Blueschist Blocks Research Articles

Cataclastic deformation and metasomatism in the subduction zone of mafic blocks-in-mélange, San Simeon, California

The primary origin of shale-matrix mélange forming the Central Belt of the Franciscan Complex is still debated. The seacliff exposures near San Simeon display numerous lensoidal blocks of graywacke, greenstone, lesser layered chert, and blueschist that are encased in a matrix of scaly shale. The boudinaging of mafic greenstone and blueschist blocks was accommodated by cataclasis and varied amounts of metamorphic reconstitution that is particularly intense along block margins, zones of necking, and intra-block shear zones. Cataclastic zones in blueschists are greenish where chlorite and pumpellyite have formed. The mineralogic assemblage produced during cataclasis in greenstones is the same as that replacing the interior of the block: albite + chlorite + pumpellyite.The cataclastic margins of many (7 of 12) greenstones gained K, Rb, Cs, and Ba. In contrast, the cataclastic margins of all blueschist blocks lost Na, several (3 of 9) with substantial decreases, and some (3 of 9) had significant increases in K, Rb, Cs, and Ba. This metasomatic alteration is indicative of fluid-rock interaction between mafic blocks and sediments. Alteration of the mafic rocks with the growth of pumpellyite and chlorite occurred when blocks were encased in the shale matrix of the mélange, which was dynamically deforming and heated to temperatures of ~150 °C. As synsubduction temperature/depth gradients were ~10 °C/km, burial of mafic blocks while encased in the mélange matrix to depths approaching 15 km is apparent. Surficial olistostromal movements alone do not account for the incorporation, deformation, and retrograde metasomatic alteration of the exotic blueschist blocks and the metamorphism of the shale matrix at such inferred depths. These attributes indicate chemical exchange with shale and smearing displacement of the margins of mafic blocks occurred in the subduction zone as the shale matrix flowed and exerted shear tractions.

Tectonic slicing and mixing processes along the subduction interface: The Sistan example (Eastern Iran)

Suture zones preserve metamorphosed relicts of subducted ocean floor later exhumed along the plate interface that can provide critical insights on subduction zone processes. Mélange-like units are exceptionally well-exposed in the Sistan suture (Eastern Iran), which results from the closure of a branch of the Neotethys between the Lut and Afghan continental blocks. High pressure rocks found in the inner part of the suture zone (i.e., Ratuk complex) around Gazik are herein compared to previously studied outcrops along the belt. Detailed field investigations and mapping allow the distinction of two kinds of subduction-related block-in-matrix units: a siliciclastic-matrix complex and a serpentinite-matrix complex. The siliciclastic-matrix complex includes barely metamorphosed blocks of serpentinized peridotite, radiolarite and basalt of maximum greenschist-facies grade (i.e., maximum temperature of 340 °C). The serpentinite-matrix complex includes blocks of various grades and lithologies: mafic eclogites, amphibolitized blueschists, blue-amphibole-bearing metacherts and aegirine-augite-albite rocks. Eclogites reached peak pressure conditions around 530 °C and 2.3 GPa and isothermal retrogression down to 530 °C and 0.9 GPa. Estimation of peak PT conditions for the other rocks are less-well constrained but suggest equilibration at P < 1 GPa. Strikingly similar Ar–Ar ages of 86 ± 3 Ma, along ~70 km, are obtained for phengite and amphibole from fourteen eclogite and amphibolitized blueschist blocks. Ages in Gazik are usually younger than further south (e.g., Sulabest), but there is little age difference between the various kinds of rocks. These results (radiometric ages, observed structures and rock types) support a tectonic origin of the serpentinite-matrix mélange and shed light on subduction zone dynamics, particularly on coeval detachment and exhumation mechanisms of slab-derived rocks.

