Geodynamic evolution of a Pan-African granitoid of extended Dizo Valley in Karbi Hills, NE India: Evidence from Geochemistry and Isotope Geology

Abstract

The Dizo Valley is an important geological province in NW Karbi Hills of Shillong Plateau (NE India). The Karbi Hills contain profusely intruded bodies of A-type granitoids that have been attributed to a post-collision setting. The Dizo Valley magmatic suite is a bimodal association of voluminous granitoid plutons with dolerite and amphibolites (metabasalts). Our present data demonstrates that there were two episodes of A-type granitoid magmatism in this part of the craton. Studied Kathalguri granitoids (KG) of Dizo Valley represents the late phase, about 515.1±3.3–515.5+2.7Ma age, dated by zircon LA-ICPMS U–Pb method. The age of emplacement represent the late Pan-African (Mid-Cambrian) event of granitoid magmatism concomitant to the Gondwana continental restructuring event with the integration of East and West Gondwana held between 570 and 510Ma. A previous granitoid emplacement episode of about 690Ma has been referred in literature. The late phase of granitoids formation tends to be massive, occasionally show flow banding, degassing pots and amphibolite breccias with chilled margin. Its mineralogy shows abundance of accessory phases like zircon, allanite, apatite, bastnaesite, monazite, sphene and xenotime.Geochemical parameters offer metaluminous to peraluminous, non-porphyritic, shoshonitic to high potassium, calc-alkaline composition. Other chemical criteria bear the character of A2-type granitoids with high SiO2 (64.00–70.75wt.%) and alkali (3.26–6.30wt.%) but poor Ca and Mg content. The binary plots of Y+Nb–Rb and Y–Nb confirm their within-plate granite (WPG) character. The pluton is enriched in total REE (av. 614.64ppm), Y and heavy REE (27.62–65.00ppm; av. 41.87ppm) compared to low calcium granite; Eu anomaly is moderately negative (∂Eu=0.43). Enrichment of incompatible elements like large ion lithophile elements (LILE) of Rb, Ba and Sr and REE are consistent with the A-type granites. The behavior of Ba, Rb and Sr suggest a progressive enrichment of Ba for the studied granitoids and a gradual depletion of Rb, Sr and Eu values. The Lu–Hf profile produced an initial 176Hf/177Hf (i) ratio of all zircons from 0.2819–0.2821 and have ɛHf(t) values −12.20 to −18.90; suggesting melting of 2.25–2.67Ga old metasomatized crust (TDM2) of magmatic origin which has been predicted to produce KG granitoids that had gained enrichment in LILE during earlier subduction events. The zircons bear evidence of oscillatory zoning visible in Cathodo Luminescence (CL) images which clearly attributes toward a magmatic origin of KG granitoids.