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Geochemical Evidence for Thickening and Thinning of Lower Crust beneath the Jiaodong Peninsula: Implications for Late Mesozoic Destruction of the Eastern North China Craton
Lithosphere ( IF 2.4 ) Pub Date : 2020-06-30 , DOI: 10.2113/2020/8831664 YaYun Liang 1 , Wenhui Guo 1 , Yao Ma 2 , Enquan Zhao 3
Lithosphere ( IF 2.4 ) Pub Date : 2020-06-30 , DOI: 10.2113/2020/8831664 YaYun Liang 1 , Wenhui Guo 1 , Yao Ma 2 , Enquan Zhao 3
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The eastern North China Craton (NCC) has been recognised as undergoing cratonic destruction during the Mesozoic; however, the mechanism of its destruction is still unclear. The main difference between the proposed models is whether the lower continental crust (LCC) underwent thinning. In this study, we conducted comprehensive analyses of Late Mesozoic felsic intrusive rocks, including Late Jurassic granites (166–146 Ma), Early Cretaceous granodiorites (136–123 Ma), and latest Early Cretaceous granites (123–108 Ma) from the Jiaodong Peninsula, located on the southeastern margin of the NCC. These rocks allowed us to investigate variations in the LCC thickness in this region and to further discuss the destruction mechanism of the eastern NCC. Here, temporal variations in crustal thickness can be tracked using whole-rock La/Yb ratios of the felsic intrusive rocks. Our study shows that the continental crust in the eastern NCC thickened during the Late Jurassic (>40 km) due to compression and the westward subduction of the Palaeo-Pacific Ocean lithosphere beneath the NCC since the Early Jurassic. The continental crust further thickened during the Early Cretaceous, caused by the steepening of the subducting slab after ~144 Ma that produced crustal underplating of mantle-derived melts in an extensional setting. However, the continental crust thinned (20–40 km) during the latest Early Cretaceous, caused by the rollback of the subducting slab after ~123 Ma. The geochemical compositions of three stages of felsic intrusions also suggest that the regional tectonic stress that affects the eastern NCC altered from a compressional to an intraplate extensional environment after ~144 Ma. Thus, the Late Mesozoic destruction of the eastern NCC and its accompanying magmatism were controlled by prolonged thermomechanical-chemical erosion due to low-angle subduction, steepening, and rollback of the Palaeo-Pacific Oceanic lithosphere.The North China Craton (NCC) is a typical ancient craton that formed during the Neoarchaean to the Palaeoproterozoic [1–3] and has been recognised as having undergone destruction during the Mesozoic. Geochemical and geophysical data have shown that the lithospheric thickness of the Eastern Block of the NCC varied from ~200 km during the early Palaeozoic to less than 80 km during the Mesozoic-Cenozoic ([4, 5]; Zhang et al., 2008; [6]). During the same time period, large-scale mafic to felsic magmatism and mineralisation occurred throughout the eastern NCC (e.g. [7–11]). However, the timing and mechanism of the destruction of the eastern NCC are still poorly understood. Two major models have been proposed, including: (1) a model of prolonged thermomechanical-chemical erosion, suggesting that the lowermost lithosphere was gradually eroded away by asthenospheric mantle convection [12, 13] or by the rollback of the Palaeo-Pacific plate [14] and (2) a model of rapid lithospheric delamination, assuming that the thick lithosphere foundered into the underlying convecting mantle [5, 15, 16], or rapid lower crustal delamination, implying that the foundering of the lower crust produced lithospheric extension and asthenospheric upwelling [17–19]. Thus, one of the main differences between these models is whether the lower continental crust (LCC) in the eastern NCC underwent thinning.Whole rock Sr/Y and La/Yb ratios of intermediate-felsic rocks in subduction-related arcs have been shown to correlate well with crustal thicknesses at global and regional scales [20, 21], as well as to the changes in crustal thickness during collisional processes [22]. Although three major periods of felsic magmatism, including Triassic, Late Jurassic, and Early Cretaceous, have been observed in the eastern NCC, it is not clear whether these magmatic events were derived from reworking of the continental crust or from the destruction of cratonic mantle (Dai et al., 2016). Studying the whole-rock La/Yb ratios of granitic magmatism is important for understanding the thickness variations in the LCC. For example, Zhu et al. [23] determined the spatial and temporal variations in crustal thickness using