ALBERT

Description: The Shyok Suture in western Himalaya preserves a record of the opening and closure of the Mesotethys Ocean between the Shyok ophiolite and Karakoram terrane prior to the India–Eurasia collision. The formation age of the Shyok ophiolite was unknown, which impeded correlation with similar rocks along the Shyok Suture in Pakistan and corresponding sutures in Tibet. We report the first zircon U–Pb ages of a newly documented suite, here named the Changmar Complex. The Changmar Complex gabbronorite and plagiogranite yielded SHRIMP U–Pb zircon Late Jurassic ages of 159.4 ± 0.9 Ma and 151.9 ± 1.5 Ma. Their highly positive initial εHf values (+14.9 to +16.9) indicate a juvenile mantle origin, without continental crust influence on the magma source. The Shyok ophiolite represents either: (1) a separate island arc that preceded formation of the Cretaceous–Eocene Ladakh Arc; or (2) the oldest magmatism and early stage of the Ladakh Arc. Intrusive and extrusive mafic rocks from the Shyok Suture analysed in this study have typical supra-subduction zone enrichment characteristics in their geochemistry and are classified as part of the volcanic-arc ophiolite. The U–Pb age and Hf isotopic signatures for the Shyok ophiolite are similar to the Late Jurassic Matum Das tonalite within the Kohistan Arc; we therefore suggest that they are part of the same intra-oceanic island-arc system that formed in the Mesotethys Ocean prior to Late Jurassic time.

Description: Integrated field structural studies and SHRIMP U–Pb zircon and monazite dating have been undertaken in Renland, west of Scoresby Sund district in the southern part of the East Greenland Caledonides. Southwest Renland is dominated by metasedimentary rocks correlated with the Krummedal supracrustal succession of East Greenland and which on Renland were intruded by augen granites. Krummedal psammite from Renland yielded a spectrum of Mesoproterozoic to Palaeoproterozoic detrital U–Pb zircon dates, the youngest of which indicate deposition of the psammite occurred c. 1000 Ma ago, thus post-dating Grenvillian continent–continent collision in North American Laurentia. These Krummedal metasediments were deformed into regional nappe-scale folds prior to metamorphism, crustal anatexis and genesis of augen granites; an example of the latter has been dated at 915±18 Ma (U–Pb zircon). This demonstrates early Neoproterozoic high-temperature tectono-metamorphism affecting rocks within the southern East Greenland Caledonides, broadly contemporaneous with similar rocks farther north in East Greenland and with Sveconorwegian events on Baltica. Still in southwestern Renland, a later thermal event led to development of uppermost amphibolite to granulite facies metamorphic assemblages, veins and patches of in situ garnetiferous melt-bearing neosome in both metasediments (432±6 Ma, U–Pb zircon) and in the augen granites, and contemporaneous biotite-bearing granite sheets in top-down-to-the-E extensional shear zones (434±5 Ma, U–Pb zircon). Monazites from southwestern Renland record Caledonian thermal events as late as 410−400 Ma. In contrast, southeastern Renland is dominated by quartzofeldspathic migmatites with a strongly Caledonian signature but enclosing relicts of augen granite and retrogressed granulite facies psammitic and pelitic metasediment. There is also a sequence of Caledonian granitoid intrusions. Two samples from a hypersthene monzonite intrusion yielded U–Pb zircon dates of 424±8 Ma and 424±6 Ma. This pluton shows the marginal effects of the regional migmatization and was intruded early in the sequence of granitoid emplacement. An amphibolite facies migmatite, textural evidence from which suggests that it had never hosted granulite facies assemblages, records zircon growth at 423±6 Ma, and closure of monazite by 402±10 Ma. High grade metamorphism, and the protracted sequence of granitoid emplacement and still younger thermal events which together span the period between 430 and 400 Ma may, in part, reflect complicated lithospheric dynamics associated with subduction outboard of the Laurentian margin. Crustal segments carrying the relict evidence of Neoproterozoic and early Caledonian events must then quickly have been thrust northwestwards in foreland-propagating, northwesterly directed thrusts over Cambro-Ordovician platformal sequences on the Laurentian margin. This records the final closure of Iapetus, encroachment of Baltica and continent–continent collision from late Llandovery times (425–430 Ma).