Mafi c enclaves in peraluminous Variscan granitoid in the Battonya Unit from Southeast Hungary

Variscan granitoids occur in the southeastern part of the Tisza Mega Unit of Hungary. The presence of amphibole, calc-alkaline-type Mg-rich biotite in metaluminous basic enclaves, and muscovite and Fe-Al-biotite in peraluminous granitoids, suggests a mixed I-S-type origin. Two types of muscovite have been identifi ed: a primary euhedral to subhedral, Ti-Na-Al rich variety, crystallized after Fe-rich peraluminous biotite in the two-mica granite and in muscovite granite, and a secondary subhedral Si enriched and Mg-bearing, Ti-poor mica formed as a hydrothermal alteration product of feldspars, and is present in all rock types. Given the compositional continuum of “white micas”, we suggest that magmatic crystallization was followed by autometasomatic and hydrothermal activity, due to a water-rich liquid trapped in the rock during the fi nal stages of magmatic activity. Based on the bulk composition of the prevailing rock-type, the abundance of primary muscovite, the majority of the granitoid magma crystallized from a watersaturated peraluminous melt for which the pressure was 490–600 MPa, the temperatures were 650–685 °C and the depth of the intrusion was a minimum of 15 km.


INTRODUCTION
Variscan granitoids occur in the Carpathian Basin (Hungary).The present geology and tectonic setting of the Carpathian or Pannonian Basin (fi g. 1/A), are the result of a multi-step evolution of geological structures (HAAS et al., 2000;KOVÁCS et al., 2000, HAAS et al., 2004;SCHMID et al., 2008 ).
The complexity is due to the tectonic position of the territory.It is situated in the collision zone of the European and African continental plates.The history began with spread-ing, followed by a series of collisions.During the Alpine orogenesis, the drifting and welding of the individual fragments was accompanied by folding processes and nappe formation.At the end of the Miocene, attenuation of the crust (upward bulging of the mantle) resulted in the development of a large basin that determines the present structural setting of the area.
The greatest proportion of the Pre-Neogene basement of the Pannonian Basin is composed of two megatectonic units: in the north the ALCAPA Mega Unit (the southern part of which is named the Pelso Composite Unit) and in the south the Tisza Mega Unit (TMU, KOVÁCS et al. 2000) or Tisia Composite Terran (KOVÁCS et al., 2010).
In the Hungarian part of the Variscan TMU (fi g. 1/B) two types of granitoids can be distinguished: an older (354 Ma, KLÖTZLI et al., 2004) deep-seated metaluminous and slightly peraluminous, K-and Mg-rich, I-and allanite-type granitod, with K-Mg-rich basic durbachitic enclaves (BUDA et al., 2004/a), occurring in the southwest part of the TMU outcropping in the Mórágy Unit (Mecsek Mts) and a slightly younger granitoid which is mostly strongly peraluminous with mafi c enclaves, I/S-and monazite-type (BUDA et al., 2009) in the southeastern part of the TMU located in the characteristic uplift of the basement of the Battonya Unit, studied only from oil prospecting drill-core samples.One of the main purposes of this study to distinguish primary muscovite from secondary muscovite in the peraluminous granitoid in order to estimate the P/T of crystallization and the depth of intrusion of the Battony granitoids.For this estimation it was important to prove the presence of primary muscovite in these altered rocks because the breakdown curve of primary muscovite (CLARKE et al., 2005), which intersects the experimentally determined watersaturated peraluminous granite solidus (JOHANNES & HOLTZ), can give a rather good estimation for the physical parameters of crystallization and depth of intrusion.Due to the volatile enriched environment, subsolidus muscovite is also widespread; con-sequently we had to use different criteria to distinguish them from primary muscovite.The role of basic enclaves in the peraluminous granitoid is not clear, and requires further research.

