The complexities of zircon crystllazition and overprinting during metamorphism and anatexis: An example from the late Archean TTG terrane of western Shandong Province, China

Accepted Manuscript The complexities of zircon crystllazition and overprinting during metamorphism and anatexis: an example from the late Archean TTG terrane of western Shandong Province, China Chunyan Dong, Hangqiang Xie, Alfred Kröner, Shijin Wang, Shoujie Liu, Shiwen Xie, Zhiyong Song, Mingzhu Ma, Dunyi Liu, Yusheng Wan PII: DOI: Reference:

S0301-9268(17)30223-1 http://dx.doi.org/10.1016/j.precamres.2017.07.034 PRECAM 4844

To appear in:

Precambrian Research

Received Date: Revised Date: Accepted Date:

28 April 2017 18 July 2017 22 July 2017

Please cite this article as: C. Dong, H. Xie, A. Kröner, S. Wang, S. Liu, S. Xie, Z. Song, M. Ma, D. Liu, Y. Wan, The complexities of zircon crystllazition and overprinting during metamorphism and anatexis: an example from the late Archean TTG terrane of western Shandong Province, China, Precambrian Research (2017), doi: http:// dx.doi.org/10.1016/j.precamres.2017.07.034

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The complexities of zircon crystllazition and overprinting during metamorphism and anatexis: an example from the late Archean TTG terrane of western Shandong Province, China

Chunyan Donga, Hangqiang Xiea, Alfred Krönera, b, Shijin Wangc, Shoujie Liua, Shiwen Xiea, Zhiyong Songb, Mingzhu Maa, Dunyi Liua, Yusheng Wana*

a

Beijing SHRIMP Center, Institute of Geology, Chinese Academy of Geological Sciences, Beijing

100037, China b

Department of Geosciences, University of Mainz, D-55099 Mainz, Germany

c

Shandong Geological Survey Institute, Jinan 250013, China

∗Corresponding author at: Beijing SHRIMP Center, Institute of Geology, Chinese Academy of Geological Sciences, Beijing 100037, China. Tel.: +86 10 68999762. E-mail address: [email protected] (Y.S. Wan).

ABSTRACT There are different viewpoints on metamorphic and anatectic zircons recording ages of 2.45–2.48 Ga or even younger in some areas of the North China Craton where both late Neoarchean and late Paleoproterozoic tectono-thermal events are well developed. These are: 1) partial resetting of the U-Pb isotopic system in the late Neoarchean zircons, 2) metamorphism lasting from the late Neoarchean to the earliest Paleoproterozoic, and 3) earliest Paleoproterozoic metamorphism as separate different event. Western Shandong Province is an area where the late Neoarchean tectono-thermal event is widely developed but the late Paleoproterozoic event has not been identified. This provides an opportunity to understand the geological processes around the Archean-Proterozoic boundary. Based on a field study, we carried out SHRIMP U-Pb zircon dating on seventeen samples of ~2.5 Ga old metamorphic and anatectic rocks, including tonalite, trondhjemite, granodiorite, gabbro, quartz diorite, granite and paragneiss with primary emplacement or depositional ages of 2.52–2.68 Ga. Anatectic zircons show some textural and compositional features: a) Homogenous or blurred oscillatory zoning, b) high U contents and low

in Th/U ratios (commonly <0.3), c) rare idiomorphic morphologies, and d) commonly containing inherited (xenocrystic) cores. We conclude that the strong late Neoarchean event is widespread in western Shandong and is limited between ~2.50 and 2.54 Ga. In contrast, apparent 2.45–2.48 Ga metamorphic zircon ages in some other areas of the North China Craton may be the result of overprinting and partial recrystallization of Neoarchean metamorphic zircons during the late Paleoproterozoic event. The Archaean rocks of western Shandong can be divided into three belts, namely Belts A, B and C from the northeast to the southwest. The difference between Belts A and B in ~2.5 Ga metamorphic and anatectic intensity may indicate that the former was elevated to a higher crustal level compared with the latter at the end of the Neoarchean.

Keywords: Neoarchean; SHRIMP U-Pb zircon dating; Tectono-thermal event; Western Shandong; North China Craton

1. Introduction The North China Craton (NCC) is characterized by extensive ~2.50 Ga magmatism, metamorphism and anatexis, which resulted in both significant juvenile addition to the crust and crustal recycling during a short period of time. In some areas, such as eastern Shandong, southwestern Liaoning, northwestern Hebei and Daqingshan, where a late Paleoproterozoic tectono-thermal event is well developed, some metamorphic and anatectic zircon ages are younger than magmatic zircon ages of crustally-derived granites, down to 2.45 Ga or even later. It is debatable whether this is the result of partial resetting of the U-Pb isotopic system in the late Neoarchean zircons due to strong late Paleoproterozoic tectono-thermal event, or whether metamorphism lasted from the late Neoarchean to the earliest Paleoproterozoic, or whether the earliest Paleoproterozoic ages reflect a different event (Dong et al., 2014; Kröner et al. 1998; Liu et al. 2011; Ma et al., 2013; Wan et al., 2011a, 2015a; Yang et al., 2016; Yang and Santosh, 2017; Zhai and Santosh, 2011; Zhao and Zhai, 2013). The superposition of the late Paleoproterozoic event on older rocks makes it difficult to clarify the geological significance of earliest Paleoproterozoic metamorphic and anatectic zircon age records. Western Shandong is one of the few areas of the NCC where the late Neoarchean event is well developed but the late Paleoproterozoic event has not been identified. This provides an opportunity to understand the

geological processes around the Archean-Proterozoic boundary. We carried out SHRIMP U-Pb zircon dating on seventeen samples of metamorphic and anatectic rocks with different protolith origins and ages based on a field study, in order to determine the spatial distribution and timing of the ~2.5 Ga event and its relationship with the formation of crustally-derived granites. We further discuss the geodynamic significance of this event for western Shandong and the NCC and the textural and compositional features of anatectic zircons.

2. Geological background Western Shandong is one of the typical areas in the NCC where Neoarchean magmatic and supracrustal rocks are well developed with an exposed area of ~10,000 km2. The terrain extends roughly in a northwest-southeast direction and is bounded by the Tanlu Fault in the east (Fig. 1). Based on numerous studies, including SHRIMP U-Pb zircon dating (Cao, 1996; Du et al., 2003, 2005, 2010; Jahn et al., 1988; Lu et al., 2008; Ren et al., 2016; Wan et al., 2010, 2011b, 2012a, 2014), some important conclusions have been arrived at concerning the crustal evolution of this region, as summarized below.

Fig. 1

The Archean basement can be subdivided into three belts (Fig. 1), namely a late Neoarchean (2.49–2.53 Ga) belt of crustally-derived granites in the northeast (Belt A), an early Neoarchean (2.60–2.75 Ga) belt of ancient rocks in the center (Belt B), and a late Neoarchean (2.50–2.55 Ga) belt of juvenile rocks (mainly granodiorire) in the southwest (Belt C). Supracrustal rocks occur as lenses and stripes within granitoids on different scales and can be subdivided into two successions: the early Neoarchean (2.70–2.75 Ga) Yanlingguan-Liuhang succession, which mainly occurs in Belt B and consists of amphibolite and metamorphosed ultramafic rocks, and the late Neoarchean (2.52–2.55 Ga) Shancaoyu-Jining succession, which occurs in all three belts and consists of fine-grained biotite gneiss, conglomerate, BIF and felsic metavolcanic rocks. Neoarchean magmatism occurred extensively with early and late Neoarchean magmatic rocks showing almost continuous formation ages from 2.75 to 2.59 Ga and 2.55 to 2.49 Ga, respectively.

The former is mainly represented by ultimately mantle-derived rocks, such as gabbro, tonalite, trondhjemite and granodiorite, whereas the latter consists of both crustally-derived intrusives (monzongranite with minor syenogranite) and juvenile rocks (granodiorite with some gabbro, diorite and tonalite). Mixing of different magma compositions is common within these intrusions. Two tectono-thermal events were recorded at ~2.5 and ~2.6 Ga. The older event was mainly observed within Belt B, represented by metamorphism, deformation and anatexis. This is regarded as evidence to consider the age of 2.6 Ga as the boundary between the early and late Neoarchean in this region (Ren et al., 2016). The younger event was mainly observed within Belt A and led to widespread anatectic rocks as well as crustally-derived granites. The anatectic rocks are cut by, and occur as enclaves at different scales within, crustally-derived granites. Compared with the ~2.6 Ga event, the ~2.5 Ga event was not studied in detail in previous publications. However, ~2.5 Ga metamorphism and anatexis have been reported in Yishui, within the Tanlu faults (Shen et al., 2004; Zhao et al., 2013; Wu et al., 2013; Li et al., 2016). Rocks in the Yishui area are considered to represent rocks in deeper crustal levels compared with those in western Shandong. It is necessary to indicate that anatexis in this paper not only includes in situ partial melting, but also short distance migration of melted leucosomes in space.The western Shandong Archaean rocks were considered to have formed in an arc environment during the late Neoarchean for the following reasons: a) magmatic rocks of different ages and compositions occur in different zones and spatially show an asymmetrical distribution; b) detrital zircons of late Neoarchean metasedimentary rocks show a similar age distribution as those from intrusive and volcanic rocks, therefore, the intrusive rocks uplifted very quickly to the surface as the source region for the late Neoarchean sedimentary basin, suggesting an active environment; c) the older rocks (2525–2550 Ma) commonly underwent stronger deformation and metamorphism than the younger rocks (2480–2525 Ma); d) magmatic rocks of different composition are depleted in Nb and Ta and thus display a subduction-related chemical signature.

3. Sampling and petrography Seventeen samples were selected for U-Pb zircon dating with ten, two and four samples being from Belts A, B and C, respectively (Fig. 1). Their field and petrographic features are described from Belt A to C as follows.

