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Introduction - San Francisco State University

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					Thermobarometry of eclogite-facies
shear zones in the Lofoten Islands,
             Norway




                      A THESIS

     SUBMITTED TO THE DEPARTMENT OF GEOSCIENCES

          OF SAN FRANCISCO STATE UNIVERSITY

     IN PARTIAL FULFILLMENT OF THE REQUIREMENTS

        FOR THE DEGREE OF BACHELOR OF SCIENCE




                         BY

                     Steven Dutra


                     May 17, 2010
I certify that I have read this thesis and find it
fully adequate, in scope and quality, as a Senior
Thesis for Bachelor of Science.


          _____________________________________________
          Prof. Mary Leech - Principal Thesis Advisor




I certify that I have read this thesis and find it
fully adequate, in scope and quality, as a Senior
Thesis for Bachelor of Science.


          ______________________________________________
           Prof. Karen Grove - Thesis Committee Member




I certify that I have read this thesis and find it
fully adequate, in scope and quality, as a Senior
Thesis for Bachelor of Science.


          _________________________________________
          Prof. David Mustart - Thesis Committee Member




                      2
Introduction

       The exhumed lower crustal rocks of the Lofoten Islands in northern Norway (Fig.

1) provide a rare opportunity to study how deep crustal processes result in the formation

of eclogite-facies shear zones during a continental collision. On the island of Flakstadøy

two sites, Flakstad and Nusfjord, were identified to have eclogite-facies shear zones.

These shear zones vary in width from 2–5 cm allowing the transition from host rock to

shear zone to appear on one thin section (Fig. 2). Some eclogite-facies shear zones

contain pseudotachylite, fault rock caused by frictional melting, indicating that both

brittle and ductile deformation took place in the lower crust.

       Previous attempts to calculate pressure and temperature conditions for the

formation of the eclogite-facies shear zones have proven difficult because of retrograde

metamorphism resulting in an amphibolite-facies overprint. Using a scanning electron

microscope (SEM) with energy dispersive spectrometry (EDS), I was able to determine

mineral composition data for thermobarometry, and to positively identify omphacite in

these shear zones confirming eclogite-facies conditions. This study focuses on using

coexisting garnet and omphacite compositions to calculate P-T conditions for eclogite

metamorphism within the narrow shear zones.

Background

       The exhumed basement rocks of the Lofoten Islands (Fig. 1) were

metamorphosed during the Caledonian orogeny when the Baltica-Laurentia plates

collided. Granulite-facies Archean-Proterozoic intrusive rocks form the basement of the

islands (Markl and Bucher, 1997; Steltenpohl et al., 2004), and were subjected to

eclogite-facies conditions during the orogenic event. Eclogite-facies metamorphism took




                                             3
place along narrow shear zones that are found throughout the Lofoten basement complex

(Steltenpohl et al., 2004). As a result of retrograde amphibolite-facies overprint it has

been difficult to date the time of metamorphism, but U-Pb data suggest eclogite-facies

metamorphism occurred c. 461 Ma (Corfu, 2004). Shear zones from two sites at Flakstad

and Nusfjord, both located on the island of Flakstadøy in the Lofoten Islands, form the

basis for this study. Pressure and temperature estimates of 1.5 GPa and 680 ºC for

eclogitization within the shear zone at Nusfjord, and 1.4 GPa and 720 °C for

eclogitization at Flakstad (Markl and Bucher, 1997) were calculated, using TWEEQ

software, for garnet and omphacite from eclogite shear zones and eclogitic lenses,

respectively. Markl and Bucher (1997) reported mm-sized omphacite around garnets and

relict omphacite crystals within garnets from both Nusfjord shear zones and the eclogitic

lens at Flakstad.

Flakstad

       The town of Flakstad is located in the northern part of the island of Flakstadøy

(Fig. 1); samples for this study are from outcrops about 1.5 km southwest of the town

along the Norwegian Sea coast (Fig. 3). Sample collection was done by S. Dutra and D.

