Mineralogy of the TK1 and TK4 'kimberlites' in the Timmasamudram cluster, Wajrakarur Kimberlite Field, India: Implications for lamproite magmatism in a field of kimberlites and ultramafic lamprophyres

A mineralogical study of the hypabyssal fades, late Cretaceous macrocrystic pulse of TK1 intrusion and the Mesoproterozoic aphanitic pulse of TK4 intrusion in the Wajrakarur Kimberlite Field of southern India shows that the rocks contain macrocrysts of forsteritic olivine, phenocrysts and microphenocrysts of Al-Na-poor diopside and phlogopite set in a groundmass mainly of Al-Na-poor diopside and phlogopite. Other groundmass minerals are spinel, perovskite and fluorapatite in TK1, and spinel, titanite, chlorite, calcite and gittinsite in TK4. K-richterite and perovskite occur only as inclusions in phlogopite and titanite, respectively in TK4. Late-stage deuteric phases include pyrite and barite in TK1, and strontianite, chalcopyrite, galena and pentlanditein TK4. Diopside microphenocrysts in TK4 exhibit oscillatory zoning with characteristics of diffusion controlled magmatic growth. TK1 spinels show magmatic trend 2 that evolves from magnesiochromite and culminates in titaniferous magnetite, whereas TK4 spinels are less evolved with magnesiochromite composition only. TK1 phlogopites show a simple compositional trend that is typical of lamproite micas, while four distinct growth zones are observed in TK4 phlogopites with the following compositional characteristics: zone I: high Cr2O3 and TiO2 and low BaO; zone II: low Cr2O3; zone III: low TiO2 and high BaO; zone IV: low BaO. Forsterite contents and trace element concentrations reveal two xenocrystic core populations and one magmatic rim population for TK1 olivines. Mineralogically, both TK1 and TK4 are classified as diopside-phlogopite lamproites rather than archetypal kimberlites. The two lamproites are considered to have formed from the same parent magma but crystallised under distinct oxygen fugacity conditions. With elevated content of Fe3+ in phlogopite, spinel and perovskite, TK1 appears to have crystallised in a relatively high oxygen fugacity environment. Multiple growth generations of phlogopite, spinel and fluorapatite in TK4 indicate a complex evolutionary history of the magma. Close spatial and temporal associations of Mesoproterozoic kimberlites and lamproites in southern India can possibly be explained by a unifying model which accounts for the generation of diverse magmas from a range of geochemical resevoirs in a continental rift setting. (C) 2016 Elsevier B.V. All rights reserved

Geology and geochemistry of giant quartz veins from the Bundelkhand Craton, central India and their implications

Giant quartz veins (GQVs; earlier referred to as 'quartz reefs') occurring in the Archean Bundelkhand Craton (29, 000 km(2)) represent a gigantic Precambrian (similar to 2.15 Ga) silica-rich fluid activity in the central Indian shield. These veins form a striking curvilinear feature with positive relief having a preferred orientation NE-SW to NNE-SSW in the Bundelkhand Craton. Their outcrop widths vary from <= 1 to 70 m and pervasively extend over tens of kilometers along the strike over the entire craton. Numerous younger thin quartz veins with somewhat similar orientation cut across the giant quartz veins. They show imprints of strong brittle to ductile-brittle deformation, and in places are associated with base metal and gold incidences, and pyrophyllite-diaspore mineralization. The geochemistry of giant quartz veins were studied. Apart from presenting new data on the geology and geochemistry of these veins, an attempt has been made to resolve the long standing debate on their origin, in favour of an emplacement due to tectonically controlled polyphase hydrothermal fluid activity

Hematite-rich concretions from Mesoproterozoic Vindhyan sandstone in northern India: a terrestrial Martian 'blueberries' analogue with a difference

We report here hematite-rich concretions observed in the sandstone of the Mesoproterozoic Vindhyan Supergroup of rocks occurring in parts of Bihariya, Uttar Pradesh, northern India. These concretions are similar to `blueberries' from Mars and their terrestrial analogues reported from the Jurassic Navajo Sandstone in Utah, USA. The presence of diagenetically formed hematite concretions gave the first confirmation of the presence of liquid water in the red planet in the past. We report here the detailed morphology, petrography, mineral chemistry, magnetic susceptibility characteristics and spectral radiometric data of hematite-rich concretions observed in the Vindhyan sandstone. These are compared with `blueberries' from Mars and other similar terrestrial analogues reported from different parts of the world. In spite of similarities, these hematite-rich concretions are strikingly distinct in having a nucleus and alternate iron-rich and iron-poor rims unlike other global occurrences. In addition, we document here outcrop scale evidence of possible fluid pathways considered responsible for the development of the concretions