Reviews on the Paleozoic-Mesozoic granitoids and sedimentary rocks in North Korea

2.1 Phyongnam Basin

The Phyongnam Basin in the Nangrim Massif is featured by Neoproterozoic to Triassic strata depositing unconformably on the Archean and Early Paleoproterozoic basement. The Lower Paleozoic sediments of the Hwangju Supergroup in the basin is exclusively of marine origin with subordinate siliciclastic rocks. The maximum thickness of this supergroup could reach 3,000 m, with an average thickness of 1,200-1,500 m. The Hwangju Supergroup was subdivided into the lower Hwangju and upper Bopdong Groups (Paek et al., 1996; Park, 2012) the former one is dominated by marine terrigenous clastic deposits, while the later one is occupied by carbonate rocks.

The Hwangju Group is subdivided into conformably laid series in ascending order, namely the Pyongsan, Junghwa, Hukgyo and Rimchon Formations. The Pyongsan Formation is mainly composed of phosphate siltstones, carbonaceous slates, nodular phosphorites and Dolomites. Among them, the phosphorous and sulphide-bearing black slate at the bottom of this formation were considered as the boundary layer (Kim et al., 2018). The Junghwa Formation consists of sandstone, siltstone, slate, and carbonate rock, with phosphate bearing siltstone at the bottom. The Hukgyo Formation is made up of terrigenous clastic rocks locally interlayered with carbonates. The Rimchon Formation is mainly composed of black phyllite with a limestone interlayer at the lowest part. The hsuapis coreanicus and redlichia belts occurred in the Pyongsan and Junghwa Formations, respectively, showing that these two formations should not be lower than Stage 3 and 4 of the Cambrian (Kim et al., 2018). Carbon isotope study show that the δ¹³C values in the Pyongsan Formation range from -3.1‰ to 0‰, Junghwa -4.7‰ to +2.0‰, Hukgyo -1.0‰ to +2.4‰, and Rimchon -2.6‰ to +0.4‰ (Kim et al., 2016, 2018). Compared to the global carbon isotope of carbonate rocks, the Pyongsan, Junghwa and Hukgyo Formations belong to Stages 3-4 of Cambrian, and the Rimchon Formation is around the boundary of Series 2 and Miaoling Series, respectively (Kim et al., 2018). The positive carbon excursion in the upper Junghwa and Hukgyo Formations and negative in the lower Rimchon Formation are comparable to the global events of the mIngxinsi carbon isotopic excursion and red-lichiid-olenellid extinction carbon isotopic excursion, respectively (Kim et al., 2018).

The Bopdong Group is subdivided into the Mujin, Kophung, Singok, Mandal, Sangsori, Koksan and Wolyang Formations, with a thickness of about 1,000 m. The first two formations are assigned to the Late Cambrian, middle three are Ordovician, and the last two are Silurian (Paek et al., 1996). The Mujin Formation is mainly composed of clay limestone, manganese-bearing limestone and layered limestone. The Kophung Formation is successive sequences of lower dolomite and limestone and upper dolomite and limestone. The Singok Formation consists mainly of dolomite with minor limestone. The Mandal Formation consists of lower limestone and upper dolomite. The Sangsori Formation consists of dolomite interlayered with mudstone, shale, and limestone. The Silurian Koksan and Wolyang Formations are only sporadically exposed in the Singye, Koksan and Popdong areas. The Koksan Formation is composed of siltstone, mudstone, shale and limestone, with a layer of siltstone at the bottom. The Wolyang Formation is mainly composed of limestone, with minor mudstone and dolomitic limestone (Paek et al., 1996). Carbon isotope study shows that the δ¹³C values in the Mujin Formation range from -1.3‰ to 0.4‰ and the Kophung Formation displays -1.0‰ to +2.4‰ (Kim et al., 2018). Synthetically, the Kophung Formation belongs to the Furong Series of Cambrian, and the positive carbon excursion in the middle of the Kophung Formation is comparable to the global events of the Steptoean positive carbon isotopic excursion (Kim et al., 2018). Combing with biostratigraphy and regional stratigraphic correlation, the Bopdong Group was dated to be Cambrian Miaoling Series to Silurian (Kim et al., 2018).

