Early cretaceous subduction-related adakite

Journal of Asian Earth Sciences 34 (2009) 298–309
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Journal of Asian Earth Sciences
journal homepage: www.elsevier.com/locate/jaes
Early cretaceous subduction-related adakite-like rocks of the Gangdese Belt,
southern Tibet: Products of slab melting and subsequent melt–peridotite
interaction?
Di-Cheng Zhu a,*, Zhi-Dan Zhao a, Gui-Tang Pan b, Hao-Yang Lee c, Zhi-Qiang Kang d, Zhong-Li Liao b,
Li-Quan Wang b, Guang-Ming Li b, Guo-Chen Dong a, Bo Liu b
a
State Key Laboratory of Geological Processes and Mineral Resources, China University of Geosciences, 29# Xue-Yuan Road, Haidian District, Beijing 100083, China
Chengdu Institute of Geology and Mineral Resources, 610082 Chengdu, China
Department of Geosciences, National Taiwan University, Taipei 106, Taiwan
d
Key Laboratory of Isotope Geochronology and Geochemistry, Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, Guangzhou 510640, China
b
c
a r t i c l e
i n f o
Article history:
Received 17 November 2007
Received in revised form 24 April 2008
Accepted 19 May 2008
Keywords:
SHRIMP zircon dating
In situ Hf isotope analysis of zircon
Subduction-related adakite
Early Cretaceous
Southern Tibet
a b s t r a c t
The limited geochronology and geochemistry data available for the Early Cretaceous igneous rocks of the
southern Gangdese Belt, southern Tibet, has resulted in the proposal of conflicting geodynamic models
for the generation of the widespread Cretaceous igneous rocks in the middle and northern parts of the
belt. To explore this issue, we present SHRIMP U–Pb zircon data and geochemical and Sr–Nd–Pb–Hf isotopic data for the Mamen andesites from the southern margin of the Gangdese Belt. The Mamen andesites, emplaced at 136.5 Ma, are sodic (Na2O/K2O = 1.2–2.3) and have geochemical characteristics
typical of adakites (i.e., high Al2O3, high La/Yb ratios and Sr contents, low Y and HREE contents, and positive Eu anomalies), except for high Cr, Ni, and MgO contents. The andesites have initial (87Sr/86Sr)t ratios
of 0.70413–0.70513, positive eNd(t) values of 3.7–5.8, and (206Pb/204Pb)t ratios of 18.37–18.51,
(207Pb/204Pb)t ratios of 15.59–15.65, and (208Pb/204Pb)t ratios of 38.43–38.72. In situ Hf isotopic analyses
of zircons that had previously been dated by SHRIMP yielded positive initial eHf(t) values ranging from
+11.0 to +15.5. A model calculation using trace element and Sr–Nd–Pb isotopic data indicates that several
percent of subducted sediment is required to generate the Mamen andesites, which were derived via the
partial melting of subducted Neo-Tethyan slab (MORB + sediment + fluid) and subsequently hybridized
by peridotite in the mantle wedge. Our data indicate that the Neo-Tethyan oceanic crust was subducted
northward beneath the Gangdese Belt during the Early Cretaceous at a high angle. Our results are inconsistent with a tectonic model that advocates the low-angle or flat-slab subduction of Neo-Tethyan oceanic crust in generating the widespread Cretaceous magmatism recorded in the Gangdese Belt.
Ó 2008 Elsevier Ltd. All rights reserved.
1. Introduction
It is traditionally accepted that the Gangdese Belt, located
between the Bangong Tso–Nujiang suture zone to the north and
the Yarlung Zangbo suture zone to the south (Fig. 1a), is not only
an archetype of a collisional orogen related to India–Asia collision,
but also a pre-Cenozoic Andean-style convergent margin associated with northward subduction of Neo-Tethyan oceanic crust
(Maluski et al., 1982; Xu et al., 1985; Coulon et al., 1986; XBGMR,
1991; Copeland et al., 1995; Yin and Harrison, 2000). Numerous
studies in recent decades on Cenozoic magmatism have helped
develop an understanding of the India–Asia collision and related
Cenozoic tectonic processes that led to the formation of the Hima* Corresponding author. Tel.: +86 10 8232 1115; fax: +86 10 8232 2094.
E-mail address: [email protected] (D.-C. Zhu).
1367-9120/$ - see front matter Ó 2008 Elsevier Ltd. All rights reserved.
doi:10.1016/j.jseaes.2008.05.003
layas and the Tibetan Plateau. However, relatively little work has
focused on pre-Cenozoic magmatism, resulting in conflicting interpretations regarding the geodynamic setting of the widespread
Cretaceous magmatism present in the middle and northern parts
of the Gangdese Belt (Fig. 1a). The magmatism has been interpreted to have originated from the southward subduction of Bangong Tso–Nujiang oceanic crust (Hsü et al., 1995; Mo et al.,
2005; Pan et al., 2006; Zhu et al., 2006, 2008a) or the northward
low-angle or flat-slab subduction of Neo-Tethyan oceanic crust
(Ding et al., 2003; Kapp et al., 2003, 2005, 2007; Leier et al., 2007).
The term ‘adakite’ is widely used to represent silica-rich, high
Sr/Y and La/Yb volcanic and plutonic rocks that form in a variety
of tectonic settings (e.g., subduction zones, continental collision
zones, and extensional environments) via various petrogenetic
processes (Defant and Drummond, 1990; Atherton and Petford,
1993; Xu et al., 2002; Chung et al., 2003; Hou et al., 2004; Wang
D.-C. Zhu et al. / Journal of Asian Earth Sciences 34 (2009) 298–309
299
Fig. 1. (a) Tectonic outline of the Tibetan Plateau (modified from Pan et al., 2006). (b) Tectonic map of the Gangdese Belt and distribution of the Sangri Group, Zenong Group,
Duoni Formation, and Linzizong volcanic rocks (modified from Zhu et al., 2008a). (c) Map showing the distribution of Mesozoic igneous rocks in the southern Gangdese Belt
(modified from Zhang et al., 2005 and Zhu et al., 2008a).
et al., 2005; Guo et al., 2007). Although adakites have been recognized in southern Tibet for several years, previously reported rocks
are all post-collision adakites (26–10 Ma) derived from the partial
melting of lower crust (Chung et al., 2003; Hou et al., 2004; Guo et
al., 2007). Their development has typically been discussed in terms
of their significance with respect to the timing of uplift of the Tibetan Plateau and the onset of east–west extension within the plateau (Chung et al., 2003, 2005; Hou et al., 2004; Guo et al., 2007).
