Rhyolite magma evolution recorded in isotope and trace element composition of zircon from Halle Volcanic Complex
Elżbieta SłodczykAnna PietranikChristoph BreitkreuzC. Mark FanningRobert AnczkiewiczBodo‐Carlo Ehling
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Abstract: Zircon U‐Pb ages and geochemical data of volcanic rocks in the Suifenhe Formation in eastern Heilongjiang Province are reported, and their petrogenesis is discussed in this paper. The Suifenhe Formation mainly consists of basalt, andesite, and dacite. Zircon from andesite and dacite are euhedral in shape and show typical oscillatory zoning with high Th/U ratios (0.18‐0.57), implying its magmatic origin. Zircon U‐Pb dating results by laser ablation inductively coupled plasma mass spectrometer (LA‐ICP‐MS) indicate that the 206 Pb/ 238 U ages of zircons from andesite range within 105–106 Ma, yielding a weighted mean age of 105.5±0.8 Ma ( n =14), and that 206 Pb/ 238 U ages of zircons from dacite are between 90–96 Ma, yielding a weighted mean age of 93.2±1.3 Ma ( n =13). The volcanic rocks from the Suifenhe Formation are subalkaline series and show a calc‐alkaline evolutionary trend with SiO 2 content of 47.69%‐65.47%, MgO contents of 1.42%‐6.80% (Mg − = 45–53), and Na 2 O/K 2 O ratios of 1.83‐3.63. They are characterized by enrichment in large ion lithophile elements (LILE) and light‐rare‐earth elements (LREE), depletion in heavy rare earth elements (HREE) and high field strength elements (HFSE) (e.g., Nb, Ta, Ti), and low initial 87 Sr/ 86 Sr ratios (0.7041‐0.7057) and positive ø Nd (t) values (0.39‐4.08), implying that they could be derived from a depleted magma source. Taken together, these results suggest that the primary magma of the volcanic rocks might originate from partial melting of the mantle wedge metasomatized by fluids derived from subducted slab under a tectonic setting of active continental margin.
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Basaltic andesite
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Precise in situ zircon U-Pb dating and Lu–Hf isotopic measurement using an LA-ICP-MS system, whole-rock major and trace element geochemistry and Sr–Nd isotope geochemistry were conducted on the volcanic host rocks of the Tongyu copper deposit on the basis of further understanding of its geological characteristics. Three zircon samples from the volcanic host rocks yielded 206Pb/238 U weighted average ages ranging from 436±4 Ma to 440±5 Ma, which are statistically indistinguishable and coeval with the ca. 440 Ma northward subduction event of the Paleo-Qinling oceanic slab. The volcanic host rocks were products of magmatic differentiation that evolved from basalt to andesite to dacite to rhyolite, forming an integrated tholeiitic island arc volcanic rock suite. The primitive mantle-normalized trace element patterns for most samples show characteristics of island arc volcanic rocks, such as relative enrichment of LILE(e.g. Th, U, Pb and La) and depletion of HFSE(e.g. Nb, Ta, Ti, Zr and Hf). Discrimination diagrams of Ta/Yb vs Th/Yb, Ta vs Th, Yb vs Th/Ta, Ta/Hf vs Th/Hf, Hf/3 vs Th vs Nb/16, La vs La/Nb and Nb vs Nb/Th all suggest that both the volcanic host rocks from the Tongyu copper deposit and the volcanic rocks from the regional Xieyuguan Group were formed in an island arc environment related to subduction of an oceanic slab. Values of ISr(0.703457 to 0.708218) and eNd(t)(-2 to 5.8) indicate that the source materials of volcanic rocks from the Tongyu copper deposit and the Xieyuguan Group originated from the metasomatised mantle wedge with possible crustal material assimilation. Most of the volcanic rock samples show good agreement with the values of typical island arc volcanic rocks in the ISr-eNd(t) diagram. The involvement of crustal-derived material in the magma of the volcanic rocks from the Tongyu copper deposit was also reflected in the zircon eHf(t) values, which range from-3.08 to 10.7, and the existence of inherited ancient xenocrystic zircon cores(2616±39 Ma and 1297±22 Ma). The mineralization of the Tongyu copper deposit shows syn-volcanic characteristics such as layered orebodies interbedded with the volcanic rock strata, thus, the zircon U-Pb age of the volcanic host rocks can approximately represent the mineralization age of the Tongyu copper deposit. Both the Meigou pluton and the volcanic host rocks were formed during the ca. 440 Ma northward subduction of the Paleo-Qinling Ocean when high oxygen fugacity aqueous hydrothermal fluid released by dehydration of the slab and the overlying sediments fluxed into the mantle wedge, triggered partial melting of the mantle wedge, and activated and extracted Cu and other ore-forming elements. The magma and ore-bearing fluid upwelled and erupted, and consequently formed the island arc volcanic rock suite and the Tongyu VHMS-type copper deposit.
