June 2008 LIP of the Month

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Speculations on the Proterozoic Large Igneous Province (LIP) Record of India

Richard E. Ernst (1) and Rajesh K. Srivastava (2)

(1)Ernst Geosciences (and Dept. Earth Sci., Carleton Univ., Ottawa), 43 Margrave Ave, Ottawa, Ontairo K1T 3Y2, Canada; Richard.Ernst@ErnstGeosciences.com
(2)Igneous Petrology Laboratory, Department of Geology, Banaras Hindu University, Varanasi, 221 005, India; rajeshgeolbhu@yahoo.com

ABSTRACT

The Large Igneous Province (LIP) record is a key tool for continental reconstruction. We consider the record of Indian LIPs and LIP-‘fragments’ from the Dharwar craton (mainly dyke swarms, 2370, 2180 and 1890 Ma, U-Pb); Bastar craton (dyke swarm, 1890 Ma, U-Pb); Singhbhum craton (mafic-ultramafic volcanics, ca. 1600 Ma); Eastern Ghats (alkaline province 1350 Ma); Eastern Dharwar (kimberlites, alkaline dykes, and rhyolite, ca. 1200-1000 Ma); and Aravalli craton (felsic volcanics and mafic dykes, 770-755 Ma, U-Pb).

Some of these magmatic events consist of possible radiating dyke swarms whose convergence point can be used to locate mantle plumes and associated continental break-up. Specifically, 2370, 2180 and 1890 Ma mantle plume centres are provisionally identified and are linked to the west, northwest, and east sides, respectively, of the Dharwar craton. Currently, in each case, the identity of the formerly conjugate margin is unknown, but ideally can be determined using the LIP record, once it has become better characterised both for India and other candidate conjugate blocks.

A few observations related to break-up and continental reconstruction are possible: (i) 2370 Ma event has not yet been identified anywhere else; any crustal block that is subsequently found to have magmatism of this age is likely to have been a nearest neighbour to the Dharwar craton, (ii) 2180 Ma event can potentially be matched with coeval events on Superior or Slave cratons, (iii) 1890 Ma magmatism is so widespread globally that it may not be a strong inter-block correlation tool, (iv) 1350 Ma magmatism appears relatively rare globally, apart from the east margin of Baltica, and northern Canada and (v) ca. 770-750 Ma magmatism of the Aravalli craton can be linked to breakup of the supercontinent Rodinia. Additional precise U-Pb & Ar-Ar dating (& coordinated paleomagnetism) are essential to definitely determine India's place in the Proterozoic world.

INTRODUCTION

Large Igneous Provinces (LIPs) and Paleocontinental Reconstructions

Large Igneous Provinces (LIPs) are large volume, short duration or pulsed igneous events, with an intraplate setting or geochemistry. The following definition is from Bryan and Ernst (2008) which is modified after the original definition of Coffin and Eldholm (1994, 2005). “Large Igneous Provinces are magmatic provinces with areal extents >1 Mkm2, igneous volumes > 0.1 Mkm3 and maximum lifespans of ~50 Myrs that have intraplate tectonic settings or geochemical affinities, and are characterised by igneous pulse(s) of short duration (~1–5 Myrs), during which a large proportion (> 75%) of the total igneous volume has been emplaced.”.  The various types of LIPs are illustrated in Figure 1.


Figure 1: Classification of Large Igneous Provinces (modified after Bryan and Ernst, 2008, to include associated carbonatites and kimberlites). The current classification builds on the initial work of Coffin and Eldholm (1994, 2005).

Recognition of LIPs (and interpreted erosional/tectonic fragments of LIPs; Ernst, 2007a) is important in solving many problems of modern geology such as understanding the timing and controls on climatic change, regional uplift, regional basin formation, Ni-Cu-PGE ore deposits, and fluctuations in the reversal frequency of the Earth’s magnetic field (e.g. Bleeker, 2004; Ernst et al., 2005). However, in the present contribution, we are most interested in using the LIP record for testing continental reconstructions (Bleeker and Ernst, 2006). In shield terrains the volcanic portion of older continental LIPs is largely removed by erosion, leaving only remnant flood basalts, and hence, it is mainly the exposed LIP plumbing system consisting of giant dyke swarms, sill provinces, and mafic-ultramafic intrusions (Ernst and Buchan, 2001) which is available for testing Precambrian paleocontinental reconstructions.

