JOURNAL OF PETROLOGY VOLUME S7 NUMBER 4 PAGES 733-765 1996

FRANK S. SPEAR*1 AND RANDALL R. PARRISH*'DEPARTMENT OF EARTH AND ENVIRONMENTAL SCIENCES, RENSSELAER POLYTECHNIC INSTITUTE, TROY, NY 12180, USA"GEOLOGICAL SURVEY OF CANADA, 601 BOOTH STREET, OTTAWA, ONT., KIA 0E«, CANADA

Petrology and Cooling Rates of theValhalla Complex, British Columbia,Canada

Rocks from the Valhalla metamorphic core complex, BritishColumbia, Canada, have experienced granulite Jadesmetamorphism at conditions of 820±30°C, 8±1 kbar. Peakmetamorphism was accompanied by dehydration melting ofmuscovite, but not biotite, followed by minor back reactionof garnet + K-feldspar + H20- sillimanite + biotite+plagio -close.' At conditions very near those of the peak, extensiveshearing produced s-c (schistositS-cisaillement) fabrics, ribbonquartz and grain size reduction of garnet at several locations.Gamet-biotite Fe-Mg exchange thermometry yields tempera-tures that range from 580 to 1051°C Low temperatures arecalculated from biotite modified dominantly by Fe-Mgexchange with garnet; high temperatures are calculated from Fe-rich biotites produced from the above retrograde reaction. Geo-thermometry is useless in these rocks to estimate peak tempera-ture a priori, but is very useful to help constrain the complexreaction history of biotites. Geochronology on monazite, zircon,allanite, titanite, hornblende, muscovite, biotite and apatite hasbeen used to constrain the timing of the metamorphic peak at67—72 Ma and the average cooling rate to 24 ± 6°C/Ma. Dif-

fusion modeling of Fe-Mg exchange between biotite inclusionsand host garnet yields cooling rates of either 3-80°C/Ma or200-2500° C/Ma, depending on the choice of diffusion coeffi-cients. The former value is consistent with the average coolingrate of 24° C/Ma for the complex determined from geochronol-ogy, but the faster rate cannot be ruled out and may indicateinitial very rapid cooling by thrusting of the complex onto coolerbasement It is suggested that cooling rates determined fromgeochronologic vs petrologic methods may not be directly com-parable because petrologic methods sample near-peak nuta-morphic cooling rates whereas geochronologic methods samplepost-peak to ambient cooling rates.

KEY WORDS geothermometry; geodavnology; gartut diffusion;cooling raits; Valhalla compUx

INTRODUCTIONCooling rates of metamorphic terranes can serve as auseful constraint on the unroofing history because, ingeneral, the more rapidly an area is denuded, themore rapidly it will cool. Geochronology, in whichminerals with different closure temperatures areanalyzed, has been used extensively to infer coolingrates in metamorphic terranes (here called geochro-nologic cooling rates). Cooling rates can also bedetermined from analysis of diffusional zoning inmetamorphic minerals (here called petrologiccooling rates). Few systematic studies have beenpublished comparing petrologic with geochronologiccooling rates. Such a comparison is criticallyimportant because petrologic cooling rates rely onextrapolations of diffusivities determined in thelaboratory over many orders of magnitude, and it isimportant to evaluate the internal consistency ofcooling rates determined by the two methods.

The application of diffusion theory to the deter-mination of petrologic cooling rates has been dis-cussed extensively (e.g. Dodson, 1973, 1986; Lasagaet al., 1977; Onorato et al., 1979, 1981; Tracy &Dietsch, 1982; Lasaga, 1983; Ozawa, 1983; Wilson& Smith, 1984, 1985; Munrill & Chamberlain, 1988;Spear, 1991; Spear & Florence, 1992; Ehlers et al.,1994), and procedures based on the shape of zoningprofiles have been presented. Most of this work hasfocused on zoning in garnet, and the closure tem-perature of the garnet-biotite thermometry is one ofthe most frequently used approaches.

In this paper, a slight variation on the garnet-biotite closure temperature method is presented inwhich closure temperatures of biotite inclusionswithin garnet are modeled as a function of biotite

•Corresponding author.Telephone (518) 276-6101 But: (518) 276-8627.e-mail: ipcar@haroligco.rpi.eduHTTP://www.geai^ebHi/laat»fl7ipear/v»lha]]a/va]ha]la.ritml © Oxford University Pros 1996

JOURNAL OF PETROLOGY VOLUME 37 NUMBER 4 AUGUST 1996

size. The method is applied to rocks from the Val-halla complex, British Columbia, and comparedwith cooling rates determined from geochronology.The Valhalla complex is an excellent area in whichto compare petrologic with geochronologic coolingrates because the geology and structure are wellcharacterized, thermal history is relatively simple,and geochronologic cooling rates have beenexamined in considerable detail (Carr et al., 1987;Parrish */a/., 1988).

