The small town of Castiglioncello is one of many seaside localities in Tuscany that are popular for bathers in the summertime. The prominent cliffs, battered by the waves of the Ligurian Sea, sustain pinewoods, restaurants, and Liberty style villas dominating small inlets and sandy coves. The rocky promontories consists of ophiolites, remnants of oceanic crust that once constituted the floor of a Jurassic ocean that closed when Europe and Africa started to collide with each other to form the Alps and the Apennines.
Some time ago, I went there for a short course of rock photography led by prof. Luca Pandolfi and I had a chance to look at the gabbro exposed on the cliffs next to the southern shore of the bay [NEW: play with the slider to see the original and the interpretation!]
To reach the bay we pass for a small paved staircase zigzagging down the cliffs through the beach clubs. The rocky wall exposes the contact between Upper Pleistocene (less than 120 thousand years old) limestones, known as the Panchina Livornese, and 160 million years old igneous gabbro, at the base of the scarp. The contact is a major unconformity that formed when the limestones deposited on top of the old, eroded gabbro in the Pleistocene: a mere 159.8 million years are missing from the geological record in this place.
The gabbros exposed here are common occurrences also in other areas of the Northern Apennines. They formed during crystallization of basaltic magma deep in the oceanic crust of the extinct Ligurian-Tethys Ocean. In the field there are evident variations in grain size, with parts formed coarser and others showing smaller grains, below the centimeters. These variations reflect the different rate at which crystals crystallized during the dynamic history of the magma chamber.
At s smaller scale, these rocks show quite an homogeneous crystalline texture with green, metallic diopside (a type of pyroxene) packed together with white to light green plagioclase crystals. If you wish, you may try to estimate visually the % of pyroxene and plagioclase grains and check on the diagram for the classification of gabbroic rocks if Apennine geologists were right in calling this rock a gabbro. Just be careful: because alteration makes the plagioclase light to dark green. How to distinguish them then?
Well, pyroxene has a high-reflectivity metallic lustre and show very well-developed cleavage that intersects at 90°. Feldspars, like plagioclase, do not reflect light so well and, especially when they are altered they appear as porcelain. Try here:
In this part of the outcrop, crystals are particularly coarse-grained. As you can see pyroxene is very reflective and appears very light-colored due to the angle of the shot, in favor of light. There are evident cleavage planes (sets of regular fractures) visible. However, both cleavage planes are visible as traces only on basal sections of the mineral. All other sections (prismatic sections) show very reflective surfaces (representing one cleavage plane) crossed by traces of the other cleavage plane. With the fingers on the outcrop, you can feel a small ‘staircase’ of cleavage planes on these crystals, intersecting in the three dimensions at tiny orthogonal steps. Note that plagioclase here is very dark green and may be mistaken for a mafic mineral if you don’t notice the porcelaneous appearance, completely different from pyroxene. Now, look at the previous image again: I am sure that you will notice that most of the dark green stuff is not really pyroxene!
I really enjoyed this outcrop and I do believe it is very didactic and interesting. Next time you visit the area, I recommend to not just look at the sea!
Bonus – Geologic Map of the bay of Castiglioncello by ISPRA.
References and further reading
A Petrographic Atlas of Ophiolite: An example from the eastern India-Asia collision zone.
The Ligurian Ophiolites. Geological Field Trips.
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