Phonotephrite and phonolite in the Tarim Large Igneous Province, northwestern China: Petrological, geochemical and isotopic evidence for contrasting mantle sources and deep carbon recycling
Introduction
Large Igneous Provinces (LIPs) are generally dominated by tholeiites with a small amount of alkaline rock series, such as alkali basalt, carbonatite, kimberlite, lamprophyre, etc., which usually occur in the marginal zone or structurally weakly zone of LIPs. In general, these rocks were formed before, after or coevally with the major tholeiitic basalt (Chalapathi-Rao and Lehmann, 2011, Foley et al., 2012, Gibson et al., 1995a, Gibson et al., 1995b, Xu et al., 2001). It has been suggested that alkaline-carbonatite complexes have mantle sources that are different from the tholeiitic basalts (Natali et al., 2017, Trumbull et al., 2003) and the heterogeneity of mantle source may be related to the contribution from recycled subducted slab (Cheng et al., 2017, Hofmann, 1997). Primitive alkaline, silica-undersaturated magmas can form at high pressure from carbonated garnet peridotite sources (Tenner et al., 2012, Till and Withers, 2012), garnet pyroxenite (Hirschmann et al., 2003, Kogiso et al., 2003), hornblendite (Niu and O'Hara, 2003, Pilet et al., 2004, Pilet et al., 2005, Pilet et al., 2008) and mixed sources (Dasgupta and Hirschmann, 2010, Prytulak and Elliott, 2007, Sorbadere et al., 2013, Yaxley, 2000). Alkaline rocks have been regarded as windows to the Earth’s interior (Menzies, 1987), and can be used to understand the mantle processes that include plume-lithosphere interaction, chemical evolution of subcontinental lithospheric mantle (SCLM) and carbon cycle in the deep mantle (Cheng et al., 2018, Hari et al., 2014, Scott et al., 2016, Torabi, 2010, Trumbull et al., 2003, Vrublevskii, 2015).
The Tarim Large Igneous Province (TLIP), located in northwestern China, is characterized by strong alkaline affinity (Cheng et al., 2015, Cheng et al., 2018, Tian et al., 2010, Yu et al., 2011, Zhang and Zou, 2013). Typical outcrops are present in the Wajilitag complex, where aillikite, mafic–ultramafic intrusions, aegirine syenite, carbonatite, nephelinite and alkaline mafic-intermediate dykes occur within less than 10 km2 area (Cheng et al., 2015, Cheng et al., 2017, Cheng et al., 2018, Li et al., 2012, Wei et al., 2014, Zhang et al., 2008, Zhang and Zou, 2013, Zhang et al., 2018; Zou et al., 2015). Previous studies have identified mafic-intermediate dykes in Wajilitag as bimodal dykes that were regarded as a whole simple dike system derived from enriched lithosphere mantle like the mafic–ultramafic intrusions (Cheng et al., 2018, Yu, 2019). However, recent studies show that the Sr-Nd isotopes of these dykes are similar to the depleted mantle (Wei et al., 2014, Zou et al., 2015). In this paper, we report Mg-Sr-Nd isotopic data on phonotephrites and phonolite dykes from the TLIP and it is the first time that phonolitic dikes with similar light Mg isotopic characteristics to carbonatite have been found in this area. The main objective of our study is to unravel the interrelationship between mafic-intermediate dykes and other lithologic components in Wajilitag and thus provide better constraints on the deep geodynamic processes associated with the TLIP.
Section snippets
Geological setting
The Tarim craton in northwestern China with an area of 600000 km2 is composed of a Precambrian crystalline basement and Phanerozoic sedimentary cover (Cheng et al., 2015, Cheng et al., 2018, Zhang and Zou, 2013). The craton is surrounded by Tianshan, Kunlun and Altun orogenic belts (Yang et al., 2013). Due to the far-field influence of the Cenozoic India-Eurasia collision, the Tarim craton developed three uplifts and five depressions which includes the Northern Tarim Uplift, Central Tarim
Petrography
The phonotephrites can be divided into pyroxene-bearing phonotephrite (PP) and kaersutite-bearing phonotephrite (KP), according to their petrographic features. The PP exhibits porphyritic texture with varying contents of phenocrysts (Fig. 3a-c). The phenocrysts are generally dominated by alkali feldspar (30–45 %), plagioclase (10–15 %), clinopyroxene (25–30 %), iron–titanium (Fe-Ti) oxide (10–15 %) and amphibole (3–5 %). The accessory minerals include titanite (~1 %) and apatite (less than
Analytical methods
Eight phonotephrite dykes (five PP and three KP) and two PH dykes are selected for detailed analysis.
Clinopyroxene
Clinopyroxenes are common in the KP, PP and PH, and occur mainly as phenocrysts and matrix microcrystalline phase. In this study, thirty-four clinopyroxene phenocrysts in KP and PP and twenty-nine clinopyroxene phenocrysts in PH were selected for analysis (supplementary data).
The clinopyroxene phenocrysts in PP exhibit restricted compositions of Wo45.25-48.50En36.23-41.56Fs11.01-15.27 falling within the field of diopside (Fig. 4a) and have Mg# ranging from 70.89 to 79.55 with an average value
Alteration effect, crustal contamination and fractional crystallization
Since samples PP, KP and PH have slightly high LOI values (1.32–4.70 wt.% and 1.54–3.78 wt.%, respectively). It is necessary to evaluate the effect of alteration on elements and isotopes. Generally, the alteration can significantly elevate Sr isotopes. PP and KP have relatively high (87Sr/86Sr)t values (0.70487–0.70574 for PP and 0.70516–0.70566 for KP), while there is no linear relationship between (87Sr/86Sr)t and LOI (Fig. 8), which can exclude the influence of alteration for KP and PP. PH
Conclusions
(1) The mineralogical and geochemical analysis of mafic dykes (phonotephrite dykes, KP and PP) and intermediate dykes (phonolite dykes, PH) indicate that they originated from different mantle sources in the Wajilitag complex. The mantle source heterogeneity is mainly reflected in the Mg isotopic characteristics. Phonotephrite dykes show mantle-like δ26Mg value (–0.295 to –0.382 ‰). However, PH is characterized by much lighter δ26Mg value (–0.851 to –0.906 ‰) than mantle value. Our geochemical
CRediT authorship contribution statement
Bowen Wei: Conceptualization, Formal analysis, Investigation, Data curation, Writing - original draft. Zhaochong Zhang: Conceptualization, Writing - review & editing, Funding acquisition. Zhiguo Cheng: Conceptualization, Writing - review & editing. Weiliang Kong: Investigation, Formal analysis. Bingxiang Liu: Investigation, Formal analysis. Xianghui Fei: Writing - review & editing. M. Santosh: Writing - review & editing.
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.
Acknowledgments
We thank Ya Cheng for field research. Financial support for this work was supported by the National Nature Science Foundation of China (42030302, 41772057, 41902048)
Funding.
This work was funded by National Nature Science Foundation of China (42030302, 41772057, 41902048)
Compliance with ethical standards.
Conflict of interest Not applicable.
Availability of data and material
Supplementary data in online resource file.
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