October 2020 LIP of the Month

Tarim large igneous province: definition and petrogenesis

Zhaochong Zhang, Zhiguo Cheng, Dongyang Zhang, Changhong Wang

State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Beijing, 100083, China

Lead author email: zczhang@cugb.edu.cn

This LIP of the month is based on the following:

Introduction

In terms of the proportion of felsic rocks, large igneous provinces (LIPs) have been divided into mafic and silicic types with the latter generally referred to as a SLIP (Bryan & Ernst, 2008; Ernst, 2014). They represent two end members in the LIP clan, whereas transitional LIPs with similar proportions of mafic and felsic rocks are largely missing (Bryan et al. 2002). The mafic LIP that is dominated by mafic rocks with minor felsic rocks (<10 %) is generally correlated to mantle plume (Bryan, 2007; Bryan & Ernst, 2008; Bryan et al., 2002; Shellnutt et al., 2012), while the origin of the voluminous felsic rocks in silicic LIPs is still debated. It has been suggested that the large volumes of felsic magma could be produced by partial melting of a highly fertile hydrous crust by the underplating basaltic magma, the underplate portion of a LIP (e.g. Bryan et al., 2002; Ernst, 2014). However, cases in which a mafic LIP shows a close temporal and spatial relationship with a silicic LIP (SLIP) have not been confirmed, and hence this linkage still lacks crucial geological evidence.

Located in northwestern China, the Tarim large igneous province (T-LIP) is considered as the second major LIP after the Emeishan LIP in China (Fig. 1a). Although the region is largely covered by the desert, drill-hole data and seismic profiles have revealed a large area of felsic volcanic rocks (~48,000 km²) and approximately 265,000 km² of basalts in the T-LIP (Fig. 1b; Liu et al., 2014; Pan et al., 2013; Xu et al., 2014; Yang et al., 2014). Due to the high proportion of the felsic rocks, T-LIP was recently proposed as a transitional LIP between the mafic and silicic LIP types (Cheng et al., 2020). In addition, T-LIP also displays several special characteristics such as complex lithology, long duration and alkaline and iron-rich affinity of the dominated lithology that are distinct from other typical mafic LIPs. The scientific issue is the genesis of T-LIP having these specific features. In the past decade, our group systematically conducted geochronological, petrologic, geochemical and isotopic research (especially non-traditional Mg isotopes) on various igneous rocks. In this contribution, we provide a summary of our recent publications on the T-LIP with a view to understand the formation of this special LIP and then build a close linkage between these rocks and deep geodynamic processes.

Tarim large igneous province

Tarim basin in northwestern China is surrounded by Tianshan Mountain to the north and Kunlun Mountain to the South (Fig. 1a). The Tarim basin has a Late Archean to Proterozoic basement overlain by a thick Phanerozoic cover. Voluminous Early Permian basalts, mafic-ultramafic layered intrusions and mafic dykes are widespread in the Tarim basin and constitute the T-LIP (Xu et al., 2014; Yang et al., 2014). Compared with other LIPs, the T-LIP has four specific characteristics: 1) Complex lithology: in addition to flood basalts, mafic-ultramafic layered intrusions, mafic dykes, rhyolite and picrite, which are common in LIPs, there are also some special rocks that are only associated with some LIPs, such as aillikite (which was previously regarded as kimberlitic rocks in Zhang et al. (2013) and Cheng et al. (2014, 2018a), but recently was identified as aillikite based on the detailed petrologic study by our group (Wang et al., 2020)), nephelinite (Cheng et al., 2015), carbonatite (Cheng et al., 2017, 2018b), sodic lamprophyre, nepheline syenite and diverse dykes in the mafic-felsic composition. The link between carbonatites and kimberlites and some LIPs are discussed in Ernst & Bell (2010) and Ernst et al. (2018). 2) While many LIP, but not all, are emplaced in a very short time of a few million years or less (Kasbohm et al., 2021; Ernst, 2014), the T-LIP has a longer duration of about 30 Ma, and consists of four pulses: a) 300 Ma aillikite, 290-285 Ma bimodal volcanic rocks including the flood basalt and rhyolite, b) 285.9-275 Ma mafic-ultramafic layered intrusions and c) ~270 Ma later various dykes (including mafic and felsic dykes and carbonatite dykes) and d) nephelinite (~268 Ma) (Zhang et al., 2013, 2016; Cheng et al., 2014; Xu et al., 2014, 2017; Shangguan et al., 2016). The flood basalts are the dominate rocks in T-LIP by volume. 3) The dominant lithology is characterized by alkaline and iron-rich affinity: According to published data from basalts and picrites, most of compositions plot in the alkaline fields and show high TFeO contents (>12.5 % and up to 18.7 wt.%; e.g., Zhou et al., 2009; Tian et al., 2010). 4) There is also a high proportion of felsic rocks: As noted above, the felsic rocks in T-LIP could occupy an area of 48,000 km² (up to 20% of the total volume) and indicate that the T-LIP could be termed as a transitional LIP between mafic and silicic LIPs (Cheng et al., 2020).

