Petrography and tectonic provenance of the Miocene Surma Group in parts of the Naga-Manipur hills, in and around Nungba,Northeast India

The Miocene molassic Surma sandstones in parts of the Naga-Manipur hills,in and around Nungba,Northeast India have been studied for their tectonic provenance using petrography and heavy mineral contents.The poorly-to-moderately-sorted sub-litharenite to lithicarkose type Surma sandstones display predominance of monocrystalline quartz(av.61%)and include undulose and non-undulose varieties and almost equal amounts of feldspars(av.22%)and rock fragments(av.17%).The heavy mineral suite of Surma sandstones is dominated by transparent varieties(96%)that include garnet,zircon,tourmaline,rutile,staurolite,scapolite,phlogopite,chondrodite,humite,wollastonite,hedenbergite,sphene,chlorite,sillimanite,glauconite,glaucophane,and chloritoid.These heavy minerals characterize the granitic and silicic metamorphic complexes(GM)as well as the basic metamorphic rocks(MT)like greenstones,green schist,and amphibolites relating to passive continental margin setup.The opaque grains constitute nearly 4%of the total heavy minerals.The presence of euhedral,as well as abraded heavy mineral grains,further supports a mixed provenance having substantial contribution from sedimentary and metamorphic rocks.A ZTR index of approximately 45%indicates mineralogically sub-mature nature of Surma sandstones.Based on the light and heavy mineral suites coupled with the type of terrain available in the vicinity of the study area,it may be visualized that the sediment supply was largely made by the Himalaya,the IMR,the Shillong plateau,and the Mikir Hills under the influence of semi-arid to semi-humid climatic conditions.An overall supply from a recycled orogen provenance has been envisaged.

Whole-rock geochemistry of Tertiary sediments of Mizoram Foreland Basin, NE India: implications for source composition,tectonic setting and sedimentary processes

Sandstones belonging to the Oligocene Barail Group and Miocene Surma Group of the Mizoram Foreland Basin have been studied geochemically to constrain their provenances,tectonic setting,and other sedimentary processes(weathering and mineral sorting etc.).Based on their mineralogical compositions,these sandstones are classified as quartzarenite and sublithic-arenite.The sandstones of Barail and Surma Groups have similar contents of most of the major elements except for SiO2 and A12O3.The Barail sandstones are relatively more siliceous and less aluminous compared to the Surma sandstones.Barail and Surma sandstones were plotted in a singular array on different geochemical discrimination diagrams.The CIA and CIW values of the sandstones of Barail(69 and 77 respectively)and Surma Groups(68 and 77 respectively)suggest that the sandstones were derived from moderately weathered source rocks.In the A-CN-K diagram,the studied samples plotted along a roughly singular trend that originates from granodiorite as well as Trans-Himalayan granitoids and also confined within the field of Siwalik sediments.Chondrite・normalized REE patterns for the Surma and Barail sandstones are identical and are similar to upper continental crust,with moderate to high LREE enrichment and prominent negative Eu anomalies(*Eu/Eu for both Barail and Surma sandstone=0.69),indicating their derivation from a felsic magmatic source.The values of,*Eu/Eu(LaN/LuN),La/Sc,La/Co,Th/Sc,Th/Co,and Cr/Th ratios of Surma and Barail sandstones are also not significantly different,and the values are similar to finefractions derived from the weathering of felsic rocks.In the K2O/Na2O versus SiO2,Th-Sc-Zr/10 and Ti/Zr versus La/Sc tectonic discrimination diagrams the studied samples of Barail and Surma sandstones plot within the fields of greywacke from continental island arcs and active continental margin.The geochemical characteristics of the studied sedimentary rocks and their similarity with the Siwalik foreland sediments thus suggest were sourced from different felsic magmatic lithounits of Himalaya and were deposited in an active continental margin.