origin of podiform chromitite, a new model based on the luobusa ophiolite, tibet
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Origin of podiform chromitite, a new model based on the Luobusa ophiolite,Tibet
Fahui Xiong, Jingsui Yang, Paul T. Robinson, Xiangzhen Xu, Zhao Liu,Yuan Li, Jinyang Li, Songyong Chen
PII: S1342-937X(14)00173-7DOI: doi: 10.1016/j.gr.2014.04.008Reference: GR 1257
To appear in: Gondwana Research
Received date: 4 September 2013Revised date: 12 April 2014Accepted date: 25 April 2014
Please cite this article as: Xiong, Fahui, Yang, Jingsui, Robinson, Paul T., Xu, Xi-angzhen, Liu, Zhao, Li, Yuan, Li, Jinyang, Chen, Songyong, Origin of podiform chromi-tite, a new model based on the Luobusa ophiolite, Tibet, Gondwana Research (2014), doi:10.1016/j.gr.2014.04.008
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Origin of podiform chromitite, a new model based on the Luobusa
ophiolite, Tibet
Fahui Xionga, Jingsui Yang
a*, Paul T. Robinson
a, Xiangzhen Xu
a, Zhao Liu
b, Yuan
Lia, Jinyang Li
a and Songyong Chen
a
a CARMA,State Key Laboratory for Continental Tectonics and Dynamics, Institute of
Geology, Chinese Academy of Geological Sciences, China.
bSchool of Earth Science and Mineral Resources, China University of Geosciences,
Beijing, China.
Corresponding Author: Jingsui Yang [email protected]
Abstract
Podiform chromitites have been interpreted as the result of melt-rock reaction and
related melt mixing in upper mantle sections of ophiolites. However, the discovery of
ultrahigh-pressure (UHP) minerals, especially diamond and coesite, in many podiform
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chromitites and host peridotites, raises fundamental questions about the validity of this
model. Chromitites in the Luobusa ophiolite of Tibet range from massive, to nodular to
disseminated. Chromite grains in both the chromitites and peridotites have variable but
relatively high MgO and are classified as magnesiochromite. Many magnesiochromite grains
in the massive chromitites contain inclusions of forsterite and pyroxene, as well as diamonds
and other unusual minerals. Forsterite inclusions have Fo numbers of 97-99 and NiO
contents of 1.11-1.29 wt%. Mg#s (=100*Mg/(Mg+Fe)) of clinopyroxene inclusions are 96-
98 and those of orthopyroxene are 96-97. X-ray studies show that the olivine inclusions have
very small unit cells and short cation-oxygen bond distances, suggesting crystallization at
high pressure. In contrast, magnesiochromite grains in nodular and disseminated chromitites
lack pyroxene inclusions and their olivine inclusions have lower Fo numbers of 94-96 and
lower NiO contents of 0.35-0.58 wt%. In addition, magnesiochromite in massive ores has
higher Fe3+
/Fetotal (0.42) than that in nodular and disseminated ores, which have ratios of
0.22. Disseminated chromitites also show systematic changes in olivine and
magnesiochromite compositions from the dunite envelope to the massive ore, indicating melt-
rock reaction. These observations suggest that the formation of podiform chromitites is a
multi-stage process. Magnesiochromite grains and perhaps small bodies of chromitite
crystallize deep in the mantle under low ambient ƒO2 from partial melts of peridotite. UHP
minerals and highly magnesian olivine and pyroxene inclusions are trapped in these
magnesiochromite grains. When oceanic crustal slabs are trapped in suprasubduction zones
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(SSZ), they are modified by island arc tholeiitic and boninitic magmas, which change the
magnesiochromite compositions and deposit chromitite ores in melt channels.
Keywords: Model; Massive chromitite; Disseminated chromitite; UHP minerals; Luobusa
ophiolite;
1. Introduction
The genesis of podiform chromitites has long been a controversial subject. The textures
and mineralogy of podiform chromitites, and comparisons with layered chromitites in large
mafic intrusions support a magmatic origin. However, the factors that control the
distribution, textures and compositions of podiform bodies are still unclear. One early model
suggests that podiform chromitites and associated lenticular dunite bodies represent small
magma chambers in which the magnesiochromite collected (Neary et al., 1979). A study of
chromitite in mantle peridotites of New Caledonia identified three categories: discordant,
concordant and subconcordant, reflecting the relationship between the chromitite and mantle
textures (Cassard and Nicolas, 1981). That study suggested that discordant chromitite bodies
formed near ocean spreading ridges and that they did not undergo significant deformation,
allowing preservation of nodular and occluded textures. In contrast, concordant and
subconcordant bodies were thought to be intensely to moderately deformed, as suggested by
pull-apart textures, and transposition of the chromitites into the foliation of the host
peridotites.
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Because of widespread interest in ophiolites during the 1980s, chromitites were studied
extensively and many additional models were developed for their formation. Cassard et al.
(1981) and Lago et al. (1982) proposed that chromitite bodies formed in small, steep, trough-
like magma chambers in the upper mantle, where magma channels widened, thus allowing
strong convection. Another model held that podiform bodies represent refractory pockets of
residual chromite left after high degrees of melting (Duke, 1982), a view similar to that of
Wang et al. (1987). Leblanc et al. (1992) suggested that podiform chromitites are
precipitated from mafic magma in open systems to form dyke-like bodies.
The recognition that suprasubduction zone (SSZ) melts migrating through the overlying
mantle wedge peridotites can dissolve pyroxene and precipitate olivine led to a new
understanding of the formation of dunite bodies in ophiolites (Kelemen et al., 1992). Because
both orthopyroxene and clinopyroxene commonly contain Cr, their dissolution would
increase both Cr and SiO2 contents of the melts. When such melts become saturated in Cr,
precipitation of chromite can occur. Because most ophiolites are formed or modified in SSZ
environments, it was thought that podiform chromitites would mostly be formed in back-arc
basins, island arcs and forearc environments (Pearce et al., 1984; Zhou et al., 1996; Proenza
et al., 1999), where hydrous mantle melts would form from subducted fluids (Roberts, 1988;
Zhou et al., 1994, 1996,1997,1998,2005; Leblanc, 1995; Melcher, 1997; Proenza, 1999,2007;
Rollinson, 2005; Rollinson and Adetunji, 2013; Tamura, 2005; Uysal et al., 2007).
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However, this model is difficult to reconcile with the presence of diamonds and other
ultrahigh pressure and highly reduced minerals in many chromitites (Diamond group of the
Institute of Geology, Chinese Academy of Geological Sciences, 1981; Bai et al., 1993, 2000;
Robinson et al., 2004; Yang et al., 2007, 2014; Xu et al., 2008) and the presence of
clinopyroxene and coesite exsolution lamellae in some chromite grains (Yamamoto et al.,
2009). These findings suggest that much, if not all, of the magnesiochromite crystallized at
depth, perhaps >300km despite strong evidence for accumulation in the upper mantle (Zhou
et al., 1996; Robinson et al., 2004). This conflicting evidence leads to a number of unresolved
questions; (1) How are UHP minerals incorporated into chromitites and why are they
preserved at shallow levels? (2) Is the formation of podiform chromitites a multi-stage
process, and if so, how can these stages be recognized? (3) Is there a petrogenetic relationship
between the chromitites and the UHP minerals or are the chromitites merely carriers of such
phases? (4) What is the relationship between the podiform chromitites and the host
peridotites, which also commonly contain UHP minerals (Xu et al., this issue)? Does the
chrome within the chromitites come mostly from dissolved clinopyroxene in the peridotites
or are there other sources? (5) What is the origin and significance of the dunite veins and
envelopes associated with many podiform chromitites.
This paper grew out of detailed studies of several ophiolite massifs in the Yarlung-
Zangbo suture zone of southern Tibet. All of these bodies have well-exposed mantle sections
but only the Luobusa ophiolite in the eastern part of the zone hosts significant chromitite ore
bodies. Other ophiolites in the western part of belt contain small orebodies as well as
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numerous dunite lenses and veins. For this study, we have analysed mineral compositions in
the chromitites and their host peridotites in the Luobusa ophiolite, investigated spatial
relationships between different ore bodies and carried out detailed studies of
magnesiochromite grains and silicate minerals for evidence of formation under UHP
conditions. We use these new data to present some constraints on the formation of podiform
chromitites and their UHP minerals, as well the nature and evolution of SSZ melts that
migrated through the mantle section.
2. Geological setting
The Yarlung Zangbo suture zone extends more than 2000 km along southern Tibet and
marks the boundary between the Indian subcontinent and Eurasia. Seven major ophiolite
massifs crop out in this zone, including from east to west, Luobusa, Zedong, Xigaze, Saga,
Dangqiong, Purang and Dongbo. The Luobusa ophiolite, located about 200 km east-
southeast of Lhasa, is the only body that contains significant chromitite deposits. It extends
~42 km along strike and has a maximum width ~ 3.7 km, resulting in an exposed area of ~70
km2
(Fig.1). On the south it is separated from Triassic flysch by a steep reverse fault and on
the north it is thrust over the Tertiary Luobusa Formation and granite of the Gangdese arc(Xu
et al., 2014 this issue). The ophiolite consists mainly of mantle peridotite and dunite with
sparse mafic cumulates. At the base is a thin mélange zone containing dismembered volcanic
rocks and cherts that crop out north of the cumulates (Yang et al., 2004). The mantle
peridotite consists chiefly of harzburgite and clinopyroxene-bearing harzburgite with minor
lherzolite and dunite, whereas the cumulate rocks include wehrlite, pyroxenite, dunite and
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gabbro (Wang et al., 1987) (Fig. 1). A transition-zone dunite, about 100-200 m thick,
underlies the entire sequence. The pseudostratigraphy of the ophiolite suggests that it is
overturned (Malpas et al., 2003; Xu et al., 2014 this issue).
