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A new meteorite find, named Khatyrka, was recovered from eastern Siberia as a result of a search for naturally occurring quasicrystals. The meteorite occurs as clastic grains within postglacial clay-rich layers along the banks of a small stream in the Koryak Mountains, Chukotka Autonomous Okrug of far eastern Russia. Some of the grains are clearly chondritic and contain Type IA porphyritic olivine chondrules enclosed in matrices that have the characteristic platy olivine texture, matrix olivine composition, and mineralogy (olivine, pentlandite, nickel-rich iron-nickel metal, nepheline, and calcic pyroxene [diopside-hedenbergite solid solution]) of oxidized-subgroup CV3 chondrites. A few grains are fine-grained spinel-rich calcium-aluminum-rich inclusions with mineral oxygen isotopic compositions again typical of such objects in CV3 chondrites. The chondritic and CAI grains contain small fragments of metallic copper-aluminum-iron alloys that include the quasicrystalline phase icosahedrite. One grain is an achondritic intergrowth of Cu-Al metal alloys and forsteritic olivine ± diopsidic pyroxene, both of which have meteoritic (CV3-like) oxygen isotopic compositions. Finally, some grains consist almost entirely of metallic alloys of aluminum + copper ± iron. The Cu-Al-Fe metal alloys and the alloy-bearing achondrite clast are interpreted to be an accretionary component of what otherwise is a fairly normal CV3 (oxidized) chondrite. This association of CV3 chondritic grains with metallic copper-aluminum alloys makes Khatyrka a unique meteorite, perhaps best described as a complex CV3 (ox) breccia.

An outstanding problem in understanding the late Proterozoic tectonic assembly of the southwest is identifying the tectonic setting associated with regional metamorphism at 1.4 Ga. Both isobaric heating and cooling, and counterclockwise looping PT paths are proposed for this time. We present a study of the Proterozoic metamorphic and deformation history of the Cerro Colorado area, southern Tusas Mountains, New Mexico, which shows that the metamorphism in this area records near-isothermal decompression from 6 to 4 kbar at ca. 1.4 Ga. We do not see evidence for isobaric heating at this time. Decompression from peak pressures is recorded by the reaction , with a negative slope in PT space; the reaction , which is nearly horizontal in PT space; and partial to total pseudomorphing of kyanite by sillimanite during the main phase of deformation. The clearest reaction texture indicating decompression near peak metamorphic temperature is the replacement of garnet by clots of sillimanite, which are surround...

The Coast Mountains of British Columbia record an increase in magmatic activity, acceleration in exhumation rate, and a change from transpression to extension between ∼60 and 52 Ma. Structural analysis of fabrics in three mid-crustal plutons and country-rocks leads to conclusions about pluton emplacement mechanisms and strain partitioning during changing tectonic conditions. The Quottoon and Kitlope plutons (∼60 Ma) have steep foliations and lineations consistent with partitioned transpressional deformation. The Chief Matthew’s pluton (∼57–55 Ma) intruded during the formation of a sub-horizontal transposition foliation, and has radially distributed lineations consistent with sub-vertical flattening during extension. The change in orientation of the foliation represents an almost orthogonal rotation of the shortening direction from sub-horizontal to sub-vertical. The Chief Mathew’s pluton is interpreted to intrude initially into gently dipping fractures perpendicular to the steeply dipping foliation. These melt-filled fractures acted as conduits for melt, triggering horizontal flow, and eventually formed kilometer-scale sills. The steeply dipping fabrics of the Coast shear-zone provided pathways for melt to ascended from lower to middle-crustal depths. Partitioning deformation into three-dimensional domains of flattening, simple shear, and constrictional strain created the space for plutons. This pattern of deformation reflects the interaction of regional deformation with magma emplacement.

Please click here to download the map associated with this article. We present a new, 1:500,000-scale geologic map of the kingdom of Bhutan, and surrounding areas of India and southern Tibet. The map is a compilation of the most complete and most recent mapping datasets available, and presents an unprecedented amount of structural data when compared to previous published geologic maps of Bhutan. The map is a combination of: 1) new data presented in this study; 2) compilation of small-scale, published geologic maps of specific areas of Bhutan, Tibet, and parts of India; and 3) compilation of specific areas of published, country-scale geologic maps of Bhutan. Mapping detail is focused primarily on Subhimalayan, Lesser Himalayan, and Greater Himalayan rocks, with a lower level of detail on Tethyan Himalayan rocks. We present new 3-part stratigraphic divisions for the Siwalik Group and the structurally-lower Greater Himalayan section, and compile detailed stratigraphic divisions and structural geomet...

Previously published and new U-Pb geochronologic analyses provide 313 zircon and 59 titanite ages that constrain the igneous and cooling history of the Coast Mountains batholith in north-coastal British Columbia. First-order findings are as follows: (1) This segment of the batholith consists of three portions: a western magmatic belt (emplaced into the outboard Alexander and Wrangellia terranes) that was active 177–162 Ma, 157–142 Ma, and 118–100 Ma; an eastern belt (emplaced into the inboard Stikine and Yukon-Tanana terranes) that was active ca. 180–110 Ma; and a 100–50 Ma belt that was emplaced across much of the orogen during and following mid-Cretaceous juxtaposition of outboard and inboard terranes. (2) Magmatism migrated eastward from 120 to 80 (or 60) Ma at a rate of 2.0–2.7 km/Ma, a rate similar to that recorded by the Sierra Nevada batholith. (3) Magmatic flux was quite variable through time, with high (>35–50 km 3 /Ma per km strike length) flux at 160–140 Ma, 120–78 Ma, and 55–48 Ma, and magmatic lulls at 140–120 Ma and 78–55 Ma. (4) High U/Th values record widespread growth (and/or recrystallization) of metamorphic zircon at 88–76 Ma and 62–52 Ma. (5) U-Pb ages of titanite record rapid cooling of axial portions of the batholith at ca. 55–48 Ma in response to east-side-down motion on regional extensional structures. (6) The magmatic history of this portion of the Coast Mountains batholith is consistent with a tectonic model involving formation of a Late Jurassic–earliest Cretaceous magmatic arc along the northern Cordilleran margin; duplication of this arc system in Early Cretaceous time by >800 km (perhaps 1000–1200 km) of sinistral motion (bringing the northern portion outboard of the southern portion); high-flux magmatism prior to and during orthogonal mid-Cretaceous terrane accretion; low-flux magmatism during Late Cretaceous–Paleocene dextral transpressional motion; and high-flux Eocene magmatism during rapid exhumation in a regime of regional crustal extension.

The High Himalayan Crystallines (HHC) of Bhutan were penetratively deformed, intruded by leucogranite and metamorphosed during the collision of the Indian and Asian plates. Metamorphic reaction textures in the HHC show that it experienced decompression while maintaining a laterally heterogeneous, and locally inverted, internal temperature range of c.600–750C. This thermal structure was produced by thrusting hot, migmatitic rocks over lower-grade rocks within the HHC and by the advection of heat from the intrusion of leucogranite dykes and sills during decompression. A variable velocity field within the HHC during exhumation and extrusion between India and Tibet caused the inversion of top to the south sense of shear present throughout most of the HHC to top down to the north shear near its top.

