Petrogenesis and T-fO2 estimates of Mt. Monzoni complex (Central Dolomites, Southern Alps): a Triassic shoshonitic intrusion in a transcurrent geodynamic setting

The Mt. Monzoni middle Triassie intrusion (Central Dolomites, Southern Alps) is elongated in ENE­ WSW direction, along a regional transcurrent fault system, and covers an area of 4.6 km 2 , with the following lithologies in surface exposure : clinopyroxenites (8.5 %), gabbros , monzogabbros, and minar olivine-gabbros (50.5 %), and monzonites (4 1 %). Quartz syenitic and shoshonitic basalt dykes occur within the intrusive massif and in the neighbouring country rocks respectively. The shoshonitie orogenie affinity of the complex is eorroborated by mineral parageneses and geochemical features : K20jNa20 0.71 . 0; LaN/YbN 81 1 ; LILE enrichment and marked Nb negative anomaly in primordial mantle-norrnalized spidergrams. The apparent discrepaney between the "orogenie" character of this magmatism and the Triassie transcurrent "anorogenie" setting of its emplacement can be explained by eonsidering mantle sources whieh inherited subduction-related geochemie al eomponents from the Hercynian orogenie eycle. The elongated shape of the intrusion and its zonal arrangement suggest that magma emplacement and fractiona­ tion were dynamically controlled by ENE-WSW transcurrent teetonics active during the Triassie time. Petrography, bulk rock and mineral chemistry, as weil as mass balance calculations, in agreement with field volume estimates, indicate that shallow level fractional erystallization in a nearly closed system played a dominant role in the petrogenesis of the eomplex. Removal from a parental gabbroie magma of 8 . 2 % solid fraction gave rise to clinopyroxenite mesoeumulates. The subsequent evolution from differentiated gabbros to monzogabbros can be aecounted for by the removal of about 5 1 . 8 % of gabbroie assemblages, whereas monzonites , eorresponding to a liquid fraction of 39 %, were in turn generated from monzogabbros. Magma fraetionation and eumulus proeesses appear to have been mainly dependent on its growth rate and filter pressing, due to eoncomitant trans­ pressive teetonics, which deterrnined squeezing of the differentiated monzonitic magma southwestward. Crystallization temperatures and oxygen fugacities have been calculated using a modified version of the Burk­ hard ( 1 99 1 ) model for biotite K-feldspar magnetite equilibrium: the obtained TO/-log f02 paths elosely fit the NNO synthetie buffer with ranges of 1 044-86TC/7 . 91 1 . 6 for gabbros, 8 1 8-804°C/l 3 . 01 3 . 5 for monzogabbros, 85 1 -769°C/1 2.01 4.7 far monzonites, and 589°C/ 1 9.0 far quartz syenite. The restricted range of Sr-Nd isotopie ratios (87Sr Sr; : 0.704 1 -0.7048; 143Nd(I44Ndm: 0.5 1 23-0.5 1 24) for the main lithologies of the eomplex confirrns that fractional crystall ization from a common gabbroie parent magma was the dominant petrogenetie process. The distinctly higher 87Srf6Sr isotopie ratios reeorded in dykes (0.70650.7079) and in the barder facies of the intrusion (0.7053-0.7055) eould be due to eontamination by the eountry rocks. Key-words: shoshonitic magmatism, anorogenic setting, geotherrnobarometry, Monzoni complex, Southern Alps. 001:10.1127/ejm/6/6/0943 0935 1 22 1 /94/0006-0943 $ 6.00 © 1994 E. Schwcizcrbart'schc Verlagsbuchhandlung. D-70176 Stuttgart