Discovery of Early Tonian Calc-alkaline and Shoshonitic Metamafic Rocks from the North Purulia Shear Zone,Chhotanagpur Gneissic Complex,Eastern India:Implications of Proterozoic Sub-continental Lithospheric Mantle

Reports of shoshonitic rocks in Precambrian terrains are relatively rare.Pl-Grt amphibolites and Hbl-Bt mafic granulites occurring in the migmatitic gneisses of the Chhotanagpur Gneissic Complex(CGC)show calc-alkaline and shoshonitic characteristics.Relict porphyritic,sub-ophitic and poikilitic textures are noted in these rocks.Their parent magma was emplaced during the waning phase of the regional metamorphism.Geochemically,these metamafics are similar to the GroupⅢpotassic and ultrapotassic rocks of Foley et al.(1987).The magma was derived from the metasomatized subcontinental lithospheric mantle(SCLM).Subduction-related sediment melts metasomatized the SCLM.Compositionally,the SCLM is a metasomatized phlogopite-amphibole-spinel-bearing harzburgite.1%–5%batch melting of the SCLM could produce the parental magma of the mafic granulites.Pressures and temperatures of metamorphic equilibration were carried out by pseudosection modeling.Peak metamorphic assemblage(M_(1):Grt-Cpx-Pl-Qz)in garnetiferous amphibolite equilibrated at 740℃and 8.7 kbar.The Cpx-Pl corona appeared around the garnet during decompression(M_(2):655℃,6 kbar).The Hbl-Pl symplectites around garnet formed during isobaric cooling(M_(3):580℃and 5.9 kbar).The emplacement of shoshonitic magma and subsequent decompression happened at the slab break-off stage of continental collision(~990 Ma).

Cenozoic potassic volcanic rocks from the Keluo and Wudalianchi volcanic districts,northeast China:origin from the new sub-continental lithospheric mantle(SCLM)metasomatized by potassium-rich fluids from delaminated lower crust

A series of Cenozoic potassium-rich volcanic rocks developed in the Xiaoguli-Keluo-Wudalianchi-Erkeshan districts,northeast China.The source region and potassium-rich mechanism of the potassic rocks remain highly disputed.In this paper,the major elements,trace elements,and Sr-Nd-Pb isotopes of the volcanic rocks in Keluo(KL)and Wudalianchi(WDLC)volcanic districts were analyzed systematically.The results show that the volcanic rocks are characterized by high K2O(4.36wt.%-6.13wt.%),remarkable enrichment in LREEs and LILEs,as well as the strong fractionation of HREEs.The isotopic characteristics with high 87Sr/86Sr(0.704990-0.705272),low 143Nd/144Nd(0.512306-0.512417),low 206Pb/204Pb(16.546-17.135)and 207Pb/204Pb(15.002-15.783)of the volcanic rocks suggest the involvement of EM-I-type mantle.On the basis of the geochemical characteristics,the potassium-rich volcanic magma originated from the new SCLM forming after delamination of the ancient SCLM,with metasomatism of the potassium-rich fluids released from the ancient lower crust during the Late Mesozoic.The proposed genetic model assumes the source which represented by a phlogopite-bearing garnet peridotite(with modal garnet in the range of 2%-10%)experienced very low degrees(i.e.,~0.5)of partial melting.During Cenozoic,the lithosphere in northeast China was affected by the extension and decompression of continental rift,and the metasomatized SCLM underwent low degree partial melting,resulting in the formation of potassium-rich primitive basaltic magma.

Off-Shore Sea Surface Electric Field Investigations around the Indian Sub-Continent during 9-20 May 1983

Sea surface electric field observations off the coast from Goa (15°25'N, 73°47'E) to Madras (13°04'N, 80°15'E) around Sri Lanka, in a distance range 25-135 km from coast, during 9-20 May 1983 were taken. In this paper we have examined the diurnal variation of electric field in the Arabian Sea, Indian Ocean and Bay of Bengal regions covered during the cruise of the research ship ORV Gaveshani. An aspect of electric field dependence on coastal distance and Aitken Nuclei concentration has also been studied. An attempt to examine the latitude dependence of field was also made. Results obtained in the above studies are presented and compared with those obtained elsewhere.

Early Devonian Ultrapotassic Magmatism in the North China Craton: Geochemical and Isotopic Evidence for Subcontinental Lithospheric Mantle Metasomatism by Subducted Sediment–Derived Fluid

The North China Craton(NCC) represents one of the oldest and largest cratons in the earth with a nearly complete record of Precambrian history. In the northern part of the NCC, the earliest phase of alkaline magmatism occurred in discrete pulses in the Early and Middle Devonian;whereas the next episode of alkaline magmatism took place in the early Mesozoic. The Gucheng pluton is exposed in the northern part of the NCC and forms a composite intrusion, consisting of K-feldspar–bearing clinopyroxenite, clinopyroxene–bearing syenite and alkali-feldspar syenite. Mineral phases in these lithologies include clinopyroxene(Wo43-48En19-35Fs18-38), sanidine(An0 Ab3-11Or89-97), and subordinate titanite, andradite and Na-feldspar. These rocks show homogeneous Sr but variable Nd isotopic compositions, and have relatively high zircon in-situ oxygen isotopes(δ18O=5.2–6.7). The Gucheng plutonic rocks formed through fractional crystallization and accumulation from ultrapotassic magmas, which were originated from partial melting of metasomatic vein systems in the subcontinental lithospheric mantle of the NCC. These vein networks developed as a result of the reactions of fluids derived from subducted pelitic sediments on the downgoing Palaeo-Asian ocean floor with the enriched, subcontinental lithospheric mantle peridotites. SHRIMP U-Pb zircon dating has revealed a crystallization age of 415 Ma for the timing of the emplacement of the Gucheng pluton that marks the early stages of alkaline magmatism associated with the Andean-type continental margin evolution along the northern edge of the NCC facing the Palaeo-Asian Ocean.

Analysis of Long Term Variation of the Total Column and Tropospheric Ozone over the Indian Region

Ozone plays a significant part in regulating climate change and the chemical characteristics of the atmosphere. Changes in atmospheric ozone can be studied in more detail using ground-based and satellite-based instruments. Studies on the long-term global changes in total column ozone have begun more than three-decade ago using satellite data. The main objective of this work is to analyze the Total Column Ozone (TCO) variations, and tropo-spheric ozone variations over different twenty locations in the Indian sub-continent by using Total Ozone Mapping Spectrometer (TOMS) and AURA OMI/MLS data. The long-term analysis of total column ozone is divided into two phases (1979-1994 and 2005-2018), and tropospheric ozone for one phase (2005-2018) in order to detect changes in the ozone trend pattern. The results of linear regression analysis show a declining trend of total column ozone, and an increasing trend of tropospheric ozone over the selected locations. The impact of wind pattern on the variation of ozone has been analyzed by using NCEP reanalysis data, and found that wind patterns played a prominent role in spatial and temporal changes in total and tropospheric ozone distribution over the subcontinent. Latitudinal variation of total column ozone from Nagarcoil to Anantanag has also been studied for the years 1979, 1994, 2005, and 2018, which indicates an increase in ozone concentration with latitude.