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This is a preface to the topical issue entitled ‘Subduction, Fluids, and Accessory Minerals: A Celebration of the Career of Sorena S. Sorensen’

Jadeitite is a rare constituent of serpentinite-matrix melange bodies from certain subduction complexes. Most jadeitite crystallizes from Na-, Al-, and Si-bearing fluids that are apparently derived from multiple subduction-zone sources. Even though jadeitite is near-end-member NaAlSi2O6 in major element composition and is volumetrically minor in subduction complexes, its trace elements and stable isotopes appear to record fluid compositions not directly seen in other subduction zone metasomatic systems. Prior to our work, how jadeitite-forming fluids interact with serpentinite host rocks and serpentinizing fluids were largely unknown, because serpentinite-to-jadeitite contacts are generally not exposed. In the Sierra de las Minas, Guatemala, we have studied a 3 m-wide pit transecting the contact between a mined-out jadeitite body and its host serpentinite. An apparent transition zone between the former jadeitite and nearby serpentinite exposed in the mine pit contains four texturally distinct rock types o...

The Guatemala suture zone is a major east–west left-lateral strike slip boundary that separates the North American and Caribbean plates in Guatemala. The Motagua fault, the central active strand of the suture zone, underwent two major collisional events within a system otherwise dominated by strike–slip motion. The first event is recorded by high-pressure/low temperature (HP/LT) eclogites and related rocks that occur within serpentinites both north and south of the Motagua fault. Lawsonite eclogites south of the fault are not significantly retrograded and give 40Ar/39Ar ages of 125–116 Ma and Sm–Nd mineral isochrons of 144–132 Ma. Eclogites north of the fault give similar Sm–Nd isochron ages (131–126 Ma) but otherwise differ in that they are strongly overprinted by a lower pressure assemblage and, along with associated HP/LT rocks, give much younger 40Ar/39Ar ages of 88–55 Ma indicating a later amphibolite facies metamorphic event. We propose therefore that all serpentinite hosted eclogites along the Motagua fault formed at essentially the same time in different parts of a laterally extensive Lower Cretaceous forearc subduction system, but subsequently underwent different histories. The southern assemblages were thrust southwards (present coordinates) immediately after HP metamorphism whereas the northern association was retrograded during a later collision that thrust it northward at ca. 70 Ma. They were subsequently juxtaposed opposite each other by major strike slip motion. This model implies that the HP rocks on opposing sides of the Motagua fault evolved along a plate boundary that underwent both dip slip and strike slip motion throughout the Late Cretaceous as a result of oblique convergence. The juxtaposition of a convergent and strike slip system means that HP/LT rocks within serpentinites can be found at depth along much of the modern Guatemala suture zone and its eastward extension into the northern Caribbean. Both sets of assemblages were exhumed relatively recently by the uplift of mountain ranges on both sides of the fault caused by movement along a restraining bend. Recent exhumation explains the apparently lack of offset of surface outcrops along a major strike slip fault.

Lawsonite eclogites preserve a record of very-low-temperature conditions in subduction zones. All occur at active margin settings, typically characterized by accretionary complexes lithologies and as tectonic blocks within serpentinite-matrix melange. Peak lawsonite-eclogite facies mineral assemblages (garnet + omphacite + lawsonite + rutile) typically occur in prograde-zoned garnet porphyroblasts. Their matrix is commonly overprinted by higher-temperature epidote-bearing assemblages; greenschist- or amphibolite-facies conditions erase former lawsonite-eclogite relics. Various pseudomorphs after lawsonite occur, particularly in some blueschist/eclogite transitional facies rocks. Coesite-bearing lawsonite-eclogite xenoliths in kimberlitic pipes and lawsonite pseudomorphs in some relatively low-temperature ultrahigh-pressure eclogites are known. Using inclusion assemblages in garnet, lawsonite eclogites can be classified into two types: L-type, such as those from Guatemala and British Columbia, contain garnet porphyroblasts that grew only within the lawsonite stability field and E-type, such as from the Dominican Republic, record maximum temperature in the epidote-stability field. Formation and preservation of lawsonite eclogites requires cold subduction to mantle depths and rapid exhumation. The earliest occurrences of lawsonite-eclogite facies mineral assemblages are Early Paleozoic in Spitsbergen and the New England fold belt of Australia; this suggests that since the Phanerozoic, secular cooling of Earth and subduction-zone thermal structures evolved the necessary high pressure/temperature conditions. Buoyancy of serpentinite and oblique convergence with a major strike-slip component may facilitate the exhumation of lawsonite eclogites from mantle depths.

