Petrological and geochemical characteristics of mafic rocks from the Neoproterozoic Sugetbrak Formation in the northwestern Tarim Block, China

1.   Introduction

Mafic dyke swarms record critical information regarding the timing and processes of continental break-ups, because they can be precisely dated, yield information on crustal thickness and mantle sources (Li ZX et al., 1996, 1999, 2003, 2008; Li XH et al., 2003, 2006; Veevers JJ et al., 1997; Wingate JA et al., 1998; Karlstrom KE et al., 2000; Li ZX and Powell CM, 2001; Direen N and Crawford A, 2003). Some researchers have noticed that a large number of mafic dyke swarms associated with the Rodinia supercontinent breakup developed in the Aksu area, northwest margin of Tarim Block (Chen Y et al., 2004; Zhang CL et al., 2004, 2006, 2007, 2009, 2011; Zhang ZY et al., 2009; Zhu WB et al., 2008, 2011; Xu B et al., 2005). The mafic rocks are distributed on different sections, almost all over the different layers of the Sugetbrak Formation. However, the widespread Neoproterozoic mafic dykes have still not been well studied. The formation ages of these mafic rocks were not well constrained, and the origin of the mafic rocks exposed in different regions or different sections in the same region and their genetic relationship are not clear, either.

In this study, the authors focus on the petrology, geochemistry, and Sr-Nd-Pb isotope geochemistry of the mafic rocks exposed in the Aksu-Wushi and Yuermeinark areas along the northwest margin of the Tarim Block and provided new constraints on the petrogenesis and their genetic relationship.

2.   Geological setting and previous works

2.1   Neoproterozoic strata in the Aksu area of the northwest Tarim Block

The Tarim Block is bound by the Tianshan Orogens to the south and Tibet Plateau to the north, covering an area of more than 0.6×10⁶ km² (Fig. 1b). The Tarim Block is a major tectonic constituent of the Central Asian collage (Fig. 1a), and it plays an important role in global tectonic framework. It is characterized by a double-layered structure, which consists of Precambrian basement and Neoproterozoic to Phanerozoic sedimentary cover sequences. The Precambrian basement rocks in the Tarim Block are mostly exposed along the northern (the Aksu and Quruqtagh areas), southwestern (the Tikelike area) and eastern (the Alytn Tagh Mountain and Dunhuang areas) margins (Fig. 1b; Lu YZ et al., 2008; Zhang CL et al., 2013; He JY et al., 2019).

Figure  1.  Geological map of Tarim Block. a–Sketch map of China showing location of the Tarim Block in China. NCB: North China Block; SCB: South China Block (modified from Lu YZ et al., 2018, China basemap after China National Bureau of Surveying and Mapping Geographical Information); b–simplified geological map of the Tarim Block showing the distribution of Precambrian-Mesozoic rocks and the location of the Aksu-Wushi area (modified from Lu YZ et al., 2018); c–simplified geological map of the Aksu-Wushi area (modified from XBGMR, 1957; Lu YZ et al., 2018). Previous geochronological data of Precambrian mafic rocks and locations of samples and Fis 4a, b are indicated. Green line: mafic dykes.

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In the Aksu area, which is located in the northwestern part of the Tarim Block (Fig. 1b), continuous outcrops of both the metamorphic basement and the Upper Neoproterozoic to Early Paleozoic successions are well exposed, and the strata were successively divided from bottom to top in the following order: the metamorphic rocks of the exposed basement of the Aksu Group, Neoproterozoic Qiaoenbrak Group, Yuermeinark Formation, Sugetebrak Formation, and Chigebrak Formation. The Aksu Group is mainly exposed in the Aksu-Wushi and Yuermeinark areas. It is composed of metasedimentary rocks and mafic schists, which undergo polyphase deformation in a subduction complex. The presence of blueschist facies attests to high-pressure and low-temperature metamorphism. The Aksu Group is considered as the oldest tectonic unit containing high-pressure metamorphic rocks. (Liou JG et al., 1989, 1996; Nakajima T et al., 1990; Zhu WB et al., 2011; Zhu GY et al., 2020). The metamorphic age of the Precambrian Aksu blueschist has been controversial, the youngest concordant detrital zircon ages in the Aksu Group indicate a maximum depositional ages of ca. 790–800 Ma (Lu YZ et al., 2017; Xia B et al., 2017) or ca. 730 Ma (Zhu WB et al., 2011). These orogenic events could have been related to the assembly of Rodinia. The Qiaoenbrak Group is exposed in the Yuermeinark area only, with a total thickness of 2000 m. It consists of a set of epimetamorphic sandstone and siltstone flysch at the lower and middle members formed in submarine gravity flow environment (Ding HF et al., 2015; Gao Z et al., 1986). The upper member of Qiaoenbrak Group is characterized by a set of diamictite and siltstone. The Yuermeinark Formation is a set of typical purplish-red diamictites considered to be continental glacial deposits and it can be generally correlated with the Tereen diamictite in the northeastern Tarim Block at top of the Cryogenian (Gao Z et al., 1986). The Sugetbrak Formation has intact strata exposed in the Aksu-Wushi and Yuermeinark areas, containing mafic rocks, red sandstones and gray mudstones, the overall thickness was approximately 400–450 m. The Sugetebrak Formation can be subdivided into two members: The lower member is composed of red conglomerates, red fluvial sandstones and gray lacustrine mudstones from bottom to top, interlaced with one to four layers of mafic rocks; the upper member is composed of gray calcirudite, quartz sandstone and red sandstone. The overlying strata of the Chigebrak Formation are a set of bedded micritic dolostone having a large number of Ediacaran stromatolites.

