Jadeitic Pyroxene Research Articles

Temporal and spatial mineralogical changes in clasts from Mariana serpentinite mud volcanoes: Cooling of the hot forearc-mantle at subduction initiation

Mafic and ultramafic clasts (mostly ~1–5 cm in size) were recovered from three different serpentinite mud volcanos in the Mariana forearc during Integrated Ocean Drilling Program (IODP) Expedition 366. Mafic clasts from drill sites distant from the trench bear lawsonite, Al-rich riebeckite, jadeitic pyroxene (~80 mol% jadeite), and aragonite as metamorphic minerals. In contrast, mafic clasts from drill sites closer to the trench are characterized by prehnite–pumpellyite-facies mineral associations and/or the presence of analcime and natrolite. An occurrence of antigorite-bearing ultramafic clasts becomes progressively more frequent with distance from the trench. One amphibolite clast from a mud volcano near the trench also has prehnite filling veins, and it also occurs as pseudomorphs after plagioclase. Amphibolite clasts at other mud volcanoes distant from the trench are partially overprinted by blueschist-facies minerals. The apparent metamorphic grades increase with distance from the trench; these metamorphic conditions represent the increasing depth from zeolite- to lawsonite–blueschist-facies conditions in a subduction zone. Considering the consistency of the low-temperature metamorphic grade of mafic and ultramafic clast mineralogy in each mud volcano, they likely reflect the thermal structure of the slab-mantle interface before the ascent. As a result, these clasts were brought up to the seafloor en masse by the serpentinite mudflow. The polymetamorphosed amphibolite clasts suggest cooling of the hot forearc-mantle at the initiation of Mariana subduction in the Eocene. The ultramafic clasts in the mud volcanoes distant from the trench frequently contain Ca amphibole and talc, which indicates hot mantle hydration by metasomatic fluids released from the slab at subduction initiation.

Geologic evolution of a Cretaceous tectonometamorphic unit in the Franciscan Complex, western California

ABSTRACTThe Franciscan Yolla Bolly terrane of the NE California Coast Ranges consists mainly of quartzose metagreywackes containing sparse high-pressure/low-temperature (HP/LT) neoblastic minerals, including ubiquitous lawsonite. Some Yolla Bolly rocks also contain one or more of the newly grown phases, pumpellyite, aragonite, glaucophane, and/or jadeitic pyroxene. These blueschist-facies metasandstones recrystallized under physical conditions of ~200–300°C and ~8 kbar at subduction-zone depths approaching 30 km. Petrologically similar Franciscan metaclastic-rich map units – Yolla Bolly terrane-like rocks, here designated the ‘YB’ unit – crop out in the central and southern California Coast Ranges. Recently published detrital zircon U‒Pb SIMS and LA-ICPMS data for 19 ‘YB’ metagreywackes indicate maximum ages of formation as follows: ~110–115 Ma (8) in the NE California Coast Ranges; ~95–107 Ma (7) in the San Francisco Bay area + Diablo Range; and ~85–92 Ma (4) in the dextrally offset Nacimiento Block. These fault-bounded ‘YB’ strata do not constitute coeval parts of a single tectonostratigraphic unit. Instead the term tectonometamorphic is proposed for such time-transgressive map units. Based on the current and likely Cretaceous 30° angular divergence between NS-palaeomagnetic stripes of the Farallon oceanic plate and the NNW-trending California convergent margin, I infer that arrival at the arc margin and underflow of a relatively thick segment of oceanic crust and its largely clastic sedimentary blanket may have resulted in progressive southeastward migration of an accreted, subducted, then exhumed HP/LT metagreywacke section. During the ~30 million year interval, ~115–85 Ma, the locus of ‘YB’ accretion, underflow, and tectonic regurgitation evidently moved SE along an ~1000 km stretch of the accretionary margin of western California.

