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Slawomir Ilnicki - One of the best experts on this subject based on the ideXlab platform.
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Amphibolites from the szklarska poreba hornfels belt west sudetes sw poland magma genesis and implications for the break up of gondwana
International Journal of Earth Sciences, 2012Co-Authors: Slawomir IlnickiAbstract:Amphibolites from the Szklarska Poreba hornfels belt (northern part of the Karkonosze-Izera Massif) represent rocks of alkali-basalt composition metamorphosed during Variscan times. Despite the intense thermal influence of the Karkonosze granite superimposed on the effects of regional Amphibolite-facies metamorphism, the geochemical affinities of the Szklarska Poreba Amphibolites are well preserved. They are similar to alkaline OIB basalts derived from an enriched (undepleted) sub-lithospheric source in the garnet stability field at depths ca 80–120 km. Trace-element characteristics and geochemical modelling indicate that the source was not modified by metasomatism in a supra-subduction zone or by alkali (silicate, carbonatitic) infiltration. Subsequent intra-crustal fractional crystallization involved olivine and clinopyroxene, and subordinate spinel and, presumably, plagioclase. The chemical composition of the rocks is most similar to that of modern magmas generated in an extensional setting (intra-continental rift). Neither geochemical characteristics nor estimated mantle temperatures only slightly higher than those of ambient mantle convincingly attest to the involvement of deep-mantle plume activity. Instead, decompression melting of passively upwelling asthenosphere beneath opening fractures in fragmented lithosphere is invoked. The origin of the Amphibolite protolith was presumably associated with the Early Palaeozoic rifting of northern Gondwana, well documented throughout the Karkonosze-Izera massif. Locally rifting must have ceased earlier (immature rift) as reflected by mafic dykes exposed in the northern part of the massif, i.e., in the Szklarska Poreba hornfelses, and by the Izera gneisses and the Stara Kamienica metapelites. A passive rift system controlled by lithosphere extension provides a plausible explanation for the origin of mafic rocks in the Karkonosze-Izera Massif and sheds light on possible mechanisms involved in the break-up of Gondwana.
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Amphibolites from the Szklarska Poręba hornfels belt, West Sudetes, SW Poland: magma genesis and implications for the break-up of Gondwana
International Journal of Earth Sciences, 2011Co-Authors: Slawomir IlnickiAbstract:Amphibolites from the Szklarska Poreba hornfels belt (northern part of the Karkonosze-Izera Massif) represent rocks of alkali-basalt composition metamorphosed during Variscan times. Despite the intense thermal influence of the Karkonosze granite superimposed on the effects of regional Amphibolite-facies metamorphism, the geochemical affinities of the Szklarska Poreba Amphibolites are well preserved. They are similar to alkaline OIB basalts derived from an enriched (undepleted) sub-lithospheric source in the garnet stability field at depths ca 80–120 km. Trace-element characteristics and geochemical modelling indicate that the source was not modified by metasomatism in a supra-subduction zone or by alkali (silicate, carbonatitic) infiltration. Subsequent intra-crustal fractional crystallization involved olivine and clinopyroxene, and subordinate spinel and, presumably, plagioclase. The chemical composition of the rocks is most similar to that of modern magmas generated in an extensional setting (intra-continental rift). Neither geochemical characteristics nor estimated mantle temperatures only slightly higher than those of ambient mantle convincingly attest to the involvement of deep-mantle plume activity. Instead, decompression melting of passively upwelling asthenosphere beneath opening fractures in fragmented lithosphere is invoked. The origin of the Amphibolite protolith was presumably associated with the Early Palaeozoic rifting of northern Gondwana, well documented throughout the Karkonosze-Izera massif. Locally rifting must have ceased earlier (immature rift) as reflected by mafic dykes exposed in the northern part of the massif, i.e., in the Szklarska Poreba hornfelses, and by the Izera gneisses and the Stara Kamienica metapelites. A passive rift system controlled by lithosphere extension provides a plausible explanation for the origin of mafic rocks in the Karkonosze-Izera Massif and sheds light on possible mechanisms involved in the break-up of Gondwana.
Peter A Cawood - One of the best experts on this subject based on the ideXlab platform.
