Tectonic Inclusions Research Articles

UPPER CRETACEOUS TRANSGRESSION OVER THE OPHIOLITES OF NORTHERN PINDOS

At the ophiolithic overthrusted masses of Northern Pindos, east of Katara, outcrops of limestones are observed. Until now these outcrops are interpreted by tectonic inclusions inside the secondary tectonic contacts. Detailed analysis of these limestones showed that they are remainings of transgressions their age determined by the microfauna as upper Campanian - early Maastrichian. The limestone series ends upwards in a karstic surface supporting doleritic lava flows with essential thickness. In place Tragopetra we can observe that these lavas clearly intrude in the caves of the paleokarst. At the same section (Tragopetra), over this lava formation, the basic conglomerate (Auversian) of the mesohellenic sediments is found. Transgression of this age whas unknown until now in Greece, but is well known in South Anatolia and Near East. An important outcome of our research is that there is no ocean trench before the 806 overthrust but land with karstification of the limestones is emerged instead. The Pindos ophiolites overthrust is related to recent land creation and developed under epiglyptic conditions. Nature reality often differs to any models humans may imaginate!

DEFORMATIONS AND THE SEQUENCE OF THE STRUCTURES FORMATION IN THE NORTHERN PART OF THE ZONE OF DEVELOPMENT OF MAKSUTOVO METAMORPHIC COMPLEX (SOUTHERN URALS)

Composition and structural evolution of the Maksutovo metamorphic complex in the its northern part has been examined. The early folds 77) plunging in the SE direction have been established to be developed only in the rocks of the Maksutovo metamorphic complex. The problem of the definition of the geodynamic nature of the 7Л deformation stage, marked by the folds 77), has not been solved yet. Tectonic inclusions of the metamorphic rocks adjacent to the western border of the Main Uralian Thrust without traces of 7Л deformation stage must not be considered as parts of the Maksutovo metamorphic complex. 7Л1 deformation stage expressed in formation of thrusts and 77. west vergent folds is connected with late Paleozoic continental collision. The third stage of deformation 7JIII is marked by development of 77) folds with steep hinges associated with post collision strike slip movements

Tectonic Inclusions in Serpentinite Landscapes Contribute Plant Nutrient Calcium

Serpentinite‐derived soils give rise to botanically distinct systems primarily because of inadequate parent material Ca content. We hypothesized that Ca content varies widely in what have been mapped as soils derived from serpentinite. An exchangeable Ca/Mg ratio <0.7 is often used to relate the imbalance of these nutrient elements in serpentinite‐derived soils. We sampled six parent materials and soils from the Coast Ranges of California in Henneke soil series (clayey‐skeletal, magnesic, thermic Lithic Argixerolls) modal location map unit polygons. Parent material total CaO content varied from 1.0 to 230 mg kg−1, and CaO/MgO varied from <0.1 to 4. A combination of x‐ray diffraction (XRD), polarized light microscopy (PLM), and electron microscopy was used to identify the Ca‐bearing accessory minerals diopside, grossularite, andradite, and tremolite. Accessory mineral content was often too low to be detected by XRD or minerals were too finely disseminated and difficult to detect in thin section by PLM. Electron microscopy, in concert with XRD and PLM, was needed to fully characterize the mineral assemblage. Two sites, Napa and Tehama, contained no serpentine minerals, were not serpentinites, and were tectonic inclusions in the serpentinite landscape. Napa rocks contained almost no Ca‐bearing minerals and would be identified as a serpentinite if relying on elemental analysis CaO/MgO ratio alone. Tectonic inclusions and Ca‐bearing accessory minerals affect Ca distribution and presumably its availability for plants. Careful mineralogical analysis may be required to identify Ca‐bearing accessory minerals.

Cretaceous accretionary complex related to Okhotsk-Chukotka Subduction, Omgon Range, Western Kamchatka, Russian Far East

