Archean
Podiform Chromitites and Mantle Tectonites in Ophiolitic Mélange, North
China Craton: A Record of Early Oceanic Mantle Processes
Jianghai Li
Department
of Geology, Peking University, Beijing 100871 China
Timothy M. Kusky*
Department
of Earth and Atmospheric Sciences, St. Louis University, St. Louis Missouri
63103, USA
Xiongnan Huang
Department
of Geology, Peking University, Beijing 100871 China
ABSTRACT
We
report 2.5 billion-year-old oceanic mantle podiform chromitite and mantle
tectonite in ophiolitic mélange in the North China craton. Tectonic
blocks of peridotite, wehrlite, pyroxenite, harzburgitic tectonite, dunite,
podiform chromitite, layered gabbro, sheeted dikes, and pillow lava are
embedded in a strongly deformed metasedimentary and metavolcanic matrix. The
blocks are traceable along strike into the relatively complete ca. 2.505 Ga
Dongwanzi ophiolite. Textures in the ultramafic blocks provide a window into
igneous and structural processes active in Archean suboceanic mantle.
Chromitites in dunitic envelopes preserve igneous nodular, orbicular,
antinodular, banded, massive, and disseminated textures. Dunite envelopes are
common features of podiform chromitites, forming almost exclusively in the
upper mantle or the crust-mantle transition zone of suprasubduction zone
(harzburgite-type) ophiolites of younger geological ages. Nodular and orbicular
chromite textures are known only from ophiolites and are interpreted to
form during partial melting of flowing upper mantle, conditions needed to keep
chromite suspended and growing concentrically into the magma. Minor orthopyroxene
porphyroclasts with asymmetrical recrystallized tails and kink-banded olivine
inclusions in chromite grains record plastic deformation and high-temperature
shearing, before or during growth of the chromite. We attribute this
deformation to flow in the Archean oceanic mantle. Later deformation is related
to dismemberment of the ophiolite and incorporation into a mélange
during collision of the Eastern and Western blocks of the North China craton.
This collision formed the 1600-km-long ophiolite-rich Central Orogenic belt,
and the 2.5–2.4 Ga Qinglong foreland basin and fold-thrust belt on the Eastern
block, and provides an important record of the operation of plate tectonics in
the Archean.
Manuscript Received by the
Society February 7, 2002
Manuscript Accepted April 30, 2002
INTRODUCTION
Return to TOC
Ophiolites
are remnants of oceanic lithosphere that have been tectonically emplaced onto
continents. They provide valuable information on the nature of seafloor
processes, global heat loss, and paleogeographic reconstructions of the
continents through ancient times. The question of whether ophiolites are
present in the earliest rock record (>2.0 Ga) is one of the most hotly
debated scientific questions in early Earth history (Kusky
and Polat, 1999 ; Karson,
2001 ). This is largely because until recently, complete ophiolite sections
consisting of (in descending stratigraphic order) pillow lava, sheeted dike
complex, gabbro, cumulate ultramafics, and tectonized mantle, had not been
documented in rocks older than 2 Ga (Kontinen,
1987 ). The recently discovered 2.505 Ga Dongwanzi ophiolite of the North
China craton (Kusky
et al., 2001 ) is a complete ophiolite, but most original mantle textures
and mineral parageneses are overprinted. In this paper, we report newly
discovered podiform chromitites from Archean mantle harzburgite tectonite and
dunite host rocks that are 60 km southwest of and related to the complete
Dongwanzi ophiolite. Podiform chromitites with nodular- and orbicular-textured
chromite balls in a harzburgite tectonite matrix are known only from ophiolitic
settings (Fig.
1), and thus serve as a diagnostic marker of oceanic mantle, potentially as
useful as the ophiolite suite itself for identifying fragments of ancient
oceanic lithosphere and asthenosphere.
The
2.5 Ga Zunhua podiform chromitites have remarkably well preserved delicate
magmatic and deformational textures that provide a glimpse into igneous and
structural processes active in the sub-oceanic mantle in the Archean. These
types of structures were known previously only from much younger oceanic mantle
rocks and, thus, preserve a unique remnant of Archean oceanic mantle.
