A study of lamprophyric dike rocks of the Velence Hills (Hungary)

Átírás

1 Fragm. Min. Pal A study of lamprophyric dike rocks of the Velence Hills (Hungary) by A. Embey-Isztin Hungarian Natural History Museum Mineralogical-Petrological Department, Budapest Introduction The granite massif of the Velence Fills is poor in dikes of basic character. Their occurrence is known at three places only, whereas granite porphyry and quartz dikes especially aplite dikes are numerous in the Velence granite,(pig. 1.). Several authors dealt with the geology and petrology of the Velence Hills, among them works of Vendl (1914), Jantsky (1957), Gokhale (1965) and Buda (1969) being the most important. A. Vendl (1914) described three kersantite dikes, two of them occur southeast of the top of the Sárhegy, in a distance of a few meters from each other, the third one is at the east edge of the Csala Forest. According to him the following minerals are present in the dike rocks: plagioclase, orthoclase, quartz, hornblende, biotite, apatite, zircon and magnetite. Besides he claims that the double-dike of Sárhegy and the Csala Forest dike are uniform, both of mineralogical and petrological point of view, only hornblende is less abundant and biotite is more a- bundant in the latter. On the basis of the dominant hypothesis of his age he regarded these formations as diaschistic dikes. M. Vendl (1923) described a weathered rock from the Székesfehérvár Quarry as a spessartite. According to her the rock is composed of plagioclase, hornblende, augite and magnetite. The

2 description is sketchy and even the author thinks re-examination to be necessary. Besides the above-mentioned dikes of basic character no others have been found. The material of the narrower dike (40 cm) of Sárhegy is completely weathered, while that of the thicker one (80 cm) is fresh at least at the central part. The rock of the Csala Forest dike is also fresh in the middle part and i t is weathered in marginal parts. In the Székesfehérvár Quarry two weathered dikes cm in diameter can be found. One of the two contains relatively fresh rock patches. Its mineral composition is quite different from that of the rock described by M. Vendl (1923). All the dikes of basic character, striking NE-SW can be traced for a few meters only. While the wall rock of dikes is granite on the Sárhegy and in the Csala Forest, i t is aplitic microgranite at the Székesfehérvár Quarry. The dike rocks contain granitic inclusions especially in marginal parts. Aim of present study and investigation methods Our knowledge about the basic dikes of the Velence Hills has not been sufficient even to decide the question whether the dikes are really of lamprophyric character and i f they are, the name kersantite is a proper one or not. In many respects 1ampropbyres seem to be "peculiar" rocks. They have a number of mineralogical, chamical and textural characteristics. It has been the author's principal aim to prove the existence or lack of these characteristics, and thus to be able to answer the questions above-mentioned. The optical properties of feldspar and hornblende crystals have been determined by means of a Leitz universal stage. At the measurement of axial angle the so-called direct method was used.

3 Pig. 1. Geological map of the Velence Hills. (After A. Vendl with slight alteration.) Lábra. A Velencei-hegység földtani térképe. (Vendl A. nyomán, kis változtatással.)

4 On the whole optical properties of 75 orthoclase, 40 plagioclase twin, and 28 hornblende crystals have been determined. Quantitative mineralogical composition of dike rocks The volume-content of essential minerals has been determined by means of an integrating stage (Table 1-3.). The mineral composition is variant, the central part of dikes is more abundant in mafic constituent s.considering that the number of inclusiens is smaller in the central part, this part can therefore be regarded as characteristic of the whole dike. The modes of samples originating from central parts, and those of average samples are demonstrated in table 4. Plagioclase is the most frequent mineral of the dike rocks. Its amount is roughly the same in the Sárhegy and the Csala Forest dike, and i t is less in the Székesfehérvár Quarry dike. Orthoclase is also a common constituent. The smallest amount of or thoclase is in the Sárhegy dike, there is some more orthoclase in the Csala Forest dike, and much more in the Székesfehérvár Quarry dike. Hornblende is the dominant mafic constituent in the Sárhegy dike but a smaller amount of biotite also occurs. In the central part of the Csala Forest dike there is some more hornblende than biotite, whereas in the average composition of the dike biotite exceeds the amount of hornblende. In the Székesfehérvár Quarry dike biotite is the only dark constituent. Quartz is present in all the dikes. In the Székesfehérvár Quarry and the Csala Forest dike the amount of quartz is high(about 20%), whereas there is much less quartz in the Sárhegy dike. As to accessories, the dike rocks contain apatite, zircon, t i - tanite, diopsidic augite, magnetite and fluorite.

