Geological Society of America Special Paper 255 1990
Comparison of early Mesozoic high-pressurerocks in the Klamath Mountains and Sierra Nevada
Bradley R. Hacker* Department of Earth and Space Sciences,University of California, Los Angeles, California 90024 John W. Goodge Department of Geological Sciences,Southern Methodist University, Dallas, Texas 75275
ABSTRACT
Early Mesozoic blueschist terranes in the Klamath Mountains (the Stuart Fork) and Sierra Nevada (the Red Ant), are IithologicaUy, petrologicaUy and structuraUy similar. - Both contain common crossite-epidote assemblages formed during subduction of oce- anic sedimentary and volcanic protoliths that had previously been metamorphosed at an ocean ridge. Both are remnants of a high-PIT metamorphic belt paired with a more easterly magmatic arc along the western margin of North America. These similarities imply that the Mesozoic blueschists in both mountain belts shared a common early history. Important differences, however, indicate that their later postsubduction histo- ries were different: blueschists in the Klamath Mountains were preserved without over- printing by higher T assemblages, whereas blueschists in the Sierra Nevada were overprinted by pumpeUyite-actinolite facies metamorphism. From these relations we infer that the Stuart Fork rocks may have been lifted upward more rapidly than the Red Ant terrane, or that the Red Ant terrane was incompletely subducted and not subject to the cooling effect of continuing subduction. The Stuart Fork blueschists were also affected by a contact metamorphic event during widespread Middle to Late Jurassic plutonism, which is not manifest in the northern Sierra Nevada blueschist terrane.
INTRODUCfION greater extent than previously appreciated, and through their multiple-generation metamorphic paragenesesand structural fab- The Klamath Mountains and the Sierra Nevada of the rics, they provide detailed records of convergent-margin processes North American Cordillera contain a variety of rocks interpreted operative during early Mesozoic growth of the Cordillera. as magmatic arcs, oceanic crust, and subduction complexes This chapter compares and contrasts the structural features, formed during different phasesand stylesof ocean-continentplate phase assemblages,and mineral chemistry of the two terranes. interactions. Magmatic arc rocks in the eastern Klamath Moun- Further details may be found in Goodge (1989a, b, 1990) and tains and eastern Sierra Nevada attest to convergence between B. R. Hacker (unpublished data). Textural relations determined continental North America and westward oceanic provinces from by back-scatteredelectron microscopy constrain the sequenceof at least Devonian to Cretaceous time (e.g., Burchfiel and Davis, deformational and metamorphic events, and phase relations and 1981). The Stuart Fork terrane in the Klamath Mountains and mineral compositions determined by electron-probe microanaly- the Red Ant terrane in the Sierra Nevada are subduction com- sis constrain the P-T conditions of metamorphism. The meta- plexesthat developedoceanward of the more easterlyearly Meso- morphic evolution of these terranes provides the petrogenetic zoic magmatic arcs. Both are coherent high-pressure belts of framework for a tectonic model linking the late Paleozoic-early Mesozoic history of the Klamath Mountains and the Sierra *Presentaddress: Department of Geology,Stanford University, Stanford, Nevada. California 94305-2115.
Hacker,B. R., and Goodge,J. W., 1990,Comparison of early Mesowic high-pressurerocks in the KlamathMountains and SierraNevada, in Harwood,D. S., and Miller, M. M., eds.,Paleozoic and early Mesozoicpaleogeographic relations; Sierra Nevada,Klamath Mountains,and relatedterranes: Boulder, Colorado, GeologicalSociety of AmericaSpecial Paper 255.
m
. , . I~'"
278 Hacker and Goodge
° 42°N
D Cretaceous and Tertiary sediments f)I:l ~ Jurassic and Cretaceous plutons
m Smith River terrane
Condrey Mountain schist
./;;:;;:;;: Rattlesnake Creek-Marble ~ //// Mountain . terrane
~{i;i!!ij~Ji~! Hayfork terrane
II North Fork-Salmon River terrane . Stuart Fork terrane Central metamorphic belt
Eastern Klamath belt terranes ~ 1°N ~ Ordovician ultramafic rocks
Figure I. Regional tectonic map of the Klamath Mountains (after Strand, 1962; Hotz, 1971; Wright, 1982; Mortimer, 1984; Wagner and Saucedo, 1988). Regional thrust faults are shown by barbed lines. Eastern Klamath belt includes the Eastern Klamath and Yreka terranes and the Trinity peridotite. Western Paleozoic and Triassic belt includes the Stuart Fork, North Fork-Salmon River, Hayfork, Rattlesnake Creek-Marble Mountain terranes. Western Jurassic belt includes the Condrey Mountain and Smith River terranes.Plutons include Russian Peak (RP), Deadman Peak (DP), China Creek (CC), English Peak (EP), and Caribou Mountain (CM). Major faults include Siskiyou thrust (ST), Salmon River thrust (SRT), Soap Creek Ridge thrust (SCRT), and Brown Meadow fault (BMF). SV = Seiad Valley; Y = Yreka; SB = Sawyers Bar; Ce = Cecilville; Ca = Callahan; E = Etna. Heavy lines delineate the area of the present study.
REGIONAL GEOLOGY lithotectonic belts in the Klamath Mountains form a gener- ally westward-younging structural sequence of units typically Field and analytical data presented in this chapter come separated by east-dipping, west-directed regional thrust faults from study of the central Stuart Fork terrane in the vicinity of the (Fig. 1; Irwin, 1966, 1981; Burchfiel and Davis, 1981). Irwin South Fork of the Salmon River in the Klamath Mountains (1966) identified four Klamath belts, from east to west, as: (I) the (Goodge, 1989a, b; Fig. 1), and the Red Ant terrane in the Yuba eastern Klamath belt, a Paleozoic to Mesozoic volcanic arc and River area of the Sierra Nevada (B. R. Hacker, unpublished data; continental margin sequence(Irwin, 1981; Miller, 1989; Brouxel Fig. 2). Previous studies of theseunits include those of Holdaway and others, 1988); (2) the central metamorphic belt, a remnant of (1963), Davis and others (1965), Hotz (1977a, b), Borns (1980, subducted Devonian oceanic crust (Davis, 1968; Cashman, 1980; 1984), Schweickert and others (1980), Hietanen (1981), and Peacock and Norris, 1989; Hacker and Peacock, this volume); Edelman and others (1989). (3) the western Paleozoic and Triassic belt, composed of various
I
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Comparison of early Mesozoic high-pressurerocks 279
° 4OoN
IN 0. 20 km
D Cretaceousand Tertiary sediments
G:;] Jurassic and Crclaccous plulons
~ Smarlvlllc terrane
~ Slate Creek IcrrarlC
~ FIddle Creek terrane
)?!~::/iCalaveras terrane . Red Ant terrarle ~ Fcathcr IUvcr terrane
0 Nortllcnl SIerra terrane 39°N Figure 2. Regional tectonic map of the northern Sierra Nevada (after Day and others, 1988; Edelman and others, 1989). Regional thrust faults are shown by barbed lines. Post-Mesozoic rocks that overlie the lithotectonic belts are not shown. Qu = Quincy; Do = Downieville; Or = Oroville; Ma = Marysville; NC = Nevada City; NYR = North Yuba River = MYR = Middle Yuba River; SYR = South Yuba River. Heavy lines delineate the area of the present study.
