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Article

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

by
Changhai Li
1,2,
Shichao Li
1,2,*,
Zhenghong Liu
1,
Zhongyuan Xu
1,
Xiaojie Dong
1,
Qiang Shi
1,
Shijie Wang
1 and
Fan Feng
1
1
College of Earth Sciences, Jilin University, Changchun 130001, China
2
Key Laboratory of Mineral Resources Evaluation in Northeast Asia, Ministry of Natural Resources, Changchun 130026, China
*
Author to whom correspondence should be addressed.
Minerals 2020, 10(7), 580; https://doi.org/10.3390/min10070580
Submission received: 7 May 2020 / Revised: 24 June 2020 / Accepted: 25 June 2020 / Published: 27 June 2020
(This article belongs to the Section Mineral Deposits)
Browse Figures
Versions Notes

Abstract

:
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, 40Ar-39Ar 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 40Ar-39Ar muscovite plateau ages of 412 ± 3 Ma and 391 ± 3 Ma, respectively. Two other 40Ar-39Ar 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 40Ar-39Ar 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 40Ar-39Ar 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.

Graphical Abstract

1. Introduction

As a Phanerozoic accretionary orogenic belt, the Central Asian Orogenic Belt (CAOB) extends from the Ural area of Russia in the west, through Mongolia, to the northeastern part of China. The CAOB records a long-lived, episodic, diachronous, and mostly Paleozoic tectonic evolution linked to closure of the Paleo-Asian Ocean and is a complex collage of microcontinents, continental-margin fragments, magmatic arcs, fore-arc and back-arc basins, seamounts, oceanic arcs, ophiolites, and accretionary complexes [1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16] (Figure 1a). The CAOB can be divided into Northwestern Kazakhstan, Northeastern Mongolia, and southern Tarim-North China Collage Systems [11]. The CAOB recorded the evolution of the Paleo-Asian oceanic domain through subduction, collision and post-collisional processes [17].
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 40Ar-39Ar 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.

