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Abstract: Intense devolatilization and chemical-density differentiation attended accretion of planetesimals on the primordial Earth. These processes gradually abated after cooling and solidification of an early magma ocean. By 4.3 or [...]

The White-Inyo Range lies within the regional transition from Paleozoic-Precambrian North American continental basement to outboard Mesozoic and younger accreted terranes and a superimposed Andean-type arc. In the central White Mountains, the metaluminous Barcroft granodiorite invaded a major NE-striking, SE-dipping high-angle reverse fault--the Barcroft break. Because it is a relatively isolated igneous body and is well exposed over an elevation range of 1,500-4,000 m, its thermal history and that of the surrounding superjacent section are clearer than those of nearly coeval, crowded plutons emplaced in the hotter Sierra Nevada belt. The Barcroft pluton was emplaced as a compositionally heterogeneous series of areally scattered melt pulses episodically injected over the approximate interval 167-161 Ma. The oldest dated rocks are relatively quartzofeldspathic, whereas the youngest is more ferromagnesian, suggesting progressive partial fusion of a relatively mafic protolith. Heavy rare earth-enriched zircons indicate that Barcroft melts were derived at mid-crustal depths from a previously emplaced metabasaltic protolith containing plagioclase but lacking garnet. Granodioritic magma genesis involved the possible mixing of mafic and felsic melts, as well as very minor assimilation of country rocks, but mainly by fractional fusion and crystallization. Bulk chemical, rare earth, and isotopic data suggest that analyzed Barcroft rocks are members of a single suite. Granodioritic rocks are slightly more magnetite-rich at higher elevations on the NE, nearer the roof of the pluton. Earlier thermobarometry chronicled cooling and re-equilibration of the Barcroft pluton from its margins inward, as well as from mid-crustal generation depths of ~25 [K.sub.m] through ascent and stalling at ~10-12 [K.sub.m]. Refractory phase assemblages crystallized along the pluton margins, whereas subsolidus minerals in the interior of the of body continued to exchange with upper crustal deuteric and surficial aqueous fluids during exhumation and cooling. Keywords Barcroft granodiorite * Magma mixing * Fractional crystallization * Igneous thermobarometry * Oxygen isotope geothermometry * Zircon U-Pb ages, Goal of the present research synthesis The NNW-trending White-Inyo Range defines the western edge of the Basin and Range province in eastern California (Stevens et al. 1997; Coleman et al. [...]

We present new U/Pb ages for detrital zircons separated from six quartzose metagraywackes collected from different Franciscan Complex imbricate nappes around San Francisco Bay. All six rocks contain a broad spread of Late Jurassic--Cretaceous grains originating from the Klamath--Sierra Nevada volcanic-plutonic arc. Units young structurally downward, consistent with models of progressive underplating and offscraping within a subduction complex. The youngest specimen is from the structurally lowest San Bruno Mountain sheet; at 52 Ma, it evidently was deposited during the Eocene. None of the other metagraywackes yielded zircon ages younger than 83 Ma. Zircons from both El Cerrito units are dominated by ca. 100-160 Ma grains; the upper El Cerrito also contains several grains in the 1200-1800 Ma interval. These samples are nearly identical to 97 Ma metasedimentary rock from the Hunters Point shear zone. Zircon ages from this melange block exhibit a broad distribution, ranging from 97 to 200 Ma, with only a single pre-Mesozoic age. The Albany Hill specimen has a distribution of pre-Mesozoic grains from 1300 to 1800 Ma, generally similar to that of the upper El Cerrito sheet; however, it contains zircons as young as 83 Ma, suggesting that it is significantly younger than the upper El Cerrito unit. The Skaggs Spring Schist is the oldest studied unit; its youngest analyzed grains were ca. 144 Ma, and it is the only investigated specimen to display a significant Paleozoic detrital component. Sedimentation and subduction-accretion of this tract of the trench complex took place along the continental margin during Early to early--Late Cretaceous time, and perhaps into Eocene time. Franciscan and Great Valley deposition attests to erosion of an Andean arc that was active over the entire span from ca. 145 to 80 Ma, with an associated accretionary prism built by progressive underthrusting. We use these new data to demonstrate that the eastern Franciscan Complex in the northern and central Coast Ranges is a classic accretionary prism, where younger, structurally lower allochthons are exposed on the west, and older, structurally higher allochthons occur to the east, in the heavily studied San Francisco Bay area. doi: 10.1130/B26399.1

Using laser ablation ICP-MS techniques, U-Pb ages are reported for 338 detrital zircons separated from four Franciscan metagraywacke samples from imbricate thrust sheets cropping out in the Pacheco Pass area, east central California Coast Ranges. Structurally higher slabs are clastic quartz [+ or -] albite-bearing rocks typified by neoblastic lawsonite + jadeitic pyroxene; the lowest exposed slab lacks jadeitic pyroxene but contains traces of newly grown pumpellyite and lawsonite in addition to abundant quartz + albite. Studied specimens contain moderate amounts of phengite + titanite [+ or -] chlorite, stilpnomelane, carbonate, iron oxides, rock fragments, and carbonaceous matter. These Diablo Range clastic sediments were deposited in mid-Cretaceous and Late Cretaceous time. The highest allochthon, unit I, was deposited after ~ 102 Ma, unit IV was deposited after ~90-102 Ma, and unit V was deposited after ~86 Ma. Metagraywacke depositional-accretionary ages within slab IV young upward, but ages of the slabs decrease progressively downward. In the four-sample aggregate, most zircons have igneous ages falling in the 135-170 Ma range, with a smaller population at 85-120 Ma. The rocks contain rare zircons of Middle Proterozoic--Late Archean ages. A minor source in northern California and NW Nevada seems likely for the Pacheco Pass metagraywackes, but the Sierran-Klamath calcalkaline arc provided most of the clastic debris. Judging by the volcanic nature of lithic clasts in these Eastern Belt metagraywackes, and because massive Sierran plutonism occurred at ~85-120 Ma, the abundant 135-170 Ma zircons probably were derived chiefly from the eroding comagmatic volcanic arc rather than from the less voluminous Jurassic arc plutons. Citation: Ernst, W. G., U. Martens, and V. Valencia (2009), U-Pb ages of detrital zircons in Pacheco Pass metagraywackes: Sierran-Klamath source of mid-Cretaceous and Late Cretaceous Franciscan deposition and underplating, Tectonics, 28, TC6011, doi: 10.1029/2008TC002352.

