Precambrian crustal components, plutonic associations ... - Springer Link

Contributions to Mineralogy and Petrology

Contrib Mineral Petrol (1988) 98:129-138

9 Springer-Verlag1988

Precambrian crustal components, plutonic associations, plate environment of the Hercynian Fold Belt of central Europe: Indications from a Nd and Sr isotopic study T.C. Liew and A.W. Hofmann

Max-Planck Institut ffir Chemic, Postfach 3060, D-6500 Mainz, Federal Republic of Germany

Abstract. Apparent crustal residence ages indicated by Nd

model age data for metamorphic rocks, sediments and granitoids of the Hercynian Fold Belt of Europe vary from 1.3 Ga to 3.0 Ga, but are mainly in the range 1.4-1.7 Ga. 2 Ga basement inliers have been documented previously in northern Spain and islands off northwestern France but, in addition to these, old ( ~ 2-3 Ga) model ages are found along the southern margin of the fold belt. These do not identify old inliers but are interpreted to represent Archeanearly Proterozoic crustal components recycled from a southern source. The Nd data, when considered together with the surface geology and recent single-grain zircon ion microprobe data, argue against a binary mixing of Archean components with new Palaeozoic crustal additions to generate the main 1.4-1.7 Ga model age population. Hercynian Europe comprise mainly recycled Proterozoic components although significant new Palaeozoic additions as well as Archean contributions are indicated. Nd and Sr isotopic data together with previous chemical and petrographic observations allow the recognition of a northern belt of continent margin I-type granitoids grading southwards to inner continent S-type plutons in the eastern half of the fold belt. This felsic plutonic association is used to infer a Hercynian plate configuration involving the attachment of the fold belt to a southern parent cratonic block that the model age data suggest may be of early Proterozoic-Archean age.

Introduction. Late Palaeozoic events that resulted in the

consolidation of a European continental configuration are collectively termed the Hercynian orogeny. The orogen comprises an east-west trending belt through central Europe extending from eastern Czechoslovakia to the Iberian Peninsula (Fig. 1). Its northern boundary is conventionally taken as the Hercynian Front and this marks the most northerly exposures affected by Hercynian deformation. For this study, the Alpine Front has been arbitrarily taken as the southern boundary but basement units of Palaeozoic age are known within the Alpine Belt (e.g. K6ppel et al. 1980). The widespread and intense Hercynian tectonothermal events have overprinted much of the pre-Hercynian geological record of central Europe. As a result of this and Offprint requests to." T.C. Liew

the large areas of post-Hercynian sedimentary cover, little is known about the extent, nature and age of the preHercynian basement. Most radiometric ages record Hercynian ( ~ 2 9 ~ 3 6 0 M a ) events but older Caledonian ( ~ 4 8 ~ 5 2 0 Ma) and Cadomian (~550~620 Ma) ages are well substantiated and are regarded to be the oldest dated events in the fold belt. As far as we know, there are two important exceptions. Calvez and Vidal (1978) reported a 2.0 Ga age for the Icart Gneiss of the Channel Islands between France and England and Guerrot et al. (1987) reported a 1.9 Ga age for granulites from northern Spain. Although other igneous or metamorphic units of this age are not known, there are indirect isotopic indications for a more extensive role involving old Precambrian crust in central Europe. The first comes from observed U-Pb zircon patterns. The discordance patterns reported by many workers (e.g. Grauert et al. 1974; Gebauer and Grunenfelder 1977; Todt and Busch 1981, Van Breemen etal. 1982) have lower intersections with the concordia curve that give Palaeozoic ages for zircon growth or recrystallisation but the upper intersections trend towards ages of around 1.5 2.5 Ga. Such patterns indicate the presence of old zircons inherited from unidentified old Precambrian basement sources. A second indication for the involvement of old crust comes from Pb-Pb isotopic studies (e.g. Wedepohl etal. 1978; Vitrac etal. 1981; Kober and Lippolt 1985). Linear arrays obtained from 2~176176 2~ K-felspar data, if interpreted to have chronological significance, yield ages exceeding 3 Ga (Vitrac et al. 1981). Wedepohl et al. (1978), however, reported an array for ore Pb data that gave an age of 1.7 Ga. Kober and Lippolt (1985) suggested that these arrays represent mixing lines with no time information and calculated ages of 2.1-2.7 Ga based on 2-stage Pb evolution models. Age estimates of 2-3 Ga, although model dependent, allow the intriguing possibility that Hercynian Europe comprise mainly reycled Archean crust. In this study, we attempt to confirm the old ages indicated by earlier isotopic work and constrain the extent and distribution of the Precambrian components using the SmNd technique. The approach is based firstly on a discussion of Nd model ages (e.g. McCulloch and Wasserburg 1978; All6gre and Ben Othman 1980; Farmer and DePaolo 1983). A model age is considered to reflect a crustal residence age (O'Nions et al. 1983). For a closed system, this corresponds to the time since the REE fraction of a crustal sam-

130

|

0 Amsterdam

'

~_._.~.~-" .I ....~..." .......... .... ~~ " ' " ~ . . . - -- -- -~26,40

nBrussel s

[] Berlin

km

'

150

.....

....... ...... ; Y ' Y / - ~

I1 ~

.ii'-

a

~8

~-w

llI

~, -

~

-

A

, LYl

e~..~.. Vieooa

L PS

Fig. 1. A Structural division and distribution of the main Hercynian

units of central Europe and the Iberian Peninsula; B Map of locations of samples from the eastern Hercynian Fold Belt. CW: Cornwall, A M : Armorican Massif, RS: Rheinisches Schiefergebirge, H: Harz, BM: Bohemian Massif, S: Schwarzwald, OS: OdenwaldSpessart, V: Vosges, MC: Massif Central, IM: Iberian Massif. I: Rhenohercynian Zone, H: Saxothuringian Zone, III: Moldanubian Zone, IV: Moravo-Silesian Zone. Numbers beside solid circles denote sample numbers ple was fractionated from the mantle. However, in view of assumptions inherent in model age calculations (e.g. Liew and McCulloch 1985) and the likelihood of recycling a crustal sample from two or more sources of differing ages, a model age is strictly an apparent crustal residence age. We report here Sm-Nd data for a wide selection of samples from eastern central Europe. The results are then reviewed together with data from Fance (Michard et al. 1985; Ben Othman et al. 1984; Bernard-Griffiths et al. 1985; Downes and Leyreloup 1986) and southwest England (Davies et al. 1985). In addition, the Nd and Sr isotopic characteristics of the West German granitoids are discussed and used to speculate on the pre-Hercynian configuration of Europe. Structural Hercynian Fold Belt

