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

ABSTRACT

The 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.     

Low-grade blueschist facies metamorphism of metagreywackes, Franciscan Complex, northern California

Abstract Metagreywackes in the Eastern Belt of the Franciscan Complex contain the assemblage: Qtz + Ab + Lws + Chl + Ph + Pmp + Fgl + Hem ° Cal/Arg or compatible subassemblages. Blue amphibole first appears in the westernmost part of the belt and pumpellyite is absent in the eastern part. The compositions of the coexisting minerals and the nature of the continuous reactions in these low-grade blueschists suggest that the distribution of blue amphibole and pumpellyite in the Eastern Belt of the Franciscan Complex reflects differences of effective bulk composition rather than differences in physical conditions of metamorphism. In rocks lacking pumpellyite, white mica may be essential to the growth of blue amphibole, but carbonate plays only a limited role. The continuous reaction that limits the appearance of blue amphibole and the disappearance of coexisting pumpellyite has the general form: Pmp + Chl + Ab + Qtz + Hem + H₂O + FeMg_-1= Fgl + Lws. This reaction requires significant hydration as pressure increases in order to produce blue amphibole. Most of the Eastern Belt of the Franciscan Complex formed in limited ranges of temperature and pressure, which are estimated to be 240—280° C, 6.5-7.5 kbar. Pressures in the westernmost part of the area were about 1 kbar lower than in the east. Pressures of about 8.5-10 kbar are estimated for tectonic blocks that contain sodic clinopyroxene.     

Elemental mobility in subduction metamorphism: insight from metamorphic rocks of the Franciscan Complex and the Feather River ultramafic belt,California

The degree of element mobility in subduction metamorphism has generated much debate; some workers advocate considerable mobility during metamorphism, whereas others postulate minimal mobility. We assess this issue by examination of major and trace element concentrations and Pb-, Nd-isotopic data for 39 mafic metavolcanic rocks from the Franciscan subduction complex, related units of coastal California, and the Feather River ultramafic belt of the northern Sierra Nevada, California; these samples span a wide range of metamorphic grade. We conclude that these rocks, despite their metamorphism up to eclogite facies, preserve protolith major and trace elemental compositions and isotopic ratios, with the exception of some mobile large ion lithophile elements such as Ba, Pb, and to a smaller extent La, U, and Sr. Thus subduction metamorphism of these metabasalts occurred in a largely closed system. Lack of light rare earth element enrichment in the rocks demonstrates lack of chemical exchange with subducted metasediments. Relatively low SiO₂ content (<48 wt.%) of many of the metamorphic rocks and the lack of correspondence between silica depletion and metamorphic grade suggests that the silica depletion resulted from seafloor hydrothermal alteration before subduction. In spite of demonstrated mobility of Pb, and possible mobility of Nd, isotopic ratios of Pb and Nd were not modified during subduction metamorphism. In contrast to our results from metabasaltic rocks, our analysis of actinolite-rich rinds from high-grade Franciscan mélange blocks suggests some chemical exchange between metachert and the overlying mantle. The increasing enrichment in Ba and Pb with increasing metamorphic grade suggests that Ba- and Pb-rich fluids interacted more intensely with metabasalt at the higher grades of metamorphism. Comparison of these results with studies of the active Mariana forearc suggests that fluids interacting with the mantle wedge up-dip of the region of magma genesis are derived from subducting sediments overlying the down-going plate.     

New evidence for polyphase metamorphism of glaucophane schist and eclogite exotic blocks in the Franciscan Complex, California and Oregon

ABSTRACT The early metamorphic history of high-grade exotic blocks in the Franciscan Complex may be more complicated than previously supposed. The different assemblages of high-grade glaucophane schist, eclogite, amphibolite and hornblende schist are commonly considered to have formed at the same time from essentially unmetamorphosed oceanic crust. However, new textural and mineralogical data presented here suggest that high-grade glaucophane schist and eclogite have replaced an earlier epidote-amphibolite facies assemblage that is identical to the primary assemblages in many of the hornblende-rich blocks. At least some of the hornblende-rich blocks may therefore be well-preserved remnants of the earlier metamorphism. Comparison of the mineral assemblages and element partitioning in the mixed-assemblage blocks suggests that the glaucophane schist and eclogite metamorphism took place at slightly lower temperatures but at the same or higher pressures than the earlier, hornblende-forming stage.     

