Petrogenesis of magmatic and hydrothermally derived late stage minerals associated with granitic plutonic complexes: A case study from the Ladakh Batholith, NW Himalayas, India

Collision of the Indian and Asian continental plates and subsequent northwards subduction of the Indian plate beneath the active Andean-type southern margin resulted in the intrusion of a 2500km long Trans-Himalayan calc-alkaline batholith known as the Ladakh Plutonic Complex, or Ladakh Batholith. The Ladakh batholiths lies sandwiched between the Indus Suture Zone in the south and is unconformably overlain by the post-collisional Indus Molasse Group. In the Chumathang area, SE of Leh, the batholith (⁓ 400 mts high) is represented by two major granitoid phases exposed on the eastern side of Indus river. The granodiorite (57.7 ± 0.2 Ma) is dark-colored, massive, medium to coarse grained composed of plagioclase, quartz, hornblende, biotite with minor titanite, apatite, zircon, epidote, magnetite, ilmenite. The younger leucogranite (47.1 ± 0.1 Ma) is a relatively fine-grained rock containing quartz, plagioclase, and biotite with minor muscovite, zircon, tourmaline, and allanite. Several pegmatite veins of variable thickness are seen cross-cutting both phases of granite and at times intrude into the older metasediments.

Compositionally, these veins are dominated by quartz, plagioclase, orthoclase, microcline and minor biotite, muscovite, and zircon. Minerals like tourmaline, chlorite, fluorite, aquamarine, baryte etc. are commonly observed along vein margins. The Chumathang granitoids exhibit pervasive hydrothermal alteration, with pronounced chloritization observed in proximity to fluorite mineralization zones. Chlorite is seen closely associated with biotite (K = > 1wt.%) with enriched Fe, Mn and Mg concentrations indicating elevated oxygen fugacity conditions. Flourite typically occurs in variable colors like green, purple, white and brown indicating different stages of fluid evolution. Ca contents vary between 55.27 wt.% to 60.85 wt. % with F varying between 41.33 to 46.39 wt.% higher than previous reports. Allanite, a REE-rich mineral belonging to the epidote group, has been identified in the present study. Allanites exhibit compositional zoning with rims enriched in Ca, Mg and Al as compared to core. Aquamarine, the blue to greenish-blue gem variety of beryl has also been identified in the pegmatites from the study area. Presence of predominant minerals like biotite, amphibole and epidote clearly suggest that both phases of granites and pegmatites were formed from a high temperature magma source. Secondary minerals like chlorite, fluorite, allanite and aquamarine found associated with the host rocks indicate derivation from a complex interplay of both late stage pegmatitic as well as hydrothermal melts. The observed accessory and secondary minerals from the study area provide key insights into magmatic differentiation, post-magmatic fluid activity, thermal history, and mineralization potential and economic potential of such plutonic complexes.

Keywords: Ladakh Batholith, Chumathang granitoids, pegmatites, magma crystallization, hydrothermal alteration