High Sr/Y magmas generated through crystal fractionation: Evidence from Mesozoic volcanic rocks in the northern Taihang orogen, North China Craton
Graphical abstract
Highlights
► Mesozoic volcanic rocks from the northern Tanghang orogen show high Sr/Y ratios. ► Chemical signature for fractional crystallization of mantle-derived melts. ► Mesozoic magma tectonics beneath the NCC explained through the MASH model.
Introduction
The North China Craton (NCC), one of the fundamental Precambrian nuclei in Asia, is characterized by an Archaean basement intruded by younger magmatic pulses carrying abundant lower crustal xenoliths (Jahn et al., 1987, Liu et al., 1992, Zheng et al., 2004, Zhang et al., 2010). It has been suggested that a significant part of the original subcontinental lithosphere in the eastern part of the NCC was removed since the early Paleozoic (Fan and Menzies, 1992, Menzies et al., 1993, Griffin et al., 1998, Fan et al., 2000, Menzies et al., 2007), and the lithosphere thinning was coupled with widespread Mesozoic magmatism (Zhang et al., 2002, Zhang et al., 2003) and basin development (Li et al., 2003). In the eastern NCC, there are widespread Mesozoic magmatic rocks with high Sr/Y and La/Yb signatures similar to those of modern arc adakites. However, the high Sr/Y rocks show potassium-rich and evolved Sr–Nd isotopic compositions, which are distinct from any slab-derived high Sr/Y rocks. Thus, these rocks are defined as adakitic or adakite-like rocks, and their petrogenesis has been the focus of a number of papers over the last decade (Xu et al., 2002, Gao et al., 2004, Wang et al., 2006a, Wang et al., 2007, Xu et al., 2006, Xu et al., 2008a, Jiang et al., 2007, Yang and Li, 2008). More importantly, these studies proposed that the Mesozoic adakite-like rocks were derived from partial melting of the lower crust and delaminated mafic crust in the underlying convecting mantle beneath the NCC. Therefore, the spatio-temporal distribution and characterization of the Mesozoic high Sr/Y rocks from the NCC are critical in evaluating the process and timing of lithospheric thinning.
The term ‘adakite’ was initially defined by Defant and Drummond (1990) for rocks with distinctive geochemical signatures (high Sr and low Y and HREE concentrations, and resultant high Sr/Y and La/Yb ratios) derived from melting of young subducted oceanic crust. The trace element signatures of these rocks are interpreted to reflect the presence of garnet (and/or amphibole) and absence of plagioclase in the source (Martin, 1999). Such source signatures are typical of young subducted oceanic crust (Defant and Drummond, 1990), slab window processes related to ridge subduction (Thorkelson, 1996, Kinoshita, 2002, Zhang et al., 2010, Eyuboglu et al., 2011b), and thickened lower crust (Atherton and Petford, 1993, Petford and Atherton, 1996). The high Sr/Y rocks from the NCC formed in continental collision or intracontinental strike-slip zones. If the geochemical signatures of adakitic rocks reflect only the source processes, a lower crustal origin for the Mesozoic adakite-like rocks of the NCC seems reasonable. However, some of the reports on Mesozoic adakite-like rocks from the NCC suggest their enriched mantle sources and magma tectonics at lower crustal levels (Chen et al., 2003, Chen et al., 2004, Chen et al., 2005, Chen et al., 2007a, Shao et al., 2006, Shao and Lu, 2008, Zhang and Shao, 2008).
From a petrological point of view, the key geochemical signatures, such as high Sr/Y and La/Yb ratios widely used as diagnostic criteria for the identification of adakites, do not likely reflect the source processes alone, because melting and fractionation are also essentially synchronous processes. Such chemical signatures can be achieved via different petrogenetic processes in a variety of tectonic settings (Moyen, 2009, and references therein). The ‘adakitic signature’ (i.e. high Sr/Y and La/Yb ratios) can be achieved by hydrous (involving amphibole) or high-pressure (involving garnet) crystal fractionation of a mafic magma (Müntener et al., 2001, Garrison and Davison, 2003, Grove et al., 2003, Grove et al., 2005, Macpherson et al., 2006, Müntener and Ulmer, 2006, Richards and Kerrich, 2007, Davison et al., 2008, Chiaradia et al., 2009, Moyen, 2009, Eyuboglu et al., 2011a). Some of these studies suggest that adakite-like signatures can result from a basaltic protolith under pH2O–T conditions in which plagioclase is not stable whereas garnet and/or amphibole are stable phases.
A series of recent works (Xu et al., 2002, Gao et al., 2004, Wang et al., 2006a, Wang et al., 2006b, Wang et al., 2007, Xu et al., 2006, Xu et al., 2008b, Yuan et al., 2006, Jiang et al., 2007, Yang and Li, 2008) suggest that adakite-like rocks in the NCC encompass a large group with common characteristics including high K2O contents and high K/Na ratios as well as evolved Sr and Nd isotopic compositions. Their chemical and isotopic signatures are obviously different from typical adakite-like rocks with a clear crustal origin (Atherton and Petford, 1993, Petford and Atherton, 1996). In particular, the Mesozoic adakite-like rocks from the NCC are often associated with normal calc-alkaline rocks without adakitic signatures in specific volcanic suites (e.g., Yixian Formation volcanic suite and Tiaojishan Formation volcanic suites, Wang et al., 2006b, Yuan et al., 2006). In geochemical discrimination diagrams such as Sr/Y versus Y, and (La/Yb)N versus LaN for adakites (Defant and Drummond, 1990), the whole datasets for Yixian Formation and Tiaojishan Formation volcanic suites exhibit well-defined and continuous trends from the adakite field into the field of normal arc magmas (Wang et al., 2006b, Yuan et al., 2006). Such trends cannot be well interpreted only by source processes in the lower crust beneath the NCC. Therefore, a proper interpretation of the petrogenesis of high Sr/Y rocks is an important requirement for the understanding the magma tectonics and its impact on the lithospheric thinning processes beneath the NCC.
