A new occurrence of (syenite-hosted) Vanadium bearing titaniferous magnetite ore body has been reported from Ganjang (26°09'35" N: 93°20' E), Karbi-Anglong, Northeastern India. The magnetite ore bodies have lumpy and sporadic occurrences within the host syenite pluton intrusive into gneissic country rocks. Ore microscopic studies reveal that magnetite is often associated with haematite and ilmenite depicting different textural patterns. Critical consideration of several elemental patterns suggests magmatic differentiation to be main ore-forming process. The ore body is suggested to have been formed as late stage segregation from a differentiating alkaline magma in a fluid enriched milieu. © GEOL. SOC. INDIA.

JYOTISANKAR RAY

Geology

S ABHISHEK

G SOHINI

D AVIK

University of Calcutta (informally known as Calcutta University or CU) is a collegiate public state university located in Kolkata, West Bengal, India.&nbsp;Within India it is recognized as a &quot;Five-Star University&quot; and accredited &quot;A&quot; Grade by National Assessment and Accreditation Council.

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University Of Calcutta

Journal of the Geological Society of India

The Samchampi (26° 13′N: 93° 18′E)-Samteran (26° 11′N: 93° 25′E) alkaline complex (SSAC) occurs as an intrusion within Precambrian basement gneisses in the Karbi-Anglong district of Assam, Northeastern India. This intrusive complex comprises a wide spectrum of lithologies including syenite, ijolite-melteigite, alkali pyroxenite, alkali gabbro, nepheline syenite and carbonatite (nepheline syenites and carbonatites are later intrusives). In this paper, we present new major, trace, REE and Sr-Nd isotope data for different lithologies of SSAC and discuss integrated petrological and whole rock geochemical observations with Sr-Nd isotope systematics to understand the petrogenetic evolution of the complex. Pronounced LILE and LREE enrichment of the alkaline-carbonatite rocks together with steep LREE/HREE profile and flat HREE-chondrite normalized patterns provide evidence for parent magma generation from low degree partial melting of a metasomatized garnet peridotite mantle source. LILE, HFSE and LREE enrichments of the alkaline-silicate rocks and carbonatites are in agreement with the involvement of a mantle plume in their genesis. Nb-Th-La systematics with incompatible trace element abundance patterns marked by positive Nb-Ta anomalies and negative K, Th and Sr anomalies suggest contribution from plume-derived OIB-type mantle with recycled subduction component and a rift-controlled, intraplate tectonic setting for alkaline-carbonatite magmatism giving rise to the SSAC. This observation is corroborated by enriched 87 Sr/ 86 Sr initial (0.705562 to 0.709416) and 143 Nd/ 144 Nd initial (0.512187 to 0.512449) ratios for the alkaline-carbonatite rocks that attest to a plume-related enriched mantle (~ EM II) source in relation to the origin of Samchampi-Samteran alkaline complex. Trace element chemistry and variations in isotopic data invoke periodic melting of an isotopically heterogeneous, metasomatized mantle and generation of isotopically distinct melt batches that were parental to the different rocks of SSAC. Various extents of plume-lithosphere interaction also accounts for the trace element and isotopic variations of SSAC. The Sr initial and Nd initial (105 Ma) isotopic compositions (corresponding to ε Nd values of − 6.37 to − 1.27) of SSAC are consistent with those of Sung Valley, Jasra, Rajmahal tholeiites (Group II), Sylhet Traps and Kerguelen plateau basalts. © 2017 International Association for Gondwana Research

Gondwana Research

Petrogenetic evolution of Cretaceous Samchampi-Samteran Alkaline Complex, Mikir Hills, Northeastern India: Implications on multiple melting events of heterogeneous plume and metasomatized sub-continental lithospheric mantle

