Download The Wulian Metamorphic Core Complex

Journal of Earth Science, Vol. 24, No. 3, p. 297–313, June 2013
Printed in China
DOI: 10.1007/s12583-013-0330-5
ISSN 1674-487X
The Wulian Metamorphic Core Complex: A Newly
Discovered Metamorphic Core Complex along the
Sulu Orogenic Belt, Eastern China
Jinlong Ni (倪金龙)
Shandong Provincial Key Laboratory of Depositional Mineralization & Sedimentary Minerals, College of
Geological Sciences & Engineering, Shandong University of Science and Technology, Qingdao 266590, China;
State Key Laboratory of Geological Processes and Mineral Resources,
China University of Geosciences, Beijing 100083, China
Junlai Liu* (刘俊来)
State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences,
Beijing 100083, China
Xiaoling Tang (唐小玲)
College of Chemistry & Environmental Engineering, Shandong University of Science and Technology,
Qingdao 266590, China
Haibo Yang (杨海波), Zengming Xia (夏增明)
State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences,
Beijing 100083, China
Quanjun Guo (郭全军)
College of Geological Sciences & Engineering, Shandong University of Science and Technology,
Qingdao 266590, China
ABSTRACT: Combined with field studies, microscopic observations, and EBSD fabric analysis, we
defined a possible Early Cretaceous metamorphic core complex (MCC) in the Wulian (五莲) area along
the Sulu (苏鲁) orogenic belt in eastern China. The MCC is of typical Cordilleran type with five
elements: (1) a master detachment fault and sheared rocks beneath it, a lower plate of crystalline rocks
with (2) middle crust metamorphic rocks, (3)
This study was supported by the National Natural Science Foun-
syn-kinematic plutons, (4) an upper plate of
dation of China (Nos. 90814006, 91214301), the Natural Science
weakly deformed Proterozoic metamorphic
Foundation of Shandong Province (No. ZR2009EQ002), the
rocks, and (5) Cretaceous volcanic-sedimentary
Foundation of the Shandong Provincial Key Laboratory of De-
rocks in the supradetachment basin. Some
positional
(No.
postkinematic incursions cut across the master
DMSM201005), and the National Key Basic Research Devel-
detachment fault zone and two plates. In the
opment Program (973 Program) of China (No. 2012CB723104).
upper plate, Zhucheng (诸城) Basin basement
*Corresponding author: [email protected]
consists of the Proterozoic Fenzishan (粉子山)
© China University of Geosciences and Springer-Verlag Berlin
Group, Jinning period granite (762–834 Ma).
Heidelberg 2013
The supradetachment basin above the
Mineralization
&
Sedimentary
Minerals
Proterozoic rocks is filled with the Early
Manuscript received July 12, 2012.
Cretaceous Laiyang (莱阳) (~135–125 Ma) and
Manuscript accepted September 28, 2012.
Qingshan (青山) groups (120–105 Ma), as well
Jinlong Ni, Junlai Liu, Xiaoling Tang, Haibo Yang, Zengming Xia and Quanjun Guo
298
as the Late Cretaceous Wangshi (王氏) Group (85–65 Ma). The detachment fault zone is developed at
the base and margin of the superposed basin. Pseudotachylite and micro breccia layers located at the
top of the detachment fault. Stretching lineation and foliation are well developed in the ductile shear
belt in the detachment faults. The stretching lineation indicates a transport direction of nearly east to
west on the whole, while the foliations trend WNW, WSW, and SE. Protomylonite, mylonite, and
ultramylonite are universally developed in the faults, transitioning to mylonitic gneiss, and finally to gneiss
downward. Microstructure and quartz preferred orientation show that the mylonites formed at high
greenschist facies to low greenschist facies as a whole. The footwall metamorphic rock series of the Wulian
MCC are chiefly UHP (ultrahigh pressure) metamorphic rocks. Syntectonic rocks developed
simultaneously with the Wulian MCC detachment and extension. Geological research has demonstrated
that the MCC is associated with small-scale intrusive rocks developing in the vicinity of the detachment
faults, for instance, dike. Geochronology results indicate that the denudation of the Wulian MCC
occurred at about 135–122 Ma. Its development and exhumation was irrelevant to the Sulu UHP
metamorphism zone rapid exhumation during Triassic Period but resulted from the crustal extension of
North China Craton and adjacent area.
KEY WORDS: metamorphic core complex, Late Mesozoic, North China Craton, crustal extension,
Sulu orogenic belt.
INTRODUCTION
The North China Craton has undergone largescale lithospheric thinning since the Late Mesozoic (Xu
and Qin, 2009; Kusky et al., 2007; Liu et al., 2005;
Deng et al., 2004; Wu et al., 2003; Xu, 2001; Menzies
et al., 1993). Petrological, geochemical, and geophysical studies led to the construction of different tectonic
models that elucidate the fundamental processes involved in the tectonic evolution and thinning of the
cratonic lithosphere (Xu and Qin, 2009; Mao et al.,
2007; Deng et al., 2004; Wu et al., 2003). The most
frequently cited ones are the delamination (Wu et al.,
2002), lithosphere derooting (Deng et al., 2004), mantle
plume (Zheng et al., 2010; Zhi et al., 2001), coupled
chemical thermo-mechanical (Xu and Qin, 2009), and
mantle-cell convection (Ren et al., 2002) models.
