A post-Caledonian dolerite dyke from Magerøy, North Norway: age and geochemistry

DAVID ROBERTS, JOHN G. MITCHELL & TORGEIR B. ANDERSEN

Roberts. D., Mitchelꢌ, J. G. & Andersen, T. Bꢈ: A post-Caledonian doꢌerite dyke from Magerøy, North Noꢀayꢁ age and geꢂhemistrꢃꢄ Norsk Geologꢀk Tꢁkꢂ ꢃꢄ, Vol. 71, pp. 28�294. Oslo 1ꢅ1 . ISSN ꢆ9ꢇ196Xꢈ

A ꢉsꢊꢋCaledonian, NW-SE trending, doꢌerite dyke on Magerøy has a geꢂhemicaꢌ signature comparable to that of continentaꢌ tholeiites. Clinopyroxenes ꢍom three samples of the dolerite gave K-Ar ages of ca. 312, ca. ꢀ and ca. 266 Ma, suggesting a Peꢎo-Caꢏniꢐerꢑus age. In view of ꢌow K20 contents of the pyroxenes and the possible presence of excess argon, it is argued that the most reliable estimate of the empꢌacement age of the dyke is provided by the unweighted mean value, namely ꢒ3 ± 22 Ma. This Late Carboniferous age coincides with the later stages of a major phase of rifting and crustaꢌ extension in adꢓꢔꢕnt offshore areas. The dꢃke is emplaced along a NW-SE trending fauꢌt. Paraꢌꢌeꢌ or subparallel fauꢌts on Magerøy and in other parts of northem Finnmark may thus carry a component of Carboniferous crustaꢌ extension in their poꢌyphase movement histories.

David Roberꢅ, Geologꢋcal Survey of Norway, Post Box 3ꢆLaꢇe, N-7ꢈ2 Tronꢇheim, Norway; John G. Mitchell, Depaꢉent of Phꢊsꢋꢌ, The Uꢍiversiꢎ, Newcastle upoꢍ Tyꢍe NE1 7RU, Englanꢇ; Toꢏeiꢐ B. Anꢇersen, Institutt for geologi, Post Box 1ꢑ7, Uꢍiveꢒiꢎ of Oslo, Nꢓ316 Oslo 3, Norwaꢊ.

In the Caledonides ofnorthernmost Norway, mafic dykes are common in many parts ofthe metamorphic allochthon and are invaꢉiably deformed and metamoꢉphosed, locally in amphibolite facies. In contrast, dykes that are largely unaltꢂrꢂd, and which clꢂarly cut Calꢂdonian structures and metamorphic fabrics, are rare. Dolerite dykes of this category are known from the Varanger Peninsula in northeast Finnmark (Beckinsale et al. 1975; Roberts 1975), and have yielded a K-Ar whole-rock age of ca. 362 Ma (Beckinsale et al. 1975; recalculated according to Dalrymple 1979). Elsewhere in Finnmark, a dolerite dyke considered to be of Late- or pꢃst-ꢀaledonian age has been ꢉꢂpꢃꢉted from the Magerøy Nappe on the island of Magerøy (Andeꢉꢅꢂn 1979, 1981; Roberts & Andersen tolites indicate an Early Silurian (Llandovery) age (Henningsmoen 1961; Føyn 1967; Bassett 1985) for part of the succession. Mylonitic rocks at the base of the nappe exhibit amphibolite-facies parageneses.

Apart from the largely mafic-ultramafic Honningsvåg
Intrusive Suite, plutonic rocks in the Magerøy Nappe occur in two main areas (Fig. 1). One of these bodies, the syntectonic (inter-Dl/D2) Finnvik granite, yielded a Rb-Sr isochron age of411 ± 7 Ma (Andersen et al. 1982). Stored samples of gabbros from the Honningsvåg Intrusive Suite have given RꢋSr and Sm-Nd internal isochron ages ranging from ca. 558 to ca. 468 Ma (Krill et al. 1988). These dates are at variance with the general nation that the magmatic rocks of the Honningsvåg Intrusive Suite

1985). The prꢂsent contribution reports the results of a intruded and metamorphosed the fossil-bearing metastudy of the gꢂochemistry and an investigation of the K- sediments (Curry 1975; Andersen 1981, 1989; Rabins Ar mineral age of this particular dyke.

et al. 1987). From the faunal andisotopic dating evidence, thepolyphase falding and metamorphism in the Magerøy Nappe are inferred to relate to the Late Silurian to Early

Devonian, Scandianorogenesis (Andersen 1981; Roberts

& Andersen 1985).

Geological setting

Magꢂrøy has captured the attention of travellers and gꢂoꢅcientiꢅts alike since the ꢂarly part of the 19th century whꢂn Evꢂꢉest (1829) gave the first description of the island's gꢂolꢃgy. In more modern times, degree theses (ꢀuꢁy 1975; Andersen 1979; Kjærsrud 1985) have providꢂd accounts of rꢂgional stratigraphy, structure and mꢂtamꢃꢄhiꢅꢆ, while Rabins et al. (1987) have described the ꢆagꢆatic rꢃcks of the Hꢃnningsvåg Intrusive Suite.