SUMMARY We processed data from a small, five-station temporary seismic network deployed from January 2002 until March 2003 within the Kingdom of Bhutan. We detected, associated, and located approximately 2,100 teleseismic, regional, and local events; approximately 900 were not in the United States Geological Survey (USGS) Earthquake Data Report catalog. We supple mented our data for these 900 events with data from the Global Seismographic Network (GSN) stations in the region. After relocation of these events, we focused on approximately 175 events that occurred near or within the borders of Bhutan. We reviewed each solution, manually timing the P- and S-waves for each event, and inverted for event locations and an average 1-D velocity model for the region. Our model was tested with other models appropriate for the region. We found a high amount of microseismicity throughout southern Bhutan and almost no seismicity under northern Bhutan and southern Tibet. Our results showed that analysis of data from small in-country seismic networks resulted in new scientific findings. In this case, we found the crust under southern Bhutan brittlely deforming, and there was evidence for strike-slip faulting, supporting previous results for the region.

Crustal growth in the Coast Mountains, along the leading edge of the Canadian Cordillera, was the result of processes associated with horizontal flow of material during transpression and subsequent transtension, and the vertical accretion of mantle derived melts. From 85 to 58 Ma, as exotic terranes were translated northward during transpression, the crust was thickened to about 55 km, and melt that originated from a mix of mantle-derived basalt with partial melt of the thickened crust intruded into crustal scale transpressive shear zones. When the orogen extended, between 58 and 50 Ma, there was large-scale decompression melting in the mantle and dehydration melting in the lower crust. Voluminous emplacement of sub horizontal sills facilitated by ductile flow of the gneissic country rocks partially filled space created as the crust was pulled apart and as 15 to 20 km of tectonic exhumation occurred across low angle normal ductile shear zones. By 50 Ma, the final crustal thickness of the new continental crust was about 34 km. Comparison of seismic data with other crustal sections suggests that the crust-forming processes identified in western British Columbia have general applicability to models for the formation of continental crust.

Over 500 previously unidentified, symmetric pointed stones of similar size, shape, color, and texture have been found on the barrier beach of Mantoloking, New Jersey, since 1940. Petrographic, stereo, and scanning electron microscopy analysis reveals that the stones are made of either a biomicritic packstone composed of 50% siliceous microfossil remains ( including sponge spicules and radiolaria) embedded in a micrite matrix or a limestone with abundant angular quartz grains (50–150 μm wide) surrounded by a calcite matrix. The distinctive shape of the Mantoloking stones is most similar to whetstones used for sharpening scythes. We conducted a worldwide search and discovered one producer of carbonate whetstones: a company in the town of Pradalunga in Northern Italy. Microscope analysis reveals that the Pradalunga source rocks are exact matches for the spicule-rich limestone and angular quartz-rich limestone found in the Mantoloking collection. The whetstones are most likely lost cargo from a wreck offshore of Mantoloking, but the exact source may never be known. © 2006 Wiley Periodicals, Inc.

We summarize our results from Bhutan and interpret the Greater Himalaya Sequence (GHS) of Bhutan, together with a portion of the underlying Lesser Himalaya Sequence, in the context of recently published channel flow models. For the GHS rocks now exposed in Bhutan the depth for beginning of muscovite dehydration melting (approximately 7508C at 11 kbar) and associated weakening of these rocks is constrained by geobarometry to be at about 35–45 km. The location of initial melting was down-dip and over 200 km to the north of Bhutan. Melt was produced and injected into ductilely deforming metamorphic rocks as they were extruded towards the south between the Main Central Thrust (MCT) and the South Tibetan Detachment zones. The lateral flow of low viscosity rocks at these depths occurred under southern Tibet between 22 Ma and 16 Ma. Subsequently, the channel rocks decompressed from 11 to 5 kbar (from 35 km to a depth of 15 km), but maintained high temperatures, between about 16 Ma and 13 Ma. The data from Bhutan are consistent with channel flow models if there were several pulses of channel flow. The first, between 22 and 16 Ma, produced the rock seen in the lower half of the GHS of Bhutan. A second pulse, which is cryptic, is inferred to have led to the uplift and exhumation of the MCT zone. A third, in central Bhutan, is exposed now as the hanging wall of the Kakhtang thrust, an out-of-sequence thrust that was active at 12–10 Ma. The latter two pulses likely broke around a plug at the head of the first pulse that was formed where the melt in the channel had solidified. The geodynamic models of Beaumont et al. (2001, 2004) and Jamieson et al. (2004) show that channel flow in the Himalaya was a likely consequence of building the Tibetan Plateau, and exhumation of the channel material was a consequence of focused erosion at the Himalaya front in concert with the channel flow. Jamieson et al. (2004) interpret most metamorphic and structural data of the central Himalayas in the context of a basic model called HT-1. We here review data from Bhutan (Fig. 1) that support a model of channel flow for formation of the Greater Himalaya Sequence (GHS). Inconsistencies between the basic model and our data can be rationalized by considering pulses of channel flow rather than the single, spatially and temporally continuous episode of channel flow as used in the basic model. The basic geodynamic model for the Himalaya calls for steady channel flow of thermally weakened rock in a channel from the Miocene to the present. This weakening is inferred to be due to the onset of melting in the midto lower crust; a few per cent of melt in a rock is known to lower the viscosity by several orders of magnitude (Rosenberg & Handy 2005). Because rock within the flowing channel, in comparison to the bounding plates, is relatively weak due to the presence of melt, when this melt solidifies (as leucogranite at about 7008C at 5 kbar; L of Fig. 2) the rock becomes stronger. Thus, the head, or the rheological tip, of the low viscosity portion of the channel is defined by the isotherm for this transition. The channel in the basic geodynamic model flowed from under southern Tibet to under the Himalaya. Under Tibet, the channel includes most of the middle crust, some 30–40 km of a 70 km thick crust. At its southern limit, the model channel narrows as it approaches the mountain front at the southern edge of the Tibetan Plateau. There, focused surface erosion leads to rapid removal of rock and to exposure of the channel as the GHS. The temperature of the channel is buffered by the presence of melt and solid phases. These phases are leucogranite melt, the remaining reactants of the melt-producing reaction, and other products of mica dehydration of metasedimentary rocks, which are in equilibrium at 11–12 kbar at 750– 8008C (muscovite þ albite þ quartz 1⁄4 aluminium silicate þ potassium feldspar þ leucogranite melt; Fig. 2). According to the basic model, the sedimentary protolith was entrained in the mid-crust as the Indian plate underthrust Tibet (Jamieson et al. 2006). The Indian lower crust and lithospheric mantle were subducted while the Late Proterozoic From: LAW, R. D., SEARLE, M. P. & GODIN, L. (eds) Channel Flow, Ductile Extrusion and Exhumation in Continental Collision Zones. Geological Society, London, Special Publications, 268, 415–423. 0305-8719/06/$15.00 # The Geological Society of London 2006. to Mesozoic sedimentary cover accreted in the middle of the thickening crust. The depth to the resulting channel corresponds to 8–12 kbar. This is the range of maximum model pressures reported from metamorphosed pelitic rocks of the GHS along the entire Himalayan chain (e.g. Brunel & Kienast 1986; Hubbard 1989; Inger & Harris 1992; Macfarlane 1995; Vannay & Hodges 1996; Vannay & Grasemann 1998; Neogi et al. 1998; Ganguly et al. 2000; Catlos et al. 2001; Daniel et al. 2003; Dasgupta et al. 2004; Harris et al. 2004). We note here that it is exceedingly difficult to extract accurate pressures (and temperatures) from the Himalayan metamorphic rocks. Davidson et al. (1997) and Daniel et al. (2003) showed that thermobarometry of rocks that are not fully characterized by using X-ray composition maps likely produces unreliable results. This is because of the near-universal chemical disequilibrium due to variable response of mineral compositions to reheating, to decompression at high temperature, and to rapid cooling. Furthermore, the uncertainty due to choice of mineral compositions outweighs analytical or thermodynamic model uncertainties. Accordingly, where, in this paper, we report the pressure in the proposed channel to be 11–12 kbar, we recognize an uncertainty that ranges from 8 to 14 kbar. And we estimate an uncertainty range from 4 to 6 kbar in our reporting of a pressure of 5 kbar. Nevertheless, maximum pressures at peak temperatures reported from the GHS along the Himalaya range only between 8 and 12 kbar. The ‘uniform’ pressure of 8–12 kbar is predicted by flow of a subhorizontal channel south from the mid-crust under Tibet. That is, the rocks of the channel do not originate from extreme depths at the base of the Tibetan or Himalayan crust (about 25 kbar). The origin of the GHS rocks from the ‘same’ depth means that the amount of exhumation depends on how far the channel has progressed Fig. 1. Geological map of Bhutan and surrounding areas, after Grujic et al. (2002) and references therein, with modifications based on continuing mapping by D. Grujic and students, and by Bhutanese colleagues. K, Klippen of Chekha Formation; KT, Kakhtang thrust; MBT, Main Boundary Thrust; MCT, Main Central Thrust; MFT, Main Frontal Thrust; STD, South Tibetan detachment. A–A’ indicates line of schematic section in Figure 5. L. S. HOLLISTER & D. GRUJIC 416