The chromitite orebodies in Luobusa are hosted in harzburgite, where they are typically
surrounded by dunite envelopes which are transitional to the peridotites (Wang et al., 1983;
Wang et al., 1987; Zhou et al., 1996). Individual orebodies are lenticular, planar podiform or
irregular, and rarely extend more than a few tens of meters. Fifteen mine groups, clustered
into three districts are recognized, largely on the basis of the nature and distribution of the
orebodies. The three districts are, from west to east, labeled Luobusa, Xiangkashan and
Kangjinla. Most of the orebodies lie near the top of the mantle section, except for one deposit
hosted in the transitional zone dunite (Wang et al., 2010).
Geochronological studies suggest that the ophiolite underwent a two-stage development,
beginning with formation at a mid-ocean ridge at 177±33Ma (Zhou et al., 2002), followed by
modification in a suprasubduction zone environment at ~126 Ma (Malpas et al., 2003). This
interpretation is supported by mineralogical and geochemical features that indicate
modification and refertilization of MORB-like mantle peridotites by SSZ melts (Xu et al.,
2011; Li et al., 2012). Many UHP minerals within the mantle peridotites and chromitites
indicate an initial formation depth of at least 150 km and most likely > 300 km (Yang et al.,
2007; 2008; Xu et al., 2009; Yamamoto et al., 2009).
3. Lithology of the Luobusa peridotite
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The Luobusa ophiolite crops out between about 4000 and 5300 m elevation and is
typically well exposed, particularly at the higher levels where vegetation is sparse to absent.
In addition to excellent outcrops, more than 15,000 m of drill core have been obtained by the
Tibet Mining Company and active mines have exposed a number of subsurface deposits.
Both surface samples and drill core were available for study.
The mantle peridotites crop out in a broad band along the southern part of the ophiolite
(Fig. 1) and make up the bulk of the ophiolite (Fig. 2). Both the drill core and surface
samples indicate that the peridotites consist chiefly of harzburgite and clinopyroxene-bearing
harzburgite with up to 3% clinopyroxene. The harzburgites are locally deformed and
serpentinized but many, particularly those in the eastern part of the ophiolite, show little sign
of alteration. Lenses of dunite, generally 1-10 m in size, are locally present in the
harzburgite, and envelopes of dunite surround many of the podiform chromitites.
The transition zone dunite, about 200-300 m thick, structurally underlies the peridotite
with a relatively sharp contact. The dunite has a somewhat banded or layered structure, but
lacks cumulate textures. These dunites are very fresh, with only traces of serpentine, and they
generally contain small amounts of magnesiochromite and clinopyroxene. Small patches of
disseminated chromitite, 1-2 m thick, are locally present and are generally oriented parallel to
the banding.
Most of the chromitite bodies in Luobusa are located in the harzburgite near the
contact with the transition zone dunite. The main orebodies are typically 5-10 m in size but
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some tabular bodies are up to 100 m long and 2-3 m thick; some podiform bodies are up to 30
m thick. Massive orebodies commonly have sharp contacts with the host peridotites, whereas
disseminated bodies have transitional boundaries. The latter are commonly surrounded by
dunite envelopes with disseminated, nodular and antinodular chromitite.
3.1 Harzburgite
Harzburgite accounts of 50-70% of the Luobusa mantle peridotite (Fig. 2). There is
little difference between harzburgites directly hosting podiform chromitites and those in
chromitite-free areas. Most of the harzburgite, both in outcrop and drillcore, is fresh, dark
green and massive. Alteration increases slightly near the chromitite orebodies and both
serpentinite and tectonic breccias are locally present. The rocks mostly have coarse-grained,
granular textures and consist chiefly of olivine (68-85 modal%) and orthopyroxene (14-30%)
with minor clinopyroxene, magnesiochromite and magnetite (Fig. 3a,b, c). Olivine crystals
are 2-6 mm in diameter and show notable deformation features, such as deformation
lamellae, kink banding and wavy extinction. Small, euhedral crystals of olivine also occur as
inclusions in magnesiochromite and orthopyroxene or along the contacts between these
minerals. Some narrow shear bands are marked by small, granular, recrystallized grains of
olivine. Most of the orthopyroxene in these rocks occurs as relatively large (5-7 mm),
typically euhedral to subhedral, tabular crystals, which also show extensive crystal flexing,
undulatory extinction and gliding twins. Small, granular crystals also occur in narrow shear
zones and as inclusions in magnesiochromite. Some of the large grains have exsolution
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lamellae of clinopyroxene. Clinopyroxene makes up less than 3 % of most samples and
occurs as small (0.1-1 mm), anhedral grains, typically between olivine and orthopyroxene.
The residual magnesiochromite in harzburgite consists of small (0.5-1 mm), subhedral to
euhedral grains, typically included in olivine and orthopyroxene or as an interstitial phase.
3.2 Dunite lenses
The dunite lenses in the Luobusa mantle peridotites are generally fresh, brown-black to
yellowish-brown in color and are about 3-8 m long. Most of the lenses consist of massive
olivine with a granular texture (Fig. 3 d, e). Individual olivine grains are typically 0.5-3mm
in diameter, but a few have been recrystallized to form fine-grained, mosaic textures. The
larger grains exhibit obvious wavy extinction and kink banding indicating local shearing.
Small amounts of chlorite, serpentine and magnetite fill small cracks and, in some cases, have
partially replaced the olivine grains, leaving only fresh cores. The dunite lenses typically
contain about 2 modal% clinopyroxene, which forms subhedral grains, 0.5-3 mm in diameter,
randomly distributed in the rock. In a few cases, relatively large clinopyroxene grains
partially surround grains of magnesiochromite. Some grains of clinopyroxene contain
inclusions of olivine and magnesiochromite. Subhedral to euhedral grains of dark brown
magnesiochromite, 0.1 to 1 mm in diameter, make up 1-2 modal% of most samples, and trace
amounts of orthopyroxene are locally present.
3.3 Dunite envelopes
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Most of the podiform chromitites in the Luobusa ophiolite are surrounded by dunite
envelopes, a few centimeters to a few meters thick. They are thickest and best developed
around disseminated chromitites. These dunites are moderately fresh, light yellowish-brown
in color and massive. Typical samples consist of 98 modal% olivine and 2%
magnesiochromite (Fig. 3f, g); neither clinopyroxene nor orthopyroxene has been identified
in these rocks, not even as inclusions in the olivine or chromite. Most olivine grains are
subhedral, granular crystals, ranging from 2 to 6 mm in diameter, which rarely exhibit
deformation structures, such as kink banding and undulatory extinction. The
magnesiochromite occurs both as small euhedral to subhedral grains and as larger (up to 5
mm), irregular, amoeboid crystals (Fig. 3f, g). Some of these contain small inclusions of
euhedral olivine.
3.4 Chromitites
Two major types of podiform chromitite are recognized in Luobusa; massive and
disseminated. Massive chromitite typically forms discontinuous layers, 0.5 to 3 m thick (Fig.
4a) or irregular masses up to 20 m thick. These are typically composed of more than 95
modal % magnesiochromite (Fig. 3h, i). Most have sharp contacts with the host peridotites,
are parallel or subparallel to the peridotite foliation and rarely have dunite envelopes (Fig.
4b), although some grade into densely disseminated deposits (Fig. 4c). Many of the
individual grains in the massive chromitites contain inclusions of olivine, orthopyroxene and
clinopyroxene ranging from 20-200 μm in size (Fig 3h, i).
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The disseminated chromitites have highly varied textures and show strong evidence of
formation from magmas in the uppermost mantle (Fig. 4c, g). These usually form irregular
patches a few centimeters to a few meters wide and sometimes occur on the margins of
massive bodies (Fig. 4c). They consist of variable proportions of magnesiochromite and
olivine and have banded, nodular, antinodular and densely disseminated textures (Fig. 4d-h).
Magnesiochromite grains in the disseminated ores do not contain inclusions of olivine,
orthopyroxene or clinopyroxene, all of which are common in the massive ores. Most of the
disseminated ores are surrounded by dunite envelopes ranging from 1cm to 1m in width (Fig.
5). The dunite envelopes consist chiefly of granular olivine with a few relatively large,
amoeboid-like grains of magnesiochromite (Fig. 3f, g) but they typically grade into the host
peridotites with increasing pyroxene.
4 Analytical methods
Systematic sampling was based on careful field studies outlining the various lithologies
and their contact relationships. Petrographic examination of 680 thin sections provided
detailed textural and mineralogical data. Selected samples were analysed with a JEOL JXA-
8100 electron microprobe at the Key Laboratory of Nuclear Resources and Environment,
East China Institute of Technology, using an Inca energy-dispersive spectrometer. The
microprobe was set to operate at a voltage of 15 kV, a beam current of 20 amps and a spot
diameter of 2 μm.
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We selected the freshest samples for whole-rock geochemical analysis. Each sample was
carefully cleaned, crushed and then ground in an agate mortar to pass a 200-mesh screen.
Major elements were determined on fused glass beads by X-ray fluorescence (XRF)
spectrometry. The analytical accuracy is estimated to be 1% relative for SiO2 and 2% relative
for the other oxides. Trace elements, including rare earth elements (REE), were determined
by inductively coupled mass spectrometry (ICP-MS). Two national standard (GSR3 and
GSR5) and three internal standards were measured simultaneously to ensure consistency of
the analytical results. Analytical uncertainties are estimated to be 10% for trace elements with
abundances ＜ 10 ppm, and around 5% for those ＞10 ppm.
Water and CO2 were determined by gravimetric techniques in which the sample is
heated in a closed container and the water vapor is collected in a separate tube, condensed
and then weighed. The detection limit for H2O and CO2 is 0.01wt%.
