The Recent Isla San Luis volcanic centre: petrology of a rift-related volcanic suite in the northern Gulf of California, Mexico

Journal of Volcanology and Geothermal Research 93 Ž1999. 31–52 www.elsevier.comrlocaterjvolgeores

The Recent Isla San Luis volcanic centre: petrology of a rift-related volcanic suite in the northern Gulf of California, Mexico Francisco A. Paz Moreno b

a,)

, Alain Demant

b,1

a UniÕersidad de Sonora, Departamento de Geologıa, ´ Apdo. Postal 847, 83000 Hermosillo, Sonora, Mexico ´ Laboratoire de Petrologie Magmatique, UniÕersite´ Aix-Marseille III, Case 441, 13397 Marseille Cedex 20, France ´

Received 28 January 1998; accepted 9 July 1998

Abstract Isla San Luis, one of the most recent eruptive centres in the Gulf of California, presents a complete magma evolution trend from basaltic andesites to rhyolites. The less-evolved lavas are palagonite tuffs, related to Surtseyan-type activity which characterized the emergent stage of the island. Subaerial lava flows and later high-energy hydromagmatic eruptions are dacites which make up the tuff rings of the southeastern corner of the island. Rhyolites are the latest erupted products and the development of the younger dome in the centre of the island was preceded by ash and pumice fallout containing quenched bombs with an obsidian crust and a pumiceous core. Basaltic andesites contain olivine ŽFo 87 – 80 ., calcium-rich plagioclase and sparse clinopyroxene microphenocrysts; the typical mineralogy of dacitic and rhyolitic lavas is plagioclaseq two-pyroxenes. Major and trace elements vary regularly with MgO taken as a differentiation index. Isla San Luis lavas are enriched in LILE and depleted in Nb, Ta and Ti. Their characteristics are intermediate between tholeiitic and calc-alkaline lavas. They are also LREE enriched and the wŽLarYb. n x ratios steepen slightly with differentiation. Such an evolution is better explained by combined fractional crystallization and assimilation processes. q 1999 Elsevier Science B.V. All rights reserved. Keywords: Gulf of California; volcanic island; volcanology; hydromagmatism; petrology; AFC process

1. Introduction Isla San Luis Ž29858X 20Y N and 114824X 14Y W at the summit point., also named Salvatierra or Encantada mayor, is located in the northern part of the )

Corresponding author. Fax: q52-62-59-21-11; E-mail: [email protected] 1 Fax: q33-04-91-28-86-27; E-mail: [email protected].

Gulf of California ŽFig. 1.. It covers an area of about 4.5 km2 and has an overall maximum elevation of 180 m. The well-preserved morphology of the volcanic structures clearly support a late Quaternary age for all the eruptive activity. A striking feature of Isla San Luis is the presence of a complete evolution trend, from basaltic andesite to rhyolite, erupted in a short time interval. Isla San Luis along with Isla Tortuga, a tholeiitic basaltic shield volcano located near Santa Rosalıa ´ ŽBatiza, 1978; Batiza et al., 1979.,

0377-0273r99r$ - see front matter q 1999 Elsevier Science B.V. All rights reserved. PII: S 0 3 7 7 - 0 2 7 3 Ž 9 9 . 0 0 0 8 3 - 9

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F.A. Paz Moreno, A. Demantr Journal of Volcanology and Geothermal Research 93 (1999) 31–52

2. Tectonic setting

Fig. 1. Main geologic and tectonic features of the Gulf of California region Žmodified from Angelier et al., 1981..

represent the youngest known volcanic centres in the Gulf of California. These two islands rise up on the continental shelf Ž- 100 m below sea level for Isla San Luis, on the bathymetric map of Henyey and Bischoff, 1973., close to the transform which connects the Guaymas and Delfin pull-apart basins ŽFig. 1.. The only previous data concerning Isla San Luis are brief reconnaissance studies of its tectonic setting and geochemistry ŽRossetter, 1970; Rossetter and Gastil, 1971; Gastil et al., 1975; Paz-Moreno and Demant, 1995.. The aim of this paper is to describe the volcanological evolution of the island, to detail the petrography, mineralogy and geochemistry of the different rock types, to estimate the composition of the primary liquid, and to discuss the petrogenetic processes responsible for the evolution of this volcanic suite.

The Gulf of California is the classical example of a transition from an active continental margin to a regime of lithospheric extension resulting in the formation of a rift and the establishment of a new boundary between the Pacific and the North American Plates. Subduction ended progressively as a result of the migration of the Pacific–Farallon–North America triple junction towards the south ŽAtwater, 1970; Mammerickx and Klitgord, 1982; Atwater, 1989.. The Gulf of California formed first as a northwest–southeast-trending continental rift Žfrom about 12 to 4.5 Ma. when the Tosco-Abreojos transform system was active off the western coast of Baja California ŽSpencer and Normark, 1979.. It evolved to an oceanic rift since 3.5 Ma, after the jump of the East Pacific Rise at the mouth of the Gulf and the development of the dextral Gulf of California–San Andreas fault system ŽDickinson and Snyder, 1979.. Oceanic crust is created in small pull-apart basins ŽFig. 1. as revealed by Leg 64 which dredged MORB-like basalts in the Guaymas basin ŽEinsele et al., 1980; Einsele, 1982; Saunders et al., 1982a.. The magmatic events in the Gulf of California region reflect this complex tectonic evolution. The subduction-related andesitic belt, reported across broad areas in the Baja California peninsula ŽGastil et al., 1975, 1979; Hausback, 1984; Sawlan, 1991. and coastal Sonora ŽGastil and Krummenacher, 1977., vanished at about 11 Ma. Synrift volcanism is represented by scarce 10-Ma-old tholeiites in the Santa Rosalıa ´ area ŽEsperanza basalts of Sawlan and Smith, 1984. and voluminous late Miocene–Pliocene rhyolite ignimbrites in the eastern side of the northern Baja California peninsula ŽGastil et al., 1975; Martın-Barajas et al., 1995.. Post-subduction Plio´ Quaternary alkali basaltic fields ŽGastil et al., 1975; Sawlan, 1991. are conspicuous in central Baja, from San Borja to Jaraguay ŽFig. 1.. In this region, peculiar high-magnesium andesites, termed bajaites, were identified among the basaltic sequence and related to slab window after ridge–trench collision ŽSaunders et al., 1982b; Rogers et al., 1985; Saunders et al., 1987.. Nevertheless, calc-alkaline volcanism has continued to Recent times in several places, as, for example, in the Tres Vırgenes area ŽDemant, 1981; ´ . Sawlan, 1981; Demant, 1984 .

F.A. Paz Moreno, A. Demantr Journal of Volcanology and Geothermal Research 93 (1999) 31–52

3. Geological framework The Mesozoic San Pedro Martir batholith, intrusive into the metamorphic basement ŽGastil et al., 1975., takes up large areas of the northern Baja California peninsula. During the Tertiary and the Quaternary, successive volcanic episodes have partly mantled this plutonic spine. From detailed mapping

33

and isotopic age determinations, Martın-Barajas et ´ al. Ž1995. have subdivided the Tertiary volcanic sequences of the Puertecitos region, located north of the area considered in this paper, into: Ž1. early and middle Miocene Ž20–16 Ma. arc-related andesitic and dacitic rocks; Ž2. synrift 6-Ma-old Miocene rhyolites and ash flow tuffs, and Pliocene ignimbrites Ž; 3 Ma old.. The synrift volcanic sequences are cut

Fig. 2. Simplified geological map of the San Luis Gonzaga bay area Žmodified from Gastil et al., 1975..

