REGIONAL SWARM EMPLACEMENT OF SILICIC ARC IN THE PENINSULAR RANGES BATHOLITH: THE SAN MARCOS DIKE SWARM (SMDS) OF NORTHERN BAJA CALIFORNIA Phil FARQUHARSON (presenter), David L. KIMBROUGH, and R. Gordon GASTIL, Department of Geological Sciences, San Diego State University

Rancho San Marcos A densely intruded, northwest-striking, predominantly silicic regional dike The SMDS occurs entirely within the western province of the PRB, which is charac- swarm is exposed over an approximately 100 km-long segment in the terized by gabbro-tonalite-granodiorite plutons with primitive island arc geochemical west-central portion of the Cretaceous Peninsular Ranges batholith (PRB) affinities (DePaolo, 1981; Silver & Chappell, 1988; Todd et al., 1988, 1994), and in northern Baja California. Dike compositions range from basalt to rhyo- Rancho El Campito U/Pb zircon ages of 120-100 Ma. The extent of the swarm is shown schematically lite and are locally strongly bimodal. The swarm is intruded into two main N on the Gastil et al. (1975) 1:250 000 map of Baja California. units; 1) Triassic-Jurassic (?) turbidite flysch and 2) older, presumably pre- The swarm is intruded into two main units; 1) Triassic-Jurassic(?) turbidite flysch of 120 Ma batholithic rocks. Cross-cutting field relationships and a prelimi- the Rancho Vallecitos Formation (Reed, 1993) that is correlated to Julian Schist and nary U-Pb zircon age of 120±1 Ma clearly establish the swarm as an inte- Middle Jurassic Bedford Canyon Formation north of the border, and 2) older, pre- gral feature in the magmatic evolution of the PRB. Surprisingly, despite Agua Blanca Fault sumably pre-120 Ma batholithic rocks for which little data is currently available. spectacular exposure of the swarm in easily accessible regions of the Low-grade greenschist facies of the Rancho Vallecitos Formation in the northwest PRB, as well as associated gold mineralization in the southern part of the 0 25 Km Tres Hermanos area of the swarm indicate shallow emplacement depths. Significantly deeper crust- swarm that has been mined off and on for over 100 years, this prominent al levels are exposed at Tres Hermanos where amphibolite facies schist occurs feature of the PRB is virtually undescribed in the literature. El Alamo (Chadwick, 1987). Cenozoic As shown on the Gastil et al. (1975) map, and supported by additional data present- Unique characteristics of the dike swarm provide important opportunities Peninsular Ranges Batholith ed below, dikes within the swarm parallel the overall trend of the swarm, which in to address two distinct classes of problems: ascent mechanisms of grani- Turonian-Maastrichtian turn is parallel to the overall ~N30°W structural grain of the PRB. At it's northern toid magma through , and regional tectonic/stratigraphic forearc deposits (modified from Gastil et. al. 1975) end, the dike swarm is intruded by younger ~120-100 Ma PRB intrusions. At it's studies of the PRB using the swarm as a strain/temporal marker. Recon- Batholithic Intrusions southern end, it is partly intruded by younger plutons and partly blanketed by Aptian- Lakeside naissance data on dike attitudes from two widely separated areas of the Map Albian supracrustal volcanic sequences, which form a nearly continuous ~10-30 km- Aptian-Albian supracrustal Location USA dike swarm suggest a regionally consistent approximately N30°W strike volcanic & sedimentary rocks MEXICO wide belt of volcanic rocks along the western margin of the PRB throughout its and 75°NE dip. The dike attitudes are consistent with a common west- Pre-batholithic units ~800-km long extent. (Tr-Jr turbidite flysch) ward tilt on the order of 15° about the N30°W longitudinal axis of the PRB. San Telmo The exposure of the SMDS within a restricted ~100 km-long segment of the 800 km- The SMDS may present clear structural evidence in support of hypothe- San Marcos Dike Swarm long PRB appears to be the result of two fortuitous circumstances; 1) the relative sized regional tilting in the PRB, hence allowing for the mechanics and Catavina paucity of ~120-100 Ma intrusions in this segment which elsewhere heavily intrude timing of this process to be understood. Localities Sampled the western zone of the PRB (e.g. Silver and Chappell, 1988; Kimbrough et al., in El Arco review.), and 2) erosional stripping of the extensive Aptian-Albian supracrustal vol- canic rocks from this region to expose a deeper structural level relative to areas along strike to the north and south. Dikes potentially correlative to the main ~100 km-long swarm occur to the north in the Lakeside area of southern San Diego County, and to the south near San Telmo, Catavina, and at El Arco near the southernmost extent of the PRB (see inset map to Fig. 1). These possible correlatives suggest the dike swarm may have been much more extensive than its present exposure suggests.