OUTLINE OF GEOLOGICAL SETTING
The TMU comprises the crystalline basement of south Hungary, east Croatia, north Serbia, and the western part of Transylvania (Romania, Fig. 1/A).It is bordered by the Mid Hungarian Lineament, the Szá va -Mosla vima -Zombor-Bečej-Lipova line, (the Northern border of the Srem -Mureş ophiolite belt), and the Someş lineament in the Northwest, South and Northeast, respectively.As a consequence of the fact that at present, the basement is covered by 1000-6500 m of Miocene-Pliocene sediments, its structure and petrology can only be investigated by geophysical methods and borehole samples.The seismic research of the past decades has shown that during the Neogene, the Pannonian Basin underwent a complex tectonic evolution that has principally modifi ed the original Variscan structures of the area (TARI et al., 1999;CSONTOS et al., 1999).
As an independent unit, the TMU existed from the Late Cretaceous, when its rotation began, until the Early Miocene (CSONTOS, 1995;MÁRTON, 2000MÁRTON, , 2001;;KOVÁCS et al., 2000).Separation from the Moldanubian Zone of Variscan Europe started in the Late Triassic-Early Cretaceous, as recorded by crystalline rocks (BUDA et al., 2004/b) and sedimentary sequences (HAAS et al., 2004;CSONTOS et al., 2004;SCHMID et al., 2008).The Hungarian part of the TMU is called the Kunságia Composite Unit and the Békésia Composite Unit (BCU).The BCU can be divided into four units: Kelebia, Csongrád, Battonya, and Sarkadkeresztúr (fi g. 1/B).The areas of interest are located in the BCU, which is part of the Békés-Codru Alpine Zone (HAAS et al., 2004).The Battonya Unit is a 15-25 km long and 10-15 km wide body forming a fl at anticline covered by 1000-1500 m of Miocene and Pannonian sediments.
The present study fi rst of all deals with the genesis of "white micas" of mostly two-mica and muscovite granitoids, occurring in the uplifted basement of the Battonya Unit.These plutonic rocks are surrounded by regional metamorphic and migmatitic rocks and are partly covered by Alpine sequences.The rocks were metamorphosed at about 6-700 MPa and 500-570°C (350-330 Ma) and are locally affected by dynamo-and retrograde metamorphism (330-315 Ma) according to SZEDERKÉNYI (2001).

SAMPLING AND ANALYTICAL METHODS
The studied samples were obtained from boreholes of the Battonya Unit around the Battonya and Mezőhegyes (Fig. 1/C/I., II.) settlements.The rocks were acquired from the rock collection of the Department of Mineralogy, Geochemistry and Petrology of the University of Szeged.The samples are drill-cores and consequently their numbers are limited.
The major and trace element analyses of the rocks were carried out using ICP-AES methods at the University of Stockholm and at ALS Chemex in Canada (Vancouver).Most of the analyses were published by PÁL-MOLNÁR et al. (2001).Detection limits for major components are 0.01Wt %, FeO content was determined by titrimetric and H 2 O -and H 2 O + by gravimetric methods.Major element compositions of minerals were determined by a CAMECA SX-100 electron probe X-ray microanalyser at the Department of Lithospheric Research of the University of Vienna.During the measurements an accelerating voltage of 15 kV and a beam current of 10 nA were used.A beam size of 1-2 μm was used for amphiboles, micas and other minerals, for feldspar minerals a beam size of 5 µm was used.The counting time was for each element ranged from 20-40 sec.For calibration, well defi ned mineral standards were used (quartz for Si, rutile for Ti, almandine for Al and Fe, a chromite for Cr, spessartine for Mn, olivine for Mg, wollastonite for Ca, albite for Na and an orthoclase for K).Afterwards, standard ZAF corrections were applied.Approximately 500 spot analyses were carried out.We analyzed 3 to 5 points in each mineral grain, but in the case of zoned crystals, as many as 20-30 points were analyzed, depending on the number of distinguishable zones.