3.1. Belt A 3.1.1. Biotite paragneiss (S0729, E36°29'01", N117°21'30") There is a belt of anatectic rocks in the Qixingtai-Dawangzhuang area, in the northwestern part of our study area, extending for at least 20 km in a NW-SE direction. It is located northeast of the late Neoarchean (2.52–2.55 Ga) Shancaoyu-Jining succession and is mainly composed of anatectic paragneiss, amphibolite and leucosomes (Fig. 2a). The banded biotite gneiss is considered to have formed as a result of anatexis of a fine-grained biotite gneiss of supracrustal origin (Wan et al., 2012a). Sample S0729 is from the belt ~3 km southwest of Qixingtai town. It is fresh and shows compositional layering with foliation-parallel leucosome bands (Fig. 2b). It is mainly composed of plagioclase, microcline, quartz and biotite (Fig. 3a). Some plagioclases show polysynthetic twinning and are partly altered to sericite. Some biotite flakes and quartz grains occur as aggregates to form a foliation.

Fig. 2 Fig. 3

3.1.2. Tonalitic gneiss (S1328, E36°26'42", N117°39'41") Ca. 2.5 Ga anatectic rocks widely occur in Belt A as enclaves of different scales in 2.5 Ga crustally-derived granites. Sample S1328 is from a large enclave of anatectic rocks west of Boshan city. The rocks are composed of anatectic tonalitic gneiss and leucosomes with variable amphibole contents, showing complexly deformed banded structures (Figs. 2c, d). A high propotion of leucosome indicates that the strong deformation may have immediately followed anatexis, rather than occurred after solidation of migmatites. The amphibole grains in the leucosome are commonly large and are considered to be of metamorphic origin in terms of field evidence. The dated sample is relatively homogenous (Fig. 2d) and mainly consists of plagioclase, quartz, biotite and amphibole (Fig. 3b). Plagioclases are partly altered to epidote. Biotite and amphibole commonly occur together and show weak orientation. The rock does not undergo strong deformation.

3.1.3. Metagabbro (S1244, E36°17'18", N118°03'20")

In Belt A, metagabbro and dioritic gneiss of ~2.5 Ga have been identified on scales of several ten meters or less. Sample S1244 is from the Lushan Geopark where both metagabbro and meta-quartz diorite occur together (>10 m in diameter) and are cut by monzogranitic dykes. The metagabbro is foliated (Fig. 2e) and is mainly composed of plagioclase, biotite and clinopyroxene (Fig. 3c). Some plagioclases occur in relatively large grain and may be magmatic in origin. Biotite and clinopyroxene commonly occur together.

3.1.4. Quartz dioritic gneiss (S1245, E36°17'18", N118°03'20") Compared with the metagabbro (S1244) at the same location, this rock shows more obvious evidence of anatexis. However, sample S1245 was taken from an outcrop where leucosoms is relatively rare (Fig. 2f). It is mainly composed of plagioclase, amphibole, biotite with quartz and microcline (Figs. 3d, e). Biotite, amphibole and clinopyroxene commonly occur together to form a foliation. The fine-grained microcline is considered to be formed as a result of anatexis.

3.1.5. Tonalitic gneiss (S1027, E36°11'25", N118°38'32") In the Yishan Geopark, northeast of Belt A, where anatexis is well developed, amphibolitite, amphibolite and tonalitic gneisses occur as small enclaves within migmatites (Figs. 2g-i). The amphibolitite enclaves are strongly altered by leucosomes (Fig. 2g). The leucosomes show complex compositional variations with some seemingly becoming to be granite due to further evolution. It is evident that some amphibole grains in the leucosomes formed by separation and recrystallization of amphibolitite or amphibolite, rather than by crystallization from a magma. This suggests that the amphibole grains in some amphibole granites are not magmatic in origin. Sample S1027 is from a leucosome-free tonalitic gneiss enclave which is cut by leucosome dykes (Fig. 2h). It is composed of plagioclase, quartz, amphibole and biotite (Fig. 3f). Some plagioclases show polysynthetic twinning. Biotite flakes form a weak foliation. Amphibole occurs as anhedral grains and shows a close association with biotite.

3.1.6. Quartz diorite gneiss (S1029, E36°10'54", N118°37'29") This sample was collected from the same location as sample S1027 and occurs as an enclave cut by leucosomes with or without large-grained amphiboles (Fig. 2i). Sample S1029 is a quartz

diorite (SiO2= 60.27 %) but with high K2O content (3.48 %) (Yusheng Wan, unpublished data). Considering that the rock underwent metamorphism and anatexis, it cannot be excluded that it was originally a tonalite or granodiorite and anatexis and alteration may have led to a decrease in SiO2 and an increase in K2O. The sample does not contain leucosomes. It is similar in petrographic features to quartz diorite gneiss sample S1245 mentioned above.

3.1.7. Banded trondhjemitic gneiss (S1334, E36°11'49", N118°37'48") As mentioned above, some leucosomes in the Yishan Geopark look like granitic rocks. At some locations, rocks with obviously different proportions of amphibolite or amphibolitite enclaves occur together (Fig. 2j), probably as a result of two magma types coming together. At the location of sample S1334 the rock looks like common granite in a large scale (Fig. 2k). However, it is very inhomogenous at a small scale as indicated by large variations in amphibole and biotite contents (Fig. 2l). The rock is considered to have resulted from a magma half way from leucosome to common granite. The sample is composed of plagioclase, quartz, microcline and biotite (Fig. 3g). Some plagioclases show polysynthetic twinning and are partly altered to epidote. Some biotite is dark in colour and loss cleavage, probably as a result of anatexis. There a few microcline grains, with some occurring as porphyroblast.

3.1.8. Biotite monzogranite gneiss (S1324, E35°53'38", N117°53'41") The sample location is near the boundary between Belts A and B, some 10 km east of Xintai where the granite exhibits anatexis with local leucosomes (Fig. 2m). The rock shows a gneissic structure, and complex folding is displayed by leucosomes, indicating that the anatectic granite underwent strong ductile deformation, probably during the anatectic process. The sample is from a homogenous portion without obvious leucosomes and mainly consists of plagioclase, microcline, quartz and biotite (Fig. 3h). Some plagioclases show polysynthetic twinning with some undergoing alteration. Orintated biotite flakes form a foliation.

3.1.9. Granodioritic gneiss (S0718, E35°35'00", N118°11'25") The Menglianggu Geopark is an area where migmatizatioin and anatexis are widely developed. In some cases it is difficult to determine the protoliths of the anatectic rocks because of very

strong melting. In the outcrop of sample S0718 the migmatite shows complex folding and is composed of granodiorite and leucosome (Fig. 2n). The sample is from a small homogenous granodiorite enclave which is mainly composed of plagioclase, feldspar, quartz and biotite (Fig. 3i). caorse-grained plagioclases show polysynthetic twinning. Some quartz grains occur as aggregates. There are fine-grained mineral aggregates which are mainly composed of plagioclase , micrpcline and quartz with some biotite, probably as a result of anatexis.

3.1.10. Banded tonalitic gneiss (S0513, E35°33'49", N118°11'17") This sample is taken from an outcrop about 1 km south of sample S0718 within the same geopark. The rock shows very strong anatexis, and we speculate that its protolith was a tonalite because of a homogenous appearance in a large outcrop, although leucosome blocks locally occur (Fig. 2o). Sample S0513 is mainly composed of plagioclase, microcline, quartz and biotite (Fig. 3j). There also are fine-grained mineral aggregates of plagioclase , micrpcline and quartz, which are distributed around caorse-grained plagioclase and microcline.

3.2. Belt B 3.2.1. Granodioritic gneiss (S1350, E35°23'20", N118°04'40") This sample is from a very large outcrop (Fig. 2p). Compared with many rocks mentioned above, this exposure shows only weak anatexis, so its granodioritic protolith is easy to determine. However, the parallel layering and tight folding of the leucosomes suggest that all rocks underwent strong ductile deformation. Sample S1350 is from a homogenous granodiorite which is mainly composed of plagioclase, quartz, microcline and biotite (Fig. 3k). Plagioclase underwent alteration, biotite flakes occur as aggregates together with fine-grained felsic minerals to form a foliation. There are some microcline porphyroblasts, probably formed as a result of anatexis and alteration.

3.2.2. Tonalitic gneiss (S1348, E35°24'46", N118°09'20") This sample is from ~10 km east of sample S1350, where the rocks are medium-grained tonalitic gneiss with fine-grained tonalitic gneiss looking like dikes (Fig. 2q). The sample shows relatively weak anatexis and deformation and was taken from a medium-grained tonalitic gneiss. It

is mainly composed of plagioclase, quartz and biotite (Fig. 3l). Both biotite and quartz occur as aggregates to form a foliation.

3.3. Belt C 3.3.1. Banded granodioritic gneiss (S0760, E35°42'44", N117°36'37") This sample is from a strongly anatectic and deformed outcrop some 15 km south of Mengjiatun village. Strong deformation resulted in parallization of different components (Fig. 2r). The sample mainly comprises plagioclase, K-feldspar, quartz with some biotite (Fig. 3m). Plagioclase occurs as anhedral to sub-euhedral grains, whereas quartz and biotite occur as aggregates.

3.3.2. Granitic gneiss (S0715, E35°27'05", N117°57'37") This sample was collected from a strongly deformed zone near the boundary between Belts B and C where trondhjemitic gneiss is interlayerd with granitic gneiss. These two rock types show sharp or transitional contact relationships (Fig. 2s). This sample is from a granite layer interlayered with trondhjemitic gneiss (Fig. 2s). It comprises quartz, microcline, plagioclase with some biotite (Fig. 3n). Almost all minerals, including quartz aggregates, show strong orientation, except for some relatively coarse feldspar grains which occur as augens.

3.3.3. Trondhjemitic gneiss (S0714, E35°27'05", N117°57'37") This sample is from a tonalite gneiss layer of relatively homogenous composition. It is similar in petrographic features to the nearby Granitic gneiss (S0715), but contain slightly more biotite.