The Khasi mafic rocks of Shillong Plateau represent the Proterozoic mafic magmatism of Shillong Cratonic Block (SCB) in Northeastern India. The Khasi mafics are best exposed at Laitlynkot, East Khasi Hills, Shillong Plateau, where they occur as lensoidal bodies that are intrusive into the Shillong group of rocks (represented by sediments or low-grade metasediments). Relict high-grade basement rocks (∼Archean) and younger porphyritic granite intrusive also occur in this area. The investigated mafic magmatic rocks petrographically belong to hornblende-gabbro clan and bear imprints of metamorphism. Geochemical characters and variation trends of major and trace elements as well as selected trace element ratios suggest that the Khasi mafic rocks have a tholeeitic lineage of arc affinity controlled by fractional crystallization with a prominent iron enrichment trend. Incompatible trace element abundance patterns of the investigated rocks marked by LILE and LREE enrichments, relative depletion of HFSE, and pronounced negative Nb anomalies conform to a subduction-related arc signature and indicate the generation of the parent melt by slab dehydration and wedge melting processes. Chondrite-normalized REE profiles with flat HREE patterns indicate melting above the stability field of garnet. Low Nb/Ta (7.67-13.7) and high Zr/Hf (44-95.2) and Zr/Sm (8.92-55.9) ratios, with respect to primitive mantle values (Nb/Ta: 17, Zr/Hf: 36, Zr/Sm: 25) are consistent with parent magma derivation from a metasomatized, enriched mantle source. Critical trace element ratios corroborate an enriched mantle source (EMI) for the magma that produced the Khasi mafics and indicate a prominent role of crustal contamination. The isotopic fingerprint of an enriched mantle reservoir (EMI) has been documented in terms of high 87 Sr/ 86 Sr ratios ranging from 0.703769 to 0.711712, while 143 Nd/ 144 Nd ratio varies between 0.511536 and 0.511596. The Khasi mafic are characterized by high Al 2 O 3 , Rb and Th enrichment and Nb/Th&lt;8 which altogether reflect crustal assimilation and fraction crystallization (AFC) of the magma. Geochemical data suggest that partial melting of a subduction-modified, metasomatized, enriched lithospheric mantle wedge of spinel-peridotite composition produced the parental melt to the Khasi mafics, which evolved and attained a continental arc signature through assimilation of crustal materials. Metasomatism of mantle wedge by slab-dehydrated, LILE-rich fluids and incorporation of subduction-derived sediments account for the enrichment of the source. The role of the subducted sediments for mantle enrichment has also been attested by Ce/Ta vs. Ce relations and Th/Ce ratios. Tectonic affiliation and petrogenetic characters of these mafic rocks point towards their generation and emplacement in a continental margin arc setting involving subduction of oceanic plates beneath continental lithosphere, slab dehydration and mantle wedge melting associated with assimilation of crustal components and fractional crystallization (AFC) processes. © 2013 Elsevier B.V.

Precambrian Research

Geochemistry and petrogenesis of Proterozoic mafic rocks from East Khasi Hills, Shillong Plateau, Northeastern India

Discussion: Petrology of mafic-ultramafic rocks along north Purulia Shear Zone, West Bengal

Ortho Magmatic Ore Deposits

Vanadium-Bearing Titaniferous Magnetite of Ramchandrapur Hill, Purulia, West Bengal - A Recycled Banded Iron Ore in the Precambrian Chhotanagpur Gneissic Complex, India

Ore petrology of the V-Ti magnetite (lodestone) layers of the Kurihundi area of Sargur schist belt, Dharwar craton

Mafic and ultramafic magmatism and associated mineralization in the Dharwar craton, southern India

The Canadian Mineralogist

EARLY MAGMATIC AND REACTION-INDUCED TRENDS IN MAGNETITE FROM THE CARBONATITES OF KERIMASI, TANZANIA

Vanadium bearing titaniferous magnetite ore bodies of Ganjang, Karbi-Anglong District, Northeastern India

Journal	Data powered by TypesetJournal of the Geological Society of India
Publisher	Data powered by TypesetSPRINGER INDIA
ISSN	0016-7622