A reinterpretation of the metamorphic Yuli belt: Evidence for a middle-late Miocene accretionary prism in eastern Taiwan

The Yuli metamorphic belt has been the topic of petrological and geochronological studies for over 40 years and has been interpreted as a Cretaceous melange. Our study utilizes zircon U-Pb dating of schist and exotic blueschist blocks in the Yuli belt. These new ages indicate that these metamorphic rocks are actually middle Miocene in age and may represent the deeper structural levels of an accretionary prism. Several distinctive detrital zircon U-Pb age populations are recognized from 14 siliceous schists of melange-hosted rocks that are similar in age population to the Cretaceous, Eocene-Oligocene, and Miocene strata of Taiwan. The wide range of ages is interpreted as a product mixing of various sedimentary strata prior to metamorphism. Three blueschists of a volcanic-arc protolith enclosed within the host rocks yield crystallization ages of 15.4 ± 0.4, 15.5 ± 0.3, and 16.0 ± 0.2 Ma based on zircon U-Pb dating. In consideration of the new data regarding the Cretaceous-Miocene host rocks and the middle Miocene exotic blueschist blocks, it strongly suggests that the Yuli belt formed at deeper levels of an accretionary wedge during subduction of South China Sea oceanic crust at the middle-late Miocene. Subsequently, the rapid uplift of the metamorphic belt was probably related to doubly vergent wedge extrusion due to the Pliocene arc-continent collision.

Low-temperature blueschist-facies mafic blocks in the Franciscan mélange, San Simeon, California: Field relations, petrology, and counterclockwiseP-Tpaths

Dozens of mafic blueschist blocks are found in the Franciscan melange, which is well exposed along 6+ km of nearly continuous sea cliffs and wave-cut benches near San Simeon, California. Thirty-four blocks were studied to discover all the varieties in this classic locality of melange. The Na-amphibole and lawsonite ± epidote–rich schists were dynamically deformed, folded, and veined (including lawsonite and aragonite) before becoming incorporated into the shale-matrix melange. Tectonically driven flowage of the matrix caused all lithologic components to pinch and swell, forming boudins that separated to become isolated ellipsoidal blocks. This megascopically ductile style of deformation was accommodated in mafic blocks by cataclasis concurrent with alteration to chlorite and pumpellyite. Shearing and alteration were most intense along block margins and within block faults. Petrographic analyses, aided by backscattered electron imaging, microprobe mineral analyses, bulk chemistry, and thermodynamic calculations, reveal these blueschists traveled along a counterclockwise pressure-temperature ( P - T ) path. Na-amphiboles have actinolitic cores that indicate an early greenschist-facies stage. Peak T and P conditions for the epidote-bearing blueschists were ∼350 °C at pressures of 5–9 kbar. Many blocks have textural evidence of lawsonite replacing epidote and Na-amphibole with Fe 3+ -rich rims. This indicates epidote replacement occurred as the rocks cooled below ∼250 °C at ∼5 kbar. It is evident that the phase of dynamic epidote-blueschist-facies recrystallization that nearly obliterated evidence of an earlier greenschist-facies stage was followed by retrograde recrystallization under much less dynamic to nearly static, but still high- P , conditions. Cooling and much of the observed veining must have happened after these rocks were deeply underplated along the bottom of the ophiolitic leading edge of the North American plate. Pieces of the underplated blueschist terrane were probably detached from the hanging wall as slabs that boudinaged and reboudinaged while entrained in shale-matrix melange upwelling from depths of at least 15 km.

Actinolitic rinds on low-T mafic blueschist blocks in the Franciscan shale-matrix mélange near San Simeon: Implications for metasomatism and tectonic history

In the mélange terrane of the Franciscan accretionary prism, Mg-rich actinolite/tremolite “rinds” have long been known to be present on many high-T, garnet ± epidote-bearing, mafic blueschist and eclogite blocks, but they were thought to be absent on low-T lawsonite + chlorite blueschist blocks. By analogy with rinds on high-T blocks, these rinds on low-T blocks must have been produced by metasomatic reaction with ultramafic rock. The rinds on high-T blocks have led to the speculation that the high-T Franciscan blueschists were uplifted to the surface in serpentinite diapirs, while other processes exhumed the volumetrically more numerous low-T blocks. This paper reports the discovery of actinolitic rinds on the margins of nine low-T, lawsonite ± epidote Franciscan blueschists that are, or were, clearly encased in the shale-matrix mélange exposed in seacliffs and wavecut benches near San Simeon, California. Petrographic and geochemical analyses show these Mg-rich rinds are similar to those found on high-T garnet-bearing blocks.The bulk composition indicates the protolith of the blueschist blocks was mafic oceanic crust. The high Fe3+/∑Fe contents indicate it was probably the uppermost hydrated volcanic part of the crust. The block/ultramafic rock association was created by faulting during subduction initiation as fragments from the top of the subducting ocean crust were imbricated with mantle materials at the base of the ophiolitic leading edge of the North American plate. The chemistry of a block margin altered to actinolitic rind indicates Si, Na, Pb, Cu, and Sr were lost from the blocks while K, Rb, Cs, and Ba were added. Most pieces of actinolitic rind are Mg- and Ni-rich because they are metasomatized ultramafic material that was enriched in Si, K, Rb, Cs, and Ba. These metasomatic changes indicate the fluids that hydrated the mantle and altered the mafic blocks ascended from subducting and/or underplated sediments. Both high-T and low-T blueschists that have actinolitic rinds were uplifted to near the surface after they sank or were plucked from the base of the overriding plate by subduction-generated mélange that upwelled from depths of >15 km.

