Gondwana Research 16 (2009) 167–169
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Gondwana Research
j o u r n a l h o m e p a g e : w w w. e l s ev i e r. c o m / l o c a t e / g r
Editorial
Geodynamic processes and metallogenesis of the Central Asian and related orogenic
belts: Introduction
Geodynamic processes and metallogenesis have been frontier
themes of various investigations in the Earth Sciences, as they provide
a better understanding of major orogenic belts and offer insights into
the origin of continental crust. Recent models for the formation,
evolution, and destruction of supercontinents have opened new
windows to the understanding of global geodynamic processes and
associated metallogenesis. Many issues have remained unresolved
regarding the relationship between orogeny, basin evolution, and
mineral deposit formation during the assembly, evolution, and
dispersal of supercontinents. Some of these aspects were discussed
in “The Gondwana 13 Conference” held in Dali, Yunnan Province, China
during September 2008, which was attended by more than 240
participants representing 16 countries (Xiao and Wang, 2009). This
special issue of Gondwana Research was initiated from some of the
relevant sessions of the Gondwana 13 Conference.
The issue begins with a GR Focus paper by G.C. Zhao et al. (2009a)
on the Xiong'er volcanic belt located along the southern margin of the
North China Craton. The authors propose a new model for the origin of
the belt and consider it to be a Paleo-Mesoproterozoic continental
magmatic arc. The Xiong'er volcanic rocks are enriched in LILE and
LREE, and show negative Nb–Ta–Ti anomalies, similar to arc-related
volcanic rocks produced by the hydrous melting of the metasomatized
mantle wedge. Their Nd-isotope composition suggests that 5–15%
older crust has been transferred into the upper lithospheric mantle by
subduction-related recycling during Archean to Paleoproterozoic
time. SHRIMP and LA-ICP-MS U–Pb zircon age data indicate that the
Xiong'er volcanic rocks erupted intermittently over a protracted
interval from 1.78 Ga, through 1.76–1.75 Ga and 1.65 Ga, to 1.45 Ga,
with the major phase of the volcanism occurring at 1.78–1.75 Ga. Such
episodic volcanism is inconsistent with a mantle plume-driven rifting
event, but is similar to that observed in continental margin arcs.
Therefore, the authors propose subduction-related and seawardmigrating arc magmatism on the continental margins of the PaleoMesoproterozoic supercontinent Columbia.
Z.Y. Zhang et al. (2009c) conducted a thermochronometric study of
the cooling history of the Precambrian Aksu blueschist facies rocks. All
of the six blueschist samples analysed yielded AFT ages spanning the
period from 107.5–62.5 Ma and confined track lengths are between
10.46 and 12.12 µm. They identified four stages for the thermotectonic evolution of the Aksu blueschist: (1) The Precambrain Aksu
blueschist was exhumed to the surface soon after its formation; (2)
the total thickness of the Late Sinian and Paleozoic strata probably
reached 10 km and resulted in the total annealing and thermal
resetting of AFT ages; (3) the AFT ages in the Cretaceous are related to
widespread uplift of the Tian Shan and adjacent regions that restarted
the AFT clock during the late Mesozoic; and (4) the Aksu blueschist
was heated to partial annealing conditions along with the overlying
Cenozoic sediments. The re-exhumation of the belt occurred during
Miocene times.
Chai et al. (2009) studied the Kangbutiebao Formation that hosts
many important iron and Pb–Zn deposits within the Altay orogenic
belt, NW China. SHRIMP analyses of zircons from three metarhyolites
of the Kangbutiebao Formation yield weighted mean 206Pb/238U ages
of 412.6 ± 3.5 Ma, 408.7 ± 5.3 Ma, and 406.7 ± 4.3 Ma, which are
interpreted as the eruption age of the Kangbutiebao silicic volcanic
rocks.
