IGCP-546 SUBDUCTION ZONES OF THE CARIBBEAN

Save this PDF as:
 WORD  PNG  TXT  JPG

Size: px
Start display at page:

Download "IGCP-546 SUBDUCTION ZONES OF THE CARIBBEAN"

Transcription

1 Tectonic and denudation along the Polochic Fault system Authemayou, C. (1), Teyssier, C. (1), Brocard, G. (1), Simon-Labric, T. (1), Lavier, L. (2), Moran, S. (3). (1) Institut de géologie et Paléontologie, Bâtiment Anthropole, University of Lausanne, CH-1015 Lausanne, Switzerland. (2)University of Texas, J.J. Pickle Research Campus, Austin, TX USA. (3) San Carlos University, Coban, Guatemala. The EW-trending Polochic-Motagua strike-slip fault system in Guatemala marks the plate boundary between the North American and the Caribbean plates. Our study focuses on the Polochic fault and its cenozoic dynamics in response to the combination of far field geodynamic forcing and near field tectonically induced capture (TIC) of the Chixoy river to 7.4 My. Combined field structural investigations and fault kinematic analyses make it possible to evaluate the kinematic framework explaining the accommodation of plate motion since Miocene and the rock exhumation along the Polochic fault induced by the TIC. The study reveals a change after 7.4 My of tectonic regime from transpression to transtension around the Polochic fault. Actually, a significant part of the Polochic fault slip is distributed along NE-trending normal faults south of the Polochic fault and along reverse faults in the Altos cuchumatanes plateau north of the Polochic fault. The other part is transferred to the Tonola fault that runs parallel to the Pacific coast. Deformation and slip rates have been determined using offsets of NS-trending pre-capture paleovalleys and a paleosurface of probable Middle Miocene age. Intriguingly, theses geomorphic features document a highly localized uplift above the Polochic fault zone that increases in the regions of high denudation. This observation draws attention on the effects of the river capture on fault dynamics. Irregular phases of erosion and exhumation produced by captures influence the structure of the fault zone. Rock exhumation enhanced by the denudation associated with the TIC is currently being tested by numerical modelling in collaboration with Prof. Lavier (University of Texas at Austin, USA). Our ultimate goal is to link these lithosphere-scale models with erosion models derived from our field studies in order to describe the feedbacks that link erosion cycles and deformation. IGCP 546 Special Contribution 1. >> Activities >> Guatemala 2007 >>. 1/13

2 Mineralogy and petrology of Hispaniolan jadeitites: First results Baese, R., Schertl, H.-P., Maresch, W.V. Institute of Geology, Mineralogy und Geophysics, Ruhr-University-Bochum, Bochum, Germany, Harlow et al. (2006) recently described jade artefacts from Antigua, West Indies, and speculated on their provenance. Jadeitite and other HP-LT rocks are now well-known (Harlow et al, 2004) from serpentinite mélanges both north and south of the Motagua fault zone in Guatemala, which separates the North American Plate from the Caribbean Plate. The Antigua jade artefacts reveal affinities with the HP-LT-rocks south of the Motagua fault. They are quartz-bearing, contain phengite, and occasionally also glaucophane and lawsonite (Harlow et al., 2006). However, Harlow et al. (2006) pointed out that serpentinite mélanges related to northern Caribbean subduction-zone complexes are common throughout the Greater Antilles, and that other sources for the Antigua jade are possible. Here we report on a newly discovered occurrence of jadeitites from serpentinite mélanges of northern Hispaniola, first identified by Schertl et al. (2007). These rocks should also be considered as potential source material for jade artefacts in the Mesoamerican/Caribbean regions. The jadeitite from this new locality occurs in the northern Dominican Republic within a unit called the "Rio San Juan Complex", which represents a fossil intra-oceanic subduction zone that was active between 120 and 55 Ma (Krebs et al., 2007). Here, serpentinite mélanges cut Upper Cretaceous to Lower Tertiary HP-LT mafic schists. These mélanges contain numerous blocks of different metamorphic rock types like blueschists, eclogites, lawsonite blueschists, jadeitites, cymrite-bearing rocks and orthogneisses. The jadeitites occur as irregular masses and layers within and closely related to surrounding lawsonite blueschists. The jadeitites are fine-grained and the colour ranges from white to greenish. The grain size varies up to 600µm. Jadeite is the main constituent, and the amount may exceed 90 vol%. Additional coexisting minerals in minor amounts are phengite, omphacite, epidote, tschermakitic amphibole, plagioclase and quartz. In some samples lawsonite, pumpellyite and stilpnomelane have been observed. Accessory minerals are titanite and rutile. In thin-section it is possible to observe a weak foliation. Generally, jadeite forms euhedral to subhedral prismatic crystals; however, anhedral massive intergrowth textures, sometimes also with inclusions of titanite may be found. The homogeneous cores of the crystals reach 98% jadeite end-member composition (the variations observed in one investigated sample are Jd Aeg Di Hd ). These are surrounded by very thin retrograde omphacite rims with significant aegirine and diopside contents (Jd Aeg Di Hd ; see also Schertl et al., 2007). Geochemically, the jadeitites reveal very high amounts of SiO 2 and Na 2 O and low amounts of K 2 O. These jadeite-rich rocks appear to have experienced a high degree of metasomatism during their formation. Harlow, G.E., Hemming, S.R., Avé Lallemant, H.G., Sisson, V.B., and Sorensen, S.S. (2004). Two high-pressure-low-temperature serpentinite-matrix mélange belts, Motagua fault zone, Guatemala: a record of Aptian and Maastrichtian collisions. Geology, 32, Harlow, G.E., Murphy, A.R., Hozjan, D.J., De Mille, C.N., and Levinson, A.A. (2006). Pre-Columbian jadeite axes from Antigua, West Indies: description and possible sources. Canadian Mineralogist, 44, Krebs, M., Maresch, W.V., Schertl, H.-P., Baumann, A., Draper, G., Idleman, B., Münker, C., and E: Trapp. (2007). The dynamics of intra-oceanic subduction zones. A direct comparison between fossil petrological evidence (Rio San Juan Complex, Dominican Republic) and numerical simulation. Lithos, in press. Schertl, H.-P., Krebs, M., Maresch, W.V. and Draper, G. (2007). Jadeitite from Hispaniola: a link between Guatemala and Antigua? Abstract Volume, 20th Colloquium on Latin American Earth Sciences, Kiel, Germany: IGCP 546 Special Contribution 1. >> Activities >> Guatemala 2007 >>. 2/13