whole-rock La/Yb ratios of intermediate intrusive rocks from the Gangdese arc during 70–10 Ma. Huang, et al. [9] also identified Late Cenozoic magmatic inflation, crustal thickening, and >2 km of surface uplift in central Tibet through studies of Late Eocene to Pliocene volcanic rocks.In the Jiaodong Peninsula, located on the southeastern margin of the NCC, Late Mesozoic felsic intrusive rocks ranging from Late Jurassic to latest Early Cretaceous are widespread and provide vitally important details for revealing not only the thickening and thinning processes of the lower crust beneath the Jiaodong Peninsula but also the mechanism that controlled the destruction of the eastern NCC during the Late Mesozoic. Although previous studies have discussed the petrogenesis of the three stages of emplaced granites, the sources of these rocks remain poorly constrained and it is unclear whether the lower crustal source of the eastern NCC underwent thinning. In this study, based on previously published data from the Late Mesozoic felsic intrusions, we provide new analyses and explanations for the geochemical data from the Late Jurassic to Early Cretaceous intrusive rocks on the Jiaodong Peninsula. The geochemical variations described herein constrain the changes in crustal source for the Late Mesozoic granites and thinning of the lower crust, which further constrains the mechanism of the destruction of the eastern NCC.The NCC is bordered by the Central Asian orogenic belt to the north, the Qinling-Dabie-Sulu orogenic belt to the south, and the Pacific Ocean to the east (Figure 1(a)). From the Triassic to the Jurassic period, the NCC and the Yangtze Craton (YC) collided, accompanied by continental subduction and crustal thickening, forming the Qinling-Dabie-Sulu orogenic belt [24]. From the Late Permian to the Early Jurassic, the northern margin of the NCC experienced compression and crustal growth as the ancient Asian Ocean closed (Xiao et al., 2003). During the Late Mesozoic, the Western Block of the NCC was relatively stable, while the eastern region underwent lithospheric thinning and delamination, accompanied by large-scale magmatic activity and the genesis of gold ore deposits [5, 14]. Since the Early Jurassic, the eastern part of the NCC and the entire eastern region of China have been influenced by the westward subduction of the Pacific plate [25].The Jiaodong Peninsula is located on the eastern margin of the NCC (Figure 1(a)) and is composed of the Jiaobei terrane and the Sulu orogenic belt, which are separated by the Wulian-Yantai fault (Figure 1(b)). The Precambrian basement in this region is composed of the Neoarchaean Jiaodong Group, the Palaeoproterozoic Fenzishan and Jinshan Groups, and the Meso-Neoproterozoic Penglai Group. The Jiaodong Group contains Neoarchaean tonalite-trondhjemite-granodiorite (TTG) gneisses. The Fenzishan and Jinshan Groups include sillimanite-biotite schists, biotite gneisses, leptynites, marbles, amphibolites, metamorphic sandstones, and graphite-bearing rocks. The Penglai Group contains fossiliferous fluvial-clastic rocks and carbonates [14]. A series of Triassic ultrahigh pressure (UHP) metamorphic rocks are spread throughout the Sulu orogenic belt [26].The Mesozoic granitic intrusive rocks are mainly distributed in the Jiaobei terrane and can be grouped into Late Jurassic plutons (166–146 Ma), Early Cretaceous intrusions (136–123 Ma), and latest Early Cretaceous intrusions (123–108 Ma) [27–30]. Additionally, a large number of Late Mesozoic mafic dykes are emplaced into the granites and the metamorphic basement (130–110 Ma; [14, 15]).Our analyses are based on a comprehensive compilation of previously published geochronological and geochemical data obtained from the felsic intrusive rocks on the Jiaodong Peninsula (Late Jurassic granites (166-146 Ma) are from [26, 31–35]; Early Cretaceous granodiorites (136-123 Ma) are from [26, 29, 30, 32, 34, 36]; latest Early Cretaceous granites (123-108 Ma) are from Huang et al., 2006; [26, 27, 32, 34, 37–42]). These datasets reveal major chemical variations over time, thus constituting a tool for tracking possible changes in the magmatic sources and lithospheric destruction. Zircon U-Pb ages, Hf-O isotopic data, whole-rock major and trace elemental concentrations, and Sr-Nd-Pb isotopic data were collected from previous studies and are listed in the supplementary tables.The Late Jurassic felsic intrusive rocks (166–146 Ma) all exhibit high SiO2 and K2O contents, mostly plotting in the high-K calc-alkaline series, with minor shoshonitic series rocks (Figures 2(a) and 2(b); Supplementary Tables 1 and 2). They also have relatively high Al2O3 and Na2O contents, with high A/CNK [molar ratio Al2O3/Na2O+K2O] values, which places them in the peraluminous series (Figure 2(c)). They have low Mg# values (11.0–48.8) and limited MgO contents (0.05–1.37 wt.