2.2   Petrography and geochronology of the mafic rocks

Different amounts of mafic rocks are exposed in the Sugetbrak Formation in the three sections in the Aksu-Wushi and Yuermeinark regions (Figs. 2a–d). Among them, the mafic rocks in the Dongergou section in the Wushi region is about 10 m thick, and four mafic dikes that intrude the sandstones of the Sugetbrak Formation are exposed in the Linkuanggou section; their thicknesses from the bottom to the top are 9 m, 20 m, 34 m, and 37 m, respectively. Three mafic dikes are exposed at the Yuermeinark section in the Yuermeinark area, with their thicknesses from the bottom to the top being 10 m, 12 m, and 50 m, respectively. They are interbedded with purplish-red sandstone, and there is no clear boundary between the various rock types, except for mafic rocks and sandstones. It is still controversial whether the mafic rocks in this area belong to extrusive origin or intrusive origin. However, based on our field investigation, the author find that the mafic rocks in the Sugetbrak Formation are most like mafic dikes intruding into surrounding rocks except the third set of mafic rocks at the top of the Yuermeinark section (Figs. 3a, b, d). In addition, there is baking phenomenon in the overlying strata and phase transformation in the edge and interior of some mafic rocks (Fig. 3c). No typical vesicular or amygdaloidal structures are observed in the Aksu-Wushi area.

Figure  2.  Field occurrence and petrography of the mafic dykes in the Aksu. a–Field occurrence of the mafic rocks with massive structure in the Dongergou section; b–the mafic rocks in the Linkuanggou section showing the mafic rocks is consistent with the strata; c–the mafic rocks intruding into strata in the Yuermeinark section; d–field occurrence of the mafic rocks in the Yuermeinark section; e, f–photomicrographic image of the mafic rocks (cross-polarized light), Abbrevation; Cpx–clinopyroxene; and Pl–plagioclase.

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Figure  3.  Field occurrence the mafic dikes in the Aksu. a–Mafic dikes in the Dongergou section; b–the mafic dikes intruding into strata in the Yuermeinark section; c–the baking phenomenon of mafic dikes to the upper strata in the Linkuanggou section; d–the mafic dikes intruding into strata in the Linkuanggou section.

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The mafic rocks show different geological characteristics, and the author also observe that there are different layers of mafic rocks in different sections. However, the mineral assemblages of these rocks are very similar (Figs. 2e, f); the majority of the mafic rocks are mainly composed of plagioclase (45%–50%), pyroxene (45%–50%), and Fe-Ti oxides (magnetite and ilmenite) (5%), rare apatite, sphene and zircon that are present as accessory minerals. The mafic rocks generally show the diabasic texture and part of them show the poikilophitic texture (Figs. 2c, d). Most of the rocks have experienced low-temperature post-magmatic alteration, such as sericitization and clay formation.

The precise formation age of the Sugetbrak mafic rocks is still not well constrained. Wang F et al. (2010) conducted the first LA-ICPMS U-Pb dating on zircons from the mafic rocks of the Sugetbrak Formation. They obtained 17 zircons for U-Pb dating, which yielded ages ranging from 1945 Ma to 755 Ma, and these ages have been interpreted as the ages of Mesoproterozoic metamorphic zircons and Late Neoproterozoic inherited magmatic zircons, Zhang ZC et al. (2012) obtained a 783 ± 2.3 Ma zircon age for the lower layers of the mafic rocks of the Sugetbrak Formation by the LA-ICP-MS method and interpreted it as the crystallization age of the Sugetbrak mafic rocks. Xu B et al. (2013) reported SHRIMP zircon U-Pb ages of 615.2 ± 4.8 Ma and 614.4 ± 9.1 Ma, and they considered these ages as eruption age of the Sugetbrak Formation. Lu YZ et al. (2018) preformed the first ⁴⁰Ar/³⁹Ar dating on separated mineral grains from the Sugetbrak mafic rocks in the Aksu area, pyroxenes form a layered mafic rock within the Sugetbrak Formation yielded a ⁴⁰Ar/³⁹Ar plateau age of 521.0 ± 2.1 Ma. They considered that the mini-plateau age of 521.0 ± 2.1 Ma was a minimum crystallization age for mafic rocks of the Sugetbrak Formation.

3.   Analytical methods

3.1   Sample preparation

Samples were taken from each set of mafic rocks in the Aksu-Wushi and Yuermeinak areas in three sections. After observation under a microscope, 18 relatively fresh samples were selected and sent to the Beijing Shougang Geological Survey Institute. The central part of the sample was used for whole-rock analysis. The sample was ground in a steel mortar using a 200 mesh in an agate grinding machine for analyzing the main and trace elements. A total of eight samples were selected from the three sections for the Sr-Nd-Pb isotopic analysis.

3.2   Major and trace element analyses

The major and trace elements of the mafic rocks were analyzed at the Wuhan Sample Solution Analytical Technology Co., Ltd., in Wuhan, China. Major element analyses of the whole rock were conducted using X-ray fluorescence (XRF; Primus II, Rigaku, Japan), and the accuracy analysis (better than 1%), FeO was performed using the chemical volumetric method. _TFe₂O₃ indicated the total amount of iron determined using the XRF spectrometer test, and the FeO content was obtained using a wet chemical analysis method. The trace element analysis of the whole rock was conducted using inductively coupled plasma mass spectrometry (ICP-MS) (Agilent 7700e), and the analysis accuracy was more than 3%. The analysis process was monitored according to international standards.