accreted oceanic assemblages(Davis and others, 1978; Wright, terrane (Blake and others, 1982; Mortimer, 1984), consists of 1982; Ando and others, 1983; Mortimer, 1984; Donato, 1987; metabasaltic and siliceous metasedimentaryrocks that originated Coleman and others, 1988); and (4) the western Jurassic belt, as late Paleozoic(?) oceanic crust (Goodge, 1990). It lies struc- containing ophiolitic rocks (Saleeby and others, 1982; Harper turally beneath Devonian amphibolite-facies rocks of the central and others, 1988), moderate- to high-P metamorphic rocks metamorphic belt. The two terranesare separatedby the Siskiyou (Donato and others, 1980; Helper, 1986), and arc-derived clastic thrust fault that was active between Devonian and Middle Juras- sedimentary rocks (Garcia, 1979). sic time, as constrained by the ages of metamorphism of the In the east-central Klamath Mountains, the western Paleo- central metamorphic belt (Lanphere and others, 1968; Hotz, zoic and Triassic belt is further subdivided (from east to west) 1977b; Cashman, 1980) and plutons that cut across the thrust into the Stuart Fork, North Fork-Salmon River, easternHayfork, fault (Harper and Wright, 1984; Wright and Fahan, 1988). The western Hayfork, and Rattlesnake Creek-Marble Mountain ter- Stuart Fork terrane overlies the low-grade, ophiolitic North ranes (Fig. 1) (Wright, 1982; Donato and others, 1982; Ando Fork-Salmon River terrane along the Salmon River thrust, and others, 1983; Mortimer, 1984; Wright and Wyld, 1985; which was active between Early and Middle Jurassic time Donato, 1987; Goodge, 1989b; Wright and Fahan, 1988). These (Goodge, 1989b). western Paleozoic and Triassic belt terranesrepresent segments of As in the Klamath Mountains, metamorphic rocks in the late Paleozoic to mid-Mesozoic oceanic or island-arc complexes northern Sierra Nevada are divisible into a number of terranes that have been deformed to varying degreesas melange or sub- with different structural, metamorphic, or depositional histories duction complex during convergencewith the western margin of (Fig. 2). The Northern Sierra terrane is composed of lower Pa- North America (Davis and others, 1978; Irwin, 1981; Burchfiel leozoic metasedimentaryrocks of the Shoo Fly Complex that are and Davis, 1981; Coleman and others, 1988). overlain by three successive magmatic arc complexes of The Stuart Fork terrane, also referred to as the Fort Jones Devonian-Mississippian, mid-Permian, and Jurassic age (e.g.,
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280 Hacker and Goodge
D'Allura and others, 1977; Schweickert, 1981; Girty and Ward- metavolcanic rocks and underlying metasedimentary rocks are law, 1985; Hannah and Moores, 1986; Hanson and Schweickert, not well exposed and may be tectonic or depositional. Major-, 1986; Harwood, 1988). The Feather River terrane has been in- trace-, and rare earth element abundancesshow that mafic meta- terpreted as a Paleozoic ophiolite (Ehrenberg, 1975; Weisenberg volcanic rocks in the Stuart Fork terrane are slightly enriched and Ave Lallement, 1977; Standlee, 1978; Edelman and others, mid-ocean tholeiites with compositions similar to modern mid- 1989). The Fiddle Creek terrane includes broken formation and ocean ridge or back-arc basin basalts (Goodge, 1990), whereas melange formed in an east-dipping subduction zone during Trias- relict pyroxenes from the Red Ant terrane share affinities with sic to Jurassic time (Hietanen, 1981), and the Calaverasand Red volcanic-arc basalts. The compositions of the metasedimentary Ant terranes are interpreted to have formed in similar environ- and metavolcanic rocks thus suggestthat deposition occurred in ments at earlier times (Hietanen, 1981; Sharp, 1988). The Slate an arc-related or larger ocean basin near a source of igneous or Creek terrane is an Early Jurassic (Hietanen, 1981; Saleeby and metamorphic detritus. others, 1989), immature intraoceanic arc (Day and others, 1965; Edelman and others, 1989). The Smartville terrane is a Late STRUCTURAL RELATIONS Jurassic rifted volcanic arc (Beard and Day, 1987) formed in Early Jurassic ophiolitic basement(Saleeby and others, 1989). A Structural relations, documented more completely by Hold- more complete description of these rock units is given by Hie- away (1963), Cebull and Russell (1979), Schweickert and others tanen (1981) and Edelman and others (1989). (1980), Hietanen (1981), Goodge (1989b), and Edelman and The Red Ant terrane lies structurally below Devonian others (1989), provide evidence of two major deformation amphibolite-facies rocks of the Feather River terrane (Fig. 2). events: D}, characterized by the formation of complex folds and These terranes are separated by a thrust fault that was active possible southwest-directed thrust faults; and D2, which formed between Devonian and Jurassic time, as constrained by agesof simple, open folds related to the emplacement of regional thrust Feather River terrane metamorphism and the age of high-angle sheets.D} structures are interpreted to have formed within a Late faults that cut the thrust fault (Edelman and others, 1989). The Triassic subduction zone, and D2 structures representa later mid- Red Ant terrane overlies the Calaveras terrane along a contact Jurassic regional contraction. also interpreted as an east-dipping thrust fault that was active b I P I . d J .. . D 1 Str UCtures etween ate a eOZOlCan uraSSlCtime, as Ind lcate. d by t he age of the Calaveras terrane and the age of high-angle faults that cut Both terranes are characterized by relatively coherent the thrust fault (Edelman and others, 1989). packets of complexly folded rocks bounded by east-or northeast- dipping reversefaults. Layered metasedimentary rocks contain a ROCK TYPES well-developed phyllitic to schistosefoliation parallel to the com- positional layering. The limbs and axial planes of early tight to The Stuart Fork and Red Ant terranes consist primarily of isoclinal, rootless intrafolial folds are parallel to the foliation. metamorphosed, interlayered siliceous sedimentary and mafic These folds, lineations, and the foliation are folded about meso- volcanic rocks (Fig. 3). Metasedimentary rocks include inter- scopic tight, similar, and parallel folds with dominantly layered quartzite, phyllitic quartzite, phyllite, and argillite. Phyl- northwest-striking axial planes and variably plunging axes. litic quartzite, which is characterizedby alternating millimeter- to Micaceous metasedimentary rocks containing D} meso- centimeter-thick layers of micaceousphyllite and quartzite, is the scopic structures display well-formed microscopic fabrics ex- most common metasedimentary lithology (Fig. 3A, B). Relict pressed by compositional layering and platy mineral preferred bedding, rare microfossils, Mn-rich bulk compositions (as indi- orientation. The foliation is commonly folded tightly to isocli- cated by spessartine-richgarnet), and the high silica content indi- nally. Lawsonite commonly occurs as elongate ellipsoids cate that the protolith was probably deep-seachert. Feldsparsof sheathedby white mica, and shows progressivedeformation from variable composition are the most common detrital grains in euhedral, unbroken blocks, to segmented,curved grains. Recrys- argillaceous layers, implying deposition proximal to a magmatic tallized quartz occurs between lawsonite microboudins. Micro- or metamorphic source. scopic D} fabrics in metabasaltic rocks include a moderate to The protoliths of mafic metavolcanic rocks include massive strong foliation. Although foliation is most commonly defined by and pillowed flows in the Stuart Fork terrane, and volcanic brec- amphibole and mica grain-shape preferred orientation, this fabric cia, flows, and tuffs in the Red Ant terrane (Fig. 3C, D). Pillowed is in places transected by zones containing recrystallized quartz metavolcanic rocks have massivecores and commonly amygda- and feldspar. Segmentsofboudinaged phasessuch as epidotes are loidal rinds; inter-pillow carbonate occurs locally. Concordant separated by strain shadows filled with recrystallized phengite depositional contacts are exposed locally between pillowed vol- and other minerals. Amphiboles and micas commonly are bent canic and overlying metasedimentary rocks of the Stuart Fork around fold hinges; in rare examples they are recrystallized to terrane (Goodge, 1989b). This relation documents that, at least form an axial planar foliation. locally, the metavolcanic rocks are not tectonic or olistostromal There are clear relations between D} structuresand minerals blocks within a metasedimentary matrix. Contacts between that grew during high-pressure metamorphism. Several petro-
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Comparison of early Mesozoic high-pressurerocks 281
Figure 3. Complexly folded interlayered phyllitic quartzite and phyllite in the Stuart Fork (A) and Red Ant (B) terranes.Metavolcanic rocks include pillowed metabasaltin the Stuart Fork terrane (C) and tuff or massiveflows in the Red Ant terrane (0).
graphic observations indicate that recrystallization took place D2 structures synkinematically, and that deformation outlasted recrystalliza- tion: (1) folia formed in mica, sodic amphibole, and lawsonite Both terranes contain decameter- to kilometer-scale open crystals are folded, and metamorphic phasesin fold hinges are folds with east-dipping axial surfaces.These open folds are mor- partially recrystallized; (2) epidote and lawsonite crystals are phologically and geometrically distinct from the complex 0) boudinaged; (3) a complete spectrum of early, strongly deformed structures, and they are interpreted to representa younger period veins to late, weakly deformed veins containing quartz, albite, of deformation-02. 02 structures occur in the Stuart Fork ter- chlorite, crossite, epidote, lawsonite, and phengite, is present; rane mostly within 1 to 2 km of the Salmon River thrust (Fig. 1), (4) fibrous vein fillings, particularly of sodic amphibole, phengite, and within the underlying North Fork-Salmon River terrane and sphene,nucleated on wall minerals and recorded movemen~ (Trexler, 1968; Ando, 1979; Goodge, I 989b). The Salmon River of the vein walls in their crystal shape.There is no indication that thrust i~elf is considered to be a 02 structure (Goodge, 1989b). recrystallization at high pressuresoccurred after deformation. Re- The kilometer-scale antiformal structure within the Red Ant ter- crystallization of phasessuch as sodic amphibole, lawsonite, and rane (Fig. 2) is interpreted as a 02 structure. Both the central phengite in the deformation indicates that the synmetamorphic metamorphic belt overlying the Stuart Fork terrane and the 0) occurred under blueschist-faciesconditions, and the deforma- Feather River terrane overlying the Red Ant terrane contain tion in general is interpreted to have taken place in a subduction- west-vergent folds that may have formed coeval with 02 struc- zone environment. 0) structures are not observed in uni~ tures. 02 structures are not associatedwith widespread metamor- adjacent to either the Stuart Fork or Red Ant terranes. phic recrytallization. . ..
