JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 87, NO. B3, PAGES 1803-1824, MARCH 10, 1982

Polygenetic Ophiolite Belt of the California Sierra Nevada' Geochronologicaland TectonostratigraphicDevelopment

JASON B. $ALEEBY

Division of Geological and Planetary Sciences, California Institute of Technology, Pasadena, California 91125

The assumptionthat ophiolite sequencesare generatedat essentiallyone point in geologictime by the processof sea-floorspreading is critical for modern conceptsin the tectonicsof ophiolites and for topicsdealing with their structureand petrology.However, this assumptionhas only been verifiedin a few locationsby an integratedgeochronological and structural-stratigraphicapproach. Many ophiolite sectionsare reconstructedfrom structurallydisrupted sequences with the idealized ocean floor model in mind. Such reconstructionsare prone to error without adequate age control on each of the reconstructedfragments. This is a significantproblem in structurallycomplex regions where more than one generationof ophiolite may be present. In this paper new Pb/U zircon agesare presentedfor key locations along a 375 km segmentof the western Sierra Nevada ophiolite belt. These age data are combined with structural-stratigraphicobservations and published ages, and significant tectonic implicationsfor the ophiolite belt emerge.Three different ophiolitic assemblagesare recognizedwith igneousages of about 300, 200 and 160m.y.B.P. Rocks of the 300 m.y. assemblageare in a completely disruptedarray of metamorphictectonite slabsand serpentinite-matrixmelange. Fragmentsof upper Paleozoic seamountsoccur in associationwith the ophiolitic melange, and togetherthese assemblages constitutethe basementframework for the western Sierra. Pb/U and K/Ar isotopic systematicsare complex within this framework and indicate a polymetamorphichistory. Systematicsin the 200 and 160m.y. assemblagesare lesscomplex and give tighter igneousage constraints.Rocks of the 200 m.y. assemblageare in a semi-intact state with only local tectonite and melange zones. Rocks of the 160 m.y. assemblageare intact, but neverthelessdeformed. Both the 200 and 160 m.y. assemblageshave equivalent age basinal volcanic-sedimentarysequences that lie unconformably above the ophiolitic melange basement. In each case the basinal sequenceslocally extend conformably into the upper stratigraphiclevels of the age-equivalentophiolite sections.These relations along with vestiges of intrusive contactsbetween the edgesof both youngerophiolites and the melangebasement indicate that the youngerophiolites underwent igneous formation in proximity to the melangebasement. The Sierran ophiolite belt is consideredto have formed by a multistageprocess initiated by the early Mesozoic tectonic accretion of upper Paleozoic sea-floor in general proximity to the ancient continental margin. Regional metamorphismand ophiolitic melange resulted. This accretionary nucleusbecame the basementof Jurassic-ageprimitive volcanic arc terraneswhich underwent rifting episodesduring the productionof the 200 and 160m.y. ophiolites.The rifting episodesresulted in the formation of sedimentarybasins which were the depositionalsites of volcanic-sedimentarysequences. Non-volcanic sourcesfor the basinalsedimentary rocks include the melangebasement and continental margin terranes. Contact zones between pre-existingbasement and the juvenile ophiolitic sequences created during the rifting episodesconsist of dynamothermalmetamorphic aureoles, protoclastic deformation zones and cross-cutting dikes. Such edge-zone assemblagesare in most localities obscurred or destroyed by superimposeddeformations resulting from convergent and perhaps transform motions along basin edges. Both the 200 and 160 m.y. basins were destroyed by compressionalorogenic episodes shortly after their formational episodes. Destruction of young ophiolite floored basins may be a common course of events when small oceanic-type plates are generatedalong continental margin environments. Such tectonic settings are ideal for the emplacement of young ophiolite sheets.

INTRODUCTION the more intact sequencessuch as the Semail ophiolite in Ophiolite sequencesconsist of ultramafic tectonites, cu- northern Oman and perhaps the Coast Range ophiolite belt mulate gabbro, sheeted dikes, and pillow basalt. These are of California [Hopson et al., 1980; Tilton et al., 1981]. Proper commonly thought to be on-land fragments of oceanic crust documentation of this critical assumptionrequires detailed and upper mantle [Coleman, 1977]. Modern studies in the field observationsof the primary structuralrelations within petrology and tectonicsof ophioliteshave relied stronglyon the ophiolite igneoussuite in conjunctionwith geochronolog- the assumption that in a given ophiolite the main igneous ical and stratigraphicstudies. masswas generatedat essentiallyone point in geologictime. This paper summarizes geochronological, stratigraphic In a plate tectonic context this is assumedto have been the and structural data from key locations along a 375-km-long time of sea-floor spreading formation of the sequence. stretch of the Sierra Nevada ophiolite belt. The area of Operating under this assumption numerous investigators interest occurs along the central and southern parts of the have looked at ophiolites with focus on the petrologic western foothills metamorphic complex which bounds the processesoperating at mid-oceanridges, and on the ancient edge of the Sierra Nevada batholith (Figure 1). Contrasting interactions between oceanic plates, and their continental metamorphic and deformational histories in different seg- margin sitesof emplacement.However, the assumptionthat ments of the ophiolite belt, along with regional stratigraphic a given ophiolite is in fact the product of one igneous relations suggest the presence of more than one age of constructionalstage has only been documentedfor a few of ophiolite. This is clearly demonstratedby the geochronologi- cal data presentedbelow. Furthermore, these investigations Copyright ¸ 1982 by the American Geophysical Union. show that the Sierran ophiolite belt developed in three Paper number 1B1764. 1803 0148-0227/82/001 B- 1764501.00 1804 SALEEBY: POLYGENETIC OPHIOLITE GEOCHRONOLOGY AND TECTONICS igneous formation stages widely spaced in time and that Chert-argillite melange belts are co-extensive with the rocks of the two youngerstages formed by rifting within a ophiolitic basementrocks, and in places intimately inter- tectonic framework constructed from rocks of the oldest mixed. Such assemblagesare generally referred to as the stage. The rifting episodesgave rise to sedimentarybasins Calaveras Formation; however, as discussedby Schweickert that underwent deformationand metamorphismshortly after et al. [1977], a formational statusfor such a diverse mixture their formation. It is not implied that such complexitiesare of rocks is inappropriate.Upper Paleozoiclimestone blocks typical of all or most ophiolites,but rather the igneousand are ubiquitous in the various chert-argillitemelange belts, stratigraphicdevelopment of each ophiolite shouldbe con- although the most distinguishingfeatures of the different sideredindependently and without a strongbias toward the belts are the ages of their youngestelements. Radiolarian idealized ocean floor model. chert and quartz-rich clastic rocks as young as Early and The approachtaken in this studywas to look in detail at possiblyMiddle Jurassicoccur in rocks west of the Melones critical structuraland stratigraphicrelations within the Sier- Fault Zone and in associationwith the Kaweah ophiolitic ran ophiolite belt and to collect samplesfor zircon Pb/U melange[Behrman, 1978a,b; Irwin et al., 1978;Saleeby and analyses with the intent of dating specific igneous and Sharp, 1980]. Such rocks are referred to as central belt stratigraphicrelations. In the discussionsthat follow, the melange.Infolded depositionalremnants and fault sliversof geochronologicaldata are consideredwithin a context that Jurassicvolcaniclastic-slate sequences also occur locally in includesthe field settingsand metamorphicpetrology of the central belt rocks. The large belt of chert-argillite melange samples.In sucha contextthe complexrelative chronology east of the Melones Fault zone is constrainedto pre-Jurassic of petrogeneticand deformationalepisodes derived from in age by Pb/U and K/Ar ages on its main metamorphic- geologicaldata can be directly related to an absolutechro- deformationfabric [Sharp et al., 1982].This belt constitutes nologybased on the zircon data. In order to fully appreciate the oldest of the chert-argillitemelange assemblages and is the geologicalsettings of the age samples,a brief introduc- referred to as the Calaveras Complex [Schweickertet al., tion to the regional setting of the ophiolite belt and its 1977]. The age constraintsof late Paleozoicto Triassic and relationswith neighboringmetamorphic units is required. Early Jurassicon the chert-argillitebelts parallel those of the 300 and 200 m.y. ophiolitic assemblages. REGIONAL SETTING OF THE OPHIOLITE BELT Ophiolitic rocks yieldingthe youngestages extend north- The aerial distribution of the major rock units of the ward from the Pine Hill area and include the Smartville western and northern Sierra Nevada are shown in Figure 1. block. These rocks yield numerouszircon ageswhich cluster Note that the east-west scale has been expanded to help between 165 and 159 m.y. Rocks of this group are generally clarify the spacialrelations between the belt-like rock units. intact and show primarily low-grademetamorphic features. Attention in this paper will focus on ophioliticrocks of the The Smartville block constitutes a broad anticlinal exposure central Sierran foothills between the Pine Hill and Penon of the plutonic and volcanic members of a continuous Blanco areas, and to a lesser extent on rocks of the Kings- ophiolite section [Menzies et al., 1975; Xenophontosand Kaweah area exposedin the southernfoothills. The main Bond, 1978]. Within its upper pillow lava section occur exposuresof ophioliticrocks in the centralfoothills occur as volcaniclastic strata which are extensions of the western a semi-continuousnorthwest-trending belt which is bounded Jurassic volcaniclastic-slate belt (Figure 1). by belts of low-grade slate, greenstone,and chert-argillite. The Jurassicvolcaniclastic-slate belts consistprimarily of Ophiolitic rocks of the southernfoothills are boundedby mafic with lesser intermediate and silicic tuffs, breccias, batholithic and high-grademetasedimentary rocks. pillowsand flows. Suchvolcanic rocks are intercalatedon all The ophioliticrocks are dividedinto two structural-setting scales with mudstone-sandstone-conglomerateflysch de- groups in Figure 1. The oldest group yields zircon agesin rived from first cycle volcaniclasticmaterial, and uplifted both the 300 and 200 m.y. range, and is characterizedby a chert-argillite, continentalmargin and ophiolitic sourceter- complex structuralhistory involving the widespreaddevel- ranes. Mudstone, sandstoneand fine tuffs of this assemblage opment of metamorphictectonites and melange.The struc- contain a well-developed slaty cleavage whereas coarse tural complexitiesof thisgroup are suchthat subdivisioninto volcanic units generallylack deformationfabrics. Much of the more significant200 and 300 m.y. age groups is not the volcaniclastic-slateassemblage is late-Middle to Late possibleat the scale of Figure 1. For the purposesof this Jurassicin age [Clark, 1964], althoughsome volcanic inter- discussion the 200 and 300 m.y. age groups are shown vals exposedin the easternbelt are of probableJura-Trias togetherbecause of their occurrenceas or closeassociation age [Sharp and Wright, 1981]. These older volcanic rocks with ophiolitic melange. Melange is used as a descriptive are similar to the Penon Blanco volcanics which, as dis- term for mappable bodies of deformed rock which lack cussedbelow, are of Jura-Trias age. An outstandingfeature internal stratigraphiccontinuity, and whoseoverall structure of both the mid- to Late Jurassic and Jura-Trias volcanic is characterizedby intact fragmentsencased within a fine- rocks is the predominanceof ankaramiticor conspicuously grained matrix. Ophiolitic melangeof the Sierran foothills clinopyroxene-phyric olivine basalt. Volcaniclastic-slate consists of mafic, ultramafic and sedimentary blocks en- units of the Mother Lode and Western belts and similar cased in a serpentinitematrix. Ophiolitic rocks which give strata exposed along the Kaweah melange (Figure 1) lie 300 m.y. agesare eitherfrom melangeblocks or occurwithin unconformably above central belt and ophiolitic basement large melange-boundedtectonite slabs. The 200 m.y. ages rocks. This relationshipin conjunctionwith the extensionof occur in melange-boundedslabs with both tectonite and such strata conformablyinto the upper levels of the Smart- well-preserved protolith features. The 300 and 200 m.y. ville ophiolite is suggestiveof a primary link between the association extends along the entire length of the Sierra older and youngerophiolitic age groups.This is one of the Nevada and constitutes the oldest basement element ex- major points of this paper and will be discussedat length posed along the western margin of the range. below. SALEEBY.' POLYGENETIC OPHIOLITE GEOCHRONOLOGY AND TECTONICS 1805

Regional deformation, metamorphismand ophiolite em- An east-dippingNevadan thrust zone separatesthe eastern placementin the westernSierra Nevada have tradi•tionally volcaniclastic-slate belt from the Calaveras Complex been viewed as the results of the Late Jurassic Nevadan [Schweickert et al., 1982], and the northern and eastern orogeny [Hinds, 1934; Moores, 1970; Bateman and Clark, margins of the Smartville block appear to be Nevadan-age 1974]. It is now clear that the Nevadan orogenyis the latest thrust faults. The Melones fault zone is traditionally viewed in a seriesof early Mesozoicorogenic pulses, each of which as a Nevadan suture. However, this fault zone is clearly not involved metamorphismand thrustingof ophioliticassem- a fundamental break inasmuch as similar Jurassic volcani- blages[Saleeby, 1980;this paper]. The main expressionof clastic-slateunits occur on both sidesof the fault (Figure 1), the Nevadanorogeny was the developmentof slatycleavage and as discussedbelow similar pre-Jurassicbasement ele- in mid- to Late Jurassicstrata and rotationsand flattening ments and Jurassicintrusive sequencesoccur on both sides. distortionsin melangefabrics of the underlyingbasement. The Melones fault zone in many localities truncates Neva-