2. 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 40Ar/39Ar 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 40Ar-39Ar ages of Early to Late Devonian metamorphism are listed in Table 1.
Minerals 10 00580 g001 550
Figure 1. (a) Sketch geological map of Central Asian Orogenic Belt (CAOB) (modified after [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 40Ar-39Ar ages).
Figure 1. (a) Sketch geological map of Central Asian Orogenic Belt (CAOB) (modified after [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 40Ar-39Ar ages).
Minerals 10 00580 g001
Table
Table 1. Summary of zircon U-Pb ages of the Early Paleozoic rocks in Bainaimiao arc, Early-Late Devonian magmatic rocks, and Ar-Ar ages of Devonian metamorphism from the northern North China Craton (NCC).
Table 1. Summary of zircon U-Pb ages of the Early Paleozoic rocks in Bainaimiao arc, Early-Late Devonian magmatic rocks, and Ar-Ar ages of Devonian metamorphism from the northern North China Craton (NCC).
LocationLatitudeLongitudeRocktypeAge (Ma)MethodRef.
Zircon U-Pb ages of the Early Paleozoic rocks in Bainaimiao arc
Deyanqimiao42°25′42″113°07′47″Amphibolite490 ± 5SHRIMP[25]
Tulinkai--Quartz diorite467 ± 13SHRIMP[26]
Tulinkai--Dacite459 ± 8SHRIMP[26]
Tulinkai--Trondhjemite451 ± 7SHRIMP[26]
Tulinkai--Anorthosite429 ± 7SHRIMP[26]
Bainaimiao--Granitic porphyry440 ± 40Sm-Nd[27]
Chaganhushao--Granodiorite430 ± 13Zircon U-Pb[28]
Bayan Obo--Diorite447–493Zircon U-Pb[29]
Damaoqi41°55′54″110°7′18″Diorite452 ± 3SHRIMP[30]
Damaoqi41°59′12″110°6′54″Diorite440 ± 2SHRIMP[30]
Damaoqi41°59′12″110°6′54″Quartz diorite446 ± 2SHRIMP[30]
Damaoqi41°55′6″110°7′12″Granodiorite440 ± 2SHRIMP[30]
Zircon U-Pb ages of Devonian magmatic rocks and Ar-Ar ages of Devonian metamorphism
Baicaigou40°56′17″110°32′54″Monzonite395 ± 3LA-ICP-MS[46]
Baicaigou40°56′47″110°32′48″Monzogranite402 ± 2LA-ICP-MS[46]
Baicaigou40°55′55″110°34′47″Monzogranite402 ± 2LA-ICP-MS[46]
Baicaigou40°55′44″110°34′46″Monzogranite399 ± 3LA-ICP-MS[46]
Gaojiacun--Hornblende syenite390 ± 5TIMS[43]
Gaojiacun41°01′34″110°31′50″Hornblende syenite399 ± 7SHRIMP[43]
Gaojiacun41°02′35″110°25′17″Hornblende syenite396 ± 2LA-ICP-MS[46]
Sandaogou--Quartz monzonite409 ± 2LA-ICP-MS[20]
Sandaogou--Pyroxene syenite408 ± 4LA-ICP-MS[20]
Sandaogou41°33′23″112°53′30″Pyroxene syenite401 ± 2LA-ICP-MS[46]
Shangdu41°34′46″112°54′50″Syenite411 ± 1LA-ICP-MS[44]
Sandaogou41°32′59″112°57′42″Monzonite401 ± 1LA-ICP-MS[46]
Sandaogou41°32′59″112°57′42″Gabbro399 ± 1LA-ICP-MS[46]
Sandaogou41°32′59″112°57′42″Pyroxenite398 ± 2LA-ICP-MS[46]
Wulanhada--Quartz monzonite382 ± 4SHRIMP[45]
Wulanhada41°35′49″113°08′18″Monzonite381 ± 2LA-ICP-MS[46]
Wulanhada41°35′33″113°10′28″Monzonite379 ± 1LA-ICP-MS[46]
Wulanhada41°35′33″113°10′28″Gabbro377 ± 1LA-ICP-MS[46]
Wulanhada41°35′33″113°10′28″Pyroxenite381 ± 5LA-ICP-MS[46]
Shuiquangou--Quartz monzonite390 ± 6SHRIMP[18]
Shuiquangou--Syenite386 ± 7SHRIMP[18]
Hongshila41°10′17″117°15′58″Pyroxenite394 ± 4LA-ICP-MS[42]
Hongshila41°10′13″117°15′52″Pyroxenite381 ± 7SHRIMP[42]
Hongshila42°10′03″117°16′11″Pyroxenite387 ± 2LA-ICP-MS[42]
Erdaogou41°08′18″117°38′15″Hornblendite395 ± 11SHRIMP[42]
Xiahabaqin42°09′17″117°43′02″Rodingite392 ± 5SHRIMP[42]
Longwangmiao41°13′32″117°46′24″Hornblendite382 ± 10LA-ICP-MS[37]
Longwangmiao41°13′44″117°46′25″Clinopyroxenite391 ± 4LA-ICP-MS[37]
Gushan41°12′05″117°48′02″Monzodiorite390 ± 5SHRIMP[38]
Hongshan--Syenogranite387 ± 4SHRIMP[40]
Hongshan42°18′03″118°58′25″Syenogranite388 ± 3LA-ICP-MS[46]
Hongmiaozi42°20′03″119°03′45″Syenogranite391 ± 4LA-ICP-MS[46]
Jiguanshan42°24′35″119°06′46″Syenogranite porphyry393 ± 3LA-ICP-MS[46]
Erfendi42°37′36″119°25′16″Alkaline granite385 ± 3LA-ICP-MS[24]
Lianhuashan42° 34′9″119°41′52″Rhyolitic tuff366 ± 2LA-ICP-MS[39]
Lianhuashan42°34’13″119°42’2″Rhyolitic tuff364 ± 2LA-ICP-MS[39]
Chehugou--Syenogranite376 ± 3LA-ICP-MS[41]
Bayan Obo--Dolostone marble425–395Amphibole Ar-Ar[47,48]
Chicheng--Mylonite399 ± 1Muscovite Ar-Ar[49]
Shangdu41°39′50″113°59′11"garnet muscovite schist412 ± 3 Muscovite Ar-ArThis study
Shangdu41°41′18″113°57′12"muscovite quartz schist392 ± 3 Muscovite Ar-ArThis study
Shangdu41°44′30″113°52′31″biotite monzogranite397 ± 2 Muscovite Ar-ArThis study
Shangdu41°43′12″113°51′56″muscovite monzogranite389 ± 2 Muscovite Ar-ArThis study
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].

