T11D-01 INVITED
Progress in our Understanding of High- and Ultrahigh-pressure Metamorphism
Fifty years ago on what we regarded as a relatively immobile Earth, the physical conditions and geologic environments that produced blueschists, eclogites, and garnet lherzolites, although surmised, were quantitatively unknown. With the advent of high-pressure synthesis equipment and more precise calorimetry, minerals such as jadeite, aragonite, pyrope, and dense polymorphs of silica and carbon were shown to be stable at elevated pressures and low-to-moderate temperatures. Geothermal gradients required by the P-T stability ranges of these phases are only explicable on a dynamic planet typified by mantle circulation and lithospheric subduction. Integration of experimental studies with plate tectonics has elucidated the genesis of both oceanic and continental crust. Combined with geochemical, geophysical, and isotopic data, high-pressure phase equilibria now are providing new constraints on the constitution and differentiation of the controlling mantle. Circumpacific-type blueschists and eclogites form in penetratively deformed allochthonous slabs and nappes that are overturned seaward, requiring profound descent of oceanic basement of at least 30-50 km during metamorphism. Neoblastic coesite and microdiamond inclusions in tough, rigid host minerals demonstrate that Alpine-type continental collision involves incomplete recovery of rocks subducted yet more deeply, up to 100-130 km. More surprising still, garnet lherzolites from the central Alps, east-central China, western Norway, and Bohemia display mineral intergrowths and exsolution lamellae reflecting the former presence of majoritic garnet and other ultrahigh-pressure phases, requiring depths of origin of host peridotites exceeding 300 km. Nano- minerals hold another key to quantifying the recovered depths of subduction and/or mantle return flow. Times of storage at great depth and exhumation rates remain current problems. Fluid-rock and lithosphere-asthenosphere interactions have recycled volatiles to the deep Earth through the subduction of both hydrous and nominally anhydrous phases. Mantle petrochemistry and the dynamics of plumes + plates control the evolving architecture of the crust and the dependent biosphere, hence investigations utilizing advanced technologies on condensed high- and ultrahigh-pressure materials will lead to a fuller understanding of the deep Earth in time and space
T11D-02 INVITED
Subduction Along the South Caribbean Deformed Belt: Age of Initiation and Backthrust Origin
The South Caribbean deformed belt (SCDB) represents a submarine prism formed at the interface between subducting oceanic material in the Colombian and Venezuelan basins and arc terranes along the northern edge of the South American continent. The SCDB and subducted Caribbean slab extends 1500 km from Panama to the Aves Ridge and is similar in scale to better studied magmatic subduction zones in the Lesser Antilles and Central America. The age of subduction of the Venezuelan basin at the SCDB has been examined by dating asymmetrical sediment wedges inferred to have formed above the bending and subducting slab of the Venezuelan basin. Ages are constrained by ties to DSDP wells drilled in the Venezuelan basin ~150 km north of the SCDB. Onlapping wedges are oldest in the western area near the Beata Ridge (middle Eocene), of intermediate age in the central area (early Miocene), and youngest in the east near the Aves Ridge (Mio- Pleistocene). Overlying younger wedges in all areas demonstrate that subduction continued following its initiation. Similar ages of deformed syn-thurst wedges are known from the now inactive continental foreland of northern South America in a belt from western Venezuela to Trinidad. Based on this correlation, we propose a simple back arc thrusting origin for the SCDB that is analogous to the eastern Sunda arc of Indonesia. The obliquity of convergence in the Caribbean case has produced a longlived, eastwardly-migrating backthrust that contrasts to more static backthrust in Sunda. The immediate onset of backthrusting following continental collision means that South America-Caribbean convergence was accommodated by newly organized and focussed subduction at the SCDB backthrust; the "collided" crust of intervening accreted arc terranes remains remarkably unaffected by regional shortening effects.
