GEOCHEMISTHY AND PETROGENESIS OF DECCAN BASALTS AROUND RAJAHMUNDRY, AP., INDIA
-v^ ^ ABSTRACT
CHAin HANUMANTHA RAO A glimpse of the literature would reveal that very meagre work has
been carried out on the petrochemical aspects of Rajahmundry basalts.
Though this area has attracted many workers because of its
palaeontological potential, it is otherwise important as these volcanic
eruptions are manifestation of the dynamic processes through which the
Indian sub-continent has passed.
The present work has been taken up to investigate petrochemical
and genetic aspects of these isolated outliers of the basaltic rocks
which are considered coeval to the Deccan flood basalts. Though the RJi
basalts are coeval to the main Deccan basalts, the former are essentially
associated with Godavari graben. In other words they are integral part cf
the lithostratigraphic succession of Godavari graben.
The work has been broadly divided into six chapters and the
present work has been sumr.arised as follows.
The basaltic traps along with Inter-trappean and Infra-trappeans
are exposed on both the banks of the river Godavari as it cuts through
the rocks. The Inter-trappeans are exposed in Kateru and Pangidi areas cf
the East and West Godavari Districts respectively, while the Infra-
trappeans are recorded in the West Godavari District only. Structurall>
the area is undisturbed and the Infra-trappean beds dip with a lou angle
of w to n degrees. The Infra-trappeans and Inter-trappeans consist of sandstones and limestones respectively and are fossiliferous in nature.
The macro, and micro-fossil studies suggest that these beds were
deposited under shallow marine to esturine conditions and are of the
Paleocene age.
The upper and lower flows are 25 m and 3 m thia respectively. The
lower trap is in unconformable relationship with Infra-trappeans and at
places it is rests directly on the formations further lower in the
succession.
The simple, blocky flow characteristics without any pyroclastic
material indicate that they are not central eruptions but are fissure
type. The presence of faults inferred from sattelite imagery passing
through the area probably suggest the closeness of the site of eruption.
The optical studies carried out on large number of samples are in
close confirmity with the microprobe data carried out on representative
samples. The presence of the two (Ca and Fe rich) pyroxenes, plagioclase
and iron oxides with Subordinate amount of interstitial altered glass and
conspicuous absence of olivine both as T.merc^l and normatixe phase is an
important feature indicating the evol\ed character of the RJ^. basalts.
The relative!) less co''--ior gloreropor:'\ritic texture, especially
in the lover floi., indicate t^e presence of intra-telluric crystals of
pyroxene and plagioclase in tne melt. Co-iplex physico-chenical conditions
such as fluctuations of Pri20 a?d tenperature, are inferred fror the zoned
crystals of plagioclase. The textural relationship of opaque nmerals and
presence of skeletal iron oxide indicate their late stage origin. The
various textural patterns seen such as ophitic, sub-ophitic,
intergranular, interstitial and occasional flow textures indicate varying cooling conditions of the basaltic flows. The ophitic to sub-ophitic texture reflects the near simultaneous crystallization of the mineral phases. The groundmass of Ca-poor pyroxenes in the interstices between the microphenocrysts reveal the rapid cooling conditions after eruption.
The geothermometric estimations (see chapter-5, Table-5.2)
suggest progressive decrease of temperatures resulting in the appearence of augite (1259**C), pigeonite (.12AEPC), and plagioclase f1169°C). The textural patterns observed are also in confirmation with the geo-thermometric estimations. Also, it has been inferred that though the lower and upper flows have similar temperatures of crystallisation of minerals, the upper flow shows slightly higher degrees of crystallisation. The plagioclase temperatures estimated from the
Glazner's (J. 984) method shows that temperature of crystallisation
increases with the increasing An % and are consistent with the basaltic
trends.
The vesicle filling mnerals such as silica, zeolites and altered glasses (palagonites and chlorophaeites) indicate influence of the low
temperature deuteric alteration. At places, the palagonitisatio-^ of clinopyroxene has also bee- a served ir these rocks. Geochemcali} , the effect of alteration i.e. ^ostl\ weathering and post consolidation
leaching on \aricj5 elements has beer investigated and the res_lts
indicate that Si02, MgO, FeO^t), A12i3, Ti02, \aZ) , \i, Cr, Zn and Y values shows reasona) le confidence while, KB , Rb, Sr, Ba and CaO \alwes
skow some alteration effects. Tne selective enrichment of Yb m RJY
basalts is attributed to break down of clinopyroxene. However, the ci served negative anomalies of Ce were attributed to seawater-roa
interaction caused by marine transgressions post dating the eruption of
the flows. "lie major and trace element variations witltirfi-'"tfie'flow (vert ically and horizontally) shows monotonous trends vdthout much differentiation. The studies show that the upper and lower flows *ow similar mineralogical and chemical features. The study of REE of these basalts indicate that they are enriched perticularly in LREE. All the samples of RJY basalts (both lower and upper flows) *ow similar enrichment of REE but only differ slightly in their * solute enrichment.
Especially, the lower flow skows higher REE contents.
On the basis of low Mg numbers, Ni and Cr contents of the RJY
basalts, it has been inferred that they are not primary melt. The present compositions presumably represent the products of magmatic evolution during its upword movement from the source, i.e. a larger degrees of partial melting at the source region and extensive differentiation-
fractionation probably in the sub-crustal magmatic chambers. It is
inferred from the LREE enrichment patterns that garnet is the principle
residual phase in the source region for the parental roa s of RJY
basalts. Ihe estimated partial melting ranges from 30!±5^= indicate higher degrees of melting. The studies suggest extensive fractional crystallization of gabbroic material in the evolution of the RJY basalts
sicilar to Deccan basalts. However, the RJY tholeiitic basalts with its characteristic iron enrichment suggest evolved character than the Deccari
basalts. Fractionation of both olivene and pyroxene phases is suggested from the major and trace element patterns. The other geochemical features such as increasing A12) 3 percent and buffering of Sr values with increasing fractional crystallization suggest the plagioclase
fractionation. Though, Sr is considered to be partly affected by
secondary mobilization, its strong negative anomaly in the spidergram would reflect the plagioclase (gabbro) fractionation.
The incompatiable trace elements enrichment in these basalts is may be due to single or combination of processes - fractionation of olivine, clinopyroxene ancf or due to partial melting of metasomatically enriched source region. The preliminary studies indicate that in the evolution of these basalts crustal component has no major role to play.
However, the crustal contamination can't be ruled out completely.
The flow characteristics as well as the geochemical characters indicate their continental condition of eruption. But oceanic character has also been suggested by some variation diagrams. Accordingly they are grouped as basalts belonging to arrbiguous tectonic setting erupted at the interface of continent and oceanic environments. Thus RJY basalts have been described as basalts belonging to initial rift tectonic setting
(Chatti and Sharma, 19 9 0).
From the above geochemical and petrogenetic studies together v»ith the limited earlier data, a synthesis of the tectono-magmatic processes of eruption of these RJ'i basalts has been attempted.
Except for the -inor ab^erent roa types within the flood basalt proMnces all o\er the '»orld, there are overwhelming evidences that
"•.ajorit\ of the basalts are tholeiitic and have erupted within e ^ort spar, of geological tine. In RJY region, the Inter-trappean sedirentar\ beds between la\a flG•v^5 are thin and sbow characteristicall) faster depositional features (Bhalla, 19 6~). The similarity of mineralogical and chemical features of the upper and lower flows suggest periodic tapping of the same magmatic chamber after a shorter gap of time. These features substantiate the idea of shorter span of magmatic event in RJY region. Regarding their origin, two alternatives seem possb le. One in which a more primitive parental magma than the RJY basalts (c.3-8X MgO range) of picritic composition as a result of partial melting of a
Fe-rich pyrolite or garnet peridotite differentiates into basalts through fractionation of olivine and clinopyroxene or both. The amount of fractionation needed to produce basalts depends upon the MgO contents of the initial picritic liquid. The higher the MgO contents, the larger are the amounts of olivine required to be removed. Approximately, 8-15X olivine loss Is sufficient to produce normal basalts (c.^SJL MgO range) from picritic compositions with 10-15% of Mg) . On the other hand, high degree or total melting of an appropriate source rock such as eclogite or amphibolite can also produce basaltic liquid in the range of 3 -8% MgO.
Since large amounts of liquids are produced, rapid flooding or eruption is possii le (Cox, 13 80). The only objection to such a scheme being the availa) ility of high degree of heat for total melting. But then, flood basalt provinces are an enigma when one considers the enormous volune of hot liquids ^,1100 ^), made availa) lie for eruption in relatively shorter span c: tir.e. Pressure relief, as a consequence of tensional forces developed during the foundering cf the Gondwanaland might have induced or assisted in large scale melti-g either total or at high degrees.
Considering the above evidences a tectono-magmatic model proposed by
Cox (I ) SJJ woi^ld explain better the RJY volcanics also. According to hirr. - the uprising magma from riantle that equili) riate in crustal magmatic chacbers in the form of sills before they eventually erupt. The high density of picritic magma (which is supposed to be parental to RJY basalts) inhibits its movement and eruption. The sill form has an advantage because it is capa le of accomodating large amounts of material without posing a space problem. An another advantage is that it provides an idea^ environment for the fractionation of large volumes of magria within comparatively restricted pressure range.
Comparatively, one would find broad similarities of mineralogical and chemical features for the time equivalent RJY basalts and Deccan basalts.
A distinct feature of RJY and Deccan tholeiitic basalts when compared to average continental tholeiites is that the former are significantly enriched in FeO(t) and Ti02, The higher content of these elements of Deccan basalts suggest higher degree of fractionation prior to extrusion. It is interesting to note that the RJY basalts and the basalts drilled from NE Indian ocean show general enrichment of iron compared to Deccan basalts from northern regions. This aspect needs special attention to study as it may indicate a general enrich-ent c£ iron in this part of the mantle or progression of Indian plate over hot spot and the subsequent enrichment of iron in the later melts.
There is a fair amount of gradation between the alKali and tholeiitic basalts for the Deccan provinance as a whole although sa-rles from individual areas have discrete groupings or show transitional features. RJY basalts show monotonous nature and does not sho- an', such
transition as seen in the Deccan basalts.
The common tholeiites of the Deccan traps are geneticall> related to primary magmas with 9 to'. 0 % MgO, which are generated in a heterogenous but dominently Indian ocean MORB type presumably at cantle depths of 35 to
45 km. Approximately 1.2 x lOr km volume of primary magma has been generated in the upper mantle of this region out of which a large fraction
of the magma crystallized as gabbros and troctolites over a wide range of 8 depths in crustal magma chambers (Sen, 19 88).
Thus the present studies reveal that geochemically and geochronologically the RJY basalts are comparable to the Deccan basalts.
However, a question remains to be answered, i.e. whether the RJY (East coast) basalts and Deccan basalts came from a single source or not'' Before finding answer to such a question it would be relevant either to relate or to unrelate the RJY volcanism with Deccan basalts proper. The fact that these RJY basalts are intrinsically associated with litho-stratigraphic sequence of the Gondwana basin lends support to deassociate it from mam
Deccan basalts proper, which are presumably erupted due to Reunion hot spot activity. Morever, the wide gap (spatial) between these two basaltic bodies raise doubt regarding the single source concept. The recent DSS profile studies reveal (V.K.Kaila, Personal discussion, I) 88) a seperate source for the RJY basalts. A mega sill underneath the RJY region has been suggested with the help of geophysical studies, carried out by ONGC during its exploratory work on On-Shore and Off-Shore of Godavari basin (Biswas,
Personal discussion, 19 58). It suggests altogether an indepen:!ent -ag-atic activity coeval to Deccan volacanisr..
TECTONICS
The relation between the type of volcanisr. and tectonic r:\e~.enLS give a positive proof of the fact that the volcanic activity in the
Rajahmundry area was invariably manifested under tensile stress conditions.
Compressive folds are singularly absent in this region (Kumar,i9 82). It is interesting to note in this context that not a single region where volcanic activity was associated with firmly established compressive structures at 9 this Mesozoic-Tertla:> boundary is cnown in the Peninsular India.
In the speculative schematic evolution of the Gondwanic belts of
India and Southwest Australia (Kat2,l9 76), four episodes are postulated - leaving the first and second Precambrian episodes, the third episode of
Phanerozoic activity along transforms of Mid Indian Ridge which include the formation of the Gondwana rift, i.e. the Godavari graben. It is important to note that the Godavari graben has been extended further eastward into the Bay of Bengal and 3 oined with the negative lineament (see Chapter-2) infered from the Satillite altimetry data map of Gahagen et al.(i9 88).
Moreover, the other faults and linements recognised in the south India can be shown to be the continental extension of m^ or contemporary transforms of the Indian Ridge (Katz, 1989). 1\e fourth and latest episode was an intense reactivation during the early Cretaceous that led to the break up and fragmentation of Gondwanaland. Strong dextral transform movements lead to the development of the new dormant 90 E ridge, which played a major role in the evolution of the Indian ocean.
The eruption of RJY basalts during the Paleocene, may be due to the reactivation of the failed rift, where tensional forces might have stretched the graben basement and the evenfjal drop of lithospheric pressure must have lead to melting of mantle and generation of melts. The other remote possibility where the cor.terporary 9Li® E hot spot plume suppling molten n;aterial to the Godavari rift tensional zone^ can' t be ruled out.
The out pouring of lavas must have taker, place during Paleocene and possibly could have formed barriers from the sea, simultaneously devoloping a coastal lagoon in which the Inter-trappean beds were deposited. The complete absence of terrigenious material in the Inter-trappean sediments 10 indicate that there was no inland drainage from the land areas and the lagoon might have been connected to the ocean only and the sedimentation of
Inter-trappeans beds could have been under calm and quite conditions with varying salinity.
iMarin'- transgressions are evident from the fauna of Inter-trappean and Infra-trappeans marine sedimentary beds associated with the basaltic flows in this region. The effect of sea water interaction with basaltic rocks during these marine transgression(s) was reflected in the mobility of some elements as discussed earlier.
FUTURE STUDIES
The limited extent of the RJY basalts erupted at the continent-ocean interface, perhaps only indicate the tip of an iceberg, i.e. the major portion of the melt generated due to the dynamic processes must have consolidated underneath the continent as well as in the ocear.ic region adjacent to it. Thus it is quite essential to consider sarples fror. the drill data in order to develop any corr.prebe".si ve model c£ this area in perticular and Godavari graben in general. The ongoing explcrati:r. of oil and gas by OXGC (Oil and Natural Gas CorL-.isior.) in the Godavari tasir. has yielde a wealth cf drill data which can possiblv widen the sccpe d such studies related to volcar.is- and associated marine sedirrier.tation. The available published information by O.N'GC indicaie that RJY basalts do extend into the Bay of Bengal continuing under the coastal Godavari Deltaic sedinients. Perhaps, these traps may show transition as oceanic crust in to
Bay of Bengal ? The possible role of these basaltic rocks as "oil and gas traps" adds to the significance of such studies. It is important to study in detail regarding the extent, timing and processes of these volcanics 11 which have documented a record of the extensive drift of the Indian plate.
The following studies can be taken up if the new drill data is made available:
Detailed studies to establish the actual thickness <£ the trap rocis throughout the subsurface; Studies c^ the basalts sampled from successive flows in the sub-surface would enable to develop better petrogenetic models; Geochronological studies to constrain the possible limits on the age of the volcanic rocks; Better comparision of these volcanics with the time equivalent Deccan basalts to develop tectono-magmatic models in order understand and to relate them it with the global tectonics processes. GEOCHEMISTRY AND PETROGENESIS OF DECCAN BASALTS AROUND RAJAHMUNDRY, A.P., INDIA
THESIS FOR THE DEGREE OF DOCTOR OF PHILOSOPHY IN GEOLOGY
CHATTI HANUMANTHA RAO
SUBMITTED TO ALIGARH MUSLIM UNIVERSITY ALIGARH (INDIA) 1990 T^555'
T4555 CERTIFICATE
This is to certify that MR. CHATTI HANUMANTHA RAO has carried out this investigation on the 'Oeochemistiy and Petrogenesis of Deccan traps around Rajahmundty, AP., India:, under our supervision for the award of the Doctor of Philosophy degree bom the Aligarh Muslim Universify, Aligarh. This work is an original contributico to the existing knowledge on the subject
He is allowed to submit the work for the PLD degree to the Aligarh Muslim Universify, Aligarh.
nillho PROF. K.K. SHARMA(SaENTISTEII) MR NOMAN GHANI (READER)
Igneous Petrology and Geochemistry Group Department cf Geology
Wadia Institute of Himalayan Geology Aligarh Muslim University
Dehradun24«001 (INDIA) Aligarh 202001 (INDIA)
DR K K SHARMA 'sciential W*dia ti^sT^.nc of Hiiiwl.iyaa Oeotogy ACKNOWLEDGEMENTS
It is iiij' jiledsure to express profound sense of yratitude to Prof. Kewal
K. Shdrmd for his continued yuidance and support throuyhout my work. I owe a yreat deal to Mr. Noman yhani, supervisor, for his constant source of inspiration and encourayeuient duriny the work. I am yrateful to the
Director, Or. V.C. Thakur, W.I.H.G. for the facilities provided to carry out the work. I am also yrateful to Prof. S.H. Israili, Chairman, Department of
Geoloyy, A.M.U., Aliyarh.
I relish to place in record my sincere yratitude to Dr. Jacob Kurien,
President, Wadia Institute of Himalayan Geoloyy, for helpiny me attendiny the workshop and field work on " Deccan flood basalts". This has benefited me in interactiny with workers of Deccan basalt community. I owe my sincere thanks to Prof. K.V. Subba Rao. It is obliyatory on my part to thank Profs.
Arnrudhd De, S.F. Sethna, K.G. Cox, P.R. Hooper, Dr. J.D. Macdouyall, Dr. b.K. Biswas and Dr. K.L. Kaila for their constructive, suyyestions.
I dill thankful to Dr. S.M. Nayvi, Dr. Govil, Dr. Balaram for allowiny me * to carry out chemical analyses in the laboratories of N.G.R.I., Hyderabad.
I am thankful to Dr. P.P. Khanna for helpiny me to carry out some analyses on ICP, at W.I.H.G., and to Dr. P.K. Mukherjee for his suyyestions duriiiy the preparation of thesis. Dr. Trilochan sinyh, Dr. Ismial Bhat, Dr.
D.R. Rao, Dr. V.M. Choubey, Dr. Hakim Rai, Dr. Talat Ahmad, Dr. Rajesh
Sharma, Dr. R. Islam, Dr. Dinesh Sati, Mr. Z.H. Fahmi, are yratefully acknowledyed for their constant encourayement. I am indebted to Mr. G.S.
Khdtri, for his help in prepdriny the diayrams. A token of deep apprecidtion to Dr. S.N. Prasad, Wild life Institute, for his help durin«j the work.
This work has been carried out with the help of fellowship duriny the tenure of C.S.I.R. project No. CSIR-2't( 167^/86- EMR-II, and is thankfully dcknuwledv^^ed.
Above all I am thankful to my family members and my Wife Mrs. Anita for the constant help and encourayement without which it would not have been possible to persue these studies.
/^•/l-90
(CHATTI HAf^UWKUTHA RAo) CONTENTS
CHAPTER 1 INTRODUCTION
1.1 Location of the area • • • 2 1.2 Previous literature • • • 3
1.3 Objectives of present study • • • 6
l.H Methodology adopted • • • 8
CHAPTER Z REGIONAL GEOLOGY AND TECTONO-STRATIGRAPHY
2.1 Reyiona! yeoloyy and
tectono-stratiyraphy • • • 10 2.Z Stratiyraphy and yeoloyical
evolution of the yraben. • • • 16 2.3 Infra-trappeans, traps
and Inter-trappeans • • • 22 2.3.a Infra-trappeans • • • 22
2.3.b Lower trap • • • 23 2.3.C Inter-trappeans • a • 2H
2.3.d Upper trap • • B 2H
CHAPTER 3 PETROMINERALOGY
3.1 Flow litholoyy 25 1.2 Textures 25 3.3 Mineraloyy 29 3.3.1 Playioclase feldspars 36 3.3.2 Pyroxenes H9 3.3.3 Opaques 52 3.3. H Secondary minerals 56
CHAPTER H GEOCHEMISTRY
H.l General chemistry of the flows 6H H.2 Differentiation Index 77 H.3 Solidifcation Index 77 t.H Correlation Coefficients 80 H.5 Mayiiia classification 85 t.6 lrd.ce e7ements 97 H.6.1 Rare earth elements 95 H.l Alterations 100 H.7.1 REE mobility 106 H.7.2 Ce anomaly 107 H.8 Tectonic environment 110 H.9 Comparision 118 CHAPTER 5 PETROGENISIS
5.1 Primary nature 123 5.2 Partial meltiny 12H 5.2.1 Sr/Ca-Ba/Ca Systamatics 12H 5.2.2 Estimation of partial meltiny 127 5.3 Fractional crystallization 128 5.H Crustal contamination IHO 5.5 Geotheniiottietry 1H7 5.5.1 French and cameron method (1981^ 1H7 5.5.2 Neil sen and Ounyan method (1983^ 7t9 5.5.3 Nathan and Vankirk method (198Hy 151 5.5.H Glazner's method OSS-^j 153
CHAPTER 6 SUMMARY, DISCUSSION & FUTURE STUDIES ... 15H
REFERENCES ... 165 CHAPTER 1
INTRODUCTION
The Deccan traps of which major portion is lost by erosion or sunk beneath the Arabian sea, still covers over 50,0000 square kilometres of
Indian land-mass and form one of the worlds largest flood basalt eruptions on the continent, in a remarkably shorter span of time. The accelerated oil exploratory drilling programme along the marginal basins and west coast of
India has also established a limited temportal span for the Deccan basalts encountered in sub-crop (Sahni, 1988). Besides the central and western
Deccan basalts, a number of outlying patches of rocks more or less similar in character and age are found in Belgam (Southern Karnataka), Jabalpur,
Sirguja, Amerakantak, Kutch and Rajahmundry (Andhra Pradesh, eastern coast). In the extreme southern part of Peninsular India, a vitric tuff of volcanic origin has recently been reported between Ariyalur and Ninnyar groups, indicating the contemporanity of the eruptive phase in wide spread regions (Sahni, 1988).
The origin of such volcanic phenomena has always been problematic and the plate tectonic paradigm has not offered a solution (Duncan and
Pyle, 1988). Alverezet (198C)and Officer and Drake (1983) have proposed meteoritic impact theory for the volcanic activity which coincided with mass extinction of fauna at the Cretaceous-Tertiary boundary. These basaltic eruptions were also linked to the on-set of hotspot related volcanism in the western Indian Ocean (Morgan, 1972). Recently, Alt et al.
(1989) have further supported the meteoritic impact theory. According to them the close association of Deccan Plateau basalt with the timing of terminal Cretaceous boundary clay suggest that it is an impact crater large enough to form a lava lake, terrestrial equivalent of lunar marie, which caused pressure relief melting in the asthenosphere and intiated
Chagos-Laccadive hotspot track.
From the above arguments regarding the origin of these volcanics, one thing would be clear that at the late Cretaceous period, the mantle under the Indian subcontinent and the adjoining areas was exceptionally active and ready to undergo such melting processes as to create voluminous flood basalts.
The present work is dealing with one of the outlying patches of such basaltic eruptions located near eastern margin of the Indian subcontinent away from the central and western provinces of Deccan basalts. In order to
distinguish the main Deccan basalts, from Rajahmundry basalts, the author would like to refer the former as Deccan basalts and the later as
Rajahmundry (RJY) basalts in the thesis.
1.1. LOCATION OF THE AREA
The study area, lies between latitude 1/00' to 17 05' N and longi
tude 81° 30' E to 81° 51' E. It includes the area from the east of
Rajahmundry to the west of Deverapalli in the Godavari District, Andhra
Pradesh. Exposures of Infra-trappeans, traps, Inter-trappeans and
Supra-trappeans (such as Rajahmundry sandstones) are exposed in this area.
The traps along with Inter-trappeans gently rise from the alluvial plains
on either side of the Godavari river and attain a height of about 250 feet above M.S.L. The area is bounded by low-lying alluvial plains in the south and
Khondalite hill ranges to the north. It is also characterized by gradually sloping and flat-topped hillocks of the Gondwana rock formations and steep-sloped hillocks of the RJY traps. The active quarring of the basaltic rock, for construction and road material, in and around
Rajahmundry has resulted in exposing the fresh rocks for present study (PI.
1 & 2).
The major river in the area is the Godavari. It flows across the
Gondwana formations, RJY basalts and Tertiary sedimentary rocks and finally drains into the Bay of Bengal in the east. The Central part of the area is characterized by red and yellow loamy soils, while in the southern and the south-eastern parts, it is covered with black cotton soil.
1.2. PREVIOUS LITERATURE
Exploration of Godavari graben began with the classical works of
Blan Ford (1856), Huges (1877), Fiestraentel (1882), Fox (1931) and Gee
(1932). The earliest reference to this area of Rajahmundry is by Benza and
Cullin (1837). They published a report on these traps and associated
Inter-trappean beds of Rajahmundry area. Subsequently, Elliot and Hislop
(1865-1866) studied the collection of fossil flora of this area and considered Pangidi Inter-trappean beds to be of esturine origin. Medicott and Blanford (1879) suggested that the Pangidi Infra-trappeans beds (West
Godavari District) showed Tertiary affinities and are marine. King (1880) described this as an outlier with important clues for the correlation of volcanic rocks with known series of fossiliferous rocks. Later, Das Gupta
(1933) visited this area and collected samples from the Infra-trappean sandstone south of Duddukuru. He described fossil Cardita beauimontid' a- Quarry section near Kateru (East Godavari Dt.,) Note: The flow is covered by Rajahmundry sand stones (Supra-trappeans) •
b. A quarry overlooking Mary temple of Gouripatnain near Pangidi (West Godavari Dt.,) Archiac on the basis of which he assigned upper Cretaceous age to the
Infra-trappean sandstones.
Sahni (1934) on the basis of fossil flora suggested that
Inter-trappean beds are of Tertiary age. According to Rao and Rao (1935,
1937, 1939) the calcareous algae of the genera Chara and Aciculeria are of early Tertiary age. From the study of foraminifera in these beds, they also arrived at similar conclusions. Crookshank (1937) also considered these traps to be of Eocene age.