Thrust sheets of the upper plate of the Saih Hatat window, NE Oman contain metasediments formed under lawsonite albite facies conditions. Quartz and Fe-Mg carpholite in these metasediments contain several types of fluid inclusions; most are two-phase at room T with an aqueous NaCl - H2O or NaCl - MgCl2 - H2O solution. Texturally early fluid inclusions display a wide range of final melting (Tmf) and homogenization (Th) temperatures. The lowest Tmf and Th values were recorded for moderately sized inclusions occurring in clusters. Some of the highest Tmf and Th values (> 320°C) are recorded for isolated inclusions in Fe-Mg carpholite and quartz. In the lower plate epidote-blueschist facies metasediments, many fluid inclusions in quartz are decrepitated, with most of the surviving inclusions being 3-phase, CO2-bearing, filled with a low salinity fluid with XCO2 < 0.05. For all samples from both plates, isochores calculated for the few texturally early inclusions with low Th values are consistent with their respective P-T estimates. We conclude that inclusions with high Th experienced post-entrapment stretching ± loss of H2O during exhumation. Larger inclusions and inclusions in Fe-Mg carpholite were more strongly affected by these changes. Small isolated inclusions in relatively unstrained quartz have escaped these modifications and are useful for constraining peak conditions.

Geochemical study of the Alta stock and adjacent contact aureole rocks provides information concerning the source, composition, and physical-chemical conditions of infiltrating fluids. Special emphasis was given to boron (B) as a tracer of fluid-rock interactions due to the occurrence of borate minerals (ludwigite, kotoite, and szaibelyite) in skarn deposits around the stock. In addition, thin section alpha-track mapping implies significant B enrichments in fluid-altered minerals within the stock, stockwork veins and related selvages, igneous sills near the stock, contact skarns, and in marbles up to 500 m from the stock. Forsterite, clinohumite, lizardite, and malachite contain between 50 and 1200 ppm B. Diopside, calcite, clintonite, phlogopite, brucite, hedenbergite, tremolite, and other minerals host B to a lesser extent. Aureole B enrichments correlate well with major and other trace-element enrichments, and support existing models of element transport in magmatic fluids with lateral down-temperature flow. Large variations in mineral B concentrations reflect changes in B concentrations of these fluids through time. Mass-balance calculations indicate that magmatic fluids emanating from the Alta pluton could supply most B in the Alta aureole. It is estimated that the emplaced magma had an initial B concentration between 7‐10 ppm; indicated exhalative losses of B from the pluton are on the order of 50%. We estimate that the exsolved fluids had a time‐integrated B concentration of 160 ± 40 ppm, although much higher concentrations may have attended local borate mineralization.

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.