5 Table 1. Mineral Composition of the Sárhegy SE Lamprophyre Dike. 1. tábla. A Sárhegy-DK-i lamprofiros telér ásványi összetétele Average 1-4 Quartz 18,8 14,2 14,8 13,7 15,4 Orthoclase 3,7 7,6 15,1 8,6 8,8 Oligoclase 46,5 54,7 48,0 44,7 48,5 Biotite 5,2 5,2 6,4 5,8 5,6 Hornblende 25,0 17,8 14,7 24,9 20,6 Accessories 0,8 0,5 1,0 2,3 1,1 Table 2. Mineral Composition of the Csala Forest Dike of Lamprophyric Character. 2. tábla. A Csala-erdei lamprofiros telér ásványi összetétele Average 1-4 Quartz 25,7 20,6 22,6 24,8 23,4 Orthoclase 9,5 14,4 4,3 11,3 9,9 Oligoclase 50,0 42,4 57,2 47,1 49,2 Biotite 12,9 9,8 11,3 7,9 10,5 Hornblende 1,2 11,7 3,5 8,2 6,1 Accessorie s 0,7 1,1 1,1 0,7 0,9

6 Table 3«Mineral Composition of Rocks of Lamprophyric Character from Székesfehérvár Quarry. 3. tábla. A székesfehérvári kőfejtő lamprofiros telérének ásványi összetétele Average 1-4 Quartz 24,6 20,9 20,6 17,8 21,0 Orthoclase 15,7 26,8 14,1 15,8 18,1 Oligoclase 36,4 41,5 49,9 49,0 44,2 Biotite 22,6 10,0 14,3 16,9 15,9 Accessories 0,7 0,8 1,1 0,5 0,8 Table 4. Modes of the Lamprophyric Dike-Rocks of the Velence Hills. 4. tábla. A Velencei-hegység lamprofiros telerednek ásványi modális összetétele. Mineral Sárhegy SE Csala Forest Székesfehérvár Quarry C A C A C A Quartz 12, 8 14,5 19,7 22,6 23,5 20,4 Orthoclase 7, 8 8,1 13 A 9,3 14,6 17,1 Oligoclase 41, 6 45,8 40,5 47,5 34,8 42, 9 Biotite 6, 5 6,4 11,3 12,2 26,1 18,6 Hornblende 28, 4 23,9 13,7 7,2 - Accessories 2, 8 1,3 1»4 1,2 0,9 1,0 C = Central part of the A = Average composition dike. of the dike. C = A telér középső része. A = A telér átlagos öszszetétele.

7 Table 5«Concentration of Trace Elements in the Lamprophyric Dike-Rocks of the Velence Hills. 5. tábla. A Velencei-hegység lamprofiros telérkőzeteinek nyomelem tartalma. Sr Ba V Cu B Mn Ga In Ti ppm OO = Sárhegy SE. 4-6 = Csala Porest = Székesfehérvár Quarry. 1-3 = Sárhegy DK. 4-6 = Csala-erdo = Székesfehérvár kőfejtő. Analyst : P. Zent ai

8 Table 6. Chemical Composition of Lamprophyric Dike-Rocks of the Velence Hills. 6. tábla. A Velence-hegységi lamprofiros telérkozetek kémiai összetétele. Constituent Si0 2 58,56 65,31 67,00 67,79 66,02 65,65 Ti0 2 1,09 0,54 0,50 0,48 0,54 0,52 A ,49 15,19 16,43 16,19 16,24 17,28 Pe ,40 1,52 2,59 2,35 4,22 3,72 FeO 4,93 3,56 0,38 0,37 0,28 0,61 MnO 0,19 0,14 0,05 0,05 0,18 0,12 MgO 2,33 1,12 0,32 0,36 0,44 0,60 CaO 4,97 2,40 1,89 1,67 1,48 1,82 Na 2 0 4,25 4,44 3,37 3,39 3,42 4,16 K 2 0 3,10 3,45 3,82 3,94 4,38 2,64 H 2 0-0,18 0,28 1,14 1,10 0,66 0,69 H ,51 2,23 2,52 2,18 2,60 2,66 co 2 0,00 0,00 0,00 0,00 0,00 0,10 P 2 5 0,35 0,16 0,29 0,29 0,27 0,26 1 = Sárhegy dike;. 2 = Csala Porest dike; 3-6 = Székesfehérvár Quarry dike Analysts: M. Emszt and Gy. Pitter