GEOLOGICAL
SETTING Return to TOC
The
North China craton is divided into two major blocks, the Western block and the
Eastern block, separated by the Neoarchean Central Orogenic belt that extends
across the craton (Fig.
2). The Western block contains a thick platformal sedimentary cover (Ordos
block) intruded by a narrow belt of 2.55–2.5 Ga arc plutons along its eastern
margin. The Eastern block contains a variety of ca. 3.80–2.5 Ga gneissic rocks
and green-stone belts locally overlain by 2.6–2.5 Ga sandstone and carbonate.
The 700-km-long Hengshan high-pressure granulite belt (2.5 Ga) is located along
the western side of the Central Orogenic belt (Li et
al., 2000a ). Isotopic ages available from the Central Orogenic belt mainly
range between 2.58 and 2.50 Ga (Li et
al., 2000a ; Wu et
al., 1998 ). A younger age peak of 1.90–1.80 Ga is related to late tectonic
extension (Li et
al., 2000a ). Central parts of the Central Orogenic belt include a complex
assemblage of ca. 2.55–2.5 Ga deformed metavolcanic, metaplutonic, banded iron
formation and metasedimentary rocks, intruded by ca. 2.5–2.4 Ga granite. A belt
of 2.5–2.4 Ga weakly metamorphosed flysch and molasse basins that extends along
the eastern margin of the Central Orogenic belt (Fig.
2) is interpreted here as remnants of a foreland basin related to the
collision of the Western and Eastern blocks. Rocks in this belt, colloquially
named the Qinglong foreland basin, are now deformed in an east-vergent
fold-thrust belt.
The
Zunhua structural belt is a mainly amphibolite-facies terrane in the northern
sector of the Central Orogenic belt, separated from an Archean granulite-gneiss
dome (3.85–2.50 Ga) of the Eastern block by a major shear zone (Fig.
3). The Zunhua structural belt contains mainly northeast striking,
intensely strained gneiss. Various thrust slices, including
tonalite-trondhjemite-granodiorite gneiss, mafic plutonic rocks, supracrustal
sequences, and granites are tectonically intercalated with each other (Kusky
et al., 2001 ; Wu et
al., 1998 ). More than 1000 ultramafic boudins have been recognized in the
Zunhua structural belt, and these range from several meters to several
kilometers in length (Figs. 3
and 4).
These were intruded by ca. 2.56–2.5 Ga tonalitic gneiss, then 2.5–2.4 Ga
granites (Wu et
al., 1998 ). Geochemical analyses reveal that mafic volcanics in the Zunhua
structural belt have an oceanic affinity. They exhibit flat rare earth element
to light rare earth element–depleted patterns that are similar to basalts from
suprasubduction and mid-oceanic ridge settings (Wu et al.,
1998 ; Zhao
et al., 1993 ; Kusky
and Li, 2002 ). A complete 2.505 Ga ophiolite has been described from the
northeastern part of the Zunhua structural belt (Kusky
et al., 2001 ). The Dongwanzi ophiolite is in the same belt as the
ultramafic blocks described in this contribution, and many blocks can be traced
into the complex mélange zone along strike with the Dongwanzi ophiolite.
The Dongwanzi ophiolite may be interpreted as the largest block in the mélange
that preserves preferentially the upper (crustal) part of the ophiolitic
sequence. The blocks described here preserve deeper parts of the ophiolitic
lithosphere than have so far been recognized in the Dongwanzi ophiolite.
Ophiolitic
mélange in the Zunhua structural belt
Detailed
field and petrographic analysis of mafic and ultramafic blocks in the
southwestern Zunhua structural belt has revealed an assemblage of typical
ophiolitic rock types. These include partly serpentinized harzburgite,
peridotite tectonite, dunite, serpentinite, podiform chromitite, hornblendite,
wehrlite, pyroxenite, metagabbro, cumulates and pillow lavas, massive
metabasalt, and greenschist (Figs. 3
and 4).
Locally, well-preserved sheeted dikes, layered gabbro, and cumulates are
recognized. All these units are intruded by ca. 2.5–2.4 Ga granite (Wu et
al., 1998 ), demonstrating their Archean ages. All the Archean units are
overlain unconformably by ca. 1.85 Ga sedimentary cover.
