Petro-geochemistry, SrNd isotopes and 40Ar/39Ar ages of fractionated alkaline lamprophyres from the Mount Girnar igneous complex (NW India): Insights into the timing of magmatism and the lithospheric mantle beneath the Deccan Large Igneous Province

doi:10.1016/j.lithos.2020.105712

Lithos

Volumes 374–375, 15 November 2020, 105712

https://doi.org/10.1016/j.lithos.2020.105712 Get rights and content

Highlights

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Petrochemistry, SrNd isotopes and age of lamprophyres from the Mt. Girnar.
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Emplacement age close to the Cretaceous-Paleogene boundary.
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Evidence for genetic relationship of litho-units.
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Trace element and isotopes similar to the Deccan lava from western India.
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Source enrichment coincides Rodinia break-up and Malagasy orogeny.

Abstract

Mount Girnar is one of the most conspicuous alkaline complexes (gabbro-diorite-syenite-lamprophyre), which intrude some of the earliest erupted basalts (ca. 69 Myr) of the Deccan Large Igneous Province (Deccan LIP) in the Kathiawar plateau of NW India. Petrography, bulk-rock geochemistry, Sr and Nd isotopes and ⁴⁰Ar/³⁹Ar mineral ages of fractionated (Mg#: 36.3–43.6) lamprophyre dykes (younger intrusives) are reported from two widely separated domains from Mt. Girnar. Petrography and mineral chemistry reveal that major mineral assemblages (pargasite and kaersutite varieties of amphibole, diopside, biotite and feldspar) in the lamprophyres are pristine and devoid of alteration. The lamprophyres belong to the alkaline variety in general and the camptonite - monchiquite series in particular. The bulk-rock major and trace elements of the Girnar lamprophyres display very good correlation with each other and also with those of associated rocks (syenites, diorites and gabbros) which support their genetic relationship. Trace-element ratios do not evidence crustal contamination and reveal derivation of the lamprophyres from partial melting of a lithospheric mantle source significantly modified by interaction with asthenospheric-derived melts, and resembles other alkaline rocks from the Deccan LIP in this regard. Initial ⁸⁷Sr/⁸⁶Sr (0.7052–0.7053) and ¹⁴³Nd/¹⁴⁴Nd (0.5125–0.5127) of the Girnar lamprophyres and associated rocks such as syenite, diorite and gabbro are tightly clustered and further attest to their derivation from a cogenetic parental melt. Their positive εNd_i values (+0.8 to +3.4) require a mantle source that has experienced moderate long-term depletion of light rare-earth elements. Neoproterozoic/Early Cambrian depleted-mantle (T_DM) Nd model ages of ~414–588 Myr are closer to the timing of break-up of the Rodinia supercontinent as well as coincide with the Ediacaran-Cambrian Malagasy orogeny. ⁴⁰Ar/³⁹Ar dating of three mineral separates (amphibole and biotite) from the lamprophyres gave precise plateau ages of 65.9 ± 0.3 Myr to 66.1 ± 0.4 Myr demonstrating that the emplacement of the Mt. Girnar igneous complex was close to the Cretaceous-Paleogene boundary. The emplacement of a range (110 Myr to 68.5 Myr) of other spatially related alkaline as well as silicic plutonic complexes, such as the Mundwara, Sarnu-Dandali, and Barda complexes prior to the main flood basalt event at ca. 66.0–65.1 Myr, highlights the role of extensional events in pre-existing rift/fault zones preceding the plume-lithosphere interaction in the Deccan LIP.

Introduction

Widespread and small-volume alkaline magmatism is an integral and important component of the end-Cretaceous (ca. 66 Myr) Deccan Large Igneous Province (Deccan LIP). Syenites, lamprophyres, carbonatites, alkali basalts and orangeites (Group II kimberlites) constitute the various intrusive and effusive alkaline rocks associated with the Deccan LIP (e.g., Basu et al., 1993; Bose, 1980; Lehmann et al., 2010; Krishnamurthy, 2020; Melluso et al., 2002; Pandey et al., 2019; Sen et al. 2009; Simonetti et al., 1998; Subba Rao, 1971) A great majority of these alkaline rocks, with the exception of the recently discovered ca. 65 Myr diamondiferous orangeites from the Bastar craton, central India, are confined to the western and north-western parts (Fig. 1A) of the Deccan LIP, and imply a ‘unique tectonic frame work’ of this terrain (see Bose, 1980).