No adakites older than 100 Ma had been reported from the Gangdese Belt until the work of Yao et al. (2006), who described the elemental geochemistry of Late Jurassic–Early Cretaceous adakites
summarizing from geological survey. However, these adakites have
been often questioned due to the lack of good quality age data and
geochemical data.
In this paper, we report the first SHRIMP zircon age data for
Early Cretaceous adakite-like rocks from eastern Zedong, southern margin of the Gangdese Belt. We also present new wholerock elemental, Sr–Nd–Pb isotopic, and in situ zircon Hf isotopic
data with the aim of gaining a better understanding of the petrogenesis and subduction history of Neo-Tethyan oceanic crust.
Our data provide valuable constraints on the geodynamic processes involved in the generation of Early Cretaceous magmas
in the Gangdese Belt.
2. Geological setting
Tibet is essentially composed of the following four continental blocks or terranes (from north to south): the Songpan–Ganzi
flysch complex, Qiangtang terrane, Gangdese Belt, and the
Himalayan Belt. These blocks are separated by the Jinsha, Bangong–Nujiang, and Yarlung Zangbo suture zones, representing
Paleo-, Meso-, and Neo-Tethyan oceanic relicts, respectively
(Fig. 1a) (cf. Yin and Harrison, 2000). The Yarlung Zangbo suture
zone comprises abundant Jurassic–Cretaceous ophiolites and
minor Late Triassic–Middle Jurassic ophiolites (Pan et al.,
2006), marking the location where the Neo-Tethyan oceanic
domains were consumed by northward subduction beneath the
Gangdese Belt during the Early Jurassic to Late Cretaceous (Xu
et al., 1985; Harris et al., 1988; Zhu et al., 2008a, and references
therein).
The Gangdese Belt consists primarily of Paleozoic–Paleogene
sedimentary strata and associated igneous rocks (Yin and Harrison, 2000). The latter include a series of volcanic suites (e.g.,
Early Jurassic volcanic rocks of the Yeba Formation, Zhu et al.,
2008a; Late Jurassic–Early Cretaceous volcanic rocks of the Sangri Group, Zhu et al., 2006), the voluminous Gangdese batholith
(ca. 103–80 Ma, Wen et al., 2008), and the Linzizong volcanic
successions (ca. 65–45 Ma, Mo et al., 2006) in the southern
Gangdese Belt, together with widespread Mesozoic igneous rocks
(e.g., Early Jurassic Amdo granitoids, Guynn et al., 2006; Late
Jurassic–Early Cretaceous volcanic rocks of the Zenong Group
and associated granitoids, Zhu et al., 2006) in the middle and
northern parts of the Gangdese Belt (Fig. 1b). These igneous
rocks define five magmatic episodes that took place at 190–
175, 120–110, 100–80, 65–45, and 25–10 Ma, with two magmatic flare-ups at ca. 110 and 50 Ma (Wen et al., 2008; Zhu
et al., 2008b). The Gangdese Belt is traditionally thought to have
detached from Gondwana and then drifted northward, finally
amalgamating with the Qiangtang terrane in the Early Cretaceous (Kapp et al., 2005). Mesozoic magmatism in the southern
Gangdese Belt is generally ascribed to the northward subduction
of the Neo-Tethyan oceanic crust beneath the Gangdese Belt;
however, the geodynamic process of the magmatism in the middle and northern parts of the belt remains a subject of debate
(e.g., Kapp et al., 2005, 2007; Zhu et al., 2008b, and references
therein).
300
D.-C. Zhu et al. / Journal of Asian Earth Sciences 34 (2009) 298–309
3. Field occurrence and petrography
4. Analytical techniques
The Late Jurassic–Early Cretaceous volcano-sedimentary
sequences of the Sangri Group, which consists of the underlying
Mamuxia Formation (the focus of the present study) and overlying Bima Formation, are sporadically exposed in the southern
Gangdese Belt from Yawa in the west to Sangri County in the
east (Fig. 1c). The general lithological features of the Mamuxia
Formation are shown in Fig. 2 and summarized in Table 1. As
a whole, a Late Jurassic–Early Cretaceous age for deposition of
the Mamuxia Formation is indicated by fossil corals, bivalves,
and gastropods observed in bioclastic limestone from the Yawa,
Salada, Rongma, Padui, and Mamen sections (Fig. 2; Table 1).
Regional comparisons of sedimentary sequence and fossils indicate that the major period of volcanism probably started during
the early stages of deposition of the Mamuxia Formation (Fig. 2),
in a shore to shallow sea or continental shelf facies (Zhu et al.,
2003). The Mamuxia Formation is concordantly overlain by the
Early Cretaceous Bima Formation, which consists mainly of volcanic rocks, sandstones and siltstones, slates, and bioclastic crystalline limestones. The volcanic rocks within the formation
(1500 m thick) vary compositionally from basaltic andesite to
andesite and dacite, with typical island-arc geochemical signatures (Li and Zhang, 1995).
To constrain the age and geochemical nature of volcanic rocks
within the Mamuxia Formation, samples were collected from the
Mamen section, where the formation was originally identified
(Fig. 2f; Badengzhu, 1979) and is easily accessible. The Mamen section is located on the south bank of the Yarlung Zangbo River,
about 3 km north of the Yarlung Zangbo suture zone (Fig. 2f). Phenocrysts within the Mamen andesites are predominantly chloritized plagioclase. Minor epidotized amphibole and rare
clinopyroxene and magnetite occur. The groundmass is dominated
by abundant plagioclase micro-crystals.
Five of the six samples described in this paper were collected in
2003 from the base of the Mamen section; the remaining sample
(T203A) was collected in 2005 from the same section (Fig. 2h).
Powdered samples were analyzed for major elements by X-ray
fluorescence (XRF) at the Analytical Center, Chengdu Institute of
Geology and Mineral Resources, China, with analytical uncertainties better than 5%. Trace element concentrations were determined
using a Perkin Elmer Elan 6000 ICP-MS at the National Geological
Analytical Center, Chinese Academy of Geological Sciences, Beijing,
China; analytical accuracy and precision were generally better than
8%. Further details of analytical methods can be found in Guo et al.
(2005). Sample T203A was analyzed for major elements by X-ray
fluorescence using a Rigaku RIX-2000 spectrometer and for trace
elements by ICP-MS using an AgilentÒ 7500s, both housed at the
Department of Geosciences, National Taiwan University, Taiwan;
further details can be found in Chung et al. (2003).
