Malani Rhyolites — A Review

Gondwana Research, L? 3, No. I, p p . 65-77. 02000 International Association for Gondwana Research, Japan. ISSN: 1342-937X

Malani Rhyolites

=

A Review

S.K. Bhushan Geological Survey of India, Jhalana Dungri, Jaipur-302 004, lndia (Manuscript received March 3,1999; accepted July 29,1999)

Abstract The late Proterozoic Malani bimodal volcanics constitute the largest suite of anorogenic acid volcanics in India. The volcanism took place during 745+ 10 Ma ago, succeeding the granitic activity of Abu pluton and ceased before the onset of Marwar sedimentation. On the basis of field evidences, three stages of igneous activity have been recognised. Volcanics of the first stage are mostly basalt with occasional andesite or trachybasalts. These are subsequently covered by the voluminous outpouring of peralkaline and peraluminous rhyolite, basalt, dacite and trachyte flows. The third stage ceased with the outburst of ash flow deposits. The dominant felsic volcanics are rhyolites and rhyodacites spread over an area of about 31,000 km2.The other rock types associated with rhyolite are trachytes, dacites, pitchstone, welded tuff, vitric, lithic and crystal ash, ignimbrite, obsidian, pyroclastic slates, agglomerate, volcanic breccia and volcanic conglomerates. Majority of the acid volcanics are high potassic and a few are calcalkaline or low potassic in composition. Feldspar geothermometry suggests the temperature of equilibrium to be above 650°C. Similar results were obtained by magnetite-ulvospinel geothermometry. Oxygen fugacity is estimated to be about 10-l8under FMQ-Ni-NiO buffer conditions. Malani volcanism was essentially under terrestrial conditions, although deposition by aqueous conditions are also indicated. The volcanic eruptions have been through fissures, shield volcanoes and central cones. The volcanism was triggered in an extensional tectonic regime of continental crust, where geotherm was raised by the repeated influx of basic magma. The initial basaltic magma was possibly generated at deeper depth by ‘hot spot’ activity. This magma while migrating upwards supplied additional heat for the partial melting of lower sialic crust resulting in the generation of felsic magma. The crustal extension has helped in the upward advancement of the felsic magma. Key words: Bimodal volcanism, Malani rhyolites, geochemistry, magma genesis, tectonic setting, Rajasthan.

Introduction Nomenclature The term ‘Malani’was introduced by Blanford (1877) for volcanic series of porphyritic lavas and ash beds occurring in parts of Barmer district, Rajasthan, formerly known as Malani area. La Touche (1902) called the volcanics as ‘Malaniseries’,while Coulson (1933) named them as ‘Malanisystem’.Pascoe (1960) called it as ‘Malani Granite and Volcanic Suite’ and grouped them under Trans-AravalliVindhyan. For long the Malani were known as ‘Malani rhyolites’. Murthy et al. (1961) described the rhyolites and associated granites as Malani Igneous Suite (MIS), which is presently in vogue.

Areal Extent The MIS occurs as residual hill tors, inselbergs and

scattered hummocks over 5 1,000 sq.km. area (including area below cover sediments) in western Rajasthan covering parts of Jaisalmer, Barmer, Jodhpur, Pali, Jalore, Bikaner and Sirohi districts (Fig. 1). More than 95% of the area is blanketed by wind blown sand and sand dunes of the Thar desert, leaving only a small portion with outcrops. Major outcrops of rhyolites can be observed at Siwana, Barmer, Pokaran, Jodhpur, Taratara, Bisala and Mandli. The volcano-plutonic association of MIS occurs in juxtaposition with middle to late Proterozoic Delhi Supergroup of rocks in its eastern limits. In southwestern Rajasthan, the volcanics have been described from Gurapratap Singh and Diri in Pali district by Srivastava et al. (1989a, 1989b). In northeastern Rajasthan the volcanics and associated granites occur in a 30 to 50 km wide zone from Jhunjhunu in Rajasthan (Basu, 1982) and

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Fig. 1. Geological map of Malhni Igneous Suite,western Rajasthan. Note : Cover sediments overlying MIS are not shown.

