Origin of the Early Permian zircons in Keping basalts and magma evolution of the Tarim Large Igneous Province (northwestern China)

Highlights

• The Early Permian zircons in Keping basalts are most likely to be xenocrysts.

• These zircons were probably from coeval igneous rocks in the South Tianshan Orogen.

• Geochemical modeling suggests variable crustal contamination of the Tarim CFBs.

• This modeling also indicates source isotopic heterogeneity of the Tarim LIP.

• It may correlate with plume–lithosphere interaction in generation of the Tarim LIP.

Abstract

The Tarim continental flood basalts (CFBs) provide important clues about the genesis and magmatic evolution of the Early Permian Tarim Large Igneous Province (Tarim LIP) in northwestern China. Here we present results of LA–MC–ICPMS Lu–Hf isotope analysis on Early Permian (ca. 290 Ma) zircons extracted from the Tarim CFBs in the Keping area, northwest of the Tarim Basin. Zircons from two sub-groups of Keping basalts (Groups 1a and 1b) have similar Lu–Hf isotopic compositions and exhibit a relatively large range of ¹⁷⁶Hf/¹⁷⁷Hf ratios between 0.282422 and 0.282568. Their negative ε_Hf(t) values (− 6.8–− 1.4) are generally lower than the whole-rock ε_Hf(t) values of their host basalts (− 2.8–2.1), and are distinct from other known intrusive rocks (− 0.3–7.1) in the Tarim LIP and their hosted zircons (4.9–8.8). Systematic studies of Hf isotopic data from Tarim and its adjacent regions reveal that these zircons are probably xenocrysts, sourced from coeval igneous rocks in the South Tianshan Orogen (e.g., the Lower Permian Xiaotikanlike Formation volcanic and pyroclastic rock suite). This, together with the presence of Precambrian zircons in Keping basalts, clearly indicates crustal contamination during their eruptions and provides hints about the potential contaminant sources. Geochemical modeling further suggests that the earlier erupted Group 1b basalts experienced more contamination, predominantly by some high Th–U–Pb rock components, most likely from the South Tianshan Orogen. The later erupted Group 1a basalts in the Keping area have been less contaminated with mainly the Tarim Precambrian rocks. Another group of the Tarim CFBs in the Northern Tarim Uplift (Group 2) appears to have undergone negligible crustal contamination but possesses evidence for variable source compositions. The modeling also indicates that the uncontaminated parental magmas of various Tarim LIP rocks (from the picrites and basalts to ultramafic–mafic and syenitic intrusive rocks) exhibit a wide range of ε_Nd(t) values (ca. − 5–5), reflecting source isotopic heterogeneity, which may be a consequence of plume–lithosphere interaction during the generation of the Tarim LIP.

Introduction

Large Igneous Provinces (LIPs) are large volumes of mainly mafic rocks emplaced within a relatively short geological timescale (Bryan and Ernst, 2008, Coffin and Eldholm, 1994). Since the late 1980s, an increasing number of LIPs, occurred in different epochs around the world, have been recognized and investigated to understand their geodynamic evolution, environmental effects and economic implications (e.g., Bryan and Ferrari, 2013, Ernst et al., 2005). The Early Permian Tarim LIP in the Tarim Block of northwestern China (Fig. 1; Li et al., 2011, Li et al., 2012, Yang et al., 2006a, Yang et al., 2013) has aroused considerable interest among geologists for petrogenetic and tectonic aspects as well as for oil–gas and mineral resources (e.g., Cao et al., 2014, Chen et al., 2006, de Boorder, 2014, Li et al., 2012a, Li et al., 2012b, Li et al., 2013, Pirajno, 2013, Qin et al., 2011, Xu et al., 2006, Yu et al., 2011, Yu et al., 2012, Yuan et al., 2012, Zhang et al., 2008, Zou et al., 2013). It comprises a diverse range of volcanic and intrusive rocks from ultramafic–mafic to felsic compositions (Liu et al., 2014a, Li et al., 2008, Li et al., 2010, Shangguan et al., 2011, Shangguan et al., 2012, Tian et al., 2010, Yang et al., 2006b, Yang et al., 2007a, Yang et al., 2007b, Zhang et al., 2010b, Zhang et al., 2010a, Zhang et al., 2013a), and is widely regarded as being genetically linked to a mantle plume (e.g., Dobretsov et al., 2010, Li et al., 2012, Liu et al., 2013, Pirajno et al., 2009, Su et al., 2011, Wei et al., 2014, Xu et al., 2013a, Xu et al., 2014, Yang et al., 2013, Zhang et al., 2012a, Zhang et al., 2013b, Zhou et al., 2009). The general temporal–spatial relations and geochemical trend of various igneous rock units in the Tarim LIP, and its adjacent areas, have now been established (Li et al., 2011, Qin et al., 2011, Xu et al., 2014, Yang et al., 2013, Zhang and Zou, 2013a, Zhang and Zou, 2013b). However, their genetic associations and source region characteristics, as well as the overall magmatic evolution of the Tarim LIP still need further investigation.

Zircon (ZrSiO₄) is a common accessory mineral in many igneous rocks (Belousova et al., 2002, Hoskin and Schaltegger, 2003, Poldervaart, 1956) and has received considerable attention in recent years as a potential tool to interpret the crust and mantle evolution history of the Earth (e.g., Belousova et al., 2010, Griffin et al., 2000, Harley and Kelly, 2007). It is notable that the Early Permian Tarim basalts in the Keping area (see Fig. 1 for the location) contain zircons, some of which exhibit features typical of a magmatic origin. SHRIMP (sensitive high resolution ion microprobe) and LA-ICPMS (laser ablation-inductively coupled plasma mass spectrometry) U–Pb dating on those magmatic zircons yielded similar ages of ca. 290 Ma (Yu et al., 2011, Zhang et al., 2012a), coincident with the formation ages of their host basalts (Li et al., 2011, Wei et al., 2014, Xu et al., 2013a, Xu et al., 2014, Yang et al., 2013). The Early Permian zircons found in Keping basalts seem to have preserved a record of the Tarim LIP event, but their precise origin has not yet been well explored. Although several studies have suggested that zircon could crystallize from basaltic magmas (e.g., Hurai et al., 2010, Luo et al., 2006, Tietz and Büchner, 2007), most of the crystals observed in the basalts are regarded as xenocrysts, derived from either crustal or mantle rocks (e.g., Belousova et al., 2002, Guo et al., 1996, Qiu et al., 2005, Siebel et al., 2009).

In this contribution, we analyzed the Lu–Hf isotopic compositions of the Early Permian zircons dated by Yu et al. (2011). The results coupled with a systematic comparison of Hf isotopic data from different rocks in the Tarim LIP and its adjacent Chinese Tianshan regions provide robust constraints on the zircon genesis, shedding further light on some of the important issues related to the magmatic evolution and petrogenesis of basalts and other mantle-derived igneous rocks in the Tarim LIP.