Whole-rock Nd and Sr isotopic compositions were determined
using a multicollector Finnigan MAT-261 mass spectrometer operated in static multicollector mode at the Laboratory for Radiogenic
Isotope Geochemistry, Institute of Geology and Geophysics, Chinese Academy of Sciences, Beijing (IGGCAS), China. Measured
87
Sr/86Sr and 143Nd/144Nd ratios were normalized to
86
Sr/88Sr = 0.1194 and 146Nd/144Nd = 0.7219, respectively, for mass
fractionation correction. During the period of data acquisition, the
mean 87Sr/86Sr ratio of NBS987 standard was 0.710254 ± 16 (n = 8),
the mean 143Nd/144Nd ratio of La Jolla standard was 0.511862 ± 7
(n = 12), and standard BCR-1 yielded 143Nd/144Nd = 0.512626 ± 9
(n = 12). Pb isotopic ratios were measured using a VG354 mass
spectrometer at the National Geological Analytical Center, Chinese
Academy of Geological Sciences, Beijing, China. The standard
204
Pb/206Pb = 0.059003 ± 0.000084
(n = 6),
NBS981
yielded
207
208
Pb/206Pb = 0.91449 ± 0.00017
(n = 6),
and
Pb/206Pb =
Fig. 2. (a–e) Stratigraphic sections of the Mamuxia Formation, showing the spatial variation of different rock types (Gao et al., 1994; Xie et al., 2003; Zhu et al., 2003). (f)
Geological sketch map of the studied area (Badengzhu, 1979). (g). Entire Mamen section, showing sample locations (modified from Badengzhu, 1979). (h) Profile of the
Mamen andesites, showing sample locations.
Badengzhu (1979), Li and
Zhang (1995), and this study
Gao et al. (1994)
Sandstone
>300 m
Andesite, dacite
Unexposed
Brecciated limestone
>400 m
180 m
301
2.16691 ± 0.00097 (n = 6). The average 2r uncertainties for
206
Pb/204Pb, 207Pb/204Pb, and 208Pb/204Pb were 0.7%, 0.3%, and
0.6% per atomic mass unit, respectively. Details of the analytical
procedures employed in measuring Sr–Nd–Pb isotopes can be
found in Guo et al. (2005).
Zircons were successfully separated from a relatively coarsegrained sample (MM02-3) using standard density and magnetic
separation techniques at the Special Laboratory of the Geological
Team of Hebei Province, China. In situ zircon U–Pb dating was carried out using a SHRIMP II at the Beijing SHRIMP Lab, Chinese
Academy of Geological Sciences, China, with analytical conditions
the same as those reported in Liu et al. (2006). U–Th–Pb isotope ratios were measured relative to the zircon standard TEMORA (Black
et al., 2003).
In situ Hf isotope measurements were subsequently performed
on the dated spots within the zircons using LA-MC-ICP-MS,
equipped with a 193 nm laser, at the IGGCAS. A stationary spot
with a beam diameter of about 63 lm was used for the analyses.
Instrumental conditions and data acquisition were generally as described by Wu et al. (2006). During analyses, the 176Hf/177Hf and
176
Lu/177Hf ratios of the standard zircon (91500) were
0.282322 ± 22 (2rn, n = 28) and 0.000318, consistent with the values (0.282307 ± 31, 2rn, n = 44) obtained previously in this laboratory (Wu et al., 2006).
5. Results
5.1. Zircon SHRIMP U–Pb data
Zircon SHRIMP U–Pb data are summarized in Table 2 and
shown in Fig. 3. Cathodoluminescence (CL) images of zircon demonstrate that the grains are mostly 100–250 lm in size (Fig. 3a). All
of the zircons show similar crystal forms, with no resorption or
inherited cores. The U and Th contents of analyzed zircons are
39–241 and 40–234 ppm, respectively, with Th/U ratios ranging
from 0.62 to 1.07. These ratios are higher than those of metamorphic zircons (typically <0.1), but consistent with those of magmatic
zircons (Hoskin and Black, 2000). Fourteen U–Pb analyses yielded
ages of 141.2 to 119.4 Ma. The concordant curve (Fig. 3b) reflects
relatively large uncertainties associated with the 207Pb/235U ages,
possibly related to correction for common lead, which is difficult
to determine precisely. This uncertainty is relatively minor for
the obtained 206Pb/ 238U ages; consequently, we refer to 206Pb/
238
U ages when considering the crystallization age of the Mamen
magma. With the exception of two discordant spots (13.1 and
14.1), 12 analyses yield a weighted mean 206Pb/238U age of
136.5 ± 1.7 Ma, with a MSWD of 0.99 at the 95% confidence interval
(2r). We therefore conclude that Mamen volcanism occurred in
the Early Cretaceous, consistent with the constraints of age-diagnostic fossils (Badengzhu, 1979).
Andesite
5.2. Whole-rock geochemistry
Mamen
Padui
Limestone breccia,
sandstone
Skarnized limestone,
brecciated limestone
Limestone
50 m
Hornblendebearing andesite
Andesite, tuff
Southeastern
Xietongmen
Northern
Jiedexiu Town
Southern
Mamen
E91°58.5310
N29°15.2100
Rongma
Duojiza
Marble, calcareous
sandstone
5m
Basic tuff
E87°380 N29°390
Salada
Bioclastic
limestone
Conglomerate
Gasteropod: Nerinea sp.; bivalve: Metaceriturn sp., Plagiostomo of
muddoerensis; coral: Diococyathus sp., etc.
Zircon SHRIMP U–Pb date: 136.5 Ma
Gao et al. (1994)
Zhu et al. (2003)
Coral: Montlivaltia sp., Discocyathus sp., Thecosmilia sp., Hexacoralla sp.,
Cyathophora sp., Actinaraea sp.; gasteropod: Nerinea sp., Maltiptyxis sp.,
etc.
Coral: Calamorphillia sp., Cyathophora sp., Cladocoropsis sp., etc.
Zhu et al. (2003)
Xie et al. (2003)
Coral: Montlivaltia sp., Distichophyllia sp., Thecosmilia sp.,
Calamophylliopsis sp., etc.