In the Cretaceous melange of the Cordillera de la Costa belt, INTRODUCTION north–central Venezuela, there are knockers of eclogite, barroisiteTwo belts of high-pressure (HP)–low-temperature (LT) bearing eclogite, and pelitic glaucophane schist. These occur in a metamorphic rocks are exposed in the Caribbean Mounmetamorphic melange matrix that locally consists of marble, sertain system of northern Venezuela, which is part of the pentinite, amphibolite, actinolite schist, feldspathic schist and gneiss, complex east–west-trending boundary zone between the graphitic schist, chloritoid schist, and garnet-bearing mica schist. Caribbean and South American plates (e.g. Menendez, The protoliths for these various rock types exhibit a wide age range 1966, 1967). These two belts are the Cordillera de la (Cambrian to Early Cretaceous?). Recently discovered knockers of Costa and Villa de Cura belts, both of which were pelitic glaucophane schist contain Mg-glaucophane + paragonite metamorphosed during the mid-Cretaceous, presumably + kyanite + garnet + talc + graphite + rutile + quartz. in a subduction zone related to the mid-Cretaceous The coexistence of kyanite and Mg-glaucophane suggests minimum Leeward Antilles volcanic arc (e.g. Pindell, 1993). From P ~2000 MPa at T > 600°C. Eclogite knockers from the same Jurassic to Eocene time North and South America were outcrop contain garnet and clinopyroxene which yield ~500°C for diverging and new proto-Caribbean sea-floor was created cores, ~700°C for rims, and P [ 1200 MPa. The assemblage between them (e.g. Pindell, 1993). Therefore, the Leegarnet–biotite–phengite–albite within schists of the melange matrix ward Antilles arc, as well as the entire Caribbean Mounof this locality indicates metamorphic conditions of T = tain system, are allochthonous and formed far to the 450–520°C at P = 1800 MPa. Because all lithologies in this west as part of the Farallon–North/South American plate outcrop record high-P conditions, this metamorphic melange formed boundary zone (e.g. Pindell, 1993). before or during peak metamorphism in a mid-Cretaceous subduction Although the two HP–LT belts have similar metazone. morphic ages, they differ greatly in lithology and metamorphic history. The Cordillera de la Costa belt consists of oceanic and passive continental-margin rocks intermixed with Paleozoic granites and granitic gneisses. In this belt, the eclogites and blueschists were retrograded to epidote–amphiboliteand greenschist-facies assemblages following an apparent P–T path typical for

The Border Ranges fault system of southern Alaska, the fundamental break between the arc basement and the forearc accretionary complex, is the boundary between the Peninsular–Alexander–Wrangellia terrane and the Chugach terrane. The fault system separates crystalline rocks of the Alexander terrane from metamorphic rocks of the Chugach terrane in Glacier Bay National Park. Mylonitic rocks in the zone record abundant evidence for dextral strike-slip motion along north-northwest-striking subvertical surfaces. Geochronologic data together with regional correlations of Chugach terrane rocks involved in the deformation constrain this movement between latest Cretaceous and Early Eocene (~50 Ma). These findings are in agreement with studies to the northwest and southeast along the Border Ranges fault system which show dextral strike-slip motion occurring between 58 and 50 Ma. Correlations between Glacier Bay plutons and rocks of similar ages elsewhere along the Border Ranges fault system suggest that as much as 700 km of dextral motion may have been accommodated by this structure. These observations are consistent with oblique convergence of the Kula plate during early Cenozoic and forearc slivering above an ancient subduction zone following late Mesozoic accretion of the Peninsular–Alexander–Wrangellia terrane to North America.

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.

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.

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 (

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.

