scholarly journals Rare allanite in granulitised eclogites constrains timing of eclogite to granulite facies transition in the Bhutan Himalaya

2021 ◽  
Author(s):  
Eleni Wood ◽  
Clare Warren ◽  
Nick Roberts ◽  
Tom Argles ◽  
Barbara Kunz ◽  
...  
Keyword(s):  
Granulite Facies ◽  
Continental Collision ◽  
Eastern Himalaya ◽  
Pb Isotope ◽  
Crustal Rocks ◽  
Metamorphic Terrane ◽  
Eclogite Facies ◽  
Lower Crustal ◽  
The Himalaya

During continental collision, crustal rocks are buried, deformed, transformed and exhumed. The rates, timescales and tectonic implications of these processes are determined by linking geochemical, geochronological and microstructural data from metamorphic rock-forming and accessory minerals. Exposures of lower orogenic crust provide important insights into orogenic evolution, but are rare in young continental collision belts such as the Himalaya. In NW Bhutan, eastern Himalaya, a high-grade metamorphic terrane provides a rare glimpse into the evolution and exhumation of the deep eastern Himalayan crust and a detailed case study for deciphering the rates and timescales of deep-crustal processes in orogenic settings. We have collected U-Pb isotope and trace element data from allanite, zircon and garnet from metabasite boudins exposed in the Masang Kang valley in NW Bhutan. Our observations and data suggest that allanite cores record growth under eclogite facies conditions (>17 kbar ~650°C) at ca. 19 Ma, zircon inner rims and garnet cores record growth during decompression under eclogite facies conditions at ca 17-15.5. Ma, and symplectitic allanite rims, garnet rims and zircon outer rims record growth under granulite facies conditions at ~9-6 kbar; >750°C at ca. 15-14.5 Ma. Allanite is generally considered unstable under granulite-facies conditions and we think that this is the first recorded example of such preservation, likely facilitated by rapid exhumation. Our new observations and petrochronological data show that the transition from eclogite to granulite facies conditions occurred within 4-5 Ma in the Eastern Himalaya. Our data indicate that the exhumation of lower crustal rocks across the Himalaya was diachronous and may have been facilitated by different tectonic mechanisms.

Protracted continental collision — evidence from the Grenville OrogenThis article is one of a series of papers published in this Special Issue on the theme Lithoprobe — parameters, processes, and the evolution of a continent.

2010 ◽  
Vol 47 (5) ◽  
pp. 591-620 ◽  
Author(s):  
Andrew Hynes ◽  
Toby Rivers
Keyword(s):  
Continental Margin ◽  
Regional Metamorphism ◽  
Granulite Facies ◽  
Continental Collision ◽  
Northwestern Part ◽  
Grenville Province ◽  
The North ◽  
Laurentian Margin ◽  
Lower Crustal ◽  
The Himalaya

The Grenville Orogen in North America is interpreted to have resulted from collision between Laurentia and another continent, probably Amazonia, at ca. 1100 Ma. The exposed segment of the orogen was derived largely from reworked Archean to Paleoproterozoic Laurentian crust, products of a long-lived Mesoproterozoic continental-margin arc and associated back arc, and remnants of one or more accreted mid-Mesoproterozoic island-arc terranes. A potential suture, preserved in Grenvillian inliers of the southeastern USA, may separate rocks of Laurentian and Amazonian affinities. The Grenvillian Orogeny lasted more than 100 million years. Much of the interior Grenville Province, with peak metamorphism at ca. 1090–1020 Ma, consists of uppermost amphibolite- to granulite-facies rocks metamorphosed at depths of ca. 30 km, but areas of lower crustal, eclogite-facies nappes metamorphosed at 50–60 km depth also occur and an orogenic lid that largely escaped Grenvillian metamorphism is preserved locally. Overall, deformation and regional metamorphism migrated sequentially to the northwest into the Laurentian craton, with the youngest contractional structures in the northwestern part of the orogen at ca. 1000–980 Ma. The North American lithospheric root extends across part of the Grenville Orogen, where it may have been produced by depletion of sub-continental lithospheric mantle beneath the long-lived Laurentian-margin Mesoproterozoic subduction zone. Both the Grenville Orogen and the Himalaya–Tibet Orogen have northern margins characterized by long-lived subduction before continental collision and protracted convergence following collision. Both exhibit cratonward-propagating thrusting. In the Himalaya–Tibet Orogen, however, the pre-collisional Eurasian-margin arc is high in the structural stack, whereas in the Grenville Orogen, the pre-collisional continental-margin arc is low in the structural stack. We interpret this difference as due to subduction reversal in the Grenville case shortly before collision, so that the continental-margin arc became the lower plate during the ensuing orogeny. The structurally low position of the warm, extended Laurentian crust probably contributed significantly to the ductility of lower and mid-crustal Grenvillian rocks.

