New eclogite localities and new 40 Ar/ 39 Ar ages within the Western Gneiss Region of Norway define three discrete ultrahigh-pressure (UHP) domains that are separated by distinctly lower pressure, eclogite facies rocks. The sizes of the UHP domains range from c. 2500 to 100 km 2 ; if the UHP culminations are part of a continuous sheet at depth, the Western Gneiss Region UHP terrane has minimum dimensions of c. 165 · 50 · 5 km. 40 Ar/ 39 Ar mica and K-feldspar ages show that this outcrop pattern is the result of gentle regional-scale folding younger than 380 Ma, and possibly 335 Ma.The UHP and intervening high-pressure (HP) domains are composed of eclogite-bearing orthogneiss basement overlain by eclogite-bearing allochthons. The allochthons are dominated by garnet amphibolite and pelitic schist with minor quartzite, carbonate, calc-silicate, peridotite, and eclogite. Sm/Nd core and rim ages of 992 and 894 Ma from a 15-cm garnet indicate local preservation of Precambrian metamorphism within the allochthons. Metapelites within the allochthons indicate near-isothermal decompression following (U)HP metamorphism: they record upper amphibolite facies recrystallization at 12-17 kbar and c. 750°C during exhumation from mantle depths, followed by a low-pressure sillimanite + cordierite overprint at c. 5 kbar and c. 750°C. New 40 Ar/ 39 Ar hornblende ages of 402 Ma document that this decompression from eclogite-facies conditions at 410-405 Ma to mid-crustal depths occurred in a few million years. The short timescale and consistently high temperatures imply adiabatic exhumation of a UHP body with minimum dimensions of 20-30 km. 40 Ar/ 39 Ar muscovite ages of 397-380 Ma show that this extreme heat advection was followed by rapid cooling (c. 30°C Myr )1 ), perhaps because of continued tectonic unroofing.

No abstract

No abstract

The Triassic Songpan‐Ganzi complex comprises >200,000 km2 of 5–15 km thick turbiditic sediments. Although surrounded by several magmatic and orogenic belts, the Triassic high‐ and ultrahigh‐pressure Qinling‐Tongbai‐Hong'an‐Dabie (QTHD) orogen, located several hundred kilometers to the east, was proposed as its major source. Middle to Late Triassic samples from the northern and southern Songpan‐Ganzi complex, studied using detrital white mica 40Ar/39Ar ages, Si‐in‐white mica content, and detrital zircon U/Pb ages, suggest that the northern Songpan‐Ganzi deposystem obtained detritus from the north: the north China block, east Kunlun, northern Qaidam, Qilian, and western Qinling; the southern Songpan‐Ganzi deposystem was supplied from the northeasterly located Paleozoic QTHD area throughout the Ladinian and received detritus from the Triassic Hong'an‐Dabie orogen during the Carnian, indicative of exhumation of the orogen at that time. The QTHD orogen fed the Norian samples in the southeastern southern Songpan‐Ganzi deposystem, signifying long drainage channels along the western margin of the south China block. An additional supply from the Emeishan magmatic province and/or the Yidun arc is suggested by the paucity of white mica in the southern Songpan‐Ganzi deposystem. Mica ages of Rhaetian sediments from the northwestern Sichuan basin best correlate with those of the Triassic QTHD orogen. Our Si‐in‐white mica data demonstrate that the high‐ and ultrahigh‐pressure rocks of the Hong'an–Dabie Shan were not exposed in the Middle to Late Triassic.

No abstract