Virginia B. Sisson

Boron depletion during progressive metamorphism: Implications for subduction processes

Systematic differences in trace element compositions of volcanic arc versus intraplate magmas are commonly attributed to involvement of subducted sediments and/or seawater-altered oceanic crust in arc magma genesis. The exact contributions from these materials is poorly understood. Metamorphic processes may significantly modify subducted oceanic slabs before they reach subarc depths. The nature and magnitude of such compositional modifications, as exemplified by the element boron, are investigated in metamorphic suites having protoliths analogous to subducted marine sediments and basalts. Our results indicate that in addition to protolith type, B content depends on metamorphic temperature. Greenschist and amphibolite facies metasediments generally have B contents at least a factor of two lower than in equivalent unmetamorphosed sediments. Moreover, four low-pressure pelitic to semipelitic metasediment suites display progressive loss of B with increasing peak metamorphic temperature (ca. 200–750°C). Metabasalts of greenschist and higher grades also have consistently lower B contents (</ 10 ppm) than estimated for altered oceanic crust. Granulites and migmatites are invariably depleted in B (typically < 3 ppm). Most subducted B initially resides in marine sediment and altered oceanic crust (i.e., within the uppermost kilometer). Our observations suggest that B may be extracted by fluids released by devolatilization reactions upon progressive heating during subduction. The relatively high B contents observed in many arc basalts imply that slab temperatures at subarc depths commonly do not exceed conditions at which B is systematically depleted. Otherwise, subducted slabs would retain insufficient quantities of B to balance its inventory in arc magmas. Using the temperature dependence of B depletion estimated from metamorphic suites, maximum temperatures are unlikely to exceed ∼ 800–900°C for uppermost portions of subducted slabs beneath arcs which show typical B enrichments. In contrast, arc basalts commonly have eruptive temperatures approaching the dry liquidus (ca. 1300°C) of subducted oceanic crust. By the time such temperatures are reached in subducted slabs, it is likely that H2O, B and other fluid-mobile elements are strongly depleted by metamorphic processes. Thus, slab melting does not appear to be a viable process for generating typical arc basalts. A more plausible scenario that is commonly invoked for production of arc basaltic magmas involves migration of aqueous fluid and fluid-mobile elements from the slab to hotter regions in the mantle wedge. Such fluids could metasomatize portions of the mantle wedge and induce melting to produce B-rich arc magmas.

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.

Geologic consequences of plate reorganization: An example from the Eocene southern Alaska fore arc

Observation of relative timing of deformation, metamorphism, and plutonism in a high-temperature-low-pressure metamorphic belt in the eastern Chugach Mountains of Alaska leads to a model of ridge subduction followed by plate reorganization to account for the abnormally high geothermal gradients in the fore arc. Between 56 and 53 Ma, a change in the direction of the Kula-Farallon spreading halted the previous southward migration of the Kula-Farallon-North American triple junction. This forced the triple junction to migrate back north along the plate margin, enlarging the slab window beneath the accretionary margin. The expanded slab window produced a large-scale thermal manifestation now recognized as the Chugach metamorphic complex. The accretionary complex responded to plate reorganization by orogen-parallel extension associated with oblique subduction of the Kula plate. Contraction began again following passage of the triple junction and subduction of the Farallon plate.

Introduction: An overview of ridge-trench interactions in modern and ancient settings

Virtually all subduction zones eventually interact with a spreading ridge, and this interaction leads to a great diversity of tectonic processes in the vicinity of the triple junction. In the present-day Pacifi c basin, there are seven examples of active or recently extinct spreading ridges and transforms interacting with trenches. In contrast, there are only a few well-documented cases of spreading ridge interactions in the ancient geologic record, which indicates this process is grossly underrepresented in tectonic syntheses of plate margins. Analogies with modern systems can identify some distinctive processes associated with triple junction interactions, yet an incomplete understanding of those processes, and their effects, remains. Additional insights can be gained from well-documented examples of ancient ridge subduction because exhumation has revealed deeper levels of the tectonic system and such systems provide a temporal record of complex structural, metamorphic, igneous, and sedimentary events. This volume focuses on ridge-trench interactions in the Paleogene forearc record of the northern Cordillera (north of the 49th parallel). Insights from this system and modern analogs suggest that there is no single unique signature of ridge subduction events, but a combination of processes (e.g., igneous associations, changes in kinematics, motion of forearc slivers, thermal events, etc.) can be diagnostic, especially when they are time-transgressive along a plate margin. Understanding these processes in both modern and ancient systems is critical to our understanding of the creation and evolution of continental crust and provides a new framework for evaluating the evolution of the onshore and offshore tectonic history of the northern North American Cordillera.

