Revista geológica de Chile
versión impresa ISSN 0716-0208
Rev. geol. Chile v.34 n.2 Santiago jul. 2007
Revista Geológica de Chile, Vol. 34, No. 2, p. 209-232, July 2007.
Jurassic to Early Cretaceous subduction-related magmatism in the Coastal Cordillera of northern Chile (18°30'-24°S): geochemistry and petrogenesis
Magmatismo asociado a subducción del Jurásico a Cretácico Inferior en la Cordillera de la Costa del norte de Chile (18°30'-24°S): geoquímica y petrogenesis.
Veronica Oliveros, Diego Morata, Luis Aguirre
Jurassic to Early Cretaceous magmatism in the Coastal Cordillera of northern Chile is represented by thick sequences of mostly basaltic-andesitic to andesitic lava flows and minor sedimentary rocks. The volcanic sucession was intruded by large plutonio bodies and smaller stocks and dikes. New geochemical data, including major and trace elements for a suite of Middle to Upper Jurassic volcanic and plutonio rocks from six localities in the Coastal Cordillera (18°30'-24°S), are presented here. The volcanic rocks are characterized by their petrological and chemical homogeneity; they are highly porphyritic basaltic-andesites and andesites with calc-alkaline to high-K calc-alkaline affinities, higher LILE than HFSE abundances, negative Nb and Ti anomalies, and LREE/HREEfractionation, which are the typical compositional features of subduction-related igneous rocks. No significant differences are observed in rocks from different areas or ages, but the plutonio rocks show subparallel, less and more enriched patterns respectively compared to volcanic rocks. The evolution and differentiation of the parental magmas is mainly due to fractional crystallization dominated by plagioclase, olivine and clinopyroxene. Assimilation of the continental crust was not important, although Th and La contents would indicate increasing sediment contribution or crustal contamination of the magmas with time. The magma source is likely to be a depleted mantle metasomatized by fluids, which originated from dehydration of the subducted oceanic crust. No evidence of slab melting was found in the studied rocks. The extensional tectonic setting that dominated the evolution of the Jurassic-Early Cretaceous arc in northern Chile would have favoured the extrusion of huge amounts of volcanic rocks during a relatively short period of time, avoiding thus a mayor interaction with the continental crust.
Key words: Geochemistry, Subduction, Jurassic, Chile.
En la Cordillera de la Costa del norte de Chile, el magmatismo del Jurásico y Cretácico Inferior está representado por potentes secuencias de rocas volcánicas y volcanoclásticas, principalmente de composición intermedia, y rocas sedimentarias. Ellas están intruidas por grandes plutones, además de diques e intrusivos menores. En este trabajo se presentan nuevos datos geoquímicos, de elementos mayores y traza, correspondientes a un grupo de rocas volcánicas y plutónicas del Jurásico Medio a Superior recogidas en seis localidades en la Cordillera de la Costa del norte de Chile (18°30'-24°S). Las rocas volcánicas son bastante homogéneas en sus características petrográficas y geoquímicas y corresponden a lavas porfiríticas de composición basáltico-andesítica a andesítica, de afinidad calcoalcalina a calcoalcalina de alto-K, presentando mayor abundancia de elementos LIL con respecto a elementos HFS y anomalías negativas de Nb y Ti. Todos estos rasgos geoquímicos son típicos de rocas generadas en ambientes de subducción. No se observan diferencias significativas entre rocas volcánicas de distintas regiones o edades. Sin embargo, las rocas plutónicas presentan patrones de REE más planos, levemente más enriquecidos o empobrecidos, con respecto a los de las rocas volcánicas. La evolución y diferenciación de los magmas parentales se debió principalmente a la cristalización fraccionada de plagioclasa, olivino y clinopiroxeno sin una asimilación importante de corteza continental. Sin embargo, los contenidos de La y Th indicarían un aumento de la contribución de los sedimentos subductados en la génesis de los magmas o un aumento del grado de contaminación cortical con el tiempo. La fuente magmática fue posiblemente un manto deprimido, posteriormente metasomatizado por fluidos provenientes de la deshidratación de la placa subducida. No se encontraron evidencias geoquímicas de fusión de la corteza oceánica en las rocas estudiadas. El régimen tectónico extensional que dominó la evolución del arco magmático del Jurásico y Cretácico Inferior en el norte de Chile habría favorecido la extrusión de grandes volúmenes de rocas volcánicas en un período de tiempo relativamente corto, evitándose así la interacción de los magmas con la corteza continental.