T11D-03
The Siuna Serpentinite Melange: An Early Cretaceous Subduction/Accretion of a Jurassic Arc
The Siuna Serpentinite Mélange (SSM), located in northeastern Nicaragua, is a small tectonic window of a large assemblage, the Mesquito Oceanic Terranes (MOT). The SSM was subducted during the Early Cretaceous at the border of continental Chortis Block (sensu stricto). The occurrence of the SSM is in conflict with the current plate tectonic schemes. The pre-Albian, N-S striking subduction-related mélange of SSM zone is exposed SW of the town of Siuna. It consists of metamorphosed, Ca-depleted ultramafic rocks. Relict Cr-rich spinel (Cr # 0.57-0.79) 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 (169-167 Ma) red- ribbon-bedded radiolarites are in sedimentary contact with greenstones (calc-alkaline metandesites) and volcanic arenites. Middle Oxfordian to late Kimmeridgian/early Tithonian (159-151/148 Ma) black radiolarian-rich cherts, minor shales and siliceous mudstones blocks were found, as well as quartzites, metasandstones and riebeckite-rich metaturbidites. Furthermore, various metamafic rocks including gabbros can be found. This indicates an oceanic setting for the mélange blocks. 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 yielding 139.2 ± 0.4 Ma 40 Ar/39Ar geochronology and blocks containing garnet with inclusions of aegirine/omphacite. Hence, the SSM resembles typical subduction zone mélanges (e.g. Franciscan, Motagua). It exposes parts of a major zone of oceanic terranes of pacific origin, the MOT, that are placed between the Chortis Block and the Caribbean Large Igneous Province (CLIP). The overlap sequence documents Aptian/Albian deep water turbiditic sequences overlain by shallow water limestones (Atima formation) associated with calc-alkaline andesitic lavas. The presence of oceanic remnants in NE Nicaragua (SSM) may radically change the current concepts of plate boundaries of the western Caribbean plate: 1-The SSM contain the oldest HP-LT suite of the circum-Caribbean metamorphic terranes, and 2- The boundary between the Paleozoic-Mesozoic Chortis Block and the oceanic terranes associated to the CLIP has been commonly placed as far south as the Costa Rica/Nicaragua border. A suture zone was interpreted to be aligned with the EW-trending Santa Elena Fault and the Hess Escarpment, based on outcrops of serpentinites that are found along the San Juan River (Costa Rica/Nicaragua border). However, newly discovered outcrops of serpentinites and oceanic sediments far north of the San Juan River suggests that Tertiary and Quaternary volcanism of Nicaragua may hide an extensive collage of oceanic terranes that we call Mesquito Oceanic Terranes.
T11D-04 INVITED
Geochemical Constraints on the pre-Cenozoic Subduction History of two Margins of the Chortis Block (Northern Central America)
The igneous forearc basement along the Pacific coast of northern Central America (between southern Mexico and Costa Rica) comprises a highly tectonized accretionary assemblage of igneous and ultramafic rocks. Volcanic and gabbroic rocks with primitive arc geochemical signatures formed between ~100 and ~180 Ma and are interpreted to have originated by arc magmatism resulting from subduction of the Pacific/Farallon plate. Additionally, the forearc contains geochemically enriched ocean island basalt (OIB)-like units that are interpreted as accreted seamounts and islands of a 100 to ~220 Ma old hotspot track, which most likely originated from a long-extinct hotspot located in the Pacific. Based on their combined Pb, Nd, Hf isotopic compositions an affiliation of these isotopically strongly enriched rocks with the Caribbean Large Igneous Province or the Galápagos hotspot appears unlikely. Rocks of similar age and geochemistry are exposed in the Santa Elena Peninsula of Costa Rica, suggesting that the same type of forearc basement is accreted to the continental Chortis block all the way from southern Mexico to Costa Rica. In contrast, gabbroic rocks of the Motagua suture zone in central Guatemala (El Tambor accretionary complex) show depleted MORB signatures and have igneous ages of ~130Ma. These MOR Gabbros were accreted on the current northern margin of the Chortis block during subduction of proto-Caribbean crust. In contrast, geochemistry of gabbros and diorites of the Sierra de Santa Cruz (SSC) indicate medium to high K-series arc affinity and their Ar/Ar ages range from ~75 to 130 Ma. The SSC is interpreted as the western extension of the early Cuban arc that collided with the Maya block in the latest Cretaceous. Arc-derived volcanic clasts were subsequently shed into the Paleocene Sepur Formation of the Maya block. No evidence of accretion of OIB-like material was found in the Motagua suture or the SSC so far. These new data suggest that proto-Caribbean crust, possibly generated during the opening of the Gulf of Mexico, was formed without the involvement of a mantle plume.
