Search results for "Underplating"
showing 10 items of 14 documents
Crust Formation and Plate Motion in the Early Archean
1992
Mounting evidence for voluminous continental crust formation in the early Archean involving intracrustal melting and selective preservation of granitoid rocks suggests that initial crust formation crust formation and growth were predominantly by magmatic underplating in plumegenerated Iceland-type settings. Collision of these early islands to give rise to larger blocks is suggested by extensive horizontal shortening in both supracrustal and granitoid assemblages. Preservation of early Archean high-grade gneisses that were once at depths of 20 to 30 kilometers implies that these blocks developed thick, subcrustal roots despite high mantle heat flow. Rigid continental plates must have existed…
Phase equilibrium constraints on a deep crustal metamorphic field gradient: metapelitic rocks from the Ivrea Zone (NW Italy)
2011
The metamorphic rocks of the Ivrea Zone in NW Italy preserve a deep crustal metamorphic field gradient. Application of quantitative phase equilibria methods to metapelitic rocks provides new constraints on the P–T conditions recorded in Val Strona di Omegna, Val Sesia and Val Strona di Postua. In Val Strona di Omegna, the metapelitic rocks show a structural and mineralogical change from mica‐schists with the common assemblage bi–mu–sill–pl–q–ilm ± liq at the lowest grades, through metatexitic migmatites (g–sill–bi–ksp–pl–q–ilm–liq) at intermediate grades, to complex diatexitic migmatites (g–sill–ru–bi–ksp–pl–q–ilm–liq) at the highest grades. Partial melting in the metapelitic rocks is consi…
High-pressure metamorphism in the Aegean, eastern Mediterranean: Underplating and exhumation from the Late Cretaceous until the Miocene to Recent abo…
2003
[1] We report 40Ar/39Ar ages from various tectonic units in the Aegean and westernmost Turkey. On the basis of published geochronologic data and our 40Ar/39Ar ages we propose that the Aegean is made up of several high-pressure units, which were successively underplated from the Late Cretaceous until the Miocene. Ages for high-pressure metamorphism range from 80–83 Ma in parts of the Vardar-Izmir-Ankara suture zone in the north to 21–24 Ma for the Basal unit in the Cyclades and the external high-pressure belt on Crete in the south. Published seismic data suggest that high-pressure metamorphism is currently occurring underneath Crete. Younging of high-pressure metamorphism in a southerly dire…
Geodynamic significance of ultramafic xenoliths from Eastern Serbia: Relics of sub-arc oceanic mantle?
2007
A suite of highly depleted peridotite xenoliths in East Serbian Palaeogene basanites represents the lithospheric mantle beneath the Balkan Peninsula. The xenoliths are harzburgites, clinopyroxene-poor lherzolites and rare dunites. They contain mostly 91), high Cr# in spinel (mostly 0.5–0.7), and by distinctively low Al2O3 contents in orthopyroxene (mostly 1–2 wt.%). They have experienced some mantle metasomatism which has slightly obscured their original composition. Nevertheless, the general characteristics of the xenoliths imply a composition which is significantly more depleted than most non-cratonic sub-continental mantle xenolith suites, as well as orogenic peridotites and abyssal peri…
Partial melting of metabasic rocks and the generation of tonalitic–trondhjemitic–granodioritic (TTG) crust in the Archaean: Constraints from phase eq…
2020
Abstract Rocks of tonalitic–trondhjemitic–granodioritic (TTG) composition preserved in Archaean terranes represent fragments of the Earth’s earliest-formed continental crust, and are thought to have formed via partial melting of hydrated metabasalt. The geodynamic environments in which such high-grade metamorphism and anatexis may have occurred in the early Earth is strongly debated. Constraining the pressure ( P ) and temperature ( T ) conditions at which melts of appropriate composition can be derived from protoliths containing plausible mineral assemblages is central to addressing this question. Phase equilibrium modelling has been undertaken for an enriched Archaean tholeiite bulk compo…
Unraveling the exhumation history of high-pressure ophiolites using magnetite (U-Th-Sm)/He thermochronometry
2020
International audience; Magnetite is a ubiquitous oxide in ultramafic and mafic rocks, which is present in a large range of geological and tectonic settings. In the case of high-pressure ultramafic rocks, exhumation timing is commonly constrained by geochronometers in nearby lithologies. The development of the magnetite (U-Th-Sm)/He method (MgHe) has opened new perspectives to refine the exhumation history of such rocks. However no thermal history has been deduced using MgHe data so far. Here we applied MgHe dating to magnetite from an HP-LT alpine ophiolite body (Rocher Blanc) from the Schistes lustres units (Western Alps, France) where part the P-T-t history is constrained to further refi…
Delamination and recycling of Archaean crust caused by gravitational instabilities
2013
The volume of Archaean crust preserved at Earth’s surface today is low. Thermodynamic calculations and geodynamic modelling show that the thick, primary crust that would have formed on a much hotter Archaean Earth was denser than the underlying mantle, and would have therefore been recycled back into the mantle as drips.