Palabras claves: Geoquímica, Subducción, Jurásico, Chile.
Huge amounts of volcanic and plutonic rocks from the Coastal Cordillera of northern Chile are related to the Jurassic-Lower Cretaceous magmatic arcwhich is thought to represent the first stages of the Andean subduction. The Jurassic-LowerCretaceous arc would have been developed in an extensional to transtensional tectonic regime due to a variably oblique subduction of the Phoenix plate under the South American Plate at the western margin of Gondwana (Jaillard et al., 1990; Scheuber and González, 1999; Grocott and Taylor, 2002). The development of the arc is characterized by a long lasting (-100 Ma) plutonic activity and a relatively short (15 to 25 Ma) volcanic activity (Oliveros et al., 2006), the latter is represented by the extrusion of thick sequences of mainly intermediate lava flows. The end of magmatic activity in the Coastal Cordillera would be the result of an eastward migration of the arc during the Cretaceous and Cenozoic until it reached its present day position in the Andean Cordillera (Mpodozis and Ramos, 1989).
Although many authors agree that the volcanic and plutonic rocks cropping out in the Coastal Cordillera were originated from likely depleted mantle-derived magmas, the geological setting of the magmas has been interpreted in various ways, e.g., an island-arc environment (Palacios, 1978), a back-arc setting in an active continental margin (Buchelt and Tellez, 1988; Rogers and Hawkesworth, 1989), magmatism triggered from decompressional melting of the lithospheric mantle without subduction-processes involved (Lucassen et al., 1996) and magmatism controlled by pull-apart structures linked to oblique subduction (Pichowiak, 1994). All these models were established mainly by geochemical and petrological evidence from the Coastal Cordillera between 22° and 25°S. An arclback-arc setting was proposed in the Arica and Iquique areas (18°30'-21°30); a westward migration of the arc and slab melt contribution to the magmas at the beginning in the Middle Jurassic and at the end in the Late Jurassic of the magmatic activity as a result of changes in the tectonic regime and subduction rate was proposed for this area (Kramer et al., 2005).
These models are supported by variable amounts of geological, geochemical and geophysical data but rely on scarce age constraints, since the age of the volcanic units from the Coastal Cordillera has been largely unknown, with the exception of the Iquique region where the strat-¡graphic record is important. In this paperwe present new geochemical data (major and trace elements) of recently dated volcanic and plutonic rocks from the Coastal Cordillera of northern Chile, between 18°30' and 24°S (Oliveros et al., 2006; Oliveros et al., in press a, b), in orderto 1) establish similarities and differences between the volcanic units along the Coastal Cordillera, and 2) constrain the magma sources of the igneous rocks, the tectonic setting for their emplacement, and the possible evolution of the magmas with time.