3.3   Sr-Nd-Pb isotope analyses

The pre-treatment and testing of the Sr-Nd-Pb isotope were completed at the Wuhan Shanghai Spectroscopic Analysis Technology Co., Ltd., Hubei, China by using the Neptune Plus Multicollector ICP-MS (MC-ICP-MS) instrument (Thermo Fisher Scientific, Dreieich, Germany). The Sr and Nd isotope ratios were standardised to be ⁸⁷Sr/⁸⁸Sr=0.1194 and ¹⁴⁶Nd/¹⁴⁴Nd=0.7219, respectively. Li CF et al. (2012) conducted a detailed test; one international National Institute of Standards and Technology (NIST; SRM 987) standard was measured for every ten samples analysed. Analyses of the NIST SRM 987 standard solution yielded ⁸⁷Sr/⁸⁶Sr ratios of 0.710244 ± 22 (2SD, n=32), which were identical to the values in previous studies (0.710241 ± 12; Thirlwall, 1991). One JNdi-1 standard was measured for every ten samples analysed. Analyses of the JNdi-1 standard yielded the ¹⁴³Nd /¹⁴⁴Nd ratio of 0.512118 ± 15 (2SD, n=31), which shows an error comparable to those found in other published studies (0.512115 ± 07; Tanaka T et al., 2000). This indicates the stability of the instrument and the reliability of the calibration strategy. Pb isotope analysis uses Hf to completely dissolve the powder sample under high temperature and pressure, and the Pb samples are separated and purified using a centrifugal column. The data were derived from a large number of tests according to a Pb standard (NBS SRM 918); one NBS SRM 981 standard was measured for every ten samples analysed. Analyses of the NBS SRM 981 standard yielded an external precision of 0.03% (2RSD) for the ^20xPb/²⁰⁴Pb ratios. All the measured ^20xPb/²⁰⁴Pb ratios of unknown samples were normalised to the well-accepted NBS SRM 981 standards of ²⁰⁸Pb/²⁰⁴Pb = 36.7262 ± 31, ²⁰⁷Pb/²⁰⁴Pb = 15.5000 ± 13 and ²⁰⁶Pb/²⁰⁴Pb = 16.9416 ± 13 (Baker J et al., 2004).

4.   Results

4.1   Major and trace element geochemistry

The major element compositions of the representative samples from the mafic rocks are listed in Table 1. Most samples showed high loss on ignition (LOI) values (2.81%–7.49%), especially the sample 20LKG-1 (LOI values of 11.42%), which indicated that they experienced significant post-magmatic alteration. The samples displayed varying contents of SiO₂ (38.35%–59.8%); contents of TiO₂ (1.47%–3.59%), _TFe₂O₃ (7.43%–16.41%), and Al₂O₃ (14.32%–16.94%); and low contents of MgO (3.52%–7.88%), K₂O (0.12%–1.21%), and P₂O₅ (0.32%–0.67%). The Na₂O content was higher than the K₂O content. The Mg^# value (=100×MgO/(MgO+FeO)) ranged from 34% to 51%. In Harker diagrams (Fig. 4), Al₂O₃, K₂O, MgO and Na₂O contents of the Sugetbrak mafic rocks display positive correlations with SiO₂ contents, whereas TiO₂, Fe₂O₃, CaO and MnO contents exhibit negative correlations. It can be seen from the Harker diagram, the chemical composition of mafic rocks in the Sugetbrak Formation in the Aksu area was basically similar and has certain correlation (Fig. 4).

Table  1.  Major element abundances (%) of the Sugetbrak mafic rocks in the Aksu area, NW Tarim Block.

Sample	SiO2	TiO2	Al2O3	TFe2O3	MnO	MgO	CaO	Na2O	K2O	P2O5	FeO	LOI	Total	Mg#	CaO/Al2O3
20LKG-1-1	38.35	2.50	14.19	10.13	1.18	4.09	12.33	4.16	0.65	0.34	2.70	11.42	99.35	42	0.87
20LKG-1-2	49.00	2.77	15.16	12.52	0.47	6.37	3.65	3.52	0.60	0.34	3.85	5.16	99.56	47	0.24
20LKG-2-1	42.09	3.11	15.73	14.13	1.89	5.76	4.50	4.48	1.11	0.48	4.90	6.39	99.66	41	0.29
20LKG-2-2	45.55	3.07	16.14	14.53	0.27	5.53	4.68	4.03	1.10	0.52	4.00	4.70	100.11	40	0.29
20LKG-3	44.84	3.01	15.88	15.30	0.30	5.26	6.23	4.21	0.92	0.50	6.00	3.43	99.87	36	0.39
20LKG-4-1	44.09	3.17	15.30	14.59	0.22	5.57	6.46	4.45	0.84	0.52	5.90	4.58	99.79	38	0.42
20LKG-4-2	44.78	2.86	15.35	15.40	0.17	5.69	6.08	4.21	0.87	0.45	6.00	3.89	99.74	38	0.40
20DRG-1-1	48.59	2.73	14.32	13.52	0.28	5.08	6.38	4.53	1.21	0.32	5.90	2.33	99.27	37	0.45
20DRG-1-2	47.04	3.00	16.25	13.81	0.21	7.88	0.95	5.20	0.72	0.40	3.55	4.32	99.78	51	0.06
20YRM-3-1	44.89	3.40	15.02	16.41	0.22	5.89	6.52	3.23	1.01	0.46	6.15	2.95	99.98	37	0.43
20YRM-3-2	48.54	2.95	16.94	12.28	0.19	5.06	7.93	3.03	0.86	0.41	3.65	2.04	100.22	42	0.47
20YRM-2-1	43.61	2.95	14.97	13.75	0.24	4.65	6.18	4.65	0.19	0.63	6.40	7.49	99.30	34	0.41
20YRM-2-2	46.35	2.96	14.25	13.70	0.19	5.80	4.88	4.38	0.34	0.57	6.25	6.60	100.01	40	0.34
20YRM-1-1	59.85	1.47	15.13	7.43	0.10	3.52	1.11	6.62	0.12	0.37	3.05	2.81	98.53	43	0.07
20YRM-1-2	44.70	3.59	14.76	14.48	0.61	5.45	5.24	4.68	0.23	0.67	6.00	5.26	99.66	38	0.35
20YRM-1-3	45.33	3.16	16.75	12.34	0.26	5.85	3.33	5.61	0.21	0.53	6.25	5.97	99.33	41	0.20
20YRM-1-4	50.06	2.29	16.19	10.73	0.26	4.08	2.91	4.90	0.64	0.43	5.45	5.35	97.83	36	0.18
Note: Mg# = molecular proportion of MgO/(MgO + FeO), assuming 85% of total iron oxides as FeO. Mg# = 100 × [MgO]/{[MgO]+[TFe2O3] × 0.8998 × 0.85}. if TFe2O3 is reported.

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Figure  4.  Variation diagrams for major oxides vs. SiO₂ contents of the mafic rocks in the Aksu.

