40Ar/39Ar geochronology and P- T-f paths from the ...ees2.geo.rpi.edu/spear/Spear_pubs_pdf/Kohn 1995... · 1. metamorphic Geol., 1995, 13, 251 -270 40Ar/39Ar geochronology and P-

1. metamorphic Geol., 1995, 13, 251 -270

40Ar/39Ar geochronology and P- T-f paths from the Cordillera Darwin metamorphic complex, Tierra del Fuego, Chile M. J. KOHN, ' * F. S . SPEAR,' T. M. HARRISON' A N D I. W. D. DALZIEL3 ' Department of Earth and Environmental Sciences, Rensselaer Polytechnic Institute, Troy, NY 12 180, USA 'Department of Earth and Space Sciences, University of California, Los Angeles, CA 90024, USA

lnstitute for Geophysics, University of Texas, Austin, TX 78759, USA

ABSTRACT 40Ar/39Ar data collected from hornblende, muscovite, biotite and K-feldspar constrain the P-T-t history of the Cordillera Darwin metamorphic complex, Tierra del Fuego, Chile. These data show two periods of rapid cooling, the first between c. 500 and c. 325" C at rates 225" C Ma-', and the second between c. 250 and C. 200" C. For high-T cooling, 40Ar/39Ar ages are spatially disparate and depend on metamorphic grade: rocks that record deeper and hotter peak metamorphic conditions have younger 40Ar/39Ar ages. Sillimanite- and kyanite-grade rocks in the south-central part of the complex cooled latest: 40Ar/39Ar Hbl = 73-77 Ma, Ms = 67-70 Ma, Bt = 68 Ma, and oldest Kfs = 65 Ma. Thermobarometry and P-T path studies of these rocks indicate that maximum burial of 26-30 km at 575-625" C may have been followed by as much as 10 km of exhumation with heating of 25-50" C. Staurolite-grade rocks have intermediate 40Ar/39Ar ages: Hbl = 84-86 Ma, Ms = 71 Ma, Bt = 72-75 Ma, and oldest Kfs = 80 Ma. Thermobarometry on these rocks indicates maximum burial of 19-26 km at temperatures of 550-580" C. Garnet-grade rocks have the oldest ages: Ms = 72 Ma and oldest Kfs = 91 Ma; peak P-T conditions were 525-550" C and 5-7 kbar. Regional metamorphic temperatures for greenschist facies rocks south of the Beagle Channel did not exceed c.300-325"C from 110Ma to the present, although the rocks are only 2 km from kyanite-bearing rocks to the north.

One-dimensional thermal models allow limits to be placed on exhumation rates. Assuming a stable geothermal gradient of 20-25" C km-I, the maximum exhumation rate for the St-grade rocks is c. 2.5 mm yr - I , whereas the minimum exhumation rate for the Ky + Sil-grade rocks is c. 1.0 mm yr-'. Uniform exhumation rates cannot explain the disparity in cooling histories for rocks at different grades, and so early differential exhumation is inferred to have occurred. Petrological and geochronological comparisons with other metamorphic complexes suggest that single exhumation events typically remove less than c. 20 km of overburden. This behaviour can be explained in terms of a continental deformation model in which brittle extensional faults in the upper crust are rooted to shallowly dipping ductile shear zones or regions of homogeneous thinning at mid- to deep-crustal levels. The P-T-r data from Cordillera Darwin (1) are best explained by a 'wedge extrusion' model, in which extensional exhumation in the southern rear of the complex was coeval with thrusting in the north along the margin of the complex and into the Magallanes sedimentary basin, (2) suggest that differential exhumation occurred initially, with St-grade rocks exhuming faster than Ky + Sil-grade rocks, and (3) show variations in cooling rate through time that correlate both with local deformation events and with changes in plate motions and interactions.

Key words: 40Ar/39Ar ages; Cordillera Darwin, Chile; exhumation rates; P-T-t paths.

I N T R O D U C T I O N

The Occurrence of Cretaceous amphibolite-grade metamorphic rocks in the Cordillera Darwin metamorphic complex is to our knowledge unique in the South American Andes south of the equator. Recent structural and petrological investigations (Dalziel & Brown, 1989;

* Present address: Department of Geology and Geophysics, IJniversity of Wisconsin, Madison, WI 53706, USA.

Kohn et al., 1993; Cunningham, 1994; Klepeis, 1994) suggest that this anomalous geological feature resulted from localized extension, that itself was caused by the development of a transform boundary at the southern end of South America in the Mid- to Late Cretaceous. This interpretation has implications about the T-r paths that are expected for different areas of the complex.

In this paper, 40Ar/'9Ar data are presented for hornblende, muscovite, biotite and K-feldspar and used in conjunction with previously published geochronological data to determine the post-peak-metamorphic cooling

251

252 M. I . KOHN E T A L

history for different areas in the complex. We then use these T-t paths in addition to previously published petrological data (Kohn et al., 1993) to develop and critically evaluate one-dimensional thermal models for areas that represent different metamorphic grades. The thermal and exhumation history of Cordillera Darwin is then compared with that observed for several other metamorphic complexes described in the literature, to draw inferences about the extensional behaviour of continental crust. Finally, a new ‘wedge extrusion’ model for the tectonic evolution of the complex is proposed, in which extension of the high-grade core at the southern rear of the thickening wedge was coeval with thrusting in the north along the toe.

GEOLOGICAL BACKGROUND

Cordillera Darwin is located at the southern terminus of the South American Andes (Fig. la), and contains rocks that were metamorphosed during the Cretaceous Andean orogeny. The structural and metamorphic history of the area, first studied by Kranck (1932), has more recently been described in detail by Nelson et al. (1980), Dalziel & Brown (1989), Kohn et al. (1993), Cunningham (1994) and Klepeis (1994) and their data and interpretations are summarized below.

The oldest unit in the region is a Palaeozoic to lower Mesozoic metasedimentary and metavolcanic basement complex that is believed to have been originally deposited as an accretionary wedge on the pre-mid-Jurassic Pacific margin of South America (Dalziel & CortCs, 1972; Nelson et al., 1980; Dalziel, 1981, 1986). These rocks were intruded by the Jurassic Darwin Granite Suite, which consists of c. 160-170 Ma felsic orthogneisses that were in turn cut by (now metamorphosed) mafic dykes of undetermined age (HervC et al., 1979, 1981; Nelson et al., 1980; Dalziel, 1981; Mukasa et al., 1988; S. Mukasa, pers. comm., 1994). In the Late Jurassic, a marginal basin opened within the rocks of the old accretionary prism between the continental margin and the subduction-related arc, and voluminous Jurassic and Cretaceous volcanic and sedimentary rocks of the Tobifera and Yahgan Formations were deposited (e.g. Hanson & Wilson, 1991). The youngest rocks of interest are referred to here as the Beagle Tonalite Suite, which consists of 70-90 Ma, relatively undeformed, I-type diorites and tonalites that cross-cut the older metamorphosed rocks (Nelson et al., 1980; HervC et al., 1984; Mukasa et al., 1988; Suarez et al., 1985; S. Mukasa, pers. comm., 1994).

The rocks of the basement complex retain relicts of a pre-Andean fabric, but metamorphic conditions evidently did not exceed lower greenschist grade prior to the Cretaceous (Nelson et al., 1980; Kohn et al., 1993). Closure of the marginal basin after Albian-Aptian time (Dott et al. , 1977) initiated the compressional Andean orogeny, and resulted in two major deformations (Nelson ef al., 1980). The first (D l ) was characterized by thrust faulting,

J

isoclinal folding, and strong fabric development as the basin floor and its cover were emplaced on the continental margin, and the second (D2) produced backfolds with a vergence towards the Pacific (Nelson et al., 1980). The main foliation, S1, shows a uniform southward dip that steepens towards the south, where it becomes strongly overprinted by steeply dipping S2. A third deformation (D3) resulted in a minor crenulation cleavage in metasedimentary rocks (Nelson et al., 1980 Kohn et al., 1993).

Along the southern margin of the complex, a shear fabric (formed by Ds) occurs in the Beagle Tonalite Suite and older rocks and cuts S2 (Dalziel & Brown, 1989; Moore, 1990; Cunningham, 1994; the relative timing of Ds to D3 is unclear). Dalziel & Brown (1989) interpret Ds to reflect significant post-D2 extension and, based on an abrupt change in metamorphic grade across the Beagle Channel (see below), suggest that extension in the south was further accommodated along a major exhumational detachment surface there. In contrast, in the north-east part of the complex, Klepeis (1994) reports structural evidence for emplacement of the basement of Cordillera Darwin onto its cover through syn- to post-Dl NE-directed, thrust-sense ductile shearing, and through post-D2 brittle thrusting and brecciation at the basement- cover contact. There is little or no evidence there for extension.

Further north and north-east in the Magallanes basin (Fig. la) , a rapid influx of coarse sediments occurred between c. 85 and c. 65 Ma with a western or cordilleran source (Dott ef al., 1982). Thrusting and folding of the basin sediments commenced at 45-50 Ma near Cordillera Darwin and continued until at least the middle Miocene (Winslow, 1982; Biddle et al., 1986; Alvarez-Marr6n et al., 1993). Left-lateral strike-slip faulting has occurred from at least the late Tertiary to the present (Winslow, 1982).

Dalziel & Brown (1989) presented a tectonic model in which development of the Patagonian orocline and core-complex-style exhumation of Cordillera Darwin resulted from changes in far-field stresses during the latest Cretaceous and earliest Tertiary. Changes in plate motions caused a shift from compression to transpression, and this in turn led to local extension at the southern bend in the Andes at Tierra del Fuego. Their model has the advantages that it explains not only why high-grade rocks occur in Cordillera Darwin and not elsewhere along the southern South American Andes, but also how extension was accommodated in the south through Ds and detachment at the Beagle Channel. Their model has the disadvantages that the principal evidence for extension is only along the southern margin of the complex (Dalziel & Brown, 1989; Moore, 1990), the major foliation everywhere has a southward dip (Nelson et al., 1980), and more recent work shows little or no evidence for extension in the north (Klepeis, 1994). Nonetheless, Dalziel & Brown (1989) clearly show that any tectonic model for Cordillera Darwin must explain the extensional fabrics in the south as well as the occurrence of Mesozoic high-grade metamorphic rocks in Cordillera Darwin at all.

f l Thrust Fault + 5 anticline +q syncline f l Strike-Slip or

. . . . Normal Fault Overturned + antiform

(Jurassic)

7 Edge of Glacier -+?- F2 SYnform Tobifera Fm.

(Jurassic)

Yahgan Fm. E 3 (Lower Cret.)

p i .....' Beagle Tonalite . - . (Upper Cret.)

Fl Darwin Granite . . . .

... . . ~ ....: .""".. ..,...

54" 30' S

I 55"OO'S

Fig. 1. (a) Geographical and geological maps of the southern part of South America, showing the location of the Cordillera Darwin metamorphic complex, major rock types and structures (geology after Nelson et al., 1980; Klepeis, 1994). MITB, Magallanes fold and thrust belt, developed in the sedimentary basin to the north of Cordillera Darwin; CM and MB,continental margin and marginal basin rocks, which comprise the deformed and metamorphosed core of Cordillera Darwin; Arc, subduction-related volcanic arc south and west of Cordillera Darwin. (b) Cross-scctions across Cordillera Darwin from south to north showing distribution and orientation of major rock types and structures. The major foliation in the basement rocks becomes progressively shallower from south to north. Emplacement of the basement onto the Tobifera Formation in the north could have occurred during ductile D1 + D2, during a later brittle event, or both (Nelson el al., 1980; Klepeis, 1994).