Tusham in Haryana, (Kochhar, 1973) and are perhaps coeval with the MIS thermal event. These volcanics are not included in the MIS since they do not occur in strike continuity with the main Malani volcanic terrain of western Rajasthan. The rocks of Diri and Gurapratap Singh are dated 779 Ma by Rathore et al. (1996) and Wsham granites at 757+ 40 Ma byvinogradov et al. (1964). These dates are not supplemented by field evidences. The characteristic boron metasomatism resulting in the formation of tourmaline is absent from Malani rhyolites or granites and is omnipresent in all other Neoproterozoic

felsic magmatic events. The pneumatolytic activity associated with tin and tungsten of Balda in Sirohi district, Degana in Nagaur district of Rajasthan and Wsham in Haryana are pre-Malani thermal events (Bhushan, 1993). Therefore Diri and Gurapratap Singh areas have not been included in the present text which is also supported by Kochhar (1998). Previous work

The only summarised account of MIS available is by La Touche (1902) who discussed the physical, GondwaiResearch, V.3, No.1,2000

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petrographic features and also nature of eruption. Most of his field observations and interpretations have remained undisputed. Pascoe (1960) attempted a generalised classification based on previous data. His stratigraphic order did not include any basic volcanics, which are now believed to prevail in many areas below the rhyolites. Murthy et al. (1961) studied the petrographic characters of sodic and peraluminous rhyolites of Banner. The acid volcanics of the same area have also been investigated by Venkataraman et al. (1968), Krishnan (1968), Mukherjee (1962) and Mukherjee and Roy (1981). Pareek (1981a, b) attempted petrochemistry of MIS. Kochhar (1984) and Eby and Kochhar (1990) studied the volcano-plutonic association and type of rnagmatism. Srivastava op.cit. elucidated the major and trace element geochemistry of felsicvolcanicsfrom Pali district. Maheshwari et al. (1995) described the crustal influence in the petrogenesis of rhyolites based on oxygen isotope studies. Dhar et al. (1996), on the basis of Sr, Pb and Nd isotope studies concluded that the Siwana alkaline magma for granites

as well as for rhyolites is mantle derived. They further concluded that initial SF7/SrE6ratio of 0.7062 -t 0.0020 indicates some crustal contamination and the presence of Archaean crust in the area. Bidwai and Krishnamurthy (1997) illustrated some of the volcanic landforms from parts of the Siwana Caldera sequence. Barring some marginal attempts a comprehensive genetic model pertaining to generation of both basic and acid magma, ascent of the fluids and emplacement of the lava piles and their physico-chemical condition of crystallization is yet to be put forward.

Fig. 2A. Xenolithic fragments of basement granite embedded in overlying rhyolite.

Fig. 2B. Pentagonal and hexagonal columnar jointing in rhyolites near Mandli.

Fig. 2C. Spherical and ellipsoidal bombs, where the liquid blebs are sufficiently fluid, the surface tension draws them into shapes approaching spheres, where less fluid blebs are present, these remain ellipsoidal.

Fig. 2D.Concentric ribbons of bombs drawn during differential cooling.

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Stratigraphic position The rocks of MIS are directly underlain by metasediments or granitoids of the mesoproterozoic Delhi Supergroup. A 98 m thick volcaniclastic boulder conglomerate defines the angular unconformity at the base of Malani rocks near Kankani, 30 km south of Jodhpur. The conglomerate is overlain by vitric and crystal ash and rhyolite flows of MIS. Near Siyana, the Mount

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Abu granitoid (800 f 50 Ma, Choudhary et al., 1984) is directly overlain by Malani rhyolites. Xenoliths of basement granite are abundant in the lower layers of rhyolites (Fig. 2A). MIS is unconformably overlain either by Pokaran boulder bed of glacial origin of Vendian age, (680 to 580 Ma) or by sediments of Marwar Supergroup (Vendian to lower Cambrian). Therefore the MIS represents a post Abu granite and pre-Vendian glaciation episode (Bhushan, 1988). Some of the rhyolites were dated by Crawford and Compston (1970) by Rb/Sr method at 745+ 10 Ma, whereas the granite intruding these felsic volcanics has been dated at 735 Ma by Aswathanarayan (1964), 731+ 14 Ma by Rathore et al. (1996) and 750+ 15 Ma by Dhar et al. (1996), reflecting a short volcano-plutonic magmatic event.

Phuses of igneous activity Based on field relationships, mode and type of magmatism, texture and compositions, three phases of igneous activity have been recognised in MIS. The first phase commenced with the eruption of basic flows, followed by voluminous acid flows which culminated with ash flow deposits. The second phase experienced intrusion of discordant peraluminous and peralkaline granites as plutons, ring dykes and bosses and the third phase registered the prolific intrusion of mafic and felsic dyke swarms. A generalised classification of MIS is given in Table 1. The present paper deals only with the first phase acid volcanics.

