Repeated modification of lithospheric mantle in the eastern North China Craton: Constraints from SHRIMP zircon U-Pb dating of dunite xenoliths in western Shandong

Four dunite xenoliths from the Tietonggou intrusion of western Shandong, China, were subjected to SHRIMP zircon U-Pb dating to constrain timing of the North China Craton (NCC) destruction, a topic of much controversy. Cathodoluminescence images revealed that 15 of the 18 zircon grains from the xenoliths display striped absorption. The rest showed oscillatory growth zoniation. All the zircons had variable contents of Th (49–3569 ppm; average, 885 ppm) and U (184–5398 ppm; average, 1277 ppm), and variable Th/U ratios (0.15–2.04). These zircon characteristics indicate a magmatic origin. The zircon age data can be divided into five groups: 131–145, 151–164, 261–280, 434–452, and 500–516 Ma. Group I (131–145 Ma) is consistent with timing of formation of the Tietonggou high-Mg diorites. Group II (151–164 Ma) is similar in age to Middle-Late Jurassic magmatism in the eastern NCC, which included both mantle-derived and intensive crust-derived magmatism. Group III (261–280 Ma) is similar in age to the Emeishan large igneous province, and Group IV (434–452 Ma) is similar in age to Paleozoic high-silica magmatism in the eastern NCC. Group V (500–516 Ma) may correspond to the global Pan-African event. Results indicate repeated modification of lithospheric mantle in the eastern NCC, and suggest that the most intensive modification occurred in the late Mesozoic (131–164 Ma).

The Archean North China Craton (NCC) is an ideal setting in which to investigate the destruction of a stable craton. The lithospheric mantle beneath the NCC underwent a dramatic change from an ancient, cold, and >200-km-thick lithospheric mantle in the Early Paleozoic to a young, hot, and 60-80-km-thick lithospheric mantle in the Cenozoic [1][2][3][4][5][6][7][8][9][10][11]. This change in thickness has been referred to as lithospheric thinning or craton destruction. However, there are uncertainties and controversy regarding the timing of this lithospheric thinning, the geodynamic context of this event, and the mechanism of the thinning. Previous studies have proposed that NCC destruction occurred during the Mesozoic [7,[12][13][14], the Late Mesozoic [15][16][17][18], or the Mesozoic and Cenozoic [19], based mainly on analyses of Mesozoic and Cenozoic magmatism in the eastern NCC and in the Dabie-Sulu orogenic belt. However, few geochronological data have been reported for the modified lithospheric mantle in this region.
Early Cretaceous high-Mg diorites in western Shandong contain harzburgite xenoliths with Archean Re-depletion model ages and abundant dunite xenoliths [20][21][22][23][24]. Trace element data of minerals and whole-rock Sr-Nd-Os isotopic data of the Tietonggou peridotite xenoliths (from Early Cretaceous high-Mg diorites) reveal that the harzburgites represent the residue of ancient lithospheric mantle; whereas, the dunites formed via a reaction between mantle peridotite and melt derived from delaminated lower continental crust [24,25]. In this case, zircons in the xenoliths are likely to have formed during modification of the lithospheric mantle by a silicate-rich melt.
The present study aims to constrain timing of the NCC destruction, based on detailed petrographic studies of four dunite xenoliths entrained by Early Cretaceous high-Mg diorites in western Shandong, as well as SHRIMP zircon U-Pb age data. The data constrain the timing of modification of lithospheric mantle beneath the eastern NCC.

Geological background and sample descriptions
The NCC -surrounded by the Central Asian Orogenic Belt (CAOB), the Qinling-Dabie-Sulu orogenic belt, and the Yangtze Craton (YC) (Figure 1(a)) -is subdivided into the Eastern Block, Western Block, and intervening Trans-North China Orogen (TNCO)/Central Orogenic Belt based on the age and lithological associations of metamorphic rocks, tectonic evolution, and the P-T-t path of metamorphism [26].