The Upper Paleozoic (Late Carboniferous)-Early Triassic sediments were named the Pyeongan Supergroup, which unconformably overlies the Lower Paleozoic Hwangju Supergroup (generally Mandal Formation of the Bopdong Group), and unconformable covered by the Upper Mesozoic strata. This supergroup, approximately 1,500m thick, consists of paralic to nonmarine clastic deposits, and is characterized by high-quality coal seams and aluminous rocks. It was subdivided into the Hongjom, Ripsok, Sadong, Kobangsan and Taezhawon Formations in ascending order.

The Hongjom and Ripsok Formations were mainly composed of shale and sandstone, with thin coal measures in the upper part. The Sadong Formation (120-150 m thick) is the main coal bearing strata, and consists of shales, siltstone and sandstone with coal measures in the lower and middle parts, and aluminum rich shale and quartz sandstone in the upper part. The Kobangsan and Taezhawon Formations are mainly composed of shale and siltstone, with minor coal measures in the lower part, and silicified wood in the upper part. Above coal-bearing strata show that the paleoclimate was probably warm and arid with somewhat humid conditions.

All formations of the Pyeongan Supergroup are fossiliferous. The Hongjom Formation contains the index fusulinid fossils of the early Middle Carboniferous (Eostaffella angusta Kir, Mediocris ovalis Vissar., Millerella kasakstanica) in the lower part and those of the late Middle Carboniferous (Profusulinella, Dagmarella and Aljutovella) in the upper part. The Ripsok Formation contains the Triticites zone and plant fossils (Neuropteris pseudovata) and the Sadong, Kobangsan and Taezhawon Formations contain the index fossils of Lower to Upper Permian (e.g., Callipteris conferta, Tingia carbonica, T. hamaguchii, Lobatannularia heianensis, P. norinii) (Paek et al., 1996). On the basis of biostratigraphy, the Hongjom and Ripsok Formations were dated to be Middle and Upper Carboniferous, Sadong Formation was Lower Permian, and Kobangsan and Taezhawon Formations were dated as Upper Permian, respectively. Detrital zircons in sandstone from the Taezhawon Formation yield ages ranging from 231 Ma to 2529 Ma (Zhang, unpublished data), and the youngest group displays a weighted mean ²⁰⁶Pb/²³⁸U age of 231 ± 5 Ma, indicating a depositional time later than 231 Ma for the Taezhawon Formation.

2.2 Imjingang Belt

The Imjingang Belt is located between the Phyongnam Basin and the Gyeonggi Massif, separated by faults on all sides from the surrounding rocks of mainly Paleoproterozoic ages. This belt had been suggested as the eastward extension of the major Chinese suture zone (the ultra-high pressure Qinling-Dabie-Sulu Belt) (e.g., Yin and Nie, 1993; Ree et al., 1996; Oh and Kusky, 2007). However, Zhai et al. (2007) proposed that the Imjingang Belt does not exhibit any metamorphic characteristics of collisional orogenic belts. Moreover, Paek and Rim (2005) suggested the belt as a Middle Devonian to Lower Carboniferous rift zone of 15-30 km wide with tens of kilometers long in the Nangrim Massif. The belt does not extend throughout the KP with an eastern terminal in the middle peninsula.

Sedimentary strata in the Imjingang Belt were divided into the Yeoncheon Complex (or Yeoncheon Group) and Imjin System (or Imjin Group) (Cho et al., 1995; Paek et al., 1996; Ree et al., 1996). The Yeoncheon Complex was cropped out in the south of the 38^th parallel in South Korea (SK), and mainly consists of metamorphosed flysch deposits (Paek et al., 1996). Detrital zircon U-Pb ages of ca. 400 Ma and 420 Ma suggested a Devonian depositional time for the Yeoncheon Complex (Cho et al., 2001, 2007; Kim et al., 2014).

