Blocks In Melange Research Articles

Microfacies and biostratigraphy of an Upper Triassic Dachstein limestone fore-reef block in the Jurassic Sirogojno carbonate-clastic Mélange (Zlatibor Mt., SW Serbia)

In the late Middle to early Late Jurassic carbonate-clastic Sirogojno Melange in the Zlatibor Mountain there is one roughly 35 m thick overturned block with an intact Late Triassic fore-reefal Dachstein Limestone succession studied for its biostratigraphic age, content and microfacies characteristics. The succession starts with coarse-grained rudstones followed by meter-sized reefal blocks intercalated in partly layered resedimented grainstones and packstones with abundant reef-building organisms like calcareous sponges, corals and encrusting organisms. Inside this part of the succession open-marine influenced layers are rare. The succession continues with a partly turbiditic sequence and chaotic rudstones, densely packed with reef-derived material like broken reefbuilding organisms and shallow-water material like gastropods, bivalves and foraminifers. Grainstones with clear open-marine influence (e.g., thin-shelled bivalves, crinoids, conodonts) appear in between those rudstones, in cases lumachelle layers be composed of halobiids were deposited. To the end of the succession some layers show turbiditic bedding with mixed shallow-water and deep-marine grains and organisms, i.e. filaments and crinoids. On base of conodonts, foraminifers, calcareous algae, holothurians and halobiids throughout the whole studied succession, a middle Norian (Alaunian) to Rhae tian 1, most probably a late Norian (Sevatian) age can be assigned to this fore-reefal Dachstein Limestone succession, with a similar sedimentation pattern like Late Triassic Dachstein fore-reef limestone facies, e.g., in the Northern Calcareous Alps or the eastern Southern Alps. The study of this block in the Sirogojno Melange closes an important gap in knowledge about the extent, facies and stratigraphy of the Dachstein Carbonate Platform evolution in the Dinarides.

Regional, Contact Metamorphism, and Autometamorphism of the Olkhon Terrane (West Baikal Area)

The rocks of the Olkhon terrane experienced high-grade regional metamorphism reaching granulite facies. In addition to regional metamorphism, they were subjected to contact metamorphism and autometamorphism (mafic dikes). Three types of regional metamorphism have been distinguished. A terrane collage north of the Orso Zone revealed two stages of regional metamorphism: early-stage granulite metamorphism (Т = 750–900°С, Р = 8–9 kbar, around 500 Ma) and late-stage amphibolite and low–amphibolite facies (Т = 550–710°С, Р = 4–6 kbar, 460–470 Ma). The Orso microterrane separating the Krestovsky island-arc subterrane from a collage of other microterranes contains high-pressure mineral assemblages (up to 10 kbar), which are characterized by the presence of high-calcium garnet in metapelites. The Orso microterrane and Krestovsky subterrane are characterized by relatively high-pressure path of metamorphism. An extended zone of contact high-temperature metamorphism related to the emplacement of gabbro of the second phase of the Ustkrestovsky Complex has been mapped in the Krestovsky subterrane. The temperature of formation of two-pyroxene hornfels after metaporphyrites of the Birkhin volcanoplutonic association is estimated at 750–850°С. Beerbachites were formed by autometamorphism of subvolcanic mafic bodies, which compose the northern part of the Tazheran composite massif (syenite, Ne-syenite, subalkaline gabbro), dikes in the Birkhin gabbro massif, as well as dikes and separate blocks in marble melange. The temperature of autometamorphism during formation of beerbachites is estimated from two-pyroxene geothermometer as 700–1000°С.

O-Hf isotope constraints on the origin of zircon in high-pressure mélange blocks and associated matrix rocks from Tinos and Syros, Greece