There are various criteria which can be employed in using LIPs to constrain continental reconstructions (modified after Ernst et al., 2008; see also Bleeker and Ernst, 2006):

(1) Key event comparisons: Identify particularly extensive and distinctive LIP events that can be matched between crustal blocks, and thereby identify that these blocks were nearest neighbors (see also point 4).

(2) Paleomagnetic studies: Compare paleomagnetic directions of coeval LIP units on different blocks to determine their relative paleolatitude and paleo-orientation.

(3) Geometry of dyke swarms: Reconstruct crustal blocks to restore the primary geometry of giant dyke swarms (belonging to LIPs): either restoring a primary radiating pattern or linear pattern.

(4) Geochemical fingerprinting: Compare the trace element geochemistry ‘fingerprints’ of LIPs on different blocks to identify matches that would suggest the blocks (and their associated LIPs) were formerly adjacent.

(5) Location of plume centre:  LIPs can be used to locate mantle plume centres (e.g. at the focus of a giant radiating dyke swarm). If plume centres of the same age can be identified on the margins of more than one crustal block, then the proper reconstruction of those blocks would result in the plume centre locations being superimposed. 

(6) Comparison of ‘barcode’ of LIP ages between crustal blocks: The LIP record for a crustal block can be summarised as a barcode and the LIP barcodes of different blocks can be compared to determine which blocks have matching barcodes and were therefore probably nearest neighbors during the interval of matching.

(7) Determination of breakup timing on a particular cratonic margin by dating the LIP associated with that margin. Cratonic margins with a similar timing of breakup were conceivably previously conjugate (or adjacent) to each other.

(8) Timing of accretion (assembly) of currently adjacent crustal blocks:  Prior to accretion, crustal blocks will have independent LIP histories, but after assembly/accretion, the LIP barcodes should match. In this way, the timing of accretion can be approximated from the LIP barcodes.

It is also important to note that reconstructions based on a single criterion are typically non-unique. The more criteria that can be applied successfully, the more confidence can be placed on the results. Herein we review the Proterozoic LIP record of the Indian shield, develop a preliminary barcode for the Indian shield, identify possible mantle plume centres for several events and consider the implications for Proterozoic reconstructions involving India. There is already an extensive scientific literature on reconstructions involving India especially during the period of proposed Paleo-Mesoproterozoic and Meso-Neoproterozoic supercontinents, Nuna (Columbia) and Rodinia, respectively (e.g. Zhao et al., 2004; Li et al., 2008 and references therein).  However, there is no consensus yet regarding the correct reconstructions and herein we wish to approach the problem freshly from the perspective of LIPs using selected criteria from the above list.

Overview of Indian Geology

The detailed geology of the Indian shield has been presented by many workers (e.g. Naqvi et al., 1974; Radhakrishna and Naqvi, 1986; Naqvi and Rogers, 1987; Naqvi, 2005; Srivastava and Ahmad, 2008; Mahadevan, 2008). The Indian shield is broadly an ensemble of dominantly Archean-early Proterozoic cratonic blocks and predominantly mid- to late Proterozoic mobile belts with Archean protoliths. The Indian shield may be divided into three protocontinents and subdivided into seven cratons viz. Southern Granulite Terrain, Western Dharwar, and Eastern Dharwar cratons in the Dharwar protocontinent, Singhbhum and Bastar cratons in the Singhbhum protocontinent and Bundelkhand and Aravalli cratons in the Aravalli protocontinent (Naqvi, et al., 1974; Naqvi and Rogers, 1987). The general nature of boundaries between crustal terranes is uncertain. Most boundaries are marked by thrust zones and/or rifts and at least two of the rifts appear to have formed along former thrust belts. However, the timing of thrusting and accretion of blocks is poorly known. Figure 2 presents generalised geologic and tectonic map of the Indian Shield.


Figure 2: Generalised geologic and tectonic map of the Indian shield (modified after French et al., 2008). Ch, Chattisgarth Basin; CIS, Central Indian Shear Zone; GR, Godavari Rift; M, Madras Block; Mk, Malanjkhand; MR, Mahanadi Rift; N, Nilgiri Block; NS, Narmada-Son Fault Zone; PC, Palghat-Cauvery Shear Zone; R, Rengali Province and Kerajang Shear Zone; S, Singhbhum Shear Zone; V, Vindhyan Basin.