3.3.4. Banded granodioritic gneiss (S0770, E35°29'13", N117°15'43") In a quarry ~15 km south of Shenshuiyu village a clear relationship between banded granodiorite and massive granodiorite can be obversed. At some outcrops the relationship appears transitional and the banded granodiorite formed as a result of anatexis and deformation of the granodiorite (Fig. 2t). At other outcropts, however, the banded granodiorite is clearly cut by massive granodiorite (Fig. 2u), and the former also occurs as enclaves in the latter, so the banded granodiorite must have formed earlier. It displays anatexis and deformation and is mainly

composed of plagioclase, microcline, quartz and biotite. The massive granodiorite shows weak deformation and is similar in composition to the banded granodiorire although it contains some K-feldspar phenocrysts. This may indicate that the massive granodiorite formed as a result of melting of the banded anatectic granodiorite (Fig. 3o). Some microcline and plagioclase occur as augens. Quartz and biotite aggregates are orientedly distributed, showing strong deformation.

3.3.5. Banded granitic gneiss (S0772, E35°28'37", N117°15'47") This sample is from a weathered outcrop several hundred meters south of sample S0770. The rock is yellow in weathered surface and shows strong anatexis and deformation, as suggested by tight folding of leucosomes (Fig. 2v). It is mainly composed of plagioclase, feldspar and quartz with little biotite (Fig. 3p). Plagioclase commonly underwent alteration. Orintated quartz aggregates and biotite flakes form a strong foliation.

4. Analytical techniques Zircon U-Pb dating was carried out using the SHRIMP II instrument at the Beijing SHRIMP Center, Institute of Geology, Chinese Academy of Geological Sciences. Age measurements were performed over a period of about ten years, but in all cases the procedures and conditions were similar to those described by Williams (1998). A primary O2− ion beam of 3–6 nA was used to bombard the surface of zircons, resulting in a 25–35 μm spot size. Each site was rastered for 120–200s prior to analysis. Mass resolution during the analytical sessions was =5000 (1 % definition). Data were generated during five scans. Standard zircons for elemental abundance calibration include 91500 (U=91 ppm), SL13 (U=238 ppm) and M257 (U=840 ppm) (Ballard et al., 2001; Nasdala et al., 2008; Williams, 1998). TEMORA, whose

206

Pb/238U age is 417 Ma

(Black at al., 2003), was analyzed for calibration of 206Pb/238U ratios after every 3 or 4 analyses. A common lead correction was applied using measured

204

Pb abundances. Data were processed and

assessed using the Squid 1.02 and Isoplot 3.00 programs (Ludwig, 2001, 2003). The data are listed in Supplementary Table 1, where uncertainties for individual analyses are quoted at 1σ, whereas uncertainties on weighted mean ages given in the text and concordia diagrams are quoted at the 95% confidence level.

5. Zircon geochronology 5.1. Belt A 5.1.1. Biotite paragneiss (S0729) The zircons are stubby or oval in shape and commonly show core-rim textures in cathodoluminescence (CL) images (Fig. 4a). The cores experienced recrystallization to different degrees with some grains displaying blurred oscillatory zoning. The rims are darker in color and some show pyramidal shapes with sparse oscillatory zoning, typical textures of anatectic zircons. There are light domains along some core margins, which are interpreted as a result of fluid alteration prior to the growth of anatectic rims. Eighteen analyses were performed on 16 zircon grains. Sixteen analyses on recrystallized cores have U contents and Th/U ratios of 37–323 ppm and 0.06–1.2, respectively. They show large variations in age (Fig. 5a). Six analyses (3.2RC, 6.1RC, 7.1RC, 9.1RC, 10.1RC, 13.1RC) plot on or near concordia, yielding a weighted mean 207

Pb/206Pb age of 2681±13 Ma (MSWD=0.65), whereas two analyses (2.1RC, 11.1RC) closest to

concordia define a weighted mean 207Pb/206Pb age of 2563±20 Ma (MSWD=1.7), and one analysis (4.1RC) close to concordia has a 207Pb/206Pb age of 3.29 Ga. Other analyses show strong lead loss. Two analyses on rims have U contents of 463–731 ppm and low Th/U ratios of 0.07–0.11. No reliable age could be obtained because of strong lead loss.

Fig. 4 Fig. 5

5.1.2. Tonalitic gneiss (S1328) The zircons are columnar or stubby with rounded terminations. They show core-rim textures in CL images, but the rims commonly are not well developed (Fig. 4b). The cores underwent strong recrystallization, resulting in almost complete disappearance of the original magmatic textures. In grain 9, an overgrowth rim joins two cores together, suggesting that they had adjacent position before the rim formed. Nine analyses were performed on 9 grains. Seven analyses on recrystallized cores have U contents of 195–580 ppm and Th/U ratios of 0.36–1.69, whereas two analyses on rims have higher U contents and lower Th/U ratios of 664–976 ppm and 0.08–0.10, respectively. Almost all zircons show lead loss, and the rims show the strongest lead loss (Fig. 5b).

Analysis 7.1RC plots closest to concordia and defines a 207Pb/206Pb age of 2518 Ma, which may be close to the time of the end-Archean tectono-thermal event, rather than defining the age of a magmatic zircon because of strong recrystallization.

5.1.3. Metagabbro (S1244) The zircons are columnar in shape and show broad, banded zoning in CL images (Fig. 4c), a feature typical of magmatic zircon from a mafic magma. They underwent recrystallization to different degrees. Thirteen analyses were performed on 11 zircon grains. Seven analyses on magmatic domains have U contents of 283–545 ppm and Th/U ratios of 0.09–0.46, three analyses on recrystallized domains have similar isotopic compositions as the magmatic domains, whereas the metamorphic rims are always low in Th/U ratios (0.04–0.10). Almost all magmatic zircons show lead loss to variable degrees (Fig. 5c) so that no presice magmatic ages could be determined. Concordant analyses 1.2R and 2.1RC yielded a weighted mean

207

Pb/206Pb age of 2480±11 Ma

(MSWD=0.62). This is considered to be close to, but slightly younger than, the age of a metamorphic event.

5.1.4. Quartz diorite gneiss (S1245) The zircons are similar in shape and texture to those from the nearby metagabbro (S1244) (Fig. 4d). Eleven analyses were performed on 10 grains. Seven analyses on magmatic domains have U contents and Th/U ratios of 239–544 ppm and 0.27–0.66, respectively. They show lead loss (Fig. 5d), and three analyses (1.1MA, 2.1MA, 7.1MA) with the smallest lead loss yielded an upper concordia intercept age at 2519±20 Ma (MSWD=2.5). Four analyses on recrystallized domains have U contents of 72–722 ppm and Th/U ratios of 0.05–0.18. They commonly show stronger lead loss than the magmatic domains. A 207Pb/206Pb age of 2663 Ma on spot 9.1RC is not considered reliable because of a large error (38 Ma). We speculate that the quartz diorite gneiss (S1245) and the metagabbro (S1244) constitute a magmatic complex that formed at around 2.52 Ga and was metamorphosed slightly later.

5.1.5. Tonalitic gneiss (S1027) The zircons are columnar or stubby in shape, and the columnar zircons show striped zoning,

whereas the stubby grains show oscillatory zoning in CL images (Fig. 4e). Both experienced recrystallization, but overgrowth rims are too narrow for SHRIMP analysis. Seventeen analyses were performed on 15 grains. Seven analyses on magmatic domains have U contents of 88–244 ppm and Th/U ratios of 0.37–0.74 with the exception of 12.1MA which has a U content of 898 ppm and a Th/U ratio of 2.24. Six of these analyses (5.1MA, 5.2MA+R, 6.1MA, 13.1MA, 14.1MA, 15.1MA) plot on or near concordia and define a weighted mean

207

Pb/206Pb age of

2666±9 Ma (MSWD=1.20) (Fig. 5e), similar as that of a tonalitic gneiss enclave (S1028) reported by Wan et al. (2014) in the area. This indicates that the ~2.66 Ga tonalitic gneisses may occur more widely in the geopark. Recrystallized zircons are similar in composition to magmatic zircons, and two analyses (1.1RC, 2.1RC) plot near concordia and yielded a weighted mean

207

Pb/206Pb

age of 2613±11 Ma (MSWD=2.7) (Fig. 5e). This age may record the time of a tectono-thermal event at the end of early Neoarchean which has recently been identified in western Shandong Province (Ren et al., 2016). Three analyses on rims yielded higher U contents of 419–739 and lower Th/U ratios of 0.06–0.13. Only analysis 9.1R plots close to concordia, and its

207

Pb/206Pb

age of 2534 Ma is interpreted as an age record of metamorphism at the end of the Neoarchean.

5.1.6. Quartz diorite gneiss (S1029) The zircons are either long-prismatic or oval in shape and show oscillatory zoning with strong recrystallization in CL images (Fig. 4f). The overgrowth rims are commonly narrow, and it is difficult to distinguish these from the recrystallized domains in some grains. Thirteen analyses were carried out on 10 zircon grains. Five analyses (1.1MA, 2.1MA, 3.1MA, 5.1MA, 9.1MA ) on magmatic domains have U contents and Th/U ratios of 130–328 ppm and 0.35–1.01, respectively. They plot on or near concordia, yielding a weighted mean

207

Pb/206Pb age of 2518±9 Ma

(MSWD=1.70) (Fig. 5f). The recrystallized domains exhibit lead loss, and three concordant analyses (2.1RC, 4.1RC+R, 6.1RC) have a weighted mean

207

Pb/206Pb age of 2518±13 Ma

(MSWD=0.37), similar to the age of magmatic zircons. We speculate that both magmatism and metamorphism occurred at ~2.52 Ga.