Western Shandong, located in the Eastern Block of the NCC (Figure 1(a)), is dominated by the Archean Taishan Group, Cambrian and Lower-Middle Ordovician series, and Carboniferous-Permian sequences. Mesozoic strata are dominant and consist mainly of sedimentary rocks in grabens, while Cenozoic strata consist mainly of alluvial and lacustrine sediments [27]. In addition to Precambrian igneous rocks, voluminous Mesozoic intrusive rocks are widespread throughout western Shandong. The Tietonggou intrusion, which is exposed over an area of approximately 5 km 2 ( Figure 1(b)), is located near Yanzhuang town in Laiwu city (117°52′E, 36°05′N), and consists mainly of early noritegabbro and later pyroxene-diorite. Results of laser ablationinductively coupled plasma-mass spectrometry (LA-ICP-MS) zircon U-Pb dating and biotite Ar-Ar dating indicate that the Tietonggou pyroxene-diorite formed in the Early Cretaceous (131-135 Ma) [28,29].
Peridotite xenoliths are abundant in the Tietonggou intrusion, and are generally ellipsoidal, ranging in size from 3 cm × 2 cm × 1 cm to 8 cm × 6 cm × 4 cm (Figure 2(a)). Based on their contents of olivine, orthopyroxene, and clinopyroxene, the xenoliths can be classified into chromitebearing dunite, spinel-bearing harzburgite, and chromitebearing wehrlite. The dunite is dominant [24]. This study is focused exclusively on chromite-bearing dunites from the Tietonggou intrusion.
The chromite-bearing dunites are green in color and are equigranular and/or porphyroclastic, or massive, and consist of olivine (~93%), chromite (~3%), orthopyroxene (~3%), and phlogopite (~1%) (Figure 2(b)-(d)). Olivines can be subdivided into two groups, based on their size. Group I consists of porphyroclastic olivines with kink bands, ranging in size from 1.0 to 4.0 mm; Group II consists of unstrained recrystallized olivines ranging in size from 0.3 to 0.6 mm. The dunites are cut by veins of orthopyroxene ± phlogopite. Secondary clinopyroxenes occur locally around chromite within the dunites. The mineralogy and petrography of the dunites have been described previously [24].

Methods
To avoid contamination of dunite xenoliths by the host rocks, a detailed petrographic study was performed initially. The weathered surfaces of the xenoliths and reaction rims between Figure 1 Geological sketch map of the western Shandong (modified after [29]). 1, Quaternary system; 2, Paleogene-Neogene system; 3, Mesozoic Erathem; 4, Paleozoic Erathem; 5, Archean Eonothem; 6, Mesozoic volcanic rocks; 7, gabbro; 8, diorite; 9, fault; 10, sampling location. the xenoliths and host rocks were removed using a diamond saw. The remaining rock was manually crushed to 100-120 mesh and washed with ethanol. After magnetic separation, zircons were concentrated using heavy liquid, and finally hand-picked under a binocular microscope. Except for the magnetic separation device, new tools were used to avoid contamination of the samples during the separation of zircon. Zircon grains were mounted in epoxy, polished, and coated with gold. The grains were examined under trans-mitted and reflected light using an optical microscope, and cathodoluminescence (CL) images were obtained using a JEOL scanning electron microscope housed at the Beijing Ion-probe Center, Chinese Academy of Geological Sciences, Beijing, China, to reveal their internal structures and to select suitable sites for SHRIMP analyses. The zircons were analyzed using a SHRIMP II at the Beijing Ion-probe Center, Chinese Academy of Geological Sciences, Beijing, China. Details of the experimental conditions and procedures have been described previously [30][31][32]. Ages were calibrated against a reference zircon (TEM) with an age of 417 Ma [33]. U, Th, and Pb concentrations were measured using the reference zircon SL13 (age of 572 Ma; U content of 238 ppm). Data were calculated using SQUID 1.0 and ISOPLOT 3.0 programs [34]. Common Pb was corrected based on the measured 208 Pb. Ablation pits were generally about 25 m × 30 m in area.

