Abstract
Two metamorphosed calc-alkaline dikes, a metadacite and metarhyolite, and their country-rock from El Quebracho hill in the Tandilia belt (TB), Rio de la Plata craton, were studied in detail. Both dikes contain chemically zoned garnet porhyroblasts with the following compositions: Alm35GrosAnd36.5Pyr0.5Spes28 (core), Alm41GrosAnd41Pyr1Spes17 (rim) in the metadacite; Alm37GrosAnd38Pyr0Spes25 (core), Alm50.8GrosAnd32Pyr0.2Spes17 (rim) in the metarhyolite. We calculated P-T pseudosections in the system Na-K-Ca-Fe-Mn-Mg-Al-Si-Ti-H-O and contoured them for chemical and modal parameters of interest with the PERPLE_X computer software to model these metamorphosed dikes. Molar fractions in garnet and mineral assemblages signalize a prograde metamorphic evolution along an apparent geothermal gradient of ~14 °Ckm−1. Syn-kinematic growth of garnet at a NE-SW compressional D1 event during burial to depths of ~43 km at around 600 °C trapped titanite, epidote, and quartz (Si) at crustal depths of 35–40 km at 535–540 °C. Afterwards, the rocks were exhumed in a continuously lasting compressional regime with S1 formation delineated by mafic minerals in the metadacite (Ca-amphibole + biotite) and metarhyolite (biotite). Biotite with Mg# contents of ~0.2 in the metadacite and Ca-rich plagioclase in both dikes point to exhumation accompanied by cooling. Local mylonitization (D2) with the development of an Sm - foliation occurred between 600-500 °C, prior to the late post-kinematic blastesis of muscovite (Si ~3.15 pfu) in the metarhyolite. LA-ICP-MS U-Th-Pb dating of zircon and its trace-element signature indicate intrusion ages of 2130 ± 10 Ma (MSWD = 4.9) for the metarhyolite and 2205 ± 7 Ma (MSWD= 1.4) for the country-rock (metagranodiorite). Based on the regional background, the metamorphic overprint is hypothesized to have taken place still in the Paleoproterozoic.
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