%) (Figures 3(a)–3(d)) (Supplementary Table 2). They have high Sr (145–1700 ppm, average 574 ppm) and low Y (1.20–14.8 ppm, average 6.87 ppm) and Yb (0.25-3.13 ppm, average 0.73 ppm) compositions, with high Sr/Y (20.4–308, average 84.5), Dy/Yb (1.03–3.36, average 1.94), and La/Yb (7.48–104, average 44.8) ratios, showing adakitic features (Figures 3(f) and 3(g); Figure 4). In addition, these granites are depleted in high field strength elements (HFSE, such as Nb, Ta, Zr, and Hf) and heavy rare earth elements (HREEs), and are enriched in large ion lithophile elements (LILEs) and light rare earth elements (LREEs), along with lack of a Eu anomaly (δEu=0.13–0.89) (Figures 5(a) and 5(b)). They have high (87Sr/86Sr)i (0.707636–0.713900) and low εNd(t) (-22.1 to -10.9) values (Figure 6(a)) and narrow ranges of (206Pb/204Pb)i (16.550–17.060), (207Pb/204Pb)i (15.380–15.470), and (208Pb/204Pb)i (36.920–37.780) isotopic ratios (Figures 6(b) and 6(c)) (Supplementary Tables 1 and 3). Zircons from the Late Jurassic granites exhibit εHf(t) values of -43.5 to -11.3 (Figure 7), with two-stage model ages (TDM2) of 2843–1914 Ma (Figure 8) (Supplementary Tables 1 and 4).The Early Cretaceous felsic intrusive rocks (136–123 Ma) also have high SiO2, Na2O, and K2O contents, mostly plotting in the high-K calc-alkaline series with minor calc-alkaline series rocks (Figures 2(a) and 2(b); Supplementary Tables 1 and 2). Most of these granitoids are metaluminous and slightly peraluminous (Figure 2(c)). They have large ranges of Mg# (22.9–71.4) and limited MgO contents (0.14–2.13 wt.%) (Figures 3(a)–3(d)) (Supplementary Table 2). This stage of intrusive rocks also shows adakitic features with higher Sr (42.9–2302 ppm, average 1159 ppm) contents and lower Y (2.41–39.0 ppm, average 8.92 ppm) and Yb (0.16-3.71 ppm, average 0.67 ppm) contents, along with higher Sr/Y (62.6–378, average 168), Dy/Yb (1.45–4.41, average 2.98), and La/Yb (8.46–206, average 96.3) ratios than the Late Jurassic intrusive rocks (Figures 3(f) and 3(g); Figure 4). These granites exhibit HFSE and HREE depletions and LILE and LREE enrichments, along with the lack of a Eu anomaly (δEu=0.06–0.64) (Figures 5(c) and 5(d)). They have high (87Sr/86Sr)i (0.710175–0.712539) and low εNd(t) (-22.3 to -11.5) values (Figure 6(a)), with narrow ranges of (206Pb/204Pb)i (16.880–17.330), (207Pb/204Pb)i (15.440–15.544), and (208Pb/204Pb)i (37.590–38.282) isotopic ratios (Figures 6(b) and 6(c)) (Supplementary Tables 1 and 3). Zircons from Early Cretaceous granites exhibit εHf(t) values of -25.1 to -11.6 (Figure 7), with TDM2 ages of 3221–1917 Ma (Figure 8) (Supplementary Tables 1 and 4).The latest Early Cretaceous felsic intrusive rocks (123–108 Ma) all have high contents of SiO2, K2O, and Na2O, belonging to the high-K calc-alkaline to shoshonitic series (Figures 2(a) and 2(b); Supplementary Tables 1 and 2). Most of these granitoids are metaluminous to peraluminous (Figure 2(c)). The granites have large ranges of Mg# (5.05–64.6) and MgO (0.04–3.87 wt.%) (Figure 3; Supplementary Table 2). These intrusive rocks have lower Sr/Y (0.27–108, average 25.3), Dy/Yb (0.39-2.49, average 1.54), and La/Yb (2.59–176, average 38.7) ratios than those of the two earlier stages (Figures 3(f) and 3(g); Figure 4). They exhibit HFSE depletion and LILE and LREE enrichments, along with a negative Eu anomaly (δEu=0.02–0.78) (Figures 5(e) and 5(f)). They have high (87Sr/86Sr)i (0.705400–0.724600) and low εNd(t) (-22.3 to -3) values (Figure 6(a)), and have narrow ranges of (206Pb/204Pb)i (16.119–17.342), (207Pb/204Pb)i (15.386–15.505), and (208Pb/204Pb)i (36.194–37.930) isotopic ratios (Figures 6(b) and 6(c)) (Supplementary Tables 1 and 3). Zircons exhibit εHf(t) values of -25.5 to -5.4 (Figure 7), with TDM2 ages of 3900–1515 Ma (Figure 8) (Supplementary Tables 1 and 4).Late Jurassic granites from the Jiaodong Peninsula have high SiO2 and Al2O3 contents, high Sr/Y and La/Yb ratios, and most of the samples plot in the adakite field (Figure 5). The geochemical characteristics of the Late Jurassic granites are similar to C-type adakitic rocks that are derived from a thickened LCC [43]. According to plots of MgO vs. SiO2 and TiO2 vs. SiO2, the Late Jurassic granites plot in the ranges for a thickened LCC (Figures 3(a) and 3(b)), which resemble the Late Mesozoic thickened LCC-derived adakitic plutons in the eastern NCC [13, 44]. These granites have high SiO2 and low MgO contents, high initial 87Sr/86Sr