Himalayan metamorphism in the Kashmir and Zanskar sector of the High Himalaya resulted from thrust- and fold-related crustal thickening within the Indian plate following the collision of India and Asia at c. 50 Ma. Interbanded metapelites, marbles, calcareous schists, amphibolites and quartzites represent metamorphosed equivalents of the Palaeozoic-Lower Mesozoic continental margin rocks. Granitic rocks include pre-collision K-feldspar megacrystic and biotite granites and post-collision two-mica ± garnet ± tourmaline granites. Average pressure-temperature conditions of equilibration using the self-consistent thermodynamic data-set of Holland & Powell (1990) are presented across a 45 km traverse of the eastern Kashmir-western Zanskar High Himalaya. Kyanite is the stable aluminosilicate phase across 35 km outcrop width in the middle of the slab. Complex microstructures indicate that prograde metamorphism up to kyanite grade and fabric development in the upper structural nappes is early and unrelated to the Main Central thrust. Diachronous metamorphism propagated southwards with the overall structural evolution. Early isograds and thrust-fold structures were carried passively in the hanging-wall of the Main Central thrust. Peak metamorphism, based on 40 Ar/ 39 Ar hornblende ages, occurred pre–30.7 ± 2.0 Ma at the top of the slab, and in the middle of the slab was pre-22 ± 1.0 Ma, probably 25-28 Ma. The lower structural levels in Zanskar record peak metamorphic conditions around 700–750 °C and 8 kbar, reflecting depths of burial of 28–30 km. A new younger schistosity, which is not present in the higher structural levels, was developed under kyanite grade conditions. The regional distribution of high temperatures recorded by thermo-barometry does not support the concept of additional heat being supplied by frictional heating along the Main Central thrust or magmatic heat resulting from anatexis. Exhumation of the Himalayan metamorphic rocks was achieved by three major processes: erosion as a result of crustal thickening, uplift of the rocks along the hangingwall of the Main Central thrust above a major frontal ramp in the Kishtwar Window area, and extensional unroofing along the footwall of a large-scale, NE-dipping normal fault at upper crustal levels, probably synchronous with compression at depth.
Pressure–temperature conditions for formation of the peak metamorphic mineral assemblages in phengite‐bearing eclogites from Dabieshan have been assessed through a consideration of Fe 2+ –Mg 2+ partitioning between garnet–omphacite and garnet–phengite pairs and of the reaction equilibrium celadonite+pyrope+grossular=muscovite+diopside, which incorporates an evaluation of the extent of the strongly pressure‐dependent inverse Tschermak's molecule substitution in the phengites. For the latter equilibrium, the calibration and recommended activity–composition models indicated by Waters & Martin (1993 ) have been employed and importantly yield results consistent with petrographic evidence for the stability at peak conditions of coesite in certain samples and quartz in others. Confirmation that in some phengite‐eclogite samples peak silicate mineral assemblages have equilibrated at confining pressures sufficient for the stability of coesite (and in some cases even diamond) rather negates previous suggestions that coesite may have been stabilized in only very localized, possibly just intracrystalline, domains. Inherent difficulties in the evaluation of peak metamorphic temperatures from Fe 2+ –Mg 2+ partitioning between mineral phases, due to uncertainties over Fe 3+ /Fe 2+ ratios in the minerals (especially omphacites), and to re‐equilibration during extensive retrograde overprinting in some samples, are also assessed and discussed. Our results indicate the existence in south‐central Dabieshan of phengite eclogites with markedly different equilibration conditions within two structurally distinct tectonometamorphic terranes. Thus our data do not support earlier contentions that south‐central Dabieshan comprises a structurally coherent continental‐crust terrane with a regional P–T gradient signalling previous deepest‐level subduction in the north. Instead, we recognize the Central Dabie ultra‐high‐pressure (coesite eclogite‐bearing) terrane to be structurally overlain by a Southern Dabie high‐pressure (quartz eclogite‐bearing) terrane at a major southerly dipping shear zone along which late orogenic extensional collapse appears to have eliminated at least 20 km of crustal section.
The mechanism of heat extraction from the lower oceanic crust near the ridge axis is poorly constrained despite its importance for understanding both the process of accretion of the plutonic complex and the mass fluxes associated with ridge hydrothermal systems. We have investigated the role of zones of focussed fluid flow in the plutonic complex of the Oman ophiolite in the near-axis cooling of the oceanic crust. Lineaments identified on aerial photographs, that occur at ∼1 km spacing, show evidence for extensive hydrothermal fluid flow through regions ∼10 to 50 m wide. Fluid flow is initiated in these regions at ∼800°C and continues at least into the lower greenschist facies. Strontium-isotope analyses indicate that the fluid flux through these zones is sufficient to transport a metasomatic front from the base of the sheeted dike complex to close to the Moho. Computed *minimum* fluid fluxes to transport a metasomatic front through the focussed fluid flow zones are ∼1x10^8^ kgm^−2^. Modeling of diffusive exchange of calcium from olivine to clinopyroxene indicates enhanced cooling rates adjacent to the focussed fluid flow zones. Heat fluxes estimated from the enhanced cooling rates are broadly consistent with the fluid fluxes determined from modeling the Sr-isotopic composition of samples from the focussed fluid flow zones. The combination of independent estimates of the fluid and heat fluxes, such as these, can provide more rigorous constraints on the thermal history than either approach used in isolation. Our results show that focussed fluid flow could play a major role in the cooling in the lower oceanic crust. Significant focussed fluid flow in the lower oceanic crust has important implications for predicting the total mass flux associated with hydrothermal circulation at mid-ocean ridges. This is because fluids flowing through channels become chemically rock-buffered at smaller fluid fluxes than those flowing pervasively through a rock mass. Thus, if focussed fluid flow is an important mechanism of heat loss from the lower oceanic crust the chemical fluxes from ridge hydrothermal systems into the oceans may be smaller than currently thought.