Trace elements in clinopyroxene were analyzed using a laser ablation inductive-coupled
mass spectrometer (LA-ICPMS) at the Institute of Geology and Geophysics, Chinese
Academy of Sciences (IGGCAS), Beijing. The system consists of a Lambda Physik LPX
120I pulsed ArF excimer laser coupled to an Agilent 7500 ICPMS. Isotopes were measured
in peak-hopping mode. A glass standard, NIST 610, was used for external calibration. For
most of the trace elements, the MPI-DING reference material GOR-132G was used as a
monitoring standard, and calcium (43
Ca) was selected as an internal standard. The CaO
contents of NIST 610 and GOR-132G are 11.5 wt% and 8.45 wt%, respectively. Reference
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values of NIST 610 and GOR-132G are from GeoREM (http:// georem. Mpch-
mainz.gwdg.de). The data were reduced using the program GLITTER 4.0 (Van Achterbergh
et al., 2001).
5 Mineral chemistry
5.1 Olivine
We obtained 150 microprobe analyses of olivine from harzburgites, dunite lenses, dunite
envelopes and chromitites, and representative data are presented in Table 1. Although all of
the olivine is highly magnesian (Fo90-98.), there are significant differences among grains from
the various host rocks.
In the harzburgites, olivine occurs as relatively large, granular crystals and as small
inclusions in both orthopyroxene and magnesiochromite. The granular olivine has the lowest
Fo (90-91.2), with NiO contents of 0.22-0.36 wt%, whereas olivine inclusions have slightly
higher Fo (90.3-92.1) and NiO contents (0.25-0.37 wt%). Both the Fo values and Ni contents
of olivine in harzburgite increase near the podiform chromitites and dunite lenses. All of the
olivine in the harzburgites has essentially the same MnO content (0.07-0.17 wt%).
Olivine in the dunite lenses is relatively uniform in composition with Fo values of 91.4-
93.3 and NiO contents of 0.25-0.42 wt%, both slightly higher than olivine in the harzburgite.
As in the other rocks, olivine inclusions in chromite are slightly more magnesian (Fo = 92.4-
93.9) and Ni rich (NiO = 0.32-0.37 wt%).
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Unlike the other lithologies, both the podiform chromitites and their dunite envelopes
show wide ranges in olivine composition. In the dunite envelopes, the granular olivine has
Fo values of 91.4-96.9 and NiO contents of 0.25-0.65 wt%, whereas olivine inclusions in
chromite grains are slightly more magnesian (Fo = 94.4-96.1), but have essentially the same
NiO contents. Within the chromitites, interstitial olivine ranges from Fo 92.8 to 96.5 wt%,
but olivine inclusions can reach Fo values as high as 98.3. Some variations in olivine
composition are observed between disseminated and massive chromitite with the latter being
more magnesian than the former and having significantly higher Ni contents (NiO = 1.1-1.4
wt%).
In general, the olivines become more magnesian from harzburgite→dunite
lenses→dunite envelopes→disseminated chromitite→massive chromitite, although there is
considerable overlap between lithologies (Fig. 6). Nickel contents show a rather steady
increase with increasing Fo values. MnO contents are rather variable, having a relatively
wide range in olivine of the harzburgites (0.07-0.17 ppm) over a narrow Fo range of olivine
and then decreasing systematically in the other lithologies (Fig. 6).
There is a sharp increase in Fo content of olivine in massive chromitite with a distinct
gap between these rocks and the dunite envelopes and disseminated chromitites. Because
these olivines are much more magnesian than magmatic olivine, they are generally
interpreted as the result of subsolidus equilibration with the chromitites in which Mg goes
into olivine and Fe into the magnesiochromite.
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5.2 Orthopyroxene
One hundred and sixty electron microprobe analyses of orthopyroxene show a narrow
range of composition ranging from En 87 to 92 (Table 2). Most of the orthopyroxene occurs
in the harzburgite, with minor amounts in the dunite lenses and almost none in the dunite
envelopes and chromitites. Thus, the compositions reported here are mostly from harzburgite
and sparse inclusions in chromitite. Orthopyroxene grains in the harzburgite range from
about 50 to 500 um long and show essentially no differences in composition related to grain
size. All of the grains are highly magnesian with compositions of En88-90 Fs9-10Wo1-2.
Alumina contents range from about 2.5 to 4 wt% Al2O3. Chrome and Ni contents are very
uniform with 0.4-0.8 wt% Cr2O3 and < 0.15 wt% NiO whereas CaO is typically <1 wt% (Fig.
7). The orthopyroxene grains plot in the fields of abyssal peridotites and fore-arc peridotites
(Fig. 8a) (Pagé et al., 2008).
A small amount of orthopyroxene is always present in the massive chromitites as
inclusions, most of which are 20-40 μm across (Fig. 3h, i). These are all more magnesian
than grains in the harzburgite with compositions in the range of En96Fs3Wo1. They have
lower Al2O3 (0.35-0.46 wt%) and CaO (0.27-0.39 wt%), but higher NiO (0.08-0.17 wt%) and
Cr2O3 (0.69-1.28%) than orthopyroxenes in the harzburgite.
The only linear compositional trend observed in the orthopyroxenes is a steady decrease
in Al2O3 with increasing Mg# (Fig. 7). Inclusions in the chromitites have very high and very
uniform Mg#s but vary somewhat in NiO and Cr2O3 (Fig. 7).
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5.3 Clinopyroxene
Clinopyroxene occurs in the harzburgites, dunite lenses and massive chromitites, but has
not yet been found in the dunite envelopes. Although all of the clinopyroxene is diopside
with En values of 46-53 (Table 3), it varies in grain size, crystal form and composition in the
different host rocks.
In the harzburgites, clinopyroxene rarely exceeds 3 modal% but ranges widely in grain
size from about 50 to 400 μm. Most of the fine-grained material is near dunite pods or
chromitite orebodies. In these rocks the clinopyroxene ranges from En46-50 Wo44-50 Fs2-4;
Al2O3 values are 2.8-4.3 wt% and Mg#s are 92-94 (Table 3). The alumina contents decrease
slightly and the Mg#s increase slightly near dunite pods and envelopes. The chrome contents
range from 0.3 to 1.3 wt% and the nickel contents from 0-0.1 wt% NiO.
Clinopyroxene in the dunite lenses is fine-grained (100-500μm) and commonly
associated with magnesiochromite. These clinopyroxenes are slightly more magnesian than
those in the harzburgite and have compositions of En47-48Fs2Wo50-51; Al2O3values are 1-1.7
wt% and Mg#s are 94-96. The chrome contents are lower than in grains hosted in the
harzburgites (Cr2O3 = 0.4-1.0 wt%) but the nickel contents are the same (NiO = 0-0.1 wt%).
Within the massive chromitites, clinopyroxene usually occurs as small (20-200μm)
inclusions in magnesiochromite grains (Fig. 3h, i). These are the most magnesian
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clinopyroxenes observed with compositions of En49-52Fs1-3Wo47-50; Al2O3 contents are low
(0.3-0.8 wt%) and Mg#s are high (96-98).
The clinopyroxenes become systematically more magnesian and less aluminous from
harzburgites to dunites to chromitites. As expected, chrome contents are high (0.74-1.46
wt% Cr2O3) and nickel contents are similar to grains elsewhere (0.02-0.1 wt% NiO). Such
variations also correlate approximately with decreasing grain size from harzburgite to
chromitite. The Mg# number of the clinopyroxenes increases significantly from coarse
grains in harzburgite to fine grains in both harzburgites and dunite lenses (Fig. 8). Also like
the orthopyroxenes, the clinopyroxenes plot in the fields of abyssal peridotites and fore-arc
peridotites (Fig. 8a) (Pagé et al., 2008). NiO contents are similar in grains from all lithologies
(Fig. 8d) but highest in the inclusions within chromitites. Clinopyroxenes in the dunite lenses
have much higher CaO contents than those in the harzburgites and there is a positive
correlation between CaO and Mg# (Fig. 8c). Interestingly, the clinopyroxene inclusions in the
massive chromitites have the same composition as exsolution lamellae in magnesiochromite
grains as reported by Yamamoto et al. (2009).
5.4 Magnesiochromite
Accessory magnesiochromite is common in the mantle peridotites but rarely exceeds 5
modal%. Although these grains are all relatively rich in Cr, some highly aluminous varieties
are present in the harzburgites. There is a wide range of substitution of Cr and Al as reflected
in their Cr#s (=100*Cr/Cr+Al)). Increasing Cr#s are typically correlated with increasing
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degrees of partial melting in the host peridotite and thus can be sensitive indicators of mantle
depletion (Dick and Bullen, 1984). Mg#s can also indicate the degree partial melting in
mantle peridotites. However, both of these indices can also be affected by melt-rock
reaction.
The magnesiochromites show a wide range of crystal form, grain size and composition
in the mantle peridotites, dunites and chromitites of the Luobusa ophiolite. In the peridotites
most of the magnesiochromite forms relatively large, irregular grains. In contrast,
magnesiochromite grains in dunites, particularly those that occur as inclusions in olivine, are
typically small and euhedral. The grain size and shape of magnesiochromites in the podiform
bodies are highly variable, but most grains are large and subhedral. On the basis of 100
microprobe analyses, magnesiochromite grains in the ophiolite have Cr2O3 contents of 16.9-
61.8 wt% and Al2O3 of 8.0-51.0 wt%, yielding Cr#s ranging from 20.2 to 84.3 (Table 4).
In the harzburgites Cr#s of the magnesiochromite range from 18.2 to 65.7, with values
increasing near dunite lenses and envelopes. The Cr#s correlate inversely with Mg#s (Fig.
9), reflecting varying degrees of partial melting. However, it is clear from textural and
mineralogical evidence that the magnesiochromites in these rocks also owe their
compositions to varying degrees of melt-rock reaction. Magnesiochromites in the
harzburgites have ferrous values (Fe2+
#=100*Fe2+
/(Mg+Fe2+
)) of 25-45 and ferric values
(Fe3+
#=100*Fe3+
/(Cr+Al+Fe3+
)) of 0.5-7.8.
