• Evolution of lithospheric mantle beneath the Tan-Lu fault zone, eastern North China Craton

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    • Abstract: represent fragments of newly accreted lithospheric mantle that makes up much of the Late Mesozoic. Cenozoic lithosphere beneath the Tan-Lu fault zone. Cpx-rich lherzolite and wehrlite re ... Combined with the data for mantle xenoliths from the adjacent regions, a highly heterogeneous ...

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Lithos 117 (2010) 229–246
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j o u r n a l h o m e p a g e : w w w. e l s e v i e r. c o m / l o c a t e / l i t h o s
Evolution of lithospheric mantle beneath the Tan-Lu fault zone, eastern North China
Craton: Evidence from petrology and geochemistry of peridotite xenoliths
Yan Xiao a,⁎, Hong-Fu Zhang a,⁎, Wei-Ming Fan b, Ji-Feng Ying a, Jin Zhang a, Xin-Miao Zhao a, Ben-Xun Su a
State Key Laboratory of Lithospheric Evolution, Institute of Geology and Geophysics, Chinese Academy of Sciences, P.O. Box 9825, Beijing 100029, China
Guangzhou Institute of Geochemistry, Chinese Academy of Sciences, P.O. Box 1131, Guangzhou 510640, China
a r t i c l e i n f o a b s t r a c t
Article history: A suite of peridotite xenoliths from Cenozoic Beiyan basalts within the Tancheng-Lujiang (Tan-Lu) wrench
Received 23 October 2009 fault zone, eastern North China Craton (NCC), has been studied to provide constraints on the nature and
Accepted 24 February 2010 evolution of the lithospheric mantle beneath this region. These xenoliths commonly have porphyroclastic,
Available online 6 March 2010
granuloblastic to resorption textures with the absence of coarse-grained texture. They can be subdivided into
three types: lherzolite, clinopyroxene (cpx)-rich lherzolite and wehrlite. Lherzolites are characterized by low
Tan-Lu fault zone
forsterite contents (Fo) (88–91) in olivines. Whole rock and cpx separates from lherzolites have convex-
Peridotite xenoliths upward rare earth element (REE) patterns except for one sample which has the highest Fo in olivine and
Lithospheric mantle shows a spoon-shaped REE pattern. The Sr–Nd isotopic compositions of cpx separates are depleted, similar to
Peridotite–melt interaction those of mid-ocean ridge basalts (MORB). These geochemical characteristics indicate that the lherzolites
Eastern North China Craton represent fragments of newly accreted lithospheric mantle that makes up much of the Late Mesozoic–
Cenozoic lithosphere beneath the Tan-Lu fault zone. Cpx-rich lherzolite and wehrlite reflect the interaction
of the lithosphere with melt, as evidenced by relatively lower Fo (b 87) than the lherzolites (Fo ∼ 90), and
higher enrichment in cpx and light rare earth elements (LREE). Shallow relics of the Archean cratonic mantle
have not been found in this region. Therefore, the abundant cpx-rich lherzolites and wehrlites could be the
result of recent modification of lherzolites by asthenospheric melt. The Tan-Lu fault zone facilitated the
ascent and migration of asthenospheric melt and enhanced lithospheric mantle—asthenospheric melt
reaction. Combined with the data for mantle xenoliths from the adjacent regions, a highly heterogeneous
and secular evolution of the lithosphere is inferred beneath the Jiaodong region during Phanerozoic times.
© 2010 Elsevier B.V. All rights reserved.
