1
Regional geology
Geochronological evidence for
existence of South Mongolian
microcontinentüüA zircon
U-Pb age of grantoid gneisses
from the Yagan-Onch Hayr-
han metamorphic core com-
plex
In South Mongolia and its border area with China
there are high-grade amphibolite facies metamorphic
rocks, Proterozoic shallow marine carbonates locally con-
taining stromatolites, Paleozoic weak metamorphic rocks
and Mesozoic sedimentary rocks. The Proterozoic car-
bonates were thrust over the Paleozoic and Mesozoic
strata[7]. The high-grade metamorphic rocks have not
been extensively studied. In Chinese maps, they are des-
ignated as Changcheng system and named the “Gashuilai
group”1). In Mongolia maps of various editions, they are
shown as a Proterozoic complex and are assigned to a
Proterozoic massif within Caledonian fold belts[1] or Pre-
cambrian basement of the South Gobi Microcontinent[2].
The recognition of the Yagan-Onch Hayrhan MCC in the
Sino-Mongolia border area shows that these rocks occur
as the metamorphic core beneath the detachment fault of
the MCC, which is distributed in the border area as an
elliptic anticlinal uplift with a total area of 90 kmh20 km.
The upper plate of the MCC is made of Permian, Jurassic
and Cretaceous rocks. The Permian experienced folding
and low greenschist facies metamorphism. The Triassic is
characterized by terrestrial redbeds and conglomerates
and did not experience regional metamorphism. The Ju-
rassic (?) and Lower Cretaceous sediments filled in syn-
extensional basins. Most biotite K-Ar and Ar-Ar ages of
the high-grade metamorphic rocks and granites are 150ü
WANG Tao1, ZHENG Yadong1,
G. E. Gehrels2 & MU Zhiguo1
1. Department of Geology, Peking University, Beijing 100871, China
(e-mail: Taowang@pku.edu.cn);
2. Department of Geology, University of Arizona, Tucson, Arizona,
AZ85721, USA (e-mail: ggehrels@geo.Arizona.EDU)
Abstract
A zircon U-Pb age of (916f16) Ma is measured
for grantoid gneisses from the Yagan-Onch Hayrhan meta-
morphic core complex and represents the crystallization age
of the grantoid magma. This age provides evidence for the
existence of the South Mongolian microcontinent, which is
consistent with the analysis of the regional geology.
ü
126 Ma[4 6], recording strong tectono-thermal events in
the final formation stage of the MCC in the late Mesozoic
(Late Jurassic-Early Cretaceous).
Keywords: zircon U-Pb age, Proterozoic, gniesses, South Mongolian
microcontinent.
A series of high-grade amphibolite facies metamor-
phic rocks and less metamorphosed dolomite and quartzite
occurs in South Mongolia and in the Sino-Mongolian bor-
der area. Russian and Mongolian geologists regard them
as Precambrian metamorphic basement and cover[1,2], re-
spectively, and presumed that there existed a Proterozoic
South Mongolian microcontinent[3] including South Gobi
microcontinent[4]. Studies in the Yagan metamorphic core
complex in the Sino-Mongolia border area indicate that
the high-grade metamorphic rocks occur as the core crys-
talline rocks of the MCC[5,6]. Recently, an American geo-
logical group confirmed it as a larger-scale metamorphic
core complex, the Yagan-Onch Hayrhan metamorphic
core complex, which stretches across the Sino-Mongolian
border. As they suggested, this carries important implica-
tions for the tectonic evolution of Asia. The recognition of
Mesozoic metamorphism at Onch Hayrhan posed a ques-
tion of the existence of the South Mongolian microconti-
nent[4]. The question concerns some important problems
on the crustal strucure and tectonic evolution in the Mid-
dle-East Asian continent. The authors restudied the MCC
and determined a set of ages. A zircon U-Pb age of (916f
16) Ma was measured for the granitoid gneiss. This pro-
vides new clues to solving the problem.
2 Composition of crystalline rocks of the Yagan-
Onch Hayrhan MCC
The core crystalline rocks exhibit variable composi-
tions. These rocks can be divided into 3 members by de-
tailed mapping: gneiss, marble-mylonitic gneiss, and
quartzite members, and the two former are the main mem-
bers. The gneiss members, distributed in the center of the
core, mainly consist of banded, augen biotite-plagioclase
gneiss, hornblende-biotite plagioclase gneiss, banded
migmatic gneiss, grantoid gneiss, and grantoid mylonites.
The marble-mylonitic gneiss member consists of banded
mylonitic marble and mylonitic gneiss, and granitoid
mylonite. In the mass, the granitoid gneiss and migmatic
gneiss, which are uniform in outcrop, are the main compo-
sitions of the crystalline rocks in the MCC. Most granitoid
gneisses have some protolith features of deformed/ meta-
morphosed palaeo-plutons. Layered paragneiss makes up
only about 10%ü20% of the core complex.
Gneissic foliation occurs widely in the granitoid
gneiss and quartzo-feldspathic gneiss. The foliation is
characterized by parallel arrangement of light-colored
quartzo-feldspathic (vein) bands and dark bands (base)
consisting mainly of biotite, quartz, and feldspar. Some
1) Geological Survey Team of Ningxia, China, 1Ή20000 Scale Geological Map of Hariaoribuhe (k-48-xx), 1982, 14.
Chinese Science Bulletin Vol. 46 No. 23 December 2001
2005