Petrological and geochemical variations of a turbidite-like metasedimentary sequence over the metatexite to diatexite transition within the Pampean Orogen, Argentina

摘要

Large masses of turbidite systems have been metamorphosed in orogenic systems during Earth's history. Under granulite-facies conditions, the transformation of turbidite sedimentary successions into metasedimentary sequences drives intracrustal differentiation by anatexis and melt-residuum separation. We report on a migmatite terrane developing from turbidite successions that were buried into the deep crust during the Neoproterozoic to Early Cambrian Pampean orogeny, in central Argentina. At the exposed middle crustal paleodepths, migmatites occur on the regional scale but lithological zones are characterized by (1) bedded migmatites, (2) metatexite, and (3) diatexite. Bedded migmatites are the low-temperature part of the migmatite terrane where the alternating metapelite and metagreywacke layers are traceable among migmatites of different protoliths. The temperature (790 degrees C) was sufficient high for melting the metapelite, but not for melting the metagreywacke. The majority of the migmatite terrane consists of stromatic metatexites in which the limit among different progenitors is either faint or erased by migmatization. Metatexite zone acts as melt transfer in the migmatite terrane. In the stromatic migmatites, the major oxide composition of leucosomes and tabular bodies of leucogranite resembles those of the glasses of experimental petrology. However, leucosomes and leucogranites are crystallized melts that have low Zr, Th and LREE contents and positive Eu anomalies resulting from accessory mineral retention in melting residues. The transformation of metatexite into diatexite is gradational over tens of metres, and related to an accumulated melt fraction that dismembers the stromatic fabric. The most abundant diatexite is mesocratic, and has little or lacks K-feldspar. A subordinate proportion of diatexites is leucocratic, contains K-feldspar phenocrysts, and shows igneous-like textures. Leucogranites and leucocratic diatexites are the potential carriers of an anatectic melt-rich component from granulite-facies migmatite sequences toward shallow crustal levels. Turbidite successions are fertile protoliths that undergo widespread melting under low granulite-facies temperature (850 degrees C), and the development of large diatexite massifs makes them suitable sources of granitic magmatism.