9 Textural features The texture of the investigated dike rocks is also variant. In the rock of the Sárhegy dike hornblende forms phenocrysts attaining 2-3 mm in length (Plate 1. Pig. 2.), but crystals of plagioclase, orthoclase and quartz forming the "groundmass" may also attain a length of 1-1,5 mm. Crystals of apatite, zircon, and sometimes, biotite and hornblende are idiomorphic, plagioclase Is hypidiomorphic, quartz is mainly xenomorphic, orthoclase is partly hypidiomorphic and partly xenomorphic. The texture is of intermediate character between porphyritic and hypidiomorphic-granular. The texture of the Csala Forest dike is,similar to the former one, only hornblende phenocrysts are smaller, thus the hypidiomorphic-granular character is more pronounced.at the same time, i t shows intermediate features to the texture of the third dike. There is no hornblende in the rock of the Székesfehérvár Quarry dike and biotite does not form phenocrysts here. Orthoclase, which is quite abundant here, may attain a length of 2 mm. I t encloses laths of plagioclase and granules of biotite giving the texture poikilitic character (Plate 1. Pig. 1.). Apatite, magnetite, and zircon formed first in the dike rocks then biotite or hornblende developed, followed by plagioclase and orthoclase, finally quartz was formed. This order of crystallization differs from that of the Velence granite where orthoclase antedates plagioclase and biotite (Buda 1969).

10 Minerals composing the dike rocks Quartz. A part of the quartz may be present as xenoliths, but the main part certainly not. I t was the last one to crystallize and i t is xenomorphic. In the rock of the Sárhegy dike quartz occupies the angular interstices between hornblende and plagioclase crystals. Orthoclase was formed at a late stage, and i t may either be xenomorphic or hypidiomorphic. In the Sárhegy dike intergranular orthoclase is also common. In the large orthoclase crystals of the Székesfehérvár Quarry dike, there are many biotite and plagioclase inclusions in random orientation. Orthoclase is either fine-perthitic or perthite-free (Plate 2. Pig. 4.). In some cases i t has a turbid aspect, otherwise its crystals are transparent. I t is important to emphasise that the crystals are untwinned almost without exeption, whereas twinning of orthoclase is common in the Velence granite. The 2V^ values vary between 33 and 75 but the average 2V x values of orthoclase crystals originating from the three dike rocks are the same, 2V^. = 52 respectively. The average 2V^. value of fine-perthitic, hypidiomorphic orthoclase crystals is 55, while that of xenomorphic, perthite-free ones is 47. Consequently the crystals that began to crystallize fomerly, have a greater degree of Al/Si distribution than those that developed at a later stage. The latter ones crystallized only after having injected between wall rocks, whereas the former ones began to develop before the emplacement. The frequency distribution of orthoclase 2V values is demonstrated in Figure 2. I t X shows two maxima with a slight minimum between them. The two maxima (45-50 and ) correspond with the average 2V x values of the two types of orthoclase above mentioned. On the basis of 2V values partly "intermediate orthoclase" structure X CM

11 partly "sanidine" structure (2V -=c 44) can be established. Plagioclase forms hypidiomorphic or idiomorphic laths which are altered in most cases, thus i t is difficult to find crystals 30 n SO Vx Pig. 2. Frequency of orthoclase 2V^ values 2. ábra. Az ortoklász 2Y^. értékeinek gyakorisága suitable for optical measurements. However crystals with an altered core and a fresh rim are fairly common. Zoning of plagio clase occurs as a rule (Plate 2. Fig. 3.). Untwinned crystals are rarely met with, they are composed of fine twin lamellae in most cases. Average An % and 2V values are as follows: Székesfehérvár Quarry Csala Forest Sárhegy Average An % V X

