The characterization of felsic medium-grade metamorphic rocks on the basis of their protoliths delivers key information on the evolution of a specific geological unit mainly composed of such rocks. Various attempts have been undertaken in the past to reach unequivocal results regarding the protolith. In our case study area in the central Erzgebirge Crystalline Complex being part of the northern Variscan Bohemian Massif in Central Europe, high-pressure metagranitoids and paragneisses dominate. It was tested to see if these previously mapped gneiss types were assigned to the correct kind of protolith using various chemical parameters obtained from X-ray fluorescence analyses of 82 samples. Except for two equivocal samples and one igneous rock (SiO2-rich saidenbachite), all others could be categorised either as metagranitoids (34 samples) or as metasediments (paragneisses + minor micaschists) based on the method of Hasterok et al. (2019), the classical AFM diagram for metamorphic rocks (after projection from quartz and K-feldspar), the vanadium or chromium contents, and the Ti/Zr (in ppm) versus the Zr (in ppm)/Al2O3 (in wt.%) diagram. The latter two methods yielded the most conclusive discrimination results. The metagranitoids were originally S-type granites. The protoliths of the metasediments were either pelites (18 samples) or psammites (27 samples), which were probably greywackes that could have been deposited in Early Palaeozoic or Precambrian times.
This paper provides a review of Late-Paleozoic skarns in the Erzgebirge/Krusne hory region (Germany/Czech Republic) with a particular emphasis on the Aue-Schwarzenberg and Geyer-Ehrenfriedersdorf Districts that host the most prolific skarn systems. Most skarns in the Erzgebirge replace carbonate units that occur intercalated with metamorphosed sedimentary and volcano-sedimentary host rock successions of pre-Variscan age. Skarn compositions range from calcic to magnesian, depending on the protolith. The timing of skarn formation is constrained by U-Pb ages of garnet and cassiterite, ranging from similar to 340 to similar to 295 Ma, broadly coinciding with different episodes of late- to post-Variscan magmatism. Textural and geochronological data implies that many of the skarns are multi-generational, with hiatuses between individual stages of up to 5 -10 Ma, indicating that they are the product of several magmatic-hydrothermal events. Early skarns are commonly characterized by skarnoid textures typical for heat-dominated metasomatism (rock-buffered), whereas younger skarns mainly show metasomatic textures and open space-infill typical for fluid-dominated metasomatism (fluid-buffered). We postulate that the relationship between timing and texture is related to uplift and exhumation of the Erzgebirge following the collisional phase of the Variscan orogeny. Most of the Sn mineralization is hosted in cassiterite, commonly intergrown with retrograde actinolite and chlorite. However, a significant portion of Sn remains locked within prograde calc-silicate phases (e.g., malayaite, Sn-bearing garnet), rendering it inaccessible for economic extraction. Indium mineralization is mainly hosted by sphalerite and less importantly other sulfides that are also associated with the retrograde stage. Scheelite is the main host of W, which shows a preferred affiliation to calcic skarns and occurs during the pro- and retrograde stage.
To constrain the petrogenesis of silicic magma along the Cameroon Volcanic Line (CVL), zircon chemistry is combined with U-Pb ages from two rhyolite samples obtained from Tertiary lithostratigraphic strata separated by a basaltic unit in the Sabga area. Ti in zircon temperatures for both rhyolites mostly vary from 636 -770 degrees C and 644 -779 degrees C for samples CMR01 and CMR02, respectively. Calculated oxygen fugacity values are below the quartz-fayalite-magnetite buffer (log fO2 = -14.5 to -10.8 for CMR01 and -14.2 to - 9.3 for CMR02). The inherited zircon from both samples yielded temperatures values ranging from 654 to 738 degrees C with a corresponding log fO2 between -12.0 to -16.2. These data suggest that both rhyolites share a common, strongly reduced deep crustal source but experienced minor, short-lived thermal variations associated with the separating basaltic unit. The evolution of magma through open-system fractional crystallization coupled with formation of REE-rich accessory minerals occurring concurrently is indicated by REE patterns and systematic co-variation between hafnium and trace element ratios (e.g., Hf vs. Th/U, Hf vs. Sm/Yb, and Hf vs. U). Epsilon Hf values for Tertiary zircon vary from -10.6 to 6.8 and also reveal Mesoproterozoic and Silurian mantle model ages (TDM) between 1144 and 440 Ma. Lu-Hf data disclose mixing between magmas from a juvenile mantle source and Palaeoproterozoic-Neoproterozoic basement rocks with TDM of 2511- 944 Ma.