Franciscan Complex olistostrome at Crescent City, northern California

An olistostrome and bounding turbidites are exposed within the late Mesozoic Franciscan Complex along the Crescent City (California) coastline. Facies grade in character from Mutti & Ricci Lucchi (1978) mixed facies B, C and D, to F (the olistostrome), to mixed A, B and E, progressing upwards within the Franciscan stratigraphic section. The facies F unit outcrop is up to 600 m thick and extends 12 km along strike. It consists of oblate to tabular blocks, up to 200 m in maximum dimension, of greenstone, tonalite, radiolarian chert, limestone, phyllite and greywacke dispersed in a scaly argillite matrix. The olistostromal origin of the unit is indicated by depositional contacts with bounding turbidites, by the presence of abundant recycled sedimentary clasts within the unit, and by the presence of sedimentary breccias and associated dismembered, slump-folded turbidites both within the olistostrome and among subjacent turbidites. Sandstones are chiefly feldspathic litharenites that were very likely derived from the partially dissected, late Mesozoic Sierran-Klamath magmatic are. Franciscan rocks record an early pervasive, layer-parallel flattening strain in such features as extensional faults, necking and pinch-and-swell structures. Several scales of extensional faulting account for the juxtaposition of turbidites of different facies and/or with varying degrees of stratal disruption, the formation of sandstone lozenges, and the formation of scaly foliation in the olistostrome matrix. The latter resulted from the juxtaposition of lenticles with varying concentrations of silt and clay. These were ultimately derived from the finer divisions of turbidite beds that were incorporated into the olistostrome. The presence of gradational contacts between some sandstone olistoliths and the olistostrome matrix, and of sandstone dykes that intrude fractures and associated drag-folded turbidite beds indicate that Franciscan sediments were not lithified during their early deformation. These sediments were deposited in either a trench or trench slope basin, and were first deformed either by gravity collapse of the trench slope cover or, less likely, by vertical loading beneath the toe of the accretionary wedge. They later were folded during internal shortening of the growing Franciscan accretionary prism.     

Metamorphic History of a High-Grade Blueschist Exotic Block from the Franciscan Complex, California

A high-grade blueschist tectonic block from the Franciscan Complexof the northeast Diablo Range shows evidence of three episodesof retrograde blueschist facies metamorphism ± deformationdeveloped under progressively declining P-T conditions. Thefirst retrograde metamorphism involved formation of an outerrind of actinolite + chlorite + rutile ± phengite, andthe growth of coarse-grained chlorite + pumpellyite within theblock. During the second event the rind and outer edge of theblueschist were folded, sheared and fractured, and primary glaucophanewithin the blueschist was replaced by albite, medium-grainedchlorite, and glaucophane-crossite. The third retrograde metamorphismwas marked by the pseudomorphic replacement of rind actinoliteby aragonite and quartz. Aragonite also crystallized extensivelywithin th block, accompanied by lawsonite, chlorite, jadeiticpyroxene, and crossite; this last mineral assemblage is identicalto that of the surrounding Franciscan metasedimentary rocks.Features characteristic of the first and second retrograde metamorphicevents are readily observed in other high-grade tectonic blocksof the Franciscan Complex and the correlative Otter Point Formationof Oregon. In contrast, evidence of a third retrograde metamorphism,matching that of presently associated Franciscan and Otter Pointrocks, has been found in some but not all blocks examined sofar. The high P/T conditions of prograde metamorphism and the availablemetamorphic age determinations suggest that the tectonic blocksoriginally formed in a pre-Franciscan subduction zone setting.Fragments of blueschist and eclogite from this metamorphic terrainwere tectonically incorporated in a serpentinite diapir, andthey developed alteration rinds through interaction with theenclosing ultramafic rock. The available data suggest that theexamined exotic block and at least some others were transferredto the Franciscan as detritus from a body of serpentinite thatreached the earth's surface. Such blocks were then resubductedand metamorphosed along with their presently associated sedimentarysequences.     