The NNE trending dextral Yangsan fault system and the WNW trending sinistral Gaum fault system comprise a conjugate set that cuts the Cretaceous Gyeongsang Basin along the SE margin of the Korean Peninsula. These faults evidently formed under compressional stress accompanied by a progressive 12[degrees] block rotation, calculated from a geometric model incorporating angles between conjugate fault sets, widths of fault domains, and measured fault displacements. The angle between paleomagnetic declinations in the set of conjugate fault domains increases with age, suggesting incremental block rotation that included overall rotation of the entire Gyeongsang Basin (or the Korean Peninsula) with respect to Eurasia. The model accounts for Yucheon Group volcanism, subsequent caldera subsidence in the Uiseong subbasin, and volcanism in the Miryang subbasin, all produced by regional tension. The tectonic regime of the Gyeongsang Basin apparently evolved from compression to extension at ~50 Ma, reflecting India-Eurasia collision and opening of the Sea of Japan (East Sea).

Fifty years ago, geologic conditions attending the formation of blueschists, eclogites, and garnet lherzolites were not known. But, with the advent of high-pressure phase-synthesis equipment and precise calorimetry, minerals like jadeite, aragonite, pyrope, and the dense polymorphs of Si[O.sub.2] and C were shown to be stable at elevated pressures and relatively low temperatures. Metamorphic conditions required by P-T stabilities of these minerals reflect the operation of plate tectonics, lithospheric subduction, and inferred mantle convection. Integration of phase equilibria with dynamic tectonic processes has illuminated the petrogenesis of the crust. Combined with geochemical, geophysical, and isotopic data, high-pressure phase equilibria are also providing new constraints on the constitution and evolution of the mantle. Circumpacific blueschists and eclogites occur in penetratively sheared nappes that are overturned seaward, indicating 30-50 km descent of an oceanic plate during metamorphism before partial exhumation of mainly low-density crustal material. Neoblastic coesite and microdiamond inclusions in tough, rigid host minerals show that continental collision involves fragmentary recovery of subducted rocks from depths of 100-130 km, far deeper than traditionally thought. Even more surprising, garnet peridotites from the central Alps, western Norway, Bohemia, and China display intergrowths and exsolution lamellae reflecting the former existence of majoritic garnet, stishovite, and other phases requiring depths of origin >300 km. Exsolved nanominerals attest to the decompression of precursor phases that had formed at profound depths preceding mantle upwelling. Times of deep-seated storage and rates of exhumation remain as major problems. Fluid-rock and lithosphere-asthenosphere interactions have recycled volatiles to the deep Earth through subduction of both hydrous and nominally anhydrous minerals. Mantle petrochemistry and plume-plate dynamics control the evolving architecture of the Earth's crust and the interdependent biosphere. Applications of advanced technologies to condensed materials are leading to a fuller understanding of the planetary interior in time and space. Keywords: Metamorphic petrology, high-pressure belts, phase equilibria, subduction-zone belts, high-pressure studies, convergent plate junctions, thermobarometry, ultrahigh-pressure rocks

[1] The NNW-trending White-Inyo Range includes intrusive and volcanic rocks on the eastern flank of the Sierran volcano-plutonic arc. The NE-striking, steeply SE-dipping Barcroft reverse fault separates folded, metamorphosed Mesozoic White Mountain Peak mafic and felsic volcanic flows, volcanogenic sedimentary rocks, and minor hypabyssal plugs on the north from folded, well-bedded Neoproterozoic-Cambrian marble and siliciclastic strata on the south. The 163 [+ or -] 2 Ma Barcroft Granodiorite rose along this fault, and thermally recrystallized its wall rocks. However, new SHRIMP-RG ages of magmatic zircons from three White Mountain Peak volcanogenic metasedimentary rocks and a metafelsite document stages of effusion at ~115-120 Ma as well as at ~155-170 Ma. The U-Pb data confirm the interpretation by Hanson et al. (1987) that part of the metasedimentary-metavolcanic pile was laid down after Late Jurassic intrusion of the Barcroft pluton. The Lower Cretaceous, largely volcanogenic metasedimentary section lies beneath a low-angle thrust fault, the upper plate of which includes interlayered Late Jurassic mafic and felsic metavolcanic rocks and the roughly coeval Barcroft pluton. Late Jurassic and Early Cretaceous volcanism in this sector of the Californian continental margin, combined with earlier petrologic, structural, and geochronologic studies, indicates that there was no gap in igneous activity at this latitude of the North American continental margin. Citation: Scherer, H. H., W. G. Ernst, and R. Brooks Hanson (2008), Geologic implications of new zircon U-Pb ages from the White Mountain Peak Metavolcanic Complex, eastern California, Tectonics, 27, TC2002, doi:10.1029/2007TC002141.

Devonian-Middle Jurassic terrane assemblies in the Klamath Mountains and Sierra Nevada Foothills consist chiefly of ophiolite-chert-argillite sequences. Mafic-ultramafic complexes are oceanic, whereas associated fine-grained deep-water terrigenous sediments were derived mainly from adjacent, previously docked Klamath-Sierran terranes. Coeval calc-alkaline arc rocks are volumetrically rare. Geologic and petrochemical relations suggest a rifted are origin for Klamath mafic metavolcanic units inter-layered with distal turbidites in the 170-200 Ma North Fork terrane; detrital zircon U-Pb ages indicate that the clastic debris had a regional eastern Klamath source. The Eastern Hayfork cherty melange contains ophiolitic scraps and distinctive olistostromal sandstone blocks evidently derived from the nearby Eastern Klamath Antelope Mountain Quartzite. The seaward 200 Ma Rattlesnake Creek terrane is an ophiolitic melange with North Fork petrotectonic affinities. The North Fork-Eastern Hayfork-Rattlesnake Creek amalgam correlates with the Calaveras Complex and the outboard Jura-Triassic arc belt in the Sierran Foothills. Geochemical bulk-rock and zircon U-Pb age data support interpretation of the 200 Ma Jura-Triassic arc as an adjacent offshore mafic belt overlying a 300 Ma ophiolitic basement. These oceanic complexes were sutured against the Central Metamorphic Belt-Eastern Klamath-Feather River-Northern Sierra terrane backstop before deposition and deformation of the outboard Upper Jurassic Galice and Mariposa formations. Klamath-Sierran terrane assemblies reflect ~230 m.y. of transpression-transtension involving only minor episodes of subduction, producing ubiquitous ophiolite-chert-argillite lithologies and rare felsic arc rocks. In contrast, the Late Jurassic to largely Cretaceous Klamath-Sierra Nevada quartzo-feldspathic volcanic-plutonic arc attests to massive calc-alkaline magmatism attending a strong eastward component of underflow by the Farallon plate. The coeval Galice-Mariposa formations, followed by the Cretaceous Great Valley forearc and Franciscan trench deposits, are first-cycle felsic debris shed mainly from the Klamath-Sierran arc. These units record ~70 m.y. of rapid sialic crustal growth attending major periods of approximately margin-normal convergence. This profound transition in northern California included Devonian-Middle Jurassic rifting, drifting, and stranding of ophiolite-chert-argillite terranes along an adjacent curvilinear continental margin, then nearly head-on Cretaceous subduction that resulted in massive calc-alkaline igneous activity, the erosion of which generated the felsic Great Valley Group forearc basin and Franciscan Complex trench clastic sedimentary units. Keywords: transpression, transtension, convergence, terrane assembly, northern California, Mesozoic plate tectonics.