Paleomagnetic data of plutons from the western Canadian Cordillera are currently interpreted to indicate thousands of kilometers of latitudinal transport, in situ tilting, or post-intrusion folding. For the Ecstall pluton of British Columbia, the inclination of magnetic remanence directions steepens to the east from 16° to 81° [Butler et al., J. Geophys. Res. 107 (2002)] between 24 and 12 km west of a thermal boundary with the Coast Mountains batholith (CMB). The CMB was at 700–800°C between 60 and 52 Ma. The stable remanent magnetization is in ilmenohematite grains with exsolved lamellae of ferrian ilmenite. It has the characteristic properties of lamellar magnetism [Robinson et al., Nature 418 (2002) 517–520] which is acquired with exsolution in ilmenohematite below 390°C [Ghiorso, Phys. Chem. Minerals 25 (1997) 28–38]; thus, it is thermal chemical remanent magnetization (TCRM). Our heat flow calculations show that the thermal effects of Eocene CMB on the adjacent 91 Ma Ecstall pluton were enough to reset the remanent magnetization directions. Reheating to the temperatures necessary for TCRM is supported by K/Ar and Ar/Ar cooling dates on hornblende and biotite, which young from west to east towards the CMB in concert with the change in inclinations. Thus, the progressive steepening of magnetization inclinations is attributed to reheating during the Eocene of the 91 Ma Ecstall pluton by the CMB. This reheating followed post-solidification northward latitudinal displacement of the pluton. The amount of translational displacement of the coastal terranes of British Columbia prior to 60 Ma hinges on the interpretation of discordant magnetizations from plutons that may have cooled slowly after emplacement in the mid to lower crust, as was the case for the Ecstall pluton. Many of these plutons contain ilmenohematite as an accessory magnetic phase. Our interpretation for the low temperature acquisition of TCRM in the Ecstall pluton may have far-reaching implications for understanding enigmatic discordant paleomagnetic directions reported from plutons of the western Canadian Cordillera.

Geothermometry and mineral assemblages show an increase of temperature structurally upwards across the Main Central Thrust (MCT); however, peak metamorphic pressures are similar across the boundary, and correspond to depths of 35-45 km. Garnet-bearing samples from the uppermost Lesser Himalayan sequence (LHS) yield metamorphic conditions of 650-675 ! C and 9-13 kbar. Staurolite-kyanite schists, about 30 m above the MCT, yield P-T conditions near 650 ! C, 8-10 kbar. Kyanite-bearing migmatites from the Greater Himalayan sequence (GHS) yield pressures of 10-14 kbar at 750-800 ! C. Top-to-the- south shearing is synchronous with, and postdates peak metamorphic mineral growth. Metamorphic monazite from a deformed and metamorphosed Proterozoic gneiss within the upper LHS yield U⁄Pb ages of 20-18 Ma. Staurolite-kyanite schists within the GHS, a few metres above the MCT, yield monazite ages of c. 22 ± 1 Ma. We interpret these ages to reflect that prograde meta- morphism and deformation within the Main Central Thrust Zone (MCTZ) was underway by c. 23 Ma. U⁄Pb crystallization ages of monazite and xenotime in a deformed kyanite-bearing leucogranite and kyanite-garnet migmatites about 2 km above the MCT suggest crystallization of partial melts at 18- 16 Ma. Higher in the hanging wall, south-verging shear bands filled with leucogranite and pegmatite yield U⁄Pb crystallization ages for monazite and xenotime of 14-15 Ma, and a 1-2 km thick leuco- granite sill is 13.4 ± 0.2 Ma. Thus, metamorphism, plutonism and deformation within the GHS con- tinued until at least 13 Ma. P-T conditions at this time are estimated to be 500-600 ! C and near 5 kbar. From these data we infer that the exhumation of the MCT zone from 35 to 45 km to around 18 km, occurred from 18 to 16 to c. 13 Ma, yielding an average exhumation rate of 3-9 mm year )1 . This process of exhumation may reflect the ductile extrusion (by channel flow) of the MCTZ from between the overlying Tibetan Plateau and the underthrusting Indian plate, coupled with rapid erosion.

The Bhutan Himalayas differ from the rest of the Himalayas in two major ways: (i) low-grade sedimentary rocks lie above the Greater Himalayan Sequence as klippen (i.e. erosional remains of the South Tibetan Detachment); and (ii) an out-of-sequence thrust, the Kakhtang thrust, lies structurally above the klippen, and it doubles the exposed thickness of the Greater Himalayan Sequence. Our field observations and geochronological data constrain the main kinematic events in the Bhutan Himalayas. Crystallisation ages of leucogranite dykes deformed by the Main Central Thrust and the South Tibetan Detachment indicate that these two structures operated together between 16 and 22 Ma. The out-of-sequence Kakhtang thrust was active at 10–14 Ma and was concurrent with reactivation of the South Tibetan Detachment. Restoration of the Bhutan Himalayas prior to the out-of-sequence thrusting shows that the Greater Himalayan Sequence was the core of a long, low-viscosity crustal channel extending under the Tibetan plateau. We propose that the gravity-driven southward extrusion of the channel material from underneath the Tibetan plateau caused the inverted metamorphic sequence in the Lesser Himalayan Sequence and in the Greater Himalayan Sequence. This process also led to occurrences of present-day surface rocks that were derived from variable distances down dip, but from similar crustal depths. Such an exhumation pattern can explain the similar peak pressures for the Greater Himalayan Sequence along the length of the Himalayas.