ratios (0.708–0.714), and negative εNd(t) (-22.1 to -10.9) and zircon εHf(t) (-43.5 to -11.3) values, indicating a continental crust-derived or highly evolved magma. The elemental correlation shows that partial melting plays a significant role in the magmatism, rather than fractional crystallisation (Figures 3(g) and 3(h)). The two-stage Nd model ages (TDM2) for these granites range from 2732 to 2047 Ma (Figure 8(a)). Magmatic zircons from the Late Jurassic granites exhibit TDM2 ages from 3221 to 1917 Ma (Figure 8(b)). Neoarchaean and Palaeoproterozoic inherited zircons (2848–1615 Ma) were also observed in these granites (Supplementary Table 3). The measured ages are consistent with granulite xenoliths captured in Late Cretaceous mafic dykes on the Jiaodong Peninsula (2.5–2.4 Ga and 2.0 Ga, respectively; [45]) and ancient basement beneath this area (2.9–2.5 Ga, 2.2–1.9 Ga, and~1.9 Ga; [3]; Wan et al., 2006). Moreover, Li et al. [32] showed that the Late Jurassic Linglong granites have high δ18O values (5.72–8.34‰), indicating an Archaean LCC source. These observations strongly argue for a Late Jurassic granitic origin related to partial melting of the thickened Archaean LCC of the eastern NCC.The Early Cretaceous granodiorites also exhibit adakitic features, implying a thickened lower crustal source. The εHf(t) values, which lie between the 1.9 and 2.5 Ga crustal evolution lines, imply a Palaeoproterozoic crustal source (Figure 7). The granodiorites also have high δ18O values (7.46–8.67‰, [32]), supporting an ancient LCC source. The TDM2 ages for εNd(t) and εHf(t) are 2931–2615 Ma and 3221–1917 Ma, respectively (Figure 8). The Neoarchaean and Palaeoproterozoic inherited zircon ages (2771–1617 Ma) from the Guojialing granodiorites are also consistent with the ancient basement rocks in the NCC (Supplementary Table 3). These characteristics indicate that the Guojialing granodiorites were sourced from partial melting of the thickened Archaean-Palaeoproterozoic LCC of the NCC. In contrast to the Late Jurassic granites, the Guojialing granitoids have higher εNd(t) and εHf(t) values (Figures 5, 6, and 7), suggesting a change in the source. Yang et al. [46] showed that the Guojialing granodiorites and dioritic enclaves occurred contemporaneously (127~123 Ma), indicating a mixed magma source. Compared to the Late Jurassic granites, the Guojialing granodiorites have a higher and wider range of Mg# (23–71) values, approaching the values of the Early Cretaceous lithospheric mantle-derived mafic dykes at Jiaodong (Mg#=55–78; [14]). The Guojialing granodiorites also exhibit approximately similar values of εNd(t) and (87Sr/86Sr)i as the Jiaodong mafic dykes (Figure 6). These geochemical characteristics reveal a crustal source with added mantle-derived components.All of the latest Early Cretaceous granites likely originated from a continental crustal source, based on their characteristic low εNd(t) (-22.3 to -3) and zircon εHf(t) (-25.5 to -5.4) values, high δ18O values (6.6 to 7.9‰), enrichment in LREEs and LILEs, and depletion in HFSEs (Figure 4). Most of the TDM2 ages derived from Hf isotopes varied from 2500 to 1515 Ma, with a few >3000 Ma (Figure 8) ages, indicating a Palaeoproterozoic crustal source and minor Neoarchaean components, which is consistent with the composition of the ancient basement of the Jiaodong Peninsula [47]. However, the ancient lower crust of the NCC is characterised by low Rb content, enrichment in Hf, and depletion in Zr, Th, and U (Gao et al., 1998), which is inconsistent with these latest Early Cretaceous granites. This shows that such granites are unlikely to have been directly or solely sourced from partial melting of the ancient NCC lower crust. Isotopically, they have similar (87Sr/86Sr)i ratios as the lower NCC crust, but their εNd(t) values plot between the ranges of the lower crust and the lithospheric mantle-derived mafic dykes at Jiaodong (Figure 6). The zircon εHf(t) values of these granites vary from -24.6 to -13.5, implying the presence of enriched sources such as sediments or an enriched mantle with/without depleted mantle components (Woodhead et al., 2011). The latest Early Cretaceous granites exhibit similar δ18O values (6.6–7.9‰ from the Aishan felsic complex, [32]; 5.65–7.78‰ and 4.68–7.08‰ from the Haiyang felsic complex, [38]) to contemporary mafic rocks (~115 Ma) from the Jiaodong Peninsula (zircon δ18O values of 5.4–6.7‰; Guo et al., 2014). Moreover, these granites have far lower O isotope components than sediments and thus have additional mantle-derived materials in the source with minor or no sediment input. Liu et al. (2019) also assumed that the felsic-intermediate