39Ar dates from the Las Tazas complex, northern Chile: Tectonic significance

Journal of South American Earth Sciences 13 (2000) 115±122

New

40

Ar/39Ar dates from the Las Tazas complex, northern Chile: Tectonic signi®cance Je€ Wilson a,*, R. David Dallmeyer b, John Grocott a

a

School of Geological Sciences, Kingston University, Penrhyn Road, Kingston-upon-Thames KT1 2EE, UK b Geology Department, University of Georgia, Athens, GA 30602-2501, USA Accepted 27 March 2000

Abstract The Atacama Fault Zone is a major Mesozoic structure that trends along the Coastal Batholith of northern Chile. Part of the fault zone underwent a kinematic change from dip-slip to strike-slip displacement during the Early Cretaceous. The Las Tazas complex intruded the fault zone during this change. New analyses of country rock protomylonites from the edge of the complex ®rmly constrain the age of the change to 130 Ma and con®rm that the complex was emplaced during active displacement along the fault zone. The intrusion heated its immediate country rocks and allowed localised ductile shearing during emplacement. Upper crustal intrusions like the Las Tazas complex are ideal targets for geochronological studies of major shear zones. 7 2000 Elsevier Science Ltd. All rights reserved.

1. Introduction A geochronological framework is an essential prerequisite to understanding the spatial and temporal evolution of magmatic arcs. A number of such studies have been integral to the study of the Coastal Batholith of northern Chile, de®ning the episodic plutonic and volcanic events that occurred during its growth (e.g., Berg and Baumannn; Brook et al., 1986; Dallmeyer et al., 1996). In this contribution we examine an aspect of the kinematic evolution of the Atacama Fault Zone, a major structure that was intimately linked with the construction of the batholith during the Mesozoic. Previous studies have identi®ed a change from dip-slip to transcurrent displacement along a segment of the fault zone between 130 Ma and 125 Ma (Brown et al., 1993; Dallmeyer et al., 1996). New geochronological data are presented from protomylonitic country rocks within the Atacama Fault Zone along the edge of the Las Tazas complex, a high-level intru* Corresponding author. Current address: 6, Abercorn Grove, Edinburgh, Scotland.

sion that was emplaced during the kinematic change. Two new samples have been analysed in order to constrain the kinematic change along the Atacama Fault Zone more closely and to con®rm whether emplacement was indeed syntectonic. Such high-level intrusions cool rapidly to ambient crustal temperatures after emplacement and have the potential to ``freezein'' geochronological evidence of short-lived deformation episodes, which are generally overprinted during regional deformation. This illustrates the unique geochronological contribution of shallow intrusions in the study of major fault zones. 2. Regional geology The Coastal Batholith of northern Chile crops out between 188S and 268S along the Coastal Cordillera (Fig. 1). It comprises a series of Permian to Early Cretaceous plutonic complexes that were emplaced into Palaeozoic metasedimentary basement (Brook et al., 1986). The complexes comprise calc-alkali hornblendebiotite gabbros, diorites, tonalites, granodiorites, and subsidiary granites that were emplaced into the upper

0895-9811/00/$ - see front matter 7 2000 Elsevier Science Ltd. All rights reserved. PII: S 0 8 9 5 - 9 8 1 1 ( 0 0 ) 0 0 0 0 8 - 0

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J. Wilson et al. / Journal of South American Earth Sciences 13 (2000) 115±122

crust (Dallmeyer et al., 1996). They have a common metaluminous character, with low Sri (0.703±0.705), and display similar geochemical patterns, suggesting a similar, possibly common mantle source (Berg and Baumann, 1985; Brown, 1991). The batholith grew episodically as a succession of volcanic and plutonic events that were separated by magmatic hiati (Brown, 1991; Dallmeyer et al., 1996). The Permo-Triassic and Jurassic plutons were emplaced as laccoliths, sometimes along low-angle extensional shear zones (Grocott et al., 1994; Grocott and Wilson, 1997). In contrast, the Early Cretaceous plutons were emplaced as vertical sheets during arc-parallel transcurrent displacement along the Atacama Fault Zone (HerveÂ, 1987; Uribe, 1987; Scheuber and Andriessen, 1990; Scheuber et al., 1995; Wilson, 1996). The Atacama Fault Zone is

Fig. 1. Geological map of the area between 25830'S and 26830'S, Atacama region, northern Chile.

