The Role of Recycled Oceanic Crust in the Generation of Alkaline A-Type Granites
Abstract
Melting and dehydration of subducted oceanic slabs dominate element recycling within the subduction factory, but the role of recycled oceanic crust in the source of intraplate magmas is poorly understood. In situ zircon O-Hf isotope data from two Early Cretaceous alkaline A-type granites that are genetically related to large-scale extension of eastern China in the Late Mesozoic (circa 125 Ma) yield low δ18O (1.8 ± 0.3‰ to 5.1 ± 0.3‰, 2σ) and positive εHf(t) (1.5 ± 1.2 to 17 ± 1.2, 2σ). This suggests the contribution of altered oceanic crust and an enriched mantle component in the source region. Elevated initial 87Sr/86Sr and 206Pb/204Pb ratios, and εHf(t) whole rock values with relatively constant εNd(t) values beyond the normal mantle array, require a component that underwent seawater interaction in the source of protolith. The geochemical data require a complex source region for the alkaline A-type granites in NE China involving more than 40% recycled oceanic crust. This altered oceanic crust beneath the Late Mesozoic lithospheric mantle likely represents remnants of multiple subduction and collision events between microblocks from the Late Paleozoic to Early Mesozoic in northeastern China. Recycling of subducted oceanic crust represents a novel exotic source for the origin of alkaline A-type granites in intraplate extensional settings.
Key Points
- Sr-Nd-Pb-Hf isotopes of two associated alkaline A-type granites suggest that the source region underwent seawater interaction
- Low-δ18O and positive-εHf(t) values of primary zircons require a novel source of altered oceanic crust
- The novel hybrid source is a new model for the genesis of A-type granites and the role of recycled oceanic crust
1 Introduction
A-type granites were originally defined as having alkaline, anhydrous, and anorogenic affinities (Loiselle & Wones, 1979). This definition was later expanded to include some hydrous or aluminous granites in the A-type group (Collins et al., 1982; King et al., 1997) with alkaline and aluminous A-type granites reported from postorogenic and extensional settings (Bonin, 2007). Eby (1992) proposed two subgroups (A1 and A2) based on geochemistry, where A1 granites formed as differentiates of magma derived from sources similar to oceanic island basalts that were emplaced in continental rifts or during intraplate magmatism, whereas A2 magmas were derived from continental crust (or underplated crust) that had been through a cycle of continent-continent collision or island-arc magmatism.
The origin of A-type granites is a long-standing geological problem with at least five genetic models proposed, including (1) fractionation from mantle-derived basaltic magmas, with interaction with continental crust in some cases (Bonin, 2007; Frost & Frost, 2010), (2) high-temperature partial melting of a granulitic residue in the lower crust formed after extraction of an orogenic granite (Collins et al., 1982; King et al., 1997; Whalen et al., 1987), (3) shallow (<15 km) dehydration melting of calc-alkaline (hornblende- and biotite-bearing) granitoids (Douce, 1997), (4) partial melting of an underplated lower crustal source at high temperature in an extensional environment (Frost et al., 2002; Wu et al., 2002), and (5) high-temperature melting of granulitic metasedimentary rocks (Huang et al., 2011).
Granitic magmas generated from either fractionation of mantle-derived magma or remelting of continental crust usually have zircon oxygen isotopes around or above the mantle zircon value (δ18O = 5.3 ± 0.6‰, 2σ; Valley et al., 1998). In contrast, low δ18O values are considered to be the result of high-temperature water-rock interaction in a subducted oceanic slab (e.g., Bindeman et al., 2005; Eiler, 2001), postcaldera volcanism (e.g., Bindeman & Valley, 2000), or a continental rifting event (e.g., Bindeman et al., 2010). Thus, zircon O isotope compositions combined with zircon Hf isotopes are powerful tracers to reveal the protolith source of granites and similar rocks (e.g., Li et al., 2010; Zhu et al., 2017). In this study, we present new zircon O-Hf isotope data for alkaline granite samples from two Early Cretaceous plutons in NE China. Combined with whole-rock Sr-Nd-Hf-Pb isotopes, these data are used to investigate the origin of the A-type granites and propose a novel hybrid source for the generation of alkaline A-type granites.