Results
We obtained 7 zircon grains from ~300 g of sample LW8-42A, 5 grains from ~280 g of sample LW8-42B, 5 from ~250 g of sample LW8-45, and 2 from ~270 g of sample LW10-2. Analytical results for the samples are given in Table 1.
Zircons from sample LW8-42A were transparent and possessed elliptical or irregular shapes. The grains were 35-110 m in length and had length/width ratios of 1.1-2.0 ( Figure 3(a)). The 206 Pb/ 238 U ages obtained for 4 of 7 analytical spots from sample LW8-42A ranged from 131 to 145 Ma. These zircons displayed striped absorption in CL images (Figure 3(a)), similar to those reported for mafic igneous rocks and the host diorite [29]. The Th and U contents of the zircons varied from 330 to 1549 ppm and from 699 to 3587 ppm, respectively, and their Th/U ratios ranged from 0.37 to 1.38 ( Figure 4). The spot 2 zircon, which yielded an age of 151 Ma, showed typical magmatic oscillatory growth zonation. The spot 1 and 3 zircons, which had striped absorption and Th/U ratios of 0.95 and 0.51, yielded 206 Pb/ 238 U ages of 261±11 and 452±17 Ma, respectively ( Figure 5(a)).
Zircons selected from sample LW10-2 were 70-150 m in length, colorless/transparent, and prismatic or irregular in shape (Figure 3(d)). Three spots were analyzed on two zircon grains from the sample. The core and rim of one grain yielded 206 Pb/ 238 U ages of 500±10 and 434±7 Ma, respectively ( Figure 5(d)). The zircon showed striped absorption in CL images (Figure 3(d)) and yielded Th/U ratios for the core and rim of 1.09 and 0.93, respectively ( Figure 4). The other zircon grain (spot 2) from the sample is structureless in a CL image, had a high U content (1011 ppm) and a high Th/U ratio (0.44), and yielded a 206 Pb/ 238 U age of 516±4 Ma.

Origin of zircon in dunite xenoliths
Zircon (ZrSiO 4 ) grows under SiO 2 -oversaturated conditions. Primary zircon does not readily form in mantle peridotite because of the extremely low Zr and Si contents of this rock type. However, zircon has been reported from ultrahighpressure garnet peridotites and mantle-derived peridotite xenoliths [35][36][37][38][39][40][41]. The growth of zircons in such xenoliths may be related to late-stage modification of mantle peridotite by silica-rich melts [35,36]. Thus it is relevant whether dunite xenoliths from the Tietonggou high-Mg diorites were modified by silica-rich melt. In the case of the Tietonggou peridotite xenoliths, evidence of such modification may be obtained from petrographic studies, in situ mineral trace element data, and whole-rock Sr-Nd-Os isotopic data. In these dunite xenoliths, metasomatized orthopyroxene and orthopyroxene + phlogopite occur as veins or zoning around chromite, suggesting that the xenoliths were indeed modified by silica-rich melt [22,24]. In addition, the orthopyroxene that occurred in veins and around chromite had higher contents of trace earth elements than primary orthopyroxene from the harzburgite xenoliths. Secondary clinopyroxene in wehrlite xenoliths was strongly enriched in light rare earth elements and depleted in heavy rare earth elements [24]. Finally, the dunite xenoliths were characterized by high initial 87 Sr/ 86 Sr ratios (0.7058-0.7212), low  Nd (t) values (19.59 to +0.18), and clear Re addition. These lines of evidence suggest that the dunite xenoliths were modified by silica-rich melt [24,25].
Combined with the existence of harzburgite xenoliths with Archean Re-depletion model ages in the same intrusion, and trace element abundances of olivines from the dunite xenoliths [22,24], we conclude that the dunite xenoliths originated in the lithospheric mantle, but were strongly modified by melt derived from the delaminated continental crust [24]. Zircons in the dunite xenoliths could be attributed to modification by such a silica-rich melt. In other words, the different groups of zircon ages may represent events in which silica-rich melts modified the lithospheric mantle.