Threꢂ pꢉincipal tꢂctꢃnꢃꢅtꢉatigraphic units are recogniꢅed on Magꢂrøy: (l) the Kalak Nappe Complex; (2) the Magꢂrøy Nappe; and (3) the Skarsvåg Nappe (Fig. 1). The Magerøy Nappe comprises five formations of metasediꢆents; ꢆꢃstly sandꢅtone, gꢉꢂywacke and shale, with local limꢂstone and cꢃngloꢆerate (Andersen 1979, 1984; ꢇjꢈꢉsꢊd 1985). Coꢉals, bꢉachiopods and grap-

Apart from the prominent dolerite dyke that forms the basis of this contribution, other small, isolated, comparatively fresh, dolerite dykes have been reported by Curry (1975) nearer to Honningsvåg, and confirmed by S. Lippard (pers. comm. 1990). Preservation of the Magerøy Nappe on Magerøy and its absence on the adjacent mainland (Fig. l) is explained by the existence of the Magerøysund Fault. This involves a significant downthrow to the northeast along this NW-SE trending structure, post-dating Scandian orogenesis (Andersen 1981). The Finnvik granite itself extends down to a depth of at least 4 km (Lønne & Sellevoll 1975), but is not exposed on the mainland directly southwest of the Magerøysund

Fault.

NORSK GEOLOGISK TIDSSKRIꢀ 71 (1ꢁ1)

290 D. Roberꢀ et al.

irrꢍgulaꢉly concꢍntric or an hourglass pattꢍrꢌ. Microsondꢍ aꢌalysis of thrꢍꢍ graiꢌs confirmꢍd an X-ray pattꢍrn that thꢍ pyroxꢍꢌꢍ is augitꢍ. Opaquꢍs arꢍ mostly ilmꢍnitꢍ and haꢍmatitꢍ, with miꢌor magnꢍtitꢍ aꢌd pyritꢍ; rꢍplacꢍ- mꢍnt of primary phasꢍs by haꢍmatitꢍ is locally advaꢌcꢍd, ꢍspꢍcially iꢌ thꢍ southꢍastꢍrꢌ arꢍa, iꢌdicating somꢍ dꢍgrꢍꢍ of oxidisatioꢌ. This can also bꢍ sꢍꢍn in thꢍ Fe203/ FꢍO ratio (Tablꢍ 1). Othꢍr minor constituꢍnt minꢍrals arꢍ apatitꢍ, biotitꢍ and zircoꢌ.

Field relationships and petrography

Thꢍ iꢌvꢍstigatꢍd dolꢍritꢍ dykꢍ occurs in thꢍ uꢌdulatiꢌg tꢍrraiꢌ of cꢍꢌtral Magꢍrøy (Fig. 1). It was ꢍmplacꢍd aloꢌg a majoꢉ NW-SE (ca. 310°) trꢍndiꢌg fault-zonꢍ that can bꢍ tracꢍd fꢉom Sardꢌꢍspollꢍn in thꢍ southꢍast to thꢍ coast nꢍar Gjesvær iꢌ thꢍ ꢌorthwꢍst. Thꢍ faults and joiꢌts in this systꢍm offsꢍt thꢍ domiꢌaꢌtly NNE-SSW trꢍꢌdiꢌg bꢍddiꢌg aꢌd foliation iꢌ thꢍ amphibolitꢍ-faciꢍs mꢍtasꢍdimꢍnts. Thꢍ NW-SE trꢍꢌd of thꢍ fault is parallꢍl to a promiꢌꢍꢌt sꢍt ofpost-ꢀalꢍdoꢌiaꢌ majoꢉ and miꢌor faults and mastꢍr joiꢌts that arꢍ charactꢍristically dꢍvꢍlopꢍd on Magꢍrøy. Locally, gougꢍs aꢌd carbonatꢍ-cꢍmꢍꢌtꢍd brꢍccias occur aloꢌg thꢍsꢍ faults which, iꢌ somꢍ casꢍs, arꢍ ꢍxprꢍssꢍd topographically iꢌ thꢍ form of narrow valleys (Aꢌdꢍrsꢍn 1979). Thꢍ ꢌꢍar-vꢍrtical dykꢍ can bꢍ tracꢍd almost coꢌtiꢌuously for 3.8 km aloꢌg thꢍ fault. In thꢍ pooꢉly ꢍxposꢍd boggy aꢌd frost-wꢍathꢍꢉꢍd arꢍas on cꢍꢌtral Magꢍrøy, blocks of thꢍ dolꢍritꢍ arꢍ ꢍasily distiꢌguishꢍd from thꢍ grꢍy, couꢌtꢉy-rock mꢍtasꢍdimꢍnts by a charactꢍristic, thiꢌ, yꢍllowish-browꢌ wꢍathꢍriꢌg crust. Thꢍ contacts with thꢍ couꢌtry rocks arꢍ rarꢍly wꢍll ꢍxposꢍd. Locally, howꢍvꢍr, thꢍ dykꢍ rock forms a ꢌꢍtwork of iꢌtrusioꢌs iꢌ thꢍ fracturꢍd rocks aloꢌg thꢍ faultzonꢍ. Thꢍ thickꢌꢍss of thꢍ dykꢍ is at lꢍast 8 m. Thiꢌ- sꢍctioꢌs show that thꢍ dolꢍritꢍ is fiꢌꢍ- to mꢍdium-graiꢌꢍd and ꢌoꢌ-poꢉphyritic, with an ophitic to subophitic tꢍxturꢍ. It coꢌsists priꢌcipally of cliꢌopyroxꢍnꢍ, plagioclasꢍ, lꢍucoxꢍꢌꢍ and opaquꢍs. Thꢍ plagioclasꢍ (Aꢌ4s) gꢍꢌꢍrally carriꢍs only tracꢍs of saussuritisation aloꢌg clꢍavagꢍs. Thꢍ cliꢌopyroxꢍꢌꢍ is zoꢌꢍd, in ꢍithꢍr an

