LA-ICPMS U–Pb Igneous and Detrital Zircon Ages from the Chile Triple Junction and the Taitao Peninsula, Chilean Patagonia
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Geochemical Journal, Vol. 47, pp. 149 to 165, 2013 Shallow-depth melt eduction due to ridge subduction: LA-ICPMS U–Pb igneous and detrital zircon ages from the Chile Triple Junction and the Taitao Peninsula, Chilean Patagonia RYO ANMA1* and YUJI ORIHASHI2 1Faculty of Life and Environmental Sciences, University of Tsukuba, Ten-nodai 1-1-1, Tsukuba, Ibaraki 305-8572, Japan 2Earthquake Research Institute, The University of Tokyo, Yayoi 1-1-1, Bunkyo-ku, Tokyo 113-0032, Japan (Received April 25, 2012; Accepted January 26, 2013) To understand the processes of melt eduction in a ridge subduction zone, we performed U–Pb dating on zircons sepa- rated from igneous and sedimentary rocks that were newly dredged from the Chile Triple Junction area and from volcanic rocks collected from the Taitao peninsula, southern Chile. The youngest fraction of the U–Pb age population was used to estimate the age of magmatism or sedimentation. Our new results indicate that the fore-arc region became volcanically active over a period of ~0.4 m.y., after obduction of the Taitao ophiolite (~5.7 to 5.2 Ma) from the west and after granite intrusions related to ridge subduction at ~6 Ma. Fore-arc volcanism produced ejecta of basaltic to dacitic compositions and migrated from offshore (~5.3 Ma) to inland (~4.6 Ma) along the Chile Margin Unit that trends northeast–southwest. The volcanism further extended east to produce the dacitic volcanic plug of Pan de Azucar (~4.3 Ma) and lavas in Fjord San Pedro (~2.9 Ma). The migration took place at a rate of ~2.3 cm/y to ~5.3 cm/y. Another intrusion of a granite pluton, widely distributed offshore of the Taitao ophiolite, took place at ~4.0 Ma. Distributions of old detrital zircon and zircon xenocryst ages were used to evaluate, respectively, the influence of subducted sediments and igneous crustal material. Our results indicate that crustal material influenced only Pan de Azucar and Fjord San Pedro dacites; other acidic magmatism shows moderate evidence for incorporation of subducted sediments. Therefore, melts were formed at shallow depths near the triple junction, due mainly to partial melting of the subducted slab and sediments, and then ejected instantaneously. Depleted oceanic materials became anhydrous, and a volcanic gap was formed along the Andean arc. Keywords: ridge subduction, slab melting, subducted sediment, U–Pb ages, zircon, Chile, LA-ICPMS to understand not only the processes of magma genesis INTRODUCTION but also the formation of initial continental crusts (Nel- Several authors have suggested that ridge subduction son and Forsythe, 1989). is important for the genesis of large granite batholiths and/ Among current ridge subduction zones (e.g., McCrory or paired metamorphic belts (Iwamori, 2000; Iwamori et et al., 2009), the Chile Triple Junction (CTJ) (Fig. 1a) is al., 2007; Aoya et al., 2009) because of its high heat flow expected to be the best example for a case study because: (DeLong et al., 1979). It has been suggested that igneous (1) the subducting ridge system is a topographically rocks with adakitic compositions are a product of partial prominent feature; (2) the half-spread-rate is ~4 cm/y, and melting of a young and hot subducted slab under garnet– a significant portion of the world’s oceanic crust has amphibolite to eclogite conditions (Rapp et al., 1991; formed under such a rate; and (3) the mid axes of the Defant and Drummond, 1990; Kay et al., 1993; Peacock Chile Ridge system are sub-parallel to the continental et al., 1994; Drummond et al., 1996; Springer and Seck, margin, and thus along-arc and across-arc variation is 1997), and hence, are related to ridge subduction. Fur- expected. ther, compared with the genesis of trondhjemite–tonalite– A chain of active volcanoes in the Southern Volcanic granodiorite (TTG) rocks, they are recognized widely to Zone (SVZ) extends from low latitudes along the Andean form initial continental crusts during Archean (Martin, Mountains (Fig. 1a) and is disrupted at Hudson Volcano 1993, 1996). Therefore, it is important to study the (H in Fig. 1a) near the latitude of CTJ. To the south from magmatism occurring in recent ridge subduction zones CJT, active volcanoes of the Austral Volcanic Zone (AVZ) are sparsely distributed (e.g., Stern et al., 1976; Ramos, 1989; Stern, 2004 and references therein). Volcanoes of *Corresponding author (e-mail: [email protected]) SVZ typically have basaltic to andesitic compositions with Copyright © 2013 by The Geochemical Society of Japan. arc-signatures whereas those of AVZ have adakitic com- 149 Fig. 1. Tectonic settings (a) and geology (b) of the study area. The Patagonian volcanic gap lies in between the Hudson (H) and Lautaro (L) volcanoes. See text for explanations. positions (Martin, 1993; Lopez-Escobar et al., 1995; Stern extends into the fore-arc region, and another chain of and Kilian, 1996; Stern, 2004; Shinjoe et al., 2013). The Middle to Late Miocene plutonism and Miocene adakitic gap between Hudson Volcano and Lautaro Volcano (L in magmatism (Kay et al., 1993; Ramos et al., 2004; Fig. 1a) at the northern tip of the AVZ is known as the Orihashi et al., 2013) appears behind the active volca- Patagonian Volcanic Gap (Orihashi et al., 2004; Stern, noes of the AVZ. Together with the Patagonian Volcanic 2004). The contrast in rock compositions between the two Fields in the extra back-arc side, this area provides a good volcanic zones and the presence of a volcanic gap are the opportunity to study both along-arc and across-arc varia- first-order along-arc variations observed around the Chile tion of magmas that are spatially and temporally con- Ridge subduction zone. strained (Stern et al., 1990; Ramos and Kay, 1992; Kay Recent geochronological studies reveal the presence et al., 1993; Gorring et al., 1997). of Miocene to Pliocene granites in the Late Jurassic to Finally, the presence of the Taitao ophiolite, exposed Eocene Patagonian Batholiths that extend from 37°S to ~50 km southeast of the current CTJ, and related activi- the southern tip of the South American continent ties of acidic magmas in the westernmost part of the Taitao (Pankhurst et al., 1999; Suarez and De la Cruz, 2001; Peninsula (Fig. 1b) provide a unique opportunity to com- Thomson et al., 2001; Thomson, 2002; Herve et al., 2007). pare the compositions of incoming materials (altered oce- The distribution of such young granite plutons almost anic crust and sediments), magmatic and hydrothermal overlaps with the SVZ volcanoes between latitudes 40°S processes occurring in the sea at present and in the time and 45°S. Hudson Volcano in the southernmost SVZ of the ophiolite obduction, and compositions of magmas marks the location of the back-arc side of the Miocene– as outcomes of these processes (see Section “Geology of Pliocene magmatism. To the south, Miocene plutonism the Taitao peninsula”). 150 R. Anma and Y. Orihashi To understand the processes of melt eduction in a ridge in the eastern sections of the batholith have Late Paleozoic subduction zone and to evaluate the influence of ages (354 to 253 Ma), whereas basement rocks distrib- subducted sediments and lower crustal components on uted in the fore-arc region (including the Taitao penin- magmatism in ridge subduction zones, we performed U– sula) have Late Permian to Jurassic ages (258 to 207 Ma). Pb dating on zircons that were separated from igneous The basement rocks also contain detrital zircons younger rocks and clastic sediments distributed near the CTJ. than 600 Ma, based on U–Pb ages (Thomson and Herve, 2002). The Chile Margin Unit is a mixture of lavas, TECTONIC SETTINGS pyroclastic rocks and semi-consolidated sediments and The study area is characterized by two oceanic plates, was supposedly distributed in the fore-arc area of the the Nazca plate in the north and the Antarctic plate in the Chilean margin during the ridge subduction events south. These plates are separated by spreading centers of (Bourgois et al., 1993; Guivel et al., 1999). Mpodozis et the Chile Ridge system and subduct beneath the South al. (1985) reported six whole-rock K–Ar ages ranging American plate. The convergent rates of the two plates, from 4.4 to 2.5 Ma from the Chile Margin Unit. with respect to the South American plate, are 9 cm/y and The Taitao ophiolite consists of a completed sequence 2 cm/y, respectively (Fig. 1a) (Herron et al., 1981; Cande typical of a Penrose-type oceanic lithosphere (Anony- et al., 1982; Cande and Leslie, 1986; Tebbens and Cande, mous, 1972); pillow lavas and sheet flows interbedded 1997; Tebbens et al., 1997). The Chile Ridge system has by clastic deposits, sheeted dike complexes, gabbros and a central axis that trends north–northwest (NNW) and ultramafic rocks from top to bottom (Mpodozis et al., consists of several segments separated by fracture zones 1985; Forsythe et al., 1986; Bourgois et al., 1992, 1993; that trend east–northeast (ENE). Nelson et al., 1993) and show evidence of hydrothermal The current plate configuration (Fig. 1a) and plate alteration that is typical of ocean-floor metamorphism kinematic data (Cande and Leslie, 1986; Breitsprecher (Shibuya et al., 2007). The ultramafic rocks provide evi- and Thorkelson, 2009) allow us to deduce that a ~700 km dence for at least two stages of melting. The first stage segment of the spreading Chile Ridge started to subduct took place at ~1.6 Ga and the other stage is recent and at high latitudes approximately 14 Ma. The position of related to magmatism in the Chile Ridge (Schulte et al., the CTJ migrated north until it reached the southern end 2009).