scholarly journals Present structure of the Near-Bug area: conditions of formation and history of development

2021 ◽  
Vol 43 (2) ◽  
pp. 96-115
Author(s):  
O.V. Usenko
Keyword(s):  
Partial Melting ◽  
Main Part ◽  
Solidus Temperature ◽  
Age Determination ◽  
Granulite Facies ◽  
Geological Structure ◽  
General Sequence ◽  
Crustal Rocks ◽  
The Moment ◽  
Lower Crustal

General sequence establishment of geological Precambrian events and associating formations, which were created in them, to the results of isotope age definition, is the task, which has no single valued solution for southwestern part of the Ukrainian Shield. Important is to create a general development model, which will describe the modern geological structure of an area, structural and textural rocks features, accounting PT-conditions in the Earth's crust during the Archean—Paleoproterozoic. Isotopic age determination demonstrates, that from the moment of protolith creation (not later than 3.75 billion years ago, up to 1.9 billion years ago), intrusion of mantle melts and partial melting of the lower crustal rocks, occurred many times over. Pobuzhie formation cannot be imagined, as a single process of accumulation, plunge, crumpling into folds and sedimentary strata metamorphism. It is necessary, to take into account, the plume (mantle) component of the general geodynamic process. In the structure of the Bug megablock and Golovanevskaya suture zone, two main structural plans are displayed. The main part of the territory displays a region of areal distribution of Archean enderbites (generated 2.8 billion years ago) and Proterozoic granites (generated 2.03 billion years ago). The paper compares the temperature distribution with depth, corresponding to the thermal model of the metamorphic temperatures found in the samples, and the solidus temperatures of the basic rocks. It is shown that at the time of the metamorphism development, 2.0 billion years ago, the rocks were at a depth of more than 20 km, and before that — at an even greater depth. During the Archean and Paleoproterozoic, the center of partial melting was repeatedly renewed here, since the temperatures were higher than the solidus temperature of gabbro. Metamorphic changes (and more often migmatization, partial melting and following crystallization in the granulite facies conditions) happened after the presence of the thermal asthenosphere on the core—mantle border, and were accompanied by bringing the substance from it. Therefore the main part of modern surface is folded by palingenic granites. In Archean and Paleoproterozoic the composition of substances were different. After 2.0 billion years ago the level of modern surface was located higher. The second structural plan is presented with vertical structures, building of which often close to concentrically zonal or linear monoclinal. They are confined to fault zones and nodes of their intersections. These structures contain rock complexes, which did not occur until 2.0 billion years ago on any craton in the world.

scholarly journals Long-lived Paleoproterozoic eclogitic lower crust

2021 ◽  
Vol 12 (1) ◽  
Author(s):  
Sebastian Buntin ◽  
Irina M. Artemieva ◽  
Alireza Malehmir ◽  
Hans Thybo ◽  
Michal Malinowski ◽  
...  
Keyword(s):  
High Velocity ◽  
Lower Crust ◽  
Crustal Layer ◽  
Crustal Rocks ◽  
Long Term Stability ◽  
Eclogite Facies ◽  
Conventional Models ◽  
Lower Crustal

AbstractThe nature of the lower crust and the crust-mantle transition is fundamental to Earth sciences. Transformation of lower crustal rocks into eclogite facies is usually expected to result in lower crustal delamination. Here we provide compelling evidence for long-lasting presence of lower crustal eclogite below the seismic Moho. Our new wide-angle seismic data from the Paleoproterozoic Fennoscandian Shield identify a 6–8 km thick body with extremely high velocity (Vp ~ 8.5–8.6 km/s) and high density (>3.4 g/cm3) immediately beneath equally thinned high-velocity (Vp ~ 7.3–7.4 km/s) lowermost crust, which extends over >350 km distance. We relate this observed structure to partial (50–70%) transformation of part of the mafic lowermost crustal layer into eclogite facies during Paleoproterozoic orogeny without later delamination. Our findings challenge conventional models for the role of lower crustal eclogitization and delamination in lithosphere evolution and for the long-term stability of cratonic crust.

