Elsevier

Precambrian Research

Volume 313, August 2018, Pages 119-133
Precambrian Research

Petrogenesis of Mesoproterozoic mafic rocks in Hainan (South China) and its implication on the southwest Hainan-Laurentia-Australia connection

https://doi.org/10.1016/j.precamres.2018.05.002Get rights and content

Highlights

  • The Baoban mafic rocks in southwest Hainan formed at 1441–1424 Ma.

  • The Mesoproterozoic Baoban Complex formed in the syn-rifting setting of the Columbia breakup.

  • southwest Hainan connected with west Laurentia and Australia in Columbia reconstruction.

Abstract

New U-Pb geochronological, elemental and Nd-Hf isotopic data for the metamorphic mafic rocks from the Baoban Complex in southwest Hainan provide new constraints on their petrogenesis and geodynamic setting. Magmatic zircons from the representative samples yielded weighted mean 207Pb/206Pb ages of 1441–1424 Ma, representing their crystallization ages. These samples show subalkaline to alkaline characteristics and can be geochemically subdivided into three groups in spite of similar formation time. Group 1 has TiO2 of 2.85–3.18 wt.%, Nb of 20.5–21.6 ppm, Nb/U of 45–49, Nb/La of 1.22–1.37, εNd (t) of +6.8–+6.9 and zircon εHf (t) of +9.3–+12.7, suggestive of the derivation from OIB-like source. Group 2 shows Nb contents ranging from 1.0 ppm to 12.6 ppm, εNd (t) values from +5.5 to +5.9 and zircon εHf (t) values from +10.3–+12.4 with right-sloping REE-normalized pattern, indicative of an E-MORB source. Group 3 is characterized by negative Nb-Ta anomalies with Nb/La ratios of 0.51–0.88 and εNd (t) value of ∼+5.1, indicative of the derivation from a previously-metasomatized mantle source. The coeval mafic and felsic rocks in the Baoban Complex are geochronologically and geochemically comparable to the Mesoproterozoic intra-plate magma in southwest Laurentia. The Mesoproterozoic Baoban igneous rocks are generated in a syn-rift setting in response to the Columbia breakup. Southwest Hainan might connected with west Laurentia and Australia at ∼1.43 Ga.

Introduction

The East Asia region, a collage of allochthonous continental blocks (i.e., North China, South China, Tarim and Indochina), is tectonically one of the most complex regions on Earth (e.g., Metcalfe, 1996, Li et al., 2002). Abundant geological relicts associated with the Precambrian supercontinent evolution from Columbia assembly to Rodinia breakup are preserved in this region (e.g., Rogers and Santosh, 2002, Zhao et al., 2002, Zhao et al., 2004, Zhang et al., 2012b). For the South China Block (SCB) in East Asia region, there are numerous geological signatures marking the early Neoproterozoic amalgamation between the Yangtze and Cathaysia blocks along the Jiangnan Orogen in response to the Rodinia assembly (e.g., Li et al., 2007, Ye et al., 2007, Zhang and Wang, 2016). The Paleo- and Mesoproterozoic igneous and sedimentary rocks as well as the Paleoproterozoic detrital and metamorphic zircons identified in the northern and western Yangtze Block, suggest that the SCB, especially the Yangtze Block, might have been involved in the Columbia assembly and its subsequent breakup (e.g., Gao et al., 1999, Wilde et al., 2002, Kusky and Li, 2003, Zhai and Liu, 2005, Zhang et al., 2006, Liu et al., 2008, Sun et al., 2009, Yu et al., 2009, Yu et al., 2012, Zhao et al., 2010, Chen et al., 2013, Yin et al., 2013). Distinct from those in the Yangtze Block, the Cathaysia Block has rare records of Mesoproterozoic igneous rocks (e.g., mafic rocks) so far. The poor understanding for the petrogenesis of the Mesoproterozoic igneous rocks and their tectonic setting resulted in the proposal of competing models (margin of vs. external) of the Columbia assembly and breakup (e.g., Zhao and Cawood, 1999, Rogers and Santosh, 2002, Zhao et al., 2002, Zhao et al., 2004, Torsvik, 2003, Zhang et al., 2012a, Wang et al., 2013, Wang et al., 2014b, Yang et al., 2014).