Carbonated sediment–peridotite interaction and melting at 7.5–12GPa

Abstract To gain a better insight into deep subduction-zone processes, the interaction between model sediment and peridotite was experimentally studied using a multianvil apparatus at 7.5–12 GPa and 900–1400 °C. Two H 2 O- and CO 2 -bearing synthetic materials similar to GLOSS of Plank and Langmuir (1998) with different K/Na ratios were used. The peridotite (harzburgite) mixture consisted of natural olivine, orthopyroxene and garnet from kimberlite-derived xenoliths. Sediment (below) and peridotite (above) were packed into a Re-lined Pt capsule and exposed to constant pressure and temperature for 24–72 h. Several experiments were conducted with pure sediment mixtures. The experimental products were analyzed by EPMA and LA ICP MS. Pure sediment melting produces Ca–alkali carbonatite melts near the solidus and liquids richer in SiO 2 at high temperatures. The residual mineral assemblage is dominated by coesite/stishovite, garnet, kyanite and jadeite. Peridotite–sediment interaction results in (1) replacement of olivine in the metaperidotite zone by low-Ca pyroxene and magnesite, (2) formation of a transitional garnet-rich zone containing low-Ca pyroxene and coesite/stishovite, and (3) disappearance of kyanite and stabilization of carbonate in the metasediment layer. The peridotite zone is transformed into a low-Ca pyroxene–garnet aggregate, and its garnet/pyroxene ratio increases with increasing temperature. The sediment zone evolves with increasing temperature from coesite / stishovite + jadeite + garnet + carbonate + rutile to coesite / stishovite + jadeite + garnet. Mass transfer is dominated by Si flux from sediment to peridotite and Mg and Fe from peridotite to sediment. The composition of the melts produced in the interaction experiments differs from that from pure sediment melting in higher Mg and Si contents and much lower Ca. Trace element partition coefficients (D) between residual minerals (pyroxenes, garnet, carbonate, zircon, rutile and SiO 2 polymorphs) and melts were determined in several experiments. Modeling using the obtained D values showed that interaction with peridotite enhances REE fractionation in sediment-derived melt owing to the replacement of kyanite by garnet. The retention of jadeitic pyroxene in the transitional and metasediment zones depresses Na mobility and leads to the formation of high-K/Na liquids. The increasing stability of carbonate in the metasediment may be favorable for deep CO 2 recycling and decoupling of H 2 O and CO 2 . It was shown that the garnet–clinopyroxene residua produced by sediment–peridotite interaction and melting are similar to high-MgO diamond-bearing eclogites reported from several kimberlite pipes worldwide.

Mineral parageneses, regional architecture, and tectonic evolution of Franciscan metagraywackes, Cape Mendocino‐Garberville‐Covelo 30′ × 60′ quadrangles, northwest California

The Franciscan Complex is a classic subduction‐zone assemblage. In northwest California, it comprises a stack of west vergent thrust sheets: westernmost Eastern Belt outliers; Central Belt mélange; Coastal Belt Yager terrane; Coastal Belt Coastal terrane; Coastal Belt King Range/False Cape terranes. We collected samples and determined P‐T conditions of recrystallization for 88 medium‐fine‐grained metasandstones to assess their subduction‐exhumation histories and assembly of the host allochthons. Feebly recrystallized Yager, Coastal, and King Range strata retain clear detrital features. Scattered neoblastic prehnite occurs in several Coastal terrane metasandstones; traces of possible pumpellyite are present in three Yager metaclastic rocks. Pumpellyite ± lawsonite ± aragonite‐bearing Central Belt metasandstones are moderately deformed and reconstituted. Intensely contorted, thoroughly recrystallized Eastern Belt affinity quartzose metagraywackes contain lawsonite + jadeitic pyroxene ± aragonite ± glaucophane. We microprobed neoblastic phases in 23 rocks, documenting mineral parageneses that constrain the tectonic accretion and metamorphic P‐T evolution of these sheets. Quasi‐stable mineral assemblages typify Eastern Belt metasandstones, but mm‐sized domains in the Central and Coastal belt rocks failed to achieve chemical equilibrium. Eastern Belt slabs rose from subduction depths approaching 25–30 km, whereas structurally lower Central Belt mélanges returned from ∼15–18 km. Coastal Belt assemblages suggest burial depths less than 5–8 km. Eastern and Central belt allochthons sequentially decoupled from the downgoing oceanic lithosphere and ascended into the accretionary margin; K‐feldspar‐rich Coastal Belt rocks were stranded along the continental edge without undergoing appreciable subduction, probably during Paleogene unroofing of the older, deeply subducted units of the Franciscan Complex in east‐vergent crustal wedges.

U‐Pb ages of detrital zircons in Pacheco Pass metagraywackes: Sierran‐Klamath source of mid‐Cretaceous and Late Cretaceous Franciscan deposition and underplating

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 ± 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 ± 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.