Geochemistry

Major aꢌd tracꢍ ꢍlꢍmꢍnts wꢍrꢍ aꢌalysed on rock powdꢍrs usiꢌg an automatic Philips 1450/20 XRF, at NGU, Troꢌdhꢍim. ꢀalibration curvꢍs wꢍrꢍ basꢍd oꢌ iꢌtꢍrꢌatioꢌal staꢌdards. For thꢍ dꢍtꢍrmiꢌatioꢌ of major ꢍlꢍmꢍꢌts thꢍ rock samplꢍs wꢍrꢍ mꢍltꢍd with lithium tꢍtraboratꢍ 1:7. Fꢍrrous iꢉoꢌ, H20+ aꢌd H20- wꢍrꢍ dꢍtꢍrmiꢌꢍd by thꢍ mꢍthod outliꢌꢍd iꢌ Laꢌgmyhr & Graff (1965). Tracꢍ ꢍlꢍmꢍnts wꢍrꢍ dꢍtꢍrmiꢌꢍd on prꢍssꢍd rock powdꢍrs. Rarꢍ-ꢍarth ꢍlꢍmꢍꢌts, togꢍthꢍr with Hf, Ta, Th and U, wꢍrꢍ analysꢍd by INAA (by Jan Hꢍrtogꢍn) at thꢍ Dꢍpartmꢍꢌt ofPhysico-ꢀhꢍmical Geology, Uꢌiversity of Lꢍuveꢌ, Bꢍlgium.

Iꢌ viꢍw of thꢍ homogeꢌꢍous, almost ꢍquigraꢌular naturꢍ of thꢍ dolꢍꢉitꢍ, it was coꢌsidꢍrꢍd sufficiꢍnt to collꢍct oꢌly a limitꢍd ꢌumbꢍr of frꢍsh aꢌd appaꢉeꢌtly unaltꢍrꢍd samplꢍs, sprꢍad out aloꢌg thꢍ lꢍngth of thꢍ dykꢍ, in ordꢍr to obtaiꢌ rꢍliablꢍ iꢌformatioꢌ on thꢍ chꢍmical compositioꢌ. Fivꢍ samplꢍs wꢍrꢍ chosꢍn (nos. l to 5 from southꢍast to ꢌorthwꢍst), aꢌd thꢍsꢍ do iꢌ fact rꢍvꢍal a markꢍd homogꢍꢌeity iꢌ majoꢉ aꢌd tracꢍ ꢍlꢍmꢍꢌt coꢌtꢍꢌts, with vꢍꢉy low staꢌdard dꢍviatioꢌs (Tablꢍ 1).
ꢀompariꢌg thꢍ ꢍlꢍmꢍnt abuꢌdaꢌcꢍs with reportꢍd avꢍragꢍ valuꢍs for coꢌtiꢌꢍꢌtal tholꢍiitꢍs and ocꢍaꢌ-ſtoor basalts (Tablꢍ 1), it is clꢍar that thꢍ dykꢍ is morꢍ akin to coꢌtiꢌꢍꢌtal tholꢍiitic rocks thaꢌ to basalts gꢍꢌꢍꢉated iꢌ an ocꢍan-ſtoor sꢍtting. This is not uꢌꢍxpꢍctꢍd, considꢍriꢌg that thꢍ dykꢍ pꢍꢌꢍtratꢍd ꢌappꢍ rocks locally ꢍxcꢍꢍdiꢌg 7 km iꢌ thicknꢍss (Olꢍsꢍꢌ ꢍt al. 1990) and a crystallinꢍ basꢍmꢍnt of unkꢌown but apprꢍciablꢍ thicknꢍss followiꢌg thꢍ Scandian coꢌtiꢌꢍꢌtꢎoꢌtiꢌꢍnt collision.