T11D-05
Upper mantle structure beneath the Caribbean – South American plate boundary from surface wave tomography
The shear wave velocity structure of the crust and upper mantle beneath the Caribbean-South American boundary was determined by analysis of fundamental mode Rayleigh waves in the 20-100s band that were recorded at the BOLIVAR/GEODINOS stations between 2003-2005. The 84 station array included 1 GSN station, 27 PASSCAL broadband (BB) seismographs, 15 OBSIP BB instruments and 8 Rice BB seismometers, as well as 34 BB stations of the Venezuelan national seismological network. Earthquakes in the distance range 20° to 60°, with magnitudes greater than 5.1, were used to invert for the upper mantle structure using the method of Forsyth and Li (2005). The model contains lateral variations that primarily correspond to tectonic provinces and plate boundaries. A clear linear velocity change parallels the plate bounding strike-slip fault systems along the northern coast of Venezuela, illustrating the differences between the continental lithosphere of the South American plate and the lithosphere of the Caribbean large igneous province. At depths up to 120 km beneath the Venezuelan Andes and the Maracaibo block there is evidence of underthrusting of the Caribbean plate a few hundred kilometers inland, but there is no other evidence of subduction of the Caribbean plate beneath the South American plate. Shallow low velocities associated with the basins near the coast are clearly imaged, as are higher velocities in the crust and upper mantle associated with the Guayana shield in southeastern Venezuela. The subducting Atlantic plate is imaged beneath the Antilles Arc as a shear tear where the subducting Atlantic plate tears away from the buoyant continental plate at the western end of the Caribbean-South American plate boundary. A low velocity "column" at the edge of the tear may be associated with asthenospheric flow from the Lesser Antilles arc mantle wedge. The complex structure of the plate boundary is best imaged and interpreted with three-dimensional modeling. We have combined the surface wave model, receiver functions, relocated local seismicity, and interpretations from active source profiling to more completely understand the geometry and upper mantle structure of the region.
T11D-06
From Aptian Onset to Danian Demise of Subduction along the Northern Margin of the Caribbean Plate (Sierra del Convento Melange, Eastern Cuba)
The serpentinite-matrix melange 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, melange 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 °C, and 14-16 kbar at peak conditions. Fluid flux at this stage triggered melting of the amphibolites to yield peraluminous tonalitic-trondhjemitic 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 113-114 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 106-97 Ma (interpreted as cooling of metamorphic/magmatic pargasite) and 87-83 Ma (interpreted as growth/cooling of retrograde overprints). 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 120-115 Ma with heating and burial rates of 150 °C/Ma and 11 km/Ma, respectively, developed shortly after onset of subduction of young oceanic lithosphere; (b) relatively fast near-isobaric cooling (25 °C/Ma) during 115-107 Ma, developed after accretion of the blocks to the upper plate mantle; (c) slow syn-subduction cooling (4 °C/Ma) and exhumation (0.7 km/Ma) in the subduction channel in a colder (mature) subduction environment during 107-70 Ma, and d) fast cooling (70 °C/Ma) and exhumation (5 km/Ma) during 70-65 Ma, when arc-continent collision occurred and subduction terminated in the region. http://www.ugr.es/~agcasco/igcp546/index.htm
T11D-07
Evolution of intrusive activity in the Greater Antilles: Zircon U-Th-Pb Ages for island arc granitoids in Puerto Rico, USA
