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Petrogenesis and geodynamic implications of the early Paleozoic potassic and ultrapotassic rocks in the South China Block

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Highlights

  • The early Paleozoic potassic and ultrapotassic rocks in SCB formed at 445–424 Ma.

  • The group 1 and 3 were generated by the partial metling of enriched mantle source.

  • The group 2 originated from the partial melting of their depleted mantle source.

  • These rocks constrained the geodynamic transition time from orogen to post-orogen.

Abstract

In this paper, some potassic and ultrapotassic rocks in the South China Block (SCB) have been recognized, according to a set of new geochronological, geochemical and Sr-Nd isotopic data. Zircon U-Pb dating from six plutons yield consistent crystallization ages of 445–424 Ma. These potassic and ultrapotassic rocks can be geochemically subdivided into three groups. Group 1, represented by the Longchuan gabbro, longmu diabase, Tangshang and Danqian diorite (445–433 Ma), have low silica contents (SiO2 = 47.38–54.16 wt.%), and high MgO (4.21–9.51 wt.%) and total alkalis (Na2O + K2O = 3.08–5.57 wt.%), with K2O/Na2O ratios of 0.62–1.82. They are enriched in LREE and depleted in Ba, Sr and Ta-Nb-Ti, and exhibit relatively high initial 87Sr/86Sr ratios (0.70561–0.71128), low εNd(430 Ma) values (−8.4 to −3.2), suggesting that they were most plausibly generated by the partial metling of enriched mantle source (EMI). Group 2, from the Huwei diorite (424 Ma), have 45.68–52.87 wt.% of SiO2, 5.79–9.25 wt.% of MgO and 52–65 of mg-number. They have significantly higher Th (9.92 ppm), Ce (88.0–115 ppm) concentration and Ce/Yb (27.6–46.8), Th/Yb ratios (2.58–7.99), and relatively low initial 87Sr/86Sr ratios (0.70501–0.70599), and high εNd(430 Ma) values (−2.1 to −1.5). We propose that they originated from the partial melting of the depleted mantle source with subsequent contamination by crustal materials. Group 3, represented by the Daning lamprophyre (∼445 Ma), has SiO2 contents ranging from 41.73 wt.% to 45.22 wt.%, MgO from 13.74 wt.% to 15.16 wt.%, and mg-muber from 73 to 77, with high K2O/Na2O ratios (>2.0). They have 87Sr/86Sr ratios of 0.62912–0.70384 and εNd(t = 430 Ma) values of −6.4 to −6.3, indicating that the source components are close to the EMI source, with significant sediments involved. These Silurian potassic and ultrapotassic rocks in the SCB can be responsible for post-orogenic delamination and intra-plate extension. And the delamination had a small size and a long duration, and a negligible impact.

Graphical abstract

Some early Paleozoic potassic and ultrapotassic rocks in the South China Block are recognized, and subdivided into three groups according to geochemical compositions. They may be originated by the partial metling of enriched mantle source and depleted mantle source, and responsible for post-orogenic delamination and intra-plate extension, respectively.

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Introduction

The nature of the early Paleozoic tectonic regime in South China Block (SCB) remains unclear (Wang et al., 2011). One school of thought is that a subduction-collision model (Yang et al., 1995, Chen et al., 2006, He et al., 2014, Peng et al., 2016). However, more and more evidences suggest that the early Paleozoic orogeny of South China was likely to be an intraplate orogen (e.g., Shu et al., 2008, Wang et al., 2007, Wang et al., 2010, Wang et al., 2011, Wang et al., 2012, Wang et al., 2013a, Wang et al., 2013b, Faure et al., 2009, Charvet et al., 2010, Wan et al., 2010, Chen et al., 2010, Chen et al., 2012, Wang et al., 2013c, Huang et al., 2013, Feng et al., 2014, Peng et al., 2015, Zhang et al., 2015), which is considered as one of the numerous examples of intraplate orogenesis in the world (Li et al., 2010). Amphibolites–facies metamorphism in the eastern Wuyi area occurred between ca. 460 Ma and 445 Ma, and the peak of tectonthermal event was possibly at 446–423 Ma (Li et al., 2011), demonstrating that the orogeny occurred between >460 Ma and ca. 415 Ma (Li et al., 2010). One of the distinct characteristics of the early Paleozoic igneous rocks in SCB is that peraluminous S- and I-type granites are dominant (Fig. 1), whereas mafic rocks are minor (Zhou, 2003, Feng et al., 2014). Recently, some early Paleozoic mafic rocks in the SCB have been recognized (Yao et al., 2012, Wang et al., 2013b, Peng et al., 2016). These early Paleozoic mafic rocks are significant to constrain the transition from syn-orogen to post-orogen in the SCB (Feng et al., 2014).

