru...
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The subduction factory: How it operates in the evolving Earth
GSA Today, 2005Co-Authors: Yoshiyuki TatsumiAbstract:The subduction factory processes raw materials such as oceanic sediments and oceanic crust and manufactures magmas and continental crust as products. Aqueous fluids, which are extracted from oceanic raw materials via dehydration reactions during subduction, dissolve particular elements and overprint such elements onto the mantle wedge to generate chemically distinct arc basalt magmas. The production of calc-alkalic Andesites typifies magmatism in subduction zones. One of the principal mechanisms of modern-day, calc-alkalic andesite production is thought to be mixing of two end-member magmas, a mantle-derived basaltic magma and an arc crust-derived felsic magma. This process may also have contributed greatly to continental crust formation, as the bulk continental crust possesses compositions similar to calc-alkalic Andesites. If so, then the mafic melting residue after extraction of felsic melts should be removed and delaminated from the initial basaltic arc crust in order to form "andesitic" crust compositions. The waste materials from the factory, such as chemically modified oceanic materials and delaminated mafic lower crust materials, are transported down to the deep mantle and recycled as mantle plumes. The subduction factory has played a central role in the evolution of the solid Earth through creating continental crust and deep mantle geochemical reservoirs.
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the petrology and geochemistry of calc alkaline Andesites on shodo shima island sw japan
Journal of Petrology, 2002Co-Authors: Yoshiyuki Tatsumi, T Nakashima, Yoshihiko TamuraAbstract:Petrographical and geochemical characteristics of calc-alkaline McLennan, 1995) that are produced at convergent plate boundaries and are characterized by lower FeO∗/MgO Andesites on Shodo-Shima Island, SW Japan, having bulk comratios than tholeiitic Andesites (Miyashiro, 1974). An positions largely identical to the continental crust, are presented. understanding of the origin of such orogenic, calc-alkaline The following petrographic observations suggest a role for magma Andesites is thus essential for comprehending not only mixing in producing such andesite magmas: (1) two types of olivine the magma flux in the modern Earth but also the role phenocrysts and spinel inclusions, one with compositions identical of subduction-related magmatism in the evolution of the to those in high-Mg Andesites and the other identical to those in solid Earth. Since the pioneering work of Eichelberger basalts, are recognized in terms of Ni–Mg and Cr–Al–Fe (1975, 1978), Anderson (1976), Sakuyama (1979, 1981, relations, respectively; (2) the presence of orthopyroxene phenocrysts 1984), and Luhr & Carmichael (1980), the majority of with mg-number >90 suggests the contribution of an orthopyroxenepetrologists have considered mixing of mafic and felsic bearing high-Mg andesite magma to production of calc-alkaline magmas to be one of the major mechanisms of generation Andesites; (3) reversely zoned pyroxene phenocrysts may not be in of calc-alkaline Andesites. Continental crust formation, equilibrium with Mg-rich olivine, suggesting the involvement of a however, may not be elucidated solely by such magma differentiated andesite magma as an endmember component; (4) the mixing processes. The reasons for this are twofold. First, presence of very Fe-rich orthopyroxene phenocrysts indicates the calc-alkaline andesite magmatism typifies continental arcs association of an orthopyroxene-bearing rhyolitic magma. Conat least on the modern Earth, suggesting that the contributions from the above at least five endmember magmas to the calctinental crust itself plays an important role in such andesite alkaline andesite genesis can also provide a reasonable explanation of formation, possibly as the source of the felsic endmember the Pb–Sr–Nd isotope compositions of such Andesites. component. Second, there is a slight but significant difference in MgO contents between the bulk continental crust and the typical calc-alkaline andesite (e.g. Kelemen,
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geochemical modeling of partial melting of subducting sediments and subsequent melt mantle interaction generation of high mg Andesites in the setouchi volcanic belt southwest japan
Geology, 2001Co-Authors: Yoshiyuki TatsumiAbstract:A possible mechanism for high-Mg andesite formation, including melting of subducting sediments and subsequent melt-mantle interaction, was examined by geochemical formulation of partial melting and melt-solid reactions. The modeling results demonstrate that a sediment-derived melt produced at 1050 °C and 1.0 GPa changes its composition from rhyolitic to andesitic as it dissolves olivine and clinopyroxene and crystallizes orthopyroxene. The resulting reaction product possesses major and incompatible trace element compositions close to high-Mg Andesites in the Setouchi volcanic belt, southwest Japan.
