Petrogenesis and T-fO (sub 2) estimates of Mt. Monzoni Complex (central Dolomites, Southern Alps); a Triassic shoshonitic intrusion in a transcurrent geodynamic setting

The shoshonitic orogenic affinity of the Mt Monzoni complex is corroborated by mineral parageneses and geochemical features: K (sub 2) O/Na (sub 2) O 0.7-1.0, La (sub N) /Yb (sub N) 8-11, LILE enrichment and marked Nb negative anomaly in primordial mantle-normalized spidergrams. The apparent discrepancy between the 'orogenic' character of this magmatism and the Triassic transcurrent 'anorogenic' setting of its emplacement can be explained by considering mantle sources which inherited subduction-related geochemical components from the Hercynian orogenic cycle. Shallow-level fractional crystallization in a nearly closed system played a dominant role in the petrogenesis of the complex; this is confirmed by the restricted range of Sr-Nd isotopic ratios ( (super 87) Sr/ (super 86) Sr (sub i) 0.7041-0.7048, (super 143) Nd/ (super 144) Nd (sub m) 0.5123-0.51224) for the main lithologies; the distinctly higher (super 87) Sr/ (super 86) Sr ratios in dykes and in the border facies of the intrusion could be due to contamination by the country rocks.