Tholeiitic lavas forming a flood basalt sequence of 870 m thickness at Toranmal in the northern Deccan Traps have a large range in isotopic ratios [ε_Nd(t) = +2·1 to –15·7, (⁸⁷Sr/⁸⁶Sr)_t = 0·70467–0·71416, ²⁰⁶Pb/²⁰⁴Pb = 16·699–20·246], similar to that of lavas in the well-studied southwestern part of the province. The basalts with the lowest ε_Nd(t) values display distinctive lows at Nb, Ta, P and Ti, and large positive Pb spikes in their primitive-mantle-normalized element patterns, indicative of significant continental lithospheric influence in their petrogenesis. As in much of the southwestern Deccan, ε_Nd(t) exhibits a rough negative correlation with Mg/Fe and SiO₂ and a positive correlation with Fe, consistent with temperature-controlled assimilation. Overall, the Toranmal section appears distinct from sections in the northwestern sector of the province; however, some Toranmal basalts are isotopically and chemically similar to flows in the northeastern Deccan, and a thick pile of lavas resembling the Poladpur Fm of the southwestern Deccan, the closest type-sections of which lie ∼380 km to the south, is present. If it indeed represents a northern remnant of this formation, the Poladpur Fm, which also extends far into the central and southeastern parts of the province, is one of the most widespread of Deccan formations, with a possible original extent ≥3 × 10⁵ km². The Ambenali Fm, which forms a thick sequence lying above the Poladpur in the southwestern Deccan, is not present at Toranmal. Several flows have broad geochemical affinities with the southwestern Bushe and Mahabaleshwar formations, which respectively lie below the Poladpur and above the Ambenali; however, these flows are not in the southwestern stratigraphic order and are probably of relatively local origin as dikes compositionally very similar to these flows are present at Toranmal and elsewhere in the vicinity.

The aim of this study is to characterise the changes in mechanical properties and to provide a comprehensive micro-structural analysis of Harcourt granite over different pre-heating temperatures under two cooling treatments (1) rapid and (2) slow cooling. A series of uniaxial compression tests was conducted to evaluate the mechanical properties of granite specimens subjected to pre-heating to temperatures ranging from 25–1000 °C under both cooling conditions. An acoustic emission (AE) system was incorporated to identify the fracture propagation stress thresholds. Furthermore, the effect of loading and unloading behaviour on the elastic properties of Harcourt granite was evaluated at two locations prior to failure: (1) crack initiation and (2) crack damage. Scanning electron microscopy (SEM) analyses were conducted on heat-treated thin rock slices to observe the crack/fracture patterns and to quantify the extent of micro-cracking during intense heating followed by cooling. The results revealed that the thermal field induced in the Harcourt granite pore structure during heating up to 100 °C followed by cooling causes cracks to close, resulting in increased mechanical characteristics, in particular, material stiffness and strength. Thereafter, a decline in mechanical properties occurs with the increase of pre-heating temperatures from 100 °C to 800 °C. However, the thermal deterioration under rapid cooling is much higher than that under slow cooling, because rapid cooling appears to produce a significant amount of micro-cracking due to the irreversible thermal shock induced. Multiple stages of loading and unloading prior to failure degrade the elastic properties of Harcourt granite due to the damage accumulated through the coalescence of micro-cracks induced during compression loading. However, this degradation is insignificant for pre-heating temperatures over 400 °C, since the specimens are already damaged due to excessive thermal deterioration. Moreover, unloading after crack initiation tends to cause insignificant irreversible strains, whereas significant permanent strains occur during unloading after crack damage, and this appears to increase with the increase of pre-heating temperature over 400 °C.

Sub-volcanic magma chambers might be a widespread component of flood basalt provinces, and their presence can be revealed in some cases by plagioclase megacrystic basalts. In at least four levels within the Deccan flood basalt sequence the generally low abundance of small plagioclase crystals increases to 5–25%, with some as large as 30 mm long. These Giant Plagioclase Basalt (GPB) flows were formed by mixing of megacryst-rich and megacryst-poor magmas. The crystal size distributions (CSD) of these megacrysts mostly plot as almost straight lines on a classic CSD diagram. For a plagioclase growth rate of 10⁻¹⁰ mm/s steady-state magma chamber models and simple continuous growth suggest residence times of 500–1500 years. However, the lack of crystals smaller than 2 mm suggests that coarsening may have been involved and crystal shape can help define the environment where this happened. Plagioclase megacrysts are very tabular and commonly form clusters of sub-parallel crystals, characteristics that are also found in the plagioclase of anorthosites formed by flotation at the top of shallow magma chambers and crystallization in a high Peclet number regime (e.g. Skaergaard; Sept Îles). A possible history is as follows. (1) Plagioclase megacrysts crystallize in a convecting magma chamber just below the lava pile. (2) Currents sweep the crystals to the top of the chamber, where they accumulate as a result of their buoyancy. The crystals coarsen in response to the continuous supply of hot magma. (3) New magma sweeps through the plagioclase mush, mingles and mixes, then erupts to form the GPBs. The residence time recorded by the megacrysts in the GPBs is that of the magma chamber where the megacrysts formed, not that of the magmas that make up the megacryst-poor part of the GPBs or the other megacryst-poor lavas. Lavas with megacrysts similar to the GPBs are uncommon but widespread (Galapagos, Surtsey, etc.), and suggest the presence of sub-volcanic magma chambers elsewhere.