Polyphase and anticlockwise P-T evolution for Franciscan eclogites and blueschists from Jenner, California, USA

High-grade exotic blocks in the Franciscan Complex at Jenner, California, show evidence for polydeformation/metamorphism, with eight distinct stages. Two parallel sets of mineral assemblages [(E) eclogite, and (BS) laminated blueschist] representing different bulk chemistry were identified. Stage 1, recorded by parallel aligned inclusions (S1) of crossite + omphacite + epidote + ilmenite + titanite + quartz (E), and glaucophane + actinolite + epidote + titanite (BS) in the central parts of zoned garnets, represents the epidote blueschist facies. The onset of a second stage (stage 2) is represented by a weak crenulation of S1 and growth of garnet. This stage develops a well-defined S2 foliation of orientated barroisite + epidote + titanite (E), or subcalcic actinolite + epidote + titanite (BS) at c. 90d? to S1, with syntectonic growth of garnet, defining the (albite-)epidote-amphibolite facies. A third stage, with aligned inclusions of glaucophane + (subcalcic) actinolite + phengite parallel to S2 in the outermost rims of large garnet grains, is assigned to the transitional (albite-)epidote-amphibolite/(garnet-bearing) epidote blueschist facies. The fourth stage represents the peak metamorphism, and was identified by unorientated matrix minerals in the least retrograded samples. In this stage the mineral assemblages garnet + omphacite + glaucophane + phengite (E) and garnet + winchite + phengite + epidote (BS) both represent the eclogite facies. Stage 5 is represented by the retrogression of eclogite facies assemblages to the epidote blueschist facies assemblages crossite/glaucophane + garnet + omphacite + epidote + phengite (E), and glaucophane + actinolite + epidote + phengite (BS), with the development of an S5 foliation subparallel to S2. Stage 6 represents a crenulation of S5, with the development of a well-defined S6 crenulation cleavage wrapping around relics of the eclogite facies assemblages. This crenulation cleavage is further weakly crenulated during a D7 event. Post-D7 (stage 8) is recorded by the growth of lawsonite + chlorite ± actinolite replacing garnet, and by veins of lawsonite + pumpellyite + aragonite and phengite + apatite. The different, yet coeval, mineral parageneses observed in rock types (E) and (BS) are probably due to differences in bulk chemistry. The metamorphic evolution from stage 1 to stage 8 seems to have been broadly continuous, following an anticlockwise P-Tpath: (1) epidote blueschist (garnet-free) to (2) (albite-)epidote-amphibolite to (3) transitional epidote blueschist (garnet-bearing)/(albite-)epidote-amphibolite to (4) eclogite to (5) epidote blueschist (garnet-bearing) to (6-7) epidote blueschist (garnet-free) facies to (8) lawsonite + pumpellyite + aragonite-bearing assemblages. This anticlockwise P-T path may have resulted from a decreasing geothermal gradient with time in the Mesozoic subduction zone of California at early or pre-Franciscan metamorphism.     

Phase relations of pyroxene and amphibole in greenstone,blueschist and eclogite of the Franciscan Complex,California

The phase relations of pyroxenes, amphiboles and associated minerals in metamorphic rocks of the Franciscan Complex can be graphically depicted on a ternary diagram which has at its apices the metamorphic clinopyroxene end members, viz NaAl-NaFe³⁺-Ca(Fe²⁺, Mg). Phases are plotted by projection from a constant subassemblage of minerals. This analysis allows interpretation of the effects of pressure, temperature, bulk rock composition and fluid composition on stability of minerals within the Franciscan.Pyroxenes in meta-igneous rocks and metagraywackes have a limited compositional range and fall into two groups: the omphacites, with 50±5% diopside +hedenbergite component; and the jadeitic pyroxenes with 10±5% diopside+hedenbergite. Pyroxenes intermediate between these two groups are unstable relative to assemblages containing Na-amphibole+other minerals.Coexisting pyroxenes and amphiboles in eclogites and associated coarse blueschists comprise equilibrium assemblages, and the proportion of pyroxene to amphibole is a function of rock composition. Eclogites are stable at higher temperature than regionally developed fine-grained greenstones and blueschists in the Franciscan, and at higher pressure than amphibolites. X _H2O ^fluid is not an important factor in the stability of Franciscan eclogite relative to amphibolite.     

The effects of sub-blocking temperature metamorphism on the K/Ar systematics of hornblendes: 40Ar/39Ar dating of polymetamorphic garnet amphibolite from the Franciscan Complex,California