PETROGRAPHY
The Battonya granitoid has light greenish grey, grey and pinkish grey colours.It is generally medium-grained, equigranular, sometimes with pinkish microcline megacrysts.The majority contains biotite and/or muscovite.Three rock types have been distinguished: Type I.: quartzmonzodiorite (Fig. 2/A, I.) is the greenish grey variety containing amphibole and biotite but no primary muscovite.The chemical composition is mostly metaluminous or slightly peraluminous (Fig. 2/B, I.).They are most probably basic enclaves in the prevailing two mica and muscovite granitoid pluton .
Sometimes biotites occur as anhedral patches in amphibole showing replacement texture.Alteration of biotite to chlorite is common.Primary muscovite is absent but a secondary variety (sericite) is common in plagioclase.The plagioclases have oscillatory zoning between andesine (An 43-40 , Fig. 3/D) and oligoclase (An 17-13 ) with secondary albitization and sericitization.Sometimes unzoned oligoclase (An 17-10 ) can also be observed.
A cross-hatched microcline is present but not common; it shows replacement texture without perthitic exsolution and has a low Na content (Or 94 , Fig. 3/D, insert).Quartz is deformed with wavy extinction, and secondary calcite is observed.Accessory minerals are acicular apatite, zircon and rare monazite.
The plagioclase is sometimes oscillatory zoned.The cores are mostly oligoclase (An 25-18 , fi g. 5/A) and the rims are albite (An 2-0 ) in the two mica granitoid and the muscovite granitoid, the plagioclases are richer in Na (An 18-0 , fi g. 5/B ).
The plagioclase cores are strongly sericitized or altered to clay minerals, partly replaced by cross-hatched microcline and albite.Albite and microcline with low Na (Or 95-97 ) is widespread, occurring in replacement textures.Secondary albite also forms vein fi llings.Sometimes large primary albite (An 3-4 ) is replaced partly by microcline (e.g.two-mica granite).Maximum or nearly maximum microcline (Δ = 0.74, BUDA, 1975) occurs as megacrysts with low Na content (Or 95-98 ) indicating a low crystallization temperature and slow rate of cooling.Quartz is ubiquitous and displays wavy extinction; it also occurs as myrmekite in plagioclase.Stubby apatite, zircon, and monazite crystals are common accessory minerals.et al., 1996).The rim is actinolite (Fig. 3/A) which additionally forms small aggregates (A/CNK=0.31-0.45).It probably formed at similar P/T conditions to secondary muscovite .