Conglomerate,
bioclastic limestone
275 m
Hornblendebearing andesite
E85°420 N30°000
Yawa
Sandy micrite,
bioclastic
limestone
Siltstone, slate
Reference
Fossil/isotopic age
Underlying
lithology
Overlying lithology
Thickness
Rock type
Locality
Section
Table 1
General lithological features of the Mamuxia Formation in the southern Gangdese Belt, southern Tibet
D.-C. Zhu et al. / Journal of Asian Earth Sciences 34 (2009) 298–309
Whole-rock geochemical data of analyzed Mamen andesites are
listed in Table 3. Major element compositions are normalized to
100% on a volatile-free basis. Mamen lavas are characterized by a
limited range in SiO2 content (56–63%), and plot in the mediumand high-potassic andesite domains on a K2O vs. SiO2 diagram
(Rollinson, 1993) (Fig. 4). All Mamen lavas record Al2O3 contents
greater than 15% and high Na2O concentrations (up to 5.2%). The
sodic character of these lavas is reinforced by Na2O/K2O ratios as
high as 2.3.
Mamen lavas display low concentrations of heavy rare earth
elements (HREEs) and Y (e.g., Yb = 0.71–1.08 ppm; Y = 8.7–
13.5 ppm). These characteristics, together with high Sr contents
(476–1755 ppm) and Sr/Y ratios (45–73), indicate that the samples
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D.-C. Zhu et al. / Journal of Asian Earth Sciences 34 (2009) 298–309
Table 2
SHRIMP zircon data of a Mamen adakite-like rock (MM02-3), southern Tibet
Spot
U (ppm)
0
Th (ppm)
Th/U
f206c (%)
206
Pb* (ppm)
207
Pb*/235U (±1r)
206
Pb*/238U (±1r)
206
Pb/238U (Ma; ±1r)
0
MM02-3 (N29°15.210 , N91°58.531 , 3638 m)
MM02-3-1.1
67
47
MM02-3-2.1
74
53
MM02-3-3.1
241
234
MM02-3-4.1
135
114
MM02-3-5.1
183
162
MM02-3-6.1
86
84
MM02-3-7.1
194
197
MM02-3-8.1
130
110
MM02-3-9.1
121
88
MM02-3-10.1
174
143
MM02-3-11.1
65
40
MM02-3-12.1
65
45
MM02-3-13.1
210
224
MM02-3-14.1
39
40
Weighted mean (without discordant spots 13.1 and
f206c denotes the proportion of common
206
0.70
9.94
1.37
0.72
4.07
1.36
0.97
2.84
4.51
0.84
1.00
2.52
0.89
1.58
3.41
0.98
2.20
1.64
1.02
1.32
3.70
0.84
4.22
2.48
0.73
0.48
2.31
0.82
0.00
3.16
0.62
5.59
1.22
0.69
3.65
1.21
1.07
1.62
3.65
1.04
4.70
0.658
14.1, 95% confidence, MSWD = 0.99)
Pb in total measured
206
0.166
0.134
0.129
0.157
0.185
0.241
0.178
0.150
0.211
0.217
0.253
0.237
0.169
0.328
(0.075)
(0.042)
(0.022)
0.014)
(0.028)
(0.020)
(0.016)
(0.053)
(0.030)
(0.009)
(0.056)
(0.043)
(0.020)
(0.072)
0.0216
0.0206
0.0212
0.0216
0.0214
0.0218
0.0219
0.0212
0.0221
0.0211
0.0205
0.0210
0.0199
0.0187
(0.0009)
(0.0006)
(0.0004)
(0.0004)
(0.0004)
(0.0005)
(0.0004)
(0.0006)
(0.0004)
(0.0004)
(0.0007)
(0.0006)
(0.0004)
(0.0006)
137.8
131.5
135.0
137.6
136.4
139.1
139.5
135.3
141.2
134.8
130.5
133.9
126.8
119.4
136.5
(5.4)
(3.6)
(2.4)
(2.6)
(2.7)
(3.0)
(2.5)
(3.7)
(2.8)
(2.3)
(4.1)
(3.6)
(2.2)
(4.0)
(1.7)
Pb*. * denotes radiogenic lead.
Fig. 3. Cathodoluminescence image (a) and concordia plot (b) of zircon SHRIMP data for the Mamen andesite (sample MM02-3) in the southern Gangdese Belt, southern
Tibet. Solid and dashed circles indicate the locations of SHRIMP U–Pb analyses and LA-MC-ICP-MS Hf analyses, respectively. The SHRIMP U–Pb ages and eHf(t) values are
given for each spot.
can be classified as adakites as defined by Defant and Drummond
(1990) (Fig. 5a), although the samples also exhibit relatively high
MgO contents (3.53–5.83%), high Mg-numbers (57.8–72.9), and
high concentrations of compatible elements (e.g., Cr = 176–
225 ppm; Ni = 105–143 ppm).
The samples display small positive Eu anomalies (Eu/
Eu* = 0.96–1.35) and have steep heavy REE (HREE) patterns (Fig.
5b). High (La/Yb)N ratios (17–24) (the subscript ‘N’ denotes that
the concentration is normalized to chondrite) indicate pronounced
LREE/HREE fractionation. The samples show strong enrichment in
large ion lithophile elements (LILEs) relative to high field strength
elements (HFSEs) and pronounced negative Nb–Ta anomalies and
positive K and Pb anomalies in primitive-mantle-normalized
incompatible element patterns (Fig. 5c).
The analyzed samples have relatively low (87Sr/86Sr)t values
(0.70413–0.70513) and moderately positive eNd(t) values (3.7–
5.8) (Table 3) relative to bulk Earth (Fig. 6a), and high (207Pb/204Pb)t
(15.59–15.65) and (208Pb/204Pb)t (38.43–38.72) values at a given
(206Pb/204Pb)t (18.37–18.51) (Table 3) compared with the Northern
Hemisphere Reference Line (not shown in figures).
Regarding trace elements, the Mamen lavas differ markedly
from the Linzizong andesites (Mo et al., 2007) in terms of their
steep slope in HREEs, and from post-collisional adakites by relative
high HREE concentrations and low concentrations of Th, U, and Pb
(Fig. 5b and c). Isotopically, Mamen lavas plot close to or overlap
with the field of 120 Ma Tethyan basalts (Mahoney et al., 1998)
(Fig. 6a and b), but are distinct from the Linzizong andesites (Mo
et al., 2007) and the majority of post-collisional adakites in southern Tibet (Hou et al., 2004; Guo et al., 2007).
5.3. Zircon Hf isotope
Thirteen in situ Hf isotope analyses were successfully carried
out on zircons within sample MM02-3 (Table 4). The zircons are
characterized by clearly positive initial eHf(t) values, with most
ranging from +11.0 to +12.9; spot 14.1 has the highest initial eHf(t)
value of +15.5 (Table 4). The positive initial eHf(t) values are comparable with those of Indian MORB (Fig. 6c) and are consistent with
a long-term depleted mantle source, in good agreement with the
Nd–Sr isotope compositions of sample MM02-3, which has a
eNd(t) value of +5.8 and initial 87Sr/86Sr ratio of 0.70413.