The Omgon Range of Western Kamchatka contains a mid to Upper Cretaceous sequence of flysch with tectonic inclusions of Jurassic–Cretaceous oceanic rocks inferred to have been imbricated together in an accretionary prism. These rocks were tectonically juxtaposed during the Cretaceous in a mélange that contains a number of elements but mainly includes: (1) Middle Jurassic–Lower Cretaceous volcanic rocks formed in an oceanic and/or marginal sea environment; and (2) Albian–Campanian terrigenous turbidites made of quartz-rich clastic sediments that accumulated near a continental-margin. The oceanic rocks are inferred to have been tectonically incorporated into the continental terrigenous unit by offscraping during subduction. The accretionary prism resulted from subduction of the Pacific paleo-oceanic plate (Izanagi) under the Eurasian continental margin, which ultimately caused volcanism in the inboard Okhotsk-Chukotka volcanic belt. Internal imbrication was completed by the Maastrichtian (∼70 Ma) as indicated by apatite fission-track ages that record cooling and exhumation of this crustal block. The Omgon accretionary wedge originated in a similar geodynamic setting and same time as the Yanranai (northern Korayk), Tonino–Aniva (southeastern Sakhalin), Hidaka (northeastern Japan) and Cretaceous part of the Shimanto belt (southwestern Japan). The similarities of ages, lithology, and tectonic setting suggest that the Omgon accretionary wedge was part of a paleo-subduction zone along the Eurasian margin during the mid to Late Cretaceous.

Tectonic Emplacement of Serpentinite Southeast of San Jose, California

The Santa Clara Formation was drastically affected by emplacement of serpentine diapirs in very recent times. Serpentine in flat thrust sheets formerly belonged to the lower unit of the Coast Range Ophiolite, which underlay the Jurassic—Cretaceous clastic sediments of the Great Valley Sequence. The regionally extensive Coast Range fault had emplaced all the aforementioned rocks, including the flat thrust sheets of serpentine, over the Mesozoic Franciscan Complex. This complex is exposed in many places in the area and presumably is basement for the entire region. Serpentine bodies within the Franciscan Complex were derived from peridotite wedges unrelated to the Coast Range Ophiolite. Exposure of the Coast Range Ophiolite serpentine may have been assisted by uplift and deep erosion combined with imbricate thrusting and westward thinning of the Great Valley sediments, broadly exposing the basal Coast Range Ophiolite serpentine. Plio-Pleistocene, non-marine Santa Clara Formation was deposited on Coast Range Ophiolite serpentine and adjacent remnants of the Great Valley sediments as well as Franciscan melange containing serpentinized peridotite wedges. Contractional deformation produced normal and reverse faults that diapirically forced the light, plastic serpentine into the Santa Clara Formation. Thrust faults rooted to the east emplaced the Coast Range Ophiolite serpentine up and onto the deformed Santa Clara Formation and its serpentine diapirs. Our inability to distinguish between serpentine from Coast Range Ophiolite or Franciscan periodtite wedges leaves open the source of the diapiric serpentinites. However, the compelling presence of tectonic inclusions in the diapiric serpentine, some of which were recrystallized during deep-seated subduction metamorphism, suggests that the diapiric serpentines in the Santa Clara are derived from peridotite wedges in the Franciscan subduction complex (now exposed as a melange).

Guatemala jadeitites and albitites were formed by deuterium-rich serpentinizing fluids deep within a subduction zone

Jadeitites and albitites from the Motagua Valley, Guatemala, are high-pressure–low-temperature metasomatic rocks that occur as tectonic inclusions in serpentinite-matrix melange. Metasomatism was driven by a fluid with a δ18OH2O value of 6‰, and a δDH2O value that is high in comparison with metamorphic fluids at other high-pressure–low-temperature localities of similar grade. We infer that the fluid was originally seawater that was entrained during subduction either as mineral-bound H2O or as free pore waters. The fluid drove serpentinization reactions in ultramafic rocks, possibly leading to deuterium enrichment of H2O, prior to forming the jadeitites and albitites at a depth of 29 ± 11 km. There are isotopic and fluid-inclusion similarities to rodingites, which are Ca-rich metasomatites found at other serpentinite localities. Our results suggest that the serpentinization process, whether it occurs within subduction zones or on the flanks of oceanic spreading ridges, may produce residual fluids that are H2O rich, have 1–8 wt% equivalent NaCl, and have high, perhaps seawater-like, δD values.

Relationships Between the North China Plate and the Tarim Plate1

Abstract The relationships and boundary between the North China and Tarim plates have been unclear for a long time; however, the two plates occupy a prominent position in the formation and evolution of the continental lithosphere of China. It is proposed that the Engger Us ophiolitic mélange zone discovered recently north of Alaxa is a typical suture between the two plates. The ophiolitic mélange zone is composed mainly of a mixture of fragments of ancient oceanic crust and sedimentary rocks of active and passive continental margins. The mélange may be divided into tectonic inclusions and matrix. The suture extends northeastwards into the Republic of Mongolia and probably westwards to meet the Altun fault. With the Engger Us ophiolitic mélange zone as the boundary the Alaxa area may be divided into two parts: the northern part (AN) belongs to the Tarim plate, while the southern part (AS) the North China plate. Geological evidence shows that the two plates were amalgamated in the Late Permian or a bit later.