5.1.7. Banded trondhjemitic gneiss (S1334) The zircons are long-prismatic with core-rim textures in CL images (Fig. 4g). The cores show

oscillatory or striped zoning, and the rims are homogenous or show blurred zoning. Twenty-five analyses were performed on 17 grains. Nine analyses on cores yielded U contents and Th/U ratios of 194–557 ppm and 0.23–0.53, and analyses plotting close to concordia have

207

Pb/206Pb

ages >2.64 Ga (Fig. 5g). Twelve analyses on rims have lower U contents of 97–291 ppm and higher Th/U ratios of 0.38–1.06. All analyses, except for 6.1R, plot on or near concordia, yielding a weighted mean

207

Pb/206Pb age of 2506±6 Ma (MSWD=0.95). Based on field features of the

rock represented by sample S1334, the zircon cores are considered to be xenocrystic in origin, and the anatectic protolith may be a 2.65–2.7 Ga magmatic rock. The rims can be subdivided into two domains. The inner domains with lighter color are considered to have formed through recrystallization of the xenocrystic zircons, whereas the outer domains with darker color are considered to have formed as a result of magma on a half way from leucosome to common granite, consistent with the field observation of the rock. In any cases, the mean age of 2506 Ma for the rims represents the time of widespread anatexis in the area, which resulted in the formation of the banded trondhjemitic gneiss.

5.1.8. Biotite monzogranite gneiss (S1324) The zircons are columnar in shape and most show striped zoning with recrystallization; narrow overgrowth rims occur on some grains in CL images (Fig. 4h). Fourteen analyses were performed on 14 grains. Eight analyses on magmatic domains have U contents of 161–1018 ppm and Th/U ratios of 0.23–0.96, five analyses on strongly recrystallized domains and overgrowth rims yielded higher U contents of 961–7990 ppm and lower Th/U ratios of 0.04–0.17. Zircon domains due to metamorphism and anatexis commonly show stronger lead loss than magmatic domains (Fig. 5h). Analyses 3.1 MA is concordant and has a

207

Pb/206Pb age of 2611±9 Ma, which is interpreted as

the crystallization age of the rock. No metamorphic age could be be obtained, but we speculate that the metamorphic overprint event occurred at the end of the Neoarchean because the sample is located in a ~2.5 Ga migmatite belt (Fig. 1).

5.1.9. Granodioritic gneiss (S0718) The zircons are long-prismatic and show oscillatory or banded zoning with some showing recrystallization in CL images (Fig. 4i). Fifteen analyses were performed on 15 zircon grains with

all data plotting on or near concordia. Eight analyses on magmatic domains yielded U contents of 207

Pb/206Pb age of 2518±11 Ma

63–209 ppm and Th/U ratios of 0.39–0.98 and a weighted mean

(MSWD=1.70; Fig. 5i). Seven analyses on recrystallized domains are similar in composition and age to the magmatic zircons. Overgrowth rims are very narrow (Fig. 4i) although the rock underwent strong metamorphism and anatexis.

5.1.10. Banded tonalitic gneiss (S0513) The zircons are stubby or oval in shape and show well-developed core-rim textures in CL images (Fig. 4j). The cores display irregular shapes and oscillatory zoning commonly with some recrystallization, and the rims show blurry oscillatory zoning. Twenty-two analyses were performed on 14 zircon grains. Eight analyses on magmatic cores have U contents of 42–376 ppm and Th/U ratios of 0.58–1.01. Some show lead loss to variable degrees (Fig. 5j). Analyses 1.2MA and 6.2MA are concordant and have a weighted mean

207

Pb/206Pb age of 2585±19 Ma

(MSWD=0.00). This is considered to represent the time of crystallization of the tonalite. Two cores (2.1X, 14.1 X) are considered as xenocrysts and have 207Pb/206Pb ages of 2735 Ma and 2693 Ma. Eight analyses on overgrowth rims yielded high U contents of 465–1113 ppm and low Th/U ratios of 0.10–0.15. Most of these show variable degrees of lead loss but form a linear array in a concordia diagram. Four analyses with the smallest lead loss (1.1R,7.1R,11.2R,12.1R) define an upper concordia intercept age of 2491±12 Ma (MSWD=2.4). Analysis 1.1 R plots nearest concordia (discordance=6%) and has a

207

Pb/206Pb age of 2496±6 Ma, similar to the upper

concordia intercept age.

5.2. Belt B 5.2.1. Granodioritic gneiss (S1350) The zircons are stubby in shape and show oscillatory zoning with recrystallization and narrow rims in CL images (Fig. 4k). Some magmatic zircons contain cores (grain 2 in Fig. 4k) as indicated by light rims along the margins of the cores, but the cores have a similar age as the magmatic zircons. This is interpreted that the cores are magmatic and formed during early magmatism, later altered by fluids to form the light rims, and then new magmatic overgrowth occurred during late magmatic processes. Nineteen analyses were performed on 14 zircon grains.

Fifteen analyses on magmatic domains have U contents and Th/U ratios of 109–922 ppm and 0.16–0.87, respectively, eleven analyses concentrating on concordia have a weighted mean 207

Pb/206Pb age of 2528±5 Ma (MSWD=2.7; Fig. 5k). One rim analysis has a

207

Pb/206Pb age of

2460 Ma but with lead loss (discordance =8%), its true age may be ~2.5 Ga.

5.2.2. Tonalitic gneiss (S1348) The zircons are columnar or stubby in shape and show core-rim textures in CL images (Fig. 4l). The cores display oscillatory zoning with some having undergone recrystallization. The rims are commonly narrow. Twenty analyses were performed on 20 zircon grains. Fourteen analyses on magmatic domains have U contents of 55–350 ppm and Th/U ratios of 0.16–0.42. Nine analyses on or near concordia yielded a weighted mean

207

Pb/206Pb age of 2683±6 Ma (MSWD=0.89, Fig.

5l). Two analyses were made on rims and have low Th/U ratios of 0.16–0.18. A concordant analysis (22.1R) has a

207

Pb/206Pb age of 2521±16 Ma, suggesting that the 2.7 Ga tonalite

underwent a tectono-thermal event at about 2.5 Ga.

5.3. Belt C 5.3.1. Banded granodioritic gneiss (S0760) The zircons are columnar in shape and show core-rim textures in CL images (Fig. 4m). The cores show oscillatory zoning with recrystallization, and the rims are dark and homogenous. Twenty analyses were performed on 16 zircon grains. Five analyses on magmatic domains have U contents of 55–258 ppm and Th/U ratios of 0.25–1.08. Among these, 4 concordant analyses yielded a weighted mean

207

Pb/206Pb age of 2562±10 Ma (MSWD=0.20) (Fig. 5m), which is

interpreted as the crystallization age of the granodiorite. Six analyses on recrystallized domains and seven analyses on rims show similar compositions, having high U contents of 285–1063 ppm and low Th/U ratios of 0.03–0.31 compared with the magmatic domains. Three analyses on rims have a weighted mean

207

Pb/206Pb age of 2514±4 Ma (MSWD=2.0), interpreted as reflecting the

late Neoarchean tectono-thermal event. Analysis 1.1X has a 207Pb/206Pb age of 2637±11 Ma and is considered to be a xenocrystic zircon core.

5.3.2. Granitic gneiss (S0715)

The zircons are columnar in shape and show core-rim textures in CL images (Fig. 4n). The cores are considered as xenocrystic in origin because the leucosome has almost become a granite. The cores show oscillatory zoning, mostly with some recrystallization. The rims are homogenous with some showing blurred zoning. Nineteen analyses were performed on 16 grains. Six analyses on xenocrystic cores plot on or near concordia (Fig. 5n) and have U contents and Th/U ratios of 125–448 ppm and 0.09–0.97, respectively. They yielded two distinctly different weighted mean 207

Pb/206Pb ages of 2619±14 Ma (2 analyses, MSWD=0.10) and 2671±9 Ma (4 analyses,

MSWD=0.73). The recrystallized domains (4 analyses) and overgrowth rims (9 analyses) show large variations in U contents (285–1284 ppm and 107–2070 ppm), and the latter has lower Th/U ratios than the former (0.10–0.26 and 0.21–0.78). The overgrowth rims also show strong lead loss, and the recrystallized domains show lead loss to different degrees. Three analyses of the latter plot close to concordia and define a weighted mean

207

Pb/206Pb age of 2541±9 Ma (1.1RC, 5.1RC and

15.1RC, MSWD=1.01), which is close to, or slightly older than, the age of metamorphism and anatexis.

5.3.23. Trondhjemitic gneiss (S0714) The zircons are columnar in shape and show core-rim textures in CL images (Fig. 4o). There are light domains between the core and rim. The cores show oscillatory zoning, whereas the rims are homogenous and some display blurred zoning. Fourteen analyses were performed on 9 grains. Seven analyses on magmatic domains have U contents and Th/U ratios of 31–475 ppm and 0.50–0.99, respectively. Six of these are concordant and yielded a weighted mean

207

Pb/206Pb age

of 2592±25 Ma (MSWD=2.2; Fig. 5o). Six analyses on rims have higher U contents of 217–595 ppm and lower Th/U ratios of 0.04–0.31, two of these (1.2R, 6.2R) are concordant with a weighted mean 207Pb/206Pb age of 2533±11 Ma (MSWD=0.95), which is considered to represent a metamorphic overprint time. It is notable that trondhjemite gneiss sample S0714 is different in zircon textures and core ages from nearby granitic gneiss sample S0715. This may indicate that the latter is not derived from anatexis of the former, although both rock types occur together in the field (Fig. 2s).

5.3.4. Banded granodioritic gneiss (S0770) The zircons are columnar in shape and show core-rim textures in CL images (Fig. 4p). The cores show oscillatory zoning and the rims are homogenous with some showing blurred zoning. There are light domains between the core and rim. Eighteen analyses were performed on 16 zircon grains. Nine analyses on magmatic domains have U contents and Th/U ratios of 117–701 ppm and 0.37–1.44, respectively. Six of these plot on or near concordia and yielded a weighted mean 207

Pb/206Pb age of 2553±9 Ma (MSWD=1.10, Fig. 5p). This is considered to represent the

formation age of the rock. Recrystallized domains (3 analyses) and overgrowth rims (6 analyses) are similar in compositions, showing large U variations (185–2355 ppm) and low Th/U ratios (0.03–0.17). Two analyses (4.1R, 16.1R) of the rims close to concordia have a weighted mean 207

Pb/206Pb age of 2534±11 Ma (MSWD=0.62), which is interpreted as metamorphic overprint

time. Only one analysis of the recrystallized domains plots close to concordia (discondance = 5%) and has a 207Pb/206Pb age of 2543±8 Ma, which is slightly older than the age of the rims.