P‐T‐t deformation framework of an accretionary prism, southern New England Orogen, eastern Australia: Implications for blueschist exhumation and metamorphic switching

The Tia Complex is located in the southern New England Orogen of eastern Australia and provides a detailed record of the structural‐metamorphic evolution of an ancient accretionary prism. This record is characterized by six stages of deformation that were accompanied by a transition from moderate‐pressure–low‐temperature to low‐pressure–high‐temperature metamorphism. The composition of jadeitic pyroxene (subduction), magnesioriebeckite (exhumation), and actinolite (heating) have been used to model P‐T conditions during these structural events, which range from P = 6.3–6.7 kbar and T = 320–350°C (subduction) to 1.5–2.0 kbar and 400–420°C (heating). On the basis of new structural and metamorphic data combined with preexisting age data, the evolution of this accretionary prism can be divided into two main stages: (1) blueschist formation and exhumation and (2) elevated heat flow and anatexis. To explain these features, a new model is presented that requires (1) wedge underthrusting and rear wedge extension associated with a stationary subduction hinge and (2) subduction hinge migration resulting in the relocation of the accretionary wedge onto the upper plate and heating during exposure to the mantle wedge. To explain the event chronology preserved in the Tia Complex, both extensional collapse and subduction hinge migration models are required.

Ion microprobe zircon U–Pb age and geochemistry of the Myanmar jadeitite

Combined geochemistry and geochronology of the Myanmar jadeitite were determined. Bulk-rock trace element compositions display U-shaped REE patterns with pronounced positive Eu anomalies. The total REE abundances are very low, less than half chondritic, and the high field strength elements and some large ion lithophile elements are moderately enriched. These features indicate a metasomatic origin. There are three groups of zircons with different interior characteristics, cathodoluminescence, mineral inclusions, chemical compositions and sensitive high-resolution ion microprobe U–Pb ages. Group-I zircons, with a mean age of 163.2 ± 3.3 Ma, mostly have distinct oscillatory zoning, highest U and Th contents, and Na-free, Mg-rich mineral inclusions, and thus indicate an igneous (formation of oceanic crust) or hydrothermal (serpentinization and/or rodingitization) event in the Middle Jurassic. Group-II zircons, with a mean age of 146.5 ± 3.4 Ma, have bright luminescence without oscillatory zoning and include jadeite and jadeitic pyroxene inclusions, suggesting that formation of the Myanmar jadeitites, as well as subduction of the eastern Indian oceanic plate, occurred in the Late Jurassic. Group-III zircons have an age of 122.2 ± 4.8 Ma, which represents a later unknown thermal event. Discovery of the Middle Jurassic zircons provides geochronological constraint on the tectonic evolution of the eastern Indo-Burman Range.

Coupling of early Tertiary extension in the Great Valley forearc basin with blueschist exhumation in the underlying Franciscan accretionary wedge at Mount Diablo, California