Metapyroxenites From New Idria, California, USA

Rocks consisting of chlorite, diopside, and titaniferous garnet from from New Idria, California, USA, are interpreted as metamorphosed pyroxenites that are part of a serpentinite massif. These rocks represent fragments of oceanic crust or underlying mantle, emplaced within the Franciscan complex during subduction of the now-defunct Farallon plate during the Jurassic. The metapyroxenites, along with other tectonic inclusions in the serpentinite, carry important geochemical clues to the nature of subduction zone processes. The metapyroxenites may themselves be tectonic inclusions. The New Idria occurrences have strong affinities to metapyroxenites from the Malenco serpentinite in the Alps.

Clastic lawsonite, glaucophane, and jadeitic pyroxene in Franciscan metagraywackes from the Diablo Range, California

Textural relationships and mineral chemistry identify a detrital origin for jadeitic pyroxene and some lawsonite and glaucophane in Franciscan metagraywackes in the Diablo Range. These clasts differ from the neometamorphic (post-depositional) assemblage by having larger grain sizes and deformational features, including bending, cracking, and optical strain polarization. Large strain-polarized jadeitic pyroxenes contain inclusions of strained lawsonite and glaucophane, but none of these minerals has grown across clast/matrix boundaries. By comparison, neometamorphic lawsonite forms small unstrained tablets crossing the margins of large clasts and is associated with quartz + albite + chlorite + white mica in recrystallized metagraywacke matrices. Similarly, authigenic glaucophane forms discrete small prisms or euhedral outgrowths on large clasts of sodic amphibole. Jadeitic clasts in contact with metagraywacke matrices may be deeply embayed and show marginal recrystallization to fine-grained mixtures of albite + chlorite + quartz or internally to pumpellyite + albite. Backscatter electron imaging and microprobe analyses consistently show that the pyroxenes have extremely variable Jd contents. Within a single clast, compositional heterogeneity can be as much as Jd 50 Ac 39 -Aug 11 to Jd 95 Ac 4 Aug 1 . Because their texture, habit, and chemical variability are similar to those of metasomatic jadeites from tectonic inclusions in serpentinite at New Idria, southern Diablo Range, or to those of some jadeite-quartz associations in extensively recrystallized metabasite knockers, such a provenance is inferred for pyroxene clasts in the Franciscan metasediments. In metaconglomerates, pebbles containing pyroxenes (Jd 59-98 ) formed original clasts that were overprinted later by the post-depositional lawsonitic metamorphism. Metabasites at Pacheco Pass have compositionally variable Na-pyroxenes (Jd 30-92 ) which indicate tectonic emplacement of the igneous bodies within enclosing (in some cases, pyroxenefree) metagraywackes. The Diablo Range metagraywackes are weakly developed blueschist-facies assemblages of lawsonite + white mica + albite + chlorite + titanite ± glaucophane ± aragonite ± pumpellyite, which indicate minimum metamorphic conditions of P = 3.5-5.0 ± 0.5 kb at T = 100-200 °C.

Shear criteria in the Grenville Province, Ontario, Canada

The Grenville Province of Canada is a major Proterozoic orogenic belt. Mapping and structural analysis shows that the central Ontario segment of this belt is composed of a stack of imbricated thrust nappes separated by mylonite zones. The mylonite zones were generated at high metamorphic grades and contain a distinctive assemblage of mesoscopic and microscopic asymmetric structures. These structures include rotated tectonic inclusions, rotated boudins and pinch-and-swell structures, sheath folds, rotated single-crystal porphyroclasts, single crystal ‘fish’, and shear band foliation. The shear asymmetry indicated by these structures is highly consistent between members of the assemblage, and over a large area of the Grenville province, and shows that the thrust nappes were transported to the northwest. The consistency of the structures is thought to be due to cyclical dynamic recrystallization leading to steady-state foliations in the mylonites.