The ACCRETE study addresses the question of continental assemblage in southeastern Alaska and western British Columbia through accretion of exotic terranes and generation of new crust by magmatic addition in a former continental arc. We present results of wide-angle P- and S-wave seismic interpretation of a 300-km long marine-land seismic line across the contacts between accreted terranes and Coast Mountains. Additional constraints on the model are obtained from correlation with geologic mapping. Our results indicate that the Coast Shear Zone (CSZ) is a nearly vertical fault zone probably related to a transpressive regime. West of the CSZ, the mid-Cretaceous (90 Ma) thrust belt is rooted in the deep crust and is truncated by the CSZ. From the interpretation of the imaged sub-vertical reflecting zones, we infer the positions of the Alexander–Wrangellia terrane boundary (AWB) and of Tertiary extensional grabens within Dixon Entrance near its intersection with the profile. The observed values of Vp and Vp/Vs in the lower crust of the Alexander terrane are similar to those of oceanic crust and distinctly different from the Coast Mountains Batholith (CMB) to the northeast. The crust under the CMB (32 km) is thinner than the average continental crust, and the Moho is sharp (∼200 m) and highly reflective. The low-velocity mantle (7.9 km/s) suggests high temperature consistent with the stability of garnet in mafic rocks in the lower crust. The lower crustal velocity of 6.9 km/s supports a lower crust composed of interlayered garnet pyroxene granulite and quartzofeldspathic-restite related to batholith generation. The crustal section under the CMB is seismically identical to the lower two thirds of normal crust, heated and inflated by intrusions of tonalite, and gabbro interlayered with restites from batholith generation and uplifted during exhumation.

The Coast Plutonic Complex is the largest magmatic arc of the North American Cordillera, extending from northwestern Washington State to eastern Alaska. It forms the transition between two tectonic domains that are suspected to have undergone several phases of large (several thousands of kilometers) orogen-parallel displacement during the Mesozoic and early Cenozoic. A compilation of fabric data, published isotopic ages, and new structural observations shows that the western Coast Plutonic Complex was affected by subvertical, orogen-parallel, crustal-scale shear zones. These shear zones mainly reflect sinistral transpression and were sequentially active from ∼110 to 87 Ma during the intrusion of voluminous batholiths. Sinistral shearing was roughly coeval with the development of the thrust belts flanking the Coast Plutonic Complex (between ∼101 and ∼85 Ma), suggesting plate-scale transpression was a first-order process in the construction of the Coast Mountains orogen. These shear zones separate panels with distinct plutonic and cooling histories, suggesting the sinistral displacements between crustal blocks were large (greater than tens to hundreds of kilometers). This transpressive shear system likely reflects the Jurassic to early Late Cretaceous migration of outboard Cordilleran terranes to the south suggested by paleomagnetic evidence and plate reconstruction models. This example from the Coast orogen shows how transpression is partitioned between a thermally weakened magmatic arc and outwardly vergent fold-and-thrust belts. Our analysis further shows that the ∼2000-km-long Late Cretaceous to early Tertiary Coast shear zone has a minimum extent toward the south to at least 51°30′N.

Structural data from the Coast Plutonic Complex, near Prince Rupert, British Columbia, are consistent with a deformational history dominated by dextral transpression from Campanian to Paleocene time. Penetrative east-side-up, southwest-directed, ductile shearing produced moderately northeast-plunging overturned kilometer-scale isoclinal folds. These folds are dextrally sheared and refolded into kilometer-scale upright northwest-plunging folds and steeply dipping transposed foliations with moderate to shallow northwest-plunging lineations along their western side. An east-side-up component to the transcurrent shearing is kinematically compatible with east-side-up shearing found within the Great Tonalite Sill. Kinematic and geometric gradients and the spatial distribution of the finite stretching direction are interpreted to result from partitioning of transpression. The location of these structures and overprinting relationships suggest the Great Tonalite Sill intruded late-kinematically into a crustal-scale dextral transpressive shear zone. These results indicate this shear zone could form part of the Baja-B.C. fault system that would have accommodated large northward displacements of the terranes making up western British Columbia and southeast Alaska. This conclusion is based on: (1) It is favorably located to accommodate the proposed displacements; (2) Deformation occurred during the time period of proposed large displacement (83–59 Ma); (3) The 15-km thickness of the shear zone indicates it records large displacements.

This study addresses the question of crustal, Moho, and uppermost mantle structure across an accreted terrane, continental arc, and fold and thrust belt in southeastern Alaska and western British Columbia. The 186-km-long Portland Canal line of the ACCRETE wide-angle seismic dataset across the Coast Mountains is analyzed using a combination of travel-time analysis techniques, including delay-time tomography, turning ray tomography, reciprocal time analysis, intercept-time inversion, and forward ray tracing. Additional constraints on the model are obtained from a coincident vertical-incidence seismic section and from correlation with the mapping of geologic structures plunging into the seismic section. The study reveals moderately high crustal velocities, low-velocity gradient in the middle crust, and decreasing average crustal velocity and a north-deepening Moho. Termination of crustal reflectivity across a vertical zone (the Coast Shear Zone, CSZ) indicates that the CSZ is most likely a strike-slip fault associated with a transpressive tectonic regime. A mid-Cretaceous thrust system mapped on the surface to the west of the CSZ is imaged by several groups of mid- to lower crustal reflectors extending close to the Moho indicating it was a thick-skinned thrust system. NE-dipping fabric imaged within the Coast Mountains batholith (CMB) is associated with a ductile deformation during Early Eocene crustal extension. The crustal section under the CMB, which has an average velocity of 6.55 km/s and shallower than average crustal thickness of 31 km, can be considered as corresponding to the lower two thirds of an average crustal section which has been inflated by intrusions of high-velocity tonalite to gabbro sills.

Quartz textures measured from deformed quartz tectonites within the Lesser Himalaya and Higher Himalaya Crystalline of Bhutan show similar patterns. Orientation and distribution of the quartz crystallographic axes were used to confirm the regional shear sense: the asymmetry of c-axis and a-axis patterns consistently indicates top-to-the-south shearing. The obliquity of the texture and the inferred finite strain (plane strain to moderately constrictional), suggest the strain regime had a combination of rotational and irrotational strain path. In most of the samples from the Bhutan Himalaya, the inferred deformation mechanisms suggest moderate- to high-temperature conditions of deformation that produced the observed crystallographic preferred orientation. Much higher temperature of deformation is indicated in the quartz veins from a leucogranite. The observed ductile deformation is pervasively developed in the rocks throughout the investigated area. The intensity of deformation increases only slightly in the vicinity of the Main Central Thrust. Simultaneous southward shearing within a large part of the Higher Himalaya Crystalline near and above the Main Central Thrust and normal faulting across the South Tibetan Detachment, is explained by the tectonically induced extrusion of a ductily deforming wedge. The process of extrusive flow suggested here can be approximated quantitatively by channel flow models that have been used to describe subduction zone processes. Channel flow accounts for some observed phenomena in the Himalayan orogen such as inverted metamorphic sequences near the Main Central thrust, not related to an inversion of isotherms, and the syntectonic emplacement of leucogranites into the extruding wedge, locally leading to an inversion of isotherms due to heat advection.

Andalusite-bearing veins formed during contact metamorphism in the aureole of the Vedrette di Ries tonalite. In the veins, quartz crystals that are completely armoured by andalusite or that occur in strain shadow areas contain three generations of fluid inclusions: low-salinity H,O-CO,-CH, mixtures with CHJ(C0, + CH,) 5 0.35 (type A); low-salinity aqueous fluids (type B); H,O-free, C02-CH, fluids with the same carbonic speciation as A (type C). Carbonic types A and C typically have a dark appearance, which is attributed to graphite coatings on inclusion walls. Microstructural analysis of the host quartz and calculated densities indicate that type A inclusions were likely trapped during vein formation. These inclusions underwent strain-assisted re-equilibration during cooling that resulted in density increases without change of composition. After the rocks had cooled below about 350" C, type C inclusions appear to have formed from one of the immiscible fractions after unmixing of the H20-C02-CH, fluid mixtures. Aqueous type B inclusions, apparently trapped between 225 and 350" C, could represent an independent fluid, or could be the H,O-rich fraction of unmixed type A fluids. Taking account of the uncertainties, the composition and density of the complex type A inclusion fluids are in good agreement with the properties of primary fluids calculated from the petrological data. The fluid inclusion data support the model of vein formation by hydrofracturing as a result of dehydration of graphitic metapelites. These new results also demonstrate the importance of considering strain in the interpretation of metamorphic fluid inclusions.