254 M. J . K O H N E T A L

Metamorphic evolution

Figure 2 shows the distribution of metamorphic index minerals within the complex as described by Kohn et nl. (1993). Because of limited accessibility, continuous isograds cannot yet be drawn. Metamorphic grade ranges from biotite/chlorite on the margins to kyanite/sillimanite in the south-central part of the complex. A metamorphic discontinuity is evident across the north-west arm of the Beagle Channel, such that kyanite-grade schists are present on the north shore, whereas chlorite/biotite-grade schists occur on the south shore. The discontinuity is probably not a result of different bulk compositions because the same sedimentary and volcanic units (Tobifera and Yahgan Formations) occur on both sides of the channel.

The petrological results of Kohn et nl. (1993) are summarized in Fig. 3. Thermobarometric calculations for garnet-bearing samples (Fig. 3a) indicate that from garnet to sillimanite grade, peak metamorphic conditions increase

~~

from c.525"C and 5-7 kbar to c.625"C and 7-9 kbar, with the notable exception of the rim pressures for two kyanite-grade garnet amphibolites (SP-1OA). Generally, the P-T conditions fall between 20 and 25"Ckm-' geotherms. P-T paths from zoning in metapelitic garnets (Fig. 3b) show that most garnet growth occurred with a small increase in pressure and a large increase in temperature during the D2 deformation (backfolding). However, the P-T paths for the SP-1OA garnet amphibolites and the pervasive development of sillimanite along plagioclase grain boundaries for kyanite-grade rocks suggest exhumation from peak metamorphic conditions by as much as 10km with an increase in temperature of

The rapid changes of grade combined with thermobarometric and P - T path analysis suggest that tectonic exhumation of the higher grade rocks relative to their lower grade margins was an important process leading to the exposure of the metamorphic core. AS noted by Dalziel & Brown (1989), the petrological discontinuity

25-50" C .

Hbl: 73.2 &El

Hbl: 75.5

US: 46-73

klometers

0 2 4 6 -

Fig. 2. Metamorphic index minerals, areas of geochronological interest, locations of samples for which 40Ar/'9Ar ages, fission track ages, or P-T paths have been collected (Nelson, 1982; Grunow el a!., 1992; Kohn et al., 1993; this study), and summary of 40Ar/'9Ar mineral ages for each area. Hbl, hornblende; Ms, muscovite; Bt, biotite; Kfs. K-feldspar; the age range for K-feldspars reflects their strong age gradients (see Fig. 9). Detail of areas A and B shown on left. The maximum metamorphic grade observed in each area is: sillimanite (A), kyanite (B and C), staurolite (D and E), garnet (F) and chloritelbiotite (G) .

GEOCHRONOLOCY OF CORDILLERA DARWIN 255

n Lc ca J3 25 a

12 P-TPaths I

n

J3 2 25

10

8

a 6

4

2 400 500 600 700 800

Fig. 3. Petrological results of Kohn et al. (1993). (a) Thermobarometric estimates for garnet-bearing rocks from areas A-F show an increase in P-T with increasing grade. Geothermal gradients of 25 and 20" C km-' are shown for reference. (b) P-T paths show early minor loading with heating, followed by late exhumation with minor heating. Synkinematic inclusion trails in the outer SO-%% of individual metapelitic garnets allow correlation of most of the early heating with the D2 deformation; a D1 origin for the lowest temperature garnet growth cannot be. ruled out.

across the north-west arm of the Beagle Channel is a muscovite and biotite have been presented by Halpern principal line of evidence for down-to-the-south exten- (1973), HervC et al. (1979), S. Mukasa (pers. comm., 1994) sional shearing (in present-day geographical coordinates). and Grunow et al. (1992) for high-grade rocks and

intrusions in southern Cordillera Darwin. Ages ranEe from

Previous geochronological results c.80 to c .65Ma, and are described in more d e t a i below for specific areas. Although these ages clearly establish

Primary crystallization ages have been determined for the post-metamorphic cooling in Cordillera Darwin in the Late Darwin Granite Suite ( 1 5 7 f t M a , Rb-Sr whole-rock Cretaceous, precise T-t curves cannot be drawn for isochron; HervC el al., 1979, 1981; 164 f 2 Ma, U-Pb individual locations, and tectonic models cannot be tested. zircon, S. Mukasa, pers. comm., 1994), intrusives into the Nelson (1982) collected fission track ages for the region base of the Tobifera Formation (150 f 1 Ma, U-Pb zircon; from titanite, zircon, and apatite, and these data are Mukasa et al., 1988; S. Mukasa, pers. comm., 1994), and summarized below for individual areas. the Beagle Tonalite Suite (c. 90 f 2 and 69 f 1 Ma, U-Pb zircon; S. Mukasa, pen. comm., 1994). K-Ar and Rb-Sr cooling ages for biotite and hornblende have also been A A A N, published for 10 Beagle Suite intrusions (Table 1). Plutons 1-9 occur in low-grade rocks south of the Beagle Channel, whereas pluton 10 is on the north shore of the channel in kyanite-grade rocks. The concordance of the biotite and hornblende ages and the lack of excess argon in most samples we have analysed from the region (see below) suggest that these ages represent crystallization times for each pluton. Individual plutons are therefore inferred to range in age from c. 110 Ma (plutons 2 and 3) to c. 70 Ma (pluton 10).

N o direct estimate of the age of peak metamorphism exists for the area. Approximately 100 km to the south-east, the youngest deformed rocks contain Albian- Aptian fossils (c. 100-120Ma; Dott et al., 1977), and a gabbroic clast from these rocks has a K-Ar hornblende age of 116-+5Ma (Halpern & Rex, 1972). The oldest metamorphic cooling age in Cordillera Darwin is 91 Ma (CD-29 K-feldspar, see below), and the oldest plutons that cross-cut the D1 and D2 fabrics are 80-90 Ma (Halpern & Rex, 1972; Halpern, 1973; HervC et al., 1984; Suarez et al., 1985; S. Mukasa, pers. comm., 1994). Thus, peak metamorphism occurred between 90 and 100-120 Ma.

K-Ar and 40Ar/39Ar cooling ages for hornblende,

40Ar/39Ar analyses were undertaken at the State University of New York at Albany (SUNY) and at the University of California, Los Angeles (UCLA), and essentially followed the analytical procedures described by Harrison & Fitz Gerald (1986) and Harrison et al. (1991). Although there are differences in the line blanks, mass spectrometers and proportions of gas inlet for analysis, the overall accuracy and precision of the two systems were quite similar for the data presented here. High purity mineral separates (?Y% pure) were prepared from crushed and sized (125-250 pm) rock powders by using conventional heavy liquid and magnetic separation techniques. Samples were wrapped in Sn foil, loaded into flat-bottomed quartz tubes with an identically prepared flux monitor interspersed at regular intervals among the samples, and irradiated in the H-5 position of the Ford Reactor at the University of Michigan.

For samples analysed at SUNY, the irradiation time was 60 h, the flux monitor was the intralaboratory standard Fe-mica (age = 307.3 Ma), and correction factors for interfering nuclear reactions were (40Ar/39Ar), = 0.0301, ('Ar/37Ar),, = 0.000216, and (39Ar/37Ar)Ca = 0.000757. J-factors varied between 0.004656 and 0.004803. Sample masses were c. 150mg for muscovite and biotite and c. 350 mg for hornblende. Argon was extracted in a double-vacuum, Ta resistance furnace (precision better than * l"C and accuracy better than +lO"C), and purified using ion

256 M. I . K O H N E T A L

Table 1. Selected geochronological results for Beagle suite intrusives of Cordillera Darwin.

Pluton Mineral analysed Radiogenic system Age * 2u (Ma)

(9)

(1) Bt Hbl

(2) Bt Bt Hbl

(3) Hbl Hbl

(4) Hbl Hbl Hbl Bt Bt Bt Zrn Hbl Bt Hbl Bt Hbl Hbl Bt Hbl Hbl Hbl Hbl Bt Bt Hbl Hbl Hbl Hbl Bt Zrn Hbl Bt Bt Bt Bt Bt

Bt

Rb-Sr K-Ar Rb-Sr Rb-Sr K-Ar K-Ar K-Ar K-Ar K-Ar K-Ar K-Ar Rb-Sr K-Ar K-Ar U-Pb K-Ar Rb-Sr K-Ar K-Ar K-Ar K-Ar K-Ar K-Ar K-Ar K-Ar K-Ar K-Ar K-Ar K-Ar K-Ar K-Ar K-Ar K-Ar U-Pb K-Ar Rb-Sr Rb-Sr K-Ar K-Ar K-Ar

84*10 8 8 i 5

1W*9 111 f 9 104*3 1 W * 3 113*6 100f4 89 f 2 9 4 f 5 86f4 89 f 7 93 f 5 Y2f2

8 8 f 2 86*7 91 f 3 97 f 4 91 f 3 94 f 3 94 f 2 M i 5 8 2 i 7 8 4 i 6 89 i 6 89 f 2 x 8 i 2 8 6 * 5 8 8 f 6 81 f 6 82f4 76 f 4 69 f I 67 f 4 62 f 6 7 0 i 6 68f4 66+4 65 f 4

