Early-Late Devonian Post-Collision Extension Due to Slab Breakoff

minerals

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

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; [email protected] (C.L.); [email protected] (Z.L.); [email protected] (Z.X.); [email protected] (X.D.); [email protected] (Q.S.); [email protected] (S.W.); [email protected] (F.F.) 2 Key Laboratory of Mineral Resources Evaluation in Northeast Asia, Ministry of Natural Resources, Changchun 130026, China * Correspondence: [email protected]

Received: 7 May 2020; Accepted: 25 June 2020; Published: 27 June 2020

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.

Keywords: North China Craton; post-collisional extension; Bayan Obo Group; 40Ar-39Ar dating; slab breakoﬀ

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 [1–16] (Figure1a). The CAOB can be divided into Northwestern Kazakhstan, Northeastern

Minerals 2020, 10, 580; doi:10.3390/min10070580 www.mdpi.com/journal/minerals Minerals 2020, 10, 580 2 of 19

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 [18–20]. 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 [14–17] is divided into the Southern Orogenic Belts (SOB) and Northern Orogenic Belts (NOB), separated by the Solonker suture zone [15,21] (Figure1b). 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) [24–30]. The representative rocks are listed in Table1. 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 [32–34]. 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 [36–46]. Furthermore, these rocks are confined in a linear zone along the northern NCC, as shown in the yellow belt in Figure1b. In addition, Late Silurian to Early Devonian metamorphism has been identified along the northern margin of NCC [47–49]. 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 Table1. The study area (Figure2a) is located in the central part of the northern margin of the NCC (Figure1b); 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 (Figure2a) [ 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 (Figure2a). Minerals 2020, 10, 580 3 of 19 Minerals 2020, 10, 580 3 of 20

FigureFigure 1. (a 1.) Sketch(a) Sketch geological geological map map of of CentralCentral Asian Asian Orogenic Orogenic Belt Belt (CAOB) (CAOB) (modified (modiﬁed after after[3]); ( [b3)]); the (b tectonic) the tectonic division division of CAOB of in CAOB central inpart central of Inner part Mongolia, of Inner China Mongolia, (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 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 Devonian alkaline rocks, those shown by red are representative muscovite 40Ar-39Ar ages ). ages of Devonian alkaline rocks, those shown by red are representative muscovite 40Ar-39Ar ages).

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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).

Location Latitude Longitude Rocktype Age (Ma) Method Ref. Zircon U-Pb ages of the Early Paleozoic rocks in Bainaimiao arc Deyanqimiao 42 25 42” 113 07 47” Amphibolite 490 5 SHRIMP [25] ◦ 0 ◦ 0 ± Tulinkai - - Quartz diorite 467 13 SHRIMP [26] ± Tulinkai - - Dacite 459 8 SHRIMP [26] ± Tulinkai - - Trondhjemite 451 7 SHRIMP [26] ± Tulinkai - - Anorthosite 429 7 SHRIMP [26] ± Bainaimiao - - Granitic porphyry 440 40 Sm-Nd [27] ± Chaganhushao - - Granodiorite 430 13 Zircon U-Pb [28] ± Bayan Obo - - Diorite 447–493 Zircon U-Pb [29] Damaoqi 41 55 54” 110 7 18” Diorite 452 3 SHRIMP [30] ◦ 0 ◦ 0 ± Damaoqi 41 59 12” 110 6 54” Diorite 440 2 SHRIMP [30] ◦ 0 ◦ 0 ± Damaoqi 41 59 12” 110 6 54” Quartz diorite 446 2 SHRIMP [30] ◦ 0 ◦ 0 ± Damaoqi 41 55 6” 110 7 12” Granodiorite 440 2 SHRIMP [30] ◦ 0 ◦ 0 ± Zircon U-Pb ages of Devonian magmatic rocks and Ar-Ar ages of Devonian metamorphism Baicaigou 40 56 17” 110 32 54” Monzonite 395 3 LA-ICP-MS [46] ◦ 0 ◦ 0 ± Baicaigou 40 56 47” 110 32 48” Monzogranite 402 2 LA-ICP-MS [46] ◦ 0 ◦ 0 ± Baicaigou 40 55 55” 110 34 47” Monzogranite 402 2 LA-ICP-MS [46] ◦ 0 ◦ 0 ± Baicaigou 40 55 44” 110 34 46” Monzogranite 399 3 LA-ICP-MS [46] ◦ 0 ◦ 0 ± Gaojiacun - - Hornblende syenite 390 5 TIMS [43] ± Gaojiacun 41 01 34” 110 31 50” Hornblende syenite 399 7 SHRIMP [43] ◦ 0 ◦ 0 ± Gaojiacun 41 02 35” 110 25 17” Hornblende syenite 396 2 LA-ICP-MS [46] ◦ 0 ◦ 0 ± Sandaogou - - Quartz monzonite 409 2 LA-ICP-MS [20] ± Sandaogou - - Pyroxene syenite 408 4 LA-ICP-MS [20] ± Sandaogou 41 33 23” 112 53 30” Pyroxene syenite 401 2 LA-ICP-MS [46] ◦ 0 ◦ 0 ± Shangdu 41 34 46” 112 54 50” Syenite 411 1 LA-ICP-MS [44] ◦ 0 ◦ 0 ± Sandaogou 41 32 59” 112 57 42” Monzonite 401 1 LA-ICP-MS [46] ◦ 0 ◦ 0 ± Sandaogou 41 32 59” 112 57 42” Gabbro 399 1 LA-ICP-MS [46] ◦ 0 ◦ 0 ± Sandaogou 41 32 59” 112 57 42” Pyroxenite 398 2 LA-ICP-MS [46] ◦ 0 ◦ 0 ± Wulanhada - - Quartz monzonite 382 4 SHRIMP [45] ± Wulanhada 41 35 49” 113 08 18” Monzonite 381 2 LA-ICP-MS [46] ◦ 0 ◦ 0 ± Wulanhada 41 35 33” 113 10 28” Monzonite 379 1 LA-ICP-MS [46] ◦ 0 ◦ 0 ± Wulanhada 41 35 33” 113 10 28” Gabbro 377 1 LA-ICP-MS [46] ◦ 0 ◦ 0 ± Wulanhada 41 35 33” 113 10 28” Pyroxenite 381 5 LA-ICP-MS [46] ◦ 0 ◦ 0 ± Shuiquangou - - Quartz monzonite 390 6 SHRIMP [18] ± Minerals 2020, 10, 580 5 of 19

Table 1. Cont.