Z.C. Zhang et al. (2009b) present major element, trace element, and
Sr–Nd isotopic analyses of 64 (ultra)mafic to intermediate volcanic
rock samples of the Eastern Junggar terrane of the Central Asian
orogenic belt. All these volcanic rocks exhibit remarkably negative Nb,
Ta, and Ti anomalies on the primitive mantle normalized trace element
diagrams, and are enriched in more highly incompatible elements
relative to moderately incompatible ones. They have subchondritic
Nb/Ta ratios, and their Zr/Nb and Sm/Nd ratios resemble those of
MORBs, which are characteristics of arc-related volcanic rocks. These
rocks are interpreted to have formed in mature island arc, immature
island arc, back-arc, and intraplate extensional settings. The authors
propose a model that involves a volcanic arc formed by northward
subduction of the ancient Junggar ocean and amalgamation of different
terranes during the late Paleozoic, with the Altai and Junggar terranes
accreted to a Cordilleran-type orogen during the end of Early to Late
Carboniferous.
Z.H. Zhao et al. (2009b) describe Permian shoshonitic series
volcanic rocks (SSVR) and adakites from the western Tian Shan, north
Xinjiang, China. Isotopic dating of the rocks yields ages of 280–250 Ma.
The SSVR include absarokite, shoshonite, and banakite, which are
characterized by enrichment of alkalis, particularly K, combined with
lower Ti, higher Al, and Fe2O3 N FeO. Interpretation of trace element,
REE, and Sr–Nd isotopic compositions suggests that both SSVR and
adakites possess similar source regions that are associated with
underplated mantle-derived basaltic materials. The authors propose
that lithosphere extension driven by magmatic underplating was
responsible for the generation of both the SSVR and adakites.
Shen et al. (2009) present new petrographic results for the orebearing porphyry stocks in the Baogutu porphyry copper belt and
discuss the copper mineralization that is hosted in diorite, diorite
porphyry, and related breccias of the diorite porphyry stocks.
Geochemical data indicate that the ore-bearing porphyries have a
predominantly intermediate composition with a transitional character
from tholeiitic to calc-alkaline, and are enriched in large ion lithophile
elements (LILE) and depleted in high field strength elements (HFSE)
with a clear negative Nb anomaly. The rocks also exhibit high initial
εNd(t) (+2.7 to + 6.3) ratios and low initial 87Sr/86Sr values (0.70359–
0.70397). The data are consistent with a transitional immature to
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168
Editorial
mature oceanic arc and suggest that the ore-bearing porphyry system
was derived from the partial melting of multiple sources including
oceanic crust and a subduction-modified mantle wedge. The melts
would have undergone significant fractionation during convergence
between terranes of the paleo-Junggar ocean and the Darbut arc of the
southern Central Asian Orogenic Belt.
L.C. Zhang et al. (2009a) investigated the Xilamulun Mo–Cu
mineral belt along the northern part of the North China Craton. The
major types of mineralization include porphyry (Chehugou Mo–Cu,
Kulitou Mo–Cu, Xiaodonggou Mo, and Jiguanshan Mo), quartz vein
(Nianzigou Mo, Xinjing Mo), and epithermal (Hongshanzi Mo–U).
Based on geochronologic data and regional geology, it is suggested
that the mineralization in the Xilamulun mineral belt was formed
during multiple events. The mineral occurrences are related to a
~ 258–210 Ma post-collisional extension with the generation of the
porphyry molybdenum–copper deposit, a ~ 185–150 Ma change in
tectonic stress from N–S to E–W that gave rise to vein and porphyry
molybdenum deposits, and a ~ 140–110 Ma lithospheric thinning stage
with formation of an additional porphyry molybdenum deposit.
Y. G. Han et al. (2009b) studied the Machaoying fault zone that
extends along the southern margin of the North China Craton (NCC)
and controlled the regional structures and hydrothermal mineral
systems in this area. The fault underwent at least two major
deformational phases. Early ductile deformation is characterized by
thrusting from north to south, which was subsequently overprinted by
late brittle faulting. Syntectonic strain shadows of biotite are preserved
surrounding rotated porphyroclasts of quartz amygdules in mylonite.
The biotite yields an 40Ar–39Ar plateau age of 524.9 ± 1.9 Ma, which is
interpreted as the time of regional thrusting along the Machaoying
fault zone. The thrusting may be temporally correlated with an Early
Cambrian discontinuity in sedimentation observed in the rock
sequences of the NCC, suggesting a compressional regime in this area
and a craton-wide tectonic event. Dating of K-feldspar from a quartz-Kfeldspar vein formed along one of the brittle faults of the Machaoying
fault zone yields a much younger 40Ar–39Ar plateau age of 119.5 ±
0.7 Ma. This is a maximum age for the brittle deformation along the
southern margin of the NCC, which also overlaps the age of widespread
gold and molybdenum mineralization in the region.