3 Erosional Disequilibrium Along Strike-slip Faults. A case study in Guatemala Brocard, G. (1), Teyssier C. (1), Authemayou C.(1), Simon-Labric T. (1) (1) Département de Géologie et Paléontologie, Université de Lausanne, Switzerland In this project we investigate how continuous movements along a strike-slip system generate and maintain irregular patterns of erosion that modify rock exhumation, fault movements, and deformation in fault zones. A variety of mechanisms can produce irregular erosion patterns, but in strike-slip environments, river diversion is one of the most common and powerful sources for these erosion patterns. Crustal anisotropy and localized exhumation along wrench zones (Tikoff and Teyssier, 1994) further maximize the effects of captures on rock denudation and deformation. We investigate the causes and effects of a large river capture on the final exhumation of rocks along the northern flank of the Sierra de Chuacùs-Sierra de las Minas orogen in Guatemala. The studied catchment has been selected among many potential sites for the quality of its markers and the large size of the captured area. The study is composed of a geomorphologic analysis (G. Brocard) and of a tectonic analysis (C. Authemayou, see following presentation). The geomorphic analysis aims at characterizing the shift in erosion pattern that followed the capture event: enhanced erosion in the captured catchment, and sharp decrease of erosion along the fault zone. Three main parameters are analyzed in the captured catchment: drainage rearrangement, catchment growth and catchment deepening. The spatial and temporal reconstruction is based on the study of newly discovered paleovalleys and their sedimentary infill. Drainage rearrangement operated by river lengthening, captures, and avulsions. Most of this reorganization was sparked by the diversion of the Chixóy River between 10 and 7.5 Ma ago, and induced a dramatic outward growth of the Chixóy River catchment, the demise of an ancestral transverse drainage and the development of new drains that converge towards the capture site. This expansion has slowed down but is still ongoing along the margins of the Chixóy basin. Spatially, it coincides with a vast zone of landscape dissection surrounded by the remnants of a pre-capture, more subdued topography. Temporally, the capture event coincides with the release of large amounts of sediments to the Macuspana Basin (SE Mexico). The capture event had reset the tectonic offset accumulated by continued displacement of the Chixoy River by the Polochic Fault. Tectonic activity since the capture event has created a new offset reach along which fluvial erosion is focused, whilst the fault segments drained by the Chixoy River before its diversion are now experiencing no erosion or sediment accumulation. This creates an alternating pattern of deep eroding valleys and shallow infilling valleys along the Polochic Fault. Surficial fault geometry seems to reflect the influence of this alternating pattern of topography and erosion rates. B. Tikoff, C. Teyssier (1994). Strain modeling of displacement field partitioning in transpressional orogens. Journal of Structural Geology, 16, IGCP 546 Special Contribution 1. >> Activities >> Guatemala 2007 >>. 3/13