Location Latitude Longitude Rocktype Age (Ma) Method Ref. Shuiquangou - - Syenite 386 7 SHRIMP [18] ± Hongshila 41 10 17” 117 15 58” Pyroxenite 394 4 LA-ICP-MS [42] ◦ 0 ◦ 0 ± Hongshila 41 10 13” 117 15 52” Pyroxenite 381 7 SHRIMP [42] ◦ 0 ◦ 0 ± Hongshila 42 10 03” 117 16 11” Pyroxenite 387 2 LA-ICP-MS [42] ◦ 0 ◦ 0 ± Erdaogou 41 08 18” 117 38 15” Hornblendite 395 11 SHRIMP [42] ◦ 0 ◦ 0 ± Xiahabaqin 42 09 17” 117 43 02” Rodingite 392 5 SHRIMP [42] ◦ 0 ◦ 0 ± Longwangmiao 41 13 32” 117 46 24” Hornblendite 382 10 LA-ICP-MS [37] ◦ 0 ◦ 0 ± Longwangmiao 41 13 44” 117 46 25” Clinopyroxenite 391 4 LA-ICP-MS [37] ◦ 0 ◦ 0 ± Gushan 41 12 05” 117 48 02” Monzodiorite 390 5 SHRIMP [38] ◦ 0 ◦ 0 ± Hongshan - - Syenogranite 387 4 SHRIMP [40] ± Hongshan 42 18 03” 118 58 25” Syenogranite 388 3 LA-ICP-MS [46] ◦ 0 ◦ 0 ± Hongmiaozi 42 20 03” 119 03 45” Syenogranite 391 4 LA-ICP-MS [46] ◦ 0 ◦ 0 ± Jiguanshan 42 24 35” 119 06 46” Syenogranite porphyry 393 3 LA-ICP-MS [46] ◦ 0 ◦ 0 ± Erfendi 42 37 36” 119 25 16” Alkaline granite 385 3 LA-ICP-MS [24] ◦ 0 ◦ 0 ± Lianhuashan 42 34 9” 119 41 52” Rhyolitic tuﬀ 366 2 LA-ICP-MS [39] ◦ 0 ◦ 0 ± Lianhuashan 42 34’13” 119 42’2” Rhyolitic tuﬀ 364 2 LA-ICP-MS [39] ◦ ◦ ± Chehugou - - Syenogranite 376 3 LA-ICP-MS [41] ± Bayan Obo - - Dolostone marble 425–395 Amphibole Ar-Ar [47,48] Chicheng - - Mylonite 399 1 Muscovite Ar-Ar [49] ± Shangdu 41 39 50” 113 59 11" garnet muscovite schist 412 3 Muscovite Ar-Ar This study ◦ 0 ◦ 0 ± Shangdu 41 41 18” 113 57 12" muscovite quartz schist 392 3 Muscovite Ar-Ar This study ◦ 0 ◦ 0 ± Shangdu 41 44 30” 113 52 31” biotite monzogranite 397 2 Muscovite Ar-Ar This study ◦ 0 ◦ 0 ± Shangdu 41 43 12” 113 51 56” muscovite monzogranite 389 2 Muscovite Ar-Ar This study ◦ 0 ◦ 0 ± Minerals 2020, 10, 580 6 of 20

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 Mineralsinclude2020 Upper, 10, 580 Ordovician limestone and Upper Permian strata, which are mainly composed6 of of 19 sandstone, slate, and volcanic tuff (Figure 2a).

FigureFigure 2. 2. SimpliﬁedSimplified geological geological map map of of the the study study area. area. (a) Geological (a) Geological map of map the ofShangdu the Shangdu Area (after Area [51]); (b) sketch geological map of the sampling area. (after [51]); (b) sketch geological map of the sampling area.

TheThe Bayan Bayan Obo Obo Group Group extendsextends aboutabout 500500 km from east to to west, west, 20 20–30–30 km km from from south south to to north, north, andand is is up up to to 7000-m 7000-m thick thick[ 51[51].]. TheThe Bayan Bayan Obo Group is adjacent to to the the Zhaertai Zhaertai and and Langshan Langshan GroupGroup in in the the west west and and connects connects withwith thethe HuadeHuade GroupGroup in the east. They They represe representnt strata strata deposited deposited inin the the Meso-Neoproterozoic Meso-Neoproterozoic Langshan-Zhaertai-BayanLangshan-Zhaertai-Bayan Obo Rift Rift System System [ [5151––5757]].. Overall, Overall, the the Bayan Bayan Obo Group consists of a succession of sedimentary strata and metapelites. From bottom to top, it is 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 divided into: (1) the lower succession of Dulahala (Chd) and Jianshan (Chj) Formations, (2) the middle middle succession of Halahuogete (Jxh)and Bilute (Jxb) Formations, (3) the upper succession of succession of Halahuogete (Jxh)and Bilute (Jxb) Formations, (3) the upper succession of Baiyinbaolage Baiyinbaolage (Qnby) and Hujiertu (Qnhj) Formations [51]. Dulahala Formation is characterized by (Qnby) and Hujiertu (Qnhj) Formations [51]. Dulahala Formation is characterized by conglomerates conglomerates and coarse-grained feldspathic quartz sandstones; the subsequent Jianshan 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 [58–60]. Minerals 2020, 10, 580 7 of 19