Rama Rao (1956) studied the fauna of Infra-trappean beds and considered them to be of Danian age, while the outliers of basalts and
Intra-trappeans to be of early Eocene age.
Sahasrabudhe (1963) regarded Rajahmundry outlier belonging to the basalts of Deccan Traps. He opines that they show normal magnetization with steeper inclination. Shastry et al, (1961-63) reported Forminefera from the
Raghavapuram shales exposed near Tirupati village in the West Godavari
District, Mathur and Evans (1964) are of the view that the Inter-trappeans are of late Cretaceous to Paleocene age.
Bhalla (1967) studied the Foraminifera and Ostrocoda from the
Inter-trappean beds and concluded that they were deposited under fluctuating marine conditions. Mahabali and Rao (1973) and Satyanarayana
(1975) described the species Dadoxylon merembriorylon and Gingo wood from the Inter-trappean beds near Rajahmundry. Mahabale (1977, 1978) also reported petrified Ginko wood from the lower series of the Rajahmundry sandstones ( Supra-trappeans) at Pangidi and suggested Miocene age. Guha and Raju (1978) recorded various microfossils from the Inter-trappean beds of Duddukuru and suggested a late Paleocene age and a shallow marine conditions of deposition for these beds. Shastry (1981) reports that the marine Inter-trappean sediments from the deep wells drilled by ONGC along east coast of India indicate an upper Cretaceous age, while the
Inter-trappeans found in the out-crops near Rajahmundry indicate Paleocene to lower Eocene age. He thus, believes that Rajahmundry Inter-trappeans show significantly marine affinity, while the Inter-trappeans elsewhere in west and central India are continental. Sastry and Lahiri (1981) were also of the similar view. They suggest esturine environment for Rajahmundry with connections to open sea while other basins, elsewhere, are in the form of small pools, lakes and marshes indicated by the fossil remains comprising freshwate. forms. Prasad (1986) recorded fish remains from the
Inter-trappean beds of Duddukuru villege (West Godavari District).
The first systematic description of the basalts of the Rajahmundry area was given by Washington (1922). Subsequently, Venkayya (1949) observed a close similarity in major element composition between the flows of this region and those of Deccan traps, supporting the veiw of Krishnan
(1941) and others that the traps of Rajahmundry are comagmatic to the
Deccan traps.
Recently, Bakshi and Krishnabrahraam (1987) with their gravity survey
data proposed two sets of fractures intersecting at depth near Rajahmundry
area.
1.3. OBJECTIVES OF PRESENT STUDY
Most of the earlier workers, have studies this area with emphasis mainly on paleontological aspects and no serious attempt has been made to evaluate the nature of the volcanic rocks or their petrogenetic aspects.
The present work therefore has been taken up to study the following aspects of Rajhamundry basalts.
1. Flow characteristics, their nature, types, extent and their associa
tion with other rocks of the area.
2. The textural and mineralogical characteristics.
3. Chemical characteristics of RJY basalts. With the help of of major,
minor and trace elements (including REE) chemistry, the following
aspects have been studied.
a. The extent of secondary alternation and weathering influence on
various elements .
b. Elemental relationships (major vs major, major vs trace, trace vs
trace) have been worked out to find out petrogentic aspects, with
the help of correlation coefficients.
c. The tectonomagmatic nature of the melt with the help of finger-print
elements.
d. Nature of the parental magma.
e. Extent of partial melting and fractional crystallization
processes to understand the petrogenesis.
f. The temparature of crystallization of various mineral phases, in 8
order to work out the sequence of crystallization in the RJY basaltic melt with the help of thennouietry.
:). And tinally, to reconstruct a tectono-inagmatic model for these
Dasait and to compare them with Ueccan basalts (both central
and western).
1.4. MhThUDOLUGY ADOPTED
Apart from the systematic field work and sample collection from airierent tlows and outcrops, detailed minerological studies were carried out ujider optical microscope using universsal sta^e techniques, reiractiveindex determination and finally microprobe analysis on selected samples to suDstantiate the optical studies.
in order to understand geochemistry of these rocks, major and trace element studies were carried out.The major and trace elements for the twelve samples of Kateru area were carried out at National Geophysical
Ketjearch institute, Hyderabad, by using the Varian Techtron Model AA-lUU of
Atomic Absorption Spectrophotometer (Shapparo and tternock, 19tt.Z). Inter national U.b.b.b standards were used for calibration. FeU was determined by titration (using, platinum crucibles). Double runs were made to check the reproducebility. The samples were also analysed by X-Kay tluorescence
Spectrometry methods (bovil, lyai) to further check the reproducebility.
Selected microprobe analyses has been carried out on pla^ioclase and ^^yroxenea by standerd wavelength dis^»ersive methods using an accelarated voltage ot 13 kV, and a Deem current of lU nA. A mixture of minerals, synthetic oxides and pure metals were em^jloyed as standards. CHAPTER:2
REGIONAL GEOLOGY AND TECTONO-STRATIGRAPHY
Pranahita-Godavari Graben, represents one of the major Gondwana basins in the Peninsular India, aligned along a well defined linear belt on the Archean platform. It has a regional trend of NW-SE and has been described as an ideal graben of which both the margins are well defined by faults (Raja Rao, 1978). Qureshy et al. (1968) suggested it to be a zone of weakness which had a tendency to subside, at least since the lower
Proterozoic times. Godavari valley earthquake of 13 April, 1969 which had epicentre near Badrachelara suggests that the graben is tectonically still active.
2.1. REGIONAL GEOLOGY AND TECTONO-STRATIGRAPHY
The present knowledge about the geology and the tectono-stratigraphy of the Pranahita-Godavari graben, and the adjoining areas is the result of sustained work of many workers, since the first preliminary study b\
Blanford (1856).
The Pranahita-Godavari graben measuring an area of 25,000 sq km is nearly 600 km long and 40 km wide zone extending from Chicholi (21° 30' N,
78°42' E) in the Northwest to Zangareddigudem (17°08' N, 80°18' E) in the
Southeast. To the further NW, the extention of the graben below the Deccan 11
traps is unknown, but to the SE it is truncated along the East coast by
ENE-NSW trending faults (Fig. 2.1). It is likely to continue below the costal sediments of Godavari delta and further extend in the Bay of Bengal.
A recent (SEASAT-GEOSAT) satillite altiraetry data map (Gahagen et al,,
1988) shows a negative lineament in the Bay of Bengal, which matches well with Godavari Graben (Fig. 2.2.). The graben being reported from the
Northern Antartica (Fig.2.3) also seems to fall in line with Pranahita-
Godavari graben or with Son-Mahanadi graben, considering the pre-split reconstruction of India and Antarctica (Greater Gondwana Land). If this correlation is correct then Pranahita-Godavari graben seem to be a major tectonic feature of Pre-split Gondwana land.
Raju (1986) suggested that this graben has more than 5,500 m thick sequence of sediments preserving more or less continuous record ranging in age from lower Proterozoic to upper Cretaceous, except for a marked break in sedimentation during the major part of the Paleozoic time. Four distinct cycles of sedimentation represented by Pakhal, Sullavai, Lower Gondwana and
Upper Gondwana. Igneous intrusions are conspicuously absent in the Gondwana sediments of this graben.
The Pranahita-Godavari graben is an Intra-cratonic basin developed over the Archean basement. The gravity surveys of Qureshy et al. (1968) and gravity, magnetic and refraction data of Oil and Natural Gas Comrassioa suggest it as a rift structure. Further, the data brings out clearly the basement configuration of the graben (Fig.2.4).
The boundary faults of graben are parallel to sub-parallel, to the basement fault directions. They have imposed a marked asymetry in the basin profile floor. The branching out of normal faults and their interference 12
LEGEND
\i I O I OUTLINE OF OONCWANA BASINS NAG PUR C7^ UO*ER GONWIftNA FORMATIONS
S(77A UPPE" GONDWANA FORMATIONS
|<^| FAULT I «. I MEAN DIRECTION OF SEDIMENT I ^^1 TRANSPORT rsm DIRECTION OF SEDIMENT I ^ I DISPERSAL AFTER REVERSAL OF PALAEO SLOPE
(AFTER RAJA RAO, 1978)
BAY RAJAHMUNDRY OF BENGAL
Fig. 2.1 Diagram showing the outlines of pranahita-
Godavari graben after Raja RaO/ 1978 Fig. 2.2 A part of tectonir fabric of ocean basin map determined by satillite (Seasat-Geosat) altimetry data (after Gahagen et al., 1988), Note the matching of Godavari graben with the negative linement in Bay of Bengal. P: positive linement, N: negative linement 14
Fij,. 2.3 Position of InC-„p sub-contin<, tjie time of split fron Antarctica and ^ijsralia. Tnc j,' \mer\Ic> shelf seens to be m line with either Godav^-r Son-Mahanadi graben. !:>
S"
• JT Sonefe"*!
~\ UPH' T'»t _' J X * -I
|flSv:x:>;;> tftttr TrapptO« r f -M^aa ••»• -#AIHA »U • BEFCBE Eft•JPTlO^ 0^ RJY BASALTS tM' f* J»rc»»»« ••««••' 0•••«••« Sali*<*. LJM».. _,^„^... D »»••• s»»««««« Fig. 2-4 Cartoon of the graben showing the position of the RJY basalts before and after eruption. In the abo\e diJi;ra- upper Gondwanas are enlarged. 16 and intersections inthe Godavari graben has further complicated its structure. Intra-basinal longitudinal faults have often formed intra-basinal troughs with thicker pile of younger sediments. These intra-basinal faults have also resulted in tilting and syn-depositional differential subsidence of different blocks, causing structural complexities (Raju, 1986). 2.2 STRATIGRAPHY AND GEOLOGICAL EVOLUTION OF THE GRABEN The entire sediments of the graben, overlying the Archean basement can be divided into two groups viz., (i) Purana Group (Proterozoic) and (ii) Gondwana Super-group (upper Carboniferous to upper Cretaceous). The generalised stratigraphy of the graben, modified after Raiverman (1985) is given in Table 2.1 and the geological map is given in fig. 2.5. The Archean basement complex comprises both Dharwarian and Eastern Ghat suites of rocks. The various rocks types that constitute the graben have been given in Table 2.1. They are mostly in faulted contact with the younger sediments along the graben margins. The Purana group rocks confined to this graben are represented by (a) Pakhal Formation and (b) Sullavai Formation. Both the lower and upper contacts of this group and also the contacts of the two formations within the group aie unconformable. The Gondwana Super-group has been subdivided (Raiverman, 1985) into four groups, viz., (i) Singareni, (ii) Kamthi, (iii) Sironcha and (iv) Peddavagu. However, Raju (1986) contends that they can only be assigned sub-group status, placing the lower and upper Gondwanas in group status. 17 Tkble 2.1: GENEKAUZH) SIMnCRAFHY CF PRANAHITA -OMVARI GRABEK i SUPER OOP SUB FORiAnCN BRIEF EESCRIPnCN THICXNESS AGE ENVXRCrWENT OOF onp (in metres)1 Recent tx> Alluvial gravel,sand clay - Holocene Continental Subrecent to IMTTMnnDtufrrV RJY Traps wit±i Basalt flows with Inter-trappais 150 Fhleocaie Inter-trappeans clays and linestcxies. to Upper - Cretaceous TWnrMCrDMTTV Laraetas Clays,limestones and Sand stones 100 Upper Marine with chert bands Q-etaceous MTir TN TTTRTTT rrVrAfT U P Chikiala Doninently red to buff late Fiedment E femgenous sand stcxies with a 200 Cretaceous ^? D few clay stones and ocxiglanerates to D early P A Cretaceous MTT TN nTPRTT rnsTTATT E V Sand stones of trown to reddish A brown,nrxierately thick bedded , R G Gangapur Chlcarious to naicalcarious, 280 Early Cretaceous U ferrv^enous with overlying \inXB and Fluvial G yellow day stc«es, occasional C Conglamerates and plant fossils. V D Kota Essentially of pale trown sand stcxies Upper Jurassic W red and greoiish grey claystcxes, thin to Fluvial A lime stales and calcarenates with Middle Fluvial- N fossil wood. 1100 lacustrine A TfrrMFTRMTTY S ^binly brown sandstones,medium to fine Lovver Jurassic I grained,hard,calcarious,to nc«i calcarious to Fluvial R Tarvai with clay pellets and occasional thin Upper Jurassic 0 beds of viiite to red clay stones and N and intematicxial ccogloiErates. 900 C H A 4JNaMTMm- Basal bright red silty clays with thin calcarious to roi-calcarious sand stones Upper Triassic I^eri and alternating seqence of sandstcxies 1350 to Fluvial and clay stcxies. Middle Triassic UNGGNFO^OTY Essentially sand staies,pink to brick red, K Upper Kamthi/ Fine to coarce grained,hard,ferruginous Chintalpudi and cross bedded,uith silicified fossil A wood. 800 IMLNFOMnY Itt M Middle Kamthi/ Alternations of greenish grey to red, Ixwer Triassic Ehutary clay stones and yellow to brown sand 5CD/1200 to Fluvial 0 T stcxies. Uf^jer Ffemrian IMENFCRMnY w H La*er Kamthi Sand stones, brown to reddish bra«ffi,niediiiTi to coerce grained,ferruginous and hard with E I minar cool streaks and clay stones. 800 LMXNFOMnY G S Sand stones,ash grey to vrfiite,mediviii to Coarce grained,hard,inoderately thick bedded 0 I Calcarious, felspathic and ndcacious,with Barakar with siltstones, carbonaceous shales and Fterraian Fluvial N N coal. 900 -UNCENFDEMnY D G Interbeds of greenish grey to olive green ncderately hard calcarious to non calcarious La«r Ffemrian A claystones and silt stones and sand stones to Upper Glarjal Talchir with basal boulder beds. 500 Carboniferous R E N I 4JNGCNFluFMnY- Dcnrinently sandstaies, pink to^«Mte, fine to P mediim grained, thin to ncderately thick bedded, Upper Deltaic Sullavai thinly laminated, hard, nc«>-calcarious,1000 Ftoterozoic to U micaceous,with vMte and pink clay stoies Esturme R -UNOMOMnY- A Dolanritic limestones, Quarzites and alteratiai I^khal of quarzitic sand stoies and {Mllitic shales Lcwer Epxneritic N shales with basal ccxiglamerates. 1500 Proterozoic A 4MI]NF(MnY- Archaean Garanites, biotite-schists, hornblende gneisses,- Early chamockites, khcmdalites with dolarite dykes Pre- Camhrian and veins of pegmatite and calcite. ly The Talchir formation occurs at different places as narrow, elongated stripe and also as isolated outliers within the Pakhal/Archean country rocks. They consist of diamictites, sandstones, conglomerates and shales/clay stones. The unequivocal evidence of striated pavement at Irai (1^53' N, 79^08' E) indicate that the tabular diamictites are of glaci- ogene origin. Barakars comprise of grey, feldspathic, coarse grained sandstones, with siltstones, carbonaceous shales and coal widely overlapped by an upper monotonous grey to white feldspathic gritty sandstone. They occur as linear patches near the graben margins. The rocks of this formation dip at 50 to 20 degrees in various directions. Kamthi sub-group continues uninterruptedly almost along the length of the graben. Rao and Pal (1983) sub-divided this unit into five formations viz., (a) Potamedugu Formation (b) Jaipuram Formation (c) Khanapur Formation (d) Maner Formation and (e) Kudurupalli Formation. The lower Kamthi Formation is characterized predominantly by sandstones with minor coal seams and hematitic claystones. The middle Kamthi Formation is comprised of alternations of claystones and sandstones with occasional basal conglomerates. The upper formation mainly comprises coarse sandstones with intra-formational conglomerates, claystones and silicified fossil wood. On the basis of plant fossil assemblage, upper formation of Kamthi sub-group is considered equivalent of Raniganj unit of Damodar valley. The post Gondwana rocks of the graben include, (1) Lameta Formation (Infra-trappeans) comprising of limestones, clays, sandstones and chert bands and (ii) Deccan traps on the western margin and RJY traps in the eastern margin, (which are also commonly called as Deccan traps) with 20 Inter-trappean limestones and clays. The Lametas are found to occur as isolated patches lying unconformably over Archeans and Kamthis in the Chandrapur sub-basin beyond Warora. The Traps with interveining Inter-trappeans occur from Asifabad to Chicholi, covering almost the northwestern part of the graben. The equivalent traps and the Infra- trappeans are noticeable also in the south eastern margin of the graben (Fig. 2.4) at Pangidi and Kateru near Rajahmundry. The commencement of sedimentation in the graben took place with the deposition of Pakhals during the lower Proterozoic time with the initiation of the rift activity along the NW-SE zone of weakness in the Archean basement. The Sullavais were subsequently deposited unconformably over the Pakhals and Archeans in the same trend due to secondary phase of rift activity. The major faults which run along the eastern margin of the Cuddapah and Bastar basins could have determined the southern limit of the Purana basin and the northern limit is concealed under Deccan Traps. The lower Gondwana sedimentation started during upper Carboniferous period due to the third phase of rift activity in the same trend. The fourth phase of rift activity at the end of the lower Gondwana period has further resulted in the structural reorganisation of the area, leading to the evolution of a narrow linear upper Gondwana basin confined to the central part of the graben. Subsequent to the deposition of upper Gondwanas, the deposition of the Infra-trappeans (equivalent to Lametas) in the coastal basin occured followed by peniplanation and eruption of lava flows with interveining Inter-trappean sedimentation during late Cretaceous to early Eocene period. A diagrant showing the tentative configuration of graben and RJY traps is given in Fig. 2.4. I7*i5 I7«00 - I«°45 Gollopall I - Bu^ivod'o In f re - "'ropptn* All uvim •j Soixlttofi* rv »~yl D»ccon Trop «ifh • • • 1 Rojohmundry \»' '^ ."H TirupC -PoKOlur 1 V » 1 Inttr-Troppuni y .• >• ^! Sonfli'en* Fig. 2.5 Geological inap and section along A A' of Ra:ann-jndry area, East and West Godavari Dt.i A.P. (After Ra^u, 1986) 22 The Geophysical investigation carried out by ONGC in Rajahmundry area reveal that the near surface thickness of the Traps including Intertrapp- eans is about 170 meters which increase (subsurface thickness) to 500 m further east. Three flows of basalts have been recognized with the help of Geophysical survey drilling data separated by marine Inter-trappeans of dark grey and variegated shales and micaceous silty sand-stones of Palaeocene age. 2.3. INFRA-TRAPPEANS, TRAPS AND INTER-TRAPPEANS 2.3a. Infra-trappeans Allied to the Laraetas are the beds occuring below the traps in the Rajahmundry region called Infra-trappeans. They are exposed only in the right bank of the Godavari river (West Godavari District). These beds occur immediately below the RJY Traps and above the Tirupati Sandstones at Duddukaru and Devarapalli villages near the base of the northern slope of RJY trap hillocks. They consist of series of sandstones which vary considerably in thickness. At some places it is two feet thick and at other places it is completely missing, with the result the traps at some places come in direct contact with beds overlying limestone. This feature suggest a possible erosional unconformity between the Intra-trappeans and the overlying basalt traps. Bhalla (1967) has given the geological sequence for the Infra-trappean sandstones. He reported that it is crowded with Turitella and other invertebrate shells, and has also yielded the maximum number of foraminifera species. 2J Structurally the area is undisturbed, the Infra-trappean beds dip with an angle of 6 to 10 degrees toward south east are overlain by the Tirupati sandstones. However, the contact between the two is concealed by alluvium in this area. On the basis of the heavy mineral characteristics of the two formations, Raju et al.(1965) suggested an unconformity between the Infra-trappean beds and Tirupati sandstones. The depositional environment of Infra-trappeans has been suggested as fluvio-deltaic conditions for the lower and esturine in the upper part (Bakshi, 1965). Bhalla (1966) described eight species of Foraminifera from the Pangidi Infra-trappean beds suggesting that these beds were deposited in a shallow marine, rather warm, inner neritic environment having open sea connections. Shallow water origin was also confirmed by the studies of Gardner (1957). . 2.3b Lower traps In Pangidi area the exposed lower basaltic flow lie non- conformably on Infra-trappeans. At Duddukuru, the trap is seen to rest both on the top most Turritella limestone beds, as well as, on the underlying coarse calcareous sandstone of the Infra-trappean bed. The trap is also resting over the formation further lower in succession, namely Tirupati sandstone and Khondalites on the right and left banks of the river respectively. The lower trap is charecterized by spheroidal weathering. It has a thickness which rearly exceeds 5 m and is best exposed in Pangidi- Gouripatnam area, where at present an active quarrying is going on. 2A 2.3c Inter-trappeans Inter-trappean limestones and mudstones lie on the lower basalt flow. The thickness of Inter-trappean beds about 5-7 metres. They are fossili- ferous and the fauna is unmistakably esturine. The mud beds and argill aceous limestones are extremely rich in macro- and micro-fossils such as Gastropods, Pelecypods, Foraminifera, Ostrocodes and Algae (Rao and Rao, 1935, 1936, 1939 and Bakshi, 1977). The composite evidences of the fauna and algae flora indicate a Paleocene age. The Inter-trappean beds in the Pangidi-Duddukura area appears to have been deposited under vary shallow, brackish marine to esturine conditions as evidenced by micro-faunal assemblages. 2.3d Upper trap The upper trap is exposed at Duddukuru-Gauripatnam, on the National High Way No.5 in West Godavari District and near Kateru village at the outskerts of Rajahmundry town of East Godavari District. The upper trap is about 25 m thick and consists of massive, dark-grey basalts. The flov shoi* occasional dip upto 4 degrees but are almost horizontal. CHAPTER:3 PETROMINERALOGY The location and the stratigraphic position of the RJY basalts in the general stratigraphy of the Godavari Graben have been discussed in the previous chapter. The petrography and the detailed mineralogy of Rajahmundry Traps is being discussed in this chapter. 3i FLOW LITHOLOGY Both the lower and upper flows are simple and blocky (PI. 2a and 2b). The upper flow is massive and fresh but, at places it is weathered to form lenses of fresh rock covered by layers of oxidised material with the margins pinching out laterally (PI. 2b, 3a and 3b). This feature gives an false impression of a compound flow. The lower flow also show such spheroidal weathering at some quarry sections near Pangidi (PI.4a and 4b). The Lower flow is fine grained compared to the upper flow which is fine to medium. The flows are generally vesicular in the upper part as noticed in the quarry section (PI. 3a). The cavities are filled with quartz, chert, zeolites and calcite. Zeolites are ubiquitous and the crypto- crystalline silica (agate, chalcedony and opal) line these cavities. Ramifications of 26 Plate 2 a. Sinple flow of basalt at Kateru b. Spheroidal weathering in top flow at Kateru 27 Plate 3 ^" Spheroidally weathered flow showing nuinber of units which look like compound flow units b. Exposed cross section showing the fresh basalt core and rim of oxidised weathered material Plate 4 a. Spheroidal weathering of the lower flow at Pangidi b. Another view of spheroidalwea- thering 29 Ramifications of thin veins of quartz measuring upto a centimeter in thickness extend in all directions, especially, into the lower horizons of the flows. The charecteristic conchoidal fracture pattern is seen in the quarry surfaces(Pl. 5b). 