The Patuki intrusive suite: closed-system fractionation beneath a slow-spreading ridge

A wide range of mafic and ultramafic rock types, together with cogenetic silicic plagiogranites, form a structurally coherent intrusive sequence within the Patuki Volcanics at south D'Urville Island, New Zealand. In addition, gabbroic rocks comprise abundant tectonic inclusions in highly-sheared, concordant serpentinite bands which intrude the Patuki suite. Chemical evidence suggests many of the gabbros, including those in which recrystallization has obliterated original textures, represent magmatic cumulates and indicates extensive closed-system fractionation analogous to that known to occur beneath slow-spreading mid-oceanic ridges. Dyke intrusion occurred throughout the generation of the suite. An early stage of spreading is suggested by the anomalously low thickness of the sequence, the non-sheeted nature of the dyke suite and chemical characteristics of the lavas which comprise the extrusive component of the ophiolite.

Plagiogranites in the Coolac ophiolite suite, New South Wales, Australia

SummaryPlagiogranites occur mainly as intrusive bodies and tectonic inclusions within mafic and ultramafic members of the Coolac ophiolite suite. Quartz diorites and trondhjemites appear to represent localized late-stage felsic differentiates of subalkaline tholeiitic magma, with their formation dominantly controlled by fractionation of hornblende and plagioclase. Albitites are interpreted as altered equivalents of quartz diorites and trondhjemites. Chemical data, while emphasizing the similarity of these rocks to ‘oceanic plagiogranite', also indicate that variation in the latter may be broader than previously defined. The Coolac rocks show less marked Fe-enrichment, higher Ni and Cr, and lower Y and Zr than many ophiolitic plagiogranites. These data are consistent with weak calc-alkaline tendencies demonstrated in the Coolac ophiolite suite.

Chrysotile asbestos at Woodsreef, New South Wales

Economic chrysotile asbestos in the Great Serpentinite belt of New South Wales is restricted to a deposit in the bifurcated Woodsreef Serpentinite. Although the Woodsreef Serpentinite is dominated by schistose serpentinite, large bodies of partially serpentinized harzburgite and massive serpentinite, as well as tectonic inclusions of dolerite, gabbro, and rodingite, occur immediately south of the Slice (the minor arm of the bifurcation) in the central part of the ultramafic body.This central area has suffered a unique stress-strain history and the asbestos deposit is localized in heavily fractured massive serpentinite which is cut by narrow S 1 and S 2 zones of schistose serpentinite. Six types of asbestos veins are recognized: all are localized in stress-induced openings--either mineralized fractures, in extension cracks, in dilation sites in folds and kink bands, or along cleavage planes in bastite grains. Within these cracks, fiber formation took place by solution and recrystallization. As growth kept pace with the opening of cracks, the fibers became elongate parallel to the direction of maximum elongation in the XY plane of the strain ellipsoid. This fiber growth took place during S 1 and S 2 formation in the surrounding schistose serpentinite and is thus an integral part of the deformation in this part of the body.

Structure and metamorphism of rocks beneath the Semail ophiolite of Oman and their significance in ophiolite obduction

ABSTRACTMetamorphic rocks showing an inverted metamorphic zonation from upper amphibolite fades immediately beneath the peridotite to greenschist fades at lower levels, crop out discontinuously along the base of the Semail ophiolite thrust sheet in the Oman Mountains of eastern Arabia. These metamorphic rocks show polyphase deformation, mylonitic fabrics, and have been disrupted, folded and imbricated, and in places form tectonic inclusions in a serpentinite melange. In the more intact sequences, garnet-clinopyroxene amphibolites, with rare hornblende-bearing marbles and banded quartzites, occur at the higher levels, whilst a wide range of metasedimentary and metabasaltic rocks occur in the greenschist fades. Low glaucophane content of amphiboles and low jadeite content of clinopyroxenes suggest relatively low pressures of crystallisation. The distribution coefficient KD for co-existing garnet and clinopyroxene suggests a temperature range of 670 to 750°C. Residual heat from the recently formed ophiolite probably provided the dominant heat source for metamorphism, although frictional heating could have supplemented this. The metamorphic rocks were produced during Cenomanian-Turonian (late Cretaceous) times.The sub-ophiolite metamorphic rocks, together with basal serpentinite, Triassic alkaline and tholeiitic basalts (Haybi volcanic group), mountain-sized ‘exotic’ limestones and an upper Cretaceous sedimentary melange, comprise a distinct thrust slice termed the Haybi complex, which always overlies marine sediments of the allochthonous Hawasina complex and underlies the Semail ophiolite. The rocks of the Haybi complex are bounded by major thrust planes, the Semail thrust above and the Haybi thrust beneath, which truncate all schistosities, fold axes, imbricate thrusts and associated features. A model for ophiolite obducton is presented based on a palinspastic reconstruction of the allochthonous rocks on the Oman continental margin.