Structural history of the Chugach metamorphic complex in the Tana River region, eastern Alaska: A record of Eocene ridge subduction

The Chugach metamorphic complex of southern Alaska is an Eocene high-temperature ( T ), low-pressure ( P ) fore-arc metamorphic belt related to subduction of the Kula-Farallon spreading center beneath western North America. The Chugach metamorphic complex has a three-phase ductile deformational history that records major changes in kinematic axes during a short interval of geologic time (∼8 m.y.). The earliest deformation (D 1 ) is a regional event recognized throughout the flysch subterrane of the Chugach terrane. D 1 is a regional layer-parallel slaty/phyllitic cleavage developed during accretion and subsequent shortening. In the Chugach metamorphic complex, D 1 predates high-temperature metamorphism. During prograde metamorphism, there were two major structural events. D 2 records orogen-parallel extensional accompanied by vertical shortening with components of pure shear and top-to-the-east simple shear. D 2 is synchronous with melt injections (Ti 2 ) in the gneissic core of the complex and large plutons throughout the complex. D 3 records a return to subhorizontal contraction perpendicular to the margin and is interpreted as a dextral transpressional event. D 3 contraction produced a dramatic thickening of the complex in a regional-scale D 3 anticlinorium. In the gneissic core, the presence of melt (Ti 3 ) strongly influenced D 3 . Finite strain data and field observations indicate that both F 2 and F 3 have axes that are parallel to the stretching direction, yet these are not sheath folds because strains are too low. Instead the structures are examples of folds that developed with their axes parallel to the elongation axis. Together these observations provide further evidence for our previous interpretations that the Chugach metamorphic complex is a manifestation of an Eocene plate reorganization at ca. 56–52 Ma. Plate models predict that before 56 Ma the Kula–Farallon–North American triple junction migrated southward and is associated with a time-transgressive fore-arc plutonic belt. After plate reorganization, the triple junction either backtracked northward (Kula Plate model) or continued southward with intermittent northward motion (Pacific-Farallon model). We interpret the D 2 -to-D 3 progression as either a result of highly oblique subduction of the Kula plate followed by more orthogonal—but still dextral-oblique—convergence of the Farallon plate (Kula Plate model), or a special case of Pacific–Farallon–North American interaction.

Boron geochemistry of the lower crust: Evidence from granulite terranes and deep crustal xenoliths

Abstract Boron contents are uniformly low in more than 100 granulites from exposed terranes in India, Norway, and Scotland and from xenolith suites in the western USA. Averages for the terranes (2.5 ppm) and for xenoliths (1.2 ppm) suggest maximum B contents of about 2 ± 1 ppm for the lower crust. Abundance distributions from exposed terranes are skewed to higher values as B has been added to some samples via retrograde fluids during decompression. The samples studied include mafic to felsic lithologies of both igneous and sedimentary origin. There is no correlation of B content with bulk composition or with protolith type in any of the suites studied. Boron is apparently depleted in all granulite protoliths during prograde metamorphism and dehydration. Similar depletions of B and other fluid-mobile elements (e.g., U, Cs) with respect to rare-earth elements (REE) Zr, Ba, Rb and K 2 O are seemingly inconsistent with origin of granulites primarily via extraction of silicate melts. These distinctive geochemical features are attributed to selective element transport in fluids released by devolatilization reactions. Anatexis is not necessarily precluded, but dehydration of the subsolidus protolith normally would precede melting and lead to depletion of the fluid-mobile elements. Compositions of any melts and restites eventually formed would reflect the effects of this antecedent process. The systematic depletion of B (and Cs) in granulites contrasts with the highly variable contents of most other incompatible elements determined. For this reason, the estimated lower crust abundance for B is relatively well constrained, whereas abundance estimates for many other elements are model dependent, have large uncertainties and are unlikely to be globally representative.

Dating blueschist metamorphism: A combined 40Ar/39Ar and electron microprobe approach

Abstract 40 Ar 39 Ar and electron microprobe examination of blueschist samples from the Iceberg Lake schist, southern Alaska suggest that phengite inclusions are the source of 40Ar in crossite. Because such finegrained inclusions may be susceptible to argon loss, caution should be exercised in interpreting K-Ar ages from this phase, and possibly other low-K amphiboles from blueschist suites. The estimated blocking temperature for phengite in the matrix (314° to 450°C), however, is close to the estimated peak metamorphic temperatures (325° ± 50°C), suggesting that phengite 40 Ar 39 Ar plateau dates may coincide closely with the time of blueschist metamorphism.

Geochemical and geochronologic constraints for genesis of a tonalite-trondhjemite suite and associated mafic intrusive rocks in the eastern Chugach Mountains, Alaska: A record of ridge-transform subduction