The area of the Slavonian Mountains in northern Croatia is considered as a natural laboratory for the study of metamorphic processes on polycyclic metamorphic rocks formed during the pre-Variscan, Variscan, and Alpine orogenies. In this area, strongly foliated micaschists and/or paragneisses metamorphosed at amphibolite-facies conditions are mainly composed of quartz, plagioclase, and biotite. In addition, white mica, opaque and accessory minerals, and 2-3 vol.% of garnet occur. Two-types of garnet crystals are present; crystals up to 200 mu m in size with composite and atoll texture belong to the first type; chemically homogenous, up to 50 mu m small crystals comprise the second type. Only the "large" garnet crystals comprise three different domains: a core (garnet I), an intermediate zone (mantle; garnet II) rich in mineral inclusions of the previous rock matrix (quartz, biotite, plagioclase) and a nearly inclusion-free rim (garnet III) compositionally identical to the small garnet crystals. The core of the "large" garnets shows higher Ca (16-28 vs. 2-3 mol.% grossular) and lower Fe contents (49-66 vs. 74-77 mol.% almandine) than the rim, whereas Mg and Mn contents are hardly variable. The garnet mantle (garnet II) is characterized with 6-11 mol.% grossular and similar to 70 mol.% almandine components. Furthermore, the three garnet domains show outer boundaries pointing to resorption. Pseudosection modelling was used to reconstruct pressure-temperature (P-T) paths. The garnet core represents a first metamorphic cycle with a clockwise P-T path recording a minimum peak pressure of similar to 800 MPa at 600 degrees C. The previously reported P-T conditions of 1000 -1200 MPa and 650 degrees C are compatible with rutile inclusions in the garnet core. These conditions were followed by a pressure decrease, which caused resorption of garnet. A second stage of garnet growth followed due to a second clockwise P-T path with peak conditions of 500 MPa and 560 - 590 degrees C. In-situ dating of matrix monazite yielded two mean ages at 514 +/- 6 and 467 +/- 13 Ma, which were assigned to the aforementioned peak events. A low-pressure, medium-temperature, andalusite-forming event (< 400 MPa, 540-580 degrees C) responsible for garnet rim growth is tentatively associated with the known Variscan event and a third clockwise P-T path overprinting metamorphic rocks from the Slavonian Mountains.
A small body of jadeite quartzite from the ultrahigh-pressure (UHP) region of the Dabie Shan was studied to contribute to the solution of the question whether the abundant UHP bodies of different rock types in this region had experienced similar or significantly different peak pressures. A sample from the jadeite quartzite body occurring at the village of Xinjian is composed of the low-variance assemblage of coexisting phengite + Na-amphibole + Na-rich clinopyroxene + garnet + rutile + quartz-coesite. Thermodynamic modelling of this sample indicates a peak pressure around 28 kbar at about 700 degrees C. This peak pressure is significantly lower than those (>= 40 kbar) determined for nearby located UHP rock lenses. It is proposed that the rocks of the UHP lenses were exhumed from different depths in a subduction channel that was generated during oceanic subduction. After exhumation in the subduction channel these rocks were inserted in Na-rich clinopyroxenefree high-pressure gneiss, which has experienced peak pressures <= 15 kbar.
