Early Cretaceous ultramafic-alkaline-carbonatite magmatism in the Shillong Plateau-Mikir Hills, northeastern India – a synthesis

A comprehensive mineralogical, geochemical and isotopic review of six ultramafic-alkaline-carbonatite magmatic intrusions of the Shillong Plateau (Sung Valley, Jasra, Swangkre-Rongjeng, and Mawpyut) and Mikir Hills (Samchampi-Samteran and Barpung) is presented here, using the published data. These intrusions emplaced ca. 115–102 Ma ago, thus are significantly younger than the tholeiitic flood basalts erupted in Rajmahal-Sylhet province (ca. 118–115 Ma). The intrusive lithologies vary from ultramafic (dunites, clinopyroxenites, melilitolites) to mafic (ijolites, gabbros sensu lato, shonkinites), to felsic (syenites, nepheline syenites) and carbonatites (mostly calcite-rich varieties). The volcanic-subvolcanic facies (lamprophyres, phonolites) are not abundant. The range of chemical compositions of the magmatic phases in the various assemblages is notable; the intrusive rocks are thus the result of crystallization of magmas from variably evolved, independent liquid-lines-of descent, generally of alkaline/strongly alkaline lineages and sodic-to-potassic in affinity. The large variations of the Sr–Nd isotopic ratios of the silicate intrusive rocks (sensu lato) suggest a role of shallow-level crustal contamination during their formation. The carbonatites of the Sung Valley and Samchampi-Samteran have different isotope ratios than the associated silicate rocks, have some isotopic affinity with the Group I tholeiitic basalts of Rajmahal Traps and have an ultimate genesis in a carbonate-bearing lithospheric mantle.


Introduction
Large Igneous Provinces (LIPs) related to Gondwana dispersal are commonly associated with alkaline igneous intrusions (Bryan and Ernst 2008;Ernst 2014;Parisio et al. 2016;Natali et al. 2018;Cucciniello et al. 2022). These alkaline rocks are often very useful to constrain the mantle composition in the area, as they are less prone to the effects of crustal contamination during the ascent through the crust than generally evolved, hotter, and poor in incompatible element, tholeiitic basalts. Nonetheless, alkaline magmas ponded in crustal reservoirs for significant periods of time and so can be subject to interaction with continental crust (e.g., De Paolo 1981). This effect, visible in the isotopic composition of evolved volcanic rocks, must be also recorded in complementary cumulates and slowly cooled intrusive rocks.
The link between alkaline igneous rocks and carbonatites is well documented and reported worldwide (cf. Woolley 1987Woolley , 1989Woolley , 2003Woolley , 2019Bell 1989;Bell et al. 1998;Mitchell 2005;Woolley and Kjarsgaard 2008a, b and references therein). However, this link is not straightforward. Carbonatite magmas are thought to be generated after three major processes: 1) fractional crystallization of primary carbonate-rich nephelinitic magma; 2) immiscible liquid products of variably carbonated nephelinitic-to-phonolitic melts and 3) direct melting of a carbonated mantle (e.g., Bell et al. 1998;Harmer 1999;Woolley 2003;Gittins and Harmer 2003;Mitchell 2005;Srivastava et al. 2005Srivastava et al. , 2019Melluso et al. 2010;Guarino et al. 2017;Beccaluva et al. 2017). A connection between carbonatites and Large Igneous Provinces (LIPs) and possible direct or indirect links with plumes has also been suggested (e.g., Simonetti et al. 1998;Bell 2002;Ernst and Bell 2010; and hypotheses on the genesis of these six UACC intrusions emplaced in the Early Cretaceous of SPMH massif.

Shillong Plateau-Mikir Hills
The SPMH massif (also known as Assam-Meghalaya Plateau or Meghalaya craton; Sharma 2009;Jain et al. 2020) is an uplifted horst-like structure, which is bordered and crosscut by fault systems (e.g., Desikachar 1974;Nandy 1980Nandy , 2001Gupta and Sen 1988). The two major, approximately E-W trending, fault systems are the Dauki to the south, the Brahmaputra to the north (see Fig. 1). Several other faults/ lineaments are also recorded around and within the SPMH; these include N-S trending Jamuna, Nongchram and Um Ngot, and NE-SW trending Badapani-Tyrsad (see Fig. 1; Gupta and Sen 1988;Nandy 2001). The major structural features are thought to be related to the Gondwana fragmentation during Jurassic-Cretaceous and Cenozoic, due to the northward migration of the Indian plate and its subsequent collision with the Asian continent, and "pop-up" of the Shillong Plateau (e.g., Desikachar 1974).