3.2 TEXTURE The basaltic flows are generally aphyric and show variable textures viz. interstitial, ophitic (PI. 6a,6b,6c), subophitic (PI. 7a). The grains of augite and opaque minerals are small enough to fit between the plagioclase microlites giving rise to interstitial texture (PI.7b,8a). Where the size of the augite crystals are much larger than those of plagioclase crystals, they tend to partly or who ly include the plagioclase microlites, resulting in ophitic texture and wherever the augite microlites are smaller than plagioclase, the latter will be partly enclosed resulting in sub-ophitic texture. Sub-ophitic texture dominates the rock relative to ophitic texture. Glomeroporphyritic texture develop where the plagioclase microlites form clusters (PI.8b). They represent early consolidated intratelluric crystals clustered together due to surface tension of the melt. At places these microlites of feldspar dispose in a sub-parallel manner with their interstices occupied by microcrystalline matrix resulting in typical trachytic flow texture (PI.9a). 30 Plate 5 a. Vesicle filled with quartz and zeolites in a quarry section near Kateru b. Conchoidal fracture in upper flow at Kateru 31 a. Pyroxene phenocrysts enclosing smaller laths of plagi- oclase resulting in ophitic texture X nicols; 25x10 b. A typical ophitic texture from upper flow of Kateru pp^- 10x10 32 Plate 6 c . Ophitic texture from upper flow of Kateru pp.; 2.5x10 33 Sub-ophitic texture in upper flow of Kateru. Note the plagioclase laths are partially enclosed in clino- pyroxenes. X nicols/ 10x10 b. Pyroxenes appearing interstitial to plagioclases. Note the palagonite around plagioclase crystals as in ophitic inter growth, pp., 10x10 Plate 8 34 a. Interstitial texture in upper flow of Kateru with some euhedral opaques. X nicols; 2.5x10 b. Augite microphenocrysts in glomeroporphyritii X nicols;«e.5xlO 3B + x An -Z z + Nc Scnr-E '. . c 1 1 O3 C c - 2 X| E c :r 3 I 5C 4 V_T 53 5 V,D 50 6 I| n 67 7 Vji 63 8 3 3 67 9 D 67 to V,5 53 II Vg4 46 12 I| I 46 3.1 Reinhard (1926) plot showing the An composition of RJY basalt on various migration curves 56 3.3 MINERALOGY The chief mineral constituents of Rajahmundry basalts are plagio- clase feldspars, pyroxenes and opaque iron ores. There are however some secondary minerals and cavity filling minerals also present in the rock. 3.3,1Plagioclase feldspars. The feldspar crystal occur as fine laths under microscope. The> constitute 30 to 35 percent of the rock by volume. The plagioclase laths occur as microphenocrysts reaching upto 1 mm in sxze (PI. 9b) and m ground mass as microlites measuring O.A to 0.7 mm m size. The maximum length to breath ratio is 5. Cleavage on (001) plane is clearly visible. The micro- phenocr>sts generallv occur as glomeroporphyritic clusters. Measurements of s\mmetricdl extention angle on (010) twin lamellae using Univesal stage gd\e \alues ranging from 32 to 3/ which correspond to labroderite conpo- sition {\i~i5D-\n70). The refractive index of plagioclase grains determined b\ the aui hor \s±th liquid immersion method gave =1.565, =1.555 and =1.662. The optic axial angle (2V), determined for various feldspar 0 o crvstals range from 78 to 79 with positive sign. About 50 cr\stals were studied with the Universal Stage Method using Remhard (1931) plot. The poles to composition planes fall on different migration curves (fig,3.1), their distribution is as follows: C.P No. of readings (100) 16 (010) 15 (110) 10 (021) 8 (001) 1 Some of the plagioclase laths studied for estimating An % (both phenocrysts and groundmass microlites) are diagramatically represented in Fig. No. 3.2a,bAc. These values suggest that plagioclase range in composition from Ar.53 to An65. According to Ritmann (1929) zone method, the compositions obtained are little high, ranging from An65-An70. Ritmann (1929) method has been used in order to over comes some of the difficulties that are faced in Reinhard method. Besides it is rapid techinique and does not require graphical reconstruction and can be employed on narrow tujn lamellae. Based on the symmetrical extinction angle measurements on (UlD; plane in ordinary microscope the composition of the plagioclases range iror. An61-An65. The probe analysis of plagioclase gives 72 % An content t..r microphenocryst and 5A to 60 % for the microlites.This further substar.:- tiates the results obtained from the optical studies. The difference in the anorthite content determined on the basis ui optic axial angle (2V) and the twin laws by using Fedeov-Nikitin graphs ;7.d> be due to structural and compositional factors like the presence c^f orthoclase molecule in the plagioclase crystal solution. Sin:,]ar discrepencies were obtained by Krishnan (1926) and Barth (1936) in the basalt feldspars and they attributed it to low content of potash freser.t • i; Reinhord Method 33 SP No V^U SP No4 5 SP No 0- Arvt? An^48 An-53 III SP No V|v SP No V SP No V, An^4 6 Anv^53 An^6 S Fig. 3.2a Diagramatically represented plagioclase crystals (bu crysts and groundmass). 39 Reinhord Method SP No )4, SP No II SP No X| II Arr^ 63 An^ 67 An--a 6 7 SP No la SP No 0^ Ani:?50 An '^ Fig. 3.2b 40 (RITTMANN ZONE METHOD ) 1 C P (0 > 0) CP ( 0 1 o ) £x angle^ 32 An » =65 A n «? ^ r 60 CPi o\o) CP(olo ) Ex aBo'J= ^ Ex angle - 38° An • =6 35 An « = 74 5 f"*- 3.2c Al plagioclases. The author calculated about 2 to 5 % orthoclase in the normative plagioclase from the chemical analysis from the RJY basalts. Further, the microprobe analysis of the plagioclases gave very low- proportions (0.1 to 0.2 percent) of orthoclase in the plagioclase microlites in contrast to charecteristic absence in microphenocrysts. Detailed studies of Perry (1968) suggest entry of Schwantke's molecule or nephelene molecule in feldspar solid solution at elevated temperatures. Twinning: Both the microlites and the microphenocrysts show twinning by various laws. Simple twins like albite (PI.9b), carlsbad and manebach and the complex twins such as albite-carlsbad (PI.10a), albite-manebach, manebach-ala and albite-ala are observed (PI.10b). Occasionally baveno (PI.11a) and baveno-carlsbad twins (PI.lib) are also observed. Observations of twinning on 30 crystals are listed as follows. C.P. Name of twin law No. of readings Percentage (100) Carlsbad B 12 40 (010) Albite 9 30 (021) Baveno left 6 20 (OOL) Manebach 3 10 Zoning: Zoning is present in plagioclase crytals in both lower and upper flows, but is significantly present in the former. Some plagioclase crytals show normal zoning (PI.12a) while the others show perfect ossilatory zoning (PI.12b). Plate 9 42 Sub-parallel arrangement of plagioclase microlites (Trachytic flow texture) X nicols; 2.5x10 b. A microphenocryst of plagioclase showing albite-carlsbad law of twinning (C-twin). The length of the phenocryst is 1 mn. Note the twinned augite microlite. X nicols 25x10 43 Plate 10 a. A lamellar twinned aggrigate showing albite-carlsbad law. X nicols; 25x10 A twinned crystal of plagioclase in centre showing albite-ala twin. Groundmass microlites show twinned and untwinned nature. X nicols; 10x10 ^ ' " 44 Plate 16" / c. Complex twin pattern in upper flow of Kateru showing albite, carlsbad and pericline laws. X nicols; 10x10 Plate 11- 45 a. Baveno twinning in a grounc3niass microlite from lower flow of Pangidi. X niceIs; 25x10 b. Baveno-carsbad complex twinning in a groundmass microlite from upper flow of Kateru. X nicols; 25x10 Plate 12 46 Normal zoning in plagioclase phenocrysts from the upper flow of Kateru. Note the inclusions of opaque ores. X nieols; 25x10 Oscillatory zoning in plagioclase phenocryst from the lower flow of Pangidi. X nieols; 25x10 47 An 64 3 An 61 8 E An 60 9 6 00 An 58 8 o An 55 4 An 55 0 An 53 0 75 r 65 55 An •/. 45 - 35 - 25 0 28 mm CORE RIM (width in mm opprcK ) Fig. 3. 3 A normal zoned plagioclase microphenocryst showing compositional gradation from core to rim 48 Normal Zoning: Fig. (3.3) shows a zoned microphenocryst of plagioclase of which An % has been computed with Reinhard method. It shows normal zoning with core having high An content (about 64.3 %) and gradually decreasing towards rim (upto An 53 %). Determinations were repeated and the reproducativity is within jf 2 %. Minimum supersaturation of plagioclase molecule in a magma is necessary to form a stable nucleus for the initiation of plagioclase crystallization. Growth of plagioclase crystals accompanied by rapid cooling and contemporaneous crystallization with falling temperature caused disequilibrium due to incomplete reaction between the growing crystals and relatively sodic residual melt. This process is largely responsible for the genesis of normal type of zoning in which the anorthite gradually decreases from core to margin as observed in the present study. Further, polysynthetic twining which is superimposed over the zoned feldspars (PI.12a) appears to be mechanical (gliding) twinning and could have nucleated in solid /semi-solid state at the points of high stress (Smith, 1974). Oscillatory zoning: The genesis of oscillatory zoning is attributed to the fact that at given temperature and pressure a minimum supersaturation is necessary to form a stable nucleus and the degree of supercooling controls the growth rate of a crystal. Crystallization of a plagioclase in equilibrium with liquid magma results in the depletion of Ca-plagioclase molecule near the interface of the ' growing crystal (Vance, 1962; Smith, 1974). This could cause composition gradient between the interface of melt and growing crystal. This in turn could lead to diffusion of An-molecule in the crystal resulting in the formation of anorthite rich zone, especially where cooling is slow. The growth rate has to be higher than the diffusion rate as otherwise the crystals would not show oscillatory zoning due to constitutional supercooling as explained by Sibley et al, (1976). Repetit ion of the process would result in the genesis of oscillatory type of zoning (PI.12b). An important observation from the plagioclase analysis is that, the overall range of plagioclase composition from lower and upper flows is remarkably same, 3.3.2 Pyroxenes The mineral next in the order of abundance is the clino-pyroxene. It is colourless to pale yellowish in colour. It shows pale greenish to pale purphish brown second order interference colours. The pyroxenes show cleavages in two directions. An important feature is that, the\ generally show alteration to chloritic material such as palagonite and chlorophaeite. The optic sign is positive and the optic axial angle varies frorr. 22 to 43 degrees with the optic plane parallel to (010). Hess (1940) groaped all the pyroxenes with optic angle 32 as pigeonite and those 32 sub-calcic augite. Since both low and high optic axial angles are obtained, it is inferred that both these mineral phases exist in the RJY basalts of this region. This has been further substantiated by the microprobe analyses (Table.3.1). The pyroxenes show cleavages in two directions. Microlites of 50 pyroxenes with twin plane (010) are not uncommon. The probe data of pyroxenes from the study area has been classified with the help of new method of classification proposed by the sub committee of Pyroxenes, International Mineralogical Association (MiTimoto, 1988) and this classification is applied for the preserit study. Inorder to avoid difference between the real and ideal site occupancies, the Ml and K., sites are considered as single M sites. The numbers of Ca, Mg and Fe2 and Na cations in M sites are plotted in the Q-J diagram where Q=Ca+Mg+Fe+2 and J=2 Na (Fig. 3.4). The lines representing the following equations are used to subdivide the Q-J diagram. (1) Q+J = 2.0 (2) Q+J = 1.5 (3) J/(Q+J) = 0.2 (4) J/(Q+J) = 0.8 The areas corresponding respectively to these sub divisions are the Ca-Mg-Fe pyroxenes, Ca-Na pyroxenes, Na pyroxenes and other pyroxenes, (quad, Ca-Na and others). The samples fall distinctly above J/Q+J =0.2 line in quad field (between Q+J =1.5 and Q+J = 2.0). Tne distinction between the augite and pigeonite in the Ca-Mg-Fe pyroxenes is primarily structural, their space groups being (2/C and P2/C) respectively. There is a miscibility gap between augite and pigeonite and man} pyroxenes with 15-25 percent Wollestanite are proved to be mixtures of the two (Mirimito, 1988). Augite with less than about 25 percent WO is often called as sub-calcic augite. On heating pigeonite undergoes a rapid 51 2-f 0 = C04-Mg<-Fe En,F«. (D.Hd),Wo J -- 2 No Fig. 3.4 Q-J diagram for pyroxene classification. RJY pyroxene analysis plot in Quad field (Morimito, 1988) 52 displasive transformation to a C2/c structure, which can't be quenched. Augite does not show such transformation. The analysis of groundmass augite from the lower trap of Pangidi area show slight iron enrichment compared to the upper trap pyroxene of Pangidi. Such trend of iron enrichment in the groundmass pyroxenes has been observed in Mahabaleswar (Deccan) basalts (Sen, 1986). The analysis of sub-calcic augite from the upper trap of Kateru shows Mg enrichment trend relative to Ca. The two analyses from the lower flow (Pangidi) and upper flow (Kateru) show pigeonitic trend. In Fig.3.5 the composition of pyroxenes are plotted, in which the dashed lines represent the solidus trend of pyroxene composition in the Skaergard intrusion after Brown (1957) Brown4Vmcent (1963). The A1203 content of Ca-rich pyroxenes in the RJY basalt varies between 1.69 to 3.78 wt percent. The samples when plotted in A1203 vs Si02 diagram after Le Bas(1962) fall in sub-alkaline field (Fig. 3.6). 3.3.3- Opaques The RJY basalts contain significant amount of opaque ores mostly of iron. They occur as euhedral crystals, needles, streaks.irregular patches (PI.13a & b) and as inclusions in other minerals. At times they also surround plagioclase laths or pyroxenes indicating their late stage origin (PI.13b). Skeletal crystals of late stage origin are also seen in the cavity filling minerals (PI.14a & b). rb o CoSi03 Co Mg SigO, Co Fe SipOft MgSiOj Fe SiO, Fig. 3.5 Composition of pyroxene in RJY basalts. Areas withdashed lines represent the solidus trend of pyroxene composition in Skaergaard intrus ion after Brown (1957; a-.i Erown and Vincent (1963) 54 54 r SUBALKALINE Si02*l 50 V. ALKALINE PARALKALINE 46 L AlgOj •) % Fig. 3.6 A1203 - Si02 diagram for RJY basalts showing sub-alkaline nature Lebas (1962) 4 , 55 Plate 13 Partially palagonitized pyroxene microlites in ophitic relationship with plagioclase. pp; 10x10 b. A magnetite crystal enclosing groundmass minerals. X nicols; 10x10 56 3. Secondary minerals The vesicular cavities and interstitial voids in the RJY basalts are filled by the minerals deposited by late magmatic fluids. The cavity filling minerals fall into three main groups; zeolites, silica (PI.14a & b) and green to yellowish brown to brown cryptocrystalline mineraloids commonly described as palagonite and chlorophaeite (Sarbadhikari, 1966). Peacock and Fuller (1925) and Peacock (1930) considered these mineraloids as potentital chlorites produced by deuteric alteration of ferromagnesian minerals and glass. They occur in the cavities ofte'n filled by chalcedony (PI.15a & b; 16a & b). Such mineraloids appear as boundles and thin fibres or aggregates of minute granules and they are cryptocrystalline or nearly amorphous. They also show zoned structures surrounding the amygdales. Many earlier workers tried to differentiate between green, yellowish and brown coloured products, but except for their colour they do not differ significantly in any other characters. Generally in these basalts pyroxene are altered but plagioclase are fresh (PI.17a). Primary glass also substantially contributed to the alteration but it would be difficult to differentiate the alteration of pyroxene from that of glass. The general presumption is that colour variation is related to the oxidation state of iron (Fermor 1925 and Peacock, 1930). Pleochroism is absent or slightly present. The alteration of pyroxenes generally release CaO (Fawcett, 1965). The presence of calcite with altered pyroxene support this process. Wilshire (1958) suggests that volatile rich fluids would cause alteration of ferromagnesian minerals by leaching out of MgO, the solution 57 would be deposited in rim and vesicles. However such phenomena was not found in RJY basalts. Phenocrysts of plagioclases (PI. 17b & c) with and without inclusions are seen. Plate 15 59 a. Plagioclase laths enclosing palagonite. X nicels; 10x10 b. Palagonite enclosing the fresh plagioclase laths. X nicols 10x10 Plate 16 60 a. Microvesicle filled with cryptocrystalline quartz lined with palagonite. X nicols; 2.5x10 b. A magnified view of above section. X niols; 25x10. Plate 17 61 a. Palagonite (in the centre) occupies the space within the plagioclase laths, pp; 10x10 Plate 11 62 Plagioclase laths without inclusions of groundmass. pp; 25x10 *•. Plagioclse phenocrysts with .inclusions fron, lower flow of Pangidi. x nicols; 25x10 '• * " •c E • c 0 cr 3 u^ CC o u oc ft, H E "4 U 1 vC r~~ o i~~ o 0 OC OJ 1 1 vC O- 3 r^ r-i rsi X (M in •>? X O X CM u 3 Q. c — f-H 1-^ •-^ r—i IM 1 i-J . vC vC • -? • in o- o O CM • • • (0 O. 1 (C o • • ri X • o r^ o CM cn in 3 1 i^ Ln -^ C '-' • — in o• 1 C • 1 • 8 in cn ^^ 0) o -^ o '^ in 1 (0 •• E 1 U vC ^ CM in cn o o o 4^ U-« 1 IT) <} -J r-j 0^ X —I o v£> X (N CM f-H X f^ •H f—< 1—< i-H « 1 iT) 1 • r>j L" • .-n . in X <7- X oc ui a» 1 1 O • • n O v£> O t—1 in• i•n CO 4^ • o • ^H« ^H• 1 a. in (N C :M O (Nl 1 ~ ON o "^ ' 1 1 1 C L'> — 1—1 a^ ^ , 1 • • r^ X C r^ • r^ r^ CM CM CM C —1 vj 3 x 1 " X 30 in vD r^ en 2C e: = 3 1 1 CO u 5 c O 3 en o 1 ^ u CNi o < •H 1 cn •• Q; o O CN O w CM CN 1-1 o ^^ c t-t j-l cr -^ a. ij -— <—1 CM U :_ — C- c oc'li CC O CO •H I—1 •H 0) c OC CO (S ll o C X! tl CD CO 1 a c zr •< H '-^ s: s: c; z :^ w ^ u 00 < H Pu s: s: o Z i^ u s: <: < o b^ E a. 3 ut; CHAPTER 4 GEOCHEMISTRY This chapter deals with the geochemical characters of the RJY basalts and comparision with the time equivalent Deccan basalts. Fifty four fresh samples representing the entire basaltic out crops in and around Rajah- mundry were selected for geochemical studies. Out of which, thirty samples collected from Kateru (East Godavari District) represent the top flow, twenty five samples represent top and bottom flows at Pangidi (West Godavari District). Samples from bottom flows were collected at closer intervals because of its relatively small thickness and its limited exposures around Pangidi. Care was taken to avoid samples with amygdaloidal and zeolitic material for chemical analysis of major and trace elements .including rare earth elements. 4.1 GENERAL CHEMISTRY OF THE FLOWS Both major and trace element contents are given in Tables 4.1.1 to 4.1.3. Silica ranges from 49.1 to 52.2 with an average value of 50.33 percent. This is low compared to the average basalt (Nockolds, 1954) but are comparable with Deccan basalts of Western and Central India. The A1203 contents range from 11.98 to 14.21, percent averaging 13.2 percent. The total iron as FeO shows a range from 11.59 to 15.46 percent with an average of 13.02 percent. 65 The Ti02 contents, which provide better discrimination power among the major oxides of the basaltic rocks (Cheyes, 1965) appears to have a variation from 1.12 to 3.03 percent, averaging at 2.22 percent. The basalts of this region give an average value of more than 1.75 which is also limiting value for the tholeiitic basalts (Cheyes, 1965). The Na20 content varies from 1.85 to 3.0, averaging at 2.44. The K20 has a comparatively lower concentration varying from 0.46 to 0.9 percent, with an average of 0.52 percent. The MgO and CaO values vary from 4.14 to 6.62 and 8.49 to 10.61 percent with an average of 5.18 and 9.57 respectively. The P205 contents ranges from 0.14 to 0.31 percent averaging 0.24 percent. The MnO content ranges from 0.11 to 0.3 percent averaging at 0.20 percent. Similarity of chemical composition for both top and bottom flows is a significant feature of RJY basalts. There is not much difference m the bulk chemistry of the lower and upper flows exposed near Pangidi. Houever, the lower flow is comparatively enriched in A1203, Ti02 and N'a20 b\ an amount of 0.31, 0.17 and 0.52 percent respectively. The upper flow, exposed at Pangidi and Kateru is enriched in Si02, FeO and K20 in contents coraparision to the lower flow at Pangidi. The oxide ratios like FeO/MgO (FeO = FeO + Fe203 x 0.85), Si02/MgO, Fe203 /FeO, K20 / CaO and Na20/ CaO are sensitive indicates of magmatic differentiation (Goldschmidt, 1954). The low Fe203/FeO ratio in RJY basalts (average 0.39) is due to relatively low oxidising conditions. This inturn may suggest a lower water content than is normally present in the parent magma of basalts (Kuno, 1968), RJi' basalts in general show a 66 CO I—1 1—t r—I i-H r>- ^ r-» in -J in in o • 1—( CN cn ON cn CN -;)• 00 vO CN CJN in o o -J in X C^l o cn KAT- 10 11 12 13 lA 15 16 17 18 Si02 49.73 A9.63 A8.71 A9.28 A9.07 50.78 A8.6A A9.59 51.A8 Ti02 2.08 1.56 2.6 2.00 2.78 1.12 2.32 2.26 .68 A1203 1A.41 13.13 13.A6 1A.03 12.A3 13.2 13.A5 12.28 13.9 Fe203 3.A3 3.5A 3.07 3.8A 3.71 2.30 3.96 A.38 3.68 FeO 9.56 9.9 9.67 9.93 10.76 10.08 9.95 8.96 10.06 MnO .1 .15 .20 .22 .2A .18 .22 .16 .08 MgO A.35 5.26 5.76 A.99 5.2 5.5A 5.28 6.61 A. 07 CaO 9.32 9.79 9.28 9.65 10. OA 10.27 10.06 9.3A 8.75 Na20 3.89 3.29 2.02 2.66 2.51 2.A2 2.59 2.36 3.06 K20 .75 .52 .58 .6 .62 .82 .6A .8A .78 P205 .20 .29 .31 .2A .27 .15 .37 .26 .36 H20 3.11 3.35 2.73 2.8A 2.76 3.18 2.85 2.92 3.18 Total 100.A3 100.Al 100.39 100.3 100.1 100.0A100.33 99.96 100.08 Norms Qz .72 .60 .39 2.9A 3.78 3.12 2.52 A.08 A.56 Or A.A5 2.78 3.3A 3.34 3.3A 5.0 3.89 5.0 A.A5 Ab 20.82 27.77 16.77 22.53 20.96 20.AA 22.0 19.91 25.68 An 21.68 19.7A 26.13 24.7A 21.13 22.52 25.07 20.29 21.00 Di 20.A2 22.74 23.A2 19.2A 22.A9 23.66 21.1 21.0 16.80 Hy 12.OA 1A.67 14.05 15.03 14.06 15.69 14.01 14.97 16.3^. Mt 4.87 5.1 4.41 5.57 4.64 3.75 5.8 6.26 5.34 II 3.06 2.89 4.06 3.80 5.32 2.13 4.41 4.26 1.22 Ap - .67 .67 - .67 .67 - .67 H20 3.11 3.35 2,73 2.84 2.78 3.18 2.85 2.02 3.18 M.I 78.94 76.26 73.56 77.6 77.38 73.87 76.74 71.56 100.9 F.I 32.03 29.12 19.73 26.39 26.09 25.23 25.46 26.84 31.82 D.I 33.99 31.15 24.01 28.81 28.06 28.56 28.41 28.99 34.69 S.I 20.25 23.37 27.30 22.64 23.1 26.18 23.56 28.5 18.8 &8 Traces(ppiT. ) Sc Ul 45 57 54 55 38 56 49 31 V 426 448 508 462 428 436 436 420 482 Cr 128 132 253 127 135 164 142 126 118 Co 36 42 47 39 42 46 43 46 38 Ni 72 74 128 68 72 46 43 46 38 Cu 264 228 405 260 431 196 324 283 87 Ga 30 30 30 25 30 30 25 25 30 Sr 231 218 192 212 210 278 290 282 241 Ba 428 426 421 365 301 464 274 456 422 Y 36 30 34 34 35 36 36 35 36 Zr 220 148 130 215 155 152 154 224 246 Zn 132 130 68 86 84 82 82 80 110 Rb 18 12 15 15 16 27 18 28 26 REE La 31.53 28.56 36.58 34.02 28.34 26.38 38.62 32.84 34.64 Ce 44.3 37.6 33.4 44.8 39.4- 30.2 57.6 46.8 50.3 Nd 33.8 30.7 39.6 39.2 30.6 28.8 41.5 34.6 36.8 7.12 6.75 6.54 6.12 5.75 6.30 7.12 7.2 7.16 Eu 2.64 2.31 2.42 2.23 2.14 2.32 2.67 2.76 2.69 Tb .63 .60 .82 .78 .61 .58 .7 .75 .77 Ho .66 .63 .85 .81 .64 .61 .82 .79 .79 Yb ..37 3.02 3.81 3.30 2.96 2.38 4.02 3.41 3.65 Lu .J8 .32 .33 .32 .31 .29 .44 .41 .42 69 Table U. 1.1 Kat 19 20 21 22 23 24 25 26 27 Si02 49.60 48.55 48.38 48.66 49.44 48.42 49.72 49.32 49.92 Ti02 2.68 2.74 2.68 2.52 2.56 2.98 1.95 2.34 1.63 A1203 12.53 13.50 13.23 14.05 12.61 12.11 13.48 13.21 12.57 Fe203 3.25 2.68 2.90 3.04 3.21 3.98 3.47 3.06 3.21 FeO 10.17 10.63 10.46 9.95 10.12 10.57 9.95 10.33 10.83 MnO .24 .29 .21 .23 .25 .27 .15 .21 .18 MgO 5.1 5.32 4.65 5.43 5.21 6.6 5.48 5.25 5.4 CaO 9.95 10.14 10.23 9.66 9.7 9.47 8.87 9.98 9.36 Na20 2.46 2.64 2.92 2.81 2.78 2.32 2.64 2.37 2.55 K20 .65 .51 .57 .54 .67 .59 .74 .68 .62 P205 .41 .32 25 .31 .23 27 .31 .29 .37 H20 2.97 2.80 2.59 2.85 3.42 2.54 3.40 3.14 3.3 TOTAL 100.01 100.12 100.07 100.05 100.20 100.12 100.16 100.18 100.02 Norms; Qz 4.92 1.96 1.26 1.5 2.76 • 3.36 3.84 4.02 3.96 Or 3.89 2.78 3.34 3.34 3.89 3.34 4.45 3.89 3.34 Ab 20.96 22.01 24.63 23.58 23.58 19.38 22.01 19.91 21.48 An 21.13 23.63 21.41 24.19 20.02 21.13 22.00 23.39 21.13 Di 21.56 20.0 24.6 10.31 21.42 20.17 10.24 20.42 1^.7 Hy 13.94 10.19 11.32 10.46 13.31 18.17 17.09 15.83 18.