4 The Nicoya Complex and other oceanic ocurrences of Costa Rica Percy Denyer (1), Esteban Gazel (2) and Guillermo Alvarado (1,3) (1) Escuela Centroamericana de Geología, Universidad de Costa Rica, P.O. Box , San José Costa Rica. (2) Department of Geological Sciences, Rutgers University, (3) Instituto Costarricense de Electricidad, One century of studies have been done on the Jurassic to Eocene complexes in Costa Rica. Those studies could be grouped in four main stage of knowledge: 1) from 1904 to 1957 were recognized the cherts, and the mafic and ultramafic igneous complexes, the first regional maps, and probably were the first to recognize of ellipsoidal basalts, now widely known as pillow lavas, 2) from 1958 to 1978, those rocks were seen under the precept of the association of ophiolites (serpentine, gabbro, diabase, basalts, and related rocks), and interpreted the radiolarites as deep-sea conditions. This stage is characterized by the seminal work of Gabriel Dengo and by the first geochemical analyses in the framework of the recent plate tectonics. 3) from 1979 to 1994, a huge amount of geochemical data, paleontological and K/Ar ages were produce, and it was the stage of more controversial papers, varied their interpretation for the same localitiy (i.e. Nicoya Peninsula) from a relative simple stratigraphic model to a very complex nappe slices, and from a simple tectonic evolution (in situ and formed by a mid-oceanic ridge volcanism) to a multistage evolution (terrains, and mid-oceanic ridge, aseismic ridge, intraplate and island arc volcanism). The situation was similar in the other Costa Rican oceanic complexes. 4) from 1995 to present, the panorama and mutual agreement between the different groups was more clear. This stage is characterized by joint collaboration, the use of modern laboratory techniques as Sr, Nd, and Pb isotopes, major, trace and rare elements, 40Ar/39Ar dating, and volcanological criteria, among detailed field mapping of some areas. The main new result of these studies was that the radiolarites ( Ma) in the Nicoya Peninsula were significatively older than the basic igneous rocks ( Ma), indicating a complex magmatic event intruding and erupting into the thick sedimentary sequence. For other areas, like Santa Elena Peninsula, Tortugal, Herradura and Quepos, the picture on these oceanic complexes are more or less clear, but it is not the case of Osa-Golfito-Burica area, in which more studies is necessary. In general, the detailed field mapping is a powerful tool in combination with the modern techniques. The similarity in age, petrochemistry and tectonic context for other oceanic complexes in Guatemala, Antilles and the north part South America is more than a coincidence, and they have similar evolution of the ideas, so in the problems. Therefore, a multidisciplinary study of the chrono and bio-stratigraphic relations, together with modern petrology, geochemical and micropaleontology approach is necessary in the all area to provide a solid basis for a more realistic plate tectonic reconstruction, and geologic history. The oceanic complexes of Costa Rica are different in their tectonic origins. Although, they are dismembered and disrupted, they are the only one available inland source of information to untangle the order of episodes of the puzzle history of this active margin. There are six main regions synthetically described in this paper. 1) Santa Elena Peninsula, constituted by the Santa Elena Nappe, formed in a supra-subduction zone setting and the Santa Rosa Accretionary Complex, which represents an igneous-sedimentary sequence of Aptian- Cenomanian age that include OIB segments. 2) The Nicoya Peninsula formed by a CLIP related sequence in which the magmatic processes had a preponderant influence in the emplacement and deformation history of it. 3) The Tortugal area formed by the Tortugal Suite with OIB signature and surrounding Nicoya Complex outcrops. 4) The Herradura Block divided in the Tulín Formation that corresponds to Maastrichtian to Lower Eocene OIB accreted oceanic island and the Nicoya Complex at the basement. 5) Quepos Block correlated with the Tulín Formation. 6) The Osa-Burica Block comprises three units: Golfito and Burica Terranes geochemical and chronological correlated to the Nicoya Complex, Rincón Block Early Paleocene to Early Eocene in age accreted seamounts, and the Miocene Osa-Caño Accretionary Complex. IGCP 546 Special Contribution 1. >> Activities >> Guatemala 2007 >>. 4/13