Jasra UACC
The Jasra intrusion emplaced within the Shillong Plateau and intrudes the Proterozoic Shillong Group and Neoproterozoic granitoids. The main rock units are pyroxenite (clinopyroxenite and olivine clinopyroxenite), shonkinite, gabbro (monzogabbro and olivine gabbro), mafic dykes and syenite/nepheline syenite ( Fig. 2b; cf. Mamallan et al.  Kumar et al. 1996;Saha et al. 2017) 1994; Sinha 2004b, 2007;Melluso et al. 2012;Srivastava et al. 2019). Small dykes/dykelets/veins of trachyte, alkali pegmatite, ijolite, carbonatite are also reported (Mamallan et al. 1994). Based on a detailed field work, Srivastava and Sinha (2004b) suggested that: 1) pyroxenite and gabbro exposures occur as distinct intrusive events; any direct field relationship between these two is difficult to identify; 2) mafic dykes are contemporaneous to the gabbro; they crosscut pyroxenites and granitic rocks, and 3) dyke/ dykelets of nepheline syenite crosscut all the formations suggesting them to be the youngest unit of the complex. These observations clearly indicate derivation of these rock units from different magma batches. Calcite clusters are also noticed in a few thin sections.

Mawpyut alkaline ultramafic-mafic complex
The ultramafic-mafic Mawpyut complex is a dome-shaped arcuate body intruded within the Shillong Group ( Fig. 2d; Chaudhuri et al. 2009Chaudhuri et al. , 2011 and shows intrusive relationships with Archean gneisses (Maitra et al. 2011). It is formed by a variety of mafic-ultramafic rocks; cumulate variants show several gabbroic facies (olivine gabbronorite, melagabbronorite, mela-gabbro, orthopyroxene gabbro and gabbro), whereas non-cumulate variants are gabbro and leucogabbro (cf. Chaudhuri et al. 2014). Syenite/nepheline syenite veins and dykelets are also reported to crosscut gabbroid bodies. No carbonatite are reported from this complex. Chaudhuri et al. (2009Chaudhuri et al. ( , 2014 and Maitra et al. (2011) infer that this complex is a differentiated product of ultramafic-mafic rocks, probably Sylhet Trap. Maitra et al. (2011) argued that the Mawpyut complex has different genetic history than the other UACCs of the SPMH.

Samchampi-Samteran UACC
The Samchampi-Samteran UACC is a plug-like intrusion intruded within the Archean Gneissic Complex (Kumar et al. 1996;Nag et al. 1999;Saha et al. 2010Saha et al. , 2017. Syenite is the most abundant rock type. As Sung Valley complex, an ijolite-melteigite intruded the syenitic body and formed a ring structure (Fig. 2e). Alkali pyroxenite, alkali gabbro, nepheline syenite and carbonatite are also found. Alkali pyroxenite and alkali gabbro are exposed as lenticular bodies within the ijolite-melteigite; field relationship suggests that they are older than the latter. Nepheline syenite and carbonatite intruded syenite rocks as a dyke/dykelets; therefore, they are likely to be youngest units. Magnetite-bearing ore bodies intruded the syenitic pluton.

Barpung alkaline-ultramafic complex
Barpung is a roughly circular body intruded within the Shillong Group; the E-W trending Kalyani lineament is likely to control its emplacement (Kumar et al. 1996). Little is known about this complex except that it is mainly formed by pyroxenite, magnetite-rich rocks, alkali syenite and fenites ( Fig. 2f). No carbonatite occurrence is known to date.