Felsic Volcanics Pyroclastics and acid lava flows constitute the most voluminous unit of MIS representing the second largest volcanic suite in India after Deccan Trap. The acid volcanics are represented by rhyolite, rhyodacite, dacite, trachyte and pitchstone. Welded tuff, vitric, lithic and partially crystallized tuffs, ignimbrites, obsidian, pyroclastic slates, agglomerates, volcanic breccia and volcaniclastic conglomerates are also associated with acid volcanic rocks. The acid volcanism occurred in three stages. The first stage is represented by pyroclastic explosions (Stage-I), the second stage by acid lava flows (Stage-11) while the final stage is marked by another violent phase of pyroclastic ash fall (Stage-111). The rhyolite flow sequence of Stage-I1is the principal member of the acid volcanic rocks. Thc pyroclastics of the initial and final stages are not much different megascopically. Acid lava flow (Stage-11)

This is the most voluminous unit of the volcanics with the maximum exposed thickness of 1950 m at Siwana. All the well known characteristics of glassy lavas can be observed. Some of them show excellent flow structure, and in many cases, original glassy texture has been retained. Columnar jointing is observed occasionally (Fig. 2B). In the hand specimens the rhyolites are rarely vesicular, generally light maroon to brick red in colour. The colour

Table 1. Generalized classification of Malani Igneous Suite. Super Group Marwar Supergroup Wendian to lower Cambrian) Malani Igneous Suite (upper Proterozoic)

Group

Formation

Mode of Magmatism

Lithology Sandstone, shale, limestone and evaporites

--------------Unconformity ---------------.................... Dyke swarms

Basic dykes; Acid dykes; Intrusive dyke Trachyte porphyry, Phase-I11 Andesite and porphyry dykes Aplite and Diorite plugs.

Granitoid plutonism

Malani granite Siwana granite Jalore granite Rhyolite, Trachyte and Basalt flows

Bimodal volcanism

Intrusive phase-I1

Extrusive phase-I

Gabbro, dolerite, basalt, granite; rhyolite porphyry Trachyte porphyry andesite porphyry porphyritic/non-porphyritic dykes &boss and aplite veins. Hornblende granite riebeckite/ aegirine granite biotite/ hornblende granite Rhyolite, dacite, trachyte and rhyodacite flows. Basalt and trachyandesite flows

--------------Unconformity -------------------- -----_--------_ Pre Malani basement (middle to lower Proterozoic)

Aravalli and Delhi Supergroup

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may vary from flow to flow, band to band and even from one cooling unit to another. In some cases they are dark grassy green, light blue, yellow or white in colour.

Structure in rhyolites The MIS has not been subjected to any deformation or metamorphism. Effect of weathering and hydrothermal activity is, however, evident from the presence of epidote and chlorite. As the area remained tectonically stable, the primary flow structures in rhyolite remain intact. On the basis of primary flow structures following observations can be made: a) The flow layers vary in thickness from 1 mm to 1 cm. They are defined by variation in colour or accumulation of phenocrysts. The flow layers are also detected with the help of intercalations of earlier cooling units in later flow layers. The earlier cooling units are often stretched in the flow direction. Some fluidal bands swirl around the phenocrysts indicating earlier formation of the latter. b) The flow layers at places display primary flow folding characterized by disharmonic fold pattern, rounded hinge, variation in wave length, amplitude and also in the thickness of the individual layers. That the flow folds are primary is also evident from the fact that such folds are not associated with any secondary structural elements. c) There are flow breccias formed by fragmentation of units undergoing cooling which were subsequently embedded in the younger layers. These breccias generally do not follow the flow direction. The fragments and matrix have same colour, texture and composition. One of the best occurrences of flow breccia is seen at Dodiyali, 40 km east of Jalore. The flow lines are generally seen by the orientation of phenocrysts along the flowage. Tabular feldspar and ferromagnesian minerals are also helpful in recognising the flow lines. Vesicles are not common in rhyolites except for their occasional presence in some flows. The vesicles are tubular, ranging from a few millimeter to several centimeters in length. Amygdules are rare, but when present are composed of secondary silica, calcite or epidote. Pisolitic structures (3 mm to 1 cm in diameter resulting from the capped vesicles) have been seen near the volcanic vents. Mega-spherulites upto 15 cm diameter have been observed near Siwana. Lapilli, bombs (Figs. 2 C and D) and blocks are commonly embedded in rhyolite lava, particularly near the source of volcanism. Pipe breccia conduits filled with rock fragments embedded in magma matrix, are found in the vents north of Mandli. Vent agglomerates and perlitic tuff ejected during the violent phase of volcanic activity Gondwana Research,

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from central cone are observed in association with rhyolite flows at Mandli. Collapse breccia formed due to collapse of the peripheral portion of the caldera during the penultimate stage of volcanism is found to be associated with the rhyolite flow near Barmer. One of the examples of intrusive breccia dyke associated with rhyolite flow is seen west of Mandli. The brecciated materials present in the fissure could not erupt but travelled with the ascending magma for some distance before cooling.