Field studies east of the Copper River provide an unusual view of the architecture of a ductile strike-slip system where the Chugach metamorphic complex plunges westward from high-grade gneiss to greenschist facies rocks of the Chugach terrane. In this down-plunge view of a transpressional system we observe a three-phase ductile deformational history similar to the Tana River region about 75 km to the east. In the Copper River area, however, spatial variations in structural development provide important insights into syntectonic flow in lithologically homogeneous rocks ranging from high-grade gneiss to greenschist facies phyllite. Early structures (D 1 ) formed prior to the Chugach metamorphic complex and produced a steeply dipping layering (So) and layer-parallel cleavage (S 1 ) following thrust imbrication and associated folding. Following D 1 , the earliest structures of the Chugach metamorphic complex (D 2 ) were associated with a vertical shortening and horizontal, margin-parallel extension. D 2 produced gently inclined to recumbent subisoclinal to isoclinal folds (F 2 ) in S 1 = S 0 layering with axes parallel to the ∼WNW-trending elongation lineation. These structures were in turn overprinted by a third set of structures (D 3 ) that produced upright, gently plunging folds (F 3 ) of S 2 that vary spatially. In the high-grade gneissic core of the complex, D 3 structures are homogeneously distributed with tight to subisoclinal mesoscopic folding throughout the core, and no significant spatial variation in fabric development. In contrast, structurally above a transition from schist to gneiss, D 3 fabric development is domainal with bands of intense S 3 foliation development separated by broad regions in which D 3 fabrics are indistinct or absent. Within these D 3 high-strain bands, numerous fabric asymmetries indicate that these bands are zones of distributed dextral shear. These observations suggest that during D 3 , upper crustal dextral strike-slip faults passed downward into discrete shear zones, but these shear zones did not penetrate to the base of the crustal column. Instead, the strike-slip motion was dispersed into a broad zone of distributed shear within the high-grade gneisses at depth, and the contact between these two crustal levels was a D 3 decoupling surface. We propose a tectonic model for the Chugach metamorphic complex that combines effects of highly oblique Kula-Farallon subduction with the coeval interactions of the Kula-Farallon-North American triple junction. Reanalysis of the plate velocities together with observations from the Border Ranges fault system immediately inboard of the Chugach metamorphic complex indicate that the Chugach metamorphic complex developed when the Chugach terrane was a forearc sliver. If slip partitioning were complete, this forearc sliver migrated northward faster than the triple junction. Thus, the triple junction moved slowly southward within the forearc sliver, which is consistent with regional geochronological trends. The proposed mid-crustal decoupling horizon formed within this evolving transform system and D 2 and D 3 probably overlapped in time. In this interpretation D 2 primarily records vertical displacement gradients related to mid-crustal decoupling that initiated during supracrustal strike slip along the Border Ranges fault. With prograde metamorphism of the Chugach metamorphic complex, the decoupling horizon rose to higher crustal levels, and strike slip stepped outboard to form D 3 dextral shear zones within the Chugach metamorphic complex. The inference of mid-crustal decoupling implies a grossly different rheologic behavior across the gneiss transition. We propose that the gneissic front represents a fundamental change in macroscopic flow behavior from homogeneous linear flow in the high-grade gneisses to strain-softening, nonlinear flow in the overlying rocks. This rheological change is interpreted as a consequence of a deformation mechanism switch from diffusion

The Chugach metamorphic complex is a high-temperature-low-pressure metamorphic belt developed in the Eocene forearc of southern Alaska. A transect along Tana Glacier shows 1 8 O/ 1 6 O depletion of quartz with increasing metamorphic grade in both rock matrix and associated veins. These 1 8 O/ 1 6 O depletions are too large to result from closed system increase in metamorphic grade (temperature). Therefore, this indicates exchange of the metasedimentary rocks with an external reservoir of lower δ 1 8 O, hot aqueous fluid and/or felsic magma, at some stage(s) in the thermal and deformational history of the Chugach metamorphic complex. The concordant decrease in δ 1 8 O values of quartz from both rock matrix and associated veins indicates that the fluid infiltration occurred prior to or during formation of the quartz veins. In turn, this fluid infiltration must have preceded the deformation of the quartz veins. The metasediments of the Chugach metamorphic complex experienced at least two stages of fluid infiltration and associated 180/160 depletion. The first stage occurred prior to or during the second deformation (D 2 ), which was near the thermal peak for intermediate-grade zones (andalusite, staurolite). The second stage occurred after D 2 but prior to the third deformation (D 3 ) and before the thermal peak of metamorphism for the high-grade core (sillimanite zone schists and gneiss). It is unlikely that this pattern of 1 8 O/ 1 6 O0 depletion is the result of a single, geometrically simple fluid flow system operating at the scale of the exposed metamorphic section at Tana Glacier and at the peak of metamorphism. Evidence against such simple flow scenarios includes (1) structural evidence for multiple phases of deformation; (2) textural relationships among deformation fabrics, multiple generations of low-δ 1 8 O quartz veins, and metamorphic index minerals; and (3) constraints on the patterns of isotopic shifts in flow systems based on transport theory. Instead, evidence suggests that the δ 1 8 O-distance profile is more likely the result of more than one generation of fluid infiltration occurring at different times, places, and orientations during the prograde metamorphic and structural development of the Tana Glacier section. General oxygen isotope data sets such as those presented here for the Tana Glacier area and those previously published for other areas may not uniquely constrain the flow geometry responsible for the depletion patterns. These data sets also need to be integrated with structural and petrological studies in order to constrain the timing of fluid flow with respect to deformation and thermal history of the metamorphic belt. The general lowering of δ 1 8 O in the intermediate- to high-grade rocks of the Chugach metamorphic complex was caused by fluid infiltration (recorded by injection of quartz veins) prior to the thermal peak of metamorphism. These fluids were most likely exchanged with hot igneous intrusions. Thus, the detailed structural, thermal, and oxygen isotope record of these rocks indicates that magmatic heating is most likely responsible for this episode of high-temperature, low-pressure, Buchan metamorphism.