Most previously dated zircons in high-pressure melange blocks and associated matrix rocks from Tinos and Syros (Cyclades), Greece, yielded ion microprobe (SHRIMP II) U-Pb ages of ca. 80 Ma. In many cases it remains unclear whether the zircons are igneous or metamorphic/hydrothermal in origin. Oxygen and hafnium isotope ratios in the dated zircons have been determined to further constrain their mode of formation. Spot analysis of zircons from metagabbro, eclogite, glaucophanite, jadeitite and chlorite schists by ion microprobe (CAMECA IMS-1280) yields a large range in δ 18 O, varying from 2.0 ‰ to 7.6 ‰ VSMOW. The average δ 18 O values for most zircons in 11 samples however fall within a relatively small compositional range between 4.7 ‰ and 5.5 ‰, which is consistent with an igneous origin. These values suggest a relationship to magmas typical for modern oceanic crust or to precursors that had been in equilibrium with primitive magma compositions or the mantle (5.3 ± 0.6 ‰, 2 SD). The δ 18 O (mantle-like) and initial epsilon hafnium values [ɛ Hf ( t ) = +10 to +24] suggest that the 80 Ma old zircons are igneous in origin and crystallised from magmas that were derived from depleted mantle. Scanning Electron Microscope cathodoluminescence imaging indicates that lower-δ 18 O zircons (grains or domains; down to 2.0 ‰), mostly from one exceptional eclogite sample (average 4.3 ± 0.8 ‰, 2 SD, n = 23), either represent cauliflower-like internal structures or weakly zoned (or porous) features in outer rims. Previous age dating of zircon domains with cauliflower-like internal structures indicated apparent ages that are considerably younger ( ca. 54 Ma and ca. 57 Ma) than the Cretaceous age determined for the majority of the zircon population. Taken together, these observations suggest that the low-δ 18 O zircons (and three high-δ 18 O zircons, > 6.0 ‰) are genetically linked to a well-documented Eocene high-pressure metamorphic event ( ca . 53–40 Ma), but post-magmatic seafloor hydrothermal alteration cannot be completely ruled out.

New radiolarian biostratigraphic age constraints on Middle Triassic basalts and radiolarites from the Inner Hellenides (Northern Pindos and Othris Mountains, Northern Greece) and their implications for the geodynamic evolution of the early Mesozoic Neotethys

The Avdella Melange in the northern Pindos Mountains and its equivalent formation, the Loggitsion Unit in the Othris Mountains expose early Mesozoic (Mid-Late Triassic) oceanic fragments beneath the Western Greek Ophiolite Belt of the Inner Hellenides, Northern Greece. The melange consists of locally interfingering blocks and slices of ribbon radiolarite, radiolarian chert and pillow basalt and is usually overthrust by Jurassic ophiolites. New Middle and Upper Triassic radiolarian biostratigraphic data are presented from radiolarites and basalt-radiolarite sequences within melange blocks. Pillow basalts associated with the radiolarites provide clues to the opening of the Neotethyan ocean basin. The radiolarians indicate a Middle Triassic age (latest Anisian, probably early Illyrian), which is documented for the first time in the northern Pindos Mountains. The new radiolarian biostratigraphic data suggest that rift-type basalt volcanism already began in pre-Ladinian time (late Scythian?—Anisian). These basalts were then overlain by Upper Anisian to Carnian (?Norian) radiolarites.

High-pressure rocks in the serpentinite melange of the Chara zone, Eastern Kazakhstan: Geochemistry, petrology, and age

The nature of the protolith of high-pressure rocks occurring as exotic blocks in serpentinite melange in the Chara ophiolite belt, Eastern Kazakhstan, is identified based on petrological and geochemical evidence. The distribution of incompatible HFSE in these rocks suggests that their protoliths were similar to E- and N-MORB tholeiites and, more rarely, to OIB alkaline basalts. This testifies to the activity of plume magmatism in the Early Paleozoic evolution of the Paleoasian Ocean. The Ar-Ar ages of phengite and Na-amphibole from the rocks date the subduction/exhumation of high-pressure rocks in the Chara ophiolite belt at the Late Ordovician (450 ± 5 Ma).

Tectonic implications of diverse igneous blocks in Franciscan melange, Northern California and southwestern Oregon

More than 50% of the igneous blocks from melanges of the Franciscan subduction complex in northernmost California and from the equivalent Dothan Formation of southwestern Oregon are more silica-rich than basalt, and some contain supra-subduction zone geochemical signatures such as elevated Th/Ta ratios. In contrast, blocks from more southerly parts of the Franciscan melange belt are more commonly mid-ocean ridge and ocean island/seamount basalts. The data indicate a north-south variation in the block population, due to differences in source terrane, or in the processes that provided the blocks and transported them into the melange, or both. We suggest that, in the northern Franciscan, a relatively greater fraction of igneous melange blocks was emplaced by block slumping from the forearc crust into the trench than was the case to the south.

