PETROGENESIS OF THE MOST RECENT VOLCANISM IN MAINLAND CHINA:EVIDENCE FROM THE ISOTOPIC CHARACTERISTICS OF ASHIKULE VOLCANIC ROCKS
The Ashikule Volcanic Cluster(AVC),located in the western Kunlun region of the northwestern Tibetan plateau,represents the most recent volcanic activity on Mainland China.This volcanic cluster,which erupted continuously from the Pleistocene to the Holocene,predominantly produced trachyandesites and trachytes,with minor occurrences of phonotephrites,basaltic trachyandesites,and rhyolites.In this study,we present zircon U-Pb-Lu-Hf and whole-rock Sr-Nd-Pb isotopic analyses for volcanic rock samples from AVC.By integrating these data with petrographic,geochronological,and geochemical findings from Yu et al.(2020),we propose further constraints on the petrogenesis of the volcanic rocks and the geodynamic evolution of the western Kunlun region from the Pleistocene to the Holocene.Zircon U-Pb-Lu-Hf isotopic analyses were conducted on five samples:Two trachyandesitic(515-01 and 518-14),two trachytic(521-1 and 521-4),and one rhyolitic(517-B-03).Together with previous 40Ar/39Ar dating,the magmatic zircon grains reveal negative εHf(t)values ranging from-8.8 to-4.4 for the trachyandesitic samples,-8.6 to-5.7 for the trachytic samples,and-9.1 to-6.7 for the rhyolitic sample,suggesting an enriched magma source.The trachyandesitic samples also contain Paleozoic to Mesozoic zircons(165-2 352Ma)with characteristics such as small oval shapes or core-rim structures,indicating that they are inherited zircons.These inherited zircons display εHf(t)values from-3.1 to 9.8,suggesting the involvement of metasedimentary components in the magma source.Whole-rock Sr-Nd-Pb isotopic analyses were conducted on eight samples(four trachyandesitic,three trachytic,and one rhyolitic),revealing 87Sr/86Sr ratios of 0.709 395-0.711 441 and 143Nd/144Nd ratios of 0.512 154-0.512 355.In the 143Nd/144Nd-87Sr/86Sr diagram,these samples plot to the right of the EM Ⅰ region in the fourth quadrant,indicating a relationship with EM Ⅱ-type magmatism.The samples exhibit 207Pb/206Pb ratios of 15.652-15.673 and 206Pb/204Pb ratios of 18.681-18.754,aligning with EM Ⅱ-type and lower crust-derived magmatism on the 207Pb/204Pb-206Pb/204Pb diagram.In the Rb/Nd-Rb diagram,the Ashikule volcanic rocks display an oblique distribution,indicating processes of partial melting or magma mixing,which is further supported by their alignment with the mixing trend on the 1/V-Rb/V diagram.Geochemical modeling results suggest that the Ashikule volcanic magmas formed primarily through a magma mixing process.Previous electron probe microanalysis studies have identified reverse zoning in plagioclase and orthopyroxene phenocrysts,providing additional evidence for magma mixing in the magma chamber.Consequently,these data reveal that Ashikule volcanic magmas originated from a mixing process between EM Ⅱ-type mantle-derived basic magmas and intermediate to acidic magmas from partial melting of ancient continental materials.Considering the tectonic setting of the Tibetan plateau,we propose that Ashikule volcanic activity likely formed in a subduction-dominated environment from the Pleistocene to the Holocene.
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