Ultramafic
and mafic pods and tectonic blocks are stretched and occur within a strongly
deformed matrix of foliated and sheared, fine-grained biotitegneiss and
hornblende-gneiss with some layers of amphibolite and banded iron formation (Figs. 3
and 5).
These blocks are intensely sheared and tectonically transposed along their
margins. In contrast, internal structures of the blocks commonly show distinct
foliation and fold patterns, discordant to the external foliations outside the blocks
in the surrounding shear zones. Gabbro and pyroxenite boudins (Fig.
5) exhibit well-preserved relict cumulate textures and cyclic cumulate
banding of clinopyroxene, olivine, and plagioclase. Within the cores of
peridotite blocks and pods, metamorphic tectonite fabrics are well preserved as
oriented orthopyroxene porphyroclasts, strings of chromite, and elongated
ribbons of olivine (Fig.
6). The early tectonic fabrics defined by compositional layering include
chromite seams, disseminated chromite, oriented nodular chromite, and flattened
antinodular chromite. In younger ophiolite complexes, these textures are
generally interpreted to form during high-temperature (>1000 °C) plastic
deformation in the mantle (Nicolas
and Arzi, 1991 ; Holtzman,
2000 ).
Late
steeply dipping shear zones parallel to tectonic contacts with country rocks
cut the early high-temperature–tectonite fabrics in the peridotite.
Serpentinization is concentrated along late shear zones and fractures cutting
across the earlier foliation. Within these shear zones, ultramafic protoliths
are separated into numerous small-scale pods and lenses, which are further
flattened and stretched. The late tectonic fabric and hydrous metamorphism that
overprints the harzburgitic mantle tectonites probably occurred during
obduction-related emplacement of the ophiolite in the Archean.
Ophiolitic
crust and mantle blocks in mélange
Peridotites
in the Zunhua structural belt are mainly composed of serpentinized olivine,
relict orthopyroxene, chromite, and minor magnetite (Fig.
6A, 6B, and 6C). Stretched orthopyroxene grains enclosed in a serpentinite
matrix with ribbon-shaped tails form augen up to 2–3 mm in diameter. Some
orthopyroxene porphyroclasts preserve embayed outlines due to corrosion by
melt. Minor subhedral to euhedral chromite is present. Some peridotite
tectonites show penetrative foliations and stretching lineations.
High-temperature metamorphic tectonite fabrics defined by aligned and stretched
olivine are well preserved in cores of blocks (Fig.
6D). Relict olivine forms extended ribbons with asymmetrical geometry.
Augen of olivine exhibit deformational kink bands (Figs.
6E and 6F). These fabrics are attributed to high-temperature flow in the
mantle. Olivine crystals are commonly serpentinized, with magnetite distributed
along the foliation planes.
Boudins
and tectonic blocks of various types of gabbro and ultramafic cumulate within
intensely sheared garnet-bearing gneiss range in size from a few centimeters to
hundreds of meters. Rarely, pods of dunite are recognized within olivine gabbro
(troctolite). These rocks are generally less deformed than the ultramafic
rocks. Alternating pyroxene-rich and plagioclaserich modal and textural
layering, 2–5 cm thick, locally preserve primary cumulate textures. The
pyroxenite and olivine pyroxenite layers commonly transformed into foliated hornblendite
along their margins as the gabbros underwent amphibolite-facies metamorphism.
Blocks
of sheeted dikes, up to 100 m along strike, occur within the Zunhua structural
belt (Fig.
4). The chilled margins are recognized as 2–3-cm-thick boundaries defined
by strong alignment of fine-grained hornblende. They have only one chilled
margin, which is a consequence of repeated intrusions in the center of a single
opening fissure. The width of individual half-dikes is generally tens of
centimeters. The mineral assemblage is plag + cpx + hb, characteristic of upper
amphibolite facies conditions.
Spectacular
pillow basalts are preserved locally in weakly deformed domains (Figs. 4
and 5E).
