Petrogenesis of Amphibolites from Northern Chotanagpur Plateau

Transcription

3rd World Conference on
Applied Sciences, Engineering & Technology
27-29 September 2014, Kathmandu, Nepal
Petrogenesis of Amphibolites from Northern Chotanagpur
Plateau, Eastern Indian Peninsular Shield
VIKASH KUMAR
Department of Geology (CE), Muzaffarpur Institute of Technology, Muzaffarpur-842003, Bihar, India
E-mail: [email protected]
Abstract: The protoliths of amphibolites belonging to northern Chotanagpur plateau including the world
renowned Koderma mica belt (Jharkhand State) of Precambrian eastern peninsular India geochemically bear
strong tholeiitic affinities of the oceanic island arc setting and has been assigned to immature or initial stages of
island arc system with no sign of volcanic ejecta and occurring as sill-flow complexes all suggesting their
emplacement in the embryonic intraarc marginal basin. The Mg-rich nature of these rocks indicates that they
have evolved from fractional crystallization of a primary magma of picritic basalt composition. Lack of spinifex
texture and CaO/Al2O3 ratio < 1 precludes the possibility of komatiite as the precursor. The various critical
constituents based on major, trace and REE data precisely support this contention. General agreement show
evolution of magmas in island arc set up is more variable and diverse in nature and is a multistage and
multisource phenomena ranging in composition from tholeiitic in the initial stage through calc-alkaline in
mature to alkaline in the late stages of their development. Amongst the various petrogenetic models of arc
magmatism the most favoured or lone viable source considered for the evolution of the Mg-rich basaltic rather
tholeiitic suites is the melting of metasomatised mantle wedge triggered by volatile influx and the convection
within upper mantle which seems the most plausible mode of genesis of the protoliths of the investigated
amphibolites under moderate degree of partial melting (15-25%) of the lherzolite (enriched hydrous peridotite)
in the subarc mantle wedge. Genetically significant REE ratios (La/Sm)N, (La/Yb)N and (La/Ce)N that reflect
mantle source rock under fairly large partial melting also support the above observations. There is conspicuous
absence of rocks of the composition of either adakite or boninite in the area which is highly important from
petrogenetic point of view in the typical subduction related island arc set-up. Olivine- normative MgO-rich
varieties (MgO~27%) of the studied rocks i.e. talc-tremolite schists evolved from tholeiitic magma by early
fractionation of ol+opx assemblages followed by cpx+plag. at lower temperature giving rise to the dominant
quartz- & hypersthene- normative basalt and basaltic andesite. Calc-alkaline series are almost absent from
amongst the studied amphibolites despite the fact that tholeiitic and calc-alkaline magma may co-exist under this
set-up.
Keywords: Petrogenesis, Amphibolites, Chotanagpur plateau, Precambrian, Eastern India
Introduction:
The Chotanagpur plateau constituting the eastern part
of Precambrian Indian shield is predominantly a
granite gneiss-migmatite complex namely CGGC
(Chotanagpur Granite Gneiss Complex) with
metasedimentay enclaves of variable sizes. CGGC
marks the close of Singhbhum/Satpura orogeny
(c.850 Ma) trending ENE-WSW to E-W. Within its
vast expanse lies supracrustal rocks which are
extension of the Iron ore series (Dunn & Dey, 1942)
later renamed Singhbhum Supergroup (Sarkar &
Saha, 1962) in the south and also a belt of high grade
metamorphic rocks popularly known as Koderma
mica belt (KMB) bordering its northern periphery
along with minor and stray occurrences of
metabasites such as calc-granulites and amphibolites.