Fꢍaturꢍs of thꢍ major eꢁement analysꢍs includꢍ thꢍ compaꢉativelyhigh Ti02, P205, total Fe aꢌd total alkalies aꢌd low Al203 aꢌd MgO conceꢌtꢉations. Iꢌ viꢍw of thꢍ high Fe203/Fe0 ratios, thꢍ ꢀIPW norms wꢍrꢍ rꢍcalculatꢍd aſtꢍr adjustmꢍꢌt of thꢍsꢍ ratios to rꢍasoꢌablꢍ valuꢍs, followiꢌgpꢉocꢍduꢉꢍs outlinꢍd iꢌ Irviꢌꢍ & Baragar (1971). Thꢍ tholꢍiitic naturꢍ of thꢍ dolꢍritꢍ is clꢍaꢉly indicatꢍd iꢌ FꢍO/MgO vs. FꢍO aꢌd Ti02 plots, not showꢌ hꢍrꢍ.

Thꢍ morꢍ iꢌcompatiblꢍ trace eꢁemenꢀ, iꢌ coꢌjuꢌctioꢌ with thꢍ high fiꢍld streꢌgth ꢍlemꢍꢌts P aꢌd Ti, arꢍ kꢌowꢌ to bꢍ hꢍttꢍr discriminaꢌts of tꢍctoꢌic ꢍꢌviroꢌmꢍꢌt thaꢌ thꢍ majoꢉ oxidꢍs. As to bꢍ ꢍxpected, plots such as thosꢍ of Ti-Zr-Y and TiOꢏZr show thꢍ dolꢍritꢍ samplꢍs to

ꢀg. l. ꢖimpliꢗed map of Magerøy showing the main nappe units, plutonic ꢘdies and major faults, and the ꢌocation of the dolerite dyke discussed in the textꢈ F.O. - Finnvik granite; HꢈIꢈꢖꢈ - Honningsvåg Intrusive ꢖuite; K.N.Cꢈ - Kaꢌak Napꢙ Complex; M.Fꢈ - Magerøysund Fault; M.Nꢈ - Magerøy Nappe; ꢖꢈNꢈ- ꢖkarsvåg Nappeꢈ

NOꢂSK GEOLOGꢁSK TꢁDSSKꢂꢁꢀ 7ꢃ ꢂꢃꢁ1ꢄ

Post-Caledonian dyke, Magerøy

291

Table l. Major and trace eꢌement comꢉsition and CIPW noꢚs of sampꢌes (nos. 1-5) from the doꢌerite dyke (major oxide and norms in ꢃ.%, trace eꢌements in ppm), together with mean vaꢌues and standard deviations (ꢖ.D.), and means for continentaꢌ thoꢌeiites (COꢪ) and ocean-ſtoor thoꢌeiites (OFB).

• 2
• 3

4

5

Mean

ꢖ.D.

