Early-Late Devonian Post-Collision Extension due to Slab Breakoff Regime along the Northern Margin of North China Craton: Implications from Muscovite 40 Ar- 39 Ar Dating

: Ordovician-Silurian subduction, Early Devonian arc-contient collision and followed post-collision extension are recorded in the north of the North China Craton. Most previous research has focused on the first two processes. Discussion on the post-collisional extension and its tetonic regime is still limited. In this study, 40 Ar- 39 Ar muscovite ages obtained from the central part of the northern North China Craton were analyzed to shed light on the timing of post-collision extension. Garnet muscovite schist and muscovite quartz schist in the Jianshan Formation yielded 40 Ar- 39 Ar muscovite plateau ages of 412 ± 3 Ma and 391 ± 3 Ma, respectively. Two other 40 Ar- 39 Ar muscovite plateau ages (389 ± 2 Ma and 397 ± 2Ma) were obtained for two Mesoproterozoic monzogranites intruding into the Jianshan Formation. Based on previous research, the northern North China Craton underwent a collision event with Bainaimiao arc at c. 415 Ma, followed by post-collision extension in early Late Paleozoic. Therefore, combined with the newly acquired muscovite 40 Ar- 39 Ar dating results, the Jianshan Formation might go through regional metamorphism at c. 412 Ma during the collision process. Subsequently, the Jianshan Formation and monzogranites intruding into it went through rapid exhumation along with metamorphism at c. 397–389 Ma in a post-collision extensional setting. The muscovite 40 Ar- 39 Ar ages provide new markers for the exhumation history and the post-collisional extension setting during Early-Late Devonian in the study area. Furthermore, slab breakoff as the cause for this extensional setting is argued by the emplacement of the Early-Late Devonian alkaline rocks.

The northern North China Craton (NCC) went through post-collision extension in the early late-Paleozoic. A series of Early-Late Devonian alkaline and mafic-ultralmafic complexes developed in this extensional setting [18][19][20]. The discussion on the regime leading to this extensional setting in northern NCC is still limited. In order to shed light on the timing and regime of this tectonic setting, muscovite 40 Ar- 39 Ar dating was conducted on garnet muscovite schist and muscovite quartz schist from the Jianshan Formation (Meso-Neoproterozoic Bayan Obo Group) and monzogranites collected from the Shangdu area, Inner Mongolia in the central part of northern NCC.