5 Siuna Serpentinite Mélange and El Castillo Mélange (Nicaragua): two examples of oceanic assemblages of the Mezquito Oceanic Terrenes Flores, Kennet (1), Baumgartner, Peter Olivier (1), Skora, Susanne (2), Baumgartner Lukas (2), Cruz, Daniel (3), Müntener, Othmar (2) David Buchs (1), Cosca, Mike (2) and Alexandre Nicolas, Bandini (1) (1) Institute of Geology and Paleontology (2) Institute of Mineralogy and Geochemistry, Anthropole, University of Lausanne, CH-1015 Lausanne, Switzerland (3) CIGEO, Universidad Nacional Autónoma de Nicaragua. P.O. Box A-131, Managua, Nicaragua The new Mesquito Oceanic Terranes (MOT) comprises the southern half of the Chortis Block (sensu classico) that has been assumed to be a continental fragment detached of the North American plate. The MOT are defined by 4 corner localities characterized by ultramafic and mafic oceanic rocks and radiolarites of Late Triassic-Jurassic age: 1. Siuna Serpentinite Mélange, 2. El Castillo Mélange, 3. The Santa Elena Complex and 4. DSDP Legs 67/84. The Siuna Serpentinite Mélange (SSM), located in northeastern Nicaragua, is a small tectonic window made of a pre-albian, N-S striking subduction-related mélange. The SSM largely consists of a serpentinite matrix that represents metamorphosed, Ca-depleted ultramafic rocks (harzburgites). Relict Cr-rich spinel (Cr # ) in serpentinites and chromite pods indicates a high degree of melting. The serpentinite matrix contains blocks of sedimentary and igneous origin. Late Bajocian-early Bathonian ( Ma) red, ribbon-bedded radiolarites are in sedimentary contact with greenstones (calc-alkaline metatuffs) and volcanic arenites. Middle Oxfordian to late Kimmeridgian/early Tithonian ( /148 Ma) black radiolarian-rich cherts, minor shales and siliceous mudstone blocks were found, as well as quartzites, metasandstones and riebeckite-rich metaturbidites. Furthermore, various metamafic rocks including gabbros and basalts can be found. This diverse assemblage indicates mixing of: 1. lower plate oceanic rocks, 2. primitive, oceanic arc rocks, and 3. detrital sediments derived from an evolved arc, in a subduction mélange. The metamafic rocks of the mélange exhibit assemblages corresponding to different metamorphic conditions. They range from typical greenschist and amphibolite facies assemblages to high pressure barroisite bearing greenschists. Possible blueschist to eclogite facies conditions are indicated by mica schist with silica-rich phengites and blocks containing garnet with inclusions of aegirine/omphacite. The phengites yielded a ± 0.4 Ma 40Ar/39Ar cooling age, indicating an Early Cretaceous exhumation of the subduction mélange. The overlap sequence documents Aptian/Albian deep water turbiditic sequences overlain by shallow water limestones (Cantarranas and Atima formations) associated with calc-alkaline andesitic lavas. El Castillo Mélange (ECM) is exposed NE and S of the town of El Castillo at the border between Nicaragua and Costa Rica. The latter was previously described as the San Juan peridiotites by Tournon et al. (1995). The ECM is a Mesozoic oceanic terrane largely composed of serpentinites, spinel peridiotites (Cr # Tournon et al., 1995) and Rhaetian ( Ma) red and green radiolarite blocks tectonically embedded in a serpentinite matrix. Tertiary turbidite sequences overlain by interbedded shallow water limestones, sandy limestones and volcaniclastic sandstones (Machuca and Venado formations) constitute the overlap sequence. The presence of oceanic remnants in NE and central S Nicaragua radically change the current concepts of plate boundaries of the western Caribbean plate: 1. The SSM and the ECM expose parts of a major zone of oceanic terranes of Pacific origin, the MOT, that are placed between the continental Chortis Block s. str. and the Caribbean Large Igneous Province (CLIP). 2. The SSM contains one of the oldest HP-LT suites of metamorphic terranes of the Circum-Caribbean realm. 3. The newly discovered Rhaetian radiolarian faunas correlate throughout Panthalassa and indicate that the ECM must be Pacific in origin, because its radiolarites predate Pangea breakup. Finally, 4. The discovery of the SSM and the ECM suggests that the Tertiary and Quaternary volcanic cover of Nicaragua hides an extensive collage of oceanic terranes that we call Mesquito Oceanic Terranes. Tournon, J., Seyler, M., and Astorga, A. (1995). Les péridiotites du Rio San Juan (Nicaragua et Costa Rica). Jalons possibles d une suture ultrabasique E-W en Amérique Centrale méridionale. C.R. Académie des Sciences Paris, t.320 serie IIa: IGCP 546 Special Contribution 1. >> Activities >> Guatemala 2007 >>. 5/13

6 History of subduction and collision: Margarita Island, Venezuela Maresch, W.V. Institute of Geology, Mineralogy & Geophysics, Ruhr-University Bochum, Bochum, Germany Pindell et al. (2005, 2006) used kinematic models and reconstructions to show in considerable detail how the Mesozoic separation of the Americas produced passive margins that were then overridden from west to east by allochthonous arc and oceanic complexes related to the "bow" of the progressing Caribbean plate (Fig. 1). These collision zones commonly entrain high-pressure rocks. Understanding the timing, origin and geodynamic setting of the high-pressure metamorphism recorded by these rocks is a key element in understanding Caribbean plate interaction with the Americas. Fig. 1: Relative eastward advance of the Caribbean plate (from Maresch and Gerya, 2005, after Pindell and Kennan, pers. commun., 2004) For the northern margin of South America, Margarita Island provides a representative sample of crust underlying as much as 70,000 km2 of coastal Venezuela (Stoeckhert et al., 1995). Margarita exposes Aptian-Albian oceanic crust as well as continental crust represented by Paleozoic basement with overlying Mesozoic sediments that were accreted and metamorphosed at high-pressure conditions ( C; kbar) in a collisional event constrained to the time interval Ma. Partial exhumation and emplacement into an intermediate to shallow crustal level in a transform plate-margin setting was followed by final uplift and cooling at c. 50 Ma. The well-constrained timing of high-pressure metamorphism on Margarita in conjunction with the documented advance of the overriding leading edge of the Caribbean plate (Fig. 1) indicate that collision and subduction must have occurred along the north-western margin of South America, with passive transport of Margaritan crust along the southern Caribbean margin since that time. This tectonic history is in marked contrast to that of other high-pressure rocks, well exposed along the northern Caribbean margin (e.g. Krebs et al., 2007), which represent obducted products of high-pressure material produced within the long-lived intra-oceanic subduction zone along the Caribbean bow itself. The Margarita HP collisional "event" represents an early stage of Caribbean-American interaction along the western margin of the Americas. Krebs, M., Maresch, W.V., Schertl, H.-P., Baumann, A., Draper, G., Idleman, B., Muenker, C., and Trapp, E. (2007). The dynamics of intra-oceanic subduction zones: A direct comparison between fossil petrological evidence (Rio San Juan Complex, Dominican Republic) and numerical simulation. Lithos, in press Maresch, W.V., and Gerya, T. (2005). Blueschists and blue amphiboles: How much subduction do they need? International Geology Review, v. 47, Pindell, J., Kennan, L., Maresch, W.V., Stanek, K.P., Draper, G., amd Higgs, R. (2005). Plate-kinematics and crustal dynamics of circum-caribbean arc-continent interactions. Geological Society of America Special Paper 394, p Pindell, J., Kennan, L., Stanek, K.P., Maresch, W.V., and Draper, G. (2006). Foundations of Gulf of Mexico and Caribbean evolution: eight controversies resolved. Geológica Acta, v. 4, Stöckhert, B., Maresch, W.V., Brix, M., Kaiser, C., Toetz, A., Kluge, R., and Krückhans-Lueder, G. (1995). The crustal history of Margarita Island (Venezuela) in detail: A constraint on the Caribbean plate-tectonic scenario. Geology, v. 23, IGCP 546 Special Contribution 1. >> Activities >> Guatemala 2007 >>. 6/13