Nature of the rhyoliteflows The rhyolite flows in well exposed areas have been shown in Fig. 1. There is, however no way to correlate the flows of one area with the other as there are no persistent marker horizons, and the outcrops occur as inselbergs, separated by long stretches of sandy terrain. Further as the lavas are very viscous, (logn of rhyolites is 7.43 poise at 1202°Cand 6.22 poise at 1304°C;Mukherjee et al., 1985), abrupt termination of flows (blocky lava) is common. A single flow may not continue for more than 10 km from its source. Several flows have been distinguished in different localities, e.g., 45 flows in Siwana caldera, where cumulative thickness of flows and pyroclastic is more than 3300 m.

Pyroclastics (Stage 1and 111) The Stage-I pyroclastics represented by lapilli tuff, vitric and crystal ash, welded tuff are seen in the basal sections of volcanic lithocolumn. At Kankani, the basal lapilli tuff of 1025 m thickness interleaving between two felsic flows has been recorded. The Stage-I11 pyroclastics reflect the deposition of vast ash flows and tuff covering 1200 sq.km in south of Pokaran. Their persistence in depth upto 60 m has been recorded. These are light yellow, green or brown thin-bedded ash flows and tuffs. They contain microscopic xenolithic fragments of basalt and rhyolite indicating that their deposition took place after the StageI1 volcanism.

Petrography In hand specimens the rhyolites are characterized by phenocrysts of quartz, feldspar and occasional ferromagne,sian minerals in a fine-grained or glassy matrix. Under microscope, the alkali feldspars are found to be K-feldspar, sanidine and anorthoclase having embayed margins, suggestive of magmatic corrosion. They sometimes contain inclusions of fluorite. They often display perthitic intergrowth or the alkali feldspar displaying marginal zoning. Plagioclases are mostly of sodic affinity. Quartz phenocrysts often have resorbed outline, but the euhedral bipyramidal grains are not

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uncommon. Secondary quartz grains sometimes splay out from the edge of the phenocrysts as minute crystals. Fluid inclusions are noted within the quartz. Rounded to hexagonal basal sections of cristobalite grains with dark interference colours and characteristic curved fractures have been recorded. Hornblende and biotite are present in some peraluminous rhyolites. The former often occurs as aggregates or as microlites. In alkali rhyolites, the ferromagnesian minerals are usually aegirine augite and in some cases riebeckite (X = blue, Y = violet, Z = yellow to bluish violet). Secondary chlorite and epidote (formed as diagenetic product) are also present in some cases in association with feldspar. Whenever amygdules are present, they are filled with chalcedony or calcite. The groundmass is devitrified acidic glass composed of quartzo-feldspathic mosaic. The feldspar phenocrysts have the same composition. Quartz is quite abundant. Feldspars are sometimes in preferred orientation. Occasionally spherulites are developed, which are at times upto 6.5 cm across. The microspherules are filled up with radiating fibres of feldspar and quartz and also by ferromagnesian minerals. The spherules follow the primary flow structures. Alkali amphiboles also occur as acicular grains or needles in groundmass. The flow layers marked by difference in granularity and colour are generally replaced by secondary iron minerals, calcite or quartz. Riebeckite and aegirine are common constituents of groundmass. In alkaline rhyolites, aenigmatite with very high relief and blood red pleochroic margins is associated with green pleochroic alkali pyroxene (aegirine) . The main accessories of rhyolite are zircon, apatite, magnetite, occasional fluorite, calcite, epidote, zoisite and some opaques. The vesicles, when present, are filled up by calcite, quartz, chlorite or epidote. Some vesicles have only fibrous quartz along rims, leaving the central portion for calcite or other secondary minerals. The interflow pyroclastics are mostly vitric and crystal tuff,although ignimbrite, volcanic breccia and agglomerate are present in central type eruptions. The vitric tuff contains angular grains of quartz, K-feldspar or plagioclase in a glassy groundmass. Calcite and chlorite are the secondary mineral associates. Glass shards are seen. The crystal tuff contains broken grains of quartz and sodic plagioclase, together with chlorite in a dense vitric mass. The lapilli tuff consists of

coarse angular grains of quartz and feldspar, with grain boundaries fractured and displaced. The groundmass often contains aggregates of feldspar, Iapilli and flame composed of finely crystalline quartz and chlorite. The large lapillis contain relict perlitic structures which control the shape of glass. The matrix also contains fibrous green biotite,zoisite and fine opaques. The vugs, when present, are internally lined by crypto crystalline quartz or by aggregate of clay minerals, sericite or calcite.