Observations and microthermometric data on fluid inclusions from a terrane that underwent deformation following peak metamorphic conditions show that grain-boundary migration recrystallization favours the entrapment of carbonic inclusions whereas microfracturing during brittle deformation favours the infiltration and eventual entrapment of aqueous fluids. Our results imply that pure CO2 fluid inclusions in metamorphic rocks are likely to be the residue of deformation-recrystallization process rather than representing a primary metamorphic fluid. Where the temperature of deformation can be deduced by other means, the densities of fluid inclusions trapped during recrystallization, which we call recrystallization-primary fluid inclusions, can be used to constrain the ambient pressure during deformation. Using these constraints, the data imply that the post-metamorphic Hercynian exhumation in Sardinia brought rocks at 300° C to within 3km of the surface. This conclusion is similar to that described for the rapidly uplifted Southern Alps in New Zealand.

We present evidence that a rock sample found in the Koryak Mountains in Russia and containing icosahedrite, an icosahedral quasicrystalline phase with composition Al 63 Cu 24 Fe 13 , is part of a meteorite, likely formed in the early solar system about 4.5 Gya. The quasicrystal grains are intergrown with diopside, forsterite, stishovite, and additional metallic phases [khatyrkite (CuAl 2 ), cupalite (CuAl), and β-phase (AlCuFe)]. This assemblage, in turn, is enclosed in a white rind consisting of diopside, hedenbergite, spinel (MgAl 2 O 4 ), nepheline, and forsterite. Particularly notable is a grain of stishovite (from the interior), a tetragonal polymorph of silica that only occurs at ultrahigh pressures (≥10 Gpa), that contains an inclusion of quasicrystal. An extraterrestrial origin is inferred from secondary ion mass spectrometry 18 O/ 16 O and 17 O/ 16 O measurements of the pyroxene and olivine intergrown with the metal that show them to have isotopic compositions unlike any terrestrial minerals and instead overlap those of anhydrous phases in carbonaceous chondrite meteorites. The spinel from the white rind has an isotopic composition suggesting that it was part of a calcium-aluminum-rich inclusion similar to those found in CV3 chondrites. The mechanism that produced this exotic assemblage is not yet understood. The assemblage (metallic copper-aluminum alloy) is extremely reduced, and the close association of aluminum (high temperature refractory lithophile) with copper (low temperature chalcophile) is unexpected. Nevertheless, our evidence indicates that quasicrystals can form naturally under astrophysical conditions and remain stable over cosmic timescales, giving unique insights on their existence in nature and stability.

Deformation in the deep crust is strongly influenced by the presence of melt. Injected melt (or magma) weakens the crust because strain will tend to localize where melt is present. The amount of strain a pluton may accommodate is dependent on the length of time it takes for a pluton to crystallize and the strain rate. For plutons that intrude into rocks which are near the solidus temperature of the melt, crystallization times can be quite long (> 1Myr). Partial melting of deep crustal rocks can lead to melt-enhanced embrittlement. This occurs because the volume change for most melting reactions is positive. Therefore, when the rate of melt production outpaces the rate at which melt can leave the system, the melt pressure increases. Eventually, the melt pressure may become sufficiently high that the melting rocks behave in a brittle fashion and fracture. Conjugate sets of dilatant shear fractures filled with melt occur in migmatite from the Central Gneiss belt (Canada); this suggests that melt-enhanced embrittlement occurred in these rocks. An expression which relates the magnitude of differential stress to the angle between conjugate dilatant shear fractures is derived. Assuming that migmatite has a small tensile strength, differential stresses are ≤ 20 MPa in migmatitic rocks at the time melt-enhanced embrittlement occurs. The occurrence of melt-enhanced embrittlement shows that a switch in deformation mechanism from plastic flow to cataclasis is possible in the deep crust during melting. Furthermore, repeated episodes of melt-enhanced embrittlement in migmatitic rocks may be an efficient mechanism for extracting melt from partially melted terrains.

In some orogenic belts, rapid pulses of uplift occur when melt is present. Detailed work on the relation of melt to structural and metamorphic features in several localities shows that hot metamorphic rocks, and especially anatectic migmatite, can be physically transported from the lower to middle crust on decollements. The decollements form in the lower crust because, even though the lower crust is structurally weak throughout due to thermally activated crystal plastic deformation mechanisms, the crust is substantially weakened where melt forms or is intruded; melt in the lower crust is present long enough to accomodate strain, whereas in the upper crust it crystallizes too fast to be a factor in strain localization. Following crustal loading due to thrusting, exhumation results in the formation of additional melt in the migmatites due to decompression melting. Support of this sequence of events includes: (1) evidence for near isothermal decompression of high temperature metamorphic rocks, with early high pressure minerals having formed pre or synchronous with penetrative deformation and with lower pressure minerals having formed after the deformation, (2) the occurrence of magma sheets that occur along decollements between high temperature metamorphic rocks and lower temperature metamorphic rocks (in either thrust or normal configurations), (3) nearly concordant KAr cooling dates for biotite and hornblende suggesting rapid cooling rates, and (4) “post-tectonic” intrusions of leucogranite or leucotonalite that may represent the products of decompression melting. The crystalline slabs of the high Himalaya, the compressional surges of the Coast orogen of British Columbia and Alaska, the Cordillera of southern British Columbia, and the southern belt of the Hercynian chain are examples where melt assisted rapid pulses of uplift appear to have occurred. During the terminal phases of uplift, and following uplift, these orogenic belts were exhumed during extension while melt was still present.

The Maclaren Glacier metamorphic belt is an exhumed portion of a deep-crustal shear zone where hot, upper amphibolite facies rocks were emplaced over cooler, lower grade rocks. It is located in south central Alaska within the collision zone between terranes previously accreted to North America and the Wrangellia superterrane. Crosscutting relationships and orientations of thin granitoid sills within the hanging wall show that melt was repeatedly intruded into the shear zone during overall compression. In addition, a 1 km thick tonalite sill (the Valdez Creek tonalite) was emplaced into the shear zone while it was active; this conclusion is based on the presence of a well-developed foliation within the sill which is concordant to the fabric of the shear zone, the alignment and tiling of plagioclase laths, the presence of highly strained mafic enclaves within a matrix which shows no evidence of any subsolidus deformation, and the preservation of a delicate magmatic texture which indicates that a magmatic fluid was still present after deformation in the sill had ceased. A one-dimensional thermal model for sill-shaped plutons of varying thicknesses, which are emplaced into country rocks at different initial temperatures, indicates that melt can be present for long periods of time in the deep crust (>1 m.y.). An important factor controlling the length of time that a sill remains molten is the temperature of the surrounding rocks which, in the lower crust, can be as high as the solidus temperature of granitoid melts. If the amount of time for melt to crystallize is sufficiently long, potentially large amounts of strain will be accommodated by zones containing melt due to the low strength of melt compared to rock. In the Maclaren Glacier metamorphic belt, the amount of time needed for the Valdez Creek tonalite to fully crystallize is calculated to be about 90,000 years. This is a minimum value since the original thickness of the sill is not known. Nevertheless, using the 90,000 year value and assuming that the bulk of convergence between North America and Wrangellia was concentrated within the Valdez Creek tonalite, a displacement of at least 10 km could have been accommodated across the sill while melt was present.

