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Melting geodynamics reveals a subduction origin for the Purang ophiolite, Tibet, China

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Abstract

The debate regarding whether the Yarlung–Zangbo ophiolite (YZO) on the south of the Qinghai-Tibet Plateau, formed in a mid-ocean ridge (MOR) or a supra-subduction zone (SSZ) setting has remained unresolved. Here we present petrological, mineralogical, and geochemical data associated with modeling melting geodynamics of the mantle peridotites from the Purang ophiolite in the western segment of the Yarlung–Zangbo Suture Zone (YZSZ) to explore its tectonic environment. The Purang lherzolites are characterized by the protogranular texture and have abyssal-peridotite-like mineral compositions, including low Cr# (20–30) and TiO2 contents (<0.1wt%) in spinel, high Al2O3 (2.9wt% – 4.4wt%) and CaO (1.9wt% – 3.7wt%) contents in orthopyroxene and LREE-depletion in clinopyroxene. Compositions of these lherzolites can be modeled by ~11% dynamic melting of the DMM source with a small fraction of melt (~0.5%) entrapped within the source, a similar melting process to typical abyssal peridotites. The Purang harzburgites are characterized by the porphyroclastic texture and exhibit highly refractory mineral compositions such as high spinel Cr# (40–68), low orthopyroxene Al2O3 (<2.2wt%) and CaO (<1.1wt%) contents. Clinopyroxenes in these harzburgites are enriched in Sr (up to 6.0 ppm) and LREE [(Ce)N = 0.02–0.4], but depleted in Ti (200 ppm, on average) and HREE [(Yb)N < 2]. Importantly, the more depleted samples tend to have higher clinopyroxene Sr and LREE contents. These observations indicate an open-system hydrous melting with a continuous influx of slab fluid at a subduction zone. The modeled results show that these harzburgites could be formed by 19% – 23% hydrous melting with the supply rate of slab fluid at 0.1%–1%. The lower clinopyroxene V/Sc ratios in harzburgites than those in lherzolites suggest a high oxidation stage of the melting system of harzburgites, which is consistent with a hydrous melting environment for these harzburgites. It is therefore concluded that the Purang ophiolite has experienced a transformation of tectonic setting from MOR to SSZ.

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Acknowledgements

We are grateful to the lab technicians, Wen-Qin Zheng, Xiang Li, and Yun Li for their assistance with the electron probe micro-analyses, and Yan-Wen Tang for his help with the LA-ICP-MS analyses and data processing. This research was financially supported by the Second Qinghai-Tibet Plateau Scientific Expedition and Research (SETP) (2019QZKK0806-02) and the National Natural Science Foundation of China (42121003, 42122024) , CAS “Light of West China” Program (xbzg-zdsys-202310), Guizhou Provincial High level Innovation Talent program (GCC[2023]057) and Guizhou Provincial 2021 Science and Technology Subsidies (No. GZ2021SIG).

Funding

This research was supported by the Second Tibetan Plateau Scientific Expedition and Research (SETP) (2019QZKK0806-02), the National Natural Science Foundation of China (42121003, 42122024), CAS “Light of West China” Program (xbzg-zdsys-202310), Guizhou Provincial High level Innovation Talent program (GCC[2023]057) and Guizhou Provincial 2021 Science and Technology Subsidies (No. GZ2021SIG).

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Authors and Affiliations

  1. State Key Laboratory of Ore Deposit Geochemistry, Institute of Geochemistry, Chinese Academy of Sciences, Guiyang, 550081, China

    Tao Ruan, Zhong-Jie Bai, Wei-Guang Zhu & Shi-Ji Zheng

  2. University of Chinese Academy of Sciences, Beijing, 100049, China

    Tao Ruan & Shi-Ji Zheng

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  1. Tao Ruan
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  2. Zhong-Jie Bai
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Tao Ruan: coming up with the theme of this manuscript, conducting analyses to obtain geochemical data, drawing figures and writing this original manuscript; Zhong-Jie Bai: applying for funding, conducting field work and revising manuscript; Wei-Guang Zhu: guiding the experiments and polishing the manuscript; Shi-Ji Zheng: conducting field work and editing the manuscript.

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Correspondence to Zhong-Jie Bai.