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 Minerals 2020, 10, 580 7 of 20 dating [50]. The granites are less deformed, lack well defined foliation, and are adjacent to the Permian graniteFormation (Figure2 b).includes slates, sericite phylllite, metamorphosed coarse sandstone, and feldsparthic Samplequartz B1193-1 sandstone. is a Halahuogete strongly deformed Formation garnet is muscovite dominated schist by glutenite from the, metamorphosed Jianshan Formation in the northeasternconglomerate, and part limestone. of Shangdu Bilute (GPS Formation position: is made 113◦ up59 0of11” sericite E, 41 ◦phyllite,39050” N).whose The protolith main minerals is includeargillite. quartz Baiyinbaolage (25 vol.%) and Formation characteristic is composed metamorphic of quartzite minerals and meta such-quartzite, as muscovite whereas (55 Hujiertu vol.%), biotite (10 vol.%),Formation and garnet comprises (10 vol.%). a succession The quartz of slate exhibits and limestone undulose [51 extinction]. Bayan Obo (Figure Group3a,b). contains Thediameters the world-famous Bayan Obo rare earth deposit, it has been the focus of numerous studies [58–60]. of the garnets are up to 1.5 cm. The muscovite and biotite define the NE-trending foliation (S1). Sample3. Samples TM33 and is Method a strongly deformed muscovite quartz schist found north of sample B1193-1 (GPS position: 113◦57012” E, 41◦41018” N). It consists of quartz (60 vol.%), muscovite (30 vol.%), and biotite 3.1. Sample Description (10 vol.%). The quartz exhibits undulose extinction and the foliation (S2) is defined by the orientation of muscoviteGarnet (Figure muscovite3c,d). schist and muscovite quartz schist were collected from the Jianshan SampleFormation, TM31 at the is abottom fine-grained of the Bayan biotite Obo monzogranite Group. They were from both a graniticstrongly deformed pluton in and the have Shangdu well-developed NE-trending foliation with dip angle at 35–40°. Two granite samples were collected area (GPS position: 113◦52031” E, 41◦44030” N). It is less deformed than sample B1193-1 and TM33. It containseach from K-feldspar a granitic (25 pluton, vol.%), which plagioclase intruded (30the Bayan vol.%), Obo quartz Group (30, emplaced vol.%), at biotite around (10 1320 vol.%), Ma and according to U-Pb zircon dating [50]. The granites are less deformed, lack well defined foliation, and muscovite (5 vol.%). Some of the K-feldspars are altered into kaolin, and the quartz seldom exhibit are adjacent to the Permian granite (Figure 2b). undulose extinctionSample B1193 (Figure-1 is a3 stronglye,f). deformed garnet muscovite schist from the Jianshan Formation in Samplethe northeastern TM32 is a part medium-grained of Shangdu (GPS muscovite position: monzogranite 113°59′11" E, 41°39′50" sampled N 3). km The southwestmain minerals of sample TM31 (GPSinclude position: quartz (2 1135 vol.%◦510)56” andE, characteristic 41◦43012” N).metamorphic It is also lessminerals deformed such aswithout muscovite foliation. (55 vol.%), Its main mineralsbiotite are (10 K-feldspar vol.%), and (30 garnet vol.%), (10 plagioclase vol.%). The (25quartz vol.%), exhibits quartz undulose (35 vol.%), extinction and (Figure muscovite 3a,b). (10The vol.%). The K-feldspardiameters isof alteredthe garnets to kaolin,are up to and 1.5 cm the. T plagioclasehe muscovite exhibits and biotite polysynthetic define the NE twinning-trending foliation (Figure 3g,h). (S1).

Figure 3. Cont.

Minerals 2020, 10, 580 8 of 19

Minerals 2020, 10, 580 8 of 20

FigureFigure 3. Representative 3. Representative outcrops outcrops and and photomicrograph photomicrograph of of garnet garnet muscovite muscovite schist schist (from (from Jianshan Jianshan Formation) and monzogranite in the study area. (a) Garnet muscovite schist (B1193-1) with garnets 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 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