Geological Background
The southernmost segment of CAOB in the central part of Inner Mongolia [14][15][16][17] is divided into the Southern Orogenic Belts (SOB) and Northern Orogenic Belts (NOB), separated by the Solonker suture zone [15,21] (Figure 1b). The SOB comprises, from north to south, the Ondor Sum subduction-accretion complex, the ophiolite belt, and the Bainaimiao arc [15]. The Ondor Sum subduction-accretion complex is composed of turbidites, ophiolite mélanges, and blueschists [22]. The phengites from quartzite mylonites of the Ondor Sum subduction-accretion complex yielded 40 Ar/ 39 Ar plateau ages of 453 ± 2 Ma and 449 ± 2 Ma [23], which are interpreted to represent the time interval in which the accretion of the complex occurred.
In the Bainaimiao-Tulinkai area, early Paleozoic magmatism with arc affinity has been identified (i.e., Bainaimiao arc) [24][25][26][27][28][29][30]. The representative rocks are listed in Table 1. The Bainaimiao arc was active until c. 420 Ma. It was built upon a Precambrian microcontinent, which had a tectonic affinity to the Tarim or Yangtze cratons, because the detrital zircon ages of Cambrian to Permian strata in Bainaimiao arc are similar to those recognised in the Neoproterozoic to Paleozoic arc terranes, which have tectonic affinity to Tarim or South China Cratons [31].
In Bainaimiao arc, the fact that the middle-late Silurian flysch of the Xuniwusu Formation and the terminal Silurian molasse of the Xibiehe Formation unconformably overlay the underlying strata supports the collision between Bainaimiao arc and NCC started in middle Silurian [32][33][34]. In addition, the 419 ± 10 Ma diorite in Baimaimiao [35] and the c. 417 Ma tonalite in Chaganhushao [30] seem to indicate that the Bainaimiao arc collided with the northern margin of the NCC in earliest Devonian [12,15].
The collision between the Bainaimiao arc and the northern margin of the NCC marks the termination of the Early Paleozoic orogeny in the northern NCC. A post-collision extension context in the northern margin of the NCC is constrained by Devonian alkaline and mafic-ultramafic complexes [36][37][38][39][40][41][42][43][44][45][46]. Furthermore, these rocks are confined in a linear zone along the northern NCC, as shown in the yellow belt in Figure 1b. In addition, Late Silurian to Early Devonian metamorphism has been identified along the northern margin of NCC [47][48][49]. Zircon U-Pb ages of these Early to Late Devonian magmatic rocks and 40 Ar-39 Ar ages of Early to Late Devonian metamorphism are listed in Table 1.  [3]); (b) the tectonic division of CAOB in central part of Inner Mongolia, China (modified after [12,46], the yellow belt is the linear zone of Devonian magmatism and metamorphism, the ages shown by black are representative zircon U-Pb ages of Devonian alkaline rocks, those shown by red are representative muscovite 40 Ar-39 Ar ages ). The study area ( Figure 2a) is located in the central part of the northern margin of the NCC ( Figure 1b); it is separated from the Bainaimiao arc belt by the Chifeng-Bayan Obo fault [12]. The northern margin of the NCC mainly comprises folded metasediments of the Meso-Neoproterozoic Bayan Obo Group and rare Mesoproterozoic granitic rocks that are intruded by Late Devonian to Early Permian magmatic rocks ( Figure 2a) [50]. The sedimentary sequences in the study area include Upper Ordovician limestone and Upper Permian strata, which are mainly composed of sandstone, slate, and volcanic tuff (Figure 2a). The Bayan Obo Group extends about 500 km from east to west, 20-30 km from south to north, and is up to 7000-m thick [51]. The Bayan Obo Group is adjacent to the Zhaertai and Langshan Group in the west and connects with the Huade Group in the east. They represent strata deposited in the Meso-Neoproterozoic Langshan-Zhaertai-Bayan Obo Rift System [51][52][53][54][55][56][57]. Overall, the Bayan Obo Group consists of a succession of sedimentary strata and metapelites. From bottom to top, it is divided into: (1) the lower succession of Dulahala (Chd) and Jianshan (Chj) Formations, (2) the middle succession of Halahuogete (Jxh)and Bilute (Jxb) Formations, (3) the upper succession of Baiyinbaolage (Qnby) and Hujiertu (Qnhj) Formations [51]. Dulahala Formation is characterized by conglomerates and coarse-grained feldspathic quartz sandstones; the subsequent Jianshan Formation includes slates, sericite phylllite, metamorphosed coarse sandstone, and feldsparthic quartz sandstone. Halahuogete Formation is dominated by glutenite, metamorphosed conglomerate, and limestone. Bilute Formation is made up of sericite phyllite, whose protolith is argillite. Baiyinbaolage Formation is composed of quartzite and meta-quartzite, whereas Hujiertu Formation comprises a succession of slate and limestone [51]. Bayan Obo Group contains the world-famous Bayan Obo rare earth deposit, it has been the focus of numerous studies [58][59][60].

Sample Description
Garnet muscovite schist and muscovite quartz schist were collected from the Jianshan Formation, at the bottom of the Bayan Obo Group. They were both strongly deformed and have well-developed NE-trending foliation with dip angle at 35-40°. Two granite samples were collected each from a granitic pluton, which intruded the Bayan Obo Group, emplaced at around 1320 Ma according to U-Pb zircon dating [50]. The granites are less deformed, lack well defined foliation, and are adjacent to the Permian granite ( Figure 2b).

