Full length articlePetrogenesis and geodynamic implications of the early Paleozoic potassic and ultrapotassic rocks in the South China Block
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.
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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2021, Journal of Asian Earth SciencesCitation Excerpt :Nowadays, a robust and effective approach is to use multiple isotope systems (such as the Sr–Nd–Hf–Os isotope system) and trace element characteristics to define whether igneous rocks came from an ancient lithospheric mantle or a new asthenospheric mantle source. For instance, ancient lithospheric mantle generally has negative εHf(t) and εNd(t) values, relatively high initial Sr values, relatively high LILE contents, and low HFSE (such as Nb and Ta) contents, which has been enriched during the amalgamation between the Yangtze and Cathaysia Blocks in the Early Neoproterozoic (825–1000 Ma, Yao et al., 2012; Wang et al., 2013b; Zhang et al., 2015; Zhong et al., 2016; Jia et al., 2017), whereas asthenospheric mantle has relatively positive εHf(t) and εNd(t) values and lower initial Sr values (Yi et al., 2014; Peng et al., 2016a, 2016b; Zhang et al., 2016; Qin et al., 2017). According to the reported Sr–Nd and Zr–Hf isotopic compositions of the Early Paleozoic magmatic rocks in the SCB, there is little evidence of magmatic products from the asthenosphere.
Origin of the Heping granodiorite pluton: Implications for syn-convergent extension and asthenosphere upwelling accompanying the early Paleozoic orogeny in South China
2020, Gondwana ResearchCitation Excerpt :Here we put forward another possibility that the asthenosphere-derived depleted magmas might be involved. Jia et al. (2017) and Ou et al. (2018) also suggested some early Paleozoic mafic rocks from the SCB with lower (87Sr/86Sr)i and higher εNd(t) values originated from partial melting of depleted mantle source with subsequent crustal contamination. Furthermore, Zhao et al. (2015, 2018) proposed that the early Paleozoic mafic rocks from Chencai, Northeast CB were derived from a depleted asthenospheric mantle source according to their N-MORB like trace element compositions, highly positive zircon εHf(t) values (+9.8–+15.1) and whole rock εNd(t) values (+8.05 − +8.63).
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2019, Journal of Asian Earth Sciences
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