Structure and Organization of the Porma Melange: Progressive Denudation of a Submarine Nappe Toe by Gravitational Collapse

The Porma Melange is a unit up to 1 km thick underlying the Forcada Nappe, in the Variscan foreland of the Iberian Peninsula. Most of the Porma Melange consists of a highly deformed, boudinaged sequence, with extension values over 300 percent. The boudins or melange blocks are the result of bed-parallel extension in two perpendicular directions giving rise to equidimensional boudins in plan view (choco- late tablet structure). In cross-section view, the boudins show different shapes (rectan- gular, rhomboid, triangular, trapezoid, or with arrowhead edges) due to various combinations of extension and shear fractures. Both the provenance of the melange blocks, mostly derived from the upper part of the Bodon Nappe, that overlies the Forcada Nappe, and flattening-type strain ellipsoid shapes support the interpretation that hanging-wall gravitational collapse of these nappes was the cause of the exten- sional deformation. The current distribution of the melange blocks from different nappe formations implies progressive unroofing of the nappe stack through subse- quent submarine slumps. The internal structure of the melange blocks depends on the degree of sediment lithification during slumping and illustrates the significance of the so-called native and exotic blocks. The source of most of the melange material (shaly matrix and native blocks or boudins) was the semilithified syntectonic sediments of the upper part of the Bodon Nappe, which underwent brittle-ductile deformation (boudinage) during slumping. However, a few and usually larger exotic blocks also came into the basin as individual slip blocks from competent well-lithified formations, originally located at the lower part of the nappe stack, with higher shear resistance and less suitable for spreading. Thus, the arrangement of major nappe fragments, traditionally named klippes and showing sizes that may reach up to several kilometers, does not agree with the geometric rules of thrusting, and are reinterpreted as megablocks resulting from gravitational sliding during denudation of advancing nappes.

Radioisotopic and biostratigraphic age relations in the Coast Range Ophiolite, northern California: Implications for the tectonic evolution of the Western Cordillera

The Coast Range ophiolite (CRO) in northern California includes two distinct remnants. The Elder Creek ophiolite is a classic suprasubduction zone ophiolite with three sequential plutonic suites (layered gabbro, wehrlite-pyroxenite, quartz diorite), a mafi c to felsic dike complex, and mafi c-felsic volcanic rocks; the entire suite is cut by late mid-oceanic-ridge basalt (MORB) dikes and overlain by ophiolitic breccia. The Stonyford volcanic complex (SFVC) comprises three volcanic series with intercalated chert horizons that form a submarine volcano enclosed in sheared serpentinite. Structurally below this seamount are melange blocks of CRO similar to Elder Creek. U/Pb zircon ages from plagiogranite and quartz diorites at Elder Creek range in age from 165 Ma to 172 Ma. U/Pb zircon ages obtained from CRO melange blocks below the SFVC are similar (166‐172 Ma). 40 Ar- 39 Ar ages of alkali basalt glass in the upper SFVC are all younger at ≈164 Ma. Radiolarians extracted from chert lenses intercalated with basalt in the SFVC indicate that the sedimentary strata range in age from Bathonian (Unitary Association Zone 6‐6 of Baumgartner et al., 1995a) near the base of the complex to late Callovian to early Kimmeridgian (Unitary Association Zones 8‐10) in the upper part. The SFVC sedimentary record preserves evidence of a major faunal change wherein relatively small-sized, polytaxic radiolarian faunas were replaced by very robust, oligotaxic, nassellarian-dominated faunas that included Praeparvicingula spp. We suggest that CRO formation began after the early Middle Jurassic (172‐180 Ma) collision of an exotic or fringing arc with North America and initiation of a new or reconfi gured east-dipping subduction zone. The data show that the CRO formed prior to the Late Jurassic Nevadan orogeny, probably by rapid forearc extension above a nascent subduction zone. We infer that CRO spreading ended with the collision of an oceanic spreading center ca. 164 Ma, coincident with the oldest high-grade blocks in the structurally underlying Franciscan assemblage. We further suggest that the “classic” Nevadan orogeny represents a response to spreading center collision, with shallow subduction of young lithosphere causing the initial compressional deformation and with a subsequent change in North American plate motion to rapid northward drift (J2 cusp) causing sinistral transpression and transtension in the Sierra foothills. These data are not consistent with models for Late Jurassic arc collision in the Sierra foothills or a back-arc origin for the CRO.