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subduction related mantle metasomatism and partial melting in the northern north china craton insights from Amphibolite enclaves siziwangqi inner mongolia
Precambrian Research, 2020Co-Authors: Nancy Huichun Chen, Guochun Zhao, Hai Zhou, Haoyang Lee, Peter A CawoodAbstract:Abstract The Siziwangqi region of Inner Mongolia lies along the northern margin of the North China Craton. It consists of diverse suites of Precambrian and Phanerozoic rock units including relicts of the Paleo-Asian Ocean, arc magmatic rocks, Amphibolites, and TTG (tonalite–trondhjemite–granodiorite). The Amphibolite rocks in the Siziwangqi area occur as enclaves embedded in granodiorite. Geochemical, Lu–Hf isotopic, and zircon U–Pb geochronological analyses on Amphibolite enclaves from Siziwangqi indicate end Archean subduction-related mantle metasomatism accompanied by partial melting. U-Pb zircon dating of Amphibolite reveals a protolith age of 2503 ± 6 Ma with an inherited zircon age of ∼ 2549 Ma. Geochemical data indicate that the Amphibolites range in composition from subalkaline basalt through basaltic–andesite and andesite, all with island arc affinities. The eHf(t) values for the Amphibolites range from ＋2.7 to ＋11.4 and their TDM1 model ages range from 2681 to 2517 Ma and peak at 2625 Ma, suggesting dominant late Neoarchean mantle input. This study shows that the Amphibolite protoliths likely formed by partial melting of spinel-bearing lherzolites from a depleted mantle, which had been metasomatized by subduction-related fluids.
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neoproterozoic subduction along the ailaoshan zone south china geochronological and geochemical evidence from Amphibolite
Precambrian Research, 2014Co-Authors: Peter A Cawood, Yongfeng Cai, Yuejun Wang, Weiming Fan, Huichuan Liu, Xiaowan Xing, Yuzhi ZhangAbstract:Abstract Lenses of Amphibolites occur along the Ailaoshan suture zone at the southwestern margin of the Yangtze Block, South China. Petrological, geochemical and zircon U–Pb geochronological data indicate that they are divisible into two coeval groups. Group 1, represented by the Jinping Amphibolite, has mg-number of 71–76 and (La/Yb)cn ratios of 7.2–7.7, and displays a geochemical affinity to island arc volcanic rocks. Group 2 Amphibolites occur at Yuanyang and are characterized by high Nb contents (14.3–18.4 ppm), resembling Nb-enriched basalts. The ɛ Nd (t) values for Group 1 range from −3.45 to −2.04 and for Group 2 from +4.08 to +4.39. A representative sample for Group 1 yields a U–Pb zircon age of 803 ± 7 Ma, whereas two samples for Group 2 give U–Pb zircon ages of 813 ± 11 Ma and 814 ± 12 Ma. Petrogenetic analysis suggests that Group 1 originated from an orthopyroxene-rich source and Group 2 from a mantle wedge modified by slab-derived melt. In combination with other geological observations, these Amphibolites are inferred to constitute part of an early Neoproterozoic (∼815–800 Ma) arc-back-arc basin system. The Neoproterozoic Amphibolites and related rocks along the Ailaoshan zone may be the southward extension of the Neoproterozoic supra-subduction zone that developed along the western margin of the Yangtze Block.
Yongfeng Cai - One of the best experts on this subject based on the ideXlab platform.
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neoproterozoic subduction along the ailaoshan zone south china geochronological and geochemical evidence from Amphibolite
Precambrian Research, 2014Co-Authors: Peter A Cawood, Yongfeng Cai, Yuejun Wang, Weiming Fan, Huichuan Liu, Xiaowan Xing, Yuzhi ZhangAbstract:Abstract Lenses of Amphibolites occur along the Ailaoshan suture zone at the southwestern margin of the Yangtze Block, South China. Petrological, geochemical and zircon U–Pb geochronological data indicate that they are divisible into two coeval groups. Group 1, represented by the Jinping Amphibolite, has mg-number of 71–76 and (La/Yb)cn ratios of 7.2–7.7, and displays a geochemical affinity to island arc volcanic rocks. Group 2 Amphibolites occur at Yuanyang and are characterized by high Nb contents (14.3–18.4 ppm), resembling Nb-enriched basalts. The ɛ Nd (t) values for Group 1 range from −3.45 to −2.04 and for Group 2 from +4.08 to +4.39. A representative sample for Group 1 yields a U–Pb zircon age of 803 ± 7 Ma, whereas two samples for Group 2 give U–Pb zircon ages of 813 ± 11 Ma and 814 ± 12 Ma. Petrogenetic analysis suggests that Group 1 originated from an orthopyroxene-rich source and Group 2 from a mantle wedge modified by slab-derived melt. In combination with other geological observations, these Amphibolites are inferred to constitute part of an early Neoproterozoic (∼815–800 Ma) arc-back-arc basin system. The Neoproterozoic Amphibolites and related rocks along the Ailaoshan zone may be the southward extension of the Neoproterozoic supra-subduction zone that developed along the western margin of the Yangtze Block.
