Pacheco Pass Research Articles

Sr–Pb isotope compositions of lawsonites in a Pacheco Pass metagraywacke, Franciscan Complex, California

Sr–Pb isotope compositions of lawsonites in a Pacheco Pass metagraywacke, Franciscan Complex, California

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.

Microfabric of folded quartz veins in metagreywackes: dislocation creep and subgrain rotation at high stress

AbstractThe microfabrics of folded quartz veins in fine‐grained high pressure–low temperature metamorphic greywackes of the Franciscan Subduction Complex at Pacheco Pass, California, were investigated by optical microscopy, scanning electron microscopy including electron backscatter diffraction, and transmission electron microscopy. The foliated host metagreywacke is deformed by dissolution–precipitation creep, as indicated by the shape preferred orientation of mica and clastic quartz without any signs of crystal‐plastic deformation. The absence of crystal‐plastic deformation of clastic quartz suggests that the flow stress in the host metagreywacke remained below a few tens of MPa at temperatures of 250–300 °C. In contrast, the microfabric of the folded quartz veins indicates deformation by dislocation creep accompanied by subgrain rotation recrystallization. For the small recrystallized grain size of ∼8 ± 6 μm, paleopiezometers indicate differential stresses of a few hundred MPa. The stress concentration in the single phase quartz vein is interpreted to be due to its higher effective viscosity compared to the fine‐grained host metagreywacke deforming by dissolution–precipitation creep. The fold shape suggests a viscosity contrast of one to two orders of magnitude. Deformation by dissolution–precipitation creep is expected to be a continuous process. The same must hold for folding of the vein and deformation of the vein quartz by dislocation creep. The microfabric suggests dynamic recrystallization predominantly by subgrain rotation and only minor strain‐induced grain boundary migration, which requires low contrasts in dislocation density across high‐angle grain boundaries to be maintained during climb‐controlled creep at high differential stress. The record of quartz in these continuously deformed veins is characteristic and different from the record in metamorphic rocks exhumed in seismically active regions, where high‐stress deformation at similar temperatures is episodic and related to the seismic cycle.

BATHYSIPHON (FORAMINIFERIDA) AT PACHECO PASS, CALIFORNIA: A GEOPETAL, PALEOCURRENT, AND PALEOBATHYMETRIC INDICATOR IN THE FRANCISCAN COMPLEX

Abstract We document a new locality for the foraminiferid Bathysiphon at Pacheco Pass, California, a classic blueschist-facies location on the eastern edge of the Franciscan Complex. The nearly featureless, silicified tubes of Bathysiphon cf. aaltoi Miller, 1986 described here are the largest reported from the Franciscan Complex; they are <153 mm long and easily recognized in the field. The rarity of fossils in the Franciscan Subduction Complex has hindered field mapping and, thus, a better understanding of its stratigraphy and structure. Bathysiphon specimens can be used as geopetal indicators, since they occur parallel to bedding at the top of argillaceous layers, as paleocurrent indicators (showing a SSE trend at Pacheco Pass), and as paleobathymetric indicators, suggesting, by their large size, mid-to-lower bathyal depths (2000–4000 m).

Microfabric of folded quartz veins in HP-LT metamorphic greywackes from Pacheco Pass, California - progressive deformation and subgrain rotation recrystallization at quasi-stationary conditions

Microfabric of folded quartz veins in HP-LT metamorphic greywackes from Pacheco Pass, California - progressive deformation and subgrain rotation recrystallization at quasi-stationary conditions

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.

Well-preserved underplating structure of the jadeitized Franciscan complex, Pacheco Pass, California

Duplex structures several hundred metres thick bounded by marker beds exhibiting basalt-chert association have been found in the Franciscan complex near Pacheco Pass, central Diablo Range, California. The terrane was regionally metamorphosed and deformed under blueschist facies conditions at 150 °C and 7–8 kbar. Detailed mapping of the basalt-chert sequence has clarified the change of environment from mid-oceanic ridge to subduction zone. Eastward convergence of duplexing indicates eastward subduction of the oceanic plate during Early Cretaceous time. The presence of duplexing and a reconstruction of oceanic plate stratigraphy recorded within the underthrust unit demonstrate the accretionary origin of protoliths of the Franciscan coherent unit, which has long been believed to have an in situ origin, interpreted as either near-trench volcanism or accreted terranes in which “geosynclinal-type” volcanism occurred.