As a significant tectonic style of continental
lithospheric extension and crust thinning, metamorphic core complexes (MCCs) provide exceptionally
important information on lithospheric evolution
(Lister and Davis, 1989). The Late Mesozoic lithospheric evolution of North China, for example, is
characterized by the formation of MCCs in the crustal
level along with regional lithospheric extension and
thinning. These geological structures include the
Liaonan (Liu et al., 2006, 2005), Yiwulüshan (Waziyu)
(Darby et al., 2004), and Xiaoqinling MCCs (Zhang et
al., 2000). Similar to regional magmatism and miner-
alization, the exhumation of the MCCs resulted from
the Late Mesozoic lithospheric evolution of North
China.
As the southeastern margin of the North China
Craton, the Jiaodong Peninsula also experienced
lithospheric extension and thinning, thereby forming a
series of extensional structures, such as the Queshan
and Linglong (Charles et al., 2011) MCCs in the
northern Shandong Peninsula. Although considerable
research has been done on the formation and evolution
of high-pressure (HP) and ultrahigh-pressure (UHP)
rocks on the southern part of the peninsula, little information on Late Mesozoic extension in the area has
been provided. In this study, we report a newly MCC
located at the northern part of the Sulu orogenic belt
near Wulian City. The discovery of this MCC and detailed analysis of its evolution in relation to the development Sulu orogenic belt can provide direct evidence for the regional lithosphere thinning process in
the Late Mesozoic.
REGIONAL GEOLOGY
The Jiaodong Peninsula principally comprises
three petrological tectonic units: the Jiaobei metamorphic block, Sulu UHP metamorphic block, and
Mesozoic Jiaolai Basin, which is superimposed on the
two blocks (Yang et al., 2002; Faure et al., 2001;
Hacker et al., 1998). The Jiaobei metamorphic block
The Wulian Metamorphic Core Complex: A Newly Discovered Metamorphic Core Complex
is possibly a segment of the Qinling microplate, composed of banded ferromagnesian-felsic gneiss of amphibolite facies, and locally granulite facies (Hacker et
al., 2006). The block is superimposed by the medium
metamorphic to nonmetamorphosed Archean, Proterozoic, and Early Paleozoic Jiaodong, Fenzishan
(Jingshan), and Penglai (Wulian) Groups (Wu et al.,
2004). The lowermost part of the Sulu UHP metamorphic block consists of UHP metamorphic rocks. The
upper section of the UHP metamorphic rocks consists
mainly of the metamorphic rocks of the Jingshan Group
(Fenzishan Group), with Neoproterozoic granites
(Faure et al., 2001).
During the Mesozoic, the north and south blocks
of the Jiaodong Peninsula evolved through multiple
stages of magma intrusion and volcanic eruption,
producing huge amounts of granitic and mafic rocks.
The volcanic activity was centered in Jiaolai Basin
(Yang et al., 2005; Zhang Y Q et al., 2005).
Tan-Lu fault is a very striking structure in the
Jiaodong Peninsula. Some parts of the main fault zone
form the western boundary of the Mesozoic Jiaolai Basin.
A series of branch faults including the WulianQingdao-Yantai fault usually considered as the northern
boundary of the Sulu orogenic belt (Zhou et al., 2008).
The Wulian MCC is located in the northwestern
section of the Sulu orogenic belt and lies to the east of
the main fault belt of the Tan-Lu fault (Fig. 1). It covers an area from Zhucheng (ZhCh), Qingdao to the
north, and Huangdun (HD), Rizhao to the south.
THE WULIAN MCC
The Wulian MCC comprises of three different
tectonic units: an NE-extending wavy-shaped detachment fault zone; a lower plate of Proterozoic granite
gneiss and Triassic ultrahigh-pressure rocks, which
were intruded by a mass of Mesozoic plutons; and an
upper plate of weakly deformed Cretaceous superposed basins and a spot of Proterozoic metamorphic
rocks (Figs. 1 and 2). These constituents form a typical Cordillera-type MCC (Lister and Davis, 1989).
Lower Plate
Proterozoic metamorphic rocks and Triassic
ultrahigh-pressure rocks
The footwall is composed primarily of the upper
299
Jingshan Group (Ar3–Pt1), Fenzishan Group (Pt1), and
Lower Triassic UHP metamorphic rocks. The
Fenzishan Groupis also called the Wulian Group in
this area. Upper rocks are distributed limitedly, with
the UHP rocks making up the majority of the composition (Yang et al., 2005).
The Jingshan and Fenzishan Group are distributed in an NW- or NEE-oriented zone or in a
lens-shaped area. The formations are intruded by Proterozoic plutonic bodies and enclosed within the rocks
as relic segments. Most of the rocks experienced intense ductile deformation, forming mylonites or
gneisses (Li et al., 2004).
The Lower Triassic UHP block primarily comprises two sets of rocks, namely, layered
meta-sedimentary rocks, and deformed and metamorphic
Neoproterozoic
granites.
The
layered
meta-sedimentary rocks are distributed chiefly near
Zhucheng in the northern part of the study area. Garnet porphyroclasts, up to 1 mm in diameter, are very
common. Large eclogite lenses are typically present in
the metamorphic rocks. The deformed and metamorphic Proterozoic granites are distributed principally in
Wulian and Huangdun, with the appearance of massive to thick-layered coarse-grained gneisses. Given
the large-scale eclogite that contains coesite in these
rocks, it is considered to have gone through UHP
metamorphism during Triassic (Enami et al., 1993).