Previous studies have reported that delamination of the lower continental crust beneath the NCC was possibly related to collision between the NCC and Yangtze blocks during the Triassic (220-240 Ma) [14,24,42], and that interaction between silica-rich melt and mantle peridotite occurred after this time. In this case, the question would remain regarding the origin of zircons in the dunites xenoliths that yielded ages of 261-280, 434-452, and 500-516 Ma. There are two possible zircon origins of these ages: (1) they originated from the delaminated lower continental crust of the NCC and/or the subducted slab of the YC; or (2) they were derived from repeated modification of the lithospheric mantle by silica-rich melts. In the first case, we would have expected to find zircons with ages of 2500 Ma, 1850 Ma (typical of the NCC) and/or 700-900 Ma (typical of the YC). However, zircons with these ages were not found in the dunite xenoliths. Thus, we conclude that these zircons record repeated modification of lithospheric mantle. This interpretation gives rise to the question of whether coeval magmatism, similar to the ages of zircons in the dunite xenoliths, existed in the eastern NCC.
Magmatic zircons are generally distinguished from metamorphic zircons based on cathodoluminescence (CL) images and Th and U contents of zircon, as well as their Th/U ratios [43]. Typically, magmatic zircons show oscillatory growth zonation (for felsic igneous rocks) or striped absorption (for mafic igneous rocks) in CL images, and have high Th and U contents, and high Th/U ratios (>0.4). Conversely, metamorphic zircons are structureless or show pudding texture on CL images and have low Th and U contents, as well as low Th/U ratios (<0.1) [43][44][45][46][47].
The group of zircons from dunite xenoliths with ages of 151-164 Ma yielded a weighted mean 206 Pb/ 238 U age of 156±7 Ma (MSWD = 0.78, n = 4). These zircons showed striped absorption and oscillatory growth zonation in CL images, and had Th/U ratios of 0.15 to 1.53, suggesting a magmatic origin. Although spot 2 in sample LW8-42A had a low Th/U ratio (0.15), it showed typical oscillatory growth zonation, again indicating a magmatic origin. Based on these findings, we conclude that the zircons with ages of 151-164 Ma are of magmatic origin. These ages (151-164 Ma) are consistent with the SHRIMP zircon U-Pb age of the Huaziyu mafic lamprophyre in eastern Liaoning province (155±4 Ma) [56], and with zircon U-Pb ages of the Jingshan granitoids in the Bengbu area, the Duogushan and Wendeng granitoids in the northern section of the Sulu ultrahighpressure metamorphic belt (155-160 Ma) [54,57], the Linglong and Luanjiahe granitoids in eastern Shandong (155-160 Ma) [53], and Late Jurassic granitoids in eastern Liaoning [58]. Late Jurassic magmatism in the eastern NCC was generally characterized by intensive felsic magmatic events, whereas little mafic magmatism occurred at this time (e.g. the Huaziyu lamprophyre in the eastern Liaoning).
The zircons with ages of 261-280 Ma were subhedral or anhedral, showed striped absorption in CL images, and had high Th/U ratios (0.39-0.96), suggesting a magmatic origin. The age group of 261-280 Ma was similar to the age of the Emeishan large igneous province (259-262 Ma) [59,60] and corresponds with timing of the mass extinction event at the end of the Permian [61]. Permian igneous rocks have not been reported from the eastern NCC, except for a small quantity of detrital zircons (ages of 273-282 Ma) extracted from Jurassic sandstones in the Mengyin and Zhoucun basins [62]. SHRIMP and LA-ICP-MS zircon U-Pb age data for felsic intrusive rocks, volcanic tuff, and mafic-ultramafic rocks indicate magmatism at 254-285 Ma along the northern margin of the NCC [63][64][65][66]. These results suggest that the global Permian event affected not only the northern margin of the NCC, but also the lithospheric mantle beneath the eastern NCC.