Method
White mica was separated from the crushed rock samples using magnetic separation methods. Aliquots were examined using a binocular microscope to ensure a purity of up to 99% at the KeDa Rock and Mineral Separation Technology Company, Langfang, Heibei Province. The white mica was washed by the ultrasonic wave method. Aliquots and the ZBH-25 biotite standard were sent to a nuclear reactor and set in hole H4 in the China Academy of Atomic Energy Sciences for neutron irradiation. The irradiation duration and neutron dose were 1444 min and 2.30 × 10 18 n·cm −2 . The ZBH-25 biotite standard produced an age of 133.3 ± 0.24 Ma [61] with a 1% relative standard deviation (1σ). The J-values for the individual samples were determined using a second-order polynomial interpolation. The Ca and K correction factors were calculated from the coirradiation of pure salts of CaF2 and K2SO4 (e.g., ( 40 Ar/ 39 Ar)K = 0.004782, ( 39 Ar/ 37 Ar)Ca = 0.000806, ( 36 Ar/ 37 Ar)Ca = 0.0002389).
The 40 Ar/ 39 Ar analyses were performed at the Isotope Geology Laboratory, Institute of Geology, Chinese Academy of Geological Sciences. The samples were loaded in aluminum packets, placed in a Christmas tree sample holder, and degassed at low temperature (250-300 °C) for 20-30 min before being incrementally heated in a double-vacuum graphite furnace. The gases released during each heating step were purified by means of Ti and Al-Zr getters. Once cleaned, the gas was introduced into a GV Instruments HELIX-MC noble gas mass spectrometer and allowed to stabilize for 4-5 min before the static analysis was done. The 40 Ar, 39 Ar, 38 Ar, 37 Ar, and 36 Ar isotopic abundances were measured at time zero through linear extrapolation of the peak intensities. The data were corrected for system blanks, mass discrimination, interfering Ca, K-derived argon isotopes, and the decay of 37 Ar since the time of irradiation. The decay constant used throughout the calculations was λ = (5.543 ± 0.010) × 10 −10 a −1 . ISOPLOT was used to do all further calculations [62]. All of the errors are reported as 2σ.

Results
The 40 Ar/ 39 Ar step-heating geochronology data are shown in Table 2. The Ca/K ratio spectra of these four samples is in Figure 4b,d,f,h. Twelve heating steps were carried out for sample B1193-1. Ten heating steps with temperatures ranging from 910 °C to 1400 °C yielded a plateau age of 412 ± 3 Ma (MSWD = 0.51), including a total of 95.1% of the released 39 Ar (Figure 4a).
Thirteen heating steps were carried out for sample TM33. Ten steps with temperatures ranging from 850 °C to 1240 °C yielded a well-defined plateau age of 392 ± 3 Ma (MSWD = 0.59) (Figure 4c), including a total of 85.9% of the released 39 Ar.
Thirteen heating steps were carried out for sample TM31. Eleven steps with temperatures ranging from 760 °C to 1400 °C yielded a well-defined plateau age of 397 ± 2 Ma (MSWD = 0.20) (Figure 4e), including a total of 99.1% of the released 39 Ar.
Thirteen heating steps were carried out for sample TM32. Twelve steps with temperatures ranging from 740 °C to 1400 °C yielded a well-defined plateau age of 389 ± 2 Ma (MSWD = 0.51) (Figure 4g), including a total of 99.7% of the released 39 Ar.  (e) and (f) diagrams of plateau and Ca/K ration spectra for biotite monzogranite (TM31); (g) and (h) diagrams of plateau and Ca/K ration spectra for muscovite monzogranite (TM32). WMPA-weighted mean plateau age.