Ali Polat - One of the best experts on this subject based on the ideXlab platform.
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a ca 2 1 ga andean type margin built on metasomatized lithosphere in the northern yangtze craton china evidence from high mg basalts and Andesites
Precambrian Research, 2017Co-Authors: Songbai Peng, Ali Polat, Timothy M Kusky, Hao Deng, Tuoyu WuAbstract:Abstract We report for the first time the presence of a suite of Paleoproterozoic (2.12 Ga) metamorphosed high-Mg basalts and Andesites in the Huangling dome, northern Yangtze craton, China, which provide new insights into crustal growth processes in the craton during the Paleoproterozoic era. The high-Mg basalts and Andesites are mineralogically and texturally amphibolites and quartz-bearing amphibolites, respectively, and occur as deformed layers within the metasedimentary rocks of the Shuiyuesi Group in the northern Huangling dome. We present new field and petrographic observations, zircon U-Pb ages, in situ zircon Hf isotope and whole-rock major and trace element and Nd isotope data, to assess the petrogenetic origin and geodynamic setting of these high-Mg rocks. The igneous zircons from the Andesites yield a weighted age of ca. 2.12 Ga that is interpreted to be the formation age of the magmatic protolith. The basalts are characterized by moderate SiO2 (49 wt.%), low TiO2 (0.63–0.65 wt.%), high MgO (13.7–14.6 wt.%) and high Mg-numbers (58–59). The Andesites have 53–60 wt.% SiO2, 0.45–0.50 wt.% TiO2 and 6.6–9.5 wt.% MgO contents, yielding high Mg-numbers (63–76). Both the basalts and Andesites are enriched in LILE and LREE, but depleted in Nb, Ta, and HREE. Zircons in the Andesites have eHf (t) values between −0.2 and +3.3 and corresponding Hf isotopic model age (TDM1) of 2.4 Ga, reflecting various degrees of crustal input. They have negative eNd (t) values, ranging from −4.4 to −2.7. Geochemical characteristics of the basalts and Andesites indicate that they are equivalent to high-Mg basalts and Andesites, respectively, occurring in Phanerozoic suprasubduction zones. The Nd and Hf isotope compositions of the high-Mg rocks suggest that they were derived from partial melting of subarc lithospheric mantle above a subducting oceanic slab. The formation age and general geochemical characteristics of the high-Mg rocks reveal the presence of a Paleoproterozoic (2.1–2.2 Ga) Andean-type continental margin developed on metasomatized lithospheric mantle in the northern Huangling dome. The geochemical and geochronological data presented in this study provide important insights into Paleoproterozoic evolution of the Yangtze craton.