6. Discussion
6.1. Nature of the source region
The primitive-mantle-normalized incompatible element patterns of the Mamen adakite-like rocks (Fig. 5c) exhibit considerable
enrichment in LILEs and negative Nb–Ta anomalies, suggesting an
affinity with magmas generated in a subduction-related tectonic
setting. Previous studies have identified two components – the
partial melts of subducted sediment and slab-derived fluids – that
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D.-C. Zhu et al. / Journal of Asian Earth Sciences 34 (2009) 298–309
Table 3
Whole-rock major, trace element and Sr–Nd–Pb isotope data of the Mamen adakite-like rocks, southern Tibet
Sample
MM02-2
MM02-3
MM02-4
MM02-5
MM02-6
XRF – major element (wt.%)
SiO2
TiO2
Al2O3
TFe2O3*
MnO
MgO
CaO
Na2O
K2O
P2O5
LOI
Mg#
56.82
0.80
15.47
4.94
0.08
5.81
10.05
3.85
1.69
0.48
2.67
70.2
55.87
0.96
18.12
6.53
0.05
5.48
5.45
4.61
2.40
0.53
1.57
62.6
59.86
0.88
17.57
5.01
0.06
3.51
7.62
3.30
1.87
0.30
1.47
58.4
57.77
0.98
17.78
4.34
0.06
5.83
5.33
5.17
2.23
0.51
1.55
72.9
62.98
0.95
16.61
6.07
0.07
4.41
4.18
2.56
1.92
0.26
2.22
59.2
58.05
1.10
18.97
5.88
0.06
4.03
6.52
2.75
2.37
0.27
ICP-MS – trace element (ppm)
Sc
13.4
V
125
Cr
190
Co
21
Ni
130
Cu
5.19
Zn
63.2
Rb
40.1
Sr
836
Y
11.5
Zr
125
Nb
9.58
Cs
10.7
Ba
1158
La
30.8
Ce
59.6
Pr
6.87
Nd
26.5
Sm
4.79
Eu
1.69
Gd
3.33
Tb
0.44
Dy
2.41
Ho
0.43
Er
1.19
Tm
0.16
Yb
0.97
Lu
0.14
Hf
3.25
Ta
0.49
Pb
16.1
Th
2.45
U
1.32
1.30
Eu/Eu*
15.5
137
176
27
133
30.3
48.9
129
530
11.9
149
10.6
61.0
577
26.0
50.3
5.77
22.7
4.03
1.66
3.49
0.46
2.39
0.44
1.25
0.16
1.00
0.15
3.40
0.60
11.2
2.85
1.43
1.35
11.4
100
225
24
105
63.1
157
62.1
631
11.2
125
10.7
30.0
477
24.1
53.1
6.43
25.0
4.52
1.26
3.21
0.45
2.45
0.44
1.21
0.16
0.99
0.14
3.43
0.56
15.2
2.48
0.62
1.01
14.8
123
190
20
143
23.6
57.4
65.4
692
11.3
158
10.8
17.7
973
22.3
44.2
5.14
20.1
3.69
1.27
3.17
0.43
2.25
0.41
1.23
0.16
0.96
0.14
3.68
0.61
11.5
3.24
1.29
1.14
8.5
85.2
225
22
139
298
62.3
57.7
476
8.7
133
11.0
26.8
475
20.0
44.1
5.04
20.0
3.79
1.09
2.62
0.37
1.99
0.35
0.94
0.12
0.71
0.10
3.78
0.58
10.5
2.22
0.38
1.06
19.3
25
220
20
108
220
58.0
86.9
633
13.5
156
10.7
44.4
724
28.5
53.4
7.21
28.3
5.03
1.38
3.89
0.54
2.76
0.49
1.28
0.18
1.08
0.16
3.60
0.68
12.0
4.03
1.06
0.96
Sr–Nd–Pb isotope compositions
87
Rb/86Sr
0.0961
87
Sr/86Sr (±2r)
0.705320 ± 11
0.70513
(87Sr/86Sr)t
147
Sm/144Nd
0.1081
143
144
Nd/ Nd (±2r)
0.512748 ± 11
143
144
0.512651
( Nd/ Nd)t
eNd(t)
3.7
206
Pb/204Pb (±2r)
18.6947 ± 12
207
Pb/204Pb (±2r)
15.6546 ± 10
208
Pb/204Pb (±2r)
38.7932 ± 27
206
204
( Pb/ Pb)t
18.57
207
204
15.65
( Pb/ Pb)t
38.72
(208Pb/204Pb)t
0.5612
0.705221 ± 13
0.70413
0.1089
0.512859 ± 14
0.512762
5.8
18.5663 ± 13
15.5988 ± 11
38.5571 ± 35
18.37
15.59
38.43
0.2370
0.705132 ± 11
0.70467
0.1081
0.512763 ± 10
0.512666
4.0
18.5725 ± 10
15.6519 ± 10
38.7642 ± 26
18.51
15.65
38.68
T203A
57.8
0.3314
0.705207 ± 11
0.70456
0.1122
0.512763 ± 12
0.512663
3.9
18.6069 ± 12
15.6487 ± 10
38.7725 ± 26
18.55
15.65
38.67
Major element oxide contents are normalized to 100 wt.% on a volatile-free basis. LOI = loss on ignition; total iron as TFe2O3*, Mg# = 100 molar Mg2+/(Mg2+ + total Fe2+)],
calculated by assuming total FeO = 0.9 TFe2O3*. Eu/Eu* = EuN/(SmN GdN)1/2, N is chondrite-normalized (Sun and McDonough, 1989). T = age-corrected initial isotopic ratios. Corrected formula as follows: (87Sr/86Sr)t = (87Sr/86Sr)m + 8 7Rb/86Sr(ekt 1), k = 1.42 1011 a1; (143Nd/144Nd)t = (143Nd/144Nd)m + (147Sm/144Nd)m (ekt 1),
eNd(t) = [(143Nd/144Nd)m/(143Nd/144Nd)CHUR(t) 1] 104, (143Nd/144Nd)CHUR(t) = 0.512638 0.1967 (ekt 1), k = 6.54 1012a1. (206Pb/204Pb)t = (206Pb/204Pb)m +
238
U/204Pb (ek1t 1), k1 = 1.55125 1010 a1; (207Pb/204Pb)t = (207Pb/204Pb)m + 235U/204Pb (ek2t 1), k2 = 9.8485 1010 a1; (208Pb/204Pb)t = (208Pb/204Pb)m +
232
U/204Pb (ek3t 1), k3 = 0.49475 1010 a1.