282 Hacker and Goodge
Summary lations among assemblages.Some samples,however, do contain low-variance assemblages,and these are emphasized in the fol- Early deformation, partly synchronous with high-pressure lowing discussion. metamorphism, produced mesoscopic and microscopic tight to Mineral paragenesesreflect three periods of recrystallization isoclinal folds and possible reverse faults in both terranes. in the Stuart Fork terrane (Goodge, 1989a): Ml, a prehnite- Younger decameter- to kilometer-scale open folds of regional actinolite to greenschist-facies,low-PIT metamorphism; Mz, an extent formed after high-pressuremetamorphism. Constraints on eclogite- and blueschist-faciesmetamorphism; and M3, a low- timing of theseevents, and whether the events were coeval in the PIT greenschist-to amphibolite-facies contact metamorphism. Klamath Mountains and Sierra Nevada, are discussedbelow. The Red Ant terrane also contains Ml and Mz assemblages,but it does not contain eclogitesor contact metamorphic rocks. METAMORPHISM M 1 low-pressure metamorphism The fine grain size of low-grade rocks makes analysis by conventional thin-section petrography problematic; consequently, Assemblages indicative of early low-pressure metamor- emphasis,inthis study was placed on determining coexisting met- phism are locally preserved within metavolcanic rocks in the amorphic minerals and the sequencesof metamorphic events by Stuart Fork terrane (Goodge, 1989a). The common Ml assem- back-scattered electron microscopy and mineral chemistry by blage is chlorite + actinolite + epidote + albite :I: biotite + quartz electron-probe microanalysis. All phaseanalyses were conducted (Fig. 4). Some samples contain relict igneous diopside to augite at the University of California, Los Angeles, with a four- phenocrysts (Goodge, 1990) that are partially replaced by spectrometer Cameca Camebax electron-probe microanalyzer chlorite + actinolite :I: biotite (Fig. 5). Patches of mimetic and a single set of well-characterized natural and synthetic min- actinolite and chlorite crystals are oriented randomly, indicating eral standards;this is important becauserocks from two different that Ml recrystallization was not accompanied by penetrative mountain rangesare compared. Spot selectionand textural analy- deformation. sis were performed in back-scatteredelectron mode. The electron Calcic amphibole cores in metavolcanic rocks in the Red beam diameter was 2 IJ.m, at 15 kV and 10 nA. One to five Ant terrane indicate an early phaseof metamorphic recrystalliza- counts of 20 seconds of peak intensities and 10 seconds of tion that predatesgrowth of sodic-calcic amphibole parageneses. background intensitieswere made for each of 10 elements.Detec- Although no other phases are observed in textural equilibrium tion limits at theseconditions are a function of mineral composi- with these early calcic amphibole cores, it is probable that the tion, but for this study, conservativelimits are approximately 0.02 amphiboles grew in equilibrium with phasessuch as plagioclase, wt % for SiOz, AlzO3, TiOz, CrZO3, and MnO; 0.03 wt % for chlorite, epidote, quartz, and a Ti-phase (Fig. 4). Metasedimen- FeO., MgO, and CaO; 0.04 wt % for KzO; and 0.05 wt % for tary rocks in both terranes do not contain any phasesthat are NazO. Analyses of inhomogeneousgrains were not averaged,but demonstrably products of MI. are presentedindividually. All mineral formulae were calculated following the method of Laird and Albee (198la, b), except for M2 high-pressure metamorphism pumpellyite, which was normalized according to Brown and Ghent (1983). Amphibole names follow the convention of Rock Minerals formed during high-pressuremetamorphism (Fig. and Leake (1984). "FeO." refers to total iron oxide, FeO + 4) grew over relict igneous or Ml minerals, and constitute the FeZO3. characteristic assemblagesof the Stuart Fork and Red Ant ter- Equilibrium mineral assemblagescan be difficult to identify ranes. The Stuart Fork terrane contains widespread crossite- in low-grade rocks, and a brief discussion of techniques used is epidote assemblagesin the central part of the terrane. Glauco- warranted. Widespread metastable igneous and sedimentary phane-lawsonite zone rocks occur locally in the Salmon River phasesindicate that complete textural and chemical equilibrium area, but they predominate in the northern part of the terrane. was not often attained, except in tuffaceousunits where recrystal- Eclogites crop out in the northern part of the Stuart Fork terrane, lization was pervasive. A combination of techniques was used to overprinting glaucophane-lawsonite assemblages;they are be- identify equilibrium mineral assemblages,including (I) recogniz- lieved to be in situ relics from high-temperature subduction-zone ing phaseswhose mutual boundaries do not show signs of reac- metamorphism (Borns, 1984). The Red Ant terrane contains prev- tion, (2) recognizing a group of phasesreplacing a single relict alent pumpellyite-actinolite paragenesesthat overprint locally phase, (3) identifying incompatible phases,(4) identifying viola- developed crossite-epidoteassemblages, and are herein described tions of the phase rule, and (5) observing systematicpartitioning as Mz (Fig. 4). Eclogites do not occur in the Sierra Nevada. of elementsamong phases.It was not assumedthat coexistenceof Diagnostic blueschist-faciesassemblages such as phengite + minerals A + B in one sample and B + C in another sample chlorite + quartz + sodic amphibole + lawsonite occur locally implied that A + B + C were stable together. Many samples within metasedimentaryrocks in both terranes (Fig. 6), but most contain high-variance assemblagesthat are not useful for con- commonly the nondiagnostic assemblagephengite + chlorite + straining conditions of metamorphism or identifying reaction re- quartz + albite :I: stilpnomelane :I: microcline is present. Lawson-
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Comparisonof earlyMesozoic high-pressure rocks 283
EVENT MI M2 MJ
TERRANE SFT/RAT RAT SFT SFT
ZONE pm-lici cr-ep cr-ep gl-lw
CClinopyroxene"bo acl acl + ph ,- a-amphl Ie I " I - ~,'" Na-amphlbole g -mn cr g . ch PI " l ab ab ab ab ab 011-and ~- aglocase qlz ~ Epidote - - cc ~ Lawsonite - - - ml/hl ~ Pumpellyite - - - ilm Biotile - - - sph Stilpnomelane
Phengite Muscovite Stilpnomelane - - - j:qlz ~ Chlorile ~ - - - ab ~ Lawsonite ~ ksp ~ Pum~llyite ~ - - ml/hl ~ E " Q ~ pid ole - - - n/llm t'J Q :'C Na-amphibole ~ lip 'Cj Ca-amphibole - - zr ~ Biolite Garnet Cordierite
Figure 4. Mineral assemblagesin the Red Ant and Stuart Fork terranes.
ite is rimmed by epidote in some samples. The common blueschist-facies assemblagein metavolcanic lithologies in the central Stuart Fork terrane is crossite + epidote + chlorite + phengite + magnetite :t stilpnomelane :t albite + quartz :t sphene (Fig. 7), whereasglaucophane-lawsonite rone rocks are predom- inant in the Yreka area (Hotz, 1973a, b; Borns, 1980). Blueschist- facies metavolcanic rocks in the Red Ant terrane contain epidote + sodic amphibole + hematite + chlorite + albite + quartz, and pumpellyite-actinolite facies rocks contain subassemblagesof ep- idote + pumpellyite + actinolite + hematite/magnetite :t lawson- ite + chlorite + albite + quartz.