RESUMEN Las rocas de alta presion encontradas en la Peninsula de La Guajira, comprenden principalmente eclogitas con la paragenesis pico: onfacita + granate + cuarzo + rutilo + mica blanca ± cianita y metapelitas con la paragenesis pico: cuarzo + mica blanca + granate + cianita + rutilo ± apatito. Estas rocas muestran evidencias de una compleja historia de evolucion tectonica: 1) Una primera fase de crecimiento progrado en dos fases, a traves de la facies anfibolita, hasta alcanzar la paragenesis pico; 2) un evento de hidratacion que permitio la formacion local de glaucofana orientada; 3) un evento no bien diferenciado que sucede en condiciones estaticas que permite la formacion de clinozoisita; y 4) la retrogradacion de estas rocas en facies anfibolita. La asociacion de metamafitas y rocas de procedencia continental sugieren una relacion con un prisma de acrecion que se genero durante la subduccion cretacica de la placa Caribe y el acercamiento de un posible fragmento continental o de la margen continental Suramericana a la placa del Caribe. Palabras Clave: Eclogitas, metapelitas, placa Caribe, quimica mineral MINERAL CHEMISTRY OF HIGH PRESSURE ECLOGITE-FACIES ROCKS, FROM THE GUAJIRA PENINSULA, COLOMBIA ABSTRACT The high pressure rocks found in the Guajira Peninsula, comprise mainly eclogites with the peak metamorphic paragenesis: omphacite + garnet + quartz + white mica ± kyanite and metapelites with the peak metamorphic paragenesis: quartz + white mica + garnet + kyanite + rutile ± apatite. These rocks register a complex history of tectonic evolution: 1) Prograde growth in two stages, through amphibolite facies, reaching the peak metamorphic paragenesis; 2) hydration of these rocks that enabled galucophane to grow locally; 3) a later not well characterized event that happened in static conditions shown by the growth of clinozoisite; 4) retrogression through amphibolite-facies. The association of eclogites with continental rocks suggest their link to an accretionary prism that was generated during the Cretaceous subduction of the Caribbean plate and the bringing together of a possible continental fragment or the South American continental margin and the Caribbean plate. Key words : Eclogites, metapelites, Caribbean plate, mineral chemistry
Two approaches to determining the high-temperature (∼1000°C to ∼600°C) cooling rate of the lower oceanic crust and upper mantle are presented and critically evaluated. The first is based on the down-temperature diffusive exchange of Ca between olivine and clinopyroxene. The second, less well-constrained, approach is based on the down-temperature diffusive exchange of Mg and Fe between olivine and spinel. Cooling rates based on olivine–spinel geospeedometry are approximately an order of magnitude faster than those from Ca-in-olivine geospeedometry. In contrast, cooling rates derived from thermochronology and remanent magnetism are approximately an order of magnitude slower than those derived by Ca-in-olivine geospeedometry; this is probably because they record cooling at lower temperatures. Using the Ca-in-olivine geospeedometer, the cooling rate of samples from the lower oceanic crust and upper oceanic mantle formed in the Oman ophiolite and in the three main ocean basins has been determined. Samples from the lower oceanic crust formed at fast-spreading ridges show a large decrease in cooling rate between the top and base of the gabbroic section, with most of the variation occurring within the upper kilometre. This is consistent with vertical heat loss (within the crustal frame of reference) dominating the thermal evolution at fast-spreading ridges. Samples from Ocean Drilling Program Hole 735B, which formed at the slow-spreading Southwest Indian Ridge, show no variation in cooling rate over 1500 m depth range and cooled substantially faster than rocks from the deeper portion of the gabbros in the Oman ophiolite, where the change in cooling rate with depth is limited. These observations are consistent with heat loss from small plutons emplaced in cool lithosphere at the slow-spreading ridge. Alternatively, they could be explained by cooling through the Ca-in-olivine closure interval during uplift towards the surface.
Green chromian andradite occurs associated with chrysotile in a chromitite layer within the Jijal Complex (northern Pakistan). The garnet contains, on average, 10.1 wt. % Cr 2 O 3 (range 9.2-11.6%) and has a formula Ca (sub 3.04) (Cr (sub 0.67) Fe (sub 3+) (sub 1.29) Al (sub 0.02) )Si (sub 2.98) O 12 . The garnet formed during retrograde greenschist-facies metamorphism of the Jijal Complex.--Modified journal abstract.
Abstract Ophiolite-related rocks accreted to Caribbean Plate margins provide insights into the understanding of the intra-oceanic evolution of the Caribbean Plate and its interaction with the continental margins of the Americas. Petrological, geochemical and isotope (K–Ar, Sr and Nd) data were obtained in serpentinites, gabbros and andesite dykes from the Cabo de la Vela Mafic–Ultramafic Complex from the Guajira Peninsula, in the northernmost Colombian Caribbean region. Field relations, metasomatic alteration patterns and whole rock–mineral geochemistry combined with juvenile isotope signatures of the different units suggest that gabbros and serpentinites formed in a slow-spreading supra-subduction zone that was brought to shallower depths and subsequently evolved to an arc setting where andesitic rocks formed with little sediment input. The tectonomagmatic evolution of the Cabo de la Vela Mafic–Ultramafic Complex involved an intra-oceanic arc that evolved from pre-Campanian time to 74 Ma. Relationships with other units from the Guajira Peninsula show either the existence of a mature arc basement or a series of coalesced allocthonous arcs, juxtaposed before accretion onto the passive continental margin of South American in pre-Eocene times.
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