Pillows vary in size from tens of centimeters to one meter. Pillowed flows are
interbedded with amygdular massive basalt. The pillow lavas are pervasively
altered to albite and chlorite assemblages. Rarely, the pillows preserve dark
cryptocrystalline margins, representing original glassy selvages. The shapes of
pillows indicate younging toward the northwest. Layers of pillow breccia and
volcanoclastic sediment are intercalated with the pillow basalt, and these
units are metamorphosed to plagioclase-biotite schist and biotite schist. Some
ultramafic lenses are intercalated with pillow lavas, indicative of large
amounts of shearing (either on the seafloor or during emplacement).
At
least six large and numerous smaller chromite-rich peridotite massifs are recognized
within the southwestern Zunhua structural belt (Figs. 4
and 7).
The chromitites are commonly lens shaped within dunite envelopes and are
concordant with the foliation of the enclosing intensely serpentinized
harzburgite. Serpentinized pods and lenses show concentric rings with
systematic variations in mineral composition and texture. Outer rings, commonly
2–10 cm thick, are composed of serpentine, whereas inner cores preserve dunite
or massive harzburgite. Narrow, branching pyroxenite dikes (1–10 cm wide) are
deformed within serpentinized harzburgite. The dikes are interpreted as
channels of melt parental to oceanic basalt. Tectonic fabrics defined by folded
chromite bands are well preserved in the cores of the serpentinized pods.
Dunite envelopes are common features of podiform chromitites. They are known to
form almost exclusively in the mantle or crust-mantle transition zone of
suprasubduction zone (harzburgite type) ophiolites of different ages (Nicolas
and Arzi, 1991 ; Zhou
et al., 1996 ; Edwards
et al., 2000 ).
Most
of the chromitites are strongly deformed by high-temperature plastic flow,
although nodular, orbicular, banded, massive, antinodular, and disseminated
chromitite textures (sensu Johnston,
1936 ) are all locally preserved, especially in discordant pods (Fig.
7). Nodular textures consist of small balls of chromite in a dunite matrix
(Fig.
8B, whereas orbicular chromites consist of thin rings of chromite
surrounding rounded to flattened cores of dunite (Fig.
8A). Nodular and orbicular chromites, with diameters of 2–10 mm and
occasionally larger than 10 mm, are generally flattened into elliptical shapes,
and some orbicules form flattened rings. Nodular and orbicular textures are the
most typical magmatic structures of ophiolitic chromitites (Nicolas,
1989 ; Nicolas
and Arzi, 1991 ). Abundant deformed olivine occurs as inclusions in the
chromite (Fig.
8E), although they are widely altered into serpentine. Preliminary work on
the crystallographic preferred orientations of olivine shows preferred slip on
(010)[100] slip systems, which occurs at temperatures >1200 °C (Nicolas,
1989 ). Orthopyroxene porphyroclasts show asymmetrical recrystallized tails
indicating high-temperature shearing.
Nodules
are locally sorted into layers by their sizes. The nodules and orbicules show
patterns of flattening and mutual impression along their contacts with each
other (Figs.
8A, 8B, and 8C), suggesting that they settled while in a melt. These
features are interpreted to be a result of rapid deposition of chromite nodules
while they were still plastic. The nodules and orbicules commonly exhibit
stretching fabrics (lineation and foliation), interpreted to have formed soon
after crystallization, while the interstitial olivine was still in liquid form.
Some are also elongated by plastic strain and show a preferred orientation.
Most nodules are oriented parallel to the foliation. The outer boundary of
single nodules is typically smooth and rounded (Fig.
8B). In contrast, the inner boundaries display individual chromite grains
that grew inward (Fig.
8A). These textures are interpreted to record dynamic magmatic flow or
partial melting conditions, needed to keep chromite suspended and growing
concentrically into the magma. The delicate magmatic structures preserved show
that they have not been significantly deformed after their formation, and they
preserve the primary interaction between Archean melts and the upper mantle.
In
some cases, nodules grade into antinodules in the same hand specimen. They
record magmatic growth and settling in the upper mantle (e.g., Zhou
et al., 1996 ; Edwards
et al., 2000 ). Rounded inclusions of olivine, orthopyroxene, and other
silicates occur within chromite grains (Fig.
