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Journal of Earth Science, Vol. 30, No. 2, p. 335–347, April 2019 ISSN 1674-487X
Printed in China
https://doi.org/10.1007/s12583-018-1203-8
Yan, J. M., Sun, G. S., Sun, F. Y., et al., 2019. Geochronology, Geochemistry, and Hf Isotopic Compositions of Monzogranites and Mafic-
Ultramafic Complexes in the Maxingdawannan Area, Eastern Kunlun Orogen, Western China: Implications for Magma Sources, Geodynamic
Setting, and Petrogenesis. Journal of Earth Science, 30(2): 335–347. https://doi.org/10.1007/s12583-018-1203-8. http://en.earth-science.net
Geochronology, Geochemistry, and Hf Isotopic Compositions
of Monzogranites and Mafic-Ultramafic Complexes in the
Maxingdawannan Area, Eastern Kunlun Orogen,
Western China: Implications for Magma Sources,
Geodynamic Setting, and Petrogenesis
Jiaming Yan 1, Guosheng Sun *1, Fengyue Sun1, Liang Li1, Haoran Li1, Zhenhua Gao2,
Lei Hua2, Zhengping Yan1, 3
1. College of Earth Sciences, Jilin University, Changchun 130061, China
2. The Eighth Institute of Shandong Geological Exploration, Rizhao 276000, China
3. Qinghai Provincial Bureau of Ferrous Metal and Geological Exploration, Xining 810007, China
Jiaming Yan: https://orcid.org/0000-0002-4065-9091; Guosheng Sun: https://orcid.org/0000-0001-7682-4995
ABSTRACT: This paper presents zircon U-Pb-Hf isotopic compositions and whole-rock geochemical
data for monzogranites and mafic-ultramafic complexes of the Maxingdawannan area in the western end
of the east Kunlun orogenic belt, western China. The data are used to determine the ages, petrogenesis,
magma sources, and geodynamic setting of the studied rocks. U-Pb zircon dating indicates that
monzogranites and gabbros of the complexes were emplaced at 399 and 397 Ma, respectively. The
monzogranites are shoshonitic, with high SiO
2
, Al
2
O
3
and total-alkali contents, and low TFeO, MgO, TiO
2
and P
2
O
5
contents. The mafic-ultramafic complexes are characterized by low SiO
2
contents. The
monzogranites display enrichment in light rare-earth elements (LREE) and large-ion lithophile elements
(LILE), depletion in heavy REEs (HREE) and high-field-strength elements (HFSE), and negative Eu
anomalies (Eu/Eu*=0.36–0.48). The mafic-ultramafic complexes are also enriched in LREEs and LILEs,
and depleted in HREEs and HFSEs, with weak Eu anomalies (Eu/Eu*=0.84–1.16). Zircon ε
Hf
(t) values
for the monzogranites and mafic-ultramafic complexes range from -6.68 to 1.11 and -1.81 to 6.29, with
zircon model ages of 1 812–1 319 Ma (T
DM2
) and 1 087–769 Ma (T
DM1
), respectively. Hf isotopic data
indicate that primary magmas of the monzogranites are originated from partial melting of ancient lower
crust during the Paleo-Mesoproterozoic, with a juvenile-crust component. Primitive magmas of the
mafic-ultramafic complexes are likely originated from a depleted-mantle source modified by slab-derived
fluids and contaminated by crustal components. Geochemical data and the geological setting indicate
that Devonian intrusions in the Maxingdawannan area are related to northward subduction of the Proto-
Tethys oceanic lithosphere.
KEY WORDS: geochronology, geochemistry, Hf isotopes, Maxingdawannan, Proto-Tethys Ocean.
0 INTRODUCTION
The east Kunlun orogenic belt (EKOB) is a part of the west-
ern segment of the central orogenic belt, China, and is one of the
primary tectono-magmatic belts of the northern Tibetan Plateau.
Multistage tectono-magmatic events have affected the region,
including the closure of ancient oceans, continental convergence,
accretionary, collisional and superimposed orogenies, and
changes in tectonic regime (Chen et al., 2018; Zhao et al., 2018;
Deng et al., 2015, 2014a, b, 2012; Dai et al., 2013). The
*Corresponding author: 691836152@qq.com
© China University of Geosciences (Wuhan) and Springer-Verlag
GmbH Germany, Part of Springer Nature 2019
Manuscript received November 13, 2017.
Manuscript accepted May 22, 2018.
EKOB is situated in a pivotal location at the junction between
the Tethyan and Pan-Asian tectonic domains (Jiang et al., 2013).