The-150 km long Bavarian Pfahl is a major inactive shear zone cutting through Late Palaeozoic basement of the Moldanubian zone of the Variscan orogen in central Europe. Due to the absence of conventional mineral geochronometers, it long presented a challenge for radiometric dating. Here, we present high spatial resolution U-Pb analyses of rare anatase crystals by large-geometry secondary ionization mass spectrometry yielding a 209 +/- 28 Ma (2 sigma uncertainty) concordia intercept age. This age is more precise than previous Rb-Sr age determinations on sericite-and fluid inclusion-bearing quartz. The youngest Variscan granite intrusions around the Pfahl zone are thus c. 100 Ma older than the Bavarian Pfahl quartz lode, ruling out magmatic fluid sources. Common 207Pb/206Pb = 0.8619 +/- 0.0039 derived from linear regression of anatase analyses overlaps with values for galena mineralization within and adjacent to the Pfahl zone, pointing towards hydrothermal leaching of common basement sources. Oxygen isotopic equilibrium thermometry for quartz and TiO2 phases suggests anatase formation at 162 +/- 4 degrees C, overlapping previously estimated quartz precipitation temperatures, albeit at their lower range. A Late Triassic extensional geodynamic environment is inferred for the emplacement of the Bavarian Pfahl, which significantly postdates late-orogenic deformation. With the caveat that only few radiometric age determinations are available, the new age for the Bavarian Pfahl falls in between reported ages for other giant quartz veins in the Variscan basement of Germany.
Utilizing laser ablation inductively coupled plasma mass spectrometry (LA-ICP-MS), the rare earth element (REE including Y) trace element chemistry of garnet and fluorapatite and the Th + U contents in fluorapatite were investigated for a series of samples across a traverse. This traverse covers an orthopyroxene-bearing localized dehydration zone in a migmatised granitic gneiss, S & ouml;ndrum stone quarry at Halmstad in SW Sweden. The rocks of the traverse were affected by two metasomatic events. While previous studies of the same traverse indicate a general homogeneity for the silicate and fluorapatite major element chemistry per sample, heavy rare earth element (Y+HREE), Th, and U show a greater variability between individual grains of garnet (Y+HREE) and fluorapatite (Y+HREE, Th, U) per sample from the dehydration zone compared to samples from the surrounding granitic gneiss. This is attributed to variable partitioning of these elements into the grain boundary CO2- and F-bearing fluid where they complexed with F during the second metasomatic event. The HREE-Th-U variability in both the garnet and fluorapatite was due to high localized variable concentrations of F in the grain boundary fluids where the source of the F was the F-bearing silicate minerals in the dehydration zone. Hence grain boundary fluids adjacent to a F-bearing silicate mineral (biotite and amphibole) would have relatively high F concentrations compared to grain boundary fluids not in the vicinity of F-bearing silicate minerals. Garnet and fluorapatite in contact with grain boundary fluids with a high F content would partition more HREE, Th, and U into the fluid. In contrast, garnet and fluorapatite grains in contact with low F grain boundary fluids would not partition these elements into the fluid. This would allow the garnet to retain its original HREE content and the fluorapatite to retain its original HREE, Th, and U content.
The Bozin gabbro pluton, located in northwestern Iran (Zanjan), is one of the basic plutons in the central Iranian Eocene magmatic arc. The pluton is nearly homogenous in mineralogy (almost gabbro, with minor diorite) but heterogeneous in textures (i.e., medium granular, intergranular, and mineral zonation). The major minerals of the igneous body, namely plagioclase (in a wide compositional range of An12.55 - 99.58) and titaniferous diopside, were crystallized under high temperature (1070 -1250 degrees C) and pressure (4 -10 kbar) conditions. Moreover, magmatic minor minerals include alkali feldspar, hornblende, ilmenite, magnetite, and titanite. The absence of any thermal effects due to contact metamorphism within the surrounding rocks (such as the Cretaceous slates, metavolcanites and metalimestones) indicates an emplacement of the pluton prior to Cretaceous sedimentation. Furthermore, the secondary mineral assemblage (tremoliteactinolite+picnochlorite+prehnite +/- oligoclase) of the gabbroic pluton points to a low-grade greenschist (T approximate to 160 - 350 degrees C) metamorphic overprint. The pluton belongs to the calc-alkaline series and shows some characteristics of the subduction setting including the positive anomalies of Pb, Th, and large ion lithophile elements. Moreover, the composition of pyroxenes verify that they were originated in an orogenic arc setting. Also, the gabbro whole rock data fall in the transitional field of mantle and volcanic arc arrays. However, they represent some alkaline tendencies, too. Petrogenetic consideration, chemical variation, and geochemical modeling diagrams of trace elements show that the primary magma originated from a deep mantle source in a subduction regime and ascended through the continental lithosphere. During ascent, the magma was slightly contaminated. Subsequently, the ascending magma was differentiated into two stages. At first, the basic minerals diopside and calcic plagioclase were crystallized in the deep under low fO2 conditions. Then, ilmenite, magnetite, sodicalkali feldspar, and hornblende were formed in a shallow depth under high fO2 conditions.
