Pinite-cordierite from spotted slate of the Brajkovac contact metamorphic aureole ( Dudovica locality , central Serbia )

The Paleozoic very low to low-grade metamorphic rocks of the Bukulja-Lazarevac Unit designated as Drina, Golija and Birač formations are contact metamorphosed by the intrusion of the Tertiary Brajkovac granodiorite into spotted slates and hornfelses. In some parts, they are slightly migmatized at the contact. In addition to their outcrops found at the western, eastern and northern parts of the formation, these rocks are also found in boreholes near Dudovica at about 8 km south-west from the pluton. There, at a depth of 110 m, the spotted slates comprise oval to ellipsoid pinite-rich spots which can be regarded as incipient cordierite porphyroblasts (up to 5 mm in diameter) overgrowing the existing regional foliation. They are composed of cryptocrystalline mixture of a very fine sericitic material ± light glassy orange „film“ (some kind of an amorphous gel-like material often mixed with limonite matter) and are abundant in inclusions: minute quartz and dusty ore minerals (magnetite) prevail. In addition, within some spots an increased number of xenotime and monazite inclusions are noted. Minute flakes of neobiotite are formed at the expense of quartz-sericite-chlorite matrix. The secondary chlorite occurring as overgrowths on pinite-cordierite spots shows variable composition (brunsvigite to diabandite). The Mg/Fe+Mg ratio of cryptocrystalline pinitic mixture ranges from 0.14–0.67. The Si vs AlIV+AlVI relations deviate from the ideal muscovite-phengite join due to Tschermak substitution towards chloritic composition or a more complex mixture, including clay minerals (which reflected a decrease of Altot and Si with increase of Fe 2+). Obtained data indicates that the cordierite-pinite spots can be related to contact metamorphic processes that occurred within the temperature range 300–450 °C.