7 SHRIMP RG U/Pb age of Chuacús complex zircon: Evidence for Cretaceous HP metamorphism in the Maya block. Martens, U (1), Liou, J (1), Solari, L (2), Mattinson, C (1), Wooden, J (3) (1) Dept. Geological & Env. Sciences, Stanford University, 450 Serra Mall, Bldg 320, Stanford, CA 94305, United States (2) Centro de Geociencias, UNAM, Campus Juriquilla, Queretaro, QRO 76001, Mexico (3) U.S. Geological Survey, 345 Middlefield road, Menlo Park, CA 94025, United States Amphibolite-facies metamorphic rocks in the Sierra de Chuacús, Central Guatemala, can be divided into two units based on structure and U/Pb geochronology. The El Tumbadero unit is composed of orthogneisses with Triassic U/Pb igneous ages, and pelitic schists, paragneisses, marbles and quartzites. Detrital U/Pb zircon geochronology of two metasedimentary samples implies post-ordovician and post-pennsylvanian deposition. The fabric of El Tumbadero unit consists of a single foliation parallel to axial planes of scarce isoclinal folds. Partial melting of orthogneisses generated pegmatites that are in part concordant and in part cut the dominant foliation. Metamorphic zircon rims from orthogneisses have ~75Ma U/Pb ages, and trace element patterns consistent with amphibolite-facies zircon recrystallization. Minor amphibolitized eclogites occur as m-sized lenses and interfoliated bands within Triassic orthogneisses. These are interpreted as mafic dikes metamorphosed to near-uhp conditions (~ C, ~22-24kbar) before crustal metamorphism and anatexis. In contrast, the El Chol unit exposes at least 3 generations of folds and foliations within banded gneisses, migmatites, and multiple generations of pegmatites; their latest foliation is parallel to the fabric of the El Tumbadero unit. Zircon U-Pb geochronology of El Chol mafic rocks reveals Pan-African protolith and Silurian- Devonian metamorphic rims. Refolded felsic bands have discordant U/Pb lower intercept ages of ~300Ma. The El Chol unit also contains eclogites as cm-scale irregular domains in amphibolite bands. A poorly constrained U/Pb age of an eclogitic zircon rim yielded a Late Cretaceous age. In summary, in the early Mesozoic the El Chol unit of the Chuacús complex constituted a metamorphosed basement over which the protoliths of the El Tumbadero unit were deposited and intruded. A late Cretaceous collision, possibly with the Cuban arc or the Chortís block, subducted both continental rock units to mantle depth. Subsequent exhumation to crustal level caused partial anatexis and ubiquitous amphibolite-facies overprint. IGCP 546 Special Contribution 1. >> Activities >> Guatemala 2007 >>. 7/13