A number of distinct structural units displaying major lithological differences and/or structural breaks can be distin-

guished in the Hercynian fold belt. In view of the important role assumed by the structural framework of the fold belt in the discussion of the present isotopic data, a brief examination of the different units is presented here. Hercynian Europe can be divided into three major eastwest trending structural belts ( see review by Behr et al. 1984). From north to south, these are the Rhenohercynian, Saxothuringian and Moldanubian zones (zones I, II, III in Fig. 1). Granitoids are present in all three zones. The Rhenohercynian zone consists mainly of thick DevonianCarboniferous flysch sediments underlain by Lower Palaeozoic clastic strata. Behr et al. (1984) consider the zone to represent a broad thrust wedge. The combination of maficultramafic units, radiolarian cherts, greywackes and pillow basalts present in the zone may be indicative of an accretionary wedge developed above a subduction zone. Sediments in the Saxothuringian Zone are of Cambrian to Carboniferous age and are mainly shallow water varieties, but abundant volcanics of Hercynian age and their detritus are associated with the sediments. Some metamorphic inliers are found in the Saxothuringian zone but it can be argued whether these represent uplifted pre-Hercynian basement. The Moldanubian zone to the south is dominated by outcrops of metamorphic basement units and granitoid-migmatite complexes (Schwarzwald, Vosges, Bohemian Massif; S, V, BM in Fig. 1). Extension of these zones into western France, although widely accepted, is not unambiguous. The Armorican Massif, for example, is not considered by Behr et al. (1984) as a Saxothuringian unit and may represent a separate tectonic block. The Moravo-Silesian zone of Czechoslovokia represents a small pre-Hercynian basement fragment attached to the southeastern margin of the Moldanubian Zone (Van Breemen et al. 1982). There is still considerable debate concerning the tectonic evolution of the Hercynian foldbelt and the significance of the recognised zonation. Many workers, including those referenced by Windley (1977) and more recently, Floyd (1982), Lorenz and Nicholls (1984), and Behr et al. (1984) tend to favour a subduction-collision mechanism. But there is no agreement concerning the number of subduction zones and their polarity. There are two possibilities for placing a Palaeozoic trench: within the Rhenohercynian zone (Anderson, 1975) or south of the Moldanubian zone and thus hidden within the Alpine belt. Such models assume central Europe to be a coherent block. In contrast, Ziegler (1984) favours the formation of central Europe by the accumulation of exotic blocks derived from Gondwana to the south, Badham (1982) rejects the evidence for wide oceans. He considers the fold belt to be a strike-slip orogen dissected by major WNW-trending strike-slip faults. It is useful to have a perspective of these problems but they are not critical to the development of the arguments presented in this paper.

Analytical

The procedure adopted are modified from those of Richard et al. (I 976) and have been previously described e.g. Patchett and Bridgwater (1984). Samples were dissolved in open beakers, then in bombs using HF-HNO3. This was followed by HC104 and HC1 treatments. Solutions were then aliquoted and aliquots for isotopedilution were mixed with a 145NdJ49gm enriched tracer. After a cation-exchange resin pass to get a REE-rich fraction, Sm and

131 Nd were separated on a second column containing Teflon powder coated with di-2-ethyl-hexyl orthophosphoric acid. A Sm-free Nd fraction is routinely obtained. Sm and Nd were loaded using H3PO4 on Re filaments and measured as the metal species on a Finnigan MAT 261 fully automated mass spectrometer in single collector mode. 15~ ratios were routinely measured to monitor data quality. Total processing blanks for Sm and Nd were less than 100 pg and were negligible.

rim of the Bohemian Massif in West Germany have model ages of 1.7- 2.0 Ga. A Caledonian granite gneiss (19) from the southern Moravo-Silesian zones has a 2.1 Ga model age, a value not significantly different from the Moldanubian zone samples. Results for the metamorphic rocks of the other central European massifs (i.e. Schwarzwald, Vosges and Spessart) are similar to the 'younger' (less than 2.1 Ga) model age population of the Bohemian Massif: their model ages are in the range 1.3-2.0 Ga.

Results

All analytical data are presented in Table 1. Sample localities are shown in Fig. 1. To facilitate a comparison of the end data at a single point in time without complications arising from radioactive decay, it was found useful to use present-day eNd values in parts of the discussion. For example, a Quarternary sample with an end value of --10 and a 500 Ma old sample with an eNd value of -- 5 have identical apparent crustal residence ages because the latter evolves to an equivalent present-day value of - 10 assuming a normal crustal Sm/Nd ratio. (An examination of the model age trajectories in Fig. 4 will reveal these relationships more clearly). Both present-day and initial ratios are listed in Table 1. A second point to bear in mind is errors in assignment of geological ages, as it is common to find that different dating techniques yield different ages. Ages in Table 1 represent assumed intrusive ages for plutonic rocks, stratigraphic ages for sedimentary rocks and age of main metamorphism for metamorphic rocks. Some of the ages have been inferred based on broad geological correlation and may be incorrect. Errors of 10-20 Ma have little effect on initial eNd values. For larger errors, initial ratios are incorrect, but the calculated model ages are still valid. Model ages are calculated with respect to a depleted mantle (footnote, Table 1).

A) Medium- and high-grade metamorphic rocks Studied samples comprise rocks ranging from mica schists to metasedimentary granulites but the majority are amphibolite-facies felsic gneisses. The sample population is fairly representative of the exposed basement lithologies of central Europe. The metamorphic rocks provide an unusually large spread in present-day end values, from --6 to --30. Calculated model ages vary from 1.3 Ga to 3.0 Ga. Caledonian units (e.g. Todt and Bfisch 198t; Hofmann and Kohler, 1973) considered to represent pre-Hercynian basement and Hercynian units both show similar Nd isotopic characteristics. Two Moldanubian zone samples from the Bohemian Massif have highly negative eNd(0) values (--24 and - 3 0 ) that clearly differentiate them from the remainder of the studied samples. Their model ages are Archean: 2.5 Ga for the Doberes gneiss (sample 15) of northern Austria and 3.0 Ga for a hornblende-gneiss (sample 22) from the Varied Series of central Czechoslovokia. Rocks of the Varied Series are apparently tectonically overlain by high-grade rocks of the Gfohl Group (sample 17) and associated granulite (14, 16, 21, 24) and ultramafic units. These high-grade rocks have significantly younger model ages, from 1.5 1.7 Ga. Similar ages are given by granitic gneisses (18, 20) from the eastern rim of the Bohemian Massif. Cordierite-bearing gneisses (10, 11, 13) and a mica-schist (12) from the western

B) Low-grade or unmetamorphosed sediments For this study, we have analysed fine-grained sediments only from West Germany and combined our data with results obtained by Taylor et al. (1983) and Stosch and Lugmair (1984). The sediments have stratigraphic ages that range from Devonian to Pleistocene. Their eNd(0) values vary from -- 6 to -- 13 (Fig. 2). Resulting model ages range from 1.3 Ga to 1.8 Ga and are similar to the 'younger' range shown by the metamorphics. Younger model ages (1.3 1.4 Ga) are obtained for greywackes (40, 41, 42) from the northern Rhenoherynian and central Saxothuringian zones whereas older model ages (1.6-1.8 Ga) are for sediments from the Saxothuringian and Moldanubian zones. These results may be interpreted to indicate that sediments are recycled from older basement sources in the south or that some immature arc-type detrital components are present in the greywackes and hence, these are recording juvenile Palaezoic crustal additions. Detailed studies of a number of local sections are required to distinguish between these possibilities. Precise provenances for the German sediments are not known but the Nd data are consistent with derivation from crystalline sources similar to the exposed metamorphic and plutonic (next section) rocks or recycled sediments of such. The sedimentary data clearly do not indicate major contributions from Archean sources. A comparison of the results for the German sediments with those of France (Michard et al. 1985) confirms the ubiquity of the 1.3-1.7 Ga model age sediment population in central Europe. There are, however, no German equivalents for a number of older model ages around 2 Ga shown by samples in the Brittany region, close to exposures of the 2 Ga Icart Gneisses.