5.3.5. Banded granitic gneiss (S0772) The zircons are columnar in shape and show banded or oscillatory zoning and developed rims with both undergoing recrystallization in CL images (Fig. 4q). Sixteen analyses were performed on 11 zircon grains. Seven analyses on magmatic domains have U contents and Th/U ratios of 57–194 ppm and 0.21–1.16, respectively, four of these plotting closest to concordia define an upper concordia intercept age of 2597±40 Ma (MSWD=1.8, Fig. 5p). This may represent formation age of anatectic protolith, however, which cannot be precisely constrained as indicated by large error. Only one analysis was carried out on anatectic rim (11.1R), which has a Th/U ratio of 0.04 and a

207

Pb/206Pb age of 2568±30 Ma (discordance = 25). We speculate that the anatectic

event occurred at about 2.5 Ga. Both the xenocrystal and anatectic zircons underwent recrystallization. Eight recrystallized domains have U contents and Th/U ratios of 7–249 ppm and 0.02–0.89, respectively. It seems that their varying Th/U ratios inherited compositional features of the zircons of diffirent origins rather than reflecting fluid alteration. The zircons show lead loss to variable degrees. No reliable age could be obtained for the recrystallized domains.

6. Discussion

6.1. Textural and compositional features of anatectic zircons Anatexis links metamorphism and magmatism, therefore, zircons crystallized during anatexis show textural and compositional features similar to both metamorphic and magmatic zircons. Note that anatexis here refers to processes including limited mobility of leucosomes for short distances with respect to in-situ partial melting. Anatectically-derived zircons commonly are homogenous or display blurred oscillatory zoning. They are darker in color than the cores because of higher U contents. Some grains show pyramidal shapes (Figs. 4a, p, n). In many cases they carry on the shapes of inherited cores. In some cases, however, the pyramidal shapes of the anatectic rims are different from the irregular shapes of the inherited cores. These indicate that the anatectic rims may be recrystalized in leucosome (magma) amount variation environments. The common existence of cores indicates that anatexis in western Shandong occurred during medium-grade rather than high-grade metamorphism. This is because anatexis in the above case is a decompresion melting process which prevents a further increase in temperature. In several cases it is difficult to distinguish between metamorphic zircon (overgrowth rims and recrystallized domains) and newly-grown anatectic zircon. However, anatectic zircon commonly shows blurred oscillatory zoning (e.g., samples S0729 and S0770), and oscillatory zoning becomes more pronounced with increasing anatexis (e.g., sample S0513), and zoning can become distinct when the leucosomes become mobile and evolve towards crustally-derived granite (e.g., sample S1334). In some samples such as S0714, S0729, S0760 and S0770, the zircons contain white domains (low-U) between cores and rims in CL images, which are interpreted to have resulted from fluid alteration prior to overgrowth of the metamorphic or anatectic rims. Metamorphic and anatectic rims are commonly high in U contents and low in Th/U ratios than the cores. In general, the Th/U ratios are <0.1 and become to be larger (commonly <0.3) when the leucosomes become mobile (patch melting), similar to those formed by recrystallization under fluid-rich conditions (Pidgeon et al., 1992; Ren et al., 2016; Rubatto et al., 2003). This suggests that anatexis commonly occurred under fluid-rich conditions in western Shandong. It is only in the leucosomes with highly increasing mobility towards granite that anatectic zircons show typical magmatic features such as high Th/U ratios and oscillatory zoning (e.g., sample S1334).

6.2. Anatectic rocks and the formation ages of their protoliths

The anatectic rocks in our study area are mainly paragneiss, tonalite, trondhjemite and granodiorite with minor gabbro, quartz diorite and granite. The protoliths have formation ages ranging from 2.52 to 2.68 Ga, but rocks with ages of 2.56 to 2.59 Ga are rare (Table 1). This is consistent with the conclusions of Wan et al. (2010, 2011a, 2014) and Ren et al. (2016) that the magmatic rocks in western Shandong mainly range in ages from 2.5 to 2.55 Ga and 2.6 to 2.75 Ga, respectively. Some rocks contain xenocrystic zircons >2.6 Ga, indicating inheritance from older continental material during their formation. A 3.29 Ga zircon grain occurs in paragneiss sample S0729, suggesting the existence of some Paleoarchean material although it is certain that most continental crust formed during the early Neoarchean. It is notable that in Belt A, which is mainly composed of late Neoarchean crustally-derived granites, there are different types of magmatic rocks with ages of 2.59 to 2.68 Ga, which commonly occur as enclaves at different but small scales and underwent anatexis. This further supports the conclusions drawn from Nd and Hf isotopic studies that the ~2.5 Ga monzogranites and syenogranites in this belt are mainly derived from early Neoarchean (2.6–2.75 Ga) juvenile rocks and an early Neoarchean juvenile rocks are widespread in western Shandong (Wan et al., 2010, 2011a, 2014; Yusheng Wan, unpublished data). On the other hand, there also are early Neoarchean rocks in Belt C.

6.3. Late Neoarchean tectono-thermal event Late Neoarchean metamorphic zircon ages have rarely been reported previously from western Shandong, but in this study we have obtained abundant late Neoarchean metamorphic zircon ages, and we constrain this tectono-thermal event between ~2.50 and 2.54 Ga (Table 1). Precise metamorphic zircon ages could not be obtained for some samples because of high U contents which resulted in variable lead loss in zircons, and metamorphic and anatectic rims were too narrow for SHRIMP analysis. However, field relationships and the spatial distribution of the dated samples indicate that they underwent a widespread late Neoarchean tectono-thermal event. A few metamorphic zircons have apparent ages of 2.46–2.48 Ga but with large errors or showing significant lead loss. Therefore, we do not consider these “ages” to be significant, and we speculate that the tectono-thermal event in western Shandong ended at ~2.5 Ga. This is supported by 2.5 Ga granite dikes intruding and enclosing the anatectic rocks (Figs. 2w, x) and is also consistent with the fact that most crustally-derived granites were emplaced between 2.49 and 2.54

Ga (mainly 2.50–2.53 Ga) in the area (Wan et al., 2010). It is common that metamorphism and anatexis occurred slightly later than deposition of supracrustal rocks and emplacement of many TTG (tonalite-trondhjemite-granodiorite) rocks and coincided the intrusion of crustally-derived granites (Wan et al., 2015b). On the other hand, western Shandong is different from many other Archean areas of the NCC as it did not experience the strong tectono-thermal event of the late Paleoproterozoic. This leads us to the conclusion that in some areas of the NCC, where both the late Neoarchean and late Paleoproterozoic tectono-thermal events are well developed, apparent metamorphic zircon ages of 2.45–2.48 Ga or even younger may be the result of partial resetting of the U-Pb system of the zircons during the late Paleoproterozoic tectono-thermal event. More work is required to confirm this conclusion, considering its importance for understanding the Archean to Paleoproterozoic geological evolution of the NCC.

Table 1

The late Neoarchean metamorphic zircon ages occur throughout Belt A, with some also in Belts B and C (Fig. 1). However, zircon dating on Archean rock samples from the northwestern and middle portions of Belt B revealed widespread metamorphism and anatexis at ~2.6 Ga but rarely at ~2.5 Ga (Du et al., 2003, 2005; Lu et al., 2008; Wan et al., 2011a; Ren et al., 2016). Only rocks in the southwestern portion of Belt B recorded ~2.5 Ga metamorphism and anatexis. This is most likely because of their lower metamorphic grade and having remained at higher crustal levels during late Neoarchean orogenesis. This is different from eastern Shandong where the 2.7–2.9 Ga rocks underwent strong metamorphism and dehydration at ca. 2.5 Ga, so the late Paleoproterozoic high-grade metamorphism occurred within a relatively dry system and did not affect the zircons (Xie et al., 2014; Yusheng Wan, unpublished data). In contrast, the widespread occurrence of ~2.5 Ga metamorphism and anatexis in Belt A indicates that this belt once occurred at low crustal levels where metamorphism and anatexis resulted in the formation of migmatites. Granites clearly cut migmatitic and anatectic rocks at many outcrops, and this suggests that the anatectic rocks had already been elevated to higher crustal levels when the granites intruded. Therefore, we speculate that at the end of the Neoarchean, prior to intrusion of crustally-derived granites, Belt A was rapidly elevated, probably due to southwestward overthrusting of Belt A along the boundary

between Belts A and B. The crustally-derived granites obviously show original relationships with the migmatites, formed as a result of further evolution of leucosomes which are formed during decompossion melting processes. The metamorphic and anatectic zircon ages mainly concentrate between 2.50 and 2.53 Ga, identical or slightly later with the change in time of the tectonic regime in western Shandong from compression to extension at around 2.525 Ga (Wan et al., 2010). Besides >2.6 Ga magmatic rocks, 2.52–2.56 Ga magmatic rocks (mainly granodiorite, gabbro and diorite) have also been identified in Belt A. This can be considered to be an important indicator of crust-mantle interaction which probably played a key role in metamorphism and anatexis at the end of the Neoarchean. In this study, we showed that a strong ~2.50 Ga tectono-thermal event occurred in western Shandong as indicated by widely distributed metamorphism, anatexis and magmatism. In fact, this event widespread all over the NCC. There are different opinions on the cause of this event. Some authors favored an underplating model (Geng et al. 2006; Yang et al. 2008; Zhao et al. 1998, 1999, 2001), whereas others favored arc magmatism (Kröner et al. 2005a, b; Kusky and Li 2003; Kusky et al. 2007; Li et al. 2002; Nutman et al. 2011; Polat et al. 2006a, b; Wan et al. 2005a, b, 2010, 2012a; Wilde et al. 2005; Wu et al. 1998; Zhang et al. 2007, 2009; Zhao et al. 2001, 2002, 2005). However, it is accepted by many authors, including those favoring the underplating and arc magmatism model, that the latest Neoarchean event was related to an extensional tectonic setting, probably triggered by magmatic underplating (Geng et al. 2006; Yang et al. 2008; Nutman et al. 2011; Wan et al., 2015a). It is indicated by this study and previous work (Li et al., 2010; Wan et al., 2010, 2012b, 2015a, b; Yang et al., 2011) that crustally-derived granites cut metamorphic and migmatitic and anatectic rocks. The formation of voluminous granite batholiths together with widespread metamorphism and anatexis are considered as hallmarks of cratonization of the NCC at the end of the Neoarchean, a convenient time to set the Archean–Proterozoic boundary at 2.5 Ga (Wan et al., 2015b; Yang et al., 2011; Zhai and Santosh, 2011).

7. Conclusions 1) A strong late Neoarchean (2.50–2.54 Ga) tectono-thermal event was widespread in western Shandong, as indicated by extensively occurring metamorphic, anatectic and magmatic rocks. The difference between Belts A and B concerning ~2.5 Ga metamorphic and anatectic intensity may

indicate that the former was elevated to a higher crustal level compared to the latter, probably through thrusting along the boundary between these belts at the end of the Neoarchean. 2) This study supports the conclusion that apparent metamorphic zircon ages of 2.45–2.48 Ga and younger in some areas of the NCC may be the result of overprinting and partial resetting of late Neoarchean metamorphic zircons during a late Paleoproterozoic tectono-thermal event, and that the latest Neoarchean event in the NCC was related to an extensional tectonic setting, probably due to magmatic underplating. 3) The rocks undergoing metamorphism and anatexis at ~2.5 Ga in western Shandong are mainly paragneiss, tonalite, trondhjemite and granodiorite; however, minor gabbro, quartz diorite and granite have also been identified. The protoliths mainly have early Neoarchean (2.6–2.68 Ga) formation ages, and some formed during the late Neoarchean (2.52–2.55 Ga). 4) Anatectic zircons show textural and compositional features similar to both metamorphic and magmatic zircons. These are: homogenous or blurred oscillatory zoning, high U contents and low in Th/U ratios (commonly <0.1), rare idiomorphic morphologies, and commonly containing inherited (xenocrystic) cores.

Acknowledgements We thank Chun Yang, Weilin Gan, Zhichao Zhang and Liqing Zhou for mount making and zircon CL imaging. We also appreciate the technical support of Jianhui Liu for the smooth operation of the SHRIMP instrument during zircon dating. We also thank Ian Williams, Chonghui Yang and Lilin Du for discussion during this study. We are grateful to editor Guochun Zhao, M. Santosh and an anonymous reviewer for their valuable comments. This study was financially supported by the National Natural Science Foundation of China (41472169, 41172172), the Key Program of the Ministry of Land and Resources of China (DD20160121-03, 121201102000150012) and the Major State Basic Research Program of China (2015FY310100).

References Ballard, J.R., Palin, J.M., Williams, I.S., Campbell, I.H., Faunes, A., 2001. Two ages of porphyry intrusion resolved for the super-giant Chuquicamata copper deposit of northern Chile by ELA-ICM-MS and SHRIMP. Geology 29, 383–386.

Black, L.P., Kamo, S.L., Allen, C.M., Aleinikoff, J.K., Davis, D.W., Korsch, R.J., Foudoulis, C., 2003. TEMORA 1: A new zircon standard for Phanerozoic U-Pb geochronology. Chemical Geology 200, 155–170. Cao, G.Q., 1996. Early Precambrian Geology of western Shandong. Geological Publishing House, Beijing, pp. 1–210 (in Chinese). Dong, C.Y., Wan, Y.S., Wilde, S.A., Xu, Z.Y., Ma, M.Z., Xie, H.Q., Liu, D.Y., 2014. Early Paleoproterozoic supracrustal rocks at Daqingshan in the Khondalite Belt of North China Craton: Evidence from SHRIMP zircon U-Pb dating. Gondwana Research 25, 1535–1553. Du, L.L., Zhuang, Y.X., Yang, C.H., Wan, Y.S., Wang, X.S., Wang, S.J., Zhang, L.F., 2003. Characters of zircons in the Mengjiatun Formation in Xintai of Shandong and their chronological significance. Acta Geologica Sinica 77, 359–366 (in Chinese with English abstract). Du, L.L., Zhuang, Y.X., Yang, C.H., Wan, Y.S., Wang, X.S., 2005. SHRIMP U-Pb zircon chronology of fine-grained amphibolite in the Mengjiatun area, western Shandong. Acta Geoscientica Sinica 26, 429–434 (in Chinese with English abstract). Du, L.L., Yang, C.H., Zhuang, Y.X., Wei, R.Z., Wan, Y.S., Ren, L.D., Hou, K.J., 2010. Hf isotopic compositions of zircons from 2.7 Ga metasedimentary rocks and biotite-plagioclase gneiss in the Mengjiatun Formation Complex, western Shandong Province. Acta Geologica Sinica 84, 991–1001 (in Chinese with English abstract). Geng, Y.S., Liu, F.L., Yang, C.H., 2006. Magmatic event at the end of the Archean in Eastern Hebei province and its geological implication. Acta Geologica Sinica 80, 819–833 (in Chinese with English abstract). Jahn, B.M., Auvray, B., Shen, Q.H., Liu, D.Y., Zhang, Z.Q., Dong, Y.J., Ye, X.J., Zhang, Q.Z., Cornichet, J., Macé, J., 1988. Archean crustal evolution in China: the Taishan complex, and evidence for juvenile crustal addition from long-term depleted mantle. Precambrian Research. 38, 381–403. Kröner, A., Cui, W.Y., Wang, S.Q., Wang, C.Q., Nemchin, A.A., 1998. Single zircon ages from high-grade rocks of the Jianping Complex, Liaoning Province, NE China. Journal of Asian Earth Sciences 16, 519–532. Kröner, A., Wilde, S.A., O‘Brien, P.J., Li, J.H., Passchier, C.W., Walte, N.P., Liu, D.Y., 2005a.

Field relationships, geochemistry, zircon ages and evolution of a late Archaean to Palaeoproterozoic lower crustal section in the Hengshan terrain of northern China. Acta Geologica Sinica 79, 605–629. Kröner, A., Wilde, S.A., Li, J.H., Wang, K.Y., 2005b. Age and evolution of a late Archean to Paleoproterozoic upper to lower crustal section in the Wutaishan/Hengshan/Fuping terrain of northern China. Journal of Asian Earth Sciences 24, 577–595. Kusky, T.M., Li, J.H., 2003. Paleoproterozoic tectonic evolution of the North China Craton. Journal of Asian Earth Sciences 22, 383–397. Kusky, T.M., Li, J.H., Santosh, M., 2007. The Paleoproterozoic North Hebei Orogen: North China Craton’s collisional suture with the Columbia supercontinent. Gondwana Research 12, 4–28. Li, J.H., Kusky, T.M., Huang, X.N., 2002. Archean podiform chromitites and mantle tectonites in ophiolitic mélange, North China Craton: a record of early oceanic mantle processes. GSA Today 12, 4–11. Li, T.S., Zhai, M.G., Peng, P., Chen, L., Guo, J.H., 2010b. Ca. 2.5 billion year old coeval ultramafic-mafic and syenitic dykes in Eastern Hebei: implicaitons for cratonization of the North China Craton. Precambr Research 180,143–155. Li, S.S., Santosh, M., Cen, K., Teng, X.M., He, X.F., 2016. Neoarchean convergent margin tectonics associated with microblock amalgamation in the North China Craton: Evidence from the Yishui Complex. Gondwana Research 38, 113–131 Liu, S.W., Santosh, M., Wang, W., Bai, X., Yang, P.T., 2011. Zircon U-Pb chronology of the Jianping Complex: Implications for the Precambrian crustal evolution history of the northern margin of North China Craton. Gondwana Research 20, 48–63. Lu, S.N., Chen, Z.H., Xiang, Z.Q., 2008. Geochronological Framework of Ancient Intrusions in Taishan Geopark, China. Geological Publishing House, Beijing, pp. 1–90 (in Chinese). Ludwig, K.R., 2001. Squid 1.02: A User's Manual. Berkeley Geochronology Centre Special Publication, Berkeley, pp. 1–19. Ludwig, K.R., 2003. User’s Manual for Isoplot 3.00, A Geochronological Toolkit for Microsoft Excel. Berkeley Geochronology Center, pp. 1–70. Ma, M.Z., Xu, Z.Y., Zhang, L.C., Dong, C.Y., Dong, X.J., Liu, S.J., Liu, D.Y., Wan, Y.S., 2013. SHRIMP dating and Hf isotope analysis of zircons from the early Precambrian basement in

