Boško Lugović

Early miocene post-collisional calc-alkaline magmatism along the easternmost segment of the periadriatic fault system (Slovenia and Croatia)

SummaryEarly Miocene calc-alkaline igneous rocks from the easternmost segment of the Periadriatic fault system can be subdivided into two series of different alkalinity: (1) Volcanics which occur in the vicinity of the Velenje-Rogatec Line range from high-alumina basalt via low-K and medium-K andesites to medium-K dacites. (2) In the Pohorje Mountains mafic rocks are lacking. A high-K tonalitic pluton crystallized at pressures of about 0.7 GPa as indicated by Al-in-hornblende barometry and accessory magmatic epidote. Rapid exhumation of the tonalite during dextral transtension along the Periadriatic fault system is indicated by tonalitic pebbles in Helvetian (?) clastic sediments. High-K andesitic to dacitic volcanics are interlayered with, and dikes cut, the clastic sediments.Compositional variations shown by the volcanics from the Velenje-Rogatec Line are consistent with fractionation of the observed phenocryst assemblages (olivine, plagioclase, clinopyroxene, orthopyroxene, titanomagnetite). Substantial fractionation of plagioclase is indicated by decreasing Sr/Nd and Sr/Y values and increasing negative Eu anomalies with increasing SiO2. All samples have chondrite-normalized HREE > 9.6 and low (Tb/Yb)n ratios (1.29−1.08). With increasing SiO2, the abundances of HREE and Y (18 to 39 ppm) increase and those of Sc (32.5 to 20.9 ppm) decrease slightly. These features, together with low Hf/Lu and Zr/Y values (10.1–5.7 and 5.7−3.6, respectively), rule out garnet as a major fractionating phase. Since (La/Yb)n values (3.24–6.59) do not increase with SiO2 and chondrite-normalized REE patterns do not show concave-upward shapes, fractionation of amphibole was probably insignificant. Although Rb/Cs values (⩽ 18) are generally low, a significant contribution by continental crustal materials to the magmas by an AFC process is suggested by negative correlations of Nb/U(7.16 to 3.14) and Nb/Th(2.14 to 0.87) values with SiO2, as well as by low Hf/Ta(7.8−3.4), and in part also by K/Rb (340-71) and Zr/Rb (5.0−1.7) values.Rocks from the Pohorje Mountains have high abundances of U. Values of Nb/U and Nb/Th are low (3.55 to 1.85 and 1.27 to 0.64, respectively) and are negatively correlated with SiO2. These features, in combination with high values of Ba/Nb (56 to 93), Ba/La (23–30), and Rb/Cs (19–56), as well as with previously published δ18O values (∼ 9‰) for the tonalite indicate a substantial contribution of the continental crust to these magmas. High Sr abundances (455 to 984 ppm) and moderate negative Eu anomalies suggest that fractionation of plagioclase was of minor importance. Although the rocks have relatively low Sc (9.3 to 3.8 ppm) and Y (21 to 14 ppm) contents, low (Tb/Yb)n values (1.67−1.14) indicate that garnet was not a major fractionating phase. Instead, fractionation of amphibole is a viable mechanism to explain the combination of high (La/Yb)n (21.8−13.1) and low (Tb/Yb)n.ZusammenfassungUntermiozäne Kalkalkali-Magmatite aus dem