formingquartz the foliationschist (TM33) (S1); with (c) development muscovite quartz of foliation schist (S2) (TM33);; (e) fine-grained (d) the biotite photomicrograph monzogranite of(TM31); muscovite quartz( schistf) phot (TM33)omicrograph with of development fine-grained biotite of foliation monzogranite (S2); ( ewithout) fine-grained being obviously biotite monzogranitedeformed (TM31); (TM31); (f) photomicrograph(g) medium-grained of fine-grained muscovite monzogranite biotite monzogranite (TM32); (h without) photomicrograph being obviously of medium deformed-grained (TM31); (g) medium-grainedmuscovite monzogranite muscovite, the foliation monzogranite is not well (TM32); developed (h) (TM32). photomicrograph Ms—muscovite; of Grt medium-grained—garnet; muscoviteQtz— monzogranite,quartz; Pl—plagioclase; the foliation Kfs—K- isfeldspar; not well Bt— developedbiotite. (TM32). Ms—muscovite; Grt—garnet; Qtz—quartz; Pl—plagioclase; Kfs—K-feldspar; Bt—biotite. Sample TM33 is a strongly deformed muscovite quartz schist found north of sample B1193-1 4. Method(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 Whiteorientation mica of was muscovite separated (Figure from 3c,d). the crushed rock samples using magnetic separation methods. Aliquots wereSample examined TM31 is using a fine- agrained binocular biotite microscope monzogranite to ensure from a agranitic purity pluton of up in to the 99% Shangdu at theKeDa area Rock and Mineral(GPS position: Separation 113°52′ Technology31″ E, 41°44′30" Company, N). It Langfang, is less deformed Heibei than Province. sample The B1193 white-1 and mica TM33. was Itwashed contains K-feldspar (25 vol.%), plagioclase (30 vol.%), quartz (30 vol.%), biotite (10 vol.%), and by the ultrasonic wave method. Aliquots and the ZBH-25 biotite standard were sent to a nuclear muscovite (5 vol.%). Some of the K-feldspars are altered into kaolin, and the quartz seldom exhibit reactorundulose and set extinction in hole H (4Figurein the 3e,f) 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 Sample TM32 is a medium-grained muscovite monzogranite sampled× 3 km· south− west of biotitesample standard TM31 produced (GPS position: an age113°5 of1′56 133.3″ E, 41°40.243′12" MaN). It [ 61is also] with less adeformed 1% relative without standard foliation. deviation Its ± (1σ). ThemainJ -valuesminerals forare K the-feldspar individual (30 vol.%), samples plagioclase were (25 determined vol.%), quartz using (35 vol.%), a second-order and muscovite polynomial (10 interpolation.vol.%). The The K Ca-feldspar and K is correction altered to factors kaolin, wereand the calculated plagioclase from exhibits the coirradiationpolysynthetic oftwinning pure salts of (Figure 3g,h). 40 39 39 37 36 37 CaF2 and K2SO4 (e.g., ( Ar/ Ar)K = 0.004782, ( Ar/ Ar)Ca = 0.000806, ( Ar/ Ar)Ca = 0.0002389). 40 39 The4. MethodAr/ 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 White mica was separated from the crushed rock samples using magnetic separation methods. Christmas tree sample holder, and degassed at low temperature (250–300 C) for 20–30 min before Aliquots were examined using a binocular microscope to ensure a purity of up◦ to 99% at the KeDa being incrementallyRock and Mineral heated Separation in a double-vacuum Technology Comp graphiteany, Langfang, furnace. Heibei The Province. gases released The white during mica 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 Minerals 2020, 10, 580 9 of 19 time of irradiation. The decay constant used throughout the calculations was λ = (5.543 0.010) 10 1 ± × 10− a− . 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 Table2. The Ca /K ratio spectra of these four samples is in Figure4b,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 (Figure4a).

Table 2. 40Ar/39 Ar step-heating geochronology data for muscovite from garnet muscovite schist, muscovite quartz schist (Jianshan Formation), and monzogranite.