2 Geological Setting
Northeastern China forms the eastern segment of the Central Asian Orogenic Belt (CAOB, also known as the Altaids), which is one of the largest accretionary orogens on Earth, that evolved over 800 Ma from the latest Mesoproterozoic to the Triassic (Sëngor et al., 1993). Since the Late Triassic, the tectonic framework of the eastern CAOB has been dominated by the closure of the Mongol-Okhotsk Ocean in the northwest, the amalgamation of the North China Craton with Asia, and subduction of the paleo-Pacific oceanic plate in the east (Wu et al., 2002; Xu et al., 2013; Yang et al., 2015). In the Late Mesozoic, regional extension took place throughout the eastern CAOB and adjacent regions, inducing the “giant igneous event” in an extensional setting that includes metamorphic core complexes, extensional basins, lithospheric thinning, and some Au-Mo-REE (rare earth element) mineralization in the Early Cretaceous (Li et al., 2017; Wu et al., 2005). The giant igneous event in NE China covers an area of ~100,000 km2 (Yang et al., 2015; Ying et al., 2010) and is mainly distributed along the Great Xing'an Mountains (Figure 1a). Rocks formed at this time include Late Jurassic andesite, andesitic tuff, and intermediate to felsic volcaniclastic rocks as well as Early Cretaceous rhyolites and andesitic tuffs (Yang et al., 2015).
Two alkaline A-type granite plutons in the Baerzhe area, as well as the Nianzishan alkaline granite pluton, were emplaced into Late Mesozoic volcanic rocks in the south of the Great Xing'an Mountains (Figure 1a). The southern pluton (Baerzhe-801) covers approximately 0.4 km2 at surface and hosts a giant Zr-REE-Nb deposit in the subsolvus phase (Yang et al., 2014), in the upper section of the pluton, whereas the hypersolvus phase in the lower section is barren (or weakly mineralized; Jahn et al., 2001). Both the mineralized and barren granites consist mainly of microcline, quartz, albite, arfvedsonite, and aegirine. The modal abundances of these minerals, their grain sizes, and crystal habits differ between the mineralized and barren counterparts (Figures 2a and 2b). The upper subsolvus granite is mainly composed of quartz (30–35 vol %), alkaline feldspar (20–25 vol %), albite (15–30 vol %), aegirine (4–6 vol %), and minor arfvedsonite. The lower hypersolvus granite is mainly composed of quartz (20–25 vol %), alkaline feldspar (50–60 vol %), arfvedsonite (10–15 vol %), and minor albite and aegirine. The occurrence of abundant miarolitic cavities in the upper portion suggests a shallow level emplacement (<2 km) and a high degree of fractionation (Jahn et al., 2001). In contrast, the northern pluton (Baerzhe-802) covers ~1.3 km2 on surface, is porphyritic and unmineralized. The phenocrysts include quartz (20–40 vol %), alkaline feldspar (40–60 vol %), and arfvedsonite (1–3 vol %), with crystal sizes of approximately 0.3–3 mm (Figures 2c and 2d).
3 Results
All analytical methods and full data sets are presented in the supporting information, with parts that are summarized in Table 1. The granites from the Baerhze-801 pluton have variable SiO2 contents, ranging from 64.8 to 75.3 wt %. They have high K2O (3.46–6.14 wt %), Na2O (2.71–6.30 wt %), and FeO(total) (2.17–7.50 wt %), but relatively low Al2O3 (9.75–16.27 wt %), MgO (0.02–0.70 wt %), and CaO (0.06–2.88 wt %) contents. They are enriched in high field strength elements (HFSE, total contents of 425–35,263 ppm), and have high FeO(t)/MgO (5.91–375) and Ga/Al ratios (2.37–11.3). In comparison, granites from the Baerhze-802 pluton have a narrow compositional range, with SiO2 varying from 76.0 to 77.0 wt %. They also have high contents of K2O (3.13–5.52 wt %), Na2O (3.03–4.74 wt %), and FeO(total) (2.47–7.50 wt %), and have high FeO(t)/MgO (49.4–144) and Ga/Al (4.30–5.74) ratios and HFSE (1450–1681 ppm).