östlichen Bereich des Periadriatischen Lineaments gliedern sich in zwei Serien unterschiedlicher Alkalinität: (1) Entlang der Velenje-Rogatec-Linie treten High-Al-Basalte, Low-K- und Medium-K-Andesite und Medium-K-Dazite auf. (2) Im Pohorje-Gebirge, wo basische Magmatite fehlen, kristallisierte ein High-K-Tonalit bei Drücken von ca. 0.7 GPa (Al-in-Hornblende-Barometer, magmatischer Epidot). Gerölle dieses Tonalits in einer untermiozänen (Helvet?) klastischeu Abfolge belegen eine schnelle Heraushebung des Tonalits durch dextrale Transtension im östlichsten Bereich des periadriatischen Lineaments. Eingeschaltet in die klastischen Sedimente sind andesitische bis dazitische Pyroklastika und Laven sowie diskordante dazitische Gänge, die alle der High-K-Serie angehören.Die chemische Variation innerhalb der Vulkanitserie von der Velenje-Rogatec-Linie läßt sich durch Fraktionierung der Einsprenglingsminerale Olivin, Plagioklas, Klinopyroxen, Orthopyroxen und Titanomagnetit erklären. Abnehmende Sr/Nd- und Sr/Y-Werte und zunehmende negative Eu-Anomalien mit steigendem SiO2 weisen auf eine bedeutende Plagioklas-Fraktionierung hin. Alle Vulkanite besitzen hohe Gehalte an schweren Seltenen Erden (chondritnormiert > 9.6) und geringe (Tb/Yb)n-Werte (1.29−1.08). Mit zunehmendem SiO2 nehmen die Gehalte an Yb(2.02 bis 4.30 ppm) und Y(18 bis 39 ppm) zu, während die Sc-Gehalte (32.5 bis 20.9 ppm) nur geringfügig abnehmen. Diese Charakteristika, zusammen mit geringen Hf/Lu- und Zr/Y-Werten (10.1−5.87 bzw. 5.7–3.6), schließen Granat als wesentliche, fraktionierende Phase aus. Da die (La/Yb)n-Verhältnisse (3.24–6.59) nicht mit SiO2 korrelieren und die chondritnormierten SeltenenErden-Muster keine nach oben konkaven Formen aufweisen, spielte die Fraktionierung von Amphibol keine wesentliche Rolle. Trotz geringer Rb/Cs-Werte (⩽ 18) wird eine beträchtliche krustale Kontamination der Magmen angenommen. Für einen AFC-Prozeß sprechen negative Korrelationen von Nb/U(7.16−3.14) und Nb/Th(2.14−0.87) mit SiO2 (bei steigenden Nb-, U- und Th-Gehalten), geringe Hf/Ta-Werte (7.8−4.3) und teilweise auch geringe K/Rb- (340-71) und Zr/Rb-Verhältnisse (5.0−1.7).Die Magmatite des Pohorje-Gebirges weisen hohe U-Gehalte sowie geringe Nb/U-und Nb/Th-Verhältnisse auf (3.55−1.85 bzw. 1.27−0.64), die eine schwache negative Korrelation mit SiO2 zeigen. Zusammen mit hohen Ba/Nb-(56–93), Ba/La-(23-30) und Rb/Cs-Verhältnissen (19–56) sowie bereits publizierten δ8O-Werten (∼ 9‰) weisen diese Eigenschaften auf einen beträchtlichen Anteil assimilierten Krustenmaterials in den Magmen hin. Hohe Sr-Gehalte (455–984 ppm) und relativ geringe negative Eu-Anomalien machen es unwahrscheinlich, daß während des AFC-Prozesses in der Unterkruste wesentliche Mengen an Plagioklas fraktioniert wurden. Obwohl die Gehalte an Sc (9.3−3.8 ppm) und Y (21−14 ppm) relativ gering sind, scheidet Granat aufgrund der geringen (Tb/Yb)n- Verhältnisse (1.67−1.14) als dominierende Fraktionierungsphase aus. Die Kombination von hohen (La/Yb)n- Verhältnissen (21.8−13.1) und geringen (Tb/Yb)n-Verhältnissen deutet eher auf Amphibol als wichtige, fraktionierende Phase hin.