Geochemistry Major element chemistry More than 300 major element analyses including 55 analyses by Pareek (1981b) on Malani volcanics carried out by wet chemical, AAS, XRF and ICPAES methods are available with the author in a diskette (to be supplied on request). Almost all samples contain H,O mainly in the form of dissolved species within glass and some are associated with the crystal structure of hydrous minerals. CO, is considered to have been released from secondary calcite. F (upto 0.1%) is related to inclusions of fluorite in K-feldspar. P,O, is present in traces and is reflected by the presence of apatite. Although the rhyolites have not suffered any metamorphism or deformation, they have been subjected to extensive alteration and devitrification. The amount of K,O and Na,O is highly variable. The peraluminous rhyolites have more K,O while peralkaline rhyolites have marked increase of Na,O over K,O. The total alkalis (Na,O + K,O) range between 6% and 10.85%. The Mg/(Mg+Fe) ratio is < 1 while SiO,/K,O ratio is generally <20. The average chemical composition of peralkaline and peraluminous rhyolites are furnished in Table 2. Since modal analysis of the mineral phases is not quite representative due to their porphyritic nature, a chemical classification is often used for the nomenclature. Peccerillo and Taylor (1976) defined rhyolites having > 70% SiO, while Ewart (1979) considered rhyolite with > 69% SiO,. Hanson (1978) treated rhyolite with > 69% SiO, and < 50% anorthite content. Barker (1981) called rhyolites having > 69% SiO, and 3.5 to 5.5% q0. Le Bas et al. (1986) kept rhyolite at > 70% SiO, with > 8% (Na,O +

Table 2. Average composition of rhyolites.

Peraluminous rhyolite #(n = 13) Peralkaline rhyolite #(n = 55)

SiO,

TiO,

A1,O

FeO(t)

K,O+Na,O

MgO

CaO

71.52 (2.05) 71.70 (0.96)

0.70 (0.18) 0.56 (0.10)

10.07 (1.42) 9.54 (0.91)

7.23 (1.33) 7.87 (0.67)

6.14 (1.35) 7.14 (1.57)

1.26 (0.90) 0.78 (0.43)

0.33 (0.18) 0.65 (0.18)

#Values in bracket indicate standard deviation. n = number of samples

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Q

A Ab

Or

50

Fig. 3A. Normative albite - anorthite - orthoclase ternary diagram. Fields after OConnor (1965). Filled circles = rhyolites; open circles = felsic tuff.

Fig. 3B. Normative quartz-alkali feldspar-plagioclase diagram of rhyolites. Fields after Streckiesen (1976); symbols as in A.

K,O). In MIS, the felsic volcanics with > 69% SiO, have been termed as rhyolites. Accordingly when the CIPW normative values are plotted in O'Connor (1965) and Streckeisen (1976) diagrams (Figs. 3A and B) majority of analyses plot in the rhyolite field of former and alkaline feldspar rhyolite field of latter. A few samples plot in dacite and rhyodacite fields also. Based on SiO, versus K,O diagram of Peccerillo and Taylor (1976) and modified by Ewart (1979), 95% of rhyolites with >69% of SiO, plot in high K-rhyolite field (Fig. 4). Most of the vitric tuff samples plot near the quartz end in QAP diagram indicating their high silica nature. The bimodal character of Malani volcanics is reflected by the percentile silica difference of 22% to as high as 67%, suggestive of wide SiO, gap.

The alkalinity of the rhyolites has been defined on the basis of (a) net amount of alkalies (Na,O + K,O), (b) Agpaitic index: Mol (Na,O+K,O)/Al,O, (c) A/CNK ratio, (d) presence of normative acmite, and (e) presence of alkali pyroxene and/or amphibole in petrographic sections. The CIPW normative composition has been used to distinguish between corundum and quartz normative and acmite normative rhyolites. However, the peralkaline rhyolites are defined by the presence of any alkali pyroxene or amphibole minerals either as phenocryst or in groundmass assemblage. The peralkaline Malani rhyolites plot in the pantellerite field (Macdonald, 1974) in association with trachyte, indicative of anorogenic setting.

6

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. I

.d

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a

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8.

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6

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I

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9.9

K- trac hy t e

c

(calc alkaline)

Andesite (calc alkaline

3

Low K-dacite

Low K-andesite I

,

1

1

1

1

Low K-rhyolite 1

I

b

~

I

Fig. 4. ~

O

&O versus SiO, (wt %) diagram P showing plots of rhyolites. Fields after Peccerillo and Taylor (1976).

