Montecatini-Orendit

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The Montecatini orendite is a lamproit intrusion that occurred during the Zancleum in the neogene sediments of Tuscany . It is closely related to the Orciatico-Orendit located 10 kilometers to the north and, like him, is counted as part of the Tuscan Magma Province .

description

The center of Montecatini Val di Cecina

The almost circular intrusion of the Montecatini orendite with a strike width of around 1000 × 800 meters on the surface took place in a laccolith-like manner immediately east of Montecatini Val di Cecina in a ramp anticline . This structure was formed in the front area of an approaching from the west ceiling thrust which Ligurian units via the north-northwest-trending, in Radicondoli-Volterra Basin located Era-ditch Val d' pushed. Its neogene sediments, predominantly marine and lacustrine mudstones and marls , were metamorphically changed by the orendite . The metamorphosis concerned strata from the late Tortonian to the early Pliocene and took place under the conditions of the pyroxene-Hornfels facies. The quenched intrusion margins show vitreous and porphyry structures, the fine to medium-grained intrusion interior is holocrystalline. A north-westerly strike direction is assumed for the conveyor dike.

mineralogy

The Montecatini orendite has recently been classified by Conticelli and colleagues (2010) as a lamprophyre ( minette ) (which, strictly speaking, is not correct, as there is no plagioclase ). The orendite has the following mineral composition with the phenocrystals :

Subordinate to the basic mass:

Thorite and zircon also appear as an accessory . The rock contains neither plagioclase nor leucite .

composition

Main and trace elements

Oxide
weight percent 		Montecatini-Orendit Montecatini-Orendit Selected
trace elements
ppm 		Montecatini-Orendit Montecatini-Orendit
SiO 2 62.04 56.90 Rb 874 792
TiO 2 0.95 1.37 Sr 362 421
Al 2 O 3 13.30 12.61 Nd 181 133
Fe 2 O 3 2.45 3.25 Sm 23.5 24.0
FeO 2.08 2.84 Ni 92 140
MnO 0.06 0.10 Y 38 28
MgO 4.24 7.15 Zr 698 491
CaO 2.17 3.74 Nb 32 30.1
Na 2 O 1.82 1.20 Ba 760 1370
K 2 O 8.91 7.91 La 60 79.8
P 2 O 5 0.60 0.92 Ce 142 206
LOI 1.36 2.43
Mg # 67.4 72.23

In the main elements, the Montecatini orendite is characterized by very high contents of 8 to 9 percent by weight K 2 O , the rock is ultrapotassic. Nevertheless, the alkalis are depleted overall compared to, for example, Sisco-Lamproit . The MgO values are also relatively high (> 4 percent by weight). The SiO 2 content fluctuates between 55 and 62 percent by weight and thus indicates an intermediate rock . Al 2 O 3 between 11 and 13 and CaO with 2 to 4 percent by weight, however, are quite low. With the trace elements enriched in incompatible elements , rubidium (800–900 ppm), barium (750–1400 ppm), chromium (380 ppm) and nickel (100–150 ppm) show significantly increased values. In addition to rubidium, positive spikes are also found in thorium (110–120 ppm) and lead (19 ppm). In comparison with other lamproites, however, in Montecatini orendite barium, zirconium , titanium , tantalum and niobium are depleted, but the ratios LREE / HREE, Ce / Sr, Nb / Sr and Th / K are increased. The high values ​​for nickel, chromium and cobalt (27 ppm) suggest a mantle origin of the magma, which is contradicted by low concentrations of scandium (20.2 ppm) and vanadium (137 ppm). The LREE show an unusual, convex upwardly curved arrangement and also have a negative Europium anomaly.

Isotope ratios

As with the other Tuscan lamproites, the isotope ratios have a high initial 87 Sr / 86 Sr ratio of 0.71580 to 0.71672 and a low initial 143 Nd / 144 Nd ratio of 0.51209 in combination with high LILE / HFSE ratios characteristic. A value of 0.28245 was determined for 176 Hf / 177 Hf, and 0.56 for 187 Os / 188 Os. The lead isotope ratios are:

  • 206 Pb / 204 Pb: 18.624-18.670
  • 207 Pb / 204 Pb: 15.638-15.642
  • 208 Pb / 204 Pb: 38.947-38.956

Petrogenesis

The magma of the Montecatini intrusion is derived from the magma of the Orciatico orendits, but has also assimilated crustal rocks during the fractionation process. It then went through a multi-stage development. At first it crystallized during its ascent, partly under still high pressures in deeper areas. Further upward movement followed by (relatively rigid crystal Engl. Crystal mush), but the places fell apart again under pressure from the advancing Residualschmelze. As a result of this process we now have intermediate to mafic magmatites with a granular structure, which are traversed by a dense, comagmatic network of veins and ocular structures (ocelli). Wire networks and Ocelli have a much higher SiO 2 content of 64 to 66 and a higher K 2 O content of 8 to 10 percent by weight, but lower values ​​of iron and magnesium oxide. They mainly consist of sanidine with subordinate phlogopite, apatite and quartz.

Associated deposit

Associated with the intrusion of the Montecatini Orendite is the Miniera di Coporciano copper mine , which is located immediately west of Montecatini Val di Cecina and has since been closed, and which was the world's largest copper producer during the 19th century.

Age

The Montecatini orendite is the same age as the Orciatico orendite and has been dated to 4.2 to 4.1 million years BP.

Individual evidence

  1. ↑ Federico Sani, Marco Bonini, Domenico Montanari, Giovanna Moratti, Giacomo Corti, Chiara Del Ventisette: The structural evolution of the Radicondoli-Volterra Basin (southern Tuscany, Italy): Relationships with magmatism and geothermal implications . In: Geothermics . 59 (Part A), 2016, p. 38–55 , doi : 10.1016 / j.geothermics.2015.10.008 ( available online at researchgate.net ).
  2. ^ A b Sandro Conticelli et al: Leucite-bearing (kamafugitic / leucititic) and -free (lamproitic) ultrapotassic rocks and associated shoshonites from Italy: constraints on petrogenesis and geodynamics . In: Journal of the Virtual Explorer . tape 36 , Article 20, 2010, doi : 10.3809 / jvirtex.2010.00251 .
  3. ↑ Sandro Conticelli, Piero Manetti, S. Menichetti: Petrology, chemistry, mineralogy and Sr-isotopic data of Pliocenic Orendites from South Tuscany . In: European Journal of Mineralogy . tape 4 , 1992, pp. 1359-1375 .
  4. ^ S. Borsi, G. Ferrara, E. Tongiorgi: 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 .