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Ti-rich granodiorite porphyries from the northeastern margin of the Klenov massif (Moldanubian Zone of the Bohemian Massif).

INTRODUCTION

Intrusions of intermediate to basic porphyries and lamprophyric dyke swarms are common throughout the whole Moldanubian Zone of the Bohemian Massif. Emplacement and evolution of intermediate to basic dyke rocks and lamprophyres in this part of the Bohemian Massif were characterised by Nemec (1970, 1974), Vrana et al. (1993) and Kosler et al. (2001). Most of intermediate to basic and lamprophyric dyke rocks of the Moldanubian Zone have calc-alkaline character and were considered to be a product of partial melting of metasediments of upper continental crust (Vrana et al. 1993) or a product of partial melting of lower crustal granulites (Malecha and Suk 1997). Various bodies of K-rich to ultra-potassic magmatic rocks in the Moldanubian Zone of the Bohemian Massif (Holub 1997, Gerdes et al. 2000a) suggest small degrees of partial melting of a hydrated phlogopite-bearing lithospheric mantle.

The objective of the present study is to evaluate the modal and geochemical characteristics of the Ti-rich granodiorite porphyries from the northeastern margin of the Klenov massif. This study is complemented by their comparison with other occurrences of intermediate to basic dyke rocks in the Moldanubian Zone for which major and some trace element determinations are available (Nemec 1970, 1973, 1974, Vrana et al. 1993, Kosler et al. 2001).

GEOLOGICAL SETTING

Ti-rich granodiorite porphyries from the northeastern margin of the Klenov massif were originally described by Kratochvil and Konta (1951). The Klenov granite massif is the greatest subsidiary magmatic body of the Central Moldanubian pluton (Fig. 1). Granodiorite porphyries occur in en-echelon, NE-SW-striking dykes filling faults oblique to younger, NNE-SSW-striking shear structures filled by hydrothermal uranium mineralization (Rene et al. 1999). Fault structures used by intermediate dyke rocks for their emplacement are parallel to the fold axes of metamorphic complexes in this part of the Moldanubian Zone. Granodiorite porphyries penetrate older dykes of the two-mica Destna granite of the Klenov massif in several cases. Higher age of the granodiorite porphyries than that of the NNE-SSW-striking shear structures is confirmed by horizontal displacement of a dyke of granodiorite porphyry recorded on a NNE-SSW-striking shear zone in the uranium deposit of Okrouhla Radoun filled with uranium mineralization. This displacement is accompanied by hydrothermal alteration of granodiorite porphyries (chloritization and albitization).

SAMPLING AND ANALYTICAL TECHNIQUES

Samples of the granodiorite porphyries were taken from the northeastern and eastern margin of the Klenov massif. The uranium deposit of Okrouhla Radoun on the northeastern margin of this granite body provided the best opportunity for the study of relations between two-mica granites of the Moldanubian batholith and various dyke rocks. Mine workings at this deposit enabled geological investigations and sampling to the depth of 600 meters. Other samples of granodiorite porphyries were taken from exploratory boreholes and an abandoned granite quarry, southern of the Divci Kopy village, in which original samples of these granodiorite porphyries were acquired by Kratochvil and Konta (1951).

Major element contents were obtained by classical wet chemical methods in the laboratory of Institute of Rock Structure and Mechanics of the AS CR (Table 1). The Si[O.sub.2] and [H.sub.2][O.sup.+] contents were determined by gravimetrically method. The contents of [Al.sub.2][O.sub.3] and FeO were determined by titrimetric method. The [Fe.sub.2][O.sub.3], Ti[O.sub.2] and [P.sub.2][O.sub.5] contents were determined by spectrophotometry method, the MnO, CaO, MgO, [K.sub.2]O and [Na.sub.2]O contents were determined by AAS. The most of the trace elements were determined by standard XRF analyses, using PW 1400 spectrometer in the laboratory of UNIGEO Brno. Uranium and thorium contents were determined by gamma-ray spectrometry using NT-512 multi-channel spectrometer at Geofyzika Enterprise, Brno. Determinations of REE and selected trace elements (Cs, Ta, Hf) were made by ICP-MS method at Analytika Ltd., Prague. Precision of all analytical methods was tested by duplicate analyses.