CALIFORNIA OPHIOLITE BELTS

// / F/9ure

•hiolitic Mel EXPLANATION ,F'• CRETACEOUSANDJURASSIC BATHOLITH

SLATES WB, MLB, EB TMWESTERN• MOTHER •!• JURASSICLODE AND VOLCANICLASTICEASTERN BELTS ROCKSAND Ophiolitic MID- TO UPPER JURASSIC OPHIOLITIC ASSEMBLAGES: LAYERED GABBRO, •n-• PLAGIOGRANITE,SHEETEDDIKES, PILLOW BASALT AND VOLCANICLASTIC ROCKS

-• MIDDLEDIORITIC JURASSICINTRUSIVE PERIDOTITIC COMPLEXES TO

:.•.•/•JURA-TRIASBASALT• CHERT,PENON VOLCANICLASTICBLANCO PILLOW ROCKS AND RELATED SHALLOW-LEVEL INTRUSIVES

UPPER PALEOZOIC TO JURASSIC CHERT- ARGILLITE ñ SANDSTONE MELANGE: • LIMESTONE•INCLUDESBLOCKSMETABASITE OFUPPER AND PALEOZOIC KingsRiver SERPENTINITES CB = CENTRAL BELTS •,••p, hi01ile cc= PRE-JURASSICCALAVERAS COMPLEX •' t' J•l••--• UPPERPALEOZOIC ANDJURA-TRIAS ULTRAMAFIC TECTONITE AND GABBRO- ß SHEETED DIKE-PILLOW BASALT SLABS • • J•D• •• BOUNDEDBYSERPENTINITE-MATRIX • • • • MELANGE '• • - • • PALEOZOICANDLOWER MESOZOIC Koweoh••••• • • .AF,CANDQUARTZ-R,CH SCHISTAND Ophio;c Melange FS= FOOTHILLSSUTURENTZ = NEVADAN THRUSTZONE ß • BNFZ= BEAR NOUNTAINS FAULT ZONE

Fig. 1. Generalizedmap showingmajor metamo•hic rock units of the westernand nowhereSiena Nevada. Note that east-westscale does not equ• noah-southscale. Geology after Jennings [1977], Saleeby [1980], Schweickert et al. [1982], and Snoke et al. [1982]. 1806 SALEEBY: POLYGENETIC OPHIOLITE GEOCHRONOLOGY AND TECTONICS dan-age cleavages, although at regional scales it is concor- overthrusting and fight-slip have been proposed for the dant with such structures.Right-slip, thrustingand combina- suturing of these two different rock regimes [Schweickert, tions of these mechanisms have been suggestedfor the 1977; Saleeby, 1980]. The Late Triassic metamorphic-defor- Melones fault zone [Clark, 1960; Saleeby, 1980;Schweickert mational episode exhibited along the Foothills suture ap- et al., 1982]. pears to correspond with the time constraints placed on The most fundamental structural break in the Sierra metamorphismand melangedevelopment in ophioliticbase- Nevada is the Foothills suture (Figure 1). The Foothills ment rocks exposed west of the suture. suture separatesrocks within a clearly oceanicor ophiolitic Structural, stratigraphicand geochronologicalrelations of basement regime to the west from rocks of continental the Bear Mountains ophiolitic melange and the Tuolumne margin affinity to the east. The continental margin rocks ophiolitic melange will serve as the primary focus of this consistprimarily of Paleozoicand lower Mesozoic quartzite, paper. The regional map patterns of these ophioliticmelange marble, deformed granitoids and silicic to intermediate vol- units are shown in Figure 1. More detailed mapswhich show canic sequences.Intense deformationand amphibolitefacies the criticalareas from which the agesamples were taken are metamorphismapparently related to the juxtapositioningof shown in Figures 2 and 3. The Tuolumne ophiolitic melange the continental margin and oceanic basement rocks took is exposed in the core area of a large south plunging place in Late (perhaps latest) Triassic time [Sharp et al., anticline. The Penon Blanco volcanics, and youngervolcani- 1982]. Various movement patterns involving westward clastic-slatesequences of the Mother Lode and westernbelts

EXPLANATION

,•r• CRETACEOUSQUARTZDIORITE

WBANKARAMITIC- WESTERN ANDBELT: LOCAL DACITIC VOLCANIC ROCKS

MLBSLATY- MOTHERFLYSCH, LODETUFF BELT: AND AN KARAM I T I C VOLCAN I CLASTIC ROCKS

•PHICOMPLE - lINE: GABBRO,HILL INTRUSIVE PYROXENITEAND LOCAL DIORITE WITH FOLDED IGNEOUS LAYERING

x• SHEETEDFDS- FOLSOM BASALT,DIKE GABBRO,SWARM' AND MICRODIORITE DIKES WITH LOCAL DIORITIC PODS AND BMOM SCREENS

.• MELANGECBMB- CENTRALAND BROKENBELT

CBM AND BMOM CUT BY ANKARAMITICFORMATION' TOINTERMIXED DACITlC HYPABYSSAL ROCKS AND OVERLAINBY MLBINFOLDS AND FAULT SLIVERS

\.•-• MELANGE:CBM- CENTRAL CHERT-BELT ARGILLITE ñ SANDSTONE MELANGE WITH LIMESTONE AND METAVOLCANIC BLOCKS AND BMOM SLIVERS N,SAR - ? X,dt NORTH• ß"•X BMOMOPHIOLITIC- BEAR MELANGE'MOUNTAINS SOUTHFORKS, CLOSELY PACKED BLOCKS AMERICAN RIVER AND SLABS OF AMPHIBOLITE TO UPPER CR - CONSUMNES GREENSCHIST FACIES RIVER METAMORPHOSED BASALT, GABBRO, CHERT• MARBLE SJ - SUPERJACENTSTRATA AND SERPENTINIZED N PERIDOTITE BOUNDED BY o 5 lO SCHISTOSE SERPENTINITE ZONES - DT = INNER [ I I DYNAMoTHERMAL CONTACT 8030 ' AUREOLE OF PHI-FDS

121ø00 '

Fig. 2. Geologic map showinggeneral settingof Bear Mountainsophiolitic melange and related rock units. Also shown are locations of zircon samplesreported in Table 1. Geology after Olmstead [1971], Clark [1976], Behrman [1978a, b], Springer [1980b], and J. B. Saleeby and W. H. Wright III. SALEEBY: POLYGENETIC OPHIOLITE GEOCHRONOLOGY AND TECTONICS 1807 successivelyenvelope the ophiolitic basementcore. A com- marized and integrated with new data presented for the plex system of longitudinal faults which complicates the Tuolumne and Bear Mountains segment.Similar patterns in hinge area and western limb of the large anticline, extends the geochronologicaldata from each segment form a basis northward from the Tuolumne region as the Bear Mountains for regional stratigraphic and tectonic correlations. In the fault zone. following section publishedand new geochronologicaldata The Bear Mountains fault zone is the locus of another will be discussedand interfaced with the geological settings anticlinal exposure of ophiolitic basement rock, the Bear of the samples. Mountains melange (Figure 1). This ophiolitic melange is GœOCI-I•tONOLOGIC^L F•,nMEWO•UC OF THE flanked on the west by the western volcaniclastic-slatebelt, OPHIOLITœ BœLT and is intermixed to the east with central belt melange which is in turn overlain and flanked on its east by the Mother Lode New zircon Pb/U age data on the Sierran ophiolite belt for belt. Northward along the Bear Mountains melangesheeted six samples related to the Bear Mountains segment, five dikes and layered gabbros of the 160 m.y. age group cross- samples related to the Tuolumne segment, and two from the cut and contact metamorphosethe older rocks. These intru- Kings-Kaweah segmentare given in Table 1. Information on sivesare similar in age to the Smartville block and may be its the samplelocations and their zircon yield are given in Table southern extremity. If so it follows that the Bear Mountains 2. In this section the zircon data are used in conjunctionwith belt may be the core of a very large north-plunginganticlinal the geologicalsettings of the samplesto develop a geochro- structurethat is envelopedby the Smartville block. Alterna- nologicalframework from which the petrogeneticand strati- tively, the Smartville block may be a completely detatched graphic relations along the belt can be viewed. Additional thrust sheetlying structurallyabove the Bear Mountains belt hornblende K/Ar data on ophiolitic mafic tectonites give and its cross-cutting sheeted dikes and gabbroic intrusives valuable insight into the metamorphic and deformational (E. M. Moores, personal communication, 1980). The field history of the belt, particularly when integrated with the and geochronologicalaspects of suchcomplexities are pres- zircon data and with field and petrographicobservations. ently under investigation (E. M. Moores and J. Saleeby). Inasmuch as metamorphic textures are widespread Ophiolitic rocks of the Kings-Kaweah segmentof the belt throughout the ophiolite belt, the possibilityof open system bear strong similarities to those of the Tuolumne and Bear behavior in the zircon Pb/U systemsis a critical issue. Open Mountains segments. The structure and geochronologyof system behavior with episodic radiogenic Pb-loss has been the Kings-Kaweah segment have been discussedin detail discussedby Wetherill [1956], Silver and Deutsch [1963], [Saleeby, 1978, 1979; Saleeby and Sharp, 1980]. Critical Davis et al. [1968], and Saleeby and Sharp [1980]. The mark relationshipsfrom the Kings-Kaweah segmentwill be sum- of such behavior is a normal discordance pattern with the

EXPLANATION

•:,,.•MLBANKARAMITIC- MOTHER VOLCANICLASTICLODEBELT: SLATYLENSES FLYSCH WITH

I•BASALT,, DIABSE ANDMICRODIORITE DIKES r,.•,.•. MELANGEUNIT CONTAINING SLICES OF

PILLOW BASALTj CHERTj . TUFFACEOUSPBV- PENON CHERT,BLANCO ANDVOLCANICS: ' ANKARAMITIC VOLCANICLASTIC ROCKS

DON PEDRO INTRUSIVES: VARI-TEXTUREDCDI- CHINESE CAMP-PYROXENE AND HORNBLENDE DIORITE AND GABBRO• MELAGABBRO• SR-STANISLAUS PYROXENITEAND WEHRLITE RIVER • TOM- TOLUMNE TR- TUOLUMNERIVER • 'b>. • t ß OPHIOLITICMELANGE: •.• • 4 SERPENTINIZED • DUNITE AND

SLABS BOUNDED SJ37ø45' - SUPERJAcENT STRATA •' • . • BYHARZBURGITE SCHISTOSE

4ß ½ • CONTAININGSERPENTINITE

0I 5I 10KMI "•:;'•'• • METAcHERTAMPHIBOLITE, AND • • MARBLE BLOCKS 120ø30' •. •

Fig. 3. Geologic map showinggeneral settingof Tuolumne ophiolitic melangeand related rock units. Also shown are locations of zircon samplesreported in Table 1. Geology after Clark [196,4],Morgan [1976], and J. B. Saleeby. 1808 SALEEBY: POLYGENETIC OPHIOLITE GEOCHRONOLOGY AND TECTONICS

•C•o o.z • o•

-H -H -H -H -H -H -H -H -H -H -H -H -H -H -H -H -H -H

-H -H -H -H -H -H -H -H -H -H -H -H -H -H -H -H -H -H SALEEBY: POLYGENETIC OPHIOLITE GEOCHRONOLOGY AND TECTONICS 1809

2ø6pb/•38Uage less than the 2ø7pb/a35Uage which is in turn Apparent concordance is defined as agreement of ages less than the 2ø7pb/2O•'pbage. Closedsystem behavior is within analytical uncertainty. The analytical uncertainties evidencedby concordanceor the equivalenceof these ages. for the zircon data are given in Table 1 along with a brief However, in the age range under consideration,the analyti- explanation of their derivation. Concordance within the cal uncertaintiesof the agesare a major inhibitingfactor in systematicsof a singlezircon analysisis consideredapparent resolvingwhether a zircon analysisis concordant,or appar- becausewith zircon populationsin the 300 m.y. and younger ently concordantbut in reality discordant. age range a moderate yet geologicallysignificant disturbance

TABLE 2. Data on Zircon Sample Location, Petrography, and Yield

Zircon Yield (mg)/ Sample Sample Number* Field Location Field Setting Petrography Size (kg)