GEOLOGICAL SETTINGThe Valhalla complex is located in the southeastcorner of British Columbia, Canada (Fig. 1, inset). Itis one of a number of exposures of fault-bounded,high-grade metamorphic core complexes that makeup the Shuswap complex (Armstrong, 1982; Brown& Read, 1983).

Figure 1 shows geologic relations of the Valhallacomplex and sample localities for the present study.The upper plate comprises largely the JurassicNelson Batholith and related intrusions, and assortedlow-grade metasedimentary and metavolcanic rocks.The lower plate comprises Paleocene-Eocenegranitoids (the Ladybird granite and Airy quartzmonzanite), Late Cretaceous orthogneiss (the Kin-naird and Mulvey gneisses), and a multiplydeformed paragneiss of uncertain age. The para-gneiss consists of quartzofeldspathic gneiss and peliticschist with minor amphibolite, calc-silicate, migma-tites and quartzite.

As shown in the cross-section in Fig. 2, thecomplex is domal with dips to the east and west. Thewestern, southern and northern margins of thecomplex are bounded by the east-directed Valkyrshear zone and the eastern margin is bounded by theeast-directed Slocan normal fault. Both the Valkyrshear zone and Slocan fault are believed to havebeen active from 59 to 54 Ma and are responsible forexposure of the core complex. For details of thegeology of the complex, the reader is directed toCarr et al. (1987) and references therein.

Thirty samples of garnet-bearing lithologies werecollected from 13 localities in the Ladybird graniteand paragneiss (Fig, 1) and examined in thinsection. Fifteen samples were evaluated for chemicalzoning in garnet and of these, five samples (V6A,V6B, V7C, V7D and V9C) were selected for detailedanalysis of petrologic cooling rates based on thesystematics of observed zoning and the number ofsuitable inclusions of biotite.

METAMORPHISMRocks from the Valhalla complex have been meta-

morphosed at sillimanite + K-feldspar grade and thetypical matrix assemblage found in metapelites isgarnet + biotite + sillimanite + K-feldspar + plagio-clase + quartz + ilmenite ± rutile. Very little chloriteis present and retrograde muscovite has only beenobserved in one thin section.

Textures and compositional zoning for the fivesamples of paragneiss used in this study are presentedin Figs 3-8; compositions of selected minerals arelisted in Table 1. Garnets range from 1 to 6 mmdiameter, are typically rounded and embayed, andlocally show reaction zones on the rims of biotite ±sillimanite (e.g. Fig. 3a, upper right corner of whitebox; Fig. 6, left side of large garnet). Included withingarnet are biotite, quartz, plagioclasc (rare), silli-manite (rare) and rutile (ilmenite ± rutile is found inthe matrix).

Garnet is zoned from core to rim. Xcn [part (b),Figs 3-7] is low in the cores (O-035-0-O76) andincreases slightly (generally <0-02) towards the rim.In some samples, the increase in grossular is dis-continuous (best seen in samples V6A and V7C,Figs. 3b and 5b, respectively), suggesting the coregrew under a different set of P—T conditions fromthe rim. A^p, is low in all samples (0-010-0-075) andzoned <0-02 from core to rim.

Almandine and pyrope arc zoned such, that Fe/(Fe + Mg) increases from core to rim. Ranges ofcore-to-rim zonations are: V6A, 0-748-0-792; V6B,0-737-0-860; V7C, 0-645-0-784; V7D, 0-696-O-811;V9C, 0-754-0-807. Fe/(Fe + Mg) zoning is symmet-rical about the rim in some samples (e.g. Fig. 4c,Fig. 6c) but more typically is confined to garnet rimsadjacent to biotite or biotite + sillimanite reactionzones. An excellent example is seen in Fig. 5c, whereFe/(Fc + Mg) is zoned along the entire perimeter ofthe larger garnet except in the lower left corner andthe middle right side where garnet is adjacent toquartz and/or K-feldspar. This rimward zoning inFe/(Fe + Mg) is interpreted as diffusion controlled inresponse to gradients set up by the continuous nettransfer reaction

sillimanite + biotite + plagioclase= garnet + K-feldspar + H2O (1)

as discussed below.Fc/(Fe + Mg) is also zoned in garnet around

biotite inclusions, as can be seen, for example, in Fig.4c (see arrow). Figure 6d is a detailed map showingthe distribution of Fe/(Fe + Mg) around biotiteinclusions in the core of a large garnet from sampleV7D. Values in garnet range from 0-710 far frombiotite inclusions up to 0-833 adjacent to inclusions.Also shown in Fig. 6d are the Fe/(Fe + Mg) of biotiteinclusions, which range from 0-277 to 0-368. Close