Intrusion of felsic batholiths in continental margin magmatic arcs is a fundamental process of continental growth. Partial melting, segregation, and ascent mechanisms of felsic magma however remain poorly understood mechanical aspects of this process. The uncertainties are highlighted by the current debate between diapir versus diking ascent mechanisms which View northeast toward densely intruded northwest-striking dike swarm exposed above began in the 1980's when several investigators concluded that the crust was too viscous to Rancho San Marcos. Hundreds of individual rhyolite and andesite dikes, locally sheeted, are exposed across the 460 meter-high skyline ridge in the distance. Rancho Vallecitos allow diapirs to rise fast, and thus would freeze after travelling only short distances (Petford, Fm turbidite flysch is hosting the swarm in this view. The most prominent dikes visible 1996). Conversely, the viscosity of felsic appeared too high to permit significant here are 5-10 m wide high-silica rhyolite dikes that are exposed along strike continuously upward propagation without freezing, a view strengthened by the observation that granitic for up to 3 km distance. composition dike swarms are rare compared with mafic swarms (Weinberg, 1996). Recent modeling and theoretical considerations now suggest that dike transport of silicic magma is an efficient, and perhaps dominant mechanism for large-scale transfer of silicic magma in the crust (Clemens & Mawer, 1992, Petford et al., 1994, Lister, 1995). Rapid ascent of silicic melts may allow them to rise adiabatically and become superheated, leading to resorption of entrained material and a reduction in effective viscosity that would accelerate ascent (Holtz & Johannes, 1994; Clemens et al. 1996). Weinberg (1996) concludes that the ideal condition for initiation of silicic dikes is in strongly extentional environments, where melt segregation produces a dense network of veins that drains within the source into a few high pressure dikes; and further, that intermediate to felsic magmas may start their ascent initially as diapirs but swap to dikes as they slow down when reaching stiffer rocks. Understanding of felsic magma ascent via diking is still badly hampered however by a paucity of actual examples where observational data can be made to constrain the various mecha- nisms that might enhance or inhibit dike transport. Here we outline a multidisciplinary investi- gation of a densely intruded bimodal but predominantly silicic regional dike swarm that is exposed over a ~100 km-long segment of the Peninsular Ranges batholith (PRB) in northern Baja California (Fig.1). This dike swarm is informally referred to here as the San Marcos Dike Swarm (SMDS) after easily accessible exposures near Rancho San Marcos in the northern Numerous dikes exposed in the Rancho El Campito area, on the northeast margin of the part of the belt (Fig. 2). Surprisingly, despite the PRB's status as one of the most intensively Ojos Negros valley. Note the granitoid dikes that have been cross-cut by the rhyolite dike.. studied Cordilleran batholiths, the SMDS has never been a focus of investigation, apart from interest in gold mineralization that is likely hosted by the dikes at El Alamo (Fig. 1) and areas farther north in the swarm that have been mined off and on dating back to the late 19th Centu- ry. 140 PENINSULAR RANGES BATHOLITH 130 Western Province Eastern Province

120 Strike and dip measurement of dike attitudes from the San Marcos, El Campito and Tres Hermanos areas are depicted below on lower hemisphere equal area stereonet plots. Trend and plunge of mean poles to dikes from each area are virtually identical and indi- 110 cate a strike of ~N30°W and a dip of 75°NE. As discussed earlier, if the dikes were emplaced originally as vertical sheets, this result indicates a consistent SW-directed tilt of 15° through this area consistent with the Butler et al. (1991) regional tilt hypothesis. Many 100 more dike attitudes however must be measured from different areas of the SMDS to con- firm this result.

Zircon U/Pb Age (Ma) Age U/Pb Zircon Silver & Chappell (1988) 90 Kimbrough et al. (in prep.) Walawender et al. (1990) 80 0 25 50 75 100 125 150 Horizontal Distance Across Batholith (km) SMDS Dikes are approximately Dikes Attitudes coeval with the start-up of San Marcos Region PRB magmatic activity

0.0204

Rancho San Marcos n = 66 128 equal area rhyolite dike 0.0200 Dike Attitudes 120±1 Ma 126 Rancho El Campito - Ojos Negros Reconnaissance whole rock analyses have been obtained from 18 SMDS samples from the Ran- cho San Marcos and Tres Hermanos regions (Table 1). Samples were chosen to represent volu- metrically abundant dikes in each area as well as the full compositional spectrum occurring in each 0.0196