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.

Fluid inclusions within the quartz veins cutting greenschist facies metasedimentary rocks of the Valdez group provide insight into the changes in deformational style displayed in two vein systems from the northern Chugachmetamorphic complex. Six samples were analyzed in order to determine temperature and pressure of vein emplacement. Metamorphic temperature estimates from chlorite geothermometry range from 325 to 350 °C. This information combined with fluid inclusion geobarometry yields a high paleogeothermal gradient ranging from 58 to 35 °C/km for the accretionary prism. This higher-than-normal geothermal gradient represents elevated temperatures associated with Eocene subduction of the Kula-Farallon ridge. Isochores calculated for each vein show little temperature variation (less than 40 °C), and large pressure variations (up to 200 MPa). Maximum pressure differences within one vein set are as much as 150 MPa. In addition, the temperature of early vein emplacement was close to the brittle-ductile transition, which is consistent with textures of veins that vary from brecciated to folded. Late stage hydrocarbon-bearing inclusions are present in the samples, and may indicate long-distance, post-peak metamorphic, fluid flow through this region. The source for the hydrocarbon-bearing fluids is unknown and poses many interesting questions about regional fluid flow distances and directions after peak metamorphism.

Eocene deformation in metasedimentary rocks of the Valdez Group was examined along the Richardson Highway between Valdez and the Copper River basin. A topographic lineament along Stuart Creek and the lower Tiekel River separates two distinct structural domains: a southern domain (Domain 1) with two fabrics, and a northern domain (Domain 2) with three. Both domains possess an early, layer-parallel phyllitic cleavage (S 1 ) associated with south-directed thrust systems. In Domain 1, S 1 is steeply dipping and overprinted by a second, steeply south-dipping cleavage (S 2 ) with a subhorizontal elongation lineation, and F 2 folds are rare to absent. To the north in Domain 2, however, S 1 was first tightly folded during D 2 into a series of gently inclined folds (F 2 ) and then refolded by tight to close, steeply inclined F 3 folds that are asymmetric to the north. Metamorphic mineral assemblages indicate greenschist facies conditions accompanied the deformation: Biotite grew during D 2 in Domain 1 and during D 3 in Domain 2; and metatuffs in Domain 1 contain actinolite-chlorite-epidote-feldspar-quartz. Fluid inclusion analyses of multiple vein sets indicate a complex fluid history but indicate a metamorphic temperature of 350-400 °C, consistent with the observed mineral assemblages. Uniform metamorphic grade across the belt together with spatial variations in fabrics suggests that D 2 in Domain 2 is time-equivalent with D 3 in Domain 2. We evaluate the kinematics of linear and planar fabric development through a combination of finite strain analysis and geometry of syn-tectonic growth-fibers developed on sand grains. Finite strains are estimated by different methods including object shapes, object centers, and stretches estimated from growth fibers. The strain data show a predominance of constrictional strains, particularly in Domain 1, with subhorizontal stretching parallel to the mesoscopic lineation. Strain superposition models indicate that the main deformation (D 2 ) in Domain 1 was a constrictional flow indicating that D 2 was marked by subhorizontal orogen-parallel elongation and simultaneous vertical and horizontal shortening. Shear sense indicators generally indicate dextral shear on steeply dipping surfaces, but are not always well developed because the constrictional flow deviates strongly from simple shear. These characteristics are typical of the strain facies referred to as lengthening shear to lengthening-narrowing shear flow. We interpret this large-scale kinematic pattern as a combined effect of distributed dextral shear and vertical shortening related to a coeval ridge-subduction event. The vertical shortening could be associated with lateral flow driven by the migrating ridge, vertical displacement gradients related to a mid-crustal decoupling zone recognized in the Chugach metamorphic complex, or both. We infer that dextral shear is distributed through a wide zone but is concentrated along the major topographic lineament that is coincident with the structural domain boundary. We informally refer to this dextral structure as the Stuart Creek shear zone and conclude that the two structural domains were juxtaposed along this structure. D 3 structures recognized north of the Stuart Creek shear zone probably represent a back-thrusting event synchronous with the dextral shear system, which may record mid-crustal strain partitioning. Locating the westward continuation of the dextral shear system is difficult due to lack of detailed structural studies and the difficulty of recognizing strike slip within the homogeneous flysch of the Chugach terrane. The dextral shear system may transfer into complex out-of sequence thrust systems if the strike-slip systems were developed during or after the Alaskan orocline.