Middle Ordovician high PT metamorphic rocks in eastern Australia: Evidence from K‐Ar ages

Phengite, separated from a metagabbro, three blueschist, and one metabasite that occur as blocks in serpentinite melange in the New England Fold Belt, eastern Australia, has yielded K‐Ar ages of 465 – 480 Ma, significantly older than the age of most components of the belt. The schists and metabasite contain either glaucophane and/or crossite intergrown with the phengite, providing the first conclusive evidence for early Paleozoic subduction accretion associated with the east Gondwana margin. The tectonic affinity of the blocks is uncertain. They and other small early Paleozoic convergent margin elements in the New England Fold Belt may be allochthonous remnants of an arc system accreted in the Devonian or Carboniferous, or they may be related to early Paleozoic convergent margin components found farther inboard in the Paleozoic fold belts of eastern Australia.

Age profile of ophiolitic rocks across the Late Palaeozoic New England Orogen, New South Wales: Implications for tectonic models

Zircon U‐Pb ages have been determined from various ophiolitic and associated rocks across the southern part of the New England Orogen in northern New South Wales. Ophiolitic rocks of the Weraerai terrane originated during the earliest Cambrian and are the oldest rocks known from this region. The Weraerai terrane is tectonically juxtaposed against Siluro‐Devonian Gamilaroi and Djungati terranes but no unambiguous pre‐Permian links exist between these three terranes. Age and geochemical characteristics of the Weraerai terrane are consistent with its interpretation as a low Ti‐tholeiitic supra‐subduction‐zone ophiolite, analogous to those within the Lachlan Fold Belt located to the west of the New England Orogen. A felsic volcanic rock which occurs as a tectonic block in melange along the Peel Fault zone is geochemically identical to intra‐oceanic island arc rocks of the Gamilaroi terrane, the only likely nearby source. U‐Pb ages for this rock indicate arc development in the Silurian. Zircon ages from a plagiogranite within ophiolitic rocks of the Yarras Complex indicate that it is Devonian. The occurrence of Devonian supra‐subduction zone ophiolitic rocks at Yarras attests to arc rifting within the associated Birpai terrane prior to its accretion to the Gondwana margin. Zircons from felsic intrusives within a highly disrupted ‘ophiolitic’ assemblage at Port Macquarie appear to be entirely detrital with no magmatic component recognized. The youngest zircons are Triassic with other age components present at 300–400 Ma (associated with magmatic activity in southeastern Australia) as well as 500–600, 1000–1200 and up to Archaean ages. The latter are typical of inheritance in Ordovician Lachlan Fold Belt turbidites and granites. Port Macquarie zircon ages are consistent with a scenario in which Lachlan Fold Belt‐type rocks underlie a thin skin of New England Orogen terranes accreted to, and thrust westwards over, the eastern margin of Gondwana during the Palaeozoic.

Primary fluids in low-temperature eclogites: evidence from two subduction complexes (Dominican Republic, and California, USA)

Eclogites occur as isolated blocks in melanges of both the Samana Peninsula, Dominican Republic, and the Franciscan Complex, California, USA. In some of these eclogites, fluid inclusions were found in omphacite and sodic-calcic amphibole grains. Textures show that non-planar populations of fluid inclusions formed during growth of clinopyroxene and amphibole. In addition, planar arrays of secondary fluid inclusions are found along healed cracks. Homogenization temperatures to liquid were used to calculate isochores for the fluid inclusions. These data were compared with petrologic geothermobarometry. Temperature conditions of 500–700° C were estimated from garnetclinopyroxene geothermometry. The jadeite contents of omphacite indicate minimum pressures of 8–11 kbar in this temperature range. The P-T estimates agree well with calculated isochores for primary fluid inclusions from the Samana Peninsula, and show some overlap for both primary and secondary fluid inclusions from the Franciscan Complex. Salinities of 1.2–5.3 wt% NaCl equiv. were estimated for both primary and secondary fluid inclusions from Samana and Franciscan eclogites. These data suggest that low-salinity aqueous fluids attended eclogite-facies metamorphism and perhaps retrograde metamorphism in both subduction complexes. The salinities and densities of fluid inclusions in eclogites from the Samana Peninsula and the Franciscan Complex resemble those of counterparts from garnet amphibolites of the Catalina Schist, southern California. An external source for such fluids is suggested by their homogeneous populations coupled with their low salinities. Geologic evidence suggests that the Samana and Franciscan eclogites may have been derived from a Catalina-like source terrane. The Catalina rocks are inferred to have interacted with large volumes of sediment-derived fluid during subduction zone metamorphism at similar P but higher T conditions than those determined for Samana and Franciscan eclogite blocks. These results contrast with data for fluid inclusions from eclogites of the Monviso area, western Alps. The Monviso eclogites yield similar estimates for metamorphic P-T to those obtained in this study, but contain fluid inclusions of brine and of other saline aqueous fluids, all of which are less dense than expected for incorporation at the reported eclogite-facies conditions. The differences between the properties of fluid inclusions from the ecologites and garnet amphibolites of the Samana-Franciscan-Catalina subduction complexes and those of Monviso probably reflect differences between fluid-flow regimes during metamorphism.