A compilation of the available, high-precision, radiometric ages from the Deccan LIP (Table 1) suggests that alkaline magmatism (i) pre-dates (Tethyan suture, Sarnu Dandali, Mundwara, and Nirwandh alkaline complexes), (ii) post-dates (Dongargaon lamprophyres) as well as is (iii) synchronous (Bhuj, Murud Jhanjira, Phenaimata, Mainpur orangeites and Amba Dongar carbonatites) to the Cretaceous-Paleogene boundary at ca. 66 Myr and the main Deccan Trap eruption in between 66.0 and 65.2 Myr with an estimated 90% of the total eruptive volume (see Sprain et al., 2019 and references therein).

Owing to their deeper depths of origin, higher abundances of incompatible trace elements, and entrainment of mantle and deep-crustal xenoliths, it has been well-acknowledged that alkaline rocks constitute ‘windows into the Earth's mantle’ (e.g., Rock, 1991). Hence, a study of the petrology, geochemistry, emplacement age and radiogenic isotopes of the alkaline rocks associated with the Deccan LIP is important as they provide significant information about the nature of their mantle source regions, relative contribution of lithosphere and asthenosphere, plume-lithosphere interaction, and tectono-magmatic setting of the Indian plate before, during, and subsequent to the eruption of the Deccan Traps.

In this study, we focus on the petrological, geochemical and radiogenic isotopic aspects of lamprophyres, which are some of the youngest intrusives in the Mt. Girnar igneous complex, Kathiawar plateau, NW India, which is spatially associated with the Deccan Traps (Fig. 1A and B). Even though the Girnar complex was extensively investigated over the years (e.g. Bose, 1971, Bose, 1973; Mathur et al., 1926; Paul et al., 1977), detailed petrogenetic studies pertaining to the lamprophyres are relatively sparse with the exception of a study by Ghosh et al. (1997). A precise age of the complex is still lacking as the available K single bond Ar and ⁴⁰Ar/³⁹Ar bulk-rock ages (69.1 ± 1.2 Myr to 56.2 Myr; Wellman and McElhinny, 1970; Paul et al., 1977; Rathore et al., 1996) show a wide range. The aims of this study are to (i) understand the petrology and geochemistry of the lamprophyres of Mt. Girnar, in relation to other Deccan-related lamprophyres, (ii) decipher the timing of their emplacement from robust ⁴⁰Ar/³⁹Ar geochronology on mineral separates, (iii) evaluate their mantle source regions, and (iv) constrain the genesis of the alkaline magmatism in the Deccan LIP. Our study also provides the first Nd isotopic data for the Girnar igneous complex.

Section snippets

Geological background

The Mount Girnar igneous complex (Figs. 1A, B and 2A), with a maximum altitude of ~1050 m above mean sea level, incorporates a range of hills located east of Junagadh city (21^o31’N; 70^o32’E) which are roughly circular in outline, occupy an area of about 174 km² within a Deccan Trap terrain, and resemble volcanic vents (Fig. 1B; Mathur et al., 1926). The complex is located within a fault zone, associated with Recent seismic activity, and is considered to be a western extension of the well known

Methodology

Exposures of freshest looking lamprophyres and their associated rocks were sampled and their petrographic study was carried out by conventional optical microscopy as well as by back-scattered electron (BSE) imaging deploying the CAMECA SX Five electron probe micro analyzer (EPMA) and EVO 18 Carl Zeiss scanning electron microscope (SEM) at the DST-SERB national facility, Department of Geology, Institute of Science, Banaras Hindu University, Varanasi, India.

