The Late Palaeoproterozoic Mayong granitoids of Assam constitute an important component of the continental crust and the Basement Gneissic Complex in the northern extension of the Shillong Plateau, north‐east India. The basement rocks of Shillong Plateau are considered as a continuation of the Central Indian Tectonic Zone. In this study, emphasis is given on the new zircon U–Pb age, Hf isotopic composition, and the tectonic implication of Late Palaeoproterozoic Mayong granitoids. The Mayong granitoids are essentially composed of quartz—K‐feldspar—plagioclase—biotite—hornblende as major minerals and titanite—zircon—allanite—bastnaesite—apatite—ilmenite as the accessory mineral phase. They show typical geochemical characteristics of A‐type granites, such as high silica (average 71.86 wt%), alkalies (average 8.01 wt%), Rb, Zr, Nb, Ga, and rare earth elements (except Eu); high FeOt/MgO and Ga/Al ratios, and low CaO, MgO, Ba, Sr, P, Eu, and Ti concentrations. Laser ablation inductively coupled plasma mass spectrometry zircon U–Pb dating indicates that they were emplaced at ~1,687 ± 35 Ma during the large‐scale magmatism of Late Palaeoproterozoic to Early Mesoproterozoic periods. Their zircon Hf model (TDM2) ages (1,833–2,257 Ma) in conjunction with trace elemental characteristics suggest that they were generated by the reworking/recycling of Early Palaeoproterozoic crust in an intraplate extensional tectonic environment. Thus, we conclude that these A‐type Late Palaeoproterozoic Mayong granitoids of the Shillong Plateau probably have a close genetic linkage with the magmatic event of the Columbia Supercontinent cycle during 1,687 ± 35 Ma.
Meteorites are the recoverable portions of asteroids that reach the surface of the Earth. Meteorites are rare extraterrestrial objects studied extensively to improve our understanding of planetary evolution. In this work, we used calibration-free laser-induced breakdown spectroscopy (CF-LIBS) to evaluate the quantitative elemental and molecular analyses of the Dergaon meteorite, a H 4-5 chondrite fall sample from Assam, India. Spectral signatures of H, N, O, Na, Mg, Al, Si, P, K, Ca, Ti, Cr, Mn, Fe, Co, Ni, andIrweredetected. Along with the atomic emission, this work reports the molecular emission from FeO molecules. The concentration of the measured elements obtained using CF-LIBS is in close agreement with earlier reports. The elements H, N, and O and their concentrations are estimated by using CF-LIBS for the first time. This study applies laser spectroscopy to establish the presence of Ni, Cr, Co, and Ir in meteorites. The elemental analysis forms the basis for the establishment of the potential molecular composition of the Dergaon meteorite. Moreover, the elemental analysis approach bodes well for in-situ analyses of extraterrestrial objects including applications in planetary rover missions.
Late intrusive Tukureswari granitoids (TKG) and the Barbhita granitoids (BBG) of Goalpara district in western Assam constitute an important component of the continental crust of the Shillong Plateau. Thus, the geochemical study of these two granitoids involving their origin, classification and petrogenetic significance would be a contribution towards a better understanding of the evolution of continental crust of the Shillong Plateau. The major oxide and trace element geochemistry reveals several genetic issues on these two granitoids. The I-type affinity of the TKG is indicated from the geochemical features such as high TiO2, P2O5 and K2O contents, low normative corundum (< 1%), high Na2O/K2O ratios, and low concentrations of Ni, Co and Cr. Further, enriched LREE-LILE and HFSE depletion, as well as the normal calc-alkaline nature of arc affinity (e.g., enhanced LILE abundance and low HFSE/LILE ratios) of the TKG indicate subduction-related magmatism. The TKG are also categorized as a deep-level pluton, being enriched in LREE and depleted in total REE and HFSE (Y, Nb, Ta, Zr, Hf). The high La/Nb ratio (1.9 – 8.6), negative Nb and Ti anomalies also suggest orogenic related magmatism. On the other hand, the geochemistry of the BBG reveal a high Niggli Si and Mg values, slightly high normative corundum values (2.16 – 3.41), high Th/Ta, Y/Nb, La/Nb, K2O/Na2O, and Rb/Sr ratios. It also shows ASI, K, Rb, and U contents, prominent depletion of Nb, Sr and Ti on the primitive mantle-normalized multi-element spider diagrams and a low concentrations of Cu, Cr, V and Na2O (> 3.2%). All these geochemical characteristics provide strong evidences in support of a sedimentary parentage for Barbhita granitoids (BBG) and are dominantly of S-type.
Meteorites represent the recoverable portions of asteroids occurring between Mars and Jupiter within the solar system that reach the surface of the Earth. Meteorites are rare extraterrestrial objects studied extensively to improve understanding of planetary evolution. In this work, calibration-free laser-induced breakdown spectroscopy (CF-LIBS) evaluates quantitative elemental and molecular analysis of the Dergaon meteorite, an H 4-5 chondrite fall sample, Assam, India. Spectral signatures of H, N, O, Na, Mg, Al, Si, P, K, Ca, Ti, Cr, Mn, Fe, Co, Ni, Ir, are measured. Along with the atomic emission, this work reports as well molecular emission from FeO molecules. The concentration of the measured elements obtained using CF-LIBS are in close agreement with earlier reports. The elements H, N and O and their concentrations are estimated using CF-LIBS for the first time. This study applies laser spectroscopy to establish presence of Ni, Cr, Co, and Ir in meteorites. Elemental analysis forms the basis for establishment of potential molecular composition of the Dergaon meteorite. Moreover, the elemental analysis approach bodes well for in-situ analyses of extraterrestrial objects including applications in planetary rover missions.
The metasomatised dioritic rocks are well exposed in and around Umsopri area (N 25°49 / and E91°39 / ), Ri-Bhoi district of Meghalaya, along with granitoid rocks. The gneissic complex represents the basement of the Shillong plateau comprising a group of high-grade metamorphic rocks including basic granulite, amphibolite, quartzofeldspathic gneiss, migmatite, calc-silicate gneiss and garnet sillimanite-bearing metapelite. The basement rocks of the Shillong plateau is intruded by Neoproterozoic granitoids of multiple phases. The granitoids are younger in age from south-west to north-east. The Shillong Plateau along with Mikir Hills experienced four major phases of felsic magmatic episodes at ~1800 Ma, ~1600 Ma, ~1400 Ma, and ~500 Ma. Petrographicaly, the rock is composed of K-feldspar, plagioclase, hornblende, quartz along with accessory phases like sphene, apatite, zircon, rutile, ilmenite, etc. The metasomatic effect has been inferred by the formation of biotite and sphene at the expanse of Hornblende. The rocks have moderate SiO 2 content (52.55 to 55.84 wt%). Trace elements Rb, Ba, Nb shows a negative trend with SiO 2 concentrations. The high field strength elements like Nb, Zr, La, Th, U show a negative correlation and Y shows a positive correlation with an increase in abundance of SiO 2 . Large ion lithophile
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