The Central Asian Orogenic Belt's ophiolites lack a full ophiolitic succession and instead occur as irregular fragments and slices in tectonic melanges and accretionary complexes. The Teskuduk and Northern Nurata ophiolites in Uzbekistan comprise extensively serpentinized mantle rocks that contain minor podiform chromitites, situated in the western area of the southern Tien Shan belt. Chromites in this region are found in a very limited area, and this study aims to determine the geochemical properties and formation environments of chromites by conducting geochemical studies on a limited number of recently discovered chromitites. In this context, mineral chemistry and platinum group element (PGE) geochemistry analyses were carried out on chromites. For Teskuduk chromitites, the Cr2O3, Al2O3, and TiO2 contents range from 53.2 to 55.7wt%, 11.1 to 12.7wt%, and 0.14 to 0.20 wt%, respectively, whereas for Nurata chromitites, the corresponding values range from 36.2 to 37.2 wt%, 23.1 to 30.2 wt%, and 0.10 to 0.30 wt%. The Teskuduk and Nurata chromitites contain a total average of 370 ppb and 117 ppb PGEs, respectively. They are classified as unfractionated chromitites, with low Pd/Ir ratios (0.03-0.20). These chromitite samples have chondrite-normalized PGE patterns that are identical to Cr-rich mantle-derived chromitites found in subduction-related ophiolites. Collectively, these data suggest that the Teskuduk ophiolites formed in a forearc setting, whereas the Nurata ophiolites evolved in a back-arc setting within the same oceanic branch.
The Early Permian (c. 290 Ma) Panjal Traps is the most extensive sequence of flood basalts within the Tethyan domains of the Himalaya. The Panjal Traps erupted during a period of tensional plate stress related to the rifting of Cimmerian terranes from the Tethyan margin of Gondwana. The majority of the mafic Panjal Traps were affected by postemplacement low-temperature deuteric alteration and/or regional deformation. Consequently, constraining the magmatic conditions of the rocks is difficult. The least altered Panjal Traps are located within the Guryal Ravine section of the western Zanskar Range and the southern Pir Panjal Range. The rock and mineral textures are preserved and they have a primary mineralogy of clinopyroxene (Wo(30.3-42.5)En(29.4-49.7 )Fs(13.8-34.2)) and plagioclase (An(61.0-43.5)) phenocrysts within an aphanitic matrix. Secondary minerals include chlorite, epidote, actinolite, quartz, albite, orthoclase, rutile, and titanite and indicate that some of the rocks underwent greenschist to sub-greenschist facies metamorphism. Clinopyroxene-liquid geothermobarometers were used to assess the equilibrium crystallization temperature and pressure of the least altered Panjal Traps. The clinopyroxene-liquid saturation conditions yielded temperatures of 1104-1184 degrees C and pressures of 1.8-7.0 kbar which are within the uncertainty of the jadeite-diopside-hedenbergite exchange thermometer (1064-1167 degrees C) and the Al exchange barometer (1.1-6.8 kbar). The equilibrium temperatures are not anomalously high and similar to lavas that erupt at a passive rift setting rather than from a mantle plume. The whole rock V/Sc and V/Ga ratios suggest that the oxygen fugacity of the lavas was likely at or below the fayalite-magnetite-quartz buffer (triangle FMQ triangle FMQ = 0 to-1). Rhyolite-MELTS modeling (triangle FMQ triangle FMQ = -1; P = 2 kbar) indicates the water content within the basalts was variable and probably did not exceed 2.25 wt% prior to eruption. The post-emplacement metamorphic conditions are constrained using Perple_X modeling and demonstrate that the rocks from both Guryal Ravine and southern Pir Panjal Range (P = < 3.0 kbar and T = 390 - 415 degrees C) underwent greenschist facies metamorphism at similar temperature. However, the pressure is not well constrained. We attribute the metamorphism to regional deformation associated with terrane accretion to the Indian plate during the Mesozoic and Cenozoic that occurred after the deposition of the Late Permian to Early Triassic Pangea megasequence but before the Oligocene.