39 40 Ar 39 40 T 40 39 36 39 37 39 Ar 14 Ar Ar Age 1σ ( Ar/ Ar)m ( Ar/ Ar)m ( Ar/ Ar)m ( 10 ± Ca/K ( C) */39Ar − (%) (%) (Ma) (Ma) ◦ ×mol) Sample number = B1193-1; mineral = muscovite; w = 27.04 mg; J = 0.004155 800 68.0003 0.0268 0.0000 60.0721 0.18 1.27 88.34 402.0 4.3 0 860 67.8880 0.0195 13.1962 63.7297 0.51 4.91 92.88 423.8 4.2 22.7502 910 65.1370 0.0104 0.0000 62.0558 1.01 12.14 95.27 413.8 3.7 0 950 64.4862 0.0091 0.0033 61.7934 2.55 30.28 95.82 412.3 3.7 0.0056 980 62.4476 0.0035 1.1001 61.5277 2.41 47.49 98.44 410.7 3.7 1.8965 1020 62.7302 0.0052 3.4097 61.5946 1.76 60.03 97.92 411.1 3.8 5.8727 1070 64.0214 0.0106 0.0000 60.8715 0.95 66.79 95.08 406.8 3.7 0 1120 64.8657 0.0129 9.9261 62.2378 0.86 72.91 95.18 414.9 4.2 17.1125 1180 64.6395 0.0103 1.7915 61.7963 1.48 83.45 95.46 412.3 3.8 3.0886 1240 62.8318 0.0036 1.4716 61.9530 1.98 97.54 98.48 413.2 3.8 2.5371 1300 62.9700 0.0121 9.9181 60.5503 0.30 99.70 95.39 405 10 17.0988 1400 102.9601 0.1404 42.3699 66.7348 0.04 100.00 62.60 442 28 73.0457 Sample number = TM33; mineral = muscovite; w = 12.05 mg; J = 0.004679 600 499.2759 1.6175 2.2808 21.5114 0.02 0.06 4.30 173 61 3.9321 700 74.5002 0.0872 0.0387 48.7319 0.51 1.61 65.41 370.5 3.5 0.0667 780 63.4705 0.0341 0.0303 53.3796 2.01 7.69 84.10 402.2 3.6 0.0522 810 55.8120 0.0092 0.0000 53.1020 1.69 12.84 95.14 400.3 3.6 0 850 54.8928 0.0083 0.0114 52.4227 3.01 21.97 95.50 395.7 3.6 0.0196 900 54.3272 0.0088 0.0056 51.7154 5.72 39.33 95.19 390.9 3.5 0.0097 940 52.6231 0.0039 0.0000 51.4546 8.68 65.66 97.78 389.2 3.5 0 980 53.4156 0.0064 0.0000 51.5051 4.70 79.93 96.42 389.5 3.5 0 1020 54.8943 0.0115 0.0091 51.4863 1.92 85.75 93.79 389.4 3.5 0.0157 1080 54.4019 0.0093 0.0000 51.6558 2.06 92.00 94.95 390.5 3.5 0 1140 53.2768 0.0036 0.0000 52.2071 1.74 97.28 97.99 394.3 3.6 0 1240 53.9998 0.0057 0.0000 52.3075 0.48 98.75 96.87 395.0 3.6 0 1400 63.1326 0.0140 0.0000 58.9888 0.41 100.00 93.44 439.7 4.0 0 Sample number = TM31; mineral = muscovite; w = 13.12 mg; J = 0.004560 600 2062.9160 6.7973 0.0000 54.2982 0.01 0.04 2.63 399 58 0 700 93.3615 0.1540 0.0396 47.8497 0.26 0.89 51.25 356.1 3.6 0.0682 760 70.1123 0.0543 0.0188 54.0787 0.51 2.57 77.13 397.6 3.6 0.0323 810 59.7989 0.0190 0.0241 54.1764 2.56 11.06 90.60 398.3 3.6 0.0415 850 56.2654 0.0075 0.0000 54.0551 1.97 17.59 96.07 397.5 3.6 0 900 54.8444 0.0032 0.0000 53.8864 4.21 31.56 98.25 396.4 3.6 0 940 54.2762 0.0020 0.0010 53.6698 6.13 51.88 98.89 394.9 3.5 0.0017 970 54.5972 0.0029 0.0083 53.7296 5.27 69.36 98.41 395.3 3.6 0.0143 1000 54.8380 0.0037 0.0000 53.7469 3.56 81.16 98.01 395.4 3.6 0 1040 54.8218 0.0035 0.0000 53.7945 1.21 85.19 98.13 395.8 3.6 0 1140 54.9719 0.0024 0.0000 54.2560 1.42 89.90 98.70 398.8 3.6 0 1200 55.4690 0.0050 0.0011 53.9751 2.96 99.73 97.31 397.0 3.6 0.0018 1400 69.0075 0.0472 0.0000 55.0600 0.82 100.00 79.79 404.1 8.7 0 Sample number = TM32; mineral = muscovite; w = 14.20 mg; J = 0.004734 680 336.9141 1.0007 0.1655 41.2319 0.13 0.30 12.24 321.0 7.9 0.2854 740 108.9103 0.1967 0.0000 50.7659 0.49 1.47 46.61 388.5 3.7 0 800 77.8633 0.0892 0.0000 51.4847 1.70 5.51 66.13 393.5 3.5 0 840 54.9206 0.0134 0.0098 50.9690 2.29 10.98 92.80 389.9 3.5 0.0169 880 51.4334 0.0026 0.0000 50.6671 3.83 20.12 98.51 387.9 3.5 0 920 50.8614 0.0008 0.0000 50.6346 2.98 27.23 99.55 387.6 3.5 0 960 51.2433 0.0008 0.0027 51.0053 8.62 47.79 99.54 390.2 3.5 0.0046 990 51.0594 0.0011 0.0000 50.7307 3.18 55.38 99.36 388.3 3.5 0 1020 51.0247 0.0026 0.0507 50.2527 1.90 59.92 98.48 385.0 3.5 0.053 1060 50.9623 0.0021 0.0198 50.3257 3.34 67.88 98.75 385.5 3.5 0.0341 1120 50.8751 0.0012 0.0063 50.5214 9.38 90.25 99.30 386.9 3.5 0.0109 1200 51.0611 0.0017 0.0188 50.5663 2.72 96.73 99.03 387.2 3.5 0.0325 1400 53.5723 0.0078 0.0387 51.2617 1.37 100.00 95.68 391.9 3.5 0.0666 * Radiogenic. Minerals 2020, 10, 580 10 of 20