Fig. 2. Geological map of the Las Tazas complex. Mylonitic branches of the Atacama Fault Zone bound the complex to the west and east. 40Ar/39Ar sample localities are indicated. Previously published data is denoted as follows; 1Naranjo et al. (1984), 2Berg and Baumann (1985), 3Brook et al. (1986) and 4Dallmeyer et al. (1996). The new dates from the eastern edge of the Las Tazas complex are underlined. Cross-section A-A' is illustrated in Fig. 3.

J. Wilson et al. / Journal of South American Earth Sciences 13 (2000) 115±122

de®ned by a number of sub-parallel brittle shear zones that trend along the Coastal Cordillera of Chile, with localised ductile segments in the vicinity of plutons (Brown et al., 1993; Scheuber et al., 1995). It became active between the Late Jurassic and Early Cretaceous, displaying a pattern of early dip-slip and later sinistral transcurrent displacement (Scheuber and Andriessen, 1990; Scheuber et al., 1995; Dallmeyer et al., 1996). It has been interpreted as a trench-linked transcurrent fault (sensu Woodcock, 1986) which formed in response to oblique subduction (Zonenshayn et al., 1984; Scheuber and Andriessen, 1990; Brown et al., 1993). Active displacement ended in the mid-Cretaceous when the arc was abandoned. The Las Tazas complex occupies a special place in the structural history of the Atacama Fault Zone since it was emplaced along it during this change from dip-slip and transcurrent displacement. 3. Las Tazas complex The Las Tazas complex crops out at 268S in the Coastal Cordillera of northern Chile and covers an area of 232 km2. It comprises a larger, elongate granodioritic pluton and a smaller, square monzonitic pluton (Fig. 2). Both plutons have a calc-alkali, metaluminous character, with initial 87Sr/86Sr values of 0.7033±0.7035 and eNd (130) values of +5.1 to +6.4, indicating that they were produced from a LREEenriched mantle or juvenile crust magmatic source (Berg and Baumann, 1985; Hodkinson et al., 1995). The Las Tazas complex forms a north-south striking vertical slab which is ¯anked by La Negra Formation andesites to the east, and tonalites of the Upper Jurassic Las Animas complex to the west (Fig. 3). Al-inhornblende geobarometry indicates that the granodioritic pluton was emplaced at 7-km depth (Dallmeyer et al., 1996). The Las Tazas complex was emplaced during the change from dip-slip to transcurrent displacement

Fig. 3. East-west cross-section A-A' across the Las Tazas complex. Horizontal and vertical scales are equal.

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along the Atacama Fault Zone. Two branches of the fault zone record this kinematic change. They crop out along the western and eastern edges of the complex (Figs. 2 and 3) and display ductile structures along its length, although structures are exclusively brittle along strike, away from the complex. The complex cuts a pre-existing ductile shear zone to the west, marked by a belt of amphibolite facies mylonites that formed during pre-emplacement shearing. The mylonites contain a vertical foliation that strikes north-south and a vertical hornblende stretching lineation (Fig. 4), indicating formation during dip-slip displacement. Asymmetric andesine s- and d- porphyroclasts (Passchier and Simpson, 1986) within the mylonites display consistent crystal imbrication (Choukroune and Lagarde, 1977), asymmetric pull-aparts (Hippertt, 1993), and asymmetric pressure shadows (Simpson and Schmid, 1983). These display an east-side-down shear sense. A ductile shear zone deforms the eastern edge of the Las Tazas complex and its country rocks, resulting from post-emplacement shearing. The shear zone comprises mylonites with a vertical foliation that strikes northsouth and a horizontal stretching lineation (Fig. 4), re¯ecting formation during transcurrent displacement. Asymmetric d-s porphyroclasts and folded veins within the main foliation display a dextral shear sense,

Fig. 4. Stereographic projections of foliations and lineations from mylonites along the western and eastern edges of the Las Tazas complex.