The mineral composition of the rocks

Petrography and mineral chemistry

Petrographic aspects of the various rocks from Mt. Girnar are relatively well known compared to those occurring in the other alkaline complexes from the Deccan LIP (Bose, 1971, Bose, 1973; Subba Rao, 1968). Salient petrographic aspects of the samples under study are summarized below along with the mineral chemistry of the lamprophyres.

Lamprophyres: Two distinct varieties of lamprophyre have been recorded in this study. The Suraj Kund occurrences are characterized by abundant needle-shaped laths

Whole-rock geochemistry

The whole-rock geochemistry of the samples (lamprophyres and their associated rocks) (Table 6) has been evaluated in conjunction with the previously published data by Paul et al. (1977) for Mt. Girnar. All the studied rocks are nepheline-normative, with the lamprophyres showing higher normative nepheline, demonstrating their alkaline nature. Silica (46.0–49.6 wt%), titania (1.13–1.80 wt%), and alumina (16.7–19.0 wt%) contents of the lamprophyres also show a restricted range (Table 6). On the

⁴⁰Ar–³⁹Ar geochronology

Step-heating data for amphibole and biotite separates from Surajkund lamprophyre and for amphibole from the Ramdas Ashram lamprophyre are presented in Supplementary Table 1. The age spectra, K/Ca and inverse isochron plots are shown in Fig. 11. The amphibole from the Surajkund lamprophyre displays a plateau age of 66.60 ± 0.35 Myr for 68% of the ³⁹Ar released, corresponding to a flat portion on the K/Ca plot, whereas the first heating steps provide evidence of excess argon resulting in

Sr–Nd isotopes

The measured ⁸⁷Sr/⁸⁶Sr (0.705625–0.736170) and ¹⁴³Nd/¹⁴⁴Nd (0.512639–0.512779) isotopic ratios (Table 7) of the seven lamprophyres and associated rocks are corrected to an emplacement age of 66 Ma. The initial ⁸⁷Sr/⁸⁶Sr ratios range from 0.70289–0.70534 and initial ¹⁴³Nd/¹⁴⁴Nd isotopic ratios range from 0.51259–0.51272 (Table 7). A remarkable similarity of the initial ¹⁴³Nd/¹⁴⁴Nd ratios demonstrates the involvement of a similar source in the genesis of all samples. The εNd values of our samples

Crustal contamination

Petrography of the studied lamprophyres did not reveal any crustal xenoliths or xenocrysts. Bulk-rock silica and alumina contents (SiO₂, 46.0–49.5 wt% and Al₂O₃, 16.6–19.3 wt% Table 6) also show a restricted range consistent with minimal crustal contamination. Trace-element ratios, involving incompatible elements, serve as very good screens to assess the contribution of crustal sources (e.g., Xia and Li, 2019). The Zr/Nb, La/Nb, Ba/Th, Rb/Nb, and Ba/La ratios of the Girnar samples are low and

Conclusions

Petrographic study reveals that the lamprophyres from Mt. Girnar are pristine, non-altered, fractionated and belong to the camptonite-monchiquite series of alkaline lamprophyre category. The bulk-rock major and trace elements of the lamprophyres display very good correlation witheach other and also with those of associated rocks (syenites, diorites and gabbros) and imply a common magma in their genesis. Their trace-element ratios preclude crustal contamination and the studied lamprophyres are

Declaration of Competing Interest

The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.

