GEOCHEMISTRY AND PETROGENESIS OF OLIGOCENE DACITES FROM THE CENTRAL BOSNIA AND HERZEGOVINA WITH INSIGHT IN THE POST- COLLISIONAL TECTONIC EVOLUTION OF CENTRAL DINARIDIC OPHIOLITE BELT

Magmatic rocks of post-Late Eocene magmatic formation are widespread in the Sava segment of Sava- Vardar suture zone and adjoin areas. The rocks formed as a response to transpressional-transtensional tectonic activity preceded by the Cretaceous-Eocene compression of the Internal Dinarides and Tisia Unit as fragments of Eurasian continental lithosphere. Central Bosnia Tertiary volcanic rocks (CBTVR), erupted as dacites in Lower Oligocene, are peculiar rocks of this formation either by their location (southernmost distal outcrops) or geological setting (extrusive within the melange of the Internal Dinaride Ophiolite Belt). Major element composition of the CBTVR reveals high-K calc-alkaline geochemical affinity whereas trace element discriminate the rocks as shoshonitic. The rocks are LILE-enriched and show negative Ta- Nb, P and Ti anomalies, and positive Pb anomalies typical of subduction related volcanic rocks. Chondrite-normalized REE patterns exhibit significant LREE/HREE enrichment [(La/Yb)cn = 21.4 - 33.4]. Geochemical affinity of the CBTVR combined with tectonic position of extrusions suggests derivation of the melts from the subcontinental mantle which had inherited strong orogenic signature during an ancient subduction.

Crustal architecture of a metallogenic belt and ophiolite belt: implications for mineral genesis and emplacement from 3-D electrical resistivity models (Bayankhongor area, Mongolia)

Crustal architecture strongly influences the development and emplacement of mineral zones. In this study, we image the crustal structure beneath a metallogenic belt and its surroundings in the Bayankhongor area of central Mongolia. In this region, an ophiolite belt marks the location of an ancient suture zone, which is presently associated with a reactivated fault system. Nearby, metamorphic and volcanic belts host important mineralization zones and constitute a significant metallogenic belt that includes sources of copper and gold. However, the crustal structure of these features, and their relationships, are poorly studied. We analyze magnetotelluric data acquired across this region and generate three-dimensional electrical resistivity models of the crustal structure, which is found to be locally highly heterogeneous. Because the upper crust (< 25 km) is found to be generally highly resistive (> 1000 Ωm), low-resistivity (< 50 Ωm) features are conspicuous. Anomalous low-resistivity zones are congruent with the suture zone, and ophiolite belt, which is revealed to be a major crustal-scale feature. Furthermore, broadening low-resistivity zones located down-dip from the suture zone suggest that the narrow deformation zone observed at the surface transforms to a wide area in the deeper crust. Other low-resistivity anomalies are spatially associated with the surface expressions of known mineralization zones; thus, their links to deeper crustal structures are imaged. Considering the available evidence, we determine that, in both cases, the low resistivity can be explained by hydrothermal alteration along fossil fluid pathways. This illustrates the pivotal role that crustal fluids play in diverse geological processes, and highlights their inherent link in a unified system, which has implications for models of mineral genesis and emplacement. The results demonstrate that the crustal architecture—including the major crustal boundary—acts as a first‐order control on the location of the metallogenic belt.

A Kinematic Reconstruction of Jurassic ocean spreading from the ophiolites of California, the western U.S. using structural geology and paleomagnetism.

&lt;p&gt;The western edge of the North America plate contains geological records that formed during the long-lived convergence between plates of the Panthalassa Ocean and North America. The geology of different segments along western North America indicates different polarities (eastward and westward) for subducted slabs and thereby various tectonic histories and settings. The western United States (together with Mexico) plays a key role in this debate, many geologic interpretations assume continuous eastward subduction in contrast to observations within proximal geologic segments and tomographic images of the lower mantle below North America and the eastern Pacific Ocean which suggest a more complex subduction history. In this study, we aim to evaluate the plate tectonic setting in which the Jurassic ophiolites of California formed. Geochemical data from these ophiolites suggest that they formed above a nascent intra-oceanic or continental margin subduction zone. We first developed a kinematic reconstruction of the western US geology back to the Jurassic based on published structural geological data. Importantly, we update the reconstruction of the various branches of the San Andreas fault system to determine the relative position of the ophiolite fragments and adopt a previous restoration of Basin and Range extension which we expand northward towards Washington state. We then reconstruct North American margin deformation associated with Cretaceous to Paleogene shortening and strike-slip faulting. We find no clear candidates in the geological record that may have accommodated major subduction between the Jurassic ophiolite belt and the North American margin and consequently concur with the school of thought that considers that the ophiolite belt, as well as the underlying subduction-accretionary Franciscan Complex, likely formed in the North American fore-arc. We collected paleomagnetic data to reconstruct the spreading direction of the Jurassic Californian ophiolites, by providing new paleomagnetic data from sheeted dykes of the Josephine and Mt. Diablo Ophiolites. These suggest a NE-SW paleo-ridge orientation, oblique to the North American margin which may be explained by partitioning of a dextral component of subduction obliquity relative to North America. We used this spreading direction in combination with published ages of the ophiolites and our restoration of the relative position of these ophiolites prior to post-Jurassic deformation to construct a ridge-transform system at which the Jurassic ophiolites accreted. The results will be used to evaluate which parts of the subduction systems that existed in the eastern Panthalassa Ocean may reside in the western US, and which parts may be better sought in the northern Canadian Segment or/and in the southern Caribbean region.&lt;/p&gt;

ACCESSORY SULPHIDES AND ARSENIDES FROM CHROMYTITES OF KHARCHERUZ ULTRAMAFIC MASSIF, THE POLAR URALS

The study is focused on accessory sulfide mineralization revealed in massive chromitites of Kharcheruz ultramafic massif, which is part of the Khadatinsky ophiolite belt closing ophiolite complexes of the Polar Urals in the north. Three paragenetic associations of accessory sulfides associated with various processes of host chromitites formation and transformation are identified. Typomorphic and chemical features of ore minerals are characterized.

Minerogenesis sequence and processes in weathering mantle of ultrabasitic rocks of the Serov-Mauk ophiolite belt (Northern Urals)

The geological structure and mineral composition in weathering mantle of ultrabasitic formations of the Serov-Mauk ophiolite belt are comprehensively studied. Considering complex geological history of the weathering mantle, the studies involved a wide range of precision research methods-optical microscopy, X-ray diffraction analysis, thermal tests, Xray microspectral analysis and Raman spectrography. As a result, the minerogenesis sequence was disclosed, and the modern genetic classification of minerals in ultrabasites and weathering mantle was developed. The classification distinguishes between hypogene and supergene minerals in weathering mantle of ultrabasitic rocks of the Serov-Mauk ophiolite belt. Hypogene minerals are divided into four groups-magmagene-relict, oceanic waste minerals, low-grade metamorphogenic-relict and hydrothermal relict minerals. Supergene minerals are grouped into residuum and rocks superimposed in infiltration. Residuum is divided into two groups of diffusion and infiltration minerals. The research findings revel the critical geological processes connected with evolution of ultrabasitic formations and weathering mantle of the Serov-Mauk ophiolite belt. The drawn conclusions prove hydrothermal processes and low-grade metamorphism in the history of rock mass and weathering mantle of the Serov-Mauk ophiolite belt, and can be used for the prognostics and prospecting of nickel ore bodies.