may metasomatize and enrich the source region of subductionrelated magmas (Elburg et al., 2002; Guo et al., 2005). Slab-derived
fluids are characterized by high contents of Ba, Rb, Sr, U, and Pb,
whereas partial melts of subducted sediment contain high concentrations of Th and LREE (Hawkesworth et al., 1997; Guo et al., 2005,
2007). The Mamen adakite-like rocks exhibit variable Ba concen-
304
D.-C. Zhu et al. / Journal of Asian Earth Sciences 34 (2009) 298–309
Fig. 4. K2O vs. SiO2 classification diagram of Rollinson (1993) showing data for the
Mamen adakite-like rocks. All samples are plotted on an anhydrous basis.
trations (475–1158 ppm) coupled with a narrow range of Nb/Y
ratio (except sample MM02-6), consistent with fluid-induced
enrichment (Fig. 7a). Present-day arc settings in which significant
amounts of sediments are subducted typically show Th/Yb
ratios P 2, whereas fluid-dominated arc environments show
Th/Yb < 1 (Woodhead et al., 2001; Nebel et al., 2007). The Mamen
adakite-like rocks have Th/Yb ratios ranging from 2.51 to 3.74, suggesting a significant contribution from sediments in their origin. It
is unlikely that bulk amounts of subducted sediment can be added
to the mantle source of subduction-related magmas (Hawkesworth
et al., 1997). This argument is supported by the linear trend of Mamen adakite-like rocks in a Th/Yb vs. Th/Sm plot (Fig. 7b), which
could be interpreted in terms of two-component mixing between
the Dazhu–Langceling basalts from the Yarlung Zangbo suture
zone (or a partial melt thereof) and a partial melt of subducted
sediment.
The foregoing interpretation is consistent with the modeling
curves defined by Mamen adakite-like rocks in Sr–Nd–Pb isotope diagrams (Fig. 6a and b), in which the Dazhu–Langceling
basalts from the Yarlung Zangbo suture zone (Fig. 1b; Zhang
et al., 2005) and Indian Ocean pelagic sediment (Ben Othman
et al., 1989) are treated as proxies for the mantle source components of the Mamen adakite-like rocks and for Neo-Tethyan
oceanic sediment, respectively. The modeling results of twocomponent mixing indicate that the origin of Mamen adakitelike rocks can be explained by mixing with contributions of
5–10% Indian Oceanic sediments to attain the measured Sr–Nd
isotopic composition, or 1–3% to attain the measured Nd–Pb isotopic composition. Previous studies have shown that a small
contribution of sediment results in a drastic increase in
206
Pb/204Pb ratios in subduction-related rocks (Vroon et al.,
1995; Rolland et al., 2002). Thus, the decreased contribution of
sediments indicated by Nd–Pb isotopic compositions for the Mamen adakite-like rocks could be attributed to the effect of
206
Pb/204Pb ratios, which are highly sensitive to any input of
oceanic sediment. In any case, we can infer with confidence that
the magma source region of the Mamen adakite-like rocks was
at least partly mixed with sediments, as well as fluids driven off
from the sediments. This interpretation is similar to the observations of Rolland et al. (2002) for the Cretaceous Ladakh arc
and of Bignold and Treloar (2003) for the Cretaceous Kohistan
island arc, for which several percent of sediments are proposed
to have become entrained into the magma source regions to
explain the measured Sr–Nd–Pb isotopic compositions.
Fig. 5. (a) Sr/Y vs. Y discrimination diagram showing data for adakites and normal
calc-alkaline rocks (Defant and Drummond, 1990). (b–c) Chondrite-normalized REE
and primitive-mantle-normalized trace element patterns for Mamen adakite-like
rocks, Linzizong andesites (Mo et al., 2007), and post-collisional adakites (Chung
et al., 2003; Hou et al., 2004; Guo et al., 2007). Data for chondrite-normalized and
primitive-mantle-normalized values and plotting order are from Sun and McDonough (1989).
6.2. Petrogenesis
Previous studies suggest that the partial melting of metabasic
igneous rocks in the eclogite to amphibolite facies, either in the
thickened lower crust or in subducted oceanic crust, can produce
melts with the geochemical characteristics of adakites (Defant
and Drummond, 1990; Atherton and Petford, 1993; Yogodzinski
et al., 1995; Rapp et al., 1999; Chung et al., 2003; Hou et al.,
D.-C. Zhu et al. / Journal of Asian Earth Sciences 34 (2009) 298–309
2004; Wang et al., 2005, 2008; Guo et al., 2007). In the case of the
Mamen adakite-like rocks, their Early Cretaceous age is consistent
with slab melting during northward subduction of the Neo-Tethyan oceanic lithosphere beneath the Gangdese Belt (Xu et al.,
1985; Coulon et al., 1986; Harris et al., 1988; Copeland et al.,
1995; Yin and Harrison, 2000). This interpretation is supported
by the following lines of evidence.
(1) The high Mg-numbers (57.8–72.9) of the Mamen adakitelike rocks are inconsistent with a slab origin. The obtained Mgnumbers are significantly higher than those for sodium-rich
305
magmas from newly underplated basaltic crust (Atherton and Petford, 1993) and are distinct from the fields in the Mg# vs. SiO2 diagram (Fig. 8a) of proposed lower-crustal melts (Condie, 2005) and
of the lower-crust-derived post-collisional adakites of southern Tibet (Chung et al., 2003; Hou et al., 2004; Guo et al., 2007).
(2) Trace element signatures for the Mamen adakite-like rocks
are more consistent with slab melting than magma originating in
the lower crust. Empirically, adakites generated in the lower crust
tend to be K-rich and are distinguished by high contents of strongly
incompatible elements such as Rb, Ba, Th, and U (e.g., Wang et al.,
2005, 2007, 2008). Compared with post-collisional adakites in the
southern Gangdese Belt (Chung et al., 2003; Hou et al., 2004), the
Mamen adakite-like rocks are sodic (Table 1) and have low Th contents and Th/Ce ratios (Fig. 8b), similar to those of Cenozoic slabderived adakites in arc settings (Wang et al., 2008).