GEOLOGIC AND TECTONIC SETTING
The Coastal Cordillera of northern Chile, between 18°30' and 23°30' (700 km) is a major geomorphologic unit mainly consisting of Mesozoic volcanic, plutonic and sedimentary rocks (Fig. 1). During the Jurassic and Early Cretaceous a sub-duction-related magmatic arc system developed along the present day Coastal Cordillera; the resulting Jurassic volcanic and sedimentary units crop out as homoclinal sequences that can reach thicknesses of 7,000 to 10,000 meters (Buchelt and Tellez, 1988; Muñoz et al., 1988). The volcanic rocks are represented by the Camaraca and Los Tarros formations (Arica area), Oficina Viz, Caleta Ligate and El Godo formations (Iquique area) and La Negra Formation (Tocopilla, Michilla Mantos Blancos, Baquedano and Antofagasta areas). They mainly consist of porphyritic lavas with phenocryst contents up to 25%. Volcanic breccias, tuffs and sedimentary rocks as well as epiclastic sandstone lenses are less abundant. Basaltic andesites and andesites are by far the main compositional types, although basalts, dacites and rhyolites (ignimbrites) have been reported from several areas (Boric et al., 1990; Cortés, 2000; Kramer et al., 2005). Large and widely distributed plutonic bodies (Coastal Batholith), gabbroic to granitic in composition, intrude the volcanic sequence, together with dikes and stocks. The volcanic and plutonic rocks have predominantly calc-alkaline affinity (Buchelt and Tellez, 1988; Rogers and Hawkesworth, 1989; Lucassen and Franz, 1994), however, tholeiitic affinities have also been found in basaltic-andesite lava flows and gabbrointrusives, representing either initial stages of the arc evolution (Palacios, 1978; Pichowiak et al., 1990; Lucassen and Franz, 1994) or a back-arc setting (Kramer et al., 2005). The Sr-Nd-Pb isotopic compositions of volcanic and plutonic rocks are uniform (87Sr/86Sr: 0.70290-0.70464; 143Nd/144Nd: 0.51250-0.51280; 206Pb/ 204Pb: 17.96-18.42; 207Pb/204Pb:15.55-15.67; 208Pb/ 204Pb: 37.83-38.44), and indicate a depleted mantle source for the magmas, though some assimilation of the Paleozoic crust could have occurred during the evolution of the arc (Roger and Hawkesworth, 1989; Lucassen et al., 2002; Kramer et al., 2005).
The age of volcanism has traditionally been established on the basis of contact relationship with sedimentary units or the fossiliferous content of interbedded sediments, from Early to earliest Late Jurassic. Nevertheless, recent 40Ar/39Ar dating indicate that volcanism took place mainly between 165 and 150 Ma (Oxfordian-Kimmeridgian, ISC 2004) (Oliveros et al., 2006), with a probable older episode at 170-175 Ma (Aaleninan-Early Bajocian; ISC, 2004) in the Iquique area (Kramer et al., 2005; Oliveros et al., 2006, Table 1).
Plutonic rocks belonging to the Coastal Batholith and several dikes have been dated by K-Ar, U-Pb, 40Ar/39Ar, and Sm-Nd methods. The ages range from ca. 106 and 200 Ma, apparently showing two phases of magmatic activity and a possible gap between ca. 160 and 170 Ma (Maksaev, 1990; Andriessen and Reutter, 1994; Pichowiak, 1994; Scheuber et al. 1995; Dallmeyer et al., 1996; Lucassen and Thirlwall 1998; Cortés, 2000; Basso, 2004; González and Niemeyer, 2005; Cortés et al., in press; Oliveros et al., 2006; Oliveros et al. in press a, b). Therefore, volcanism and the second phase of plutonism (ca. 160-106 Ma) were at least partly contemporaneous.
Structural and geochemical studies carried out between 23°S and 28°S suggest that throughout the Jurassic and Early Cretaceous, the subduction system developed under an extensional and later transtensional tectonic regime (Scheuber and González, 1999). The arc itself accommodated most of the deformation, as evidenced by the structures observed in syn-tectonic plutonic bodies that intruded at relative deep crustal levels, whereas tectonic quiescence dominated in the back-arc basins located farther to the east (Prinz et al., 1994; Scheuber and González, 1999; Grocott and Taylor, 2002). The shift from extensional to transtensionalregime occurred likely at the Jurassic-Cretaceous boundary (Scheuber and González, 1999; Grocott and Taylor, 2002) after the deposition of the whole volcanic sequence (Oliveros et al., 2006) and the later activation of the Atacama Fault System (AFS), a 1000 km trench-parallel structure that accommodated strike-slip displacements, at -135-120 Ma. The magmatic arc and fault system were abandoned after ca. 118 to 106 Ma, the magma foci shifted to the east and a compressive tectonic regime started during the Late Cretaceous (Grocott and Taylor, 2002; Cembrano et al., 2005).
