Detrital-zircon geochronology of Paleozoic sedimentary ...
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Tectonophysics 451 (2008) 290 每 311
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Detrital-zircon geochronology of Paleozoic sedimentary rocks in the
Hangay每Hentey basin, north-central Mongolia: Implications for the
tectonic evolution of the Mongol每Okhotsk Ocean in central Asia
Thomas K. Kelty a,?, An Yin b,c,d , Batulzii Dash e , George E. Gehrels f , Angela E. Ribeiro a
a
Department of Geological Sciences, California State University at Long Beach, Long Beach, CA 90840-3902, United States
b
Department of Earth and Space Sciences, University of California, Los Angeles, CA 90095-1567, United States
c
Institute of Geophysics and Planetary Physics, University of California, Los Angeles, CA 90095-1567, United States
d
Structural Geology Group, China University of Geosciences, Beijing, People's Republic of China
e
Department of Geology, Mongolian University of Science and Technology, Ulaan Baatar 210646, Mongolia
f
Department of Geosciences, University of Arizona, Tucson, Arizona 85721, United States
Received 15 October 2007; accepted 6 November 2007
Available online 8 December 2007
Abstract
Understanding the development of the Central Asian Orogenic System (CAOS), which is the largest Phanerozoic accretionary orogen in the
world, is critical to the determination of continental growth mechanisms and geological history of central Asia. A key to unraveling its geological
history is to ascertain the origin and tectonic setting of the large flysch complexes that dominate the CAOS. These complexes have been variably
interpreted as deep-marine deposits that were accreted onto a long-evolving arc against large continents to form a mega-accretionary complex or
sediments trapped in back-arc to fore-arc basins within oceanic island-arc systems far from continents. To differentiate the above models we
conducted U每Pb geochronological analyses of detrital-zircon grains from turbidites in the composite Hangay每Hentey basin of central Mongolia.
This basin was divided by a Cenozoic fault system into the western and eastern sub-basins: the Hangay Basin in the west and Hentey basin in the
east. This study focuses on the Hentey basin and indicates two groups of samples within this basin: (1) a southern group that were deposited after
the earliest Carboniferous (‵ 339 Ma to 354 Ma) and a northern group that were deposited after the Cambrian to Neoproterozoic (‵504 Ma to
605 Ma). The samples from the northern part of the basin consistently contain Paleoproterozoic and Archean zircon grains that may have been
derived from the Tuva每Mongol massif and/or the Siberian craton. In contrast, samples from the southern part of the basin contain only a minor
component of early Paleozoic to Neoproterozoic zircon grains, which were derived from the crystalline basement bounding the Hangay每Hentey
basin. Integrating all the age results from this study, we suggest that the Hangay每Hentey basin was developed between an island-arc system with a
Neoproterozoic basement in the south and an Andean continental-margin arc in the north. The initiation of the southern arc occurred at or after the
early Carboniferous, allowing accumulation of a flysch complex in a long-evolving accretionary complex.
? 2007 Elsevier B.V. All rights reserved.
Keywords: Mongol每Okhotsk Ocean; Hangay每Hentey basin; Detrital-zircon geochronology; Central Asian Orogenic System; Central Asian Orogenic Belt
1. Introduction
The Central Asian Orogenic System (CAOS) or the Central
Asian Orogenic Belt was a site of significant continental growth
in the Phanerozoic (Zonenshain et al., 1990; Seng?r et al., 1993;
? Corresponding author. Fax: +562 985 8638.
E-mail address: tkelty@csulb.edu (T.K. Kelty).
0040-1951/$ - see front matter ? 2007 Elsevier B.V. All rights reserved.
doi:10.1016/j.tecto.2007.11.052
Seng?r and Natal'in, 1996; Jahn et al., 2004; Windley et al.,
2007). Understanding its tectonic history has important
implications for the growth mechanisms of continental crust
in Earth's history (Kovelenko et al., 2004). The development of
the CAOS has been attributed to the following competing
processes: (1) progressive duplication of a long-evolving arc
with its original length exceeding 5000 km, which was subsequently shortened in map view by syn-subduction and strike-
T.K. Kelty et al. / Tectonophysics 451 (2008) 290每311
slip faulting between ‵620 and 360 Ma (Seng?r et al., 1993;
Seng?r and Natal'in, 1996), (2) collision of multiple island arcs
with Siberia and China (Chen and Hs邦, 1995; Badarch et al.,
2002; Xiao et al., 2003, 2004a,b; Windley et al., 2007), and
291
(3) collision of micro-continents rifted from Gondwanaland
onto the Siberian craton (e.g., Dobretsov et al., 1996). Although
these competing hypotheses have distinctive predictions regarding the paleogeographic and tectonic origins of individual
Fig. 1. Location (A) and tectonic (B) maps of the central part of the Central Asian Orogenic System, simplified from Seng?r and Natal'in (1996).
292
T.K. Kelty et al. / Tectonophysics 451 (2008) 290每311
terranes across the orogen, differentiating them has been a
challenge. The low-grade flysch complexes dominate the
orogen and their tectonic settings are poorly understood (e.g.,
Seng?r et al., 1993; Badarch et al., 2002) (Fig. 1). The flysch
complexes have been interpreted to represent large accretionary
complexes fringing a long-evolving arc along continental
margins (Seng?r et al., 1993; Seng?r and Natal'in, 1996).
They have also been interpreted to represent back-arc/fore-arc
basin deposits within arcs formed in an intra-oceanic setting
(Badarch et al., 2002) or along continental margins (Hs邦 and
Chen, 1999).
