The first find of kimberlitic accessory minerals in mafic-ultramafic dikes in Spitsbergen

Autor: A. N. Evdokimov, E. S. Radina, A. N. Sirotkin, M. Yu. Burnaeva
Rok vydání: 2006
Předmět:
Zdroj: Doklady Earth Sciences. 407:275-279
ISSN: 1531-8354
1028-334X
DOI: 10.1134/s1028334x06020280
Popis: Since the discovery of diamond in kimberlites, the interest of geologists in these specific rocks has not faded away. The term “kimberlite” was proposed at the end of XIX century by H. Luis for diamondiferous rocks (volcanic peridotite with the porphyritic basaltic texture) from the Kimberley district in South Africa. The majority of researchers believe that kimberlite is a hypabyssal porphyritic rock. According to V.A. Milashev, one of the leading specialists in diamond geology, ultramafic and alkaline-ultramafic igneous rocks of the kimberlite facies must include barophile minerals, i.e., diamond and (or) pyrope garnet [6]. Therefore, finds of pyrope and other kimberlitic accessory minerals in ultramafic and lamproite-type mafic magmatites are very important for the discovery of new kimberlite fields. The kimberlites are located on West Spitsbergen Island (Norway) in the Devonian graben filled with redrocks [3]. The kimberlites are represented by five extended dikes of altered picrites in the northern area of the graben (eastern part of Andre Land) and by one serpentinized dike (Fig. 1) in the southern area of the graben (coast of the Ekman fjord, James I Land). Based on the low silica content and presence of mica, such rocks were previously referred to the lamprophyre association of the archipelago [4, 5]. The nearly vertical dikes (average thickness 0.5 m) crosscut Devonian terrigenous rocks of the Gray Hook Formation and extend in the NNW direction. They are composed of rocks with porphyritic texture and massive structure. In picrites, the porphyritic phenocrysts are mainly composed of olivine, clinopyroxene, and biotite. Olivine phenocrysts are almost completely replaced by talc, serpentine, and carbonate. The groundmass is composed of chloritized and carbonatized glass with microlites of primarily pyroxene and biotite compositions. Phenocrysts in the carbonatized basalt dike are composed of partly albitized calcic plagioclase. Phenocrysts of pyroxene and completely carbonatized olivine are also found. The groundmass is composed of up to 0.5-mm-long plagioclase laths, between which one can see xenomorphic segregations of pyroxene (near the dike center) or glass (near the contact). Specific features of the composition of dikes are shown in Table 1. They are characterized by low contents of SiO 2 and MgO. However, contents of Al 2 O 3 , CaO, and TiO 2 are high. They can be referred to the group of alkaline rocks based on the sum of alkali metals [7]. With respect to alkalinity, picrites in Andre Land belong to the K‐Na and K types, whereas carbonatized basalt of James I Land belong to the Na type. Identification of these rocks is based on texture, miner
Databáze: OpenAIRE
Externí odkaz: