Mantle plume or something else?, zircon U-Pb dating and Lu-Hf isotopic data combined with whole-rock Nd and Sr isotopes related to Early Chon Aike Igneous Province, Northpatagonian region, Argentina.

The origin of the Chon Aike igneous province is currently a matter of intense debate mainly due to two principal issues: its extreme temporal extension (c.a. 30 Ma) and the contrasting related geodynamic scenarios that include intraplate mantle plume activity and protracted arc magmatism in an active margin. However, geodynamic characteristics throughout the Early Jurassic interval are far from uniform. Typically, the subductive features throughout the Jurassic were variable and the massive extension conditions of the lithosphere were un-uniform.

The northern Patagonia region registers the ancient activity of the Chon Aike magmatic province constituting the best and widespread outcrops of such activity. In this sense, the early events related to the Chon Aike Igneousc Province cover two different volcanism types with consecutive U-Pb ages (Pavón Pivetta et al. 2020). The first volcanic event (V0) has U-Pb in zircon ages of 192.6 Ma (PRJ 18a), 190 Ma (MD 1b),189.5 Ma (ARV 5) and 190 Ma (F14), while the second volcanic event (V1) ages vary between 185 Ma (PRJ 16b), 186.9 Ma (AVM 3b) and 182.3 Ma (ARV 43).

U-Pb zircon geochronology and Lu-Hf isotopic records are nowadays essential in understanding the manthe-crust dynamics. For this purpose, eight representative samples from the volcanic units of the Marifil Complex were selected to perform U-Pb and seven for Lu-Hf in situ analysis in zircon grains. This information was complemented with five representative samples of Sm-Nd isotopes. U-Pb and Lu-Hf analytical procedures were conducted in the MultiLab while Nd isotopes were analyzed in the LAGIR, State University of Rio de Janeiro (UERJ).

The obtained data reveal mantle-derived fluids for the V0 and V1 events similar εHf(t) values in the Marifil Complex (V0) vary from 4.9 to 15.7 Ma while in the zircon grains from (V1), volcanic events are between 6.8 to 15.4. However, the TDM ages can be divided also into two groups: the older samples belonging to stage V0 have TDM ages of 190 to 1860 Ma. The extrusive younger rocks belonging to V1 have TDM ages of 200 and 700 Ma, indicating a younger source. Furthermore, the calculated ƐNd(0 Ma) are negative in all five samples and vary from -7 to -6.6. In contrast, the ƐNd(t) varies from -4.6 to -4.9, very similar to the values proposed by Pankhurst and Rapela, 1995 for the V1 rhyolites.

Positive εHf(t) values indicate a mantle-derived origin for the magmas related to the early activity of the Chon Aike Igneous province at northeastern Patagonia, typical of restraining orogen. On the other hand, this data is partially contradictory with the negative ƐNd and ƐSr values that indicate a crustal provenance for the same magmatic suite. The decoupling of these isotopic systems is assigned to lower crustal contamination during the magma storage and upwelling, in similar terms to Pankhurst and Rapela, 1995.