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F.A. Paz Moreno, A. Demantr Journal of Volcanology and Geothermal Research 93 (1999) 31–52

by numerous N–S faults and tilted towards the east. West of Bahıa ´ San Luis Gonzaga, basaltic mesas capped the ignimbritic sequences, and locally fluvial or marine Tertiary deposits ŽFig. 2.. The Encantadas Islands have been considered as part of the Mio-Pliocene acidic sequence by Gastil et al. Ž1975.. Isla San Luis, the largest and the more recent island of this NW–SE-trending archipelago, is in fact Quaternary, and Isla Poma, a small spur of 0.10 km2 lying 1 km to the NNE, also belongs to this sequence.

4. Volcanic history of Isla San Luis The arid climate which prevails in the Gulf of California region favours the preservation of good exposures of the different rock types and facilitates the reconstruction of the volcanological evolution of Isla San Luis. Since no topographic map was available, aerial photographs and GPS were used during field work, done in autumn 1994 and spring 1995. The most prominent features, clearly seen on the air photos, are the central and northwestern rhyolitic domes, and the partly destroyed tuff ring of the southeastern corner of the island ŽFig. 3.. These three vents are aligned along a NW–SE direction. Such a direction is parallel to the transform which runs between Isla Angel de la Guarda and the Baja California peninsula, connecting the Guaymas and Delfin basins ŽFig. 1.. From their petrography and the dynamics of the eruptions, Isla San Luis volcanic rocks were assigned to four stratigraphic units. 4.1. Surtseyan episode The oldest unit, covering about 2% of the total surface of the island, is a palagonite horizon which crops out sporadically in the sea cliffs and arroyos of the southwestern side of the island ŽFig. 3.. The upper member of the palagonite succession consists of thin-bedded ash and lapilli size tuffs; the lower member is coarser and contains cauliflower bombs, up to 20 cm in diameter. All the juvenile fragments forming the palagonite are glassy and vesicular; they are embedded in a yellowish brown matrix made of hydrated glass. Accidental components are very scarce. These layers are well lithified, and have a

maximum thickness of 10 m above the present sea level, but the basement is never exposed. Palagonite tuffs highlight high energy hydromagmatic eruptions; this lithological unit corresponds, therefore, to the emergent stage of the island. At that time, the dynamics of the eruption was certainly similar to that of Surtsey volcano in 1963 ŽThorarinsson et al., 1964.. Dips of the palagonite strata are almost near horizontal Ž3 to 78.. This is mainly because the fragmented material was transported by pyroclastic surges, but probably also because the volcanic basement, formed during the initial stage of construction of the island had, like Tortuga island, a shield morphology. The palagonite vent could not be located precisely as it was buried below the more recent volcanic coverings. Lavas involved in the emergent stage of Isla San Luis are in part basalts, as indicated by the abundance of quenched olivine phenocrysts in the palagonitic tuffs. Nevertheless, the only nonfragmented juveniles that could be analysed, i.e., the moderately vesicular cauliflower bombs, have slightly more differentiated compositions Žfrom 53 to 59 wt.% silica.. 4.2. Subaerial dacitic actiÕity The second lithological unit easily recognizable on the field, corresponds to dacitic lavas. These blocky glassy flows contain sparse plagioclase laths discernible in hand specimen. They are well exposed on the western side of the island but limited outcrops are also present in the northern sea cliffs and in the inner wall of the Plaza de toros tuff ring, below the pumice layers ŽFig. 3.. At the northern limit of the main western outcrop, the dacite flow Ž; 20 m thick. overlies stratified marine deposits made of grey cinders and accretionary lapilli, fossils Žmostly pectinids. and gypsum. This marine coastal horizon lies in turn, on top of the palagonite. The stratigraphic position of the dacite is therefore well established. The northern outcrop is limited by a fault towards the south ŽFig. 3., and has been slightly uplifted. The location of the outcrops, and the topography, indicates that the dacite lavas flowed away from a vent located in the centre of the island. This vent, partly destroyed during the emplacement of the younger rhyolitic dome, is still clearly visible to the southwest of this central dome. It has a typical cone

F.A. Paz Moreno, A. Demantr Journal of Volcanology and Geothermal Research 93 (1999) 31–52

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Fig. 3. Geologic map of Isla San Luis, showing the major rock types and volcanological features, established from the 1:50,000 scale DETENAL air photographs. Ž1. main palagonite outcrops; Ž2. dacitic vent wax and flows wbx; Ž3. tuff rings; Ž4. northern and central rhyolitic domes wax and pumice falls related with the central tuff cone wbx; Ž5. alluvium wax and mangrove-type vegetation wbx.

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F.A. Paz Moreno, A. Demantr Journal of Volcanology and Geothermal Research 93 (1999) 31–52

shape, and was built by successive vulcanian explosions as indicated by the abundance of poorly vesiculated sharp glassy dacitic blocks on its flanks. 4.3. Dacitic tuff rings The third volcanic episode is represented by the Plaza de toros tuff ring, 1 km in diameter, that crops out in the southeastern corner of the island. Remnants of dacite flows are preserved in the inner cliffs and at the base of the northern rim ŽFig. 3.. The material forming the tuff ring is stratified and composed of thin alternating ash and pumice layers, well exposed in the high Žup to 50 m. and steep southeastern and southern sea cliffs. The overall fine grain size of the tephra, the presence of megaripples, antidune structures, low-angle cross-stratification and sags below ballistic blocks and cauliflower bombs are characteristic of surges due to hydromagmatic activity ŽSelf and Sparks, 1978; Wright et al., 1984.. The white to light brown pumice fragments, less than 10 cm in diameter, represent the juvenile dacitic magma involved in the eruption. Accidental components are centimetric fragments of palagonite or dacitic lava. Only one third of the Plaza de toros tuff ring is still preserved; the southern and eastern flanks collapsed in relation with NE–SW high-angle normal faults. Further marine erosion and longshore movement of the material build a spit about 2 km long at low tide ŽFig. 3.. As revealed by the complex bedding of the ash and pumice layers, the Plaza de toros tuff ring developed on top of two older badly destroyed hydromagmatic vents located to the north and to the southwest ŽFig. 3.. Cauliflower bombs belonging to the southwestern vent are observed near the base of the sea cliffs, directly overlying the palagonite. They are less vesiculated and darker than the pumices from the tuff ring, and have an andesitic composition. This volcanic episode preceded the eruption of the dacitic lavas; the grey cinders and accretionary lapilli forming the marine strata found below the dacite flow to the north, are probably related to this event. The paleo-topography explains well the distribution of the dacitic lavas on the island. Small light-coloured inclusions of poorly vesiculated dacite are frequent in the cauliflower bombs. The presence

of both andesitic and dacitic juveniles in the tuff ring reflects the compositional zonation of the magma chamber at the beginning of the hydromagmatic eruptions. Isla Poma, 1 km north of Isla San Luis, is a volcanic remnant that can be correlated with the highly explosive stage which built the tuff ring. Indeed, pumices from the Plaza de toros and Isla Poma have similar petrography and geochemical signatures. 4.4. Rhyolitic domes The fourth stratigraphic unit corresponds to the emplacement of rhyolitic domes. The central dome has a very well-preserved blocky and rough surface and seems to be very recent. The extrusion of the viscous magma was preceded by early explosive phases, which blanketed the whole surface of the island with ashes and pumices. This sequence of fall deposits is well exposed in the canyons which gully the southwestern flank of the island. A peculiar horizon is the uppermost 2-m-thick nonwelded air-fall deposit, composed of nonsorted pumice tephra Ž1 to 40 cm in size., and small accidental fragments of the volcanic basement, yellowish brown palagonite and black dacite. This upper layer also contains abundant rhyolitic bombs ŽFig. 4. which have Ž1. a black obsidian crust, cracked as in the bread crust bombs, and Ž2. a light slightly vesiculated pumiceous inner part. In the smaller bombs Ž20 cm. the obsidian crust can be continuous. The biggest ones Žup to 1.5–2 m. are much more inflated, and the rapid increase of volume, due to gas bubbles nucleation and growth, has broken and stretched the obsidian crust in a mechanism which resembles the formation of pop corn. These peculiar obsidian bombs clearly result from the intrusion of water into the vent. The contact between magma and water occurred at relatively deep levels as it quenched the rhyolitic magma prior to its vesiculation. The central vent, built mainly by tephra fallout, can be defined as a tuff cone ŽWohletz and Sheridan, 1983; Sohn, 1996.. The difference with the tuff rings of the southern coast lies mostly in the lesser abundance of surface water available, and this is in clear relation with its location in the center of the island. This explosive magma– groundwater interaction phase preceded the emplace-

