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~~ INSTITUTUL GEOLOGIC AL ROMANIEI

-~- GEOLOGICAL SURVEY OF ROMANIA

IGII - 'I...The 90th Anniyersary of the

GEOLOGICAL INSTITUTEOF ROMANL\June 12-19.1996

DACITEfrom tlte (vpe-Iocali(v

POIENI, VHideasa Massif, ROMANIA

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/':4;J;..." INSTITUTUL GEOLOGICAL ROMANIEI[41.11..) GEOLOGICAL SURVEY OF ROMANIA

~

DACITE FROM TYPE LOCALITY:GENEALOGY AND DESCRIPTION

Avram ~tefan, Alexandru Szakacs, loan Seghedi

DACITE is one of the fundamental volcanic rock types -beside basalt, andesite and rhyolite - of the calc-alkaline igneousseries. It occurs worldwide, as a common petrotype in continental orisland arc volcanic settings.

The rock name DACITE derives from DACIA, the Romanname for the actual Romanian territory.

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The term DACITE was introduced in the scientific literatureby the Austrian geologist Guido Stache, and firstly used in the bookGeologie Siebenbiirgens (The Geology of Transylvania) of F.v.Hauer and G. Stache, Wien, 1863, pages 56 - 72. The reason todesignate a new igneous rock type was to discriminate between theorthoclase-phyric and plagioclase (i.e. oligoclase)-phyric rocks ofthe "quartz-trachytic" group (today's ca1c-alkaline series), so thatthe oligoclase-bearing rocks (dacites) be distinguishable from theorthoc1ase-bearing rocks (rhyolites) on petrographic grounds. Theterm was then generally assumed in the petrographic literature(Johannsen, 1958). The modern definition of DACITE is based onchemical diagnosis in QAPF (Streckeisen, 1978), K20 vs. Si02 orK2O+Na20 vs Si02 (Peccerillo and Taylor, 1976, Le Maitre et al.,1989) diagrams (e.g. 63-69% Si02, 1.2-3.1% K2O).

The TYPE-LOCALITY of DACITE is Gizella quarrylocated along the Cri~ul Repede river, near the village of POIENI,VlMeasa Mts. in the northern Apuseni Mountains, Romania (Fig.!).Upper Cretaceous (Maastrichtian) to Paleogene (Paleocene) igneousrocks crop out in the old quarries - now merged in a single hugemodern quarry - on the southern river-side (Fig.!). They consist ofeffusive dacites pierced by intrusive porphyritic microgranodiorites.

The DACITES from the type-locality are dark-grey to black incolour, consisting of small phenocrysts (0.1-3.0 mm) of quartz,zoned and twinned plagioclase feldspar (26-49 % An), hornblende,biotite and sparse pyroxene (both augite and hyperstene) noating ina glassy-microcrystalline groundmass. In contrast, the porphyriticmicrogranodiorites are lighter-coloured (grey), including largerphenocrysts (1-8 mm) of plagioclase feldspar, quartz, hornblende,

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biotite and occasionally orthoc1ase, embedded in a microgranulargroundmass of interlocked quartz and K-feldspar. In his originaldescription, Stache did not distinguish between the two rock types.~tefan (in Cioflica et al., 1973) has recognised the intrusive originof the lighter-coloured and coarser-porphyritic rock-type, discardingit from the category of dacites.

The modal composition, as well as the major and trace elementcompositions of a typical DACITE from the type-locality are givenbelow (Tables 1 and 2).

TABLE 1Modal composition of the Dacite

(~tefan, 1980)

Component mm %

Plagioc1ase 3.10/1.60 - 0.20/0.10 30.08 - 40.59Quartz 2.10/1.20 - 0.07/0.05 4.61 - 7.96Amphibole 1.20/0.50 - 0.20/0.05 2.27 - 4.28Biotite 2.70/0.90 - 0.20/0.05 2.43 - 4.88Pyroxene 1.50/0.50 - 0.50/0.20 0 - 4.28Opaque minerals 0.20/0.20 - 0.05/0.05 0.58 - 1.10Apatite 0.50/0.10 - 0.05/0.05 0.06Zircon 0.10/0.05 - 0.05/0.05 0.70Groundmass 42.31 - 54.44

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TABLE 2Major and trace element composition of the Dacite

(~tefan et aI., 1992)

Major element % Trace element ppmSiO') 65.90 Pb 12Ti02 0.62 Cu 10AI')01 15.70 Zn 64Fe')01 3.96 Ga 13FeO ;fr.;0.51 Ni 14MnO 0.06 Co 8.5MgO 1.87 Cr 17CaO 4.10 V 67Na20 4.00 Sc 9.5K20 2.46 Zr 130P205 0.20 Yb 2.4LOI 0.51 Y 29Total 99.91 Sr 280

Ba 1100Rb 110Nb 20.5U 3.7Th 10.8La 27Ce 43Srn 2.9Eu 1.12Tb 0.90Hf 6.5

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Fig.l Sketch-map of the Poieni area with the Iyp~-locali!y (Giz~lIa quarry) of DACITE. Ins~t showslocaLion in Romania. 1: AJluvia; 2: Fluvial t~rrace d~posits; 3: Dacilcs; 4: Porph)'riLicmicrogranodioriles; S: Metamorphic rocks; 6: 1973 outline of the quarries and the: sile: in Gizdlaquarry wh~re the t~rm DACITE has be:~n ddin~d; 7: 1996 outline of the quarrics.

Fig.2 Photomicrograph (crossed nicols) of a Iypical Poieni <I.cite with quartz (q). plagioclase: (pi).bioLile (bi) and amphibolc: (am) phe:nocr)'sts.

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REFERENCES

Cioflica G., Savu H., Borc~ M., ~tefan A., Istrate G. (1973)A]pine voleanism and metal]ogenesis in the Apuseni Mountains.Guide to Excursion 2A, Symp. Vole. and MetalIogenesis, ]973,

.Geo]ogica] Institute, Bucure~ti.Hauer F.v., Stache G. (1863) Geo]ogie Siebenbiirgens. Wien.Johannsen A. (1958) A descriptive petrography of the igneous rocks.

Vol. II: The quartz-bearing rocks. The University of ChicagoPress, 428 p.

Le Maitre, Bateman P., Dudek A., Keller J., Lameyre J., Le BasM.J., Sabine P.A., Schmid R., Sorensen H., StreckeisenA., W oolley A.R., Zanettin B. (989) A classification ofigneous rocks and G]ossary of terms. B]ackwell ScientificPublications, ]93 p.

Peccerrillo A., Taylor S.R. (1976) Geochemistry of Eocene cale-alkaline volcanic rocks from Kastamonu area, northern Turkey.Contrib. Mineral. PetroL, 58, p.63-8].

~tefan A. (1980) Petrographic study of the Eastern part of the Vl1ideasaMassif. An. Inst. Geol. Geofiz., LV, p.207-325.

~teran A., Ro~u E., Andar A., Robu L., Robu N., Bratosin I.,Grabari G., Stoian M., Viijdea E. (1992) Petro]ogical andgeochemical features of Banatitic magmatites in Northern ApuseniMountains. Rom. J. Petrology, 75, p. 97-] IS.

Streckeisen A. (1978) lUGS Subcommission on the Systematics ofigneous rocks. Classification and nomenclature of volcanic rocks,]amprophyres, carbonatites and melilite rocks. Recommendationsand suggestions. Neues Jahrbuch fUrMinera]ogie, v. ]43, p.1-14.