Zircons of the present study with ages of 434-452 Ma showed striped absorption in CL images and had high Th/U ratios (0.51-0.93), suggesting a magmatic origin. These ages are consistent with the LA-ICP-MS U-Pb ages of captured zircons from the Xiachangzhuang magnetite-amphibolite intrusive rock (450-484 Ma) [67], SIMS U-Pb ages of captured magmatic zircons with oscillatory growth zonation from Cenozoic basalt in eastern Liaoning (419-487 Ma) [68], and the U-Pb age of perovskites from the Mengyin kimberlite in western Shandong Province (456±8 Ma) [69], as well as phlogopite Rb-Sr and Ar-Ar ages obtained for the Fuxian kimberlite in the Liaoning region (463-466 Ma) [70,71]. These results point to the occurrence of Paleozoic magmatic events in the eastern NCC. The ultramafic nature of kimberlites hampers the formation of zircon. However, the presence of zircons that grew within kimberlitic magma in western Shandong and eastern Liaodong indicates the occurrence of a silica-rich magmatic event in the eastern NCC, in addition to early Paleozoic silica-poor ultramafic magmatism. Early Paleozoic zircons from dunite xenoliths, as analyzed in the present study, may have resulted from metasomatism of a silica-rich melt.
As mentioned above, zircons with ages of 500-516 Ma were of magmatic origin. These ages were similar to those of the Pan-African tectono-thermal events, indicating that the lithospheric mantle underneath the eastern NCC was affected by this event. Until now, this period of magmatism had only been reported in captured zircons from the Xiachangzhuang magnetite-amphibolite intrusion (505±10 Ma) [67] and in detrital zircons from Cretaceous sedimentary rocks (497±13 Ma) in the Pingyi Basin of western Shandong [62].

Repeated modification of the lithospheric mantle in the eastern NCC
Results of SHRIMP zircon U-Pb dating of the Tietonggou dunite xenoliths indicate that lithospheric mantle in western Shandong records multiple episodes of mantle magmatism ranging in age from the early Paleozoic to the late Mesozoic (131-516 Ma). This observation indicates that the lithospheric mantle was subjected to various degrees of meltrelated modification, and that the most intensive modification occurred in the late Mesozoic (131-164 Ma).
Recent studies of peridotite xenoliths from Paleozoic diamond-bearing kimberlites and Cenozoic basalts have revealed that the lithospheric mantle in the NCC has experienced a complex evolutionary process [72][73][74][75]. For example, Li, Sr, and Nd isotopic data for peridotite xenoliths from the Hannuoba, Fanshi, and Hebi Cenozoic basalts within the NCC suggest that lithospheric mantle in the NCC experienced multiple interactions between melt/fluid and peridotite [72]. The repeated modification of lithospheric mantle in the NCC is indicated by zircon U-Pb dating, trace element data, and Hf isotopic data for garnet/spinel pyroxenite veins that formed via reactions between a silica-rich melt and peridotite in the Cenozoic Hannuoba basalts [73], and by in situ Re-Os isotopic data on sulfides from peridotite xenoliths in these basalts [74]. Petrographic and mineral chemical data for pyroxenes from garnet peridotite xenoliths in the Mengyin kimberlites revealed that the ancient lithospheric mantle in the eastern NCC has been repeatedly overprinted [75].
Results reported herein suggest that the lithospheric mantle in the eastern NCC has been repeatedly modified and that the most intensive modification occurred in the late Mesozoic (131-164 Ma).

Conclusions
(1) Zircons from dunite xenoliths in the Tietonggou intrusion of western Shandong formed during repeated modification of the lithospheric mantle by silica-rich melt.
(2) SHRIMP zircon U-Pb age data indicate that all the zircons are of magmatic origin, and yield ages that define five groupings: 131-145, 151-164, 261-280, 434-452, and 500-516 Ma, consistent with the occurrence of multiple magmatic-thermal events in the eastern NCC.
(3) The lithospheric mantle in the eastern NCC was subjected to repeated modification, with the most intensive modification occurring in the late Mesozoic (131-164 Ma).