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Volume 31 Issue 6
Dec.  2020
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Songjie Wang, Lu Wang, Yue Ding, Zhuocheng Wang. Origin and Tectonic Implications of Post-Orogenic Lamprophyres in the Sulu Belt of China. Journal of Earth Science, 2020, 31(6): 1200-1215. doi: 10.1007/s12583-020-1070-y
Citation: Songjie Wang, Lu Wang, Yue Ding, Zhuocheng Wang. Origin and Tectonic Implications of Post-Orogenic Lamprophyres in the Sulu Belt of China. Journal of Earth Science, 2020, 31(6): 1200-1215. doi: 10.1007/s12583-020-1070-y

Origin and Tectonic Implications of Post-Orogenic Lamprophyres in the Sulu Belt of China

doi: 10.1007/s12583-020-1070-y
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  • Lamprophyre dykes that crosscut different types of ultrahigh pressure (UHP) metamorphic rocks are widely distributed in the Triassic Sulu UHP orogenic belt. Although abundant studies have been performed on these dykes, their origin and petrogenesis remain topics of controversy. This study presents the results of a detailed field-based study of petrology, whole-rock geochemistry and zircon U-Pb and Lu-Hf isotopes on lamprophyre dykes exposed in the central Sulu UHP zone, aiming at shedding lights on their petrogenesis and providing clues on the geological evolution of eastern continental China during the Cretaceous. The lamprophyres are typically porphyritic, with phenocrysts dominantly composed of amphibole and clinopyroxene set in a lamprophyric matrix. The dykes have moderate SiO2 (47.70 wt.%-60.44 wt.%), variably high MgO (2.58 wt.%-8.28 wt.%) and Fe2O3T (4.88 wt.%-9.26 wt.%) contents with high Mg# of 49-66. Geochemically, they have enriched light rare earth element (REE) and flat heavy REE patterns ((La/Gd)N=5.14-10.56; (Dy/Yb)N=1.43-1.54) with negligible Eu anomalies (Eu/Eu*=0.83-1.10), and they show enrichment in large ion lithophile elements (e.g., Ba and K) but depletion in high-field strength elements (e.g., Nb, Ti and P). In-situ zircon U-Pb geochronology reveals that the lamprophyres have concordant ages of 120-115 Ma, demonstrating that the dykes emplaced in the Early Cretaceous. These zircons have εHf(t) values ranging from -26.0 to -11.0. Inherited zircons that occur in the dykes are dated to be Neoproterozoic, in line with the protolith ages of their host (i.e., the UHP rocks). An integration of these data allows us to propose that the lamprophyres were generated during the Cretaceous, by melting of subcontinental lithospheric mantle-derived metasomatite with enriched chemical compositions underneath the North China Craton. The metasomatite was formed mainly by peridotite-fluid/melt reactions, with the fluids/melts mainly liberated from subducted Yangtze continental crust during the Triassic. Regional extension, lithospheric thinning and mantle upwelling caused by rollback of the subducted paleo-Pacific plate is considered to account for the generation of the lamprophyres as well as the extensive arc-like magmatic rocks in eastern continental China during the Early Cretaceous.
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Origin and Tectonic Implications of Post-Orogenic Lamprophyres in the Sulu Belt of China

doi: 10.1007/s12583-020-1070-y

Abstract: Lamprophyre dykes that crosscut different types of ultrahigh pressure (UHP) metamorphic rocks are widely distributed in the Triassic Sulu UHP orogenic belt. Although abundant studies have been performed on these dykes, their origin and petrogenesis remain topics of controversy. This study presents the results of a detailed field-based study of petrology, whole-rock geochemistry and zircon U-Pb and Lu-Hf isotopes on lamprophyre dykes exposed in the central Sulu UHP zone, aiming at shedding lights on their petrogenesis and providing clues on the geological evolution of eastern continental China during the Cretaceous. The lamprophyres are typically porphyritic, with phenocrysts dominantly composed of amphibole and clinopyroxene set in a lamprophyric matrix. The dykes have moderate SiO2 (47.70 wt.%-60.44 wt.%), variably high MgO (2.58 wt.%-8.28 wt.%) and Fe2O3T (4.88 wt.%-9.26 wt.%) contents with high Mg# of 49-66. Geochemically, they have enriched light rare earth element (REE) and flat heavy REE patterns ((La/Gd)N=5.14-10.56; (Dy/Yb)N=1.43-1.54) with negligible Eu anomalies (Eu/Eu*=0.83-1.10), and they show enrichment in large ion lithophile elements (e.g., Ba and K) but depletion in high-field strength elements (e.g., Nb, Ti and P). In-situ zircon U-Pb geochronology reveals that the lamprophyres have concordant ages of 120-115 Ma, demonstrating that the dykes emplaced in the Early Cretaceous. These zircons have εHf(t) values ranging from -26.0 to -11.0. Inherited zircons that occur in the dykes are dated to be Neoproterozoic, in line with the protolith ages of their host (i.e., the UHP rocks). An integration of these data allows us to propose that the lamprophyres were generated during the Cretaceous, by melting of subcontinental lithospheric mantle-derived metasomatite with enriched chemical compositions underneath the North China Craton. The metasomatite was formed mainly by peridotite-fluid/melt reactions, with the fluids/melts mainly liberated from subducted Yangtze continental crust during the Triassic. Regional extension, lithospheric thinning and mantle upwelling caused by rollback of the subducted paleo-Pacific plate is considered to account for the generation of the lamprophyres as well as the extensive arc-like magmatic rocks in eastern continental China during the Early Cretaceous.