The Indian shield, as with most shield areas around the world (e.g. Buchan and Ernst 2004), is transected by numerous, crosscutting Precambrian mafic dyke swarms (Murthy, 1987, 1995), and also contains sills, mafic-ultramafic intrusions, and volcanic supracrustal sequences. Many of the largest of these magmatic events are probably LIPs or LIP ‘fragments’ (Ernst, 2007a). Current concepts of the evolution of the shield fall in the categories of both collision and non-collision models, but such models are at present in their infancy. A new dimension to understanding the evolution of the shield is available when magmatism (especially of LIP-scale) through time is integrated with tectonics and stratigraphy, and with the evolving understanding of the thermal and chemical regimes of the sub-crustal lithospheric and asthenospheric mantle. A beginning has been made in this direction with regard to dyke swarms and some alkaline plutons (Mahadevan, 2008).

Available geochronological data on Proterozoic LIPs of India are summarised in Table 1. Only a few precise U-Pb or Sm-Nd ages exist, so the Proterozoic LIP record of India is currently poorly constrained. The purpose of this contribution is to evaluate available ages on the Proterozoic magmatism in the Indian shield, consider preliminary implications for continental reconstructions, and point the way forward for a more robust and expanded use of LIPs to interpret India’s place in the Proterozoic world.

PROTEROZOIC LIP RECORD OF INDIAN SHIELD

2370 Ma Bangalore LIP (Dharwar craton)

Numerous paleomagnetic sites in the east-west trending Bangalore dyke swarm (swarm name from H.C., Halls, 2007, pers. comm.) of the Dharwar craton carry a consistently steep remanence which has been interpreted as primary (Halls et al., 2007). The age of this swarm is constrained by two precise U-Pb baddeleyite ages (Table 1; Site Nos. 1 and 2), as well as a Sm-Nd age (Table 1; Site No. 3). Compositionally the dykes are iron-rich tholeiites. The swarm is about 300 km wide and more than 350 km long. Halls et al. (2007) note that this swarm converges to the west and can be linked with a mantle plume located at the convergence point (Fig. 3A). It is expected that further study of a vast reservoir of undated mafic units in the Dharwar craton, will reveal sills, and other intrusions, and remnant volcanics also belonging to this widespread early Paleoproterozoic LIP event.

2180 Ma Mahbubnagar LIP (Dharwar craton)

Northwest trending dykes in the Mahbubnagar area consist of three textural petrographic types, gabbro (dominant type), dolerite and metapyroxenite. Dykes vary in width from 5 to 30 m, and geochemically they are quartz or olivine normative, tholeiitic and sub-alkalic in composition and show a continental (within plate) affinity (Pandey et al., 1997). This swarm has an imprecise Sm-Nd age of 2173±64 Ma (Table 1; Site No. 4; Pandey et al., 1997). We interpret that the two dykes precisely dated by the U-Pb baddeleyite method as 2180 Ma (French et al., 2004) extend this “Mahbubnagar” swarm throughout the Dharwar craton (Table 1; Site Nos. 5 and 6).  Furthermore, the distribution and orientation of these precisely-dated dykes suggests a radiating pattern: “U-Pb baddeleyite and zircon dating of two dolerites dykes sampled ~500 km apart, including a NW-SE trending dyke from the western edge of the Shimoga supracrustal belt and an E-W trending dyke from the NE Dharwar craton yield identical ages of 2.18 Ga” (French et al., 2004). If the convergence of these two precisely dated dykes is representative of the broader swarm, then this would suggest that a 2180 Ma mantle plume was located on the northwest side of the Dharwar craton (Fig. 3B). The Bijli rhyolites have an Rb-Sr age of 2180±25 Ma (Table 1; Site No. 7) (Divakara Rao et al., 2000) and are possibly also related.

Table 1: The Proterozoic record of LIPs (and possible LIPs) for the Indian shield.

Site No.

Magmatic Unit Name (Location)

Magmatic unit description

Age and method

Age Reference (s)

A. 2366 Ma Bangalore LIP (Dharwar craton)

1.

(Henur, near Mysore)

WNW trending dolerite dyke

2366±1 Ma; U-Pb baddeleyite

Halls et al. (2007)

2.

(Near Bangalore)

Dolerite dyke

2365±1.1 Ma; U-Pb baddeleyite

French et al. (2004)

3.