rocks (<200 Ma) in the NCC were the result of crustal reworking without a significant addition of depleted mantle melt. Many mafic microgranular enclaves (MME) are observed in the latest Early Cretaceous granites, indicating the occurrence of magma mixing [27, 37, 39]. In addition, coeval emplacement of the MME and the host monzogranite further supports the occurrence of magma mixing [27].We need to first evaluate the effects of assimilation and fractional crystallisation (AFC) on the adakitic features. AFC processes would produce magmas with low Sr/Y ratios and negative Eu anomalies (Ma et al., 2015), which are inconsistent with these granites. High-pressure garnet fractionation processes could generate the positive correlation observed between Sr/Y and Dy/Yb with SiO2, as well as the overall low HREE contents [48], which also are inconsistent with the Late Jurassic granites and Early Cretaceous granodiorites (Figures 3(e) and 3(f)). Moreover, the contemporary basaltic magma has not been observed in the Jiaodong Peninsula. Thus, high-pressure fractionation processes of hydrous basaltic melts also cannot explain the origin of the Late Jurassic granites and Early Cretaceous granodiorites. The adakitic features were derived from the source. Although the genesis of typical adakite was originally ascribed to the partial melting of subducted basaltic oceanic crust (Defant and Drummond, 1990), the Late Jurassic granites and Early Cretaceous granodiorites have much lower Nd and higher Sr isotopic compositions (Figure 6(a)). In addition, they exhibit different Pb isotopic (Figures 6(b) and 6(c)) and trace elemental compositions than those of mid-ocean ridge basalt (MORB) (Figure 4(b)). Thus, the adakitic features also were not derived from the partial melting of a subducting oceanic crust.High Sr/Y and La/Yb ratios are characteristic of adakite signatures and likely result from the preferential partitioning of HREEs (e.g., Y and Yb) into garnets or amphiboles while Sr and La enter the liquid phases [11, 21]. The La/Yb and Sr/Y ratios of arc-derived intermediate rocks are indicative of changing crustal thickness ([20]; Chiaradia, 2015; [21, 23]). The Late Jurassic granites from Jiaodong Peninsula exhibit adakitic characteristics, including high Sr/Y (20.4–308, average of 84.5) and La/Yb (7.48–104, average of 44.8) ratios and low HREE compositions, which suggest a magma source that contains residual garnet and/or amphibolite, but lacks plagioclase. Garnet could be stable within the residual assemblage (amphibolite-eclogite and/or garnet-amphibolite) in thickened continent crust (>40 km; ~1.2 GPa; [49]). However, plagioclase would disappear at pressures above 1.2–1.5 GPa [49]. From these data, we postulate that the Late Jurassic granites may have been sourced from a plagioclase-free amphibolite-eclogite or a garnet-amphibolite crustal source in the eastern NCC, with an overall thickness of >40 km.The Early Cretaceous Guojialing granodiorites also exhibit adakitic characteristics (Figure 4) that are similar to the Late Jurassic granites, also suggesting a thickened lower crustal source that is a plagioclase-free amphibolite-eclogite or garnet-amphibolite source. However, the Guojialing granodiorites have higher Sr/Y (62.6–378, average of 168) and La/Yb (8.46–206, average of 96.3) ratios than the Late Jurassic granites from the Jiaodong Peninsula (Figure 4), indicating that the continental crust thickened further during the Late Jurassic.In contrast, the latest Early Cretaceous granites exhibit lower Sr contents, higher Y and Yb contents, lower Sr/Y (0.27–108, average of 25.3) and La/Yb (2.59–176, average of 38.7) ratios, and negative Eu anomalies, as compared to the earlier two magmatic stages in the Jiaodong Peninsula (Figure 5). This indicates more plagioclase, less amphibole, and the absence of garnet in the residual mineral assemblages during partial melting [50–52]. The negative Ba anomalies of these granites could also have resulted from the presence of plagioclase or small amounts of biotite in the residue (Figure 4(f); [51]). The enrichment of HREEs and flat HREE patterns indicate that garnet and amphibole were stable as residual phases at crustal thicknesses of 20–40 km, corresponding to 0.8–1.2 GPa (Figure 4(e)) [53]. These geochemical characteristics imply that the latest Early Cretaceous granites were sourced from a shallower crustal depth.The Late Jurassic to Early Cretaceous felsic intrusive rocks from the Jiaodong Peninsula present a geologic record of variations in thickness of the LCC and the destruction of the eastern NCC. As mentioned above, there are large variations in the major