Miocene intrusions in Southern Patagonia - a possible link to ridge subduction ?
A series of subvolcanic and plutonic igneous bodies of mostly calc-alkaline to alkaline composition form a line of Miocene magmatic activity in Patagonia (southern Chile and Argentina).This trench-parallel belt is located in a transitional position between the calc-alkaline rocks of the Patagonia Batholith and recent volcanoes in the east, and the alkaline Cenozoic plateau lavas in the west. It has been suggested that the emplacement of plutons in this distinctive position during the Miocene might be related to the subduction of the Chile Ridge at these latitudes in Miocene to recent times (Michael 1983,1991; Welkner et al. 2002). Ridge subduction is an extensively studied process (Johnson & O’Neil 1984; Gorring & Kay 2001; Sisson et al. 2003).
In southern Chile and Argentina the timing of the ridge collision at the Chile triple junction, the presence of high Mg-back-arc plateau basalt, rapid mountain uplift, the development of a fold and thrust belt and the intrusion of these Miocene plutons show an approximate coincidence. The coexistence of primitive mafic and felsic magmas, as well as the calc-alkaline to alkaline character of these plutons suggest a subduction related genesis.The few available geochemical data indicate an extensive interaction between mantle-derived magmas and the crust. These observations are consistent with ridge subduction processes, however, this hypothesis has never been fully explored. Precise correlations are hampered by a lack of geochronological and geochemical data, since the nature of the magmatic bodies (Cerro Balmaceda, Cerro Donoso, Cerro Torres del Paine, Cerro Fitz Roy and Cerro San Lorenzo) is in general not well known. Only a few modern petrological, geochemical or chronological studies are available (Michael 1984, 1991; Vargas & Hervé 1995; Welkner 1999, 2000, Welkner et al. 2002; Kosmal & Spikermann 2001, 2002). Thus, the causal relationship between ridge-collision and the intrusion of these Miocene plutons has yet to be established in this region between 47°S and 52°S.
The objective of the proposed project is to study of the intrusives in the Fitz Roy region(Argentina) along with Cerro Donoso and Cerro Balmaceda (Chile) to further examine the ridge subduction theory. Very few data exist for these three plutons and with our study where we aim to (a) document the geochemical signature of this intrusive body to determine source regions and magmatic evolution and (b) to better constrain intrusion ages, and to unravel the ages of individual intrusion cycles. A petrological characterization in combination with major and trace element studies will form the basis for more advanced geochemical investigations using rare earth elements and radiogenic isotopic (Sr, Nd, Pb) analyses. Stable isotope (O, H, C) analyses will complement this analytical approach. A second important aspect of the proposed research is age determination; here Ar-Ar dating will form the backbone of our geochronological work, and we will combine these data with U-Pb zircon geochronology for selected samples.