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J. Wilson et al. / Journal of South American Earth Sciences 13 (2000) 115±122

while quartz c-axis fabrics contain a component of dextral asymmetry (Law, 1990). Geochronological dating has been integral to previous studies of the Las Tazas complex. Naranjo et al. (1984), Brown et al. (1993), and Dallmeyer et al. (1996) have identi®ed concordant K/Ar, Rb/Sr mica and 40Ar/39Ar hornblende cooling ages of 0130 Ma for the Las Tazas complex and the pre-emplacement mylonites along its western edge (Fig. 2). They suggested from the concordant ages that the complex was emplaced during active displacement. Brown et al. (1993) and Dallmeyer et al. (1996) also identi®ed a switch from dip-slip to transcurrent displacement along the Atacama fault Zone. They stated that it occurred at 0126 Ma, based on an 40Ar/39Ar isotope correlation age of 125.7 2 0.6 Ma from transcurrent mylonites exposed along a parallel fault branch of the Atacama Fault Zone to the east. Grocott et al. (1994) postulated that the kinematic switch occurred as the result of a change in the direction of plate convergence vectors, at the end of a period of Mesozoic extension. 4.

40

Ar/39Ar analytical methods

The techniques used during the 40Ar/39Ar analysis of mineral concentrates generally followed those described by Dallmeyer and Takasu (1992). Optically pure (>99%) mineral concentrates and sized wholerock powders were wrapped in aluminium foil packets, encapsulated in sealed quartz vials, and irradiated in the TRIGA Reactor at the U.S. Geological Survey in Denver. Variations in the ¯ux of neutrons along the length of the irradiation assembly were monitored with several mineral standards, including Mmhb-1 hornblende (Sampson and Alexander, 1987). The samples were incrementally heated until fusion in a double-vacuum, resistance-heated furnace following methods described by Dallmeyer and Gil-lbaruchi (1990). Measured isotopic ratios were corrected for total system blanks, the e€ects of mass discrimination and interfering isotopes produced during irradiation. 40 Ar/39Ar ages were calculated from corrected isotopic ratios using the decay constants and isotopic abundance ratios listed by Steiger and JaÈger (1977). Intralaboratory uncertainties have been calculated by statistical propagation of uncertainties with measurement of each isotopic ratio (at two standard deviations of the mean) through the age equation. Interlaboratory uncertainties are ca. 2 1.25±1.5% of the quoted age. Total-gas ages have been computed for each sample by appropriate weighting of the age and percent 39Ar released within each temperature increment. A ``plateau'' is considered to be de®ned in the ages recorded by two or more contiguous gas fractions (with similar apparent K/Ca ratios), each repre-

senting >4% of the total 39Ar evolved (and together constituting >50% of the total quantity of 39Ar evolved), and mutually similar within a 21% intralaboratory uncertainty. Analyses of the Mmhb-1 monitor indicate that apparent K/Ca ratios may be calculated through the relationships 0.518 (20.005)  (39Ar/37Ar) corrected, plateau portions of the analyses have been plotted on 36Ar/40Ar isotopic correlation diagrams. Regression techniques followed the methods of York (1969). A mean square of the weighted deviates (MWSD) has been used to evaluate isotopic correlations. 5. Results Multigrain hornblende concentrates were prepared from two samples collected within the Atacama Fault Zone along the eastern edge of the Las Tazas complex. Sample GCH-69 is a tonalite with a magmatic fabric, from the country-rock along the eastern side of the fault zone, located at 70821 '170W 26833 '500S (Fig. 2). Sample GCH-68 is a penetratively deformed, protomylonitic tonalite that was collected 030 m to the west of sample GCH-69. Petrographic descriptions of the samples are provided in the Appendix. The 40Ar/39Ar analytical data are provided in Table 1 and are displayed in Fig. 5 as apparent age and K/Ca spectra. The two hornblende concentrates display variably discordant apparent age spectra. The relatively small volume low-temperature gas fractions record considerable variation in the apparent ages. These are matched by ¯uctuations in apparent K/Ca ratios, which suggest that experimental evolution of argon occurred from compositionally distinct, relatively non-retentive phases. These could have been represented by: 1) very minor, optically undetectable mineralogical contaminants in the hornblende concentrates; 2) petrographically unresolvable exsolution or compositional zonation within constituent hornblende grains; 3) minor chloritic replacement of hornblende; and/or 4) intracrystalline inclusions. Most intermediate- and high-temperature gas fractions display little intrasample variation in the apparent K/Ca ratios, suggesting that experimental evolution of gas occurred from compositionally uniform sites. The intermediate- and hightemperature gas fractions experimentally evolved from the massive tonalite samples GCH-69 record generally similar apparent 40Ar/39Ar ages which de®ne a plateau age of 196.5 2 0.3 Ma (Fig. 5). 36Ar/40Ar v. 39Ar/40Ar isotope-correlations of the plateau data are well de®ned, and de®ne an inverse ordinate intercept (40Ar/36Ar ratio) of 294.6. This is similar to that of the present-day atmosphere and suggests no signi®cant intracrystalline contamination with extraneous (``excess'') argon components. Using the inverse

