A journey through a contact metamorphic aureole:
 
Metamorphism of the Kluane metamorphic assemblage, SW Yukon

If you are interested to to find out what happened to metasedimentary rocks that were previously metamorphosed under regional greenschist grade metamorphism and then endured the intrusion of a thick granodioritic sill-like pluton, then you are at the right place!




Introduction: The North American Cordillera and the Kluane metamorphic assemblage

The Kluane metamorphic assemblage (KMA) is located in the northwestern part of the North American Cordillera. In Canada the Cordillera is subdivided in five belts, from east to west the the Foreland, the Omineca, the Intermontane, the Coast and the Insular Belts. The belts coincide roughly with different geological regions, that are subdivided into several terranes (definition of terranes). The belts are the result of accretion of various island arcs onto the North American continent starting from the Middle Jurassic with the collision of the terranes forming the Intermontane belt. The miogeocline, a passive continental margin from the Middle Proterozoic on, was detached, folded and thrusted eastward over the craton. Thus the Foreland Belt, including the Rocky Mountains, was created. Intense metamorphism and plutonism took place along the suture, the Omineca Belt. A second plutonic-metamorphic belt, the Coast Belt, resulted from the collision of the Insular Superterrane approximately from the mid-Cretaceous on. To further complicate matters, lateral displacement of terranes occurred after accretion. However, the exact timing and the amount is still a matter of hot debate. Up to 3000 km of northward displacement is postulated from paleomagnetic data.

Simplified Geology of the Canadian Cordillera
Geology of the Kluane metamorphic assemblage
Terrane and belt map of the Canadian Cordillera, covering British Columbia Yukon Territory, western Alberta and southeastern Alaska (modified after Wheeler et al. 1991, DNAG G-2). Geological map of the southwestern Yukon east of the Denali fault zone. Outlined are the Kluane metamorphic assemblage, various pericratonic assemblages associated with the Yukon-Tanana-Terrane (Aishihik Metamorphic Suite, Nisling assemblage, Nasina assemblage, White River assemblage), the Late Jurassic-Early Cretaceous Dezadeash Formation, and various Jurassic to Cretaceous plutonic suites east of the Eocene Ruby Range batholith. Modified after Mezger (1997).

The KMA forms a 12km thick sequence of homogeneous, N- to NE-dipping metapelitic rocks, interlayered with slices of olivine serpentinites. They are exposed in a 150km long, NW-SE striking belt. In the geological frame work it is situated at the northwestern margin of the Coast Belt, in contact with the Ruby Range granodiorite, part of a late Mesozoic magmatic belt. To the southwest it is truncated by the Denali fault, separating the KMA from accreted volcanic island arcs.

The sedimentary protolith of the KMA was deposited in a backarc basin. The geochemical and isotopic data which led to this conclusion are discussed in detail on the following page. The backarc basin collapsed in the late Mesozoic during eastward subduction underneath the North American continental margin, and its sediments were deformed and metamorphosed under lower amphibolite facies conditions at depth greater than 20 kilometers. At the same time the mylonized schist were accreted onto the overriding North American plate, represented by the Aishihik metamorphic suite. Encroachment of the Insular Superterrane led to the development of a magmatic arc, the Coast Plutonic Complex, which was active in the Late Cretaceous to early Eocene. Its northernmost portion, the Ruby Range Batholith, intruded along the suture between KMA and North America, and thermally overprinted the schist of the KMA at about 57 Ma, resulting in a wide contact metamorphic aureole (J. Metamorph. Geol. abstract). The KMA is correlated by some authors to the Mclaren terrane southwest of the Denali fault in east central Alaska, thus inplying a major dextral offset of (> 400 km) along the Denali fault (Nokleberg et al., 1985, GSA Bulletin 96). I am not convinced, though.

In the geographical framework, the KMA underlies the Ruby and Dezadeash Ranges west of the Alaska Highway and the Haines Road , which also provide the best access. Haines Junction is the central village. The KMA has its name from Kluane Lake , its natural western boundary. Southwest of Kluane Lake , across the Shakwak trench, the magnificent Kluane Ranges rise up to 2000 m above the valley floor.


