We present the whole‐rock and mineral chemical data of the Khasi meta‐mafic/basic‐trondhjemite intrusives (KMI) and meta‐tonalite‐trondhjemite (MTT) that coevally emplaced the Proterozoic Shillong Group (SG) of Shillong Plateau, northeast (NE) India and discuss their petrogenetic‐tectonic origin. The calc‐alkaline to tholeiite KMI are compositionally transitional between hornblende‐gabbro; hornblende‐diorite and minor‐hornblendite. They show Ta–Nb troughs and strong decoupling large‐ion lithophile/high field strength elements (HFSE) signature consistent with subduction zone‐related basic magmas. The calcic to calc‐alkaline, low‐Al2O3, and I‐type metaluminous Fe‐hedenbergite‐trondhjemite (KMT) exhibits similar trace and rare earth elements (REE) distribution to the KMI. The MTT on the other hand are depleted in trace elements and REE, and correspond with the I‐type granitoids, and are comparable to Proterozoic to Phanerozoic tonalite‐trondhjemite‐granodiorites. Gentle negative sloping of REE in KMI ([La/Lu]CN = 4.04 to 6.35) with moderate LREE, suggests partial melting or, fractional crystallization from a transitional source between garnet and spinel lherzolite. Although almost a flat pattern ([La/Lu]CN = 3.70 to 4.05) for MTT entails a high degree of partial melting from sources similar to Archaean Mafic Composite or Archaean Tholeiite. Trace elemental modellings ([Sr/Y] vs. Y and [La/Yb]N vs. YbN) for MTT infer low pressure (<15 kbar) and high degree partial melting of >50% in equilibrium with 10% garnet +90% amphibole residue. A relatively flat REE pattern with some elevated HREE of MTT compared to KMI suggests for depleted source. Bimodal plutonism and the prevalence of meta‐dacite‐rhyolite in SG invokes for tonalite‐trondhjemite‐dacite setting of a continental arc or, active continental margin tectonic setting. Emplacement of KMI initiated in a temporary relax‐extensional phase and MTT in a compressional regime. This is possibly caused by an over‐thickened crust later subjected to orogenic collapse, exhumation‐metamorphism, and decompressional melting. This event might have taken place between 1.0 to 0.5 Ga during the Pan‐African Orogenic crustal growth, linked to the amalgamation of Gondwana.

Hitherto unknown rare orbicular carbonatites and associated nodular structures occur in the southeastern parts of the Cretaceous Sung valley ultramafic-alkaline-carbonatite complex (SUACC), Shillong plateau, Meghalaya, northeast India. The orbicular structures are formed around fragmented pyroxenite xenoliths and uralitized sodium-richterite nodular patches during emplacement of a calcium-carbonatite plug. The orbicular structures surrounding the xenoliths were formed in at least three stages of chemical disequilibrium represented by three distinct sectors. They are: (i) an innermost shell of sodium-richterite developed around the core of xenoliths. These shells are associated with radial forsterite and/or fine to medium grained granular forsterites. The xenoliths are partially and at times completely metasomatized, due to the metasomatic interaction of the xenolith and the carbonatite melt (antiskarn); (ii) an intermediate zone resembling a “zebra” striped pattern comprising of alternating white calcite-rich and black forsterite-rich bands. These are formed through recrystallization of contaminated carbonatite melt after complex partial resorbtion and assimilation of the pyroxenite xenolith rims; and (iii) an outermost relatively thicker carbonate rich olivine layer formed by late phase intrusion of an ultramafic melt. This is followed by the development of sodium-richterite porphyroblastic grains representing post crystallization sodium metasomatism. Further, proto-orbicules in carbonatite with associated nodular minerals are present adjacent to the orbicular carbonatite suggesting local instability during intrusion at the contact of the pyroxenite xenolith with the calcium-carbonatite host. Apart from reporting the first occurrence of the orbicular carbonatites from the SUACC, petrogenetic implications involving antiskarn due to metasomatic processes induced by carbonatite magmatism is also presented.

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Metamorphic to magmatic ortho/clinopyroxene bearing, massive to foliated charnockites sensu lato are exposed in the central part of the Shillong-Meghalaya Gneissic Complex (SMGC), Northeast India. The two pyroxene bearing metamorphic (group-1) charnockites are tonalitic to granodioritic in composition and together with associated high grade rocks pre-date all deformational episodes and successive metamorphic events. On the contrary, the orthopyroxene bearing (group-2) and fayalite + quartz bearing (group-3) magmatic charnockites post-date all deformational and metamorphic episodes. Both group-2 and-3 charnockites are granitic in composition. Thermobarometric estimations and quantitative P-T pseudosection modeling constrain the peak metamorphism of Group-1 charnockites at 900°C/6–6.5 kbar indicating a clockwise P-T trajectory. The ferroan to magnesian charnockites are calc-alkalic to calcic, and weakly peraluminous/metaluminous. The A2-type charnockites with strongly fractionated REE patterns [(La/Lu)N : 3.95‒27.87] and negative Eu anomalies (Eu/Eu*: 0.28‒0.46) are depleted in Nb, Ta, Sr and Ti abundances that correspond with a post-collisional setting. By comparison, in the associated mafic granulites as enclaves/bands, the REEs are 10-60 times enriched relative to chondrite, have flat patterns, and with no negative Eu anomalies. The charnockite suite is derived by the partial melting of varied protoliths including garnet-bearing amphibolites and crustal sources. Tightly-constrained chemical dates in chemically-zoned monazites, hosted within recrystallized grains of pyroxenes and plagioclase in three charnockites samples vary between 463±40 and 526±37 Ma (mean: 503±4 Ma). The magmatic to metamorphic charnockites attest to the prevalence of high-T during the period of post-collisional East Gondwana assembly related accretion at a late stage of Pan-African orogenic cycle, arguably continuous with the Pan-African Prydz Bay suture (East Antarctica) within the East Gondwanaland.

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