Petrogenesis of the mafic rocks

Geochemical compositions show that the basalts are characterized by enrichment in large ion lithophile elements and light rare earth elements, which are similar to OIBs. Furthermore, two categories of flood basalts were identified in the T-LIP: Group 1 basalts that are the dominant type display lithospheric mantle signatures with weak negative Nb-Ta anomalies, high (⁸⁷Sr/⁸⁶Sr)_i and low ¹⁴³Nd/¹⁴³Nd ratios; Group 2 basalts from NTU show positive Nb-Ta anomalies, lower (⁸⁷Sr/⁸⁶Sr)_i and higher ¹⁴³Nd/¹⁴³Nd ratios, suggesting a possible asthenospheric origin with potential interaction with the lithosphere (Yu et al., 2017).

Compared to the early flood basalts, the later mafic-ultramafic layered intrusions exhibit positive Nb-Ta anomalies and depleted Sr-Nd-Hf isotopic compositions, and were hence suggested to be the product of the mantle plume-derived magmas by fractional crystallization (Xu et al., 2014; Wei et al., 2014). However, zircon SIMS oxygen isotopes measured in situ are still more enriched than those of the depleted mantle, indicating a lithospheric mantle source for the mafic-ultramafic layered intrusions (Zhang et al., 2016, 2018). Based on the relationship between the Sr-Nd-O-Mg isotopes and the partial melting degrees, Cheng et al. (2018a) proposed that both the Group-1 basalts and mafic-ultramafic layered intrusions should have originated from the lithospheric mantle, while the enriched components in the mantle sources were waning over time from basalts (290-285 Ma) through layered intrusions (285.9-275 Ma) to the later mafic dykes (~270 Ma) (Fig. 2).

Notably, the basalts (including the picrite) in T-LIP are typically characterized by high alkali and iron contents (most of TFeO contents >12.5 wt.% and up to 18.7 wt.%) and hence could be termed ferropicrite/ferrobasalts (Cheng et al., 2018a). On the other hand, based on the reconstruction, the primitive magma of the Wajilitage and Puchang mafic-ultramafic intrusions are suggested to be ferropicrite in composition (Zhang et al., 2016, 2018). In addition, the iron-rich feature is also displayed by the aillikite and nephelinite (Cheng et al., 2015). The generation of the iron-rich rocks is suggested to be related to the involvement of recycled pyroxenite/eclogite components in the mantle sources (Zhang et al., 2016, 2018; Cheng et al., 2018a). Systematic Mg isotopic studies on the major rocks of T-LIP (including basalts, mafic-ultramafic layered intrusions, mafic dykes, aillikite, nephelinite and carbonatite) revealed lighter Mg isotopic compositions (<0.25‰) than the normal mantle (Fig. 3), indicating the contribution of ancient recycled sedimentary carbonate rocks in their mantle sources (Cheng et al., 2015, 2017, 2018a). The carbonated mantle sources and the involved pyroxenite/eclogite components are considered to be the main controlling factors for the generation of the alkaline and iron-rich mafic rocks (Fig. 4a). Furthermore, according to the Sr-Mg isotopic modeling, two distinct mantle domains are recognized for T-LIP: 1) a lithospheric mantle source for the basalts and mafic-ultramafic layered intrusions which were modified by the calcite/dolomite and eclogite-derived high-Si melts, as evidenced by enriched Sr-Nd-O and light Mg isotopic compositions; 2) a plume source for carbonatite, nephelinite and aillikite which were related to the magnesite or periclase/perovskite involvement as reflected by the depleted Sr-Nd-O and extremely light Mg isotopes (Fig. 4b; Cheng et al., 2018a).