CB1 Fiddletown 7.5' quad: Creek bottom exposureof Metaplagiogranite: partially granulated and fine- 85 / 104 (CB 1) lat 38034' 19"N, monolithologicblock in ly recrystallized oligoclaseand quartz, with long 120ø52'26"W sliver of serpentinite me- relict granophyric texture; accessorychlorite, lange calcite, and sericite. BMO1 Latrobe 7.5' quad: Hilltop exposureof 50 m Metaplagiogranite:almost completely recrystal-• 15 / 109 (BMO1) lat 38ø33'04"N, thick concordantlayer in lized quartz and Na-plagioclasewith relict oli-• long 120ø56'42"W amphibolite tectonite goclaseigneous grains and accessorymeta- morphic green hornblende, epidote and magnetite. TOM1 Melones Dam 7.5' quad: Knobly exposure of 10 m Metaplagiogranite: granulatedquartz and oligo- 9/90 (TO9) lat 37ø56'37"N, diameter monolithologic clase with relict hypidiomorphic granular tex- long 120ø31'04"W block in serpentiniteme- ture; secondaryaccessory chlorite and calcite. lange KSM10 Piedra 7.5' quad: Concordant layer within ul- Plagiogranite gneiss: gneissicbanding defined by -25 / 41 (P12) lat 36ø47'21"N, tramafic schist model and grain-size variation in quartz, Na- long 119ø25'23"W plagioclase,garnet, and sphenewith granob- lastic epidote overprint. CDI1 Sonora 7.5' quad: Road cut exposure in ho- Pyroxene diorite: granulated and partly recrystal- -250 / 88 (TO!4) lat 37ø52'36"N, mogeneousdiorite lized intermediateplagioclase with relict diop- long 120ø26'24"W sidic augite grains, secondarychlorite, serpen- tine, actinolite, epidote, and fine white mica. CDI2 Penon Blanco Peak 7.5' Hillside exposure of vari- Hornblende diorite: coarse brown-greenhorn- -150 / 86 (TO11) quad: lat 37ø43'58"N, textured intrusive body blende and intermediate plagioclasein hypidio- long 120ø19'37"W morphic granular texture, relict clinopyroxene and secondaryepidote, white mica, green am- phibole, and quartz. CDI3 Penon Blanco Peak 7.5' Road cut exposure in vari- Pyroxene diorite: highly recrystallized remnants 12/98 (TO35) quad: lat 37ø42'02"N, textured diorite of hypidiomorphictexture with remnantsof long 120ø16'04"W clinopyroxenegrains largely replaced by chlo- rite-actinolite clusters. Plagioclasecompletely altered to sericite, calcite, and albite. MLB1 Chinese Camp 7.5' quad: Cobble in conglomerate Pyroxene diorite: granulated and recrystallized 3/6 (TO21) lat 37ø5l'02"N, long bed exposedalong road intermediate plagioclaseand clinopyroxene; 120ø23 '39' 'W cut secondaryepidote, chlorite, pumpellyite, and white mica. KRO10 Pine Flat Dam 7.5' quad: 30 cm wide screenin basal- Metaplagiogranite: hornfelsic Na-plagioclase, 6/74 (PF250) lat 36ø46'33"N, long tic sheeteddikes at base quartz, and minor green hornblende; relict 119ø15'32"W of cliff granophyric texture. FDS1 Auburn 7.5' quad: 25 cm wide screenin basal- Plagiogranite/quartzdiorite: fine-grainedNa-pla- 18/81 (SV1) lat 38ø53'39"N, tic sheeteddikes along gioclase, quartz and green amphibole with sec- long 121ø04'31"W road cut ondary epidote, chlorite, and fine white mica. FDS2 Pilot Hill 7.5' quad: Boudinageddike in de- Plagiogranite: distorted and recrystallized grains -30 / 89 (FD28B) lat 38ø50'32"N, formed mafic sheeted of quartz and Na-plagioclasewith granoblastic long 121ø04'03"W dikes chlorite and actinolite. PHI Coloma 7.5' quad: 10 cm wide deformed dike Plagiogranite: coarse-grainedhypidiomorphic 7/70 (PH2) lat 38ø46'07"N, w•thin gabbro host along granular intermediate plagioclasequartz and long 121ø00'00"W stream gorge hornblendewith incipient recrystallization with sparce epidote and chlorite. PHI2 Shingle Springs7.5' quad: Low-color index phaseof Pyroxene diorite: hypidiomorphicgranular clino- 18 / 75 (PH10) lat 38ø38'57"N, long mixed zone in pluton pyroxene and intermediate plagioclase,minor 120ø58'32"W orthopyroxene, deep red biotite and magnetite, trace quartz. *Original field number in parentheses. 1810 SALEEBY: POLYGENETIC OPHIOLITE GEOCHRONOLOGY AND TECTONICS dispersesthe agesonly within a range comparableto that of between Table 1 and the geologicmaps and diagrams.The the analytical uncertainty. Thus additional criteria are need- Pine Hill intrusive complex has been studied in detail by ed for the recognitionof true concordance. Springer [1980a, b], and the Folsom dike swarm has been The analysis of multiple zircon fractions split from a mapped by Olrnstead [1971] and J. Saleeby, W. H. Wright cogenetic population based on physical properties such as III and R. K. Springer(ongoing field studies).The Pine Hill size, shape, color and magnetic susceptabilityis helpful in complex consistsof a main layered intrusive body composed resolving apparent from true concordance. Open system of gabbrowith lesserclinopyroxenite and diorite, and small- behavior within a cogeneticzircon populationhaving a wide er massive satellitic intrusives with a similar compositional range of physical properties should result in a noticeable range. The Folsom dike swarm consistsof basalt, gabbro, dispersion of ages between fractions, particularly the diorite and rare plagiograitedikes with large sheeteddo- 2øtpb/238Uages which are the mostprecisely determined mains. [Silver, 1963, 1964, 1969; Silver and Deutsch, 1963; Saleeby The Pine Hill complex and the Folsom dike swarm are and Sharp, 1980].Thus concordancewithin analyticaluncer- intruded into the northern end of the Bear Mountains tainty of 2øtpb/238Uages between multiple fractions is an ophiolitic melange. A narrow screen of the melange sepa- important criterion for recognizingtrue concordance. rates the Pine Hill complex from the dike swarm, and a An additional criterion for the recognitionof true concor- number of similar screens occur within the dike swarm. A dance is the analysis of zircon populations from different wide range of wallrock compositions,which include ultra- locations within a cogenetic suite of rocks. Except for the mafic, mafic and carbonateassemblages, display homblende caseof compete resetting, atleast a moderatedispersion of hornfels to pyroxene hornfels facies mineral assemblages ages,partict/larly 2øtpb•a38U ages, should be recognizable [Springer, 1974, 1980a, b]. As discussedbelow, however, throughout a large rock body in the event of an episodic the contact metamorphicfabrics are largely tectoniticrelated disturbance. Further insight into zircon behavior is also to synplutonic and hot sub-solidusdeformation in the Pine derived from geologicalobservations including stratigraphic Hill and Folsom intrusives. relations, metamorphicmineral assemblagesand basic field Zircon data on the Pine Hill complex include a 162 m.y. relations. concordantage on a late-stageplagiogranite dike (PHI1), and By the use of all the criteria outlined above assumptions a164m.y.• ,•on•,l•^-•- uant ' age on a pyroxene diorite phaseof the can be made about the behavior of the zircon for which data main plutonic mass(PHI2). Concordantages are givenin the are given in Table 1. From these data igneousage constraints text asrounded off 2øtPb/a38Uages. Similar concordant ages can be deduced. In the case of discordant populationsonly were obtained for the Folsom dike swarm. Sample FDS 1 is broad age constraintscan be made, but there are a significant from a plagiogranitescreen within sheetedbasaltic dikes; its number of concordantages for which tight constraintscan be age is 163 m.y. Sample FDS2 yields 165 m.y. concordant made. Viewing the concordantand discordantages together ages within and between multiple fractions. This sampleis in a context which includes the K/Ar and geologicaldata from a boudinagedplagiogranite dike which lies concordant- results in a coherent and geologically soundpicture for the ly within deformed mafic sheeted dikes. geochronologicaldevelopment of the Sierran ophiolite belt. Zircon ages reported above for the Pine Hill intrusive Isotopic age data for ophiolitic rocks of central and complex and the Folsom dike swarm are consideredreason- southernSierra are graphicallysummarized in Figure 4. The able igneousage determinations.This is supportedby una- 2øtpb/a38Uand 2ø7pb/:øtPbages are shownfor eachzircon nimity of apparent concordance,tight agreementof ages analysis.The analyticaluncertainties in the2ø7pb/2øtpb ages over a wide geographicrange, and concordancebetween are shown by thick lies; overlap with the large points which multiple fractions of sample FDS2. Furthermore, regional symbolizethe 2øtpb/238Uages indicates apparent concor- amphibolite and greenschistfacies tectonites of the Bear dance. The age data in Figure 4 are divided into four groups Mountains ophiolitic melangeshow a resettingof their K/Ar based on field locations. The data for each location are hornblende systemsto a reasonableage range within the arranged into age groups which correspondto structural- aureole of the Pine Hill intrusive complex and the adjacent stratigraphic sequences. Relations between sequencesfor Folsom dike swarm (Figures 2 and 4). which a relative chronologycan be derived, such as uncon- A commonconcern in usingplagiogranites to date ophioli- formities or intrusivecontacts, are also symbolizedin Figure tic assemblagesis whether the subordinate felsic rocks 4. Hornblende K/Ar data on ophiolitic amphibolitesare also significantly post-date the mafic igneous sequence.In the summarized in Figure 4. Zircon ages on samplesrelated to case of the Folsom dike swarm sample FDS1 is from a the ophiolite belt fall into three groups,300,200 and 160m.y. screenwhich is cut by the mafic dikes. Furthermore, sample The 300 m.y. group includes a number of discordant sam- FDS2 is from a domainof sheeteddikes which showsstrong ples, and thus it can only be broadly defined. The younger protoclasticdeformation, and suchdeformation is sharedby groups are more tightly defined in that they yield predomi- the plagiograniteas well as the predominate mafic dikes. nantly concordantages. Discussion of the agedata will begin Protoclastic deformation in the dike swarm consists of with the 160 m.y. group inasmuchas this group has experi- widespread boudinage,homogeneous flattening, streaking enced a simplemetamorphic history relative to the other two out and local folding of dikes. Contradictory relations be- groups. tween deformation features and intrusive contacts indicate a protoclasticor synplutonicdeformational regime. Protoclas- 160 m.y. Age Group tic deformationfeatures are also widespreadin the Pine Hill Zircon ages in the 160 m.y. range have been determined complex [Springer, 1980a, b]. Of importancewith respectto for rocks of the Pine Hill intrusive complex and the Folsom the age data is the presenceof local massesof plagiogranite Lake dike swarm (Figure 2). Note that the samplenumbers that were mobile during suchdeformation along with remo- have been coded by letter prefixes for easy referencing bilized cumulate-texturedpyroxenite and gabbro.This indi- SALEEBY:POLYGENETIC OPHIOLITE GEOCHRONOLOGYAND TECTONICS 1811

100mybp 150 200 250 300 350mybp INNER WALLROCKS• • CB1 • OFPHI-FDS

BMO]

• PHIl • PHI2 • FDS•

FDS2

• CDI• q:) •k• '• -- CDI3 • ,• MLB1

KRO 10

-• [• [• •, DIORITE • MYLONITE

• -- QUARTZ • • DIORITE • • GNEISS

SILICIC RANGE ,• MAFICTO OF 30 ANDPLUTONS DIKES CONCORDANTZ I RCON O v AGES ON CROSS- BATHOCUTT LI NGI TH • LOWERAND• .,• INTERCEPT •. (• OFRANGEKSM •• INTERCEPTUPPER KSM • CHORD A• ... • OFRANGE KSM• METAPLAGIOGRANITEBLOCKS • • CHORD

m %\!/ DIORITE

ANDESITE •__,•O : DIKEDIKE

O• AGESJZIRCONMULTIPLE206pB/238U FRACTIONSAND207pB/206pB LINKED • TECTONITE$/HORNBLENDE K/AROPENAGES BOXES ON FORAMPHIBOLITE SAMPLES SYMBOLS O • ANDINTENSEOPEN CONTACTCIRCLES METAMORPHICFORSAMPLES OVERPRINTSHOWING • OVERPRINTSHOWING STRONG STATIC TEXTURAL / UNCONFORMABLEANDINTRUSIVE RELATIONS BETWEEN YOUNGER ANDOLDER SEQUENCES