C. M. Han et al. (2009a) present results from a Re–Os geochronological study of the Lanjiagou Mo deposit in the eastern part of the
North China Craton. Molybdenite was analyzed from moderately to
strongly silicified granite porphyries Rhenium concentrations in
molybdenite samples are between 33 and 48 µg/g. Analyses of eleven
molybdenite samples yield an isochron age of 181.6 ± 6.5 Ma (2σ).
Based on the geological history and spatial-temporal distribution of
the granitoids, the Mo deposits in the eastern part of the North China
Craton are interpreted to be related to the Jurassic subduction of the
Paleo-Pacific plate.
C. W. Oh et al. (2009) report two types of Neoproterozoic
metabasites that occur together with regionally extensive, arc-related,
ca. 850–830 Ma granitoids in the Hongseong area, southwestern
Gyeonggi Massif, South Korea. This area is the extension of the Dabie–
Sulu collisional belt described in China. The first type of metabasite
(the Bibong and Baekdong metabasites) is a MORB-like back-arc basin
basalt or gabbro formed at ca. 890–860 Ma. The second type of
metabasite (the Gwangcheon metabasite) formed in a plume-related
intra-continental rift setting at 763.5 ± 18.3 Ma and is geochemically
similar to oceanic island basalt (OIB). These data indicate a transition
in tectonic setting in the Hongseong area from arc to intra-continental
rift between ca. 830 and 760 Ma. This transition is correlated with the
Neoproterozoic transition from arc to intra-continental rift settings of the
margin of the Yangtze Craton and corresponds to the amalgamation and
breakup of the Rodinia Supercontinent.
Tseng et al. (2009) studied ~450–430 Ma adakitic intrusive rocks of
in the North Qilian orogenic belt. These adakitic rocks were lower crust
melts, rather than slab melts, as indicated by their crustal Ce/Pb, Nb/U,
Ti/Eu, and Nd/Sm ratios and radiogenically enriched (87Sr/86Sr)i of
0.7053–0.7066 and εNd(t) of −0.9 to −1.7. Whereas they are all
characterized by low Yb (b1.1 ppm) and Y (b11.5 ppm) abundances, and
high Sr/Y (N65) and (La/Yb)N (N13.7) ratios, these adakitic rocks are
classified into low-MgO–Ni–Cr and high-MgO–Ni–Cr groups. The lowMgO samples were derived from partial melting of thickened lower
crust, whereas the high-MgO samples were formed from melts from
delaminated lower crust, which subsequently interacted with mantle
peridotite upon ascent. A comparative study across the North Qilian and
North Qinling orogenic belts shows that they formed a N1000-km-long
early Paleozoic orogenic belt.
Li et al. (2009) present data for quartz c-axis fabrics and marco- to
micro-scale structures in an attempt to evaluate the regional shear
sense across the western Dabie orogen. The asymmetry of c-axis
patterns consistently indicates top-to-the-southeast thrusting across
the orogen in early structural stages. Later stages of deformation show
different senses of movement in the northern and southern parts of
the orogen, with top-to-the-northwest sinistral shearing recorded in
rocks north of the Xinxian HP-UHP eclogite facies belt, and top-to-thesoutheast dextral shearing south of the same unit. A model involving
two different stages and types of extrusion and exhumation of HPUHP rocks is envisaged for east-central China, one during Middle
Triassic and the second during Late Triassic to Early Jurassic. These two
extrusions are correlative with two stages of rapid exhumation of the
Dabie HP-UHP rocks.
Hara et al. (2009) reconstruct the accretion process related to
Paleo-Tethys subduction recorded in northern Thailand. This is based
upon mélange and thrust structures, and metamorphic temperatures
derived from illite crystallinity data. Mélange formation was characterized by hydrofracturing and cataclastic deformation, with mud
injection under semi-lithified conditions, followed by shear deformation and pressure solution. Illite crystallinity data suggest metamorphic temperatures below 250 °C during mélange formation.
Asymmetric shear fabrics in mélange indicate top-to-south shear.
The authors proposed that the exposed rocks related to the PaleoTethys subduction reflect shallow levels within an accretionary prism.