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nd isotope systematics of 2 7 ga adakites magnesian Andesites and arc basalts superior province evidence for shallow crustal recycling at archean subduction zones
Earth and Planetary Science Letters, 2002Co-Authors: Ali Polat, Robert KerrichAbstract:Abstract An association of adakite, magnesian andesite (MA), and Nb-enriched basalt (NEB) volcanic flows, which erupted within ‘normal’ intra-oceanic arc tholeiitic to calc-alkaline basalts, has recently been documented in ∼2.7 Ga Wawa greenstone belts. Large, positive initial ϵNd values (+1.95 to +2.45) of the adakites signify that their basaltic precursors, with a short crustal residence, were derived from a long-term depleted mantle source. It is likely that the adakites represent the melts of subducted late Archean oceanic crust. Initial ϵNd values in the MA (+0.14 to +1.68), Nb-enriched basalts and Andesites (NEBA) (+1.11 to +2.05), and ‘normal’ intra-oceanic arc tholeiitic to calc-alkaline basalts and Andesites (+1.44 to +2.44) overlap with, but extend to lower values than, the adakites. Large, tightly clustered ϵNd values of the adakites, together with Th/Ce and Ce/Yb systematics of the arc basalts that rule out sediment melting, place the enriched source in the sub-arc mantle. Accordingly, isotopic data for the MA, NEBA, and ‘normal’ arc basalts can be explained by melting of an isotopically heterogeneous sub-arc mantle that had been variably enriched by recycling of continental material into the shallow mantle in late Archean subduction zones up to 200 Ma prior to the 2.7 Ga arc. If the late Archean Wawa adakites, MA, and basalts were generated by similar geodynamic processes as their counterparts in Cenozoic arcs, involving subduction of young and/or hot ocean lithosphere, then it is likely that late Archean oceanic crust, and arc crust, were also created and destroyed by modern plate tectonic-like geodynamic processes. This study suggests that crustal recycling through subduction zone processes played an important role for the generation of heterogeneity in the Archean upper mantle. In addition, the results of this study indicate that the Nd-isotope compositions of Archean arc- and plume-derived volcanic rocks are not very distinct, whereas Phanerozoic plumes and intra-oceanic arcs tend to have different Nd-isotopic compositions.
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magnesian Andesites nb enriched basalt Andesites and adakites from late archean 2 7 ga wawa greenstone belts superior province canada implications for late archean subduction zone petrogenetic processes
Contributions to Mineralogy and Petrology, 2001Co-Authors: Ali Polat, Robert KerrichAbstract:Magnesian Andesites (MA) occur with 'normal' tholeiitic to calc-alkaline basalt-andesite suites in four greenstone belts of the 2.7 Ga Wawa subprovince, Canada. Collectively, the magnesian Andesites span ranges of SiO2=56–64 wt%, Mg-number=0.64–0.50, with Cr and Ni contents of 531–106 and 230–21 ppm, respectively. Relative to 'normal' Andesites, the magnesian Andesites form distinct trends on variation diagrams, with relatively high Th and LREE contents, uniform Yb over a range of MgO, more fractionated HREE, and lower Nb/Thpm and Nb/Lapm ratios. Niobium-enriched basalts and Andesites (NEBA; Nb=7–16 ppm), and an Al-enriched rhyolite (adakite) suite are associated in space and time with the magnesian Andesites. Nb-enriched basalts and Andesites are characterized by high TiO2, P2O5, Th, and Zr contents, variably high Zr/Hf (36–44) ratios, and more fractionated HREE (Gd/Ybcn=1.3–4.1) compared to the 'normal' tholeiitic to calc-alkaline basalt-andesite suites. The adakite suite has the high Al (Al2O3=16–18 wt%), high La/Ybcn (21–43), and low Yb (0.4–1.2 ppm) of Archean tonalite-trondhjemite-granodiorite (TTG) suites and Cenozoic adakites, indicative of liquids derived mainly from slab melting. The basalt-andesite suites are not characterized by normal tholeiitic or calc-alkaline fractionation trends of major or trace elements. Rather, compositional trends can be accounted for by some combination of fractional crystallization and variable degrees of metasomatism of the source of basalt and/or Andesites by adakitic liquids. The occurrence of magnesian Andesites, Nb-enriched basalts/Andesites, and adakites has been described from certain Phanerozoic arcs featuring shallow subduction of young and/or hot oceanic lithosphere. Adakites likely represent slab melts, magnesian Andesites the product of hybridization of adakite liquids with mantle peridotite, and Nb-enriched basalts/Andesites melts of the residue from hybridization. Geological similarities between the late-Archean Wawa greenstone belts and certain Cenozoic transpressional orogens with the MA-NEBA-adakite association suggest that subduction of young, hot oceanic lithosphere may have played an important role in the production of this arc-related association in the late Archean.