1. Introduction refractory lithospheric mantle survived until the mid-Ordovician
(Menzies et al., 1993; Harris et al., 1994; Meyer et al., 1994; Zheng
Refractory mantle roots, often preserved beneath cratons due to et al., 1998; Wang and Gasparik, 2001; Zheng et al., 2001, 2007; Menzies
their inherent buoyancy and high viscosity, can be modified by more et al., 2007), consistent with the model that Archean crust is underlain
fertile materials through asthenosphere–lithosphere and crust– by thick, cold and refractory Archean lithospheric mantle (Boyd and
mantle interactions (Griffin et al., 1998; Zheng et al., 1998; Downes, Nixon, 1978; Erlank et al., 1987; Griffin et al., 1998; Pearson, 1999;
2001; O'Reilly et al., 2001; Zhang, 2005; Zhang et al., 2007a; Zheng Zhang et al., 2008). However, peridotite xenoliths from the Cenozoic
et al., 2007; Tang et al., 2008). Mantle-derived xenoliths are direct basalts suggest a hot (60–80 mW/m2), thin (80–60 km) and fertile
samples of the lithospheric mantle, and can provide direct informa- “younger” lithospheric mantle beneath the eastern NCC (Menzies et al.,
tion about these mantle processes. 1993; Griffin et al., 1998; Zheng et al., 1998; Fan et al., 2000; Hu et al.,
The North China Craton (NCC) is one of the oldest cratons on Earth 2000; Xu, 2001; Zheng et al., 2001; Rudnick et al., 2004; Chu et al., 2009;
(3.8 Ga) (Liu et al., 1992) and is also one of the major Archean cratons in Zhang et al., 2009a). These observations indicate that the lithospheric
eastern Eurasia. Previous studies on mantle xenoliths, xenocrysts and mantle of the NCC has not been only considerably thinned but also
solid diamond inclusions in Paleozoic diamondiferous kimberlites from compositionally changed from highly refractory to more fertile mantle
Mengyin County, Shandong Province and Fuxian County, Liaoning during Phanerozoic times. Systematic investigation of the petrology,
Province, show that a thick (∼200 km), cold (∼40 mW/m2) and highly mineralogy and Sr–Nd isotopic geochemistry on the spinel lherzolite
xenoliths entrained in the Cenozoic basalts of eastern China, show that
these lherzolite xenoliths have an “oceanic” affinity and thus may
⁎ Corresponding authors. No. 19, Beitucheng Xi Road, Chaoyang District, 100029,
Beijing, PR China. Tel.: + 86 10 82998514; fax: + 86 10 62010846.
represent newly accreted lithospheric mantle formed by low degree
E-mail addresses: [email protected] (Y. Xiao), [email protected] partial melting (Boyd, 1989; Fan et al., 2000). However, more recent
(H.-F. Zhang). studies on mantle xenoliths entrained in the late Mesozoic Junan
0024-4937/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
230 Y. Xiao et al. / Lithos 117 (2010) 229–246
Fig. 1. Simplified geological map showing major tectonic units of the NCC and the xenolith localities mentioned in the text. (a) The tectonic subdivisions of the NCC (after Zhao et al.,
2001). (b) Distribution of Mesozoic intrusions and volcanics in Shandong Province, as well as Paleozoic Mengyin diamondiferous kimberlite, with emphasis on basalts and related
rocks (after Zhang and Sun, 2002). (c) Distribution of Cenozoic volcanoes in the Changle-Linqu volcanic field (after Wang et al., 2003 and Yang et al., 2009).
basaltic rocks and Qingdao mafic dykes, Shandong Province, show that newly accreted lithospheric mantle. This implies that the newly
these volcanic rocks contain two types of mantle peridotite xenoliths, i.e. accreted lithospheric mantle was widespread beneath the eastern
high-Mg# (Fo N 91) and low-Mg# (Fo ≤ 91) peridotite (Ying et al., 2006; NCC in the Late Mesozoic, much earlier than the previous estimate as
Zhang et al., 2009b). Detailed petrological and geochemical studies indicated by the xenoliths entrained in Neogene and Quaternary basalts
suggest that the high-Mg# peridotites are samples of the old (Fan et al., 2000; Zheng et al., 2001; Rudnick et al., 2004). In addition,
lithospheric mantle which was severely modified by peridotite–melt peridotite–melt interaction was also widespread in the Mesozoic
interaction, and the low-Mg# peridotites represent fragments of the lithospheric mantle, as evidenced by Mesozoic basaltic rocks, mantle
Y. Xiao et al. / Lithos 117 (2010) 229–246 231
Table 1 xenocrysts and xenoliths entrained in the Late Cretaceous volcanic rocks
Modal composition of mantle xenoliths entrained in Beiyan basalts (vol.%). (Zhang et al., 2002, 2003; Zhang, 2005; Ying et al., 2006; Zhang et al.,
Sample Texture Ol Opx Cpx Sp Amp Feld Ap Phl 2007b, 2009a). Thus, the evolution of the lithospheric mantle beneath
the eastern NCC is complicated and the mantle has experienced
CLB05-03 Porphyroclastic 68 18 13 1 widespread peridotite–melt interaction, perhaps during both the
CLB05-13 Porphyroclastic 68 17 14 1 Mesozoic and Cenozoic. The objectives of this paper are to further
CLB05-23 Porphyroclastic 68 20 11 1 constrain the evolution of newly accreted lithospheric mantle since the
CLB05-24 Granuloblastic 76 15 8 1
late Mesozoic and to describe the reaction processes recorded in the
CLB05-29 Porphyroclastic 70 18 11 1
CLB05-30 Porphyroclastic 60 22 17 1 Trace Trace mantle-derived xenoliths.