The Metamorphic Age of Jianshan Formation, Bayan Obo Group
The metamorphic minerals in sample B1193-1 (garnet muscovite schist) include muscovite, biotite, and garnet. The mineral assemblage indicate greenschist facies metamorphism. Sample B1193-1 yields muscovite 40 Ar- 39 Ar ages at 412 ± 3 Ma, which corresponds, within errors, with the collision between Bainaimiao arc and the northern NCC at c. 415 Ma. Therefore, it is believed that Jianshan Formation metamorphosed during this collision event with the development of foliation S1.
The metamorphic minerals of sample TM33 (garnet muscovite schist) are mainly muscovite and biotite, indicating a greenschist facies metamorphism with metamorphic temperature roughly between 400 °C and 500 °C regarding microstructure of the deformed quartz [63]. The boundaries of some of the quartz grains in the two monzogranite samples (TM31 and TM32) are irregular, which is a classic characteristic of bulging recrystallization. Furthermore, the bulging recrystallization might occur at a temperature of 350-450 °C [64]. The photomicrographs of sample TM31 and TM32 show that the size of muscovite is 0.5-1.0 mm; the diffusion dimension is comparable with grain size for well crystalized muscovite [65]. Based on this, the metamorphic temperature is roughly comparable to the closure temperature of muscovite (420-520 °C, diffusion dimensions 0.5-1.0 mm, cooling rates of 1-100 °C/Ma, pressures of 0.5 GPa [66]; 500-550 °C for muscovite that is not deformed [67]). This suggests that cooling from closure temperature was achieved almost at the same time as the peak of metamorphism. Therefore, the muscovite 40 Ar- 39 Ar ages of c. 397-389 Ma from TM31, TM32 and TM33 could represent the time of another metamorphic event. This event should be the metamorphism of Jianshan Formation and the monzogranites intruding into it during Early-Late Devonian rapid exhumation under post-collision extension setting. Meanwhile, the foliation S2 observed in TM33 developed during this metamorphic event.