F.A. Paz Moreno, A. Demantr Journal of Volcanology and Geothermal Research 93 (1999) 31–52

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Fig. 4. Photograph of a ‘‘pop corn’’ rhyolitic bomb showing the cracked black obsidian crust and the light expanded pumiceous centre.

ment of the degassed magma as a lava dome. The rhyolitic dome developed first as a regular plug filling the crater but, as the feeding from below continued, a new coulee ´ grew south of the previous plug, and overflowed the southeastern rim of the tuff cone ŽFig. 3.. A 40-m-high piton formed on top of the second dome and fell, but it is still visible on the highest point of the plug. Obsidian is frequent near the margin of the dome, and is interlayered with finely vesicular pumiceous rhyolite giving a typical flow-banding to the lava. At the northern end of the island, another rhyolitic dome formed. It is slightly older as it is covered by the ash and pumice falls from the central rhyolitic vent. This dome is composed of a near circular plug Žabout 500 m in diameter. and two flat lobed flows, one to the north, the other towards the east and southeast ŽFig. 3.. The presence of pillows with a quenched glassy margin, breccias, and a central highly vesiculated dark rhyolitic material are morphological evidence indicating that these flows entered the sea and that their flat tops correspond to a marine abrasion surface. The northern dome was formed, therefore, during a period of slightly higher

sea level. Marine terraces, about 6 m above the present sea level, were recognised in Bahıa ´ San Luis Gonzaga ŽOrtlieb, 1987. and attributed to the late interglacial Ž110 to 130 ka.. We consider that this could be the age of emplacement of the northern dome. In summary, the dynamics of the eruptions in Isla San Luis has been largely controlled by the interaction of seawater with magma. The first palagonitic cycle coincided with the less differentiated magmas, and has all the characteristics of Surtseyan-type eruptions occurring during the emergent stage of an island. Next was the only completely subaerial phase, which built a volcanic cone in the centre of the island, and erupted radially distributed dacitic lava flows that cover most of the island. The highly explosive episodes which formed the tuff rings of the southern coast are related to high energy hydromagmatic eruptions with high water–magma ratios. The northern rhyolitic dome was emplaced near the shore line, and entered the sea. Finally, the presence of bombs with obsidian crust in the uppermost pumice falls associated with the central rhyolitic dome indicates that intrusion of water into the vent persisted

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F.A. Paz Moreno, A. Demantr Journal of Volcanology and Geothermal Research 93 (1999) 31–52

Table 1 Representative microprobe olivine analyses. P: phenocrysts) 0.5 mm; mp: microphenocrysts from 0.5 to 0.1 mm Sample number: Rock type: Analysis: Mineral: SiO 2 Al 2 O 3 FeO MnO MgO CaO Total Fo

91-23 Pala 82 mp

91-23 Pala 142 mp

91-23 Pala 130 mp

91-23 Pala 139 Prcore

91-23 Pala 148 Prcore

94-21b Pala 42 Prcore

94-21b Pala 46 Prrim

94-21b Pala 66 Prrim

94-21b Pala 77 Prcore

94-21b Pala 79 Prcore

94-21b Pala 84 Prcore

39.13 0.01 17.61 0.28 42.91 0.19 100.13 81.29

39.01 0.03 16.62 0.27 44.34 0.21 100.48 82.63

39.27 0.05 16.02 0.25 44.54 0.24 100.37 83.20

39.73 0.02 13.25 0.16 47.09 0.17 100.40 86.37

39.41 0.10 12.46 0.32 47.81 0.31 100.41 87.24

39.70 0.01 18.50 0.30 42.39 0.19 101.09 80.33

39.39 0.00 18.57 0.32 42.75 0.21 101.23 80.40

39.55 0.00 18.57 0.35 42.63 0.21 101.32 80.36

39.29 0.03 18.95 0.28 41.91 0.21 100.67 79.77

39.14 0.01 18.19 0.28 43.17 0.19 100.97 80.88

39.24 0.00 18.77 0.28 42.22 0.22 100.73 80.04

until the end of the volcanic activity on Isla San Luis. In that case, a lower supply of water at a greater depth, and its interaction with a nonvesiculated magma, favoured fallout processes instead of surges, building a tuff cone instead of a tuff ring. 5. Analytical methods Electron microprobe mineral analyses were performed on a Cameca–Camebax electron microprobe at the ‘‘Service microsonde’’ of the USTL ŽMontpellier, France., using wavelength-dispersive spectrometers and natural material as standards. The operating conditions were typically 15 kV accelerating voltage, 10-nA beam current, 1-mm spot size, and integrated counting times ranging from 6 to 20 s depending on the analysed elements. For feldspar analyses, sodium was determined first to prevent its tendency to volatilize under the electron beam. A new accurate computer correction program ŽMerlet, 1994. was used to calculate the elemental concentrations. Major-, minor- and trace-element concentrations in whole rocks were determined by ICP-AES at the University of Aix-Marseille III, except for Na, K and Rb which were determined by Atomic Absorption Spectrophotometry. Additional trace elements ŽCs, Hf, Ta, Th, U and Pb., including the rare-earth elements ŽREE., were analysed on nine selected samples by ICP-MS at the USTL Montpellier. Preci-

Fig. 5. Ca–Fe–Mg Žmol%. plot of pyroxenes and Mg–Fe Žmol%. plot of olivine from the basaltic andesite to rhyolite suite of Isla San Luis. Representative olivine analyses are reported in Table 1, and pyroxenes analyses in Table 2. Basaltic andesites: sample 91-23 Žfilled triangles.; sample 94-21 Žempty squares.. Dacites Žempty circles. and rhyolites Žempty rhombs..

Sample number: Rock type: Analysis: Mineral: SiO 2 Al 2 O 3 FeOt MnO MgO CaO Na2O TiO 2 Cr2 O 3 Total Wo En Fs

