Late Paleozoic Mantle Source Nature of Tianshan Orogen, Northwest China: Evidence from the Geochemistry, Zircon U‐Pb Dating, Hf and Whole Rock Sr‐Nd‐Pb Isotopes of the Mafic Dykes
Authors: ZHANG Chengli , XU Xueyi , WANG Hongliang , MA Zhongping , GAO Xiaofeng , CHEN Junlu 
Published: November 2020

The Tianshan Orogen (TO) is one of the largest typical accretionary orogenic belts in the world. Of which, the late Paleozoic was a critical era to understand the tectonic and geodynamic transition from accretion to collision. However, the late Paleozoic tectonic evolutionary history, especially for the time of the ocean‐continent transition, is still debated although the origin and tectonic settings for the Paleozoic volcanic, felsic igneous magmatism in TO and reginal geology have been done in the last decades. In contrast, the researches on the mafic dykes in TO was not systematically carried out till now. Reginal‐scale mafic dykes are commonly regarded as the products created in a extensional setting, and used to identify the major tectonic events such as rifting and continental break‐up and further trace the mantle natures and geodynamic mechanism (Halls, 1982; Bleeker and Ernst, 2006; Li et al., 2008; Ernst et al., 2010; Srivastava, 2011; Hou, 2012; Peng, 2015; Peng et al., 2019). There are widespread late Paleozoic mafic dykes beside the huge of intermediate‐acid igneous rocks in the TO, being an idea object to reveal the extensional events, tectonic evolution and the mantle nature and geodynamic processes. We present the ICP‐MS in situ zircon U–Pb dating, Lu‐Hf and whole‐rock Sr‐Nd isotopes as well as the geochemistry data for these mafic dykes to better constraint their petrogenesis and mantle nature. New zircon U‐Pb dating for 12 samples from the representative basic dykes and basalts yield three distinct stages of ∼332 Ma, 316–302 Ma and 288–282 Ma, respectively. In which, the first stage of mafic dykes is mainly occurred in both East Tianshan Orogen (ETO) and West Tianshan Orogen (WTO), and composed of dolerite with minor basalts. The second stage of mafic dyke also can be found in both ETO and WTO. However, in contrast to the first stage of mafic dykes, they have relatively variable rock types from the dolerite/or gabbros to gabbroic diorite. The third stage of mafic dykes are slightly intermediate in composition, and chiefly consist of andesitic‐basaltic dolerite with some diorites. They are widely developed not only in both ETO and WTO, but also in the Beishan area to the east of the ETO, indicating a large‐scale mafic magmatism in Tianshan and adjacent areas.

Geochemically, first stage of mafic dykes is characterized by low SiO2 content of 47.23–47.09 wt% in ETO and 48.69–51.31 wt% in WTO with the variable Mg# values varying from 48.24 to 60.8 in ETO and from 54.26 to 59.14 in WTO. They evolved towards Fe‐depletion along the calc‐alkaline trend. Some of dykes display a slightly negative Sr anomalies and minor negative Eu anomalies (δEu=0.84–0.96), indicating a calc‐alkaline series with a the clinopyroxene‐ and plagioclase‐dominated fractionation. While, those sharing the positive Eu and Sr anomalies are most likely indicative of some degree of plagioclase accumulation. All mafic dykes have relative higher and variable LREE {(La/Yb)N=1.61–9.39} as well as LILEs (Rb, Ba and K) and depletion in HFSE with evident peak of Pb and valley of Nb. They show coherent zircon Hf and whole rock Sr‐Nd‐Pb isotopic composition, with a low (206Pb/204Pb)i (18.79) and variable (87Sr/86Sr)i (0.7033–0.7060) and positive variable ∊Nd(t) from 3.69 to 10.14 and ∊Hf(t) from 3.24 to 13.09. In addition, these mafic dykes have high Zr/Nb (13.6–48.6, average 30.9), and low La/Nb (2.1–5.1, average 3.2) ratios, showing a similarity to those of MORB mantle melts (Zr/Nb=30, La/Nb=1.07) (Sun and McDonough, 1989) rather than OIB asthenospheric mantle melts (Zr/Nb=5.3, La/Nd=0.77, Sun and McDonough, 1989). Their higher Zr/Y (2.5–6.8, average 4.0) and Nb/Y (0.1–0.5, average 0.2) ratios further suggest that they were derived from partial melting of a depleted lithospheric mantle (Bradshaw and Eugene, 1993; Condie, 2005). However, their Nb/La (0.20–0.49, average 0.35), Nb/U (3.3–17.8, average 8.6) and Ce/Pb (1.1–11.8, average 6.84) ratios are very low, much close to those of the continental crust (Nb/La=0.4, Nb/U=6.15, Ce/Pb=3.9, Rudnick and Gao, 2003), indicating crustal contamination (Xia and Li, 2014). All geochemistry and isotope composition favor that they were derived from a heterogeneous mantle source mixed by both depleted lithosphere mantle and EMI, and then experienced crustal contamination during the magma ascending.