elemental, trace elemental, and radiogenic isotopic compositions of these granites from 166–108 Ma (Figures 6–10). Firstly, a series of dramatic changes offer a window into the temporal changes in the Late Mesozoic lower crustal source beneath the Jiaodong Peninsula, as recorded by the three stages of granite emplacement derived from partial melting of the LCC. Late Jurassic granitoids originated from the partial melting of the eastern NCC Archaean lower crust. Early Cretaceous granodiorites were derived from the partial melting of the eastern NCC Archaean-Palaeoproterozoic lower crust, including the mixing of mantle-derived materials. However, the latest Early Cretaceous granites were derived from the mixing of lower crustal-origin felsic magmas with mantle-derived mafic melts. Secondly, the variations in the La/Yb (or Sm/Yb and Gd/Yb) ratios from the Late Jurassic to the latest Early Cretaceous granites (Figures 9 and 10) indicate a thickening of the LCC during the Late Jurassic, continued thickening of the LCC in the Early Cretaceous, but thinning of the LCC in the latest Early Cretaceous. These changes in the magma source also reflect regional changes in tectonic stress. The early-stage (166–144 Ma) granites are mainly typical I-type granites, but the latter two stages (136–123 Ma and 123-108 Ma) shift to A-type and/or highly fractionated I-type granites. This change suggests that the regional tectonic stress regime that affected the eastern NCC altered from compression to intraplate extension after ~144 Ma (Figure 10).The Late Jurassic granites (166–146 Ma) from the Jiaodong Peninsula exhibit high Sr/Y and La/Yb ratios, indicating a thickened lower crustal source (>40 km) (Figure 10). At the southern margin of the eastern NCC, the Wuzhangshan felsic pluton (157–156 Ma) also has high SiO2, Al2O3, and Sr contents, low Y and Yb contents, and high Sr/Y (16–44.5) and La/Yb (16.1–31.8) ratios [52]. In the Liaodong Peninsula in the northeastern NCC, Late Jurassic granites (179–156 Ma) also have similar adakitic characteristics (e.g., high Sr/Y=33–2010112 and La/Yb=10.3–110.3) [5] (Figure 5). Moreover, the Late Triassic Longtou-Chaxinzi-Xiaoweishahe and Nankouqian-Xidadingzi granitoids from the northern Liaodong Peninsula, which have been recognised as being derived from the partial melting of ancient lower crustal materials with additional mantle components [29, 30], are characterised by strong negative and variable whole rock εNd(t) and εHf(t) and zircon εHf(t) values, but lack adakitic features. This implies that the Late Jurassic lower crust of the eastern NCC has been modified and thickened. It has been shown that the Late Jurassic magmatism in the eastern NCC likely either occurred in (1) a postcollisional extensional setting following the collision of the NCC and the YC [26, 31] or (2) a compressional setting resulting from the subduction of the Palaeo-Pacific oceanic lithosphere [5, 33]. We can exclude the former hypothesis for the following reasons: (1) the subduction and collision of the NCC and the YC (240–200 Ma) occurred prior to the Late Jurassic magmatism in Jiaodong at ~100–80 Ma ([54]; Zheng et al., 2003); (2) the large differences between the Sr-Nd isotopic compositions of Late Triassic crust-derived syenites (~215 Ma) from the Sulu orogenic belt and those of the Late Jurassic granites from Jiaodong, indicating different sources (Figure 6); and (3) the Late Jurassic Linglong granites from the Jiaodong Peninsula are predominantly typical I-type granites, indicating a compressional setting rather than a postcollisional extensional setting (Lameyre, 1988; Xia et al., 2015). Moreover, both Jurassic granitoids from the Liaodong Peninsula and from the southern margin of the NCC are also typical I-type granites [5, 52]. Therefore, the Late Jurassic thickened lower crust (>40 km depth) and associated magmatism in the eastern NCC did not have a direct genetic relationship with the Triassic collision of the NCC and YC.Previous studies have shown that the Palaeo-Pacific oceanic lithosphere subducted westward under the Asian continent since the early Jurassic (~200–190 Ma; [25]; Zhou and Long, 2017; [2]). Wu et al. [5] showed that Jurassic granitic magma in the Liaodong Peninsula was derived from the partial melting of thickened ancient crust resulting from Pacific plate subduction. Thus, the thickening of Middle-Late Jurassic lower crust in the eastern NCC may have resulted from compression and subduction of the Palaeo-Pacific oceanic lithosphere since the Early Jurassic. Liu et al. (2019) suggested that the initial low-angle subduction of the Palaeo-Pacific oceanic lithosphere likely lowered