It is demarcated from north Singhbhum mobile belt
(NSMB) i.e. the type area of Proterozoic Singhbhum
Supergroup rocks in the south by Tamar Khatra
lineament or south Purulia shear zone and extends
upto Indogangetic plain in the north. The present
work is a part of the major project dealing with the
petrological and geochemical studies of various
lithologic units of the Koderma mica belt and
adjoining areas with a bias towards the understanding
of the genesis and mineralization of the pegmatites
associated with the different host rocks. It is apt to
mention here that some of the pegmatites intruding
into the hornblende schists of the area are also
mineralized. As such to have an idea of the chemical
nature of these amphibolitic rocks which have
intrinsic potential to throw light on the tectonic
setting under which their parentage evolved is a
prerequisite for better understanding of the problems
of pegmatites in the context of which the main theme
of the present study is to investigate the geologic,
petrochemical and petrotectonic evolution to unravel
their petrogenesis.
Regional & Local Geologic set-up:
The select area of study falls in the regional geologic
set up of Singhbhum region, one of the highly
mineralized sectors of the world, belonging to eastern
India peninsular shield the geological knowledge of
which is of modern character that has evolved
through decades of geological, geochemical and
structural works combined with radiometric dating of
the rocks of the area (Fig.1).
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VIKASH KUMAR
Fig. 1 Regional Geological Map of the area (Modified after GSI map)
The field work was carried in select areas around
Koderma on National highway No.-31 and Bagodar
on NH-2 under Koderma & Hazaribagh districts of
Jharkhand state of India (Fig.2) belonging to
Koderma mica belt in the north and northern CGGC
in the south respectively which are two distinct units
separated by a fault zone and they in fact represent
somewhat different geologic settings within the same
geological domain. The area is covered under the
toposheets 72H/10-12 & 16 bounded by Lat 2400` to
24037`N and Long. 85030` to 8600`E and is
dominantly an undulating plain, where the
amphibolites being comparatively more resistant than
the surrounding rocks mica schist and gneisses stand
out prominently as hillocks, ridges and mounds like
intrusive granites in addition to some younger
doleritic dykes. As mentioned, the area is a part of
Chotanagpur crystalline complex and its northern
extremities, the KMB, the structural base of which is
provided by granites and gneisses associated with
thick piles of high grade metasediments as well as
amphibolitic rocks both foliated and non-foliated
(Fig.2). The KMB, world’s largest producer of ruby
mica, basically include undifferentiated group of
pelitic schists intercalated with bands of schistose
quartzites and amphibolites in addition to some
granitic and gneissic exposures of variable
dimensions. All these rocks are invaded by swarms
of pegmatite dykes and veins which are genetically
related to the synkinematically emplaced intrusive
granites. Some of them contain workable lodes of
mica besides number of rare minerals. CGGC, on the
other hand, is primarily a gneiss-migmatite-granitoid
assemblage with metasedimentary enclaves of
variable sizes along with pyroxene as well as calcgranulite rocks besides some concordant and
discordant bands of metabasics like peridotite,
pyroxenite, gabbro, epidiorites, dolerites including
amphibolites.
Proceedings of the 3rd World Conference on Applied Sciences, Engineering and Technology
27-29 September 2014, Kathmandu, Nepal, ISBN 13: 978-81-930222-0-7, pp 594-606
Petrogenesis of Amphibolites from Northern Chotanagpur Plateau, Eastern Indian Peninsular Shield
Fig. 2 Sample map of the area
The amphibolitic rocks of the study area constituting
mafic to ultramafic suites are identified in the field as
Talc-tremolite-schists, foliated and non-foliated
amphibolites. They form sill-flow complexes
occurring as linear bands, strips, concordant lenses
and oval shaped hills. They have been simultaneously
intercalated, deformed and co-folded with the
associated metasediments (Fig.3a). In CGGC, talctremolite-schists
(Greenschists)
are
scarcely
occurring and unevenly distributed in the area and
their presence has genetic values. In fact, there is no
conspicuous field difference in the amphibolites from
gneissic as well as mica belt but whether they belong
to the same phase of magmatic activities is still a
matter of detailed investigation. The area appears to
be devoid of pyroclastics which is significant from
the point of view of petrogenesis. The supracrustal
rocks have undergone polyphase deformation and
metamorphic episodes under amphibolites facies
conditions.
