A geochronological investigation of two rocks with an eclogitic assemblage (omphacite-garnet-quartz-rutile) from the High Himalaya using the Sm/Nd, Rb/Sr, U/Pb and AdAr methods is presented here. The first three methods outline a cooling history from the time of peak metamorphism at 49k6 Ma recorded by S d N d in garnet-clinopyroxene to the closure of Rb/Sr in phengite at 4 3 f l Ma and U/Pb in rutile at 39-40 Ma. The Sm/Nd isotopic system was fully equilibrated during eclogitization and has not been disturbed since; its mineral ages may date the peak metamorphic conditions (650250°C at 13-18 kbar: Pognante and Spencer, 1991). The AriAr data reveal the presence of substantial amounts of excess 40Ar in hornblende, and yield a statistically acceptable but geologically meaningless phengite plateau age of 81.4k0.2 Ma, inconsistent with Sm/Nd, Rb/Sr and U/Pb. This questions the use of such a chronometer for the dating of high-pressure assemblages. The results imply a Late Palaeocene or Early Eocene subduction of the northern Indian plate margin in NW Himalaya. The fact that eclogites are restricted to NW Himalaya may be the result of a peculiar p-T-t path associated with a high convergence rate during the first indentation, in contrast to the later and slow subduction in Central and Eastern Himalaya.

The sou thern Sesia zone in the western Italian Alps consists of fou r tectonic su b-units separated by maj or thru sts. These su b-units inclu de : "gneiss minu ti" (GM) , blu eschist-facies "polymetamorphie paraschists" (PP) similar to the "11 zona diorito-k inzigitica" , "eclogitic micaschists" (E M) and blu eschist-facies "Rocca Canavese thru st sheets" (RCT). Du ring Alpine metamorphism these su b-units underw ent a polyphase early Alpine history at high-P, follow ed by a late-Alpine low-P event. Evidence for an early-Alpine prograde path has been only fou nd in the EM and GM. The early-Alpine metamorphic peak s occu rred at T = 500-550 °C and P = 13-20 kbar in the EM, T = 400-450°C and P � 11-12 kbar in the GM and at T = 300-400°C and P � 8-10 kbar in the PP and RCT. Decompression du ring the early-Alpine event occu rred at decreasing T in the EM (trom T = 500-550 °C to T < 450-500 0C) and at more or less constant T in the other su b-u nits. In the sou thern EM, the early-Alpine post-eclogitic recrystallization produ ced law sonite porphyroblasts which grew in eq uilibriu m with quartz ± jadeite. Formation of law sonite occu rred at low XC0 2 ; it was favou red by a temperatu re decrease after the HP climax and probably the syn-metamorphic infiltration of H20-rich flu ids along preferential channels. Law sonite also occu rs in the other Sesia su b-u nits, where it crystallized du ring the metamorphic climax. The post-eclogitic T decrease inferred here for the sou thern EM is less evident in more northerly parts of this su b-u nit ; it may be related to a process of condu ctive-cooling du ring early-Alpine ex hu mation and the stack ing at deep cru stal levels with the relatively cooler blu eschist-facies su b-u nits. After stack ing and recrystallization under fairly comparable blu eschist-facies conditions, the different Sesia su b-units underw ent a low-P event of probable late-Alpine age. Su ch a low-P event cau sed a metamorphic zonation, with low er T in the stru ctu rally high er and eastern RCT.

A polyphase evolution and a thermal and mineral zonation characterize metamorphism of the High Himalayan Crystallines (HHC) in the Everest transect. After an early Barrovian-type metamorphism (M1: T = 550–680° C; P = 8–10 kbar) caused by subduction and crustal thickening during India-Eurasia collision, exhumation of the HHC occurred at rates which allowed the isotherms to rise and the thickened crust to suffer partial melting. In the upper structural levels, the thermal conductivity contrast between crystalline and upper sedimentary rocks together with the infiltration of metamorphic fluids released during exhumation, favoured migmatization and formation of leucogranite magmas through partial melting of muscovite-bearing metasediments chiefly at medium (M2: T = 650–750° C; P = 4–7 kbar) but also at low-pressure (M3: T = 600–700° C; P = 2–4 kbar). The leucogranite melts segregated as dykes and kilometre-sized bodies into extensional shear zones situated at the uppermost levels of the HHC and at the base of the overlying Tibetan Series. During exhumation, a metamorphic zonation was imposed on the HHC in response to the complex interplay between rise of the isotherms in the thickened crust, emplacement of leucogranites in the uppermost levels of the HHC and conductive cooling near the ‘cold’ Lesser Himalaya and Tibetan Series coupled with thrust imbrication.

-In the High Himalayan belt of northwest India, crustal thickening linked to Palaeogene collision between India and Eurasia has led to the formation of two main crystalline tectonic units separated by the syn-metamorphic Miyar Thrust: the High Himalayan Crystallines sensu stricto (HHC) at the bottom, and the Kade Unit at the top. These units are structurally interposed between the underlying Lesser Himalaya and the very low-grade sediments of the Tibetan nappes. They consist of paragneisses, orthogneisses, minor metabasics and, chiefly in the HHC, leucogranites. The HHC registers: a polyphase metamorphism with two main stages designated as Ml and M2; a metamorphic zonation with high-temperature recrystallization and migmatization at middle" structural levels and medium-temperature assemblages at upper and lower levels. In contrast, the Kade Unit underwent a low-temperature metamorphism. Rb-Sr and U-Th-Pb isotope data point to derivation of the orthogneisses from early Palaeozoic granitoids, while the leucogranites formed by anatexis of the HHC rocks and were probably emplaced during Miocene time.Most of the complicated metamorphic setting is related to polyphase tectonic stacking of the HHC with the 'cooler' Kade Unit and Lesser Himalaya during the Himalayan history. However, a few inconsistencies exist for a purely Himalayan age of some Ml assemblages of the HHC. As regards the crustal-derived leucogranites, the formation of a first generation mixed with quartzo-feldspathic leucosomes was possibly linked to melt-lubricated shear zones which favoured rapid crustal displacements; at upper levels they intruded during stage M2 and the latest movements along the syn-metamorphic Miyar Thrust, but before juxtaposition of the Tibetan nappes along the latemetamorphic Zanskar Fault.

Eclogites of the Higher Himalaya were analysed for their bulk rock geochemistry and Sr-Nd isotopic composition. These results are compared to lower grade metamorphic equivalents of rocks of the same age as weil as to unmetamorphosed basalt. Major and trace element diagrams indicate tholeiitic to alkalic magma compositions wh ich evolved by fractional crystallisation. 87 Srf 6 Sr initial values for the eclogite are 0.7053-0.7073 with values increasing for metabasites of lower metamorphic grade. 1 4 7 Sm/ 44 Nd values range from lower than chondritic to slightly higher. 1 4 3 Nd/ 44 Nd measured values range from 0.51201 to 0.5 1283, with tNd(l) from +4.9 to-10. The lowest values occur in the least metamorphosed rocks. Based on their bulk geochemistry and isotope data, the eclogites could be the equivalents of the Permian Panjal Traps. The observed isotopic "trends" are here suggested not to be explained by variations of metamorphic grade, but indicate some protoIith contamination with the evolved Higher Himalayan continental crus!.