the temperature gradient of the mantle wedge and the overlying subcontinental lithospheric mantle (SCLM), leading to limited asthenospheric mantle melting and thus insignificant crust-mantle mixing. This is consistent with the Late Jurassic granites observed in the Jiaodong Peninsula (Figure 11(a)).The Early Cretaceous granodiorites (136–123 Ma) from the Jiaodong Peninsula are A-type and/or highly fractionated I-type granites, suggesting an extensional tectonic environment ([5, 16]; Xia et al., 2017). The contemporary metamorphic core complexes (MCCs), pull-apart basins, A-type granites, and dyke swarms that occur in the eastern NCC evidence a regional extensional tectonic environment (Figure 10). Early Cretaceous A-type alkaline granites (131–118 Ma) have also been observed in the Liaodong Peninsula and on the southern margin of the NCC (132–125 Ma) [5, 16, 52]. Early Cretaceous pull-apart basins (140–120 Ma), including the Songliao, Jiaolai, and Hefei Basins [55, 56], also developed. Early Cretaceous MCCs (135–115 Ma) are present in the eastern NCC, including the Liaonan, Yunmengshan, and Hohhot MCCs on the northern margin, and the Xiaoqinling MCC on the southern margin [57]. Therefore, the eastern NCC became a regional extensional setting at ~136 Ma.The Early Cretaceous granodiorites exhibit higher Sr and lower Y contents, with higher Sr/Y and La/Yb ratios than the Late Jurassic granites (Figures 5 and 10), implying thicker crust. Because the La/Yb (or Sm/Yb and Gd/Yb) ratios show a positive relationship with crustal thickness [21, 23, 51, 58], the higher La/Yb values from the Early Cretaceous granodiorites, compared to those from the Late Jurassic granites, further indicate crustal thickening after 146 Ma. This has also been observed on the southern margin of the eastern NCC, such as in the Huashani complex (~132–125 Ma), with higher Sr/Y (17.6–76.8) and La/Yb ratios (18.5–43.2) than those of the Wuzhangshan pluton (~157–156 Ma) [52]. Numerical modelling and geophysical studies have shown that slab steepening (a precursor of rollback) and rollback are important in producing topographic uplift (e.g., [51, 59, 60]). In particular, if the temperature distribution in the lithospheric mantle is heterogeneous during rollback, then crustal uplift can occur in the inboard region due to the removal of lithospheric mantle (or arc root), whereas crustal subsidence will develop in the trenchward region [51, 59, 60]. The crustal thickening of the arc likely resulted from basaltic magma underplating caused by slab rollback and slab breakoff, such as in the Gangdese arc at 70–45 Ma [11]. The Guojialing granodiorites (136–123 Ma) from the Jiaodong Peninsula are also evidence of interaction between lithospheric mantle-derived melts and crust-derived magma. Wu et al. [61] showed that the Early Cretaceous (131–118 Ma) high-temperature granitoids from the Liaodong Peninsula are likely related to mantle plume turbulence within the mantle and decompression partial melting of the upwelling asthenosphere.Niu et al. [62] proposed that the oldest age of the Zhangjiakou Formation (135 Ma) in the eastern NCC represents the time at which tectonic stress altered from a compressional to an extensional setting. Liu et al. (2017) indicated that the trench retreat of the western Pacific occurred at 137–130 Ma. [10] studied Mesozoic-Cenozoic mafic igneous rocks and inferred that the subduction angle of the Palaeo-Pacific plate changed from flat to steep at ~135 Ma. Zheng et al. (2018) proposed that the Palaeo-Pacific oceanic lithosphere initially subducted westward at a low angle (flat slab subduction), then began to sink or roll back to allow asthenospheric upwelling at ~144 Ma. A higher subduction angle would lead to a decoupling between the subducting Palaeo-Pacific slab and the mantle wedge, which would result in not only rollback of the downgoing slab but also upwelling of asthenospheric mantle, generating mantle-derived melts [10]. The occurrence of Early Cretaceous lithospheric mantle-derived mafic dykes (144–143 Ma) in the Zichuan area of the eastern NCC suggests the presence of mantle thermal anomalies [18].We propose that a steepening of the subducting Palaeo-Pacific oceanic lithosphere took place after ~144 Ma, and that this was the first step in the process of slab rollback, which then triggered basaltic magma underplating in the region and the subsequent crustal thickening of the Jiaodong Peninsula (Figure 11(b)).The late-stage granites (123–108 Ma) from the Jiaodong Peninsula, characterised by low Sr/Y and La/Yb ratios (Figure 10), were derived from lower crustal depths (20–40 km) than the earlier two stages of