Chapter 28 Heavy and Light Mineral Fractions Indicate Polygenesis of Extensive Terra Rossa Soils in Istria, Croatia

Terra rossa is the most widespread soil type in Istria, Croatia, a region that has been affected by karst processes, (neo)tectonic activity and sediment supply since the Late Tertiary. Analysis of the heavy and light mineral fractions of the terra rossa in Istria reveals a polygenetic origin. For comparative purposes, the heavy and light mineral composition in the insoluble residue of underlying limestones and dolomites, and that in the insoluble residue of other potential source formations such as fl ysch and loess, have been analysed. The extent of pyroclastic infl uence on these terra rossa soils was also evaluated. Amphiboles, zircon, tourmaline, garnet, kyanite, clinopyroxene and orthopyroxene, present in both loess deposits in Istria and in the terra rossa, prove that detritus mineralogically similar to that found in Late Pleistocene loess has been incorporated into the terra rossa. Heavy mineral data indicate that material was also derived from Istrian fl ysch during terra rossa formation. Air-fall of particles from the Roman- Campanian Volcanic Province may also have contributed, and hypidiomorphic Kfeldspars accompanying clinopyroxene and orthopyroxene in some loess and terra rossa samples might be of this volcanic origin. It is diffi cult to estimate to what extent materials other than the insoluble residue of limestones and dolomites have contributed to terra rossa in Istria, though analytical data available to date suggest that this might have reached 50%. We conclude that the main external contributor is Middle Pleistocene loess, followed by fl ysch and tephra. Late Pleistocene loess may have become incorporated in the upper parts of already formed terra rossa. In the terra rossa on the island of Korcˇ ula, devitrifi ed pumice clasts, sanidine, hyalophane, Ti-andradite, potassic hastingsite, along with orthopyroxene and black mica, clearly represent volcanic input.

A TECTONO-MAGMATIC CORRELATION OF BASALTIC ROCKS FROM OPHIOLITE MÉLANGES AT THE NORTH-EASTERN TIP OF THE SAVA-VARDAR SUTURE ZONE; NORTHERN CROATIA; CONSTRAINED BY GEOCHEMISTRY AND PETROLOGY

Ophiolite melanges of Mts. Kalnik and Ivanscica form the northern sector of the Kalnik Unit and represent the northeasternmost exposed tip of the Sava- Vardar Suture Zone. Petrological; geochemical and isotopic characteristics of extrusive rock blocks incorporated in the melanges; particularly of pillow basalt altered to spilite; in association with radiolarian cherts; allow the rocks to be separated into five distinct groups. Each group has an age-marker based on the radiolarian assemblage in the associated cherts and/or on isotope dating which; along with previously published data on the Kalnik Unit; provide detailed analysis of the approximately 80 Ma long geodynamic evolution of the Repno Oceanic Domain - a part of the Meliata-Maliak ocean system. The rock groups are: (1) Late Anisian alkali basalts [(La/Lu)cn = 5.9-7.2; eNd(T) = +2.96; Cpx-TiO2 = 3-6wt%] related to a terminal phase of intracontinental rifting; (2) enriched E-MORB-type basalts [(La/Lu)cn = 2.4-2.8; eNd(T) = +4.74; Cpx-TiO2 = 2.1-2.9wt%] and depleted E-MORB-type basalts [(La/Lu)cn ~ 1.5; eNd(T) = +5.26; Cpx- TiO2 = 1.9-2.3wt%] which reflect successive proto-oceanic crust formed at the ridge during the early Ladinian and late Ladinian; respectively; (3) middle Carnian T-MORB-type basalts [(La/Sm)cn = 0.79-0.87; eNd(T) = +6.63; Cpx-TiO2 = 1.2-2.2wt%]; representing crust from an evolved spreading center; (4) NMORB- type basalts [(La/Sm)cn = 0.67-0.87; eNd(T) = +6.58; Cpx-TiO2 = 0.6-1.9wt%] with inherited supra-subduction signature that diminish from the Late Carnian basalts [(Nb/La)n = 0.81] to the Bajocian basalts [(Nb/La)n = 1.1]; and Middle Jurassic (Bathonian?) N-MORB-type basalts with recurrent subduction related influence [Nb/La)n = 0.72; (La/Sm)cn = 0.76; eNd(T) = +6.23]; (5) late Bathonian medium-Ti [(Nb/La)n = 0.24-0.72; eNd(T) = +6.30; Cpx-TiO2 = 0.6- 1.1wt%] to late Oxfordian low-Ti IAT-type basalts [(Nb/La)n = 0.12-0.22; eNd(T) = +6.31; Cpx-TiO2 = 0.2-0.6wt%] which reflect melts derived from a metasomatized and successively depleted mantle wedge in an extensional proto-arc setting.

