No abstract

No abstract

We investigated rutile needles with a clear shape preferred orientation in garnet from (ultra-) high pressure metapelites from the Kimi Complex of the Greek Rhodope by electron microprobe, Electron Back Scatter Diffraction and TEM-techniques. A definite though complex crystallographic orientation relationship between the garnet host and rutile was identified in that Rt[001] is either parallel to Grt<111> or describes cones with opening angle 27.6° around Grt<111>. Each Rt[001] small circle representing a cone on the pole figure displays six maxima in the density plots. This evidence together with microchemical observations in TEM, when compared to various possible mechanisms of formation, corroborates a precipitate origin. A review of exchange vectors for Ti-substitution in garnet indicates that rutile formation from garnet cannot occur in a closed system. It requires that components are exchanged between the garnet interior and the rock matrix by solid state diffusion, a process we refer to as "open system precipitation" (OSP). The kinetically most feasible reaction of this type will dominate the overall process. The perhaps most efficient reaction involves internal oxidation of Fe 2+ to Fe 3+ and transfer from the dodecahedral to the octahedral site just vacated by Ti 4+ : 6

This study combines new data on tectonostratigraphy, macrostructures and microstructures, petrology, and geochronology to propose a comprehensive model for the tectonometamorphic evolution of the Rhodope orogen from the Jurassic to the early Paleogene. Rocks from two study areas in the central and eastern Greek Rhodope represent a continental suture zone (Rhodope Suture Zone), with the included material most likely forming an extensional allochthon south of the European continent during Permo/Triassic times that was subsequently subducted beneath Europe in the Early Jurassic (≥180 Ma). On the basis of comparable metamorphic ages and coherent structures but differences in metamorphic conditions and lithologies, the rocks of the Rhodope Suture Zone are subdivided into an upper and a lower part. The prograde history is linked with subduction‐related structures in the lower part (uniaxial stretching, deformation stage D1). In metapelites, the earliest stage of metamorphism recorded at circa 180 Ma occurred at least under ultrahigh‐pressure metamorphic conditions. The rocks of the upper part experienced isothermal decompression with partial anatexis. Exhumation paths of both parts differ in temperature because of the relative tectonic position within the exhuming wedge. Exhumation was forced by the Nestos Shear Zone that controlled the early phase of normal displacement by SW shearing at the base (lower part) and NE shearing on top (upper part) from the Late Jurassic to the Late Cretaceous (deformation stage D2). An intervening stage of mineral recrystallization and thermal reequilibration in the upper part was followed by a common exhumation history of both parts at pressures lower than about 12 kbar (35–40 km depth). During this stage, exhumation was controlled by southwest directed shearing and folding (deformation stage D3). Slab retreat to the south led to subsequent extension (deformation stage D4) and final exhumation coeval with the formation of basement domes, thus making the Rhodope a classic core complex juxtaposed with former structurally higher units.

This study deals with microtextures and fluid inclusions from veins and vesicles hosted in the Troodos Pillow Lavas that enable a conclusive model for vein formation during the post-magmatic stage of the Troodos supra-subduction zone. Three different types of veins from the Upper and Lower Pillow Lavas are distinguished and imply different modes of fracturing, fluid flow, and precipitation. (1) Syntaxial calcite-, quartz-, and zeolite-bearing veins are interpreted as mineralized extension fractures that were pervaded by seawater. This advective fluid flow in an open system changed later into a closed system characterized by geochemical self-organization. (2) Blocky and (3) antitaxial fibrous calcite veins are associated with host rock brecciation due to hydrofracturing and diffusion-crystallization processes, respectively. Based on aqueous fluid inclusion chemistry with seawater salinities in all studied vein types, the representative fluid isochores crossed with minimum hydrostatic pressure conditions yield vein mineral precipitation temperatures between 180 and 210 °C at 250 bar, independently of the Pillow Lava units. This points to a heat source for the circulating seawater and implies that vein and vesicle minerals precipitated shortly after pillow lava crystallization under dominant isobaric cooling conditions. Compared to previous suggestions derived from secondary mineral parageneses, significant higher temperatures of vein formation in the Troodos Pillow Lavas are proposed.

International Ocean Discovery Program Expeditions 352 and 351 drilled into the Western Pacific Izu‐Bonin forearc and rear arc. The drill cores revealed that the forearc is composed of forearc basalts (FAB) and boninites and the rear arc consists of FAB‐like rocks. These rocks are pervaded by calcite veins. Blocky vein microtextures enclosing host rock fragments dominate in all locations and suggest hydrofracturing and advective fluid flow. Significant diffusion‐fed and crystallization pressure‐driven antitaxial veining is restricted to the rear arc. The lack of faults and presence of an Eocene sedimentary cover in the rear arc facilitated antitaxial veining. Rare earth element and isotopic (δ18O, δ13C, 87Sr/86Sr, and Δ47) tracers indicate varying parental fluid compositions ranging from pristine to variably modified seawater. The most pristine seawater signatures are recorded by FAB‐hosted low‐T (<30 °C) vein calcites. Their 87Sr/86Sr ratios intersect the 87Sr/86Sr seawater curve at ~35–33 and ~22 Ma. These intersections are interpreted as precipitation ages, which concur with Pacific slab rollback. Boninite‐hosted low‐T (<30 °C) vein calcites precipitated from seawater that was modified by fluid‐rock interactions. Mixing calculations yield a mixture of >95% seawater and <5% basaltic 87Sr/86Sr. In the rear arc, low‐T rock alteration lowered the circulating seawater in δ18O and 87Sr/86Sr. Thus, vein calcites precipitated from modified seawater with up to 20–30% basaltic 87Sr/86Sr at temperatures up to 74 ± 12 °C. These results show how the local geology and vein growth dynamics affect microtextures and geochemical compositions of vein precipitates.

No abstract

In metabasites from the high-grade metamorphic Kimi Complex of the Greek Rhodope, lamellae of quartz and amphibole were observed together with either rutile or titanite precipitates in the cores of clinopyroxene that underwent a prolonged exhumation history from eclogite via granulite and amphibolite facies conditions. Lamellae are crystallographically oriented along clinopyroxene cleavage planes (010) and interpreted as the result of precipitation during a two-step intracrystalline compositional adjustment from an omphacite-to a Ca-Tschermak-rich clinopyroxene and finally to diopside. Even though retrograde diffusion profiles develop in clinopyroxene, the diffusion rates of highly charged ions like Si 4+ , Al 3+ and Ti 4+ are too slow, so precipitates rich in Si 4+ (quartz), Al 3+ (amphibole) and Ti 4+ (rutile or titanite) form as viable kinetic alternatives. These precipitation reactions are not possible isochemically, and additional exchange of ions with the matrix is required for precipitation to occur. This exchange of mainly rapidly diffusing low-charge ions is postulated to proceed by bulk diffusion (self-diffusion) and does not require special pathways or fluid, even though these may play an additional role during metamorphism. This hitherto undescribed process of precipitation made possible by diffusional exchange with the matrix through an intact host crystal lattice is termed here as Ôopen system precipitationÕ (OSP). No prior Ca-Eskola-rich (ultrahigh-pressure, UHP) composition of omphacite is required to explain the observed precipitates. It is proposed that many precipitation phenomena that were hard to explain in a closed-system scenario can now be explained more readily by OSP. Calculation of an original solid solution composition, as in an UHP phase, from precipitate and host mineral analyses becomes imprecise to the degree that bulk diffusion and hence OSP have been important.