The blueschists along the Indus Suture Zone in Ladakh, NW Himalaya

Abstract

Blueschists occur along the Indus Suture Zone in Ladakh as tectonic thrust slices, as isolated blocks within mtlange units and as pebbles within continental detrital series. In the Shergol-Baltikar section high-pressure rocks within the Mtlange unit lie between the Dras-Naktul-Nindam nappes in the north and the Lamayuru units in the south. The blueschists are imbricated with melange formation of probably upper Cretaceous age. They are overlain discordantly by the Shergol conglomerate of post Eocene (Oligo-Miocene ?) age. Blueschist lithologies are dominated by volcanoclastic rock sequences of basic material with subordinate interbedding of cherts and minor carbonates. Mineral k m b l a g e s in metabasic rocks are characterized by lawsonite-glaucophane/cr~site-Na-pyroxene~hlorite-phen~te-tit~te f albite f stilpnomelane. In the quartz bearing assemblages garnet is present but omphacite absent. P-T estimates indicate temperatures of 350 to 420°C and pressures around 9-11 kbar. Geochemical investigations show the primary alkaline character of the blueschist, which suggests an oceanic island or a transitional MORB type primary geotectonic setting. K/Ar isotopic investigations yield middle Cretaceous ages for both whole rocks and minerals. Subduction related HP-metamorphism affecting the Mesozoic Tethyan oceanic crust developed contemporaneously with magmatism in the Dras volcanic arc and the Ladakh batholith. Subsequent collision of India with Asia obducted relics of subduction zone material which later became involved in nappe emplacement during the Himalayan mountain building.

A tectonic model reconciling evidence for the collisions between India, Eurasia and intra-oceanic arcs of the central-eastern Tethys

“…3), exhibit geochemical signatures that suggest they are co-genetic and could be relicts of Ladakh's (intra-oceanic) Dras Arc (Maheo et al, 2006). The Sapi-Shergol mélange contains mid Cretaceous blueschist-bearing blocks of alkaline affinity (Honegger et al, 1989), some with calc-alkaline protoliths suggesting melt derived from a supra subduction zone setting (Maheo et al, 2006), and Albian-aged radiolarians (Kojima et al, 2001), suggest an oceanic (backarc) basin existed during the mid Cretaceous.…”

Petrology of Indus River sands: a key to interpret erosion history of the Western Himalayan Syntaxis

“…The forearc-basin succession includes Lower Cretaceous carbonates, volcaniclastic turbidites, syn-collisional Nummulite-bearing clastics, and eventually post-collisional arkosic alluvial fans [33]. Basaltic to dacitic lavas, volcaniclastic turbidites, and ophiolitic mélanges with blueschists are exposed along the Indus Suture [34].…”

Geochemistry and geodynamic implications of magmatic rocks from the Trans-Himalayan arc.

“…These ophiolitic rocks consist of chaotic blocks of schistose basic volcanics, agglomerates, amygdaloidal basalt, chlorite schist, deformed conglomerate schist, glaucophane schist, slates and large lenses of pe lagic limestones dispersed in a turbiditic matrix (Thakur and Misra, 1984;Sinha and Upadhyay, 1990). The basaltic rocks show predominantly OIB-E-MORB and minor N-MORB trace-element characteristics (Honegger et al, 1989, Ahmad et al, 1996 Harris et al ., 1988). Dashed rectangles show the outlines of study areas in eastern Ladakh ( Fig .…”