A DETAILED STUDY ON ZIRCON AND MONAZITE REVEAL THE METAMORPHIC EVOLUTION OF EASTERN PAMPEAN RANGES, ARGENTINA

ESCAYOLA M.; PIMENTEL M.; RAMOS V.; ARMSTRONG R.

Rimini

Congreso; EMC 2016; 2016

European Mineralogical Society

The Eastern Pampean Ranges comprise high-grade supracrustal sequences with linear belts of mafic-ultramafic bodies representing ophiolite remnants. U-Pb and Nd isotopic data suggest that the tectonic evolution of the Pampean Ranges started ca. 640Ma with deposition of supracrustal sequences in a back-arc basin.Exposed within the Athos Pampa Migmatitic Massif there is a boudin shape body made of kinzigite (Gordillo 1984), bounded by shear zones. Its particular mineral association (Qtz+Grt+Crd+Pl+Bt+Mag+-Ath) and the lack of Kf distinguishes this rock unit from the migmatites which encircle them. The thermobarometric data calculated by Gordillo (1984) on garnet-cordierite pairs yield 7,5Kb and 750°C..Gordillo (1979), Rapela & Gordillo (1981) and Otamendi & Rabia (1996) interpreted them as metasedimentary metamorphic rocks that suffered a high-grade anatectic event which produced a leucocratic alkalic rich granodiorite leaving behind a restitic ?degranitized? rock. One kinzigite sample was selected for zircon and monazite conventional TIMS and SHRIMP U-Pb analyses, as well as for whole-rock, garnet and cordierite Sm-Nd analysis. Monazite grains are euhedral, pale yellow, clear, homogeneous. Zircon crystals are mostly euhedral, prismatic, transparent and fine grained (50 to 150 ìm). Rims of new zircon around older cores are ubiquitous. Cathodoluminiscence images reveal that virtually all zircons contain cores of low luminescence. Uranium concentrations of the zircon grains vary between 162 and 591 ppm and Th/U ratios are from 0.018 to 1.317. The lowest Th/U ratios are for one rim with the age of ca. 510 Ma whereas higher Th/U ratios correspond to cores with ages ranging from ca. 576 to 510 Ma. Both cores and rims have almost the same age and possibly originated during a single temperature-progressive high grade metamorphic event. Monazite age is of 517+-4 Ma. and it must have grown concomitantly with the low Th/U ratio zircon rims during the highest temperature metamorphic event.Zircon ID-TIMS U-Pb method yielded the age of 517+/- 5 Ma for the high-grade metamorphic event . Some grains, however, clearly show an older core surrounded by metamorphic zircon and a few analyses of such grains indicated ages ranging grossly from ca. 622 to 2200 Ma. The pattern is, therefore, compatible with a detrital sedimentary protolith for the kinzigite..Sm-Nd isotopic analyses indicated high 147Sm/144Nd ratio (0.166) and TDM model age of ca. 2.6 Ga. The high value of the Sm/Nd ratio maybe explained by: (i) the original sediment was originally depleted in the LREE, (ii) the rock suffered strong LREE depletion during melting and extraction of the granitic melt, resulting in some fractionation between Sm and Nd.Several authors (Sims et al., 1998; Rapela et al., 1998) studied metamorphic metapelitic rocks at the northern sector of the Cordoba Pampean Ranges. Their U-Pb SHRIMP data on zircon and monazite yielded ages of 530-525Ma, and were interpreted as the age of the ?in situ? metamorphic peak. On the previous models the metamorphic peak is related to crustal thickening and compression with P-T conditions of 8.6Kb and 810°C; culminating in isothermal uplift and low pressure anatexis linked to the generation of peraluminous discordantly emplaced granites (520Ma. 4-5Kb, 715°C). Our data reveal that high grade metamorphism followed collision-uplift and medium grade metamorphism and mark the final stages of evolution of the Eastern Pampean Orogen at 517Ma.