C) Granitoids Large areas of the exposed crystalline massifs in the Moldanubian zone of the Hercynian fold belt consist of granitoid plutons that display many mineralogical and chemical characteristics of the S-type granitoid class of White and Chappell (1983).Such granitoids are considered to be derived by partial melting of metasediments and as such, can be used to provide isotopic images of their deep crustal metasedimentary sources. We analysed samples of S-type granitoids from the Schwarzwald (samples25-33) and Bohemian Massifs (34-38) in the Moldanubian zone and one sample from the Harz (26) in the Rhenohercynian zone. Their eNd(0) values fall within a narrow range, from - 6 to - 10 (Fig. 2). Plutons of uncertain origin like the Brocken Granite (25), a pink felspar-bearing pluton from the Harz, and the 'des Cretes' Granite (39), an enigmatic actinolite-bearing pluton from the Vosges, have eNd(0) values of --7.8 and - 9 . 3 respectively and also fall within this range. The data for

Table

1. Nd and Sm analytical data for rocks from the Hercynian Fold Belt of central Europe

Sample

Assumed Nd Sm Age (Ma) + ppm

x47Sm 144Nd

143Nd ( 0 ) # 14'*Nd

eNd ( 0 )

143Nd (T) eNa (T) l'*'*Nd

TDM (Ga)

500 500 500 500 500 380 500 500 500 320 480 480 480 320 480 480 320 320 340 490 480 320 350

39.19 30.03 32.88 23.92 36.43 18.97 39.15 11.67 57.06 53.99 17.88 27.42 21.24 21.06 33.65 32.94 26.49 26.08 29.99 14.80 16.04 14.15 32.11

7.50 5.70 5.76 5.67 7.49 3.46 7.58 3.83 9.27 9.49 3.73 4.69 4.78 5.20 5.05 4.96 5.83 5.66 6.45 3.18 2.57 3.39 5.58

0.1156 0.1147 0.1060 0.1434 0.1242 0.1102 0.1170 0.1983 0.0982 0.1062 0.1261 0.1033 0.1359 0.1492 0.0907 0.0910 0.1330 0.1312 0.1300 0.1299 0.0970 0.1446 0.1091

0.512027_+15 0.511861_+16 0.512237_+ 18 0.512338_+22 0.512211 _+25 0.512220_+26 0.511942__+26 0.512289_+14 0.512155__+21 0.511913_+19 0.512079-+ 10 0.511866_+16 0.512056_+13 0.512229_+13 0.511413_+16 0.511408_+22 0.512117_+22 0.512126_+26 0.512175_+22 0.512205_+16 0.511709-+18 0.512239__+32 0.512005-+25

-12.0 -15.2 -7.8 -5.9 8.3 8.2 -13.6 -6.8 -9.4 -14.2 -10.9 -15.1 11.4 -8.0 -23.9 -24.0 10.2 -10.0 -9.0 -8.5 -18.1 -7.8 -12.3

0.511648 0.511485 0.511889 0.51/868 0.511804 0.511946 0.511558 0.511639 0.511833 0.511691 0.511683 0.511542 0.511629 0.511917 0.511128 0.511122 0.511839 0.511852 0.511881 0.511792 0.511401 0.511937 0.511764

-6.8 -10.0 -2.1 -2.4 -3.7 -4.0 -8.5 -7.0 -3.2 -10.5 -6.6 -9.3 -7.6 -6.0 -17.4 -17.5 -7.6 -7.3 -6.1 -4.3 11.9 -5.7 -8.0

1.7 2.0 1.3 1.4 1.5 1.4 1.8 1.7 1.4 1.9 1.7 1.9 1.8 1.5 2.5 2.5 1.6 1.6 1.5 1.5 2.1 1.5 1.7

350 350

30.38 33.57

4.40 6.34

0.0876 0.1141

0.511123_+30 -29.6 0.511947_+14 13.5

0.510922 -24.5 0.511686 -9.8

3.0 1.8

350

12.92

3.39

0.1586

0.512197_+22

-8.6

0.511834

-6.9

1.6

290 290 315 330 320 290 330 320 330 320 320 320 320 320 330

46.20 38.09 36.19 30.21 37.55 20.07 38.85 26.12 26.13 38.30 52.54 76.73 58.80 35,11 70.16

9.47 8.48 6.36 5.55 6.05 4.58 6.84 4.84 5.05 6.96 9.41 14.31 9.94 7.53 14.41

0.1240 0.1346 0.1062 0.1110 0.0974 0.1381 0.1065 0.1120 0.1169 0.1099 0.1083 0.1127 0.1021 0.1296 0.1242

0.512239__+14 0.512204-+18 0.512197-+18 0.512231-+32 0.512221__+14 0.512128__+25 0.512193_+18 0.512227_+25 0.512238__+23 0.512259_+29 0.512283+25 0.512188_+21 0.512181_+32 0.512226_+28 0.512160__+14

-7.8 8.5 -8.6 -7.9 -8.1 10.0 -8.7 -8.0 -7.8 -7.4 6.9 -8.8 -8.9 -8.0 9.3

0.512007 0.511952 0.511975 0.511991 0.512017 0.511870 0.511963 0.511993 0.511985 0.512029 0.512057 0.511952 0.511968 0.511955 0.511892

-5.1 -6.2 -4.9 -4.3 -4.1 -7.8 -4.9 -4.6 -4.4 -3.9 -3.3 5.4 -5.1 -5.3 6.3

1.4 1.5 1.4 1.4 1.4 1.6 1.4 1.4 1.4 1.4 1.3 1.5 1.4 1.5 1.5

350 350 350 280 280 280 150 150 150

22.82 21.03 18.31 31.43 40.59 98.72 36.58 27.32 35.49

4.53 4.40 3.84 6.52 6.86 18.84 7.36 5.43 6.76

0.1201 0.1266 0.1268 0.1254 0.1021 0.1154 0.1215 0.1201 0.1151

0.512305__+32 0.512267__+25 0.512283+14 0.512076_+28 0.512074__+35 0.512132-+25 0.512085__+20 0.512078__+19 0.511972__+18

-6.5 -7.2 -6.9 -11.0 -11.0 9.9 -10.8 10.9 -13.0

0.512030 -3.1 0.511981 -4.1 0.511996 -3.8 0.511842 -8.4 0.511883 -7.6 0.511916 -6.9 0.511966 -9.3 0.511960 9.5 0.511859 -11.4