Both volcanic and sedimentary units are largely affected by several hydrothermal alteration and/or non-deformational metamorphism processes that likely occurred between 160 and 100 Ma. Some of these processes were partially contemporaneous with the extrusion of volcanics and the emplacement of large and small plutonic bodies (Losert, 1974; Oliveros, 2005). Stratabound and breccia-style Cu (±Ag) deposits, hosted in volcanic rocks, together with vein-type Cu deposits hosted in plutonic rocks are mayor economic deposits in the region. Hydrothermal alteration processes accompanied the generation of these deposits, also resulting in the formation of secondary mineral phases (Losert, 1974; Oliveros et al., 2003).
ANALYTICAL METHODS AND PETROGRAPHIC CHARACTERIZATION OF THE STUDIED SAMPLES
Forty samples of volcanic and plutonic rocks were selected for major, trace and rare earth elements chemical analyses. Samples of volcanic rocks containing amygdales and veinlets were avoided except for 6 strongly altered lavas, one for each studied area, analyzed in order to evaluate the impact of alteration. The analyses were carried out by ICP-AES and ICP-MS at the Centre de Recherches Pétrographiques et Géochimiques, Vandoeuvre les Nancy (France). Uncertainties are under 2% for major elements and under 5% for trace and rare earth elements, exceptforCo, Cs, Hf, Ho, Ni, Rb, Th and Tm, with uncertainties under 10%. Chemical compositions of plagioclase and pyroxene phenocrysts were obtained using a CAMECA SX-100 electron microprobe with 20kV, 0,001 nA and 2-5 |im as analytical conditions, at the Institut des Sciences de la Terre de l'Environnement et de I'Espace of the Université de Montpellier (France).
The studied volcanic rocks correspond exclusively to lava flows from the Camaraca (Arica area), Oficina Viz (Iquique area) and La Negra (Tocopilla, Michilla, Mantos Blancos-Baquedano and Antofa-gasta areas) formations (Fig. 1). These flows are 1 to 10 m in thickness and present a typical morphology with an amygdaloidal bottom, a massive center with porphyritic or aphyric texture and a more altered brecciated top with high amygdale content. Plagioclase, clinopyroxene, idiomorphic olivine, and Fe-Ti oxides (magnetite) and a micro-crystalline matrix (former glass) are the mineralo-gical components of the lavas (Table 1a). Phenocrysts are more abundant in the massive centers whereas glass predominates in the top and bottom of the flows.
The plagioclase phenocrysts composition is labradorite (An6749Ab4931Or31) and is rather homogeneous for rocks of all studied areas (Table 2, Fig. 2a); rims are normally less calcic than cores (An4955 versus An5567) as a consequence of magmatic zonation and not because of Ca-loss due to alteration. Clinopyroxene phenocrysts are predominantly augite (En5334Fs2811Wo49 27), although some pigeonite isfound in recrystallized rims of these phenochrysts; theTi-Ca contents correspond to those of calc-alkaline basalts (Table 3, Fig. 2b,c). The groundmass is composed by plagioclase microlites, normally flux-oriented near the bottom and tops of the lava flows, clinopyroxenes, Fe-Ti oxides and crystallized glass, its texture is intersertal or intergranular (Table 1 b).
All samples were altered to some extent, but those taken from massive centers are much fresher than tops and bottoms. Typical alteration features are partial albitization and/or sericitization of the plagioclase, partial or complete replacement of mafic minerals by chlorite, and glass crystallization to titanite and chlorite. Epidote, quartz, calcite, K-feldsparand minoramountsof pumpellyite, prehnite, actinolite and zeolite can be found depending on the alteration degree (Table 1b).
The composition of the plutonic rock varies from diorite to granite (Table 1a). These rocks are much fresher than the volcanic rocks, although in some samples sericite, chlorite, titanite, actinolite, tourmaline and biotite are present as secondary mineral phases. Partial replacement of clinopyroxene by hornblende occurred as a result of the cooling process of these rocks. Sample RS18 from Michilla is a diorite with ortho and clinopyroxene, with rather homogeneous Mg# (0.77 to 0.69, Table 1a, Fig. 2b).
