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Structure of hadaasan metamorphic core complex
1. Structural analysis and deformation characteristics of the Hadaasan metamorphic
core complex in the junction of Hangay and Hentii mountain ranges
Javkhlanbold D., Chuluun D., Bayanmunkh B., Bayartsengel Ts.,
Introduction
The geological setting of Mongolia has been explained as a result of a long period of north-
south convergence, leading to the formation of one of the largest accretionary complexes in
between the Siberian Craton to the North and the Sino-Korean and Tarim Cratons to the South,
and extends from the Urals in the West to the Sikhote-Alin Range in the Russian Far East the,
which called as Central Asian Fold/ Orogenic Belt (CAOB) [Zonenshain et al., 1990; Mossakovsky
et al., 1993; Badarch et al., 2002], also known as the Altaids [Suess, 1908; Şengör et al., 1993;
Şengör and Natal’in, 1996; Yakubchuk et al., 2001]. The CAOB is composed of different tectono-
stratigraphic terranes which accreted during the Paleozoic–Late Triassic [Sengör et al., 1993;
Sengör and Natal'in, 1996; Khain et al., 2002]. During the Jurassic–Early Cretaceous, large-scale
NW-SE extension that occurred throughout eastern Asia. This major tectonic event is marked by
the widespread occurrence of continental sedimentary basins [Watson et al., 1987; Allen et al.,
1998; Khand et al., 2000; Ren et al., 2002; Meng et al., 2003] described as NE-SW trending rift
basins [e.g.,Graham et al., 2001; Meng, 2003; Meng et al., 2003] and mainly filled with late
Jurassic to early Cretaceous sediments and volcanics and metamorphic core complexes
documented in Transbaikalia-northern Mongolia and northeastern China [Sklyarov et al., 1994,
1997; Davis et al., 1996, 2002; Webb et al., 1999; Darby et al., 2004; Liu et al., 2005; Mazukabzov
et al., 2006; Donskaya et al., 2008; Daoudene et al., 2009; Charles et al., 2011] Geochronological
studies point to core complex development during early Cretaceous times [Sklyarov et al., 1997;
Donskaya et al., 2008; Webb et al., 1999; Davis et al., 2002; Zhang et al., 2002, 2003; Lin et al.,
2008], with a likely peak of exhumation in a narrow time range between 130 Ma and 125 Ma
[Daoudene et al., 2011].
This paper presents new structural data on the Hadaasan metamorphic core complex in the
Harhorin area at the junction of Hangay and Hentii mountain ranges. The Hadaasan metamorphic core
complex means Dashinchilen and Hadaasan intrusive massifs that intrudes lower-middle carboniferous
terrigenic and Triassic molass units accumulated in the Orkhon reactivated depression [Zonenshain
1967;Antipin 1977].
3. Lithology and structure
Hanging wall: Coarse grained sediments including conglomerates and sandstones
of early-middle Triassic Avzaga formation classified in hanging wall block. This formation
is composed of conglomerates, gravelstones, sandstones, and siltstone. Conglomerate
is classified as basal conglomerates with well to middle rounded, weekly and non-sorted,
alluvial origin, and with gravel of Permian semi-alkaline granite and light yellow, yellowish
coarse grained sandy cement. Gravels within the conglomerate are oriented, which
indicates water flow direction during the sediment accumulation. Middle to coarse, rarely,
fine grained sandstones and brownish-green siltstone thin beds and lenses are marked
within the conglomerates. Some middle and poor preserved floras are founded in
siltstone’s thin beds and lenses. Hanging wall unit is mostly intercepted by imbricately
extended normal fault system often shows subsidiary normal faults either dipping in the
same direction. There are also some antithetic normal faults observed to the far north of
detachment fault, is forms roll-over antiform and small half graben filled lower Cretaceous
uncemented conglomerates. Normal faults closer to the Hadasan massif is more likely synthetic
listric behavior with curved fault plane and tilted blocks with sedimentary units dipping opposite
direction to the fault plane. Sediments of Avzaga formation is mostly horizontal and gently dipping
around 10-20°, and somewhere close to normal fault plane sedimentary unit is dipping around
30°. Quartz veins and aplite dykes are rarely mapped along the normal faults and joints.
Picture 2. The hanging-wall unit or conglomerates of the Avzaga formation
Detachment fault or ductile shear zone:
The ductile shear zone along NW flanks of the granitic and gneissic core is approximately 30 in
length and around 0.8 km in thickness (Fig. 2). This zone separates core units from overlying non-
metamorphic or weakly metamorphosed upper units. Mylonitization mainly occurred in the and
Carboniferous metasediments. Shallow-dipping foliations and gently dip-slipping lineations characterize
the mylonitic shear zone, which contains a large number of micro- and outcrop-scale kinematic indicators,
including oblique foliation, S-C structures, asymmetric folds, porphyroclasts or augens, extensional
crenulation cleavages (C0 ), stretching lineations, and sheared magmatic veins or boudins. C0 - foliation is
identifiable in most of the mylonites. The mylonitic shear zones on both flanks of the dome exhibit a
4. uniform top-tothe-northwest shear sense, and are regarded as two components from the same shearing
deformation. The northwest-dipping mylonitic shear zone is capped by normal faults to form the Yingba
detachment fault, with nearly down-dip slickenside striations on its surface. Fault breccias are
characterized by fragments of chlorite breccias and gouges, and clasts derived from Mesozoic sandstones,
quartzites from Proterozoic strata, granitic rocks and mylonitic rocks. Normal faults within the ductile
shear zone itself expectedly overprint its mylonitic fabrics.