The above models on the origin of flysch basins in the CAOS
have specific predictions about the provenance and age
distribution of their sedimentary detritus. For example, the
continental-margin-arc model predicts the basins to have
received significant Precambrian sediments (Fig. 2A). In
contrast, the intra-oceanic-arc model predicts that the flysch
sediments were derived exclusively from a nearby arc (Fig. 2B).
Finally, the rifted-continental-arc model predicts the arc to be
sandwiched by flysch basins (back-arc and fore-arc basins), all of
which contain a significant Precambrian signature (Fig. 2C).
To test the above models we conducted U每Pb detrital-zircon
geochronology on late Paleozoic meta-sedimentary rocks from
the Hangay每Hentey basin in north-central Mongolia. Because
this basin was the largest flysch basin in the CAOS and located
at its core (Fig. 1), understanding its provenance, timing of
deposition, and tectonic setting has important implications for
testing the competing hypotheses with regard to the origin of the
Hangay每Hentey basin and the overall evolution of the CAOS.
2. Regional geology
2.1. Hangay每Hentey Basin
The 200- to 300-km wide and 1000-km long Hangay每
Hentey basin in central and eastern Mongolia was part of the
Fig. 2. Three end-member models for the formation of large flysch basins in the Central Asian Orogenic System. (A) Andean-type continental-margin-arc model.
(B) Island-arc model. (C) Marginal arc rifted from nearby continent. See text for discussion.
T.K. Kelty et al. / Tectonophysics 451 (2008) 290每311
293
Fig. 3. Tectonic map of Mongolia, simplified from Badarch et al. (2002). Sample locations for this study and the major age provinces surrounding the Hangay每Hentey
basin are also indicated. The VD and VD每VC map symbols indicate the locations of Devonian and Devonian每Carboniferous igneous rocks, respectively. The Hangay每
Hentey basin for this figure was defined from Seng?r and Natal'in (1996) as a Vendian to Carboniferous subduction每accretion complex formed in the ※Khangai每
Khantey Ocean.§ Badarch et al. (2002) have a more spatially and time restricted definition of the Hangay每Hentey basin to be a Devonian to Carboniferous turbidite
basin.
2500-km long Mongol每Okhotsk Ocean that extends from
central Mongolia in the west to the Okhotsk Ocean in the east
(Seng?r and Natal'in, 1996; Yin and Nie, 1996) (Fig. 1). The
western termination of the Mongol每Okhotsk Ocean was abrupt
and its cause remains uncertain (see discussion on tectonic
models below). The Hangay每Hentey basin has two subdomains separated by a northwest-striking Cenozoic fault
system: the Hangay basin in the west and the Hentey basin in
the east (Fig. 3). The basin consists mainly of Devonian to
Carboniferous turbidites that were folded and faulted and
intruded or overlain by Mesozoic and Cenozoic igneous rocks
(Tomurtogoo, 2006). The basement of the Devonian每Carboniferous turbidites was uncertain, as the contact between the
sequence and the older rocks are tectonic (Badarch et al., 2002).
Fragments of Ordovician and Silurian chert were tectonically
mixed with Devonian and Carboniferous strata in the Hangay每
Hentey basin (e.g., Kurihara et al., 2006). They were also in
fault contact against a sequence of Ordovician strata along the
northeastern margin of the basin (Badarch et al., 2002). Because
the basin has been extensively intruded by Permian granites,
Jahn et al. (2004) used geochemical tracers to suggest that the
Hangay每Hentey basin was either floored by an enriched mantle
or a Precambrian basement.
Although marine sedimentation ceased in the late Permian in
the Hangay每Hentey basin, marking the closure of the PaleoAsian ocean, the eastern segment of the Mongol每Okhotsk
Ocean continued to receive marine sedimentation and its
oceanic floor was subducting below North China until the
Jurassic due to diachronous closure of this large and complex
oceanic basin (Zorin et al., 1993; Yin and Nie, 1996; Halim
et al., 1998).
2.2. Tuva每Mongol continental block
The Hangay每Hentey basin was sandwiched by the Precambrian Tuva每Mongol massif (also known as the central Mongolian
massif) (Figs. 1 and 3). The massif forms a tight ※V§ in map view,
opening towards the east (Seng?r and Natal'in, 1996). The Tuva每
Mongol massif was considered either as an isolated microcontinent in the Paleo-Asian ocean in the Late Proterozoic and
Cambrian (Zorin et al., 1993; Mossakovsky et al., 1994; Zorin,
1999), a Precambrian continental strip connecting the much larger
Siberian craton (Seng?r and Natal'in, 1996), or a composite
tectonic unit that was composed of several smaller continental
blocks with uncertain tectonic relationships between each other
(Badarch et al., 2002).
294
T.K. Kelty et al. / Tectonophysics 451 (2008) 290每311
Fig. 4. Schematic diagrams showing models for the evolution of the Hangay每Hentey basin. (a) Model proposed by Seng?r et al. (1993) and Seng?r and Natal'in (1996)
assumes the Tuva每Mongol massif was originally a linear belt that was later oroclinally folded for ‵ 180∼ during the closure of the basin in the Devonian to the late
Jurassic. (b) Model proposed by Zorin et al. (1993) requires that the Tuva每Mongol massif to have collided with Siberia in the late Proterozoic and was later rotated
oroclinally for ‵ 90∼ to close the Hangay每Hentey basin. (c) The back-arc basin model of Badarch et al. (2002) predicts limited (b40∼) rotation of the Tuva每Mongol
massif to close the Hangay每Hentey basin. See text for details of comparisons among the model predictions.
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