[FIGURE 1 OMITTED]

Analyses of various rock-forming minerals were made by microprobe Jeol JXA-50 A with EDAX PV 9400 system (accelerating voltage 15-20 kV, spot diameter 1-2 microns, beam current 25 nA) and by microprobe CamScan 4DV equipped with LINK AN 10000 and Microspec 2A (accelerating voltage 20 kV, spot diameter 2 microns, beam current 50 nA). All microprobe analytical data were corrected using the ZAF method. Calculations and presentations of the analytical results were aided by the Minpet software.

PETROGRAPHY

The examined granodiorite porphyries are porphyric, black to grey-black rocks with fine-grained texture of the groundmass. The groundmass often contains plagioclase phenocrysts, several millimetres, sometimes up to 1 cm in size. The phenocrysts are often zoned, with a basic core ([An.sub.54]) and acid rim ([An.sub.14]). The groundmass is predominantly formed by plagioclase ([An.sub.25-50]), subordinate amount of K-feld-spar and quartz. Dark minerals, often chloritized, are represented by pyroxene (Fe-augite) and biotite Fe/(Fe + Mg) = 0.66 - 0.72. Amphibole (cummingtonite) occurs rarely. Accessory minerals are sometimes very abundant ilmenite, less abundantly by apatite, rutile and zircon and very rare allanite. Sphene also rarely occurs and sometimes replaces ilmenite. Texture of the groundmass is most often ophitic. Examined granodiorite porphyries were also characterized by content of normative minerals (CIPW norm). Compared to other granodiorite, tonalite and diorite porphyries of the Moldanubian Zone of the Bohemian Massif (Nemec 1970, 1974), granodiorite porphyries from the eastern margin of the Klenov massif have lower content of normative quartz (8.5-10.6 %), higher content of normative hypersthene (8.1-16.6%) and complete absence of normative corundum. A typical feature of the examined granodiorite porphyries is also the high content of normative apatite (0.2-1.3%) and ilmenite (0.4-3.8%) (Fig. 2.).

GEOCHEMISTRY

Granodiorite porphyries from the eastern margin of the Klenov massif have a metaaluminous character with value of aluminium saturation index (mol. [Al.sub.2][O.sub.3]/CaO+[Na.sub.2]O+[K.sub.2]O) of 0.66 to 0.87. This value attributed the examined granodiorite porphyries to the group of I-type granitoids (Chappel and White 1974). The AFM diagram (Fig. 3) shows significant difference in the content of FeOt between the examined granodiorite porphyries and other intermediate to basic dyke rocks of the Moldanubian Zone of the Bohemian Massif. For granodiorite porphyries from the margin of the Klenov massif is significant high content of Ti[O.sub.2] (1.85-1.95 wt.%) and relatively low mg-values (MgO/(MgO+FeO)x100 mol.; Fig. 4).

Representative incompatible trace element concentrations normalized to primitive mantle values are plotted in Fig. 5. The contents of Rb, Ba, Th and U are significantly enriched compared to those in primitive mantle. A depletion in Nb, Ta and Sr was observed compared to their amounts in primitive upper mantle. The examined porphyries typically show significantly low contents of Ni and Cr, but still a strong positive correlation between the two elements. Some enrichment compared to primitive mantle occur by Nb, Sr, Zr and Y.

The examined granodiorite porphyries are characterised by negative europium anomaly (Eu/[Eu.sup.*] 0.36-0.79) and relatively high LREE/HREE ratio ([La.sub.N]/[Yb.sub.N] 4.9-8.9; Fig. 6). The negative Eu anomaly is very probably controlled by fractionation of plagioclase in the original melt of these rocks. Granodiorite porphyries from the margin of the Klenov massif show significantly higher contents of HREE than the related dyke rocks from margin of the Sevetin massif, analysed by Vrana et al. (1993).