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Trace element geochemistry The average trace element contents (in ppm) of peralkaline and peraluminous rhyolite are listed in Table 3. Table 3. Average trace element abundances of rhyolite. Elements Ta Nb Hf Zr Th Y La Ce Nd Sm Eu Tb Yb

Lu Ba Sr

Peraluminous rhyolite (n=44) 15 186 113 6853 41 331 3 78 862 413 78 6.9 15 60 7.7 120 15

Peralkaline rhyolite (n=16) 32.5 252 215 8079 101 613 497 1098 545 137 11 29 128 15 76 15

I,

' ' ' ' L o C e Nd

I

' ' Eu T b '

I

'

I

I

Yb

Lu

'

Fig. 5A. Primordial mantle normalised trace element spider diagram for rhyolites. 10000

1

1

1

1

1

1

1

1

1

1

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(b) a, 1000 +

c

CJ

The peralkaline rhyolites are enriched in all the trace elements listed above except Ba, when compared to peraluminous rhyolites. The higher enrichment of HFS elements (Zr, Nb, Hf, Ta) is considered as a diagnostic feature of the alkaline magma by Salvi and Jones (1990). Wide variation in Zr/Nb, Y/Nb, Zr versus (Ce/Yb)N ratios is suggestive of non-magmatic fractionation for the generation of felsic magma from a basic magma parentage. Both mafic as well as felsic lava flows plot in the 'within plate lavas' of Pearce et al. (1984), well away from convergent or divergent plate tectonic fields. Leat and Thorpe (1986) opined that the bimodal volcanic suites with peralkaline rhyolites represent zones of crustal extension. Fig. 5A. exhibits primordial mantle normalised trace element variation spidergram for the average peralkaline and peraluminous rhyolites. The relative enrichment of Th, La, Ce, Zr and Y is indicative of involvement of crustal component in the melt. The negative Ba and Sr anomaly and anomalous Zr enrichment is considered as characteristic of rift related volcanics (Taylor et al., 1981). The low Sr is due to its compatibility with plagioclase. The high Zr is due to original alkalinity of the parent magma or source rock. In Fig. 5B. chondrite normalised REE patterns of peralkaline and peraluminous rhyolites are shown. Both types of rhyolite have strongly depleted Eu, giving rise to negative anomalies. Such Eu depletion is caused by partitioning into feldspar, fractionation of which is an important process in developing peralkalinity. Storey (1981) has suggested that the extended feldspar

-

.-0 7 0

10

.-E L

Q 1

1°

~0

L

aI

Peralkaline rhyolite I

I

I

I

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1

fractionation is important in producing peralkaline rhyolites. The higher concentration of LREE indicates excessive presence of zircon, apatite and sphene which have not moved out of the melt. The flat HREE pattern suggests their incompatibility during later stage of crystal fractionation.

Malani Volcanism The Neoproterozoic magmatism in Rajasthan during Delhi orogenesis led to the intrusion of syntectonic Erinpura granite/ granodiorite (900 Ma : Chaudhury et al., 1984), with pneumatolytic phase enriched in Sn and W (Chattopadhyay et al., 1982). This was followed by late tectonic granitoid plutonism of Mt. Abu batholith (800 k 50 Ma; Chaudhury et al., 1984). This continuum of felsic magmatism culminated with the large scale outpour of bimodal volcanism and emplacement of anorogenic 'A' Gondwana Research, V. 3, No. 2,2000

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type granites of MIS between 750 to 730 Ma (Bhushan, 1995).

Mode of eruption The viscosity which gives resistance against fluidity does not permit lavas to flow over a large distance. Although the amount of volatiles, volume of lava, the surface temperatures, paleoslope etc., are the factors responsible for movement, but none can dominate over the high viscosity and enhance movement of flow considerably. Thus, rhyolitic lava flows over large distance from central type of eruption does not seem possible. Although presence of H,O rich volatiles can reduce the viscosity to a great extent (log, = 12.5 poise at 800°C when rhyolite contains 0.1% H,O to log, = 5.5 poise at 800°C when it has 6.2% H,O; Shaw, 1980), still it is hard to visualize flowage beyond a few kilometers. Rhyolitic rocks in the Malani area therefore represent an acid volcanic complex comprising (1) ignimbrite eruption (Jodhpur Fort) (2) rhyolite dome (Barmer, Taratara) (3) hot avalanche eruptions forming partially welded or unconsolidated deposits (Phalsund) (4) rhyolite flows (constitutes ~ 6 0 % of the volcanics) (Agolai, Korna, Bisala, Siwana, Jasol etc.) and (5) ash fall eruptions (Phalsund - Lava). Eruptions through closely spaced multiple fissure systems might be responsible for such a large volcanic pile. Central type of eruptions at places also produced voluminous eruptions. Siwana and Barmer calderas are the examples of such central type of eruptions. The Siwana caldera (32 x 25 km2) covers about 800 sq.km and has 45 volcanic flows comprising 6 of basalt, 20 flows of rhyolite, alkali rhyolite and rhyodacite, 15 of trachyte, one of dacite and three of trachyandesite composition (Bhushan and Chittora, 1999). At Barmer only a portion of a crescent-shaped caldera is exposed, the length of which is 30 km. One prominent central cone is located near Mandli 60 km west of Jodhpur (Bhushan, 1978). Type of eruption

