Melt-rock interaction between granitic pegmatites and hosting amphibolites from the Chiavenna Ophiolitic Unit (Tanno Pegmatitic Field, Central Alps, North Italy)

Abstract

The Tanno pegmatitic field, placed southward of Chiavenna (Central Alps, Sondrio, Italy), develops a large number of subplanar dykes that crosscut the Chiavenna Unit, an ophiolitic complex mainly composed, in the study area, of amphibolite rocks. This study focuses on the contact between a pegmatitic dyke and the amphibolitic country rock. We distinguished 4 zones across the contact: I) inner amphibolite, II) contact amphibolite, III) contact pegmatite, IV) inner pegmatite. The inner amphibolite, not affected by melt-rock interaction, is composed of amphibole, phlogopite, ilmenite, titanite and rutile. Two amphibole generations occur, both of them showing a patchy compositional zoning. Amphibole I are Mg-hornblende, whereas Amphibole II have a pargasitic composition. The contact amphibolite shows an enrichment of mica belonging to the phlogopite-biotite series, titanite and the appearance of fuorapatite and plagioclase (Ab45-60), that is absent in the inner amphibolite. Close to the contact, amphiboles display no zoning and gain a higher Mg-horneblenditic composition. The contact pegmatite has quartz, albitic plagioclase, garnet (almandine-spessartine series), muscovite, K-feldspar and fluorapatite. It shows a comb texture, with elongation of plagioclase crystals normal to the contact itself. Far from the contact, the inner pegmatite has an increasing grain-size and a less organized texture. In this zone several accessory phases occur, including gahnite, columbite-(Fe), monazite-(Ce), xenotime-(Y), uraninite and betafite. Whole rock analyses suggest that a chemical exchange, concerning both major elements and trace elements, occurred between the pegmatitic melt and the hosting amphibolite. A considerable increase of SiO2, Na2O and, to a lesser extent, of Al2O3 is observed from the amphibolite towards the pegmatite; K2O and CaO show a decrease at the same extent. The REE pattern in the pegmatite highlights an enrichment in HREE at the contact. Mineral chemistry confirms this trend with variations observable in plagioclase, gradually more albitic from the amphibolite to the pegmatite. Mineralogical characters and geochemical features allow to classify the Tanno pegmatite in the LCT (lithium, cesium, tantalum) family. Based on the metamorphic peak conditions reported from the Lepontine Dome the ambient conditions during pegmatite intrusion were ca. 550°C and 5 kbar. The reduced thermal difference between pegmatite and wall rock explains the diffuse contact observed by X-ray micro-computed tomography. The collected data suggest a chemical interaction between melt and wall rock, according to the following reaction taking place in the amphibolite:

Amphibole I + Amphibole II + Ilmenite + Pegmatitic melt →Amphibole III+Plagioclase+Phlogopite+Titanite+Apatite.

 (First online: 3 January 2020)