Tectonic history of a Jurassic backarc-basin sequence (the Gran CanÄon Formation, Cedros Island, Mexico), based on compositional modes of tuffaceous deposits Salvatore Critelli* Dipartimento di Scienze della Terra, UniversitaÁ degli Studi della Calabria, 87036 Arcavacata di Rende (CS), Italy Kathleen M. Marsaglia² Department of Geological Sciences, California State University, Northridge, California 91330-8266, USA Cathy J. Busby§ Department of Geological Sciences, University of California, Santa Barbara, California 93106, USA ABSTRACT ancient volcaniclastic deposits suspected of that re¯ect eruption style, in addition to those having formed in backarc-basin settings. that relate to magma composition. The modal The Jurassic Gran CanÄon Formation analysis of textures makes use of descriptive (Cedros Island, Baja California, Mexico) Keywords: backarc basins, Baja California, classes of shard morphology commonly re- constitutes an unusually well preserved and Jurassic, magmatic arcs, Mexico, prove- ferred to by volcanologists (e.g., Heiken and exposed example of ancient backarc-basin nance, sandstone petrology, volcanology. Wohletz, 1985) in SEM (scanning electron mi- ®ll. Petrofacies analysis conducted on tuff- croscope) studies and only recently quanti®ed INTRODUCTION aceous sandstone and tuff samples from in modal analysis of lithi®ed rocks by De this formation complement and reinforce Rosa (1999). The modal analysis of magmatic Few detailed studies have been conducted prior lithofacies interpretations, but with components to determine their composition on ancient, uplifted, intraoceanic backarc- was developed by sedimentary petrologists some modi®cation. When temporal and basin assemblages, probably because of their spatial trends in petrographic data (detrital (e.g., Dickinson, 1970; Ingersoll and Cavazza, intraoceanic setting and poor preservation dur- 1991; Marsaglia, 1992, 1993; Critelli and In- modes) are analyzed and compared to mod- ing subduction and terrane accretion. The vol- gersoll, 1995; Marsaglia and Devaney, 1995) els based on data collected from Deep Sea caniclastic ®ll of a late Middle Jurassic back- to help ®ngerprint ma®c, intermediate- Drilling Project and Ocean Drilling Pro- arc basin on Cedros Island (Baja California) composition, and felsic components and their gram cores, the trends indicate a second, (Fig. 1), the Gran CanÄon Formation, is re- relative proportions in reworked tuffaceous heretofore unrecognized, phase of backarc markable for its low degree of structural mod- sediments. To quantify these parameters in rifting. Basalt lavas interstrati®ed with da- i®cation, its low to moderate alteration and combination provides a unique approach to in- citic pyroclastic rocks of the primary vol- metamorphism, and its unusually good expo- terpreting the provenance of pyroclastic sedi- canic lithofacies, previously interpreted to sure. Detailed facies analysis of this backarc ments deposited in marine settings, where re- record the eruption of differentiated mag- basin was previously conducted by Busby- mas at the climax of growth of the Gran Spera (1987, 1988). In this paper, we present working and mixing can be common CanÄon island arc, are now as a result of this new petrographic data from the Gran CanÄon processes. study considered to be the product of arc Formation and compare these with petro- Results from the Deep Sea Drilling Project extension and rifting. graphic data from modern backarc basins We (DSDP) and Ocean Drilling Program (ODP) Our method of modal analysis uniquely re®ne the earlier-published model for the tec- have greatly improved our understanding of combines the quanti®cation of textural at- tonic evolution of the Jurassic arc-backarc backarc-basin facies relationships and sedi- tributes of pyroclastic and epiclastic debris system on Cedros Island. In this re®ned mod- ment provenance (Karig, 1983; Taylor and that re¯ect eruption style and magma com- el, we infer that a second phase of arc rifting Karner, 1983; Klein and Lee, 1984; Klein, position, as well as the effects of reworking has been recorded in the backarc-apron se- 1985; Nishimura et al., 1991; Klaus et al., and mixing in marine settings. This study quence (subsequent to the phase that formed 1992; Tappin et al., 1994; Arculus et al., 1995; demonstrates that detailed petrographic the basin). The second rifting event resulted Cambray et al., 1995; Clift, 1995; Clift and analysis is useful in the interpretation of in conversion of the active backarc basin into ODP Leg 135 Scienti®c Party, 1995; Haw- a remnant backarc basin. kins, 1995; Marsaglia, 1995; Marsaglia et al., 1995). However, these widely spaced drill *E-mail: [email protected]. Our petrographic methods include modal ²E-mail: [email