| Citation: |
Hong-zhe Xie, Xiang-kun Zhu, Xun Wang, Yuan He, Wei-bing Shen, 2023. Petrological and geochemical characteristics of mafic rocks from the Neoproterozoic Sugetbrak Formation in the northwestern Tarim Block, China, China Geology, 6, 85-99. doi: 10.31035/cg2021067
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Petrological and geochemical characteristics of mafic rocks from the Neoproterozoic Sugetbrak Formation in the northwestern Tarim Block, China
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Abstract
The Neoproterozoic Sugetbrak Formation in the Aksu area, which is located at the northwest margin of Tarim Block, comprises mafic rocks and provides key records of the evolution of the Rodinia supercontinent. However, the genetic relationship among these mafic rocks exposed in different geographical sections are still unclear. In this study, the petrology, geochemistry, and Sr-Nd-Pb isotope geochemistry of the mafic rocks exposed in the Aksu-Wushi and Yuermeinark areas have been studied in some detail along three sections. The authors found that the mafic rocks in these three typical sections were mainly composed of pyroxene and plagioclase, containing a small amount of Fe-Ti oxides and with typical diabasic textures. All the mafic rocks in this region also showed similar geochemical compositions. They were characterised by high TiO2 contents (1.47%–3.59%) and low MgO (3.52%–7.88%), K2O (0.12%–1.21%). Large ionic lithophile elements (LILEs) (Rb, Sr, and Cs) were significantly depleted. Meanwhile, high field strength elements (HFSEs) were relatively enriched. In the samples, light rare earth elements (LREEs) were enriched, while heavy rare earth elements (HREEs) were depleted. Based on the Zr/Nb, Nb/Y, and Zr/TiO2 ratios, the Aksu mafic rocks belong to a series of sub-alkaline and alkaline transitional rocks. The mafic rocks along the three typical sections showed similar initial values of 87Sr/86Sr (ISr) (0.7052–0.7097) and εNd(t) (–0.70 to –5.35), while the Pb isotopic compositions with 206Pb/204Pb, 207Pb/204Pb and 208Pb/204Pb values of 16.908–17.982, 15.487–15.721, 37.276–38.603, respectively. Most of the samples plot into the area near EM-Ⅰ, indicating that the magma of the mafic rocks might have derived from a relatively enriched mantle with some crustal materials involved. The geochemical element characteristics of most samples showed typical OIB-type geochemical characteristics indicating that the source region had received metasomatism of recycled materials. Combining with the regional geological background and geochemical data, we inferred that the mafic rocks of the Sugetbrak Formation in the Aksu area were formed in an intraplate rift environment. Summarily, based on our study, the mafic rocks of the Sugetbrak Formation in the Aksu area were derived from a common enriched mantle source, and they were product of a magmatic event during the rift development period caused by the breakup of the Rodinia supercontinent.
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References
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Hong-zhe Xie, Xiang-kun Zhu, Xun Wang, Yuan He, Wei-bing Shen, 2023. Petrological and geochemical characteristics of mafic rocks from the Neoproterozoic Sugetbrak Formation in the northwestern Tarim Block, China, China Geology, 6, 85-99. doi: 10.31035/cg2021067
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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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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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Figure 4.
Variation diagrams for major oxides vs. SiO2 contents of the mafic rocks in the Aksu.
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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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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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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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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. εNd(t) diagrams. All mafic rocks exhibit not correlation between (87Sr/86Sr)i and SiO2 and between εNd(t) and SiO2.
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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–εNd(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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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.