J. Wilson et al. / Journal of South American Earth Sciences 13 (2000) 115±122

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Table 1 40 Ar/39Ar analytical data for incremental heating experiments on hornblende concentrates from the Las Tazas complex, northern Chile Release temp (8C)

(40Ar/39Ar)a

(36Ar/39Ar)

Sample GCH-69 J=0.009832 Tonalite 610 18.71 0.02702 710 16.45 0.01427 735 24.37 0.03720 760 25.69 0.02934 785 25.75 0.03373 810 26.48 0.04247 835 20.45 0.03082 860 18.90 0.02628 885 17.01 0.01983 910 13.76 0.00860 940 14.46 0.01031 Fusion 116.35 0.00160 Total 17.32 0.01888 Total without 6108C-8108C and fusion Sample GCH-68 J=0.009832 Protomylonitic 610 8.40 0.00470 710 8.45 0.00234 735 9.59 0.00662 760 9.68 0.00516 785 10.20 0.00517 810 10.36 0.00745 835 10.09 0.00617 860 9.33 0.00310 885 8.69 0.00312 910 8.30 0.00174 940 8.31 0.00161 970 8.45 0.00265 Fusion 12.11 0.01462 Total 8.92 0.00362 Total without 6108C-8608C and fusion a

Measured.

C

(37Ar/39Ar)

39

0.828 0.649 0.835 1.014 1.441 2.457 3.009 3.876 5.786 6.659 5.610 4.213 4.171 Tonalite 0.330 0.216 0.200 0.256 0.244 0.395 0.447 0.613 0.834 0.635 1.059 1.534 1.225 0.752

Corrected for post-irradiation decay of

37

%40Ar non-atmos+

36

Apparent Age (Ma)

Location: 70821 '170W 26833'500S 9.69 57.66 6.22 74.65 1.68 55.15 1.71 66.55 1.76 61.71 1.92 53.32 11.48 56.616 13.15 0.54 12.07 68.24 18.49 85.36 17.37 82.02 4.45 99.14 100.00 71.55 72.56

0.83 1.24 0.61 0.94 1.16 1.57 2.66 4.01 7.93 21.05 14.80 71.58 11.68

182.020.3 205.620.3 224.020.5 280.520.3 262.020.5 234.820.3 194.820.3 192.820.3 195.620.1 197.920.2 199.720.4 267.420.3 202.620.3 196.520.3

Location: 70821 '170W 26833'500S 4.34 83.70 10.41 91.93 3.66 79.70 2.77 84.39 2.68 85.15 2.74 78.99 5.64 82.23 6.08 90.65 15.73 90.11 13.95 94.35 16.08 95.21 11.77 92.14 4.15 65.10 100.00 89.25 61.68

1.91 2.50 0.82 1.35 1.28 1.44 1.97 5.38 7.27 9.93 17.84 15.77 2.28 8.27

119.920.4 131.920.1 129.920.2 138.520.1 146.920.2 138.720.5 140.620.2 143.220.2 133.020.2 133.120.1 134.320.2 132.420.3 133.920.3 134.120.2 133.320.2

Ar% of total

Ar (35.1 day half-life).