(West of the Cuddapah basin)

Dolerite dyke

2454±100 Ma; Sm-Nd

Zachariah et al. (1995)

B. 2180 Ma Mahbubnagar LIP (Dharwar craton)

4.

Mahbubnagar

Gabbro and dolerite dyke

2173±64 Ma; Sm-Nd

Pandey et al. (1997)

5.

(Shimoga region)

NW trending dolerite dyke

2180 Ma; U-Pb baddeleyite and zircon

French et al. (2004)

6.

(NE Dharwar)

E-W trending dolerite dyke

2180 Ma; U-Pb baddeleyite and zircon

French et al. (2004)

7.

Bijli (northern margin Dharwar)

rhyolites

2180±25 Ma Rb-Sr

Divakara Rao et al. (2000)

C. 1891-1883 Ma Southern Bastar – Cuddapah LIP

8.

BD2 swarm (Dantewara, South Bastar)

NW-trending dolerite dyke

1891.1±0.9 Ma; U-Pb baddeleyite

French et al. (2008)

9.

BD2 swarm (Bastanar, South Bastar)

NW-trending dolerite dyke

1883.0±1.4 Ma; U-Pb baddeleyite and zircon

French et al. (2008)

10.

Pulivendla  (Cuddapah basin)

mafic sill

1885.4±3.1 Ma; U-Pb baddeleyite

French et al. (2008)

11.

Pulivendla and Tadpatri (Cuddapah basin)

mafic sills

1899±20 Ma; Ar-Ar phlogopite

Anand et al. (2003)

12.

(West of the Cuddapah basin)

E-W mafic dyke

1894 Ma; U-Pb baddeleyite

Halls et al. (2007)

13.

(SW of Cuddapah basin)

E-W mafic dyke

1879±5 Ma; Ar-Ar

Chatterjee and Bhattacharya (2001)

D. 1600 Ma Dalma LIP? (Singhbhum craton)

14.

Dalma

gabbro-pyroxenite intrusive

1619±38 Ma; Rb-Sr

Roy et al. (2002)

15.

Base of the Semri Group

Igneous zircons from tuff units (~porcelanite)

1631±5 Ma; U-Pb zircon

Rasmussen et al. (2002)

E. 1350 Ma Prakasam alkaline province (part of LIP?) (Eastern Ghats)

16.

Errakonda.

ferrosyenite

1351.5±2 Ma; U-Pb zircon

Vijaya Kumar et al. (2007)

17.

Errakonda.

ferrosyenite

1352.1±1Ma; Pb-Pb zircon

Vijaya Kumar et al. (2007)

18.

Uppalapadu

nepheline syenite

1356±6.5 Ma; U-Pb zircon

Vijaya Kumar et al. (2007)

19.

Elchru

nepheline syenite

1321±17 Ma; U-Pb zircon

Upadhyay et al. (2006)

20.

Kondapalle

monzosyenite

1263±23 Ma; U-Pb zircon

Upadhyaya and Raith (2006)

F. 1200-1000 Ma LIPs? (Dharwar and Bastar cratons)

21.

Harohalli

N-S alkaline dykes

1192±10 Ma U-Pb, zircon

Pradhan et al. (2008)

22.

Wajrakarur

Kimberlite (Pipe-2)

1124+5/-3 Ma; U-Pb perovskite

Kumar et al. (2007)

23.

Wajrakarur

Kimberlite (Pipe-6)

1102±23 Ma; Rb-Sr

Kumar et al. (2007)

24.

Siddanpalli

Kimberlite (SK-1)

1093.4±4.4 Ma; Rb-Sr

Kumar et al. (2007)

25.

Majhgavan

Kimberlite

1073.5±13.7; Ar-Ar phlogopite

Gregory et al. (2006)

26.

(Chattisgarh Basin, Bastar craton)

rhyolite protolith

990-1020 Ma U-Pb SHRIMP, zircon

Patranabis-Deb et al. (2007)

G. 770-750 Ma Malani LIP (Aravalli craton and Seychelles)

27.

Malani first stage

Rhyolite tuff

771±5 Ma; U-Pb zircon

Gregory et al. (2007)

28.