From the Early Paleozoic to Late Triassic, it was affected by the
Proto- and Paleo-Tethys orogenies (Chen et al., 2015; Yang et
al., 1996). Multistage tectono-magmatic events occurred during
subduction of the Proto-Tethys oceanic lithosphere. The EKOB
was subsequently affected by the Paleo-Tethys orogeny, which
performed a key role in the generation of Permian–Triassic gra-
nitic magmatism (Yang et al., 1996). The Paleozoic tectonic evo-
lution of the EKOB has been the focus of much research (Zhu C
B et al., 2018; Liu et al., 2012; Zhang et al., 2010; Chen et al.,
2008; Zhao et al., 2008; Li et al., 2006; Zhu Y H et al., 2006,
2005; Pan et al., 1996), although uncertainties remain. For ex-
ample, Mo et al. (2007) put forward that subduction of the Proto-
Tethys continued until the Late Ordovician, while Dong et al.
(2017) considered that the Proto-Tethys Ocean has closed during
336 Jiaming Yan, Guosheng Sun, Fengyue Sun, Liang Li, Haoran Li, Zhenhua Gao, Lei Hua and Zhengping Yan
the Devonian. Neither view can emphasize the post-collision
magmatism, while the granites and mafic-ultramafic complexes
associated with post-collision extension can provide an effective
method to deal with these contradictions.
The Maxingdawannan area in the Qimantagh region rec-
ords the early tectonic evolution of the region and contains vo-
luminous magmatic rocks. Therefore, an understanding of the
geology of the area is vital in elucidating the tectonic evolution
of the EKOB. Although some detailed studies of the Qimantagh
region have been undertaken (Feng et al., 2012; Wang et al.,
2012, 2009; Cui et al., 2011; Ma et al., 2010; Liu et al., 2006;
Lu et al., 2006; Tan et al., 2004; Shen et al., 1999), its Paleozoic
evolution remains unclear. Here we report new data for
monzogranites and mafic-ultramafic complexes of the Maxing-
dawannan area, with the aim of establishing an evolutionary
model of Early Devonian magmatism in the EKOB and further
constraining the tectonic evolution of the EKOB.
1 GEOLOGICAL BACKGROUND AND SAMPLE DE-
SCRIPTION
The EKOB is situated in the north section of Tibetan Plat-
eau (Ju et al., 2017; Mo et al., 2007), bordered by the Qaidam
Basin to the north and the Songpan-Ganzi Basin to the south
(Fig. 1), extending 1 500 km from east to west and south-north
of 50–200 km (Yang et al., 1996; Jiang et al., 1992). Sun et al.
(2009) regarded the EKOB as a collage of blocks that experi-
enced multiple stages of orogeny and accumulated northward
to the Qaidam Block. Major E-W-trending faults delineate
several belts, which from north to south are the Caledonian
back-arc rift belt (north EKOB), the basement uplifted and
granite belt (middle EKOB), and the composite collage belt
(south EKOB)(Fig. 1; Sun et al., 2009). The northern EKOB
comprises mainly basic volcanic rock that is locally overlain
by Devonian sedimentary molasse, and the middle EKOB is
characterized by intense granitic magmatism that covers an
area of ~48 400 km
2
. In the southern EKOB, exposed strata
range from Paleoproterozoic to Triassic in age (Jiang et al.,
1992), are generally intensely deformed, and are overlain by
extensive Triassic strata. Sun et al. (2009) considered the
marine basic volcanic rocks of the Mesoproterozoic Anbaogou
Group to constitute an “Ocean Basalt Plateau”. The middle and
northern EKOB are located at the northern end of the middle
Kunlun fault, and consist of a Paleoproterozoic crystalline
basement (Jinshuikou Group). The Meso-Neoproterozoic
southern EKOB comprises an oceanic plateau and a belt of
tectono-stratigraphic terranes.
The study area is located in the middle EKOB (Fig. 1)
91°27′39″E–91°29′41″E and 36°55′23″N–36°58′01″N. Quater-
nary sediments are mainly exposed strata (Fig. 2). Northwest-
southeast- and NE-SW-trending faults dominate the area. The
NW-SE faults are strike-slip faults. Intrusive rocks are wide-
spread and are dominated by monzogranite, granodiorite and di-
orite, as well as mafic-ultramafic complexes. The monzogran-
ites are mainly Mesozoic and Paleozoic in age, and exposed
area is ~6 km
2
. The Mesozoic monzogranites occur in the north-
western part of the area, while the Paleozoic monzogranites oc-
cur in the eastern and middle parts. The granodiorites and dio-
rites are found in the southwest and central parts of the area,
respectively. The mafic-ultramafic complexes occur in the
southern end of the district, where they outcrop over an area of
~0.73 km
2
. Dikes, including granite porphyry, diorite and gab-
bro dikes, are also present in the area (Fig. 2).
The samples used for study were gained from outcrops in
the southern and eastern Maxingdawannan area (Fig. 2). The
monzogranites, collected from 36°55′56″N, 91°29′45″E (Figs. 2,
3a, 3c), are pale red, medium- and coarse-grained, massive and
contain quartz (~35%), plagioclase (~27%), orthoclase (~30%),
biotite (~5%), and minor zircon and titanite (~3%) (Figs. 4a, 4b).