8E), and some inclusions of olivine show kink bands that record plastic
deformation before or during growth of the chromite. A few fluid inclusions
have also been observed within the silicate inclusions in chromite. These
original magmatic structures are commonly destroyed with higher shear strain.
For example, nodular and orbicular textures are strongly stretched and
transposed into layering, folded bands, or antinodular chains. Compared with
nodules, orbicules are more strongly stretched, their ratio of X/Z being up to
5:1 (Figs
7E and 8A).
We attribute this deformation to mantle flow in the oceanic mantle, as
suggested for similar structures in many younger ophiolites (e.g., Nicolas,
1989 ).
Antinodular
chromitites consist of flattened dunite aggregates with lengths of 2–10 mm
surrounded by fine-grained chromite chains or flattened networks (Figs. 7
, 8F,
and 8H). Flattened antinodular texture is typical of high-temperature
plastic deformation in oceanic mantle, which is a result of straining of weaker
olivine inclusions in a rigid chromite-rich matrix. Alignment of needle-like
chromite also indicates strong shearing. Layered and banded textures consist of
anastomosing 2–4 cm thick bands and chains of chromite surrounding ovoids of
olivine (Figs.
7F and 7H), which were generated by shearing of antinodular and nodular chromite
layers, rather than by crystallization and accumulation. Tight folds are common
in the banded chromitite. In a few places, narrow pyroxenite, dunite, and
gabbroic dikes crosscut them. Some chromite layers occur as rootless folds or
asymmetric lensoidal boudins, and other layers and lenses consist of nodules.
Pullapart textures are common in the massive and layered chromitite deposits (Fig.
8G). These form extensional veins perpendicular to the chromite layers,
filled by serpentinized dunite. Nicolas
(1989) and Holtzman
(2000) show that such textures form only at temperatures >1000–1200 °C.
DISCUSSION
AND CONCLUSIONS Return to TOC
We
interpret the mafic and ultramafic blocks in the biotite gneiss matrix to
represent a strongly dismembered ophiolite in a metasedimentary and
metavolcanic matrix. Relationships are strongly reminiscent of younger
ophiolitic mélange terranes, where blocks of ophiolite are preserved in
a metasedimentary accretionary prism and/or trench complex (e.g., Kusky
et al., 1997 ). The Zunhua ophiolitic blocks in mélange do not
preserve an overall younging direction, although a few of the blocks show
younging directions toward the west. Similarly, the Dongwanzi ophiolite to the
northeast preserves a westward-younging sequence.
These
relationships suggest, although do not require, that the ophiolites were
emplaced into the mélange during westward directed subduction, then
thrust over the Eastern block during closure of the intervening ocean basin (Fig.
9). In this model the contemporaneous arc would be located to the west of
the Zunhua structural belt. We interpret a narrow belt of deeply eroded and
strongly metamorphosed 2.55–2.5 Ga arc-type tonalite-trondhjemite-granodiorite
plutonic rocks and a greenstone belt in the Wutai-Hengshan-Taihang Mountains to
the southwest to represent the remnants of this arc (Li et
al., 2000b ; Wilde
et al., 1998 ). The ca. 2.5–2.4 Ga Qinglong, Hutuo, and Dengfeng sedimentary
sequences and other similar basinal deposits east of the Central Orogenic belt
(Figs.
2 and 9)
may represent the foreland basin sequence resulting from the collision of the
east and west blocks. These basin sequences consist of lower turbidite and
upper molasse sequences, with more intense thrusting and folding in the west
adjacent to the Central Orogenic belt. The 2.5–2.4 Ga granitoids that intrude
the base of the ophiolite and much of the Central Orogenic belt could represent
collisional-to-postcollisional granites formed during crustal thickening during
orogenesis. This model also explains the exhumation of ca. 2.5 Ga high-pressure
granulites and retrograde eclogites in the Hengshan belt to the west (Li et
al., 2000a ) (Fig.
9).
Harzburgite
blocks in the mélange host podiform chromitites with dunite envelopes.
