Kaszab Zoltán (szerk.): A Magyar Természettudományi Múzeum évkönyve 69. (Budapest 1977)

Embey-Isztin, A.: The Szigliget amphibolitelherzolite compound xenolith as an evidence for diapiric uprise in the mantle below Hungary

The centre of secondary euheral amphiboles is chemically very similar to the primary ones (Mg/27Fe = 2.70; 100 Mg/Mg + 2Fe = 72.70) but at the borders the composition becomes different; higher in Ti, Fe, Ca and lower in Si, Al, Mg, Na and K. Accordingly, the Mg/i:Fe = 1.68; 100 Mg/Mg + £Fg = 62.64 ratios become smaller. As the number of Ti per half unit cell becomes 0.82, it is no more a titaniferous pargasite but a kaersutite. The traversing of a secondary amphibole crystal by electron beam shows clear chemical zoning (Fig. 1). The green spinel differs from the brown spinel of the lherzolite in having higher Al, Ti and especially Fe, and smaller Mg and Cr, this last one becoming only a trace element. In SOKOLOV'S classification the green spinel (Alf,J Fe%

+ CTQ

+

; Mgj-J Fell is a magnesianpicotite (Fe

3 + /Fe

2+ calculated from structural formula). Because of its heterogeneity, the glass has been analyzed with wide electron beam moved across the sample. It is highly oversaturated, high in Si, Al and K, low in in Na, Mg. Discussion The Szigliget amphibolite/lherzolite composite xenolith is an extreme curiosity. Strictly speaking, it has only one exact equivalent petrographically ; the amphibolite type-LZA in lherzolite at Lherz (Pyrenees, France) described by CONQUERE (1971). However, the geological environment is different; Lherz is an alpino-type peridotite body that has been elevated high in the crust. Here we have a very complicated vein system among which there are both complex and simple veins. The complex veins may contain anhydrous rock types (pyroxenites without amphibole) and amphibole-bearing rocks such as garnetiferous amphibole ariegite or garnetiferous amphibole-bearing websterite. The different rock-types of veins are arranged in a symmetric manner (CONQUÉRÉ 1971). Amphibolites ("lherzites" in the French literature) which only contains essentialy hydrous minerals may occur in two different manners : either in complex veins generally in a central position, or as distinct simple veins cutting the host lherzolite. This last one may again be one of the following two types: Amphibolites without mica (CONQUÉRÉ'S type-LZA) and amphibolites with mica: type-LZB. Between the Szigliget amphibolite/lherzolite composite xenolith and the type-LZA of the ultrabasic massif of Lherz there is petrographically an astonishing similarity. In both cases the width of the amphibolite veins are of the same order : the texture with signs of cataclysis, the lack of evidence in favour of a cumulus origin, the proportions of its minerals, the sharp contacts toward the host lherzolite are all identical. Even the occurrence of small opaque inclusions is a common feature, like that of the green spinel: however, this last one has only been detected in the type-LZB of the Pyrenees. Another find, very similar to the Szigliget one, is described by DICKY (1968) from the Kakanui (New Zealand) mineral breccia, which is a composite amphibolite/lherzolite xenolith, like the Szigliget one, but the amphibolite is clearly of a LZB-type (60% amphibole, 38% phlogopite, 2% opaque). From the amphibole-bearing xenoliths of the Grand Canyon (Arizona), BEST mentions the presence of 7 amphibolite inclusions from basanitic lava. BEST'S sample (BEST 1973, Nor. 29, Fig. 9) is also a coarse-grained amphibolite of type-LZB. Amphibolites of the Grand Canyon are thought to have originated in a somewhat different manner from those of Lherz, judging from their larger size and cumulus fabric. Very interesting mode of occurrence of amphibole, yet different from the former ones and to a smaller extent that of phlogopite, apatite, and plagioclase, are described by VYTLSHTRE and TRASK (1971) from peridotite inclusions of Dish Hill, California. Among them there are very thin amphibole selvages and veins, and interstitial amphiboles. A common mode of occurrence of this last one is as thin mantles on spinel grains. (The author found