The surrounding rocks are thought to have undergone
such metamorphism together with the eclogites (Ye et
al., 2000).
Syntectonically and posttectonically emplaced rock
mass
Syntectonic rock mass
The study site is composed of numerous but small-scale syntectonic rocks
that occur primarily in the form of dikes or apophysis.
The major rock type making up these stocks is diorite
(Fig. 3a).
Both the foliations and stretching lineations are
developed with different grades at varied locations in
the ductile detachment zone. The lineations and foliations in the rocks are similar in the wall rocks and
pluton, and the refraction phenomenon took place at
the contact area of the rock body and wall rocks. Micrographs (Figs. 3b and 3c) and quartz EBSD fabric
Jinlong Ni, Junlai Liu, Xiaoling Tang, Haibo Yang, Zengming Xia and Quanjun Guo
300
120 o
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Fenzishan and Jingshan groups
( marble and amphibolite )
Archaean basement, containing
Jiaodong Group (mafic and felsic gneiss)
Sulu UHP metamorphic unit (gneiss and
quartzite, mafic rock bearing eclogite)
High-presure unit (gneiss)
Attitude of foliation and
stretching lination
Detachment fault
Normal fault
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YQWF. Yantai-Qingdao-Wulian fault
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ZhCh. Zhucheng
Figure 1. Simplified geological map of the Jiaodong Peninsula (modified from Lin et al., 2005).
testing (Fig. 3d) show that the intrusive rocks underwent syntectonic deformation (Schofield and
D’Lemos, 1998).
Based on the lattice-preferred orientations of the
quartz c-axis, three types of pole density exist in rock
mass, namely, types II, III, and IV (Fig. 3d). Types II
and IV pole density surround the y-axis and denote
mid-temperature deformation caused by simple shear
movement during magma crystal formation. The
included angle of the Type III pole density asymmetrically distributed in the Z-axis is 58°. These features
characterize mid-lower temperature deformation,
which may be caused by detachment extension
movement during later stage or after magma crystal
formation.
Post-tectonic plutons
Post-tectonic plutons outcrop in a large area in the footwall, showing massive
structural characteristics. The field survey shows that
these rocks did not undergo ductile shear as did the
Fangzi, Wulianshan, and Maershan plutons
(Fig. 2).
The Fangzi and Maershan plutons intruded into
the footwall gneisses and mylonites, cutting the ductile shear zone, which was similarly cut by later NNE
or NE brittle faults. The rock types that make up the
0
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43 146±0.9 Ma (Yang et al., 2002)
Guojiacun
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(m) 0
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The projection of scratche
(a)
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Gr.
(d)
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The projection of limbs of
CT antiform
2 145.3±6.6 and 128.2±0.7 Ma ( Webb et al., 2006)
1 136.3±2.3 Ma ( GSFSP, 2002)
1
The projection of limbs of
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5 km
+ Wulian
+ +
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+ + + + +FZ+
+ + 30+17+ + +
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+ + + ++ +
+ + ++III +30 +20+CF+ +
+JZSh
+ ++ + ++ +
+ + +++++++++XBBG
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+ + 0++25
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10
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300-1 500 m
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(c)
K lf
548 m
Zhucheng
36 o00'N 65 Ma
K 2w
85 Ma
105 Ma
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K 1q
Jiaonan
++
Laiyang Gr.
K lq
K ll w
0-600 m >300 m
(b)
119 o40'E
Guojiacun
10
K 1l
K lz
Pt 1f 127 m 330 m
11
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20
119 o30'
301
Q sh Gr. WS Gr.
'
The Wulian Metamorphic Core Complex: A Newly Discovered Metamorphic Core Complex
Qbgg
Shantuan
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K 2zb
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Figure 2. Structural outline and geological cross-section of the Wulian MCC. (a) Geological map of the
Wulian MCC; (b) stereographic projection of foliations; (c) point and contoured diagram for all Wulian
(WL) MCC lineations; (d) the simplified Cretaceous columnar section in Zhucheng Basin; (e) simplified
geological cross-section of the northern WL MCC. 1. Mesozoic cover in Zhucheng Basin; 2. Laiyang Group
in Zhucheng Basin; 3. conglomerate; 4. pebbled sandstone; 5. sandstone; 6. siltstone; 7. mudstone; 8. volcanic rock; 9. Late Cretaceous granite; 10. Late Cretaceous intrusions; 11. Early Cretaceous quartz syenite;
12. Early Cretaceous intrusions; 13. Triassic intrusions; 14. Neo-Proterozoic Qingbaikou gneiss; 15. Triassic
metamorphic eclogite; 16. Fenzishan (or Wulian) Group; 17. Sulu UHP metamorphic unit (gneiss, quartzite,
mafic rocks with enclaves of eclogite); 18. transpressional fault; 19. detachment fault; 20. ductile shear zone;
21. gneissosity; 22. stretching lineation; and 23. mylonite foliation. FZ. Fangzi; HD. Huangdun; ShCh.