3. Samples and Method

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).
Sample B1193-1 is a strongly deformed garnet muscovite schist from the Jianshan Formation in the northeastern part of Shangdu (GPS position: 113°59′11″ E, 41°39′50″ N). The main minerals include quartz (25 vol.%) and characteristic metamorphic minerals such as muscovite (55 vol.%), biotite (10 vol.%), and garnet (10 vol.%). The quartz exhibits undulose extinction (Figure 3a,b). The diameters of the garnets are up to 1.5 cm. The muscovite and biotite define the NE-trending foliation (S1).
Sample TM33 is a strongly deformed muscovite quartz schist found north of sample B1193-1 (GPS position: 113°57′12″ E, 41°41′18″ N). It consists of quartz (60 vol.%), muscovite (30 vol.%), and biotite (10 vol.%). The quartz exhibits undulose extinction and the foliation (S2) is defined by the orientation of muscovite (Figure 3c,d).
Sample TM31 is a fine-grained biotite monzogranite from a granitic pluton in the Shangdu area (GPS position: 113°52′31″ E, 41°44′30″ N). It is less deformed than sample B1193-1 and TM33. It contains K-feldspar (25 vol.%), plagioclase (30 vol.%), quartz (30 vol.%), biotite (10 vol.%), and muscovite (5 vol.%). Some of the K-feldspars are altered into kaolin, and the quartz seldom exhibit undulose extinction (Figure 3e,f).
Sample TM32 is a medium-grained muscovite monzogranite sampled 3 km southwest of sample TM31 (GPS position: 113°51′56″ E, 41°43′12″ N). It is also less deformed without foliation. Its main minerals are K-feldspar (30 vol.%), plagioclase (25 vol.%), quartz (35 vol.%), and muscovite (10 vol.%). The K-feldspar is altered to kaolin, and the plagioclase exhibits polysynthetic twinning (Figure 3g,h).

4. 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 × 1018 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., (40Ar/39Ar)K = 0.004782, (39Ar/37Ar)Ca = 0.000806, (36Ar/37Ar)Ca = 0.0002389).
The 40Ar/39Ar 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 40Ar, 39Ar, 38Ar, 37Ar, and 36Ar 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 37Ar 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.

5. Results

The 40Ar/39Ar 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 39Ar (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 39Ar.
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 39Ar.
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 39Ar.

6. Discussion

6.1. 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 40Ar-39Ar 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 40Ar-39Ar 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.

6.2. 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 87Sr/86Sr 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.
Subsequent to initial collision, several regimes are regarded as triggers for post-collision magmatism: (1) delamination of lithospheric mantle [71,72,73]; (2) convective thinning of the lithosphere [74] and (3) breakoff of subduting slab [75,76,77,78]. Both delamination and convective thinning models would cause some recognizable effects, such as rapid uplift of overriding plate, temperature rise due to upwelling of mantle asthenosphere and accompanying magmatism transfer from early asthenospheric mantle melts, followed by increasingly crustally contaminated calc-alkaline melts to crustal partial melts [71,72,73]. However, the two models would lead to diffuse and nonlinear zones of magmatism, which is incompatable with the linear distribution of Devonian rocks along the northern NCC.
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 40Ar-39Ar 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 40Ar-39Ar dating will be useful for the recognition of slab breakoff in ancient orogeny.

7. Conclusions

The 40Ar-39Ar 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 40Ar-39Ar 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.

Author Contributions

Conceptualization, C.L. and Z.L.; investigation, C.L., Z.L., Z.X., S.L. and Q.S.; writing—original draft preparation, C.L.; writing—review and editing, Z.L., Z.X., S.L. and X.D.; visualization, C.L., S.W., F.F.; funding acquisition, Z.L., Z.X., S.L. All authors have read and agreed to the published version of the manuscript.