| Isotopes | Baerzhe-801 pluton | Baerzhe-802 pluton |
|---|---|---|
| Zircon U-Pb ages | 124 ± 1 Ma (2σ) | 122 ± 1 Ma (2σ) |
| Zircon δ18O values | +2.8‰ to +5.1‰ | +1.8‰ to +2.7‰ |
| Zircon εHf(t) values | +1.5 to +13 | +6.9 to +17 |
| Initial 87Sr/86Sr values | 0.7048 to 0.7061 | 0.7055 |
| εNd(t) values | +1.0 to +2.5 | +1.3 to +2.0 |
| εHf(t) values | +6.1 to +7.4 | +6.3 to +7.5 |
| Initial 206Pb/204Pb values | 17.95–18.30 | 18.24–18.29 |
- Note. Zircon U-Pb ages and O-Hf isotopic data of Baerzhe-801 pluton were previously reported in Yang et al. (2013).
Zircon grains from the two plutons show oscillatory magmatic zoning under cathodoluminescence (CL) imaging and lack notably altered textures (Figure 3). Magmatic zircon grains from the Baerzhe-801 pluton have low U (87–249 ppm) and Th (48–237 ppm) content, with Th/U ratios of 0.37–1.0. They yielded concordia U-Pb ages of 124 ± 1 Ma (Figure 4a). Zircon from two samples of the Baerzhe-801 pluton yielded δ18OVSMOW (Vienna Standard Mean Ocean Water) values between 2.8‰ and 5.1‰ and εHf(t) values between 1.5 and 13 (Figure 5), with two-stage Hf model ages (TDM2) of 373 to 1,083 Ma. Similarly, zircon grains from the Baerzhe-802 pluton have low U (110–417 ppm) and Th (46–275 ppm) contents, with Th/U ratios of 0.39–0.85 and concordia U-Pb ages of 122 ± 1 Ma (Figure 4b). They yielded δ18OVSMOW values between 1.8‰ and 2.7‰ and εHf(t) values between 6.9 and 17 (Figure 5), with TDM2 of 78 to 661 Ma.
Granite samples from both plutons have evolved Sr-Nd-Hf-Pb isotope compositions (Table 1). Initial 87Sr/86Sr ratios ranging from 0.7048 to 0.7061 were obtained for least fractionated samples with relatively low Rb/Sr ratios from the Baerzhe-801 pluton. Sample 8023H5 from the Baerzhe-802 pluton with the lowest Rb/Sr ratio (14) has initial 87Sr/86Sr ratios of 0.7055, similar to those of the Baerzhe-801 pluton. Epsilon Nd(t) values for the Baerzhe-801 pluton range from +1.0 to +2.5, corresponding to two-stage Nd model ages of 714 to 837 Ma. Epsilon Nd(t) values of the Baerzhe-802 pluton range from +1.3 to +2.0, corresponding to two-stage Nd model ages of 754 to 808 Ma. Epsilon Hf(t) values range from +6.1 to +7.4 in Baerzhe-801 and from +6.3 to +7.5 in the Baerzhe 802 pluton, with two-stage Hf model ages of 703 to 786 Ma and 700 to 774 Ma, respectively. The initial 206Pb/204Pb ratios of the Baerzhe-801 and Baerzhe-802 pluton range from 17.95 to 18.30 and 18.24 to 18.29, respectively.
4 Discussion
4.1 Petrogenesis of the Baerzhe Granites
Granites from the two Baerzhe plutons show A-type affinities based on both petrography and geochemistry. They have high alkali and HFSE contents, high FeO(t)/MgO and Ga/Al ratios, low Sr and Eu contents, and plot within the A1 subtype of the A-type granites (e.g., Frost & Frost, 2010). The occurrence of the sodic-amphibole (arfvedsonite) suggests that they are alkaline A-type granites (Jahn et al., 2001).
