EM1 signature from within the post-Archaean subcontinental lithospheric mantle: Isotopic evidence from the potassic volcanic rocks in NE China.

The Lookout Volcanics are the remnants of an extensive sheet of mid-Cretaceous (ca. 96 Ma) continental intraplate volcanic rocks erupted just prior to the rifting of New Zealand from Gondwana. Preserved in a fault angle depression bounded by the Awatere Fault located in Marlborough, South Island, New Zealand, the volcanic rocks cover an area of ca. 50 km2 with exposed thicknesses up to 1000 m. On the basis of stratigraphic evidence the dominantly terrestrial lavas flows are inferred to have erupted from dykes of a coeval radial dyke swarm. A detailed sampling of the lava flows of the Lookout Volcanics has been undertaken with a ca. 700 m composite stratigraphic section being constructed, largely based on a continuous sequence of lava flows outcropping in Middlehurst Stream. New Rb-Sr age constraints for the Lookout Volcanics (97.6 plus or minus 3.4 Ma) and Blue Mountain Igneous Complex (97.1 plus or minus 0.7 Ma) are consistent with previous radiometric dates of plutonic complexes in the Central Marlborough Igneous Complex, and suggest a rapid accumulation of volcanic material from ca. 98-96 Ma during the initial extension of proto-New Zealand. The predominantly mafic and alkaline samples include basalt, picrobasalt, basanite, trachybasalt and basaltic trachyandesite rock types. No samples represent primary magmas with all samples having undergone fractionation (or accumulation) of olivine plus clinopyroxene plus or minus plagioclase plus or minus Fe-Ti oxides. Initial Sr-Nd-Hf-Pb isotopic variations (87Sr/86Sr = 0.7030-0.7039; 143Nd/144Nd = 0.51272-0.51264; 176Hf/177Hf = 0.28283-0.28278; 206Pb/204Pb = 20.32-18.82) reflect mixing between melts of a HIMUlike mantle component with up to 25-30% of an Early Cretaceous upper crustal component. Oxygen isotope ratios determined by laser fluorination analysis from 6 lava flows yielded delta 18O = 4.7-5.0 per thousand for olivine, 4.8-5.4 per thousand in clinopyroxene cores, 3.9-5.5 per thousand in clinopyroxene rims. Average olivine (4.8 per thousand) and clinopyroxene core (5.1 per thousand) values are 0.4-0.5 per thousand lower than those of average mantle peridotite but comparable to those of HIMU OIB, and are consistent with New Zealand intraplate magmas being generated by a low delta 18O mantle. However, oxygen isotopic disequilibrium between clinopyroxene cores and rims (Delta 18O = -1.4 to +0.3) records the overprinting of this signature by crustal processes. Negative disequilibrium between clinopyroxene rims and cores in primitive samples suggests these phenocrysts grew in a shallow crustal magma chamber with an active meteoric water system. The effects of crustal assimilation can also be observed with clinopyroxene phenocrysts from the most evolved sample exhibiting coupled elevated delta 18O and 87Sr/86Sr. Variations in incompatible trace element ratios are consistent with the Lookout Volcanics being the small degree (2-5%) partial melts of an amphibole-bearing garnet pyroxenite. Furthermore, the elevated NiO contents of olivine phenocrysts are consistent with melting of a pyroxenitic mantle source. The presence of residual amphibole constrains melting to the hydrous subcontinental lithospheric mantle. The Lookout Volcanics and coeval plutonic complexes are the oldest occurrences of HIMU magmatism in Zealandia. This source was generated by small degree silicate melts from recycled oceanic lithosphere that metasomatised the base of the subcontinental lithospheric mantle beneath East Gondwana over 200 Ma ago.

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Petrogenesis of Early Cretaceous volcanic rocks of the Manketouebo Formation in the Wuchagou region, central Great Xing’an Range, NE China, and tectonic implications: geochronological, geochemical, and Hf isotopic evidence

International Geology Review ◽

10.1080/00206814.2015.1095132 ◽

2015 ◽

Vol 58 (5) ◽

pp. 556-573 ◽

Cited By ~ 25

Author(s):

Zheng Ji ◽

Wen-Chun Ge ◽

Qing-Hai Wang ◽

Hao Yang ◽

Guo-Chun Zhao ◽

...

