Zusammenfassung
Metasedimentäre Einschlüsse in den krustal anatektischen Rhyodaziten des vulkanischen Komplexes Monte Amiata lassen sich untergliedern in eine Silica-arme Gruppe, reich an Aluminium-Mineralien, und eine kleine Silica-reiche Gruppe mit Quarz im Übermaß. Die beobachteten Texturen, unterstützt von der Mineralchemie weisen auf drei metamorphe Ereignisse. Die Regionalmetamorphose M0 ist häufig verbunden mit Deformation, während zwei nachfolgende progressive Thermometamorphosen M1 und M2 verursacht worden sind von der magmatischen Wärme. Die Mineralgesellschaften des M1 sind representativ für die Pyroxen-Hornfels Fazies und die Gesellschaften des M2, in Kombination mit Anweisungen für partielle Aufschmelzung und Sanidinisation, suggerieren Umstände der Sanidinit-Fazies. Die Einschlüsse werden interpretiert als xenolithische Fragmente einer Kontaktaureole, die nochmals metamorphosiert wurden nach der Aufnahme ins Magma. Die Aureole entstand in pre-mesozoischen Formationen während der Akkumulation des Schmelzes in einer Magmakammer. Die geschätzten Druckbedingungen für M1 sind im Einklang mit der Möglichkeit einer ziemlich tiefen Position der Wärmequelle gehörend zum Geothermalfeld des Monte Amiata.
Abstract
Metasedimentary inclusions in the crustal anatectic rhyodacites of Mt. Amiata volcanic complex can be subdivided into a silica-poor group, rich in aluminous minerals, and a small silica-rich group containing abundant quartz. Textural observations, supported by mineral chemistry provide evidence for three metamorphic events. The regional metamorphic M0 is often associated with deformation, and two subsequent progressive thermometamorphic events, M1 and M2, are caused by the magmatic heat. Mineral assemblages of M1 are indicative for the pyroxene-hornfels facies, and assemblages of M2, in combination with evidence for partial melting and sanidinization, suggest sanidinite facies conditions. The inclusions are interpreted mainly as xenolithic fragments of a contact aureole, which were remetamorphosed after incorporation by the magma. The aureole formed in pre-mesozoic formations during accumulation of the melt in a magmachamber. Constraints on pressure conditions for M1 indicate the possibility of a fairly large depth of the heat-source giving rise to Mt. Amiata geothermal field.
Résumé
Les enclaves métasédimentaires des rhyodacites anatectiques crustales du complexe volcanique du Monte Amiata se laissent subdiviser en deux groupes: l'un pauvre en silice avec des minéraux alumineux abondants, et l'autre, moins important, contenant un excès de quartz. Les textures, supportées par la minéralogie chimique témoignent de trois événements métamorphiques. Le métamorphism régional M0 est souvent associé avec une déformation tandis que deux événements successifs de métamorphisme progressif, M1 et M2, ont été causés par la chaleur magmatique. Les assemblages des minéraux de M1 représentent le faciès des cornéennes à pyroxene, et les assemblages de M2, en combinaison avec indications de fusion partielle et sanidisation, suggèrent les conditions du faciès des sanidinites. Les enclaves sont interprétées comme des fragments xénolithiques d'une auréole de contact, qui ont été remétamorphosés après incorporation par le magma. L'auréole s'est formée dans les formations pré-mésozoiques pendant l'accumulation du magma dans un réservoir. Les conditions P-T de M1 permettent de supposer que la source thermique donnant lieu au champ géothermique du Monte Amiata, occupe une position assez profonde.
Краткое содержание
Метаседиментные вкл ючения в анатектичес ких кварцевых латитах вулканического комп лекса Монте Амиата мо жно подразделить на две с виты: бедную кремнием, но бо гатую алюминиевыми м инералами и на небольшую богатую кремнием, сод ержащую ещё и кварц. Из учение текстуры зтих пород, а также их химическог о состава говорит о то м, что они были подвергнуты трёхркатному воздей ствию метаморфическ их процессов. Региональный метамо рфизм М0 часто связан с дефо рмацией, а последующи е два возрастающие термометаморфизма — M1 и M2 — вызваны магмати ческим нагревом. Минералогические ас социации M1 указывают на пироксеново-роговик овый Фаций, а частичное опл авление и санидиниза ция в М2 разрешает предполаг ать условия, приводящие к образованию санидин ового Фация. Включения в зтих породах интерпретир уют чаще всего, как ксе нолитовые Фрагменты контактного ореола, к оторые были повторно метаморФизированы п осле нового внедрения в них магмы. Зтот ореол образовал ся в до-мезозойский перио д, во время накопления спл ава в магматической к амере. Исходя из оценки давл ения для M1, считают довольн о вероятным, что источ ник тепла в Районе Монте Амиата находился на б ольшой глубине, что со здало известное геотермич еское поле.
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van Bergen, M.J. Polyphase metamorphic sedimentary xenoliths from Mt. Amiata volcanics (Central Italy); evidence for a partially disrupted contact aureole. Geol Rundsch 72, 637–662 (1983). https://doi.org/10.1007/BF01822086
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DOI: https://doi.org/10.1007/BF01822086