Fig. 4. Summaryof geochronologicaldata for centraland southern segments of SierraNevada opholite belt. Sample numbersare givenfor zircondata reported in thispaper. Additional zircon data from Saleeby and Sharp[ 1980],Chen and Moore [1982]. K/Ar hornblendedata on marietectonites from Morgan [1976],Behrrnan [1978a, b], Saleebyand Sharp [1980], and J. B. Saleebyand L. A. Silver (work in progress). 1812 SALEEBY: POLYGENETIC OPHIOLITE GEOCHRONOLOGY AND TECTONICS

cates co-magmaticrelations between the plagiograniteand cobble-sizeclast of Chinese Camp-Don Pedro-like pyroxene the main mass of the pluton as suggested by the near diorite from the overlying Mother Lode fiysch sequencealso equivalence of ages between the PHI1 plagiograniteand yields a concordantage of 198 m.y. (MLB1). PHI2 pyroxene diorite. The age data along with the geologi- In a regional study of K/At systematics,W. D. Sharp cal relations show that the Pine Hill intrusive complex and (personalcommunication, 1980) obtained a 200 m.y. igneous the Folsom dike swarm were magmatically emplaced into hornblende age on sample CDI2. Furthermore, Morgan the Bear Mountains ophiolitic melangebetween 163 and 165 [1976] reports a 200 ___10 m.y. igneoushornblende age from ChineseCamp-Don Pedro wehrlite. A 200 ___10 m.y. amphi- Igneous assemblageswith roughly comparablezircon ages bole age is also reported by Morgan [1976], on a garnet occur in associationwith the Kings-Kaweah segmentof the amphibolite block from the Tuolumne ophiolitic melange. belt (Figure 4 and Saleeby and Sharp [1980]). Such igneous The amphibolite block shows a pervasive static-textured rocks include plutonic bodies of dioritic to pyroxenitic green amphibole overprint. Static-textured overprints are composition,with grosssimilarities to the Pine Hill complex, also commonin the ultramaficrocks of the Tuolume ophioli- and a homogeneous quartz diorite which crosscuts the tic melange as well as in mafic, chert, carbonate and dioritic to pyroxenitic body. Zircon ages with apparent plagiogranitemelange blocks. Contact relationsbetween the concordance of 170 m.y. and 157 m.y., respectively, have main exposuresof the Chinese Camp-Don Pedro intrusives been determined on these bodies. Additional Middle to and the Tuolumne ophiolitic melangeare obscuredby Juras- Upper Jurassicages have been determinedon plagiogranite sic deformation. However, disrupted dikes of Chinese and andesitic dikes which cross-cut ophiolitic rocks and Camp-Don Pedro gabbroand diorite cross-cutthe basement, which occur in association with volcaniclastic strata that lie and the widespreadstatic-textured overprint in the basement unconformably above ophiolitic melange. has no apparent source other than heat related to the Zircon ages in the 155 to 170 m.y. range from the Kings- intrusive complex. Kaweah region, and the 163 to 165 m.y. agesfrom the Pine Concordanceof ageswithin and between zircon fractions, Hill-Folsom associationare collectively referred to as mid- consistency of ages from different sample locations, and to Late Jurassicin later stratigraphicdiscussions. Peridotitic agreementwith the K/At data all indicate an igneousage of to dioritic intrusive complexesand a regionalswarm of mafic about 200 m.y. for the Chinese Camp-Don Pedro complex. dikes which yield numerousPb/U and K/At ages in the 170 Field and petrologic data which show that such an age is to 157 m.y. range cut the Calaveras Complex and adjacent applicable for the Penon Blanco volcanics are discussedwith continental margin assemblageeast of the Foothills suture the stratigraphic relations. [Sharp and Saleeby, 1979; Sharp, 1980]. The mafic dikes Rocks in the 200 m.y. range also constitute an important also cut ophiolitic melangeand 200 m.y. rocks exposedalong component of the Kings-Kaweah segment. Sample KR010 the Tuolumne segmentof the belt. Thus the entire central from the Kings River ophiolite yields a 199 m.y. zircon age and southern Sierra foothill region was the site of igneous with apparent concordance.A poor zircon yield prohibited activity involving the emplacementof ultramaficto dioritic multiple fraction analysis. Sample KR010 is from a plagio- plutons and widespread emplacement of primarily mafic granite screen within a sheeted dike sequence. The dike compositiondikes in mid- to Late Jurassictime. The igneous sequence lies beneath pillow basalt and above gabbro and age of the Smartville ophiolite falls within this age group ultramafic-mafic tectonite which together constitute a com- [Saleeby et al., 1979], and as discussedbelow stratigraphic plete ophiolite section [Saleeby, 1978]. and paleontologicaldata constrainmost volcaniclastic-slate Prior to the age determination on KR010 an approximate sequencesto a similar age. 200 m.y. age was interpreted for the Kings River section based on discordantzircon ages, K/At data and geological 200 m.y. Age Group relations. Discordant age arrays from diorite mylonite and Zircon ages in the 200 m.y. range have been determined quartz diorite gneisslayers present in the lower levels of the for rocks of the Chinese Camp-Don Pedro intrusive complex ophiolite are shown in Figure 4. The diorite mylonite layer along the Tuolumne segmentof the belt (Figure 3). The area occurs as a thin segregation in protoclastically deformed between samplesCDI1 and 2 was mappedby Morgan [1976]. gabbro which itself is encasedin dunite-harzburgitictecton- Topical mapping within this area and its extensionsto the ite. The ultramafic tectonites also encasea layer of coarsely northwest and southeastwas done in conjunctionwith this banded leucoquartzdiorite and diorite gneissfrom which the study. The Chinese Camp-Don Pedro intrusivesare consid- quartz diorite fraction was sampled. Unfortunately the rare ered to be the shallow-level feeder system for the Penon zircon-bearing dioritic layers lie within the metasomatic Blanco volcanics which lie unconformably above the Tuol- contact aureole of a cross-cuttingCretaceous pluton. Zircon umne ophiolitic melange(B. A. Morgan, personalcommuni- samplesfrom suchfield settingsalong the entire lengthof the cation, 1976). Field and petrographic data obtained in this Kings-Kaweah segmentshow episodicPb-loss due to hydro- study strongly supportMorgan's interpretation. thermal attack during contact metamorphism. K/At ages The ChineseCamp-Don Pedro intrusive complexconsists from the amphibolitic gabbro host of the diorite mylonite primarily of vail-textured pyroxene and hornblendediorite layer and similar amphibolitic rocks within the aureole are with subordinate amounts of wehrlite, clinopyroxenite and completely reset to Cretaceous ages (Figure 4). Comparison gabbro. Three of five dioritic members sampled yielded of the zircon systematicsand hydrothermal attack effects zircon. Sample CDI1 is a pyroxene diorite which has yielded between the Kings River diorites and more discordant concordant ages of 199 m.y. within and between multiple plagiogranitezircon from Kaweah melange blocks led Sa- fractions. Similar singlefraction concordantages of 200 and leeby and Sharp [1980] to conclude a relatively moderate 196 m.y. were determined on a hornblende diorite (CDI2) disturbance for the diorite zircon populations and thus a and another pyroxene diorite (CDI3) respectively. A large Jura-Trias or approximate 200 m.y. igneousage. This inter- SALEEBY: POLYGENETIC OPHIOLITE GEOCHRONOLOGY AND TECTONICS 1813 pretation is further supportedby a 190 m.y. K/Ar age on CB 1 was split into three size fractions, whereas the poorer igneous hornblendefrom a protoclasticallydeformed am- and overall finer grain-size yield of BM01 was hand sorted phibolitic gabbro layer located well outside the Cretaceous into deeply colored and relatively clear fractions. contact aureole [Saleeby and Sharp, 1980]. The two coarser fractions of CB1 and the BM01 clear Ophiolitic melange of the Kaweah River area is uncon- fraction yield concordant ages in the 304 to 311 m.y. range formably overlain by a volcanic-chert sequence which is (Table 1, Figure 4). The deeply colored BM01 fraction is similar to the Penon Blanco volcanics of the Tuolumne significantlydiscordant, and the fine CB 1 fraction is moder- segment.This sequenceis in turn unconformablyoverlain by ately discordant within its own systematics but has its a volcaniclastic-slateassemblage with strong similarities to 2ø6pb/238Uage in agreementwith the other concordant the Mother Lode belt, which in the Tuolumne region lies fractions. The CB1 fine fraction is similar to the BM01 dark above the Penon Blanco volcanics. Thus regional strati- fraction in that both are deeply colored and include an graphic relations suggesta Jura-Trias age for the Kaweah abundanceof poorly faceted grains with cracks and surface volcanic-chertsequence. Mafic dikes which cut the ophioli- defects, and both contain numerous inclusions and rare tic melange basement and extend up into the volcanics darkened core areas. Such features may help explain prefer- appear to be the volcanic feeder system. A dioritic dike rock ential disturbanceof the Pb/U systematics,but the perplex- associated with this system yields a slightly discordant ingaspect of thediscordances are the older 2ø7pb/2ø6pb ages. zircon age in the 200 m.y. range (Figure 4 and Saleeby and Two alternative explanationsare offered for the aberrant Sharp [1980]). Poor zircon yield again prohibited multiple 2ø7pb/2ø6pbages. fraction analysis.The slightdiscordance, weak metamorphic 1. Impurities introduced into the darker zircon fractions features in the diorite and stratigraphicrelations all support at the time of crystallization included Pb which cannot be an age assignmentof about 200 m.y. for the diorite and its properly accounted for by the common Pb correction used associated mafic dikes. (Table 1). Such initial Pb would have to be radiogenic- From the above discussion of age data and geological enrichedinasmuch as high 2ø•pb/2ø4pb ratios (1313 and 1701) relations a 200 m.y. age group can be tightly defined along were measured on the discordant fractions. The error would the Tuolumne segmentof the belt. Such rocks consistof a havea significanteffect on the 2ø7pb/2øapbages, but only a volcano-plutonic complex built on an ophiolitic melange minoreffect on the 2ø6pb/238Uages. Furthermor e, suchan basement. Along the Kings-Kaweah segmentthis age group error would not be introduced into the systematics of the is more loosely definedas rocksin the 200 m.y. or Jura-Trias concordantfractions which lack the impurities. range which occur as a completeophiolite sequence,and as 2. Rare grains with darkened core areas reflect inheri- mafic volcanic-hypabyssalrocks and chert also built on an tance of older U-rich zircon which introduced old ophiolitic melangebasement. Zircon ages in this age range 2ø7pb/2øapbages into the systematics.Either interpretation have not been obtained on rocks of the Bear Mountains given above is difficult to reconcile with an idealized ridge- segment. However, central belt melange rocks (Figures 1 crestmodel for thegeneration of theophiolitic protolith, and and 2), which include elements of the Bear Mountains implies a multi-stage history for the 300 m.y. assemblage. ophiolitic melange, contain large fault-bounded slabs of Such early-stage complexitiesare further suggestedby the dioritic and mafic hypabyssal rock in sequence with a 300 m.y. KJAr age obtainedon an amphibolitetectonite slab volcanic-chertsection. Field and preliminarygeochronologi- from an ophiolitic melange exposure in the central b,elt cal studies indicate a Jura-Trias age for such slabs (W. H. [Behrman, 1978a,b]. Furthermore,both the CB1 and BM01 Wright III and J. Saleeby). zircon populations are U-enriched relative to well-studied ophiolite sections [Martinson, 1975, 1976; Hopson et al., 300 m.y. Age Group 1980; Tilton et al., 1981]. Such a problem exists for zircon Field settingsof samplesfrom the 300 m.y. age group are from other plagiograniteblocks, and suggeststhe presence distinctly different from those of the younger groupswhich of magmabodies that are U-enriched relative to typical mid- are from intact igneousbodies and stratigraphic sequences. ocean ridge basalt. All 300 m.y. samples are from plagiogranitebodies that The concordantages on CB1 and BM01 fractionswhich do occur as metamorphosedmelange blocks or as layers within not appear to have chemical and morphologicalcomplexities amphibolitic melange blocks. The zircon populations from leads to the interpretation of these ages being igneous in the 300 m.y. group are also distinctly different from those of origin. Whether this igneous event represents sea-floor the younger groups. Variable and high U concentrations, spreadinggeneration of an ophiolitic protolith or someother deep coloring, crystal surface defects, and complex discor- oceanicigneous process is irresolvableowing to the structur- dance patterns are common in the older rocks, but almost al and metamorphicstate of the rocks sampled.The younger absent in the younger rocks (Table 1, Figure 4, and Saleeby 2ø6pb/238Uage in the BM01 coloredfraction may reflect Pb- and Sharp [1980]). SamplesCB1-and BM01 yield the only loss in imperfect grains during Mesozoic metamorphicepi- zircon populationsfrom the 300 m.y. group with any sem- sodes. blance of concordance.These samples are from the Bear Serpentinitemelange exposures of the Tuolumne segment Mountains segment of the belt where blocks and slabs of (Figure 3) also contain a plagiogranitemelange block (Sam- ophiolitic material are denselypacked with narrow interven- ple TOM1). The block is situated in an area which lacks ing serpentinite matrix zones and where ophiolitic melange obvious high grade metamorphic features. The only meta- zones extend into central belt melange. morphic features displayed in the plagiograniteare intense Sample CB1 is from a monolithologic block within a tectonicgranulation and a 1ow-greenschistfacies statictex- central belt serpentinite melange sliver. Sample BM01 is tured overprint. This overprint is attributed to the thermal from a structurally concordantlayer within one of the larger event related to the Chinese Camp-Don Pedro intrusive amphibolite slabs of the Bear Mountains melange. Sample complex. Sample TOM1 produceda poor zircon yield pro- 1814 $ALEEBY: POLYGENETIC OPHIOLITE GEOCHRONOEOG¾ AND TECTONICS hibiting multiple fraction analyses.Fortunately the popula- concordant zircon ages measured on batholithic rocks re- tion is quite homogeneouswith a predominanceof clear sponsiblefor the contact metamorphicepisode [Saleebyand euhedral grains having smooth generally well-developed Sharp, 1980; Chen and Moore, 1982]. The upper intercept faces and terminations. Analysis of the total population range is 270 to 310 m.y. which is similar to concordant or yields slightly discordantages in the 300 m.y. range(Table 1, near concordant ages measured on similar plagiogranites Figure 4). Characteristicsdiscussed above and in the work from the Tuolumne and Bear Moutains segmentsof the belt. by $aleeby and Sharp [1980] which would suggestinheri- Rocks of these northern segmentsdid not experience Creta- tance of older radiogenicPb or significantepisodic Pb-loss ceous contact metamorphism.A straightforwardinterpreta- are lacking. Thus minor Pb-loss is favored for the discor- tion of the concordia data is an igneousage in the 300 m.y. dance mechanismof sampleTOM1 and a primary age in the range and episodic Pb-loss during Cretaceous batholith 300 m.y. range is most reasonable. emplacement. Zircon data from the KSM10 felsic gneiss Plagiograniteoccurs in blockswithin serpentinitemelange layer fall on the same chord, and an approximate 300 m.y. at several localities along the Kings-Kaweah segmentof the protolith age is interpreted for all such felsic layers and belt (Figure 1). The ophiolite belt in this region has under- blocks within the Kaweah ophiolitic melange. gonea near penetrativehornblende hornfels facies metamor- Discordant zircon systematicsfrom Kings-Kaweah diorit- phism which was impartedby early Cretaceousplutonism of ic samplesthat are interpreted as about 200 m.y. in age can the Sierra Nevada batholith [$aleeby and Sharp, 1980]. be contrasted with those from the 300 m.y. plagiogranite Contact metamorphismof the serpentinitematrix gave rise blocks. The dioritic zircon data as a group are slightly offset to a complexfamily of metamorphicand metasomaticrocks, to the high side of the plagiogranite chord. This relation including antigorite and talc hornfels and jasper-magnesite translates into the Figure 4 plot as a more consistent and skarns. Zircon populations from eight metaplagiogranite restricteddispersion in the 2ø6pb/238Uand 2øTpb/2ø6pb ages. bodies encasedwithin the metaserpentinitesshow a complex Inspection of Figure 4 also shows the dioritic fractions pattern in chemicalattack and alteration and a wide rangeof clustering in younger groups of ages than the plagiogranite discordant ages. The chemical activity of the contact meta- fractions. morphic and metasomaticenvironment, in addition to high Zircon data on metaplagiogranitefrom melange blocks temperatures,is cited for the causeof the zircon disturbance scattered along the entire 375 km length of the ophiolite belt [Saleeby and Sharp, 1980]. consistently indicate igneous ages in the 300 m.y. range. New zircon Pb/U data are presentedfor one such sample Such felsic bodies occur as monolithologic blocks, as (KSM10) in Table 1. This sample is from a quartz-plagio- screens within blocks of mafic dike complex, and as struc- clase-garnetgneiss layer within ultramaficschist. The ultra- turally concordant layers within mafic tectonite blocks and mafic schist bounds the western edge of the Kings River ultramafic schist. The predominant block assortment con- ophiolite and actually extends into the ultramafic-mafic sists of ultramafic and mafic tectonites, metachert and tectonite portion of the ophiolite as screen-likebodies [Sa- marble. Locally preservedprotolith features suchas pillows, leeby, 1978]. The protolith of the schistand its inclusionsis sheeted dikes and radiolaria tests in bedded chert indicate the northern end of the Kaweah serpentinitemelange. Much the melange was derived from a complete ophiolite suite. of the melange assemblagein the Kings River area has been The added presenceof thick carbonatebuildups on massive polymetamorphosed,first by dynamothermalprocesses re- and vesicular basaltic flows indicates that seamount-type lated to the Jura-Trias igneoushistory of the Kings River assemblageswere closely associated with the ophiolitic ophiolite, and then by contactmetamorphic processes relat- fragments. The presenceof seamountassemblages offers an ed to the Cretaceous batholith. The dynamothermal event explanation for possible multi-stage igneous processesand will be discussed in the next section. The results of the the presenceof magmathat may differ from mid-oceanridge contact metamorphic event are a static-texturedtalc over- basalt. Other oceanic environments that may offer such print and patchy developmentof jasper-magnesiteskarn in complexities along with the early development of metamor- the ultramafic schist. The KSM10 felsic geniss shows gran- phic tectonitesare large fracture zones.The importantpoint oblastic growth of epidote minerals related to the contact here is that the plagiogranite blocks are part of a complex event, but the older gneissicfabric derived from the dyna- ocean floor association. Chert and upper Paleozoic lime- mothermal event is well-preserved. stone sequenceswithin this associationrange up to hundreds Sample KSM10 exhibits a discordancepattern that is of meters in thickness, and thus a substantial oceanic comparableto the discordancepatterns of multiplefractions sedimentationhistory is represented.The discordance-relat- from three meta-plagiøgranitemelange blocks located fur- ed uncertainties in the zircon ages also leave room for a ther south along the Kaweah River (Figure 4 and Saleeby substantial range in igneous ages (up to 50 m.y.). Such a and Sharp [1980]). Not all metaplagiogranitesamples of the range may reflect a time and spaceinterval between the sea Kings-Kaweah segmentare representedin Figure 4. Those floor spreadingformation of juvenile crust, the subsequent omitted have extremely high U concentrations(up to 3400 growth of a seamount on such crust, or some form of off- ppm), show extreme surface defects, and have undergone ridge magmatismsuperimposed on juvenile crust. The broad late-stageand/or modem disturbances.Samples included in age constraints outlined above lead to a late Paleozoic age Figure 4 do not fall into sucha category.Nevertheless, these assignmentfor this complex ocean floor assemblage. samplesstill displaysignificant discordances both within and Regional metamorphismin upper greenschistto primarily between multiple fractions. amphibolite facies conditionsmarks the disruptionof the late The eleven fractions shown in Figure 4 from the Kaweah Paleozoic ocean floor assemblageand its mixing into ophili- melangedefine a tight best fit chord on a 2ø6pb/238U-tic melange. Time constraintson this important processcan 2ø7pb/235Uconcordia diagram. An early Cretaceouslower be derived from geological relations and K/Ar data on intercept for the chord coincideswith the rangeof numerous amphibolite tectonite melange blocks. The K/Ar ages are SALEEBY: POLYGENETIC OPHIOLITE GEOCHRONOLOGY AND TECTONICS 1815 summarized in Figure 4. They cluster in the 180 to 200 m.y. Upper Paleozoic Ophiolitic Basement range, although geologicalrelations show that the amphibo- lites and their melange host pre-date the 200 m.y. igneous Upper Paleozoic ophiolitic rocks of the central and south- age group. Textural overprints suggestingresetting of the ern Sierra show a remarkable degree of consistencyboth in K/Ar systemsare present only in sampleslocated within geologicalcharacter and geochronologicaldevelopment. The contact aureoles that are superimposedover the melange. associationof rock types in the ophiolitic melange is con- Aureoles of 200, 160 and about 110 m.y. age are present stant along the entire 375 km length of the belt, and thus it along different segmentsof the ophiolite belt. In each case appears that all segmentsof the belt were derived from a inner aureole amphibolite blocks have totally regenerated common protolith. Such basementcontinuity establishesat amphibole with K/Ar agesthat are similar to zircon ageson least broad tectonostratigraphicties between Jura-Trias and the cross-cutting intrusives. Amphibolites from locations later Jurassic rock assemblagesthat were either intruded outside obvious aureolesand with K/Ar agesin the 180 m.y. into or deposited on top of the basement. range show incipient textural and compositionalchanges in The geologic history of the upper Paleozoic basement their amphibole grains. Overprinting features are almost prior to petrogenesisof the early Mesozoic ophiolites and lacking in sampleswhich fall in the 190 to 200 m.y. range, volcanic-sedimentary sequencesis of great importance in except along the Tuolumne segmentwhere the only contact understandingthe early development of the ophiolite belt. metamorphic event was at 200 m.y. It is unlikely that a Upper Paleozoic ophiolite had been disrupted, metamor- regional resetting event would occur without leaving a phosedand mixed into serpentinitemelange in Triassictime. textural imprint, and thus the 190 to 200 m.y. ages are The remnants of a regional unconformity between the base- interpreted as a near minimum age constraintfor the devel- ment and Jura-Trias volcanic-sedimentary sequences are opment of the amphibolites. This is similar to the Late preserved along the Kaweah and Tuolumne segmentsof the Triassic metamorphic-deformationpeak determinedfor oce- belt. Detailed relations along the Kaweah segment are anic and continental margin rocks juxtaposed along the discussed in Saleeby [1979] and summarized in Figure 5. Foothills suture [Sharp et al., 1982].Thus regionalmetamor- Here Jura-Trias pillow lava was erupted across a substrate phism and ophiolitic melangedevelopment may be linked in consistingof ophiolitic sedimentarydebris. The debris was time to the suturing of the late Paleozoic ocean floor locally derived and appears to have sat directly on its assemblage to rocks of continental margin affinity. serpentinite melange source. Along the Tuolumne segment the Jura-Trias unconformity surfaceappears to have been an uplifted block of serpentinizedperidotite that gradesto the TECTONOSTRATIGRAPHIC FRAMEWORK OF THE north and west into serpentinite-matrixmelange. The rem- OPHIOLITE BELT nants of this surfaceare preservedalong the lower Tuolumne In previous sectionsthe regional setting of the ophiolite River (Figure 3) where non-deformedPenon Blanco pillow belt wa• reviewedand a geochronologicalframework was basalt lies directly on highly deformed ultramafic tectonite. established.In this sectiondetailed structural-stratigraphic A similar unconformity between ophiolitic melange and relations within the belt are discussedwith an emphasison Jura-Trias strata of the central belt presumably extended how the igneous history of the belt relates to regional northward into the Bear Mountains segment(Figures 1 and patterns in sedimentation and deformation. Detailed rela- 2). However, Jurassic deformations have obliterated any tions along four segmentsof the belt are summarizedin a remnants of such a surface. seriesof tectonostratigraphiccolumns (Figure 5). In contrast The submarinelandscape prior to depositionof the Jura- to conventional stratigraphiccolumns these columns are Trias strata apparently consistedof fault bounded ridges of tectonostratigraphic in nature in that deformational and serpentinizedperidotite, metamorphosedmafic rock, chert metamorphicepisodes as well as igneousand sedimentary and carbonates,and serpentinitemelange. Locally derived sequencesare represented. The columns are diagrammatic serpentinite debris flows and mafic, chert and carbonate reconstructionsbased on map-scaleand detailed field rela- talus breccias are the only remnantsof a sedimentaryrecord tions with time constraintsgiven by geochronologicaland followingthe disruptionand metamorphismof the basement biostratigraphicdata. Zircon agesfor the two youngerophio- and preceding the deposition of the Jura-Trias strata. Mod- litic age groups cluster near important period and epoch ern oceanic environments which display such a landscape boundaries. Considering the discussionsof Harland et al. include large oceanic fracture zones [Bonatti and Honnorez, [1964], Lambert [1971], and Armstrong [1978] agesof about 1976; Fox et al., 1976] and the inner walls of sediment-poor 200 m.y. are considered Jura-Trias, and the boundary be- trenches [Evans and Hawkins, 1979; Hawkins et al., 1979]. tween Middle and Late Jurassic time is considered to be On the basis of a wide range of regional geologicalconsider- about 160 m.y. Spatial variationsalong the ophiolite belt for ations it was suggestedthat disruptionand emplacementof these two critical slicesin time are representedalong hori- the late Paleozoic ocean floor assemblage occurred by a zontal rows in Figure 5. combination of transform and convergent tectonics [Sa- Lower Mesozoic volcanic-sedimentary sequences show leeby, 1980]. The importance of transform tectonics along an overall consistency in lithologic character along the the ancient California margin is suggestedby tectonic trun- western Sierra region. However, rapid lateral variations cations, regional patterns in aberrant paleomagneticpole over short distances due to both local facies changesand positions, and displacedfaunal belts. Evidence for conver- tectonicdisruption inhibit detailedstratigraphic correlations. gence is more direct, and includes early Mesozoic magmatic Nevertheless broad correlations can be based on regional belts in eastern California and the apparent remnants of an continuitybetween similar rock associationswith similarage early Mesozoic accretionarywedge. This wedge includesthe constraints.A critical basisfor establishingthe correlations ophiolitic melange basement rocks and upper Paleozoic to is the demonstrationof basementcontinuity. Triassic rocks of the central belt and Calaveras Complex. 1816 SALEEBY:POLYGENETIC OPHIOLITE GEOCHRONOLOGY AND TECTONICS