1.3 1.4 1.4 1.7 1.6 1.6 1.6 1.6 1.8

METAMORPHICS i 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 17 18 19 20 21 22 23 24

Feldberg paragneiss, Schwarzwald, M N * Hirschbach orthogneiss, Schwarzwald, M N Hausach paragneiss, Schwarzwald, M N Glottertal granulite, Schwarzwald, M N Kinzigtal granulite, Sehwarzwald, M N Haibach granite gneiss, Spessart, SX M6mbris schist, Spessart, SX Felsic granulite, Central Vosges, M N Kayserburg migmatite, Central Vosges, M N Eck gneiss, W Bohemian Massif, M N Brennes gneiss, W Bohemian Massif, M N Micaschist, W Bohemian Massif, M N Sch6nbach gneiss, W Bohemian Massif, M N Meidling granulite, S Bohemian Massif, M N Doberes Gneiss, S Bohemian Massif, M N - replicate Steinegg granulite, S Bohemian Massif, M N - replicate Gfohl Gneiss, E Bohemian Massif, M N Sneznik Gneiss, E Bohemian Massif, SX? Bites Gneiss, E Bohemian Massif, MS Vir Gneiss, E Bohemian Massif, M N Bory Granulite, E Bohemian Massif, M N Hornblende gneiss, Rajov, E Bohemian Massif, MN Paragneiss, Strazkovice, E Bohemian Massif, M N Felsic granulite, Rokytno, E Bohemian Massif, MN

GRANITOIDS 25 26 27 28 29 30 31 32 33 34 35 36 37 38 39

Brocken Granite, Harz, R H Oker Granite, Harz, R H Triberg Granite, Schwarzwald, M N Oberkirch Granite, Schwarzwald, M N St. Blasien Granite, Schwarzwald, M N Seebach Granite, Schwarzwald, M N Albtal Granite, Schwarzwald, M N Blauen Granite, Schwarzwald, M N Malsburg Granite, Schwarzwald, M N Viechtach Granite, W. Bohemian Massif, M N Saldenburger Granite, W Bohemian Massif, M N Kristallgranit 1, W. Bohemian Massif, M N Kristallgranit 2, W. Bohemian Massif, M N Joching Granite, S. Bohemian Massif, M N 'des Cretes' Granite, central Vosges, M N

SEDIMENTS 40 41 42 43 44 45 46 47 48

Greywacke, Harz, R H Greywacke, S Spessart, SX Greywacke, N o r t h e r n Vosges, SX Permian siltstone, Middle Rhine, SX Permian sandstone, Middle Rhine, SX Permian siltstone, Odenwald, SX Jurassic shale, Swabian Jura, M N Jurassic shale, Swabian Jura, M N Jurassic shale, Swabian Jura, M N

* MN

:

M o l d a n u b i a n zone, SX : Saxothuringian Zone, R H : Rhenohercynian Zone, MS : Moravo-Silesian zone.

+ Ages have been inferred from previous dating studies which were not always conclusive or assumed on the basis of broad geological correlation. Errors in assignment of geological ages (main metamorphism? for metamorphic rocks, intrusive ages for granitoids, stratigraphic ages for sediments) have little effect on model age calculations. # Ratios normalised to I46Nd/144Nd=0.7219; errors are 2 a means; 143 Nd/ 1 4 4 N d = 0 . 5 1 1 8 4 7 _ 2 1 was obtained for the La Jolla standard during period of study. 1 F r143Nd/144Nd~ rear l'~rfx47sm/144Nd'~ ~'1478m/i44Nd'~ ~ ;t43Nd/144Nd', q T - - I n / 1 + Jt m ~ / a ~ ~ t - - )/~. / ]meas--k / /CC~--k / 7DM_I DM - - ..~

[

(1478m/1,,4Nd)cc_

(147Sm/I,~4Nd)DM

where ( 143 Nd/ i4-4 Nd)DM=0.513151, ( 147 Sm/ 1 4 4 Nd)DM=0.219, ( 147 Sm/ 1 4 4 N d ) c c = 0 . 1 2

133 9 Sediments [] Hercynian granitoids [] High- grade metamorphics

,cN N

-4

ENd(O) Fig. 2. Histogram plot of present day eNd values for analysed samples from eastern central Europe. Also included are the sediment data of Taylor et al. (1983) and Stosch and Lugmair (1984)

the plutonic rocks, when taken together with the results for the metamorphic rocks of the fold belt, can be used to infer that end(0) values of --6 to --12 and apparent crustal residence ages of ~ 1.4-1.7 Ga are typical of the eastern central European crystalline basement. In addition to the crustal residence age information, important insights into the tectonic environment of Hercynian Europe can be obtained from a characterisation of the Hercynian granitoid plutons along a transect across the fold belt. Sufficient data are avalaible to do this for the plutons of the Schwarzwald, Odenwald-Spessart and Harz. Together, they provide a critical 300 km north-south transect across the west German segment of the Hercynian fold belt. (i) The Schwarzwald Complex consists mainly of Hercynian granitoids and pre-Hercynian gneisses. The granitoids have been the subject of numerous studies. Especially relevant to the arguments developed in this paper are the studies of Emmermann (1977), Brewer and Lippolt (1974), Hofmann (1979) and Hoers and Emmermann (1983). Details of the intrusive ages of the plutons still remain to be worked

out. Following Hoefs and Emmermann (1983), we consider the majority to have been emplaced within the interval 310-330 Ma ago, but it is possible that some intrusions may be as old as 360 Ma. In addition, Schleicher (1978) described suites of late porphyry dykes that may be as young as 270 Ma but which are apparently unrelated to the main Schwarzwald plutons. The descriptions and chemical data of these earlier studies lead us to consider the Schwarzwald plutonism as primarily S-type in nature. The plutons are mostly peraluminous units of granite-granodiorite composition; muscoviteand, less commonly, cordierite- bearing units are present; the plutons are low in CaO and have high K 2 0 / N a 2 0 and high Rb/Sr ratios. Highly differentiated rocks have high Rb (>500 ppm) and low Sr ( < 5 0 ppm). Initial Sr ratios are high (0.709-0.719, Table 2) and 61so values exceed + 10 where these have not been altered by reaction with meteoric waters. The high and variable Sr initials, and negative end initials (-- 4 to -- 8) of the Schwarzwald plutons are compatible with their derivation by melting of pre-Hercynian basement rocks that have had a long crustal residence history. In this sense, they are isotopically similar to S-types like those of the Lachlan Fold Belt (McCulloch and Chappell 1982) but stand in contrast to the S-types of the New England Batholith of eastern Austrailia (e.g. Hensel et al. 1985) which were derived from juvenile (i.e. short crustal residence times after derivation from the mantle) sources. (ii) In the Odenwald-Spessart, just north of the Schwarzwald, a number of small hornblene-bearing plutons of granodiorite to diorite composition associated with minor gabbros are found. In view of their metaluminous compositions, high CaO, low K 2 0 / N a 2 0 ratios, high Fe3+/Fe 2+ ratios and low Rb/Sr ratios (see Okrusch 1983; and Table 3), these plutons are considered to belong to the broad class of granitoids often termed I-type for which one possible mode of origin is derivation from lower crustal igneous source rocks (White and Chappell 1983; but see also Pitcher 1983). Unlike the Schwarzwald S-types, the isotopic data for these plutons are not compatible with a simple derivation by melting of pre-Hercynian high-grade basement rocks alone: a juvenile, possible mantle, component is indicated. The Sr initial ratios range from 0.706 to 0.708. eNd initials range from --6 to 0 i.e. between the regional base-