Multi-stage dehydration–decompression in the metagabbros from the lower crustal rocks of the Serre (southern Calabria, Italy)

2008 ◽  
Vol 145 (3) ◽  
pp. 397-411 ◽  
Author(s):  
PASQUALE ACQUAFREDDA ◽  
ANNAMARIA FORNELLI ◽  
GIUSEPPE PICCARRETA ◽  
ANNARITA PASCAZIO
Keyword(s):  
Southern Italy ◽  
Granulite Facies ◽  
Crustal Thickening ◽  
Metamorphic Evolution ◽  
Crustal Rocks ◽  
Multi Stage ◽  
Crustal Thinning ◽  
The Matrix ◽  
Lower Crustal ◽  
Peak Assemblage

AbstractPorphyroblastic garnet-bearing metagabbros from the base of the lower crust section of the Serre (southern Italy) exhibit multi-stage dehydration and decompression after the Panafrican emplacement of their protoliths. The first dehydration event produced Am–Opx–Cpx–Pl–Grt as the peak assemblage. Two decompression stages are documented by: (1) coronas of Opx–Pl and Opx–Am, and symplectites of Opx–Am–Pl around clinopyroxene within the porphyroblastic garnet as well as in the matrix and (2) symplectites of Pl–Am–Opx–Grt having different textures around the porphyroblastic garnet. During the second decompression stage, a new local, somewhat intense, dehydration occurred and produced rims of Opx+Pl around the porphyroblastic amphibole, or lenses of Pl–Opx–Am–Spl±Bt between layers of dominant amphibole. A deformation stage separates older from younger reaction textures. The porphyroblastic garnet, its inclusions and the matrix are affected by fractures, which have been overgrown by coronas and symplectites around the porphyroblastic garnet and the amphibole of the matrix. PreferredP–Testimates are: ∼900 °C and ∼1.1 GPa at the metamorphic peak; ∼850 °C and 0.8–0.9 GPa during the formation of corona around clinopyroxene; 750–650 °C and 0.7–0.8 GPa during the formation of corona around garnet. All these textures formed under granulite-facies conditions. The subsequent metamorphic evolution consists of rehydration under amphibolite-facies conditions. TheP–T–tpath agrees with the path shown by the uppermost migmatites of the Serre section, and theP–Testimates at the top and the bottom of the section are consistent with the thickness (7–8 km) of the lower crustal segment. A contractional regime, which caused a crustal thickening of about 35 km, was followed by an extensional one producing significant crustal thinning; the change of tectonic regime probably occurred about 300 Ma ago when the emplacement of voluminous granitoids and the initial stages of exhumation of the lower crustal section had taken place.

A garnet peridotite and a garnet-amphibole pyroxenite from South Harris, Outer Hebrides, and their bearing on the South Harris eclogite facies status

1967 ◽  
Vol 36 (279) ◽  
pp. 380-388 ◽  
Author(s):  
A. Livingstone
Keyword(s):  
Optical Properties ◽  
Granulite Facies ◽  
Garnet Peridotite ◽  
Ultramafic Rocks ◽  
Chemical Analyses ◽  
The South ◽  
Outer Hebrides ◽  
Eclogite Facies

SummaryA garnet-olivine metaperidotite and a garnet-amphibole pyroxenite are described. Chemical analyses are presented for six rocks and optical properties and chemical analyses are tabulated for clinopyroxene, almandine-pyrope garnet, and hastingsitie amphibole from the garnet-amphibole pyroxenite. A possible origin for the garnet peridotite and chemically similar granulite facies ultramafic rocks is suggested. The eclogite facies in South Harris is reinstated in the light of the data presented.

scholarly journals Diversity and distribution of microlichens in the state of Arunachal Pradesh, Eastern Himalaya, India

Check List ◽  
2015 ◽  
Vol 11 (6) ◽  
pp. 1807 ◽  
Author(s):  
Pushpi Singh ◽  
Krishna Pal Singh ◽  
Ajay Ballabh Bhatt
Keyword(s):  
Biodiversity Hotspot ◽  
The State ◽  
Eastern Himalaya ◽  
New Combinations ◽  
Arunachal Pradesh ◽  
The Himalaya

The paper reports the occurrence of 404 species of microlichens belonging to 105 genera and 39 families known so far, from the state of Arunachal Pradesh, a part of the Himalaya biodiversity hotspot. Twelve species, namely Arthopyrenia saxicola, Arthothelium subbessale, Diorygma macgregorii, D. pachygraphum, Graphis nuda, G. oligospora, G. paraserpens, G. renschiana, Herpothallon japonicum, Megalospora atrorubricans, Porina tijucana and Rhabdodiscus crassus, are new distributional records for India. Astrothelium meghalayense (Makhija & Patw.) Pushpi Singh & Kr. P. Singh and Astrothelium subnitidiusculum (Makhija & Patw.) Pushpi Singh & Kr. P. Singh are proposed as new combinations and 66 species marked by an asterisk (*) are new distributional records for the state.

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