The western Indian volcanic rifted margin, and its large-scale tectonic feature called the Panvel flexure, formed at 62.5 Ma during the late stages of Deccan Traps flood volcanism. We present a geological account of late-stage ($\leq$ 62.5 Ma) Deccan volcanism and plutonism in the relatively poorly studied Thane–Vasai region in the Panvel flexure zone. The study area shows west-dipping basaltic sequences up to hundreds of meters thick, overlain by pyroclastic deposits of various types. The volcanic units are intruded by gabbro plutons, and all these units are in turn intruded by dykes of varied compositions (including tholeiitic basalt, lamprophyre, and granophyre). There are also early tholeiitic dykes, some of which may be feeders to the basaltic sequence. We focus on the gabbro intrusions and provide extensive petrographic and mineral chemical data on them. The gabbros are tholeiitic, and of considerable interest in commonly containing interstitial silicic melts (granophyre or silicic glass). One of the intrusions, the Chena pluton, shows clear outcrop transitions from gabbro, and gabbro with interstitial granophyre, to transitional gabbro-granophyre, and then to a distinct upper zone of granophyre and microgranite. Granophyre is common in mafic intrusions in continental flood basalt provinces of the world (e.g., the Palisades Sill and the Skaergaard Intrusion), where its genesis is ascribed to mechanisms such as fractional crystallisation, liquid immiscibility, or crustal melting, typically based on geochemical data. The Chena gabbro outcrops are valuable in providing direct evidence for a fractional crystallisation origin of the granophyres (and silicic glasses) found in the Thane–Vasai gabbro intrusions, with which the mineral chemical compositions are also consistent.

Multivariate statistical analysis involving hierarchical clusters was carried out for basaltic samples (and associated units) from Khandwa (21°49'N, 76°21'E). ‘Highly significant’ or ‘significant’ linear correlation coefficient values (r) corresponding to different minerals (namely, olivine, clinopyroxene and plagioclase), denote several oxides (as for example, MgO, FeO, SiO$_2$, Al$_2$O$_3$, Na$_2$O, CaO and TiO$_2$) which were used for construction of dendrograms. Critical analysis of hierarchical patterns revealed that at the outset of magmatic crystallization, heterogeneous (${\sim}$greater symmetry) clusters are present. For the crystallization of the lava flows, the ‘bulk level of crystallization’ (in respect of clinopyroxene and plagioclase) varies from ${\sim}$30 to ${\sim}$60%, whereas their ‘ultimate crystallization’ appears to be quite high (${\sim}$80 to ${\sim}$97%). The bulk crystallization of the lava flows shows a broad control of ambient temperature. The dyke system (feeder dyke and chilled dyke) also shows bulk crystallization pattern similar to that of lava flows. Cluster analyses for basement gabbroic rock suggest that there is a wide compositional spectrum for the accumulate portion, whereas the intercumulus portion is marked by relatively restricted compositions. In general, the present CA (cluster analysis) clearly indicates progressive amalgamation of clusters (and their concomitant fall of symmetry) with advancing differentiation.

$\bf{Highlights}$

$\bullet$ Multivariate statistical analysis from a portion of eastern Deccan volcanic province deciphers distinctive nature of crystallization.

$\bullet$ Cluster patterns indicate distinct ‘bulk level of crystallization’ and ‘ultimate crystallization’ for the lavas.

$\bullet$ Rapid quenching of dyke rocks only allows them to crystallize up to the bulk level.

$\bullet$ Ambient temperature controls bulk level of crystallization for the lavas.

Continental flood basalts of the world are represented either by picrobasalt/picrite layers (with abundant growth of olivine) or by (almost) olivine-depleted tholeiite basalts. We have taken a case study from the olivine-depleted basaltic rocks from Khandwa (21°49'N: 76°21'E) Eastern Deccan Volcanic Province. Our research shows that recent cooling experiments (available in the literature) involving a dry basalt and construction of some selected binary major element variation diagrams (entailing lever rules) can explain (and quantify) the insignificant level (${\sim}$2–4%) of olivine crystallization (${\sim}$olivine decadence) in tholeiite basalts. Such decadent olivine retains a skeletal crystallographically-oriented geometry. We contemplate that our simplistic model of olivine decadence can be extended to other flood basalt provinces of the world as well.

$\bf{Highlights}$

$\bullet$ First-time cogent model of olivine decadence in Deccan basalts

$\bullet$ Integration of newly obtained experimental data and binary variation diagrams

$\bullet$ Petrographic support validates the model

$\bullet$ Model can be extended to other continental Flood basalts