9 0 k 2

~~~

Note: Rb-Sr biotite ages assume an initial mSr/86Sr ratio of 0.705 f 0.002. Rb-Sr ages were recalculated to A@ = 1.42 X lo-" year (Steiger & Jager, 1977). Both U-Pb zircon ages are from S.'Mukasa, pen. comm., 1994. Other referencesfordataare:Pluton(l) = Halpern(l973),Plutons(2)-(7) = Hervter o/.(1984).Plutons(8)and(9) = Suarezera/.(198S),and Pluton(l0) = Halpern (1973).

pumping of H,, two stages of SAES Zr-Ti getters, and the gettering action of the hot furnace walls. 100% of the gas was transferred using activated charcoal to an automated Nuclide 4.5-60-RSS mass spectrometer and analysed in Faraday-cup mode. The line blank contained less than 1 X 10-'4moles 40Ar below looo" C, and had an atmospheric argon ratio.

For samples analysed at UCLA, the irradiation time was 30 h, the flux monitor was Fish Canyon Sanidine (age = 27.8 Ma; Miller et al., 1985), and correction factors for interfering nuclear reactions were ("Ar/39Ar)K = 0.0198, ('Ar/37Ar),, = 0.00023, and (39Ar/37Ar),, = 0.0007. I-factors varied between 0.005404 and 0.005525. Sample sizes were c. 5 mg for muscovite and biotite, and c. 15 mg for K-feldspar and hornblende. Argon was also extracted with a double-vacuum, Ta resistance furnace, and the gas was purified by the furnace walls and with a single SAES getter. The 40Ar line blank below looo" C was less than 1 X moles, and

had an atmospheric argon ratio. Expansion allowed c. 50% of the argon to be transferred to an automated VG 1200s mass Spectrometer, and analysed in electron multiplier mode.

Corrected 40Ar/39Ar, 37Ar/39Ar, =Ar/"Ar, % o ' ~ A ~ released, ratio of radiogenic 40Ar to potassium-derived 39Ar, apparent age, and its standard deviation are presented in the Appendix (Table A l ) for each heating increment, along with the increment temperatures, sample mass, J-factor, moles of 39Ar released, corrected 40Ar/36Ar and 39Ar/36Ar ratios, and their uncertainties. Age uncertainties do not include the uncertainty in the J-factor (c. 0.5%). Unless otherwise specified, uncertainties listed below are at the 2 r level.

A temperature-cycling technique (Lovera er ol., 1991) was used for the K-feldspar separates in order to delineate better their potential domain-size distributions. Precise cooling histories can then be fit to the age spectra provided that kinetic parameters can be determined. Computer programs (Lovera, 1992) were used to obtain fits of ( 1 ) domain size distributions, activation energies, and D,,/d to the 39Ar release data via log(r/ro) plots (see Lovera et al., 1989, 1991; Harrison et al., 1991), and (2) the individual cooling histories (in combination with the heating schedule, domain size distributions, and kinetic parameters) to the age spectra.

A S S I G N M E N T OF C L O S U R E T E M P E R A T U R E S

The assignment of closure temperatures to the ages obtained from minerals assumes that diffusion dominates the loss of daughter isotopes during cooling in nature. We assumed that argon diffusion behaviour was similar to that documented experimentally by Harrison (1981) for hornblende, Robbins (1972) for muscovite and Harrison el al. (1985) for biotite. Diffusion geometries were based on those assumed in the studies: hornblende = sphere, muscovite = plane sheet, and biotite = infinite cylinder. Recasting the experimental data in terms of different geometries does not lead to significantly different results. The length scales for hornblende and biotite were based on the experimentally derived estimates (60 pm and 150 pm, respectively; Harrison, 1981; Harrison et al., 1985). A typical length scale for muscovite of 2 0 p m was based on the average half-width of muscovite flakes observed in thin section for these rocks (see below). For the cooling rates encountered in this study (15-60" C Ma-'), these assumptions lead to closure temperature estimates of c. 505" C for hornblende, c. 370" C for muscovite and c. 320" C for biotite. The closure temperature of biotite to Rb-Sr diffusional re-equilibration was assumed to be c. 300" C (e.g. Harrison & McDougall, 1980). The closure temperatures of titanite, zircon and apatite with respect to fission track annealing were assumed to be c. 250" C, c. 175" C and c. loo" C, respectively (e.g. Wagner, 1968; Naeser & Faul, 1969; Gleadow, 1978; Harrison et al., 1979). Track length distributions were not published for apatite, and no refinements to apatite closure temperature or late-stage cooling history are possible. Uncert- ainties in all these closure temperatures were assumed to be i 50" C except for apatite (*25" C).

For K-feldspar, temperature significance for the measured ages was assigned according to the approach of Lovera ef a / . (1989, 1991) and Harrison et al. (1991) (see also Richter et'ol., 1991). This approach assumes that "Ar loss during laboratory step heating is diffusion-controlled, and that the same diffusion mechanisms and boundaries for 40Ar loss were present as the sample cooled in nature. Thus, the release of "Ar during laboratory step heating is assumed to allow a direct experimental determination of the diffusional parameters that governed the

CEOCHRONOLOCY OF CORDILLERA DARWIN 257

natural closure of 'the K-feldspar to 4"Ar loss, including domain frequency terms (Do/r2), relative abundances of domain sizes, and activation energies (Lovera ef al., 1989, 1991; Harrison ef al., 1991). Once these parameters have been determined, a cooling history specific to the sample can be fit to the observed age spectrum (e.g. Richter er al., 1991). For the K-feldspars studied, the individual domains have closure temperatures in the range of 200-325" C (see Table A2, Appendix). The sizes and distributions of the domains are probably the result of high-T exsolution, fracturing and dissolution-reprecipitation mechanisms (Lovera ef al., 1991, 1993; Fitz Gerald & Harrison, 1993), and so were probably already present before the K-feldspar started to retain argon. An uncertainty of *25"C was assigned to the absolute placement of the K-feldspar cooling curves associated with the uncertainties in extrapolating the K-feldspar kinetic data down temperature, in fitting the Iog(r/r") plots and age spectra, and in extracting the argon (e.g. uncertainties in absolute furnace temperature, furnace hysteresis, absolute amount of 39Ar released for each step, etc.). The parameters used to fit the log(r/r") plots and age spectra are presented in the Appendix (Table A2). For each K-feldspar, the domains were assumed to have a single activation energy (Lovera ef al., 1989, 1991; Richter et al., 1991); each activation energy was determined from the first 20-25% of the 39Ar released, as this corresponded to a well-correlated array on Arrhenius plots. The insensitivity of the modelled age spectra to the choice of diffusion geometry and number of domains, and sensitivity to cooling history have been documented by Lovera er al. (1991) and Richter ef al. (1991).

40Ar/39Ar results

Seven areas of geochronological interest are outlined in Fig. 2, and the locations of samples are indicated for which either new data arc reported here or results particularly significant to this study were obtained by Nelson (1982) or Grunow et al. (1992). Thcse areas also have petrological and structural interest, as each corresponds generally to a single grade of metamorphism, and so represents a region of nearly constant burial depth. Ages for biotite, muscovite and hornblende are presented in Table 2, and our preferred ages correspond to standard inverse isochron regressions on isotope correlation diagrams (i.e. regressions of 3"Ar/40Ar vs. '9Ar/40Ar, with the *-intercept reflecting the age). Because of the likelihood that K-feldspar has a distribution of domain sizes, each with its own characteristic closure temperature and age, no isochron age is listed in Table 2. The relevant data from Nelson (1982) and Grunow er al. (1992) are also summarized in Table 2 for each area.

Age spectra and assignment of mineral ages

Figures 4-9 show age spectra for hornblende, biotite, muscovite and K-feldspar vs. %39Ar released and isotope correlation diagrams for hornblende, biotite and muscovite. Apparent K/Ca ratio vs. %39Ar released is shown for hornblende analyses and illustrates the low K/Ca that is expected for hornblende lhat is uncontaminated by high-K minerals (K/Ca estimates are based on '7Ar/40Ar measurements: 37Ar is almost entirely produced from Ca during irradiation). With only a few exceptions, the spectra for hornblende, biotite and muscovite are quite flat, and the isotope correlation diagrams show little evidence for a non-atmospheric trapped component. This suggests that previously collected K-Ar ages on biotite and hornblende may have chronological significance.

Table 2. Summary of 40Ar/'9Ar and fission-track mineral ages for specific areas in Cordillera Darwin.

Total gas age Preferred age Sample ID Mineral (*20) ( i2U) [Steps. "C] Reference

Arm A. sillimontre-grade CD-163G Hbl 74.3 (0.6)

CD-163H CD-I63H PIA-70 PIA-74 PIA-66 PIA-57 PIA-74 PIA-70

Mus 70.5 (03) Bt 69.3 (02) Ttn Zrn Zrn Zm AP AP

73.2 (0.6) [970-1000.

69.7 (0.1) [600-1180] 68.0 (2.4) [670-3350] 52.2 (5.9) 50.1 (5.6) 41.7 (4.5) 43.6 (4.8) 21.9 (2.7)

1050-1450]

28.8 (4.7)

This Study This Study This Study Nelson (1982) Nelson (1982) Nelson (1982) Nelson (1982) Nelson (1982) Nelson (1962)

Area 8, kyanire-grade CD-167A Hbl 79.8 (11.5) 75.5 (2.0) [650-1450] This Study PIA-83A Mus 70 (1) Grunow er4l. (1992) PIA-I3A Bt 70 (1) Grunow cr a/. (1992) CD-168 K-fs 57.3 (0.3) This Study

Area C. kyanae-grade VI-IG Hbl

CD-NIB Mus CD-.3OIB Bt NB-76 Ttn NB-88 Zrn NB.76 Zrn NB-84 Ap NB-76 Ap

87.4 (3.6) 77.0 (2.2) [650-1050] 77.6 (1.2) [950-1450]

66.9 (0.1) 66.7 (0.6) [600-1040] 72.9 (0.2) 73.8 (0.6) [600-1100]

56.6 (9.5) 58.5 (10.5) 45.1 (3.7) 29.4 (4.4) 35.9 (3.9)

This Study This Study This Study Nelson (1982) Nelson (1982) Nelson (1982) Nelson (1982) Nelson (1982)

Area D, suturolile-grade CD-171A Hbl 83.0 (2.4) 84.2 (2.0) [750-850,

950-1015, 1110-1450] This Study CD-171A Rt 73.3 (0.1) 73.2 (0.5) (600-1350] This Study CD-171B Bt 71.6 (0.1) 71.8 (0.4) [Mo-I(Ho] This Study CD-I75 K-fs 64.6 (0.4) This Study

Area E, rraurolire-grade CD-31A Hbl 83.1 (1.2)

CD-MF Mus 71.7 (03)

CD-29 K-fs 65.6 (0.7) BP-122 Zrn BP.122 Ap HP-102 Ap

CD-10F Bt 74.5 (0.2)

85.8 (2.7) [800t950-1000t This Study This Study

74.9 (4.0) [670-l150] This Study This Nelson Study (1982)

45.1 (4.6) Nelson (198.7)

27.2 (5.8)

71.3 1 IM- (0.3) 1200l [600-1180]

18.5 (3.3) Nelson (1982)

Area F, garner-gradc

CD-28 K-fs 80.0 (0.1) BP-131 Ttn 62.8 (9.2)

BP-l29D Mus 73 (1)

Area G. biorir=lch/on5e.gmde CD-207B. Hbl XY.7 (1.0) CD-207Bt Hbl 90.5 (1.0) CD-212 €31 90.3 (0.4) SB-I2 Ttn SB-I2 Zrn SR-8 Zrn SB-14 Ap SR-12 Ap

89.3 (1.4) [800-1450] 93.5 (0.3) (850-1450] 88.9 (0.4) [600-1350] 68.8 (7.9) 47.0 (6.5) 48.9 (S.0) 55.8 (12.1) 43.5 (7.2)

Grunow era/. (1992) This Study Nelson (1982)

This Study This Study This Study Nelson ( 19x2) Nelson (1Y82) Nelson (1982) Nelson ( I 982) Nelson (1982)

Note: [Steps, 'C] refen to the specific gas increments expressed in terms of the temperature of extraction that were regressed on irotopc correlation diagrams 10

determine the preferred mArAr/"Ar age for samples analysed in this study. *Hornblende from a finer-grained layer (typical grain size = 0.6 mm x 0.15 mm). tHornblende from a coarser-grained layer (typical grain size = 4.0 mm X 1.5 mm).

Area A , sillimanite grade (Fig. 4). Hornblende was separated from a mafic garnet amphibolite (CD-163G; grain size typically 600 p m long and 60 p m across). Coarse-grained biotite and muscovite (typically 600pm wide and 60pm thick) were separated from a

258 M. J . KOHN E T A L

.004- a I I , , 8 8 1 1 8

.=CD-l63G Hbl A=CD- I63HMs

Slcps 1-5.7-13 Steps 1-13 Agc=73.2+0.6 Age=69.7+0.1

4!4r1'6Ar,=332i26 4!4d36Ar,=302k5

C - -

*= CD- 163H Bt Steps 2 - I I - Age=68.0+2.4 \ 4%r136Ar,=616t404 -

'P. 4 .4., -

. .

100 ' I I I I I I I ' a Area A -

2 90 (Sil-Grade) -

80

70

60

CD- 163G Hornblende I

I Muscovite

0 20 40 60 80 100 Cumulative % 39Ar released

.OO .02 .04 .06 .08 .10 .12 .14 39AAd40Ar