Samples from mafic-ultramafic complexes, collected at
36°55′43″N, 91°29′09″E (Figs. 2, 3b), are olivine gabbros and
olivine pyroxenites. The olivine gabbros are pale-grey, fine- to
medium-grained, massive, and contain plagioclase (~40%), cli-
nopyroxene (~30%), olivine (~25%), and minor talc and biotite
(~5%)(Figs. 3d, 4c). The olivine pyroxenites are dark-grey, fine-
to medium-grained, massive, and contain clinopyroxene (~65%),
olivine (~25%), plagioclase (~5%), and minor talc and biotite
(~5%) (Figs. 3e, 4d). Early crystallized clinopyroxene was later
transformed into talc by autometamorphism.
Figure 1. Geological map showing the regional structure of the east Kunlun orogenic belt.
Geochronology, Geochemistry, and Hf Isotopic Compositions of Monzogranites and Mafic-Ultramafic Complexes in the Maxingdawannan 337
Figure 2. Geological map of the Maxingdawannan area.
2 ANALYTICAL METHODS
2.1 U-Pb Zircon Geochronology
Zircons were separated and extracted at the Langfang Re-
gional Geological Survey, Hebei Province, China, according to the
standard procedures. All zircons were examined carefully and their
internal structures were revealed by cathodoluminescence (CL) im-
ages. Zircon U-Pb isotope analysis were implemented at the
Yanduzhongshi Geological Analysis Laboratories, Beijing, China,
following the method of Yuan et al. (2004), corrected method seen
Andersen (2002). Isotopic data were calculated by the ICP-MS-
DATECAL program (Liu Z Q et al., 2011; Liu Z X et al., 2008).
Age calculations and generate concordia plots used Isoplot pro-
gram (Ludwig, 2003). These data are listed in Table S1.
2.2 Whole-Rock Geochemistry
Whole-rock samples were performed in the Yanduzhongshi
Geological Analysis Laboratories Ltd., Beijing, China. After re-
moving altered surfaces, fresh samples were chosen and then grind
to less than 200 mesh (0.520 0±0.000 1 g) for analysis. Sample
powders were performing V8C automatic fusion machine
procedure. The precision of major and trace elements are better than
1% and 5%, respectively. These data are shown in Table S2.
2.3 Zircon Hf Isotopes
Zircon Hf isotope analyses were carried out using a New-
Wave UP213 laser system attached to a Neptune multi-collector
ICP-MS, at the Institute of Mineral Resources, Chinese Acad-
emy of Geological Sciences, Beijing, China. The spot size of in-
strument is 55 μm. Analytical methods and process, data acqui-
sition techniques see Guo et al. (2012) and Wu et al. (2006). Data
are presented in Table S3.
3 RESULTS
3.1 U-Pb Zircon Geochronology
3.1.1 Monzogranites
Zircons from monzogranite sample MXDWN-16-DB-N4
are 100–150 μm long with aspect ratios of 1 : 1.5 to 1 : 3, and
exhibit oscillatory zoning (Fig. 5). They have variable Th (58
ppm–249 ppm) and U (139 ppm–599 ppm) contents, with Th/U
ratios of 0.34–0.79 (Table S1), consistent with a magmatic origin
(Corfu et al., 2003; Hoskin and Schaltegger, 2003). The 24 anal-
yses yield a mean age of 399±1 Ma (MSWD=1.07; Fig. 6a), in-
terpreted as the crystallisation age of MXDWN monzogranites.
3.1.2 Mafic-ultramafic complexes
Cathodoluminescence images reveal that the zircon crystals
from a gabbro sample (MXDWN-16-DB-N12) are mostly subhe-
dral with noinherited cores. Most crystals are stubby prisms or sub-
rounded with aspect ratios of 1 : 1.5 to 1 : 2.5 (Fig. 5). The samples
have variable Th (384 ppm–2 191 ppm) and U (431 ppm–2 109
ppm) contents, and Th/U ratios of 0.54–1.4, which, together with
internal textures, indicate a magmatic origin (Corfu et al., 2003;
Hoskin and
Schaltegger, 2003). Thirty-two analyses yielded ages
of 402–393 Ma, with a weighted-mean age of 397±1 Ma
(MSWD=0.42; Fig. 6b), interpreted as the crystallization age of
MXDWN gabbros.
Figure 3. Photographs showing (a) outcrop of monzogranites in the Maxingdawannan area, (b) outcrop of mafic-ultramafic complexes in the Maxingdawannan
area, (c) hand specimen of monzogranites, (d) and (e) hand specimen of mafic-ultramafic complexes, (d) olivine gabbro, (e) olivine pyroxenite.
338 Jiaming Yan, Guosheng Sun, Fengyue Sun, Liang Li, Haoran Li, Zhenhua Gao, Lei Hua and Zhengping Yan
Figure 4. Photomicrographs showing monzogranites and mafic-ultramafic complexes of the Maxingdawannan area (crossed-polarized light). (a), (b) Monzogranite;
(c) troctolite; (d) olivine pyroxenolite. Pl. Plagioclase; Q. quartz; Cpx. clinopyroxene; Or. orthoclase; Ol. olivine; Bi. biotite.