Mantle-derived garnet peridotites are a minor component in many very high-pressure metamorphic terranes that formed during continental subduction and collision. Some of these mantle rocks contain trace amounts of zircon and micrometer-sized inclusions. The constituent minerals exhibit pre- and postsubduction microstructures, including polymorphic transformation and mineral exsolution. Experimental, mineralogical, petrochemical, and geochronological characterizations using novel techniques with high spatial, temporal, and energy resolutions are resulting in unexpected discoveries of new phases, providing better constraints on deep mantle processes.

The NNW-trending White-Inyo Range represents the eastern continuation of the Sierran volcanic-plutonic are along the central California-Nevada state line. South of White Mountain Peak, the NE-trending, steeply SE-dipping Barcroft structural break transects the orogenic belt. This high-angle reverse fault downdropped deformed, weakly recrystallized, mid-Mesozoic, mildly alkaline, White Mountain Peak mafic and felsic metavolcanic flows, interlayered volcanogenic metasedimentary strata, and minor subvolcanic plugs against a folded Neoproterozoic-lower Paleozoic quartzite and carbonate metasedimentary section on the south. The Middle Jurassic Barcroft granodioritic pluton later intruded the Barcroft break; the Middle Jurassic Cottonwood, and Late Cretaceous McAfee Creek + Pellisier Flat granitoids also invaded the superjacent section on the NE. Heating of the White Mountain Peak metavolcanic complex by the Barcroft and nearby plutons caused a southeastward progressive increase in metamorphic grade, producing neoblastic biotite, epidote, and hornblende in mafic volcanic rocks + minor hypabyssal stocks, and biotite + epidote [+ or -] andalusite in felsic volcanic rocks + volcaniclastic strata. Gradual increases in biotite Ti and plagioclase An contents reflect this SE metamorphic zonation. Utilizing hornblende-actinolite, muscovite-celadonite, and biotite solid solutions, thermobarometry indicates lithostatic pressures of 2-3 kbar during recrystallization, with temperatures ranging from a background value of ~350 [degrees]C far from the Barcroft pluton, to >500 [degrees]C along the intrusive contact. Physical conditions were similar to those developed in the Neoproterozoic-lower Paleozoic platform strata directly south of the Barcroft granodiorite. The region constitutes a typical example of the P-T evolution of the late Mesozoic Californian crustal margin, where episodic magma emplacement caused widespread contact metamorphism.

Crustal thickness is related to climate through precipitation-induced erosion. Along the Andes, the highest mountains and thickest crust ([approximately equal to] 70 km) occur at 25 [degrees] south, a region of low precipitation. Westerly winds warm passing over the Atacama Desert; precipitation is modest in the High Andes and eastward over the Altiplano. Severe aridity, hence low erosion rates, helps to account for the elevated volcanogenic contractional arc and high, internally draining plateau in its rain shadow. Weak erosion along the north-central arc provides scant amounts of sediment to the Chile-Peru Trench, starving the subduction channel. Subcrustal removal might be expected to reduce the crustal thickness, but is not a factor at 25 [degrees] south. The thickness of the gravitationally compensated continental crust cannot reflect underplating and/or partial fusion of sediments, but must be caused chiefly by volcanism-plutonism and contraction. Contrasting climate typifies the terrane at 45 [degrees] south where moisture-laden westerly winds encounter a cool margin, bringing abundant precipitation. The alpine landscape is of lower average elevation compared with the north-central Andes and is supported by thinner continental crust ([approximately equal to] 35 km). Intense erosion supplies voluminous clastic debris to the offshore trench, and vast quantities are subducted. However, the southern Andean crust is only about half as thick as that at 25 [degrees] south, suggesting that erosion, not subcrustal sediment accretion or anatexis, is partly responsible for the thickness of the mountain belt. The Himalayas plus Tibetan Plateau, the Sierra Nevada plus Colorado Plateau, and the Japanese Islands exhibit analogous relationships between crustal thickness and climate.

To assess the effects of regional climate change using remote-sensing methods, we conducted integrated geologic, botanical, and field-radiometer baseline studies on the ground in the central White-Inyo Mountains. Vegetation on contrasting geologic substrates in 58, 50 x 50 m sites was studied annually in early July and in late September 1997-2001. We quantified areal abundances of plant species by using field inventories and ground-based values of the normalized-difference vegetation index (NDVI). Distributions and abundances of species are complex functions of microclimate (elevation, slope, and facing direction), seasonality, soil development, bedrock lithologies, and albedo. X-ray diffraction studies demonstrate that the soils consist chiefly of in situ weathering components such as illite, montmorillonite, chlorite, quartz, carbonates, and feldspars. Soils along higher ridges contain modest amounts of wind-blown grains derived from glaciated granodioritic plutons and volcanic roof pendants of the Sierra Nevada. Light-colored limestone and dolomite support only sparse amounts of Sagebrush at any altitude, whereas this drought-tolerant species is widespread on dark argillite, quartzite, phyllite, and granitic rocks. Grasses grow luxuriantly on dark granite and quartzite, but poorly on carbonate bedrock. Trees are numerous on carbonate rocks, but are uncommon on dark granite and quartzite. Higher-albedo rocks and soils retain moisture better than darker substrates and in general support greater vegetative cover and biomass. These studies provided ground control for hyperspectral data collected by ER-2 aircraft along three Airborne Visible and InfraRed Imaging Spectrometer (AVIRIS) flight lines. Because of the ~16 m spatial resolution of the remotely sensed data, only plant communities could be mapped from AVIRIS imagery. On the basis of the dominant species, we identified the following communities: Shadscale/desert scrub; Pinyon-Juniper woodlands; Sagebrush meadows; Mountain Mahogany woodlands; Aspen woodlands; Bristlecone Pine-Limber Pine woodlands; and Alpine fell-fields. When judged by correlation with the 50 x 50 m ground-based inventoried sites, botanical community assignments for the June 2000 flight lines achieved an accuracy of 81%; in contrast, October 1996 assignments were only 65 % correct, reflecting low sun angle and senesced vegetation. Keywords: hyperspectra, microclimate, vegetation, White-Inyo Mountains, remote sensing, geobotany.