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Magnesiochromites in the dunite lenses have higher Cr#s than those in the harzburgites,
ranging from 56-84 with an average of 67, ferrous values of 36-61 and ferric values of 0.9-
5.3. The Cr# numbers are even higher in the dunite envelopes, ranging from 75-84 with an
average of 77. The ferrous and ferric values are also higher, 25-56 and 2.6-12.5, respectively.
Both the FeO contents and Cr#s of the magnesiochromite increase from harzburgite to
dunite lenses, whereas MgO, Mg#s and Al2O3 decrease (Fig. 9). As seen in Figure 9, the
Cr#s and Mg#s of the magnesiochromites are negatively correlated, a feature common in
most Alpine ultramafic rocks (Leblanc, 1980).
Magnesiochromites in the podiform chromitites vary in composition depending on
whether the chromitites are disseminated or massive (Fig. 9). In disseminated varieties Cr2O3
compositions are 54-63 wt% with an average of 59, and Al2O3 compositions are 5.7-12.1
wt% with an average of 10.6. MgO is relatively constant (12-15 wt% with an average of
13.5), yielding Mg#s of 49-71 and Cr#s of 76-83 (Table 4). Forty analyses of
magnesiochromite in massive chromitite reveal Cr2O3 contents of 55-59 with an average of
58, and Al2O3 contents of about 10-15 with an average of 14 (Table 4). The different varieties
have similar values of 100*Fe3+
/(Cr+Al+Fe3+
) but the ferrous values are significantly higher
in the disseminated chromitites than in the massive varieties (Fig. 9c).
6 Discussion
6.1 Different types of chromitite
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Podiform chromitites are generally classified as discordant, subconcordant or concordant
depending on their relationship to the host peridotites (Cassard et al., 1981). Cassard et al.
(1981) suggested that the discordant chromitite formed at mid-ocean spreading ridges,
whereas the concordant varieties were produced by later compressional deformation. In
general, discordant bodies are characterized by disseminated, nodular and anti-nodular
textures, whereas concordant bodies consist chiefly of massive chromitite. Nicolas (1989)
suggested that textures in the discordant chromitites were obliterated by later deformation and
recrystallization that produced the concordant bodies. Ahmed and Arai (2002) suggested that
concordant chromitites at Wadi Hilti in the northern Oman ophiolite were formed from mid-
ocean ridge magmas and that the later discordant bodies were produced by melt-rock reaction
between SSZ boninitic melts and the host harzburgites. As outlined above, we document
significant compositional differences between massive and disseminated chromitites in the
Luobusa ophiolites.
The massive chromitites in Luobusa contain many small (20-200 μm), subhedral
inclusions of high-Fo olivine, orthopyroxene and clinopyroxene (Fig. 3h, i). In contrast,
disseminated and nodular chromitites have only olivine inclusions and these grains have
lower Fo values and NiO contents than those in the massive bodies (Fig. 6). Bai et al (2001)
reported that the high-Fo (97-98) olivine inclusions in the massive chromitite have much
shorter bond lengths than normal mantle olivine, a feature typical of synthetic forsterite
(Smyth and Hazen, 1973). On the basis of their crystal structure, these olivines are thought to
have formed at pressures as high as 14 GPa and temperatures of 1600°C (Bai et al., 2001).
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The covariation trends of NiO–MnO–Fo in the olivine inclusions resembles an olivine
fractionation trend (Fig.6) (Nakamura, 1995), but one starting from a very primitive, Ni-rich
liquid. However, the Fo values of these olivines are too high to be magmatic and must reflect
subsolidus equilibration between magnesichromite and olivine. The very high NiO contents
of olivines suggest crystallization in the transition zone or lower mantle (McCammona et al.,
2004). On the basis of their structure and high-Ni contents, these olivines may have formed
in the deep mantle.
The subhedral clinopyroxene inclusions in the massive chromitite (Fig. 3h, i) have the
same compositions as the acicular and spherical clinopyroxene exsolution features reported
from the massive chromitites (Yamamoto et al., 2009) (Fig.8). These clinopyroxenes, like the
olivines and coesite lamellae are UHP minerals formed at great depth, and thus are quite
different from pyroxenes in the host harzburgites. Their presence indicates that the Luobusa
mantle peridotites and chromitite may have formed in different stages. Similar inclusions
have been reported in the massive chromitites of the Oman ophiolite (Miura et al., 2012). The
clinopyroxene inclusions in the massive chromitite in Luobusa show slight depletion of light
rare earth elements (Fig. 10) suggesting 2-5% partial melting, which means the clinopyroxene
has undergone only weak melt-mineral reaction or metamorphism.
Like the pyroxenes, magnesiochromites in the massive and disseminated chromitites
also show significantly different characteristics. In the disseminated chromitites, the
magnesiochromite Mg#s are 49-71 and the Cr#s are 77-84, whereas the massive chromitites
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have a narrower range of Mg#s but more variable Cr#s (Fig. 9). Massive chromitite has
Fe3+
/∑Fe=0.42 and disseminated chromitite Fe3+
/∑Fe=0.22, probably because the massive
chromitite formed in a UHP environment of low oxygen fugacity, whereas the disseminated
and nodular chromitites formed in the higher oxygen fugacity environment (Ruskov et al.,
2010), again suggesting two stages of formation. The Fe3+
⁄∑Fe = 0.42 results of chromitite
from massive ores would normally suggest that they formed under or were subsequently
altered under conditions of high oxygen fugacity. However, high oxygen fugacity is
inconsistent with Fe0, as well as the wide variety of other reduced phases that includes SiC.
Moreover, there also are no other positive indications of strong oxidation, for example no
chemical zoning that would record the presence of a more oxidized rim compared with a
more reduced chromite core (Ruskov et al., 2010). Inclusions of TiO2 and nitrideS in the
massive chromitite also provide evidence of ultra-high pressure and low oxygen fugacity
conditions (Dobrzhinetskaya et al., 2009). In addition, many other UHP minerals have been
separated from the Tibetan ophiolites (Geological Research Institute of the Chinese Academy
of Geological Sciences - Diamonds, 1980; Bai et al., 2000; Robinson et al., 2004; Yang et al.,
2007). Coesite and clinopyroxene exsolution lamellae in magnesiochromites from the
massive chromitites have been reported by Yamamoto et al. (2009) and in-situ diamonds in
the chromitites have been confirmed by Yang et al. (2012; 2014). Both discoveries indicate a
deep formation for magnesiochromite grains and perhaps some small bodies of massive
chromitite.
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Another significant difference between the massive and disseminated chromitites of
Luobusa is that diamonds and other UHP minerals have not been found in the disseminated
varieties, however, they are common in the host peridotites (Xu et al., this issue). In addition,
most of the dunite envelopes are found only around the disseminated chromitites, not the
massive varieties. The prevailing view that podiform chromitites form only in subduction
zone (SSZ) settings by reaction between hydrous boninitic melts and mantle peridotites in the
shallow mantle (Roberts, 1988; Zhou et al., 1996, 1997, 1998, 2005; Rollinson, 2005;
Tamura, 2005; Uysal et al., 2007) is based largely on study of the disseminated bodies and
does not readily explain the many unusual features of the massive orebodies.
We investigated the transition from dunite envelopes to disseminated chromitite using
thin sections about 4.5 by 3 cm (Fig.5) to analyse changes in mineral compositions. In all 6
samples investigated both the Cr#s of magnesiochromites (Fig. 11) and the Fo numbers of
olivine (Fig. 12) show significant increases from the dunite envelopes to the disseminated
chromitites. Similar trends were reported by Zhou et al. (1996) for Luobusa chromitites and
were interpreted as the result of melt-rock reaction between MORB-like mantle peridotites
and later boninitic melts formed in an SSZ environment. This interpretation is supported by
similar REE patterns that differ only in the total REE in the different samples (Fig. 13).
These patterns are slightly U-shaped showing significant LREE enrichment, interpreted as the
result of interaction with water-rich, SSZ magmas. The systematic variations in Cr#s and
Mg#s in the residual peridotites of the Luobusa ophiolite are thought to reflect both variable
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degrees of partial melting of the original peridotites (Dick and Bullen, 1984) and variable
degrees of melt-rock reaction in the mantle wedge (Zhou et al., 1996).
6.2 Model for the formation of podiform chromitites
On the basis of our study of the Luobusa and other ophiolites, we propose a 4-stage
model for the formation of podiform chromitites (Fig.14).
Stage a: Over a long period of time lithospheric slabs are subducted to the mantle
transition zone (410-660km) and perhaps even deeper. Because of the ongoing subduction,
the transition zone is thought to be slightly cooler and more hydrous than the overlying
MORB mantle (Maruyama et al., 2007) and to consist of a mixture of subducted crustal
material (Yang et al., 2001; Yamamoto et al., 2003; 2011; Robinson et al, 2012, 2014 this
issue) and highly reduced phases from the lower mantle (Rudashevsky, 1987; Stachel et al.,
1998; Bai et al., 2000; Robinson et al., 2004). The wide range of UHP and highly reduced
minerals in the Luobusa ophiolite provides evidence for formation of these grains at
T=>1000°C and P＞9 GPa (＞300km depth).
Stage b: Formation of magnesiochromite grains and perhaps small podiform bodies
within the diamond stability field. On the basis of the UHP, highly reduced and crustal
silicates found in massive chromitites, we suggest crystallization somewhat above the top of
the mantle transition zone where these various components have mixed. Chromite
crystallization is thought to result from decomposition of an earlier UHP phase (CF calcium
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ferrite; CaFe2O4) (spinel) at a pressure greater than 12.5Ga (＞380km depth) (Yamamoto et
al., 2009). This magnesiochromite would have formed under low oxygen fugacity and higher
Fe3+
/∑Fe and would have encapsulated diamonds and other UHP minerals from carbon-rich
fluids. It seems clear that such minerals can survive exhumation and melt-rock reaction only
if they are protected by resistant minerals. Stages a and b may be a continuum, rather than
separate processes
Stage c: Mantle convection beneath mid-ocean spreading ridges carries the
magnesiochromites and host peridotites to shallow levels（Yang et al., this issue）. During
this stage some minerals may undergo phase changes, such as stishovite to coesite (Yang et
al., 2007), and exsolution of coesite and clinopyroxene from the chromite may occur
(Yamamoto et al., 2009) but grains encased within chromite crystals are preserved.