Introduction
The basic feature of the majority of the outer contact metamorphic zones is the production of spotted slates formed at the expense of argillaceous rocks or their very low to low grade regionally metamorphosed products (slate/schist).Generally, the spotted texture is the result of the metamorphic reactions that take place due to heat released from the adjacent magma body: the quartz-clayish or quartz-sericite-chlorite foliated matrix becomes reactive in this condition.At certain places, the ions mobility can cause it to change into a completely new composition where mineral phases with high activation energy of nucleation (e.g.cordierite) can start to crystallize.At the beginning they will form oval spots up to a few millimeters in size and then, due to temperature increase, they turn into porphyroblasts towards the contact.According to a number of authors (e.g., CHANDLER 1975;HASLAM 1983;NÉDÉLEC & PA-QET 1981;CLEMENS & MCMILLAN 1982;OGIERMANN 2002;DEER et al. 1962;ÇERNY & POVONDRA 1967;SCHENK & ARMBRUSTER 1985;HASLAM 1983) the composition of these spots is still uncertain.They can represent either the very fine-grained crystalline mixture of phyllosilicates (occasionally accompanied by a light yellowish amorphous isotropic material), formed as the first product of low temperature contact metamorphic reactions (often similar to pinite), or the incipient growth of cordierite.Moreover, there is a doubt whether these pinite-like spots are formed as prograde metamorphic phases or as retrograde products of cordierite porphyroblasts (see DEER et al. 1962;PATTISON & TRACY 1991;MIYASHIRO 1994;RUIZ CRUZ & GALAN 2002;OGIERMANN 2002).Also, these pinite-like spots can be composed of hydrous alkali-bearing phyllosilicates (CHANDLER 1975;HASLAM 1983;NÉDÉLEC & PA-QET 1981;CLEMENS & MCMILLAN 1982;OGIERMANN 2002) or mixture of chlorite + muscovite ± clay minerals (DEER et al. 1962) or clay mineral-bearing assemblages and isotropic alteration products (ÇERNY & PO-VONDRA 1967;SCHENK & ARMBRUSTER 1985;HASLAM 1983).In addition, it is still uncertain at which P-T-X conditions begin the transformation of cordierite to pinite and whether it is controlled by local fluid-involved reactions.
The focus of this study is to present petrographic characteristics of spotted slates first found in the contact metamorphic aureole of the Brajkovac granodiorite during exploratory drilling at a depth of 110 m in the area of Dudovica (Fig. 1).For this study, we used a combination of macroscopic, microscopic, and SEM data, as well as microprobe major-element mineral analyses to infer the composition and origin of the pinitecordierite spots.Mineral abbreviations used in this paper follow those recommended by KRETZ (1983).
on the limbs of the larger fold structures; D3 affected D1 and D3 in a brittle manner.It should be stressed out that the age of these sub-units and phases of deformations has not yet been fully resolved (see TRIVIĆ et al. 2010;MAROVIĆ et al. 2007, and reference there in).
The contact metamorphic changes of various degree, took place due to the intrusion of the Brajkovac granodiorite, are recorded in the DF, GF and BF.
According to KOSTIĆ & PAVLOVIĆ (1978) the effects of contact metamorphism which could be seen even at a distance of 5 to 8 km from the exposed granodiorite mass indicates the existence of a much greater undiscovered body that could be linked to the Bukulja granite.The spread of the Brajkovac granodiorite to the northeast is supported by geophysical exploration in the area between Baroševac and Rudovci (∼ 5-6 km from the exposed mass) where it was found at a depth of about 80 m (VUKAŠINOVIĆ 1970).In addition, the discovery of spotted slates in exploration boreholes at a depth of 110 m (Dudovica locality, about 8 km south of the exposed granodiorite; SREĆKOVIĆ-BATOĆANIN et al. 2013), also points to its much wider distribution.The above mentioned authors emphasized that the contact metamorphic changes within the basement?rocks took place at moderate P-T conditions recorded in feldspatic biotite-muscovite rich hornfelses with rare porphyroblasts of andalusite adjacent to contact, and feldspatized or metasomatized muscovite-biotite schists at some distance from it.FILIPOVIĆ et al. (1978) andFILIPOVIĆ et al. (1980) also recognized various types of hornfelses, sericite ± chlorite or biotite-muscovite schist (± andalusite) and feldspatized or metasomatized schists, at similar distance from the contact; data related to the conditions of contact metamorphism are lacking.
The Brajkovac granodiorite is not well studied yet.The available data on its composition and associated contact aureole are very scarce as can be seen from the previous discussion.It is classified as medium-to fine-grained, rarely porphyritic, hornblende-biotite granodiorite with local transition to tonalite (KOSTIĆ & PAVLOVIĆ 1978;FILIPOVIĆ et al. 1980;KNEŽEVIĆ et al. 1994).Small intrusions of aplitic granite and very rare aplite and pegmatite dykes are also noted.The main rock-type is granodiorite and is composed of quartz, plagioclase (32-38 % An), microcline, biotite and hornblende; accessories are epidote, allanite, titanite, apatite, zircon and Fe-Ti oxides; secondary minerals are calcite, chlorite, sericite and epidote.

Analytical techniques
The samples were examined in thin sections using a Leica DMLSP petrographic microscope with digital camera Leica DC 300.Chemical compositions of mineral phases were identified using a JEOL JSM-6610LV Scanning Electron Microscope that was connected to an X-Max Energy Dispersive Spectrometer.The samples were covered with carbon using a BALTEC-SCD-005 Sputter coating device, and the results were recorded under high vacuum conditions, with an accelerating voltage of 20 kV and a beam current of 0.5-1.8nA.The scanning electron microscope is also used for imaging of specimens.

