Gulab C. Gautam

Geochemical characteristics and petrogenesis of four Palaeoproterozoic mafic dike swarms and associated large igneous provinces from the eastern Dharwar craton, India

Palaeoproterozoic mafic dike swarms of different ages are well exposed in the eastern Dharwar craton of India. Available U-Pb mineral ages on these dikes indicate four discrete episodes, viz. (1) ~2.37 Ga Bangalore swarm, (2) ~2.21 Ga Kunigal swarm, (3) ~2.18 Ga Mahbubnagar swarm, and (4) ~1.89 Ga Bastar-Dharwar swarm. These are mostly sub-alkaline tholeiitic suites, with ~1.89 Ga samples having a slightly higher concentration of high-field strength elements than other swarms with a similar MgO contents. Mg number (Mg#) in the four swarms suggest that the two older swarms were derived from primary mantle melts, whereas the two younger swarms were derived from slightly evolved mantle melt. Trace element petrogenetic models suggest that magmas of the ~2.37 Ga swarm were generated within the spinel stability field by ~15–20% melting of a depleted mantle source, whereas magmas of the other three swarms may have been generated within the garnet stability field with percentage of melting lowering from the ~2.21 Ga swarm (~25%), ~2.18 Ga swarm (~15–20%), to ~1.89 Ga swarm (~10–12%). These observations indicate that the melting depth increased with time for mafic dike magmas. Large igneous province (LIP) records of the eastern Dharwar craton are compared to those of similar mafic events observed from other shield areas. The Dharwar and the North Atlantic cratons were probably together at ~2.37 Ga, although such an episode is not found in any other craton. The ~2.21 Ga mafic magmatic event is reported from the Dharwar, Superior, North Atlantic, and Slave cratons, suggesting the presence of a supercontinent, ‘Superia’. It is difficult to find any match for the ~2.18 Ga mafic dikes of the eastern Dharwar craton, except in the Superior Province. The ~1.88–1.90 Ga mafic magmatic event is reported from many different blocks, and therefore may not be very useful for supercontinent reconstructions.

Petrology, Geochemistry and Petrogenesis of Early Precambrian Mafic Dyke Swarm from Dondi-Bhanupratappur-Keshkal Area, Central Bastar Craton, India

Different generations of Precambrian mafic dyke swarms are well documented in different parts of the Archaean Bastar craton. Mafic dykes reported from southern part of the Bastar craton is well studied but those exposed in central part have limited petrological and geochemical information. This paper presents petrological and geochemical characteristics of mafic dykes exposed around Dondi, Bhanupratappur and Keshkal areas in the central Bastar craton. These mafic dykes trend in NW–SE and intrude Archaean granite/granite-gneiss and metamorphic rocks of Bengpal and Bailadila Groups. Petrographically most of the dykes show metamorphic textures and amphibolites facies mineral assemblage but few samples retain original igneous texture and mineralogy which classifies them as metabasites and metadolerites, respectively. Geochemically these rocks show either sub-alkaline thoeliitic basaltic or basaltic andesitic nature. Further, they show variation from high-iron tholeiites to high-magnesium tholeiites. Due to the post-magmatic alteration major oxides do not show pronounced crystallization trends but trace elements, particularly high-field strength elements (HFSE), indeed display good crystallization trends. This suggests a possible co-genetic relationship between the studied samples. Ni-Zr modeling suggests that ~25% melting of a depleted mantle source followed by 30–40% fractional crystallization fed the mafic dykes of present study.

Nd-isotope and geochemistry of an early Palaeoproterozoic high-Si high-Mg boninite–norite suite of rocks in the southern Bastar craton, central India: petrogenesis and tectonic significance

ABSTRACT Nd-isotope and lithogeochemistry of an early Palaeoproterozoic high-Si high-Mg boninite–norite (BN) suite of rocks from the southern Bastar craton, central India, are presented to understand their nature, origin, and tectonic setting of emplacement. Various types of evidence, such as field relationships, radiometric metamorphic ages, and the global distribution of BN magmatism, suggest emplacement in an intracratonic rift setting, commonly around 2.4–2.5 Ga. On the basis of geochemistry these high-Si high-Mg rocks are classified as high-Ca boninites, high-Mg norites, and high-Mg diorites. Nd-isotope data indicate that the high-Mg norite and the high-Mg diorite samples are similar, whereas the high-Ca boninites have a different isotopic character. The high-Mg norite and the high-Mg diorite samples have younger TDM model ages than the high-Ca boninites. Geochemical and Nd-isotopic characteristics of the studied rocks indicate some prospect of crustal contamination; however, the possibility of mantle metasomatism during ancient subduction event cannot be ignored. Trace-element modelling suggests that the high-Ca boninites may have crystallized from a magma generated by a comparatively greater percentage of melting of a lherzolite mantle source than the source for the other two varieties. Furthermore, the high-Ca boninite rocks are most likely derived from an Archaean subduction process (the Whundo-type), whereas the other two types are the products of the interaction of subduction-modified refractory mantle wedge and a plume, around the Neoarchaean–Palaeoproterozoic boundary. The emplacement of the high-Mg norites and the high-Mg diorites may be linked to crustal thickening and associated cratonization at the end of the Archaean.