Elsevier

Lithos

Volumes 396–397, September 2021, 106202
Lithos

Research Article
Contributions of slab-derived fluids to ultrapotassic rocks indicated by K isotopes

https://doi.org/10.1016/j.lithos.2021.106202Get rights and content

Highlights

  • Baoji ultrapotassic rocks have isotopically heavier K than the depleted mantle.

  • Baoji ultrapotassic rocks were derived from an amphibole lherzolite mantle source.

  • The mantle source was modified by slab-derived fluids or supercritical fluids.

  • Potassium isotope system is a powerful tracer of crustal-mantle interactions.

Abstract

Mantle-derived ultrapotassic rocks (K2O/Na2O > 2, K2O > 3 wt%, and MgO > 3 wt%) are crucial to reveal crust-mantle interactions. Here, we present petrographic compositions, whole-rock geochemical and potassium (K) isotopic compositions of Baoji ultrapotassic rocks from North Qinling Orogen, China, to trace their origin. The high Ba/La, Ba/Th, Ba/Rb and low Rb/Sr ratios of the investigated ultrapotassic rocks, together with partial melting modelling, reveal a garnet-facies amphibole lherzolite mantle source modified by slab-derived fluids. In particular, the δ41K values of the ultrapotassic rocks (−0.57‰ to −0.06‰) are higher than that of the depleted mantle and display positive correlations with K/Th, Ba/Th and Ba/Rb ratios, which indicate that slab-derived fluids have contributed heavy K isotopic signatures to the mantle source from which the ultrapotassic rocks were derived. Collectively, the data demonstrate that slab-derived fluids, or supercritical fluids play important roles in the generation of post-collisional ultrapotassic rocks and K isotopes have the ability to trace the deep processes in subduction zones.

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),

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