The Middle Paleozoic Maksyutov Complex is an important component of the Eurasian collisional orogeny. It consists of dominant mica-rich garnet schist and mica-poor quartzofeldspathic gneiss enclosing minor mafic eclogite boudins (unit no. 1). Employing Raman spectroscopy, we identified three cuboidal microdiamond inclusions ([Tilde]2-3 micrometers in diameter) in garnet hosts from two different mica-poor gneissic samples. Broad spectral bands and high magnification SEM images suggest that the cuboids are fine-grained nanocrystalline diamond aggregates characterized by limited long-range ordering. Their poor crystallinity is compatible with relatively low-temperature, solid-state growth in the absence of both melt and a C-O-H-N fluid. Poor crystallinity, and small grain size suggest that such aggregates may represent the lowest temperature microdiamonds yet identified in nature. Their formation required ultrahigh-pressures (UHP) at a minimum of 3.2 GPa, and a metamorphic temperature of [Tilde]650 [degrees]C. Blocky graphite up to 10+ mm across in the matrix of mica-rich carbonaceous garnet schist may represent pseudomorphs after much larger neoblastic diamonds. Thermobarometric calculations for analyzed coexisting garnet + omphacite + phengite from six Maksyutov unit no. 1 mafic eclogites indicate retrograde physical conditions of 610-680 [degrees]C, 1.7-2.6 GPa, slightly lower-pressure conditions than the coesite stability field. Complete conversion of diamond to blocky graphite in the mica-rich schists, and recrystallization of coesite to quartz in the schists, quartzofeldspathic gneisses, and eclogite pods reflect relatively slow exhumation from [Tilde]110 km depth to upper crustal levels over 60-90 m.y. Phengite inclusions in zircon and garnet hint at modest activity of [H.sub.2]O during prograde UHP metamorphism of the eclogites and mica-poor gneisses. The latter have retained rare, tiny microdiamond inclusions in garnet on decompression. Abundant white mica in the carbonaceous garnet schists probably reflects a C-O-H-N fluid-mediated, kinetically enhanced prograde production of diamond, and efficient obliteration of this phase accompanying leisurely ascent of the subduction complex. In contrast, associated micapoor gneisses and eclogites were relatively dry during exhumation, so retained rare nanocrystalline microdiamond inclusions in garnet.

The north-northwest--trending White-Inyo Range locally defines the western edge of the Great Basin. The northeast-trending Barcroft structural break lies astride the province boundary. Along this preintrusive, high-angle reverse fault, middle Mesozoic White Mountain Peak alkaline volcanic and intercalated volcaniclastic rocks on the north are separated from uppermost Proterozoic-Lower Cambrian miogeoclinal quartzite and carbonate strata on the south by the 165 [+ or -] 1 Ma (SHRIMP-RG, [sensitive, high-resolution ion microprobereverse geometry], U-Pb zircon) Barcroft pluton. Although locally faulted, the subparallel southeast and northwest borders of the body display intrusive contacts. Finer-grained comagmatic metadiorite occurs as early-stage dikes in the wall rocks. Eastward, the 100 Ma McAfee Creek granite intrudes the Barcroft pluton. Tertiary diabase dikes crosscut the section. The two granitic series, reflecting local evolution of the Mesozoic arc system, are described in this paper: (1) mafic granitoid rocks of the calc-aikaline Barcroft series, including chemically intergradational granodiorite, gabbro/diorite, metadiorite, and rare alaskite-aplite--all rich in large ion lithophile elements (LILEs) and depleted in high field strength elements (HFSEs), and (2) granite of the felsic McAfee Creek series, which has even greater enrichment of LILEs and greater depletion in HFSEs than the Barcroft pluton. Rocks rich in hornblende clinopyroxene and belonging to the Ca-rich, metaluminous Barcroft series exhibit a broad range of chemical and mineral compositions and represent products of both mixing between high- and low-silica members of the series to generate intermediate compositions and fractional crystallization to generate the most mafic (cumulate) rocks. The younger, more homogeneous, K-rich McAfee Creek-type muscovite granite possesses mildly peraluminons, minimum-melt compositions. Field evidence for magma mixing and isotopic data for the Barcroft series ([[epsilon].sub.Nd(t)] = -1.53 to -5.50, [sup.87]Sr/[sup.86][Sr.sub.(i) = 0.7053 - 0.7063) and the McAfee Creek granite ([[epsilon]Nd(t)] = -5.64 to -9.76, [sup.87]Sr/[sup.86][Sr.sub.(i) = 0.7062 - 0.7116) require open-system processes and involvement of preexisting crustal rocks in their genesis. The Barcroft series is one of several shallow-level magma systems now recognized in the Sierran arc that include significant amounts of reworked, preexisting crustal material. These magma series are distinct from those that represent demonstrably deeper levels of Mesozoic magma systems and lack isotopic evidence for significant involvement of older crust. Despite these differences in magma sources and evolution, both magma series evolved along indistinguishable petrologic and chemical trends. Igneous rocks in the White Mountains record post-Paleozoic growth of this part of the Californian margin: (1) Jurassic and younger subduction, partial fusion of the overlying mantle wedge, and/or deep-seated mafic crust, ascent of mildly alkaline and later calc-aikaline magmas attending Andean-style voicanic-plutonic arc production, with significant involvement of preexisting crustal rocks; (2) Late Cretaceous thickening, heating, and crustal contamination, followed by rise of peraluminous granite-minimum melts; and (3) Neogene Basin and Range lithospheric transtension, tapping of upper-mantle diabasic melt, and dike emplacement. Keywords: Barcroft pluton, caic-alkaline magmas, metaluminous granodiorite, peraluminous granite, Andean arc, Mesozoic margin.