91-23 Pala 86 Prcore

91-23 Pala 87 Prcore

91-23 Pala 114 Prcore

91-23 Pala 133 Prcore

91-23 Pala 147 mp

91-22 Dac 15 Prcore

91-22 Dac 23 Prcore

91-22 Dac 28 Prcore

91-22 Dac 30 Prcore

91-22 Dac 50 Prcore

91-26 Rhyo 202 mp

91-26 Rhyo 178 mp

91-26 Rhyo 212 mp

91-26 Rhyo 203 Prcore

51.14 3.23 6.28 0.16 16.99 21.37 0.53 0.73 0.56 100.99 42.71 47.24 10.05

52.20 2.29 6.88 0.25 17.16 20.75 0.00 0.63 0.10 100.26 41.34 47.56 11.10

52.01 2.88 5.85 0.22 17.19 21.09 0.00 0.63 0.25 100.12 42.40 48.07 9.53

52.32 2.33 5.47 0.10 17.48 21.59 0.17 0.40 0.58 100.44 42.96 48.39 8.65

50.83 3.75 5.95 0.08 16.74 21.24 0.02 0.75 1.00 100.36 43.14 47.30 9.56

54.02 1.49 11.95 0.44 29.52 1.89 0.10 0.17 0.19 99.77 3.59 78.03 18.38

51.78 1.41 9.50 0.39 14.85 20.93 0.49 0.23 0.04 99.62 42.44 41.89 15.67

52.41 1.52 6.65 0.18 18.04 20.15 0.32 0.24 0.31 99.82 39.84 49.62 10.54

52.32 1.50 6.14 0.16 17.87 20.49 0.22 0.38 0.26 99.34 40.76 49.45 9.79

53.51 1.70 11.82 0.37 29.65 1.58 0.03 0.27 0.23 99.16 3.02 78.80 18.18

51.61 0.90 12.62 0.34 13.50 20.34 0.73 0.25 0.00 100.29 41.31 38.14 20.55

50.81 0.98 15.40 0.46 12.36 19.55 0.00 0.27 0.00 99.83 39.79 35.00 25.21

50.26 0.47 28.55 0.15 17.83 1.42 0.47 0.13 1.46 100.74 2.92 51.01 46.07

50.15 0.29 30.12 1.18 16.72 1.50 0.00 0.11 0.07 100.14 3.05 47.27 49.68

F.A. Paz Moreno, A. Demantr Journal of Volcanology and Geothermal Research 93 (1999) 31–52

Table 2 Representative microprobe pyroxenes analyses. P: phenocrysts) 0.5 mm; mp: microphenocrysts from 0.5 to 0.1 mm

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F.A. Paz Moreno, A. Demantr Journal of Volcanology and Geothermal Research 93 (1999) 31–52

sion and accuracy of these methods are given by Govindaraju and Mevelle Ž1987.. All the samples were pulverised in an agate mill. 6. Petrography and mineral chemistry Volcanics from Isla San Luis are characterized, as the rocks from the Cerro Prieto geothermal area ŽHerzig, 1990., by relatively low K 2 O contents, even in the more silica-rich end members. Nevertheless, as no iron enrichment is observed in the intermediate lavas, San Luis series is more akin to calc-alkaline rocks. Therefore, in the following descriptions, we will used the IUGS terminology proposed by Le Maitre Ž1989.: slightly differentiated lavas Ž52–57 wt.% silica. are referred to as basaltic andesites, 57–63 wt.% silica lavas as andesites, 63–72 wt.% silica lavas as dacites, and rocks with more than 72 wt.% silica as rhyolites. 6.1. Basaltic andesites and andesites The most primitive rocks analysed with the microprobe are quenched fragments Žgenerally less than 1 cm., present in the palagonite tuffs. These fragments correspond to sparsely porphyritic lavas con-

taining olivine and plagioclase phenocrysts Žup to 1 mm. and microphenocrysts Ž0.2 to 0.5 mm., in a ratio of about 1:5. Clinopyroxene phenocrysts are much less abundant. Crystals are frequently fragmented when scattered in the palagonite, but euhedral with quenched morphologies in the glassy fragments. Iron–titanium oxides are completely lacking either as a phenocrystic phase or in the matrix; this is not a common characteristic of calc-alkaline andesitic liquids. Fresh glass from the matrix Žsideromelane. is partly vesicular and contains about 20% spherical vesicles. In one of the analysed samples Ž91-23. olivine phenocrysts ŽTable 1. have a homogeneous composition in the range Fo 87 – 85 , whereas microphenocrysts are slightly less magnesian ŽFo 83 – 81 .. In the other sample Ž94-21b., both phenocrysts and microphenocrysts have the same Fo 81 – 80 composition. The larger plagioclase phenocrysts Žup to 1.5 mm in size. are normally zoned with a wide bytownitic core ŽAn 86 – 83 . surrounded by a limited more sodic outer rim ŽAn 78 – 75 .. A few phenocrysts, nevertheless, are reversely zoned, with rims having up to 10% more anorthite component than the cores; these crystals contain, furthermore, abundant glass inclusions reminding the sieve texture. Plagioclase is also present

Fig. 6. Or–Ab–An Žmol%. plots for feldspar of the main rock types of Isla San Luis. Representative analyses are reported in Table 3.

F.A. Paz Moreno, A. Demantr Journal of Volcanology and Geothermal Research 93 (1999) 31–52

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Table 3 Representative microprobe plagioclase analyses. P: phenocrysts) 0.5 mm; mp: microphenocrysts from 0.5 to 0.1 mm Sample number: Rock type: Analysis: Mineral: SiO 2 Al 2 O 3 FeO CaO Na 2 O K 2O Total An Ab Or

91-23 Pala 92 Prcore

91-23 Pala 106 Prcore

94-21b Pala 58 Prrim

91-23 Pala 88 Prcore

91-23 Pala 95 Prcore

94-21b Pala 82 mp

91-22 Dac 13 Prcore

91-22 Dac 11 mp

91-22 Dac 18 mp

91-22 Dac 12 mp

91-22 Dac 4 mp

91-26 Rhyo 171 Prcore

91-26 Rhyo 167 Prrim

91-27 Rhyo 225 mp

46.90 34.00 0.23 17.56 1.57 0.02 100.28 85.98 13.88 0.14

48.41 32.85 0.42 16.28 2.25 0.05 100.26 79.75 19.96 0.29

49.76 32.04 0.52 15.52 2.70 0.04 100.57 75.89 23.88 0.23

51.03 30.68 0.37 14.22 3.24 0.09 99.62 70.45 29.00 0.54

52.00 30.29 0.47 13.43 3.84 0.09 100.12 65.55 33.91 0.55

54.71 28.59 0.61 12.00 4.62 0.13 100.66 58.51 40.73 0.75

49.99 30.81 0.55 14.37 3.21 0.01 98.94 71.15 28.80 0.05

52.01 29.73 0.52 12.55 4.17 0.18 99.17 61.81 37.14 1.04

53.26 29.23 0.55 11.67 4.75 0.15 99.60 57.08 42.03 0.89

55.54 27.84 0.58 10.04 5.58 0.13 99.71 49.49 49.74 0.77

56.95 26.21 0.64 8.89 6.25 0.17 99.11 43.56 55.46 0.98

58.18 25.84 0.93 7.91 6.67 0.23 99.76 39.05 59.61 1.35

59.14 25.20 0.40 7.06 7.14 0.29 99.23 34.73 63.58 1.69

59.85 25.08 0.34 6.85 7.54 0.32 99.97 32.81 65.39 1.80

as quenched laths with swallow-tail terminations in the matrix, the composition of which decreases regularly from An 70 to An 58 , as a function of size. FeO contents of plagioclase are around 0.5 wt.%. Clinopyroxenes are poorly zoned augite phenocrysts Ž- 1 mm . with compositions in the range Wo 40 – 44 En 49 – 46 Fs 9 – 13 ŽFig. 5, Table 2.. The glassy fragments from the palagonite tuff were too small to be isolated for bulk-rock analyses. Their mineralogy Žpresence of magnesium-rich olivine and bytownite phenocrysts. suggests, however, that basaltic lavas exist at the bottom of the volcanic edifice. Glass of the sideromelane fragments Ž91-23, Table 4., perfectly clear and isotropic in thin section, has compositions similar to the bulk rock analysis of a cauliflower bomb sampled in the palagonitic sequence Žsample 95-18, Table 5.. Composition of the yellowish palagonite glass shows that low-temperature hydration and alteration of sideromelane result in a drastic increase of water content Žfrom 1.5 wt.% in the clear glass to ; 15 wt.% in the palagonite., a loss of MgO, CaO and in a lesser extent Na 2 O Žanalysis 91, Table 4.. 6.2. Dacitic laÕas The dacitic lavas are aphyric Žless than 1% phenocrysts. and often glassy, with elongated irregularly shaped vesicles. They contain euhedral plagioclase and pyroxene microphenocrysts Ž0.2–0.5 mm.. Plagioclase laths are predominant and responsible for

the flow-banding visible in hand specimen. They are optically zoned and range in composition from An 72 to An 62 . Lath-shaped feldspars of the groundmass, with typical quenched morphologies and pilotaxitic or felty textures, are more sodic ŽAn 60 to An 40 .. All the plagioclases, even the less calcic, have a very low orthoclase component ŽFig. 6, Table 3 ; this is in relation with the low K 2 O contents of the bulk rocks. Most pyroxene microphenocrysts are intimately associated with plagioclase in small glomerocryst assemblages. They correspond to: Ž1. Mg-rich augite ŽWo 40 – 43 En 49 – 47 Fs 9 – 11 ., with compositions