Figure 5. Cathodoluminescence (CL) images of zircons selected for analysis from the Early Devonian rocks of the study area. The numbers on these images
indicate individual analysis spots, and the values below the images show zircon ages and their T
DM
(Hf) ages.
Figure 6. U-Pb isotope concordia diagrams for analyzed zircons from (a) monzogranite and (b) gabbro (insets show weighted mean age diagrams).
Geochronology, Geochemistry, and Hf Isotopic Compositions of Monzogranites and Mafic-Ultramafic Complexes in the Maxingdawannan 339
3.2 Whole-Rock Geochemistry
3.2.1 Monzogranites
The monzogranites contain 64.8 wt.%–66.8 wt.% SiO
2
,
14.1 wt.%–14.6 wt.% Al
2
O
3
, 2.56 wt.%–3.28 wt.% Na
2
O, 4.20
wt.%–5.95 wt.% K
2
O (Na
2
O/K
2
O=0.43–0.78), 1.97 wt.%–2.76
wt.% CaO, 0.76 wt.%–0.81 wt.% TiO
2
, 4.59 wt.%–5.22 wt.%
TFeO, 0.92 wt.%–1.13 wt.% MgO, and Mg
#
=28.0–30.0
(Mg
#
=100×MgO/(MgO+TFeO)). The monzogranites are related
to quartz monzonite series in the TAS diagram (Fig. 7a), and
within the shoshonite series in the K
2
O vs. SiO
2
diagram (Fig.
7b), with A/CNK ratios of 0.94–1.06. The samples have right-
declining REE patterns (Fig. 8a) with (La/Yb)
N
ratios of 3.67–
6.94 and negative Eu anomalies (Eu/Eu*=0.36–0.48). Primitive-
mantle-normalized variation patterns (Fig. 8b) indicate Rb, U
and Sr enrichment and strong Nb, Ta and Ti depletion.
3.2.2 Mafic-ultramafic complexes
The loss-on-ignition (LOI) values of 1 wt.%–5 wt.% indicate
significant weathering and/or alteration of the olivine gabbros and
olivine pyroxenites (Li et al., 2018). The olivine gabbros contain
47.3 wt.%–52.0 wt.% SiO
2
, 16.1 wt.%–21.3 wt.% Al
2
O
3
, 0.32
wt.%–3.15 wt.% TiO
2
, 6.07 wt.%–11.82 wt.% TFe
2
O
3
, 7.54 wt.%–
12.7 wt.% CaO, 1.80 wt.%–4.59 wt.% Na
2
O+K
2
O, 5.00 wt.%–8.87
wt.% MgO, and Mg
#
=46.88–77.33. The olivine pyroxenites contain
41.3 wt.%–42.9 wt.% SiO
2
, 7.26 wt.%–9.13 wt.% Al
2
O
3
, 0.45
wt.%–0.82 wt.% TiO
2
, 11.7 wt.%–12.7 wt.% TFeO, 4.96 wt.%–
6.05 wt.% CaO, 0.99 wt.%–1.38 wt.% Na
2
O+K
2
O, 23.0 wt.%–24.7
wt.% MgO, and Mg
#
=79.5–81.5. The LREE contents and (La/Yb)
N
ratios of the olivine gabbros are 25.20 ppm–73.57 ppm and 4.44–
5.03, respectively, and those of the olivine pyroxenites are 17.39
ppm–35.52 ppm and 2.96–3.18, respectively (Fig. 8c). Both rock
types are depleted in Nb and Ta (Fig. 8d) and show weak Eu anom-
alies (Eu/Eu*=0.85–1.16; Fig. 8c; Sun and McDonough, 1989).
Figure 7. (a) SiO
2
-(Na
2
O+K
2
O) diagram (Irvine and Baragar, 1971), and (b) SiO
2
-K
2
O diagram (Peccerillo and Taylor, 1976)
of the Maxingdawannan monzogranite.
Figure 8. Trace elements characteristics of monzogranite showing (a) chondrite-normalized REE patterns, (b) primitive-mantle-normalized trace elements abun-
dances (Sun and McDonough, 1989), and mafic-ultramafic complexes (c) and (d) of the Maxingdawannan monzogranite.
340 Jiaming Yan, Guosheng Sun, Fengyue Sun, Liang Li, Haoran Li, Zhenhua Gao, Lei Hua and Zhengping Yan
3.3 Zircon Hf Isotopes
Fifteen spot analyses were gained from the MXDWN
monzogranite samples, yielding
176
Yb/
177
Hf ratios of 0.016 555–
0.039 955 and
176
Lu/
177
Hf ratios of 0.000 584–0.001 388.
176
Hf/
177
Hf ratios are ranging from 0.282 343 to 0.282 560,
which corresponds to ε
Hf
(t) values of -6.68 to 1.11 (weighted
mean -1.99; Fig. 9). T
DM2
model ages vary from 1.32 to 1.81 Ga
(weighted mean 1.52 Ga; Table S3).