Geochronology
All geochronological data available in the published literature are summarized in the Table 1. The age of the Sung Valley intrusion varies from 115.1 ± 5.1 Ma (perovskite of an ijolite) to 101.7 ± 3.6 Ma (perovskite of an uncompahgrite). The ages of Jasra vary from 106.8 ± 0.8 Ma (zircon from a syenite) to 101.6 ± 1.2 Ma (perovskite in a clinopyroxenite). The age of a lamprophyre of Swangkre-Rongjeng is 107 ± 3 Ma. The age of apatite separated from a carbonatite of Samchampi-Samteran is ~ 105 Ma. No geochronological data are available in the literature for Mawpyut and Barpung. Mawpyut is thought to be part of the Early Cretaceous magmatism (Chaudhuri et al. 2009(Chaudhuri et al. , 2014Maitra et al. 2011). Magmatism emplaced in the SPMH (ca. 115-102 Ma) is significantly younger than the tholeiitic flood basalts of Rajmahal-Sylhet volcanic province, erupted ca. 118-115 Ma ago (e.g., Baksi 1995;Kent et al. 2002).

Mineral chemistry
Generalities Table 3 reports the list of mineral phases of each rock type identified in all UACC intrusions. No chemical mineral analyses are available for Barpung, where the main identified minerals are clinopyroxene, olivine, rutile, magnetite and apatite in pyroxenites, ilmenite, titanomagnetite, titanite, clinopyroxene and perovskite in magnetite-rich rock and alkali feldspar, clinopyroxene, and haematite in alkali syenite (Kumar et al. 1996). Detailed descriptions and chemistry of minor phases such as oxides, apatite and accessories can be additionally found in Melluso et al. (2010Melluso et al. ( , 2012.

Feldspars
Alkali feldspar is the main feldspar, with subordinate plagioclase at Sung Valley, Jasra, Swangkre-Rongjeng, Mawpyut and Samchampi-Samteran (Fig. 3c). Alkali feldspar in the Sung Valley intrusion is the main cumulus crystal of nepheline syenites; it also occurs as an interstitial phase in clinopyroxenites and has potassic composition (An 0-1 Ab 18-4 Or 82-95 in nepheline syenites and An 0-0 Ab 29-4 Or 71-96 in clinopyroxenites). Secondary albite is also identified (An 0 Ab 99 Or 1 ). The concentration of Ba and Sr is generally low (BaO < 0.67 wt%, SrO < 0.75 wt%). In the Jasra intrusion, alkali feldspar is interstitial in the clinopyroxenites and a cumulus phase in the syenites and nepheline syenites and varies from anorthoclase to alkali feldspar (An

Bulk-rock geochemistry
The magmatism in the SPMH massif has a slight potassic affinity at Jasra and Swangkre-Rongjeng, a sodic affinity at Sung Valley and Mawpyut intrusions, and variable from slightly potassic to highly potassic at Samchampi-Samteran (see Table 2 for references).
The variations of MgO, SiO 2 and Al 2 O 3 highlight the distribution of the different lithologies of the SPMH intrusions, which are mainly the result of accumulus of mafic silicates and feldspars or feldspathoids (Fig. 5). A general decrease of MgO of silicate rocks (s.l.) corresponds to Sr increase, more pronounced in the Jasra rocks (Fig. 6). The concentrations of Rb, Ba, Nb, and Zr increase with Sr, where in the silicate rocks (s.l.) these elements increase in all the intrusions (Fig. 6).
The SPMH silicate rocks (s.l.) have variable concentration of high-field strength elements (HFSE) and large-ion lithophile elements (LILE), with variable peaks in these elements, as seen in the multi-elemental patterns (Fig. 7). All these patterns are influenced by the mineral assemblages of the rocks and cannot be taken as evidence of magmatic liquids. The pronounced peaks at Ba and Sr are related to accumulus of alkali feldspar, whereas the peaks are much less marked where alkali feldspar crystallized and accumulated from evolved magmatic liquids, such as trachytes of phonolites; similarly, the different enrichment pattern observed in the Zr-Hf and Th-U pairs are associated to the accumulation or removal of phases such as perovskite, zircon, zirconolite and so on.
The Sr concentration is high in the Sung Valley and Samchampi-Samteran carbonatites but Rb, Ba, and Nb are not very enriched. The Zr concentration in the Sung Valley and Samchampi-Samteran carbonatites is widely variable (Zr > 400 ppm at Sung Valley and Zr < 90 ppm at Samchampi-Samteran) (Fig. 6). The multi-elemental patterns of Sung Valley and Samchampi-Samteran carbonatites have a trough at Zr and Hf relative to the neighboring REE (Fig. 9), as commonplace of igneous carbonatites worldwide. This feature is mainly related to poor solubility of Zr and Hf in carbonatitic magmas and/or strong partitioning of these elements in immiscible silicate rather than conjugate carbonated magmas rather than to effects of fractional crystallization of a specific phase (Andrade et al. 2002;Chakhmouradian 2006;Guarino et al. 2017). The chondritenormalized REE patterns of carbonatites are more fractionated at Samchampi-Samteran (La N /Yb N = 49-101) than at Sung Valley (La N /Yb N = 33-57) (Fig. 9).

