Geochronology and geochemistry of Mesoproterozoic porphyry granitoids in the northern Karbi Hills, NE India: Implications for early tectonic evolution of the Karbi Massif

摘要

Extensive Mesoproterozoic magmatism occurred in the Karbi Hills Massif, NE India, providing a key to understanding the early geodynamic evolution of Karbi Craton during the Columbia (Nuna) supercontinent cycle. We present here LA-ICP-MS zircon U-Pb ages, Lu-Hf isotopic and geochemical data of Panbari-Geleki-Dallamara and Kuthori granitoids in the Kaziranga area in northern Karbi Craton. These granitoids occur as stock like bodies intruded into the Late Archean-Paleoproterozoic Basement Gneissic Complex, supracrustals and previously emplaced mafic magmatics. Due to subsequent tectono-thermal activities, the older magmato-sedimentary sequences suffered variable degrees of metamorphism and deformations. The U/Pb zircon geochronology indicates that these granitoids were emplaced over a span of about 94 m.y., subsequent to the emplacement of mafic dykes, sills and stocks in a post collision rift basin, now represented by the 'Shillong Basin'. The Karbi Hills granitoids are younger (1644 +/- 33-1550 +/- 25 Ma) than the Meghalaya felsic magmatics (1778 +/- 37-1620 +/- 9.2). Basin formation and sedimentation began in the Shillong Basin (Shillong Group) as early as 1900 Ma and continued till 1400 Ma, coinciding with the assembly and dismantling of Columbia supercontinent. Early Proterozoic psammopelitic sedimentation prior to and/or after the mafic-felsic intrusions is now represented by the 'Shillong Group'. The mafic magmatics occur in the craton, present a continental flood basalt character. The chemical makes up state that the granitoids bear mostly metaluminous but weakly per-aluminous, A-type, within plate characters. The granitoids are highly potassic and silica saturated (up to 74.3 wt % SiO2), REE deficient, K, Rb and Ba enriched but Nb, Ti and P depleted. The zircon epsilon(Hf(t)) for granitoids show a variation up to 7 eHf units (+1.5 to + 9.9), however, 13% grains possess negative epsilon Hf(t) values. This is particularly ascribed to minor mixing of magmas sourced from depleted mantle and metasomatised continental crust. There is, however, no record of any vestige of ophiolite preserved in the craton that would support an active subduction mechanism responsible for this part of early Mesoproterozoic magmatic episode.