M 3 contact metamorphism Middle to Late Jurassic plutons intrude the Stuart Fork terrane and are surrounded by 1- to 2-km-wide aureolescontain- ing muscovite + biotite . :t garnet :t cordierite. in.. semipelitic... rocks, F19ure. 5. Ba c k -scatter ed e Iectron Images. 0 f Stuart Fork metabasa1ts and homblend~ + plagIoclase :t epidote :t dlopsld~ :t biotite m showing M1 assemblages formed by replacement of primary igneous mafic rocks (Fig. 4). In many samples,euhedrallDlcas and am- clinopyroxeneand plagioclaseassemblage. Scale bar is 100Jlm. Sample phiboles, designated as M3, are randomly oriented and unde- GC84-13, containing nearly completelyreacted clinopyroxene (cpx) formed, indicating a lack of penetrative deformation during phenocrystenclosed by an assemblageof actinolite(act), chlorite (ch), recrystallization. Contact metamorphism is unknown in the Red albite.(ab), and biotite (not in vie~). Ma~rixphases (top, bottom,and left .. . .. margInsof photograph)surroundIng relict phenocryst(center of photo- Ant terrane. Clu~ps of r~dlatmg stilpnomelane and a~oh.te ~re graph)include epidote (mottled), albite, quartz, chlorite, and fibrousM2 present on M2 mmerals m the Red Ant terrane, but thelT slgnifi- crossite(cr). Crossiteappears to be growing acrossactinolite-ehlorite cance is unknown. assemblage.
, I . 284 Hacker and Goodge
MINERAL CHEMISTRY
As described above, metasedimentary and metavolcanic rocks from the Stuart Fork and Red Ant terranes contain numer- ous phases related to each metamorphic event. Of these, the compositions of amphibole and mica provide some constraintson the conditions of metamorphism, and these are discussedhere. Compositions of all phases are discussed in greater detail by Goodge (1989a) and Hacker (in review). M I assemblagesin the Stuart Fork terrane include chlorite, actinolite, epidote, albite, biotite, and quartz, whereas only am- phibole is demonstrably an M I phasein the Red Ant terrane. For this reason,only Ml amphiboles are compared, whereas M2 am- phiboles and phengitesare discussed.
Amphibole
Ml calcic amphiboles in metavolcanic rocks are uniformly actinolitic (Table I), and M2 sodic-calcic amphiboles are prima- rily winchite in both metasedimentary and metavolcanic rocks (Table 2). Sodic amphiboles span compositions from magnesio- riebeckite through glaucophane.M2 blueschist assemblagesin the Stuart Fork terrane contain only one amphibole, rather than coexisting sodic or sodic-calcic amphibole and actinolite as in some high-pressureterranes (Fig. 4). Amphibole compositions plotted in Figures 8 and 9 (after Laird and Albee, 1981b) illustrate that calcic amphiboles .in M I assemblagesare distinctly different from sodic-calcic amphiboles in M2 assemblages.M1 Ca-amphiboles are uniformly low in M4-siteNa (.;;;;0.25atoms per formula unit, pfu), and they show variable but significant A-site occupancy and tetrahedral AI; these amphiboles plot in the medium- to low-pressure facies-seriesfield of Laird and Albee (198Ib) and Hynes (1982). Ca-amphiboles in theseassemblages also show strong tschermakite substitution, and minor edenite substitution. The weakly tschermakitic composi- tions of M I actinolites indicate conditions well below those of the garnet isograd in pelitic rocks (Laird and Albee, 1981b). In contrast, sodic amphiboles from M2 assemblagescontain much Na in the M4 site (0.5 to 2.0 atoms pfu) but generally low A-site occupancy and tetrahedral AI; amphiboles from these as- semblagesare compositionally similar to those from the Sanbag- awa and Franciscan high-pressuremetamorphic belts.
Phengite White mica in M2 assemblagesis phengite with only minor Figure6. Optical photomicrographsof metasedimentaryrocks contain- paragonite (.;;;;0.02Na atoms pfu) o~ ~argarite «:0.02 Ca atoms ing M2 assemblages.Field of view in all is approximately1 mm. A, Fold pfu) components (Table 3). Phengtte ID metasedimentary rocks in amphibole-micaquartzite from Stuart Fork terranecontaining inter- containing chlorite :J:stilpnomelane :J:lawsonite contains high Si grown crossite(cr) and phengite,in additionto quartz(qtz); kink bands (3.4 to 3.8 atoms pfu in the Stuart Fork terrane and 3.3 to 3.6 pfu of amphibolei~ fold hingesi?dicate that the.deformation .was syn-.t° in the Red Ant terrane). Phengite in metavolcanic rocks also postmetamorphic.B, Lawsomtetablets (lw) IDtergrownWIth phengtte ... . (ph) in phyllitic quartzitefrom Stuart Fork terrane;note boudinageof contains h~ghSI (3.6 to 3.9 pfu ID the Stuart Fork ten:aneand .3.3 lawsoniteand growth of quartzbetween segments. C, Contortedfolia of to 3.6 pfu ID the Red Ant terrane), and octahedral cation substltu- intergrownphengite and chloritein Red Ani terranephyllitic quartzite. tion for Al (0.8 to 1.0 atoms in the Stuart Fork terrane and 0.2 to 0.6 in the Red Ant terrane). . -.
Comparisonof earlyMesozoic high-pressure rocks 285
CONDmONS AND TIMING OF METAMORPffiSM
M 1 low-pressureevent
Stuart Fork M 1 assemblagescontain actinolite, epidote, and albite, but lack zeolites,pumpellyite, hornblende, or intermediate- composition plagioclase.Temperatures inferred from phaseequi- libria are 300° to 450°C, and pressures are unconstrained (Goodge, 1989a); we term M1 "low-pressure" metamorphism principally to distinguish it from the blueschist-faciesmetamor- phism. M 1 amphiboles in both terranes (Fig. 8) are similar to medium- to low-pressure facies-series rocks metamorphosed below amphibolite grade (Laird and Albee, 1981a; Hynes, 1982). Basedon the geobarometerof Brown (1977b), lessthan 0.20 Na in the amphibole M4 site indicates a pressureof approximately 200 MPa; becauseof metamorphic overprinting, we are unable to evaluate whether these amphiboles coexisted with an iron oxide, hence these may be minimum pressures.These conditions are compatible with the lowest pressure region of the greenschist facies,and overlap in part with the prehnite-actinolite facies(Liou and others, 1985). Thus, Ml in both terranes is interpreted as a low-PIT greenschist-faciesevent. Ml recrystallization may have been attended by moderate LIL-element enrichment in the Stuart Fork terrane suggestiveof weak hydrothermal alteration or meta- somatism (Goodge, 1990), which likely occurred near an oceanic spreading ridge or transform fault.
M 2 high-pressure event
Phase equilibria relevant to the high-pressure metamor- phism are shown in Figure 10. Metamorphic pressures and temperatures can be estimated from coexisting minerals, as fol- lows. The conditions of formation for the lawsonite + sodic am- phibole rocks are limited by five of the experimentally determined equilibria shown in Figure 10 (reactions 1,3,5,7, 10). The absenceof jadeite and the presenceof lawsonite and glaucophane indicate pressuresless than 800 to 1,000 MPa, and more than 400 MPa (Newton and Smith, 1967; Liou, 1971; Maresch, 1977), and temperatures less than 350° to 400°C (Heinrich and Althaus, 1980). Brown (1977b) and Brown and Ghent(1983) have estimated the P-T stabilityof lawsonite+ Sodic amphibole + epidote assemblages. as approximately 600 to 100~m 1,000MPa and 200° to 300°C,usmg natural assemblages whose c' - pressuresand temperatures were inferred from experimentally *ftc.,'BJ_=_I_- determined reactions and independent thermobarometerssuch as ~igure7. Photo~icrographsof characteristiccrossite-epid~te meta~asa.I- sphalerite composition. tiCassemblage~ m the St~rt Fork and RedAnt terra?es.Flel.d of view m ...... (A) and (B) IS approximately I mm; scale bar m (C) IS 100 p.m. The condl~lo~s of formation f?r the epidote + S.odIC am~~I- A, Optical photomicrograph of foliated Stuart Fork crossite-epidote bole rocks are limited by two experImentally determmed equillb- blueschistcontaining fibrous crossite(cr), epidote(ep), phengite(ph), ria shown in Figure 10 (reactions 1, 5). The absenceof jadeite and quartz (qtz). Kinks in crossitein lower left and lower right cor- and the presenceof glaucophane indicate pressuresless than 800 ne~s.B, Optical. photomi~r.ographo~ foliated ~tuart Fork crossite- to 1000 MPa, and greater than 400 MPa (Newton and Smith, epidote blueschist.containing boudmaged epidote (e~) segments . ° separatedby phenglte(ph). C, Back-scatteredelectron micrograph of 1967, Maresch, 1977), and temperatures less than 450 C (Ma- vein in Red Ant volcaniclasticrock, showingelongate sphene (s), por- resch, 1977). Brown (1977b), Brown and Ghent (1983), Brown phyroblasticepidote(e), phengite(ph),chlorite (c), sodicamphibole(g), and Blake (1987) have estimated the P-T stability of epidote + and albite (a).