U 124 area. Dike compositions from the two areas vary significantly. Rancho San Marcos dikes are dominated by rhyolite and andesite along with subordinate basaltic andesite. Dikes in the Tres

238 Hermanos region in contrast comprise a strongly bimodal basalt-rhyolite suite. The difference may / 0.0192 122 n = 10 relate to differences in depth of exposure in these two regions. Rhyolites in both regions neverthe- Pb Intercepts at equal area less are chemically similar, and resemble Late Tertiary high-silica rhyolite from bimodal volcanics fields in the Basin & Range/ Plateau region (Moyer & Nealey, 1989).

206 748±99 and 120±1 Ma 120 0.0188 (MSWD = 1.8) Data from the SMDS samples are displayed in a SiO2 vs. K2O plot that suggests medium-K calc- Tres Hermanos alkaline affinities typical of orogenic plate settings. Dikes with unusually high K2O contents may be fraction indicative of potassium metasomatism. A pronounced silica gap occurs between ~63-75%. The two exceptions to this are samples TH947 (71.6%) and TH975 (70%) which were both collected 0.0184 from composite dikes (cf. Snyder et al., 1997) that occur as ~8-meter thick rhyolite dikes with 0.123 0.127 0.131 0.135 basalt-rich enclave zones in their central portions. Magma mixing/mingling therefore may have trend & plunge of mean pole to dikes lowered SiO2 contents of rhyolite in these dikes. 207Pb/235U 242.1° 14.9° Subalkaline arc-related volcanic rocks of Aptian-Albian volcanic rocks to the west are chemically similar to SMDS samples, and display a silica gap from 63 to 67% (Herzig, 1991; Kimbrough et al., A preliminary U/Pb zircon age of 120±1 Ma has been obtained from a 6-meter in review). wide rhyolite dike in the densely intruded Rancho San Marcos region of the swarm (Figs. 2). A single fraction from an 8-meter wide rhyolite dike in the Dike compositions in the Independence Dike Swarm also tend to be bimodal and vary along the Tres Hermanos region ~60 km to the south displays virtually identical U/Pb length of the swarm from the northern Sierra to southeastern California (Carl et al., 1998). systematics to the Rancho San Marcos sample although additional data is Mafic Independence dikes in the central Sierra Nevada yield Late Cretaceous ages and may be necessary to confirm this age. trend & plunge of mean pole to dikes related to Late Cretaceous mafic plutons in the region (Coleman et al., 1994). The voluminous The reconnaissance U/Pb data provides significant support to the idea that 237° 16.6° high-silica rhyolites of the SMDS however, is a feature that distinguished it from the predominantly rhyolite dikes which dominate the swarm were emplaced in a relatively brief Rancho San Marcos fine-grained diorite porphyry to granodiorite dikes of the Independence swarm (Moore and Hopson, igneous episode. Interestingly, the preliminary 120±1 Ma zircon age from the 1961). dikes coincides closely with the inception of voluminous intrusive activity in the PRB which ranges from ~120-90 Ma (Silver and Chappell, 1988; Kimbrough et al., in review.). Dike Attitudes Tres Hermanos Region Rancho El Campito N

n = 10 Agua Blanca Fault equal area

0 25 Km Tres Hermanos

El Alamo Cenozoic

Peninsular Ranges Batholith

Turonian-Maastrichtian forearc deposits (modified from Gastil et. al. 1975)

