Granulite

About: Granulite is a research topic. Over the lifetime, 6763 publications have been published within this topic receiving 268925 citations.

Rapid synconvergent exhumation of Miocene-aged lower orogenic crust in the eastern Himalaya

Abstract: Rare granulitized eclogites exposed in the eastern Himalaya provide insight into conditions and processes deep within the orogen. Sensitive high-resolution ion microprobe (SHRIMP) U-Pb, Ti, and rare earth element (REE) data from zircons in mafic granulitized eclogites located in the upper structural levels of the Greater Himalayan Sequence in Bhutan show that zircon was crystallized under eclogite-facies metamorphic conditions between 15.3 ± 0.3 and 14.4 ± 0.3 Ma, within a couple million years of the later granulite-facies overprint. In conjunction with pressure estimates of the eclogite- and granulite-facies stages of metamorphism, the age data suggest that initial exhumation occurred at plate-tectonic rates (cm yr−1). These extremely rapid synconvergence exhumation rates during the later stages of the India-Asia collision require a revision of theories for the transportation and exhumation of crustal materials during continental collisions. In contrast to western Himalayan examples, the eastern Himalayan eclogites cannot be tectonically related to steep subduction of India beneath Asia. Instead, they more likely represent fragments from the base of the overthickened Tibetan crust. Based on the zircon age and trace-element data, we hypothesize that the protolith of the mafic granulites was middle Miocene mafic intrusions into the lower crust of southern Tibet, linked to Miocene volcanism in the Lhasa block. We suggest that a transient tectonic event—possibly the indenting of a strong Indian crustal ramp into crust under southern Tibet that had been weakened by partial melting—may have promoted exhumation of the eclogitized lower crust under Tibet. The mafic magmatism and volcanism themselves may have been related to the convective thinning of the lithospheric mantle triggered by a reduction in the India-Eurasia convergence rate during the middle Miocene, which in turn could have facilitated the rapid extrusion of the lower crust over the earlier-exhumed middle crust.

Subduction zone metamorphism during formation and emplacement of the Semail ophiolite in the Oman Mountains

Abstract: The metamorphic sole along the base of the Semail ophiolite in Oman records the earliest thrust slice subducted and accreted to the base of the ophiolite mantle sequence. In the Bani Hamid area (United Arab Emirates) a c. 870 m thick thrust slice of granulite facies rocks includes garnet + diopside amphibolites, enstatite + cordierite + sillimanite + spinel ± sapphirine quartzites, alkaline mafic granulites (meta-jacupirangites) quartzo-feldspathic gneisses and calc-silicates. The latter contain garnet + diopside + scapolite + plagioclase ± wollastonite. P–T conditions of granulite facies metamorphism are in the range 800–860 °C and 10.5 ± 1.1 kbar to 14.7 ± 2.8 kbar. Garnet + clinopyroxene + hornblende + plagioclase amphibolites from the metamorphic sole record peak P–T conditions of 840 ± 70 °C and 11.6 ± 1.6 kbar (THERMOCALC average P–T mode) and 840–870 °C and 13.9–11.8 kbar (conventional thermobarometry) with low degrees of partial melting producing very small melt segregations of tonalitic material. Pressure estimates are equivalent to depths of 57–46 km beneath oceanic crust, much deeper than can be accounted for by the thickness of the ophiolite. 40Ar/39Ar hornblende ages from the amphibolites range from 95–93 Ma, synchronous with formation of the plagiogranites in the ophiolite crustal sequence (95 Ma), eruption of the Lasail (V2) volcanic sequence and deposition of Cenomanian–Turonian radiolaria in metalliferous sediments between the Geotimes (V1) and Lasail (V2) lavas. Protoliths of the metamorphic sole were Triassic–Jurassic and early Cretaceous Haybi volcanic rocks, Exotic limestones and quartzites and were clearly not equivalent to the Semail ophiolite rocks, showing that initiation of subduction could not have occurred at the ridge axis. Heat for metamorphism was derived from the mantle sequence harzburgites and dunites which were at or around 1100–1500 °C. All data from the sub-ophiolite metamorphic sole in Oman and the United Arab Emirates indicate that the ophiolite was formed in a Supra-Subduction zone setting and that obduction occurred along a NE-dipping high-temperature subduction zone during Late Cretaceous times.

Eclogite-facies relics and inferred ultrahigh-pressure metamorphism in the North Dabie Complex, central-eastern China

Abstract: Mineral assemblages and microstructures of a newly identified (retrogressed) eclogite (sensu lato) in the North Dabie Complex (NDC), central-eastern China, indicate early very high-pressure metamorphic relics preserved in a dominant amphibolite-facies host, where no eclogite (sensu stricto) has been positively identified before. The investigated eclogitic rock shows distinct multistage recrystallization, with granulite- and amphibolite-facies assemblages overprinting eclogite-facies relics. The minimum temperature for the eclogite-facies metamorphism is estimated to be ~800−820 °C. A spectacular microstructure of oriented quartz needles (~2–20 μm wide, ~5–200 μm long) in matrix Ca-Na clinopyroxene implies the prior existence of a non-stoichiometric “supersilicic” omphacite stabilized at ultrahigh-pressure (UHP, ≥25 kbar) conditions, although no coesite or coesite pseudomorphs have been found in the samples. The absence of coesite may be due to the lack of free silica at UHP conditions or the consumption of silica during retrograde reactions. The inferred UHP conditions metamorphism is further supported by Sm-Nd ages that are equivalent to Triassic metamorphic ages from UHP eclogites in the southeastern Dabie Mountains. This finding expands the UHP terrane northward about 50 km; spatial distribution of subduction/collision-related UHP rocks includes parts of the NDC.