Calc-alkaline to shoshonitic Eocene and alkaline Oligocene volcanic rocks are exposed in Godar-e-Siah and Toveiereh areas, respectively, northwest of the Central-East Iranian Microcontinent (CEIM). Granulitic xenoliths have been found in these volcanic rocks. The Godar-e-Siah xenoliths comprise the Ca-poor plagioclase (An33) 33 ) + phlogopite + corundum + sillimanite + spinel +/- garnet. This mineral assemblage corresponds to conditions characteristic of peak of granulite facies metamorphism. The Toveireh xenoliths consist of spinel and plagioclase as major minerals and corundum, rutile, ilmenite and magnetite as accessory ones. The presence of Al-rich minerals and the absence of quartz suggest the Al-saturated but Si-undersaturated nature for the xenoliths. Mineralogical characteristics, thermobarometry estimates and use of experimental petrogenetic grids indicate that the estimated P-T conditions for Toveireh (8 -10 kbar, 800 - 900 degrees C) and Godar-e-Siah (7.8 kbar, 780 degrees C) xenoliths are consistent with the granulite facies rocks near anatectic condition. The Toveireh xenoliths are Al-rich granulites (Al2O3 2 O 3 = 33 - 34 wt%) and have LREE-enriched patterns with large positive Eu anomalies (Eu/Eu* = 3 - 5). These patterns indicate that the plagioclase rich restites of the lower continental crust developed as a result of the removal of Neoproterozoic-Cambrian S-type granitic magma. The Aeirakan S-type granites and the xenolith bearing rocks are located at the northwest of the CEIM along the Great Kavir Fault. The parental magma of the Aeirakan S-type granite which is located at northeast of the xenolith bearing sites (Toveireh and Godar-e-Siah) is formed by anatexis and dehydration melting of such Al-saturated Si-undersaturated crustal granulites during Pan-African orogeny. It is probable that some parts of these dehydrated materials are brought to the surface as granulitic xenoliths by Eocene and Oligocene volcanism in Godar-e-Siah and Toveireh areas, respectively.
Petrography and mineral chemistry of mantle xenoliths recovered from Early Miocene trachybasaltic lavas near Sabga village in northwest Cameroon reveal the complex evolution of the underlying lithospheric mantle. The xenoliths are distinguished into harzburgite and lherzolite that are ovoid and angular in shape and show protogranular and equigranular textures, respectively. Major mineral phases are olivine, orthopyroxene and clinopyroxene. Electron microprobe analyses indicate high-Mg olivine (Fo 88.5 - 89.7 for lherzolite and 91.1- 91.2 for harzburgite). Variations in major oxide abundances, Mg#, Cr# and Ca/Al ratios of clinopyroxene are present within each xenolith type, leading to distinction of two clinopyroxene groups. Moreover, spinel is Al-rich in lherzolite (Cr/[Cr+Al] = 0.09) and Cr-rich (Cr/[Cr+Al] = 0.46 - 0.47) in harzburgite. Calculated equilibrium temperatures (793 -1211 degrees C for lherzolite and 822 -1067 degrees C for harzburgite) agree with mantle xenolith temperatures from other locations worldwide. Estimated equilibrium pressures are 1.1- 3.5 GPa for lherzolite and similar to 1.3 GPa for harzburgite. Corresponding sampling depths are 36 -115 km and similar to 43 km for lherzolite and harzburgite, respectively. The lithospheric mantle below Sabga volcano has experienced melt extraction that is superimposed on cryptic metasomatism involving silicate and carbonate fluid phases. Because of this metasomatism recorded in the Sabga xenoliths, it is conceivable that the lithospheric mantle beneath the western highlands of Cameroon will continue to produce CO2-rich 2-rich magmas where outgassing during volcanic eruptions is particularly hazardous.