Songjie Wang, Lu Wang, Yue Ding, Zhuocheng Wang. Origin and Tectonic Implications of Post-Orogenic Lamprophyres in the Sulu Belt of China. Journal of Earth Science, 2020, 31(6): 1200-1215. doi: 10.1007/s12583-020-1070-y
Citation: Songjie Wang, Lu Wang, Yue Ding, Zhuocheng Wang. Origin and Tectonic Implications of Post-Orogenic Lamprophyres in the Sulu Belt of China. Journal of Earth Science, 2020, 31(6): 1200-1215. doi: 10.1007/s12583-020-1070-y
  • Subduction of continental crust is an important process to recycle evolved crustal materials back into the mantle, which may significantly modify the nature and compositions of the overlying mantle wedge (Zheng, 2012; Stern and Scholl, 2010). Although continental subduction zones, in comparison with oceanic subduction zones, are characterized by the absence of syn-subduction magmatism, many scholars have documented the widespread occurrence of post-orogenic magmatic rocks in collisional orogens and the nearby regions (e.g., Wang et al., 2020a, 2019; Zheng et al., 2018; Zhao et al., 2013; Liu S et al., 2009, 2008, 2006). These rocks may record key evidence of crustmantle interactions in a subduction zone (Wang X et al., 2020; Dai et al., 2015; Zhao et al., 2013; Zheng, 2012; Schaltegger and Brack, 2007; Bonin, 2004; Altherr et al., 2000; Hegner et al., 1998). Therefore, it is important to track the origin of post-orogenic rocks if we are to further understand the recycling and reworking of continental lithosphere at convergent plate margins.

    The Dabie-Sulu orogenic belt born from the Triassic amalgamation of the Yangtze Craton (YC) and the North China Craton (NCC) in eastern China is one of the world's most classic continental collisional orogens (Ernst et al., 2007; Hacker et al., 2000). Coesite and rare micro-diamond have been recognized from exposed crustal metamorphic rocks from this orogen since the 1990s (Xu et al., 1992; Wang and Liou, 1991; Okay et al., 1989), making it a representative natural site to probe into the processes related to deep continental subudction. After terminal collision of the two blocks, this UHP terrain and a vast number of areas in the surrounding NCC as a whole were reactivated due to subduction of the paleo-Pacific Plate beneath the eastern Asian continent in the Jurassic (Kusky et al., 2014; Zhao and Ohtani, 2009; Maruyama et al., 1997). Affected by these two crucial convergent events, Jurassic to Early Cretaceous magmatic rocks widely occur in these regions, in which the Early Cretaceous rocks with compositions varying from mafic to felsic are volumetrically dominant (e.g., Wang S J et al., 2020a, 2019; Wang X et al., 2020; Wan et al., 2019; Li et al., 2018; Deng et al., 2017; Guo et al., 2014, 2005, 2004; Zhang et al., 2012; Liu S et al., 2008, 2006). What concerns more is the Early Cretaceous magmatic rocks have a peak formation age (ca. 120 Ma) that is accompanied with a significant loss of subcontinental lithospheric mantle (SCLM) of the NCC from ~200 to ~60–120 km (e.g., Wang X et al., 2020; Wu et al., 2019; Griffin et al., 1998; Menzies et al., 1993). Thus, these rocks, especially the mafic ones that may represent mantle-derived products, would add new information concerning reworking of ancient lithospheric mantle and the mechanism of craton destruction in eastern China.

    In the last twenty years, a considerable number of investigations have been carried out to decode the origin of volumetrically minor but scientifically important Early Cretaceous mafic rocks within this orogen and its ambient areas (as reviewed by Zheng et al., 2018; Zhao et al., 2013). These work via different geochronological methods, including zircon U-Pb, whole-rock and biotite K-Ar/Ar-Ar dating, has constrained the emplacement time of these mafic dykes at ca. 130–120 Ma. However, the source nature, petrogenesis and geodynamic mechanism that generated these magmas are debatable, with diverse models such as: (1) partial melting of enriched SCLM peridotite in the NCC (Wang X et al., 2020; Deng et al., 2017; Cai et al., 2015; Ma et al., 2014; Guo et al., 2004; Jahn et al., 1999); (2) mixing of a mantle-derived and a crust-derived andesitic-dacitic magma, or between different batches of mafic magmas sourced from the SCLM (Liang et al., 2018; Dai et al., 2015); or (3) melting of the YC lithospheric mantle (e.g., Zhao et al., 2005). These debates show that further investigations are essentially required to help advance our knowledge concerning the origin of these mafic rocks. In order to clarify the formation time, source nature and petrogenetic processes of post-orogenic mafic rocks in the Sulu belt, here we show the results of a detailed field mapping, geochemical, and zircon U-Pb and Lu-Hf isotopic study on a suite of lamprophyre dykes that intruded into migmatized UHP rocks from the central Sulu belt.

  • The Sulu belt, offset northward from the Dabie belt for ~500 km due to the development of the Tanlu fault (Fig. 1a), marks the suture zone of the YZ and the NCC during the Triassic (e.g., Wu and Zheng, 2013; Ernst et al., 2007; Hacker et al., 2006). The Sulu belt tectonically belongs to the Jiaodong Peninsula and is separated from the NCC by a series faults, e.g., the southern Jiashan- Xiangshui fault and the northern Wulian-Yantai fault (Hacker et al., 2009). The Jiaobei and Luxi terrains within the Shandong Peninsula that border the Sulu belt mainly comprise Precambrian metamorphic basement and Mesozoic magmatic rocks, both belonging to the eastern part of the NCC (Tang et al., 2007; Wan et al., 2006). According to field geological and petrological observations, the Sulu belt can be further divided into a HP zone in the south and a UHP zone in the north (see Liu et al., 2004). The UHP zone is dominated by ortho- and paragneisses with subordinate coesite-bearing eclogite, garnet peridotite, kyanite quartzite and marble (e.g., Wang et al., 2018; Zhang et al., 2009; Liu F L et al., 2006). These UHP rocks record successive stages of metamorphism during the Triassic orogeny, including UHP eclogite-facies metamorphism at ca. 235–225 Ma, HP eclogite-facies recrystallization at ca. 225–215 Ma and amphibolite-facies overprinting at ca. 215–208 Ma (see Liu and Liou, 2011). Most of these metamorphic rocks were transformed from bimodal magmatic rocks in a rifting environment during the Neoproterozoic, possibly related to the breakup of the supercontinent Rodinia (Tang et al., 2008).

    Figure 1.  (a) Sketch geological map of the Sulu belt (Yoshida et al., 2004), with the inset showing that the Sulu belt is located in eastern China; the framed area points out locations of the study areas—Yangkou Bay and General's Hill outcrops—within the Sulu belt. WYF. Wulian-Yantai fault. (b), (c) The geological maps of two discontinuous outcrops (location of the outcrop shown in (b) is ~1 km to the north of that of (a)) at General's Hill, showing major lithological units and the occurrence of lamprophyre dykes for this study. The dotted box in (b) shows the mapping area in Fig. 3. The geological map of (b) was mapped based on Landsat ETM+ and 1 : 500 topographic survey provided by Laoshan National Park, and the geological map shown in (c) is modified after Wang et al. (2014).