Acknowledgements

In 1926 Prof K.K. Mathur, founder Head of the Department of Geology, BHU, published a seminal paper on the differentiation aspects of the Mount Girnar complex in the ‘Journal of Geology’ which is cited till today. We humbly dedicate our paper to his memory on the occasion of the completion of 100 years of existence of the Department of Geology, BHU. The authors thank Head, Department of Geology, BHU for extending facilities. This work is supported by two major research projects granted to NVCR

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Citation Excerpt :
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2020, Lithos
Citation Excerpt :
Three dykes in the Central Saurashtra swarm have yielded 40Ar/39Ar ages of 65.6 ± 0.2 Ma, 66.6 ± 0.3 Ma, and 62.4 ± 0.3 Ma (2σ errors), indicating swarm development continuing for several million years (Cucciniello et al., 2015). Alkaline lamprophyre dykes intruding the diorite-gabbro plutonic complex of Mount Girnar (Fig. 1b) have 40Ar/39Ar ages of 65.9–66.1 Ma (Sahoo et al., 2020). The Southeastern Saurashtra dyke swarm studied here is exposed between the towns of Jesar and Mahuva (Fig. 1b).
The Deccan Traps continental flood basalt (CFB) province of India contains several dyke swarms, which are dominated by tholeiitic basalts and basaltic andesites. The Southeastern Saurashtra dyke swarm, containing mainly these rock types, also contains an andesite and several rhyolites. Based on petrographic, mineral chemical, and whole-rock major and trace element and Sr-Nd isotopic data, we discuss the magmatic evolution and pressure-temperature conditions of crystallisation of these dyke magmas. The tholeiitic basalts and basaltic andesites have low to moderate TiO₂ contents. The andesite and the rhyolites have low CaO, MgO, Fe₂O₃t, TiO₂ and P₂O₅, and high K₂O, Rb, Ba and light rare earth element contents. Thermobarometric calculations for equilibrium mineral–whole-rock pairs indicate plagioclase crystallisation at 1200–1170 °C, overlapping with clinopyroxene crystallisation at 1181–1143 °C, and a pressure range of 0.1–3.6 kbar indicating crystallisation during magma ascent or storage at shallow crustal levels. Major and trace element modelling is consistent with the rhyolites being produced by advanced fractional crystallisation of basaltic parental magmas. However, a broad trend of increasingly radiogenic Sr and nonradiogenic Nd isotopic ratios, from the mafic rocks through the andesite to the rhyolites, suggests a combined assimilation and fractional crystallisation (AFC) process between basaltic magma and ancient granitic crust. The rhyolites contain the largest crustal contributions and display the most “enriched” Sr-Nd isotopic characteristics yet recorded from the Deccan Traps (ε_Ndt = −20.6, (⁸⁷Sr/⁸⁶Sr)_t = 0.74855). The widespread occurrence of mafic enclaves in the rhyolite dykes suggests that processes such as magma mixing may also be responsible for some of the andesitic rocks known in Saurashtra.
The physical volcanology of large-scale effusive and explosive silicic eruptions in southeastern Saurashtra, Deccan Traps
2023, Geological Magazine

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Research ArticlePetro-geochemistry, SrNd isotopes and 40Ar/39Ar ages of fractionated alkaline lamprophyres from the Mount Girnar igneous complex (NW India): Insights into the timing of magmatism and the lithospheric mantle beneath the Deccan Large Igneous Province

Highlights

Abstract

Introduction

Section snippets

Geological background

Methodology

Petrography and mineral chemistry

Whole-rock geochemistry

40Ar–39Ar geochronology

Sr–Nd isotopes

Crustal contamination

Conclusions

Declaration of Competing Interest

Acknowledgements

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Research Article
Petro-geochemistry, SrNd isotopes and ⁴⁰Ar/³⁹Ar ages of fractionated alkaline lamprophyres from the Mount Girnar igneous complex (NW India): Insights into the timing of magmatism and the lithospheric mantle beneath the Deccan Large Igneous Province

⁴⁰Ar–³⁹Ar geochronology

Early and late alkali igneous pulses and a high-³He plume origin for the Deccan flood basalts

⁴⁰Ar/³⁹Ar geochronology and geochemistry of the central Surashtra mafic dyke swarm, insights into magmatic evolution, magma transport, and dyke-flow relationships in the northwestern Deccan Traps

Mineralogy, geochemistry and ⁴⁰Ar³⁹Ar geochronology of the Barda and Alech complexes, Saurashtra, northwestern Deccan Traps, early silicic magmas derived by flood basalt fractionation