Shulman in 2009. There are at least ten 2-5 cm-wide eclogite-facies shear zones (Fig. 2)

that extend up to 10-20 m cross-cut the basement gabbronorite at Flakstad (Steltenpohl et

al. (2004). The shear zones vary in strike and seem to not be correlated to any larger

structures. The gabbronorite is composed of Grt, Pl, En, Di, Mag, Ilm, Amp, and <1% of

Bt and Ky. There is an eclogitic lens ~50 m west of the shear zone outcrop that Markl

and Bucher (1997) calculated P-T conditions for.

Nusfjord




                                             4
       The Nusfjord site is located between two small lakes 1 km west of the town

Nusfjord on the island of Flakstadøy (Fig. 1). Sample collection at Nusfjord was done by

S. Dutra and D. Shulman in 2009 and Mary Leech in 2000. Unlike Flakstad, the eclogite-

facies shear zones at Nusfjord are 5-30 cm wide and formed in basement gabbroic gneiss

comprised of Grt, Pl, Bt, Di, Mag, Ilm, Amp, and <1% Qz. There are ~3 parallel shear

zones that cut across the area (Kullerud, 2001).

Methods

Petrography

       I used a petrographic microscope to identify minerals and textures within the host

rocks and eclogite-facies shear zones to locate areas on each thin section for further

investigation using a Scanning Electron Microscope (SEM).

Scanning Electron Microscope

       Using the new SFSU field-emission SEM equipped with an Energy Dispersive

Spectrometer (EDS), I was able to collect detailed mineral chemistries to compare

eclogite shear zones and the host gabbros. I used the mineral chemistries of garnets and

clinopyroxenes for P-T calculations for eclogite-facies metamorphism in Flakstadøy

shear zones. Because of the fine-grained texture in the shear zones, I used high-

magnification, between 100x and 3,000x, for this study to locate and analyze garnet and

clinopyroxene as well as 20-40 m diameter relict omphacite inclusions within garnets

(Fig. 3). Omphacite was also present along garnet grain boundaries, but the size of the

grains were to small to analyze.

P-T Estimates




                                             5
       I used mineral chemistries of garnet and omphacite that were collected using

EDS, to calculate P-T conditions for eclogite-facies metamorphism within the narrow

shear zones at Flakstad and wider shear zone at Nusfjord. Temperatures were calculated

using Fe-Mg exchange thermometers for garnet and omphacite (Raheim and Green,

1974; Ellis & Green 1979; Krogh, 2000; Powell, 1985). The importance of using

omphacite for the Grt/CPX thermometers was to insure that the calculations were for the

eclogite-facies metamorphism and not for another event.

Results

Mineral Chemistries

Host Rock

       The host rock at Flakstad is a gabbronorite (Steltenpohl et al., 2004). Under the

petrographic microscope the rock is dominated by ~65% anorthite plagioclase. The

plagioclase has classic polysynthetic twinning. The mineral composition of the

plagioclase attained by EDS analysis is ~An60 or labradorite (Table 1). Corona textures

also dominate the gabbronorite (Fig. 5). In the core is OPX enstatite with composition

~En70 (Table 1). The outer rim of the corona texture is garnet with composition

Alm40Grs30Prp30Sps1. Adjacent to the outer rim of the garnets are amphibole and CPX

diopside. The amphiboles are ~1-2 mm long prismatic crystals that are always adjacent

to the outer rim of the coronas and are distinguishable by their shape and by the presence

of K. The amphibole is Fe rich. The diopside also is adjacent to the outer rim of the

garnet and forms semi-pentagonal crystals. The diopside is also ~10% Fe in composition

(Table 1).




                                            6
       The host rock of Nusfjord is of gabbroic composition and has been identified by

Markl and Bucher (1997) as leucogabbronorite and by Kullerud (1996) as gabbronorite.