WHITE MICA PETROGRAPHY AND CHEMISTRY
We distinguished the primary (magmatic) muscovite from secondary muscovite in order to obtain information of the P/T conditions of crystallization and depth of the intrusion of a mainly peraluminous granitoid melt in the Battonya area, where knowledge is limited as these rocks are only studied in drill-cores from 1000-1500 m depth.
There are textural and chemical criteria for distinguishing primary (magmatic) muscovite from secondary muscovite (SPEER, 1984): 1) Textural criteria: the grains of primary muscovite are subhedral to euhedral with sharp grain boundaries; grain sizes are similar to those of other, coexisting magmatic minerals.Often the textural evidence is uncertain, and chemical criteria are also needed.2) Chemical cri-teria: the magmatic muscovite is enriched in Ti, Na, and Al and contains lower amounts of Si and Mg compared to secondary muscovite.
Primary and secondary muscovites can be distinguished in the granite-granodiorite of the Battonya Unit based on these criteria.
The primary muscovite grains are large, up to a few millimetres in size, with euhedral and subhedral outlines.In the two-mica granitoids they usually grew across biotite (Fig. 4./1,2) , and in the muscovite granite they also form large euhedral and subhedral grains with sharp boundaries that clearly separate them from the grain boundaries of neighboring minerals, such as feldspars and quartz (Fig. 4./3).The secondary muscovites are smaller than the primary grains (100-250 μm or less), and have subhedral shapes and, in most cases, they occur in plagioclases or microcline as alteration products (Fig. 4./4-7), usually together with secondary anhedral albite and microcline .
Chemically, the primary muscovite (Fig. 6) has more Ti (Table III, avg.: 0.12 pfu), Na (avg.: 0.14 pfu), and Al (avg.: 5.21 pfu) and less Mg (avg.: 0.20 pfu) and Si (avg.: 6.16 pfu) than the secondary muscovite, which has avg.: Ti 0.05 pfu, Na 0.07 pfu, Al 5.06 pfu, Mg 0.28 pfu, Si 6.28 pfu.A high Ti content is a good indicator of primary muscovite because it is not affected by secondary processes such as subsolidus oxidation or exchange of cations (ZEN, 1988).The Mg, Fe, Na, Al, and Si contents of the two types of muscovite can overlap according to SPEER (1984).A very slight enrichment in Fe can be observed in the secondary muscovite (Fe in secondary: 0.46 pfu; Fe in primary: 0.43 pfu).According to MONIER et al. (1984), the ratio of Na/ (Na+K) is also a good indicator for distinguishing the origin of muscovite (primary: Na/(Na+K) = 0.06-0.12;post-and late-magmatic: 0.01-0.07;hydrothermal: < 0.04).In the case of the Battonya granitoids, primary muscovite has a higher Na/(Na+K) ratio (0.07) compared to secondary muscovite (0.04, Table III).Therefore the primary muscovites can be distinguished from secondary muscovite both texturally and compositionally.Textures show that primary muscovite crystallized from the melt after biotite.The secondary muscovite originated by low-temperature hydrothermal processes compared with late-to post-magmatic muscovites.The primary (magmatic) muscovite crystallization was followed by secondary (subsolidus) mica formation as a continuous postmagmatic process during the Variscan orogeny.This is also supported by K/Ar, Sr/Rb ages (average white mica age 350-330 Ma) published by SZEDERKÉNYI (2001).

DISCUSSION AND CONCLUSION
The relationhip of quartzmonzodiorite to the two mica and muscovite granitoid is not clear but it is most probably a basic enclave showing I-type characters e.g.low initial 87 Sr/ 86 Sr 330Ma ratio (0.7005) and the presence of amphibole.Similar enclaves are known in the other variscan granitoids in Hungary (Mórágy Unit) or in the other granitoids of Moldanubian zone of the Variscan orogenic belt.
The average composition of the Battonya granite-granodiorite generally corresponds to the composition of the watersaturated peraluminous near eutectic granite melt called "granite low" (Fig. 7/A , WINKLER et al., 1974, CLARKE, 1992).
The increasing ƒ H2O and high a KAlSi3O8 increases the stability of muscovite (CLEMENS et al., 1988).Muscovite is stable if the An content of coexisting plagioclase is low (albite-oligoclase).At higher An content, the solidus temperature of the granitoid melt is higher, and primary muscovite cannot crystallize as in the case of quartzmonzodiorite.

Figure 1 :
Figure 1: A -Sketch map of the main tectonic units of Central Europe (CBP= Central Bohemian Pluton, SBP=South Bohemian Pluton, MHL=Mid Hungarian Lineament).B -Structural units of the Hungarian part of the Tisza Mega Unit.Inset: simplifi ed geological map of Békesian Composite Unit after KOVÁCS et al. (2000), modifi ed by PÁL-MOLNÁR et al. (2001).C. Location of bore-holes of the Battony Unit.

Figure 5 :
Figure 5: Plot of plagioclase and microcline composition in two mica (A) and in muscovite granitoid (B).

Figure 8 .
Figure 8. A, B, C -Plot of muscovite compositions in the Battonya granitoids (arrows show the changing of composition from primary to secondary muscovites).

Table II :
Representative compositions of biotites from Battonya Unit determined by electron microprobe.

Table III :
Representative compositions of primary (magmatic) and secondary (hydrothermal) muscovites from the Battonya Unit determined by electron microprobe.