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Appendix

Appendix

1.1 The mass balance equations used in modeling melting geodynamics

1.1.1 Fractional and dynamic melting

Fractional melting can effectively extract incompatible trace elements from peridotites into melts and has been used to quantitatively model the melting history of abyssal peridotites in modern oceanic lithospheric mantle (Johnson et al. 1990; Hellebrand et al. 2002; Warren 2016). However, the more realistic geological fact is that some partial melts might not be completely extracted from the source and trapped within the mantle (e.g., Hellebrand et al. 2002; Brunelli et al. 2006), which is defined as dynamic melting (Zou 1998; Shaw 2000). The equations for fractional and dynamic melting are from Zou (1998):

$${{\varvec{C}}}_{{\varvec{r}}{\varvec{e}}{\varvec{s}}}=\frac{{{\varvec{C}}}_{{\varvec{o}}}}{1-{\varvec{X}}}{\left\{1-\frac{{\varvec{X}}[{\varvec{P}}+\boldsymbol{\Phi }(1-{\varvec{P}})}{{{\varvec{D}}}_{0}+\boldsymbol{\Phi }(1-{\varvec{P}})}\right\}}^{1/[\boldsymbol{\Phi }+\left(1-\boldsymbol{\Phi }\right){\varvec{P}}}$$
(1)
$${{\varvec{C}}}_{{\varvec{f}}}=\frac{{{\varvec{C}}}_{{\varvec{r}}{\varvec{e}}{\varvec{s}}}}{{\varvec{\Phi}}\left(1-{\varvec{F}}\right)+({{\varvec{D}}}_{0}-{\varvec{F}}{\varvec{P}})}$$
(2)
$$\overline{{{\varvec{C}} }_{{\varvec{L}}}}=\frac{{{\varvec{C}}}_{0}}{{\varvec{X}}}\left\{1-{[1-\frac{{\varvec{X}}[{\varvec{P}}+{\varvec{\Phi}}(1-{\varvec{P}})}{{{\varvec{D}}}_{0}+{\varvec{\Phi}}\left(1-{\varvec{P}}\right)}]}^{1/[{\varvec{\Phi}}+\left(1-{\varvec{\Phi}}\right)\mathbf{P}}\right\}$$
(3)

where Cres: the concentration of an element in the residual mantle (residual solid + retained melt); C0: the concentration of an element in the initial mantle source (DMM); X: the mass ratio of melt to solid; P: the weighted proportions of minerals entering melt. \({\varvec{P}}=\boldsymbol{ }\sum {{\varvec{p}}}^{{\varvec{i}}}{{\varvec{K}}}_{{\varvec{d}}}\), where \({{\varvec{p}}}^{{\varvec{i}}}\) is the melting mode of mineral phase i, and \({{\varvec{K}}}_{{\varvec{d}}}\) is the mineral/melt partitioning coefficient of an element; Φ: the critical mass porosity, which determines the mass fractions of retained melt in the source. When Φ = 0, these equations are suitable for fractional melting. When F < Φ, no melt is escaped from the source (i.e., X = 0), and the melting mode becomes batch melting. When F > Φ, X = (F-Φ)/(1-Φ); D0: the initial bulk partitioning coefficient of an element. \({{\varvec{D}}}_{0}=\sum {{\varvec{K}}}_{{\varvec{d}}}{{\varvec{X}}}_{0}^{{\varvec{i}}}\), \({{\varvec{X}}}_{0}^{{\varvec{i}}}\) is the initial modal proportion of mineral phase I; Cf: the composition of an element in the last drop of extracted melt in equilibrium with mantle minerals; \(\overline{{{\varvec{C}} }_{{\varvec{L}}}}\): the concentration of an element in the accumulated melt (i.e., the total of extracted melt).

1.1.2 Open-system fluid-influx melting (OSFM)

OSFM has been proposed to quantitatively model the melting history of mantle wedge at subduction zones (Zou 1998; Shaw 2000). The most striking feature of such a melting process is that many incompatible trace element compositions might buffered by the slab-fluid, especially for fluid-mobile elements (e.g., Sr and LREE). Therefore, LREE- and Sr-enriched clinopyroxenes might be expected in this melting process (e.g., Ozawa and Shimizu 1995; Suhr and Edwards 2000; Bizimis et al. 2000; Le Roux et al. 2014). On the other hand, minerals will become more refractory because of higher degrees of melting promoted by the introduction of slab-fluid at subduction zones. Geochemically, the degrees of mineral refractory (such as spinel Cr#) are positively correlated with clinopyroxene LREE and Sr (e.g., Ozawa and Shimizu 1995; Lin et al. 2020).