High-pressure silicate inclusions and diamonds are encapsulated in crystallizing
magnesiochromite grains within the peridotites. Therefore, the olivine, orthopyroxene and
clinopyroxene compositions in the ophiolitic mantle peridotites are different from those in the
massive chromitite and the magnesiochromites show different trends in Cr# and Mg# (Fig.
9).
Stage d: Once the mantle peridotites and massive chromitites with UHP minerals reach
the upper mantle, they are carried away from the spreading ridge by seafloor spreading. Most
of these lithospheric slabs are subducted back into the mantle but some are entrapped as
mantle wedges in suprasubduction zones. Hydrous partial melting of the mantle wedge
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produces island arc tholeiitic and boninitic magmas which react with the overlying peridotites
(Zhou et al., 1996, 2005, this issue) and mobilizes chromite in fluid-rich magmas. During
melt-rock reaction, pyroxene is partly to completely removed from the peridotites, olivine
becomes richer in MgO and magnesiochromite becomes richer in Cr2O3, but depleted in
Al2O3 and MgO. Crystallization of such magmas produces disseminated and nodular
chromites and extreme melt-rock reaction produces dunite pods and dunite envelopes around
the disseminated chromitites. Remobilized magnesiochromite grains are transported along
melt channels and are finally deposited as both massive and disseminated chromitites. The
disseminated chromitites have few, if any, UHP minerals, probably because they crystallized
in the SSZ mantle wedge, rather than at depth. Diamonds and other UHP minerals are
common in the residual peridotites, but their mode of preservation is not yet clear. We
suspect that they are encased in separate chromite grains that crystallized at depth during
stage b. Many of the peridotites also show refertilization by SSZ melts resulting in a second-
stage generation of clinopyroxene with relatively high TiO2 and REE (Xu et al., 2011), same
as reported in the other ophiolite and chromitite formation as Turkey(Uysal et al., this
issue).
Some osmium isotope data on the massive chromitites suggest that they are older than the
peridotite. Gabbro in the Luobusa mantle peridotite has a Sm-Nd isochron age of 177±31Ma
(Zhou et al., 2002) and Zhong et al. (2006) reported a SHRIMP U-Pb age of 162.9±2.8Ma for
a diabase dyke. However, Shi et al. (2007) reported that the Ru-Os-Ir minerals of Luobusa
massive chromitite have model ages of about 234±3Ma, and that the Os isotopic
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compositions have not been reset by a later process. Thus, both the age of ophiolite formation
(160-170Ma) and the age of modification in a SSZ environment (about 120Ma) are later than
the Re-Os isotope model age.
7 Conclusions
We recognize two types of chromitite in the Luobusa ophiolite; massive and
disseminated. The massive chromitites are interpreted as bodies formed both within in-situ
oceanic mantle and in a SSZ mantle wedge, whereas the disseminated ores are the result of
melt-rock reaction in a suprasubduction zone environment. Magnesiochromite grains and
some chromitites are considered to have formed at depth under high oxygen fugacity and to
have entrapped UHP and highly reduced minerals upon crystallization. In contrast, the
disseminated chromitites formed at shallow depths under relatively low oxygen fugacity by
redistribution and reprecipitation of the chromites within the peridotites. These bodies
typically lack UHP minerals. The massive chromitites in any given ophiolite have the
highest Cr#s and a narrow range of Mg#s, suggesting that their compositions have also been
modified by melt-rock reaction even though their UHP minerals have been preserved. We
suggest that the formation of high-Cr and high-Al chromitites depends largely on the
composition of the suprasubduction zone magmas moving through the mantle wedge. High-
Al chromitites may reflect melt-rock reaction between MORB-like arc tholeiitic melts
whereas high-Cr chromitites probably reflect the presence of boninitic melts.
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We conclude that the formation of podiform chromitites in ophiolites is a mutli-stage
process involving subduction of lithospheric slabs into the transition zone, crystallization of
magnesiochromite and some massive chromitite at depth, incorporation of UHP and highly
reduced phases into the chromitites and entrapment of oceanic lithospheric slabs above
subduction zones where they undergo varying degrees of reaction with hydrous, SSZ melts.
Acknowledgements
We thank Guolin Guo, Fei Liu, WenDa Zhou, Fenghua Lang and Lan Zhang for
assistance in the field work, the Key Laboratory of Nuclear Resources and Environment (East
China Institute of Technology) for the microprobe analyses and the China National Research
Center for the geochemical analyses. Yildirim Dilek is thanked for his comments and
suggestions that helped to improve the paper. This research was funded by grants from
Sinoprobe-05-02 from the Ministry of Science and Technology of China, the NSF China (No.
40930313, 40921001, 41202036), and the China Geological Survey (No. 1212010918013).
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Table Captions
Table 1. Representative microprobe analyses of olivine in the peridotites and chromitites
Table 2. Representative microprobe analyses of orthopyroxene in the peridotite and
chromitite
Table 3. Representative microprobe analyses of clinopyroxene in peridotites and chromitites
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Table 4. Representative microprobe analyses of magnesiochromite in the peridotite and
chromitite
Table 5. LA-ICP-MS analyses of clinopyroxene that are as inclusions in massive chromitite
(ppm)
Table 6. Major and trace element compositions of whole-rock samples of peridotite in
Luobusa
Figure Captions
Fig.1. Generalized geological map of the Luobusa ophiolite, Tibet.
Fig.2. Representative lithologic column of the Luobusa ophiolite
Fig.3. Photomicrographs of various lithologies of the Luobusa ophiolite.
(a), (b) Harzburgite (c) Harzburgite showing mineral distribution; (d), (e) Dunite
lenses; (f), (g) Dunite envelope with disseminated chromitite;(h); (i) Massive
chromitite with many inclusions of Ol-olivine; Cpx-clinopyroxene; Opx-
orthopyroxene. Cr = Cr spinel.
Fig.4. Different textures of chromitites in the Luobusa mantle peridotites
(a) Layers of massive chromitite in harzburgite. (b) Massive chromitite in sharp
contact with harzburgite. Note the absence of a dunite envelope. (c) Densely
disseminated chromitite. (d) Outcrop of complex chromitite hosted in dunite. The
dunite contains nodules and small massive zones of chromitite. At a later stage the
dunite was fractured and the spaces filled with moderately disseminated chromitite.
(e) Deformed bands and layers of disseminated chromite hosted in dunite. (f)
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Banded and layered disseminated chromitite. (g) Moderately disseminated chromitite
in contact with nodule material. (h) Well-developed nodular chromitite in contact with
dunite containing thin layers of disseminated chromitite.
Fig.5. Photomicrographs of dunite envelopes around disseminated chromitites of the Luobusa
ophiolite. Arrows point in direction of olivine and magnesiochromite analyses
shown in Figures 11 and 12. Sample numbers correspond to those in Figures 11 and
12.
Fig. 6. Olivine compositional diagrams in the different lithologies of the Luobusa district.(a)
Forsterite(Fo) vs.NiO and (b) Fo vs.Cr2O3; The mantle olivine array is from
Takahashi(1986), partial melting trends are from Ozawa(1994) and Nakamura(1995).
ABP= abyssal peridotites and FAP=fore-arc peridotites(from Pagé et al.,
2008);Granular in transition zone dunite are from Li et al.(2012).
Fig.7. Orthopyroxene compositional diagrams for the different lithologies of the Luobusa
ophiolite.(a) Mg# vs.Al2O3;(b) Mg# vs.Cr2O3;(c) Mg# vs.CaO; (d) Mg# vs.NiO;
The melting trend is from Smith and Elthon(1988). The 10% orthopyroxene
fractionation trend is from Varfalvy et al.(1996,1997). ABP= abyssal peridotites and
FAP=fore-arc peridotites (from Pagé et al., 2008).
Fig.8. Clinopyroxene compositional diagrams in the different lithologies of the Luobusa
district. (a)Mg# vs.Al2O3; (b)Mg# vs.Cr2O3; (c)Mg# vs.CaO; (d)Mg# vs.NiO; The
melting trend (line with arrow) is from Smith and Elthon(1988). ABP= abyssal
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peridotites and FAP=fore-arc peridotites(from Pagé et al., 2008); Values for granular
clinopyroxene in the transition zone dunites and dunite envelopes are from Li et
al.(2012)
Fig.9. Magnesiochromite compositional diagrams for the different lithologies of the Luobusa
ophiolite:.(a) Cr2O3 vs.Al2O3;(b) 100*Fe3+/(Cr+Al+Fe3+) vs.
100*Fe2+/(Mg+Fe2+);(c) MgO vs.FeO; (d) Mg# vs.Cr#.
Fig.10. Chondrite-normalized REE patterns for inclusions of Cpx (Mg#96-98) in the massive
chromitite Sample 904-1.
Fig.11. Magnesiochromite compositional variations across contacts between chromitites and
dunites. See Fig. 5 for sample numbers and locations. Samples 900-1 and 911-c are 3
cm wide; samples 916-a, 916-c, 916-e and 916-f are 4.5 cm wide.
Fig.12. Olivine compositional variations across contacts between chromitites and dunites.
See Fig. 5 for sample numbers and locations. Samples 900-1 and 911-c are 3 cm
wide; samples 916-a, 916-c, 916-e and 916-f are 4.5 cm wide.
Fig.13. Primitive mantle-normalized REE patterns of peridotite in Luobusa. Samples 904,
925, 927 and 932 are harzburgite, samples 934, 952, 968 and 970 are harzburgite
envelopes around chromitites, samples 900, 950 and 976 are dunite envelope around
chromitites and samples 950, 976 and 982 are dunite lenses.