Silicic dikes in the SMDS range tremendously in texture from glassy aphyric to Batholithic Intrusions Lakeside porphyritic to medium-grained hypabyssal plutonic. Magmas with phenocryst Map Aptian-Albian supracrustal Location USA contents more than about 50-60 vol% will have a rheology controlled by defor- volcanic & sedimentary rocks MEXICO mation rather than magma flow; phenocryst-rich magmas therefore have viscosi- trend & plunge of mean pole to dikes ties too high to be intruded as dikes (Wada, 1994; Kerr & Lister, 1995). This Pre-batholithic units 238.4° 16.2° (Tr-Jr turbidite flysch) indicates that granitic textures in silicic dikes in the SMDS must reflect in-situ San Telmo crystallization of magma after dike emplacement. Flow-banding in some rhyolite dikes in the Rancho San Marcos region strongly San Marcos Dike Swarm resemble eutaxitic textures in welded tuffs suggesting that dike emplacment here Catavina was shallow enough to allow for violent vesciculation of magma within the dike Localities Sampled conduit at present levels of exposure. Local evidence of rheomorphic deforma- El Arco tion in flow-banded rhyolite dikes may represent late stage flow features as magma transport comes to a halt in the conduit. Rubin (1995) calculates that dikes can begin to vesciculate at 4 km depth in a 10 m dike supporting these ten- tative interpretations. Mafic dikes in general have much more uniform aphyric to sparsely porphyritic textures. Chill margins on basaltic-andesite dikes in the San Marcos region are common. Butler, R.F., Dickinson, W.R., and Gehrels, G.E., 1991, Paleomagnetism of coastal California and Baja California: alternatives to large-scale northward transport: Tectonics, v. 10, p. 561-576. Carl, B.S., Glazner, A.F., Bartley, J.M., Dinter, D.A., Coleman, D.S., 1998, Independence dikes and mafic rocks of the eastern Sierra: Guidebook to Field Trip #4, 94th Annual Meeting, Cordilleran Section of the Geological Society of America, Pub- lished by CSU Long Beach, Dept of Geological Sci., 26p. Chadwick, B., 1987, The , petrography, geochemistry, and geochronology of the Tres Hermanos-Santa Clara region, Baja California, Mexico. M.Sc. thesis, San Diego State University library. Clemens, J.D., and Mawer, C.K., 1992, Granitic magma transport by fracture propagation: Tectonophysics, v. 246, p. 339-360. Clemens, J.D., Petford, N., and Mawer, C.K., 1996, Ascent mechanisms of granitic ascent: causes and mechanisms. Min. Soc. Special Pub. Coleman, D.S., Bartley, J.M., Glazner, A.F,.and Carl, B.S., 1994, Late Cretaceous dikes in the Independence swarm, California: EOS, Trans. Amer. Geophys. Union, v. 75, p. 686. DePaolo, D.J., 1981, A neodymium and strontium isotopic study of the Mesozoic calc-alkaline granitic batholiths of the Sierra Nevada and Peninsular ranges, California: Journal of Geophysical Research, v. 86, p. 10470-10488. Dunning, G.R., and Hodych, J.P., 1990, U/Pb zircon and baddeleyite ages for the Palisades and Gettysburg sills of the northeastern United States: Implications for the age of the Triassic/Jurassic boundary: Geology, v. 18, p. 795-798. Gastil, R. G., 1993, Prebatholithic history of Peninsular California, in Gastil, R. G., and Miller, R. H., eds., The prebatholithic stratigraphy of Peninsular California: Geological Society of America Special Paper 279, p. 145-156. Gastil, R. G., Phillips, R. P., and Allison, E. C., 1975, Reconnaissance geology of the state of Baja California including reconnaissance geologic map of the state of Baja California: Geological Society of America Memoir 140, 170 p. Halls, H.C., Fahrig, W.F., eds., 1987, Mafic Dyke Swarms, Geological Association of Special Paper 34, 503pp. Herzig, C.T., 1991, Petrogenetic and tectonic development of the Santiago Peak Volcanics, northern Santa Ana Mountains, California: Ph.D. dissertation, University of California, Riverside, 376pp. Holtz, F., and Johannes, W., 1994, Maximum and minimum water contents of granitic melts: implications for chemical and physical properties of ascending magmas: Lithos, v. 32, p. 149-159. Karson, Jeffrey A., et al., 1992, Tectonic rotations of dikes in fast-spreading oceanic crust exposed near Hess Deep, Geology, v. 20, No. 8, p. 685-688. Kerr, R.C., and Lister, J.R., 1995, Comment on "On the relationship between dike width and magma viscosity" by Yukata Wada: Journal of Geophysical Research, v. 100, p. 15,541. Kimbrough, D.L., Herzig, C.T., Anderson, C.A., Reed, B.C., Carrasco, A., Taylor, M., [in review], U-Pb and 40Ar-39Ar