SHRIMP ZIRCON AGES AND PETROLOGY OF LOWER CRUSTAL GRANULITE XENOLITHS FROM THE LETSENG-LA-TERAE KIMBERLITE, LESOTHO: FURTHER EVIDENCE FOR A NAMAQUA-N…
2013
Recrystallized zircons extracted from intermediate granulites of the Letseng-la-Terae Diamond Mine in northeastern Lesotho provide evidence that the Tugela Terrane of the Mesoproterozoic high-grade Namaqua-Natal mobile belt may extend ~200 km farther west than previously thought. The rock-forming minerals in the investigated lower crustal granulites contain garnet, clinopyroxene, plagioclase and quartz or garnet, wollastonite, plagioclase and quartz. Calculated values for peak granulite-facies metamorphism are ~13 kbar and ~850°C. Thermal overprint and therein granulite-facies metamorphism occurred during the late Mesoproterozoic and is documented by SHRIMP metamorphic zircon ages of 1018 ±…
Tethyan vs. Cordilleran ophiolites: a reappraisal of distintctive tectono-magmatic features of supra-subduction complexes in relation to the subducti…
2004
Abstract Supra-subduction zone (SSZ) ophiolites deserve special attention because they represent fundamental markers of intraoceanic convergence and generation of new lithosphere above subduction zones. Moreover, owing to their structural characteristics and location in the overriding plate, these complexes are far better represented and preserved than Mid-Ocean-Ridge-Basalt (MORB) ophiolites in orogenic belts. In terms of their structure, tectonics, and magmatic features, SSZ ophiolites may be classified in two main types: (1) “Tethyan complexes” (such as those of the Albanide-Hellenide belt), which mostly consist of complete and extensive volcanic, dyke, plutonic, and mantle sections with…
Reworking of the Tarim Craton by underplating of mantle plume-derived magmas: Evidence from Neoproterozoic granitoids in the Kuluketage area, NW China
2011
Abstract Most Neoproterozoic granitoids in the Kuluketage area, northern Tarim Craton are characterized by strongly depleted HREE abundances and high Sr/Y and (La/Yb) N ratios, showing typical geochemical features of adakitic rocks. Zircon U–Pb dating of three adakitic plutons yielded Neoproterozoic ages (754 ± 4, 790 ± 3 and 798 ± 3 Ma). The adakitic granitoids exhibit low MgO and TiO 2 contents. Their low Cr, Co and Ni abundances, low ɛ Hf ( t ) values (−21 to −11) and high FeO T /MgO ratios (mostly 2.2–2.9) preclude the possibility of being derived from partial melts of delaminated lower crust. The relatively low ɛ Hf ( t ) values and old Hf two-stage model ages ( T DM 2 = 2 . 42 – 3 .0 …