Fig. 14 The new model illustrating the formation of the Podiform Chromite
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Figure 1
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Figure 4
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Figure 5
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Figure 6
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Figure 7
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Figure 8
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Figure 9
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Figure 10
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Figure 11
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Figure 12
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Figure 13
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Figure 14
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Table 1. Representative microprobe analyses of olivine in the peridotites and chromitites
Lithology Granular in harzburgite Inclusions in Harzburgite Granular in dunite envelopes Inclusions in dunite envelopes
Sample 905.1 905.24 905.25 905.3 905.31 912.11 912.1 912.1 905.13 905.15 900.2 900.2 900.2 900.2 900.2 917.1 917.2 917.5 917.21 916b.5
SiO2 40.89 41.09 41.00 41.11 41.09 40.92 41.21 41.05 40.78 41.18 41.61 41.73 41.41 41.88 41.61 41.66 41.93 41.84 41.86 41.27
TiO2 0.01 0.02 0.00 0.00 0.04 0.00 0.01 0.00 0.03 0.02 0.00 0.00 0.00 0.00 0.00 0.00 0.02 0.00 0.02 0.04
Al2O3 0.21 0.00 0.00 0.01 0.00 0.00 0.02 0.00 0.00 0.02 0.00 0.00 0.00 0.00 0.02 0.00 0.00 0.01 0.00 0.01
Cr2O3 0.12 0.00 0.05 0.00 0.01 0.03 0.24 0.00 0.30 0.04 0.03 0.06 0.01 0.01 0.02 0.18 0.35 1.03 0.53 0.86
FeO 9.17 8.95 9.40 9.39 9.55 9.16 9.24 9.35 8.79 9.21 5.88 5.86 5.90 5.84 5.82 4.70 4.39 3.77 5.56 4.31
MnO 0.08 0.14 0.14 0.12 0.15 0.10 0.12 0.13 0.14 0.07 0.10 0.07 0.08 0.09 0.07 0.07 0.06 0.07 0.08 0.06
NiO 0.28 0.28 0.32 0.31 0.23 0.35 0.29 0.39 0.25 0.25 0.43 0.43 0.48 0.38 0.48 0.55 0.51 0.58 0.45 0.45
MgO 48.89 49.11 48.67 49.32 49.01 48.80 49.35 48.71 48.80 49.39 51.86 51.92 51.71 51.76 52.27 52.29 52.61 52.33 52.20 52.04
CaO 0.02 0.00 0.00 0.01 0.01 0.00 0.03 0.03 0.00 0.00 0.02 0.05 0.02 0.05 0.08 0.06 0.01 0.02 0.03 0.02
Na2O 0.11 0.00 0.00 0.01 0.00 0.01 0.01 0.00 0.01 0.00 0.01 0.01 0.02 0.00 0.00 0.00 0.00 0.01 0.01 0.08
K2O 0.04 0.00 0.00 0.00 0.00 0.00 0.01 0.00 0.02 0.00 0.03 0.00 0.00 0.00 0.02 0.00 0.01 0.01 0.01 0.03
Total 99.81 99.58 99.58 100.26 100.09 99.36 100.51 99.65 99.13 100.19 99.97 100.12 99.64 100.02 100.39 99.49 99.88 99.66 100.76 99.17
Fo 90.5 90.7 90.2 90.4 90.2 90.5 90.5 90.3 90.8 90.5 94.0 94.1 94.0 94.1 94.1 95.2 95.5 96.1 94.4 95.6
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Table 1 (con't a)
Lithology Granular in dunite lenses Inclusions in dunite lenses Granular in disseminated
chromitite
Sample 913.14 913.14 913.15 914.71 914.73 913.14 914.57 914.6 914.66 914.68 916a.4 916a.3 916a.5 916a.6 916a.8
SiO2 41.47 41.40 41.12 41.17 41.35 41.25 41.57 41.53 41.51 41.53 41.59 41.63 41.24 41.32 41.59
TiO2 0.01 0.00 0.00 0.00 0.00 0.00 0.01 0.01 0.02 0.02 0.00 0.00 0.01 0.00 0.00
Al2O3 0.01 0.00 0.00 0.01 0.00 0.00 0.01 0.00 0.01 0.01 0.01 0.00 0.00 0.00 0.00
Cr2O3 0.09 0.00 0.00 0.06 0.05 0.31 0.03 0.18 0.06 0.06 0.02 0.03 0.02 0.00 0.00
FeO 7.50 8.01 7.94 7.10 7.06 7.36 5.90 6.67 6.37 6.37 6.58 6.93 6.83 7.06 6.81
MnO 0.09 0.11 0.10 0.08 0.10 0.14 0.11 0.12 0.04 0.04 0.13 0.10 0.14 0.10 0.10
NiO 0.30 0.34 0.32 0.31 0.42 0.32 0.37 0.36 0.34 0.34 0.43 0.43 0.46 0.43 0.37
MgO 50.62 49.94 50.04 50.72 50.62 50.49 51.41 50.61 50.71 50.84 51.19 51.02 51.26 50.63 50.68
CaO 0.08 0.07 0.06 0.02 0.01 0.09 0.01 0.03 0.02 0.06 0.02 0.06 0.05 0.07 0.05
Na2O 0.01 0.00 0.00 0.00 0.00 0.01 0.00 0.00 0.00 0.00 0.02 0.01 0.00 0.01 0.00
K2O 0.01 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.01 0.01 0.00 0.00 0.00
Total 100.19 99.88 99.57 99.45 99.61 99.98 99.42 99.52 99.09 99.28 99.99 100.23 100.02 99.62 99.60
Fo 92.3 91.7 91.8 92.7 92.7 92.5 94.0 93.1 93.4 93.5 93.3 92.9 93.1 92.7 93.0
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Table 1 (con't) b
Lithology Inclusions in disseminated chromite Granular in massive chromite Inclusions in massive chromite
Sample 916a.24 916a.32 916a.34 916a.37 916a.38 951.58 951.59 1091.6 951.61 1091.62 145.11 145.12 145.13 145.14 145.15
SiO2 41.95 42.09 41.66 42.04 41.72 41.86 42.22 42.40 42.05 42.06 41.51 40.98 41.55 42.26 41.91
TiO2 0.00 0.05 0.00 0.02 0.04 0.00 0.00 0.00 0.00 0.00 0.01 0.00 0.00 0.00 0.01
Al2O3 0.00 0.00 0.02 0.01 0.00 0.01 0.01 0.02 0.00 0.00 0.00 0.02 0.01 0.00 0.01
Cr2O3 0.68 0.21 0.80 0.45 0.46 0.20 0.63 0.23 0.10 0.03 0.84 0.78 0.72 0.73 0.85
FeO 3.88 4.15 3.00 3.92 3.91 3.85 2.85 3.06 3.13 4.60 1.73 1.96 1.84 1.85 1.79
MnO 0.07 0.11 0.05 0.07 0.06 0.07 0.01 0.05 0.03 0.06 0.01 0.04 0.01 0.01 0.04
NiO 0.60 0.54 0.49 0.45 0.45 0.50 0.49 0.48 0.49 0.70 1.22 1.22 1.29 1.01 1.23
MgO 52.88 52.46 52.75 52.72 52.44 52.76 53.04 52.81 52.80 52.19 53.91 54.22 53.88 53.74 53.97
CaO 0.00 0.02 0.00 0.04 0.02 0.01 0.01 0.02 0.00 0.00 0.01 0.01 0.02 0.00 0.00
Na2O 0.01 0.03 0.03 0.01 0.03 0.02 0.00 0.02 0.01 0.00 0.00 0.00 0.00 0.01 0.00
K2O 0.01 0.00 0.00 0.01 0.01 0.01 0.00 0.01 0.01 0.00 0.00 0.01 0.00 0.01 0.01
Total 100.08 99.66 98.80 99.73 99.13 99.11 99.26 99.09 98.61 99.64 99.25 99.25 99.32 99.63 99.81
Fo 96.0 95.8 96.9 96.0 96.0 96.1 97.1 96.9 96.8 95.3 98.2 98.0 98.1 98.1 98.2
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Table 2. Representative microprobe analyses of orthopyroxene in the peridotite and chromitite
Lithology Granular in harzburgite Inclusions in harzburgite Inclusions in massive chromitite
Sample 905.6 905.7 905.11 905.12 905.16 904.1 904.25 968.27 968.37 970.15 1012.12 1012.13 1012.15 1012.17 1012.19