Robert Kerrich - One of the best experts on this subject based on the ideXlab platform.
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nd isotope systematics of 2 7 ga adakites magnesian Andesites and arc basalts superior province evidence for shallow crustal recycling at archean subduction zones
Earth and Planetary Science Letters, 2002Co-Authors: Ali Polat, Robert KerrichAbstract:Abstract An association of adakite, magnesian andesite (MA), and Nb-enriched basalt (NEB) volcanic flows, which erupted within ‘normal’ intra-oceanic arc tholeiitic to calc-alkaline basalts, has recently been documented in ∼2.7 Ga Wawa greenstone belts. Large, positive initial ϵNd values (+1.95 to +2.45) of the adakites signify that their basaltic precursors, with a short crustal residence, were derived from a long-term depleted mantle source. It is likely that the adakites represent the melts of subducted late Archean oceanic crust. Initial ϵNd values in the MA (+0.14 to +1.68), Nb-enriched basalts and Andesites (NEBA) (+1.11 to +2.05), and ‘normal’ intra-oceanic arc tholeiitic to calc-alkaline basalts and Andesites (+1.44 to +2.44) overlap with, but extend to lower values than, the adakites. Large, tightly clustered ϵNd values of the adakites, together with Th/Ce and Ce/Yb systematics of the arc basalts that rule out sediment melting, place the enriched source in the sub-arc mantle. Accordingly, isotopic data for the MA, NEBA, and ‘normal’ arc basalts can be explained by melting of an isotopically heterogeneous sub-arc mantle that had been variably enriched by recycling of continental material into the shallow mantle in late Archean subduction zones up to 200 Ma prior to the 2.7 Ga arc. If the late Archean Wawa adakites, MA, and basalts were generated by similar geodynamic processes as their counterparts in Cenozoic arcs, involving subduction of young and/or hot ocean lithosphere, then it is likely that late Archean oceanic crust, and arc crust, were also created and destroyed by modern plate tectonic-like geodynamic processes. This study suggests that crustal recycling through subduction zone processes played an important role for the generation of heterogeneity in the Archean upper mantle. In addition, the results of this study indicate that the Nd-isotope compositions of Archean arc- and plume-derived volcanic rocks are not very distinct, whereas Phanerozoic plumes and intra-oceanic arcs tend to have different Nd-isotopic compositions.
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magnesian Andesites nb enriched basalt Andesites and adakites from late archean 2 7 ga wawa greenstone belts superior province canada implications for late archean subduction zone petrogenetic processes
Contributions to Mineralogy and Petrology, 2001Co-Authors: Ali Polat, Robert KerrichAbstract:Magnesian Andesites (MA) occur with 'normal' tholeiitic to calc-alkaline basalt-andesite suites in four greenstone belts of the 2.7 Ga Wawa subprovince, Canada. Collectively, the magnesian Andesites span ranges of SiO2=56–64 wt%, Mg-number=0.64–0.50, with Cr and Ni contents of 531–106 and 230–21 ppm, respectively. Relative to 'normal' Andesites, the magnesian Andesites form distinct trends on variation diagrams, with relatively high Th and LREE contents, uniform Yb over a range of MgO, more fractionated HREE, and lower Nb/Thpm and Nb/Lapm ratios. Niobium-enriched basalts and Andesites (NEBA; Nb=7–16 ppm), and an Al-enriched rhyolite (adakite) suite are associated in space and time with the magnesian Andesites. Nb-enriched basalts and Andesites are