Hainan Island is adjacent to the Indochina Block and separated from the SCB mainland by the Qiongzhou Strait. It is traditionally considered as a southward continuation of the Cathaysia Block in the SCB (Fig. 1a). The Mesoproterozoic gneissic granites and associated metasedimentary sequences have been traditionally defined as the Baoban Group but recently named the Baoban Complex (e.g., Hou et al., 1992, Liang, 1995, Chen et al., 1997). These granitic rocks in the complex have been considered to be characterized by the ∼1430 Ma cal-alkaline monzonitic granites and a small number of alkaline A-type granites (e.g. Ding, 1995, Xu et al., 2001, Xu et al., 2006, Li et al., 2008, Zhang et al., 2017). Our field investigation recently identified Mesoproterozoic mafic rocks in the Baoban Complex, which are more suitable targets than the granitoids for constraining the SCB Mesoproterozoic tectonic setting and reconstructing its paleogeographic location in Columbia (e.g., Li et al., 2008, Yao et al., 2017). This paper presents a set of new zircon U-Pb geochronological, Lu-Hf isotopic, whole-rock elemental and Nd isotopic data of the Mesoproterozoic mafic rocks from the Baoban Complex in southwest Hainan. Our results, along with the available geological data, provide the possibility of reconstructing Mesoproterozoic linkage of southwest Hainan with west Laurentia and Australia in Columbia.

Section snippets

Geological setting and petrography

The SCB consists of the Yangtze Block in the northwest and the Cathaysia Block in the southeast (Fig. 1a), and is united by the Neoproterozoic amalgamation along the Jiangnan orogenic zone (e.g., Zhao and Cawood, 1999, Wang et al., 2013, Wang et al., 2014b, Zhang and Wang, 2016 and reference therein). The oldest crystalline basement in the Yangtze Block is characterized by the ∼3.45–2.90 Ga Kongling Complex, which is surrounded by late Mesoproterozoic to early Neoproterozoic folded belt in the

Analytical methods

The fresh samples were crushed to 200-mesh in a steel mortar and ground in a steel mill for the elemental and isotopic analyses. Major oxides were determined by a wavelength X-ray fluorescence spectrometer using a Rigaku ZSX100e instrument at the Guangzhou Institute of Geochemistry (GIG), the Chinese Academy of Sciences (CAS), with the relative standard derivations controlled within 5%. Trace elements were analyzed by ICP-MS in the GIG, CAS and the international standard BCR-1 was chosen to

Geochemical characteristics

The samples show SiO2 = 48.71–50.71 wt.% (volatile free), MgO = 6.06–8.46 wt.%, with mg-number ranging from 51 to 61. They have Al2O3 ranging from 12.75 wt.% to 15.36 wt.%, FeOt from 10.65 wt.% to 13.10 wt.%, TiO2 from 1.02 wt.% to 3.36 wt.% and P2O5 from 0.06 wt.% to 0.47 wt.% (Table 1). Their Cr contents are in the range of 47–282 ppm and Ni contents of 25–120 ppm. On the plot of Zr/TiO2 and Nb/Y (Fig. 4), these samples fall into the field of the subalkaline and alkaline basalt. Based on the

Magma process and source characteristics

It is important to assess whether these samples have undergone low-temperature alteration and crustal contamination before speculating on their source nature (e.g. DePaolo, 1981). Our samples might have been subjected to small degrees of alteration due to the variable loss on ignition (LOI) of 0.67–5.12 wt.%. However, the correlations of LOI with Na2O, K2O, Nb/La ratios and εNd (t) values are insignificant, contradicting to the significant alteration effect (e.g., Deniel, 1998). These

Conclusions

The comprehensive geochemical, zircon U-Pb geochronological and Nd-Hf isotopic data of the mafic rocks in SW Hainan lead to the following conclusions:

  • (1)

    Metamorphic mafic rocks from the previously-mapped Baoban Group formed at 1441–1424 Ma, indicative of Mesoproterozoic origin.

  • (2)

    These mafic rocks are subdivided into Group 1, 2 and 3, geochemically resembling the derivation of OIB, E-MORB and previously-metasomatized source, respectively.

  • (3)

    The Mesoproterozoic mafic rocks in the SW Hainan were generated

Acknowledgments

We would like to thank Dr. G-F Zhao for his help during field work and zircon U-Pb analyses. We thank the National Science Foundation of China (U1701641 and 41672213) and Ministry of Science and Technology of China (2016YFC0600303 and 2014CB440901) for financial support and SYSU for the Fundamental Research Funds for the Central Universities.

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