Geochemistry, petrology and tectonomagmatic significance of basaltic rocks from the ophiolite mélange at the NW External-Internal Dinarides junction (Croatia)

Geochemistry, petrology and tectonomagmatic significance of basaltic rocks from the ophiolite mélange at the NW External-Internal Dinarides junction (Croatia) At the NW inflexion of the Sava-Vardar Suture Zone ophiolite mélanges, known as the Kalnik Unit, form the surface of the slopes of several Pannonian inselbergs in the SW Zagorje-Mid-Transdanubian Zone. The Mt Samoborska Gora ophiolite mélange, thought to be a part of the Kalnik Unit, forms a separate sector obducted directly onto Dinaric Triassic carbonate sediments. Basaltic rocks, the only magmatic rocks incorporated in the mélange, include Middle-Triassic (Illyrian-Fassanian) alkali within-plate basalts and Middle Jurassic (uppermost Bathonian-Lower Callovian) tholeiitic basalts. The latter sporadically constitute composite olistoliths, and are geochemically divided into N-MORB-like (high-Ti basalts) and transitional MORB/IAT (medium-Ti basalts). These geochemically different rocks suggest crystallization at various tectonomagmatic settings, which is also indicated by the rock paragenesis and host clinopyroxene compositions. Alkali basalts reflect melts derived from an OIB-type enriched mantle source [Ti/V= 62.2-82.4; (La/Lu)cn= 6.4-12.8] with Nd-Sr isotope signatures close resembling the Bulk Earth [εNd(T=235 Ma)= + 1.6 to + 2.5]. They are recognized as preophiolite continental rift basin volcanic rocks that closely predate the opening of the Repno oceanic domain (ROD) of the Meliata-Maliac ocean system. The high-Ti and medium-Ti basalts from composite blocks derived from a similar depleted mantle source (εNd(T=165 Ma) = + 6.01 vs. + 6.35) succesively metasomatized by expulsion of fluids from a subducting slab leading to a more pronounced subduction signature in the latter [Ti/V=31.6-44.8 and (Nb/La)n=0.67-0.90 vs. Ti/V=21.5-33.9 and (Nb/La)n=0.32-0.49]. These composite blocks indicate crust formation in an extensional basin spreading over the still active subducting ridge. The majority of high-Ti basalts may represent the fragments of older crust formed at a spreading ridge and incorporated in the mélange of the accretionary wedge formed in the proto-arc-fore-arc region. The Mt Samoborska Gora ophiolite mélange represents the trailing edge of the Kalnik Unit as a discrete sector that records the shortest stage of tectonomagmatic evolution related to intraoceanic subduction in the ROD.

Petrology, geochemistry and tectono-magmatic affinity of gabbroic olistoliths from the ophiolite mélange in the NW Dinaric-Vardar ophiolite zone (Mts. Kalnik and Ivanščica, North Croatia)