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.

Neoblastic jadeitic pyroxene in Franciscan metagreywackes from Pacheco Pass, central Diablo Range, California, and implications for the inferred metamorphic P‐T trajectory

Abstract Textural, chemical, and areal relations of jadeitic pyroxene‐bearing, quartzose metagreywackes near Pacheco Pass, east‐central California Coast Ranges, indicate that: Na‐clinopyroxene is a newly generated mineral rather than clastic material; pumpellyite is a minor or nonexistent neoblastic phase in associated albite‐bearing metagreywackes, hence growth of jadeitic pyroxene must have resulted from a reaction of the sort chlorite + Na‐plagioclase + iron oxide = jadeitic pyroxene + glaucophane + lawsonite + quartz ± H2O; and, because of sluggish reaction rates, sodic plagioclase persisted in a metastable condition into the higher pressure, nearly end‐member jadeitic pyroxene + quartz P‐T field. Irregular zoning within single jadeitic pyroxene prisms and variable clino‐pyroxene/albite modes unrelated to Na‐clinopyroxene compositions are consequences of kinetics rather than reflecting equilibrium recrystallisation within a P‐T transition zone; high‐pressure overstepping of the stability boundary can ...

Neoblastic jadeitic pyroxene in Franciscan metagreywackes from Pacheco Pass, central Diablo Range, California, and implications for the inferred metamorphic P‐T trajectory

Neoblastic jadeitic pyroxene in Franciscan metagreywackes from Pacheco Pass, central Diablo Range, California, and implications for the inferred metamorphic P‐T trajectory

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

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

Low-temperature recrystallization of Franciscan greywackes from Pacheco Pass, California

AbstractLow-temperature metamorphism of the Franciscan complex at Pacheco Pass occurs at P-T conditions near the so-called jadeite isograd. Along Highway 152 in the Diablo Range, from west to east, four distinct prograde metamorphic zones are defined: (I) albite-quartz zone, (II) albite-rich, albite-quartz-clinopyroxene assemblage, (III) clinopyroxene-rich, albite-quartz-clinopyroxene assemblage, and (IV) clinopyroxene zone. Metamorphic pyroxenes are restricted to zones II, III, and IV in metagreywackes, but metabasites contain augite-rich pyroxenes even in zone I. With increasing grade, the compositions of pyroxene in metagreywackes change in XJd from nearly 100 in zone II to about 60 in zone III. The diopside component has little effect on the XJd of pyroxene. The apparent change of XJd reflects differences in pressure and temperature rather than in bulk rock composition. In metagreywackes, at the onset of zone II, albite breaks down to form Jd100 according to the reaction Ab = Jd+Qz. With increasing grade, this reaction leads to less jadeitic pyroxene. From zones II to III, a continuous reaction is delineated: Qz+2 pyroxenes (Jd80Ac15Aug5)=pyroxene (Jd60Ac30Aug10) + Ab.The metamorphic temperatures are estimated to be about 170°C in zones I and II, and about 230°C in zones II and III. The temperature variation recorded in a single rock is probably less than 30°C The pressure estimate depends on the choice of experimental data for the jadeite-albite-quartz curve; it ranges from 4.5 to 8 kbar. A gently folded thermal structure of the Cretaceous subduction zone metamorphism is postulated for this area.

