Revisiting the Santa Rosa garnet-cordierite-anthophyllite granulite, Argentina: a protracted story in a deep magmatic arc during the early Cambrian Pampean orogeny

The Santa Rosa kinzigite is a Cambrian Pampean orogeny granulite containing cordierite, garnet, and anthophyllite. It forms a lensoid body with tectonic foliation and sharp concordant contacts (parallel foliations) amidst migmatites (stromatites and diatexites) containing cordierite and garnet but lacking anthophyllite. It has long been interpreted as a metasedimentary restite. In this revision, using new isotopic (Sr), mineralogical, and geochronological (U-Pb zircon) data and new metamorphic modelling, we propose that this unique rock originated from a metapelite that underwent migmatization and complete extraction of the K-rich anatectic melt at approximately 6-7 kbar. Subsequently, the resulting restite (sillimanite + garnet ± cordierite) was invaded by intermediate to mafic magmas (gabbroic and tonalitic) from the Pampean syn-orogenic magmatic arc (ca. 900 °C). The present-day rock resulted from mingling of the restite and magma whose crystallization produced the kinzigite matrix (quartz, plagioclase, and orthopyroxene). The mingled restite and magma underwent further cooling to subsolidus temperature (ca. 750 °C) under isobaric conditions, followed by burial to 10 kbar, which gave rise to kyanite. These processes took place between ca. 540 and 520 Ma of the collisional Pampean orogeny. The kinzigite underwent a stage of exhumation and water infiltration at ca. 510 Ma when anthophyllite and abundant cordierite formed from garnet after a long period (ca. 10 My) of hot residence at depth. The Santa Rosa kinzigite is a good example of the complexity of processes at the roots of magmatic arcs during subduction and eventual collision.