Mike Villeneuve - One of the best experts on this subject based on the ideXlab platform.
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geochemistry and geochronology of the metamorphic sole underlying the xigaze ophiolite yarlung zangbo suture zone south tibet
Lithos, 2009Co-Authors: Carl Guilmette, Rejean Hebert, Chengshan Wang, Mike VilleneuveAbstract:Abstract Strongly foliated Amphibolite clasts are found embedded within the ophiolitic melange underlying the Xigaze Ophiolite near Bainang and Angren, Yarlung Zangbo Suture Zone, Southern Tibet. These high-grade Amphibolites are interpreted as remnants of a dismembered subophiolitic metamorphic sole that would have formed during the inception of a subduction. They include garnet-clinopyroxene Amphibolites, clinopyroxene Amphibolites and common Amphibolites. Petrographic descriptions, mineral chemistry and thermobarometry of these rocks can be found in a companion paper [Guilmette, C., Hebert, R., Dupuis, C., Wang, C.S., Li, Z.J., 2008. Metamorphic history and geodynamic significance of high-grade metabasites from the ophiolitic melange beneath the Yarlung Zangbo Ophiolites, Xigaze area, Tibet. Journal of Asian Earth Sciences, 32, 423–437.]. The geochemistry of the Amphibolites confirms that their protoliths were igneous mafic rocks of basaltic to pyroxenitic composition that were likely part of an upper oceanic crust. Rare Earth Elements contents are suggestive of an N-MORB origin. However, enriched LILEs and depleted Nb–Ta–Ti when compared to N-MORBs rather suggest a suprasubduction zone influence. A large proportion of the overlying ophiolitic mafic rocks share the same geochemical characteristics, suggesting the protoliths of the Amphibolites might have crystallized in the same environment as the Xigaze ophiolitic crust, likely in a back-arc basin. 40 Ar/ 39 Ar step-heating dating of hornblende concentrates from three samples yielded ages of 123.6 ± 2.9 Ma, 127.7 ± 2.2 Ma and 127.4 ± 2.3 Ma. These cooling ages are slightly younger or overlapping magmatic and sedimentary ages obtained from the overlying ophiolite. All these new data support a model in which the ophiolitic crust and the protolith of the Amphibolites were formed along the same spreading center above a subduction zone. The demise of the early subduction circa 130 Ma forced the inception of a new subduction zone at the SSZ spreading axis, burying SSZ mafic rocks underneath a SSZ ophiolitic mantle.
Yuzhi Zhang - One of the best experts on this subject based on the ideXlab platform.
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neoproterozoic subduction along the ailaoshan zone south china geochronological and geochemical evidence from Amphibolite
Precambrian Research, 2014Co-Authors: Peter A Cawood, Yongfeng Cai, Yuejun Wang, Weiming Fan, Huichuan Liu, Xiaowan Xing, Yuzhi ZhangAbstract:Abstract Lenses of Amphibolites occur along the Ailaoshan suture zone at the southwestern margin of the Yangtze Block, South China. Petrological, geochemical and zircon U–Pb geochronological data indicate that they are divisible into two coeval groups. Group 1, represented by the Jinping Amphibolite, has mg-number of 71–76 and (La/Yb)cn ratios of 7.2–7.7, and displays a geochemical affinity to island arc volcanic rocks. Group 2 Amphibolites occur at Yuanyang and are characterized by high Nb contents (14.3–18.4 ppm), resembling Nb-enriched basalts. The ɛ Nd (t) values for Group 1 range from −3.45 to −2.04 and for Group 2 from +4.08 to +4.39. A representative sample for Group 1 yields a U–Pb zircon age of 803 ± 7 Ma, whereas two samples for Group 2 give U–Pb zircon ages of 813 ± 11 Ma and 814 ± 12 Ma. Petrogenetic analysis suggests that Group 1 originated from an orthopyroxene-rich source and Group 2 from a mantle wedge modified by slab-derived melt. In combination with other geological observations, these Amphibolites are inferred to constitute part of an early Neoproterozoic (∼815–800 Ma) arc-back-arc basin system. The Neoproterozoic Amphibolites and related rocks along the Ailaoshan zone may be the southward extension of the Neoproterozoic supra-subduction zone that developed along the western margin of the Yangtze Block.