The P-T-t history of blocks in serpentinite-matrix mélange, west-central Baja California

Thermochronologic, petrologic, and geochemical analyses of epidote-amphibolite-, amphilbolite-, eclogite-, and blueschist-facies blocks in serpentinite-matrix melanges from East San Benito Island and Cedros Island, west-central Baja California, provide constraints on the P-T-t history of this disrupted terrane. Results of {sup 40}Ar/{sup 39}Ar step heating experiments on minerals separated from these blocks vary according to metamorphic grade and indicate different P-T-t histories. Mid-Jurassic (160-170 Ma) epidote-amphibolite- and amphibolite-facies blocks were probably derived from oceanic crust and associated sediments that were metamorphosed during limitation of subduction. Coarse-grained blueschist blocks were likely metamorphosed during continued subduction in late Early Cretaceous time (that is, 115-100 Ma). Some epidote-amphibolite blocks are partially overprinted by blueschist-facies mineral assemblages and apparently record both metamorphic events. Fission-track analyses of apatites indicate that the blocks underwent significantly different post-metamorphic cooling histories. Epidote-amphilbolite and amphibolite blocks cooled below {approximately}100 {degrees}C form mid-Jurassic to Paleocene time; blueschist blocks cooled below {approximately}100 {degrees}C in Oligocene-Miocene time. In general, mafic blocks have trace-element concentrations and REE patterns characteristic of ocean-flood basalts. This study demonstrates that samples with very different P-T-t histories can evidently occur over relatively small length scales (<<1 km) in serpentinite-matrix melanges. 67 refs., 16 figs., 4 tabs.

Overview of Early Paleozoic Magmatism in the Eastern Klamath Mountains, California

Igneous rocks of the Yreka and Trinity terranes record a complex history of magmatic events that occurred discontinuously over a 200-m.y. period between the Early Cambrian and the Middle Devonian. Lower Cambrian rocks occur as fault-bounded massifs along the northwestern margin of the Trinity terrane, and as tectonic blocks in melange of the Yreka terrane. The textures and structures of the strongly foliated, ductilely deformed Cambrian rocks indicate a complicated tectonic history prior to their juxtaposition with younger rocks of the Trinity and Yreka terranes. Samples dated as Cambrian (570-565 Ma) consist of tonalite and metagabbro; one tonalite contains inherited Precambrian zircon. Ordovician ultramafic and mafic rocks of the Trinity terrane range in age from 472 to 435 Ma. Two plagiogranites dated at 475 and 469 Ma are presumably also related genetically to the oceanic lithosphere represented by the ultramafic rocks. Basaltic to andesitic pillow lavas, flows and dikes occur locally within the Yreka and Trinity terranes. Within melange, these poorly dated occurrences may be Cambrian, Ordovician, or Silurian in age. However, volcanic rocks that overlie the melange are constrained to be Early to Middle Devonian in age and may be a minor northern manifestation of the well developed Devonianmore » magmatic arc in the Redding terrane to the south. These minor Lower to Middle Devonian volcanics and the dikes that presumable fed them are undeformed, indicating that they were erupted after the Yreka and Trinity terranes were amalgamated. Small swarms of sheeted dikes related to this volcanism suggest eruption during post-amalgamation extension.« less

Diverse Sources for Igneous Blocks in Franciscan Melanges, California Coast Ranges

