Magmatic, detrital, and metamorphic ages in ...

Magmatic, detrital, and metamorphic ages in metamorphic rocks from south-central Sweden Ulf B. Andersson1,2, Karin Högdahl1,2, Håkan Sjöström1, and Stefan Bergman3 1) Institutionen för geovetenskaper, Uppsala universitet, Villavägen 16, SE-752 36 Uppsala, Sweden 2) Laboratoriet för isotopgeologi, Naturhistoriska riksmuséet, Box 50007, SE-104 05 Stockholm, Sweden 3) Sveriges geologiska undersökning, Box 670, SE-751 28 Uppsala, Sweden H95:17b Charnockite east of Hudiksvall

TIMS ages of H97:2b (monazite), H95:17b and STB92:119a (zircon), yielding upper age estimates (with some scatter) for metamorphism in these high-grade rocks.

Introduction

0.36

H95:17b) This granulitegrade charnockite shows a large range in ages of magmatically zoned crystals. A discordia through those not obviously older yields an approx. age of the igneous protolith of c. 1.87 Ga. Even older (1.971.91 Ga) material is evident. Crystals and overgrowths apparently related to the high-grade metamorphism fall in the range 1845-1830 Ma.

data-point error ellipses are 68.3% conf.

0.38

Mostly magmatically zoned crystals

Intercepts at 95 ± 120 & 1870 ± 6 [±9] Ma MSWD = 3.6

1980 1940 1900

0.34

206Pb/238U

The Bergslagen area in the southern part of the Svecofennian Domain displays a metamorphic zonation with upper amphibolite to locally granulite facies in the south grading to lowermiddle amphibolite facies in the north, and a small area of greenschist facies rocks in the NW (Fig. 1). Going north from the Bergslagen area a rapid increase in metamorphic grade to upper amphibolite facies is observed in the southern part of the south-central Svecofennian province (Ljusdal batholith area; Fig. 2). Within the Ljusdal batholith region several granulitic areas have been recorded.

Diffusely zoned overgrowths and crystals 1845-1830 Ma

1860 1820

1780

0.32 1740 1700 0.30 1660

Magmaticaly zoned crystals and cores 1.97-1.91 Ga

0.28

0.26 4.0

4.4

4.8

5.2

5.6

6.0

207Pb/235U

One point at 2674±10

0.31

1740

8b 1767±7

1700 1660 0.29

1620 1580

0.27 3.6

4.0

4.4

4.8

207

5.2

5.6

235

Pb/

U

H97:2b High-grade metasediment, Hudiksvall

0.42

2300

18a 2125 ±8

2200 0.40

2100

U

1820

1780

206

Pb/238U

1860

data-point error ellipses are 68.3% conf

0.38

238

Weighted average = 1798 ± 5 [0.28%] 95% conf. Wtd by data-pt errs only, 0 of 10 rej. MSWD = 1.4, probability = 0.16

0.33

We here report U-Pb TIMS and SIMS inherited, magmatic, and metamophic ages from variably metamorphosed metavolcanic, metasedimentary, and metaintrusive rocks from these areas. Only analyses with low common Pb has been used (206Pb/204Pb > 5000). Part of these data were presented in preliminary version in Anderson et al. (2000).

1900

Majority of detrital zircons are 1900-2050 Ma, six are Archaean: 2.5-3.3 Ga

23a 1828 ±2

Pb/

STB92:119a Amphibolite-facies metasupracrustal, NW Edsbyn

STB92:119a) Zircons are short prismatic with low Th/U (< 0.10), except one core which is 2.67 Ga. The overlapping analyses yield the age of high-grade metamorphism at 1.80 Ga. One analysis is slightly younger (1.77 Ga).

2000

Youngest metamorphic zircons

0.36

206


0.35

H97:2b) Detrital grains (with relatively higher Th/U) in this sheared metasediment yield ages 1.90-2.12 and 2.49-3.31 Ga, while lower Th/U grains and rims stretches down to 1830-1815 Ma.