VIKASH KUMAR
Fig. 3 (a) Field photograph showing anticlinal pitching fold in amphibolite gneiss sequence with pegmatitic material along
the nose of the fold.
Microphotographs : (b) Talc-tremolite schist with brown iddingsite and platy talc (grey greenish) and prismatic laths of
tremolite with brown stains under polarized light,
(c) Foliated amphibolites with corona-like structure i.e. relicts of pyroxene rimmed by green hornblende under x-nicol,
(d) Porphyroblast of fractured garnet surrounded by vermicular growth of rod shaped hbl within saussuritized calcic
plagioclase under pol. light.
Petrographic types:
Petromineralogically the rocks of the study area have been classified as follows:
Petrographic type
Mineral composition
Non-foliated amphibolite (i) Hbl + Calcic plag. + Cpx
Koderma Mica Belt
Foliated amphibolites
(i) Hbl + Calcic plag.
(ii) Calcic plag. + hbl + augite + gar.
Non-foliated amphibolites
CGGC
Foliated amphibolites
(i) Hbl + Calcic plag.
(ii) Hbl + Calcic plag. (hornblendite)
(i) Hbl + calcic.plag. + Cpx + Opq.
```````````````````````````````````` (ii) Hbl + Opx + Cpx (di) + calcic.plag.
Talc-tremolite-schist (Greenschist) Talc+tremolite+actinolite+opaque
The rocks are massive to well foliated with
amphibole + calcic plagioclase + pyroxene together
constituting more than 90 by volume. Petrogahically
and mineralogically, the amphibolites of both the
units are quite similar and conspicuously
homogeneous. Based on relict texture (ophitic to subophitic) and mineralogy (Opx, Cpx) they exhibit
undoubted signs of igneous parentage (Fig.3 b & c)
and belong to basalt - basaltic andesite - rhyolite
petrotectonic association which is typical of the
modern volcanic suites encountered in the subduction
related island arc set-up.
Geochemical features:
Whole rock as well as trace including REE analysis
of the selected samples was done in the geochemical
laboratories of NGRI, Hyderabad using XRF (Philips
Magix PRO Model 2440) and ICP-MS (Model
ELAN DRC II, Perkin-Elmer Sciex Instrument)
respectively with high degree of precision and
comparable accuracy. For analytical method and
techniques adopted papers of Govil (1985); Balaram
& RaO (2003) can be seen. Major oxides along with
their CIPW norms and trace element including REE
data are depicted in Tab.-1 & 2 respectively for
convenience of evaluation and interpretation.
Table 1. Chemical Composition (Major Oxides, CIPW Norms) of the Amphibolitic Rocks of the Area
The compositional data (Tab.-1) when plotted on
standard
variation diagrams
show
normal
differentiation trend within narrow range. Talc tremolite schist (A-77) with SiO2 content 40.54 wt%
and MgO = 27% has been described as early formed
ultramafite. Foliated and non-foliated amphibolites
exhibit compositional similarities of basic character.
AFM compositional diagram (Fig.4) shows that rocks
of the area fall close to the early stage of
fractionation trend of Skaergaard as well as the
Palisades tholeiitic suites indicating an initial Feenrichment and igneous origin for these rocks.
Exceedingly low alkalies (Na2O+K2O = 0.78-3.39,
with Na2O>K2O) coupled with iron enrichment at the
early stage of differentiation indicate the tholeiitic
nature and mafic character of these rocks. These
amphibolites are exclusively quartz and hypersthene
normative except a few ultramafic type being olivine
normative in addition to a narrow range of variation
in critical constituents Fe2O3/FeO (0.33-0.49),
FeO0/MgO (0.97-2.08), FeO0/FeO0+MgO (0.49-0.68)
where FeO0 is total iron as FeO along with their
metaluminous mineralogy and mg number (54-72,
except 84 in the ultramafic rock) all supporting a
comagmatic nature of these rocks (Kumar, 2014).
Ortho- nature of these amphibolites has also been
delineated using the chemical criteria adopted by
Leake, 1964 (Fig.5) while dealing with the debatable
question of distinguishing the ortho- and para- nature
of the amphibolites in general.