Бүхэлдээ маш хүчтэй занаржсан, зарим хэсэгтээ ялангуяа зүсэлтийн доод
хэсэгт гнейслэг текстуртэй болсон байхаас сегрегацийн кварц, аплит, грейзен,
пегматойд судлуудаар жишүү болон нийцлэгдүү байдлаар хэрчигдсэн. Хэдэн
сантиметрээс хэдэн метр хэмжээтэй жижиг атираа элбэг ажиглагдахаас гадна том
хэмжээний флексур маягийн изгиб (нугларал), хэвтээ байрлалтай хөнтрүү
атираанууд тохиолдоно. Атирааны тэнхлэг унал болон сунал нь янз бүрийн
чиглэлтэй байх ба зонхилох уналын азимут нь хойд, баруун хойд, зүүн хойд чиглэлд
байна. Мөн налархай деформацийн нөлөөгөөр битүү атираа (sheath fold) үүсгэсэн
байдаг онцлогтой. Эхэнд үүссэн хөнтрүү атираа нь дараагийн шатны эсхүл
деформацийн төгсгөл шатанд дахин атираажсан байгаа нь үүгээр батлагдахаас
гадна дараагийн атираажлийн нөлөөгөөр үүссэн шүдлэг төрлийн (crenulation
foliation) фолиашн хөгжсөн байдаг. Микро атирааны тэнхлэгийн унал занаржилтийн
хавтгайтай ижил, зарим хэсэгт кварцын судланцарууд атираажиж тасарсан
байдалтай ажиглагдана (Зураг 3).
Эвэрхуурмаг ба мусковитын сунасан тэнхлэг тэдгээрийн чиглэлийн
хуваагдлаар хянагдаж байна. Хуваагдлын чиглэл зурвас текстурт 70-75° өнцөг
үүсгэжээ. Эрдсүүдийн сунал фолеишны хавтгайн чиглэлтэй жишүү байрласан байх
байна. Шлиф 4031 –д кварцын жижиг мэшил хэлбэртэй мөхлүүд занаржилтийн
хавтгайг дагасан байдалтай ажиглагдах ба унтралтын өнцөгөөр хэмжсэн кварцын
тэнхлэгийн сунал занаржилтийн хавтгайтэй жишүү байрлалтай хөгжсөн байдаг.
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![Structural analysis and deformation characteristics of the Hadaasan metamorphic
core complex in the junction of Hangay and Hentii mountain ranges
Javkhlanbold D., Chuluun D., Bayanmunkh B., Bayartsengel Ts.,
Introduction
The geological setting of Mongolia has been explained as a result of a long period of north-
south convergence, leading to the formation of one of the largest accretionary complexes in
between the Siberian Craton to the North and the Sino-Korean and Tarim Cratons to the South,
and extends from the Urals in the West to the Sikhote-Alin Range in the Russian Far East the,
which called as Central Asian Fold/ Orogenic Belt (CAOB) [Zonenshain et al., 1990; Mossakovsky
et al., 1993; Badarch et al., 2002], also known as the Altaids [Suess, 1908; Şengör et al., 1993;
Şengör and Natal’in, 1996; Yakubchuk et al., 2001]. The CAOB is composed of different tectono-
stratigraphic terranes which accreted during the Paleozoic–Late Triassic [Sengör et al., 1993;
Sengör and Natal'in, 1996; Khain et al., 2002]. During the Jurassic–Early Cretaceous, large-scale
NW-SE extension that occurred throughout eastern Asia. This major tectonic event is marked by
the widespread occurrence of continental sedimentary basins [Watson et al., 1987; Allen et al.,
1998; Khand et al., 2000; Ren et al., 2002; Meng et al., 2003] described as NE-SW trending rift
basins [e.g.,Graham et al., 2001; Meng, 2003; Meng et al., 2003] and mainly filled with late
Jurassic to early Cretaceous sediments and volcanics and metamorphic core complexes
documented in Transbaikalia-northern Mongolia and northeastern China [Sklyarov et al., 1994,
1997; Davis et al., 1996, 2002; Webb et al., 1999; Darby et al., 2004; Liu et al., 2005; Mazukabzov
et al., 2006; Donskaya et al., 2008; Daoudene et al., 2009; Charles et al., 2011] Geochronological
studies point to core complex development during early Cretaceous times [Sklyarov et al., 1997;
Donskaya et al., 2008; Webb et al., 1999; Davis et al., 2002; Zhang et al., 2002, 2003; Lin et al.,
2008], with a likely peak of exhumation in a narrow time range between 130 Ma and 125 Ma
[Daoudene et al., 2011].
This paper presents new structural data on the Hadaasan metamorphic core complex in the
Harhorin area at the junction of Hangay and Hentii mountain ranges. The Hadaasan metamorphic core
complex means Dashinchilen and Hadaasan intrusive massifs that intrudes lower-middle carboniferous
terrigenic and Triassic molass units accumulated in the Orkhon reactivated depression [Zonenshain
1967;Antipin 1977].](https://image.slidesharecdn.com/structureofhadaasanmetamorphiccorecomplex-211207043901/85/Structure-of-hadaasan-metamorphic-core-complex-1-320.jpg)