SiO2 54.11 54.17 53.86 53.87 54.73 56.59 57.01 56.19 55.46 55.52 57.77 58.05 59.09 58.73 58.51
TiO2 0.03 0.04 0.05 0.03 0.01 0.03 0.01 0.03 0.01 0.03 0.03 0.09 0.06 0 0.04
Al2O3 4.07 3.74 4.33 4.33 3.89 0.93 1.13 2.49 2.23 2.69 0.46 0.37 0.37 0.37 0.35
Cr2O3 0.75 0.65 0.7 0.74 0.62 0.27 0.37 0.63 0.33 0.76 0.97 1.28 0.79 0.77 0.69
FeO 6.6 6.98 6.82 6.74 6.81 6.51 6.14 6.12 6.41 6.07 2.35 2.31 2.26 2.37 2.3
MnO 0.14 0.18 0.13 0.11 0.14 0.15 0.13 0.08 0.11 0.14 0.09 0.04 0.06 0.05 0.05
MgO 33.02 33.38 32.75 32.77 33.07 34.99 35.13 33.37 33.51 32.83 36.88 37.19 37.8 37.81 37.39
CaO 0.93 0.55 0.7 0.71 0.77 0.25 0.39 0.91 0.48 1.27 0.35 0.27 0.34 0.39 0.36
Na2O 0.00 0.01 0.00 0.00 0.00 0.02 0.01 0.07 0.02 0.01 0.02 0.02 0.00 0.00 0.02
K2O 0.00 0.01 0.00 0.00 0.00 0.02 0.01 0.01 0.00 0.01 0.00 0.01 0.01 0.00 0.00
NiO 0.11 0.06 0.06 0.06 0.04 0.04 0.03 0.09 0.05 0.15 0.08 0.1 0.08 0.11 0.17
Total 99.75 99.76 99.41 99.35 100.08 99.79 100.35 99.97 98.61 99.47 98.98 99.72 100.85 100.59 99.88
En 88.3 88.6 88.3 88.4 88.3 90.1 90.4 89.1 89.5 88.4 95.9 96.2 96.2 95.9 96.0
Fs 9.9 10.4 10.3 10.2 10.2 9.4 8.9 9.2 9.6 9.2 3.4 3.4 3.2 3.4 3.3
Wo 1.8 1.0 1.4 1.4 1.5 0.5 0.7 1.7 0.9 2.5 0.7 0.5 0.6 0.7 0.7
Mg# 89.9 89.5 89.5 89.7 89.7 90.6 91.1 90.7 90.3 90.6 96.6 96.6 96.8 96.6 96.7
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Table 3. Representative microprobe analyses of clinopyroxene in peridotites and chromitites
Lithology Granular in harzburgite Inclusions in harzburgite Granular in dunite lenses
Sample 905.1 905.2 905.4 905.23 905.28 904.2 904.2 904.21 904.21 904.2 913.15 913.15 913.11 914.59 914.1
SiO2 51.49 51.59 51.25 51.23 51.60 53.79 53.95 54.12 53.85 54.51 53.52 53.68 53.08 54.06 53.16
TiO2 0.11 0.06 0.18 0.18 0.13 0.04 0.01 0.04 0.02 0.02 0.04 0.09 0.04 0.00 0.00
Al2O3 3.28 3.43 3.40 3.46 3.23 0.46 0.47 1.11 0.57 0.75 1.36 1.56 1.69 1.37 1.04
Cr2O3 0.70 0.65 0.75 0.86 0.71 0.32 0.20 0.62 0.19 0.39 0.69 0.69 0.79 0.82 0.65
FeO 2.68 2.67 2.65 2.25 2.18 1.66 1.83 2.13 1.84 1.85 1.59 1.58 1.53 1.80 1.68
NiO 0.01 0.06 0.04 0.01 0.07 0.03 0.02 0.03 0.06 0.00 0.00 0.03 0.06 0.03 0.03
MnO 0.06 0.06 0.10 0.07 0.09 0.08 0.08 0.10 0.10 0.00 0.01 0.04 0.04 0.06 0.06
MgO 16.83 17.22 17.80 15.93 15.91 17.58 20.01 18.13 17.73 18.02 17.31 16.88 16.65 17.84 17.39
CaO 23.47 23.67 22.63 24.81 24.67 25.32 21.70 24.55 25.29 25.06 25.92 25.70 25.14 24.56 25.36
Na2O 0.06 0.10 0.05 0.05 0.05 0.06 0.12 0.12 0.04 0.08 0.09 0.10 0.09 0.20 0.16
K2O 0.00 0.03 0.01 0.00 0.02 0.03 0.03 0.01 0.00 0.00 0.00 0.00 0.00 0.00 0.00
Total 98.69 99.54 98.85 98.84 98.65 99.37 98.42 100.96 99.69 100.68 100.53 100.36 99.10 100.75 99.53
En 47.8 48.2 50.1 45.5 45.6 47.9 54.6 49.1 48.0 48.6 47.0 46.6 46.8 48.9 47.6
Fs 4.3 4.2 4.2 3.6 3.5 2.5 2.8 3.2 2.8 2.8 2.4 2.4 2.4 2.8 2.6
Wo 47.9 47.6 45.8 50.9 50.9 49.6 42.6 47.7 49.2 48.6 50.6 51.0 50.8 48.4 49.9
Mg# 91.8 92.0 92.3 92.7 92.9 95.0 95.1 93.8 94.5 94.6 95.1 95.0 95.1 94.6 94.9
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Table 3.
(con't)
Lithology Inclusions in dunite lens Inclusions in massive chromitite
Sample 914.10 913.14 913.14 913.14 914.63 901.80 901.80 901.80 901.81 901.81
SiO2 53.60 53.86 54.24 54.51 54.22 53.85 54.30 54.07 55.14 54.89
TiO2 0.02 0.04 0.06 0.02 0.05 0.00 0.08 0.07 0.07 0.08
Al2O3 0.79 0.91 0.56 0.73 0.91 0.40 0.41 0.50 0.32 0.55
Cr2O3 0.40 0.46 0.29 0.73 0.65 0.90 1.30 0.92 1.10 0.79
FeO 1.48 1.35 1.39 1.45 1.87 0.90 1.02 0.97 0.76 0.98
MnO 0.05 0.04 0.03 0.04 0.03 0.05 0.00 0.05 0.05 0.04
NiO 0.08 0.05 0.00 0.06 0.01 0.07 0.05 0.03 0.05 0.09
MgO 16.65 17.07 17.27 17.38 17.59 17.93 18.10 18.12 18.64 18.34
CaO 26.26 25.68 25.66 25.21 23.85 25.20 24.67 24.95 23.79 23.42
Na2O 0.14 0.10 0.10 0.12 0.15 0.18 0.16 0.17 0.19 0.18
K2O 0.01 0.00 0.00 0.01 0.00 0.02 0.00 0.02 0.00 0.00
Total 99.46 99.55 99.62 100.25 99.33 99.49 100.08 99.87 100.11 99.36
En 45.8 47.1 47.3 47.9 49.2 49.1 49.7 49.5 51.6 51.3
Fs 2.3 2.1 2.1 2.2 2.9 1.4 1.6 1.5 1.2 1.5
Wo 51.9 50.9 50.5 49.9 47.9 49.6 48.7 49.0 47.3 47.1
Mg# 95.3 95.8 95.7 95.5 94.4 97.3 96.9 97.1 97.8 97.1
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Table 4. Representative microprobe analyses of spinel in the peridotite and chromitite
Lithology In harzburgite In dunite lenses In dunite envelopes
Sample 977.12 977.17 977.19 977.2 977.23 913.14 913.14 913.14 913.15 913.15 911.2 911.2 911.2 911.21 911.21
SiO2 0.01 0.00 0.00 0.02 0.00 0.02 0.05 0.00 0.02 0.02 0.04 0.03 0.03 0.00 0.02
TiO2 0.07 0.04 0.08 0.03 0.08 0.07 0.09 0.14 0.09 0.07 0.23 0.25 0.24 0.25 0.18
Al2O3 46.10 40.53 38.77 46.59 40.70 22.23 22.67 21.23 22.33 22.52 11.72 12.05 11.60 11.94 11.22
Cr2O3 21.81 28.70 28.48 22.33 28.06 44.21 45.63 48.45 44.55 44.53 54.27 54.72 54.25 54.39 55.47
FeO 13.91 13.36 13.98 12.73 13.47 19.51 18.12 16.89 18.61 18.87 19.78 19.41 20.25 19.17 18.51
MnO 0.15 0.18 0.16 0.14 0.23 0.35 0.27 0.29 0.24 0.28 0.33 0.35 0.31 0.36 0.31
NiO 0.13 0.14 0.20 0.20 0.13 0.11 0.12 0.02 0.05 0.10 0.05 0.05 0.11 0.03 0.00
MgO 17.51 17.51 17.48 18.44 17.58 12.94 13.26 13.45 13.40 13.05 12.23 12.23 12.54 12.51 12.74
CaO 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00 0.00
Na2O 0.05 0.03 0.00 0.01 0.01 0.01 0.03 0.01 0.00 0.03 0.01 0.01 0.00 0.00 0.00
K2O 0.00 0.00 0.01 0.00 0.00 0.03 0.00 0.00 0.01 0.00 0.00 0.00 0.02 0.01 0.01
Total 99.72 100.50 99.17 100.49 100.26 99.48 100.23 100.48 99.30 99.47 98.65 99.09 99.34 98.65 98.46
Mg# 69.6 70.2 69.2 72.2 70.3 54.5 56.9 59.2 56.3 55.5 53.0 53.4 53.0 54.1 55.5
Cr# 24.1 32.2 33.0 24.3 31.6 57.2 57.5 60.5 57.2 57.0 75.7 75.3 75.8 75.4 76.8
Fe2+
# 28.4 27.3 26.2 25.3 26.9 40.2 39.4 38.5 38.5 39.9 40.8 41.0 39.8 39.6 38.3
Fe3+
# 1.8 2.0 3.5 1.8 2.3 5.3 3.6 2.4 4.8 4.5 6.7 5.9 7.6 6.4 6.2
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Table 4. Con't.