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.

Preliminary results of an on-going study of the Cordillera de la Costa belt between Puerto Cabello and Choroni, north-central Venezuela, indicate that the deformational history is far more complicated than expected from simple plate-tectonic models. The Cordillera de la Costa belt consists of oceanic rocks (e.g., serpentinites, amphibilites, with lenses of eclogite and blueschist) intimately intermixed with metamorphosed continental margin deposits (e.g., mica and graphite schist, quartzite, marble). Locally, large granitic (basement ) complexes of Lower Paleozoic age are included as well. In late Cretaceous time, the entire belt was involved in four synmetamorphic deformations phases (D[sub 1a] to D[sub 1d]); the first (D[sub 1a]) occurred at depths of at 35-40 km and the later ones at successively shallower depths. This deformation occurred in a subduction zone, related to right-oblique convergence of the Farallon and Atlantic plates. The most penetrative structures resulted from (all in present coordinates) north-south contraction and east-west dextral simple shear (D[sub 1b]). During an Early Tertiary ( ) event (D[sub 2]), the belt was emplaced southward onto the South American continental margin. Subsequent deformational structures (D[sub 3]) resulted in cross folds and faults (with small pull-apart basins) which are consistent with the eastward passage of themore » Caribbean past the South American plate.« less

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.

Felsic intrusive rocks in the Chugach metamorphic complex (Alaska) have clear geochemical signatures indicating mixing of magmas from different sources. In this region, the relative sequence of intrusion for four generations of these rocks is well constrained by crosscutting relations. They were intruded during a four-phase deformational history that may be related to changes in the Kula–North America–Farallon plate configuration and subduction of a ridge axis. The felsic intrusions are predominantly tonalitic; some have adakite-like geochemical signatures. Both rare earth element (REE) abundance and initial Sr and Nd isotopic ratios vary, from values similar to metasedimentary rock to more isotopically depleted values with lower REE abundance. This evidence suggests two sources for the Chugach metamorphic complex magmas: metasedimentary rocks of the accretionary prism and a mafic (mantle-derived) source, possibly underplated oceanic material. Furthermore, the adakite-like magmas probably did not form in response to melting of the slab.

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.

Observations of crystal growth in supersaturated KH 2 PO 4 solutions indicate that it duplicates natural crystallization processes and formation of fluid inclusions in free liquids similar to vein formation or igneous systems. Continuous crystal growth by surface nucleation results in elongate fluid inclusions forndng as hollow tubes within kinked growth steps. Large, flat fluid inclusions also form either by sealing of etch pits and other surface irregularities or along cracks propagated by crystal growth. Microscopic observation of real-time crystal growth documents the formation of fluid inclusions along a surface or within a plane, inclusions that would otherwise be identified as secondary. These primary fluid inclusions resemble pseudosecondary inclusions or secondary "necked-down" inclusions commonly observed in geologic samples. Furthermore, these fluid inclusions, and many or most in natural samples, are not generated by spiral growth processes. This simple analogue experiment serves as a model for crystal growth and fluid-inclusion entrapment in hydrothermal and igneous systems.