The presence of pillow lavas, interbedded ferruginous chert and poorly sorted and tuffaceous rocks with graded ~ subaqueous eruption. There is bedding L I S L I ~ I Iindicates no such evidence in case of the basalts or rhyolite flows of the Malani area, although there are evidences of subaqueous conditions of eruption at Siwana and Jasol. At Siwana, in the central part of the subsidence structure where the lava flows are horizontal, the older flow is overlain by three conglomeratic beds presumably of aqueous origin interbedded with coarse to finely graded tuffs (La Touche, 1902). The conglomerate might have been deposited under aqueous condition when subsidence was taking place. Most of the cementing material of these conglomerates is Gondwana Research, K 3, No. I , 2000

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ferruginous tuff. The interbedded tuffs with these conglomeratesshow graded bedding, tabular cross bedding and near symmetrical ripple marks, all indicating their aqueous environment of deposition. However, aqueous conditions did not prevail for long in this caldera since the overlying vitric ash beds are fine, thinly bedded without any feature typical of deposition under aqueous condition. At Jasol, once again mega ripple marks were observed in finely laminated rhyolitic tuff over an area of 40m2, indicating aqueous condition of deposition (Gathania et al., 1984). Ripple marks are also observed in ash flow tuff near Kankani (30 km south of Jodhpur) and Lava (15 km SE of Pokaran). The subaerial bedded deposits of ash with typical channeling and occasional cross bedding are present. Therefore, Malani volcanism is essentially of sub-aerial origin although locally they were deposited under subaqueous environments.

Crustal anatexis for the production of rhyolitic melt It has been noted that in case of all the areas showing bimodal distribution of basalt and rhyolites, the felsic volcanics dominate over mafic volcanics, (e.g. Yellowstone, and Medicine lake in Western USA, Iceland, Western Scotland and Southern Queensland of Australia). No single parent magma can, therefore, be responsible for the generation of such suites of both basaltic and rhyolitic lavas. The author considers that fractional crystallization may have played only a minor role. Evidently, the generation of silicic magmas should be related to partial melting of a preexisting continental crustal material. Similar views have been expressed by Pareek (1981b) suggesting upper crust as the source material for generation of Malani rhyolites. The geophysical studies on Yellowstone caldera, USA indicate that thermally disturbed zone exists even below Moho. This gives indication of heat plumes accompanying even high level felsic volcanism. Since no geophysical data on these lines are available from Siwana or other known calderas of Malani area, it can only be suggested that the depth of magma reservoir might have penetrated below Moho discontinuity, causing greater inhomogeneity of the silicic magma composition.

Pressure-temperature condition of equilibration of the rhyolites Venkataraman e t al. (1968) used Barth's geothermometer to determine the temperature of equilibrium of feldspars in peralkaline granites of Siwana and found it to range between 660"C-740°C. Tuttle and Bowen (1958) have shown by experimental studies that only one feldspar crystallizes above 660°C. The presence of bipyramidal quartz and sanidine as phenocrysts is also indicative of high temperature of crystallization.

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Mukherjee and Roy (1981) showed that crystallization under a P,,, of 3-3.5 Kb occurs at about 650°C under NiNiO buffer conditions. However, under QFM buffer, arfvedsonite may react with liquid to form aegirine. Aegirine is also formed by reaction of aenigmatite and liquid under low f0, conditions (Grapes et al., 1979). f0, and temperature of crystallization of a magnetite from a peralkaline lava was determined using Buddington and Lindsley’s (1964) method. The magnetites with 18 mol.% ulvospinel (corresponding to 7 wt% TiO,) crystallized at 650°C at a f0, of 10 l8 correspond to crystallization under QFM-Ni-NiObuffer condition.