Research Article| November 01, 2007 Coupling of early Tertiary extension in the Great Valley forearc basin with blueschist exhumation in the underlying Franciscan accretionary wedge at Mount Diablo, California Jeffrey R. Unruh; Jeffrey R. Unruh 1William Lettis & Associates, Inc., 1777 Botelho Drive, Suite 262, Walnut Creek, California 94596, USA Search for other works by this author on: GSW Google Scholar Trevor A. Dumitru; Trevor A. Dumitru 2Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305, USA Search for other works by this author on: GSW Google Scholar Thomas L. Sawyer Thomas L. Sawyer 3Piedmont Geosciences, 10235 Blackhawk Drive, Reno, Nevada 89506, USA Search for other works by this author on: GSW Google Scholar Author and Article Information Jeffrey R. Unruh 1William Lettis & Associates, Inc., 1777 Botelho Drive, Suite 262, Walnut Creek, California 94596, USA Trevor A. Dumitru 2Department of Geological and Environmental Sciences, Stanford University, Stanford, California 94305, USA Thomas L. Sawyer 3Piedmont Geosciences, 10235 Blackhawk Drive, Reno, Nevada 89506, USA Publisher: Geological Society of America Received: 13 Jun 2006 Revision Received: 15 Jan 2007 Accepted: 24 Jan 2007 First Online: 08 Mar 2017 Online ISSN: 1943-2674 Print ISSN: 0016-7606 The Geological Society of America, Inc. GSA Bulletin (2007) 119 (11-12): 1347–1367. https://doi.org/10.1130/B26057.1 Article history Received: 13 Jun 2006 Revision Received: 15 Jan 2007 Accepted: 24 Jan 2007 First Online: 08 Mar 2017 Cite View This Citation Add to Citation Manager Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Search Site Citation Jeffrey R. Unruh, Trevor A. Dumitru, Thomas L. Sawyer; Coupling of early Tertiary extension in the Great Valley forearc basin with blueschist exhumation in the underlying Franciscan accretionary wedge at Mount Diablo, California. GSA Bulletin 2007;; 119 (11-12): 1347–1367. doi: https://doi.org/10.1130/B26057.1 Download citation file: Ris (Zotero) Refmanager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentBy SocietyGSA Bulletin Search Advanced Search Abstract In the California Coast Ranges east of San Francisco, post–3.5 Ma folding and erosion of the Mount Diablo anticline have created unusual three-dimensional exposures of late Mesozoic–Cenozoic stratigraphic and structural relations among rocks of the Franciscan subduction complex, Coast Range ophiolite, and Great Valley forearc basin. These relations offer new insights into the kinematics of Late Cretaceous–early Tertiary synsubduction extension, attenuation, and blueschist exhumation within the ancestral California forearc region. Map relations and subsurface data reveal that marine strata of the Great Valley forearc basin northeast of Mount Diablo accumulated in an ∼100-km-long, north-south–trending, extensional graben system. Normal faults in the graben system cut steeply downsection through Late Cretaceous and early Tertiary growth strata and terminate against the Clayton–Marsh Creek fault, which is at a low angle to bedding and is interpreted to be the basal detachment for part of the graben system. The Clayton–Marsh Creek fault in turn is linked to a system of faults that juxtapose blueschist facies rocks of the underlying ancestral Franciscan accretionary prism against attenuated remnants of forearc crust that were never deeply buried, including the Coast Range ophiolite and basal Great Valley strata. These faults are the local expression of the Coast Range fault in this area. Franciscan rocks exposed beneath the faults were metamorphosed at ≥20 km depths after 108 Ma, based on jadeitic pyroxene and 108 Ma detrital zircons (U-Pb) in metagraywackes. Apatite fission-track data indicate that these blueschist facies rocks were subsequently uplifted and exhumed from ∼9 km to ∼3 km depths in latest Cretaceous–early Tertiary time, coeval with the subsidence and extension in the structurally overlying forearc graben system. Much of their earlier rise from ≥20 km to 9 km depth presumably also occurred coeval with graben development, although our data cannot determine this directly. These relations demonstrate that the California forearc around Mount Diablo was in an extensional regime undergoing active sedimentation during major Franciscan exhumation, and thus support models for exhumation of Franciscan blueschist facies rocks via synsubduction extensional processes, rather than via erosional processes. You do not have access to this content, please speak to your institutional administrator if you feel you should have access.

UHP-metamorphic rocks from Dora Maira/Western Alps and Kokchetav/Kazakhstan: New insights using cathodoluminescence petrography

Thin sections of ultrahigh pressure (UHP) metamorphic rocks from the Dora Maira Massif (Italy) and the Kokchetav Massif (Kazakhstan) were investigated using the hot cathode cathodoluminescence (CL) technique. Coloured images of important, but otherwise invisible growth features could be easily identified with this tool within seconds. These features are in excellent correlation with chemical variations of minerals revealed by electron microprobe (EMP). Generally, CL is induced by activator-elements (e.g. Mn and REE) and lattice defects whereas so-called quencher-elements like Fe may reduce or even extinct luminescence. Since X-ray-intensity mapping images (MAPS) of minerals can take up to 50 hours, the CL-method represents an ideal and rapid approach prior to chemical characterization. In addition to typical carbonates such as calcite, Mg-bearing calcite and dolomite, a number of rock forming and accessory minerals including Mg- and Mg-Ca-garnets, diopsidic and jadeitic pyroxenes, kyanite, K-feldspar, quartz, coesite, diamond, zircon, apatite, and bearthite were examined. Features observed in garnets include small-scale oscillatory zoning patterns, changes in morphology during growth as well as different crack generations which were partly annealed. SiO2 phases (coesite, quartz, chalcedony) as well as exsolution textures of dolomite and Mg-bearing calcite are easy to distinguish due to their different CL-colours. Pyroxene displays complex zonation patterns and -to some extent- exsolution-textures of K-feldspar. Kyanite reveals distinct growth zones; in combination with mineral inclusion studies it is possible to discriminate between different kyanite-forming reactions. The different crystallographical orientation of twinned kyanite crystals leads to various luminescence colours, thus, the suture of the twin plane is well defined. Prior to SHRIMP analyses, knowledge of the internal structures of zircon is indispensable. Even very tiny coesite crystals are easy to distinguish from quartz or chalcedony by their disparate luminescence colours. Accessory luminescent minerals like diamond, apatite, bearthite are easy to identify in thin section even if they occur in very small abundance within the matrix or as inclusions. The CL method presented here for UHP-metamorphic rocks is recommended as a pathfinder for the discovery of internal structures of minerals prior to their chemical characterization using EMP.