The effects of sub-blocking temperature metamorphism on the K/Ar system in hornblende, as revealed by ⁴⁰Ar/³⁹Ar release spectra, have been studied in a polymetamorphic knocker from the Franciscan Complex (FC), California. A primary amphibolite assemblage of horn-blende +rutile+epidote+apatite±garnet±sphene is variably overprinted by a blueschist facies assemblage of blue amphibole +lawsonite+chlorite+white mica+pumpellyite±sphene. The secondary assemblage formed at a temperature of 370° C, below that at which rapid Ar diffusion is expected in hornblende. Hornblendes from three, variably-altered samples of garnet amphibolite yield total gas ages of 147 to 161 Ma, but the corresponding plateau ages of 163.0±2.8, 160.6±2.2, and 161.8±2.2 Ma are identical within error. Hornblende separates with lower total gas ages come from more highly overprinted rocks, have excess K compared to that expected on the basis of electron probe analyses, and exhibit anomalously high K/Ca ratios in the low-temperature fractions of their incremental heating spectra. The reduced total gas ages result from the presence of thin (2 m) sheets of younger white mica in hornblendes from the moderately and highly altered amphibolites. The secondary micas are difficult to detect because of their small size and low abundance (2%), but because their K content is 50 to 100 times that of the host hornblende, they contribute significantly to the K and Ar budgets of the sample. The mica intergrowths are not removed by normal sample preparation, but because the mica inclusions degas at lower temperatures than hornblende during vacuum extraction, incremental heating analyses can provide precise cooling ages for the hornblendes as well as useful estimates of the age of the mica inclusions. The hornblende separate from the most altered sample contained 20±10% younger blue amphibole replacing hornblende, but its plateau age was not significantly affected. This is consistent with a replacement process in which K and Ar loss from the hornblende are coupled, leaving the K/Ar system undisturbed in relict primary grains. The K and Ar budgets of the sample are not strongly affected by the blue amphibole because of its very low K content. Because partial replacement of primary amphibole by high-K phyllosilicates occurs in many geological environments, effects like those described here could be widespread.     

Designating tectonostratigraphic terranes versus mapping rock units in subduction complexes: perspectives from the Franciscan Complex of California,USA

Accretionary complex histories are broadly understood. Sedimentation in seafloor and trench environments on drifting subducting plates and in associated trenches, followed by (1) deformation and metamorphism in the subduction zone and (2) subsequent uplift at the overriding plate edge, result in complicated stratigraphic and structural sequences in accretionary complexes. Recognizing, defining, and designating individual terranes in subduction complexes clarify some of these complicated relationships within the resulting continent-scale orogenic belts. Terrane designation does not substitute for detailed stratigraphic and structural mapping. Stratigraphic and structural mapping, combined with radiometric and palaeontologic dating, are necessary for delineation of coherent, broken, and dismembered formations, and various mélange units, and for clarification of the details of subduction complex architecture and history. The Franciscan Complex is a representative subduction complex that has evolved through sedimentation, faulting, folding, and low-temperature metamorphism, followed by uplift, associated deformation, and later overprinted deformation. Many belts of Franciscan rocks are offset by strike-slip faults associated with the dextral San Andreas Fault System. In the Franciscan Complex, among the terrane names applied widely, are the ‘Yolla Bolly Terrane’ and the ‘Central Terrane’. Where detailed mapping and detrital zircon ages exist, data reveal that the two names have been applied to rocks of similar general character and age. In the northeastern Diablo Range, Franciscan Complex rocks include coherent units, broken and dismembered formations, and various types of mélanges, all assigned at various times to the Yolla Bolly and other terranes. The details of stratigraphic and structural history revealed by large-scale mapping and radiometric dating prove to be more useful in clarifying the accretionary complex history than assigning a terrane name to the rocks. That history will assist in resolving terrane assignment issues and allow discrimination of subduction-associated and post-subduction events, essential for understanding the overall history of the orogen.     

西准噶尔玛依勒地区枕状玄武岩年代学、地球化学及岩石成因

通过对造山带内洋岛玄武岩的时代及地球化学性质研究,不仅可以进行古海山/大洋高原的识别,而且还可以进行古洋盆演化及古构造格局恢复。笔者等最新在西准噶尔玛依勒山北侧识别出一套枕状玄武岩,其与火山碎屑岩、硅质岩共生。枕状玄武岩斜长石微晶普遍发育中空骸晶结构,是在水下熔岩急剧萃冷条件下迅速结晶的产物。通过LA-ICP-MS锆石U-Pb测年,获得枕状玄武岩206Pb/238U加权平均年龄为437.2±2.2Ma,该年龄的获得填补了志留纪碱性玄武岩的空白。岩石地球化学分析结果显示,玛依勒枕状玄武岩为碱性玄武岩系列,岩石具有中等Si O2(44.89%~47.81%),高Ti O2(3.28%~4.12%)及P2O5(0.50%~0.70%),低Mg O(3.49%~6.79%),轻、重稀土元素分异较为明显((La/Yb)N=5.5~7.3),无明显Eu异常(Eu/Eu*=0.96~1.06),相对富集Rb、Th、U,亏损Ba、K、Sr,没有明显Nb、Ta负异常,这些地球化学特征与洋岛玄武岩(OIB)极其相似。微量元素含量及反映源区性质的比值表明,枕状玄武岩来源于富集地幔源区,主要组成为尖晶石和石榴石二辉橄榄岩,并发生了5%±的部分熔融,其形成于大洋板内与地幔柱有关的海山/大洋岛屿环境。结合前人研究,认为西准噶尔乃至古亚洲洋在中古生代洋内俯冲的同时,大洋板内可能存在地幔柱活动。     