The blocks grade up-section into wehrlite, pyroxenite, olivine gabbro
(troctolite), and gabbro. Podiform chromitites are a normal component of
ophiolites of different ages. They are located in the transition zone between
layered gabbro and peridotite tectonite, and the lherzolite-harzburgite
transition in ophiolites (Nicolas
and Arzi, 1991 ). Their geological occurrence is closely associated with
oceanic spreading processes (Nicolas
and Arzi, 1991 ). Late Proterozoic podiform chromitites in ophiolites have
been described in several areas, and Phanerozoic examples are numerous (Fig.
1). The oldest relatively intact podiform chromitite previously recognized
is that from the Jourma and Outokumpu ophiolite complexes (2 Ga), Finland (Kontinen,
1987 ; Vuollo
et al., 1995 ). The Zunhua chromite ores exhibit remarkable similarities to
the podiform ores described from the examples mentioned above.
The
Zunhua nodular and orbicular chromites are characteristic of alpinetype
peridotites or ophiolitic chromite ores (Nicolas,
1989 ; Peters
et al., 1991 ; Dilek
et al., 2000 ). It is now believed that this type of chromite accumulated
below the transition between oceanic crust and mantle based on numerous investigations
in ophiolites. The origin of the podiform chromitites is attributed to
melt-rock reaction, or dynamic magmatism within melt channels in the upper
oceanic mantle (Nicolas
and Arzi, 1991 ; Zhou
et al., 1996 ). The presence of water in the melt is thought to be
important for the formation of podiform chromite (e.g., Edwards
et al., 2000 ). Inclusions within chromites, olivine, and orthopyroxene of
the host peridotites in the Zunhua structural belt record high-temperature
plastic deformation. The flattening and elongation of chromite parallel to
foliation and lineation are indicators of intensive high-temperature shear
strain. These textures probably record the plastic flow of the upper mantle,
now mainly preserved in the core of tectonic blocks. These early lineations
defined by deformed magmatic inclusions and the elongation of ore zones are not
parallel to later lineations related to the emplacement of the blocks along
shear zones, supporting the idea that they represent early mantle-deformation–related
fabrics. Podiform chromitites are remarkably resilient to later deformation and
metamorphism since they are generated at high temperatures (1200–1300 °C) and
become very rigid when cooled, thus resisting later shear. These asthenospheric
chromite pods are miniature time capsules preserving extraordinary amounts of
information about the Archean mantle that we have only begun to tap and
understand.
Coupled
with the presence of a full ophiolite sequence in the Dongwanzi complex, the
documentation of the Zunhua chromitites provides evidence for the operation of
seafloor spreading and plate tectonics during the Archean before 2.5 Ga. We
prefer to ascribe a faster-to-moderate spreading rate to the formation of the
Zunhua podiform chromitites, as podiform chromite is mainly associated with
harzburgite-type ophiolites (Nicolas
and Arzi, 1991 ). Although the field and petrographic observations are
consistent with the Neoarchean ophiolites of the Central Orogenic belt
preserving relatively hot mantle features, we do not have evidence that this
mantle record was any hotter than the present-day range of mantle temperatures.
However, the hot Archean North China mantle is consistent with some of the
higher heat production during the Archean being accommodated by faster creation
of oceanic lithosphere from a slightly hotter oceanic asthenosphere.
ACKNOWLEDGMENTS
This research
was supported by the National Science Foundation of China (No. 49832030),
Peking University (Project 985), the U.S. National Science Foundation
(Tectonics Program EAR-0221567), and by St. Louis University. Li thanks X.L.
Qian for discussions and his extensive help. Jesse Dann, Claude Herzberg, John
Encarnacion, and Igor Puchtel provided thoughtful reviews of this work. This is
a contribution to International Geological Correlation Program Project 453.
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Zhou, M.F., P.T. Robinson, J. Malpas, and Z. Li. 1996, Podiform chromitites in
the Luobusa ophiolite (southern Tibet): Implications for melt-rock interaction
and chromite segregation in the upper mantle: Journal of Petrology. v. 37, p.
3–21.
*Corresponding
author: E-mail: kusky@eas.slu.edu.