12 According to the values the composition correspond to that of an oligoclase, basic oligoclase. The most acid plagioclase occurs in the Sárhegy dike rock.examining the An % and 2V x values on the migration curves of C. Burri-R. L. Parker-E. Wenk (1967) an intermediate state of temperature at the crystallization process can be deduced. This fact as well as the relative small orthoclase 2V values X and its partly sanidinic character indicate lower temperature of origin or slower consolidation than that of volcanic rocks but higher temperature of origin and faster consolidation than that of plutonic rocks. These intermediate features correspond well to a narrow dike rock consolidating at depth. 'H J M 4 ^ *IH Pig. 3. Frequency of plagioclase twins. 3- ábra. A plagioklász ikrek gyakorisága. 1. albite 4. albite-ala 2. acline 5. ala 3. Carlsbad 6. albite-carlsbad

13 As mentioned above,twinning of plagioclase is very common. Frequency distribution of plagioclase feldspar twins is illustrated in Figure 3«The frequency of complex and parallel twins is 85%. The ratio of C-twins to A-twins is also great (C:A - 2:1). According to Gorai (1951)» A-twins are commonly present in the igneous as well as in the metamorphic rocks, while C-twins are confined to or are characteristic of the volcanic and plutonic rocks. Albite and acline twins belong to the former one, while the other twins belong to the latter one. Albite-ala has been found to be the most common twin law in the dike rocks. Mafic constituents. The dike rocks carry three kinds of mafic minerals, these are: hornblende, biotite, and diopsidic augite. The chief mafic mineral in the Sárhegy dike is hornblende, whereas the amount of hornblende and that of biotite is about the same in the Csala Forest dike, finally in the Székesfehérvár Quarry dike biotite is the only mafic mineral. A very small amount of diopsidic augite occurs in the Sárhegy and Csala Forest dike.it is associated with hornblende and biot i t e, or i t is enclosed in these minerals. I t has a rounded shape due to resorption. I t is probable that diopsidic augite had crystallized before other mafic constituents, but reacting with the liquid phase volatile-bearing ferromagnesian minerals (hornblende, biotite) developed. Hornblende is present in two generations. Those belonging to the f i r s t generation form euhedral and subhedral phenocrysts of 1-3 mm size (Plate 3. Fig. 2.). Twinning on (100) is common at the larger crystals (Plate 3. Pig. l.).the pleochroism is fairly intense:t - dark brown, ß - pale brown, «= yellow. The extinction angle is generally small: (<p : C = ), but greater and smaller values were also recorded. In some cases the extinction angle is as small as 8. The average value of optic

14 axial angles is 69. Hornblende crystals belonging to the second generation form pale green needles of size. Generally biotite has smaller size than hornblende,and i t forms euhedral hexagonal plates in some cases (Plate 3«Pig. 4»). Chloritization both of hornblende and biotite is a common phenomenon. Other mineralogical constituents. In the neighbourhood of dark silicates altered to chlorite, titanite may occur as an accessory. I t forms anhedral crystals of A* size (Plate 1. Pig. 3, 4.). In some cases i t shows mosaic structure as well as twinning. Titanite is obviously a secondary mineral here. The S'rhegy dike is the most abundant is secondary titanite. Apatite crystals of ^ size are relatively common (Plate 2. Pig. 2. ). Zircon present.in small amounts forms euhedral crystals (Plate 2. Pig. 1.). Anhedral crystals of fluorite are very scarce. They were formed by submagmatic processes. (Fluorite abundant in some varieties of the Velence granite is of similar origin.) The amount of opaque minerals is insignificant. The occurrence of titanite, diopsidic augite and fluorite is not mentioned in works of earlier authors. Discussion The quantitative mineralogical and chemical composition of the Velence dikes of basic character vary over a fairly wide range, yet their close relationship is undoubted.this fact is favoured

