Visible and invisible complexities in low-to medium grade metamorphic rocks: mineralogical and petrological constraints on the Variscan metamorphic gradient in the Southalpine metamorphic basement (Brixen quartzphyllites, Northern Italy)

The metamorphic basement of the Southern Alps occurs in the Brixen unit (Meran – Brixen – Timau, “Brixner Quarzphyllite”), the Valsugana Unit (Trient – Borgo Valsugana – Agordo) and the Recoaro Unit (Recoaro Terme – Schio). The associated Variscan P-T conditions correspond to a greenschist-facies metamorphic overprint, which exhibits a metamorphic gradient that extends from the lower greenschist-facies in the South to the amphibolite-facies in the North. The aim of this study was to provide mineralogical and mineral-chemical constraints of major mineral phases as well as accessories such as apatite and tourmaline on this gradient and obtain P-T conditions along a North-South profile

Quartzphyllite samples were collected along a traverse from Reccoaro in the South to Brixen in the North. Petrographic investigations revealed that the metapelites contain quite a complex polyphase mineral assemblage. The mineral assemblage in the South is represented by chlorite + muscovite + albite + quartz. Towards the center of the traverse, biotite occurs in the mineral assemblage, which has subsequently been replaced by chlorite. Samples in the vicinity of the Permian Cima d’Asta intrusion show petrographic evidence for contact metamorphism. In the North the mineral assemblage is chlorite + muscovite + plagioclase + quartz + garnet. Therefore, the metapelite zones of chlorite, biotite and garnet were observed along the traverse from South to North.

Mineral chemical investigations of samples without the contact metamorphic overprint reveal additional hidden traces of the polymetamorphic nature of some of the samples. Although the chemical compositions of muscovite, chlorite and plagioclase vary continuously with increasing P-T conditions from South to North, the chemical data also reveal that the southernmost sample shows for instance chemical evidence for a later T-accentuated overprint texturally not visible.

The chemical composition of apatite changes continuously from South to North with slightly increasing F and FeO and Y₂O₃ contents. Tourmaline shows an increase in Ca(X) from the biotite to the garnet zone. Reliable multi-equilibrium geothermobarometry yielded P-T conditions of 554 ± 11°C and 6.49 ± 1.3 kbar in the northernmost sample. In contrast, muscovite-chlorite-quartz geothermobarometry shows considerable scatter in the data due to pervasive later retrogression.

Additionally, we applied low temperature thermochronology to the samples to reveal the post Variscan to Neoalpine thermal history of the rocks. Zircon U/Th-He (ZHe) data suggest cooling of the Valsugana Unit in the upper Carboniferous below 160°C whereas cooling in the Brixen Unit occurs only at the border of Middle to Upper Triassic. The latter can be interpreted as cooling after a thermal event related to the Ladinian Volcanism, which also reset the Apatite Fission Track (AFT) system in the Brixen Unit. This Ladinian AFT reset does not occur in the quarzphyllites of the Valsugana Unit. AFT data and time-Temperature models suggest Permian and Triassic cooling to 70 ± 10°C before 240 Ma in the Valsugana Unit, and post-Ladinian cooling of the Brixen Unit before 70 Ma.

This study shows that quartzphyllites are able to record complex metamorphic histories hidden in petrographic, geochronological and mineral chemical data.