8 The Chuacús Complex of Guatemala: deep and hot processes at the Caribbean-North American plate boundary Ortega Gutiérrez, Fernando (1), Solari, Luigi A. (2) (1) Instituto de Geología, UNAM, México DF, México (2) Centro de Geociencias, UNAM, Querétaro, México The oldest basement rocks of Nuclear Central America had been traditionally represented by the Chuacús group, which has been variously described as a high to medium-grade metamorphic unit formed by schist, gneiss, amphibolite and local migmatites. Because of this highly crystalline state, it was assigned a Precambrian or early Paleozoic age based on its apparent unconformable relationships with overlying fossiliferous rocks of the late Paleozoic Santa Rosa Group. Garnet, white micas, and kyanite were often mentioned as conspicuous phases in schists and gneisses, and even barroisite, and rutile were also noticed, but the temperature-pressure conditions were never quantified. Nevertheless, since the discovery of eclogite facies metamorphism as a common relict feature of the Chuacús Complex, and the application of modern analytical and thermobarometric techniques to these rocks (Ortega-Gutiérrez et al., 2004), unsuspected high temperatures and pressures were measured, opening new questions regarding the real tectonic meaning for the Caribbean evolution of these deep-seated rocks, which even showed intriguing petrologic evidence of having reached ultrahigh pressure conditions at temperatures in excess of 800º C. The metamorphic conditions attained and preserved locally in the Chuacús Complex include unusual assemblages and textural patterns that reinforce the possibility that the rocks were buried beyond the quartzcoesite boundary, and that partial melting at high pressure produced sodium-titanium rich trondhjemitic melts preserved within the eclogitic rocks as sodic amphibole-rutile-garnet-albite-quartz-zoisite/epidote ± biotite domains with igneous-like textures. Although UHPM coesite has not been identified yet as inclusions in appropriate phases (zircon, omphacite, garnet, epidote, and kyanite) of Chuacús Complex rocks, the following textural and mineralogical assemblages are not only consistent with such physical condition, but altogether support its former existence and possible actual presence. 1. Radial cracks in garnet, kyanite, and rarely staurolite around quartz inclusions, as well as conspicuous kink-bands nucleated on elongate tablets of quartz inclusions in kyanite. 2. Mantles of fibrous quartz (palisade-like) around quartz in impure metacarbonates with relict aragonite, white mica, dolomite, clinopyroxene, and rutile. 3. Lamellar inclusions of a SiO2 phase or plagioclase in rare omphacites. 4. Lamellar inclusions of rutile needles oriented in three different crystallographic planes within garnet. 5. Lamellar inclusions of rutile needles oriented in different crystallographic planes in zoisite and sometimes omphacite. 6. Polycrystalline (rutile, zircon, tourmaline, biotite, omphacite?) abundant inclusions in zircon. 7. Rutile lamellae in serpentinized olivine (?) in impure metacarbonates On the other hand, high temperatures in the Chuacús Complex are evidenced by sharp metamorphic banding, ubiquitous partial melting of metapelitic and mafic lithologies, as well as by phases such as Fe-Ti oxides with relict dense exsolution textures, and high-ti amphiboles and biotites. Subduction of continental edges to mantle depths where they undergo HP-UHPM recrystallization and their later exhumation require collision of two continental plates, which for the Chuacús Complex case probably involved the Maya and Chortís blocks during the Mesozoic evolution of the Caribbean-North America tectonic boundary, or during the assembly of Pangea. Ortega-Gutiérrez, F., Solari, L.A., Solé, J., Martens, U., Gómez-Tuena, A., Morán-Ical, S., Reyes-Salas, M., and Ortega-Obregón, C., 2004, High temperature eclogite facies in the Chuacús complex, central Guatemala: petrology, geochronology and tectonic implications: International Geology Review, v. 46, p IGCP 546 Special Contribution 1. >> Activities >> Guatemala 2007 >>. 8/13

9 From Aptian Onset to Danian Demise of Subduction along the Northern Margin of the Caribbean Plate (Sierra del Convento Melange, Eastern Cuba). García-Casco, Antonio (1), Lázaro, Concepción (1), Rojas Agramonte, Yamirka (2), Kroener, Alfred (2), Iturralde-Vinent, Manuel (3), Neubauer, Franz (4), Blanco Quintero, Idael (1), Núñez, Kenya (6) (1) Departamento de Mineralogia y Petrologia. Universidad de Granada. Avda. Fuentenueva s/n, Granada, Spain. (2) Institut fur Geowissenschaften, Universitat Mainz, Mainz, Germany. (3) Museo Nacional de Historia Natural. Obispo no. 61, Plaza de Armas, La Habana 10100, Cuba (4) Fachbereich Geographie, Geologie und Mineralogie, Universitat Salzburg, Hellbrunner Strasse 34, A Salzburg, Austria. (5) Instituto de Geología y Paleontología. Via Blanca y Carretera Central, La Habana, Cuba. The serpentinite-matrix mélange of the Sierra del Convento, eastern Cuba, represents an oceanic subduction channel related to Mesozoic subduction in the Caribbean realm which provides evidence for a long-lasting history of subduction, accretion, mélange formation, and uplift, and for Aptian onset of subduction in the region. Exotic blocks of MORB-derived plagioclase-free epidote±garnet amphibolite followed a hot subduction-related prograde P-T path, reaching ca. 750 ºC, and kbar at peak conditions. Fluid flux at this stage triggered melting of the amphibolites to yield peraluminous tonalitictrondhjemitic melts, which appear intimately associated with the amphibolites. Trondhjemitic-granitic varieties richer in K2O suggest the local participation of a sedimentary source, likely diluted through the infiltrating fluid. Calculated conditions for the magmatic assemblages (plagioclase, quartz, epidote, ±paragonite, ±pargasite, ±muscovite) of the siliceous rocks yield pressures of ca. 15 kbar, indicating crystallization at depth in the subduction environment. SHRIMP U-Pb zircon dating of two granitoid samples gives crystallization ages of Ma. Partial melting of subducted oceanic crust in eastern Cuba is unique in the Caribbean realm and is interpreted as the result of onset of subduction of young oceanic lithosphere during the Aptian (ca. 120 Ma), in agreement with regional geological data. Calculated P-T conditions for the retrograde blueschist-facies overprints present in all rocks indicate counterclockwise P-T paths during exhumation in a colder, syn-subduction scenario. Ar-Ar amphibole dating yielded two groups of cooling ages of Ma (interpreted as cooling of metamorphic/magmatic pargasite) and Ma (interpreted as growth/cooling of retrograde overprints). Fig. 11. T-t and P-t paths derived from geochronological and petrological data of the Sierra del Convento mélange and neighbouring geologic bodies. The above P-T-t data and additional ages of other rocks from the area suggest the following stages of evolution:(a) hot subduction during Ma with heating and burial rates of 150 ºC/Ma and 11 km/ma, respectively, developed shortly after onset of subduction of young oceanic lithosphere; (b) relatively fast near-isobaric cooling (25 ºC/Ma) during Ma, developed after accretion of the blocks to the upper plate mantle; (c) slow syn-subduction cooling (4 ºC/Ma) and exhumation (0.7 km/ma) in the subduction channel in a colder (mature) subduction environment during Ma, and d) fast cooling (70 ºC/Ma) and exhumation (5 km/ma) during Ma, when arccontinent collision occurred and subduction terminated in the region. IGCP 546 Special Contribution 1. >> Activities >> Guatemala 2007 >>. 9/13

