May 2007 LIP of the Month

May 2007 LIP of the Month

Corresponds to event #6 in LIP record database.

Geochemistry of Deccan Traps dikes: insights into the evolution of a flood basalt feeder system

Loÿc Vanderkluysen1, John J. Mahoney1, Peter R. Hooper2, Hetu C. Sheth3, Ranjini Ray3

(1) SOEST, University of Hawai’i at Manoa,, (2) Washington State University, Pullman, (3) Indian Institute of Technology, Bombay


The Deccan Traps is a 500,000 km2 flood basalt province in west-central India (Fig. 1). The original volume of the province has been estimated at approximately 1.5 x 106 km3 (Wadia, 1975; Eldholm & Coffin, 2000), but there are large uncertainties in estimates of original volume and area because the amount of erosion and the volume of the lava pile that has subsided below sea level along the western continental shelf are known only rather poorly.

Figure 1: Schematic map of the Deccan Traps (green) showing the locations of the three major dike systems (red bars). Modified after Hooper (1990) and Deshmukh & Sehgal (1988)

The thickest lava sections are exposed along the Western Ghats escarpment, located 40-70 km inland from the west coast. The maximum stratigraphic thickness in this area is in excess of 3000 m (single sections reach as much as 1500 m), thinning to 200 m or less at the fringes of the province. In most areas, flows are almost flat-lying (e.g. West, 1959; Raja Rao et al., 1978).

The lava flows are dominantly tholeiitic basalts, with notable picritic and alkalic occurrences in the northwestern Deccan (e.g., Krishnamurthy & Cox, 1977). Several alkalic, acidic and carbonatitic complexes occur in scattered areas, often cutting through or overlying tholeiitic lavas (see review of Mahoney, 1988).

Some debate remains as to the total duration of Deccan volcanism, but flows of the main lava pile have been dated consistently between 64 and 67 Ma. Most appear to have erupted during paleomagnetic chrons 29R and 29N, and the main phase of volcanism may have occurred in as little as 1 Myr or even less (e.g., Courtillot et al., 1988; Duncan & Pyle, 1988; Vandamme et al., 1991; Baksi, 1994; Allègre et al., 1999; Sen, 2001; Pande et al., 2004). The presence of an iridium anomaly in inter-lava sediments (Bhandari et al., 1995; Courtillot et al., 2000), although contentious, may indicate that the eruption straddled the Cretaceous-Tertiary boundary.

The lava stratigraphy in the Western Ghats has been divided into eleven flow formations (Table. 1) on the basis of major and trace element compositions and Pb, Nd and Sr isotopic ratios (e.g., Cox & Hawkesworth 1985; Beane et al., 1986; Lightfoot et al. 1990; Peng et al. 1994; Subbarao et al., 1994). The more extensive of these formations have been shown to extend far to the southeast, east, and/or northeast, although their chemical and isotopic variability is somewhat greater than seen in the Western Ghats sections (Mitchell & Widdowson, 1991; Peng et al., 1998; Mahoney et al., 2000).

Table 1: Stratigraphic Summary of the Western Deccan formations (see Peng et al., 1998, and references therein)

Subgroup Formation
(Maximum thickness
Wai Panhala (>175 m)
Mahabaleshwar (280 m)
Ambenali (500 m)
Poladpur (375 m)
Lonavala Bushe (325 m)
Khandala (140 m)
Kalsubai Bhimashankar (140 m)
Thakurvadi (650 m)
Neral (100 m)
Igatpuri-Jawhar (>700 m)

Three main dike systems are recognized (Beane et al., 1986; Deshmukh & Sehgal, 1988; Hooper, 1990; 1999): (1) the Narmada-Tapi swarm, trending roughly ENE-WSW and parallel to the graben of the Narmada and Tapi rivers; (2) the coastal swarm, which is largely N-S oriented (although dikes of different orientations are also common) and parallel to the coast; and (3) the Nasik-Pune swarm, in which dikes exhibit no single preferred orientation (Fig. 1). The Narmada-Tapi swarm and coastal swarm have generally been discounted as major feeder systems (Hooper, 1990), despite a lack of systematic geochemical studies. On the basis of reconnaissance major and trace element analyses of 42 Nasik-Pune dikes, Beane et al. (1986) postulated that the Nasik-Pune swarm served as the principal locus of feeders for the lava pile. Hooper (1990, 1999) interpreted the lack of preferred orientation in the Nasik-Pune system as strong evidence that the main phase of eruptive activity was not accompanied by significant directed extension of the regional lithosphere. This conclusion, in turn, has often (e.g., Campbell, 1998) been used as a key argument in favor of a plume-head (e.g., Campbell & Griffiths, 1990) origin, and as evidence against rifting-based models (e.g. White & McKenzie, 1989) and non-plume models (e.g. King & Anderson, 1995; Smith & Lewis, 1999).