Table 2. Rb and Sr isotopic data for selected granitoids of the Schwarzwald and Harz Sample

Inferred Age (Ma)

Rb (ppm)

Sr (ppm)

87Rb S6Sr

S7Sr 86St (meas)

STSr 86Sr (T)

Triberg 1 Triberg 2 Oberkirch 1 Oberkirch 2 St. Blasien Seebach Albtal 1 Albtal 2 Blauen Malsberg Brocken Oker

315 315 330 330 320 290 330 330 320 320 290 290

344.6 295.6 223.7 198.6 199.9 315.9 201.8 196.3 248.4 245.7 271.9 264.2

17.4 262.9 164.6 262.4 416.1 68.7 345.8 416.4 273.5 274.8 50.0 41.5

58.85 3.26 3.94 2.19 1.39 13.40 1.69 1.36 2.63 2.59 15.85 18.55

0.9744 0.7251 0.7293 0.7197 0.7157 0.7743 0.7169 0.7154 0.7206 0.7205 0.7767 0.7889

0.7108 0.7105 0.7108 0.7094 0.7094 0.7190 0.7090 0.7090 0.7086 0.7087 0.7113 0.7123

134

Table 3. Nd and Sr isotopic data for the I-type plutons of Odenwald-Spessart Sample

Rb

Sr

Sm

(ppm) Odenwald Granodiorite Odenwald Diorite Spessart Diorite Spessart Granodiorite

109.2 50.4 93.5 118.8

Nd

8VRb S6Sr

1478m 144Nd

S7Sr ~ (0)

i43Nd SVSr 1,4Nd (0) ~ (T)*

aNd(T)*

41.51 14.59 79.88 29.89

0.41 0.57 0.24 1.32

0.0996 0.1442 0.1055 0.0941

0.7081 0.7100 0.7074 0.7143

0.512262 0.512500 0.512316 0.512133

-3.2 -0.5 -2.4 -5.5

(ppm) 768.1 254.3 1110.9 259.6

6.84 3.48 13.94 4.65

0.7062 0.7073 0.7063 0.7081

* Ages of plutons assumed to be 330 Ma (e.g. Kreuzer and Harre, 1975).

+2

9 Schwarzwatd

SpessorfOdenwatd (~ Harz Iv- - - -

_2

"10

z

L.LJ -I,

-r-t---g

-8

\\ i

0.70t~

.................

metiapetites I

0.708

I

I

0.712

I

I

0.716

87Sr/86SF(T) Fig. 3, Plot of initial SVSr/S6Sr vs initial end values for Hercynian granitoids of West Germany

ment values and positive values that would identify a purely mantle-derived end-member (Fig. 3). These results suggest that the I-types probably represent mixes of mantle and crustal components. Additionally, the intermediate Sr and negative end initials are highly suggestive of a mature continent-margin I-type plutonic setting (cf. Sierra Nevada Batholith in DePaolo 1981). (iii) In the Harz of northern Germany, small post-tectonic plutons are found with ages of ca. 290 Ma (Mecklenburg et al. 1985). These are younger than most of the plutons of the Sehwarzwald and Odenwald-Spessart that have ages of 310-330 Ma. The peraluminous, two-mica Oker Granite can be recognised as S-type. The main intrusive body consists of a mildly peraluminous, highly silicic, pink-feldspar granite called the Brocken Massif. We suggest that this body be considered distinct from the older 'orogenic' intrusions discussed earlier. It may represent an example of an 'A-type granite' in the sense of Collins et al. (1982) in view of the 'somewhat Alkaline, Anorogenic and Anhydrous' characteristics it shares with granites of this class. Both the Oker and Brocken plutons show crustal Nd and Sr isotopic values. Initial eNd values are --6.2 and --5.1 and Sr initial ratios are 0.712 and 0.711 respectively. It is our contention, based on the nature of the Hercynian felsic plutons of the Schwarzwald and Spessart-Odenwald complexes, that there is a north-south change from

I-type in the Saxothuringian zone to mainly S-type in the Moldanubian zone in the West German segment of the fold belt. This does not appear to be a local feature - the pattern can also be inferred from previous studies of other central European massifs. For example, plutons of the northern Vosges (Fluck 1980; Fluck et al. 1980) consist of 'calc-alkaline' diorite to granodiorite suites that we consider equivalent to the I-type granitoids of the Spessart-Odenwald. In the central and southern Vosges, two-mica and cordierite-bearing units with high Sr initial ratios (Brewer and Lippolt 1974) are associated with I-type units as well as an actinolite-bearing 'des Cretes' series which has Sr and Nd initials that permit an entirely crustal origin. In the Czechoslovokian part of the Bohemian Massif, Van Breemen et al. (1982) described a northern belt of diorite-tonalite-granodiorite plutons (often hornblende-bearing, and inferred to be I-types) and a southern group of two-mica plutons that are very likely S-type. Lastly, recent and continuing work on the plutons of the southern Bohemian Massif in Austria (Finger and H t c k 1985; Liew, Finger and H t c k , in prep.) further demonstrates the importance of S-type granitoids in the Moldanubian zone. Certainly the negative eNd values of the granitoids in the southwestern portion of the Bohemian Massif (samples 34-38, Table 1) attest to their S-type origin. These observations suggest that the occurrence of Saxothuringian I-types grading southwards to Moldanubian S-types can be followed across eastern central Europe. The significance of this will be discussed further in the Synthesis section. France

There are many Nd data avalaible from the published literature for rocks from France. The relevant works are those of Michard et al. (1985) on French sediments, BernardGriffiths et al. (1985) on the leucogranites of Brittany, Ben Othman et al. (1984) on granitoids and basement rocks of the Pyrenees, and Downes and Leyreloup (1986) on deep crustal xenoliths of the Massif Central. The French data are consistent with the ubiquity of rocks in central Europe having crustal residence ages of 1.4-1.7 Ga, except for the Brittany area where some older ages of ~ 2 Ga were found. An important constraint on probable basement signatures may be inferred from the sedimentary data of Michard et al. (1985). Although their results indicated juvenile crustal additions during the Hercynian, Caledonian and Cadomian orogenies, such mantle Nd contributions were rapidly swamped by the pre-existing crustal Nd components because inter-orogenic sediments evidently reverted to the preexisting 'equilibrium' signatures. This is most simply interpreted to indicate that juvenile crustal additions during the Phanerozoic have not drastically affected the 'regional equi-

J35 +i,

Hercynian

0 Mefamorphic rocks o

~ , .