Fig. 4. 40Ar/39Ar data for area A (sillimanite-grade). (a) Age spectra for hornblende, muscovite and biotite. Spectra are quite flat, and similarity of ages suggests rapid cooling. (b) Apparent K/Ca ratio vs. % o ~ ~ A ~ released for hornblende showing no evidence for high-K contaminant in mineral separate. (c) lsotope correlation plots for hornblende, muscovite and biotite. Lines represent inverse isochron regressions of data for hornblende (solid) and the micas (dashed). Open symbols represent data not used in regressions. Note that the x-intercept ('9Ar/40Ar at MAr/40Ar = 0) does not directly reflect relative ages between minerals because the irradiation parameters (J-factors) are different.

garnet + sillimanite-bearing metapelite (CD-163H). No alteration was observed in either sample. All spectra are quite flat, inverse isochron regressions indicate only an atmospheric trapped argon component, and ages are 73.2 f 0.6 Ma (hornblende), 69.7 f 0.1 Ma (muscovite) and 68.0 f 2.4 Ma (biotite).

Area B, kyanite grade (Fig. 5). Unaltered hornblende was separated from a titanite-rich garnet-bearing metabasite (CD- 167A), and its spectrum shows a slight gradient in apparent age from c. 76 Ma at c. 20% gas release to c. 80 Ma at 100% gas release. Inverse isochron regression of all the data suggests an age

100

n

? W 90 & 80 6 5 70 2 3 60

I

CD-301B 6 Biotite

50 I I I I I I I I I

. l o L C : P I , I 1-g

VI-IG Hornblende Hornblende

.01 0 20 40 60 80 100

Cumulative % 39Ar released

.OO .02 -04 .06 .08 .10 .I2 .14 39Ar/40Ar

Fig. 5. Hornblende, biotite and muscovite "Ar/@Ar data for areas B and C (k anite-grade). (a) Age spectra. (b) Apparent K/Ca ratio vs. % Ar released for hornblendes. (c) Isotope correlation plot for hornblende and muscovite, with inverse isochron regressions (dashed, VI-1G hornblende; solid, CD-167A hornblende and CD-301 B muscovite). VI-1G hornblende from area C apparently has a resolvable non-atmospheric trapped argon component (lower dashed regression line).

r9

of 75.5 f 2.0 Ma (Fig. 5b) and an initial 40Ar/36Ar ratio that is not statistically different from that of air. The inverse isochron ages rcported by Grunow et al. (1992) for muscovite (70 f 1.0 Ma) and biotite ( 7 0 f 1.0Ma) from area B are in good agreement with poorly constrained data from a submilligram muscovite separate from a metapelite at location SP-9 (total gas age = 72.9 f 2.6 Ma; inverse isochron age, all steps = 69.3 f 0.8 Ma; see Appendix), as well as with the muscovite and biotite inverse isochron ages of c. 70 Ma from area A nearby.

Area C, kyanire grade (Fig. 5). A hornblende separate was obtained from a pristine garnet metabasite (sample VI-lG), and its age spectrum is slightly disturbed. Regression of steps 1-9 suggests an age of 77.0 f 2.2 Ma with an initial wAr/36Ar ratio

CEOCHRONOLOGY OF CORDILLERA DARWIN 259

I I I ' I I I ' a -130 - Area E -

- (St-Grade) 4 -

150 I I I I I I I I I a Area D

W 130 I 1 (St-Grade)

- CD- 17 1A - - Hornblende -

. 0 l L I I I I I I I I


.OO .02 .04 .06 .08 .10 .12 .14 39Ar/40Ar

Fig. 6. Hornblende and biotite 40Ar/39Ar data for area D (staurolite grade). (a) Age spectra. (b) Apparent K/Ca ratio vs. Yo3'Ar released for hornblende. (c) Isotope correlation plot. Inverse isochron regressions shown by solid lines (micas) and dashed line (hornblende). Open symbols represent data not used in inverse isochron regressions.

that is indistinguishable from air (294 f 18; Fig. 5c); regression of steps 8-13 suggests a non-atmospheric trapped argon component (initial 40Ar/36Ar ratio = 327 f 1 0 Fig. Sc), and the resulting age of 77.6 f 1.2 Ma is not statistically different from that derived from steps 1-9. Coarse-grained biotite (200-1500 p m wide and 60-750 p m thick) and fine-grained muscovite (150 p m wide and 20 p m thick) were separated from a garnet-bearing mctapelite (CD-301B), and their age spectra are flat. Biotite from the mctapelite may contain a non-atmospheric trapped argon component because its apparent age (c.74Ma) exceeds that of muscovite in the same hand sample (66.7 f 0.6 Ma). Alternatively, the small grain size of the muscovite may have resulted in a lower closure temperature than for the coarse-grained biotite. The muscovite age for CD-301B is similar to a 40Ar/39Ar plateau age of 68.1 f 0.4 Ma obtained from fine-grained muscovite from

= 110

c, 90

Q) PD

G

a 70 ' 50

0 20 40 60 80 100


.OO .02 .04 .06 .08 .10 .12 .14 39Ar/40Ar

Fig. 7. Hornblende, biotite and muscovite 40Ar/39Ar data for area E (staurolite grade). (a) Age spectra. (b) Apparent K/Ca ratio vs. Yo'~A~ rcleased for hornblende. (c) Isotope correlation plot. Inverse isochron regressions shown by solid line (hornblende) and dashed lines (micas). Open symbols represent data not used in regressions.

sheared Beagle tonalite collected east of area C (S. Mukasa, pers. comm., 1994).

Area D, staurolite grade (Fig. 6). Hornblende (typically 3 mm long and 400 p m across) and biotite (typically 1 mm wide and 200 p m thick) were separated from a mafic amphibolite (CD-l71A), and biotite (typically 600pm wide and lOOpm thick) was separated from a garnet-bearing metapelite (CD-171B). The only alteration observed was a slight chloritization of biotite rims in CD-171A. The age spectrum for hornblende is disturbed, and an inverse isochron regression of selected steps suggests an age of 84.2 f 2.0 Ma with an initial 40Ar/36Ar that is indistinguishable from air. 'I'he biotite spcctra are flat, and inverse isochron regressions of all steps lead to ages of 73.2 f 0.5 Ma (CD-171 A) and 71.8 f 0.4 Ma (CD-171B).

260 M. J . KOHN E T A [

100 Area G

(BtIChl-grade) w

$110 6

2 90

70 cd

CD-207B Hornblendes

.01 0 20 40 60 80 100


.OO .02 .04 .06 .08 .10 .I2 .14 39Ar/40Ar

Fig. 8. Hornblende and biotite q’Ar/3yAr data for area G (biotite/chlorite-grade). (a) Age spectra. (b) Apparent K/Ca ratio vs. %39Ar released for two layers with different sized hornblende crystals. (c) Isotope correlation plot. Inverse isochron regressions shown by solid lines (CD-2WB coarse hornblende and CD-212 biotite) and dashed line (CD-207B fine hornblende). The biotite age is similar to K-Ar and Rb-Sr ages for biotite and hornblende from the same pluton (Hervk er al.. 1984), and likely reflects rapid cooling of the pluton in relatively cold country rock.

Area E, sraurolite grade (Fig. 7). Hornblende was separated from a deformed metabasic dyke (CD-31A; grain size typically 300 p m to 1 mm long and 30-160 p m across), and is rarely altered to reddish-brown biotite. Biotite and muscovite were obtained from a garnet-staurolite metapelitic schist (CDJOF) and are unaltered. Biotite is coarse-grained (200-1200 p m wide and 50-150pm thick), whereas muscovite is relatively fine-grained (150 p m wide and 20 prn thick). The age spectrum for the CD-31A hornblende is imprecise and disturbed. An inverse isochron regression of selected steps suggests an age of 85.8f2.8Ma, and a trapped @Ar/=Ar ratio that is indistinguishable from that of air. The age spectra for the micas are flat, and inverse isochron regressions result in ages of 71.3 f 0.3 Ma (muscovite) and 74.9 f 4.0 Ma

n 2 80 Q) bD

7 t (area 201 I I I I ’ I I I ’


Fig. 9. Age spectra for K-feldspars from areas B (kyanite-grade; CD-168), D (staurolite-grade; CD-175), E (staurolite-grade; CD-29) and F (garnet-grade; CD-28). During argon extraction the furnace was cycled to low temperature to resolve the Arrhenius behaviour of the smaller domains, and the resulting age determinations for these low-T steps are quite uncertain. For ease of visualizing the age spectra, these low-T steps are not included in the plot, resulting in small breaks in the spectra at ”Ar gas release of 25 and 40% (CD-28), 20 and 50% (CD-29). 30 and 60% (CD-175), and 35 and 50% (CD-168). Age gradients for samples CD-28,CD-29 and CD-175 span c. 40 Ma, and for CD-168 the gradient is 20 Ma.

(biotite). The old age of biotite relative to muscovite could indicate a non-resolvable, non-atmospheric trapped argon component in the biotite, or a relatively low closure temperature for the fine-grained muscovite.

Area F, gurnet grade. N o new data were collected for hornblende, muscovite or biotite from area F. but Grunow er al. (1992) report a muscovite 40Ar/’9Ar age of 73 f 1 Ma.

Area G, biotifelchlorite grade (Fig. 8 ) . Two hornblende separates and a biotite separate were analysed from igneous rocks in the biotite-grade region south of the Beagle Channel. Hornblende in sample CD-207B occurs in two habits: in a coarsely crystalline 95% pure hornblende layer (‘coarse’; grain size = 4 mm long and 1.5 mm across), and in a finer grained hornblende + plagioclase layer (‘fine’; grain size = W p m long and 150pm across). The two separates yield flat spectra with similar inverse isochron ages of 90.5 f 0.3 Ma (coarse) and 89.3 f 1.4 Ma (fine). The biotite separate (CD-212) was obtained from a Beagle Suite tonalite that is probably the same intrusion as analysed by Hew6 el ol., (1984; pluton #4, Table 1). Our biotite separate gave a flat release spectrum, and an inverse isochron age of 88.9 f 0.4 Ma. This age corresponds well with biotite (K-Ar and Rb-Sr) and hornblende (K-Ar) ages of c.90zt4Ma reported by Hervk et al. (1984) and with discordant zircon U-Pb ages (c. 90 f 2 Ma; S. Mukasa, pers. cornm., 1994), and suggests that the pluton crystallized and cooled rapidly through biotite closure to argon diffusion.

K-feldspars (Fig. 9 ) . K-feldspar separates were obtained from four plutonic rocks of Beagle and Darwin Suite affinities from areas B, D, E and F. All spectra show excess argon over the first 20% of 3yAr released, and contamination of samples CD-28 and CD-29 is sufficiently significant to be seen through isochron analyses that suggest initial “Ar/36Ar of c . 1670 (not shown). Of particular


600

500

"L 400 o ̂i? 3 300 TJ L 0 $ 200

loo 0 100 80 60 40 20 0

Time (Ma) , I

rlnlt = Mi17 'CMa 600

O100 80 60 40 20 0 Time (Ma)

- ri.\-s.:.* \

100 80 60 40 20 0 Time (Ma)

T,,,, = 6Oilh 'CMn

- C D 2 9

100 80 60 40 20 0 Time (Ma)

I t--J . . . . . . . . I

BP 122 BP-102

100 80 60 40 20 0 Time (Ma)

100 so 60 40 20 0 Time (Ma)

Time (Ma)

._.... - =Constrained by .._ . . . peochronolwy.

.. I . . . . '.

500

"v 400 0

3 300 $ $ 200

100

0 120 100 80 60 40 20 0

on petrology an mgional ages

100 80 60 40 20 0 Time (Ma) Time (Ma)

Fig. 10. Thermochronolo ical constraints on T - f paths for areas A-G (a-g) and summary of calculated cooling curves (h). Boxes show individual data for 40Ar/ Ar on hornblende (closure temperature c. 505" C), muscovite (c. 370" C), and biotite (c. 320" C), for Rb-Sr on biotite (c. 300°C) and for fission track on titanite (250" C), zircon (175" C) and apatite (100" C) at 20 confidence level. Dashed lines in l l b , l l d , lle, and l l f show results of K-feldspar modelling. Patterned boxes represent data collected in other studies. Initial cooling rates (Tni,) from c. 500" C to c. 300" C were rapid for amphibolite facies areas, but the rocks cooled progressively later with increasing metamorphic grade (e.g. hornblende ages are youngest for sillimanite-grade area A, whereas the last-extracted K-feldspar ages are oldest for garnet-grade area F). K-feldspar models indicate that rapid initial cooling changed to much slower cooling until c. 50 Ma. At this time, increased cooling occurred, followed again by more uniform slow cooling.

A

interest is that the initial gas release shows alternating older and younger ages, particularly for samples CD-28 and CD-29. This oscillation is due to the fashion in which the gas was extracted, which included a replication of a heating step at certain temperatures (see Table Al). Although we usc these isothermal duplicate steps primarily to establish whether trends on Arrhenius plots are robust, it has recently been documented that K-feldspars heated in the range 400-800" C release CI-correlated excess argon from decrepitation of fluid inclusions (Harrison er ol., 1994). Thus, the initial step is anomalously old because first attainment of a new peak heating temperature causes decrepitation of inclusions and release of CI-correlated excess 40Ar*. The second step is correspondingly younger because most of the inclusions that were