Fig. 2 (from Bergman & Sjöström 1994)

0.34

22b 1814 ±9

0.32

1800

0.30 1700 4

1900

5

6

7

8

9

207

Pb/235U


data-point error ellipses are 68.3% conf data-point error ellipses are 68.3% conf

1900

206

Pb/238U

1860 1820

0.33

1780 8a 1934± 15

1740 1700 1660

0.31

0.29

Intercepts at 1899 ± 5 Ma & 172 +140/-130 Ma MSWD = 1.20

0.27

0.25 4.0

4.4

4.8 207

5.2

Pb/

235

5.6

Upper intercept 1904 ± 14 [±15] Ma MSWD = 0.14 Weighted average 1895 ± 5 Ma MSWD = 0.65

0.34

UB98:29) Lower-middle amphibolite facies metabasite. Th/U lie in the range 0.38-1.18. Crystal with two youngest analyses have the lowest U, and may represent a younger (metamorphic?) growth. Six overlapping analyses give the intrusive age (c. 1.90 Ga), and one is older (1.94 Ga).

1870±20

1900

1800 0.32

1700 0.30

04a 1944±14

1600

Intercepts at 388 ± 130 & 1894 ± 13 [±14] Ma MSWD = 5.4

4.2

4.6

5.0 207

U

Pb/

5.4 235

5.8

1900

1800 0.32

1700 0.30

1600 0.28 3.8

3.8

U97:1) Lower-middle amphibolite facies. Crystals oscillatory zoned, often rich in common Pb, and discordant. No detectable overgrowths. Th/U > 0.3. Upper intercept interpreted as extrusion age, although scatter is large.

0.34

0.28

6.0

2000

U97:1 Felsic metavolcanite Ramhäll, Uppland

0.36

Pb/238U

14b 1857±12

1940

UB98:29 Amphibolite Ludvika NE

0.36

2000

Weighted average

206

11a 1855±15

Pb/238U

0.35

Weighted average 1895 ± 4 [0.23%] 95% conf. Wtd by data-pt errs only, 0 of 27 rej. MSWD = 1.5, probability = 0.046

1980

EGU980074) Partly garnet-bearing metaquartzdiorite. Th/U 0.15-0.65. No systematic relation with age. 27 crystals overlap and yield an intrusive age of c. 1895-1900 Ma. Two are younger c. 1855 Ma, and at least one is older (1.93 Ga).

206

0.37

EGU980074 Meta-quartz diorite Larsbo formation, Ludvika SE

4.2

4.6

5.0 207

6.2

Pb/

5.4 235

5.8

6.2

U data-point error ellipses are 68.3% conf

U U97:3 Supracrustal rock Borggårde, Uppland

1980

0.36

TIMS ages of samples: Fin2 (monazite), EGU980074, and UB98:25 (zircon).

Weighted average 1891.5 ± 6.8 [0.36%] 95% conf. Wtd by data-pt errs only, 0 of 6 rej. MSWD = 2.5, probability = 0.026


7 analyses 1915-1940 Ma

1940 1900

0.34

1860 1820 1780

0.32

1750

0.31

1950

13a 1868±10

1850

Intercepts at 1901 ± 8 [±10] & 456 ± 220 Ma MSWD = 0.86

0.33

206

Pb/238U

0.35

Pb/238U

Mean = 1894 ± 5 [0.24%] 95% conf. Wtd by data-pt errs only, 0 of 19 rej. MSWD = 1.2, probability = 0.22

206

0.37

UB98:25 'garnet tonalite' Uvberget, Ludvika SE

U97:3) Amphibolebearing supracrustal gneiss. Crystals oscillatory zoned, with no overgrowths (Th/U > 0.2). Coherence of age data may indicate magmatic rather than sedimentary origin.