Funding

This research was funded by National Natural Science Foundation of China (Grant 41872203; 41872194, and 41872234) and Supported by Opening Foundation of Key Laboratory of Mineral Resources Evaluation in Northeast Asia, Ministry of Natural Resources(DBY-ZZ-18-08).

Acknowledgments

Thanks to Wen Chen of Isotope Geology Laboratory, Institute of Geology, Chinese Academy of Geological Sciences for providing assistance in muscovite 40Ar-39Ar dating. We also thank LetPub (www.letpub.com) for its linguistic assistance during the preparation of this manuscript.

Conflicts of Interest

The authors declare no conflict of interest.

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Figure 2. Simplified geological map of the study area. (a) Geological map of the Shangdu Area (after [51]); (b) sketch geological map of the sampling area.
Figure 2. Simplified geological map of the study area. (a) Geological map of the Shangdu Area (after [51]); (b) sketch geological map of the sampling area.
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Figure 3. Representative outcrops and photomicrograph of garnet muscovite schist (from Jianshan Formation) and monzogranite in the study area. (a) Garnet muscovite schist (B1193-1) with garnets up to 1.5 cm in size; (b) the photomicrograph of garnet muscovite schist (B1193-1) with muscovite forming the foliation (S1); (c) muscovite quartz schist (TM33); (d) the photomicrograph of muscovite quartz schist (TM33) with development of foliation (S2); (e) fine-grained biotite monzogranite (TM31); (f) photomicrograph of fine-grained biotite monzogranite without being obviously deformed (TM31); (g) medium-grained muscovite monzogranite (TM32); (h) photomicrograph of medium-grained muscovite monzogranite, the foliation is not well developed (TM32). Ms—muscovite; Grt—garnet; Qtz—quartz; Pl—plagioclase; Kfs—K-feldspar; Bt—biotite.
Figure 3. Representative outcrops and photomicrograph of garnet muscovite schist (from Jianshan Formation) and monzogranite in the study area. (a) Garnet muscovite schist (B1193-1) with garnets up to 1.5 cm in size; (b) the photomicrograph of garnet muscovite schist (B1193-1) with muscovite forming the foliation (S1); (c) muscovite quartz schist (TM33); (d) the photomicrograph of muscovite quartz schist (TM33) with development of foliation (S2); (e) fine-grained biotite monzogranite (TM31); (f) photomicrograph of fine-grained biotite monzogranite without being obviously deformed (TM31); (g) medium-grained muscovite monzogranite (TM32); (h) photomicrograph of medium-grained muscovite monzogranite, the foliation is not well developed (TM32). Ms—muscovite; Grt—garnet; Qtz—quartz; Pl—plagioclase; Kfs—K-feldspar; Bt—biotite.
Minerals 10 00580 g003aMinerals 10 00580 g003b
Minerals 10 00580 g004a 550Minerals 10 00580 g004b 550
Figure 4. 40Ar-39Ar weighted mean plateau and Ca/K ration spectra for muscovite from schist and monzogranite. (a,b) diagrams of plateau and Ca/K ration spectra for garnet muscovite schist (B1193-1); (c,d) diagrams of plateau and Ca/K ration spectra for muscovite quartz schist (TM33); (e,f) diagrams of plateau and Ca/K ration spectra for biotite monzogranite (TM31); (g,h) diagrams of plateau and Ca/K ration spectra for muscovite monzogranite (TM32). WMPA—weighted mean plateau age.
