Metamorphic and magmatic consequences of subduction of young oceanic lithosphere and exhumation in a subduction channel.Eastern cuba

Laburpena

Ophiolitic rocks associated with high pressure blocks occur in serpentinite matrix mélanges in the north-western Antilles (northern part of Cuba and Hispaniola). These mélanges represent fragments of a well preserved oceanic subduction channel related to subduction of the Protocaribbean (Atlantic) lithosphere below the Caribbean plate during the Cretaceous. The mélanges occur in three tectonic domains of the north-western Antilles: western and central Cuba occupying the western portion, eastern Cuba (east of the Nipe-Guacanayabo fault) in the center, and Hispaniola representing the eastern portion. The high pressure blocks from mélanges of the western and eastern portions of the north-western Antilles indicate cold subduction represented by eclogite, blueschist as well as lawsonite-eclogite/blueschist as the main lithologies. On the other hand, the high pressure blocks from mélanges of the central portion (eastern Cuba) indicate hot and cold subduction, with epidote-amphibolite rocks containing the tonalitic-trondhjemitic products of partial melting of the slab, and blueschists. The Sierra del Convento and La Corea mélanges, being located in the central portion, are geologically similar and provide a good example for studying hot subduction processes and, in particular, partial melting of the slab. Partial melting of a subducted young oceanic lithosphere in eastern Cuba is unique in the Caribbean realm (and probably in the world). This scenario can only be compared with the Catalina schist (a fragment of the Franciscan mélange) in south western USA and has significant consequences for the plate tectonic interpretation of the region. This is the focus of this PhD thesis in the La Corea mélange. The La Corea mélange is located in the Sierra del Cristal mountain range. It occurs as a tectonic window between the Mayarí-Cristal ophiolite massif, on top, and the Cretaceous Santo Domingo volcanic arc formation, below. Exotic blocks, mainly of garnet-amphibolite, blueschist and greenschist, occur in the mélange within a serpentinite-matrix. The metamorphism is related to subduction processes and evolved under medium to high-pressure and high- to low-temperature. A unique characteristic of the mélange, shared by the Sierra del Convento mélange, is the presence of dikes and veins of intermediate to acid (tonalitic-trondhjemitic-granitic) composition which occur intimately associated with the amphibolites. The integrated field, petrological, and geochemical study of the amphibolite blocks and the associated tonalitic-trondhjemitic materials indicate that the latter formed by partial melting of the former under water-saturated conditions. The main (peak) mineral assemblage of the amphibolite blocks is pargasitic amphibole + epidote + quartz + rutile + titanite ± garnet ± phengite, lacking primary (peak) plagioclase. The amphibolites have variable grain size and degree of deformation. The magmatic mineral assemblage of the tonalite-trondhjemite rocks is plagioclase + quartz + phengite ± epidote ± paragonite ± pargasite. These rocks show variable grain size and a mild deformation. The leucocratic bodies appear associated with the amphibolites and do not crosscut other types of exotic blocks or the serpentinitic matrix. Major and trace element geochemistry of the amphibolites show a basaltic composition within the subalkaline series and a tholeiitic low-K affinity. The REE patterns indicate N-MORB composition. N-MORB signatures are locally modified by infiltrating fluids evolved from dehydration of the subducting Protocaribbean lithosphere. Therefore the mafic protoliths (amphibolites) represent subducted Protocaribbean lithosphere exhumed in the subduction channel. The tonalitic-trondhjemitic-granitic rocks are of andesitic to rhyolitic composition in the TAS diagram, have a peraluminous character, and display distinctive REE patterns with a negative slopes and positive Eu anomalies. The positive Eu anomalies suggest an important contribution of plagioclase to the partial melting processes, consistent with the lack of peak plagioclase in the amphibolites. Thermobarometric determinations by means of the multiequilibrium and pseudosection approaches indicate that partial melting of the amphibolites took place at. 700ºC and 14-16 kbar during subduction and accretion to the upper plate mantle, while crystallization of the tonalitic-trondhjemitic melts occurred within the blocks at similar depths but during cooling in the upper plate (ca. 680-700ºC and 13-15 kbar). At these conditions (ca. 50 km depth; > 600 ºC), antigorite is not stable, preventing the formation of the serpentinitic subduction channel. Upon further cooling of the subduction system due to continued subduction, antigoritite