characterized by high TiO2, P2O5, Th, and Zr contents, variably high Zr/Hf (36–44) ratios, and more fractionated HREE (Gd/Ybcn=1.3–4.1) compared to the 'normal' tholeiitic to calc-alkaline basalt-andesite suites. The adakite suite has the high Al (Al2O3=16–18 wt%), high La/Ybcn (21–43), and low Yb (0.4–1.2 ppm) of Archean tonalite-trondhjemite-granodiorite (TTG) suites and Cenozoic adakites, indicative of liquids derived mainly from slab melting. The basalt-andesite suites are not characterized by normal tholeiitic or calc-alkaline fractionation trends of major or trace elements. Rather, compositional trends can be accounted for by some combination of fractional crystallization and variable degrees of metasomatism of the source of basalt and/or Andesites by adakitic liquids. The occurrence of magnesian Andesites, Nb-enriched basalts/Andesites, and adakites has been described from certain Phanerozoic arcs featuring shallow subduction of young and/or hot oceanic lithosphere. Adakites likely represent slab melts, magnesian Andesites the product of hybridization of adakite liquids with mantle peridotite, and Nb-enriched basalts/Andesites melts of the residue from hybridization. Geological similarities between the late-Archean Wawa greenstone belts and certain Cenozoic transpressional orogens with the MA-NEBA-adakite association suggest that subduction of young, hot oceanic lithosphere may have played an important role in the production of this arc-related association in the late Archean.
M Santosh - One of the best experts on this subject based on the ideXlab platform.
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magnesium isotopic composition of continental arc Andesites and the implications a case study from the el laco volcanic complex chile
Lithos, 2018Co-Authors: Zhaochong Zhang, M Santosh, Eduardo Campos, Zhiguo Cheng, Shan Ke, Lijuan XuAbstract:Abstract Continental crust can dramatically modify the geochemical and isotopic compositions (e.g., Sr-Nd, Pb) of mantle-derived lavas, and has important implications in understanding magmatic processes in continental arcs which involve subducted materials. In this paper, we report the Mg isotopic compositions of continental arc Andesites from El Laco in northern Chile, and evaluate the contribution of the subducted slab to the formation of continental arc lavas. The Andesites in the El Laco volcanic complex (ELVC) display relatively high (87Sr/86Sr)i ratios and negative age-corrected eNd(t) (t = 1.6 Ma) values. The δ26Mg values of the ELVC Andesites range from −0.26 ± 0.05‰ to −0.15 ± 0.04‰ (average δ26Mg = −0.18 ± 0.05‰), slightly heavier than that of the primitive mantle. The δ26Mg values of clinopyroxene, orthopyroxene and magnetite grains separated from Andesites are −0.27 ± 0.03‰ to −0.20 ± 0.04‰, −0.24 ± 0.06‰ to −0.18 ± 0.03‰ and +0.09 ± 0.06‰ to +0.33 ± 0.06‰, respectively. Our results suggest that Mg isotope fractionation occurred during the fractional crystallization of El Laco andesite, and imply complex Mg isotopic fractionation at high-temperature conditions than previously inferred. Combing the Mg isotopes with the geochemical and Sr-Nd isotopic data, we confirm that neither the deep process of partial melting nor the shallow process of crust contamination contributed to the heavier Mg isotopes of the Andesites. The heavier Mg isotopic features of these lavas were mainly inherited from aqueous fluids derived from the subducted slab, which reacted with the primitive mantle wedge to produce a metasomatic mantle source for the continental arc lavas.