Twelve spot analyses on MXDWN gabbro samples yielding
176
Yb/
177
Hf ratios of 0.019 772–0.030 968 and
176
Lu/
177
Hf ratios
of 0.000 680–0.001 117.
176
Hf/
177
Hf ratios are 0.282 480–
0.282 711 with ε
Hf
(t) values of -1.81 to 6.29 (Fig. 9). T
DM1
model
ages are 1 087–769 Ma (weighted-mean 865 Ma; Table S3).
4 DISCUSSION
4.1 Early Devonian Magmatism in the EKOB
There is a general consensus that intrusive rocks in the
EKOB were mostly emplaced in Early Paleozoic and Early Mes-
ozoic (Dai et al., 2013; Feng C Y et al., 2012, 2010; Cao et al.,
2011; Cui et al., 2011; Chen et al., 2006). However, voluminous
intrusive rocks within the Maxingdawannan area have not yet be
precisely dated. Based on rock associations and lithostratigraphic
relationships with surrounding rocks, it was initially identificated
as monzogranites and mafic-ultramafic complexes by the China
Aero Geophysical Survey and Remote Sensing Center (AGRS)
during regional geological mapping. This study firstly provides
precise U-Pb dating of the monzogranites and mafic-ultramafic
complexes in the Maxingdawannan area.
The ages of the Early Paleozoic Maxingdawannan
monzogranites and mafic-ultramafic complexes (399 and 397
Ma, respectively) are consistent with those of Early Paleozoic
magmatic activity in other parts of the EKOB (Fig. 10; Wang
et al., 2013; Liu et al., 2012; Chen et al., 2006). For example,
~407 and 406 Ma granodiorites and gabbros are found in the
Yuejinshan area (Liu et al., 2012), ~406 Ma websterite and
~405 Ma gabbronorite in the Xiarihamu (Song et al., 2016),
~391 Ma syenogranites occur in the Xiarihamu area (Wang et
al., 2014, 2013), and ~403 and 394 Ma gabbros and syenogran-
ites are present in the Kayakedengtage area (Chen et al., 2006).
The new U-Pb ages for the Maxingdawannan area, together
with previously published data, indicate a widespread Early
Devonian igneous event in the EKOB.
4.2 Magma Sources and Petrogenesis
4.2.1 Petrogenesis
4.2.1.1 Monzogranites
The geochemical data of the Early Devonian monzogran-
ites collected from the Maxingdawannan area can be utilized to
trace their magma sources. The monzogranites have elevated
SiO
2
, Al
2
O
3
, and alkali element concentrations. In contrast, the
concentrations of TFe
2
O
3
, MgO (Mg
#
=28.04–30.00), P
2
O
5
, and
TiO
2
are relatively low. The monogranites have high
Zr+Nb+Ce+Y contents (>698 ppm) and 10 000 Ga/Al ratios
(>2.46), consistent with A-type granites (Whalen et al., 1987).
In the (Zr+Nb+Ce+Y)×10
-6
vs. (K
2
O+NaO)/CaO,
(Zr+Nb+Ce+Y)×10
-6
vs. TFeO/MgO, 10 000Ga/Al vs.
(K
2
O+NaO)/CaO, and 10 000Ga/Al vs. TFeO/MgO diagrams
(Fig. 11; Whalen et al., 1987), most samples plot in the A-type
granite field. Furthermore, in the Nb vs. Y vs. Ce and Y/Nb vs.
Rb/Nb diagrams (Fig. 12), the Maxingdawannan A-type granites
plot in the A
2
subgroup, indicating their formation in a post-
collisional setting. Zircon saturation thermometry (Watson and
Harrison, 1983; Watson, 1979) can be used to estimate the tem-
peratures of felsic magma. The zircon saturation temperature
(827–865 °C; mean 842 °C) is close to the A-type granite tem-
perature of 839 °C (Chen et al., 2018).
Figure 9. Zircon Hf isotopic features for the Early Devonian intrusive rocks
in Maxingdawannan area.
Figure 10. Probability curve of ages for magmatic zircons from the east
Kunlun orogenic belt.
Figure 11. (a) Zr+Nb+Ce+Y(×10
-6
)-(K
2
O+Na
2
O)/CaO diagram; (b)
Zr+Nb+Ce+Y(×10
-6
)-TFeO/MgO diagram; (c) 10 000Ga/Al-(Na
2
O+K
2
O)/CaO
diagram and (d) 10 000Ga/Al-TFeO/MgO diagram (after Whalen et al., 1987)
of the Maxingdawannan monzogranite. FG. Fractionated felsic granites; OGT.
unfractionated M-, I-and S-type granites; A. A-type granites.