+3 analyses at: 2084±6, 2803±3, 3358±12

1740

1650

0.29

1700 0.30 4.2

0.27

4.6

5.0 207

5.4

Pb/

235

5.8

6.2

U

0.25 4.2

4.6

5.0 207

5.4

5.8

6.2


2400

Pb/235U

0.44

UB98:25) Apparently anatectic, ’garnet tonalite’, cross-cutting metasediments of the Larsbo formation (left), NW Bergslagen. Th/U 0.16-0.86. No systematic relation with age. Majority of crystals are short prismtic and faintly zoned. 19 analyses overlap and yield c. 1.89 Ga – age of migmatisation. One is slightly younger, and 7 appear to be older 1.91-1.94 Ga. Three inherited crystals are much older

R89:14) Mafic sill in felsic metavolcanic rocks, both in lower amphibolite facies. All crystals have Th/U > 0.65. Five overlapping analyses yield the intrusive age (1887 Ma), while two represents somewhat older, inherited material (1.91 and 1.92 Ga).

U Pb/

238 206

Nyk (STB96:2025A) High-grade metasediment. Four zircon SIMS analyses with Th/U < 0.09 yield overlapping age with two monazite TIMS analyses. Age of metamorphism. TIMS data from Andersson (1997). See also Väisänen et al. (2004-this meeting). data-point error ellipses are 68.3% conf  Nyk: M etasediment Nyköping, Södermanland

1840

0.33

Pb/ 238U

Pb/238U 206

1960

SIMS (blue) intercepts: 16 ± 510 & 1804 ± 10 [±11] Ma MSWD = 1.5 TIMS (red) w eighted av.: 1805 ± 3 Ma

0.32

1820 1800 1780

1760 1740

0.31 1720 03a 1910±7

1700

4.8

5.0

5.2

5.4 207

5.6

Pb/

235

5.8

6.0

6.2

0.30 4.3

U

4.5

4.7

4.9

207

Pb/

235

Pb/238U

0.44

5a 1924±5

Weighted average 1855 ± 7 [0.37%] 95% conf. Wtd by data-pt errs only, 0 of 3 rej. MSWD = 0.18, probability = 0.84

1900 6a 1908±6

2a 1822±4

1800

+ one Archaean zircon

0.32

2777±5

2400 5a 2447±6

2200

0.40

Ma

10b 2474±15

2000 0.36

1700

1800

0.32

0.30

12a 1967±9

1600

0.28

1600

9b 2021±15

11a 2032±17

206

206

7a 1827±8

SIMS (blue) youngest: weighted average 1793 ± 5 Ma MSWD = 0.25 TIMS (red) weighted av.: 1796 ± 1 Ma

0.34

Fin2 Augen gneiss (TIB) Finspång, Östergötland

Pb/238U

0.36


0.48

2000

5.1

U


Kga: Gt-Crd gneiss Karlskoga, Värmland

0.28 3.8

4.2

4.6

5.0 207

Pb/

5.4 235

5.8

1800

1898-1919

4.5

5.5

6.5

7.5

8.5

U97:6) Lower-middle amphibolite facies metasediment. Oscillatory zoned detrital crystals, often high in common Pb and discordant. Most are in the range 1.9-2.08 and three are c. 2.5 Ga. One overgrowth has a max. age of 1866±4 Ma.

9.5

Pb/235U

(modified from Stålhös 1991)

1840

0.32

15a 2449±6

2a 1967±16

207

1880

1800

5b 2069±13

9b 1866±4

0.281600 3.5

02a 1920±8

0.33

17a 2527±7

2000

4a 2074±12

1920

0.34

18a 1930 ±13

0.36

0.32

R89:14 Amphibolite Rishöjden, Filipstad NE

0.35

8a 2005±5

overgrowth

2000

Weighted average 1887 ± 5 [0.28%] 95% conf. Wtd by data-pt ers only, 0 of 5 rej. MSWD = 0.64, probability = 0.63