Cause of variation in the chemistry of acid volcanic rocks The Malani silicic volcanics are alkaline, sub-alkaline or pcraluminous types. Peralkaline and peraluminous flows occur simultaneously in the same place, sornctimes alternating with each other. The trachytc flows arc also associated with thc pcralkalinc rhyolites. Ewart (1979) observed the presence of meta aluminous rhyolites with peralkaline rhyolites in many bimodal distribution. He suggests a late stage fractionation of meta aluminous rhyolites to produce peralkaline rhyolites. The development of alkalinity has been taken as evidence of the crustal assimilation during fractionation by Ewart et al. (1977). The peralkaline rhyolites are generally present and associated with the central type of eruptions due to either (1) Central type of eruptions last for a long period as compared to fissure-type eruption giving more time for development of alkalinity in the magma chamber and during dormant stage or (2) the increase of volatile content during paucity may contribute towards alkalinity in the melt. Bailey (1969) indicated that lowering of f0, can yield alkaline-rich liquid due to breakdown of aegirine to sodic amphibole. He concludes that fractional crystallization under high vapour pressure will generate an alkaline magma. Mukherjee and Roy (1981) suggested earlier formation of sodic amphibole and later formation of aegirine by sudden lowering of oxygen fugacity and depressurization of magma chamber in Siwana area. The decrease in temperature increases the alkali content and volatiles, thus changing the oxidation reduction equilibrium causing oxidation of the melt. The volatiles greatly enhance the lowering of melting of mineral phases, thus suggesting possible melting at even lower temperature. In case of Malani rhyolite, the peralkalinity of the melt might have been controlled by the vapour pressure rather than inhomogeneity of the starting materials.

Magma genesis The development of silicic magma can take place by (1) fractional crystallization from parental mafic magma

without assimilation of wall rock, (2) with mixing, assimilation or chemical equilibrium with wall rock during or after fractional crystallization, (3) mixing of two or more already fractionated magmas and (4) partial fusion of acid - intermediate crustal rocks with or without superimposed crystal fractionation. The anatexis will result from extensive high temperature mafic magma emplacement. The bimodal basalt-rhyolite associations occur in orogenic belts, above subduction zones, in continental ‘hot spot’ settings, in areas of crustal extension and even in oceanic volcanic provinces like Iceland (Bevins et al., 1991). The heat for melting is provided either (1) by continental ‘hot spot’, where the plate remains stationary over the mantle for more time or (2) by partial melting of underplatcd basic rocks at the base of crust, associated with lithosphcric thinning. According to Crccraft ct al. (1981), the high heat flow zone (hot spot) is crcatcd by injcction of mantle derived basalts into thc crust, which results in partial melting and generation of silicic cornponcnts. Whcn fair amount of felsic magma is accumulated, it ascends into the uppcr crust, which helps accumulation of extensional stress field. The felsic volcanics will find passage through NW trending normal faulting which is perpendicular to the direction of extension. Alternatively, underplating of basalt can also help in providing heat source for melting. Pareek (1981b), on the basis of major element chemistry suggested that Malani magmatism is lineament controlled and resulted from the contamination of tholeiitic magma with sialic crust. Kochhar (1984) attributed ‘hot spot’ magmatism to the generation of felsic magma and considered it as ‘A‘ type magmatic activity. Eby and Kochhar (1990) considered the peralkaline magma with high REE content to have been derived either as a high temperature melt from an anhydrous granulitic source or by melting of metasomatized lower crustal material. Maheshwari et al. (1996) also supported their ‘A‘ type affinity and suggested crustal involvement in the genesis of rhyolites. In bimodal volcanic suites, both mafic and felsic rocks are K,O rich and indicate rift environment (Condie, 1982). The rhyolites are low in CaO and high in alkalies and REE indicating ‘A‘ type magmatism as defined by Whalen et al., (1996). The gravity anomaly map of Rajasthan (Reddy and Ramakrishna, 1988) shows the presence of granites (density 2.87) at a very shallow depth. The density of this basement granite is very close to that of Archaean Banded Gneissic Complex (BGC). Dhar et al. (1996) also suggested the presence of Archaean basement below MIS. Therefore the possibility of generation of silicic magma of the Malani area by partial melting of pre-existing granitic basement i.e. the BGC, is not ruled out. A sketch Gondwana Research, V. 3, No. 1,2000

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Harsani

K a n kani

Siwano

Fig. 6. Schematic prcscntation showing the location of rifting, hotspot, dcvelopment of basic magma chamber in Archaean basement (BGC) and

subscqucnt formation of felsic magma reservoir and their eruptions.

model across MIS is shown in Fig. 6.The development of 'hot spot', starts updoming of the continental crust and hclps in propagation of continental rifts. The ascending basaltic magma produces a wide zone of melting in the overlying crustal rocks (BGC ?). These acidic melts from this zone feed thc volcanic activity by central and fissure eruptions, the conduit being active rifts. The basaltic magma chamber is occasionally tapped separately to feed surface flows. Pareek (1981b) also considered that upper continental rocks are the source material for fclsic melts. Wyllie (1983) experimentally showed that by partial melting of continental gneiss in presence of H,O (2%) will generate water undersaturated liquids of rhyolitic composition between 650"and 1000°C.With the increase of pressure (-- 10 Kb), the melting can start at 600°C. However, to prove that convincingly, studies involving different degrees of fusion on BGC under low vapour pressures at variable temperatures are required.

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