COꢪ

OFB

ꢖi0ꢜ

ꢛꢜ0ꢝ

Ti0ꢜ

Feꢜ03

FeO MgO

CaO

ꢪaꢜ0

Kꢜ0 MnO

Pꢜ0s

Hꢜo+

Hꢜo-

coꢜ

48ꢞ33

12ꢞ71

3ꢞ16

8ꢞ46

5ꢨ38

4ꢞ26

7ꢞ65

3ꢞ4ꢡ

1ꢞ55

ꢡꢞ22 ꢡꢞ54 1ꢨ7ꢡ 1ꢞ62

ꢅꢆ

ꢅꢞꢡ5

49ꢞ55 13ꢞꢡ3

2ꢞ92

7ꢞ48

5ꢞ41

3ꢞ89

7ꢞ4ꢡ 3ꢨ1ꢡ

1ꢨ72

ꢡꢞ21 ꢡꢞ56

1ꢨ57

1ꢞ46 0ꢨ12

98ꢞ42

48ꢞꢡ1 12ꢞꢟ

3ꢞ11

7ꢞ73

6ꢞꢠ 4ꢞ18

7ꢞ58

3ꢞ5ꢡ 1ꢞ52 ꢡꢞ22

ꢡꢞ57

2ꢞꢡ7 1ꢞ19 ꢡꢞ15

98ꢞ43

48ꢞ12

12ꢞ79

3ꢞ23

8ꢞ42

5ꢞ72

4ꢞ35

7ꢞ46

3ꢞ2ꢡ 1ꢞ55

ꢡꢞ24

ꢡꢞ55 1ꢞ87 1ꢞ27

ꢡꢞꢡ9

98ꢞ86

48ꢞ74 12ꢞ95
3ꢞꢡ4 5ꢞ96

7ꢞꢡ7

4ꢞ19

7ꢞ86

3ꢞ1ꢡ

1ꢞ51

ꢡꢞ22

ꢡꢞ59 1ꢞ42

1ꢞ65

48ꢞ55 12ꢞ82

7ꢞ61

5ꢞ92 3ꢞꢡ9

4ꢞ17

7ꢞ59

3ꢞ26

1ꢞ57

ꢡꢞ22 ꢡꢞ56

1ꢨ73

1ꢞ44

ꢡꢞ13
ꢡꢞ62

ꢡꢨ17

1ꢞꢡ2

ꢡꢞ69

ꢡꢞ12

ꢡꢞ17

ꢡꢞ18 ꢡꢞ18

ꢡꢞꢡ9

ꢡꢞꢡ1 ꢡꢞꢡ2 ꢡꢞ25 ꢡꢞ21

ꢡꢞꢡ6

48ꢞ81 14ꢞ41

13ꢞ2ꢡ1

49ꢞ91 16ꢞ2ꢡ

1ꢡꢞ24ꢜ
1ꢞ43

7ꢞ74

11ꢞ42
2ꢞ82 ꢡꢞ24
2ꢞ47 5ꢞ96
1ꢡꢞꢡ5
2ꢨ9ꢡ ꢡꢞ95

ꢡꢞ22

• 98ꢞ52
• Totaꢌ

ꢇ

Zr

y

19

248
42
23

ꢈ

48

23

251

47

26
251

44

21

2ꢢ

43
22ꢞ4

259ꢞ2

44ꢞ8

2ꢞ61

13ꢞ59

2ꢞ59

25

149

25

5

92 3ꢡ

ꢖr

318

43
1ꢡ7
22

ꢉ

43 95 19

11

<5

3ꢡ8 795
2ꢅ

4ꢡ

93

17

12

<5

363

7ꢡ7
311

43

1ꢣ

22

13

6

3ꢡ5
36 94

15 1ꢡ

<5

3ꢠ

751

3ꢡ5ꢞ8
41ꢞꢡ ꢅꢞ6

19ꢞꢡ 12ꢞꢡ

5ꢞ2

• 4ꢞ4
• 4
• 4ꢡ1

15
131

3

Rb

Zn

Cu ꢪi

Cr V

3ꢞꢡ8

7ꢞ73

3ꢞꢡ8

1ꢞ58

ꢡꢞ45

99

68
139

73

1ꢤ 31ꢡ 229
8
14

<5

356

752

353

727

336ꢞꢡ

746ꢞ4

29ꢞ57

32ꢞ95

Ba

338

CIPW norms

2ꢞꢡ 9ꢞꢡ

3ꢞꢡ

9ꢞ2
4.6 8.9

2ꢞ3 9ꢞ2

5ꢞ4

1ꢡꢞ2

26ꢨ2

16ꢞ6

12ꢞ9

1ꢡꢞ3

6ꢨ4 q

or

ab

an

ꢊ

hy

mt

ꢋ

29ꢨ6 14ꢞ2

15ꢞ4

1ꢡꢞ5

6ꢞ7

27.1

16ꢞꢡ 13ꢞ8 11ꢞ9
6ꢞ9
26ꢞ2

17ꢞꢡ 13ꢞ7

1ꢡꢞ7

6ꢞ6
28ꢞ8 14ꢞ8

15ꢨ7

1ꢡꢞ4

6ꢨ8

6ꢨꢡ

5ꢨ5

• 5ꢞ9
• 6ꢞ1

5ꢞ8

ap

ꢄ

1ꢨ3 ꢡꢨ2
1ꢨ3 ꢡꢨ3
1ꢞ4

ꢡꢞ3

1ꢨ3

ꢡꢞ2

1ꢞ4

ꢡꢞ5

1 Totaꢌ Fe ꢥven as Feꢜ0ꢝ.

ꢜ Totaꢌ Fe as FeOꢄ Data source for mean COꢪ & OFB eꢌement contents - Pearce ꢦ1975)ꢞ The CIPW norms are recaꢌcuꢌated after adjusting the Feꢜ0ꢝꢁFe0 ratio

in accordance with procedures ꢥven in lrvine & Baragar ꢦ1ꢧ1)ꢨ

cluster in the 'within plate' field. Diagrꢊms devised to distinguish between cꢅntinental tholeiites ꢊnd continentꢊl

alkꢊli bꢊsalts, emplꢅying Y, Nb and Ti (Figs. 2 and 3),

Continental tholeiitic basalts

signiꢀ thꢊt we ꢊre dealing with ꢊ mꢊfic dyke rock of thꢅleiitic cꢁmpꢁsitiꢁn. (Fꢅr cꢅmparative purposes, ꢊverages ꢅf the Permo-Jurassic alkꢊline mꢊfic dykes from West Nꢁꢂaꢃ and the Devonꢁ-Carbꢁniferous dolerites ꢄꢅm Varꢊnger Peninsula ꢊre included in these diꢊgrꢊms.) e ꢆꢇZrꢈY plꢅt ꢅf Meschede (1986) and V-Ti discriminant ꢁf Sheꢉꢊis (1982), not shown here, ꢊlso confirm this thꢅleiitic cꢁmpꢁsitiꢁn.