Evolution processes within the various intrusions in SPMH areas
The Sung Valley is a plagioclase-free ultramafic alkaline intrusion. The data resulting from field-setting, bulk-rock geochemical, mineral chemistry, and Sr-Nd isotopes highlighted the absence of a genetic correlation between silicate rocks (s.l.) and carbonatites (Melluso et al. 2010;Srivastava et al. 2005Srivastava et al. , 2019Srivastava and Sinha 2004a). The silicate rocks (s.l.) were generated by different pulses of magmas, each with its own composition formed the cumulitic intrusive rocks. The silicate rocks (s.l.) are ultramafic perovskite-bearing and olivine-bearing rocks,  Table 2 for references uncompahgrites, ijolites, clinopyroxenites and nepheline syenites, which formed after the accumulation of different mineral assemblages from variably evolved magmas, with incompatible-element nature, ranging from melilititic and nephelinitic to phonolitic types.
The Sung Valley carbonatites are not likely to be formed after immiscibility of carbonated "nepheline syenitic" or carbonated "ijolitic" melts or their equivalents, due also to markedly different isotopic compositions (cf. Srivastava et al. 2019). Their mineral assemblages, characterized by highly magnesian olivine, phlogopite and ilmenite (plus clinohumite and dolomite), and some high concentration of Cr in spinel, indicate a different genesis from the surrounding silicate rocks (cf. Srivastava et al. 2005Srivastava et al. , 2019. The shallow intrusion of Jasra comprises ultramafic to felsic lithotypes. The genesis of ultramafic rocks is likely due to mineral accumulation from magmas of different compositions, likely ranging from basalt/basanite. The gabbroic rocks and nepheline syenites were originated by mineral setting of clinopyroxene, kaersutite, sodic plagioclase, relatively Fe-rich olivine, phlogopite and nepheline, from slightly evolved alkaline magmas (tephrites, trachybasalts), and syenites formed after accumulus of alkali feldspar and Fe-rich phases from trachytic magmas (s.l.). Some of the gabbroic rocks have a subalkaline affinity, related to the presence of olivine with orthopyroxene rims and augite clinopyroxene. An interesting mineralogical feature observed at Sung Valley and Jasra is the peritectic replacement of perovskite by titanite (see Fig. 4a, b; Table 3; cf. Melluso et al. 2010Melluso et al. , 2012. At Sung Valley, the presence of perovskite with garnet and titanite rims in an ijolitic rock (sample SV58) are indicative of increase of silica activity in the equilibrium melt of larnite-normative affinity, stabilizing titanite. More interestingly, perovskite was found as corroded grains within euhedral titanite in a Jasra clinopyroxenite, hence never in contact with alkali feldspar, testifying the same peritectic reaction in a more evolved rock type (cf. Melluso et al. 2012).  Table 2 for references 1 3 The Swangkre-Rongjeng potassic lamprophyres were likely generated during partial melting of hydrous mantle source (Srivastava et al. 2016), whereas, the Mawpyut intrusion is characterized by the presence of two different ranges in the Sr-Nd isotopes for mafic ultramafic cumulate and non-cumulate rock types (Fig. 10) that suggest a different role of upper crustal contamination as evident in the higher initial 87 Sr/ 86 Sr ratio of non-cumulate rocks.
The Samchampi-Samteran UACC has markedly different isotopic composition between alkaline silicate rocks and carbonatites, like at Sung Valley; hence these rocks cannot be derived by simple fractional crystallization and accumulus of phases of a single parental melt composition. Fractional crystallization and upper crustal contamination have played a marked role in the high variability of Sr-Nd isotopes. In the Samchampi-Samteran intrusion, the isotopic similarity between one carbonatite and the associate silicate rocks (s.l.) are indicative of a similar petrogenetic process (Fig. 10). View the strong isotopic affinity between Samchampi-Samteran and Sung Valley carbonatites, it is possible to consider that the Samchampi-Samteran carbonatites may be derived by melt originated at greater depths in the mantle.
The Barpung intrusion is formed by pyroxenites, alkali syenites and fenites; despite the low amount of data available, the silicate rocks may have formed through fractional crystallization/accumulus from mantle-derived magmas, mainly associated with upper crustal contamination as highlighted by their higher 87 Sr/ 86 Sr ratios (Fig. 10).
It is remarked that none of these intrusions can be considered as feeders of the tholeiitic lavas of the Rajmahal-Sylhet traps, for the highly alkaline affinity of the outcropping lithotypes and the ages, making any link with the tholeiitic magmatism of the area fully unjustified. At the same time, there is no evidence of crystallization in the mantle of any lithotype and its phases found in the various intrusions, from the silicate rocks to the carbonatites.