Shichang; SanZh. Sangzhuang; XBLG. Xiaobuluogu; CF. Chuanfang; CT. Chetuan; I. Chetuan to Shimengou Reservoir Section; K1l. Laiyang Gr.; Klls. Linshansi F.; Klz. Zhifengzhuang F.; Kly. Yangjiazhuang
F.; Kllw. Longwangzhuang F.; Klq. Qugezhuang F.; Klf. Fajiaying F.; K1q. Qingshan Gr.; K2w. Wangshi Gr..
302
Jinlong Ni, Junlai Liu, Xiaoling Tang, Haibo Yang, Zengming Xia and Quanjun Guo
Figure 3. Macroscopic and microscopic features of syntectonic and post-tectonic rocks. (a) Syntectonic intrusive rocks, mylonitization diorite (from Dayuchang in Wulian, Sample 10-JDW47); (b) and (c) are micrographs of Sample 10-JDW47, with (b) plane polarization and (c) cross polarization; and (d) the c axis
fabric of quartz in Sample 10-JDW47. Kf. K-feldspar; Qtz. quartz; Cal. calcite.
Fangzishan rock mass are mainly monzodiorite, monzonite, and monzogranite. Maershan rock is composed
primarily of inequigranular hornblende-bearing biotite
monzogranite, with the hornblende content decreasing
locally to form biotite monzogranite.
The Wulianshan pluton is situated far from the detachment fault zone. This intrusion outcrop inside the
Sulu UHP metamorphic belt occurs in the form of stock.
In the field, this rock intrudes into granite gneiss, with
the granite gneiss inclusions visible in the rock.
The ages of different post-tectonic plutons are
relatively close. Zircon U-Pb, hornblende, and biotite
40
Ar-39Ar chronology testing show that the rocks were
formed at 116±4 to 125±4 Ma (Huang et al., 2006;
Yang et al., 2005). Thus, the rocks can be regarded as
indicative of contemporaneous emplacement. Later
emplaced rock that cut the ductile shear zone can aid
in determining the chronological constraints of the
denudation history of the Wulian MCC.
Detachment Fault Zone
Field structural observations and microstructure
analyses demonstrate that the Huangdun-Wulian-
Zhucheng (HD-WL-ZC) fault is an important detachment fault zone in the northwest section of the Sulu
orogenic belt. This detachment fault zone aids the understanding of the tectonic distribution, evolutionary
history, and extension-thinning mechanism in this region. It is distributed primarily throughout Huangdun,
Rizhao City, Wulian City, and Zhucheng City, cutting
through the Paleoproterozoic metamorphic rock series
and UHP metamorphic rock mass. It also controlled
the development of Early Cretaceous sedimentary
rocks.
HD-WL-ZC detachment fault and fault-related
tectonites
The strike of the HD-WL-ZC detachment fault
changes from NNE to NE from south to north, forming a spacious antiform in a plane graph, whose sides
are cut by the Maershan pluton at the fold axial trace
(Fig. 2). As indicated by the change in geometric
shapes and occurrences along the strike, the detachment fault has a basic wavy-shaped structure, with its
southern part more apparent (Fig. 2).
The data on and analyses of the planar and linear
The Wulian Metamorphic Core Complex: A Newly Discovered Metamorphic Core Complex
structure of the Chuangfang (CF) and Chetong (CT)
antiforms show the hinge orienting 272°∠22° and
278°∠34°, respectively. The field measuring slickenside occurrence is oriented 274°∠20°, which is in
accordance with the 270°∠21° direction of regional
stretching lineation (Fig. 2).
Crossed by later transpressional faults, the wavy
shape of the northern detachment fault was slightly
destroyed; the data on foliation occurrence show that
the preferred orientations of the strike are NE and
NNE (Fig. 2). These orientations still reflect the wavy
feature of the previous detachment fault.
The HD-WL-ZC detachment fault zone has an
outcrop spanning 1–2 km. The fault tectonites observed along the detachment fault have certain differences in lithological association.
The differences in tectonite associations are
manifested primarily in the integrity of the tectonite
series in the detachment fault zone. The tectonites developed perpendicular to the detachment fault strike in
eastern Chetong, Wulian City, and occurred in sequence as follows: fault gouge (Fig. 4a) >microbreccia (Fig. 4b) >cracked mylonite (Fig. 4c) >mylonite
(Figs. 4d and 4e) >mylonitic gneiss (Fig. 4f). These
are the products of different evolutionary layers when
the detachment fault footwall withdrew to the surface;
they also indicate whether there is a detachment fault
structure (Lister and Davis, 1989).
The integrity of tectonite combination varies depending on area; such differences are possibly related
to the dip angle of mylonitic gneiss. The tectonite series are generally integrated in segments with a gentle
dip, such as the Chetong-Shimengou Reservoir Section in Wulian County. In the segments with a steep
dip, the tectonite series are generally not intact, indicating that they are void of fault gouge or microbreccia in the upper detachment fault zone. The differentiation of the tectonite series may be relevant to the
uplift range during footwall uplift. The areas characterized by large-scale uplift generally have higher terrain elevation and a steeper dip angle, which causes
weathering remolds and destroys the tectonites.
Tectonite microstructures of the HD-WL-ZC
detachment fault zone
The progressive deformation of detachment
303
faults is recorded by microstructural features. These
features include low-temperature deformation at the
shallow crust level and medium- to high-temperature
deformation characteristics at the middle crust level
(Table 1 and Fig. 5).