Ophioliticbasement rocks are intimatelymixed with these volcanicsubstratum. Thus the accreted oceanic assemblages chert-argillite melange belts as are remnantsof accreted appearto be substantiallyout of place. upperPaleozoic seamaunts. Such seamaunt fragments occur The upperPaleozoic oceanic assemblage of the western in the centralbelt with a majorophiolitic melange exposure Sierrarepresents the accretionary nucleus for thegrowth of (Clark[1976]; W. H. WrightIII andJ. B. Saleeby,ongoing a complexassociation of Mesozoic igneous and sedimentary field studies)and are encasedin upperPaleozoic to Triassic rocksof ensimaticcharacter. Triassic-age regional metamor- chert-argillitemelange of the CalaverasComplex. phism,melange development and suturing with continental Late Paleozoicocean floor assemblagesform a major margin rocks presumablymarks accretionof the oceanic accretionary belt that stretches from the southern Sierra assemblageto the continentalong a convergentplate junc- Nevadainto northernBritish Columbia [Davis et al., 1978]. ture. Importantelements of suchassemblages include upper Pa- leozoic ophioliteand seamauntfragments, Triassic meta- Jura-TriasVolcanic and SedimentarySequences morphic blocks and upper Paleozoic to lower Mesozoic Volcanicand sedimentary sequences of Jura-Triasage chert-argilliteassemblages. The geographiclocation of whichformed unconformably above ophiolitic melange wherethe oceanfloor formed is unknown,but paleobiogeo- basement are preservedalong the Kaweah,Tuolumne and graphicevidence suggests that it was a substantialdistance Bear Mountainssegments of the belt. The tightestage from the ancientcontinental margin. Upper Paleozoic lime- constraintsexist on rocksof the Tuolumnesegment where stoneblocks that occur throughout this regional accretionary the PenonBlanco volcanic sequence sits above ultramafic belt, andin melangebelts of the westernSierra [Douglass, tectonitesand serpentinitemelange of the basement.The 1967;Saleeby, 1979] contain fossils that are thoughtto be baseof thePenon Blanco Formation consists of greaterthan diagnosticof an equatorialoceanic faunal belt [Danner, 1000m of aphyricpillow basalt. Above the basalt lies up to 1976, 1977; Nestel, 1980]. This faunal belt was for the most 30 m of radiolarianchert with numerousfine greenish part out of communicationwith North Americanequatorial tuffaceousintervals. Radiolaria from the chert intervalare forms. The limestoneblocks appear to have beenderived early Mesozoicin age(D. L. Jones,personal communica- from the seamauntfragments, and in somelocations the tion, 1979).The chert intervalgrades upward into fine fossilsare from carbonatebuildups still attachedto their crystaland lapilli tuff which is overlainby a thicksequence