First 40Ar-39Ar Ages from Low-T Mafic Blueschist Blocks in a Franciscan Mélange near San Simeon: Implications for Initiation of Subduction

The Franciscan Complex of California is a type example of an accretionary prism with widespread high-P/T subduction-zone metamorphism. Low-T, fine-grained, lawsonite-bearing mafic blueschists encased in a shale-matrix mélange near San Simeon have been dated for the first time. 40Ar-39Ar ages were obtained for phengite separated from four blocks and from an actinolitic rind on the margin of one of the blocks. The three blocks with lawsonite + epidote yield ages of between 154 and Ma, while the actinolitic rind yields an age of Ma. These ages are from the part of the Franciscan Complex west of the San Andreas Fault that moved northward at least 300 km with respect to the extensive Franciscan exposures east of the fault. The ages obtained in this study show that some fine-grained, low-T (lawsonite + epidote) Franciscan blueschist blocks formed at the same time as coarse-grained, high-T (garnet ± epidote) Franciscan blueschist blocks. These dated rocks indicate that both high- and low-T mafic blueschists are coeval and probably formed along 1000+ km of the North American Plate margin at ∼160–155 Ma. The episode of dynamic blueschist metamorphism was soon followed by high-Mg rind formation and a long period of nearly static high-P/T conditions.

New occurrences of jadeitite, jadeite quartzite and jadeite-lawsonite quartzite in the Dominican Republic, Hispaniola: petrological and geochronological overview

New occurrences of jadeitite and jadeite-rich rocks have been discovered in the Rio San Juan Complex (RSJC) of the northern Dominican Republic in serpentinite melanges associated with a former intra-oceanic subduction zone. Allochthonous blocks in lag deposits developed on the melange outcrops or boulders in river beds are common. A very unusual feature for the RSJC is the occurrence of concordant layers and discordant veins of cm to dm thickness in blocks of jadeite±lawsonite- or omphacite-garnet-bearing blueschist of the melange. Two suites of jadeite-rich rocks can be recognized. The first is represented by quartz-free jadeitite s.str . (>90 vol% jadeite) found so far only as blocks and boulders. The second suite comprises quartz-bearing jadeitite s.str . grading into jadeitite quartzite (JQ), jadeite-lawsonite quartzite (JLQ) and jadeite-free lawsonite quartzite (LQ). The second suite is found both as blocks and boulders as well as layers and veins in blueschist blocks. One single occurrence of a cross-cutting omphacitite vein in blueschist has also been observed. Additional important phases so far found in both suites are omphacite, phengite, glaucophane, epidote, albite, calcite, titanite and zircon. Apatite and pumpellyite have only been identified in quartz-free jadeitite s.str .; almandine-rich garnet has so far been observed only in JLQ. The two suites of jadeite-bearing rocks occur in various shades of green, are fine- to coarse-grained, and usually equigranular. Mineral distribution is commonly homogeneous, but may be patchy in JLQ, giving this rock type a distinctly mottled appearance. Cathodoluminescence (CL) images show oscillatory zoning patterns in jadeite, zircon, apatite and calcite; this is evidence for crystallization from an aqueous fluid under open-system conditions. Zircons separated from a sample of quartz-free jadeitite s.str . contain primary inclusions of high-pressure matrix minerals such as jadeite and omphacite, indicating coeval zircon growth. The cores of the zircons yield ages of 114.9 ± 2.9 Ma, thus defining a crystallization age close to the initiation of subduction in the Rio San Juan Complex, when “warm” geotherms of ≈15°/km prevailed. These ages are in contrast with the crystallization ages of the blueschists hosting the second, quartz-bearing suite of jadeite-rich rocks. These range from 80 to 62 Ma, towards the end of subduction-zone activity at ≈55 Ma and “cool” geotherms of 8–9 °C/km. For the younger quartz-bearing suite, the combination of phengite compositions with the available P-T-t paths of the host blueschists suggests crystallization temperatures of ≈ 350 to ≈ 500 °C at minimum pressures of 15–16 kbar. The P-T conditions for the older quartz-free suite are more difficult to constrain, but the combination of phengite compositions with the prevailing geotherms in the young and warm subduction zone suggest minimum conditions of at least 500 °C and 11 kbar. However, temperatures and pressures as high as 600 °C and 15 kbar, as documented for jadeitites of similar age in the same subduction zone exposed in neighbouring eastern Cuba, are possible. Jadeitites and jadeite-rich rocks of the RSJC are thus interpreted to have crystallized over a time-span of ≥ 60 Myr at initial temperatures of at least 500 °C, later evolving down to 350 °C in a single, thermally self-organizing, cooling subduction zone. The P-T conditions suggested for the younger quartz-bearing suite correlate well with those of jadeitite formation in Guatemala south of the Motagua Fault Zone, the only other occurrence world-wide where jadeitite with both lawsonite and quartz appears to be common. Further evidence is needed to corroborate that the older quartz-free suite represents another example of rare high-temperature jadeitite as documented in Cuba.