Table 4 Representative microprobe analyses of glass. Prglasss Palagonite glass. n s Number of analyses Sample number: Rock type: Analysis: Mineral: SiO 2 TiO 2 Al 2 O 3 FeO MnO MgO CaO Na 2 O K 2O Total

91-23 Pala 91 Prglass

91-23 Pala ns6 Glass

94-21b Pala ns 5 Glass

49.41 1.65 14.77 8.10 0.15 2.66 3.95 2.46 1.31 84.45

55.54 56.09 1.28 1.42 16.14 15.48 7.30 7.88 0.15 0.16 5.22 4.22 8.51 8.03 3.54 2.55 0.73 0.68 98.41 96.51

91-22 Dac ns 7 Glass

91-27 Rhyo ns6 Glass

91-26 Rhyo ns6 Glass

70.74 73.42 75.23 1.24 0.39 0.31 12.62 13.62 13.42 4.35 2.75 2.15 0.06 0.04 0.05 0.28 0.39 0.12 1.64 1.62 1.14 3.73 3.11 2.91 2.84 2.71 2.85 97.51 98.05 98.18

42

Sample number: Rock type:

94-31 B-and

95-18 B-and

95-33 And

95-32 And

94-25aa And

94-25b a And

94-24ab Dac

94-27 Dac

94-15 Dac

94-20 Dac

91-22 Dac

95-25 Dac

95-34 Dac

94-22 Dac

94-23 Dac

SiO 2 Žwt.%. TiO 2 Al 2 O 3 Fe 2 O 3 FeO MnO MgO CaO Na 2 O K 2O P2 O5 LOI Total Mg-number

53.53 1.23 16.79 2.31 5.81 0.13 5.84 9.40 3.54 0.55 0.19 1.19 100.51 61.05

55.84 1.07 16.89 1.43 5.70 0.11 4.83 8.69 3.86 0.65 0.19 0.60 99.86 59.41

57.45 1.32 16.27 1.14 6.11 0.12 3.77 7.13 3.69 0.85 0.22 0.68 98.75 52.80

59.14 1.03 16.28 2.02 4.25 0.10 3.39 6.06 4.82 1.02 0.22 1.95 100.28 54.19

59.63 1.19 16.42 4.11 1.98 0.09 2.90 6.05 4.74 1.31 0.30 0.78 99.50 51.96

60.36 0.80 17.40 2.48 2.61 0.07 2.96 6.27 4.64 0.96 0.16 1.42 100.13 56.42

62.90 1.10 15.23 2.06 3.75 0.10 1.56 4.08 4.84 1.49 0.30 3.24 100.65 37.09

63.30 0.92 16.05 2.18 3.88 0.10 1.76 4.39 4.86 1.44 0.22 1.13 100.23 38.95

64.02 0.98 15.83 2.92 2.68 0.10 1.93 4.93 5.06 1.53 0.23 0.20 100.41 43.50

64.30 0.99 15.70 3.25 2.39 0.11 1.90 4.62 5.07 1.46 0.23 0.34 100.36 43.09

64.31 0.94 15.52 1.29 4.24 0.10 1.60 4.17 5.05 1.51 0.22 0.38 99.33 38.55

64.78 0.90 15.73 1.04 4.37 0.09 1.70 4.26 5.04 1.54 0.23 0.40 100.08 40.42

65.04 0.94 15.49 1.10 4.28 0.09 1.71 4.20 5.12 1.59 0.23 0.43 100.22 40.73

65.07 0.89 15.88 2.92 2.19 0.09 1.61 3.76 4.96 1.40 0.22 0.38 99.37 41.44

65.85 0.88 15.09 1.88 3.38 0.09 1.33 3.76 5.23 1.56 0.25 0.50 99.80 35.70

CIPW norm Qz Or Ab An Di Hyp Mt Ilm Ap Rb Žppm. Sr Ba Ni Cr V Zr Y Nb

1.85 3.27 30.13 28.47 14.12 17.31 1.73 2.36 0.45 18 365 185 40 146 215 123 28 4

5.00 3.87 32.88 27.02 12.47 14.52 1.53 2.05 0.45 15 370 240 23 72 183 106 25 6

10.70 5.12 31.81 25.80 7.35 14.38 1.59 2.56 0.53 12 424 294 22 53 206 157 26 9

8.51 6.12 41.44 20.09 7.52 12.18 1.34 1.99 0.53 24 325 293 10 37 144 188 32 4

9.59 7.83 40.59 19.90 7.16 10.20 1.25 2.29 0.72 30 807 533 19 38 145 219 28 6

11.13 5.74 39.74 24.11 5.27 10.70 1.07 1.54 0.38 24 620 406 18 33 128 118 17 3

16.89 9.03 42.01 15.83 2.63 9.24 1.25 2.15 0.73 42 330 483 2 5 90 296 42 11

15.55 8.58 41.46 17.87 2.34 10.36 1.28 1.77 0.53 44 325 465 6 11 83 310 42 7

14.55 9.01 42.69 15.92 5.94 7.99 1.15 1.86 0.54 46 299 429 7 19 81 322 45 7

15.48 8.62 42.85 15.75 4.83 8.50 1.16 1.88 0.54 48 319 439 9 18 86 312 45 6

16.54 9.01 43.15 15.37 3.54 8.69 1.19 1.81 0.53 21 318 460 12 11 80 285 40 8

16.52 9.12 42.75 15.79 3.39 8.86 1.16 1.72 0.55 36 290 461 6 11 88 302 42 9

16.46 9.40 43.38 14.61 4.11 8.40 1.15 1.79 0.55 37 298 459 4 10 84 312 42 9

18.66 8.35 42.36 17.09 0.36 9.53 1.06 1.71 0.53 44 315 470 9 23 82 312 44 7

18.31 9.27 44.53 13.17 3.47 7.61 1.11 1.69 0.60 46 340 519 5 4 65 312 43 5

F.A. Paz Moreno, A. Demantr Journal of Volcanology and Geothermal Research 93 (1999) 31–52

Table 5 Whole-rock analyses and CIPW-normative compositions of Isla San Luis volcanic suite. Norms calculated on a water-free base

Table 5 Žcontinued. Sample number: Rock type:

94-21c Dac

95-29 Dac

94-24b b Dac

95-30 Rhyo

95-23 Rhyo

95-30b Rhyo

94-30 Rhyo

94-12 Rhyo

94-13 Rhyo

91-25 Rhyo

95-22 Rhyo

94-28 Rhyo

94-14 Rhyo

91-26 Rhyo

65.88 0.84 14.79 1.70 3.37 0.09 1.49 3.48 5.55 1.84 0.23 1.17 100.43 39.17

66.00 0.84 14.96 1.20 3.67 0.08 1.25 3.50 5.28 1.66 0.22 1.25 99.91 35.79

66.29 0.89 15.12 1.75 3.45 0.08 1.23 3.61 5.16 1.57 0.23 1.11 100.49 34.14

69.19 0.44 13.08 1.42 1.60 0.05 0.69 1.89 5.80 2.12 0.07 3.90 100.25 33.67

71.92 0.44 13.50 0.71 2.28 0.05 0.48 1.81 5.48 2.31 0.07 0.57 99.62 25.83

72.05 0.45 13.26 1.16 1.91 0.06 0.60 1.96 5.46 2.40 0.07 1.19 100.57 30.08

72.06 0.45 13.90 1.36 1.75 0.06 0.42 1.80 5.11 2.12 0.05 0.93 100.01 23.02

72.13 0.43 13.77 0.99 1.99 0.06 0.42 1.80 5.15 2.15 0.04 0.80 99.73 23.59

72.24 0.42 13.56 1.14 1.79 0.06 0.44 1.82 5.10 2.17 0.04 0.84 99.62 24.86

72.52 0.42 11.94 1.17 1.83 0.04 0.33 1.88 4.91 1.90 0.09 3.12 100.15 19.51

72.54 0.42 13.45 0.68 2.33 0.05 0.45 1.76 5.34 2.28 0.05 0.37 99.72 24.47

72.65 0.47 13.82 1.01 2.11 0.06 0.42 1.90 5.07 2.10 0.05 0.32 99.98 22.76

72.92 0.45 13.98 0.96 2.12 0.06 0.47 1.84 5.17 2.16 0.06 0.43 100.62 25.01

75.62 0.39 12.35 0.58 2.21 0.04 0.15 1.63 4.98 2.08 0.06 0.38 100.47 10.42

CIPW norm Qz Or Ab An Di Hyp Mt Ilm Ap Rb Žppm. Sr Ba Ni Cr V Zr Y Nb

16.18 10.94 47.27 10.08 4.99 7.08 1.08 1.61 0.55 41 284 475 1 14 59 329 44 6

18.64 9.93 45.25 12.37 3.29 7.15 1.05 1.62 0.53 35 299 535 2 3 62 283 40 10

19.36 9.32 43.90 13.53 2.63 7.68 1.10 1.70 0.55 46 305 512 6 15 58 310 42 8

22.85 12.99 50.89 3.52 4.89 2.93 0.65 0.87 0.17 53 180 670 2 4 16 338 36 11

26.06 13.77 46.77 5.46 2.72 3.40 0.64 0.85 0.17 61 170 677 3 3 15 305 38 8

25.69 14.25 46.45 4.61 4.05 3.07 0.65 0.86 0.17 57 172 658 8 1 16 350 40 6

28.43 12.63 43.60 8.71

28.34 12.83 44.01 8.19 0.51 4.37 0.63 0.83 0.10 60 169 685 3 10 10 335 41 6

28.79 12.97 43.65 7.79 0.95 4.13 0.62 0.81 0.10 66 169 687 3 11 11 350 40 7

32.55 11.56 42.78 5.08 3.45 2.69 0.65 0.82 0.22 10 129 514 4 7 10 306 37 7

27.45 13.55 45.44 6.03 2.11 3.69 0.65 0.80 0.12 52 172 690 4 1 15 342 39 8

29.02 12.44 43.01 8.77 0.33 4.56 0.66 0.90 0.12 62 180 691 1 3 15 326 38 7

28.38 12.72 43.63 8.54 0.18 4.72 0.65 0.85 0.14 62 168 631 4 14 14 331 41 8

34.07 12.27 42.06 5.19 2.22 2.56 0.59 0.74 0.14 14 146 634 4 5 3 320 39 7

4.73 0.65 0.86 0.12 68 179 693 4 5 16 331 35 6

F.A. Paz Moreno, A. Demantr Journal of Volcanology and Geothermal Research 93 (1999) 31–52

SiO 2 Žwt.%. TiO 2 Al 2 O 3 Fe 2 O 3 FeO MnO MgO CaO Na 2 O K 2O P2 O5 LOI Total Mg-number

a

Accidental andesitic fragment from the Isla Poma tuff ring. Pumice samples from Isla Poma.

b

43

44

F.A. Paz Moreno, A. Demantr Journal of Volcanology and Geothermal Research 93 (1999) 31–52

similar to that of clinopyroxenes from the palagonite; and Ž2. orthopyroxene ŽWo 3 – 4 En 79 – 76 Fs 17 – 20 .. Scattered crystals are slightly more iron-rich ŽWo 42.5-

En 42 Fs 15.5 and Wo 4.5 En 65 Fs 30.5 , respectively; Fig. 5.. Scarce titanomagnetite is also present in the glomerocrysts but never found as isolated crystals.

F.A. Paz Moreno, A. Demantr Journal of Volcanology and Geothermal Research 93 (1999) 31–52

However, microlitic size Ž- 0.01 mm. grains are abundant in the groundmass. The brown glassy matrix has a rhyolitic composition ŽTable 4.. 6.3. Rhyolites All the rhyolites are predominantly glassy and some dense flow-laminated obsidian layers are observed at the margin of the domes. Most of the crystals Žless than 1%. in the rhyolitic lavas, obsidian or pumice, are euhedral and normally zoned plagioclases. Their compositions range from andesine ŽAn 43 – 31 . for the microphenocrysts to oligoclase for the lath shaped crystals ŽAn 25 – 23 ; Fig. 6.. Some plagioclase microphenocrysts contain large glass inclusions corresponding to liquid trapped during rapid growth. Mafic minerals are small dispersed euhedral clinopyroxenes ŽWo 41 – 37 En 38 – 35 Fs 20 – 25 . and orthopyroxenes ŽWo 3 En 51 – 47 Fs 46 – 50 .. These crystals are more iron-rich than the pyroxenes analysed in the dacite ŽFig. 5.. Scarce titanomagnetite and ilmenite grains are also present but difficult to analyse. Glass of the near vent obsidian bombs, which corresponds to the initial stage of degassing of the rhyolitic magma before the emplacement of the central dome, is slightly less differentiated than that of the lava dome Ž91-27, Table 1.. Microprobe analyses of glass are close to the bulk rock analyses, except for the sodium, which has lower values in the probe analyses. This is due to high dispersion of this element below the electron beam. The petrography and mineralogy of the different rock types from Isla San Luis show that: Ž1. all the rocks are mostly aphyric lavas; Ž2. hydrous minerals are lacking even in the rhyolites; Ž3. plagioclase compositions evolve towards Na-rich members with differentiation but their orthoclase components re-

45

main negligible; Ž4. Fe–Ti oxides are absent in the less differentiated lavas; Ž5. neither quartz nor Kfeldspar crystallized in the rhyolites. All of these characteristics are more typical of tholeiitic liquids than calc-alkaline series which are generally much more porphyritic. On another hand, the progressive evolution observed in the mineral chemistry of plagioclases and pyroxenes, from basalts to rhyolites, suggests that fractional crystallization was a major petrogenetic process. The gradual change is corroborated by the crystallization temperatures obtained with different geothermometers. Olivinerliquid cation partition coefficients for Mg and Fe 2q ŽRoeder and Emslie, 1970; Ford et al., 1983. indicate that olivine in equilibrium with glass of the quenched fragment, in the palagonite tuff Žsample 91-23., has a Fo 82 composition. Mg-rich olivine phenocrysts ŽFo 87 . were probably in equilibrium with a more primitive basaltic liquid. This suggests the presence of a potential basaltic shield, corresponding to the initial building stage, below Isla San Luis. Temperatures obtained with olivine–glass pairs Ž K D s 0.3 " 0.03., are in the range 1160–11108C, using the method of Leeman and Scheidegger Ž1977.. Opx– Cpx geothermometer ŽWood and Banno, 1973; Wells, 1977. gives crystallization temperatures in the range 1110–10808C for the dacite Ž91-22., and between 1020 and 9908C for the rhyolite Žsample 91-26.. 7. Geochemistry Twenty-nine samples from Isla San Luis and Isla Poma, covering the entire range from basaltic andesite to rhyolite, have been analysed for whole rock geochemistry Žsee Table 5.. On the total alkali vs. silica ŽTAS. diagram, Isla San Luis lavas plot in the subalkaline field and