15 by investigations of the Al/Si distribution of feldspars which indicate a uniform cooling history of all dikes. The chief mineral constituents and their order of crystallization are also the same. All these indicate similar genetic cicumstances. Nevertheless, i f the dike rocks are considered from the standpoint of nomenclature, distinctions must be made. Mafic minerals make up about the one-third of the composition only In the Sárhegy dike, accordingly here is the highest content of PeO+MgO. On the basis of this fact and other characteristics (field occurrence,recurrence of hornblende in two generations and its alteration to chlorite, occurrence of secondary titanite, high volatile and alkali content)the Sárhegy dike rock can be regarded as a lamprophyr, though i t is not a typical one. The amount of quartz is relatively high and i t contains less PeO+MgO than lamprophyres generally do, there is no C0 in the composition, and finally the texture has a less pronounced porphyritic character. Considering tha't the hornblende-plagioclase assemblage is the dominant one in the Sárhegy dike, the rock ought to be named spessartite instead of kesantite. Obviously the other dike rocks can not be regarded as lamprophyres.since the features characteristic of the Sárhegy dike are more or less characteristic of these rocks too, they can be considered as dike rocks of "lamprophyric character". In the rock of the Székesfehérvár Quarry the biotite-orthoclase assemblage is also important with a higher amount of quartz.the Csala Porest dike shows intermediate features. Thus the composition seems to have shifted from a lamprophyric starting state towards a more granitic one. Lamprophyres were once thought to have developed through differentiation processes of granitic-dioritic magmas, and they were supposed to have associated with certain classes of plutonic rock (minette and vogesite with granite, and kersantite as well as spessartite with diorite). Now these suppositions are generally denied and lamprophyres are thought to have risen from an independent source along fractures formed in response to previous emplacement of granitic or dioritic plutons.some potassic

16 lamprophyres carry a considerable amount of quartz, the suggestion has therefore been made that these rocks developed through partial assimilation of granitic country rocks by basic magma. On the basis of the present investigation, this assimilation process could have played an important part in the Velence Hills, moreover the lamprophyric magma may have mingled with granitic one (hybridization). This supposition is favoured by the fact that the intrusion of lamprophyric magma generally follows the emplacement of the granatic body after a relatively small intervall. On the other hand, magmatic evolution in the Velence granite massif must have taken long time judging from Zfisp f * *> _ 6 0 ^ I Maf. A Sarhegy SE Dike Granite of //?e Ve/ence H/'//s 4- Csa/a Forest D/ke O D/ke occur/ng ar Székesfehérvár Quarry Pig. 4. Mineral composition of lamprophyric dikes compared with that of the granite. 4. ábra. A lamprofiros telérek ásványi összetétele a gránittal összehasonlítva.

17 the great abundance of granite porphyry, aplite and quarz dikes. Consequently the probability of such a hybridization process could have been great. The 2V^ frequency distribution of orthoclase crystals showing two maxima confirms this supposition. The subhedral crystals of orthoclase with greater 2V^ values might have begun to develop in the granitic magma, while the anhedral ones having "sanidine" optic began to crystallize only after the intrusion of the dikes had taken place.supposing a fairly intense hybridization process, the more granitic composition of the Csala Forest and Székesfehérvár Quarry dike can well be explained. With increasing K-content more and more biotite develops instead of hornblende in these dikes and the a- mount of quartz and that of orthoclase increases at the same time. The fact that the composition became more granitic is shown in Figure 4. The values representing the composition of the dikes and that of the Velence granite lie along a straight line. Acknowledgement The author wishes to thank Professor K. I. Sztrókay for his critical review of the manuscript. A Velencei-hegység lamprofiros kőzeteinek vizsgálata Embey-Isztin Antal Természettudományi Múzeum Asvány-Kőzettára, Budapest Összefoglalás. - Hungarian summary. A Velencei hegységben három helyről irtak le bázisos, lamprofiros jellegű telérkőzetet: a Sárhegy csúcsától DK-re, a Csalai