unstable at that temperature have already decrepitated. In sample CD-29, isothermal duplicate steps taken over the first 15% of gas release show a strong correlation of ACI/K versus AWAr*/K (for details, see Harrison et ol., 1994) that indicate a CI-correlated component of excess radiogenic argon of *Ar,/CI = 1.26 f 0.06 X This allows us to 'see through' and subtract out the contaminating effects of the CI-correlated excess 40Ar*. For example, the two steps extracted at 600" C that yield ages of 88.9 and 38.7 Ma CI-correct to 34.1 f 2.7 and 32.2 f 0.4 Ma, respectively. In general, the correction to the second isothermal steps is small and simply extrapolating between the young apparent ages yields a spectrum that is within c. 1-2 Ma of the presumed true value. Nonetheless, it is interesting that the CI-correction

262 M. J . KOHN E T A l

approach permits precise recovery of thermal history information over a temperature range (c. 250-200" C) otherwise not routinely accessible by such K-feldspars.

K-feldspars from samples CD-175, CD-28 and CD-29 reveal gradients in their apparent age spectra that span c. 40 Ma for gas that appears to be uncontaminated by excess 40Ar*. The age spectrum for K-feldspar from sample CD-168 shows a smaller gradient of only c. 20Ma, suggesting either a different kinetic behaviour for argon retention or a different cooling history. Samples CD-29 and CD-175 have similar ages for the last 70% of their gas released, whereas CD-168 and CD-175 have similar ages for the first 30% of the gas released.

Thermochronology and determination of T-t paths

Figure 10 shows the resulting thermochronological constraints on the cooling histories for the different areas and a summary of the cooling curves using closure temperatures as described above. In addition to the 40Ar/39Ar data described above, we have incorporated some fission track data of Nelson (1982), @ArfYAr data of Grunow et al. (1992) and Rb-Sr data of Halpern (1973). Initial cooling rates were estimated by regressing the closure temperatures of hornblende, muscovite, biotite and oldest K-feldspar against their ages using the approach of York (1%9). Areas that reached sillimanite and kyanite grade (areas A-C) cooled initially at rates of c.25"CMa-' (37 f 19, 26 f 14 and 15 f 7" C Ma-', 2a), whereas staurolite-grade areas (D and E) appear to have cooled more rapidly (66 f 17 and 60 f 36" C Ma-'). Of additional interest is a quite systematic dispersion of ages with grade, such that staurolite-grade rocks cooled much earlier than kyanite- and sillimanite-grade rocks. The old ages measured for the most retentive domains in garnet-grade K-feldspar (sample CD-28; area F) imply that these rocks cooled even earlier than the staurolite-grade rocks. K-feldspar modelling for all areas further suggests that cooling below 300°C was not uniform. Specifically, a renewed phase of rapid cooling occurred between c.250 and c. 200" C that was followed again by slower cooling. A 'knee' in the cooling history at c. 250" C was also suggested by Nelson (1982) for area G based on fission track data, and the 40Ar/39Ar and fission track data in area A are consistent with a cooling curve of this general shape, although the resolution of the fission track ages is much poorer than the K-feldspar data. A systematic dispersion of cooling histories with grade was not observed at lower temperature.

The correspondence between the K-feldspar models and the muscovite and biotite data is typically not better than about *50"C . In areas B and D, the K-feldspar cooling histories are most similar to those determined using the mica '"'Ar/3yAr data, whereas in areas E and F the correspondence is much poorer. Because we directly estimate the K-feldspar kinetic parameters but assume characteristic values for the micas, we are more confident of the cooling histories derived from the K-feldspar modelling.

In biotite/chlorite-grade area G, ages for hornblende (K-Ar) and biotite (K-Ar, 40Ar/39Ar and Rb-Sr) for

individual intrusions are indistinguishable and range from c. 110 Ma to at least 85 Ma and possibly to 80 Ma (plutons 1-9, Table 1). 40Ar/39Ar analyses of hornblende (CD-207B) and biotite (CD-212) from area G show no indication of excess argon (Fig. 8). The concordancy of minerals with different closure temperatures from single plutons provides strong evidence that the country rock temperature has been below biotite closure for the K-Ar and Rb-Sr systems (300-320°C) since 110Ma, in agreement with the interpretation of Nelson (1982). The low temperature during and immediately after the Andean orogeny contrasts with the petrological and geochronological data for kyanite-grade area C, only 2 km away across the north-west arm of the Beagle Channel. Peak metamorphic temperatures there were 2600" C (Kohn er a/., 1993), and temperatures remained in excess of 500°C until at least 80Ma. Thus, a distinct palaeothermal and petrological discontinuity exists across the north-west arm of the Beagle Channel (see also Nelson, 1982). The present-day juxtaposition could have occurred through down-to-the-south extensional shear and/or strike-slip motion (Dalziel & Brown, 1989; Cunningham, 1993).

ONE-DIMENSIONAL THERMAL MODELS

In order to test whether there is internal consistency among the geochronological and petrological data and to determine what magnitude initial exhumation rates plausibly explain the high-T data (c. 500 to c. 300" C), one-dimensional thermal models were constructed for rocks from the staurolite and kyanite + sillimanite metamorphic zones (areas A-E). The reason we believe 1-D models might have some validity over this temperature range is that the differences in cooling histories for different areas appear to be a function of peak metamorphic grade or burial depth, rather than distance from geological features such as faults or plutons. This suggests a 1-D exhumation process that relates to vertical uplift rather than a 2-D process that relates to local heat sources or sinks. Areas A-E were chosen because they have the most complete and consistent petrological and geochronological data. Simple, smooth cooling curves were fitted to the data between 500°C and c.275"C, emphasizing the hornblende and K-feldspar data. No attempt was made to fit the details of the K-feldspar models at lower temperatures, and the thermal and tectonic evolution below c. 275" C clearly had additional complexity.

The principal observations we sought to reproduce in the models were: (1) the highest grade (Ky + Sil) rocks had maximum temperatures of 575-625" C , were buried to depths of 30 km (8 kbar), had initial cooling rates between 500°C and 320°C of c. 25"CMa-', and cooled c. 10Ma later than staurolite-grade rocks; (2) the staurolite-grade rocks had maximum temperatures of 550-580" C, were buried to depths of 25 km (6.7 kbar), and had initial cooling rates 225" C Ma-'.

The two most significant assumptions were: (1) thickening by thrusting (Dl) and backfolding (D2)


occurred at an average rate of 1 mm yr-', resulting in only a small perturbation to the geotherm within the top 30 km of the crust; (2) the stable geotherm was on average 20-25" c km-'. Assumption 1 was based on the flatness of the P-T paths during backfolding, which suggests relatively fast thermal re-equilibration compared with any D2 loading. Assumption 2 was based on the observations that peak P-T conditions fall between 20 and 25°C km-' geotherms (Fig. 3), and the geotherms could have had as long as 30Ma to equilibrate. Furthermore, if the exhumation paths derived from garnet amphibolites are correct for area B, then the small increase in temperature suggests that the geotherm was not strongly perturbed when exhumation began.

Many combinations of the thermal and physical properties of the rocks allow a stable geotherm of 20-25" C km-' to be constructed. Although nine combinations of parameters were tested to see their effects on predicted T-t and P-T paths, the results were found to be insensitive to the choice of parameter set. The principal thermal model results are illustrated well by the following parameters: surface heat production (A) = 2 X

W I J - ~ , characteristic length scale for exponential decrease of heat production with depth (I) = 23.5 km, mantle heat flux (q,,,) = 30 mW m-*, thermal conductivity ( k ) = 2.25 W m-' K-', and thermal difisivity (k) = 0.8 mrn's-'. In each model, at 100-120 Ma a 35-km-thick section of crust at thermal equilibrium was loaded with 20km of material and allowed to equilibrate for 20Ma. Between 15 and 25 km of material was then stripped off the top of the thickened crust at rates of 0.5-5 mm yr-' for 3-50 Ma. Exhumation rates were then decreased to 0.05- 0.2mmyr-', so that rocks that were buried to maximum depths of 25-30 km would just reach the surface at 0 Ma. The P-T-t histories of rocks that reached depths of 25 and 30km were recorded at various time intervals and compared with the observations. We adjusted the absolute start time for each model to fit both the staurolite-grade and kyanite + sillimanite-grade data as well as possible. Thus, the absolute time of each model T - f curve shown in Fig. 11 reflects a compromise of fitting data for two different grades.

The sensitivity of the P-T and T-r paths to exhumation amounts and rates is illustrated well by the results of two thermal models. In the first model, a uniform exhumation rate of 1.0mmyr-' (solid lines) was assumed until the rocks reached 7.5km depth (durations of 22.5Ma for kyanite + sillimanite-grade rocks and 17.5 Ma for staurolite-grade rocks), folloded by slow differential erosion (<0.2 mm yr..') so that both rocks reached the surface at the same time. In the second model (dashed lines), it was assumed that initial exhumation of staurolite- and kyanite + sillimanite-grade rocks began simultaneously but at different rates: 17.5 km was exhumed from the staurolite-grade rocks at a rate of 2.5mmyr-' (7Ma duration), whereas 20.0 km was exhumed from the kyanite + sillimanite-grade rocks at a rate of 1.0 mm yr-' (20 Ma duration). After initial exhumation, the staurolite- grade rocks were assumed to remain stationary at 7.5 km

depth for 13 Ma until the kyanite + sillimanite-grade rocks reached the same level. At that point, slow uniform exhumation (c. 0.15 mm yr-I) of both staurolite- and kyanite + sillimanite-grade rocks was assumed.

Figure 11 shows that the P-T paths provide few

I I I I I I-D models

200 400 600 800

600

500

Temperature ("C)

0 100 80 60 40 20 0

Time (Ma) Fig. 11. (a) Comparison of predicted F-T paths from thermal models with observed P-Tpaths. The I-D P-Tpaths (solid lines) are relatively insensitive to exhumation rates and are consistent with the clockwise path determined from amphibolites from area B (samples at SP-IOA). Two-dimensional juxtaposition of hot lower plate rocks with cooler upper plate rocks would result in a path with substantial cooling during exhumation (dashed line), and this contrasts with the observed path for the SP-10A amphibolites. (b) Comparison of predicted T-f paths from thermal models with observed T-t paths. Because models 1 and 2 have similar initial exhumation rates and durations for kyanite + sillimanite-grade rocks (1 mm yr-' for 20-22.5 Ma), the T - f curves are similar in shape. However, the absolute ages for the two curves are slightly different, as a result of minimizing the differences between model curves and data for both staurolite- grade and kyanite + sillimanite-grade rocks. Simple uniform exhumation models (solid lines) do not produce sufficient disparity in T - f paths for different metamorphic grades to explain the observed T-r paths. Differential exhumation models (dashed lines) reproduce the observed paths more closely, but consideration of lateral heat flow effects indicates that the differences in the modelled exhumation rates are overestimated. See text for details.

264 M. J. K O H N € T A L