    There are mainly two episodes of magmatism in the Sulu belt and the adjacent regions during the Late Mesozoic (Zhao et al., 2013), including (1) Late Jurassic granitic magmatism (ca. 160–150 Ma) and (2) Early Cretaceous mafic to felsic magmatism (ca. 140–110 Ma). While Late Jurassic granitic rocks occupy only a small volume, Early Cretaceous magmatic rocks are volumetrically dominant and occur in different locations throughout the orogen, as well as in a variety of regions within the NCC, including the Jiaobei and Luxi terrains, Liaoning Peninsula, Taihang Mountain and Jining area of Inner Mongolia (Wang S J et al., 2020a, 2019; Feng et al., 2019; Zhao et al., 2013; Sun et al., 2007; Yang et al., 2005). The Early Cretaceous rocks include predominant granitoids and subordinate intermediatemafic igneous rocks, with the mafic rocks consisting of many lithologies, such as sillite, dolerite, lamprophyre, gabbro, basalt and hornblendite.

    This paper deals with dykes of lamprophyre exposed at Yangkou Bay and the nearby General's Hill (~2 km to the north of Yangkou) localities in the central Sulu UHP zone (Fig. 1a), ~35 km to the north of Qingdao City. The coastal outcrops at Yangkou Bay and General's Hill are discontinuous and extend for more than 5 km (Figs. 1b, 1c). The outcrop along Yangkou Bay consists of metagabbro, variably retrogressed coesite-bearing eclogite and serpentinized garnet peridotite that are enclosed in ortho- and paragneisses (see Fig. 3a in Wang et al., 2010). Partially transitional relationships from protolith gabbro to UHP eclogite are well preserved in this outcrop, as documented by petrological and geochemical observations (Zhang and Liou, 1997). The UHP eclogite may have subducted to a depth of > 150–200 km, as indicated by P-T metamorphic conditions from phase equilibrium modeling results (Xia et al., 2018) and recovery of majoritic garnet in eclogite (Ye et al., 2000). Wang et al. (2016) recognized five microstructural types of barite in eclogite at Yangkou, which recorded multi-stage fluid/melt flow as P-T changed from subduction to subsequent exhumation. The outcrop at General's Hill mainly comprises strongly foliated and complexly folded UHP eclogite enclosed by granitic gneiss; the eclogite is variably retrogressed and preserves evidence of intensive migmatization (Wang S J et al., 2020b, 2017; Wang L et al., 2014). Leucosome crystallized from hydrous melts derived from the eclogite and country gneiss records P-T conditions ranging from ~3.5 to 2.1 GPa and ~850 to 770 ℃ at 224–219 Ma (Wang et al., 2020b). All of the UHP units in these two outcrops are cut by different types of dykes, including lamprophyre, andesite and granite porphyry (Wang et al., 2020a, 2019).

    Figure 3.  Geological map of a part of the coastal outcrop at General's Hill showing the interrelation of a lamprophyre and an andesite dyke, hosted in migmatitic ultrahigh pressure eclogite (after Wang et al., 2019).

  • The exposed lamprophyre dykes mostly striking SWWNEE are brown to grey with widths and lengths ranging of ~0.5–2 and ~10–60 m, respectively (Figs. 2a2e). They commonly exhibit vesicular structure and show clear chilled margins against the host rocks (e.g., Figs. 2a, 2f, 3). As shown in Fig. 3, the lamprophyre occasionally cuts through andesite dykes. Eleven samples, including two samples that crosscut UHP gneiss at Yangkou and the other nine samples crosscutting migmatitic eclogite at General's Hill, were collected for this study. Except for sample YK137-11 that was taken from the center of a lamprophyre dyke at General's Hill (Fig. 2d), all the other samples constitute five rock pairs selected from the central and marginal parts of five different lamprophyre dykes, respectively (Figs. 2a, 2b, 2c, 2e).

    Figure 2.  Representative field photographs of lamprophyre dykes at Yangkou (a) and General's Hill (b)–(f); solid circles represent the locations where samples were collected. The lamprophyre dykes are generally ~0.5 to ~2 m in width and exhibit vesicular structure (f) and show clear chilled margins (e), (f) against the host rocks.

    The lamprophyre samples show porphyritic textures with amphibole (~10%–15%), clinopyroxene (~5%–10%) and minor amounts of plagioclase and biotite as phenocrysts (e.g., Figs. 4a4d). The matrix (~70%–80%) exhibiting typical lamprophyric texture consists of amphibole, clinopyroxene, biotite and two feldspars with accessory minerals, including apatite, magnetite, titanite, and zircon (Fig. 4e). Calcite with alteration at the rim exists in the lamprophyres as subhedral-unhedral grains (Fig. 4f). Amphibole phenocryst is euhedral-subhedral with sizes up to several tens-hundreds micrometers across, and clinopyroxene phenocrysts are euhedral with sizes ranging from ~500 µm to several millimeters in diameter. Plagioclase phenocryst commonly shows strong sericitization (Fig. 4d).

    Figure 4.  Photomicrographs exhibiting typical mineral assemblage of the lamprophyre dykes. (a)–(c) Subhedral amphibole (a) and euhedral clinopyroxene (b), (c) phenocrysts embedded in a lamprophyric matrix (a), (c) are taken in cross-polarized light and (b) is taken in plane-polarized light. (d) Occurrence of plagioclase grains as a phenocryst phase that is commonly altered to sericite (cross-polarized light). (e) A cross-polarized light microphotograph showing the microstructures of the matrix with typical lamprophyric texture. (f) Occurrence of calcite in the lamprophyres (cross-polarized light). Amp. Amphibole; Cpx. clinopyroxene; Pl. plagioclase; Bt. biotite; Cal. calcite.

  • In order to constrain the formation age, source nature and petrogenesis of the lamprophyres, this study combined whole-rock geochemistry with zircon U-Pb and Lu-Hf isotope analyses. The whole-rock analyses were completed at the Wuhan Sample Solution Analytical Technology Co., Ltd., Wuhan, China, and cathodoluminescence (CL) imaging and zircon analysis were carried out at the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, Wuhan. The whole-rock major and trace element compositions were determined by using the same methods and procedures as described by Wang et al. (2020b).