The Middle Jurassic Barcroft mafic granodiorite and Late Cretaceous, ternary-minimum McAfee Creek Granite are important components of the igneous arc sited along the SW North American margin. Bulk-rock analyses of 11 samples of the metaluminous, I-type Barcroft comagmatic suite have an average [delta][sup.18]O value of 7.4 [+ or -] 0.6[per thousand] (all values [+ or -] 1[sigma]). Four Barcroft specimens average [[epsilon].sub.Nd] = -3.6 [+ or -] 1.8, [sup.87]Sr/[sup.86]Sr = 0.707 [+ or -] 0.001. The pluton consists of petrochemically gradational, Ca-amphibole-rich gabbro/diorite, granodiorite, metadiorite, and rare alaskite-aplite; for most of the pluton, oxygen isotope exchange of quartz, feldspar(s), biotite, and Ca-amphibole accompanied local deuteric alteration. Eight specimens of slightly peraluminous granitic rocks of the muscovite-bearing McAfee Creek series have an average [delta][sup.18]O of 8.6 [+ or -] 0.5[per thousand]. Four McAfee-type samples average [[epsilon].sub.ND] = -7.8 [+ or -] 1.7, [sup.87]Sr/[sup.86] Sr = 0.711 [+ or -] 0.004. For both plutons, bulk-rock evidence of exchange with near-surface water is lacking, suggesting ~5-10 km cooling depths. Barcroft minerals exhibit regular oxygen isotopic partitioning from high to low [delta][sup.18]O in the sequence quartz > plagioclase > K-feldspar > > amphibole [greater than or equal to] biotite. Along the SE margin of the pluton, quartz and biotite in Lower Cambrian quartzites are higher in [delta][sup.18]O, and show slightly larger fractionations than igneous analogues. Exchange with fluids derived from these heated, contact-metamorphosed country rocks increased bulk [sup.18]O/[sup.16]O ratios of Barcroft border rocks (and constituent plagioclase + subsolidus tremolite-actinolite), especially of granitic dikes transecting the wall rocks. Oxygen isotope thermometry for seven Barcroft pluton quartz-amphibole and six quartz-biotite pairs indicate apparent subsolidus temperatures averaging 519 [+ or -] 49[degrees]C. Quartz-plagioclase pairs from two Barcroft granodiorites yield values of 519 and 515[degrees]C. A quartz-biotite pair from a quartzite adjacent to the Barcroft pluton yields an apparent temperature of 511[degrees]C, in agreement with estimates based on contact metamorphic parageneses. Except for its SE margin, Barcroft pluton silicates evidently exchanged oxygen isotopes under local deuteric conditions. Compatible with Ca-amphibole thermobarometric analyses, areal distributions for quartz-plagioclase, quartz-amphibole, and quartz-biotite pairs reveal that putative annealing temperatures are lowest in NE-trending axial portions of the Barcroft body, so it simply cooled inwards. Intrusion ~70 million years later by the McAfee Creek Granite had no discernable effect on [delta][sup.18]O values of Barcroft minerals and bulk rocks.

Mid-Mesozoic to Tertiary igneous activity reflects arc evolution along the Sierra + White-Inyo sector of the continental margin. The Ca-amphibole + biotite [+ or -] augite-bearing Middle Jurassic Barcroft pluton consists of intergradational, quartzose gabbro-diorite, metadiorite, voluminous mafic-to-felsic granodiorite, and rare alaskite-aplite. It was emplaced along the Barcroft structural break--a major ENE-striking suture juxtaposing mid-Mesozoic bimodal metavolcanics on the north against metamorphosed Lower Cambrian platform strata on the south. The Cretaceous, two-mica McAfee Creek Granite intruded both wall rocks and the Barcroft granodioritic pluton on the east. Geologic mapping and petrochemical studies have documented the structure and metaluminous geochemistry of the Barcroft body. Analyses of Ca-clinoamphiboles from 16 Barcroft rocks allow the assignment of P-T conditions of crystallization/recrystallization based on experimentally determined and calculated clinoamphibole thermobarometers. Combined with earlier studies of country-rock metamorphism, and oxygen isotopic investigations of mineral assemblages in both plutons and wall rocks, four stages in the P-T evolution of the Barcroft pluton have been identified: Stage 1, partial melting of a mafic protolith at ~25 km or greater depth, then onset of crystallization of refractory hornblende and other ferromagnesian phases at ~915 [+ or -] 25 [degrees] C attending magma ascent. Stages 2 [right arrow] 3, crystallization of progressively more actinolitic hornblende during decompression cooling and stalling of the pluton at a depth of 10-12 km, as temperature declined over the interval ~760-650 [degrees] C. Stage 4, subsolidus recrystallization/exsolution of nearly stoichiometric actinolite [+ or -] grunerite attending deuteric alteration over the range ~395-545 [degrees] C at a depth of 4-5 km. Mineral assemblages of stages 1 [right arrow] 4 are disposed progressively inward, indicating that the Barcroft pluton lost heat along intrusive contacts with the wall rocks, cooling, annealing, and re-equilibrating in the process. The Cretaceous McAfee Creek Granite transected this ENE-trending thermal zonation, but exerted little effect on Ti + Al contents (and [[delta].sup.18]O values) of Barcroft amphiboles.

Andesitic arcs are sited over convergent lithospheric plate junctions. Aqueous fluids driven off the downgoing slab are responsible for partial fusion of the warming subducted plate and/or the overlying mantle wedge, and the ascent of calc-alkaline melts. Various hydrous minerals have been proposed to be the source of this [H.sub.2]O. Experimental equilibrium studies show that, under subduction-zone geothermal gradients of 5-7 [degrees] C/km, clinoamphibole constitutes a major phase in deep-seated (>75 km) metabasalts; other hydrous minerals are absent or are of very minor abundance. Clinoamphiboles dehydrate at pressures of [approximately]2.2-2.4 GPa under equilibrium conditions (pressure overstepping is probable), so mafic blueschists and amphibolites expel [H.sub.2]O at magmagenic depths. Partly serpentinized mantle beneath the oceanic crust devolatilizes at comparable to slightly higher pressures. However, micas remain stable in pelitic and granitic gneisses to pressures far exceeding 4.0 GPa, so at subduction depths >100 km, micaceous lithologies characterizing the upper and middle sialic crust fail to evolve significant [H.sub.2]O. Deep underflow of hydrated oceanic lithosphere thus generates most of the volatile flux along and/or above a subduction zone prior to continental collision. As large masses of quartzofeldspathic material enter a suture zone, volatile evolution at deep levels nearly ceases. While small amounts of peraluminous, S-type anatectic melts may be produced, I-type calc-alkaline arcs - and the continents - owe their formation over geologic time to the sustained underflow of oceanic lithosphere.