Geochronology, Geochemistry, and Hf Isotopic Compositions of Monzogranites and Mafic-Ultramafic Complexes in the Maxingdawannan 341
4.2.1.2 Mafic-ultramafic complexes
It is unavoidable that mantle-sourced magma will undergo
contamination before the magma was emplaced. LILEs in en-
richment and HFSEs in depletion, combined with the REE pat-
terns of the Maxingdawannan complexes, unlike those of N-
MORB originated from asthenospheric mantle source (Fig. 8c).
Some elements ratios would not be affected during fractional
crystallization and partial melting, due to the similar distribution
coefficient (MacDonald, 2001; Campbell and Griffiths, 1993).
Among these elements and their ratios (e.g., K
2
O/P
2
O
5
, Ce/Pb,
Ta/Yb, Nb/Ta and Th/Yb), their covariant relationships can effi-
ciently detect whether crustal components were incorporated
into the magma. Positive correlation between the Th/Yb-Nb/La,
and La/Yb-Ce/Yb are indicated in Figs. 13a, 13b. Nb/U and
Ce/Pb ratios will remain constant during magmatic evolution;
accordingly, implying their origin. Hofmann (1988) suggest that
Nb/U ratios of MORB and OIB are 47±10. By comparison,
Maxingdawannan mafic-ultramafic complexes have Nb/U ratio
range from 3.57 to 14.62, which is close to the mean value of
the net continental crust (~9.7; Campbell, 2002). In addition,
Ce/Pb ratio in the mantle is 25±5, as for the continental crust is
less than 15 (Furman et al., 2004). The Maxingdawannan mafic-
ultramafic complexes have Ce/Pb ratio range from 1.24 to 3.52
(mean of 2.35), suggesting that incorporated significant crustal
material. (La/Nb)
PM
and (Th/Ta)
PM
value for the upper and lower
crust are markedly different, meaning these trace element ratios
can be utilized to ascertain the contaminants (Neal et al., 2002).
In Fig. 14, samples either plot near the upper crust or near the
lower crust, implying that both the upper and lower crust were
the sources of contamination.
4.2.2 Magma sources
4.2.2.1 Monzogranites
A-type granites commonly originate from partial melting of
felsic crust under low pressure and have negative Eu anomalies
and relatively flat HREE patterns. Their Rb/Sr and Rb/Nb values
differ from those of coeval mafic-ultramafic complexes, and they
contain no mafic enclaves. Based on field and microscopic obser-
vations and geochemical analyses, we consider that the Maxing-
dawannan A-type granites are in this category. Moreover, the
Rb/Sr ratios (0.50–0.86) and Rb/Nb ratios (6.0–7.6) of
monzogranites are higher than the mean values for global conti-
nental crust (0.32 and 4.5, respectively), which also supports a
crustal origin for the magmas. Besides, the monzogranites are
LILEs in enrichment and HFSEs in depletion, suggesting that
these magma was originated from partial melting of the crust (Wu
et al., 2000). Depletions in Ti, Nb and Ta indicate that the source
for the magma are contained residual ilmenite, rutile and titanite
(McKenzie, 1989; Sun and McDonough, 1989). The negative Eu
anomalies indicate that these magmas either fractionated plagio-
clase or were derived from a source containing residual plagio-
clase.
Figure 12. (a) Nb-Y-Ce diagram, and (b) Y/Nb-Rb/Nb diagram of the Maxingdawannan monzogranite. Modified after Eby (1992).
A1. A1-type granites; A2. A2-type granites.
Figure 13. Plots of selected trace element for checking contamination of the Maxingdawannan mafic-ultramafic complexes.
342 Jiaming Yan, Guosheng Sun, Fengyue Sun, Liang Li, Haoran Li, Zhenhua Gao, Lei Hua and Zhengping Yan
Figure 14. (La/Nb)
PM
vs. (Th/Ta)
PM
diagrams for Maxingdawannan mafic-
ultramafic complexes (after Neal et al., 2002).
The zircons from the monzogranites have ε
Hf
(t) values of
-6.68 to 1.11 (mean value of -1.99; Table S3) and
176
Hf/
177
Hf ra-
tios of 0.282 343 to 0.282 560, reflecting that the zircons didn’t
crystallize from a homogeneous magma (Andersen et al., 2007).
Negative ε
Hf
(t) values are consistent with crustal melt, while pos-
itive values may indicate mantle components or juvenile crust
(Wang et al., 2014, 2013). The older T
DM2
ages of ca. 1.81 Ga for
the monzogranites resemble the formation age of the Baishahe
Group (Wang et al., 2007), indicating that older continental crust
materials might do duty for the principal source. The positive Hf
for monzogranites suggest that the source might have incorpo-
rated into a high-proportioned juvenile materials. In conclusion,
we conclude that the monzogranites are probably originated
from partial melting of ancient lower crust during the Paleo-
Mesoproterozoic, including juvenile crustal materials.
4.2.1.2 Mafic-ultramafic complexes
The ε
Hf
(t) values of zircons are intermediate between those
of chondrites and depleted mantle (Fig. 9), implying the gabbros
were mainly associated with partial melting of depleted mantle.