However, the Imjin Group, located in the north of the 38^th parallel, is a volcano-clastic sedimentary stratum with the thickness more than 2,000 m (Paek et al., 1996). It is in tectonic contact with Proterozoic or even older strata, and the rocks near the margin of the Imjingang Belt have commonly suffered strong deformation and mylonitization. The metamorphic grade of this group increases from north to south, spanning from unmetamorphosed through sericite to garnet, staurolite and kyanite zones, defining a Barrovian-type facies series. This group is about 15 to 30 km in width and several hundred kilometers in length, with the eastern and western parts that divided by the Ryesonggang fault zone. The eastern part stretches as continuous volcanoclastic sedimentary strata from the estuary of the river Ryesong through Kaesong to the Cholwon area, while the western part is distributed in the Ongjin, Kangryong and Paechon districts, with outcrops at Kangryong extending into the South Yellow Sea.

The lower part of the Imjin Group mainly consists of epicontinental-shallow water terrigenous clastic sedimentary strata with thin terrigenous intercalations, while the upper part comprises volcano-sedimentary strata with terrigenous clastic rocks. It can be divided as three formations with conformable relationships, the Anhyop, Puap and Sannyong Formations, in ascending order.

The Anhyop Formation covers an area of ~10 km² in the northern margin of the sedimentary basin, and consists of calcareous schist, chlorite schist, phyllite, psammitic conglomerate, calcareous conglomerate, quartz sandstone and limestone. On the basis of the biostratigraphy, this formation was confirmed to be Lower-Middle Devonian (Kim, 2012). The Puap Formation consists of shale, siltstone, sandstone and conglomerate with the intercalation of carbonate rocks, which contains Middle-Late Devonian standard fossils, e.g., charophyta fossil sycidium melo var. puskowensis, and Cyrtospirife (Kim, 2012). The Sannyong Formation comprises grayishgreen massive siltstone, biotite spotted and massive siltstone, aleuritic shale, dark green shale and sandstone. Zircons from the upper siltstone and sandstone of the Puap Formation yield the youngest ages at an interval of 393 ± 8 Ma and 423 ± 5 Ma (Li et al., 2004; Zhang et al., 2018), which indicate that the Puap Formation was deposited at a maximum age of ca. 400 Ma. Moreover, the Puap Formation shows similar detrital zircon age spectra features to those of representative Paleozoic sequences in the Pyongnam and Taebaeksan Basins of the KP and Paleozoic basins in the NCC, suggesting that the NCC and KP might experience similar tectonic evolution, and the Imjingang Belt might not be the eastward extension of the Qinling-Dabie-Sulu Collisional Belt (Zhang et al., 2018).

2.3 Tumangang Belt and Kwanmo Submassif

The Upper Paleozoic Tuman Group is well developed in the Tumangang Belt (also called the Hambuk Massif), and is also exposed sporadically in the Kwanmo Submassif (Fig. 1, 3). This group consists of the Amgi, Kyeryongsan and Songsang Formations in ascending order, with an exposure area around 1,600 km² (Paek et al., 1996; Kim, 2012).

Fig. 3.

Sketch geological map of the Tumangang Belt and Kwanmo Massif (modified after Paek et al., 1996).

The Amgi Formation (360-950-m-thick) is mainly distributed in the Chongjin-Najin area, unconformably overlying the Precambrian rocks. It is a suit of terrigenous and volcanic clastic rocks, which consists of shale, slate, biotite siliceous schist, metamorphic sandstone, amphibolite, siliceous limestone and crystalline limestone lens. The Kyeryongsan Formation is a 570-1200-m-thick sequence of volcanic sedimentary rocks, and almost located in the Chongjin, Hoeryong and Kyongwon areas. The lower part is dominated by the tuffite and tuffaceous rocks, with minor shale, siliceous schist and limestone, whereas the upper part consists mainly of light metamorphic mafic volcanic rocks, porphyry and tuff. At the basal part, volcanic breccia was well developed, which was considered as the boundary with the underlying Amgi Formation. Limestone lenses and tuffaceous sandstones are an occasionally observed in the upper part. The Songsang Formation consists of terrigenous clastic rock, including siltstone, mudstone, shale with minor tuffaceous sandstone and limestone. Many fossils were hosted in these Formations, including fusulinids, brachiopods, crinoids, bryozoans, and plant fossils (Paek et al., 1996).