 

 
Northern Kluane Lake with Ruby Range View from Jacquot island towards the southern Kluane Lake
The Ruby Range along the northern Kluane Lake provide a cross section through the contact metamorphic aureole. The structurally deepest and metamorphic lowest grade rocks are in the south (on the right). The contact with the Ruby Range batholith is just to the left (north) of the picture. Kluane Lake is the biggest lake in the Yukon Territory and means 'big whitefish lake' in the Southern Tutchone language. View from Jacquot island towards the southern Kluane Lake. The hills on the left are underlain by greenschist grade muscovite-chlorite schist. To the right (southwest) the impressive Kluane Ranges rise 1500m above the lake level. They are the northeastern front of the St. Elias Mountains, which are actively rising along a reverse fault, a reactivation of the Denali fault. The are underlain by low grade Paleozoic-Cenozoic vulcanic arc and oceanic sediments that comprise the gigantic Insular Superterrane. We are on the former continental margin of North America.

Illustrated guide through the KMA contact aureole

This page is a guide through a contact metamorphic aureole: metamorphism imposed on sedimentary rocks by the intrusion of a pluton , in this case the latest Cretaceous-early Tertiary Ruby Range batholith. The map below is a geological map with the emphasis on metamorphic mineral assemblages. The mineral isograds are lines that mark the first prograde occurrence of a metamorphic mineral. Red numbers show the location of the images below.


 
Metamorphic mineral isograds of the Kluane metamorphic assemblage

Below is a generalized cross section through the western part of the Kluane metamorphic assemblage. Thermobarometry results are shown on a different page.


 
cross section trough a metamorphic succession

 

Start

We begin in the lowest grade metamorphic rocks, those that have not been overprinted by contact metamorphism. They can be found along the northeastern shore of Kluane Lake : They contain Na-rich plagioclase feldspar, albite [NaAlSi3O8], muscovite [KAl2AlSi3O10(OH)2], chlorite [(Fe, Mg, Al)6(Si,Al)4O10(OH)8], quartz [SiO2], calcite [CaCO3], graphite [C], and locally epidote [Ca2(Al,Fe)3Si3O12] and clinozoisite [Ca2Al3Si3O12]. The following four pictures show their typical appearance.


 
Field appearance of greenschist facies muscovite-chlorite schist
Mylonitic muscovite-chlorite-quartz schist with characteristic plagioclase porphyroblasts in the center
Field appearance of greenschist facies muscovite-chlorite schist. Dark spots indicate plagioclase porphyroblasts with characteristic graphite inclusion trails. See microphotographs to the left and below for details. Mineral assemblage 1 from the east shore of Kluane Lake (location 1 on map).
 
Mylonitic muscovite-chlorite-quartz schist with characteristic plagioclase porphyroblasts in the center. Graphite inclusion trails preserve an older crenulation cleavage Crossed polarized light (XPL); length of slide ~3mm (location 2 on map).
 
Graphit Inclusions
Rare twinning in plagioclase with distinct graphite inclusion trails, which display a slight curvature, an indication for syngrowth rotation. Schistosity is defined by muscovite (reddish to light blue interference colours) XPL. Length of slide ~3mm. Thin section cut parallel to the mineral stretching lineation. Note the abundant plagioclase porphyroblasts with their characteristic graphite inclusion trails. Several mineral grains touch each other, and some have rotated up to 90° with respect to the main schistosity (horizontal). Light coloured layers represent quartz-rich layers. The schistosity is undulating and is cut by several shear bands. Location 3 on map. XPL. Length of slide ~10mm.

 

Biotite [K(Mg,Fe)3(Al,Fe)Si3O10(OH)2] appears 13km away from the contact with the batholith. With increasing biotite abundance chlorite is decreasing in percentage:

The first garnets [(Fe,Ca,Mg,Mn)3Al2(SiO4)3] are compositionally zoned with Ca-rich cores (grossular component: 22-34%) and graphite inclusions that can be spiral-shaped, implying synkinematic growth (Figure).

