Paleoproterozoic Kimberlites of Kimozero: Petrographic Facies Reconstruction of Kimberlite Pipe Overcoming Tectonic and
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OLOGY
Paleoproterozoic Kimberlites of Kimozero: Petrographic Facies Reconstruction of Kimberlite Pipe Overcoming Tectonic and Metamorphic Modification A. V. Kargina,*, A. A. Nosovaa, I. I. Babarinaa, A. Ya. Dokuchaeva, and I. A. Kondrashova Presented by Academician V.V. Yarmolyuk February 16, 2020 Received February 17, 2020; revised April 9, 2020; accepted April 10, 2020
Abstract—Based on a detailed petrographic investigation and geological observations of the Paleoproterozoic Kimozero kimberlite (Karelia, Russia), we present a new model of kimberlite pipe with multiphase and mono-crater structure. We recognised volcanoclastic and coherent kimberlite series that filled the inner and outer zones of the kimberlite crater. The multiphase structure, emplacement style, petrography and reconstructed size of the Kimozero kimberlite correspond to Phanerozoic kimberlite pipes. Keywords: kimberlite pipe, pyroclastic kimberlite, crater zone, diatreme zone, macrocryst, an ancient kimberlite, Precambrian, Karelian Craton DOI: 10.1134/S1028334X20070090
Temporal distributions of global kimberlite occurrences have shown that kimberlites with Paleoproterozoic emplacement ages are extremely sparse. A few kimberlite occurrences with Early Precambrian ages are found in Central and South Africa and Eastern Australia [1, 2]. The Kimozero metakimberlite is a unique Paleoproterozoic 1.92 Ga age kimberlite occurrence in the Karelian Craton [3–6]. Nearly all Precambrian kimberlites can be found as macrocristic hypabyssal varieties, which occur as dykes or rarely as small bodies that represent a root part of the kimberlite pipes [1, 2]. High erosion and alteration of ancient kimberlites have significantly complicated their volcanological reconstructions and have led to conclusions about the ‘unusual’ features of their explosive emplacement style and kimberlite composition [5, 7], which differentiate them from Phanerozoic kimberlites. On the basis of geological and petrographical investigations of the Paleoproterozoic Kimozero kimberlite, we provide evidence that Precambrian kimberlites are characterized by the same compositions and emplacement mechanisms as Phanerozoic kimberlites. The Kimozero kimberlite contains not only pipe a Institute
of Geology of Ore Deposits, Petrography, Mineralogy, and Geochemistry, Russian Academy of Sciences, Moscow, 119017 Russia *e-mail: [email protected]
roots but also crater volcaniclastic facies, which include resedimented ones that allow the reconstruction of the architecture of the ancient kimberlite pipe. Study of the Kimozero kimberlite is complicated by metamorphism under conditions of a prehnite-pumpellyite facies [8] and by tectonic and neo-tectonic deformations [6]. Previous studies did not undertake special petrographic investigations of kimberlite facies and reconstructed the pyroclastic kimberlite tuffs of the outer-crater zones as diatreme-filled kimberlites. This misleading suggestion led to numerous small pipes of the Fort a la Corne type within Kimozero occurrence being identified (e.g. [4, 5]
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