Santosh et al. (2009) propose a model involving Pacific-type
orogeny to explain the Palghat-Cauvery Suture Zone (PCSZ) and the
Neoproterozoic evolution of southern India and its final amalgamation
within Gondwana assembly. Evidence for the southward subduction
and subsequent northward extrusion are preserved in the PCSZ, where
the orogenic core carries high-pressure and ultrahigh-temperature
metamorphic assemblages with ages corresponding to the Cambrian
collisional orogeny. The close association of eclogites with ultramafic
rocks having abyssal signatures, together with linear belts of iron
formation and metachert in several localities within the PCSZ, probably
represent a subduction–accretion setting. Fragments of the mantle
wedge were brought up through extrusion tectonics within the orogenic
core, which now occur as suprasubduction zone/arc assemblages. The
crustal flower structure mapped from PCSZ supports the extrusion
model, and the large-scale, north-verging thrusts towards the north of
the orogenic core may represent a fold-thrust belt. Towards the south of
the PCSZ, there is a long-lived Neoproterozoic magmatic arc within a
N200-km-wide belt of magmatic rocks preserved. All these magmatic
units were subsequently metamorphosed, when the accretionary-type
orogeny switched to collision-type in the Cambrian during the final
phase of assembly of the Gondwana supercontinent. The final phase of
the orogeny witnessed the closure of the extensive Mozambique Ocean
and the amlagamation of continental fragments within the Gondwana
supercontinent.
Liu et al. (2009) present SHRIMP U–Pb analyses on zircons from
felsic orthogneisses and mafic granulites of East Antarctica. The
protoliths of these rocks were emplaced during four episodes of ca.
1380 Ma, ca. 1210–1170 Ma, ca. 1130–1120 Ma, and ca. 1060–1020 Ma.
Subsequently, these rocks experienced two episodes of high-grade
metamorphism at N970 Ma and ca. 930–900 Ma. Most of these were
Editorial
subjected to high-grade metamorphic recrystallization at ca. 535 Ma.
Two suites of charnockites intruded at N955 Ma and 500 Ma. These,
together with associated granites of similar ages, reflect late- to postorogenic magmatism occurring during the two major orogenic events.
The similarity in age patterns suggests that the EAIS-Prydz Bay region
may have undergone the same high-grade tectonothermal evolution
as the Rayner Complex and the Eastern Ghats of India. The authors
also identify a late Mesoproterozoic/early Neoproterozoic orogen that
relates to ca. 1380 to 1020 Ma magmatism and eventual collision
before ca. 900 Ma between India and the western part of East
Antarctica.
The fifteen papers assembled in this issue provide a cross-section of
the geodynamic processes and metallogenic events covering various
segments of Asia and part of East Antarctica. We thank all the authors
for their contributions and cooperation. We acknowledge all the
reviewers for their valuable time and patronage and for providing
scholarly reviews that helped to maintain high quality of the special
issue: A.J. Barber, Hanlin Chen, Nengsong Chen, Koen De Jong, Richard
Goldfarb, Simon Harley, Dazhi Jiang, Sanghoon Kwon, Jingwen Mao,
Ian Metcalfe, Franco Pirajno, H.M. Rajesh, V.J. Rajesh, M. Santosh, David
Selby, Reimar Seltmann, A.A. Sorokin, Svetlana Tessalina, Qiang Wang,
Yuejun Wang, Fuyuan Wu, D.A. Wyman, Daping Yan, Chao Yuan,
Guochun Zhao, Yue Zhao, Jianbo Zhou, and Taofa Zhou. We appreciate
Editor-in-chief, M. Santosh for his great effort during the various stages
of the special issue. This study was financially supported by the Major
State Basic Research Development Program of China (2007CB411307)
and the National Natural Science Foundation of China (40725009,
40523003). Contribution to ILP (Topo-Central Asia, and ERAs) and
IGCP 480 project.
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Guest Editors
Wenjiao Xiao
State Key Laboratory of Lithospheric Evolution,
Institute of Geology and Geophysics, Chinese Academy of Sciences,
P.O. Box 9825, Beijing 100029, China
Corresponding author. Tel.: +86 10 8299 8524;
fax: +86 10 6201 0846.
E-mail address: [email protected].
Timothy Kusky
Department of Earth and Atmospheric Sciences,
Saint Louis University, St. Louis MO 63108, USA
2 May 2009