Zhang et al. (2016) reported the SIMS U-Pb age of 257 ± 3 Ma from the basaltic andesite (13NK-1) in the Kyeryongsan Formation, and suggested that this formation was formed in the Late Permian. Moreover, basaltic andesite are characterized by positive zircon ε_Hf(t) values (+10 to +16), and fairly young T_DM (262-513 Ma). Synthesizing other geochemical features, the generation of the basaltic andesite most likely involves a precursory metasomatism stage of mantle peridotites by melts from subduction-related sediments and an immediate partial melting stage (Zhang, X.H. et al., 2016).

4.1 Late Permian-Triassic rocks

Twenty five Late Permian-Triassic ages was obtained. Among them, three Late Permian ages (265-263 Ma) were obtained from the Rimmyongdong biotite granite and Guanmobong fo-liated granite and biotite granite, other ages are Triassic, ranging from 248 Ma to 211 Ma. These granites are mainly distributed in the Kwanmo Submassif, Nangrim Submassif and south margin of the Phyongnam Basin, and divided into calc-alkaline igneous rocks and alkaline rocks. The calc-alkaline igneous rocks include diorite, quartz diorite, tonalite, granodiorite, and monzogranite. Representative plutons are the Guanmobong batholith in the Kwanmo Submassif, the Pukjin-Unsan, Komsanyong, and Jongphyong batholithes in the Nangrim Massif.

The Guanmobong batholith in the Kwanmo Submassif is exposed area over 3,000 km², and it extends northward into China, where it is called the Bailiping-Gaoling batholith. This batholith shows a wide range of lithological types, including gabbro, through diorite, biotite granite, granodiorite and monzogranite. Among the eight dated samples, one foliated granite and one biotite granite yielded Paleozoic U-Pb zircon ages of 265 ± 2 Ma and 263 ± 2 Ma, and two monzogranites revealed Early Triassic ages of 248 ± 2 Ma and 241 ± 2 Ma, and the remaining 4 samples (diorite, biotite granite, granodiorite and monzogranite) yielded Jurassic ages, ranging from 190 ± 3 Ma to 173 ± 1 Ma. Above age data show that the Guanmobong batholith was a composite one which formed from Late Paleozoic to Jurassic. Similarity, four magmatic stages are distinguished from the Bailiping-Gaoling batholith: Permian (285 ± 6 Ma), Triassic (248 ± 2~245 ± 6 Ma), Jurassic (192 ± 2~170 ± 3) and Cretaceous (119-116 Ma) (Zhang et al., 2004). The main part of the batholith was formed during Triassic to Jurassic. In addition, Jurassic diorite, biotite granite and granodiorite from the Guanmobong batholith are characterized by positive ε_Hf (t) values (4.9 ~ 10.6), fairly young T_DM2 (Hf) (500-784 Ma) and uniform O isotopic composition (δ¹⁸O: 5.7 ~ 6.9‰) (Fig. 6). Nevertheless, most of the Permian and Jurassic biotite granite and monzogranite present lower ε_Hf(t) values (-10.9 ~ 3.8), older T_DM2 (Hf) (889-1651 Ma) and large variation of O isotopic composition (δ¹⁸O: 5.6 ~ 9.2‰) (Zhang, Y.B. et al., 2016).

Fig. 6.

Hf-O isotopes of the zircons for Phanerozoic granitoids in the Kwanmo Massif, Korean Peninsula (after Zhang, Y.B. et al., 2016).