BEAR MTNS TUOLUMNER. uppermostJurassic ' Nevadan oroqeny' foldinq, thrustinq, •l !•--••-:•La.: . '.• NKARAMITICIHICSAND VOLCANICLASTICS•:•TURBIDITES-MLBI-----:- ::•• LIIHICANKARAMITIC SAND TURBIDITESVOLCANICLASTIC NITH '••k •.••a a aa -- 'WITH LOCAL ANDESITIC AND ••]-]L_.•...... LENSES- MLB •] ?*••f.•SILICIC LAYERSMLB-WB ]• ••q CONGLOMERATELENSESWITH • ' /HI,• a • ' AnARATC TO •L2 •--• BASEMENT-DERIVEDCLASTS • •,•( uncf.A.SSA.S ., =-- .nc •l•,•,'J/•t/tU//'/I{•'½•'••SHEETED,••. •AGABBRO-PYROXENITE-DIORITEDIKES- FDS dURA-TRIASROCKS- PBVBASEMENT- -- [/[I]h'•;'•'•• • I NTRUSI VE COMPLEX- PHI •• I'l/{[/•/•'•//••=PROTOCLASTIC/• 'r DEFORMATION MAFICBASEMENTDIKES CUTTING

, ME - W•ROCK, , MELANGEROCKS - TOMBASEMENT eorl tomid-Jurassic' deve]open •QUARTZSANDSTONE AND SlLICEOUS-ARGILLITE • • _ •,•,. -- ANKARAMI T I C CHAOTICCHERT-ARGILLITE fi A• •., VOLCANICLASTICSAND ••• SANDSTONE•ITH PALEOZOIC • F%• • CHERTYTUFF - PBV LIMESTONEBLOCKS BEDDEDfi CHAOTICCHERT fi - TUFFACEOUS CHERT - PBV - GREENSTONEVOLCANI CLAST I C • •• DRADIOLARIAN I ORI TE-GABBRO-CHERT PYROXENANDI TE ••uncf? LENSES- CB-PBV INTRUSIVES - CDI PALEOZOICOPHIOLITIC •APHYRICPILLOW BASALT-PBV BASEMENT- BMOM: •PERIDOTITE TECTONITE SLABS •, •, GABBROINTRUSIVES - CDI ANDMETAcHERT ••,,.•'•_•"• •••IPALEOZOICORI TE-WEHRL OPHIOLITIC ITE- AMPHIBOLITICMETAGABBRO ,•-. ,.• SERPENTINITEBASEMENT-MELANGETOM' lateto TriassicPoleozoic.gvolutio nof primary ophiolitic basement bysea-floor Fig.5. Diagrammatictectonostratigraphic columnsfor four key locations along the ophiolite belt. Note that pre- existingbasement for each time frame is shownby slightlylighter shades. SALEEBY: POLYGENETIC OPHIOLITE GEOCHRONOLOGY AND TECTONICS 1817 of ankaramitic volcaniclastic rocks (coarse clinopyroxene beds. The volcano-plutonicassociation of the Penon Blanco phyric-olivine basalt). Subordinateplagioclase and/or horn- Formation and the Chinese Camp-Don Pedro intrusives is blende phyric basaltic-andesitealso occurs in the volcani- interpreted as the remnants of a Jura-Trias volcanic center. clastic sequence.This sequenceis in excess of 2 km thick As shown in Figure 3 the Chinese Camp-Don Pedro intru- and consists of pyroclastic flow breccias, massive and sives are concentratedin the lower pillow lava sectionof the bedded crystal and lapilli tuffs, and local pillows and broken volcanic sequence, but they also extend well up into the pillow breccia. ankaramitic section. Some massive ankaramitic horizons Plutonic rocks that are genetically related to the ankara- may actually be sills that are linked to the intrusives. The mite-rich volcaniclastic sequence consist of the Chinese eruptive history of the volcaniccenter began with cappingof Camp and Don Pedro intrusives (informal units used by the basementby aphyric pillow lava not unlike ocean ridge Morgan [1976]). The genetic link was first suggestedby B. basalt. Following a modest amount of radiolarian chert and A. Morgan (1977, personalcommunication), based on simi- fine tuff sedimentation, a thick ankaramitic volcaniclastic lar clinopyroxenecompositions in both plutonicand volcan- cone was constructed. Intrusive contacts between the sub- ic rocks. Field and petrographicinvestigations of this study volcanic intrusives and the basement are only locally pre- strongly support Morgan's suggestion.The Chinese Camp- served due to Jurassicdeformations, but a widespreadstatic- Don Pedrointrusives consist of a myriadof smallinjections textured metamorphicoverprint and reset K/At systematics which range from diorite to wehrlite in composition.Intense in amphiboliteof the basementmark a broad thermal imprint shattering,synplutonic veining by low-grademineral assem- related to the volcano-plutoniccenter. The remnants of a blagesand complex agmatitesare all suggestiveof a shallow steep metamorphicgradient are also preservedin the Penon subvolcanicorigin. Some phasesare coarselyclinopyroxene Blanco volcanic section. Here hornblende hornfels assem- porphyritic, and very similar in appearanceto the main clast blages of the lower pillow section grade upward into static- population in the ankaramitic breccias. Furthermore, scat- textured greenschistand then progressivelyinto very low- tered clinopyroxeneand hornblendemegacrysts of id•entical grade greenschist assemblagesin the upper volcaniclastic character occur in both the intrusives and volcanics. Finally, sequence. The total thickness of the Penon Blanco volcanic cognate xenoliths of wehrlite, clinopyroxenite and diorite pile is unknown due to the unconformitywith the overlying that are identical to some Chinese Camp-Don Pedro intru- Mother Lode belt. sive phases occur locally in Penon Blanco volcaniclastic The Penon Blanco volcanics run northward from the

KAWœAHR. KINGS slatycleavage and low-grade metamorphism JQUARTZ AND LITHIC SAND JURA-TRIAS AND PALEOZOIC TURBIDITES OPHIOLITIC WALLROCKS L•?KARAMITICVOLCANICLASTICS AND PILLOWS GABBRO-DIORITE-PYROXENITE OPHIOLITIC OLISTOSTROMES INTRUSIVE COMPLEX REWORKED CHERT,ARGILLITE AND VOLCANICLASTICS -uncf LOCALIZED PROTOCLASTIC •INTERMEDIATE TO MAFIC DIKES DEFORMATION ZONES

QUARTZ DIORITE xJURA-TRIASAND INTRUSIVE SHEET PALEOZOIC OPHIOLITIC BASEMENT PLAGIOGRANITE DIKES foldinq,thrusting, Iocol uplift ond erosion --•-•-•-2_"•)_VOLCAN I Cß LIMESTONE AND '•:•-'__•-_• SA N D'S TO N E BL 0 C K S •.•ANKARAMITIC PILLOWSAND •APHYRICPILLOW BASALT WITH ••••CHERT-ARGILLITE-QUARTZ••..•• VOLCANICLASTICS • SHEETEDRARECHERTDIKES & ANKARAMITE ••••• SANDSTONEOLISTOSTROMES GABBROAND PERIDOTITE •'•••I••••• WITHLIMESTONEPALEOZOIC BLOCKS • CUMULATES •••'•RADIOLARIANCHERT "•-•-L'•APHYRICPILLOW BASALT •• • • /PROTOCLASTICDEFORMATION '•,',[{•:•_•,''?-•'-• SEDIMENTARYANDOPH I CALC SERPENTINITEI TE • •/ ANDDYNAMoTHERMAL •H••Dcf ••••WITHMETAMORPHMETAGABBROISM LAYERS :•,•'{?•,••,:3F• MAFPALEOZOICIC DIKES OPHIOLITIC ,•• •//•]•//•/PALEOZOIC OPH IOLITIC •,•':, •• MELANGEBASEMENT •1/I:7•,//• •ELANGEWALLROCKS spreodin,polymetamorphism ondmelane developmenl