    Zircon grains separated from three lamprophyre samples (YK137-6-1, YK137-21-1, and YK137-35-1) using conventional separating methods were used for U-Pb and Lu-Hf isotope analyses. CL imaging was undertaken using the same equipment and working conditions as those described by Wang et al. (2020b). Guided by CL images, zircon U-Pb and Lu-Hf isotope compositions were successively determined by LA-ICP-MS and LA(MC)-ICP-MS with spot sizes of 32 and 44 µm, respectively. Zircon 91500 was selected as external standard for U-Pb dating, and was ablated twice bracketing each batch of 5–6 unknowns. Trace element compositions of zircons were calibrated against multiple-reference materials (BCR-2G and BIR-1G) without applying internal standardization (Liu Y S et al., 2008). The analytical methods, working conditions and data reduction protocol are similar to those given by Liu Y S et al.(2010, 2008) and Hu et al. (2012), respectively.

  • Major and trace element compositions of the analyzed lamprophyres are summarized in Table S1 and graphically illustrated in Figs. 5, 6 and 7. Major oxide compositions were recalculated to 100% on a volatile-free basis before plotting in the geochemical diagrams. The chemical compositions are similar for the lamprophyres taken from the central and marginal parts of one dyke, although they vary a lot among different dykes. Overall, the samples generally have variable contents of SiO2 (47.70 wt.%–60.44 wt.%), Fe2O3T (4.88 wt.%–9.26 wt.%), MgO (2.58 wt.%–8.28 wt.%), high contents of total alkali (4.81 wt.%–8.07 wt.%), CaO (3.95 wt.%–7.06 wt.%) and low TiO2 (0.88 wt.%– 1.21 wt.%) values with high Mg numbers (Mg#=100×(Mg/ (Mg+Fe) molar) of 49–66. In a total alkali vs. silica (TAS) plot (Middlemost, 1994), these samples mostly fall in the basaltic trachyandesite and trachyandesite fields, mainly with alkaline signature (Fig. 5a). In a Zr/TiO2×0.000 1 vs. Nb/Y diagram after Winchester and Floyd (1977), the samples fall in the fields of alkaline basalt and trachyandesite (Fig. 5b). According to the K2O vs. SiO2 discrimination diagram of lamprophyre after Rock (1987), most of the studied lamprophyres are calc-alkaline with high-K to shoshonitic features (Fig. 5c). On Harker diagrams (Figs. 6a6i), the concentrations of SiO2 and Na2O+K2O decrease, whereas Fe2O3T, CaO, TiO2, Cr and Ni generally show increasing trends from low to high MgO; the values of other oxides, including Al2O3 and P2O5, remain constant with varying MgO.

    Figure 5.  (a) Na2O+K2O vs. SiO2 diagram (after Middlemost, 1994), (b) Zr/TiO2×0.000 1 vs. Nb/Y diagram (after Winchester and Floyd, 1977), and (c) SiO2 vs. K2O diagram (after Rock, 1987) for classification of the lamprophyre dykes. CAL. Calc-alkaline lamprophyres; UML. ultramafic lamprophyres; AL. alkaline lamprophyres; LL. lamproites.

    Figure 6.  Bivariate plots of MgO vs. selected major oxide (a)–(g) and trace element (h), (ⅰ) compositions for the lamprophyres.

    Figure 7.  Chondrite-normalized rare earth element (a) and primitive mantle-normalized multi-element patterns (b) of the lamprophyres. Data of contemporaneous mafic rocks (with lithospheric mantle origin) in the Sulu belt and the adjacent North China Craton are presented (see Fig. 6 in Wang et al., 2019 for data sources). Average compositions of normal- & enriched mid-ocean ridge basalts (N- and E-MORB), bulk continental crust (BCC) and ocean island basalt (OIB) are also plotted for comparison. Normalization values of chondrite and primitive mantle, as well as N- and E-MORB and OIB data are from Sun and McDonough (1989), and BCC data are from Rudnick and Gao (2003).

    The lamprophyres have moderate rare earth element (REE) compositions, with total REE contents of 130 ppm–344 ppm. They have chondrite-normalized REE patterns that are enriched in L- (light-) REE and flat in H- (heavy-) REE ((La/Gd)N=5.14– 10.56; (Dy/Yb)N=1.43–1.54) with negligible Eu anomalies (Eu/Eu*=0.83–1.10; Fig. 7a). In a primitive mantle-normalized multi-element plot (Fig. 7b), the samples are enriched in large- ion lithophile elements (LILE; such as Ba, K and Pb) and are depleted in high-field strength elements (HFSE; including Nb, Ta, P and Ti). These geochemical signatures make them obviously different from those of the N- and E-MORB and OIB, but are consistent with the BCC.

  • Zircons from three lamprophyre samples (YK137-6-1, YK137-21-1 and YK137-35-1) were selected for U-Pb isotope and trace element analyses, with the results listed in Tables S2 and S3, respectively. Most zircon grains have prismatic forms with lengths of 100–300 µm and widths of 50–100 µm, yielding length-to-width ratios of 1 : 1 to 4 : 1. They generally show euhedral to subhedral forms, and are colorless and transparent in plane polarized light. In CL images, most zircon grains have moderate luminescence and show typical oscillatory zoning (Figs. 8a8c), consistent with the morphological features of magmatic zircons (Rubatto, 2017; Wu and Zheng, 2004; Hoskin and Ireland, 2000). While in some cases, zircons with core-rim structures can be observed under CL images. The cores have low to moderate luminescence and are characterized by oscillatory or blurry zoning, whereas the thin rims are mostly unzoned and are moderately luminescent (Figs. 8a8c). These zircons are texturally comparable with those within UHP rocks from the study areas and other localities throughout the orogen (e.g., Wang et al., 2020b, 2017), and consequently are considered as inherited zircons.

    Figure 8.  Cathodoluminescence images of zircons from three lamprophyre samples at Yangkou (YK137-6-1) (a) and General's Hill (YK137-21-1, YK137-35-1) (b), (c). White and black circles represent the locations for U-Pb and Lu-Hf isotope analyses, respectively, with respective 206Pb/238U and ɛHf(t) values.