geochronology and petrology of the western Peninsular Ranges batholith and associated volcanic rocks, in Todd, V.R. & Kimbrough, D.L., eds., Intrusive rocks of the Peninsular Ranges batholith, Geological Society of America Special Publication. Krogh, T.E. & 8 others, Precise U-Pb isotopic ages of dykes and mafic to ultramafic rocks using trace amounts of baddeleyite and zircon: in Halls, H.C. and Fahrig, W.F.eds., Mafic Dyke Swarms, Geological Association of Canada Special Paper 34, p. 147-152. Leeman, W.P., and Fitton, J.G., 1989, Magmatism Associated with Lithospheric Extension: Introduction: Journal of Geophysical Research, v. 94, p. 7682-7684. Le Maitre, R.W., (ed.), 1989, A classification of Igneous Rocks and Glossary of Terms, Blackwell, Oxford, 193pp. Lister, J.R., 1995, Fluid-mechanical models of the interaction between solidification and flow in dykes: in Baer, G., and Heimann, A., eds., Physics and chemistry of dykes, Rotterdam: Bakema, p. 115-124. Lothringer, C.J., 1993, Allochthonous Ordovician Strata of Rancho San Marcos, Baja California: in Gastil R. G., and Miller, R. H., eds., The Prebatholithic Stratigraphy of Peninsular California, Geological Society of America Special Paper 279, p. 11- 22. Lund, S.P., and Bottjer, D.J., 1991, Paleomagnetic evidence for microplate tectonic development of southern and Baja California, in J.P. Dauphin and B.R.T. Simoneit, eds., The Gulf and Peninsular Province of the Californias, AAPG Memoir 47, p. 231-248. Moore, J. G., and Hopson, C. A., 1961, The Independence Dike Swarm in eastern California: American Journal of Science, v. 259, p. 241-259. Moyer, T.C., and Nealey, L.D., 1989, Regional Compositional Variations of Late Tertiary Bimodal Rhyolite Lavas Across the Basin and Range/Colorado Plateau Boundary in Western Arizona: Journal of Geophysical Research, v. 94, p. 7799-7816. Petford, N., 1996, Dykes and diapirs?: in Brown et al., eds., The Third Hutton Symposium on the Origin of Granites and Related Rocks, Geological Society of America Special Paper 315, p. 105-114. Petford, N., Kerr, R.C., and Lister, J.R., 1994, The ascent of felsic magmas in dykes: Lithos, v. 32, p. 161-168, Pollard, D. D., 1987, Elementary Fracture Mechanics Applied to the Structural Interpretation of Dykes, in Hall, H.C., and Fahrig, W. F., eds., Mafic Dyke Swarms, Geological Association of Canada Special Paper 34, p. 5-24. Reed, J., 1983, Rancho Vallecitos Formation, Baja California Norte, Mexico: in Gastil R. G., and Miller, R. H., eds., The Prebatholithic Stratigraphy of Peninsular California, Geological Society of America Special Paper 279, p. 119-134. Ron, H., and Nur, A., 1996, Vertical axis rotations in the Mojave: Evidence from the Independence dike swarm: Geology, v. 24, p. 973-976. Rubin, A.M., 1995, Getting granite dikes out of the source region: Journal of Geophysical Research, v. 100, p. 5911-29. Silver, L.T., and Chappell, B., 1988, The Peninsular Ranges batholith: an insight into the Cordilleran batholiths of southwestern North America: Transactions of the Royal Society of Edinburgh, Sciences, v. 79, p. 105-121. Snyder, D., Crambes, C., Tait, S., and Wiebe, R.A., 1997, Magma mingling in dikes and sills: Journal of Geology, v. 105, p. 75-86. Tarling, D.H., and Hrouda, F., 1993, The magnetic anisotropy of rocks: Chapman & Hall, pp. 217. Todd, V.R., Erskine, B.G., and Morton, D.M., 1988, Metamorphic and Tectonic Evolution of the northern Peninsular Ranges batholith, southern California: in Ernst, W.G., ed., Metamorphism and Crustal Evolution of the Western United States, Rubey Volume VII, Prentice Hall, p. 894-937. Todd, V.R., Kimbrough, D.L., and Herzig, C.T., 1994, The Peninsular Ranges Batholith from Western Volcanic Arc to Eastern Mid-Crustal Intrusive and Metamorphic Rocks, San Diego County, California: in McGill, S.F., and Ross, T.M., eds., Geologi- cal Investigations of an Active Margin: Geol. Soc. Am. Cordilleran Section Meeting Guidebook, p. 227-235. Turcotte, D.L., Emerman, S.H., and Spence, D.A., 1987, Mechanics of dyke injection: in Halls, H.C. and Fahrig, W.F.eds., Mafic Dyke Swarms, Geological Association of Canada Special Paper 34, p. 25-30. Wada, Y., 1994, On the relationship between dike width and magma viscosity: Journal of Geophysical Research, v. 99, p. 17,743-17,755. Weinberg, R.F., 1996, Ascent mechanism of felsic magmas: new and views: in Brown et al., eds., The Third Hutton Symposium on the Origin of Granites and Related Rocks, Geological Society of America Special Paper 315, p. 95-104.