18 erdő K-i széléről és a székesfehérvári kőfejtőhői. Az első kettőt Vendl A. (1914) kerzantitnak, a harmadikat Vendl Mária (1923) spesszártitnak irta le. Vizsgálataink alapján megállapítottuk, hogy ezek közül csak a Sárhegyen húzódó telér kőzete sorolható a lamprofirok közé, mivel a mafikus elegyrészek mennyisége csak i t t éri el, illetve közelíti meg az egyharmados részarányt. Azonban még ez a kifejlődés sem tipikus,mivel a FeO+MgO mennyisége valamivel kevesebb a lamprofirok megfelelő átlagánál, viszonylag magas a kvarctartalma, és bár illóanyagtartalma magas, COp-t nem tartalmaz, továbbá szövete Kevésbé kifejezetten porfiros jellegű. Tekintettel arra, hogy a sárhegyi telérkőzetben az amfibol-plagioklász együttes az uralkodó, ezért ezt a kőzetet nem kerzantitnak, hanem spesszártitnak kellett volna elnevezni. A másik két telérkőzetet nem sorolhatjuk a lamprofirok közé, mert kevés mafikus elegyrészt és szokatlanul sok kvarcot tartalmaznak. Mivel azonban az előző telérre jellemző "lamprofiros" sajátságok jórésze többé-kevésbé e telérekre is jellemzőek, e- zért ezeket "lamprofiros jellegű" telérkőzetéknek tartjuk. Földpát-vizsgálataink alapján a három telérkőzet megszilárdulás i körülményei egyformák v o l t a k, éspedig átmeneti jellegű keletkezési hőszintet állapítottunk meg mindenhol. Véleményünk szerint a telérkőzetek képződésében jelentős szerepet játszott a gránit asszimilálása, sőt a lamprofiros magma és a gránitos magma bizonyos fokú keveredése is (hibridizáció;.ezt látszik bizonyitani az ortoklászkristályok 2V értékeinek kétmaximumos eloszlása (2. ábra). Tekintettel arra, hogy a lamprofiros telérek kialakulása a gránit benyomulását viszonylag hamar követi, és mivel a Velencei hegységben a magmás tevékenység hosszan elhúzódott, a hibridizáció valószínűsége nagy lehetett. Amint az a 4. ábrán látható, a hibridizáció illetve asszimiláció mértékének növekedésével a Csala-erdei telér és a székesfe-

19 hérvári kőfejtőben lévő telén kőzetének összetétele a lamprofirostól a gránitos felé tolódott el. References - Irodalom BUDA, Gy. (1969): Genesis of the granitoid rocks of the Mecsek and Velence Mountains on the basis of the investigation of the feldspars. - Acta Geol. Ac. Sei. Hung., 13, ( ). BURRI, C. - L. R. PARKER - E. WEEK (1967): Die optische Orientierung der Plagiklase. - Basel (1-334). GOKHALE, N.W. (1965): A Velencei-hegység gránit és metamorf kőzeteinek ásványtani, kőzettani és kőzetszerkezeti vizsgálata. - Manuscript. GORÁI, M. (I95l)i Petrological studies on plagioclase twins. - Amer. Mineralogist, 36, ( ). JANTSKY, B. (1957): A Velencei-hegység földtana. - Geol. Hung., 10, (1-170). VENDL, A. (1914): A Velencei-hegység geológiai és petrográfiai viszonyai. - M.Kir. Pöldt.Int. Évkönyve, 22, (1-170). VENDL, M. (1923)2 Ujabb adatok a Velencei-hegység kőzeteinek ismeretéhez. - Ann. Mus. Nat. Hung., 20, (81-84).

20 Explanation of plates. Plate I. 1. Poikilitic texture, Székesfehérvár Quarry, +N, M=32x. 2. Hornblende phenocrysts, Sárhegy, UN, M=32x. 3. Titanite, Sárhegy, IÍN, M=85x. 4. Titanite, Sárhegy, UN, M=85x. Plate I I. 1. Zircon, Sárhegy, UN, M=200x. 2. Apatite, Sárhegy, UN, M=200x. 3. Zoned plagioclase, Csala Porest, +N, M=85x. 4. Fine-perthitic orthoclase, Székesfehérvár Quarry, +N, M=32x Plate I I I. 1. Hornblende twin, Sárhegy, +N, M=200x. 2. Hornblende, Sárhegy, UN, M=85x. 3. Hornblende and biotite, Csala Porest, +N, M=32x. 4. Biotite, Csala Forest, UN, M=200x.

21 PLAT F,I

22

23 PLATE IL

24

25 PLATE 111.

26