In this study, we report new petrographic, geochemical and isotopic (Sr, Nd, Pb) data from the Mesozoic volcanic rocks from the northern Taihang orogen in the eastern part of the NCC. Our new data show that the adakite-like signatures (e.g., high Sr/Y and La/Yb ratios) of these rocks are primarily the result of fractional crystallization of mantle-derived melts, rather than of partial melting in a thickened lower crust. Thus, the occurrence of Mesozoic adakite-like rocks does not provide a robust evidence for the lower crust delamination beneath the northern Taihang orogen. Our study has important implications in evaluating the geodynamic framework of the NCC lithospheric evolution, and the diverse mechanisms of origin of Mesozoic adakitic rocks in the eastern part of the NCC.
Section snippets
Geological setting
The North China Craton is one of the oldest continental nuclei in the world (Jahn et al., 1987, Liu et al., 1992, Zhai and Santosh, 2011). It is bound on the south by the Qinling–Dabie–Sulu orogenic belt (Li et al., 1993) and the north by the Central Asian orogenic belt (Sengör et al., 1999, Xiao et al., 2010, Rojas-Agramonte et al., 2011). The NCC consists of an Archean and Paleoproterozoic metamorphic basement and an overlying Mesoproterozoic to Phanerozoic unmetamorphosed sedimentary cover.
Field characteristics and petrology
The Mujicun caldera is one of the numerous edifices exposed in the northern part of the Taihang orogenic belt, and consists mainly of welded breccias, ignimbrite sheets and lava flows. This caldera depression has a NE elongate elliptical shape, and is cut across by NE–SW trending faults (Fig. 1c). The volcanics unconformably lie over Paleozoic strata exposed in this area. The lava flows erupted from cinder cones and domes within the caldera and on the external flanks of the collapsed caldera.
Analytical methods
Zircons from the andesite samples were separated using gravitational and magnetic sorting, and handpicked under a binocular microscope. The zircon crystals were mounted in an epoxy disk and polished. Their internal morphology was examined using cathodoluminescence prior to U–Pb isotopic analysis. The U–Pb isotopic analyses were performed using the LA–ICP-MS housed at the State Key Laboratory of Continental Dynamics, Northwest University, China, following the analytical procedures described in
Zircon U–Pb geochronology
The LA–ICP-MS zircon U–Pb data are listed in Table 1. The cathodoluminescence images of the analyzed grains and concordia plot are shown in Fig. 3. Most of the zircons show similar crystal forms, with no resorption or inherited cores. The zircons are transparent, colorless and show clear oscillatory zoning, indicating a magmatic origin (Fig. 3a). They possess high Th/U ratios (0.4–1.7; Table 1) further confirming their magmatic origin. Eight analyses on zircon crystals from LMT-3 andesite yield
Fractional crystallization and/or differentiation processes
As discussed in an earlier section, the Tiaojishan volcanic suite shows a large diversity of lithologies from basaltic though andesitic to rhyolitic composition, with SiO2 contents ranging from 52 wt.% to 70 wt.% (Fig. 4). These volcanic rocks reveal remarkable trends in nearly all their major and trace elements against increasing SiO2 content (Fig. 7, Fig. 8). The major elements such as MgO, Fe2O3T, Al2O3, TiO2, CaO and P2O5 decrease with increasing SiO2 content, reflecting a fractional
Conclusions
Our study leads to the following major conclusions:
- 1.
The magmatic zircons in the Tiaojishan volcanic rocks yield a 206Pb/238U age range of 152 Ma to 138 Ma, with a weighted mean of 145.6 Ma, representing the timing of crystallization of the volcanic rocks. The cores of inherited zircon xenocrysts yield discordant ages of 1840 Ma and 2013 Ma suggesting derivation from ancient crustal sources.
- 2.
The volcanic suite shows a large diversity of lithologies from basaltic though andesitic to rhyolitic
Acknowledgements
This project is financially supported by the National Natural Science Foundation of China (no. 41172068) and the Geological Exploration Project of Hebei Province Department Land & Resources, China (2010–005). We thank Yener Eyuboglu and two anonymous reviewers for constructive comments.
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2021, Journal of Asian Earth SciencesCitation Excerpt :Part of Jurassic igneous rocks with adakitic affinities in the eastern NCC have ever been interpreted as the partial melting of a thickened crust as the result of Middle-Late Jurassic contraction (Zhang et al., 2008; Yang et al., 2012; Chen et al., 2016; Cui et al., 2020). But some authors considered the adakitic signatures could be inherited from the magma sources or fractional crystallization of mantle-derived melts, instead of the partial melting of thickened crust over 50 km (Yang and Li, 2008; Gao et al., 2012; Qian and Hermann, 2013; Ma et al., 2015; Dai et al., 2017). In fact, adakitic rocks are common in the extensional tectonics during NCC destruction of the Early Cretaceous and are farfetched to relate to a thickened crust coeval with a compressional setting (Su et al., 2007; Chen et al., 2013; Zhao et al., 2018).