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.

Nature and Composition of the Lower Continental Crust in Central Spain and the Granulite–Granite Linkage: Inferences from Granulitic Xenoliths

Abstract: Xenolith-bearing alkaline ultrabasic dykes were intruded into the of the Hercynian basement of the Spanish Central System in early Mesozoic times. The suite of lower-crustal xenoliths in the dykes divided into three groups: felsic peraluminous granulites, metapelitic granulites and charnockitic granulites. The felsic granulites form ~95% of the total volume of the xenoliths, whereas the charnockitic and metapelitic granulites are much less abundant (~0·01 5%, respectively). Thermobarometric calculations based on mineral paragenesis indicate equilibration conditions around 850–950°C, 7–11 kbar; thus the xenoliths represent lower continental crustal material. Superimposed on this high-T high-P assemblage is a high-T low-P paragenesis represented mainly by kelyphitic coronas, reflecting re-equilibration during transport in the clearly restitic mineral assemblages, with up to 50% garnet and 37% sillimanite. Major and trace element modelling supports the idea that the late-Hercynian peraluminous granites of central Spain represent liquids in equilibrium with restitic material of similar composition to the studied lower-crustal xenoliths. 87Sr/86Sr and eNd of the felsic xenoliths, calculated at an average Hercynian age of 300 Ma, are in the range 0·706–0·712, and –1·4 to –8·2, respectively. These values match the isotopic composition of the outcropping late Hercynian granites. The Sr isotopic composition of the xenoliths is lower than that of the outcropping mid-crustal lithologies (orthogneisses, pelites). A major contribution from the lower crust to the source of Hercynian granites greatly reduces the necessity of invoking a large mantle contribution in models of granite petrogenesis. The felsic nature of the lower continental crust in central Spain contrasts with the more mafic lower-crustal composition estimated in other European Hercynian areas, suggesting a non underplated crust in this region of the Hercynian orogenic belt.

Zircon U-Pb geochronology in the Bergen arc eclogites and their Proterozoic protoliths, and implications for the pre-Scandian evolution of the Caledonides in western Norway

Abstract: The Proterozoic crystalline Lindas nappe of the Bergen arc, Caledonides of western Norway, displays local Caledonian eclogite facies overprint along fractures and shear zones. The overprint results from fluid infiltration in a dry granulite facies protolith under eclogite facies conditions. Formation of a low Th/U zircon overgrowth in eclogites is interpreted to be coeval with crystallization of the eclogite facies assemblage. A lower intercept age of 456 ± 7 Ma obtained from combined zircon U-Pb data for an eclogite and an associated granulite are believed to reflect the eclogite facies fluid infiltration event. An alternative minimum age of 419 ± 4 Ma can be extracted from the data. A charnockite and a mafic granulite yield Proterozoic intrusion ages of 1237 +43/–35 and 951 ± 2 Ma, and a garnet- bearing granulite provides an age of 929 ± 1 Ma for the late Sveconorwegian granulite facies metamorphism. The Proterozoic ages are compatible with a correlation between the Lindas and Jotun nappes and an origin of the Lindas nappe in the Sveconorwegian province of Baltica. We propose a possible model featuring subduction of the Lindas nappe at 456 ± 7 Ma attached to the Baltica plate during a Late Ordovician (Caradoc) collision between Baltica and an outboard island arc in the Iapetus Ocean. Accretion of some of the ophiolite-bearing terranes preserved in the Caledonides of western Norway may have taken place during this event.