The interrelationships between metamorphism, deformation, magma intrusion, and 40Ar/39Ar geochronology were determined for a low-pressure/high-temperature metamorphic complex which formed from an accretionary prism in the Chugach Mountains, southern Alaska. Compressional deformation, which first produced south verging folds and associated thrusts, was followed by magma intrusion and development of north verging folds. Synmetamorphic southward directed thrusting of metamorphosed flysch over flysch produced increased load in the footwall, as documented by the distribution of mineral assemblages and by pressure-temperature modeling of garnet growth. The initial heating to the greenschist facies may have been accomplished by a combination of advective heating from aqueous fluids and of conductive heating from subducted young oceanic crust. Regionally developed amphibolite facies metamorphism followed intrusion of felsic sills. The peak metamorphic conditions derived from geothermobarometry, mineral assemblages, and fluid inclusions ranged from 400° to 600°C at a depth of ∼10 km. The increased heat from associated synmetamorphic concordant felsic sills raised the ambient temperatures to produce a regional distribution of andalusite and cordierite with a core zone of sillimanite-bearing migmatites. Subsequent cooling was initially rapid (≈55°C/Ma) to ∼350°C based on 40Ar/39Ar dates of 53 Ma for hornblende and 50 Ma for biotite and may have slowed to ∼11°C/Ma until 200°C based on an 40Ar/39Ar date of 35 Ma for plagioclase. Intrusions of felsic sills and at least one pluton were along the initially north dipping foliations associated with south verging folds. This suggests the source region for the melts may have been downdip in the subduction zone rather than from directly below within the accretionary prism.

During hydrothermal fluid activity associated with the intrusion of the Ponder pluton, Coast Plutonic Complex, British Columbia, Canada, the halogen composition of biotite, amphibole and apatite is controlled by the composition of the metamorphic fluid. The core of the pluton was not affected by infiltrating fluids, and the fluorine composition of biotite was controlled by the biotite Fe-Mg ratio. The edge of the cooling, crystallized pluton interacted with a chlorine-rich, fluorine-poor fluid which changed the halogen composition of biotite and amphibole. The composition of the metamorphic fluid derived from biotite and apatite compositions was relatively high in chlorine and low in fluorine. The concentration of chlorine in the metamorphic fluid increases towards the pluton, whereas fluorine remains constant. This suggests that the metamorphic fluid near the pluton had the greatest potential for masstransfer of metals.

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.

The 40Ar/39Ar step-heating analyses of three hornblende samples from the Border Ranges ultramafic-mafic assemblage yield plateaus with slightly discordant dates of 180.2±0.7, 177.5±0.7, and 181.1±1.7 Ma. A plot of the plateau temperature data for all three hornblende samples on an 36Ar/40Ar versus 39Ar/40Ar diagram reveals the presence of a non-atmospheric “initial” argon component with an apparent 40Ar/36Ar of 379±16 and yields a date of 175.7±1.0 Ma. A potassium-poor hastingsite from the same complex yields a complicated spectra with dates as high as ∼900 Ma. These older dates reflect contamination by argon with an isotopic composition similar to that found for the initial argon in the hornblende and do not record a Paleozoic component in the age of the Border Ranges ultramafic-mafic complex. Hornblende from an Eocene tonalite sill in the nearby Chugach Metamorphic Complex also contains an initial argon component (40Ar/36Ar = 314 ± 4). The composition of this initial argon is presumably dictated by the argon isotopic composition of the host rock fluid at the time of amphibole/fluid equilibration. Approximately 2×10−7 cm3/g of this initial argon is sufficient to perturb the 40Ar*/39ArK “model” plateau dates. Although 40Ar/39Ar stepwise degassing at low temperatures removes an additional “atmospheric” argon component, the initial argon in these samples is released at a rate that closely coincides with the degassing rate of radiogenic argon from the amphibole, and only through application of the 36Ar/40Ar versus 39Ar/40Ar diagram can this nonatmospheric argon component be detected. For the Border Ranges ultramafic-mafic and Chugach Metamorphic complexes, the range in K-Ar amphibole dates could be interpreted as recording differential uplift and an extended duration of intrusive activity, respectively. The results suggest, however, that nonatmospheric initial argon contributes significantly to the observed age variations.

The low-pressure/high-temperature metamorphism of the Chugach metamorphic complex (Alaska) occurred in an ocean-continent convergence zone. To achieve the high temperatures at a relatively shallow depth in an accretionary prism, we propose the large-scale transport of heat by fluids, which preheated the metamorphic belt by tectonic focusing of fluids followed by injection of melts, both of which were generated downdip in a shallow subduction zone.