Abstract Ultrahigh-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?

Abstract The Lhasa terrane, forming one of the main tectonic components of the Himalayan–Tibetan orogen, has received a lot of attention as it records multiple episodes of plate spreading, subduction and collision within the realm of the Paleo-Tethys Ocean. A review of the mineralogical, petrological, geochemical and geochronological data of eclogites, associated blueschists and garnet-bearing mica schists from the Sumdo high- and ultrahigh-pressure metamorphic (HP/UHP) complex in the central/southern part of the Lhasa terrane, is present here so that the origin and tectono-metamorphic evolution of this important suture can be deduced. By re-evaluating the original published P – T conditions for the metamorphic rocks of the Sumdo Complex, we consider that the Sumdo Complex has experienced low temperature HP/UHP metamorphic conditions, characteristic of fast subduction (and exhumation) in a typical oceanic subduction zone setting. The original wide spread in the maximal peak P – T conditions could be reduced in size due to thus far unknown inconsistencies in the usage of applied geothermobarometric techniques. The remaining spread in the maximal P – T conditions ( c. 200°C/10 kbar) of the HP/UHP regions can be explained by a mechanism that the rocks from individual tectonic slices were subducted to different depths and followed by juxtaposition on their way back to the surface. A re-consideration of the isotopic ages of eclogites from the Sumdo Complex demonstrates that the opening of the Paleo-Tethys Ocean, located in between the two major components of the Lhasa terrane, was initiated prior to c. 280 Ma and the eclogite facies metamorphism is likely to be of late Permian ( c. 260 Ma) to early Triassic age (245–225 Ma), recording different ages of subduction from individual slices of the oceanic crust. The closure of the Paleo-Tethys Ocean resulted, no earlier than 210 Ma, in the final collision between the northern and southern Lhasa blocks. This final collision event may have been triggered by the initial subduction of the Bangong–Nujiang Tethys Ocean in the north.

Early Cretaceous lawsonite eclogites and related high-pressure rocks occur as tectonic inclusions within serpentinite mélange south of the Motagua fault zone, Guatemala. Petrologic and microtextural analyses of mafic high-pressure rocks reveal three metamorphic stages linked to several deformational textures. The prograde stage represents an incipient eclogitization and is preserved in prograde garnet, along with an older S 1 –S 2 foliation. The prograde assemblage is garnet ( X Mg = ∼0.22) + omphacite (∼52 mol% jadeite) or jadeite (∼83 mol % jadeite) + lawsonite + chlorite + rutile + quartz ± phengite (3.6 Si p.f.u.); some rocks also have ilmenite and rare ferro-glaucophane. Lawsonite in garnet of some lawsonite eclogites contains rare pumpellyite inclusions. The presence of synmetamorphic brittle deformation, inclusions of pumpellyite, Fe 2+ -Mg distribution coefficients between omphacite inclusions and adjacent garnet with Ln( K D ) = 2.7–4.5, and garnet-clinopyroxene-phengite thermobarometry suggest that eclogitization initiated at temperature ( T ) = ∼300 °C and pressure ( P ) > 1.1 GPa, and continued to T = ∼480 °C and P = ∼2.6 GPa. In contrast, the retrograde eclogite-facies assemblage is characterized by reversely zoned garnet rims and omphacite ± glaucophane + lawsonite + rutile + quartz ± phengite (3.5 Si p.f.u.) along the S 3 foliation. Garnet-phengite-clinopyroxene thermobarometry yields P = ∼1.8 GPa and T = ∼400 °C. The youngest, blueschist-facies assemblage (glauco-phane + lawsonite + chlorite + titanite + quartz ± phengite) locally replaces earlier mineral assemblages along S 4 crenulations. The inferred prograde P - T trajectory lies near a geotherm of ∼5 °C km −1 , comparable to the calculated thermal and petrologic structure of the NE Japan subduction zone. These petrologic characteristics indicate: (1) the basalt-eclogite transformation may occur at T = ∼300 °C in cold subduction zones, (2) glaucophane-bearing prograde assemblages are rare during incipient eclogitization in cold subduction zones, and (3) the chlorite-consuming reactions that form Fe-Mg-Mn garnet are more effective than the lawsonite-consuming reaction that forms a grossular component. At depths of ∼100 km in cold subduction zones, dehydration embrittlement may be caused by such chlorite-consuming reactions.