Two high-pressure–low-temperature serpentinite-matrix mélange belts, Motagua fault zone, Guatemala: A record of Aptian and Maastrichtian collisions

Left-lateral motion along the North American-Caribbean plate boundary has juxta- posed two high-pressure-low-temperature (HP-LT) belts from separate Cretaceous colli- sions. These two belts have quite different ages and different suites of high-pressure as- semblages, yet they both contain jadeitite, a relatively rare rock type. This part of the plate boundary zone follows the Motagua River Valley in Guatemala, where it separates the Maya block (North American plate) from the Chortis block (Caribbean plate). On both sides of the bounding Motagua fault, tectonic slices of serpentinite-matrix melange host the HP-LT rocks. South of the fault, the melange slices contain eclogite, lawsonite eclogite, glaucophane eclogite, and blueschist blocks. North of the fault, the melange slices contain omphacite metabasite, albitite, and garnet amphibolite blocks, but lack intact eclogite. In addition to the dissimilar rock assemblages, 40 Ar/ 39 Ar geochronology of phen- gitic micas yields 77-65 Ma for northern and 125-113 Ma for southern blocks. These data suggest that the southern belt formed during Early Cretaceous (Aptian), northeastward- dipping subduction of the Farallon plate and collision of the Chortis block with western Mexico. The block was then displaced southeastward along this suture. In contrast, the northern belt records subduction related to the Maastrichtian collision of an extension of the Chortis block, perhaps the Nicaraguan Rise, with the Maya block.

Occurrence and significance of blueschist in the southern Lachlan Orogen

Serpentinite/talc‐matrix mélanges, bearing blocks of blueschist metavolcanics, occur within the Heathcote and Governor Fault Zones of the southern Lachlan Orogen. In the Heathcote Fault Zone, serpentinite‐matrix mélange consists of blocks or small pods of boninite, andesite, ultramafic rocks, chert and volcanogenic sandstone variably metamorphosed to prehnite‐pumpellyite, greenschist, or greenschist to blueschist facies. In the Governor Fault Zone, blueschist metavolcanics occur as blocks within serpentinite/talc matrix that is interleaved with prehnite‐pumpellyite to greenschist facies, intermediate pressure slate and phyllite. Ar/Ar dating of white mica from slaty mud‐matrix (broken formation) indicates that the main fabric development occurred at 446 ± 2 Ma. U–Pb (SHRIMP) dating of titanite from blueschists in the Governor Fault Zone indicates that metamorphism occurred at approximately 450 Ma, close to the time of mélange formation. Previously published, Ar/Ar dating of white mica from phyllite and biotite from metadiorite in the Heathcote Fault Zone suggest that blueschist metamorphism occurred at a similar time. These ages are supported by field relationships. Illite crystallinity and b0 data from white mica, and the preservation of blueschist blocks indicate that these fault zones maintained low temperatures both during and after intermediate‐ to high‐pressure metamorphism. Occurrences of blueschists in the Arthur Lineament of the Tyennan (Delamerian) Orogen in Tasmania, and in the New England Orogen, have different ages, and in conjunction with the occurrences described here, suggest that subduction‐accretion processes contributed significantly to the development of the Tasmanides from Cambrian through to Carboniferous times.