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Titanite
Igneous differentiation
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The Ela Mountain area is located at the easternmost point of the East Kunlun Orogen, in which voluminous igneous rocks developed in the Triassic period, and it is a good place to investigate the tectonic evolution of the Paleo-Tethys Ocean. In this study, petrological, geochemical, zircon U-Pb geochronology and zircon Hf isotope studies were carried out on the volcanic rocks in the Ela Mountain area. Dacite (239.3 ± 1.4 Ma) exhibits calc-alkaline I-type characteristics, and rhyolite (237.8 ± 2.1 Ma) is similar to high-K calc-alkaline highly fractionated I-type volcanic rock. The petrogenesis shows that both rhyolite and dacite originated from the partial melting of the mafic lower crust of the Mesoproterozoic under relatively high temperature and low pressure. Dacite and rhyolite were derived from the same or similar parent magma, and they are volcanic rocks with different differentiation degrees formed in the same magmatic pulse activity. Differing from rhyolite and dacite, basaltic andesite shows a relatively young age (234 ± 1.2 Ma), mainly originating from the partial melting of the lithospheric mantle modified by subducted slab-derived fluids; the magma was contaminated with a small amount of crustal source components and experienced the fractional crystallization of mafic minerals before the eruption to the surface. This study on the tectonic environment of these volcanic rocks shows that they were formed in the environment of slab failure in the late stage of syn-collision, and that they are different types of volcanic rocks from different sources under similar tectonic environments. The volcanic rocks of the Ela Mountain area in this contribution provide important evidence for Middle Triassic to Late Triassic syn-collisional magmatism in the slab failure stages. The results of this study constrain the lower age limit of the closure of the Paleo-Tethys Ocean and the initial time of extension of the late stage of syn-collision, providing important information regarding regional tectonic evolution processes and volcanic activity history. They can be applied to regional tectonic evolution, petrology, volcanic stratigraphy and mineral deposits related to volcanic rocks.
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Felsic
Fractional crystallization (geology)
Igneous differentiation
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Igneous differentiation
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Abstract Precise in situ zircon U‐Pb dating and Lu–Hf isotopic measurement using an LA‐ICP‐MS system, whole‐rock major and trace element geochemistry and Sr–Nd isotope geochemistry were conducted on the volcanic host rocks of the Tongyu copper deposit on the basis of further understanding of its geological characteristics. Three zircon samples from the volcanic host rocks yielded 206 Pb/ 238 U weighted average ages ranging from 436±4 Ma to 440±5 Ma, which are statistically indistinguishable and coeval with the ca. 440 Ma northward subduction event of the Paleo‐Qinling oceanic slab. The volcanic host rocks were products of magmatic differentiation that evolved from basalt to andesite to dacite to rhyolite, forming an integrated tholeiitic island arc volcanic rock suite. The primitive mantle‐normalized trace element patterns for most samples show characteristics of island arc volcanic rocks, such as relative enrichment of LILE (e.g. Th, U, Pb and La) and depletion of HFSE (e.g. Nb, Ta, Ti, Zr and Hi). Discrimination diagrams of Ta/Yb vs Th/Yb, Ta vs Th, Yb vs Th/Ta, Ta/Hf vs Th/Hf, Hf/3 vs Th vs Nb/16, La vs La/Nb and Nb vs Nb/Th all suggest that both the volcanic host rocks from the Tongyu copper deposit and the volcanic rocks from the regional Xieyuguan Group were formed in an island arc environment related to subduction of an oceanic slab. Values of I Sr (0.703457 to 0.708218) and ∊ Nd ( t ) (–2 to 5.8) indicate that the source materials of volcanic rocks from the Tongyu copper deposit and the Xieyuguan Group originated from the metasomatised mantle wedge with possible crustal material assimilation. Most of the volcanic rock samples show good agreement with the values of typical island arc volcanic rocks in the I Sr –∊ Nd ( t ) diagram. The involvement of crustal‐derived material in the magma of the volcanic rocks from the Tongyu copper deposit was also reflected in the zircon ∊ Hf ( t ) values, which range from –3.08 to 10.7, and the existence of inherited ancient xenocrystic zircon cores (2616±39 Ma and 1297±22 Ma). The mineralization of the Tongyu copper deposit shows syn‐volcanic characteristics such as layered orebodies interbedded with the volcanic rock strata, thus, the zircon U–Pb age of the volcanic host rocks can approximately represent the mineralization age of the Tongyu copper deposit. Both the Meigou pluton and the volcanic host rocks were formed during the ca. 440 Ma northward subduction of the Paleo‐Qinling Ocean when high oxygen fugacity aqueous hydrothermal fluid released by dehydration of the slab and the overlying sediments fluxed into the mantle wedge, triggered partial melting of the mantle wedge, and activated and extracted Cu and other ore‐forming elements. The magma and ore‐bearing fluid upwelled and erupted, and consequently formed the island arc volcanic rock suite and the Tongyu VHMS‐type copper deposit.
Dacite
Geochronology
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Island arc
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