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SPEAR AND PARRISH PETROLOGY AND COOLING OF VALHALLA COMPLEX

Columbia Riverfault zone

LowerArrowLake

87-52, V6[-V7 Pas more paragneissji

Valkyrextensionalshear zone%

49°15'

Hanging wall of Slocan Lakeand Valkyr extensional faults

Middle EoceneCoryell syenite

Late Cretaceousplutonic rocks

Middle Jurassicplutonic rocks

Metamorphic rocks ofmainly greenschist facies,Late Pafeozoic-EarlyMesozoic in age

Geochronological andpetrological samples

Fig. 1. Geologic map of the Valhalla complex, British Columbia. Inset shows location of Valhalla complex in western North America.Numbers refer to sample numbers discussed in text.

Valhalla Complex

Paleocene-Early Eoceneleucogranitic rocks

100-110 Magranodiorite gneiss

Metamorphic rocks ofupper amphibolite facies,age uncertain

Figure 1

10

5

SL

-5

Slocan Lafcefault

km204B-

121B-84&529 -83G w i l l i m Creek

shear zones

Mulvey gneiss

Paragneiss

Paleogene granitoids

Focenc Corvoll svenitf

10

5

SL

-587-52 , Passmore, km

V6, V7 , V8

Q Late CretaceousgranitoidsJurassic granitoids and

Fig. 2. Schematic east-west cross-section at latitude 49°45'N of central Valhalla complex with geochronological and petrological samplelocalities projected into the section. Locality for sample V9 is not shown because it is uncertain where it resides relative to Gwillim Creek

shear zone owing to the probable absence of Mulvey gneiss at the latitude of ~ 49°3OTSJ.

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JOURNAL OF PETROLOGY VOLUME 37 NUMBER AUGUST 1996

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s l

4

(a;Fig. 3. Sample V6A. (a) Photomicrograph showing large garnet with thin reaction zone in upper right corner. Foliation is composed ofsillimanite ± minor biotite. (Note the numerous small garnet crystals elongated in the foliation plane.) Width of field is 8 mm. White boxshows area of (b)-(d). (b) and (c) X-ray composition maps showing distribution of Ca and Fe/(Fe + Mg) in upper right part of garnet in(a). Ca is higher near the rim (Jfg,, = 0-053) than the core (X^, = 0-045). Fe/(Fe + Mg) is practically unioned in the core (0-742-0-748),and increases slightly near the rim where late biotite is present, (d) Sketch of garnet showing values of Fe/(Fe + Mg) in garnet (filledcircles: numbers range from 0-742 to 0-792) and biotite (filled squares: numbers range from 0-494 to 0-403). Numbers in boxes aretemperatures calculated from garnet—biotite Fe—Mg thermometry using either the garnet core composition [Fe/(Fe + Mg) = 0-748] plusbiotite indicated by the arrow or the garnet-I-biotite pair indicated by the two arrows. Numbers in parentheses refer to analytical spots

listed in Table 1. Plagiodase analyses 100 and 103 from Table 1 are out of the figure area.

inspection will reveal that there is a correlationbetween biotite Fe/(Fe + Mg) and biotite size:smaller crystals have lower Fe/(Fe + Mg). Thisobservation is consistent with the interpretation thatthe Fe—Mg zoning in garnet, as well as the dis-tribution of biotite Fe/(Fe + Mg), is the product ofdiffusion in response to gradients caused by Fe—Mgexchange between biotite inclusions and adjacent

garnet host. These data will be used below to inferpetrologic cooling rates.

Biotite Fe/(Fe + Mg) and Ti contents are afunction of location in the sample. The lowest Fe/(Fe + Mg) values are observed in biotites that areincluded in garnet, and the lowest values are associ-ated with the smallest crystal inclusions (see Fig. 6d).The highest Fe/(Fe + Mg) is generally observed in

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