! "" - "
286 Hacker and Goodge
sodic amphibole + hematite assemblagesas approximately 600 to The pumpellyite-actinolite facies metamorphism did not 900 MPa and 250° to 400°C. Maruyama and others (1986) occur at very low pressures(less than -200 MPa), becausethe bracketed the equilibrium epidote + magnesio-riebeckite= tremo- rocks lack characteristic low-pressure assemblagessuch as preh- lite + hematite at 300°C between 310 and 470 MPa. This bracket nite + epidote + actinolite or oligoclase + actinolite (e.g., Liou and was derived from unreversed experiments, and may be in error others, 1987). Assemblages containing lawsonite and lacking because the run products were not fully characterized. If this sodic amphibole are limited to temperatures greater than 150°C determination is correct, the stability field of epidote + sodic and less than 350°C (Nitsch, 1968; Heinrich and Althaus, 1980; amphibole extends downward to at least 470 MPa, and this Schiffman and Liou, 1980) and to pressuresgreater than 300 provides a minimum crystallization pressure of the Red Ant MPa (Liou, 1971), by reactions 3, 6, 7, and 10, and bracket 4 in terrane. Figure 10. Maximum pressuresare less than -600 to 900 MPa
TABLE 1. REPRESENTATIVE CALCIC AMPHIBOLE ANALYSES FROM
STUART FORK AND RED ANT TERRANE M1- METABASALTIC ROCKS
Stuart Fork terrane Red Ant terrane Sample 84-14 8 118 13 214 22 23 28 20 15 19
SiO2 54.34 53.15 54.62 53.55 53.64 42.98 45.08 49.23 43.16 40.58 47.87 TiO2 0.60 0.09 0.04 0.13 0.04 2.86 2.18 1.03 2.98 0.03 0.06 AI203 1.46 2.28 1.70 2.12 2.15 9.55 8.46 4.38 9.20 12.73 6.46 Cr203 0.05 0.08 0.35 b.d. b.d. b.d 0.05 0.06 0.05 b.d. 0.06 Fea" 10.87 13.22 9.37 13.65 13.54 19.20 18.43 17.83 20.77 21.09 22.55 MnO 0.24 0.26 0.25 0.27 0.28 0.35 0.29 0.36 0.30 0.50 0.35 MgO 16.54 14.29 17.73 14.99 14.71 8.50 9.76 12.34 8.50 6.80 7.94 CaD 12.35 11.93 12.83 12.50 11.66 11.15 11.02 10.29 10.81 11.62 11.31 Na20 0.41 0.37 0.25 0.33 0.73 2.11 1.91 0.94 2.50 1.70 1.26 K20 0.05 0.06 0.06 0.09 0.13 0.67 0.66 0.28 0.66 1.20 0.38
Total 96.91 95.73 97.20 97.63 96.88 97.37 97.84 96.74 98.93 96.25 98.24
Cations based on 23 oxygen atoms Si 7.81 7.81 7.78 7.72 7.78 6.54 6.74 7.21 6.48 6.30 7.19 AIIV 0.19 0.19 0.22 0.28 0.23 1.46 1.26 0.76 1.52 1.71 0.81 L 8.00 8.00 8.00 8.00 8.00 8.00 8.00 7.96 8.00 8.00 8.00
AIVI 0.06 0.20 0.07 0.08 0.14 0.26 0.23 0.00 0.11 0.62 0.34 Ti 0.07 0.01 0.00 0.01 0.00 0.33 0.25 0.11 0.34 0.00 0.01 Fe3+ 0.08 0.10 0.13 0.19 0.22 0.15 0.32 1.05 0.40 0.46 0.38 Cr 0.00 0.00 0.01 b.d. b.d. b.d. 0.01 0.01 0.00 b.d. 0.00 Fe2+ 1.23 1.53 0.99 1.46 1.42 2.29 1.98 1.13 2.21 2.27 2.46 Mn 0.03 0.03 0.03 0.03 0.03 0.05 0.04 0.05 0.04 0.07 0.05 Mg 3.54 3.13 3.77 3.22 3.18 1.93 2.18 2.69 1.90 1.57 1.78 E 5.01 5.00 5.00 4.99 4.99 5.01 5.01 5.04 5.00 4.99 5.02
Ca 1.90 1.89 1.96 1.93 1.81 1.82 1.77 1.61 1.74 1.93 1.82 NaM4 0.10 0.11 0.04 0.07 0.19 0.18 0.23 0.27 0.26 0.07 0.18 L 2.00 2.00 2.00 2.00 2.00 2.00 2.00 1.88 2.00 2.00 2.00
NaA 0.02 0.00 0.03 0.02 0.02 0.44 0.32 0.00 0.47 0.44 0.19 K 0.01 0.01 0.01 0.02 0.02 0.13 0.13 0.05 0.13 0.24 0.07 L 0.03 0.01 0.04 0.04 0.04 0.57 0.45 0.05 0.60 0.68 0.26
Total 15.04 15.01 15.04 15.03 15.03 15.58 15.46 14.93 15.60 15.67 15.28
"All Fe as FeO. b.d. = below detection.