VIKASH KUMAR
Fig. 4 AFM diagram for the amphibolitic rocks
of the area;  : tholeiitic trend, skaergaard
intrusion, after McBirney, 1996
 : tholeiitic trend, the Palisades sills,
after Shirley, 1987,  : calc-alkaline
trend, the Medicine Lake, after Grove &
Baker, 1984.
Fig. 5 The amphibolites of the area showing
negative correlation on TiO2 vs
MgO/MgO+FeO plot after Leake
(1964) indicating that their protoliths
are basaltic in nature.
Table 2. Trace and REE Compositions of the Amphibolitic Rocks of the Area
The trace element distribution pattern (Tab.-2) and
systematic variation in relevant ratios like Cr/Ni,
Ni/Co and K/Rb also support fractional
crystallization model. A high value of Cr (~2897
ppm), and Ni (~624) coupled with high MgO (
>27%) and SiO2 < 50% characterised by talctremolite schist (A-77) indicate the primary nature of
basaltic magma derived from partial melting of the
upper mantle. The MORB- normalized trace element
patterns of these rocks (Fig.-6) using normalizing
constants from Pearce (1983) have revealed that the
amphibolites of the area are enriched in LIL elements
such as Sr, K, Rb, Ba & Th as compared to HFSEs
such as Ta, Nb, Ce, P, Zr, Hf, Sm, Ti, Y & Yb
coupled with a -ve Nb-Ta-Ti & Zr-Hf anomaly
reflecting their diverse nature characteristic of
subduction related island arc setting (Kumar, 2014).
Fig. 6 MORB normalized trace element
patterns using normalizing constants
from Pearce (1983)
Fig. 7 Chondrite normalized rare earth element
patterns. Normalizing constants after
Sun & McDonough ((1989)
The REE abundances and the Chondrite-normalized
patterns of the rocks studied (Fig.7) using
normalizing constants of Sun & McDonough (1989)
are broadly similar, negatively sloping and invariably
marked by a trough at Yb indicating a strong
enrichment in LREE and depletion in HREE almost
1/5 times the LREE that could probably result from
lower degree of melting of a normal/depleted mantle
(Best, 2006) or a moderate degree of melting of an
enriched mantle (Rathna et al., 2000). This is also
supported by (Ce/Yb)N ratios that vary from 2.66 7.54. Moreover, (La/Sm)N ratio > 1 (1.43–3.21) also
reveal a source related LREE enriched nature of these
rocks (Hart & Blusztajn, 2006). (La/Yb)N ratios range
from 1.16-8.98 suggesting a strongly fractionated
REE pattern. A systematically increasing variation in
ΣREE from talc-tremolite-actinolite schist (13.97
ppm), foliated amphibolites (av.57.96 ppm) to non-
foliated amphibolites (139.8 ppm) also suggest the
fractional crystallization model. Talc-tremolite schist
indicates absence of plagioclase as a small –ve Eu
anomaly occurs in its chondrite normalized pattern
(Fig.7). This is also supported by Eu/Eu* values
which are remarkably <1 (0.74) for rocks with –ve
Eu anomaly and >1 (1.01-1.29) for rocks with +ve Eu
anomaly.