The major, trace and rare earth (REE) elements abundances for the studied volcanic and plutonic rocks are listed in table 4.
All the studied rock samples are altered to some extent. Alteration is much stronger in volcanic than in plutonic rocks and in the top and bottom of the lava flows than in the centers. Although samples containing amygdales and veinlets were avoided, no wholly fresh rocks were analyzed. Therefore, in order to observe the mobility of the elements due to the alteration processes, six strongly altered rocks, with high LOI, abundant sericite and chlorite in phenocrysts, abundant calcite, pumpellyite or actinolite andlor oxidation of the groundmass, were sampled and analyzed to be compared with fresher rocks. A probable alkali addition by alteration processes is inferred when comparing the total alkali versus silica classification (TAS) diag ram with the Zr/Ti02 versus Nb/Y classification diagram for altered rocks. In the first diagram an important number of samples plot in the trachy-andesite, basaltic trachy-andesite and trachy-basalt fields, in the second diagram they only plot in the andesite and basaltic andesite fields (Fig. 3). Therefore, the alkali enrichment of these samples is likely due to the albitization/sercitization of the plagioclase phenocrysts but notto magmatic processes. The altered samples also showa higher dispersion for large ion lithophile elements (LILE) than high field strength elements (HSFE) relative to the fresher samples, suggesting that HFSE are less mobile. The REE show a global enrichment (Iquique, Mantos Blancos, Antofagasta areas) or depletion (Arica, Tocopilla and Michilla areas), but patterns of altered and fresh samples have no significant differences.
The most altered sample (V0164) was collected in the Iquique area and corresponds to a highly altered top of a lava flow, where pumpellyite, epidote, quartz, chlorite and K-feldspar have almost completely replaced phenocrysts and groundmass. This sample shows a high dispersion in oxides, except by Ti02 and P205, significant depletion in LILE and less important in Sr and Pb, while HSFE and REE contents are relatively similar to the fresh samples. The discussion of the geochemical data will be focused on the HSF and RE elements, which are thought to be immobile during the alteration.
Si02 content (anhydrous base) in the volcanic rocks varies between 50.9% and 57.6%, except V0164 that has a silica content of 63.4% (Fig. 4). The total alkali content varies between 4.1 and 7.6% and in the diagram K20 versus Si02 (not shown), the lavas plot mainly on the high-K calc-alkaline series. Nevertheless, as explained above, the high K20 content of the lavas, also recognized by Palacios (1978) and Buchelt and Tellez (1988) as a distinguishing feature of these rocks, could be due to potassium enrichment during alteration processes.
MgO content is low (3.0-5.4%) and Al203 is high (19.7-14.9%). The variations of major elements with respect to the Si02 content are plotted in figure 4. In spite of the small range in the silica content some correlations are observed as the decreasing MgO, CaO, Al203 and FeO and increasing of K20 abundances with increasing Si02 content, i.e., increasing differentiation. No systematic differences in majorelements abundances are observed neither in lavas from different areas nor in rocks with different ages.
Intrusives and a dike show a wider range of Si02 content, 50.1 to 69.2%, and better defined trends with decreasing Al203, FeO, MgO, Ti02, CaO, increasing K20 and Na20 and rather homogeneous P205. According to their total alkali content they are equivalent to basalt, basaltic andesite, trachy-andesite, trachydacite and rhyolite (Fig. 3). The K20 versus Si02 contents for intrusive rocks from Tocopilla, Michilla and Antofagasta area indicate they belong to the calc-alkaline series whereas those from Iquique belong to the high-Kcalc-alkaline series.