Igneous blocks in Franciscan melanges are of three chemical-petrologic types: (1) tholeiitic basalts of both arc and spreading center origin, with depletions in light relative to heavy rare-earth elements, $$3\%> TiO_{2} >1\%$$, high Y/Zr and Y/Ti ratios, and relict augites that generally have low Al and Ti and well-defined iron-enrichment trends; (2) basalts of probable seamount origin with marked enrichments in light relative to heavy rare-earth elements, $$5\%> TiO_{2} >1\%$$, lower Y/Zr and Y/Ti than (1), and Ti-Al-rich augites showing little if any iron-enrichment trends; and (3) hypabyssal intrusives having $$SiO_{2} >52\%, TiO_{2} <1\%$$, flat or only slightly fractionated rare-earth-abundance patterns, and diopsidic augites that are very low in Ti and Al and show no iron-enrichment trends. All of the blocks are metamorphosed; most are undeformed pumpellyite-bearing greenstones, and a few contain sodic amphibole ± lawsonite ± sodic pyroxene. The melanges are probably olistostromal in origin, deriving their igneous block detritus both from the downgoing Pacific plate (ocean floor basalts and seamounts) and from the hanging wall of the Franciscan trench (basalts and arc-related silicic intrusive rocks). The silicic intrusive rocks and some of the basalts are eroded fragments of the fore-arc crust that ultimately became the Coast Range Ophiolite. These fragments were incorporated into the Franciscan trench fill and subducted. Our results suggest that the igneous blocks in ophiolitic melanges provide important information about melange formation and about the tectonics and paleogeography of the regions in which the melanges are found.

Geochronology of Blueschists from West-Central Baja California and the Timing of Uplift in Subduction Complexes

A geochronologic study of blueschists from west-central Baja California has provided constraints on the timing of high-pressure, low-temperature metamorphism and the subsequent uplift of this subduction complex. Mineral separates from coherent blueschists and blueschist blocks enclosed in serpentinite melange were analyzed by {sup 40}Ar/{sup 39}Ar, fission track, and Rb/Sr methods. {sup 40}Ar/{sup 39}Ar age spectra for white mica separated from four blueschist blocks and an overprinted eclogite block in serpentinite-matrix melange show slow cooling gradients between 115-95 Ma. Rb/Sr apparent ages for one block are concordant with these results. {sup 40}Ar/{sup 39}Ar ages of blue amphibole are primarily the result of the degassing of white mica intergrowths as confirmed by microprobe analyses. Apatite fission track ages indicate a blueschist block and an eclogite block cooled to a temperature below {approximately}100 C at 22 Ma and 32 Ma, respectively. Subduction-related metamorphism of the coherent blueschists occurred in late Early Cretaceous time, based on a {sup 40}Ar/{sup 39}Ar age or 109 Ma for metamorphic white mica from a metasandstone. {sup 40}Ar/{sup 39}Ar analysis of partially albitized K-feldspar from a plutonic clast in a metaconglomerate indicates this sample cooled to a temperature of {approximately} 145 C at approximately 20 Ma. Geochronologic and petrologicmore » data for coherent blueschists indicate an average uplift rate of 0.1 mm/yr for one portion of the subduction complex. The relatively slow uplift rate and lack of any higher temperature overprinting assemblages in the coherent blueschists suggest that synsubduction uplift was gradual and proceeded through a dynamic accretionary wedge characterized by low geothermal gradients. An increase in uplift rate (to 1.0 mm/yr) during post-Miocene time coincides with a change from a convergent to a transform plate boundary.« less

Juxtaposition of Tethyan and non-Tethyan Mesozoic radiolarian faunas in melanges, Waipapa terrane, North Island, New Zealand

Red cherts from Kawakawa Bay near Auckland have yielded Late Triassic and Early Jurassic radiolarians. The cherts occur as blocks in melanges that have a green argillite matrix containing Middle and Late Jurassic (Callovian-Oxfordian) radiolarians. The melanges mark fault zones along which the Waipapa terrane has been imbricated. Callovian-Oxfordian green argillites are also found in stratigraphic contact with overlying Kimmeridgian-Tithonian green argillites and gray terrigenous clastics. The Late Triassic and Early Jurassic radiolaria are Tethyan, but the Middle and Late Jurassic faunas are dominantly non-Tethyan because the two sets of faunas are of low-latitude and of high-latitude origin, respectively, or because they originated in areas of differing ocean productivity. Occurrence of both Late Triassic and Early Jurassic radiolarians in one red chert horizon indicates sedimentary reworking.