Sample number = TM33; mineral = muscovite; w = 12.05 mg; J = 0.004679 600 499.2759 1.6175 2.2808 21.5114 0.02 0.06 4.30 173 61 3.9321 700 74.5002 0.0872 0.0387 48.7319 0.51 1.61 65.41 370.5 3.5 0.0667 780 63.4705 0.0341 0.0303 53.3796 2.01 7.69 84.10 402.2 3.6 0.0522 810 55.8120 0.0092 0.0000 53.1020 1.69 12.84 95.14 400.3 3.6 0 850 54.8928 0.0083 0.0114 52.4227 3.01 21.97 95.50 395.7 3.6 0.0196 900 54.3272 0.0088 0.0056 51.7154 5.72 39.33 95.19 390.9 3.5 0.0097 940 52.6231 0.0039 0.0000 51.4546 8.68 65.66 97.78 389.2 3.5 0 980 53.4156 0.0064 0.0000 51.5051 4.70 79.93 96.42 389.5 3.5 0 1020 54.8943 0.0115 0.0091 51.4863 1.92 85.75 93.79 389.4 3.5 0.0157 1080 54.4019 0.0093 0.0000 51.6558 2.06 92.00 94.95 390.5 3.5 0 1140 53.2768 0.0036 0.0000 52.2071 1.74 97.28 97.99 394.3 3.6 0 1240 53.9998 0.0057 0.0000 52.3075 0.48 98.75 96.87 395.0 3.6 0 1400 63.1326 0.0140 0.0000 58.9888 0.41 100.00 93.44 439.7 4.0 0 Sample number = TM31; mineral = muscovite; w = 13.12 mg; J = 0.004560 600 2062.9160 6.7973 0.0000 54.2982 0.01 0.04 2.63 399 58 0 700 93.3615 0.1540 0.0396 47.8497 0.26 0.89 51.25 356.1 3.6 0.0682 760 70.1123 0.0543 0.0188 54.0787 0.51 2.57 77.13 397.6 3.6 0.0323 810 59.7989 0.0190 0.0241 54.1764 2.56 11.06 90.60 398.3 3.6 0.0415 850 56.2654 0.0075 0.0000 54.0551 1.97 17.59 96.07 397.5 3.6 0 900 54.8444 0.0032 0.0000 53.8864 4.21 31.56 98.25 396.4 3.6 0 940 54.2762 0.0020 0.0010 53.6698 6.13 51.88 98.89 394.9 3.5 0.0017 970 54.5972 0.0029 0.0083 53.7296 5.27 69.36 98.41 395.3 3.6 0.0143 1000 54.8380 0.0037 0.0000 53.7469 3.56 81.16 98.01 395.4 3.6 0 1040 54.8218 0.0035 0.0000 53.7945 1.21 85.19 98.13 395.8 3.6 0 1140 54.9719 0.0024 0.0000 54.2560 1.42 89.90 98.70 398.8 3.6 0 1200 55.4690 0.0050 0.0011 53.9751 2.96 99.73 97.31 397.0 3.6 0.0018 1400 69.0075 0.0472 0.0000 55.0600 0.82 100.00 79.79 404.1 8.7 0 Sample number = TM32; mineral = muscovite; w = 14.20 mg; J = 0.004734 680 336.9141 1.0007 0.1655 41.2319 0.13 0.30 12.24 321.0 7.9 0.2854 740 108.9103 0.1967 0.0000 50.7659 0.49 1.47 46.61 388.5 3.7 0 800 77.8633 0.0892 0.0000 51.4847 1.70 5.51 66.13 393.5 3.5 0 840 54.9206 0.0134 0.0098 50.9690 2.29 10.98 92.80 389.9 3.5 0.0169 880 51.4334 0.0026 0.0000 50.6671 3.83 20.12 98.51 387.9 3.5 0 920 50.8614 0.0008 0.0000 50.6346 2.98 27.23 99.55 387.6 3.5 0 960 51.2433 0.0008 0.0027 51.0053 8.62 47.79 99.54 390.2 3.5 0.0046 990 51.0594 0.0011 0.0000 50.7307 3.18 55.38 99.36 388.3 3.5 0 1020 51.0247 0.0026 0.0507 50.2527 1.90 59.92 98.48 385.0 3.5 0.053 1060 50.9623 0.0021 0.0198 50.3257 3.34 67.88 98.75 385.5 3.5 0.0341 1120 50.8751 0.0012 0.0063 50.5214 9.38 90.25 99.30 386.9 3.5 0.0109 1200 51.0611 0.0017 0.0188 50.5663 2.72 96.73 99.03 387.2 3.5 0.0325 Minerals 2020, 10, 580 10 of 19 1400 53.5723 0.0078 0.0387 51.2617 1.37 100.00 95.68 391.9 3.5 0.0666 *Radiogenic

Minerals 2020, 10, 580 11 of 20

Figure 4. 40Ar-39Ar weighted mean plateau and Ca/K ration spectra for muscovite from schist and Figure 4. 40Ar-39Ar weighted mean plateau and Ca/K ration spectra for muscovite from schist and monzogranite. (a) and (b) diagrams of plateau and Ca/K ration spectra for garnet muscovite schist monzogranite.(B1193- (1);a, b(c)) diagramsand (d) diagrams of plateau of plateau and and Ca/ Ca/KK ration ration spectra spectrafor for garnetmuscovite muscovite quartz schist schist (TM33); (B1193-1); (c,d) diagrams(e) and of(f) plateau diagrams and of plateau Ca/K rationand Ca/K spectra ration for spectra muscovite for biotite quartz monzogranite schist (TM33); (TM31); ( e(g,f) )and diagrams (h) of plateau anddiagrams Ca/K ration of plateau spectra and for biotiteCa/K monzogranite ration spectra (TM31);for muscovite (g,h) diagrams monzogranite of plateau (TM32). and Ca/K ration spectraWMPA for—weighted muscovite mean monzogranite plateau age. (TM32). WMPA—weighted mean plateau age.

6. Discussion

Minerals 2020, 10, 580 11 of 19

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) (Figure4c), ◦ ◦ ± 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) (Figure4e), ◦ ◦ ± 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) (Figure4g), ◦ ◦ ± 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 [19,20,38,41,44–46]. 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 (Figure5a). Nearly all of them are plotted in the fields of A-type granite in the Na O + K O vs. 10,000 Ga/Al diagram (Figure5b) and show a post-orogenic a ffinity 2 2 × (A2 type granite) (Figure6). 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 [43–45]. 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. Minerals 2020, 10, 580 12 of 20

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 [19,20,38,41,44–46]. 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 [43–45]. Besides, the reasons leading to thermal Mineralschange2020 of , 10 overriding, 580 plate and the resultant melting of lithospheric mantle as well as 12 lower of 19 continental crust are not clear either.

Minerals 2020, 10, 580 13 of 20 Figure 5. (a) Total alkali (K2O + Na2O) vs. silica (SiO2)[68]; (b) total alkali (K2O + Na2O) vs. Figure 5. (a) Total alkali (K2O + Na2O) vs. silica (SiO2) [68]; (b) total alkali (K2O + Na2O) vs. 10,000 Ga/Al [69] (data from [19,20,38,41,44–46]). × 10,000×Ga/Al [69] (data from [19,20,38,41,44–46]).