(3) Isotopic evidence also supports a slab origin for the Mamen adakite-like rocks. The results of recent studies indicate that
the Mesozoic lower crust of the southern Gangdese Belt is juvenile and was probably dominated by underplated magmas of
similar composition to the Yeba mafic rocks (Chu et al., 2006;
Zhu et al., 2008a). The highest eNd(t) value (+5.8), obtained for
sample MM02-3, is distinct from those of the Yeba mafic rocks
(Fig. 6a and b) in the southern Gangdese Belt. In terms of
Sr–Nd–Pb–Hf isotopic compositions, the Mamen adakite-like
rocks are comparable with the Tethyan basalts (Zhang et al.,
2005) (Fig. 6a and b) and Indian MORB (Fig. 6c), which probably
represented newly formed oceanic crust at the time of the Mamen adakite-like magmatism.
The Mamen adakite-like rocks are enriched in Zr and HREE (Fig.
9), with higher Cr, Ni, and MgO contents than typical adakite (Defant and Drummond, 1990). Experimental results show that during
ascent through the mantle wedge, slab melt assimilates peridotite
and undergoes metasomatic reactions involving orthopyroxene
and garnet. This process has the potential to significantly modify
SiO2, MgO, Ni, and Cr contents and increase the abundance of trace
elements in hybridized slab melts, although most element ratios
(e.g., La/Yb, Sr/Y, Sr/Nd, Nb/La, and K/La) remain largely unchanged
(Rapp et al., 1999). This process might be invoked to explain the
geochemical characteristics of the Mamen adakite-like rocks, as
their primitive-mantle-normalized incompatible element patterns
are consistent with the patterns observed in experimental melt
(Fig. 9; Martin et al., 2005). Such an interpretation of the origin
of the Mamen adakite-like rocks is similar to those of Stern and Kilian (1996) and Yogodzinski and Kelemen (1998), who suggested
that bajaites and related rocks are derived from reactions between
slab partial melt and overlying mantle peridotite.
In summary, the Mamen adakite-like rocks are interpreted to
have been derived directly from the partial melting of subducted
Neo-Tethyan slab (MORB + sediment + fluid), subsequently having
been hybridized by peridotite in the mantle wedge.
3
Fig. 6. eNd(t) vs. (87Sr/86Sr)t, eNd(t) vs. (206Pb/204Pb)t, and eHf(t) vs. eNd(t) diagrams
for Mamen adakite-like rocks. Data sources are as follows: Tethyan basalts (150 Ma
and 120 Ma; Mahoney et al., 1998), Dazhu–Langceling basalts (including sample
DZ98-1G) from the Yarlung Zangbo suture zone (Nd = 6.66 ppm, eNd(t) = 8.9,
Sr = 180.7 ppm, (87Sr/86Sr)t = 0.70354; Zhang et al., 2005), Indian Ocean pelagic
sediment (V28-343, Nd = 23.05 ppm, eNd(t) = –9.3, Sr = 119 ppm, (87Sr/86Sr)t =
0.71682, Pb = 32.68 ppm, 206Pb/204Pb = 18.99; Ben Othman et al., 1989), Yeba mafic
rocks (Zhu et al., 2008a), field of Hf–Nd isotopic data for Indian MORB, and Juvenile
Rock Array (Chauvel and Blichert-Toft, 2001; Ingle et al., 2003). The star at the top of
(b) is the average value for Langceling basalts (Nd = 3.53 ppm, eNd(t) = 8.8,
Sr = 103.7 ppm, (87Sr/86Sr)t = 0.70451, Pb = 0.43 ppm, 206Pb/204Pb = 17.68; Zhang et
al., 2005). Other data are as in Fig. 5. Indian Ocean pelagic sediment is used as a
proxy for Neo-Tethyan sediment. Note that the Sr–Nd and Nd–Pb isotopic
compositions of the Mamen adakite-like rocks can be attained by mixing with 5–
10% and 1–3% of Indian Oceanic sediments, respectively.
306
D.-C. Zhu et al. / Journal of Asian Earth Sciences 34 (2009) 298–309
Table 4
Hf isotopic data for zircons from a Mamen adakite-like rock (MM02-3), southern Tibet
Spot
176
Yb/177Hf
176
Lu/177Hf
1.1
2.1
3.1
4.1
5.1
6.1
7.1
8.1
9.1
10.1
11.1
12.1
13.1
14.1
0.022640
0.023769
0.044673
0.022723
0.042328
0.042767
0.000458
0.000471
0.000844
0.000437
0.000772
0.000784
0.048164
0.020913
0.027656
0.043378
0.022197
0.057601
0.088543
0.000866
0.000376
0.000610
0.000831
0.000419
0.001045
0.001714
176
Hf/177Hf
2r
176
0.283024
0.283033
0.283004
0.283038
0.283029
0.283038
0.000026
0.000020
0.000020
0.000015
0.000017
0.000018
0.283040
0.283004
0.283052
0.283004
0.283020
0.283060
0.283140
0.000025
0.000015
0.000027
0.000018
0.000023
0.000023
0.000018
Hf/177HfT
eHf(0)
eHf(t)
TDM1 (Ma)
TDM2 (Ma)
fLu/Hf
0.283023
0.283032
0.283002
0.283037
0.283027
0.283036
8.9
9.2
8.2
9.4
9.1
9.4
11.9
12.1
11.1
12.4
12.0
12.4
318
306
349
298
314
300
431
414
479
398
421
398
0.99
0.99
0.97
0.99
0.98
0.98
0.283037
0.283003
0.283050
0.283002
0.283019
0.283057
0.283137
9.5
8.2
9.9
8.2
8.8
10.2
13.0
12.4
11.3
12.8
11.0
11.7
12.9
15.5
300
346
280
349
323
272
160
399
474
369
482
440
359
183
0.97
0.99
0.98
0.97
0.99
0.97
0.95
*: eHf(t) = 10000 {[(176Hf/177Hf)S (176Lu/177Hf)S (ekt 1)]/[(176Hf/177Hf)CHUR,0 (176Lu/177Hf)CHUR (ekt 1)] 1}.
TDM1 = 1/k ln{1 + [(176Hf/177Hf)S (176Hf/177Hf)DM]/[(176Lu/177Hf)S (176Lu/177Hf)DM]}.
