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Geochemistry and Origin of Middle Miocene Volcanic Rocks from Santa Cruz and Anacapa Islands, Southern California Borderland Peter W

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Geochemistry and Origin of Middle Miocene Volcanic Rocks from Santa Cruz and Anacapa Islands, Southern California Borderland Peter W Geochemistry and Origin of Middle Miocene Volcanic Rocks from Santa Cruz and Anacapa Islands, Southern California Borderland Peter W. Weigand Department of Geological Sciences California State University Northridge, CA 91330 Cenozoic volcanism that began in the eastern Abstract - Major-oxide and trace-element Mojave Desert about 30 m.y. ago and swept compositions of middle Miocene volcanic rocks irregularly west and north. This extensive from north Santa Cruz and Anacapa Islands are extrusive activity was related closely to complex very similar. In contrast, they are geochemically tectonic activity that included subduction of the distinct from the volcanic clasts from the Blanca Farallon plate whose subduction angle was Formation, of similar age but located south of steepening, and interaction of the Pacific and the Santa Cruz Island fault, which implies North American plates along a lengthening significant strike-slip movement on this fault. transform boundary; this activity additionally The island lavas are also compositionally involved rotation and possible northward distinct from the Conejo Volcanics located translation of crustal blocks. onshore in the Santa Monica Mountains. The The origin of the volcanic rocks in this area island lavas are part of a larger group of about has been variously ascribed to subduction of the 12 similar-aged volcanic suites from the Farallon plate (Weigand 1982; Crowe et al. California Borderland and onshore southern 1976; Higgins 1976), subduction of the Pacific- California that all belong to the calc-alkaline Farallon spreading center (Dixon & Ferrar magma series. This group is interpreted to have 1980), mantle diapirisrn into a slab-free window originated in a subduction environment in one (Dickinson & Snyder 1980), extensional of two possible scenarios: 1) their magmas were melting at a migrating triple junction (Hurst produced from the subduction of the Cocos 1982), upper mantle and crustal dilation related plate south of the Rivera triple junction, which to the East Pacific Rise (Hawkins 1970) and would imply that the Baja-Borderland extension related to the formation of coastal allochthon onto which they were emplaced California Cenozoic basins (i.e., Crowell 1987). experienced northwestward translation A knowledge of the chemical composition of subsequent to eruption or 2) their magmas were the volcanic rocks may provide a better produced from warm, young subducted slab understanding of the specific tectonic located north of the Rivera triple junction, environment of their magma genesis. The which would imply that slab persisted in the chemical characterization of these lavas will aid Los Angeles area after the triple junction had also in the correlation between different migrated to tire south. volcanic areas. This information would be useful in placing constraints on amounts of northward translation, strike-slip offset and rotation experienced by adjacent crustal blocks. Introduction A better knowledge of the correlation between The volcanic rocks on Santa Cruz and volcanic units also might be useful in helping to Anacapa Islands (Fig. 1) are part of a larger direct exploration efforts for oil and gas in group of volcanic centers that erupted in the offshore southern California. Los Angeles and California Borderland areas For this project, I determined the major- about 13-18 m.y. ago. These areas represent oxide and trace-element concentrations in the culmination in southern California of samples from north Santa Cruz and Anacapa Third California Islands Symposium 21 andesite) and more felsic near the top (andesite differences in the stress regime at a triple and dacite). junction largely control the petrologic Crowe & co-authors (1976) studied the processes that operate. Locally variable and petrology and chemical composition of the SCI mild extension and compression at the Volcanics. They noted a slight iron-enrichment Mendocino triple junction resulted in magmas trend and a similarity in major-oxide dominated by crustal anatexis, whereas composition to island-arc tholeiites, and substantial extension at the Rivera triple suggested that they may have formed part of an junction produced rocks derived primarily by island-arc sequence that recorded Miocene crystal fractionation of mantle-derived basalt subduction off southern California. They with only limited crustal contamination. For speculated that the East Pacific Rise played a samples of SCI volcanics, they reported significant but unknown role in the generation 818O(magma) values of 6.7 to 7.3%o and of these magmas. Higgins (1976) included the 87Sr/86Sr(initial) ratios of 0.70253 to 0.70361. SCI Volcanics in his study of Miocene Figure 1. Sketch map showing some of the Miocene volcanic areas (lined) mentioned in the text. The Santa Cruz Island The SCI Volcanics are classified subalkaline Volcanics and Blanca Formation crop out respectively north and south of the Santa Cruz Island fault (SCIF), volcanic rocks volcanism in the greater Los Angeles Basin area on a SiO2-alkalis plot and calc-alkaline on the comprise all of Anacapa Island, and the Conejo Volcanics and Zuma Volcanics crop out respectively north and south of the and supported the conclusion that this basis of AFM ratios (Crowe et a!, 1976, Fig.2; Malibu Coast fault (MCF). volcanism was probably related to active Higgins 1976, Fig. 3). They also are classified Islands in order to 1) evaluate possible subduction. Higgins also noted that the SCI Canyon Member consists of flows and as calc-alkaline according to the Si02- Volcanics 1) lack a distinctive iron-enrichment correlations between the two lava sequences epiclastic volcanic breccias of basaltic and FeO/MgO criterion of Miyashiro (1974) and and other nearby volcanic units of similar age trend on an AFM diagram, 2) are generally andesitic composition that were deposited in a the Al203-normative plagioclase criterion of and 2) evaluate the origin of the lavas and the higher in K20 man are the Conejo Volcanics subaerial environment. The overlying Stanton Irvine & Baragar (1979). Silica-K20 SCI Volcanics and 3) are both calc-alkaline and tectonic setting of their emplacement. Ranch Member is composed of andesite flows, relationships show that these rocks are tholeiitic on the basis of Si02-K20 flow breccias and subordinate tuff breccias relationships. predominantly medium-potassium andesite Previous Work erupted on the flanks of a volcanic edifice. Next using the scheme of Gill (1981, Fig. 1.2), which Hurst & co-authors (1982) and Hurst (1983) Santa Cruz Island: The volcanic sequence younger is the Devil's Peak Member is in agreement with petrographic estimates containing a variety of scoriaceous andesitic included the SCI Volcanics in their studies of (Nolf & Nolf 1969; Crowe et al. 1976). exposed north of the Santa Cruz Island fault Miocene volcanism in the Southern California was first correlated with the Conejo Formation and dacitic flows, flow breccias, and reworked Turner (1970) reported K/Ar age dates of pyroclastic rocks. These rocks were apparently Borderland. They reviewed the available 16.1 ± 0.9 Ma on a sample probably from the (now the Conejo Volcanics) of the western geochemical data and presented Sr-isotopic emplaced on the slopes of, and adjacent to, a upper part of the Devil's Peak Member, and Santa Monica Mountains on the basis of data for 10 samples of SCI Volcanics. Initial volcanic center. The uppermost Prisoner's 16.5 ± 0.8 Ma on a sample probably from the lithologic similarities and stratigraphic position 87Sr/86Sr ratios range from 0.70253 to 0.70318, Harbor Member is composed of andesitic and middle of the Prisoner's Harbor Member (Kleinpell 1938). The volcanic section was overlap the range of Leg 63 DSDP samples renamed the Santa Cruz Island (SCI) Volcanics dacitic flows, flow breccias and tuffaceous (these and subsequent dates have been from the California Borderland (Pal Verma corrected to new IUGS constants). Crowe & and subdivided into four members by Nolf & volcaniclastic beds, probably deposited in a 1981), and are generally lower than those from co-authors (1976) reported dates of 16.0 ± 0.7 Nolf (1969). It forms a north-dipping submarine environment. Basalt, andesite and Conejo or Santa Catalina Island volcanic rocks. Ma and 19.9 ± 0.9 Ma on the same sample from homocline composed of interbedded flows, dacite are most commonly porphyritic and all Hurst & co-authors (1982) concluded on the a dike cutting the lower part of the Devil's flow breccias and volcaniclastic rocks that have contain phenocrysts of plagioclase, opaque basis of a variety of petrographic, chemical and Peak Member. The lower part of the sequence a cumulative thickness of 2,400 m and that are oxides and variable amounts of augite, isotopic evidence that the SCI Volcanics has not been dated. San Onofre Breccia of cut by numerous shallow intrusions at an hypersthene and pigeonite. An-contents of exhibit a mixture of tholeiitic and calc-alkaline probable Saucesian age was encountered inferred eruptive center near Devil's Peak. The plagioclase decrease from An65 to An28 in the characteristics, and resulted from mantle-ridge beneath the volcanics in the Union Oil Gemini formation overlies inferred San Onofre Breccia three rock types. Basalt contains areas of melting due to complex ridge-trench-transform No. 1 well drilled at the east end of the island in the subsurface and is overlain by the alteration (iddingsite) presumed to be interactions in the vicinity of the Rivera triple (Weaver & Meyer 1969), which probably limits Monterey Formation. pseudomorphic after olivine, and dacite junction. contains variable amounts of highly altered the onset of volcanism to less than 23-25 Ma The following brief summary of stratigraphy Johnson & O'Neil (1984) measured trace- (Crouch & Bukry 1979). hornblende. The sequence is mafic near the and petrography is from Nolf & Nolf (1969), element abundances and isotopic ratios of Sr Anacapa: Being nearly entirely volcanic, the base (basalt and andesite), intermediate in with additional information from Crowe & co- and O in numerous Neogene volcanic centers geology of Anacapa Island is much simpler than composition in the middle members (largely authors (1976).
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