I , '- r
Comparisonof earlyMesozoic high-pressure rocks 287
O~I/) .I'-NI/) 8 ~o~ 8 ~~ N v 01 ~f6~~~~:8g~~ ~ 888 ~o~8~8~~ RI~~ 8~~ ~ t::<0 vNOOvO W NOOI .0000101. 01 ~O~ ~~ ~-I- 0 ~ ~Nv .~vN~ ~~OI .. ~ 0~ <-~z ~ NNI'-C')v~~I'-C')~ ~< E < N Ov 0 .~ (/)< as w~ OIC')I/)C')~OIvC')OI. C') °1'-C') 8 0I~C') 8 ~N~~ ~ I-() .- (/)- g OII/)I'- ~I/)I/) .vv c 0 ~ C') C') . v 0 I'- I/) C') <0 01 <0 v 0 <0 a 01 . ~ C') ~ 0 01 N ~ a ~ ~ N .." < I , , ~ " -" 288 Hacker and Goodge TABLE 3. REPRESENTATIVE PHENGITE ANALYSES FROM STUART FORK AND RED ANT TERRANE M2 ASSEMBLAGES STUART FORK TERRANE Metabasalts Metasediments Sample 178 18 35C V60 11A 22 33A 350 219C 268A V6A V9 SiO2 55.47 56.34 55.30 57.42 52.54 53.92 51.43 53.50 50.70 54.62 57.00 53.02 TiO2 0.04 0.04 0.05 0.06 0.06 0.10 0.11 0.05 0.12 0.03 0.03 0.06 AI203 16.58 18.21 19.71 18.54 24.74 25.87 26.67 21.65 24.51 22.59 18.24 25.01 Cr203 0.06 0.16 b.d. b.d. b.d. 0.04 0.02 b.d. b.d. 0.06 b.d. b.d. FeO' 5.14 4.30 4.45 4.28 2.49 2.78 2.84 3.08 4.44 3.01 3.97 3.23 MnO 0.09 0.07 0.03 0.05 b.d. 0.12 0.05 0.12 0.18 b.d. 0.09 0.03 MgO 5.72 6.55 5.81 6.93 3.68 3.84 3.68 4.42 3.39 5.23 6.29 3.84 CaO b.d. 0.03 0.06 0.29 b.d. b.d. 0.04 b.d. 0.05 0.03 0.11 0.04 Na20 b.d. b.d. b.d. 0.09 b.d. b.d. b.d. b.d. b.d. b.d. 0.30 b.d. K20 11.18 9.82 9.26 7.77 9.86 7.84 8.47 10.80 9.31 9.78 8.03 8.30 Total 94.28 95.52 94.67 95.43 93.37 94.51 93.31 93.62 92.70 95.35 94.06 93.53 Cations based on 11 oxygen atoms Si 3.80 3.76 3.71 3.78 3.53 3.49 3.40 3.63 3.43 3.58 3.75 3.50 AIIV 0.20 0.24 0.29 0.22 0.47 0.51 0.60 0.37 0.57 0.42 0.25 0.51 L 4.00 4.00 4.00 4.00 4.00 4.00 4.00 4.00 4.00 4.00 4.00 4.00 AIVI 1.14 1.19 1.27 1.22 1.49 1.47 1.47 1.37 1.39 1.32 1.16 1.44 Ti 0.00 0.00 0.00 0.00 0.00 0.01 0.01 0.00 0.01 0.00 0.00 0.00 Fe3+ 0.00 0.00 0.00 0.00 0.00 0.03 0.12 0.00 0.17 0.10 0.09 0.06 Cr 0.00 0.01 b.d. b.d. b.d. 0.00 0.00 b.d. b.d. 0.00 b.d. b.d. Fe2+ 0.30 0.24 0.25 0.24 0.16 0.12 0.04 0.18 0.09 0.07 0.13 0.12 Mn 0.01 0.00 0.00 0.00 b.d. 0.01 0.00 0.01 0.01 b.d. 0.01 0.00 Mg 0.59 0.65 0.58 0.68 0.37 0.37 0.36 0.45 0.34 0.51 0.62 0.38 L 2.03 2.10 2.10 2.14 2.02 2.00 2.00 2.01 2.01 2.00 2.01 2.00 Ca b.d. 0.00 0.01 0.02 b.d. b.d. 0.00 b.d. 0.00 0.00 0.01 0.00 Na b.d. .b.d b.d. 0.01 b.d. b.d. b.d. b.d. b.d. b.d. 0.04 b.d. K 0.98 0.84 0.79 0.65 0.85 0.65 0.71 0.94 0.80 0.82 0.67 0.70 L 0.98 0.84 0.80 0.68 0.85 0.65 0.71 0.94 0.80 0.82 0.72 0.70 Total 7.01 6.94 6.90 6.82 6.87 6.65 6.71 6.95 6.81 6.82 6.73 6.70 'All Fe as FeO. b.d. = below detection. (Brown, 1977b). If the experimentsof Maruyama and others lawsonite-outcurve of Liou (1971) is displacedan unknown (1986) representthe minimum pressurestability of epidote + amountto higherpressures in the presenceof other phases,and sodic amphibole, then assemblagescontaining lawsonite and doesnot effectivelylimit the maximumpressure of theseassem- lackingsodic amphibole may be stableto pressuresas high as470 blages.Schiffman and Liou (1983) have suggestedthat Fe- MPa.Maresch's (1977) experimentsdelineate a maximumpossi- bearingpumpellyite breaks down at temperaturesperhaps 100° ble stability field for glaucophane,and do not providean upper to 150°Clower thanthe thermalstability limit of Mg-pumpellyite pressurelimit to assemblageslacking sodic amphibole.Note, such that - 250°C may be a more realistic upper temperature however,that Maresch'slower pressurelimit is consonantwith limit. Liou and others(1985) reportedpreliminary results for the the bracketof Maruyamaand others(1986). reactionpumpellyite + chlorite= clinozoisite + tremolite, indicat- Pumpellyite-bearingassemblages lacking lawsonite and ing that the upper thermal stability of pumpellyite+ chlorite is sodic amphibole are stable up to maximum temperaturesof about 325° to 350°C, but detailsof their experimentshave not 350°C (reaction6; Schiffmanand Liou, 1980),and to pressures beenpresented. at leastas high as 400 MPa (reaction5; Maresch,1977). The Red Ant terranesamples contain the four-phasereaction ! i ~~ .. Compo",". of earlyMesozoic high-p'es3"," rock,. 289 TABLE 3. REPRESENTATIVE PHENGITE ANALYSES FROM STUART FORK AND RED ANT TERRANE M2 ASSEMBLAGES (continued) RED ANT TERRANE Metabasalts Metasediments Sample 26 25 76 3 5 9 37 55/56 92 63 61/62 57/58 SiO2 52.24 51.30 52.91 54.03 53.94 53.70 54.63 51.87 49.31 50.35 50.97 51.27 TiO2 0.07 0.05 0.08 0.14 0.07 b.d. 0.04 0.09 b.d. 0.21 0.35 0.08 AI203 25.49 27.90 21.63 22.59 19.50 22.20 21.38 22.88 30.24 26.61 25.04 25.38 Cr203 0.28 0.03 0.05 0.04 0.03 b.d. b.d. 0.05 0.03 0.05 0.07 0.03 FeO' 3.21 2.70 6.06 5.39 7.63 5.60 3.14 4.59 3.19 2.53 3.11 2.85 MnO b.d. 0.03 0.12 0.03 0.08 0.10 0.27 0.07 0.14 0.03 0.09 0.04 MgO 4.01 3.45 5.33 4.51 5.00 4.10 5.69 4.08 1.98 3.85 3.05 3.18 CaD 0.06 0.08 0.05 0.07 0.08 b.d. b.d. 0.05 0.04 0.04 0.05 b.d. Na20 0.09 b.d. b.d. b.d. b.d. 0.48 b.d. b.d. 0.11 0.11 b.d. b.d. K20 10.40 10.57 10.06 10.50 10.16 9.88 10.82 10.33 10.29 9.53 10.11 10.26 Total 95.85 96.11 96.29 97.30 96.49 96.06 95.97 94.01 95.34 93.31 92.84 93.09 Cations based on 11 oxygen atoms Si 3.44 3.36 3.47 3.52 3.56 3.55 3.59 3.50 3.26 3.36 3.47 3.48 AIIV 0.56 0.64 0.53 0.48 0.44 0.45 0.41 0.50 0.74 0.64 0.53 0.52 L 4.00 4.00 4.00 4.00 4.00 4.00 4.00 4.00 4.00 4.00 4.00 4.00 AIVI 1.41 1.51 1.13 1.26 1.08 1.28 1.25 1.32 1.62 1.46 1.49 1.51 Ti 0.00 0.00 0.00 0.01 0.00 b.d. 0.00 0.00 b.d. 0.01 0.02 0.00 Fe3+ 0.14 0.13 0.33 0.20 0.35 0.17 0.15 0.18 0.12 0.14 0.04 0.01 Cr 0.01 0.00 0.00 0.00 0.00 b.d. b.d. 0.00 0.00 0.00 0.00 0.00 Fe2+ 0.04 0.02 0.00 0.09 0.07 0.14 0.02 008 0.06 0.00 0.14 0.15 Mn b.d. 0.00 0.01 0.00 0.00 0.01 0.01 0.00 0.01 0.00 0.00 0.00 Mg 0.39 0.34 0.52 0.44 0.49 0.40 0.56 0.41 0.20 0.38 0.31 0.32 L 1.99 2.00 2.00 2.00 2.00 2.00 1.99 1.99 2.01 2.00 2.00 1.99 Ca 0.00 0.01 0.00 0.00 0.01 b.d. b.d. 0.00 0.00 0.00 0.00 b.d. Na 0.01 b.d. b.d. b.d. b.d. 0.06 b.d. b.d. 0.01 0.01 b.d. b.d. K 0.87 0.88 0.84 0.87 0.86 0.83 0.91 0.89 0.87 0.81 0.88 0.89 L 0.88 0.88 0.84 0.87 0.87 0.89 0.91 0.89 0.88 0.82 0.88 0.89 Total 6.87 6.88 6.84 6.87 6.87 6.89 6.90 6.88 6.89 6.82 6.89 6.88 'All Fe as FeO. b.d. = below detection. assemblagepumpellyite + hematite + epidote + actinolite. Nitsch been determined (Schiffman and Liou, 1980; Liou and others, (1971) bracketed the equilibrium pumpellyite = epidote + actino- 1985). lite at 700 MPa between 3450 and 435°C (bracket 4 in Fig. The inferred conditions of recrystallization based solely on 10)-and not at any other pressuresor temperaturesas has been experimental determinations of reactions shown in Figure 10 are implied by numerous authors; the bracket was derived from un- 1500 to 350°C and 200 to 400(?) MPa for pumpellyite-actinolite reversed experiments on unbuffered natural materials, and may facies rocks lacking lawsonite, 1500 to 350°C and 300 to 400(?) be in error because the experiments are truly only studies of MPa for pumpellyite-actinolite facies rocks containing lawsonite, reaction kinetics and not equilibrium. Unfortunately, this bracket and 1500 to 400°C and 400(?) to 1,000 MPa for the blueschist- does not provide information on the high temperature limit of facies metamorphism. The pressuresseparating the blueschistand pumpellyite beyond that provided by Schiffman and Liou (1980). pumpellyite-actinolite facies are uncertain becausethere is only Pumpellyite and actinolite do not coexist in low-pressure rocks, one experimental bracket on the stability of epidote and sodic but an experimental low-pressure limit for their stability has not amphibole. Empirical calibrations of Brown (1977a), Brown and - - '" I 290 Hacker and Goodge (A) (C) 2 1 "2 . Stuart Fork ~ ~ [J Red Ant ~~ ~ Z Z "-' ~z [J !:b [J . [J [J t:r~ . [J . [J 0 [J 0 .00 N~+K .75 0 AI/(AI+Si) .4 (B) (D) &.. 