Insights into the Neoarchean Magmatic Evolution of the Western Nilgiri Block, Southern India.

Abstract

The Southern Granulite Terrane (SGT) is an integral part of the Neoproterozoic Gondwana supercontinent. The Nilgiri block situated in the northern part of the SGT is characterized by Neoarchean magmatism. The major rock types found in the western part of the Nilgiri block include charnockite, hornblende biotite gneiss (HBG), pyroxenite, gabbro, amphibolite, mafic granulites, and granite. This study focuses on the geochemical signatures of major lithologies in the western Nilgiri block to understand the geological evolution within the broader Southern Granulite Terrane. Oxide data show charnockite samples with SiO2 from 51 to 67 wt% and HBG with SiO2 from 63 to 71 wt%. Normative Ab-An-Or classification diagrams indicate tonalitic and granodioritic signatures for charnockite and HBG, respectively. Both exhibit high Al2O3 (>15 wt%) and low K2O (<3 wt%). Charnockites and HBG display a magnesian and calcic-to-calc-alkalic composition, while mafic granulite shows a low-K tholeiite and calc-alkali composition. Zr vs. Ti diagrams reveal a volcanic arc signature for charnockite, HBG, and mafic granulites. In the study area, Peralimala Granite (PG), Ambalavayal Granite (AG), and Kalpetta Granite (KG) are present as plutons. KG and PG are magnesian, while AG is ferroan. KG and PG have an alkali-calcic composition, which contrasts with the calc-alkalic nature of AG. Comparative analysis with charnockites from the eastern Nilgiri block shows variations in SiO2 and Al2O3 content, with the western region exhibiting lower SiO2 and higher Al2O3. HBG in the western part shows a slightly elevated Na2O+K2O value. The charnockite consistently shows depletion in Y, Cs, Ta, Th, and U, coupled with Zr, Ba, Pb, and La enrichment. Similarly, HBG displays depletion in Y, Ta, Th, and U and enrichment in Rb, Sr, Zr, and Ba. Both rock types consistently exhibit enrichment in LREE and depletion in HREE. Mafic granulites show depletion in Cs, Th, Ta, and U, along with Nb, Ba, and Pb enrichment, displaying a flat REE pattern without significant fractionation. KG and PG exhibit enriched LREE and depleted HREE, whereas AG displays a strong negative Eu anomaly and a flat REE pattern. KG and PG are enriched in Ba, K, and La, with depletion in Ta, Nb, Ti, and Y. Conversely, AG is enriched in Ba, K, and Sr, with minimal Rb content and depletion in Th, U, Ta, Nb, and Ti, suggesting AG partial melts formed from the deep crustal level or upper mantle region. Charnockite and HBG show an affinity towards Archean TTG, suggesting their formation through the partial melting of Archean-hydrated, low K-metabasic source rocks, resulting in garnet amphibolite restite. U-Pb zircon data reveals an age of 2555±15 Ma for charnockite and 2512±12 Ma for HBG, indicating Neoarchean magmatism. Discordant ages from HBG align on a Pb-loss trend toward the lower intercept age of ~1100 Ma, suggesting a Late-Mesoproterozoic thermal event. U-Pb zircon data of Kalpetta Granite gives an age of 544±5 Ma, implying a Pan-African thermal event. These findings highlight distinct magmatic and thermal events in the western Nilgiri block, including Neoarchean activity and notable Late-Mesoproterozoic and Neoproterozoic thermal events.