The tectonomagmatic evaluation based on
discrimination diagrams (Fig.8 & 9) after Glassley
(1974) and Miyashiro (1975) also show that the rocks
of the area are mainly comparable to island arc
tholeiites. Thus, the major as well as trace element
including REE variations have supported a fractional
crystallization model within a narrow range varying
from ultramafic to dominantly mafic types that
invariably range in composition from basalt to
basaltic andesite (Fig.10)
VIKASH KUMAR
Fig. 8 TIO2 vs FeO0 / MgO tectonic disciminant
diagram (after Glassley, 1974)
Fig. 9 Ni vs FeO0 / MgO diagram
(after Miyashiro, 1975)
Fig. 10 Zr/TiO2 vs Nb/Y variation diagram
for the amphibolitic rocks of the area
after Winchester & Floyd, 1977
Petrogenesis:
In the concept of plate tectonics the processes
involved in generation of magma in subduction
related island arc environment is more variable and
diverse in nature than any tectonic setting and hence
the magma evolution under this environment is not
only complex but also a multistage and multisource
phenomena (Hawkesworth & Powell 1980, Wyllie
1984, Arculus & Powell, 1986). Despite the obvious
correlation between the subduction of lithosphere and
magma generation, the role of subducted lithosphere
is quite varying and significant. Early models
favoured partial melting of subducted oceanic crust
(Marsh & Carmichael, 1974 ; Green & Ringwood,
1968) whereas the more recent models have favoured
partial melting involving mantle wedge (Wilson &
Davidson, 1984; Wyllie, 1984; Arculus & Powell,
1986 and others). However, the contributing factors
for their petrogenetic and geochemical variation are
large enough including heterogeneities of mantle
source, degree of partial melting, varying P-T-X,
high level crystal fractionation in magma chambers,
contamination and mixing of magmas, spatial and
temporal distribution of rock types in trench - arc
setting. The various models of magma generation in
this tectonic setting, proposed so far, have attempted
to assess the relative role of the potential source
regimes in different island arc system as well as
involvement of sedimentary components. A brief
discussion of these models is very pertinent to
understand precisely the processes involved in
genesis of the studied amphibolites.
(i) The earliest models consider the melting of
subducted basaltic slab to yield island arc
magmas:
At convergent plate margin the subducted
lithospheric slab releases significant amount of
aqueous fluid and silicate melt as a result of
progressive dehydration of the hydrothermally
altered basaltic crust including the uppermost part of
the upper mantle at the depth of about 100 Km
through metamorphic reactions under existing P-T
conditions where amphibole bearing assemblage
changes to pyroxene bearing eclogite. The fluid so
generated is confined to the upper part of the slab and
also infiltrate into overlying peridotitic mantle wedge
below island arcs. Under hydrous condition the
solidus temp. of the rocks usually lowers down far
below the solidus temp. under dry condition.
Experimentally, it has been observed that at a depth
of 125-150 Km the prevailing temp. surpass the
modified lower temp. under hydrous condition to
facilitate melting of eclogite in the slab or even the
metasomatised mantle to finally yield the arc
magmas. Further, experimental and geochemical
studies, however, apprehends about the melting of
basaltic rock to the extent of producing arc magmas.
(ii) Subsequent new models suggesting melting of
overlying water influxed peridotite wedge --- a
more viable source :
These explain generation of magma exclusively from
hydrated mantle wedge more or less on similar lines
as stated above. However current researches do not
approve addition of water to the wedge as the only
factor for such arc magma to form. As such the
magma would have been formed at much lower
temp. with correspondingly different composition
than other settings. Infact, what has been noticed
frequently is that there is hardly any difference in the
major oxides composition of basalts from various
tectonic setting and are nearly identical (Plank &
Langmuir, 1986). At the same time the available
chemical data and experimental results indicate that
the primary magmas were generated at temp.
>13000C (Tatsumi, 2003). Thus, besides fluxing of
water, there must be some other factors involved in
the generation of arc magmas.
(iii) The third model involving melting of mantle
wedge triggerd by volatile influx and
convection within the upper mantle.
Recent works relating to modelling of the thermal
and mechanical properties of the material constituting
subduction zone including the various processes
working within the mantle wedge have favoured the
operation of some sort of convection within the
overlying mantle wedge because of the drag effect of
the subducted slab and pulling down of mantle
material in the upper layers immediately over it. As a
result the hotter and deeper mantle material is piled
up towards the ‘corner’ of the wedge (Fig.11) as
depicted in the model suggested by Schmidt and Poli
(1998). Thus, melting may be induced as a result of
decompression in this setting as happens in the midoceanic ridges which infact is the combined effect of
addition of water and decompression. The
metasomatised and enriched mantle wedge on
substantial degree of melting yield certainly a nearly
dry olivine tholeiitic magma which is incapable of
being extruded to the surface and occurs as a suite of
low K-tholeiite in young and immature arcs. With
little cooling tholeiitic and basaltic andesite form as a
result of limited fractional crystallization which may
penetrate the thin crust at this stage.