Results
The contact metamorphic rocks found at a depth of 110 m in the drillhole at Dudovica locality correspond to porphyroblast-bearing spotted slates (Fig. 2) formed under the influence of Brajkovac granodiorite (30-25 Ma) on low grade regionally metamorphosed argilaceous sediments of Paleozoic age.Their primary metamorphic fabric is completely preserved.The main mineral assemblage is quartz, muscovite (sericite), chlorite, biotite and cordierite-pinite; dusty ore minerals (magnetite), xenotime and monazite are accesories; secondary chlorite is also present.The grain sizes of foliated matrix minerals (white mica +quartz + chlorite ± neobiotite + dusty ores) range from <0.01 mm to 0.15 mm.Cordierite, occurring in oval to ellipsoidal poikilobalsts up to 5 mm in diameter, contains numerous inclusion of matrix minerals (white mica + quartz) ± neobiotite.Within all examined samples cordierite is almost completely altered to a bright yellowish pinite -a mixture of very tiny white micas, chlorite and probably clay minerals.Within some poikiloblasts a small accumulation (up to 0.25 mm) of secondary chlorite was found as overgrowths on pinite (Fig. 2А) Moreover, some porphyroblast are transformed into a bright orange-brown vitreus isotropic material probably formed as a weathering product.The relationships between cordierite poikiloblasts and regional foliation indicate their posttectonic growth with respect to the Paleozoic (post-Variscan) deformation, i.e. cleavage (Fig. 2 A, B).Quartz, except as matrix mineral, makes small lensshaped fine-grained mosaic aggregates parallel to schistosity.Biotite occurs in light brown flakes (< 0.3 mm in size) associated with matrix white micas (sericite-phengite) and chlorite.It is also found as inclusion in cordierite.Coarse calcite grains (up to 1 mm in size) are noted in some samples.
For the purpose of the present work only pinitized cordierite poikiloblasts and secondary chlorite overgrowths on pinite were analyzed; the results of the chemical analyses are shown in Table 1.
Pinite as a cordierite breakdown product is almost represented by a mixture of hydrous phyllosilicates i.e. phengite + chlorite + clay mineral phases (smectite?).The mixture of chlorite-white mica pinite is a known assemblage (e.g.DEER et al. 1962).The involvement of clay minerals is also reported (e.g.ÇERNY & POVONDRA 1967).
Generally, the Mg/Fe+Mg ratio of the micro-to crypto-crystalline pinitic mixture (phengite prevails) ranges from 0.14 to 0.63.The Na-free mixture (see analyses S2 & S3) with the Mg/Fe+Mg range of 0.27-0.35shows deficiens in Al tot (4.14-4.38 p.f.u.) as compared with mixture having Na (0.287-0.361 p.f.u.) -there Al tot range varies between 4.97-5.19p.f.u.The Si vs Al IV +Al VI relations deviate from the ideal muscovite-phengite join due to Tschermak substitution towards chloritic composition or more complex mixture including clay minerals which is reflected in the decrease of Al tot and Si with increase of Fe 2+ (Fig. 3A,  B).The K content varies from 1.385 to 1.759 p.f.u.

Disscusion and Conclusions
In exploration boreholes, at a depth of 110 m, pinite-cordierite spotted slates were found at Dudovica locality, about 8 km south of the Brajkovac granodiorite.Mineral assemblage (white mica + neobiotite + cordierite ± chlorite) and textural features indicate that these rocks were formed probably from low regionally metamorphosed argillaceous rocks of the DF in the temperature range 350-450 °C.
Pinite fractions within cordierite porphyroblast are composed of micro-to cripto-crystalline mixtures of  Table 1.Representative microprobe analyses of secondary chlorite and mixture phases from pinitized cordierite poikiloblasts.phengite, chlorite, clay minerals (smectite?)and quartz as well as of some amorphous gel-like material mixed with fine limonite material.Their formation could be considered as a retrogressive process involving hydration reaction caused by fluid infiltration released during the heating of the aureole (e.g.transformation of mixed-layered clay minerals into chlorite and rearrangement of illite into sericite i. e. phengite due to coupled substitution of Si and Fe 2+ or Mg for 2Al), or from the intrusion of the granitic body.This water with dissolved ions leached from the unstable clay and phyllosilicate minerals, or fluids released during the cooling of the granitic body can cause occurrence of various retrogressive products within the cordierites.
The brunsvigite-diabandite chlorite overgrowths on pinitized cordierite are probably formed in the temperature range 250-350 °C due to circulation of still hot hydrothermal K-bearing fluids according to the reaction: So far, the composition of pinite and its petrological significance has not been resolved yet.The SEM analyses done in this study are not sufficient for complete identification of pinitic phases.Detailed model reaction can only be done using more sophisticated methods as for example, TEM (Transmission Electron Microscopy), RS (Raman spectroscopy), and FTIR (Fourier Transform Infrared spectroscopy).
The first finding of pinite-cordierite spotted slates within the contact metamorphic aureole of the Brajkovac granodiorite contributes to a better understanding of granodiorite history as well as of its relationship with Paleozoic formations.

Fig. 3 .
Fig. 3.Chemical composition of pinitic mixture phases in Si vs. Al IV +Al VI (A) and Fe+Mg vs. Si (B) diagram; (C) composition of secondary chlorite overgrowths on pinitized cordierite poikiloblasts in the classification diagram of HEY (1954).