constraints on 1-D exhumation amount or rate, but that the T-t results are sensitive to 1-D thermal models. Uniform exhumation at a rate of 1 mm yr-' does not fit the T-t data well. The predicted differences in age between staurolite- and kyanite + sillimanite-grade rocks for any temperature are not more than 2-5 Ma, whereas observed differences are 10 Ma. Although models involving slower uniform exhumation rates lead to greater disparity in the cooling ages, predicted cooling rates between 500 and 325°C are much too slow, and predicted hornblende closure to argon loss is not reached until 40-60Ma after initial thickening. This long delay exceeds the limits imposed by the palaeontological and K-Ar provenance data. The results of the second thermal model involving initial differential exhumation show a significantly im- proved fit to the data. Differences in cooling ages are at least 10 Ma, and cooling rates are uniformly high. Thus, we believe that models of initial differential exhumation of 15-20km of material at rates of 1-2.5mmyr-' are the most internally consistent with the petrological, geochronological and palaeontological data.

The close association of staurolite- and kyanite + sillimanite-grade rocks in the field poses a problem for the acceptance of simple 1-D thermal modelling results. If we accept initial exhumation rates of 1.0 mm yrf ' for kyanite + sillimanite-grade rocks and 2.5 mm yr'.' for staurotite-grade rocks, and examine the geotherm through time for areas A + B and D, then lateral thermal gradients of 25-50' C km-' are implied. Obviously, such strong lateral thermal gradients imply significant lateral heat flow between areas D and A + B , and invalidate 1-D models. Because there are so few constraints on the geometry and location of structures that could have accommodated differential uplift between the areas, we chose not to fit 2-D models to the data. Nonetheless, some comments about the first-order effects of lateral heat flow are warranted. Initial rapid exhumation of the staurolite-grade rocks in area D will juxtapose them with the cooler upper section of area A + B . This will tend to cool the staurolite-grade rocks earlier and at a greater rate than indicated by 1-D models. In contrast, the juxtaposition of the hot lower crust in area D with the mid-crust in area A + B will tend to maintain higher temperatures longer for the kyanite + sillimanite-grade rocks. Kyanite + sillimanite-grade rocks will therefore cool later and at slower rates than indicated by 1-D models. The fundamental implication of lateral heat flow is that early rapid exhumation of staurolite-grade rocks will produce greater disparity in T-t curves for staurolite- and kyanite+ sillimanite-grade rocks than suggested by 1 -D models. With respect to Fig. 11, this implies that 2.5 mm yr-' is too fast for staurolite-grade rocks, whereas 1.0 mm yr-' is too slow for kyanite + sillimanite-grade rocks. That is, average exhumation rates are bracketed between I .O and 2.5 mm yr-'.

The possibility that 2-D juxtaposition of hot high-grade rocks in areas A-D with the cold low-grade rocks of area G caused the observed initial cooling was also considered. The characteristic time for thermal re-equilibration of

rocks located within 10km of a thermal boundary is < 1 Ma, or essentially instantaneous geologically. There- fore, if fault movement occurred between 85-90 and 70 Ma and caused cooling of all rocks to c. 325' C , then for peak temperatures of c.625'C (area A), c.600°C (B and C) and c. 575" C (D), we would expect to find nearly linear and constant cooling rates of c . 15-20' C Ma-' (area A), c. 14-18' C Ma-' (B and C) and c . 12-16' C Ma-' (D). We would also expect P - T paths to involve substantial cooling during exhumation, so that their retrograde paths (dashed line, Fig. 11) would nearly track back down their prograde paths. Although the observed cooling rates for areas A-C can be reconciled with this model, the observed cooling rate for area D is much faster than predicted. More importantly, the P -T paths from amphibolites in area B (SP-IOA) show exhumation with heating, and this strongly argues against a simple 2-D shear model to explain the high-T cooling (see Fig, 11). Possibly, the present juxtaposition of low-grade and high-grade rocks across the Beagle Channel occurred at lower temperatures.

DISCUSSION

Comparison with T-t paths from other metamorphic complexes: implications for crustal rheology . The cooling history of Cordillera Darwin was compared with that of the Waterman terrane (Henry & Dokka, 1992), the Funeral Mountains (DeWitt et al., 1986; Holm & Dokka, 1991), the northern Chemehuevi Mountains (Foster et af., 1990), the Old Woman Mountains (Foster et al., 1989) and the Big Maria Mountains (Hoisch et a[., 1988), all in southeastern California; the Ruby Mountains in north-eastern Nevada (Dallmeyer el al., 1986); the Valhalla + Monashee complexes in south-eastern British Columbia (Sevigny et al., 1990, Heaman & Pamsh, 1991, and references therein); the Salmon River suture zone in western ldaho (Lund & Snee, 1988; also see summary of Selverstone et of., 1992); the Diancang Shan in the Yunnan province of China (Leloup et af . , 1993); and the D'Entrecasteaux Islands of Papua New Guinea (Baldwin el al., 1993). These areas were chosen for three reasons. First, like Cordillera Darwin they all exhibit high-grade metamorphic cores that rapidly decrease in metamorphic grade structurally upward and outward. Second, high- quality geochronological data for many minerals with different closure temperatures have been collected for a single outcrop or small area in each complex, so that precise T-t curves could be drawn. Third, they all have been studied petrologically to determine pre-exhumational burial depths and temperatures. The compilation is intended to be illustrative of metamorphic complexes in general rather than an exhaustive documentation of published T-t histories for core complexes. Most of the areas chosen have additional geochronological data that bear on regional cooling patterns, but are less specific for cooling histories of single outcrops or groups of outcrops. Furthermore, the areas are not all metamorphic core complexes, in that some do not show a simple jump in metamorphic grade across low-angle detachment faults.

Tem

pera

ture

(˚C

)

Time (Ma)100 80 60 40 20 0120

0

200

400

600

500

300

100

ValhallaComplex

SalmonRiverSuture

WatermanTerrane

DiancangShan

NorthernChemehuevi

MtnsCD

10 ˚C/Ma

20 ˚C/Ma

50 ˚C/M

a

Ruby Mtns

Old WomanMtns

BigMariaMtns

FuneralMtns

St-Grade

D'EntrecasteauxIslandsKy-Grade Monashee

Complex

266 M. I . KOHN ET AL

cooling event also cannot be ruled out for the Salmon River suture, and the late tectonic and cooling history for at least the Monashee complex may have involved thrusting as well as normal faulting (Sevigny et al., 1990).

It is interesting to explore the possibility that major fault systems excise less than c. 20 km of overburden, because this limit can be readily explained in terms of the fundamental rheological variations in continental crust that control the shape and behaviour of extensional systems. Lister & Davis (1988) have summarized many of the relevant observations and discussions that relate crustal rheology to fault geometry and style in continental extensional areas, and the following discussion is largely based on that paper and the references therein (especially Chen & Molnar, 1983; Jackson & McKenzie, 1983). The most important feature of continental crustal rheology is an increase of strength with depth towards the brittle/ductile transition and a subsequent decrease of strength within the ductile regime towards the crust/mantle boundary. Major earthquakes nucleate in a zone of maximum strength at the brittle/ductile transition (c. 8-12 km depth), and resulting brittle slip occurs between the surface and 15-16km depth. Imaging of extensional faults shows them to be planar to a depth of 10-16 km, and below c. 15 km brittle movement along a major planar fault clearly cannot be the means by which continental crust extends.

One explanation of these observations is that lower- crustal ductile deformation is diffuse and distributed. If so, then mid- to lower-crustal rocks will tend to deform laterally along low-angle, distributed shear zones or through homogeneous thinning rather than through high-angle shear. In terms of uplift rates, brittle extension in the upper crust is an efficient exhumative agent, because extension directly translates into fault movement, and a significant component of that fault movement results in exhumation. In contrast, distributed extension in the middle and lower crust is an inefficient exhumative agent, because extension there has a very small vertical component (i.e. it is almost entirely lateral). These considerations imply that tectonic exhumation of 215- 20km of material requires operation of more than one fault system or shear zone, and that shallowly and deeply metamorphosed complexes will typically have single-stage and multiple-stage exhumation histories, respectively. These thermochronological implications are in general accord with T-t histories described above.

If instead lower crustal deformation is accommodated along lithosphere-scale discrete ductile shear zones (e.g. Wernicke, 1985), then there is no theoretical limit on the amount of exhumation that can occur in a single event. In this case, we would expect to see less correlation between the number of exhumation events and depth of burial. This model better explains the single-stage thermal histories of the deeply metamorphosed D'Entrecasteaux Islands, Salmon River Suture and possibly the Valhalla + Mona- shee complexes, but overall there is less geochronological and petrological support for lithosphere-scale discrete ductile shear zones than for mid-crustal distributed shear.

Regional tectonic implications for Cordillera Dawin . The geochronological and petrological data from Cordillera Darwin combined with thermal modelling allow a new evaluation of the metamorphic core complex model of Dalziel & Brown (1989). The transition from rapid, spatially disparate cooling rates (c. 25" C Ma-') to slower, more uniform rates (average of c.4"CMa-') and the observed P-T paths involving exhumation with heating are consistent with initial rapid exhumation (1-2 mm yr-') that was succeeded by much slower exhumation (average of c. 0.1 mm yr-I). Although rapid erosion rather than extension can equally well explain the geochronological and petrological data, the Occurrences of intermediate-T (300-500" C) kinematic indicators with extensional shear senses along the southern margin of the complex (Dalziel & Brown, 1989; Moore, 1990; Cunningham, 1994) clearly allow extension to be linked temporally with cooling and exhumation. Thus, the simplest explanation of the high-T data from the southern part of the complex is one involving Late Cretaceous extensional exhumation, as suggested by Dalziel & Brown (1989). In the north, however, extensional fabrics are extremely rare or absent. Instead, thrust-sense deformation may have continued into a low-T brittle regime (Klepeis, 1994) and, consequently, we prefer to consider alternatives to the core-complex model.

Cunningham (1994) recently proposed a lithospheric root detachment model for the exhumation history of Cordillera Darwin, similar to that described by Selverstone et al. (1992) for the Salmon River Suture zone of Idaho. In Cunningham's model, closure of the marginal basin during D1 caused partial southward subduction of the northern basin floor and possibly the edge of the South American margin. Detachment of this dense lithospheric root at 80-95 Ma caused isostatic rebound and rapid exhumation of southern Cordillera Darwin, whereas thrusting was unaffected in the north. Cunningham infers the Beagle Channel to mark a major structure that accommodated differential uplift between southern Cordillera Darwin and basin rocks to the south. The prediction of different deformation styles for southern and northern Cordillera Darwin is in better accord with structural observations (Nelson et af., 1980 Moore, 1990; Cunningham, 1994; Klepeis, 1994), but implications for cooling histories are not supported by the geochronological data. For example, Selverstone et al. (1992) describe how lithospheric root detachment would likely cause a monotonic change in cooling ages and rates away from the structure that accommodates differential uplift between an isostatically rebounding block and a relatively stable adjacent block, and they summarized compelling geochronological evidence for just such systematic changes away from the Salmon River suture zone in Idaho. In contrast, there is no monotonic change in cooling rates and ages away from any geographical or geological feature of Cordillera Darwin, including the Beagle Channel, and this suggests that accommodation of uplift along a major structure during initial cooling of the complex did not occur. Furthermore, a lithospheric root detachment model does not explain why