[FIGURE 2 OMITTED]

[FIGURE 3 OMITTED]

[FIGURE 4 OMITTED]

[FIGURE 5 OMITTED]

[FIGURE 6 OMITTED]

COMPOSITION OF MINERALS

Results of microprobe analyses of some rock-forming and accessory minerals are listed in Table 2. The examined granodiorite porphyries are characterized by large plagioclase phenocrysts, sometimes up to 1 cm in size. Some larger plagioclase phenocrysts form older tables with very thin reactions borders. These borders and oval corners of some phenocrysts suggest possible melting of the latter during crystallisation of fine-grained matrix. In some cases, reactions zones are developed on borders between plagioclase phenocrysts and groundmass, with small grains of quartz in rim zone of plagioclase phenocrysts. K-feldspar in the groundmass is orthoclase with 93-99% orthoclase molecule. Microprobe analyses of pyroxenes document a composition of augite according to IMA classification (Morimoto 1988). The relatively rare sphene shows a higher content of Al in comparison with sphene from magmatic rocks (see Ribbe 1982).

DISCUSSION

Time constraints on intermediate to basic dyke emplacement

The emplacement of granodiorite porphyry dykes occurred after the solidification of granites of the Klenov massif, including the youngest dykes of two-mica granites and aplites. In view of the fact that the contact of granodiorite porphyry dykes with the surrounding metamorphic and granitic rocks is always sharp, it is possible to suppose that a longer time interval elapsed between the emplacement of granitic rocks of Klenov granite body and basic dyke rocks. On the other hand, upper limit of the time interval for emplacement of investigated dykes is defined by the origin of uranium hydrothermal mineralization, which caused chloritization and albitization of granodiorite porphyries. The crystallization of melt of investigated dyke rocks was not in one stage. The relatively complicated origin of dykes can be documented by remelting of marginal parts of some plagioclase phenocrysts. Plagioclase phenocrysts formed in the early phase of magma evolution were later broken or partial-remelted on their margins during crystallization of groundmass melt.

In view of this geologic and tectonic dating of examined porphyries, a few more precious radiometric datings of basic dyke rocks from area of the Moldanubian Zone have been available. The isotopic age of the related basic dykes on margin of the Sevetin granite massif has been broadly constrained by K-Ar dating of bulk-rock samples, which yielded ages of 263 and 222 Ma, respectively (Vrana et al. 1993). A more precise Ar/Ar dating for primary titanian pargasite of 270 [+ or -] 2 Ma of the same rocks was interpreted as the minimum emplacement age of these dyke rocks by Kosler et al. (2001).

This time constraint is also supported by the determination of the age of lateral movements on the most significant NNE-SSW-striking shear zone (Rodl Zone, 281 [+ or -] 0.6 Ma; Brandmayr et al. 1995). The other constraint for the age of the emplacement of granodiorite porphyries on the northeastern margin of the Klenov massif is the age of uranium mineralization (U/Pb, 255 [+ or -] 3 Ma; Anderson et al. 1989) of the Okrouhla Radoun uranium deposit. As it was mentioned above, some of the dykes of granodiorite porphyries are altered by this uranium mineralization. The above mentioned ages give probably time constraint for the emplacement of investigated dykes--250-280 Ma.

Potential magma sources

The importance of the role of late Variscan low-pressure high-temperature regional metamorphism in crustal melting and granite magma generation of extensive granite batholiths in the Moldanubian Zone of the Bohemian Massif is widely accepted (Buttner and Kruhl 1997, Kalt et al. 1999). The older models of evolution of the Moldanubian batholith presumed gradual fractionated crystallisation of granite melt and splitting of basic dyke rocks in consequence of this fractionation (Fuchs and Thiele 1968). Based on a detailed study of modal and major element compositions in syenite-, diorite-, granodiorite porphyries and lamprophyres of the Moldanubian Zone of the Bohemian Massif, Nemec (1970, 1972, 1975, 1977) preferred the origin of the above mentioned intermediate to basic dyke rocks from contaminated tholeiitic magma.