The fabric features of the deformation of the medium and shallow levels show that the detachment
fault underwent deformation and metamorphism,
transforming from low amphibolite facies and high
greenschist facies to low greenschist facies (Lister and
Davis, 1989). These changes reveal that the rocks of
the main detachment fault footwall experienced progressive exhumation and gradual extraction to the
surface from deep to shallow crust levels.
Quartz EBSD fabric analysis of tectonites in the
HD-WL-ZC detachment fault zone
Polished thin sections were cut along the directions perpendicular to the foliations and parallel to the
lineations. The samples were taken to the State Key
Laboratory of Geological Processes and Mineral Resources of the China University of Geosciences (Beijing). Subsequently, the lattice-preferred orientation
data were obtained using a Hitachi S-3400N-II scanning electronic microscope (connected to a Nordlys
EBSD Model NL-II probe) operated in interactive
mode at an accelerating voltage of 15 kV and operating distance of 23 mm. The LPO was counted using
HKL CHANNEL5 software. Lower hemisphere projection was adopted, with foliations parallel to the XY
plane and lineations parallel to the X axis.
Six samples were collected from the section
measured at the Chetong-Shimengou Reservoir (Fig. 6).
The microstructures of the samples from the
Chetong-Shimenzi Reservoir show that the detachment fault tectonites transformed from mylonitic
gneiss to mylonite and microbreccia. The deformation
of feldspar is manifested in various forms, such as
ductile elongation (Sample 10-JDWL12), dynamic
recrystallization (samples 10-JDWL9 and 10JDWL2),
and intracrystal fractures (Sample 10-JDWL4). Quartz
pervasively developed dynamic recrystallization, producing a series of subgrains or polycrystalline quartz
aggregates that make up the mylonitic gneiss. The
deformation features revealed by the microstructures
shows that the deformation of the detachment fault
304
Jinlong Ni, Junlai Liu, Xiaoling Tang, Haibo Yang, Zengming Xia and Quanjun Guo
Figure 4. Macroscopic features of the HD-WL-ZC detachment fault zone. (a) Fault gouge in the vicinity of
the detachment fault plane (Chetong, Wulian), in gray-yellow, locally gray, siliceous lens is interbedded; (b)
felsic microbreccia, smaller than 1 mm, containing pseudotachylite, blue-gray, extremely dense fractures
are developed, indicating that the footwall moved upward (Sanzhuang, Rizhao City); (c) chloritization
breccia, which is gray-green (Huangdun, Rizhao City); (d) felsic mylonite (Chetong, Wulian City) and
feldspar porphyroclasts are visible, quartz are found around feldspar in the form of threadiness, indicating
NW shearing; (e) A-type fold in the detachment fault (Sanzhuang, Rizhao City); (f) tight folds in mylonitic
gneiss (Sanzhuang, Rizhao City).
tectonites followed a medium-to-low temperature pattern (Fig. 6 and Table 1).
The measurement of the preferred orientation of
quartz demonstrates that the tectonites in this section
embody many pole density types (Fig. 6). Among these,
the c axis pole density (Schmid and Casey, 1986) region of the quartz fabric is located on the Y
axis or on both sides of the axis. These axis sides are
chiefly medium-temperature prism slip or intermediate
fabric formed by rhombohedrons, with the slip system
{10-10}<a> or {1011}<a> (Okudaira et al., 1995).
In addition, V-type pole density (Fairbairn and
Robson, 1942) developed near the Z axis, while
Type-III pole density developed on both sides of the Z
The Wulian Metamorphic Core Complex: A Newly Discovered Metamorphic Core Complex
Table 1
Sample number
Sec. I
10-JDWL1
305
The microstructure of tectonite from HD-WL-ZC detachment fault zone
Rock type
Mylonite
Mineral assemblage
P (40%): Qz+Fsp
Type of quartz
Type of feldspar
Deformation
recrystallization
recrystallization
temperature (℃)