CLB05-31 Granuloblastic 75 14 9 2 This paper focuses on a suite of the newly discovered mantle
CLB05-32 Granuloblastic 75 15 8 2 peridotite xenoliths from the Beiyan locality of the Cenozoic Changle-
CLB05-34 Porphyroclastic 77 15 7 1 Linqu volcanic field within the Tan-Lu fault zone in Shandong Province,
CLB05-47 Porphyroclastic 69 19 11 1
eastern NCC. The petrological and geochemical data reported in this
CLB05-48 Porphyroclastic 68 19 12 1
CLB05-50 Porphyroclastic 67 18 14 1 paper are used to probe the nature and evolution of the lithospheric
CLB05-51 Porphyroclastic 65 19 15 1 mantle beneath the Tan-Lu fault region, and the processes involved in its
CLB05-55 Porphyroclastic 63 21 15 1 evolution. Previously published mantle xenolith data (Xu et al., 1996b,
1998; Zheng et al., 1998; Ying et al., 2006; Zheng et al., 2007; Zhang et al.,
Cpx-rich lherzolite
CLB05-07 Granuloblastic 70 9 20 1 Trace 2009b) have been incorporated for comparison, and to demonstrate the
CLB05-15 Granuloblastic 70 8 20 2 modification of the newly accreted lithospheric mantle.
CLB05-22 Granuloblastic 70 11 17 2
CLB05-25 Granuloblastic 66 14 19 1
CLB05-45 Granuloblastic 70 13 16 1
2. Geological background
CLB05-53 Porphyroclastic 72 9 17 2
Wehrlite The NCC is bounded by the Central Asia Orogenic belt to the north
CLB05-01 Granuloblastic 72 27 1 and the Qinling–Dabie–Sulu high–ultrahigh pressure metamorphic
CLB05-02 Granuloblastic 75 24 1 belt to the south, which separates the craton from the Yangtze Block
CLB05-26 Granuloblastic 74 25 1
(Fig. 1a). The craton has been divided into three blocks, i.e. a Western
CLB05-35 Granuloblastic 73 25 2
CLB05-46 Granuloblastic 71 28 1 Trace Trace Block, a Trans-North China Orogen and an Eastern Block, based on
CLB05-80 Resorption texture 75 25 geology, geochronology, tectonic evolution and P–T-t paths for the
Note: Ol: olivine; Cpx: clinopyroxene; Opx: orthopyroxene; Sp: spinel; Amp: metamorphic rocks (Zhao et al., 2001) (Fig. 1a). The Western Block
amphibole; Feld: feldspar; Phl: phlogopite; Ap: apatite. consists of Late Archean to Paleoproterozoic metasedimentary belts
that unconformably overlie the Archean basement (Zhao et al., 2001).
The Trans-North China Orogen is a Proterozoic orogenic belt that
Fig. 2. Petrological classification of mantle xenoliths from the Beiyan locality. The on-craton Cenozoic mantle xenoliths in eastern China are from Fan et al. (2000). The data of
Shanwang, Qixia, Nüshan, Yitong, Qingdao and Junan are from Zheng et al. (1998), E and Zhao (1987), Zhang et al. (2009b), Ying et al. (2006).