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paleoproterozoic crustal evolution in the east sarmatian orogen petrology geochemistry sr nd isotopes and zircon u pb geochronology of Andesites from the voronezh massif western russia
Lithos, 2016Co-Authors: R A Terentiev, K A Savko, M SantoshAbstract:Abstract Andesites and related plutonic rocks are major contributors to continental growth and provide insights into the interaction between the mantle and crust. Paleoproterozoic volcanic rocks are important components of the East Sarmatian Orogen (ESO) belonging to the East European Craton, although their petrogenesis and tectonic setting remain controversial. Here we present petrology, mineral chemistry, bulk chemistry, Sr–Nd isotopes, and zircon U–Pb geochronological data from Andesites and related rocks in the Losevo and Vorontsovka blocks of the ESO. Clinopyroxene phenocrysts in the Andesites are depleted in LREE, and enriched in HFSE (Th, Nb, Zr, Hf, Ti) and LILE (Ba, Sr). Based on the chemistry of pyroxenes and whole rocks, as well as Fe–Ti oxides, we estimate a temperature range of 1179 to 1262 °С, pressures of 11.3 to 13.0 kbar, H2O content of 1–5 wt.%, and oxygen fu gacity close to the MH buffer for the melts of the Kalach graben (KG) and the Baygora area (BA) Andesites. Our zircon U–Pb geochronological data indicate new zircon growth during the middle Paleoproterozoic as displayed by weighted mean 207Pb/206Pb ages of 2047 ± 17 Ma and 2040 ± 16 Ma for andesite and dacite–porphyry of the BA, and 2050 ± 16 Ma from high-Mg basaltic andesite of the KG. The Andesites and related rocks of the KG and BA are characterized by high magnesium contents (Mg # up to 0.68). All these volcanic rocks are depleted in LREE and HFSE, and display negative Nb and Ti anomalies relative to primitive mantle. The high-Mg bulk composition, and the presence of clinopyroxene phenocrysts suggests that the parent melts of the KG and BA suite were in equilibrium with the mantle rocks. The rocks show positive eNd(T) values and low initial 87Sr/86Sr, suggesting that the magmas were mostly derived from metasomatized mantle source. The geochemical differences between the two andesite types are attributed to: the predominance of fractional crystallization, and minor role of contamination in the case of KG Andesites in contrast to the higher degree of contamination with continental crustal lithologies and/or dacitic melts for the BA Andesites. Furthermore, the upper mantle source of the KG Andesites is inferred to be metasomatically enriched in comparison with the hydrous mantle source of the BA magmas. We propose post-collisional setting for both the high-Mg Andesites of the KG and normal Andesites of the BA, with magma generation through partial melting of enriched mantle sources.
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crustal reworking in the north china craton at 2 5 ga evidence from zircon u pb age hf isotope and whole rock geochemistry of the felsic volcano sedimentary rocks from the western shandong province
Geological Journal, 2013Co-Authors: Wei Wang, M Santosh, Mingguo Zhai, Shijin Wang, Lilin Du, Bin LvAbstract:The western Shandong Province (WSP) is one of the typical Neoarchaean granite–greenstone belts in the Eastern Block of the North China Craton (NCC) and is an important region to investigate the early Precambrian evolutionary history of the NCC. The Taishan association, consisting of a ~2.7 Ga komatiite–tholeiite sequence and a ~2.5 Ga felsic volcano-sedimentary sequence, is the major lithological assemblage in the WSP. In the Qixingtai area, the felsic volcano-sedimentary sequence, partly subjected to anatexis, is composed of hornblende gneiss, voluminous fine-grained biotite gneiss and biotite plagioclase gneiss. SHRIMP zircon dating shows that the protolith of the biotite plagioclase gneiss formed after ca. 2.53 Ga. The tonalite intrusion into the volcanic protoliths of the fine-grained biotite gneiss and biotite plagioclase gneiss occurred at 2.52 Ga. Our age data constrain the time of formation of the felsic volcano-sedimentary sequence in the Taishan area as 2.53–2.52 Ga. The majority of zircons from the felsic volcano-sedimentary rocks have intermediate eHf(t) values (−1.2 to +2.1). The whole rock Nd isotopes of the Taishan felsic volcano-sedimentary rocks yield the eNd(t = 2522 Ma) values of +2.6 to −1.8 and TDM2 ages 3.03–2.68 Ga. These values indicate the pre-existing crust was reworked at the end of the Neoarchaean (~2.5 Ga). The Taishan felsic volcano-sedimentary rocks are rhyodacite–dacite and andesite in composition. These rocks and the tonalite display similar high SiO2 (65–72 wt.