Relict epidote is found as inclusions, suggesting garnet was also produced by breakdown of epidote

The pressure and temperature during the beginning of garnet growth can only be estimated with the program Gibbs 4.7 by Frank Spear: 7.5 kbar (ca. 25 km depth) and 508°C.
 
9 km from the intrusive contact staurolite [Fe2Al9O6 (SiO4)4(OH)2] appears and marks the beginning of the amphibolite facies. Staurolite grows along garnet rims. Not having any spiral-shaped inclusion trails, it overgrows previously rotated garnets. The stress field has changed: from non-coaxial deformation (simple shear) to coaxial deformation (flattening or pure shear).


Welcome, now we have entered the contact aureole!

Garnet with spiral-shaped inclusion trails
Euhedral staurolite
Garnet with spiral-shaped inclusion trails indicating synkinematic growth, and a dextral sense of shear. Yellow pleochroic staurolite grows at grain boundary of garnet with indication of consumption of garnet, according to reaction(4). Brown pleochroic mineral is biotite. Plane polarized light (PPL). Location 4 on map. Length of slide ~ 2.5mm. Euhedral staurolite (yellow pleochroic) overgrows shear zone between large light coloured plagioclase porphyroblasts. Graphite inclusions outline shear zone. Staurolite crystals grew postkinematic. PPL. Length of slide ~10 mm.

 

Along with staurolite, biotite also increases in abundance, while muscovite and chlorite diminish. Thus we can deduce the following reaction:

The next minerals to appear are andalusite and needle-shaped sillimanite (fibrolite), the low-temperature and high-temperature phases of Al2SiO5. Andalusite forms poikiloblastic porphyroblasts with inclusions of biotite and graphite. Fibrolite occurs along the rims of andalusite grains. With a sharp increase of andalusite and fibrolite staurolite disappears less than one kilometer “later” and can only be found as relict inclusions in andalusite. By now chlorite has also dissappeared:


 
Staurolite as relict within poikiloblastic andalusite
Staurolite (St) as relict within poikiloblastic andalusite (And). Indication for reaction (5). Difficult to see, at contact with andalusite at the lower left is fibrolitic sillimanite. Biotite is distinguished by green and brown interference colors. Location 5 on map. XPL.
Length of slide ~3.5 mm.

In some parts no andalusite is observed, only sillimanite. There, the temperature (or pressure) was a bit higher. A significant change in the appearance of the rocks occurs with the next mineral, cordierite [(Mg,Fe)2 Al4Si5O18]: The schist becomes coarse and looses its distinct schistosity. The rock is now a gneiss. This is due to the replacement of biotite, which defines the schistosity, by cordierite. At the same time, andalusite disappears, and sillimanite and garnet decrease. Cordierite, with its low relief and lack of colour, can be distinguished by yellow alteration rims, called pinitization. Several cordierite-forming reactions can be inferred:

biotite-cordierite gneiss Field shot of a cordierite gneiss
Plane polarized microphotograph of a typical biotite-cordierite gneiss. Cordierite generally lack graphitic inclusions of the plagioclase porphyroblasts and does not produce euhedral crystals. Nevertheless, at first glance it can not be easily distinguished from plagioclase, since cordierite is also colourless and of low relief. Notice that the amount of biotite has decreased, and the nice schistosity is gone. Field shot of a cordierite gneiss of the Dezadeash Range (Location 6 on map). Notice the wider spaced gneissic foliation and the absence of distinguishable porphyroblasts compared to the muscovite-chlorite schist of the Kluane Lake shore shown above.

 
extremely corroded garnet
Extremely corroded garnet, partly overgrown by cordierite, which can be distinguished on this microphotograph only by its yellow alteration rim (pinitization),  indicating the activity of reaction (6). Location 7 on map. PPL. Length of slide ~3 mm.