(Seychelles Island, Indian Ocean)

Dolerite dyke

750.2±2.5 Ma; U-Pb zircon

Torsvik et al. (2001)

1890 Ma Southern Bastar – Cuddapah LIP

Recently French et al. (2008) have presented precise U-Pb baddeleyite and zircon dating for the BD-2 swarm of the Bastar craton (Table 1; Site Nos. 8 and 9) and the Pulivendla sill from the Cuddapah basin (Table 1; Site No. 10), in the Dharwar craton, and based on this geochronology they defined a “Southern Bastar – Cuddapah LIP”. Anand et al. (2003) have also dated Pulivendla and Tadpatri sills from the Cuddapah basin using the Ar-Ar method and found similar ages (Table 1; Site No. 11). The presence of 1890 Ma magmatism in both Bastar and Dharwar cratons suggests that both cratons were already connected by this time (French et al., 2008). A U-Pb baddeleyite age of 1894 Ma was also reported for a E-W trending mafic dyke exposed in southwestern margin of Cuddapah basin in the Dharwar craton (Table 1; Site No. 12). On the basis of the precise baddeleyite ages from the NW-trending BD-2 dykes in the Bastar craton (French et al., 2008) and the single precise baddeleyite age for an E-W dyke in the Dharwar craton west of the Cuddapah basin (Halls et al., 2007), (and assuming no 45 degree relative rotation between the Bastar and Dharwar cratons), we hypothesize a possible radiating dyke swarm. The convergence point would be to the east side of the Dharwar craton and would mark an 1890 Ma mantle plume (Fig. 3C).  The ca. 1879 Ma E-W dyke (Table 1; Site No. 13) in the Dharwar craton dated using the Ar-Ar system by Chatterjee and Bhattacharji (2001) could also be part of this LIP. It would be useful to obtain additional U-Pb ages dykes from west of the Cuddapah basin to confirm the E-W trend of the swarm in this area.

In addition to the sills of the Cuddapah basin it is also possible that a major layered intrusion in the Cuddapah basin belongs to this 1890 Ma event (Radhakrishna et al., 2007). An oval shaped gravity high of +60 mgals is interpreted by Grant (1983), to be caused by a sill-like body ~4 km thick at 7 km depth. Given the absence of an aeromagnetic anomaly, Krishna Brahmam and Dutt (1992) interpreted the body to be Mg-rich and dominated by olivine cumulates. However, it should be noted that there is no direct geochronology on this intrusion and so the age could be different than 1890 Ma.

1600 Ma Dalma LIP? (Singhbhum craton)

Tholeiitic volcanics and related komatiites and intrusive gabbro-pyroxenite bodies are reported from the Dalma volcanic belt. Roy et al. (2002) have dated a gabbro-pyroxenite intrusive by the Rb-Sr method at 1619±38 Ma (Table 1; Site No. 14). Later Misra and Johnson (2005) dated Dalma volcanics and suggested a 2396±110 Ma (Rb-Sr) emplacement age for this unit. But Roy and Sarkar (2006) identified flaws in the methodology, and therefore, doubted this result. Bose et al. (1989) also suggested an ~1600 Ma (K-Ar) age for the Dalma volcanics. Interestingly, Rasmussen et al. (2002) have dated “igneous zircons from tuff units (~porcelanite) near the base of the Semri Group (Lowermost Vindhyan) by the U-Pb SHRIMP method and found ages between 1628 and 1631 Ma (Table 1; Site No. 15). If we assume that Chattisgarh sediments (Bastar craton) and Vindhyan sediments (Bundelkhand craton) were more or less contemporaneous, then the base of the Chattisgarh could be ~1650 Ma  (Patranabis-Deb et al., 2007). We conclude that these tuffs that are dated at 1631±5 Ma belong to the Dalma igneous event, and hence we believe that the Dalma event occurred at 1631±5 Ma, not ~2300 Ma.

1350 Ma Prakasam alkaline province (part of a LIP?) (Eastern Ghats)

The Prakasam alkaline province of the Eastern Ghats has a wide range of ages (1100-1600 Ma) suggesting that more than one event is represented (Vijaya Kumar et al., 2007). However, the most precise ages for the largest grouping of units (see Table 1; Site Nos. 16 to 20; Figure 3D) are 1350 Ma and for this reason we focus on the 1350 Ma magmatic stage. Alkaline and carbonatitic magmatism is associated with many LIPs (e.g. Bell, 2001; Ernst, 2007b). Therefore, we provisionally include the 1350 Ma Prakasam alkaline event in this review of Proterozoic LIPs, based on the likelihood that geochronology on the many undated mafic magmatic units in the region will yield coeval LIP units sensu stricto.

