Research ArticleContributions of slab-derived fluids to ultrapotassic rocks indicated by K isotopes
Introduction
Mantle-derived ultrapotassic rocks are widely distributed in collisional belts (e.g., Alpine-Himalayan, Soder and Romer, 2018; Zhao et al., 2009, and references therein). Detailed investigation on ultrapotassic rocks may yield crucial information regarding to the composition of the lithospheric mantle and the chemical cycling that occurs at convergent margins (Foley et al., 1987; Liu et al., 2015; Palmer et al., 2019; Soder and Romer, 2018; Zhao et al., 2009; Zheng et al., 2011). Petrogenetic models of these rocks generally involve K-rich melts or fluids derived from subducted crustal materials that react with the surrounding mantle to form amphibole- and phlogopite-bearing veins prior to partial melting (Foley, 1992b; Förster et al., 2019; McCoy-West et al., 2010; Soder and Romer, 2018). Subsequently, the enriched mantle may get partially melted through tectonic processes such as slab rollback and breakoff, strike-slip faulting, or orogenic collapse and lithosphere delamination (Dalslåen et al., 2020; Palmer et al., 2019; Sun et al., 2002a; Thirlwall et al., 1994). Lithospheric mantle metasomatized by slab-derived melts or fluids has been generally taken as the source for ultrapotassic igneous rocks in collisional zone (Liu et al., 2014; Palmer et al., 2019; Soder and Romer, 2018); however, it is still debated (Gao et al., 2007; Murphy et al., 2002). For instance, Murphy et al. (2002) found that Gaussberg lamproites (ultrapotassic) from the East Antarctica exhibit very unusual Pb isotope compositions and proposed that their sources could be melts derived from continental sediments that were subducted to and remained isolated in the transition zone or lower mantle.
As a large ion lithophile element (LILE), K is highly mobile during fluid-related processes. Therefore, K and its isotopes are promising tracers for processes and pathways of fluid transfer and crustal material recycling in the subduction zone (Becker et al., 2000; Hu et al., 2020; Hu et al., 2021a; Liu et al., 2020; Parendo et al., 2017; Santiago Ramos et al., 2020; Sun et al., 2020; Teng et al., 2020). In particular, large K isotope fractionations were observed during various fluid-rock interactions, which offer further potential of using K isotopes to trace the contribution of the melts or fluids that metasomatized the mantle source (Chen et al., 2020; Hu et al., 2020; Hu et al., 2021a; Li et al., 2019a; Liu et al., 2020; Parendo et al., 2017; Santiago Ramos et al., 2020; Sun et al., 2020; Teng et al., 2020; Wang et al., 2021). Especially, studies on eclogitic rocks demonstrated that subduction-related dehydration during prograde metamorphism causes large K isotopic fractionation and that slab-derived fluids with isotopically heavy K (δ41K up to +1.4‰) are released into the mantle wedge (Liu et al., 2020). In contrast, due to continental weathering and submarine diagenetic alteration, subducted sediments generally have light K isotopic compositions (δ41K as low as −1.3‰), with the low end nearly 1‰ lighter than the average mantle value (Hu et al., 2020). Consequently, addition of subducted sediments to the mantle source would produce a negative shift in δ41K values (Hu et al., 2020).
Given that magmatic differentiation does not cause detectable K isotopic fractionation (Hu et al., 2021b; Tuller-Ross et al., 2019a; Tuller-Ross et al., 2019b), the K isotopic compositions of mantle-derived ultrapotassic rocks should be controlled by the K-rich metasomes (Foley et al., 1987; Förster et al., 2019). Therefore, the study of K isotopic characteristics for ultrapotassic rocks may have the ability to identify distinct contributions of subducted sediments or slab-derived melts or fluids.
In this study we present petrographic observations, major element, trace element, and K isotope compositions to constrain the petrogenesis of the Baoji ultrapotassic rocks from North Qinling block, China. Our results show that the rocks have considerably heavier K isotopic compositions than both the depleted mantle and upper continental crust. The positive correlations between δ41K values and K/Th, Ba/Th and Ba/Rb ratios indicate that slab-derived fluids or supercritical fluids with heavy K isotopic signatures have contributed to the mantle source of the ultrapotassic rocks. Therefore, our study provides crucial information about the crust-mantle interactions and the cycling of K at convergent margins.