232 Y. Xiao et al. / Lithos 117 (2010) 229–246
amalgamated the Archean Western Block and Eastern Block into an Luxi part and the eastern Jiaodong part. Jiaodong differs from the Luxi
integrated craton. The orogen comprises Late Archean to Paleoproter- due to the occurrence of the Sulu ultrahigh pressure metamorphic
ozoic tonalitic–trondhjemitic–granodioritic (TTG) gneisses and gran- belt. Cenozoic volcanism in Shandong Province is distributed in the
itoids that are interleaved with abundant sedimentary and volcanic Changle-Linqu, Yishui, Penglai, Qixia and Wudi fields (Chen and Peng,
rocks which developed in magmatic arc and intra-arc basin environ- 1985; E and Zhao, 1987; Wang et al., 1987; Zheng et al., 1998)
ments (Zhao et al., 2001). The basement of the Eastern Block is (Fig. 1b). The Changle-Linqu (10.6–18.8 Ma) and Yishui (12.1–
primarily composed of Early to Late Archean TTG gneisses and 2.5 Ga 14.0 Ma) (Jin, 1985) volcanoes occur within the Tan-Lu fault zone.
syntectonic granitoids, with Early to Late Archean granitic gneisses The Penglai (5.6 Ma) and Qixia (6.2 Ma) (Jin, 1985) volcanoes cover
and supracrustal rocks. the Jiaodong Group metamorphic rock in eastern Shandong and the
Unlike other Archean cratons worldwide, the NCC has experienced Wudi volcano in northwestern Shandong. Volcanoes in eastern
widespread tectonothermal reactivation during Phanerozoic times. Shandong erupted in the Miocene (11–19 Ma) and in western
Kimberlites were emplaced during the Ordovician (Chi and Lu, 1996; Shandong in the Pliocene (5–8 Ma) with the youngest volcano
Zhang and Yang, 2007). After a long magmatic hiatus, volcanism (0.7 Ma) in Wudi (Chen and Peng, 1985). The Cenozoic basalts in
resumed in the Late Jurassic and intensified in the Early Cretaceous and Shandong Province are dominantly composed of alkali olivine basalts,
Cenozoic as shown by the emplacement of voluminous Early Cretaceous olivine nephelinites and basanites (Chen and Peng, 1985; Xu et al.,
volcanic rocks, alkaline and granitoid rocks (Fan et al., 2000; Xu, 2001; 2000). Hundreds of Cenozoic volcanoes distributed in the Changle and
Zhang and Sun, 2002; Zhang et al., 2004, 2005) and extensive Tertiary to Linqu counties (Fig. 1c) and basalts in the Changle-Linqu volcanic field
Neogene basalts carrying abundant mantle and crustal xenoliths (Zhou directly overlie the Early Tertiary coal-bearing lacustrine sedimentary
and Armstrong, 1982; Cao and Zhu, 1987; Zheng et al., 1998; Tang et al., rocks of the Wutu Formation (Wang et al., 2003). Beiyan volcano is
2006). These magmas and their entrained mantle and crustal xenoliths one of the volcanoes in the Changle-Linqu volcanic field and is next to
provide us with an opportunity to understand the evolution of the sub- the Shanwang volcano (Zheng et al., 1998) (Fig. 1c).
continental lithosphere beneath the eastern NCC. The Tan-Lu fault zone is a major wrench fault in northeastern Asia
Shandong Province is situated in the central part of the eastern and extends from Nikolayevsk in Russia to the Yangtze Craton in
NCC and is separated by the Tan-Lu fault into two parts: the western South China with a strike length of more than 5000 km (Xu et al.,
Fig. 3. Photomicrographs and backscattered electron micrographs of representative texture of the Beiyan peridotites. (a) and (b) Porphyroclastic texture. (c) and (d) Granuloblastic
texture. (e) and (f) Resorption texture. (a), (c), (d) and (e) are plane-polarized light. (b) is cross-polarized light.