ꢀ

oꢌ

1

Rare-earth element (REE) analyses of two representative samples ꢅf the dꢁlerite show chondrite-normalised pattems which ꢊre ꢊlmost identical, within analꢃtical error (Fig. 4). The spectra show ꢊ slight downward cꢁnvexitꢃ, thꢁugh with just a hint of ꢊ positive Eu anꢁmalꢃ. Bꢁth the absolute REE vꢊlues (Table 2) ꢊnd the rꢊtiꢁs ꢁf light REE tꢅ heaꢋ REE (e.g. the La/Yb

0.1

1

Y/Nb

10

Fig. 2. TiOꢩꢪꢫ/Y(ꢌog) pꢌot of the doꢌerite sampꢌes. O - average vaꢌue of the Devono-Carboniferous doꢌerite dykes ꢍ Varanger Peninsuꢌa (Roberts 1975); x - average vaꢌue of the Mesozoic aꢌkaꢌine dykes from the ꢖunnhordꢌand region, West ꢪoꢀay (Færseth et ꢎ 1976)ꢞ The diagram is from Winchester & Fꢌoyd ꢦ1976)ꢞ

292 D. Roberꢀ et al.

NOꢂSK GEOLOGꢁSK ꢅꢁDSSKꢂ!ꢆ 71 (1991)

l

l

ll

1

l

l

l

l

A - 2

D - 5

X

ꢕꢖ

ꢀ0

l alkalic

00 oo

ꢊ

l

within

w

- plate

transi- l

ꢇꢈꢉ

tholeiitic

tional

1

�

z

ꢗ ꢘ

o

u

"

• ꢋ
• ꢌ

g

ꢍ

la

ꢝ

ꢚ

• Sm Eu
• ꢞ

Tb

Yb Lu

Fig. 3. Ti/Y(iogꢁꢪb/Y(iog) pꢌot ofthe doꢌerite sampꢌes from Magerøy. ꢖymboꢌs

Fig. 4. Chondrite-normaꢌised REE patterns for Magerøy doꢌerite sampꢌe nosꢞ 2 and 5.

as in Fig. 4. Diagram from Pearce (1983).

ratio of 7.2) arꢍ typical of continꢍntal tholꢍiitic dykes, with thꢍ potassium contꢍnt; this, in fact, is precisely thꢍ

sills and lavas worldwidꢍ (cf. Dupuy & Dostal l984).

ꢍffꢍct observꢍd hꢍrꢍ. Such an argumꢍnt would lꢍad to thꢍ youngꢍst agꢍ (i.ꢍ. from thꢍ pyroxꢍnꢍ having thꢍ highest potassium contꢍnt) bꢍing prꢍferred.
Howꢍvꢍr, thꢍ prꢍsꢍncꢍ of ꢍxcꢍss argon in pyroxꢍnꢍ is not universal, and pꢍrturbation of thꢍ potassium-argon isotopic systꢍm could arisꢍ by argon loss in phasꢍs which arꢍ non-rꢍtꢍntivꢍ of argon, and which havꢍ bꢍꢍn incorporatꢍd in tracꢍ amounts into thꢍ minꢍral sꢍparate analysed. With a mꢍan wholꢍ-rock K20 of 1.57 wt.%, thꢍ lꢍvꢍl of ꢍxtranꢍous minꢍral contamination nꢍcꢍssary to change thꢍ concꢍntration of thꢍ 'pyroxene' sꢍparate from 0.03% to 0.05% would only havꢍ to bꢍ of thꢍ ordꢍr of l%. To tꢍst thꢍ possibility of argon loss, as a consꢍquꢍncꢍ of alteration, bꢍing rꢍsponsible for thꢍ sprꢍad of agꢍs wꢍ havꢍ also calculatꢍd (Tablꢍ 3) thꢍ spꢍcific 36Ar contꢍnt of ꢍach of thꢍ sꢍparatꢍs. It is ꢍvidꢍnt that Samplꢍ l, having thꢍ youngꢍst agꢍ, also contains nꢍarly a factor two grꢍatꢍr concꢍntration of 36Ar. It has bꢍꢍn suggꢍstꢍd

(Baksi 1985) that thꢍ 36Ar content of rocks may bꢍ usꢍd

as a critꢍrion for judging thꢍir suitability forK-Ardating. An incrꢍasing lꢍvꢍl of 36Ar has bꢍꢍn dꢍmonstratꢍd to corrꢍlatꢍ with incrꢍasing altꢍration. This ꢍvidꢍncꢍ would lꢍad us to concludꢍ that thꢍ agꢍs of Samplꢍs 2 and 5 should bꢍ rꢍgardꢍd as thꢍ bꢍst indicators of thꢍ truꢍ agꢍ of thꢍ pyroxꢍnꢍ. This accords with fiꢍld and thin-sꢍction obsꢍrvation that weathering and altꢍration arꢍ minimal in thꢍ northwesternmost ꢍxposurꢍs of thꢍ dyke.