10 The Rio San Juan serpentinite complex and its jadeitites (Dominican Republic) Schertl, H.-P (1), Maresch, W.V.(1) Krebs, M.(1), and Draper, G.(2) (1) Institute of Geology, Mineralogy and Geophysics, Ruhr-University Bochum, Bochum, Germany (2) Department of Earth Sciences, Florida International University, Miami, FL 33199, U.S.A. The Rio San Juan Complex (RSJC) of the northern Dominican Republic consists of subduction-related mafic schists which were cut by diapir-like serpentinite melanges. These mélanges contain knockers of various metamorphic rock types such as blueschists, eclogites, lawsonite blueschists, jadeitites, cymrite-bearing rocks, and orthogneisses. Comprehensive petrological studies demonstrate a broad diversity of PTt-paths which, however, are closely related. The mélanges very likely represent deep-seated roots of serpentinite mud volcanoes and seamounts, as observed on the sea floor within trenches of several present-day subduction zones. In the early stages of development of the subduction zone, the PTt-paths typically are anticlockwise with shallow ( hot ) P/T-gradients. Maximum PT-conditions derived from eclogites are 750 C/24 kbar; the related Lu-Hf-age is Ma (Grt-Ep-Amp-Omp-WR). Continuous cooling and steepening of the subduction zone PT-gradient is documented by omphacite blueschists, which experienced peak metamorphic conditions of C/16-18 kbar at 80.3 Ma (Rb-Sr on Phe-Amp-WR). Jadeite-blueschists typically are constrained to very steep cold P/T-gradients; Rb-Sr-ages (Phe-Amp-WR) of 62.1 Ma date the peak metamorphic conditions of about 380 C/18 kbar (see also Krebs et al., 2007). The jadeitites from the RSJC form irregular isolated masses as well as layers within surrounding lawsonite blueschists. The jadeitites are fine-grained, whitish-green, and contain jadeite as main constituent; the amount of jadeite may exceed 90 vol.%. Phengite, omphacite, epidote, Na-amphibole, plagioclase, and quartz occur in minor amounts. Lawsonite, pumpellyite, and stilpnomelane have locally also been observed; titanite and rutile are accessories. It is important to note that, in comparison to other blocks in the mélanges, the jadeite-rich rocks appear to have experienced a high degree of metasomatism during their formation. On the basis of these data, it may be concluded that the RSJC jadeitites are analogous in both mineralogy and fabric to jadeitites described from occurrences south of the Motagua Fault Zone in Guatemela (Harlow and Sorensen, 2005). More detailed information on the RSJ jadeitites, including first electron microprobe measurements, will be presented by Baese et al. (2007). Baese, R., Schertl, H.-P., Krebs, M., and Maresch, W.V. (2007). Mineralogy and petrology of Hispaniolan jadeitites: First results (this meeting). Harlow, G.E. and Sorensen, S.S. (2005). Jade (nephrite and jadeitite) and serpentinite: metasomatic connections. Int. Geol. Rev. 47: Krebs, M., Maresch, W.V, Schertl, H.-P., Baumann, A., Draper, G., Idleman, B., Münker, C., and E. Trapp (2007). The dynamics of intraoceanic subduction zones: A direct comparison between fossil petrological evidence (Rio San Juan Complex, Dominican Republic) and numerical simulation. Lithos doi: /j.lithos (available online). IGCP 546 Special Contribution 1. >> Activities >> Guatemala 2007 >>. 10/13