The compositional diversity of A-type granites is usually explained as the result of crustal assimilation or magma mixing (Kemp et al., 2005; Monani & Valley, 2001; Yang et al., 2006). However, despite the postmagmatic alteration, there is no field evidence for assimilation in the Baerzhe granites, such as enclaves and/or xenoliths. Additionally, both of the plutons lack xenocrystic zircon, which argues against the assimilation of ancient felsic crust. The narrow range of εNd(t) values (+1.0 to +2.5), and elevated initial 87Sr/86Sr (0.7048 to 0.7075) and 206Pb/204Pb values (17.95 to 18.30) are better explained by seawater alteration within the protolith rather than crustal assimilation (e.g., Menzies & Seyfried, 1979). The geochemistry of these granites does not support assimilation but instead indicates a dominant role for fractionation. For example, Eu anomalies decrease sharply with increasing SiO2 contents and Rb/Sr ratios, consistent with the fractionation of alkali feldspars (e.g., Borchert et al., 2010). As REE (except Eu) and other HFSE are incompatible in plagioclase (Fujimaki et al., 1984), their contents will increase during magmatic fractionation. In the case of the Baerzhe-801 granites, Zr, REE and Nb are strongly concentrated in the subsolvus granites, which suggests that they are highly fractionated (Jahn et al., 2001). Therefore, the geochemical variations of the alkaline granites from the two plutons support fractionation rather than significant crustal assimilation.
The variation of zircon O-Hf isotopes is not consistent with a homogeneous source for the Baerzhe granites. Given that there is no significant evidence for inherited or xenocrystic zircons, the magmatic zircons from Baerzhe are characteristic of the primary magma. Zircon δ18O and εHf(t) values of both plutons are not very restricted (Figure 5), which could have resulted from the mixing of two or more protolith sources. Compared to the zircon O-Hf isotopes of the Baerzhe-801 pluton, the magmatic zircons from the Baerzhe-802 pluton have lower δ18O and higher εHf(t) values (Figure 5), which require a low-δ18O source component with very positive εHf(t) values.
The generation of low-δ18O magma has typically been explained by invoking high-temperature tectonic scenarios, including continental rifts, caldera collapse, and subducted slabs (e.g., Bindeman & Valley, 2000; Wei et al., 2008). High-temperature interaction between rock and fluid are essential for the formation of low-δ18O magma (e.g., Bindeman et al., 2010; Bindeman & Valley, 2000; Zheng et al., 2007). Late Mesozoic A-type granites in eastern China are widely distributed from north to south along the margin of the Pacific Ocean, including some alkaline and low-δ18O granites (e.g., Martin et al., 1994; Wei et al., 2008; Wu et al., 2002). The low δ18O and positive εHf(t) isotopic signatures of the Baerzhe zircons suggest the protolith of the alkaline granitic magmas was juvenile and subjected to high-temperature interaction with seawater. Thus, the recycling of juvenile oceanic crust via slab subduction is probably the main source for the Baerzhe granites rather than continental rifting or caldera collapse. Accordingly, the subducted oceanic crust, which had interacted with seawater components, is the most likely candidate for the protolith of the alkaline A-type granites in Baerzhe.
4.2 Contribution of Previous Altered Oceanic Crust
Petrogenetic models of A-type granite predominantly focus on differentiation or partial melting of mafic magma (Bonin, 2007; Frost & Frost, 2010; Litvinovsky et al., 2015), or partial melting of crustal components (Douce, 1997; Huang et al., 2011), with possible hybrid models (Papoutsa & Pe-Piper, 2014; Yang et al., 2006). Zircon in equilibrium with melts that originated from any of these magma sources would have δ18O values equal or higher than that of mantle zircon (5.3 ± 0.6‰) (e.g., Huang et al., 2011; Kemp et al., 2007; Valley, 2003). The two alkaline granite plutons in the Baerzhe area have much lower zircon δ18O values (+1.8‰ to +5.1‰) than that of mantle zircon, mostly between 2‰ and 4‰ (Figure 5a). Since the pristine magmatic zircons preserve their δ18O value from the time of crystallization (Muñoz et al., 2012; Valley, 2003; Yang et al., 2014), the low-zircon δ18O values in the Baerzhe granites require a significant contribution of low-δ18O source. One such source would be melts of hydrothermally altered gabbros (+2‰ to +5‰) from subducted slabs as proposed by Bindeman et al. (2005).
