July 2007 LIP of the Month

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Paleomagnetism and Geochronology of the Malani Igneous Suite, Northwest India: Implications for the configuration of Rodinia and the assembly of Gondwana

Laura C. Gregory, Department of Geological Sciences, University of Florida, 241 Williamson Hall, Gainesville, FL 32611, lalaura@ufl.edu

Joseph G. Meert, Department of Geological Sciences, University of Florida, 241 Williamson Hall, Gainesville, FL 32611

Bernard Bingen, Geological Survey of Norway, N-7491, Trondheim, Norway

Manoj K. Pandit, Department of Geology, University of Rajasthan, Jaipur, 302004, Rajasthan, India

Trond H. Torsvik, Geological Survey of Norway, N-7491, Trondheim, Norway

Paleomagnetic data from mafic dikes and a U-Pb zircon age from rhyolitic flows in the Malani Igneous Suite (MIS) near Rajasthan, NW India (Figure 1) provide an improved, demonstrably primary, paleomagnetic pole, with a reliable temporal constraint for India in the late Neoproterozoic.  Magmatism in the MIS occurred in three intrusion phases to form the third largest felsic igneous province in the world (Pareek, 1981) that spans 51,000 km2 (Bhushan, 2000). When the Malani suite is reconstructed alongside the Seychelles and Madagascar (see discussion below), the province can be estimated at greater than 100,000 km2 in area (Ashwal et al., 2002).  Activity commenced with an initial volcanic phase made up of basaltic then felsic flows.  The second phase is characterized by the intrusion of granitic plutons.  Predominately felsic and minor mafic dike swarms form the third and final phase of the igneous cycle.  Malani felsic rocks are unmetamorphosed, but slightly tilted and folded.  Vertical to sub-vertical late stage dolerite dikes crosscut all of the other components and thus mark the termination of magmatism.  These mafic dikes intrude the Jalore Granite plutons south of Jodhpur and can be wide, up to 5 meters in extent (Figure 2).

Figure 1: Map showing Precambrian stratigraphic units of the Aravalli Mountain Region in NW India with sampling area boxed (adapted from GSI publications and other published work).

Figure 2: Satellite photo from Google Earth® of the mafic dikes in the Malani Igneous Suite near the city of Jalore.  Large dikes trend  NNW-SSE.

The configuration of the Precambrian supercontinent Rodinia and the subsequent assembly of Gondwana are under considerable debate due to a paucity of high quality paleomagnetic data.  Some (Windley et al., 1994; Piper, 2000; Yoshida and Upreti, 2006; Paulsen et al., 2007) argue that these cratons came together in a single collisional event around or even earlier than 1300 Ma, and stayed in the same configuration within Rodinia and up to the formation and breakup of Gondwana.  However, this scenario is viewed with skepticism.  The alternative formation of Gondwana as a polyphase assembly of cratonic nuclei that had previously dispersed from the Rodinia supercontinent seems to be more consistent with available geologic, paleomagnetic and geochronologic data (Meert et al., 1995; Meert and Powell, 2001; Meert and Torsvik, 2003; Boger et al., 2002; Fitzsimons, 2000; Pisarevsky et al., 2003; Collins and Pisarevsky, 2005, Meert and Liberman, 2008).  The Indian continent is crucial to this topic and plays an essential role in the history of East Gondwana amalgamation.

The MIS provides the best paleomagnetic pole for the Indian subcontinent at ~750-770 Ma, with a combined pole that plots at 69.0˚ N, 83.2˚ E (dp=8.8˚, dm=10.9˚; Figure 3).  Our study further strengthens the case for a primary magnetization from the MIS with a reversal found in mafic dikelets and a baked contact test, in which a ‘reverse-polarity’ small dike is overprinted by a larger E-W trending ‘normal-polarity’ dike.  The first documented U-Pb age of 771 ±5 Ma from an early-stage rhyolitic tuff provides a more accurate and concordant lower age limit for Malani volcanism (Figure 4).  In Figure 3, published paleopoles determined from the initial rhyolitic magmatism in the MIS (Athavale, 1963; Klootwijk, 1975; Torsvik et al., 2001a) are compared to the paleopole derived from the third stage of magmatism of the suite (mafic dikes, this study).  All studies show equivalent results.  Thus India did not likely undergo a large extent of latitudinal movement during Malani magmatism, even with errors taken into account.  With reference to paleomagnetic data, the duration of the Malani magmatism occurred without much change in location relative to the earth’s spin axis. 

Figure 3: Stereoplot of means from this study and previous studies from the Malani Igneous Suite, with our reverse polarity mean and the C component from Torsvik et al., (2001a) indicated.  Closed circles represent positive inclinations.

Figure 4: Inverse concordia diagram showing U-Pb analyses of zircon and CL image of one zircon crystal from a rhyolitic tuff representing the first stage of magmatism in the Malani Igneous Suite.  The concordia age of 771 ±5 Ma reflects magmatic crystallization of the rock.

The MIS is often stated to be the result of a rift setting or a mantle plume (Singh et al., 2006; Bhushan, 2000), mantle heating event.  Malani rocks are also commonly attributed to Anorogenic-type (A-type) volcanism, but the mechanism for this eruption type has not been explicitly stated.  Subduction of an ancient ocean, the Mozambique, has been suggested as a source for activity, but this has also been disputed because of the lack of deformation in the area.  However, it is certainly possible that the undeformed Malani magmatic rocks represent the inboard result of a subduction zone, and that deformed margin material has been eroded or buried.  Madagascar is commonly placed alongside India in Gondwana-fit reconstructions, and Seychelles has been paleomagnetically placed adjacent to India’s margin (Figure 5, Torsvik et al., 2001b).  Both micro-continents have volcanism attributed to the subduction of a Neoproterozoic ocean (Torsvik et al., 2001b, Tucker et al., 2001, Ashwal et al., 2002).  Trond et al. (2001b) reported an age of ~750 Ma for Mahe mafic dikes from the Seychelles, and these dikes are likely coeval with Malani activity, thus indicating a ~20 million year duration for the MIS.  Considering the nature and timing of magmatism in the Seychelles and India, it appears that the bulk of granitic and subsequent mafic magmatism in those regions was constrained to the interval from ~771-751 Ma.  

India was previously stated to be a part of East Gondwana from about 1.1 Ga until the Mesozoic breakup of Gondwana (Yoshida and Upreti, 2006).  However, paleomagnetic data place India and the Seychelles at higher latitudes than coeval poles from Australia (Wingate and Giddings, 2000; Figure 5).  These three robust paleomagnetic results (Mundine Well dikes, Malani Igneous Suite and Mahe Dikes) argue strongly against an amalgamated East Gondwana at 750 Ma and explain the younger Pan-African belts between these cratons.  Thus, we argue that if paleomagnetism is to make any contribution to Neoproterozoic plate tectonic models, the new Malani pole must be seriously considered in any geodynamic explanation for the assembly of Gondwana.

Figure 5: Reconstruction at 770 Ma of pertinent East Gondwana components.  Grey India outline is plotted as a VGP from this study, with the Seychelles  Euler rotation fit from Torsvik et al., (2001b) and Madagascar is placed according to the Gondwana fit.  Australia is plotted according to the Mundine Well dikes VGP, and Antarctica and India are placed in their Gondwana fit locations, in the Australia reference frame.  There is >20º of latitudinal displacement between the Malani and Mundine Well study sites.

Adapted from Gregory et al., 2007 (submitted).


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