TRACE ELEMENTS AND REE
Multielement patterns for all the studied samples are similar showing a stronger enrichment in LILE than in HFSE with respect to the primitive mantle and variable LILE contents (Fig. 5), since these elements are more mobile during alteration and incompatible during early stages of crystal fractionation. Negative Nb (Nb-Ta trough) and Ti anomalies, and highly variable but ratherflat Th-U paths are also common features in the studied rocks. Volcanic rocks from different areas and ages show no significant differences in their trace element patterns except a generally lower LILE enrichment in the lavas from the Arica area. Trace element contents in basic plutonic rocks are lower than in the volcanic rocks but the trace element patterns of both are sub-parallel, with steeper Nb and Ti anomalies in the plutonic rocks. In spite of the general large scatter in LILE abundances some systematic patterns are observed: Sr<Ba<Rb and Bagenerally lowerthan Th. Ta, Nb, Zr have a well defined positive correlation with Si02 content, whereas Th has also positive correlation but with higher dispersion. Compatible elements such as Cr, Ni and V have negative correlation with Si02 (Table 4), Sr behavior seem coupled with Ca as it has also a negative correlation with Si02 (Table 4). Cu content does not show correlation either with Si02 nor with Mg#, and it is noteworthy to point out that the most altered samples, e.g., V0144, V0177, V047, V0187, V0164, normally show lower Cu contents than fresher samples (10-70 ppm versus 50-200 ppm). Rare earth elements (REE) (Fig. 6) display fractionated patterns in the volcanic rocks with a higher enrichment in light REE (LREE, La to Sm) than heavy REE (HREE, Gd to Lu) relative to chondrite abundances. (La/Yb)N ratios range between 2.2 and 5.0. The HREE show a rather flat pattern and have abundances 13-37 times chondritic values, whereas the LREE are 30-80 times the chondriticvalues. Eu negative anomalies are observed in most of the analyzed lava flows in the volcanic rocks. Some samples from Antofagasta, Mantos Blancos and Iquique areas show slight or no Eu anomaly; they would correspond to the less differentiated rocks because they also have a low Zr content (Zr being an indicator of the differentiation degree) and EuN has a clear negative correlation with Zr content. YbN and (La/Lu)N ratios have slight positive correlations with Si02 and Zr, though the scatter of the Si20 content is large. Basic plutonic rocks from Tocopilla and Michilla areas show less enriched trace element patterns, positive Eu anomalies and less fractionated LREE/HREE than volcanic rocks. Nevertheless, three samples from Mantos Blancos-Baquedano area, with low (49%) to normal (54%) Si02 content, show a similar pattern (Fig. 6). Positive Eu anomalies characterize also these rocks. The opposite is observed in the granitic intrusives (V0184 in Iquique area and V0153 in Antofagasta area) for which LREE are strongly enriched, whereas the HREE maintain the flat pattern.
The dike from the Iquique area (sample V0161) shows a distinctive pattern relative to the contemporaneous plutons (V0184 and V0185) and to the Oficina Viz lavas. It is slightly less enriched in LILE and has significant lower content in HFSE, very pronounced Th, Nb and Ti02 negative anomalies and a Pb positive anomaly. It has a very flat REE pattern, as much as the basic intrusives in Tocopilla and Michilla, and an Eu positive anomaly.
A remarkable feature of the studied volcanic samples, and in general of the volcanic rocks from the Coastal Cordillera of northern Chile, is their textural, mineralogical and chemical homogeneity, as pointed out by Lucassen et al. (2006) and supported by this work. Porphyritic intermediate lavas are by far the more abundant rocks and their geochemical characteristics are shared by rocks of extreme areas (Arica-Antofagasta, 700 km apart) and ages (Iquique, Tocopilla and Antofagasta areas, 175-170, 165-160 and 150-154 Ma, respectively). The general higher enrichment in LILE than HFSE, LREE/HREE fractionation, Nb-Ta trough and the enrichment of Pb relatively to Ce are common features in all studied samples; such characteristics are considered as representative of a subduction-related origin for the magmas (Pearce, 1982). Nevertheless, some differences are observed in the trace element abundances between plutonic and volcanic rocks, as well as in volcanic rocks with low Si02 content. These differences are mainly represented by subparallel but more or less enriched trace element patterns, depending on whether the rocks are more or less differentiated relatively to the main group of volcanic rocks. These chemical variations indicate therefore that the rocks from the Coastal Cordillera were likely to have evolved and differentiated mainly by fractional crystallization. The Pearce ratio diagrams show that crystal fractionation was dominated by the crystallization of plagioclase and olivine and, probably, clinopyroxene to a lesser extent (Fig. 7).