Possible Ca. 1200-1000 Ma event(s) (Eastern Dharwar and Bastar cratons)

There are possibly three as yet poorly constrained events in the time range 1200-1000 Ma. The oldest is represented by the alkaline Harohalli dykes which have a new U-Pb age of 1192+/-10 Ma (Table 1; Site No. 21). However, this age should be considered provisional (Pradhan et al., 2008).


Figure 3: Schematic distribution (in red) of various Proterozoic mafic units. Red-filled star shows location of inferred mantle plume. Numbers marked in each figure show location of the Proterozoic magmatism; for details see Table 1. (A) 2370 Ma Bangalore swarm; (B) 2180 Ma Mahbubnagar swarm; (C) 1890 Ma Southern Bastar – Cuddapah LIP;  (D) Other events: 1600 Ma Dalma LIP? (Singhbhum craton); 1350 Ma Prakasam alkaline province (LIP?) (Eastern Ghats); 1200-1000 Ma events (eastern Dharwar and Bastar cratons); and 770-750 Ma Malani LIP (Aravalli craton).

The next group comprises kimberlites from the eastern Dharwar which have ages between 1124 and 1074 Ma (Table 1; Site Nos. 22 to 25), the most precise estimate being a 1124 Ma U-Pb perovskite age. Some kimberlites are associated with LIPs, especially those kimberlites exhibiting a clustering of ages; two important examples are from Siberia: the 250 Ma Siberian trap event and the 360 Ma Yakutsk event (e.g. Ernst, 2007b). Other kimberlites are associated with plume tails, especially those kimberlites that exhibit an age progression; (e.g. Heaman and Kjarsgaard, 2000).

A possible third event may be represented by a newly recognised succession of rhyolitic ignimbrite, ash beds, and volcaniclastic sandstones near the top of the ~2.2-km-thick sedimentary fill of the Chattisgarh Basin (Bastar craton) (Patranabis-Deb et al., 2007) (Table 1; Site no. 26). Euhedral igneous zircons from these units give U-Pb SHRIMP ages of 990-1020 Ma, and provide an age estimate for the presumed rhyolite protolith.

770-750 Ma (U-Pb) Malani LIP (Aravalli craton)

The Malani igneous suite (Table 1; Site No. 27) is one of the largest felsic LIPs in the world, and when reconstructed with related magmatism in formerly adjacent Seychelles (Table 1; Site No. 28) and Madagascar, has an areal extent greater than 100,000 km2 (Gregory et al., 2007). This LIP formed in three stages. Initial bimodal volcanism (basaltic overlain by felsic volcanism), was followed by a stage of felsic plutonism, and finally by emplacement of dykes of both mafic and felsic composition. Precise geochronology (Table 1) indicates a 20 myr span for the event, but additional geochronology is required to determined whether two pulses are involved or the age distribution is more continuous.

LIP BARCODE OF INDIA

The history of Large Igneous Provinces for a crustal block can be efficiently summarised in a ‘barcode’ diagram (Bleeker and Ernst 2006). It is clear that the record for the Indian shield (Figure 4) is very preliminary, and that many more units (especially dyke swarms) in the Indian Shield will need to be precisely dated before the LIP barcode for India is sufficiently robust for effective comparison with other crustal blocks and for assessing regional distribution of LIPs within the Indian shield. However, based on the available data, a few preliminary conclusions are possible.

Links with other blocks

The Paleoproterozoic links between the Indian Shield and other crustal blocks are speculative. Ca. 2180 Ma LIPs (mainly dyke swarms) are also found in the Superior craton, and in the Rae and Slave cratons, and also possibly occur in West Africa associated with the Birimian magmatic event (Ernst and Buchan, 2001).

The 2370 Ma age is so-far unique in the world. If and where a 2370 Ma age is obtained elsewhere we can be confident that we have found the nearest neighbor for the Dharwar craton. Halls et al. (2007) suggested a possible link with the Yilgarn craton based on the rough similarity in ages between the Bangalore dykes and the Widgiemooltha dykes (2410-2418 Ma) and a plausible paleomagnetic reconstruction. However, this proposal must be regarded as preliminary until the Widgiemooltha swarm has received a new paleomagnetic study, since the available study is old (Evans, 1968), and should be redone with modern paleomagnetic demagnetisation techniques. Also, the 45 Ma age mismatch between the two swarms weakens the argument for the reconstruction of Widgiemooltha and Bangalore dyke swarms into a single LIP event.

