The geology of the Southern Adamello Massif, Italy
The Adamello Massif is a composite calc-alkaline batholith of Tertiary age in the Italian Alps. This thesis concerns the field geology, petrology and geochemistry of the oldest and southernmost plutonic unit, the Val Fredda Unit (VFU), which is exposed over 7 square kilometres at the southern contact of the massif with Triassic sedimentary rocks. A 1:5,000 scale geological map and cross-sections of the area are presented. The VFU comprises a complex suite of acid to basic intrusive rocks emplaced at a shallow crustal level in a body of broadly lopolithic form. Geobarometry from the VFU aureole rocks reveals an emplacement pressure of 1.6±0.3kb. The VFU exploited a distinct stratigraphic horizon within a broad, pre-intrusive synclinorial structure in the country rocks. Upwards flexure and rupture of the roof rocks, accompanied by passive overhead stoping were important processes in its emplacement. Petrologically, the VFU evolved as a succession of discrete magmatic pulses in the order: porphyritic granodiorite; tonalite; plagioclase-phyric quartz-diorite; fine-grained hornblende gabbro and diorite; and hornblende-phyric pyroxene-gabbro. The first three pulses intruded in close succession to form a reversely zoned felsic pluton with granodiorite at the margin and quartz-diorite at the core. At the time of gabbro emplacement this pluton was sufficiently consolidated to undergo brittle deformation. Gabbroic magmas ascended in the north of the VFU and spread southwards through the pluton along laterally propagating fractures to form a series of extensive, sub-horizontal mafic sheets between 0.5 and 100m thick. The efflux of heat and water from the cooling mafic sheets remelted and remobilised the felsic host giving rise to a wealth of magma commingling textures including crenulate chilled margins to mafic sheets, the inclusion of ellipsoidal mafic enclaves and wispy schlieren in the felsic host and the incorporation of felsic xenocrysts and rock fragments into the mafic magma. Commingling was responsible locally for generating heterogeneous hybrid rocks. The final pulse of hornblende-phyric gabbro magma intruded along the unconsolidated cores of the mafic sheets and experienced minimal mingling with the felsic host. Concentration of hornblende phenocrysts in these sheets generated hornblendite (cortlandite) layers. Migration of remobilised felsic and hybrid magmas led to felsic back-veining and dyking of the earlier pulses and caused textural modifications in some basic rocks. Prior to its complete solidification the VFU was intruded by the Re di Castello Unit (RCU) to the north. The RCU evolved as a series of concentric diapiric pulses of quartz-gabbro to tonalite in composition, each pulse deforming its partially consolidated envelope, including the northern margin of the VFU. Strain analysis is compatible with a model of ballooning emplacement. RCU rock types at the pluton margin are extremely heterogeneous and reveal a high degree of magma mingling on a centimetric scale due to forced convection. During a pause in emplacement a series of diorite ring-dykes were emplaced into the partially molten carapace. The dykes became the locus of flattening deformation during the final, climactic pulse of tonalite. Proto-mylonitic rocks develop in zones of local high shear, tangential to the balloon. A late suite of cross-cutting lamprophyre dykes cross-cuts both units. Geochemically the VFU spans a substantial range of silica content (from 40-71 wt%) and reveals major and trace element variations consistent with a fractionation link between all VFU rock types. Magma mixing was locally important during emplacement. The fractionation trend is distinctly calc-alkaline and resembles that of many volcanic arc sequences. The five intrusive pulses represent discrete magma batches from the overall fractionation sequence (through gabbro to granodiorite), including fragments of entrained cortlandite cumulates. The evolution of the suite can be modelled in terms of HBL-dominated fractional crystallisation of a hydrous picrite parent, similar in composition to the late lamprophyre dykes. Early fractionation (at 6-10 kbars) involved olivine, chrome spinel and pyroxenes, followed by brown hornblende with minor bytownite and magnetite. Final crystallisation (during ascent and emplacement) involved green hornblende and andesine in subequal proportions. Biotite crystallised from the felsic and intermediate rocks. Low strontium isotope ratios (0.7036 to 0.7056) indicate minimal crustal involvement. The observed intrusion sequence from felsic to mafic suggests that the VFU evolved by sequential discharge of a vertically zoned mid-crustal magma system into the upper crust. Mineralogically, VFU rocks have many features in common with calc-alkaline plutonic rocks worldwide, notably in the presence of mantled plagioclases with calcic cores. Major and trace element variations (measured by ion-microprobe) in plagioclase indicate that this mineral was retained during fractionation, growing by zoning overgrowth. Hence individual plagioclase crystals are shown to have been in equilibrium with a considerable range of derivative liquids. Major discontinuities in anorthite content were generated during decompression of the magmas from ~25km to 2km. Compositional variations in hornblende indicate progressive oxidation during cooling and consolidation. Quantitative estimates of the T-fo, evolution of the VFU (using new and published thermodynamic calibrations) reveals that the magmas were oxidised from the QFM buffer at over 1050oC and 8kb to the HM buffer at subsolidus temperatures at the emplacement level. Experimental petrology on mafic enclaves infers gabbro emplacement temperatures around ~1070°C, suggesting that mafic magma ascent was rapid and even adiabatic. Crystallisation experiments confirm that the acicular texture of mafic enclaves can be generated by rapid cooling. It is concluded that calc-alkaline I-type granitoids can be generated by protracted fractionation of mantle-derived magmas. The abundance of hornblende in the fractionating assemblage rapidly drives derivative liquids to silica-rich compositions. A granitoid:gabbro/cumulate ratio of 1:3 is determined from the fractionation calculations. The real ratio will be slightly lower as the felsic rocks contain entrained cumulate plagioclase. The cumulate rocks that are not exposed at the surface are probably lodged at ~25km depth, a conclusion consistent with geophysical profiles of the Adamello and other granitoid batholiths.