As zircon is a robust mineral that is typically unaffected by alteration, the initial low-δ18O values of the Baerzhe zircons suggest derivation from a source region that was modified by earlier interaction with low-δ18O fluids, such as meteoric water or seawater (e.g., Bindeman & Valley, 2000). Given that all zircon δ18O values from this study are positive, we exclude the possibility of meteoric water, which would have extremely negative δ18O values (e.g., Bindeman et al., 2010; Yang et al., 2013). Thus, the required fluid with low-δ18O values likely represents seawater components from a subducted slab, which is consistent with the evidence for seawater alteration suggested by the whole-rock Sr-Nd-Pb-Hf isotopes (Figure 5). A slab-derived fluid interacting with juvenile gabbro under high temperatures would sharply decrease the δ18O value of altered gabbro, which could then represent a possible source for the low-δ18O granitic magma (e.g., Bindeman et al., 2005). However, altered juvenile gabbro would typically have very positive εHf(t) values much higher than the corresponding Baerzhe zircons in this study, because the Hf content of seafloor fluids is significantly lower than that of oceanic crust. For example, gabbro modified by seawater-derived fluids from the Juassic Liguran ophiolites has δ18O values (+1.0‰ to +0.7‰) close to seawater, with an initial εNd(t) value of +9.2, corresponding to estimated εHf(t) value of +15 (Tribuzio et al., 2014). Assuming that the ratio of Hf contents between juvenile gabbro and seafloor fluid is 107 (based on the data of Rickli et al., 2009), we calculated a mixing curve for the possible O-Hf isotopes of altered oceanic crust (Figure 6). In comparison, the most altered oceanic crust reported by Tribuzio et al. (2014) show similar trend with the estimated curve, indicating that the calculated model is feasible. However, the zircon O-Hf isotopes from the Baerzhe granites are mostly beyond the calculated curve between juvenile gabbro and seafloor fluid (Figure 6). Therefore, partial melts of altered juvenile gabbro alone would not have O-Hf isotopes similar to those of the Baerzhe alkaline granites.
The involvement of enriched mantle components, with evolved Nd-Hf isotopes and normal depleted mantle δ18O value (δ18O = 5.3 ± 0.6‰) (Valley et al., 1998), is required in the source region of the Baerzhe alkaline granites. The enriched lithospheric mantle component can be constrained by the regional mantle-derived Early Cretaceous basalts and gabbros (Guo et al., 2010), with εNd(t) values between -2 and +3 (Figure 7d). Thus, we assume that the enriched mantle component, with a zircon δ18O value of +5.3‰ and εHf(t) value of 0, represents one end-member (source B) in the source region, with the juvenile gabbro altered by 50% seawater-derived fluid representing the second (source A). Binary mixing of these two sources can account for the variations of zircon O-Hf isotopes in the Baerzhe alkaline granites, as well as the Nianzishan alkaline granites (Figure 6). The Baerzhe-802 can be modeled with a contribution of more than 80% altered oceanic crust whereas Baerzhe-801 granite requires 40%–70% altered oceanic crust. In contrast, the Nianzishan alkaline granites can be modelled with less than 40% altered oceanic crust (Figure 6). In summary, the O-Hf isotopic variations of zircons from alkaline granites in NE China can be explained by melting of a hybrid source consisting of different proportions of altered oceanic crust and enriched mantle components.
Aspects of the regional geology are consistent with an altered oceanic crust component in the source region of the granites, including the Late-Paleozoic Hegenshan ophiolitic complex which is composed of serpentinite, metagabbros, metabasalt, and metadiabase (Miao et al., 2008; Nozaka & Liu, 2002). The Hegenshan ophiolitic complex has high positive εNd(t) values (+8 to +11), consistent with derivation from a subduction-modified N-MORB-like source (mid-ocean ridge basalt) (Miao et al., 2008). In addition, regional Early Cretaceous basalts and gabbros are characterized by enriched Sr-Nd-Pb isotopic signatures (Figure 7), indicating that the Early Cretaceous regional lithospheric mantle was metasomatized by slab-derived fluids. Thus, the previously altered oceanic crust and enriched mantle components proposed to be the source of the Baerzhe alkaline granites likely existed during the Early Cretaceous beneath NE China.