 
 
 
REM picture of a high grade gneiss

potassium-feldspar [KAlSi3O8] is difficult to determine by optical microscopy, since it lacks twinning like plagioclase in the high grade gneisses of the KMA, and its other optical properties do not allow unambiguous distinction. For that we need the electron microprobe. They can be visualized by images that show the density of mineral phases by their relative brightness, which correlates to the amount of electrons reflected. This method is called backscatter imagery. The backscatter image on the right shows that dark grey cordierite (Crd) and lighter grey potassium-feldspar (Kfs) is consuming biotite (Bt), just as it is expected according to reaction (7). The scale bar is 100 micron.

In the eastern part of the Kluane metamorphic assemblage spacing of mineral isograds increases (see map above). In the Dezadeash Range the KMA is underlain by granodiorites. Some of them crop out and form little plugs of a few hundred metres to one kilometre in diameter. IN the contact areas schlieren fabrics are evidence for beginning of migmatization, or partial melting of the gneiss. Now, we are very close to the contact of the Ruby Range granodiorite.


 
Starting Migmatisation - Part 1 Starting Migmatisation - Part 2
It is getting hot! Near the contact with the granodiorite plug in the Dezadeash Range (southern part of the KMA - see location 6 on map) fragments of KMA cordierite gneiss are surrounded by the granodioritic melt. Leucosomes within the gneiss (red arrow) are indicators that the gneiss itself is starting to melt: the beginning of migmatization.

 
A few hundred meters above the cordierite-in isograd muscovite disappears, and new sillimanite nucleates (along with more potassium-feldspar):

A green, worm-shaped mineral can be observed in cordierite: hercynite, a chromium-rich spinel. The occurrence of anhedral garnet inclusions in some cordierite crystals and sillimanite surrounding hercynite suggest that spinel was formed by the reaction:


 
Cordierite with inclusions of sillimanite, hercynite (dark green) and garnet (center) Cordierite porphyrpblasts with inclusions of green spinel (hercynite) and fibrolite (sillimanite)
Cordierite with inclusions of sillimanite, hercynite (dark green) and garnet (center). Embayment of the garnet suggests that cordierite and spinel were formed by a garnet and sillimanite consuming reaction (9). PPL. Length of slide ~3mm. (Location 8 on map) Cordierite porphyrpblasts with inclusions of green spinel (hercynite) and fibrolite (sillimanite). Note the sillimanite-free zone around hercynite. The reddish brown mineral is biotite. PPL. Length of slide ~1cm.

Next to the intrusive contact a new generation of garnet, free of inclusions and consuming biotite, can be observed:

Cordierte-Biotite-Schist Cordierte-biotite schist above the second garnet isograd, close to the contact with the Ruby Range Batholith in the eastern Dezadeash Range (location 9 on map). Inclusion-free garnet (notice the difference to the corroded garnets shown above) consumes biotite, marking the transition from uppermost amphibolite to lower granulite facies. Compositional cross-section of this garnet can be viewed here. PPL; length of slide ~1.2mm.
 

Now we have reached the transition zone between the amphibolite and the granulite facies, the highest metamorphic grade that is preserved in the Kluane metamorphic assemblage. Through our little journey we have experienced a decrease of 3-4kbar or 12km of movement upwards in the crust to a depth of 13km and a heating of 250C to 750C near the contact with the Ruby Range batholith. You may have recognized that temperatures increase at higher levels, contrary to the expected. However, remember that in the case of the KMA we don't see a typical regional metamorphism, but a contact metamorphic overprinting. Since the Ruby Range batholith intruded above the KMA, it produced an inverse contact metamorphic aureole.

And so ends the metamorphic story of the Kluane metamorphic assemblage rocks. Now, all the future holds for them is alteration and weathering. If this sounds too pessimistic, just think that there are more rocks to come up, and maybe they will tell us more... .

If you are interested in more details, you can look it up in these articles:

Additional literature:

Pages of further interest: