Carboniferous granitic plutons from the northern margin of the North China block: implications for a late Palaeozoic active continental margin

We report four late Palaeozoic zircon sensitive high-resolution ion microprobe (SHRIMP) U–Pb ages for granitic plutons from the Inner Mongolia Palaeo-uplift on the northern margin of the North China block. These cast a new light on the poorly understood tectonic history of the northern margin of the North China block and the Central Asian Orogenic Belt during the late Palaeozoic. The plutons have for a long time been considered to belong to the early Precambrian basement of the North China block. Our new SHRIMP U–Pb zircon dating of four plutons at Longhua, Daguangding, Boluonuo and Hushiha has yielded intrusive ages of 311 ± 2 Ma, 324 ± 6 Ma, 302 ± 4 Ma and 310 ± 5 Ma, respectively. Geochemical data suggest that these granitoids have a calc-alkaline, subduction-related I-type signature, indicating the existence of an Andean-style continental arc along the northern margin of the North China block during the late Palaeozoic. Our results also indicate that the Palaeo-Asian Ocean still existed during latest Carboniferous–earliest Permian time, and that the final collision between the southern Mongolia composite terranes and the North China block occurred later than c. 290 Ma. We suggest that the northern margin of the North China block was an active continental margin and the Inner Mongolia Palaeo-uplift is a deeply exhumed mid-crustal ‘root’ of a late Palaeozoic Andean-style continental arc.

uplift, except for some Mesoproterozoic sedimentary rocks along a narrow belt of the Fengning-Longhua fault (Figs 1 and 2).
Traditionally, the North China block (including the Inner Mongolia Palaeo-uplift) has been considered stable since the end of rifting in the Neoproterozoic, with no crustal reactivation until the Triassic (Ren et al. 1990;Song 1999). Therefore, the possible existence of late Palaeozoic plutons on the northern margin of North China block has been debated for many decades. It has been reported that Permian granitoids exist in northern Hebei Province (HBGMR 1989;Hong et al. 1995;Cui & Wu 1997) and western Liaoning Province (LBGMR 1989;Pan 1996) on the northern margin of the North China block, but their accurate ages and petrogenesis are still not clear. Recently, some Permian zircon U-Pb ages were obtained from so-called early Precambrian basement rocks in northern Hebei Province (Kröner et al. 2004;and unpubl. data) and from so-called 'Caledonian' granitoids at Yanbian in Jilin Province ), but their petrogenesis is poorly constrained. Late Palaeozoic plutons, commonly overlain unconformably by Jurassic-Cretaceous volcanic and sedimentary rocks, reflect the uplift and exhumation history of the Inner Mongolia Palaeo-uplift and are crucial for understanding its tectonic history and the late Palaeozoic-early Mesozoic history of the northern margin of the North China block and the Central Asian Orogenic Belt.

Field occurrence and sample description
Field occurrence of the investigated plutons Longhua pluton. The long axis of the Longhua pluton trends east-west, and its area is in excess of 60 km 2 . The pluton, which is located on the northern side of the Fengning-Longhua fault (Fig. 2), consists of strongly foliated and partly migmatized quartz diorite, quartz monzodiorite and diorite, which are overlain unconformably by Late Jurassic-Early Cretaceous sedimentary and volcanic rocks to the north, and are intruded into Archaean metamorphic rocks to the SE. Previous studies (HBGMR 1974(HBGMR , 1994a interpreted the pluton as migmatites or migmatized metamorphic diorite of Palaeoproterozoic or earlier age. The diorite is strongly deformed with a NE-SW-to ENE-WSW-trending steep foliation. Its main minerals are plagioclase (An 24-43), quartz, hornblende, biotite and K-feldspar; subordinate minerals are magnetite, titanite, apatite and zircon, and occasional pyroxene and secondary chlorite and epidote. Detailed petrographic descriptions are available online at http:// www.geolsoc.org.uk/SUP18258. A hard copy can be obtained from the Society Library. Daguangding pluton. The Daguangding pluton is located between the Fengning-Longhua and Damiao faults (Fig. 2). It is composed of quartz diorite, diorite and gabbroic diorite. Its long axis trends east-west, and its area is about 200 km 2 . The strongly deformed diorite has a welldeveloped foliation, especially in the western part of the pluton, but the foliation strike and dip are not steady. The host rocks of the pluton are Archaean metamorphic rocks, and it is unconformably overlain on its southeastern side by Late Jurassic-Early Cretaceous strata. It has previously been interpreted as an Archaean (HBGMR 1989), Archaean-Proterozoic (HBGMR 1974), or Palaeoproterozoic (HBGMR 1994b pluton. Its minerals are very similar to those of the Longhua pluton (see the Supplementary Publication). Boluonuo pluton. The long axis of the pluton trends ENE-WSW, and its area is about 80 km 2 (Fig. 2). The main components are quartz diorite, diorite and granodiorite. Foliation is locally developed, but deformation is less severe than that of the Longhua and Daguangding plutons. As in the case of the Daguangding quartz diorite, this pluton has previously been viewed as Archaean (HBGMR 1989), Archaean-Proterozoic (HBGMR 1974) or Palaeoproterozoic in age (HBGMR 1988  subordinate minerals are magnetite, titanite, apatite and zircon, with secondary chlorite, epidote and sericite (see the Supplementary Publication). Hushiha pluton. The Hushiha granodiorite is a newly discovered small pluton within Archaean biotite-plagioclase-hornblende gneiss (Fig. 2). Because the outcrop of the pluton is small, it was not shown on previous geological maps (HBGMR 1989). Its main minerals are plagioclase (An c. 30), quartz, K-feldspar, biotite and muscovite; subordinate minerals are magnetite, apatite and zircon, with secondary sericite and calcite (see the Supplementary Publication).
Although some rocks of the Carboniferous plutons were foliated, they have not undergone high-grade metamorphism. The secondary chlorite, epidote, sericite or calcite in these rocks is a result of thermal alteration or low-grade metamorphism caused by their deep emplacement. Because contents of the secondary minerals are very low (,1-2 vol%), they have only slight influence on the geochemical compositions of the Carboniferous rocks, and the present geochemical compositions of the plutons are very similar to the original plutonic compositions.
More detailed zircon CL images and a summary of all the granitoid samples from the late Palaeozoic plutons are given in the Supplementary Publication.

SHRIMP U-Pb dating
Zircons were extracted using conventional separation techniques and were then handpicked under a binocular microscope. Together with the TEMORA zircon standard they were mounted in epoxy and polished to expose the cores of the grains for CL and SHRIMP U-Pb analyses.
U-Pb dating was performed on the SHRIMP II at the Beijing SHRIMP Centre following the method of Williams (1998). The mass resolution was c. 5000 at 1% peak height. The spot size of the ion beam was 25-30 ìm, and five scans through the mass range were used for data collection. The SL13 (572 Ma, U ¼ 238ppm) and TEMORA (417 Ma) standards were used in the analyses as discussed by Black et al. (2003). Sites for dating were selected on the basis of CL and microscope images. To maintain precision, one TEMORA analysis was performed after every three or four spots on the sample zircons during data collection. Ages and concordia diagrams were produced using the programs SQUID 1.03 (Ludwig 2001) and ISOPLOT/Ex 2.06 (Ludwig 1999).

Major and trace element analyses
Fourteen samples (six from the Longhua pluton, four from the Daguangding pluton, three from the Boluonuo pluton and one from the Hushiha pluton) were analysed for major and trace elements at the Key Laboratory of Orogenic Belts and Crustal Evolution in the School of Earth and Space Sciences, Peking University. Major elements were analysed on fused glass discs by X-ray fluorescence spectrometry (XRF) following the method of Harvey (1989). Trace elements (including REE) were analysed by inductively coupled plasma mass spectrometry (ICP-MS) using a VG Axiom multicollector system. Rock powder (0.1 g) was digested with concentrated HF-HNO 3 -HClO 4 in tightly sealed 15 ml Teflon screw-cap bombs by heating to 110 8C on a hot plate, then evaporating to dryness. The residue was dissolved with 1 ml HNO 3 by heating; subsequently, the final solution was prepared by adding 1% HNO 3 with internal standard and diluted with de-ionized water to 30 ml. An aliquot of 10 ml was taken and also diluted to 30 ml to bring down count rates for detected elements. The composition of the internal standard contains 10 ppb indium to correct for instrumental drift during the ICP-MS analysis; all acids used were ultra-pure. The Chinese national standards GSR-1 were used to monitor analyses. The results of analyses were consistent with the reference values, with the differences for major elements being within 1%, except for P 2 O 5 (5%), and trace elements (including REE), which were within 5-10%.

SHRIMP U-Pb dating results
The SHRIMP U-Pb analyses of zircons from the Longhua quartz diorite (sample SD020-3) are listed in Table 1 and plotted on the concordia diagram of Figure 4a. Twenty-six spots on 17 zircon grains were measured, and most are concordant. The errors on spots 1.2 and 8.1 are large because of low U contents. Four rounded cores (2.1, 6.1, 9.1, 12.1) give a 207 Pb/ 206 Pb mean age of 1809 AE 21 Ma (95% confidence, MSWD ¼ 1:9), and are interpreted as inherited zircons (Fig. 4a inset). Spot 5.1 is rejected because it is far from the concordia curve. Spot 6.2 probably reflects a mixed age of an inherited grain and a magmatic grain because it is near the boundary between the rounded inner core and the magmatic oscillatory overgrowth. Except for these eight anomalous spots, the remaining spots yield a weighted mean 206 Pb/ 238 U age of 311 AE 2 Ma (95% confidence, MSWD ¼ 2:1, n ¼ 18). Their well-developed oscillatory zoning and high Th/U ratios (.0.4) indicate a magmatic origin. Therefore, the mean age of 311 AE 2 Ma dates the time of crystallization of the Longhua quartz diorite in the late Carboniferious.
Trace element compositions. The trace element compositions of all the plutons are broadly similar, with constant low Rb/Sr and Rb/Ba values (Table 2). On primitive mantle-normalized diagrams (Fig. 6), all samples display a moderate to strong depletion in P, Nb, Ta and Ti, which is a characteristic feature shared by many subduction-related magmatic rocks (Briqueu et al. 1984;Rogers & Hawkesworth 1989;Sajona et al. 1996;Chen et al. 2000;Zhou et al. 2002). They also have high Th/Ta ratios (3.37-33.11), which is again indicative of subduction-related magmas (Rogers & Hawkesworth 1989;Chen et al. 2000;Gorton & Schandl 2000). On Sr/Y v. Y and La N /Yb N v. Yb N diagrams, the Daguangding, Boluonuo and Hushiha plutons have an adakitic composition, and the Longhua pluton has the composition of tholeiitic and calc-alkaline island-arc rocks (Fig. 7). The Rb, Nb and Y contents of all samples are low. In the tectonic discrimination diagrams of Pearce et al. (1984), they all fall in the VAG (volcanic-arc granitoids) field ( Fig. 8a and b). On the ternary tectonic discrimination diagram of Harris et al. (1986), most samples also fall in or close to the VAG field (Fig.  8c). For magmatic arc granitoids, a diagram of Rb/Zr v. Nb can be used as a qualitative indicator of arc maturity (Brown et al. 1984). Most samples fall within the field of normal continental arcs, or near the boundary between primitive arcs and normal continental arcs (Fig. 8d). Therefore, we interpret these rocks as having formed in a primitive-normal continental magmatic arc environment that was probably related to southward subduction of the Palaeo-Asian oceanic crust beneath the North China block.

Tectonic setting of the granitic plutons
Our new SHRIMP zircon ages indicate that the Longhua, Daguangding, Boluonuo and Hushiha plutons are late Palaeozoic in age, and not Archaean or Palaeoproterozoic as previously thought (e.g. HBGMR 1989(e.g. HBGMR , 1994a. They were intruded into the basement of the North China block during Carboniferous time. Geochemical analyses indicate that the Carboniferous plutons are calc-alkaline, I-type granitoids that show features of subduction-related magmatism. Our recent field investigation and lithological comparison indicate that this type of dioritic plutons may occur widely in the area to the east and west of these four investigated plutons, which together form a Carboniferous arc magmatic belt in the Inner Mongolia Palaeo-uplift along the northern margin of the North China block. Tuff layers in Upper Palaeozoic sedimentary rocks are widely distributed near Daqingshan, Datong, Tangshan, western Beijing and Jinzhou in the North China block (e.g. Zhong et al. 1995;Jia et al. 1999;Zhou et al. 2001). Geochemical analyses of the tuffs indicate they have a calc-alkaline, volcanic arc composition (Jia et al. 1999). Sedimentary and tectonic analysis indicates a northern provenance (e.g. Zhong et al. 1995;Jia et al. 1999), specifically the northern margin of the North China block and especially the Inner Mongolia Palaeo-uplift. Therefore, these tuffs may record volcanic activity at the northern margin of the North China block in late Palaeozoic time.
SHRIMP zircon dating of a tuff layer from western Beijing We suggest that the Carboniferous granitoid plutons and latest Carboniferous-earliest Permian volcanic rocks resulted from southward subduction of the Palaeo-Asian oceanic lithosphere beneath the northern margin of the North China block. The existence of late Palaeozoic magmatic arc plutons and possible arc volcanic rocks demonstrates that the northern margin of the North China block was an active continental margin (e.g. Wang 1996;Davis et al. 2001;Xiao et al. 2003) instead of a passive continental margin (e.g. Xu & Chen 1997;Shi et al. 2004), presumably of Andean type because of some Precambrian inheritance in zircons of some of these granitoids (Fig. 9).

Implications for exhumation of the Inner Mongolia Palaeo-uplift
Aluminium-in-hornblende barometry results indicate that the Longhua, Daguangding and Boluonuo plutons were emplaced at depths of 15.7-18.7 km (Zhang et al. 2006). Because some of these Carboniferous plutons are unconformably overlain by Early Jurassic to Cretaceous volcanic and sedimentary rocks (HBGMR 1989), at least some of them were already exposed at the surface prior to the early Jurassic. We infer that the exhumation of the Inner Mongolia Palaeo-uplift was at least 15.7 km; this includes the removal of Archaean-Palaeoproterozoic basement rocks,  Taylor & McLennan (1985 Sun & McDonough (1989). Symbols are in as Figure 5.  Evensen et al. (1978); the fields of island-arc volcanics and adakites in the Sr/ Y v. Y diagram are after Martin (1999); the fields of island-arc volcanic rocks and adakites in the La N /Yb N v. Yb N diagram are after   China block has long been controversial: latest Silurian to Devonian (e.g. Yue et al. 2001), middle to late Devonian (e.g. Tang 1990;Xu & Chen 1997), late Devonian to early Carboniferous (e.g. Hong et al. 1995;Shao & Zhan 1998), late Carbounferous (e.g. Dobretsov et al. 1995), early Permian (e.g. Wang et al. 1999), late Permian (e.g. Hsu et al. 1991;Wang & Mo 1995;Sengör & Natal'in 1996;Wang 1996;Yin & Nie 1996;Xiao et al. 2003), Permo-Triassic , Middle-Late Triassic (e.g. Dorjnamjaa et al. 1993;Badarch & Orolmma 1998), or even Cretaceous (Nozaka & Liu 2002). The existence of late Carbonifereous magmatic arc granitoids on the northern margin of the North China block and latest Carboniferousearliest Permian tuff and possible volcanic activity indicates that the Palaeo-Asian Ocean still existed during the latest Carboniferous-earliest Permian time, and the final collision between the southern Mongolia composite terranes and the North China block occurred later than c. 290 Ma. Combined with the newly discovered radiolarians in an argillite bed of the Middle Permian Zhesi Formation in Zhesi and Xilinhot, Inner Mongolia (Shang 2004), we suggest that the Palaeo-Asian Ocean was not closed until the late Permian (Guadalupian, Shang 2004), which supports the Permian-Triassic collision model of two active continental margins of Xiao et al. (2003).

Conclusions
Variably foliated plutons on the northern margin of the North China block, which were once assigned to the Precambrian crystalline basement, have now been identified as being Carboniferous in age. The Longhua, Daguangding and Boluonuo quartz diorites and the Hushiha granodiorite crystallized at 311 AE 2 Ma, 324 AE 6 Ma, 302 AE 4 Ma and 310 AE 5 Ma, respectively. These calc-alkaline, I-type plutons formed in a continental magmatic arc of Andean type during southward subduction of the Palaeo-Asian oceanic crust beneath the North China block. The Inner Mongolia Palaeo-uplift was not always an uplifted domain between the Neoproterozoic and the early Triassic. The present Inner Mongolia Palaeo-uplift is the consequence of major uplift and erosion between the Carboniferous and the Early Mesozoic. Exhumation of the deep (.15.7 km) plutons prior to early Jurassic time was accompanied by the removal of higher levels of Precambrian basement and supracrustal rocks of the late Palaeozoic Andean-style continental arc. The Inner Mongolia Palaeo-uplift along the northern margin of the North China block is therefore a deeply eroded mid-crustal 'root' of a late Palaeozoic Andean-style continental arc.