Late Triassic alkaline (syenite) plutons in NK are known as the Phyonggang Complex, which previously thought as formed in Permian, with the K-Ar ages ranged from 300 Ma to 190 Ma (Paek et al., 1996). These plutons mainly distributed in the south margin of the Phyongnam Basin, with minor sporadic exposed in the Nangrim Massif. In the Phyongnam Basin, the Phyonggang Complex are mostly distributed along major faults, and consist of more than ten syenite plutons and some small satellite bodies (e.g., Tokdal, Yonan, Ryeui, Kangbuk, Solhwasan, Sokthandong, Jodoksan), with the total area of about 530 km². The syenite plutons composed of Na-pyroxene syenite, quartz syenite, nepheline syenite, alkali amphibolite, gabbro, pyroxenite, and kimberlite, with composition changed from sodic to potassic from west to east (Paek et al., 1996). Peng et al. (2016) divided these plutons into the northern (in the Nangrim Massif) and southern (in the Phyongnam Basin) syenite belts. The Pukjin pluton, belonging to the northern syenite belt, shows a U-Pb zircon age of 234 ± 2 Ma (Wu et al., 2007). The Tokdal and Kangbuk Complexes crop out in the Hwanghae province, belonging to the southern syenite belt. The for-mer having an exposed area about 30 km², is consist of alkali pyroxenite, gabbro, alkali amphibole, clinopyroxene, biotite and/or nepheline syenite. A SHRIMP U-Pb zircon age of 224 ± 4 Ma was obtained from a biotite syenite sample (Peng et al., 2008). The latter having an exposed area about 50 km², is mostly composed of biotite syenite, with minor quartz syenite and pyroxene syenite. Peng et al. (2016) reported the Triassic age (229 ± 1 Ma, SIMS U-Pb) of the biotite syenite. In generally, these syenite plutons formed at ca. 234-220 Ma.

4.2 Jurassic rocks

The Jurassic granites are well developed in NK, with the ages ranged from 199 Ma to 163 Ma (Table 1). Early Jurassic granites are mostly exposed in the Tumangang Belt and Kwanmo Submassif, and Middle Jurassic granites are mainly distributed in the central KP (Fig. 3).

The Tumangang Belt and Kwanmo Submassif were characterized by large volumes of granitoids, which are known as the Tumangang Complex (Paek et al., 1996). New age data indicate that these granites were mostly emplaced during the Early Jurassic (199-183 Ma), which differs greatly from the previous view that most of them were in the Late Permian and Early Triassic age (Paek et al., 1996). Representative plutons are the Daedokri, Wonsanki, Gongsim, Sungam, and Musanri plutons. The rock types include diorite, quartz diorite, granodiorite, and biotite granites. These granites are I- and highly fractionated I-types, and most rocks belong to high-K calc-alkaline (Zhang, Y.B. et al., 2016). Zircons from these Jurassic granites are characterized by positive ε_Hf(t) values (+4.7 to +13.5), relatively young T_DM2(Hf) model ages (784-367 Ma), and uniform O isotopic compositions (δ¹⁸O = 5.7‰-7.4‰) (Zhang, Y.B. et al., 2016) (Fig. 7). Such Hf-O isotopic characteristics are likely indicative of a high proportion of juvenile material in their petrogenesis.

Fig. 7.

Hf-O isotopes of the zircons for Phanerozoic granitoids in the Tumangang Belt, Korean Peninsula (after Zhang, Y.B. et al., 2016).

In the central KP, the Jurassic granites are also well developed, with the emplacement ages ranged from 191 Ma to 163 Ma. The representative plutons include the Manthapsan, Tanchon, Ragwon, Sampyong-ri, Yangdok, Kosan, and Pukjin-Unsan plutons in the Nangrim Massif and Phyongnam Basin, and the Mulkilri, Kyejong-ri, Kaesong plutons in the Imjingang Belt.

The large Tanchon batholith is considered to be representative of the Jurassic granites, and the “Tanchon Complex-J₂” is the terminology used for all the Jurassic granites in the KP (Paek et al., 1996). However, a zircon age of 250 ± 5 Ma was obtained within this granitic pluton, and therefore it has been classified as the Triassic magmatism (Paek et al., 1996). The batholith consists of diorite, fine-grained biotite granite, medium- to coarse-grained biotite granite, and porphyritic biotite granite. New zircon U-Pb ages from 4 samples indicate the Tanchon batholith was a composite one, and mainly formed during Triassic (247-222 Ma) and Jurassic (181-168 Ma).