CONSTRAINTS ON MAGMA SOURCES AND EVOLUTION
Despite the quantity of data, the Mg# (45-74) and the MgO% (1.01-5.27) of the analyzed rocks (Table 4) indicate that no primitive magmas were analyzed and suggest previous olivinefractionation. Sr isotopic signatures for the volcanic rocks from the Iquique, Arica, Tocopilla and south of Antofagasta areas indicate that the magmas were derived from a depleted mantle source (Rogers and Hawkesworth, 1989; Lucassen and Franz, 1994; Kramer et al., 2005). Zr/Nb ratios are indicative of mantle depletion as they are not influenced by fluid enrichment orfractional crystallization; Nb/Zr ratios from the studied samples have a narrow range similar to the normal mid-ocean ridge basalts (N-MORB) composition and indicate either slight depletion or enrichment relative to N-MORB (Fig. 8a). These ratios do not show a significant variation with time. Lowchondriticnormalized(La/Yb)N ratios (2.2-5.0) or flat REE patterns are interpreted as the result of high degrees of partial melting from a mantelic source. YbN>10 excludes the possibility of the garnet as a residual phase in the source of the magmas.
The poor radiogenicSrsignaturesforthe Jurassic volcanic and plutonic rocks would rule out crustal contamination as a majorfactor in differentiation and evolution of the parental magmas; nevertheless, Pb signatures in lavas from the Iquique area indicate that the Pb system is dominated by crustal signature (Kramer et al., 2005). In the Th/Yb versus Ta/Yb diagram, the studied samples follow either the subduction component or the crustal contamination trends (Fig. 9).
The Ba, K and Sr abundances relative to HFSE elements such as Nb and Ta would favour the hypothesis of fluid enrichment in the slab/mantle boundary (Peate et al., 1997). Addition of slab melts into the mantle wedge has been invoked as a process in the generation of the Oficina Viz Formation lavas in Iquique area (Kramer et al., 2005). In the case of the samples studied here, Nb/ Ta ratios are generally lower than the MORB values and Sr/Y ratios are relatively low (<30) (Fig. 8b), therefore a slab melt component is unlikely in the genesis of these rocks.
Th/La and Sm/La ratios can be used to trace sediment addition to the mantle wedge (Plank, 2005); in figure 8c, samples plot directly along the sediment component vector, suggesting that this process was involved in the generation of the parental magmas. In addition, the Th/La ratios for volcanic rocks exhibit a slight positive correlation with age (Fig. 8d), whereas the lavas from Antofagasta, Arica and Tocopilla areas have in general higherTh/La ratios than thosefrom Iquique area. In the Ce/Pb versus Nb/U diagram the samples plot in the arcs field but also in the marine sediments field (Fig. 8e).
Ba/Ta and Ce/Pb ratios show no correlation when plotted versus age of the rocks (Fig. 8f,g). The same is valid for major elements, only Ti presents a slight negative correlation, and other trace element ratios such as (La/Lu)N, Nb/Ta, Zr/Y, (La/Yb)N and Sr/Y. Therefore, the sources that originated the parental magmas and the processes which dominated theirevolution and differentiation did not change significantly from 175-170 (Oficina Viz, Iquique area) to 150 Ma (La Negra Formation, Antofagasta). Only the amount of the sediment component added to the mantle wedge during slab/mantle interaction in the subduction zone seems to have increased as the arc evolved. In this sense, and based on Nd, Pb and Sr isotope composition, Lucassen et al. (2006) also postulated a rather uniform depleted subarc mantle source over the entire studied region.