Correlation and provenance of upper Mesozoic chert-rich conglomerate of California

In the central coastal region of California, conglomerate that is rich in chert and quartzite clasts occurs as channel deposits in thin-bedded turbidites of the lower part of the Great Valley sequence of Late Jurassic and Early Cretaceous age (Tithonian to Valanginian). Very similar conglomerate occurs in displaced blocks in melange of the Franciscan assemblage. The Franciscan conglomerate, however, is richer by about 8% in vein-quartz pebbles and requires a slightly different source area. Thick-bedded lithic sandstone in Franciscan melange is also petrographically similar to sandstone of the lower part of the Great Valley sequence, but it is less rich in lithic grains, and, likewise, requires a slightly different source area. The Franciscan conglomerate and sandstone compare most closely with the southernmost exposures of the lower part of the Great Valley sequence near Santa Barbara and may have had a more southerly source area. Younger Franciscan sedimentary rocks are less similar to rocks of the Great Valley sequence, possibly because of the influence of an enlarging subduction complex. Correlative conglomerates of the Great Valley sequence of the Sacramento Valley are very rich in chert clasts, whereas those of the San Joaquin Valley contain more clasts of other sedimentary and volcanic rocks. A speculative model, similar to one proposed previously, suggests that the Great Valley forearc basin formed in an embayment with promontories north and south. The promontories contributed pebbles rich in chert that were transported longitudinally north and south into the basin, then westward. Less extensive, volcanic-rich conglomerates were derived from the east. Comparisons of clast compositions suggest that the chert-rich conglomerates of the Great Valley sequence of the central coast area were deposited about 300 km north of their present positions. Deposition was followed by 700 to 800 km of mid-Cretaceous sinistral strike slip on the Sur-Nacimiento fault and 400 to 500 km of Cenozoic dextral strike slip on the San Andreas fault.

Franciscan blueschist-facies metaconglomerate, Diablo Range, California

Certain Franciscan conglomerate units are shown to be metaconglomerate and to exhibit selective blueschist-facies metamorphism of various constituents. Petrographic and x-ray studies were made on pebbles and matrix of 18 Franciscan conglomerate units forming blocks in melanges and lenses in stratiform bodies of graywacke in the vicinities of Mount Hamilton and Pacheco Pass. The conglomerate ranges from little-deformed varieties (textural grade 1) to tectonite with pronounced secondary planar structures (textural grade 2). Significant minerals in conglomerate of textural grade 1 are quartz + albite ± lawsonite ± minor glaucophane + CaCO 3 , and those of textural grade 2 are lawsonite + glaucophane + quartz ± jadeitic pyroxene ± albite ± aragonite. Thus the most intense metamorphism is empirically associated with internal deformation; however, the converse is not invariably true. Lawsonite in granitic clasts favors feldspar, but its abundance varies within single clasts, from clast to clast, and between adjacent conglomerate units; it commonly forms crystallographically oriented prisms in albite and is locally concentrated near the margins of albite crystals. Glaucophane, which is scarce in conglomerate of textural grade 1, is locally concentrated as rinds in some clasts of fine-grained basalt or it forms rare fibrous prisms in granitic clasts. In rocks of textural grade 2, it commonly forms beards on green amphibole in the matrix and in granitic clasts and completely replaces some mafic clasts. Jadeitic pyroxene replaces albite in conglomerate of textural grade 2 and is concentrated in the outer parts of certain clasts. Carbonate, commonly aragonite, shows crosscutting relations with lawsonite, glaucophane, or jadeitic pyroxene. Paragenetic relations between lawsonite and glaucophane are equivocal, but probably both minerals formed earlier than jadeitic pyroxene. Albite is apparently the only feldspar in the metaconglomerate units. The chemistry of granitic clasts is reflected in their metamorphosed mineral assemblages. They are deficient in potassium and enriched in sodium relative to average granite of similar silica content. The mineral assemblages of the metaconglomerate are consistent with those reported from nearby metagraywacke units, although there is generally no structural continuity with the metagraywacke. Petrographic features indicate postdepositional blueschist-facies metamorphism of the conglomerate units and demonstrate the presence of a fluid phase during metamorphism.