SR
B+BLG
450–500
GBM
BLG
450–550
M (60%): Qz+Fsp+Ms+Ch
10-JDWL2
Mylonite
P (35%): Qz+Fsp+Ms
M (65%): Qz+Ms+Bi+Cal+Ch
10-JDWL4
Mylonite
P (40%): Qz+Fsp
SR
B+BLG
450–500
10-JDWL7
Mylonite
P (40%): Qz+Fsp
SR
BLG+SR
450–650
SR
B+BLG+SR
450–650
BLG
BLG+SR
400–550
GBM
BLG
450–500
SR+GBM
BLG+SR
450–650
M (60%): Qz+Fsp+Ms+Cal+Ch
10-JDWL9
Mylonite
P (15%): Qz+Pl
M (85%): Qz+Pl+Ms+Ch+Ep
Sec. II
10-JDW11
Mylonite
P (35%): Qz+Fsp+Ms
M (65%): Qz+Ms+Bi+Cal+Ch
10-JDW12
Ultramylonite P (10%): Qz+Fsp
M (90%): Qz+Ms+Bi+Ch
10-JDW13
Mylonite
P (25%): Qz+Fsp+Ms
M (80%): Qz+Ms+Cal+Ch
10-JDW15
Mylonitised P (10%): Fsp
gneiss
Sec. III
10-JDW46
H-GBM
500–650
M (90%): Qz+Fsp+Ms+Bi
Ultramylonite P (10%): Fsp
GBM+SR
BLG+SR
450–650
SR
BLG
300–550
SR
BLG+SR
400–650
GBM+SR
BLG+SR
400–650
M (90%): Qz+Fsp+Ms
10-JDW47
Protomylonite P (50%): Qz+Fsp
M (50%): Qz+Fsp+Ms+Cal+Ch
10-JDW51
Mylonite
P (20%): Fsp
M (80%): Qz+Fsp+Bi+Ch
10-JDW53
Mylonite
P (20%): Fsp+Bi+Hb
M (80%): Qz+Fsp+Bi+Ms+Chl
P. Porphyroclast; M. matric; B. brittle fracture; BLG. bulging recrystallization; GBM. grain boundary sliding; H-GBM.
high-temperature grain boundary sliding; SR. subgrain rotation; situation of sections in Fig. 2.
axis. The angle of type-III pole density is ca. 60°,
embodying the fabric of a low-temperature basal slip
system, whose slip system and formation temperature
are {0001}<a> (Festa, 2009). The superposition of
two kinds of pole densities is typically visible in the
same sample. For instance, Sample 10-JDWL4 exhibits the superposition of the Y axis (type-I pole density)
and Z axis (V-type pole density) pole densities; the
former represents a medium-temperature deformation
environment, while the latter is indicative of a
low-temperature environment. Overall, the quartz fabric is manifested as a transition from prism slip <a> or
rhombohedron slip <a> to basal slip <a>. The tectonites corresponding to this manifestation transitioned
from high greenschist facies to low greenschist facies,
illustrating that the footwall underwent gradual denudation (from deep to low of crust) until it rose to the
surface.
306
Jinlong Ni, Junlai Liu, Xiaoling Tang, Haibo Yang, Zengming Xia and Quanjun Guo
Figure 5. Microstructure features of tectonites in the Huangdun-Wulian-Zhucheng (HD-WL-ZC) fault zone.
(a) The bulging recrystallization of quartz; (b) domino-like structures of feldspar; (c) intracrystal crack and
bulging recrystallization of K-feldspar; (d) and (e) σ porphyroclast, bulging and subgrain rotating recrystallization, and core-mantle structure of K-feldspar; (f) stress fringe and bulging and subgrain rotating recrystallization of K-feldspar; (g) polycrystal quartz ribbon aggregation and subgrain rotation recrystallization; and (h) ductile stretching of K-feldspar.
N=200
Min=0.00, Max=3.29
N=200
Min=0.00, Max=5.85
X0
N=206
Min=0.00, Max=3.02
X0
Fault
100 m
X0
330 o
48 o
N=200
Min=0.00, Max=2.84
Z0
10-JDWL9
Mylonite gneiss
0
330 o
41 o
X0
N=200
Min=0.00, Max=4.51
Z0
10-JDWL12
335 o
32 o
Shimengou Reservoir
(35 o41'28''N, 119 o14'9''E)
10-JDWL12
Figure 6. Measured cross-section along Chetuan (CT) to Shimengou Reservoir, microstructure images, and LPOs of the quartz < c > axis in
the tectonites of the Huangdun-Wulian-Zhucheng (HD-WL-ZC) detachment fault.
N=200
Min=0.00, Max=4.49
X0
Z0
Z0
Eyeball-type
mylonite
Z0
Mylonite
308 o
45 o
Z0
X0
297 o
45 o
Microbreccia
284 o
25 o
10-JDWL7
Sandstone
284 o
28 o
10-JDWL7
10-JDWL4
284 o
264 o 16 o
25 o
10-JDWL4
10-JDWL9
10-JDWL2
K1
10-JDWL1 10-JDWL2
Chetuan Village
(35 o41'51''N, 119 o13'26''E)
10-JDWL1
150
(m)
200
250
109 o
The Wulian Metamorphic Core Complex: A Newly Discovered Metamorphic Core Complex
307
308
Jinlong Ni, Junlai Liu, Xiaoling Tang, Haibo Yang, Zengming Xia and Quanjun Guo
Kinematic features of the detachment fault
The direction of motion about the detachment
fault is determined according to macroscopic, microstructural, and fabric features. As determined through
the field observation, the silica lenses in the fault
gouge (near the major fault plane of the HD-WL-ZC
detachment fault) and the en echelon extensional
fractures (which grew with the microbreccia) indicate
that the footwall was rising (Figs. 4a and 4b, respectively).
Fairly clear foliation and lineation structures,
which exhibit a W-E extension motion as reflected by
stereographic projection, are present in the mylonite of
the HD-WL-ZC detachment fault (Fig. 2). In the detachment fault tectonite, asymmetric fabrics generally
developed, revealing the direction of shear motion.
These fabrics include σ porphyroclast in feldspar (Figs.
5d and 5e), δ porphyroclast and domino structure (Fig.
5b), quartz subgrain obliquely arraying in a polycrystal quartz aggregation (Fig. 5g), and asymmetric fold
(Fig. 4e). In addition, the lattice-preferred orientation
of quartz significantly guided the determination of the
shear strain direction. The numerous quartz fabrics
indicate that the tectonite of the detachment fault exhibited a top-to-the-west sense of shear (Fig. 6). The
above-mentioned asymmetric fabrics all suggest that
the footwall moved eastward. Thus, the kinematics of
the detachment fault system points to a nearly E-W
extension, which is consistent with regional extension.
