April 2012 LIP of the Month

Abor Volcanics, northeast India

Diane Chung & Jason R. Ali*
Department of Earth Sciences, University of Hong Kong, Pokfulam Road, Hong Kong, China
* Email: jrali@hku.hk

Abor volcanics – key information

The Abor Volcanics comprise a series of tholeiitic to alkaline basalts (Singh, 2007), that are exposed in the Siang “window” in Arunachal Pradesh, NE India. The unit, which is up to 1500 m thick (Bhat, 1984; Singh, 2007), sits a short distance south of the eastern Himalayan Syntaxis (Fig. 1).

The volcanic succession forms part of the Lesser Himalayan sequence (Figs 2 and 3), the southernmost of three major thrust-bounded litho-tectonic packages that formed the northern part of the Indian craton prior to their structural disruption following the sub-continent’s collision with Asia in the mid-Cenozoic (Aitchison et al., 2007).

Main Central Thrust; STDF, Southern Tibet detachment fault; YTS, Yarlung Tsangpo suture. Of particular importance are the structural sites of the various mafic suites.

Abor Volcanics age-dating controversy

Over the last decade or so, controversy has arisen concerning the age of the Abor Volcanics. Traditionally, the formation was viewed as being Late Paleozoic in age, with some consensus on Late Carboniferous-Early Permian. In recent years, however, S.K. Acharyya and colleagues have argued for the unit being erupted in the Early Eocene (for a full account, see Ali et al., 2012, p. 105). Critically, though, the basalt geochemistry underpins two very different models, the first related to the eastern Gondwana’s Late Paleozoic break-up (e.g. Sinha Roy & Furnes, 1978; Bhat, 1984; Garzanti et al., 1999; Chauvet et al., 2008), the second the India–Asia collision system in the early Paleogene (e.g. Sengupta et al., 1996; Acharyya & Sengupta, 1998; Acharyya, 1999, 2001, 2007).

Interestingly, two recent works have been published that are relevant to the Abor series’ age-assignment debate. Kesari (2010) argues that the suite represents two discrete magmatic episodes: an Early Permian one is considered equivalent to a nearby unit called the Lichi Volcanics; a second dates from the Early Eocene and sees the ‘‘Abor Volcanics’’ sitting within the Yongkiang Group. Unfortunately, the accompanying geological map has a scale of 1:2,000,000, thus it is difficult to visualize the detailed geographical distributions and field relations of the two packages. In a second investigation, as part of a multi-disciplinary study, Liebke et al. (2011) generated various types of radiometric data: K–Ar whole-rock on five basalt specimens and two samples from an overlying clastic unit (Yinkiong Formation shales) together with low-temperature fission track and (U–Th)/He analyses of zircons extracted from the bounding sedimentary packages. Unfortunately, the results were somewhat ambiguous, and the Late Paleozoic versus early Cenozoic age debate was thus left unresolved.

Towards a resolution of the age-dating issue

Ali et al. (2012) adopted a different approach to the age-dating problem. Based on paleomagnetism theory, they argued that if a primary remanance could be recovered from the Abor Volcanics, because the Indian block was in very different positions in the Late Paleozoic (mid-latitude Southern Hemisphere) and Cenozoic (equatorial) the magnetizations should reflect this (Fig. 4). In total, 34 cooling-unit directions were obtained (from eight exposures), with the mean inclination being 72.7° (a95 = 6.2°, k = 18.3), which equates to a formation latitude of 58.1°S (if the magnetization had been acquired in the Ypresian, the inclination would have been appreciably shallower, 5–10°). This provides compelling evidence supporting the notion that the series is of Late Paleozoic age (and combined with various other arguments, Early Permian). As Ali et al. (2012) pointed out, though, the studied sections are all from the lower Siang River basin, thus it is possible that Eocene volcanic rocks are present elsewhere in the Siang window.

Geodynamic Implications

Based on a Late Paleozoic age assignment, several workers have correlated the Abor Volcanics with broadly coeval mafic volcanic suites in Oman, NE Pakistan–NW India and southern Tibet–Nepal, which developed in response to the Cimmerian block peeling-off eastern Gondwana in the Early-Middle Permian. There is, however, a problem with relating the Abor Volcanics (as well as a suite in southern Sikkim, Fig. 2) to the rifting of the Cimmerian terrane. The other bodies form part of the Himalayan Tethys sequence (Fig. 3), and were thus erupted close to the Indian block’s northern edge. There, the volcanics are covered by a relatively complete sequence of Mesozoic shelf deposits. In contrast the series at Abor and Sikkim are part of the Lesser Himalayan sequence and would have thus sat a considerable distance inboard from the Indian block’s ‘‘northern’’ edge (Fig. 3). This probably explains why the stratigraphic record at those locations is very different to that of the Tethyan Himalayas; above the Permian Abor Volcanics the next youngest clearly mappable unit is the Yinkiong Formation, from which Middle Eocene marine fossils have been reported (e.g., Singh & Singh, 1983; Tripathi & Mamgain, 1986; Sengupta et al., 1996). A relatively recent evaluation of Greater India’s size, shape and pre-breakout position within Gondwana (Ali & Aitchison, 2005) suggests that the Abor and Sikkim eruptions would have been 1400–1500 km south of the rifting zone, which had to run north of the continental basement forming the Wallaby and Zenith plateaus west of Australia (Fig. 5; Colwell et al., 1994; Stilwell et al., 2012). As a consequence, we suggest that the Abor Volcanics in the lower Siang River result from the approximately coeval rifting that was also taking place between India/Antarctica and India/Australia, for example, Veevers & Tewari (1995), Harrowfield et al. (2005), and Veevers (2006), as well as extension within the Indian craton (Ghosh et al., 2012) (Fig. 13). Extension in these areas terminated soon afterwards, but 120-plus million years later on in the Early Cretaceous some of the rifting loci formed the break-up margins between eastern India and eastern Antarctica, and northern Greater India and western Australia (e.g. Ali & Aitchison, 2008).

Acknowledgements

Research Grants to JRA (CERG – HKU7002/05, HKU7001/10) partially supported the fieldwork and laboratory studies. We thank Jonathan Aitchison, Alan Baxter, Scott Bryan and Sam Chik for discussions and insights.

References