New data on fluid flow through the Southern Marginal Zone of the Limpopo Belt are in the papers for this volume by Baker et al., Hoerns and Van Reenen, Vennemann and Smith and Van Schalkwyk et al. These data appear to support an hypothesis that a small amount of CO 2 -rich fluid entered the Southern Marginal Zone at high (but subsolidus) temperatures and participated in retrograde reactions. However, there remain unexplained inconsistencies between conclusions based on the petrologic results and those based on the stable isotopic results. The new data are here reviewed and are considered in the context of contemporary discussions on the origins and roles of fluids in high-grade metamorphism.

A >700-km-long and 5- to 15-km-wide aggregation of sill-like Tertiary plutons separates low- to medium-grade (greenschist to amphibolite facies) metamorphic rocks to the west from medium- to high-grade (upper amphibolite to granulite facies) gneisses and granitic plutons to the east along the western boundary of the Coast Plutonic Complex in southeastern Alaska and British Columbia. 40Ar/39Ar analyses of hornblende, biotite, and plagioclase samples from a sea-level traverse and a 1- to 2-km-elevation traverse across one of the sills, near Holkham Bay, southeastern Alaska, yield dates that vary systematically with position. Samples collected on the eastern margin of the sill are 3 to 6 m.y. younger than samples of the same mineral collected at the same elevation from the western margin of the sill, indicating that the western margin cooled earlier. Samples collected at sea level yield 40Ar/39Ar dates on hornblende, biotite, and plagioclase that are consistently younger than those collected at 1- and 2-km elevation. Dates from the center and eastern margin of the sill define isotherm-migration rates (km/m.y.) of 0.8-1.3 at 60-56 Ma (hornblende), 0.8-0.6 at 56-53 Ma (biotite), and 0.4 at 54-50 Ma (plagioclase). 40Ar/39Ar dates from a single sea- level sample collected 15 km farther south indicate no significant variation in dates parallel to the southeasterly strike of the sill for at least 30 km. The younger dates calculated for the east side relative to the west side and metamorphic temperatures suggest that the country rock was hotter to the east than to the west when the pluton cooled through mineral blocking temperatures. The rapid isotherm migration rates calculated from the vertically distributed samples and the suggestion of hotter country rock to the east during cooling is consistent with an interpretation that the pluton intruded during uplift of the rocks to the east. Because the relative precision of the 40Ar/39Ar technique is sufficient to detect subtle difference in the cooling ages, its application to spatially distributed samples within a homogeneous plutonic body, as in this study, may yield new insights into the timing of metamorphism, magmatism, and tectonism of young mountain belts.

Ductile strain-induced leakage of H2O from mixed CO2 + H2O fluid inclusions in quartz is proposed as a mechanism for producing occurrences of pure CO2 fluid inclusions in metamorphic rocks. The H2O needed for hydrolytic weakening of quartz under stress may be provided by the H2O in fluid inclusions. With dislocation creep, as dislocations nucleate on the walls of fluid inclusions or intersect them, H2O can be transported with the dislocations from the inclusion to the grain boundaries. The process should continue so long as there is stress on the host quartz and the inclusion contains H2O. The H2O taken to the grain boundaries would, due to its wetting properties, be wicked out along the grain boundaries. If the H2O is totally removed from an inclusion by a flux of dislocations through the quartz during the crystal-plastic flow, then a residual inclusion bearing components other than water should remain. A mixed CO2 + H2O inclusion would, as a consequence of the process, become a pure CO2 fluid inclusion. During dynamic recrystallization, the CO2 would collect at grain boundary triple junctions and, following grain boundary migration, would become incorporated into the recrystallized quartz as fluid inclusions. This mechanism of generating pure CO2 inclusions could result in CO2 fluid inclusions having densities appropriate for the pressure—temperature conditions during deformation. Using estimates of temperature of deformation, the total confining pressure during deformation could be determined from the density of the CO2 fluid in these inclusions.

[1] We describe the crustal structure of the Coast Mountains batholith between 54° and 55°N, within the Canadian Cordillera, with emphasis on emplacement of the 7 km thick Kasiks sill complex (KSC). Kinematic patterns that developed during emplacement of the KSC are the result of interactions between magma transport, magma accumulation and regional deformation. The sills were emplaced during NW directed normal shearing and flattening of country rocks that host the KSC. A ∼2 km thick shallowly NE dipping mylonite zone cuts the eastern side of the KSC. Kinematic indicators within the mylonite zone record top to the east normal displacements. Structural analysis shows that mylonite formation occurred during subvertical shortening and east-northeast, subhorizontal extension. U/Pb zircon age dates show that ENE directed normal shearing along the eastern side of the KSC and WNW directed normal shearing within the KSC occurred contemporaneously between ∼54 and 51 Ma, indicating strong strain partitioning between the mylonite and the KSC. This pattern of strain partitioning is interpreted to have been driven by return flow of melt-laden crust in response to tectonic denudation of the upper crust. Seismic profiling shows that many of these structures extend to mid and lower crustal depths. Comparison of our results with other regions within the Canadian Cordillera indicates that orogen-scale right-lateral strike-slip faults deformed synchronously with wide spread magmatism and formation of extensional gneiss domes. Thus the crustal structure of the Coast Mountains batholith was the result of early Tertiary batholith construction during dextral oblique convergence and synorogenic collapse.

[1] The ACCRETE project studies continent formation by assemblage of exotic terranes and arc magmatism. A marine-land seismic profile crosses the terranes with dense sampling and remarkably strong S waves as well as P waves. Unique, high-quality S-wave seismic data provide further constraints on interpretation and are used together with new laboratory P- and S-wave velocity measurements (corrected for high temperatures within and at the base of the crust) to make a lithologic and petrologic interpretation. VP/VS ratios indicate three distinct terranes that were assembled to form continental crust. These terranes are the outboard, accreted Alexander-Wrangellia terrane, the Coast Mountains Batholith (CMB) magmatic arc, and the inboard Stikinia terrane. Below the CMB, VP and VP/VS to a depth of 10–15 km are appropriate for tonalite and diorite. The VP and VP/VS increase with depth indicating that the rocks become more mafic, but VP/VS is not high enough for gabbro. The VP and VP/VS of the lower crust are consistent with a mixture of mafic garnet granulite and restite, whose quartz content lowers the VP/VS ratio below that of gabbro. When corrected for high temperature and uplift due to exhumation, the crust under the CMB exhibits seismic properties of an average continental crust. These results suggest that gabbro could have intruded a metasedimentary pile in the deep crust to cause melting of the metasediments to form intrusions of the CMB, leaving behind a mixture of mafic garnet granulite and sillmanite-garnet-quartz restite and generating the deeper part of what becomes an average continental crustal section.