the Xi Ulanbulang area, Wuchuan, Inner Mongolia. Acta Petrologica Sinica 29, 501–516 (in Chinese with English abstract). Nasdala, L., Hofmeister, W., Norberg, N., Mattinson, J.M., Corfu, F., Dor, W., Kamo, S.L., Allen, K., Kennedy, A.K., Kronz, A., Reiners, P.W., Frei, D., Kosler, J., Wan, Y.S., Goze, J., Hoer, T., Kröner, A., Valley, J.W., 2008. Zircon M257–a homogeneous natural reference material for the ion microprobe U-Pb analysis of zircon. Geostandards and Geoanalytical Research 32, 247–265. Nutman, A.P., Wan, Y.S., Du, L.L., Friend, C.R.L., Dong, C.Y., Xie, H.Q., Wang, W., Sun, H.Y., Liu, D.Y., 2011. Multistage late Neoarchaean crustal evolution of the North China Craton, eastern Hebei. Precambrian Research 189, 43–65. Pidgeon, R.T., 1992. Recrystallisation of oscillatory zoned zircon: some geochronological and petrological implications. Contributions to Mineralogy and Petrology 110, 463–472. Polat, A., Li, J., Fryer, B., Kusky, T., Gagnon, J., Zhang, S., 2006a. Geochemical characteristics of the Neoarchean (2800–2700 Ma) Taishan greenstone belt, North China Craton: Evidence for plume–craton interaction. Chemical Geology 230, 60–87. Polat, A., Herzberg, C., Münker, C., Rodgers, R., Kusky, T., Li, J.H., Fryer, B., Delaney, J., 2006b. Geochemical and petrological evidence for a suprasubduction zone origin of Neoarchean (ca. 2.5 Ga) peridotites, central osorgenic belt, North China Craton. Bulletin of the Geological Society of America 118, 771–784. Ren, P., Xie H.Q., Wang, S.J., Nutman A., Dong C.Y., Liu S.J., Xie S.W., Che X.C., Song Z.Y., Ma M.Z., Liu D.Y., Wan Y.S., 2016. A ca. 2.60 Ga tectono-thermal event in Western Shandong Province, North China Craton from zircon U–Pb–O isotopic evidence: Plume or convergent plate boundary process. Precambrian Research 281, 236–252. Rubatto, D., Hermann, J., 2003. Zircon formation during fluid circulation in eclogites (Monviso, Western Alps): Implications for Zr and Hf budget in subduction zones. Geochimica et Cosmochimica Acta 67, 2173–2187. Shen, Q.H., Song, B., Xu, H.F., Geng, Y.S., Shen, K., 2004. Emplacement and metamorphism ages of the Caiyu and Dashan igneous bodies, Yishui County, Shandong Province: zircon SHRIMP chronology. Geological Review 50, 275–284 (in Chinese with English abstract). Wan, Y.S., Song, B., Yang, C., Wu, J.S., 2005a. Zircon SHRIMP U-Pb geochronology of Archaean

rocks from the Fushun-Qingyuan area, Liaoning Province and its geological significance. Acta Geologica Sinica 79, 78–87 (in Chinese with English abstract). Wan, Y.S., Song, B., Geng, Y.S., Liu, D.Y., 2005b. Geochemical characteristics of Archaean basement in the Fushun-Qingyuan area, northern Liaoning Province and its geological significance. Geological Review 51, 128–137 (in Chinese with English abstract). Wan, Y.S., Liu, D.Y., Wang, S.J., Dong, C.Y., Yang, E.X., Wang, W., Zhou, H.Y., Ning, Z.G., Du, L.L., Yin, X.Y., Xie, H.Q., Ma, M.Z., 2010. Juvenile magmatism and crustal recycling at the end of the Neoarchean in western Shandong Province, North China Craton: Evidence from SHRIMP zircon dating. American Journal of Science 310, 1503–1552. Wan, Y.S., Liu, D.Y., Wang, S.J., Yang, E.X., Wang, W., Dong, C.Y., Zhou, H.Y., Du, L.L., Yang, Y.H., Diwu, C.R., 2011a. ~2.7 Ga juvenile crust formation in the North China Craton (Taishan-Xintai area, western Shandong Province): Further evidence of an understated event from zircon U-Pb dating and Hf isotope composition. Precambrian Research 186, 169–180. Wan, Y.S., Liu, D.Y., Dong, C.Y., Liu, S.J., Wang, S.J., Yang, E.X., 2011b. U-Th-Pb behavior of zircons under high-grade metamorphic conditions: a case study of zircon dating of meta-diorite near Qixia, eastern Shandong. Geoscience Frontiers 2, 137–146. Wan, Y.S., Wang, S.J., Liu, D.Y., Wang, W., Kröner, A., Dong, C.Y., Yang, E.X., Zhou, H.Y., Xie, H.Q., Ma, M.Z., 2012a. Redefinition of depositional ages of Neoarchean supracrustal rocks in western Shandong Province, China: SHRIMP U-Pb zircon dating. Gondwana Research 21, 768–784. Wan, Y.S., Dong, C.Y., Liu, D.Y., Kröner, A., Yang, C.H., Wang, W., Du, L.L., Xie, H.Q., Ma, M.Z., 2012b. Zircon ages and geochemistry of late Neoarchean syenogranites in the North China Craton: A review. Precambrian Research 222–223, 265–289. Wan, Y.S., Dong, C.Y., Wang, S.J., Kröner, A., Xie, H.Q., Ma, M.Z, Zhou, H.Y., Xie, S.W., Liu, D.Y., 2014. Middle Neoarchean magmatism in western Shandong, North China Craton: SHRIMP zircon dating and LA-ICP-MS Hf isotope analysis. Precambrian Research 255, 865–884. Wan, Y.S., Liu, D.Y., Dong, C.Y., Xie, H.Q., Kröner, A., Ma, M.Z., Liu, S.J., Xie, S.W., Ren, P., 2015a. Formation and evolution of Archean continental crust of the North China Craton. In: Zhai MG (Ed.) Precambrian geology of China. Springer, 59–136.

Wan, Y.S., Wang, S.J., Ren, P., Xie, H.Q., Dong, C.Y., Liu, D.Y., 2015b. Neoarchean magmatism in the Culaishan area, western Shandong: Evidence from SHRIMP zircon U-Pb dating. Acta Geoscientica Sinica 36, 634–646 (in Chinese with English abstract). Wilde, S.A., Cawood, P.A., Wang, K.Y., Nemchin, A.A., 2005. Granitoid evolution in the Late Archean Wutai Complex, North China Craton. Journal of Asian Earth Sciences 24, 597–613. Williams, I.S., 1998. U-Th-Pb geochronology by ion microprobe. In: McKibben, M.A., Shanks, W.C., Ridley, W.I. (Eds.), Applications of Microanalytical Techniques to Understanding Mineralizing Processes. Review of Economic Geology 7, pp. 1–35. Wu, M.L., Zhao, G.C., Sun, M., Li, S.Z., He, Y.H., Bao, Z.A., 2013. Zircon U-Pb geochronology and Hf isotopes of major lithologies from the Yishui Terrane: Implications for the crustal evolution of the Eastern Block, North China Craton. Lithos 170–171, 164–178. Xie, S.W., Xie, H.Q., Wang, S.J., Kröner, A., Liu, S.J., Zhou, H.Y., Ma, M.Z., Dong, C.Y., Liu, D.Y., Wan Y.S., 2014. Ca. 2.9 Ga granitoid magmatism in eastern Shandong, North China Craton: Zircon dating, Hf-in-zircon isotopic analysis and whole-rock geochemistry, Precambrian Research 255, 538–562. Yang, C.H., Du, L.L., Ren, L.D., Song, H.X., Wan, Y.S., Xie, H.Q., Liu, Z.X., 2011. Petrogenesis and geodynamic setting of Jiandeng potassic granite at the end of the Neoarchean in Zanhuang Complex, North China Craton. Earth Science Frontiers 18, 62–78 (in Chinese with English abstract). Yang, J.H., Wu, F.Y., Wilde, S.A., Zhao, G.C., 2008. Petrogenesis and geodynamics of Late Archean magmatism in eastern Hebei, eastern North China Craton. Geochronological, geochemical and Nd–Hf isotopic evidence: Precambrian Research 167, 125–149. Yang, Q.Y., Santosh, M.,Collins, A.S., Teng, X.M., 2016. Microblock amalgamation in the North China Craton: Evidence from Neoarchaean magmatic suite in the western margin of the Jiaoliao Block. Gondwana Research 31, 96–123. Yang, Q.Y., Santosh, M., 2017. The building of an Archean microcontinent: Evidence from the North China Craton. Gondwana Research, doi.org/10.1016/j.gr.2017.01.003 (in press). Zhai, M.G., Santosh, M., 2011. The early Precambrian odyssey of the North China Craton: A synoptic overview. Gondwana Research 20, 6–25. Zhang, J., Zhao, G.C., Li, S.Z., Sun, M., Wilde, S.A., Liu, S.W., Yin, C.Q., 2009. Deformational

history of the Fuping Complex and new U-Th-Pb geochronological constraints: implications for the tectonic evolution of the Trans-North China Orogen. Journal of Structural Geology 31, 177–193. Zhang. J., Zhao, G.C., Li, S.Z., Sun, M., Liu, S.W., Wilde, S.A., Kröner, A., Yin, C.Q., 2007. Deformation history of the Hengshan Complex: implications for the tectonic evolution of the Trans-North China Orogen. Journal of Structural Geology 29, 933–949. Zhao, G.C., Wilde, S.A., Cawood, P.A., Lu, L.Z., 1998. Thermal evolution of Archean basement rocks from the eastern part of the North China carton and its bearing on tectonic setting. International Geological Review 40, 706–721. Zhao, G.C., Wilde, S.A., Cawood, P.A., Lu, L.Z., 1999. Tectonothermal history of the basement rocks in the western zone of the North China Craton and its tectonic implications. Tectonophysics 310, 37–53. Zhao, G.C., Wilde, S.A., Cawood, P.A., Sun, M., 2001. Archean blocks and their boundaries in the North China Craton: lithological geochemical, structural and P–T path constraints and tectonic evolution. Precambrian Research 107, 45–73. Zhao, G.C., Wilde, S.A., Cawood, P.A., Sun, M., 2002. SHRIMP U-Pb zircon ages of the Fuping complex: implications for Late Archean to Paleoproterozoic accretion and assembly of the North China Craton. American Journal of Science 302, 191–226. Zhao, G.C., Sun, M., Wilde, S.A., Li, S.Z., 2005. Late Archean to Paleoproterozoic evolution of the North China Craton: key issues revisited. Precambr Research 136:177–202. Zhao, G.C., Zhai, M.G., 2013. Lithotectonic elements of Precambrian basement in the North China Craton: Review and tectonic implications. Gondwana Research 23: 1207–1240. Zhao, Z.R., Song, H.X., Shen, Q.H., Song, B., 2013. Zircon SHRIMP U-Pb dating of ultramafic rock and mafic granulite from Qinglongyu in Yishui County, Shandong Province. Acta Petrologica Sinica 29, 551–563.