A

+ 0oo,o0,

(I

[] Sediments

]

I

~ateaonian

\

/

I

\la

/l_....~

_t+ ~,Be

-8

I

~

r .......

I, -------~----"'--I

' I l ~ C-I-O,~ m , ' +.b,__

_-----

- - - ~ -

-,

L._I'

I

I

_~ - C

L__LJ____L__,

I~'--' c~

~

r-l.___.T-)

u r---l;::l.---- i

~

I

~

'

......

_P_L,

O

GI

O ~ O

. . . . . . . . . . .

', PY !'

v

z LID

~,

O

-I~

-16

i -20!

J

~o_

-2t,.

160

260

360

~00

560

AGE(Mo) Fig. 4. Plot of ~Nd(T) VS geological age displaying data from this study and those of Michard et al. (1985), Bernard-Griffiths et al. (1985) (granitoids of Armorican Massif, AM), Davies et al. (1985) (granites of Cornwall, CW), Taylor et al. (1983), Stosch & Lugmair (1984) and Downes & Leyreloup (1986) (metasedimentary xeno]ith population marked MC). Fields for Lorraine (LB), Brittany (BR) and Pyrenees (PY) denote sediment data with the oldest crustal residence ages inferred form Michard et al. (1985). These ages are

inferred to be the 'equilibrium basement signatures' of these regions i.e. the model ages are least affected by juvenile Palaeozoic additions. Broad arrows indicate deviations from equilibrium values during the Hercynian, Caledonian and Cadomian orogenies when new crust additions occurred. Reference TDMage evolution lines for 1.4 Ga, a.7 Ga, 2 Ga, 2.5 Ga and 3 Ga average continental crust were calculated assuming a typical crustal 1gram/ 144Nd ratio = 0.12.

librium signatures' and these then reflect the average regional basement values. Synthesis

a) Crustal residence ages of the central European crust

The end isotopic data for eastern central Europe summarised in the histogram of Fig. 2 show a main population with values of - 6 - - 1 2 (model ages are 1.4-1.7 Ga), a secondary population with values between - 1 2 - - 1 6 (model ages of 1.7-2.0 Ga) and outliers with values of - 18, - 2 4 and - 3 0 (model ages are 2.1 Ga, 2.6 Ga and 3.0 Ga). At face value, our Nd data and those of previous studies would exclude the case that Hercynian Europe represents recycled Archean crust or that it is made up entirely of new crust formed during the Hercynian and other Palaeozoic orogenies. The data are broadly consistent with deductions made on the basis of inherited zircon and Pb-Pb data that central Europe has had an old Precambrian history but the Nd model ages are, for the main part, largely younger than ages indicated by extrapolation of discordant U-Pb zircon arrays ( ~ 2 Ga, but with large errors) (Gebauer and Grfinenfelder J977), Pb-Pb arrays ( > 3 Ga) (Vitrac et al. 1981) and modelled U-Pb evolution (2.1-2.7 Ga) (Kober and Lippolt 1985). Although an old Precambrian ancestry for the central European crust seems difficult to refute, interpretation of these ages are to various degress, model dependent. Depending on how the few samples with model ages of 2-3 Ga are interpreted, an Archean contribution albeit a minor one is allowed by the Nd data. Unlike the Icart

Gneiss and granulites of northern Spain, the old Nd model ages do not identify previously unrecognised inliers or tectonic slices of old basement. The model ages are much older than the Palaeozoic ages of metamorphism. If these model ages are the result of a mixture of source components, as we think most likely, then they only provide a minimum estimate for the model age of the oldest component rather than dating a crust addition event. Model ages of 2-3 Ga would imply relatively large amounts of an Archean component. More precise estimates of the ages of such Archean components are better made by ion microprobe U-Pb data for individual detrital zircon grains rather than Nd model ages. Archean zircon ages of ~ 2.5-2.7 Ga have in fact been reported by Grfinenfelder (J984), Gebauer (1986) and Kr6ner et al. (in prep.) for gneisses and granulites from the central Alps and the Bohemian Massif. Variable contributions from such late Archean components can readily explain the 2-3 Ga Nd model ages but we cannot rule out the possibility that the protoliths for samples 15 and 22 retained unusually low 147Sm/144Nd ratios for extended periods of their crustal history. If this is the case, and if an extreme ratio of 0.08 is used to calculate model ages instead of a typical crustal 147Sm/144Nd ratio of 0.12 (see Table l), the model ages decrease to 1.9 Ga (from 2.5 Ga for 15) and 2.2 Ga (from 3.0 Ga for 22). This then places samples in the same ~ 2 Ga group as 2, J0, 12, 19 and some French sediments. The role of an Archean source is then downgraded and the importance of ~ 2 Ga crust, as exposed near Brittany and northern Spain and also detected by ion microprobe and conventional U-Pb zircon data, is increased. Samples with model ages older than 1.8 Ga are apparently restricted to the southern Moldanu-

136 100 ~ _

2a

.L..I

\\,,, 2-5

2'.0

.rc

eon,

1'.5

Age of 'otd component' (6a)

Fig. 5. Results of 2-component (2a, 2b) and 3-component (3a, 3b) Nd mixing models (e.g. Patchett and Bridgwater 1984). Curve 2a shows the amount of 0.5 Ga depleted mantle-derived arc-type components (e~d= + 6, 10 ppm Nd) required to mix with an 'old component' which is typical upper crust of Archean-mid Proterozoic age (eNd at 500 M a = - - 8 to --/8, 30 ppm Nd) to obtain a model age of 1.6 Ga. For curve 2b, the same calculations were repeated using 15 ppm for the arc component and 25 ppm for typical upper crust. Curves 3a, 3b are the results for 3-component mixtures: a arc-type component with 10 ppm Nd, an Archean upper crustal component with 25 ppm Nd and Proterozoic upper crust (25 ppm Nd) whose average age is allowed to vary between 1.6 Ga and 2 Ga. Curves 3a and 3b assume the presence of 10% and 5% of a 2.5 Ga upper crustal component respectively bian and Moravo-Silesian zones. These zones a p p e a r to have preferentially i n c o r p o r a t e d larger a m o u n t s o f old components. The simplest way to explain this is to have the old c o m p o n e n t s recycled from a former southern cratonic source o f A r c h e a n - early Proterozoic age. Mixing between the old crustal c o m p o n e n t s discussed above and early Palaeozoic additions from the mantle can theoretically explain the m a i n 1.4-1.7 G a model age p o p u lation. The large n u m b e r o f such ages does not prove an i m p o r t a n t mid-Proterozoic juvenile addition event. Results o f simple t w o - c o m p o n e n t N d mixing calculations between an 'old c o m p o n e n t ' with average u p p e r crustal characteristics and a 0.5 G a mantle-derived m a g m a are shown in Fig. 5 (solid curves 2a, 2b). F o r the case involving an 'old c o m p o nent' with an age o f 2.5 Ga, 70-80% o f Palaeozoic juvenile contributions are required to obtain a mixture with a 1.6 G a model age. Such large a m o u n t s o f new Palaeozoic crust are not c o m p a t i b l e with the geological record: field evidence for widespread basalt-andesite volcanic strata and voluminous I-type batholiths are lacking. W e have also pointed out earlier that the absence of large shifts in the basement N d signatures implied by the d a t a o f M i c h a r d et al. (1985) for F r e n c h sediments is not consistent with very large a m o u n t s o f new Palaeozoic crust. The arguments against a binary mixing model suggest that the 1.4-1.7 G a model ages m a y in fact d o c u m e n t juvenile mid-Proterozoic components. Recent ion m i c r o p r o b e d a t a certainly allows this interpretation: G e b a u e r (1986) and K r 6 n e r (pers. comm.) reported n e a r - c o n c o r d a n t zircon ages o f 1.1 G a and 1.9 G a a n d 1.1-1.5 G a respectively. W e have a t t e m p t e d to quantify such additions in Fig. 5 by using simple three-component mixing calculations with endmembers having 2.5 Ga, 0.5 G a and 1.5-2.0 G a model ages,