Second stage of mafic dykes have variable SiO2 content of 46.71–54.92 wt% in ETO and 48.53–52.59 wt% in WTO and variable Mg# values from 45.94 to 55.74 in ETO and from 27.04 to 53.64 in WTO and FeOT/MgO values higher than 1 (1.75–2.33 in ETO, 1.71–5.35 in WTO), showing a calc‐alkaline series in ETO and tholeiitic series in WTO associated with a certain extent magmatic evolution. However, many of the chemical and Sr‐Nd‐Hf isotopic features are in accord with those of first stage of mafic dykes. They are characterized by the right deviation REE pattern with slightly high LREE, moderate fraction between LREE and HREE {(La/Yb)N =2.55–7.52} and Eu (δEu =0.85–1.10) anomalies as well as the enrichment in LILEs (Rb, Ba and K) and poor HFSEs with obvious depletion of Nb and peak of Pb. Furthermore, they, just like first stage mafic dyke, have relatively depleted zircon Hf and whole rock Nd isotopic composition with a higher ∊Nd(t) (2.68–9.73) and ∊Hf(t) (2.55–15.46) but variable (87Sr/86Sr)i of 0.7036–0.7157 and high (206Pb/204Pb)i of 19.26‐19.78. They also display a higher Zr/Nb (10.3‐50.7, average 22.3), La/Nb (1.1‐3.7, average 2.51), Zr/Y (2.4–10.3, average 5.3) and Nb/Y (0.05–0.5, average 0.3) as well as low Nb/La (0.27–0.91, average 0.46), slightly high Nb/U (4.59–26.80, average 13.28) and Ce/Pb (2.21–16.94, average 7.71) ratios. All geochemistry and isotopic composition suggest they have a mantle source similar to those of first stage of mafic dykes, and originated from partial melting of a heterogeneous mixing source composed of the depleted lithosphere mantle and EMII.

The third stage of mafic dykes, like the second stage of mafic dyke, have variable SiO2 ranging from 46.39 to 52.69 wt% in ETO and 49.62 to 54.68 wt% in Beishan area, and belong to a calc‐alkaline series. They have moderate Mg# values from 41.86 to 55.24 in ETO and from 41.7 to 55.28 in Beishan area, and exhibit negative Sr anomalies and medium negative or slight positive Eu anomalies (Eu/Eu=0.56–1.08), indicating plagioclase‐dominated fractionation during magma evolution. They are enriched in LILEs and LREE {(La/Yb)N = 3.16–6.38} and depleted in HFSEs with the valleys of Nb and Ti and a peak of Pb. They have narrow (87Sr/86Sr)i (0.7040 to 0.7050) and low (206Pb/204Pb)i (18.41). Their eNd(t) values are positive and variable, ranging from 5.36 to 10.36. While, their eHf(t) range from −7.13 to 18.33. Furthermore, like early mafic dykes, they are also characterized by the higher Zr/Nb (13.0‐29.8, average 20.57), La/Nb (1.0‐3.4, average 2.35), Zr/Y (3.2–6.8, average 4.8) and Nb/Y (0.1–0.4, average 0.3) as well as low Nb/La (0.29–1.00, average 0.50), Nb/U (3.21–17.53, average 7.92) and Ce/Pb (1.73–10.79, average 6.75) ratios. All suggest they were derived from a source inherited from early heterogeneous depleted lithosphere mantle involved in EMI and subjected crustal contamination during magma ascending.

In summary, the late Paleozoic mafic dykes widely developed in the TO can be subdivided three distinct stages of mafic magmatism, and shear the consistent rock associations, petrogeochemistry and isotopic compositions. Three stages of mafic dykes are relatively enriched in LREE, LILEs and depleted in HFSEs with the valleys of Nb and Ti and a peak of Pb and slightly fractionation between LREE and HREE. They are coherently characterized by positive and variable ∊Nd(t) and ∊Hf(t) values, low (206Pb/204Pb)i and variable (87Sr/86Sr)i isotope ratios. Furthermore, they have higher Zr/Nb, La/Nb, Zr/Y and Nb/Y and low Nb/La, Nb/U and Ce/Pb ratios. All suggest that these late Paleozoic mafic dykes were mainly originated from a long‐lived unitary heterogeneous lithospheric mantle mixed by EMI and possible minor MEII and experienced varying degrees crustal contamination during the magma ascending. Consequently, based our new geochemical and Sr‐Nd‐Pb‐Hf isotopic data and combined with the previous studies on the reginal geology and widespread late Paleozoic post‐collisional magmatism (Xu et al., 2013; Xia et al., 2004, 2013), it can be proposed that the arc‐microcontinents collision in the TO started at least before the early late Carboniferous (Serpukhovian), and the mafic dykes with three distinct stages of ∼332 Ma, 316–302 Ma and 288–282 Ma were generated in a lithosphere extensional settings after arc‐microcontinents collision from late Carboniferous to early Permian.