Most of other plutons also have complex rock types. For instance, the Yangdok pluton consists mainly of biotite granite, two-mica granite, and leucogranite; the Pukjin-Unsan batholith contains abundant rock types, including the diorite, biotite granite, two-mica granite, monzogranite, and syenite. Geochemically, biotite monzogranites from the Kyejong-ri pluton and porphyritic granite from the Yangdok pluton have tetrad REE patterns with large negative Eu anomalies, suggestive of extensive magmatic differentiation (Zhang et al., 2021). Some samples (e.g., the Kosan diorite and Tanchon and Yangdok biotite granites) are characterized by high Ba-Sr and low Y and heavy REE contents, high Sr/Y ratios, and strong light and heavy REE fractionation, which suggest that they are high Ba-Sr granites (Zhang et al., 2021).

In general, the Jurassic granites in the central KP are I-type or highly fractionated I-type ones, with small plutonic shapes or a part of composite batholiths, such as the Tanchon batholith.

4.3 Cretaceous rocks

The Cretaceous granites are distributed mainly in the eastern and western parts of the Nangrim Massif (Fig. 4), with the ages ranging from 127 Ma (granite sample PC-1 from the Pukjin-Unsan pluton) to 83 Ma (Table 1), represented by the Nampho and Kuwolsan batholithes in the west, and the Kosan and Kumgangsan ones in the east.

The Nampho and Kuwolsan batholithes were traditionally thought to be Triassic or Triassic-Jurassic age (Paek et al., 1996). However, recent study reveals that the rock types of the Nampo batholith include diorite, tonalite, monzogranite and syenogranite. Three magmatic stages were distinguished as Triassic (234-215 Ma), Jurassic (174 Ma) and Cretaceous (114-109). Seven samples (dioritic enclave, biotite granite, and porphyritic granite) from the Kuwolsan batholith yield uniform Early Cretaceous ages between 113 ± 2 and 109 ± 2 Ma (Wu et al., 2007), which suggests this batholith were formed in the Early Cretaceous.

The huge Kosan and Kumgangsan batholithes located in the east of NK, and were considered as Jurassic age (Paek et al., 1996). Nevertheless, medium-grained two-mica granite from the Kosan batholith yields zircon U-Pb age of 106±2 Ma, and Quartz diorite and biotite granites from the Kumgangsan batholith yield emplacement ages ranging from 100 Ma to 83 Ma. The ages of the Kumgangsan batholith show that the Late Cretaceous magmatism also occurred in the NK.

Combined with ages of volcanic rocks, the Cretaceous magmatism in NK mainly occurred from 136 to 83 Ma, with an age peak of ~110 Ma.

5.1 Tectono-stratigraphic correlation between the Phyongnam Basin, Taebaeksan Basin and some Paleozoic Basins in the NCC

It is an important geological issue that the relationship between the Paleozoic strata in the Phyongnam Basin and those in it’s neighboring tectonic blocks, e.g., the NCC. The Paleozoic sedimentary strata are mainly developed in the Hyesan-Rowon and Phyongnam basins in the northern KP, and the Taebaeksan Basin in the southern KP.

The Lower Paleozoic sediments, named the Hwangju and Joseon Supergroups in the Phyongnam and Taebaeksan basins, respectively, are exclusively of marine origin. Their general stratigraphic successions are very similar with each other (e.g., Lee and Lee, 2003; Li et al., 2016; Zhai, 2016; Zhai et al., 2019; Peng et al., 2020), with inner continental shelf carbonate depositional features. The Upper Paleozoic littoral coal-bearing sediments were named the Pyeongan Supergroup in both basins, and the paleoclimate was probably warm and arid with somewhat humid conditions when they were deposited. As a result, many large coalfields were formed, e.g., the Samcheok and Yeongweol ones in the Taebaegsan region, the Ripsok one in the Phyongnam Basin.