ArCa%

+ 40

[ Artotÿ (36Ar atmos) (295.5)]/40Artot.

Fig. 5. 40Ar/39Ar apparent age and apparent K/Ca spectra for hornblende concentrates GCH-68 and GCH-69. Analytical uncertainties (2s, intralaboratory) are indicated by vertical width of bars. Experimental temperatures increase from left to right. Plateau increments are delineated with horizontal lines.

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J. Wilson et al. / Journal of South American Earth Sciences 13 (2000) 115±122

abscissa intercept (40Ar/39Ar) ratio in the age equation yields a plateau isotope-correlation age of 197.2 2 0.3 Ma (Fig. 5). Because calculation of isotope correlation ages does not require assumption of a present-day 40 Ar/36Ar ratio, they are considered more signi®cant than those directly calculated from the analytical data. The 197 Ma isotope-correlation age recorded by the hornblende concentrate from samples GCH-69 is considered geologically signi®cant and is interpreted to date post-magmatic cooling through temperatures required for intracrystalline retention of argon in constituent hornblende grains. Harrison (1981) suggested that temperatures of 0500 2 258C are appropriate for argon retention within most hornblende compositions in the range of cooling rates likely to characterise most geological settings. In view of the high-level intrusive character of the Mesozoic complex, it is likely that post-magmatic cooling was relatively rapid. Therefore the 197 Ma isotope-correlation age is interpreted to probably closely date initial pluton emplacement. The hornblende concentrate prepared from protomylonitic sample GCH-68 displays more extensive internal spectra discordance. However, the 8858C-fusion increments record similar apparent ages that de®ne a plateau of 133.320.2 Ma (Fig. 5). Isotope-correlation of the plateau data is well de®ned, with an inverse ordinate intercept of 297.8. A plateau isotope-correlation age of 133.3 2 0.4 Ma is de®ned (Fig. 5). This is interpreted as dating cooling following development of the protomylonitic fabric. 6. Discussion The tonalitic sample GCH-69 has yielded a Ar/39Ar hornblende cooling age of 197 Ma. The rock has a magmatic fabric (Appendix), which suggests that this age re¯ects the primary, post-crystallisation cooling. This links it with Lower Jurassic rocks of similar 202-188 Ma age that are exposed farther west within the Coastal Batholith (Brook et al., 1986; Dallmeyer et al., 1996), and demonstrates that it cannot be linked with either the Las Tazas or Las Animas complexes. The lack of comparable rocks in the vicinity suggests that these tonalites form part of a fault-bounded sliver within the Atacama Fault Zone, detached from an as yet unexposed intrusion at depth and exhumed during dip-slip displacement. Such fault-bounded basement slivers are a common feature in major transcurrent fault zones, and large-scale vertical displacement is common in compressional restraining bends (Sylvester, 1988). The protomylonitic sample GCH-68 has yielded a 40Ar/39Ar hornblende cooling age of 133 Ma. This age is markedly younger than that of its undeformed protolith, suggesting that the original magmatic cooling age has been totally overprinted during later solid40

state deformation. However, the age is concordant with U/Pb zircon crystallisation ages from the Las Tazas complex, together with 40Ar/39Ar hornblende cooling ages from country rock mylonites along the western edge of the complex (Naranjo et al., 1984; Dallmeyer et al., 1996). The complex was emplaced at 7-km depth, and rocks at this depth are likely to have a temperature of 02008C at normal geothermal gradients. However, microstructures in GCH-68 record amphibolite facies conditions (Appendix) which are well above ambient temperatures. Only rocks close to the complex record these elevated temperatures, as mylonites are not exposed along the fault zone away from the complex. This combination of elevated temperatures, concordant ages, and localised exposure strongly indicates that the Las Tazas complex intruded its country rocks and heated them to amphibolite facies conditions, allowing localised ductile shearing along the Atacama Fault Zone. Pre-emplacement dip-slip and post-emplacement strike-slip deformation along the Atacama Fault Zone are both dated at 0130 Ma. This indicates that the kinematic switch was relatively rapid, perhaps over a period of 1-2 million years. The concordant timing of plutonic crystallisation and ductile shearing also con®rms that the Las Tazas complex was emplaced syntectonically, during displacement along the fault zone, over this period. The remarkable constraint on these dates directly re¯ects the shallow emplacement depth. Ambient temperatures of 02008C are well below 40 Ar/39Ar hornblende closing temperatures. After emplacement, it is likely that the pluton and its wall rocks cooled rapidly, over perhaps 105-106 years (Paterson and Fowler 1992), a short enough period to preserve concordant dates. Such short-lived heating and ductile deformation is only possible in the upper crust in the vicinity of syntectonic intrusions. As such, high-level plutons such as the Las Tazas complex provide a unique constraint on the kinematic history of major shear zones.