TDM2 = TDM1 (TDM1 t) [(fcc fs)/(fcc fDM)].
fLu/Hf = (176Lu/177Hf)S/(176Lu/177Hf)CHUR 1, where k = 1.867 1011 year1 (Soderlund et al., 2004); (176Lu/177Hf)S and (176Hf/177Hf)S are the measured values of the samples;
(176Lu/177Hf)CHUR = 0.0332 and (176Hf/177Hf)CHUR,0 = 0.282772 (Blichert-Toft and Albarède, 1997); (176Lu/177Hf)DM = 0.0384 and (176Hf/177Hf)DM = 0.28325 (Griffin et al., 2000);
(176Lu/177Hf)mean crust = 0.015; fcc = [(176Lu/177Hf)mean crust/(176Lu/177Hf)CHUR] 1; fs = fLu/Hf; fDM = [(176Lu/177Hf)DM/(176Lu/177Hf)CHUR]fLu/Hf = (176Lu/177Hf)S/(176Lu/177Hf)CHUR 1; t = crystallization time of zircon.
Fig. 7. Ba vs. Nb/Y and Th/Yb vs. Th/Sm plots for the Mamen adakite-like rocks. Other data are as in Fig. 5.
Fig. 8. (a–b) Mg# vs. SiO2 and Th/Ce vs. Th diagrams of the Mamen adakite-like rocks. Data sources: crustal AFC (Stern and Kilian, 1996), TTG (lower-crustal melts) and
adakite (slab melts) (Condie, 2005), Cenozoic crust-derived adakite of the Songpan-Ganzi block (intracontinental setting) and Cenozoic slab-derived adakites (arc setting)
(Wang et al., 2008). Other data are as in Fig. 5.
D.-C. Zhu et al. / Journal of Asian Earth Sciences 34 (2009) 298–309
307
would have occurred if flat subduction had continued for several
million years (Gutscher et al., 2000). Accordingly, we argue that
the generation of Early Cretaceous magmatism throughout the
Gangdese Belt can be attributed to a distinct geodynamic process
that is beyond the scope of this paper will be discussed in a future
study.
7. Conclusions
Fig. 9. Primitive-mantle-normalized trace element patterns of the Mamen adakitelike rocks, experimental melt (Martin et al., 2005), and adakite (Defant et al., 1991).
6.3. Geodynamic implications
SHRIMP zircon dating and geochemical data presented in this
study provide the first solid evidence for the existence of subduction-related adakite-like rocks in the southern Gangdese Belt during the Early Cretaceous. This arc volcanism significantly predates
India–Asia collision, which began at 65 Ma (Mo et al., 2006, and
references therein), thereby recording the northward subduction
of Neo-Tethyan oceanic crust prior to 130 Ma.
Given that adakites can only form at temperatures above 700 °C
and depths greater than 70–85 km, regardless of whether subduction occurs at normal dips or shallower angles (Defant et al., 1992;
Sajona et al., 1993; Gutscher et al., 2000), the Neo-Tethyan oceanic
crust is likely to have subducted beneath the southern Gangdese
sub-arc mantle to depths of 70–85 km during the Early Cretaceous.
This depth, together with the location of the Mamen adakite-like
rocks, which are exposed about 3 km to the north of the fossil
trench represented by the Yarlung Zangbo ophiolites (Fig. 2f), indicates that the Neo-Tethyan oceanic crust was subducted northward beneath the Gangdese Belt at a steep angle, similar to that
seen in the western Aleutians (Yogodzinski et al., 1995). In this
case, identification of the Mamen adakite-like rocks in the southern Gangdese Belt provides valuable constraints on the geodynamic process of widespread Early Cretaceous magmatism in the
middle and northern parts of the Gangdese Belt (Fig. 1a).
The widespread nature of Early Cretaceous magmatism in the
Gangdese Belt led some investigators (Coulon et al., 1986; Copeland et al., 1995; Ding et al., 2003; Kapp et al., 2003, 2005, 2007;
Leier et al., 2007) to suggest that low-angle or flat-slab subduction,
analogous to that observed in the modern Andes (Allmendinger et
al., 1997), may have occurred in southern Tibet prior to India–Asia
collision. Evidence in support of this model is based on the scarcity
of Early Cretaceous igneous rocks in the southern Gangdese Belt
(Kapp et al., 2007, and references therein); however, SHRIMP zircon age date for the Mamen adakite-like rocks reported in this
study, along with the regional comparison shown in Fig. 2, indicate
that significant arc volcanism was active in the southern Gangdese
Belt at 136 Ma, coeval with the initial volcanism recorded in the
Zenong Group (130 Ma; Zhu et al., 2008c) and associated Early
Cretaceous plutonism in the middle and northern parts of the
Gangdese Belt (133 Ma; Zhu et al., 2008b). These recently published age data suggest that extensive magmatism occurred contemporaneously throughout the Gangdese Belt during the Early
Cretaceous (Fig. 1b). Such an observation is inconsistent with a tectonic model that advocates the low-angle or flat-slab subduction of
the Neo-Tethyan oceanic crust, as a period of volcanic quiescence
(1) Early Cretaceous Mamen andesites (136.5 ± 1.7 Ma) in the
southern Gangdese Belt, southern Tibet, are sodic and show
geochemical affinities with adakite.
(2) The Mamen adakite-like rocks were probably derived from
partial melting of subducted Neo-Tethyan slab (MORB + sediment + fluid), subsequently hybridized by peridotite in the
mantle wedge.
(3) The Mamen adakite-like rocks probably resulted from the
northward subduction of Neo-Tethyan oceanic crust
beneath the southern Gangdese Belt at a relatively steep
angle during the Early Cretaceous.
(4) Our data are inconsistent with a tectonic model in which the
widespread Early Cretaceous igneous rocks of the middle
and northern parts of the Gangdese Belt were derived from
the low-angle or flat-slab subduction of Neo-Tethyan oceanic crust.
Acknowledgements
We thank Q.R. Geng and C.Y. Zhou for their assistance in the
field; H. Tao and B. Song for help with SHRIMP dating; and F.K.
Chen, C.F. Li, L.W. Xie, and Y.H. Yang for their assistance with
Sr–Nd–Hf isotopic analyses. We are grateful for helpful discussions
with Dr. S.L. Chung and Z.F. Guo, constructive reviews by Catherine
Chauvel and an anonymous reviewer, and insightful comments
and careful editorial handling by Bor-ming Jahn. This study benefited from financial support by ongoing NSFC projects (40503005,
40572051, and 40473020), the Programme of Excellent Young
Scientists of the Ministry of Land and Resources, the National
Key Project for Basic Research of China (Project 2002CB412600),
and the Integrated Study of Basic Geology of Qinghai–Tibetan
Plateau project.
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