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Rare earth elements (REEs) and other trace elements contents of mafic rocks are listed in Table 2. All the samples showed similar trends. Since most of the samples have been altered to different degrees in the later period, some of the samples showed highly negative anomalies of K and Sr. The content of REEs in all the samples was relatively high, as shown in the chondrite-normalized REE patterns (Fig. 5a), which indicated all the samples were enriched in light rare earth elements (LREEs) relative to heavy rare earth elements (HREEs). Additionally, some samples showed a slightly negative Eu anomaly (δEu=0.82).

Table  2.  Trace element abundances (10⁻⁶) of the Sugetbrak mafic rocks in the Aksu area, NW Tarim Block.

Sample	20LKG-1-1	20LKG-1-2	20LKG-2-1	20LKG-2-2	20LKG-3	20LKG-4-1	20LKG-4-2	20DRG-1-1	20DRG-1-2	20YRM-4-1	20YRM-4-2	20YRM-2-1
Cs	0.34	0.70	0.39	0.28	0.97	1.64	2.09	0.20	0.21	0.26	0.12	0.31
Rb	7.60	9.69	21.52	12.98	13.35	12.23	13.77	15.96	5.67	12.71	14.88	3.69
Ba	170	158	412	454	550	511	517	905	1482	459	439	763
Th	2.17	2.03	2.33	2.54	2.67	2.05	2.30	2.19	2.37	2.18	2.05	2.52
U	0.55	0.62	0.73	0.73	0.67	0.52	0.60	0.68	2.14	0.55	0.47	0.75
Nb	19.04	18.45	25.01	27.77	27.17	25.33	25.20	20.10	21.24	25.16	22.14	26.81
Ta	1.11	1.06	1.49	1.67	1.66	1.52	1.50	1.19	1.26	1.53	1.32	1.63
K	5354	4997	9206	9089	7620	6998	7197	10061	5976	8400	7164	1560
La	24.90	23.10	26.32	28.60	28.25	28.66	26.70	25.08	19.15	27.36	23.44	32.77
Ce	51.90	48.86	56.19	62.31	61.13	60.98	57.01	51.43	42.95	57.12	50.03	70.69
Pb	379	2768	193	12.07	6.38	6.03	4.82	4.71	19.18	6.41	5.89	7.81
Pr	6.48	6.35	7.25	8.09	8.00	7.90	7.32	6.52	5.74	7.56	6.61	9.31
Sr	281	289	331	476	542	486	572	711	88.38	377	472	162
P	1483	1470	2112	2260	2195	2247	1950	1401	1758	1985	1798	2749
Nd	28.39	27.13	31.56	34.75	33.95	34.39	31.46	27.72	25.29	32.21	28.38	39.65
Zr	193	189	221	240	248	208	225	199	221	226	201	268
Sm	6.58	6.60	6.98	7.76	7.63	7.61	7.09	6.51	6.45	7.41	6.73	8.64
Eu	2.20	2.25	2.21	2.40	2.40	2.48	2.38	2.14	1.76	2.37	2.19	2.59
Ti	14987	16624	18632	18386	18050	18998	17133	16336	17961	20383	17679	17709
Dy	5.86	5.98	5.35	5.97	5.74	5.68	5.40	5.81	6.24	6.36	5.65	7.66
Y	31.77	32.27	27.72	30.79	30.03	29.57	28.39	31.42	33.93	33.20	29.15	42.10
Yb	2.59	2.50	2.18	2.43	2.42	2.24	2.30	2.56	2.84	2.89	2.48	3.74
Lu	0.37	0.36	0.32	0.35	0.35	0.32	0.32	0.37	0.40	0.42	0.35	0.55
Hf	4.75	4.71	4.99	5.35	5.47	4.89	5.12	4.89	5.44	5.59	4.87	6.33
Gd	6.41	6.45	6.13	6.95	6.79	6.93	6.44	6.23	6.50	6.95	6.44	8.16
Tb	1.03	1.07	1.00	1.09	1.05	1.03	0.98	1.00	1.04	1.11	0.97	1.27
Ho	1.15	1.13	0.99	1.10	1.11	1.06	1.01	1.14	1.23	1.25	1.11	1.49
Er	3.03	2.99	2.62	2.91	2.85	2.70	2.67	2.93	3.25	3.18	2.81	4.04
Li	58.66	107	73.58	51.86	52.92	65.80	47.98	46.72	121.55	35.42	28.43	59.62
Be	1.22	1.43	1.79	1.65	1.63	1.39	1.32	1.19	1.12	1.48	1.26	1.22
Sc	29.72	33.64	18.50	17.52	17.88	19.02	17.82	29.57	36.47	27.54	22.65	20.94
V	319	356	187	172	186	196	185	336	372	290	226	223
Cr	59.64	70.66	29.71	32.82	34.87	38.76	46.19	56.41	74.31	67.23	60.91	2.15
Co	44.53	52.46	44.13	48.87	55.14	52.07	58.27	48.41	50.70	52.37	47.82	37.63
Ni	54.34	60.19	55.77	84.59	91.77	89.14	114	59.92	64.66	78.80	135	37.44
Cu	98.44	99.83	39.14	52.17	52.85	51.98	51.41	89.76	57.72	277.58	497.62	11.04
Zn	540	515	471	142	144	136	142	130	221	170	143	853
Ga	21.62	24.35	23.13	21.80	23.09	21.01	20.83	21.67	25.96	23.89	23.30	22.33
Sn	1.54	1.64	1.84	1.99	1.83	1.82	1.76	1.52	1.94	1.80	1.78	1.94
Tm	0.41	0.42	0.36	0.39	0.38	0.36	0.35	0.40	0.45	0.45	0.39	0.57
Tl	0.06	0.06	0.10	0.11	0.06	0.09	0.14	0.18	0.23	0.05	0.03	0.03
Nb/Y	0.60	0.57	0.90	0.90	0.90	0.86	0.89	0.64	0.63	0.76	0.76	0.64
Lu/Yb	0.14	0.14	0.15	0.14	0.15	0.14	0.14	0.15	0.14	0.14	0.14	0.15
Ta/Th	0.51	0.52	0.64	0.66	0.62	0.74	0.65	0.55	0.53	0.70	0.64	0.65
La/Sm	3.79	3.50	3.77	3.69	3.70	3.77	3.77	3.85	2.97	3.69	3.48	3.79
Th/Nb	0.11	0.11	0.09	0.09	0.10	0.08	0.09	0.11	0.11	0.09	0.09	0.09
Nb/La	0.76	0.80	0.95	0.97	0.96	0.88	0.94	0.80	1.11	0.92	0.94	0.82

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Figure  5.  Normalization plots. a–Chondrite-normalized REE patterns; b–primitive mantle-normalized multi-element diagrams of the mafic rocks in the Aksu. Chondrite and primitive mantle values are from Sun SS and McDonough WF (1989).