The sample I analyzed did not contain OPX; however, large ~3-4 cm diameter OPX

crystals within the host rock were observed in the field. Like the gabbronorite from

Flakstad, the gabbro from Nusfjord is dominated by ~50-60% plagioclase. The

plagioclase here is ~An40 or Andesine (Table 2). Corona texture is present but not as

prevalent as it is in the Flakstad norite. Garnets are large and form the outer rims of any

coronas. The composition of the garnets is Alm56Grs25Prp16Sps1 (Table 2).

Amphibole forms distinguishable 1-2 mm large grains and is located between and around

garnet crystals. Under 200-300x magnification on the SEM, the amphibole forms

distinctive prismatic crystals. From the EDS chemical analysis I concluded that the

amphibole type is hornblende and K and Ti are present (Table 2). Biotite is present

within fractures in garnet crystals and forms ~20-30 m sized grains (Table 2).

Shear Zones

       The shear zones at Flakstad are 2-5 cm wide and are visibly ‘greener’ than the

host rock and display shearing texture (Fig 2). The shear zone is very fine grained

compared to the host rock. Under the petrographic microscope and SEM, the shear zones

display foliation and the matrix flows around remnant corona structures from the host

gabbro (Fig 6). Corona textures within the shear zone indicate that the eclogite

metamorphism was incomplete. The matrix of the shear zone is amphibole and

plagioclase, with the presence of the amphibole due to the amphibolite-facies overprint.

There are garnets not associated with remnant corona textures that represent eclogite

metamorphism. The garnets are compositionally Alm46Prp32Grs20Sps1 (Table 3).




                                             7
Small ~1-3 m omphacite zones surround the garnets. These grains are compositionally

omphacite but their size and relative location to the garnet mean that any mineral

chemistry analysis is unreliable. Relict omphacite grains found within garnet grains

represent continued garnet growth after the eclogite metamorphism took place (Fig 3).

Omphacite composition is () (Table 3). Plagioclase surrounding the garnet has a

composition of ~An32 or andesine.

       The shear zone at Nusfjord displays a broader sense of shearing than its Flakstad

counterpart (Fig 7). Here the shear zone is ~5-30 cm in width and displays clear foliation

and shearing texture.

Shear Zone Vs Host Rock

       Flakstad eclogite-facies shear zones have minerals that differ compositionally

from their counterpart minerals in the host gabbronorite. The garnet composition from

shear zone to host rock differs by approximately +6% Fe, +2% Mg, and -10% Ca. In

plagioclase we see a decrease in An by approximately 28%. For CPX composition, there

is omphacite in the shear zone and diopside in the host rock. Compositionally they can

be compared by how much Fe has substituted in their crystal structure. Fe percentage in

both is approximately 10%. The most significant difference is the decrease in Ca within

the eclogite-shear zone, compared to the host rock. The loss in Ca is likely due to the

amphibolite-facies overprint.

       Compositional differences between the shear zone and host rock at Nusfjord are

not as pronounced as they are at Flakstad.

Calculations

Flakstad




                                             8
        The Flakstad eclogite-facies shear zones presented a difficult challenge in

determining the P-T conditions of eclogitization. This was due to the amphibolite-facies

overprint which erased almost all evidence of eclogite mineralization. The little evidence

of eclogitization comes from relict omphacite crystals within garnet crystals. I calculated

P-T conditions using the four geothermometers; results are:

Geothermometers         Pressure (Kba)             Temperature (°C)      Error +/- (°C)
Krogh                   14                         872.8                 183.7
Powell                  14                         886.1                 185.8
Ellis & Green           14                         901.3                 188.3
Raheim                  14                         836.1                 177.8