TECTONIC SETTING FOR THE VOLCANIC AND PLUTONIC ROCKS
The projection of the chemical features of the studied samples on different geotectonic discrimination diagrams confirms the dominant calc-alkaline orogenic signature of these rocks. In the La/10-Y/5-Nb/8 discrimination diagram (Fig. 10), the volcanic rocks cluster near the calc-alkaline/ volcanic arc tholeiites transition zone. The compositions of the clinopyroxene phenocrysts show the same pattern (Fig. 2.c) and in the Th/Yb versus Ta/Yb diagram (Fig. 9) the volcanic rocks plot near the boundary between the oceanic arc and active continental margin fields of Pearce (1983) or in the active continental margin field ofActive 'Alkaline' Oceanic arcs continental + oceanic margins arcs Gorton and Schandl (2000). In the Iquique area a back-arc setting for the Middle to Upper Jurassic sedimentary and volcanic series has been recognized in a region where the lavas have tholeiitic affinities and bimodal volcanism occurs (Kossler, 1998; Kramer et al., 2005). In this context, the lavas from the Oficina Viz Formation, the oldest unit in this area, are thought to have erupted in a frontal arc position and a westward migration of the arc front has been proposed (Kramer et al., 2005). However, this tectonic setting would have developed only locally, since most of the volcanic and plutonic rocks, with the exception of those in the Oficina Viz Formation studied here, are contemporaneous with the back-arc series described by Kramer et al. (2005) (-165-155 Ma) but their geochemical characteristics suggest that they were emplaced in an intra-arc setting.
An extensional to transtensional tectonic regime has been proposed forthe development of the magmatic arc in the Coastal Cordillera of northern Chile. A convergent margin dominated by oblique subduction led to strain partitioning, resulting in the development of arc-normal extension and sinistral strike-slip shearzonesthat would have controlled the magmatic activity and deformation of the arc (Scheuber and González, 1999; Grocott and Taylor, 2002). The fissural volcanism typical for the Coastal Cordillera of northern Chile would have taken place in a weakened trench-parallel sinistral shear zone (Scheuber and González, 1999) or in subsiding intra-arc basins related to arc normal extension in a retreating (low convergence rate) boundary (Grocott and Taylor, 2002). Lower Cretaceous volcanic rocks from the Coastal Cordillera of Central Chile are thought to have extruded also in a setting dominated by subduction and controlled by intra-arc extension due to low-spreading rate of 5 cm yr1 (Áberg et al., 1984; Morata and Aguirre, 2003). These rocks have textural, mineralogical, geochemical and isotopic similarities with the Jurassic volcanic rocks from the Coastal Cordillera of northern Chile, consistent with similar geotec-tonic conditions.
The short time interval of the volcanic activity relative to the whole magmatic activity of the Jurassic-Early Cretaceous arc in northern Chile (Oliveros et al., 2006) and the relatively homogeneous geochemical character of the volcanic rocks would suggest that a distinctive tectonic setting dominated the continental margin from Middle to Late Jurassic (170 to 150 Ma). During this time, the magmas would have ascended rapidly reaching the surface without major interaction with the continental crust.
A suite of Middle to Upper Jurassic volcanic and plutonic rocks from the Coastal Cordillera of northern Chile is characterized by chemical and mineralogical homogeneity. The volcanic rocks are highly porphyritic basaltic-andesites and andesites, whereas the intrusive rocks are diorites, granodiorites and granites.
These volcanic and plutonic rocks have calc-alkaline to high-K calc-alkaline affinities, high LILE contents relative to HFSE, Nb-Ta throughs and Ti anomalies, LREE/HREE fractionation and enrichment in Pb over Ce, indicating that they result from subduction-related magmatism. Some differences in trace element patterns between the volcanic and plutonic rocks are explained by chemical differentiation from parental magmas due to plagioclase-olivine-clinopyroxene dominated fractional crystallization.
Assimilation of the continental crust did not play a major role in the evolution of the magmas. However, Th and La content in the studied rocks would indicate increasing sediment contribution or crustal contamination with time. The magma source was likely to have been a depleted mantle metasomatized by slab fluids. No evidence of slab melting involved in the genesis of magmas was found. Finally, the relatively short time interval for volcanism with respect to the whole activity of the arc favoured the extrusion of relatively homogeneous magmas.
This study was supported by the Institut de Recherche pour le Développement (IRD) grant and fellowship. S.M. Kay, J. Cembrano and F. Lucassen are thanked for their helpful reviews which greatly improved the original version of this manuscript.
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Manuscript received: May 29, 2006; accepted: April 3, 2007