Y. Xiao et al. / Lithos 117 (2010) 229–246 233
1987, 1993; Zheng et al., 1998). This fault cuts through the eastern grained orthopyroxene (opx) porphyroclasts (N1 mm) surrounded by
part of the NCC. Previous studies (Peng et al., 1986; Fan and Hooper, small neoblasts (≤0.5 mm) (Fig. 3a and b). Beiyan lherzolites with
1989; Xu et al., 1993, 1996a; Zheng et al., 1998, 2007) show that this porphyroclastic texture are often foliated and generally contain b10%
fault extends deep into the lithospheric mantle. It may therefore have opx porphyroclasts. The opx porphyroclasts often show fractures,
influenced lithospheric mantle evolution. curved or zigzag grain boundaries and are commonly strained. Some
of them show fine exsolution lamellae of cpx and sometimes they
3. Xenolith petrography contain small and rounded olivine inclusions (Fig. 3b), suggesting that
these small olivine grains are crystallized prior to opx recrystalliza-
The Beiyan alkaline basalts contain abundant deep-seated xeno- tion. Olivine and cpx occur as small neoblasts which are strain-free
liths of dominantly spinel lherzolites and wehrlites with minor recrystallized grains (Fig. 3a and b). Some deformed olivine neoblasts
pyroxenites and megacrysts of augite and anorthoclase. The peridotite also show undulatory extinction or asymmetric extinction and kink
xenoliths are very fresh and large (10–15 cm in diameter). Most bands. Rare spinel typically occurs as interstitial and vermicular-
peridotite xenoliths are unfoliated or weakly foliated. Beiyan perido- shaped crystals, and is sometimes included in opx.
tite xenoliths can be subdivided into three types: lherzolite, cpx-rich
lherzolite and wehrlite, which can also be recognized in other xenolith 3.2. Granuloblastic texture
suites of the eastern NCC such as Shanwang, Qixia, Nüshan, Yitong,
Fuxin, Qingdao and Junan (Table 1 and Fig. 2) (E and Zhao, 1987; Zheng The granuloblastic texture occurs in the Beiyan peridotites.
et al., 1998; Ying et al., 2006; Zheng et al., 2007; Zhang et al., 2009b). Peridotites with granuloblastic texture are strongly tectonized with
The Beiyan peridotite xenoliths are highly enriched in cpx, different a grain-refined fabric, and are often tabular with abundant triple
from normal xenoliths from other basaltic localities worldwide. junctions and straight grain boundaries (Downes, 1990). All the
The texture of ultramafic mantle xenoliths worldwide can be divided minerals in the granuloblastic peridotites belong to the same
into coarse-grained (representing undeformed mantle with coarse generation and are recrystallized (Fig. 3c and d). They display
grains (N4 mm), curved and indented grain boundaries and large equilibrated texture with nearly 120° triple junctions and curved
anhedral spinels), porphyroclastic and granuloblastic textures accord- grain boundaries (Fig. 3d). The grain sizes are relatively fine from
ing to the grain sizes and the deformation and recrystallization features 0.2 mm to 0.5 mm. Some deformed grains show undulatory extinction
(Downes, 1990). The textures of mantle xenoliths within the Tan-Lu and kink bands. Cpx in most lherzolites have rims with sieve texture
fault, such as the Beiyan locality, have their own characteristics. Due to (Fig. 4a, b, c). In some samples, the opx was clearly replaced by cpx
strong deformation and recrystallization, they have complex features (Fig. 4d).
(Fig. 3) and mainly show porphyroclastic and granuloblastic textures.
3.3. Resorption texture
3.1. Porphyroclastic texture
An unusual resorption texture occurs in mantle xenoliths from
Porphyroclastic texture occurs only in the Beiyan lherzolite and Beiyan, especially in the wehrlite xenoliths. In sample CLB05-80, all
cpx-rich lherzolite and is characterized by the occurrence of large- the cpx have sieve texture and occur as interstitial grains between
Fig. 4. Photomicrographs (plane-polarized light) and backscattered electron micrographs of sieve-textured cpx from the Beiyan xenoliths.