The 1890 Ma event (southern Bastar – Cuddapah LIP; French et al. (2008) is an important LIP, but because 1880-1900 Ma LIPs are found on so many different blocks, it probably means that from a global perspective there was more than one centre of mantle upwelling (e.g. plume) of this age, and therefore 1890 Ma magmatism may not in general be very useful for reconstructions. 1880-1900 Ma LIPs are found in the Superior craton (“Circum-Superior” LIP), Slave, Kaapvaal, Siberian, and possibly East European cratons (French et al., 2008;  www.largeigneousprovinces.org/lom.html [May2005]). However, it does seem likely that, as already shown by French et al. (2008), the tracing of 1890 Ma magmatism between Bastar and eastern Dharwar cratons is strong evidence for these two cratons already being adjacent by this time.

LIP magmatism of 1385-1350 Ma is known elsewhere in the world (e.g. Kalahari craton, Laurentia, eastern Baltica, Antarctica; Ernst et al., 2008) and on this basis it is possible that one or more of these cratons were nearest neighbors to the east side of the Indian Shield.

During the broad time period 1200-1000 Ma there are some important global events outside India, such as the ca. 1100 Ma Keweenawan and 1100 Ma Umkondo LIPs of Laurentia and Kalahari cratons, respectively, and the 1076 Ma Warakurna event of Australia (see summary in Ernst et al., 2008). Other important events are the Sette Daban sills of the Verkoyansk belt of Siberia (974 and 1005 Ma, U-Pb), and the Bahia dykes of the São Francisco craton of South America, with Ar-Ar ages between about 1020-1080 Ma (Ernst et al., 2008). However, until the Indian LIP record is better dated and defined in this time interval, LIP-barcode comparisons with other blocks during this interval will be nearly meaningless.

The Neoproterozoic link between the 770-750 Ma Malani magmatism in India and the 825-755 Ma LIPs in Laurentia, Kalahari, Siberia, Tarim and South China is compelling and can be interpreted within the context of a Rodinia breakup superplume (Ernst et al., 2008; Li et al., 2008).

Radiating Dyke Swarms

Another potentially significant result of this contribution is the possible identification of two plume centres at 2180 Ma and 1890 Ma in addition to the 2370 Ma plume centre (previously recognized by Halls et al., 2007), all identified on the basis of radiating dyke swarms (Figure 3A-C).

If this conclusion is sustained by further work, it would become a framework for evaluating the breakup history of the respective margins of the Dharwar craton, and any associated regional uplift (expected in association with a mantle plume), and possibly definition of new metallogenic belts especially of the Ni-Cu-PGE type (e.g. Pirajno, 2000; Ernst, 2007b).


Figure 4: Summary of the LIP barcode for the Indian Shield as discussed in this contribution. Geochronlogical details shown as follows, Each filled box has vertical position corresponding to the age, and the box vertical width corresponds to the age uncertainty. The horizontal width is arbitrary (shorter when multiple individual age determinations are available).  Black boxes are based on the U-Pb dating method and grey boxes are based on the Ar-Ar method and, in some cases, the Sm-Nd method. Data are mainly listed in Table 1 (keyed by site number), but some additional imprecise Ar-Ar data are from Radhakrishna et al. (1999); Chatterjee and Bhattacharji (2001), Mallikharjuna Rao (2004) and Radhakrishna et al., (2007). U-Pb age uncertainty for units 5, 6 and 12 (Table 1), are not given in the original references, and in this diagram have been arbitrarily assigned an uncertainty of +/-10 myr. .

Future Work

As high-precision U-Pb geochronology is increasingly applied to the multitude of poorly dated swarms of diabase dykes and sills of India, it will be expected that barcode comparisons between the Indian Shield and other crustal blocks will become increasingly more robust for testing paleocontinental reconstructions. The expanded database of Indian LIPs will also help determine the timing of assembly and breakup of component blocks/terranes of the Indian shield. The key is expanded precise geochronology on LIP units of the Indian shield (and from other blocks/terranes around the world), which will rapidly contribute to revealing India’s place in the Proterozoic world.

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