The average calculated temperature is 874.1°C +/- 183.9 which effectively places the

depth of eclogitization at ~29-35 km. However, there is large uncertainty in the data

which can be explained partly by the fact that the number of relict omphacite is low and

that reliable mineral chemistry data were difficult to attain due to the small size of the

grains. With the size of the omphacite grains being ~20-40 m and at a magnification of

nearly 2,000 x, the mineral chemistries recorded by EDS may have been influenced by

the garnet grains. Markl and Bucher (1997), reported 1.4 GPa (14 Kba) and 720 °C for

eclogitization of an eclogitic lens at Flakstad. With the amount of uncertainty in the

calculated average temperature, it is difficult to say if their eclogitic lens and the eclogite-

facies shear zones of this study coincide within the same metamorphic event. Markl and

Bucher (1997) do not report an uncertainty on their calculated temperature and it is

therefore not possible to determine how well the two calculations overlap.

Nusfjord

        The shear zone at Nusfjord is an example of a completely retrogressed shear zone.

Analysis on the SEM revealed no evidence of omphacite grains surrounding or within




                                               9
garnet crystals. Amphibole was most prevalent throughout the sample, illustrating the

retrograde amphibolite-facies metamorphism. Studies done by Markl and Bucher (1997)

and Steltenpohl et al. (2006) discovered omphacite grains in samples at Nusfjord.




Conclusion

       This study focused on producing evidence that the shear zones at Flakstad and

Nusjford reached eclogite-facies conditions and what the P-T conditions were at the time

of eclogitization.

References

Corfu, F., 2004, U-Pb geochronology of the Leknes Group: An exotic early-Caledonian

       metasedimentary assemblage stranded on Lofoten basement, northern Norway

Ellis, D.J., Green, D.H.. 1979. An experimental study of the effect of Ca upon garnet-

       clinopyroxene Fe-Mg exchange equilibria

Krogh, E. 2000. The garnet-clinopyroxene Fe2+ -Mg geothermometer: an updated

       calibration.

Kullerud, K., Flaat, K., Davidsen, B.. 2001. High-Pressure Fluid-Rock Reactions

       involving Cl-bearing fluids in Lower-crustal Ductile Shear Zones of the

       Flakstadøy Basic Complex, Lofoten, Norway

Markl, G., and Bucher, K., 1997. Proterozoic eclogites from the Lofoten Islands,

       northernNorway




                                            10
Powell, R.. 1985. Regression diagnostics and robust regression in

       geothermometer/geobarometer calibration: the garnet-clinopyroxene

       geothermometer revisited

Raheim, A., Green, D.H.. 1974. Experimental determination of the temperature and

       pressure dependence of the Fe-Mg partition coefficient for coexisting garnet and

       clinopyroxene

Steltenpohl, M.G., Hames, W.E., and Andresen, A., 2004. The Silurian to Permian

       history of a metamorphic core complex in Lofoten, northern Scandinavian

       Caledonides

Steltenpohl, M.G., Kassos, G., Andresen, A. 2006. Retrograded eclogite-facies

       pseudotachylytes as deep crustal paleoseismic faults within continental basement

       of Lofoten, north Norway

Steltenpohl, M.G., Hames, W., Andresen, A., Markl, G.. 2003. New Caledonian eclogite

       province in Norway and potential Laurentian (Taconic) and Baltic links




                                          11
Figures

   1) Location map of Lofoten islands (Markl and Bucher, 1997)




Figure 1. ……




                                        12
   2) Image of Flakstad Shear zone and sample Li-5. Circles indicate location of drill
      cores. (Taken by S.Dutra)




Figure 2.




                                          13
3) Image of relict omphacite crystal in Flakstad garnet. Picture taken using SEM.




Figure 3. ……




                                       14
   4) Image of Flakstad gabbronorite with 2 parallel shear zones. (Taken by S.Dutra)




   5) Pm image of corona from Flakstad
   6) Pm image of gabbro
   7) Image of Nusfjord shear zone


Tables

   1)    Mineral Chemistries for Flakstad host rock
   2)    Mineral Chemistries for Nusfjord host rock
   3)    Mineral Chemistries for Flakstad shear zone
   4)    Mineral Chemistries for Nusfjord shear zone




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