granites. The thinning of the LCC source for these late granites (126–110 Ma) also occurred at the southern margin of the eastern NCC. Zou et al. [52] showed that the latest Early Cretaceous granitoids (~125–110 Ma) from the southern margin of the NCC, characterised by low Sr and high Y contents, with low Sr/Y (0.25–13.2) and La/Yb (5.48–57.8) ratios, originated from the partial melting of a thinned crust (<33 km depth). Gao et al., [50] concluded that the 126–112 Ma granites from the southern margin of the NCC are also A-type and/or highly fractionated I-type granites with low Sr contents, δEu values, and Sr/Y ratios. Thus, they formed in an intracontinental extensional environment.Lithospheric extension should occur after oceanic lithosphere rollback ([59] and references therein). If this had occurred, the driving force for thrusting in the NCC lithosphere should have been removed. In this study, we found that the eastern NCC was a dominantly regional intracontinental extensional environment since the Early Cretaceous, which is likely relevant to the rollback of the Palaeo-Pacific plate after ~144 Ma. Wang, et al., [10] showed that higher angle subduction of the western Pacific plate would induce not only rollback of the downgoing slab but also asthenospheric mantle convection beneath the retreating slab. Intense asthenospheric mantle convection likely eroded the weakened overlying lithospheric mantle, generating a larger scale of lithospheric mantle-derived melts and resulting in the thinning of the lithosphere. Meanwhile, decompression and melting of upwelling asthenospheric mantle also occurred, as indicated by the mantle-derived mafic dykes observed in the southwestern Jiaodong Peninsula (122–121 Ma; [15]). The prolonged thermomechanical-chemical erosion model suggests that the lowermost lithosphere was gradually eroded due to the rollback of the Palaeo-Pacific plate [14]. Previous studies have shown that large-scale formation of lithospheric mantle-derived mafic dikes occurred at 130–110 Ma, with a peak at ~123 Ma, throughout the Jiaodong Peninsula [14]. Following this event, upwelling and underplating of the mantle-derived magma led to partial melting of the lower crust, producing massive volumes of felsic intrusive rocks at 123–108 Ma. The stress regime during this period was dominated by extension, generating metamorphic core complexes, pull-apart basins, A-type granites, and dyke swarms. Therefore, the rollback of the subducting Palaeo-Pacific Ocean lithosphere occurred after ~123 Ma (Figure 11(c)).This study presents a comprehensive analysis of the geochronology and geochemistry of Late Jurassic to Early Cretaceous granites from the Jiaodong Peninsula, in the eastern NCC. Late Jurassic granites (166–146 Ma) originated from the partial melting of thickened Archaean lower crust (>40 km) in the eastern NCC produced by the compression and westward subduction of the Palaeo-Pacific oceanic lithosphere since the early Jurassic. The Early Cretaceous granodiorites (136–123 Ma) originated from the partial melting of a thicker Archaean-Palaeoproterozoic lower crust in the eastern NCC, with added mantle-derived magma. The thicker lower crust resulted from mantle-derived magmatic underplating in an extensional setting related to the steepening of the subducting Palaeo-Pacific oceanic lithosphere after ~144 Ma. The latest Early Cretaceous granites (123–108 Ma) are derived from mixing between mantle-derived mafic magmas and thinned lower crust-derived felsic magmas. The lower crust of the eastern NCC experienced underplating and thinning through upwelled mantle-derived mafic magma that produced by the rollback of the subducting Palaeo-Pacific plate after ~123 Ma. Therefore, we postulate that the destruction of the eastern NCC was related to the subduction and rollback of the westward subducting Palaeo-Pacific oceanic lithosphere beneath the Asian continent.The authors declare no conflict of interest.This research was supported by the Fundamental Research Funds for the Central Universities (Grant No. FRF-TP-19-023A1) and the National Natural Science Foundation of China (Grant No. 41802077).Supplementary Table 1: the summary of the Late Mesozoic felsic rocks in the Jiaodong Peninsula. Supplementary Table 2: LA-ICP-MS and/or SHRIMP zircon U-Pb ages of the Late Mesozoic felsic rocks in the Jiaodong Peninsula. Supplementary Table 3: whole rock geochemical compositions of Late Mesozoic felsic rocks from the Jiaodong Peninsula. Supplementary Table 4: zircon Hf-O isotopic compositions of Late Mesozoic felsic rocks from the Jiaodong Peninsula.
更新日期：2020-06-30
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