^- Mt 4.64 3.94 4.57 4.41 4.69 5.8 5.0 4.41 4.'.- 11 5.02 5.17 5.01 4.71 4.86 5.62 3.65 4.41 3.11- Ap 1.01 .67 - .67 - .67 .67 .67 .67 H20 2.47 2.80 2.59 2.85 3.42 2.54 3.4 3.14 3.38 M.I 76.8 75.97 74.36 75.07 76.3 74.24 75.46 76.26 76.61 F.I 24.96 24.75 27.1 26.85 27.43 24.63 28.88 24.58 26.47 D.I 29.77 26.77 29.23 28.42 30.23 26.09 30.3 27.82 28.7s S.I 23.58 24.43 20.67 24.94 23.69 27.43 24.60 24.3 23. '^^ TRACES: Sc 49 58 63 58 50 61 48 53 45 V 468 474 482 474 438 418 396 464 436 Cr 132 146 165 126 126 118 116 132 128 Co 41 43 39 44 42 46 45 42 45 Ni 69 80 86 68 66 64 62 60 65 Cu 427 429 428 372 394 445 231 326 198 Ga 3U 30 25 30 30 25 30 30 30 Sr 219 221 219 215 217 206 238 218 232 Ba 370 421 279 370 423 421 446 384 342 \ 3b 30 34 31 35 34 35 36 34 Zr 152 154 184 155 202 238 247 183 224 Zn 82 85 IDS 98 96 78 85 82 84 Rb 18 12 14 13 19 15 24 19 17 REE La 31.68 28.76 26.48 25.85 32.65 36.76 34.52 38.85 42.28 Ce 40.5 37.2 33.4 30.9 45.3 52.8 49.6 61.2 64.3 Nd 33.7 31.3 28.8 27.3 34.5 38.3 35.8 40.6 44.1 Sm 6.78 6.24 6.14 6.78 6.98 . 6.56 6.42 5.94 6.18 Eu 2.51 2.32 2.24 2.52 2.59 2.42 2.39 2.18 2.27 Tb .74 .69 .59 .50 64. .69 .67 .73 .78 Ho .72 .71 .62 .61 .6" .72 .70 .76 .81 Yb 3.34 3.06 2.82 2.72 3.41 3.83 3.64 4.02 4.84 Lu .39 .37 .37 .36 .?i .47 .41 .45 .48 71 Table A.1.2 Pangidi basalts (Upper flow) Pl-1 Pl-2 Pl-3 Pl-4 Pl-5 Pl-6 PI-7 PI-8 Pl-9 Si 02 49.69 48.57 50.04 47.02 50.37 50.69 40.76 48. 48.26 Ti02 2.08 2.43 2.16 2.03 2.26 2.35 2.18 2.88 2.95 A1203 13.48 13.2 12.33 12.58 11.9 13.08 13.54? 13.28 14.25 Fe203 3.63 3.17 3.64 3.28 3.45 3.02 3.87 3.83 3.73 FeO 9.59 11.23 8.91 10.93 10.7 9.04 9.54 9.43 9.39 MnO .12 .14 .19 .17 .23 .2 .13 .25 .21 MgO/ 4.85 5.11 5-22 s iq z. A3 4.7 5.2 5.2 4.3 CaO 9.37 9.06 10.24 1051 9.01 9.23 10.05 10.43 9.73 Na20 3.13 3.6 3.1 2.8 2.24 3.42 3.72 2.96 3.31 K20 .67 .56 .27 .24 .41 .46 .46 .42 .55 P205 .21 .33 .42 .37 .28 .21 .33 .48 .39 H20 3.22 3.08 3.51 2.87 3.54 3.72 3.14 2.85 2.91 Total 100.04 99.94 100.03 99.99 100.02 100.14 100.12 100.01 99.98 Norms Qr 1.14 .78 4.02 2.28 4.14 3.3 2.4 1.74 1.44 Or 3.89 3.34 1.67 1.11 2.22 2.78 2.78 2.22 2.37 Ab 26.2 25.68 26.2 23.58 27.24 28.82 23.58 25.15 27.77 An 26.41 20.57 18.9 21.3 16.96 18.9 23.07 _; ji^ 22.52 Di 16.7 19.02 24.6 24.6 21.8 22.49 22.28 2J. 95 19.48 Hy 14.84 17.54 10.87 15.09 14.16 11.02 12.56 ^ - . - 10.59 Mt 5.34 4.64 5.34 4.64 5.1 4.41 5.67 5. 57 5.34 11 3.95 4.56 4.1 3.8 4.26 4.41 4.1 5.47 5.62 Ap - .67 1.01 .67 .67 - .67 1.01 1.01 H20 3.22 3.08 3.51 2.87 3.54 3.72 3.14 2.85 2.91 M.I 77.36 78.04 76.06 77.52 79.36 7633 76.35 7o. J 3 79.26 F.I 29.9 29.71 26.65 23.27 29.89 30.69 30.44 25.4G 29.53 D.I 31.23 29.8 31.89 26.97 33.6 34.9 28.76 29. 32.5 S.I 22.18 22.09 24.69 23.13 20.64 22.77 22.82 23.51 20.21 72 ,Traces (ppm) Sc 48 61 45 55 42 43 58 65 63 V A38 408 486 494 338 364 438 462 426 Cr 129 118 163 208 116 127? 136 186 125 Co 38 43 45 44 38 39 43 43 36 Ni 68 63 88 105 62 68 72 96 65 Cu 252 356 279 254 284 326 280 426 445 Ga 25 30 25 30 25 30 25 25 30 Sr 226 237 221 193 221 220 218 195 217 Ba 386 423 199 91 P 486 492 491 489 425 r y 35 33 28 26 30 30 30 31 33 Zr 218 234 150 146 216 194 183 154 215 Zu lU 108 112 98 128 134 146 106 132 Rb 19 14 6 5 10 11 11 10 13 REE La 36.84 25.46 28.54 31.05 34.82 32.58 38.54 36.38 42.28 Ce 52.9 32.2 37.3 41.3 52.3 45.5 58.2 54.3 65.8 Nd 38.7 27.2 30.4 33.5 36.4 34.7 40.8 39.2 45.3 Sm 7.06 7.14 6.48 7.14 7.23 7.04 6.96 6.75 6.24 Eu 2.63 2.71 2.39 2.69 2.74 2.61 58 2.51 2.31 Tb .63 .58 .58 .63 .67 .66 .73 .71 .83 Ho .66 .61 .61 .67 .71 .69 . / / .74 .88 Yb 3.85 2.76 3.05 3.29 3.71 3.43 4.02 3.84 4.41 ^Lu .45 .39 .34 .39 .39 .38 .3'^ .37 .39 Table 4.1.2 :(P1- Pangidi upper flow, P2- lower flow) • Pl-10 Pl-1] Pl-12 Pl-13 P2- 1 P2- 2 P2- 3 P2- 4 P2- 5 Si02 50.22 50.05 50.05 48.40 47.39 50.52 49.85 49.74 49.21 Ti02 2.63 2.24 3.21 3.28 3.49 1.36 1.41 1.5 3.35 A1203 12.04 12.26 11.97 14.06 14.44 13.93 14.21 13.82 11.5 Fe203 3.45 3.89 4.58 4.69 4.4 3.74 2.65 3.26 3.12 FeO 9.6G 9.2 8.68 8.78 9.03 10.41 10.51 10.31 11.34 MnO .18 .23 .08 .11 .24 .15 .05 .06 .24 MgO 5.38 5.18 4.66 4.29 4.15 4.00 5.35 4.55 4.92 TaO Q.Ol 9.17 9.02 9.29 9.53 9.38 9.52 10.36 10.58 Na20 2.97 3.75 2.94 3.11 3.34 2.74 2.67 2.91 2.25 K20 .65 .80 .67 .64 .68 .64 .84 .73 .63 P205 .19 .29 .16 .21 .15 .21 .05 .21 .25 H20 3.63 2.98 3.81 2.87 2.74 3.01 2.7 2.84 2.74 Total 100.01 100.04 99.99 100.03 100.12 99.98 99.81 100.29 100.13 Norms: Qz 4.08 .48 7.32 1.98 .96 4.56 1.14 .42 5.22 Or 3.89 4.45 3.89 3.89 3.89 3.89 5 3.89 3.89 Ab 25.15 31.44 24.63 28.82 28.3 23.06 22.53 24.02 18.86 An 17.51 14.46 17.51 21.15 22.52 23.91 22.63 22.52 19.46 Di 22.14 24.6 22.74 20.88 20.63 18.78 19.7 24.12 28.20 Hv 17.72 11.03 6.9^ ~.32 7.09 14.74 18.21 14.91 12.28 Mt 5.1 5.57 6.7:, ^,.73 7.19 5.34 3.94 4.64 4.41 I? 5.02 4.26 6..- • .23 6.69 2.58 2.74 2.89 6.38 Ap - 0.67 ------.67 H20 3.63 2.98 3.81 2.87 2.74 3.81 2.7 2.84 2.74 M.I 75.36 75.97 77.99 79.02 80.72 81.62 75.65 76.96 78.76 F.I 29.88 34.47 29.82 31.07 30.89 27.68 28.25 27.2 22.49 D.I 33.12 36.37 35.84 _.4.09 33.15 31.51 28.67 28.33 27.97 S.I 24.39 22.78 21.64 19.47 19.74 18.58 24.3 20.91 22.1 74 Traces (ppm) Sc 43 45 44 61 72 42 45 47 53 V 328 342 330 356 384 364 382 448 442 Cr 114 125 115 134 143 126 132 174 208 Co 42 40 30 36 35 35 44 38 41 Ni 62 68 62 71 75 70 71 92 108 Cu 422 278 459 468 205 196 202 214 485 Ga 30 25 25 26 25 30 30 25 30 Sr 221 258 225 231 245 228 285 278 ??'^ Ba 335 364 368 335 222 279 485 426 279 Y 36 37 35 35 35 35 36 35 35 Zr 216 208 218 232 215 224 215 152 142 Zn 109 148 106 142 116 95 84 88 75 Rb 18 26 19 17 11 17 28 24 17 REE La 39.36 36.84 35.63 32.87 30.64 36.68 38.42 35.43 32. Ce 59.5 54.1 52.2 46.3 41.6 54.5 58.2 42.3 45.: Nd 42.8 38.4 37.2 30.2 32.1 37.8 39.3 36.4 34.6 Sni 7.25 6.68 6.34 6.0 6.84 7.14 5.7 5.89 6.67 Eu 2.76 2.46 2.34 2.01 2.56 2.65 2.12 2.18 2.45 TD .81 .90 .78 .58 .73 .79 .82 .78 .74 ::4 .81 .80 .78 .73 .73 .79 .82 .78 .64 Yb 4.12 3.84 3.72 3.41 3.24 3.81 4.03 3.73 4.32 Lu .42 .41 .40 .24 .25 .28 .42 .41 .39 /D T^ble 4.1.3 (Pangidi lower flow) P2-6 P2-7 P2-8 P2-9 P2-10 P2-11 P2-12 P2-13 Si02 49.39 49.08 50.68 49.76 49.8 50.14 49.92 50.96 Ti02 1.18 3.18 2.74 3.18 2.28 2.47 2.25 1.72 A1203 13.48 11.78 11.12 11.82 11.83 12.5 11.96 12.52 Fe203 3.67 2.97 4.74 2.85 3.15 4.99 4.55 3.41 FeO 9.95 11.73 9.16 11.84 10.9 9.19 9.83 10.05 MnO .21 .22 .21 .24 .21 .18 .15 .04 Mgo 5.48 4.78 5.16 5.14 5.6 5.42 5.87 5.66 CaO 10.25 10.55 10.2 10.35 9.66 9.97 9.82 9.48 Na20 2.68 2.46 2.28 2.75 2.39 2.42 2.53 2.82 K20 .72 .78 .87 .63 .53 .56 .68 .84 P205 .46 .32 .25 .28 .31 0,30 .21 .64 H20 2.67 2.21 2.64 1.62 2.44 2.12 2.92 2.36 Total 100.14 100.06 100.12 100.46 100.14 1000.16 100.69 100.5 Norms Qz 1.32 3.24 7.62 2.16 4,98 6.84 4.14 3.6 Or 4.45 4.45 5.0 3.89 3.34 3.34 3.89 5.0 Ab 22.53 20.96 19.39 23.06 19.91 20.44 21.48 23.58 An 22.52 18.63 17.51 18.07 20. C'2 21.41 19.18 19.18 Di 21.56 26,66 27.84 27.84 21.b 22.02 24.81 20.42 Hi 16.52 12.83 7.69 13,27 I".'"'- 11.47 13.26 16.83 Mt 5.34 4.18 6.96 4.18 4.6- 7.19 6.5 4.87 11 2.28 .08 5.17 6.08 -4. , . ^ 4.71 4.26 4.29 Ap 1.01 .67 - - ,67 .67 - 1.34 H20 2.67 2.21 2.64 1.62 3,44 2.12 2.92 2.36 M.I 75.65 79.53 77.03 78.32 75,97 76.49 75.36 74.93 F.I 26.19 24.7 24.76 25.74 24.28 24.09 25.83 29.17 D.I 28.3 28.65 32.01 29.11 28.:,. 30.62 29.51 32.19 S.I 24.36 31.04 23.23 22.15 24.5: 24.00 25.02 24.85 76 TRACES Sc 51 54 38 46 47 44 45 36 V 436 452 464 478 424 432 422 436 Cr 165 204 158 162 173 154 128 116 Co kU 40 43 42 43 44 46 44 Ni 88 105 83 86 91 79 68 62 Cu 164 452 428 440 324 363 282 184 Ga 24 30 25 30 30 25 25 30 Sr 206 292 299 211 214 216 216 284 Ba 420 446 462 289 420 421 384 464 Y 36 36 37 35 31 33 36 37 Zr 152 148 150 155 215 184 196 212 Zn 86 82 76 95 78 80 84 98 Rb 24 26 29 17 14 15 20 28 REE La 31.32 24.52 28.68 24,63 26.78 28.82 26.46 32.35 Ce 44.6 30.1 37.2 30.5 34.6 38.3 33.2 45.8 Nd 33.5 25.8 29.6 25.8 27.6 30.2 28.3 34.5 Sm 6.89 •7.04 6.58 6.94 6.81 6.38 6.89 7.12 Eu 2.57 2.61 2.42 2.59 2.54 2.37 2.58 2.66 • Tb .59 . I) Jj .64 .62 .67 .59 .58 .83 Ho .62 . 3 J .67 .65 .71 .63 .63 .86 Yb 3.31 '•" ; 3.08 2.65 2.86 3.05 2.84 3.43 Lu .38 .30 .32 .31 .32 .35 .32 .38 higher Si02 /MgO ratio (average 9.75) the FeO/MgO, ratio (average 2.5) indicating an advanced stage of fractionation of primary magma compared to Deccan basalt and basalts of other regions. Norms have been calculated with the help of a computer programme in Fortran IV. Normative values are given in Tables 4.1.1 to 4.1.3. RJY basalts are hypersthene normative suggesting tholeiitic in character. Being saturated in silica do not allow the nepheline to appear in the norm. Normative quartz content of RJY basalts also indicate saturated tholeiitic nature- 4.2 DIFFERENTIATION INDEX Differentiation index is important to differentiate between the early, middle and late stage of fractionation with limiting values as 20, 30 and 50, respectively (Thornton and Tuttle, 1960). The average D.I. of RJY basalts is 30j+2 suggesting middle to late stage fractination which is further substantiated by D.I. versus major oxides (Fig.4.1). Differe ntiation index plotted against Si02 confirms saturated nature of RJ\ basalts (Fig. 4.2). The D.I. values are given in Tables- 4.1.1 to 4.1.3. The D.I. values of the RJY basalts compare well with that of the Deccan basalts (Ghosh, 1976). v ''' 4.3 SOLIDIFICATION INDEX • T^555 Solidification Index (S.I.) is a better index to use in variation diagrams than Si02, which changes steadily throughout fractionation. The 7P T102 5 . «o%Y ^'0 ' Fe,03 '° oogS* f'^° 20 FeO to .•c'%^ MgO '° .»o"'4 "fee 20 CoO 10 »°^* 0 NQ^O '0 oc*=; ^00 0 K^O '° »k^ 10 20 30 40 50 60 70 80 D L Fig. 4 1 Plot cf differentiation index vs various oxides indicating middle to late stage origin of RJY basalts 79 ... ,, .. ,, —^_ 70 Ab Or OVERSATURATED 60 SlO; 22 50 i;^====^^^^ UNDI^SATJRATED 1 1 1 1 1 1 1 10 20 30 4 0 50 6 0 70 80 O.I. NORM (Q*Or*Ab) Fig.4j: Si02/norm (Q-Or + Ab) showing saturated trend for RJY basalts du S.I. value represents a measure of the amount of residual liquid relative to crystallized solid phase, as the magma fractionates. Primary magmas appear to have S.I. about 40 or little higher (Kuno, 1968). The RJY rocks with S.I, around 25 (i.e 60 % S.I. of primary basalt) would indicate middle stage fractionation. The S.I. values are given in Tables-4.1.1 to 4.1.3. Various oxide percentages plotted against S.I. (Fig.4.3) broadly show clustering of data points except FeO and Na20 which show negative trend while MgO and CaO show positive trend. The K20 and Ti02 values are independent of solidification index. 4.4 CORRELATION COEFFICIENTS The correlation coefficients (CC) allow the examination of overall compositional relationships of a rock and can range from +1 to -1. A CC value of +1 indicates sympathetic relationship, while a -1, indicate a perfect antipathic relationship. Values between +1 and -1 would therefore indicate the degree of correlation. Correlation coefficients for both Kateru and Pangi basalts are computed with the help of Correlation Coefficients Matrix programme in Fortran (Tables 4.2.1, 4.2.2 and 4.2.3). Elements with similar ionic radii and ionic charge substitute for each other in a crystal structure. Many major and trace element correlations can be explained by similar charge or similar size. This inturn, reflects the change in the bulk composition of the liquid which is either enriched or depleted in a few elements. The correlation coefficients for the RJY basalts indicate certain 81 O r) O -^ tN • • • --99 ^ O O fO CN • • • • -HO OO ^ o f^ ^^ n fo •-I O O O O g O "^ fO CM -H CM -^ d d o d d ^ n O vt ST SJ CN in VJ- '^999°99 a en cNi r-i ro O r^ ro 2°999°99 -^ O cncs<-mocM»3- pi ^•ooooooo ^ _^ CNCNOmOvC-* r~) • CT^ • • • • • • • ^•9d°9°9°99 d c cN oo-^oO"—'CO in — •O'l. OOQOOOO C5i ^ 9 ' ^ (j • ^—1 • fSj • • a • • • • C J ^O-C'OOOOOOO g t3 O i-""! rOvC-—itMCMcno O ^9^9999°9°9°° 1 O L'^ C*^ • CC • O^ • • • • • • • Q ^•9-'9d9d°999°99 6 — r^ .—irsimmono ' "sj ^N] '•^ • ;NJ • ^"^ • • • • • • • ^> cs c") CM o n m _ C^rC"^"'— --~~ •"" •L'~i»m_> • • o o 8 • O 0) - q: 9 c q; c c c - ; - 9c^ oco o cx. ^ 9" 9* 9 9' 9' - 9' - 9 9 9 9 ° 9 9 9 °' ^ -^ cp d d d d d 9 cp 9 d 9 d d cp 9 9 cp cp 9 P -^ Cp d cp* cp' cp' cp' Cp' d d d cp* d cp* 9 d d d d d d oo 82 in ' in ti: O 00^- oc c)5 C DC 9~ O (N m 'M ^ d r5 9d "99 9d t ocr^ "9 9 9 99 99 fe O r^ tn \C vC r^ L"" ^ d ^ -Q 9 9 99 vc in N d d 9 9' CM CN 9 9 n CM O CN CN ^ "9 d d 9° d d O t~si rsi 9° --^ d d d d in vD O r^ cN -^ CNl ^ 52 :j^ ^ O O — C^L'^^NJ— - O^'—i~3'^r^^-cNr-HrOLnncsisrr-~om Ln-^-, L^_ -^(£cSddcDdcSc6(DdddcScDCD OC^^^O-^ — ^J-OCNLOr^— CNCN.—l^Hr-HSj CNCMCN-- &&K • O -3 -' • • * — o o >- O f^ fO r~- -^ d o d c)5 O fO lA ^ fO ^ — o o o o E o < ro m . r^ LO CN O O f^ —• 0990099 z o O "-jfo-^orsi-^in _ —1OOOO990 O ^ 5 O-—I o.r^iOr^oor-^ ^ j^ • • ; iH • • • • • • * C^ J '-lO 0990099 r-^ d 9' d d 9* 9 9' 9* 9 9 cH o • ^^v^OLOvi^c^^^l-^ o •cMr-iO'—1>—ii.'~iLncr>crmiri o • [" 3 •• •• •• •• •• • OJ --^199099000090 i^ ~ -' o" d d 9" d d d 9 d d d d 9 d 9 9 ti ^x-- — — <3- — nvccNi^m^ Ororsir^Xr\i—,v0^^cNOQOOoo-d-0'-<00-*^ OJ -Jddd9*dd9dd9-^9d99dddddd ^ B4 SS so ZS 20 15 10 S 70 SS 30 2S 2C IS 10 5 0 1 9 1 £ Til 1 II • 60 10 o 50 «•P ? • 0 M" o 9 + •+ u • \ 8 7 16 4 ./ + i" 3 4.A 2 '/•'• /••• 1 0 • 0 8 o \ • 0. o o + 0 e • •• <» Z 5 • 0 + !• 0 4 1 + • • + •. \ 0 2 0 / 2 9 o o • + T '* • n 2 6 O + w 13 - • • + o 2 S + • * 12 0 + O /• 2 0 Fig. 4.3 Sclidification index various major oxides plot for RJY basalts 85 chemical characteristics. Si02 shows a positive correlation with Na20 and K20 and negative correlation with CaO and MgO. It is observed that there exists a positive correlation between Ni (olivene) and Cr (clinopyroxene) and Ti02 and FeO and beteen K and Sr and Ba. The absence of good cofrelations between any elements in RJY basalts does however preclude the possibility of any simple mechanism being responsible for the observed variation. Polybaric fractionation processes, crustal contamintion and irregulrity in the degree of enrichment in various phenocysts are all factors which might diminish good correlations. 4.5 THE MAGMA CLASSIFICATION The most commonly used method to distinguish magma types in basic igneous rocks on the basis of chemistry (Fig. 4.4) is the alkali-silica diagram (Kuno, 1957; Mecdonald and Katsura, 1964; Mecdonald, 1968; Irvine and Barakar,1971; Cox et al., 198 and Lebas et al., 1986). Majority of samples from RJY plot in tholeiitic field except, few samples of lower flow which lie in alkali field. In AFM diagram (Nockolds and Allen, 1954; Irvm and Barakar, 1971) also the RJY basalt samples plot distinctly in tholeiitic field (Fig. 4.5). In Fig.No 4.6 tholeiitic nature of RJY basalts is further confirmed by A1203 versus Na2o+K20 plot (Kuno, 1957) and FeO/ FeO + MgO vs Ti02 plot (Fig. 4.7). Miashiro and Shido (1974) suggested use of relatively less mobile elements and their elemental ratios to discriminate between various magma types and the RJY basalts accordingly fall in typical tholeiitic field (FeO/MgO vs Si02; FeO/MgO vs FeO; FeO/MgO vs Ti02; FeO/MgO vs V and Cr vs Si02; of Figs 4.8a, b, c; 4.9 and 4.10 86 ALKALI BASALTS r ^ c »o^ c ^ 5 - 0-0 , 4- • ^2 • • •. 2 / y THOLEIITES y /*; 1 1 1 J 1 t 1 1 1 i :. 1 1 .. 45 50 55 Si O2 % o P Top Flow -Kottru Area • P Top Flow - Pangidi Area + P, Bottom Flow - Pongidi Area FIG 4.4 Plot of Na20-K20 vs Si02 showing tholeiitic nature for RJY basalts 87 F«O^Ft,2^03 MgO MopO + KoO Fig, 4.5 AFM plot showing tholeiitic nature for RJY basalts- + Pangidi lower flow, O Kateru upper flow, O Pangidi upper flow 88 AljOj 15 - fig- -^-^ Na20+K20 vs AL203 plot showing tholeiitic nature 4 t- ^^ o ^^' o o . 25 36 45 55 65 75 85 lOO F«0 / F»0 MgO ( wt V. Vokjtite f ree ) Fig.4 plot of 100FeO*/FGO*+MgO versus Ti02 indicating rncleaitic trend 82c Si02 B I 5 <^0 13 3 ••• 11 • 9 ; 'x X F.O 7 - \ \ T1O2 \ 2 \ \ \ ' \ - \ 1 \ > I . JL . 1 . Ju. _.l _ I 2 S s c F«0/MgO Fig. 4.8 Variation of Si02, FeO and Ti02 with F eO/MgO (Miyashiro and Shido, 1974) for RJY basalts 90 vxo Fig. 4.9 Feo/MgO vs V ppm plot indicating tholeiitic nature 70 65 CALC. ALKALINE 60 SlO^'/c 55 50 *m 45 THOLEIITE 40 _i I I I L I I L_JL. • I J 200 100 90 SO 50 105 Cr(»p«) Fig. 4.]: Si02 vs log Cr plot showing tholeiitic nature of RJY basalts (Miyashiro and shido, 1974) 9: respectively)). Recently, Jensen (1976) also suggested use of less mobile major elements to discriminate the type of tholeiites and the RJY basalts show Fe-rich tholeiitic trend (Fig. 11). The plot A1203 versus normative plagioclase also suggest tholeiitic charecter for RJY basalts (Fig. 4. 12). The diagram has been used because it has the advantage of relative immunity to alkali loss as the later produces sympathetic changes in both plotted parameters which tend to shift samples semi-parallel to the line dividing the two fields (Bowen et al., 1986). Floyd and Winchester (1975) have also used immobile major and trace elements to discrimiante between tholeiite and alkaline fields. In the plot Ti02 as a function of Zr/P205 a few samples such as 4, 7, 8, 9, from lower flow and samples 4, 5, 7, 8, 9, 10, 12 and 15 from Pangidi upper flow and samples 16, 19, 20, 21, 22 and 26 from Kateru upper flow fall in Alkali field and rest of the samples fall in tholeiitic field (Fig. 4.13). This is due to the enrichment of incompatiable elements such as P205, Ti02 and Zr in the RJY basalts. 4.6 TRACE ELEMENTS Trace element data of the RJY basalts is scarce compared to the voluminous data produced on the Deccan basalts. In the present study trace element data has been used to understand various processes responsible for the evolution of RJY basalts. The compatiable and incompatiable element enrichment and variations are due to a number of factors such as degree of partial melting, crystal-liquid fractionation (both high and low pressures) 92 Fc+Ti Mg Fig. 4. i: Ternary plot of Al-Fe+Ti-Mg cation percentplot after (Jensen, 1976) showing distinct tholeiitic nature 93 18 - "^^-^^^ CALC-ALKALIC 16 - AljOjVe THOLEIITE ^^^-^ 14 -- •+ •• •* ^^--^ - o o 1 i I 1 1 1 .. 1 1 90 80 70 60 50 40 30 20 NORM. PLA6. COMPOSITION Fig. 4. 12 Normative plagioclase vs A1203 plot showing tholeiitic trend for RJY basalts ALKALI + °o • • + • o / • oV •.• -»- TiO; \ 2 - ••-V/ ^ W o + THOLEIITIC _L JL X X _L J_ _L J_ J {X02 ao4 0.06 ooe o.io 0.12 0.14 aie OB 0.2 Zr / P2O5 Fig.4.13 Zr/P205 versus Ti02 indicating alkali-tholeiitic transition for RJY basalts 94 and wall rock reaction, assuming a chemically homogenous or heterogenous (with regard to minor and trace elements) source compositon. (Krishnamurthy and Udas, 1981). In the fallowing sections the possible operations of the above mentioned processes are discussed. The strong partitioning charecter of trace elements between liquid and crystal makes them senitive indicator of the degree and mechanism of differentiation. Various elements such as Sc, V, Cr, Co, Ni, Cu, Ga, Sr, Ba, Y, Zr, Zn, and Rb are analysed for the present study and the results are presented in Tables(4.1.1 to 4.1.3). Nickel is considered to be very sensitive element to fractional crystallization in the early stages of differentiation. Ni enters in the early formed crystals (conipatiable with olivene) and upto middle stage of crystallization it is almost completely removed from the melt. The Ni contents range from 38 to 128 ppm. Cr is analogous to Ni. and their contents range 36 to 48 ppm in RJY basalts. Both NI and Cr decrease rapidly with increasing FeO/MgO (Miyashiro and Shido, 1975). They show perfect positive correlation (Table. 4 2.1 to 4.2.3). The Cobalt contents of RJY basalts show narrow range of variation between 35 to 45 ppm. The Co like Ni and Cr has preference for the octahedral coordination sites (Burnes and Fyfes, 1964). It enters the mineral along with Ni, but, in a lesser amount (Wager and Mitchell, 1951). The Rubidium concentration in RJY basalts is only in detectable limits. It ranges from 5 to 30 ppm. The Sr contents of RJY range between 192 to 322 ppm. The Rb vs Rb/Sr, K vs K/Sr, K vs K/Rb, relations show uniformly positive correlations (Plots are not shown). K/Rb and K/Sr ratios of these basalts have been used to estimate the depth of the wall rock reaction and 95 also an indicator of crustal contamination (Krishnamurthy and Udas, 1981). The Barium contents range from 218 to A92 ppm. The Ba and Sr values coupled with Ca values are systematically used to understand and estimate the partial melting percentages.(chapter 5, petrogenesis). The incompatiable elements such as Zr, Y are also used in various plots in order to understand the paleotectonic environment of eruption of the RJY basalts. These elements were used with greater confidence because of their high immobile nature. The Zr contents of the RJY basalts show a range of 120 to 247 ppm. The variation in Zr and other incompatiable contents of magma as it evolves by fractional crystallization, can be interpreted interms of the nature and proportion of crystallizing phases, a factor which is also directly related to tectonic setting during the magma genesis (Pearce and Norry, 1979). 4.6.1 Rare earth elements: All the samples of RJY basalts show more or less s.milar pattern of the rare earth elements (REE) with slight differences in their absolute enrichment (Figs. 4.14 and 4.15). The total REE contents of lower flow of Pangidi area, ranges from 142 to 160 ppm, averaging 145 ppm. On the other hand, the upper flow of Kateru and Pangidi areas show lower total REE contents which ranges from 95 to 136 ppm and 91 to 144 ppm, respectively, A significant feature that emerges from this study is that the higher REE in the lower flow as compared to the upper flows is consistent with the relative enrichment of total alkalis and the ratio between the LREE and the HREE (La/Lu). The REE ratios are given in table 4.3. 96 in CM en o o o r-- o o • X CO o r^ IN CO cn CJ•N ON — 30 o in St ON r^ X CM CM CO CO X —> C3N o^ ^ »-H in sr X • CN X O X CO —t 'St X X -1 . ON —1 CM ON CO rsi LO 30 30 :^ p~- >i; m • ^ r~~. r^ X ^ ro CN • co sT CN CM — C sr x' —1 CO in CN 1^ X o in r-~ —H r^ o o in CO OA St• CO C"J en -1 O • rs • O CN _( o 30 oo 0^ o X ON in • CO o r^ CO • CO o CO — • sr m X ^ X St ON CO CO in 00 CN r^ vX! ^^ . m X .—[ sr • ro CN CO T—* .—t CO CM CM X r^ • .—1 (J\• ^ X X —I > • • 03 in J3 r^ CM O v£) X —I X r~~ ,-1 ON • CO ^ —I CM C CO CO CM • CN n CO C3N • X '-< X n ,o o CO rs. QO sr CN r^ X ON X o O • m X r- 1—( CO• St• fs. CO CN ^H o CM CN o o co • -H X as ON X a CN CO ^H ON X in OS CO r^ • CN r-- CO CN CNl co ,—t • o . X •J- I 01 -J 3 J3 I 1> S T3 3 J3 0) S •O 3 J3 1—( CM 2 -J CN O CO Z O X Z J >- I ;3 < 3 H -a -yo o iJ CO u ^ 'jJ cB c!