Mélanges and associated rocks, Livingstone Mountains, Southland, New Zealand

Abstract Four parallel belts of rocks west of North Mavora Lake, northern Southland, New Zealand, are mapped and described. The easternmost belt, the West Burn semischists, consists of reconstituted spilites, phyllites, and semischists, whose textural grade of metamorphism increases up-section to the west. The two central belts are melanges, one with serpentinite matrix and small tectonic inclusions (Upukerora melange), while the other has a spilitic matrix and large (up to several kilometres) tectonic inclusions (Windon melange). The westernmost belt consists of overturned volcanogenic Upper Permian sediments (Bryneira Group). Regional metamorphism ranges from pumpellyite-actinolite facies, over most of the area, to lawsonite-albite-chlorite facies in Bryneira Group. Localised bodies of greenschist facies rocks occur in fault zones. Metasomatism is very pronounced in the serpentinite melange. Pectolite occurs abundantly as a low temperature metasomatic vein and replacement mineral. The area has been comp...

A thrust plate of ophiolitic rocks in the Preston Peak area, Klamath Mountains, California

In the Preston Peak area, Klamath Mountains, California, a regional thrust fault separates metasedimentary rocks of the Late Jurassic Galice Formation from an overlying plate of older ophiolitic rocks. The ophiolite consists of a basal sheet of ultramafic tectonite overlain and intruded by a heterogeneous mafic complex that in turn is overlain by metabasaltic and metasedimentary rocks. Field relations indicate that the ophiolite is polygenetic, with a major temporal hiatus separating the tectonitic ultramafic rocks and the associated mafic rocks. Mineral assemblages and textures in the ultramafic rocks suggest high-temperature recrystallization and penetrative deformation. In contrast, diabase and diabase breccia, the most abundant constituents of the mafic complex, are nonschistose rocks metamorphosed to lower greenschist facies. Contacts between ultramafic rocks and rocks of the mafic complex are fault contacts, intrusive contacts, or both. Mafic rocks occur in the ultramafic rocks as diabase dikes with chilled margins and as tectonic inclusions. Piecemeal growth of the ophiolite is also indicated by minor features: scarce jackstraw-textured talc-olivine rocks in tectonitic peridotite, cognate xenoliths of gabbro and olivine clinopyroxenite in diabase, and scattered dikes of intermediate composition in both ultramafic and mafic rocks. Field aspects of the ophiolite appear more compatible with a primitive island-arc setting than with a spreading oceanic ridge or marginal-basin model. The temporal relations between the ultramafic and mafic rocks, the presence of pyroclastic breccias, and the character of associated epiclastic rocks support this hypothesis. On the basis of this interpretation, the tectonic history of this segment of the Klamath Mountains during late Paleozoic to Jurassic time was dominated by island-arc genesis and westward extensional rifting. The ultimate collapse of this system occurred during the Late Jurassic (Nevadan orogeny) when the Galice Formation (Jurassic island arc and associated sedimentary basin) was thrust beneath the Preston Peak ophiolite, a Permian-Triassic remnant arc.

Amphibole‐olivine‐spinel, cordierite‐anthophyllite and related hornfelses associated with metamorphosed serpentinites in the goobarragandra district, near Tumut, New South Wales

Abstract A large variety of hornfelses is developed in and around the Coolac Serpentinite, in the aureole of the Bogong Granite. Essentially isochemical metamorphism of partly serpentinized harzburgites has produced metaserpentinites containing variable amounts of talc, tremolite, olivine, orthopryoxene, and other phases: Chromian spinel, partly changed to magnetite and/or ferrichromite, remains as a relict primary phase very near the granite contact. Alumino‐ferromagnesian hornfelses containing amphiboles, spinel, and magnetite, with or without olivine, chlorite, orthopyroxene, and sulphides, as well as cordierite‐antho‐phyllite hornfelses, various calc‐silicate hornfelses and mafic hornfelses, are intimately associated with the metaserpentinites. The chemical compositions of the amphibole‐olivine‐spinel hornfelses can be matched with those of chlorite‐rich and chlorite‐tremolite‐rich rocks outside the thermal aureole produced by reaction‐zone metasomatism of tectonic inclusions within the Coolac Serpent...

