The sedimentary record of the exhumation of a granitic intrusion into a collisional setting: the lower Gonfolite Group, Southern Alps, Italy

The elastic wedge of the Gonfolite Lombarda Group (GLW) accumulated during Oligocene-Miocene times in the Southern Alps foreland basin, formed on the southern, inner side of the Alpine belt. It represents the depositional cou nterpart of the exhumation and erosion of the Central Alps metamorphic-magm atic units. Among the Central Alps units, the Tertiary Bergell Intrusion (TBI) is one o f the principal sources of pebbles occurring within the GLW. Geochronologic data, both from intrusive pebbles and present-day outcrops of intrusive ro cks, document the rapid uplift history of the GLW source area. The lower Gonfolite elastic wedge (Como Conglomerate and Val Grande Sandsto ne Formations, Oligocene-Early Miocene) has been investigated through the s tudy of sandstone and conglomerate petrology for detecting the effects in t he sedimentary record of this collision-related event. The main results are: (i) sandstone petrology of the Come Conglomerate reco rds an evolution from feldspatholithic to feldspathic sandstones; (ii) the related Q/F-F/L ratios suggest an evolution from a mixed plutonic-metamorph ic to a mainly plutonic source; (iii) consistently, conglomerate petrology records a progressive increase of plutonic pebbles (from nearly 0-50% of th e total), a corresponding decrease of metamorphic clasts (from nearly 80 to nearly 50%) and the disappearance of cover rock fragments. Considering the high relief/short transport setting of the GLW elastic routing system, the se values probably resemble the real proportions of such rocks in the Gonfo lite catchment area. During the Aquitanian, the return to a metamorphic-rich source is recorded both by sandstones and conglomerates at the top of the Come Conglomerate an d in the Val Grande Sandstone. This last signal is interpreted as the resul t of the reorganisation of the Gonfolite source area, possibly related to t he northward shift of the main Alpine divide. (C) 2001 Elsevier Science B.V . All rights reserved.