Igneous rocks in the Chugach metamorphic complex of southern Alaska form part of the Sanak-Baranof belt, a series of forearc plutons believed to have been formed by subduction of a spreading ridge. The intrusives are predominantly tonalite-trondhjemite-granodiorite suite. There are some associated mafic plutonics close to the eastern end of the belt near Yakutat and Glacier Bay National Park. Trace element and Nd-Sr isotopic data suggest that tonalitic magmas were derived from the mixing of two sources: melted accretionary wedge sedimentary rocks and mafic material underplated at the base of the wedge as the ridge was subducted. The mafic material could include metabasaltic amphibolites at Nunatak Fjord that have a mid-ocean ridge basalt-composition protolith. Early Tertiary ridge migration along the Alaskan margin was generally toward the east. New U-Pb and 4 0 Ar/ 3 9 Ar geochronology data indicate synchronous magmatism at 54 Ma in the western Chugach metamorphic complex and Nunatak Fjord region. There is also a younger 49 Ma intrusive at Nunatak Fjord. The Nunatak Fjord region is subdivided into two blocks: (1) Chugach block has young amphibole cooling ages of 20 Ma, whereas (2) Boundary-Fairweather block has older cooling ages of ca. 53 Ma, which are contemporaneous with an U-Pb age on an adjacent tonalitic pluton. This shows that within the Chugach metamorphic complex, ridge subduction was probably complicated by subduction of a transform, causing the locus of magmatism to appear to jump ∼130 km eastward from Van Cleve Glacier to Nunatak Fjord near Yakutat at 54 Ma. In addition, forearc slivers record different emplacement, thermal, and exhumation processes along the margin.

Mid-Cretaceous extensional tectonics of the Yukon-Tanana terrane, trans-Alaska crustal transect (TACT), east-central Alaska

Mid-Cretaceous crustal extension played a fundamental role in the structural evolution of the Yukon-Tanana terrane (YTT) in the northern Cordilleran interior. In the central portion of the YTT northwest of Delta Junction, Alaska, a mylonitic shear zone juxtaposes greenschist facies rocks in the upper plate against middle to upper amphibolite facies metamorphic rocks in the lower plate, a juxtaposition suggesting elimination of as much as 10 km of crustal section. The mylonites form a partial sheath enveloping a domal footwall structure and kinematic analysis of the mylonite zone yields a uniform transport direction of hanging wall to ESE. These relations suggest analogies to the metamorphic core complexes of the southern Cordillera. However, the YTT structures are entirely ductile, suggesting either a relatively deep erosional level or relatively high geothermal gradients during extension. In the study area remnants of an older preextensional thrusting event are preserved at the highest structural levels at the base of the Seventymile terrane and the leading edge of YTT in the Wickersham terrane. However, most areas display a regional, subhorizontal fabric that is superimposed on older fabrics, and in the study area this latest fabric is subparallel to the mylonitic sheath of the apparent extensional structure. Thus the conventional viewpoint that this latest fabric is related to thrusting needs to be reevaluated and this fabric may be entirely extensional in origin. Further evidence for extension is provided by clear similarities between YTT and characteristic features of other extensional terranes. Thus we suggest that the YTT is a deeply eroded view of highly extended continental crust. The tectonic mechanism for the extensional event and the magnitude of the extension is uncertain because of complications in regional timing relationships and in alternative interpretations of the reconstruction of the crustal section. Three end-member models based on analogies with Neogene extensional systems are presented as working models to accommodate the alternative interpretations: (1) a Jurassic collision and Cretaceous extension model based on comparisons with the Neogene history of the Mediterranean region; (2) an Early to mid-Cretaceous syncollisional model analogous to the Carpathian Mountains of eastern Europe; and (3) a syncollisional plateau uplift model with extension driven by gravity spreading.

TWO CONTRASTING PRESSURE-TEMPERATURE-TIME PATHS IN THE VILLA DE CURA BLUESCHIST BELT, VENEZUELA : POSSIBLE EVIDENCE FOR LATE CRETACEOUS INITIATION OF SUBDUCTION IN THE CARIBBEAN

The Villa de Cura blueschist belt is one of several east-west–trending allochthonous belts comprising the Caribbean Mountain system of northern Venezuela. This blueschist belt consists of four structurally coherent subbelts that also trend east-west; from north to south these are characterized by: (1) pumpellyite-actinolite, (2) glaucophane-lawsonite, (3) glaucophane-epidote, and (4) barroisite. The retrograde pressure-temperature ( P-T ) path of the northern three subbelts generally parallels their prograde path. Such P-T paths are typical for Franciscan-style subduction settings and are characterized by relatively low geothermal gradients indicative of refrigeration during subduction-zone-parallel ascent and exhumation of these rocks. The barroisite subbelt formed at high pressures similar to those of the glaucophane-epidote subbelt, but at substantially higher temperatures, and followed a counterclockwise P-T path. New 40 Ar/ 39 Ar ages record peak metamorphism at 96.3 ± 0.4 Ma for the barroisite subbelt and 79.8 ± 0.4 Ma for the northern three subbelts. The Caribbean plate is thought to have been a fragment of the Farallon plate, which together with the “Great Arc of the Caribbean” (Greater Antilles–Aves Ridge–Lesser Antilles–Leeward Antilles) migrated northeastward after a subduction polarity reversal and overrode the young Proto-Caribbean lithosphere that had formed by spreading between North America and South America. The more silicic barroisite subbelt may have been part of the arc that was subducted immediately after polarity reversal, whereas the other three belts formed much later when the geothermal gradient had decreased substantially. The Villa de Cura belt was exhumed in two stages, first by Late Cretaceous arc-parallel extension, and second by Miocene southward thrusting onto the South American continent.