Fig. 5. (continued) 1818 SALEEBY: POLYGENETIC OPHIOLITE GEOCHRONOLOGY AND TECTONICS

Tuolumne region and appear to extend into the central belt Paleozoic limestone blocks and quartz-rich clastic rocks. melange (Figure 1). The structural and stratigraphicrelation- Both have nearby sources in the seamountremnants of the ships are not clear along this part of the central belt due to oceanicbasement complex, and in continentalmargin rocks the presence of infolds and fault slivers of mid- to Late exposed east of the Foothills suture. Blocks of ankaramitic Jurassic strata. Part of this complex area is shown as central volcanic rock possess a local source within the Kaweah belt melange and broken formation in Figure 2. Greenstone section. Such blocks may also have been derived from a and lesser ankaramitic volcanic rocks which appear to be the major volcanic constructional center such as the Penon northern extension of the Penon Blanco sequenceare mixed Blanco section. with chert-argillite-sandstone melange along the southern The Paleogeographic setting envisioned for Jura-Trias part of the central belt. Upper Paleozoic limestone blocks time is a complex basinalframework with submarineridges also occur in the mixture [Douglas, 1967], and in some cases or fault scarps exposing ophiolitic melangebasement rocks they are encased in pebble-cobblemudstone suggestiveof a and the deformed remnants of upper Paleozoic seamounts. slide-block origin. Pebble to cobble-size clasts of chert, More distant exposuresof continentalmargin rocks lie to the metabasalt and marble, and the remnants of sedimentary east. Within this framework chert and argillite sedimentation serpentinitebeds mark local contributionsto the centralbelt predominated except where ankaramitic volcanic construc- from the ophiolitic basement.Other importantadmixtures to tional piles were built. Abundant slide blocks derived from the melange include bedded intervals and blocks of quartz- such constructionalcenters, and from basementexposures, rich sandstone and quartzite [Behrman, 1978a, b]. This and the resulting chaotic mixtures with chert, mud and sand diverse melangeassemblage probably originated unconform- are considered a reflection of the tectonic instability of the ably above the Bear Mountains ophiolitic basement or at basinalenvironment. Important insightsinto the evolutionof least formed in a position where it could receive detritus the basin complexare derivedfrom relationshipsexposed in from the basement. Unfortunately, remnantsof any primary the Kings River ophiolite. contactsbetween these assemblagesare lacking as a result of Jurassic deformations. Such deformations include folding Kings River Ophiolite: Implications for Jura-Trias Basin and thrusting episodes which both pre- and post-dated The Kings River ophiolite representsthe only complete depositionof the youngervolcaniclastic-slate units [Duffield and reasonably intact ophiolite sequencerecognized in the and Sharp, 1975; Behrman, 1978a, b]. A similar structural Sierra Nevada. The ophiolite's tectonite-cumulate-sheeted chronology exists for similar central belt rocks of the dike-pillow sequence is represented in Figure 5. As dis- Kaweah segment,however, someprimary relationswith the cussed in Saleeby [1978], the apparent stratigraphicthick- basement are preserved [Saleeby, 1979]. nessesare in reasonable agreement'with the idealized ophio- The highest structural-stratigraphiclevels of the Kaweah lite model [Coleman, 1977], althoughthe top of a relatively melangeconsist of locally-derivedserpentinite debris flows thick pillow section is not exposed. The important aspect of with fragments of metabasite, peridotite and chert. Jura- a complete ophiolite sequence stressedhere is that it pre- Thas strata above and interbeddedwith the ophiolitic debris sumably represents the products of sea-floor spreading include abundant pillow lava and broken pillow breccia, [Dewey and Bird, 1971; Coleman, 1977]. chert, limestone, and ophicalcite(carbonate-cemented ser- The focus of this discussionis the questionof where did pentinite breccia). Dikes and smallirregular intrusive bodies sea-floor spreading generation of the Kings River section of mafic to intermediate composition,which can locally be occur relative to the rest of the ophiolite belt. This is mappeddirectly into the pillowedrocks as their feeders,cut addressedby consideringthe subtle yet significantcomplex- the underlying serpentinite melange as well as the lower ities in the relations between the ophiolite section and the intervals of the Jura-Trias section. Above the pillow-rich adjacent Kaweah melange. Rocks of the Kaweah melange sequencelies a 200 m sequenceof argillaceousand tuffa- bound the western margin of the ophiolite and extend into ceous chert which gradesinto a chaotic sequenceof olistos- the basal ultramafic tectonites as narrow screen-like bodies. tromes. The olistostromesconsist of clasts and slabs of The entire melangeassemblage in this area hasbeen penetra- chert, upper Paleozoic limestone, quartz sandstoneand tively metamorphosedat amphibolite grade. The result of ankaramite within a siliceousargillite and chert-pebblemud- this metamorphism has been the transformation of mafic, stone matrix. The thickness of the olistostrome sequenceis chert and limestone blocks into amphibolite, metaquartzite unknown due to an unconformitywith the overlyingJurassic and marble layers which lie concordantlywithin an ultramaf- volcaniclastic-slate unit. However, it must have measured ic schist host. The one plagiograniteblock located in this on the order of thousands of meters inasmuch as some area was transformed into the quartz-plagioclase-garnet limestoneblocks range up to 500 m in diameterand a 400 m gneiss layer of sample KSM10. The original metamorphic thick ankaramitic volcanic sequencelies within it. mineralogy of the ultramafic host is difficult to resolve due to Jura-Trias strata of the Kaweah River area consists of a widespread static-textured talc growth related to Cretaceous combination of elements present in the Jura-Trias assem- contact metamorphism. However, nematoblastic antigorite blages exposed along the Tuolumne and Bear Mountains and tremolite appear to have been important phases and segmentof the belt (Figure 5). Basaltic volcanism along the local vestigesof anthophylliteare present.The metamorphic Tuolumne segmentspread across a basementexposure that tectonite fabrics of the ultramfic schist and its inclusions was apparently swept clean of ophiolitic debris. In the map out in continuity with the pervasive mylonitic fabric Kaweah region the pillow lavas were depositedabove and present in the lower levels of the Kings River ophiolite. This interbedded with such debris. Both the Kaweah and Bear is showndiagramatically in Figure 5, and is clear in mapsand Mountains assemblagescontain a basinal assemblagedomi- structure diagrams in Saleeby [1978]. Furthermore, the nated by chert, argillite and olistostromalunits. Externally mylonitic fabric of the lower Kings River sectionis manifest derived admixtures to the basinal assemblagesinclude upper by intense protoclastic deformation in the lower igneous SALEEBY: POLYGENETIC OPHIOLITE GEOCHRONOLOGY AND TECTONICS 1819 sequence, shown by countradictory intrusive and deforma- considered significant.This discussionwill focus primarily tional relations between gabbroicmasses within peridotite, on the Mother Lode belt inasmuch as there is greater and rare dioritic and felsic segregationswithin gabbroic stratigraphic control and more petrographic data for this masses. belt, and in general it is less deformed than the western belt. The relations outlined above indicate that the Kings River The remnants of a regional unconformity between the ophiolite underwent its 200 m.y. igneous petrogenesisin volcaniclastic-slatesequences and older Jura-Trias and up- close proximity to the pre-existing Kaweah melange, and per Paleozoicassemblages exist alongthe baseof the Mother that the melangeexhibits a dynamothermalcontact aureole Lode belt. This belt is roughly an east-dippinghomoclinal presumably related to the primary heat of the Kings River sequencewith its basement exposed along the Tuolumne- section. The metamorphosedmelange remnants may repre- Bear Mountains segmentof the ophiolite belt. In the Tuol- sent a wallrock border facies that was intruded by hot sub- umne area Penon Blanco volcanics and the Chinese Camp solidus peridotite and protoclastically deforming gabbroic intrusives are in contact with slaty flysch of the Mother Lode magma. Such intrusive relations may mark a fossil margin of belt (Figure 3). Northward along the contact from the the rift basin which gave rise to the Kings River ophiolite Tuolumne River area Penon Blanco ankaramite is cut out of section, or occur where screens of wallrock were stranded the section by pre-Late Jurassicfaults which disrupt the within such a rift basin. Alternatively, the Kings River Jura-Trias assemblageinto a narrow melange unit. These section may have been tectonically transported into its faults place Chinese Camp intrusives to a higher structural contact relations with the Kaweah melangewhile it was still level so that slaty mudstone of the Mother Lode belt lies hot enough to have patches of gabbroic magma remaining immediately on diorite. Further north Penon Blanco volcan- along its lower stratigraphiclevels. In either case, generalor ics and relatedintrusives emerge from a cover of superjacent direct proximity is required between the site of genesisfor strata in essentially the same stratigraphic position. The the Kings River ophio!ite and the older Kaweah melange. contact at the base of the Mother Lode belt was originally Such a spatial relation between the sea-floor spreading mappedby Morgan [1976] as a thrust fault. Further investi- genesis site of the Kings River section and the significantly gation has revealed that it is a faulted unconformity. In areas older melange complex is not likely to form at a mid-ocean of critical exposure well-preserved bedding in the slaty ridge. Furthermore, during the Jura-Trias formation of the mudstone is parallel to the contact and only locally sheared. Kings River section, the adjacentmelange was basementfor Shearing is absent in the underlying igneousrocks, and a the basinalvolcanic-sedimentary sequences. The interpreta- hummocky altered horizon may be the remnantsof a paleo- tion favored here is that the basinal conditions that were weathered zone. Locally derived clasts of greenstone, an- superimposedover the melange basement are a result of karamite and diorite occur as rare admixtures in the overly- rifting. Jura-Triaspillow sectionserupted directly acrossthe ing mudstone. A large cobble-size diorite clast, which is basement may be manifestationsof the rifting, but the petrographically similar to Chinese Camp diorite, yielded a extreme product was the generationof juvenile oceaniccrust 198 m.y. zircon age (sample MLB1). This age is the same as represented by the Kings River section. Original spatial ages determined on the Chinese Camp-Don Pedro intrusive relations between the Kings River sectionand the Jura-Trias complex (Table 1, Figure 4), and together with the petro- basinal strata are unknown due to Jurassic deformations, graphic similarities substantiatesthe existence of Chinese uplift and erosion. It is possiblethat the basinal strata sat Camp-Don Pedro type intrusivesin the Mother Lode base- above the Kings River pillow section, but were removed ment regime. The diorite clast was situated within a channel tectonically or by erosion. Such a relation may also account conglomeratebed with scatteredgreenstone clasts and abun- for the local occurrenceof ankaramitein the upper levels of dant chert, argillite and metaquartziteclasts. The sourcefor the Kings River pillow section. The important point here is the channel conglomerateappears to have been a terrane the primary link between the ophiolite section and the with both basinalvolcanic-sedimentary and continentalmar- basinal strata based on the age data and basementrelations. gin-type assemblages.First cycle and reworked material The most remarkable aspect of geologic history in the from a central belt-type assemblagewould produce such a western Sierra region is that the process outlined above mixture. repeated itself in mid- to Late Jurassictime. The Mother Lode belt in the Tuolumne area is well over 1000 m thick, but the top of the section is not exposed. It Mid- to Upper Jurassic Basinal Strata consists primarily of thin bedded turbidites with a mixed The most complete and intact history of basinal sedimen- provenance of first cycle volcaniclasticmaterial and uplifted tation in the western Sierra Nevada lies in the mid- to Upper central belt-type assemblages[Clark, 1964; Behrman and Jurassicvolcaniclastic-slate sequences. The grosspattern of Parkison, 1978]. Basin plain, slope and channel facies of a intercalated volcanic and sandstone-mudstone units is de- classic submarine fan complex are all present within the picted in Figure 1, but interstratificationoccurs down to very slaty strata. Such strata are Oxfordian to Kimmeridgian in small scales involving individual sedimentation units. The age, as indicated by bivalve and ammonite faunal assem- Mother Lode and Western belts are very similar, and appear blages [Clark, 1964]. Northward from the Stanislaus River to be complimentarylimbs of major anticlinal structures. area the slaty strata interfingerwith volcanic units that range The only notable differencein the two belts is the occurence up to 1000 m in thickness. The volcanic units are predomi- of local dacitic volcanic dome-like assemblagesin the West- nantly ankaramiticin compositionand consistof pillow lava, ern belt [Clark, 1964;Behrman, 1978b;Behrrnan and Parki- pillow breccia, crystal and lapilli tuff and tuff-breccia. The son, 1978] and a lack of such in the Mother Lode belt. lowermost volcanic unit reaches its maximum thickness in However, inspection of Figure 1 shows a much larger the CosumnesRiver area (Figure 2), where it is Callovian in samplingavailable for rocks of the western belt, and thus the age. lack of local silicic analoguesin the Mother Lode belt is not A depositional contact was originally mapped between 1820 SALEEBY: POLYGENETIC OPHIOLITE GEOCHRONOLOGY AND TECTONICS