The NCC was stable during the Paleozoic. The well-developed Cambrian and Ordovician sedimentary rocks consist predominately of marine carbonate with minor fine clastic rocks (Chen and Qin, 1989). Late Ordovician strata are locally developed, and volcanic rocks from the constituent formations (the Zhaolaoyu, Pingliang, and Jinsushan Formations) yielded the eruption ages ranging from 449 Ma to 466 Ma (Chen et al., 2012; Wu et al., 2014; Wang et al., 2015). After a depositional hiatus in the Early Carboniferous, the NCC subsided again and then transgressional marine sediments with coal-bearing clastics and continental clastics deposited during the Late Carboniferous and Permian, respectively. The Late Paleozoic strata are the Benxi and Taiyuan Formations with Late Carboniferous age, whereas the Shanxi and Xiashihezi Formations are Early Permian strata, and the Shangshihezi and Shiqianfeng Formations are Late Permian strata (Chen and Qin, 1989).

Detail comparative studies show that the NCC (the Western Hill and Dalian basins in Beijing and Liaodong Peninsula, respectively) and the KP (the Taebaeksan and Pyongnam basins) have similar Paleozoic stratigraphic sequences (Zhai et al., 2007 and references therein). Furthermore, detrital zircon age spectra for representative Paleozoic sequences in the Pyongnam Basin, Taebaeksan Basin, Imjingang Belt, and the NCC show similar features, suggesting that the KP and NCC may share congruous provenances evolutional history during the Paleozoic (Zhang et al., 2018).

5.2 Tumangang Belt: a part of the Central Asian Orogenic Belt

The Tumangang (Domangang) Belt, situated in the northeastern part of the KP, was regarded as the southeastern extension of the eastern Central Asian Orogenic Belt (CAOB) (Paek et al., 1996; Kim, 2012; Zhang, X.H. et al., 2016; Zhang, Y.B. et al., 2016), or called the Hambuk Block comparable with the Jiamusi Block in the NE China (Lyang et al., 2009; Kim et al., 2012). This belt is characterized by the Late Paleozoic low-grade metamorphic volcano-sedimentary strata (Tumen Group) and granites. The mafic-ultramafic Chongjin Complex in this belt was considered as the ophiolite (Paek et al., 1996), and consists of a rock assemblage of peridotite, clinopyroxenite, gabbro, gabbroic diorite, hornblendite and doleritic dykes. Moreover, zircon SIMS U-Pb dating on distinct rock units document a prolonged emplacement ages from ca. 250 to ca. 170 Ma (Zhang et al., 2019). In addition, they exhibit a compact elliptical geometry, with linear array of stock clusters and crude lithological zonation from cores of peridotites to margins of hornblendites and gabbroic rocks. Furthermore, their geochemical characteristics are very similar to those of the classic Phanerozoic Alaskan-type complexes, e.g., SiO₂ ranged from 39.4% to 57.4%, enrichment in light rare earth elements (LREE), depletion in high field strength elements (HFSE), whole- rock (I_Sr(t) = 0.703062 to 0.705507, ε_Nd(t) = +3.80 to +12.2) and ε_Hf(t) values from + 12.5 to + 24.8, and zircon ε_Hf(t) values from -17.3 to +12.8 and δ¹⁸O from 4.92 to 10.4 ‰ (Zhang et al., 2019). Above features indicate the Chongjin Complex belongs to the Alaskan-type complexes, and their petrogenesis most likely involved precursory metasomatism of mantle peridotites by melts from subduction-related sediments with subsequent partial melting.

Large volumes of granitoids distributed in the Tumangang Belt, and the plutons extend into the Kwanmo Massif. The Jihei (Jilin-Heilongjiang) Orogenic Belt, part of the CAOB, which is connected with the Tumangang Belt, also distinguished by extensive distribution of Phanerozoic granites and volcanic rocks. Our age data show that the Phanerozoic magmatism in the Tumangang Belt can be divided into three stages, i.e., Late Permian (265-263 Ma), Early Triassic (248-240 Ma) and Early-Middle Jurassic (199-173 Ma). From the presently available data, the granites in the Jihei Orogenic Belt show a broadly similar chronological sequence at 285-230 Ma, 228-150 Ma and 130-110 Ma, and the Triassic and Jurassic granites distributed all over the area, occupying about 95% of the exposed granites. Consequently, we may infer that the Triassic and Jurassic magmatism took place synchronously across these tectonic units. Furthermore, these granites have the similar mineralogical and geochemical features; e.g., i) most of granites are I- and high fractionated I-types and belong to high calc-alkaline series, ii) although granites located in the north margin of the NCC (or the Nangnim Massif) yield relatively lower ε_Hf(t) values and older T_DM2 ages, most others display positive ε_Hf(t) values and fairly young T_DM2 ages. Such Hf isotopic characteristics of these granitoids are likely indicative of a high proportion of juvenile material in their petrogenesis. Overall, above similarity between granites in the Jihei Orogenic Belt and those in the Tumangang Belt suggested that the Tumangang Belt is a part of the CAOB.