GEOCHRONOLOGY OF CORDILLERA DARWIN 267 ~

other areas of the southern South American Andes that also experienced closure of the marginal basin, such as the Sarmiento complex to the north, do not contain similarly high-grade metamorphic rocks.

As an alternative to both the core complex model of Dalziel & Brown (1989) and the lithospheric root detachment model of Cunningham (1994), we propose a 'wedge extrusion' model (i.e. internal extension f underplating; Platt, 1986) as a viable explanation of the kinematic and P-T-t data. As described by Dahlen (1984) and Yin (1993), within an overall compressional regime both non-cohesive critical coulomb and elastic wedges can exhibit rearward extension or thickening, with the type of deformation depending on the mechanical properties of the wedge and its boundaries. For example, if the basal friction or basal thrust angle changes, then the upper portion of a deforming wedge can stop thickening and fail extensionally. We believe that such systematic changes in mechanical properties, geometries and/or stresses through time caused systematic changes in deformation styles in southern Cordillera Darwin from early thickening to later extension, as summarized in Fig. 13 and as follows.

(a) r 9 0 M a . Overthrusting and isoclinal folding of the marginal basin sediments produced a strong shallowly inclined fabric in the rocks (Fig. 13a), caused most of the thickening and initiated the formation of the wedge. A shallowly inclined basal thrust is shown (dashed line) to emphasize the overall wedge geometry; there is no direct evidence for or against such a thrust.

(b) 290 Ma. South-directed backfolding (Fig. 13b) was synchronous with peak amphibolite facies metamorphism of the metapelites, but P-T paths suggest there was little thickening in southern Cordillera Darwin (Kohn el af. , 1993; Fig. 3b). Instead, rocks were either passively warped at deep structural levels or backfolded at shallow structural levels with minor shallowly dipping backthrusts (structural data from Nelson et al., 1980). At this time, the rear of the wedge may have reached some critical thickness beyond which it was mechanically unstable to thicken or thin.

(c) 70-90 Ma. Initial rapid cooling and extension of at least the southern part of the high-grade metamorphic core likely resulted from a change in the mechanical behaviour of the wedge or its boundaries. Because cooling patterns are not consistent with extension along a discrete major detachment fault, we believe extension was distributed at relatively high structural levels (Fig. 13c). Thrusting continued at lower structural levels and towards the toe of the wedge in the north.

(d) 60-4OMu. The detailed models of K-feldspar argon release spectra show a complex T-t history in this period (temperatures of 325-200" C), including rapid cooling at c. 50 Ma between two slow cooling segments. A specific structural mechanism for rapid exhumation and cooling at c. 50 Ma remains somewhat enigmatic, but a normal component of movement on t h e Beagle Channel fault system could have stripped the remnants of the marginal

Simplified Tectonic Model Southern Northern

Cordillera Darwin Cordillera Darwin

3verthrusting A A - c (>90Ma)

L

Exhumation 2 (60-40 Ma)

Fig. 13. Model of the tectonic evolution of Cordillera Darwin. Note shift in structural style from (a) major thickening to (b) predominantly lateral movement and backfolding to (c) extension. This is interpreted to be the result of changes in the mechanical properties of the deforming wedge and its boundaries through time. The Beagle Channel is interpreted to have accommodated only late extension and strike-slip movement (d).

basin from the high-grade core (Fig. 13d), and thinned the rear of the wedge to a new stable geometry. This episode of cooling corresponds with lengthening of the wedge through the initiation of thrusting in the Magallanes basin in the north (Winslow, 1982; Biddle et af . , 1986).

0-40Mn (not shown). A shift to slow cooling at average rates of 3-5"CMa-' suggests a change from extension in the south to simple block uplift and mechanical stabilization of the wedge. Thrusting until at least 15 Ma is documented in the sedimentary rocks of the Magallanes basin (Winslow, 1982; Biddle et a!., 1986; Alvarez-Marr6n el al., 1993).

Our new interpretation of a progressive shift from thickening and backfolding (290 Ma) to 'wedge extrusion'

268 M. I . K O H N E T A L

(40-%Ma) has the advantages that it explains: (1) the extensional textures, P-T paths and rapid initial cooling observed in the south; (2) the continued thrusting in the north; and (3) the southward dip of the dominant fabric. The location of anomalously high-grade rocks in Cordillera Darwin but not elsewhere along the Andes could then still be genetically related to far-field stresses and the development of the Patagonian orocline in the latest Cretaceous and earliest Tertiary as first suggested by Dalziel & Brown (1989; see also Cunningham et al., 1991; Grunow et al., 1992; Cunningham, 1993). Corresponding changes of local stress and strain geometry in Cordillera Darwin could have led to a shift in basal shear stresses or basal thrust slope, and thus initiated extension in the southern part of the complex while maintaining thrusting in the north.

Within the larger context of plate motions and interactions, a shift from slow to fast spreading rates in the South Atlantic and Pacific during the mid- to Late Cretaceous (e.g. Larson, 1991; see also Dalziel, 1986) probably initiated closure of the marginal basin, producing the Andean orogeny and forming the thickened wedge in Cordillera Darwin. Similarly, changes in far-field stresses accompanying a shift towards diffuse transform and divergent motion between the South American and Antarctic plates between 90 and 40 Ma (e.g. Cunningham & Dalziel, 1992) caused changes in the mechanical behaviour of the thickened wedge to produce extension and rapid cooling in the southern part of the complex and thrusting in the north into the Magallanes basin. Finally, development within the last 40 Ma of discrete faults between South America and Antarctica (e.g. the North Scotia Ridge transform fault immediately east of Cordillera Darwin and the Bransfield Strait spreading centre immediately north of the Antarctic Peninsula) caused a change to relatively quiet block uplift of Cordillera Darwin. Thus, we find that there is good internal consistency among structural, petrological, geochronological and geodynamic data for this area, and that a relatively simple wedge model involving crustal thickening followed by initial rearward extension and final block uplift can be interpreted in terms of tectonic interactions from local to oceanic/continental plate scales.

ACKNOWLEDGMENTS

Special thanks are due M. Heizler for his assistance and training with argon isotopic analysis and interpretation. We thank the captains and crews of the R / V Polar Duke for their help in collecting samples, and S. Mukasa and A. Grunow for divulging U-Pb and 40Ar/3yAr ages from the area'before publication and for supplying crushed rock for K-feldspar separates. M.J.K. thanks: M. Roden, 0. Lovera, M. Grove, D. Foster and P. Copeland for their advice and help in separating minerals, collecting analyses, interpreting data and modelling K-feldspar spectra; the Heizler and Hamson families for their generous hospitality in Los Angeles; K. Klepeis and D. Cunningham for

discussing the structural geology of Cordillera Darwin; and T. Spell for restarting a dead Honda. D. Henry, J. Selverstone, and an anonymous reviewer helped significantly to clarify the presentation and concepts of the paper. This work represents a portion of the PhD and post-doctoral research of M.J.K., and was funded by National Science Foundation grants DPP 8643441 and EAR 9005495 (I.W.D.D.), EAR 8803785 and EAR 9004241 (F.S.S.), and by National Science Foundation doctoral and post-doctoral fellowships to M.J.K. Final preparation of the manuscript was supported by NSF grant EAR 9316349 (M.J.K).

REFERENCES

Alvarez-Marr6n, J., McClay, K. R., Harambour, S., Rojas, L. & Skarmeta, J., 1993. Geometry and evolution of the frontal part of theMagallanes foreland thrust and fold belt (Vicuiia area), Tierra del Fuego, southern Chile. American Association of Petroleum Geologists Bulletin, 77, 1904-191.

Baldwin, S. L., Lister, G. S., Hill, E. J., Foster, D. A. & McDougall, I.. 1993. Thermochronologic constraints on the tectonic evolution of active metamorphic core complexes, D'Entrecasteaux Islands, Papua New Guinea. Tectonics, U,

Biddle, K. T.. Uliana. M. A., Mitchum, R. M., Fitzgerald, M. R. & Wright, R. C., 1986. The stratigraphic and structural evolution of the central and eastern Magallanes basin, southern South America. In: Foreland Basins (eds Allen, P. A. & Homewood, P.), International Association of Sedimentologists Special Publication, 8,41-61.

Chen, W-P. & Molnar, P., 1983. The depth distribution of intracontinental and intraplate earthquakes and its implications for the thermal and mechanical properties of the lithosphere. Journal of Geophysical Research, 88,4183-4214.

Cunningham, W . D., 1993. Strike-slip faults in the southernmost Andes and the development of the Patagonian orocline. Tectonics, U, 169-186.

Cunningham, W. D., 1994. Orogenesis at the southern tip of the Americas: the structural evolution of the Cordillera Darwin metamorphic complex, southernmost Chile. Tectonophysics, in press.

Cunningham, W. D. & Dalziel, I. W. D., 1992. Quantification of southern South America-Antarctic Peninsula relative motion since 90 Ma: implications for Scotia arc tectonics. EOS, 73, 507.

Cunningham, W. D., Klepeis, K., Gose, W. & Dalziel, I. W. D., 1991. The Patagonian orocline: New paleomagnetic data from the Andean magmatic arc in Tierra del Fuego. Journal of Geophysical Research, %, 16061-16067.

Dahlen, F. A., 1984. Noncohesive critical coulomb wedges: an exact solution. Journal of Geophysical Research, 89, 10125- 10133.

Dallmeyer, R. D., Snoke, A. W. & McKee, E. H., 1986. The Mesozoic-Cenozoic tectonothermal evolution of the Ruby Mountains, East Humboldt Range, Nevada: a cordilleran metamorphic core complex. Tectonics, 5 , 931-954.

Dalziel, 1. W. D., 1981. Back-arc extension in the southern Andes: A review and critical reappraisal. Philosophical Transactions of the Royal Society of London, A m , 319-335.

Dalziel, 1. W. D., 1986. Collision and Cordilleran orogenesis: an Andean perspective. In: Collision Tectonics (eds Coward, M. P. & Ries, A. C.), Geological Society Special Publication, 19,

Dalziel, 1. W. D. & Brown, R., 1989. Andean core complex evolution related to marginal basin collapse: implications for Cordilleran tectonics. Geology, 17, 699-703.

Dalziel, 1. W. D. & Cortks, R., 1972. Tectonic style of the

611-628.

389-404,.

GEOCHRONOLOGY OF CORDILLERA DARWIN 269

southernmost Andes and the Antarctandes. 24th International Geological Congress, section 3, 316-327.

Davies, H. L. & Warren, R. G., 1988. Origin of eclogite-bearing, domed, layered metamorphic complexes (‘core complexes’) in the D’Entrecasteaux Islands, Papua New Guinea. Tectonics, 7, 1-21.

DeWitt, E., Sutter, J. F., Wright, L. A. & Troxel, B. W., 1986. Ar-Ar chronology of early Cretaceous regional metamorphism, Funeral Mountains, California - a case study of excess argon. Geological Society of America Abstracts with Program, u), A16.