The Central Gneiss Complex of British Columbia, between lat 54° and 55°N, consists mainly of orthogneiss, rusty-weathering migmatite, leucogneiss, amphibolite, and minor calc-silicate. It forms the country rock for the large intrusions of tonalite and granodiorite that constitute the Coast Mountains batholith. With the exception of calc-silicates, all lithologies intruded by the Paleogene plutons have been partially melted to varying degrees. Metamorphic grade within the complex varies from upper amphibolite facies to granulite facies. The granulite facies rocks are in close proximity to the margins of the large tonalite-granodiorite plutons, suggesting that the plutons locally heated gneisses to the granulite facies. The geologic structure of the Central Gneiss Complex is the result of sequential regional deformations related to the emplacement of voluminous tonalite-granodiorite plutons. Much of the structure can be interpreted to be due to progressive deformation during dextral transpression, with a final imprint produced by flattening during extension. Kilometer-scale tight to isoclinal folds with steeply east-dipping axial planes and shallow to moderately north-plunging hinges were formed during east-side-up shearing with a dextral component along the eastern side of the complex. Transfer of strain from the eastern side of the complex to the western side was accommodated by top-to-the-west shearing that produced reclined isoclinal folds with east-plunging fold axes and shallowly north-dipping axial planes that constitute a flat between two ramps. A domain of steeply dipping foliation dominates the western side of the complex and is cored by the 58.6 Ma Quottoon pluton. Dextral strike-slip shearing occurred in the country rock along the eastern side of Quottoon pluton, and east-side up shearing occurred within the Quottoon pluton. The Quottoon pluton intruded late during transpressive deformation. Intrusion of the Kasiks sill (53 Ma) resulted in flattening of the underlying gneisses, and the interference pattern of this late flattening with foliations produced by the earlier transpressive deformation produced a foliation triple point to the south of the sill. The occurrence of a distinctive lithologic package crossing the Central Gneiss Complex restricts any major zones of strike-slip displacement to be located along its eastern side or along its western side; dextral strike slip strains are recorded along the western side. These features suggest that if the Baja British Columbia fault system passes through the Central Gneiss Complex, it is likely to be where the Quottoon pluton is currently located.

Recent fluid-inclusion and 40 Ar/ 39 Ar cooling-age data show that currently exposed basement rocks in the Raikhot glacier valley of the Nanga Parbat-Haramosh massif, Pakistan Himalaya, were at temperatures of 350 ± 50 °C at depths of 6 ± 2 km (hydrostatic pressure correction). These data imply the presence of a steep thermal gradient in the upper crust at 1 Ma (29-100 °C/km) and denudation rates over the past 1.0 m.y. of 3-6 mm/yr, providing independent corroboration of previous estimates of rapid denudation at Nanga Parbat (4.5 mm/yr over 3.3 m.y.). Our data provide direct documentation of near-surface compaction of isotherms under conditions of rapid denudation, a result that has long been supported by thermal modeling.

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.

The pressure and temperature conditions through which five metamorphic terranes may have passed during uplift have been estimated using the measured compositions and densities of fluids in fluid inclusions, the rupture strength of quartz, and the minimum temperature (~380° C) of CO2-H2O miscibility in natural solutions. The pressures and temperatures of final mineral equilibration are taken as starting points. For four localities, thermal gradients of approximately 27, 33, 54 and 66° C/km existed at some stage during uplift after the rocks had cooled below 380° C. For a fifth locality, a gradient of about 43° C/km existed prior to cooling below 380° C. The P-T path during uplift in four localities was convex towards the temperature axis., Les conditions de température et de pression réalisées pendant la remontée de cinq régions métamorphiques ont été estimées à partir des déterminations de composition et de densité des fluides piégés dans les inclusions, des contraintes de rupture du quartz et de la température minimale (~380° C) de miscibilité de H2O et CO2 dans les solutions naturelles. Les pressions et températures d'équilibre ultime entre minéraux ont été prises comme conditions métamorphiques initiales. Dans quatre localités, les gradients thermiques d'environ 27, 33, 54 et 66° C/km caractérisent cette remontée après que les roches aient déjà été refroidies en dessous de 380° C. Dans une cinquième localité, un gradient d'à peu près 43° C/km existait avant que la série atteigne 380° C. Dans quatre localités le gradient d'ensemble est convexe vers l'axe des températures., Hollister Lincoln S., Burruss Robert C., Henry David L., Hendel Eva-Marie. Physical conditions during uplift of metamorphic terranes, as recorded by fluid inclusions. In: Bulletin de Minéralogie, volume 102, 5-6, 1979. Minéraux et minerais.

The Coast Plutonic Complex between Prince Rupert and Terrace, British Columbia, developed in two stages between mid-Cretaceous and mid-Eocene time. The early stage (>100–70 Ma) involved crustal thickening as the amalgamated Alexander and Wrangellia terranes were emplaced against the Stikine terrane to the east. This thickening resulted from tectonic stacking of crustal slabs, lubricated by intrusion of melt generated at the base of the thickened crust. Emplacement of westward directed thrust slabs along the western margin of the orogen was accompanied by intrusion of a high pressure epidote-bearing tonalite pluton and associated sills. Thrusting resulted in inversion of the metamorphic sequence as supracrustal rocks buried early in the collision were tectonically exhumed. Pluton emplacement also accompanied eastward directed thrusting of high temperature gneisses over low-grade rocks on the eastern margin of the orogen as the tectonic welt was backthrust over Stikinia. The earliest recognized events in the core of the orogen involve pervasive ductile deformation accompanied by emplacement of tonalite sills between about 85 and 50 Ma. Uplift of the orogen core between 60 and 48 Ma coincided with and was facilitated by emplacement of large volumes of tonalitic magma, accompanied by anatexis and the development of ductile shear zones. One of these shear zones, the Work Channel lineament, presently separates the core of the orogen from the schists to the west. The main orogenic cycle ended by 48 Ma when the rocks cooled rapidly through the biotite and hornblende Ar blocking temperatures. The time and space associations of deformation, metamorphism, and plutonic intrusion imply that substantial differential movement occurred across zones which contained fluid during deformation of the Coast Plutonic Complex. In the region of westward directed thrusting and inverted metamorphism, melts generated in the tectonically thickened lower crust and fluids released during metamorphic recrystallization of supracrustal rocks, which were rapidly buried during tectonic thickening, acted to reduce the strength of the crust. In the core zone of the complex, injections of tonalite magma into the lower crust induced pervasive anatexis. The weakened core zone yielded along both low angle and steep shear zones that were further lubricated by melt as the relatively buoyant hot crust rose rapidly in response to continued compression and underthrusting.

The Khtada Lake. British Columbia, metamorphic complex consists of high grade amphibolite and metasedimentary units with development of gneiss, migmatite and homogeneous autochthonous plutons. Maximum metamorphic conditions are estimated to have exceeded 5 kbar and 700°C. Fluid inclusions in matrix quartz are highly variable in density and composition, ranging from apparently pure CO 2 (gas or liquid or both at room temperature) through CO 2 + H 2 O ± CH 4 mixtures to inclusions which are entirely aqueous. They occur along cracks, in groups without planar features and as isolated inclusions. The latter and some which occur in groups, are interpreted to most nearly approximate, in density and composition, the fluids present during the peak of metamorphism. The density and fluid composition data are derived from direct observations of phase changes between − 180 and + 380°C and from the application of published experimental data in the system CH 4 -CO 2 -H 2 O-NaCl. The most dense, pure CO 2 inclusions indicate a pressure of entrapment at 5 kbar, if a temperature of 700°C is assumed. This is in close agreement with the minimum P - T estimates from the mineral assemblages. Methane was positively identified in inclusions in graphite-bearing specimens. Salt content is concluded to be about 5–6 wt% NaCl equivalent in the aqueous phase in both aqueous and CO 2 + H 2 O inclusions. There is evidence of immiscible separation of CO 2 -rich and H 2 O-rich fluids at temperatures at least as high as 375°C.