Generally, OSFM consists of three parts (Shaw 2000): (1) the supply of slab fluid. To simplify the calculation, it is assumed that this fluid is added into the source at a constant rate β (mass fractions, relative to F), which builds a bridge to connect the mantle with slab fluid, (2) the extraction of arc magmas, and (3) the residual source (residual solid and retained melt). After every incremental melting, the newly produced melt will mix with slab fluid to form a mixing melt. Then the mixing melt is distributed between the mantle (small melt was retained in the source) and crust (most melt was expelled to form the crust). The equations for open-system fluid-influx melting are from Shaw (2000):

$${{\varvec{C}}}_{{\varvec{f}}}=\left[{\varvec{A}}-{\varvec{x}}{{\varvec{C}}}_{{\varvec{s}}{\varvec{l}}{\varvec{a}}{\varvec{b}}}\right]\times {[\frac{1-{\varvec{z}}{\varvec{F}}}{1-{\varvec{z}}{\varvec{\Phi}}}]}^{{\varvec{y}}}+{\varvec{x}}{{\varvec{C}}}_{{\varvec{s}}{\varvec{l}}{\varvec{a}}{\varvec{b}}}$$
$${{\varvec{C}}}_{{\varvec{r}}{\varvec{e}}{\varvec{s}}}={{\varvec{C}}}_{{\varvec{f}}}\frac{\boldsymbol{\alpha }+{{\varvec{D}}}_{{\varvec{b}}{\varvec{u}}{\varvec{l}}{\varvec{k}}}}{1+\boldsymbol{\alpha }}$$
$$\overline{{{\varvec{C}} }_{{\varvec{L}}}}=\frac{{{\varvec{C}}}_{0}+{{\varvec{C}}}_{{\varvec{s}}{\varvec{l}}{\varvec{a}}{\varvec{b}}}{\varvec{\beta}}{\varvec{F}}-{{\varvec{C}}}_{{\varvec{s}}}(1-{\varvec{F}})}{{\varvec{F}}(1+{\varvec{\beta}})}$$
$$\mathbf{A}=\frac{{{\varvec{C}}}_{0}+{\varvec{\Phi}}{\varvec{\beta}}{{\varvec{C}}}_{{\varvec{s}}{\varvec{l}}{\varvec{a}}{\varvec{b}}}}{{{\varvec{D}}}_{0}+{\varvec{\Phi}}\left(1+{\varvec{\beta}}\right)\left(1-{\varvec{P}}\right)};\boldsymbol{ }{\varvec{x}}=\frac{{\varvec{\beta}}}{\left(1+{\varvec{\beta}}\right)\left(1-{\varvec{P}}\right)};\boldsymbol{ }{\varvec{y}}=\frac{\left(1+{\varvec{\beta}}\right)\left(1-{\varvec{P}}\right)}{\boldsymbol{\alpha }+{\varvec{P}}\left(1-{\varvec{\beta}}\right)};\boldsymbol{ }{\varvec{z}}=\frac{\boldsymbol{\alpha }+{\varvec{P}}(1-{\varvec{\beta}})}{{{\varvec{D}}}_{0}+\boldsymbol{\alpha }}$$

where, β: the supply rate of continuous influx of slab fluid, relative to F; \({{\varvec{C}}}_{{\varvec{s}}{\varvec{l}}{\varvec{a}}{\varvec{b}}}\): the concentration of an element in slab fluid; α: the mass ratio of mixing melt (partial melt + slab fluid) to residual solid. \(\boldsymbol{\alpha }=\frac{\boldsymbol{\Phi }(1+{\varvec{\beta}})}{1-\boldsymbol{\Phi }}\); \({{\varvec{D}}}_{{\varvec{b}}{\varvec{u}}{\varvec{l}}{\varvec{k}}}\): the bulk partitioning coefficient of an element when the melting degree is at F.

$${{\varvec{D}}}_{{\varvec{b}}{\varvec{u}}{\varvec{l}}{\varvec{k}}}=\frac{{{\varvec{D}}}_{0}-{\varvec{F}}{\varvec{P}}(1+{\varvec{\beta}})}{1-{\varvec{F}}}=\frac{{{\varvec{C}}}_{{\varvec{s}}}}{{{\varvec{C}}}_{{\varvec{f}}}}.$$

Other parameters are noted in Sect. 1.

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Ruan, T., Bai, ZJ., Zhu, WG. et al. Melting geodynamics reveals a subduction origin for the Purang ophiolite, Tibet, China. Acta Geochim (2024). https://doi.org/10.1007/s11631-024-00687-1

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