Hanging Wall
The superposed basin, which is an important part
of the Wulian MCC, is the Zhucheng sag. It is one of
the secondary tectonic elements in the south of Jiaolai
Basin. The basin basement consists of the Proterozoic
Fenzishan Group, Jinning Period granite, separated to
the east by a master detachment zone, from a lower
unit composed of the middle crust metamorphic rocks
and syn-kinematic pluton (see details in the
above-mentioned part about the lower plate).
The basin is covered with the K1 Laiyang, Qingshan, and K2 Wangshi groups (Fig. 2). The Laiyang
Group is a set of fluvial-lacustrine sediments, with
broadly developed parallel bedding, graded bedding,
rhythmic stratification, and trough cross-bedding. Its
rocks consist primarily of gray sandstone followed by
shale and rudite (Fig. 2), which partly comprise volcanic rocks. According to the chronology test on
hornblende and zircon in the basalt invading this set of
rocks and fossil data, the rock formation occurred at
approximately 135–125 Ma (Zhang Y Q et al., 2003).
The Qingshan Group, a set of volcanic eruptive
rocks, is composed of andesitic volcanic breccia, rudite breccia, brecciated lava, and rudite brecciabearing tuff, with the bottom consisting of andesite
tuffaceous rudite and sandstone. Numerous geochronological tests show that the age of this set of rocks
range from 120 to 105 Ma (Ling et al., 2009; Zhang L
C et al., 2003; Zhang Y Q et al., 2003).
The Wangshi Group, which has an unconformable
contact under the Lower Cretaceous, is composed of a
set of fluvial-lacustrine sediments, including mauvebrick red sandy conglomerate with marl. On the basis of
geochronological data, the age of the rocks was determined to be 85–65 Ma (Zhang Y Q et al., 2003).
DISCUSSION
Chronological Age of the MCC Exhumation
MCC exhumaiton is generally accompanied by
the intrusion of rocks bodies and the ductile deformation of detachment faults, as was observed in the
Liaonan MCC (Liu et al., 2005). Thus, the exhumaiton
age of this MCC can be determined by the syntectonic
rock chronology or by looking into the period at
which the ductile deformation of the detachment fault
occurred.
The analysis of the microstructures of the detachment fault zone and EBSD testing on the preferred
orientation of quartz show that the tectonites of the
detachment fault zone were deformed and metamorphosed from high greenschist facies to low greenschist
facies. The deformation and metamorphic conditions
of these facies are lower than those of the amphibolite
facies during the rapid exhumation of the Sulu orogenic belt in the Late Triassic. Thus, the development
and exhumaiton of the Wulian MCC is unrelated to
the rapid exhumation of the Sulu UHP belt during the
Late Triassic.
The muscovite 40Ar/39Ar chronology of the mylonites of the ductile shear zone reveals varied ages:
146.7±0.9 (Xu et al., 2003), 145.3±0.6, 128.2±0.7
(Webb et al., 2006), and 136.3±2.3 Ma (GSFSP, 2002).
The Wulian Metamorphic Core Complex: A Newly Discovered Metamorphic Core Complex
The first two are relatively close and agree with the
compression-shear movement of Tan-Lu fault at
Jiaodong and Liaodong peninsulas (Li et al., 2004;
Yang et al., 2004), which may not be able to as the
initial detaching movement time. The age of 136 Ma
(GSFSP, 2002) is basically consistent with the formation age of the Laiyang Group of Jiaolai Basin
(~135–125 Ma) (Zhang Y Q et al., 2003) and maybe
more suitable for the initial development and denudation of the Wulian MCC. The age of 128 Ma (Webb et
al., 2006) may show two extension detachment events
of the ductile shear zone in the course of the Wulian
MCC denudation. The formation of the Laiyang
Group of Zhucheng Basin indicated that the footwall
rocks underwent gradual exhumaiton from deep to
shallow crust with the extension of Wulian area.
The chronology data demonstrate that the emplacement age of post-tectonic rocks is from 115±1 to
122±2 Ma, with the peak age at 122 Ma (Gao et al.,
2008; Huang et al., 2006; Yang et al., 2005). The
post-tectonic rocks cut the HD-WL-ZC detachment
fault zone and maintained the features of the massive
structure, showing that the post-tectonic rocks were
not sheared by ductile deformation after intrusion.
This phenomenon may also indicate that the Wulian
MCC was no longer exhumed after 122 Ma.
Wulian MCC and Dabie-Sulu Orogen
Dabie and Sulu orogenes, separated by Tan-Lu
fault, are suture zones between Yangtze and North
China cratons. Thus, these orogenes possessed similar
plate subductions, rapid exhumations, and other tectonic deformations. Wulian MCC, which developed in
Sulu orogen, was an important tectonic type of Sulu
orogen that formed under strong extensional setting
during the Early Cretaceous (K1). Hence, two possible
phenomena should be discussed, i.e., whether Dabie
orogen underwent lithosphere thinning during K1 and
whether Wulian MCC resulted from the rapid exhumation of Sulu orogen during the Late Triassic (T3)
Period.