    Twenty-eight analyses were performed on different grains for sample YK137-6-1. Ten spots on inherited zircons have Neoproterozoic 206Pb/238U ages of 819±8 to 617±6 Ma, while the other eighteen analyses on magmatic zircons are concordant and give 206Pb/238U ages of 134±2 to 111±3 Ma, with a weighted mean of 116±2 Ma (1σ, MSWD=2.9; Fig. 9a). Similarly, thirty analyses were conducted on thirty grains for sample YK137-21-1. Ten analyses on the cores of inherited zircon yield Neoproterozoic 206Pb/238U ages of 883±13 to 576±7 Ma, while the other twenty analyses on magmatic zircons are characterized by high Th/U ratios (0.99–2.46) and give 206Pb/238U ages of 131±1 to 107±2 Ma, yielding a weighted mean of 120±2 Ma (1σ, MSWD=3.6; Fig. 9b). Consistently, twenty-four analyses were made for sample YK137-35-1. Eleven spots on inherited zircons with high Th/U ratios (0.52–1.61) have Neoproterozoic 206Pb/238U ages of 817±6 to 756±7 Ma, and the other thirteen spots on magmatic zircons give concordant ages of 134±2 to 111±2 Ma that return a weighted mean of 115±2 Ma (1σ, MSWD=3.2; Fig. 9c).

    Figure 9.  U-Pb concordia plots (a)–(c) and corresponding chondrite-normalized rare earth element patterns (d)–(f) of zircons from the lamprophyre dykes at Yangkou (a), (d) and General's Hill (b), (c), (e), (f). The insets in (a)–(c) show weighted mean 206Pb/238U ages for newly crystallized magmatic zircons. Normalization values in (d)–(f) are from Sun and McDonough (1989). Error ellipses in the Concordia plots and uncertainties on the mean ages are 1σ. U-Pb dates that are < 95% concordant or lie far from the weighted mean line are not included in the mean age calculation and are shaded grey.

    The inherited and magmatic zircons have variably high total REE contents of 580 ppm–21 867 ppm and 1 782 ppm–8 904 ppm, respectively. On the one hand, the magmatic zircons show left-leaning chondrite-normalized REE patterns with depletion of LREE and enrichment of HREE ((Dy/Yb)N=0.09–0.26) with moderately negative Eu anomalies (Eu/Eu*=0.14–0.67). While on the other hand, the inherited zircons that also exhibit steep HREE patterns ((Dy/Yb)N=0.13–0.22) have more negative Eu anomalies (Eu/Eu*=0.01–0.14).

  • Sixty-seven Lu-Hf isotope analyses were performed on magmatic and inherited zircons from three samples (YK137-6-1, YK137-21-1 and YK137-35-1) to determine their compositions (Table S4). According to the zircon U-Pb data, the ɛHf(t) values and Hf model ages are recalculated to 118 Ma and to the apparent ages for the magmatic and inherited zircons, respectively.

    For sample YK137-6-1, on the one hand, the magmatic zircons (n=18) possess 176Hf/177Hf and 176Lu/177Hf values of 0.281 997–0.282 315 and 0.000 646–0.003 198, respectively, giving ɛHf(t) values of -24.9 to -13.8 and TDM2 ages of 2 737–2 042 Ma (Figs. 10a, 10b). On the other hand, the inherited zircons (n=6) possess Hf isotope compositions with 176Hf/177Hf ratios of 0.282 017–0.282 164 and 176Lu/177Hf ratios of 0.001 062–0.001 418, yielding ɛHf(t) values of -10.3 to -5.5 and TDM2 ages of 2 315–2 017 Ma (Figs. 10a, 10b). Similarly, nineteen and five analyses were conducted for magmatic and inherited zircons, respectively, from sample YK128-21-1. The magmatic zircons (n=19) give 176Hf/177Hf and 176Lu/177Hf values of 0.282 037–0.282 377 and 0.000 708–0.004 195, respectively, yielding ɛHf(t) values of -23.5 to -11.7 and TDM2 ages of 2 650–1 912 Ma (Figs. 10c, 10d). In comparison, the inherited zircons (n=5) have 176Hf/177Hf ratios of 0.281 995– 0.282 146 and 176Lu/177Hf ratios of 0.000 977–0.002 021 that return ɛHf(t) values of -13.6 to -5.9 and TDM2 ages of 2 373–2 048 Ma (Figs. 10c, 10d). The magmatic zircons (n=12) for sample YK128-35-1 have ɛHf(t) values of -26.0 to -11.0 and TDM2 ages of 2 809–1 871 Ma, while the inherited zircons (n=7) have less negative ɛHf(t) values of -11.9 to -6.9, yielding TDM2 ages of 2 401–2 202 Ma (Figs. 10e, 10f).

    Figure 10.  Histograms of zircon ɛHf(t) values (a)–(c) and two-stage Hf model ages (TDM2) (d)–(f) for the lamprophyres at Yangkou (a), (d) and General's Hill (b), (c), (e), (f).

  • Based on the dataset obtained from this study, here we attempt to provide constraints on the following four issues: (ⅰ) what was the timing for emplacement of the mafic rocks? (ⅱ) what was the source nature of the lamprophyre; (ⅲ) how did the source rocks achieve their enriched geochemical signatures; and (ⅳ) what were the petrogenetic process and geodynamic implications of these mafic rocks for the evolution of eastern China during the Mesozoic.

  • The lamprophyre dykes contain magmatic zircons that are euhedral and show well-developed oscillatory zoning (Fig. 8). The high Th/U and low Hf/Y values of these zircons, together with enriched HREE relative to LREE patterns and negative Eu anomalies (Figs. 9d9f), are consistent with those derived from magmatic rocks (Wu and Zheng, 2004; Corfu et al., 2003).

    Therefore, it is suggested that the magmatic zircons crystallized from the parent magmas producing the dykes (cf., Xiong et al., 2020; Du et al., 2019; Song et al., 2019). The newly crystallized zircons yield weighted mean 206Pb/238U ages of 120–115 Ma (Figs. 9a9c), confining the formation time of the mafic rocks as Early Cretaceous. The above age information is comparable to the peak timing (ca. 120 Ma) of the widespread Early Cretaceous magmatism developed within the orogen and the nearby regions (e.g., Zheng et al., 2018; Zhao et al., 2013; Wu et al., 2005), indicating that the studied lamprophyres also witnessed the largest magmatic activity in eastern China during the Late Mesozoic.