2 2 U + E= ~ ++ [J [J ~~ f")~ . [J [J Z ~ [J [J ~+ -..s. ~~ .. . []Gj [J """" 0 0 [J 0 Al IV 2 0 AfI +Ft?++ Ti+Cr 2 Figure 8. Compositionsof M( calcic amphiboles.Common assemblage consists of, or is assumedto consist of, amphibole + epidote + chlorine + plagioclase+ quartz. In each diagram, M I calcic amphiboles plot in fields defined by Laird and Albee (198Ib) for the Abukuma low-pressure terrane of Japan. MI calcic amphiboles are low in Na M4 and show variable contents of A-site cations and tetrahedral AI. Ca amphiboles of M ( assemblagesshow tschermak substitution (b), a coupled substitution representedby (AlVI+Fe3++Ti+Cr), Allv - (Fe2++Mg+Mn), Si. Ghent (1983), and Brown and Blake (1987) suggestconditions of In summary, estimated metamorphic temperaturesand pres- approximately 275° to 375°C and 200 to 600 MPa for sures are roughly 250° to 400°C and 500 to 700 MPa for pumpellyite-actinolite facies rocks, 250° to 400°C and 600 to crossite-epidoteassemblages, 200° to 300°C and 500 to 700 MPa 900 MPa for epidote + sodic amphibole, and 200° to 300°C and for glaucophane-lawsonite assemblages,and 150° to 350°C and 600 to 900 MPa for lawsonite + sodic amphibole. 200 to 400 MPa for pumpellyite-actinolite assemblages(Fig. 11). Na occupancy of the M4 site in amphibole coexisting with Note that although the pumpellyite-actinolite facies assemblages iron oxide, albite, chlorite, epidote, and H2O is a function of have been described with the blueschists, metamorphic condi- pressure (Brown, 1977b). Brown's empirical calibration yields tions of the pumpellyite-actinolite facies are poorly constrained pressuresof 500 to 700 MPa for the sodic and sodic-calcic am- and include low PIT conditions. phiboles in the Red Ant and Stuart Fork terranes. Phengite compositions are also compatible with high- Timing pressure crystallization. Phengite white mica can indicate high pressure(Ernst, 1963; Guidotti and Sassi, 1976), and the Si con- Known agesof rock-forming, deformational, and metamor- tent of phengite coexisting with K-feldspar, quartz, and biotite is phic events place few constraints on the petrotectonic evolution apparently a function of pressurein unreversedexperiments in the of the Stuart Fork or Red Ant terranes.The agesof sedimentary K20-MgO-AI203-SiO2-H20 (KMASH) system conducted in and volcanic protoliths in both terranes are unknown, as is the the temperature range 350° to 600°C (Massone and Schreyer, age of Ml metamorphism. 1987). Two brackets for this same reaction were previously ob- Phengite from Stuart Fork lawsonite-glaucophaneblueschist tained on natural materials by Velde (1965). Assemblagesfrom in the Yreka area yields K-Ar and 4OAr/39Ar ages of approxi- the Stuart Fork and Red Ant terranes do not contain biotite, and mately 220 Ma (i.e., Late Triassic) (Hotz and others, 1977). K-Ar the biotite-absent reaction relating chlorite + microcline = celado- geochronology was performed on the Red Ant terrane by nite + muscovite has a steep dP/dT slope (Bucher-Nurminen, Schweickert and others (1980); two whole-rock and three 1987), rendering it a poor choice for barometry at low muscovite-separatedates range from 157 :t 5 to 190 :t 8 Ma. temperatures. Because the Red Ant blueschists were overprinted by pump- I i I , I .,. .. Comparison of early Mesozoic high-pressure rocks 291 (A) (C) 2 [J 1 . .- ~ [J U ..,. + ~~ . ~ Z [J Z . :::::::. StuartFork Z~ [JRed Ant 0 0 .00 NaA+K .7S 0 AI/(Al+Si) .4 (B) (D) '- 2 2 U [J .-+ ~ ..,. ++ ~~ [J ~~ Z ~ . + . . .. >- -< 0 0 0 AllY 2 0 AfI +FIl++ Ti+Cr 2 Figure 9. Compositions of amphiboles in M2 assemblages(after Laird and Albee, 1981a, b). Common assemblageconsists of amphibole + epidote + chlorite + plagioclase + quartz. M2 sodic amphiboles are similar in composition to those from the Sanbagawa and Franciscan high-pressuremetam°[J'hic belts. M22.0 sodicpfu. Sodicamphiboles amphiboles are lowin in M2A-site assemblages cations andshow tetrahedral glaucophane AI; theysubstitution show a range(d), ofNaM represented to nearlyby - NaM4,(Alvl+Fe3++Ti+Cr) - Ca, (Fe2++Mg+Mn). ellyite-actinolite recrystallization, which may have influenced the 50°C/km). Although no agesare available that directly date M}, K-Ar ages,the meaning of these dates is unclear. We tentatively ocean crust formation and M} metamorphism must have oc- infer that M2 in both units is of a roughly comparable Late curred prior to M2, providing a minimum age of Late Triassic. Triassic(?) age. Consequently, O} may have been concurrent in Randomly oriented M} minerals indicate that M} was not ac- the Red Ant and Stuart Fork terranes, but 02 need not have companied by penetrative deformation. We suggest that M} been. metamorphism occurred during hydrothermal alteration near an ocean ridge, intra-oceanic arc, or other shallow oceanic heat P-T EVOLUTION source. The cooling oceanic crust then passed after an unknown Rocks of the Stuart Fork and Red Ant terranes were meta- period into a Late Triassic or older subduction zone at the west- morphosed during two or three distinct episodes,each of which ern edge of North America. Rocks in the subducting slab were was characterizedby different P-T conditions (Fig. 11). The rela- recrystallized (MV during deformation (OJ) at high pressuresand tive timing of these events was establishedby textural relations, moderate to low temperatures before or during Late Triassic and the P-T conditions of metamorphism were inferred from time. The subduction zone probably dipped eastward, as indi- phaserelations and mineral chemistry. cated by west-vergent O} structures and the position of the sub- Protoliths consisted chiefly of pelagic radiolarian-bearing duction complex with respectto contemporaneousEarly Triassic muds and oozes,and mafic volcanic rocks. We consider it likely to Middle Jurassic arc volcanism and plutonism in the eastern that the Stuart Fork and the Red Ant protoliths represent crust Klamath Mountains and Sierra Nevada (e.g., Miller, 1989; Har- from the same ocean basin, although they may be somewhat wood, 1988). M2 blueschist assemblagesdeveloped at relatively diachronous. low temperaturesand high pressures(500 to 1,100 MPa), reflect- The first episodeof metamorphism, M}, formed assemblages ing deep crustal levels (on the order of 17 to 38 km). at relatively high temperaturesand low pressures,reflecting shal- Blueschist-faciesrocks of the Sierra Nevada are overprinted low crustal levels and high geothermal gradients (more than by pumpellyite-actinolite faciesminerals, whereasthe Stuart Fork I I I '" 292 Hacker and Goodge 1000 1. The Stuart Fork and Red Ant terranes formed in pre- 900 Late Triassic(?) time in a back-arc or larger ocean basin near a detrital volcanic source. BOO 2. Prior to Late Triassic time, the ocean crust of these ter- ~ ranes were metamorphosed under static conditions at low pres- ~ 7 sures in an ocean-ridge, intraoceanic arc, or other oceanic '-;;' hydrothermal environment. .. ~ 5 3. Theoceanic lithosphere, composed in partby theStuart ~ Fork and Red Ant terranes,was subducted eastward beneath an 400 upper plate containing the Eastern Klamath-Northern Sierra magmatic arcs, and metamorphosed at depths of 17 to 38 km. By 3 Late Triassic time, the rocks had ascendedand cooled. This high- PIT metamorphism represents the characteristic metamorphic episode of both terranes in their evolution as subduction com- 200 250 300 350 400 plexes. Strong folding and thrust imbrication occurred after and Temperature(Oc) during the high-pressuremetamorphism. Figure 10. Phaseequilibria relationsof M2 assemblagesin metabasalt. 