^ 37 3 -I 1—1 5 xa > 3 o rH U a D m IS) J- 1—1 m m o K) I XI GJ I JJ n o 1- c 4-1 i 3 J-l UJ c 0) E a; r—1 'D 0) (L E 4J (/) J:: 4- -p c 'a 0) 0) (I^! a; m •a 0^ z 1—1 m Ea •H ro f—1 K E a !u 0 •I->' C -U 9 0) 4-1 (J -p c u m TJ OJ c ^ o o a _l x; o o u -L_L _i I L I I I i_ O O O o €> O o o m ro CVJ I— BimoNOHO / nvsvg x: -U O D3 CO >- 01 ro ^ ^ •^ ^ ^ lO If) lO fO •H I I I cr> c CD CD 03 to OD O m CL O Q. Q. Q I (X 3 -I 3 3 _J ? t f o M-l w c i-l 0) 4-) •Ua c 01 B QJ 3 UJ j-J E QJ (A C w o o •D 03 E 0/ z o o c (U T3 4J C S-i c QJ a :s 0 a x: u in o « 'a- -1—1 I L__l_ o o o w o o o 3J.iaUN0H3 / )IDOU yy REE variations have been systematically related to petrochemistry of different basaltic types (Shimizu and Aruches, 1975). REE differences between MORB tholeiites and continental tholeiities have been related to distinct thermal regimes (Schilling, 1971). The MORB patterns show no signs of garnet as a residual phase in their source regions and accordingly pastulated to be derived from the shallow depths in the mantle. In contrast, the OIB, with heavy depleted/light enriched patterns as in continental basalts bear the signature of equilibration with garnet as a residual phase in the zone patial melting. Thompson (1980) also observes a close similarity between OIB and continental basalts. The Galopogas Island basalts and Cape Gamaroon basalts show similar enrichment patterns as that of RJY basalt except in their absolute enrichment. Various processes may enrich the LREE in these basalts such as: 1) Very small amounts of partial melting of fertile mantle (Thompson et al. 1984). 2) Partial melting of metasomatically enriched mantle (Le Roex et al, 1982) associated with mantle plumes and 3) Wall rock reaction. The possibility of small degrees of parial melting has been ruled out in the case of RJY tholeiite basalts because tholeiiteS are produced by higher degrees of partial melting only (Ringwood, 1974; O'Hara, 1968). It has been estimated that around 30 jf5 % of partial melting of upper mantle source which produced parental basalts of RJY basalts. The possibility of crustal contamination was discussed in detail in chapter 5, and it has been xOu concluded that contamiantion of crust did not play a major role in changing the chemistry of these rocks. The only alternative could be partial melting of metasomatically enriched mantle. 4.7 ALTERATIONS Different processes may alter the rocks to variable degree and accordingly produce variations in major and trace element distrubution (Mecdonald, 1980). In order to use geochemical data for proper petrogenetic interpretation it is neccessary to know the extent to which the data is reliable. Many schemes have been suggested in recent years to discriminate the geochemical effect of such alteration processes, such as sea-water interaction, regional raetaraorphism and simple deuteric alterations. The petrographic studies of RJY basalts reveals that the glassy matrix in these basalts is commonly altered to palagonite and chlorophaeite and the clino-pyroxenes to chlorite with release of Ca produceing raicro- veinlets of calcite. Peacock and Fuller (1928) and Peacock (1930) suggest that the chlorite is produced by deuteric alteration of ferromagnesian minerals and glass. Chemically, clinopyroxene to chlorite transformation should involve release of CaO (Fawcett, 1965) which in case of RJY basalts is evident from the presence of free calcite. The samples of the RJY basalts were plotted in ternary diagram CaO/A1203-MgO/10-Si02/100 (Daneis et al., 1978). It has been obsereved that out of 5A samples analysed most of the samples are unaltered except for Sample Nos. 4, 5, 6, 9, 21 from Kateru and Sample Nos. 5, 6, 7, 8 (upper flow) and 4, 8, 11 (lower flow) from Pangidi which fall in altered field lOi (Fig.4.16). In another diagram H20 versus Na20 majority of the samples fall in fresh to altered basalt field (Fig. 4.17). As the H20 and Na20 values of the samples are not disturbed it indicates that the alteration, if any, did not mobilise Na. However, Ca in some of the samples has been mobilised as seen in H20 versus CaO diagram (Fig.4.18). This may be due to the altera tion of pyroxenes to chlorite. In view of this the caution has been taken to restrict the use of CaO for petrogenetic interpretations. A recent approach, which has become useful in recognizing elemental mobility due to alteration in volcanic rocks is the use of molecular proportion ratio (MPR) (Beswick, 1982; Barnes, 1985). Based on the fact, the relaibility of data can be checked by the distribution of whole rock compositional trends that are consistent with primary magmatic processes. The molecular proportion of Si02 and FM (FM = FeO + MgO, where FeO is expressed as total iron) normalised to oxides incompatible with olivine (such as Ti02, A1203, CaO, Zr, V, Y) are plotted against each other (Fig.4.19). The sample lie along the trend of clino-pyroxene having a slope of 2:1 which imply that the observed compositional variation in RJY basalts is consistent with primary magmatic process such as fractionation or melting of Cpx. It is therefore, apparent that subsequent secondary processes such as metamorphism (if any) and sea-water interaction have not noticebly altered the chemistry of the samples. From the above investigation, Si02, MgO, FeO , A1203, Ti02, Na20, Ni, Cr, Zr and Y values are in reasonable confidence. K20, CaO, Rb, Sr, and Ba values are to be used with caution. 102 CoO/ AljOj •«- P-1 Lower Flow o P-2 Top Flow • K-2 Top Flow MgO/ 10 SIO2/IOO Fig.4.16 A ternary plot to recognise alteration on the basis of major elements (Schweitzer and Kroner, 1986) \ 103 \ \ METAMORP^HOSEOR I D BASALTS. \ \ \ H20% \ \ A tf» \ / 2 - I . • / I • / \ ALTERED AND I FRESH BASALTS 10 Na20% Fig.4.17 Na20 vs H20 plot showing fresh to altered nature of RJY basalts lOr \ \ METAMORPHOSED \ \ BASALTS \ \ \ H,0% \ \ \ / \ I WEATHERED BASALTS + FRESH BASALTS \ / _L _L ^ I 2 4 6 8 10 12 14 C a 0 w^ % Fig. 4.18 H20 vs CaO variation plot for RJY basalts indicating slightly metamophosed nature, fields are drawn for MORE samples (Melson and Van Andel,1966; Bonatti et al,1971) 1C4 N 0.8 80 120 02 0 4 0 6 08 10 1.2 Tm/ TiO ?tn/2r 0 5 5 - 06 Fm/CaO Fm / Y Fig. 4.19 Molecjlar Proportion Ratio (MPR) of major oxideS/ traces vs FM (FeD+MgO, where FeO is Total). Note the samples lie along the clinopyroxene trend 105 180 r -"" ~>N ^*""'* \ / .\ 160 - / 1 Z 4-J.+ / • 1 ^^ /' • ^^ ^ 140 / \ X / + • / Z REE + o V '-' ^ r. y > ^ •• 120 - ^^^/C-^ O y/ • / o* o# +° 100 - 80 ! 1 1 1 1 1 26 28 30 32 34 36 D.I Fig. 4.20 Differentiation Index vs total REE ir.diraring altered and unaltered nature of RJY basalts. Tcntinuous line indicate field of unaltered basalts; Broken line field indicate altered nature Iu6 4.7. REE Mobility Alteration effects on major and trace elements of RJY basalts have been discussed in the earlier part of this chapter. Here, the effects of weathering on the mobility of the REE are being discussed. Such effects together with those of the low grade metamorphism and sea water-rock interaction have been well documented (Condie et al., 1977; Hellmann.l97S Mengies, 1979 The deviation (relative enrichment) in the total REE contents of some of the RJY basalts (Sample Nos.'2, 3, 7, 8, 9 and 13 From lower flow and Nos. 3, 13 from upper flow of Pangidi and 13, 26, 27 from Kateru ) from the straight line approximation (SLA) in D.I. vs total REE diagram (Fig.4.20) seems to be due to alteration. The wide variations in La/Lu ratios of these samples also indicate the LREE mobility. The LREE enrichment in these samples may be due to micro-vesicles containing calcite and chlorite material in the samples analysed^ Elemental mobility is often enhanced by ves-icularity (Smith, 1976). Many workers (Humphris et al., 1978) contributed seemingly hapahazard variations of REE enrichment processes. They have postulated that the amount of glass with its REE contents, the position and nature of the glass (Valence, 1969; Humphris et al., 1978) will have a dominent control on REE mobility in basaltic lavas. Variation in these factors in conjugation with the sensitivity of adsorption to PR (Roaldset, 1973, 1975) also enhance the mobility. Genarally La, Ce, Eu, Yb shows preferential mobility (Hellman, 1979).In RJY basalts Ce and Yb shows selective depletion and enrichment trends respectively where as Eu enrichments are doubtful. The selective enrichment iu7 of Yb (Figs.4.14 and 4.15) in RJY basalts might have resulted due to the break down of clinopyroxene (Robertson et al., 1976). 4.7. . Ce anomaly The study of REE patterns of RJY shows a marked Ce-depletion trends for all the samples but relatively few samples No 11 and 12 from Kateru flow and samples 54 and 56 from Pangidi upper flow and sample 36 from lower flow, all collected from upper part of the flow show significant negative Ce-anomalies (after considering the instrumental errors) in comparision to others samples taken from middle and lower parts of the flow. The possible explanations for the Ce-anomaly in the igneous rocks are by (i) preferential partitioning of Ce into a crystal phase rather than in residual melt, (ii) Inheritence of the anomaly from the source, and (iii) alteration. It is well known fact that Europium exists in either +2 or +3 vallency state and that the valency state is a function of temperature and oxygen fugacity in the magma during the crystallization (Weill and Drake, 1973). However, no such study on the behaviour of the Ce+4 does exist (Shribel et al., 1980). Besides no primary igneous mineral phase »ith sufficient positive Ce in basaltic system, by its fractionation can IrTipart negative Ce to the rocks. Ce anomaly is reported from island arc lavas (Masuda&Nagasawa, 1976; Taylor, 1968) and pillow lavas from Ocean floor (Ludden and Thompson, 1978). The continental rift environment of RJY basalts differ from the above said oceanic island arc conditions and in turn is likely to differ in iOo source characteristics. Ce anomaly has not been reported from the rift regimes. From the above inferences it would not be preferable to attribute the Ce-anomaly to the source characteristics either. The mobility of the LREE either during the hydrothermal alteration and in zeolite facies was thoroughly discussed by Menzies (1977) and Humphris et al,.(1978). In this studies it was generally argued that the lanthenides move as a group, but were not internally fractionated from one another (staudigel and Hart, 1983). But preferential mobility of Ce can't be explained by this process. Leaching of sea water circulating through basaltic rocks has been demonstrated by earlier workers (Masuda and Nagasawa, 1976; and Ludden and Thompson, 1978). Sea water in general has a negative Ce- anomaly due to its removal and incorporation in Mn-nodules (Frayer, 1977; Buta Manard and Chesselet, 1979, and Goldberg, 1963). Under oxidizing conditions Ce +3 is converted to Ce +4 state and gets removed thus causing a depletion in the source (De Baar et al., 1985). The observed Ce anomalies can be attributed to the seawater-rock interaction. Marine transgressions took place in RJY regions in past. The existence of shallow marine conditions are evident from fauna in the Infra-trappeans and Inter-trappeans of Pangidi areas of Rajahmundry (Bhalla, 1967; Shastry, 1981). Along with the three main mineral phases in RJY basalts there is subordinate amount of interstitial glass which is altered to palagonite and ferromagnesian minerals like pyroxenes were also altered to such chlorotoid minerals. REE contents are rich in glasses because all the early phases in i^ these basalts havt partition coefficients less than one. In addition the glass will have high HREE/LREE ratio. Of the two main mineral phases namely plagioclases and pyoxenes - only augites have the highest partition coefficients averaging about 0.7 HREE and 0.3 for LREE. So LREE tend to remain in the melt (glass). Hence due to alteration of interstial glass there will be mobilization of REE and preferential mobilization of LREE (Humphris et al., 1978; Frey et al., 1974; Staudigel et al., 1979; Staudigel and Hart, 1983). The less prominent anomaly observed for the samples 11, 12 from Kateru and samples 54 and 36 from upper and lower flows of Pangidi, would have been due to limited time of rock exposure to the sea water i.e., less water rock ratio due to the shallow and fluctuating marine conditions (Menzies, 1977). It is also evident from the above discussion that the marine transgressions are post-dating the eruption and effected only the upper part of the flow through perculation and interaction. Elemental mobility is often enhanced by vescicularity (Smith, 1976). One possibility would be simple weathering (Nesbitt, 1979) for the observed Ce anomalies but one should get the complimentary enrichment trends of cerium in weathered products which are obviously missing in the RJY region (assuming that weathering feeble and weathered and residual products are confined to the weathering profile). Hence the observed Ce-anomaly in the RJY basalts can be attributed to the sea-water rock interaction only. iiu 4.8 TECTONIC ENVIRONMENT Several attempts have been made to classify and discriminate between tectonic environment of magma eruption using major and trace elements (Pearce and Cann, 1973; Floyd and Winchester, 1975; Pearce et al. 1975; Wood et al., 1979; Muller, 1983). The RJY basalt flow characteristics and the field relations display a clear continental setting but it is customary to prove such setting with the help of discrimination diagrams which can differntiate between various volcanic suites. Various plots such as (Si/Al, Fe/Mg, Ti/Fe) after Hynes and Gee (1986) indicate typical continental nature (Fig.4.21). In a binary plot (Ti02/Zr) and ternary plot (Ti/lOO-Y/3-Zr) of Pearce and Cann (1973), most of the RJY basalt samples plot in the within plate basalt field (Fig. 4.22 and 4.23). Same feature is recorded in a Zr vs Zr/Y (Fig. 4.24)_ Although within plate continental nature of the RJY basalt is evident from the above plots, however, an ambiguous tectonic setting is suggested from 1000 Ti ppm vs V ppm plot. (Fig.4.25). Ocean island affinity (Figs, 4.26 and 4.27) is suggested from Ti02 vs P205 and Si02 vs Ti02 plots. In a plot of discriminant functions Fl against F2 for clinopyroxene analysis (Fig. 4.28), RJY basalts fall in within plate basalt to ocean floor basalt transition field. This is further confirmed by Ti02-K20-P205 diagram (Fig. 4.29) of Pearce et al., (1975). The oceanic affinity displayed by otherwise continental basalt suites may be a result of an "abortive" attempt to generate new oceanic floor suggesting intial rift origin for RJY basalts (Pearce et al., 1975). The rift environment for RJY basalts is further substantiated by K20/K20+Na20 vs Ti02/P205 plot (Chandrashekaram and Ill 22 20 - sv^ 18 16 - "X \'^^^„ \ 14 S - 12 1 1 i 40 45 50 55 12 Si v.—*- Fe O Vo Ti •/. 3 Or 2 5 - / 2.0 - 1 ^+ o I 5- 1.0 L o 0.5 - f 1 1 1 t ..1.1 1 J Fig. 4.21 Plots of a) Al-Si, b) Fe-Mg, c) Fe-Ti for RJY basalts highlighting its typical tholeiitic nature. Field boundaries demarked by : solid line -MORB; dashed line - continental tholeiites. Modified after (Hyr.es and Gee. ,1982) ; z X X \ / \ \ / \ 4;! VC WPB / / / •- ' o I t- / / / / / \ / / / \ 0 S " / \ \ lAB \ 0 UPPER FLCM-1 KATERU ^ \ • UPPER fLOfc-l PANGIDI > 1 + LOWERFLWV-2 PANGIDI 1 1 1 1 10 too 330 300 lOOC Z f (ppm ) Fia „^ vs Ti02 percent plot shcui'-g .n plate basalt setting 113 Ti/100 Zr Y/3 Fig. 4.23 A ternary plot Ti/lOO-Y/3-Zr showing within plate basalt setting for RJY basalts (Pearce and cann.,1973) 114 Zr/Y _L J l_J to 100 too SCO 400 SOO Z r { p pm ) Fig. 4.24 Zr ppm vs Zr/Y plot indicating within-plate basalt trend for RJY basalts •00 TOO - »00 - 500 «00 SOO 200 - 1O0 - fig- 4.25 Ti ppm/ 1000 vs V ppm plot showing ambiguous tectonic setting for RJY basalts. Samples plot outside the field of MORB and OIB-CFB fields 0 8 115 0 6 P,0, 04 4^ • \ OCEANIC ISLAND / BASALT / Co • rf^ t ^ /^. 0 2 7^ OCEANIC RIDGE , / BASALT _aJ —e:J J I 2 3 Ti02 % Fig. 4.26 P205 vs Ti02 plot showing oceanic island basalt trend for RJY basalts A- Octanic islond alkali basalt. 5 - B- Floor thoieiite C Calc alkaline basalt t • / A \ TiOz 3^- / i,| \ - '^ f^r ^^^^ a J (^ B "'V^ 1 - C I 1 1 1 N1 1 i 1 1 X 40 48 52 5€ 60 64 68 72 76 SiOt rig. 4.27 Si02 vs Ti02 plot indicating oceanic island oceah floor transition nature for RJY basalts tl6 23 -r O Katereu upp iiu» O Pongidi upp flow 4- Pangidi lower flow Pia4.28piot of disrimination functions Fl againest F2 fcr clinopyroxene analysis of RJY basalts Fl = (0.0026 A12O3-0.0087 MgO) - (0.012 SiO2-0.0807 TiO2+0.0012FeO*-0.0026 MnO +0.0128 CaCH-0.0419 Na20) . Fe = (0.0085 CaOO.016 Na20) - (0.0469 SiO2+0.0818 TiO2+0.0212 Al2O3-^0.0041 FeO* + 0.1435 MnO0,0025 MgO). Fields are from Nisbet and Pearce (1977) in CONTNENTAL THOLEIITES K5O P2 05 Fig. 4.29 ri02 - F2 % ~**^2 *^ Diogrom (TH Pearce ef al , 1975) Discr.mmotmg Between Continental and Oceonic (Rift) Regimes Most of the RJY Somples Plot in Oceonic ThoJeiitic Field Suggesting Initial Rift Origin 0 7 0 6 0 5 o o 0 4 + 0 3 o 0 2 o 0 I Fig. 4.30 Plot showing rift volcanic nature for RJY basalts. Fields are after (Chandrashekaram and Parthasarathv., 19 ) lie Parthasarathy, 1979) in Fig. 4.30. Major problem still involved in the separation of within plate basalts from the continental and oceanic environments (Pearce and Cann, 1973; Floyd and Winchester, 1975; Holm, 1982). They contend that, although the bulk partition coefficients for the elements in the discrimination diagram vary considerably, expressing the different behaviour of the elements in the melting process, most diagrams are binary or ternary and accordingly partial features of melting geochemistry are "pictured" in a single diagram. In the construction of a reliable discrimination daigram, it is now clear that multi-element (Spider) diagrams must be considered (Saunder et al, 1979; Sun et al., 1979; Wood et al., 1979). Using upto 15 hygromagraato- phyile (incompatable) elements, with bulk partition coefficients<1, varia tions in the elemental behaviour closely reflect the tectono-magmatic regime (Wood et al., 1979). The RJY basalt (Fig. A.31) resembles an open inverted U, compared to basalts of other tectonic settings. The negative slope for the segment Sm-Y should be noted as those characteristics important in distinguishing initial rift tholeiites from continental tholeiites. 4.9 COMPARISION Comparatively, one would find broad similarities of raineralogical and chemical features for the time equivalent RJY basalts and Deccan basalts. k comparision of the average values of RJJ basalts with those of average tholeiites of Deccan and other basaltic provinces are given in (Table 4. tl9 o o S 5 ^i. •2 c « »- < ^ 2 t> c . >- hr W ••"* O C h- **- _ O «Q- P.s ^—' o O - \^ (/> 00 e "lA5 0) o>. D > >»- E CD o E • w x: = i/> " — o QL £ 5 5 o o - _J c ^ o o c w o c o •- O o QC _ Q. ~" • ic -i «/> D ' • I 1 1—I T T- -r- I I 1—I—I—r——I 1— O u. 120 ^0 S332: basalts (KrisnaT^jrtr/ , Pawa;:arr ' Tiwar: . ]••'"1 KJ::- Kr.sh'.a-. :-•-_. >'- laatpjr, ',B:>6'_£ e: a:..l^bc Bort«a\ basalts • sjthesvala aria ?clder\-aart Me^ai-aleswar basalts (Na3a:. et al. , 196: RT; Dasalts - Present stJT/ 9C ciearees ridae basalts s-te 21- , Itiompstr et al. / 19~z 9'1 de::rees ridge oasalt s^te 114 Itiaopsor et al., 15~2 !•. . rej-:or basalts {Upto" aro WadswDrth, 15" Fig. 4.32 Plot showing enrichment of FeO (as total Iron) in the NE Indian ocean and the Southern region of Deccan plateau compared to Notrhern regions. Note the Iron enrichment in RJY basalts also 1/i 4). A distinct feature of RJY and Deccan tholeiitic basalts when compared to average continental tholeiites is that the former are significantly enriched in Ti02 and total FeO and Fe203. This higher content of iron and titania of Deccan basalts suggest higher degree of fractionation prior to extrusion. In general the northern part of the Indian ocean character- stically has elevated Iron levels in basalts (Kashintsev,1981). Fig. No A.32 depicts this trend of Iron enrichment in the southern part of the Peninsula, (including RJY region) and nothern part of the Indian ocean. There is a fair amount of gradation between the alkali and tholeiitic basalts for the Deccan provinance as a whole although samples from individual areas have discrete groupings or show transitional features (for example Dhanduka bore hole flows). Such discrete tholeiitic nature is observed in RJY basalts without any transition. There is marked contradiction for field and chemical evidences for Deccan basalts (Holm, 1985). Deccan basalts were classically regarded as typical continental tholeiites (Carmichael et al., 1974). Recently, however, this view has changed as.additional chemical data revealed their marked oceanic affinity (Pearce, 1975; Chandrashekaram and Parthasarathy, 1978). The RJY basalts also show this nature. Ill CO •H cn (U CO CO CM 00 CN in oD • « • in CO i-H CO (X) •—I -J- M 8) s ^ • \0• -^» 1—I• C3^• o• 8 \^ ;^ d CD vO T-H in '>o o m r-i CN I •H a i-i •oH LO i-l 1 1-4 4-J 4-) CO in CO CM r-- .—I « S vO 1—1 S3- o CN • • CO CO CM in 21 J 3 i 5 1—I CN vD vO cxj U-1 d d o \D in CN CJ^ in CN CN ^ a^ in • in 5 CO CM sf CN CM 5 <^ • _ en 14-1 —t o m C30 Cd fQ CM CO g r-- CO O in C3^ CO . • \oCN (XDi) r—1 CM 1 r^ n CN CO in in CO CO S ^ S 2 5 • r^ GC3 CM 0 in •J CM CN CN CT> O vD O " • • -=. CM O O CM tn CO CO vO in 1—I C3^ r^ CO Q CO • • ^. i5 S 5 ^ En -w 1—1 CO o^ in CT' CN o c CO -J c c 0) 8 R S a CO 3gssiJ§is§ -4 r^ CHAPTER 5 PETROGENESIS It is important to arrive at an agreement about the nature of the RJY basalts before discussing the petrogenisis in detail. 5.1 PRIMARY NATURE Earlier, workers like Sukheshwala and Poldervaart (1958) and Subba Rao (1965) were of the view that the tholeiites are parental to picritic basalts. But, the growing evidences strongly support that the tholeiites are not primary in nature but have undergone extensive differentiation from more basic parental melt i.e., picrites which in turn are derived from the upper mantle peridotitic source with extensive partial melting (Green and Ringwood 1967; Cox, 1980). The conclusion, that the commonly erupted basaltic rocks are derived from picritic magma is based on the following observations. 1. There is geophysical evidence for the existence of significant magma chambers under currently actiye valcanoes, mid-oceanic ridges i.e., chambers in which crystal fractionation can take place. 2. Low pressure phenocryst assemblages such as, pyroxenes, plagioclases are prevalent in common basalts, indicating the liquid compositions are equilibriated to low pressure crustal conditions rather than that of high pressure mantle conditions ( O'Hara et al., 1975 ). LZH 3. From the experimental petrology studies it is known that the tholeiites with even 9-10 percent MgO are not in equilibrium with olivene at mantle pressures (O'Hara et ai-^ 1975). The commonly accepted values of Mg number for mantle parental melts is 70 to 72, according to the report on "Basaltic volcanisra project" (1981). The RJY basalts vith low Mg-number ranging from 38 to 47 and having low values of Ni, Cr, CO therefore indicate their derivation from more basic parental melts with Mg number higher than 60 as suggested by Cox (1980). 5.2 PARTIAL MELTING 5.2.1 Sr/Ca-Ba/Ca (SB) systematics. The parental melts of RJY tholeiites have undergone large degrees of partial melting before they reached crustal magmatic chambers and were subjected to extensive differentiation-fractionation processes during their evolutionry trend. Inorder to estimate partial melting percentages, SB systamatics as suggested by Onuma et al.