Dokka, R. K., 1989. The Mojave Extensional Belt of southern California. Tectonics, 8, 363-390.

Dott, R. H., Winn R. D., Dewit M. J. & Bruhn R. L., 1977. Tectonic and sedimentary significance of Cretaceous Tekenika beds of Tierra del Fuego. Nature, 266,620-622.

Dott, R. H., Jr, Winn, R. D., Jr & Smith, C. H. L., 1982. Relationship of Late Mesozoic and Early Cenozoic sedimenta- tion to the tectonic evolution of the southernmost Andes and Scotia arc. In: Antarctic Geoscience (ed. Craddock, C.), pp. 193-202. University of Wisconsin Press, Madison, WI.

Fitz Gerald, J. D. & Harrison, T. M., 1993. Argon diffusion domains in K-feldspar I: Microstructures in MH-10. Contributions to Mineralogy and Petrology. 113, 367-380.

Foster, D. A., Harrison. T. M. & Miller, C. F., 1989. Age, inheritance, and uplift history of the Old Woman-Piute Batholith, California and implications for K-feldspar age spectra. Journal of Geology, 97,232-243.

Foster, D. A., Harrison, T. M., Miller, C. F. & Howard, K. A,, 1990. The 40Ar/39Ar thermochronology of the eastern Mojave Desert, California, and adjacent western Arizona with implications for the evolution of metamorphic core complexes. Journal of Geophysical Research, 95,20005-20024.

Gleadow, A. J. W., 1978. Anisotropic and variable track etching characteristics in natural sphenes. Nuclear Track Detection, 2,

Grunow, A. M., Dalziel, 1. W. D., Harrison, T. M. & Heizler, M. T., 1992. Structural geology and geochronology of subduction complexes along the margin of Gondwanaland: New data from the Antarctic Peninsula and southernmost Andes. Geological Sociery of America Bulletin, 104, 1497-1514.

Halpern, M., 1973. Regional geochronology of Chile south of 50“ latitude. Geological Society of America Bulletin, 84, 2407-2422.

Halpern, M. & Rex, D. C., 1972. Time of folding of the Yahgan Formation and age of the Tekenika Beds, Southern Chile, South America. Geological Sociery of America Bulletin, 83,

Hanson, R. E. & Wilson, T. J., 1991. Submarine rhyolitic volcanism in a Jurassic proto-marginal basin; southern Andes, Chile, and Argentina. In: Andean Magmatism and its Tectonic Setting (eds Harmon, R. S. & Rapela, C. W.), GSA special paper, 2.65, 13-27.

Harrison, T. M., 1981. Diffusion of 40Ar in hornblende. Contributions to Mineralogy and Petrology, 78, 324-331.

Harrison, T. M., Armstrong, R. L., Naeser, C. W. & Harakal, J. E., 1979. Geochronology and thermal history of the Coast Plutonic complex, near Prince Rupert, British Columbia. Canadian Journal of Earth Sciences, 16,400-410.

Harrison, T. M., Duncan, I. & McDougall, I. , 1985. Diffusion of 40Ar in biotite: Temperature. pressure and compositional effects. Geochimica et Cosmochimica Acfa, 45,2461-2460.

Harrison, T. M. & Fitz Gerald J. D., 1986. Exsolution in hornblende and its consequences for 40Ar/’9Ar age spectra and closure temperatures. Geochimica et Cosmochimica Acta, 50,

Harrison, T. M., Heizler, M. T., Lovcra, 0. M., Chen, W. & Grove, M., 1994. A chlorine disinfectant for excess argon released from K-feldspar during step heating. Earrh and Plonetary Science Letters, l23, 95- 104.

Harrison, T. M., Lovera, 0. M. & Heizler, M. T., 1991. 40Ar/39Ar results for alkali feldspars containing diffusion domains with differing activation energy. Geochirnica et Cosmochimica Acta,

105-117.

1881-1886.

247-253.

55, 1435-1448.

Harrison, T. M. & McDougall, I., 1980. Investigations of an intrusive contact, northwest Nelson, New Zealand - I. Thermal chronological and isotopic constraints. Geochimica et Cos- mochimica Acta, 44, 1985-2003.

Heaman, L. & Parrish, R., 1991. U-Pb geochronology of accessory minerals. In: Applications of Radiogenic Isotope Systems to Problem in Geology. Short Course Handbook 19 (eds Heaman, L. & Ludden, J. N.), pp. 59-102. Mineralogical Association of Canada, Toronto, Canada.

Henry, D. J. & Dokka, R. K., 1992. Metamorphic evolution of exhumed middle to lower crustal rocks in the Mojave Extensional Belt, southern California, USA. Journal of Metamorphic Geology, 10.347-364.

HervC, F., Nelson, E. & Suarez, M., 1979. Radiometric ages of granitic and metamorphic rocks of the Cordillera Darwin, region XII, Chile. Rev. Geol. Chile, 7 , 31-40.

HervC, F., Nelson, E., Kawashita, K. & Suarez, M., 1981. New isotopic ages and the timing of orogenic events in the Cordillera Darwin, southernmost Chilean Andes. Earth and Planetary Science Letters, 55, 257-265.

Herv6, M., Suarez, M. & Puig, A., 1984. The Patagonian Batholith S of Tierra del Fuego, Chile: timing and tectonic implications. Journal of the Geological Society of London, 141,

Hill, E. J. & Baldwin, S. L., 1993. Exhumation of high pressure metamorphic rocks during crustal extension in the D’Entrecasteaux region. Papua New Guinea. Journal of

. Metamorphic Geology, 11,261-277. Hodges, K. V., 1988. Metamorphic and geochronologic constraints

on the uplift history of cordilleran metamorphic core complexes. Geological Society of America Abstracts with Program, 20, A18.

Hodges, K. V. & Walker, J. D., 1990. Petrologic constraints on the unroofing history of the Funeral Mountain metamorphic core complex, California. Journal of Geophysical Research, 95, 8437-8445.

Hodges, K. V., Snoke, A. W. & Hurlow, H. A., 1992. Thermal evolution of a portion of the Sevier hinterland: the northern Ruby Mountains - East Humboldt Range and Wood Hills, northeastern Nevada. Tectonics, 11, 154-164.

Hoisch, T. D., Miller, C. F., Heizler, M. T., Harrison, T. M. & Stoddard, E. F., 1988. Late Cretaceous regional metamorphism in southeastern California. In: Metamorphism and Crustal Evolution in the Western Cordillera, Conterminous United States: Rubey volume 7 (ed. Ernst, W. G.) , pp. 538-571. Prentice-Hall, Englewood Cliffs, NJ.

Holm, D. K. & Dokka, R. K., 1991. Major late Miocene cooling of the middle crust associated with extensional orogenesis in the Funeral Mountains, California. Geophysical Research Letters, 18,1775-1778.

Jackson, J. & McKenzie, D., 1983. The geometrical evolution of normal fault systems. Journal of Structural Geology, 5,471-482.

Klepeis, K., 1994. Relationship between uplift of the metamorphic core of the southernmost Andes and shortening in the Magallanes foreland fold and thrust belt, Tierra del Fuego, Chile. Tectonics, 13, 882-904.

Kohn, M. J., Spear F. S . & Dalziel 1. W. D., 1993. Metamorphic P-T paths from Cordillera Darwin, a core complex in Tierra del Fuego, Chile. Journal of Petrology, 34, 519-542.

Kranck, E. H., 1932. Geological investigations in the Cordillera of Tierra del Fuego. Acta Geographica, HeLsinki, 4, 1-231.

Larson, R. L., 1991. Latest pulse of Earth: evidence for a mid-Cretaceous superplume. Geology, 19,547-550.

Leloup, P. H., Hamson, T. M., Rycrson, F. J., Wcnji, C., Qi, L., Tapponnier, P. & Lacassin, R., 1993. Structural, petrological and thermal evolution of a Tertiary ductile strike-slip zone, Diancang Shan, Yunnan. Journal of Geophysical Research, 98, 67 15-6743.

Lister, G. S. & Davis, G. A., 1988. The origin of metamorphic core complexes and detachment faults formed during Tertiary continental extension in the northern Colorado River region, U.S.A. Journal of Sfructural Geology, 11, 65-94.

909-917.

270 M. J . KOHN E T A L

Lovera, 0. M., 1992. Computer programs to model 40Ar/39Ar diffusion data from multidomain samples. Computers & Geosciences, 7,789-813.

Lovera, 0. M., Heizler, M. T. & Harrison, T. M., 1993. Argon dihsion domains in K-feldspar 11: Kinetic properties of MH-10. Contributioru to Mineralogy and Petrology, 113, 381-393.

Lovera, 0. M., Richter, F. M. & Harrison, T. M., 1989. The “Ar/”Ar thennochronometry for slowly cooled samples having a distribution of diffusion domain sizes. Journal of Geophysical Research, 94, 17917-17935.

Lovera, 0. M., Richter, F. M. & Harrison, T. M., 1991. Diffusion domains determined by 39Ar released during step heating. JOUrMl of Geophysical Research, 96,2057-2069.

Lund, K. & Snee, L. W., 1988. Metamorphism, structural development, and age of the continent-island arc juncture in west-central Idaho. In: Metamorphirm and Crustal Evolution in the Western Cordillera, Conterminous United States, Rubey volume 7 (ed. Ernst, W. G.) , pp. 297-331. Rentice-Hall, Englewood Cliffs, NJ.

Miller, D. S., Duddy, P. F., Hurford, A. J. & Naeser, C. W., 1985. Results of interlaboratory comparison of fission-track age standards. Fission-Track Workshop-1984: Nuclear Tracks, 10,

Moore, T. K., 1990. A metamorphic and kinematic study of the Cordillera Darwin metamorphic core complex, southwestern Chile. BSc Thesis, Carleton University.

Mukasa, S. B., Dalziel, 1. W. D. & Brueckner, H. K., 1988. Zircon U-Pb constraints on the kinematic evolution of the northern Scotia Arc. Geological Society of America, Abstracts with Program, 20, A12.

Naeser, C. W. & Faul, H., 1969. Fission-track annealing in apatite and sphene. Journal of Geophysical Research, 74,705-710.

Nelson, E. P., 1982. Post-tectonic uplift of the Cordillera Darwin orogenic core complex: evidence from fission-track geochronology and closing temperature-time relationships. Journal of the Geological Society of London, 139,755-761.

Nelson, E. P., Dalziel, 1. W. D. & Milnes, A. G., 1980. Structural geology of the Cordillera Darwin - collisional-style orogenesis in the southernmost Chilean Andes. Eclogue Geologicae Helvefiae, 73,727-751.

Platt, J. D., 1986. Dynamics of orogenic wedges and the uplift of high-pressure rocks. Geological Society of America Bulletin, 97, 1037-1053.

Richter, R. M., Lovera, 0. M., Harrison, T. M. & Copeland, P., 1991. Tibetan tectonics from 40Ar/39Ar analysis of a single K-feldspar sample. Earth and Planetary Science Letters, 105,

383-391.

266-278.

Robbins, G. A., 1972. Radiogenic argon diffusion in muscovite under hydrothermal conditions. MS Thesir, Brown University.

Selverstone, J., Wernicke, B. P. & Aliberti, E. A., 1992. lntracontinental subduction and hinged unroofing along the Salmon River suture zone, west central Idaho. Tectonics, 11, 124-144.

Sevigny, J. H. & Ghent, E. D., 1986. Metamorphism in the northern Adams River area, northeastern Shuswap metamorphic complex, British Columbia. Current Research: Geological Survey of Canada Paper, 87-1B, 693-698.

Sevigny, J. H. & Ghent, E. D., 1989. Pressure, temperature, and fluid conditions during amphibolite facies metamorphism of pelitic rocks, Howard Ridge, British Columbia. Journal of Metamorphic Geology, 7,497-505.

Sevigny, J. H., Parrish, R. R., Donelick, R. A., & Ghent, E. D., 1990. Northern Monashee Mountains, Omineca crystalline belt, British Columbia: timing of metamorphism, anatexis, and tectonic denudation. Geology, 18,103-106.

Steiger, F. H. & Jager, E., 1977. Subcommission on geochronology: convention on the use of decay constants in geo- and cosmochronology. Earth and Planetary Science Letters, 36,

Stewart, J. H., 1967. Possible large right-lateral displacement along fault and shear zones in the Death Valley - Las Vegas area, California and Nevada. Geological Society of Americu Bulletin, 78, 131-142.

Suarez, M., Hervk, M. & Puig, A., 1985. Plutonismo diapirico del Cretacico en Isla Navarino. IV Congreso Geologico Chileno,

Wagner, G. A., 1968. Fission-track dating of apatites. Earth and Planetary Science Letters, 4, 41 1-415.

Wernicke, B., 1985. Uniform-sense normal simple shear of the continental lithosphere. Canadian Journal of Earth Science, 22,

Winslow, M. A., 1982. The structural evolution of the Magallanes basin and neotectonics in the southernmost Andes. In: Anfarctic Geoscience (ed. Craddock, C.), pp. 143-154. University of Wisconsin Press, Madison, WI.

Yin, A., 1993. Mechanics of wedge-shaped fault blocks 1. An elastic solution for compressional wedges. Journal of Geophysi- cal Research, 98, 14245-14256.

York, D., 1969. Least squares fitting of a straight line with correlated errors. Earth and Planetary Science Letters, 5 ,

359-362.

4-549-4-563

108- 125.

320-324.

Received 7 January 1994; revision accepted 5 July 1994.

Documents

40Ar/39Ar geochronology and P- T-f paths from the ...ees2.geo.rpi.edu/spear/Spear_pubs_pdf/Kohn 1995... · 1. metamorphic Geol., 1995, 13, 251 -270 40Ar/39Ar geochronology and P-