Late Cretaceous volcanic rocks in the Halab-Avaj-Nobaran region of the northern Urumieh-Dokhtar Magmatic Arc in Iran are relicts of Mesozoic activity when subduction-related magmatism migrated inland. They include subalka-line (transitional) basalts, basaltic trachyandesites, and dacites, which formed in association with various pyroclastic and sedimentary rocks. Their trace element composition is characterized by enrichment in large ion lithophile relative to high field strength elements, sometimes showing negative anomalies in Nb and Ta when normalized to primitive mantle, and rare earth element (REE) chondrite-normalized patterns are enriched in light REE with nearly flat heavy REE segments. Initial 87Sr/86Sr ratios and eNd(i) of the studied rocks range from 0.7040 to 0.7061 and - 2.91 to +4.14 calculated for an age of 80 Ma, respectively. The whole set of geochemical information is compatible with generation of the parental magma by decompression and partial melting of metasomatized peridotite in the mantle wedge that experienced local extension within a regional compressional tectonic regime caused by subduction of Neo-Tethyan oceanic lithosphere. The local extensional conditions may have been responsible for the formation of pull-apart structures in the northern Urumieh-Dokhtar magmatic arc that channeled magma rising towards the surface.
The Emmelberg scoria cone in the West Eifel Volcanic Field, Germany, contains crystal-rich enclaves of ultramafic and evolved composition that were ejected during the terminal phase of the eruption. Chemical compositions of clinopyroxene in ultramafic enclaves (clinopyroxenites +/- phlogopite and amphibole) partly overlap with those in the olivine-nephelinite host lava, suggesting crystallization from consanguineous magmas that occurred near the crust-mantle boundary (29 +/- 3 km depth) as constrained by clinopyroxene-melt barometry. Fluid inclusion barometry of ultramafic enclaves, by contrast, indicates temporary magma ponding at shallower crustal levels (11 +/- 2 km). Evolved enclaves are syenites with a paragenesis of alkali feldspar (dominantly anorthoclase), nosean, magnetite, and biotite with comparatively low Mg-numbers (Mg# = 41- 51). Although no direct barometric constraints are available for the syenite enclaves, evolved melt generation in upper-crustal magma reservoirs is inferred from published experimental phase relations at depths that are similar to those indicated by fluid inclusion barometry for ultramafic enclaves. Accessory zircon in syenite enclaves reveals multiple differentiation and zircon crystallization episodes between c. 235 and 85 ka, predating the published erup- tion age of c. 49 ka. In some cases, early formed zircon subsequently recrystallized, leading to resetting of U-Th ages, trace elemental and O isotopic compositions, whereas Hf isotopic compositions remained unchanged. This suggests that evolved upper crustal intrusions were affected by hydrothermal alteration, where trace element enriched magmatic zircon was replaced presumably at subsolidus conditions via dissolution-reprecipitation processes. These results indicate a complex, polybaric, and multi-episode magma plumbing system underneath the Emmelberg scoria cone, where magma intrusion and differentiation occurred in the subsurface without contemporaneous eruptions.
Ilmenite is a widespread accessory mineral in tonalite-trondhjemite-granodiorite (TTG) gneisses of the Banded Gneissic Complex (BGC-I) from the Aravalli Craton. Studied ilmenites are found as an inclusion in micas, magnetite and has low Mg (similar to 0.00 - 0.21 wt%) and high Mn contents (3.94 -10.29 wt%) in contrast to the ilmenites from igneous rocks. Studies that record the transformation of Fe-Ti oxides under low- to medium-grade metamorphic conditions are very restricted. The rocks of BGC-I of the Aravalli Craton have experienced amphibolite-facies metamorphism under medium grade metamorphic conditions and the effects of this metamorphism can be studied in the Fe-Ti oxides. Petrographic, bulk rock and mineral chemical data reveal that the primary magmatic composition of ilmenite has been diversely modified during metamorphism by diffusion re-equilibration with a co-existing silicate mineral. Biotite being the dominant mafic mineral in the TTG gneisses, has re-equilibrated during metamorphism and the Ti-in-biotite geothermometer indicates that re-equilibrated primary biotite crystallized at temperatures between 530 degrees and 680 degrees C. Therefore, enhanced Mn diffusion out of biotite into ilmenite under oxidizing conditions provides an explanation for high Mn contents in ilmenite. This further infers short-range equilibration during medium-grade metamorphism of the BGC-I from the Aravalli Craton.