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In the primitive mantle-normalized spidergram (Fig. 5b), samples from different areas of the Aksu exhibit similar characteristics. The samples showed depletion in large-ion lithophile elements (LILEs) and enrichment in high field elements (HFSEs), which was identical to the features of ocean island basalt (OIB)-type mafic rocks. In view of the alteration effects proved by high LOI values, to further determine the properties of the magmatic rocks, immobile trace elements (Nb and Y) were used to assess their alkali character. However, the use of the alkali vs. SiO₂ diagram (Le Bas MJ et al., 1986) to make related judgements has been inaccurate. In the Zr/TiO₂-Nb/Y diagram (Winchester JA and Floyd PA, 1977), given that the Nb/Y ratios range from 0.49 to 1.01, plotted of the five elements (Ti, P, Zr, Y, and Nb) in the mafic rocks at the boundary line between sub-alkaline basalts and alkali basalts (Fig. 6a) In the Nb/Y-Zr/(P₂O₅×10000) diagram, they also plotted near the boundary line between alkali basalts and tholeiitic basalts (Fig. 6b). Therefore, the authors concluded that this is a transitional magmatic series.

Figure  6.  Lithological classification diagrams of the mafic rocks in the Aksu. a–Zr/TiO₂-Nb/Y diagram (afterWinchester JA and Floyd PA, 1977); b–Nb/Y-Zr/(P₂O₅×10000) diagram (afterWinchester JA and Floyd PA, 1976).

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4.2   Sr-Nd-Pb isotopic geochemistry

The Sr-Nd isotopic data for the samples in the Aksu mafic rocks are listed in Table 3. The results showed that the Sr-Nd isotopic characteristics of the mafic rock samples collected in the Aksu area were similar. The initial Sr and Nd isotopic ratios were corrected using the age of 615 Ma (Xu B et al. 2013 obtained the zircons from two samples of the Sugetbrak mafic rocks yield weighted mean ages of 615±4.8 Ma and 614.4±9.1 Ma). The mafic rock samples had initial ⁸⁷Sr/⁸⁶Sr (t=615 Ma) ratios ranging from 0.7052 to 0.7097. The ¹⁴³Nd/¹⁴⁴Nd values of the samples were similar (0.5121 to 0.5124), with ε_Nd(t) values of –0.70 to –5.35. The Pb isotopic data of the rock samples are listed in Table 3. The samples exhibit concordant variations in Pb isotopic compositions, with (²⁰⁶Pb/²⁰⁴Pb)_i of 16.9084 to 17.9817, (²⁰⁷Pb/²⁰⁴Pb)_i of 15.4782 to 15.6273, and (²⁰⁸Pb/²⁰⁴Pb)_i of 37.2760 to 38.6033.

Table  3.  Sr-Nd-Pb isotopic compositions of mafic rocks in the Aksu, NW Tarim Block.

Sample	20LKG-1-2	20LKG-2-1	20LKG-3	20LKG-4-1	20YRM-3-2	20YRM-2-2	20YRM-1-1	20YRM-1-3
Rb/×10−6	9.69	21.5	13.4	12.2	14.9	5.82	1.64	4.49
Sr/×10−6	289.55	331.65	542.96	486.80	472.57	178.61	245.93	207.89
87Rb/86Sr	0.094	0.183	0.069	0.071	0.089	0.092	0.019	0.061
87Sr/86Sr	0.706577	0.710175	0.706860	0.707487	0.705960	0.710225	0.709817	0.709139
(87Sr/86Sr)i	0.705749	0.708569	0.706251	0.706865	0.705181	0.709419	0.709652	0.708604
Sm(ppm)	6.60	6.98	7.63	7.61	6.73	8.90	6.45	8.76
Nd(ppm)	27.13	31.56	33.95	34.39	28.38	40.71	27.81	36.62
147Sm/144Nd	0.1534	0.1394	0.1417	0.1395	0.1495	0.1378	0.1463	0.1508
143Nd/144Nd	0.512384	0.512354	0.512339	0.512334	0.512412	0.512127	0.512173	0.512305
εNd(t)	−1.55	−1.0	−1.5	−1.4	−0.7	−5.35	−5.11	−2.89
206Pb/204Pb	16.9890	17.0100	17.6128	17.9422	18.4766	17.5791	19.2960	17.8263
207Pb/204Pb	15.4783	15.4780	15.4818	15.5260	15.6597	15.5128	15.7210	15.5716
208Pb/204Pb	37.4238	37.4386	38.1486	38.5041	39.1985	38.1213	40.0849	38.4060
(206Pb/204Pb)i	16.9875	16.9848	16.9084	17.3591	17.9396	17.1739	17.9817	17.7685
(207Pb/204Pb)i	15.4782	15.4765	15.4393	15.4908	15.6273	15.4884	15.6417	15.5681
(208Pb/204Pb)i	37.4223	37.4135	37.2760	37.7961	38.4697	37.7022	38.6033	38.3615
Note: (147Sm/144Nd)CHUR=0.1967, (143Nd/144Nd)CHUR=0.512638 (Hamilton PJ et al., 1983); (147Sm/144Nd)DM=0.2137, (143Nd/144Nd)DM=0.51315(Goldstein SL et al., 1984); λRb=1.42×10−11 year−1, λSm=6.54×10−12 year−1 (Lugmair GW et al., 1978). λU238=1.55125×10−10 year−1, λU235=9.8485×10-10 year−1, λTh232=4.9475×10-11 year−1 (Steiger RH and Jäger E, 1977).