Except for two samples, Zr/Nb and Sm/Nd ratios are 22.31–
29.74 and ~0.26, respectively, similar to MORB values (10–60
and ~0.32, Davidson, 1996; Anderson, 1994). But Hf model ages
T
DM1
(1 087–769 Ma) of the zircons are much older. If the prim-
itive magma of the zircons came from depleted mantle, Hf model
ages would represent the time of separation of zircon from
source. Under the circumstances, the crystallization ages (397
Ma) should be comparably equal to the Hf model ages (Wu et al.,
2007, 2004). Therefore, we consider that the enriched materials
have contaminated the magma source.
Nb/U ratios are generally defined to reflect the mantle
source characteristics, on account of uniform partition coeffi-
cient during magmatic crystallization process. MORB and OIB
have Nb/U ratios of ~50 (McDonough and Sun, 1995) and con-
tinental upper crust ~9 (Rudnick and Fountain, 1995), but arc
volcanic rocks have lower ratios of 0.3–9 (Chung et al., 2001).
If mantle rocks are generated by contamination of the crust ma-
terials, Nb/U ratios should be between ~9 and ~50. We’ve found
that three rocks have Nb/U ratios of 9.97–14.64, this may be the
result of contamination. The other three rocks have lower Nb/U
ratios (3.57–4.93) than those of continental upper crust but fall
into the range of arc volcanic rocks. This signifies that these
rocks aren’t formed by contamination, but stemmed from a met-
asomatic mantle source.
Previous researches have suggested that the bulk of fluid-
soluble elements would be affected by an influx of water from
the subducting plate in the mantle wedge, but HFSEs are rela-
tively depleted (Regelous et al., 1997). Thus, the LILEs in en-
richment and HFSEs in depletion are due to the metasomatism
of the mantle wedge (Peng et al., 2016), as observations of the
Maxingdawannan mafic-ultramafic complexes. In the Th/Yb vs.
Nb/Yb diagram, the mafic-ultramafic complexes, plotting close
to the volcanic arc array (Fig. 15a), which reflects the input of
subduction materials (Pearce and Peate, 1995). Ba/Th and Th/Nb
ratios can effectively distinguish between slab fluid and sedi-
ment melts. Maxingdawannan mafic-ultramafic complexes have
Th/Nb and Th/Yb ratios of 0.20–1.27 and 0.32–3.64, respec-
tively. Figure 15b shows that large volumes of fluids were in-
jected into the source region of the parental magmas (Hanyu et
al., 2006). In conclusion, we consider that the Maxingdawannan
mafic-ultramafic complexes are resulted from magma originated
from a depleted mantle source, which had previously been met-
asomatized by slab fluids.
Figure 15. (a) Nb/Yb vs. Th/Yb, (b) Th/Nb vs. Ba/Th diagrams of the Maxingdawannan mafic-ultramafic complexes (after Hanyu et al., 2006; Pearce and Peate, 1995).
Geochronology, Geochemistry, and Hf Isotopic Compositions of Monzogranites and Mafic-Ultramafic Complexes in the Maxingdawannan 343
4.3 Tectonic Implications
4.3.1 Post-collision magmatism in the EKOB
A-type granites are normally considered to form in an ex-
tension setting (Eby, 1992; Whalen et al., 1987), with A
2
-type
granites representing a post-orogenic setting. Mafic-ultramafic
complexes are also considered A-type granites. Granites in the
western EKOB have received little attention, and study of their
petrogenesis may help constrain the dynamical evolution of the
Proto-Tethys Ocean. As mentioned above, we suggest that
Maxingdawannan monzogranites have a A
2
-type origin. The
Maxingdawannan A
2
-type granites (399 Ma) and associated
mafic-ultramafic complexes (397 Ma) indicate the onset of an
Early Devonian post-collision setting in the EKOB.
Many granitic rocks in the EKOB have been generated in a
post-collision extensional setting (419.0±1.7 Ma, Yan et al., 2016;
420.6±2.6 Ma, Hao et al., 2015; 407±1.7 Ma, Liu et al., 2012; 432
Ma, Gao et al., 2010; 420±4 Ma, Hao et al., 2003). Massive mag-
matic Ni-(Cu) sulfide deposits linked with mafic-ultramafic com-
plexes (424–394 Ma; Peng et al., 2016; Song et al., 2016; Wang et
al., 2014; Li et al., 2012) and 391 Ma post-orogenic A
2
-type gran-
ites in the Xiarihamu area (Wang et al., 2013) are indicative of
strong extension. The ages of the Maxingdawannan monzogranites
and associated mafic-ultramafic complexes overlap those of the
Xiarihamu Ni-(Cu) Deposit (394–424 Ma; Peng et al., 2016; Song
et al., 2016; Wang et al., 2014; Li et al., 2012), which is consistent
with post-collisional extension. Given the facts above, the authors
inclined to support the view of Dong et al. (2017), that is, EKOB
was in the post-collisional setting during the Early Devonian.