El Complejo Metamórfico Bahía Mansa en la cordillera de la Costa del centro-sur de Chile (39°30'-42°00'S): geocronología K-Ar, 40Ar/39Ar y U-Pb e implicancias en la evolución del margen sur-occidental de Gondwana

El Complejo Metamorfico Bahia Mansa (CMBM) esta formado por esquistos peliticos, metagrauvacas y esquistos maficos de afinidades oceanicas, con menor proporcion de cuerpos maficos y ultramaficos tectonicamente emplazados, milonitas a ultramilonitas y escasos cuerpos intrusivos traquiticos e intercalaciones de metaignimbritas. Las asociaciones de minerales metamorficos observados tanto en las secuencias metapeliticas como en las metavolcanicas indican que todo el conjunto litologico fue afectado por una fase de deformacion y metamorfismo principal (D2) en la facies de esquistos verdes, de caracter ductil y penetrativo, que ha obliterado completamente las caracteristicas originales de las rocas, exhibiendo ellas una tipica fabrica caracterizada por el desarrollo de foliacion (S2). La presencia local de anfibolas azules en esquistos maficos, de glaucofano en bloques de esquistos azules y de zussmanita en esquistos peliticos ricos en hierro, senalan la existencia de una fase de deformacion y metamorfismo previa (D1) en condiciones de alto gradiente P/T, en la facies de esquistos azules, actualmente evidenciada por texturas relictas (S1), tales como micropliegues y foliacion interna en albita, microfoliacion intrafolial y pliegues isoclinales desraizados de cuarzo. Las edades de enfriamiento 40Ar/39Ar y K-Ar en minerales indican que la deformacion y metamorfismo principal ocurrio durante el Permico-Triasico, en el lapso 260-220 Ma, mientras que la fase de alta presion-baja temperatura lo hizo, probablemente, durante el Carbonifero Superior, en el lapso 320-300 Ma. Una edad U-Pb en circones de 396 Ma obtenida en un cuerpo traquitico, que intruye a esquistos maficos, entrega indirectamente una edad minima devonica inferior para una porcion de estas rocas. Edades U-Pb en circones detriticos senalan edades maximas de deposicion en el Devonico Medio y el Permico Inferior para algunos componentes sedimentarios del complejo, lo cual implica que el metamorfismo y deformacion en la facies de esquistos azules habria afectado solo a las rocas con edades maximas de deposicion mas antiguas. Asi, en el CMBM se verifican, al menos, dos episodios de sedimentacion y similar numero de eventos de deformacion y de metamorfismo. El CMBM habria evolucionado durante el lapso Devonico-Triasico en el borde sur-occidental de Gondwana. Mas al sur, otros complejos metamorficos habrian evolucionado en el Triasico-Jurasico y en el Jurasico-Cretacico, lo cual implica un diacronismo en la acrecion en el borde continental de Gondwana y Sudamerica.

Blueschist‐facies metamorphism during Paleozoic orogeny in southwestern Japan: Phengite K–Ar ages of blueschist‐facies tectonic blocks in a serpentinite melange beneath early Paleozoic Oeyama ophiolite

