PALEOZOIC COLLISIONAL GRANITOIDS OF UNDINSKY COMPLEX ( EASTERN TRANSBAIKAL REGION )

There are several geodynamic models of the Central Asian Orogenic Belt (CAOB) development [Şengör et al., 1993, Zorin, 1999; Parfenov et al., 1999, 2003; Willem et al., 2012; and others]. The Mongol-Okhotsk Orogenic Belt (MOB) represents important part of CAOB. All geodymanic models of Late Riphean to Paleozoic structures of CAOB emphasize significance of subduction processes along Northern Asian craton margin at that time. Collage of CAOB terrains formed as a result of accretion of island arc, accretionary wedge, turbidite, and continental margin terrains to the Siberian paleocontinent. These terrains became a substrate for the further granitoid magmatism. Formation of large volumes of the granite magmas within orogenic belts is often related to widespread terrain displacement, when the settings of active continental margin and collision turn into transform margin of lithospheric plates. Such geodynamic change probably occurred within MOB during Permian to Early Triassic [Parfenov et al., 2013]. At that time, extended Gobi-Khingan belt of palingenic granitoids, attributed to the Undin complex in Eastern Transbaikalia, formed along northern margin of the Argun superterrain [Kozlov et al., 2003; Parfenov et al., 2003]. Numerous massifs of these granitoids of granodiorite-granite series and genetically related small leucogranite intrusions are located both within the Onon accretionary wedge terrain of the MOB and the Argun superterrain (Fig. 1). New Rb-Sr and U-Pb geochronological data specify the age of the Undin complex. Based on Sr-Nd isotope systematics, involvement of different crustal protoliths into petrogenesis of collisional granitoids is evaluated. GEODYNAMICS & TECTONOPHYSICS

There are several geodynamic models of the Central Asian Orogenic Belt (CAOB) development [Şengör et al., 1993[Şengör et al., , Zorin, 1999;;Parfenov et al., 1999Parfenov et al., , 2003;;Willem et al., 2012;and others].The Mongol-Okhotsk Orogenic Belt (MOB) represents important part of CAOB.All geodymanic models of Late Riphean to Paleozoic structures of CAOB emphasize significance of subduction processes along Northern Asian craton margin at that time.Collage of CAOB terrains formed as a result of accretion of island arc, accretionary wedge, turbidite, and continental margin terrains to the Siberian paleocontinent.These terrains became a substrate for the further granitoid magmatism.Formation of large volumes of the granite magmas within orogenic belts is often related to widespread terrain displacement, when the settings of active continental margin and collision turn into transform margin of lithospheric plates.Such geodynamic change probably occurred within MOB during Permian to Early Triassic [Parfenov et al., 2013].At that time, extended Gobi-Khingan belt of palingenic granitoids, attributed to the Undin complex in Eastern Transbaikalia, formed along northern margin of the Argun superterrain [Kozlov et al., 2003;Parfenov et al., 2003].Numerous massifs of these granitoids of granodiorite-granite series and genetically related small leucogranite intrusions are located both within the Onon accretionary wedge terrain of the MOB and the Argun superterrain (Fig. 1).New Rb-Sr and U-Pb geochronological data specify the age of the Undin complex.Based on Sr-Nd isotope systematics, involvement of different crustal protoliths into petrogenesis of collisional granitoids is evaluated.
Therefore, time period of granitoid formation covers Late Permian to Early Triassic.Notable, age of regional metamorphism in the Onon accretionary wedge terrain of MOB was regarded as Permian [Rutshtein, 1997].Probably, granitoid massifs of the Undin complex intruding the Onon terrain formed during retrogressive stage of the regional metamorphism.The origin of the Krasnokovylin and Margutseksk massifs located within internal part of the Argun terrain is less evident.This part of the Argun superterrain is supposed to be stable in Paleozoic and was not affected by the processes which took place in Mongol-Okhotsk suture during Late Paleozoic.The magma generation processes in this stable part probably were influenced by transpressiontranstension regime characteristic of transform margin of lithospheric plates [Khanchuk, 2006].In this case the sources of melts formed within metamorphosed accretionary wedge (Onon terrain) and stable block (Argun terrain) should be different.
Sr-Nd isotope systematics of the Undin complex granitoids is shown on Fig. 4. The granitoids emplaced into the accretionary wedge form rather extended trend of increasing radiogenic Sr isotope composition at nearly constant moderately negative εNd, corresponding to the trend of metasedimenary rocks of the MOB accretionary wedge, and to the granitoids of SW Japan, originated from subduction-accretionary com-plexes [Jahn, 2010].This indicates participation of metasedimentary rocks in the granitoid genesis.Compositions of the leucogranites within the Argun terrain have lower Sr isotope ratios at moderately negative or close-to-zero εNd.This can argue for a mafic lower crustal source of these granites.

Fig. 1 .
Fig. 1.Geological scheme of the Undin complex location in the Transbaikalian sector of the Mongol-Okhotsk Belt (MOB).
Mongol-Okhotsk Orogen led to formation of the Gobi-Khingan belt of palingenic granitoids along northern margin of the Argun superterrain.In the Eastern Transbaikalia these granitoids, known as the Undin complex, formed in Late Permian to Early Triassic (from new Rb-Sr and U-Pb geochronological data), constraining time of collision in the Mongol-Okhotsk Orogenic Belt.Similar Sm-Nd and Sr-Nd isotope characteristics of the granitoids and Mongol-Okhotsk accretionary wedge metasediments indicate participation of the latter in the sources of granite magmas.