Plastic Deformation and Recrystallization of Garnet: A Mechanism to Facilitate Diffusion Creep

Elongate and deformed garnets from Glenelg, NW Scotland, occurwithin a thin shear zone transecting an eclogite body that hasundergone partial retrogression to amphibolite facies at circa700°C. Optical microscopy, back-scattered electron imaging,electron probe microanalysis and electron back-scatter diffractionreveal garnet sub-structures that are developed as a functionof strain. Subgrains with low-angle misorientation boundariesoccur at low strain and garnet orientations are dispersed, aroundrational crystallographic axes, across these boundaries. Towardshigh-strain areas, boundary misorientations increase and thereis a loss of crystallographic control on misorientations, whichtend towards random. In high-strain areas, a polygonal garnetmicrostructure is developed. The garnet orientations are randomlydispersed around the original single-crystal orientation. Somegarnet grains are elongate and Ca-rich garnet occurs on thefaces of elongate grains oriented normal to the foliation. Commonly,the garnet grains are admixed with matrix minerals, and, wherein contact with other phases, garnet is well faceted. We suggestthat individual garnet porphyroclasts record an evolution fromlow-strain conditions, where dislocation creep and recoveryaccommodated deformation, through increasing strain, where dynamicrecrystallization occurred by subgrain rotation, to higheststrains, where recrystallized grains were able to deform bydiffusion creep assisted grain boundary sliding with associatedrotations. KEY WORDS: diffusion creep; EBSD; garnet; plastic deformation; recrystallization     

Kerguelen Hotspot Magma Output since 130 Ma

The Kerguelen hotspot (Indian Ocean) has produced basalt for     

The Prinsen af Wales Bjerge Formation Lavas, East Greenland: the Transition from Tholeiitic to Alkalic Magmatism during Palaeogene Continental Break-up

We present elemental and isotopic (Sr–Nd–Pb–Hf–Os–He)data on primitive alkalic lavas from the Prinsen af Wales Bjerge,East Greenland. Stratigraphical, compositional and ⁴⁰Ar–³⁹Ardata indicate that this inland alkalic activity was contemporaneouswith the upper parts of the main tholeiitic plateau basaltsand also post-dated them. The alkalic rocks show a marked crustalinfluence, indicating establishment of new magmatic plumbingsystems distinct from the long-lived coastal systems that fedthe relatively uncontaminated plateau basalts. The least contaminatedlavas have high ³He/⁴He isotope ratios (R/R_A 12·4–18·5),sub-chondritic ¹⁸⁷Os/¹⁸⁸Os_i (0·120–0·126),low     

Petrogenesis of metamorphic rocks within a subduction-accretion terrane, Signy Island, South Orkney Islands

Abstract Whole-rock and mineral analyses of polydeformed mica-schist, quartzite, marble and amphibolite are presented from Signy Island, South Orkney Islands, part of the Scotia metamorphic complex. Whole-rock chemistry suggests that the amphibolites are the metamorphosed equivalent of enriched tholeiitic and alkali basalts of an oceanic intraplate basalt series. These, together with limestones and Mn-rich cherts of an oceanic island assemblage were tectonically mixed with trench or trench inner slope basin sediments in a subduction zone environment. Variation in mineral chemistry indicates an increase in temperature and decrease in pressure during metamorphism; pressures of 8 kbar and temperatures of approximately 545°C were reached during amphibolite facies metamorphism in the latter stages of deformation. These new data provide good evidence to support the previous interpretation of the Scotia metamorphic complex as a subduction complex.     

An Isotope and Trace Element Study of the East Greenland Tertiary Dyke Swarm: Constraints on Temporal and Spatial Evolution during Continental Rifting

Dykes of the East Greenland Tertiary dyke swarm can be dividedinto pre- and syn-break-up tholeiitic dykes, and post-break-uptransitional dykes. Of the pre- and syn-break-up dykes, themost abundant group (Tholeiitic Series; TS) has major elementcompositions similar to the main part of the East Greenlandflood basalts. A group of high-MgO tholeiitic dykes (Picrite–AnkaramiteSeries; PAS) are much less common, and are equivalent to someof the oldest lavas of the East Greenland flood basalts. Isotopiccompositions of the TS and PAS dykes partly overlap with thosefor Iceland, but Pb isotopic compositions extend to less radiogenicvalues than those seen in either Iceland or North Atlantic mid-oceanridge basalt (MORB). The isotopically depleted source requiredto account for this isotopic variation is interpreted as subcontinentallithospheric mantle with low ⁸⁷Sr/⁸⁶Sr and ²⁰⁶Pb/²⁰⁴Pb and high