Our recent work is aimed at locating the principal Deccan feeder dike system(s) by comparing the major element, trace element, and Nd-Sr-Pb isotopic compositions of dikes with those of the lava formations.


The formational affinity of more than 400 samples from all three dike systems was evaluated using disciminant-function analysis of major and several trace elements, following Peng et al. (1998). The results are summarized in Fig. 2. A larger suite of trace elements was compared using normalized incompatible element patterns and inter-element ratios, and a subset of 60 samples was analyzed for Nd, Sr and Pb isotope ratios (Fig. 3).

Figure 2: Histogram of formational matches determined by discriminant-function analysis of major and selected trace elements for each dike system. Note that a different scale is used for the Narmada-Tapi dikes because the number of samples from this area is relatively small.

Figure 3: Pb-Sr-Nd isotopic signatures of dikes analyzed in this study compared with fields defined by the different lava formations (after Peng et al., 1994). 87Sr/86Sr and eNd values are age-corrected to 65 Ma.

A clear difference in chemical composition is present between the Narmada-Tapi dikes and those of the coastal and Nasik-Pune swarms. Major and trace element characteristics of the Narmada-Tapi dikes dominantly resemble those of the lower and middle lava formations (Thakurvadi and Khandala, in particular) whereas many of the coastal and Nasik-Pune dikes have strong affinities with the upper three formations (Poladpur, Ambenali, Mahabaleshwar).

Strikingly, however, nearly half of the analyzed samples have isotopic signatures that fall outside the fields of the lava formations, despite having strong chemical similarities. Although clearly petrologically related to the formations, roughly half the dikes we analyzed isotopically must thus be discounted as feeders to known flows; the same is presumably true of many of the samples not analyzed for isotopes. Some of these dikes may have fed now-eroded flows, but many are probably hypabyssal (cf. Bondre et al., 2006).

In the coastal and Nasik-Pune swarms, most of the remaining dikes have isotopic signatures of one of the three principal upper formations, the Mahabaleshwar, Ambenali, or Poladpur, consistent with their major and trace element affinities. These dikes are probable feeders. In addition, rare dikes with middle- and lower-formation characteristics are present. The isotopic signatures of the remaining Narmada-Tapi dikes likewise reflect their elemental characteristics, possessing predominantly middle- and lower-formation affinities, although some with upper-formation signatures are also present.

In summary, the highest concentration of potential feeders for the lower and middle formations is in the Narmada-Tapi region, whereas potential feeders for the upper formations are concentrated in the Nasik-Pune system extending over to the coast, with scattered isolated examples found in the Narmada-Tapi area and as far south as Goa.

Dikes with major and trace element characteristics similar to those of the thick Thakurvadi formation are present throughout the province. However, only one has been found to have a Thakurvadi-type Nd-Pb-Sr isotopic signature. This result squares with observations made by previous authors (e.g., Beane et al., 1986; Mahoney et al., 2000; Hooper & Widdowson, 2007) that many chemically Thakurvadi-like dikes cut through flows and dikes that are younger than the Thakurvadi formation. Thus, late-stage, chemically Thakurvadi-like magmas were widespread in the Deccan, but apparently unrelated to the Thakurvadi formation itself.

Implications and summary

1) At the time of lower and middle formation eruptions, the Narmada-Tapi swarm was the most important locus of feeder dikes in the Deccan. The length, spacing and straightness of the dikes imply that they were emplaced under conditions of regional directed (roughly N-S) extension, and were not controlled predominantly by underlying geological structures (Delaney et al., 1986; Ray et al., 2007).

2) At the time of upper formation emplacement, the majority of feeder dikes were concentrated in the Nasik-Pune and coastal areas, with other potential feeders scattered across the province. Taken as a whole, these dikes do not display any preferred orientation, implying that their emplacement was not controlled by regional directed tectonic stress.

3) Dikes continued to be emplaced in all three systems during the late stages of volcanism. Although broadly similar to some of the earlier dikes and lavas in their major and trace element characteristics, these dikes have distinct isotopic signatures and cut through flows of the upper formations and dikes similar to those formations. They are also accompanied by alkalic dikes along the coast south of Bombay and in the Narmada-Tapi area. In the coastal zone, these dikes are strongly N-S oriented, and their emplacement can be linked to rifting that resulted in separation of the Seychelles Bank from western India, followed by full-fledged seafloor spreading at ~62 Myr (Dyment, 1998).

The general absence of a preferred trend among probable feeders for the upper formations in the Nasik-Pune and coastal areas implies that significant E-W extension is unlikely to have begun before the late stages of Deccan volcanism. As a result, E-W extension was probably not the trigger for the massive volcanic event. In contrast, the Narmada-Tapi area was under extension at the time the lower and middle formations were emplaced. Thus, we cannot rule out the possibility that N-S extension triggered large-scale volcanism. However, it appears that another mechanism(s) must be invoked to account for the voluminous upper formations.


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