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5.   Discussion

5.1   Magmatic consanguinity of the mafic rocks

In this paper, studies on mafic rocks in the Sugetbrak Formation in the Aksu area along three sections are presented. Whether the mafic rocks of the Sugetbrak Formation in this area are extrusive or intrusive is still controversial. On the basis of detailed fieldwork, the author observed phenomenon of the intruded strata in most layers of mafic rocks (Fig. 3), especially for the intruded upper strata, meanwhile, the red baked zone was observed at the contact boundary of overlying sandstone strata, the mafic rocks here have obvious phase transformation characteristics, and the crystalline particles of the mafic rocks near the formation contact boundary are very fine, while the mineral crystalline particles inside the mafic rocks become coarse, in other word, the crystal particles of the mafic rocks are transformed from marginal facies, transitional facies to mesophase facies. In addition to the above-mentioned macroscopic output characteristics, these mafic rocks are generally do not develop amygdaloidal structure and vesicular structure, with significant local spherical weathering, it is found that the mafic rocks have a typical diabasic structure and poikilophitic texture (Figs. 2e, f), These evidences strongly prove that these mafic rocks are actually formed by magma intrusion (Ren KX et al., 2009; Xiao YY et al., 2011; Lu YZ et al., 2018).

The Sugetbrak mafic rocks in the Aksu area are developed in similar spatial locations, basically belonging to the same tectonic setting, with similar geological characteristics in the field. Generally, the mafic rocks are developed in different layers in the Sugetbrak Formation. In addition, the mafic rocks have different degrees of invasion of the upper and lower sandstone layers. Further research show that these mafic rocks have similar petrographic characteristics and mineral composition. The mineral composition is mainly plagioclase (45%–50%), pyroxene (45%–50%) and Fe-Ti oxides (5%) and most of them have suffered slight alteration, and their main structure is diabasic structure and poikilophitic texture. The geochemical characteristics of the mafic rocks are basically similar. In the chondrite-normalized REE patterns and the primitive mantle-normalized spidergram, all the samples display similar elemental characteristics. In the Harker diagrams, the Sugetbrak mafic rocks exhibit strong correlations (Fig. 5), meanwhile, the mafic rocks of the three typical sections have generally similar initial values of Si-Nd-Pb isotopes, further confirming their genetic correlation. Based on the geology, petrology, element geochemistry and isotope geochemistry characteristics of the above the mafic rocks in the Sugetbrak Formation, the authors infer that the mafic rocks distributed in different sections of the Aksu area have a common mantle source.

5.2   Age assessment of the mafic rocks

Abundant mafic rocks crop out in different lithological section of the Aksu-Wushi and Yuermeinark areas. According to the regional stratigraphic research, many researchers have carried out detailed chronological studies on these mafic rocks. Wang F et al. (2010) used the LA-ICPMS U-Pb dating on the zircons from the mafic rocks in the Dongergou section which yielded ages ranging from 1945 Ma to 755 Ma (17 zircon analyses), indicating that these zircons were inherited from the crustal materials during the ascending of the magma. Thus, the ages of the mafic rocks are therefore considered to be younger than 755 Ma. Besides, Zhang ZC et al. (2012) reported a LA-MC-ICP-MS U-Pb zircon age of 783.7 ± 2.3 Ma (23 zircon analyses) form the upper mafic rocks in the south of Yuermeinark section, these radiometric dates suggested it as the crystallization age of the mafic rocks in the Sugetbbrak Fomation, however, this result was inconsistent with the fact that the strata belonged to the Ediacaran. According to previous reports (Gao Z et al., 1985), the Chigebrak Formation, overlaying the Sugetbrak Formation, developed a large number of Ediacaran indicative stromatolite assemblages, therefore the Chigebrak Formation and the Sugetbrak Formation were jointly classified as the Ediacaran. Furthermore, these ages are even older than the metamorphic age of the underlying Aksu Group (Liou JG et al., 1996; Wen B et al., 2015). Recently, Lu YZ et al. (2018) studied the geochronology of the mafic rocks in the Aksu area again, and he performed ⁴⁰Ar/³⁹Ar analyses on the plagioclase and pyroxene from most layer of the mafic rocks the Sugetbrak Formation yield, due to the alteration of sericitization in the plagioclases, the ⁴⁰Ar/³⁹Ar age of plagioclase was not obtained, and the ⁴⁰Ar/³⁹Ar plateau age of 521.0 ± 2.1 Ma and the mini--inverse isochron age of 520.6 ± 2.1 Ma were obtained only from a pyroxene mineral. Since the two results are consistent, it can be concluded that there is no excess Ar in pyroxenes. However, it should be noted that pyroxene is a mineral with very low potassium content, so it is usually difficult to obtain reliable ⁴⁰Ar/³⁹Ar age results. Moreover, according to detailed microscope observation, the pyroxene in the mafic rocks in the study area had undergone relatively weak alteration. Additionally, since the ⁴⁰Ar/³⁹Ar age results of pyroxene indicated the Cambrian, according to the stratigraphic records of Cambrian in Tarim Block, there is no record of magmatism in the regional strata of Cambrian, so whether this result can be recognized as the diagenetic age of the mafic rocks is still questionable. Xu B et al. (2013) reported a U-Pb zircon age of 615.2 ± 4.8 Ma and 614.4 ± 9.1 Ma providing an age constraint on the timing of the Sugetbrak Formation. Based on the stratigraphic relationship with the Chigebrak dolomite, the Chigebrak Formation and the Sugetbulak Formation are collectively classified into the Ediacaran Formation, moreover, the presence of 615 ± 15 Ma volcanic rocks in the Mochia-Khutuk area of the northeast Tarim Block (Xu B et al., 2009), and 588–619 Ma detrital zircons in the Aksu area of the northwest Tarim Block (Zhu WB et al., 2011), Xu B et al. (2013) reported this age of 615 Ma correspond to them in time. To sum up, the author infered that the age of 615 Ma may be the most reliable age of the mafic rocks in the Sugetbrak Formation in the Aksu area.