protected]. analysis designed to quantify the textural at- holes provide only a limited view of largely §E-mail: [email protected]. tributes of pyroclastic and epiclastic debris submerged, tectonically complex basins, leav- GSA Bulletin; May 2002; v. 114; no. 5; p. 515±527; 9 ®gures; Data Repository item 2002059. For permission to copy, contact [email protected] q 2002 Geological Society of America 515 CRITELLI et al. which appear to form in 10 m.y. or less (Tay- lor and Karner, 1983). METHODS Volcaniclastic Terminology Volcaniclastic units in the Gran CanÄon For- mation are composed of sand pyroclasts and matrix that are inferred by their textural attri- butes and sedimentary structures to have been transported and deposited in a submarine en- vironment by turbidity currents. If a classi®- cation scheme based on transport and depo- sitional processes (McPhie et al., 1993) is used, these samples would be called sand- stones. According to a classi®cation scheme based on the origin of the particles (Fisher and Schmincke, 1984), however, these samples would be called tuffs because they are mostly composed of compositionally homogeneous pyroclasts that show little to no textural mod- i®cation. Therefore, as described in the pre- vious section and as shown in Figure 2, these deposits are referred to as tuffs, but for pur- poses of comparison with modern backarc- basin deposits described in the literature, they are referred to as tuffaceous sandstones. Petrographic and Modal Analysis We selected 32 medium- to coarse-grained Figure 1. Generalized geologic map of Cedros Island (modi®ed from Kilmer, 1977, 1984) tuffaceous sandstone samples from the Gran and location of the measured sections shown in Figure 2. CanÄon Formation for thin-section preparation and modal analysis. These cover the entire formation in both proximal and distal sections ing much to learn about facies and deposi- ation was immediately followed by progra- (Fig. 2; Table DR11 presents ®eld description tional processes in these settings. Our new dation of a deep-water pyroclastic apron into of samples used for petrographic study and petrographic data complement the data set the backarc basin, contemporaneous with the lithofacies subdivision of Gran CanÄon For- from modern basins by documenting the evo- growth of an oceanic arc from deep water to mation). We also examined thin sections of lution of a backarc apron in a more arc- sea level or above. Busby-Spera (1987, 1988) lithic blocks from the tuff breccias, as well as proximal setting than most of the drill sites in divided this pyroclastic apron into tuff, lapilli thin sections of basalt lava ¯ows, to charac- the western Paci®c. Proximal coarse-grained tuff±tuff breccia, and primary volcanic litho- terize better the mineral assemblages and tex- volcaniclastic packages, such as the Gran CanÄ- facies (Fig. 2). Whole-rock analyses and mi- tures of smaller fragments within the tuffa- on Formation, provide more detailed infor- croprobe studies show that the basalts in the ceous sandstones. Some thin sections were mation on the tectonic and volcanic evolution primary volcanic lithofacies are chemically etched and stained for plagioclase and potas- of an intraoceanic arc during backarc-basin and petrographically homogeneous tholeiitic sium feldspar. formation (Marsaglia and Devaney, 1995). basalts that are distinctly more alkalic than The tuffaceous sandstone samples were tholeiitic rocks of the ophiolitic or arc base- point-counted through the use of a petrograph- GEOLOGIC AND STRATIGRAPHIC ic microscope equipped with an automated SETTING OF THE GRAN CANÄ ON ment to the Gran CanÄon Formation (Kim- stage. Five hundred points were counted for FORMATION brough, 1982, 1984). The pyroclastic apron was then blanketed by a relatively thin sheet each sample by using the Gazzi-Dickinson method (Ingersoll et al., 1984; Zuffa, 1985, A rifted-arc and ophiolite assemblage and of siltstone to ®ne-grained sandstone turbi- 1987). Grain parameters (Zuffa, 1985, 1987; overlying volcaniclastic rocks, all of late Mid- dites (``epiclastic lithofacies''; Fig. 2), inter- Critelli and Le Pera, 1994) are de®ned and dle Jurassic age, represent a fragment of the preted to record abrupt extinction and erosion, arc side of a backarc basin, now exposed on but no uplift, of the island arc within 10 m.y. 1 of ophiolite generation. This cycle was in- GSA Data Repository item 2002059, Tables Cedros Island in Baja California (Kilmer, DR1±DR4, is available on the Web at http:// 1977, 1984; Kimbrough, 1984; Busby-Spera, ferred to re¯ect the episodic nature of pro- www.geosociety.org/pubs/ft2002.htm. Requests 1987, 1988; Fig. 1). Backarc ophiolite gener- cesses in backarc basins (Busby-Spera, 1988), may also be sent to [email protected].