  • Magmatic zircons from the lamprophyres have variable ɛHf(t) values of -26.0 to -11.0, implying that the lamprophyres may have undergone crustal contamination during magma ascent. However, we assume that crustal assimilation had an insignificant influence on parent magma evolution of the lamprophyres based on the following observations. First, the lamprophyres have higher concentrations of Ba (900 ppm–2 419 ppm) and Sr (410 ppm–982 ppm) than those of the average continental crust (Ba 390 ppm, Sr 325 ppm; cf., Rudnick and Fountain, 1995). They also possess Th/Nb (0.14–0.31; with two exceptions both of 0.63), Ba/La (22.86–51.03), Ce/Pb (4.01–13.46) and Nb/U ratios (17.12–26.19; with two exceptions of 8.26 and 8.44) that are either higher or lower when compared to the bulk continental crust (Rudnick and Fountain, 1995; Taylor et al., 1981). These key elemental compositions and ratios indicate these incompatible components were not significantly influenced by contamination of crustal materials from either the NCC or YC. Second, zircons from continental crustal rocks of the NCC generally have two representative groups of ages at ca. 2.5/1.8 Ga (e.g., Wei et al., 2020). If the parent magma suffered apparent affect during its rising through the ancient crust of the NCC, the resulted products would likely contain xenocrystal zircons with Neoarchean or Paleoproterozoic ages. However, no captured zircons with such ages have been identified in the studied rocks, arguing for a limited degree of crustal contamination. Although the lamprophyres contain relict zircons of Neoproterozoic ages that are consistent with those from the YZ, we argue that they were involved into the magma source prior to the start of magmatism, as further discussed below. Third, previous studies also revealed that crustal assimilation was inconspicuous during the generation of mafic- andesitic dykes in the ambient regions (e.g., Jiaodong and Jiaobei terrains) (Wang et al., 2019; Ma et al., 2014; Liu S et al., 2009, 2008, 2006; Guo et al., 2004). Therefore, we assume that the parent magma responsible for crystallization of the lamprophyres suffered a minor rather than a prominent extent of crustal contamination and the variable zircon Hf isotope compositions may reflect the heterogeneity of the magma source.

    On Harker diagrams (Fig. 6), Fe2O3T, CaO, Cr, Ni and V (not shown) are positively correlated, and SiO2 and total alkali are negatively correlated with MgO in the studied rocks, indicating that the magma underwent a series of fractionation of olivine and/or pyroxene to feldspars. The positive relations between MgO and TiO2 and the consistently low P2O5 contents argue for slight fractionation of Fe-Ti oxides but insignificant fractionation of apatite.

  • The lamprophyres have low to moderate SiO2 (47.70 wt.%–60.44 wt.%) and variably high MgO (2.58 wt.%–8.28 wt.%) concentrations with high Mg# values of 49–66; they contain high contents of compatible elements, including Cr of 46 ppm–365 ppm and Ni of 30 ppm–224 ppm. These features contrast to the products derived from crustal melting, as documented by systematic experimental petrology (Qian and Hermann, 2013; Patiño Douce and Beard, 1995; Rapp and Watson, 1995). Therefore, it is inferred the lamprophyres sourced from a mantle source of ultramafic-mafic lithology.

    The studied dykes display arc-like REE and multi-element patterns that are enriched in LREE and LILE but depleted in HFSE, consistent with the patterns of the BCC (Fig. 7); the magmatic zircons have pronounced negative ɛHf(t) values of -26.0 to -11.0 (Fig. 10). These evidences imply that the source rocks for production of the dykes did not originate from MORB- or OIB-type asthenospheric mantle (Hofmann, 1988), but was from a SCLM. Considering that the mafic dykes occur within the Sulu belt that was formed by deep burial of the YC beneath the NCC, both the subducted continental lithosphere of the YC and the overlying SCLM of the NCC could be potential source rocks (Ma et al., 2014; Zhao et al., 2005). However, as presented in Fig. 7 and reviewed in Wang et al. (2019), mafic-andesitic igneous rocks with similar ages and whole-rock compositions widely occur in the Sulu belt, as well as in the hinterlands of the NCC. Therefore, the subducted lithospheric mantle of the YC may not be able to account for generation of mafic rocks thousands of kilometers far from the fossilized subduction zone. Accordingly, we suggest that the ancient SCLM of the NCC was the most appropriate source rocks for these dykes.

    The lamprophyres contain abundant amphiboles, and they have variably high K2O (2.01 wt.%–4.17 wt.%) concentrations, suggesting that they were originated from a potassium-rich (hornblende or phlogopite) mantle source (Ionov et al., 1997; Foley et al., 1996). All the samples have scattered Rb/Sr ratios of 0.08–0.25 and Ba/Rb ratios of 8.86–25.77, which plot in the transition zone from amphibole to phlogopite fields in the Rb/Sr vs. Ba/Rb plot (Fig. 11a), requiring the existence of both amphibole and phlogopite in the melting source. In addition, in the K/Yb vs. Dy/Yb plot that is appropriate to constrain whether partial melting of the source rocks did occur in the spinel or garnet stability field of an amphibole- and/or phlogopite-bearing lherzolite (Jiang et al., 2010; Duggen et al., 2005), the lamprophyres exhibit a limited range of Dy/Yb ratios (2.14–2.30) and fall in the transition domain from spinel-facies to garnet-facies lherzolite (Fig. 11b), indicating that the melting potentially has occurred in the transition zone of spinel and garnet. This inference is coincident with that of the calc-alkaline lamprophyres in the Jiaodong Peninsula, as reported by Wang X et al. (2020) and Ma et al. (2014).

    Figure 11.  (a) Rb/Sr vs. Ba/Rb plot (after Furman and Graham, 1999), and (b) K/Yb vs. Dy/Yb diagram for the lamprophyres at Yangkou and General's Hill. Melting curves for garnet and spinel lherzolite, garnet-facies phlogopite/ amphibole lherzolite and spinel-facies amphibole lherzolite are after Duggen et al. (2005). PM. Primitive mantle.