Deformation and Metamorphism of the Franciscan Subduction Zone Complex Northwest of Pacheco Pass, California

The Franciscan Complex northwest of Pacheco Pass, California, includes three fault-bounded units, each characterized by a different deformational style and suite of metamorphic mineral assemblages. Structurally highest is jadeitic pyroxene-bearing metagraywacke semischist. The areally extensive Garzas tectonic melange separates the semischist from the structurally lowest pumpellyite-bearing Orestimba metagraywacke. The Garzas melange is representative of Franciscan melanges in general. These mappable bodies have an internal fabric dominated by penetrative, mesoscopic shear fractures and contain tectonic inclusions of all sizes immersed in a pervasively sheared, generally fine-grained matrix. The shear fractures record brittle deformation of consolidated rock bodies. Many melanges contain exotic inclusions, clearly not derived from adjacent units, and metamorphic mineral assemblages and textures indicate that they were metamorphosed before being tectonically mixed with more voluminous, generally lower grade, metagraywacke inclusions. The structural units are grossly sheetlike in external form and are separated from one another by gently to steeply dipping major faults. Unlike low-angle thrusts in imbricated Cordilleran terranes, the faults do not systematically repeat or offset a normal stratigraphic sequence but rather juxtapose rock units that bear no apparent stratigraphic, deformational, or metamorphic relation to one another. The structural units were separately deformed and metamorphosed under a variety of conditions prior to their tectonic juxtaposition during late Mesozoic continental margin subduction. Field and petrographic evidence permit, but do not prove, the hypothesis that both the semischist and exotic, high-grade melange inclusions once were more deeply buried and have been emplaced upward into their present anomalously shallow structural positions.

Petrogenesis of sulphide-bearing reaction zones in the Coolac ultramafic belt, New South Wales, Australia

The Coolac ultramafic belt consists dominantly of variably serpentinised harzburgite and contains a diversity of tectonic inclusions. Reaction zones of chlorite-, talc- and Ca-Al silicate-rich rocks are commonly developed between serpentinites and either tectonic inclusions or country rocks. The chlorite-and talc-rich parts of the reaction zones typically contain sparsely disseminated to rarely massive Cu- and Fe-bearing sulphides, variable sphalerite, and minor Ni- (-Co-Fe) sulphides, arsenides and sulpharsenides, Pb and Bi minerals. The reaction zones have formed concomitantly with the serpentinisation of the harzburgite at temperatures of 100°–350°C and at pressures of <6 kb. Migration of Ca, Al, Ti, V, Sc, Cu and Zn has occurred from the ultramafic rocks to the reaction zones. The sulphur content of the ultramafic rocks increased during serpentinisation, but decreased markedly in the final stage of the process owing possibly to rising oxygen fugacity. The availability of sulphur during serpentinisation may have enabled sulphide minerals to form from the concentration of base metals in the reaction zones.

Paragenetic Relationships in an Amphibolitic Tectonic Block in the Franciscan Terrain, Panoche Pass, California

Two tectonic inclusions within the Franciscan mélange of California display a relict epidoteamphibolitic assemblage that has been modified by rather continuous degrees of retrogressive replacement to blueschist facies minerals. The relict assemblage consists of plagioclase +hornblende+epidote+sphene±rutile±calcic pyroxene±garnet; superimposed during later blueschist metamorphism is the asemblage sodic amphibole+lawsonite±sodic pyroxene±pumpellyite±phengitic mica ±albite+sphene±Ca-carbonate. In an attempt to evaluate chemical redistribution accompanying blueschist metamorphism, electron microprobe analyses have been determined for all major minerals from both assemblages. Hornblende commonly exhibits replacement by crossite around grain boundaries and replacement patches of more riebeckitic amphibole adjacent to mafic grains or inclusions; actinolitic amphibole occurs in veins cross-cutting hornblende and in some cases these veins are zoned to include a central core of sodic amphibole. Omphacitic pyroxene occurs in veinlets and replaces calcic pyroxene in some cases; a more aluminous omphacite variety is commonly associated with the breakdown of epidote. Garnet is highly retrogressed to chlorite. In general, relict Ca-Al silicates are in various stages of breakdown, being largely converted to lawsonite and pumpellyite; concurrently Na-Al silicates formed by replacement where early calcic minerals participated in blueschist reactions. Reactants and products may be separated by phases not participating in the reactions. Whole rock compositions point strongly to a closed system with respect to all major cations, and the principal source for Na participating in replacement reactions appears to be from the breakdown of albite. The mechanism for replacement of the older assemblage is primarily attributed to a contemporaneous series of coupled reactions among nearly all the early minerals and a fluid phase. Sea-floor spreading models may explain the tectonic emplacement of the amphibolite from a relatively high temperature-high pressure regime into a comparatively low temperature-high pressure environment leading to the observed mineralogical re-adjustments.