assuming that A r c h e a n contributions are m i n o r (less than 10%). Curves 3a and 3b depict how the quantity o f Palaeozoic crust required to obtain a mixture with a 1.6 G a model age varies with the average of the Proterozoic component. The calculations show that mixtures can contain large amounts (20-60%) o f new Palaeozoic contributions, yet still retain relatively old 1.6 G a model ages. In terms o f the a m o u n t s o f a mid-Proterozoic component, the model allows for a range o f 30-75%. We consider 50-75% to be a m o r e reasonable estimate in view o f the French sediment N d data, the geological record and the scarcity o f detrital zircon d a t a that m a y indicate new Palaeozoic crust as the d o m i n a n t c o m p o n e n t o f the central E u r o p e a n crust. A l t h o u g h we do not wish to attach excessive importance to the actual numbers, it appears that mid-Proterozoic components d o m i n a t e or comprise m a j o r p r o p o r t i o n s o f the continental crust o f Hercynian Europe. However, important Palaeozoic juvenile additions and m i n o r A r c h e a n contributions are also indicated.

b) Hercynian granitoid types G r a n i t o i d s ranging in age from 300-330 M a and found along a transect across the Hercynian F o l d Belt in West G e r m a n y display a transition from I-type in the Saxothuringian zone to S-type in the M o l d a n u b i a n zone. The Spessart-Odenwald plutons are suggested to represent continent margin I-type plutons. The Schwarzwald S-types a p p e a r to be analogous to inner continent S-types (see review o f distribution o f granite types by Pitcher 1983). The j u x t a p o sition o f a Saxothuringian I-type belt and a southern Mold a n u b i a n S-type belt can be inferred along the entire length o f eastern central Europe.This deduction, if correct, is consistent with (but does not prove) a tectonic configuration involving southward-dipping subduction from a Rhenohercynian trench system. In addition, the paired development o f coeval continent margin I-type and inner continent Stype belts over distances exceeding 500 k m appears to be restricted to the margins o f large crustal plates, as is best exemplified by Circum-Pacific batholiths. This is likely to be the case o f the Hercynian granitoids also: in terms o f tectonic enviromnent, we would b r o a d l y correlate the M o P d a n u b i a n S-types with the Inner Cordilleran S-types o f Miller and Bradfish (1980), and the Saxothuringian I-types are b r o a d l y analogous to the Sierra N e v a d a Batholith (DePaolo 1981). By emphasising the c o m p a r a b l e petrological similiarities o f the Hercynian and Cordilleran granitoids in this manner, we have d o w n g r a d e d the i m p o r t a n c e o f one feature i.e. the absence o f voluminous, linear I-type batholiths parallel to the trench that seems such a conspicuous characteristic o f the Mesozoic plutonism o f N o r t h and South America. W e suspect this is not critical for our interpretation. I f we are correct, it would imply that the eastern half o f the Hercynian fold belt represents a detached orogen, and a former craton is indicated south o f the fold belt rather than a p a l a e o T e t h y a n ocean. A crude analogue m a y be the Japanese islands, which were attached to the Asian m a i n l a n d prior to the opening o f the Sea o f Japan.

Acknowledgements. We are very grateful to past and continuing workers of the Hercynian Fold Belt who have kindly donated samples for this study, especially R. Emmermann, O. van Breemen, P. Bliimel, A. Kr6ner, P.J. Patchett, H. Schleicher and E. Rettmann. A. Kr6ner, W. Todt and C. Chauvel are thanked for useful discussion and providing useful literature.

137 References All+gre CJ, Ben Othman D (1980) Nd-Sr isotopic relationship in granitoid rocks and continental crust development: a chemical approach to orogenesis. Nature 286:335-342 Anderson TA (1975) Carboniferous subduction complex in the Harz Mountains, Germany. Bull Geol Soc Am 86:77 82 Badham JPN (1982) Strike-slip orogens - an explanation for the Hercynides. J Geol Soc London 139:493 504 Behr HJ, Engel W, Franke W, Giese P, Weber K (1984) The Variscan Belt in central Europe: Main structures, geodynamic implications, open questions. Tectonophysics 109:15-40 Ben Othmann D, Fourcade S, All6gre CJ (1984) Recycling processes in granite-granodiorite complex genesis: the Querigut case studied by Nd-Sr isotope systematics. Earth Planet Sci Lett 69 : 290-300 Bernard-Griffith J, Peucat JJ, Sheppart S, Vidal Ph (1985) Petrogenesis of Hercynian leucogranites from the Southern Armorican Massif: Contribution of REE and isotopic (Sr, Nd, Pb and O) geochemical data to the study of source rock characteristics and ages. Earth Planet Sci Lett 74:235-250 Brewer MS, Lippolt HJ (1974) Petrogenesis of basement rocks of the Upper Rhine region elucidated by Rb-Sr systematics. Contrib Mineral Petrol 45:125-141 Collins WJ, Beams SD, White AJR, Chappell BW (1982) Nature and origin of A-type granites with particular reference to southeast Australia. Contrib Mineral Petrol 80:189-200 Davies G, Gledhill A, Hawkesworth C (1985) Nd and Sr isotope studies bearing on the crustal evolution of southern Britain. Earth Planet Sci Lett 75:1-12 DePaolo DJ (1981) A N D and Sr isotopic study of the Mesozoic calc-alkaline batholiths of the Sierra Nevada and Peninsular Ranges, California. J Geophys Res 86:10470-10488 Downes H, Leyreloup A (1986) Granulitic xenoliths from the French Massif Central - petrology, Sr and Nd isotopic systematics and model age estimates. In: The Nature of the Lower Continental Crust. (Eds: Dawson JB, Carswell DA, Hall J, Wedepohl KH). Geol Soc Lond Spec Pub 24:319-330 Emmermann R (1977) A petrogenetic model for the origin and evolution of the Hercynian granite series of the Schwarzwald. N Jb Mineral Abh 128:219 253 Finger F, H6ck V (1985) Zur Geochemie von S-typ Granitoiden aus dem westlichen ober6sterreichischen Moldanubikum. Fortschr Mineral 63 : 280 Floyd PA (1982) Chemical variation in Hercynian basalts relative to plate tectonics. J Geol Soc London 139:505-520 Fluck P (1980) Les granitoides des Vosges. In: Autran A, Dercourt J (eds) Evolutions g6ologiques de la France. M6moire du B R G M 107:83-86 Fluck P, Maass R, von Raumer JF (1980) The Variscan Units east and west of the Rhine graben. Colloque C6: Geology of Europe, from Precambrian to post-Hercynian sedimentary basins. M~moire du B R G M 108:112-131 Gebauer D, Grfinenfelder M (1977) U-Pb systematics of detrital zircons from some unmetamorphosed to slightly metamorphosed sediments of central Europe. Contrib Mineral Petrol 65 : 29-37 Gebauer D (1986) The development of the continental crust of the European Hercynides since the Archean based on radiomettic data. Proceedings of the 3ra workshop on the European Geotraverse Project. 15-23 Grauert B, H/inny R, Soptrijanova G (1974) Geochronology of a polymetamorphic and anatectic gneiss region: The Moldanubicum of the area Lam-Deggendorf, E. Bavaria, Germany. Contrib Mineral Petrol 45 : 37 63 Grfinenfelder MH; Williams IS, Compston W (1984) Use of the ion-microprobe in deciphering complex U-Th-Pb systems in zircons from the pre-Alpine basement, Switzerland. Terra Cognita Spec. Vol ECOG VIII:26-27 Guerrot C, Peucat JJ, Capdevila R (1987) The oldest granulitic crust involved in the Hercynian Belt: Preliminary U-Pb and