Petrologic Reconnaissance of Franciscan Metagraywackes from the Diablo Range, Central California Coast Ranges

The complexly folded and faulted Diablo antiform representsa core of pervasively metamorphosed Late Mesozoic Franciscanrocks surrounded by roughly contemporaneous, less deformed,only feebly recrystallized Great Valley strata. The contactbetween the two lithologic series is nearly everywhere a high-anglethrust, the Ortigalita fault. Thin sections of 679 metaclastics from the 3000 km² Franciscanarea of the Diablo Range have been studied. A very rough correlationseems to exist between the degree of textural reconstitutionand phase assemblage. Many feebly metamorphosed Franciscan rockscontain relict clastic biotite; chlorite, white mica, and minoramounts of pumpellyite appear to be newly generated from claysand detrital plagioclase which has been albitized. In more thoroughlyrecrystallized Franciscan rocks, lawsonite has grown by hydrationof the minor An component of plagioclase, but more commonlyby the inferred interaction of interstitial clays+calcium carbonate;some of these rocks carry metamorphic aragonite. Intensely recrystallizedFranciscan metagraywackes contain jadeitic pyroxene±glaucophane.The observed changes in mineral assemblages are thought to reflectprogressive pressure increment at nearly constant temperature. In the north-western portion of the Range, jadeitic metaclasticrocks are located along the faulted margin of the antiform.Elsewhere there appears to be no clear relationship betweensystematic Franciscan parageneses and the tectonic contact withrocks of the Great Valley sequence. Evidently Diablo Range metaclasticassemblages do not owe their present areal distribution to apostulated process involving tectonic overpressures accompanyingthrusting of the Great Valley strata over the Franciscan alongthe Ortigalita fault. Jadeitic pyroxenebearing metagraywackesalso appear to be unrelated to the emplacement of alpine-typeperidotites.     

Anatomy of a subduction complex: architecture of the Franciscan Complex,California, at multiple length and time scales

The Franciscan Complex of California records over 150 million years of continuous E-dipping subduction that terminated with conversion to a dextral transform plate boundary. The Franciscan comprises mélange and coherent units forming a stack of thrust nappes, with significant along-strike variability, and downward-decreasing metamorphic grade and accretion ages. The Franciscan records progressive subduction, accretion, metamorphism, and exhumation, spanning the extended period of subduction, rather than events superimposed on pre-existing stratigraphy. High-pressure (HP) metamorphic rocks lack a thermal overprint, indicating continuity of subduction from subduction initiation at ca. 165 Ma to termination at ca. 25 Ma. Accretionary periods may have alternated with episodes of subduction erosion that removed some previously accreted material, but the complex collectively reflects a net addition of material to the upper plate. Mélanges (serpentinite and siliciclastic matrix) with exotic blocks have sedimentary origins as submarine mass transport deposits, whereas mélanges formed by tectonism comprise disrupted ocean plate stratigraphy and lack exotic blocks. The former are interbedded with and grade into coherent siliciclastic units. Palaeomegathrust horizons, separating nappes accreted at different times, appear restricted to narrow zones of <100 m thickness. Exhumation of Franciscan units, both coherent and mélange, was accommodated by significant extension of the hanging wall and cross-sectional extrusion. The amount of total exhumation, as well as exhumation since subduction termination, needs to be considered when comparing Franciscan architecture to modern and ancient subduction complexes. Equal dextral separation of folded Franciscan nappes and late Cenozoic (post-subduction) units across strands of the (post-subduction) San Andreas fault system shows that the folding of nappes took place prior to subduction termination. Dextral separation of similar clastic sedimentary suites in the Franciscan and the coeval Great Valley Group forearc basin is approximately that of the San Andreas fault system, precluding major syn-subduction strike-slip displacement within the Franciscan.     