Calcic amphiboles were synthesized from a natural mid-ocean ridge basalt (MORB) in 39 experiments representing 24 sets of pressure-temperature (P-T) conditions ranging from 650-950 [degrees] C, 0.8-2.2 GPa, at [f.sub.[o.sub.2]] controlled by the fayalite-magnetite-quartz (FMQ) buffer, and [P.sub.aqueous fluid] = [P.sub.total]. Experiments lasted up to 1630 h at low temperatures; in all cases, synthesized hornblendes were coarse-grained (5-7 x 10-15 [[micro]meter]) and chemically homogeneous. Over the investigated pressure range, Ca-amphibole coexisting with phases rich in Al and Ti gradually changes composition from sodic-calcic, Si-rich at low temperatures to calcic, Si-poor at high temperatures: it is barroisite at 650 [degrees] C, edenite at 700 [degrees] C, and pargasite at 800-950 [degrees] C. Electron microprobe data were combined with 41 comparable analyses from the literature for Ca-amphiboles synthesized from MORBs at intermediate [f.sub.[o.sub.2]] in order to erect a petrogenetic grid for the experimental range 0.0-2.2 GPa, 450-1050 [degrees] C. Isopleths for [Al.sub.2][O.sub.3] in Ca-amphibole exhibit markedly negative P-T slopes, indicating increasing [Al.sub.2][O.sub.3] contents with both P and T. In contrast, Ti[O.sub.2] isopleths are nearly independent of P, demonstrating that Ti[O.sub.2] in Ca-amphibole increases almost exclusively as a function of T. For natural metabasaltic assemblages that contain coexisting Al-rich and Ti-rich phases, and closely approached chemical equilibrium under crustal or uppermost mantle conditions, this semiquantitative petrogenetic grid allows the simultaneous assignment of attendant P and T employing Ca-amphibole [Al.sub.2][O.sub.3] and Ti[O.sub.2] contents. However, during slow cooling, natural Ca-amphiboles may exsolve Ti[O.sub.2] as rutile, titanite, and/or ilmenite, but in general do not redistribute [Al.sub.2][O.sub.3], so this thermobarometer must be applied with caution to inhomogeneous specimens.

The chemical microstructure of five jadeitic pyroxenes from three Franciscan quartzose metagraywacke samples was investigated by backscattered electron imagery, element mapping, and electron microprobe microanalysis. These clinopyroxenes are neoblastic, subhedral grains or subradial aggregates replacing albite. For each compositionally zoned Cpx grain, a series of up to five parallel, polished sections were cut essentially normal or parallel to the crystallographic c axis. Five investigated grains, single and subradial aggregates, were examined quantitatively and three-dimensional chemical sections of the grains were constructed. Chemical zoning progresses from the Na-Cpx core to rim in four distinct regions (Q, L, Acm, and T). Region Q consists of microcrystalline blebs of quartz and jadeite (xjd = 0.95) and has a bulk composition of nearly pure albite. Very rarely it contains albite. L is jadeite (xjd = 0.80) that contains lawsonite inclusions. Acm is acmiterich Cpx (xjd = 0.65), and T consists of Ti[O.sub.2]-bearing jadeire (1-2 wt% Ti[O.sub.2], xjd = 0.95). Isolated Cpx grains and prismatic aggregates display the same chemical architecture regardless of crystallographic orientation. The earliest growth stage, Q, represents a volume-for-volume replacement of pre-existing albite by nearly stoichiometric jadeite plus quartz. As prisms grew, diffusion from the lithic matrix progressively enriched outer Na-Cpx zones in [Fe.sup.3-], Ca, and Mg. Terminal stages of high-pressure growth are represented by Ti[O.sub.2]bearing jadeitic pyroxenes, possibly reflecting a temperature increase or relatively longterm annealing under the same conditions in the presence of titanite and rutile. The core Q and rim T regions represent a jadeite plus quartz assemblage and confirm our earlier notion that the jadeite zone of Franciscan metamorphic rocks was subducted to depths where the assemblage jadeite and quartz was stable, at pressures of more than 10 kbar.

Preservation of ultrahigh-pressure (UHP) minerals formed at depths of 90-125 km require unusual conditions. Our subduction model involves underflow of a salient (250 [+ or -] 150 km wide, 90-125 km long) of continental crust embedded in cold, largely oceanic crust-capped lithosphere; loss of leading portions of the high-density oceanic lithosphere by slab break-off, as increasing volumes of microcontinental material enter the subduction zone; buoyancy-driven return toward midcrustal levels of a thin (2-15 km thick), low-density slice; finally, uplift, backfolding, normal faulting, and exposure of the UHP terrane. Sustained over [approximately equal to]20 million years, rapid ([approximately equal to]5 mm/year) exhumation of the thin-aspect ratio UHP sialic sheet caught between cooler hanging-wall plate and refrigerating, downgoing lithosphere allows withdrawal of heat along both its upper and lower surfaces. The intracratonal position of most UHP complexes reflects consumption of an intervening ocean basin and introduction of a sialic promontory into the subduction zone. UHP metamorphic terranes consist chiefly of transformed, yet relatively low-density continental crust compared with displaced mantle material - otherwise such complexes could not return to shallow depths. Relatively rare metabasaltic, metagabbroic, and metacherty lithologies retain traces of phases characteristic of UHP conditions because they are massive, virtually impervious to fluids, and nearly anhydrous. In contrast, [H.sub.2]O-rich quartzofeldspathic, gneissose/schistose, more permeable metasedimentary and metagranitic units have backreacted thoroughly, so coesite and other UHP silicates are exceedingly rare. Because of the initial presence of biogenic carbon, and its especially sluggish transformation rate, UHP paragneisses contain the most abundantly preserved crustal diamonds.

Research into the metamorphic parageneses of the thin mafic dikes which transect uppermost Proterozoic-Cambrian metasedimentary strata of the White-Inyo Range in eastern California has produced a number of interesting findings with regard to subsolidus recrystallization of the mafic metadikes. Diachronous suites of mafic metadikes are clearly present in the central White-Inyo Range. It remains unclear whether these are correllative with the Independence dike swarm. It was established that neoblastic mineral assemblages are systematic in both sequence of phases and their areal disposition.