Keyword(s):

Early Cretaceous ◽

Volcanic Rocks ◽

Ne China ◽

Isotopic Evidence ◽

Tectonic Implications ◽

Great Xing’An Range

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Fe-Cu-S rich melts in the subcontinental lithospheric mantle: insight from the Lower Silesian (SW Poland) xenoliths

10.5194/egusphere-egu2020-18544 ◽

2020 ◽

Author(s):

Hubert Mazurek ◽

Jakub Ciążela ◽

Magdalena Matusiak-Małek ◽

Jacek Puziewicz ◽

Theodoros Ntaflos

Keyword(s):

Lithospheric Mantle ◽

Volcanic Rocks ◽

Oceanic Lithosphere ◽

Mantle Xenoliths ◽

Subcontinental Lithospheric Mantle ◽

Origin And Evolution ◽

Depleted Mantle ◽

Group A ◽

Sw Poland ◽

Group B

Migration of strategic metals through the lithospheric mantle can be tracked by sulfides in mantle xenoliths. Cenozoic mafic volcanic rocks from the SW Poland (Lower Silesia, Bohemian Massif) host a variety of subcontinental lithospheric mantle (SCLM) xenoliths. To understand metal migration in the SCLM we studied metal budget of peridotites from the Wilcza G&#243;ra basanite and their metasomatic history.The Wilcza G&#243;ra xenoliths are especially appropriate to study metasomatic processes as they consist of 1) peridotites with OlFo=89.1-91.5 representing depleted mantle (group A); 2) peridotites with OlFo=84.2-89.2 representing melt-metasomatized mantle (group B), as well as 3) hornblende-clinopyroxenites and websterites with OlFo=77.2-82.5 representing former melt&#160; channels (group C; Matusiak-Ma&#322;ek et al., 2017). The inherent sulfides are either interstitial or enclosed in the silicates. High-temperature exsolutions of pyrrhotite (Po), pentlandite (Pn) and chalcopyrite (Ccp) indicate magmatic origin of the sulfides.The three peridotitic groups differ by sulfide mode and composition. The sulfide modes are enhanced in group C (0.022-0.963 vol.&#8240;) and group B (<0.028 vol. &#8240;) with respect to group A (<0.002 vol.&#8240;). The sulfides of group C are Ni-poor and Fe-Cu-rich as reflected in their mineral composition (Po55-74Ccp1-2Pn24-44 in group A, Po67-85Ccp1-6Pn14-33, in group B and Po80-97Ccp1-7Pn2-20 in group C) and major element chemical composition. Ni/(Ni+Fe) of pentlandite is the lowest in group C (~0.25) and the highest in group A (0.54-0.61). Cu/(Cu+Fe) of chalcopyrite is 0.32-0.49 in group C contrasting to~0.50 in groups A and B.&#160;The sulfide-rich xenoliths of group C indicate an important role of pyroxenitic veins in transporting Fe-Cu-S-rich melts from the upper mantle to the crust. However, the moderately enhanced sulfide modes in melt-mantle reaction zones represented by xenoliths of group B demonstrate that the upper continental mantle is refertilized with these melts during their ascent. Hence, significant portion of S and metals remains in the mantle never reaching the crust, as has been previously observed in the oceanic lithosphere (Ciazela et al., 2018).&#160;Acknowledgments: This study was supported by the NCN project no. UMO-2014/15/B/ST10/00095. The EPMA analyses were funded from the Polish-Austrian project WTZ PL 08/2018.&#160;References:Ciazela, J., Koepke, J., Dick, H. J. B., Botcharnikov, R., Muszynski, A., Lazarov, M., Schuth, S., Pieterek, B. & Kuhn, T. (2018). Sulfide enrichment at an oceanic crust-mantle transition zone: Kane Megamullion (23 N, MAR). Geochimica et Cosmochimica Acta, 230, 155-189Matusiak-Ma&#322;ek, M., Puziewicz, J., Ntaflos, T., Gr&#233;goire, M., Kuku&#322;a, A. & Wojtulek P.&#160;&#160; M. (2017). Origin and evolution of rare amphibole-bearing mantle peridotites from Wilcza G&#243;ra (SW Poland), Central Europe. Lithos 286&#8211;287, 302&#8211;323.

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