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Polygenetic Ophiolite Belt of the California Sierra Nevada' Geochronologicaland Tectonostratigraphicdevelopment

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Polygenetic Ophiolite Belt of the California Sierra Nevada' Geochronologicaland Tectonostratigraphicdevelopment 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 together these 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
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