A complete section of the Bitlis Massif crops out in the Avnik and unaldi regions (E Turkiye). It comprises a core of Precambrian-Early Cambrian gneisses, mafic metavolcanic rocks with Kiruna-type magnetite-apatite ores and metagranite bodies. Unconformably overlying this core is a cover succession made up of Silurian to late Cretaceous micaschists and marbles. The Kiruna-type ore beds, focused on this paper, range from centimetres up to 20 m in thickness and are concordantly interbedded with mafic metalavas, detrital magnetite-rich metavolcanoclastic rocks, and metachert beds. Geological characteristics of the metamorphosed ore-bearing volcanic and volcanosedimentary succession indicate that the magnetite-apatite ores were formed in a shallow marine environment by volcanic exhalative and sedimentary processes. U-Pb zircon ages obtained from the interlayered basaltic flows are 578 - 542 Ma, marking the maximum age of ore formation. Geological and petrological characteristics of the basaltic association in the Precambrian units indicate that their formation took place in a forearc environment along the northern margin of Gondwana. This active magmatic margin was formed above the south dipping Prototethys oceanic crust, which was carrying a small oceanic or continental fragment. When this microcontinent collided with Gondwana, south-verging compressional deformation, high-grade regional metamorphism, and syntectonic intrusion of the Avnik and Yayla metagranites occurred. U-Pb zircon ages of 521- 506 and 521- 503 Ma for the Avnik and Yayla metagranites, respectively, date this collision, and thus the termination of ore deposition in the Bitlis Massif.
This study presents petrological and geochemical characteristics of various rock types from the Nidar ophiolite complex (NOC), eastern Ladakh, India, which include dunites, orthopyroxenites, clinopyroxenites, gabbros, basalts, an-desites, rhyolites, and minor plagiogranite. The aim of this study was to understand the geological processes which control the genesis, and tectonic affinity of the NOC, and its correlation with other nearby ophiolite complexes. The mafic to acidic NOC rock types are subalkaline in nature and shows affinity towards the spinel-lherzolite depleted mantle source, with partial melting degree of 5 to 30 %. The ultramafic cumulates display overall depleted patterns with respect to the primitive mantle and positive anomalies of large ion lithophile elements (LILE: Rb, Ba, K, Pb, and Sr) and negative anomalies of Nb, La, and Eu compared to other trace elements. However, gabbros, basalts, rhyolites, and plagiogranite show fractionated multi-element patterns with LILE enrichment and depletion of high-field strength elements (HFSE: Nb, P, and Ti). These geochemical characteristics in addition to high LILE to HFSE ratios such as La/Nb (> 1.4), Ba/Nb (2.36 -109.68), Rb/Nb (0.30 - 45.12), and La/Nb (1.06 - 3.99) indicate their origin from a depleted mantle source, which was metasomatized by fluids from the subducting slab. From the geochemical characteristics, it is concluded that the NOC evolved in a subduc-tion zone environment, similar to other Neo-Tethyan ophiolites such as the Nagaland-Manipur (India), Sabzevar (Iran), and Dras-Thasgam-Suru-Shergol (western Ladakh) ophiolites.
The Sariseki mafic-clastic type VMS deposit is in the central parts of the Pontides, north of the Anatolian segment of the Alpine-Himalayan orogenic belt. It is hosted by a volcano-sedimentary sequence of submarine mafic dykes/ sills and deep marine sedimentary rocks, which represent a metallogenic province for massive sulfide deposits. The mineralization with a well-developed gossan zone in a peneplain topography was selected to investigate the soil geochemical characteristics of a mafic-clastic type VMS deposit in the Jurassic metallogenic belt. The applicability of the soil geochemistry was tested using different statistical methods and spatial anomaly patterns. The concentrations of 21 elements and relationships between them were investigated based on a total of 337 soil samples. Basic and traditional statistical methods were carried out to determine the relations among the selected elements mostly associated with the nature and type of the mineralization. Elemental threshold and anomaly values of Cu: 3.77 ppm, Zn: 2.62 ppm, Co: 1.47 ppm in log-based were examined using quantile-quantile plots and median absolute deviation values. The anomalies of Cu, Zn and Co in sampled soils developed over the gossan profile are most probably indication of weathering processes through pre-existent outcropping ore zones. Enrichment of these elements can be associated with the Fe-oxides and hydroxides following oxidation processes. Cu-Zn-Co and Au falling into the same main cluster seem to be the best indicators and pathfinder elements over the deposit, relatively close to the surface, located in the Central Pontides. Similar distribution patterns of Cu-Zn-Co and Au in the soil sampling area are also compatible with the mineralization. Structural elements i.e. thrusts and relatively younger faults control the limits of the mineralization and the distribution of the gossan zones, therefore, physical dispersions are clearly associated with the faults. The generated anomaly maps of Cu-Zn-Co show that there may be another mineralization or that the existing ore zone may continue towards the southwest of the known mineralization zone or downslope effect is present through the anomalies in the residual soils. Manganese, Sr, Li, Ba, Ag, Cr, Ni, Pb and V, on the other hand, are not anomalous in the sampled soils and these are not suitable to be pathfinders. This soil geochemistry study becomes an exploration guide for potential mineralization for mafic-clastic/pelitic-mafic/Besshi type VMS that show similar soil geochemistry characteristics.