Heating from hot, young, subducting oceanic lithosphere and from a subducting spreading center has been proposed to explain the metamorphism and associated plutonism observed in former accretionary prism sedimentary rocks in the Gulf of Alaska. We evaluated the thermal effects on the overlying accretionary wedge by heat conduction from a hot, young, subducting slab through numerical modeling. The temperature-limiting parameter in the modeling is the age of the subducted crust. The rate of convergence, angle of subduction, amount of radiogenic heating, and values of thermal conductivity are of secondary importance in determining temperatures in the accretionary prism. A possible former setting for the subduction of young oceanic crust is recorded in the Valdez metasedimentary rocks in the Chugach Metamorphic Complex of the Chugach terrane in southern Alaska. Petrologic data indicate that rocks now at the surface were at 10-km depth when they were metamorphosed under greenschist facies conditions (350–400°C) and subsequently intruded by felsic plutons. Using parameter values appropriate for Chugach terrane rocks, our results indicate that, for accretionary prisms less than 20 km thick, oceanic lithosphere aged 1.0 Ma or less must be subducted in order to obtain temperatures greater than 300°C at 10-km depth within the prism. We also modeled the transient heating of the accretionary prism due to the cessation of subduction of young oceanic lithosphere. Recent seismic refraction surveys from the Trans-Alaska Crustal Transect have been interpreted to show underplated sedimentary rocks and relict oceanic crust beneath the Chugach Mountains. Transient heating due to cessation of subduction or underplating of hot oceanic crust does not appreciably relax the requirement of subducting oceanic crust aged 1.0 Ma or less in order to attain temperatures greater than 30O°C at 10-km depth. However, at depths greater than 25 km, it may provide the heat necessary to generate hot fluids and felsic magma, which can augment the temperatures at shallow depths by advection of heat, as well as produce the postmetamorphism or synmetamorphism emplacement of the anatectically derived plutons observed in the Chugach Mountains. Our modeling requires subduction of such young crust that subduction of a spreading ridge was probable, although underplating of extremely young oceanic crust would also have provided sufficient heat to cause the formation of the Chugach Metamorphic Complex.

Nearly pure CO2 fluid inclusions are abundant in migmatites although H2O-rich fluids are predicted from the phase equilibria. Processes which may play a role in this observation include (1) the effects of decompression on melt, (2) generation of a CO2-bearing volatile phase by the reaction graphite + quartz + biotite + plagioclase = melt + orthopyroxene + CO2-rich vapour, (3) selective leakage of H2O from CO2+ H2O inclusions when the pressure in the inclusion exceeds the confining pressure during decompression, and (4) enrichment of grain-boundary vapour in CO2 by subsolidus retrograde hydration reactions.

Luna 16 sample G36 is a microbasalt containing skeletal olivine, plagioclase, ilmenite, and interstitial pyroxene. It apparently resulted from very rapid crystallization of a highly fractionated, totally liquid mafic magma. Although different in many details, G36 is generally similar to the ferromagnesian-rich Apollo 11 and 12 basalts. In this respect, it emphasizes the continuing problem of identifying a process on the moon which generated highly mafic magmas.

The vitrophyric nature and textural appearance of the Apollo 15 sample 15597, and the unusual compositions of its pigeonite centers and chromites, as well as the extreme chemical zonation of its pyroxenes, give strong evidence that this rock arrived on the lunar surface in an entirely liquid state. It may represent a 'primitive melt,' or at least a precursor from which mare lavas were derived.

Mineral assemblages diagnostic of the granulite facies of metamorphism occur between Terrace and Prince Rupert, British Columbia. The estimated pressure (5–8 kb) and temperature (750–850 °C) of metamorphism are important constraints in unravelling the geologic history of the Coast Range batholithic complex.

The anorthosite slide 15415,18 contains 98 percent subhedral plagioclase (97 mole percent anorthite), two pyroxenes: diopsidic augite (46 percent wollastonite, 39 percent enstatite, 16 percent ferrosilite) with subsidiary (100) lamellae and grains of hypersthene (2.5 percent wollastonite, 58 percent enstatite, 39.5 percent ferrosilite), and traces of ilmenite. The pyroxene occurs interstitial to, and as small grains enclosed within, plagioclase. The textures and compositions of the phases appear compatible with an origin by concentration and adcumulus growth of plagioclase from a gabbroic anorthosite (or hyperaluminous) magma in a "plutonic" environment.

Detailed microscopic studies of rocks from virtually all terrestrial environments show that one or more minerals in these rocks contain small quantities of liquid (or glass) and/or vapor trapped in cavities generally less than 50 microns in diameter. These are fluid inclusions, and they provide us with samples of the fluid phase present in the rock at some time during its evolution. Early petrographers (Sorby, 1858; Rosenbusch, 1923; Zirkel, 1873) included descriptions of these features. More recently, several individuals (notably Lemmlein, Dolgov, and Ermakov in the U.S.S.R.; Poty, Touret, Touray, and Weisbrod in France; and Roedder in the United States) have emphasized the petrologic information that can be obtained from these fluid inclusions. Two recent publications synthesize and summarize much of the previous work on fluid inclusions: the Mineralogical Association of Canada short course handbook Fluid Inclusions: Applications to Petrology (edited by Hollister and Crawford, 1981) and the Mineralogical Society of America Review in Mineralogy entitled Fluid Inclusions (Roedder, 1984). In this chapter, we emphasize important contributions toward understanding metamorphic processes that have been established through study of fluid inclusions, and we review several areas of ongoing research in which the study of fluid inclusions contributes to discussions of unresolved problems.

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.

A prominent topographic lineament along the western margin of the Coast Plutonic Complex of southeast Alaska and adjacent parts of British Columbia separates terranes of contrasting metamorphic histories where it enters British Columbia near Prince Rupert as the Work Channel lineament. The western terrane preserves a regional medium-high pressure metamorphic facies series, increasing from chlorite grade in the west to kyanite + muscovite migmatite near the lineament. The metamorphic, structural and age relations of the western terrane are consistent with a model of crustal thickening accompanied by westward transport of higher over lower grade metamorphic units during the Jurassic. This terrane was uplifted and cooled by Cretaceous time (∼85 Ma). East of Work Channel lineament, high temperature, relatively low pressure assemblages are overprinted onto high pressure assemblages. The low pressure metamorphic assemblages developed in the Eocene during rapid uplift (2 mm/yr) of rocks which initially may have been metamorphosed at the same time and along the same PT gradient as those of the western terrane. Structures along the Work Channel lineament suggest vertical motion during uplift of the eastern terrane and after metamorphism of the western terrane. Because the metamorphic discontinuity across the lineament is 3–5 kbar at about 700°, the lineament in the Prince Rupert area marks a major crustal break.

Convergent tectonics between continental crustal blocks result in deep burial and anatexis of supracrustal rocks. Anatectic and/or mantle-derived melts combine to form a melt-weakened zone in the thickened lower crust. The accumulation of melt eventually leads to crustal failure along melt-lubricated shear zones. Rapid (>1 mm/yr vertical component) movements of large crustal blocks result. We refer to these crustal displacements as tectonic surges. The melt-lubricated shears are characterized by the close association of sheared country rocks with foliated or massive igneous sills and plutons. Rocks that formed at different crustal levels are juxtaposed across these shear zones. One result of surges with large lateral component of movement is metamorphic inversion with high-pressure and high-temperature metamorphic rocks structurally over lower pressure and temperature assemblages. Large and rapid vertical surges may displace crust containing abundant melt and may result in high T/P (including granulite facies) metamorphism and preservation of metamorphic textures caused by rapid decompression.