7. Conclusions A switch from ductile dip-slip to transcurrent displacement occurred along part of the Atacama Fault Zone at 130 Ma. This occurred over the course of 1±2 million years, coinciding with the syntectonic emplacement of the Las Tazas complex. The complex intruded the fault zone and heated its immediate country rocks to amphibolite grade, inducing localised ductile shearing. The preservation of such tightly constrained ages is a function of the upper crustal emplacement depth of the Las Tazas complex.

J. Wilson et al. / Journal of South American Earth Sciences 13 (2000) 115±122

Acknowledgements Many thanks to Pete Treloar and Mike Brown for all their support and inspiration. Also thanks to Richard Allmendinger and an anonymous reviewer for helpful reviews. Logistical support was received from the Servicio Mineria y Geologia de Chile (SERNAGEOMIN). Je€ Wilson was funded by a Kingston University research studentship. John Grocott gratefully acknowleges funding by NERC research grants GR9/476 and GR3/11199. Appendix. Sample petrography Sample GCH-69 is a tonalite with a 1-3 mm grainsize. The rock has a magmatic-state fabric, although solid-state microstructures are developed in most mineral phases. It displays a well-developed magmaticstate foliation, de®ned by the preferred alignment of plagioclase crystals, coexisting with non-aligned interstitial sub-poikilitic magmatic phases (Hutton, 1988; Paterson et al., 1989). Plagioclase consists of subhedral squat-elongate andesine prisms with extensive deformation twins, grain-bending, and domainal extinction. Hornblende is the dominant ma®c mineral, forming subhedral-interstitial poikilitic crystals with intensive disequilibrium microstructures in some grains, indicating the former occurrence of retrogressed clinopyroxene cores. Biotite forms subhedral, equant ¯akes with intensive grain-bending and kinking. Quartz consists of interstitial, sub-poikilitic crystals with well-developed undulose extinction, deformation bands, and serrated sub-grain boundaries. Magnetite forms occasional anhedral polygonal-rounded crystals. Accessory minerals comprise titanite, apatite, and zircon. Sample GCH-68 is tonalitic protomylonite. It comprises plagioclase and hornblende grains that display a well-de®ned planar fabric, together with an interstitial, recrystallised quartz matrix. Plagioclase forms subhedral, squat to elongate prisms and augen, 0.5-2 mm long, with deformation twins, grain bending, and fracture-induced domainal extinction. Augen-shaped hornblende crystals de®ne a mineral lineation, with tails of ®ne recrystallised crystals, <0.2-mm grain-size. Occasional biotite grains form subhedral, <0.5 diameter, ¯akes that exhibit intensive grain bending, kinking, fracturing, and fragmentation to form clusters of euhedral crystals of 0.05-0.1 mm grain-size. Quartz is totally recrystallised, forming a ®ne recrystallised mosaic, 0.005-0.02 mm grain-size. Deformation features re¯ect mid- to low-temperature ductile deformation. The dominant hornblende-andesine-quartz assemblage indicates amphibolite facies conditions, while the ductile deformation required to form augenshaped plagioclase and hornblende crystals requires

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conditions between upper greenschist (Simpson, 1985; Gapais, 1989) and mid-upper amphibolite facies (Olsen and Kohlstedt, 1985; Pryer, 1993; Tribe and D'Lemos, 1996). The ®ne recrystallised mortar texture suggests that dislocation creep continued down to lower temperatures (5008C to 3008C).

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