Chemical fine structure of Franciscan jadeitic pyroxene from Ward Creek, Cazadero area, California

Jadeitic pyroxenes in two Franciscan samples, one, a possible segregation vein in metabasite, and another, a metabasite, both from Ward Creek, Cazadero area, western California, were examined with the electron microprobe. The samples were collected from the area of Type III rocks, the highestgrade portion of the pumpellyite zone. Jadeitic pyroxenes from both samples are very heterogeneous and cannot be ascribed to crystallization under conditions of surface equilibrium with the coexisting phases. Two possibilities for their origin are suggested: one is as tectonic blocks, and the other as the highest-grade rocks from the zone that crystallized close to or within the P-T stability field of pure jadeite + quartz

Dehydration and melting experiments constrain the fate of subducted sediments

Geochemical tracers demonstrate that elements are cycled from subducted sediments into the arc melting regime at subduction zones, although the transfer mechanism is poorly understood. Are key elements (Th, Be, Rb) lost during sediment dehydration or is sediment melting required? To investigate this question, we conducted phase equilibria and trace element partitioning experiments on a pelagic red clay for conditions appropriate to the slab beneath arc volcanoes (2–4 GPa, 600°–1000°C). Using both piston cylinders and multianvils, we determined the solidus, phase stabilities, and major element compositions of coexisting phases. The solidus (H2O + Cl fluid‐saturated) was located at 775 ± 25°C at 2 GPa, 810 ± 15°C at 3 GPa, and 1025 ± 25°C at 4 GPa with noevidence for complete miscibility between melt and fluid. This sediment composition produces a profusion of phases both above and below the solidus: garnet, jadeitic pyroxene, alkali‐rich amphibole, phengite, biotite, magnetite, coesite, kyanite, apatite, zircon, Cl‐rich fluids, and peraluminous to peralkaline granitic melts. At 2 GPa the phengite dehydration solidus is at 800°–825°C, while biotite breaks down between 850° and 900°C. To explore trace element partitioning across the solidus at 2 GPa, we used diamonds to trap fluids and melts. Both the bulk sediment residues and diamond traps were analyzed postexperiment by inductively coupled plasma–mass spectrometry (ICP‐MS) and inductively coupled plasma–atomic emission spectrometry (ICP‐AES) for 40 elements for which we calculated bulk partition coefficients (D = Csolid/Cfluid). Below the solidus, Rb, Sr, Ba, and Pb showed the greatest mobility (D ∼ 0.5–1.0), while at the solidus, Th and Be became notably partitioned into the melt (D values changing from >2.0 to <1.0). K and Rb D values fall below 1.0 when the micas breakdown. Only at the solidus do Th and Rb attain similar partition coefficients, a condition required by arc data. Taken together, the experimental results indicate that critical elements (Th and Be) require sediment melting to be efficiently transferred to the arc. This conclusion is at odds with most thermal models for subduction zones, which predict slab temperatures more than 100°C lower than sediment solidi. Thus the condition of sediment melting (with oceanic crust dehydration) may provide new constraints on the next generation of thermal/geodynamical models of subduction zones.

Structure and Metamorphism of the Franciscan Complex, Mt. Hamilton Area, Northern California

Truncation of metamorphic isograds and fold axes within coherent terranes of Franciscan meta-graywacke by intervening zones of melange indicate that the melange is tectonic and formed after the subduction-related metamorphism and folding. These relations are expressed in two terranes of blueschist-facies rocks of the Franciscan Complex in the Mt. Hamilton area, northern CaliforniA&Mdash; the Jurassic Yolla Bolly terrane and the structurally underlying Cretaceous Burnt Hills terrane. Local preservation in both terranes of basal radiolarian chert and oceanic basalt beneath continent-derived metagraywacke and argillite demonstrates thrust repetition within the coherent terranes, although these relations are scarce near Mt. Hamilton. The metagraywackes range from albite-pumpellyite blueschists to those containing well-crystallized jadeitic pyroxene, and a jadeite-in isograd can be defined in parts of the area. Primary bedding defines locally coherent structural orientations and folds within the metagraywacke units. These units are crosscut by thin zones of tectonic melange containing blocks of high-grade blueschist, serpentinite, and other exotic rocks, and a broader, but otherwise identical melange zone marks the discordant boundary between the two terranes.