Review: The Battle of Santa Clara, by Dorothy F. Regnery and Francisco Pacheco of Pacheco Pass, by Albert Shumate

Book Review| April 01 1979 Review: The Battle of Santa Clara, by Dorothy F. Regnery and Francisco Pacheco of Pacheco Pass, by Albert Shumate The Battle of Santa ClaraDorothy F. RegneryFrancisco Pacheco of Pacheco PassAlbert Shumate Seonaid McArthur Seonaid McArthur Search for other works by this author on: This Site PubMed Google Scholar California History (1979) 58 (1): 87. https://doi.org/10.2307/25157897 Views Icon Views Article contents Figures & tables Video Audio Supplementary Data Peer Review Share Icon Share Facebook Twitter LinkedIn MailTo Tools Icon Tools Get Permissions Cite Icon Cite Search Site Citation Seonaid McArthur; Review: The Battle of Santa Clara, by Dorothy F. Regnery and Francisco Pacheco of Pacheco Pass, by Albert Shumate. California History 1 April 1979; 58 (1): 87. doi: https://doi.org/10.2307/25157897 Download citation file: Ris (Zotero) Reference Manager EasyBib Bookends Mendeley Papers EndNote RefWorks BibTex toolbar search Search Dropdown Menu toolbar search search input Search input auto suggest filter your search All ContentCalifornia History Search This content is only available via PDF. Copyright 1979 California Historical Society Article PDF first page preview Close Modal You do not currently have access to this content.

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.

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

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

Metaconglomerate in the Franciscan Formation near Pacheco Pass, California

Metaconglomerate in the Franciscan Formation near Pacheco Pass, California

Widespread occurrence of jadeite, lawsonite, and glaucophane in central California

The Pacheco Pass area, 80 mi SE. of San Francisco, has the most extensive development of metamorphic jadeite and lawsonite yet reported. The host rocks are metamorphosed geosynclinal sedimentary and igneous rocks in the Franciscan Formation (Jura-Cretaceous), and are of 2 types: 1) partially recrystallized rocks that appear in hand specimen relatively unaltered ("subcrystalline" rocks); and 2) thoroughly recrystallized schists, gneisses, and hornfelses ("crystalline" rocks). Subcrystalline rocks predominate; crystalline rocks are common locally, especially in major shear zones. Microscopic and X-ray analyses of the subcrystalline rocks indicate a progressive metamorphic sequence not apparent elsewhere. Lawsonite crystallized first, replacing calcium plagioclase, muscovite, biotite, and possibly other minerals. It occurs in almost all samples from the eastern half of the map area (140 sq mi), and in approximately one-half of those from the western part. Jadeite crystallized next, replacing albite in clastic sedimentary rocks, and both albite and augite in greenstones. Glaucophane, replacing chlorite and hornblende, began to crystallize typically before the crystallization of jadeite was completed. Jadeite and glaucophane are present in almost all samples from the eastern half of the area. Jadeite, lawsonite, and glaucophane are common also in the crystalline rocks, along with other minerals characteristic of the glaucophane schist facies. Greenschist facies rocks and eclogite occur locally in shear zones. The consistent distributional pattern of the subcrystalline rock types probably reflects a regional variation in the physical conditions of metamorphism. The local development of crystalline rocks might indicate sharp local changes in temperature, pressure, or solution action, or possibly the carrying in of these rocks by faulting. The high P-T condition of metamorphism may have been caused by rapid subsidence in a deep marine environment, coupled with widespread tectonic pressure. The absence of syntectonic intrusions probably reflects the low temperature condition of metamorphism, but whether this is a cause or effect relationship is not clear. Finally, the widespread nature of this type of metamorphism at Pacheco Pass suggests new possibilities for seismic interpretation of parts of the crust which may have had a similar geologic history.

Bio-Ecology of Some Eurygaster Species in Central California (Pentatomidae-Scutellerinae)

In Central California, Eurygaster amerinda Bliven and E. minidoka Bliven occur in open woodlands in the foothill area, and feed principally on wild oats. Under certain conditions, they shift to other host plants ( Amsinckia, Ranunculus , and Lolium ). Both species have one generation per year, with a developmental period of 70 to 90 days; from June or July until March or April they are in adult diapause. In the Berkeley Hills, the habitat of E. amerinda is relatively undisturbed, with dormancy shelter sites and feeding areas so close together that migratory movement is insignificant. In Pacheco Pass, the grasslands in the vicinity of the dormancy sites are grazed early in February and March, and at the time of emergence in April the bugs fly to lower rangelands. The adaptation of E. amerinda and E. minidoka to the wild oat, an introduced grain, shows that they have the adaptive ability to become pests of cultivated cereals in the United States.