Figure 6. Plots of Nb-Y-Ce for the Devonian alkaline and mafic-ultramafic rocks in the northern

Figure 6. PlotsNCC of (areas Nb-Y-Ce of A1 and for A2the modified Devonian after [70]; alkalinedata from [19 and,20,38 mafic-ultramafic,41,44–46]). rocks in the northern NCC (areas of A1 and A2 modified after [70]; data from [19,20,38,41,44–46]). Subsequent to initial collision, several regimes are regarded as triggers for post-collision magmatism: (1) delamination of lithospheric mantle [71–73]; (2) convective thinning of the Subsequentlithosphere to initial [74 collision,] and (3) breakoff several of regimes subdutingare slab regarded [75–78]. Both as triggers delamination for and post-collision convective magmatism: (1) delamination ofthinning lithospheric models would mantle cause [71 some–73 ];recognizable (2) convective effects, thinning such as rapid of uplift the lithosphere of overriding [ plate74], and (3) breakoff of temperature rise due to upwelling of mantle asthenosphere and accompanying magmatism transfer subduting slabfrom [75 – early78]. Bothasthenospheric delamination mantle and melts, convective followed by thinning increasingly models crustally would contaminated cause some recognizable effects, such ascalc rapid-alkaline uplift melts of to overridingcrustal partial plate,melts [71 temperature–73]. However, risethe two due models to upwelling would lead to of diffuse mantle asthenosphere and accompanyingand nonlinear magmatism zones of magmatism, transfer from which earlyis incompatable asthenospheric with the linear mantle distribution melts, of Devonian followed by increasingly rocks along the northern NCC. crustally contaminatedThe slab calc-alkaline breakoff, also melts referred to to crustal as slab partial detachment, melts occurs [71 when–73]. part However, of a subducted the two models would lead to diffuselithospheric and nonlinear plate detaches zones and of magmatism, sinks into the mantle which [79 is,8 incompatable0]. Slab breakoff will with lead the to linear the distribution of upwelling of hot and dehydrated subslab asthenosphere and resultant heating up of the overriding Devonian rockslithosphere along;the thermomechanical northern NCC. modeling of the slab-detachment shows large (500 °C) transitory The slab breakoheating offf the, also base referredof the upper to plate as slab for several detachment, million years occurs due to whenupwelling part mantle of afollowing subducted lithospheric the slab detachment [81]. Therefore, the melting of enriched layers and lower continental crust will plate detachesbe possible, and sinks causing into the formation the mantle of Devonian [79, 80alkaline]. Slab magma breako and maficff -willultramafic lead rocks to. Besides, the upwelling of hot and dehydratedthe propagation subslab of asthenosphere slab breakoff will produce and resultant a linear zone heating of lithospheric up heating of the and overriding the area lithosphere; thermomechanicaloverlying modeling the breakoff of line the is slab-detachment therefore predestined to shows form a large tectonic (500 fault [75C)]. transitory The magmatism heating of the base also occurs along a narrow linear zone roughly parallel to the trench and is confined◦ in time, which of the upper platemakes forslab severalbreakoff distinct million from years other duemodels to for upwelling syncollisional mantle and post following-collsional magmatism, the slab detachment [81]. Therefore, thesuch melting as subduction of enriched melting, delamination, layers and or thermal lower boundary continental layer detachment crust will [75]. beRegarding possible, causing the the fact that the Devonian alkaline rocks develop in a linear zone along the northern NCC (Figure formation of Devonian1b), the slab breakoff alkaline will magmabe the most and suitable mafic-ultramafic model for this distribution rocks. of Besides,the alkaline therocks. propagation In of slab breakoff willaddition produce, the aslab linear breakoff zone is followed of lithospheric by a period of uplift heating that reaches and theabout area 1.5 km overlying within about the breakoff line 20 Ma [79]. Slab breakoff has caused uplift and exhumation because the dynamic subsidence related is therefore predestinedto subduction stopped to form coevally a tectonicwith the removal fault of [ 75large]. loads The of magmatism lithosphere mantle also [76]. occursThe rare along a narrow linear zone roughlypreservation parallel of Devonian to the strata trench in the northern and is NCC confined also proves in time,the existence which of uplift makes event slab [82]. breakoff distinct from other modelsBesides, forin comb syncollisionalination with the and heat post-collsional input from upwelling magmatism, asthenosphere, suchthe uplift as subduction and melting, exhumation would lead to thermal metamorphism in the overriding plate. However, this relevant delamination,Devonian or thermal metamorphism boundary has rarely layer been detachmentreported before. T [he75 muscovite]. Regarding 40Ar-39Ar ages the presented fact that the Devonian alkaline rockscould develop constrain in the a lineartime interval zone of alongthe Devonian thenorthern metamorphism NCC and (Figureexhumation1b), process the in slab the breako ff will be study area. Therefore, the magmatism, deposition history, and metamorphism during Early-Late the most suitable model for this distribution of the alkaline rocks. In addition, the slab breakoff is