Vrana et al. (1993) supposed the origin of pyroxene microgranodiorite dykes evolved in the Sevetin impact structure by impact melting of regionally metamorphosed metapelites of the Moldanubian Zone. On the other hand, Malecha and Suk (1997) preferred the origin of this rock by anatectic melting of granulites. According to Gerdes et al. (2000b), lamprophyre magma in the Moldanubian Zone was separated from a hydrated lithospheric mantle as a result of thickening of the crust and of intensive melting of subducted deeper parts of lithospheric crust and rocks at the crust/mantle boundary. Gerdes et al. (2000a) supposed that mafic rocks of K-rich granitoid group and lamprophyre group of the Moldanubian Zone suggest a heterogeneous lithospheric mantle source, which was possibly contaminated by crustal components during pre-Variscan subduction.

The examined granodiorite porphyries from the margin of the Klenov massif show significantly low mg-number, together with relatively high content of titanium. Other significant feature of geochemical composition of these mafic dyke rocks are their enrichment by Rb, Ba, Th and U in relation to the composition of primitive mantle (Fig. 5). The enrichment in Rb, Ba, Th in relation to primitive mantle abundances of these elements is usually regarded as presence of mantle metasomatism (Stille et al. 1989) or evidence of mixing of crustal rocks with mantle melt (White and Dupre 1986, Hegner et al. 1998). Mafic dyke rocks could more plausibly have been generated through partial melting of an enriched portion of mantle (Turpin et al. 1988). Similar origin of mafic dyke rocks was proposed for calc-alkaline lamprophyres of the Central Swiss Alps by Oberhansli et al. (1991).

CONCLUSION

Dykes of granodiorite porphyries from the margin of the Klenov massif may by considered evidence for the communication of the upper crust in the Moldanubian Zone with the upper mantle. The emplacement of basic dykes occurred after solidification of post-collisional granites of the Moldanubian batholith, but before the origin of Early Permian uranium mineralization. The examined granodiorite porphyries are characterised by a high FeO/MgO ratio, relatively low mg-numbers and high Ti contents. The distribution of trace elements suggests origin of granodiorite magma from enriched lithospheric mantle.

ACKNOWLEDGEMENTS

The present paper was completed thanks to financial support by the Grant Agency of the Czech Republic (Project No. 205/97/0514). D. Matejka, F. Finger and A. Dudek are thanked for their suggestions to the matter of this paper. I also thank P. Sulovsky and V. Srein for microprobe analyses of minerals. The two anonymous reviewers are acknowledged for some comments, which considerably improved the manuscript. Author would also like to thank J. Adamovic for improving the English of this manuscript.

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Milos RENE

Institute of Rock Structure and Mechanics, Academy of Sciences of the Czech Republic, CZ-182 09 Praha 8, V Holesovickach 41, Czech Republic
Table 1 Major and trace element concentrations for granodiorite
porphyries from the eastern margin of the Klenov massif (wt.%).

Sample                1/23-32   4/23-33   No-8049   Re-532   Re-637

Si[O.sub.2]            56.16     56.30     55.67    55.23    54.21
Ti[O.sub.2]             1.94      1.91      1.93     1.95     1.85
[Al.sub.2][O.sub.3]    15.05     15.68     15.73    15.11    15.64
[Fe.sub.2][O.sub.3]     1.11      1.99      1.99     1.53     0.31
FeO                     8.11      7.86      7.94     8.24     7.87
MnO                     0.15      0.14      0.14     0.16     0.14
MgO                     2.92      3.07      3.10     3.10     2.78
CaO                     6.48      5.61      5.77     5.70     8.00
[Na.sub.2]O             3.19      3.19      3.14     3.29     3.92
[K.sub.2]O              2.49      2.47      2.43     2.46     2.55
[P.sub.2][O.sub.5]      0.57      1.61      1.85     1.64     2.28
[K.sub.2]O              0.25      0.24      0.23     0.32     0.02
[P.sub.2][O.sub.5]      0.56      0.54      0.54     0.76     0.54
C[O.sub.2]              n.d.      n.d.      n.d.     0.10     0.25
Total                  98.98    100.61    100.46    99.59   100.36