  • Since crustal contamination during evolution of the parent magma was limited, the enriched chemical compositions of the lamprophyres should be inherited from the mantle source, implying that their source rocks have been metasomatized by addition of crustal materials via source mixing before the start of mafic magmatism (cf., Zhao et al., 2013, 2012). The lamprophyres contain inherited zircons that are dated to be Neoproterozoic, comparable with the protolith ages of the UHP rocks from the Dabie-Sulu Orogen (e.g., Wang et al., 2020b, 2017; Liu and Liou, 2011; Hacker et al., 2006). Therefore, it is evident that the subducted crustal rocks from the YC were added to the overlying SCLM of the NCC during the Triassic orogeny. Recent studies have widely verified that liberation of fluids/melts from UHP rocks was conspicuous mainly during exhumation of a subducted continental slab, e.g., in the Kokchetav Complex in Kazakhstan (e.g., Stepanov et al., 2016; Korsakov and Hermann, 2006), the Western Gneiss Region of Norway (Ganzhorn et al., 2014; Gordon et al., 2013; Labrousse et al., 2011), and the Dabie-Sulu Orogen in China (e.g., Ferrando et al., 2005). Anatexis of UHP rocks during the Triassic orogeny, including gneiss, eclogite, kyanite quartzite and marble, in the study area and the nearby regions within the Sulu belt has been a research focus in the last ten years and has been systematically documented by petrology, geochemistry and geochronology (e.g., Feng et al., 2020; Wang S J et al., 2020b, 2017, 2016; Xia et al., 2018; Wang L et al., 2014). Thus, the released fluids/melts may act as an efficient medium to transport ancient materials into the overlying mantle wedge in a continental subduction channel, reacting with mantle peridotite to form enriched source rocks for the production of mafic magmatism (Zheng et al., 2015; Zheng, 2012). This process has been also applied to interpret the enriched signatures of coetaneous mafic-andesitic rocks in the adjacent regions (e.g., Dai et al, 2016; Zhao et al., 2013). However, it should be noted that fluids/melts discharged from marine sediments on top of the subducted paleo- Pacific Plate might have further modified the nature of the SCLM beneath the NCC during the Late Mesozoic (cf., Wang X et al., 2020; Kong et al., 2019; Wang S J et al., 2019; Ma et al., 2014; Tang et al., 2012).

    Since crustal contamination during evolution of the parent magma was limited, the enriched chemical compositions of the lamprophyres should be inherited from the mantle source, implying that their source rocks have been metasomatized by addition of crustal materials via source mixing before the start of mafic magmatism (cf., Zhao et al., 2013, 2012). The lamprophyres contain inherited zircons that are dated to be Neoproterozoic, comparable with the protolith ages of the UHP rocks from the Dabie-Sulu Orogen (e.g., Wang et al., 2020b, 2017; Liu and Liou, 2011; Hacker et al., 2006). Therefore, it is evident that the subducted crustal rocks from the YC were added to the overlying SCLM of the NCC during the Triassic orogeny. Recent studies have widely verified that liberation of fluids/melts from UHP rocks was conspicuous mainly during exhumation of a subducted continental slab, e.g., in the Kokchetav Complex in Kazakhstan (e.g., Stepanov et al., 2016; Korsakov and Hermann, 2006), the Western Gneiss Region of Norway (Ganzhorn et al., 2014; Gordon et al., 2013; Labrousse et al., 2011), and the Dabie-Sulu Orogen in China (e.g., Ferrando et al., 2005). Anatexis of UHP rocks during the Triassic orogeny, including gneiss, eclogite, kyanite quartzite and marble, in the study area and the nearby regions within the Sulu belt has been a research focus in the last ten years and has been systematically documented by petrology, geochemistry and geochronology (e.g., Feng et al., 2020; Wang S J et al., 2020b, 2017, 2016; Xia et al., 2018; Wang L et al., 2014). Thus, the released fluids/melts may act as an efficient medium to transport ancient materials into the overlying mantle wedge in a continental subduction channel, reacting with mantle peridotite to form enriched source rocks for the production of mafic magmatism (Zheng et al., 2015; Zheng, 2012). This process has been also applied to interpret the enriched signatures of coetaneous mafic-andesitic rocks in the adjacent regions (e.g., Dai et al, 2016; Zhao et al., 2013). However, it should be noted that fluids/melts discharged from marine sediments on top of the subducted paleo- Pacific Plate might have further modified the nature of the SCLM beneath the NCC during the Late Mesozoic (cf., Wang X et al., 2020; Kong et al., 2019; Wang S J et al., 2019; Ma et al., 2014; Tang et al., 2012).

    Since crustal contamination during evolution of the parent magma was limited, the enriched chemical compositions of the lamprophyres should be inherited from the mantle source, implying that their source rocks have been metasomatized by addition of crustal materials via source mixing before the start of mafic magmatism (cf., Zhao et al., 2013, 2012). The lamprophyres contain inherited zircons that are dated to be Neoproterozoic, comparable with the protolith ages of the UHP rocks from the Dabie-Sulu Orogen (e.g., Wang et al., 2020b, 2017; Liu and Liou, 2011; Hacker et al., 2006). Therefore, it is evident that the subducted crustal rocks from the YC were added to the overlying SCLM of the NCC during the Triassic orogeny. Recent studies have widely verified that liberation of fluids/melts from UHP rocks was conspicuous mainly during exhumation of a subducted continental slab, e.g., in the Kokchetav Complex in Kazakhstan (e.g., Stepanov et al., 2016; Korsakov and Hermann, 2006), the Western Gneiss Region of Norway (Ganzhorn et al., 2014; Gordon et al., 2013; Labrousse et al., 2011), and the Dabie-Sulu Orogen in China (e.g., Ferrando et al., 2005). Anatexis of UHP rocks during the Triassic orogeny, including gneiss, eclogite, kyanite quartzite and marble, in the study area and the nearby regions within the Sulu belt has been a research focus in the last ten years and has been systematically documented by petrology, geochemistry and geochronology (e.g., Feng et al., 2020; Wang S J et al., 2020b, 2017, 2016; Xia et al., 2018; Wang L et al., 2014). Thus, the released fluids/melts may act as an efficient medium to transport ancient materials into the overlying mantle wedge in a continental subduction channel, reacting with mantle peridotite to form enriched source rocks for the production of mafic magmatism (Zheng et al., 2015; Zheng, 2012). This process has been also applied to interpret the enriched signatures of coetaneous mafic-andesitic rocks in the adjacent regions (e.g., Dai et al, 2016; Zhao et al., 2013). However, it should be noted that fluids/melts discharged from marine sediments on top of the subducted paleo- Pacific Plate might have further modified the nature of the SCLM beneath the NCC during the Late Mesozoic (cf., Wang X et al., 2020; Kong et al., 2019; Wang S J et al., 2019; Ma et al., 2014; Tang et al., 2012).