formed due to hydration of the upper plate peridotite, allowing the formation of the subduction channel and, henceforth, the blocks started exhumation. The retrograde overprints in the amphibolites and trondhjemites are made of combinations of actinolite + glaucophane + tremolite + paragonite + lawsonite + albite + clino-zoisite + chlorite + phengite, formed during retrograde blueschist/greenschist facies conditions. Calculated P-T conditions indicate counterclockwise P-T paths (i.e., -hot Franciscan type- exhumation). Occasionally, the blocks indicate large-scale convective circulation in the channel, which is consistent with predictions of thermo-mechanical models. This finding constitutes the first report of large-scale convective circulation of tectonic blocks in a subduction channel in the literature. Formation of amphibolite-trondhjemite rocks at high pressure - high temperature conditions documents high thermal gradient at the slab-wedge interface. The counterclockwise P-T paths followed by these rocks indicate, in turn, cooling of the subduction system upon continued subduction. This thermal history suggests onset of subduction of a young oceanic lithosphere, consistent with predictions of thermo-mechanical models. However, a similar thermal evolution is possible after subduction of young lithosphere followed by subduction of older lithosphere, whether or not onset of subduction is involved, during oblique subduction of a ridge. The thermo-mechanical models permit also considered the partial melting process in eastern Cuba as a cold plume aborted and crystallized in the slab-mantle wedge interface. Ba-rich phengite-bearing rocks, including amphibolite, trondhjemite, pegmatite and Qtz+Ms rocks, have major and trace elements composition that indicates circulation of Ba-rich fluids and melts in the subduction channel. Fluid infiltration at high temperature of Ba-rich fluids, likely evolved from subducting sediments, transformed subducted/accreted MORB material (amphibolites). This process led to the formation of Ba-rich trondhjemites with adakitic signature during fluid-fluxed melting of amphibolite. The partial melts represent pristine slab melts that did not react with the upper mantle. The pegmatites are interpreted as magmatic products after differentiation of trondhjemitic melts, while Qtz-Ms rocks probably represent material crystallized from a primary sediment-derived fluid. The chemistry of phengite is primarily governed by the celadonite (tschermak) ((Mg,Fe)Si(VIAlIVAl)-1) and celsian (AlBa(SiK)-1) exchange vectors. The preservation of zoning in phengite indicates sluggish diffusion. This finding is important because crystallization of phengite at depth makes the behaviour of Ba (and K and other LILE) compatible, preventing the transfer of these elements to the mantle wedge and reinforcing the importance of phengite stability in the subduction channel for the transference of elements from the slab to the arc environment. Two main groups of ultramafic material appear in the mélange associated with the high pressure blocks: antigorite- and antigorite-lizardite-serpentinites. The strong alteration (i.e., serpentinization, Cr-spinel transformation) of ultramafic protholiths indicate pervasive fluid flow, best explained if the rocks experienced hydration during infiltration of fluids evolved from the subducted slab. Antigorite serpentinites have harzburgitic protolith and most likely formed at depth after hydration of the mantle wedge (Caribbean lithosphere) by fluids derived from the SW-dipping subducted slab (Protocaribbean lithosphere). Antigorite-lizardite serpentinites are of harzburgitic-lherzolitic composition and are best explained as abyssal (meta)peridotites (Protocaribbean lithosphere) accreted to the subduction channel developed in the Caribbean-Protocaribbean plate interface. Antigorite-serpentinites document large-scale hydration of the Caribbean plate mantle wedge and the formation of a thick subduction channel which allowed exhumation of accreted subducted material during Cretaceous times. U-Pb SHRIMP and Ar-Ar dating constrain the evolution of the subduction zone. Zircon from trondhjemitic rocks provided U-Pb crystallization ages ranging 105-110 Ma. Ar-Ar phengite data from the same trondhjemitic samples yielded cooling ages of 83-86 Ma (interpreted as the closure-cooling temperature of phengite). These data, combined with the calculated counterclockwise P-T paths indicate a very slow syn-subduction exhumation history in the channel (~1mm/yr). The leucocratic rocks and the host amphibolite slowly cooled and exhumed from 700 ºC and 15 kbar to 350 ºC and 8 kbar during 25 My. Final exhumation in the mélange occurred during the late Cretaceous (70-65 Ma) obduction of the supra-subduction ophiolitic and volcanic arc rocks as determined by the presence of syn-tectonic deposits.