The Montecristo monzogranite (Northern Tyrrhenian Sea, Italy): a collisional pluton in an extensional setting
Article first published online: 4 DEC 1998
Copyright © 1997 John Wiley & Sons, Ltd.
Volume 32, Issue 2, pages 131–151, June 1997
How to Cite
INNOCENTI, F., WESTERMAN, D. S., ROCCHI, S. and TONARINI, S. (1997), The Montecristo monzogranite (Northern Tyrrhenian Sea, Italy): a collisional pluton in an extensional setting. Geol. J., 32: 131–151. doi: 10.1002/(SICI)1099-1034(199706)32:2<131::AID-GJ735>3.0.CO;2-F
- Issue published online: 4 DEC 1998
- Article first published online: 4 DEC 1998
- Manuscript Accepted: 26 NOV 1996
- Manuscript Received: 24 JUN 1996
- Centro di Studio per la Geologia, Strutturale e Dinamica dell'Appennino, CNR. Grant Number: NSF EPSCOR OSR 9350540
- Montecristo Island;
- Apenninic Orogeny;
- collisional granitoid;
- mafic microgranular enclave
The Montecristo monzogranite (MM) is a near-circular peraluminous monzogranite pluton occupying the entire 10 km2 of Montecristo Island. Outcrops of country rock are scarce, and are mainly roof pendants of metagabbros and calcsilicate hornfels of the Apenninic ophiolite sequence. Emplacement of the pluton (Rb–Sr age=7·1±0·2 Ma), following the early Miocene onset of continental collision, occurred during an extensional phase which migrated eastward via a combined process of subduction–delamination. The MM rocks are strongly porphyritic, the assemblage being composed of alkali-feldspar, quartz, plagioclase (all occurring as mega- or phenocrysts), biotite and minor cordierite. Accessory minerals include tourmaline, apatite, zircon, ilmenite, allanite, monazite, rutile and hellandite. Reconstructed crystallization histories for the mineral phases reveal a polybaric crystallization starting at about 5 kb. Textural variations of MM occur in sharp contact with each other; darker types often form globular masses containing fewer megacrysts and more abundant mafic microgranular enclaves. Geochemical, isotopic and petrographic data indicate that the MM magma was produced by anatectic melting of an intermediate to deep pelitic crustal source. On the basis of the geochemical and mineralogical characteristics of the enclaves, modification of their parent magma occurred by crystal fractionation coupled with mixing and mingling of components from the MM magma. The limited geochemical variation in MM is interpreted as due to crystal fractionation processes during the magma's ascent. Younger porphyritic dykes with more potassic and alkaline affinities cut the pluton; these dykes are concentrated in a major fracture zone and are associated with contemporaneous pseudotachylites. © 1997 John Wiley & Sons, Ltd.