Isotopic signature and mantle source characteristics beneath the Shillong Plateau-Mikir Hills
The marked similarity in the initial 87 Sr/ 86 Sr and εNd i of Sung Valley and Samchampi-Samteran carbonatites is remarkable. Their similarity suggests an origin from a mantle source with an isotopic composition similar to that of the mantle source of Rajmahal Group I basalts, and different from source of present-day Indian MORB (Fig. 10). The melt that have crystallized the Sung Valley carbonatites is originated through low degree of partial melting of a metasomatized carbonated mantle; by analogy, a similar process may be assumed for the formation of Samchampi-Samteran carbonatites. This assumption suggests the role played by a metasomatized carbonated mantle throughout the whole province (Fig. 1). The presence of a carbonate-bearing mantle, which produce melts of olivine melilititic, olivine nephelinitic and basanitic composition, was responsible for the crystallization of various  (2013) and Srivastava et al. (2019); Jasra data from Srivastava and Sinha (2007) and Srivastava et al. (2019); Samchampi-Samteran data from Ghatak and Basu (2013) and Saha et al. (2017); Barpung data from Ghatak and Basu (2013); Mawpyut data from Chaudhuri et al. (2014). Isotopic variations in Kerguelen basalts are from Storey et al. (1992), Mahoney et al. (1992Mahoney et al. ( , 1995Mahoney et al. ( , 2002, Kumar et al. (2003); compiled in Srivastava et al. (2005), Srivastava and Sinha (2007), Ghatak and Basu (2013), and Mattielli et al. (2002), Ingle et al. (2002), Doucet et al. (2005); Rajmahal basalts Group I and II are from Storey et al. (1992), Baksi (1995) and Kent et al. (1997); Sylhet tholeiitic basalts from Ghatak and Basu (2011); Jharia lamproites from Kumar et al. (2003). The continental clasts from IODP site 1137 are from Ingle et al. (2002). The Indian MORB is from Mahoney et al. (2002) types of intrusive alkaline rocks, often in the same intrusive complex. The range of initial Sr and Nd isotopes of the silicate rocks (s.l.) belonging to these six UACC intrusions suggests the presence of an open magmatic system in throughout the whole SPMH province (Fig. 10). The increase in the initial 87 Sr/ 86 Sr and decrease in the ε Ndi in the silicate rocks (s.l.) define a trend towards the underlying SPMH crust suggesting that during their ponding and emplacement in the SPMH area there is the involvement of an upper continental crust component, rather than isotopic heterogeneity within the mantle, during the crystallization processes related to the formation of these silicate rocks (s.l.), as happen for Rajmahal Group II (Kent et al. 1997) and some Sylhet basalts (Ghatak and Basu 2013;Srivastava et al. 2005;Srivastava and Sinha 2007;Veena et al. 1998).