Chemical microstructure of Franciscan jadeite from Pacheco Pass, California

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 TiO 2 -bearing jadeite (1-2 wt% TiO 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 volumefor-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 3+ , Ca, and Mg. Terminal stages of high-pressure growth are represented by TiO 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.

Thermobaric Structure of the Franciscan Complex in the Pacheco Pass Region, Diablo Range, California

Examination of the thermobaric structure of jadeite-glaucophane-bearing Franciscan rocks in the Pacheco Pass region of the Diablo Range show that they are subhorizontal, and the structurally intermediate horizon has the highest metamorphic grade. Mineral parageneses of more than 700 metagraywackes, plus 38 metagraywacke samples from up to 1200 vertical feet of drill core were examined. Modal compositions of about 100 metagraywackes were determined, and more than 2600 microprobe analyses were completed to quantify compositions of jadeitic pyroxene from about 80 metagraywackes. The jadeite (Jd) zone is defined by the assemblage jadeitic clinopyroxene + quartz $\pm$ albite + lawsonite $\pm$ sodic amphibole $\pm$ aragonite. This zone occurs at an intermediate structural position, sandwiched between subhorizontal jadeitic clinopyroxene-free albite (Ab) zones. Transition zones occur both above and below the Jd zone. The Jd zone covers an area >20 km wide and <400 m thick. The nearly flat-lying boundaries betwee...

Very low-temperature metamorphism of shales from the Diablo Range, Franciscan Complex, California: New constraints on the exhumation path

We have determined illite crystallinity, coal rank, percent 2M 1 white K-mica polytype, and phase assemblages for shales and shale-matrix melange from various areas in the Diablo Range. Additional information concerning metamorphic conditions and fluid composition was derived from stable isotope work and fluid inclusion measurements. This study presents new constraints on the pressure-temperature evolution of the Franciscan units from the Diablo Range. Several shales indicate a lower pressure, possibly higher temperature overprint on the peak pressure assemblages. Peak pressure assemblages include jadeitic pyroxene, sodic amphibole, lawsonite, aragonite, and phengite. Reactions, which indicate a lower pressure, possibly higher temperature overprint, are the transition from aragonite to calcite, the formation of albite from jadeitic pyroxene + quartz, the compositional gradient from phengite to muscovite in white K-mica, and the transition from sodic to calcic amphibole. The assemblage albite, calcic amphibole, lawsonite, and chlorite found in a single shale from Pacheco Pass indicates the transition from blueschist to greenschist facies conditions. We suggest that this transition occurred at a pressure and temperature of 5–8 kilobars and 300–350°C. Fluid inclusion data from post-kinematic quartz veins from the Mount Hamilton area support the idea of a lower pressure, possibly higher temperature metamorphic event. Pressure-temperature conditions of trapping early H 2 O- and CH 4 -rich fluids were 4.5 kilobars and ≈230°C. Geothermal gradients for the transition from blueschist to greenschist facies and the formation of early fluid inclusions are estimated to be ≈14°C/km. Illite crystallinity for all areas indicates diagenetic to high-grade anchizonal conditions; coal ranks are between 1.6% and 2.5% mean Ro. These estimates are not consistent with metamorphic conditions deduced for two different shale assemblages from Pacheco Pass, suggesting that illite crystallinity and coal rank are retarded. Poor illite crystallinity is thought to result from small coherent scattering domain sizes along c* of white K-mica crystals rich in a phengite component (Dalla Torre et al., 1994a, in press). The exact cause of the retardation of coal rank is not clear. The high pressure in the Diablo Range may have affected the maturation of organic matter. Our results imply that coal rank data from high-pressure terranes must be interpreted with caution if used as an absolute paleothermometer.

