Geochemical characteristics of mafic and ultramafic rocks from the Naga Hills Ophiolite, India:Implications for petrogenesis

The Naga Hills Ophiolite(NHO) represents one of the fragments of Tethyan oceanic crust in the Himalayan Orogenic system which is exposed in the Phek and Kiphire districts of Nagaland, India. The NHO is composed of partially serpentinized dunite, peridotite, gabbro, basalt, minor plagiogranite,diorite dyke and marine sediments. The basalts are mainly composed of fine grained plagioclase feldspar, clinopyroxene and orthopyroxene and show quenching and variolitic textures. The gabbros are characterized by medium to coarse grained plagioclase, orthopyroxene and clinopyroxene with ophitic to sub-ophitic textures. The ultramafic cumulates are represented by olivine, Cpx and Opx.Geochemically, the basalts and gabbros are sub-alkaline to alkaline and show tholeiitic features.The basalts are characterized by 44.1-45.6 wt.% of SiO_2 with 28-38 of Mg#, and the gabbros by38.7-43.7 wt.% of SiO_2, and 26-79 of Mg#. The ultramafic rocks are characterized by 37.4-52.2 wt.% of SiO_2, and 80-88 of Mg#. In multi-element diagrams(spidergrams) both basalts and gabbros show fractionated trends with strong negative anomalies of Zr. Nb. Sr and a gentle negative anomaly of P.However, the rare earth element(REE) plots of the basalts and gabbros show two distinct patterns. The first pattern, represented by light REE(LREE) depletion, suggests N-MORB features and can be interpreted as a signature of Paleo-Tethyan oceanic crust. The second pattern, represented by LREE enrichment with negligible negative Eu anomaly, conforms to E-MORB, and may be related to an arc tectonic setting. In V vs. Ti/1000, Cr vs. Y and AFM diagrams, the basalts and gabbros plot within Island Arc Tholeiite(IAT) and MORB fields suggesting both ridge and arc related settings. The ultramafic rocks exhibit two distinct patterns both in spidergrams and in REE plots. In the spidergram, one group displays highly enriched pattern, whereas the other group shows near flat pattern compared to primordial mantle. In the REE plot, one group displays steeper slopes [(La/Yb)N = 4.340-4.341], whereas the other displays moderate to flat slopes [(La/Yb)N = 0.97-1.67] and negative Eu-anomalies. Our study suggests that the ultramafic rocks represent two possible mantle sources(fertile and refractory).

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.

Neoproterozoic highly fractionated Ⅰ-type granitoids of Shillong Plateau, Meghalaya, Northeast India: geochemical constraints on their petrogenesis

There are several Pan-African granitoid plutons widely distributed in Shillong Plateau,NE India.Nongpoh(506.7±7 Ma)and Mylliem(480-430 Ma)plutons were chosen for the petrological and geochemical study to constrain their petrogenesis.Nongpoh pluton consists of coarse-grained porphyritic quartz-monzogranite(NQM)and Mylliem pluton consists of medium to coarse-grained porphyritic granite(MG).The constituent minerals are K-feldspar,plagioclase,quartz,and biotite in both granitoids with accessory minerals of hornblende,zircon,sphene,and ilmenite.Both NQM and MG are metaluminous to weakly peraluminous(A/CNK=0.82-1.06),exhibit varied ranges of SiO2(NQM:58.4-64.9 wt%;MG:66.9-69.9 wt%),and display a clear compositional gap in the Harker variation diagram.NQM contains higher abundances of CaO,MgO and Fe2O3t and similar K2O and total alkali contents compared to MG.They have distinctive geochemical features typical of highly fractionated I-type granitoids such as higher abundances of K2O,Al2O3,MgO,CaO,Al2O3+CaO>15 wt%and A/CNK<1.1,low P2O5 content,enrichment in LILE,depletion in HFSE and HREE and highly fractionated REE patterns with moderate Eu anomalies,implying magma generation in a post-collisional extension setting and thinning induced asthenosphere upwelling,accompanied by the partial melting of the overlying enriched lithospheric mantle.The multi-element diagram of both NQM and MG shows pronounced negative anomalies at Ba,Nb,Sr,P,Zr,and Ti which implies a major role of crystal fractionation in their petrogenesis.High concentrations of Th,U,and Pb in the granitoid types point to additional involvement of crustal components in their generation.However,MGs with more pronounced positive spikes at U,Th,and Pb compared to NQM in multi-element diagram suggests the involvement of more felsic crustal material.The observed geochemical features of the granitoid types thus suggest that they are genetically unrelated to each other and their parental magmas were modified during magmatic differentiation processes.We suggest that the NQM and MG were formed as a result of fractional crystallization of compositionally diverse hybrid magmas produced due to mingling and mixing of an enriched lithospheric mantle-derived melts with lower crust-and middle crust-derived melts respectively during a period of extension late in the cycle of PanAfrican orogeny.