4.3.2 Regional geodynamics
Systematic studies of the opening and expansion of the
Early Paleozoic oceans related to the rifting and prolonged
break-up of the Rodinia supercontinent at 860–570 Ma in the
EKOB were presented by Feng J Y et al. (2010), Li et al. (2008),
Lu (2001) and Yang et al. (1996). Early Paleozoic ophiolites
represent Proto-Tethyan oceanic crust along the central Kunlun
suture (Liu et al., 2011; Yang et al., 1996) that formed mainly at
525–509 Ma (Kong et al., 2017; Liu et al., 2011; Lu, 2002).
Subsequently, along the middle Kunlun fault, the Proto-Tethys
oceanic lithosphere began subducting to the north, meaning that
the southern margin of Qaidam Block switched from passive to
active property. A series of volcanic events may have occurred
along the middle Kunlun active epi-continental arc in this period
(Liu et al., 2013a, b; Cui et al., 2011; Chen F et al., 2002; Chen
N S et al., 2002). Gabbro from Qingshuiquan area (452±5 Ma;
Sang et al., 2016), basalt from the Kuangou-Xiaolangyashan
area (440±2 Ma; Wang et al., 2012) and basalt from Qimantagh
area (439±1.2 Ma; Li et al., 2008) were the products of north-
ward subduction of Proto-Tethys oceanic lithosphere. Along the
middle Kunlun suture zone, these mafic plutons and dikes may
represent the earliest stage of magmatism relevant to the Early
Paleozoic subduction of oceanic crust (Peng et al., 2016; Liu et
al., 2013a, b). Monzogranite and rapakivi were formed in conti-
nental collision setting that yield crystallization ages of
430.8±1.7 and 428.5±2.2 Ma in the EKOB, respectively (Cao et
al., 2011). Moreover, eclogites with ages of ~428 Ma were
formed in the course of closure of the Proto-Tethys Ocean (Meng
et al., 2013). Furthermore, the deposition of molasse sediments
of Maoniushan Formation (423–400 Ma; Lu, 2002, 2001) indi-
cate that subduction did not continue to Devonian. Collectively,
the ages of these rocks indicate that continent-continent collision,
including post-collision extension and closure of the Proto-
Tethys, finished at ~430 Ma.
Northward subduction of the Proto-Tethys oceanic plate was
resulted in metasomatism and enrichment of the subcontinental
lithospheric mantle by slab fluids during the Early Paleozoic. At
the same time, the Proto-Tethys oceanic lithosphere was accreted
to the Qaidam Block and then the closure of Proto-Tethys Ocean.
By reason of its great thickness (25–35 km), subduction of the
Proto-Tethys oceanic lithosphere was impeded, whereas the pre-
viously subducted oceanic crust continued sinking into the mantle.
Thus, the subducting slab broke due to the asymmetrical tensional
forces, resulting in the formation of a slab window. Astheno-
spheric mantle upwells through the slab window and then partial
melting. Overlying enriched lithospheric mantle would be incor-
porated into the primary magmas during their uprise. Fractional
crystallisation plus crustal contamination was resulted in S satu-
ration and sulphide liquation, and then magmatic differentiation.
These differentiated magmas ascended into the crust under buoy-
ancy, forming the mafic-ultramafic complexes. Subsequently, the
overlying felsic crust was heated by upwelling and melting of the
asthenosphere, resulting in partial melting of the felsic crust at
low pressure and formation of the monzogranites (Fig. 16).
Figure 16. Geodynamic sketch map of monzogranite and mafic-ultramafic complexes, modified after Wang et al. (2014a).
344 Jiaming Yan, Guosheng Sun, Fengyue Sun, Liang Li, Haoran Li, Zhenhua Gao, Lei Hua and Zhengping Yan
5 CONCLUSIONS
(1) U-Pb zircon dating shows that the monzogranites
(398.8±1.4 Ma) and mafic-ultramafic complexes (396.65±0.92
Ma) were formed during the Early Devonian.
(2) The monzogranites were probably generated by partial
melting of ancient lower crust during the Paleo-Mesoproterozoic,
involving juvenile crustal melt. The mafic-ultramafic complexes
were probably originated from a depleted mantle source that had
previously been modified by slab fluids. Crustal materials were
incorporated into the primitive magmas during their ascent.
(3) The Early Devonian intrusions were associated with
northward subduction of the Proto-Tethys oceanic lithosphere.
ACKNOWLEDGMENTS
We thank the staff of the Yanduzhongshi Geological Anal-
ysis Laboratories Ltd., Institute of Mineral Resources, Chinese
Academy of Geological Sciences, for helping in the analysis.
This work was supported by the National Natural Science Foun-
dation of China (No. 41272093), and China Geological Survey
(No. 12120114080901). The final publication is available at
Springer via https://doi.org/10.1007/s12583-018-1203-8.
Electronic Supplementary Materials: Supplementary materi-
als (Tables S1, S2, S3) are available in the online version of this
article at https://doi.org/10.1007/s12583-018-1203-8.
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