5.3 Tectonic implications of Triassic granitoids in North Korea

The Triassic granitoids in NK can be divided into two groups: east-west-trending alkaline rocks (234-220 Ma) at the south margin of the Phyongnam Basin, and calc-alkaline igneous rocks (248-211 Ma) in the Nangrim Massif. These Triassic granitoids, especially alkaline rocks, have been considered to be formed in Syn- to post-collisional tectonic setting after collision between the North and South China blocks by some studies (e.g., Chough et al., 2000; Peng et al., 2008, 2016; Seo et al., 2010). However, Triassic granitoids are mainly distributed in the south part of the Dabieshan-Sulu Orogenic Belt, and the alkaline and gabbroic intrusive bodies (the Jiazishan complex) only outcropped in east margin of the Sulu Orogenic Belt and yield little younger isotopic ages of ~211-205 Ma (Guo et al., 2005). Furthermore, the basement of different massifs (the Nangrim, Gyeonggi and Yeongnam Massifs) are closely correlated to the NCC, and the Paleozoic basins in the KP have a similar Paleozoic tectono-stratigraphy to those in the NCC. These geological observations indicate that the NCC and KP might share congruous tectonic history in the Paleozoic (Zhai, 2016; Zhai et al., 2019), and further inferences that the Hongseong Complex might be an allochthonous tectonic slab and not the tectonic zone cut cross the KP (Zhai et al., 2007, 2016, 2019). Therefore, the influence of the continental collision between the North and South China blocks on the KP may be a local and not be as intensive as previously thought.

It should be noted that the Triassic granitoids are widespread in the NCC (Fig. 8). In addition, the Triassic alkaline rocks are distributed in east-west-trending belts along the north margin of the NCC, while calc-alkaline igneous rocks are mainly in the eastern NCC (Zhai, 2016; Zhai et al., 2016, 2019; Wu et al., 2019). By comparison in petrological observations, chronology and geochemical features, the Triassic granitoids and igneous rocks in the KP and NCC indicate that they were most likely formed in the same tectonic setting. Previous studies on the Triassic granitoids in the NCC proposed different models. While two main models ascribed them to post-orogenic extension related to the CAOB or the Sulu Belt, a third one resorted to intracontinental rifting within the NCC (e.g., Zhang et al., 2009; Yang et al., 2012; Wu et al., 2018; Zhu et al., 2020). The first model advocated the continuation of the CAOB orogeny until the Late Triassic. However, some geological and chronological data indicate that the CAOB orogenic event might have ended before the Permian, with the granite-alkaline-gabbro belt in the northern margin of the NCC representing a rift type. Other studies suggested that the Triassic magmatic rocks are related to the Dabie-Sulu orogeny (Oh and Kusky, 2007; Peng et al., 2016). However, since the 250-230 Ma granite-alkaline granite-gabbro rocks are widely distributed from the north through the middle to the south of the KP, and are older than the Jiazishan complex in the eastern Sulu belt, this makes them incompatible with the Sulu orogenic belt (Zhai et al., 2016b, 2019). Therefore, this favored the intracontinental rifting model. In a word, the same tectonic regime may serve as one of the mechanisms for forming the Triassic granitoids in both NK and the NCC. Given the tectonic position of NK in the Triassic, the formation of Triassic igneous rocks in the KP should be influenced by surrounding blocks including the South and North China blocks as well as the Pacific plate.

Fig. 8.

Distribution of Triassic magmatic rocks in the NCC and KP (modified from Peng et al., 2016; Zhai et al., 2016).