Lithology In dissiminated chromitite In massive chromitite
Sample 916a.27 916a.28 916a.29 916a.30 916a.35 1046.12 1046.13 1046.4 1046.15 1065.12
SiO2 0.02 0.05 0.07 0.00 0.03 0.03 0.03 0.03 1.18 0.03
TiO2 0.12 0.20 0.19 0.15 0.20 0.21 0.22 0.16 0.18 0.14
Al2O3 10.90 11.64 11.81 11.04 11.29 11.36 11.36 11.20 10.75 11.28
Cr2O3 57.19 57.41 57.25 57.47 56.93 58.64 58.71 59.10 57.21 58.71
FeO 17.49 16.28 16.29 17.26 17.11 11.62 11.53 11.53 11.44 13.28
MnO 0.30 0.33 0.27 0.25 0.32 0.20 0.16 0.21 0.22 0.23
NiO 0.06 0.06 0.03 0.03 0.05 0.11 0.14 0.20 0.21 0.14
MgO 13.37 13.69 14.06 12.90 13.17 14.58 15.49 16.74 17.17 16.29
CaO 0.00 0.00 0.00 0.00 0.00 0.00 0.02 0.00 0.01 0.02
Na2O 0.05 0.08 0.05 0.00 0.04 0.00 0.02 0.01 0.01 0.02
K2O 0.01 0.03 0.01 0.01 0.00 0.00 0.00 0.01 0.00 0.01
Total 99.50 99.78 100.02 99.11 99.13 96.75 97.67 99.18 98.37 100.13
Mg# 58.2 60.7 61.4 57.8 58.2 71.1 74.4 78.9 81.9 76.2
Cr# 77.9 76.8 76.5 77.7 77.2 77.6 77.6 78.0 78.1 77.7
Fe2+
# 35.8 34.7 33.5 37.8 36.6 28.9 25.6 21.2 18.1 23.8
Fe3+
# 5.8 4.6 5.0 4.6 4.9 1.5 2.8 4.8 6.4 5.6
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Table 5. LA-ICP-MS analyses of clinopyroxene inclusions in
massive chromitite
Sample 900-8.1 900-8.2 900-8.3 900-8.4
SiO2 53.85 54.07 55.14 54.89
TiO2 0.00 0.08 0.07 0.08
Al2O3 0.40 0.50 0.32 0.55
Cr2O3 0.90 0.92 1.10 0.79
FeO(t) 0.90 0.97 0.76 0.98
MnO 0.05 0.05 0.05 0.04
MgO 17.93 18.12 18.64 18.34
CaO 25.20 24.95 23.79 23.42
Na2O 0.18 0.17 0.19 0.18
K2O 0.02 0.02 0.00 0.00
Total 99.49 99.87 100.11 99.36
Trace elements (ppm)
Sc 50 48 52 58
V 39 37 41 36
Co 8 7.9 10.3 9.4
Ni 687 822 750 756
Cu 0.5 0.0 0.0 0.1
Zn 0.9 3.1 0.0 1.4
Ga 1.1 0.1 1.0 0.1
Rb 0.3 0.1 0.0 0.3
Sr 0.0 2.7 0.0 0.0
Y 7.2 6.8 7.1 7.6
Nb 0.01 0.01 0.02 0.04
Ag 0.00 2.33 0.00 3.53
In 0 0 0.17 0
Sn 0.00 0.23 1.31 0.00
Sb 0.06 0.00 0.05 0.00
Cs 0.07 0.00 0.00 0.06
Ba 0.00 0.73 2.93 0.00
La 0.00 0.05 0.12 0.08
Ce 0.00 0.00 0.15 0.00
Pr 0.00 0.03 0.08 0.10
Nd 0.00 0.22 0.39 0.00
Sm 0.13 0.00 0.30 0.41
Eu 0.17 0.10 0.00 0.07
Gd 1.01 0.00 1.03 2.32
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Tb 0.17 0.17 0.09 0.07
Dy 1.35 0.47 1.36 1.09
Ho 0.30 0.30 0.34 0.38
Er 0.83 1.51 0.69 1.24
Tm 0.07 0.15 0.11 0.09
Yb 0.74 0.88 1.33 0.86
Lu 0.26 0.02 0.03 0.00
Hf 0.32 0.00 0.19 0.00
Ta 0.00 0.08 0.00 0.00
W 0.00 0.00 0.10 0.09
Pt 0.00 0.09 0.00 0.00
Au 0.00 0.00 0.07 0.00
Bi 0.01 0.00 0.00 0.00
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Table 6. Major and trace element compositions of whole-rock samples of peridotite in Luobusa
Lithology Harzburgite Harzburgite envelopes Dunite envelopes Dunite Lenses
Sample 904 925 927 932 934 952 968 970 900 950 976 982 971 973
Major oxides (wt%)
SiO2 38.80 43.00 40.50 40.90 39.60 41.00 41.40 41.70 40.90 38.50 38.10 42.30 41.10 40.80
TiO2 0.05 0.07 0.06 0.05 0.05 0.04 0.07 0.04 0.06 0.07 0.04 0.04 0.03 0.04
Al2O3 4.50 2.28 2.42 2.43 2.56 3.02 4.10 3.34 1.90 2.09 2.05 4.51 2.83 1.92
Cr2O3 0.52 0.56 0.56 0.58 0.67 0.61 0.55 0.45 0.82 0.55 0.61 0.42 0.45 0.49
TFe2O3 7.83 7.92 7.79 7.77 7.60 7.88 7.30 7.32 7.82 7.36 8.11 7.54 7.76 8.11
FeO 3.54 5.85 3.99 4.37 3.60 4.50 4.12 4.89 4.50 4.37 4.82 3.92 5.15 5.40
MnO 0.11 0.12 0.11 0.11 0.11 0.12 0.11 0.13 0.13 0.10 0.12 0.11 0.12 0.17
NiO 0.26 2.70 0.31 0.25 0.26 0.27 0.26 0.26 0.25 0.32 0.31 0.30 0.26 0.26
MgO 38.80 43.50 43.10 40.80 39.60 41.50 39.80 40.90 40.90 46.20 47.10 40.20 42.20 43.30
CaO 0.38 0.85 0.33 1.26 1.05 0.90 1.36 1.11 1.55 0.10 0.27 0.24 0.99 1.24
Na2O 0.01 0.02 0.01 0.01 0.02 0.01 0.02 0.02 0.03 0.01 0.01 0.04 0.03 0.02
K2O 0.00 0.01 0.00 0.00 0.01 0.00 0.00 0.01 0.00 0.01 0.01 0.01 0.00 0.00
P2O5 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01 0.01
H2O+ 9.13 2.19 4.82 5.66 8.38 4.58 5.88 5.24 6.11 4.99 4.38 5.13 4.55 3.74
CO2 0.66 0.22 0.29 0.77 0.58 0.54 0.58 0.49 0.54 0.46 0.45 0.33 0.26 0.61
Total 100.70 102.87 100.23 100.02 100.17 100.34 100.75 100.47 100.56 100.27 100.81 100.51 100.35 100.10
LOI 9.40 1.85 4.99 5.81 8.60 4.94 5.85 5.23 6.15 4.98 4.15 4.75 4.53 3.77
Trace elements (ppm)
La 1.21 0.14 0.14 0.21 2.24 0.32 0.4 0.82 0.72 0.3 2.7 1.31 1.87 3.53
Ce 2.18 0.38 0.35 0.5 4.59 0.8 0.81 1.72 1.55 0.76 5.92 2.85 4.14 6.57
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Pr 0.21 0.02 0.03 0.04 0.45 0.06 0.06 0.14 0.13 0.06 0.52 0.24 0.34 0.66
Nd 0.75 0.08 0.12 0.14 1.7 0.23 0.23 0.49 0.48 0.23 1.99 0.91 1.21 2.41
Sm 0.13 0.02 0.03 0.03 0.32 0.05 0.05 0.1 0.08 0.04 0.4 0.18 0.22 0.44
Eu 0.03 0.01 0.01 0.01 0.05 0.01 0.01 0.02 0.02 0.01 0.06 0.04 0.05 0.08
Gd 0.13 0.02 0.02 0.03 0.3 0.05 0.06 0.11 0.09 0.04 0.38 0.18 0.21 0.41
Tb 0.02 0 0.00 0.01 0.05 0.01 0.01 0.02 0.02 0.01 0.06 0.03 0.03 0.06
Dy 0.13 0.03 0.03 0.05 0.3 0.08 0.11 0.16 0.13 0.04 0.36 0.19 0.2 0.37
Ho 0.03 0.01 0.01 0.01 0.06 0.02 0.03 0.04 0.03 0.01 0.07 0.04 0.05 0.07
Er 0.09 0.03 0.03 0.05 0.2 0.07 0.09 0.12 0.11 0.03 0.19 0.13 0.15 0.22
Tm 0.01 0.01 0.00 0.01 0.03 0.01 0.02 0.02 0.02 0.00 0.03 0.02 0.02 0.03
Yb 0.1 0.04 0.04 0.07 0.22 0.09 0.11 0.15 0.14 0.03 0.16 0.14 0.16 0.22
Sc 5.4 9.1 5.8 7.9 9.8 8.4 10 9.3 13.4 3.6 4.9 4.4 9.5 8.9
Co 99 106 104 91 96 100 96 100 108 104 112 107 98 97
Ni 1724 1857 2031 1582 1733 1893 1833 1844 2021 2090 2277 2113 1839 1714
Nb 6.54 8.31 20.22 1.48 5.87 14.28 2.06 6.79 6.56 11.12 5.1 5.06 6.62 0.93
Y 1 1 1 1 1 1 1 1 1 1 1 1 1 1
Lu 0.02 0.01 0.01 0.01 0.04 0.02 0.02 0.02 0.02 0.01 0.02 0.02 0.03 0.04
Ta 0.16 0.24 0.78 0.1 0.19 0.68 0.16 0.41 0.12 0.42 0.33 0.35 0.46 0.07
Th 0.74 0.39 0.45 0.16 2.11 0.47 0.27 0.59 1.33 0.7 1.18 0.92 0.74 1.13
U 0.15 0.09 0.09 0.1 0.17 0.12 0.08 0.12 0.08 0.12 0.19 0.18 0.14 0.17
Rb 3 3 2 3 3 4 4 3 3 3 2 3 3 3
Ba 1 1 1 8 1 1 1 4 1 1 1 1 2 6
V 12 20 14 23 25 23 27 22 33 11 11 13 22 23
Hf 0.23 0.23 0.27 0.09 0.42 0.18 0.1 0.13 1.11 0.19 0.17 0.15 0.15 0.14
Cu 13.2 7.9 7.7 12.2 16 8 19.2 10.4 21.7 7.2 9.6 10 10.5 9.7
Zn 36 45 39 38 35 42 43 44 52 39 44 37 39 77
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Graphical abstract
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Highlights
The Luobusa ophiolite contains both massive and disseminated chromitite
High – pressure mineral inclusions are only found in the massive chromitite
The formation of podiform chromitites in ophiolites is a mutli-stage process