Sm-Nd data. Abst Vol E U G IV Strasbourg, Terra Cognita 7:159 Hensel HD, McCulloch MT, Chappell, BW (1985) The New England Batholith: constraints on its derivation from Nd and Sr isotopic studies of granitoids and country rocks. Geochim Cosmochim Acta 49: 369-384 Hoefs J, Emmermann R (1983) The oxygen isotope composition of Hercynian granites and pre-Hercynian gneisses from the Schwarzwald, SW Germany. Contrib Mineral Petrol 83 : 320-329 Hofmann AW (1979) Geochronology of the crystalline rocks of the Schwarzwald. In: J/iger E, Hunziker JC (eds) Lectures in Isotope Geology. Springer Berlin Heidelberg New York, pp 215-221 Kober B, Lippolt HJ (1985) Pre-Hercynian Pb transfer to basement rocks as indicated by Pb isotopes of the Schwarzwald crystalline, SW Germany, I + II. Contrib Mineral Petrol 90:162-178 K6ppel V, Gfinthert A, Grfinenfelder M (1980) Patterns of U-Pb zircon and monazite ages in polymetamorphic units of the Swiss Alps. Schweiz Mineral Petrogr Mitt 61:97 119. Kreuzer H, Harre W (1975) K/Ar Altersbestimmungen an Hornblenden und Biotiten des kristallinen Odenwaldes. In: Amstutz GC, Meisl S, Nickel E (eds) Mineralien und Gesteine im Odenwald. AufschluB, Sonderband 27 (Odenwald), Heidelberg Liew TC, McCulloch (1985) Genesis of granitoid batholiths of Peninsular from a Nd-Sr isotopic and U-Pb zircon study. Geochim Cosmochim Acta 49 : 587-600 Lorenz V, Nicholls IA (1984) Plate and intraplate processes of Hercynian Europe during the late Palaeozoic. Tectonophysics 107 : 25-56 McCulloch MT, Chappell, BW (1982) Nd isotopic characteristics of S- and I-type granites. Earth Planet Sci Lett 58:51-64 McCulloch MT, Wasserburg GJ (1978) Sm-Nd and Rb-Sr chronology of continental crust formation. Science 200 : 1003-1011 Mecklenburg S, Vinx R, Baumann A (1985) Altersbestimmungen und Isotopenuntersuchungen an Harzer Magmatiten. Fortschr Mineral [Beih 1] 63:287 Michard A, Gurriet P, Soudant M, Albar6de F, (1985) Nd isotopes in French Phanerozoic shales: external vs internal aspects of crustal evolution. Geochim Cosmochim Acta 49:601-610 Miller CF, Bradfish LJ (1980) An inner Cordilleran belt of muscovite-bearing plutons. Geology 8 : 412-416 Okrusch M (1983) The Spessart Crystalline Complex, northwest Bavaria (Excursion Guida). Fortschr Mineral [Beihe2] 61 : 135-169 O'Nions RK, Hamilton PJ, Hooker, PJ (1983) A Nd isotope investigation of sediments related to crustal development in the British Isles. Earth Planet Sci Lett 63:229-240 Patchett PJ, Bridgwater D (1984) Origin of continental crust of 1.9-1.7 Ga age defined by Nd isotopes in the Ketilidian terrain of south Greenland. Contrib Mineral Petrol 87:311-318 Pitcher WS (1982) Granite type and tectonic environment. In: Hsu K (ed) Mountain Building Processes. Academic Press, London, pp 19-40 Richard P, Shimizu N, All~gre CJ (1976) 143Nd/146Nd, a natural tracer: An application to oceanic basalts. Earth Planet Sci Lett 31 : 269 278 Schleicher H (1978) Petrologie der Granitoporphyre des Schwarzwaldes. N Jb Miner Abh 132(2): 153 181 Stosch HG, Lugmair GW (1984) Evolution of the lower continental crust: granulite facies xenoliths from the Eifel, West Germany. Nature 311:368 370 Taylor SR, McLennan SM, McCulloch MT (1983) Geochemistry of loess, continental crustal composition and crustal model ages. Geochim Cosmochim Acta 47:1897-1905 Todt WA, Bfisch W (1981) U-Pb investigations on zircons from pre-Variscan gneisses-I: A study from the Schwarzwald, West Germany. Geochim Cosmochim Acta 45 : 1789-1801 Van Breemen O, Aftalion M, Bowes DR, Dudek A, Misar Z, Povondra P, Vrfina S (1982) Geochronological studies of the Bohemian massif, Czechoslovakia, and their significance in the

138 evolution of central Europe. Trans Roy Soc Edinburgh: Earth Sc 73:89-108 Vitrac AM, Albar+de F, All6gre CJ (1981) Lead isotope compositions of Hercynian granitic K-felspars constrain continental genesis. Nature 291:460464 Wedepohl KH, Delevaux MH, Doe B (1978) The potential source of Pb in the Permian Kupferschiefer Bed of Europe and some selected Palaeozoic mineral deposits in the Federal Republic of Germany. Contrib Mineral Petrol 65 : 273-281 White AJR, Chappell BW (1983) Granitoid types and their distri-

bution in the Lachlan Fold Belt, southeastern Australia. Geol Soc Am Mem 159:21-34 Windley BR (1977) The evolving continents. John Wiley & Sons, London pp 193-201 Ziegler PA (/984) Caledonian and Hercynian crustal consolidation of the European Alpine foreland. Terra Cognita 4:51-58 Received August 19, 1987 / Accepted November 25, 1987 Editorial responsibility: J. Hoefs