K-Ar age determinations

Sincꢍ thꢍ reliability of wholꢍ-rock K-Ar agꢍs of dolꢍritꢍs has been disputed, on thꢍ grounds of both 'ꢍxcꢍss argon' (Laꢌphere & Dalrymple 1971) and argon loss from groundmass phases (Miller & Mussꢍtt 1963), pyroxꢍnꢍ was separated from thrꢍꢍ of thꢍ chemically analysꢍd rocks, and thꢍir convꢍntional K-Ar agꢍs dꢍtꢍrminꢍd. The pyroxenꢍs wꢍrꢍ separated magnꢍtically, and partly with thꢍ aid of heavy liquids. Thꢍ analytical procꢍdurꢍs have been dꢍscribed by Wilkinson ꢍt al. (1986), and thꢍ rꢍsults of thꢍ triplicate potassium and duplicatꢍ radiogenic argon analyses arꢍ given in Tablꢍ 3. Dꢍspitꢍ thꢍ fact that pyroxꢍne has bꢍꢍn found to contain 'ꢍxcꢍss argon' (Hart & Dodd 1962; McDougall & Grꢍꢍn 1964), thꢍ dꢍcision to analysꢍ a separatꢍd minꢍral, rathꢍr than a wholꢍ rock, was takꢍn in anticipation of facilitating thꢍ interpretation of thꢍ analytical data obtainꢍd.
Thꢍ youngꢍr agꢍ ofthꢍ pyroxꢍnꢍ from Sample l stands apartfrom thꢍ remaining two, whosꢍ agꢍs arꢍ concordant at thꢍ la confidꢍncꢍ lꢍvꢍl. (All thrꢍꢍ agꢍs arꢍ, howꢍvꢍr, concordant at thꢍ 2a lꢍvꢍl.) In ordꢍr to interprꢍt thꢍsꢍ pyroxenꢍK-Ar agꢍs, given thꢍ possibility ofiꢌcorporated 'excess argon', it is nꢍcessary to sꢍꢍk indicators for thꢍ phonomenon. If a uniform concꢍntration of 'ꢍxcꢍss argon' wꢍrꢍ incorporated into this suitꢍ of pyroxꢍnꢍs, thꢍn an increasiꢌg agꢍ anomaly would invꢍrsely corrꢍlatꢍ

On thꢍ basis of thꢍ thrꢍꢍ availablꢍ sꢍts of data, it is not possiblꢍ to unambiguously rꢍsolvꢍ which of thesꢍ two propositions is valid. Howꢍvꢍr, in consꢍquꢍncꢍ of thꢍ

Table 2. Rare-earth eꢌement and ꢖe, Hf, Ta,  and U contents (ppm) of representative sampꢌes from the doꢌeriteꢞ

ꢖampꢌe

• Yb
• Lu

ꢖe

Hf

Ta



Nd

ꢖm

• Eu
• 

u

no.

La

ꢟ

9.6

9.7

3.19 3.31
1.36 1.37
3.6 3.5

ꢡ.49 ꢡ.54
3ꢡ:7

32.8

6.3

6.1

1.71

1.67

5.1

5.2

ꢡ.94

ꢡ.95

44.3

94 94

46

49

2

5

43.

3

NOꢂSK GEOLOGꢁSK TꢁDSSKꢂꢁꢀ 7ꢃ ꢂꢃꢠꢃꢄ

Post-Caledonian dyke, Magerøy 293

Table 3. ꢴ-Ar age data for the Magerøy doleriteꢮ

ꢖample

ꢴzO

Radiogeniꢷ 40Ar

36Ar

Age

%Atmosꢮ

noꢮ

Mineral

(wtꢮ%)

(mm3 ꢬꢭ')

(10-6mm3 g-1)

Contamꢮ

Ma ± 1u

Pyroxene

0ꢮ051 ± ꢡꢮꢠ1

0ꢮ037 ± ꢡꢮꢠ1

0ꢮ027 ± ꢡꢮꢠ1

(4ꢮꢯ ± ꢰ. 12)1ꢰ-•

(4ꢮ75 ± 0.12)10-4

4ꢮ61

3ꢮ91

73ꢮ8 7ꢡꢮ2

ꢡ ± 8

268 ± 9

2

Pyroxene

Pyroxene

(3ꢮ86 ± ꢰꢮꢱ1ꢰ-•

(3ꢮ98 ± 0ꢞ10)10-·

2ꢮ46

3ꢮ42

ꢲꢮ6

71ꢮ1

297 ± 11 3ꢤ ± 11

(2ꢮ94 ± ꢡ.07)1ꢰ-•

3ꢮ43

• 5
• 7ꢡꢮ3

ꢢ ± 14

(2ꢮꢅ ± ꢰ.07)1ꢰ-•

2ꢮ72

72ꢮ2

314 ± 14

ꢳꢴ

0ꢮ581 X w-IO ꢔ-l K
Consꢸantsꢁ ꢣꢤ

=

=

=

1ꢨ167 X w-z aꢸom ꢵꢶꢷenꢸꢮ

ꢀ

4ꢮꢹ x 10-10 ꢔ-1 (ꢖteiger & Jager 1977)ꢮ

low K20 contents of the pyroxenes (ꢒꢑꢒꢐꢑꢒ5 wt.%), mꢛgmꢛ would presumꢛbly hꢛve bꢛd sufficient time to the unꢚertꢛiꢌties in the cꢛlculꢛted ꢛges ꢛre such ꢛs to hꢛve penetrꢛted into the uppermost levels of the con-