%) and low MgO (1–2 wt.%) content. In contrast, the minor Andesites in the area have lower SiO2 (61–62 wt.%) and higher MgO (5.3–6.3 wt.%), suggesting more mantle contribution during their petrogenesis. However, the Mg-rich Andesites have identical rare earth element and multi-element patterns with the rhyodacite–dacites and distinguish them from the Taishan sanukitoids. We suggest that the 2.53–2.52 Ga felsic volcanics of the Taishan association are the products of partial melting of a subducted oceanic crust. The Andesites were produced by minor modification of the ascending tonalite–trondhjemite–granodiorite-type magma by the mantle wedge. Our study indicates that (1) the tectonic regime of the WSP greenstone belt was dominated by an arc-subduction system at ~2.52 Ga and (2) the 2.53–2.52 Ga felsic volcano-sedimentary rocks and intrusive tonalite are the products of crustal reworking. The felsic volcano-sedimentary sequence of the Taishan association and other ~2.5 Ga arc-like assemblages in the NCC provide important clues to understand the nature of the tectonothermal events at the end of the Neoarchaean. Copyright © 2013 John Wiley & Sons, Ltd.
Yoshihiko Tamura - One of the best experts on this subject based on the ideXlab platform.
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the petrology and geochemistry of calc alkaline Andesites on shodo shima island sw japan
Journal of Petrology, 2002Co-Authors: Yoshiyuki Tatsumi, T Nakashima, Yoshihiko TamuraAbstract:Petrographical and geochemical characteristics of calc-alkaline McLennan, 1995) that are produced at convergent plate boundaries and are characterized by lower FeO∗/MgO Andesites on Shodo-Shima Island, SW Japan, having bulk comratios than tholeiitic Andesites (Miyashiro, 1974). An positions largely identical to the continental crust, are presented. understanding of the origin of such orogenic, calc-alkaline The following petrographic observations suggest a role for magma Andesites is thus essential for comprehending not only mixing in producing such andesite magmas: (1) two types of olivine the magma flux in the modern Earth but also the role phenocrysts and spinel inclusions, one with compositions identical of subduction-related magmatism in the evolution of the to those in high-Mg Andesites and the other identical to those in solid Earth. Since the pioneering work of Eichelberger basalts, are recognized in terms of Ni–Mg and Cr–Al–Fe (1975, 1978), Anderson (1976), Sakuyama (1979, 1981, relations, respectively; (2) the presence of orthopyroxene phenocrysts 1984), and Luhr & Carmichael (1980), the majority of with mg-number >90 suggests the contribution of an orthopyroxenepetrologists have considered mixing of mafic and felsic bearing high-Mg andesite magma to production of calc-alkaline magmas to be one of the major mechanisms of generation Andesites; (3) reversely zoned pyroxene phenocrysts may not be in of calc-alkaline Andesites. Continental crust formation, equilibrium with Mg-rich olivine, suggesting the involvement of a however, may not be elucidated solely by such magma differentiated andesite magma as an endmember component; (4) the mixing processes. The reasons for this are twofold. First, presence of very Fe-rich orthopyroxene phenocrysts indicates the calc-alkaline andesite magmatism typifies continental arcs association of an orthopyroxene-bearing rhyolitic magma. Conat least on the modern Earth, suggesting that the contributions from the above at least five endmember magmas to the calctinental crust itself plays an important role in such andesite alkaline andesite genesis can also provide a reasonable explanation of formation, possibly as the source of the felsic endmember the Pb–Sr–Nd isotope compositions of such Andesites. component. Second, there is a slight but significant difference in MgO contents between the bulk continental crust and the typical calc-alkaline andesite (e.g. Kelemen,