234 Y. Xiao et al. / Lithos 117 (2010) 229–246
rounded olivine crystals with xenomorphic granular texture (Fig. 3e water and then leached with purified dilute HCl before isotopic
and f). This may suggest that the sieve-textured cpx formed at the analysis.
expense of primary olivine and opx through peridotite–melt reaction. Modal mineralogy of the peridotites was determined by point
The interstitial sieve-textured cpx and rounded olivine form a counting techniques (Table 1). Major element compositions of
resorption texture (Fig. 3e and f). minerals (Table 2) were determined with a Cameca SX50 electron
In addition, a few metasomatic minerals such as mica, apatite, microprobe at the Institute of Geology and Geophysics (IGG), Chinese
amphibole, feldspar and calcite also occur in the Beiyan peridotites Academy of Sciences. Analyses were carried out with a beam of 15 keV
(Fig. 5a–d). Phlogopite occurs as a vein in sample CLB05-30 and shows and 10 nA and focused to a spot of ∼ 2 μm diameter. Natural mineral
zigzag grain boundaries surrounded by small minerals such as olivine standards were used for calibration and the PAP correction procedure
and cpx (Fig. 5a and b), suggesting multiple metasomatic events. was applied to the data (Pouchou and Pichoir, 1991).
Apatite, amphibole and feldspar crystals rarely occur in microcracks or For trace element analyses, whole rocks and cpx separates
on the boundaries of olivine and pyroxenes (Fig. 5c and d). Feldspar (100 mg) were weighed and dissolved in distilled HF–HNO3 in
occurs in the reaction zone as a matrix of anhedral grains in which other 15 ml Savillex Teflon screw-cap capsules at 100 °C for 2 days, dried
secondary minerals are embedded (Fig. 5d). Calcite is relatively common and then digested with 6 M HCl at 150 °C for 4 days. Dissolved
in the Beiyan peridotites and occurs mainly as veins (Fig. 5e and f). samples were diluted to 100 ml before analyses. A blank solution was
prepared and the total procedural blanks were b50 ng for all the trace
4. Analytical methods elements reported in this paper (Table 3). Three duplicates and two
standards were prepared using the same procedure to monitor the
After thin-section observation representative lherzolite and analytical reproducibility. Trace elements were analyzed with an Elan
wehrlite xenoliths were selected and crushed to b30 mesh. Cpx sep- 6100 DRC ICP-MS at IGG. The discrepancy between the triplicates is
arates were carefully handpicked under a binocular microscope to a less than 5% for all the elements given in Table 3. Analyses of
purity of N99%. The cpx was cleaned in an ultrasonic bath in distilled standards are in agreement with the recommended values.
Fig. 5. Photomicrographs (plane-polarized light) and backscattered electron micrographs of metasomatic minerals and carbonate veins.
Y. Xiao et al. / Lithos 117 (2010) 229–246 235
For Sr and Nd isotope analyses, cpx separates were rinsed several are dark green diopsides. They have relatively low-Mg# (b92) and
times in deionised water after leaching with sub-boiling distilled 6 M wide compositional ranges in Al2O3 (2.77–8.73 wt.%), Cr2O3 (0.07–
HCl for 30 min. Then they were powdered with an agate mill to 1.50 wt.%), TiO 2 (0.04–2.36 wt.%) and Na 2 O (0.69–2.05 wt.%)
200 mesh. About 100 mg of sample powder was weighed out and a (Table 2). Al2O3 contents in these cpx show a good correlation with
known quantity of mixed 84Sr, 85Rb, 149Sm and 145Nd spike solution Mg# numbers from lherzolites to cpx-rich lherzolites to wehrlites
was added to each sample. Samples were digested in Teflon beakers (Fig. 6c), but their TiO2 contents are relatively scattered (TiO2 vs. Mg#
with a mixture of concentrated HF and HNO3 at 150 °C for 7 days. The plot, Fig. 6d). In addition, the exsolution lamellae and sieve-textured
decomposed samples were then dried on a hot plate and the residue cpx rims have slightly higher Al2O3 and TiO2 contents compared with
was re-dissolved and dried down with HClO4 to remove the HF. The the clean cores (Table 2).