4.3 Implications for the Recycled Oceanic Crust
A consequence of plate tectonics is that geochemical recycling occurs via subduction zone processes (e.g., Walter et al., 2011). The presence of recycled sediments from the subducted paleo-Pacific slab in the Late Mesozoic-Cenozoic continental basalts of eastern China has been advocated based on low-δ26Mg and heavy-δ66Zn isotopic compositions (Huang et al., 2015; Liu et al., 2016). Since the Late Mesozoic, the residual mantle wedge underneath eastern Asia has been metasomatized by the melting of the carbonate-bearing oceanic crust of the paleo-Pacific plate (Liu et al., 2016). The recycled components from the subducted oceanic crust may have been derived from the seismically detected stagnant Pacific slab within the mantle transition zone (Xu et al., 2012). The signature of subducted oceanic crust has been recognized in continental basalts from the Late Mesozoic to Cenozoic across eastern China (Huang et al., 2015; Liu et al., 2016; Xu et al., 2012), but it has not been documented in the associated granitoids.
The Early Cretaceous of eastern China has been proposed to be a peak period of intracontinental extension as a result of gravity-induced collapse, likely related to the far-field effect of the paleo-Pacific plate subduction (Li et al., 2017). The Early Cretaceous large-scale igneous event in NE China and the destruction of the North China Craton were probably induced by intense lithospheric thinning (Wu et al., 2005; Xu et al., 2009). In this scenario, the trapped slabs of Paleozoic oceanic crust were preserved for tens of millions of years under the lithosphere (Li et al., 2017), before becoming unstable due to conductive heating from the underlying lower mantle in response to Early Cretaceous extension, releasing volatiles and slab melts into the crust triggering fluxed melting of the overlying mantle wedge. This model is permissive of partial melting of a hybrid source, composed of altered oceanic crust and enriched mantle-derived mafic rocks, for the generation of alkaline A-type granites in the Early Cretaceous extensional setting of eastern China. Consequently, the recycling of subducted oceanic crust can be recognized in both the continental basalts and the intraplate granitoids.
5 Conclusions
The combined zircon hafnium and oxygen isotope data, as well as the whole-rock Sr-Nd-Pb-Hf isotopes presented here, demonstrate that the Early Cretaceous alkaline A-type granites at Baerzhe are derived from an exotic source which is much more complicated than previously thought. The range of zircon δ18O (+1.8‰ to +5.1‰) and εHf(t) (+1.5 to +17) for the two plutons requires a complex and hybrid source, consisting of altered oceanic crust and enriched mantle-derived components. In addition, the elevated initial 87Sr/86Sr, 206Pb/204Pb, and εHf(t) values of granites are consistent with a subducted oceanic crust component in the source region. The isotopic characteristics of the majority of samples can be accounted for by a greater than 40% contribution of altered oceanic crust in the source region. The recycling of subducted oceanic crust in the source of the granitoids suggests that element recycling within the subduction factory was the dominant process in the generation of intraplate granites in extensional settings.
Acknowledgments
Presented data are available in the supporting formation file or by request from the corresponding author. We sincerely thank Xian-Hua Li and Qiu-Li Li, Hong Zhang and Xiang-Lin Tu for the help with SIMS zircon O isotopes and U-Pb dating, MC-ICP-MS zircon Hf isotope and whole rock analyses, respectively. The constructive comments from two anonymous reviewers and editorial handling by Michael Walter are greatly appreciated. This study was financially supported by the National Key R&D Program of China (2016YFC0600408 and 2017YFC0602301), National Natural Science Foundation of China (grants 41472062 and 41373031), and the Youth Innovation Promotion Association CAS to W. B. Yang. This is contribution IS-2464 from GIGCAS.