Geochemistry of metamorphosed pillow basalts of the Chara Zone,NE Kazakhstan

Geochemical study of the metamorphosed pillow lavas of the Chara Zone revealed that their protoliths have N-MORB compositions. Elevated concentrations of K, Rb, Cs, and lower Ca may be related not to blueschist metamorphism, but rather they may be inherited from underwater alteration of parental basalts. Comparative analysis of the compositions of massive glaucophanites and vein rocks demonstrated the relative mobility of Sr, U, P, Ba, Rb, K, Cs, Ca, and LREEs, at least on the local scale. Their mobility has been provided by fluids circulating in the subduction zone, where the fluids have been generated through slab dehydratation processes. These studies of metamorphosed pillow lavas of the Chara Zone displayed only local alterations in the bulk composition of their protoliths, which evidences a low intensity of fluid flows: this allowed preservation of the geochemistry of oceanic basalts.     

Lithic-volcanic sandstones derived from oceanic crust in the Franciscan Complex of California: 'sedimental memories' of source rock geochemistry

The Permanente terrane, part of the Franciscan accretionary complex of California, contains an unusual fault-bounded packet bearing lithic-volcanic sandstones and conglomerates. Based on petrographical, sedimentological, stratigraphical and geochemical studies, it is concluded that these rocks were deposited on the flank of an oceanic rise or plateau by sediment gravity flows probably below the carbonate compensation depth. The bulk geochemistry of the sandstones is similar to the chemistry of greenstones in the Permanente terrane, suggesting that the greenstones or rocks similar to them represent the source of the lithic-volcanic sandstones, and that the sandstones geochemically ‘remember’ the chemistry of their source rocks. This ‘sedimental memory’ is impressive: geochemical variation in the sandstones is co-linear with presumable magmatic variation trends in the greenstones. Sandstone geochemistry, plotted on Zr/Y vs. Y and V vs. Ti diagrams, shows that compositions occupy similar fields as greenstones in the Permanente terrane and oceanic crustal compositions in general. Because oceanic crustal sequences can become accreted to continental margins, oceanic crust should be considered a potential source of sediment, especially in continental margin settings. However, differentiating sandstones derived from oceanic arcs and from oceanic crust is extremely difficult unless discriminating geochemical criteria are used.     

西准噶尔克拉玛依OIB型枕状玄武岩地球化学及其地质意义研究

克拉玛依西山的枕状玄武岩与浊积岩-凝灰岩共生,厚度大于400米的枕状玄武岩层被火山角砾岩-安山岩-硅质岩．凝灰岩覆盖。岩枕之间充填着硅质泥岩。锆石SHRIMP定年结果表明,枕状玄武岩可能在早寒武世形成（〉517Ma,这套地层曾经一直被认为属于石炭系）。枕状玄武岩的稀土元素含量（117．4×10^-6～153．6×10^-6）和配分模式与洋岛玄武岩（OIB）基本一致。枕状玄武岩中大离子亲石元素（Cs、Rb、Ba、K、Pb和Sr）的含量变化较大（明显偏离OIB）,高场强元素（Nb、Ta、Zr、Hf、Ti和P）相对OIB和原始地幔没有表现出明显异常[e．g．,（Nb／Ta）PM=0．92-O．98,（Zr／Hf）PM=1．08-1．18]。西准噶尔地区存在这套OIB型海相火山．沉积建造说明古亚洲洋在西准噶尔地区于寒武纪就已经存在。这套海相玄武岩岩枕中存在大量古元古代一新太古代（1883—2536Ma）岩浆锆石的事实说明,早古生代洋岛玄武岩岩浆源区存在古老大陆地壳物质。     

Contact metamorphism of roof pendants at Hope Valley,Alpine County,California, USA