4. Regional shortening formed possible southwest-directed Two bracketedreactions, 4 and 9, are shown by two-headedarrows. thrust faults andlor west-vergent, kilometer-scale folds by mid- Reactions shown are (1) jadeite + quartz = albite (Newton and Smith, Jurassic time. In the Klamath Mountains this tectonic event is 1967~;(2) a~a~onite= calci~e(Joh.an~es and Puhan, 1971); (3) lawsonite associatedwith contraction acrossthe west~rn Hayfork magmatic + albite= ZOlslte P paragonIte(Hemnch and Althaus,1980); (4) pumpel- .. . lyite = epidote + actinolite (Nitsch, 1971); (5) maximum glaucophane arc. of the Middle Jurassic convergent marg~n (Harper and stabilityfield (Maresch,1977); (6) Mg-pumpellyite= clinowisite+ gros- Wnght, 1984; Coleman and others, 1988; Wnght and Fahan, sulaire(Schiffman and Liou, 1980); (7) lawsonite= wairakite (Liou, 1988). 1971);(8) Mg-pumpelly~te+ chlorite =.cli~ozois~te+ grossul.arite (Liou 5. The Stuart Fork terrane is bounded on the east by the a.ndothers, 1985); (9) epidote+ magneslonebeckl~e= tremoht~ + he?Ia- central metamorphic belt which contains mafic igneous rocks tlte (Maruyamaand others,1986); (10) laumontite= lawsonIte(LIOU, . ' . . 1971);(11) heulandite= lawsonite(Nitsch, 1968).The positionof the metamorphos~ dunng De.vom~n time (Cashman, 1980;. Pe.a- reactionas determinedby Heinrich and Althaus (1980) is essentially cock and Noms, 1980). LIkewISe, the Red Ant terrane lies 10 coincidentwith a positioncalculated by Holland (1979) from thermo- thrust contact with the Feather River terrane to the east which is dynamicdata. In general,the P-T limits of blueschistand pumpellyite- actinolitefacies are poorly known and the shadedregions in this figure shouldbe regardedwith caution. gl-lw 1000 :t.. lIe terrane lacks any pervasive overprint. Either the Stuart Fork terrane returned toward the surface along a path with a high-PIT 800 Zone slope, or the kinetics of recrystallization were sufficiently slow to preservethe higher pressureparageneses. By Middle Jurassic time, Stuart Fork rocks reached signifi- '2 600 cantly lower pressure and higher temperature conditions when ~ they were intruded by granitoid plutons of the widespread west- 6 em Hayfork magmatic arc (Harper and Wright, 1984; Wright ~ 400 and Fahan, 1988). CONCLUSIONS 200 The Stuart Fork terrane of the central Klamath Mountains and the Red Ant terrane of the Sierra Nevada have important similarities and differences. They represent either (1) unrelated 0 rocks that formed independently under closely similar deposi- 200 400 600 tional and metamorphic conditions, or (2) separatefragments of T (OC) the same. crustal element that .were physically contiguous. during FIgure. 11. Schematlc. P-T -time. diagramshoWIng . sequence 0 f M I-M3 f~rmatlon and at least two e?lSodesof metamorph.IS~.~.e con- metamorphicevents. Constraints on metamorphicconditions provided sider the latter to be most likely because of the slmllantles be- by data presentedin this study,and from Borns(1980). Arrows show tween the Stuart Fork and Red Ant terranes,including: possibleP- T pathsfor metamorphicevents. I I ""' , Comparison of early Mesozoic high-pressurerocks 293 also composedof mafic metaigneousrocks metamorphosedin the 2. High-pressure paragenesesof the Stuart Fork terrane Devonian (Edelman and others, 1989). The central metamorphic consist of eclogite- and blueschist-faciesminerals, whereas the belt and the Feather River terrane share a number of petrologic Red Ant terrane contains only blueschists.Pumpellyite-actinolite characteristics,and they have been consideredrelicts of the same facies paragenesesin the Red Ant terrane may belong to this metamorphic belt (Hacker and Peacock, this volume). high-pressureevent or to a later unrelated metamorphism. 6. The Stuart Fork terrane is bounded immediately to the In this chapter we have attemptedto show the lithologic, west by the Late Paleozoic to Early Jurassic North Fork-Salmon structural, and petrologic equivalenceof the Stuart Fork and Red River terrane, a relatively intact ophiolitic sequence(Ando, 1979; Ant terranes.Our new data are a confirmation of the correlation Ando and others, 1983). Farther west and tectonically beneath between these metamorphic belts proposed earlier by Davis the North Fork-Salmon River terranelies the Late Triassicto (1969) and expandedupon by others. Evidencepresented herein Early Jurassic siliceous melange of the eastern Hayfork terrane suggeststhat these blueschist belts are relict pieces of a more (Wright, 1982).The Red Ant terraneis boundedon the westby broadlycontinuous convergent-margin trench complex. As such, the Calaveras terrane, a package of disrupted metasedimentary they provide a tectonic and paleogeographiclink betweenthe and minor metavolcanicrocks that may be tectonicallyequiva- early Mesozoichistory of the Klamath Mountains and Sierra lent to the North Fork-Salmon River and eastern Hayfork ter- Nevada. Although we are certain that these rocks signify the ranesof the KlamathMountains. These assemblages probably are existenceof a convergentplate boundary,future studiesmust oceanic materials accreted outboard of the Stuart Fork-Red Ant addressthe critical problem of where thesebelts fit in early Meso- subduction complexes. zoic paleogeographicreconstructions. Important differences between the early Mesozoic high- pressurerocks in the Klamath Mountains and Sierra Nevada are ACKNOWLEDGMENTS probably representativeof the spatial heterogeneityinherent in an arc-trench system that may have stretched for hundreds of Support for this research was provided by grants from the kilometers: National Science Foundation (EAR83-12702, to W. G. Ernst), 1. The Middle to Late Jurassic M3 contactmetamorphism the GeologicalSociety of America,Sigma Xi, and UCLA Gradu- recognized in the Stuart Fork terrane is absent from the Red Ant ate Fellowships. This work was stimulated by the 1988 Penrose terrane.This differencemay simply be a resultof the small out- Conferenceon Paleozoic-EarlyMesozoic Paleogeographic Rela- crop of the Red Ant terrane; Cretaceous and Jurassic plutons tions between the Klamath Mountains, Northern Sierra Nevada, were emplacednorth, east,west, and southof the Red Ant ter- and North America, sponsoredby the GeologicalSociety of rane, but apparently none intruded close enough to cause America. Helpful reviews were provided by E. H. Brown, D. S. recrystallization. Harwood, and R. Haugerud. REFERENCES CITED Ando, C. J., 1979, Structural and petrologic analysis of the North Fork terrane, -, 1977h, The cr~ite content of Ca-amphibole as a guide to pressure of central Klamath Mountains, California [Ph.D. thesis]: Los Angeles, Univer- metamorphism: Journal of Petrology, v. 18, p. 53-72. sity of Southern California, 197 p. Brown, E. H., and Blake, M. C., Jr., 1987, Correlation of Early Cretaceous Ando, C. J., Irwin, W. 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