Mafic intrusive rocks are subordinately represented fragments of the oceanic crust in the ophiolite melange exposedat Mts. Kalnik and Ivanscica located in the NW Dinaric-Vardar ophiolite zone. This ophiolite melange occurs in thenorthern area of the Kalnik Unit and represents the SW surface boundary of the Zagorje-Mid-Transdanubian ShearZone. The melange, except for mafic intrusive rocks, consists of a chaotic mixture of various extrusive rocks formedin different tectonic settings of the Repno Oceanic Domain (ROD). The ROD was the segment of Neo-Tethys thatconnects the Meliata-Maliak and Dinaric-Vardar oceanic systems. Previous study of mafic extrusive sequences suggestedan 80 Ma period of tectono-magmatic evolution of the ROD from intra-continental rifting during the Anisian,to the formation of proto-arc crust during the Callovian-Oxfordian. The domain exposes ophiolitic rocks in four melangeareas. Isotropic gabbroic rocks that are abundant in two northern areas (Mts. Kalnik and Ivanscica), can be discriminatedinto three distinct geochemical groups: (A) N-MORB-type gabbro [(Th/Nb)n = 0.99–1.10; (Nb/La)n =0.95–0.99], (B) IAT-type amphibole gabbro with clear supra-subduction characteristics [(Th/Nb)n = 6.04–8.16; (Nb/La)n = 0.32–0.42] and (C) BABB-type amphibole-bearing gabbro [(Th/Nb)n = 2.88–4.02; (Nb/La)n = 0.58–0.69].Representative gabbro samples of each geochemical group were dated by the Ar-Ar and/or the K-Ar dating method.The Early Jurassic N-MORB-type gabbros (geochemical group A), ~185 Ma old, signifies a peculiar stage of Palaeo-Tethyan slab break-off. The Late Jurassic IAT-type gabbros (geochemical group B), ~147 Ma old, is the vestige of anascent intra-oceanic arc, whilst the Early Cretaceous BABB-type gabbros (geochemical group C), ~100 Ma old,provides evidence of magmatism in the back-arc marginal basin. The analyzed gabbroic rocks enable refinement andcompletion of the geodynamic evolution of the ROD, from the opening of an ensialic back-arc basin during the Ladinianand a continuous spreading event until the Bajocian. Intra-oceanic convergence was initiated in the Bathonian,with the formation of a nascent island-arc during the Tithonian, leading to formation of a Cretaceous ensimatic backarcmarginal basin. There are many lines of evidence that correlate the geodynamic evolution of the ROD with theAlbanide-Hellenide Neo-Tethyan oceanic segment.

Evidence of the spreading culmination in the Eastern Tethyan Repno oceanic domain, assessed by the petrology and geochemistry of N-MORB extrusive rocks from the Mt. Medvednica ophiolite mélange (NW Croatia)

Eastern Tethyan Repno oceanic domain (ROD) from Zagorje-Mid-Transdanubian Shear Zone bounds Meliata-Maliak and Dinaric-Vardar oceanic systems. The domain lacks ophiolite complexes and oceanic rocks are restricted into Early Callovian to Late Valanginian four ophiolite melanges integrated in Kalnik Unit. Previous researchs on ophiolitic rocks from Kalnik Unit provided high-resolution tectonomagmatic evolution of crust formation at different settings from Late Anisian to Cretaceous. N-MORB-type crust with peculiar suprasubduction signatures formed on spreading ridge from Late Carnian to Late Pliensbachian and first intraoceanic subduction rocks were dated to Late Bathonian. The newly discovered basaltic rocks in coherent slices with Latest Bajocian-Early Bathonian radiolarian cherts from Mt. Medvednica ophiolite melange sector completed the Late Pliensbachian-Late Bathonian chemostratigraphic gap in the domain. The analyzed unfractionated extrusive rocks lack any influence of a subduction related component [(Nb/La) n = 1.16-1.34; Th/Ta = 0.99-1.05] and are akin to proper N-MORB compositions [(Nd/Lu) N-MORB ~ 1.1; (La/Lu) cn = 0.82-0.92] that were derived from slightly depleted mantle source [e Nd(T=170 Ma) = +6.21 to +6.27; ( 87 Sr/ 86 Sr) i = 0.703365 to 0.703511]. The rocks are interpreted as vestiges of the middle ocean ridge crust formed during culmination stage of spreading in the ROD.

Si-P impure Al-Goethite Mineralization on the island of Dugi Otok (Central Adriatic, Croatia)

At the western seaward coast of the island of Dugi Otok, unusual goethite mineralization occurs as the infilling of a system of thin crevices in the Upper Cretaceous limestones of the Adriatic carbonate platform. Goethite is exceptionally well ordered and shows a variety of chemical compositions ranging from almost pure FeO·OH to Al-substituted composition accompanied by high Si and P impurity. The most characteristic textural type assemblages are goethite and coarse grained calcite in composite aggregates. These aggregates contain Al-substituted goethite with metamorphic silicate minerals embedded in the core, (black goethite) armoured by the XRD-pure, almost stoichiometric goethite (brown goethite). The geochemical and mineralogical features of the mineralization suggest an ascending solution from a hidden metamorphic source. We speculate that the source could have been activated by the thermal influx linked to the Pliocene intraplate magmatism in the eastern segment of Adria.