Calcareous hornfelses and marbles all contain calcite+K-feldspar+quartz+sphene±diopside±plagioclase ±scapolite±clinozoisite. In addition, rocks on one side of a fault contain combinations of biotite, amphibole, and muscovite while those on the other side contain combinations of grossular, wollastonite, and axinite. At bars, mineral-fluid equilibria in biotite and amphibole-bearing rocks record T= 440° C and garnet-bearing rocks record T=540° C and Conventional volumetric fluid-rock ratios were calculated using measured progress of prograde decarbonation reactions and the conditions of metamorphism: marbles, 0–0.4; amphibole-bearing hornfelses, 1.0–1.4; garnet-bearing hornfelses, 2.8–6.7. Decarbonation reactions were driven by pervasive infiltration of rock by reactive aqueous fluids. Differences in fluid-rock ratio between interbedded marble and hornfels and lack of correlation between fluid-rock ratio and whole-rock Cl-content, however, argue for channelized fluid flow along lithologic layers. A new analysis of reaction progress allows estimation of time-integrated fluxes for a specified temperature gradient along the direction of flow. Results are: marbles, 0–0.1×10⁵ cm³/cm²; amphibole-bearing hornfelses, 0.8–1.3×10⁵ cm³/cm²; garnet-bearing hornfelses, 1.2–2.5 × 10⁵ cm³/cm². Fluid flowed from regions of low to regions of high temperature. Using a simple thermal model for the area, the duration of contact metamorphism was estimated as 10⁵ years. Assuming the time of fluid flow was the same as the duration of the thermal event, the first measurements of average metamorphic fluxes (q) and permeabilities (k) are: average marbles, q=0–0.3×10^–8 cm/s and k =2×10^–6 darcy; hornfels, q=3–8×10^–8 cm/s and k =20–53×10^–6 darcy. Estimated premeabilities are within the range of values measured for metamorphic rocks in the laboratory. Fluxes, permeabilities, and whole-system fluidrock ratios are similar to those estimated for the Skaergaard hydrothermal system by Norton and Taylor (1979).     

Petrogenesis of Franciscan glaucophane schists and associated metamorphic rocks,California

Rocks of the glaucophane-schist facies are widely though irregularly developed in the Franciscan formation of California. Minerals critical of the facies are lawsonite, aragonite, jadeite and omphacitic pyroxenes associated with quartz; amphiboles of the glaucophane-crossite series are almost ubiquitous. The most widely distributed rock, occurring over areas of many square kilometers, is jadeite-lawsonite metagraywacke, commonly veined with aragonite. More spectacular, but occurring mainly in isolated blocks are coarse-grained glaucophane-lawsonite Schists of many kinds. Commonly, but by no means invariably, they are closely associated with bodies of serpentinite. Also common in the vicinity of serpentinite masses are blocks of amphibolite and eclogite.All the metamorphic rocks are considered to be Franciscan sediments and basic volcanics metamorphosed and metasomatized in the deep levels of a folded geosynclinal prism. Experimental data on the stability fields of jadeite-quartz, aragonite, and lawsonite show that the glaucophane-schist facies represents metamorphism at pressures of between 5 and 10 kb and temperatures of 150–300° C. Such conditions could develop at depths greater than 15 km provided a very low geothermal gradient (10°/km) were maintained. The metagray-wackes are considered to represent a regional response to such conditions.The role of serpentinites in glaucophane-schist metamorphism is discussed in terms of a tentatively proposed model: — In very deep levels — perhaps at depths as great as 30 km, bodies of hot ultramafic magma develop restricted aureoles' in which temperatures of 400–600° C are maintained fer perhaps 100–1000 years. The products of metamorphism, which also involves desilication under the influence of the ultramafic magma, are eclogite and amphibolite. Later, and perhaps at higher levels serpentinization of the now solid ultramafic masses (near 400° C), causes renewed metamorphism at lower grades. Marginal development of glaucophane Schists and prehnite and hydrogarnet rocks, and retrogressive alteration of eclogite and amphibolite to glaucophane-schist assemblages is attributed to this period.     

Late Quaternary deformation and slip rates in the northern San Andreas fault zone at Olema Valley, Marin County, California