Fig. 11 Diagram illustrating features of
typical subduction zone in an island arc
setting with stability region of various
hydrous minerals in the subducting oceanic
crust. The magma generation takes place in
the shaded region as combined effects of the
infiltration of fluid from dehydrating slab
and the hotter material moving up from
beneath towards shallower level in a mantle
wedge after Schmidt & Poli, 1998.
Successive primary magmas represent smaller
degrees of melting of the mantle wedge and, thus,
contain some what larger amount of water, alkalies
and incompatible elements (Keith, 1978) with the
maturity of arcs. The rising magma may encounter
thicker crust and have more difficulty penetrating to
the surface and thus underplating takes place in
chambers beneath the arcs where they undergo
fractional crystallization. At this stage of the arc
evolution voluminous calc-alkaline andesite is
produced followed by more evolved lighter density
types like dacite and rhyolite that characterize island
arc set-up and are capable of violent eruption because
of great buoyancy and release of volatiles on cooling
and differentiation. The composition of evolved
magmas such as andesite is so diverse that no single
magma generation process can account for range of
the observed chemistry. The island arcs commonly
have younger and more K-rich rocks successively
farther from the trench as they successively come
from deeper level of melting. The model is well
exemplified by the spatial and temporal distribution
VIKASH KUMAR
of arc volcanic found in young active island arc
province of Japan. The volcanic sequence of rocks
starting from volcanic front – tholeiite-calc-alkaline - calc-alkaline --- K-calc-alkaline --- Alkalic calcic -- Alkalic series was found in Izu-Bonin arc, an
offshoot of Japan arc (Miyashiro,1972).
(iv) The fourth model favoured melting of the
subducted basaltic oceanic crust --Adakite
The idea of subducted slab melting was revived by
Drummond and Defant (1990). The adakite rocks
from the type area Adak island in the Aleutian arc
generally contain > 56 wt % SiO2 and have higher
mg-number and high La/Yb ratio than typical calcalkaline rocks. However, both experimental results
and geochemical data indicate that chemical
composition especially trace-element abundance of
adakite can better be explained by basaltic rocks
rather than peridotite (lherzolite) that is either garnet
amphibolite or eclogite in the descending slab. All
known adakitic magmas are found in relatively
young (< 20 Ma) and still hot subducting plate. In
contrast to this, the average age of the present
subducted lithosphere is about 60 Ma. This accounts
for the paucity of the modern adakitic magmatism.
However, such rocks resembling adakite in
composition are far more common in the Archaean
terranes that might suggest that the subduction of
young hot slab was frequent in early Earth’s history
(Martin, 1999).
(v) The fifth model favoured partial melting of
metasomatised refractory lithospheric slab --boninite
Boninite is an unusual MgO-rich orthopyroxene
bearing rock of andesitic composition which are
derived from an ultra-depleted harzburgite source
which occur below the oceanic crust or lithospheric
part of mantle wedge. However, these rocks also
exhibit the characteristic of island arc enrichment of
LIL elements like K, Ba, Rb, Sr suggesting that an
incompatible element enriched fluid was to
metasomatise the harzburgite prior to the melting
event either from the dehydrating subducting source
and/or from the convection triggered asthenospheric
source. The metasomatised and veined harzburgite
represent the lithospheric part of the mantle wedge
the partial melting of which produced the magma of
boninite composition.
In the light of the foregoing discussion regarding the
petrogenesis of arc magmatism and in view of the
fact that there is no rock of the composition of either
adakite or boninite in the area under investigation the
lone viable source to be considered for the genesis of
the studied amphibolites is the melting of mantle
wedge over the descending slab triggered by volatile
influx and convection (Schmidt & Poli, 1988). The
Jensen’s cation plot which is commonly used to
classify the metavolcanics for their magma types
(Fig.12) the majority of the amphibolitic rocks plot
within the field of Mg-rich tholeiite. In addition, the
major element chemistry has supported the
importance of crystal fractionation of parental
magma in a restricted compositional range. Since the
earliest formed rock represented by Talc-tremoliteactinolite schists contained MgO > 27 %; Ni, ~ 624
ppm; Cr, ~ 2897 ppm along with mg-number (84)
and FeO0/ MgO (0.50), indicate that the nature of the
magma was primary and picritic basalt in
composition generated from hydrous peridotite in the
subarc mantle wedge. However, relatively low MgO,
Ni, Cr, mg-number content of the more evolved types
suggest that the magma was not primary and are the
result of extensive differentiation after leaving their
source. Their evolved composition can be
appreciated in CaO–MgO diagram (Fig.13). The
MgO- rich varieties (MgO >12 %) exhibit an
inflection dominated by Olivine + Opx assemblage
followed by Cpx + Plagioclase at lower temperature
from residual melt derived from previously olivine
fractionated magma. This has given rise to the
dominant rock types ranging in composition from
tholeiitic basalt to basaltic andesite which are quartz
and hypersthene normative (SiO2 saturated). Arc
volcanism is generally characterised by either
tholeiitic or calc-alkaline magmatism. If they coexist
in close temporal and spatial context the latter follow
with the maturity of the arc evolution. It is pertinent
to note from the petrogentic point of view that rocks
which follow the magmatic trend of differentiation of
calc-alkaline series are almost absent from amongst
the study amphibolites. Thus the protolith of the
studied amphiboltes can be safely assigned to the
immature and initial stage of development of island
arc system with no sign of volcanic eruption and sill
flow mode of emplacement. The latter is also
supported chemically on the basis of the av.
oxidation ratio after Chinner (1960). In addition to
this the Sugimura index (1968) values (av. ~37)
indicate intraarc spatial position of the basin.
Fig. 12 Al-Fe+Ti-Mg ternary cation plot after
Jensen (1976) showing fields of
komatiitic, tholeiitic and calc- alkaline
suite of rocks
Typical spiky arc signature in normalized trace
element patterns (Fig.6) are thought to be contributed
by slab generated fluid. Another distinctive feature of
these patterns is the occurrence of a – ve Nb-Ta-Ti
anomaly, perhaps, because of either depletion in the
source or their retention in the refractory phase in the
mantle source to which these elements are highly
compatible during partial melting of the source. The
pattern also show greater abundance of LILEs than
HFSEs this is because LILEs have low ionic potential
and are readily dissolved and transported in aqueous
fluid at high P&T (Tatsumi & Eggins, 1995) as
compared to HFSEs.
Fig. 13 MgO-CaO diagram showing composition of
island arc volcanic rocks (light shaded) from west
Pacific and continental arc rocks (dark shaded) from
central Andes.
The fractionating phases indicated by diagonal
control lines. The more Mg-rich types experienced
olivine and Opx fractionation whereas less Mg types
representing fractionation of Cpx + plag. during
evolution of magma. Dark circles indicate
experimentally produced partial melt fraction (1838%) at 10Kb and 1200-13500C of hydrous lherzolite
(Hirose & Kawamoto, 1995). Most arc rocks solidify
from evolved magmas resulting from differentiation,
fractionation and contamination with felsic crustal
rocks. Magma mixing line redrawn after Davidson,
1996)
The mafic and ultramafic amphibolites of both the
areas show similar Ce/Nd ratio of immobile elements
(1.13 to 1.89) which are highly incompatible in mafic
system (Balakrishnan et al.,1990) suggest that they
have evolved from the same magma and are
uncontaminated. Some other REE ratios such as
(La/Sm)N, (La/Yb)N and (La/Ce)N (Tab.-2) are
particularly significant from petrogenetic point of
view because they reflect the ratios in the mantle
source rock of the magma provided partial melting is
fairly large (>10%) in which case REE fractionation
could not have taken place (Wilson, 1989). The
estimated melting in the present case varies from 1525 % (Fig.-13).