Geobarometry of low-temperature eclogites: applications of isothermal pressure-activity calculations

Estimation of metamorphic pressures in low temperature eclogite (Type C) is difficult because of the high variance mineral assemblages and problems in geothermometry, solution properties of low-temperature omphacite, and the thermodynamic properties of clinozoisite. We have considered equilibria in the CaO−FeO−MgO−TiO2−Al2O3−SiO2−H2O (CFMTASH) system involving the phase components, quartz, rutile, kyanite, ilmenite, almandine, pyrope, grossular, clinozoisite, sphene, diopside, and H2O-fluid There are four linearly independent equilibria involving the phase components in this system. Because kyanite can crystallize as a nearly pure phase, the lack of kyanite in a rock indicates that a Al2SiO5 is<1.0. If we can estimate temperature independently, we can solve for a Al2SiO5 and pressure by using two of the equilibria in isothermal pressure-activity diagrams. We have applied this approach to eclogites from New Caledonia and from southwestern Oregon. For the New Caledonia eclogites, calculated pressures range from 11.2 to 13.6 kbar at 500°C, and are consistent with the minimum pressures based upon the presence of jadeitic pyroxene+quartz and the lack of stable albite. Oregon eclogites come from different tectonic blocks and calculated minimum pressures of 11–12 kbar are based upon the presence of jadeitic pyroxene+rutile+garnet and lack of stable albite and ilmenite at reduced values of a SiO2 (0.7–0.9).

Petrotectonic Significance of High and Ultrahigh-Pressure Metamorphic Belts: Inferences for Subduction-Zone Histories

The 900-km-long Franciscan accretionary prism of the California Coast Ranges consists chiefly of greywackes and dark shales of late Mesozoic age. The complex is characterized by monotonic growth toward the edge of the continent and by the systematic, ocean ward decrease in metamorphic intensity. High-pressure (HP) mineral assemblages include lawsonite, aragonite, glaucophane, and/or jadeitic pyroxene. Systematic seaward vergence and younging of the Franciscan, as well as progressively lower metamorphic grade, indicate that landward parts of the belt have been exhumed more profoundly than sections farther outboard. Mapped metamorphic/structural discontinuities reflect juxtaposition of spatially associated, co-evolving continental margin lithotectonic units, and, except for far-travelled oceanic entities, do not imply amalgamation of exotic, unrelated terranes. Pacific-type underflow, metamorphism, and tectonic imbrication characterize belts in which oceanic lithosphere has returned to the mantle. Resurrected, largely metasedimentary, trench complexes display mineral parageneses suggesting maximum retrieved burial depths of ∼ 25-30 km. Analogous HP metamorphic belts fringe the Pacific Basin (e.g., Indonesian Archipelago, Sanbagawa belt of SW Japan, Kamuikotan terrane of Hokkaido and Sakhalin Island, Chugach-Kenai-Kodiak belt of southern Alaska, and Western Series of the Chilean Coastal Range). The Dabie Mountains of east-central China represent a segment of the more than 2000-kmlong Qinling-Dabie-Sulu suture zone between the Sino-Korean and Yangtze cratons. Eastern extensions of the belt apparently cross the Korean Peninsula, and crop out along the Japan Sea margin of Kyushu and western Honshu; a NE tract may be present in the Sikhote-Alin of the Russian Far East. This orogenic belt reflects the latest Paleozoic/early Mesozoic ocean-floor consumption and the Triassic collision of two Precambrian continental massifs. The preservation of coesite and diamond as inclusions in strong, refractory mineral constituents in eclogite facies ultrahigh-pressure (UHP) metamorphic crustal rocks (mafic eclogites, quartzofeldspathic gneisses, and marbles) attests to profound subduction of the Dabie-Sulu area, a leading salient of the old, cold Yangtze craton. The northward increase in intensity of subsolidus recrystallization of the suture complex is comparable to the internal progression in grade of HP and UHP metamorphism in the Western Alps. Plate-tectonic histories of both regions evidently included continental collision and underflow, ultrahigh-pressure metamorphism, and then return toward midcrustal levels of low-density (but coherent) microcontinental blocks as a result of decoupling and continued descent of the higher-density oceanic plate. Other UHP metamorphic complexes include the Kokchetav complex of Kazakhstan, the Maksyutov massif of southern Russia, and the Western Gneiss Region of Norway. In all cases, fragments of ancient sialic-crustcapped lithosphere subducted to depths in excess of 80-100 km have been recovered. Pacific-style HP metamorphic belts now exposed at the surface recrystallized under physical conditions up to about 400-500 °C and 12-14 kbars; Alpine-type UHP parageneses include similar phases and assemblages but, in addition, contain traces of others requiring conditions of 600-800 °C and 28-35 kbars or more. Broad tracts of buoyant, quartzofeldspathic debris carried downward on oceanic lithosphere (Pacific-type subduction) tend to choke trenches with voluminous, weak materials that decouple at 25-30 km depth. In contrast, as they enter the suture zone, narrow salients of subducted continental crust (Alpine-type subduction) appear to be negatively buoyant in regional terms, are therefore carried to much greater depths, remain intact, and may rise differentially as coherent blocks. In both cases, low-density quartzofeldspathic material displaces denser mantle, hence the return to mid and upper crustal levels is propelled by body forces.