Figure captions

Fig. 1. Geological map of western Shandong Province. Modified after Cao (1996) and Wan et al.

(2011a). Also shown are the locations of samples in this study. NCC in inset means North China Craton.

Fig. 2. Field photographs of Neoarchean rocks in western Shandong. (a) Biotite paragneiss with amphibolite and leucosomes, ~3 km southwest of Qixingtai village; (b) biotite paragneiss (S0729), showing compositional layering with leucosome bands, ~3 km southwest of Qixingtai village; (c)-(d) tonalitic gneiss (S1328), showing a complex layered structure, west of Boshan city; (e) metagabbro (S1244), showing a gneissic structure, Lushan Geopark; (f) quartz diorite gneiss (S1245), showing anatexis, Lushan Geopark; (g) amphibolitite strongly altered by leucosomes, Yishan Geopark; (h) tonalitic gneiss (S1027) cut by leucosomes, Yishan Geopark; (i) quartz diorite gneiss (S1029) cut by leucosomes, Yishan Geopark; (j) Leucosomes with and without amphibolite and amphibolitite enclaves occurring together, Yishan Geopark; (k)-(l) banded trondhjemitic gneiss (S1334), Yishan Geopark; (m) biotite monzogranitic gneiss (S1324) showing anatexis and deformation, 10 km east of Xintai city; (n) granodioritic gneiss (S0718), showing anatexis and complex folding, Menglianggu Geopark; (o) banded tonalitic gneiss (S0513) showing strong anatexis, ~1 km south of sample S0718 within the Menglianggu Geopark; (p) granodioritic gneiss (S1350) showing anatexis and strong deformation, ~25 km southwest of Menglianggu Geopark; (q) tonalitic gneiss (S1348), showing anatexis and deformation, ~10 km east of sample S1350; (r) banded granodioritic gneiss (S0760), showing anatexis and strong deformation, ~15 km south of Mengjiatun village; (s) trondhjemitic gneiss (S0714) and granitic gneiss (S0715) showing sharp or transitional contact relationships, ~15 km south of Guimengding Geopark; (t) weakly deformed granodiorite and banded granodioritic gneiss showing transitional contact relationship, ~15 km south of Shenshuiyu town; (u) banded granodioritic gneiss (S0770) cut by weakly deformed granodiorite, ~15 km south of Shenshuiyu town; (v) Banded granitic gneiss (S0772) showing strong anatexis and deformation, south of sample S0770; (w) 2.5 Ga granite dyke intruding anatectic rocks, Qixingtai village; (x) 2.5 Ga granite containing an anatectic rock enclave, Yishan Geopark.

Fig. 3. Photographs showing petrographic features of Neoarchean rocks in western Shandong. (a) Biotite paragneiss (S0729); (b) tonalitic gneiss (S1328); (c) metagabbro (S1244); (d) and (e)

quartz diorite gneiss (S1245); (f) tonalitic gneiss (S1027); (g) banded trondhjemitic gneiss (S1334); (h) biotite monzogranitic gneiss (S1324); (i) granodioritic gneiss (S0718); (j) banded tonalitic gneiss (S0513); (k) granodioritic gneiss (S1350); (l) tonalitic gneiss (S1348); (m) banded granodioritic gneiss (S0760); (n) granitic gneiss (S0715); (o) granodioritic gneiss (S0770); (p) Banded granitic gneiss (S0772). (+) and (-) mean cross and plane polarized light. Mineral symbol: Cpx: Clinopyroxene; Am: Amphibole; Bi: Biotite; Ms: Muscovite; Pl: Plagioclase; Kfs: K-feldspar; Mc: Microcline; Q: Quartz; Ep: Episode.

Fig. 4. Cathodoluminescence images of zircons from late Neoarchean rocks in western Shandong. (a) Biotite paragneiss (S0729); (b) tonalitic gneiss (S1328); (c) metagabbro (S1244); (d) quartz diorite gneiss (S1245); (e) tonalitic gneiss (S1027); (f) quartz diorite gneiss (S1029); (g) banded trondhjemitic gneiss (S1334); (h) biotite monzogranitic gneiss (S1324); (i) granodioritic gneiss (S0718); (j) banded tonalitic gneiss (S0513); (k) granodioritic gneiss (S1350); (l) tonalitic gneiss (S1348); (m) banded granodioritic gneiss (S0760); (n) granitic gneiss (S0715); (o) trondhjemitic gneiss (S0714); (p) granodioritic gneiss (S0770); (q) Banded granitic gneiss (S0772). C, MA, RC, X and R in brackets mean core, magmatic, recrystallized, xenocrystic and overgrowth (rim) zircons, respectively.

Fig. 5. Concordia diagrams showing SHRIMP U-Pb zircon analyses of late Neoarchean rocks in western Shandong. (a) Biotite paragneiss (S0729); (b) tonalitic gneiss (S1328); (c) metagabbro (S1244); (d) quartz dioritic gneiss (S1245); (e) tonalitic gneiss (S1027), red and black ellipses represent magmatic zircon and recrystallized zircon + overgrowth respectively ; (f) quartz dioritic gneiss (S1029), black and red ellipses represent magmatic zircon and recrystallized zircon + overgrowth respectively; (g) banded trondhjemitic gneiss (S1334); (h) biotite monzogranitic gneiss (S1324); (i) granodioritic gneiss (S0718), red and black ellipses represent magmatic zircon and recrystallized zircon respectively; (j) banded tonalitic gneiss (S0513); (k) granodioritic gneiss (S1350); (l) tonalitic gneiss (S1348); (m) banded granodioritic gneiss (S0760), red and green ellipses represent magmatic zircon and overgrowth for age calculation, respectively; (n) granitic gneiss (S0715), red ellipses represent xenocrystic zircon for age calculation;; (o) trondhjemitic gneiss (S0714); (p) granodioritic gneiss (S0770), red, blue and black ellipses represent magmatic

zircon, overgrowth and recrystallized zircon, respectively; (q) Banded granitic gneiss (S0772), red ellipses represent xenocrystic zircon for age calculation.

Dong et al.: The complexities of zircon crystllazition and overprinting during metamorphism and anatexis: an example from the late Archean TTG terrane of western Shandong Province, China Table 1 Summary of SHRIMP zircon U-Pb ages for late Neoarchean rocks in western Shandong Province exnocrystal zircon

magmatic zircon overgrowth or anatectic zircon

No

Sample

Rock

Location

exnocrystal

recrystallized

magmatic

recrystallized

zircon age

zircon age

zircon age

zircon age

age

1

S0729

Biotite paragneiss

2681±13 Ma

2563±20 Ma

(6)

(2)

Belt A

~2.50 Ga

2518±8 Ma 2

S1328

Tonalite gneiss

Belt A

~2.50 Ga (1) 2486±9 Ma

3

S1244

Metogabbro

Belt A

2476±8 Ma (1) (1)

4

5

6

S1245

S1027

S1029

Quartz diorite gneiss

Tonalite gneiss

Quartz diorite gneiss

S1334

2519±20 Ma

(1)

(3) 2613±11 Ma

(6)

(2)

2518±9 Ma

2518±13 Ma

(5)

(3)

2534±6 Ma (1)

Belt A

2518±13 Ma (1)

2685±54 Ma Belt A

gneiss

2506±6 Ma (13) (1)

Biotite monzogranite 8

2666±9 Ma Belt A

Banded trondhjemite 7

2663±38 Ma Belt A

S1324

2611±9 Ma Belt A

~2.50 Ga

gneiss

9

S0718

Granodiorite gneiss

(1)

Banded tonalite 10

S0513

2518±11 Ma

2513±12 Ma

(8)

(7)

2735±16 Ma

2585±19 Ma

2520±13 Ma

2491±12 Ma

(1)

(2)

(2)

(4, lead loss)

Belt A

Belt A gneiss

2528±6 Ma 11

S1350

Granodiorite gneiss

Belt B

2460±21 Ma (1, lead loss) (11) 2683±6 Ma

12

S1348

Tonalite gneiss

Belt B

2521±16 Ma (1) (9)

Banded granodiorite 13

S0760 gneiss

14

S0715

2637±11 Ma

2562±10 Ma

(1)

(4)

Belt C

Granitic gneiss

2514±4 Ma (3)

2671±9 Ma

2541±9 Ma

/2619±14 Ma

(3)

Belt C

~2.50 Ga

2592±25 Ma 15

S0714

Trondhjemitic gneiss

Belt C

2533±11 Ma (2) (6)

Banded granodioritic 16

S0770

2553±9 Ma Belt C

2534±11 Ma (2)

gneiss

(6)

Banded granitic 17

S0772

2668±10 Ma

2570±8 Ma

2597±40 Ma

(1)

(1)

(4)

Belt C gneiss

~2.50 Ga

Note: Arabic numerals in brackets mean the spots of analyses.

Dong et al.: The complexities of zircon crystllazition and overprinting during metamorphism and anatexis: an example from the late Archean TTG terrane of western Shandong Province, China Highlights A strong late Neoarchean (2.50–2.54 Ga) tectono-thermal event was widespread in western Shandong Privonce. The rocks undergoing this event include tonalite, trondhjemite, granodiorite, gabbro, quartz diorite, granite and paragneiss. Belts A and B are different in ~2.5 Ga metamorphic and anatectic intensity. This may indicate that the former was elevated to high crustal levels compared with the latter at the end of the Neoarchean. In some areas of the NCC, 2.45–2.48 Ga or younger metamorphic zircon ages may be due to superposition of the late Paleoproterozoic event.