  • In light of the above discussion, we argue that the lamprophyres stemed from melting of an enriched domain of the SCLM beneath the NCC in the spinel-garnet facies transition zone. However, how did the SCLM get melted is an important question if we are to fully clarify the petrogenetic process and geological implications of the extensive Early Cretaceous magmatism in eastern continental margin of China. Since the Jurassic, subduction of the paleo-Pacific Plate with a low angle was the dominant dynamic process that controlled the geological structure and framework of the eastern Asian continental margin (e.g., Kusky et al., 2014; Maruyama et al., 1997). Slab rollback at ca. 145–120 Ma might not only induce extension of the lithospheric mantle, but might give rise to asthenospheric flux to the 'gap' that was generated between old and new slab positions (Niu, 2018; Kusky et al., 2014), supplying the required heat for melting of the overlying SCLM of the NCC. This geodynamic process has been widely used to account for the large-scale magmatism and decratonization of the NCC during the Early Cretaceous (e.g., Kong et al., 2019; Niu, 2018; Zhao et al., 2013; Sun et al., 2007; Wu et al., 2005). In this study, we interpret the formation of the lamprophyres as a consequence of the same tectonic event.

    Based on a comprehensive data compilation of post-orogenic mafic dykes in the Dabie-Sulu Orogen and the adjacent NCC, Zhao et al. (2013) proposed a five-step process to illustrate their generation in these regions, including (1) deep burial of the Yangtze crust to a depth of > 100 km during the Triassic; (2) anatexis of the subducted slab in the subduction channel, generating hydrous melts with enriched LILE/LREE and depleted HFSE features; (3) SCLM wedge peridotite-melt reactions to form metasomatites that show fertile characteristics; and (4), (5) storage of the metasomatites for millions of years until sufficient heating was provided for their partial melting, generating 'arc-like' mafic igneous rocks. According to the geochemical consistence of the lamprophyres with these rocks, as well as the geological setting where the lamprophyres are developed, a similar process is suggested to illustrate the generation of the lamprophyres at Yangkou and General's Hill. First, our earlier studies on eclogites and gneisses that directly host the studied lamprophyres revealed the UHP rocks underwent extensive partial melting during exhumation from UHP conditions (e.g., Wang S J et al., 2020b, 2017; Wang L et al., 2014). It is obvious from field observations that the melts formed meter-scale leucosome sheets along former melt channels (Wang et al., 2020b). Therefore, we argue that melts released from the subducted Yangtze continental crust may have transferred to the overlying manlte wedge of the NCC along the continental subduction channel, metasomatizing the ancient SCLM peridotite to form an enriched metasomatite (Fig. 12, Stage 1). Second, Early Cretaceous dykes ranging in composition from mafic (lamprophyre) to felsic (granite porphyry) are widely distributed in the study area, which show similar geochemical features to contemporaneous magmatic rocks widely exposed in eastern China, suggesting that they were generated under the same background. As we have briefly introduced above, paleo-Pacific subduction has been proven as the most significant tectonic movement during the Late Mesozoic (e.g., Kusky et al., 2014; Windley et al., 2010). Therefore, we assume that rollback of the subducted paleo-Pacific slab at ca. 130–110 Ma contributed to melting of the metasomatites, producing the lamprophyre dykes (Fig. 12, Stage 2).

    Figure 12.  A three-dimensional model to exhibit the evolution of the Dabie-Sulu Orogen and the adjacent North China Craton (NCC) from the Early Paleozoic to Tertiary (modified from Windley et al., 2010), highlighting that (1) the subducting Yangtze slab metasomatized the sub-continental lithospheric mantle (SCLM) of the NCC during the Triassic (lower left inset for a closer view, Stage 1); (2) the SCLM from both the eastern NCC (including the Dabie-Sulu belt) and the Yangtze Craton underwent further metasomatism (Stage 2) during the Jurassic to Early Cretaceous, due to roll-back of subducted paleo-Pacific Plate beneath eastern continental China.

    Based on the discussion above, we may be able to summarize that the Early Cretaceous lamprophyres record post-orogenic tectono-magmatic movement of the Sulu belt. The similar formation time and geochemical features of these dykes with the extensive Late Mesozoic magmatic rocks suggest that they were genetically related to the paleo-Pacific subduction, but were not formed due to the orogenic collapse of the Dabie and Sulu belts. Therefore, further studies should make a more careful distinction between post-collisional and post-orogenic/anorogenic magmatism (cf., Song et al., 2015).

  • This study of petrology, geochemistry and zircon geochronology on a suite of lamprophyre dykes in the Sulu belt adds information to understand the origin of extensive Early Cretaceous magmatism in eastern continental China. The major findings we draw from this study are concluded as follows.

    (1) The lamprophyres were emplaced in the Early Cretaceous (120–115 Ma), witnessing the largest magmatic activity in eastern continental China during the Mesozoic.

    (2) The lamprophyres originated from melting of the SCLM beneath the NCC; the SCLM has been remoulded by fluids/melts released from the Yangtze continental crust due to the Triassic collisional orogeny.

    (3) Rollback of the subducted plaeo-Pacific slab was the dominant mechanism causing melting of the overlying SCLM underneath the NCC.

  • This study was financially supported by the National Natural Science Foundation of China and Shandong Province (Nos. ZR2018BD019, 41572182, 41803031, 41272225), the MOST Special Fund from the State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences (Wuhan) (No. MSFGPMR02-3) and the Youth Innovation Team Development Plan of the Universities in Shandong Province. We thank Dr. Junpeng Wang for help with sample collection, Hongfang Chen for help with whole-rock geochemical analyses, and Profs. Yongsheng Liu and Zhaochu Hu for the helps in LA(MC)-ICP-MS zircon analyses. We appreciate the editors for their efficient editorial work and two anonymous reviewers for their insightful comments. The final publication is available at Springer via https://doi.org/10.1007/s12583-020-1070-y.

    Electronic Supplementary Materials: Supplementary materials (Tables S1–S4) are available in the online version of this article at https://doi.org/10.1007/s12583-020-1070-y.

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