Sedimentary rocks of the White-Inyo Range have been metamorphosed by episodically emplaced Jurassic-Cretaceous calc-alkaline arc plutons, reflecting stages in the thermotectonic history of this evolving continental margin. A north-northwest-trending anticlinorium is manifested in strata ranging in age from latest Precambrian to middle Paleozoic. Most middle and late Mesozoic (80-180 Ma) intrusive igneous bodies crosscut or deflect the regional fold pattern and incipient axial-plane cleavage in the country rocks - hence granitoid emplacement was largely postkinematic and, accordingly, isogradic surfaces transect structural levels. Retrograde andalusite [+ or -] cordierite-bearing metapelitic assemblages and diopside + grossular-bearing calc-silicate wall-rock skarns at plutonic contacts reflect high-temperature thermal maxima in the inner aureoles. Bulk-rock chemistries, mineral parageneses, and compositions of coexisting phases for several noncalcareous, weakly foliated sequences, the uppermost Proterozoic Wyman meta-argillites, the Lower Cambrian Deep Spring and Andrews Mountain metaquartzites, and the Montenegro phyllites, demonstrate that the overall metamorphic zonation is due to advective heat transport by the postkinematic granitoids. The higher grade parageneses do not represent a synchronous thermal event, but instead constitute a composite of local recrystallization episodes attending calc-alkaline pluton emplacement over an [approximately equal to]100 m.y. interval, overprinting a pervasive, weak, chloritic metamorphism. The latter may have been produced during late Paleozoic and/or early Mesozoic folding. Bulk-rock X-ray fluorescence analyses demonstrate that Wyman meta-argillites are rich in CaO, [Al.sub.2][O.sub.3], Cr, Th, and Sr, and low in Si[O.sub.2], Ti[O.sub.2], [P.sub.2][O.sub.5], [Na.sub.2]O, and [K.sub.2]O, indicating the former presence of carbonate, abundant clays, and minor detrital feldspars and iron oxides. Deep Spring and Andrews Mountain metaquartzite analyses exhibit high concentrations of titania, iron, silica, alkalies, Zr, Nb, and phosphate, but are poor in [Al.sub.2][O.sub.3], CaO, Cr, Th, and Sr, reflecting original protoliths enriched in alkali feldspars, quartz, and magnetite. Montenegro phyllites have compositions intermediate between the other two investigated metasedimentary units; however, they are relatively low in CaO and Sr, and high in [Al.sub.2][O.sub.3], suggesting noncalcareous precursor sediments containing abundant clay minerals, quartz, and alkali feldspars. Metaquartzites and phyllites are more oxidized than the sparsely carbonaceous meta-argillites. Except for a gradual decrease in volatile contents, the analyzed rocks do not exhibit systematic chemical variation with increasing metamorphic grade. Contrasts in phase compositions and proportions reflect protolith chemistries as well as inferred conditions of recrystallization. The areal disposition of visually estimated neoblastic biotite is mapped as isopleths transecting the three investigated sequences. Titanium contents of biotite and white mica, and An contents of plagioclase increase toward the larger, more marie Barcroft, Cottonwood-Beer Creek, and Joshua Flat plutons, whereas the cation proportions of Si in white micas decrease. Taking into account the ferric iron contents of white micas, [Fe.sup.2+][Mg.sub.-1] fractionation becomes less pronounced adjacent to these thermal highs. Metamorphic grade west of the range crest is greenschist facies chlorite zone ([approximately equal to]300 [degrees]C), but rises abruptly on the extreme north adjacent to the Barcroft granodiorite, and gradually increases eastward toward the Beer Creek-Cottonwood and Joshua Flat plutons. An ill-defined metamorphic culmination is directly north of the Papoose Flat granite. Along contacts with plutons, hornfelsic wall rocks approached temperatures of 500-600 [degrees] C. On the basis of rare preservation of cordierite and/or andalusite, phase chemistry, and mineralogic thermobarometry, pressures attending regional and thermal recrystallization were about 3 [+ or -] 1 kbar. The activity of [H.sub.2]O was moderate (2 [+ or -] 1 kbar) during recrystallization of carbonate-poor rocks. Compositions of coexisting phases exhibit systematic, sympathetic variations, compatible with a close approach to local chemical equilibrium.

The Dabie Mountains are part of the >2000-km-long Qinling-Dabie-Sulu suture zone juxtaposing the Sino-Korean and Yangtze cratons. An eastern extension apparently crosses Korea and lies along the Japan Sea side of Honshu as the Imjingang and Sangun terranes, respectively; a northeastern segment may be present in Sikhote-Alin, Russian Far East. This orogenic belt records late Paleozoic ocean-floor consumption and the Triassic collision of two Precambrian continental massifs in east-central China. Coesite and microdiamond inclusions in strong, refractory minerals of eclogite facies ultrahigh-pressure (UHP) metamorphic rocks in the Dabie-Sulu area attest to profound subduction of a leading salient of the old, cold Yangtze craton, now recovered through tectonic exhumation and erosion. Northward increase in intensity of subsolidus recrystallization of the suture complex is analogous to the internal progression in grade of high-pressure (HP) and UHP metamorphism documented in the Western Alps. In both regions, subduction of narrow prongs of continental material, UHP metamorphism, and return toward midcrustal levels of relatively lower density, buoyant microcontinental blocks resulted from delamination of these rocks from the descending, higher density, oceanic-crust-capped lithospheric plate. Such salients of continental crust represent an integral structural part of the downgoing slab, remain intact, and may be dragged to great depths before disengaging and rising differentially as coherent blocks. UHP parageneses include trace mineralogic relics requiring peak metamorphic conditions of 700-900 [degrees] C and 28-35 kbar or more. In contrast, Pacific-type HP metamorphic belts, as represented by the Franciscan Complex of western California, recrystallized under physical conditions up to 200-500 [degrees] C, 10 [+ or -] 3 kbar. In this setting, voluminous quartzo-feldspathic and graywacke debris was carried downward on oceanic-crust-capped lithosphere, choking the subduction zone with incompetent material. Sited between both plates, and strongly adhering to neither, this buoyant, largely sedimentary complex decoupled at 25-30 km depth, and ascended toward the surface. In both Alpine-type intracontinental collision and Pacific-type underflow, light sialic material displaced dense mantle; thus, the return to midcrustal levels was propelled dominantly by body forces.