(1968) and Matsui (1977) diagram has been used. Of the three elements, Ca Sr and Ba, the large cation Ca is contained mainly in clinopyroxene, while the other cations Sr and Ba do not enter ipto the three major phases (olivene, orthopyroxene and clinopyroxene) and in the two minor phases (garnet and spinel) by crystal structure control in the mantle conditions. When the mantle materials are gradually heated, small degrees of partial melting produces strong 123 enrichment of Sr and Ba in the melt and therefore, Sr/Ca and Ba/Ca ratios of the melt are the largest one. With increasing degrees of partial melting, Sr and Ba contents of the early melt is diluted with further melting of the Sr and Ba-poor mineral phases. At a stage, when garnet and clinopyroxene begin to melt, Ca content of the melt increases with addition of Ca from them and the Sr/Ca and Ba/Ca ratios of the melt accordingly decreases with a constant Sr/Ba ratio. Orthopyroxene having small amounts of Ca and lack of Sr and Ba hardly changes Sr/Ca and Ba/Ca ratios of the melts with a still constant Sr/Ba ratio. The melt evolved by complete melting of the mantle materials has the same Sr/Ca and Ba/Ca ratios as in the mantle materials. Therefore, a series of melts derived directly from the mantle materials with different degree of partial melting will follow a Sr/Ca-Ba/Ca evolutionary line with a slope of 45 through the mantle material as shown in Fig (5.1) At any stage of partial melting, if the melt starts crystal fractination it will follow the following trend. The fractinating clinopyroxene will preferentially take Ca and exclude Sr and Ba thus increasing Sr/Ca and Ba/Ca ratio. (Onuma et al., 1968; Matsui et al., 1977). But the increase in the Ba/Ca is certainly much greater than the Sr/Ca in the melt. In the case of plagioclase fractionation Sr/Ca ratio in the melt does not change, while Ba/Ca ratio increases greatly, This is because it takes both Ca and Sr with similar degree and excludes Ba by the crystal structure control (Higuchi and Nagasawa, 1969; Mastui et al., (1977). Thus, a series of melts via clinopyroxene and plagioclase fractionation make another SB systematics with a gentler slope through the primary magma as shown in Fig (5.1). It should be noted, however, that 10 V 1% Primary Melt -2 (Smoller Degree) 10 o O (/) Crystol Froctionation Line -3 10 \<^ 10 10^ i6' Bo/ Co Fig.5.1 Ba/Ca vs Sr/Ca systamatics plot showing partial melting percentages for RJY basalts. The slope corresponds to nearly constant Sr/Ba ratio with variable Ca in the plot. After (Qnuma and Montoya, 1982) + Pangidi lower flow; o Pangidi upper flow and o Kateru upper flow ill olivene, orthopyroxene and magnetite fractionation process do not change both Sr/Ca and Ba/Ca ratios, since these minerals do not accept the triad (Ca,Sr and Ba). But the SB systematica does not rule out the possibility of olivene fractionation in the primary magaraa during its ascent. A rough estimate of 30 to 35 percent of partial melting can be attributed for the derivation of parental melts of Rajhamundry basalts. The SB systamatics also indicates that plagioclase and pyroxene are the possible fractionating phases in the evolution of RJY basalts. However, the effect of secondary alteration on these elements Ca, Sr and Ba are considered before drawing conclusions. It is observed that the alteration in RJY basalt is limited and do not influence the results above. The results were also cross checked using the eqations proposed by Chen-Hsia-Chen,(1988). 5.2.2 Partial melting estimations (Chan-Hsia-Chen, 1988) Equation 1 FQ- (Na^O + KpO)s_o_0,^g 0.957 (Na20 + K20)m 0.9348 (Al203/Si02)s/(Al203/Si02)nT-0.1532 Equation 2 Fc= ; O.I08(Al203/Si02)s / (Al203/Si02)n,+ 0.597 The subscripts 'S' and 'm' represent source and supposedly primary magma or melt respectively. A correct estimate or choice of i2a source composition and composition of primary magma are important for the calculation. Direct estimation of partial melt percentages are not possible because parental magma compositons are unknown in the present study area. However, with the help of mass balance equation, the supposidly parental magma (m) composition are calculated, by adding dunite compositions to the average RJY basalts compositions in various proportions. The estimated compositions are similar to that of picritic basalt compositions, which are supposed to be parental to RJY basalts. The estimated partial melting percentages thus calculated for Equation No.l and 2 of Chao-Hsia-Chen (1988) range from 28.7 to 32.9 and 31.7 to 34.6 respectively (Table. 5.1). These estimates are in agreement with the estimates using SB systematics. It can also be suggested from this, that major element Ca in RJY basalts has not been removed out of the system during the alteration of pyroxene to chlorite and that the alterations has effected only on minor scale. 5.3 FRACTIONAL CRYSTALLIZATION Extensive fractional crystallization of gabbroic material is well documented for Deccan basalts (Sen, 1988). The model would also hold good in the case of RJY basalts also. The inset of Fig. (5.2a) shows how the overall trend of RJY basalts is best explained by gabbro fractination. The encloser 01-Cpx-Pl represents plausible compositional ranges of the fractionating gabbroic assemblages. 5.3.1 Oxide-oxide plots (Fig 5.2 a,b,c and d) of RJY tholeiites are t20 TABLE 5.1 Estimation of degree of partial melting by equations proposed by CHAO-HSIA CHEN (1988). Compositions used in the calculation of parental melt.(in mole percent) Basalt(Average IXinite Plagioclase(RJY of 20 samples) Microprobe data) Si02 .4937 ,3829 .505 Ti02 .023"^ .0009 .0004 A1203 .1272 .0182 .31 Fe203 .0385 .0319 .0 FeO .1006 .0938 .0051 MnO .0014 .0071 .0 MgO .0536 .3794 .0027 CaO .0996 .0101 .152 Na20 .0283 .0020 .0311 K20 .0063 .0008 .0001 P205 .0025 .0020 .0 Proportions of Basalt + Dunite + Plagioclase Partial melting percentages with Percentages Equation I Equation III 62.0 + 38.0 + 0 30.0 % 33.72 % 64.0 + 36.0 + 0 30.4 Z 34.24 % 64.5 + 35.5 + 0 28.7 % 34.37 % 64.9 + 35.5 + 0 29.13 % 34.40 % 55.0 + 45.0 + 0 32.85 % 31.67 % 63.0 + 37.0 + 0 34.3 % 34.0 % 62.0 + 37.0 + .009 29.3 % 34.4 % 62.4 + 37.5 + .1 29.34 % 33.91 % 65.9 + 35.0 + .1 29.7 % 34.63 % 62.5 + 37.4 + .1 29.3 % 33.93 % 54.0 + 45.0 + 1.0 32.92 % 32.31 % 20 KOttru Upp«r Row Oeccon basalt field Pongidi UPPtr Flow 10 Pongidi CoO 10 - .': Lo«tr Flo« •^^ CoO '^KJy^^^ 30 9- -o Refer* 1o ^ pressures ir Kb (A) I- t 15 2o CoO 10 - F«0 Fig. 5.2 Oxide - oxide plot (Wt%) of RJY basalts compared with the near solidus trends of the rrjlricomponent system HK-66-KRB studied bv Takshashi and Kushiro (1983) 131 2C (D) p30 I p25 10 FIELD OF .tj^ DECAAN BASALTS FIELD OF RJY BASALTS FeO iSl compared with the near solidus trend of the multicomponent system MK -66 - KRB studied by Takashahi and Kushiro (1983). Fields of western and central Deccan basalts are given for coraparision in the plot (Sen, 1988). In these plots, the RJY samples plot away from the low pressure kink region 'in these plots suggesting more evolved character at low pressures than the Deccan basalts. 5.3.2 Various incompatiable elements such as Ti02 , K20, show enrichments with increasing differentiation (decreasing Mg-Number) suggesting fractional crystallization to be the dominant process in the evolution of RJY basalts (Fig. 5.3). These basalts show a strong enriched LREE distrubution patterns, suggesting garnet to be the principle residual phase in the source. Positive correlation between total REE and Si02 (Fig.5.4) suggests the role played by fractional crystallization mechanism for these basalts. REE contents also increases with the increase of Ti02 and P205 contents (Fig. 5.5) in these rocks. Titanium is present in pyroxenes. Therefore the contents of pyroxenes have great influence on the concentration of REE in RJY basalts. Si02 shows positive correlation with Na20 and K20 and negative correlation with CaO and MgO. This is charecteristic of clinopyroxene fractionation from the melt and this further supported by positive correlaton of Ni and Cr and positive correlation of Cr and Sc against Mg number also indicate fractionation of clinopyroxene. Consistency in the normlised (Ce/Yb) and HREE abundances are clear indicators of pyroxene fractionation (Kay et al., 1979; Saunders, 1984). RJY basalts do not show any Europiam anamolies . 5.3.3 Increase of A1203 wt percent with increasing fractionation UJ (decreasing Mg number) suggest plagioclase fractionation (Fig.5.6). The plagioclase fractionation is further substatinated in the plot of Mg-nuraber versus Sr ppm (Fig. 5.7). Sr is notably partitioned into plagioclase rather than other phases. Buffering of Sr to almost a constant value is absolutely typical of gabbro fractionation because the partitiion coefficents (plagioclase/liquid) is about 2 and since half the extract is typically plagioclase the bulk partition coefficent is about one (Cox, 1980; Cox and Howkesworth 1985). Though Sr is considered to be partially effected by secondary mobilization, its conspicuous negative anamoly in the spidergrams (Fig.5.8) would strongly reflect the plagioclase fractionation. Absence of Europium negative anamoly in RJY basalts alone does't rule out the possibility of plagioclase fractionation. Generally continental basalts does not show Eu anomalies (Culler and Graph., 1984; Cox et al., 1980) 5.3.4 In A1203/Ti02 versus Ti ppm plot (Fig.5.9) after Pearce and Flower (1977), vectors for 50 percent fractionation are drawn. RJY samples show fractionation of pyroxenes dominanting over plagioclase phase. Since olivene, orthopyroxene and plagioclase have very low Kd values for Ti, it should be effected mainly by the fractionation of clinopyroxene (Pearce and Norry, 1979). 5.3.5 Al-Si Cation Plot: An approximate melt field in the Al-Si space that could coexist with pyrolite-3 is shown with contours for different pressures, temperatures and percentages of melting (Francis et al., 1983). The majority of the samples fall in 1250°- 1300° C temperature grid. However, two samples show 1250 C temperature. The plot also suggests negative relationship between Al and Si indicating plagioclase Fig. 5.3 Mg nuittoer vs various elemental plots for RJY basalts Ni and Cr shows positive correlation and Ti02, K20/ Sc and Ba show negative relationship. 135 Si 02 Fig.5.- Total REE as a function of Si02 indicating the role of differentiation -fractionation for RJY basalts 13i TiOz _L _L 50 100 150 200 I REE Fig. 5.5a Plot indicating the tre.id between total REE and Ti02 for RJY basalts 15 1 - P^O, 0.5 1 50 •00 150 200 250 Z REE Fig. 5.5b Plot indicating the trend between the total REE and P205 1' J/ 15 14 - 13 - O N 12 11 t»9' Fig. 5, 6 A1203 wt% variation as a function of Mg number suggesting plagioclase fractionation of RJY basalt (Cox, 1980) 400 • • - • —. — , „ , - 300 - o o o o * o OS 2 00 100 I I I i 30 40 50 60 70 Mg^ Fie. D.7 Sr ppm variation as a function of Mg # indicating _i ;^„i^^^ •P^a,-<-S/-inat--inn ffnx, 1980^ 13S ^> ^f^ hi- M . E 10 T3 C ha. c o Q. Q CO CO "c o c CO cr c ? o x: to o in D CD l^ > ct o£ c o X3 «> •o o. CO o 00 o —I— -f o o o o m CM 13S fractionation (Fig. 5.10) 5.3.6 Mgo-FeO Plot: Fig. 5.11 shows that all the samples plot well below the compositions of the melts that could be in equilibrium with pyrolite at 101 Kpa. They show large variation of cation percent FeO for a limited range of cation percent MgO. There is not much difference in samples from both lower and upper flows and they plot around 1200 C temperature contour. The position of majority of samples indicate that they could not have been derived through differentiation from a melt in equilibrium with a pyrolite source. As indicated by the olivene fractionation curves, below 1300 C both FeO and MgO start decreasing in the melt if only olivene fraction ates. This is because below 1300 C, distribution coefficients of FeO between olivene and basaltic melt is 1 (Langmuir&Hanson , 1980). As suggested from the plot, the increase of FeO is due to joining of plagioclase and pyroxenes as liquidus phases with olivene. 5.4. CRUSTAL CONTAMINATION There is abundant evidence that continental tholeiitic magmas may have undergone extensive crustal contamination (Carter et al., 1978; Thompson et al., 1984; Beane, 1986). It would be essential to investigate the effect of this important process on RJY basalts which would alter its primary chemistry. Various elemental ratios such as K/Sr and Rb/Sr are used to inve stigate the crustal contamination during the ascent of magma. It can be seen that K/Sr and Rb/Sr ratios for RJY basalts fall in 140 10 r e '^.-^o "^P^ 7 6 ^« o 5 ^QCX°^ P 4 + a&0 ^ o N 2 - Ti ipp"^' Fig. 5.9 Ti02 - A1203 plot for RJY basalts showing vectors of 50% fractionation is indicated (after Pearce and Flower, 1977) 141 A\-Si GRID 20 r 15 10 PYROLlTE-3 OUIVENE -o- 35 40 45 50 Si Fig. 5. lOPlot of cation percent Al-Si space which could coexist with pyrolite- 3 is shown with contours of different pressures, temperatures and extent of melting (Francis/ 1983). The plot suggests negative relation ship between Al-Si. This relationship could indicate plagioclase satu ration and fractionation 142 Fig. 5.11 Plot of MgO vs FeO (cation mole percent ) for RJY basalts. Description of the plot given in (Hanson and Langmuir, 1978). Note the FeO enrichment charecter of the basalts and the samples lie out side the melt field 143 the range of mantle values. The absence of any correlation between Ni (compatiable) and K20 (incorapatiable) elements also suggest no role of crustal component in changing RJY basalt chemistry. Higher MgO content is characteristic of magmas genarated at higher temperatures and can assimilate more crustal material during their ascent (Campbell, 1985). The lower MgO contents of RJY basalts would suggest that assimilation by the magma, if any, has taken place only before the magma entered the chamber and not in the feeder dykes which connects magma chambers to surface. Longhi (1981) has made a key observation which allows us to further investigate whether crustal components have played any role in changing the chemistry of the RJY besalts or not. He points out that Ca-poor pyroxene (pigeonite) is an important phase in continental basalts and its crystallization preceeds that of Ca-rich pyroxene (augite). The reverse is true in case of oceanic basalts. He argues that primary melts derived from the mantle that fractionates at pressures less than 5 kbars, should crystallize Ca-rich pyroxene before Ca-poor pyroxene. Thus, the oceanic crystallization sequence is normal and the continental crystallization sequence is anomalous. Longhi (1981) showed that the difference between the continental and oceanic tholeiites can be explained if continental tholeiites are contaminated by a silicic crustal melt. The relative dominence of Ca-rich pyroxene phase to Ca-poor pyroxene in RJY basalts and higher temperature of crystallization for Ca-rich pyroxenes compared to Ca-poor pyroxenes, indicate that the silicic crustal component is absent in the melt. However, the presence of pigeonite (although in minor quantity) in RJY basalts does not rule out 1^4 the complete possibility of crustal contamination. This may be further substantiated by the higher K/Sr value of these rocks in comparsion to the mantle values. The principle behind the Irvine-Longhi arguement can l)e illustrated using the system Forsterite-Diopside-Silica (Fig.5.12). The path ABCD represent true fractional crystallization path for RJY basalts, where Ca-rich pyroxene crystallizes before Ca-poor pyroxene. If the melt assimilates silicious rock before crystallization, the liquid follows the AEFD path and Ca-poor pyroxene starts crystallizing first. When compared to MORE, the continental basalts are enriched in LREE along with some incompatiable trace elements, as evident in RJY basalts (present study) Columbian river basalts (Carlson, 1981); Still water complex, (Selpes et al., 1983); and Deccan basalts (Subba Rao et al., 1988). Such enrichment may be attributed to the assimilation of LREE enriched crustal component (Taylor, 1980; Depolo, 1981; Grove and Baker, 1984),and to the melting of LREE enriched mantle itself (Frey and Green, 1974; Stosch and Seek, 1980). Since crustal contamination played an insignificant role in case of RJY basalts, the enrichment of LREE and incompatiable elements can be attributed to the partial melting of LREE enriched upper mantle. Plots such as P205 vs Ti02, and Zr vs Y (Fig. 5.13 a & b) for RJY basalts shows source enrichment in incompatiable elements. Sen (1986) points out that even if trace amounts of an earlier enriched crust is melted together with a largely peridotitic (depleted) upper mantle, the basaltic magmas so produced may have a higher concentration of incompatiable trace elements. In case of Deccan basalts, components such as N-MORB type 145 Fo SiOa (A) Fig. 5.12Eqilibriuin phase diagram for the system diopside - foresterite - silica showing only the fields in which various crystals appear as primary phases (after Bowen/ 1914; Kushirc, 1"~2) A ^ 4C 5 • (TiO,/P,Oa= 101 o / • ,o/(Upp» Monti* Volu«) TiO, 5 Chondrittt (TiOj / f^ O5 0 105 ) 04 I 2 PjOs *0 + 0 a) 0 0 8f # ° so %oOOo * °° ° + + 0 + E 0 0 20 10 / Chondrite / ' ^ /r = 2 56) 1 1 : 1 1 1 • 0 160 240 320 400 4B0 560 Zr (ppm ) Fig. 5.13 Plots showing P205 versus Ti02 and Zr versus Y indicating enrichment cf these elements relative to chondrites in RJY basalts 147 depleted mantle and old enriched subcontinental mantle are being suggested to have controlled the compositions of primary magma before it reached crustal raagmatic chamber (Mahoney et al., 1982; Cox and Howkesworth, 1985; Beane et al., 1986; Sen, 1986). Such an assumption of an enriched mantle source for RJY basalts simplifies the problem of trace element enrichment. Better insight into this problem may be possible by isotopic study that needs to be carried out. 5.5. GEOTHERMOMETRY Various methods for calculating the temperature of crystal lization of mineral phases are employed and the estimations by different methods gave similar results (Table.5.2.). The methods eraploved arc summarised as fallows. 5.5.1 French and Cameron (1981) method: The RJY basalt data belongs to class I of MgO/A1203 .- pec~r.- plot of Cameron and French (1977). Thus coefficient of class I are _5e. for calculating the temperature of cryastallization of plagioclases ^n pyroxenes. The euqation used for estimations is as fallows: T°c = alxl + a2x2 + a3x3 + anxn + K Where al a2 are coefficients of xl x2 (which are wt percentages of -.-e 148 o CO 0£ u O CD u O cr^ o in CN > SI CNl CN CNl \0 CN 3 O 0) cr — u u u a zx. a o CM -C in a. - o r-4 CN CN o CN OJ c X o >1 CJ — 1- X CJ O CJ u DC - u in r\i '-TS r^ CN o X ZC ZC i- ::C c E CO C3 OJ U > X E- rt c CX) X in CO c a 0) o c N S CO tj 149 twelve oxides ) and K is a constant. The cefficients and constants of basalts class I used in the method are from (French and Cameron,1981). The aver:agc e error in the temperature calculations for plagioclase is +3.3' C .0 and for clinopyroxenes is ± 10.5 C. 5.5.2 Nielsen and Dungen (1983) method: It has been demonstrated that the modelling of mafic phase equilibria using a two-lattice melt component activity model, based on the Boltinga and Weill (1972) approch to the mixing properties of silicate melts can be successfulluy implimented to modelling the crystallization of mafic magmas. This model assumes that the melt is composed of two independent quasi-lattices, i.e., the net work formers and the net work modifiers. The significant reduction of the effects of composition on major and minor element distribution relations makes this model superior for the calculation of raineral-melt equilibria to models in which melt components are assumed to be simple cation mole fractions. Empirical, single component distribution coefficents have been determined by the calculation of linear least-squares regressions of experimental mineral-melt pair compositions. These functions have been combined with an alterative algorithm in a computer model of crystalli zation processes in mafic systems. This model reproduces the input data set within 20'"^C and 3.1% (1.) for oxides in olivene, plagioclase, pyroxene, spinel and ilmenite. Calculated crystallization sequences for 52 natural mafic compositions were compared to with published experimentally-data mineral 150 crystallization sequences. The 1..precisions for temperature were within 15° C for the initial crystallization temperatures of olivene, high Ca pyroxene and plagioclase. The net work forming components are Si02, N'aA102 and KA102 , and the network modifying components are MgO, FeO, CaO, A101.5,Ti02, MnO and CrOl.5. The activities of the melt components are calculated assuming that the components mix ideally within their quasi-lattices vith no mixing between quasi-lattices. As examples, the activites of XaA102, MgO and A101.5 are defined as: 1 wNF 1 I ,1 . ,1 » 1 a.NaAiaZ.X =X k^ ^^X ) NaA102 N/ Ma K Si 1 VNM wl MgP 1 v.NM vl ,,1 1 a =X 4 -X -X > A101.5 Al 'k ' Na ^'K NF Where as _ Si02 is the activity of Si02 in tne li;uid. YM, Y VNM and Y are cation normalized mole fractions of melt com:;nent M in the Liquid, network formers, and network modifiers respectivelv. The generalized formula for pyroxene is (Ml) (M2' 7206. Ml is the smaller of the two octahedral M sites and is preferentially occupied by Mg, Fe, Al, Ti and Cr. M2 is preferentially occupied by la, Mu, and Xa along with Mg and Fe. T is a tetrahedraly coordinated site :ccupied by Si and Al. The mixing properties of both high-Ca and low-Ca-tyroxene may be 151 approximated by evaluating the contribution to the entropy and enthalpy of mixing on and between all three sites, Ml, M2 and T, with a two M site distribution model. The pyroxene component activity model used for the purpose of this investigation assumes that at magmatic temperatures cations mix ideally on each M site, with complete disorder of Al between the Ml and T sites. In this case, the activity of the enstatite component in either pyroxene may be expressed as : Mg2Si206 Mg ^}^\ S±) 5.5.3 Nathan and Vankirk (1984) method: The study by the these authors demonstrates that a relatively simple model simulates many of the features of fractional crystallizacion of magmas through a broad range of liquid composition at one atmosphere pressure. As a liquid cools and crystallizes, the mineral assemblages evolves by progressive addition of new minerals occasionally \vich the replacement of an old minerals. The fundamental premise for this study is that equilibrium phase relations of a liquid-crystal mixture are governed solely by the chemical composition of the liquid, given a fixed pressure. The liquid composition then determines the mineral assemblage, the mineral composition, and even the temperature of the liquid-crystal mix. The dependence of temperature and minerals on liquid composition is a consequence of restriction to equilibrium liquid-Crystal relations at constant pressure. 152 Mineral-temperature equation : The Crystallization temperature of each mineral is assumed to vary in a continuous manner with liquid composition. The principle of relative crystallization temperatures has been extended in this study to complex nine-component systems at one atraorphere pressure. Liuquidus is conceived as a set of smooth surfaces each o fwhich describes for one mineral the dependence of crystallization temperature on liquid composition. The surfaces are described by mineral temperature equations. For each crystallizing liquid, the equations are solved for the nine minerals of the model and only the mineral with the highest crystallization temperature may be in equilibrium with the liquid. Equilibrium of several minerals with a liquid requires that their cryst- ization temperature be higher than any others and equal to one another. The crystallization temperature of each mineral has been approximated by equation of the form. +3 +2. T = a^ + 'a A1+ a_Ti + a.Fe + a_ Fe + a Mg+ a Ca+ a Na + a K + ^1 -^ 3 4 567 8 agClogg n ) + a^Q-^kl -(Na + K) Where the chemical symbols represent cation fractions in the liquid. The first term (^o) is an emphirical constant. The next eight terms account for the specific liquid compositions, in which the nine cations of constant sum have eight degrees of freedom. Arbitrarily, the ions of lesser abundance were selected to decribe the composition of silicate liquids. The very significant tenth term is a (log^ II) where II is the geometric mean of the fractions of the essential cations multiplies according to their ratios in the mineral. 