Petrogenesis of the felsic rocks

The geochemical studies show that felsic rocks in the T-LIP are commonly characterized by two distinct groups, i.e. one crustal-like group with negative Nb-Ta anomalies, crustal La/Nb, Ce/Pb and Nb/U ratios and radioactive Sr-Nd-Hf isotopic compositions, and the other one is a mantle-like group with positive Nb-Ta anomalies, chondritic La/Nb and Nb/U ratios and depleted Sr-Nd-Hf isotopic compositions, which were suggested to be related to crustal anatexis and fractional crystallization coupled with crustal assimilation, respectively (Liu et al., 2014; Tian et al., 2010; Cheng et al., 2020). Cheng et al. (2020) further noted that the crustal-like group has ε_Nd values that are much lower than those of the Tarim basement, indicating the involvement of mantle-derived magma, and hence they proposed a MASH model to explain the formation of the crustal-like felsic rocks (Fig. 5). It is suggested that a key factor to yield the large volumes of felsic magma is the prior formation of a highly fertile, hydrous crust (Bryan et al., 2002). However, the high-water contents in felsic suites associated with LIPs still lack evidence. In order to evaluate the role of hydrous crust in the formation of felsic rocks in LIPs, we used the updated plagioclase-liquid hydrometer proposed by Waters & Lange (2015) to estimate the water contents for the felsic rocks in T-LIP and also some other mafic LIPs (NAIP, Madagascar, Columbia River, Paraná-Etendeka and Karoo) and a silicic LIP (Sierra Madre Occidental) to make a comparison. The results show that the H₂O contents of felsic rocks from silicic LIP are significantly higher than those of the mafic LIPs, while the felsic rocks in mafic LIPs could display both low and high H₂O contents (Fig. 6a; Cheng et al., 2020). In addition, the water contents of the felsic rocks in mafic LIPs display a positive relationship with the (⁸⁷Sr/⁸⁶Sr)_i values, indicating the contribution of the hydrous crust (Fig. 6b).

Subducted slabs-lithosphere-mantle plume interaction model for T-LIP

Based on the discussion above, two distinct mantle domains were recognized in T-LIP, which were derived from different metasomatized processes (Fig. 7). The mantle sources for the basalts and mafic-ultramafic layered intrusions were proposed to be modified by the calcite/dolomite and eclogite-derived high-Si melts, while the sources for the carbonatite, nephelinite and aillikite units were related to the magnesite or periclase/perovskite metasomatism. With the progressive partial melting of the mantle, the components with enriched Sr-Nd-O isotopes and light Mg isotopes were waning over time. These observations lead us propose a subducted slab-lithosphere-mantle plume interaction model to explain the generation of T-LIP (Fig. 7).

In summary, the carbonated mantle sources and the involved pyroxenite/eclogite components are responsible for the alkaline and iron-rich affinity of the dominant rocks. Different degrees of partial melting of the distinct mantle sources could lead to the generation of the complex lithology of the T-LIP. On the other hand, the recycled carbonated eclogite in the mantle plume could also account for the long duration, which is inferred from the numerical modeling by Cordery et al. (1997) and Lin & Kekin (2005) that eclogite entrained in plumes could contribute to multiple episodes of LIPs and extend the plume-related melting to a much longer period of up to ~50 Ma. On the other hand, while the MASH and AFC model could explain the crustal-like and mantle-like felsic rocks, our study revealed hydrous crustal components could significantly contribute to producing the voluminous felsic rocks in LIPs.

Acknowledgement

The authors greatly appreciate R.E. Ernst for encouragement and the invitation to write this article for largeigneousprovinces.org.

The research was carried out in the China University of Geosciences, Beijing, with the financial support of National Natural Science Foundation of China (41472060, 41772057 and 41702064), China Postdoctoral Science Foundation funded project (2017M610104) and 111 Project (B07011).

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