Cr (ppm)                  72        42        42       65       38
Ni (ppm)                   5        14        10       35       10
V (ppm)                  176        97       102      200       96
Pb (ppm)                   5        13         5        4        4
Rb (ppm)                  84        70        97       97      103
Cs (ppm)                                              9.0
Ba (ppm)                 699       772       627      612      618
Sr (ppm)                 212       201       215      292      215
Ta (ppm)                                             0.72
Nb (ppm)                  11        10        14       15       15
Hf (ppm)                                              6.5
Zr (ppm)                 250       332       260      280      258
Y (ppm)                 n.d.        43        46       50       45
Th (ppm)                15.4      11.7      12.6     12.1     11.8
U (ppm)                  1.2       7.2       4.6      4.1      4.5
La (ppm)                                   51.24    48.54
Ce (ppm)                                   94.90    98.43
Pr (ppm)                                    9.98    11.75
Nd (ppm)                                   41.58    45.59
Sm (ppm)                                   10.18    10.96
Eu (ppm)                                    2.34     1.32
Gd (ppm)                                    8.02    11.51
Tb (ppm)                                    1.36     1.99
Dy (ppm)                                    8.45    11.65
Ho (ppm)                                    1.61     2.44
Er (ppm)                                    4.06     6.91
Tm (ppm)                                    0.60     0.98
Yb (ppm)                                    3.91     6.17
Lu (ppm)                                    0.71     0.97
[La.sub.N]/[Yb.sub.N]                       8.86     5.32
Eu/Eu *                                     0.79     0.36

Table 2 Chemical compositions of rock-forming and accessory
minerals of granodiorite porphyries (wt.%).

Mineral           augite   augite   biotite   ilmenite   ilmenite

Analysis          637/15   637/16   637/13      637/1      637/2

Si[O.sub.2]       47.71    48.65     35.41       0.00       0.00
Ti[O.sub.2]        0.86     0.30      3.64      49.59      48.19
[Al.sub.2]
  [O.sub.3]        5.64     5.01     14.34       2.30       3.18
FeO               25.58    25.48     30.53      42.81      42.90
MnO                0.47     0.53      0.46       3.53       3.59
MgO                7.24     7.86      6.37       1.21       1.52
CaO               11.69    11.42      0.27       0.23       0.24
[Na.sub.2]O        0.56     0.48      0.39       0.18       0.15
[K.sub.2]O         0.26     0.26      8.59       0.15       0.23
Total             100.01   99.99    100.00     100.00     100.00
Number            (O=6)    (O=6)    (O=24)      (O=6)      (O=6)
of atoms
Si                1.880    1.913     5.687         --         --
Ti                0.025    0.009     0.399      1.868      1.815
Al                0.261    0.231     2.712      0.136      0.187
[Fe.sup.2+]       0.843    0.838     4.100      1.793      1.796
Mn                0.016    0.018     0.063      0.150      0.152
Mg                0.425    0.461     1.525      0.090      0.113
Ca                0.493    0.481     0.046      0.012      0.013
Na                0.043    0.037     0.121      0.017      0.015
K                 0.013    0.013     1.760      0.010      0.015

Mineral           sphene   plagioclase   plagioclase
                               matrix    phenocryst

Analysis          637/3     No-8049/2         637/5

Si[O.sub.2]       31.53         58.83         54.79
Ti[O.sub.2]       33.61          0.17          0.00
[Al.sub.2]
  [O.sub.3]        4.75         25.95         28.79
FeO                0.91          0.55          0.90
MnO                0.00          0.18          0.13
MgO                0.59          0.71          0.87
CaO               28.05          7.25          9.72
[Na.sub.2]O        0.33          5.38          4.38
[K.sub.2]O         0.23          0.99          0.24
Total             100.00       100.01         99.82
Number            (O=5)         (O=8)         (O=8)
of atoms
Si                1.021         2.628         2.470
Ti                0.818         0.006         0.000
Al                0.181         1.365         1.528
[Fe.sup.2+]       0.022         0.021         0.034
Mn                0.000         0.007         0.005
Mg                0.028         0.047         0.058
Ca                0.973         0.347         0.469
Na                0.021         0.466         0.383
K                 0.009         0.056         0.014
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Author:Rene, Milos
Publication:Acta Montana. Serie A: Geodynamics
Date:Jul 1, 2003
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