5.3   Nature of the mantle source

The presence of voluminous phenocrysts (clinopyroxene and plagioclase) in the mafic rocks, suggesting fractional crystallization of clinopyroxene and plagioclase (Frey FA and Prinz M, 1978). The Aksu Sugetebrak Formation mafic rocks showed low MgO contents (3.52%–7.88%) and Mg^# (34–51) as well as relatively low contents of compatible elements, such as Ni (19×10⁻⁶–136×10⁻⁶) and Cr (1.4×10⁻⁶–74×10⁻⁶). The correlation of Ni, Cr, and CaO/Al₂O₃ with Mg^# in these mafic rocks indicated in the early crystallization of olivine and clinopyroxene, respectively (Figs. 7a, b). The petrological and geochemical characteristics indicated that they represent evolved magmas rather than primary magmas.

Figure  7.  Elemental variation diagrams of the mafic rocks in the Aksu. a–Ni vs. Cr diagram; b–CaO/Al₂O₃ vs. Mg^# diagram. Arrows showing fractional crystallization trends in the evolution of the mafic rocks in the Aksu. Partition coefficients are from Rollision HR (1993). Arrows in the diagrams indicate data trends. Cpx: clinopyroxene; Ol: olivine.

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Most of the samples showed Ba contents of 110×10⁻⁶–1483×10⁻⁶ and Sr contents of 85.8×10⁻⁶–711×10⁻⁶, much higher than those of the primitive mantle on average (Ba = 5.1×10⁻⁶, Sr = 19.9×10⁻⁶; Taylor SR and McLennan S, 1995), which indicated that the magma source of the mafic rocks involved some crustal materials. In the process of fractional crystallization, the incompatible elements are relatively enriched in the residual melt, while the Sr-Nd-Pb isotope is not affected. Therefore, their relatively high (⁸⁷Sr/⁸⁶Sr)_i ratios (0.7052–0.7097), negative ε_Nd(t) values of (–0.70 to –5.35) and high (²⁰⁶Pb/²⁰⁴Pb)_i ratios (16.9084–17.9817) implied the involvement of crustal materials which were enriched in these isotope ratios. Crustal materials can incorporate in the formation of these mafic rocks via two ways, i.e., the crust contamination and source enrichment.

Mantle-sourced magma may be affected by crustal contamination as it rises or temporarily settles in magma chambers in the continental crust. Some incompatible elements have similar geochemical characters, such as Ta and Th, La and Sm, hence Ta/Th and La/Sm ratios cannot be significantly modified by partial melting or fractional crystallization. Mantle-derived magmas are characterized by low La/Sm ratios and high Ta/Th (Sun SS and McDonough WF, 1989). The Ta/Th vs. La/Sm diagram (Fig. 8) showed that the Sugetbrak mafic rocks plotted between upper crust and OIB, suggesting some crustal contamination. However, the Lu/Yb value of the characteristic rock of mantle-derived magma (0.14–0.15) was low (Sun SS and McDonough WF, 1989); this was consistent with the results obtained for the Sugetbrak mafic rocks (0.14–0.15). In chondrite-normalized REE patterns and primitive mantle normalized spidergrams, the lack of negative Eu and Ti anomalies also argued against the possibility of extensive crustal contamination (Rudnick R and Gao S, 2003). Meanwhile, the Nb and Ta contents in the samples showed no obvious depletion, and most of the Sugetbrak samples had chondritic Nb/Ta (16–17), Zr/Hf (40–45) and Nb/U (30–47) ratios, which suggested that crustal contamination had less effect in magma source (Sun SS and McDonough WF, 1989). Moreover, In the SiO₂ vs. initial Sr isotopic ratio and ε_Nd(t) diagram (Fig. 9), the Sugetbrak mafic rocks exhibit no correlations between (⁸⁷Sr/⁸⁶Sr)_i vs. SiO₂ and between ε_Nd(t) vs. SiO₂. All these above demonstrated that the magma was not significantly affected by crustal contamination during their ascent to the continental crust. In summary, all the evidence suggests that the parental magma of the Neoproterozoic mafic rocks in the Aksu experienced slight crustal contamination as it ascended the surface.

Figure  8.  Ta/Th vs. La/Sm diagram of the Sugetbrak mafic rocks. OIB data from Sun SS and McDonough WF (1989), and upper crust and lower crust data from Taylor SR and McLennan S (1995).

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Figure  9.  SiO₂ vs. Sr-Nd isotopic ratios. a–SiO₂ vs. (⁸⁷Sr/⁸⁶Sr)_i, b–SiO₂ vs. ε_Nd(t) diagrams. All mafic rocks exhibit not correlation between (⁸⁷Sr/⁸⁶Sr)_i and SiO₂ and between ε_Nd(t) and SiO₂.

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As discussed above, the Sugetbrak mafic rocks had undergone insignificant crustal contamination, the crustal materials might have been involved in the source region. In the ⁸⁷Sr/⁸⁶Sr vs. ²⁰⁶Pb/²⁰⁴Pb isotope correlation diagram and the ε_Nd(t) vs. ²⁰⁶Pb/²⁰⁴Pb isotope diagram, most of the sample plots were observed near EM-Ⅰ (Figs. 10a, b), Additionally, Pb isotope source discriminant diagram (Fig. 11) show that the data of Sugetbrak mafic rocks plotted near the lower crust, combining to eliminate the influence of crustal contamination, indicating that the magma of the mafic rocks might be derived from a relatively enriched mantle with some lower crustal materials involved.

Figure  10.  Source divisional diagrams of Sr-Nd isotope of mafic rocks in the Aksu. a–⁸⁷Sr/⁸⁶Sr vs. ²⁰⁶Pb/²⁰⁴Pb isotope correlation diagram; b–ε_Nd(t) vs. ²⁰⁶Pb/²⁰⁴Pb isotope correlation diagram. Both diagrams show the positions of the mantle reservoirs identified by Zindle A and Hart SR (1986): DM, depleted mantle; HIMU, mantle with high U/Pb ratio; EMⅠ and EMⅡ, enriched mantle sources; MORB, mid-ocean ridge basalt (after Zindle A and Hart SR, 1986). Fields of MORB, EMI and EMII with shadow-hatching. In above figures all Pb isotopes plotting are the measured value.