Figure 1. Distribution of Archean cratons, areas underlain by
Precambrian crust, and Phanerozoic crust and podiform chromite deposits .Filled
patterns show locations of major ophiolitic podiform chromite deposits in
relation to crust and accretionary orogens of different ages. Squares—Archean;
triangles—Proterozoic; circles—Phanerozoic. Zunhua podiform chromitites are
associated with the 2.505 Ga Dongwanzi and related ophiolites of North China
craton
Figure 2. Tectonic map of North China craton showing division of North
China craton into Eastern and Western blocks, separated by the Central Orogenic
belt. (Modified after Li et
al., 2000a , 2000b ;
Kusky
et al., 2001
Figure 3. Structural sketch map showing distribution of Archean
ophiolites and major structural boundaries of Zunhua Structural belt, eastern
Hebei
Figure 4. North Zunhua area showing
distribution of ophiolitic blocks in a matrix of biotite gneiss and
hornblende-biotite (modified from local geological maps and remapping by the
authors)
Figure
5. Field photos of podiform chromite and related outcrops, North Zunhua area.
A. Flattened lenses and pods of dunite in a foliated serpentinized harzburgite
matrix. B. Serpentinized harzburgite block in mélange, matrix of
weathered biotite gneiss. C. Gabbroic block within sheared biotite gneiss. D.
Mafic and ultramafic boudins as inclusions with dioritic gneiss. E. Pillow lava
from block in mélange
Figure
7. Polished surfaces of hand specimens illustrating principal microstructures
and textures of chromitites ores. A. Banded chromitites with dunite envelop
preserved at base of sample. B. Rootless fold showing thickening of chromitite
band in a serpentinized dunite matrix. C. Chromite vein within serpentinized
dunite. D. Nodular chromite in weakly deformed domain. E. Nodular and orbicular
chromite. F, G. Antinodular chromite, showing flattening of dunite in cores of
chromite rings. H. Layered antinodular chromites
Figure
6. Structures and microscopic textures of peridotite blocks in mélange.
A, B. Polished surfaces of hand specimens illustrating principal textures of
flattened harzburgite core with outer rings of serpentinite. Early
high-temperature mantle-tectonite foliations are preserved in cores of pods. C.
Harzburgite showing orthopyroxene (OPX), chromite (CMT), and olivine (OL)
crystals (field of view is 3.2 mm horizontally). D. Asymmetrical olivine
porphyroclast with recrystallized tail (outlined in white) within harzburgite
tectonite (field of view is 3.2 mm horizontally). E, F. Kink bands within
relict olivine (OL) in serpentinized harzburgite (field of view is 3.2 mm
horizontally in both photomicrographs)
Figure
8. Microscopic textures of chromite ores. A. Flattened orbicule of chromite
(reflected light). B. Flattened nodules of chromite and igneous contact between
two nodules (reflected light). C. Flattening of contact between orbicule and
nodule of chromite. D. Flattening of chrome nodule with asymmetrical tail. E.
Inclusion of orthopyroxene (OPX) and olivine (OL) showing kink bands within
chromites. F. Deformation textures of chromitite ore showing stretched olivine
crystals in dunite core (strain ellipse outlined in white) with antinodular
chromite. G. Pulled-apart chromites. H. Flattened antinodule forming concentrated
bands of chrome and apparent folds from initial chrome network structure that
originally surrounded antinodule. Field of view is 8 mm (horizontally) in A, B,
C, D, and F, 3.2 mm for G, and 1.6 mm in E and H
Figure
9. Model for the Late Archean tectonic evolution of the North China craton. An
arc terrane built on the Western block at 2.55 Ga collides with the Eastern
block at 2.5 Ga, forming an ophiolitic mélange with fragments of forearc
and other ophiolites in an accretionary wedge during closure of the intervening
ocean basin. The Central Orogenic belt overrides the passive margin of the
Eastern block during attempted subduction of the margin of the Eastern block,
forming the Qinglong foreland basin and foreland fold-thrust belt. Numerous
2.50–2.40 Ga granites intrude during and slightly after the collision. The
partially subducted margin of the Eastern block rebounds isostatically and
flexurally, causing the rapid uplift, extensional collapse, and exhumation of
high-pressure granulites of the Hengshan belt at 2.5–2.4 Ga, along with the
intrusion of an extensional mafic dikes swarm