Quaternary sedimentary deposits along the structural depression of the San Andreas fault (SAF) zone north of San Francisco in Marin County provide an excellent record of rates and styles of neotectonic deformation in a location near where the greatest amount of horizontal offset was measured after the great 1906 San Francisco earthquake. A high-resolution gravity survey in the Olema Valley was used to determine the depth to bedrock and the thickness of sediment fill along and across the SAF valley. In the gravity profile across the SAF zone, Quaternary deposits are offset across the 1906 fault trace and truncated by the Western and Eastern Boundary faults, whose youthful activity was previously unknown. The gravity profile parallel to the fault valley shows a basement surface that slopes northward toward an area of present-day subsidence near the head of Tomales Bay. Surface and subsurface investigations of the late Pleistocene Olema Creek Formation (Qoc) indicate that this area of subsidence was located further south during deposition of the Qoc and that it has migrated northward since then. Localized subsidence has been replaced by localized contraction that has produced folding and uplift of the Qoc. This apparent alternation between transtension and transpression may be the result of a northward-diverging fault geometry of fault strands that includes the valley-bounding faults as well as the 1906 SAF trace. The Vedanta marsh is a smaller example of localized subsidence in the fault zone, between the 1906 SAF trace and the Western Boundary fault. Analyses of Holocene marsh sediments in cores and a paleoseismic trench indicate thickening, and probably tilting, toward the 1906 trace, consistent with coseismic deformation observed at the site following the 1906 earthquake.New age data and offset sedimentary and geomorphic features were used to calculate four late Quaternary slip rate estimates for the SAF at this latitude. Luminescence dates of 112–186 ka for the middle part of the Olema Creek Formation (Qoc), the oldest Quaternary deposit in this part of the valley, suggest a late Pleistocene slip rate of 17–35 mm/year, which replaces the unit to a position adjacent to its sediment source area. A younger alluvial fan deposit (Qqf; basal age 30 ka) is exposed in a quarry along the medial ridge of the fault valley. This fan deposit has been truncated on its western side by dextral SAF movement, and west-side-down vertical movement that has created the Vedanta marsh. Paleocurrent measurements, clast compositions, sediment facies distributions, and soil characteristics show that the Bear Valley Creek drainage, now located northwest of the site, supplied sediment to the fan, which is now being eroded. Restoration of the drainage to its previous location provides an estimated slip rate of 25 mm/year. Furthermore, the Bear Valley Creek drainage probably created a water gap located north of the Qqf deposit during the last glacial maximum 18 ka. The amount of offset between the drainage and the water gap yields an average slip rate of 21–30 mm/year. Finally, displacement of a 1000-year-old debris lobe approximately 20 m from its hillside hollow along the medial ridge indicates a minimum late Holocene slip rate of 21–25 mm/year. Similarity of the late Pleistocene rates to the Holocene slip rate, and to previous rates obtained in paleoseismic trenches in the area, indicates that the rates may not have changed over the past 30 ka, and perhaps the past 200–400 ka. Stratigraphic and structural observations also indicate that valley-bounding faults were active in the late Pleistocene and suggest the need for further study to evaluate their continued seismic potential.     

Graphite‐schist blocks in the Franciscan Mélange,San Simeon,California: Evidence of high‐P metamorphism

The seacliff exposure at San Simeon, California, contains graphite‐schist blocks in a shale‐matrix, an undocumented lithology within the Franciscan mélange. Thirty graphite‐schist blocks were studied to discover all the varieties in this classic locality of mélange. Based on their mineralogical assemblage and composition, and textural characteristics the graphite‐schists in San Simeon are subdivided into two main types (Type I and II) with two subdivisions each (A and B). Type IA and IIA blocks are the most abundant. Type IA graphite‐schists are siltstone/fine greywacke‐like, preserve sedimentary textures, and lack lawsonite. Type IB graphite‐schists are mineralogical and texturally similar to Type IA schists, but are finer grained siltstone and shale. Type IIA graphite‐schists are compositionally layered and contain quartz‐ and albite‐rich layers and dark graphite‐ and intergrown mica/chlorite‐rich layers. Nine out of the 15 Type IIA blocks contain lawsonite. Two Type IIA blocks also contain aragonite (+calcite) in veins. Type IIB graphite‐schists are mostly composed of quartz and minor graphite, intergrown chlorite and white mica, and white mica pseudomorphs after lawsonite. The phengite content of mica in Type IIA blocks is higher than that of mica in Type IA graphite‐schists, confirming they were metamorphosed under high‐P/low‐T conditions. Type IA blocks were recrystallized between 200 and 250 °C at <~3 kbar; whereas, Type IIA blocks were metamorphosed under higher pressure conditions, probably at 250–300 °C and 3–5 kbar. Most likely both types of graphite‐schists were derived from a similar layered siltstone/fine greywacke/shale protolith. Organic matter‐rich sediments deposited in the trench axis were subducted along with oceanic crust during Franciscan subduction. Type I graphite‐schists were subducted to depths <10 km, whereas Type II graphite‐schists were subducted to depths ~15 km where they were underplated under high‐P conditions. The graphitic metasedimentary rocks were juxtaposed with mafic lithologies from the subducted oceanic crust that were metamorphosed to blueschist facies and retrograded to greenstone as they returned to the surface in the subduction channel shear zone.