Understanding the timing of island arc construction in the Greater Antilles is central to unraveling the complex history and evolution of the Caribbean plate. In this study, we present zircon U-Pb ages for oceanic island arc granitoids from Puerto Rico to address both the compositional variation and timing of intrusive activity along the northeast margin of the Caribbean plate. Prior geochronology studies of plutonic rocks in PR have used the K-Ar or Ar-Ar methods on minerals and WR, which are susceptible to thermal resetting (Cox et al., 1977; Smith et al., 1998). In this study 14 plutons were analyzed from the three main igneous provinces of Puerto Rico, the southern (SIP) central (CIP), and northeast (NIP) igneous provinces, and include the following stocks/batholiths: SIP: Las Tunas, Tibes, Tea Road; CIP: Utuado, Ciales, Morovis, Barranquitas, Coamo Arriba, Cuyon, Caguas, San Lorenzo, Vieques; NIP: Rio Blanco. While the igneous bodies vary in size (1 to 500 km2), whole-rock analyses show little variation, as the plutons are primarily intermediate in composition (56 to 66 wt. % SiO2), metaluminous (avg. A/CNK = 0.91, n=135), and range from diorite to granodiorite. Exposures of gabbros associated with the above plutons are rare. Zircons were mounted in epoxy rounds, and imaged for internal zoning characteristics prior to U-Pb analysis. Cathodoluminescence images show that the zircons are mostly oscillatory and/or sector zoned, and record simple growth histories, with no single grain demonstrating evidence of inheritance. Zoning patterns are similar within a population, but differ from among samples, regardless of WR composition or pluton size. Zircons were analyzed for age by MC-ICPMS at the Univ. of Arizona. Spot analyses were made on 20-30 grains per sample. The zircon U-Pb ages record a nearly continuous interval of Cretaceous to Eocene intrusive activity from 85 to 39 Ma. Three main pulses of magmatism are distinguished: (1) intrusion of small, intermediate to felsic stocks from 85-83 Ma (Santonian) in the CIP (n=3); (2) intrusion of large granodiorite batholiths from 75 to 67 Ma (Campanian/ Maastrichtian) in the CIP (n=5); and (3) intrusion of small intermediate stocks at ca. 48 Ma (Middle Eocene) in the CIP and NIP (n=3). The three small intermediate stocks from the SIP yield U-Pb ages of 79, 60, and 39 Ma, ages not recorded in the CIP and NIP. The difference in pluton crystallization ages suggests the SIP records a disparate history of arc evolution from the CIP and NIP. In both the CIP and NIP, an unexpected correlation was found between the average Th/U ratio for the population of zircons analyzed (20 to 30 grains per sample), and the composition of the arc basement intruded by the pluton. Plutons intruded into ¡®primitive island arc' rocks (the PIA series of Donnelly and Rogers, 1980) uniformly have avg. Th/U(zircon) ¡Ü0.45 (n=7); plutons intruded into younger calc-alkaline basement have avg. Th/U(zircon) >0.45 (n=6). The observed correlation of zircon Th/U composition with basement lithology occurs over the entire age range of plutons analyzed in the two provinces, irrespective of size and major element composition, and thus does not appear to be related to any temporal evolution or compositional changes in the granitoid source region through time. This unusual correlation suggests that the composition of the arc basement influenced the evolution of the granitoids, and is thus interpreted as a previously unrecognized record of crustal recycling during granitoid genesis in the Puerto Rico section of the Greater Antilles island arc.
T11D-08
Petrotectonic Evolution of the Chuac\'us Complex, Central Guatemala, by U/Pb Geochronology
Amphibolite-facies metamorphic rocks in the Sierra de Chuac\'us, 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 (~700-750°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 Chort\'is block, subducted both continental rock units to mantle depth. Subsequent exhumation to crustal level caused partial anatexis and ubiquitous amphibolite-facies overprint.