Fig. 14 Zr/Y vs Ce/Yb plot for the amphibolitic rocks
of the area
Fig.15 MgO-CaO-Al2O3 plot for the rocks of the area
in relation to the fields for tholeiitic and komatiitic
lavas of Munro township (Arndt et al., 1977)
VIKASH KUMAR
Apart from the complications owing to crustal
contamination the fundamental attributes of the
magma Zr/Y change very little with stratigraphic
level or the distance from the source. The plot of
Zr/Y vs. Ce/Yb (Fig.-14) exhibit the uncontaminated
nature of the studied amphibolites and also their
derivation from the metasomatised mantle source.
Similarly, high Th/Yb ratio has been used as an index
of contamination of basaltic magmas ascending
through the crustal layer. Since these ratios are small
(0.47-1.24) in these rocks the possibility of crustal
contamination is ruled out.
To resolve the problem of the nature of primary /
parental magma of the studied amphibolites whose
protoliths have been delineated as basalts of tholeiitic
affinity the question that arises whether these
tholeiites have been derived from komatiitic or
picritic primary (?) magma. Experimental results
(Jaques & Green, 1980) on both depleted and
enriched lherzolite source composition have
indicated that tholeiitic basalt magma can be
produced by moderate degree of partial melting 2030% of either source at pressure 15-20 Kbar i.e. at a
presumed depth of 50-60 Km. and at high pressure
picritic liquid are produced at the same degree of
partial melting whereas the liquid of the composition
of peridotitic Komatiite can be produced by 40-50%
partial melting of a fertile lherzolite. It has been
found that the rocks akin to the composition and
texture of komatiite in the area under study are not in
existence despite the fact that a few samples plot in
the field of peridotitic komatiite on MgO-CaO-Al2O3
diagram (Fig.15). Nevertheless lack of spinifex
texture and other chemical milieu like CaO/Al2O3
ratio (< 1) preclude the rock to be classified as
komatiite. Picrite, on the other hand, may produce
basaltic magma similar to those found as
amphibolites present in the area. There is the
possibility that the dense picrite magma are
underplated at the base of less dense crust where they
undergo fractional crystallization either in situ or
may ascent as differentiated magma to higher levels
either as intrusive bodies or lava flows.
& Nesbitt (1977) have, thus, concluded that
komatiites are derived by 60-80 % partial melting of
the mantle. As the Mg/Mg+Fe2+ ratio for the
amphibolites from the study area is quite low 0.540.72 except the ultramafic type in which case it is a
bit high (0.84) it only suggests a moderate degree of
partial melting of mantle source for the production of
magma of basaltic composition and the ultrabasic
rock may be taken as a good index of olivine
fractionation the fact which has otherwise been
inferred on other considerations as well. Moreover,
the occurrence of talc-tremolite schists forming the
basal portion of the differentiation masses followed
by pyroxenite and other mafic rocks well support a
differentiation theory for the occurrences of
ultramafic rocks associated with these mafic suites
representing the amphibolitic rocks of the area.
Conclusion:
The geochemical characteristics and the trend of
evolution of the ortho-amphibolites of the study area
constituting mafic – ultramafic complexes associated
with either the Chotanagpur granite gneiss terrane or
the supracrustal schistose formation of the adjacent
Koderma mica belt are quite similar i.e.
homogeneous and co-magmatic suite of rocks of
tholeiitic affinity. It has also been well demonstrated
that they bear the petrographic and chemical
signature of the subduction related oceanic island arc
set-up. Further, the dominance of tholeiite under this
set-up is indicative of the immature or initial stage of
evolution of island arc system.They have evolved
from primary olivine tholeiite (picritic) by moderate
degree of partial melting (15-25%) of hydrated and
enriched peridotite (asthenospheric) in the sub-arc
mantle triggered by volatile influx and convection.
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Petrogenesis of Amphibolites from Northern Chotanagpur Plateau

Transcription

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