11 Timing of the HP metamorphism of the Escambray massif, Central Cuba Klaus Peter Stanek1 and Walter V. Maresch2 Institut für Geologie, TU Bergakademie Freiberg, Freiberg, Germany Institut für Geologie, Mineralogie und Geophysik, Ruhr-Universität Bochum, D Bochum, Germany In the Early Cretaceous, after separation of the North and South American continental plates, an intraoceanic island arc was initiated by subduction along the paleo-pacific-atlantic boundary. Today, remnants of the subduction-accretionary complex of this island arc are widespread throughout the Caribbean plate margins, and record the geotectonic history of this plate boundary. The subduction-accretion complex of the GAA follows the northern Caribbean suture, and in part crops out in dome-like massifs in the hinterland of the thrust front. The p-t-t-d data set from the Escambray Massif in Central Cuba comprises at least four metamorphic nappe sheets (Stanek et al. 2006), of which at least two clearly indicate highpressure (HP/LT) metamorphism. Boudins of eclogite and blueschist-facies rocks corroborate maximum conditions of kbar and C. An island-arc unit (Mabujina unit - MU) with LP/HT metamorphism of ~7 kbar at C was thrust over the HP-nappes. Dating of Late Cretaceous undeformed granitoids cross-cutting sheared MU and foliated granitoids allows deformation to be bracketed between 88 and 90 Ma (Grafe et al. 2001; Rojas-Agramonte et al. 2006). SHRIMP zircon ages of ~220 to 106 Ma likely date eclogite protolith. Conventional U/Pb and Lu-Hf data on metamorphic minerals gave an uniform time of about 70 Ma fort the peak metamorphism of the HP. The origin of the subducted rocks (south of Yucatán), the arc polarity, the timing of metamorphism, and preliminary paleomagnetic data support a Pacific origin of the Cretaceous GAA. Grafe, F., Stanek, K.P., Baumann, A., Maresch, W.V., Hames, C., Grevel, C., Millan, G. (2001), Journal of Geology, 109, Rojas-Agramonte, Y.; Kröner, A.; Garcia-Casco, A.; Iturralde-Vinent, M.A.; Wingate, M.T.D.; Liu, D.Y. (2006), Geophys. Res., vol. 8, García-Casco, A., Torres-Roldán, R.L., Iturralde-Vinent, M.A., Millán, G., Núnez Cambra, K., Lázaro, C., Rodríguez Vega, A. (2006), Geol. Acta, Barcelona, vol. 4, Stanek, K.P.; Maresch, W.V.; Grafe, F. Grevel, Ch.; Baumann, A. (2006), Geologica Acta, vol.4, Nr.1-2, IGCP 546 Special Contribution 1. >> Activities >> Guatemala 2007 >>. 11/13

12 Geological Evolution of the NW Corner of the Caribbean Plate Valls Alvarex, R.A.(1) (1) Nichromet Extraction Inc. The NW corner of the Caribbean Plate is complicated by the presence of a continental type block, the Chortis Block, within a mostly oceanic plate and a combination of a slip-strike boundary to the north running from the Belize-Guatemala border with a subduction zone to the west where the Cocos Plate is subducted beneath the Caribbean Plate, and an extinguished subduction zones to the north and south, were the Caribbean Plate was temporarily subducted beneath the Maya and Chortis Block. Fig. 1 The Chuacús Orogeny. The migration of the Chortis block in an S-SW and then N direction was one of the mechanisms responsible for the changes observed among the ophiolitic complexes in Guatemala. We are introducing the idea of the pre-existence of a trench associated with the Motagua-Jalomáx slipstrike fault system near the north border of Honduras, currently filled up and destroyed by the northward migration of the Chortis Block. Also we introduce the idea of an orogenic event - The Chuacús Orogeny - probably the same age as the Laramide Orogeny in North America. We postulate that the Chuacús Orogeny pushed younger ophiolites complexes in Guatemala to the surface and is responsible for the metamorphic basin of Central Guatemala - The Chuacús Series. The obduction of the oldest ophiolites on the western end of the belts may have being caused by the passing by of the Jamaica block on its way to its present position south of Cuba. Anderson, T.H. (1967). Geology of the central third of La Democracia quadrangle. M.A. Thesis, Austin, Texas University. Blount, D. (1967). Geology of the Chiantla quadrangle, Huehuetenango, Guatemala. Ph.D. Thesis, Lousiana State University. Boyd, A. (1966). Geology of the western third of La Democracia quadrangle. M.A. Thesis, Austin, Texas University. Davis, G.H. (1966). Geology of the eastern third of La Demcracia quadrangle. M.A. Thesis, Austin, Texas University. Giunta, G., et al., (2002). The Motagua Suture Zone in Guatemala, Field Trip Guide Book, publication supported by Ofioliti Int. Journal (M. Marroni and L. Pandolfi eds.). McBirney, A. R. (1963). Geology of a part of the Central Guatemalan Coordillera, in University of California Publication Geological Science, Vol. 38, pp. Millan, S.M. (1985), Preliminary Stratigraphic Lexicon North and Central Guatemala (a compilation under a contract with the United Nations Development Program). Vinson, G.L. (1962). Upper Cretaceous and Tertiary stratigraphy of Guatemala, in American Association of Petroleum Geologists Bulletin, Vol. 44, pp. Richard, H.G. (1963). Stratigraphy of the earliest Mesozoic sediments in southwestern Mexico and western Guatemala, in American Association of Petroleum Geological Bulletin, Vol. 47, pp. Sapper, K. (1937). Hanbuch der regionalen geologie: Mittelame-rika, Heidelberg, Bd. 8, Abt. 4a, Heft. 29. Termer, F. (1932). Geologie von Nordwest-Guatemala Zeit-Schr. Grssel: f. Erdkunde. Berlin, num 7-8, pp Walpe, J.L. (1960). Geology of the Coban-Purulha Area, Alta Verapaz, Guatemala, in Bulletin of the American Association of Petrology Geologists, Vol. 44, No. 8, pp. IGCP 546 Special Contribution 1. >> Activities >> Guatemala 2007 >>. 12/13