Figure 4. 40Ar-39Ar weighted mean plateau and Ca/K ration spectra for muscovite from schist and monzogranite. (a,b) diagrams of plateau and Ca/K ration spectra for garnet muscovite schist (B1193-1); (c,d) diagrams of plateau and Ca/K ration spectra for muscovite quartz schist (TM33); (e,f) diagrams of plateau and Ca/K ration spectra for biotite monzogranite (TM31); (g,h) diagrams of plateau and Ca/K ration spectra for muscovite monzogranite (TM32). WMPA—weighted mean plateau age.
Minerals 10 00580 g004aMinerals 10 00580 g004b
Minerals 10 00580 g005 550
Figure 5. (a) Total alkali (K2O + Na2O) vs. silica (SiO2) [68]; (b) total alkali (K2O + Na2O) vs. 10,000×Ga/Al [69] (data from [19,20,38,41,44,45,46]).
Figure 5. (a) Total alkali (K2O + Na2O) vs. silica (SiO2) [68]; (b) total alkali (K2O + Na2O) vs. 10,000×Ga/Al [69] (data from [19,20,38,41,44,45,46]).
Minerals 10 00580 g005
Minerals 10 00580 g006 550
Figure 6. Plots of Nb-Y-Ce for the Devonian alkaline and mafic-ultramafic rocks in the northern NCC (areas of A1 and A2 modified after [70]; data from [19,20,38,41,44,45,46]).
Figure 6. Plots of Nb-Y-Ce for the Devonian alkaline and mafic-ultramafic rocks in the northern NCC (areas of A1 and A2 modified after [70]; data from [19,20,38,41,44,45,46]).
Minerals 10 00580 g006
Minerals 10 00580 g007a 550Minerals 10 00580 g007b 550
Figure 7. Simplified cross-sections illustrating the tectonic evolution of the northern margin of the NCC. (a) Ordovician-Silurian, the southward subduction of the Paleo Asian Ocean lithospheric plate; (b) Early Ordovician, the collision between the Bainaimiao arc and the NCC, the lithosphere mantle thickened, and Jianshan Formation went through regional metamorphism at c. 412 Ma; and (c) Early-Late Devonian, post-collision extension, Jianshan Formation, and associated monzogranites were metamorphosed at c. 397–389 Ma.
Figure 7. Simplified cross-sections illustrating the tectonic evolution of the northern margin of the NCC. (a) Ordovician-Silurian, the southward subduction of the Paleo Asian Ocean lithospheric plate; (b) Early Ordovician, the collision between the Bainaimiao arc and the NCC, the lithosphere mantle thickened, and Jianshan Formation went through regional metamorphism at c. 412 Ma; and (c) Early-Late Devonian, post-collision extension, Jianshan Formation, and associated monzogranites were metamorphosed at c. 397–389 Ma.
Minerals 10 00580 g007aMinerals 10 00580 g007b
Table
Table 2. 40Ar/39 Ar step-heating geochronology data for muscovite from garnet muscovite schist, muscovite quartz schist (Jianshan Formation), and monzogranite.
Table 2. 40Ar/39 Ar step-heating geochronology data for muscovite from garnet muscovite schist, muscovite quartz schist (Jianshan Formation), and monzogranite.
T
(°C)		(40Ar/39Ar)m(36Ar/39Ar)m(37Ar/39Ar)m40Ar */39Ar39Ar
(× 10−14 mol)		39Ar
(%)		40Ar
(%)		Age
(Ma)		±1σ
(Ma)		Ca/K
Sample number = B1193-1; mineral = muscovite; w = 27.04 mg; J = 0.004155
80068.00030.02680.000060.07210.181.2788.34402.04.30
86067.88800.019513.196263.72970.514.9192.88423.84.222.7502
91065.13700.01040.000062.05581.0112.1495.27413.83.70
95064.48620.00910.003361.79342.5530.2895.82412.33.70.0056
98062.44760.00351.100161.52772.4147.4998.44410.73.71.8965
102062.73020.00523.409761.59461.7660.0397.92411.13.85.8727
107064.02140.01060.000060.87150.9566.7995.08406.83.70
112064.86570.01299.926162.23780.8672.9195.18414.94.217.1125
118064.63950.01031.791561.79631.4883.4595.46412.33.83.0886
124062.83180.00361.471661.95301.9897.5498.48413.23.82.5371
130062.97000.01219.918160.55030.3099.7095.394051017.0988
1400102.96010.140442.369966.73480.04100.0062.604422873.0457