Blueschist‐bearing Osayama serpentinite melange develops beneath a peridotite body of the Oeyama ophiolite which occupies the highest position structurally in the central Chugoku Mountains. The blueschist‐facies tectonic blocks within the serpentinite melange are divided into the lawsonite–pumpellyite grade, lower epidote grade and higher epidote grade by the mineral assemblages of basic schists. The higher epidote‐grade block is a garnet–glaucophane schist including eclogite‐facies relic minerals and retrogressive lawsonite–pumpellyite‐grade minerals. Gabbroic blocks derived from the Oeyama ophiolite are also enclosed as tectonic blocks in the serpentinite matrix and have experienced a blueschist metamorphism together with the other blueschist blocks. The mineralogic and paragenetic features of the Osayama blueschists are compatible with a hypothesis that they were derived from a coherent blueschist‐facies metamorphic sequence, formed in a subduction zone with a low geothermal gradient (~ 10°C/km). Phengite K–Ar ages of 16 pelitic and one basic schists yield 289–327 Ma and concentrate around 320 Ma regardless of protolith and metamorphic grade, suggesting quick exhumation of the schists at ca 320 Ma. These petrologic and geochronologic features suggest that the Osayama blueschists comprise a low‐grade portion of the Carboniferous Renge metamorphic belt. The Osayama blueschists indicate that the ‘cold’ subduction type (Franciscan type) metamorphism to reach eclogite‐facies and subsequent quick exhumation took place in the northwestern Pacific margin in Carboniferous time, like some other circum‐Pacific orogenic belts (western USA and eastern Australia), where such subduction metamorphism already started as early as the Ordovician.

Fluid flow in subduction zones: evidence from Nd- and Sr-isotope variations in metabasalts of the Franciscan complex, California

Greenstone, blueschist and eclogite metabasaltic blocks from the Franciscan complex of California preserve extensive petrographic and chemical evidence for interaction with hydrous fluids at high-P, low-T metamorphic conditions. The Nd and Sr isotope variations within and among the blocks constrain the origin of the basaltic protoliths, the nature of the fluid metasomatism that occurred within the upper levels (15–45 km) of the paleosubduction zonc, and the character and provenance of the rock that generated the hydrous fluids within the paleosubduction zone. Samples with little or no petrographic evidence of retrograde alteration and unaltered garnet separates have ɛNd. With increasing degrees of retrograde alteration, Nd isotope compositions are consistently lower, ranging down to ɛNd(160)=5. Actinolitic alteration rinds which are present on some blocks have the least radiogenic compositions with ɛNd=1.6 to 6.1. While Nd isotope compositions of unaltered blockes are in the range expected for basalt derived from normal depleted mantle, the Sr isotope compositions are more radiogenic ranging from ɛSr(160)=−5 to +11. Compositions of unaltered eclogite and blue-schist blocks are consistent with a protolith origin in normal oceanic crust derived from depleted mantle. The Sr isotopy systematics indicate that the protoliths were modified by seawater alteration in an ocean-floor hydrothermal system. Isotopic compositions of samples from parts of blocks that have a retrograde metamorphic overprint show a strong correlation between less radiogenic Nd compositions and the extent of retrograde metamorphism. Maximum Nd isotope ratios of the metasomatizing fluid are provided by analyses of actinolitic rinds, and range from ɛNd(160)=1.6 to 6.1. A possible source for fluids of this composition is subducted sediment that was derived from a continental craton. Because rind formation occurred while the basaltic blocks were within an ultramafic matrix, the fluids must have migrated from sediments in the accretionary wedge into an overlying wedge of mantle material imbricated with blocks of oceanic crust. This suggests possibly km-scale movement of fluids that carry an amount of the rare-earth elements sufficient to significantly modify the trace-element budget of subducted basalt.

The P-T-t history of blocks in serpentinite-matrix mélange, west-central Baja California

Thermochronologic, petrologic, and geochemical analyses of epidote-amphibolite-, amphilbolite-, eclogite-, and blueschist-facies blocks in serpentinite-matrix melanges from East San Benito Island and Cedros Island, west-central Baja California, provide constraints on the P-T-t history of this disrupted terrane. Results of {sup 40}Ar/{sup 39}Ar step heating experiments on minerals separated from these blocks vary according to metamorphic grade and indicate different P-T-t histories. Mid-Jurassic (160-170 Ma) epidote-amphibolite- and amphibolite-facies blocks were probably derived from oceanic crust and associated sediments that were metamorphosed during limitation of subduction. Coarse-grained blueschist blocks were likely metamorphosed during continued subduction in late Early Cretaceous time (that is, 115-100 Ma). Some epidote-amphibolite blocks are partially overprinted by blueschist-facies mineral assemblages and apparently record both metamorphic events. Fission-track analyses of apatites indicate that the blocks underwent significantly different post-metamorphic cooling histories. Epidote-amphilbolite and amphibolite blocks cooled below {approximately}100 {degrees}C form mid-Jurassic to Paleocene time; blueschist blocks cooled below {approximately}100 {degrees}C in Oligocene-Miocene time. In general, mafic blocks have trace-element concentrations and REE patterns characteristic of ocean-flood basalts. This study demonstrates that samples with very different P-T-t histories can evidently occur over relatively small length scales (<<1 km) in serpentinite-matrix melanges. 67 refs., 16 figs., 4 tabs.