5a 2080±10

0.40


0.36

U97:6 Metasediment Odensala, Uppland

2200

206

3.8

6.2

1400

0.24

U

2

4

6

207

Kga) Strongly mobilized metasediment, remelted in granulite facies within a contact aureole to TIB rocks (cf. Andersson et al. 1992). The youngest (Th/U < 0.07) zircons yield an age overlapping with the monazite TIMS age of the same sample. This gives the time of contact migmatisation and TIB intrusion in the area (c. 1795 Ma). Inherited crystals and cores recorded at 1.91, 1.92,and 2.78 Ga. Analyses at c. 1825 Ma appear to be mix-ages. TIMS data from Andersson (1997).

8

10

Pb/235U

Fin2) Peraluminous porphyritic orthogneiss. Youngest crystals are elongated and strongly oscillatory zoned, with no younger overgrowths. Th/U is usually low, < 0.2, suggesting co-precipitation with monazite. The age of these crystals, 1855±7 Ma, gives the igneous age of the rock, confirming that it belongs to the earliest TIB generation (TIB-0). Older cores are common (1.97, 2.02, 2.03, 2.45, & 2.47 Ga). Metamorphic ages are indicated by the monazite TIMS data at 1.83-1.82 Ga (see upper left)

Summary Magmatic ages recorded for Svecofennian rocks in northern Bergslagen are 1.90-1.88 Ga. Metamorphic ages obtained from southern Bergslagen are 1.83-1.79 Ga. The garnet tonalite from NW Bergslagen seems to record an early Svecofennian anatectic event. High-grade metamorphism and anatexis in S. Norrland may have started as early as 1.86 Ga and continued at least until 1.80 Ga. The igneous protolith of the charnockite formed c. 1.87 Ga, partly from material 1.97-1.91 Ga in age. Detrital, sedimentaryderived zircons fall in two groups: 1.90-2.12 and 2.50-3.36 Ga. Inherited crystals in the range 1.97-1.91 Ga also in metaigneous rocks confirm earlier postulations of such crustal components in these areas. The Finspång augen gneiss belongs to the earliest TIBgeneration. References: Andersson, U.B. 1997: The late Svecofennian, high-grade contact and regional metamorphism in southwestern Bergslagen (central southern Sweden). Final report 970519, SGU-project 03-819/93, 36 pp (unpubl.). Andersson, U.B.; Larsson, L. & Wikström, A. 1992: Charnockites, pyroxene granulites and garnet-cordierite gneisses at a boundary between Early Svecofennian rocks and Småland-Värmland granitoids, Karlskoga, southern Sweden. GFF 114: 1-15. Andersson, U.B., Högdahl, K., Sjöström, H. & Bergman, S. 2000. Geochronologic constraints on metamorphic, intrusive and detrital ages of rocks from the Bergslagen area, southern Sweden. Sveriges Geologiska Undersökning, report, July 2000, contract nr 031025/97, 12 p. + figures. Bergman, S. & Sjöström, H. 1994. The Storsjön-Edsbyn Deformation Zone, central Sweden. Research Report, Geological Survey of Sweden, 46 pp. Claesson, S., Huhma, H., Kinny, P.D. & Williams, I.S., 1993. Svecofennian detrital zircon ages - for the Precambrian evolution of the Baltic Shield. Precambrian Research 64, 109-130. Sjöström, H. & Bergman, S. 1998. Svecofennian metamorphic and tectonic evolution of east central Sweden. Sveriges Geologiska Undersökning-report, 50 pp. + figs. Stålhös, G. 1991. Beskrivning till berggrundskartorna Östhammar NV, NO, SV, SO. Sveriges Geologiska Undersökning Af 161, 166, 169, 172, 249 p. Väisänen, M., Andersson, U.B., Huhma, H. & Mouri, H. 2004: Age of late Svecofennian regional metamorphism in southern Finland and south-central Sweden. 26th Nordic Geological Wintermeeting, Uppsala, 040106-09. GFF 126: 40-41.