The Pontides are located to the north of the Anatolian segment of the Alpine-Himalayan orogeny. The Central Pontides are distinguished from the Western and Eastern Pontides with a large turbiditic fan in the north, and an area composed of thrust-accretion complex which is defined as Central Pontide Structural Complex (CPSC) hosting volcanogenic massive sulfide (VMS) deposits in the south. VMS deposits discovered in the CPSC recently have been classified into three types: Mafic-type, mafic-siliciclastic type and bimodal-mafic type. The Buyuk Hill deposit is a Cu-Zn-dominated VMS with a well-developed gossan zone, and has ores showing semi-massive to massive textures constituting of clastic sulfide minerals. The ore mineral paragenesis is predominantly composed of pyrite, chalcopyrite and lesser amounts of sphalerite and magnetite. The volcanic units in a wide region comprise of dacite (69 -73.3 SiO2 wt%) hosting the mineralization, and basaltic andesite, both of which have undergone metamorphism at greenschist facies conditions. The zircon U-Pb geochronology of the dacite (Th/U: 0.2 - 0.6; U/Yb: 0.1- 0.2) indicates presence of an oceanic island arc during the Early Jurassic (182 & PLUSMN; 8 Ma). The structure-texture and geochemical data of the Buyuk Hill deposit, together with the petrogenetic properties of the wall-rocks, imply that the mineralization is a bimodal-mafic type VMS. Correlation of the Buyuk Hill deposit with other VMS deposits discovered in the Central Pontides suggest temporal and spatial association within an Early Jurassic arc-back arc rift system developed over the same oceanic crust.
Adakitic rocks in the Alpine-Himalayan belt are indicative for the geodynamic evolution of collision zones and often associated with mineralization. This study reports the presence of Quaternary low-SiO2 andesitic to trachyandesitic (Mg# 51- 56) adakites of medium-K calc-alkaline composition that are coeval with normal calc-alkaline rocks of northwestern Iran. The adakitic rocks are characterized by high Sr (998 -1250 ppm) and low Y (6.5 - 8.4 ppm), whereas the normal calc-alkaline rocks are low in Sr (296 - 530 ppm) and high in Y (14.3 - 22.6 ppm). The adakitic samples have an average of similar to 56 wt.% SiO2, similar to 3 wt.% MgO, and similar to 12 wt.% CaO+Na2O, indicating that they fall into the low-SiO2 adakite group. The steep rare earth elements patterns with chondrite-normalized (La/Yb)n of 35 - 46 and absent to minor positive Eu anomalies, low contents of Y, Nb, and Ti, as well as high ratios of Sr/Y indicate the presence of garnet +/- amphibole +/- Fe-Ti oxides and the absence of plagioclase in the melt residue. Although these rocks belong to a post-collisional environment, they have inherited geochemical signatures of a subduction zone. The plausible scenario for the simultaneous occurrence of adakitic and non-adakitic magmas in northwestern Iran is partial melting at different depths within the mantle wedge that was metasomatized by slab-derived melts. The important drivers of post-collisional magmatism were probably slab break-off and/ or sub-lithospheric convection. Semi-parallel active strike-slip faults most likely played a key role in the Plio-Quaternary magmatism by facilitating crustal magma ascent. Despite known Cu-Mo-Au mineralizations in the form of porphyry and epithermal deposits in connection with Oligocene-Miocene sub-volcanic stocks and dykes that share the adakite-like and high-K calc-alkaline compositions with the Plio-Quaternary rocks in the study area, no related mineralization has been observed.