Geochronology of Blueschists from West-Central Baja California and the Timing of Uplift in Subduction Complexes

A geochronologic study of blueschists from west-central Baja California has provided constraints on the timing of high-pressure, low-temperature metamorphism and the subsequent uplift of this subduction complex. Mineral separates from coherent blueschists and blueschist blocks enclosed in serpentinite melange were analyzed by {sup 40}Ar/{sup 39}Ar, fission track, and Rb/Sr methods. {sup 40}Ar/{sup 39}Ar age spectra for white mica separated from four blueschist blocks and an overprinted eclogite block in serpentinite-matrix melange show slow cooling gradients between 115-95 Ma. Rb/Sr apparent ages for one block are concordant with these results. {sup 40}Ar/{sup 39}Ar ages of blue amphibole are primarily the result of the degassing of white mica intergrowths as confirmed by microprobe analyses. Apatite fission track ages indicate a blueschist block and an eclogite block cooled to a temperature below {approximately}100 C at 22 Ma and 32 Ma, respectively. Subduction-related metamorphism of the coherent blueschists occurred in late Early Cretaceous time, based on a {sup 40}Ar/{sup 39}Ar age or 109 Ma for metamorphic white mica from a metasandstone. {sup 40}Ar/{sup 39}Ar analysis of partially albitized K-feldspar from a plutonic clast in a metaconglomerate indicates this sample cooled to a temperature of {approximately} 145 C at approximately 20 Ma. Geochronologic and petrologicmore » data for coherent blueschists indicate an average uplift rate of 0.1 mm/yr for one portion of the subduction complex. The relatively slow uplift rate and lack of any higher temperature overprinting assemblages in the coherent blueschists suggest that synsubduction uplift was gradual and proceeded through a dynamic accretionary wedge characterized by low geothermal gradients. An increase in uplift rate (to 1.0 mm/yr) during post-Miocene time coincides with a change from a convergent to a transform plate boundary.« less

Distribution and Age of High-Grade Blueschists, Associated Eclogites, and Amphibolites from Oregon and California

Isolated blocks of high-grade blueschist and amphibolite facies metamorphic rocks occur within the Jurassic and Cretaceous eugeosynclinal deposits of the Coast Ranges of southwestern Oregon and California. The blocks range in size from individual rock masses commonly 5 to 1,000 ft in diameter to a few larger masses as much as 7 mi long and 2 mi wide. The high-grade blocks are predominantly basaltic in composition and include glaucophane schists, eclogites, and gneissic rocks of the amphibolite facies. Field relationships indicate that the blocks are closely associated with serpentine, that high-grade blueschist and amphibolite blocks, lower grade blueschists, volcanic rocks, and cherts occupy disturbed zones that may be related to thrusting, and that there is no exposed in situ provenance for the high-grade blueschists, eclogites, and amphibolites. Potassium-argon mineral ages of white mica and actinolite from the blueschists and of hornblende from the amphibolites indicate that these minerals crystallized approximately 150 m.y. ago, but the ages measured on glaucophane from the blueschist blocks are commonly younger. These data suggest that the high-grade blue-schist and amphibolite blocks represent fragments of a cryptic metamorphic terrane of pre-Tithonian age that have been tectonically mixed with younger rocks of the Franciscan Formation in California and Otter Point Formation in Oregon. The younger ages for glaucophane probably reflect metamorphic episodes in which lower grade in situ blueschist facies mineral assemblages were developed in the blocks after their emplacement within the Franciscan Formation. This pre-Tithonian cryptic metamorphic terrane probably developed as a result of interaction between oceanic and continental plates. The occurrence of tectonic blocks of this terrane within melange zones in Oregon and California may be related to later plate interaction.