dried salts were dissolved again in 1 ml of dilute HCl and then loaded
onto columns containing AG50W-X8 resins for separation and 5.1.4. Spinel
purification of Rb, Sr and REE, with the REE cut finally loaded on to Spinel from the Beiyan xenoliths is characterized by large
HDEHP columns for separation of Nd and Sm, using HCl eluants. Nd compositional variability such as Cr# (1.67–41.8) and Al2O3 contents
and Sr isotopic compositions were determined using a Finnigan MAT- (31.9–62.1 wt.%) (Table 2 and Fig. 6e–f). Most Beiyan peridotite
262 thermal ionization mass spectrometer at IGG. The mass spinels plot within the field for Cenozoic peridotites (Fig. 6e). The Cr#
fractionation corrections for Sr and Nd isotope ratios were based on in spinel is positively correlated with the Mg# in cpx (Fig. 6f).
Sr/88Sr = 0.1194 and 146Nd/144Nd = 0.7219. Repeat analyses yielded
Sr/86Sr of 0.710245 ± 0.000011 for the NBS-987 standard and 143Nd/ 5.1.5. Metasomatic minerals
Nd of 0.511853 ± 0.000012 for the La Jolla standard. The results are Phlogopite occurs as a vein in sample CLB05-30 and has Mg#
given in Table 4. (88.3) and major element compositions similar to that found in the
phlogopite-bearing lherzolites in Hannuoba, Sanyitang and Hebi
5. Analytical results (Zhao et al., 2007). One amphibole grain was found in the wehrlite
CLB05-46 and is pargasite with high TiO2, resembling amphibole from
5.1. Mineral chemistry peridotite xenoliths from the NCC (Zheng et al., 2001). Feldspars that
occur in association with the amphibole in sample CLB05-46 are
Multiple analyses on the cores and the rims of the constituent sanidines, which have rarely been reported in mantle peridotites (Xu
minerals, olivine, opx and cpx show that all the mineral phases are et al., 1996b). The carbonate minerals in these peridotites are calcites.
homogeneous within and between grains. The average compositions Dolomites have not been found. Peridotites with carbonate veins do
of the minerals are reported in Table 2. not show a big difference in major elements and trace element
compositions compared to those without carbonate veins.
5.1.1. Olivine
Olivine from the Beiyan peridotites is low in MgO and NiO and high 5.2. Trace element concentrations
in FeO and MnO contents, leading to extremely low Fo contents,
especially for cpx-rich lherzolites and wehrlites (Table 2 and Fig. 6a, In anhydrous spinel-facies mantle peridotites, cpx is a main
b). Although olivine compositions from lherzolite, cpx-rich lherzolite repository of trace elements and its trace element composition can be
and wehrlite overlap, their Fo contents decrease from 91.0 to 88.8 in used in the petrogenetic interpretation of mantle processes (Rampone
lherzolites identical to late Mesozoic–Cenozoic peridotites (Zheng et al., 1991; McDonough et al., 1992). However, some peridotites in
et al., 1998; Ying et al., 2006; Zhang et al., 2009b), from 87.6 to 82.2 in Beiyan are hydrous and trace element concentrations of cpx alone
cpx-rich lherzolites, then from 86.9 to 81.0 in wehrlites. The highest may not be enough to represent the whole rock situation. So, trace
Fo numbers observed in the Beiyan peridotites are similar to those in element compositions of whole rock and cpx separates from the
the low-Mg# (≤91) peridotites in Junan, but not the high-Mg# (N91) Beiyan peridotites are reported in Table 3.