Section snippets
Geological background and samples
The Qinling Orogenic Belt (QOB), the western counterpart of the Dabie ultrahigh-pressure metamorphic terrane, consists primarily of four major units. These are, from north to south (Fig. 1) the southern margin of North China Craton (S-NCC), the North Qinling Belt (NQB), the South Qinling Belt (SQB), and the northern margin of South China Craton (N-SCC) (Dong et al., 2016; Sun et al., 2002a; Sun et al., 2002b; Wu and Zheng, 2013). The QOB formed during the early Mesozoic and was created by a
Whole rock major and trace elements
Whole rock major elements were determined on fused glass discs with an X-Ray Fluorescence Spectrometer at the laboratory of ALS minerals at Guangzhou. Loss on ignition (LOI) was obtained by weight difference after ignition at 1000 °C using an electronic balance. Trace elements were analyzed by ICP-MS after high pressure and acid dissolution at the laboratory of ALS minerals at Guangzhou. The analytical precision and accuracy for major and trace elements are better than ±1% and ± 5%,
Whole rock major and trace elements
Baoji ultrapotassic rocks have low SiO2 (51.8–61.9 wt%, with an average of 55.4 ± 3.5 wt%) and Na2O contents (1.3–2.4 wt%, with an average of 1.6 ± 0.4 wt%), while high MgO (2.9–10.3 wt%, with an average of 6.6 ± 2.1 wt%) and K2O contents (6.4–8.0 wt%, with an average of 7.3 ± 0.3 wt%) (Fig. 3, Fig. 4 and S2, Table S5), consisting with the published data (Xue et al., 2018). The investigated rocks fall in the monzo-diorite and monzonite fields in the total alkalis (K2O + Na2O) versus SiO2
Petrogenesis of the Baoji ultrapotassic rocks
The Baoji ultrapotassic rocks have high MgO (Mg#: 57– 76) and high compatible elements (e.g., Sc, Cr, Ni) concentrations, indicating that they are mantle-derived rocks (Fig. 6) (Turner et al., 1996; Zhao et al., 2009). On the other hand, they are enriched in LREE and LILE (Fig. 5), and have high Ba/Nb, La/Nb, Th/Yb and Nb/Yb ratios (Fig. 8), which suggest that they must have formed from a highly enriched lithospheric mantle (Foley, 1992a; Thirlwall et al., 1994; Turner et al., 1996;Xue et al.,
Conclusions
Our petrographic, geochemical and K isotopic data from the Baoji ultramafic rocks lead to the following conclusions:
- (1)
The rocks have high MgO, high compatible elements (e.g., Sc, Cr, Ni), and display significant enrichment in LREE and LILE, indicating that they were derived from a highly enriched lithospheric mantle source.
- (2)
Baoji ultrapotassic rocks have high Ba/La, Ba/Th, Ba/Rb and low Rb/Sr ratios. This, together with partial melting modelling, reveal that their mantle source was a garnet-facies
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
We acknowledge Dr. Heng Chen and Zhen Tian for their assistance during the K isotope measurements. Kai Wu is thanked for his help during field work. English polishing by Professor Wolfgang Siebel is gratefully acknowledged. We thank Dr Xian-Hua Li and two anonymous reviewers for their thorough and helpful reviews that greatly improved the manuscript. This study was financially supported by the Strategic Priority Research Program of the Chinese Academy of Sciences (XDA22050103, XDB42020303),
References (79)
- et al.
Trace element fractionation during dehydration of eclogites from high-pressure terranes and the implications for element fluxes in subduction zones
Chem. Geol.
(2000) - et al.
Potassium isotope fractionation during chemical weathering of basalts
Earth Planet. Sci. Lett.
(2020) - et al.
Trace elements and Sr–Nd–Pb isotopes of K-rich, shoshonitic, and calc-alkaline magmatism of the Western Mediterranean Region: genesis of ultrapotassic to calc-alkaline magmatic associations in a post-collisional geodynamic setting
Lithos
(2009) - et al.
Ordovician shoshonitic to ultrapotassic volcanism in the central Norwegian Caledonides: the result of sediment subduction, mantle metasomatism and mantle partial melting
Lithos
(2020) - et al.