Tectonic regime of post-collision extension
Based on previous studies, the Early-Late Devonian intrusions in the Northern NCC are mainly alkaline and mafic-ultra mafic complexes [19,20,38,41,[44][45][46]. The alkaline rocks show wide range of silica content (54-76 wt.%) and the mafic-ultramafic rocks have low silica content (<45% wt.%), however, they all have high contents of total alkalis. In a total alkali vs. silica (TAS) plot, these rocks mainly show syenite compositions (Figure 5a). Nearly all of them are plotted in the fields of A-type granite in the Na2O + K2O vs. 10,000× Ga/Al diagram (Figure 5b) and show a post-orogenic affinity (A2 type granite) ( Figure 6). According to whole rock Sr-Nd and zircon Lu-Hf analysis on these rocks, they have low initial 87 Sr/ 86 Sr ratio and low negative εNd(t). They show low negative zircon εHf(t) with Hf isotope two stage model ages ranging between c. 2.0 Ga and c. 3.5 Ga. Based on the above geochemical data, there is still no consensus concerning the petrogenesis and source areas of these Devonian alkaline rocks, such as: (1) partial melting of enriched lithospheric mantle sources with involvement of ancient lower crust [42,46] and (2) partial melting of the Archaean lower continental crust beneath the NCC [43][44][45]. Besides, the reasons leading to thermal change of overriding plate and the resultant melting of lithospheric mantle as well as lower continental crust are not clear either.  [68]; (b) total alkali (K2O + Na2O) vs. 10,000×Ga/Al [69] (data from [19,20,38,41,[44][45][46]).
The slab breakoff, also referred to as slab detachment, occurs when part of a subducted lithospheric plate detaches and sinks into the mantle [79,80]. Slab breakoff will lead to the upwelling of hot and dehydrated subslab asthenosphere and resultant heating up of the overriding lithosphere; thermomechanical modeling of the slab-detachment shows large (500 °C ) transitory heating of the base of the upper plate for several million years due to upwelling mantle following the slab detachment [81]. Therefore, the melting of enriched layers and lower continental crust will be possible, causing the formation of Devonian alkaline magma and mafic-ultramafic rocks. Besides, the propagation of slab breakoff will produce a linear zone of lithospheric heating and the area overlying the breakoff line is therefore predestined to form a tectonic fault [75]. The magmatism also occurs along a narrow linear zone roughly parallel to the trench and is confined in time, which makes slab breakoff distinct from other models for syncollisional and post-collsional magmatism, such as subduction melting, delamination, or thermal boundary layer detachment [75]. Regarding the fact that the Devonian alkaline rocks develop in a linear zone along the northern NCC ( Figure  1b), the slab breakoff will be the most suitable model for this distribution of the alkaline rocks. In addition, the slab breakoff is followed by a period of uplift that reaches about 1.5 km within about 20 Ma [79]. Slab breakoff has caused uplift and exhumation because the dynamic subsidence related to subduction stopped coevally with the removal of large loads of lithosphere mantle [76]. The rare preservation of Devonian strata in the northern NCC also proves the existence of uplift event [82]. Besides, in combination with the heat input from upwelling asthenosphere, the uplift and exhumation would lead to thermal metamorphism in the overriding plate. However, this relevant Devonian metamorphism has rarely been reported before. The muscovite 40 Ar- 39 Ar ages presented could constrain the time interval of the Devonian metamorphism and exhumation process in the study area. Therefore, the magmatism, deposition history, and metamorphism during Early-Late Devonian in the northern NCC testified slab breakoff would be a suitable model for the post-collsion evolution.
Based on the previously reported alkaline magmatism and muscovite ages obtained in this study, the Devonian post-collisional extension could be caused by slab breakoff, and an evolutionary model in northern NCC from Ordovician to Early-Late Devonian is proposed ( Figure  7). The Paleo-Asian Ocean lithospheric plate subducted southward during the Ordovician-Silurian (c. 485-420 Ma), at the same time that the calc-alkaline magmatism with arc affinities developed in Bainaimiao island arc. A back-arc basin might exist at this time (Figure 7a). Later, the Bainaimiao island arc collided with the northern NCC, and as direct consequence the continental crust and lithosphere mantle thickened. The collision caused regional metamorphism of Jianshan Formation at c. 412 Ma. Meanwhile, the lower part of oceanic slab might neck (Figure 7b). Finally, slab breakoff led to the upwelling of asthenosphere mantle as well as the intrusion of alkaline rocks and the mafic-ultramafic complexes. The resultant uplift caused the Jianshan Formation and monzogranites intruding into it to be metamorphosed at c. 397-389 Ma (Figure 7c). Overall, the alkaline magmatism as well as uplift and extension revealed by muscovite 40 Ar-39 Ar dating will be useful for the recognition of slab breakoff in ancient orogeny.

Conclusions
The 40 Ar- 39 Ar datings of garnet muscovite schist and muscovite quartz schist from the Jianshan Formation of the Bayan Obo Group yielded ages of 412 ± 3 Ma and 392 ± 3 Ma, respectively, wheras two muscovite monzogranites yielded 40 Ar- 39 Ar ages of 397 ± 2 Ma and 389 ± 2 Ma. Based on those findings and available information from literature we assume that the Jianshan Formation experienced regional metamorphism at c. 412 Ma due to the collision between the Bainaimiao arc and the northern NCC. Later, the Jianshan Formation and associated were metamorphosed during an exhumation process at 397-389 Ma linked to post-collision extension.
The northern NCC was in a post-collision extension setting in the Early Devonian, which was probably induced by slab breakoff. The slab breakoff led to the upwelling of subslab asthenosphere, causing the emplacement of Devonian alkaline and mafic-ultramafic complexes as well as the development of thermal metamorphism in a linear zone of the northern NCC.