peridotites (Ying et al., 2006; Zhang et al., 2009b). The extremely low
Fo (b87) olivine in the Beiyan cpx-rich lherzolites and wehrlites have 5.2.1. Lherzolites and cpx-rich lherzolites
also been observed in mantle peridotitic xenoliths along the Tan-Lu The whole rock and cpx separates from the Beiyan lherzolites and
fault zone at Shanwang (Wang et al., 1987; Zheng et al., 1998), cpx-rich lherzolites display a large variation in trace element
Nüshan (Xu et al., 1998) and Fuxin (Zheng et al., 2007). At Beiyan, the abundances and chondrite-normalized rare earth element (REE)
NiO content decreases from lherzolites (0.32–0.47 wt.%) to cpx-rich patterns (∑ REE = 4.62–84.7 ppm and (La/Yb)N = 1.58–20.2 in
lherzolites (0.31–0.41 wt.%) and to wehrlites (0.19–0.37 wt.%) ac- whole rocks and ∑REE = 19.6–103 ppm and (La/Yb)N = 0.91–6.54
companied by a decrease in Fo (Table 2 and Fig. 6b). In general, the in cpx separates) (Table 3 and Fig. 7). Three types of chondrite-
Beiyan lherzolites fall into the field of Cenozoic peridotites (Fig. 6a), normalized REE patterns occur in lherzolites and cpx-rich lherzolites.
but cpx-rich lherzolites and wehrlites plot far from the field of
Cenozoic peridotites. (1). Spoon-shaped REE pattern: lherzolite CLB05-31 with the
highest Fo is characterized by a spoon-shaped REE pattern
5.1.2. Opx with Eu or Pr at the minimum in both the whole rock and cpx
Opx occurs only in lherzolites and cpx-rich lherzolites, but not in (Fig. 7a). This sample shows low REE abundances and LREE/
wehrlites. They have low-Mg# (83.8–91.4), Al2O3 (2.22–4.63 wt.%) HREE fractionation both in the whole rock and cpx separates
and CaO (0.19–0.87 wt.%) contents and show a decreasing trend from ((La/Yb)N = 1.58 and 0.91, respectively). Apart from La-Pr
lherzolites to cpx-rich lherzolites. Such a compositional feature slight enrichment, this cpx shows a depletion in middle rare
broadly corresponds to that of the coexisting olivine, suggesting a earth elements (MREE), which is similar to the LREE-depleted
general equilibrium between olivine and opx. No significant variation patterns of cpx in the Junan low-Mg# peridotites (Fig. 7b). In
between large porphyroclasts and fine-grained ones has been primitive mantle-normalized trace element diagrams, cpx
observed (Table 2). separates show significantly negative Ba and slightly negative
HFSE, (i.e. Nb, Zr, and Hf) and Ti anomalies, but the whole rock
5.1.3. Cpx exhibits negative Ti and positive Sr and Pb anomalies (Fig. 7b).
Cpxs from the Beiyan lherzolites and cpx-rich lherzolites are (2). LREE-enriched and convex-upward REE patterns: most lherzo-
generally bright emerald-green Cr-diopsides and from wehrlites they lites and cpx-rich lherzolites from Beiyan display LREE-enriched
236 Y. Xiao et al. / Lithos 117 (2010) 229–246
Table 2
Representative electron microprobe analyses (wt.%) of minerals from Beiyan mantle xenoliths.
Sample CLB05-03 CLB05-07 CLB05-13 CLB05-15
Rock Sp lherzolite Sp cpx-rich lherzolite Sp lherzolite Sp cpx-rich lherzolite
Mineral Ol Cpx Opx Sp Ol Cpx Opx Sp Ol Cpx Opx Sp Ol Cpx Opx Sp
SiO2 41.4 53.8 56.7 40.7 52.2 55.6 41.0 53.2 55.9 39.7 49.9 53.9
TiO2 0.20 0.08 0.37 0.33 0.55 0.38 0.02 0.08 0.62 0.16 0.33
Al2O3 3.60 3.04 45.9 2.91 2.76 31.9 6.05 4.25 54.1 7.11 4.43 54.5
Cr2O3 0.96 0.54 21.0 1.50 0.42 34.1 0.90 0.37 13.6 0.74 0.29 10.2
FeO 10.9 2.89 6.63 13.9 11.7 3.22 7.79 17.4 9.97 2.27 6.14 10.9 16.9 4.80 10.5 17.7
MnO 0.14 0.10 0.15 0.03 0.21 0.08 0.22 0.23 0.18 0.12

Use: 0.2179