Mesozoic intracontinental orogeny in the Qinling Mountains, Central China
Gondwana Res.
(2016) Petrological characterization of the source components of potassic magmas: geochemical and experimental constraints
Lithos
(1992)Vein-plus-wall-rock melting mechanisms in the lithosphere and the origin of potassic alkaline magmas
Lithos
(1992)- et al.
The ultrapotassic rocks: characteristics, classification, and constraints for petrogenetic models
Earth Sci. Rev.
(1987) - et al.
Erosion of lithospheric mantle beneath the East African Rift system: Geochemical evidence from the Kivu volcanic province
- et al.
Subduction of continental crust to mantle depth
Treat. Geochem.
(2014)
High-precision analysis of potassium isotopes by HR-MC-ICPMS
Chem. Geol.
Potassium isotopic evidence for sedimentary input to the mantle source of Lesser Antilles lavas
Geochim. Cosmochim. Acta
Heterogeneous potassium isotopic composition of the upper continental crust
Geochim. Cosmochim. Acta
One view of the geochemistry of subduction-related magmatic arcs, with an emphasis on primitive andesite and lower crust
Treat. Geochem.
Sm–Nd and Rb–Sr isotopic chronology and cooling history of ultrahigh pressure metamorphic rocks and their country rocks at Shuanghe in the Dabie Mountains, Central China
Geochim. Cosmochim. Acta
First-principles investigation of the concentration effect on equilibrium fractionation of K isotopes in feldspars
Geochim. Cosmochim. Acta
First-principles investigation of equilibrium K isotope fractionation among K-bearing minerals
Geochim. Cosmochim. Acta
Recent progresses in plate subduction and element recycling
Solid Earth Sci.
Postcollisional potassic and ultrapotassic rocks in southern Tibet: mantle and crustal origins in response to India–Asia collision and convergence
Geochim. Cosmochim. Acta
Identifying mantle carbonatite metasomatism through Os-Sr-Mg isotopes in Tibetan ultrapotassic rocks
Earth Planet. Sci. Lett.
Potassic volcanic rocks and adakitic intrusions in southern Tibet: INSIGHTS into mantle–crust interaction and mass transfer from Indian plate
Lithos
Extremely light K in subducted low-T altered oceanic crust: Implications for K recycling in subduction zone
Geochim. Cosmochim. Acta
Polyphase inclusions in garnet–orthopyroxenite (Dabie Shan, China) as monitors for metasomatism and fluid-related trace element transfer in subduction zone peridotite
Earth Planet. Sci. Lett.
The chemical composition of subducting sediments
Treat. Geochem.
The depth of the spinel to garnet transition at the peridotite solidus
Earth Planet. Sci. Lett.
Composition of the continental crust
The subduction-zone filter and the impact of recycled materials on the evolution of the mantle
Treat. Geochem.
Low-temperature oceanic crust alteration and the isotopic budgets of potassium and magnesium in seawater
Earth Planet. Sci. Lett.
Incompatible element-rich fluids released by antigorite breakdown in deeply subducted mantle
Earth Planet. Sci. Lett.
Mid-paleozoic collision in the north Qinling: Sm–Nd, Rb–Sr and 40Ar/39Ar ages and their tectonic implications
J. Asian Earth Sci.
Constancy of Nb/U in the mantle revisited
Geochim. Cosmochim. Acta
Tracing subducted oceanic slabs in the mantle by using potassium isotopes
Geochim. Cosmochim. Acta
Potassium isotope fractionation during continental weathering and implications for global K isotopic balance
Geochim. Cosmochim. Acta
Potassium isotope systematics of oceanic basalts
Geochim. Cosmochim. Acta
Potassium isotope fractionation during magmatic differentiation of basalt to rhyolite
Chem. Geol.
An estimate of the bulk silicate earth potassium isotopic composition based on MC-ICPMS measurements of basalts
Geochim. Cosmochim. Acta
Fluid-controlled element transport and mineralization in subduction zones
Solid Earth Sci.
Deep subduction of continental crust in accretionary orogen: evidence from U–Pb dating on diamond-bearing zircons from the Qinling orogen, Central China
Lithos
Early cretaceous lamprophyre dyke swarms in Jiaodong Peninsula, eastern North China Craton, and implications for mantle metasomatism related to subduction
Lithos
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