Petrogenesis of Gabbros of the South Bohemian Batholith: Geochemical Evidence for the Presence of Mantle-Derived Magmas During Batholith Formation

C. Vellmer Mineralogisch-Petrographisches Institut, Universität Hamburg, Grindelallee 48,

D-20146 Hamburg, Germany

F. Koller Institut für Petrologie, Universität Wien, Austria

The genesis and emplacement of huge volumes of granitoid magmas as composite batholiths is a characteristic feature of the Variscan belt of central Europe and provide information on the tectonometamorphic evolution of the Variscan orogen. One of these late Palaeozoic batholiths is the 6000 km2 Carboniferous South Bohemian Batholith at the southeastern margin of the Bohemian Massif. The rocks within the batholith range from volumetrically subordinate gabbros and quartz diorites to predominant granites and granodiorites. Intermediate 87Sr/86Sr320 (0.7053 to 0.7123) and low eNd320 (-5.0 to -7.6) of the felsic granitoid rocks (Liew et al., 1989; Vellmer and Wedepohl, 1994) assign no fundamental role to mantle contributions in their genesis. However, these authors consider an input of mantle-derived magmas indirectly necessary as a heat source to generate large amounts of magma. The occurrence of gabbros and quartz diorites documents the existence of mantle-derived magmas as parental magmas and/or mafic end members in AFC processes during batholith formation.

The primary mineral assemblage of the least evolved gabbros is plagioclase (~An70 to An46), clinopyroxene (±Wo47En41Fs12), orthopyroxene (±Wo2En60Fs38), biotite, quartz, ilmenite, phyrrhotite and apatite. Ubiquitous amphibole (actinolite, cummingtonite) which formed late in the crystallization history often contains cores of relict orthopyroxene and clinopyroxene. Two-pyroxene thermometry (Lindsley, 1983) yield typical magmatic temperatures of about 1000°C using pyroxene-core compositions.

The gabbros are quartz normative and range in SiO2 from ~ 52 to 58 wt.%. Abundances of Cr (182 to 101 ppm), V (234 to 100 ppm), Sc (30 to 22 ppm) decrease whereas TiO2 (0.64 to 1.1 wt.%), K2O (0.82 to 2.4 wt.%), Rb (29 to 104 ppm) increase with decreasing Mg/(Mg+Fe) (0.67 to 0.53) indicating that fractionation has occurred. Chondrite-normalized REE patterns are moderately fractionated (CeN/YbN= 4.6
to 7.8) with low YbN values (4.8-8.9) and variable Eu
anomalies (Eu/Eu*= 1.61 to 0.77) due to plagioclase accumulation and separation, respectively. Compared with MORB the gabbros are relatively enriched in Pb, Th, alkali and alkaline-earth elements relative to REE and HFSE resulting in high Rb/Ce (1.14 to 1.76), Sr/Nd (12.8 to 24.7), Ba/Ce (8.0 to 20.9), Pb/Ce (0.21 to 0.38), Th/Ce (0.12 to 0.19) and Ba/Nb (37 to 131) ratios which are characteristic of orogenic magmatic series. Primitive-mantle-normalized, incompatible-element patterns exhibit well-pronounced negative Nb and Ti anomalies and resemble patterns of calc-alkaline andesites from a continental margin setting.

The gabbros have enhanced (87Sr/86Sr)320 values (0.7069 to 0.7073) compared with primitive mantle and relatively low eNd320 values (-5.3 to -4.6) which may indicate a major crustal contribution to their parent magma. However, large scale contamination of a MORB-source derived melt within the continental crust by an AFC process is not conformable with low d18O values (SMOW) of both whole rock (6.6”) and separated feldspar (6.6”) in the least evolved sample as well as high Sr concentrations and high Mg/(Mg+Fe) ratios even in the more evolved samples. Crustal-like eNd320 values (-5.4 and -5.1) of garnet pyroxenites (Carswell and Jamtveit, 1990) within tectonically emplaced fragments of subcontinental lithosphere probably provide evidence for a heterogeneous and enriched mantle source beneath the Bohemian Massif during Variscan time.


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