Mother Lode belt rocks and rocks of the central belt [Clark, continentalmargin environmentduring the time that elapsed 1964]. Later studies consideredthis contact to be tectonic in between the two basinal environments. As with the Jura- origin primarily on the basisof the older assemblagebeing a Thas basin, the mid- to Late Jurassicbasin can be linked to melange[Duffield and Sharp, 1975].This critical relationship rifting and ophiolite formational processes. was reinterpretedby Behrman [ 1978b]as a faulted unconfor- mity which is consistentwith the relationspresent along the Mid- to Upper Jurassic Ophiolitic and Plutonic Rocks Tuolumne segment. The unconformablerelations in the Evidence of rifting and related ophiolite formation during Cosumnes River area are supported by the presence of mid- to Late Jurassic time is widespread in the western central belt and ophiolitic detritus in Mother Lode clastic Sierra Nevada, and throughout much of California. The units [Behrman, 1978b; Behrman and Parkison, 1978], and remnants of a rift margin are exposed along the Bear the presence of ankaramitic to dacitic hypabyssalrocks Mountains segmentof the belt where the Folsom dike swarm which cross-cut rocks of the melange. Such hypabyssal intrudes the Bear Mountains ophiolitic melange. The pre- rocks are presumably related to the ankaramitic to dacitic dominanceof sheetingstructure in the dike swarm indicates volcanic units present in strata of the Mother Lode and a tremendouscomponent of extensionduring dike emplace- western belts. Infolds and fault slivers of Mother Lode-type ment. The occurrence of similar age mafic dikes in great rocks that occur in the central belt melange and broken abundancein chert-argillitemelange and continentalmargin formation (Figure 2) may represent stratified remnants rocks exposed east of the Melones fault [Sharp, 1980], and above the transposedunconformity. Such a relationshipis locally within opholitic melange and the Jura-Trias se- well-preservedalong the Kaweah segmentof the belt. quencesof the Tuolumne and Feather River segmentsof the Mid- to Upper Jurassicstrata exposedalong the Kaweah ophiolite belt, indicate that mid- to Late Jurassicextensional segment of the ophiolite belt consist of slaty flysch with tectonics was widespread throughout the western Sierra ankaramitic and andesitic volcaniclastic interbeds. These region. Furthermore, the extensionof ankaramitic volcani- strata are preserved as tightly folded erosional remnants clastic strata of the western Jurassic belt into the upper which lie unconformablyabove ophiolitic and chert-argillite stratigraphiclevels of the Smartville ophiolite ties the pro- melange [Saleeby, 1979]. Abundant chert, argillite, quartz duction of juvenile crust to the proximity of the pre-existing sandstone and lesser greenstonedetritus constitute an im- ophiolite basement and its overlying basinal sequences. portant Jura-Trias detrital component,and sedimentaryser- Thus the mid- to Late Jurassic basinal environment can be pentinite and ophiolitic olistostromebeds represent debris directly related to rifting and ophiolite formation as can the derived from the ophioliticbasement. The ophioliticmaterial Jura-Trias basin. was derived from serpentinite diapiric protrusions which Unlike the remnantsof the Jura-Triasrifting episode,mid- carried blocks of other ophioliticlithotypes. Local interme- to Late Jurassic ophiolitic assemblagescovered by basinal diate-compositionhypabyssal rocks, which are presumably strata are widespread in other regions of California which related to the volcanic intervals, cut the ophiolitic melange include the western Klamath Mountains and Coast Ranges basement and the Jura-Trias strata. The contact between the [Saleeby et al., 1979;Blake and Jones, 1980;Harper, 1980; Kaweah volcaniclastic-slatesequence and the older assem- Hopson et al., 1980]. It has been suggestedthat the Klamath blages is highly interdigitatedparallel to the slaty cleavage and Coast Range exposuresare the main vestigesof juvenile direction and to axial surfaces of folds in the flysch beds crust created by the mid- to Late Jurassic rifting event [Saleeby, 1979]. Such structuralrelations are similarto those recorded in the western Sierra [Saleeby, 1980]. It follows presentin the central belt melangeand broken formationunit that significant quantities of the Late Paleozoic and Jura- to the north. In the Kaweah region this relation is shown to Thas assemblagesmay have been removed from the evolv- be a transposedunconformity by the direct connectionof ing continentalmargin during rifting and basinopening. This sedimentary serpentinitebeds with basement source rocks is in concert with the observation that definitive information by the remnants of protrusion dikes. and critical relations are sparceand fragmentalfor the older Stratigraphic relations in mid- to Upper Jurassicbasinal assemblages. sequencesdeposited above the Sierran ophiolite belt are The Pine Hill intrusive complex posesan important ques- summarizedin Figure 5. A similarpaleogeographic setting to tion concerningthe rift-edge exposedalong the Bear Moun- that of the Jura-Trias basin is envisaged, although in later tains belt. The intrusive complexcould be interpretedas the Jurassictime exposuresof Jura-Trias assemblagesadded to deformed cumulate sequenceof a typical ophiolite section. the sediment flux supplied by active volcanic centers, and Of course this cannot be the case due to its intrusive continental margin and oceanic basementrocks. The flysch relations with the Bear Mountains melange. Furthermore, sequenceswhich gave rise to the slate units are the remnants the dioritic end members of the Pine Hill complex contain of coalescing submarine fans which experienced interrup- minor but significant amounts of igneous biotite which is tions in depositionalstyle due to the growth of predominant- unlike typical ophiolitic cumulate sequences [Coleman, ly ankaramitic volcanic centers within the basinal setting 1977]. The early abundanceof diopsidic-augitein the Pine [Behrman and Parkison, 1978; Saleeby et al., 1978]. The Hill intrusion along with subordinateolivine in ultramafic main difference between the mid- to Late Jurassic and the and gabbroic compositionsmakes the intrusion a suitable Jura-Trias basin was a greater influx of externally derived plutonic analogue to the ankaramitic volcanic units of the sediment in the younger basin. In the Jura-Trias basinal Mother Lode and western belts.-Clinopyroxenite xenoliths sequenceshemipelagic strata are in greater abundanceover similar to phases of the Pine Hill complex occur in the clastic strata of possiblesubmarine fan origin, whereasin the lowermost Mother Lode belt volcanic unit. Furthermore, the younger sequencessubmarine fan strata greatly predomi- Callovian faunal age for this volcanic unit is essentiallythe nate. This perhapsreflects a componentof maturingin the same as the 163 m.y. zircon age for the Pine Hill complex $ALEEBY' POLYGENETIC OPHIOLITE GEOCHRONOLOGY AND TECTONICS 1821

[Armstrong, 1978], and in addition, ankaramitic units in both Bell, 1969; Carmichael et al., 1974]. Separatingthe frontal the Mother Lode and western belts increase in thickness as and remnantarc segmentsis a partiallyclosed inter-arc basin the Pine Hill region is approached. system floored by oceanic-typecrust [Karig, 1972]. Major It is suggestedthat the Pine Hill complex and the ankara- transcurrent fault zones have disrupted the non-volcanic mitic hypabyssal rocks of the central belt are the sub- remnant arc into en echlon ridges and basins, and deep volcanic roots of a major ankaramitic volcanic construction- fracturesand fault zonesoccur alongthe frontal arc region. al center. The close spatial and temporal associationof the Such deep fracturesmay be related to the tappingof picritic Pine Hill complex and the Folsom dike swarm is consistent and ankaramitic lava sources [Snoke et al., 1982]. with the extension of western belt ankaramitic volcaniclastic It is suggestedthat a complexfractured and rifted ensima- strata into the Smartville ophiolite volcanic section. It fol- tic arc environmentsuch as the SolomonIslands is the .type lows that a major volcanic constructionalcenter was proba- of tectonic setting where multiple ophiolite formation epi- bly built over the edge of the mid- to Late Jurassicrift basin. sodesmay occur in associationwith the growth of ankarami- Mid- to Late Jurassicage intrusive complexescharacter- tic volcaniccenters and the formationof complexsedimenta- ized by clinopyroxene-rich ultramafic phases and biotite- ry basins. Such an arc system presumably begins its bearing pyroxene diorite occur in a number of locations evolution within a primary oceanic or ophiolitic basement a.longthe western Sierra Navada-Klamath Mountains region regime. The upper Paleozoic oceanic assemblagesof the [Snoke et al., 1982]. The Sierran locations are shown in western Sierra represent such a primary basementregime. Figure 1. The Pine Hill complex resemblessuch a plutonic The disruptionof thisassemblage into a regionalmelange-

association,although unlike Pine Hill, many of the complex- tectonite belt presumably records the establishment of.. a es also contain late-stagegranitoid bodies. Such an associa- plate juncture system that would lead to the growth of tion occurs in the lower Kings River area where a 170 m.y. primitive volcanicarc constructioncenters and the opening clinopyroxeniticto dioritic plutoncuts the ophiolite,but is in of inter-arcbasins. Disruption of the 200and 160m.y, basio turn cut by a 157 m.y. quartz dioritic intrusive sheet(Figures systemsshortly following their formation points to an inher• 4 and 5). It has been suggestedthat the peridotitic to dioritic en,tinstability in suchbasins. However, the repetitionof intrusive complexes represent the plutonic remnants of nearly identical petrotectonicassemblages in the 200 and 160 island arc volcanic centers with overall calc-alkaline trends, m.y. basin systemssuggests that basinalframework 'destruc- but with a peculiar tendency to also erupt ankaramiticlavas tion can be an intra-orogenicprocess without the complete [Snoke et al., 1982]. Such considerationslead to the question reorganizationof the plate juncture system. of a modern plate tectonic analoguefor the production of Importantquestions arise concerning basinal framework ankaramite-rich volcanic sequencesin concert with the destructionfrom structural relations preserved along •both formation of polygeneticophiolitic assemblages. the 200 and 160 m.y. rift margins. In both the Kings Riyer ophiolite and Folsom dike swarm-PineHill complex intense POLYGENETIC OPHIOLITE AND PLATE TECTONICS protoclastic deformation of juvenile material accompanied The Sierra Nevada ophiolite belt and related basinal dynamothermal metamorphism of the adjacent wallrOck v0icanic-sedimentarysequences exhibit a repetitionin tec- complex. Such deformation regimes may mark major trans- tonic processesover a wide time span with a remarkable form segmentsalong the rift margins.Evidence suggesting degreeof consistency.At both200 m.y. and 160m.y. a pre- Jurassictransform motions along the California continental existingophiolitic basementcomplex was rifted apart with marginregime is reviewedby Saleeby[ 1980].Alternatively, theformation of juvenile ophiolitic crust, while the basement thesedistinct deformational regimes may indicatethat the was covered with basinal Volcanic-sedimentarysequences. edgesof normal rifts begin to undergocompressional defor- Both basinal sequenceswere characterizedby the eruption mations almost immediately following the onset of rifting. of ankaramitic volcaniclasticrocks. Furthermore, both sys- Sucha settingwould be idealfor the obductionof YoUng tems includedcomplex basement uplifts which added local- ophiolitesheets. Perhaps the easternand northernmargins ly-derived detritus and large slide masses to the basinal of the Smartvilleophiolite were thrusteastward ove{'it s strata.The 200 m.y. basin framework was disrupted during a boundingmetamorphic wall in responseto such a deforma- compressionalphase of orogenyprior to the developmentof tionalregime. The pointstressed here is thatthe basi n edges the 160m.y. framework and possiblyprior to the end of early underwent compressional-typedeformations at essentially Jurassictime. Within 10 to 15 m.y. after the formation of the the same time that the basin was growing, and shortly younger basin another compressionalorogenic phase dis- thereafterthe basinframework was disrupted by orOgenic rupted that framework. This phase is recognized as the pulses. Such an inherent instability may be a common ClassicalNevadan orogeny [Hinds, 1934]. Thus neither basin featurein smalloceanic-type plates generated along complex systempersisted'as a stable tectonic entity for a significantcontinental margin environments. time spanfollowing rifting and formationalstages. The closestmodem analoguethat can be recognizedfor CONCLUSIONS such a complex tectonic and petrogenetichistory is the Igneous assemblagesthat comprise the Sierra Nevada SolomonIslands system.This complexarc systemconsists ophiolite belt are po,lygeneticin origin. The amount of time of non-volcanicislands or remnant arc fragmentsexposing elaspedbetween the 300,200 and 160 m.y. igneousformation upper Cenozoic volcanic-sedimentarysequences and meta- stagesis significant.Complex sedimentation, deformation, morphosed mafic and ultramafic basement rocks [Coleman, metamorphismand erosionalprocesses modified the evOlv- 1970;Hackman, 1973]. Volcanic islandsof the New Georgia ingophiolite belt between each igneous stage. It is Clearthat Group sit in a frontal arc Positionand are characterizedby the Sierra Nevada ophiolite belt does not represent a frag- ankaramitic and picritic constructionalcenters [Stanton and ment•of open ocean crust and upper mantle. Perhapsthe 300 1822 $ALEEBY' POLYGENETIC OPHIOLITE GEOCHRONOLOGY AND TECTONICS m.y. assemblagerepresents the vestigesof such an ideal reviews of the manuscriptby R. G. Coleman,J. M. Mattinsonand ophiolite, but any clear evidenceof suchan origin has been G. E. Gehrelswere helpful.Support from NSF grantsEAR7708691 and EAR 7925998 is gratefully acknowledged.Contribution 3583 of obscured. the Division of Geological and Planetary Sciences,California Insti- The Sierra Nevada ophiolite belt representsan ensimatic tute of Technology. basementcomplex. Just as continentalbasement complexes / develop through an overprint history of the igneous, sedi- REFERENCES mentary and metamorphicprocesses of the ensialicdomain, Armstrong, R. 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Geol., 71, 721-758, 1963. Geol. Soc. Am. Abstracts Programs, 7, 352, 1975. Snoke, A. W., W. D. Sharp, J. E. Wright, and J. B. Saleeby, Morgan, B. A., Geology of Chinese camp and Moccasin quadran- Significance of mid-Mesozoic peridotitic to dioritic intrusive gles, Tuolumne County, California, Misc. Field StudiesMap MF- complexes, Klamath Mountains-Western Sierra Nevada, Califor- 840, scale 1:24,000, U.S. Geol. Surv., Reston, Va., 1976. nia, Geology, in press, 1982. Nestel, M. K., Permian Fusulinacean provinces in the Pacific Springer, R. K., Contact metamorphosedultramafic rocks in the northwest are tectonic juxtapositions of ecologically distinct western Sierra Nevada foothills, California, J. Pet., 15, 160-195, faunas (abstract), Geol. Soc. Am. Abstracts Programs, 12, 144, 1974. 1980. Springer, R. K., Geology of the Pine Hill intrusive complex, a Olmsted, F. 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Wright Uranium-lead 1824 SALEEBY: POLYGENETIC OPHIOLITE GEOCHRONOLOGY AND TECTONICS

iSotOpicages of the Semailophiolite, Oman, with applicationto phy of the Western United States, Paleogeogr. Syrup.2, edited by Tethyanocean ridge tectonics, J. Geophys.Res., 86, 2763-2775, D. E. Howell and K. A. McDougall, pp. 291-302, Society of 1981. Economic Paleontologistsand Mineralogists,Pacific Section, Los Wetherill, G. W., Discordant uranium-lead ages, 1, Eos Trans. Angeles, Calif., 1978. AGU, 37, 320-326, 1956. Xenophontos,C., and G. C. Bond,Petrology, sedimentation, and (Received March 5, 1981; pale0geographyof the Smartvilleterrane (Jurassic)--Bearing on revised October 22, 1981; the genesisof the Smartvilleophiolite, in MesozoicPaleogeogra- accepted November 13, 1981.)