Fig. 7. Ža. TAS diagram ŽLe Bas et al., 1986. for the volcanic rocks of Isla San Luis Žgrey squares.. For comparison are also plotted the basaltic suite of Isla Tortuga Žempty circles. wBatiza et al., 1979x, basalts of the Guaymas basin Žempty triangles. wSaunders et al., 1982ax, the basaltic andesite to dacitic suite of the Cerro Prieto geothermal area Žempty squares. wHerzig, 1990x, and the Pliocene calc-alkaline sequence of Isla San Esteban Žempty rhombs. wDesonie, 1992x. Žb. K 2 O vs. silica diagram ŽPeccerillo and Taylor, 1976; Le Maitre, 1989.. Same symbols as in Fig. 7a. The shaded area corresponds to the Isla San Esteban series ŽDesonie, 1992.. Žc. AFM diagram for some Plio-Quaternary volcanic series of the northern Gulf of California. A s Na 2 O q K 2 O; F s Fe 2 O 3 P 0.9 q FeO; M s MgO q MnO. Same symbols as in Fig. 7a. Notice the transitional position of the San Luis series between the calc-alkaline trend of Isla San Esteban ŽDesonie, 1992. and the tholeiitic trends of Isla Tortuga-Guaymas basin basalts, or the basaltic to dacitic sequence of Cerro Prieto ŽHerzig, 1990..

46 F.A. Paz Moreno, A. Demantr Journal of Volcanology and Geothermal Research 93 (1999) 31–52 Fig. 8. Major- and trace-element variation diagrams of Isla San Luis volcanic sequence, using MgO as an index of differentiation. Microprobe analyses of glass Žfilled circles. are also plotted.

F.A. Paz Moreno, A. Demantr Journal of Volcanology and Geothermal Research 93 (1999) 31–52

follow a regular trend from basaltic andesite to rhyolite ŽFig. 7a.. It is apparent from this diagram that dacites and rhyolites predominate in the series, while andesites and basaltic andesites are scarce and true basalt lacking. This is in part due to the difficulty in sampling the palagonite episode, however, the volume of acidic products is effectively much greater on the emerged part of the island. For comparison we also report on this diagram: Ž1. the tholeiitic basalts from Isla Tortuga ŽBatiza et al., 1979. and the Guaymas basin ŽSaunders et al., 1982a.; Ž2. the Pliocene porphyritic orogenic lavas of Isla San Esteban ŽDesonie, 1992., another island of the northcentral Gulf of California; and Ž3. the tholeiitic basaltic to dacitic sequence of the Cerro Prieto geothermal area, a Quaternary volcanic field located in the Salton Trough ŽHerzig, 1990.. It is clear that the TAS diagram is not useful to differentiate the tholeiitic and calc-alkaline series of the northern Gulf of California. The K 2 O vs. silica diagram ŽPeccerillo and Taylor, 1976; modified by Le Maitre, 1989. seems a better discriminant ŽFig. 7b., and calc-alkaline rocks

47

from Isla San Esteban plot separately, mostly in the high-K field. Isla San Luis lavas classified as medium-K series, while basaltic andesites plot near the limit with the low-K series. K 2 O is not abundant even in the more differentiated lavas Ž2% K 2 O for 75% SiO 2 .. The Na 2 OrK 2 O ratio decreases from 6.3 in the less evolved lavas to 2.3 in the rhyolites. No peculiar iron enrichment is observed in the intermediate lavas from Isla San Luis, as in the classical tholeiitic series of the Galapagos Islands ŽGeist et al., 1995. or Iceland ŽJonasson et al., 1992.. Neverthe´ less, as seen on the AFM diagram ŽFig. 7c., total iron is higher in San Luis dacites than in the equivalent rocks from Isla San Esteban, while Al 2 O 3 contents are about the same in both series. In summary, major element geochemistry shows the transitional signature, between calc-alkaline and tholeiitic series, of Isla San Luis lavas. All the samples are quartz normative, and even the basaltic andesite Ž94-31. does not correspond to a primary liquid as indicated by its low-Ni contents Ž40 ppm. and Mg-number Ž61.. Major-element variation diagrams ŽFig. 8. exhibit a regular decrease of

Table 6 Whole-rock REE and incompatible trace-element analyses of selected samples determined by ICP-MS Sample number: Rock type: La Žppm. Ce Pr Nd Sm Eu Gd Tb Dy Ho Er Tm Yb Lu Cs Hf Ta Th U Pb

94-31 B-and

94-25a And

94-24a Dac

94-27 Dac

94-15 Dac

94-23 Dac

94-24b Dac

94-12 Rhyo

94-28 Rhyo

9.10 22.50 3.06 14.50 3.55 1.25 4.56 0.70 4.58 0.97 2.73 0.41 2.43 0.40 0.42 3.44 0.43 1.11 0.42 3.44

18.90 44.10 5.63 24.10 4.74 1.36 4.78 0.72 4.37 0.91 2.49 0.38 2.29 0.37 0.44 4.96 0.46 2.62 0.76 7.13

19.10 45.60 5.78 26.10 5.96 1.44 6.50 1.04 6.50 1.35 3.81 0.59 3.61 0.58 1.68 7.60 0.48 3.91 1.21 6.06

19.60 45.50 5.85 25.60 5.86 1.48 6.49 1.04 6.55 1.40 3.93 0.63 3.84 0.62 1.72 7.79 0.51 4.10 1.26 8.02

18.50 44.50 5.67 25.10 5.72 1.43 6.41 1.01 6.60 1.36 3.91 0.61 3.78 0.63 1.77 8.03 0.46 4.14 1.28 13.80

19.60 47.00 5.92 25.90 5.93 1.42 6.27 1.00 6.47 1.32 3.82 0.59 3.69 0.62 1.87 8.16 0.52 4.33 1.35 6.81

19.90 47.60 6.12 27.10 5.91 1.41 6.47 1.03 6.60 1.42 4.03 0.62 3.82 0.64 1.87 8.00 0.51 4.48 1.38 9.22

21.20 49.40 6.05 25.20 5.44 1.12 5.86 0.96 6.18 1.30 3.75 0.60 3.74 0.61 2.72 9.33 0.53 5.77 1.83 9.07

23.00 49.00 5.96 25.00 5.27 1.08 5.60 0.91 5.87 1.28 3.61 0.59 3.46 0.60 2.74 8.65 0.45 5.63 1.76 10.28

48

F.A. Paz Moreno, A. Demantr Journal of Volcanology and Geothermal Research 93 (1999) 31–52

FeOt, CaO, TiO 2 , and Al 2 O 3 with decreasing MgO contents, and an increase of SiO 2 , K 2 O and Na 2 O, even if a slight decrease in sodium is observed in the more silica-rich rhyolites. Analysed glasses from the groundmass plot on the same general trends, except sodium. Different mechanisms are classically invoked for the generation of rhyolitic liquids: Ž1. fractional crystallization of a basaltic magma, with or without crustal assimilation; Ž2. fusion of the continental crust; or Ž3. as suggested recently for the Icelandic rhyolites, remelting of intrusive granophyric bodies or hydrated oceanic crust ŽMarsh et al., 1991.. The low potassium content of Isla San Luis rhyolites precludes a fusion of the continental crust, and the tectonic setting of the Gulf of California precludes the hypothesis of remelting oceanic material. Assimilation and fractional crystallization ŽAFC. seems likely therefore to be the major petrogenetic process that produced Isla San Luis rhyolites. Trace-element variation diagrams are consistent with a fractional crystallization model, as most of the elements, even mobile ones such as Ba, show a linear correlation with MgO considered as a differentiation index ŽFig. 8.. The ThrTa ratio increases regularly with differentiation, and this is due mainly to an increase of the Th values ŽTable 6.. The same kind of evolution observed at Paricutın ´ volcano during its 9-year activity, was interpreted as a combination of fractional crystallization and concurrent assimilation of continental crust ŽMcBirney et al., 1987.. The pattern of the basaltic andesite Žsample 9431., on a Pearce Ž1983. spidergram, exhibits a marked enrichment in large-ion-lithophile elements ŽRb, Ba and Th. and a slight negative Nb anomaly ŽFig. 9., characteristic of subduction-related magmatism. However, even the basalts from the Guaymas pullapart basin have such a depletion in Nb and enrichment in Ba ŽBatiza et al., 1979; Saunders et al., 1982a.. Isla San Luis lavas correlate well with the Plio-Pleistocene synrift series of the Puertecitos area, which differs from the Mio-Pliocene porphyritic arc-related rocks in being less enriched in incompatible elements and LREE ŽMartın-Barajas et al., 1995.. ´ Dacitic and rhyolitic lavas from Isla San Luis present similar humped trends, suggesting a common source, but rhyolites are slightly more enriched in Rb, Ba