According to Ratschbacher et al. (2000), a
Cordilleran-type MCC occurred during K1 in the
Dabie orogen. Suo et al. (2012) considered a tectonic
dome similar to the Cordilleran-type MCC. The MCC
or dome created in the Dabie orogene proved that this
309
orogen underwent strong stretching, similar to that of
the Sulu orogen. Li et al. (2002) also conformed that a
tectonic dome in the Dabai orogen, and considered it
was resulted from lithophere delamination and thinning event during K1 (130–110 Ma).
The other phenomenon is whether Wulian MCC
resulted from the rapid exhumation of the Sulu ultrahigh pressure zone.
A Cordilleran-type MCC comprises five parts.
The supradetachment basin was an essential tectonic
unit among them. However, no extensional basin was
present during T3, thereby denoting that the Wulian
MCC was not caused by the rapid exhumation of the
Sulu UHP zone.
The seismic section perpendicular to the Wulian
MCC long axis (Xu et al., 2003), and the general attitude of the foliation planes show that the structure of
the Wulian MCC is consistent with a dome-like structure.
Plate collision or rapid exhumation of UHP zone
may result in ductile shear zone in the deep lithosphere. Nevertheless, the thermochronologic data,
combined with field and microstructural observations,
suggest that the HD-WL-ZC detachment shear zone
was active as an Early Cretaceous top-to-the-W detachment fault.
Chronology data revealed multiperiod deformations in the detachment fault zone during K1. The
chronology of the Laiyang Group in Zhucheng sag coincided with the era of deformation in the HD-WL-ZC
detachment shear zone. The preferred orientation of
stretching lineation and cold scratches were in accordance with the stretching direction during K1, demonstrating that the exhumation of Wulian MCC resulted
from strong crustal stretch movements in the Sulu orogen during K1 and not from the rapid exhumation of the
Sulu orogen during T3.
Regional Tectonic Significance of the Development
of the Wulian MCC
The denudation of the Wulian MCC was not an
isolated geological event in Jiaodong Peninsula and
adjacent North China Craton. Its denudation was accompanied by a series of MCC denudation events, the
rapid uplifting and denudation event of Jiaobei Block,
and a change in the properties of numerous other in-
310
Jinlong Ni, Junlai Liu, Xiaoling Tang, Haibo Yang, Zengming Xia and Quanjun Guo
trusive masses in the North China Craton and its margin. It also followed the tectonic system changing
from compressional shearing to transtension of the
Tan-Lu fault zone (Wang, 2006; Li et al., 2004; Yang
et al., 2004; Zhang Y Q et al., 2003; Zhu et al., 2001).
The similarities in denudation periods with that
of the Wulian MCC are given as follows: the Linglong
MCC (150–124 Ma) located in the north of Jiaodong
Peninsula (Charles et al., 2011), Yagan MCC
(150–126 Ma) on the north margin of the North China
Craton (Wang et al., 2002), Xiaoqinling MCC
(135–123 Ma) (Zhang et al., 2000), Liaonan MCC
(130–110 Ma) (Liu et al., 2005), and Yunmengshan
MCC of the Yanshan orogen (151–127 Ma) (Shi et al.,
2009; Passchier et al., 2005).
As discussed in the front of this article, the development and denudation of Wulian MCC has nothing do with the rapid exhumation Sulu orogen during
Triassic Period. Obviously, it was one of the regional
extensional events. What led to these events?
We noted that along with the exhumation of a series of MCCs, the type of intrusive granitic magmas
had changed.
Intrusive granitic magmas in the North China
Craton indicate that there might be a thickened crust
during 165–127 Ma but thinned after 127 Ma
(Xiong et al., 2011; Hu et al., 2010; Wang et al., 2000).
In view of this result, the denudation event of the
Wulian MCC may be directly related to the crustal
thickening and partial melting of the crust in the
Jiaodong Peninsula and North China Craton. This
large-scale crust melting event may have brought
about the delamination of thickened crust and reduction of the lithosphere (Gao et al., 2009).
Tectogenesis at the crustal level generally signifies a shallow response to extensive lithospheric processes (Shao and Han, 2000). The formation and denudation of the Wulian MCC may be an important form
of the extension and thinning of the lithosphere in the
North China Craton and its adjacent area.
cal Cordilleran MCC possessing three typical factors
and five components (hanging wall and superposed
basin, detachment fault zone, footwall, syn-extension
dykes, and post-extension plutons). Given the late
compressional-shear fracture and rock mass invasion,
the detachment fault zone was crossed along its strike
and dip, making it a dismembered structure.
(2) The exhumation of the Wulian MCC was
possibly from ~135 to 122 Ma. The superposed basin
of the MCC is covered with the K1 Laiyang Group
(~135–125 Ma), Qingshan Group (120–105 Ma), and
the K2 Wangshi Group (85–65 Ma).
(3) The attitudes, microstructure, and preferred
orientation of the mylonites in the detachment fault
belt indicated that the footwall had gradually transformed from middle to shallow crustal level under the
nearly W-E extensional kinematics.
(4) The development and exhumation of the
Wulian MCC were unrelated to the rapid exhumation
of Sulu UHP metamorphism zone during T3 yet not
isolated events, which may be an important form of
the extension and thinning of the lithosphere in the
North China Craton and its eastern margin.
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