Orciatico-Orendit

The Orciatico-Orendit is a lamproitic storage corridor that penetrated into the Pliocene sediments of Tuscany in the Zancleum . The Orendit (or Diopside-Sanidin-Phlogopit-Lamproit) belongs to the Tuscan Magma Province .

Occurrence

The meadows around Orciatico

The camp corridor penetrated about 500 meters northeast below the center of Orciatico, which belongs to Lajatico and is about 60 kilometers south of Pisa , into the Pliocene clay layers that dipped slightly at 15 to 30 ° to the northeast. It is located directly below the SS Anunziata church. The orendit is composed of three structural units: a conveyor passage and two lobed storage passages. The conveyor dike, which dips steeply to the northeast, strikes north-north-west to north-west and thus follows the local trench structures. The two lobes were fed from the centrally located conveyor tunnel, with the lobe sent in the direction of Orciatico being 40 to 50 meters thicker than the part that advanced to the northeast. The penetration depth was between 100 and 200 meters.

Petrology

The Orciatico orendite is a dark dike rock with deterrent structures and a glassy to slightly porphyry structure at its edge. Inside, the corridor is solid and holocrystalline. The neighboring rocks were overprinted with contact metamorphosis by the vein intrusion in the pyroxene-Hornfels facies.

mineralogy

The micro-phenocrystals of orendite consist of olivine (Fo _75-90 ), phlogopite and low -aluminum diopside with an Al ₂ O ₃ content of approximately 0.5 percent by weight. In the matrix, these phases occur again, plus there are Sanidin , (mainly), glass , accessory-potassium richterite ( amphibole ) and zircon , rutile , ilmenite , spinel ( chromite ), and silicates of rare earths .

Some of the olivine crystals show corrosion and kinking and are therefore likely xenocrystals that may have come from disintegrating ultramafic high pressure xenolites. Another important structural feature is the presence of large gas bubbles, which can be up to 17.5 centimeters long and are mainly concentrated in the conveyor aisle and in the connecting zone to the storage aisles.

Chemical composition

The following rock analyzes should clarify the chemical composition of the Orciatico orendite:

Oxide wt.%	Orciatico-Orendit 2E	Orciatico-Orendit 7	Orciatico-Orendit 1	CIPW standard	Orciatico-Orendit 2E	Orciatico-Orendit 7	Orciatico-Orendit 1
SiO 2	56.74	57.61	58.11	Q	0.90	1.36	7.09
TiO 2	1.42	1.43	1.60	Or	45.33	45.16	43.38
Al 2 O 3	11.25	11.51	10.95	From	11.50	11.16	12.60
Fe 2 O 3	2.00	2.05	1.77	On	1.91	2.38	1.48
FeO	2.92	2.96	3.32	Ns	0.00	0.00	0.00
MnO	0.08	0.08	0.06	Ks	0.00	0.00	0.00
MgO	8.29	8.95	6.96	Ac	0.00	0.00	0.00
CaO	4.40	3.42	3.84	Tuesday	12.16	7.98	10.02
Na 2 O	1.36	1.32	1.49	Hy	12.29	23.03	16.92
K 2 O	7.68	7.82	7.35	Mt	1.16	1.18	1.21
P 2 O 5	0.70	0.69	0.76	Il	2.70	2.72	1.89
LOI	3.16	2.17	2.26	Ap	1.66	1.63	1.80

The Orciatico orendite is an intermediate rock with an SiO ₂ content of 56 to 59 percent by weight. It is ultrapotassic and is oversaturated with silicon (since it is normative to quartz). It also has a high magnesium number Mg # = 75-80 as well as increased values ​​of nickel (up to 350 ppm ) and chromium (up to 800 ppm). Its aluminum, calcium and sodium levels are low.

As with the other Tuscan lamproites, the isotope ratios are characterized by an unusually high initial ⁸⁷ Sr / ⁸⁶ Sr ratio of 0.7160 and a low initial ¹⁴³ Nd / ¹⁴⁴ Nd ratio of 0.5121 in combination with high LILE / HFSE ratios.

Petrogenesis

The high contents of iron and magnesium coupled with the low values ​​of the poorly fusible elements calcium, aluminum and sodium indicate that the orendite originates from a metasomatically influenced mantle spring, which partially melted under relatively low pressure at a depth of less than 50 kilometers. Basalt melt had already been withdrawn from the mantle source before this melting event . The high enrichment in LILE together with the radioisotope ratios indicate that a depleted Harzburgite had been enriched by metasomatic fluids which came from upper crustal material and which had reached the upper mantle area by subduction during the Alpine-Apennine Cycle. The Orciatico orendite is just one example among the various mantle magmas within the Tuscan magma province.

Dating

The Orciatico orendite was dated to 4.1 million years BP in 1967 by Borsi and colleagues using the potassium-argon method. However, this very old dating should be viewed critically, as the intruded clay layers were assigned an age from the Piacenzium . If this turns out to be true, the orendit cannot be older than 3.6 million years.

Individual evidence

1. ↑ Conticelli, S., Manetti, P. and Menichetti, S .: Mineralogy, geochemistry and Sr isotopes in orendites from South Tuscany, Italy: constraints on their genesis and evolution . In: European Journal of Mineralogy . tape 4 , 1992, pp. 1359-1375 .
2. ↑ Conticelli, S. and Peccerillo, A .: Petrology and geochemistry of ultrapotassic volcanism in central Italy: petrogenesis and inferences on the evolution of the mantle sources . In: Lithos . tape 28 , 1992, pp. 221-240 .
3. ↑ Peccerillo, A., Poli, G. and Serri, G .: Petrogenesis of orenditic and kamafugitic rocks from central Italy . In: Canadian Mineralogist . tape 26 , 1988, pp. 45-65 .
4. ↑ Borsi, S., Ferrara, G. and Tongiorgi, E .: Determinazione con il metodo del K / Ar della età delle rocce magmatiche della Toscana . In: Bollettino della Società Geologica Italiana . tape 86 , 1967, pp. 403-410 .