Economic aspects of the SPMH UACCs
An important aspect in these six UACCs is their marked enrichment in some elements as P, Ti, Ba, Th, U, Nb, Ta, and LREE. These elements are hosted in some typical mineral observed and analyzed in these complexes, and looking the chemical analysis of bulk-rocks, they reveal a high potential of substantial resources of these elements ( Table 3). The main minerals of economic significance are pyrochlore, apatite, magnetite and vermiculite at Sung Valley (Kumar et al. 1996); magnetite, ilmenite, perovskite, Nb-Th-phases, and the occurrence of sulfides (pyrite, chalcopyrite, covellite) in association with pyroxenites and high amounts in REE (up to 9500 ppm) in the soils above titanomagnetite-rich rocks at Jasra (Kumar et al. 1996). The Sung Valley carbonatites have high economic potential view their enrichment in REE carbonates and phosphates associated with REE-Nb bearing pyrochlore (Sadiq et al. 2014). The residual soil related to alteration processes over the Sung Valley carbonatite are extremely enriched too; it contains about 1300 tons of Nb 2 O 5 in 6.75 million tons ore, mainly due to pyrochlore (8.50% ThO 2 and 2.2% U 3 O 8 ) (Singh 2020 and references therein). The soils above Barpung complex contain high concentrations of LREE and Y (Kumar et al. 1996).
High absolute concentrations in P (41.68%), REE (62.60%) and Nb (60.19%) have been noted in the Samchampi-Samteran intrusion (Hoda and Krishnamurthy 2016a). These authors estimated a reserve of 15 million metrics tons of phosphatic ore, and the possibility of associate uranium and rare-earth elements as possible by-products, all economically viable. Instead the residual soil contains 10,970 tons Nb and 3740 tons of Ta in an area of 10.94 km 2 (Hoda et al. 1997) and 3644 tons Y resource in 41.88 million tons ore (Hoda and Krishnamurthy 2016b).

Final considerations
By combining mineral chemistry, bulk-rock geochemical and Sr-Nd isotopic composition of the UACC intrusions in the SPMH area, several different processes acted in the petrogenesis of the alkaline province above SPMH, such as upper crustal contamination and fractional crystallization. They are synthesized as follows: (i) a carbonate-bearing lithospheric mantle is proposed to produce different melts for the genesis of these six SPMH intrusions, (ii) the marked similarity in the initial 87 Sr/ 86 Sr and εNd i of Sung Valley and Samchampi-Samteran carbonatites, and the high variability in the initial Sr-Nd isotopes of silicate rocks (s.l.), are indicative of different processes of formation, (iii) the geochemical features of silicate rocks (s.l.), related to their ponding and emplacement, are indicative of different parental magmas (alkali basalts, basanites, nephelinites, melilitites), associated with crustal contamination during their ascent, (iv) the melts that crystallize carbonatite in Sung Valley and Samchampi-Samteran could have been at the base of lithospheric mantle in areas far from the influence of mantle plumes and high geothermal gradients, (v) the absolute REE abundance observed in the Sung Valley and Samchampi-Samteran carbonatites are related to the melts from which they crystallized, melts ultimately generated in the lithospheric mantle, and (vi) the presence of economic minerals and soils in the UACC intrusions bearing REE, P, Ti, Ba, Th, U, Nb, Ta will be useful for their expanding applications in various strategic and high technological fields.
Acknowledgements We are grateful to Kirtikumar R. Randive for inviting us to contribute a paper in this special issue dedicated to the late Prof. Lalchand G. Gwalani. Fu-Yuan Wu is also greatly acknowledged for his interest to study the alkaline rocks of NE India. Constructive comments of two anonymous reviewers, the scientific advice of Guest Editor Peter J. Downes and scientific and editorial input of Editor-in-Chief Lutz Nasdala are gratefully acknowledged. The Heads of the Department of Geology, Banaras Hindu University, succeeding in the years, are thanked for extending all necessary facilities during this review work.
Funding Open access funding provided by Università degli Studi di Napoli Federico II within the CRUI-CARE Agreement. LM is thankful to the Ministero dell'Università e della Ricerca for funding through Progetti di Ricerca di Interesse Nazionale (PRIN) 2017 [fund ID Number: 20178LPCPW-004 to Ciro Cucciniello].
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