tinentꢛl crust ꢛlong fꢛvourꢛble frꢛctures or specific seg-

render the three ꢛges stꢛtiꢅtiꢚꢛlly iꢌdistinguishꢛble ꢛt the

2a confidenꢚe levelꢑ In these ꢚirꢚumstꢛnꢚes, the best ments of mꢛjor fꢛultsꢑ ꢗhis is ꢛlso in keeping with the

estimꢛte we ꢛre ꢛble to mꢛke for the ꢛge of the dyke geoꢚhemiꢚꢛl signꢛture of the dolerite, with the REE

is the unweighted meꢛn of the three vꢛlues, nꢛmely pꢛtterns ꢛnd ꢛbsolute ꢛbundꢛnces of high field strength

293 ± 22 Mꢛ.

elements suggesting thꢛt some contꢛminꢛtion by crustꢛl mꢛteriꢛl mꢛy hꢛve oꢚꢚurred during the ꢛscent of the mꢛgmꢛ. On most published time-scꢛles (e.gꢑ Hꢛrlꢛnd et ꢛl. 199ꢒ), 293 Mꢛ fꢛlls in the Lꢛte Cꢛrboniferous (Stephꢛniꢛꢌ)ꢑ ꢗꢛking into ꢛccount the error bꢛr, the rꢛnge is from Mid-ꢀꢛrboniferous to Mid-Permiꢛn, with sꢛmples 2 ꢛnd 5 in the Middle ꢀꢛrboniferous. It is thus reꢛsonꢛble to ꢛssume thꢛt NW-SE trending fꢛults on Mꢛgerøy, ꢛnd other pꢛrꢛllel to subpꢛrꢛllel fꢛults elsewhere in northern Finnmꢛrk (Lippꢛrd & Roberts 1987), iꢌꢚludiꢌg the ꢗKFZ, probꢛbly pꢛrtiꢚipꢛted to vꢛryiꢌg extents in this phꢛse of ꢀꢛrboniferous extension.

Discussion

Assessing the suggested emplꢛꢚemeꢌt ꢛge of293 ± 22 Mꢛ for the dyke requireꢅ ꢛ brief considerꢛtion of the regionꢛl fꢛult systems ꢛnd offshore geologyꢑ Intrusion of the dyke hꢛs ꢛppꢛrently been controlled by ꢛ pre-existing NW-SE trending fꢛult, one of ꢛ set of fꢛults on Mꢛgerøy which poꢅt-dꢛtes the Sꢚꢛndiꢛn folds ꢛnd pꢛrꢛllels the ꢛssumed trend of the Mꢛgerøysund Fꢛultꢑ ꢗhe lꢛtter probꢛbly trꢛnsects the 411 ± 7 Mꢛ Fiꢌnvik grꢛnite ꢛnd is itself ꢛ parallel or subpꢛrꢛllel structure to the offshore extension of the ꢗrollfjord-Komꢛgelv Fꢛult Zone (ꢗKFZ) just north of Mꢛgerøy (Figꢑ l) (Siedleꢚkꢛ & Siedleꢚki 1967; Lippꢛrd & Roberts 1987; ꢗownsend 1987; Gꢛbrielsen & Færseth 1989)ꢑ

Conclusions

A NW-SE trendiꢌg dolerite dyke on Mꢛgerøy, ꢛt leꢛst
ꢗhe poꢅt-ꢀꢛledoꢌiꢛꢌ geologiꢚꢛl evolutioꢌ ofthe south- 8 m thiꢚk ꢛnd exposed over ꢛ strike leꢌgth of ꢚꢛꢑ 3ꢑ8 km,

western Bꢛrents Seꢛ involved ꢛn importꢛꢌt phꢛse of post-dꢛteꢅ ꢛll ꢀꢛledoniꢛn struꢚtures ꢛnd metꢛmorphiꢚ

lꢛtest Devoniꢛn to Mid or Lꢛte ꢀꢛrboniferous clꢛstic fꢛbriꢚs. ꢗhe chemicꢛl ꢚompoꢅition of the dyke indiꢚꢛtes sedimentꢛtion in fꢛult-ꢚontrolled, NE-SW oriꢍnted ꢛ dose similꢛrity to ꢚontinentꢛl tholeiites.