153 for example plagioclase is shown as V (Na+Ca) (Al)(Si)3 and augite as V (Ca) (Mg+Fe ) (Si). Coefficient for the mineral temperature equation are given Nathon and Vankirk (1984). 5.5.4 Glazner's (1984) method: Activities of plagioclase were calculated by using Glazner's (1984) method. It has been adopted to estimate temperature of crystalli zation of plagioclases. The activities of plagioclase component in basaltic melt can be calculated from the compositions of coexisting melt and crystals, provided that 1) the component is indipendsntly variable component of the crystal, and 2) approximate thermodynenic data for the component are known. This approch is used to calibrate the compositional dependence of the activities of anorthite. Glazner^ (1984) activity models anorthite provide excellent geothermometers with standered deviation temperatures residuals of approximately _+10 C. The activity of anorthite can be calculated by first determin ing the mole fractions of the oxides in the melt and tr.an computing the equation using the activity models as given for anorthite m Glazner's, (1984) paper. The temperature estimations of mineral phases suggest that Ca-rich pyroxene (augite) is first to crystallize fallovei by Ca-poor pyroxene (pigeonite) and plagioclase. The consistency of results by the various methods shows that the estimations are relaible. CHAPTER 6 SUMMARY AND CONaUSION A glimpse of the literature would reveal that very meagre work has been carried out on the petrochemical aspects of Rajahmundry basalts. Though this area has attracted many workers because of its palaeontological potential, it is otherwise important as these volcanic eruptions are manifestation of the dynamic processes through which the Indian sub-continent has passed. The present work has been taken up to investigate petrochemical and genetic aspects of these isolated outliers of the basaltic rocks which are considered coeval to the Deccan flood basalts. Though the RJ": basalts are coe\al to the man Deccan basalts, the former are essentiall\ associated \sith Goda\ari graber. In other words they are integral part of the lithostratigraphic succession of Godavari graben. The present i^ork has been summarised as follous. The basaltic traps along \Nith Inter-trappean and Infra-trappeans are exposed on both the banks of the river Godavari as it cuts through the rocks. The Inter-trappeans are exposed m Kateru and Pangidi areas of the East and Uest Godavari Districts respectively, while the Infra- trappeans are recorded in the Uest Godavari District only. Structurallv the area is undisturbed and the Infra-trappean beds dip with a lov* angle of Kj to 10 degrees. The Infra-trappeans and Inter-trappeans consist of 155 sandstones and limestones respectively and are fossiliferous in nature. The macro, and micro-fossil studies suggest that these beds were deposited under shallow marine to esturine conditions and are of the Paleocene age. The upper and lower flows are 25 m and 3 m thicc respectively. The lower trap is in unconformable relationship with Infra-trappeans and at places it is rests directly on the formations further lower in the succession. The simple, blocky flow characteristics without any pyroclastic material indicate that they are not central eruptions but are fissure type. The presence of faults inferred from sattelite imagery passing through the area probably suggest the closeness of the site of eruption. The optical studies carried out on large number of samples are in close confirmity with the microprobe data carried out on representative samples. The presence of the two (Ca and Fe rich) pyroxenes, plagioclase and iron oxides with subordinate amount of interstitial altered glass and conspicuous absence of olivine both as mineral and normative phase is an important feature indicating the evolved character of the RJY basalts. The relatively less common glomeroporphyritic texture, especially in the lower flow, indicate the presence of intra-telluric crystals of pyroxene and plagioclase in the melt. Complex physico-chemical conditions p such as fluctuations of H20 and temperature, are inferred from the zoned crystals of plagioclase. The textural relationship of opaque minerals and presence of skeletal iron oxide indicate their late stage origin. The various textural patterns seen such as ophitic, sub-ophitic, intergranular, in^^erstitial and occasional flow textures indicate varying cooling conditions of the basaltic flows. The ophitic to sub-ophitic 156 texture reflects the near simultaneous crystallization of the mineral phases. The groundmass of Ca-poor pyroxenes in the interstices between the microphenocrysts reveal the rapid cooling conditions after eruption. The geothermometric estimations (see chapter-5, Table-5.2) suggest progressive decrease of temperatures resulting in the appearence of augite (1259°C), pigeonite (.124^C), and plagioclase (1169°C). The textural patterns observed are also in confirmation with the geo-thermometric estimations. Also, it has been inferred that though the lower and upper flows have similar temperatures of crystallisation of minerals, the upper flow shows slightly higher degrees of crystallisation. The plagioclase temperatures estimated from the Glazner's 0.9 84) method shows that temperature of crystallisation increases with the increasing An % and are consistent with the basaltic trends. The vesicle filling minerals such as silica, zeolites and altered glasses (palagonites and chlorophaeites) indicate influence of the low temperature deuteric alteration. At places, the palagonitisation of clinopyroxene has also been ci served in these rocks. Geochemically, the effect of alteration i.e. mostly weathering and post consolidation leaching on various elements has been investigated and the results indicate that Si02, MgO, FeO(t), A12i3, Ti02, NaS , Ni, Cr, Zn and Y values shows reasona) le confidence while, K2) , Rb, Sr, Ba and CaO values siow some alteration effects. The selective enrichment of Yb in RJY basalts is attributed to break down of clinopyroxene. However, the ci served negative anomalies of Ce were attributed to seawater-roa interaction caused by marine transgressions post dating the eruption of the flows. 157 Tki e major and trace element variations within the flow (vert ically and horizontally) shows monotonous trends without much differentiation. The studies show that the upper and lower flows ^ow similar mineralogical and chemical features. The study of REE of these basalts indicate that they are enriched perticularly in LREE. All the samples of RJY basalts (both lower and upper flows) ^ow similar enrichment of REE but only differ slightly in their d) solute enrichment. Especially, the lower flow shows higher REE contents. On the basis of low Mg numbers, Ni and Cr contents of the RJY basalts, it has been inferred that they are not primary melt. The present compositions presumably represent the products of magmatic evolution during its upword movement from the source, i.e. a larger degrees of partial melting at the source region and extensive differentiation- fractionation probably in the sub-crustal magmatic chambers. It is inferred from the LREE enrichment patterns that garnet is the principle residual phase in the source region for the parental roa s of RJY basalts. The estimated partial melting ranges from 3Q±5^ indicate higher degrees of melting. The studies suggest extensive fractional crystallization of gabbroic material in the evolution of the RJY basalts similar to Deccan basalts. However, the RJY tholeiitic basalts with its characteristic iron enrichment suggest evolved character than the Deccan basalts. Fractionation of both olivene and pyroxene phases is suggested from, the major and trace element patterns. The other geochemical features such as increasing A12) 3 percent and buffering of Sr values with increasing fractional crystallization suggest the plagioclase fractionation. Though, Sr is considered to be partly affected by secondary mobilization, its strong negative anamoly in the spidergram 158 would reflect the plagioclase (gabbro) fractionation. The incompatiable trace elements enrichment in these basalts is may be due to single or combination of processes - fractionation of olivine, clinopyroxene antf or due to partial melting of metasomatically enriched source region. The preliminary studies indicate that in the evolution of these basalts crustal component has no major role to play. However, the crustal contamination can't be ruled out completely. The flow charecteristics as well as the geochemical characters indicate their continental condition of eruption. But oceanic character has also been suggested by some variation diagrams. Accordingly they are grouped as basalts belonging to ambiguous tectonic setting erupted at the interface of continent and oceanic environments. Thus RJY basalts have been described as basalts belonging to initial rift tectonic setting (Chatti and Sharme, 19 9 0). From the above geochemical and petrogenetic studies together with the limited earlier data, a synthesis of the tectono-magmatic processes of eruption of these RJY basalts has been attempted. Except for the minor abjerent roa types within the flood basalt provinces all over the world, there are overwhelming evidences that ffl.ajority of the basalts are tholeiitic and have erapted within e *ort span of geological time. In RJY region, the Inter-trappean sedimentary beds between lava flows are thin and. sliow characteristically faster depositional features (Bhalla, 19 6""). The similarity of mineralogical and chemical features of the upper and lower flows suggest periodic tapping of the same magmatic chamber after a sborter gap of time. These features substantiate the idea of shorter span of magmatic event in RJY region. 159 Regarding their origin, two alternatives seem possi) le. One in which a more primitive parental magma than the RJY basalts (c.5-8X MgO range) of picritic composition as a result of partial melting of a Fe-rich pyrolite or garnet peridotite differentiates into basalts through fractionation of olivine and clinopyroxene or both. The amount of fractionation needed to produce basalts depends upon the MgO contents of the initial picritic liquid. The higher the MgO contents, the larger are the amounts of olivine required to be removed. Approximately, 8-15X olivine loss is sufficient to produce normal basalts (c.5-8^ MgO range) from picritic compositions with 10-15% of Mg) . On the other hand, high degree or total melting of an appropriate source rock such as eclogite or anphibolite can also produce basaltic liquid in the range of 5-8% MgO. Snce large amounts of liquids are produced, rapid flooding or eruption is possi) le (Cox, I.} 80). The only obj ection to such a scheme being the availa) ility of high degree of heat for total melting. But then, flood basalt provinces are an enigma when one considers the enormous volume of hot liquids i,110 0 C), made availa) ile for eruption in relatively shorter span of time. Pressure .relief, as a consequence of tensional forces developed during the foundering of the Gondwanaland might have induced or assisted in large scale melting either total or at high degrees. Considering the above evidences a tectono-magmatic model proposed by Cox (1)80) would explain better the RJY volcanics also. According to him - the uprising magma from mantle that equili) riate in crustal magmatic chambers in the form of sills before they eventually erupt. The high density of picritic magma (which is supposed to be parental to RJY basalts) inhibits its movement and eruption. The sill form has an adventage because it is capaa le of accomodating large amounts of material without posing a 160 space problem. An another advantage is that it provides an ideal environment for the fractionation of large volumes of magma within compar^itively restricted pressure range. Comparatively, one would find broad similarities of mineralogical and chemical features for the time equivalent RJY basalts and Deccan basalts. A distinct feature of RJY and Deccan tholeiitic basalts when compared to average continental tholeiites is that the former are significantly enriched in FeO(t) and Ti02. The higher content of these elements of Deccan basalts suggest higher degree of fractionation prior to extrusion. It is interesting to note that the RJY basalts and the basalts drilled from NE Indian ocean show general enrichment of iron compared to Deccan basalts from northern regions. This aspect needs special attention to study as it may indicate a general enrichment of iron in this part of the mantle or progression of Indian plate over hot spot and the subsequent enrichment of iron in the later melts. There is a fair amount of gradation br-tween the alkali and tholeiitic basalts for the Deccan provinance as a whole although samples from individual areas have discrete groupings or show transitional features. RJY basalts show monotonous nature and does not show any such transition as seen in the Deccan basalts. The common tholeiites of the Deccan traps are genetically related to primary magmas with 9 toO % MgO, which are generated in a heterogenous but dominently Indian ocean MOJ^ type presumably at mantle depths of 35 to 6 "i 45 km. Approximately 1.2 x 10 km volume of primary magma has been generated in the upper mantle of this region out of which a large fraction of the magma crystallized as gabbros and troctolites over a wide range of 161 depths in crustal magma chambers (Sen, 19 88), Thus the present studies reveal that geochemically and geochronologically the RJY basalts are comparable to the Deccan basalts. However, a question remains to be answered, i.e. whether the RJY (East coast) basalts and Deccan basalts came from a single source or not? Before finding answer to such a question it would be relevant either to relate or to unrelate the RJY volcanism with Deccan basalts proper. The fact that these RJY basalts are intrinsically associated with litho-stratigraphic sequence of the Gondwana basin lends support to deassociate ±t from main Deccan basalts proper, which are presumably erupted due to Reunion hot spot activity. Morever, the wide gap (spatial) between these two basaltic bodies raise doubt regarding the single source concept. The recent DSS profile studies reveal (V.K.Kaila, Personal discussion, V 3 88) a seperate source for the RJY basalts. A mega sill underneath the RJY region has been suggested with the help of geophysical studies, carried out by ONGC during its exploratory work on On-Shore and Off-Shore of Godavari basin (Biswas, Per ^.onal discussion, 19 38). It suggests altogether an independent magmatic activity coeval to Deccan volacanism. TECTONICS The relation between the type of volcanism and tectonic movements give a positive proof of the fact that the volcanic activity in the Rajahmundry area was invariably manifested under tensile stress conditions. Compressive folds are singularly absent in this region (Kumar, i9 82). It is interesting to note in this context that not a single region where volcanic activity was associated with firmly established compressive structures at 162 this Mesozo.c-Tertiary boundary is cnown in the Peninsular India. In the speculative schematic evolution of the Gondwanic belts of India and Southwest Australia (Kat2,l9 76), four episodes are postulated - leaving the first and second Precambrian episodes, the third episode of Phanerozoic activity along transforms of Mid Indian Ridge which include the formation of the Gondwana rift, i.e. the Godavari graben. It is important to note that the Godavari graben has been extended further eastward into the Bay of Bengal andj oined with the negative lineament (see Chapter-2) infered from the Satillite altimetry data map of Gahagen et al.(l3 88). Moreover, the other faults and linements recognised in the south India can be shown to be the continental extension of maj or contemporary transforms of the Indian Ridge (Katz, 1989). 'he fourth and latest episode was an intense reactivation during the early Cretaceous that led to the break up and fragmentation of Gondwanaland. Strong dextral transform movements lead to the development of the new dormant 90 E ridge, which played a major role in the evolution of the Indian ocean. The eruption of RJY basalts during the Paleocene, may be due to the reactivation of the failed rift, where tensio-al forces might have stretched the graben basement and the eventual drop of lithospheric pressure must have lead to melting of mantle and generation of melts. The other remote possibility where the contemporary 90 E hot spot plume suppling molten material to the Godavari rift tensional zones can't be ruled out. The out pouring of lavas must have taken place during Paleocene and possibly could have formed barriers from the sea, simultaneously devoloping a coastal lagoon in which the Inter-trappean beds were deposited. The complete absence of terrigenious material in the Inter-trappean sediments 163 indicate thai there was no inland drainage from the land areas and the lagoon might have been connected to the ocean only and the sedimentation of Inter-trappeans beds could have been under calm and quite conditions with varying salinity. Marine transgressions are evident from the fauna of Inter-trappean and Infra-trappeans marine sedimentary beds associated with the basaltic flous m this region. The effect of sea water interaction with basaltic rocks during these marine transgression(s) was reflected in the rrobility of some elements as discussed earlier. FUTORE STUDIES The limited extent of the RJY basalts erupted at the continent-ocean interface, perhaps only indicate the tip of an iceberg, i.e. the mq or portion of the melt generated due to the dynamic processes must have consolidated underneath the continent as uell as m the oceanic region adjacent to it. Thus it is quite essential to consider samples from the drill data in order to develop any comprehensive model cf this area m perticular and Goda\ari graben m general. The ongoing exploration of oil and gas b\ 0\GC (Oil and Natural Gas Comraision) m the Goda\ari oasm has \ielded a v^ealth of drill data uhich can possibly v^iden the scope a such studies related to \olcanism and associated marine sedimentation. The available published information b\ 0\GC indicate that RJY basalts do extend into the Ba\ of Bengal continuing under the coastal Godavari Deltaic sediments. Perhaps, these traps mav show transition as oceanic crust m to Ba\ of Bengal '' The possible role of these basaltic rocks as "oil and gas traps" adds to the significance of such studies. It is important to stud> m detail regarding the extent, timing and processes of these volcanics 164 which have documented a record of the extensive drift of the Indian plate. The following studies can be taken up if the new drill data is made available: Detailed studies to establish the actual thickness of the trap rocis throughout the subsurface; Studies of the basalts sampled from successive flows in the sub-surface would enable to develop better petrogenetic models; Geochronological studies to constrain the possible limits on the age of the volcanic rocks; Better comparision of these volcanics with the time equivalent Deccan basalts to develop tectono-magmatic models in order understand and to relate them it with the global tectonics processes. 165 REFERENCES Alt, D., bears, M. and tlyndiaan, D. W., iyay. Terstrial maria: The origin of large Dasalt ^jlateaus, hotspot tracks and spreading ridges. 96, 6-^7-66/. Aiverez, L.W., Aiverez, W., Asaro, F. and rticnei, ri.V., 1980. extraterrestrial cause tor the Lretaceous-Tertiary extinction, bcience, 2uo, iuy:)-iiUB. BaedecKer, D.A., Kowe, J. and Steinnes, E., 1977. Application ot epitnenual neutraou activation in multielement anally sis ot silicate roctcb employing botn coaxial bd (Li) and low energy pnoton detector system. J. Kadioanal. Chern., 40, 115-1^6. liaKsni, S.IC., 1963. Gluconitic mudstone trom Kagnavapuram shale, west Godavari District, Andhra Pradesh, Curr. Sci,, 34(11), 35l. BaKshi, S.K., 1977. Marine transgression, beaimentat.iou and cectuiiicb ot the west Godavari coastal rocKs ot Andhra Praoesn. Indicni. Jour. Earth Sciences, 67-94. BaKshi, S.K. and Krishnabrahmam, M., l987. Kajaniaundry Traps. Age, Geochemistry and trauieworK ot eruption. WGKl Report. Bnandari, L.L., l984. Oil bearing sediraentar> basini= of Inaia, Jour .inaiciii. Assoc. Sediment, 4, 1-66. Barnes, S.J., iy85. The petrograpny and Geocneiaistry ot Komatite tlows from tne Abitibe greenstone belt ana a model for tneir toruiation. Lithos, 18, 241-270. Bartn, l936. Tne crystallization process or Odsaltt,. Ani. Jour. Sci. 31, 321-51. 166 Badaitic volcanism study project, l^Si. Volcaniaiu on tne terrestrial Planets, i^ew York. Pergamon, i2a6. Beane, J.E., Turner, C.A., Hooper P.K., Subba Rao, K.V. and Walsh, J.W., iy86. btratigraphy. Composition and term of the Deccan basalts, "Western gnats, India, tiull, Volcanol., 4o, 6i-aJ. Benza and Cuilen iy37. "Notes cheifly Geological ot a Journey through the northern circles in the year lyj:?," :3:)J-:)7u. BeskvicR, /i.E., iyo2. Some geochemical asijects ot alteration and 5enetic relations in Komatitic suites. In: Komatites, N.T. Arndt and t.G. Nisbet. London, tngland, 283-308. Bnalla, S.N., 1966. Foraminitera trom the Intra-trappean Beds ot the Pangidi area, J. Paleo., 40(2), 343-345. Bhaila, S.N., 1967. Forminitera from the inter-trappean beds (lower Eocene) oi the Pangidi area. India. Micropalaeontology, 13, 351-368. Blauford, Iv'.I., Blanlord, H.F. and Teobald, W., 19.56. On the geological structure and relations ot the Talchir coal tieid in the District of CuttdCK. Mem. Geol. Surv. India, 1. Bowen, N.L., lvi4. The ternary system diopside-toresterite-silica. Am. Jour. Sci., 33, 551-573. Bowen, T.B., l-larsn, J.S. , Bowen, M.P. and Fales, H.V., i9b6. Volcanic rocKs of. tne unit water stand Triad, Southafrica-Uescription, classification and geochemical stratigraphy. Precambrian Res., 31, 297-324. Brovn, G.M., 1957. Pyroxenes from the early and middle stages of fractionation or tne ;3Kaeroaard Intrusion. East Greenland, Miner. Mag., 31, 511-543. 167 Brown, G.n. and Vincent, E.A., 1963; pyroxenes from the late stages ot fractionation ot SKaergaard intrusion, east Greenland. Jour. Petrol., <* , 175-197. Buat-Menard, P. and Cnesslet, R., 1979. VariaDle intluence ot the atmosphere plux of the trace metal chemistry of oceanic suspended matter. Earth Planet Sci. Lett., '•Z, 399-411. BurKe, K.. and itewey, J.b., iy/j. Plume generated triple Junction, Key indicators in applying plate tectonics to old rocks. Jour. Geol., 8i, 406-^33. CauipDell, I.h., 1985. The difference between oceanic and continental tnoleiites a fluid dynamic explanation Contrib Mineral Petrol., 91, 37-43. Carlson, R.W., Luguiair, G.W. and NcDougall, J.D., lySl. ColuuiDia river volcanism : the question ot mantle heterogenity or crustal contamination, ijeochem. Cosmochera. Acta, 4D, ^40:5-ZDUU. Uirmicnael, I.S.E., Turner, F.J", and Vernoogen, J., 1974. Igenous petrology, wcGraw-Hill New York, \iA., 7jy, (see especially pages 427-453). Calter, S.R., Evensen, W.M., Hamilton, P.J. and U'Mons. R.k., 1978. Na-and Sr isotopic evidence for crustal contamination ot continental volcanics. Science, 2u2, 743-747. Cnaiidraseknaram, D. and Partnasarathy, A., 1978. Geochemical and Tectonic studies on the coastal and inland Deccan trap volcanics and a model for the evolution ol Deccan trap volcanism. Neues Jahrb, Mineral. Abh., 132, 214-229. Chatti, H.R. and Sharma, K.k., 1990. Petrochemistry of Deccaa oasalts uf Rajahmandry, East coast (A.P) India. Ind. Jour. Earth. Sci., 17, 6-77. 