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Figure  11.  Source divisional diagram of Pb isotope in the Sugetbrak mafic rocks. Dashed lines enclose probable average values (a–upper crust; b–lower crust; base map after Doe BR and Zartman RE, 1979)

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In the diagram of (⁸⁷Sr/⁸⁶Sr)_t vs. ε_Nd(t), most samples were close to the OIB field (Fig. 6), meanwhile, the trace element characteristics of the samples were roughly similar to those of the OIB-like trace element patterns and REE patterns characterized by enrichment of large-ion lithophile elements (LILE), high field strength elements (HFSE), and LREE with no significant negative Nb or Ta anomalies, typical of OIB. It may indicate that the source region has received metasomatism of recycled materials.

5.4   Tectonic implications

Ti, Zr, Y, and Nb are immobile trace elements during the metamorphic process of the zeolite to greenschist facies and can be used to determine the tectonic environment of mafic rocks (Pearce JA and Cann JR, 1973; Winchester JA and Floyd PA, 1976). In the Zr/4-Nb×2-Y triangle diagram (Meschede M, 1986), all the sample input points were within the areas of intra-plate alkali basalt and intra-plate tholeiite, meanwhile, in the Zr/Y-Zr diagram (Pearce JA and Norry MJ, 1979), all the samples plotted in the field of intraplate tectonic setting. (Figs. 12a, b). Both Zr/4-Nb×2-Y and Zr/Y-Zr diagrams suggested the intra-plate environment.

Figure  12.  Tectonic discrimination diagrams of mafic dikes in the Aksu. a–The Zr/Y vs. Zr diagram (after Pearce JA and Norry MJ, 1979); A–Island arc basalts; B–Mid ocean ridge basalts; C–Within plate basalts. b–the Nb-Zr-Y diagram (after Meschede M, 1986). AⅠ+AⅡ– Intra-plate alkaline basalts; (AⅡ+C)–Intra-plate tholeiite; B–P-Mid ocean ridge basalts; (C+D)–Volcanic arc basalts; D–N-Mid ocean ridge basalts.

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Neoproterozoic magmatic records have been reported in the Tarim Block (Li ZX et al., 1999; Xu B et al., 2005; Zhang ZC et al., 2009, 2012). Four stages of Neoproterozoic magmatic events have been reported in the Tarim Block: 820–800 Ma, 780–760 Ma, 740–735 Ma and 650–635 Ma (Li XH et al., 2003; Li ZX et al., 1996, 2003, 2008; Zhang CL et al., 2011; Zhang ZC et al., 2012), including the age of bimodal volcanic rocks from the Beiyixi Formation of the Quruqtagh area was 755 ± 15 Ma (Xu B et al., 2005) and that of the volcanic agglomerate from the Beiyixi Formation was 739±6 Ma (Gao Z et al., 2010); the basalt age in the Beiyixi Formation was 740±7 Ma and 725±10 Ma, while the andesite age was 615±6 Ma in the Zhamokti Formation (Xu B et al., 2009). Recently, some scholars have proposed that the mafic rocks of the Sugetebrak Formation are caused by the mantle plume activity under the Neoproterozoic Rodinia supercontinent and are evidence of the pyrolysis of the Tarim plate from the Rodinia supercontinent (Wang F et al., 2010). Xu B et al. (2013) evaluated the zircon age of the Sugetebrak Formation; the youngest rocks exhibited an age of 615 Ma, this age interpreted as the eruption of the Sugetbrak mafic rocks Furthermore, the Sugetebrak mafic rocks were associated with a plume-related magmatism during the Neoproterozoic may represent the waning stage of plum volcanism during a long-lasting continental breakup. Previous studies have shown that the Sugetbrak Formation in the southwest Aksu areas has been interpreted as fluvial and lacustrine facies sediments (Turner S, 2010, Wang F et al., 2010). Combined with several horizons of mafic rocks that record episodic volcanism, these sedimentary facies are thought to indicate the development of a Neoproterozoic rift system the Aksu area. In our research, the Sugetbrak mafic rocks belong to intra-plate environment, so it suggests that the Sugetbrak mafic rocks may represent the last stage of the Neoproterozoic rifting associated with the breakup of the Rodinia Supercontinent.

6.   Conclusions

Based on the petrological, geochemical and Sr-Nd-Pb isotopic studies of the mafic rocks in the Aksu area of the northwest Tarim block, the author made several conclusions:

(i) Based on the similar geological, petrological, element geochemical and isotope geochemical characteristics of the mafic rocks in the Sugetbrak Formation, the author infer that these mafic rocks distributed in different sections of the Aksu area have a common mantle source.

(ii) Petrological and geochemical characteristics indicated that the Sugetbrak mafic rocks in the Aksu represent evolved magmas rather than primary magmas, and they had not undergone significant crustal contamination during ascent to the surface, most of the sample plots were observed near EM-Ⅰ, indicating that the magma of the mafic rocks might be derived from a relatively enriched mantle with some crustal materials involved. They showed OIB-like geochemical characteristics indicating that the source region has experienced metasomatism of recycled materials.

(iii) The mafic rocks of the Sugetbrak Formation in the Aksu area were formed in an intraplate rift environment, which might result from the breakup of the Rodinia supercontinent and represent the waning stage of plume volcanism during a long-lasting continental breakup.

CRediT authorship contribution statement

Xiang-kun Zhu conceived of the presented idea. Hong-zhe Xie, Xun Wang, Yuan He, Wei-bing Shen wrote the manuscript in consultation. All authors discussed the results and contributed to the final manuscript.

Declaration of competing interest

The authors declare no conflicts of interest.

Acknowledgements

This work was funded by the Key Laboratory of Deep-Earth Dynamics of Ministry of Natural Resources (J1901-20-1), and China Geological Survey Program (D20090002). The authors are grateful to Dr. Zhao-fu Gao and Dr. Qiu-hong Xie and three anonymous reviewers for their constructive comments and suggestions which substantially improved this work in science and presentation.