The Pacheco Pass area, well known for the relatively high-pressure, low-temperature mineral assemblages developed in a well-ordered section of chiefly metaclastic strata, was restudied in order to elucidate the geologic, paragenetic, and textural relationships among the various lithologic units. The mapped 7 1/2 minute quadrangle extends from 4 km west of the range crest eastward to the Ortigalita fault, tectonic contact of the Franciscan assemblage with coeval Great Valley strata. Semicontinuous metachert beds, underlain by scraps of Na-cpx- and/or Na-amphibole-bearing metabasaltic rock, mark the base of terrigenous tectonostratigraphic units, evidently decoupled from the downgoing, oceanic-crust-capped plate. Underplating along newly recognized bedding-plane faults has juxtaposed members of an apparently conformable Franciscan sedimentary sequence. Four depositional units, each consisting of monotonous metagraywacke and interlayered metashale, are stacked within the accretionary section: from top down, these are tectonostratigraphic members A, B, C, and D. Unit D differs from overlying members in possessing a greater abundance of bluestone/greenstone pods, minor serpentinite bodies, and rare metaconglomerate lenses; varying degrees of stratal disruption indicate that it has been arrested in the transformation to broken formation. The Ortigalita fault is nearly vertical, strikes north-northwest, and exhibits apparent dextral offset; it truncates the east-west-striking Gonzaga fault and other Franciscan structures. The topographic low occupied by San Luis Reservoir is a Cenozoic tectonic depression sited at a releasing bend on the Ortigalita fault. Newly acquired textural, chemical, mineralogic, and areal relations of albitic and jadeitic pyroxene-bearing Franciscan metagraywacke near Pacheco Pass demonstrate that (1) Na-cpx and Na-amphibole are metamorphic minerals, not clastic materials; (2) pumpellyite is a minor neoblastic phase in albite-bearing metagraywacke west of Pacheco Pass, and in the northwestern corner of the quadrangle, but it does not occur in jadeitic pyroxene-bearing metaclastic rocks; (3) higher textural grade metagraywacke specimens generally contain higher modal proportions of jd; (4) abundance, textural grade, and chemistry of Na-cpx are not related to tectonostratigraphic units; and (5) because of sluggish reaction rates, albite persisted metastably into the higher pressure, jadeitic pyroxene + quartz P-T field. Production of Na-cpx resulted from a reaction of the sort ab + chl = jd + lws + qtz + or - gln + or - H2O. Irregular, oscillatory zoning within single jadeitic pyroxene prisms and variable jd/ab modes for rocks of the same bulk composition reflect growth governed by diffusion rather than recrystallization within a P-T transition zone; high-pressure overstepping of the phase boundary explains variable modal proportions of ab and jd, as well as chemical heterogeneity of the jd. Elevated P attended metamorphism of Na-cpx-bearing rocks exposed in the Pacheco Pass area, as indicated by the widespread coexistence of quartz and NaAlSi2O6-rich pyroxene. Inferred physical conditions of prograde metamorphism were 150 + or - 50 degrees C at 7-8 kbar or more. Franciscan rocks in the east-central Diablo Range underwent subduction-zone metamorphism accompanying paleo-Pacific lithospheric plate descent during mid-Cretaceous convergence. Underplating, contraction, and sequestering of the tectonostratigraphic assembly at depths of 25-30 km during underflow, followed by gradual rise, took place under conditions of refrigeration due to protracted descent of the oceanic slab. Paleogene diapiric uplift and extension followed, indicating more complete decoupling of the buoyant trench complex from the downgoing plate. Finally, passage of the East Pacific Rise triple junction in Neogene time initiated the present dextral-slip regime in the California Coast Ranges.

The central Klamath Mountains of California were geologically analyzed by using major petrological, geochronological and structural studies and by dilineating a north-south belt of rocks that are observed from one end of the Klamaths to the other end. An accretionary wedge, which began forming in the Permian time and was totally developed by the Middle Jurassic, is the Eastern Hayfork that partly constitutes the Sawyers Bar Terrane of the Klamath Mountains. A volcanoplutonic arc was demonstrated by the Salmon River Units and the North Fork Terranes of the Klamath mountains.

Mafic meta-igneous supracrustal rocks in the east-central part of the western Triassic and Paleozoic belt are interlayered with and appear to predominantly overlie fine-grained clastic and cherty metasedimentary strata. This complex constitutes a lithostratigraphic terrane exposed in the vicinity of Sawyers Bar, California. Basaltic flows, dikes, and sills are informally referred to as the 'Yellow Dog greenstones.' Dark-colored, amygdaloidal flow breccias rich in titanium, iron, phosphorus, and light rare-earth elements (LREE's) constitute the lowest, mildly alkalic members of the mafic flow series, intimately interlayered with fine-grained terrigenous detritus. Element proportions suggest that the dark, Ti + [Fe.sup.*] + P-rich basaltic lavas may have been extruded in an oceanic intraplate setting. Interstratification with distal turbiditic strata indicates concurrent submarine volcanism and deep-sea sedimentary deposition. The main mass of the stratigraphically higher extrusive sequence is a light-colored, massive tholeiite series, poorer in Ti + [Fe.sup.*] + P + LREE's. Hypabyssal rocks belong exclusively to this second igneous suite. Compositions of the pale-colored, overlying basaltic/diabasic mass of the Yellow Dog greenstone section are consistent with eruption in an immature magmatic arc. Dikes presumably comagmatic with terminal-stage Yellow Dog volcanism cut the overlying, more eastern Stuart Fork terrane. The Yellow Dog greenstones contain relict magmatic clinopyroxene and/or pargasitic hornblende. Both melt series include Mg-rich units ([ave.sub.17]: MgO = 14.4 [+ or -] 1.9 wt. %; Cr = 650 [+ or -] 200 ppm; Ni = 270 [+ or -] 100 ppm). Sea-floor alteration apparently affected these mafic rocks in the marine environment following eruption, as reflected by [[delta].sup.18.O], MgO, and CaO/[Al.sub.2.O.sub.3] values and by minor alkali and silica metasomatism. Yellow Dog igneous activity was extinguished during mid-Jurassic suturing of the oceanward Sawyers Bar terrane against and beneath the pre-existing landward Stuart Fork blueschist complex of Late Triassic metamorphic age. Subsequently, the terrane amalgam was invaded by Middle/Late Jurassic granitoids. Minor chemical alteration, including [sup.18.O] enrichment, also probably accompanied later regional, and especially contact, metamorphism. Nevertheless, many major-, minor-, and rare-earth-element variations in the Yellow Dog greenstones are compatible with igneous fractional crystallization trends but exclude the assimilation of significant amounts of sialic crust.