Minerals 2020, 10, 580 13 of 19 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 beMinerals a suitable 2020, 10 model, 580 for the post-collsion evolution. 14 of 20 Based on the previously reported alkaline magmatism and muscovite ages obtained in this study, Devonian in the northern NCC testified slab breakoff would be a suitable model for the the Devonian post-collisional extension could be caused by slab breakoff, and an evolutionary model post-collsion evolution. in northern NCCBased from on the Ordovician previously reported to Early-Late alkaline Devonian magmatism andis proposed muscovite (Figureages obtained7). The in this Paleo-Asian Ocean lithosphericstudy, the plate Devonian subducted post-collisional southward extension during could the be Ordovician-Silurian caused by slab breakoff (c. 485–420, and an Ma), at the same timeevolutionary that the calc-alkaline model in northern magmatism NCC from Ordovician with arc atoffi Earlynities-Late developed Devonian is inproposed Bainaimiao (Figure island arc. 7). The Paleo-Asian Ocean lithospheric plate subducted southward during the Ordovician-Silurian A back-arc basin might exist at this time (Figure7a). Later, the Bainaimiao island arc collided with (c. 485–420 Ma), at the same time that the calc-alkaline magmatism with arc affinities developed in the northernBainaimiao NCC, andisland as arc. direct A back consequence-arc basin might the exist continental at this time crust (Figure and 7a). lithosphere Later, the Bainaimiao mantle thickened. The collisionisland caused arc collided regional with metamorphism the northern NCC, of Jianshanand as direct Formation consequence at c.the 412 continental Ma. Meanwhile, crust and the lower part of oceaniclithosphere slab mightmantle neckthickened. (Figure The 7collisionb). Finally, caused slab regional breako metamorphismff led to the of upwelling Jianshan Formation of asthenosphere at c. 412 Ma. Meanwhile, the lower part of oceanic slab might neck (Figure 7b). Finally, slab breakoff mantle asle welld to as the the upwelling intrusion of asthenosphere of alkaline mantle rocks as and well the as mafic-ultramaficthe intrusion of alkaline complexes. rocks and The the resultant uplift causedmafic the-ultramafic Jianshan complexes Formation. The resultant and monzogranites uplift caused the Jianshan intruding Formation into it and to bemonzogranites metamorphosed at c. 397–389intruding Ma (Figure into7 c).it to Overall, be metamorphosed the alkaline at magmatism c. 397–389 Ma as well (Figure as uplift7c). Overall, and extension the alkaline revealed by muscovitemagmatism40Ar-39Ar as dating well as uplift will beand useful extension for revealed the recognition by muscovite of 40 slabAr-39Ar breako datingff willin ancientbe useful orogeny.for the recognition of slab breakoff in ancient orogeny.

Figure 7. Cont.

Minerals 2020, 10, 580 14 of 19 Minerals 2020, 10, 580 15 of 20

Figure 7. Simplified cross-sections illustrating the tectonic evolution of the northern margin of the Figure 7. Simplified cross-sections illustrating the tectonic evolution of the northern margin of NCC. (a) Ordovician-Silurian, the southward subduction of the Paleo Asian Ocean lithospheric the NCC. (platea) Ordovician-Silurian,; (b) Early Ordovician, the the collision southward between subductionthe Bainaimiao of arc the and Paleo the NCC Asian, the Oceanlithosphere lithospheric plate; (b) Earlymantle Ordovician,thickened, and theJianshan collision Formation between went through the Bainaimiao regional metamorphism arc and the at NCC,c. 412 Ma the; and lithosphere mantle thickened,(c) Early-Late and Devonian, Jianshan post Formation-collision extension went, throughJianshan Formation regional, an metamorphismd associated monzogranites at c. 412 Ma; and (c) Early-LatewereDevonian, metamorphosed post-collision at c. 397–389 Ma extension,. Jianshan Formation, and associated monzogranites were metamorphosed7. Conclusions at c. 397–389 Ma. 7. ConclusionsThe 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 The 40twoAr- muscovite39Ar datings monzogranites of garnet yield muscoviteed 40Ar-39Ar schist ages of and 397 ± muscovite 2 Ma and 389 quartz ± 2 Ma. schist Based from on those the Jianshan Formationfindings of the Bayan and available Obo Group information yielded from ages literature of 412 we 3 assume Ma and that 392 the Jianshan3 Ma, respectively, Formation wheras experienced regional metamorphism at c. 412 Ma due to± the collision between± the Bainaimiao arc two muscovite monzogranites yielded 40Ar-39Ar ages of 397 2 Ma and 389 2 Ma. Based on those and the northern NCC. Later, the Jianshan Formation and associated± were metamorphosed± during findings andan exhumation available informationprocess at 397– from389 Ma literature linked to post we-collision assume extension that the. Jianshan Formation experienced regional metamorphismThe northern at NCC c. 412 was Main a duepost- tocoll theision collision extension between setting in the Early Bainaimiao Devonian arc, which and was the northern NCC. Later,probably the Jianshan induced by Formation slab breakoff. and The slab associated breakoff led were to the metamorphosed upwelling of subslab during asthenosphere, an exhumation causing the emplacement of Devonian alkaline and mafic-ultramafic complexes as well as the process atdevelopment 397–389 Ma of linkedthermal tometamorphism post-collision in a extension.linear zone of the northern NCC. The northern NCC was in a post-collision extension setting in the Early Devonian, which was probably inducedAuthor Contributions: by slab breako Conceptualization,ff. The slab C.L. breako and Z.L.ff led; investigation, to the upwelling C.L., Z.L, ofZ.X., subslab S.L., and asthenosphere, Q.S.; writing—original draft preparation, C.L.; writing—review and editing, Z.L., Z.X., S.L., and X.D.; visualization, causing theC.L., emplacement S.W., F.F.; funding acquisition of Devonian, Z.L., Z.X., alkaline S.L. All authors and have mafic-ultramafic read and agreed to the complexes published version as wellof as the developmentthe manuscript. of thermal metamorphism in a linear zone of the northern NCC. Funding: This research was funded by National Natural Science Foundation of China (Grant 41872203; Author Contributions:41872194, and 41872234Conceptualization,) and Supported by C.L. Opening and Foundation Z.L.; investigation, of Key Laboratory C.L., of Z.L., Mineral Z.X., Resources S.L. and Q.S.; writing—originalEvaluation draft in Northeast preparation, Asia, Ministry C.L.; writing—reviewof Natural Resources(DBY and- editing,ZZ-18-08).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 Acknowledgments: Thanks to Wen Chen of Isotope Geology Laboratory, Institute of Geology, Chinese the manuscript. Academy of Geological Sciences for providing assistance in muscovite 40Ar-39Ar dating. We also thank LetPub Funding: This(www.letpub.com) research was for funded its linguistic by National assistance Naturalduring the Science preparation Foundation of this manuscript. of China (Grant 41872203; 41872194, and 41872234)Conflicts and of Supported Interest: The by authors Opening declare Foundation no conflict of interest. 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. Minerals 2020, 10, 580 15 of 19

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