Fig. 9. MORB-normalized variation diagram Žspidergram. of representative samples from Isla San Luis. Normalization values from Pearce Ž1983.. Basaltic andesite, sample 94-31 Žfilled triangles.. Dacites: sample 94-15 Žempty squares.; sample 94-23 Žempty circles.; sample 94-27 Žempty triangles.. Rhyolites: sample 94-12 Žfilled squares.; sample 94-28 Žfilled circles.. The thick black line corresponds to an accidental andesitic fragment from the Isla Poma tuff ring Žsample 94-25a..

and Th, and more depleted in Ti, P and Cr. An accidental andesitic fragment from the Isla Poma tuff ring Ž94-25a. has a very distinctive trend, more typical of the orogenic series from Isla San Esteban ŽDesonie, 1992.. On a chondrite-normalized REE diagram ŽFig. 10., Isla San Luis andesite is slightly enriched wŽLarYb. n s 2.52x and differs from the LREE depleted MORB-like patterns of Isla Tortuga and Guaymas basin basalts. Dacites and rhyolites present similar trends, but with an increasing negative Eu anomalies in the more silica-rich lavas, indicative of plagioclase crystallization. REE patterns steepen slightly with increasing differentiation wŽLarYb. n s 3.3–3.6 in the dacite, 3.8 in the rhyolitesx, and this probably reflects assimilation of crustal material as indicated also by an increase of the ThrLa ratios from 0.12 in the basaltic andesites to 0.27 in the rhyolites. The xenolithic andesitic fragment from Isla Poma has a peculiar REE pattern characterized by LREE values similar to the dacites and rhyolites, but HREE similar to the andesite ŽFig. 10.. Such a trend

F.A. Paz Moreno, A. Demantr Journal of Volcanology and Geothermal Research 93 (1999) 31–52

Fig. 10. Chondrite-normalized REE patterns of representative Isla San Luis rock types. Normalization values from Boynton Ž1984.. The shaded area represents the field of the Guaymas basin basalts ŽSaunders et al., 1982b.. Same symbols as in Fig. 9.

resembles again that of the San Esteban andesites ŽDesonie, 1992.. REE patterns and trace-element geochemistry confirm therefore the peculiarity of Isla San Luis series in the regional context of the northern Gulf of California.

8. Discussion The Isla San Luis lavas do not display a simple geochemical signature. The overall aphyric character of the series, the absence of Fe–Ti oxides in the less evolved rocks, the major element compositions, particularly the low-K 2 O contents of the rhyolites and their high temperature of crystallization show tholeiitic affinity. However, high contents of Rb, Ba, Th and negative anomalies in Ta, Nb, and Ti are indicative of a calc-alkaline component in the source region. This dual character could reflect either a magma source characteristic or a late contamination process in a shallow reservoir. An inheritance from a vanishing subduction component was invoked to explain some of the geochemical particularities of the basalts dredged in the Guaymas basin ŽEinsele, 1982; Saun-

49

ders et al., 1982a.. The influence of such a subduction component would be more prominent at Isla San Luis, due to its tectonic setting at the margin of the peninsular block, and the presence of a thicker lithospheric mantle. A shallow-level magma chamber may have underlain Isla San Luis. The progressive enrichment in silica observed in the lavas erupted to the surface, and the regular trends of most of the elements with differentiation, may be ascribed to closed-system fractional crystallization of a small batch of magma. Nevertheless, increasing ThrTa, or ThrLa ratios with differentiation, the high rhyoliterbasalt volume ratio, the short time interval between the different eruptive events and the overall aphyric character of the lavas suggest that assimilation of crustal material at shallow depth was also an important contributor for the evolution of the series, in a way similar to that inferred from Sr isotopic data for the recent Quaternary tholeiitic suite of Cerro Prieto ŽHerzig, 1990.. It is obvious that this contaminant had to be the huge northern Baja California batholith. The characteristics of Isla San Luis lavas also provide evidence for a strict control of the tectonic setting on the geochemistry of the lavas erupted since the upper Miocene in the Gulf of California area. Quaternary tholeiitic lavas are located only in the Gulf area and the Salton Trough, that is in regions where the crust has been largely stretched and thinned by the strike slip movement related to the Gulf of California–San Andreas rift system. Voluminous Plio-Pleistocene acidic volcanic sequences are present on the faulted margin of the continental Baja California peninsular block or in coastal Sonora. The Mio-Pliocene sequences of Puertecitos and Isla San Esteban have the typical petrography and geochemical signatures of arc-related lavas, but the latest events, Plio-Pleistocene in the Puertecitos region or Quaternary on Isla San Luis, present evidence of a shift of the geochemical signatures towards more tholeiitic affinities.

9. Conclusion Isla San Luis, a recent small volume volcanic centre in the northern Gulf of California, is charac-

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terized by the successive eruption of progressively more differentiated lavas. The first building stage is Surtseyan-type eruptions producing brownish yellow palagonites layers present as scattered and limited outcrops on the island. Magmas erupted during this stage were basaltic andesites and andesites. Next, subaerial dacitic lava flows covered most of the surface of the island, while towards the southeast, highly explosive hydromagmatic dacitic eruptions were building overlapping tuff rings. Finally, rhyolitic magmas were erupted with the emplacement of domes. The overall small volume of magma, the progressive increase of silica with time and the eruption of the magma in the order of differentiation are in favour of a small batch of magma which evolved in a shallow reservoir by combined fractional crystallization and crustal assimilation ŽAFC., as revealed by the increase of the ThrTa, or ThrLa ratios with increasing silica contents. The crustal contaminant was most probably the granitic rocks of the Baja California peninsular batholith. The transitional characteristics of these lavas reflect the vanishing influence of subduction, and the progressive influx of rift-related magma types on the edge of the Gulf of California. Acknowledgements This research was supported by an interuniversity convention between Universidad de Sonora ŽUNISON. Mexico, and Universite´ d’Aix-Marseille ´ III, France. Field work benefited from invaluable logistical support kindly provided by the team of fishermen of Senor Hernandez, from ˜ Cleto Velazquez ´ ´ Mexicali, and hospitality of Don Natividad Rıos ´ Sanchez, and Leoncio Rıos from camping ´ ´ Hernandez ´ Las Encantadas, in Punta Bufeo. The authors are grateful to M.O. Trensz ŽMarseille., L. Savoyant and S. Pourtales ŽMontpellier. for the chemical data and to C. Merlet for technical assistance during microprobe work. R. Batiza and W. Rose are kindly thanked for their review of the manuscript. References Angelier, J., Colletta, B., Chorowicz, J., Ortlieb, L., Rangin, C., 1981. Fault tectonics of the Baja California peninsula and the

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