168 Cndo-hsid Cnen., 1988. Estimation ot tne degree ot partial melting by (Na20+K20) and Al203/Si02 of babic aiagmab. Cneinical geology, 7i, 355-36^. Cne^es, F., 1964. A petrograpnic dibtinction between Cenozoic volcanics m and around ttie Upeb ocean. Jour. Lieophys. Research, 69, Cneyes, f., iy6j. iitania and aluiama content ot oceanic and circum- oceanic Dasaitb. Winerdl I'lagazine, J4, iZ6-iJi. Condie, K.C., iy77. tttects ot alteration on element distribution m Arcneau thoieiiteb troiii the aerDerton greenstone belt, South rttrica. Hrcnean beochemistry, symjj. rtyderabdd, India, Abstr.No.J. Cox, "K.G., i^SO, A modal tor tlood babalt volcanism. Jour. Petrol., Zi, 6zy-6DU. Cox, K.G. and HawKesworth, C.J., iVaS. Geochemical stratigraphy ot the ueccan traps at ildhabaleswar, Western ghats, India, with impiicdtions tor open system, magnetic processes. Jour. Petrol., /6, JD3-J77. CrooKananK, H. , 1936. Geology ot the northern slopes of the Satpuras between "lorand and Sher Kivers, Mem. G.S.I., 66(2). Cullers, R.L. and Grat, J.L., igS't. Rare Earth elements m igneous rocks ot tne continental crust ; Predommently basic and ultrabdsic rocKs, In : Rare Earth Elementb Geochemistry, ed. D. Handersoii (ElvebVier Aiuesterdam), 237-27^. Ddb Gupta, H.C., iy33. On the age ot the 1obsiliterous beas at Duddukur hadras Presidency. Geol. Mm. & Met. Soc. India., 5, 59-66. Udvis, A., Blackburn, ¥.h., Brown, W.K. and Ehmann, W.D., 1978. Trace eiement geochemistry and origin ot late Precambrian-early Cambrian Cdtoctm greenstones ot the Appalachian rtountams. Univ. of ro Cdiitornid dt Ddvies, Calit., Rep. (unpublished). De'baar, H.J.'W., Bacon, M.P., Brewer, P.G. and Brilland, K.W., 1985. Rare eartn elements in Pacitic and Altantic oceans. Geochein. Cosmochem. Acta, 47, iy43-iyD9. De Paolo, U.J., I98i. Trace elements and isotopic et'tects ot combined wall-rock assimilation and tractional Crystallization. Eartn Planet. Sci. Lett., 53, 189-202. Duncan, R.A. and Pyle U.G., 1988. Rapid eruption ot the Deccan flood basalts at the Cretaceous/Tertiary boundary. Nature, 333. Elliot, "W., 1854. Jour. As. Soc. Bengal. XXIII, 399. Fawcett, J.J., 1965. Alteration products ot olivene and pyroxene in basaltic lavas trom the Isle of Mull. Mineral. Magazine, 35, 1965-66. Fermor, L.L., 1^25, Kec. Geol. Surv. India, 58, 93. fiestuientel, 0., 1882. Tne flora ot the Damuda and Panchet divisions, Pal. Indica, Geol. Surv. India. Series, No. 5, 3. Flo>d, P.A. and Wmcnester, J.A., 1975. nagma Type and lectonic setting discritumation using lannobile elements. Earth Planet. Sci. Lett., 27, 211-218. Fox, C.S., l93l. The Gondwana system and related Formations, Geol. Surv. India, 58. Francis, D., Ludden, J, and Hynes, A., 1983. Magma evolution in a Proterozoic ritting environment. Jour. Petrol., 24(4), 556-582. French, W.J. and Cameron, E.P., 1981. Calculation ot the temperature of crystallization ot silicates trom basaltic melts. Mineral Wae,aziae, Ht*, ly-z6. ^70 Frey, F.A., ndbkin, in.A., Poetz, J.A. and rtdbkin, L.A., iy6a. Kare earth dDouudaaceb in some Dasic rocKb. Jour, beopnys. Kes., 7j, 6085-6098. Fre^, F.A., tsrayan, W.b. and Tnoiutison, G., 1974. Aitantic Ocean tloor Geochemistry and petrology ot basalts trora legs 2 and 3 ot the deep sea drilling project. Jour. Geophys. Kes., 79, 5507-5529. Frey, F.A. and Green D.ti., 1974. The minerology, geochemistry and origin ot Iherzolite inclusions in Victorian basanites. Geochem. Cosmochim. Acta, JO, lU/J-lUi9. Frayer, B.J., 1977. Rare earth evidence in iron formations tor changing Precamorian oxidation state^j. Geochem. Cosmochem. Acta, 41, 36i-j67. Garauer, J., iy57. Littie ituve LreeR Aiabama-Paiaeoecological study, beol. 5oc. Ajjier, rJem. 67, I, j7j-500. ijahagan, L.i'l, bcotese, L.K, Koyer, ^A., bandwell, U.T., Winn, J.K., fomlins, K.L., Koss, 11.1., iNewmann, J.S., rtullar, K.D., ilayes, C.L., Ldwver, L.A. and Heubeck L.h., 19bcS. Tectonic tabric map or the ocean Da^ins trom satellite altimetry data. I'ectonoph^sics, JD, i-26. Gee, £.R., 1932. The Geology and coal resources ot Raniganj Coal tield, i4em. beul, bur v. Inaia, 01. Gno&e, N.C., iy76. Composition and origin ol Deccan basalts. Lithos, 9, 65-73. Giazuer, A.I., 1984. Activitiet^ ot olivene and plagiociase components m silicate meiti and their application to geothermometry. Contrib. Mineral. Petrol., 00, z6u-z6o. GulUDerg, E.D., ^uico, d., Scnmitt, K.A. and Smith, R.ri., iy6J. Rare earth 171 QistriDution in tne marine enviromnent. Jour. Geophys. Kes., tti, GoiascnmiQt, V.ri., IVD^. Geocheiuistry (td. Muir, A.) Oxtord Univ. Preas, 514. Govil, P.K.., l9S5. A-rdy tlourescence analysis ot major, minor and selected tBce elements in new iWG reterence rock samples. Jour beol. Soc. India, Z6, So-AZ. Green, i».H. and Kingwood, A.t,., 1967. Tne genesis ot basaltic magmas. Contrib. Ilineral. Petrol., !:>, lUJ-iyu. Grove, T.L. and Baxer, M.B., 1904. Phase equilibria controls on the tnoleiitic versus calc-alkaline differentiation trend. J. Geophy. Kes,, 89, 3253-3274. Guha, D.'K. and Raju, D.S.N., 1978. Stratigraphy and microfauna of the Inter-trappean beds near Rajahmundry, Andhra Pradesh. Vll th Indian coll. micropal. Strat. Abst. 3. HasKiu, L., Hd^sKia, M.A., Frey, F.A. and "Wildeman, T.R., i96c5. Relative and dbsolate abounaances ot the rare earths. In: Ahrens (hditor) Origin and distriDution ot the elements. Pergamon, London, (bee p. 889-912). Hellioan, P.L., 1979. The geochemistry and phase chemistry of a spilitic piiiow Iroui tne Bhoiwada section. Bombay Island, India. In : Prol. K.h. iUKheswdid's cominemoration volume. Hess, h.S. and Polaervaart, A., 1947. Basalt vol.1 and 2. The Poldervaart treatise uii rocks of Dasaltic compostion. interscience pUDiication. ni^ucni, Ff. and i»iagcibdwa, H., iy6y. Partition ot trace elements t)etween rock-tormiug minerals and the host volcanic rocks. Earth Planet Sci. Lett., 7, 281-287. 172 Hoiiii, P.E., 1982. Noa-recogiiitioa of cuutiiieutdi tfioieiites using tne Ti-Y-2r didgrdiii. Contrio. Wiuerai. i'etrui., 79, 308-310. rtoim, P.£., i985. Tne Geochemical Finger-prints ot different tectunoiiiagiiidtic environments using nygromagnidtopniie element aDounddaces ot tnoleiitic Ddsaits dnd Ddsditic andesites. Cneinicdl Geology, 5i, 303-323. •huiuphris, S.E., Morrison, M.A. and Thompson, R.N., 1978. Influence of rock cr>stdliizdtion nistory upon subsequent idnthenide moDility during hydrotneraidl alteration of basalts. Cnem. Geol, 23, 125-137. Hugnes, T."W.h"., i877. Tne Wardna valley coalfield, rtem. Geol. Soc.India, 13. riynes, A. and Gee, R.D., 1986. Geological setting and petrocnemistry of the i\arracoota. Volcanics Capricorn Urogen, "Western Australia. Precarab. Ret.earcn, 31, 1U7-132. irvme, T.;N. and Bar agar, W.K.fl., 197l. A guide to tne cneiTrical ciassiticdtioii of tne common volcanic rocKs. Can. Jour, liartn ici., l8, 5Z3-5^8. Jen:?eu, L.S., 1975. A new method ot classifying sub-alkalic volcanic ruCKs, Ontario divis,ion ot mines. Misc. pa^^er iNo. 56. K.d£jhintdv, G.I., Rudni, G.B. and boDolov, S.f., i98l. iHaguiatites and metaniorpnites. In. Geologia igeotizikandna vostochnoy chesti IndiyoiKogo oKeana (Geology and geopnysics ot the tloor in the eastern part ot the Indian ocean) 166-193. iNanKa press, Moscow. tCatz, M.B., 1976. Early Frecambrian granuiites- grenstones, transform mobile oelts,and ridge-rifts on early crust. In. Windly, B., ed., edrly nibtory ot tne earth: New York, "Wiley, 147-15«. Katz, ?l.S., 1989. Sri Lanka-lndiaji eastern ghats-tast Antarctica and the Australian Albany Fraser loobile belts: Gross geometry, age relatioiisnips, aad tectonics in PrecauiDrian Gondwanaland. Jour. Geology, 97, 6^6-6^8. king, 'W., 188O. Tne upper Gondwanas and other formations of the coastal region of tne Godavari districts, Mem. Geol. Surv. India, 16, 195-26-^. Xrishnan, M.b., ly26. Tne petrology of the rocKs from the Girnar and Osham hills, Kathiwar, India. Rec. Geol. Surv. India, 56, 4380-4424. Krishnan, M.S., 1941. Fort 31 George Gazette development dept., G.U. No. 493. Krishnauiurtny, P. and Udas, &.R., 1981. Regional geochemical charecter- istics of the Deccan trap lavas and their genetic implications. In. Deccaa volcanism and related basalt provinces in other parts of tne world. Geol. Soc. India. Mem.3, 3y4-4l8. Kumar, S.P., 1982. Krishna-Godavari basins. Petrolliferous basins of India. Edited by L.L. Bandari., KHMIPE, Published by Himachal Times. Kuno, R., llamasaKi, K. , lida, C. and iMagashima, K., 1957. Differentiation of Hawaiian magmas. Jap. J. Geol. Geogr., 28, 179-218. Kuno, H., iy6o. Differentiation of basaltic magmas in basalts hd. riess, H.h. aad Poldervaart, A., 2, 623-688. Interscience puD., ftewyurk. Kusniro, I., 1972. Tne system toresterite-diopside silica with and without water at Kgh pressure. Am. Jour, bci., 267-A, 269-2y4. Laagmuir, C.H. aad Hanson, G.N., 1980. An evaluation of major element heterogeneity in the mantle sources of basalts. Phil. Trans. R. Soc. Lond., A 297, 383-407. Le Bas, M.J., 1962. The role ot aluminium in igneous clinop/roxenes with 174 relation to tneir parentage. hn\. Jour. Sci., 260, 275-88. Le Bab, M.J., Le iMditre, R.'W., Streckheiben, S. and Zanettm, B., iy86. A cnemical ciasitication ot volcauics rockb basad on the total alKdli-silica diagrams, Jour. Petrol., 27, 7-^:3:730. Le Roex, A.F. and Erlank. A.J., 1982. Quantitative evaluation ot fractional crystallization m iJouvet Island lavas. Jour. Volcanol. Geotnerm. Res., 13, 309-338. Longhi, J., l98l. Multicomponent phase diagrams and the phase equilibria of basalts In: workshop on inagmatic processes ot Larly Planetar-y crusts. LFI Technical report number 82-01, 90-9A. Luddea, J.N. ana Tnompson, G., 1978. Behaviour ot REE during subuiarine weatneriiig of tholeiite basalts. ITature, 274, 147-149. Wacdonala, G.A. and Katsura, T., 196^. Chemical composition ot "Hawaiian lavas. Jour. Petrol., 5, 82-133. Macdonald, G.A., 1963. Lomposition and origin ot Hawaiian inagiuas, Geol. Soc. Aiuer, Mem., 116, 477-J22. Macdonald, R., l980. Trace element evidence tor mantle neterogeatv Deaeatn the Scottish midland vallev in the Larboniterous ana Permian. Phil. Trans. Koyal. Soc. Lond., A, 297, 245-257, NanaDale, T.S. and Rao, S.V., 197j. Fossil flora ot Rajdhmundr> area. In: bym. On Decean traps country. I. i^. b.A. Bull., 45, 192-214. Manaoale, T.S. and Satyanarajna, T., 1977. Two species ot the genus Taxaceoxvloii tCrausal and Jam From Decean Inter-trappean beds near Kajahmuadry in Ananra Prade&h, India., Biovigyanain, 3, 131-140. Wdhaoale, T.S. ana Datyanarayna, T., 197d. Petrilied Zinkgo wood trom fangidi in AuQnra Prauesh, Inaia., Recent researches m Geology, 4, '+6Z-'459. 175 Mcinoiie>, J.J., Wdcaougaii, J.D., Lugmair, G.W., Murali, A.V., SanKar Das. and Gopalau, K., 1982. Urigin ot the Deccan trap flows at ttanabdieswar intered from rtd and br isotopic and cnemical evidence. Eartn Planet. Sci. Lett., l60, 47-60. rtbSuda, H. and wagasawa, b., i976. Rocks with negative Ce anamoly dredged from snatsKy rise, oeochem. Jour., 9, 227-233. Matnar, L.P. and Evans, P., 196^. Oil in india. IXII Int. Geol. Congr., a5ii. rfew Delhi. natsai, Y., Unuma, N., Wagasawa, H., ttiguchi, H. and Uanno, S., 1977. Crystal structure control in trace element partition between crystal and magma. Bull. boc. Fr. Mineral. CristallOgr., 100, 315-324. Medlicott, H.u. and tilanford, W.T., 1879. A manual of Geology of India Geol. Surv. India, Meiizies, M., 1977. Rare eartn and trace element geochemistry of metabasdlts from the point sal ophiolite, California. Earth Planet. 5ci. Lett., 37, 203-215. wi-yasniro, A. and Shidu, f., iy74. Tholeiitic and calc-alkaline series in reldtiua to the behaviours ol Titanium, Vanadium, Chromium and rticKel, Aiuer. Jour. Sci., 275 (3), 255"-277. Morgan, V.J., 1972. Plate motions and deep mantle convection. Ceol. boc. Amer. Mem., 132, 7-22. Morimoto, r». IrtA., i988. (buDcommnittee ol pyroxenes) Nomenclature of {ryroxent (try CNMMW). Mineraiogical nagazine, 52, 535-550. Mailer, £.D., 1^83. ^iilU/Ti02/P205; A minor element discrimination for Ddsaltic rocKs ot oceanic environments and its implications for petrogenesis. Earth Planet, Sci. Lett., 62, 53-62. 176 Ndtnau, tl.D. and Van Kirk, C.K.., iy7o. A model ot magmatic crystal lization. Jour. Petrol., 19, 66-94. Neilsen, K.L. and Uun^an, M.A., 1983. Low pressure mineral-melt equilibria in natural anhydrous matic systems. Contrib. Mineral. Petrol., 8^, JlO-326. Nesbitt, H.W., 1979. Mobility and fractionation of Rare earth elements during weathering of a granodiorite. "Nature, 279, 206-2i0. Nisbet, E.G. and Pearce, J.A., 1977. Clinopyroxene compositions in mafic Idvatj Irom different tectonic settings, Contrib. Mineral. Petrol., 63, 1^9-160. rtocKoids, S.R. and Allen, R., 1954. The Geochemistry of some igneous rock series, pt, J, Geochim. Cosmochim. Acta, 9, J4-77. Officer, C.B. and Drake, C.L., 1983. The Cretaceous-Tertiary transition. Science, Zi9, Ij83-l390. O'H'ara, h.J., 1958. Arc Ocean floor basalts. Primary Magma? Nature, 220, 683-686. Oauiiid, i>i., Higndi, ri., Vakita, rt. and Ndgasawa, H., 1968. Trace element i^dttern between the two pyroxenes and the host lava, harth Planet, bci. Lett., 5, 47-51. Pearce, J. and Worry rt.J., iy7y. Petrogenetic implications of Ti, Y and Nb variations in volcanic rocks. Contrib. Mineral. Petrol., 69, JJ-47. Pearce, l.A. and Flower, M.F.J., 1977. The relative importance of petrogenetic variables in magma genesis ot accreting plate margins a preliminary investigation. Jour. Geol. Soc. Lond., 134, 103-127. 177 Pearce, J.A. ana Cann J.R., iy7j. Tectonic setting of the Dasic volcanic rocKd aetermined using trace element analysis. Earth Planet. Sci. Lett., i9, 290-300. Pearce, T.H., Goran, B.E. and Bincett, T.C., 1975. The Tx02-K20 -P2Ui diagram, a new method ot discriminating between oceanic and non-oceanic basalts. Earth Planet. Sci. Lett., 24, 2l9-ZZ6. PeacKock, M.A. and Fuller, R.E., 1928. American nin., 13, 360. PeacocK, M.A., 1930, Geological Magezine, 67, 170. Perry, K.., 1968. Representation ot raineral chemical analysis in II- dimensioual space : I feldspars. Lithos, 1, 201. Prasad, G.V.P., 1987. Fossil fish remains from the Inter-trappean beds of uuQduKuru, A.P. The Indian Geologist association, 20, 33-40. Queresny, n.h., Branmam, K.N., Garde, S.C. and ilathur, B.K., 1968. Gravit> aiiauioiies and the Godavari rift (India). "Bull. Geol. Soc. Atii., 79, 1221-1230. Raiverniaa, V., iy85. Stratigraphy and structure ot the Pranahita-Godavari Graoen, UnjjuD. rep. ONGC reported by D.b.R. Kaju, iy«6. beology and tTydrocarbon prospects of Pranahita-bodavari-Grabea. Jour. ASSOC, txp. Geophysics., 3, 131-14&. Raja Kao, C.S. ana Mitra, N.D., 1978. Sedimentation and Tectonics ot Gondwana basins ot peninsular India, third Regional Conf. on Geoiog-y aaa Mineral resources ot SE asia Bankok, Thailand., 14-lB, O5-90. Kaju, D.S.:\., Rau, C.N. and beugupta, B.K., 1965. Paleo-currents in the Kajdhmmmry formation, Andhra Pradesh, India. Jour. Sed. Petrol., 5, 758-762. Kaju, D.5.K., iy86. Geology and nydrocarbon prospects of Pranahita- 178 GoQavari Graben. Jour. Assoc. tXijl. Geophys., J5, i3i-l45. Rama Kao, L., iy56. Kecent contributions to our knowledge of the Cretaceous rocks ot South India. Proc.Indian. Acad, bci., 44, Rao, b.K.N. and iiri^jadarao, IC., 1935. Un the occurence of Foraminifera and Kadioiaria in the Infra-trappean limestones near Pangidi, Kajanmundry. Cur. Sci., 13, J65. Rao, S.K.iN. and ijrii^ada Kao, K., ly36. Foraminifera of the Inter-Trappean beds near Kajahmundry. Kec. Geol. Surv. India, 71, 36y-Jy6. Rao, b.K.w. and bripada Kao, K., 1939. Forminitera of the Inter-trap^iean beds nears Kajahmundry Pal. Ind. New. Ser., 29, Mem. 2. Reeds, J.a., 1975. Electron microprobe anaysis. Cambridge University press. London, 400. pp. Reinhard, M., 1931. iJniversaldrehtichmethoden Wepf and CI. Basel. Riiigwood, A.L., 1974. The petrological evolution or island arc systeiu. Jour. beol. boc. Lond., 13U, iaJ-2U4. Roaidset, t., 1973. Rare earth elements in quarternary clays of the iNujuedai area, bouthern Norway. Lithos, 6, 349-37^. Roalaset, n,, 1975. Kare earth element deistribution in some t^recambrian rocKs and their pnyllosilicates. I'Jumedal. Norway. Ceochim. Cosmocnim. Acta, 39, 455-469. RoDerttoon, A.n.F. and Fleet, A.J., 1976. The origin of rere earths in metailiterous sediments of the Troodos masssif. Cy^jrus. Earth Planet, bci. Lett., 28, 385-394. Sartni, B., 1934. ben. Kep. Kec. Geol. Surv. India, 68, 22-2J. 179 Sdhiii, A., iy88. Cretaceous tertiary boundary events: mass extinctions of iriaiuiii eiirictuuent and Deccan voicanisin, Curr. :icience. D7, 10. SanasraDudne, P. 'W., 1963. Paleoiiiagnetism and the geology of Ueccan tra^js. Salt-as, P.A., Haskin, L.A. and llcCaiiuin, i.S., 1^83. Still water anortnosites a lunar analog ? Proc. lunar. i;lanet. Sci. Conf. l^tfi, beoptiys, Kes. (supplement), 88, 1527-b3y. SarbadliiKari, T.R., iy66. Secondary clay in Rajmahal basalts of India and Its relation to palagonite, cnloropherite. Min. rtag.,JD , 7:)U-77D. bastri, V.V., iy6l. Foraminifera and Ustracode from the Inter-trappean beds near Rajahmundry. Ind. Min., 1:3(2), 197-1^0. bdstry, V.V., iy53. A note on the Forminifera and Ostracoda from the inter-trappean beds near Kajahmundry. Rec. Geol. Surv, India, 92, /99-3I0. Sastri, V.V., I98i. Observations on the age of Deccan traps and related irap activity in India. In. Deccan volcanism, and related provinces in other parts ol the world. Geol. Soc. Inaia, i*iem. No. 3, bastrv , I'l.V.A. and Lahari, T.C., 1981. Lite during Ueccan trap episode. In: Deccan volcanism and related basalt provinces in other parts ot the world. Geol. Soc. India, Mem. 3. Sdtyanarayana, T., 1973. Fossil llora ot Andhra Pradesh (India) Godavari District, Ph.D. fhesis submitted to Poona University, Poona. baunders, A.D., larney, J., Weaver, S.D. and barker, P.F., 1982. Scotia sea floor Geochemistry ol basalts from the Drake passage and south sand whicn Spreading centre. In : C. Craddock (editor), Antarctic beoscience. int. Lfnion. beol. Sci., 4, Zid-2ZZ. bduiidera, A.U., i904. The rare earth element charecteristics of igneous rocKs trom the ocean basin, In: Rare JKarth elements Geochemistry, 180 ed. P. Headesson. (Elsevier, Amsterdam), 2UD-2J6. bcniiiin^, J.W., iy7l. Sea-tloor evolution. Rare eartn evidence. Phil. Trans. Koy. Soc. London, 26«, 66JS-7U6. Schreiber, ti.D., Lander, H.V. and Thanyasiri, T., 19a0. The redox state of cerium in basaltic magiiias an experimental study ot iron-cerium interactions in silicate melts. Geochem. Cosmochim. Acta, 44, iDyy-ieii. bchweitzer, J. and lironer. A., IW5. Geochemistry and petrogenisis of early proterozoic intracratonic volcanic rocks of the Ventersdorp buper group, bouth Africa. Chem. Geol., 51, 26^-2^8. aen, b., iyd6. I'linerology and petrogenisis of the Ueccan traps lava flows around rtahabaleswar, India. Jour. Petrol., 27, 3627-66J. ben, b., i98'8. Possible depth of origin of primary Deccan tholeiite magma in: Workshop on Ueccan flood basalts. Geol. boc. India, JD-D2. bnapiro, L. and Brannock, W.W., 1962. Rapid analysis uf sioicates, carbonates and phosphate rocks. Contrib. Geochem. Geol. Surv. bull., ii44-A, USbS, US bovt. Printing office. Wasington. bhiiiii^u, N. and Arcuius, R.J., 1975. Rare earth elements concentrations ill d suite of tSasanitoids and alkali Olivine basalts rroin Grenada, Lesser Antilles. Contrib. Mineral. Petrol., 50, 2Jl-24u'. Siioey, D.f., Vogei, T.A., WalKer, H.n. and Byerly, G., 197&. The origin o£ osciiiatary zoning in plagioclase : a diffusion and growth coatroiied moaei. Aaier. Jour. Sci., 276(3), 273-284. Smitn, J.v., 197^. Feldspar minerals 2. chemical and textural properties. bpinger-veria'3 tierlin, see page 6««. bmith, K.£., iy76. beggregation vescicles in basaltic lava. Am. Jour, bci., /65, 6y6-713. 181 btaudigei, H.. frey, f.A. and hart, S.A., 1979. In compatiable trace element geocheuiistry and tJr«7/Sr/«6 in basalts and corresponding glasses and palagonite. In. T. Uonnetty, J. Franche/team et al., Initial reports of tne Deep sea drilling project, 51, 52, 53 part. US. Goverruiient Drilling office, Washington D.C., 1137-1144. Stosch, H.G. and Seek, H.A., 19a0. Geocnemistry of two spinel i^eridolite suites from drfeiser Weiher, West Germany. Geochim. Cosmoctiim. Acta, 44, 457-470. SuDDa Kao, S., iy65. Origin and evolution of tne Ueccan trap Igneous activity. Jour, of the Geological Soc. University of Saugar, 1. bubba Kao, K.V., ijreenivasa Rao, Ramasubba Keddy, M., Prasad, C.V.R.K. and Mrs. tlariharan, M., lyaa. Use of correlation coefficients patterns in cnemical statigraphy and petrology - A case study of the Deccan volcanics in parts of tne Naruiada Region. In. Deccan flooa basalts Geol. iioc. India. Mem. 10, 199-212. SuKneswald, K.N. and Polder Vaart, A., 19:)a. Deccan basalts of tne Bombay area, India. Geol. Soc. Aiuer. Bull., 69, 147:3-1494. Suji, S.S., i'Jesbitt, R.W. and Sharskin, A.ifa., 1979. Chemical charectersticb of mid-ocean ridge basalts. Earth Planet. Sci. Lett., 44, 119-i3o. Taylor, b.K., twart, A. and Capps, A.t., 196ti. Leucogranites and Rnyolites? trace element evidence tor fractional crystdllization and i^artiai meltin5. litnos, 179-la6. Takasashi, £. and Kushiro, I., 1983. rteltin^ of a dry peridotite at hign pressures and basalt magma genisis. Aui. Min., u«, »59-a79. Tiwari, a.D., 1971. rtagmatic differentiations of the volcanic rocks at Favagarh, Gujarat, India. Bull. Volcan., 35, 1129-1177. Tnompsoii, G., Brayan, "W.ii., frey, f.A. and. ^un^, CM., 1974. Petrology and 182 geocnemistry ot basalts and related rocks from sites 2I4, 21:), 216, UbUP leg, 24 Indian ocean. Initial reports ot the Deey sea drilling t^roject. AXil, 4:)y-4v.ci. Thompson, K.N., Gibsin, I.L., Warriner, G.F. Mattey, U.P. ana Morrison, M.A., i^ttU. Trace element evidence ot uiultistate mantle tusion and polybasic tractional crystallization in the Palaeocene lavas of bkye, N.W. Scotland. Jour. Petrol., 21, 26i-29J. Tnoiii^json, K.i^., rtorrison, M.A., Uickin, A.P., Bibson, I.L. and Harmon, K.b., 1954. Diverse styles ot interaction between basic marinas and continental crust in tne British Tertiary volcanic province. ISEM. Proceeaiu^s ot the conference on ot bkye, w.W. Scotland. Jour. Petrol., 21, -159. Thorton, C.P. and Turtle, O.F., 1956. Application ot the ditteratiation index to tne petrolo^ical problems, beol. Soc. Amer. Bull., 67, 12, 1738-1739. Upton, B.G.J, and Worasworth, W.J., 1972. Aspects ot magmatic evolution on Reunion island. Phil. Trans. Koy. Soc. Land., 271, 105-lJO. Vallence, T.G., i9vj9. Spilltes a^ain : some consequences ot the degradation ot basalts. Proc. Linn. Soc. N.S.V. 94, 8-51. Vance, J.S., 1962. Zoning m igneous plagioclases normal and oscillatory zoning. AEier. Jour. Sci., 2oU, 740-760. VeuKayya, £., 1949. Ueccan Trap outliers ot the Godavari Districts. Proc. Ino. Acad. bci., 29, 431. Weill, U.F. .and Urake, ri.f., 1975. Luropium anamoly in plagiclase teldspart.. Experimental results and semi quantitative moael. bcieuce, 1801, 1UD7-Iu6i. Wiisnire, H.G., 19Da. Am. tlin., 43, see p. 120. 183 •Wood, D.A., Joron, 3.1. and Treuil, H., 1979. A reappraisal of the use of trace elements to classify and discriminate between uiagma series erupted in aifferent tectonic settings. Earth Planet, bci. Lett., 45, SZ6-S:i^. Wasnin^ton, h.S., 1922. Averase of il analysis of Deccan Traps. Bull, beoi. boc. Am., JJ:J, 7V^^-8U4.