Kónya Péter (szerk.): A Bakony-Balaton-felvidék vulkáni terület ásványai - TQS Monographs 1. (Miskolc - Budapest, 2015)

Kónya P. - Papp G. - Földvári M.: A Bakony-Balaton-felvidék vulkáni terület Mg-Ca-szilikátos kőzetzárványainak ásványai

Kónya P. (szerk.) (2015): A Bakony-Balaton-felvidék vulkáni terület ásványai.

TQS Monographs 1. Miskolc-Budapest: Herman Ottó Múzeum és Magyar Földtani és Geofizikai Intézet, pp. 63-80.

A Bakony-Balaton-felvidék vulkáni terület Mg-Ca-szilikátos kőzetzárványainak ásványai

Minerals of Mg-Ca-silicate xenoliths from the Bakony-Balaton Highland Volcanic Field, Hungary

Kónya Péter1*, Papp Gábor2, Földvári Mária3

'Magyar Földtani és Geofizikai Intézet, 1143 Budapest, Stefánia út 14.

2Magyar Természettudományi Múzeum, Ásvány- és Kőzettár, 1083 Budapest, Ludovika tér 2. 31036 Budapest, Nagyszombat u. 4.

"e-mail: konya.peter@mfgi.hu

Abstract

The first finds that may correspond to Mg-Ca-silicate xenoliths were originally described several decades ago from the Balaton Highland as opal (by Béla Mauritz from Guides Hill and János Erdélyi from that of Tátika Hill). Serpentine, which later turned to be one of the dominant components of these xenoliths, had been mentioned only as a cavity-lining mineral by Mauritz from several quarries. By the description of “Hydroamesite”, Erdélyi and his co-workers proved that (aluminous) serpentine may be a major constituent of the inclusions of the Balaton Highland basalts. They explained the formation of the mineral association by the interaction of bauxitic clay and the olivine of the basalt by the influence of hydrothermal fluids (hot aqueous vapours). Some 25 years later Gábor Papp contributed to the knowledge of this peculiar paragenesis by the investigation of several xenoliths. He originated the mineral association from the multistage transformation of dolomite fragments, which came from the basement rocks into the basaltic magma. Since then, further xenoliths have been found, and their XRD, thermal and EDS investigation considerably broadened the knowledge of the paragenesis.

Xenoliths are usually oval in diameter and 4 to5 cm in length, but sometimes they can reach even 10 cm. Based on their external features, they can be classified into the four groups as follows: (1) White, light grey or pink serpentinebearing xenoliths with thin or thick contact zone. (2) Green xenoliths, rarely with contact zone. The dominant components of (1) and (2) are smectite and serpentine minerals. (3) Small, massive, black, lustrous or dull brucitebearing xenoliths. (*) Dark green, dull serpentine-bearing spherules (up to 5 mm) are on the wall of cavity.

The most conspicuous feature of XRD pattern of the serpentin-bearing xenoliths is the two basal reflections, 7.3 к and 3.65Ä that are typical of common (magnesium) serpentines. Based on the presence and shape of other reflections in the ranges 4.70-3.40, 2.70-2.34 and 1.54—1.48 Ä, five groups (R1-R5) can be outlined, which do not always correspond to the grouping based on external features. In the XRD group (Rl) the patterns are similar to that of “hydroamesite”. Characteristic features of this group are the presence of the reflections at around 4.25 (small) and 3.89Á (well-defined) and especially four sharp, well-defined peaks in the range 2.70-2.37Ä at around 2.64,2.58, 2.49,2.39Ä; two reflections (-1.535 and - 1.505Á) can be observed in the range 1.54-1.48Ä. XRD group (R5) represents the other extreme. The peak at 3.89Á is practically inseparable from the broad 3.65Ä reflection; in the range 2.70-2.37Ä only one peak at around 2.495Ä overlies on the broad, asymmetric “tail” fromed by the merged reflections; a single broad reflection appear at 1.540 Ä in the 1.54-1.48 Ä range. Patterns of XRD groups (R2-R4) can be formally evaluated as mixtures of clinochrysotile (sometimes orthochrysotile) and lizardite, accompanied most frequently by smectite, calcite and brucite. It is to be noted that, according to ТЕМ observations made on (mostly crushed and replica) samples of a few xenoliths, a part or even the dominant part of their serpentine components is not made of the “ordinary” thin chrysotile tubes or lizardite flakes but of “rods” of polygonal serpentine and isometric grains of what called earlier by Papp as “spheroidal” or “polyhedral serpentine”.

In the contact zone at the margin of the xenoliths, Ca-Mg-silicates (äkermanite, gehlenite, hydrogrossulare, pyroxenes, thaumasite, tobermorite, wollastonite etc.), oxides (lime, magnetite, perovsikte etc.) can be identified.

Fluorite, hydromagnesite, perovskite and thaumasite have been detected for the first time in the paragenesis of the BBHVF xenoliths. The accompanying clay minerals were found to be mostly smectite, rarely chlorite, chlorite/smectite irregular or illite/smectite regular mixed layer clay minerals.

The mineral paragenesis of the Mg-Ca-silicate xenoliths has been developed in several stages. In the first stage, the dolomitie fragments (from the basement rocks) was affected by contact metamorphism. Dolomite dissociated according to the process CaMg(C03)2 —> CaCO, + MgO + CO,. Periclase (MgO) was transformed into brucite by aqueous fluids below 600 °C. A small part of calcite was consumed during reaction with siliceous fluids above 400 °C, resulting wollastonite in the contact zone. In the contact zone of xenoliths, hydrogarnets crystallised due to additional fluids below 420 °C. In the later stages, tobermorite formed (below 140 °C), thaumasite may be contemporaneous with tobermorite. Brucite altered into serpentine minerals by a retrograde alteration due to the siliceous hydrothermal fluids, or into hydromagnesite due to fluids containing CO,. Hydrotalcite may have been formed by alteration of calcite and serpentine. In the centre of the xenoliths, fluorite formed from late hydrotermal fluids containing F-.