Abstract
AbstractUltrahigh-pressure eclogites of the Tso Morari area, NW Himalaya (Ladakh, India), have been intensively investigated petrographically and petrologically with surprisingly different results. Metamorphic subduction paths based on mineral isopleths in pressure–temperature pseudosections in some studies claim concave (to the temperature axis) pressure–temperature paths predicting significant Ca–Mg–Fe garnet growth in the lawsonite and glaucophane fields: a prediction at odds with abundant epidote/clinozoisite and sodic-calcic amphibole inclusions in garnet interiors more probable along a convex path. One study deduced strong heating still at high pressures and proposed a felsic diapir rising through the mantle wedge: an explanation strongly at odds with well-documented glaucophane cores to barroisite replacing matrix omphacite requiring a cold exhumation most likely back up the subduction channel. In addition, matrix magnesite rimmed by dolomite suggests pressures well into the coesite (if not diamond) stability field: something neglected in most studies. Despite the application of modern analytical and thermodynamic modelling tools, the peak conditions attained by Tso Morari ultrahigh-pressure rocks are often poorly deduced and at odds with simple observations. Is this problem perhaps hindering the reliable identification of new ultrahigh-pressure terranes?
Highlights
The most spectacular testing ground for geological models of subduction–collision, plateau growth, erosion- or tectonically-driven exhumation, or even relief–climate interaction is the Himalayan–Tibetan region
Lawsonite-bearing blueschists (Groppo et al 2016) and carpholite-bearing metasediments (Oberhänsli 2013) in the former accretionary prism incorporated into the Indus-TsangpoSuture Zone (ITSZ) of NW India and Pakistan record lowtemperature subduction of Tethys oceanic crust below the Asian margin during Cretaceous times (Fig. 2, stage 1) i.e. at the same time as magmatism in the Transhimalayan Batholith (Honegger et al 1982, 1989)
With further convergence the weak, anatectic crust is liable to flow – a process extensively modelled for Himalayan-type orogens (e.g. Beaumont et al 2001, 2004, 2006; Jamieson et al 2004) – and could extrude to shallower crustal levels – a process invoked to explain the anatectic, sillimanite-grade Higher (or Greater) Himalaya Crystalline (HHC) below a detaching roof (Searle & Rex 1989; Searle et al 2006: Fig. 2 stage 4) and the characteristic inverted metamorphism induced in the footwall of the hot extruded bodies (LeFort 1975)
Summary
The metamorphic evolution of the Himalaya can be closely tied to the evolving stages of a classic subduction–collision orogeny that involves a transition from oceanic subduction to continental collision via a short-lived deep continental subduction (O’Brien 2001; Warren et al 2008; Kohn 2014; Searle 2015). Geochronological data from the eclogites point to deep subduction and substantial, extremely rapid exhumation all in Eocene (40–50 Ma) times (Tonarini et al 1993; de Sigoyer et al 2000; O’Brien & Sachan 2000; Kaneko et al 2003; Schlup et al 2003; Treloar et al 2003; Parrish et al 2006; Wilke et al 2010b, 2012; Donaldson et al 2013; Rehman et al 2013, 2016) These rocks are located today directly adjacent to the ITSZ (Fig. 1) supporting exhumation along the subduction channel. Even today the base of the thickened crust is at eclogite facies conditions so it is not surprising that several locations of anatectic HHC gneisses, in some cases c. 200 km south of the ITSZ, contain eclogites overprinted by granulite-facies assemblages at Oligocene to Miocene times (Groppo et al 2007; Cottle et al 2009; Grujic et al 2011; Warren et al 2011a, b; Lombardo et al 2016; Wang et al 2015, 2017; Corrie et al 2016)
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