Granulite

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

Single Zircon Evaporation and SHRIMP Ages for Granulite‐Facies Rocks in the Mozambique Belt of Tanzania

Abstract: We report zircon Pb‐Pb evaporation and U‐Pb SHRIMP ages on high‐grade gneisses from the Mozambique belt of eastern Tanzania in an attempt to understand the Neoproterozoic evolution of this belt. Several orthogneisses of granitoid composition have igneous emplacement ages between 2740 and 2608 Ma and document the presence of Late Archean crust of unknown dimension that is either part of the Tanzania craton farther west or constitutes a separate ancient crustal domain, perhaps related to Archean rocks in Madagascar. Paleo‐ and Mesoproterozoic ages, ranging from 1880 to 1095 Ma, are documented for granitoid gneisses in the eastern Usambara Mountains of northeast Tanzania, and the significance of these rocks in the geodynamic evolution of the Mozambique belt is equally uncertain. Ages between 843 and 665 Ma document a period of magmatic and depositional activity in the Neoproterozoic that is comparable to other parts of the Mozambique belt, notably Madagascar. Metamorphic zircons extracted from high ...

A reappraisal of the Lewisian Gneiss Complex: geochronological evidence for its tectonic assembly from disparate terranes in the Proterozoic

Abstract: New U-Pb single-zircon geochronology undertaken on tonalitic gneisses, granite sheets, migmatites and metasediments from the Lewisian Gneiss Complex on the mainland and the northern part of the Outer Hebrides, NW Scotland, have been used to test the correlation of so-called Laxfordian events across the complex from the Outer Hebrides to the mainland, and the current model for the evolution of the complex as a whole. The study has revealed that the granite sheets originated in two quite different melting events. Those on the mainland at Loch Laxford are ca. 1,855 Ma old whereas those on Harris and Lewis, with which they are presently correlated, are ca. 1,675 Ma old. Grey gneisses associated with granites on the south side of Loch Laxford are confirmed to belong to the 'northern region'. A migmatitic grey gneiss on Harris has given a protolith age of ca. 3,125 Ma, the currently oldest recognised in the complex. Detrital zircons in the Leverburgh and Langavat belts range in age from 2,780 to 1,880 Ma and unequivocally demonstrate deposition in the Palaeoproterozoic. The granulite facies metamorphism in this block is dated from zircon overgrowths at ca. 1,880 Ma. The Laxford Shear Zone which separates the northern and central regions is interpreted to have evolved post-1,860 Ma, during amphibolite facies metamorphism accompanying deformation which took place at ca. 1,740 Ma in both regions. On Harris, the Langavat-Finsbay shear zone developed after 1,675 Ma when a ca. 1,880-Ma granulite facies Proterozoic arc was juxtaposed against amphibolite facies Archaean rocks to the north. Therefore, the shear zones which bound tectonic blocks in the Lewisian Complex evolved at different times and can be interpreted as terrane boundaries. The new data confirm that the Lewisian Complex was not constructed from one contiguous piece of Archaean crust reworked in the Proterozoic but was progressively assembled from several discrete terranes during the Proterozoic. Accordingly, the former regional divisions of the Lewisian Complex are here renamed as follows. On the mainland, the northern region is called the Rhiconich terrane, and the central region the Assynt terrane. On the Outer Hebrides, the Archaean gneisses of Lewis and the northern part of Harris comprise the Tarbert terrane, whereas the newly accreted Proterozoic blocks are called the Roineabhal terrane in Harris and the Niss terrane in the north on Lewis. Wider correlations show that the geology of the Outer Hebrides has more in common with East Greenland than mainland Scotland on the eastern side of the Minch Fault.

Reavaliação da evolução geológica em terrenos pré-cambrianos brasileiros com base em novos dados U-Pb SHRIMP, parte II: Orógeno Araçuaí, Cinturão Mineiro e Cráton São Francisco Meridional

Abstract: This paper discusses new zircon SHRIMP (Sensitive High Resolution Ion Microprobe) U-Pb geochronological data for 19 key-exposures of several geological units exposed at the eastern border of the Southern Sao Francisco Craton and at the adjacent Proterozoic Mineiro and Aracuai orogens. Samples were collected along several E-W transects, aiming at tracing the precise limit of the Sao Francisco Craton Archean basement, as well as assessing the extension of the successive proterozoic orogenic collages. Due to the complex geologic history and/or high grade metamorphism which most of the rock units investigated have undergone, zircon morphology and the U-Pb analytical data exhibit very complex patterns. These are characterized by a combination of inheritance, partial resetting and new zircon growth during high-grade metamorphism. As a consequence, very careful and detailed analyses of cathodoluminescence imagery were required to allow distinction between inheritance, newly melt-precipitated zircon and partially reset zircons, as well as between the ages of magmatic and metamorphic events. In the southeastern border of the craton 5 units yielded Archean crystallization ages ranging from ca . 3000-2700 Ma, with poorly constrained metamorphic ages ranging from ca . 2850 to 550 Ma. The TTG gneissic complex exposed to the east and south of the Quadrilatero Ferrifero, formerly ascribed to the Archean basement, have crystallization ages from ca . 2210 Ma to 2050 Ma, and can now be interpreted as representing pre- to syn-collisional magmatic phases of the Mineiro Belt. Metamorphic ages of ca . 2100 Ma and 560 Ma are also well constrained in zircon populations from these gneisses. The crystallization age of ca 1740 Ma observed for an alkaline granite of the Borrachudos Suite (intrusive into the Archean basement east of the Southern Espinhaco Range) confirmed previous conventional U-Pb data for this Paleoproterozoic rift-related magmatism. One of the major basement inliers within the Aracuai Orogen, the Caparao charnockitic granulite, revealed magmatic crystallization age of ca . 2195 Ma. Zircons from an amphibolite of the Pocrane Complex, also a basement inlier within the Aracuai Orogen, yielded an age of ca . 1500 Ma. Both inliers furnished well-constrained neoproterozoic metamorphic overprint at ca . 590 Ma. The subalkaline gneissic granite from the Salto da Divisa Suite yielded a magmatic crystallization age of ca . 870 Ma, close to U-Pb ages of the continental rift magmatism found in the African counterpart of the Aracuai Orogen. Four gneissic granites from the Aracuai Orogen (three of them previously interpreted as part of the Archean/Paleoproterozoic basement) were dated in the range of ca . 560-570 Ma, and were correlated to the g2 2 syn-collisional magmatism. Borders of some zircon grains from the older ( ca. 570 Ma) dated unit (Nanuque Granite) yielded ages around 500 Ma, similar to U-Pb ages obtained from titanite, monazite and zircon in other localities from the Aracuai Orogen. This youngest ages could be related to heating effects caused by melting of country rocks and ascent of magmas formed during the collapse of the orogen.