Mantelsources in Hercynian Granitoids?
A Trace Element and Isotope Study

A. Gerdes Geochemisches Institut, Goldschmidtsr. 1, 37077 Göttingen, Germany

G. Wörner Geochemisches Institut, Goldschmidtsr. 1, 37077 Göttingen, Germany

F. Finger Institut für Mineralogie, Hellbrunnerstr. 34, 5020 Salzburg, Austria

Cause and sources of the voluminous Hercynian granitoid plutonism is still a matter of debate. Several large post-collisional batholiths in the central part of the Hercynian fold belt are generally taken to show a short-lived, high-T -low-P event at the end of the orogeny, possibly caused by magmatic underplating. The South Bohemian Batholith (SBB) with nearly 10.000 km2 surface area and estimated volume of >80.000 km3 represents one of the largest batholiths. The magmatic activity occurred from 330 to 300 Ma (Friedl et al., 1993) post-dating the metamorphic peak in the country rocks by 20 Ma and more (Friedl et al., 1993). Estimates of the intrusion depths range from 18-20 km in the SW to 7-9 km in the E (Büttner and Kruhl, 1995; Knop et al., 1995). Crustal thickness thus may have reached 50 km or more during granite formation. In the SBB, four major magma types are defined based on age, trace elements, and radiogenic isotopes (Sr, Nd). Durbachites in the E (Rastenberg + Trebic pluton, ~10.000 km3) may perhaps represent the oldest Type 1 (× 335 - 323 Ma). Its mafic components (dikes, enclaves, bodies till 300 m thickness, Mg-No = 65 - 80) have high Cr and Ni (150 - 800, 60 - 180 ppm) but also high incompatible element contents (LREE, Th, U, Rb, K). These incompatible and compatible elements show linear negative correlations with indices of differentiation (SiO2). This suggests mixing of enriched (lamprophyric) mantle magmas and crustal melts. Both appear to be isotopically similar
(eNd = -4.3 to -5.6, (87Sr/86Sr)i = 0.707 - 0.709). Type 2 (~35.000 km3) are peraluminous (A/CNK: 1.0-1.17) biotite granites and are represented by the large Weinsberg granite (WbG × 315 - 330 Ma). Major and trace elements may be explained by melting immature sediments under high-T fluid-absent conditions (Finger and Clemens, 1995). Country-rocks of Plag-Bt-gneisses (meta-greywackes) surrounding the deepest part of the SBB in the southwest have compositions (high K, U, Th, Zr, LREE, slightly peraluminous) and isotope ratios (eNd = -10 to -3.2, (87Sr/86Sr)i = 0.715-0.707) which make them suitable source rocks for the WbG (eNd = -4 to -6.2, (87Sr/86Sr)i = 0.708 to 0.712). An influence of asthenospheric melts in the formation of type 2 granites is unlikely because the WbG lacks igneous mafic inclusions indicative of mixing, and mafic (dioritic) rocks which are rarely associated with the WbG show an enriched signature (eNd = -4.6 to -5.3, (87Sr/86Sr)i = 0.707-0.708: Vellmer et al., 1995 + own data). Slightly younger (ca. 310-325 Ma) relatively homogenous two-mica granites of Type 3 (ca. 28.000 km3, mainly Eisgarner) are strongly peraluminous crustal melts (A/CNK = 1.15-1.3) which have evolved isotope ratios (eNd = -6 to -7, (87Sr/86Sr)i =0.712-0.718) and form a separate group in Sr-Nd isotope space overlapping Hercynian crustal rocks. High-K calc-alkaline fine-grained metaluminous to slightly peraluminous (A/CNK = 0.95-1.15) biotite-granites represent the relative small Type 4 (ca. 300 Ma, < 10% of the SBB, Freistadt + Pfahl granitoids). They intruded as small bodies along major faults and each shows distinct geochemical trace element trends and isotopic ratios. Their distinctly low-(87Sr/86Sr)i suggests incorporation of a different crustal component (lower crust ?). The Pfahl granitoids have the most primitive isotope compositions of the SBB (eNd = -3.6 to -1.9) which could indicate the involvement of an asthenospheric (?) mantle component.

Our results show that the SBB formed from crustal and mantle sources which changed through time. Enriched mafic magmas suggest the involvement of lithospheric mantle melts. Clear indications for an asthenospheric influence during the phase of maximum heating and major crustal melting until 310 Ma are absent. Enriched mantle melts between 345 and 325 Ma, however, are widespread in the Hercynian belt (North and South Bohemia, Aar Massif Central Alps, Corsica, Black Forest and Vosges, Meißen syenite Lausitz Massif up to the Amorican Massif) and may constitue in the granitoids an important mantle component from the lithosphere.

A long time interval exists between resulting crustal stacking (>345 Ma) and widespread crustal melting (325-310 Ma). Crustal thickness reached more than 50 km. Most crustal melts derived from immature sediments which must have had high radiogenic heat production. There is no clear indication of asthenospheric influence. These observations lead to the conclusion that widespread asthenospheric melting and magmatic underplating may not be an obvious explanation for the high-T, low-P melting event in the Hercynian. It may be argued that crustal stacking and
radiogenic heat production have heated the Hercynian crust near its solidus and that additional heating was then provided by minor volumes of lithospheric mantle melts.


Büttner, S. & Kruhl. J., Terra Nostra, in press (1995).

Finger, F. & Clemens, Contrib. Mineral. Petrol., 120, 311-326 (1995).

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Knop, E, Büttner, S., Haunschmid, B., Finger, F. & Mirwald. P.W., Terra Nova, 7, 316 (1995).

Vellmer, C., Wedepohl, K.H., Koller, F., Knöller, K. & Arth. J.G., Beih. Eur. J. Mineral., 7, 256 (1995).