Sample number = TM33; mineral = muscovite; w = 12.05 mg; J = 0.004679
600499.27591.61752.280821.51140.020.064.30173613.9321
70074.50020.08720.038748.73190.511.6165.41370.53.50.0667
78063.47050.03410.030353.37962.017.6984.10402.23.60.0522
81055.81200.00920.000053.10201.6912.8495.14400.33.60
85054.89280.00830.011452.42273.0121.9795.50395.73.60.0196
90054.32720.00880.005651.71545.7239.3395.19390.93.50.0097
94052.62310.00390.000051.45468.6865.6697.78389.23.50
98053.41560.00640.000051.50514.7079.9396.42389.53.50
102054.89430.01150.009151.48631.9285.7593.79389.43.50.0157
108054.40190.00930.000051.65582.0692.0094.95390.53.50
114053.27680.00360.000052.20711.7497.2897.99394.33.60
124053.99980.00570.000052.30750.4898.7596.87395.03.60
140063.13260.01400.000058.98880.41100.0093.44439.74.00
Sample number = TM31; mineral = muscovite; w = 13.12 mg; J = 0.004560
6002062.91606.79730.000054.29820.010.042.63399580
70093.36150.15400.039647.84970.260.8951.25356.13.60.0682
76070.11230.05430.018854.07870.512.5777.13397.63.60.0323
81059.79890.01900.024154.17642.5611.0690.60398.33.60.0415
85056.26540.00750.000054.05511.9717.5996.07397.53.60
90054.84440.00320.000053.88644.2131.5698.25396.43.60
94054.27620.00200.001053.66986.1351.8898.89394.93.50.0017
97054.59720.00290.008353.72965.2769.3698.41395.33.60.0143
100054.83800.00370.000053.74693.5681.1698.01395.43.60
104054.82180.00350.000053.79451.2185.1998.13395.83.60
114054.97190.00240.000054.25601.4289.9098.70398.83.60
120055.46900.00500.001153.97512.9699.7397.31397.03.60.0018
140069.00750.04720.000055.06000.82100.0079.79404.18.70
Sample number = TM32; mineral = muscovite; w = 14.20 mg; J = 0.004734
680336.91411.00070.165541.23190.130.3012.24321.07.90.2854
740108.91030.19670.000050.76590.491.4746.61388.53.70
80077.86330.08920.000051.48471.705.5166.13393.53.50
84054.92060.01340.009850.96902.2910.9892.80389.93.50.0169
88051.43340.00260.000050.66713.8320.1298.51387.93.50
92050.86140.00080.000050.63462.9827.2399.55387.63.50
96051.24330.00080.002751.00538.6247.7999.54390.23.50.0046
99051.05940.00110.000050.73073.1855.3899.36388.33.50
102051.02470.00260.050750.25271.9059.9298.48385.03.50.053
106050.96230.00210.019850.32573.3467.8898.75385.53.50.0341
112050.87510.00120.006350.52149.3890.2599.30386.93.50.0109
120051.06110.00170.018850.56632.7296.7399.03387.23.50.0325
140053.57230.00780.038751.26171.37100.0095.68391.93.50.0666
* Radiogenic.

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Li, C.; Li, S.; Liu, Z.; Xu, Z.; Dong, X.; Shi, Q.; Wang, S.; Feng, F. Early-Late Devonian Post-Collision Extension Due to Slab Breakoff Regime along the Northern Margin of North China Craton: Implications from Muscovite 40Ar-39Ar Dating. Minerals 2020, 10, 580. https://doi.org/10.3390/min10070580

AMA Style

Li C, Li S, Liu Z, Xu Z, Dong X, Shi Q, Wang S, Feng F. Early-Late Devonian Post-Collision Extension Due to Slab Breakoff Regime along the Northern Margin of North China Craton: Implications from Muscovite 40Ar-39Ar Dating. Minerals. 2020; 10(7):580. https://doi.org/10.3390/min10070580

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Li, Changhai, Shichao Li, Zhenghong Liu, Zhongyuan Xu, Xiaojie Dong, Qiang Shi, Shijie Wang, and Fan Feng. 2020. "Early-Late Devonian Post-Collision Extension Due to Slab Breakoff Regime along the Northern Margin of North China Craton: Implications from Muscovite 40Ar-39Ar Dating" Minerals 10, no. 7: 580. https://doi.org/10.3390/min10070580

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