Potassium-rich igneous rocks with K2O > Na2O-2 wt.% (Le Maitre et al., 1989) or 0.5 < K2O/Na2O < 2.0 (Turner et al., 1996), and ultrapotassic rocks with K2O > 3 wt.%, MgO > 3 wt.% and K2O/Na2O > 2 (Foley et al., 1987) are classified as potassic and ultrapotassic, respectively. Potassic and ultrapotassic mafic rocks are significant to constrain the regional tectonic settings. Our recent investigations identify several early Paleozoic mafic rocks in the SCB, and they have geochemical characteristics of potassic and ultrapotassic rocks. In this paper, We present geochronological results, combined with whole rock chemical and Sr–Nd isotope data of these potassic and ultrapotassic in order to: (1) document the emplacement age of these rocks; (2) investigate their magma sources and petrogenetic processes; (3) constrain the geodynamic transition time from orogenic compression to post-orogenic extension; and (4) discuss the space-time connection between the delamination of lithosphere and tectonic process of the early Paleozoic orogeny of South China.

Section snippets

Geological setting, samples and petrography

The SCB is composed of the Yangtze and Cathaysia Blocks, which are separated by the Jiangshan-Shaoxing Fault in the northeastern (Fig. 1), but the southwestern extension of the boundary that remains controversial (Zhang and Wang, 2007, Wang et al., 2011). Either the Anhua-Luocheng or the Chenzhou-Linwu Fault has been suggested as the southwest boundary (e. g., Chen and Jahn, 1998, Wang et al., 2010, Wang et al., 2011, Wang et al., 2012, Wang et al., 2013a, Wang et al., 2013b, Zhang et al., 2012

Analytical methods

Zircons are separated using standard density and magnetic separation techniques. The samples N16-1, N17-1 and HW-1 from the Longchuan, Longmu and Huwei plutons are selected for SHRIMP zircon U–Pb dating and the samples LM-1, DQM-1, NX12-1 and NX12-2 from the Tangshang, Danqian and Daning plutons are selected for LA-ICPMS zircon U–Pb dating, respectively.

Cathodeluminescenence (CL) imaging are carried out at the State Key Laboratory of Continental Dynamics in Northwest University, Xi'an

Zircon U-Pb ages

Seven representative samples are selected for zircon U-Pb ages. Most zircon grains exhibit oscillatory zoning or wide tabular, and high Th/U ratios (>0.16) of typical magmatic zircon.

Eighteen zircon grains from the Longchuan grabbroic sample (N16-1) show Th/U ratios ranging from 0.44 to 1.34 and yield the weighted mean 206Pb/238U age of 439 ± 2 Ma (MSWD = 1.5) (Fig. 4a), this age is interpreted as the estimate of the time of crystallization.

Seventeen zircon analyses from the Longmu diabase (N17-1)

Petrogenesis

Several models have been proposed for the origin of potassic igneous rocks, including: (1) derivation from depleted mantle, mixed by silicic melts, or contaminated by crustal materials (Benito et al., 1999, Battistini et al., 2001, Hébert et al., 2014); (2) derivation from partial metling of enriched mantle, including crustal recycling mantle (EMI) or metasomatic mantle (EMII) (Weaver, 1991, Turner et al., 1996, Schiano et al., 2004).

Group 1 and 2 have the similar Nb/Ta ratios to primitive

Concluding remarks

  • (1)

    Some potassic and ultrapotassic mafic rocks in South China have been recognized, with emplacement ages of 445–424 Ma.

  • (2)

    Geochronological, geochemical and Sr-Nd isotopic data demonstrate that potassic rocks from group 1 and 3 were most plausibly generated by the partial metling of enriched mantle source. Whereas group 2 originated from the partial melting of their depleted mantle source with subsequent contamination by crustal materials.

  • (3)

    The potassic and ultrapotassic rocks can be responsible for

Acknowledgments

We would like to thank Drs. D. Zhou, X.-F Qiu and H.-L. Yuan for their help during the fieldwork and geochronology analyses. This study was supported by the National Natural Science Foundation of China (Grant Nos. 41302046) and the China Geological Survey Project (nos. 12120113063600).

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