Biostratigraphic Constraints on Formation and Timing of Accretion in a Subduction Complex: An Example from the Franciscan Complex of Northern California

The determination of the total age coverage of pelagic bedded chert is particularly important in studies of ancient accretionary complexes because the time span represents the minimum travel time of an oceanic plate before accretion at an island arc or continental margin. The Yolla Bolly terrane of the Franciscan Complex consists of rare metabasalt overlain by bedded radiolarian chert which in turn is overlain by metagraywacke and subordinate slaty mudstone and conglomerate. The bedded chert ranges in age from Aaleian (early Middle Jurassic) to Tithonian (latest Jurassic). A lithologic gradation from pelagic bedded chert through hemipelagic siliceous mudstone occurred during the Tithonian and thus marks the arrival of the oceanic sediments at the continental margin. The Tithonian age also agrees with previously reported Tithonian to Valanginian (Early Cretaceous) fossils in the overlying terrigenous metagraywacke. Both chert and metagraywacke are intruded by gabbroic sills, suggesting off-ridge volcanism, probably in a near-trench environment. Following the intrusive activity, all of the rocks were subducted to depths of 20-30 km, imbricated, and recrystallized to the lower blueschist facies (lawsonite-aragonite ± jadeitic pyroxene). Metamorphic ages, using a variety of isotopic methods, range from 90-105 Ma and thus indicate a long interval (ca. 30-40 Ma) between accretion and subduction. Two possible models include: (1) a long period of storage in an accretionary prism prior to subduction, or (2) accretion and subsequent northward transport.

Jadeitites, albitites and related rocks from the Motagua Fault Zone, Guatemala

ABSTRACTJadeitites from Guatemala are found as weathered blocks in tectonized serpentinite in a 15‐km zone north of the Motagua Fault Zone. Rock types found with jadeitite include albitites, albite‐mica rocks, omphacite/taramitic amphibole‐bearing metabasites, chlorite‐actinolite schists, talc‐carbonate rocks and antigorite schists. In addition to the predominant jadeitic (Jd93_100) pyroxene, common phases in jadeitite include micas (paragonite and/or phengite ± rarer phlogopite), omphacite, albite, titanite /Pm zircon, apatite and graphite. Conditions of jadeitite formation are 100‐400d̀ C, 5‐11 kbar with 0.0 &gt; log10asio2≥= 0.7. Fluid inclusions, coarse textures, vein structures, and rhythmic zoning of pyroxene indicate an aqueuos fluid was involved. Jadeitites are either (1) metasomatic modifications of former felsic‐to‐pelitic inclusions that have undergone silica depletion plus efficient soda exchange and enrichment, or (2) solution precipitations derived from such a source. The close spatial relationship of faults and shear zones, serpentinites, and jadeitites suggests jadeitites form in a relatively high‐P/T setting with substantial flow of sodic fluid in a tectonized zone.Most Guatemalan jadeitites are extensively altered to analcime, albite, taramitic amphibole, (clino)zoisite ± nepheline and preiswerkite. This alteration reflects depressurization /Pm heating to below the jadeite + fluid = analcime reaction at high aNa. With progressive alteration, analcime and nepheline are replaced by albite; the increase in silica content may result from fluid flowing up a tectonized zone reaching saturation with an albite assemblage. Albitite phases, albite, actinolite, zoisite, /Pm chlorite, phengite, K‐feldspar and quartz, record conditions of c. 3‐8 kbar at T &lt; 400d̀ C, indicating a clockwise P‐T trajectory of the blocks.Barium aluminosilicates—banalsite, celsian, cymrite and hyalophane—are common minor late‐stage phases in jadeitites and albite‐rich rocks. Barian phengite is common in albite‐mica rocks.

Metamorphism of Franciscan tectonostratigraphic assemblage, Pacheco Pass area, east-central Diablo Range, California Coast Ranges

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 7½ 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 north-western 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 + Iws + qtz ± gln ± H 2 O. 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 wide-spread coexistence of quartz and NaAlSi 2 O 6 - rich pyroxene. Inferred physical conditions of prograde metamorphism were 150 ± 50 °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.