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.

PAVON PIVETTA, CECILIA; BENEDINI LEONARDO; GREGORI, DANIEL A.; STREMEL, AGUSTIN; LEONARDO STRAZZERE; BARROS, MERCEDES; COSTA DOS SANTOS, ANDERSON; GERALDES, MAURO CÉSAR

Buzios

Simposio; SSAGI; 2024

UERJ

The Marifil Complex in the North Patagonian Region is considered part ofthe Chon Aike Magmatic province. Within the Marifil Complex, there are twodifferent volcanic types with different U-Pb ages (Pavón Pivetta et al 2020). Thefirst 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 nowadaysessential in the understanding of lithosphere dynamics. For this purpose, eightrepresentative samples from the volcanic and volcano-sedimentary units of theMarifil Complex were selected to perform U-Pb and seven for Lu-Hf in situanalysis in zircon grains (sample PRJ 16b gave ). Also, five representativesamples were chosen for whole-rock Nd isotopes. U-Pb and Lu-Hf analytical procedures were conducted in the MultiLab whileNd isotopes were analyzed in the LAGIR. Both laboratories are located at theState University of Rio de Janeiro (UERJ).The samples were collected from theoutcrops near both margins of the Arroyo Verde creek. Sample PRJ 18A is part of the coulée facies described in Pavón Pivettaet al, (2020); it is the host rock of the Arroyo Verde low sulphidationepithermal deposit and has a Concordia age of 192.6± 2.5 Ma, MSDW of 0.98 and aprobability of concordance of 0.32. Sample MD 1b represents volcanic shardsbearing siltstones with plant imprints. U-Pb in zircon age is 189.5 ± 2.2 Ma,MSDW 1 and probability of 0.32. Sample F14 represents a rhyolitic lava flowlocated west of the Arroyo Verde area. The zircon grains yield a Concordia ageof 187.1 ± 2.7 Ma, with MSWD of 0.0057 and probability of concordance of 0.94. ARV5 is a dacitic lava flow, lateral to the coulée facies with a Concordia age of189.5 ± 2.6 Ma, an MSWD of 0.17 and a probability of 0.68. This facies has noevidence of epithermal veining, which allows to constrain the vein age between192.6 and 189.5 Ma. AVM 27 is a rhyolitic lava similar to AVM 27, has aConcordia Age of 188.8 ± 2.5 Ma, MSWD of 0.89 and probability of 0.35. Sample AVM3b represents the first pyroclastic flow (mLT) and four zircons arebrittle-fractured, indicating the presence of a high-energy explosive processin the deposit formation. The Concordia Age is 186.9 ± 1.9 Ma, MSWD 0.23 andprobability of 0.63. Sample ARV 43 represents a subvolcanic body of 182.3 ± 2Ma, MSWD 0.62 and probability of 0.43.The samples analyzed for ƐNd (t) in Arroyo Verde (F37, F34 andPRJ 16b) belong to the massive lapilli tuff facies, which may include smallbasement clasts. The other two samples (F14 and F4) belong to lavas andsubvolcanic bodies with no evidence of basement clast.The distributions of εHf(t) values in the Marifil Complex (V0) vary from4.92 to 15.73 Ma while in the zircon grains from (V1) volcanic events arebetween 6.86 to 15.49.Model ages can be divided also into two groups: the older samples belongingto stage V0 have TDM ages of 190 to 1860 Ma. The extrusive younger rocksbelonging to V1 have TDM ages of 200 and 700 Ma, indicating a younger source.The calculated ƐNd(0 Ma) are negative in all fivesamples and vary from -7 to -6.6, and the ƐNd(t) variesfrom -4.6 to -4.9, very similar to the values proposed by Rapela et al (2005)for the V1 rhyolites. In the Marifil Complex at the Arroyo Verde area, all εHf(t) are allpositive, which generally indicates a mantle origin. This is contradictory tothe negative ƐNd that indicates crustal provenanceof the isotopic ƐNd, possibly contaminated during theupwelling. This isotopic contradiction behaviour in Northeastern Patagonia iscomparable with the same evidence presented in Sierras Pampeanas (Dahlquist etal 2020), where the ƐNd (t) and Sr/Sr concentrations aresimilar to the metasedimentary basement of Cambrian Puncoviscan Series and the ƐHf are positive values indicative of mantle juvenile material. In theArroyo Verde sector, the metasedimentary sequence is composed by CambrianNahuel Niyeu and El Jaguelito Fm. Here we propose that the same pattern isrepeating in Sierras Pampeanas and eastern North Patagonia.